Amorphization-induced energy loss of amorphous Si anodes for Li-ion batteries

The thermodynamic stability and energy loss (kinetic overpotential) of amorphous Si (a-Si) anodes during electrochemical cycling are investigated by intensive free energy analysis based on molecular dynamics simulations. We first demonstrate the trade-off between energy loss and specific capacity that the enhancement of maximum Li concentration (specific capacity) is compensated with the increase of kinetic overpotential. The effects of external pressure and reaction rate on the formation enthalpy and kinetic overpotential of a-Si anodes are also explored. We find that both effects destabilize amorphous Li-Si (a-LixSi) phases with raised formation enthalpies, and result in the increased kinetic overpotential of a-Si anodes during electrochemical cycling. Our thermodynamic analysis predicts the constant value of overpotential, due to the neglect of ohmic contribution and possible formation of intermediate crystalline LixSi phases. These computational results provide a better understanding of the amorphization effect on the energy loss of alloying-type anodes.