Recent Advances in Co₃O₄ as Anode Materials for High-Performance Lithium-Ion Batteries

1. Introduction

Currently, enormous efforts have concentrated on the exploitation of new energy storage to effectively use intermittent renewable energy for replacing traditional non-renewable energy sources, which cause increasingly serious environmental problems such as acid rain, smog, greenhouses effect and so on.^[1-6] Lithium ion batteries (LIBs) are dominating the present battery market due to the high energy density, wide operation temperature range, rapid charge/discharge capability, long cycle life and no memory effect.^[7-9]

Rechargeable LIBs are regarded as one of the most successful energy storage devices since the first commercial application by Sony Corporation.^[8,10] LIBs consist of three main components: anode electrodes, cathode electrodes and electrolyte. Anode and cathode electrodes are separated by polypropylene film that is saturated with electrolyte. Nowadays, LiMn₂O₄, LiNiO₂, LiCoO₂, LiFePO₄, and Lithium-rich layered xLi₂MnO₃-(1-x)LiNi_1/3Co_1/3Mn_1/3 materials have been widely used as cathode electrodes in commercial LIBs.^[11-13] However, the mature anode materials are only limited to graphite materials including silicon/graphite  composite containing no more than 15% Si, due to its merits of excellent cycling performance, inexpensive and relatively low working voltagevs. Li/Li⁺.^[14] A schematic illustration of a working LIB based on a LiCoO₂ cathode and a graphite anode is shown in Fig. 1. During the charging process, Li ions are de-intercalated from LiCoO₂ electrode and travel towards to graphite anode across polypropylene film through the electrolyte, and then insert into graphite electrode; at the same time, the electrons move to anode side from cathode side through the external circuit in order to maintain the electrical neutrality of the material. Vice versa, Li ions are de-intercalated from the graphite electrode and move back to the LiCoO₂ cathode during the discharge process, and electrons move back to the cathode side from the anode side. The efficient conversion of chemical energy to electric energy inside a lithium-ion battery is implemented by the de-intercalation/intercalation processes of Li⁺ accompanied with an equal charge quantity of electron transfer. The electrochemical reactions can be described by the following Equations:

Anode:

xLi⁺ + 6C + xe^- ↔ Li_xC₆                                                                                   (1)

Cathode:

LiCoO₂↔ Li_1-xCoO₂ + xLi⁺ + xe^-                                           (2)

Overall reaction:

LiCoO₂ + 6C ↔ Li_1-xCoO₂ + Li_xC₆                                                          (3)

    Currently, LIBs are dominantly used in portable electronics, the ever-growing applications that need high energy/power capacities, such as high-power electric vehicles, large spacecraft and national grid, etc. They place greater demands on LIBs with better electrochemical behaviors, which depend on the breakthroughs in electrode materials.^[15]

2. Overview of Anode Electrodes

The conventional graphite offers a relatively low theoretical capacity of 372 mAh g^-1.^[14] Therefore, different carbon based and non-carbon based materials with higher lithium storage capacity have been exploited, as shown in Fig. 2. Carbon-based materials include, but are not limited to, graphene, amorphous carbon, carbon nanotubes, and carbon nanofibers.^[14] Non-carbon materials mainly include lithium metal, silicon, titanium dioxide, germanium, transition metal oxides, transition metal nitride, transition metal sulfide, transition metal phosphides, and transition metal carbides.^[16]

The state-of-art anodes materials are classified into three categories according to the electrochemical lithiation/delithiation mechanism as shown in Fig. 3. One type is alloy anodes based on an alloying/dealloying reaction (M+ xLi⁺↔ Li_xM), which mainly contain silicon, germanium, silicon monoxide, and stannic oxide; This family materials exhibit high energy density, higher specific capacity and good safety, but with huge volume changes during cycling processes, resulting in capacity fading and poor cycling performance.^[17] The second type is intercalation anodes based on the lithium insertion/extraction mechanism(MX₂+ Li⁺↔ LiMX₂), which mainly include carbon based materials, titanium dioxide, Li₄Ti₅O₁₂, and niobium pentoxide; They usually present good working potential, long cycle lifespan and low price, but with relatively low capacity and low energy density. The third type is conversion anodes based on the redox reaction mechanism (MX+ 2Li⁺↔ Li₂X+ M), which is most commonly referred to transition metal oxides, nitride, sulfide, phosphides, carbides, etc. They commonly show high specific capacity and low operation potential, but with relatively low coulombic efficiency, poor cycling lifespan and unstable SEI.^[17,18]

