Enhanced structural and cycling stability of Li2CuO2-coated LiNi0.33Mn0.33Co0.33O2 cathode with flexible ionic liquid-based gel polymer electrolyte for lithium polymer batteries

Abstract

Recently, ionic liquid-based gel polymer electrolytes (IL-GPEs) attracted increasing attention because of its use in developing safe and flexible rechargeable lithium-based batteries. The IL-GPE composed of microporous polymer PVdF-HFP incorporating different weight percentages of ionic liquid PYR13FSI with 20 wt.% lithium salt LiTFSI is prepared. These prepared films are investigated in detail by thermogravimetric analysis, impedance spectroscopy, cyclic voltammetry and linear sweep voltammetry measurements for battery application. The 70 wt.% IL containing GPE shows excellent thermal stability up to ∼240 °C and high lithium ion conductivity (1.6 × 10−3 S cm−1 at 30 °C) with wide electrochemical stability window (∼4.3 V vs. Li/Li+ at 30 °C). Furthermore, the Li+-ion conductive Li2CuO2 is coated on LiNi0.33Mn0.33Co0.33O2 (Li2CuO2@LNMC) cathode particle by using a wet chemical method. The structural and electrochemical properties of pristine and Li2CuO2@LNMC cathode are investigated using XRD, SEM, TEM and electrochemical analysis. The XRD pattern shows that no impurity phase is present in the Li2CuO2@LNMC cathode. Thin Li2CuO2 coating layer (20–25 nm) on the surface of LNMC particle is confirmed by TEM image. The optimized electrolyte is used to fabricate Li-cells (Li/LNMC and Li/Li2CuO2@LNMC). The charge-discharge results show that the initial specific discharge capacity of Li2CuO2@LNMC is ∼196 mAh g−1 at 0.1C (25 mA g−1), whereas, the pristine LNMC has ∼182 mAh g−1 under same condition. The Li2CuO2 coating layer improves electrochemical performance and cyclic stability of pristine cathode up to 100 cycles at 1 C-rate. The capacity retention is ∼69% for Li2CuO2@LNMC over 100 charge-discharge cycles at 1C, whereas, the capacity retention is only ∼30% for pristine LNMC. After cycling, the EIS results also indicate that the impedance of Li/LNMC cell reduces after Li2CuO2 modification.