Weak coulomb interaction between anions and Na+ during solvation enabling desirable solid electrolyte interphase and superior kinetics for HC-based sodium ion batteries

Abstract

Electrolyte plays an important role in conducting sodium ions and carrying out chemical/electrochemical reactions. However, the impact mechanism of microscopic solvation structure remains vague but is significant for the electrolyte design. Here we reveal that the inner relationship between solvation structure and electrochemical reaction is manifested intuitively by manipulating the anions in the solvated shell, which determines Na storage kinetics and SEI evolution process. The result elucidates the weak coulomb interaction between PF6− and Na+ could promote Na+ transport number in bulk liquid and Na storage kinetics. And it could induce PF6− to reduce preferentially and facilitate additive to enter solvation structure, thereby forming a thin&compact&protective SEI with layered morphology. The excellent SEI would protect anode effectively, inhibiting decomposition of solvents and improving battery performances. As expected, 5 Ah Na3V2(PO4)3-hard carbon pouch-type batteries with well-matched PF6−-based electrolyte display a high specific energy of 129 Wh kg−1, a superior rate capability with capacity retention of 90 % at 5 C and cycling stability with retention of 86.82 % at 1 C after 700 cycles.