lithium-ion batteries (LIBs) is vital to meeting the requirements for large-scale ... lithium-ion batteries (LIBs) have been considered as the main power source for.
Review
Nanostructured Conversion-type Anode Materials for Advanced Lithium-Ion Batteries Yan Lu,1 Le Yu,1,* and Xiong Wen (David) Lou1,*
The development of high-performance anode materials for next-generation lithium-ion batteries (LIBs) is vital to meeting the requirements for large-scale applications ranging from electric vehicles to power grids. Conversion-type transition-metal compounds are attractive anodes for next-generation LIBs because of their diverse compositions and high theoretical specific capacities. Here, we provide an overview of the recent development of some representative conversion-type anode materials (CTAMs) in LIBs. In this review, we start with an introduction to typical CTAMs and their lithium storage mechanisms. Then, we present the obstacles to their widespread implementation and the corresponding nanoengineering strategies for high-performance CTAMs, including the use of low-dimensional nanostructures, hierarchical porous nanostructures, hollow structures, and hybridization with various carbonaceous materials. Particularly, we highlight the relationship between these nanostructures and the lithium storage properties. Lastly, we present some perspectives on the current challenges and possible research directions for nanostructured CTAMs.
INTRODUCTION Nowadays, modern society is facing an unprecedented energy challenge as a result of the fast depletion of fossil fuels and the accompanying environmental pollution. To address this urgent issue, numerous research efforts have been devoted to exploring cost-effective, efficient, and sustainable energy storage and conversion systems.1 Among the available energy storage technologies, rechargeable lithium-ion batteries (LIBs) have been considered as the main power source for various portable consumer electronics over the past two decades.2 To meet the requirements for large-scale applications from electric vehicles to smart electric grids, potential breakthroughs are needed to further improve the energy density and power performance of LIBs. The existing LIBs with graphite-based anodes and lithiummetal-oxide- or lithium-phosphate-based cathodes are reaching their theoretical limits with regard to energy and power densities.3 Hence, the development of high-capacity anode materials offers a great opportunity for the successful implementation of advanced LIBs.4 So far, broadly three types of reaction mechanisms have been reported for anode materials.4,5 Specifically, the mechanism for commercial anodes is based on the reversible intercalation and extraction of Li+ ions in the lattice of a host anode material with layered structures, known as the intercalation mechanism. Graphite is the most successful intercalation-type material as the current anode in commercialized LIBs. Non-graphitized hard carbon materials possess high capacity (200– 600 mAh g1) and good power capability, but suffer from poor electrical
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The Bigger Picture Despite the successful commercialization of lithium-ion batteries (LIBs) in portable electronic devices, intensive research on high-energy density batteries is still ongoing to meet the energy demand for upcoming large-scale applications ranging from electric vehicles to power grids. Hence, high-energy density electrode materials have become the research emphasis for nextgeneration LIBs. Among various candidates, transition-metal compounds based on the conversion reaction mechanism have attracted great interest because of their high theoretical specific capacities. In this review, recent advances on the design and synthesis of nanostructured conversion-type anode materials (CTAMs) in LIBs are presented. The CTAMs covered in this review are transition-metal oxides, sulfides, selenides, fluorides, nitrides, and phosphides. Various advanced strategies toward highperformance CTAMs in LIBs, including structural engineering and carbon hybridization, are discussed.
conductivity and large irreversible capacity.6 In addition, Ti-based oxides, such as TiO2 or Li4Ti5O12, are regarded as another important class of anode materials on the basis of the intercalation reaction mechanism.7–9 Because of the limited number of intercalated Li+ ions in the crystal structure, intercalation-type anodes undergo small volume change during cycling with low theoretical capacities. The second mechanism for anode materials is based on alloying reactions between lithium and the anode materials (such as Si, Sb, Sn, Zn, In, Bi, and Cd).10 The capacities achieved from alloying reactions can reach extremely high values at low operating potentials (e.g., 4,200 mAh g1 for Si, Si + 4.4 Li+ + 4.4 e 4 Li4.4Si, working potential sulfides > nitrides > phosphides.12,14 In addition, CTAMs suffer from poor rate capability and fast capacity fading as a result of the low intrinsic conductivity and the pulverization issue during repeated cycles.30 Apart from the above issues, the highly active M nanoparticles formed during lithiation might induce electrolyte decomposition, resulting in partially reversible side reactions.12,33 Hence, there is still a long way to go to make those conversion-type materials practical alternative anodes. To circumvent the problems, tremendous efforts have been made over the past two decades. Particularly, nanoengineering has proved to be one of the most effective strategies to enhance the lithium storage of CTAMs (Figure 2). Generally speaking, downsizing electrode materials to nanoscale turns the highly reversible storage of lithium via the conversion mechanism from theory into reality, because the metal
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Figure 2. Schematic Illustration of the Nanoengineering Strategies for High-Performance CTAMs for Next-Generation LIBs
nanoparticles formed during the reaction between Li+ and nanosized MaXb have shown much enhanced electrochemical activity toward the decomposition of LinX.15,16,32 Moreover, nanostructures offer reduced diffusion length to mitigate the poor intrinsic conductivity of electrodes, thus improving their reaction kinetics at a high rate charging-discharging process.34 In addition, the various structural features of nanostructures bring diverse advantages. Specifically, one-dimensional (1D) and two-dimensional (2D) nanostructures facilitate the transport of electrons within the electrodes.33,35,36 Three-dimensional (3D) hierarchical structures combine the multiple benefits from low-dimensional building blocks and complex pore configurations.37,38 Furthermore, hollow structured nanomaterials have demonstrated their unique role in addressing volume expansion for CTAMs.39–41 In addition, the introduction of carbonaceous species provides a great opportunity to boost their lithium storage properties by ameliorating the conductivity of CTAMs and preventing electrolyte decomposition.12,33 Apart from these prominent merits, nanostructured CTAM electrodes, especially those supported on carbons, tend to increase their capacity with cycling. A highly probable explanation for this phenomenon is the highly reversible cycling-induced formation or dissolution of a polymer gel around the SEI surface.42–45 Surface adsorption might also be responsible for the extra capacity.46,47 Nevertheless, these strategies contribute little to largely reduce the voltage hysteresis because this issue is related more to the intrinsic nature of the materials. The introduction of alloying-type materials has been proven beneficial to lower the polarization.12
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To gain a comprehensive understanding of the relationship between electrode engineering and optimized electrochemical performance of CTAMs, we review and discuss the recent progress on the above-mentioned nanoarchitectures for future electrode design in next-generation LIBs.
