Hong Jihyun, Gent William E, Xiao Penghao, Lim Kipil, Seo Dong-Hwa, Wu Jinpeng, Csernica Peter M, Takacs Christopher J, Nordlund Dennis, Sun Cheng-Jun, Stone Kevin H, Passarello Donata, Yang Wanli, Prendergast David, Ceder Gerbrand, Toney Michael F, Chueh William C
Department of Materials Science and Engineering, Stanford University, Stanford, CA, USA.
Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA, USA.
Nat Mater. 2019 Mar;18(3):256-265. doi: 10.1038/s41563-018-0276-1. Epub 2019 Feb 4.
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 LiIrSnO 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 Å metal-oxygen π bonds and approximately 1.4 Å O-O dimers during oxygen redox, which occurs in LiIrSnO 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.
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。氧阴离子氧化还原因其在多种氧化物材料中相对于Li/Li在超过4V时提供大量氧化还原容量，在这类应用中，特别是在锂离子电池中，引起了广泛关注。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相关，这目前阻碍了其广泛应用。通过全面研究LiIrSnO模型体系，该体系在循环过程中随着y表现出可调的氧化态和结构演变，我们揭示这种结构-氧化还原耦合源于氧氧化还原过程中短的约1.8Å金属-氧π键和约1.4Å O-O二聚体的局部稳定，这在LiIrSnO中通过配体到金属的电荷转移发生。至关重要的是，这些氧化氧物种的形成需要通过在相邻阳离子位点形成空位使氧去配位到单个共价键合伙伴，从而导致阳离子无序。这些见解为为什么在循环过程中阴离子氧化还原常常伴随着局部结构无序和电压滞后提供了一个点缺陷解释。我们的发现为富锂层状氧化物的独特电化学性质提供了解释，对采用氧氧化还原化学的材料设计具有普遍意义。