Fig. 1 Schematic illustration of a typical LIB (cathode: LiCoO₂; anode: graphite).

All the mentioned three types of anode electrodes have shown huge application potential for the next generation LIBs. The intercalation anodes show good electrochemical properties of long-cycling life, low price, low working potential, and extreme safety during charge/discharge processes. However, this type of anodes also shows the relatively low capacity and low density compared with alloy and conversion anodes.^[18-20] The alloy anodes, such as Si material, show superior high capacity (4200 mAh g^-1) and high energy density. However, this kind of anodes presents large irreversible capacity, huge volume changes during cycling (Si, 400 %), which cause the huge capacity fading and poor cycle life performance compared with intercalation and conversion anodes.^[17-20] The conversion anodes own relatively higher theoretical capacity compared with intercalation anodes, and relatively low volume changes during the cycling compared with alloy anodes. In addition, these anodes show the merits of environmental benignity and low cost, attracting more and more attention as the substitute for the current anode material graphite of the next generation LIBs.^[21] 

Recently, transition metal oxides, as a kind of conversion anodes, including MnO, SnO₂, MnO₂, Fe₃O₄, Fe₂O₃, NiO, etc., are becoming the star anodes for LIBs, due to the high capacity, large reserves, low cost and environment-friendliness.^[22] Among various electrode materials,  Co₃O₄  has been  investigated  for  many  years in virtue of its advantages of high theoretical specific capacity (890 mAh g^-1), wide availability, easy preparation and stable chemical properties since its first applied as the anode electrode for LIBs by Poizot at 2000.^[23,24]More than 1000 papers about Co₃O₄-based electrodes for LIBs have been published in the last 5 years according to the database of Web of Science, confirming the continued hot attention. More and more works were carried out to obtain the capacity higher than the theoretical specific capacity of Co₃O₄, better rate capability and longer cycling performance due to the unique designed nanostructures. Here, we will present the recent advances in rationally designed nanostructures of high performance Co₃O₄-based anodes that focus on the illustration of structure-property relationship. We will also present our perspectives on the methods to enhance the electrochemical performance of nanostructured Co₃O₄-based anodes for the new generation of LIBs.

Fig. 2 Schematic illustration of anode electrodes for the LIBs.

3. Nanostructured high-performance Co₃O₄-based Anodes

Despite that Co₃O₄-based anodes are considered as the star materials for LIBs anode, they still suffer from some drawbacks including relatively low electronic conductivity and huge volume changes during the intercalation/deintercalation of Li⁺, impeding the practical application. Constructing different nanostructures for Co₃O₄ electrodes is an efficient strategy to solve the mentioned problems and enhance the electrochemical performance due to the shortened Li⁺ transport path, increased specific surface area and increased number of active sites.^[18,25] As a consequence, different morphologies of Co₃O₄ electrodes have been developed, including zero dimensional material, one dimensional material, two dimensional materials and three dimensional materials.

3.1 Zero Dimensional Co₃O₄-based Anodes

Zero Dimensional (0D) nanostructures usually refer to the particles at the nano scale with some special characters.^[26,27] The 0D Co₃O₄-based materials include nanospheres, nanocrystals, nanoparticles, nanocages and nanocubes (Fig. 4), which can offer more active sites, shorten transport pathways, increase the contact area between electrolyte and electrode, and promote the intercalation/deintercalation of Li⁺ process. Thus, the electrochemical performance of Co₃O₄-based electrodes has been greatly improved. 

