Abstract:

Natural graphite possesses advantages such as high capacity, stable voltage platform, and low cost; however, it suffers from defects and poor electrolyte compatibility, limiting its widespread application. In the digital field, the anode material of lithium-ion batteries primarily comprises artificial graphite prepared from needle coke or petroleum coke. This study investigates the effect of a composite anode consisting of natural graphite and artificial graphite on the performance of digital lithium-ion batteries, using pouch cells as the research object. The anode materials and assembled batteries were analyzed using scanning electron microscopy (SEM), laser particle size analysis, specific surface area analysis, and electrochemical impedance spectroscopy, among other methods. The electrochemical performance of batteries utilizing composite graphite (C-Gr; 70% artificial graphite + 30% natural graphite) and pure artificial graphite (A-Gr) anodes was evaluated. The results show that the direct current internal resistance, solid electrolyte interphase (SEI) film impedance, and charge transfer impedance of composite graphite anodes are relatively high, leading to greater internal temperature rise, significant polarization, and reduced performance during high-rate, constant power, and low-temperature discharge conditions. The large specific surface area of the composite graphite anode increases the side reactions with the electrolyte, resulting in decreased residual capacity and recovery capacity after full-charge storage at high temperatures. After 500 cycles at room temperature, the capacity retention of the composite graphite anode was 76%, which was 11.6% lower than that of the artificial graphite anode. Furthermore, the thickness swelling rate and internal resistance growth rate were 1.8% and 26.3% higher than those of the artificial graphite anode, respectively. SEM analysis reveals significant thickening and increased cracking of the SEI film on the composite graphite anode after cycling, leading to reversible capacity fading. In addition, the degradation of the anode material indirectly affects the structural stability of the cathode material. After cycling with the composite graphite anode, more Ni, Co, and Mn elements dissolved from the cathode material, with Mn content approximately double that of the artificial graphite anode. Severe particle cracking and skeleton structure destruction were also observed in the ternary cathode material.

Key words: lithium-ion battery, artificial graphite, composite graphite, impedance, SEI film