Microscopic insights into glassy solid electrolytes

Recently the scientific community, with significant contribution from the SOLBAT project, has made great progress in understanding the origins of metallic lithium "dendrite" formation and propagation in solid electrolytes [Ning et al. 2021]. All evidence points toward the detrimental role of reactive interphases at the Li-SSE electrolyte interface [McDowell et al. 2019] and the grain boundaries in crystalline solids [Sakamoto et al. 2017].
Glassy (non-crystalline) solid-state electrolytes (SE) have attracted much attention due to advantages over their crystalline counterparts: isotropic ionic conduction, large available composition scales, thermal history manipulations, weak electronic contribution to the conductivity, ease of fabrication into films and, most importantly, absence of grain-boundaries and associated resistance.

Some of the most promising SEs have glass or glass-ceramic microstructures, including LiPON [Dudney et. al. 2018] and thiophosphates [Wang et al. 2021]. Nevertheless, the interplay between atomic structure, microstructure, ionic and electronic conductivity, and electro-chemo-mechanical properties of the Li|SE interphase remains elusive. Therefore, the scientific community is still struggling to design SEs with high Li-ion conductivity, able to prevent dendrite formation at commercially relevant current densities and processable as thin film at scale.
In this project, the student will combine advanced experiments and machine-learning-driven simulations to explore the synthesis and characterization properties (structural, physico-chemical, electrochemical and mechanical) of solid glass electrolytes. Oxy-halide glasses will be used as a model system because of their promising ionic conductivity and compositional tunability [Goodenough et al. 2016, Lunz et al. 2019]. In the first part, structure and mechanism of ion conduction in the bulk electrolyte will be explored experimentally (X-ray PDF, solid-state NMR and impedance spectroscopy) and computationally, using ML-based methodology. In the second part, leveraging existing expertise in XPS, AFM and advanced electron microscopy characterisation (including cryo-FIB cross sectioning and low-dose TEM), the Li-SE interphase will be probed. Combining these experiments with ML-driven simulations on the ten-nanometre length scale, the project promises new insight into the effect of composition and morphology on mechanical properties and ion transport.
The Pasta group has recently investigated the effect of crystal structure and microstructure on ionic conduction in Li2OHX (X=Cl, Br) solid electrolytes and has developed a unique expertise in melting-solidification processes to control grain size [Pasta et. al. 2021]. Rapid quenching of the molten salt will produce glasses that will then be characterized structurally (X-ray PDF in collaboration with Dr. Maria Diaz-Lopez, beamline scientist at Diamond Light Source), mechanically, and electrochemically. The Deringer group specialises in the development and application of machine-learning-based force fields for simulations of structurally complex materials [Deringer et al. 2021].
The combined experimental-computational approach will be first validated on a well-defined model system, viz. lithium halide antiperovskites (for which the melt-quenching can be directly described with established simulation protocols) - but the more ambitious long-term aim is for computation to support and accelerate the material selection and processing conditions workflows more generally, ultimately leading to a combined approach for the discovery of glassy solid-state electrolytes.

This is a 4-year Faraday Institution Studentship (part of the course fee paid from Oxford Materials funds)