Li-storage Behavior Research Articles

Understanding the influence of sulfur configuration on the electrochemical reaction pathway for lithium storage in molybdenum sulfides

Molybdenum sulfides are highly regarded as conversion-type electrodes with exceptional performance potential in lithium-ion batteries (LIBs). However, elucidating their lithium (Li) storage behavior has proven challenging due to the intricate sulfur (S) coordination with molybdenum atoms, which has impeded the realization of high energy densities in future Li storage systems. In this study, we investigate the electrochemical redox reaction pathways of molybdenum sulfide, specifically exploring the impact of various S configurations, including terminal, bridging, and apical S. We employ a comprehensive approach, involving extensive electrochemical measurements and depth X-ray photoelectron spectroscopy (XPS) profiling. In contrast to the multi-step conversion reactions observed in MoS2, sulfur-enriched molybdenum sulfides exhibit a unique single-step Li-ion storage mechanism. As a consequence of the pronounced effect of S configuration on the activation energy barrier during charging and discharging, the redox reaction voltage with Li ions exhibits variations. This versatility verifies that molybdenum sulfides have the potential to serve as both anode and cathode materials in lithium-ion batteries. Furthermore, we propose the incorporation of reduced graphene oxide (rGO) composites to address inherent limitations of molybdenum sulfides, including low electrical conductivity and irreversible reactions. The robust interaction between molybdenum sulfides and oxygen functional groups in rGO enhances rate capability and ensures a stable cycling lifespan. Through this systematic investigation, we suggest a comprehensive insight into the electrochemical reaction pathways governing Li storage behavior in molybdenum sulfides with distinct S configurations. This contributes to the fundamental understanding of these materials and their potential applications in advanced energy storage systems.

Metal-organic framework-derived Nickel Diselenide grown in situ on carbon for high-performance Li storage

The creation of innovative anodes to replace graphite in lithium-ion batteries (LIBs) is necessary to meet the rising demand for energy storage devices. Based on the large Li storage capacity, transition metal selenides (TMSs) draw considerable interest for LIBs. However, volume expansion of TMSs during the cycling process hinders their large-scale commercial application for LIBs. Herein, the NiSe2 particle grown in situ on the porous carbon framework (NiSe2@C) is prepared by selenization of metal-organic frameworks (MOFs) in an argon atmosphere. NiSe2@C exhibits a high reversible Li storage capacity (770 mAh g−1 at 200 mA g−1after 100 cycles), long cycling life (308 mAh g−1 at 1 A g−1 after 1000 cycles), and good rate capability (214 mAh g−1 at 5.0 A g−1). The long cycling performance can be attributed to the fact that NiSe2@C can withstand volume expansion during the Li+ insertion and desertion process. Furthermore, cyclic voltammetry (CV) tests at various scan rates are conducted to investigate the kinetics of NiSe2@C for LIBs, indicating that the Li storage behaviour of NiSe2@C is primarily controlled by the capacitive process. The fabrication of the TMS@C composites by selenization of MOFs is a potential way to obtain anodes for high-performance Li storage.

The application of hard carbon with internal quasi-lithium metal deposition in high-energy Li-ion/Li-metal hybrid batteries

The energy density of lithium-ion batteries (LIBs) with graphite as the negative electrode reaches the upper limit. The lithium ion/lithium metal hybrid batteries (HLBs) constructed by lowering the n/p of LIBs with graphite as the anodes effectively improves the energy density, but the massive plating of lithium metal (Li-M) on the surface of the graphite anode leads to rapid cell failure. In this paper, hard carbon (HC) is proposed as a better choice for anode in HLBs than that of graphite, and the difference in Li storage behaviors between HC and artificial graphite (AG) is systematically investigated. At low n/p, part of the “excess Li” is stored as quasi-lithium metal (Li-QM) in the sub-nanometer pores inside HC in a highly reversible way, which reduces Li-M plating on the surface of anodes, effectively increasing the cycle stability of HLBs. The HLBs with HC anode mass loading of 5.1 mg cm−2 and 7.8 mg cm−2 have 41.7% and 30.3% higher stack energy density compared to LIBs. Moreover, HLB-type pouch cells using HC anode with a mass loading of 7.8 mg cm−2 achieves high capacity retention of 81% after 450 cycles, while the capacity retention drops to 55% for the AG anode with the same mass loading.

