Electrode materials with tailored facets for electrochemical energy storage

Abstract

In recent years, the design and morphological control of crystals with tailored facets have become hot spots in the field of electrochemical energy storage devices. For electrode materials, morphologies play important roles in their activities because their shapes determine how many facets of specific orientation are exposed and therefore available for surface reactions. This review focuses on the strategies for crystal facet control and the unusual electrochemical properties of electrode materials bound by tailored facets. Here, electrode materials with tailored facets include transition metal oxides such as SnO₂, Co₃O₄, NiO, Cu₂O, and MnO₂, elementary substances such as Si and Au, and intercalation compounds such as Li₄Ti₅O₁₂, LiCoO₂, LiMn₂O₄, LiFePO₄, and Na_0.7MnO₂ for various applications of Li-ion batteries, aqueous rechargeable lithium batteries, Na-ion batteries, Li–O₂ batteries and supercapacitors. How these electrode materials with tailored facets affect their electrochemical properties is discussed. Finally, research opportunities as well as the challenges in this emerging research frontier are highlighted.

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1. Introduction

The specific exposed facets and their related surface energies exert significant influences on the kinetics and thermodynamics of heterogeneous reactions occurring at the interface.^1–3 Thus, the design and morphological control of crystals with tailored facets are becoming hot spots in scientific research.^4–9 In the case of anatase TiO₂, for example, it is usually exposed with low index facets such as (001) and (101) as shown in Fig. 1. Specifically, its most common crystal shape is the truncated octahedral bipyramid comprising eight (101) facets (94%) on the sides and two (001) facets (6%) on the top and bottom truncated facets.⁵ However, theoretical studies indicate that the (001) facet of anatase TiO₂ is much more reactive than its other surfaces.⁹ Surface energy is closely related to the density of undercoordinated Ti atoms. The right panel in Fig. 1 summarizes the possible shapes that may be produced. Synthesis of high energy facets of TiO₂ has been achieved to improve its properties and extend its applications in the past decade.

Fig. 1 Equilibrium crystal shape of anatase TiO₂ through the Wulff construction and the other evolved shapes (modified from ref. 5, copyright permission from American Chemical Society).

At the same time, rechargeable batteries and supercapacitors are emerging as two important classes of electrochemical energy storage devices.^10,11 Electrochemical energy storage involves physical interaction and/or chemical reaction at the surface or interface. The discharge/charge processes in rechargeable batteries and supercapacitors are accompanied by the transport of active ions across the surface of crystals. Thus, the interaction between active ions in the electrolyte and the surface of crystals is essential in the field of batteries and supercapacitors. To develop electrode materials with specific facets is of vital significance to control the interaction between active ions and surfaces of crystals. Different facets with different surface atomic structures may exhibit distinct abilities to host guest materials. In this regard, electrode materials with preferred orientation and exposure of facets have received widespread attention for electrochemical energy storage in recent years. For example, our group reported the synthesis of LiMn₂O₄ nanotubes with the exposed (400) plane,¹² LiFePO₄ nanoparticles with the major (010) exposed plane,^13,14 and a hollow Li₃VO₄ microbox with the exposed (200) plane.^15,16 Interestingly, based on density functional theory calculations and in situ mass measurements (electrochemical quartz crystal microbalance), we found that when the LiFePO₄(010) surface is brought into contact with an aqueous solution, there is always a water molecule chemisorbed near the Fe site on the surface. This water molecule strengthens Li binding on surface sites and increases the binding energy, which lowers the energy barrier for Li diffusion from the subsurface to the (010) surface.¹³

However, most reviews on the unusual properties of crystals with exposed highly reactive facets mainly focus on the field of catalysis and gas adsorption.^4–6 Those catalytic or adsorption processes critically depend on the surface atoms' arrangement and the number of dangling bonds on different crystal planes. Even though there are a few reviews dealing with crystals with tailored facets for electrochemical energy storage, they usually mainly focus on specific electrode materials such as TiO₂,^4,17,18 Co₃O₄,¹⁹ and positive electrode materials²⁰ with predominantly exposed facets. Therefore, a comprehensive summary of crystals with tailored facets for electrochemical energy storage is lacking and highly desirable in order to rationally promote the further development of rechargeable batteries and supercapacitors.

In this review, we will summarize the latest developments in the applications of electrode materials with tailored facets for electrochemical energy storage in five fields, namely, Li-ion batteries, aqueous rechargeable lithium batteries, Na-ion batteries, Li–O₂ batteries and supercapacitors. It will be shown that there is some relationship between crystals with exposed facets and their unusual electrochemical properties. The particular focus is directed to strategies for their shape control and the uniqueness of electrode materials with preferred orientation and facets in energy storage applications. To facilitate further research and development in this promising field, some future trends or directions are also discussed.

2. Electrode materials with tailored facets for Li ion batteries

Lithium is the third lightest element and has the lowest redox potential of all known elements (−3.05 V vs. the standard hydrogen potential). Though lithium has been incorporated into battery systems since the late 1960s, rechargeable lithium ion batteries were commercialized by Sony Corporation only in 1991.^21,22 A Li-ion battery consists of three functional components (a negative electrode, a positive electrode and an electrolyte) and functions by converting the chemical potential into electrical energy via Faradaic reactions. This process includes heterogeneous charge transfer occurring at the surface of an electrode. Therefore, the crystal structure and morphology are critical to the reaction rate and transfer processes.^23,24

2.1 Negative electrode materials with tailored facets

Negative electrode materials for Li-ion batteries can be classified into three different categories (1) intercalation compounds such as TiO₂, Li₄Ti₅O₁₂ and Li₃VO₄; (2) elementary substances based on alloy reactions, such as Si, Sn and Ge; and (3) metal oxides with conversion reactions, such as NiO, Co₃O₄ and Fe₂O₃. Some of their characteristics are summarized in Table 1.

Table 1 Some negative materials with tailored facets and their primary characteristics

Electrode	Tailored facets	Preparation method	Specific capacity/mA h g^−1	Rate/A g^−1	Cycle	Ref.
TiO2	(001)	Solvothermal method	200	3.4	100	25
Li4Ti5O12	(011)	Hydrothermal + calcination	205	4	100	26
SnO2	(110)	Precipitation + calcination	918	5	50	27
NiO	(110)	Hydrothermal + calcination	700	40C	1000	28
Cu2O	(001)	Hydrothermal method	841	6.7	1000	29
Co3O4	(110)	Hydrothermal method	946	2	50	30

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2.1.1 TiO₂ with tailored facets. As one of the most promising negative electrode materials for Li ion batteries, TiO₂ exhibits excellent chemical stability, non-toxicity, low cost, low volume expansion as well as the advantage of high operation safety with an operating voltage higher than 1 V above the potential at which most types of electrolytes or solvents are reduced.^31–33

Several years ago, anatase single crystal TiO₂ with 47% highly reactive (001) facets was prepared by using hydrofluoric acid (HF) as a capping agent under hydrothermal conditions.¹ The choice of capping agents is essential for controlling the facets grown in the TiO₂ crystals. The adsorption of the capping agents reduces the surface free energy of materials with more active sites inhibiting the crystal growth along the corresponding direction. This breakthrough has attracted great interest in various synthesis methods to achieve TiO₂ with specific facets, including hydrothermal,^34–36 solvothermal,^25,37,38 and template methods.³⁹

For anatase TiO₂, the Li⁺ ion diffusion coefficient is approximately 2.0 × 10⁻¹³ cm² s⁻¹ along the [001] orientation while it is only 7 × 10⁻¹⁴ cm² s⁻¹ along the [101] orientation.⁴⁰ The energy barriers calculated based on density functional theory for Li⁺ ion insertion into (001) and (101) surfaces are 1.33 and 2.73 eV, respectively.⁴¹ Thus, the charge transfer and chemical diffusion coefficient for TiO₂ are greatest along the (001) facet, and exposing this facet can result in a lower energy barrier for faster and more Li⁺ ion intercalation. As shown in Fig. 2, anatase TiO₂ nanosheets composed of 80% exposed (001) facets demonstrate a high-rate of insertion/extraction of Li ions over extended cycling compared to anatase TiO₂ with dominant (101) facets. These two samples have very different percentages of exposed (001) facets, about 80% for the nanosheets (Fig. 2a) and only about 2.2% for the octahedral sample dominated by (101) facets (Fig. 2b). At 10 C, anatase TiO₂ nanosheets with 80% exposed (001) facets show a higher specific capacity compared to (101) dominated anatase TiO₂ nano-octahedra (Fig. 2c). More Li⁺ ion insertion/extraction can occur through the less thermodynamically favored (001) surface, which agrees well with the theoretical prediction of a lower barrier for the surface transmission of Li⁺ ions across this facet compared with the (101) surface.

Fig. 2 TEM micrographs of the anatase TiO₂ with dominant (a) (001) and (b) (101) surfaces (insets are corresponding geometrical models of the anatase single crystals), and (c) the cycling behaviors with dominant (001) (upper) and (101) (lower) facets at 10C (modified from ref. 41, copyright permission from The Royal Society of Chemistry).

Ultrathin anatase TiO₂ nanosheets with nearly 100% exposed (001) facets were also synthesized.²⁵ The individual nanosheet adopts a random orientation and forms a three-dimensional highly nanoporous structure with a high specific surface area (170 m² g⁻¹). The short diffusion path length in these ultrathin TiO₂ nanosheets leads to highly efficient solid-state diffusion of Li⁺ ions. So, even at a high current rate of 20 C (3.4 A g⁻¹), a reversible capacity of 95 mA h g⁻¹ could still be achieved. Similar results have also been demonstrated by sandwich-like TiO₂/C composite hollow spheres³⁴ and TiO₂/graphene aerogel composites.³⁵ Apart from the reactive (001) surface, the strong connection between TiO₂ and conductive carbon also improves the electron transport efficiency. All these results clearly demonstrate the significance of tailoring the facets of TiO₂ crystals for improving the lithium storage capability of negative electrode materials.

