First-principles investigation on structural and electrochemical properties of NaCoO₂ for rechargeable Na-ion batteries

SU Jing-cang(粟劲苍)¹, ZHOU Guang(周广)^{1, 2}, PEI Yong(裴勇)¹, YANG Zhen-hua(杨振华)³, WANG Xian-you(王先友)¹

(1. Key Laboratory of Environmentally Friendly Chemistry and Applications of Minister of Education,
(School of Chemistry, Xiangtan University), Xiangtan 411105, China;
2. School of Materials Science and Engineering, Central South University, Changsha 410083, China;
3. Faculty of Materials, Optoelectronics and Physics, Key Laboratory of Low Dimensional Materials &
Application Technology of Ministry of Education, Xiangtan University, Xiangtan 411105, China)

Abstract:Na_xCoO₂ is a commonly used cathode material for sodium ion batteries because of its easy synthesis, high reversible capacity and good cyclability. the structural and electrochemical properties of Na_xCoO₂ during sodium ion insertion/extraction process are studied based on first principles calculations. The calculation results of crystal structure parameters and average intercalation voltage are in good agreement with experiment data. Through calculation of the geometric structure and charge transfer in charging and discharging processes of Na_xCoO₂, it is found that the oxygen atom surrounding Co of the CoO₆ octahedral screens the coulomb potential produced by sodium vacancy in Na_xCoO₂, and the charge is removed from the entire Co-O layer instead of the Co atom adjacent to sodium vacancy when sodium ions are extracted from the NaCoO₂ lattice. Thus, during the insertion/extraction of sodium ion from NaCoO₂, the CoO₆ octahedral structure undergoes small lattice distortion, which makes the local structure quite stable and is beneficial to the cycling stability of the material for the application of sodium ion batteries.

Key words:sodium ion batteries; first principles calculation; cathode material; electronic structure