Enhanced Electrochemical Performance of LiNiMnO Composite Cathodes for Lithium-Ion Batteries by Selective Doping of K/Cl and K/F.

K/Cl and K/F co-doped LiNiMnO (LNMO) materials were successfully synthesized via a solid-state method. Structural characterization revealed that both K/Cl and K/F co-doping reduced the LiNiO impurities and enlarged the lattice parameters compared to those of pure LNMO. Besides this, the K/F co-doping decreased the Mn 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 LiKNiMnOCl (LNMO-KCl0.03) exhibited the best rate capability with discharge capacities of 116.1, 109.3, and 93.9 mAh g at high C-rates of 5C, 7C, and 10C, respectively. Moreover, the K/F co-doped LiKNiMnOF (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.