Investigation of capacity fade for 18650-type lithium-ion batteries cycled in different state of charge (SoC) ranges

18650-type cells comprising of LiNi0.5Co0.2Mn0.3O2 and LiNi0.9Co0.05Al0.05O2 as cathode blend and graphite as anode are cycled in various SoC ranges. Differing capacity fade is found, indicating that cycling in a medium SoC range results in lower capacity loss and behaves better than including high or low SoC in the cycling ranges. Cycling to low SoC tends to have nonlinear capacity fade (sudden capacity drop). Non-destructive methods, i.e. in situ neutron powder diffraction (NPD) and alternating current (AC) impedance, are employed to study the degradation mechanisms. Lithiated cathode loss and loss of lithium inventory (LLI) are calculated from crystal structure parameters refined from the in situ NPD. LLI is the dominating degradation factor for the differing capacity fade of cells. Lithiated cathode loss and solid electrolyte interphase (SEI) growth are deemed to be main fatigue reasons behind the LLI. By investigating the sensitivity of impedance parameters concerning the change in battery capacity, both ohmic resistance (R-0) and SEI resistance (R-1) present a linear relationship with the change of capacity for both, linear and nonlinear degradation, i.e. R-0 and R-1 follow the sudden capacity drop, which is ascribed to the formation of new SEI as evidenced by scanning electron microscopy (SEM) images.

Automated summary

By cycling 18650-type cells with cathode blend LiNi0.5Co0.2Mn0.3O2 and LiNi0.9Co0.05Al0.05O2 and graphite anode in various SoC ranges, differing capacity fade was observed, with cycling in a medium SoC range showing lower capacity loss. Lithiated cathode loss and loss of lithium inventory (LLI) were found to be the dominating degradation factors, with SEI growth considered as one of the main fatigue reasons behind the LLI. Ohmic resistance (R-0) and SEI resistance (R-1) were found to have a linear relationship with the change in battery capacity.