An overview of AB₂O₄- and A₂BO₄-structured negative electrodes for advanced Li-ion batteries

Abstract

Energy-storage devices are state-of-the-art devices with many potential technical and domestic applications. Conventionally used batteries do not meet the requirements of electric or plug-in hybrid-electric vehicles due to their insufficient energy and power densities. Graphite is used for the conventional anodes of Li-ion batteries. However, the specific capacity (372 mA h g⁻¹) of a graphite electrode is not sufficient for high-power applications. Therefore, Co-, Ni-, Mn-, and Zn-based simple oxides have been investigated as anode components due to their high specific capacities (500–1000 mA h g⁻¹). Among these, Co-based anodes have demonstrated the best electrochemical performances; however, Co's high cost and toxicity limit its use as an ideal anode component. Recently, mixed-metal oxides with AB₂O₄ (A = Cu, Co, Ni, Mn, and Zn; B = Co, Mn, and Fe) and A₂BO₄ (A = Co, Mn, and Fe; B = Sn, Si, Ti, and Ge) structures have received much interest, and their electrochemical performances have been extensively studied. This type of mixed-metal oxide affords the following main advantages: they store charge through conversion as well as alloying–dealloying processes, and they exhibit higher electronic conductivities than that obtained with simple metal oxides. The above points indicate the importance of AB₂O₄- and A₂BO₄-structured materials. The present review emphasizes the recent literature on the electrochemical performance of AB₂O₄- and A₂BO₄-structured materials and their composites and feasible ways to implement these materials in Li-ion batteries in the near future.

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1. Introduction

In recent years, the increasing usage of fossil fuels in transport vehicles and the refinery industry has produced more toxic gases such as CO, CO₂, and hydrocarbons, which cause severe health problems in human beings, including pneumonia and the blocking of oxygen from the brain, heart, and other vital organs by CO. Importantly, the continued release of CO₂ increases global warming, leading to severe climate change.^1–3 In order to reduce the emissions of greenhouse gases, the exploitation of green-energy sources, including solar and wind energy, instead of fossil fuels is necessary in the present scenario.

Hence, to reduce air pollution, people are focusing on green-energy sources such as solar and wind energy; they can be considered as important alternative energy sources for sustainable economic growth. However, solar and wind energy as well as electric cars require highly efficient energy-storage devices. In recent decades, batteries have been viewed as the most promising energy-storage devices, as they can store electrical energy as electrochemical energy. Batteries are classified into two types: primary and secondary batteries. A primary battery is used in its charged state once it converts its chemical energy into electrical energy; it is then discarded as it is not rechargeable. A secondary battery is electrically rechargeable after discharge. To date, the most investigated secondary batteries have been lead–acid, Ni–Cd, Ni–metal hydride, and Li-ion batteries (LIBs). Among these batteries (Table 1), LIBs have exhibited the best electrochemical performances in terms of long cycle life, low self-discharge, high cell voltage, and no memory effects, and their energy densities are two times and their power densities five times greater than those of current Pb–acid and Ni–Cd batteries.⁴ Hence, LIBs are widely used in different electronic devices, including laptops, mobile phones, and other such electronic devices.^5–7

Table 1 Comparison of lead–acid, Ni–Cd, Ni–metal hydride, and Li-ion batteries

Specifications	Lead–acid	Ni–Cd	Ni–MH	Li-ion batteries
				LiCoO2	LiMn2O4	LiFePO4
Cell voltage	2.1 V	1.2 V	1.2 V	3.7 V	3.7 V	3.3 V
Energy density (W h kg^−1)	30–50	45–80	60–120	150–190	100–135	90–120
Power density (W kg^−1)	180	150	250–1000	1800	1200–1400	1400
Cycle life (cycles)	400	500	500	>500	>500	>2000
Energy efficiency (%)	60	70	90	75	90	95
Self-discharge (%) per month	20	30	35	10	10	8
Memory effect	No	Yes	Little	No	No	No

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2. Construction and working principle of LIBs

The LIB is an energy-storage device that converts chemical energy into electrical energy through a controlled, thermodynamically favorable chemical reaction. Generally, such a battery consists of a number of interconnected cells. Each individual cell has a cathode, anode, and electrolyte. Both the cathode and anode are electrically isolated by the electrolyte and a separator in order to avoid a short circuit. The electrolyte is an ionically conductive medium, such as a polymeric/organic electrolyte, that facilitates Li ion transfer between the anode and cathode. Currently, such batteries consist of a layered structure of LiCoO₂ and graphite, which are used as the cathode and anode, respectively.

In rechargeable batteries, the storage mechanism involves a reversible insertion–extraction of Li ions into and out of the electrode material during the charge–discharge process, based on the rocking-chair concept. While charging (which involves the loss of electrons and Li ions), Li ions are deintercalated from the cathode and intercalated into the anode. In the discharge process, Li ions are deintercalated from the anode and intercalated into the cathode, delivering the energy to an external circuit. This discharge process continues until the potential difference between the two electrodes becomes too small at which point the cell is fully discharged. Fig. 1 depicts a schematic diagram of their function, and the reversible-reaction mechanism of LIBs is described as follows,^7–9

Anode: 6C + xLi⁺ + xe⁻ ↔ Li_xC₆, (1)

Cathode: LiCoO₂ ↔ Li_1−xCoO₂ + xLi⁺ + xe⁻. (2)

Fig. 1 Charge–discharge process of a Li-ion battery (LIB).

Currently, commercially used cathode materials include layer structured LiCoO₂ (140–160 mA h g⁻¹); olivine-type LiFePO₄ (140–160 mA h g⁻¹); and spinel-type LiMn₂O₄ (100–120 mA h g⁻¹), and their practical energy densities versus a graphite anode are 584, 398, and 424 W h kg⁻¹, respectively. On the other hand, while using combinations of Li₄Ti₅O₁₂ and LiMn₂O₄ the obtained energy density is 200 W h kg⁻¹, which is half of the reported value when using graphite as the anode, since the specific capacity of spinel Li₄Ti₅O₁₂ is 175 mA h g⁻¹.¹⁰ The currently used graphite anode has several fascinating properties such as a low working potential versus Li, long cycle life, and low cost. However, its primary drawback is its low reversible capacity, as the diffusion rate of Li is in between 10⁻⁹ and 10⁻⁶ cm² s⁻¹, resulting in the low power density of this type of battery.¹¹ Therefore, to further improve the power density of this component, it is mandatory to adopt new strategies and to identify novel materials to replace the graphite anode. Another well-known anode material is Li metal, which possesses a high specific capacity of 3860 mA h g⁻¹ and low working potential. However, the working potential of Li metal exists outside the window of the electrolyte, so the transfer of Li ions into the electrode material is achieved through a solid-electrolyte-interphase (SEI) passivation layer that is formed by way of an electrolyte-decomposition process. However, for a fast charge–discharge process, a larger number of Li ions build up on the surface of the Li metal; moreover, changes in the electrode volume break the SEI layer, forming Li plating that leads to dendrite formation. On subsequent cycles, Li dendrites grow across the electrolyte, causing these batteries to short circuit.^8,12 Therefore, based on safety concerns, Li metal is highly restricted to the application of anodes in LIBs. The most important issue is that commercially used LIBs do not have sufficient energy and power densities for implementation in hybrid-electric and plug-in hybrid-electric vehicles. Generally, the energy and power densities are directly proportional to the cell voltage and specific capacity of a cell (energy density = VIt (W h kg⁻¹)). Hence, to achieve high-power- and high-energy-density LIBs, appropriate anode and cathode materials with high specific capacities, cell voltages, and Li-diffusion coefficients are needed. After the pioneering work of Poizot et al., transition-metal-oxide (MO, M = Co, Ni, Cu, Fe) nanoparticles have been explored as potential anodes for LIBs. They demonstrated an excellent electrochemical property of 700 mA h g⁻¹ with 100% capacity retention after 100 cycles at high charging rates and have suggested that the high electrochemical reactivities of transition-metal oxides might lead to the improved performances of such batteries.¹³ The electrochemical reactions of transition-metal oxides are

MO + 2Li⁺ + 2e⁻ ↔ Li₂O + M (M = Co, Ni, Fe, and Mn) (3)

Here, the redox reactions between metal oxides and Li ions are thermodynamically more favorable and involve multiple electron transfers per metal atom, leading to high specific capacities in the range of 500 to 1000 mA h g⁻¹.^14,15 Even though simple metal oxides have shown high reversible capacities and energy densities, they suffer from low coulombic efficiencies in the first cycle, unstable SEI-layer formation, large potential hysteresis, and poor capacity retention.¹⁶

Hence, these drawbacks can be overcome by adopting mixed-metal oxides such as ZnFe₂O₄, NiCo₂O₄, Co₂GeO₄, and MnCo₂O₄ as possible anodes for LIBs due to their high electronic conductivities, fast ionic-transport rates, and easy methods of synthesis. Their different architectures and crystallinity controls may prove these mixed-metal oxides to be good electrochemical-energy-storage materials for LIBs. Recently, Wei et al. demonstrated that complex binary- and ternary-metal oxides possess better electronic conductivities compared to simple metal oxides, and they demonstrated that NiCo₂O₄ has a larger electrochemical activity and an electronic conductivity two orders of magnitude greater than those of simple Ni and Co oxides.¹⁷ These kind of mixed metal oxides has special features like complex chemical composition, interfacial and synergistic effects leads to enhanced electrochemical performance and electronic conductivity.^18,19 So in order to further emphasize the importance of this type of binary spinel compounds, an attempt is made here to review the electrochemical performance of the mixed metal oxides especially AB₂O₄ and A₂BO₄ structures on the application of Li-ion anode materials.

3. AB₂O₄ structure

The general formula of the spinel structure (AB₂O₄) is [A²⁺][B₂³⁺][O₄²⁻], where A is a divalent cation and B is a trivalent cation. This structure consists of a closely packed array of 32 oxygen ions that form 64 tetrahedral cations and 32 octahedral cations in a single unit cell. The spinel structure can be classified into three types: normal, inverse, and mixed spinel based on the cation occupancy.^20,21 In a normal spinel, the A cations preferentially occupy tetrahedral sites, and the B cations occupy octahedral sites. Examples of such normal spinels include MgAl₂O₄, ZnFe₂O₄, and CdFe₂O₄. In the inverse-spinel structure, the octahedral sites are occupied by the A cations and half of the B cations, while the other half of the B cations occupy the tetrahedral sites. It is denoted as [B³⁺][A²⁺B³⁺]O₄. CoFe₂O₄ and NiFe₂O₄ are examples of the inverse-spinel structure.^22,23 In a mixed spinel, both A and B cations occupy both octahedral and tetrahedral sites, and the molecular formula is [M_x²⁺Fe_1−x³⁺][M_1−x²⁺Fe_1+x³⁺]O₄. Examples of mixed spinels include NiCo₂O₄, MnCo₂O₄, and CoMn₂O₄.

There are three different possible charge-storage mechanisms involved in anode materials: intercalation–deintercalation, alloying–de-alloying, and conversion reactions.^24–26

(1) Normally, intercalation–deintercalation mechanisms occur in carbon-based materials such as carbon nanotubes (CNTs), graphene, porous carbon, TiO₂, and Li₄Ti₅O₁₂.

MO_z + xLi⁺ + xe⁻ ↔ Li_xMO_z (M = Ti and W) (4)

(2) Similarly, the alloying–de-alloying mechanism occurs with Si, Ge, Sn, Bi, SnO₂, SiO₂, GeO₂, and so on.

MO_x + 2xLi⁺ + 2xe⁻ → M + xLi₂O (5)

M + xLi⁺ + xe⁻ ↔ Li_xM (M = Si, Sn, Sb, and Ge) (6)

In the case of pure Zn, Ge, Sn and Si electrodes, the charges are stored through alloying/de-alloying mechanism by forming Li_xZn, Li_xGe, Li_xSn and Li_xSi, respectively, according to eqn (6). On the other hand, the metal oxides including ZnO, GeO₂ and SnO₂ based electrodes involving both conversion and alloying/de-alloying mechanism, according to eqn ((5) and (6)). This different charge storage mechanism mainly arises from the oxygen molecules.

(3) The conversion-reaction mechanism is applicable to transition-metal oxides such as MnO₂, Mn₂O₃, NiO, Fe₂O₃, Fe₃O₄, Co₃O₄, CoO, and CuO.

M_zO_x + 2xLi⁺ + 2xe⁻ ↔ zM + xLi₂O (M = Co, Ni and Fe) (7)

The pure Co, Fe, Mn and Ni elements are electrochemically inactive. But, it involved in the conversion reaction, while in the form of metal oxides. So the main contribution arises from the decomposition and reformation of Li₂O amorphous matrix according to eqn (7).