Cr2S3 nanosheet grafted with porous N-doped carbon architecture as an anode with enhanced Li-storage property

Chromium-based transition metal sulfide such as chromium hemi trisulfide (Cr2S3) gradually emerges as a potential lithium-ion host material considering its relatively large specific capacity along with low cost advantage. However, some excruciating issues such as serious particle aggregation, large volume expansion and insufficient inherent conductivity always result in an inferior Li-storage behavior such as low reversible capacity, limited cycling life and poor high-rate performance. Rational structure design and effective combination with suitable component are crucial to achieve superior Li-storage performance. Herein, a heterostructured composite with small Cr2S3 nanosheets embedded into phenanthroline-derived N-doped porous carbon skeleton (Cr2S3/NC) has been synthesized through a calcination and sulfuration route. In as-prepared Cr2S3/NC composite, Cr2S3 nanosheets are well confined by carbon matrix, which is conductive to an effective suppression on aggregation and volume change. Meanwhile, porous conductive carbon framework supplies desirable expressway for fast electron delivery and reduces the distance for lithium ions diffusion. Consequently, the Cr2S3/NC anode delivers a remarkably elevated Li-storage performance.

γ-Graphyne adjusted diffusion–capacitance behavior of lithium titanate for boosting high-rate and wide-temperature lithium storage

Pursuing high power density and wide-temperature environmental adaptability is vital to broadening the utilization of lithium titanate (LTO) anodes in start-stop batteries of electric vehicles. Here, we introduce γ-graphyne, an emerging carbon allotrope composed of sp- and sp2-hybridized carbon, to encapsulate LTO particles via a one-pot ball milling. γ-Graphyne with a two-dimensional π-conjugated structure contains abundant electron-rich acetylene linkages in favor of fast electron conduction and, more importantly, it is a promising Li-ion host delivering a certain capacitance within the typical potential window of LTO. The optimum LTO/γ-graphyne composite shows outstanding rate capability (83 % retention of 10C compared to 0.5C), long-term cycling stability (91 % retention over 1000 cycles at 2C), and high low-temperature retentivity (102 mAh g−1 at −20 °C after 500 cycles at 1C, 63 % utilization of that at 25 °C). The impressive performance arises from adjusted diffusion–capacitance Li storage behavior with significantly reduced plateau-potential polarization and enhanced interfacial ionic migration capability. This work highlights an electrochemical active, highly conductive alkyne carbon candidate endowing full-climate applications of LTO anodes.

Quenching-induced construction of graphene-wrapped Fe3O4 composite as ultrahigh-performance Li-storage electrode

Iron oxide/graphene hybrids are outstanding electrodes with high specific capacity for secondary batteries. However, in practice there still remains challenges in alleviating the poor electrical conductivity and significant volume changes of FeOx during the lithiation/delithiation process. Herein, we describe the discovery of fabricating graphene-wrapped Fe3O4 (3D-rGO/Fe3O4) via a solution-based self-assembly process. During which, GO/Fe3O4/Na2SO4 precursor was first formed by the corrosion reaction of metallic Fe in GO aqueous solution and antisolvent self-assembly, which was then quenched into water to dissolve the Na2SO4 template and meanwhile enable the GO to shrink and encapsulate the Fe3O4 nanoparticles spontaneously, followed by a final thermal reduction. Fe3O4 nanoparticles, with a diameter of tens of nanometers, are homogeneously enwrapped into rGO framework, guaranteeing the “plane-to-plane” interconnection. The obtained 3D-rGO/Fe3O4 demonstrates unprecedented Li-storage behaviors when utilized as Li-ion battery anode. Furthermore, quantitative kinetic analysis revealed that both capacitance and diffusion mechanisms account for Li-storage behavior. This contribution not only presents a ultrahigh-performance Li-storage electrode but also shows a novel methodology to design and construct other graghene-based metallic compounds for various applications.

Precise Construction of Sn/C Composite Membrane with Graphene-Like Sn-in-Carbon Structural Units toward Hyperstable Anode for Lithium Storage.

