Mitigation of resistance increase in positive electrodes of lithium-ion batteries during floating–cycling by operating voltage control

Abstract

As the use of lithium-ion batteries (LIBs) is widespread, understanding the degradation factors as well as life extension of LIBs is of great importance. While the LIBs are often continuously charged (float-charged) and occasionally discharged, such a mixture of floating and cycling causes severe capacity loss, which is much worse than the sum of mere floating and mere cycling. In this work, durability tests of commercially available 18 650 LIB cells were performed for more than 2 years in the floating–cycling mode, and the effects of charging voltage limitation and depth of discharge restriction on degradation were evaluated. The aged cells were analyzed using voltage profiles, impedance measurements and operando neutron diffraction, which showed that the resistance increase in the positive electrode as the main origin of degradation can be alleviated with the charging voltage limitation rather than the depth of discharge limitation. The post-mortem analysis of an aged positive electrode indicated that the microcracks caused by cycling were covered with resistive films formed by the floating operation, which was mitigated by the charging voltage limitation. These results will give insights into the capacity loss in LIBs and contribute to LIB life extension.