Enhanced Electrochemical Performance of LiNi0.5Mn1.5O4 Composite Cathodes for Lithium-Ion Batteries by Selective Doping of K+/Cl- and K+/F.

K+/Cl- and K+/F- co-doped LiNi0.5Mn1.5O4 (LNMO) materials were successfully synthesized via a solid-state method. Structural characterization revealed that both K+/Cl- and K+/F- co-doping reduced the LixNi1-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 Li0.97K0.03Ni0.5Mn1.5O3.97Cl0.03 (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 Li0.98K0.02Ni0.5Mn1.5O3.98F0.02 (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.