Effect of Stack Pressure on Morphological Stability of Lithium Metal Anodes
Authors: Mosayebi Mehdi , Hebert Kurt 
Published: December 2024

Morphological instability of the lithium-electrolyte interface is a critical problem limiting the development of lithium-metal anodes for batteries. At current densities typical of liquid cells, unstable deposition produces filaments (or "dendrites") and dense arrays of filaments termed "moss" (1,2). Filaments grow by addition of Li atoms to their base or "root" (3). Experimental evidence indicates that filament growth is fed by solid-state diffusion, and that filaments relieve compressive stress in the metal generated by electrodeposition (4). Instability may be suppressed by application of stack pressure in the range 0.1 - 1 MPa, within which Li metal deforms by creep (5,6). Mathematical models for unstable deposition have typically not included base growth, instead assuming that filaments grow by direct addition of Li atoms at the surface (7).
A model is presented for morphological stability of the Li metal-electrolyte interface during plating and stripping cycles in liquid cells. The model includes stress-driven diffusion of Li in and out of the anode during plating and stripping, respectively (8). Diffusion-induced strain is accommodated by creep deformation of the metal. Stack pressure is applied uniformly at the metal-electrolyte interface. Morphology evolution is tracked by linear stability analysis; that is, small periodic disturbances are applied at the interface, and the growth rate of the disturbance amplitude is predicted as a function of disturbance wavelength. The model can be adapted to solid-state cells with additional accounting for solid electrolyte elasticity.
Calculations show that in the absence of stack pressure, the Li-electrolyte interface is unstable during both plating and stripping. Instability arises because diffusion in the metal is impeded at "peaks" of the interface profile during plating, and enhanced at interface "valleys" during stripping. The emergent interface roughness is determined by destabilization due to diffusion at large wavelengths, in combination with stabilization by surface energy-driven creep at small wavelengths. Calculations further reveal that the interface is stable when a sufficient stack pressure is applied. Stack pressure induces shear stress at interface peaks that drives creep away from peaks and toward valleys. The effectiveness of stack pressure diminishes at small anode thickness, because of resistance to creep due to friction at the lithium-current collector interface. These effects of stack pressure and anode thickness are also observed experimentally (5, 9).
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