No abstract

Silicon is widely used in the semiconductor industry and has recently become very attractive as a lithium ion battery anode due to its high capacity. However, volume changes associated with repeated lithiation–delithiation cycles expose fresh silicon surfaces to the electrolyte, causing irreversible side reactions. Moreover, silicon suffers from a poor electronic conductivity at a low lithium content. Carbon impurities originating at synthesis or resulting from subsequent contact with other electrode components are often neglected. However, atomistic simulations reveal that dissolved carbon decreases the local potential energy surface by drawing the electron density from silicon to form polar covalent C–Si bonds that are stronger than the non-polar covalent Si–Si bonds they replace. This leads to a higher density and elastic stiffness, regardless of the interstitial lithium concentration. Substitutional carbon also reduces the mobility of silicon self-vacancies and interstitial lithium by increasing their diffusion barriers by 24.7 and 27.3 kJ mol−1, respectively. Moreover, the [carbon, silicon vacancy] complex is basically stable, while the [carbon, lithium] complex is found to become stable against both single defects at a spacing of 4.72 Å. The minimum energy paths ultimately demonstrate that both the interstitialcy and dissociative mechanisms are mainly responsible for carbon diffusion in silicon.

Lithium stabilizes silicon–graphite interfaces, making hollow core–shell structures favorable in the presence of lithium and yolk–shell configurations favorable in its absence.

No abstract

Silicon–carbon (Si–C) nanocomposites have recently established themselves among the most promising next-generation anode materials for lithium (Li)-ion batteries. Indeed, combined Si and graphitic (sp2-C) phases exhibit high energy densities with reasonable electrochemical responses, while covalently bonded silicon carbide (SiC) compounds offer high mechanical stiffness to withstand structural degradation. However, to form and utilize a Si–C composite structure that effectively satisfies specific battery energy, power, and lifetime requirements, it is necessary to understand the underlying atomistic mechanisms at work within its individual phases and at their interfaces. Here, we develop and validate a reactive interatomic potential (ReaxFF) for the Li–Si–C system, trained and tested against a large, diverse collection of ab initio data. In conjunction with molecular dynamics and Monte Carlo simulations, our new force field links macroscale phenomena to the nanostructures of various hybrid Si–C systems in a wide range of lithiation, temperature, and stress conditions. As an illustration, it demonstrates that the high capacity of SiC (5882 mA h g–1) is caused by amorphous lithium carbides (e.g., a-Li4.4C), which soften the overall a-Li4.4(SiC)0.5 system while swelling it volumetrically up to 668%. Furthermore, our force field accurately depicts step-wise lithiation mechanisms and volume changes in Si–sp2-C composites throughout the lithiation domain of each host subsystem. It reveals the formation of a Li-rich interphase at their grain boundary, which favors their adhesion and increases the local Li (de)insertion voltage up to 1 V. These examples demonstrate the ability of the new Li–Si–C ReaxFF potential to provide atomistic-scale insights required for designing and optimizing a wide range of Si–C-based anode candidates for upcoming Li-ion battery technologies.