Recent advances in Mn-based oxides as anode materials for lithium ion batteries

Abstract

The development of new electrode materials for lithium-ion batteries (LIBs) is of great interest because available electrode materials may not meet the high-energy demands for electronic devices, especially the demands for good cyclic and rate performance. Mn-based oxides have received substantial attention as promising anode materials for LIBs due to their high theoretical specific capacities, low charge potential vs. Li/Li⁺, environmental benignity and natural abundance. Herein, the preparation of Mn-based oxide nanomaterials with various nanostructures and chemical compositions along with their applications as negative electrodes for LIBs are reviewed. The review covers MnO, Mn₃O₄, Mn₂O₃, MnO₂, CoMn₂O₄, ZnMn₂O₄ and their carbonaceous composite/oxide supports with different morphologies and compositions. The aim of this review is to provide an in-depth and rational understanding of the relationships among the chemical compositions, morphologies and electrochemical properties of Mn-based anode materials and, to understand how electrochemical performance can be improved using materials engineering strategies. Special attention has been paid to the discussion of challenges in the practical applications of Mn-based oxides in LIB full cells.

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1. Introduction

Since the beginning of the 21^st century, the call for more energy and a cleaner environment has forced many countries to invest large amounts of money in developing environmentally-friendly energy storage systems.¹ Compared to lead–acid, nickel–cadmium and nickel–metal hydride batteries, rechargeable lithium-ion batteries (LIBs) have the advantages of high energy density, long cycle life and memory-free effect, making them widely applicable in the portable electronic and electric tool market.^2–4 In the near future, they are expected to play even more important roles in electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs).⁴ However, they currently still cannot meet the demands dictated by the powering of EVs and PHEVs or by the large-scale energy storage systems needed for renewable energies. Therefore, a significant improvement in the lithium storage capacity of LIBs is urgently needed to enhance their feasibility for the aforementioned large-scale applications.

The charge–discharge mechanism of LIBs is based on the rocking-chair concept. A typical commercial LIB consists of a cathode (e.g., LiCoO₂) and an anode (e.g., graphite) together with an electrolyte-filled separator that allows lithium (Li) ion transfer but prevents direct contact between the electrodes. When the battery is charging, Li ions deintercalate from the cathode and intercalate into the anode. Conversely, the Li ions intercalate into the cathode via the electrolyte during discharge. During charge–discharge, Li ions flow between the anode and the cathode, enabling the conversion of chemical energy into electrical energy and the storage of electrochemical energy within the battery. The reactions in a typical battery are LiCoO₂ + 6C ↔ Li_1−xCoO₂ + Li_xC₆ (0 < x < 1).^5–8

LIB energy density depends on the capacities and operating potentials of the respective electrode materials. The identification of electrode materials that can yield higher capacity and energy density, e.g., cathode materials with higher operating voltages (≥4.0 V vs. Li/Li⁺)^9–12 and anode materials with lower operating voltages (≤1.0 V vs. Li/Li⁺), while having low environmental cost is essential.⁸ Research on anode materials for LIBs has been directed towards new materials that involve three different mechanisms: (1) intercalation–deintercalation mechanism, e.g., compounds with a two-dimensional (2D) layer structure¹³ or a three-dimensional (3D) network structure¹⁴ that can reversibly intercalate/deintercalate Li⁺ into/out of the lattice; (2) elements (M) that can form alloys with Li metal (Li_xM), e.g., Si,^15–17 Sn/SnO₂,^18–23 Ge,²⁴ Sb,²⁵ Zn,²⁶ In, Bi, and Cd;²⁷ and (3) materials that can act as an anode in the so-called redox or “conversion” reaction with Li.²⁸ These materials are mainly oxides, fluorides, oxyfluorides, sulfides, nitrides and phosphides.²⁹ As a consequence, a large number and wide variety of metal-containing oxides including binary and complex oxides have been explored for Li cycling.^8,30–32 In addition, modern nanotechnology has been employed to prepare many metal oxides as anode materials for LIBs; these materials contain various morphologies and structures for Li cycling.⁷ As a result, research on metal oxides as anodes for LIBs has made significant progress in recent years.⁸

Among the metal oxides used as anode materials for LIBs, Mn-based oxides have obvious advantages because of their high specific capacities and low toxicity, cost, and operating voltage (average discharge and charge voltages of 0.5 V and 1.2 V vs. Li/Li⁺, respectively) compared to the iron,^33–35 cobalt^36–38 and Ni-based oxides.^39,40 Table 1 lists the experimental values of the plateau potentials and theoretical specific capacities associated with conversion reactions in Mn-based oxides.^41–45 In a Li/Mn_xO half-cell, the electrochemical processes can be summarized by the following equation:

MnO_x + 2xLi⁺ + 2xe⁻ ↔ Mn + xLi₂O

Table 1 Experimental values of potential for the plateaus and theoretical specific capacities associated with conversion reactions in Mn-based oxides

Mn-based oxides	Potential (Li^+/Li)	Theoretical specific capacity (mA h g^−1)	Ref.
MnO	0.2	756	41
Mn3O4	0.37	937	42
Mn2O3	0.3	1019	43
MnO2	0.4	1223	43
CoMn2O4	0.3	921	44
ZnMn2O4	0.3	1008	45

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This reaction for Li storage in Mn-based oxides based on the “conversion mechanism” has four major obstacles for practical application:²⁹ (1) the strong structural reorganization that must take place to accommodate the chemical changes induces large changes in volume, resulting in severe particle agglomeration and unsatisfactory cycling performance; (2) the low rate performance arising from their low conductivity; (3) an unacceptably large voltage hysteresis in terms of round-trip energy density loss, which is observed between the discharge and charge steps; and (4) a virtually ubiquitous large coulombic inefficiency observed in the first cycle.

Different strategies have been proposed to resolve the aforementioned problems of Mn-based anodes, and four of them are widely practiced. The first strategy is to fabricate nanosized particles to shorten the diffusion length for electrons and lithium ions.⁴⁶ The second strategy is to synthesize Mn-based oxides with porous structures containing porous 1D nanorods/nanowires⁴⁷ and hollow nanotubes,⁴⁸ 2D nanoflakes,⁴⁹ and 3D hollow or hierarchical porous nanostructures.⁵⁰ The vacant space provided by the hollow or porous structures can accommodate the structural strain to facilitate fast Li⁺ insertion/extraction kinetics by enabling electrolyte uptake through the nanoparticles; this leads to improved rate and cycling performance. The third strategy is to introduce a considerable proportion of carbon into the Mn-based oxides to form homogenous oxides/carbon nanocomposites.⁴⁶ It has been suggested that the carbon-based materials can provide a cushioning effect against the volume strain because of their elastic character. Meanwhile, carbon-based materials could also increase the electrical conductivity of the nanocomposites, which is essential to the rate performance and cyclability. Carbon is also active at the low voltages at which some of the conversion reactions occur, partially compensating for the decrease in capacity associated with the dilution of the active phase. The fourth strategy is to optimize the electrodes by adjusting the ratios of active materials, binders and conductive agents or by choosing novel binders/conductive agents.⁵¹ The proper selection of the ratios of these materials has been demonstrated to generate significant improvement in the rate and cyclic performance of metal oxides-based⁵² and other anode materials.⁵¹ Because of the potential applications of nano-scale Mn-based oxides as anode materials for LIBs, it is of great importance to have a more detailed discussion on these kinds of materials along with their various nanostructures, chemical compositions and electrode fabrication processes.

Cabana et al.²⁹ gave an excellent review of the different conversion reactions of metal oxides, sulfides, nitrides, phosphides, and fluorides in 2010; the development of Mn-based oxides as anode materials for LIBs before 2010 were extensively reviewed in this work. Wu et al.³⁰ gave another good review on the topic of metal oxides as anode materials for LIBs in 2012. Reddy et al.⁸ reviewed the detailed development of metal oxides and oxysalts as anode materials for LIBs. In addition to highlighting some of the recent advances in Mn-based oxides as anode materials for LIBs, these three excellent review papers demonstrated that the field of metal oxides as anode materials is a very active and important LIB research area. Although some recent advances on this topic have been reviewed in the aforementioned papers, continuous updates are still required to reflect the rapid developments of strategies for improving the electrochemical performance of Mn-based oxides. In order to better understand the status of research on this topic, this review mainly discusses the recent developments, current status and major strategies related to the performance enhancement of Mn-based oxides with various structures and chemical compositions. The oxides reviewed in this work include MnO, Mn₃O₄, Mn₂O₃, MnO₂, CoMn₂O₄ and ZnMn₂O₄ as well as their carbon/metal oxides-based composites. The key challenges and opportunities for their future developments and applications are also discussed. Given the broad nature and rapid development of the field of Mn-based oxides, the authors apologize for the inevitable oversights of some important contributions.

2. Binary Mn-based oxides as anode materials

Mn-based binary oxides have been widely investigated as anode materials for LIBs in the past decades after the works of Tarascon et al. in 2000.²⁸ Many binary Mn-based oxides including MnO, Mn₃O₄, Mn₂O₃, MnO₂ and their carbon-based nanocomposites with different nano-structures have found useful applications in LIBs.⁸

2.1 MnO and its nanocomposites

Although MnO has a high theoretical capacity of 755.6 mA h g⁻¹, which is twice as the capacity of graphite, and a lower electromotive force (1.032 V vs. Li/Li⁺) than other transition metal oxides (TMO) anodes such as Fe₂O₃, Co₃O₄, NiO, CuO, and so forth, few examples of the Li cyclability of pure MnO anode materials have been reported in recent years.^46,49,53,54 This may be attributed to the fact that manganese is one of the first row transition metals, which are very difficult to be reduced into the metallic state.²⁹ Zhong et al.⁴⁶ investigated commercial MnO powders as anode-active materials for LIBs. According to the results of ex situ XRD, TEM and a galvanostatic intermittent titration technique, they suggested that lithium is stored reversibly in MnO through the conversion reaction and an interfacial charging mechanism. In addition, they found that a layer of the solid electrolyte interphase with a thickness of 20–60 nm is covered on MnO particles after full insertion. The cyclic performance of MnO is significantly improved by decreasing particle size. Following this work, MnO anode materials have been fabricated with microsphere,⁵³ nanoflake⁴⁹ and thin-film⁵⁴ forms to improve their cycling and rate performance. In 2011, Zhong et al.⁵³ synthesized porous MnO microspheres by decomposing MnCO₃ under argon atmosphere at 600 °C for 1 h. The as-prepared MnO material shows a reversible capacity of 800 mA h g⁻¹, and an initial coulombic efficiency of 71%. It can deliver 600 mA h g⁻¹ at a rate of 400 mA g⁻¹. They further confirmed the conversion reaction mechanism by Mn K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies, indicating that pristine MnO is reduced to Mn⁰ after discharging to 0 V, and that part of the reduced Mn⁰ is not oxidized to Mn²⁺ after charging to 3 V. Li et al.⁴⁹ developed a novel method for the preparation of porous MnO nanoflakes on nickel foam (Fig. 1a–d) by the reduction of a hydrothermally-synthesized MnO₂ precursor under hydrogen. The porous MnO nanoflakes showed good capacity retention capability and rate performance (Fig. 1e), with a capacity of 568.7 and 708.4 mA h g⁻¹ at the 2^nd and 200^th charge–discharge cycles, respectively, at a current density of 246 mA g⁻¹; the capacity was 376.4 mA h g⁻¹ at a current density as high as 2460 mA g⁻¹. Thin films of MnO were successfully deposited on Cu foils by radio-frequency (RF) sputtering under an Ar : H₂ (95 : 5 vol%) reduction atmosphere at 500 °C.⁵⁴ These films have approximately 0.5 mm thicknesses and exhibit an initial coulombic efficiency of 75%; after 100 cycles, their reversible specific capacity is 700 mA h g⁻¹ and 428 mA h g⁻¹ at 0.05 C and 20 C, respectively. These values demonstrate that the sputter-grown MnO films exhibit excellent cyclability and rate performance in comparison with MnO powders. This study suggests that pure phases with low oxidation states and certain porosities might be favourable, accounting for the improved electrochemical performance.

