Comprehensive Approach about Time-Relevant Degradation of Ni-Rich Cathode Materials for Electric Vehicles

Abstract

With the prevalence of electric vehicles, Ni-rich layered cathodes have been extensively developed to increase the energy density of Li-ion batteries (LIBs). However, cathodes with a high Ni content suffer from inherent structural and chemical instabilities, which lead to rapid capacity fading and thermal instability.1 In particular, the rapid capacity fading of Ni-rich layered cathodes is largely caused by microcracking and the resultant microstructural instability.2 In addition to microcracking, the time during which highly reactive Ni4+ ions are exposed to the electrolyte when the cathode is in a highly charged state also affects the deterioration of the cathode. Although the exposure time is not significant for cathodes with Ni contents of <60%, the calendar aging test of Ni-rich cathodes reveals that the exposure time, especially at high SoCs, critically affects the degradation of these high-Ni cathodes, as the internal surfaces of the particles are also exposed to the electrolyte via the microcracks.3 The exposure time is particularly pertinent for electric vehicle (EV) LIBs, as they remain stationary for long periods in the highly charged state and are operated intermittently. Consequently, the upper SoC limit of EV LIBs is commonly limited to ≈85%, but this limits the driving range and increases the cost of the batteries. Therefore, a holistic approach considering real-use behavior, as well as the properties of the material itself, is needed to overcome the limitations of Ni-rich cathodes.Here, 1 mol% B doping of a core–shell concentration gradient (CSG) Li[Ni0.88Co0.10Al0.02]O2 cathode (CSG-NCA88) is shown to dramatically alter the microstructure of the cathode and effectively protect the particle interior from parasitic electrolyte attack. The B-doped CSG-NCA88 cathode, CSG-NCAB87, maintains its original microstructure even after holding for 500 h in the fully charged state, whereas irreversible structural damage occurs in the pristine CSG-NCA88 cathode during the prolonged electrolyte exposure. The long-term cycling results confirm that the capacity retention of the cathodes is determined by the electrolyte exposure time at a high SoC and that microstructural modification can effectively suppress the time-dependent degradation from electrolyte attack. The proposed CSG-NCAB87 cathode can be utilized at full capacity without restricting the SoC, thus realizing the development of economical high-energy-density LIBs. Reference s : [1] H.-J. Noh, S. Youn, C. S. Yoon, Y.-K. Sun, J. Power Sources, 2013, 233, 121.[2] H.-H. Ryu, K.-J. Park, C. S. Yoon, Y.-K. Sun, Chem. Mater. 2018, 30, 1155.[3] H.-H. Ryu, G.-T. Park, C. S. Yoon, Y.-K. Sun, Small, 2018, 14, 1803179.