Soft Carbon as Cathode with High Rate Performance for Dual-Ion Batteries via Fast PF6 - Intercalation Improved by Surface Effect

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.

Electrochemical intercalation of anions into graphite: Fundamental aspects, material synthesis, and application to the cathode of dual-ion batteries

In this review, fundamental aspects of the electrochemical intercalation of anions into graphite have been first summarized, and then described the electrochemical preparation of covalent-type GICs and application of graphite as the cathode of dual-ion battery. Electrochemical overoxidation of anion GICs provides graphite oxide and covalent-fluorine GICs, which are key functional materials for various applications including energy storage devices. The reaction conditions to obtain fully oxidized graphite has been mentioned. Concerning the application of graphite for the cathode of dual-ion battery, it stably delivers about 110 mA h g-1 of reversible capacity in usual organic electrolyte solutions. The combination of anion and solvent as well as the concentration of the anions in the electrolyte solutions greatly affect the performance of graphite cathode such as oxidation potential, rate capability, cycling properties, etc. The interfacial phenomenon is also important, and fundamental studies of charge transfer resistance, anion diffusion coefficient, and surface film formation behavior have also been summarized. The use of smaller anions, such as AlCl4 -, Br- can increase the capacity of graphite cathode. Several efforts on the structural modification of graphite and development of electrolyte solutions in which graphite cathode delivers higher capacity were also described.