A new-type binder-free Sn/C composite membrane with densely stacked Sn-in-carbon nanosheets was prepared by vacuum-induced self-assembly of graphene-like Sn alkoxide and following in situ thermal conversion. The successful implementation of this rational strategy is based on the controllable synthesis of graphene-like Sn alkoxide by using Na-citrate with the critical inhibitory effect on polycondensation of Sn alkoxide along the a and b directions. Density functional theory calculations reveal that graphene-like Sn alkoxide can be formed under the joint action of oriented densification along the c axis and continuous growth along the a and b directions. The Sn/C composite membrane constructed by graphene-like Sn-in-carbon nanosheets can effectively buffer volume fluctuation of inlaid Sn during cycling and much enhance the kinetics of Li+ diffusion and charge transfer with the developed ion/electron transmission paths. After temperature-controlled structure optimization, Sn/C composite membrane displays extraordinary Li storage behaviors, including reversible half-cell capacities up to 972.5 mAh g-1 at a density of 1 A g-1 for 200 cycles, 885.5/729.3 mAh g-1 over 1000 cycles at large current densities of 2/4 A g-1, and terrific practicability with reliable full-cell capacities of 789.9/582.9 mAh g-1 up to 200 cycles under 1/4 A g-1. It is worthy of noting that this strategy may open up new opportunities to fabricate advanced membrane materials and construct hyperstable self-supporting anodes in lithium ion batteries.

Ambient‐Condition Strategy for Production of Hollow Ga2O3@rGO Crystalline Nanostructures Toward Efficient Lithium Storage

Crystalline γ‐Ga2O3@rGO core–shell nanostructures are synthesized in gram scale, which are accomplished by a facile sonochemical strategy under ambient condition. They are composed of uniform γ‐Ga2O3 nanospheres encapsulated by reduced graphene oxide (rGO) nanolayers, and their formation is mainly attributed to the existed opposite zeta potential between the Ga2O3 and rGO. The as‐constructed lithium‐ion batteries (LIBs) based on as‐fabricated γ‐Ga2O3@rGO nanostructures deliver an initial discharge capacity of 1000 mAh g−1 at 100 mA g−1 and reversible capacity of 600 mAh g−1 under 500 mA g−1 after 1000 cycles, respectively, which are remarkably higher than those of pristine γ‐Ga2O3 with a much reduced lifetime of 100 cycles and much lower capacity. Ex situ XRD and XPS analyses demonstrate that the reversible LIBs storage is dominant by a conversion reaction and alloying mechanism, where the discharged product of liquid metal Ga exhibits self‐healing ability, thus preventing the destroy of electrodes. Additionally, the rGO shell could act robustly as conductive network of the electrode for significantly improved conductivity, endowing the efficient Li storage behaviors. This work might provide some insight on mass production of advanced electrode materials under mild condition for energy storage and conversion applications.

Theoretical prediction on net boroxene as a promising Li/Na-ion batteries anode.

Novel two-dimensional (2D) electrode materials have become a new frontier for mining electrode materials for Li-ion batteries (LIBs) and Na-ion batteries (NIBs). Herein, based on first-principles calculations, we present a systematic study on the Li and Na storage behaviors in Calypso-predicted completely flat 2D boron oxide (l-B2O) with large mesh pores. We start our calculations from geometrical optimization, followed by a performance evaluation of Li/Na adsorption and migration processes. Finally, the specific capacity and average open-circuit voltage are evaluated. Our study reveals that l-B2O has good electrical conductivity before and after Li/Na adsorption and the Li/Na diffusion barrier height and average open-circuit voltage are both low, which is beneficial to the rate performance and full-cell operation voltage, respectively. Furthermore, it suffers a small lattice change (<1.7%), ensuring good cycling performance. In particular, we find that the Li and Na theoretical specific capacities of l-B2O can reach up to 1068.5 mA h g-1 and 712.3 mA h g-1, respectively, which are almost 2-3 times higher than graphite (372 mA h g-1). All the above outcomes indicate that 2D l-B2O is a promising anode material for LIBs and NIBs.