Chen et al. prepared Co₃O₄ nanocages by heat treatment of cobalt-based Prussian blue analogues. The obtained unique designed nanocages samples showed large specific surface area, resulting in excellent electrochemical performance. They confirmed that the better crystallinity, moderate particle size and multihole structure of samples enhanced the electrochemical properties of initial coulomb efficiency and cycle stability.^[28] In Xiao’s research (Fig 4a), single-crystalline Co₃O₄ cubes have been successfully obtained via a simple hydrothermal method, which displayed a discharge capacity of 877 mAh g^-1 at 0.2 A g^-1 after 110 cycle-life.^[29] Kim et al prepared Co₃O₄ nanoparticles and obtained a reversible capacity of ~1100 mAh g^-1 at 1.0 A g^-1 after 150 cycles.^[30]

In order to further improve the electrochemical performance of Co₃O₄-based electrodes, conductive carbon^[39] as additives or supports have been introduced such as carbon nanotubes,^[31] porous carbon,^[32] graphene oxide.^[40] The choice of carbon materials is based on the following merits. First, conductive carbon additives themselves have been widely applied as electrodes for energy storage devices due to its advantages of giveaway prices, good chemical and physical stability, relatively high capacity, high conductivity, and light weight.^[41] Second, conductive carbon additives can effectively alleviate volume changes during cycling performance.^[21] Han et al. has shown that the compounds of Co₃O₄ nanoparticles and porous carbon nanotube sponge obtained by intense densification, which light up a high efficiency and inexpensive method to develop high areal capacity and high mass loading electrodes for LIBs^[31] (Fig 4b).

Fig. 3 Three categories of anode electrodes and their characteristics. Reproduced with permission.^[17] Copyright 2018, Springer.

Fig. 4 0D nanostructured Co₃O₄ with different morphologies: SEM image of Co₃O₄ cubes (a), Reproduced with permission.²⁹ Copyright 2018, Springer. SEM image of Co₃O₄-CNT nanocomposites (b), Reproduced with permission.^[31] Copyright 2018, WILEY-VCH. SEM image of Co₃O₄ nanoparticles@C cube (c), Reproduced with permission.^[32] Copyright 2018, Elsevier. TEM image of Co₃O₄ /graphene composites (d), Reproduced with permission.^[33] Copyright 2017, Elsevier. SEM image of Co₃O₄ nanospheres@N-C framework (e), Reproduced with permission.^[34] Copyright 2016, American Chemical Society. SEM image of sandwich-shape Co₃O₄/Graphene composites (f), Reproduced with permission.^[35] Copyright 2017, American Chemical Society. SEM image of Co₃O₄/Co/carbon nanocages (g), Reproduced with permission.^[36] Copyright 2017, Elsevier. TEM image of hollow Co₃O₄ nanoparticles (h), Reproduced with permission.^[37] Copyright 2015, American Chemical Society. SEM image of Co₃O₄ nanoparticles/Carbon nanotube (i), Reproduced with permission.^[38] Copyright 2016, Wiley-VCH.

In addition, building up hollow structure or mesoporous structure has been proved to improve the electrochemical performance of nanosized Co₃O₄ electrodes. Wang et al. reported hollow Co₃O₄ nanoparticles supported by carbon matrix prepared by a method of impregnation-reduction coupled with the following oxidation process in air atmosphere. The composites exhibited superior electrochemical performance, thanks to the unique hollow structure that benefits the transportation of electrons and Li⁺ as well as buffers the stress caused by the huge volume changes during the cycling process, as shown in Fig 1h^[37] (Fig 4h). The composites consisting of hollow Co₃O₄ nanoparticles and carbon nanotubes were reported by Lou’s group. The as-prepared composites delivered a high capacity of 1281 mAh g^-1 at 0.1 A g^-1 after 200 cycles due to the merits of the distinctive structures, including that the hollow structure offered more active sites, enhanced structural stability, and the carbon nanotubes improved the conductivity of electrons and effective inhibition of particle aggregation^[38] (Fig 4i).

