Local structure of layered oxide electrode materials for lithium-ion batteries

Formulating Local Environment of Oxygen Mitigates Voltage Hysteresis in Li-Rich Materials

Li-rich cathode materials have emerged as one of the most prospective options for Li-ion batteries owing to their remarkable energy density (>900 Wh kg-1). However, voltage hysteresis during charge and discharge process lowers the energy conversion efficiency, which hinders their application in practical devices. Herein, the fundamental reason for voltage hysteresis through investigating the O redox behavior under different (de)lithiation states is unveiled and it is successfully addressed by formulating the local environment of O2-. In Li-rich Mn-based materials, it is confirmed that there exists reaction activity of oxygen ions at low discharge voltage (<3.6 V) in the presence of TM-TM-Li ordered arrangement, generating massive amount of voltage hysteresis and resulting in a decreased energy efficiency (80.95%). Moreover, in the case where Li 2b sites are numerously occupied by TM ions, the local environment of O2- evolves, the reactivity of oxygen ions at low voltage is significantly inhibited, thus giving rise to the large energy conversion efficiency (89.07%). This study reveals the structure-activity relationship between the local environment around O2- and voltage hysteresis, which provides guidance in designing next-generation high-performance cathode materials.

On the Elusive Crystallography of Lithium-Rich Layered Oxides: Novel Structural Models

Lithium-rich layered oxides (LRLOs) are one of the most attractive families among future positive electrode materials for the so-called fourth generation of lithium-ion batteries (LIBs). Their electrochemical performance is enabled by the unique ambiguous crystal structure that is still not well understood despite decades of research. In the literature, a clear structural model able to describe their crystallographic features is missing thereby hindering a clear rationalization of the interplay between synthesis, structure, and functional properties. Here, the structure of a specific LRLO, Li1.28 Mn0.54 Ni0.13 Co0.02 Al0.03 O2 , using synchrotron X-ray diffraction (XRD), neutron diffraction (ND), and High-Resolution Transmission Electron Microscopy (HR-TEM), is analyzed. A systematic approach is applied to model diffraction patterns of Li1.28 Mn0.54 Ni0.13 Co0.02 Al0.03 O2 by using the Rietveld refinement method considering the R3 ¯ $\bar{3}$ m and C2/m unit cells as the prototype structures. Here, the relative ability of a variety of structural models is compared to match the experimental diffraction pattern evaluating the impact of defects and supercells derived from the R3 ¯ $\bar{3}$ m structure. To summarize, two possible models able to reconcile the description of experimental data are proposed here for the structure of Li1.28 Mn0.54 Ni0.13 Co0.02 Al0.03 O2 : namely a monoclinic C2/m defective lattice (prototype Li2 MnO3 ) and a monoclinic defective supercell derived from the rhombohedral R3 ¯ $\bar{3}$ m unit cell (prototype LiCoO2 ).

Stacking Faults Inducing Oxygen Anion Activities in Li2 MnO3

Controllable anionic redox for a transformational increase in the energy density is the pursuit of next generation Li-ion battery cathode materials. Its activation mechanism is coupled with the local coordination environment around O, which posts experimental challenges for control. Here, the tuning capability of anionic redox is shown by varying O local environment via experimentally controlling the density of stacking faults in Li₂ MnO₃ , the parent compound of Li-rich oxides. By combining computational analysis and spectroscopic study, it is quantitatively revealed that more stacking faults can trigger smaller LiOLi bond angles and larger LiO bond distance in local Li-rich environments and subsequently activate oxygen redox reactivity, which in turn enhances the reactivity of Mn upon the following reduction process. This study highlights the critical role of local structure environment in tuning the anionic reactivity, which provides guidance in designing high-capacity layered cathodes by appropriately adjusting stacking faults.