DFT investigation of Li storage behavior of sceamless α-graphyne and β-graphyne nanotubes

Graphynes have great potential -as anode materials for lithium-ion batteries (LIBs) owing to their advanced structural and electronic properties. Previously, the curvature of γ-graphyne folded into nanotubes (γGyNTs) caused a significant increase in its Li storage capacity and diffusion ability. Here, using density functional theory (DFT) calculations, we report the same curvature effect in α- and β-graphyne nanotubes (αGyNTs and βGyNTs), whose maximum storage capacities are predicted to be -up to 3488 and 3070 mAh/g, respectively, similar to that of pure Li (3861 mAh/g). Such high capacities are attributed to the graphyne nanotubes having larger specific surface areas and pores than those of other carbon materials. Additionally, climbing -image- nudged-elastic- band calculations reveal that Li atoms can diffuse almost freely through acetylenic rings on the concave side of the αGyNTs, indicating an excellent charge -discharge rate ascribed to weaker adsorption on the 18-membered ring than that on the 12-membered rings of the βGyNTs and γGyNTs. Our results suggest that the αGyNTs are promising as the anode material for LIBs with exceptional overall performance. This study also provides a strategy to design anode materials based on their curvature and topological morphology.

Effective transportation of electrons/Li ions in V2O3 nanoparticle/carbon-coated stainless steel composite electrodes for lithium-ion storage

The performance of V2O3 is threatened as a result of relatively sluggish electrons/ion transportation and structural pulverization in lithium-ion batteries. The rational design of the electrode structure to exhibit great lithium storage of V2O3 is crucial. In this work, we report stable composite electrodes by integrating V2O3 nanoparticles with an uneven carbon-coated stainless-steel oxide (V2O3/C/SS) current collector via an in-situ growth method. They allow fast electrical conductivity through the carbon coating and reduction strategy. Meanwhile, the small nanoparticle size of V2O3 on V2O3/C/SS as well as the undestroyed 3D V–V skeleton are helpful to Li-ion diffusion. The weak interaction between Li ions and V2O3 achieves fast and high Li storage behavior, which proves by electrochemical kinetics analysis and DFT calculation. The electrode delivers high rate capacitive and long cycling stability with 564 mAh g−1 and 310 mAh g−1 at 100 mA g−1 and 1 A g−1, respectively, and a specific capacity of 118 mAh g−1 is achieved after 1000 cycles at 5 A g−1. The excellent electrochemical performance may be ascribed to the unique design for effective electron and Li-ion transportation. This work offers a new prospect for the development of high-performance binder-free V2O3-based electrodes.

Mechanistic and nanoarchitectonics insight into Li–host interactions in carbon hosts for reversible Li metal storage

Li metal has been regarded as a promising anode for rechargeable batteries with high energy densities. However, the growth of Li dendrites and severe volume changes in the Li anode still hinder its practical use. Three-dimensional (3D) host structures have recently attracted significant attention as an effective strategy to resolving these problems. Herein, we demonstrate reversible Li metal storage in carbon hosts with strong Li–host interactions derived from metal-organic frameworks (MOFs). The combined experimental and computational modeling studies reveal that galvanically displaced Ag enhances Li–host interactions and the spatial distribution characteristics of Ag play a crucial role in controlling Li storage behavior and reversibility. The atomic Ag clusters trigger the outward growth of Li from the internal pores of the host and enables stable battery cycling, whereas the surface-anchored Ag nanoparticles induce uneven Li plating on the outer surface of the carbon host, resulting in a rapid performance drop. This work provides new insights into the development of advanced host materials for reversible Li anodes by utilizing strong Li–host interactions.

Superior Li-storage behaviors of single crystal for Zr-substituted LiNi0.815Co0.15Al0.035O2 as cathode material for Li-ion battery

Superior Li-storage behaviors of single crystal for Zr-substituted LiNi0.815Co0.15Al0.035O2 as cathode material for Li-ion battery

Revealing the size-dependent electrochemical Li-storage behaviors of SiO-based anodes

The size-dependent electrochemical behaviors in terms of the irreversibility/reversibility of electrochemical lithiation/delithiation, interfacial/bulk stability and the interplays between SiO and graphite particles are investgated.