3.2 One Dimensional Co₃O₄-based Anodes

One Dimensional (1D) nanostructures usually refer to nanowires, nanofibers, nanorods and nanotubes.^[46-51] The 1D nanostructures have been extensively used in the field of energy storage on account of the characteristics of shortened electrons and ions transmission pathway, more active sites, increased contact area between electrodes and electrolyte. For Co₃O₄-based materials, the 1D nanostructures can also restrain the volume changes during the intercalation/deintercalation process of Li⁺ and limit the agglomeration of nanoparticles^[52] (Fig 5a).

Chen et al. synthesized hollow Co₃O₄ fibers by the method of chemical precipitation and thermal decomposition and the as-prepared samples showed high capacity of 1177.4 mAh g^-1 at 0.1 A g^-1 and a relatively high initial coulombic efficiency of 82.9 %. The enhanced electrochemical performance was ascribed to the unique 1 D structure that increased the specific surface area to offer more active sites and contact area and the void space to accommodate the volume change during cycling^[53] (Fig 5b). Zhang’ group distributed Co₃O₄hollow nanoparticles onto porous carbon fiber to form the 1 D structure, and the composite exhibited a high reversible capacity of 1100 mAh g^-1 at 0.1 A g^-1 after 200 cycle-life^[54] (Fig 5c). The Co₃O₄nanofibers were also prepared by electrospinning technology and showed outstanding capacity and cycling performance^[55] (Fig 5d). Yan et al. reported carbon-doped Co₃O₄hollow nanofibers and obtained superior performance, the structure-property relationship was evidenced by density functional theory (DFT) calculations^[56] (Fig 5e). The Co₃O₄nanofibers can also be synthesized by the hydrothermal method^[57,64] (Fig 5f), template-based engineering^[65] and hydrolysis strategy^[58] (Fig 5g), and all the as-prepared samples delivered outstanding electrochemical performance.

In addition to nanofibers, Co₃O₄ nanotubes and nanorods have also been developed as anode electrodes for LIBs. Co₃O₄@Carbon nanotube arrays were synthesized by Zhao’s group and showed desirable performance in capacity, rate capability and cycle stability.^[52] The formation mechanism of Co₃O₄mesoporous nanotubes was controlled by releasing of the gas originated from organic oxidation reaction and kokendar effect and the electrochemical performance were enhanced^[59] (Fig 5h). The Co₃O₄ hollow nanotubes were synthesized by Yan et al. They introduced the concept of the local built-in electric field, causing the imbalance of charge distribution due to the C-doping, promoting the Li⁺ imbedding and deblocking process, to obtain high lithium storage properties^[60] (Fig 5i). The compounds of Co₃O₄ and carbon nanotubes in a hierarchically porous structure were prepared by Li and colleagues, and this distinctive structure exhibited a high reversible capacity of 1083 mAh g^-1 at 0.5 mA g^-1 after 140 cycles, excellent rate capability of 521mAh g^-1 at 8.0 A g^-1 and superior cycle stability at 1.0 A g^-1 for 200 cycles^[61] (Fig 5j).

Besides the mentioned 1D nanostructures, the Co₃O₄ nanowires were also used as LIBs anodes.^[66]  Mesoporous Co₃O₄ nanowires on the nickel foam were formed by the decomposition of a metal-organic framework structure, and these composites possessed a high reversible capacity of 1609 mAh g^-1 at 0.5 A g^-1 and excellent rate capability without the use of conductive additives. This unique structure offered more active sites, shortened the pathway for Li⁺ and electrons and promoted the rapid transmission of electrons^[62] (Fig 5k). Wu et al. reported Co₃O₄/α-Fe₂O₃ nanowire heterostructure structure, theseCo₃O₄ nanowire arrays were grown on the Ti matrix that promoted the transmission of electrons, and the α-Fe₂O₃ improved the capacity, relieved the volume changes and inhibited the aggregation of Co₃O₄ electrodes, resulting in outstanding electrochemical performance^[63] (Fig 5l).

