WEB Progress in High-Capacity Concentration Gradient Cathode for Next-Generation Electric VehiclesAudimax
Li-ion batteries (LIBs) have been positioned as the main portable energy source for electric vehicles (EVs) based on their high energy densities, high power densities, and practical cycle lives. However, the current state-of-the-art LIBs applied for EVs is still insufficient for the requirement of a driving range per single charge. Among components, the performances of their cathodes largely limit both the energy density and service life of EV batteries. Therefore, the increasingly strong demand for the higher energy density of LIBs has pushed the development of high-capacity cathodes. The typical strategy used to improve the cathode performance involves the progressive replacement of Co and Mn/Al with Ni in Li[NixCoyMnz]O2 (NCM) and Li[NixCoyAlz]O2 (NCA) (x > 0.8). The specific capacity of Ni-rich cathodes increases with the Ni content, but the capacity gain from Ni enrichment is negated by the fast capacity fading. One of the most dominant mechanisms for the poor cycling stability of Ni-rich cathodes is the formation of microcracks within cathode particles. An abrupt anisotropic volume changes accompanied by H2/H3 phase transition near the charge–end accumulate mechanical stress on the particle and the magnitude of the internal strain proportionally intensifies with the Ni content. Once the internal stress reaches a certain standard, microcrack initiates and propagates along the grain boundaries from the particle center. During repetitive cycling, the propagated microcracks extend to the particle surface and provide channels for electrolyte penetration which accelerates the degradation of the exposed primary particles through parasitic reactions with the electrolyte. Many researchers have proposed various doping or surface coating techniques to improve the stability of Ni-enriched cathode materials. However, these attempts have left room for questions in improving the long–term cycling stability of Ni-rich layered cathodes, because they hardly overcome the formation of a microcrack. Inevitably, a common strategy of limiting a depth of discharge of 60% has been adopted despite significant decreases in the energy density and cost competitiveness.
Thus, for an improvement in the cycling stability of the Ni-rich cathodes, it is necessary to suppress the formation of microcracks by dissipating the internal stress developed by H2/H3 phase transition. One of the effective methods is to control the microstructure of primary particles, such as an elongation of the primary particles with radial orientations, so that the primary particles contract and expand uniformly rather than in random directions. In this presentation, the changes in morphology and microstructure of Ni-rich NCM cathodes are introduced. The most recent and promising results concerning concentration gradient NCM cathodes will be intensively reviewed. One typical example, resulting from a preferential growth of particular facet, concentration gradient Li[Ni0.9Co0.05Mn0.05]O2 cathode delivers a discharge capacity of 229 mAh g-1 and exhibits capacity retention of 88% after 1000 cycles in a pouch-type full cell (compared to 68% for the conventional NCM cathode). The superior cycling stability clearly demonstrates the importance of the particle microstructure in mitigating the abrupt internal strain caused by H2/H3 phase transitions in the deeply charged state. The proposed cathodes signpost to the rational design of microstructure for enhancing the cycle life of all class of Ni-rich layered cathode.