2.1.2 Li₄Ti₅O₁₂ with tailored facets. Spinel Li₄Ti₅O₁₂ is regarded as another ideal negative electrode material with long cycling stability due to zero volume change when Li ions intercalate into and de-intercalate out of the electrodes.⁴² Li₄Ti₅O₁₂ can accommodate three Li ions with a theoretical specific capacity of 175 mA h g⁻¹ at a relatively high operating potential of 1.55 V (vs. Li⁺/Li), corresponding to a chemical formula of Li₇Ti₅O₁₂.⁴³ However, its rate capability is relatively low because of large polarization at high charge/discharge rates resulting from the poor electrical conductivity and sluggish Li ion diffusion.

Spinel Li₄Ti₅O₁₂ has plenty of porous channels along the [011] direction. These porous channels are favorable for fast Li ion diffusion. Therefore, the spinel Li₄Ti₅O₁₂ negative electrode with the preferentially exposed (011) plane should be of benefit to high electrochemical performance due to the directly accessible channels for the intercalation of Li ions. With this consideration in mind, Li₄Ti₅O₁₂ hollow spheres composed of nanoflakes with preferentially exposed (011) facets were fabricated via a facile hydrothermal process followed by calcination.²⁶Fig. 3 shows the top view HRTEM micrographs of Li₄Ti₅O₁₂ nanoflakes. Two sets of lattices form a dihedral angle of approximately 70.5° with each other with an equal inter-fringe spacing of 0.49 nm, corresponding to their (111) planes (Fig. 3a). The corresponding SAED pattern of the same region can be indexed to diffraction spots of the (011) zone, indicating that nearly 100% exposed surfaces are (011) planes. A more detailed crystal structure of the spinel Li₄Ti₅O₁₂ with projection along the (011) direction can be found in partially enlarged HRTEM micrographs (Fig. 3b).

Fig. 3 (a) The top view HRTEM micrographs of the nanoflake (inset is the corresponding SAED pattern); (b) partial enlarged details of the red box part in (a). The inset in b shows the corresponding crystal structure of spinel Li₄Ti₅O₁₂ along (011); and (c) schematic illustration of Li⁺ ion diffusion in spinel structured Li₄Ti₅O₁₂ (modified from ref. 26, copyright permission from American Chemical Society).

Cycled at a high current density of 4 A g⁻¹, Li₄Ti₅O₁₂ nanoflakes show a high discharge capacity of 148 mA h g⁻¹ in the first cycle, giving a retention rate of approximately 74% after 100 cycles.²⁶ The preferentially exposed (011) planes of Li₄Ti₅O₁₂ nanoflakes provide directly accessible channels for the intercalation/deintercalation of Li ions along the (011) direction (Fig. 3c). Besides, Li₄Ti₅O₁₂ nanoflakes, with a thickness of approximately 10 nm, significantly decrease the diffusion distance for Li ions compared with the bulk materials. Additionally, the hollow structure of the Li₄Ti₅O₁₂ spheres facilitates the permeation of the electrolyte within the electrode.

2.1.3 Si with tailored facets. Silicon (Si) as a negative electrode for Li ion batteries has a high theoretical capacity of 4200 mA h g⁻¹ when Li_4.4Si is formed, which is more than 10 times that of commercial graphite. Its discharging potential, about 0.2 V (vs. Li⁺/Li), is lower than most of the other alloy-type and metal oxide negative electrode materials.⁴⁴ Such a low operation potential can lead to a high energy density of full cells. Besides, Si is environmentally friendly and abundant in the Earth crust. However, Si exhibits significant volume changes (>400%) during Li alloying and dealloying. These changes cause cracking and crumbling of the electrode material and a consequent loss of electrical contact between individual particles and hence severe capacity fading.^45,46

The intercalation of a Li atom into the surface and subsurface layers of Si(100) and Si(111) planes was studied by density functional theory calculations.⁴⁷ It is found that once the Li atom is incorporated into the Si surface, Li diffuses faster by at least two orders of magnitude along the [100] direction than along the [111] direction. Similar results were also obtained through experimentally studying the shape and volume changes of Si with different orientations upon first lithiation.⁴⁸ The Si nanopillars with axial orientations along (100), (110) and (111) planes were fabricated by etching a Si wafer surface with SiO₂ nanospheres as the etch mask.

The nanopillars stand vertically on a fixed substrate as shown in Fig. 4. When they were discharged at 0.12 V (vs. Li⁺/Li) (partially lithiation), all of them show obvious anisotropic cross-sectional expansion. As shown in Fig. 4(a)–(f), the initially circular (100), (110), and (111) pillars transform into a cross, an ellipse, and a hexagon, respectively. With further discharge at 0.01 V (vs. Li⁺/Li) (full lithiation), the cross and elliptical cross-sectional shapes of the (100) and (110) nanopillars become more significant and the cross section of the 〈111〉 nanopillar is slightly hexagonal. In all three cases, the nanopillars expand most significantly along the (110) family of directions (Fig. 4d) because this channel is much larger than those along the (100) and (111) directions (Fig. 4e). Besides, the (111) and (100) nanopillars shrink in height after partial lithiation, while (110) nanopillars increase in height. It was suggested that Li enters the crystalline Si nanostructure through 〈110〉 ion channels and induces the collapse of some (111) facets by breaking Si–Si bonds. These findings provide guidelines for developing high power Si electrodes by increasing the prevalence of (110) facets to promote fast diffusion of Li ions. However, unfortunately few efforts have been put into the development of Si with tailored (110) facets.

Fig. 4 Top-view SEM micrographs of Si nanopillars with crystal orientation along (a) (100), (b) (110), and (c) (111) planes upon lithiation and each lithiation state, (d) schematic diagram of the crystallographic orientation of the facets on the sidewalls of each of the pillars, (e) view along three different directions of the diamond cubic lattice, and (f) schematic explaining anisotropic expansion of Si nanopillars (modified from ref. 48, copyright permission from American Chemical Society).

2.1.4 SnO₂ with tailored facets. SnO₂ is one of another most extensively studied negative electrode materials because of its abundance, safe lithiation potential and high theoretical capacity (782 mA h g⁻¹).^49,50 SnO₂ reacts with lithium in a two-step process (SnO₂ + 4Li⁺ + 4e⁻ → Sn + 2Li₂O and Sn + xLi⁺ + xe⁻ ↔ Li_xSn). Similar to Si, the practical application of SnO₂ typically suffers from a large volume expansion (up to 250%) and agglomeration during the Li-alloying/dealloying process, resulting in pulverization of the electrode and rapid capacity fading.

Ultrathin SnO₂ nanosheets along the [110] direction were assembled by oriented attachment of SnO₂ nanoparticles with the help of additives (ethanol, NH₃·H₂O and urea).⁵¹ The –OH group of ethanol, the –NH₂ group of urea, and the NH₄⁺ group of aqueous ammonia play important roles as surface-modifying agents in the reactions. These functional groups can lead to the variation in the surface energy of SnO₂ nanocrystals due to their different abilities to bind Sn ions or form hydrogen bonds. In another report, N-doped graphene–SnO₂ sandwich films were fabricated via a three-step route.²⁷ First, the 7,7,8,8-tetracyanoquinodimethane anion (TCNQ⁻) was adsorbed on the surface of graphene under electrostatic repulsion preventing the inter- or intra-π–π stacking of graphene. After addition of the Sn(II) salt, the graphene layers containing Sn–TCNQ self-assembled into a sandwich structure because of the strong electrostatic interactions between Sn²⁺ and TCNQ⁻. After that, the obtained precipitate was dried and annealed under an inert gas atmosphere to obtain the sandwich-type graphene–SnO₂. During the synthesis process, the interlayer space of graphene acting as a nanoscale reactor effectively restricts the growth of the SnO₂ nanoparticles and further drives the fusion of two or more nanoparticles into a 1D nanostructure within the lamellar superstructure. Most particles are epitaxially fused together towards [110] because of the continuity of the lattice (110) planes across the interface, suggesting that the oriented attachments occur along the (110) facets.

A much higher reversible capacity of 534 mA h g⁻¹ was achieved for highly oriented SnO₂ nanosheets along the (110) direction even after 50 cycles.²⁷ The SnO₂ nanosheets show a greatly enhanced lithium storage capability compared to commercial SnO₂ nanoparticles. The specific capacity is maintained as high as 683 and 619 mA h g⁻¹ at 1 and 2 A g⁻¹, respectively. Even at a current density of 5 A g⁻¹, they still deliver a capacity of 504 mA h g⁻¹. As for the commercial SnO₂ nanoparticles, the capacity rapidly decays to 153 mA h g⁻¹ at 1 A g⁻¹, and to 12 mA h g⁻¹ at 2 A g⁻¹. In the sandwich structure, the electronic transport length in graphene–SnO₂ is effectively shortened to a level comparable to the particle size of the nanocrystals. Moreover, the ultra-small SnO₂ nanocrystals (2–5 nm) with the preferred orientation of (110) planes render a very short transport length for Li ions during insertion/extraction processes.

2.1.5 Transition metal oxides based on a conversion reaction mechanism with tailored facets. A series of transition metal oxides has received growing interest as potential negative electrode materials for next-generation Li ion batteries. The lithium storage mechanism of these transition metal oxides is based on the redox conversion reaction, where the metal oxides are reduced to the corresponding metallic nanoclusters dispersed in a Li₂O matrix upon lithiation and are then reversibly restored to their initial oxidation states during delithiation. These transition metal oxides usually suffer from poor cyclability associated with the large volume change during the charge/discharge process. And the low conductivity results in poor rate behavior.

The properties of transition metal oxide crystals are largely determined by exposed external surfaces. NiO crystals with dominantly exposed (110) reactive facets were obtained by the thermal conversion of hexagonal Ni(OH)₂ nanoplatelets.²⁸ The prepared NiO crystals preserve the single crystalline feature and hexagonal shape of the precursor Ni(OH)₂ nanoplatelets. This may be because the (111) crystal facets of Ni(OH)₂ and the (110) crystal facets of NiO have low crystal mismatch. The (110) lattice plane of Ni(OH)₂ crystals has a d-spacing of 0.29 nm, which is very close to that of the (111) plane of NiO crystals (0.24 nm). The control of the crystal mismatch within a low range is highly favorable for monocrystallization.