3.3 Two Dimensional Co₃O₄-based Anodes

Generally, Two Dimensional (2D) nanostructures usually refer to nanosheets, nanofoils, nanoflakes and films.^[67-70] The electrons can move rapidly in the 2D nanostructures and are almost unrestricted between layers, thus accelerating reaction kinetics of electrodes for good rate capability and cycle stability.

Zhao et al. reported porous Co₃O₄ films that were tightly attached to Ti nanowire arrays as anode electrodes for LIBs. The as-prepared samples showed a highly desirable cycle stability of ca. 400 mAh g^-1 at 20.0 A g^-1 after 2000 cycles without significant decline.^[71] Meanwhile, a kind of graphene-like, porous 2D Co₃O₄ nanofoils was synthesized and the as-prepared composites exhibited a high capacity far beyond its theoretical capacity^[72] (Fig 6a).

Fig. 5 1D nanostructured Co₃O₄ with different morphologies: SEM image of Peapod-like Co₃O₄@C nanotube (a), Reproduced with permission.^[52] Copyright 2015, WILEY-VCH. SEM image of microfibers (b), Reproduced with permission.^[53] Copyright 2017, Elsevier. TEM image of porous fibers (c), Reproduced with permission.^[54] Copyright 2018, Royal Society of Chemistry. SEM image of Co₃O₄ Nanofiber (d), Reproduced with permission.^[55] Copyright 2017, American Chemical Society. TEM image of Co₃O₄ hollow nanofibers (e), Reproduced with permission.^[56] Copyright 2016, WILEY-VCH. TEM image of Co₃O₄ crystal nanofibers (f), Reproduced with permission.^[57] Copyright 2016, Springer Nature Limited. SEM image of Co₃O₄@TiO₂ core-shell nanofibers (g), Reproduced with permission.^[58] Copyright 2017, Elsevier. TEM image of Co₃O₄ mesoporous nanotubes (h), Reproduced with permission.^[59] Copyright 2016, WILEY-VCH. TEM image of Co₃O₄ hollow nanotubes (i), Reproduced with permission.^[60] Copyright 2018, WILEY-VCH. TEM image of Co₃O₄ nanorods/carbon nanotubes (j), Reproduced with permission.^[61] Copyright 2018, WILEY-VCH. SEM image of Co₃O₄ nanowires (k), Reproduced with permission.^[62] Copyright 2018, WILEY-VCH. SEM image of Co₃O₄ nanowires (l), Reproduced with permission.^[63] Copyright 2013, Tsinghua University Press and Springer.

Nulu et al. prepared hierarchical Co₃O₄ nanoflakes arrays, which presented superior anode performance based on the rate capability and lifespan^[73] (Fig 6b). Du’ group reported the porous Co₃O₄ nanoplates/graphene composites, which the evolution process of the composites electrode was investigated by in situ TEM, found that the volume changes during cycling performance were smaller than the theoretical volume expansion due to the unique nanoplates structure, giving reasons for the stable cycling performance and high reversible capacity^[74] (Fig 6c).

Fig. 6 2D nanostructured Co₃O₄ with different morphologies: TEM image of the 2D Co₃O₄ nanofoils (a), Reproduced with permission.^[72] Copyright 2016, WILEY-VCH. TEM image of Co₃O₄ Nanoflakes (b), Reproduced with permission.^[73] Copyright 2017, ESG. TEM image of Co₃O₄ nanoplates (c), Reproduced with permission.^[74] Copyright 2014, Elsevier. SEM image of Co₃O₄ nanosheets (d), Reproduced with permission.^[75] Copyright 2016, WILEY-VCH. SEM image of Co₃O₄ nanosheets (e), Reproduced with permission.^[76] Copyright 2016, Elsevier. SEM image of Co₃O₄ nanosheet networks (f), Reproduced with permission.^[77] Copyright 2014, Elsevier. SEM image of Co₃O₄ nanosheets (g), Reproduced with permission.^[78] Copyright 2018, American Chemical Society. SEM image of Co₃O₄ nanohybrids (h), Reproduced with permission.^[79] Copyright 2016, Springer Nature Limited. SEM image of Co₃O₄ nanosheet arrays (i), Reproduced with permission.^[80] Copyright 2018, American Chemical Society.