Generally, the mixed metal oxides store the lithium ions through both conversion reaction as well as alloying/de-alloying mechanism.^27–29 In M₂M′O₄ (M = Co, Fe, Mn and Ni; M′ = Sn, Ge and Si) system, during first discharging process the active material disintegrated into its constituent elements (M and M′) followed by the formation of Li₂O amorphous matrix and electrolyte decomposition at 0.6 to 0.7 V vs. Li/Li⁺.³⁰ Subsequently, the M′ reacts with lithium to form Li_xM′ (Li-alloying formation) at 0.1 to 0.3 V vs. Li/Li⁺.³⁰ During charging process, de-alloying reaction takes place at 0.5 to 0.8 V vs. Li/Li⁺, and yielded individual metals (M and M′). Subsequently, this metal particles act as catalyst to decompose the Li₂O amorphous matrix and formed a corresponding oxides (MO_x and M′O_x), according to conversion reaction mechanism.¹³ In the case of MM₂′O₄ (M = Ni, Mn, Co and Fe; M′ = Ni, Mn, Co and Fe), the elements M and M′ are involved in conversion reaction mechanism. During the first discharging process, the crystal structure destructed into individual metal particles accompanying with the formation of Li₂O matrix. As produced metal particles facilitate the electrochemical activity by means of formation/decomposition of Li₂O that give the way for conversion reaction mechanism.³¹

In the following sections, the main significance and electrochemical performances of these different types of structured spinel materials such as ferrites (AFe₂O₄), cobaltites (ACo₂O₄), manganites (AMn₂O₄) (where A = Mg, Zn, Cu, Mn, Ni, and Co), and A₂BO₄ (where A = Co, Zn, Mn, and B = Sn, Ge, and Si)-structured materials are discussed.

3.1. AFe₂O₄ (A = Mg, Zn, Cu, Mn, Ni, and Co)

Generally, AFe₂O₄-based spinel ferrites possess good electrical and magneto-optical properties, according to the A- and B-site cation distribution, and they have been widely investigated for applications in different fields, including the electronics industry, magnetic recording, magnetic-resonance imaging, and ferrofluids.^32–34 Ferrite-based materials are abundant, cheap, and environmentally friendly. The spinel-ferrite MFe₂O₄ (M = Mg, Zn, Cu, Mn, Ni, and Co)-based materials exhibit significant discharge capacities of 1000 mA h g⁻¹, which is two to three times greater than that of commercially used graphite anodes.³⁵

3.1.1. MgFe₂O₄. MgFe₂O₄ has a mixed-spinel structure, and its formula is (Mg_1−xFe_x)[Mg_xFe_2−x]O₄, where x is the inversion parameter and denotes the fraction of B cations in the tetrahedral sites. It consists of a face-centered close-packed oxygen sublattice in which the tetrahedral sites are filled by Mg²⁺ ions and a fraction of the Fe³⁺ ions and the octahedral sites are filled by the Fe³⁺ ions and a fraction of the Mg²⁺ ions.³⁶ Pan et al. prepared MgFe₂O₄ nanoparticles via a sol–gel method and reported a discharge capacity of 474 mA h g⁻¹ at 90 mA g⁻¹ over 50 cycles. Sadly, it delivered only 293 mA h g⁻¹ after 50 cycles when the current density was increased to 900 mA g⁻¹.³⁷ On the other hand, Gong et al. prepared carbon-coated MgFe₂O₄ nanoparticles (Fig. 2(a and b)) for which the thickness of each carbon layer was 4 nm.^37​ According to the electrochemical reaction (eqn (8) and (9)), MgO does not react with Li, preventing the aggregation of Fe oxides during the charge–discharge process and also acting as a buffer matrix during the insertion–deinsertion process,^37,38

Li_0.56MgFe₂O₄ + 5.44Li⁺ + 5.44e⁻ → MgO + 2Fe + 3Li₂O, (8)

2Fe + 3Li₂O → Fe₂O₃ + 6Li⁺ + 6e⁻. (9)

Fig. 2 (a–c) Carbon-coated MgFe₂O₄ (MFO/C-600) nanoparticles and their rate-capability curves compared with Fe₃O₄ (FO/C-600) _​(reprinted with permission from ref. 38. Copyright 2013 Elsevier).

The carbon-coated MgFe₂O₄ electrode demonstrated a high specific capacity of 466 mA h g⁻¹ at a high current density of 1600 mA g⁻¹ and a greater rate capability than carbon-coated Fe₃O₄ (Fig. 2(c)). This high rate capability as well as its good cycling stability was achieved through its smaller grain size, MgO matrix, and carbon coating. These factors facilitated a short diffusion-path length for the Li⁺ ions, preventing particle agglomeration and enhancing the electrical conductivity. The carbon coating not only enhanced the electrical conductivity of the material but also accommodated the volume expansion during the charge–discharge process. It is well known that utilizing graphene composites is an important strategy to improve the electrical conductivity and to prevent volume expansion during the insertion–deinsertion process, and it can provide a large number of accessible active sites for Li⁺ ion insertion and a short diffusion-path length for both Li⁺ ions and electrons due to the synergistic effects between the active material and graphene. The prepared MgFe₂O₄–graphene-nanocomposite electrode showed a high reversible discharge capacity of 764.4 mA h g⁻¹ at 0.04C over 60 cycles and retained a capacity of 219.9 mA h g⁻¹ at a high current rate of 4.2C.³⁹ The carbon-coated and graphene–composite MgFe₂O₄ particles demonstrated better electrochemical performances than did the regular MgFe₂O₄ nanoparticles.

3.1.2. ZnFe₂O₄. ZnFe₂O₄ has a normal-spinel structure in which the Zn²⁺ cations occupy the tetrahedral sites and the Fe³⁺ cations occupy the octahedral sites.^40,41 ZnFe₂O₄ is considered to be a good candidate for anode materials, given its high specific capacity around 1000 mA h g⁻¹. The charge-storage mechanism of ZnFe₂O₄ results primarily from the alloying–de-alloying- and conversion-reaction mechanisms. Sharma and coworkers synthesized ZnFe₂O₄ using a combustion method, examined the charge-storage properties, and explained the charge-storage mechanism through TEM images and selected area electron diffraction (SAED) patterns, given the following reactions,⁴²

ZnFe₂O₄ + 0.2Li⁺ + 0.2e⁻ → Li_0.2ZnFe₂O₄ (10)

Li_0.2ZnFe₂O₄ + 7.8Li⁺ + 7.8e⁻ → Zn + 2Fe + 4Li₂O (11)

Zn + Li⁺ + e⁻ ↔ LiZn (12)

Zn + Li₂O ↔ ZnO + 2Li⁺ + 2e⁻ (13)

2Fe + 2Li₂O ↔ 2FeO + 4Li⁺ + 4e⁻ (14)

During the first discharge process, i.e., from OCV to 0.8 V, 0.2 mol of Li ions intercalated into the crystal structure, according to eqn (10). Below the potential of 0.8 V, intercalation of Li ions led to the deterioration of the crystal structure of ZnFe₂O₄ into LiZn, ZnO, FeO, and Li₂O matrices in which Li₂O was involved in the reversible reaction. Generally, the first cycle was considered to be an irreversible reaction, and it possessed a high irreversible capacity due to the formation of the SEI layer, the side reaction of the electrolyte with the active material layer, and the amorphous-Li₂O formation. In the second cycle, the ZnFe₂O₄ nanoparticles possessed a discharge capacity of 800 mA h g⁻¹ at 60 mA g⁻¹, which decreased to 615 mA h g⁻¹ after 50 cycles due to the volume expansion of the active material, which in turn was due to the alloying–de-alloying- and conversion-reaction mechanisms. To overcome this problem, Teh and coworkers⁴³ prepared one-dimensional (1D) ZnFe₂O₄ nanofibers that exhibited a high specific capacity of 925 mA h g⁻¹ in the first cycle at 60 mA g⁻¹ and 733 mA h g⁻¹ after 30 cycles. Consequently, they had a capacity of 400 mA h g⁻¹ at 800 mA g⁻¹, a value that was high compared with that of the commercially used graphite anode. It has been reported that this enhanced capacity arises mainly from this morphology of ZnFe₂O₄ as the ZnFe₂O₄ nanofibers are well connected with nanoparticles and they act as electronic wiring during the charge–discharge process.⁴³ Hence, tuning the morphology and size of an electrode material effectively enhances its specific capacity and rate capability. Various researchers have proposed further increasing the rate capability of the ZnFe₂O₄ material [Table 2] with different carbonaceous-material composites, morphological features, and binder effects.^44–57 Mesoporous, ZnFe₂O₄ carbon-composite microspheres demonstrated good electrochemical performance as well as a high reversible capacity of 1100 mA h g⁻¹ at a current density of 0.05 A g⁻¹ over 100 cycles.⁵² The carbon composite not only enhanced their specific capacity but also facilitated a superior rate capability that provided a discharge capacity of 600 mA h g⁻¹ at a high current density of 1 A g⁻¹. Recently, Yao and coworkers prepared a mesoporous ZnFe₂O₄–graphene composite, which exhibited excellent electrochemical performance due to the synergistic effects between the large surface areas of the ZnFe₂O₄ nanoparticles and the excellently flexible and highly electronically conductive graphene.⁵⁴ The mesoporous ZnFe₂O₄–graphene composite had a high specific capacity of 870 mA h g⁻¹ at 1 A g⁻¹ after 100 cycles and demonstrated a good rate capability. Nonetheless, it retained a specific capacity of 713 mA h g⁻¹ at 2 A g⁻¹, indicating the ultrafast charging capability and good cycling stability of this mesoporous ZnFe₂O₄–graphene composite.⁵⁴ Table 2 lists the overall electrochemical performances of ZnFe₂O₄ and its composites, synthesized by various techniques.^42–57

Table 2 A summary of recent electrochemical studies on Zn₂SnO₄ based anodes for LIBs

Sample	Experimental methods	Specific capacity	Rate capability	Ref.
ZnFe2O4	Combustion synthesis	615 mA h g^−1 at 60 mA g^−1, 50 cycles	—	42
ZnFe2O4 nanofibers	Electrospinning technique	733 mA h g^−1 at 60 mA g^−1, 30 cycles	400 mA h g^−1 at 800 mA g^−1	43
ZnFe2O4/graphene hybrid	Solvothermal route	600 mA h g^−1 at 200 mA g^−1, 90 cycles	400 mA h g^−1 at 800 mA g^−1	44
MWCNT–ZnFe2O4 nanocomposites	Solvothermal route	1152 mA h g^−1 at 60 mA g^−1, 50 cycles	580 mA h g^−1 at 600 mA g^−1	45
ZnFe2O4-nanorod	Template assisted method at 700 °C	888 mA h g^−1 at 150 mA g^−1, 80 cycles	625 mA h g^−1 at 1000 mA g^−1	46
ZnFe2O4/C hollow sphere	Solvothermal route	625 mA h g^−1 at 500 mA g^−1, 30 cycles	450 mA h g^−1 at 700 mA g^−1	47
ZnFe2O4 nano-octahedrons	Hydrothermal synthesis	910 mA h g^−1 at 60 mA g^−1, 80 cycles	730 mA h g^−1 at 1000 mA g^−1	48
ZnFe2O4–carbon conductive network	Carbonothermal method using sucrose	1100 mA h g^−1 at 50 mA g^−1, 60 cycles	—	49
Yolk–shell ZnFe2O4	Spray-drying process	862 mA h g^−1 at 500 mA g^−1, 200 cycles	—	50
ZnFe2O4/graphene nanoparticles	Hydrothermal method	956 mA h g^−1 at 100 mA g^−1, 50 cycles	600 mA h g^−1 at 1000 mA g^−1	51
Mesoporous ZnFe2O4/C composite microspheres	Hydrothermal method using sucrose	1150 mA h g^−1 at 50 mA g^−1, 100 cycles	600 mA h g^−1 at 1100 mA g^−1	52
Carbon coated ZnFe2O4 nanoparticles	Carbonothermal method using sucrose	1300 mA h g^−1 at 40 mA g^−1, 100 cycles	525 mA h g^−1 at 3890 mA g^−1	53
Mesoporous ZnFe2O4/graphene composite	Ambient pressure method	1259 mA h g^−1 at 100 mA g^−1, 30 cycles	726 mA h g^−1 at 2000 mA g^−1	54
ZnFe2O4@C/graphene nanocomposite	Hydrothermal method followed by carbon coating and graphene composite process	705 mA h g^−1 at 232 mA g^−1, 180 cycles	403 mA h g^−1 at 5C	55
ZnFe2O4/graphene hybrid films	One-step electrophoretic deposition subsequent thermal annealing	881 mA h g^−1 at 200 mA g^−1, 200 cycles	510 mA h g^−1 at 3200 mA g^−1	56
ZnFe2O4–C nanocomposite	Sol–gel followed by ball milling technique	681 mA h g^−1 at 0.1C, 100 cycles	469 mA h g^−1 at 4C	57

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Another method to improve specific capacity is by forming mesoporous microspheres, since they form from well-interconnected nanoparticles. Therefore, they increase the interfacial surface reactions with Li and shorten the diffusion path for Li ions and electrons, leading to high specific capacities. The interconnected nanoparticles create these mesopores in between neighboring particles, thereby accommodating the volume expansion during the charge–discharge process.⁵² Furthermore, the interconnected nanoparticles embedded in the carbon matrix also enhanced the intrinsic electrical conductivity and thus speeded the transport of Li⁺ ions and electrons. Therefore, this type of mesoporous-carbon-composite structure facilitates high reversible specific capacities, good cycling stabilities, and good rate capabilities of electrodes.

