Development of Solid Electrolytes Using a Combined in-Operando XPS-Theroetical Approach

Lithium-ion battery systems are beginning to approach the theoretical limits of their performance and as a result, developments in new Li-ion technologies are becoming increasingly more dependent on gaining a deeper understanding of surface chemistry and its impact on electrochemical response. For renewable energy technologies to become more widely available, it is pertinent that safe, scalable, and reliable methods of energy storage continue to be developed. Battery systems are interface devices and the chemical processes occurring at the interfaces are crucial to overall performance and capacity of the battery.

Techniques like x-ray photoelectron spectroscopy (XPS) are increasingly crucial for advancing our understanding of surface and interface chemistry. XPS facilitates the tracking of electrode material oxidation state changes during charge and discharge cycles, and it adeptly detects alterations in chemical environments and the formation of new species at interfaces.
Cycling of the battery induces the development of a solid-electrolyte interface (SEI) on the anode. This electronically insulating yet ion-conductive film consists of electrolyte decomposition products, and its impact on cell performance varies based on composition, potentially limiting, or enhancing overall functionality.

Surface sensitive techniques are capable of characterising battery interphases, and XPS is surface sensitive as photoelectrons can only escape the sample from the surface region without losing energy and specific chemical and electronic information.

In-Operando studies allow for understanding of the growth, composition, and kinetics of forming interphases in solid-state batteries through observation and comparison of shifts in observed XPS peaks. Shifts can be attributed to changes in oxidation state, changes in chemical environment, changes to the energy levels due to sample doping of presence of an applied potential. When used alongside other surface- sensitive techniques, a broad range of information can be obtained about a sample. Techniques such as FTIR and Raman spectroscopy can be used to further understanding of the chemistry occurring at the cathode and the nature of and species formed upon charging or discharging.

Comparison of experimental data to computational studies will allow for evaluation of the causes of peak shifts, as theoretical data for the density of states and XPS spectra can be obtained and compare this to what experimental findings through use of in-Operando and post-mortem studies of battery cycling. DFT can give key insights into chemical properties of materials and can be used to assess viability as battery materials. Molecular dynamics simulations will also show key insights into the kinetics of formation of interphases.

Initial aims of the project are to compare and reproduce results of existing solid electrolytes, starting with lithium lanthanum titanium oxide and lithium lanthanum zirconium oxide, with this new technique, focusing on avoidance of artefacts in the XPS data and reproduction of accurate electrochemical data, which gives information about capacity, capacity fading, redox potential, and other battery data.

From focusing on charge and discharge rates, as this alters the compounds formed and the overall performance of the cell, understanding of dynamics of the cell and interphase formation can be improved, and the results can be applied to developing new electrolyte materials. The effects of alterations to existing solid electrolytes, for example to effect of halogenation, on the performance of the cell can then be studied and applied to production of new materials such as sulfides and composite solid-electrolyte materials.