Surface coating is an effective way to suppress the structural collapse and surface side reactions of LiCoO 2 (LCO) at high voltage. Herein, KAlF 4 is used as the raw material for coating modification and combining the wet chemical method and high-temperature solid-phase method to form a dense LiF, KF, and LiCo 1−x Al x O 2 composite coating layer on the surface of LCO. The fluoride composite coating layer can stabilize the surface of the material, and the solid solution phase can accelerate the transport of Li + while stabilizing the surface. The synergistic effect of the composite coating phase has a positive effect on mitigating the surface side reactions and structural collapse of LCO at high cutoff voltages above 4.5 V. The modified sample had a first discharge specific capacity of 216.3 mAh/g at 0.5 C in the high-voltage range of 3.0−4.7 V and still had capacity retention of up to 60.4% after 200 cycles, while only 5.8% of unmodified LCO samples remained after 160 cycles. The improved electrochemical performance is attributed to the stabilized surface and phase structure, improved lithium ion diffusion coefficient induced by composite coating as evidenced by electrochemical impedance spectroscopy, cyclic voltammetry, and scanning electron microscopy.
The existing CO2 absorption by deep eutectic solvents is limited by the unavoidable water absorption problem during use. In this study, we prepared three deep eutectic solvents with different alcohol aminations and added different water contents to discuss the effect of water content on the absorption of carbon dioxide by deep eutectic solvents. All deep eutectic solvents have a low melting point at room temperature as a liquid and have high thermal stability, where the choline chloride-diethanolamine deep eutectic solvents have a high viscosity. Anhydrous choline chloride-monoethanolamine deep eutectic solvents have the largest CO2 absorption, reaching 0.2715 g/g, and the absorption of CO2 by anhydrous choline chloride-N-methyldiethanolamine deep eutectic solvents is only 0.0611 g/g. Water content inhibited the absorption of CO2 in primary amine and secondary amine systems, whereas it enhanced the absorption of CO2 in tertiary amine systems, which was related to the reaction process of deep eutectic solvent and CO2.
The
surface and interface stability of the electrode is an important
factor affecting the electrochemical performance of the battery, and
surface modification is an effective means to stabilize the electrode
surface interface. In this paper, a TiO2–LiF composite
coating layer with very stable chemical properties on the LiCoO2 (LCO) surface is prepared in one step. At the same time,
surface doping is realized, which provided a stable surface structure
for LCO and stabilized the interface between the electrode and electrolyte.
Electrochemical test results show that in the range of 3–4.5
V, the capacity retention rate of the sample with a coating amount
of 1% is 97.4% after 110 cycles at 0.2 C and a discharge capacity
of 151.5 mAh g–1 at 5 C, while the bare electrode
is only 67% (110th cycle) and 50.7 mAh g–1 under
the same conditions. Even at 0.5 C within 3–4.6 V, the capacity
retention rate of the coated sample is still as high as 88.6% after
100 cycles, showing excellent high-voltage cycle stability. Studies
such as cyclic voltammetry and electrochemical impedance spectroscopy
show that the improvement in electrochemical performance is due to
the coating layer effectively stabilizing the LCO surface composition
and structure, alleviating the structural degradation of LCO, and
optimizing the lithium-ion transport channel.
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