Unraveling the Mechanism of Phase Transformation and Structural Evolution in Si Anode with Deep-Potential Molecular Dynamics

Silicon has been extensively studied as one of the most promising anode materials for next-generation lithium-ion batteries because of its high specific capacity. However, a direct understanding of the atomic-scale mechanism associated with the charging and discharging processes for the Silicon anode is still lacking, partly due to the fact that the electrochemical reaction and structural evolution in the silicon anode involve complex phase transformation between crystalline (c-) and amorphous (a-) phases. In the past, such theoretical studies were limited by the capability of ab initio molecular dynamics. In this work, employing a newly developed deep potential model, we calculate and analyze voltage curves and microstructure evolution pathways starting from c-Si/a-Si/c-Li3.75Si/a-Li4.5Si with lithium insertion/extraction. Our simulations not only reproduce the key experimental phenomena, but also allow us to study the atomic scale mechanism associated with these phenomena. In particular, the voltage plateaus in both of the c-Si and a-Si lithiation processes are reproduced with a predicted plateau difference close to experimental measurements, which indicates the two-phase reaction mechanism at the initial lithiation stage. The latent heat of the phase transformation from c-Li15-δSi4 to a-Li15-δSi4 phases along the delithiation paths is evaluated, and agrees well with experimental value. Furthermore, the structural evolution from the crystal Si to the Li-Si solid solution and subsequently to the a-LixSi phase is captured, and the change of Si-Si bond distribution matches well with experiments. The simulation shows that the stress in the a-Si lithiation is lower than that in the c-Si lithiation, which is consistent with the experimental observation that the a-Si phase exhibits better stability than the c-Si phase. Our results provide much-needed insights into the thermodynamics of the phase transitions and structural evolution between the crystalline and amorphous LixSi phases, which play a key role in silicon anode optimization for battery applications.