Investigation of hysteresis and relaxation behaviour in graphite and LiNi_0.33Mn_0.33Co_0.33O₂ electrodes

Li-ion battery electrode materials exhibit hysteresis between lithiation and delithiation due to different thermodynamic equilibria at the same state of charge (SOC). The acquisition time for the open circuit potential (OCP) and the hysteresis behaviour of graphite and LiNi_0.33Mn_0.33Co_0.33O₂ (NMC) has been investigated using galvanostatic intermittent titration technique in Lithium metal anode cells. The OCP at various stages of relaxation after each current pulse have been recorded during lithiation and delithiation. The voltage hysteresis is the difference between the potential at equilibrium after lithiation and delithiation. Hysteresis is ∼600 and ∼300 mV below 0.1 SOC for graphite and NMC electrodes respectively, above 0.1 SOC the hysteresis is much lower 35-33 and 45-42 mV, with short rest requirements of 5–120 min. The error in hysteresis voltage with shorter relaxation times is compared to 120 min rest. This analysis shows that 5 min has an error of 16 mV for graphite and 2 mV for NMC between 0.1 and 1 SOC. Below 0.1 SOC, 90 min (for graphite/NMC) and between 0.1 and 1 SOC, 30 min (for graphite) and 5 min (for NMC) at charge rates below C/20 are appropriate relaxation times and can shorten the OCP parameter acquisition test time significantly.