Improved performance of LiFePO4 cathode for Li-ion batteries through percolation studies

This study evaluates the relationship established between active/inactive material of LiFePO4 (LFP) composites and the contributions arising from electronic conduction/Li+ ionic transport during electrochemical performance of a Li-ion cell. A traditional electrode preparation technique is used to assembly four electrode compositions (LFP/C-SP: 94/06, 86/14, 80/20 and 74/26), selected around the percolation threshold, which are subsequently characterized using voltammetry, rate capabilities, electrochemical impedance spectroscopy and scanning electron microscopy. ∼7 wt% of carbon Super P (C-SP) is determined as the electric percolation threshold from the conductivity curves collected for binary components: LFP:C-SP and PVDF:C-SP. Cyclic voltammetry and rate capability plots reveal that electronic conduction (∼10−2 S cm−1) of composites (80/20 and 74/26) above the percolation threshold do not present any impact in the rate capabilities of LFP cathode, whence this increase of C-SP only shrinks capacity, which is more emphasized at high C-rates. Thus, it is suggested that the excess of binder and conductive carbon beyond the percolation threshold generates the ion-blocking effect of PVDF, and ionic transport pathways are extended. A well interconnected network with an efficient amount of binder avoiding particle segregation and carbon disconnection is determined for the 86/14 ratio, likewise, obtaining better rate capabilities, electrode stability and properties during 50 cycles. This differs from the traditional composition used to assembly these composites (80/20).