CuO nanoplatelets with exposed (001) facets and hollow hierarchical Fe₂O₃ spheres self-organized from the ultrathin nanosheets of Fe₂O₃ were prepared by the hydrothermal process.^29,52 The ultrathin Fe₂O₃ nanosheet subunits possess an average thickness of around 3.5 nm and show preferential exposure of (110) facets. The highly exposed (110) facets of Fe₂O₃ are largely dominated by the high density of Fe atoms. The crystal facet engineering in the formation of Fe₂O₃ mesocrystals was also studied in rhombic hematite mesocrystals by a facile solvothermal approach using N,N-dimethylformamide (DMF) and methanol as the mixed solvent.⁵³ The reactants chemically transform into active particles to form the hematite crystals according to the following reactions:

HCON(CH₃)₂ + H₂O → NH(CH₃)₂ + HCOOH (1)

HCOOH + CH₃OH → HCOOCH₃ + H₂O (2)

2Fe³⁺ + 3H₂O + 6NH(CH₃)₂ → Fe₂O₃ + 6NH₂(CH₃)₂⁺ (3)

The Fe₂O₃ particle units have a common crystallographic orientation sharing (012) facets, as the cationic thermal DMF hydrolysis product can adsorb the O-terminated layer of (012) facets, which could stabilize the high-energy (012) facets. Then, crystallographic fusion happens because high interface energy leads to high colloidal attraction, where bonding between the particles allows the system to gain a substantial amount of energy by eliminating two high-energy surfaces. In this case, the exposed facet can be easily tuned by controlling the composition of the mixed solvent. Increasing the amount of CH₃OH can accelerate the hydrolysis of Fe³⁺ ions by the esterification reaction (eqn (2)), while decreasing the amount of DMF results in less NH₂(CH₃)₂⁺ required for protecting primary particles with exposed high-energy (012) facets, leading to the formation of the particles with low-energy facets. So the fusion process can be hindered since the attraction between low-energy facets cannot provide sufficient energy.

The shape control of Co₃O₄ with different well-defined crystal planar structures was facilely achieved by simply changing the content of NaOH and Co(NO₃)₂·6H₂O without using a capping agent by a one-step hydrothermal method. Three kinds of Co₃O₄ crystals, a cube with the (001) plane, a truncated octahedron with (001) and (111) planes, and an octahedron with the (111) plane, were reported.³⁰

NiO crystals retained a lithium storage capacity of 468 mA h g⁻¹ at the 20C rate and 322 mA h g⁻¹ at the 40C rate, respectively.²⁸ The most stable crystal plane for NiO crystals is the (100) plane with the lowest surface energy of 0.958 J m⁻². The (110) and (101) planes have relatively high values, larger than 1.47 J m⁻². Because of the relatively high surface energy, the (110) crystal planes provide reactive sites for reaction with Li⁺ ions, which can facilitate fast conversion reaction during the charge and discharge process. Likely, CuO single crystals with exposed high-energy (001) facets also show high rate capability.²⁹

A reversible discharge capacity as high as 815 mA h g⁻¹ after the 200th cycle was delivered at a current density of 0.5 A g⁻¹ when ultrathin nanosheets of Fe₂O₃ with preferentially exposed (110) facets were used as the negative electrode materials.⁵² The highly exposed (110) facets of Fe₂O₃, largely dominated by the high density of Fe atoms, play an important role in the Fe/Li₂O interface. Fe₂O₃ mesocrystals obtained by crystal facet engineering also show improved cycling behavior.⁵³

Electrochemical tests suggested that the electrochemical performance of Co₃O₄ is ranked as: octahedron > truncated octahedron > cube.³⁰ The (111) plane is more beneficial to Li⁺ ion transport than the (001) plane. Co₃O₄ octahedra with (111) planes possessed the highest charge/discharge capacity and best cycling behavior. Through analyzing the surface atomic configurations in the (001), (111), and (110) planes of the Co₃O₄ unit cell, it can be clearly seen that the (001) plane contains only 2Co(II) but the (111) plane contains 3.75Co(III). There is a direct relationship between the electrochemical performance of Co₃O₄ and the redox reaction of Co^m+/Co. The Co₃O₄ octahedron has a larger area of exposed (111) planes and thus faster Co²⁺/Co redox reaction. The authors also predicted that Co₃O₄ with (110) planes would exhibit better electrochemical performance than Co₃O₄ with (001) and/or (111) planes because the (110) plane contains 5Co(II) and 4Co(III).

2.1.6 Other negative electrode materials with tailored facets. Apart from the above negative electrode materials, the effects of surface facet control are also clear for Li₃VO₄^15,16 and Ca₂Ge₇O₁₆.⁵⁴

Hollow-structured Li₃VO₄ microboxes with the exposed (200) plane were synthesized via an in situ hydrothermal method reported by our group.¹⁶ We found that the hollow Li₃VO₄ microboxes were formed via an oxygen-engaged oxidation process, as well as by Ostwald ripening. The low ratio of vanadium shows an enhancement in the (200) plane, as we can see from the peak intensity of the XRD pattern of Li₃VO₄ (Fig. 5a). The trace oxygen dissolved in the solution may gradually oxidize the V₂O₃ surface to VO₄³⁻. Then reacting with LiOH, Li₃VO₄ precipitated on the surface could ensure that the surfaces of the V₂O₃ cubes are effectively covered, making the surface of V₂O₃ cubes less reactive than the freshly exposed interior. Actually, three lithium atoms and three oxygen atoms can form a radially arranged hexagon in the (100) plane (Fig. 5b), whereas two lithium atoms and two oxygen atoms can form a radially arranged rectangle in the (001) plane (Fig. 5c). Considering the diagonals of the hexagons and rectangles, the (100) plane has wider interatomic spacing for lithium. Therefore, lack of V cations could tend to increase the intensity of the (200) plane and decrease the intensity of the (002) plane. Similarly, the Ostwald ripening process was also reported to grow Ca₂Ge₇O₁₆ nanowires with a preferred (001) growth direction.⁵⁴

Fig. 5 (a) XRD patterns of Li₃VO₄ synthesized from different ratios of Li and V, structure of the orthorhombic Li₃VO₄ viewed from the (b) (100) plane and (c) (001) plane. The blue atoms are O atoms and the grey atoms are Li atoms. The atoms in the red tetrahedron cages are V atoms (modified from ref. 16, copyright permission from Wiley).

Li ions mainly intercalate into Li₃VO₄ in the potential range between 0.5 and 1.0 V (vs. Li⁺/Li), lower than the potential of Li₄Ti₅O₁₂ and higher than that of graphite. Its theoretical capacity is 394 mA h g⁻¹, in accordance with Li₅VO₄.⁵⁵ The Li₃VO₄ electrode with the exposed (200) plane delivered a discharge capacity of 323 mA h g⁻¹,¹⁶ close to that of commercial graphite⁵⁶ and much higher than that of Li₄Ti₅O₁₂ (in Section 2.1.2). It can also retain a reversible capacity of 83 mA h g⁻¹ at 20 C. The Ca₂Ge₇O₁₆ nanowires with a preferred (001) growth direction exhibit a reversible capacity of 420 mA h g⁻¹.⁵⁴ It also shows extremely long stable cycling. Actually, after the conversion reaction during the initial lithium uptake process, the in situ formed active Ge nanoparticles are highly dispersed within the mixed matrix of Li₂O and CaO, which not only provides an elastic buffer to accommodate the volume changes, but also prevents the agglomeration of nanosized Ge particles.

2.2 Positive electrode materials with tailored facets

The insertion or intercalation compounds are among the most useful positive electrode materials for Li ion batteries, and some of their characteristics are summarized in Table 2.

Table 2 Some positive electrode materials with tailored facets and their primary characteristics

Electrode	Tailored facets	Preparation method	Specific capacity/mA h g^−1	Rate/A g^−1	Cycle	Ref.
LiCoO2	(100)	Hydrothermal + calcination	189	1.4	200	57
LiMn2O4	(111)	Self-sacrifice template	115	3	500	58
LiNi1/3Co1/3Mn1/3O2	(010)	Precipitation + calcination	179	15C	100	59
LiFePO4	(010)	Solvothermal method	164	20C	60	60
Li1.2Ni0.2Mn0.6O2	(010)	Precipitation + calcination	230	20C	80	61
LiMnPO4	(010)	Hydrothermal method	130	0.5C	20	62

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2.2.1 LiCoO₂ with tailored facets. For the past two decades, LiCoO₂ has been the dominant positive electrode material for commercial Li-ion batteries owing to its high energy density and long cycling life.⁶³ Although the theoretical capacity of LiCoO₂ is 274 mA h g⁻¹, its practical capacity is only around 140 mA h g⁻¹ because its reversible amount of Li ion in LiCoO₂ is usually below 0.5 per unit. Over-delithiation will result in a phase transition from the hexagonal to monoclinic phase, which leads to an abrupt shrinkage along the c-axis direction (9% volume change).⁶⁴

LiCoO₂ has a layered α-NaFeO₂ type structure (Fig. 6a).⁶⁵ By analyzing the crystal structure of LiCoO₂, it is obvious to find that Li ions can fluently shuttle back and forth in the LiCoO₂ crystal along the [100] or [010] directions (Fig. 6b and c). Li ions are hard to diffuse in the crystal along [001] (Fig. 6d) because a mass of atoms such as oxygen or cobalt obstruct the passing route. Thus, the (010) and (100) planes perpendicular to the (001) plane are favorable for the transportation of the Li⁺ ion.

Fig. 6 (a) The crystal structure of LiCoO₂ and perspective views from (b) (001), (c) (100) and (d) (001) planes, respectively (modified from ref. 65, copyright permission from Springer).

