Different solid electrolyte interface and anode performance of CoCO3 microspheres due to graphene modification and LiCoO2||CoCO3@rGO full cell study

Hollow and urchin-like CoCO₃ dumbbell microspheres (HCCM) and CoCO₃@reduced graphene oxide composite microspheres (CC@rGO) were synthesized by one-step solvothermal methods. RGO modification significantly improves the cycling stability of CoCO₃ microsphere anodes at low current densities. CC@rGO delivers 1067 and 997 mAh/g after 50 cycles at 0.1 C and 100 cycles at 0.2 C, respectively. Interestingly, HCCM shows rising cycling stability with increased rates while CC@rGO exhibits slight and continuous capacity fading at almost all rates thus declining advantage with rising rates. A competition theory between "protective force" mainly from solid electrolyte interface (SEI) or rGO layer and "destructive force" from volume swing applied on the microspheres was raised to clarify this phenomenon. The changing volume swing degree and SEI formation rate with increased rates, as well as different protective effects of HCCM and CC@rGO's SEI film or rGO layer, determined various competition results of the two forces at each rate thus various structural stability and cycling performance of the two samples. Furthermore, the application prospect of CoCO₃ anodes in lithium ion full cells has been explored and the assembled LiCoO₂||CC@rGO exhibits an initial coulombic efficiency of 92.9%, a discharge mid-voltage of 2.3 V and stable cycling performance.