Fig. 1 (a and b) Low-magnification SEM images of the samples before and after reduction; (c and d) high-magnification SEM images of the samples before and after reduction; and (e) the discharge capacity of MnO nanoflakes at various current densities. Reproduced with permission.⁴⁹ Copyright 2012, RSC.

In addition to phase-pure MnO, MnO-based nanocomposites have also been intensively studied as negative electrodes due to their improved electrochemical properties (Table 2).^46,55–74 The group of Li⁴⁶ synthesized carbon-coated commercial MnO (MnO-L) by ball milling with sugar followed by pyrolysis at 600 °C in Ar atmosphere (C/MnO-L). These materials have a reversible capacity of ∼650 mA h g⁻¹ at 50 mA g⁻¹ (0.08 C), and are almost stable up to 150 cycles in the voltage range of 0.01–3.0 V. A good rate performance (400 mA h g⁻¹ at a rate of 400 mA g⁻¹) was also found. Additionally, they demonstrated that the C/MnO-L nanocomposites have a lower voltage hysteresis (<0.7 V with a charging voltage of 1.2 V) during the discharge–charge cycles compared to other metal oxides.

Table 2 A summary of recent studies on MnO-based anodes for LIBs

Materials	Preparation method	1^st discharge–charge capacity (mA h g^−1)	Capacity retention	Rate performance (mA h g^−1)	Ref.
MnO powder	Commercial + ball milling	∼1266/766 at 50 mA g^−1	550 (10 cycles)	400 at 400 mA g^−1	46
MnO microspheres	Precipitation + annealing	1126/800 at 50 mA g^−1	704 (50 cycles)	600 at 400 mA g^−1	53
Porous MnO nanoflakes	Hydrothermal method	781/570 at 246 mA g^−1	648 (100 cycles)	376 at 2460 mA g^−1	49
Crystalline MnO films	Radio-frequency (RF) sputtering + annealing	550/413 at 0.05 C	350 (100 cycles)	150 at 20 C	54
Amorphous MnOx/C	Aerosol spray pyrolysis	1083/650 at 200 mA g^−1	601 (130 cycles)	500 at 800 mA g^−1	55
MnOx/OMC nanocomposite	Wet-impregnation + annealing	1390/860 at 100 mA g^−1	950 (50 cycles)	500 at 2000 mA g^−1	56
Mesoporous MnO–C	Microwave-polyol process + annealing	1456/1000 at 200 mA g^−1	1224 (200 cycles)	731 at 1500 mA g^−1	57
Monodisperse MnO–C hollow microspheres	A facile biotemplating technique	1021.9/755.6 mA g^−1	702.2 (50 cycles)	230 at 3000 mA g^−1	58
(MnO/MWNTs)	Hydrothermal + annealing	1050/686 at 14.4 mA g^−1	484 (200 cycles) at 144.1 mA g^−1	455 at 310 mA g^−1	59
MnO-attached graphene	Impregnation + annealing	685/635 at 0.2 C	490 (30 cycles) at 1C	410 at 5 C	60
MnO nanoparticles/graph-ene composite	In situ carbothermal reduction	1080/700 at 100 mA g^−1	782 (60 cycles)	402 at 1000 mA g^−1	61
Nitrogen-doped MnO/graphene nanosheets	Hydrothermal method + ammonia annealing	1193/829 at 50 mA g^−1	772 (90 cycles) at 100 mA g^−1	202 at 5000 mA g^−1	62
(MnO/RGOS)	Liquid phase deposition + annealing	1001/648 at 100 mA g^−1	666 (50 cycles)	454 at 400 mA g^−1	63
MnO/graphene	Liquid phase deposition + annealing	900/891 at 200 mA g^−1	2014 (150 cycles)	843 at 2000 mA g^−1	64
Coaxial MnO–C nanotubes	Self-templates method	1014/678 at 75.5 mA g^−1	600 (10 cycles)	500 at 188.9 mA g^−1	65
MnO–C core–shell nanorods	In situ reduction method	1090/780 at 200 mA g^−1	600 (40 cycles)		66
MnO@carbon core–shell nanowires	Solvothermal + annealing	1196/840 at 100 mA g^−1	801 (200 cycles) at 500 mA g^−1	462 at 2000 mA g^−1	66
MnO@1-D carbon composites	Annealing of C4H4MnO6	1249/738 at 500 mA g^−1	660 (1000 cycles)	250 at 6 Ag^−1	67
Porous MnO–C nanotubes	Hydrothermal method + annealing	1129/810 at 100 mA g^−1	763 (100 cycles)	303 at 3200 mA g^−1	68
MnO–C nanorods	Hydrothermal method + annealing	1408/707 at 200 mA g^−1	481 (50 cycles)	252 at 1000 mA g^−1	69
Porous carbon-modified MnO disks	Microwave-polyol process + annealing	1387/997 at 100 mA g^−1	1044 (140 cycles)	535 at 1000 mA g^−1	70
Nano-MnO–C composites	An alcoholysis process + annealing	1650/900 at 75.5 mA g^−1	939 (30 cycles)	589 at 7550 mA g^−1	71
MnO@C core–shell nanoplates	Hydrothermal method + annealing	1365/770 at 200 mA g^−1	563 (30 cycles)	550 at 300 mA g^−1	72
MnOx/C nanocomposite	Sol–gel method + annealing	1055/700 at 200 mA g^−1	500 (100 cycles)	320–345 at 2000 mA g^−1	73
Hierarchical micro/nanostructured MnO	Hydrothermal method	1051/750 at 98 mA g^−1	783 (200 cycles)	350 at 1572 mA g^−1	74

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Inspired by the positive effects of carbon-coating, the MnO–C nanocomposites as anode materials for LIBs have received considerable interest.^55–59 For example, unique amorphous MnO_x–C nanocomposite particles with interspersed carbon have been synthesized using aerosol spray pyrolysis.⁵⁵ The amorphous MnO_x–C nanoparticles showed a high reversible capacity of approximately 650 mA h g⁻¹ under a current density of 200 mA g⁻¹, with an exceptional capacity retention of 93% after 130 cycles. The coulombic efficiency of the amorphous MnO_x–C remained stable during cycling after the first few cycles. They suggested that the observed 100% coulombic efficiency is attributed to the absence of new SEI formation during cycling, where carbon blocks liquid electrolyte penetration into MnO_x–C particles, reducing the contact area between MnO_x and the electrolytes. Chae et al.⁵⁶ obtained an MnO_x/OMC (OMC = ordered mesoporous carbon) nanocomposite by a simple wet-impregnation of Mn(NO₃)₂ aqueous solution onto OMC nanorods followed by thermal treatment at 450 °C in an Ar flow (Fig. 2a). The MnO_x/OMC composite exhibited a high reversible capacity (>950 mA h g⁻¹) after 50 deep charge–discharge cycles; it also exhibited excellent cycling stability (Fig. 2b), acceptable coulombic efficiency (97%) and good rate capability (750 mA h g⁻¹ at 1000 mA g⁻¹) (Fig. 2b and c). The incorporation of insulating and high density MnO_x into OMC nanorods resulted in synergistic benefits including higher volumetric and specific capacities, and a smaller redox voltage hysteresis compared to OMC nanorods.

Fig. 2 (a) Schematic diagram for the preparation of MnO_x-loaded OMC nanorods; (b) specific capacities of OMC nanorods and MnO_x/OMC with cycle number; and (c) rate performances of MnO_x/OMC, OMC nanorods and graphite cycled at different current densities. Reproduced with permission.⁵⁶ Copyright 2012, RSC.

Very recently, Luo et al.⁵⁷ developed an efficient microwave-polyol process with subsequent thermal treatment for the synthesis of MnO nanoparticles encapsulated uniformly in 3D mesoporous interconnected carbon networks (MnO-MICN). Sodium citrate plays an important role in the morphology of the Mn-based precursors. Interestingly, the specific capacities (∼30% of the overall capacity) of the nanocomposite anode materials gradually increases after 100 cycles. The authors suggested that a new electrochemical reaction might be occurring; for instance, Mn²⁺ may be oxidized to a higher oxidation state. After 200 cycles at 200 mA g⁻¹, the specific discharge capacity reaches 1224 mA h g⁻¹ with a coulombic efficiency of ∼99%. Moreover, a specific capacity of 731 mA h g⁻¹ is retained after 200 cycles at 1500 mA g⁻¹, indicating good rate performance of the nanocomposites. Inspired by natural microalgae that have desirable characteristics (easy availability, biological activity, and a carbon source), Xia et al.⁵⁸ developed a green and facile biotemplating method to fabricate monodisperse MnO–C microspheres. Due to the unique hollow porous structure where MnO nanoparticles were tightly embedded in a porous carbon matrix, forming a penetrative shell, the MnO–C microspheres exhibited a high reversible specific capacity of 700 mA h g⁻¹ at 0.1 A g⁻¹, excellent cycling stability with 94% capacity retention after 50 cycles, and good rate performance of 230 mA h g⁻¹ at 3 A g⁻¹. More recently, manganese monoxide nanocomposites and multi-walled carbon nanotubes (MnO/MWNTs) have been hydrothermally prepared by MWNTs and KMnO₄ followed by a sintering process.⁵⁹ These nanocomposites also show improved electrochemical performances as anode materials for LIBs, with a reversible capacity as high as 770.6 mA h g⁻¹ at the current density of 7.21 mA g⁻¹ and good capacity retention (over 92.4% after 200 cycles at 216.14 mA g⁻¹ between 0.01 and 3.0 V).

The strategy of preparing MnO/graphene composites has been demonstrated to be an effective route to obtain enhanced lithium-storage properties because graphene has superior conductivity, large surface area, structural flexibility, and chemical stability. In 2011, a composite of graphene nanosheets (GNs) supported by MnO nanocrystals was fabricated through a chemical-wet impregnation followed by thermal reduction.⁶⁰ The MnO-attached GN anode delivers a reversible capacity of 635 mA h g⁻¹ at 0.2 C at voltages between 0.01 and 3.5 V, a high coulombic efficiency (92.7%) during the 1^st cycle and good rate capability [capacity retention (5/0.2 C: >70%)]. They also investigated the effect of Mn/C atomic ratio on coulombic efficiency and found that the reversibility of Li⁺ intercalation/deintercalation gradually increases with the Mn/C atomic ratio. This result may be due to two major reasons: (i) Mn ions tend to terminate dangling bonds located at the edges of GNs, forming MnO-terminated edges of graphene fragments, and (ii) the appropriate amount of MnO spacers tend to stabilize the GN network, offering more active sites for reversible Li-storage. After this work, MnO/graphene nanocomposites with different MnO structures have been synthesized by different routes and used as anode materials for LIBs. Qiu et al.⁶¹ prepared a MnO nanoparticle/graphene composite (the diameters of the MnO nanoparticles range from 20 to 250 nm) via in situ carbothermal reduction of Mn₃O₄ on the surface of graphene nanosheets. The detachment and agglomeration of MnO nanoparticles were effectively prevented by the tight interface between the MnO nanoparticles and graphene. The MnO nanoparticle/graphene composites show a high specific capacity of approximately 700 mA h g⁻¹ at 100 mA g⁻¹, excellent cyclic stability, and good rate capability as an anode material for LIBs. Zhang et al.⁶² synthesized a nitrogen-doped MnO/graphene nanosheet (N-MnO/GNS) hybrid material by a hydrothermal method followed by ammonia annealing (Fig. 3a). Because of its unique N-doped nanostructure and efficiently-mixed conducting network, this hybrid nanostructure exhibits a reversible electrochemical lithium storage capacity as high as 772 mA h g⁻¹ at 100 mA g⁻¹ after 90 cycles (Fig. 3b), and an excellent rate capability of 202 mA h g⁻¹ at a high current density of 5 A g⁻¹ (Fig. 3c).

Fig. 3 (a) Schematic illustration of the preparation of N-MnO/GNS hybrid materials; (b) comparison of cycling performance of N-MnO, N-GNS, and N-MnO/GNS hybrids at a current density of 100 mA g⁻¹; and (c) rate capability of N-MnO and N-MnO/GNS at various current densities. Reproduced with permission.⁶² Copyright 2012, ACS.