In addition, other than carbon composites and mesoporous structures, a binder can also effectively alleviate the volume-expansion problem. Bresser et al. used sodium carboxymethyl cellulose (NaCMC) as a binder for carbon-coated ZnFe₂O₄ nanoparticles, and Fig. 3 depicts its corresponding cycling-stability curve.⁵³ This binder promoted superior electrochemical performance as a result of the stable reversible capacity (1300 mA h g⁻¹) at 0.02 A g⁻¹ over 100 cycles and 525 mA h g⁻¹ at a high current density of 3.89 A g⁻¹. The main advantages of the CMC binder are that it is nontoxic, eco-friendly, and water soluble; therefore, it provides better contact between the active material and the current collector and modifies the SEI formation due to the continuous reaction of CMC with organic electrolytes, enhancing the cycling stability and high rate capability.

Fig. 3 Rate capabilities of carbon-coated ZnFe₂O₄ nanoparticles using different binders (circles: ZnFe₂O₄–C, triangles: ZnFe₂O₄–CMC, and stars: ZnFe₂O₄–PVDF-HFP) (reprinted with permission from ref. 53. Copyright 2013, Wiley-VCH Verlag GmbH & Co. KGaA).

3.1.3. CuFe₂O₄. CuFe₂O₄ possesses an inverse-spinel structure, and it has been widely investigated as an anode material due to its low cost, high abundance, environmental friendliness, and high theoretical specific capacity of 895 mA h g⁻¹. A number of factors limit the implementation of CuFe₂O₄ as an anode material, including its low inherent electronic conductivity and volume expansion during lithium insertion and extraction process. To overcome this problem down the particle size from bulk to nanolevel and making composite with the carbonaceous material is required.⁵⁸ The spinel-CuFe₂O₄ structure undergoes a conversion reaction during the charge–discharge process as follows,^59–62

CuFe₂O₄ + 8Li⁺ + 8e⁻ → 4Li₂O + Cu + 2Fe (15)

Cu + 2Fe + 4Li₂O ↔ CuO + Fe₂O₃ + 8Li⁺ + 8e⁻ (16)

Ding et al. prepared the CuFe₂O₄ nanoparticles which possess a high specific capacity of 869 mA h g⁻¹ in the second cycle; further increasing the number of cycles decreases the capacity to 551 mA h g⁻¹.⁵⁸ Normally, the nanoparticles have high surface energies, which often result in self-aggregation and lead to less surface area overall as well as the easy diffusion of the electrolyte ions, in turn leading to severe capacity fading. Significantly, Jin et al. reported the preparation of hollow CuFe₂O₄@C nanospheres and CuFe₂O₄@C nanoparticles via a polymer-templated hydrothermal method, followed by calcination to create the carbon shells.⁶⁰ Among these compounds, hollow, carbon-coated CuFe₂O₄ nanospheres demonstrated a good cyclic performance of 550 mA h g⁻¹ at 100 mA g⁻¹ over 70 cycles, whereas hollow CuFe₂O₄ only showed 120 mA h g⁻¹ and CuFe₂O₄@C nanoparticles only showed 300 mA h g⁻¹. The carbon layer and hollow space influenced the electrochemical performance of hollow CuFe₂O₄@C, effectively accommodating the volumetric changes during the cycling process, mitigating the agglomeration between the particles, and facilitating the electron transport that enhanced the specific capacity and good rate performance of the electrode material.⁶⁰

3.1.4. MnFe₂O₄. The spinel manganese ferrite is a promising candidate for several applications, including semiconductors, biosensors, photocatalysts, supercapacitors, and medical applications.^63–67 However, only a sparse number of reports are available concerning MnFe₂O₄ with regards to its application to anode materials in LIBs.^68,69 MnFe₂O₄ stores Li ions through a conversion reaction (MnFe₂O₄ + 8Li⁺ + 8e⁻ ↔ Mn + 2Fe + 4Li₂O), and it exhibits a high theoretical specific capacity of 928 mA h g⁻¹. Zhang et al. synthesized mesoporous manganese ferrite microspheres using a solvothermal method and examined their electrochemical performance versus Li metal. These microspheres showed a high reversible discharge capacity of 712 mA h g⁻¹ at a rate of 0.2C over 50 cycles. Only feeble capacity fading was observed over each cycle, and they retained a capacity of 552 mA h g⁻¹ even at a high current density at a rate of 0.8C.⁶⁸ This reported capacity fading was an acceptable limit and was achieved without fabricating any sort of composite with carbon, CNTs, or graphene. As explained above, the porous nature of these microspheres could accommodate the volume changes during the conversion reaction. The appropriate grain size with crystallinity might improve the surface area of the active material, and the electronic conductivity of the electrode material thus could lead to both an improved specific capacity and cycling stability.^69–71 Furthermore, Xiao et al. prepared MnFe₂O₄–graphene nanocomposites via an ultrasonication process to improve their electrochemical properties.⁷² A graphene composite has several advantages such as its enhanced inherent conductivity and high surface area (2600 m² g⁻¹), and it stores Li⁺ ions through an insertion–de-insertion process, thereby demonstrating its high theoretical specific capacity of 744 mA h g⁻¹ at the same time that it can accommodate volume expansion.⁷³ The MnFe₂O₄–graphene composite possessed a high specific capacity of 1017 mA h g⁻¹ at 100 mA g⁻¹ over 90 cycles and 767 mA h g⁻¹ at 1000 mA g⁻¹. It also showed an excellent rate capability.⁷²

3.1.5. NiFe₂O₄. NiFe₂O₄ is also considered a promising anode material for LIBs, since it can accommodate 8 mol of Li per formula unit; therefore, it has a high theoretical specific capacity of 914 mA h g⁻¹. It stores Li⁺ ions through an electrochemical-conversion reaction as follows,^74–77

NiFe₂O₄ + 8Li⁺ + 8e⁻ → Ni + 2Fe + 4Li₂O (17)

Ni + Li₂O ↔ NiO + 2Li⁺ + 2e⁻ (18)

2Fe + 3Li₂O ↔ Fe₂O₃ + 6Li⁺ + 6e⁻ (19)

However, there are numerous difficulties in implementing NiFe₂O₄ as an anode material because of the low diffusion of Li ions in the bulk material, low conductivity, and large volume expansion during the charge–discharge process.^74–76 Several reports have proposed solutions to overcome this issue so as to synthesize nanosized particles with porous structures and conducting carbon networks to improve the electrochemical performance of NiFe₂O₄.^77–79 Macroporous NiFe₂O₄ particles were prepared via a sol–gel method using a citrate precursor. The sample, calcined at 800 °C, showed better cycling stability and a better rate capability due to its smaller particle sizes with larger pore sizes, as compared to a sample calcined at 1000 °C, and it maintained a capacity of 600 mA h g⁻¹ after 80 cycles at a rate of 1C.⁷⁶ Morphological features also play an important role in enhancing the electrochemical performance. Recently, NiFe₂O₄ nanofibers were synthesized via an electrospinning technique, and their electrochemical performance was compared with that of NiFe₂O₄ nanoparticles.⁷⁷ The cycling-stability curve indicated that the NiFe₂O₄ nanofibers exhibited a superior electrochemical performance as compared to the nanoparticles. The NiFe₂O₄ nanofibers showed only feeble capacity fading in the initial 15 cycles due to the structural rearrangement of the active material, as they were well connected with conductive carbon and had stable SEI formation. After 15 cycles, the capacity stabilized at 870 mA h g⁻¹ at 100 mA g⁻¹ for another 40 cycles. In the subsequent cycles, the capacity increased steeply to 1000 mA h g⁻¹ over 100 cycles. This reported specific capacity was greater than the theoretical specific capacity; it was attained through a kinetically activated electrolyte degradation.^80,81 Imperatively, binders also effectively contribute to the electrochemical performance. Many authors have reported using different binders such as polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), CMC, and sodium alginate for electrode preparation.^82–85 Ramesh and coworkers reported that they synthesized nanocrystalline NiFe₂O₄ via a sol–gel-combustion route and examined its electrochemical performances using PVDF and sodium alginate as binders. The cycling-stability curve clearly illustrated the severe capacity fading of the NiFe₂O₄ nanoparticles at a rate of 0.1C, which indicated the inadequate interfacial stability of the PVDF binder.⁸⁵ However, in the case of the NiFe₂O₄ nanoparticles with sodium alginate, the nanoparticles showed a stable discharge capacity of 880 mA h g⁻¹ over 30 cycles at a current density of 92 mA g⁻¹ without any degradation of the discharge capacity. Further studies of the rate performance of this electrode material were carried out using a sodium alginate binder at different current densities. The discharge-capacity values decreased to 740, 620, 504, 420, and 380 mA h g⁻¹ when the current density was increased to 1C, 2C, 5C, 10C, and 20C, respectively. The NiFe₂O₄ nanoparticles with sodium alginate had a discharge capacity of 380 mA h g⁻¹ at a high current density of 18 300 mA g⁻¹ at a rate of 20C, as shown in Fig. 4, and they demonstrated a good rate capability as compared with the ternary composite.⁸² The sodium alginate binder effectively improved the cycle life and exhibits good rate capability, compared to commercially used PVDF, as the PVDF could not accommodate any volume strain during the cycling process. On the other hand, sodium alginate contains carboxylic groups evenly distributed in the polymer chain that improve the transport of Li ions and increase the stablility of the SEI-layer formation. It also possesses polar molecules that improve the interfacial interaction of the polymer binder and facilitate the self-healing process during the charge–discharge process. Furthermore, it provided strong adhesion between the electrode layer and the Cu substrate.^84,86,87 The NiFe₂O₄ nanoparticles with sodium alginate binder showed superior electrochemical performance, as compared to previously reported NiFe₂O₄–graphene heteroarchitectures.⁸⁸

Fig. 4 Rate capabilities of the NiFe₂O₄ nanoparticles with sodium alginate binder at different current densities (reproduced from ref. 85. Copyright 2013, with permission from the Royal Society of Chemistry).

3.1.6. CoFe₂O₄. Cobalt ferrite is a thoroughly investigated anode material used for LIBs due its low cost, environmental friendliness, and high chemical stability. It also stores Li ions through a conversation reaction similar to that of NiFe₂O₄, and it possesses a high theoretical specific capacity of 914 mA h g⁻¹.^89–91 However, it has severe drawbacks such as a large volume change that induces the pulverization and aggregation of the active material and low conductivity that leads to poor cycling stability and a low rate capability of the CoFe₂O₄ particles.^92–94 To achieve better performance, CoFe₂O₄ nanoparticles have been synthesized with different morphologies, including hollow nanospheres, mesoporous nanospheres, and nanorods, and as composites with different conducting matrices.^46,95–98 In particular, Xiong et al. prepared different morphologies of CoFe₂O₄, such as irregular particles, microspheres, and flower-like microspheres (Fig. 5(a)), via a hydrothermal method using various concentrations of ascorbic acid. Among these morphologies, the flower-like microspheres had the largest surface areas of 51.0 m² g⁻¹ compared to the regular microsphere and the irregular particles. Similarly, the flower-like microspheres (Fig. 5(b)) had a high reversible specific capacity of 790 mA h g⁻¹ at a current density of 200 mA g⁻¹, and even when the current density increased to 1000 mA g⁻¹, they still maintained their high discharge capacity of 744 mA h g⁻¹.⁹⁹ The high reversible capacity and rate capability attained through good electronic conductive wetting of the active materials thus facilitated a rapid charge-transfer process during the insertion–deinsertion process.¹⁰⁰ Recently, Zhang et al. reported the superior electrochemical performance of mesoporous CoFe₂O₄ nanospheres with cross-linked CNTs.⁹⁶ The CNT-weight ratios varied between 10, 20, and 30 wt%. Among these values, the CoFe₂O₄ with 20 wt% CNTs is shown in Fig. 5(c) which possessed the highest specific capacity of 1137 mA h g⁻¹ after 10 charge–discharge cycles at a current density of 200 mA g⁻¹ and increase the current density to 1200 mA g⁻¹ which has stable specific capacity of 648 mA h g⁻¹ with a good cycling stability and rate capability (Fig. 5(d)), as compared to the CoFe₂O₄–graphene composite.^96,98 The SEM results of after charge–discharge cycles indicated that the formation of a small amount of SEI film on the active surfaces led to a fast conversion reaction and that the CNTs accommodated the volume changes during the charge–discharge process.¹⁰¹ On the other hand, Kumar et al. reported that porous CoFe₂O₄–rGO with an alginate binder showed an excellent rate performance (Fig. 5(e)) compared to both CNTs and graphene composite.⁹¹ Therefore, a novel binder and a conductive matrix both play essential roles in improving the cycling stability as well as the rate capability of an electrode material. As such, it can be concluded that MgFe₂O₄, CoFe₂O₄, and NiFe₂O₄ are not suitable anode materials. After the first lithiation process, MgFe₂O₄ converts into MgO, Fe, and a Li₂O matrix in which MgO does not participate in the electrochemical reaction, leading to a low specific capacity. So too, while CoFe₂O₄ has been considered as an important anode material, it is costly, and its high working potential (2.1 V vs. Li/Li⁺) reduces the cell voltage. Finally, NiFe₂O₄ is both highly expensive and toxic. Therefore, these inherent characteristics restrict the use of MgFe₂O₄, CoFe₂O₄, and NiFe₂O₄ in potential applications requiring high-energy-density LIBs. ZnFe₂O₄ and MnFe₂O₄ are environmentally friendly and have low working potentials, and their high specific capacities could pave the way for the construction of efficient high-energy-density batteries.

