Understanding the Behaviour of High-Nickel NMC Cathodes with Respect to the Vinylene Carbonate Additive

Abstract

The demand for secondary batteries has shown continual and rapid growth, driven by the societal transition to electric and hybrid modes of transportation, with the lithium-ion battery finding extensive application throughout the energy storage industry.1–3 To attain the energy density, lifetime and reliability required to satiate the needs of society, research efforts have largely focused on the development of novel transition metal oxide based cathode materials.4–6 The battery performance is intrinsically tied to the composition of its constituent parts, with understanding the chemical interactions and implications of a given cell chemistry being critical to ensuring fully optimised performance.Additives are commonplace in cell chemistries, with the performance benefits of chemicals such as vinylene carbonate and fluoroethylene carbonate proving instrumental in the formation of a thin, stable, and robust graphite solid-electrolyte interphase.7–9 Vinylene carbonate has been shown to improve the performance of a huge variety of cells containing a range of cathode chemistries, from the trailblazing lithium cobalt oxide, through to the next generation high-nickel NMC materials and, as such, has achieved extensive commercial success, demonstrated by the recent material price increases driven by amplified global demand.6,8,10,11 It is important to remember that altering the cell chemistry, results in changes in the intrinsic electrochemical and chemical reactions that occur at the electrode-electrolyte interfaces and in the electrolyte solution, with the key to enhanced performance lying in the understanding of these processes.Here we consider and analytically examine the role of vinylene carbonate in cells containing a high nickel-NMC cathode material. Previous research has suggested that vinylene carbonate is formed in early stage cycling of these materials, however, the proposed formation route and detection with adequate sensitivity are areas of contention.12,13 We have combined a range of methods including cell cycling, OEMS, NMR, GC-MS, and fluorimetry to understand the feasibility of vinylene carbonate formation at high nickel-NMC cathodes and have been successful in identifying its presence in extracted electrolyte. The results of these studies will be discussed, along with their implications for cell performance and our understanding of the role of vinylene carbonate.