Performances and thermal stability of LiCoO2 cathodes encapsulated by a new gel polymer electrolyte

Abstract

A unique approach for improving the thermal stability of delithiated LiCoO2 cathodes has been presented, which is based on the encapsulation of LiCoO2 by a new cyano-substituted polyvinylalcohol (cPVA)-based gel polymer electrolyte. In a bid to maximize the effect of encapsulation, the cPVA-based gel polymer electrolyte was applied to the LiCoO2 cathode with a predetermined degree of porosity. Through this new process, the gel polymer electrolyte is expected to locate preferentially onto the LiCoO2 at well-controlled thickness, with the overall porous structure of the modified cathode being little influenced. Due to the presence of cyano ({single bond}CN) groups, the cPVA shows high dielectric constant (ε = 15 at 1 kHz at 20 °C), which is thus expected to enhance the Li-salt dissociation, leading to the excellent ionic conductivity (around 7 mS cm at 25 °C). Under the assumption that the LiCoO2 could be fully covered with the cPVA, the encapsulated thickness is calculated around 10 nm, which was further evidenced by the FE-SEM results. Meanwhile, compared to the pristine LiCoO2 cathode (ΔH = 413 J g-1), the modified LiCoO2 cathode exhibited the superior thermal stability (ΔH = 31 Jg-1) and also presented the satisfactory C-rate performances and cyclability. Such a remarkable enhancement in the thermal stability and the electrochemical performances has been discussed on the basis of the morphology of the modified LiCoO2 cathode and the electrochemical properties of the cPVA-based gel polymer electrolytes.