Thermodynamics of Lithium-Ion Batteries

Li-ion batteries (LiB) of coin-cell form factor (2032) were subjected to three different accelerate aging modes; i. overcharge ageing up to 4.9 V end of charge cut-off voltages (COV), ii. thermal aging at 60 °C and 70 °C for a period of up to 8 weeks after being charged to 4.2 V at ambient temperatures , and iii. long cycling at a C/2 rate at ambient temperature for up to 1000 cycles. We found the entropy (ΔS) and enthalpy (ΔH) profiles varied with aging conditions much more readily than open-circuit potential (OCP) profiles. We also found that aged batteries keep a thermodynamic fingerprint of the process by which they aged. This memory effect can be revealed by measuring the entropy and enthalpy of aged cells at two particular OCP of 3.87 V and OCP = 3.94 V, which are specific to LiB based on graphite anode and lithiated cobalt oxide (LCO) cathode material. In fact 3.87 V and 3.94 V are potentials of phase transition in the anode and cathode materials, respectively. Another aspect of the thermodynamics memory effect is a characteristic change of entropy and enthalpy profiles of cells having incurred the same capacity loss under different aging modes as compared to almost identical OCP and discharge profiles.