Engineering the morphology/porosity of oxygen-doped carbon for sulfur host as lithium-sulfur batteries

Utilizing the oxygen-doping carbon, rational tailoring of morphology and pores were designed for Li-S battery, resulting in the negative charge surface carbon matrix with fascinating pore structure. Carbon with mesopore and big zeta potential was constructed to improve lithium-storage capabilities the restrain of soluble polysulfide and the kinetic during solid-state. Despite the intriguing merits of lithium-sulfur (Li-S) systems, they still suffer from the notorious “shuttling-effect” of polysulfides. Herein, carbon materials with rational tailoring of morphology and pores were designed for strong loading/adsorption with the controlling of energy-storage ability. Through rational tailoring, it is strongly verified that such engineering of evolutions result in variational of sulfur immobilization in the obtained carbon. As expected, the targeted sample delivers a stable capacity of 925 mAh g −1 after 100 loops. Supporting by the “cutting-off” manners, it is disclosed that mesopores in carbon possess more fascinated traits than micro/macropores in improving the utilization of sulfur and restraining Li 2 S x (4 ≤ x ≤ 8). Moreover, the long-chain polysulfide could be further consolidated by auto-doping oxygen groups. Supported by in-depth kinetic analysis, it is confirmed that the kinetics of ion/e - transfer during charging and discharging could be accelerated by mesopores, especially in stages of the formation of solid S 8 and Li 2 S, further improving the capacity of ion-storage in Li-S battery. Given this, the elaborate study provide significant insights into the effect of pore structure on kinetic performance about Li-storage behaviors in Li-S battery, and give guidance for improving sulfur immobilization.

Interfacial Design on Graphene-Hematite Heterostructures for Enhancing Adsorption and Diffusion towards Superior Lithium Storage.

Hematite (α-Fe2O3) is a promising electrode material for cost-effective lithium-ion batteries (LIBs), and the coupling with graphene to form Gr/α-Fe2O3 heterostructures can make full use of the merits of each individual component, thus promoting the lithium storage properties. However, the influences of the termination of α-Fe2O3 on the interfacial structure and electrochemical performance have rarely studied. In this work, three typical Gr/α-Fe2O3 interfacial systems, namely, single Fe-terminated (Fe-O3-Fe-R), double Fe-terminated (Fe-Fe-O3-R), and O-terminated (O3-Fe-Fe-R) structures, were fully investigated through first-principle calculation. The results demonstrated that the Gr/Fe-O3-Fe-R system possessed good structural stability, high adsorption ability, low volume expansion, as well as a minor diffusion barrier along the interface. Meanwhile, investigations on active heteroatoms (e.g., B, N, O, S, and P) used to modify Gr were further conducted to critically analyze interfacial structure and Li storage behavior. It was demonstrated that structural stability and interfacial capability were promoted. Furthermore, N-doped Gr/Fe-O3-Fe-R changed the diffusion pathway and made it easy to achieve free diffusion for the Li atom and to shorten the diffusion pathway.

Polymorphic Consideration in Conversion Reaction-Based MnO2 Anode Materials for Next-Generation Li Rechargeable Batteries