Mai et al.⁶³ synthesized a MnO/reduced graphene oxide sheet (MnO/RGOS) hybrid by a two-step electrode design consisting of a liquid phase deposition of MnCO₃ nanoparticles on the surface of graphene oxide sheets followed by heat treatment in nitrogen flow. The MnO/RGOS hybrid electrode shows a reversible capacity of 665.5 mA h g⁻¹ after 50 cycles at a current density of 100 mA g⁻¹ and delivers 454.2 mA h g⁻¹ at a rate of 400 mA g⁻¹. The MnO/RGOS hybrid electrode exhibits an enhanced reversible capacity with respect to that of the MnO electrode, which is attributed to the re-oxidation of the Mn/Li₂O nanocomposite to MnO nanoparticles. In addition, the authors suggested that the incorporation of RGOS is not very effective in decreasing the hysteresis voltage of the MnO electrode by the galvanostatic intermittent titration technique (GITT) as well as by the electrochemical impedance spectroscopy (EIS). This remarkable hysteresis has both thermodynamic and kinetic origins. In 2013, Sun et al.⁶⁴ reported a facile solution-based method combined with a subsequent reduction process for the large-scale fabrication of a MnO/graphene nanocomposites. The prepared MnO/graphene hybrid anode exhibits an initial reversible capacity of around 890.7 mA h g⁻¹ at a current density of 200 mA g⁻¹ and a reversible capacity as high as 2014.1 mA h g⁻¹ after 150 discharge–charge cycles (Fig. 4). In addition, the composite shows excellent rate capability (625.8 mA h g⁻¹ at 3000 mA g⁻¹) and superior cyclability (843.3 mA h g⁻¹ even after 400 cycles at 2000 mA g⁻¹ with only 0.01% capacity loss per cycle). Its specific capacity is much higher than the theoretical capacities of both MnO (∼756 mA h g⁻¹) and graphene (∼744 mA h g⁻¹). The lithiation and delithiation behaviours suggest that the further oxidation of Mn(II) to Mn(IV), similar to the recent report of Luo et al.,⁵⁷ and the interfacial lithium storage upon cycling contribute significantly to the enhancement of specific capacity.

Fig. 4 (a) Discharge and charge voltage profiles of the synthesized MnO/graphene anode at a current density of 200 mA g⁻¹; (b) cycling performance of the prepared MnO/graphene hybrid electrode cycled at 200 mA g⁻¹ and (c) at various current densities of 200, 400, 800, 1200, 1600, 2000, and 3000 mA g⁻¹; and (d) discharge and charge curves at a current density of 2000 mA g⁻¹. All were carried out in the voltage range 3–0.01 V vs. Li. Reproduced with permission.⁶⁴ Copyright 2013, Wiley.

One dimensional (1D) nanostructures have attracted considerable attention as anode materials for LIBs since they allow better accommodation of volume changes during repetitive charge–discharge cycles. 1D nanostructures also possess direct 1D electronic pathways for efficient charge transport. In 2011, 1D coaxial MnO–C nanotubes with an average diameter of approximately 450 nm, a wall thickness of approximately 150 nm, a length of 1–5 μm and a 10 nm thick carbon layer were first reported by Ding et al.⁶⁵ using β-MnO₂ nanotubes as self-templates in acetylene at 600 °C. This sample exhibits a reversible capacity of about 500 mA h g⁻¹ at a current density of 188.9 mA g⁻¹ and a capacity retention of 83.9%. Although the as-prepared sample shows better lithium storage properties compared to bare MnO nanotubes (58.2%) and MnO nanoparticles (25.8%), its electrochemical performance is still not satisfactory due to a quick fading in capacity. The enhanced reversible capacity and cycling stability for coaxial MnO–C-NTs are attributed to the conductive carbon layer on the MnO nanotubes, which can provide improved electronic transport to the MnO nanotube core and can be considered as a buffering layer to alleviate the volume changes caused by repetitive cycling. In recent years, the reduction of MnO_x (x > 1) with special nanostructural features has been more widely practiced. The group of Wang⁶⁶ synthesized MnO–C core–shell nanorods by an in situ reduction method using MnO₂ nanowires as a precursor and block copolymer F127 as a carbon source in a gas flow of 5 vol% H₂ in Ar. As anode materials for lithium ion batteries, the as-prepared MnO–C core–shell nanorods exhibit a higher specific capacity than MnO microparticles; they have a specific discharge capacity of approximately 700 mA h g⁻¹ after 40 cycles under a current density of 200 mA g⁻¹. Li et al.⁶⁷ reported a facile synthetic method for the preparation of MnO@carbon core–shell nanowires with a jointed appearance based on the reduction of Mn₂O₃ precursors followed by graphitization of C₂H₂. The MnO@carbon core–shell nanostructures could deliver reversible capacities as high as 801 mA h g⁻¹ at a high current density of 500 mA g⁻¹, with excellent electrochemical stability over 200 cycles. The remarkable electrochemical performance is mainly attributed to the highly-uniform carbon layer around the MnO nanowires, which is not only effective in buffering the structural strain and volume variations of anodes during repeated electrochemical reactions, but also greatly enhances the conductivity of the electrode material. They also suggested that their strategy is simple but very effective, and appears to be sufficiently versatile, with the ability to be extended to other high-capacity electrode materials with large volume variations and low electrical conductivities. More recently, Zhu's group⁶⁸ developed a novel route to synthesize MnO@1D carbon composites through a pyrolysis process using C₄H₄MnO₆ as the single-molecule precursor. The MnO nanoparticles are uniformly dispersed inside or adhered to the surface of the 1D carbon nanotubes, which overlap each other to form carbon scaffolds. This special structural feature not only improves the electronic conductivity, but also provides a support for loading MnO nanoparticles, resulting in a high specific capacity (1482 mA h g⁻¹ at 200 mA g⁻¹), good rate performance (420 mA h g⁻¹ at 2.0 A g⁻¹) and excellent capacity retention capability (810 mA h g⁻¹ at 1460 mA g⁻¹ after 1000 cycles).

The solvo/hydrothermal approach has been demonstrated as a suitable route for the preparation of 1D MnO–C nanocomposites. For examples, Sun's group⁴⁸ developed and employed a facile hydrothermal method followed by thermal annealing to synthesize porous MnO–C nanotubes (Fig. 5). Electrochemical results demonstrate that the porous MnO–C nanotubes can deliver a reversible capacity as high as 763.3 mA h g⁻¹ after 100 cycles at a charge–discharge current density of 100 mA g⁻¹ (0.13 C, 1 C = 755.6 mA g⁻¹); the capacity is 618.3 mA h g⁻¹ after 200 cycles at a rate of 0.66 C (Fig. 6). The authors suggested that the superior cyclability and rate capability are attributed to the hollow interior, porous structure, the 1D structure and the uniformly-dispersed carbon in the porous MnO–C nanotubes. Su et al.⁶⁹ reported a two-step hydrothermal treatment and subsequent sintering at 600 °C to prepare hierarchical MnO@C nanorods using KMnO₄ and citric acid as raw materials. The ultra-small MnO nanocrystals (<5 nm) were homogeneously dispersed in a carbon matrix and further coated with a well-proportioned carbon shell. This sample exhibits a reversible capacity of 481 mA h g⁻¹ after 50 cycles at a current density of 200 mA g⁻¹, which is higher than that of carbon-coated MnO nanoparticles.

Fig. 5 (a) Low and (b) high magnification SEM images; (c) TEM image and (d) XRD pattern of the porous MnO–C nanotubes. Reproduced with permission.⁴⁸ Copyright 2012, RSC.

Fig. 6 (a) Charge–discharge curves and (b) cycle performances of commercial MnO particles (short dash dot) and the porous MnO–C nanotubes (solid) at a charge–discharge rate of 0.13 C; (c) cycle performance of the porous MnO–C nanotubes at a charge–discharge rate of 0.66 C; (d) rate performance of the porous MnO–C nanotubes. Reproduced with permission.⁴⁸ Copyright 2012, RSC.

The 2D nanostructures of MnO-based nanocomposites are not as well-researched as their 1D counterparts, likely due to the difficulty in fabricating well-defined MnO-based nanocomposites. Sun et al.⁷⁰ prepared porous carbon-modified MnO disks with approximately 50 nm average thicknesses and up to 3 mm diameters by a microwave-polyol process followed by thermal annealing of the disk-like Mn-complex precursor. In their work, they demonstrated that each C–MnO disk has a single-crystal-like nature and is built up by the assembly of ∼12 nm carbon-modified MnO nanocrystals having the same crystallographic orientations. Because the unique assembled nano-architecture involves three-dimensionally interconnected nanopores and carbon modification as well as small MnO nanocrystal particle sizes, the as-synthesized C–MnO nanocomposites exhibit high capacities (an initial capacity of 1387.2 mA h g⁻¹ with a coulombic efficiency of 71.9% at a current density of 100 mA g⁻¹ in the first cycle) and excellent cycling stability (a reversible capacity of 1044.2 mA h g⁻¹ after 140 cycles at 100 mA g⁻¹). It is also worthy to note that the capacity drops firstly and then increases upon cycling, which has been observed in the case of MnO–C,⁵³ MnO/reduced graphene oxide⁶³ and MnO_x/C.⁵⁵ The authors suggested that the mechanism behind the U-shaped capacity retention curve is not clear, but may be attributed to the formation of high oxidation state products⁴⁹ or the mixed effects of Mn cluster aggregation and reversibility improvement of the conversion reaction in manganese oxide due to the formation of defects and deformations.⁵⁵ Although the carbon materials (amorphous carbon, carbon nanotube and graphene) have apparent beneficial effects on the electrochemical cycling and rate performance of MnO-based composites, they have very low electrochemical activity and limited lithium storage capacity. Therefore, the carbon content in the composites needs to be carefully adjusted in order to find a balance between electrode capacity and cycle life. Chen's group⁷¹ synthesized carbon-coated nano-MnO powders (MnO–C) with different carbon contents by the reduction of Mn₃O₄ with a particle size of about 20 nm, using glucose as both a carbon source and a reducing agent. The electrode fabricated using 10.7 wt% carbon-coated MnO–C shows the best cycling stability and rate performance, with a high reversible capacity of 939.3 mA h g⁻¹ after 30 cycles at 0.1 C. By controlling the treatment temperature and reaction time, Zhang et al.⁷² synthesized MnO@C core–shell nanoplates with controllable carbon shell thicknesses via the thermal treatment deposition of acetylene on Mn(OH)₂ nanoplate precursors (Fig. 7a). By controlling the temperature of carbon deposition and the reaction time, the thickness of the carbon shell can be tuned from 3.1 to 13.7 nm. The electrochemical experimental results show that the MnO@C nanoplates with a carbon shell thickness of 8.1 nm display a higher reversible capacity (770 mA h g⁻¹ at a current density of 200 mA g⁻¹) and better cyclability than those with carbon shell thicknesses of 3.1, 4.0, 4.2, 10.9 and 13.7 nm (Fig. 7b).

Fig. 7 (a) Schematic illustrating the formation process of MnO@C; (b) discharge capacity vs. cycle number for MnO@C products synthesized at different conditions at a current density of 200 mA g⁻¹; and (c) the rate performance of the MnO@C products synthesized at 550 °C for 10 h. Reproduced with permission.⁷² Copyright 2012, RSC.

In summary, results from this series of studies demonstrate the high capacities of MnO-base composites and the improved cyclic and rate performances generated by many strategies; however, these data were obtained using Li/MnO half-cells. At present, few papers^73,74 have reported full cells using MnO-based oxides as anodes and LiMn₂O₄ or LiNi_0.5Mn_1.5O₄ as cathodes. A lithium ion battery using LiMn₂O₄ as the cathode and MnO_x/mesoporous carbon as the anode was reported by Lee et al.⁷³ The capacity of the MnO_x/C anode decayed gradually from 700 mA h g⁻¹ to 500 mA h g⁻¹ after 100 cycles, and the MnO_x/mesoporous carbon/LiMn₂O₄ full cell operates at an average working voltage of 3.3 V with a capacity of 105 mA h g⁻¹ (Fig. 8).

