Hollow structured cathode materials for rechargeable batteries

Hollow structured cathode materials for rechargeable batteries

Journal Pre-proofs Review Hollow structured cathode materials for rechargeable batteries Xiaobo Zhu, Jiayong Tang, Hengming Huang, Tongen Lin, Bin Luo...

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Journal Pre-proofs Review Hollow structured cathode materials for rechargeable batteries Xiaobo Zhu, Jiayong Tang, Hengming Huang, Tongen Lin, Bin Luo, Lianzhou Wang PII: DOI: Reference:

S2095-9273(19)30706-6 https://doi.org/10.1016/j.scib.2019.12.008 SCIB 903

To appear in:

Science Bulletin

Received Date: Revised Date: Accepted Date:

25 November 2019 29 November 2019 2 December 2019

Please cite this article as: X. Zhu, J. Tang, H. Huang, T. Lin, B. Luo, L. Wang, Hollow structured cathode materials for rechargeable batteries, Science Bulletin (2019), doi: https://doi.org/10.1016/j.scib.2019.12.008

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© 2019 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.

Received 25-November-2019; Revised 29-November-2019; Accepted 02December-2019 Hollow structured cathode materials for rechargeable batteries Xiaobo Zhu a, Jiayong Tang a, Hengming Huang a,b, Tongen Lina, Bin Luo a, Lianzhou Wang a* a

Nanomaterials Centre, School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, QLD 4072, Australia E-mail: [email protected] b

College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China

Abstract Hollow structuring has been intensively studied as an effective strategy to improve the electrochemical performance of the electrode materials for rechargeable batteries in terms of specific capacity, rate capability, and cycling performance. To date, hollow structured anode materials have been extensively studied, while hollow structured cathode materials (HSCMs) are relatively less explored because of the difficulties in morphological control as well as the concern of reduced volumetric capacities. In this review, we provide an overview of the research advances in the synthesis and evolution of HSCMs for metal (Li, Na, etc.) ion batteries. Attributing to the advantages of hollow structures including high surface area, excellent accessibility to active sites, and enhanced mass transport and diffusion, hollow structuring can significantly improve the performance of highcapacity cathode materials with low kinetics, such as lithium rich layered oxides, silicates, and V2O5. It is anticipated that the precise and flexible control of the spatial configuration can balance the electrochemical performance of HSCMs and the volumetric capacities of HSCMs, leading to practical high-performance batteries. Keywords: rechargeable batteries, cathode materials, hollow structures, electrochemical performance.

1. Introduction

Hollow structures are micro/nanostructures incorporating large void space. As schematically shown in Fig. 1a, hollow structures can be based on different shapes, such as tubes, plates, cubes, or more commonly spheres. In terms of spatial configuration, they can be single-shelled, yolk-shelled, and multi-shelled. Due to their large surface areas, enormous accessible spaces, low densities, and high loading capacities, hollow structured materials show advantages for light harvesting, electron and ion transport, and mass loading and diffusion. As a result, hollow structured materials have evolved into an important family of functional materials for diverse applications including energy storage, photocatalysis, heterogeneous catalysis, gas sensing, environmental remediation and biomedicine [1-7]. High-performance electrical energy storage is increasingly demanded along with technological advancement. As a state-of-the-art energy storage system, lithium-ion batteries (LIBs) continue to power consumer electronics and are found in defense, automotive, and aerospace applications owing to their high energy density [8]. On the other hand, some other rechargeable batteries based on alternative charge-carrying ions, such as sodium-ion batteries (SIBs) [9], magnesium-ion batteries (MIBs) [10], zinc-ion batteries (ZIBs) [11], and aluminum-ion batteries (AIBs) [12], have entered into the spotlight due to the high abundance of these guest ions. In particular, thanks to the very similar electrochemistry with that of LIBs, SIBs have been considered as the most promising complements to LIBs for large-scale electrical storage applications [13, 14]. LIBs and SIBs can be termed as “rockingchair batteries”, of which the electrodes store the charge carriers via topotactic redox reactions [15]. Such a mechanism determines that the electron and ion migration in the solid electrode can be a ratedetermining step. Therefore, the development of high-capacity, high-rate, and stable electrode materials is a research focus. The key features of hollow structures make them structurally desirable as high-performance electrode materials. First, the large area guarantees sufficient contact between electrode and electrolyte and hence high accessible capacities. Second, hollow structures are normally assembled by nanosized particles, which can reduce the migration pathway of electrons and ions together with the electrolyte penetrable cavities, leading to boosted rate performance. Moreover, the rationally designed void space can accommodate the local volume changes of the electrode materials upon the ion insertion and extraction, thereby improving the cycling performance. The past years have witnessed a surge of interest in developing hollow structured electrode materials, especially the anode materials, which generally suffers huge volume change and problematic ion diffusion [16-18]. There are some comprehensive review papers summarizing the advancement of hollow structured materials for anode application [19-26]. In comparison, the study of hollow structured cathode materials (HSCMs) is relatively limited. The first obstacle is the difficult synthesis. As shown in Fig. 1b, different kinds of hollow structures are generally prepared by hard-template, soft-template, and self-template methods [2]. While the synthesis of most cathode materials requires high-temperature calcination to form certain crystal phases, which easily leads to the irregular aggregation of particles. Second, serious

concern exists in the volumetric capacities of HSCMs, which are undoubtedly lower than those of commercial solid counterparts. Nevertheless, researchers have made great efforts in designing and advancing HSCMs for rechargeable batteries. Hollow structuring can greatly improve the electrochemical performance of many cathodes especially for the emerging high-capacity cathode materials, such as lithium rich cathode materials, silicates, and V2O5, which undergo considerable strain change and low kinetics during charge/discharge. Herein, we summarize the development of HSCMs for different kinds of rechargeable batteries and discuss the key challenges and opportunities.

2. HSCMs for LIBs A typical LIB is a hermetic device consisting of a cathode, an anode, a separator, and a certain amount of electrolyte. Upon charging, the oxidization of the cathode enables Li+ extraction and electron output. The Li ions then migrate to the anode across the electrolyte and the separator. Simultaneously, the electrons move to the anode via external conduction. This leads to a reduction of the anode along with Li+ insertion. The reverse process occurs over discharging. The performance of the batteries, therefore, depends intimately on the electrode materials (capacities, kinetics, and reversibility). As suggested above, hollow structuring is a powerful strategy to advance the electrode performance, because a hollow structure guarantees a large contact area between the electrode and the electrolyte, good infiltration of the electrolyte, short ion diffusion distance, and excellent structural integrity. All these apply to both the anode and cathode materials. In this sector, we summarize the development of HSCMs in LIBs. We discuss the hollow structures separately based on their component, namely, layered oxides, spinel-structured oxides, polyanionic compounds, and vanadium oxide. Note that hollow structures have also been widely used as sulfur hosts for lithium sulfur batteries [27-31]. Several recent articles have reviewed this topic [3235]; therefore, they are not covered here. 2.1. Layered oxides Layered oxides represented by LiCoO2 are the most widely used cathode materials since the 1990s [36]. The theoretical capacity of LiCoO2 is as high as 282 mAh g−1. However, the oxygen atoms will lose from the lattice when more than half of the Li ions are removed, because the t2g band of redox active Co4+/Co3+ overlaps with the top of the 2p band of O2− [37]. To improve the practical capacity and structural stability, Co is partially replaced by Ni and Mn. It is also an economic and sustainable choice as Ni and Mn are cheaper, more abundant, and less toxic than Co. Therefore, the mixed transition metal (TM) layered oxides have attracted much interest, especially the high-capacity Ni-rich layered oxides. However, they exhibit inferior rate capability and cycling stability due to the sluggish electron and Li ion migration as well as increased cation mixing. Constructing hollow structures has been investigated along with other strategies, such as lattice doping and surface modification, to improve the rate performance and cycling stability of the mixed TM layered oxides.