Uniform Co(OH)₂ nanoplates were synthesized by co-precipitation and then transformed into LiCoO₂ nanoplates by solid state reaction at 750 °C.⁶⁵ Al doped LiCoO₂ with the highly exposed (100) plane was synthesized via solid state reaction with Co₆Al₂CO(OH)₁₆·H₂O and LiOH powder.⁵⁷

The initial discharge capacity of the Al doped LiCoO₂ electrode is 189 mA h g⁻¹ at 0.1C, higher than that of the conventional LiCoO₂.⁵⁷ After 200 cycles, it delivers a reversible capacity of 183 mA h g⁻¹ (equal to 96.8% capacity retention). The rapid Li ion diffusion planes of (100) or their equivalent planes have a large exposure ratio up to 100%, which remarkably improves the cyclability and rate capability of Al doped LiCoO₂. As discussed above, the (001) plane of LiCoO₂ is not electrochemically active. However, the LiCoO₂ nanoplates with the exposed (001) plane present a stable discharge capacity of 113 mA h g⁻¹ at 1 A g⁻¹ after 100 cycles.⁶⁵ It was found that there are many cracks on the nanoplates which are perpendicular to the (001) plane and favor fast Li⁺ ion transportation.

2.2.2 LiMn₂O₄ with tailored facets. Spinel LiMn₂O₄, owing to its low cost, environmental friendliness, and the abundance of manganese resources, is regarded as one of the most prospective candidates for high-power positive electrode materials for Li ion batteries.⁶⁶ Li ion extraction occurs at approximately 4 V (vs. Li⁺/Li) in a two-stage process (LiMn₂O₄/Li_0.5Mn₂O₄ and Li_0.5Mn₂O₄/λ-MnO₂).⁶⁷ Further lithiation of LiMn₂O₄ occurs at around 3 V (vs. Li⁺/Li), which leads to the formation of a rock salt phase Li₂Mn₂O₄. However, the dissolution of Mn easily happens in the spinel electrodes via a disproportionation reaction (2Mn³⁺ → Mn²⁺ + Mn⁴⁺) in the presence of acid impurities at high potential, which affects its cycling stability.⁶⁸

LiMn₂O₄ epitaxial films with (111) and (110) orientations were synthesized by pulsed laser deposition using SrTiO₃(111) and (110) substrates, respectively.⁶⁹ LiMn₂O₄ nanosheets composed of single-crystals with exposed (111) facets were synthesized via a template-engaged reaction using ultrathin MnO₂ nanosheets as a self-sacrificial template.⁵⁸ Similarly, LiMn₂O₄ nanotubes,⁷⁰ nanorods⁷¹ and nanowires⁷² with exposed (110) planes were also prepared using the corresponding shaped MnO₂ as a self-sacrificial template.

Recently, structural changes of (111) and (110) surfaces for LiMn₂O₄ during the (de)intercalation process were reported.⁶⁹ It is suggested that surface stability could be related to variations in the surface termination arrangements of oxygen ions and/or manganese ions with the valence changing during redox reactions. As shown in Fig. 7a, the (111) plane of LiMn₂O₄ is terminated by a cubic closed-packed oxygen arrangement in the spinel structure, while no closed-packed arrangement of oxygen appears in the (110) plane (Fig. 7b). The Mn ions are less densely arranged at the (110) surface and can easily come into close contact with the electrolyte, which makes Mn ions highly reactive towards solvents in the electrolyte. Therefore, the (110) plane in the spinel LiMn₂O₄ structure is less stable than the (111) surface. However, the orientation of the (110) plane is obviously aligned to Li⁺ ion diffusion, which can support fast Li ion insertion/extraction. So the above discussion may be the reason why all of the LiMn₂O₄ (nanotubes,⁷⁰ nanorods⁷¹ and nanowires⁷²) with (110) planes presents high-rate capabilities while LiMn₂O₄ nanosheets with exposed (111) facets exhibit good cycling behavior. For example, LiMn₂O₄ nanowires had a discharge capacity of about 80 mA h g⁻¹ even at 150 C (22.2 A g⁻¹),⁷² while nearly 100% of the initial capacity could be retained after 500 cycles using the LiMn₂O₄ nanosheets with exposed (111) facets.⁵⁸

Fig. 7 Atomic stacking of LiMn₂O₄ along (a) (111) and (b) (110) orientations (modified from ref. 69, copyright permission from American Chemical Society); schematic illustration of LiMn₂O₄ with various crystal shapes for (c) the truncated octahedron (Oh^T), (d) the bare octahedron (Oh), (e) platelets (Pl), and (f) their rate capability measured at various discharge C-rates, and cycling behaviors at 10C for discharge at (g) 25 and (h) 55 °C (modified from ref. 73, copyright permission from American Chemical Society).

Following the above discussion, in order to obtain both excellent rate capability and cycling life, a truncated octahedral structure (Fig. 7c) was prepared through the solid state reaction of lithium hydroxide monohydrate with an Mn₃O₄ truncated octahedron.⁷³ Its most surfaces are aligned along the (111) orientation activating minimal Mn dissolution, while a small portion of (110) planes is truncated along the directions that support Li ion diffusion. Octahedral structures enclosed by (111) planes (Fig. 7d) and nanoplates with even smaller dimensions (Fig. 7e) were also synthesized for comparison. Electrochemical test results (Fig. 7f–h) show that the truncated octahedral structure exhibits far better performance in both power density and cycle life compared to octahedral and nanoplate structures. Therefore, the concept of truncating a small portion of surfaces to support Li ion diffusion while leaving most remaining surfaces aligned along the crystalline orientations with minimal Mn dissolution enables excellent rate performance and cycle life simultaneously.

2.2.3 LiNi_0.5Mn_1.5O₄ with tailored facets. The nickel-doped spinel-type material LiNi_0.5Mn_1.5O₄ is an attractive positive electrode material for next generation lithium-ion batteries as it offers high energy density with an operating voltage of 4.7 V (vs. Li⁺/Li) involving the oxidation of Ni²⁺ to Ni⁴⁺.^74,75

In contrast to the results of LiMn₂O₄ with the (110) plane,^70–72 the truncated LiNi_0.5Mn_1.5O₄ sample with the presence of (110) planes exhibits poor electrochemical performance while the octahedral LiNi_0.5Mn_1.5O₄ material with high exposure of (111) planes demonstrates the highest reversible capacity and the best rate capability.⁷⁶ From the analysis in the above sections, we know that the (111) plane in the spinel structure is more stable than the (110) planes, while (110) planes can support fast Li ion insertion/extraction. So the electrochemical properties of LiNi_0.5Mn_1.5O₄ are very sensitive to the dissolution of Mn(III). Actually, most prepared LiNi_0.5Mn_1.5O₄ still contains a small amount of Mn(III), which can be shown by a small redox peak at 4.0 V (vs. Li⁺/Li). The Mn ions are less densely arranged at the (110) surface, which can easily come into close contact with the electrolyte, making the Mn ions highly reactive towards solvents in the electrolyte.

2.2.4 LiNi_1/3Co_1/3Mn_1/3O₂ with tailored facets. LiNi_1/3Co_1/3Mn_1/3O₂, another commercial positive electrode material, has a more stable structure than the basic layered oxides such as LiCoO₂, LiNiO₂ and LiMnO₂.^77,78 In LiNi_1/3Co_1/3Mn_1/3O₂, divalent nickel ions and trivalent cobalt ions are electroactive, involving Ni²⁺/Ni⁴⁺ and Co³⁺/Co⁴⁺ redox couples. The electrochemical inactivity of tetravalent manganese ions benefits the structure stability and effectively prohibits capacity fading.

LiNi_1/3Co_1/3Mn_1/3O₂ with a hexagonal α-NaFeO₂ layer structure, which is similar to that of LiCoO₂, is made up of MO₂ oxygen layers perpendicular to the c axis, indexed to (001) facets that include the (001) and (001) planes at the top and bottom.⁵⁹ The close packed (001) facets are therefore not electrochemically active for Li⁺ ion transportation due to their close-packed structure. The (010) facets are perpendicular to the (001) facets and have an open structure with a wide window between the layers for Li⁺ ion migration. Therefore, producing LiNi_1/3Co_1/3Mn_1/3O₂ crystals with a high percentage of exposed (010) facets will facilitate the fast and efficient transportation of Li⁺ ions.

Single crystalline LiNi_1/3Co_1/3Mn_1/3O₂ nanobricks with a high percentage of exposed (010) facets were synthesized by a solid-state reaction of LiOH·H₂O with nickel–cobalt–manganese hydroxide precursors.⁵⁹ When a plate-like structure is formed, PVP molecules immediately adsorb on its negatively charged (001) surfaces via the amine groups, thus reducing the growth rate along the (001) direction and leading to the formation of (001)-plane-dominated nanoplates. Then, the produced precursor Ni_1/3Co_1/3Mn_1/3(OH)₂ was mixed with a lithium salt and calcined at high temperature. The Ni_1/3Co_1/3Mn_1/3(OH)₂ hexagonal nanosheets with the reaction of the Li salt along the (010) direction result in a significant increase in the percentage of lateral (010) facets. Besides, a polyol medium (ethylene glycol) can also be used to control the preparation of the LiNi_1/3Co_1/3Mn_1/3O₂ nanoplates with exposed (010) active facets.⁷⁹ Hierarchical cubed LiNi_1/3Co_1/3Mn_1/3O₂ with the enhanced growth of (010) facets was synthesized by using cube structured MnCO₃ as a self-sacrificial template.⁸⁰

In case of single crystalline LiNi_1/3Co_1/3Mn_1/3O₂ nanobricks with a high percentage of exposed (010) facets, the initial discharge capacities are 159, 151, 136 and 130 mA h g⁻¹ at 2, 5, 10 and 15C rates, respectively.⁵⁹ Both LiNi_1/3Co_1/3Mn_1/3O₂ nanoplates and microcubes with exposed (010) active facets display a high initial discharge capacity of above 200 mA h g⁻¹ at a low current density.^79,80 All of them exhibited excellent cycling behaviors. Moreover, the LiNi_1/3Co_1/3Mn_1/3O₂ crystal with a high percentage of exposed (010) facets also ensured an ordered atomic arrangement, which improves the stability of the crystallographic structure upon cycling.⁵⁹

2.2.5 LiFePO₄ with tailored facets. The olivine-structured lithium iron phosphate (LiFePO₄) is also a good positive electrode material for commercial usage in Li-ion batteries because of its high theoretical capacity (172 mA h g⁻¹), one dimensional diffusion channel along the (010) direction, excellent safety attribute, attractive cost competitiveness and environmental friendliness.^81–85 However, LiFePO₄ is a poor conductor of both electrons and Li⁺ ions.^82,83

Many theoretical calculations and atomistic models were applied to study the surface energy of LiFePO₄ structure. Through studying Li⁺ ion transportation in olivine LiFePO₄ by first-principles calculations, it was found that the (010) plane possesses a lower Li ion migration energy and a higher Li ion diffusion coefficient, up to several orders of magnitude, than that of the (001) plane.⁸⁶ Therefore, considerable attempts have been made to enhance their electrochemical performances by construction of LiFePO₄ materials with maximal exposure of the (010) plane over other planes. Solvothermal and hydrothermal reactions are two effective strategies to prepare LiFePO₄ with an increased percentage of (010) planes.^60,87–90 The selection of chelating agent, reaction temperature and time, composition and concentration of surfactant plays critical roles in the growth of LiFePO₄ nanocrystals.

