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Guanchen Li1 Charles Monroe1

1, University of Oxford, Oxford, , United Kingdom

Doped Li7La3Zr2O12 (LLZO) garnet has room-temperature Li-ion conductivity of 0.4 mS/cm [1] and elastic modulus sufficiently large to suppress the morphological instability of Li metal during plating [2]. Although it shows very good cycle life in low current scenarios, the Li/LLZO interface still suffers from dendrite formation and eventual failure above a critical current [3]. Such failure events may owe in part to mechanical phenomena, so understanding the electromechanical response of polarized Li/LLZO interfaces has great significance to the development of solid-state Li batteries. We have produced a model that shows the impacts of mechanical effects on the DC response and electrochemical impedance of LLZO.

Both theoretical research [4] and TEM observations [5] of metal/solid-electrolyte interfaces have probed the structure of space-charge layers (SCLs) and their importance in determining battery performance. A theoretical study by Braun et al. [4] highlights the influence of deformation stress on SCLs within solid electrolytes between biased blocking electrodes. For dynamic scenarios where Faradaic currents can flow through solid electrolytes, we are still pursuing a clear picture that can advance the understanding of dendrite nucleation.

To achieve this goal, Newman’s concentrated solution theory has been generalized in a thermodynamically consistent way [6] to form a dynamic electrochemomechanical model of solid electrolytes. Poisson’s equation releases the constraint of local electroneutrality and brings in the Lorentz force. Onsager–Stefan–Maxwell multicomponent-diffusion theory provides relationships between current flow and thermodynamic driving forces such as gradients in ion concentration, pressure, and voltage. A momentum balance couples the electrical, mechanical, and electrochemical processes, all of which contribute substantially to the structure of SCLs. This new framework has been used to model the steady-state response of galvanostatically polarized Li/LLZO/Li cells.

Interfacial impedance impacts the dynamic behaviour of solid electrolytes substantially. We will discuss how it affects the distribution of Li-ion concentration, stress, and voltage under both direct and alternating currents.

Bulk impedance can be extracted from the model by linear perturbation analysis. Impedance reveals information about structural changes in SCLs; it also exhibits several novel features associated with stress, such as the appearance of acoustical waves at high frequency. The discussion will close with an analysis of how electrochemomechanical interactions impact the impedance spectra of Li/LLZO/Li cells.

Reference:
1.R. Murugan, et al. Angew. Chem 46, 7778 (2007).
2.C. Monroe and J. Newman, J. Electrochem. Soc. 152, A396 (2005).
3.A. Shara, et al. J. Power Sources 302, 135 (2016).
4.S. Braun, et al. J. Phys. Chem. C 119, 22281 (2015).
5.K. Yamamoto, et al. Angew. Chem 49, 4414 (2010).
6.P. Goyal and C. W. Monroe, J. Electrochem. Soc. 164, E3647 (2017).

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