For example, Qian’s group [38] synthesized LiNi1/3Co1/3Mn1/3O2 hollow microspheres using binary metal oxide (Mn1.5Co1.5O4) as the self-templates. Fig. 2a and b show the scanning electron microscopy (SEM) images of the product. The product delivered 196.2 and 114.2 mAh g−1 at 0.2 and 5 C, maintaining 80.3% of its capacity after 100 cycles at 0.2 C (Fig. 2c). Meanwhile, Cao and co-workers [39, 40] prepared hollow spherical LiNi1/3Co1/3Mn1/3O2 and LiNi0.5Mn0.5O2 from single metal oxide (MnO2). The LiNi0.5Mn0.5O2 microspheres showed discharge capacities of 181.5 and 152.2 mAh g−1 at 1 and 10 C, with an impressive capacity retention of 95.5% after 1000 cycles at 15 C. Yang et al. [41] and Zhang et al. [42] reported the use of carbon spheres as the hard templates to prepare LiNi1/3Co1/3Mn1/3O2 hollow microspheres and nanospheres. With the support of the hollow structure, Ni-rich layered oxides can maintain high specific capacities for longer calendar life at higher rates. Zhang’s team [43] developed a series of multi-shelled Ni-rich LiNixCoyMnzO2 (x = 0.8, 0.7, 0.65, and 0.5) hollow fibers by using alginate fibers as a template (Fig. 3d). The nickel-rich LiNi0.8Co0.1Mn0.1O2 hollow fiber delivered capacities of 229.9 and 172.7 mAh g−1 at 0.5 and 10 C, respectively. Moreover, it showed a capacity retention of 84.36% after 300 cycles at 0.5 C (Fig. 3e). Meanwhile, a hollow corn-like LiNi0.8Co0.1Mn0.1O2 was synthesized by employing hydrothermally prepared mixed carbonate precursor [44], which displayed a capacity of about 187 mAh g−1 at 200 mA g−1 with a capacity retention of 85.71% after 100 cycles. Clearly, hollow structuring could improve the electrochemical performance for those layered oxides in terms of specific capacity, rate performance, and cyclability. However, the volumetric capacity, as a crucial metric for practical application, cannot be neglected especially for these commercially viable materials. In view of the difficulty of realizing satisfactory electrochemical stability in high-capacity layered oxides, lithium rich layered oxides (LRLOs) have entered into the spotlight [45]. Attributing to the extraordinary deliverable capacities above 250 mAh g−1 and Co-poor constitution, LRLOs, described as xLi2MnO3⋅(1−x)LiMO2 or Li(LiyM1−y)O2 (M = Mn, Co, Ni, etc., 0 < x < 1, 0 < y ≤ 0.33), have been considered as ones of the most promising cathode candidates for next-generation LIBs. However, Li-rich cathode materials face several critical challenges hindering their practical application, especially the large initial irreversible capacity, voltage and capacity decay, and poor rate capability. The activation of the Li2MnO3 component is kinetically slow and largely determines the electrochemical properties of LRLOs [46]. In the first charge process, the removal of Li ions from Li2MnO3 is charge compensated by the oxidation of O2−, causing lattice evolution with TM migration, which is responsible for the capacity loss, voltage fading, and poor rate capability. Design LRLOs with micro/nanostructures, especially hollow structures, has been proved an effective strategy to enhance their electrochemical kinetics. First, the hollow cavities can facilitate the electrolyte penetration, significantly shorten the Li+ diffusion pathway and enhance the rate performance. Second, additional active sites in the hollow structured electrode material are beneficial for Li+ storage and further increase the specific capacity. Moreover, the hollow structures can accommodate the local volume change upon charge/discharge, guaranteeing excellent structural stability.

Researchers have explored a range of hollow structured LRLOs with enhanced electrochemical performance. A typical process is using single or mixed TM oxides or carbonates as the self-template. For

example,

Jiang

et

al.

[47]

reported

the

self-template

synthesis

of

hollow

0.3Li2MnO3⋅0.7LiNi0.5Mn0.5O2 microspheres by a nanoscale Kirkendall effect from porous MnO2. The hollow structured LRLOs exhibited superior cycling stability and rate capability in comparison with nanoparticles and bulk LRLOs, which maintained 295 mAh g−1 after 100 cycles at 15 mA g−1 and retained 125 mAh g−1 at 1 A g−1. Li and co-workers [48] introduced a molten salt process into the selftemplate synthesis of hollow spherical Li1.2Mn0.56Ni0.16Co0.08O2, the molten salt can reduce the annealing temperature and better preserve the morphology of the insoluble precursor. The resultant cathode material showed a higher specific capacity, better rate capability, more stable cycling behavior, and a higher first cycle Coulombic efficiency compared with the control sample without a NaCl flux. The role of different templates has been extensively investigated. Tu’s group [49] employed binary Co0.33Mn0.67CO3 as the precursor to prepare hollow Li1.2Mn0.5Ni0.25Co0.05O2 microcubes (Fig. 3a, b). The product showed discharge capacities of 272.9 mAh g−1 at 20 mA g−1 and 110 mA h g−1 at 2 A g−1, retaining 85.87% of its capacity after 100 cycles at 200 mA g−1. Wang’s team [50] further developed a series of hollow spherical LRLOs, xLi2MnO3⋅(1−x)LiNi1/3Co1/3Mn1/3O2, starting from binary CoyMn3−yO4 porous microspheres. When x is 1/3, the LRLOs presented the highest capacity and rate capability. Wu et al. [51] investigated the role of PVP during the hydrothermal-assisted precipitation of metal oxalates. PVP could direct the formation of hollow spherical Li1.2Mn0.54Ni0.13Co0.13O2 assembled by nanoplate-like particles. The product showed a discharge capacity of 170.5 mAh g−1 after 112 cycles at 250 mA g−1, equivalent to 86.2% of its initial capacity. Using sulfonated polystyrene nanospheres (PS) gel as the template, Zhang et al. [52] synthesized lamellar Li1.2Mn0.54Ni0.13Co0.13O2 hollow nanospheres (Fig. 3d and e), which showed discharge capacities of 281.7 mAh g−1 at 20 mA g−1 and 136.6 mAh g−1 at 2 A g−1. After 200 cycles at 2 A g−1, the nanosphere cathode maintained 80% of its initial capacity. The pronounced rate and cycling performance were attributed to both the surface lamellar units and the hollow architectures. Li et al. [53] reported a PEG-assisted solvothermal process to obtain carbonate precursors incorporating lithium and TM ions, which were converted into uniform Li1.2Mn0.54Ni0.13Co0.13O2 hollow microspheres with excellent cycle stability and rate capability. The microspheres delivered 287 mAh g−1 at 25 mA g−1 and 150 mAh g−1 at 1250 mA g−1, with capacity retentions of 85.7% after 100 cycles at 25 mA g−1. Apart from the enhanced cycling and rate performance, Li1.2Mn0.54Ni0.13Co0.13O2 hollow microspheres were reported to demonstrate better voltage stability [54]. The average voltage retention of Li1.2Mn0.54Ni0.13Co0.13O2 hollow microspheres was 93.5% after 200 cycles at a rate of 1 C, compared to 86.3% for the infarctate one. The improvements were attributed to the hollow secondary particle structure, which released the stress caused by Li+ insertion and extraction and suppressed the migration of TM ions into the lithium layers and the transformation from layered phase to spinel-like phase. Researchers have also combined hollow structuring with other strategies, such as metal doping