Besides the above mentioned 2D nanostructures that can improve the electrochemical performance of Co₃O₄-based materials, more attention is paid for nanosheets structures. Dou’ group reported the atomically thin mesoporous Co₃O₄ nanosheets/graphene composites, which delivered the capacity as high as 2014.7 mAh g^-1 at 0.1 A g^-1 and 1134.4 mAh g^-1 at 2.0 A g^-1. The distinguished electrochemical performance was ascribed to the proper thickness and the mesoporous structure of nanosheets that shortened the transportation pathway of electrons and increased the contact interfaces of electrolytes and electrodes. Meanwhile, the obtained higher capacity than theoretical specific capacity was ascribed to the pseudocapacitive contribution as the extra capacity due to unique designed layer structure, defects, edges et al^[75] (Fig 6d). The porous Co₃O₄ nanosheets grown on the Ni foam was applied as the binder-free anode for LIBs, and delivered excellent electrochemical properties, including the high reversible capacity of 543 mAh g^-1 even at the high current density of 20.0 A g^-1 and cycle stability of 2000 lifespan at 5.0 A g^-1 and 20.0 A g^-1[76] (Fig 6e). The electrochemical properties of 2D Co₃O₄ nanosheets were enhanced by the 3D graphene networks, where the conductivity and the interface between electrolyte and electrode were improved and the transportation pathway was shortened, which resulted in satisfactory performance.^[81] Ultra-thick mesoporous Co₃O₄ nanosheets were also grown on the Ni foam by Zhang’ group, and also demonstrated desired electrochemical behaviors, which further implying the advantages of 2D nanosheets as electrodes for energy storage^[77] (Fig 6f). Co₃O₄/graphene foam electrodes with good flexibility and mechanical strength were reported by Yao et al. The pore structure can well relieve the volume changes that keep the integrity of the electrodes during the cycling process, the nature on the electrochemical behaviors was explored, and the full cell measurements were carried out to identify the as-prepared composites^[78] (Fig 6g). A Co₃O₄ hierarchical nanostructure consisting of Co₃O₄ nanoparticles and Co₃O₄ nanosheets was developed as anode electrodes and exhibited superior lithium storage behaviors^[79] (Fig 6h). Li et al. prepared Co₃O₄ ultra-thin nanosheets to improve the kinetics of electrons and Li⁺ transport of Co₃O₄-based materials and got the ideal electrochemical results^[80] (Fig 6i).

3.4 Three Dimensional Co₃O₄-based Anodes

Three Dimensional (3D) nanostructures usually refer to the composites materials that are composed of one or more basic units of 0D, 1D and 2D materials. They mainly include micro-sized cages, hollow structures, mesoporous materials, flower-like structures, etc.^[22,82-86] The 3D architecture electrodes have demonstrated excellent electrochemical performance due to good compaction, abundant mesoporous and large specific surface area.

Constructing 3D hollow structures is regarded as an effective strategy to speed up the electrochemical performance of Co₃O₄-based materials. Firstly, a hollow structure can effectively accommodate the volume changes of Co₃O₄ electrodes during the charge-discharge processes. Secondly, a hollow structure provides a large surface area, which increases the interfaces between electrodes and electrolytes and improves the number of active sites. Last but not the least, a hollow structure would shorten the diffusion pathway, offer more transportation channels for electrons and Li⁺. Wang et al. reported multi-shelled Co₃O₄ hollow microspheres by adjusting the synthesis process parameters, which delivered satisfactory reversible capacity, rate capability and cycle performance^[87] (Fig 7a). Similarly, the Co₃O₄ hollow sphere structure was constructed by Sun and his colleagues, and the as-prepared electrodes displayed an excellent capacity of 924 mAh g^-1 at 1.0 A g^-1, enhanced rate capability and ultra-long cycle stability of 7000 cycles at a high current density of 5.0 A g^-1 without obvious capacity decline^[88] (Fig 7b). The porous Co₃O₄ hollow structure with adjustable porosity was prepared by a simple method of gas template-assisted spray pyrolysis, which showed excellent electrochemical performance.^[96]

