Investigations On The Interface Issues Of High Voltage Cathode In Lithium Ion Battery

In order to satisfy various needs of human being,the demand for batteries with high energy density increases gradually.on the cathode side,the realization of higher voltage is an effective approach to improve energy density and it is under wide investigation.However,the introduction of high-voltage cathode also causes problems like electrolyte decomposition or structure failure of active materials since their working voltage usually exceeds the stable window of commonly-used electrolyte,further resulting in a lower coulombic efficiency as well as poor cycling stability.Fortunately,these processes could be partially inhibited by effective surface modification as they usually happen at the interface between the cathode and electrolyte.Based on the increasing understanding on the interfaces between electrolyte and various cathodes,combining with the unique advantage of atomic layered deposition in oxide,this thesis aims at providing some new insights on both active material-electrolyte interface and conductive additive-electrolyte interface.Firstly,an improved coulombic efficiency of spinel high voltage cathode LiNi_0.5Mn_1.5O₄ was obtained at an elevated temperature?55??,this is due to the formation of Ta₂O₅ coating layer with a thickness of about 2-3 nm which protects it from structural degradation.It can be concluded from the results of X-ray diffraction,X-ray photoelectronic spectrum and transmission electron microscopy,at a high temperature above 900?,Ta would slightly diffuse into LiNi_0.5Mn_1.5O₄ and result in a Li_xNi_1-xO rock salt phase on the outer surface during the treatment process.This rock salt surface has a more stable oxygen framework and lower valent transition metal ion compared to original spinel phase,thus the coated material shows a higher coulombic efficiency?98.4%vs 97.5%?than that with initial material.Besides,half cells with materials containing surface rock salt phase show a lower interface resistance after cycling,which is owing to the protection of rock salt phase that could suppress the decomposition of electrolyte as well as the accumulation of products along the cathode surface.Secondly,considering the large surface area of conductive additive in cathode that may bring side reactions,oxide coating was performed on commercial carbon black super P with atomic layered deposition to protect it.Three kinds of oxides?Al₂O₃,Ti O₂and Ta₂O₅?are chosen as coating layers to make comparison.Including the uncoated super P,we prepared four kinds of electrodes with different coating layers.A step potential method is carried out to investigate the oxidation current under high voltage?5 V?.All modified super P shows the lower oxidation current than the uncoated one.Nevertheless,due to the different stability with the hydrofluoric,three oxides display the different optimized performance,which are confirmed by the following reaction energy calculation and soaking experiments.The origin of the oxidation current investigated by Raman and XPS indicates that it is the electrolyte decomposition that corresponds to the observed current while not the anion intercalation.Ta₂O₅-coated super P shows smallest oxidation current in step potential test and helps to improve the coulombic efficiency of LiNi_0.5Mn_1.5O₄ half cells from 98.3% to 99.5%.Thirdly,in view of current situation that step potential and linear scanning potential are employed to investigate anti-oxidation ability of pure carbon electrode at the same time,their area of application was analyzed.A simple series of resistor and capacitor was chosen as the equivalent circuit after the illustration of charge transfer process with the pure carbon electrode.Assuming that the voltage has a time-varying form and this form is random,a general current respond then can be acquired by differentiating and further solving the voltage conservation equation.Exact solutions under step potential and linear scanning potential were obtained by substituting each voltage function.Experimental results with pure super P electrode show that current under step potential test is consistent with theoretical analysis while linear scanning potential is far away from that of equation solution.The higher current than theory observed in linear scanning potential method may come from a contradiction between fast scanning rate and large surface of porous electrode.In conclusion,modification of interfaces is the key factor to achieve high voltage cathode and high energy density lithium ion battery.The ALD technique made it possible to coat a thin and uniform oxide layer onto the surface of the cathode actives as well as the conductive additives,and then improving the coulombic efficiency of the high voltage cathode.Experimental results combining with equivalent circuit analysis gave new insight on detection of weak current for carbon electrode upon charging process.Our results also provided methodology foundation for checking the effect of surface coating on the conductive additives.