[55] and heterostructure design [56, 57]. For instance, Ma et al. [57] synthesized double-shell Li-rich layered oxide hollow microspheres with sandwich-like [email protected]@[email protected]@carbon shells (Fig. 3g and h), the composites deliver an initial charge capacity of 312.5 mAh g−1 with an initial Coulombic efficiency of 89.7%. After cycling 200 times, they still show discharge capacities of 228.3 and 196.1 mAh g−1 can be obtained at 1 and 5 C, respectively. As a group of emerging highenergy-density cathode materials, LRLOs still face a couple of critical challenges that need to be overcome prior to its practical application. Hollow structuring could be effective strategy together with other modifications. Meanwhile, the underlying mechanism of enhanced cycling stability of the hollow structured layered oxides requires to be further depicted with the aid of higher-level measurement techniques such as in-situ an ex-situ characterizations. 2.2. Spinel-structured oxides. Spinel-type LiMn2O4 is an important type of cathode materials due to its environmental benignity, low-cost and abundance of Mn [58]. Ni-substituted LiNi0.5Mn1.5O4 is considered as a promising cathode candidate for next-generation LIBs owing to the much higher voltage [59]. Moreover, the three-dimension (3D) spinel framework is very stable upon lithium extraction/insertion. However, the electrochemical performance of the spinels was restricted by its kinetic problems such as low electronic conductivity and small lithium diffusion coefficient (10−9−10−11 cm2 s−1) [60], also inferior cyclability caused by TM dissolution in the electrolyte. Although hollow structuring cannot change the intrinsic conductivity and diffusion coefficient, it provides extra active sites and shortened electron and Li transfer distance. According to the following equation: t=L2/D, where the diffusion time t can be significantly decreased at reduced diffusion length L without altering the diffusion constant D. Various spinel HSCMs have been synthesized from different methods or precursors and show improved electrochemical properties. For example, Wu et al. [61] prepared hollow porous LiMn2O4 microcubes using MnO2 as the self-template. The hollow microcubes show improved specific capacities as well as enhanced rate and cycling performance. Meanwhile, Lou’s group [62] reported the preparation of LiNi0.5Mn1.5O4 hollow microspheres and microcubes from MnO2 self-templates (Fig. 4a−d). The resultant LiNi0.5Mn1.5O4 hollow structures deliver a discharge capacity of about 120 mAh g−1, with good cycling stability and rate capability up to 20 C. Similarly, Xiang’s team [63] prepared ordered LiNi0.5Mn1.5O4 hollow microspheres, of which the electrochemical performance is disclosed to be better than that of the disordered counterpart. To prepare highly nanoporous Mn2O3 precursors, Zhu et al. [64] introduced a CaCO3-template method, which is to obtain coprecipitation of Mn-Ca-carbonates followed by a controlled decomposition–dissolution process. The spinel hollow structures achieve a discharge capacity of 115 mAh g−1 at a 10 C rate with a capacity retention of 94%

after 800. Using the same method, they [65] also prepared LiNi0.5Mn1.5O4 hollow microspheres with favorable electrochemical performance. He et al. [66] reported the preparation of hollow polyhedron MnF2 precursor via an ionic liquid-assisted solvothermal method and its further conversion into hollow polyhedron Mn2O3 and hollow LiMn2O4 microspheres. The products show impressively high rate performance at 20 C. Combining hollow structures with other strategies, such as facet control, can lead to outstanding stable high-rate performance. For example, Wu et al. [67] and Sun et al. [68] reported hollow structured LiMn2O4 cubes and LiNi0.5Mn1.5O4 spheres (Fig. 4e, f) with exposed {111} facets, respectively. The LiMn2O4 hollow cubes can maintain over 70% of the discharge capacity for up to 1000 cycles at 50 C. The urchin-like LiNi0.5Mn1.5O4 hollow spheres achieve a capacity retention of about 92% after 1500 cycles at 30 C. The outstanding stability is due to the low surface energy and Mn dissolution of the exposed {111} facets as well as the favorable hollow structures. In addition to the single-shelled hollow structures, researchers also developed complex hollow structures, such as double-shelled, multi-shelled and yolk-shelled hollow structures, which have some advantages including the balanced design of adequate voids for volume change and more electrochemically active species in certain volume size. For instance, Xie’s group [69] and Wang’s group [70] prepared double-shelled LiMn2O4 and LiNi0.5Mn1.5O4 hollow microspheres, respectively. The double-shelled LiMn2O4 shows a discharge capacity of 127 mAh g−1 at 0.1 C and a capacity retention of 80% after 800 cycles at 5 C. And the LiNi0.5Mn1.5O4 retains 98.3% of its capacity after 100 cycles at 0.5 C. Chen’s group [71] and Kang’s group [72, 73] developed LiMn2O4 (Fig. 4g) and LiNi0.5Mn1.5O4 hollow microspheres with yolk-shell structures from a self-template method and a spray pyrolysis process, respectively. The yolk-shelled spinels demonstrate stable high rate performance. To facilitate the Li+ diffusion and the penetration of electrolyte, Li’s group [74] developed multi-shelled LiMn2O4 hollow microspheres by an aerosol spray pyrolysis method (Fig. 4h, i). Wang et al. [75] also reported the preparation of multi-shelled LiMn2O4 hollow microspheres using carbonaceous microspheres as the hard template, in which the shell number, shell thickness, and porosity could be controlled by adjusting the heating rate and adsorption duration (Fig. 4j, k). Compared to single-shelled HSCMs, multi-shelled structures could be designed for higher volumetric capacities via geometric manipulation. However, volumetric capacities have not been given in most cases. In addition, economic and scalable production of multi-shelled structures is necessary in future application.

2.3. Polyanionic compounds Polyanionic materials contain a series of polyhedrons anion units (XxOy)n– (X = B, S, P, Si, etc.) instead of single O2− [76]. Typical polyanion cathode materials include phosphates LiMPO4, silicates

Li2MSiO4, borates LiMBO3 (M = Fe, Mn, Co, and Ni), and superionic conductor (NASICON)structured Li3V2(PO4)3. In polyanionic compounds, each O ion is covalent-bonded by both an electrochemical inactive X ion and a metal ion, leading to better thermal stability and safety upon lithium (de)insertion compared to metal oxides. In addition, the existing of X−O also increases the iconicity of the M−O bond, contributing to higher voltages. Nevertheless, polyanionic compounds face a critical challenge of inherently low electronic and ionic conductivities, resulting in poor rate performance. In view of the advantages of hollow structures in boosting the kinetics of electrode materials, constructing different types of hollow structures has been studied as a remedy to address the issues in the application of polyanionic compounds together with other solutions especially the carbon coating. LiFePO4 is the most important polyanionic compound because of its high capacity (170 mAh g−1),

the abundance of raw materials, and safety. However, it suffers from low electrical conductivity