On the disparity in reporting Li-rich layered oxide cathode materials

Lithium-rich layered oxides are considered one of the most promising cathode materials for next generation lithium-ion batteries due to their extraordinary specific capacity of over 280 mA h g^-1 and superior energy density of over 1000 W h kg^-1. Despite the excellent performance, LRLOs still suffer from low Coulombic efficiency, serious capacity/voltage decay upon cycling, voltage hysteresis, short lifespan, and poor rate capability. Driven by the thirst for high-energy-density battery technologies, various strategies have been developed to address these issues with great progress being achieved in the past several years. However, the emerging disparity among the published results severely precludes meaningful comparisons between different LRLOs and material modification strategies, which has become an impediment to the development and commercialization of LRLOs. Although the significance of standardization has been recognized in the battery community, the standardization of LRLOs is worth particular attention due to their complicated compositions and unique electrochemical properties. This perspective analyzes the underlying parameters that can cause varied and even controversial results observed in LRLOs, from the synthesis procedure to the electrochemical evaluation procedure, followed by preliminary suggestions for the standard protocols of chemical compositions, synthesis pathways, calcination conditions, electrode preparation, battery fabrication, and battery testing. Hopefully, this perspective can help build a reliable baseline for LRLO research, thus aligning the huge research effort toward the practical applications of LRLOs.

Understanding and Control of Activation Process of Lithium-Rich Cathode Materials

Lithium-rich materials (LRMs) are among the most promising cathode materials toward next-generation Li-ion batteries due to their extraordinary specific capacity of over 250 mAh g−1 and high energy density of over 1 000 Wh kg−1. The superior capacity of LRMs originates from the activation process of the key active component Li2MnO3. This process can trigger reversible oxygen redox, providing extra charge for more Li-ion extraction. However, such an activation process is kinetically slow with complex phase transformations. To address these issues, tremendous effort has been made to explore the mechanism and origin of activation, yet there are still many controversies. Despite considerable strategies that have been proposed to improve the performance of LRMs, in-depth understanding of the relationship between the LRMs’ preparation and their activation process is limited. To inspire further research on LRMs, this article firstly systematically reviews the progress in mechanism studies and performance improving attempts. Then, guidelines for activation controlling strategies, including composition adjustment, elemental substitution and chemical treatment, are provided for the future design of Li-rich cathode materials. Based on these investigations, recommendations on Li-rich materials with precisely controlled Mn/Ni/Co composition, multi-elemental substitution and oxygen vacancy engineering are proposed for designing high-performance Li-rich cathode materials with fast and stable activation processes.

Graphical abstract

The “Troika” of composition adjustment, elemental substitution, and chemical treatment can drive the Li-rich cathode towards stabilized and accelerated activation.

Synthesis-structure relationships in Li- and Mn-rich layered oxides: phase evolution, superstructure ordering and stacking faults

Li- and Mn-rich layered oxides are promising positive electrode materials for future Li-ion batteries. The presence of crystallographic features such as cation-mixing and stacking faults in these compounds make them highly susceptible to synthesis-induced structural changes. Consequently, significant variations exist in the reported structure of these compounds that complicate the understanding of how the crystallographic structure influences its properties. This work investigates the synthesis-structure relations for three widely investigated Li- and Mn-rich layered oxides: Li₂MnO₃, Li_1.2Mn_0.6Ni_0.2O₂ and Li_1.2Mn_0.54Ni_0.13Co_0.13O₂. For each compound, the average structure is compared between two synthetic routes of differing degrees of precursor mixing and four annealing protocols. Furthermore, thermodynamic and synthesis-specific kinetic factors governing the equilibrium crystallography of each composition are considered. It was found that the structures of these compounds are thermodynamically metastable under the synthesis conditions employed. In addition to a driving force to reduce stacking faults in the structure, these compositions also exhibited a tendency to undergo structural transformations to more stable phases under more intense annealing conditions. Increasing the compositional complexity introduced a kinetic barrier to structural ordering, making Li_1.2Mn_0.6Ni_0.2O₂ and Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ generally more faulted relative to Li₂MnO₃. Additionally, domains with different degrees of faulting were found to co-exist in the compounds. This study offers insight into the highly synthesis-dependent subtle structural complexities present in these compounds and complements the substantial efforts that have been undertaken to understand and optimise its electrochemical properties.

Impact of Stacking Faults and Li Substitution in LixMnO3 (0 ≤ x ≤ 2) Structural Transformations upon Delithiation

Lithium-rich layered oxides appear in most roadmaps as next generation Li-ion cathode materials owing to their superior capacity. Within this family, Li₂MnO₃ represents the archetype material and is often taken as model compound to better understand the complex structural modifications occurring in the first charging cycle. In this work, density functional theory (DFT) calculations have been used to understand the impact of stacking faults in the structural transformations occurring in Li₂MnO₃ upon delithiation, which are found to hinder the phase transformations leading to structural degradation. The formation energies of both ideal and defective Li_xMnO₃ compositions and the analysis of the encountered ground states have been used to rationalize the predicted differences in terms of structural evolution. From the understanding of the origin in the O1 phase transformation, Mg substitution is proposed as alternative strategy to improve the structural stability in this family of materials.

