• Energy Research

AbstractEarth-abundant cathode materials are urgently needed to enable scaling of the Li-ion industry to multiply terawatt hours of annual production, necessitating reconsideration of how good cathode materials can be obtained. Irreversible transition metal migration and phase transformations in Li-ion cathodes are typically believed to be detrimental because they may trigger voltage hysteresis, poor kinetics and capacity degradation. Here we challenge this conventional consensus by reporting an unusual phase transformation from disordered Li- and Mn-rich rock salts to a new phase (named δ), which displays partial spinel-like ordering with short coherence length and exhibits high energy density and rate capability. Unlike other Mn-based cathodes, the δ phase exhibits almost no voltage fade upon cycling. We identify the driving force and kinetics of this in situ cathode formation and establish design guidelines for Li- and Mn-rich compositions that combine high energy density, high rate capability and good cyclability, thereby enabling Mn-based energy storage.

This review summarizes the history and critical working mechanisms of Li-rich oxides with a special focus on anionic redox reactions.

LiCoO2 is a dominant cathode material for lithium-ion (Li-ion) batteries due to its high volumetric energy density, which could potentially be further improved by charging to high voltages. However, practical adoption of high-voltage charging is hindered by LiCoO2’s structural instability at the deeply delithiated state and the associated safety concerns. Here, we achieve stable cycling of LiCoO2 at 4.6 V (versus Li/Li+) through trace Ti–Mg–Al co-doping. Using state-of-the-art synchrotron X-ray imaging and spectroscopic techniques, we report the incorporation of Mg and Al into the LiCoO2 lattice, which inhibits the undesired phase transition at voltages above 4.5 V. We also show that, even in trace amounts, Ti segregates significantly at grain boundaries and on the surface, modifying the microstructure of the particles while stabilizing the surface oxygen at high voltages. These dopants contribute through different mechanisms and synergistically promote the cycle stability of LiCoO2 at 4.6 V. LiCoO2 is a widely used cathode material in Li-ion batteries for applications such as portable electronics. Here, the authors report multiple-element doping to enable stable cycling of LiCoO2 at high voltages that are not yet accessible with commercial Li-ion batteries.

Anionic redox chemistry has emerged as a new paradigm to design higher-energy lithium ion-battery cathode materials such as Li-rich layered oxides. However, they suffer from voltage fade, large hysteresis and sluggish kinetics, which originate intriguingly from the anionic redox activity itself. To fundamentally understand these issues, we decided to act on the ligand by designing new Li-rich layered sulfides Li1.33 – 2y/3Ti0.67 – y/3FeyS2, among which the y = 0.3 member shows sustained reversible capacities of ~245 mAh g−1 due to cumulated cationic (Fe2+/3+) and anionic (S2−/Sn−, n < 2) redox processes. Moreover, its negligible initial cycle irreversibility, mitigated voltage fade upon long cycling, low voltage hysteresis and fast kinetics compare positively with its Li-rich oxide analogues. Moving from the oxygen ligand to the sulfur ligand thus partially alleviates the practical bottlenecks affecting anionic redox, although it penalizes the redox potential and energy density. Overall, these sulfides provide chemical clues to improve the holistic performance of anionic redox electrodes, which may guide us to ultimately exploit the energy benefits of oxygen redox.

AbstractDoping an indium sulfide buffer layer with sodium is a promising route to replace the “state‐of‐the‐art” CdS buffer layer in chalcopyrite‐based thin‐film solar cells, as it achieves efficiencies as high as 17.9% for large‐area devices (30 cm × 30 cm). We report on the chemical and electronic structure of the InxSy:Na/CuIn(S,Se)2 (CISSe) interface for thin‐film solar cells by means of photoelectron, soft x‐ray emission, and inverse photoemission spectroscopy. For as‐deposited InxSy:Na buffer layers, we find a sulfur‐poor surface and, in comparison to undoped InxSy and the standard CdS buffer, derive a large electronic surface band gap of 2.60 ± 0.11 eV. The conduction band offset at the buffer/absorber interface is a spike of 0.32 ± 0.10 eV. After annealing at 200°C to simulate the thermal load of subsequent cell manufacturing processes, an additional diffusion of copper and selenium from the absorber towards the buffer layer surface is observed, leading to a distinct electronic surface band gap decrease of the InxSy:Na buffer layer (to 2.11 ± 0.11 eV). We speculate that the diffusion of absorber elements causes a band gap widening at the former absorber surface and that both effects lead to a reduction of the conduction band spike for the buried InxSy:Na/CISSe interface after annealing.

Energy storage has become more and more a limiting factor of today's sustainable energy applications, including electric vehicles and green electric grid based on volatile solar and wind sources. The pressing demand of developing high-performance electrochemical energy storage solutions, i.e., batteries, relies on both fundamental understanding and practical developments from both the academy and industry. The formidable challenge of developing successful battery technology stems from the different requirements for different energy-storage applications. Energy density, power, stability, safety, and cost parameters all have to be balanced in batteries to meet the requirements of different applications. Therefore, multiple battery technologies based on different materials and mechanisms need to be developed and optimized. Incisive tools that could directly probe the chemical reactions in various battery materials are becoming critical to advance the field beyond its conventional trial-and-error approach. Here, we present detailed protocols for soft X-ray absorption spectroscopy (sXAS), soft X-ray emission spectroscopy (sXES), and resonant inelastic X-ray scattering (RIXS) experiments, which are inherently elemental-sensitive probes of the transition-metal 3d and anion 2p states in battery compounds. We provide the details on the experimental techniques and demonstrations revealing the key chemical states in battery materials through these soft X-ray spectroscopy techniques.