LiFePO₄ nanoplates with exposure of different crystal planes of (010) and (100) demonstrate similar discharge capacities at low current densities but quite different ones at high current densities (5 and 10C-rates).⁶⁰ For example, LiFePO₄ nanoplates with (010) planes delivered 156 and 148 mA h g⁻¹ at 5 and 10C-rates, respectively, while the latter delivered 132 and only 28 mA h g⁻¹ at the 5C-rate and the 10C-rate, respectively. It was reported that the crystallographic plane of LiFePO₄ nanoplates was controlled by the mixing procedure of the starting materials. In addition, the b-axis thickness plays a critical role in the percentage of (010) planes and the electrochemical performance of LiFePO₄. With a decreased b-axis thickness, LiFePO₄/C nanoplates present increasingly improved electrochemical properties in comparison with those at larger b-axis thickness, owing to the higher percentage of (010) planes than that at smaller b-axis thickness.⁸⁸ The key point is to allow LiFePO₄ to grow along the ac plane and decrease the thickness of the b-axis as much as possible, leading to more exposure of (010) planes.

2.2.6 xLi₂MnO₃·(1 − x)LiMO₂ or Li(Li_1/3−2x/3 M_xMn_2/−x/3)O₂ (M = Mn, Ni, Co, Fe, Cr, etc.) with tailored facets. Lithium-rich layered oxide materials, xLi₂MnO₃·(1 − x)LiMO₂, have attracted much attention in recent years because their capacities can be larger than 250 mA h g⁻¹.^91,92 However, the low initial coulombic efficiency, unsatisfactory rate performance and cycling stability of these materials are still the main problems preventing their utilization in practical Li-ion batteries.⁹³ Due to the removal of oxygen above 4.5 V (vs. Li⁺/Li), this kind of positive electrode material usually suffers from a huge irreversible capacity loss and rearrangement of surface microstructure.⁹⁴

The benefits of faceting have also been observed for lithium-rich layered oxide materials. In this positive electrode material with α-NaFeO₂ structure, it was widely reported that Li⁺ ions prefer to intercalate along the direction parallel to the Li⁺ ion layers, as discussed in Sections 2.2.1 and 2.2.4. The (010) facets can facilitate the migration of Li⁺ ions between the MO₆ octahedron interlayers. Two different synthetic approaches were employed to synthesize lithium-rich layered oxide materials with deliberately (010) planes.^62,95 It has been demonstrated that a different precursor and a shorter hydrothermal time both enable the Li_1.17Ni_0.25Mn_0.58O₂ nanoplates to grow simultaneously along the (010) and (001) directions, leading to the formation of (010)-facet-dominated nanoplates.⁹⁵ Another layered lithium-rich material, hierarchical Li_1.2Mn_0.6Ni_0.2O₂ quasi-spheres, whose surface is constructed with (010) planes, was prepared by using the Ni_0.2Mn_0.6(OH)_1.6 precursor as a self-sacrificial template.⁶¹ The hierarchical nanostructured Ni_0.2Mn_0.6(OH)_1.6 precursors were synthesized through a co-precipitation reaction. After heat-treatment with the lithium salts at 900 °C, the nanoplates are developed with round edges, shrunk lateral dimensions, and expanded thickness, leading to a decrease in the non-electrochemically active (001) facets.

At a 6C-rate, the reversible capacity of Li_1.17Ni_0.25Mn_0.58O₂ nanoplates could reach around 200 mA h g⁻¹ and 186 mA h g⁻¹ after 50 cycles.⁹⁵ In comparison with that in the conventional thermodynamic equilibrium nanoplate material of lithium-rich layered oxides, the active surface area in this habit-tuned nanoplate material of the Li_1.17Ni_0.25Mn_0.58O₂ sample is increased by about 50% although the proportion of the (010) nanoplates is only about 1/7. The hierarchical structured Li_1.2Mn_0.6Ni_0.2O₂ yields high initial specific discharge capacities of 230.8 and 141 mA h g⁻¹ at the 1C and 20C rate, respectively, demonstrating an outstanding high-rate performance,⁶¹ which is attributed to the increased active surface area for Li⁺ ion transportation in the lithium-rich layered oxide. This unique hierarchical structure combines the advantages of a hierarchical architecture with electrochemically active (010) planes. The special directional alignment of nanoplates provides paths for Li⁺ ion rapid insertion/extraction, while the hierarchical structure gives an efficient 3D electron transport network, which enables both efficient ion and electron transport for fast Li⁺ ion transport kinetics.

2.2.7 Other positive electrode materials with tailored facets. LiMnPO₄, a member of the phosphate olivine family, is also a promising positive electrode material with a higher intercalation potential at 4.1 V (vs. Li⁺/Li) than that of LiFePO₄. However, LiMnPO₄ also suffers from intrinsically low electronic conductivity (<10⁻¹⁰ S cm⁻¹) and sluggish Li ion diffusion in the bulk resulting in poor rate performance.⁹⁶ Since the 1950s, a class of polyanionic materials such as Li₂MSiO₄ (M = Mg, Zn, Ca, Co, Fe, and Mn) have been intensively studied as glass-ceramics materials for the traditional ceramic industry. Among these materials, Li₂FeSiO₄ and Li₂MnSiO₄ were studied as positive electrode materials for Li-ion batteries.^97,98

LiMnPO₄ microspheres with different crystallographic orientations were assembled via a facile hydrothermal route.⁶² Na₂S·9H₂O is employed as a sole additive for controlling the phase, shape and crystallographic orientation of LiMnPO₄ microspheres. The Na₂S·9H₂O additive can be rapidly hydrolyzed into sulphions and hydroxyl ions, which selectively adsorb on different crystallographic facets of a LiMnPO₄ crystal in aqueous solution. Li₂FeSiO₄ and Li₂MnSiO₄ nanosheets with growth oriented along the a-axis were prepared by a rapid one pot supercritical fluid synthesis method.⁹⁹ The surface tension of supercritical fluids completely vanishes above the critical point of the fluid, which is of particular utility in controlling the surface and interface chemistries of the nanostructured materials.

The synthesized LiMnPO₄ microspheres assembled with nanoplates exhibit discharge capacities of 130 mA h g⁻¹ at 0.05C and 76.8 mA h g⁻¹ at 0.5C, respectively, which showed a superior electrochemical performance over microspheres assembled with edges and prisms.⁶² This is due to the exposure of (010) facets. The (010) direction is the thinnest part of the crystal allowing for fast Li⁺ ion diffusion. Ultrathin Li₂MnSiO₄ nanosheets show a discharge capacity of 340 mA h g⁻¹.⁹⁹ For the first time, two Li ions were successfully extracted/inserted using the Li₂MnSiO₄ nanosheets. The sheet-like morphology oriented along the a-axis plays a significant role in achieving the two Li ion insertion/extraction of Li₂MnSiO₄. Both the orthorhombic and monoclinic structures of the Li₂MSiO₄ family are based on the slightly distorted hexagonal close packing of oxygen ions with all cations in the tetrahedral voids and a pseudohexagonal plane parallel to (001).

3 Electrode materials with tailored facets for aqueous rechargeable lithium batteries

Research on Li-ion battery technology shows that the use of flammable organic electrolytes might lead to thermal run-away and safety accidents. Moreover, the cost of a Li-ion battery is relatively high due to the organic electrolytes and special cell assembly technology such as the requirement of a strictly dry environment during manufacturing processes. In this regard, aqueous rechargeable lithium batteries (ARLBs) were proposed in 1994.¹⁰⁰ However, the inferior capacity retention and low energy density limit their practical application. In 2007, ARLBs using various electrode materials based on redox reactions were reported by our groups and have attracted wide attention as promising systems in the past several years.¹⁰¹ ARLBs can overcome some disadvantages of Li-ion batteries. They are inherently safe by avoiding the use of flammable organic electrolytes. Moreover, the cost of the aqueous electrolyte is low because expensive salts can be replaced by cheap salts. In addition, the ionic conductivity of aqueous electrolytes is high, about two orders of magnitudes higher than that of organic electrolytes, which ensures high rate capability and thus high specific power.^102–104

3.1 Negative electrodes with tailored facets

As for the negative electrode materials of ARLBs, vanadium oxides, molybdenum oxides and some lithium intercalation compounds have been reported. Among them, LiV₃O₈,¹⁰⁵ H₂V₃O₄¹⁰⁶ and VO₂¹⁰⁷ with preferred orientation and exposure of facets were studied in the past several years.