Reaction Mechanisms of Layered Lithium-Rich Cathode Materials for High-Energy Lithium-Ion Batteries

Layered lithium-rich cathode materials have attracted extensive interest owing to their high theoretical specific capacity (320-350 mA h g^-1 ). However, poor cycling stability and sluggish reaction kinetics inhibit their practical applications. After many years of quiescence, interest in layered lithium-rich cathode materials is expected to revive in answer to our increasing dependence on high-energy-density lithium-ion batteries. Herein, we review recent research progress and in-depth descriptions of the structure characterization and reaction mechanisms of layered lithium-rich manganese-based cathode materials. In particular, we comprehensively summarize the proposed reaction mechanisms of both the cationic redox reaction of transition-metal ions and the anionic redox reaction of oxygen species. Finally, we discuss opportunities and challenges facing the future development of lithium-rich cathode materials for next-generation lithium-ion batteries.

Influence of Synthesis Routes on the Crystallography, Morphology, and Electrochemistry of Li2MnO3

With the potential of delivering reversible capacities of up to 300 mAh/g, Li-rich transition-metal oxides hold great promise as cathode materials for future Li-ion batteries. However, a cohesive synthesis-structure-electrochemistry relationship is still lacking for these materials, which impedes progress in the field. This work investigates how and why different synthesis routes, specifically solid-state and modified Pechini sol-gel methods, affect the properties of Li₂MnO₃, a compositionally simple member of this material system. Through a comprehensive investigation of the synthesis mechanism along with crystallographic, morphological, and electrochemical characterization, the effects of different synthesis routes were found to predominantly influence the degree of stacking faults and particle morphology. That is, the modified Pechini method produced isotropic spherical particles with approximately 57% faulting and the solid-state samples possessed heterogeneous morphology with approximately 43% faulting probability. Inevitably, these differences lead to variations in electrochemical performance. This study accentuates the importance of understanding how synthesis affects the electrochemistry of these materials, which is critical considering the crystallographic and electrochemical complexities of the class of materials more generally. The methodology employed here is extendable to studying synthesis-property relationships of other compositionally complex Li-rich layered oxide systems.

High-Performance Lithium-Rich Layered Oxide Material: Effects of Preparation Methods on Microstructure and Electrochemical Properties

Lithium-rich layered oxide is one of the most promising candidates for the next-generation cathode materials of high-energy-density lithium ion batteries because of its high discharge capacity. However, it has the disadvantages of uneven composition, voltage decay, and poor rate capacity, which are closely related to the preparation method. Here, 0.5Li₂MnO₃·0.5LiMn_0.8Ni_0.1Co_0.1O₂ was successfully prepared by sol–gel and oxalate co-precipitation methods. A systematic analysis of the materials shows that the 0.5Li₂MnO₃·0.5LiMn_0.8Ni_0.1Co_0.1O₂ prepared by the oxalic acid co-precipitation method had the most stable layered structure and the best electrochemical performance. The initial discharge specific capacity was 261.6 mAh·g⁻¹ at 0.05 C, and the discharge specific capacity was 138 mAh·g⁻¹ at 5 C. The voltage decay was only 210 mV, and the capacity retention was 94.2% after 100 cycles at 1 C. The suppression of voltage decay can be attributed to the high nickel content and uniform element distribution. In addition, tightly packed porous spheres help to reduce lithium ion diffusion energy and improve the stability of the layered structure, thereby improving cycle stability and rate capacity. This conclusion provides a reference for designing high-energy-density lithium-ion batteries.

Metastability and Reversibility of Anionic Redox-Based Cathode for High-Energy Rechargeable Batteries

Great focus has recently been placed on anionic redox, to which high capacities of Li-rich layered oxides are attributed. With almost doubled capacity compared with state-of-the-art cathode materials, Li-rich layered oxides still fall short in other performance metrics. Among these, voltage decay upon cycling remains the most hindering obstacle, in which defect electrochemistry plays a critical role. Here, we reveal that the metastable state of cycled Li-rich layered oxide, which stems from structural defects in different dimensions, is responsible for the voltage decay. More importantly, through mild thermal energy, the metastable state can be driven to a stable state, bringing about structural and voltage recovery. However, for the classic layered oxide without reversible anionic redox, thermal energy can only introduce cation disordering, leading to performance deterioration. These insights elucidate that understanding the structure metastability and reversibility is essential for implementing design strategies to improve cycling stability for high-capacity layered oxides.

