Simulations of a LiF Solid Electrolyte Interphase Cracking on Silicon Anodes Using Molecular Dynamics
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2018 The Electrochemical Society. The main advantage of silicon-based anodes for lithium-ion batteries is their high volumetric and gravimetric capacity to store Li. However, swelling upon lithiation causes cracks on the anode protective solid electrolyte interphase (SEI), creating channels for the electrolyte to enter and disturb the anode, leading to capacity fading, short- or open-circuit failures. We study the mechanical properties of LiF as a model SEI layer of a SiLi nanoparticle (NP) swollen by a time-dependent expanding potential (TDEP) simulating the lithiation of Si. The stress on the SEI increases until reaching its maximum at LiF bond breaking and cracking onsets. Then, stress decreases and the number of bonds in the SEI approaches a minimum, confirming the cracking. However, at the lowest currents, atoms have enough relaxation time that a surface reconstruction is observed and the stress reaches a second and higher maximum, then the SEI layer breaks into pieces. The microscopic amorphization mechanism is found to be independent of the charging rate; however, the value of the tensile strength depends on the expanding rate. LiF bonds breaking begins with those nearest to the SiLi-LiF interface, along a radial direction, then forming larger LiF rings until the SiLi alloy loses its protective shell.
