Simulation-Based Analysis of Structural and Thermodynamic Aspects of the Electrochemical Lithiation of Silicon Nanoparticles

摘要

Lithium-ion batteries are among the most predominant energy storage systems for portable to stationary electronic devices. Lithium-ion batteries are indispensable to laptops, mobile phones, and electric vehicles due to their high energy/power density and long cycle life. Silicon has been intensively pursued as one of the most promising anode materials because of its high specific capacity (4200 mAh/g for Li_(22)Si_5), in comparison with the conventional graphite (372 mAh/g for LiC_6), and its abundance. Despite its high capacity, silicon-based anode materials suffer from fast capacity loss caused by its large volume change (>300%), unstable solid electrolyte interphase and the physical disintegration, such as racking and crumbling, of the electrode structure during lithiation and delithiation processes. Therefore, there are various research activities to control the electrochemical performance of silicon anode materials. The engineering of silicon nanostructures proved to be an effective method for improving capacity and cycling stability, since nano-sized silicon can alleviate mechanical fractures during volume changes. In particular, silicon nanoparticles can be added into the void region in the polycrystalline graphite matrix, resulting in an effective increase in the overall energy density of the anode. In this work, we designed reasonable structural models of silicon nanoparticles in terms of particle size and lithiation ratio through Monte Carlo simulations, and we conducted a series of first-principle simulations to understand their intrinsic electrochemical properties. We reported the theoretical understandings of the detailed structural and thermodynamic mechanism of the actual lithiation process of silicon nanoparticle systems based on atomistic simulation approaches. We found that the rearrangement of the silicon bonding network is the key mechanism of the lithiation process, and that it is less frequently broken by lithiation in the smaller sizes of silicon nanoparticles. The decreased lithiation ability of the silicon nanoparticles results in the lithiation potential being significantly lower than that of crystalline silicon phases, which impedes the full usage of the theoretical maximum capacity. Thus, nanosized slicon could have a negative effect on performance if they become too fine-sized. These findings provide a detailed view of the electrochemical lithiation process of silicon nanoparticles and engineering guidelines for designing new silicon-based nanostructured materials for the anode of high capacity lithium ion batteries.