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

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.