Fig. 5 (a and b) Schematic diagram of the reaction producing various morphologies of CoFe₂O₄ particles and rate capability curve, (reprinted with permission from ref. 99. Copyright 2014 Elsevier). (c and d) TEM image of CoFe₂O₄ with 20 wt% CNT and its rate capability curve, (reproduced from ref. 96. Copyright 2013, with permission from the Royal Society of Chemistry) and (e) rate capability curve of CoFe₂O₄ + 20 wt% graphene with a sodium alginate binder (reproduced from ref. 91. Copyright 2014, with permission from the Centre National de la Recherche Scientifique (CNRS) and the Royal Society of Chemistry).

3.2. AMn₂O₄ (A = Co, Ni, and Zn)

It is well known that Mn is environmentally benign, highly safe, low in cost, and abundant in nature compared to Co. With respect to anode materials, Mn-based oxides have lower operating voltages (∼1.5 V), reduce the source of Co in mixed-transition-metal oxides, and correspondingly increase the energy density as well as the output cell voltage of such anodes. Therefore, investigating the peculiar properties of manganite-based mixed-transition-metal oxides [AMn₂O₄ (A = Co, Ni, Zn)] for applications to LIBs is very necessary.^102–124 CoMn₂O₄ spinels have been synthesized with different morphologies, including hierarchical microspheres, double-shelled hollow microcubes, CNF@CoMn₂O₄ nanocables, nanorods, and nanofibers, in attempts to achieve stable capacities and good rate capabilities for electrode materials.^105–110 Significantly, Hu et al. prepared hierarchical CoMn₂O₄ microspheres, assembled with porous nanosheets, which possessed large pores between the neighbouring nanosheets [Fig. 6(a)], via a solvothermal method.¹⁰⁵ These microspheres exhibited a slight decrease of their specific capacity only a few cycles after that capacity reached 894 mA h g⁻¹, a value that was close to their theoretical specific capacity of 920 mA h g⁻¹, over 60 cycles with excellent cycling stability [Fig. 6(b)]. It has been reported that generally at low current densities, the electrolyte ions easily penetrated into the inner parts of the active material and utilized all the active sites, and over subsequent cycles, the observed increase in capacity resulted from the full conversion reaction of the active material. These hierarchical microspheres also had a high specific capacity of 559 mA h g⁻¹ at a rate of 4C [Fig. 6(c)], ensuring a good rate capability of the active material. It has been concluded that porous CoMn₂O₄ hierarchical microspheres provided a short diffusion length for both Li ions and electron transport within the electrolyte–electrode interface and accommodated the volume expansion and easy diffusion of the electrolyte into open pores, while the well-interconnected nanosheets enhanced the energy and power densities. Zhang et al. recently synthesized carbon-nanofiber–metal-oxide coaxial nanocables using two steps, a polyol method and subsequent thermal annealing.¹⁰⁷ The morphology of the CNF@CoMn₂O₄ nanocables and their cycling stability are shown in Fig. 6(d)–(f). The cycling-stability curve of these nanocables provided a high specific capacity of 870 mA h g⁻¹ at a current density of 200 mA g⁻¹ over 150 cycles, and they maintained a capacity of 650 mA h g⁻¹ even at a high current density of 1000 mA g⁻¹. These results confirmed the superior electrochemical performance of CNF@CoMn₂O₄, as the outer shells of the CoMn₂O₄ cables were strongly anchored to the carbon nanofibers and the inner cores of the CNFs could accommodate the volume change during the Li insertion–extraction process, providing good electrical conductivity and thus leading to a high capacity, long cycle life, and good rate performance.¹⁰⁷

Fig. 6 (a)–(c) Hierarchical microsphere of CoMn₂O₄ and its cycling stability and rate-capability curve, respectively (reprinted with permission from ref. 105. Copyright 2012, Nature Publishing Group). (d)–(f) HRSEM images of CNF@CoMn₂O₄ nanocables and their cycling-stability curve, respectively (reproduced from ref. 107. Copyright 2014, with permission from the Royal Society of Chemistry).

NiMn₂O₄ has also been considered as an attractive anode material for LIBs. Lee's group recently synthesized hierarchical tremella-like NiMn₂O₄/C nanostructures through a simple solvothermal method followed by a calcination process.¹¹¹ The tremella-like nanostructures demonstrated a discharge capacity of 2130 mA h g⁻¹ after 350 cycles at a current density of 1000 mA g⁻¹, a value that is greater than the theoretical specific capacity of NiMn₂O₄ (∼922 mA h g⁻¹), and they retained a high discharge capacity of 1773 mA h g⁻¹ at 4000 mA g⁻¹ after 1500 cycles. This high discharge capacity arose due to an interfacial-storage mechanism, which originated from the reversible formation–dissolution of an organic polymeric gel-like layer via electrolyte decomposition that induces the extra capacity in the electrode material by way of pseudocapacitive behavior. The small amount of carbon facilitated this high rate capability, prevented the destruction of the hierarchical structure, and provided for continuous electron transport.^111,112

Compared with CoMn₂O₄ and NiMn₂O₄, ZnMn₂O₄ has several attractive features,^113–124 including the fact that it is environmentally friendly; Zn and Mn are also more abundant and lower in cost compared to Ni and Co. In terms of applications, Zn (1.2 V) and Mn (1.5 V) exhibit lower oxidation potentials than does Co (2.2–2.4 V). The low oxidation potentials of Zn and Mn eventually increase the cell voltage and energy density of a battery. ZnMn₂O₄ has a high theoretical capacity of 1008 mA h g⁻¹ in the first lithiation process due to the conversion and alloying-reaction mechanisms,^113–116

ZnMn₂O₄ + 9Li⁺ + 9e⁻ → ZnLi + 2Mn + 4Li₂O (20)

ZnLi + 2Mn + 3Li₂O ↔ ZnO + 2MnO + 7Li⁺ + 7e⁻ (21)

However, in the second reversible reaction, only 7 mol of Li is contributed to the energy storage, and therefore, this compound has a specific capacity of 784 mA h g⁻¹. As such, its capacity is twice as large as that of a conventional graphite anode. Yang et al. synthesized nanocrystalline ZnMn₂O₄ (Fig. 7(a)) using a polymer-pyrolysis route and reported the first investigation of this compound for use as an anode material for LIBs.¹¹³ In the initial ten cycles, the discharge capacity decreased up to 20% due to the formation of an SEI film on the surface of the active material is shown in Fig. 7(b). However, after ten cycles, only 0.20% capacity fading was observed, and a stable capacity of 569 mA h g⁻¹ was maintained after 50 cycles. To implement ZnMn₂O₄ in LIB applications, its electrochemical performance was enhanced with different morphologies and sizes, using various synthetic techniques that are listed in Table 3. Courtel et al. synthesized ZnMn₂O₄ particles via a simple coprecipitation route and optimized their electrochemical performance by varying the different reaction conditions such as the sintering temperature, binder, and electrolyte.¹⁰² Among the sintered (400, 600, 800, and 1000 °C) samples, the one sintered at 800 °C showed a stable capacity of 620 mA h g⁻¹ over 70 cycles at a rate of 0.1C. The optimized sample was used as an anode material with different binding agents such as polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (NaCMC), lithium carboxymethyl cellulose (LiCMC), Xanthan Gum (XG), and poly-3,4-ethlenedioxythiophene-poly-styrenesulfonate (PEDOT/PSS or Baytron). Among these agents, CMC-based salts and Baytron delivered the most stable capacities. In contrast, the use of PVDF and XG resulted in severe capacity fading [Fig. 7(c)]. However, the Baytron binder, while highly conductive (10–500 mS cm⁻¹), is also expensive. The PVDF and XG binders could not accommodate the volume strain during the insertion–extraction process, so the electrical connections between the active material and current collector broke, leading to capacity fading. CMC-based salts alleviated the volume expansion during the cycling process, thus facilitating the improvement of the cycling stability. Li-based CMC resulted in a capacity of 690 mA h g⁻¹ over 70 cycles at a rate of 0.1C, which is greater than the capacity that resulted from the use of Na-based CMC as it only provided a specific capacity of 615 mA h g⁻¹ over 70 cycles. Furthermore, to analyze the effects of the electrolyte composition, a sample sintered at 800 °C and a LiCMC binder were used with different compositions of the electrolyte propylene carbonate (PC)–1 M lithium hexafluorophosphate (LiPF₆), ethylene carbonate (EC) : dimethyl carbonate (DMC) (1 : 1)–1 M LiPF₆, and ethylene carbonate (EC) : diethylene carbonate (DEC) (3 : 7)–1 M LiPF₆]. The specific capacity was increased for both EC : DMC (1 : 1) and EC : DEC (3 : 7) when the cycle number increased, but such a change did not occur with the PC-based electrolyte because PC does not contribute to a stable SEI layer and is also inactive for Li storage.¹¹⁷ The EC : DEC electrolyte-composition capacity increased faster, as compared to that of the EC : DMC, as the former compound retained a specific capacity of 800 mA h g⁻¹ after 100 cycles. This observation indicated that EC : DEC electrolyte promoted the faster formation of an SEI layer on the surface of the active material. Finally, the optimized sintered sample, LiCMC binder, and EC : DEC (3 : 7)–1 M LiPF₆ (electrolyte) were used to fully fabricate a cell.

Fig. 7 (a)–(b) ZnMn₂O₄ nanoparticles synthesized via polymer pyrolysis route and its cycling stability, (reprinted with permission from ref. 113. Copyright 2008 Elsevier). (c) Cycling stability of ZnMn₂O₄ calcined at 800 °C, and its cycling-stability curves with different binders, respectively (reproduced from ref. 102. Copyright 2011, with permission from the Royal Society of Chemistry). (d)–(f) FESEM image of ZnMn₂O₄ microspheres, their cycling-stability curve, and different current densities, respectively (reprinted with permission from ref. 119. Copyright 2014 Elsevier). (g)–(i) TEM image of twin microspheres of ZnMn₂O₄, their cycling stability, and their rate-capability curves, respectively (reproduced from ref. 120. Copyright 2014, with permission from the Royal Society of Chemistry). (j)–(l) TEM image of ZnMn₂O₄–graphene, its cycling-stability curves, and different current densities shows the comparison of 2D ZMO-G, ZMO-SD (physical mixing of Super P carbon) and control ZMO-G (physical mixing of rGO) respectively (reprinted with permission from ref. 122. Copyright 2014, American Chemical Society).

Table 3 A summary on recent electrochemical performance of MMn₂O₄ (M = Co, Zn) anodes for LIBs

Sample	Experimental methods	Reversible capacity	Rate performance	Ref.
CoMn2O4 hierarchical microspheres	Solvothermal method	894 mA h g^−1 at 100 mA g^−1, 65 cycles	344 mA h g^−1 at 7200 mA g^−1	105
Double-shelled CoMn2O4 hollow microcubes	Co-precipitation and annealing process	624 mA h g^−1 at 200 mA g^−1, 50 cycles	406 mA h g^−1 at 800 mA g^−1	106
CNF@CoMn2O4 nanocables	Polyol method and subsequent annealing treatment	870 mA h g^−1 at 200 mA g^−1, 150 cycles	390 mA h g^−1 at 2400 mA g^−1	107
CoMn2O4 nanorods	Hydrothermal and subsequent annealing process	520 mA h g^−1 at 200 mA g^−1, 50 cycles	450 mA h g^−1 at 800 mA g^−1	108
CoMn2O4 nanofibers	Electrospinning technique	526 mA h g^−1 at 400 mA g^−1, 90 cycles	463 mA h g^−1 at 800 mA g^−1	109
CoMn2O4 quasi-hollow spheres	Solvothermal route	706 mA h g^−1 at 200 mA g^−1, 25 cycles	450 mA h g^−1 at 400 mA g^−1	110
Nanocrystalline-ZnMn2O4	Polymer-pyrolysis route	569 mA h g^−1 at 100 mA g^−1, 50 cycles	—	113
Flower-like ZnMn2O4	Solvothermal process	626 mA h g^−1 at 100 mA g^−1, 50 cycles	—	114
Nano-ZnMn2O4	Single precursor route	640 mA h g^−1 at 100 mA g^−1, 50 cycles	405 mA h g^−1 at 600 mA g^−1	115
ZnMn2O4 nanoparticles	Hydrothermal method	430 mA h g^−1 at 78 mA g^−1, 100 cycles	120 mA h g^−1 at 784 mA g^−1	116
ZnMn2O4-tubular arrays	Template route	784 mA h g^−1 at 100 mA g^−1, 100 cycles	363 mA h g^−1 at 1600 mA g^−1	118
Porous ZnMn2O4 microsphere	Solvothermal method	800 mA h g^−1 at 500 mA g^−1, 300 cycles	395 mA h g^−1 at 2000 mA g^−1	119
Quasi-mesocrystal ZnMn2O4 twin microsphere	Solvothermal method	860 mA h g^−1 at 500 mA g^−1, 130 cycles	329 mA h g^−1 at 5000 mA g^−1	120
Loaf-like ZnMn2O4 nanorods	Solid state reaction	517 mA h g^−1 at 500 mA g^−1, 100 cycles	457 mA h g^−1 at 1000 mA g^−1	121
2-D hybrid ZnMn2O4–graphene nanosheet	Reflux method	800 mA h g^−1 at 500 mA g^−1, 100 cycles	650 mA h g^−1 at 2000 mA g^−1	122
ZnMn2O4 hollow microsphere	Co-precipitation and annealing method	607 mA h g^−1 at 400 mA g^−1, 100 cycles	361 mA h g^−1 at 1600 mA g^−1	123
MWCNT/ZnMn2O4	Polyol method subsequent thermal annealing process	847 mA h g^−1 at 400 mA g^−1, 100 cycles	527 mA h g^−1 at 1600 mA g^−1	124

