Xueyi Luo

Structural Exfoliation of Layered Cathode under High Voltage and Its Suppression by Interface Film Derived from Electrolyte Additive

Layered cathodes for lithium-ion battery, including LiCo1-x-yNixMnyO2 and xLi2MnO3·(1-x)LiMO2 (M = Mn, Ni, and Co), are attractive for large-scale applications such as electric vehicles, because they can deliver additional specific capacity when the end of charge voltage is improved to over 4.2 V. However, operation under a high voltage might cause capacity decaying of layered cathodes during cycling. The failure mechanisms that have been given, up to date, include the electrolyte oxidation decomposition, the Ni, Co, or Mn ion dissolution, and the phase transformation. In this work, we report a new mechanism involving the exfoliation of layered cathodes when the cathodes are performed with deep cycling under 4.5 V in the electrolyte consisting of carbonate solvents and LiPF6 salt. Additionally, an electrolyte additive that can form a cathode interface film is applied to suppress this exfoliation. A representative layered cathode, LiCoO2, and an interface film-forming additive, dimethyl phenylphosphonite (DMPP), are selected to demonstrate the exfoliation and the protection of layered structure. When evaluated in half-cells, LiCoO2 exhibits a capacity retention of 24% after 500 cycles in base electrolyte, but this value is improved to 73% in the DMPP-containing electrolyte. LiCoO2/graphite full cell using DMPP behaves better than the Li/LiCoO2 half-cell, delivering an initial energy density of 700 Wh kg -1 with an energy density retention of 82% after 100 cycles at 0.2 C between 3 and 4.5 V, as compared to 45% for the cell without using DMPP.

Diethyl(thiophen-2-ylmethyl)phosphonate: a novel multifunctional electrolyte additive for high voltage batteries

Carbonate-based electrolytes used in Li-ion batteries encounter various challenges in extreme electrochemical environments, and hence their application requires various additives, especially when used with high voltage cathode materials. These additives are designed to form protective interphases that prevent parasitic carbonate oxidation, while in certain cases they stabilize electrolytes from reduction at anode surfaces or even serve as flame-retardants that postpone the thermal runaway during overcharge. However, most of these additives casts negative effects, lowering ionic conductivity of electrolyte or impairing the compatibility between cathode and electrolyte. An ideal solution of minimizing the presence of these inert molecules is to identify an additive that structurally integrates these multiple functions into a single compound. In this work, we report a novel additive, diethyl(thiophen-2-ylmethyl)phosphonate (DTYP). Its 0.5% presence in a base electrolyte dramatically improves the capacity retention of a high voltage Li-ion cell using LiNi0.5Mn1.5O4 from 18% to 85% after 280 cycles at 1C at 60 °C, increases the endothermic reaction onset temperature from 193 °C to 223 °C, and reduces the self-extinguishing time of the electrolyte from 88 s to 77 s. Thus, such a multifunctional additive presents a cost-efficient solution to the issues often faced in high voltage lithium-ion batteries.