Fig. 8 (a) Voltage profile; (b) cycling performance of LMO–MnO_x/C (A) cell at 0.2 C for 100 cycles and then at 0.1 C for up to 188 cycles; (c) cycling performance of LMO–MnO_x/C (B) cell at 0.2 C for up to 195 cycle; and (d) rate performance of LMO–MnO_x/C (B) cell. Voltage cut-off range is between 2.0 and 4.1 V. Reproduced with permission.⁷³ Copyright 2013, Elsevier.

In a recent paper, Sun's group⁷⁴ successfully developed a facile route to synthesize a hierarchical micro/nanostructured MnO anode and a spinel LiNi_0.5Mn_1.5O_4−δ cathode using olive-shaped hierarchical micro/nanostructured MnCO₃ as a precursor. Both the MnO and LiNi_0.5Mn_1.5O_4−δ materials possess excellent lithium storage properties due to their unique hierarchical micro/nanostructures. The MnO in a MnO/Li half-cell can deliver a reversible capacity of 782.8 mA h g⁻¹ after 200 cycles at a rate of 0.13 C along with a stable reversible capacity of 350 mA h g⁻¹ at a high rate of 2.08 C. A full lithium ion battery using MnO as anode and LiNi_0.5Mn_1.5O_4–δ as cathode demonstrates a high discharge specific energy ca. 350 W h kg⁻¹ after 30 cycles at 0.1 C, an average working voltage at 3.5 V and long cycle stability. The MnO/LiNi_0.5Mn_1.5O_4−δ full cell can maintain a discharge specific energy of 227 W h kg⁻¹ after 300 cycles at a higher rate of 0.5 C (Fig. 9). Considering these two successful examples, full cells assembled with MnO-based nanocomposite anodes may present a promising potential application for next-generation LIBs.

Fig. 9 (a) Charge–discharge profiles and (b) cycling performance of the MnO/LNMO full cell at a rate of 0.1 C; (c) cycling performance of the MnO/LNMO full cell at a rate of 0.5 C; and (d) the discharge specific energies of the MnO/LNMO full cell at different rates (0.05 C charge). Reproduced with permission.⁷⁴ Copyright 2013, ACS.

2.2 Mn₃O₄ and its nanocomposites

In 1983, the group of Goodenough⁷⁵ reported the electrochemical insertion of Li into Mn₃O₄. However, few studies^75–78 have reported this material as an anode for LIBs, partially due to its extremely low electrical conductivity (∼10⁻⁷ to 10⁻⁸ S cm⁻¹), limiting its capacity to ∼400 mA h g⁻¹, even with Co doping. The optimization of synthetic conditions is therefore necessary to obtain single-phase Mn₃O₄ materials. In 2010, Li's group⁷⁹ reported a facile solvothermal process for the controllable synthesis of Mn₃O₄ nanocrystals with different sizes and shapes including dots, rods, and wires in the presence of the surfactants dodecanol and oleylamine. The as-prepared monodisperse nanocrystals, acting as ideal building blocks, can be rationally assembled into 3D Mn₃O₄ colloidal spheres using a facile ultrasonication strategy. This paper gave a good example of the preparation of pure Mn₃O₄ nanomaterials; subsequently, this mixed-valent oxide (Mn₃O₄) and its nanocomposites have been extensively investigated as anode materials by the “conversion reaction” of lithium, and they continue to be studied (Table 3).^80–89 Shen et al.⁸⁰ synthesized Mn₃O₄ nanorods with diameters of 50–150 nm and lengths of approximately 1–2 μm via the heat-treatment of MnCO₃ nanorods, which were obtained from a facile hydrothermal method, in nitrogen atmosphere. When applied as anode materials for LIBs, the Mn₃O₄ nanorods exhibited a reversible lithium storage capacity of 1050 mA h g⁻¹ in the first few cycles. However, the pure Mn₃O₄ nanorods show a rapid capacity loss with increasing cycle number; the capacity decreased to 108 mA h g⁻¹ at the 100^th cycle. Engineering the morphology of Mn₃O₄ nanomaterials has been an effective strategy to enhance their electrochemical performance. Therefore, other pure Mn₃O₄ materials with different morphologies and microstructures including octahedral Mn₃O₄ crystals with exposed high-energy facets,⁸¹ sponge like Mn₃O₄ nanostructures,⁴³ mesoporous stacked Mn₃O₄ nanosheets,⁸² foam-like porous Mn₃O₄,⁸³ mesoporous Mn₃O₄ nanotubes,⁸⁴ order-aligned Mn₃O₄ nanostructures⁸⁵ and Mn₃O₄ octahedrons with typical diameters around 300–400 nm (ref. 86) have been synthesized and intensively investigated as anode materials for LIBs. In these Mn₃O₄ nanomaterials, the mesoporous Mn₃O₄ nanotubes⁸⁴ and Mn₃O₄ octahedra⁸⁶ show high specific capacities and excellent cycling performances. Specifically, the mesoporous Mn₃O₄ nanotubes⁸⁴ with a high surface area (42.18 m² g⁻¹) and an average pore size of 3.72 nm obtained from the hydrogen reduction of β-MnO₂ nanotubes under a H₂/Ar atmosphere at 280 °C for 3 h exhibit a high specific capacity of 641 mA h g⁻¹ after 100 cycles at a high current density of 500 mA g⁻¹ (Fig. 10). The Mn₃O₄ octahedra,⁸⁶ which were straightforwardly prepared at room temperature using a one-step de-alloying method (Fig. 11a), have ultra-long cycle lifetimes with capacity retentions of 81.3% and 77.8% after 500 cycles at 100 and 300 mA g⁻¹, respectively (Fig. 11b and c).

Table 3 A summary of recent studies on Mn₃O₄-based anodes for LIBs

Materials	Preparation method	1^st discharge–charge capacity (mA h g^−1)	Capacity retention	Rate performance (mA h g^−1)	Ref.
SpongelikeNanosized Mn3O4	Precipitation method	1327/869 at 40 mA g^−1	800 (40 cycles)	500 at 10C	43
Mn3O4 nanorods	Hydrothermal + annealing method	1050/555 at 0.1 C	108 (100 cycles)		80
Mn3O4 octahedral	Hydrothermal method	1009/700 at 50 mA g^−1	404 (20 cycles)	269 at 500 mA g^−1	81
Mesoporous stacked Mn3O4 nanosheets	Chemical bath deposition (CBD) route	824/300 at 0.1 C	400 (60 cycles)	298 at 1.5 C	82
Porous MnxCo3−xO4 nanocubes	A New facile strategy	discharege1395 at 200 mA g^−1	733 (30 cycles)	473 at 1600 mA g^−1	83
Mesoporous Mn3O4 nanotubes	Hydrothermal + hydrogen reduction	1065/641.5 at 100 mA g^−1	641 (100 cycles) at 500 mA g^−1	525 at 1000 mA g^−1	84
Order-aligned Mn3O4 nanostructures	Electrochemically depositing method	919/622 at 936 mA g^−1	882 (85 cycles)	500 at 40 Ag^−1	85
Mn3O4 octahedrons	Dealloying of MnAl alloy	918.3/536.8 at 100 mA g^−1	746 (500 cycles)	240 at 1500 mA g^−1	96
Fe-doped MnxOy	Nanocasting technique + controlled calcination	1358/800 at 200 mA g^−1	620 (100 cycles)	409 at 1600 mA g^−1	87
Fe2O3–Mn3O4 nanocomposite	Electrostatically-derived selfassembly method	1050/919.8 at 90 mA g^−1	230 (25 cycles)		88
Mn3O4–graphene hybrid	Two-step solution-phase reactions	1350/950 at 40 mA g^−1	730 (40 cycles)	390 at 1600 mA g^−1	91
Mn3O4–graphene composite	Ultra-sonication + annealing	1375/750 at 75 mA g^−1	720 (100 cycles)		92
Mn3O4–graphene nanocomposite	Microwave hydrothermal technique	1354/901 at 40 mA g^−1	900 (50 cycles)	400 at 1000 mA g^−1	93
Mn3O4–graphite nanosheet	Solvothermal route	1079/762 at 50 mA g^−1	∼600 (50 cycles)	∼250 at 600 mA g^−1	94
Mn3O4/ordered mesoporous carbons	Precipitation + annealing method	1843/773 at 100 mA g^−1	802 (50 cycles)		95
Mn3O4/(MWCNTs) nanocomposite	Solvothermal route	1380/750 at 100 mA g^−1	592 (50 cycles)	387 at 1000 mA g^−1	96
Mn3O4/C nanorod	Solvothermal route	1246/723 at 40 mA g^−1	473 (50 cycles)		97
Mn3O4 nanoparticles/graphene	An in situ transformation method	1600/700 at 60 mA g^−1	500 (40 cycles)	200 at 1500 mA g^−1	98
Mn3O4 nanocrystals/(RGO)	Ultra-sonication-assisted method	1630/1050 at 40 mA g^−1	900 (20 cycles)	200 at 1600 mA g^−1	99
Mn3O4/SACNT	Thermal decomposition	≈1900/1108.4 at 1 C	390 (100 cycles)	342 at 10 C	100
Mesoporous Mn3O4 nanosheet/graphene	Precipitation	1553/973 at 400 mA g^−1	910 (230 cycles)	360 at 8 A g^−1	101

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Fig. 10 (a) TEM and (b) HRTEM images of mesoporous Mn₃O₄ nanotubes; (c) cycle performance and coulombic efficiency versus cycle number of Mn₃O₄ nanotubes at a current density of 500 mA g⁻¹ and (d) rate capabilities with increasing current density. Reproduced with permission.⁸⁴ Copyright 2013, RSC.

Fig. 11 (a) Schematic for the fabrication of Mn₃O₄ octahedra; (b) cycling behaviour; and (c) coulombic efficiency of the Mn₃O₄ anode at different current densities. Reproduced with permission.⁸⁶ Copyright 2014, RSC.

In order to tackle the problem of the low capacity retention capability of pure Mn₃O₄ materials, researchers have designed M-doped Mn₃O₄ (M = Co, Fe and Cu) samples.^76,87–89 In 2005, Pasero et al.⁷⁶ synthesized a Co-doped Mn₃O₄ sample to improve the cycling performance of the pure Mn₃O₄ phase. The Co-doped Mn₃O₄ sample delivers a specific capacity of 400 mA h g⁻¹, which is much higher than the specific capacity of pure Mn₃O₄ (∼200 mA h g⁻¹). More recently, Ma et al.⁸⁷ prepared Fe-doped Mn_xO_y with connected macro- and mesopores to provide hierarchical porosity by a facile and scalable nanocasting technique followed by controlled calcination using a novel macroporous amine-functionalized bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) membrane as the sacrificial template (Fig. 12). The macropores in the final product were mainly formed by the reversible replication of the membrane porous structure and interconnected branches in the pore system. The macropores cushioned the volume change during the conversion reaction of the lithium during cycling, and also acted as reservoirs and thoroughfares to improve Li⁺ transport in the electrolyte phase. The mesopores between primary nanocrystalline particles were formed upon heat treatment and provided a large interfacial contact between electrolyte and electrode active material to support a high Li⁺ flux across the solid/liquid interface. The selection of iron as a dopant was intentional: the redox potential of iron (1.0 V for discharge/1.7 V for charge) is higher than that of manganese (0.5 V for discharge/1.2 V for charge).⁹⁰ Therefore, the evenly-distributed iron remains in the zero-valent state during a large part of the Mn_xO_y conversion reaction, conceptually serving the dual function of (1) electrical wiring of the electro-active material to lower the resistance for electron transport, and (2) promoting ionic conduction by creating narrowly-spaced interfaces at the oxide-grain boundary. As anticipated, the Fe-doped Mn_xO_y composites show good capacity retention and high rate capabilities, delivering a capacity of ∼400 mA h g⁻¹ at 1500 mA g⁻¹ after 900 cycles (Fig. 13). The authors also synthesized manganese oxides doped with different amounts of iron and investigated the effect of Fe content on phase structures and electrochemical performance of the as-synthesized samples. These results indicate the importance of a moderate level of iron doping to balance its effects on interfacial charge transfer and the structural integrity of the mesopores. Further studies are required to explore the effect of metal dopants on the rate and long-term cycling performance as well as the columbic efficiency of Mn₃O₄ composites.