A promising Mo-based lithium-rich phase for Li-ion batteries

Herein, we demonstrate a composite Mo-based lithium-rich Li₂MoO₄·LiNi_0.4Mn_0.4Co_0.2O₂ material, which exhibits a higher practical capacity of 270 mA h g⁻¹, and better capacity retention (61% after 50 cycles) when compared with the well-known Li₂MnO₃.

A Mo-based composite material was demonstrated as a cathode for Li-ion batteries.

Understanding voltage decay in lithium-excess layered cathode materials through oxygen-centred structural arrangement

Lithium-excess 3d-transition-metal layered oxides (Li_1+xNi_yCo_zMn_{1−x−y−z}O₂, >250 mAh g⁻¹) suffer from severe voltage decay upon cycling, which decreases energy density and hinders further research and development. Nevertheless, the lack of understanding on chemical and structural uniqueness of the material prevents the interpretation of internal degradation chemistry. Here, we discover a fundamental reason of the voltage decay phenomenon by comparing ordered and cation-disordered materials with a combination of X-ray absorption spectroscopy and transmission electron microscopy studies. The cation arrangement determines the transition metal-oxygen covalency and structural reversibility related to voltage decay. The identification of structural arrangement with de-lithiated oxygen-centred octahedron and interactions between octahedrons affecting the oxygen stability and transition metal mobility of layered oxide provides the insight into the degradation chemistry of cathode materials and a way to develop high-energy density electrodes.

There is growing interest in the fundamental understanding of the voltage decay mechanism in Li-excess layered cathode materials. Here, the authors report a multilateral and macroscopic analysis that considers interaction between oxygen and atomic arrangement of Li_1+xNi_yCo_zMn_{1−x−y−z}O₂.

Enhanced electrochemical performance of Li-rich cathode materials through microstructural control

Structural defects are used as a design opportunity to prepare better battery materials: limiting capacity and voltage fadings in Li₂MnO₃.

Anionic Redox in Rechargeable Lithium Batteries

The extraordinarily high capacities delivered by lithium-rich oxide cathodes, compared with conventional layered oxide electrodes, are a result of contributions from both cationic and anionic redox processes. This phenomenon has invoked a lot of research exploring new kinds of lithium-rich oxides with multiple-electron redox processes. Though proposed many years ago, anionic redox is now regarded to be crucial in further developing high-capacity electrodes. A basic overview of the previous work on anionic redox is given, and issues related to electronic and geometric structures are discussed, including the principles of activation, reversibility, and the energy barrier of anionic redox. Anionic redox also leads to capacity loss and structural degradation, as well as voltage hysteresis, which shows the importance of controlling anionic redox reactions. Finally, the techniques used for characterizing anionic redox processes are reviewed to aid the rational choice of techniques in future studies. Important perspectives are highlighted, which should instruct future work concerning anionic redox processes.

Operando EPR for Simultaneous Monitoring of Anionic and Cationic Redox Processes in Li-Rich Metal Oxide Cathodes

Anionic redox chemistry offers a transformative approach for significantly increasing specific energy capacities of cathodes for rechargeable Li-ion batteries. This study employs operando electron paramagnetic resonance (EPR) to simultaneously monitor the evolution of both transition metal and oxygen redox reactions, as well as their intertwined couplings in Li₂MnO₃, Li_1.2Ni_0.2Mn_0.6O₂, and Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ cathodes. Reversible O^2-/O₂^n- redox takes place above 3.0 V, which is clearly distinguished from transition metal redox in the operando EPR on Li₂MnO₃ cathodes. O^2-/O₂^n- redox is also observed in Li_1.2Ni_0.2Mn_0.6O₂, and Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ cathodes, albeit its overlapping potential ranges with Ni redox. This study further reveals the stabilization of the reversible O redox by Mn and e^- hole delocalization within the Mn-O complex. The interactions within the cation-anion pairs are essential for preventing O₂^n- from recombination into gaseous O₂ and prove to activate Mn for its increasing participation in redox reactions. Operando EPR helps to establish a fundamental understanding of reversible anionic redox chemistry. The gained insights will support the search for structural factors that promote desirable O redox reactions.

