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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<i>_x</i>Ni₁₋<i>_x</i>O 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.