Abstract
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.
Highlights
The structures, morphologies, and electrochemical performances of pure LiNi0.5 Mn1.5 O4 (LNMO), K+ /Cl− codoped Li1−x Kx Ni0.5 Mn1.5 O4−x Clx, and K+ /F− co-doped Li1−x Kx Ni0.5 Mn1.5 O4−x Fx were comprehensively compared in this study
A simple solid-state ball-milling process followed by a high-temperature calcination procedure was used to synthesize pure LNMO, K+ /Cl− co-doped, and K+ /F− co-doped samples
XRDresults results of of the LNMO-KCl0.02, LNMO-KCl0.03, LNMO-KF0.01, TheThe the pure pureLNMO, LNMO, LNMO-KCl0.02, LNMO-KCl0.03, LNMOand LNMO-KF0.02 are shown in Figure diffraction peaks of all the were
Summary
Ion co-doping is an effective strategy to simultaneously stabilize the crystal structure and enhance the electrochemical properties of LNMO materials. Cation and anion co-doping has a unique advantage in that both ions can play a synergistic role in the impact of LNMO on the structure and properties This approach has been widely applied to improve the rate capability and cycling stability of LNMO. Cl− (349 kJ mol−1 ) ions, with larger electron affinities, are commonly used elements for O2− (141 kJ mol−1 ) site substitution in LNMO cathodes, and they can stabilize the crystal structure and reduce the Lix Ni1 − x O or NiO impurity generation due to the stronger. The structures, morphologies, and electrochemical performances of pure LNMO, K+ /Cl− codoped Li1−x Kx Ni0.5 Mn1.5 O4−x Clx , and K+ /F− co-doped Li1−x Kx Ni0.5 Mn1.5 O4−x Fx were comprehensively compared in this study
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