Temperature and Pressure Effects on Unrecoverable Voids in Li Metal Solid-State Batteries

Abstract

All-solid-state batteries (ASSB) can potentially achieve high gravimetric and volumetric energy densities (900 Wh/L) if paired with a lithium metal anode and solid electrolyte. However, there is a lack in critical understanding about how to operate lithium metal cells at high capacities and minimize unwanted degradation mechanisms such as dendrites and voids. Herein, we investigate how pressure and temperature influence the formation and annihilation of unrecoverable voids in lithium metal upon stripping. Stack pressure and temperature are effective means to initiate creep-induced void filling and decrease charge transfer resistances. Applying stack pressure enables lithium to deform and creep below the yield stress during stripping at high current densities. Lithium creep is not sufficient to prevent cell shorting during plating. Three-electrode experiments were employed to probe the kinetic and morphological limitations that occur at the anode−solid electrolyte during high-capacity stripping (5 mAh/cm 2 ). The role of cathode−LLZO interface, which dictates cyclability and capacity retention in full cells, was also studied. This work elucidates the important role that temperature (external or in situ generated) has on reversible operation of solid-state batteries.

Assessment of Critical Stack Pressure and Temperature in Li‐Garnet Batteries

“…Furthermore, these findings reveal that even a relatively low temperature of 50°C enables the stabilization of the Li/LLZO interface, even when applying the minimum pressure of 1.25 MPa (Figure 3d,f). These results align with the observations made by Hatzell et al, [20] highlighting the substantial impact of temperature in mitigating void formation. It should also be noted that the observed quasi-stable lithium stripping conditions, i.e., a temperature of 50°C and a stack pressure of 1.25 MPa, are within the requirements for practical application in electric vehicles.…”

“…Since void formation during Li stripping is significantly influenced by the external operating conditions, in particular the stack pressure applied to the cell, their effect on the Li/LLZO interface was recently investigated. [19][20][21][22] Namely, Sakamoto et al [19] in his pioneering study aimed to determine the critical stack pressure, which represents the minimum pressure required for electrochemical Li stripping without the occurrence of voids at the Li/LLZO interface. The proposed methodology involved monitoring the overpotential of Li/LLZO/Li symmetrical cells under a constant applied current density (ranging from 0.005 to 0.4 mA cm −2 ) while the stack pressure was gradually reduced from 3.5 MPa to 1 MPa in 0.5 MPa steps.…”

“…Stripping Li at lower pressures resulted in significant polarization of the Li/LLZO/Li symmetrical cell. It is worth noting that while many other studies have explored the impact of pressure and temperature on void formation, [20][21][22] they have mainly focused on the general trend of mitigated void formation with increasing pressure and temperature, rather than determining the critical stack pressure. Notably, the influence of pressure on the formation of void was also assessed computationally.…”

Evaluating the Use of Critical Current Density Tests of Symmetric Lithium Transference Cells with Solid Electrolytes

“…When 5 MPa of stack pressure is applied at moderate 35 °C to 50 °C, it already facilitates the collapse of pores induced by inhomogeneous stripping with 0.3 mA cm −2 and increases the CCD as discussed recently by Zaman et al. [ 47 ] Furthermore, temperatures above the melting point of lithium can help to negate the issue of pore formation completely, since vacancy diffusion in molten lithium is orders of magnitude faster than in the solid. [ 48 ] A general trend for higher CCD values is expected with increasing temperature and even small temperature differences in the order of 10 K may cause significant changes.…”

Reference Electrode Reveals Insights on Sodium Metal/Solid Electrolyte Interface Cycling and Voiding Behaviors at High Current Densities and Areal Capacities

“…We also note that much of the work in this field uses two-electrode, symmetric cell configurations (i.e., metal/SE/metal configurations, without a reference electrode). Three electrode systems are a known and well used method for cycling investigations in Li-ion research, − but we have identified less than a dozen papers on solid state three electrode cycling. ,,− Two electrode system interpretations often presume cell polarization is only from the electrode undergoing stripping (where voids are expected), and the plating electrode acts as a stable pseudoreference electrode, but this is often not investigated carefully. Adding a reference electrode, albeit difficult to implement and not without its own limitations, can decouple the interface behaviors in symmetric cells.…”