Designing Electrolyte Solvation Structure for High Voltage Dual-Ion Batteries

Abstract

Graphite is a potential cathode material for dual-ion batteries (DIBs) as anions can be intercalated into and deintercalated from it at a high potential of more than 4.5 V vs. Li/Li⁺. There have been many research papers reporting the high rate capability of the process with various electrolyte salts and solvents. However, several scientific aspects of the electrodes such as the thermal stability of the charged cathode, compatibility with the electrolyte salt and solvents, and the low temperature performance remain unclear. These topics are studied in this thesis.

In Chapter 3, I investigated the thermal stability of the graphite cathode with various lithium salts such as LiFSI, LiPF₆, LiTFSI, and LiBF₄ and solvents, such as ethyl methyl carbonate (EMC) and fluoroethylene carbonate (FEC). The results are also compared with thermal stability of common cathodes for lithium-ion batteries such as LiCoO₂ and LiNi_0.5Mn_1.5O₄ and graphite anode.

As LiFSI was found to be a salt with better thermal stability, in Chapter 4, I focused on studying suitable LiFSI-based electrolyte for graphite cathode. Specifically, I found that the electrochemical performance of graphite, side reaction and Al current collector corrosion are susceptible to the salt concentration. The composition of the electrolyte also affects its viscosity, ionic conductivity, as well as the corresponding kinetics of anion transport into graphite. In Chapter 5, I continued to study the solvation structure of the electrolyte, as well as the effect of weakly solvating solvent such as methyl acetate (MA), to understand the governing factors affecting the electrochemical performance.

Since graphite cathodes in DIBs can provide high-rate performance at room temperature, they have great potential as cathode materials for batteries used in low-temperature applications. In Chapter 6, I further tailored the solvation structure and viscosity of the electrolyte using low-viscosity, low-freezing-point solvent such as fluoroethyl acetate (FEA).

Overall conclusions on the mechanism of anion intercalation into and deintercalation from graphite as well as the effect of solvation structure of the electrolyte, are provided in Chapter 7 of the thesis. Further scientific questions and proposed future studies are also discussed in Chapter 7.

Date of Award	25 Aug 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Patrick SIT (Supervisor) & Denis YU (External Co-Supervisor)