The Solvation Structure, Transport Properties and Reduction Behavior of Lithium-Ion Battery Electrolytes

Novel cathode materials and anode materials emerge as promising candidates for realizing next-generation energy storage. Along with the innovation of electrode materials, electrolytes that enable Li+ transport between electrodes during charge and discharge require to be redesigned as well. This is particularly important because the as-formed electrode-electrolyte interphase (SEI) is found crucial for the full cell operation. In Chapter 1, the components of conventional electrolytes and the formation mechanism of the SEI are briefly introduced. Moreover, some of the promising anode systems, i.e., Si anode and Li metal anode, are reviewed. Finally, approaches to stabilize the interphase of the anodes using novel liquid electrolytes and solid-state electrolytes (SSEs) are thoroughly discussed.In order to obtain a fundamental understanding of the solvation structure, transport properties, and reduction behavior of electrolyte systems of LIBs, modeling and simulation techniques including classical molecular dynamics (MD) and quantum chemistry calculations are widely utilized. In Chapter 2, the related theory and methods for building a consistent theoretical framework for evaluating both commercial and novel battery electrolytes are introduced. First, the procedures for modeling an electrolyte system using molecular dynamics are thoroughly discussed based on Frenkel and Smit. Second, the analytical equations to obtain transport properties from molecular dynamics trajectories are derived. Finally, the methodologies of calculating reduction and solvation properties from quantum chemical calculations are briefly introduced. Fluoroethylene carbonate (FEC) has been proposed as an effective electrolyte additive that enhances the stability and elasticity of the SEI. However, uncertainties still remain on the exact mechanism through which FEC alters the electrolyte decomposition and SEI formation process. In Chapter 3, the influence of FEC on a LiPF6/ethylene carbonate (EC) electrolyte is investigated through classical MD, Fourier-transform infrared spectroscopy, and quantum chemical calculations. FEC is found to significantly modify the solvation structure and reduction behavior of the electrolyte while being innocuous to transport properties. Even with limited 10% of FEC, the Li+ solvation structure exhibits a notably higher contact-ion pair ratio than the parent EC electrolyte. Moreover, FEC itself, as a new fluorine-containing species, appears in 1/5 of the Li+ solvation shells. The Li+-coordinated FEC is found to reduce prior to EC and uncoordinated FEC which will passivate the anode surface at an early onset by forming LiF. The critical role of FEC in tailoring the Li+ solvation structure and as-formed protective SEI composition provides mechanistic insight that will aid in the rational design of novel electrolytes. Despite the extensive employment of binary/ternary mixed-carbonate electrolytes (MCEs) for Li-ion batteries, the role of each ingredient with regards to the solvation structure, transport properties, and reduction behavior is not fully understood. In Chapter 4, the Gen2 (1.2 M LiPF6 in EC and ethyl methyl carbonate (EMC)) and EC-base (1.2 M LiPF6 in EC) electrolytes, as well as their mixtures with 10 mol% FEC, are investigated by atomistic modeling and transport property measurements. Due to the mixing of cyclic and linear carbonates, the Gen2 electrolyte is found to have a 60% lower ion dissociation rate and a 44% faster Li+ self-diffusion rate than the EC-base electrolyte, while the total ionic conductivities are similar. Moreover, we propose for the first time the anion–solvent exchange mechanism in MCEs with identified energetic and electrostatic origins. For electrolytes with additive, up to 25% FEC coordinates with Li+, which exhibits a preferential reduction that helps passivate the anode and facilitates an improved SEI. The work provides a coherent computational framework for evaluating mixed electrolyte systems. The novel intrinsically anionic Metal–Organic Frameworks (MOFs) with a superior ionic conduction performance has opened up a new possibility for the development of SSEs. Given the numerous materials space with almost unlimited possibilities of MOFs, it is important to develop a theoretical method that can predict the transport properties of SSEs based on MOFs. In Chapter 5, classical molecular dynamics, grand canonical Monte Carlo, and quantum chemistry are utilized to model the diffusion and ionic conduction phenomena of a novel MOF-688 material and its derivatives. The main ionic conduction mechanism is identified as solvent-assisted Li hopping by calculating the ionic conductivity using theories based on different levels of simplification. Moreover, the Li+ distribution in the SSE is found to be highly correlated to the charge distribution on the POM cluster. A hypothetical non-interpenetrated MOF-688 derivative is proposed with improved ionic conduction performance, providing insights into the design rules of the novel type of SSEs.

Advisor: Persson, Kristin A.