The need for higher energy density rechargeable batteries has generated interest in alkali metal electrodes paired with solid electrolytes. However, metal penetration and electrolyte fracture at low current densities have emerged as fundamental barriers. Here, we show that for pure metals in the Li-Na-K system, the critical current densities scale inversely to mechanical deformation resistance. Furthermore, we demonstrate two electrode architectures in which the presence of a liquid phase enables high current densities while preserving the shape retention and packaging advantages of solid electrodes. First, biphasic Na-K alloys show K + critical current densities (with K-β″-Al2O3 electrolyte) that exceed 15 mA⋅cm -2 . Second, introducing a wetting interfacial film of Na-K liquid between Li metal and Li6.75La3Zr1.75Ta0.25O12 (LLZTO) solid electrolyte doubles the critical current density and permits cycling at areal capacities exceeding 3.5 mAh⋅cm -2 . These design approaches hold promise for overcoming electro-chemo-mechanical stability issues that have heretofore limited performance of solid-state metal batteries.

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The need for higher energy-density rechargeable batteries has generated interest in alkali metal electrodes paired with solid electrolytes [1]. However, metal penetration and electrolyte fracture at low current densities [2], as well as impedance growth at the metal-solid electrolyte interface due to void formation during cycling at practical current densities (> 0.5 mA cm-2), have emerged as fundamental barriers [3]. Here we demonstrate two semi-solid electrode architectures in which the presence of a minor liquid phase enables high current densities while it preserves the shape retention and packaging advantages of solid electrodes [4]. First, biphasic Na–K alloys selected for controlled liquid fraction between the state-of-charge limits show K+ critical current densities (with a K-β″-Al2O3 electrolyte) that exceed 15 mA cm‒2. Second, introducing a wetting interfacial film of Na–K liquid between a Li metal electrode and Li6.75La3Zr1.75Ta0.25O12 solid electrolyte doubles the critical current density and permits cycling at areal capacities that exceed 3.5 mAh cm‒2. These design approaches hold promise for overcoming electrochemomechanical stability issues that have heretofore limited the performance of solid-state metal batteries.We acknowledge support from the US Department of Energy, Office of Basic Energy Science, through award no. DE-SC0002633 (J. Vetrano, Program Manager).[1] Albertus, P., Babinec, S., Litzelman, S. & Newman, A. Nat. Energy 3, 16–21 (2018).[2] Porz, L. et al. Adv. Energy Mater. 7, 1701003 (2017).[3] Kasemchainan, J. et al. Nat. Mater. 18, 1105–1111 (2019).[4] Park, R.J-Y. et al. Nat Energy 6, 314–322 (2021). Figure 1

The use of alkali metal electrodes is widely considered to be an enabler for the next generation of high energy-density rechargeable batteries [1]. In all-solid-state systems, the most critical interface appears to be that between the alkali metal and the solid electrolyte, from which metal-filled cracks can initiate and grow into single-crystal, polycrystal, and glassy electrolytes alike [2] under sufficiently high electrochemical stress. However, failure can be mitigated by softening of the metal electrode, whether through increases in temperature (including melting) or changes in composition (including changing alkali metals [3]). Here, we discuss semi-solid metal electrode design approaches in which a minor liquid phase fraction is deliberately introduced to produce a self-healing function that enables high current densities [3]. A bulk semi-solid electrode approach is demonstrated using Na–K alloys with controlled liquid fraction between the state-of-charge limits; these show potassium ion critical current densities (using the a K-β″-alumina electrolyte) that exceed 15 mA cm‒2. An interfacial wetting approach uses a thin interfacial film of Na–K liquid between a Li metal electrode and an LLZTO solid electrolyte; here the critical current density is doubled, and cyclable areal capacities exceed 3.5 mAh cm‒2. Moreover, evidence from both approaches suggest that void formation in solid metal electrodes during cycling at practical current densities (>0.5 mA/cm2) [4], manifested as impedance growth at the metal-solid electrolyte interface, can be largely mitigated through these semi-solid design strategies.Support from the US Department of Energy, Office of Basic Energy Science, through award no. DE-SC0002633 (J. Vetrano, Program Manager), is gratefully acknowledged.[1] Albertus, P., Babinec, S., Litzelman, S. & Newman, A. Nat. Energy 3, 16–21 (2018).[2] Porz, L. et al. Adv. Energy Mater. 7, 1701003 (2017).[3] Park, R.J-Y. et al. Nat Energy 6, 314–322 (2021).[4] Kasemchainan, J. et al. Nat. Mater. 18, 1105–1111 (2019). Figure 1