(10−9−10−8 S cm−1) as well as low ionic diffusivity (below 10−12 cm2 s−1) [60]. A range of hollow structures has been explored to boost the kinetics of LiFePO4. For instance, Cho’s group [77] prepared nanowire and hollow LiFePO4 cathodes using the hard templates KIT-6 and SBA-15. The hollow LiFePO4 cathode showed 153 mAh g−1 at 15 C. Song and co-workers [78] developed a hollow sphere structure of spherical carbon-coated LiFePO4 nanoparticles from a sequential precipitation method followed by hydrothermal treatment. The hollow product delivered 100 mAh g−1 at a rate of 50 C. With a solvothermal process, LiFePO4 hollow microspheres was prepared using spherical Li3PO4 as self-template, the resultant discharged 72 mAh g−1 at a 50 C [79]. A solvothermal process was also developed to synthesize hollow melon-seed-shaped olivine-type LiFePO4 microplates with a large exposure of ac surfaces (Fig. 5a, b) [80]. The combination of the hollow structure and the favorable crystal facet exposure enabled the product higher capacities. Similar to layered oxides, LiFePO4 has already been widely used in commercial products. The penalty of volumetric capacities can be a critical issue. Hollow structuring also improves electrochemical properties of other polyanionic cathode materials, which generally show even lower intrinsic kinetics and larger volume change [81]. For instance, the theoretical capacity of LiMnPO4 is as high as isostructural LiFePO4, but the nearly insulating nature and the much lower Li mobility of LiMnPO4 make the high capacity much less achievable [82]. To tackle the challenge, Fu et al. [83] developed a glucose assisted solution method followed by carbon coating to synthesize hollow spindle LiMnPO4/C nanocomposites. The nanocomposites discharged 161.8 mAh g−1 at 0.05 C and 110.8 mAh g−1 at 0.2 C, maintaining 92% after 100 cycles at 0.2 C. Silicate cathode materials have very theoretical capacities over 300 mAh g−1 but very low deliverable ones. The complete delithiation leads to huge volume change (23% and 17% for Pmn21-cycl and P21/n-cycl LixFeSiO4 polymorphs compared to ~7% and ~11% for LiFePO4 and LiMnPO4 [84]). Researchers have devoted many efforts in improving the real

performance of silicates. With a spray drying-assisted method, Huang and co-workers [85] synthesized

hollow

microspherical

Li2FeSiO4/C particles

consisting

of

carbon-coated

primary nanoparticles. The composites delivered discharge capacities of 165 and 96 mAh g− 1 at 0.2 and 10 C rates, with a capacity retention of 94.5% after 100 cycles at 2 C. Hollow structured Li2FeSiO4 including mesocrystals [86], microspheres [87], and nanospheres (Fig. 5c) [88] were also reported to be prepared by hydrothermal processes. Among them, Li2FeSiO4/C hollow nanospheres showed 168.1 and 50.5 mAh g− 1 at 0.1 and 10 C with capacity retentions of 93% and 72% after 100 cycles, respectively [88]. The relatively inferior performance of hydrothermal products was ascribed to unfavorable carbon coating compared with in-situ carbon coating generated by other techniques. Ying’s group [89] reported [email protected] hollow nanoboxes made from a wet-chemistry method followed by a solid-state reaction (Fig. 5d), during which hollow structure and carbon coating layered were in-situ formed. The composites were further modified by graphene oxide nanosheets, resulting in capacities of 290 and 220 mAh g−1 in the initial and 50th cycles at 0.02 C at 40 ℃. At this stage, silicates are still far from real applications, as the appreciable capacities are generally tested at high temperatures as well as very slow rate. The charge/discharge curves also manifest huge overpotential. It requires us to develop more effective strategies to combat the extremely low kinetics. Borates face the same challenge. Chen et al. [90] synthesized mesoporous LiFeBO3/C hollow spheres, the composites deliver an initial reversible capacity of 190 mAh g−1 at 10 mA g−1 with a capacity retention of 77% after 100 cycles. New polyanionic compounds hold great promise for their high theoretical energy densities. However, it could be difficult to achieve satisfactory electrochemical performance relying on a single strategy.

2.4. Vanadium oxides V2O5 has attracted considerable attention due to its higher lithium storage capacities originated from multi-electron reactions. For example, the theoretical capacity of V2O5 attains 294 or 441 mAh g−1 based on an insertion of two or three Li per formula unit, respectively [91]. On the other hand, the multi-electron reactions trigger pronounced strain change of host structure, which can ruin the structure upon charge/discharge. Moreover, Li ions have lower mobility in V2O5, rendering limited charge/discharge rates. The implement of hollow micro/nanostructures can circumvent both the challenges of kinetics and structural stability. As an example, the V2O5 hollow microspheres produced by a wet-chemistry method showed a high capacity of 319 mAh g−1 at 80 mA g−1 [92]. However, only 160 mAh g−1 was retained after 50 cycles. Liu et al. [93] synthesized double-shelled V2O5-SnO2 hollow nanocapsules and investigated them as both anode and cathode materials for LIBs. As cathode materials, the capacity of nanocomposites dropped 18% to 100 mAh g−1 after 50 cycles at 100 mA g−1. Significant improvement of the electrochemical performance was achieved by porous multilayer V2O5

hollow sphere arrays on graphite paper substrates made by a PS template assisted electro-deposition method [94]. The free-standing electrode delivered 293 mAh g−1 at 147 mA g−1, maintaining 285 mAh g−1 even after 300 cycles. The electrode also showed impressively high rate performance, retaining 263 and 152 mAh g−1 at 1470 and 5880 mA g−1. The outstanding electrochemical performance was attributed to the hollow sphere arrays architecture as well as the free-standing architecture that facilitated charge transfer and reduced the structure degradation upon cycling. By varying the thermal treatment conditions on a vanadyl glycerolate precursor, Zou’s group [95] prepared of single-shelled and double-shelled V2O5 hollow nanospheres. The double-shelled V2O5 hollow nanospheres showed better electrochemical properties, which delivered an initial capacity of 256.7 mAh g−1 at 500 mA g−1, with a capacity retention of 77% after 50 cycles. Meanwhile, Xu’s group [96] reported the control preparation of hollow V2O5 microspheres with different architectures (flower-like, yolk-shell, doubleshell and triple-shell) by controlling the recipe and duration of a solvothermal reaction. Among the products, the flower-like hierarchical hollow microspheres exhibited best electrochemical performance, which discharged capacities of 146.8 and 107.2 mAh g−1 at 147 and 2940 mA g−1 in a working window of 2.5–4 V. A capacity of 73.5 mAh g−1 was maintained after 3000 cycles at 2940 mA g−1. Wang’s group [97] proposed a concept of competitive anion adsorption by carbonaceous microsphere templates followed with a Trojan catalytic combustion process (Fig. 6). It was used to synthesize a series of multi-shelled metal oxides, of which the multi-shelled V2O5 hollow microspheres deliver a specific capacity of 447.9 and 402.4 mAh g−1 for the first and 100th cycle at 1000 mA g−1 in a working window of 1.5–4 V, respectively. It is undeniable that HSCMs show improved specific capacities, rate capability, and cycling stability attributing to their structural properties. Table 1 compares the key features of some HSCMs compared with solid counterparts in LIBs. The role of hollow structuring is prominent for highcapacity and low-conductivity cathode materials that undergo significant volume change and problematic ion and electron transport, such as lithium rich materials, silicates, and V2O5. Note that HSCMs are undoubtedly lower in volumetric density compared with commercial solid cathode materials, although this metric is generally neglected in the literature. The problem may be addressed by controlling the void volume, focusing on new high-capacity cathode materials, and finding specific applications. Table 1 The comparison of electrochemical properties between hollow and solid structures of some cathode materials for LIBs. Materials

Hollow structures

Synthesis

Electrochemical

method

performance

Electrochemical performance of solid counterparts

157.3 mAh g−1 at LiNi1/3Co1/3 Mn1/3O2 [38]

