Silicon is an important electrode material for next generation high performance lithium ion batteries due to its order of magnitude higher charge carrying capacity compared to conventional graphite electrodes. The main obstacle of using Si electrodes in commercial lithium batteries is the massive volume expansion of the Si electrode under repeated charge cycling, which leads to delamination of the Si electrode from the Cu current collector and inevitably results in capacity fade. Using first principle calculations based on density functional theory and ab-initio molecular dynamics simulations, this thesis focuses on the structure of the Cu/Si interface and aims to provide a complete picture of the intermixing at the Cu/Si interface during lithiation processes. The hypothesis, supported by existing experiments, is that the Cu/Si interface is not pristine and comprises of an interdiffused Li-Si-Cu interphase structure. To test this hypothesis, the barrier energies for Li diffusion into the assumed crystalline silicide interphase structure separating crystalline Si and Cu is studied. Results show that the barrier energies for Li ion diffusion decreases towards the interphase structure, which suggest that Li ions can diffuse into the silicide structure even during early stages of lithiation. Several interdiffused Li-Si-Cu interphase structures with varying Li to Si content are subsequently modeled using rapid heating and quenching process. The atomic structure of interdiffused Li-Si-Cu phase reconstructed from rapid heating and quenching are in good agreement with previous experiment results. The work of separation of these interdiffused phase are also examined.
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Structure and properties of the interdiffused phase between lithiated silicon and copper current collector
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