Study of lithiation mechanisms of high performance carbon-coated Si anodes by in-situ microscopy

Carbon coated Si (Si/C) composites with a high Si content of 81 wt% are synthesized by one-pot carbonization of the mixture containing commercial Si particles and polyvinylidene fluoride (PVDF) at an optimized temperature. The Si/C electrodes deliver a high cyclic capacity of 2003 mA h g^-1 at 0.5 A g^-1 after 50 cycles and an enhanced rate capability of ~750 mA h g^-1 at 4 A g^-1 for over 200 cycles. The effect of ultrathin carbon coating on lithiation mechanisms of Si particles is evaluated using the in-situ transmission electron microscopy (TEM). It is revealed that the carbon-coated Si particles undergo an isotropic to anisotropic transition during the initial lithiation, whereas such transition is not observed for the uncoated Si particle. The lithiation rate of Si/C is 3-4.5 times faster than that of uncoated Si with the same diameter, a testament to high rate capacities of Si/C in real batteries. The flexible, amorphous carbon coating favorably alters the damage mode of Si particles from pulverization by multiple cracking to fracture by a single crack. The above findings offer fundamental understanding and practical guideline for designing carbon coatings of Si-based electrodes with much enhanced electrochemical performance.