Self-template from Mn0.5Co0.5CO3

0.2 C

after

cycles

123.8 mAh g−1 at 100 0.2 C and cycles

120.5 mAh g−1 at 0.5 C

after

after

100 and

89.6 mAh g−1 at 200 0.5 C

cycles

after

200

cycles

208.9–135.9 mAh g−1 ~180–105 mAh g−1 LiNi0.5Mn0.5

Self-template

O2 [40]

from MnCO3

at 0.5–10 C, 85.1% at 0.5–10 C, 69.3% capacity

retention capacity

retention

after 40 cycles at 0.1 after 40 cycles at 0.1 C

C

229.9–172.7 mAh g−1 Ni-rich

Hard-template

at 0.5–10 C, 84.36%

LiNixCoyMnz

using alginate capacity

O2 [43]

fibers

retention Not given

after 300 cycles at 0.5 C (x=0.8)

0.3Li2MnO3·

295 mAh g−1 after 130 mAh g−1 after

0.7LiNi0.5Mn

Self-template

100 cycles at 15 mA 100 cycles at 15 mA

0.5O2

from MnO2

g−1, 125 mAh g−1 at g−1, 30 mAh g−1 at

[47]

1000 mA g−1

1000 mA g−1

Self-template

272.9–110 mAh g−1 259.4–68 mAh g−1 at

from

at

Co0.33Mn0.67C

85.87%

O3

cycles at 200 mA g−1

[email protected]

Self-template

291.7–191.6 mAh g−1

[email protected]

from

at 0.2–5 C, 84.4%

@[email protected]

hydroxide

after 200 cycles at 1

rbon [57]

precursor

C

Li1.2Mn0.5Ni0. 25Co0.05O2

[4

9]

Hard-template LiNi0.5Mn1.5

using

O4 [68]

polystyrene spheres

0.02–2

A

g−1, 0.02−2

A

g−1,

after

100 82.82%

after

100

cycles at 200 mA g−1

Not given

135–99 mAh g−1 at 135–10 mAh g−1 at 1–50 C, 85% after 1–50 C, 76% after 1500 cycles at 30 C

50 cycles at 1 C

110–94.7 mAh g−1 at 87.7-30.8 mAh g−1 LiMn2O4 [74

Aerosol spray

0.2–1 A g−1, 91.9% at

]

pyrolysis

after 400 cycles at 86.2% 0.2 A g−1

LiFePO4 [78]

[88]

g−1,

after

400

cycles a t 0.2 A g−1

150–100 mAh g−1 at 150–0 mAh g−1 at

from Li3PO4

0.1–50 C

using polystyrene spheres

Li2MnSiO4/C

Self-template

[89]

from MnCO3

Soft template LiFeBO3/C

method using

[90]

copolymer F127 Hard-template using

V2O5 [97]

A

Self-template

Hard-template Li2FeSiO4/C

0.2–1

carbonaceous microsphere templates

0.1–50 C,

168.1–66.1 mAh g−1 at 0.1–5 C, 93% after 100 cycles at 1 A g−1

77.4%

after

cycles at 1 A

100

g−1

290 mAh g−1 at 0.02 C, 75.9% after 100 Not given cycles at 1 A g−1

190 mAh g−1 at

10

mA g−1, 77% after Not given 100 cycles

564–331 mAh g−1 at 390–100 mAh g−1 at 0.05–2 A g−1, 89.8% 0.05–2 A g−1, 60% after 100 cycles at 1 after 100 cycles at 1 A g−1

A g−1 (nanosheets)

3. HSCMs for SIBs Na sits below Li in the periodic table and is physiochemically similar to Li. By using Na ions as the charge carriers, SIBs adopt the identical electrochemical mechanism of LIBs [98, 99]. However, the size of Na+ (1.02 Å) is much larger than that of Li+ (0.69 Å), which would cause the inferior ionion coulomb repulsion for Na+ and subsequently lead to lower rate capability [100, 101]. Bulk cathode

materials usually suffer large strain during the charge-discharge process and hence to pulverization, and the dense structure cannot provide adequate diffusion paths for Na+ ion insertion/extraction. The hollow sphere structure has attracted great interests because this architecture can buffer the volume changes, tolerate expansion and shrinkage during the cycling process, and make a larger contact area between the electrode and electrolyte. In the second part of this paper, we summarize the advance of HSCMs designed for SIBs. We discuss the hollow structures separately based on their component, namely, oxide compounds, polyanionic compounds, and other positive electrode compounds. 3.1. Oxide compounds TM oxides have attracted wide interest as cathode materials for SIBs, owing to their characteristics of controllable synthesis and high electrochemical activity [102]. V2O5 is a typical intercalation metal oxide for SIBs. Its dimensions, morphology, porosity, and texture are important for the electrochemical performance [103, 104]. Su et al. [105] prepared hierarchical hollow V2O5 nanospheres with predominantly exposed {110} facets (Fig. 7a) . When applied as the cathode material for SIBs, the V2O5 hollow nanospheres exhibited good cycling reversibility at different current densities (Fig. 7b). Shao et al. [106] prepared VOOH hollow microspheres with low crystallinity as the cathode for SIBs through a hydrothermal method (Fig. 7c). VOOH hollow microspheres presented a capacity of 150 mAh g−1, along with appreciable rate capability (84 mAh g−1 at a discharge current of 300 mA g−1) and long cycling life (126 mAh g−1 after 200 cycles) (Fig. 7d). Recently, considerable efforts have been made in exploring new rechargeable aqueous SIB systems with low cost, good safety, and abundant resource. Liu et al. [107] prepared hollow K0.27MnO2 nanospheres by using PS as a template. A small amount of K could preserve structural stability whilst the Na ions are inserted/extracted. When fabricated into a coin cell with a NaTi2(PO4)3 anode, the cathode materials displayed an initial discharge capacity of 83.0 mAh g−1, and remained stable within the first 5 cycles. Moreover, a discharge capacity of 56.6 mAh g−1 can be retained at 600 mA g−1. The full device showed good cycle performance with 17% drop after 100 cycles. 3.2. Polyanionic compounds Polyanionic compounds have been extensively investigated as Na+ host cathodes for SIBs owing to their structural diversity and stability, as well as the strong inductive effect of the anions. They commonly feature high operating potentials and excellent cycling performance [108, 109]. However, the poor electron conductivity of polyanionic compounds limits the rate of electron transfer and increases the polarization of electrochemical reactions. As a result, bare polyanionic compounds are subject to low rate performance. Combining carbon modification, particle nanosizing, and pore design to fabricate porous or hollow micro/nanostructures can dramatically enhance the rate performance due

