Shoichi Matsuda

Lithium–oxygen batteries (LOBs) have emerged as a compelling next-generation energy storage technology for aviation due to their high theoretical energy densities and the use of abundant oxygen as a cathode reactant. Unlike conventional lithium-ion batteries (LIBs), which are limited by the mass of intercalation materials and one-electron redox processes, LOBs leverage multielectron oxygen redox chemistry to achieve gravimetric energy density exceeding 3,500 Wh kgLi2O2–1 based on lithium peroxide and up to 5,200 Wh kgLi2O–1 for lithium oxide. However, practical implementation of LOBs remains constrained by significant challenges including low round-trip efficiency and low system-level energy density, the latter arising from the need for oxygen management in both open and closed architectures. The electrolyte viscosity, oxygen solubility, volatility, and chemical stability toward reactive oxygen species can profoundly influence discharge capacity, rate capability, reversibility, and long-term stability. This review provides a comprehensive assessment of recent progress in LOBs, focusing on electrolyte designs, electrode engineering, and strategies for mitigating parasitic reactions as well as the interplay between material-level properties and system-level constraints, offering a critical perspective on the path toward realizing high-energy, practical LOB systems.

In neutral and mildly alkaline water electrolysis, electrolytes are primarily viewed as media controlling macroscopic solution properties such as pH and ionic conductivity. Beyond this conventional role, electrolyte species can interact with metal ions and dynamically reconstruct electrode surfaces through dissolution and reformation. Although such reconstruction is widely recognized, the extent to which operating surfaces retain the initial oxidation history or are governed by the electrolyte environment remains unclear. Here, a high-throughput electrochemical platform evaluated cell voltages under 108 conditions using 304 stainless steel anodes, independently varying anodic pretreatment, electrolyte pH (9.5, 10.5, and 11.5), phosphate concentration (1 and 3 M), and electrolysis regime. Analysis of variance showed that pH accounted for 70–81% of cell-voltage variation, whereas anodic pretreatment accounted for only 6–7%, demonstrating the dominant influence of the electrolyte environment on the electrochemical response. Surface analyses revealed widespread nickel depletion and heterogeneous chromium-rich films under lower-pH, high-phosphate conditions. Iron behavior was consistent with the equilibrium solubility trend, whereas nickel depletion did not follow the equilibrium solubility trend, indicating the involvement of processes beyond equilibrium thermodynamics. Together, these results reveal that multicomponent electrode surfaces do not reconstruct through a uniform pathway. Instead, the operating surface emerges from distinct element-specific dissolution and retention processes. The surface formed during operation should therefore be treated as a materials-design target rather than considering the as-prepared electrode and electrolyte independently.

Cation-disordered rocksalt oxides offer broad compositional flexibility and high theoretical capacities, but their electrochemical performance is often limited by uncontrolled competition between transition metal and oxygen redox processes. In particular, controlling the balance between Mn and O redox activities remains a key challenge in Mn-based disordered oxides. Here, a previously unexplored compositional space in the Li−Mn−Mo−O system is investigated, revealing that Mo incorporation systematically alters the electronic structure and redox behavior of Mn-based disordered rocksalt oxides. Mo doping lowers the average Mn oxidation state, thereby stabilizing a Mn-dominated charge-compensation mechanism. Combined spectroscopic characterization and first-principles calculations reveal a shift in charge compensation from oxygen-centered to Mn-centered redox processes with Mo doping. As a result, Li1.25Mn0.5Mo0.25O2 delivers a reversible capacity exceeding 300 mAh g−1 with enhanced electrochemical stability. These findings demonstrate that targeted compositional and electronic structure modulation enables effective control of redox pathways in cation-disordered oxides and offers design principles for stable, high-capacity Mn-based cathode materials.

