Understanding The Li Dendrite “Soft Short” in Solid-State Li Metal Batteries by Phase-field Method

Lithium (Li) dendrite growth in Li metal batteries is a long-standing problem, which causes critical safety concerns and severely limits the advancement of rechargeable Li batteries. Replacing conventional liquid electrolyte with solid electrolyte (SE) of high mechanical strength and rigidity was once thought as an effective strategy to suppress the Li dendrite growth. However, defects in SEs (such as cracks, voids, grain boundaries etc.) and the imperfect contact between the electrode and the SE can even facilitate the Li dendrite growth. Recently, it was postulated that the Li dendrite growth in SEs could be the reason for “soft short” of the cell. Compared to “hard short” in which the voltage of the battery decreases suddenly during the charging process, in “soft short” the voltage is dynamically stable but cannot increase during the charging process. However, the underlying mechanism of “soft short” is still not well understood. In this work, we developed a phase-field model coupled with electrochemistry and solid mechanics to unravel the dynamics of Li dendrite growth in a porous SE under the applied external pressure during charging, and its correlation with the “soft short”. It is revealed that Li dendrite growth causes a hydrostatic pressure inside the Li metal, which increases with the external pressure and is responsible for the suppression of Li dendrite growth. Without external pressure, Li dendrite growth is less restricted by the SE and can fill the small pores inside the SE to form a tree-like morphology. The growth of multiple Li dendrites eventually penetrates through the entire SE, causing “hard” short circuit of the cell. This is evidenced by the sudden increase of leakage current during charging process in the simulation. When the external pressure increases to 10MPa, the hydrostatic pressure is maximized at the pore-SE interface which blocks the Li dendrite to fill into the pores, and eventually breaks parts of the Li dendrites to form dead Li. Our simulation results agree with the X−ray computed tomography of the deposited Li metal and cracks inside the SE by our collaborators. More importantly, the simulated fracture of Li dendrite (disconnection to the anode) and the subsequent reconnection to the Li anode can well explain the “soft short” in experiments, as they prevent the Li dendrite from completely penetrating the SE. This is further evidenced by the leakage current fluctuations in our simulation, indicating a typical "soft short" behavior. Our work thus provides a deeper understanding of the Li dendrite growth dynamics in porous solid electrolyte and the “soft short” phenomenon in solid state batteries.