Capacity recovery in commercial lithium iron phosphate cells: Experimental investigations under varying state of health and state of charge conditions

Lithium-ion batteries are widely used in electronic devices due to their high energy density, fast charging capabilities, and long cycle life. Since degradation is inevitable, accurate state-of-health (SOH) estimation is essential to ensure safety, reliability, and long-term performance. However, batteries exhibit a capacity increase or decrease caused by the passive electrode effect (PEE) after short- to long-term storage. This unexpected change can lead to inaccuracies in capacity-based SOH estimation. This paper investigates the capacity recovery using 14 commercial 18650 lithium iron phosphate (LFP) cells stored at elevated temperatures for 30 days at five different state-of-charge (SOC) conditions with initial SOH ranging from approximately 65% to 90%. The rates and characteristics of the capacity recovery effect were evaluated through a comprehensive analysis of cell behavior. The batteries stored at high SOC presented a discernible capacity increase after 19 cycles regardless of the initial SOH. Incremental capacity analysis (ICA) revealed changes in peak amplitude across specific regions of the incremental capacity curve. These changes indicate capacity recovery resulting from both increased lithium-ion availability and thinning of the calendar-aged solid electrolyte interphase (SEI) layer during storage. The voltage-SOC profiles showed a measurable increase in SOC error when stored under high-SOC conditions. Additionally, Coulombic efficiency (CE) calculations produced CE values greater than 1 during the initial cycles, indicating lithium-ion diffusion between the overhang area and the active area when stored at high SOC. As a result, the analysis indicates the capacity recovery effect is influenced more by storage SOC than by SOH.