Universal chemomechanical design rules for solid-ion conductors to prevent dendrite formation in lithium metal batteries

Abstract

Dendrite formation during electrodeposition while charging lithium metal batteries compromises their safety. 1-6 While high shear-modulus (G s ) solid-ion conductors (SICs)have been prioritized to resolve pressure-driven instabilities that lead to dendrite propagation and cell shorting, it is unclear whether these or alternatives are needed to guide uniform lithium electrodeposition, which is intrinsically density-driven. 7-9 Here, we show that SICs can be designed within a universal chemomechanical paradigm to access either pressure-driven dendrite-blocking or density-driven dendrite-suppressing properties, but not both. This dichotomy reflects the competing influence of the SIC's mechanical properties and partial molar volume of Li + (V Li+ ) relative to those of the lithium anode (G Li and V Li ) on plating outcomes. 9 Within this paradigm, we explore SICs in a previously unrecognized dendrite-suppressing regime that are concomitantly "soft", as is typical of polymer electrolytes, but feature atypically low V Li+ , more reminiscent of "hard" ceramics. Li plating (1 mA cm -2 ; T = 20 ˚C) mediated by these SICs is uniform, as revealed using synchrotron hard x-ray microtomography. As a result, cell cycle-life is extended (>300 cycles vs. ~100 cycles for control cells), even when assembled with thin Li anodes (~30 µm) and high-voltage NMC-622 cathodes (1.44 mAh cm -2 ), where ~20% of the Li inventory is reversibly cycled.Heterogeneous nucleation and ramified growth of lithium metal electrodeposits while charging lithium metal batteries is tied to uneven Li + transport across the anode-electrolyte interface. 7-11 Whereas the increasingly fractal character of this interface during battery cycling accelerates electrolyte degradation, rare events associated with dendrite formation, if left unchecked, can lead to device shorting and in some cases thermal runaway. 1-6 Both early-and late-stage instabilities associated with dendrite formation and propagation can be modeled using Butler-Volmer physics, 8

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“…In cases where the separator is not penetrated, as Li dendrites continue to form, they are continuously encapsulated by the SEI; this leads to electrochemically inert Li, decreased CE, and deteriorated cycling performance. [115] Additionally, the dead and uneven Li dendrites can increase the diffusion pathway and resistance of Li ions and electrons, rendering a large polarization. [116][117][118] Suppressing dendrite formation is, therefore, of paramount importance, and a variety of techniques have been investigated, including material tailoring, electrolyte optimization, functionalizing the separator, and building artificial anode/electrolyte interfaces.…”

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“…Synchrotron X-ray tomography is a potential method to resolve three-dimensional morphological transformations with adequate spatial and temporal resolutions relevant to solid state batteries. [25][26][27][28][29] Currently, it is very challenging to discern lithium electrode kinetics at solid electrolyte interfaces. This work uses imaging techniques to track morphological transformations at lithium metal/solid electrolyte interfaces.…”

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