Ion transport in solid electrolyte interphases

锂金属电池（LMB）以锂金属作为阳极，作为高端电动汽车和包括电动飞行在内的新型应用中传统锂离子电池（LIB）的更高能量密度替代品，最近引起了人们的极大兴趣。然而，LMB的成功商业化将需要具有高比能（高于500 Wh/kg）且成本低至100美元/kWh的电池。此外，电池必须在1000次循环中保持其容量的80-90%，从而需要超过99.99%的库仑效率（CE）。实现这些目标还需要实现高能阴极和开发与两种电极兼容的新型电解质。液体电解质是理想的，因为它们确保了良好的电极接触和与现有LIB制造路线的兼容性。然而，锂金属阳极在任何电解质的电化学稳定性窗口之外运行，当阳极的电化学电势超过电解质的最低未占分子轨道（LUMO）时，会导致电解质自发还原。动力学稳定性是通过形成由不溶性反应产物组成的表面层来实现的，Peled在1979年首次将其命名为固体电解质中间相（SEI）。目前，LMB的循环寿命受到不均匀的锂电镀/剥离的限制，这使额外的锂暴露于电解质中，并由于形成SEI和电化学隔离的“死”锂而导致“活性”锂损失。这降低了CE，需要使用锂箔形式的过量锂以将循环寿命延长至实用值，从而降低了比能量。这种不均匀的锂镀覆和剥离行为受到液体电解质和金属锂的基本性质的影响。电解质的传输和热力学性质决定了电解槽运行过程中盐浓度梯度和过电位的发展。在极端情况下，电解质在充电期间可完全耗尽阳极表面处的盐，导致分形锂枝晶的成核和相关的安全问题。最近的研究还表明，电荷转移动力学影响沉积形态，观察到快速界面电荷转移与CE呈正相关。此外，锂金属的微观结构和各向异性纳米机械性能影响循环行为。事实上，非均匀剥离的影响，晶体织构，和更均匀的沉积形貌下施加的堆栈pressure.However，观察到的LMB中的退化现象不能完全描述的锂和电解质的属性单独。最终，SEI通过调节锂形态和“死”锂形成来控制循环性能。这就需要更好地了解SEI的性质及其对循环性能的影响，从而使SEI的合理设计能够指导未来电解质的发展。本项目旨在首先通过表面/界面表征技术，包括原子力显微镜，X射线光电子能谱，电子显微镜和电化学阻抗谱，检查SEI的纳米结构。仅对SEI纳米结构的了解不足以预测电池性能，因为还不了解SEI性质如何受到其结构和组成的影响。因此，本项目的第二个目标是研究结构与性能的关系，以促进合理的SEI设计，并指导未来的电解质开发。这一主题的目标是让英国实现其环境和能源目标。

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.