First-principles calculations of bulk, surface and interfacial phases and properties of silicon graphite composites as anode materials for lithium ion batteries

The high energy density offered by silicon along with its mineralogical abundance in the earth's crust, make silicon a very promising material for lithium-ion-battery anodes. Despite these potential advantages, graphitic carbon is still the state of the art due to its high conductivity and structural stability upon electrochemical cycling. Composite materials combine the advantages of silicon and graphitic carbon, making them promising materials for the next generation of anodes. However, successfully implementing them in electric vehicles and electronic devices depends on an understanding of the phase, surface and interface properties related to their performance and lifetime. To this end we employ electronic structure calculations to investigate crystalline silicon-graphite surfaces and grain boundaries exhibiting various orientations and degrees of lithiation. We observe a linear relationship between the mixing enthalpies and volumes of both Li-Si and Li-C systems, which results in an empirical relationship between the voltage and the volume expansion of both anode materials. Assuming thermodynamic equilibrium, we find that the lithiation of graphite only commences after LixSi has been lithiated to x = 2.5. Furthermore, we find that lithium ions stabilize silicon surfaces, but are unlikely to adsorb on graphite surfaces. Finally, lithium ions stabilize silicon-graphite interfaces, increasing the likelihood of adhesion as core@shell over yolk@shell configurations with increasing degree of lithiation. These observations explain how lithium might accelerate the crystallization of silicon-graphite composites and the formation of smaller nanoparticles with improved performance.