Fig. 7 3D nanostructured Co₃O₄ with different morphologies:  TEM image of the multi-shelled Co₃O₄ hollow microspheres (a), Reproduced with permission.^[87] Copyright 2013, Wiley-VCH. SEM image of hollow spheres (b), Reproduced with permission.^[88] Copyright 2014, Macmillan Publishers Limited. TEM image of the multi-shelled Co₃O₄ hollow structure (c), Reproduced with permission.^[89] Copyright 2017, Elsevier. SEM image of 3D flower-like Co₃O₄ (d), Reproduced with permission.^[90] Copyright 2018, Tsinghua University Press and Springer. SEM image of flower-like Co₃O₄ (e), Reproduced with permission.^[91] Copyright 2018, Springer. SEM image of bowknot-like Co₃O₄ (f), Reproduced with permission.^[92] Copyright 2017, Royal Society of Chemistry. SEM image of 3D Co₃O₄ microspheres/N-doped C (g), Reproduced with permission.^[93] Copyright 2018, Elsevier. SEM image of straw-sheaf-like Co₃O₄ (h), Reproduced with permission.^[94] Copyright 2018, Elsevier. TEM image of 3D mesoporous network of Co₃O₄ (i), Reproduced with permission.^[95] Copyright 2017, WILEY-VCH.

The uniquely designed multi-shelled Co₃O₄ hollow structure with a controllable number of internal shells was synthesized and obtained good electrochemical properties^[89] (Fig 7c). In addition, the hierarchical 3D flower-like structure also plays an important role in improving the performance of Co₃O₄-based materials. Cao et al. reported a kind of flower-like Co₃O₄ and applied as anodes for LIBs with a desirable capacity of 920 mAh g^-1 at 1.0 A g^-1, excellent rate capability and cycling behaviour^[90] (Fig 7d). The 3D flower-like Co₃O₄ particles were also constructed by Zhai et al. and exhibited good electrochemical behaviours^[91] (Fig 7e). A mesoporous bowknot-like structure was built up via hydrothermal method and heat treatment process, which displayed a high reversible capacity of 1388.8 mAh g^-1 at 0.18 A g^-1 after 100 cycles^[92] (Fig 7f). The 3D Co₃O₄ microspheres modificated with N-doped amorphous carbon showed high reversible capacity of 830 mAh g^-1 under a high current density of 1.0 A g^-1 after a lifespan of 500 cycles^[93] (Fig 7g). The hierarchical straw-sheaf-like Co₃O₄ was fabricated via the hydrothermal method and the mechanism of preparation was well explored, which demonstrated superior anode electrodes properties, thanks to the unique 3D structure^[94] (Fig 7h). Zhu et al. reported a 3D interconnected Co₃O₄ structure and the magical structure presented remarkable lithium storage performance^[95] (Fig 7i). In our group, we manufactured mesoporous hollow Co₃O₄ microspheres via a facial two-step hydrothermal strategy and introduced oxygen vacancies into the structure by the heat treatment process (Fig. 8).^[97] The delivered outstanding electrochemical performance of the prepared samples can be explained by the concept of local built-in electric field，which caused by the unbalanced charge distribution around the oxygen vacancy region, promoting the Li⁺ intercalation/ deintercalation process. This strategy cast light on developing Co₃O₄ and other transition metal oxides as anode electrodes for LIBs.^[98]

