Electrochemical Performance and Capacity Retention of Lithium Iron Phosphate and Lithium-Ion Battery Cells for Grid-Scale Renewable Energy Storage

The accelerating global deployment of intermittent renewable energy sources — solar photovoltaic and wind — creates a critical need for large-scale, long-duration electrochemical energy storage systems capable of decoupling generation and demand profiles at the grid level. Lithium-iron phosphate (LFP, LiFePO₄) batteries have emerged as the leading chemistry for grid-scale stationary storage applications due to their superior cycle life, thermal stability, and absence of cobalt in the cathode composition. This study presents a systematic comparative electrochemical evaluation of commercially available 18650-format LFP (nominal capacity 1500 mAh) and NMC (Nickel-Manganese-Cobalt, LiNi₀.₆Mn₀.₂Co₀.₂O₂, nominal capacity 2200 mAh) cells over 3000 charge-discharge cycles under controlled laboratory conditions. Electrochemical characterisation including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), galvanostatic intermittent titration technique (GITT), and galvanostatic cycling at C/2 rate was performed at 25°C and 45°C to evaluate temperature effects on degradation mechanisms. LFP cells retained 91.3% of initial capacity after 3000 cycles at 25°C versus 82.7% for NMC cells, demonstrating substantially superior cycle-life stability attributable to the olivine crystal structure's resistance to structural phase transformation during lithiation-delithiation. EIS analysis reveals that NMC degradation is dominated by increasing charge transfer resistance at the cathode-electrolyte interface (CEI) layer, while LFP degradation is primarily driven by lithium plating at the graphite anode under elevated temperature and high rate conditions.