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Furthermore, to attain a stable capacity and high rate capability, ZnMn₂O₄ has been synthesized in various morphologies such as mesoscale tubular arrays, porous microspheres, quasimesocrystal twin microspheres, loaf-like nanorods, and hybrid ZnMn₂O₄–graphene nanosheets.^118–122 Wang and coworkers reported porous ZnMn₂O₄ microspheres prepared through the calcination of metal carbonates via a solvothermal reaction [Fig. 7(d)].¹¹⁹ BET analysis indicated that the surface area and pore volume of these porous ZnMn₂O₄ microspheres were 17.7 m² g⁻¹ and 0.27 cm³ g⁻¹, respectively, indicating that the pores were in the range of 45 to 90 nm. Fig. 7(e) shows the cycling performance of the ZnMn₂O₄ microspheres, analyzed in a potential window of 0.01–3 V at 500 mA g⁻¹. Over the first 50 cycles, the specific capacity decreased to 700 mA h g⁻¹ after it had gradually increased, and it reached a value of 800 mA h g⁻¹ on the 100^th cycle, where it was maintained over 300 cycles. Fig. 7(f) indicates that at a higher current density of 2000 mA g⁻¹, the capacity was maintained at 395 mA h g⁻¹. Overall, this excellent cycling stability and rate performance were achieved through the porous structures of the ZnMn₂O₄ microspheres, which provided high accessibility of the electrolyte over the electrode surface, high surface area that facilitated both charge transfer and a short diffusion distance for Li ions, and the alleviation of the pulverization problems during the insertion–extraction of Li⁺ ions.

Compared to porous microspheres, solvothermally prepared quasimesocrystal ZnMn₂O₄ twin microspheres [Fig. 7(g)] demonstrated better electrochemical performance [Fig. 7(h) and (i)]. They demonstrated a high specific capacity of 484 mA h g⁻¹ at a current density of 2 A g⁻¹,¹²⁰ and current density return to 0.2 mA g⁻¹ they exhibited a capacity of 1084 mA h g⁻¹. This excellent electrochemical performance was mainly attributed to the structural assembly, i.e., the twin microspheres that consisted of a 3D, porous, hierarchical structure composed of smaller primary nanoparticles that increased the number of active sites available for Li ions. The pores on the surfaces and within the interiors alleviated strain during the insertion–extraction process and provided a short diffusion path for Li ions, so an oriented mesocrystal could facilitate more electronic conductivity and in turn excellent specific capacity, cycling stability, and high rate capability. Xiong et al. reported the high rate capability of a ZnMn₂O₄ anode material using a 2D graphene nanoarchitecture prepared via a reflux method.¹²² The ZnMn₂O₄ nanoparticles uniformly distributed on the 2D nanosheet, forging a synergistic effect between the nanoparticles and graphene sheets that enhanced the conductivity and large active surface area, which maintained the structural integrity during the cycling process [Fig. 7(j)]. Fig. 7(k) and (l) show the astonishing properties of this 2D ZnMn₂O₄–graphene that had a stable capacity of 806 mA h g⁻¹ at a low current density of 200 mA g⁻¹, but it still maintained a high specific capacity of 568 mA h g⁻¹ even when the current density was increased to 3200 mA g⁻¹, indicating the high rate capability of these ZnMn₂O₄ nanoparticles with a graphene nanoarchitecture.¹²² For simple comparison, the electrochemical performances of CoMn₂O₄ and ZnMn₂O₄ with various morphologies, synthesized via different techniques, are provided in Table 3.

Among the manganites, ZnMn₂O₄ exhibits a lower oxidation potential and higher cell-output voltage than NiMn₂O₄ and CoMn₂O₄. However, ZnMn₂O₄ exhibits a higher cell-output voltage than ZnFe₂O₄, since Fe has a higher oxidation potential (∼1.61 V). Therefore, the synthesis of a large surface area with a proper composite is necessary to enhance the electrochemical performance of ZnMn₂O₄, and it may be expected to become a valuable source for Li ion anode materials for applications to LIBs.

3.3. ACo₂O₄ (A = Mg, Cu, Fe, Zn, Mn, and Ni)

Spinel cobaltites are an important group of spinel-type materials and have been widely investigated for use in applications in different fields such as sensors, bifunctional electrocatalysts, supercapacitors, and LIBs.^125–128 ACo₂O₄ has demonstrated excellent performances in LIBs, including such characteristics as high specific capacities, long cycle lives, and good rate capabilities, and it is slightly less toxic than Co₃O₄. Table 4 lists the electrochemical performances of spinel cobaltites.^128–165 For the most part, spinel cobaltites store Li ions through a conversion-reaction mechanism except for ZnCo₂O₄, since it follows both the conversion and alloying–de-alloying mechanisms. Sharma et al. first investigated the electrochemical performance of CuCo₂O₄, MgCo₂O₄, and FeCo₂O₄, synthesized via a low-temperature urea-combustion method and an oxalate-decomposition method, for applications in LIBs.^128,129 The electrochemical reaction was analyzed using cyclic-voltammetry analysis in the potential range of 0.005–3.0 V, and their reaction mechanisms are written as follows,^128–130

MCo₂O₄ + 8Li⁺ + 8e⁻ → M + 2Co + 4Li₂O (M = Cu, Fe Ni, Mn) (22)

MgCo₂O₄ + 6Li⁺ + 6e⁻ → MgO + 2Co + 3Li₂O (23)

M + Li₂O ↔ MO + 2Li⁺ + 2e⁻ (24)

2Co + 2Li₂O ↔ 2CoO + 4Li⁺ + 4e⁻ (25)

Table 4 A summary of recent studies of spinel cobaltites (MCo₂O₄; M = Zn, Mn, Ni and Fe) for LIB anode materials

Sample	Experimental method	Reversible capacity	Rate performance	Ref.
Nanophase ZnCo2O4	Combustion method	894 mA h g^−1 at 60 mA g^−1, 60 cycles	400 mA h g^−1 at 0.7C	131
Porous ZnCo2O4 nanotubes	Electrospinning technique	1454 mA h g^−1 at 100 mA g^−1, 30 cycles	794 mA h g^−1 at 2000 mA g^−1	132
ZnCo2O4 nanowires/carbon cloth	Hydrothermal method	1278 mA h g^−1 at 200 mA g^−1, 100 cycles	605 mA h g^−1 at 5C	133
Double-shelled ZnCo2O4 hollow microspheres	Citrate route followed by calcination	1019 mA h g^−1 at 90 mA g^−1, 120 cycles	570 mA h g^−1 at 5C	134
Hierarchical ZnCo2O4/Ni current collector	Hydrothermal process	1050 mA h g^−1 at 100 mA g^−1, 60 cycles	240 mA h g^−1 at 2778 mA g^−1	135
Mesoporous ZnCo2O4 microspheres	Solvothermal route	721 mA h g^−1 at 100 mA g^−1, 80 cycles	382 mA h g^−1 at 5000 mA g^−1	136
Yolk–shelled ZnCo2O4 microsphere	Refluxing route	949 mA h g^−1 at 200 mA g^−1, 100 cycles	331 mA h g^−1 at 1000 mA g^−1	137
Hierarchical ZnCo2O4 nanostructure on Ni foam	Hydrothermal process	1122 mA h g^−1 at 0.5 A g^−1, 50 cycles	542 mA h g^−1 at 2000 mA g^−1	138
Mesoporous rose-like ZnCo2O4	Microwave assisted synthesis	1000 mA h g^−1 at 50 mA g^−1, 50 cycles	800 mA h g^−1 at 800 mA h g^−1	139
ZnCo2O4/graphene nanosheets	Auto-combustion method	755 mA h g^−1 at 0.1C, 70 cycles	302 mA h g^−1 at 4.5C	140
Fiber bundle structure ZnCo2O4	Co-precipitation method	1026 mA h g^−1 at 100 mA g^−1, 100 cycles	606 mA h g^−1 at 2000 mA g^−1	141
Flake-by-flake ZnCo2O4	Co-precipitation	1275 mA h g^−1 at 100 mA g^−1, 50 cycles	730 mA h g^−1 at 3000 mA g^−1	142
Porous ZnCo2O4 nanowires	Sacrificial template route	1197 mA h g^−1 at 100 mA g^−1, 20 cycles	—	143
Porous ZnxCo3−xO4 hollow polyhedra	ZIFs templating approach	990 mA h g^−1 at 100 mA g^−1, 50 cycles	575 mA h g^−1 at 10C	144
Yolk–shell ZnCo2O4	Gas-phase reaction	753 mA h g^−1 at 3000 mA g^−1, 200 cycles	—	145
Flower-like ZnCo2O4 nanowires	Hydrothermal method	900 mA h g^−1 at 200 mA g^−1, 50 cycles	347 mA h g^−1 at 800 mA g^−1	146
1D-MnCo2O4 nanowires	Hydrothermal	895 mA h g^−1 at 100 mA g^−1, 50 cycles	345 mA h g^−1 at 1000 mA g^−1	147
Hierarchical MnCo2O4 nanowires	Hydrothermal/calcination approach	1038 mA h g^−1 at 200 mA g^−1, 45 cycles	388 mA h g^−1 at 1600 mA g^−1	148
Hierarchical MnCo2O4 nanosheets/carbon cloth	Hydrothermal method	3 mA h cm^−2 at 800 μA cm^−2, 60 cycles	2 mA h cm^−2 at 1600 μA cm^−2	149
MnCo2O4 microsphere	Solvothermal route	722 mA h g^−1 at 200 mA g^−1, 25 cycles	320 mA h g^−1 at 900 mA g^−1	150
Mn1.5Co1.5O4 core–shell microsphere	Urea assisted solvothermal route	618 mA h g^−1 at 400 mA g^−1, 300 cycles	455 mA h g^−1 at 800 mA g^−1	151
Porous NiCo2O4 microflowers	Solvothermal route	952 mA h g^−1 at 100 mA g^−1, 60 cycles	720 mA h g^−1 at 500 mA g^−1	153
Uniform NiCo2O4 hollow spheres	Solvothermal route	885 mA h g^−1 at 150 mA g^−1, 50 cycles	533 mA h g^−1 at 2000 mA g^−1	154
NiCo2O4 mesoporous microspheres	Solvothermal route	1198 mA h g^−1 at 200 mA g^−1, 30 cycles	705 mA h g^−1 at 800 mA g^−1	155
Mesoporous NiCo2O4	Co-precipitation	1000 mA h g^−1 at 0.5C, 400 cycles	718 mA h g^−1 at 10C	156
Mesopoprous NiCo2O4 nanosheets	Microwave method	767 mA h g^−1 at 100 mA g^−1, 50 cycles	487 mA h g^−1 at 1000 mA g^−1	158
Porous NiCo2O4 nanobelts	Hydrothermal technique	981 mA h g^−1 at 500 mA g^−1, 100 cycles	1062 mA h g^−1 at 2000 mA g^−1	159
Porous NixCo3−xO4 nanosheets	Thermal decomposition	844 mA h g^−1 at 500 mA g^−1, 200 cycles	293 mA h g^−1 at 1600 mA g^−1	160
Hollow NiCo2O4 nanospheres	Template method	1346 mA h g^−1 at 100 mA g^−1, 20 cycles	695 mA h g^−1 at 2000 mA g^−1	161
NiCo2O4 nanocubes	Etching and precipitation	1160 mA h g^−1 at 200 mA g^−1, 200 cycles	750 mA h g^−1 at 1000 mA g^−1	162
Porous NiCo2O4@C nanocomposite	Hydrothermal	1389 mA h g^−1 at 0.55C, 180 cycles	625 mA h g^−1 at 4.4C	163
Peapod-like NiCo2O4–C nanorods	Hydrothermal	1183 mA h g^−1 at 100 mA g^−1, 200 cycles	664 mA h g^−1 at 2000 mA g^−1	164
FeCo2O4 nanoflakes	Hydrothermal	905 mA h g^−1 at 200 mA g^−1, 170 cycles	1222 mA h g^−1 at 800 mA g^−1	165

---

After the first discharge, MgCo₂O₄ converted into MgO, which was then no longer involved in the reaction mechanism, since it acted as an inactive matrix and avoided the volume expansion. Galvanostatic-cycling studies indicated that MgCo₂O₄ demonstrated serious capacity fading, i.e., it retained only a very small capacity of 108 mA h g⁻¹ after 50 cycles.¹²⁹ On the other hand, CuCo₂O₄ and FeCo₂O₄ possessed reasonable discharge capacities of 747 and 752 mA h g⁻¹, respectively, after 50 cycles. These results might be due to the conversion reactions of both the A- and B-site metal cations. In contrast, only the B-site cations contributed to the charge storage in MgCo₂O₄.

