Modification of Transition-Metal Redox by Interstitial Water in Hexacyanometalate Electrodes for Sodium-Ion Batteries

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...

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Bibliographic Details
Published in:Journal of the American Chemical Society 2017-12, Vol.139 (50), p.18358-18364
Main Authors: Wu, Jinpeng, Song, Jie, Dai, Kehua, Zhuo, Zengqing, Wray, L. Andrew, Liu, Gao, Shen, Zhi-xun, Zeng, Rong, Lu, Yuhao, Yang, Wanli
Format: Article
Language:English
Subjects:
Batteries
Electrodes
ENERGY STORAGE
MATERIALS SCIENCE
Quantum mechanics
Redox reactions
Transition metals
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Summary: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 Na x MnFe­(CN)6·zH2O. 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.
ISSN:0002-7863
1520-5126
DOI:10.1021/jacs.7b10460