| Description | High-Energy Cathodes for Next-Generation Electric Vehicles General electromobility is increasingly becoming a necessity as a part of the global efforts to reduce carbon emissions. Although investments in electric vehicle (EV) technology and infrastructure has increased tremendously in recent years, market acceptance of EVs has been rather slow mostly because of the inadequate energy density of EV battery which is currently based on Li-ion battery (LIB). Increasing the energy density of an LIB makes it lighter and cheaper. Most importantly, it also increases the driving range which has limited the consumer appeal for EVs. To reach the driving range of 300 mile per single charge, which is considered as the threshold for increasing the EV market share, LIB requires its cathode to deliver a discharge capacity well above 200 mAh g-1. Although myriad LIB cathode materials have been proposed, layered-structured Li[NixCoyMnz]O2 (NCM) and Li[NixCoyAlz]O2 (NCA) are the most successful cathode materials that have been employed in recent EVs. Currently, Tesla, which may be the most prominent EV producer, uses the Panasonic Li[Ni0.84Co0.12Al0.04]O2 cathode in its Model S and Model X. However, reaching the threshold discharge capacity using these cathode materials has been challenging because increased discharge capacity gained by Ni-enrichment is quickly traded off by rapid capacity fading during cycling.
In this presentation, we review comprehensive picture of the electrochemical performances and propose capacity fading mechanism of the NCM and NCA cathodes. Furthermore, to overcome the inherent chemical instability of Ni-rich layered cathodes and to improve capacity retention, we have hybridized Ni-rich and Ni-deficient NCM cathodes at the particle level by establishing concentration profiles within a single particle such that the Ni-rich center maximizes the capacity while the Ni-deficient particle surface ensures high thermal and structural stability; one example, 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 proposed concentration gradient cathodes provide an opportunity for the rational design and development of a wide range of multifunctional cathodes, especially for Ni-rich NCM and NCA cathodes, by compositionally partitioning the cathode particles and thus optimizing the microstructural response to the internal strain produced in the deeply charged state. |
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