NANOENGINEERING TOWARD HIGH-PERFORMANCE CTAMs Construction of Low-Dimensional Nanostructures In general, low-dimensional materials at nanoscale can be categorized into zerodimensional (0D), 1D, and 2D nanostructures. Compared with bulk-sized materials, these low-dimensional nanostructures with high electrocatalytic ability turn CTAMs from theory into reality.35 Their distinguished structural superiority has stimulated tremendous research interest as advanced electrode designs for CTAMs. 0D Nanoparticles Early reports on 0D nanostructured CTAMs can be traced back to Tarascon’s work in 2000, which initially revealed that the formation of metal nanoparticles improved the reversibility of conversion reactions at room temperature.32 Other than transitionmetal oxides, some 0D nanostructured transition-metal sulfides, selenides, fluorides, nitrides, and phosphides have also been reported as CTAMs.18,48–50 With the development of in situ techniques, it is possible to observe lithium storage of CTAM nanoparticles at atomic resolution. For instance, Luo et al.51 have investigated the lithiation and delithiation process of Co3O4 nanocubes with a particle size of about 5 nm (Figures 3A–3C) by in situ high-resolution transmission electron microscopy (HRTEM). During the lithiation process, a lithium-inserted Co3O4 phase (LixCo3O4) and a phase consisting of nanosized Co-Li-O clusters can be identified as the intermediate products (Figure 3D). At the final stage of lithiation, the Co3O4 nanocube is completely converted into a hybrid consisting of Co clusters grown in the Li2O matrix with prominent volume expansion and crystalline lattice collapses. Such in situ observations may shed some light on our understanding of the reaction mechanisms of CTAMs. 1D Nanostructures 1D nanostructures such as nanorods and nanowires have attracted considerable attention as advanced electrode designs for LIBs. Specifically, continuous 1D configurations can enable fast electron transport, enhance ion diffusion rates, and offer large specific surface area for effective active mass and electrolyte contact.33 Thus, a large number of 1D nanostructured CTAMs have been reported for LIBs.52,53 For example, Li et al.52 have prepared CuP2 nanowires via an unconventional supercritical fluid-liquid-solid growth strategy. Morphological observations clearly indicate the 1D feature of the prepared solid CuP2 nanowires (Figures 4A and 4B). As expected, the calcinated CuP2 nanowire sample shows outstanding capacity and cycling performance with a capacity retention of 945 mAh g1 after 100 cycles at 100 mA g1, indicating its potential as an anode material for LIBs (Figure 4C). More importantly, a pouch-type full cell based on a CuP2 nanowire anode and a commercial LiFePO4 cathode offers a capacity over 60 mAh (Figure 4D). Such a powerful cell can drive some electronic devices such as mobile phones (Figure 4E) and mini 4-wheel drive (WD) cars (Figure 4F), indicating its practical significance. In addition, the conversion reaction mechanism of CuP2 in this work has been confirmed via transmission electron microscopy (TEM) analysis. 2D Nanostructures Specific facets preferentially exposed in the 2D structure can enable fast ion and electron transfer for efficient lithium storage.35,54 Furthermore, the ultrathin
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Figure 3. Morphological Characterizations and Reaction Mechanism Investigations (A–C) Scanning electron microscopy (SEM) image (A), selected area electron diffraction (SAED) pattern (B), and high-resolution transmission electron microscopy (HRTEM) image (C) of Co 3 O4 nanocubes. (D) In situ HRTEM images and schematic atomistic models of the lithiation process of a single Co 3 O 4 nanocube. Reprinted with permission from Luo et al.51 Copyright 2014 American Chemical Society.
thickness of 2D nanomaterials offers good mechanical flexibility, making them suitable for the development of thin, flexible, and stretchable LIBs.54 To date, numerous 2D nanostructured CTAMs have been fabricated via various chemical and/or physical methods.55,56 Recently, researchers have been trying to figure out the underlying lithiation pathways and the structural advantages of 2D CTAMs by using in situ analysis methods. For example, He et al.56 have investigated the kinetics of the lithiation of NiO nanosheets through real-time electron microscopy observations (Figure 5A). The authors believe there are two possible pathways for the redox reaction transition from the near-surface (NS) region to the interior (I) region (Figure 5B). The classic shrinking-core mode proceeds rather slowly perpendicular to all surfaces of the NiO nanosheet, including (111) surfaces (Figure 5C). In the other pathway, named the finger mode, lithiation fingers nucleate on h112i facets, penetrate perpendicularly to [111] until they reach the opposite surface of the NiO nanosheet, and thicken into the bulk of these nanosheets (Figure 5D). According to the real-time results, the transition from NS to I is more likely to happen via the finger mode than
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Figure 4. Morphological Characterization and Electrochemical Performance Investigations (A and B) SEM (A) and TEM (B) images of CuP 2 nanowires. (C) Cycling performance of CuP 2 nanowires in a half cell at a current density of 100 mA g 1 . (D) Galvanostatic charge-discharge profiles of a pouch-type full cell using CuP 2 nanowires as anode and LiFePO 4 as cathode. (E and F) Application demonstrations of CuP 2 -based pouch-type full cells in a mobile phone (E) and a mini 4-WD car (F). Reprinted with permission from Li et al.52 Copyright 2016 American Chemical Society.
