Metal-oxygen decoordination stabilizes anion redox in Li-rich oxides
Metal-oxygen decoordination stabilizes anion redox in Li-rich oxides
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DOI：
10.1038/s41563-018-0276-1
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发表时间：
2019-03-01
期刊：
影响因子：
41.2
通讯作者：
Chueh, William C.
中科院分区：
文献类型：
--
作者：
Hong, Jihyun;Gent, William E.;Chueh, William C.
可逆高压氧化还原化学是许多电化学技术的重要组成部分，从（电）催化剂到锂离子电池。氧阴离子氧化还原已经引起了此类应用的强烈兴趣，特别是锂离子电池，因为与各种氧化物材料中的Li/Li+相比，它提供了超过4 V的大量氧化还原能力。然而，氧的氧化几乎普遍与不可逆的局部结构转变、电压滞后和电压衰减有关，这阻碍了氧的广泛应用。通过对Li2-xIr1-ySnyO3模型体系的全面研究，我们发现这种结构-氧化还原耦合是由于Li2-xIr1-ySnyO3在氧氧化还原过程中，通过配体到金属的电荷转移，产生了大约1.8埃的短金属-氧pi键和大约1.4埃的O-O二聚体的局部稳定。至关重要的是，这些氧化氧的形成需要氧通过在邻近阳离子位点形成空位来配位到单个共价键伙伴，从而驱动阳离子紊乱。这些见解建立了一个点缺陷解释，为什么阴离子氧化还原经常发生在局部结构紊乱和电压滞后循环过程中。我们的发现为富锂层状氧化物独特的电化学特性提供了解释，对采用氧氧化还原化学的材料设计具有普遍意义。
Reversible high-voltage redox chemistry is an essential component of many electrochemical technologies, from (electro) catalysts to lithium-ion batteries. Oxygen-anion redox has garnered intense interest for such applications, particularly lithium-ion batteries, as it offers substantial redox capacity at more than 4 V versus Li/Li+ in a variety of oxide materials. However, oxidation of oxygen is almost universally correlated with irreversible local structural transformations, voltage hysteresis and voltage fade, which currently preclude its widespread use. By comprehensively studying the Li2-xIr1-ySnyO3 model system, which exhibits tunable oxidation state and structural evolution with y upon cycling, we reveal that this structure-redox coupling arises from the local stabilization of short approximately 1.8 angstrom metal-oxygen pi bonds and approximately 1.4 angstrom O-O dimers during oxygen redox, which occurs in Li2-xIr1-ySnyO3 through ligand-to-metal charge transfer. Crucially, formation of these oxidized oxygen species necessitates the decoordination of oxygen to a single covalent bonding partner through formation of vacancies at neighbouring cation sites, driving cation disorder. These insights establish a point-defect explanation for why anion redox often occurs alongside local structural disordering and voltage hysteresis during cycling. Our findings offer an explanation for the unique electrochemical properties of lithium-rich layered oxides, with implications generally for the design of materials employing oxygen redox chemistry.