Among these cobaltites, ZnCo₂O₄ and its composite has received much attention as a potential material for anodes in LIBs.^131–146 In the ZnCo₂O₄-spinel structure, Zn²⁺ ions occupy the tetrahedrally coordinated sites, and Co³⁺ ions occupy the octahedrally coordinated sites, and this structure involves an overarching electrochemical reaction through a conversion reaction between the Co and Zn ions and an alloying reaction between the Zn and Li that provides the overall charge-storage capacity. This reaction mechanism is given as,^131–135

ZnCo₂O₄ + 8Li⁺ + 8e⁻ → Zn + 2Co + 4Li₂O (27)

Zn + Li⁺ + e⁻ ↔ LiZn (28)

Zn + Li₂O ↔ ZnO + 2Li⁺ + 2e⁻ (29)

2Co + 2Li₂O ↔ 2CoO + 4Li⁺ + 4e⁻ (30)

ZnCo₂O₄ has been widely investigated, and to study its electrochemical performance, researchers have reported various morphologies, including nanotubes,¹³² 3D hierarchical nanowires,¹³³ uniform mesoporous microspheres,¹³⁶ and yolk–shelled microspheres,¹³⁷ and they have also grown it on different current collectors, including Ni foam and carbon cloth.^135,138 Importantly, porous ZnCo₂O₄ nanotubes, synthesized via an electrospinning technique with subsequent heat treatment. Fig. 8(a) and (b) show the nanotubes (∼200 nm), which consist of interconnected primary nanocrystals (∼30 nm) and nanopores (∼3 nm) in the walls which exhibited a capacity of 1454 mA h g⁻¹ at 100 mA g⁻¹ over 30 cycles with 100% coulombic efficiency (Fig. 8(c)).¹³² Even at high current densities of 500, 1000, and 2000 mA g⁻¹, they demonstrated high reversible capacities of 1011, 841, and 794 mA h g⁻¹, indicating that these porous nanotubes could accommodate the volume strain and that the well-interconnected nanoparticles shortened the pathway of Li ion diffusion and enhanced the electronic conductivity, and they possessed large surface areas that led to an increase in the active sites for Li ion diffusion. Liu et al. reported a binder- and current-collector-free 3D hierarchical ZnCo₂O₄ nanowire array grown on flexible carbon cloth using a hydrothermal method.¹³³ Fig. 9(a) depicts its schematic diagram, while Fig. 9(b)–(e) depict the morphologies of the well-ordered woven structure of the array on carbon cloth in which the ZnCo₂O₄ nanowires have uniform diameters of 80–100 nm and lengths of 5 μm. Fig. 9(f) shows the cycling-stability curve at 200 mA h g⁻¹. This array had a high irreversible capacity of 1530 mA h g⁻¹ over the initial cycles. After 50 and 100 cycles, it possessed capacities of 1280 and 1278 mA h g⁻¹, respectively, with 99% capacity retention, and still it retained a stable specific capacity of about 1200 mA h g⁻¹ after 160 cycles. The reported capacity was very high when compared with the theoretical specific capacity due to the reversible growth of a polymeric/gel-like film on the surface of the active material. The current density rises upto 5C rate it maintain the stable capacity of 605 mA h g⁻¹ (Fig. 9(g)). The superior electrochemical performance of ZnCo₂O₄ on carbon cloth depended primarily on the following factors. The ZnCo₂O₄ nanowires strongly adhered to the carbon cloth, facilitating good electronic conductivity, and the open spaces between the nanowires accommodated the volume strain and allowed electrolyte penetration into the active materials, leading to an increase in the number of active sites. The nanowires also shortened the Li⁺ ion diffusion path, enhancing the rate capability. It has been suggested that binder- and current-collector-free flexible ZnCo₂O₄ nanowires on carbon cloth are a good candidate for flexible LIBs.

Fig. 8 (a and b) Porous ZnCo₂O₄ nanotubes and (c) their cycles at different current densities (reproduced from ref. 132. Copyright 2012, with permission from the Royal Society of Chemistry).

Fig. 9 (a) Schematic diagram of a ZnCo₂O₄ nanowire array on carbon cloth. (b)–(e) Morphologies of these ZnCo₂O₄ nanowires. (f) and (g) Their cycling stability and different current densities (reprinted with permission from ref. 133. Copyright 2012, American Chemical Society).

In addition to that of ZnCo₂O₄, the Co-based-spinel structures of MnCo₂O₄ and NiCo₂O₄ have also been widely examined,^{147–151,153–163} as Mn-based cobaltite (MnCo₂O₄) is considered a promising candidate for LIBs due to its low cost, environmental benignity, and lower operating voltage.¹⁴⁹ Recently, Fu et al. reported the electrochemical performance of stoichiometric MnCo₂O₄-spinel mesoporous microspheres prepared via a one-step low-temperature solvothermal method. The microspheres showed a discharge capacity of 722 mA h g⁻¹ over 25 cycles at 200 mA g⁻¹. Due to the hierarchical structure of MnCo₂O₄,¹⁵⁰ they demonstrated a superior rate-capability performance. Li et al. prepared core–shell Mn_1.5Co_1.5O₄ with a nonstoichiometric, spinel structure via a solvothermal route and reported its discharge capacity of 618 mA h g⁻¹ at 400 mA g⁻¹ with good cycling stability even after 300 cycles.¹⁵¹ Interestingly, Hou et al. fabricated the hierarchical structure of MnCo₂O₄ nanosheet arrays on carbon cloth [Fig. 10(a)–(c)] that demonstrated excellent electrochemical performance, which was even better than that of Co₃O₄ on carbon cloth¹⁴⁹ since the carbon cloth assured good electronic conductivity between the active material and the current collector. Therefore, this type of binder- and additive-free electrode is a promising anode material for LIBs.

Fig. 10 (a)–(c) Morphologies of MnCo₂O₄ nanosheet arrays on carbon cloth. (d) Their cycling stability and (e) rate-capability curve (reproduced from ref. 149. Copyright 2014, with permission from the Royal Society of Chemistry).

Similarly, NiCo₂O₄ has also been exploited as an efficient energy-storage material for diverse applications, including bifunctional electrocatalysts, supercapacitors, and LIBs.^{126,152,153–164} It exhibits a high electronic conductivity due to the mixed valences of the same cations in the metal oxides. NiCo₂O₄ [Co_1−x²⁺Co³⁺[Co³⁺Ni_x²⁺Ni_1−x³⁺]O₄] exhibits two- to three-fold increases in the electrical conductivity, compared to simple metal oxides. For example, the electrical conductivity of Co₃O₄ is 3.1 × 10⁻⁵ S cm⁻¹, and that of Ni_xCo_3−xO₄ is 0.1–0.3 S cm⁻¹. Hence, the high electronic conductivity of NiCo₂O₄ enhances the rate capabilities and cycle lives of LIBs.^156,157 Park and his group recently reported the excellent electrochemical performance of urchin-like, mesoporous [Fig. 11(a)] NiCo₂O₄ spinel, which was synthesized via a simple, cost-effective coprecipitation method followed by a low-temperature calcination process.¹⁵⁶ It possessed a discharge capacity of 1000 mA h g⁻¹ after 400 cycles at a current rate of 0.1C with a high coulombic efficiency greater than 98%, and it showed an extraordinary rate capability [Fig. 11(b) and (c)]. Instead of conventional PVDF, they used PAA as a binder as it could accommodate the volume strain due to its elastic nature and could assist the efficient electronic conductivity between the active material and the current collector.

Fig. 11 (a)–(c) Urchin-like morphology of NiCo₂O₄, its cycling stability curve at a 0.1C rate, and its current densities at different C rates (reproduced from ref. 156. Copyright 2014 with permission from the Royal Society of Chemistry).

From the observation of these cobaltites, urchin-like mesoporous NiCo₂O₄ demonstrated an excellent electrochemical performance without any composites because its good electronic conductivity and mesoporous structure led to a high rate capability and high specific capacity. To further improve the electronic and Li ion transports of the urchin-like NiCo₂O₄, a composite containing carbonaceous material is necessary.

4. Germanates, stannates, and silicates, A₂BO₄ (A = Zn, Co, Mn, and Co; B = Ge, Sn, Si, and Ti)

It is believed that Zn₂GeO₄ is an efficient anode material, since ZnO and GeO₂ are electrochemically active for Li ion storage through the alloying–de-alloying-reaction mechanism and they exhibit high reversible capacities of 978 and 1100 mA h g⁻¹, respectively.^167,168 Ge has 400 and 10 000 times greater Li diffusivity and electronic conductivity, respectively, than Si, but during the alloying–de-alloying process, it undergoes a volume change of up to 260% that leads to cracks in the electrode material.^168,169 Therefore, to avoid the aggregation of Ge particles and reduce the cost of the electrode material, some active metal oxides such as BaO, CaO, and ZnO have been implemented.^170,171 Among these compounds, BaO and CaO act as inactive matrices that reduce the capacity of the material, but ZnO facilitates Li storage and prevents particle aggregation. The alloying–de-alloying-reaction mechanism of Zn₂GeO₄ is,^{168,169,172–178}

Zn₂GeO₄ + 8Li⁺ + 8e⁻ → 2Zn + Ge + 4Li₂O (32)

Zn + xLi⁺ + xe⁻ ↔ Li_xZn (0 ≤ x ≤ 1) (33)

Ge + yLi⁺ + ye⁻ ↔ Li_yGe (0 ≤ y ≤ 1) (34)

Zn + Li₂O ↔ ZnO + 2Li⁺ + 2e⁻ (35)

Ge + 2Li₂O ↔ GeO₂ + 4Li⁺ + 4e⁻ (36)

Feng and coworkers analyzed the electrochemical performance of Zn₂GeO₄ nanorods, synthesized via a hydrothermal method,¹⁶⁸ and obtained a discharge capacity of 1820 mA h g⁻¹ in the first cycle, but this value linearly decreased to 616 mA h g⁻¹ after 100 cycles. Therefore, to further enhance the specific capacity and cycling stability, they synthesized amorphous nanoparticles and graphene composite with the Zn₂GeO₄ particles in order to overcome the volume expansion and enhance the conductivity of the electrode material.^{169,172–174} The amorphous phase of the Zn₂GeO₄ nanoparticles showed a high reversible capacity of 1250 mA h g⁻¹ at 400 mA g⁻¹ over 500 cycles, and the capacity was maintained at 470 mA h g⁻¹ at 6400 mA g⁻¹. The high specific capacity and rate capability resulted from the facts that the amorphous structure did not suffer from stresses that mitigated the pulverization of the material and that the incorporation of Zn and oxygen acts as a buffering matrix, thus contributing to the reversible capacity.¹⁷²

The Huang group reported sandwiched Zn₂GeO₄–graphene-oxide nanocomposites using an ion-exchange-reaction method [Fig. 12(a)].¹⁶⁹ Among these composites, 12.1 wt% of the Zn₂GeO₄–graphene-oxide nanocomposite showed a good cycling stability, had a specific capacity of 1150 mA h g⁻¹ at 200 mA g⁻¹ over 100 cycles, and maintained a stable capacity of 522 mA h g⁻¹ at a high current density of 3.2 A g⁻¹, as shown in Fig. 12(b) and (c). These Zn₂GeO₄ nanorods were completely wrapped in the graphene oxide, which thereby accommodated the strain from the volume expansion and preserved the integrity of the active material, thus leading to the improved mobilities of the ions and electrons. Recently, the Wang group explored Fe₂GeO₄ as an anode material for LIBs, as it has shown excellent electrochemical performance. Fe₂GeO₄ exhibits a high theoretical specific capacity of 1119 mA h g⁻¹, according to the following electrochemical reactions,²⁷

Fe₂GeO₄ + 8Li⁺ + 8e⁻ → 2Fe + Ge + 4Li₂O (37)

2Fe + 3Li₂O ↔ Fe₂O₃ + 6Li + 6e⁻ (38)

Ge + 4.4Li⁺ + 4.4e⁻ ↔ Li_4.4Ge (39)

Fig. 12 (a) Schematic diagram of Zn₂GeO₄–graphene-oxide nanocomposites. (b) Cycling-stability curve at 200 mA g⁻¹ at different weight percentages of carbon and (c) the rate-capability curve (reproduced from ref. 169. Copyright 2014, with permission from the Royal Society of Chemistry). (d) TEM image of an Fe₂GeO₄–graphene composite and (e) its rate-capability curve (reprinted with permission from ref. 27. Copyright 2014 Elsevier).

