Study On Cycle Characteristics Of LiFePO₄ Material

LiFePO₄/C cathode materials were synthesized by carbothermal reduction method using Li2CO₃, FePO₄ and C₆H₁₂O₆·H₂O as raw materials. In this paper, the capacity fading mechanism of LiFePO₄/C material at high rate were studied preliminarily. And the cycle characteristics of LiFePO₄/C materials synthesized in different preparation conditions, such as calcination temperature, calcination time, carbon content and lithium content, were analyzed. The structure, morphology and electrochemical properties of LiFePO₄/C materials were characterized by using XRD, SEM, FTIR and galvanostatic charge-discharge tests.The particles of LiFePO₄/C material appeared cracks after 300 times cycles, which may be caused by high internal strain within b-c plane along a-axis upon Li⁺-extraction/insertion. The cracks would weaken electrical contact between active material and conductive agent, and lead to electrode polarizatiion, causing the capacity fading. In the XRD patterns after 300 cycles, the diffraction peak shifted right, and the impurity peaks (FePO₄ and Fe₄(P₂O₇)₃) were found. This showed the interplanar spacing became smaller. And the impurities not only damaged the crystallinity of LiFePO₄ phase and may cause electrical contact reduced between particles, but also reduced the utilization of active material and increased internal resistance, resulting in capacity loss.The initial discharge capacity and the discharge capacity after 300 times cycles of LiFePO₄/C material prepared at 750°C were about 115mAh/g and 114mAh/g at 3C rate, respectively. This was due to good crystallinity, uniform and tiny particles, which had large specific surface area, conduced to contact with the electrolyte fully and shorten deintercalation distance of lithium ion, thus improving the electrochemical properties and active material utilization. The initial discharge capacity and the discharge capacity after 300 times cycles of LiFePO₄/C material synthesized at 800°C were about 120mAh/g and 115mAh/g at 3C rate, respectively. It was because of uniform and tiny particles, as well as good carbon coating layer and carbon floc structure that improved the conductivity of LiFePO₄/C material, provided electronic transmission channels, and shortened deintercalation distance of lithium ion.When the mole ratio of FePO₄, Li₂CO₃, C₆H₁₂O₆·H₂O was 2:1:1.75, the initial discharge capacity and the discharge capacity after 300 times cycles of prepared materials were about 118mAh/g and 116mAh/g at 3C rate, respectively. The materials displayed good cycle performance. However, the XRD pattern showed low crystallinity, probably because the amorphous carbon content of sample increased, and the crystalline of material decreased, leading to diffraction intensity decreased. The SEM image showed that uniform and tiny particles, as well as lots of carbon particles and carbon floc structure. When the mole ratio of FePO₄, Li₂CO₃, C₆H₁₂O₆·H₂O was 2:0.98:1.75, the discharge capacity of prepared material L_0.98FP was to 114mAh/g after 100 cycles until the end of 300 times cycles. This was probably because low lithium content produced lithium-ion vacancy, promoting lithium-ion transport. The discharge capacities of samples L_1.08FP, L_1.06FP, L_1.04FP, L_1.02FP were small in the beginning. This may be because excessive Li⁺ occupied lithium vacancy and diffusion channel, and affected the deintercalation of lithium-ion, resulting in capacity loss.