Solid‐state lithium metal batteries, assumed to show improved safety and high energy density, have garnered growing interest. Nevertheless, establishing an effective ionic conduction pathway within the composite positive electrode remains challenging, especially with high‐active‐ materials mass‐loading in the electrode. Herein, we develop a high‐active‐mass loading positive electrode for solid‐state Li metal rechargeable batteries, optimized for operation at an elevated temperature of 60°C. The electrode is based on NMC622 as active material, incorporating the ionic liquid Pyr ₁₄ TFSI and LiTFSI (combined‐electrolyte, cE). The positive electrode membranes are successfully prepared by preventing undesired gelation of the slurry solution and seamlessly integrating them with a thin‐film multilayer hybrid solid electrolyte. Notably, the NMC622‐based electrode with 10 wt% of cE achieves a specific capacity exceeding 160 mAh g ⁻¹ at a high mass loading of 40 mg cm ⁻² . The methodology demonstrated in the present study highlights the potential of incorporating a cE into cathode compositions for high energy density solid‐state Li metal batteries.

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The energy–power trade‐off in lithium–oxygen batteries (LOBs) arises from sluggish oxygen (O ₂ ) transport in the porous positive electrode and pore clogging by lithium peroxide (Li ₂ O ₂ ). While increasing porosity enhances electrolyte accessibility and Li ₂ O ₂ storage, it also increases electrolyte demand, compromising the overall energy density of the cell and necessitating alternative strategies to boost power capabilities without sacrificing energy density. In this study, theoretical simulations of O ₂ transport reveal that reducing tortuosity by improving pore interconnectivity has a more significant impact on O ₂ transport than porosity itself. Based on this insight, a freestanding graphene‐based electrode with a highly interconnected macroporous network is fabricated via a non‐solvent‐induced phase separation approach using polyacrylonitrile (PAN) as a carbon scaffold and polyethylene oxide (PEO) as a sacrificial porogen. The selective decomposition of PEO creates spatially interconnected macropores, effectively reducing tortuosity. The resulting electrode enables LOB cells to achieve >2500 mAh g ⁻¹ at 1.0 mA cm ⁻² under lean‐electrolyte conditions. Stable cycling at 4 mAh cm ⁻² is maintained with only 3.25 g Ah ⁻¹ electrolyte, and high‐rate performance persists over 90 cycles at 1.5 mA cm ⁻² . This work demonstrates a robust strategy to simultaneously improve energy and power performance in practical LOBs through rational electrode architecture.

Autonomous high-throughput combinatorial experimentation is a key approach for accelerating materials discovery. However, achieving a fully closed-loop system remains a challenge due to the lack of effective optimization strategies for combinatorial experimentation. Here, we developed a Bayesian optimization method specifically designed for composition-spread films, enabling the selection of promising composition-spread films and identifying which elements should be compositionally graded. Using this approach, we demonstrated an autonomous closed-loop exploration of composition-spread films to enhance the anomalous Hall effect (AHE). Our method optimized the composition of a five-element alloy system consisting of three 3 d ferromagnetic elements of Fe, Co, and Ni and two 5 d heavy elements from Ta, W, or Ir to maximize the AHE. Through our autonomous exploration, we achieved a maximum anomalous Hall resistivity of 10.9 µΩ cm in Fe _44.9 Co _27.9 Ni _12.1 Ta _3.3 Ir _11.7 amorphous thin film on thermally oxidized Si substrates deposited at room temperature.

The operation of lithium-oxygen (Li-O ₂ ) batteries under lean electrolyte conditions offers higher energy density but leads to rapid degradation and short cycle life. Although cathode passivation and electrolyte decomposition occur in all regimes, we show that under lean electrolyte conditions, failure is primarily driven by progressive electrolyte consumption at the lithium/solid electrolyte interphase (SEI), rather than by irreversible cathode passivation. Poor wetting of the lithium surface results in heterogeneous SEI growth and high local current densities, which accelerate electrolyte loss and cell failure. Strategies aimed at improving interfacial stability, including optimized wetting and SEI forming additives, significantly extend cycle life without compromising energy density. Our results establish anode-electrolyte interactions as the dominant degradation mechanism under lean electrolyte conditions, and emphasize the need to engineer a stable Li/SEI interface for long-lasting Li-O ₂ batteries.