via the shrinking-core mode, which might be due to the lower energy barrier. These findings could provide guidance for 2D nanostructured CTAMs with enhanced rate capabilities. Design of Hierarchical Configurations Hierarchical nanostructures possess great advantages in LIBs: (1) the nanosized building blocks ensure short electronic and Li+ transport lengths to gain better electrochemical activity, (2) the porous structures guarantee efficient impregnation of the electrolyte and sufficient contact area between electrode and electrolyte to enhance rate performance, and (3) the hierarchical configuration can accommodate the strain caused by lithium insertion or removal to prolong cycle life.13,37,38 Hierarchical Structures Assembled from 0D Nanoparticles Usually, hierarchical nanostructures assembled from CTAM nanoparticles are composed of numerous primary building blocks with dimensions of tens of nanometers, resulting in a porous structure with relatively large surface area.40 For example, Shi et al.57 have developed a nanocasting strategy by using mesoporous silica SBA-15 as a hard template for highly ordered mesoporous crystalline MoSe2. The as-obtained MoSe2 nanoparticles inherit rod-like morphology and ordered mesostructure from the SBA-15 templates (Figures 6A and 6B). Given the unique structural features, the mesoporous MoSe2 sample delivers a high discharge capacity of 630 mAh g1 after cycling for 35 cycles, which is much higher than its theoretical capacity through the four-electron conversion reaction (Figures 6C and 6D). In addition, the rate performance of the MoSe2 materials is much better than that of the mesoporous MoS2 with similar structure, indicating the compositional importance (Figures 6E and 6F). Hierarchical Structures Assembled from 1D or 2D Nanosized Subunits Hierarchical structures assembled from 1D or 2D building blocks could inherit structural advantages from both the overall hierarchical configurations and the
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Figure 5. In Situ Characterization and Conversion Mechanism Investigations (A and B) Schematic illustrations of the in situ electrochemical cell setup (A) and the heterogeneous pathways of NS-to-I transitions (B). (C and D) Structural evolution during in situ lithiation of NiO nanosheets: shrinking-core mode (C) and finger mode (D) observed by time-sequenced SEM and TEM snapshots. Dashed lines and arrows indicate reaction fronts and propagation directions, respectively. Reprinted with permission from He et al.56 Copyright 2015 American Chemical Society.
low-dimensional building blocks, which endow these advanced structures with enhanced cycling stability and faster reaction kinetics.35,37 Various hierarchical nanostructures of CTAMs with certain geometric shapes, such as urchin-like nanospheres and flower-like nanostructures, have been reported with remarkable lithium storage properties.58,59 For example, Zhang et al.59 have reported the synthesis of MoS2 hierarchical structures constructed from radially arranged nanosheets through a poly(vinylpyrrolidone) (PVP)-assisted hydrothermal method. As illustrated in Figure 7A, the PVP molecules can protect the 2D MoS2 nanosheets from restacking and enable the radially oriented assembly of these nanosheets into hierarchical nanospheres. Field-emission scanning electron microscopy (FESEM) and TEM images show that these uniform hierarchical nanospheres are composed of well-organized ultrathin MoS2 nanosheets (Figures 7B–7D). When applied as an anode material for LIBs, these highly oriented MoS2 nanospheres deliver a high reversible capacity of 1,095.7 mAh g1 over 110 cycles at the current density of 100 mA g1 (Figure 7E). PVP-free MoS2 nanosheets suffer from continuous and quick capacity decay with cycling. The above results suggest that the hierarchical structures can effectively buffer the volume change during cycling, leading to the enhanced cycling stability.
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Figure 6. Morphological Characterizations and Performance Investigations of Mesoporous MoSe2 (A and B) FESEM (A) and TEM (B) images of mesoporous MoSe 2 . (C and D) Charge-discharge curves of the first three cycles (C) and cycling performance (D) of a mesoporous MoSe 2 electrode at a current rate of 0.05 C (0.05 C refers to 4 mol Li+ uptake into MoSe 2 per formula unit in 20 hr). (E) Rate performance of a mesoporous MoSe 2 electrode. (F) Rate comparison between mesoporous MoSe 2 and MoS 2 electrodes. Reprinted with permission from Shi et al. 57 Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
Recently, hierarchical nanoarrays of CTAMs constructed from 1D or 2D nanosized building blocks have stimulated extensive research interest as binder-free electrodes for next-generation LIBs considering their exceptional structural superiority.60,61 Specifically, low-dimensional (1D nanowires and nanorods or 2D nanosheets
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Figure 7. Synthesis Strategy, Morphological Characterization, and Performance Investigations of Hierarchical MoS2 Nanospheres (A) Illustration of the formation of 3D radially oriented MoS 2 nanospheres. (B–D) FESEM (B) and TEM (C) images of MoS 2 nanospheres. (D) HRTEM image of a MoS 2 shell. (E) Cycling performance of MoS 2 nanospheres and MoS 2 nanosheets at 100 mA g 1 . Reprinted with permission from Zhang et al. 59 Copyright 2015 American Chemical Society.
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Figure 8. Morphological Characterization and Performance Investigations of Hierarchical CoxMn3xO4 Arrays (A–C) FESEM image (A), TEM image (B), and cycling performance (C) of a CoMn 2 O 4 nanowire array. (D–F) FESEM image (D), TEM image (E), and cycling performance (F) of a MnCo 2 O4 nanosheet array. Reprinted from Yu et al., 62 published by the Royal Society of Chemistry.
