Effect of slight structural difference in polyvinylidene fluoride binders on the electrochemical performance of single-crystal LiNi0.5Co0.2Mn0.3O2 cathode

Abstract

Polyvinylidene fluoride (PVDF) binder is an important part of lithium-ion batteries, where a slight structural difference can significantly affect the capacitance and service life of batteries. Herein, ATR-FTIR technology and a rheological test were employed to investigate the crystal forms and rheological behavior of three different PVDF binders, respectively. The results showed that PVDF-CG has low molecular weight, high α/β phase ratio, strong adhesive action and stripping strength, which are closely associated with the durability and electrochemical properties of LiNi_0.5Co_0.2Mn_0.3O₂ single-crystal (denoted as SC) materials. The SCs with PVDF-CG binder exhibited a capacity retention of 93.44% after 100 cycles at a rate of 1C and high rate performance (92 mA h g⁻¹ at 10C), which was higher than that of PVDF-HSV900 (90.31%, 86 mA h g⁻¹ at 10C) and PVDF-Solvay 5130 (90.52%, 72 mA h g⁻¹ at 10C). The XPS and HR-TEM analysis showed that the cycled SCs bonded with HSV900 and Solvay 5130 as binders suffered from severe interface structural degradation, while SCs with PVDF-CG maintained a well-layered structure and the formation of a surface NiO-like rock-salt phase was greatly inhibited. This work demonstrated that the capacity, rate capability and interface stability of SC cathodes are significantly affected by the mode and strength of the adhesion and stripping strength of PVDF binders, which are closely associated with α/β phase ratios, providing a useful reference for the development of efficient binders.