Multiphysical modelling and experimental parameterisation of solid-state lithium-metal batteries

Abstract

Owing to the non-flammability and the potential to replace graphite negative electrodes with lithium metals, solid-state lithium-metal batteries show promise to be next-generation batteries. However, several critical bottlenecks such as a high impedance, a low ionic conductivity of the solid electrolyte, the scalability and manufacturability of large-format solid-state batteries limit the widespread adoption of these batteries for real-world applications.

In this thesis, an in-depth review of the possible multiphysical interactions affecting the performance and degradation of solid-state batteries is first presented. The behaviour of solid-state batteries under a wide range of operating conditions are intrinsically multiphysical, where solid-state electrochemistry and chemical degradations are often coupled to the thermal gradients and mechanical cracks. A comprehensive summary of different multi-physics mechanisms is especially relevant to detect potential fault behaviours before a complete failure in the future development of solid-state batteries. The impacts of solid-state physics on cell architecture engineering were also highlighted. Here, this research discusses how lithium metal’s low melting temperature could limit the manufacturability of solid-state lithium-metal batteries and pave a new research direction towards using cell configurations without negative electrodes.

Based on a near-unity transference number due to negligible concentration overpotentials across the solid electrolyte, electrochemical models were developed to predict solid-state lithium-metal cells’ performance under pulse operating conditions. A model-based approach was proposed to upscale the single-layer cell to forming large-format solid-state bipolar and parallel stacks by determining the scaling factor from dimensionless governing equations. Here, the model predicts how the choice of current collectors and volumetric change of lithium metal could limit the gravimetric and volumetric densities of large-format solid-state lithium-metal batteries. A bipolar aluminium solid-state cell stack is shown to have higher power and energy densities than a parallel lithium solid-state cell stack. Nevertheless, the performance of bipolar stacks is limited by their lower Coulombic stack capacity. This limitation indicates that an advanced electrode design is required for the future development of a bipolar stack. The stack model was further extended to study the impacts of heat generation, where the temperature-dependent model parameters were estimated based on different measurements of a single-layer cell. Although the model predictions show that heat generations due to Joule heating are insignificant for bipolar solid-state cell stacks, the Joule heating contributions cannot be neglected for parallel solid-state cell stacks. A good thermal management system is required for parallel solid-state cell stacks to mitigate adverse implications due to dominant heat generations. The thermally-coupled solid-state stack models proposed in this thesis can be used as design guidelines to advance large-format solid-state batteries’ future prototyping.

For a wide range of electrolytes, including liquid and solid polymer electrolytes, an independent measurement or calculation of both electrolyte conductivity and diffusion coefficient is often time-consuming and challenging. As a result, Nernst-Einstein’s relation has been used to relate the ionic conductivity to ionic diffusivity after determining either parameter. Although Nernst-Einstein’s relation has been used for different electrolytes, the short perspective in this thesis demonstrates that this relation is not directly transferable to describe the ionic mobility in an inorganic solid electrolyte. The fundamental physics of Nernst-Einstein’s relation shows that the relationship between the diffusion coefficient and electrolyte conductivity is derived for ionic mobility in a viscous or a gaseous medium. This postulation contradicts state-of-the-art experimental studies measuring the mechanical behaviour of inorganic solid electrolytes, showing that inorganic solid electrolytes are usually brittle rather than viscoelastic at ambient room temperature. The measurement of loss tangent is required to justify the use of Nernst-Einstein’s relation. The outcome of such measurement has two implications. First, if the loss tangent of inorganic solid electrolytes is less than unity in the range of batteries operating temperatures, the impacts of using Nernst-Einstein’s relation in modelling the ionic mobility should be quantified. Secondly, if the measured loss tangent is comparable to that of solid polymers and lithium metal, inorganic solid electrolytes may behave in a viscoelastic manner instead of the brittle behaviour usually suggested.