Fig. 12 (a) Schematic illustration of the preparation of hierarchical macro/mesoporous metal oxide structures; (b) FESEM images at low and high (inset) magnifications of Fe-doped Mn_xO_y-2; (c) TEM images at low and high (inset) magnifications of Fe-doped Mn_xO_y-2; (d) SEM image of Fe-doped Mn_xO_y-2 and EDX element maps of Mn, Fe, and O; and (e) nitrogen adsorption–desorption isotherms of Fe-doped Mn_xO_y-2 and the corresponding pore size distribution (inset). Reproduced with permission.⁸⁷ Copyright 2013, Wiley.

Fig. 13 (a) The 1^st and 2^nd discharge–charge curves of the Fe-doped Mn_xO_y-2 electrode; (b) cycle stability of the Fe-doped Mn_xO_y-0, Fe-doped Mn_xO_y-1, Fe-doped Mn_xO_y-2, Fe-doped Mn_xO_y-3 and Fe-doped Mn_xO_y-4 electrodes measured at 200 mA g⁻¹; (c) cycling stability of the Fe-doped Mn_xO_y-2 electrode at different current densities; and (d) cycling performance of the Fe-doped Mn_xO_y-2 electrode at 1500 mA g⁻¹. Reproduced with permission.⁸⁷ Copyright 2013, Wiley.

Mn₃O₄–graphene hybrids as high capacity anode materials for LIBs were first investigated by Cui's group.⁹¹ They developed a two-step solution-phase synthesis route to form hybrid materials of Mn₃O₄ nanoparticles with diameters around 10–20 nm on 10 wt% reduced graphene oxide (RGO) sheets (Fig. 14). A stable capacity of ∼900 mA h g⁻¹ (∼810 mA h g⁻¹ based on the total mass of the composite) was obtained for the first five cycles when cycled at a current of 40 mA g⁻¹ in the voltage range of 0.1–3.0 V vs. Li/Li⁺. Very good rate capability and cycling stability were observed; for example, a capacity of ∼720 mA h g⁻¹ at 400 mA g⁻¹ was retained after 40 cycles of charge–discharge at various C-rates. However, the bare Mn₃O₄ nanoparticles exhibit inferior electrochemical performance, with an initial reversible specific capacity of 300 mA h g⁻¹ at 40 mA g⁻¹. The high capacity, good rate capability and cycling stability of this Mn₃O₄/RGO hybrid material are attributed to the precisely-synthesized nanocomposites of Mn₃O₄ (20 nm in diameter) wrapped in graphene sheets (GSs) via a facile, effective, energy-saving, and scalable microwave hydrothermal technique. The composite shows a high specific capacity of more than 900 mA h g⁻¹ at 40 mA g⁻¹, and an excellent cycling stability with nearly no capacity decay up to 50 cycles. They first give evidence that there is charge transfer between the graphene and the Mn₃O₄ nanoparticles by theoretical calculations. Li et al.⁹⁵ developed a facile one-step route to generate interactions between the graphene substrates and the Mn₃O₄ nanoparticles grown directly on them, which makes Mn₃O₄ electrochemically active since charge carriers can be effectively and rapidly conducted back and forth from the Mn₃O₄ nanoparticles to the current collector through the highly conductive 3D graphene network. In addition, the graphene–nanoparticle interactions afford good dispersion of Mn₃O₄ nanoparticles on the RGO sheets, which could avoid Mn₃O₄ nanoparticle aggregation to result in better cycle stability.

Fig. 14 (a) Schematic of the two-step synthesis of Mn₃O₄/RGO; (b) SEM image of a Mn₃O₄/RGO hybrid; (c) XRD spectrum of a packed thick film of Mn₃O₄/RGO; (d) TEM image of Mn₃O₄/RGO; inset shows the electron diffraction pattern of the Mn₃O₄ nanoparticles on RGO; (e) high-resolution TEM image of an individual Mn₃O₄ nanoparticle on RGO; (f) charge (red) and discharge (blue) curves of Mn₃O₄/RGO for the first cycle at a current density of 40 mA g⁻¹; (g) representative charge (red) and discharge (blue) curves of Mn₃O₄/RGO at various current densities; (h) capacity retention of Mn₃O₄/RGO at various current densities and (i) capacity retention of free Mn₃O₄ nanoparticles without graphene at a current density of 40 mA g⁻¹. Reproduced with permission.⁹¹ Copyright 2010, ACS.

This growth-on-graphene approach offers a new technique for the design and synthesis of high performance anode materials based on highly-insulating Mn₃O₄ materials. After this work, Mn₃O₄–carbon-based hybrid materials have been intensively studied due to their potential for improving their cycle stability and rate performance as LIB anode materials. The specific capacity values and cycling performances are listed in Table 3.^92–101 For example, Li et al.⁹³ successfully synthesized 3D hybrids with Mn₃O₄ nanoparticles homogeneously embedded in ordered mesoporous carbon (OMC). The Mn₃O₄/OMC hybrids display a high specific capacity up to 802 mA h g⁻¹ and a high coulombic efficiency of up to 99.2% after 50 cycles at a high current density of 100 mA g⁻¹. This capacity is 1.6 times higher than the discharge capacity for pure ordered OMC materials (512 mA h g⁻¹), and more than 5.4 times higher than that of pure Mn₃O₄ nanoparticles (148 mA h g⁻¹). The authors suggested that the enhanced capacity and cycling performance of the Mn₃O₄/OMC hybrids can be attributed to their unique, robust 3D composite structures and the synergistic effects between the Mn₃O₄ nanoparticles and OMC. The ordered mesostructured channels of Mn₃O₄/OMC hybrids are expected to effectively buffer against the local volume change during Li uptake/removal reactions, thus enhancing structural stability. The OMC matrix wall with a thickness of <10 nm greatly reduces the solid-state transport length for Li diffusion. Moreover, the hierarchically-ordered mesoporosity facilitates liquid electrolyte diffusion into the bulk of the electrode material, providing fast conductive ion transport channels for the conductive Li⁺ ions. The improved cycling performance can also be attributed to good electrical contact between Mn₃O₄ and OMC in the 3D nanocomposites during the phase transformation of Mn₃O₄ upon lithiation/delithiation that usually leads to capacity fading. Wang et al.⁹⁷ developed a facile one-step solvothermal reaction route for the large-scale synthesis of carbon-homogeneously-wrapped manganese oxide (Mn₃O₄@C) nanocomposites for anode materials using manganese acetate monohydrate and polyvinyl pyrrolidone as precursors and reactants. The synthesized Mn₃O₄ with tetragonal structures (space group I4₁/amd) display nanorod-like morphology with widths of 200–300 nm and thicknesses of 15–20 nm. The Mn₃O₄@C nanocomposites display enhanced capacity retention on charge–discharge cycling, with a stable specific capacity of 473 mA h g⁻¹ after 50 cycles (as much 3.05 times as that of pure Mn₃O₄ sample). Wang et al.⁹⁹ fabricated colloidal Mn₃O₄ nanocrystals supported by graphene oxide (GO) and reduced graphene oxide (RGO) (Mn₃O₄–GO and Mn₃O₄/RGO nanocomposites) through a facile ultrasonic-assisted synthetic route in an ethanol amine (ETA)–water system (Fig. 15a). The integration of GO with abundant O-containing groups and colloidal Mn₃O₄ crystals endows this composite with excellent electrochemical performances including high reversible capacity, good cycle stability, and high rate performance, making it suitable for use as an electrode material in electrochemical capacitors (ECs). Compared with Mn₃O₄–GO nanocomposites, Mn₃O₄/RGO exhibits excellent electrochemical performance in rechargeable LIBs due to the high electrical conductivity of RGOs and the structure of Mn₃O₄ nanocrystals wrapped in RGOs (Fig. 15b). In addition, the authors explained how the synergetic structural composition of Mn₃O₄–GO and Mn₃O₄/RGO nanocomposites plays an important role in their properties for ECs or LIBs (Fig. 15c). Luo et al.¹⁰⁰ recently reported the Li cycling behaviour of a Mn₃O₄–carbon nanotube hybrid material, which is a composite of Mn₃O₄ nanoparticles anchored on continuous super-aligned carbon nanotube (SACNT) films. They reported that the electrochemical performance of the Mn₃O₄/SACNT composite electrode is significantly affected by the Mn₃O₄ content and particle size. Mn₃O₄/SACNT composites with Mn₃O₄ sizes under 10 nm demonstrated higher capacity, lower polarization extent, and better cycle stability than those with Mn₃O₄ sizes of 165 nm. The optimized Mn₃O₄/SACNT composite displays a capacity of more than 700 mA h g⁻¹ at 0.1 C (based on the total mass of the electrode). A large capacity of 342 mA h g⁻¹ can still be obtained at a high rate of 10 C, and a capacity retention of 95% was achieved for 100 cycles at 1 C. Despite improvements in the anodic properties of Mn₃O₄ by conductive coating/composites, further studies on the effects of conductive coatings on cycling stability and rate performance are still needed.

Fig. 15 (a) Preparation routes for Mn₃O₄ nanoparticles, Mn₃O₄–GO and Mn₃O₄/RGO nanocomposites; (b) cycling performance of these samples at 40 mA g⁻¹; (c) the charge mechanism of Mn₃O₄–GO; and (d) Mn₃O₄/RGO for ECs and LIBs. Reproduced with permission.⁹⁹ Copyright 2013, RSC.

2.3 Mn₂O₃ and its nanocomposites

There were only a few reports on Mn₂O₃ nanomaterial-based anodes for LIBs before 2010.^102,103 Liu et al.¹⁰² reported preliminary studies of Li cycling behaviours on Mn₂O₃; they found a first discharge and charge capacity of 1080 and 480 mA h g⁻¹, respectively, in the voltage range of 0.2–3.0 V vs. Li/Li⁺ at 0.25 mA cm⁻². Lavela et al.¹⁰³ prepared Mn₂O₃ by a sol–gel method followed by heat treatment at different temperatures. They reported that Mn₂O₃ showed a discharge potential (1^st cycle) of ∼0.25 V vs. Li/Li⁺. The Mn₂O₃ delivered reversible capacities of 500–700 mA h g⁻¹ depending on synthesis temperature when cycled in the voltage range of 0.005–3.0 V Li/Li⁺. However, capacity fading was noted in all cases with capacities ranging from 250 to 400 mA h g⁻¹ after 20 cycles. Porous 1D Mn₂O₃ nanostructures containing nanofibers, nanowires and nanorods were successfully fabricated via a facile hydrothermal treatment and sequential thermal decomposition without any template or surfactant.¹⁰⁴ These as-prepared porous Mn₂O₃ nanomaterials manifested high initial capacities; however, capacity fading was also observed in all cases with capacities of ∼250 mA h g⁻¹ after 30 cycles. A similar phenomenon was also found by Shen's group.¹⁰⁵ By means of morphology-conserved transformation, Qiu et al.¹⁰⁶ synthesized hierarchically-structured Mn₂O₃ nanomaterials with different morphologies (oval- and strawsheaf-shaped) and pore structures by a hydrothermal method followed by heating in air at 600 °C. Both of the as-prepared mesoporous Mn₂O₃ nanomaterials deliver high reversible capacities and excellent cycling stabilities compared with the commercial Mn₂O₃ nanoparticles. The authors observed a reversible capacity of 380 and 320 mA h g⁻¹ at the end of the 150^th cycle for straw-sheaf- and oval-shaped nanostructured Mn₂O₃, respectively, at a current of 200 mA g⁻¹ when cycled in the voltage range of 0.01–2.5 V vs. Li/Li⁺. The good capacity retention capability of the straw-sheaf-shaped Mn₂O₃ was attributed to the relatively high surface area and peculiar nanostrip structure, which resulted in a reduced length for lithium ion diffusion. Recently, our group¹⁰⁷ has synthesized porous Mn₂O₃ microspheres by the morphology-controlled decomposition of spherical MnCO₃ precursors at 600 °C. The porous Mn₂O₃ microspheres show a specific capacity as high as 796 mA h g⁻¹ at a current density of 100 mA g⁻¹ after 50 cycles. A high rate performance (∼600 mA h g⁻¹ at 1200 mA g⁻¹) was also observed. Based on the CV curves, discharge–charge curves and ex situ XRD analysis, a novel lithium storage mechanism for the synthesized porous Mn₂O₃ material after the first discharge process was suggested as follows:

Mn + xLi₂O ↔ 2xLi⁺ + MnO_x + 2xe⁻ (1.0 < x < 1.5)

After our work, porous Mn₂O₃ hierarchical microspheres^108,109 and Mn₂O₃ nanocones,¹¹⁰ polyhedrons,¹¹¹ porous sheets,¹¹² cubes and spindles¹¹³ have been synthesized and investigated as anode materials by other groups. The results of these investigations are listed in Table 4. Although some advances in Mn₂O₃ anode materials have been made via morphology optimization, further studies on the improvement of the cycling stability and rate performance are still required.

