Understanding the sodium ion transport properties, deintercalation mechanism, and phase evolution of a Na2Mn2Si2O7 cathode by atomistic simulation.

Molecular dynamics (MD) together with the first principles method (DFT) reveal that Na+ is capable of migrating three dimensionally in a Na2Mn2Si2O7 cathode material. Migration along the a-axis and c-axis have the same mechanism, that is, alternating between the Na1 and Na2 route with a similar local environment and distance. Long-distance hopping between two Na2 atoms or between Na1 and Na2 atoms is crucial for continuous migration along the b-axis. Also, the anti-site phenomenon is identified, and it facilitates the migration of the Na ions. Four intermediate phases are determined according to the formation energy curve and, as a result, the voltage profile is predicted accurately. The state of charge (SOC) dependency of the Na+ energy shows that the mobility of Na+ is highly inhibited in the fully discharged state. Upon the deintercalation of sodium ions, Na+ is activated immediately. A maximal DNa+ value of 3.6 × 10-9 cm2 s-1 and a low energy barrier of ca. 0.26 eV at the deintercalation level of x = 0.25 are observed. Because of the scarcity of Na+, DNa+ experiences a sharp decrease at the end of deintercalation. Despite the low level of Na+ mobility in the range of 0.25 < x < 1, Na2Mn2Si2O7 is still a potential cathode material for use in sodium ion batteries (SIBs).

Citation

Yuting Xie, Keshu Dai, Qinyun Wang, FanPei Gu, Miao Shui, Jie Shu. Understanding the sodium ion transport properties, deintercalation mechanism, and phase evolution of a Na2Mn2Si2O7 cathode by atomistic simulation. Physical chemistry chemical physics : PCCP. 2021 Jan 21;23(2):1750-1758

PMID: 33427832

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