1. What is voltage hysteresis?
Voltage hysteresis refers to the phenomenon where the electrode potential evolution curves with capacity do not coincide during the charging-discharging process, thus forming a voltage hysteresis loop. Voltage hysteresis leads to energy dissipation and reduced charging/discharging efficiency during lithium-ion battery charging and discharging.
The equilibrium point of the charging curve is lower than the terminal voltage, while the equilibrium point of the discharging curve is higher than the terminal voltage. The main reason for this phenomenon is the charging, discharging, and mass transfer processes of the double layer.
Performing a complete charge-discharge cycle (full charge and discharge) on the battery yields the primary hysteresis loop; performing an incomplete charge-discharge cycle (partial charge and discharge) yields the secondary hysteresis loop. The two curves constituting the primary hysteresis loop are the boundary curves of all hysteresis loops, forming the largest closed loop. The charge-discharge curves of the secondary hysteresis loop lie within the boundary curves and tend to converge towards them.
2. Causes of Voltage Hysteresis in Lithium-ion Batteries
The main reasons for voltage hysteresis in lithium-ion batteries are the entropy effect, mechanical stress, and microscopic distortion within the active electrode material during lithium insertion and extraction. Lithification and delithiation have different lattice constants, generating mechanical stress in the phase barrier, thus causing a potential drop within a single particle from charging to discharging.
Thermodynamic effects are also a factor that cannot be ignored.
3. What aspects are affected by the hysteresis effect?
According to the relaxation effect principle, the voltage after charging and discharging should converge to the true OCV value at that SOC point. However, due to the hysteresis effect, the voltage after discharging always relaxes to a value lower than the true OCV, while the voltage after charging relaxes to a value higher than the true OCV. This introduces a significant error in the measurement and calculation of polarization voltage, thus affecting the prediction of battery voltage and power.
