Unraveling the Li Penetration Mechanism in Polycrystalline Solid Electrolytes

Abstract

Lithium dendrite penetration has been widely evidenced in ceramic solid electrolytes (SEs), which are expected to suppress Li dendrite formation due to their ultrahigh elastic modulus. This work aims to reveal the mechanism of Li penetration in polycrystalline SEs through electro‐chemo‐mechanical phase‐field model, using Li7La3Zr2O12 (LLZO) as the model material. The results show the Li penetration patterns are influenced by both mechanical and electronic properties of the microstructures, i.e., grain boundaries (GBs). Li nucleates at the GB junctions on the Li/SE interface and propagates along the GB, at which the interfacial compressive stress is small due to the GB softening. Moreover, the excess trapped electrons at the GB may trigger isolated Li nucleation sites, abruptly increasing the Li penetration depth. High‐throughput simulations yield a phase map of Li penetration patterns under different trapped electrons concentrations and GB/grain elastic modulus mismatch. The map can quantitatively inform whether the mechanical or electronic properties dominate Li penetration morphologies, providing a strategy for the design of improved SE materials.

Revealing Atomic-Scale Ionic Stability and Transport around Grain Boundaries of Garnet Li7La3Zr2O12 Solid Electrolyte

“…51,55,56 The current study focuses on the impact of the microstructure structure of GB, and provides a potential reason for the high electronic conductivity of LLZO, which has been widely reported as another major origin of the dendrite formation within SE. 17,33,34,53 Our study shows that the decomposition of ZrO6 octahedron at the GBs plays an important role in the dendrite growth in LLZO. Specifically, the presence of the ZrO5 pyramid decreases the Ef of Li i × , and induces the electron localization, and the existence of the isolated O stabilizes the Schottky-like defect at the GB.…”

“…These electrons, whose states are close to the conduction bands, would show the potentially high mobility, and high capability to combine with the excess Li ions to trigger the nucleation of Li 0 inside the SE. 33,34,53 Meanwhile, these excess electrons may lead to the redistribution of the electric field. 54 The GB with high electronic conductivity will facilitate the Li penetrations.…”

Unlocking the Electrochemical–Mechanical Coupling Behaviors of Dendrite Growth and Crack Propagation in All‐Solid‐State Batteries

“…Lithium dendrites and cracks in SEs are reported to initiate at the lithium electrode/SE interface, [ 7 ] mainly within the initial defects, such as open pores, voids, cracks, and grain boundaries. [ 6d,8 ] Above the critical current density, the driving force is strong enough for Li dendrite growth to oppose the mechanical resistive force. [ 6d ] Generally, the dendrite growth drives cracks to propagate within SEs, and the newly formed crack provides the vacant space for dendrites to grow further.…”

“…Plenty of pioneering efforts from the perspective of material science have addressed the dendritic and interfacial issues, mainly with respect to three aspects, that is, 1) advanced structure design of the electrode or current collector to accommodate the newly grown Li dendrite and release the stress; [ 10 ] 2) interfacial modification to improve the solid‐solid contact property between the electrode and SE; [ 11 ] and 3) improvement of SE electrochemical/mechanical properties to suppress dendrite growth and improve battery performance. [ 12 ] From the mechanical perspective, stacking pressure is found to significantly influence dendrite growth, crack propagation, and interface stability [ 8b,13 ] such that the concept of applying residual compressive stress in SEs is introduced to prevent dendrite penetration. [ 14 ] Although the performance of ASSBs has been greatly enhanced, inevitable dendrite growth still occurs during charging/discharging, [ 9 ] and the critical current density and cyclability performance need to be improved as well.…”

“…[ 21 ] The coupled phase‐field model showed that excess surface electrons significantly affect the initiation positions of Li dendrites within the grain boundaries of polycrystalline LLZO SE. [ 8a,22 ] By direct numerical simulation of restructured SE microstructure, the effective SE properties were obtained, and the effects of operating conditions, including temperature and external pressure, were parametrically studied. [ 23 ] The stack pressure and SE electrochemical properties were found able to influence the interfacial deposition and mechanical stability.…”