and nanoplates) subunits directly grown on conductive substrates could guarantee a direct electron transport pathway for enhanced rate performance. In addition, these self-supported electrodes could avoid the use of extra binder or conductive additive to further improve the specific capacity. Moreover, the open space between subunits would facilitate the penetration of electrolyte and address the volume expansion during the Li+ uptake or removal process. As a typical example, Yu et al.62 have reported a facile two-step method to fabricate hierarchical CoxMn3xO4 arrays on stainless steel with tailorable morphologies. Controlling the solvent components in the solvothermal reactions can tune the subunits of the arrays from nanowires to nanosheets with different chemical compositions. After a follow-up annealing treatment in air, CoMn2O4 nanowire arrays (Figures 8A and 8B) and MnCo2O4 nanosheet arrays (Figures 8D and 8E) can be obtained with highly porous textures. These hierarchical CoxMn3xO4 arrays display morphology- and composition-dependent electrochemical performance as anode materials in LIBs. Particularly, CoMn2O4 nanowire array delivers a much higher specific capacity of 600 mAh g1 at a current density of 800 mA g1 in the second cycle (Figure 8C). MnCo2O4 nanosheet array shows a lower capacity of 460 mAh g1 at a current density of 800 mA g1 with better cycling retention (Figure 8F). Design of Hollow Structures Hollow structures have been considered advanced designs to effectively accommodate the volume expansion for anode materials in LIBs.63 Nevertheless, the low tap density of active materials for simple hollow structures limits their practical applications. To boost the electrochemical performance of hollow nanostructured CTAMs, their geometric morphology, shell architecture, and chemical composition need to be optimized.39,40,64 Rationally increasing the complexity of hollow nanostructures can endow them with new functionalities for better lithium storage. To date, diverse hollow structures of CTAMs with different configurations, including simple single-shelled, yolk-shelled, and multi-shelled hollow nanostructures, have been synthesized.40,65
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Single-Shelled Hollow Structures Single-shelled hollow structures with an interior void enclosed by a single layer of active material are the basic form of hollow structures. The synthetic strategies for these structures usually involve template-engaged or template-free methods.41,63 For example, Ma et al.66 have reported the synthesis of single-shelled hierarchical hollow spheres constructed from ultrathin Fe3O4 nanosheets. Fe-containing precursor solid spheres based on iron ions and isopropyl alcohol (denoted as Fe-IPA) are selected as the self-engaged template (Figure 9A). During the one-pot solvothermal process, Fe-IPA spheres gradually transform into hierarchical Fe-glycerate hollow spheres composed of sheet-like subunits. After a subsequent annealing treatment, the resultant hierarchical Fe3O4 hollow spheres could preserve the original singleshelled configuration of the Fe-glycerate precursors without apparent structural deterioration (Figures 9B–9D). Benefitting from the highly porous texture and the sheet-like subunits, the single-shelled Fe3O4 hollow spheres exhibit remarkable structural stability to withstand the volume changes during repeated cycles. As a result, these hollow spheres deliver a high specific capacity of 1,046 mAh g1 without notable fading over 100 cycles (Figure 9E). Compared with single-shelled hollow nanospheres, 1D tubular nanostructures can ensure better electron transport and enhanced structural robustness. Thus, many 1D nanostructured templates have been explored for the synthesis of single-shelled tubular structures.69,70 Other than the templating methods, Wang and co-workers have demonstrated a typical template-free route for the formation of hierarchical nanotubes assembled from MoS2 nanosheets.67 Different from conventional templating strategies, the construction of MoS2 nanotubes is based on dipole-dipole interaction-induced assembly behavior of the nanosheets. As seen from the morphological observations, the assembled MoS2 nanotubes are homogeneous in size with a hollow interior and an uneven surface (Figures 9F and 9G). Separated ‘‘threads’’ are observed in the HRTEM image of the MoS2 shell, corresponding to disordered MoS2 slabs with a single-layer character (Figure 9H). The hierarchical MoS2 tubular structures exhibit much higher electrochemical activity with enhanced cycling performance as an LIB anode compared with MoS2 nanosheets and commercial MoS2 powder, highlighting the superiority of the designed structure (Figure 9I). Because of the lack of templates and an effective coating method on high-curvature surfaces, nanoboxes or hollow polyhedrons are relatively less reported. So far, only a few of particular materials have been used as templates for those hollow structures, such as MnCO3 microcubes,68 Prussian blue (PB)-based microcubes,71 and polyhedral metal-organic frameworks (MOFs).72 Here, we take the synthesis of hierarchical MoS2 microboxes as a representative example.68 MnCO3 microcubes are used as the template for growing a MoS2 nanosheet shell via an L-cysteine-assisted hydrothermal method. Via the selective removal of the inner templates, single-shelled MoS2 microboxes with a well-defined hollow interior are obtained. The FESEM image indicates that the MoS2 sample constructed from numerous nanosheets can inherit the cubic shape from the MnCO3 microcube (Figure 9J). Further characterizations reveal the hollow feature of these hierarchical MoS2 microboxes (Figures 9K and 9L). Compared with MoS2 microparticles, the hierarchical MoS2 microboxes can deliver a much higher capacity with better capacity retention over 50 cycles (Figure 9M). Hollow Nanostructures with Multi-level Architecture Notwithstanding the advantages for single-shelled hollow structures, the excess empty space greatly reduces the tap density of the active materials, resulting in
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Figure 9. Synthesis Strategy, Morphological Characterization, and Performance Investigations of Single-Shelled Hollow Structures (A–E) Illustration of the formation process of Fe-glycerate hollow spheres (A). Low-magnification FESEM image (B), high-magnification FESEM image (C), TEM image (D), and the corresponding cycling performance (E) of hierarchical Fe 3 O 4 hollow spheres. Reprinted with permission from Ma et al. 66 Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (F–I) FESEM (F) and TEM (G) images of the nanotubes assembled from single-layered MoS 2 , HRTEM image of the shell of an individual assembled MoS 2 nanotube (H), and cycling performances of the assembled MoS 2 nanotubes, MoS 2 nanosheets, and MoS 2 powder (I). Reprinted with permission from Wang et al. 67 Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (J–M) FESEM images of hierarchical MoS 2 microboxes (J) and a broken particle (K), TEM image of an individual MoS 2 microbox (L), and cycling performances of the hierarchical MoS 2 microboxes and MoS 2 microparticles (M). Reproduced from Zhang et al. 68 with permission of the Royal Society of Chemistry.
