Analysis of PF₆⁻ Anion Intercalation/De‐Intercalation Mechanisms in Graphite Cathodes of Sodium Dual‐Ion Batteries

Abstract

A detailed study of the intercalation/de‐intercalation mechanisms of PF6− anions in a graphite cathode of sodium dual‐ion batteries is carried out by cyclic voltammetry. The influence of the continuous anion intercalation/de‐intercalation on the graphite structure is investigated by ex‐situ XRD after cycling. It was concluded that the intercalation/de‐intercalation occurs through a combination of diffusion‐controlled and pseudocapacitive mechanisms. Initially, the contribution of the diffusion‐controlled is significant, specifically at the highest voltage and at the lowest scan rates which agrees with the slow kinetic of this mechanism and its usual prevalence in carbon materials with a graphitic structure. However, the continuous anion intercalation/de‐intercalation causes some deterioration of the graphite structural order, even from the initial cycles, as it was demonstrated by the evolution of the crystalline parameters, interlayer spacing, d002, and crystallite size, Lc. This deterioration hinders the diffusion‐controlled anion intercalation/de‐intercalation and, as a consequence, the pseudocapacitive becomes the main mechanism along cycling. Therefore, since graphite is capable of intercalating PF6− anions at high voltages through a capacitive mechanism, with its consequent rapid kinetics, this carbon material is an excellent candidate for using as cathode in high power sodium dual‐ion batteries, in which, kinetically fast mechanisms are required.