Enhanced High-Temperature Performance of LiNi_xCo_yMn_1-x-y  O₂ Cathode by Polymeric Artificial Solid Electrolyte Interphases

Abstract

Layered Li ternary transition metal oxides, LiNixCoyMn1-x-yO2 (denoted as NCM) is an important cathode material for lithium ion batteries because of its high capacity, good thermal stability, low cost, and low toxicity. However, its cycle life deteriorates rapidly with increasing operating temperature, because of strong interaction at the solid-electrolyte interface (SEI) causing rapid SEI formation and dissolution of the transitional metal ions. To solve these problems, some of researchers proposed to partially substituting the transitional metals of NCM with different metals, such as Mg2+, Al3+, etc., to stabilize a cation disordered structure after the transitional metals dissolve from the material. However, during long-term cycling, particularly at elevated temperatures, the LiPF6 based electrolyte can readily decompose and reacts with residual water in the electrolyte to generate hydrofluoric acid (HF). The acidic attacking from HF and other side reactions from electrolyte would still deteriorate the active material. On this account, surface coating with metal oxides such as MgO, TiO2, ZrO2, ZnO, Al2O3, etc., had been employed to prevent direct contact with the electrolyte solution. Unfortunately, these approaches also lead to additional drawbacks, such as cracking of the oxide layers, decrease of ion-conductivity and increase of production costs from additional complex steps. In this research, a simple approach using polymeric artificial SEIs to modify the surface of cathode materials to enhance the cyclic and rate performance of the Li-ion batteries at elevated temperatures is proposed. Various polymeric blends were employed for modification of the cathode surface. The results show that the polymeric SEIs can substantially suppress the deteriorating reactions between the electrolyte and electrode, improve the structural stability and decrease the polarization of the material during the electrochemical operations. Comparing to the pristine cell, the modified cells show better performances in both cycle life and rate capability, maintaining lower cell impedance due to the reduced electrolyte decomposition and enhanced structure stability. The underlying mechanism is discussed. Figure 1