Unveiling the Roles of Key Dopant Elements in Tuning Electrochemical, Thermal, and Air Stabilities of High-Nickel Layered Oxide Cathodes

Abstract

High-nickel (Ni) layered oxides, LiNixM1–xO2 (M = Mn, Co, Al, etc.), are regarded as promising cathode candidates for next-generation lithium-based batteries in electric vehicles. Current successfully adopted LiNi0.8Mn0.1Co0.1O2 and LiNi0.8Co0.15Al0.05O2 cathodes with an energy density of nearly 750 Wh kg–1 will be inadequate for easing the driving range anxiety of customers in the foreseeable future. Increasing the Ni content further to > 90% is deemed as a feasible solution as it can simultaneously provide more energy density and reduce the raw material cost. Unfortunately, as the Ni content increases, structural, interfacial, thermal, and air stabilities of high-Ni cathodes become significantly worse, hindering their practical applications.1 In this regard, a rational compositional design with a precise use of dopant elements and their concentrations is needed to achieve the best stabilities of a high-Ni cathode. Evidently, this cannot be realized without a clear understanding of the roles of key dopant elements, such as Co, Mn, and Al, in tuning the stabilities of high-Ni cathodes.In this work, three cathode materials with a 5% single-element doping, viz., LiNi0.95Co0.05O2 (NC), LiNi0.95Mn0.05O2 (NM), and LiNi0.95Al0.05O2 (NA), along with undoped LiNiO2 (LNO) are synthesized in-house to systematically examine the influences of Co, Mn, and Al on cycle, thermal, and air stabilities of high-Ni cathodes. From the aspect of cycle stability, it is unveiled that a critical role of dopants is regulating the state-of-charge and, more importantly, the extent of H2-H3 phase transition, which have a dominate role in dictating the cycle stability. In addition, Co offers important benefits of reducing irreversible capacity/energy loss and facilitating Li+ diffusivity. With respect to thermal stability, although Co, Mn, and Al doping can all mitigate the heat release of thermal runaway reactions, only Al doping can effectively delay the onset of thermal runaway reactions. Furthermore, Al doping can also delay the onset of outgassing (mainly O2 and CO2) at deep charge due to its ability to perturb long-range metal-metal interaction and make the metal-O bonds more ionic and stronger.2 Finally, in terms of air stability, compared to Co and Al, Mn doping can more effectively reduce the formation and accumulation of surface residual Li species (LiOH and Li2CO3) due to a higher concentration of stable Ni2+ in NM counterbalancing Mn4+. Our systematic investigation into the roles of key dopant elements can help precisely predict the most effective high-Ni cathode compositions according to the specific application requirements. References W. Li, E. M. Erickson, and A. Manthiram, Nat. Energy, 5, 26 (2020).A. Manthiram, Nat. Commun., 11, 1550 (2020).