Self-Polymerized Cr–Sr Co-Doped LiNi 0 . 5 Mn 1 . 5 O 4 : Synergistic Enhancement of Structural Stability and CV-Quantified Li + Diffusivity for 5 V Long-Cycle High-Rate Performance

Cr–Sr codoped LiNi0. 5Mn1. 5O4 (LNMO) cathode materials with different compositions, LiNi0. 5–xCrxSryMn1. 5–yO4 (x = 0, 0. 02; y = 0, 0. 03) were synthesized using a self-polymerization method. The materials were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM) to investigate their structural properties. Electrochemical performance was evaluated through the initial charge–discharge capacity, rate capability, cycling discharge capacity, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) tests. The study provides an in-depth analysis of the effects of crystal structure, microstructure, and the distribution of doped elements on the rate performance and cycling stability of the Cr–Sr codoped LNMO samples during the charge–discharge process. The results revealed that the Cr–Sr codoped material LiNi0. 48Cr0. 02Mn1. 47Sr0. 03O4 exhibited a disordered Fd3m space group, which reduced electrochemical polarization and charge transfer resistance. The material effectively suppressed the formation of the LixNi1–xO impurity phase, ensuring product purity and structural stability. The crystal structure transitioned from spinel octahedral to truncated octahedral, promoting the exposure of 100 crystal planes, which facilitated the Li+ diffusion rate and improved rate performance. Furthermore, the doping also reduced the Mn3+ content on the crystal surface, avoiding the dissolution side reactions of manganese at the electrode–solution interface, thereby enhancing cycling capacity stability during charge–discharge processes. At 1C and 5C rates, the discharge capacities were 132 mAh/g and 127 mAh/g, respectively. After 200 cycles at 1C, the capacity remained at 121 mAh/g, with a capacity retention of 91. 67%, indicating that Cr–Sr doping via the self-polymerization method significantly improves the electrochemical performance.