Modification Studies Of Low-temperature Petroleum Coke As Anode Material For Lithium Ion Batteries

Low temperature petroleum coke anode material is one of the most potential negative materials because of its high specific capacity,excellent rate performance,good safety,readily available material and cheap price.However,its composition is very complex,including a variety of aromatic,aliphatic,heterocyclic compounds,or polycyclic aromatic hydrocarbons,benzene fused heterocyclic compounds and fused heterocyclic compounds by another classification,and heteroatomic functional groups composed of O,S,N,P and other atoms,which make the current-producing reaction of the electrode extremely complex.On the one hand,the raw coke has high lithium storage capacity,which is beneficial for its adoption as a candidate for the high specific capacity active materials;on the other hand,it also leads to the low initial coulombic efficiency and reversible cycling capacity,which would limit its commercialization process.Therefore,it is necessary to further improve the initial coulombic efficiency and the reversible cycling specific capacity of the material by a series of effective methods.In this thesis,the low temperature petroleum coke was selected as the anode material for lithium ion battery.The solvent and inorganic salts were used to treat the raw cokes,respectively.After the charge/discharge performance testing,the charge/discharge curve was analyzed by a differential curve analysis method.Through the investigation of the effects of solvent leaching and inorganic salts coating on the shape and changing trend of charge/discharge curves connected with current-generating reaction of various chemical components in petroleum coke,the corresponding relationship between the composition of petroleum coke,the type of functional group and the performance of reversible lithium storage is explored in order to provide the solid basis and feasible solution for the development of the material with high cycling specific capacity and high initial coulombic efficiency.According to the different experimental data,it is concluded that cyclohexane,nitric acid and hydrogen peroxide can improve the capacity and efficiency of petroleum coke.The final stable specific capacity achieves 240 mA·h·g^-1 and the efficiency achieve 34%.Al₂O₃,LiF and PAN-Li₂CO₃can improve the capacity and efficiency of petroleum coke significantly.The final stable specific capacity exceeds 330 mA·h·g^-1 and the efficiency achieves 50%.Through the differential curve analysis between the initial charging curves of the raw material and treated low temperature petroleum coke,together with the identification of the the active compounds,composition and structure of the current-generating components and the difference analysis between them,the electrochemical behaviors,such as the potential response interval and reversibility,of the lithium storage reaction of these compounds,compositions and structures can be understood clearly.On the other hand,through the differential curve analysis between the first and second charging curves of a single material of either the raw or the treated low temperature petroleum cokes,we can understand the specific potential interval range where the current-generating reaction has occurred,which triggered the initial irreversible capacity,and its capacity proportion in the whole irreversible capacity,further clarify the chemical nature that leads to the current-generating reaction,and eventually provide feasible solutions for significantly reducing irreversible capacity and improving reversible capacity.On a voltage-specific capacity curve,each variation trend of the voltage with the specific capacity corresponds to one or several specific actively reactive components in the material,such as the lithium storage behavior of a particular compound or functional group.According to the slope or changing rate of the curve,the voltage-specific capacity curve can be divided into different current-generating reaction regions.By analyzing the effects of different treatment methods on the capacity across the potential response region of the corresponding current-generating reaction on the curve,the species category,content and electrochemical behavior of the actively reactive compound can be identified.Therefore,the differential curve analysis method is a simple,effective and practical tool for studying their contribution and role for anode current-generating reaction of a variety of complex chemical components and functional groups in low-temperature petroleum coke.In this thesis,for the first charged voltage-specific capacity curves of the raw and chemically treated low temperature petroleum coke samples,the differential curve analysis method are used to obtain the residual curves between the pristine and the variously treated ones,in order to understand which components of the irreversible capacity have been eliminated by the chemical treatment,and based on these curves,we further discussed the effects of the chemical treatments on the various current-generating reaction voltage intervals or platforms in their first charging curves.Here,the curve will be termed as capacity loss curve.The voltage-specific capacity residual curves were made based on the first and second charged curves for the raw or the treated petroleum cokes,and the amplitude and constitution of the residual irreversible capacity after chemical treatments were analyzed.Here,this kind of curves is called as irreversible capacity curve.The work provided a useful attempt in order to selectively and intelligently deal with petroleum coke,e.g.,to reduce the components that cause irreversible capacity,and retain or especially increase the compounds that produce reversible capacity.Based on the data analysis of all experiments,the petroleum coke was point perturbated with hydrogen peroxide or potassium permanganate in the first batch and then treated in the second batch with Li₂CO₃ or LiF.The differential irreversible capacity analysis and capacity loss analysis were further carried out to gradually improve the initial coulombic efficiency.