To satisfy the energy density requirement, layered Ni-rich LiMO₂ (M = Ni, Co, and Mn (NCM) or Ni, Co, and Al (NCA)) cathodes have been developed extensively for the last two decades because of their high theoretical capacity of 275 mAh g^-1.^1,2 Currently, Li[Ni_0.8Co_0.15Al_0.05]O₂ and Li[Ni_0.6Co_0.2Mn_0.2]O₂ cathodes are adapted for EVs (Tesla Model 3 and GM Bolt) that have a driving range of 380 km (236 miles) for a single charge, which is still short of the recommended threshold. To further increase the driving range, the Ni content in the cathodes should be increased gradually so that the energy density of the LIBs is increased. However, it is well acknowledged that the increase in the Ni concentration of LiMO₂ leads to an almost linear increase in the reversible capacity but a proportional decrease in the cycling performance and thermal safety.^3,4 The fast capacity fading of Ni-enriched NCMs is ascribed to the H2–H3 phase transition in the highly charged state of approximately 4.2 V Li/Li⁺, inducing an anisotropic lattice volume change that generates internal microcracks in the cathode particles.⁴ The microcracks facilitate electrolyte infiltration into the particle interior along the grain boundaries. The penetrated electrolyte accelerates surface degradation of the primary particles through the reaction of unstable Ni⁴⁺ with electrolyte to form NiO-like rocksalt impurity phases, and thereby increases the cell impedance.⁴

The currently deployed cylindrical full cell based on the NCA cathode (Li[Ni_0.76Co_0.14Al_0.1]O₂) and graphite anode demonstrates poor long-term cycling performance (capacity retention of 50% after 2000 cycles) when cycled at 100% depth of discharge (DOD). On the other hand, the full cell demonstrates an outstanding Li intercalation stability at DOD of 60% during the same cycling period, indicating that the DOD should be limited to 60% for long-term cycling.^5,6 However, limiting the DOD during cycling reduces the energy density of the battery significantly, and consequently, increases the EV’s weight and battery cost.

Currently, Tesla, which is one of the most prominent EV producers, uses the Panasonic Li[Ni_0.84Co_0.12Al_0.04]O₂ cathode in its Model S and Model X. However, with the increasing demand for higher energy density and the ever-increasing cost of raw cobalt, which has more than doubled in the last five years, increasing the Ni content in the cathode active material has become mandatory. In this regard, we performed a systematic comparative study of the standard Li[Ni_0.8Co_0.16Al_0.04]O₂ (herein after denoted as NCA80), Li[Ni_0.88Co_0.10Al_0.02]O₂ (NCA88), and Li[Ni_0.95Co_0.04Al_0.01]O₂ (NCA95). Based on this result, we explore the fundamental battery performance and capacity fade mechanisms of the three NCA cathodes to understand the impact of the increased Ni content on the electrochemical performance of the highly Ni-enriched NCA cathodes.

References

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(2) Y.-K. Sun, S.-T. Myung, B.-C. Park, J. Parakash, I. Belharouak, K.Amine, Nat. Mater. 2009, 8, 320–324.

(3) C. S. Yoon, M. H. Choi, B.-B. Lim, E.-J. Lee, Y.-K. Sun, J. Electrochem. Soc. 2015, 162, A2483–A2489.

(4) H.-H. Ryu, K.-J. Park, C. S. Yoon, Y.-K. Sun, Chem. Mater. 2018, 30, 1155–1163.

(5) S. Watanabe, M. Kinoshita, T. Hosokawa, K. Morigaki, K. Nakura, J. Power Sources 2014, 260, 50–56.

(6) S. Watanabe, M. Kinoshita, T. Hosokawa, K. Morigaki, K. Nakura, J. Power Sources 2014, 258, 210–217.