Furthermore, 3D Co₃O₄ cube structures have been developed to improve electrochemical performance. Micro-/nanostructured Co₃O₄ cube structures were synthesized by Huang et al. and the electrode with such unique morphology showed a high capacity and capacity retention.^[99] Deng et al. reported a Co₃O₄-based composite that Co₃O₄ nanoparticles were covered with double carbon coating of 3D porous carbon network and cubic N-doping carbon shell. The as-prepared electrodes presented outstanding electrochemical performance of 1208 mAh g^-1 at 0.1 A g^-1 after 100 cycles,

excellent rate capability of 539 mAh g^-1 at 10.0 A g^-1, and 516 mAh g^-1 under 5.0 A g^-1 for 400 cycles due to the merits of structure, including high porosity, high surface area, high conductivity and physical stability.^[100] The cubic Co₃O₄ composites structure was also constructed by Wang’s group, where the Co₃O₄ nanoparticles were dispersed in the cubic hierarchically porous carbon matrix, delivered prominent capacity property.^[32] The metal-organic framework-derived Co₃O₄ particles were protected by MoS₂ nanosheets, which accommodated the volume changes during the charge-discharge process, resulting in remarkable lithium storage properties.^[101] Meanwhile, Lou’s group prepared multishelled Co₃O₄@Co₃V₂O₈ nanoboxes, and showed satisfactory electrochemical characteristics.^[102]

4. Conclusions and Perspectives

  With the huge demands for energy storage devices, especially for lithium ion batteries with outstanding performance, electrode materials with even higher energy and power densities will continue to be developed. Co₃O₄-based materials, traditional transition metal oxides, are still charming as anode electrodes for LIBs due to the huge potential for the next generation anodes to meet the requirements of high performance.

Fig. 8 SEM image of Urchin-like Co₃O₄ microspheres (a), cycling performance at 0.1 (b) and 1.0 A g^-1(c) and the schematic illustration of the mechanism of local built-in electric field caused by oxygen vacancies. Reproduced with permission.^[98] Copyright 2018, Royal Society of Chemistry.

In this work, we have summarized the recent developments in high performance Co₃O₄ and the composites as anode materials for LIBs. The morphology and electrochemical performance are discussed through different dimensions, including 0D (nanospheres, nanocrystals, nanoparticles, nanocages and nanocubes), 1D (nanowires, nanofibers, nanorods and nanotubes), 2D (nanosheets, nanofoils, nanoflakes and films) and 3D structures (microsized cages, hollow structures, mesoporous materials, flower-like structures). Since Co₃O₄-based materials are faced with low electrical conductivity and huge volume expansion during charge-discharge processes, which hinder the commercial application. Different strategies have been applied to overcome these drawbacks. The preparation of the nanosized particles (various nanostructures), introducing conductive carbon matrix (carbon nanofibers, graphene, carbon nanotubes, N-doping porous amorphous carbon et al.), constructing special structures (hollow structure, flower-like structures, mesoporous materials, core-shell structure et al.) have been proved to be effective in improving the electrochemical performance of Co₃O₄-based materials. With the unique designed nanostructure, the practical application of Co₃O₄-based electrodes with the enhanced electrochemical performance for next-generation LIBs may be realized in the near future, and the development will be promoted by the emerging machine-learning techniques.

To fit the practical application, the lithium storage properties of Co₃O₄-based electrodes, including rate capability and stable long-lifespan still have plenty of room to enhance. In order to better meet the needs of commercial applications of Co₃O₄-based electrodes for LIBs, the facial, inexpensive, environmentally friendly, and mass-producible methods are needed to be developed. Meanwhile, the size and morphology of Co₃O₄-based electrodes, especially the microsized and uniform particles that are more conducive to large-scale production, need to be controlled strictly. In addition, the understanding of the mechanisms behind the performance and the materials’ formation play an important role in unraveling the improved electrochemical performance, which will provide the guideline to design new materials for energy storage electrodes.

Acknowledgements

This work was supported by research program of the Key Laboratory of Biomedical Effects of Nanomaterials and Nanosafety, Chinese Academy of Sciences (No: NSKF201908) and the research program of Top Discipline in Materials Science of Shandong Province.

Conflict of interest

There are no conflicts to declare.

Supporting information

Not applicable.

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