K⁺/Cl⁻ and K⁺/F⁻ co-doped LiNi_0.5Mn_1.5O₄ (LNMO) materials were successfully synthesized via a solid-state method. Structural characterization revealed that both K⁺/Cl⁻ and K⁺/F⁻ co-doping reduced the Li_xNi_1−xO impurities and enlarged the lattice parameters compared to those of pure LNMO. Besides this, the K⁺/F⁻ co-doping decreased the Mn³⁺ ion content, which could inhibit the Jahn–Teller distortion and was beneficial to the cycling performance. Furthermore, both the K⁺/Cl⁻ and the K⁺/F⁻ co-doping reduced the particle size and made the particles more uniform. The K⁺/Cl⁻ co-doped particles possessed a similar octahedral structure to that of pure LNMO. In contrast, as the K⁺/F⁻ co-doping amount increased, the crystal structure became a truncated octahedral shape. The Li⁺ diffusion coefficient calculated from the CV tests showed that both K⁺/Cl⁻ and K⁺/F⁻ co-doping facilitated Li⁺ diffusion in the LNMO. The impedance tests showed that the charge transfer resistances were reduced by the co-doping. These results indicated that both the K⁺/Cl⁻ and the K⁺/F⁻ co-doping stabilized the crystal structures, facilitated Li⁺ diffusion, modified the particle morphologies, and increased the electrochemical kinetics. Benefiting from the unique advantages of the co-doping, the K⁺/Cl⁻ and K⁺/F⁻ co-doped samples exhibited improved rate and cycling performances. The K⁺/Cl⁻ co-doped Li_0.97K_0.03Ni_0.5Mn_1.5O_3.97Cl_0.03 (LNMO-KCl0.03) exhibited the best rate capability with discharge capacities of 116.1, 109.3, and 93.9 mAh g⁻¹ at high C-rates of 5C, 7C, and 10C, respectively. Moreover, the K⁺/F⁻ co-doped Li_0.98K_0.02Ni_0.5Mn_1.5O_3.98F_0.02 (LNMO-KF0.02) delivered excellent cycling stability, maintaining 85.8% of its initial discharge capacity after circulation for 500 cycles at 5C. Therefore, the K⁺/Cl⁻ or K⁺/F⁻ co-doping strategy proposed herein will play a significant role in the further construction of other high-voltage cathodes for high-energy LIBs.

Spinel-type LiNi_0.5Mn_1.5O₄ (LNMO) materials have attracted broad attention as components of rechargeable lithium-ion batteries (LIBs) due to their high energy density. However, continuous decomposition of electrolyte at high working voltage hinders their practical application in carbonate-solvent-based electrolytes. In this work, 4-methyl-1,3,2-dioxathiolane-2,2-dioxide (MDTD) was added as a functional electrolyte additive to the standard electrolyte (i.e., 1 M LiPF₆–ethylene carbonate/ethyl methyl carbonate/dimethyl carbonate) to enhance the electrochemical performances of LNMO-based LIBs. Specifically, the addition of a 0.5% additive to the standard electrolyte significantly improved the capacity retention of LNMO/Li half-cells from 78.5 to 95.8% after 500 cycles at 5C, increased the capacity retention of LNMO/graphite full-cells from 70.8 to 80.9% after 100 cycles at 0.5C, and enhanced the capacity retention of LNMO/LTO full cells from 37.2 to 81.8% after 200 cycles at 5C. In addition, the standard electrolyte containing 0.5% MDTD also improved the cycling performance of LNMO/Li half-cells at 45 °C, expanding the applicable temperature range. Comprehensive characterization combined with theoretical calculations allowed us to reveal the action mechanism of the additive. In this mechanism, MDTD is decomposed and subsequently involved in the decoration of the electrode–electrolyte interfacial films (for the LNMO, Li, and graphite electrodes), endowing it with improved conductivity, protection, and resistance to electrolyte oxidation/reduction decomposition features. These findings suggest that the use of the MDTD additive is a promising strategy to optimize high-voltage electrolytes for the industrialization of LNMO-based LIBs.