(Invited) Fluorinated Electrolytes for 5-V Li-Ion Chemistry
Authors: Zhang Zhengcheng , Hu Libo , He Meinan , Su Chi-Cheung , Tornheim Adam 
Published: February 2020

The lithium-ion battery is an ideal power source for electrified vehicles due to its long cycle life and high energy and power density. To further increase the energy density, the general approach is to use cathode materials with high operating voltages (5 V vs. Li+/Li) and high specific capacity (250 mAh/g). Tremendous attention has been paid to raising the specific capacity and the discharge voltage of the cathode materials. For example, two-Li+-ion material such as silicate Li2FeSiO4 and fluorophosphate Li2FePO4F has been proposed and investigated as a high capacity cathode. The theoretical capacity of these two materials is high, 323 and 292 mAh/g, respectively. There also have been many studies on expanding the operating voltage. Cathode materials with a potential of more than 5 V have been proposed, for example, olivine-type LiNiPO4 and LiCoPO4, and normal spinel-type LiNi0.5Mn1.5O4  and LiCoMnO4. However, these cathodes require an electrolyte with high oxidation stability to deliver the full capacity. Okada et al. proposed Li2CoPO4F as both a high-capacity and a high-voltage cathode material. However, its practical capacity is 120 mAh/g, and its energy density showed no advantage over LiCoPO4 because of electrolyte decomposition above 5 V.

The conventional lithium-ion battery employs organic carbonate esters as the electrolyte solvent, in particular, mixtures of ethylene carbonate (EC) with dimethyl carbonate (DMC), diethyl carbonate (DEC), and/or ethyl methyl carbonate (EMC) dissolved in LiPF6 salt. This electrolyte continuously decomposes above 4.5 V vs. Li+/Li, limiting its application to a cathode chemistry that delivers capacity at a high charging voltage. Therefore, the demand for a high-voltage electrolyte has become a high priority for the development of lithium-ion batteries with high energy density.

Several studies have been dedicated to the development of high voltage electrolytes such as sulfones, ionic liquids, and dinitriles, as well as electrolyte additives that stabilize the charged cathode surface to afford a reversible Li+-intercalation chemistry in the coveted 5-V region. These electrolyte materials can provide high anodic stability, but they suffer from their intrinsic high viscosity, low dielectric constant, and low conductivity. More importantly, they do not form a solid-electrolyte interphase (SEI) on carbonaceous anode material. Moreover, it is a major challenge to develop an electrolyte additive for these new electrolytes that will provide the SEI formation required for extended cycling performance, especially under abuse conditions. Although an additive for cathode SEI formation was reported to be able to suppress the reactivity of the charged electrode and electrolyte, a superior electrolyte system comprising solvents with intrinsic anodic stability is the ultimate solution for high voltage electrolytes.

In this presentation, new electrolyte based on fluorinated carbonate solvents were designed, syntehsized and evaluated with high voltage cathode materials at elevated temperature. The theoretically high anodic stability of these new electrolytes was supported by electrochemical evaluation results using LiNi0.5Mn1.5O4 spinel cathodes and LiNi0.5Co0.2Mn0.3O2 cathodes. Fluorinated carbonate appears to be a suitable electrolyte candidate for transition metal oxide cathodes at high voltage.