Enhanced Cycling Stability through Erbium Doping of LiMn₂O₄ Cathode Material Synthesized by Sol-Gel Technique

In this work, LiMn_2-xEr_xO₄ (x ≤ 0.05) samples were obtained by sol-gel processing with erbium nitrate as the erbium source. XRD measurements showed that the Er-doping had no substantial impact on the crystalline structure of the sample. The optimal LiMn_1.97Er_0.03O₄ sample exhibited an intrinsic spinel structure and a narrow particle size distribution. The introduction of Er³⁺ ions reduced the content of Mn³⁺ ions, which seemed to efficiently suppress the Jahn⁻Teller distortion. Moreover, the decreased lattice parameters suggested that a more stable spinel structure was obtained, because the Er³⁺ ions in a ErO₆ octahedra have stronger bonding energy (615 kJ/mol) than that of the Mn³⁺ ions in a MnO₆ octahedra (402 kJ/mol). The present results suggest that the excellent cycling life of the optimal LiMn_1.97Er_0.03O₄ sample is because of the inhibition of the Jahn-Teller distortion and the improvement of the structural stability. When cycled at 0.5 C, the optimal LiMn_1.97Er_0.03O₄ sample exhibited a high initial capacity of 130.2 mAh g^-1 with an excellent retention of 95.2% after 100 cycles. More significantly, this sample showed 83.1 mAh g^-1 at 10 C, while the undoped sample showed a much lower capacity. Additionally, when cycled at 55 °C, a satisfactory retention of 91.4% could be achieved at 0.5 C after 100 cycles with a first reversible capacity of 130.1 mAh g^-1.