Beginning with the commercialization by Sony in the 1990s, Li-ion batteries (LIBs) have been used in many electronic devices. In commercialized LIBs, insertion reaction-based electrode materials, such as LiCoO2, LiFePO4, LiMn2O4 for the cathode and graphite, Li4Ti5O12, TiO2 for the anode, have been dominantly used as active materials. In these materials, Li-ions can reversibly be stored and extracted at the specific interstitial site in the crystal without severely collapsing the initial crystal structure. As so, the crystallographic phase significantly affects the electrochemical performances because the local environments around the interstitial site are all different among these phases. For example, LiFePO4 has stable FePO4 framework even after most Li-ions have been extracted, although some Li-ions confined in the tunnel block the migration paths due to the one-dimensional tunnel inside the crystal. In contrast, Li-ions can move easier in LiCoO2 when comparing with LiFePO4 even if the Li-ions are trapped in the Li slab, but over lithiation results in the collapse of the layered structure leading to capacity fading. Even in the same composition, the electrochemical Li storage behavior varies much by the polymorphs. For instance, the layered LiCoO2 exhibit larger reversible capacity and higher redox potential compared to spinel structured Li2Co2O4 [1], and in case of TiO2, despite the similar amount of Li-ion storage, in detail, the voltage profiles are different in anatase, rutile and brookite TiO2 [2].Since the demand for high capacity energy storage devices is continuously growing, there have been extensive researches on high energy density electrode materials based on alloying and conversion reaction due to the limited capacity of insertion-based electrode materials. Manganese dioxide (MnO2) has been considered as a potential candidate due to its low cost, environmentally benignity, and especially large theoretical capacity of ~1233 mAh g-1. Accordingly, various types of polymorphs in MnO2 (α, β, γ, δ, ε, λ, etc.) have been adopted as the anode materials in LIBs. However, unlike the insertion-based electrode, the effect of crystal structure in electrochemical conversion reaction was not clearly identified because it is hard to identify the detailed phase transition process as the initial crystalline structures transformed into the amorphous state after lithiation. Recently, some researchers compared the Li storage performance of different polymorphs of MnO2 [3,4], but the morphologies of the materials are quite different or the reversible capacities are far below the theoretical value based on the conversion reaction, which could not sufficiently explain the polymorphic effects in conversion reaction. In this work, we have prepared 4 different polymorphs of MnO2 (α, β, γ, and λ) in similar hollow urchin-like morphology and compared the Li storage behaviors to discover the influence of polymorphs in conversion-based electrode materials. Among those 4 polymorphs, λ-MnO2 exhibits the highest reversible capacity of ~1200 mAh g-1 and the fastest reaction kinetics. To demonstrate the structural advantages of λ-MnO2, we observed structural changes during electrochemical reactions using synchrotron-based X-ray absorption spectroscopy and X-ray crystallography. As a result, we discovered that the overall Li storage behaviors in the 4 different polymorphs are nearly the same. However, the structural similarity between the initial polymorphs and the final polymorph after reversible conversion reaction affects the electrochemical performance. A more detailed discussion will be presented at the time of the meeting.

Highly Mesoporous Carbon Derived from Silica-Embedded Zeolitic Imidazolate Framework As a Potential Reservoir of Metallic Li

During the last decade, lithium ion batteries (LIBs) have shown great promise as a potential power source for various applications such as portable electronic devices and electric vehicles (EVs) [1]. Responding to the growing demand for high-energy LIBs, it is crucial to develop advanced materials which provides a higher energy density than commercial materials currently available. Li metal has long been considered as a future anode material for LIBs because of its extremely high theoretical capacity (~3,860mAh/g) and low potential for electrochemical reactions (-3.04V vs. standard hydrogen electrode). Despite these advantages, the practical use of Li metal is hindered by several technical issues as follow; i) unavoidable dendritic growth of Li and ii) infinite volume changes during cycling. Such issues are still regarded as main drawbacks of Li metal causing rapid performance fading and safety hazards [2]. Recently, the feasibility of various porous materials with a well-defined pore structure has been examined as potential reservoirs for Li storage to suppress unfavorable dendritic growth as well as volume changes of Li metal [3-4]. Herein, we design a highly mesoporous carbon (MC) as a potential Li storage material, based on the dual-phase reaction mechanism (i.e. lithiation and metallization). It can be synthesized via a direct carbonization of silica-embedded zeolitic imidazolate frameworks (SiO2@ZIFs) combined with a chemical etching process. Colloidal silica nanoparticles (~20nm) are employed as a mesopore former during the synthesis of ZIFs and then completely removed by a hydrofluoric acid. The formation of mesopores is effective for accommodating a large amount of metallic Li, showing a stable cycle performance over 100 cycles. It is confirmed that Li can be reversibly stored in the structure without significant dendritic growth of Li and volume changes based on various electrochemical and structural analyses. Furthermore, we thoroughly investigate a correlation between pore structure and Li storage behavior of MC as the next generation anode for high-energy LIBs.

Water-soluble salt-templated strategy to regulate mesoporous nanosheets-on-network structure with active mixed-phase CoO/Co3O4 nanosheets on graphene for superior lithium storage

