The crystal structure and electrochemical performance of Li1.167Mn0.548Ni0.18Co0.105O2 composite cathodes doped and co-doped with Mg and F

Enhanced Performance of Li-Rich Manganese Oxide Cathode Synergistically Modificated by F-Doping and Oleic Acid Treatment

Even lithium-rich manganese oxides (LRMOs) are considered as promising cathode materials for next-generation lithium-ion batteries, their commercialization is hindered mainly by the low initial Coulombic efficiency, poor cyclability and unexpected capacity fade. Here, a synergistic modification strategy by using both F doping and weak organic acid surface treatment is proposed to improve the electrochemical performances of LRMOs significantly. Optimized Li1.2 Mn0.54 Ni0.13 Co0.13 O1.95 F0.05 sample with surface oxygen vacancy defects and thin carbon coating layer exhibits profound electrochemical performances, for example, discharging capacities of 298.6 and 212.5 mAh g-1 at 0.1 C and 1 C rate, respectively. In addition, it can own an initial Coulombic efficiency of 84.4%, which is much higher than that of untreated sample. In situ X-ray diffraction analysis implies that synergistic modification can enhance the skeleton stability of LRMOs , especially at a high state of charge. Galvanostatic intermittent titration technique analysis suggests that as-developed synergistic modification can accelerate the lithium ions diffusion. Theoretical calculations reveal that substituted F and oxygen vacancy defects can diminish the diffusion energy barrier of Li+ ions. This work provides a new synergistic modification strategy to improve the comprehensive properties of LRMO cathode effectively.

Enhanced Electrochemical Performance of the Lithium-Manganese-Rich Cathode for Li-Ion Batteries with Na and F CoDoping

The lithium-manganese-rich layered oxide cathode (LMR-NMC), xLi₂MnO₃·(1 - x)LiMO₂ (M = Co, Ni, and Mn), is on demand because of its high specific capacity of over 250 mA h g^-1 between the voltage range 2.0-4.8 V (vs Li/Li⁺). Because of the requirement of activating the Li₂MnO₃ phase in the first cycle, oxygen extraction from the lattice structure occurs. Consequently, capacity fading and voltage fading during cycling are still major obstacles to the commercialization of LMR-NMC in battery applications. Here, codoping Na and F into LMR-NMC via facile hydroxide coprecipitation followed by solid-state reaction is introduced. Na and F are partially substituted into Li and O sites, respectively. These dopant ions enlarge the Li slab, which in turn eases Li diffusion and minimizes oxygen loss, thereby stabilizing the structure. The codoped sample exhibits both high capacity retention (97%) and high voltage retention (91%) over 100 cycles with an initial discharge capacity of 260 mA h g^-1 at 0.1 C. Compared to other reports on LMR-NMC as obtained by coprecipitation, results from this study show the best capacity retention. The developed codoping approach may provide a new strategy for designing high-performance LMR-NMC cathodes for next-generation lithium ion batteries.

Improved Cycling Stability and Fast Charge-Discharge Performance of Cobalt-Free Lithium-Rich Oxides by Magnesium-Doping

Layered Li-rich, Co-free, and Mn-based cathode material, Li_1.17Ni_0.25-xMn_0.58Mg_xO₂ (0 ≤ x ≤ 0.05), was successfully synthesized by a coprecipitation method. All prepared samples have typical Li-rich layered structure, and Mg has been doped in the Li_1.17Ni_0.25Mn_0.58O₂ material successfully and homogeneously. The morphology and the grain size of all material are not changed by Mg doping. All materials have a estimated size of about 200 nm with a narrow particle size distribution. The electrochemical property results show that Li_1.17Ni_0.25-xMn_0.58Mg_xO₂ (x = 0.01 and 0.02) electrodes exhibit higher rate capability than that of the pristine one. Li_1.17Ni_0.25-xMn_0.58Mg_xO₂ (x = 0.02) indicates the largest reversible capacity of 148.3 mAh g^-1 and best cycling stability (capacity retention of 95.1%) after 100 cycles at 2C charge-discharge rate. Li_1.17Ni_0.25-xMn_0.58Mg_xO₂ (x = 0.02) also shows the largest discharge capacity of 149.2 mAh g^-1 discharged at 1C rate at elevated temperature (55 °C) after 50 cycles. The improved electrochemical performances may be attributed to the decreased polarization, reduced charge transfer resistance, enhanced the reversibility of Li⁺ ion insertion/extraction, and increased lithium ion diffusion coefficient. This promising result gives a new understanding for designing the structure and improving the electrochemical performance of Li-rich cathode materials for the next-generation lithium-ion battery with high rate cycling performance.