Hierarchical LiV₃O₈ nanofibers, assembled from nanosheets that have exposed (100) facets, were fabricated by using electrospinning combined with calcination.¹⁰⁵ During calcination, PVA was used and decomposed to release CO₂, whilst NH₄VO₃ reacted with Li(CH₃COO)·H₂O to produce LiV₃O₈ nanoparticles. Besides acting as the template for forming the fibers, PVA could prevent LiV₃O₈ nanoparticles from aggregating into larger ones, making them grow into small nanosheets with exposed (100) facets owing to the self-limitation properties of LiV₃O₈. Single-crystal H₂V₃O₈ nanowires along the [001] growth direction were obtained through a facile method by one-step hydrothermal treatment of commercial V₂O₅ powder.¹⁰⁶ Flowerlike VO₂ (B) micro-nanostructures assembled by single-crystalline nanosheets have been successfully synthesized via a hydrothermal route using polyvinyl pyrrolidone (PVP) as a capping agent.¹⁰⁷

Compared with the other LiV₃O₈ micro/nanostructures, the hierarchical LiV₃O₈ with exposed (100) facets clearly shows better electrochemical performance in ARLBs. When the pH value of the electrolyte was adjusted to below 5, it was found that nanosheets with exposed (100) facets could effectively alleviate proton co-intercalation into the LiV₃O₈ electrode. In the crystal structure of LiV₃O₈, the (010) and (001) facets provide a lot of channels while the (100) facet has fewer and smaller channels, which makes it more difficult for H⁺ ions to intercalate into this material. A small amount of H⁺ ions intercalating into these channels instead of Li⁺ ions will lead to capacity fading. The pH value of the electrolyte has little influence on the cycling performance of the hierarchical LiV₃O₈ nanofibers with exposed (100) facets.¹⁰⁵

The post-treated flowerlike VO₂ (B) electrode shows an initial discharge capacity reaching 81 mA h g⁻¹, which is higher than that of the flowerlike VO₂ (B) sample before annealing.¹⁰⁷ Compared to V₂O₅, H₂V₃O₈ (or V₃O₇·H₂O) has a higher electronic conductivity arising from a mixed-valence of V⁴⁺/V⁵⁺. The H₂V₃O₈ nanowires allow a full intercalation of Li⁺ ions in preference to hydrogen evolution, and thus can deliver a specific capacity of 234 mA h g⁻¹,¹⁰⁶ much higher than that of any other vanadium oxides (VO₂¹⁰⁷ and V₂O₅¹⁰⁸) in aqueous electrolytes. In H₂V₃O₈, the V₃O₈ layers are held together by van der Waals interactions together with hydrogen bonding, in which this weakly bonded layer structure can favor the mobility of Li⁺ ions between the layers.

3.2 Positive electrodes with exposed facets

As discussed in Section 2.2, intercalation compounds (LiCoO₂, LiNi_1/3Co_1/3Mn_1/3O₂, LiMn₂O₄ and LiFePO₄) are used as positive electrode materials for lithium ion batteries. In aqueous electrolytes, lithium intercalation and de-intercalation can also occur in/from these materials as in organic electrolytes.

LiMn₂O₄ nanotubes with a preferred orientation of (400) planes were prepared by using multiwalled carbon nanotubes as sacrificial templates.¹² The oriented MnO₂ was deposited on the CNTs. Then the as-prepared LiMn₂O₄ showed a typical crystal orientation after reacting with a Li salt at 700 °C. In the standard LiMn₂O₄, the intensity of (111) planes is much stronger than that of (400) planes.^66,68 For the prepared LiMn₂O₄ nanotubes, their intensities are almost the same, suggesting that LiMn₂O₄ has a crystal orientation. LiFePO₄ crystals with major (010) exposed facets were prepared by the reflux route in ethylene glycol solution under atmospheric pressure.¹³

Both LiMn₂O₄ with a preferred orientation of (400) facets and LiFePO₄ crystals with major (010) exposed facets present superfast second-level charge capability (up to 1000C).^13,65 In the case of the crystal orientation of the LiMn₂O₄ nanotube, there are more (001) or (010) planes on the edges of these planes or vertical to (400) planes, which is similar to the LiCoO₂ nanoplates with exposed (001) planes.⁶⁵ The 8a sites for lithium intercalation and deintercalation are situated at the (001) or (010) planes. Therefore, more Li sites are exposed to the aqueous electrolyte due to the preferred growth of (400) planes.

As for LiFePO₄ with exposure of (010) facets, the reason for fast charge in an aqueous electrolyte is a little relatively complicated. H₂O adsorption at different atomic sites on the LiFePO₄(010) surface was calculated and the most stable structure was identified when three H₂O molecules adsorb at three different sites, as illustrated in Fig. 8a and b (the 1, 2, 3 sites). These sites are the exact locations of the O vacancies at the corners of FeO₆ and LiO₆ octahedra in a stoichiometric (010) LiFePO₄ surface. Such unique arrangement of water molecules at those LiFePO₄(010) sites was verified using an accurate in situ mass measurement (EQCM) and Fourier transform infrared spectroscopy (FTIR). Each Li⁺ ion in the aqueous electrolyte is always coordinated by 4 water molecules in its primary solvation sheath. Two water molecules will be stripped away from this complex in order for a Li⁺ ion to intercalate into the nanoparticle (i to ii, and to iii in Fig. 8c and d). Then the resultant Li⁺(H₂O)₂ can approach the LiFePO₄ surface with almost no additional barrier and docks at the site (iii to iv in Fig. 8c and d) to form a structure similar to the scenario of 3H₂O on top of LiFePO₄ in Fig. 9b. Such interfaces (HSLE) are effective in promoting fast mass transfer, which results in high rate capability (Fig. 8e). After this Li⁺ ion diffuses into the LiFePO₄ bulk along the Li channel, these two H₂O molecules will desorb from the surface.

Fig. 8 (a) Top and (b) side views of LiFePO₄/vacuum, LiFePO₄/H₂O, and FePO₄/H₂O along the (010) direction. The four LiO₆ octahedra correspond to four Li diffusion channels. The dashed red circles denote the O vacancies at the surface. (c) The reaction profiles for Li⁺ ion transport across the FePO₄/water interface in the discharge process and (d) their energies at each step (right hand panels). (e) Charge and discharge curves at different current densities (1C = 170 mA g⁻¹) between 0.2 and 0.75 V vs. SCE in a 0.5 M Li₂SO₄ aqueous electrolyte (modified from ref. 13, copyright permission from American Chemical Society).

Fig. 9 Charge–discharge curves of the cubic Au NCs@SP (red line), T-OCT Au NCs@S P (blue line), TOH Au NCs@SP (green line) and bare SP electrodes (black line) at 100 mA g⁻¹ in the first cycle. The right images are their FEM or TEM micrographs and structure models (modified from ref. 122, copyright permission from Nature Publishing Group).

4 Electrode materials with tailored facets for Na-ion batteries

Historically, progress in Na-ion batteries is parallel to that of lithium ion batteries.¹⁰⁹ Unfortunately, Na-based systems rapidly fell into oblivion because Na⁺ ions are much larger in radius than Li⁺ ions and it is more difficult to find a suitable host material to accommodate Na⁺ ions. However, for sustainability reasons, Na-ion batteries have recaptured the scientific community's attention in recent years. The breakthroughs in materials science open new horizons for new energy storage and conversion devices.¹¹⁰

4.1 Negative electrodes with exposed tailored facets

In this section, recent research progress on negative electrodes with tailored facets for Na ion batteries is summarized, such as SnO₂ octahedral nanocrystals consisting of dominantly exposed (221) high energy facets¹¹¹ and SnO nanocrystals with exposed (001) facets.¹¹²

SnO₂ with exposed (221) high energy facets and SnO with exposed (001) facets were synthesized by a hydrothermal method using PVP and Na₂SO₄ as a morphology directing agent, respectively.^111,112 Sulfate ions are most strongly adsorbed onto faces perpendicular to the c-axis of the crystal through bridging-bidentate adsorption, leading to the retarded growth along the c-axis and the formation of the facet crystals.¹¹²

SnO₂ nanocrystals with exposed (221) facets demonstrated a good high rate performance.¹¹¹ It was found that Na ions first insert into SnO₂ crystals at the voltage range from 3 to 0.8 V (vs. Na⁺/Na), and that the exposed (1 × 1) tunnel-structure could facilitate the initial insertion of Na ions. Then, Na ions react with SnO₂ to form Na_xSn alloys and Na₂O in the low voltage range from 0.8 to 0.01 V (vs. Na⁺/Na). The SnO with exposed (001) facets delivered specific capacities of 525, 438, and 421 mA h g⁻¹ at current densities of 40, 80, and 160 mA g⁻¹, respectively.¹¹² Similar to SnO₂, SnO nanocrystals are dominated by (001) facets, exposing the 2D diffusion pathways for the insertion of Na⁺ ions into SnO crystals and the facilitated reaction towards sodium at high current densities.

4.2 Positive electrodes with tailored facets

Positive electrode materials are considered in this section including V₂O₅ hollow nanospheres constructed from hierarchical nanocrystals with predominantly exposed (110) crystal facets.¹¹³ Na_0.54Mn_0.50Ti_0.51O₂ with the growth direction perpendicular to the c-axis,¹¹⁴ Na_0.7MnO₂ single crystals with predominantly exposed (100) facets¹¹⁵ and β-MnO₂ nanorods with exposed (111) crystal facets¹¹⁶ have been reported.

