Hiromasa Shiiba

High-capacity electrode materials for rechargeable lithium batteries: Li3NbO4-based system with cation-disordered rocksalt structure.

Significance This study describes new and promising electrode materials, Li3NbO4-based electrode materials, which are used for high-energy rechargeable lithium batteries. Although its crystal structure is classified as a cation-disordered rocksalt-type structure, lithium ions quickly migrate in percolative network in bulk without a sacrifice in kinetics. Moreover, the large reversible capacity originates from the participation of oxide ions for a charge compensation process, which has been confirmed by first-principles calculations combined with X-ray absorption spectroscopy. This finding can be further expanded to the design of innovative positive electrode materials beyond the restriction of the solid-state redox reaction based on the transition metals used for the past three decades. Rechargeable lithium batteries have rapidly risen to prominence as fundamental devices for green and sustainable energy development. Lithium batteries are now used as power sources for electric vehicles. However, materials innovations are still needed to satisfy the growing demand for increasing energy density of lithium batteries. In the past decade, lithium-excess compounds, Li2MeO3 (Me = Mn4+, Ru4+, etc.), have been extensively studied as high-capacity positive electrode materials. Although the origin as the high reversible capacity has been a debatable subject for a long time, recently it has been confirmed that charge compensation is partly achieved by solid-state redox of nonmetal anions (i.e., oxide ions), coupled with solid-state redox of transition metals, which is the basic theory used for classic lithium insertion materials, such as LiMeO2 (Me = Co3+, Ni3+, etc.). Herein, as a compound with further excess lithium contents, a cation-ordered rocksalt phase with lithium and pentavalent niobium ions, Li3NbO4, is first examined as the host structure of a new series of high-capacity positive electrode materials for rechargeable lithium batteries. Approximately 300 mAh⋅g−1 of high-reversible capacity at 50 °C is experimentally observed, which partly originates from charge compensation by solid-state redox of oxide ions. It is proposed that such a charge compensation process by oxide ions is effectively stabilized by the presence of electrochemically inactive niobium ions. These results will contribute to the development of a new class of high-capacity electrode materials, potentially with further lithium enrichment (and fewer transition metals) in the close-packed framework structure with oxide ions.

Origin of stabilization and destabilization in solid-state redox reaction of oxide ions for lithium-ion batteries

Further increase in energy density of lithium batteries is needed for zero emission vehicles. However, energy density is restricted by unavoidable theoretical limits for positive electrodes used in commercial applications. One possibility towards energy densities exceeding these limits is to utilize anion (oxide ion) redox, instead of classical transition metal redox. Nevertheless, origin of activation of the oxide ion and its stabilization mechanism are not fully understood. Here we demonstrate that the suppression of formation of superoxide-like species on lithium extraction results in reversible redox for oxide ions, which is stabilized by the presence of relatively less covalent character of Mn4+ with oxide ions without the sacrifice of electronic conductivity. On the basis of these findings, we report an electrode material, whose metallic constituents consist only of 3d transition metal elements. The material delivers a reversible capacity of 300 mAh g−1 based on solid-state redox reaction of oxide ions.

Effect of A-site cation disorder on oxygen diffusion in perovskite-type Ba0.5Sr0.5Co1−xFexO2.5

Molecular dynamics simulations of the effect of A-site cation disorder on oxygen diffusion in (Ba0.5Sr0.5)CoO2.5, (Ba0.5Sr0.5)FeO2.5 and (Ba0.5Sr0.5)Co0.8Fe0.2O2.5 were conducted to understand the oxygen diffusion mechanism. The diffusion coefficients of oxygen were strongly dependent upon the degree of A-site Ba/Sr cation ordering. The oxygen diffusion coefficient decreased and the oxygen diffusion activation energy increased with Ba/Sr cation ordering in the alternating (001) layers of the perovskite structure. The ordering of Ba/Sr cations also caused oxygen/vacancy ordering. In particular, vacancy location in the oxygen layers parallel to the Ba-rich layers significantly increased oxygen diffusivity in BSCF-related materials.

New Insight for Surface Chemistries in Ultra-thin Self-assembled Monolayers Modified High-voltage Spinel Cathodes

The electrochemical properties of the interface between the spinel LiNi0.5Mn1.5O4-δ (LNMO4-δ) cathodes and ethylene carbonate−dimethyl carbonate (EC-DMC) electrolyte containing 1 M of LiPF6 have been investigated to achieve high-voltage durability of LNMO4-δ/graphite full cells. Coating the LNMO4-δ crystal surface by a fluoroalkylsilane self-assembled monolayer with a thickness below 2 nm resulted in a capacity retention of 94% after 100 cycles at a rate of 1 C and suppression of capacity fading for both the cathode and anode of the full cell. The observed effect is likely caused by the inhibited oxidative decomposition of EC−DMC electrolyte and vinylene carbonate (VC) species at the LNMO4-δ crystal surface and formation of a stable VC solid electrolyte interface near the anode. Moreover, the results obtained via photoelectron spectroscopy and density-functional calculations revealed that the increase in the work function of the LNMO4-δ crystal surface due to the formation of Si−O−Mn species primary contributed to the inhibition of the oxidative decomposition of the electrolyte and VC molecules at the cathode/electrolyte interface.

Thin and Dense Solid-solid Heterojunction Formation Promoted by Crystal Growth in Flux on a Substrate

In this work, we demonstrate the direct growth of cubic Li5La3Nb2O12 crystal layer on the LiCoO2 substrate through the conversion of ultra-thin Nb substrate in molten LiOH flux. The initial thickness of the Nb layer determines that of the crystal layer. SEM and TEM observations reveal that the surface is densely covered with well-defined polyhedral crystals. Each crystal is connected to neighboring ones through the formation of tilted grain boundaries with Σ3 (2–1–1) = (1–21) symmetry which show small degradation in lithium ion conductivity comparing to that of bulk. Furthermore, the sub-phase formation at the interface is naturally mitigated during the growth since the formation of Nb2O5 thin film limits the whole reaction kinetics. Using the newly developed stacking approach for stacking solid electrolyte layer on the electrode layer, the grown crystal layer could be an ideal ceramic separator with a dense thin-interface for all-solid-state batteries.

Impact of trace extrinsic defect formation on the local symmetry transition in spinel LiNi0.5Mn1.5O4-δ and their electrochemical characteristics

Many fundamental studies have been conducted on the electrochemical and electronic structures in transition metal cation-substituted LiNi0.5Mn1.5O4 systems. These systems have potential use as 5 V-level high voltage cathode materials for lithium ion batteries, but there are only a few reports regarding the control of their symmetry transitions which contribute the electronic structures and Li+ transport efficiency. We address this solid chemistry and the corresponding electrochemical characteristics using both systematic experimental and theoretical approaches. Trace substitution of Cu2+ with Mn4+ (CuMn) can promote the symmetry transition from Fdm to P4332 phase in oxygen-deficient LiNi0.5Mn1.5O4−δ. This behavior is detectable by Fourier-transform infrared and Raman spectroscopies but undetectable by X-ray diffraction, suggesting that the symmetry transition was localized in the space near the point of extrinsic defects CuMn in the spinel framework. Notably, a very small amount of Cu2+ substitution not only affects the local atomic arrangement but also remarkably affects the macroscopic electrochemical redox responsiveness, including the inactivation of Mn3+/Mn4+ redox couples, suppression of Mn ion dissolution, and enhancing the C rate capability (increasing the electron conductivity, and reducing the activation barrier for lithium ion hopping along the most energetically preferable 8a–16c–8a route) and cyclability.