Conventional Li-ion cathodes store charge by reversible intercalation of Li coupled to metal cation redox. There has been increasing interest in new materials capable of accommodating more than one Li per transition-metal center, thereby yielding higher charge storage capacities. We demonstrate here that the lithium-rich layered iron sulfide Li2FeS2 as well as a new structural analogue, LiNaFeS2, reversibly store ≥1.5 electrons per formula unit and support extended cycling. Ex situ and operando structural and spectroscopic data indicate that delithiation results in reversible oxidation of Fe2+ concurrent with an increase in the covalency of the Fe-S interactions, followed by reversible anion redox: 2 S2-/(S2)2-. S K-edge spectroscopy unequivocally proves the contribution of the anions to the redox processes. The structural response to the oxidation processes is found to be different in Li2FeS2 in contrast to that in LiNaFeS2, which we suggest is the cause for capacity fade in the early cycles of LiNaFeS2. The materials presented here have the added benefit of avoiding resource-sensitive transition metals such as Co and Ni. In contrast to Li-rich oxide materials that have been the subject of so much recent study and that suffer capacity fade and electrolyte degradation issues, the materials presented here operate within the stable potential window of the electrolyte, permitting a clearer understanding of the underlying processes.

中文翻译：

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富锂硫化铁阴极中的多电子、阳离子和阴离子氧化还原

传统的锂离子阴极通过可逆嵌入与金属阳离子氧化还原偶联的锂来储存电荷。人们对能够在每个过渡金属中心容纳超过一个锂的新材料越来越感兴趣，从而产生更高的电荷存储容量。我们在这里证明了富锂层状硫化铁 Li2FeS2 以及一种新的结构类似物 LiNaFeS2，每个配方单元可逆地存储≥1.5 个电子并支持延长循环。异位和原位结构和光谱数据表明，脱锂导致 Fe2+ 的可逆氧化，同时 Fe-S 相互作用的共价键增加，随后是可逆的阴离子氧化还原：2 S2-/(S2)2-。S K-edge 光谱明确地证明了阴离子对氧化还原过程的贡献。发现 Li2FeS2 对氧化过程的结构响应与 LiNaFeS2 不同，我们认为这是 LiNaFeS2 早期循环中容量衰减的原因。此处介绍的材料具有避免使用 Co 和 Ni 等资源敏感过渡金属的额外好处。与最近研究的主题并遭受容量衰减和电解液降解问题的富锂氧化物材料相比，此处介绍的材料在电解液的稳定电位窗口内运行，从而可以更清楚地了解潜在过程. 此处介绍的材料具有避免使用 Co 和 Ni 等资源敏感过渡金属的额外好处。与最近研究的主题并遭受容量衰减和电解液降解问题的富锂氧化物材料相比，此处介绍的材料在电解液的稳定电位窗口内运行，从而可以更清楚地了解潜在过程. 此处介绍的材料具有避免使用 Co 和 Ni 等资源敏感过渡金属的额外好处。与最近研究的主题并遭受容量衰减和电解液降解问题的富锂氧化物材料相比，此处介绍的材料在电解液的稳定电位窗口内运行，从而可以更清楚地了解潜在过程.