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Summary The realization of lithium-ion battery (LIB) anodes with high volumetric energy densities and minimal Li plating at high rates remains a key challenge for emerging technologies, including electric vehicles and grid-level energy storage. Here, we present graphene-functionalized Li2TiSiO5 (G-LTSO) as a high volumetric energy and power density anode for LIBs. G-LTSO forms a dense electrode structure with electronically and ionically conductive networks that deliver superior electrochemical performance. Upon lithiation, in situ transmission electron microscopy reveals that graphene functionalization yields minimal structural changes compared with pristine LTSO, resulting in high cycling stability. Furthermore, G-LTSO exhibits not only high charge and discharge capacities but also low overpotentials at high rates with minimal voltage fading due to reduced formation of a solid-electrolyte interphase. The combination of highly compacted electrode morphology, stable high-rate electrochemistry, and low operating potential enables G-LTSO to achieve exceptional volumetric energy and power densities that overcome incumbent challenges for LIBs.

Irreversible phase transformation of layered structure into spinel structure is considered detrimental for most of the layered structure cathode materials. Here we report that this presumably irreversible phase transformation can be rendered to be reversible in sodium birnessite (NaxMnO2·yH2O) as a basic structural unit. This layered structure contains crystal water, which facilitates the formation of a metastable spinel-like phase and the unusual reversal back to layered structure. The mechanism of this phase reversibility was elucidated by combined soft and hard X-ray absorption spectroscopy with X-ray diffraction, corroborated by first-principle calculations and kinetics investigation. These results show that the reversibility, modulated by the crystal water content between the layered and spinel-like phases during the electrochemical reaction, could activate new cation sites, enhance ion diffusion kinetics and improve its structural stability. This work thus provides in-depth insights into the intercalating materials capable of reversible framework changes, thereby setting the precedent for alternative approaches to the development of cathode materials for next-generation rechargeable batteries., The irreversible layered-to-spinel phase transformation is detrimental for many cathode materials. Here, the authors show that reversibility can be realized in crystal water containing sodium birnessite by controlled dehydration, leading to enhanced ion diffusion kinetics and improved structural stability.

Layered, nickel-rich lithium transition metal oxides have emerged as leading candidates for lithium-ion battery (LIB) cathode materials. High-performance applications for nickel-rich cathodes, such as electric vehicles and grid-level energy storage, demand electrodes that deliver high power without compromising cell lifetimes or impedance. Nanoparticle-based nickel-rich cathodes seemingly present a solution to this challenge due to shorter lithium-ion diffusion lengths compared to incumbent micrometer-scale active material particles. However, since smaller particle sizes imply that surface effects become increasingly important, particle surface chemistry must be well characterized and controlled to achieve robust electrochemical properties. Moreover, residual surface impurities can disrupt commonly used carbon coating schemes, which result in compromised cell performance. Using x-ray photoelectron spectroscopy, here we present a detailed characterization of the surface chemistry of LiNi0.8Al0.15Co0.05O2 (NCA) nanoparticles, ultimately identifying surface impurities that limit LIB performance. With this chemical insight, annealing procedures are developed that minimize these surface impurities, thus improving electrochemical properties and enabling conformal graphene coatings that reduce cell impedance, maximize electrode packing density, and enhance cell lifetime fourfold. Overall, this work demonstrates that controlling and stabilizing surface chemistry enables the full potential of nanostructured nickel-rich cathodes to be realized in high-performance LIB technology.

Ni-rich LiNi0.8Co0.1Mn0.1O2 layered oxide cathodes have been highlighted for large-scale energy applications due to their high energy density. Although its specific capacity is enhanced at higher voltages as Ni ratio increases, its structural degradation due to phase transformations and lattice distortions during cycling becomes severe. For these reasons, we focused on the origins of crack generation from phase transformations and structural distortions in Ni-rich LiNi0.8Co0.1Mn0.1O2 using multiscale approaches, from first-principles to meso-scale phase-field model. Atomic-scale structure analysis demonstrated that opposite changes in the lattice parameters are observed until the inverse Li content x = 0.75; then, structure collapses due to complete extraction of Li from between transition metal layers. Combined-phase investigations represent the highest phase barrier and steepest chemical potential after x = 0.75, leading to phase transformations to highly Li-deficient phases with an inactive character. Abrupt phase transformations with heterogeneous structural collapse after x = 0.81 (~220 mAh g−1) were identified in the nanodomain. Further, meso-scale strain distributions show around 5% of anisotropic contraction with lower critical energy release rates, which cause not only micro-crack generations of secondary particles on the interfaces between the contracted primary particles, but also mechanical instability of primary particles from heterogeneous strain changes.