Table 4 A summary of recent investigations on Mn₂O₃-based anodes for LIBs

Materials	Preparation method	1^st discharge–charge capacity (mA h g^−1)	Capacity retention	Rate performance (mA h g^−1)	Ref.
Mn2O3 nanorods	Hydrothermal + annealing	1509/689 at 100 mA g^−1	374 (30 cycles)		104
Mn2O3 nanofibers		1694/795 at 100 mA g^−1	404 (30 cycles)
Mn2O3 nanowires		1611/707 at 100 mA g^−1	384 (30 cycles)
Mn2O3 nanorods	Hydrothermal + annealing	998/349 at 100 mA g^−1	∼50 (100 cycles)		105
Straw-sheaf-shaped Mn2O3	Solvothermal + annealing	1179/680 at 200 mA g^−1 (0.01–2.5 V)	400 (150 cycles) at 400 mA g^−1	270 at 800 mA g^−1	106
Oval-shaped Mn2O3		1159/∼650 at 200 mA g^−1 (0.01–2.5 V)	320 (150 cycles) at 400 mA g^−1
Porous Mn2O3 microsphere	Solvothermal + annealing	1310/882 at 100 mA g^−1	796 (50 cycles) at 100 mA g^−1	470 at 2400 mA g^−1	107
Porous Mn2O3 microspheres	Solvothermal + annealing	2134/1400 at 50 mA g^−1	748 (45 cycles)	423 at 1600 mA g^−1	108
Mesoporous Mn2O3 microspheres	Hydrothermal + annealing	668/∼630 at 200 mA g^−1	524 (200 cycles)	∼150 at 50 mA g^−1	109
Hollow Mn2O3 nanocones	Hydrothermal + annealing	1875/909 at 50 mA g^−1	280 (200 cycles) at 200 mA g^−1	380 at 400 mA g^−1	110
Porors Mn2O3 nanosheets	Solvothermal + annealing	1518/987 at 300 mA g^−1	521 (100 cycles)	500 at 500 mA g^−1	111
Cubic Mn2O3	Hydrothermal + annealing	1680/780 at 100 mA g^−1	∼300 (55 cycles)	240 at 600 mA g^−1	113
Spindle Mn2O3		1530/620 at 400 mA g^−1	∼200 (50 cycles)
Fusiform Mn2O3		1402/400 at 100 mA g^−1	155 (40 cycles)
Cu-doped hollow Mn2O3 spheres	Hydrothermal + annealing	1998/1190 at 100 mA g^−1	642 (100 cycles)	308 at 815 mA g^−1	89

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Recently, a good report on Cu-doped hollow spherical Mn₂O₃ samples by Li et al.¹¹³ indicated a higher specific capacity of 642 mA h g⁻¹ at a current density of 100 mA g⁻¹ after 100 cycles. They suggested that the significant enhancement of the electrochemical lithium storage performance can be attributed to improvements in the electronic conductivity and lithium diffusivity of electrodes.

Xia's group¹¹⁴ prepared Mn₂O₃ nanowires by thermally annealing MnOOH nanowire precursors, which were transformed from hydrothermally-grown MnO₂ nanoflakes and directly attached on Ti foils via reaction with poly(vinyl pyrrolidone) (Fig. 16a). The Mn₂O₃ nanowires (Fig. 16b) show an initial discharge capacity of 815.9 mA h g⁻¹ at 100 mA g⁻¹ and maintains a capacity of 502.3 mA h g⁻¹ after 100 cycles (Fig. 16c), and a good rate performance of 220 mA h g⁻¹ at 1000 mA g⁻¹ (Fig. 16d). Furthermore, the authors first fabricated a flexible Mn₂O₃//LiMn₂O₄ lithium ion full cell using the Mn₂O₃ nanowires as the anode and LiMn₂O₄ nanowires as the cathode (Fig. 17). The full cell has an output voltage of >3 V, a low thickness of 0.3 mm, high flexibility, and a specific capacity of 99 mA h g⁻¹ based on the total weight of the cathode material. It also exhibits good cycling stability with a capacity of ∼80 mA h g⁻¹ after 40 charge–discharge cycles. This successful example of the Mn₂O₃//LiMn₂O₄ full cell and further optimization and development of the nanowire design and fabrication will lead to new opportunities in high power-density energy storage devices.

Fig. 16 (a) Schematic of the synthesis of Mn₂O₃ nanowires (NMs) and fabrication the Mn₂O₃//LiMn₂O₄ LIB full cell; (b) SEM images of Mn₂O₃ NWs grown on Ti foils; (c) cycling performance of Mn₂O₃ NWs (red curve) for the first 100 cycles; and (d) rate performance of Mn₂O₃ NWs (1 C = 1000 mA g⁻¹). Reproduced with permission.¹¹⁴ Copyright 2013, ACS.

Fig. 17 Flexible Mn₂O₃//LiMn₂O₄ full cells. (a) 3D structure scheme and (b) optical image of a flexible Mn₂O₃//LiMn₂O₄ full cell; (c) optical images showing a light emitting diode (3 V, 10 mW) powered by a flexible Mn₂O₃//LiMn₂O₄ full cell; (d) optical image of the voltage output of a three-cell battery in tandem; (e) charge–discharge curves of a flexible Mn₂O₃//LiMn₂O₄ full cell at the first cycle and (f) cycling performance (red curve) and coulombic efficiency (black curve) of a representative flexible Mn₂O₃//LiMn₂O₄ full cell. Reproduced with permission.¹¹⁴ Copyright 2013, ACS.

2.4 MnO₂ and its nanocomposites

Manganese dioxide (MnO₂) possesses a high theoretical capacity of 1233 mA h g⁻¹ based on the conversion mechanism.^44,115 Moreover, it is environmentally-benign and naturally abundant, making it a promising material for application in LIBs. However, due to the low conductivity of MnO₂ and large volume expansion during battery operation, rather low coulombic efficiencies in the first cycle and rapid capacity decays with cycling were observed.^116–119 For example, Li et al.¹¹⁷ synthesized novel α-MnO₂ hollow urchins on a large scale by a facile and efficient low-temperature (60 °C) mild reduction route. The α-MnO₂ hollow urchins show a high capacity of 746.0 mA h g⁻¹ in the first discharge process, but the capacity of the 40^th cycle is only 481 mA h g⁻¹, about 64.4% of the initial capacity. As reported by Reddy et al.,¹¹⁹ a coaxial MnO₂/CNT array material can give an initial capacity of approximately 2000 mA h g⁻¹ at a rate of 50 mA g⁻¹; however, the capacity sharply decays to 500 mA h g⁻¹ after only 15 cycles. The authors suggested the initial high capacity and enhanced lithium storage capacity can be attributed to the in situ deposition of MnO₂ on CNTs, while the fast capacity decay might result from a structural deterioration of a thick MnO₂ layer with a compact structure deposited on the outside surface of the CNTs. One of the best results, which reported a capacity of 600 mA h g⁻¹ after 100 cycles for a γ-MnO₂ film deposited on nickel metal, was reported before 2010.¹¹⁸ Subsequently, systematic efforts have been dedicated to optimize MnO₂ electrodes by combining materials synthesis, processing and engineering techniques to improve their electrochemical performance.

Recently, attempts have also been made to fabricate MnO₂ composites with unique nanostructures,^120–122 MnO₂-based nanocomposites consisting of MnO₂ nanostructures and electrically conductive carbon such as carbon nanotubes^123–125 amorphous carbon¹²⁶ or graphene,^127–131 its nanocomposites including MnO₂ and other transition metal oxides,^132–134 and MnO₂@polymers.¹³⁵ The results of these studies are listed in Table 5. Kundu et al.¹²² reported the direct growth of mesoporous MnO₂ nanosheet (NS) arrays on a nickel (Ni) foam current collector by electro-deposition followed by low temperature thermal annealing (170 °C) in a nitrogen atmosphere. When using the Ni foam-supported mesoporous MnO₂ NS arrays as an anode in LIBs, the electrode can deliver a reversible capacity as high as 1690 mA h g⁻¹ even after 100 cycles at a charge–discharge current density of 100 mA g⁻¹. Two anodic peaks located at 1.3 and 2.4 V vs. Li/Li⁺ are observed, and the re-oxidation of manganese was suggested to take place in two steps. It is also interesting to note that the capacity gradually increases with increasing cycle number, which is the opposite trend observed in previous studies.^{116–120,135} The increase in capacity might originate from the increased number of vacancies and grain boundaries, where Li ions could be stored, due to the pulverization of the MnO₂ NSs upon cycling. This example suggests that the electrochemical performance of MnO₂ may be optimized by nanostructural tuning. Guo et al.¹²⁷ reported a novel approach to fabricate a hierarchically-nanostructured composite comprised of 1D rod-like layered birnessite-type manganese oxides (LMOs), 2D graphene nanosheets, and a 3D porous structure (LMO/PEDOT/graphene). As shown in Fig. 18a, the 3D graphene material is used as a supporting matrix on which in situ polymerized poly(3,4-ethylenedioxythiophene) (PEDOT) is assembled for the direct growth of uniformly-distributed 1D rod-like LMOs. The composites exhibit a large first discharge capacity of 1835 mA h g⁻¹ and reversible discharge capacities of 1105 mA h g⁻¹ and 948 mA h g⁻¹ after 1 and 15 cycles, respectively, at a rate of 50 mA g⁻¹ (Fig. 18b and c). The authors suggested that the enhanced performance of LMO/PEDOT/graphene in LIBs can be explained as follows: (1) the LMO in LMO/PEDOT/graphene has characteristics of an open structure with an interlayer space of approximately 7.2 Å, which promotes efficient lithium intercalation/deintercalation to achieve high specific capacity; (2) the use of highly conductive 3D graphene is favourable in electron transport, resulting in a fast charge–discharge rate and (3) the PEDOT coating directs the growth of relatively-ordered, textured LMO; it also prevents LMO aggregation to produce a uniform distribution, which can accommodate large volume expansion during battery operation.