Fig. 12(d) shows the TEM image of an Fe₂GeO₄–graphene composite that had a capacity of 980 mA h g⁻¹ at 360 mA g⁻¹ after 175 cycles and a high reversible capacity of 340 mA h g⁻¹ at 4800 mA g⁻¹ is shown in Fig. 12(e).²⁷ This type of A₂BO₄ structure has some advantages, as the first lithiation evenly distributed between the Li₂O matrix and the metal particles. Here, Li₂O acts as a buffering matrix, as it can effectively accommodate the volume strain, and the secondary metal nanoparticles enhance the electrical conductivity of the electrode material. These factors lead to electrode materials with both good cyclabilities and good rate performances.¹⁷⁹ Zhang et al. first reported a Co₂GeO₄@reduced-graphene composite as a Li ion anode material that demonstrated a specific capacity of 1085 mA h g⁻¹ over 100 cycles with cycling stability superior to that of Co₂GeO₄. Reduced-graphene oxide (rGO) served as a good electronic conductor between the particles and an excellent buffering matrix to accommodate the volume strain during the charge–discharge process. Co₂GeO₄ combined with rGO specifically demonstrated a high specific capacity, good rate capability, and cycling stability.³⁰ Among the germanates, Zn₂GeO₄ is considered an ideal anode material for LIBs because of its low working potential and high energy density, and the reduced usage of Ge is also beneficial in reducing the cost of the material (27% of Ge in Zn₂GeO₄) compared to other Ge-based materials.¹⁷⁸

A different set of mixed-metal oxides that contain Sn is another group of important prospects for improving the specific capacity of an electrode material. Metal stannates (M₂SnO₄; M = Mn, Co, Zn) store Li ions through both a conversion-reaction mechanism and an alloying–de-alloying mechanism as follows,^180–189

M₂SnO₄ + 8Li → 2M + 4Li₂O + Sn (M = Mn, Co) (40)

Sn + 4.4Li ↔ Li_4.4Sn (41)

Sn + 2Li₂O ↔ SnO₂ + 4Li (42)

M + Li₂O ↔ MO + 2Li (43)

Lei et al. prepared ultrafine Mn₂SnO₄ nanoparticles using the thermal decomposition of a MnSn(OH)₆ precursor.¹⁸³ The Mn₂SnO₄ nanoparticles showed severe capacity fading upon cycling due to volume expansion. During the alloying–de-alloying process, Sn metal expands up to 200–300% (volume expansion), causing peeling of the active material and breaking the electronic conductivity between the active material and the current collector. A similar type of capacity was observed for Co₂SnO₄ nanoparticles, synthesized via a facile hydrothermal method, which had a capacity of 555 mA h g⁻¹ after 50 cycles at 30 mA g⁻¹ with a capacity retention of 50.3%, and even after 50 cycles, they still could not attain a stable capacity.¹⁸⁴ To overcome this drawback, a Co₂SnO₄@C core–shell structure, a composite with MWCNTs and Co₂SnO₄ wrapped with graphene, was analyzed to enhance the electrochemical performance of the Co₂SnO₄.^185,188 Qi and coworkers reported that the core–shell structure of Co₂SnO₄@C, prepared through a hydrothermal process, with different thicknesses of carbon coating (10.5 and 25.2 wt%) was achieved using glucose as the carbon source. Among these materials, the one with the thick carbon coating showed a high discharge capacity of 474 mA h g⁻¹ with a capacity retention of 60.4% after 75 cycles, a value that was greater than that of the one with the thin carbon coating (173 mA h g⁻¹).¹⁸⁵ The cycling stability of Co₂SnO₄ was attributed to the core–shell structure of Co₂SnO₄@C, and it had several advantages such as stable SEI-film formation, a lack of aggregation with the active material, the enhancement of the electrical conduction with the active material, and functioning as a buffering matrix during the charge–discharge process.¹⁸⁶ CNTs are also considered to be important carbonaceous materials that enhance the electrochemical performances of metal oxides, and they have various attractive features, including excellent electrical conductivity, a large length versus diameter ratio, a large surface area, structural flexibility, and chemical stability.^{187,189–192} Liu and his group proposed a multiwalled-carbon-nanotube (MWCNT) composite with Co₂SnO₄ nanoparticles that possessed a stable capacity of 898 mA h g⁻¹ at 50 mAg⁻¹ over 50 cycles.¹⁸⁷ Compared to the Co₂SnO₄ nanoparticles, the Co₂SnO₄–MWCNT composite showed good reversibility with feeble capacity fading that was due to its highly conductive matrix of MWCNTs that facilitated contact between the nanoparticles, accommodated the volume expansion, and prevented the aggregation of nanoparticles during the cycling process.^191,192 Recently, Qian and his groups reported Co₂SnO₄ hollow cubes@rGO, synthesized via the pyrolysis-induced transformation from the hollow, cubic precursor prepared via a hydrothermal method, where the composite with graphene sheets was attained through an electrostatic-interaction mechanism.¹⁸⁸ Fig. 13(a–f) show the schematic diagram and TEM images of hollow Co₂SnO₄ cubes with graphene composite, which exhibit excellent electrochemical performance, as compared to the core–shell structure and the MWCNT composite.^187,188 The BET analysis indicated that the Co₂SnO₄ hollow cubes@rGO exhibited a high surface area of 62.88 m² g⁻¹ that facilitated an interfacial electrochemical reaction. Fig. 13(g) depicts the discharge capacity versus cycle number and indicates that the specific capacity of Co₂SnO₄ hollow cubes@rGO was 1126 mA h g⁻¹ at 100 mA g⁻¹ after 50 cycles. This value was greater than the theoretically calculated value (1105 mA h g⁻¹), a discrepancy that resulted from either the interfacial-charge storage or the thick-SEI-film formation,^111,193 and this material maintained its capacity of 1016 mA h g⁻¹ after even 100 cycles. For practical applications, they varied the mass loading from 1–2 mg to 5–6 mg, but no obvious change was observed in the cycling process; the discharge capacity was only reduced to 40 mA h g⁻¹, compared to that of the low mass loading. In Fig. 13(h), the Co₂SnO₄ hollow cubes@rGO had a greatly enhanced rate capability of 775 mA h g⁻¹ at a current density of 500 mA g⁻¹.

Fig. 13 (a–d) and (e and f) Schematic diagram of Co₂SnO₄–graphene-composite formation and its corresponding TEM images respectively. (g) The cycling stability and (h) rate capability of Co₂SnO₄ nanocrystals, hollow cubes, and graphene composite (reproduced from ref. 188. Copyright 2014, with permission from the Royal Society of Chemistry).

The electrochemical performance was mainly attributed to the hollow cubes of Co₂SnO₄ and the combined effects of the graphene nanosheets. Both the hollow cubes and the graphene could accommodate the volume expansion and contraction during the charge–discharge process, thereby facilitating the extra capacity from the interfacial storage of Li ions and graphene, improving the conductivity between the particles, and thus leading to a good rate capability of the active material.

Zn₂SnO₄ belongs to an inverse-spinel group (space group Fd3m) with a band gap of 3.6 eV, and it possesses high electron mobility, high electron conductivity, and low visible absorption.^194,195 Due to these astonishing properties, it has been widely investigated for applications in different fields such as gas sensors, solar cells, and photocatalysts.^196–198 Apart from these studies, Zn₂SnO₄ has mainly been investigated as a Li ion anode material due to the low working potentials of ZnO (0.05–1.50 V) and SnO₂ (0–1.0 V) as well as their high specific capacities of 980 and 780 mA h g⁻¹, respectively.^199,200 Both Zn and Sn are electrochemically active and are involved in the charge-storage mechanism through the alloying mechanism (Li–Zn and Li–Sn), and they could accommodate 14.4 Li ions in the first cycle, demonstrating an irreversible capacity of 1231 mA h g⁻¹ and exhibiting a reversible capacity of 547 mA h g⁻¹, according to the electrochemical-reaction mechanism,^201–203

Zn₂SnO₄ + 4Li⁺ + 4e⁻ → Sn + 2Li₂O + 2ZnO (44)

Zn₂SnO₄ + 8Li⁺ + 8e⁻ → 2Zn + Sn + 4Li₂O (45)

Sn + xLi⁺ + xe⁻ ↔ Li_xSn, (x ≤ 4.4) (46)

Zn + yLi⁺ + ye⁻ ↔ Li_yZn, (y ≤ 1) (47)

During the charge–discharge process, the Zn₂SnO₄ material undergoes a large volume change, but it cannot accommodate the volume strain during the cycling process, thus leading to severe capacity fading.^201–203 Therefore, many efforts have been made to improve the cycling stability and specific capacity of Zn₂SnO₄, so particles have been synthesized in different shapes, including cubes, hollow cubes, wires, nanoplates, hollow nanospheres, composites with carbon, graphene metal–organic frameworks, and conducting polymers,^{195,204–213} and their electrochemical performances are listed in Table 5. Zhang et al. developed hollow Zn₂SnO₄ nanospheres [Fig. 14(a)] via a hydrothermal method followed by an annealing process, and these hollow nanospheres exhibited a high surface area of 42.53 m² g⁻¹ and pore volume of 0.24 cm³ g⁻¹.²⁰⁵ Fig. 14(b) and (c) depict these hollow nanospheres with a capacity of 602 mA h g⁻¹ at 100 mA g⁻¹ over 60 cycles and 442 mA h g⁻¹ even at 1000 mA g⁻¹ after 60 cycles. Therefore, this superior electrochemical performance of the hollow nanospheres resulted from different factors, including their large surface areas, enlarging the electrolyte–electrode-contact area and porous structure that allowed the electrolyte into the electrode, increasing the active sites for Li storage and accommodating the volume strain during the charge–discharge process. Recently, hollow Zn₂SnO₄ boxes@carbon/graphene ternary composite was prepared via a hydrothermal method followed by a calcination process.²⁰⁶ Fig. 14(d) and (e) show the TEM image of the ternary composite and its electrochemical performance, and it exhibited high irreversible discharge capacities of 1863 and 1256 mA h g⁻¹ on the first two cycles. The discharge-capacity values were large compared to the theoretical specific capacity, a result that is due to the extra Li storage on the active material through a pseudocapacitance mechanism.²¹³

Table 5 A summary on electrochemical performance of M₂M′O₄ (M = Zn and Co, M′ = Sn and Ge) based anodes for LIBs

Sample	Experimental methods	Reversible capacity	Rate performance	Ref.
Ultrathin 2D Co2GeO4 nanosheets	Hydrothermal method	1026 mA h g^−1 at 0.22 A g^−1, 150 cycles	600 mA h g^−1 at 1.16 A g^−1	29
Mn doped Zn2GeO4 nanosheet array	Hydrothermal synthesis	1301 mA h g^−1 at 100 mA g^−1, 100 cycles	500 mA h g^−1 at 2 A g^−1	175
Coaxial Zn2GeO4@C nanowires	Single-step chemical vapour deposition	790 mA h g^−1 at 2000 mA g^−1, 100 cycles	465 mA h g^−1 at 10 A g^−1	176
Carbon coated Zn2GeO4 on Ni foam	Chemical vapour deposition method	1100 mA h g^−1 at 400 mA g^−1, 1000 cycles	396 mA h g^−1 at 9.6 A g^−1	177
Zn2SnO4 nanowires	Vapour transport method	660 mA h g^−1 at 120 mA g^−1, 50 cycles	—	28
Zn2SnO4 cubic crystals	Solid state reaction	689 mA h g^−1 at 50 mA g^−1, 50 cycles	—	194
Zn2SnO4 cubes	Hydrothermal method	775 mA h g^−1 at 50 mA g^−1, 20 cycles	—	195
Zn2SnO4	Supercritical water	856 mA h g^−1 at 0.75 mA cm^−2, 50 cycles	—	199
Cube shaped-Zn2SnO4 particles	Hydrothermal method	580 mA h g^−1 at 100 mA g^−1, 50 cycles	—	201
Zn2SnO4/C composite	Hydrothermal method	563 mA h g^−1 at 60 mA g^−1, 40 cycles	—	202
Crystalline Zn2SnO4 nanoparticles	Hydrothermal method	521 mA h g^−1 at 50 mA g^−1, 40 cycles	—	203
Hollow Zn2SnO4@N–C composite	Co-precipitation method followed by calcination	616 mA h g^−1 at 300 mA g^−1, 45 cycles	354 mA h g^−1 at 1000 mA g^−1	204
Zn2SnO4 hollow nanosphere	Hydrothermal method	602 mA h g^−1 at 100 mA g^−1, 60 cycles	442 mA h g^−1 at 1000 mA g^−1	205
Hollow Zn2SnO4 boxes@C/graphene ternary composite	Hydrothermal process followed by calcination process	726 mA h g^−1 at 300 mA g^−1, 50 cycles	396 mA h g^−1 at 1800 mA g^−1	206
Zn2SnO4/graphene composite	Hydrothermal method	688 mA h g^−1 at 200 mA g^−1, 50 cycles	300 mA h g^−1 at 1600 mA g^−1	207
Hollow Zn2SnO4 boxes wrapped graphene	Co-precipitation and electrostatic interaction	678 mA h g^−1 at 300 mA g^−1, 45 cycles	355 mA h g^−1 at 2000 mA g^−1	208
Flower-like Zn2SnO4 composite	Green hydrothermal synthesis	501 mA h g^−1 at 300 mA g^−1, 50 cycles	—	209
PPY coated Zn2SnO4	Microemulsion polymerization	478 mA h g^−1 at 60 mA g^−1, 50 cycles	—	211
PANI coated Zn2SnO4	Emulsion polymerization method	491 mA h g^−1 at 600 mA g^−1, 50 cycles	—	212
Ex situ carbon coated Zn2SnO4	Hydrothermal synthesis	533 mA h g^−1 at 700 mA g^−1, 50 cycles	—	214

---

Fig. 14 (a)–(c) Hollow Zn₂SnO₄ nanospheres, their cycling stability at 100 mA g⁻¹, and different current densities, respectively (reproduced from ref. 205. Copyright 2014, with permission from the Royal Society of Chemistry). (d)–(f) Zn₂SnO₄ ternary composite, cycling stabilities of different morphologies of Zn₂SnO₄ and its composites, and their stabilities at different current densities of the ternary composite (reprinted with permission from ref. 206. Copyright 2014 Elsevier).

