Learning from Overpotentials in Lithium Ion Batteries: A Case Study on the LiNi1/3Co1/3Mn1/3O2 (NCM) Cathode

The practically available specific energy of Li ion batteries (LIB) is highly depending on the used specific charge/discharge current, since the respective overpotentials of each electrode affect the two vital specific energy parameters, specific capacity and voltage. Focusing on the positive composite electrode as the specific energy bottleneck, the overall nature of the overpotential is discussed for the LiNi1/3Co1/3Mn1/3O2 (NCM) active material. It is shown that the characteristic overpotentials during charge (delithiation) and discharge (lithiation) is state of charge (SOC) and depth of discharge (DOD) dependent, respectively. It was demonstrated that the discharge characteristics are intertwined with the previous charge conditions, particularly with the charging time and the specific charge capacity. Increasing both in parallel can even lead to a deterioration of the subsequent specific discharge capacity. Furthermore, Li+ transport pathways within the NCM composite electrode are discussed and their influence on the observed overpotential evaluated. Changes of the overpotential are found to be mainly associated with changes within the NCM crystal structure, which is experimentally supported by the correlation of the SOC dependent overpotential with the XRD determined c-axis lattice parameter. Consequently, the Li+ transport within the active material is mostly responsible for limiting the practically available specific energy.