Controlling the oxidation degree of graphene for improving the electrochemical performance of Ni-rich cathodes

Abstract

Incorporating graphene into Ni-rich cathode materials, such as the LiNi_0.815Co_0.15Al_0.035O₂ (NCA) cathode, has been frequently reported as an effective strategy to improve the performance of lithium-ion batteries. However, the influence of the oxidation degree of graphene on the electrochemical performance of the mixed NCA/graphene cathode remains largely unexplored. In this work, the oxidation degree of graphene was tuned by varying the electrolyte concentration during the electrochemical exfoliation process, and its impact on the performance of NCA/graphene cathodes was evaluated. The results show that the exfoliated graphene (EG) synthesized using 0.3 M (NH₄)₂SO₄ (denoted as EG 0.3) possesses the lowest oxidation degree among all the samples, which leads to the highest electrical conductivity and the lowest moisture content. When incorporated into the NCA cathode, EG 0.3 produced a composite (NCA/EG 0.3) with the best rate capability and cycling stability owing to the low degree of cation mixing and the lowest charge-transfer resistance in the resulting cathode. Electrochemical measurements revealed that the NCA/EG 0.3 cathode delivers the highest specific capacity of 191.32 mA h g⁻¹ at 0.1C, 67.72% capacity retention rate at 5C compared with 0.1C, and good cycling stability (85.13% capacity retention after 100 cycles at 0.2C), which outperformed other composite cathodes. This study demonstrates that the oxidation degree of graphene can govern the cation mixing degree and charge-transfer resistance in Ni-rich cathodes, offering insights into the design of graphene-based additives and protective layers for lithium-ion battery cathodes.