High-voltage positive electrode materials for lithium-ion batteries

The ever-growing demand for advanced rechargeable lithium-ion batteries in portable electronics and electric vehicles has spurred intensive research efforts on high-voltage positive electrode materials over the past decade.

One-Pot Synthesis of Lithium-Rich Cathode Material with Hierarchical Morphology

Lithium-rich transition metal oxides, Li_1+xTM_1-xO₂ (TM, transition metal), have attracted much attention as potential candidate cathode materials for next generation lithium ion batteries because their high theoretical capacity. Here we present the synthesis of Li[Li_0.2Ni_0.2Mn_0.6]O₂ using a facile one-pot resorcinol-formaldehyde method. Structural characterization indicates that the material adopts a hierarchical porous morphology consisting of uniformly distributed small pores and disordered large pore structures. The material exhibits excellent electrochemical cycling stability and a good retention of capacity at high rates. The material has been shown to be both advantageous in terms of gravimetric and volumetric capacities over state of the art commercial cathode materials.

Nano-Crystalline Li1.2Mn0.6Ni0.2O₂ Prepared via Amorphous Complex Precursor and Its Electrochemical Performances as Cathode Material for Lithium-Ion Batteries

An amorphous complex precursor with uniform Mn/Ni cation distribution is attempted for preparing a nano-structured layered Li-rich oxide (Li_1.2Mn_0.6Ni_0.2O₂)cathode material, using diethylenetriaminepentaacetic acid (DTPA) as a chelating agent. The materials are characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrochemical tests. The crystal structure of Li-rich materials is found to be closely related to synthesis temperature. As-obtained nano materials sintered at 850 °C for 10 h show an average size of 200 nm with a single crystal phase and good crystallinity. At a current density of 20 mA·g⁻¹, the specific discharge capacity reaches 221 mAh·g⁻¹ for the first cycle and the capacity retention is 81% over 50 cycles. Even at a current density of 1000 mA·g⁻¹, the capacity is as high as 118 mAh·g⁻¹. The enhanced rate capability can be ascribed to the nano-sized morphology and good crystal structure.

Improved electrochemical performances of layered lithium rich oxide 0.6Li[Li1/3Mn2/3]O2·0.4LiMn5/12Ni5/12Co1/6O2 by Zr doping

HRTEM patterns of undoped and Zr4% doped samples, (0a) x = 0 before cycle, (0a1) x = 0 after 101 cycles, (4a) x = 4% before cycle, (4a1) x = 4% after 101 cycles.

Unravelling structural ambiguities in lithium- and manganese-rich transition metal oxides

Although Li- and Mn-rich transition metal oxides have been extensively studied as high-capacity cathode materials for Li-ion batteries, the crystal structure of these materials in their pristine state is not yet fully understood. Here we apply complementary electron microscopy and spectroscopy techniques at multi-length scale on well-formed Li1.2(Ni0.13Mn0.54Co0.13)O2 crystals with two different morphologies as well as two commercially available materials with similar compositions, and unambiguously describe the structural make-up of these samples. Systematically observing the entire primary particles along multiple zone axes reveals that they are consistently made up of a single phase, save for rare localized defects and a thin surface layer on certain crystallographic facets. More specifically, we show the bulk of the oxides can be described as an aperiodic crystal consisting of randomly stacked domains that correspond to three variants of monoclinic structure, while the surface is composed of a Co- and/or Ni-rich spinel with antisite defects.

Hexagonal BC3: A Robust Electrode Material for Li, Na, and K Ion Batteries

We have investigated the stability, maximum intercalation capacity, and voltage profile of alkali metal intercalated hexagonal BC3 (MxBC3), for 0 < x ≤ 2 and M = Li, Na, and K. Our calculations, based on dispersion-corrected density functional theory, show that these intercalation compounds are stable with respect to BC3 and their bulk metal counterparts. Moreover, we found that among all MxBC3 considered, the maximum stable capacity corresponds to an x value of 1.5, 1, and 1.5 for Li, Na, and K, respectively. These values are associated with large gravimetric capacities of 572 mA h/g for Na and 858 mA h/g for Li and K. Importantly, we show that metal intercalated hexagonal BC3 has the advantage of a small open-circuit voltage variation of approximately 0.49, 0.12, and 0.16 V for Li, Na, and K, respectively. Our results suggest that BC3 can become a robust alternative to graphitic electrodes in metal ion batteries, thus encouraging further experimental work.

