PF6- and Na+ Co-intercalation in Graphite Cathode of Sodium Dual-Ion Batteries by Enhanced Surface Concentration Effect.

Dual-ion batteries are attracting much attention due to the joint participation of anions and cations in the energy storage process. However, this unique battery configuration imposes high demands on the cathode, which typically presents an inferior rate performance. Herein, we employ graphite in different microcrystalline sizes as cathodes, associated with high concentration electrolyte to construct sodium dual-ion batteries. The results of in-situ XRD and Raman evidence that the surface effect is enhanced by suitably small graphite microcrystals, where a greater surface involvement affords more electro-activated regions for the ions. Furthermore, the analysis of sputtering XPS confirms that the PF6- is accompanied by the co-intercalation of Na+ into cathodes by constructing the model of concentration effect, thus accelerating the kinetic process. In conclusion, the co-intercalation of PF6- together with Na+ is demonstrated under the influence of enhanced surface concentration effect in cathodes, and thus the cathodes exhibit a superior rate performance with a capacity of 103.6 mAh g-1 at a rate of 2 C and a rate retention of 94.8% even at 50 C. This work provides new insights to explain the mechanism of ion intercalation in dual-ion batteries and offers a perspective for the construction of high energy storage systems.