low volumetric energy and power density.40,41 To circumvent the drawbacks, hollow nanostructures with multi-level architecture, including yolk-shelled, multi-shelled, and multi-chambered structures, have been proposed.69,73–81 These complex designs make better use of the inner cavity in the hollow structure to effectively increase the weight fraction of the electrochemically active components.40 In addition, the multi-level architecture could serve as physical supports to further enhance the structural stability.39 Moreover, the void spaces within these hollow structures could
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ensure the penetration of electrolyte into the inner region and accommodate the volume changes during repeated charging-discharging cycles. To date, several efficient synthetic strategies for yolk-shelled structures of CTAMs, such as Ostwald ripening,79 spray pyrolysis,80 and heterogeneous contraction,73 have been reported. For example, Yu et al.73 have reported the preparation of a series of yolk-shelled Ni-Co mixed oxide nanoprisms through a thermally driven heterogeneous contraction process. Monodisperse Ni-Co precursor solid prisms with tailorable composition have been exploited as starting materials. As a typical illustration, the as-obtained Ni0.37Co oxide sample maintains a prism-like morphology with a pyramid-shaped apex at both ends (Figure 10A). TEM observations elucidate the yolk-shell feature of these nanoprisms composed of numerous polycrystalline nanoparticles (Figure 10B). Galvanostatic measurement of the Ni0.37Co oxide nanoprisms indicates that the yolk-shelled sample can deliver a high reversible capacity of 1,028.5 mAh g1 over 30 cycles at 200 mAh g1, illustrating good stability of the electrodes (Figure 10C). With the rapid development of nanotechnology, the design and synthesis of these complex hollow structures of CTAMs have become more and more realizable. To date, the synthetic strategies for these complex hollow structures have been extensively reviewed and summarized by several research groups.40,41 Carbonaceous material (CM)-based hard-templating methods and inorganic-organic hybrid-based template-engaged methods are among the most reported methodologies for well-defined hollow structures of CTAMs with multiple shells and chambers. CM-based strategies usually involve the adsorption and penetration of metal precursors into the porous nanocarbons with several functional groups (–OH and –C=O, etc.) and subsequent heat treatment in air to combust the carbon templates. During the consumption of CM, the adsorbed metal ions are oxidized into metal oxide to form a rigid shell. More importantly, the CM-based templates are gradually removed as a result of the temperature gradient along the radical direction, providing possibilities for the formation of multi-shelled hollow structures. With elaborate control of the synthesis conditions (metal ion concentration, solvent type, pH value, heating temperature and rate, and atmosphere type, etc.), various features of the multishelled hollow structure, such as shell number, shell thickness, porosity, and chemical composition, can be tuned.78,82 So far, quite a few multi-shelled hollow structures of conversion-type binary metal oxides and mixed metal oxides have been synthesized through CM-based strategies.39,69,75,78,82 For example, Lou’s group has reported a modified CM-based strategy for constructing a series of multi-shelled mixed metal oxide hollow spheres and tubes by adding a solidification step before the annealing treatment.69,75 After the penetration of metal ions, the solidification process can form the metal-glycolate in the interior and outer surface of the CMs as the intermediate products for the hollow structures (Figure 10D). TEM images clearly show typical examples of mixed metal oxides (CoMn2O4, Co1.5Mn1.5O4, and MnCo2O4) with identical multi-shelled hollow sphere-like configurations from this ‘‘penetration-solidification-annealing’’ method (Figures 10E– 10G). Benefitting from the unique structural features, these hollow spheres of CTAMs with multiple shells demonstrate remarkable lithium storage properties with high specific capacity and long cycle life. Recently, self-templated methods have enriched the synthesis field of multi-shelled or multi-chambered complex hollow structures.77,83,84 Among the available selfengaged templates, inorganic-organic hybrids composed of metal ions and/or
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Figure 10. Synthesis Strategy, Morphological Characterization, and Performance Investigations of Complex Hollow Structures (A–C) FESEM image (A), TEM image (B), and the cycling performance (C) of Ni 0.37 Co oxide prisms. Reprinted with permission from Yu et al. 73 Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (D–G) Illustration of the synthesis of multi-shelled metal oxide hollow spheres (D) and TEM images of multi-shelled CoMn 2 O4 (E), Co 1.5 Mn 1.5 O 4 (F), and MnCo 2 O 4 (G) hollow spheres. Scale bars, 500 nm. Reprinted with permission from Zhang et al. 75 Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (H–J) TEM image of CoS 4 nanobubble hollow prisms obtained after sulfidation in ethanol (H) and TEM image (I) and the corresponding rate capability (J) of CoS 2 nanobubble hollow prisms obtained after annealing in nitrogen at 350 C. Reprinted with permission from Yu et al. 76 Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (K–M) TEM image of T-Co 3 O 4 @Co 3 V 2 O 8 nanoboxes (K) and TEM image (L) and elemental mappings (M) of an individual T-Co 3 O 4 @Co 3 V 2 O 8 nanobox. Reprinted with permission from Lu et al. 81 Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
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clusters and organic components have attracted increasing interest. Especially, metal alkoxides or metal-polymer composites and MOFs are considered to be attractive precursors to prepare complex hollow structures of CTAMs.71,74,85 With rational selection of precursor and synthetic route, delicate hollow structures with multi-level internals can be obtained. As an example, Yu et al.76 have developed a two-step diffusion-controlled method to fabricate a hierarchical prism-like hollow nanostructure assembled from CoS2 nanobubbles. Prism-like cobalt acetate hydroxide solid precursors are chosen as the self-engaged templates, which are successively transformed into imidazolate framework-67 (ZIF-67) hollow structures and then CoS4 nanobubble hollow nanoprisms (Figure 10H). After an annealing treatment, these hierarchical structures composed of interlinked CoS2 nanobubbles preserve the prism morphology and the multi-level hollow interiors (Figure 10I). Benefitting from the hierarchical hollow structures, these CoS2 hollow prisms display remarkable reversible rate capabilities at current densities of 200–5,000 mA g1 (Figure 10J). Apart from the configurations, the chemical compositions between different shells can be manipulated during the preparation. Compared with single-component hollow structures, the hybridization of two electrochemically active components in one electrode might provide extra benefits in structural stability, conductivity, and electrochemical activity. Particularly, Lu et al.81 have reported the synthesis of multi-shelled Co3O4@Co3V2O8 hybrid nanoboxes through a MOF-engaged strategy based on cubic ZIF-67 precursors. By controlling the amount of vanadium oxytriisopropoxide (VOT) in the reaction system, they could transform the ZIF-67 nanocubes into double-shelled and triple-shelled Co3O4@Co3V2O8 composite nanoboxes (denoted as D-Co3O4@Co3V2O8 NBs and T-Co3O4@Co3V2O8 NBs). More importantly, these structures have different chemical compositions in different shells. As illustrated in TEM