Understanding the Impact of K-Doping on the Structure and Performance of LiFePO₄/C Cathode Materials

The K-doped Li_1-KFePO₄ (x = 0, 0.005, 0.01, and 0.02) samples were synthesized successfully via a solid-state method, and the electronic structures of the samples were calculated by the first-principles based on density functional theory. Theoretical calculations show that the bandwidth of Li_1-KFePO₄ decreases with the increase in K⁺ doping, which is consistent with the experimental results. It was demonstrated that Li_0.995K_0.005FePO₄ delivers higher capacity retention with 92.7% after 100 cycles compared with LiFePO₄ (86.3%) at 1 C and shows better high-rate performance with capacities of 151.9, 151.8, 149.2, 128.3, and 84.6 mAh·g^-1 at current densities of 0.1 C, 0.2 C, 0.5 C, 1 C, and 3 C; the corresponding values for LiFePO₄ were 153.2, 136.5, 125.9, 111.5, and 66.0 mAh·g^-1. Owing to the expanded Li ion diffusion pathway, EIS analysis showed that the lithium ion diffusion coefficient of LiFePO₄ doped with K ion was significantly improved compared to LiFePO₄; the values were 1.934×10^-13 and 1.658×10^-12 cm²·s^-1, respectively. Additionally, Li_0.995K_0.005FePO₄ showed a lower charge transfer resistance (300.2 Ω compared to 407.1 Ω of LiFePO₄).