Theoretical Study of Electrode and Electrolyte Materials in Graphite-Based Dual-Ion Batteries

石墨基二元離子電池電極材料及電解質的理論研究

Student thesis: Doctoral Thesis

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Award date13 Jan 2021

Abstract

The graphite-based dual-ion battery (DIB) has shown promising prospects for energy storage applications. It has the potential to compete with the current state-of-the-art lithium-ion battery due to its potential low cost, high energy capacity, and enhanced environmental friendliness. However, many key issues in this class of batteries still need to be addressed before it can be adopted for widespread application. Although experimental studies have provided important insights into the development of DIBs, their microscopic understanding is still limited. Density functional theory and molecular dynamics simulations are useful tools to fill this knowledge gap, a feat that is difficult to accomplish by experiments alone.

This thesis reports a systematic study of DIBs using ab initio methods and classical molecular dynamic simulations to provide in-depth knowledge of the effects of electrolyte salts, the staging processes of graphite-based materials, and electrolyte solvents on the battery properties. First, the effects of the salt ion on the onset voltage of graphite-based DIBs were studied in detail. The overall discharging process was analyzed by dividing it into three steps: extraction, ionization, and solvation. Second, the staging mechanism of graphite intercalated compounds (GICs) during the charging process and its role in determining the battery properties were examined. Finally, classical molecular dynamics simulations were used to explore the details of how different electrolyte solvents affect the battery performance.

In the study of salt ions, we performed density functional theory calculations to investigate the effects of different salt ions, including four anions (BF4, ClO4, PF6, and bis[trifluoromethanesulfonyl]imide [TFSI]) and three cations (Li+, Na+, and K+) on the onset voltage of the DIB. We found that the extraction and ionization energetics for each anion nearly compensate each other, resulting in small differences between their net voltage contributions. Therefore, the solvation energy of the anion is the main contribution to the voltage difference between the different anions. However, the strong interaction between the cations and the graphene layers results in a significant contribution from the extraction and ionization steps to the onset voltage.

In the study of the staging mechanism, we studied the intercalation of multiple intercalants at different stages using full graphite models. We systematically investigated the properties of staging in GICs. For example, the favorability of the intercalation process of different species was compared. In addition, for the same intercalation stage, we found that the initial intercalation is the least favorable step because of the larger energy required to expand the graphene interlayer distance. The electronic structural results show that the electronic bands shift down and up relative to those of the graphite host for the donor GIC and acceptor GIC, respectively. Moreover, the contribution near the Fermi level for the different GICs was identified. The relationship between the cell voltage, the intercalation energy, and the stages of intercalation were also analyzed in detail.

In the solvent work, solvents such as diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and their binary mixtures were treated explicitly using classical molecular dynamics simulations. The solvation structures of the LiPF6 salt in the electrolytes were studied using radial distribution functions. The effects of different solvent types, salt concentrations, and solvent mixing ratios on the solvation structure were thoroughly examined. The solvation free energy of the salt was also analyzed to compare the effect of the solvent type and salt concentration at the solvation step.

In summary, we have carried out a systematic theoretical study on the active materials in graphite-based DIBs, including the electrolyte salts, the electrode materials, and the solvents. This thesis advances the microscopic understanding of DIBs to contribute to the development of this class of energy storage devices.