Geballe Laboratory for Advanced Materials, Stanford University , Stanford, California 94305, United States.
Advanced Light Source, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
J Am Chem Soc. 2017 Dec 20;139(50):18358-18364. doi: 10.1021/jacs.7b10460. Epub 2017 Dec 5.
A sodium-ion battery (SIB) solution is attractive for grid-scale electrical energy storage. Low-cost hexacyanometalate is a promising electrode material for SIBs because of its easy synthesis and open framework. Most hexacyanometalate-based SIBs work with aqueous electrolyte, and interstitial water in the material has been found to strongly affect the electrochemical profile, but the mechanism remains elusive. Here we provide a comparative study of the transition-metal redox in hexacyanometalate electrodes with and without interstitial water based on soft X-ray absorption spectroscopy and theoretical calculations. We found distinct transition-metal redox sequences in hydrated and anhydrated NaMnFe(CN)·zHO. The Fe and Mn redox in hydrated electrodes are separated and are at different potentials, leading to two voltage plateaus. On the contrary, mixed Fe and Mn redox in the same potential range is found in the anhydrated system. This work reveals for the first time how transition-metal redox in batteries is strongly affected by interstitial molecules that are seemingly spectators. The results suggest a fundamental mechanism based on three competing factors that determine the transition-metal redox potentials. Because most hexacyanometalate electrodes contain water, this work directly reveals the mechanism of how interstitial molecules could define the electrochemical profile, especially for electrodes based on transition-metal redox with well-defined spin states.
钠离子电池（SIB）在电网规模的电能存储方面具有吸引力。由于其易于合成和开放框架，低成本的六氰合金属配合物是 SIB 有前途的电极材料。大多数基于六氰合金属配合物的 SIB 采用水电解质工作，并且已经发现材料中的间隙水强烈影响电化学轮廓，但机制仍难以捉摸。在这里，我们基于软 X 射线吸收光谱和理论计算，对具有和不具有间隙水的六氰合金属配合物电极中的过渡金属氧化还原进行了比较研究。我们发现水合和脱水的 NaMnFe（CN）·zHO 中存在明显不同的过渡金属氧化还原序列。水合电极中 Fe 和 Mn 的氧化还原是分开的，并且处于不同的电位，导致两个电压平台。相反，在无水体系中发现相同电位范围内的混合 Fe 和 Mn 氧化还原。这项工作首次揭示了电池中的过渡金属氧化还原如何受到间隙分子的强烈影响，这些分子似乎是旁观者。结果表明，基于三个决定过渡金属氧化还原电位的竞争因素的基本机制。由于大多数六氰合金属配合物电极都含有水，因此这项工作直接揭示了间隙分子如何定义电化学轮廓的机制，特别是对于基于具有明确定义自旋态的过渡金属氧化还原的电极。
