Cracks and Dendrites in Solid Electrolyte Interphases in Lithium-Ion Nanobatteries: Molecular Dynamics Simulations
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Lithium-ion batteries are widely used as power sources for a variety of electronic devices; however, still further developments are expected for power applications in transportation and electrical grid backups, among several others. A passivating solid electrolyte interphase (SEI), naturally grow on the electrolyte-anode interface of a new battery or sometimes is artificially built to cover the anode. One way or the other, the SEI, once formed, protects the anode and electrolyte from mutually reacting each other, thus playing a critical role in the performance of batteries. For high-capacity intercalation anode materials, when they suffer a large volumetric increase, such as silicon, the SEI may crack and lead to the formation of dendrites, which are directly linked with the capacity fading of the battery over cycles, and even worse, dendrites may contact the cathode producing a short-circuit, one of the major concerns in lithium-ion batteries. A multiscale computational approach using molecular dynamics techniques will be presented for the simulation of full nanobattery test-beds that provides relevant information of silicon-based anode lithiation growth, including interfacial mechanical effects, and lithium-dendrite formation and growth.Therefore, we study the mechanical properties of LiF as a SEI layer of a SiLi nanoparticle (NP) expanded by a time-dependent expanding potential (TDEP) simulating the lithiation of Si and allowing to keep the number of Li and Si atoms of the NP constant. The stress on the SEI increases until reaching its maximum (tensile strength) at the LiF bond breaking and cracking onsets. As the expansion continues, the stress decreases and the number of bonds in the LiF SEI approaches to a minimum, confirming the SEI cracking. However, for the lowest currents, atoms have enough relaxation time that a surface reconstruction is observed with the stress reaching a second and higher maximum, then the SEI layer divides into pieces. The cracking mechanism is independent of the charging rate. The breaking of LiF bonds begins with the nearest ones to the SiLi-LiF interface, along a radial direction pointing to the outside of the NP, then forming larger and larger LiF rings until the SiLi alloy totally loses its protective shell.Then, we attempt to predict the mechanism of dendrite growth by simulating possible behaviors of charge distributions in the anode of an already cracked solid electrolyte interphase of a nanobattery under the application of an external field representing the charging of the battery; thus, elucidating the conditions for dendrite growth. Although a dendrite may take a few hours to grow, the simulation can be performed at totally different time scales because the extremely slow drift velocity of the Li-ions, ~1mm/hour in a typical commercial Li-ion battery and the so small time for the important chemistry to take place (~ps). The conditions before the growth are assumed and conditions that do not lead to the growth are ignored. This part of the work responds to the question: if dendrites grow at some point, what are the possible conditions that favored such a growth? We respond this question by using molecular dynamics simulations of a pre-lithiated silicon anode with a Li:Si ratio of 21:5, corresponding to a fully charged battery. The electrolyte is composed of 1M LiPF6 salt solvated in ethylene carbonate (C2H4CO3). We simulate the dendrite growth by testing a few charge distributions in a nanosized square representing a crack of the solid electrolyte interphase, which is where the electrolyte solution gets in direct contact with the LiSi alloy anode. Using selected charge distributions for such anode surface, the dendrites grow during the simulation when an external field is applied. We found that dendrites grow when strong deviations of charge distributions take place on the surface of the crack. In addition to our estimates of stress limits in the anode during charging, results from this work are important to find ways to constrain lithium dendrite growth using tailored coatings or pre-coatings covering the LiSi alloy anode.