New Insights into Improving Rate Performance of Lithium-Rich Cathode Material

A Sn doping strategy is demonstrated to improve lithium diffusion kinetics by expanding the spacing of (003) planes without destroying the layered structure. A large (003) spacing will significantly decrease the energy barrier associated with lithium diffusion. Besides better rate capability, the Sn-doped material exhibits an unexpected much improved capacity above 3.0 V (vs Li(+) /Li), which is highly desired in future applications.

Surface-orientation-dependent distribution of subsurface cation-exchange defects in olivine-phosphate nanocrystals

Atomic-scale exchange between two different cations of similar size in crystalline oxides is one of the major types of point defects when multiple cations in oxygen interstitials are arrayed in an ordered manner. Although a number of studies have been performed on a variety of Li-intercalation olivine phosphates to determine the distribution of exchange defects in bulk, understanding of the thermodynamic stability of the defects in subsurface regions and its dependency on the crystallographic orientation at the surface has remained elusive. Through a combination of small-angle neutron scattering, atomic-scale direct probing with scanning transmission electron microscopy, and theoretical ab initio calculations, we directly demonstrate that the antisite exchange defects are distributed in a highly anisotropic manner near the surfaces of LiFePO4 crystals. Moreover, a substantial amount of cation exchanges between Li and Fe sites is identified as an energetically favorable configuration in some surface regions, showing excellent agreement with the calculation results of negative defect formation energies. The findings in this study provide insight into developing better ways to avoid degradation of lithium mobility through the surface as well as scientifically notable features regarding the distribution of exchange defects in olivine phosphates.

Electrochemical properties of Li2MnO3 nanocrystals synthesized using a hydrothermal method

Li₂MnO₃ nanocrystals are synthesized with controlled size and morphology via a hydrothermal method with the oxidant KMnO₄, controlling the amount of KMnO₄ leads to a decrease of grain size from 28.7 nm to 9.8 nm and increased discharge capacities in a linear relationship.

LaF3-coated Li[Li0.2Mn0.56Ni0.16Co0.08]O2 as cathode material with improved electrochemical performance for lithium ion batteries

The LaF₃-coated Li_1.2Mn_0.56Ni_0.16Co_0.08O₂, compared with pristine Li_1.2Mn_0.56Ni_0.16Co_0.08O₂, exhibits an enormous improvement in the initial coulombic efficiency and rate capability.

Mitigating voltage fade in cathode materials by improving the atomic level uniformity of elemental distribution

Lithium- and manganese-rich (LMR) layered-structure materials are very promising cathodes for high energy density lithium-ion batteries. However, their voltage fading mechanism and its relationships with fundamental structural changes are far from being well understood. Here we report for the first time the mitigation of voltage and energy fade of LMR cathodes by improving the atomic level spatial uniformity of the chemical species. The results reveal that LMR cathodes (Li[Li0.2Ni0.2M0.6]O2) prepared by coprecipitation and sol-gel methods, which are dominated by a LiMO2 type R3̅m structure, show significant nonuniform Ni distribution at particle surfaces. In contrast, the LMR cathode prepared by a hydrothermal assisted method is dominated by a Li2MO3 type C2/m structure with minimal Ni-rich surfaces. The samples with uniform atomic level spatial distribution demonstrate much better capacity retention and much smaller voltage fade as compared to those with significant nonuniform Ni distribution. The fundamental findings on the direct correlation between the atomic level spatial distribution of the chemical species and the functional stability of the materials may also guide the design of other energy storage materials with enhanced stabilities.

