Improving Toleration of Volume Expansion of Silicon-Based Anode by Constructing a Flexible Solid-Electrolyte Interface Film via Lithium Difluoro(bisoxalato) Phosphate Electrolyte Additive

The silicon (Si) anode is considered one of the most promising candidates among many novel anode materials in lithium-ion batteries owing to its high theoretical capacity and earth abundancy. Nonetheless, a large volume expansion of Si particles appears with cycling, prompting unceasing breakage/reformation of the solid-electrolyte interface (SEI) and fast capacity degradation in traditional electrolytes. For the purpose of tolerating volume expansion for the Si anode, lithium difluoro(bisoxalato) phosphate (LiDFBOP) was adopted in the standard (STD) electrolyte based on LiPF6. Density functional theory (DFT) calculations, Young's modulus from atomic force microscopy, potential-resolved in situ electrochemical impedance spectroscopy (PRI-EIS) measurement, and other characterizations proved that the formed SEI can inhibit volume expansion of a Si@Graphite@C anode. In the STD+2% LiDFBOP electrolyte, the solvation structure of Li(EC)2(PF6)1(DFBOP)1 is more likely to be produced, and this kind of solvation structure has stronger reducibility and easily participates in SEI formation. The 2% LiDFBOP additive increases the inorganic LiF component of SEI, which yields great advantages in regulating the uniform diffusion of Li+ ions passing through the SEI. Besides, the organics containing P and F atoms are also abundant in SEI, which has greater flexibility and can tolerate volume expansion of the Si@Graphite@C anode. Therefore, the STD+2% LiDFBOP electrolyte can improve the electrochemical performances of Si@Graphite@C/Li half-cells. This work has practical implications not only for the molecular design of novel lithium salt but also for the constructions of SEI and electrolyte systems compatible with the Si@Graphite@C anode.