Improving the high-voltage electrochemical performance of LiNi 0.5 Co 0.2 Mn 0.3 O 2 cathode material by Al 2 O 3 /graphene co-modification

The poor performance of LiNi 0.5 Co 0.2 Mn 0.3 O 2 cathode materials in lithium-ion batteries under high cut-off voltage, such as fast capacity fading and poor cycle capability, hinders the development of high energy density lithium-ion batteries. Surface modification is generally conducted on the electrode to improve high cut-off voltage performance. In this study, Al 2 O 3 and graphene are co-coated on the surface of LiNi 0.5 Co 0.2 Mn 0.3 O 2 . The Al 2 O 3 /graphene co-coating layer effectively reduces side reactions and HF erosion and facilitates lithium-ion diffusion/electronic conduction. The degree of cation mixing and electrode polarization is also decreased by the co-coating from the XRD and cyclic voltammetry results. The modified sample exhibits improved electrochemical properties under high cut-off voltage (3–4.5 V): The initial discharge capacity and coulombic efficiency of the modified sample reaches 187.2 mAh g −1 and 87.0%, which is higher than the pristine NCM (161.0 mAh g −1 , 82.9%). The co-coating electrode possesses a high cycling retention of 88.2% with 10% improvement at a current density of 180 mA·g −1 (1 C) after 200 cycles, and the rate capability reaches 119.6 mAh·g −1 at 1800 mA·g −1 (10 C), compared to 78.6 mAh·g −1 of the pristine sample. The modification effect in different voltage ranges (3–4.5 V and 3–4.3 V) was compared, which shows a more significant improved effect under high voltage than low voltage: The cycle retention after 200 cycles is increased by 10% at 3–4.5 V, higher than 7.7% (3–4.3 V), which is due to the intensified side reactions under high voltage being suppressed by co-coating.