Effect of Low Cell Voltages on the Performance of MCMB Anode and LiNi0.8Co0.2O2 Cathode

Lithium-ion cells employing graphitic anodes (graphite or MCMB) and lithiated metal oxide cathodes, Li(Ni,Co,Al)O2 are generally intolerant to low cell voltages, which is often attributed to the side reactions on the anode. Even though dissolution of the copper substrate at the anode is not thermodynamically favored at the anode potentials positive to lithium, mild copper dissolution was often reported to occur at positive potentials. Furthermore, the cell performance appears to degrade upon exposure to low voltages, even without any visible copper dissolution. One of the engineering batteries for the NASA Mars Science Laboratory (MSL) mission sustained a high impedance short due to a failure in the harness insulation, which resulted in the deep-discharge of multiple cells to low voltages, and an overall performance loss in these cells. Results from the laboratory test cells confirm the trend in the performance degradation as a function of low voltage exposure and point to, surprisingly, a degradation of the cathode, more than the anode. Electrochemical measurements, both ac impedance (EIS) and DC polarization show an unexpected trend that the cathode kinetics are affected by the deep-discharges to low voltages.