V₂O₅ with predominantly exposed (110) crystal planes was synthesized via a polyol-induced solvothermal process.¹¹³ The preferred orientation of V₂O₅ nanocrystals is not triggered by the preparation conditions, but influenced by the vanadyl ethylene glycolate precursor's crystal structure. Tunnel-structured Na_0.54Mn_0.50Ti_0.51O₂ nanorods were synthesized by a facile molten salt method.¹¹⁴ These nanorods are grown in the direction normal to the Na-ion tunnels, which could greatly shorten the diffusion distance of Na ions and benefit the transfer kinetics. Na_0.7MnO₂ nanoplates with exposed (100) crystal planes and β-MnO₂ nanorods with exposed (111) crystal planes were synthesized by a hydrothermal method.^115,116 The individual Na_0.7MnO₂ nanoplate shows a perfect rhombus shape. Its corresponding SAED spot pattern could be well-indexed to the orthorhombic crystal structure along the (100) zone axis. And the facet vertical to the incident electron beam is the (100) crystal plane. The two basal planes of the rhombus nanoplates are the (100) facets, which are the predominantly exposed facets. For Na_0.7MnO₂ with layered structure, each layer perpendicular to the c axis is indexed to the (001) crystal plane. The (001) crystal plane is not electrochemically active for Na⁺ ion transport while the (100) crystal plane is an active plane for Na⁺ ion insertion/extraction, owing to the existence of a 2D channel for Na⁺ ion transport. So Na_0.7MnO₂ nanoplates with predominantly exposed (100) facets could easily facilitate the insertion/extraction of Na⁺ into/from the crystal structure. At a current density of 0.18 A g⁻¹, the Na_0.7MnO₂ nanoplate electrode still delivers an initial capacity of 125 mA h g⁻¹.¹¹⁵

Beta-MnO₂ nanorods deliver a high initial discharge capacity of 350 mA h g⁻¹. Although the discharge capacity decreases gradually upon cycling, it still maintains a high specific capacity of 200 mA h g⁻¹ after 100 cycles.¹¹⁶ These β-MnO₂ nanorods have exposed (111) crystal planes with a high density of (1 × 1) tunnels. The (1 × 1) tunnel not only provides facile transport for Na-ion insertion and extraction but also accommodates Na-ions.

5 Electrode materials with tailored facets for Li–O₂ batteries

The first Li–O₂ battery was introduced in 1996, and it was composed of lithium, an organic-impregnated polyacrylonitrile electrolyte, and a carbon electrode.¹¹⁷ A decade later, the Li–O₂ battery using organic carbonates as electrolytes was demonstrated.¹¹⁸ After that, research on Li–O₂ batteries quickly became a hot topic.^119–121 A typical rechargeable non-aqueous Li–O₂ battery is comprised of a Li metal as the negative electrode, a Li conducting organic electrolyte and a catalyst with oxygen as a positive electrode. Catalytic processes critically depend on the surface atoms' arrangement and the number of dangling bonds on different crystal planes.

The catalytic properties of polyhedral Au nanocrystals (NC) with different index facets were studied in Li–O₂ batteries, including cubic gold (Au) NCs enclosed by (100) facets, truncated octahedral Au NCs enclosed by (100) and (110) facets, and trisoctahedral (TOH) Au NCs enclosed by 24 high-index (441) facets.¹²² This preparation system has three species including a reducing agent (ascorbic acid), a gold precursor (HAuCl₄) and a capping agent (CTAB or CTAC surfactant). It has been found that CTAB molecules can bind more strongly to the (100) than the (111) facets.¹²³ Different CTAB concentrations can produce shapes with (100) or (111) facets. When CTAB is replaced by CTAC at the same concentration for the synthesis, TOH Au NCs bound by (441) facets dominate in the final product. The controlled synthesis of Co₃O₄ with different shapes and crystal planes and their catalytic properties for Li–O₂ batteries have also been systematically studied, including nanocubes, pseudo octahedra, nanosheets, hexagonal nanoplatelets and nano-laminar.¹²⁴ They are exposed with (100), (110), (111), and (112) crystal facets, respectively.

Compared to the carbon black (Super-P, SP), all these Au NCs significantly reduce the charge potential and show high reversible capacities (Fig. 9). Particularly, TOH Au NC catalysts demonstrate the lowest charge/discharge overpotential and the highest capacity of 20 298 mA h g⁻¹. Density functional theory calculations on the different Au crystal planes and their interaction with the Li and O atoms show that the interaction energy between the Au and the Li and O atoms decreases as the surface energy of the Au crystal planes increases. The oxygen adsorption energy on the surface of the (441) crystal planes is lower than those of the (100) and (111) crystal planes, which makes TOH Au NCs more active toward the oxygen evolution reaction (OER), thus leading to an enhanced electrochemical performance.¹²²

For the Co₃O₄ catalyst, the essential factor promoting the OER is its surface crystal planes.¹²⁴ The correlation between different Co₃O₄ crystal planes and their effect on reducing charge–discharge over-potential is established in the following order: (111) > (112) > (110) > (100). Similar to the above Au crystal planes, Co₃O₄(111) crystal planes with highest surface energy have the largest interaction with Li and O atoms, leading to the highest catalytic properties for the Li and O reaction.

6 Electrode materials with exposed facets for supercapacitors

Supercapacitors have some advantages over batteries such as short charge–discharge time, high power density, high charge–discharge efficiency and long cycling life. According to different charge-storage mechanisms of electrode materials, supercapacitors can be categorized into electrochemical double layer capacitors (EDLCs) and Faradaic pseudocapacitors.^125,126 In terms of the electrode configuration, supercapacitors can be fabricated as symmetric and asymmetric supercapacitors. The well-studied materials for EDLCs are usually based on nanostructured carbon materials. Pseudocapacitive materials cover transition metal oxides, conducting polymers and intercalation compounds. Among them, carbon materials and conducting polymers are amorphous, so these electrode materials are not included in this section.

6.1 Negative electrodes with tailored facets

Hierarchical nanotubular titanium nitride (TiN) was fabricated by magnesiothermic reduction of titania replicas of ordinary filter paper (cellulose substance).¹²⁷ The preferred growth direction is perpendicular to [200]. The Fe₃O₄@SnO₂ core/shell nanorod film was achieved by fabricating Fe₃O₄ pristine nanorod film from FeOOH followed by the hydrothermal coating with SnO₂ nanoparticles.¹²⁸ Fe₃O₄ nanorods possess single-crystalline nature and grow along the [110] direction. MoO₃ nanoplates with a crystal orientation along [001] were prepared via a sol–gel method by our group.¹²⁹ The prepared MoO₃ consists of nanoplates which are on average about 1 μm × 1 μm × 100 nm. A core/shell structure of PPy grown on V₂O₅ nanoribbons was fabricated by using SDB-(dodecylbenzenesulfonate) as a surfactant by our group.¹³⁰ V₂O₅ nanoribbons were prepared by the hydrothermal treatment of NH₄VO₃ and the poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) copolymer in acid solution at 120 °C.

TiN possesses attractive properties such as thermal conductivity, corrosion resistance and chemical stability. The shape of its CV curve is approximated to rectangular in a 1 M KOH electrolyte, showing an ideal double layer capacitive behavior.¹²⁷ The specific capacitance of TiN is very low, only 74.2 F g⁻¹ at 0.16 A g⁻¹. When Fe₃O₄@SnO₂ core/shell nanorod film is tested in the potential range of −0.8 to 0.2 V (vs. Ag/AgCl) in 1 M Na₂SO₃ solution, 82.8% of the initial capacitance could be stabilized after 2000 cycles.¹²⁸ It is claimed that the capacitance of magnetite Fe₃O₄ originates from the surface redox reaction of sulfur in the form of sulfite anions, as well as the redox reactions between Fe²⁺ and Fe³⁺ accompanied by intercalation of sulfite ions to balance the extra charge with the iron oxide layers. Besides, SnO₂ absorbs solvated cations (Na⁺) on the electrode surface from the electrolyte.

MoO₃ nanoplates with a crystal orientation along [001] have a specific capacitance of 280 F g⁻¹ in the Li₂SO₄ aqueous electrolyte, which is higher than that of the bulk MoO₃ (208 F g⁻¹).¹²⁹ The nanoplate structure makes the solvated Li⁺ ions reach the MoO₃ surface more easily than the bulk MoO₃. Our group also investigated the insertion/extraction of K⁺ ions into/from V₂O₅ occurring in the (001)-plane-constituted interlayer space.¹³⁰ V₂O₅ retains a layered structure with distinct diffraction peaks of (001), (003), and (004) planes. When K⁺ ions insert into the V₂O₅ electrodes possessing a K/V ratio of 0.35, the calculated interlayer space of the (001) plane decreases to 9.45 Å in comparison with the original value of 10.5 Å. This probably results from the reinforced interaction between K⁺ and the V₂O₅ skeleton. At the end of charge and discharge, the interlayer space expands slightly at the end of the charge accompanied by K⁺ ion extraction. The specific capacitance of V₂O₅ nanoribbons is 162 F g⁻¹ at 100 mA g⁻¹. The PPy shell coated on the V₂O₅ core further improves the charge transfer process and prevents vanadium dissolution into the aqueous electrolyte.

6.2 Positive electrodes with tailored facets

The capacitance of Co₃O₄ is directly linked to surface properties and its electrochemical performance is greatly influenced by any change related to the surface morphology of this electroactive material.¹³¹ A Co₃O₄@CNT hybrid was prepared by depositing Co₃O₄ on CNTs through a simple hydrothermal method. The size of the Co₃O₄ nanoparticles with preferentially exposed (220) planes is 10–15 nm.¹³² Co₃O₄ nanomesh with large-scale exposed (112) crystal planes is obtained based on the formation of single-crystal (NH₄)₂Co₈(CO₃)₆(OH)₆·4H₂O nanosheets as the precursor and their subsequent conversion upon heating in air.¹³³ The monocrystallization and conversion process from (NH₄)₂Co₈ (CO₃)₆(OH)₆·4H₂O to Co₃O₄ is considered as thermodynamically supported recrystallization. The exposed crystal plane of Co₃O₄ on the largest scale is changed from (110) to (112) with the precursors changing from Co(CO₃)_0.5(OH)_0.11·H₂O to (NH₄)₂Co₈(CO₃)₆(OH)₆·4H₂O, which is convinced by XRD measurements.