and corresponding elemental mapping results, T-Co3O4@Co3V2O8 NB has a Co3V2O8 outermost shell and two Co3O4 inner shells (Figures 10K–10M). As expected, the triple-shelled nanocomposite demonstrates enhanced lithium storage properties as a result of a synergistic effect from complex configurations and compositions. Hybridization with Carbonaceous Materials Beyond structural engineering, carbon hybridization is another effective route for improving the lithium storage of CTAMs.13,31,33,86 Through careful design of the hybrid nanostructures, nanocarbons with good electronic conductivity can largely enhance the reaction kinetics. In addition, good elasticity of carbon materials would effectively accommodate the strain of volume change during Li+ insertion or extraction, offering extra benefits for the cycling stability of the active materials. On the basis of the location of the carbon species and CTAMs, nanocomposites of these two components can be grouped into three categories: carbon nanocoating on CTAMs, growth of CTAMs on nanocarbons, and embedding CTAM nanoparticles into carbonaceous matrixes. Carbon Nanocoating on CTAMs Carbon nanocoating is one of the most effective surface modification approaches to improve the electrochemical performance of electrode materials. Carbon nanocoating can serve as a physical barrier to prevent aggregation of active materials and direct contact between the CTAMs and the electrolyte. Thus, the electrode-electrolyte interface can be stabilized, and electrolyte decomposition can be partly addressed.12,33 Generally, carbon nanocoating on CTAMs is accomplished through chemical coating of a polymer layer with a subsequent carbonization step under an inert atmosphere or physical coating of graphene-based carbon species.33,87 In the chemical coating process, carbohydrate solutions containing glucose or sucrose
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are frequently exploited to generate a layer of carbon-rich polysaccharide on various materials.33 Recently, great advances in nanocoating strategies have been made and some other organic monomers have been used to generate carbon layers with rich chemistry. For example, dopamine hydrochloride has been reported as a fascinating carbon source to form a polydopamine (PDA) layer around many metal-containing materials, which can be further evolved into a nitrogen-doped carbon shell.88,89 As an example, Jiang et al.90 have demonstrated the fabrication of a novel nanopeapod-like MnO/carbon hybrid structure by using dopamine hydrochloride as the coating agent (Figure 11A). After a self-polymerization process under alkaline conditions, a PDA shell is coated on the surface of the poorly crystalline MnO nanowire (denoted as MnO-P NW) to obtain the MnO-P/PDA core-shell structure. During the subsequent carbonization treatment, the inner MnO-P NW is broken into nanoparticles encapsulated in the PDA-derived carbon layer to form the nanopeapod structure. As a comparison, only the normal MnO/C core-shell structure is generated with single-crystalline MnO NWs as the precursor. The FESEM image indicates that the MnO/C nanopeapod sample retains the 1D structure (Figure 11B). TEM mapping results reveal dispersive MnO nanoparticles in the carbon shells, confirming the peapod-like morphology (Figure 11C). When tested under a current density of 500 mA g1, the MnO/C nanopeapod sample delivered a much higher reversible capacity (up to 1,119 mAh g1) than MnO/C core-shell NWs and pure MnO NWs (Figure 11D). Even cycled at a high current of 2,000 mA g1 over 1,000 cycles, the capacity of the peapod-like sample still reached a high value of 525 mAh g1 with no obvious electrode pulverization or size variation (Figure 11E). Other than the layer-by-layer coating process, the development of MOF-based strategies enables carbon nanocoating on CTAMs through a more facile route. For example, Hu et al.91 have reported the formation of hybrid nanoboxes with a CoSe-enriched inner shell intimately confined within a carbon-enriched outer shell (denoted as CoSe@carbon nanoboxes) by a facile MOF-engaged strategy. During the synthesis, the ZIF-67 nanocubes serve as the source of both carbon and metal species, which are successively converted into CoSe2/carbon nanoboxes with a homogeneous shell and then to CoSe@carbon nanoboxes. FESEM and TEM images indicate the hollow nature within the uniform cubic-like CoSe@carbon nanoboxes (Figures 11F–11H). The mapping results further demonstrate the inhomogeneous feature within the complex CoSe@carbon shell (Figure 11I). Different from the layer-by-layer strategy, the combined effects of chemical transformation and carbonization of the CoSe2/carbon shell result in the formation of the carbon-coated CoSe nanoshell. Growth of CTAMs on Nanocarbons With good conductivity, light weight, and porous structure with large surface area, carbon materials have been considered ideal supports for CTAMs to improve their kinetics for charge transfer and ionic diffusion and cycling stability. Among various carbon matrixes, highly conductive and flexible carbon nanotubes (CNTs) have been widely utilized for constructing tube-like metal compound nanocomposites with CTAMs.31,33 Particularly, Chen et al.92 have developed a hierarchical MoS2 tubular nanostructure internally wired by a single CNT (denoted as CNT/ MoS2 nanohybrid) through an effective multi-step route with the assistance of an electrospinning process (Figures 12A–12C). In this case, electrospun CNT/polyacrylonitrile (PAN) tube-in-fiber composite is used as the starting material. After a CoSx
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Figure 11. Synthesis Strategy, Morphological Characterization, and Performance Investigations of Carbon Nanocoating on CTAMs (A–E) Schematic illustration of the preparation of heterostructured MnO/C nanopeapods and MnO/C core-shell NWs (A), SEM image (B) and TEM-EDX mappings (C) of MnO/C nanopeapods, comparative cycling performances of the MnO/C nanopeapods and the reference samples (D), and cycling performance of MnO/C nanopeapods at 2,000 mA g 1 and a TEM image of the MnO/C nanopeapod electrode after 1,000 cycles (inset) (E). Reprinted with permission from Jiang et al. 90 Copyright 2014 American Chemical Society. (F–I) FESEM (F and G) and TEM (H) images of CoSe@carbon nanoboxes and TEM image and corresponding element mappings of an individual CoSe@carbon nanobox (I). Reprinted with permission from Hu et al. 91 Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
protective layer and a MoS2 nanosheet layer are loaded in sequence with follow-up acid etching, the final CNT-in-tube nanostructure of CNT/MoS2 nanohybrid is obtained. Moreover, altering the initial amount in the CNT/PAN composite can steadily control the number of CNTs in each MoS2 nanotube. From the structural observations, the coaxial CNT is confined in the MoS2 tubular nanostructure
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Figure 12. Synthesis Strategies, Morphological Characterization, and Performance Investigations of CTAMs Grown on Nanocarbons (A–C) Illustration of the synthesis procedure (A), FESEM image (B), and TEM image (C) of the CNT/MoS 2 nanohybrid. Reprinted from Chen et al. 92 (D–G) FESEM image (D) and TEM image (E) of the HC-MoS 2 @GF sample, schematic illustration of the structural advantages of HC-MoS 2 @GF for lithium storage (F), and cycling performances of HC-MoS 2 @GF, HS-MoS 2 @GF, and HS-MoS 2 electrodes (G). Reprinted with permission from Wang et al. 93 Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (H–J) Illustration of the synthesis procedure (H) and FESEM image (I) and TEM image (J) of bowl-like NiO NSs@C hollow particles. Reproduced from Liang et al. 94 with permission of the Royal Society of Chemistry.
