Effect of Additives on Rechargeable NCA/Graphite 18650 Li-Ion Batteries

Abstract

Electrolyte additive is one of the most important parts in the development of stable Lithium-ion batteries (LIBs) with high efficiency and durability. In particular, a commonly used nickel cobalt aluminum (NCA) cathode with graphite anode battery is receiving growing interest from their high specific energy (275 mAhg-1), long life span, and good energy density. However, Ni-rich cathode material trends to suffer from the unstable solid electrolyte interface (SEI) layer on the anode, leading to capacity fading after charge-discharge cycles. Among several parameters affecting on the SEI formation, electrolyte additive is a crucial and practical factor capable of being optimized in a large-scale battery production process. In this work, vinyl carbonate (VC) and fluoroethylene carbonate (FEC) were used as additives in an electrolyte of 1 M LiPF6 in a mixed solvent system consisting ethylene carbonate (EC), Ethylene methyl carbonate (EMC), and dimethyl carbonate (DMC) at a volumetric ratio of 1:1:1. The effect of each additive on the creation of SEI structure and its further effect on the battery cycle stability was investigated. A systematic study of additive contents in the control electrolyte showed that the highest coulombic efficiency of the NCA/graphite battery was achieved when 2wt% VC was used, while an optimum FEC content can be up to 4wt%. The combination of VC and FEC at different ratios were also studied and their synergistic effect on the battery performances was revealed by SEI investigation.