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Abstract

Major obstacles in the realization of next-generation All Solid-State Batteries (ASSBs) are degradation and deterioration processes at solid-solid interfaces in the Solid-State Electrolyte (SSE). Dendrite nucleation and penetration along the mechanically softer grain boundary network results in performance loss and ultimately cell failure. The buried nature of these interfaces and their inherently finite width obstruct experimental characterization and in operando analyses. Therefore, a combined theory-experiment approach is crucial to resolve the length and time scales of interfacial processes and gain a deeper mechanistic understanding.
As a promising SSE candidate, the ceramic Li_1+xAl_xTi_2-x(PO₄)₃ (LATP) exhibits a high dendrite suppression capability despite a bulk electronic conductivity which exceeds a postulated critical threshold by orders of magnitude. A novel computational approach yields realistic atomistic structural models of glass-amorphous LATP grain boundaries, based on experimental insights from Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT). These models reveal a nanometer-sized complexion, effectively protecting the grains by constituting a sizable electronic barrier. A local separation of mobile Li⁺ and excess electrons at the nanoscale thus adds to a more nuanced assessment of SSE performance than macroscopic material properties alone.
The established atomistic model of a realistic grain boundary serves as a basis for active interfacial engineering. Encouraged by APT findings, Mg²⁺ is doped locally into the amorphous grain boundary phase. An adopted Monte-Carlo (MC) based protocol suggests a local confinement of Mg²⁺ in the bulk interphase. The resulting minimal dopant bleeding does not significantly compromise adjacent crystalline LATP performance and inherent 3D interconnected pathways for charge carrier migration stay intact. Aliovalent doping allows for the deliberate reduction of Ti⁴⁺ Transition Metal (TM) centers while increasing the Li ion content. As a result, the residual electronic conduction via polaron hopping is reduced and the local Li⁺ conductivity is increased. Local doping with Mg²⁺ therefore presents a promising route towards active engineering of buried solid-solid interfaces from a computational perspective. Due to non-trivial effects upon doping, a confident assignment of target stoichiometries requires experimental input.
Combining insights at different length and time resolutions from theory and experiment is a powerful ansatz to progress in the functional design of high-performing battery materials.