Understanding Electrode-Electrolyte Interfaces for Next-Generation Batteries

The production capacity of batteries is set to rise drastically within the coming decades, precipitated by the projected expansion in renewable (yet intermittent) power generation, the upcoming bans on diesel and petrol engines that are driving a rapid increase in electric car manufacture, and the permeation of decentralised power networks, among many other influences. The scale of resource extraction required to furnish such storage needs necessitates the development of a new generation of batteries with improvements in sustainability, performance, and safety characteristics over current battery technologies.

The substitution of conventional liquid electrolytes (LEs) with novel solid electrolytes (SEs) in glassy, ceramic, and other polycrystalline phases has shown promise in addressing these requirements. Potential SEs have demonstrated characteristics such as cycling stabilities on the order of supercapacitors, high room temperature ionic conductivities, wide electrochemical windows, and improved safety under compromisation of cell structure (when electric cars crash, for example) due to their thermodynamic stability in air. Despite these advantages, the incorporation of SEs into operational, all-solid-state batteries (ASSBs) presents a myriad of constraints to be mitigated. For example, where LEs enable fast interfacial kinetics due to a high degree of electrode wetting, SE-electrode interfaces induce a rate-limitation due to high resistance, thereby inhibiting ion transport. This high resistance is considered to originate owing to crystal lattice mismatch, poor interfacial contact, ion-deficient space-charge layers, and interphase formation. Hence, such complex challenges, amongst many others, make finding suitable SE materials a highly non-trivial task.

To fully optimise these candidate materials as electrolytes, a comprehensive understanding of the interfacial and bulk ion dynamics is needed. However, owing to the buried nature of interfaces and transport mechanisms, the direct observation of ion mobility is difficult and the complete dynamic picture, which is crucial to the design of high-performance SSBs, therefore remains elusive. In this vein, this project aims to elucidate the structural motifs responsible for improved ionic conductivity; carefully combining results from materials synthesis and characterisation with those from density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations.

One family of materials hosting many promising candidate SEs are sulphides, where the polarisability of the sulphide anion provides a low electrostatic barrier to ion migration, with, for example, Li10GeP2S12 (LGPS) and Li6PS5Cl showing room temperature ionic conductivities on the order of, and surpassing, many organic LEs. Recent work on lithium-boron-sulphur-type systems suggest better ionic conductivities and electrochemical stabilities than LGPS, and it is within this family where this research project will begin. The effects of different synthetic methods and compositional doping on structure and the resulting conductivity of candidate SEs will be explored using a combination of X-ray and neutron powder diffraction with multinuclear solid-state NMR spectroscopy. It is anticipated that DFT and AIMD calculations will assist in elucidating ion mobility mechanisms in such systems, and provide valuable insights into potential optimisation routes for future high-performance SE materials and interfaces.

ReNU's enhanced doctoral training programme delivered by three uniquely co-located major UK universities, Northumbria (UNN), Durham (DU) and Newcastle (NU), addresses clear skills needs in small-to-medium scale renewable energy (RE) and sustainable distributed energy (DE). It was co-designed by a range of companies and is supported by a balanced portfolio of 27 industrial partners (e.g. Airbus, Siemens and Shell) of which 12 are small or medium size enterprises (SMEs) (e.g. Enocell, Equiwatt and Power Roll). A further 9 partners include Government, not-for-profit and key network organisations. Together these provide a powerful, direct and integrated pathway to a range of impacts that span whole energy systems.

Industrial partners will interact with ReNU in three main ways: (1) through the Strategic Advisory Board; (2) by providing external input to individual doctoral candidate's projects; and (3) by setting Industrial Challenge Mini-Projects. These interactions will directly benefit companies by enabling them to focus ReNU's training programme on particular needs, allowing transfer of best practice in training and state-of-the-art techniques, solution approaches to R&D challenges and generation of intellectual property. Access to ReNU for new industrial partners that may wish to benefit from ReNU is enabled by the involvement of key networks and organisations such as the North East Automotive Alliance, the Engineering Employer Federation, and Knowledge Transfer Network (Energy).

In addition to industrial partners, ReNU includes Government organisations and not for-profit-organisations. These partners provide pathways to create impact via policy and public engagement. Similarly, significant academic impact will be achieved through collaborations with project partners in Singapore, Canada and China. This impact will result in research excellence disseminated through prestigious academic journals and international conferences to the benefit of the global community working on advanced energy materials.