Nanoscale lamellar monoclinic Li(2)MnO(3) phase with stacking disordering in lithium-rich and oxygen-deficient Li(1.07)Mn(1.93)O(4-δ) cathode materials

The powdered crystalline samples of nominal composition Li1.07Mn1.93O4-δ have been investigated by transmission electron microscopy (TEM) combined with X-ray powder diffraction (XRD) at room temperature. As suggested by the TEM observation, the dominant phase of the particles is a cubic spinel Li1+αMn2-αO4-δ with space group Fd3̅m. A monoclinic Li2MnO3 phase with C2/m space group was also identified. Furthermore, the occurrence of nanoscale rotational twinning domains in Li2MnO3 with 120° rotation angles, stacked along the [103]m/[111]c ("m" and "c" represent the monoclinic and cubic descriptions, respectively) axis was also observed. These nanoscale rotational twining domains are responsible for the pseudo-3-fold axis and their formation is supported by the superstructure reflections in selected-area electron-diffraction (SAED) patterns. Similar patterns were reported in the literature but may have been misinterpreted without the consideration of such domains. Consistent with the TEM observation, the XRD results reveal the increasing percentage of monoclinic Li2MnO3 with increasing annealing time, associated with more oxygen vacancies. In addition, the electron beam irradiation during TEM studies may cause the nucleation of nanoscale cubic spinel Li-Mn-O crystallites on the monoclinic Li2MnO3 grains. These results provide the detailed structural information about the Li1.07Mn1.93O4-δ samples and advance the understanding of corresponding electrochemical properties of this material as well as other layer structured cathode materials for lithium-ion batteries.

First evidence of manganese-nickel segregation and densification upon cycling in Li-rich layered oxides for lithium batteries

Lithium-rich manganese-based layered oxides Li[Li(x)Mn(y)TM(1-x-y)]O2 with TM standing for Ni, Co, or Fe are of great interest as cathode materials for lithium ion batteries. Indeed, among all of the materials, they offer the highest rechargeable capacity and energy density. However, when used, they suffer from complex evolutions that need to be understood before their practical use. Here we report on such evolutions studied using advanced transmission electron microscopy. Structural modifications are directly observed at the atomic scale using Cs corrected STEM HAADF imaging technique, and the chemical modifications are probed by the means of STEM EELS experiments. For the first time, segregation between nickel and manganese close the particle surface is pointed out. Finally, observed evolutions are correlated within a proposed mechanism that leads to the densification of the material. Our results allow understanding the link between the decrease of electrochemical performance and these evolutions occurring into the material upon cycling.

High-Energy Cathode Materials (Li2MnO3-LiMO2) for Lithium-Ion Batteries

Lithium-rich layered oxide materials xLi2MnO3·(1-x)LiMO2 (M = Mn, Ni, Co, Fe, Cr, etc.) have attracted much attention for the use of cathode materials in lithium-ion batteries in recent years. However, there are many issues still unclear (the structure and reaction mechanism are ambiguous until now), and numerous scientific challenges (low initial Coulombic efficiency, poor rate capability, and voltage degradation during cycling) of these materials that must be overcome to realize their utilization in commercial lithium-ion batteries. This Perspective focuses on the challenges and prospects associated with the current researching results of these lithium-rich layered cathode materials. Specifically, their average/local structures, reaction mechanisms, and electrochemical properties are discussed.

Conflicting roles of nickel in controlling cathode performance in lithium ion batteries

A variety of approaches are being made to enhance the performance of lithium ion batteries. Incorporating multivalence transition-metal ions into metal oxide cathodes has been identified as an essential approach to achieve the necessary high voltage and high capacity. However, the fundamental mechanism that limits their power rate and cycling stability remains unclear. The power rate strongly depends on the lithium ion drift speed in the cathode. Crystallographically, these transition-metal-based cathodes frequently have a layered structure. In the classic wisdom, it is accepted that lithium ion travels swiftly within the layers moving out/in of the cathode during the charge/discharge. Here, we report the unexpected discovery of a thermodynamically driven, yet kinetically controlled, surface modification in the widely explored lithium nickel manganese oxide cathode material, which may inhibit the battery charge/discharge rate. We found that during cathode synthesis and processing before electrochemical cycling in the cell nickel can preferentially move along the fast diffusion channels and selectively segregate at the surface facets terminated with a mix of anions and cations. This segregation essentially can lead to a higher lithium diffusion barrier near the surface region of the particle. Therefore, it appears that the transition-metal dopant may help to provide high capacity and/or high voltage but can be located in a "wrong" location that may slow down lithium diffusion, limiting battery performance. In this circumstance, limitations in the properties of lithium ion batteries using these cathode materials can be determined more by the materials synthesis issues than by the operation within the battery itself.