Ion transport in solid electrolyte interphases

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Lithium Metal Batteries (LMBs), with lithium metal as the anode, have recently garnered significant interest as a higher energy density alternative to conventional lithium-ion batteries (LIBs) for high-end electric vehicles and novel applications, including electric flight. However, the successful commercialisation of LMBs will require batteries with high specific energies (above 500 Wh/kg) at a low cost of US $100/kWh. Additionally, batteries must retain 80-90% of their capacity over 1000 cycles, necessitating a coulombic efficiency (CE) of over 99.99%. Achieving these targets also demands the implementation of high-energy cathodes and the development of novel electrolytes compatible with both electrodes. Liquid electrolytes are ideal, as they ensure good electrode contact and compatibility with existing manufacturing routes established for LIBs.Nevertheless, lithium metal anodes operate outside of the electrochemical stability window of any electrolyte, leading to spontaneous electrolyte reduction when the electrochemical potential of the anode exceeds the lowest unoccupied molecular orbital (LUMO) of the electrolyte. Kinetic stability is achieved through the formation of a surface layer composed of insoluble reaction products, first named the solid electrolyte interphase (SEI) by Peled in 1979.Currently, the cycle life of LMBs is limited by inhomogeneous lithium plating/stripping, which exposes additional lithium to the electrolyte and results in 'active' lithium loss due to the formation of SEI and electrochemically isolated 'dead' lithium. This reduces the CE, requiring the use of excess lithium in the form of lithium foil to extend cycle life to practical values, thus reducing specific energy. This non-uniform lithium plating and stripping behavior is influenced by the fundamental properties of the liquid electrolyte and metallic lithium. Electrolyte transport and thermodynamic properties govern the development of salt concentration gradients and overpotentials during cell operation. In extreme cases, the electrolyte can be entirely depleted of salt at the anode surface during charge, leading to the nucleation of fractal lithium dendrites and associated safety concerns. Recent studies have also shown that charge-transfer kinetics influence deposition morphology, with fast interfacial charge-transfer observed to positively correlate with CE. Additionally, the microstructure and anisotropic nanomechanical properties of lithium metal affect cycling behaviour. In fact, inhomogeneous stripping is influenced by crystallographic texture, and more uniform deposition morphologies are achieved under applied stack pressures.However, the degradation phenomena observed in LMBs cannot be fully described by the properties of lithium and the electrolyte alone. Ultimately, it is the SEI that controls cycling performance by regulating lithium morphology and 'dead' lithium formation. This demands a better understanding of the properties of the SEI and their influence on cycling performance, enabling the rational design of SEIs to guide future electrolyte development.This project aims to first examine the nanostructure of the SEI through surface/interfacial characterisation techniques, including atomic force microscopy, x-ray photoelectron spectroscopy, electron microscopy, and electrochemical impedance spectroscopy. Knowledge of SEI nanostructure alone is inadequate to predict cell performance, as it is not yet understood how SEI properties are affected by its structure and composition. Therefore, the second objective of the project is to investigate the structure-property relationships to facilitate rational SEI design and guide future electrolyte development.This project falls within the EPSRC Energy research area. The goal of this theme is for the UK to meet its environmental and energy targets.