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A Theoretical study on the charge and discharge states of Na‐ion battery cathode material, Na 1+ x FePO 4 F
Author(s) -
Shinagawa Chikashi,
Morikawa Yusuke,
Nishimura Shinichi,
Ushiyama Hiroshi,
Yamada Atsuo,
Yamashita Koichi
Publication year - 2019
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.25582
Subject(s) - cathode , battery (electricity) , diffusion , phase diagram , phase (matter) , work (physics) , ion , chemistry , materials science , thermodynamics , physics , power (physics) , organic chemistry
Na 2 FePO 4 F is a promising cathode material for a Na‐ion battery because of its high electronic capacity and good cycle performance. In this work, first principle calculations combined with cluster expansion and the Monte Carlo method have been applied to analyze the charge and discharge processes of Na 2 FePO 4 F by examining the voltage curve and the phase diagram. As a result of the density functional theory calculation and experimental verification with structural analysis, we found that the most stable structure of Na 1.5 FePO 4 F has the P 2 1 / b 11 space group, which has not been reported to date. The estimated voltage curve has two clear plateaus caused by the two‐phase structure composed of P 2 1 / b 11 Na 1.5 FePO 4 F and Pbcn Na 2 FePO 4 F or Na 1 FePO 4 F and separated along the c ‐axis direction. The phase diagram shows the stability of the phase‐separated structure. Considering that Na 2 FePO 4 F has diffusion paths in the a ‐ and c ‐axis directions, Na 2 FePO 4 F has both innerphase and interphase diffusion paths. We suggest that the stable two‐phase structure and the diffusion paths to both the innerphase and interphases are a key for the very clear plateau. We challenge to simulate a nonequilibrium state at high rate discharge with high temperature by introducing a coordinate‐dependent chemical potential. The simulation shows agreement with the experimental discharge curve on the disappearance of the two plateaus. © 2018 Wiley Periodicals, Inc.