Table 5 A summary of recent studies on MnO₂-based anodes for LIBs

Materials	Preparation method	1^st discharge–charge capacity (mA h g^−1)	Capacity retention	Rate performance (mA h g^−1)	Ref.
Phase-pure β-MnO2 nanorods	Hydrothermal + annealing	1728/488 at 0.1 mA cm^−2			43
Interconnected MnO2 nanowires	Hydrothermal + annealing	1650/800 at 85 mA g^−1	∼580 (100 cycles)		116
Hollowα-MnO2 urchin	Low-temperature (60 °C) mild reduction route	746/650 at 270 mA g^−1	481 (40 cycles)		117
Nanoporous γ-MnO2 hollow microspheres	Hydrothermal + annealing	1289/1071 at 100 mA g^−1	657 (20 cycles)		118
Nanoporous γ-MnO2 nanocubes		1992/1042 at 100 mA g^−1	602 (20 cycles)
Coaxial MnO2/carbon nanotube	Templates + infiltration + annealing	2500/880 at 50 mA g^−1	500 (16 cycles)		119
α-MnO2 nanotubes	Exfoliation and scrolling approach	∼1200/1000 at 200 mA g^−1	512 (300 cycles) at 800 mA g^−1		120
Hierarchical hollow microspheres constructed with MnO2 nanotubes	A Bubble template-based self-scrolling method	983/830 at 300 mA g^−1	700 (30 cycles)		121
Mesoporous MnO2 nanosheet	Electrodeposition + annealing	1643/1225 at 50 mA g^−1	1690 (100 cycles) at 100 mA g^−1	900 at 1000 mA g^−1	122
Nanoflaky MnO2/carbon nanotube	Hydrothermal method	1402/816 at 200 mA g^−1	620 (50 cycles)	320 at 4000 mA g^−1	123
Mesoporous γ-MnO2 particles/carbon nanotube	Ultrasound irradiation	1278/741 at 50 mA g^−1	934 (150 cycles)	∼380 at 1000 mA g^−1	124
Coaxial MWNTs@MnO2@PPy	Immersing + in situ polymerization	1250/730 at 100 mA g^−1	820 (120 cycles)	530 at 1000 mA g^−1	125
MnO2 nanosheets/core-leaf onion-like carbon	Hydrothermal method	1278/854 at 50 mA g^−1	541 (100 cycles)	102 at 2000 mA g^−1	126
MnO2/conjugated polymer/graphene	In situ polymerization + Immersing	1835/1050 at 50 mA g^−1	948 (15 cycles)	698 at 400 mA g^−1	127
MnO2 nanotube/graphene	Ultra-filtration technique	1250/686 at 100 mA g^−1	495 (40 cycles)	208 at 1600 mA g^−1	128
Graphene-wrapped MnO2–graphene nanoribbons	Hydrothermal + electrostatic interaction	880/600 at 100 mA g^−1	612 (250 cycles) at 400 mA g^−1	580 at 1000 mA g^−1	129
α-MnO2–graphene 3D network	Hydrothermal approach	1875/1150 at 60 mA g^−1	998 (30 cycles)	590 at 12 A g^−1	130
Porous MnO2–3D graphene nanocomposites	A redox process + annealing	1554/917 at 100 mA g^−1	836 (200 cycles)	358 at 1600 mA g^−1	131
Porous α-Fe2O3 branches on β-MnO2 nanorods	Hydrothermal + annealing	1480/1147 at 100 mA g^−1	1028 (200 cycles) at 1000 mA g^−1	881 at 4000 mA g^−1	132
TiO2–C/MnO2 core−double-shell nanowire	In situ chemical redox	865/500 at 33.5 mA g^−1	218 (150 cycles) at 3350 mA g^−1	230 at 3350 mA g^−1	133
ZnO/MnO2 sea urchin-like sleeve array	Electro-deposition	2023/878 at 200 mA g^−1	1259 (100 cycles)	150 at 5000 mA g^−1	134
Polythiophene (PTh)-coated ultrathin MnO2 nanosheets	A Redox process + in situ polymerization	700 at 500 mA g^−1	500 (100 cycles)		135

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Fig. 18 (a) Schematic representation of the hierarchically-nanostructured composite fabrication; (b) cycling variation in charge–discharge capacity vs. cycle number for different LIBs; and (c) capacity of LMO/PEDOT/graphene batteries under various current densities. Reproduced with permission.¹²⁷ Copyright 2011, Wiley.

Li et al.¹²⁹ designed a unique, hierarchical, sandwiched-structured graphene–MnO₂–GNRs (GMG) composite in which the graphene wraps the porous MnO₂ that is grown directly from graphene nanoribbons (GNRs). Fig. 19a depicts the synthetic procedures for the GMG composites. Graphene and GNRs are in good contact with MnO₂, improving the electrical conductivity of the GMG composites. More importantly, graphene and GNRs can buffer the volume changes and prevent the loss of MnO₂ during the Li ion conversion reaction with Li, improving the composite's electrochemical stability. As an anode material, the GMG composites demonstrate a reversible specific discharge capacity of 890 mA h g⁻¹ at 0.1 A g⁻¹ after 180 cycles with various current rates ranging from 0.1 to 1.0 A g⁻¹; the anode shows about 24% improvement compared to its initial capacity after 245 cycles at a current density of 0.4 A g⁻¹ (Fig. 19b).

Fig. 19 (a) Schematic illustration of the synthesis of the GMG composite; (b) rate performance of GMG at various current rates from 0.1 to 1.0 A g⁻¹; and (c) cycling performance of MnO₂, MG, and GMG at 0.1 A g⁻¹ for the first five cycles and at 0.4 A g⁻¹ for the following cycles. Reproduced with permission.¹²⁹ Copyright 2013, Wiley.

In a recent paper, Li et al.¹³¹ prepared MnO₂/3D porous graphene-like (PG) (3D PG–xMn) composites via a simple and cost-effective redox process. Typically, the self-controlled reaction between 3D PG and KMnO₄ under neutral pH conditions led to the homogeneous deposition of MnO₂ on the 3D PG networks. They suggested that the porosities of the 3D PG–xMn composites can be adjusted by modulating the initial concentration of KMnO₄. The surface areas and total pore volumes of 3D PG–xMn composites are gradually decreased with the increase of the concentration of KMnO₄. In addition, after the deposition of MnO₂ on 3D PG, there was no obvious decrease in powder conductivity of the 3D PG–xMn (x = 0.5 and 1.0) composites, while the 3D PG–1.5Mn composite shows an obvious decrease. Among the three 3D PG–xMn composites, the 3D PG–1Mn composite displayed the best electrochemical performance. It delivered a discharge capacity of 988 mA h g⁻¹ in the second cycle, and the discharge capacity remained as high as 836 mA h g⁻¹ after the 200^th cycle (∼84.6% capacity retention). The excellent electrochemical performances of the as-prepared 3D PG–1Mn composites can be attributed its reasonable MnO₂ content (62.7 wt% MnO₂) and high conductivity. Branched nanocomposites with β-MnO₂ nanorods as the backbone and porous α-Fe₂O₃ nanorods as the branches were synthesized by Gu et al.¹³² using a high-temperature annealing process to achieve epitaxial growth of FeOOH on the β-MnO₂ nanorods. The branched nanorods of β-MnO₂/α-Fe₂O₃ exhibit reversible specific capacities of 1028 mA h g⁻¹ and 881 mA h g⁻¹ at current densities of 1000 mA g⁻¹ and 4000 mA g⁻¹, respectively, up to 200 cycles. The presence of α-Fe₂O₃ in the branched nanorods greatly promotes the charge transfer at the electrode/electrolyte interface, which is beneficial to electrochemical performance. These results indicate the importance of the selection of proper chemical components and the control on their construction. The combination of advantages in structure and components would effectively improve the performance of electrode materials in LIBs.

3. Ternary Mn-based oxides as anode materials

Ternary Mn-based oxides containing AMn₂O₄ (A = Co and Zn) with a spinel or spinel-like crystal structure have been investigated as anode materials for LIBs. The Li cycling mechanism involves the “alloying and conversion reaction” for ZnMn₂O₄ and the “conversion reaction” for CoMn₂O₄.⁸ Depending on the structure, morphology, particle size and composition, large and stable reversible capacities have been reported.

3.1 ZnMn₂O₄-based anodes

ZnMn₂O₄ has been proposed as a practical alternative anode material for LIBs because of its high theoretical capacity of 1008 mA h g⁻¹ (the Zn metal can form an alloy with Li and participate in the conversion reaction along with manganese) and the low oxidation potentials (i.e., delithiation potential) of zinc and manganese (1.2 and 1.5 V vs. Li/Li⁺, respectively). These low oxidation potentials could eventually increase the battery output voltage compared to ZnFe₂O₄ and ZnCo₂O₄, allowing higher energy density delivery. Furthermore, zinc and manganese are abundant, environmentally-friendly, and relatively inexpensive compared to cobalt.^45,136

Yang et al.⁴⁵ synthesized ZnMn₂O₄ nanocrystals by a polymer pyrolysis method and first used the obtained nanocrystals as lithium storage anode materials. The nanocrystalline ZnMn₂O₄ electrode delivers an initial reversible capacity of 766 mA h g⁻¹ when cycled at 100 mA g⁻¹ in the voltage range of 0.01–3.0 V vs. Li/Li⁺. Cycling studies up to 50 cycles show almost stable cycling performance between 10 and 50 cycles with an average capacity fading of only 0.20% per cycle. The electrode still maintains a capacity of 569 mA h g⁻¹ after 50 cycles. Subsequently, ZnMn₂O₄ with different particle sizes and morphologies including flower-like ZnMn₂O₄ superstructures,¹³⁶ ZnMn₂O₄ nanoparticles,^44,137,138 ZnMn₂O₄ nanowires,¹³⁹ ZnMn₂O₄ nanoplates,¹⁴⁰ ZnMn₂O₄ hollow microspheres,^141–143 ball-in-ball ZnMn₂O₄ hollow microspheres,¹⁴⁴ loaf-like ZnMn₂O₄ nanorods,¹⁴⁵ ZnMn₂O₄ nanofibers¹⁴⁶ and ZnMn₂O₄ mesoscale tubular arrays¹⁴⁷ have been studied and applied as anode materials for LIBs. The corresponding results are listed in Table 6. Specifically, the ball-in-ball ZnMn₂O₄ hollow microspheres¹⁴³ were prepared by a novel template-free method (Fig. 20a–d). The hollow microspheres typically consist of small nanoparticles with an average size of 30 nm, and exhibit mesoporous features. When used as an anode material for LIBs, the ZnMn₂O₄ hollow microspheres show a high specific capacity of 750 mA h g⁻¹ after 120 cycles at 400 mA g⁻¹ (Fig. 20e and f). Despite the improved cyclic and rate performance of ZnMn₂O₄ nanomaterials obtained by different strategies, further studies are still required to achieve long cycling stability and the high rate performance for these materials.

Table 6 A summary of recent studies on ZnMn₂O₄-based anodes for LIBs

Materials	Preparation method	1^st discharge–charge capacity (mA h g^−1)	Capacity retention	Rate performance (mA h g^−1)	Ref.
Nanocrystalline ZnMn2O4	Polymer pyrolysis route	1302/766 at 100 mA g^−1	569 (50 cycles)		45
Flower-like ZnMn2O4	Solvothermal method	1350/763 at 100 mA g^−1	626 (50 cycles)		136
Nanocrystalline ZnMn2O4	Coprecipitation method	1145/800 at 100 mA g^−1	690 (70 cycles)	365 at 1000 mA g^−1	44
Nanocrystalline ZnMn2O4	A single-source precursor route	1088/680 at 100 mA g^−1	650 (200 cycles)	405 at 600 mA g^−1	137
Nanocrystalline ZnMn2O4	Hydrothermal method	1200/680 at 100 mA g^−1	430 (50 cycles)	75 at 2000 mA g^−1	138
ZnMn2O4 nanowires	Solid-state reaction	1400/891 at 60 mA g^−1	650 (40 cycles)	350 at 1000 mA g^−1	139
ZnMn2O4 nanoplates	An “escape-by-crafty-scheme” method	1277/730 at 100 mA g^−1	502 (30 cycles)	324 at 1800 mA g^−1	140
ZnMn2O4 hollow microspheres	Coprecipitation + annealing method	1325/772 at 400 mA g^−1	607 (100 cycles)	361 at 1600 mA g^−1	141
ZnMn2O4 hollow microspheres	Solvothermal method	1335/750 at 100 mA g^−1	433 (50 cycles)	245 at 400 mA g^−1	142
ZnMn2O4 hollow microspheres	Coprecipitation + annealing method	1260/830 at 100 mA g^−1	599 (50 cycles)	450 at 1000 mA g^−1	143
Ball-in-ball ZnMn2O4 hollow microspheres	Solvothermal + annealing method	945/6622 at 400 mA g^−1	790 (120 cycles)	396 at 1200 mA g^−1	144
Loaf-like ZnMn2O4	Hydrothermal + annealing	1357/839 at 100 mA g^−1	517 (100 cycles) at 500 mA g^−1	457 at 1000 mA g^−1	145
ZnMn2O4 nanorods	Electrospinning	1257/680 at 60 mA g^−1	318 (50 cycles)		146
ZnMn2O4 nanofibers		1469/716 at 60 mA g^−1	705 (50 cycles)
ZnMn2O4 nanowires		1526/891 at 60 mA g^−1	530 (50 cycles)
ZnMn2O4 mesoscale tubular arrays	A Reactive template route + annealing	1198/784 at 100 mA g^−1	784 (100 cycles)	364 at 1600 mA g^−1	147
rGO–ZnMn2O4 composite	A bottom-up wrapping route	1412/960 at 100 mA g^−1	707 (100 cycles)	450 at 2000 mA g^−1	148

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Fig. 20 Typical (a), (b) FESEM and (c), (d) TEM images of ZnMn₂O₄ ball-in-ball hollow microspheres; (e) cycling performance of the ZnMn₂O₄ hollow microcubes in the voltage range of 0.01–3.0 V vs. Li/Li⁺ at current densities of 400 mA g⁻¹ and (f) rate performance at different current densities for the same cell after 120 cycles. Reproduced with permission.¹⁴³ Copyright 2012, Wiley.