Ni-rich LiNi0.8Co0.1Mn0.1O2 layered oxide cathodes have been highlighted for large-scale energy applications due to their high energy density. Although its specific capacity is enhanced at higher voltages as Ni ratio increases, its structural degradation due to phase transformations and lattice distortions during cycling becomes severe. For these reasons, we focused on the origins of crack generation from phase transformations and structural distortions in Ni-rich LiNi0.8Co0.1Mn0.1O2 using multiscale approaches, from first-principles to meso-scale phase-field model. Atomic-scale structure analysis demonstrated that opposite changes in the lattice parameters are observed until the inverse Li content x = 0.75; then, structure collapses due to complete extraction of Li from between transition metal layers. Combined-phase investigations represent the highest phase barrier and steepest chemical potential after x = 0.75, leading to phase transformations to highly Li-deficient phases with an inactive character. Abrupt phase transformations with heterogeneous structural collapse after x = 0.81 (~220 mAh g−1) were identified in the nanodomain. Further, meso-scale strain distributions show around 5% of anisotropic contraction with lower critical energy release rates, which cause not only micro-crack generations of secondary particles on the interfaces between the contracted primary particles, but also mechanical instability of primary particles from heterogeneous strain changes.

2D materials have shown great promise to advance next-generation lithium-ion battery technology. Specifically, tin-based chalcogenides have attracted widespread attention because lithium insertion can introduce phase transformations via three types of reactions-intercalation, conversion, and alloying-but the corresponding structural changes throughout these processes, and whether they are reversible, are not fully understood. Here, the first real-time and atomic-scale observation of reversible phase transformations is reported during the lithiation and delithiation of SnSe2 single crystals, using in situ high-resolution transmission electron microscopy complemented by first-principles calculations. Lithiation proceeds sequentially through intercalation, conversion, and alloying reactions (SnSe2 → Lix SnSe2 → Li2 Se + Sn → Li2 Se + Li17 Sn4 ) in a manner that maintains structural and crystallographic integrity, whereas delithiation forms numerous well-aligned SnSe2 nanodomains via a homogeneous deconversion process, but gradually loses the coherent orientation in subsequent cycling. Furthermore, alloying and dealloying reactions cause dramatic structural reorganization and thereby consequently reduce structural stability and electrochemical cyclability, which implies that deep discharge for Sn chalcogenide electrodes should be avoided. Overall, the findings elucidate atomistic lithiation and delithiation mechanisms in SnSe2 with potential implications for the broader class of 2D metal chalcogenides.

Li-rich oxide cathode is one of the most promising high capacity cathode materials for the next commercialized Li-ion battery application, and its composition can be described as a mixture of Li2MnO3 and Li(TM)O2(TM: Transition metal). Li-rich cathode material has large capacity based on relatively cheap Mn oxide, and it is easy to synthesize. However, the Li-rich cathode material has critical material challenges such as capacity degradation and voltage drop. It has been reported that the main source of the material challenge is related to phase transformation inside the bulk cathode material during charge/discharge processes. In this work, detailed phase transformation mechanism of Li2MnO3 is investigated by combining the experimental and computational approaches to develop fundamental understanding on the atomic scale processes. The structure of Li-rich oxide is a composite of Li2MnO3 and layered Li(TM)O2, and the monoclinic layered structure of Li2MnO3 is known to be the source of the degradation problems. We have observed the evidence of phase transformation in Li2MnO3experimentally, and a theoretical analysis based on density functional theory calculations is combined with experimental data for a systematic comparative study. For experimental study, we have synthesized Li2MnO3 powder by solid state method at low temperature following the literature procedures. Using this powder, we made a coin cell with standard Li reference electrode, and the CV measurement shows the phase transformation during charge/discharge processes. First, basic powder characterization was conducted through XRD, and SEM analyses. Second, coin cell was assembled for cyclic performance test, and charge/discharge profile and cyclic voltammogram were obtained. From the experimental investigation, synthesized Li2MnO3is identified as monoclinic C2/m space group structure and their particle size is around 200~300 nm. The evidences of phase transformation are found from cyclic charge/discharge profile as well as cyclic voltammogram. As reaction cycle is progressed, charge/discharge capacity operated under 4.6 V has a steady increase indicating that, the initial active material is getting transformed to another phase with lower reaction voltage than 4.6 V. As described in Fig. 1, we could observe the changes of charge/discharge profile and reaction voltage fundamentally caused by phase transformation consistent with similar previous experiment studies. As observed before, the first charge voltage is around 4.6 V, but the charge voltage changes to around 3.2 V as cycle goes on. Remarkably, discharge voltage reveals at three different values around 2.8 V, 3.3 V, and 4.0 V, suggesting that there are three different transformed phases inside the active material as the cycle goes on. To understand this and similar experimental observations of phase transformation, we have examined how and why it could happen in terms of thermodynamics and kinetics based on density functional theory investigation. In case of thermodynamic study, phase stability, intercalation voltage, electronic charge, and electronic structures are studied. From phase stability and intercalation voltage analyses, we would estimate when initial structure could be transformed. The electronic charge and partial density of state for both initial and transformed structure (whose Mn ion is migrated) are investigated for structure stability and physical/chemical characters. We found that thermodynamic stability of structures and the changes in bonding characters between Mn and O ions are to the main cause of phase transformation. For kinetic analysis, we investigated the migration barriers of Li and Mn ions in the Li2MnO3framework with controlled delithiation. Based on the kinetic calculation results, we could show the possibility of Li and Mn ion migrations with different Li contents in active material. As shown in Fig 2, not only the possibility and the delithiation effect of phase transformation, but also the detailed Mn migration mechanism could be predicted providing atomic scale explanation of phase transformation. As a result, detailed phase transformation mechanism could be quantitative understood, and it would be possible to suppress such phase transformation based on theoretical studies on material design such as doping on effect the electronic structure analyses. By understanding detailed phase transformation mechanism of Li2MnO3, we are developing material modification strategy to solve capacity degradation and voltage drop problems. The developed strategy will be critically validated by experimental implementation of the designed material modification. Such combined material design and experimental validation approaches will accelerated the high capacity cathode material development based on atomic scale understanding. This work was supported by the Industrial Strategic technology development program(10041589) funded by the Ministry of Knowledge Economy(MKE, Korea)