(12C). The good organization of ultrathin MoS2 nanosheets and the 1D porous tubular structure can enable fast Li+ and electron transport along the whole hierarchical structure by the interior CNTs with stable structural integrity and a sufficiently large electrode-electrolyte interface. Remarkably, the CNT/MoS2 tubular nanohybrid exhibits striking electrochemical performance with a very high specific capacity up to 1,300 mAh g1, outstanding rate capability, and an ultralong cycle life up to 1,000 cycles. Graphene has also attracted intense research interest as another appealing type of carbonaceous support for loading numerous CTAMs. For example, Shen and coworkers have developed a facile strategy for growing honeycomb-like MoS2 nanoarchitectures anchored into 3D graphene foam (denoted as HC-MoS2@GF).93 3D GF has been exploited as the backbone for loading MoS2 nanosheets through a self-assembly process with the assistance of P123 as the structure-directing agent. The as-prepared honeycomb-like MoS2 nanostructures constructed from highly
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interconnected nanosheets are uniformly distributed on 3D GF (Figures 12D and 12E). Beyond the highly conductive carbon matrix, the numerous crossing points within the staggered MoS2 nanosheets might provide dislocations, vacancies, and distortions as massive active sites for lithium storage (Figure 12F). As shown in Figure 12G, the HC-MoS2@GF electrode demonstrates much higher capacity and bettercycling performance for lithium storage than MoS2 hollow spheres coated on GF (HS-MoS2@GF) and Cu foil (HS-MoS2). Besides CNTs and graphene, other carbon nanostructures with diverse configurations (such as nanospheres, nanoboxes, and nanobowls, etc.) have been utilized as effective supports to load various active materials for enhanced lithium storage.88,94,95 As a typical example, Liang et al.94 have reported a bowl-like hybrid structure composed of NiO nanosheets supported on flat carbon hollow particles (denoted as NiO NSs@C). The synthesis process of this hybrid structure involves the growth of a layer of Ni-precursor nanosheets on the bowl-like hollow polystyrene particles with subsequent carbonization treatment in N2 (Figure 12H). The FESEM image shows that the resultant bowl-like NiO NSs@C particles can be stacked to enhance the packing density (Figure 12I). TEM displays the hollow cavity of the hierarchical bowl-like structure, where the carbon shell is surrounded by the NiO nanosheets (Figure 12J). Because of the combined features from the internal void space, sheet-like subunits, and denser packing carbon shell, the bowl-like NiO NSs@C sample exhibits excellent rate performance and good cycling stability as an anode material in LIBs. CTAMs Embedded in Carbon Matrixes As a single layer of carbon atoms in a hexagonal lattice, graphene with p electrons and a graphitic plane exposed has many merits, such as light weight, large surface areas, high conductivity, and structural flexibility.86 Therefore, graphene and its derivatives are widely chosen as promising nanoscale building blocks for constructing various novel 3D porous carbon matrixes with fast mass and electron transport kinetics. Furthermore, incorporating nanomaterials of CTAMs into these carbon matrixes could enhance their lithium storage properties as a result of the diverse functionalities and synergistic effects in the composites. For example, Mu¨llen’s group has incorporated graphene-encapsulated Fe3O4 core-shell nanospheres into 3D graphene to fabricate hierarchical Fe3O4/graphene hybrids.87 As illustrated in Figure 13A, coating negatively charged graphene oxide (GO) on the surface of positively charged Fe3O4 nanospheres is an electrostatic assembly process, and the resultant Fe3O4@GO core-shell particles can be dispersed into GO suspension. After a series of follow-up treatments, 3D graphene foams cross-linked with pre-encapsulated Fe3O4 nanospheres are obtained (Fe3O4@GS/GF). Characterizations clearly reveal the intimate contact between Fe3O4 nanoparticles and graphene nanosheets in the 3D graphene networks (Figures 13B and 13C). Such a geometric confinement of electrochemically active materials within the graphene coating and exterior porous graphene networks is expected to provide multiple levels of protection against the aggregation and volume changes of Fe3O4 and ensure favorable transport kinetics for both electrons and Li+ ions. Compared with Fe3O4@GS and Fe3O4 nanospheres, the prepared 3D Fe3O4@GS/GF electrode manifests remarkable cycling performance with a capacity of 1,059 mAh g1 over 150 cycles (Figure 13D). Other than the direct physical combination of carbon matrixes with CTAMs, advanced material design of carbon-based frameworks with embedded nanoparticles of CTAMs can also be achieved via thermal annealing of inorganic-organic hybrids.72,84,97,98 Moreover, the rich chemistry of organic components in the precursors can bring heteroatom dopants to further improve the electrochemical
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Figure 13. Synthesis Strategy, Morphological Characterization, and Performance Investigations of CTAMs Embedded in Carbon Matrixes (A–D) Fabrication process and photograph (A), SEM image (B), and HRTEM image (C) of Fe 3 O4 @GS/GF and comparative cycling performances of Fe 3 O 4 @GS/GF, Fe 3 O 4 @GS, and Fe 3 O4 nanospheres (D). Reprinted with permission from Wei et al. 87 Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (E–H) Illustration of the formation process (E), TEM image (F), HRTEM image (G), and the corresponding cycling performance (H) of NiO/Ni/graphene composite. Scale bar in (G), 2 nm. Reprinted with permission from Zou et al.96 Copyright 2016 American Chemical Society.
