Si is an attractive negative electrode material for lithium ion batteries due to its high specific capacity (~3600 mAh/g). However, the huge volume swelling and shrinking during cycling leads to several coupled mechanical and chemical degradation at the material/electrode/cell level, including fracture of Si particles, unstable solid electrolyte interphase (SEI), and low Coulombic efficiency. In this talk, we will discuss how to improve the cycle efficiency from those aspects, starting with the fundamental understanding of the relationship between the structure and properties of SEI to pinpoint the SEI failure mechanisms using advanced in-situ electrochemical tools. Then we will discuss how to develop artificial SEI, combining the design of Si nanostructure and electrode architecture to enable the high cycle efficiency and the extended life. At last, we will discuss how the nanostructure design at material level can impact the electrochemical-mechanical behaviors at electrode and cell level and the trade-off between gravimetric energy density and volumetric energy density for EV applications.