Enhancing high-voltage performance of LiNi_0.5Co_0.2Mn_0.3O₂ cathode material via surface modification with lithium-conductive Li₃Fe₂(PO₄)₃

Increasing Ni content and (or) elevating the upper cut-off operating voltage are the most frequently utilized methods for enhancing the energy density of Ni-based layered cathode materials in lithium-ion batteries. However, both methods will lead to the structure instability and aggravate the unwanted side reactions between electrode and electrolyte. Aiming at mitigating this problem, lithium-conductive Li₃Fe₂(PO₄)₃ is employed as a coating layer to enhance the high-voltage performance of LiNi_0.5Co_0.2Mn_0.3O₂ cathode material for lithium-ion batteries. A homogeneous Li, Fe and P-containing colloidal suspension is prepared, via a facile wet chemical method, and used as the precursor in preparing the coating layer. X-ray diffraction and scanning electron microscope results indicate that the surface coating do not alter the structure and morphology of the material particles. Energy dispersive spectrometry and elemental mapping results confirm that the coating layer is uniformly distributed on the surface of the matrix material. Electrochemical characterizations demonstrate that all the surface-modified samples exhibit slower capacity fading than the bare one at elevated upper cut-off voltages. For instance, the one with 1.0 wt. % Li₃Fe₂(PO₄)₃ coating has a remaining discharge capacity of 135.3 mAh g⁻¹ after 100 charge-discharge cycles at 1 C rate upon the voltage range of 2.8–4.5 V, with a capacity retention of 75.33%, whereas the uncoated one exhibits the discharge capacity and the capacity retention of only 91.8 mAh g⁻¹ and 51.92%, respectively, under the same experimental conditions. Furthermore, 1.0 wt. % Li₃Fe₂(PO₄)₃-coated material shows a film resistance (R_sf) of 40.47 Ω and the charge-transfer resistance (R_ct) of 35.69 Ω after 100 cycles, whereas the values of the uncoated one are 102.43 Ω and 42.76 Ω demonstrating that the surface coating can lead to a more stable solid-electrolyte interphase (SEI) layer.