Solvation structure, transport and stability in Li-ion battery electrolytes

Thesis

The electrolyte is a key component in LIB, which will play a crucial role in mitigating the global dependence on fossil fuels through the electrification of vehicles and the storage of energy from renewable sources. Our understanding of these devices continually evolves as we delve into the reactions occuring at the interface between the electrodes and the electrolyte. To further improve LIB energy densities for vehicle electrification and grid-scale energy storage applications through the use of high voltage cathode materials, a thorough understanding of electrolyte solvation structure, transport and electrochemical stability must be achieved.

The effect of salt concentration on the electrochemical stability of LiPF₆ in EC : EMC (vol:vol 3:7) electrolytes at high voltage, Ni-rich cathode interfaces has been investigated. Initially, we investigate the CEI, examining the influence of LiPF6 salt concentration on its composition in NMC811 and LTO cells under constant potential holds. We propose reaction mechanisms describing the degradation of electrolyte components on the NMC811 surface by considering the observed degradation and CEI composition post cycling.

Next, the relationship between electrolyte solvation structure and continuum scale transport properties are linked by investigating the behaviour of electrolytes under applied currents. We calculate D_ij, and Gam_ij directly from MD simulations for LIB electrolytes in the MS diffusion formulation. This approach combines MD with operando confocal Raman microspectroscopy to test the accuracy of these parameters against experimental concentration gradients formed under a constant current. We present the computed values of these parameters for electrolytes investigated herein.

Finally, we study the relationship between electrolyte solvation structure and electrochemical stability using MD, DFT, XAS and Raman spectroscopy. MD, Raman spectroscopy and XAS reveal the influence of salt concentration on the solvation environments in LIB electrolytes. DFT reveals the effect of salt concentration on the HOMO and LUMO states. The inferred impact of these changes on both the oxidative and reductive stabilities of the electrolyte are compared to experimental findings.

This comprehensive study offers new insights into the correlation between electrolyte solvation structure and salt concentration, elucidating its consequential effects on transport properties, electrochemical stability and CEI composition, particularly concerning high voltage nickel-rich NMC cathode materials in carbonate based LIB electrolytes. Such insights pave the way for informed strategies aimed at mitigating NMC degradation at elevated potentials, such as through the judicious selection of electrolyte additives to aid the widespread adoption of high voltage cathodes in electric vehicles and grid-scale energy storage systems.

Authors: Phelan, C

• DOI: 10.5287/ora-qry9qen8r
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: highly concentrated electrolytes; Maxwell-Stefan diffusion; molecular dynamics; cathode-electrolyte interphase
• Subjects: Materials Science; Chemistry; Electrochemistry; Physics