NiO hexagons with exposed (110) facets on metallic Ni backbones were prepared via a simple hydrothermal method followed by annealing at 300 °C for 1 h.¹³⁴ The presence of SO₄²⁻ anions in the solution is primarily responsible for the restricted crystal growth in the perpendicular direction as they are strongly adsorbed onto the surfaces perpendicular to the c-axis through bridging-bidentate adsorption, thereby resulting in oriented hexagonal nanoplatelets of Ni(OH)₂ on the surface. Upon annealing, the hexagon shapes are preserved and the exposed facets are the (110) facets which allow a lower lattice mismatch with Ni(OH)₂. The NiO crystals have dominantly exposed (110) facets on both the hexagonal surfaces together with (002) and (111) facets as edges, forming a close-packed hexagonal nanoplatelet structure.

Alpha-MnO₂ nanowires grown on flexible carbon fabric were synthesized by a hydrothermal approach.¹³⁵ The growth direction of these MnO₂ nanowires is very close to the normal direction of the (112) plane. Mn₃O₄ octahedral nanoparticles with (101) facets were prepared by a simple controlled oxidation method,¹³⁶ similar to the growth mechanism of Li₃VO₄.^15,16 Mn₃O₄ exists naturally as hausmannite, a distorted spinel in which Mn(II) and Mn(III) occupy the tetrahedral and octahedral sites, respectively. The most stable phase of Mn₃O₄ is a truncated tetragonal bipyramidal structure in which the (101) facets are primarily exposed with a small percentage of (001) and (100) facets. Growth of the (101) plane depends on the availability of Mn(III), which is formed by the oxidation of Mn(II), because of the greater density of Mn(III) in the (101) plane than that of the (001) plane.

The hybrid of Co₃O₄ nanocrystals coupled with CNTs can cycle over 9000 cycles in 1 M KOH aqueous solution.¹³² When assembled into an asymmetric supercapacitor by using activated carbon as the negative electrode, the hybrid capacitor shows excellent cycling performance between 0 and 1.8 V with an energy density of 31 W h kg⁻¹ and a power density of 3 kW kg⁻¹. Co₃O₄ nanomesh shows a capacitance of 297 F g⁻¹ when scanned at 2 A g⁻¹ in 3 M KOH aqueous solution though the voltage window is only in the range from 0.25 to 0.5 V (vs. Ag/AgCl),¹³³ and 288 F g⁻¹ could be achieved even at 10 A g⁻¹. However, Co₃O₄ nanostructures with dominant (111) or (100) crystal planes do not have highly expected capacitive performance, and their specific capacitances are below 20 F g⁻¹ and degrade rapidly upon increasing the scan rates. Actually, the dominant (112) crystal plane in the Co₃O₄ nanomesh has much higher surface energy than the conventional (111) and (100) crystal planes, leading to higher activity in supercapacitors.¹³³ The Ni/NiO composite electrode with exposed high surface energy facets exhibits a specific capacitance as high as 2100 F g⁻¹.¹³⁴ When assembled as an asymmetric supercapacitor with mesoporous carbon as the negative electrode, the energy and power densities are calculated to be 17 W h kg⁻¹ and 3.5 kW kg⁻¹, respectively. Besides, this asymmetric supercapacitor also delivers good cycling performance over 2000 cycles, as the mesoporous wire-like network structure can uniformly distribute the stress across the electrode.

The areal capacitance of the MnO₂ nanowire electrode can be 150 mF cm⁻² at a current density of 1 mA cm⁻².¹³⁵ The specific capacitance corresponds to 197.4 F g⁻¹ at an equivalent current density of 1.3 A g⁻¹. A solid-state flexible asymmetric supercapacitor was assembled with MnO₂ nanowires and Fe₂O₃ nanotubes as the electrodes using a gel electrolyte. It demonstrates excellent stability in a large potential window of 1.6 V and exhibits an excellent energy density of 0.55 mW h cm⁻³. The Mn₃O₄ octahedral nanoparticles show a high capacitance of 260 F g⁻¹ at a scan rate of 1 mV s⁻¹.¹³⁶ Based on density functional theory calculations, Na preferentially binds to the (101) surface with a binding energy of 2.04 eV, compared to that of 1.4 eV on the (001) surface.

7 Conclusion and outlook

Over several years, the increasingly large amount of efforts devoted to electrode materials with tailored facets has resulted in a rich database for their synthesis, characterization and wide range of applications. This review has summarized recent advances in the synthesis chemistries and distinctive electrochemical properties of electrode materials with tailored facets for Li-ion batteries, aqueous rechargeable lithium batteries, Na-ion batteries, Li–O₂ batteries as well as supercapacitors. The relationship between the crystal facets and their electrochemical activities has been analyzed and discussed.

Generally, the synthesis can be achieved by a wet-chemistry route with addition of a surfactant. More specifically, through preferential binding of the surfactants with certain crystal planes, the shape of the crystals can be finely tuned. A surfactant is the most important preparation parameter as a morphology-directing agent (or a template, or a capping agent) in the formation of the desired electrode materials with tailored facets. Besides, the exposure of crystal facets can be controlled to some extent by selecting the appropriate precursor, crystal structure and ratio of precursors, and other reaction conditions such as temperature. The precursors can control both the chemical compositions and morphologies of the crystals while temperature can significantly control the speed of crystal growth in some cases.

On the other hand, the relationship between the crystal facets and their electrochemical activities can be summarized as follows:

(1) The charge transfer resistance and chemical diffusion coefficient are high along some facets, and exposing these facets can result in a lower energy barrier for faster transport of active ions across the surface of crystals such as TiO₂ as the negative electrodes for Li-ion batteries.^40,41

(2) Some facets have relatively high surface energy and these crystal facets provide reactive sites for fast redox reaction during the charge and discharge process, such as transition metal oxides based on conversion reaction for Li-ion batteries,^{28–30,52,53} catalysts for Li–O₂ batteries,^122–124 and electrode materials for supercapacitors.¹³⁶

(3) Active ions can quickly shuttle back and forth in crystals along some directions while they cannot diffuse in the crystal along other directions because a mass of other atoms obstruct their passage. Thus, those facets, perpendicular to the directions that allow transportation of active ions, render a very short transport length for ions during insertion/extraction, such as most intercalation compounds for Li-ion batteries and Na-ion batteries.^57,115

(4) There are many cracks on the exposed crystal planes and these cracks can favor fast ion transportation, such as LiCoO₂ with exposed (001) planes and LiMn₂O₄ with exposed (400) planes.^12,65

(5) A small amount of non-active ions intercalating into the channels of electrode materials will lead to capacity fading. Some exposed facets can effectively alleviate non-active ion co-intercalation into the electrode materials because these facets have fewer and smaller channels which only allow active ion intercalation into the material such as LiV₃O₁₈ with exposed (100) planes for aqueous rechargeable lithium batteries.¹⁰⁵

(6) There will be a water molecule chemisorbed near some sites of some facets. This water molecule strengthens the active ion binding on surface sites and increases the binding energy, which lowers the energy barrier for active ion diffusion from the subsurface to this facet, such as LiFePO₄ with exposed (010) planes for aqueous rechargeable lithium batteries.¹³

(7) Some ions in the crystals are less densely arranged at some facets and can easily come into close contact with the electrolyte, which makes these ions highly reactive towards solvents in the electrolyte, such as LiMn₂O₄ with exposed (111) planes for Li-ion batteries.⁷³

The mechanism for the enhancement of electrochemical performances varies with materials and applications. In the case of most electrode materials for batteries, some planes possessing relatively low migration energy for active-ions like Li⁺ and Na⁺ are needed. To predict the electrochemical properties, the active-ion transport in some complex structures should be confirmed at first. Thus some well-established atomic modeling techniques are of utmost significance for electrode material optimization. The arrangement of surface atoms and surface energy also mean a lot especially for those electrode materials used in Li–O₂ batteries and supercapacitors. For example, the surface energy of some planes of electrode materials can be calculated with the help of the Vienna ab initio simulation package (VASP).¹⁹ Currently, despite these significant advances achieved in this field, research on electrode materials with exposed tailored facets is still in its preliminary stage. Great opportunities and huge challenges coexist in this field. Throughout this review, we hope to generate more interest in them and boost extensive investigation in related areas. Thus, new vital interest and some challenges listed below are expected to motivate future studies.

To begin with, the technology for the synthesis of electrode materials with tailored facets is facing a challenge in establishing some theories that enable us to predict the type of surface or plane produced by a certain method. In fact, the selection of surfactant still remains empirical currently. Even with the use of same adsorbent as the surface controller, it is possible to prepare nanocrystals with different morphologies. Therefore, the surface binding structures of molecular adsorbents need to be well characterized at the molecular level. Besides, most synthesis strategies involve the use of morphology-controlling agents that must eventually be removed in order to obtain clean facets. This process might lead to some uncontrollable changes in the surface atomic structure of the crystal.

In addition, the integration of several instruments based on in situ characterization techniques is necessary. Obviously, in situ observations are essential to acquire a true understanding of the electrode surfaces. For example, in situ X-ray diffraction¹³⁷ and in situ TEM techniques¹³⁸ are strongly recommended to investigate the mechanism and structural evaluation of electrode materials.

Last but not least, developing new materials and structures is always expected, especially for Na-ion batteries and Li–O₂ batteries. Exploring reliable electrode materials with suitable structures that can allow the intercalation/deintercalation of Na ions with high efficiency and meanwhile possessing excellent cycling stability is needed for the further development of Na-ion batteries. Designing different structured and efficient catalysts towards both oxygen reduction and/or evolution reactions is one of key strategies to improve Li–O₂ battery performance. Meanwhile, many new electrochemical energy storage devices have emerged in recent years, including Mg-ion batteries,¹³⁹ Al-ion batteries,¹⁴⁰ Zn-ion batteries,¹⁴¹ F- and Cl-ion batteries,^142,143 Na- and K–O₂ batteries,^144,145 Li–CO₂ batteries,¹⁴⁶ Li–Br₂ batteries,¹⁴⁷ metal ion capacitors¹⁴⁸ and so on. However, there are few reports on their electrode materials with tailored facets. Therefore, seeking new electrode materials with tailored facets will become a hot spot in the field of electrochemical energy storage devices.

Acknowledgements

Financial support from the Distinguished Young Scientists Program of the National Natural Science Foundation of China (NSFC51425301) is gratefully appreciated.

Notes and references

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