The development of rechargeable lithium-ion batteries (LIB) has progressed rapidly to meet the demand for consumer electronic devices such as cellular phones and laptop computers, and the recent demands for larger energy storage applications (xEV and grid storage) require further development of LIB or alternative energy storage technology. Among possible alternative cathode materials compared to the currently used material LiCoO2, Li-rich compounds xLi2MnO3·(1-x)LiMO2 (M = Ni, Co, Mn) are the most promising candidate with higher capacity, lower cost and absence of toxic element. Within the two-component composite compounds, Li2MnO3 plays a central role in increasing the capacity of the compounds and independently possess a high theoretical capacity of 460 mAh/g (if the entire lithium is utilized). Although the Li2MnO3 is not electrochemically active between 2.0V and 4.4V, it is active to extract lithium ions over 4.5V. During the initial delithiation process, Li2MnO3 is known to transform into layered LiMnO2structure, and the subsequent charge/discharge cycling induces a gradual phase transformation to the spinel structure, resulting in lowering voltage plateau and a large irreversible capacity accompanied by oxygen loss. To overcome these capacity loss problems, diverse approaches have been made to improve electrochemical properties of the cathode materials: adjusting the relative ratio of different transition metal ions, coating layers on the particles, and synthesizing at various temperature to control specific surface area. Even though the bulk properties of cathode materials have been studied by many research groups, the surface properties of cathode have seldom been researched so far. Specifically, theoretical study on cathode surface has been rarely performed even though major reactions and chemical transformations occur near the surface and interfaces between different phases. Recently, several research groups highlighted and reported experimental observations of a phase transformation from the layered structure to spinel phase at the particle surface upon the first charge cycle. However, it is still unclear and controversial for theoretical and fundamental understanding on the mechanism of phase transformations. Without the basic understanding based on detailed atomic scale theoretical analysis, many problems of LIB materials will take extreme efforts and time to resolve the problems. In the present paper, we established a fundamental understanding on the origin of phase transformation at the surface through first principles study of interface models. The theoretical findings suggest unusual redox activities of the surface layers of Li2MnO3which is confirmed by detailed experimental study. These theoretical and experimental findings provide basic understanding to improve performance degradation by phase transformation. For the experimental study, coin cells are made with Li2MnO3 cathode and Li reference electrode, and variation of voltage between 2.0 V and various limiting voltage is applied to the electrochemical test with a constant current at room temperature and under 0.05 C-rate during 10 cycles. There is a small amount of redox capacity (4 mAh/g) at lower voltage as shown in Fig. 1a inset, and Figs. 1d-f show 10 mAh/g capacity during subsequent 10 cycles with lower limiting voltage of 4.1-4.3 V. Fig. 1a shows voltage plateau at 4.6 V extracting Li in the bulk Li2MnO3 with accompanying phase change to layered LiMnO2with lower voltage during subsequent cycles. For the modeling study, in order to elucidate the low voltage redox mechanism observed in Fig. 1, we examined the Li intercalation potential at the surface using the model shown in Fig. 2. The model interface contains stoichiometric ratio of constituents (Li : Mn :O = 2 : 1 : 3) split into two regions of a semi-infinite bulk (SIB), which is inactive at low voltage (4-4.5 V), and a interface layer (IFL) exposed to vaccum, which is calculated to be active at low voltage (~4 V). Finally, detailed electronic structures of the interface model show different oxidation state of surface Mn atoms compared to the fully oxidized bulk Mn4+ within the bulk. Since the high activation potential of 4.6 V of bulk Mn is known to be related to electron extraction from oxygen atoms coordinating Mn4+, any lower Mn oxidation state on the surface would reduce the delithiation potential comparable to those of layered LiMnO2 and spinel (3-4 V). Furthermore, delithiation potential at the surface show strong dependence on the Li location in Mn-layer (4.6 V) and Li-layer (4V) at the surface of Li2MnO3. The preferential delithiation of Li from the surface Li-layer would facilitate the Mn atom migration to the Li-layer causing spinel phase transformation at the surface. These underlying understanding mechanisms of the surface will provide a conceptual basis to develop diverse approached to suppress phase transitions in the cathode materials in LIB, which is our current modeling and experimental research topics. This work was supported by the Industrial Strategic technology development program(10041589) funded by the Ministry of Knowledge Economy(MKE, Korea)