to the large improvement of the reaction kinetics [110, 111]. Wang et al. [112] fabricated honeycombstructured hierarchical porous Na3V2(PO4)3 (NVP)/C microballs by a sol-gel method. As shown in Fig. 8a and b, the composite had enormous voids and interconnected nanochannels, which facilitated electrolyte penetration and ion transport. Electrochemical impedance spectroscopy (EIS) measurements demonstrated that the charge transfer resistance of the honeycomb-structured microball was much decreased compared to that of the carbon-coated NVP, leading to high discharge capacities of 97 and 80 mAh g −1 at the rates of 5 and 20 C, respectively (Fig. 8c). Mao et al. [113] introduced a scalable spray method for the synthesis of NVP/carbon porous hollow spheres as cathode materials for SIBs. In this structure, NVP particles were uniformly decorated on the inner and outer surfaces of the porous hollow carbon spheres (Fig. 8d, e). Biochemistry strategies are also useful in synthesizing nanomaterials with interconnecting pore channels or hollow porous architecture for the rapid transmission of electron and Na ions. A microalgae-based biochemistry-directed bottom-up strategy was employed by Lin and co-workers [114] to construct 3D hollow porous NVP microspheres. The as-synthesized NVP sample had a highly hierarchical porous structure with abundant interconnected small pores, facilitating easy electrolyte penetration and fast ion transport. Langrock et al. [115] synthesized carbon-coated hollow Na2FePO4F spheres via spray pyrolysis. This nanostructured material presented a 500 nm average diameter (from 100 nm to 1 μm) and 80 nm wall thickness, with micro-sized cavities and nanopores inside the outer wall of about 2.5 nm size (Fig. 8f). The material presented in this work displays good rate capability and one of the longest cycling stabilities reported for this compound, with a capacity retention of 80% (60 mAh g−1) after 750 cycles. Ling et al. [116] reported double-shelled hollow Na2FePO4F/C spheres from the solvothermal process. They exhibit a discharging capacity of 120 mAh g–1 at 0.1 C and a capacity retention of 92.5% at 1 C over 200 cycles. Wu et al. [117] prepared carbon‐coated Na2MnPO4F hollow spheres via a spray-drying technique with enhanced electrochemical performance. Peng et al. [118] reported Ru-doped Na3V2O2(PO4)2F hollow microspheres using the low-temperature solvothermal method and further improved the electrochemical performance by coating thin conductive RuO2 layer. The hollow microspheres exhibited reversible capacities of 102.5 and 44.9 mAh g−1 at 20 and 100 C, respectively, and around 55 mAh g−1 capacity retention could be achieved after 7500 cycles (Fig. 8e). Zhang et al. [119] reported the synthesis of Mn2+-doped Na3V2(PO4)2F3 hierarchical hollow microspheres via a polyol-assisted hydrothermal route and coated them with a thin conductive carbon layer by CVD. Na3V1.95Mn0.05(PO4)[email protected] delivered an initial capacity of 122.9 mAh g−1. Even after 500 cycles, a discharge capacity of 109 mAh g−1 was maintained with a Coulombic efficiency of 99.1%. Qi et al. [120] developed a scalable fabrication of Na3(VOPO4)2F multi-shelled microspheres. The 3D multi-shelled hierarchical architectures were formed based on the in-situ generated bubbles as soft templates and the controlled release of vanadium without further carbon coating and hightemperature treatment (Fig. 9a and b). The as-prepared cathode materials delivered a discharge

capacity of 111 mAh g−1 at 0.1 C and even maintained 98.6 mAh g−1 at 10 C current rate (Fig. 9c). Moreover, they exhibit excellent cycling performance with about 70% capacity retention after 3000 cycles at a 15 C current rate (Fig. 9d). The extensive studies of olivine LiFePO4 as the cathode of LIBs have stimulated the development of NaFePO4 in SIBs [121, 122]. However, the practical capacity of olivine NaFePO4 is far lower than the theoretical capacity, due to the poor electron conductivity and the one-dimension (1D) diffusion channel [121]. Compared with oxides, some polyanionic compounds demonstrate excellent cycling stability for SIBs, which are promising for future application of energy storage systems. However, the preparation of hollow polyanionic compounds by hard-template or hydrothermal-assisted self-assembling usually involves multistep processes, violating the low-cost promise of SIBs. Therefore, developing costeffective synthetic processes for mass production of cathode materials could be a potential research direction in the future. 3.3. Other compounds Prussian blue (PB) and Prussian blue analogous (PBAs) are a large family of TM cyanides and considered to be promising cathode materials for SIBs. PB and PBAs, with the general formula AaBxB′y(CN)6·nH2O (A = alkali metal ions, B/B′ = TM ions), present fast-ion diffusion kinetics in the solid state, which is contributed by their open hollow frameworks and robust structures [123125]. However, PBs and PBAs generally suffer from poor cycling stability and low rate properties, which can be ascribed to low conductivity and structural defects by vacancies and coordination water [126]. To overcome these problems, Na-rich sodium iron hexacyanoferrate (Na1.58Fe[Fe(CN)6]0.92) hollow nanospheres were prepared as cathodes for SIBs from a self‐sacrificial template with the assistance of ascorbic acid as a reducing agent (Fig. 10a) [127]. The as‐prepared Na1.58Fe[Fe(CN)6]0.92 hollow nanospheres showed high initial charge and discharge capacities of 133 and 142 mAh g−1 and superior cycling performance (~90% retention after 800 cycles) (Fig. 10b). Huang et al. [128] prepared a hollow core-shell Na2MnxNiyFe(CN)6 heterostructures by simple chemical precipitation as superior cathode materials for SIBs (Fig. 10c) . The materials exhibited a reversible capacity of 102 mAh g−1 after 600 cycles, 50% of which was maintained at a high rate of 20 C (Fig. 10d). Redox-active polymers are low-cost, environmentally friendly, and possibly accessible from abundant biomass resources. Therefore, they are promising electrode materials for SIBs [129, 130]. Su and co-workers [131] prepared the polypyrrole (PPY) hollow nanospheres by in-situ polymerization of pyrrole monomer on polymethyl methacrylate (PMMA) surface and acetone washing that can remove the PMMA core. PPY hollow spheres exhibited a specific capacity of 100 mA h g-1, stable cyclability, and superior rate capability through reversible doping/de-doping reactions during the charge/discharge processes. Han et al. [132] introduced polyaniline hollow nanofibers by using PMMA nanofibers as a sacrifice-template via an in-situ polymerization with a subsequent dissolution process. As cathode material for SIBs, the polyaniline hollow nanofibers exhibited a high

reversible capacity of 153 mAh g−1 at 0.3 C (1 C =150 mA g−1), high cycling stability (73.3% capacity retention after 1000 cycles), and decent rate capability (70 mAh g−1 at 8 C). The performance was attributed to the 1D hollow nanostructures with unique morphologies and highly reversible doping/dedoping property of the conducting polymer, which guaranteed good structural stability and good electrical/ionic conducting connectivity. SIBs could be the most promising candidate for large-scale energy storage system owing to the economic constitution, good safety, and the similar chemistry to that of commercial LIBs. Table 2 summarizes the structures and synthetic methods of some HSCMs in SIBs and compares their electrochemical properties with solid counterparts. Obviously, HSCMs perform much better over solid counterparts. However, they still cannot compete with LIB cathodes. When taking the anode part in consideration as well, the energy density of sodium ion full batteries could be much lower than that of commercial LIBs. The future direction of developing practical SIBs could be realizing ultralong cycle life at moderate rates for large energy storage system [133]. The role of hollow structuring could be prominent for long cycle Na-ion host materials together with other strategies including lattice and surface engineering. Table 2 The comparison of electrochemical properties between hollow and solid structures of some cathode materials for SIBs. Materials

V2O5 [105]

Hollow structures

Synthesis

Electrochemical

method

performance

performance of solid counterparts

Soft-template

112−159 mAh g−1 at

68 mAh g−1 at 80

with the

20−640 mA g−1, 89%

mA g−1, 86% after

assistance of

after 100 cycles at 40

100 cycles at 80 mA

PVP

mA g−1

g−1

Self‐template VOOH [106]

Electrochemical

from V(OH)2NH2

84−145 mAh g−1 at 40−300 mA g−1, 87% after 200 cycles at 40

Not given

mA g−1

Soft‐template

54−116 mAh g−1 at

11−114 mAh g−1 at

Na3V2(PO4)3/

with the

0.2−50 C, 93.6%

0.2−50 C, 67.1%

C [112]

assistance of

after 200 cycles at 1

after 200 cycles at 1

CTAB

C

C

9.6−100 mAh g−1 at

3.9−88.6 mAh g−1 at

Na3V2(PO4)3/

Ultrasonic

20−1000 mA g−1,

20−1000 mA g−1,

C [113]

spray pyrolysis

90% after 300 cycles

73.4% after 300

at 20 mA g−1

cycles at 20 mA g−1

89−112 mAh g−1 at

65−109 mAh g−1 at

0.2−20 C, 96.2%

0.2−20 C, 81.2%

after 500 cycles at 20

after 200 cycles at

C

10 C

Hard template

79−104 mAh g−1 at

56−91 mAh g−1 at

using

0.5−5 C, 93.1% after

0.5−5 C, 67.4% after

microalgae

500 cycles at 10 C

200 cycles at 5 C

26−90 mAh g−1 at

15 mAh g−1 at 8 C,

0.1−9 C, 80% after

75% after 20 cycles

750 cycles at 1 C

at 1 C

Na3V2(PO4)3 [114]

