Unveiling the Impact of the Polypyrrole Coating Layer Thickness on the Electrochemical Performances of LiNi0.5Co0.2Mn0.3O2 in Li-Ion Battery

Coating polypyrrole (PPy) on the surface of cathode materials could effectively enhance the electrochemical performances of lithium ion batteries (LIBs). The effect of the coated PPy layer thickness on the electrochemical performance, however, has not been well revealed. In this work, we study experimentally the impact of the PPy coating layer thickness on the electrochemical performance of the cathode materials in LIBs. The surface-coated LiNi0.5Co0.2Mn0.3O2 (NCM) with different PPy layer thickness was synthesized by in situ growing PPy on the surface of NCM. When these coated-samples were tested as cathodes of LIBs, the sample PPy5 with a coating layer thickness of about 3 nm exhibits an optimum electrochemical performance. More detailed analysis of electrochemical impedance spectra and structures for the cycled electrodes show that the coating layer of about 3 nm endows the PPy5 cathode with the smallest resistance and the most stable structure, ensuring its excellent cycle stability and rate performance. Comparatively, coating layer thickness of about 1 nm is too thin to maintain the structural stability of PPy1 cathode during a long-term cycle, and an over thick coating layer (approximately 5 nm) significantly increases the resistance due to the shedding of the electrode, which results in their poor electrochemical performance. The results given in present paper will provide the guidance for optimizing the electrochemical properties of conductive polymer coated cathode materials.