Performance optimization of freestanding MWCNT-LiFePO4 sheets as cathodes for improved specific capacity of lithium-ion batteries

Abstract

The typical lithium-ion-battery positive electrode of "lithium-iron phosphate (LiFePO4) on aluminum foil" contains a relatively large amount of inactive materials of 29 wt% (22 wt% aluminum foil + 7 wt% polymeric binder and graphitic conductor) which limits its maximum specific capacity to 120.7 mA h g-1 (71 wt% LiFePO4) instead of 170 mA h g-1 (100 wt% LiFePO4). We replaced the aluminum current-collector with a multi-walled carbon nanotube (MWCNT) network. We optimized the specific capacity of the "freestanding MWCNT-LiFePO4" positive electrode. Through the optimization of our unique surface-engineered tape-cast fabrication method, we demonstrated the amount of LiFePO4 active materials can be as high as 90 wt% with a small amount of inactive material of 10 wt% MWCNTs. This translated to a maximum specific capacity of 153 mA h g-1 instead of 120.7 mA h g-1, which is a significant 26.7% gain in specific capacity compared to conventional cathode design. Experimental data of the freestanding MWCNT-LiFePO4 at a low discharge rate of 17 mA g-1 show an excellent specific capacity of 144.9 mA h g-1 which is close to its maximum specific capacity of 153 mA h g-1. Furthermore, the freestanding MWCNT-LiFePO4 has an excellent specific capacity of 126.7 mA h g-1 after 100 cycles at a relatively high discharge rate of 170 mA g-1 rate.