Na3.12Fe2.44(P 2O7)2

[114]

Hard template using microalgae

Na2FePO4F/

Ultrasonic

C [115]

spray pyrolysis

Spray drying, Na2MnPO4F/ C [117]

and in-situ pyrolytic carbon coating process

Ru-doped Na3V2O2(PO 4)2F

[118]

Na3V1.95Mn0. 05(PO4)2F3/C

[119]

Selfassembling under solvothermal

Polyol-assisted Soft‐template

27−102 mAh g−1 at 0.05−1 C, 72.4% after 30 cycles at 0.05 C

45−117 mAh g−1 at 1−100 C, 54% after 7500 cycles at 20 C

60−126 mAh g−1 at 0.2−10 C, 99% after 500 cycles at 0.2 C

41 mAh g−1 at 0.05 C, 72.4% 30 cycles at 0.05 C

3−104 mAh g−1 at 1−100 C

18−97.5 mAh g−1 at 0.2−10 C, 82.1% after 60 cycles at 0.2 C

Soft-template Na3(VOPO4) 2F

[120]

using in situ generated bubbles

73−111 mAh g−1 at 0.1−15 C, 70% after 3000 cycles at 15 C

Na1.58Fe[Fe(

Self-template

101−142 mAh g−1 at

CN)6]0.92

from reduced-

0.1−5 C, 90% after

[127]

FeIIOx

800 cycles at 2 C

Polypyrrole [131]

Polyaniline

[132]

Hard-template using PMMA nanospheres Hard-template using PMMA nanofibers

Not given

57−120 mAh g−1 at 0.1−5 C

68−100 mAh g−1 at 20−320 mA g−1, 78.5% after 1000

Not given

cycles at 400 mA g−1 55−153 mAh g−1 at 0.3−10.7 C, 73.3% after 1000 cycles at

Not given

5.3 C

4. HSCMs for other rechargeable batteries beyond Li/Na-ion batteries Recently, tremendous efforts have been devoted to other battery systems based on the intercalation chemistry of multivalent cations, such as Zn2+, Mg2+, and Al3+ [10-12]. First, these multivalent ions are far more abundant in the earth’s crust than Li ion, resulting in wider availability and lower cost. Second, the multivalent metal anodes are much safer and higher in volumetric capacity. However, the higher charge density of multivalent guest ions poses great solid-state diffusion challenges. The quest for ideal cathode materials is, therefore, a crucial but tough task for multivalent ion batteries. Hollow structuring presents as a remedy towards the problematic kinetics. Wu and co-workers [134] reported ZIBs using hollow porous spinel ZnMn2O4 as the cathode material. It achieved the highest capacity of ~220 mAh g−1 at 100 mA g−1 after the addition of 0.05 mol L−1 of MnSO4 into the electrolyte, maintaining 100 mAh g−1 after 300 cycles. Meanwhile, it discharged a high capacity of 70.2 mAh g−1 at 3200 mA g−1. Qin et al. [135] reported V2O5 hollow spheres synthesized via a solvothermal method for aqueous rechargeable ZIBs. V2O5 hollow spheres displayed a high specific discharge capacity of 132 mAh g−1 with a capacity retention of 82.5% after 6200 cycles at the current density of 10 A g−1. Tao et al. [136] fabricated tetragonal-spinel MgMn2O4 (T-MgMn2O4) hollow spheres via a solvothermal assisted solid phase reaction for using as the cathode

material for MIBs. Electrochemical characterization demonstrated that Mg ions could be reversibly intercalated into the T-MgMn2O4 host with an operation plateau of 3.2 V versus Mg2+/Mg. A high reversible capacity of 261.5 mAh g−1 was delivered at a current of 100 mA g−1. Even at a very high rate of 300 mA g−1, the T-MgMn2O4 hollow spheres retained a specific capacity of 92.6 mAh g−1 with superior long-term cycling stability. Pan and co-workers [137] presented a new type of hybrid battery using hollow MoO2 microsphere cathode, magnesium metal anode, and an electrolyte containing Mg2+ and Li+ ions. The hollow MoO2 microsphere cathode delivered an initial discharge capacity of 217.2 mAh g−1 at 20 mA g−1, retaining ca. 150 mAh g−1 after 50 cycles. 5. Summary and outlook Electrode materials with hollow structures have attracted increasing interest. In this review, we summarized the development of advanced HSCMs for rechargeable batteries. In particular, the recent progress made in the application of HSCMs in rechargeable batteries is reviewed according to the types of intercalation ions (Li ion, Na ion, and some multivalence ions). Clearly, hollow structuring improves the specific capacity, rate capability, and cycling stability for a variety of cathode materials simultaneously and substantially (Tables 1 and 2). Hollow structures offer large surface areas for reactions, good accessibility for guest ions, short ion and electron diffusion paths, and abundant voids for buffering the volume change. These features remedy emerging high-capacity cathode materials with low kinetics, such as lithium rich materials, silicates, V2O5, which suffer significant strain during ion extraction/insertion, inferior electron conductivity, and frustrated ion mobility. Although hollow structures have provided many new opportunities for cathode materials in rechargeable batteries, challenges still exist within both the material synthesis and the practical application. First, scalable, cost-effective, and controllable methods are still in demand for synthesizing high-quality HSCMs with precisely controlled geometric and compositional parameters. Compared to solid cathode materials, the preparation of HSCMs generally requires the sequential template methods, which can be costly and complicated. The challenge now involves the transformation of HSCMs from lab-scale research towards industry-compatible production. To tackle the challenge, novel cost-effective and scalable synthesis processes should be further developed. For example, the spray pyrolysis method is a powerful and industrially viable synthetic method that has the ability to generate a myriad of electrode materials with various structures. The fast decomposition of organic and/or inorganic materials during spray pyrolysis will generate a large amount of gas, enabling the formation of materials with hollow and porous structures. Second, the structureperformance relationship of HSCMs for rechargeable batteries is still ambiguous. Advanced insitu and ex-situ characterizations are still needed to fully understand the structural advantages of hollow structures for use in batteries. In addition, basic theoretical calculations and modeling may help to unravel the detailed reaction processes. Finally, the volumetric capacities of the hollow

structures have rarely been investigated in the literature. However, it can be the bottleneck preventing the practical application of HSCMs, which should be rigorously studied in the future. . Conflict of interest The authors declare that they have no conflict of interest. Acknowledgments The authors gratefully acknowledge the financial support from the Australian Research Council Discovery and Linkage Programs, Queensland-Chinese Academy of Sciences (Q-CAS) Collaborative Science Fund, and BAJC Grant. Author contributions Xiaobo Zhu, Jiayong Tang and Lianzhou Wang conceived the idea and drafted the manuscript. All the authors discussed the results and commented on the manuscript.

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Xiaobo Zhu is a postdoctoral research fellow at Prof. Lianzhou Wang’s group the Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland (UQ). He attained his B.Sc. degree (2011) from Heilongjiang University, M.Sc. degree (2014) from University of Science and Technology of China and Ph.D. degree (2018) from UQ. His research interests focus on the synthesis and characterization of low-cost and high-performance cathode materials for metal ion batteries.

