Mikhailova Daria, Karakulina Olesia M, Batuk Dmitry, Hadermann Joke, Abakumov Artem M, Herklotz Markus, Tsirlin Alexander A, Oswald Steffen, Giebeler Lars, Schmidt Marcus, Eckert Jürgen, Knapp Michael, Ehrenberg Helmut
Karlsruhe Institute of Technology (KIT) , Institute for Applied Materials (IAM), Hermann-von- Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany.
IFW Dresden , Institute for Complex Materials, Helmholtzstr. 20, D-01069 Dresden, Germany.
Inorg Chem. 2016 Jul 18;55(14):7079-89. doi: 10.1021/acs.inorgchem.6b01008. Epub 2016 Jul 1.
Layered Li(M,Li)O2 (where M is a transition metal) ordered rock-salt-type structures are used in advanced metal-ion batteries as one of the best hosts for the reversible intercalation of Li ions. Besides the conventional redox reaction involving oxidation/reduction of the M cation upon Li extraction/insertion, creating oxygen-located holes because of the partial oxygen oxidation increases capacity while maintaining the oxidized oxygen species in the lattice through high covalency of the M-O bonding. Typical degradation mechanism of the Li(M,Li)O2 electrodes involves partially irreversible M cation migration toward the Li positions, resulting in gradual capacity/voltage fade. Here, using LiRhO2 as a model system (isostructural and isoelectronic to LiCoO2), for the first time, we demonstrate an intimate coupling between the oxygen redox and M cation migration. A formation of the oxidized oxygen species upon electrochemical Li extraction coincides with transformation of the layered Li1-xRhO2 structure into the γ-MnO2-type rutile-ramsdellite intergrowth LiyRh3O6 structure with rutile-like [1 × 1] channels along with bigger ramsdellite-like [2 × 1] tunnels through massive and concerted Rh migration toward the empty positions in the Li layers. The oxidized oxygen dimers with the O-O distances as short as 2.26 Å are stabilized in this structure via the local Rh-O configuration reminiscent to that in the μ-peroxo-μ-hydroxo Rh complexes. The LiyRh3O6 structure is remarkably stable upon electrochemical cycling illustrating that proper structural implementation of the oxidized oxygen species can open a pathway toward deliberate employment of the anion redox chemistry in high-capacity/high-voltage positive electrodes for metal-ion batteries.
层状Li(M,Li)O2（其中M为过渡金属）有序岩盐型结构被用作先进金属离子电池中锂离子可逆嵌入的最佳主体材料之一。除了在锂脱出/嵌入时涉及M阳离子氧化/还原的传统氧化还原反应外，由于部分氧氧化产生氧空位会增加容量，同时通过M-O键的高共价性在晶格中保持氧化态的氧物种。Li(M,Li)O2电极的典型降解机制涉及部分不可逆的M阳离子向锂位置迁移，导致容量/电压逐渐衰减。在此，我们首次以LiRhO2作为模型体系（与LiCoO2同结构且等电子），证明了氧氧化还原与M阳离子迁移之间存在紧密耦合。电化学锂脱出时氧化态氧物种的形成与层状Li1-xRhO2结构转变为γ-MnO2型金红石-兰多石共生结构LiyRh3O6同时发生，该结构具有沿[1×1]通道的金红石状结构以及通过大量且协同的Rh向锂层中空位迁移形成的更大的类似兰多石的[2×1]隧道。氧-氧距离短至2.26 Å的氧化态氧二聚体通过局部Rh-O构型在该结构中得以稳定，这种构型类似于μ-过氧-μ-羟基Rh配合物中的构型。LiyRh3O6结构在电化学循环过程中非常稳定，这表明氧化态氧物种的适当结构实现可为在金属离子电池的高容量/高电压正极中有意利用阴离子氧化还原化学开辟一条途径。
