Studies On Composite Cathodes For Lithium Ion Battery

Lithium ion batteries are thought to be the most promising energy storage systems by virtue of their higher energy densities,which have been widely applied in consumer electronic devices,electrical vehicles,smart grids and stationary electricity storage.Cathodes are one of the key components of lithium ion batteries so that exploitation of cathodes with high voltage and high capacity is plausible to improve the energy density and achieve the large-scale manufacture of lithium ion batteries.Besides,additives can also improve the anodic stability of electrolyte or film-forming ability and further facilitate the electrochemical performance of cathodes.In detail,this work mainly contains four parts as follows.Spherical LiNi_0.5Mn_1.5O₄ with spinel structure was fabricated through hydrothermal method followed by a solid state calcination process,and different content of Cr₂O₃ was used to modify the surface by treatment with various amount of Cr（CH3COO）2.Cr₂O₃ was used as a coating material for the first time to fabricate high voltage LiNi_0.5Mn_1.5O₄.The optimum amount of Cr₂O₃ is 3 wt.%.Cr₂O₃ coating can avoid the direct contact between the active material and the electrolyte without altering the structure,retard the side reaction on the interface and protect from the dissolution of metal,thus alleviate capacity deterioration.Cr₂O₃ is proved to improve the cycling stability of the original cathode.The capacity retention is 94.7% after 50 cycles,which is assigned to the protection of the coating layer.Cr₂O₃/LiNi_0.5Mn_1.5O₄ composite electrode can be served as a competitive candidate to meet demands of future applications.Layered Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ was synthesized by co-precipitation using Na₂CO₃ as precipitator,and the ratio of precipitator and metal ions was also optimized.Equal molar ratio of metal ions and OH-was beneficial to form uniform distribution and simultaneous sediment.The electrochemical tests show that the initial discharge capacity is 195.1 mAh/g and the capacity retention can reach to 87.2% after 50 cycles.From the cyclic voltammetry curves,the proposed Li_1.2Mn_0.54Co_0.13Ni_0.13O₂/Li battery presents good cycling reversibility,and the remarkable electrochemical properties are attributed to less agglomeration and better morphology.To further investigate the alternative method to enhance the properties of cathode,two kinds of cathode with different structure were mixed physically.That is,spinel LiNi_0.5Mn_1.5O₄ and layered Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ were mixed with different ratios and characterized by electrochemical tests.This is a facile and novel method to improve the electrochemical performance of batteries.Introduction of layered component can improve the electrochemical performance of spinel material.With the increasing ratio of Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ to the composites,both the specific discharge capacity and columbic efficiency of the initial cycle were improved.When the ratio of layered component is 0.75,the initial columbic efficiency reaches 83.7%.New fluorinated solvents and phenyl compounds were used as electrolyte additives to improve the electrochemical performance of lithium ion batteries.The binding energies of those additives were calculated by frontier molecular orbital theory and the initial voltages of decomposition were tested by linear sweep voltammetry.Due to the electronegativity and electron-withdrawing ability of fluorine,the oxidative stability of organic compounds and traditional carbonate-based electrolyte could be enhanced.The optimized formula of electrolyte is 0.7 M LiBOB EC/FEC/DMC.In particular,partial phenyl compounds can be oxidized before the other components of traditional LiPF6-based electrolyte and form a passivation layer on the surface of cathode,which can restrict the further decomposition of electrolyte and thus improve the electrochemical performance of lithium ion batteries.At 5 C,the average discharge capacity of batteries with the addition of biphenyl is 94.58 mAh/g.