Carbon coating on the current collector and LiFePO<inf>4</inf> nanoparticles - Influence of sp² and sp³-like disordered carbon on the electrochemical properties

The charging/discharging cycles, rate capabilities, cyclic voltammetry and electrochemical impedance spectra (EIS) of sol-gel prepared C-coated LiFePO4 (C-LFP-SG) and a commercial LiFePO4 (LFP-comm), in combination with two different current collectors, viz. Al foil and C-coated Al foil (Al-C) are studied. A flat voltage profile at ∼3.4 V was seen during the charging/discharging cycle of C-LFP-SG/Al. However, C-LFP-SG/Al-C showed a continuous decrease in voltage during discharging and an increase during charging cycles. On contrary, LFP-comm/Al-C showed a constant flat voltage profile, whereas LFP-comm/Al shows a slight decrease during the discharge. C-LFP-SG/Al and LFP-comm/Al-C cells exhibit better electrochemical performance compared to C-LFP-SG/Al-C and LFP-comm/Al respectively. This is also reflected from a direct correlation between the flat potential behavior and their rate capabilities. From EIS measurements, we discuss the effect of C-coating on the charge-transfer resistances of LFP nanoparticles and current collectors. Raman analysis of carbon modes in all the cathodes and current collectors reveal that a large contribution from sp2 bonded carbon gives better electrochemical properties. Increasing sp3-like disordered carbon at the C-C interface leads to large polarization effects during the charging and discharging cycles.