Investigation into Mechanical Degradation of Lithium-Ion Batteries with Nickel-Rich Cathode Materials - A Long-Term Cycling Study

Layered lithium transition-metal oxides of type LiNi x Co y Mn z O2 (NCM, with x + y + z = 1) are well established and represent the current state of the art cathode materials for different lithium-ion battery applications. The ultimate goal in the area of NCMs is to increase the nickel fraction in the material as much as possible, as this allows for higher specific capacities (for a given charge cutoff voltage). However, recent studies have demonstrated that with increasing practically used capacity the material suffers increasingly from volume changes, eventually leading to mechanical failure. Apart from volume changes, adverse side reactions like structural/morphological changes and/or phase transitions during cycling operation are also possible causes of degradation, affecting capacity retention and overall lifetime of a battery cell. In this work, we report on a long-term study on the structural, electronic and morphological changes of high-nickel NCM/graphite full-cells from operando X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy and electrochemical impedance spectroscopy. The acquired data demonstrate that the capacity loss can be attributed predominantly to the cathode side. Using XRD we observed the presence of two differently active NCM phases. However, the NCM cathode material was still found capable of virtually complete Li (de-)lithiation, but with significantly slower kinetics due to, in part, built-up of resistive surface layers with cycling. Figure 1. a) Charge/discharge profiles of LiNi0.85Co0.1Mn0.05O2/graphite full-cells for various cycles at 1C rate and in the voltage range between 2.8 and 4.2 V. b) Phase fractions of the less and more active NCM phases after charging to 4.2 V (including a 1 h CV step). Figure 1