Lianzhou Wang is currently a Professor in School of Chemical Engineering, Director of Nanomaterials Centre (Nanomac), and Senior Group Leader of Australian Institute for Bioengineering and Nanotechnology, the University of Queensland. He received his Ph.D. degree from Shanghai Institute of Ceramics, Chinese Academy of Sciences in 1999. Before joining UQ in 2004, he has worked at two leading national research institutions (NIMS and AIST) of Japan as a research fellow for five years. Since joining UQ, he has worked/working as ARC Queen Elizabeth II Fellow (2006), Senior Lecturer (2007), Associate Professor (2010), and Professor (2012-) in School of Chemical Engineering and Nanomac. His research focuses on the synthesis, characterization and application of semiconductor nanomaterials for use in renewable energy conversion/storage systems.

Captions for figures Fig. 1 (a) Schematic illustration of different kinds of hollow structures. (b) Typical synthetic methods of hollow structures: hard-, soft-, and self-template. Reprinted with permission from Ref. [2]. Copyright © 2018 Wiley-VCH. Fig.

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LiNi1/3Co1/3Mn1/3O2 hollow microspheres. Reprinted with permission from Ref. [38], Copyright © 2013 Elsevier. SEM image (d) and electrochemical properties (e) of Ni-rich LiNixCoyMnzO2 (x = 0.8, 0.7, 0.65, and 0.5) hollow fibers. Reprinted with permission from Ref. [43], Copyright © 2017 WileyVCH. Fig. 3 SEM images (a, b) and cycling performance (c) of hollow spherical Li1.2Mn0.56Ni0.16Co0.08O2. Reproduced with permission from Ref. [49]. Copyright © 2014 Elsevier. SEM (d) and transmission electron

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of

lamellar

Li1.2Mn0.54Ni0.13Co0.13O2 hollow nanospheres. Reproduced with permission from Ref. [52], Copyright © 2017 Elsevier. TEM images (g, h) and electrochemical performance (i) of double-shell Li-rich layered oxide hollow microspheres with sandwich-like shells. Reprinted with permission from Ref. [57], Copyright © 2019 Elsevier. Fig. 4 SEM images of LiNi0.5Mn1.5O4 hollow microspheres (a, b) and microcubes (c, d). Reprinted with permission from Ref. [62], Copyright © 2012 Wiley-VCH. SEM (e) and TEM (f) images of urchin-like LiNi0.5Mn1.5O4 hollow spheres with exposed {111} facets. Reprinted with permission from Ref. [68], Copyright © 2018 Elsevier. (g) TEM image of yolk-shelled LiMn2O4. Reproduced with permission from Ref. [72], Copyright © 2013 The Royal Society of Chemistry. SEM (h) and TEM (i) images of multi-shelled LiMn2O4 hollow microspheres from an aerosol spray pyrolysis [74], TEM images (j, k) of multi-shelled LiMn2O4 hollow microspheres from a hard template [75]. Reproduced with permissions from Refs. [74,75], Copyright © 2016 The Royal Society of Chemistry. Fig. 5 SEM and TEM images (a, b) of hollow LiFePO4 microplates and sub-microplates. Reprinted with permission from Ref. [80], Copyright © 2014 Wiley-VCH. TEM image (c) of Li2FeSiO4/C hollow nanospheres. Reprinted with permission from Ref. [88], Copyright © 2019 Springer Nature. TEM image (d) of [email protected] hollow nanoboxes. Reproduced with permission from Ref. [89], Copyright © 2015 Elsevier. Fig. 6 TEM images of thin single-shelled (a), thin double-shelled (b), thick triple-shelled (c), thick single-shelled, thin triple shelled (e), and multi-cavitied (f) V2O5 hollow microspheres. (g, h) Cycling

and rate performance of V2O5 hollow microspheres. Reprinted with permission from Ref. [97], Copyright © 2016 Nature Publishing Group. Fig. 7 SEM images of V2O5 nanospheres (a) and discharge capacities of V2O5 hollow nanospheres versus cycle number at different current densities (b). Reprinted with permission from Ref. [105]. Copyright © The Royal Society of Chemistry, 2014. (c) SEM images of VOOH hollow microspheres. (d) Cycling performance of VOOH and V2O5 hollow microspheres, and the comparison with the capacity of other cathode materials at the corresponding cycle number. Reproduced with permission from Ref. [106], Copyright © 2013 The Royal Society of Chemistry. Fig. 8 SEM images (a, b) and rate performance (c) of hierarchical porous Na3V2(PO4)3/C microballs. Reproduced with permission from Ref. [112]. Copyright © The Royal Society of Chemistry, 2015. (d, e) SEM images of the aerosol synthesized Na3V2(PO4)3/C composites aerosol. Reproduced with permission from Ref. [113]. Copyright © The Royal Society of Chemistry, 2015. SEM image (f) of a broken shell of C/Na2FePO4F particles with an average particle size of 0.5 μm. Reproduced with permission from Ref. [115]. Copyright © 2012 Elsevier. (g) Long-term cycling stability of the Rudoped Na3V2O2(PO4)2F hollow microspheres a current rate of 20 C, inserts are SEM and TEM images of the hollow microsphere. Reproduced from Ref. [118] with permission. Copyright © Elsevier, 2016 Fig. 9 SEM (a) and TEM (b) images of Na3(VOPO4)2F multi-shelled hollow microspheres at the final reaction time of 144 h. The rate (c) and cycling (d) performance of the Na3(VOPO4)2F microspheres. Reproduced with permission from Ref. [120]. Copyright © 2018 Cell Press. Fig. 10 TEM image (a) and cycling performance (b) of the Na1.58Fe[Fe(CN)6]0.92 hollow nanospheres. Reproduced with permission from Ref. [127]. Copyright © Tsinghua University Press and Springer, 2018. TEM image (c) and cycling profile (d) of hollow core-shell Na2MnxNiyFe(CN)6 sample. Reproduced with permission from Ref. [128]. Copyright © 2018 Wiley‐VCH.

可充电电池中具有空心结构的正极材料 朱晓波 a, 唐佳勇 a, 黄亨明 a,b, 林同恩a, 罗彬a, 王连洲 a*

a昆士兰大学澳大利亚生物工程和纳米技术研究所与化学工程学院,

纳米材料中心, 澳大

利亚

b

南京工业大学材料科学与工程学院, 南京210009, 中国

随着可充电电池应用的普及, 人们对可充电电池的性能(包括质量比容量、倍率性能和 循环稳定性)提出了更高的要求。设计合成具有微纳米结构的电极材料是提升可充电电 池的电学性能一个重要的研究方向, 其中空心结构设计因其有效性引起研究人员的重视。 目前空心结构的应用主要集中在负极材料上, 相比之下具有空心结构的正极材料因其较 难的形貌控制以及较低的体积比容量, 研究相对较少。本文总结了具有空心结构的正极 材料在可充电电池, 主要是在锂离子电池和钠离子电池中应用, 分析讨论了此类正极材 料研究中取得的成果以及存在的问题和挑战。由于空心结构具有更多的反应位点、利 于电解液的渗透以及能缓解充放电过程中的体积变化, 空心结构化能明显改进下一代高 比容量正极材料(比如富锂层型材料、钒基氧化物、硅酸盐等)的电学性能。这些材料 往往表现出较低的电子电导率、离子扩散系数和较高的体积变化率, 我们相信通过精确 控制空心结构的空间结构、开发低成本可放大的合成策略, 空心结构能为未来的高性能 正极材料提供较为实用化的解决方案。

关键词: 可充电电池, 正极材料, 空心结构, 电学性能