Facile construction of high-energy Li-ion batteries (LIBs) anode materials with robust heterostructure remains great challenges, especially for the fine integration of active materials with conductive carbon. Here, vertically aligned mixed-phase CoO/Co3O4 nanosheets anchored on wrinkled graphene (CoO/Co3O4/Gra) are controllably synthesized with the assistance of water-removable NaCl-templates through facile freeze-drying and calcination processes. In-depth investigations disclose that the wrinkled graphene induces the bonding with nucleating precursors, and the nanosheets vertically grow along the crystal planes of multilevel-NaCl. The as-formed mesoporous nanosheets-on-network structure endows the hybrids with efficient exposure of mixed-phase CoO/Co3O4 active sites and rapid charge-transfer channels. And the multiple composites can integrate the advantages of their long complements and fully exert their synergistic effects to solve the problems of the single metal oxide electrode. Furthermore, the electrochemical performance of hierarchically mesoporous architecture is beyond that of the single individual structure on account of the sufficient electrode/electrolyte interfaces, rapid Li+ diffusion and electron conduction, and stable structural integrity upon cycling. Accordingly, the capacitive kinetic of Li-storage behaviors is greatly enhanced by these synergistic effects. Serving as a LIBs anode material, the mesoporous CoO/Co3O4/Gra-4 hybrid yields superior reversible capacity (1390 mAh g−1 at 0.1 A g−1), high rate performance (1130 mAh g−1 at 0.2 A g−1), and impressive cycling stability (811 mAh g−1 after 500 cycles at 1 A g−1). This work may inspire the design of high-performance electrodes with advanced 3D hierarchical structures.

Polymorphic Consideration in Conversion Reaction-Based Anode Materials for Next-Generation Li Rechargeable Batteries

Beginning with the commercialization by Sony in the 1990s, Li-ion batteries (LIBs) have been used in many electronic devices. In commercialized LIBs, insertion reaction-based electrode materials, such as LiCoO2, LiFePO4, LiMn2O4 for the cathode and graphite, Li4Ti5O12, TiO2 for the anode, have been dominantly used as active materials. In these materials, Li-ions can reversibly be stored and extracted at the specific interstitial site in the crystal without severely collapsing the initial crystal structure. As so, the crystallographic phase significantly affects the electrochemical performances because the local environments around the interstitial site are all different among these phases. For example, LiFePO4 has stable FePO4 framework even after most Li-ions have been extracted, although some Li-ions confined in the tunnel block the migration paths due to the one-dimensional tunnel inside the crystal. In contrast, Li-ions can move easier in LiCoO2 when comparing with LiFePO4 even if the Li-ions are trapped in the Li slab, but over lithiation results in the collapse of the layered structure leading to capacity fading. Even in the same composition, the electrochemical Li storage behavior varies much by the polymorphs. For instance, the layered LiCoO2 exhibit larger reversible capacity and higher redox potential compared to spinel structured Li2Co2O4 [1], and in case of TiO2, despite the similar amount of Li-ion storage, in detail, the voltage profiles are different in anatase, rutile and brookite TiO2 [2].Since the demand for high capacity energy storage devices is continuously growing, there have been extensive researches on high energy density electrode materials based on alloying and conversion reaction due to the limited capacity of insertion-based electrode materials. Manganese dioxide (MnO2) has been considered as a potential candidate due to its low cost, environmentally benignity and especially large theoretical capacity of ~1233 mAh g-1. Accordingly, various types of polymorphs in MnO2 (α, β, γ, δ, ε, λ, etc.) have been adopted as the anode materials in LIBs. However, unlike the insertion-based electrode, the effect of crystal structure in electrochemical conversion reaction was note clearly identified because it is hard to identify the detailed phase transition process as the initial crystalline structures transformed to the amorphous state after lithiation. Recently, some researchers compared the Li storage performance of different polymorphs of MnO2 [3,4], but the morphology of the materials are quite different or the reversible capacities are far below the theoretical value based on the conversion reaction, which could not sufficiently explain the polymorphic effects in conversion reaction. In this work, we have prepared 4 different polymorphs of MnO2 (α, β, Ramsdellite, and λ) in similar hollow urchin-like morphology and compared the Li storage behaviors to discover the influence of polymorphs in conversion-based electrode materials. Among those 4 polymorphs, λ-MnO2 exhibits the highest reversible capacity of ~1200 mAh g-1 and the fastest reaction kinetics. To demonstrate the structural advantages of λ-MnO2, we observed structural changes during electrochemical reactions using synchrotron-based X-ray absorption spectroscopy and X-ray crystallography. As a result, we discovered that the overall Li storage behaviors in the 4 different polymorphs are nearly the same, however, the structural similarity between the initial polymorphs and the final polymorph after reversible conversion reaction affects the electrochemical performance. More detailed discussion will be presented at the time of meeting.