Abstract:

In recent years, LiFePO₄ (LFP) has garnered significant attention because of its low cost, high safety, and long cycle life. However, conventional LFP energy storage devices typically have a cycle life of approximately 2000 cycles at a rate of 0.1—2 C. To further develop high-power and long-life LFP energy storage devices, pouch-type energy storage devices based on different anode materials (hard carbon/soft carbon/graphite) with a capacity of 9 Ah were designed. Examining the cycling performance at a high rate of 4 C revealed that after 4000 cycles, the capacity retentions of energy storage devices using hard and soft carbons as anodes were 83.0% and 78.9%, respectively, outperforming those using graphite, which had a retention of 51.6%. Analytical techniques such as XRD and EIS analyses, incremental capacity analysis, and different voltage analyses revealed that the primary causes of capacity fading include increased contact resistance, SEI film resistance, and lithium-ion consumption in the anode due to structural changes. Further investigation into the poor cycle life of graphite as an anode material revealed that graphite exhibits a lower Coulomb efficiency, higher operating temperature, greater displacement, and a decrease in peak intensity in the dQ/dV peak during cycling. The interlayer spacing of graphite was 0.335 nm, which is smaller than that of soft carbon (0.360 nm) and hard carbon (0.395 nm). Graphite is more susceptible to structural changes and volume expansion during repeated lithium removal and insertion processes. Thus, graphite exhibits higher contact resistance and SEI film resistance for lithium ions and poorer diffusion kinetics. Compared with graphite, hard and soft carbons exhibit longer cycle lives as anode-active materials in high-power LFP energy storage devices.

Key words: LiFePO₄, hard carbon, soft carbon, graphite, cycle life