Recently, in an attempt to improve the electrochemical properties of ZnMn₂O₄, Zheng et al.¹⁴⁸ prepared thermally-reduced graphene oxide (rGO)-wrapped ZnMn₂O₄ nanorods via a facile bottom-up approach. They suggested that this unique architecture can provide 1D interconnected ZnMn₂O₄ nanoparticle networks to facilitate rapid diffusion of lithium ions within the electrode material. Meanwhile, the rGO sheets could act as a conductive layer for electron transfer and an efficient buffer layer to enable the structural stabilization of ZnMn₂O₄ crystals. The results indicate a high and stable reversible capacity (707 mA h g⁻¹ at 100 mA g⁻¹ over 50 cycles) and an excellent rate capability (440 mA h g⁻¹ at 2000 mA g⁻¹) for this composite.

As mentioned in the introduction, the reasonable selections of binders and electrolytes are necessary to improve the rate and cyclic performance of metal oxides-based electrodes. Courtel et al.⁴⁴ have studied the effects of binder and electrolyte on the ZnMn₂O₄ cycling performance in a recently-published paper. The investigation of the electrochemical performance of ZnMn₂O₄ electrodes using five binders (PVDF dissolved in NMP and four other water-soluble binders: NaCMC, LiCMC, XG and Baytron; CMC = carboxymethylcellulose) show that LiCMC and NaCMC are the best binders (Fig. 21). Specifically, the electrode fabricated by ZnMn₂O₄ nanoparticles (<150 nm) sintered at 800 °C with the LiCMC binder exhibits a capacity of 690 mA h g⁻¹ after 70 cycles at C/10 and a good rate capability (450 mA h g⁻¹ for 0.5 C, 340 mA h g⁻¹ for 1 C, 230 mA h g⁻¹ for 2 C, and 150 mA h g⁻¹ for 3 C). In addition, studies on the effect of the electrolyte on ZnMn₂O₄ electrode cycling performance showed that a carbonate (EC)–dimethyl carbonate (DEC) mixture (3 : 7 by vol) is the best electrolyte, while PC (propylene carbonate) is the worst (Fig. 22). These results could be explained by the fact that PC does not form a polymeric organic layer, or that the layer formed with PC is inactive for lithium storage. Furthermore, the authors assembled a full cell for the first time using ZnMn₂O₄ as the anode material. LiMn_1.5Ni_0.5O₄ was the cathode material, and 1 M LiPF₆ in EC–DEC (3 : 7 by vol) was the electrolyte. The full cell was cycled between 1.5 V and 4.7 V at 0.1 C, and the capacity was reported versus the limiting electrode (cathode). The initial capacity at a cycling rate of 0.1 C is 108 mA h g⁻¹, and there is a plateau at 4.1 V during the first charge. However, in the subsequent discharge and charge cycles, the charge vs. voltage curve exhibits a sloping voltage between 3.5 V and 3 V and 3.5 and 4.1 V, respectively. Unfortunately, after 20 cycles, the battery exhibits a capacity of only 75 mA h g⁻¹.

Fig. 21 Discharge capacities of ZnMn₂O₄ electrodes made using five different binders: PVDF, NaCMC, LiCMC, XG, and Baytron. Reproduced with permission.⁴⁴ Copyright 2011, RSC.

Fig. 22 Discharge capacities of ZnMn₂O₄ electrodes. The cells were cycled at room temperature using the same lithium salt, 1 M LiPF₆, in three different electrolytes: EC–DMC (1 : 1 by vol), EC–DEC (3 : 7 by vol), and PC. Reproduced with permission.²¹ Copyright 2011, RSC.

3.2 CoMn₂O₄ anodes

CoMn₂O₄ has an incomplete normal-type spinel with a tetragonal structure (c/a = 1.14) because of the Jahn–Teller effect of Mn³⁺ (3d⁴). Co²⁺ mainly occupies the A sites, while Mn³⁺ mainly occupies the B sites. It can store Li⁺ through the oxidation of metallic Co and Mn to CoO and MnO, respectively, leading to a high initial discharge capacity of 921 mA h g⁻¹ and a reversible capacity of 691 mA h g⁻¹.^44,149 Courtel et al.⁴⁴ first reported sub-micrometer-sized CoMn₂O₄ particles with a reversible capacity of about 515 mA h g⁻¹ at a current density of 69 mA g⁻¹. However, a capacity of only 330 mA h g⁻¹ after 50 cycles was obtained for this material.

Zhou et al.¹⁵⁰ prepared double-shelled CoMn₂O₄ hollow microcubes via a facile co-precipitation and annealing method (600 °C with a ramping rate of 2 °C min⁻¹) using Co_0.33Mn_0.67CO₃ as the precursor. These materials had initial discharge and charge capacities of 1282 and 806 mA h g⁻¹, respectively, and a reversible capacity of 624 mA h g⁻¹ after 50 cycles at 200 mA g⁻¹ (Fig. 23). It is interesting that the initial charge capacity is much higher than the theoretical value (691 mA h g⁻¹) based on the oxidation reactions of metallic Co and Mn nanoparticles to CoO and MnO: 3Li₂O + Co + 2Mn ↔ 2MnO + CoO + 6Li⁺ + 6e⁻. They suggested that the extra reversible capacity in their studies is attributed to the reversible formation/dissolution of polymeric gel-like films. Single-crystalline CoMn₂O₄ nano/submicrorods with approximately 100 nm diameters and lengths up to tens of micrometers were assembled¹⁵¹ using hydrothermally-synthesized β-MnO₂ nanorods as templates and cobalt hydroxide as the cobalt source; the samples were subsequently annealed at 650 °C in air for 10 h with a heating rate of 5 °C min⁻¹. Reversible capacities of 512 and 400 mA h g⁻¹ at the current densities of 200 and 1000 mA g⁻¹, respectively, were obtained after 100 cycles. Recently, Kim et al.¹⁵² synthesized yolk–shell and hollow CoMn₂O₄ powders by spray pyrolysis from aqueous spray solutions of the cobalt and manganese components with different concentrations of sucrose. The single and double-shelled yolk–shell powders prepared from spray solutions with 0.4 and 0.7 M sucrose have high discharge capacities of 519 and 573 mA h g⁻¹, respectively, in the voltage range of 0.01–3 V vs. Li/Li⁺ after 40 cycles at a high discharge rate of 800 mA g⁻¹. The group of Lou¹⁵³ obtained hierarchical CoMn₂O₄ array micro-/nanostructures with tunable morphologies on conductive stainless steel using a facile solvothermal route with subsequent annealing. The CoMn₂O₄ nanowires show reversible capacities of 530–215 mA h g⁻¹ when cycled at 1–10 C (1 C = 700 mA g⁻¹). Additional recent studies on CoMn₂O₄ as anode materials include investigations of multiporous CoMn₂O₄ spinel quasi-hollow spheres reported by Li et al.¹⁵⁴ and hierarchical nanosheets reported by Hu et al.¹⁵⁵

Fig. 23 (a) Illustration of the fabrication process for double-shell CoMn₂O₄ hollow microcubes; typical (b), (c) FESEM and (d), (e) TEM images; (f) discharge–charge profiles; and (g) cycling performance of the double-shelled CoMn₂O₄ hollow microcubes in the voltage range of 0.01–3.0 V vs. Li/Li⁺ at current densities of 200 and 800 mA g⁻¹. Reproduced with permission.¹⁵⁰ Copyright 2013, Wiley.

4. Conclusion and outlook

Recent progress in the design, synthesis and electrochemical evaluation of Li storage and cycling properties of binary, ternary, and complex Mn-based metal oxides have been reviewed. The motivations of the reviewed studies came from the eager desire to replace the graphite anode to increase energy density, decrease cost, and improve operational safety of the existing LIBs.

Two mechanisms were reviewed: the “conversion reaction” mechanism for binary Mn-based oxides (MnO, Mn₃O₄, Mn₂O₃ and MnO₂) and some ternary oxides (CoMn₂O₄), as well as the “conversion reaction involving alloying” mechanism for ZnMn₂O₄. As discussed previously, many strategies including morphology control of the micro-/nanostructures, carbon-coating and composition optimization of the Mn-based oxides have been extensively developed with the aims of improving rate and cycling performance and advancing the practical application of these oxides in LIBs. Despite some important advances in cycling and rate performance of Mn-based oxides as anode materials, the majority of the promising reported results relating to the applications of these oxides in LIBs are experimental results. Furthermore, different experimental results have been obtained even when the same synthetic methods were used to prepare the Mn-based anode materials with the same morphologies. A comprehensive and in-depth understanding of the relationship between the composition, micro/nano-structures and electrochemical Li storage performance of these oxides is therefore still demanded. Achieving this understanding requires in situ characterizations including in situ X-ray diffraction (XRD), in situ X-ray absorption spectroscopy (XAS) and in situ high-resolution transmission electron microscopy (HTEM) to verify the fine structure and reaction pathways of anode materials, and also to assign electrochemical features within specific reactions.

Because of the large electrode polarization and solid electrolyte interphase (SEI) film formation, Mn-based oxides suffer from significant irreversible capacity loss (ICL) during the first cycle. Moreover, the mechanical and chemical properties of SEI films have significant impacts on columbic efficiency, cycling performance and the operational safety of LIBs. Efforts should be made to modify the mechanical properties of SEI films by adding electrolyte additives and/or modifying electrode surfaces with the goal of forming flexible SEI films which can accommodate large volume variation during charging–discharging cycles to achieve improved cycling performance and columbic efficiencies.

The fabrication of full cells using Mn-based oxides as anode materials has not been thoroughly investigated, although several examples have been reported in recent years. Mn-based oxides are suitable to assemble full cells by combining with 4 V and 5 V cathodes. To obtain safe full cells with high cycling and rate performance, significant developments need to be made to optimize anode preparation parameters, cathodes and electrolytes along with their interfaces. Such progress will boost the development of full cells based on Mn-based oxides as anode materials in the coming years.

In spite of the aforementioned challenges, the research area of Mn-based oxide anodes, especially MnO, Mn₃O₄ and ZnMn₂O₄, for LIBs has a very bright future due to the extensive growth and application of nanotechnology and in situ characterization techniques.

Acknowledgements

This work was supported by the special financial grant from the China postdoctoral science foundation (2013T60795), the Guangzhou Scientific and Technological Planning Project (2013J4100112), the Guangdong Province Science & Technology Bureau (Industry-Education-Research Project, grant no. 2011B050300008, 2012B050300004), and the Fok Ying Tung Foundation (NRC07/08.EG01).

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