PNNL

Abstract

Side reactions in any battery are ubiquitous but are often prominent during the initial cycles and gradually “fade” upon cycling if the electrolyte and additives effectively passivate the reactive interfaces. However, if new surfaces or phase boundaries are continuously exposed to the electrolyte, those side reactions will not be sufficiently suppressed. The end results are the accelerated cell failure and/or gas evolution, sometimes raising serious safety concerns. A good example is Ni-rich cathode e.g., LiNixMnyCo1-x-yO2 (NMC, x>0.8) material that has a high energy but suffers from limited cycle life, moisture sensitivity and gas generation, fundamentally rooted in the pulverization of agglomerated nano particles. Single crystalline Ni-rich NMC is expected to address the common issues that polycrystals have, but a quantifiable understanding on the gas generation from different Ni-rich NMC crystals is lacking. Herein, the origin, evolution and accumulation of different gases detected in batteries with polycrystalline and single crystalline Ni-rich cathodes are studied. Interestingly, no oxygen is detected from single crystalline Ni-rich NMC within regular electrochemical window, indicating its intrinsically safe nature. In contrast, when cycling polycrystals, the continuous and simultaneous occurrence of H2 and O2 is revealed to be the fundamental reason for Ni-rich NMC safety. New insights have been provided in understanding and addressing the gas generation challenge which is the main obstacle that delays the large scale application of Ni-rich NMC cathodes for future battery technologies.