The ternary composite retained a capacity of 726 mA h g⁻¹ at 300 mA g⁻¹ over 50 cycles, and when the current density increased to 1200 mA g⁻¹, it maintained a capacity of 438 mA h g⁻¹ (Fig. 14(f)). The ternary composite exhibited an excellent cycling stability and rate performance, as compared to those of hollow Zn₂SnO₄ boxes wrapped with flexible graphene.^206,208 Therefore, their excellent electrochemical performance was attributed to their hollow structure, carbon layer, and graphene composite, which act as a triple buffering structure, efficiently accommodating the volume strain during alloying–de-alloying process. Additionally, the carbon layer and graphene composite enhanced the electronic conductivity of the composite and thus improved the rate capability of the material. Overall, it can be concluded that the Zn₂SnO₄@graphene composite showed linear capacity fading during the cycling process but that Co₂SnO₄ showed a high reversible capacity. Nonetheless, the synthetic process of Co₂SnO₄ is difficult because of the high temperature it requires.¹⁸⁶

Silicate-based mixed-metal oxides have also been analyzed for their potential applications to Li ion anode materials. Only a few reports are currently available that analyze silicate-based materials like Zn₂SiO₄ and Co₂SiO₄, but these materials do not exhibit high specific capacities compared to that of Co₃O₄.^215,216 Zhang et al. reported Zn₂SiO₄ nanorods using a facile hydrothermal method, but they possessed a capability of only 388 mA h g⁻¹ at 50 mA g⁻¹ after 20 cycles with that capacity fading on subsequent cycles.²¹⁵ Mueller and coworkers first reported an orthorhombic α-Co₂SiO₄ structure, prepared through a solid-state reaction, and examined its electrochemical performance as a Li ion anode material.²¹⁶

From the cyclic voltammetry, in situ XRD analysis, and XPS results, they deduced the following reaction mechanism,

Co₂SiO₄ + 4Li⁺ + 4e⁻ → 2Co + Li₄SiO₄, (48)

xCo + Li₄SiO₄ ↔ 4Li⁺ + 4e⁻ + Co_xSiO₄. (49)

The reported reversible formation of Li₄SiO₄ enhances the Li ion conductivity that facilitates the high rate capability of the active material and yields a capability of 370 mA h g⁻¹ at a high current density of 1580 mA g⁻¹. However, exploring the detailed electrochemical performances of such silicate-based mixed-metal oxides is still necessary. For instance, Co₂SiO₄ and Zn₂SiO₄ materials yield SiO₂ and Li₄SiO₄ during the first discharge processes, respectively, so they are not involved in the electrochemical reactions, leading to low specific capacities and difficulties attaining pure phase formations.^217,218 Therefore, it is mandatory to discover alternative anode materials apart from those mentioned above.

5. Role of binder

In Li-ion batteries, binder plays a crucial role to improve the electrochemical performances. It is used as glue, which interconnected the active material and conducting agent that ensures the electronic conductivity between the active materials and the current collector.²¹⁹ The commercially used PVDF has several drawbacks, i.e., low electronic conductivity, toxic and it dissolves in expensive organic solvent of NMP. Similarly, it could not accommodate volume change during charging–discharging process and adsorb more electrolyte ions that lead to high irreversible capacity loss during first cycle. Therefore, it creates an new avenue to identify an alternate binder instead of PVDF having the characteristics features including water soluble, low cost, high elasticity nature, high electronic conductivity and environmentally friendly.^220,221 In this regard, recently many groups have identified different water soluble binders such as CMC (carboxy methyl cellulose), SBR (styrene butadiene rubber), PAA (polyacrylic acid) and SA (sodium alginate) for the applications of Li-ion batteries. CMC is a polymeric derivative of cellulose which consists of carboxylate anion and hydroxyl functional groups and these groups makes CMC as a water-soluble binder. Similarly, SBR is an elastomer, which has high flexibility, stronger binding force and better heat resistance than PVDF.²²² On the other hand, PAA is an environmentally friendly and it not only soluble in water it also soluble in organic solvent like ethanol. It has high concentration of functional groups which can give spacing between them through copolymerization with other monomers. The copolymerization of PAA offer many properties including, elastic modulus, maximum elongation, swelling in electrolyte, surface chemistry and the strength of adhesion between the active material and current collector.⁸³ The SA is a high modulus polysaccharide derived from brown algae which mainly consists of copolymer of 1 → 4 linked β-D-mannuronic acid (M) and α-L-guluronic acid (G) residues.²²³ It has several advantages including ∼6.7 times higher stiffness than dry films of PVDF, small swelling of electrolyte in its structure which restricts the undesirable reaction with the electrode/electrolyte interface, polar hydrogen bonds between the carboxyl groups in the binder that leads to strong adhesive between the active material and current collector and exhibits self healing effect during charging–discharging process.^223,224 The following literatures are well depicted the importance of binders. Wei groups have reported that electrochemical performance of ZnFe₂O₄ using both PVDF and SBR/CMC as binders.²²² ZnFe₂O₄–SBR/CMC delivered high capacity of 627.6 mA h g⁻¹ at 1C rate, which was much larger than that of ZnFe₂O₄–PVDF (144.9 mA h g⁻¹). Similarly, yolk–shelled microsphere ZnCo₂O₄, delivered the stable specific capacity of 718 mA h g⁻¹ and 325 mA h g⁻¹, while using CMC and PVDF, respectively as binders over 100 cycles. The result assured that SBR/CMC is a better choice for replacing PVDF, since SBR/CMC makes a unique three dimensional network between the active materials, which enhanced the stability of the electrode during cycling process.^137,225 Wang group have analyzed the electrochemical performance of amorphous Zn₂GeO₄ nanoparticles using PAA as a binder.¹⁷² The amorphous ZnGe₂O₄ nanoparticles showed the high reversible specific capacity of 1250 mA h g⁻¹ over 100 cycles and it exhibited the high discharge capacity of 610 mA h g⁻¹ at the high current density of 3.2 A g⁻¹. This excellent electrochemical performance was mainly attributed to the combined effect of amorphous Zn₂GeO₄ nanoparticles and PAA binder. Yushin group has first time introduced the sodium alginate as a binder for high capacity silicon nanopowder based lithium ion batteries.⁸⁴ Mitra groups recently evaluated the electrochemical performance of spinel CoFe₂O₄ nanoparticles using sodium alginate binder which exhibits the stable discharge capacity of 890 mA h g⁻¹ over 50 cycles but using PVDF it stabilized at 160 mA h g⁻¹ over 50 cycles. On the other hand, PAA used CoFe₂O₄ electrode delivered 470 mA h g⁻¹ even at high current density of 20C.²²⁶ Similarly, Wang group reported the Co₂GeO₄ nanosheets for anode materials using sodium alginate as a binder that could possess stable capacity of 1026 mA h g⁻¹ over 150 cycles. So, this excellent electrochemical performance such as cyclability and rate capability mainly attributed to the peculiar properties of sodium alginate binder.²⁹ Overall binders, the sodium alginate shows the good cycling stability and high rate capability compared to other so it is one the best binder to construct the Li-ion batteries.

6. Summary and outlook

In this overview, numerous research has been examined that focuses on the electrochemical performances of AB₂O₄- and A₂BO₄-structured materials and their corresponding pristine materials and that expounds on the large electrode polarization, high irreversible capacity loss, and unstable SEI-film formation that lead to poor cycling stability and rate capability. Hence, to achieve better electrochemical performances, researchers have focused on different strategies as summarized here. (i) The electrode materials have been prepared in different morphologies such as nanoparticles, hierarchical structures, nanowires, and nanotubes using different synthetic methods, including hydrothermal, solvothermal, thermal decomposition, and coprecipitation. (ii) Pristine materials have been composited with carbonaceous compounds such as core–shells of carbon and graphene. (iii) Binders such as CMC, PAA, and sodium alginate also play an important role in facilitating the electrochemical performance. (iv) Finally, electrolyte additives such as fluoroethylene carbonate (FEC) also contribute to cycling stability.

The first potential approach, i.e., while scaling down the electrode materials in to nanoscale, the large surface area will be obtained. This nanosize provides an access to the maximum number of reaction sites at the electrode–electrolyte interface as well as creating short diffusion-path length. Also it minimizes the volume expansion during Li-insertion when compared to the bulk-material counterparts, thus enhancing the specific capacities. However, this large surface area will leads to some unwanted side reactions with electrode–electrolyte interface which in turn the formation of a large amount of SEI layer that degrades the capacity. Mainly nanostructured electrode material has isolated often repeated cycling that inhibits the efficient flow of ions and electrons which leads to fast capacity fading on, especially at high current densities.^227–230

The next approach involves hierarchical structures. Here, the secondary nanoparticles that tend to aggregate themselves in oriented manner to form hierarchical microstructures. These are potentially reducing the side reactions with the electrolyte, increases the tap density and the available nanoscale building blocks increases the kinetics of lithium ion as well as the electrical properties when compared to nanoparticles.^105,231,232 However, unstable SEI-film formation causes high irreversible capacities and low coulombic efficiencies in the initial cycles.

Another approach involves the formation of composites with carbonaceous materials. The main advantages is that this prevents the side reactions with electrode/electrolyte interface and accommodate more volume strain. Among the composite, core–shell structure is the simplest strategy to enhance the electrochemical performance. Core consists of active material to involve the electrochemical reaction and shell consists of amorphous carbon which can act as protection layer to strengthen the performance of core particles. The shell has several advantages including, protecting the core from the unwanted side reactions with electrolyte, due to its flexible nature it can accommodate the volume strain and maintain the integrity, prevent the aggregation between the particles. The carbon shell has high electronic conductivity that leads to enhance the lithium ion diffusion rate. The shell thickness is also important factor that influences the electrochemical performance of core particles. So to make a proper thickness of carbon coating on the active material enhances the electrochemical performances.^4,214 Binders are another important strategy that can overcomes the core–shell structure and graphene composite, because binders such as CMC, PAA, and sodium alginate are capable of completely covering the active material and flexibly accommodate the swelling of the electrode material.^219,233 They also restrict the peel of active material from the current collector, facilitating the tight attachment of the active material on the current collector and enhancing the electronic conductivity. However, the binder PVDF does not.²¹⁹ The aforementioned key factors improve the electrochemical performance by accommodating the volume strain and enhancing the electronic conductivity and adhesion to the current collector. However, several problems are also persistent; stable SEI-film formation and large irreversible capacity loss mainly cause poor cycling stabilities and low coulombic efficiencies in the first cycles, thus leading to the safety problems of using LIBs. But it can be moderately overcome by using electrolyte additives such as fluoroethylene carbonate. FEC electrolyte additives facilitate to thin SEI film formation and reducing the irreversible capacity. Therefore, it is increasing the cycle life by means of modifying the surface chemistry of the SEI film.^87,234

In summation, it is concluded that the electrochemical performance of the mixed-transition-metal oxides, including characteristics such as the cycling stability and rate capability, are enhanced in the following ways: synthesized mixed-metal oxides with large surface areas with porosity, composites with carbonaceous materials (core–shells and graphene), highly flexible binders (CMC, PAA, and sodium alginate), and electrolyte additives such as fluoroethylene carbonate that reduce both the irreversible capacity loss and the formation of stable SEI films. The above modifications not only make mixed-metal oxides promising high-energy-density anode materials for LIBs, but due to their multivalent states and better electronic conductivities compared to corresponding simple metal oxides, they may also be excellent candidates for bifunctional electrocatalysts and supercapacitors. Therefore, mixed-metal oxides are some of the most important forthcoming materials for the next generation of energy-storage devices.

Acknowledgements

One of the authors (RKS) is grateful to UGC (No. 41-838/2012 (SR)) for their financial support under UGC-MRP. S. Yuvaraj would like to thank UGC-BSR (No. G2/5357/2013) for providing the fellowship to carry out this work successfully.

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