activity by modifying the band gap and/or changing the surface characteristics.98,99 Specifically, a hierarchical hollow ball-in-ball structure of NiO/Ni/graphene composite, which is derived from Ni-based MOFs through a two-step thermal treatment, has been developed by Zhu and co-workers.96 As shown in Figure 13E, the Ni-MOF ball-in-ball particles are first carbonized into Ni nanoparticle embedded graphene frameworks under N2. Then, the Ni particles are partially oxidized to NiO to form the final NiO/Ni/graphene hollow composite via further annealing in air. TEM images clearly illustrate the hierarchical hollow structure of the NiO/Ni/graphene hybrid, in which the NiO/Ni nanoparticles are wrapped by ultrathin onion-like graphene shells (Figures 13F and 13G). This unique structural design could effectively address the volume expansion of NiO and provide a highly conductive framework for enhanced rate capability. After being fully activated at a small current in the first five cycles, the as-prepared hybrid electrode can deliver a
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Table 1. Lithium Storage Performance of Different CTAMs Classification
Typical Examples
Voltage Plateau (V versus Li/Li+)
Capacity (mAh g1)
0D to 2D nanostructures
Co3O4 nanosheets55
1.0
1,291 after 25 cycles
445
MnS nanocrystals48
0.75
288 after 600 cycles
100
a-MnSe nanocubes49
0.6
150 after 5,000 cycles
800
Cu3N nanoparticles50
0.58
260 after 150 cycles
1,000
CuP2 nanowires
0.6
945 after 100 cycles
100
MnCo2O4 nanosheet array62
0.8
460 after 30 cycles
800
MoS2 nanospheres59
0.6
1,096 after 110 cycles
100
1.0
630 after 35 cycles
21
52
Hierarchical configurations
Mesoporous MoSe257 Hollow structures
Hybridization with carbonaceous materials
66
Current Density (mA g1)
Fe3O4 hollow spheres
0.7
1,046 after 100 cycles
500
yolk-shelled Ni0.37Co oxide prisms73
1.0
1,029 after 30 cycles
200
MoS2 nanotube67
0.6
839 after 50 cycles
100
CoS2 nanobubble hollow prisms76
1.25
737 after 200 cycles
1,000
MnO/C nanopeapods90
0.5
525 after 1,000 cycles
2,000
NiO/Ni/graphene composite96
0.75
962 after 1,000 cycles
2,000
0.6
800 after 1,000 cycles
5,000
1.3
660 after 100 cycles
1,000
CNT/MoS2 nanohybrid92 CoSe@carbon nanoboxes
91
reversible specific capacity of 962 mAh g1 over 1,000 cycles at a high current density of 2 A g1 (Figure 13H).
CONCLUSIONS AND OUTLOOK As promising alternatives to the currently used graphite anode in next-generation LIBs, CTAMs are of great significance because of their high theoretical capacity, tunable operation voltages, and the diversity of chemical composition and phases. Nevertheless, many obstacles, including poor intrinsic conductivity and severe pulverization during conversion reactions, need to be addressed before the widespread implementation of CTAMs. In this regard, tremendous efforts have been devoted to exploring effective strategies for circumventing these issues. Here, we have summarized the recent advances in nanoengineering designs for CTAMs in LIBs. Specifically, these designed nanostructures can be divided into four categories according to the complexity of configuration and chemical composition: (1) low-dimensional materials resulting from nanostructuring, including 0D nanoparticles, 1D nanorods and nanowires, and 2D nanosheets and nanoplates; (2) hierarchical porous nanostructures assembled from low-dimensional subunits; (3) hollow architectures with different shell structures and interiors; and (4) nanocomposites of CTAMs and various CMs. These advanced designs endow CTAMs with shortened ion and electron transport paths, extra active sites, enhanced reaction kinetics, and improved structural stability. As illustrated in Table 1, the use of these delicate strategies could significantly improve the lithium storage performance of CTAMs, endowing them with high specific capacity, superior rate capability, and prolonged cycle life.
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Notwithstanding this great progress, many challenges in the research and application of CTAMs in LIBs remain. To be more specific, the current exploration of CTAMs has mainly focused on transition-metal oxides and sulfides. Research on metal selenide-, fluoride-, nitride-, and phosphide-based materials is still at the embryonic stage and requires further systematic investigations. From a synthesis point of view, the current methods for obtaining well-defined nanostructured CTAMs are still quite expensive and time consuming. Facile and cost-effective strategies are urgently required, especially for metal fluoride-, nitride-, and phosphide-based CTAMs. As for the structural designs, more in situ observations and theoretical simulations and calculations are needed for us to understand the intrinsic electrochemical mechanisms and structural advantages for further performance optimization. Furthermore, although carbon compositing brings great benefits for CTAMs, the addition of CMs decreases the specific capacity and reduces the tap density of the electrode to some extent. Therefore, further efforts are still required for optimizing the carbon content for CTAMs. In addition, although we can realize the formation of nanomaterials for commercialization in many applications, we must further reduce the production cost of well-defined nanostructures as CTAMs by simplifying the synthetic routes and using low-cost precursors for widespread implementation. Although breakthroughs are unlikely to come in the near future, we are optimistic that CTAMs with rational designs and delicate controls in structure and composition have potential as anode materials for high-performance LIBs.
ACKNOWLEDGMENTS X.W.L. acknowledges funding support from the National Research Foundation (NRF) of Singapore via the NRF Investigatorship (NRF-NRFI2016-04).
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