Application Of Polymer Electrode Materials In The Electrochemical Energy Storage

The successful and wide application of Li-ion battery in electronics make people expect more on the electrical vehicle powered by lithium secondary battery, and more requirement on capacity and energy density has been imposed on Li battery accordingly. Therefore, electrode material with high energy density has been pursued. Now, it is realized that the capacity or energy breakthrough will not be achieved by the conventional inorganic transient oxide due to the rigid crystal cell, which supply preferred Li+diffusion path, however, greatly restrict the amount of inserted Li+. On the other side, scientists start to consider more about the sustainability of the electrode material. So, more and more attention has been paid to develop high energy, high power density and green battery.It has been proved that, organic material, as a candidate for cathode in Li secondary battery, its capacity and rate performance is comparable even better than the inorganic material. Another advantage of organic material over inorganic compound is that its diversified structure makes it possible to tailor the material property by adjusting the molecular structure, and thus supply more space for performance improvement. Basing on this consideration, the work presented in this dissertation will focus on polymer electrode material for energy storage. Several typical polymer electrode materials and their application in non-aqueous secondary battery have been investigated, besides, the application of polymer material in novel aqueous secondary battery and membrane battery has been proposed and explored. The main result is as follows:1. High energy polyimide materials.Experimental and modeling methods have been used to study the factors that influence the redox voltage of polyimide electrode. It is shown that, in the voltage range of1.5-3.5V(vs Li+/Li), no matter what kind of starting material is used, the resulting polyimide products will present a two-electrons redox process, generating a capacity at about half of the theoretical value. Further comparison indicates that the average discharging voltage as well as the hysteresis between charging and discharging varies according to the molecular structure. The trend can be summarized as that, when the diamine moiety in polyimide contains the electron-withdrawing function group, such as-F, or m-phenylenediamine is used as the raw material, the average discharging voltage of the sample will increase; on the other side, when the dianhydride moiety has better conjugacy, the sample shows better kinetic property. To better understand the dependence of the electrochemical property on the molecular structure, we first optimized the unit structure of polyimide by B3LYP/6-31G*method, and then a further analysis were conducted through frontier orbital theory. It has been revealed that, the LUMO energy changes in the same mode as the average discharging voltage, namely, the lower the LUMO energy is, the higher the average discharging voltage will be. Meanwhile, from the energy gap between LUMO and HOMO, the gap between the charging and discharging plateau can be predicted. The finding provides very useful clues for exploring polyimide with high discharging voltage. It can be reasonably predicted that the polyimide with strong electro-withdrawing group such as-CN or-F in the dianhydride moiety will have a discharging voltage as high as3V (vs. Li+/Li)2. Polyimide as cathode material for sodium secondary batteryDue to the reaction mechanism of polyimide material, it is conceived that polyimide can also be used as cathode material for sodium secondary battery. To verify our conception, polyimide material with1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTCDA) moiety was chosen and its electrochemical property has been investigated. Although modeling calculation shows that its average discharging voltage will be lower than the analogues with-F group, its relatively lower molecular weight and super conjugated structure can lead to higher capacity and better reaction reversibility. It has been proved in our experiment that, the electrochemical performance of this polyimide is quite similar to our estimation. In1M NaClO4/EC+DMC electrolyte, its discharging capacity is as high as200mAh/g, and a capacity of150mAh/g can be maintained after200cycles, with the efficiency of above99%in all the cycling process. Additionally, when the current rate is-2C (C=405.8mAh/g), its capacity is still about150mAh/g, the excellent performance indicates that polyimide is an promising material for sodium secondary battery.3. Polymer material as anode for alkaline-ion non-aqueous secondary batteryWe propose a new generation of aqueous rechargeable alkali-ion battery involving organic anode material based on conjugated carbonyl. NTCDA-based polyimide is demonstrated as high performance anode material for both ARLB (aqueous rechargeable lithium battery) and ARSB (aqueous rechargeable sodium battery). The electrochemical measurements indicate that the type of alkali-ion has little impact on the electrochemical performance of PI anode. Compared with previous ARLB systems based on inorganic intercalation anodes, the PI/LiNO3/LiCoO2system shows greatly improved comprehensive battery performance. The full-cell can achieve a specific capacity of71mAh/g and a specific energy of80Wh/kg, both of which are the highest among all the reported ARLB systems. Other advantages include high coulombic efficiency, cycling stability and rate capability. The excellent performance is ascribed to the use of PI anode, which presents high specific capacity, appropriate redox potential and good cycling stability. For ARSB, a PI/NaNO3/NaVPO4F system has been investigated. Although the battery performance is not as good as the analogous ARLB because of the low capacity and poor cycling stability of NaVPO4F, a more successful ARSB is still reasonably anticipated by developing better cathode materials for Na-ion batteries. Encouraged by the excellent electrochemical performance of PI in both LiNO3and NaNO3solution, we believe that the performance of this kind of aqueous rechargeable alkali-ion battery can be further improved by developing other organic anode materials based on conjugated carbonyl with higher performance.4. The relationship between the electrochemical properties of polyaniline (PANi) and electrolytePANi was prepared by chemical oxidation method, and its electrochemical property was tested in two different electrolytes. The results show that, in1M LiC104/EC+DMC, when the upper voltage limit of4.0V,4.1V and4.2V is applied, the initial discharging capacity will increase accordingly, while the efficiency is kept above98%, but overall it still shows a doping/de-doping reaction characteristic. In1M LiTFSI/DOL+DME, when the upper voltage limit is higher than4.0V, PAN shows some extra capacity which cannot be explained by doping.de-doping reaction, additionally, more extra capacity can be released when the upper voltage limit further increases. Because of the instability of1M LiTFSI/DOL+DME electrolyte at high voltage, it is thought that the "extra" capacity may relate to the oxidative polymerization of the electrolyte at high voltage and the interaction between thus-resulted product and PAN. Therefore, it is suggested that, although1M LiTFSI/DOL+DME electrolyte shows very good compatibility with most organic material, a suitable voltage of below4V (vs. Li+/Li) is recommended.5. Application of polyimde as cathode for membrane batteryPolyimide (name as PMDA-ODA) sample was synthesized through the polycondensation reaction between Pyromellitic dianhydride (PMDA)å'Œ4,4’-Diaminodiphenyl ether (ODA). Elctrochemical measurement of PMDA-ODA powder shows that, the sample can release capacity through a process similar to enolization reaction, with the redox voltage between1.8V and2.6V. Charging/discharging tests indicate that, the max capacity of80mAh/g can be obtained when the voltage limit of1.5-3.5V (vs Li+/Li) is applied, but with not good cycling stability. While in the potential range of1.0-3.0V (vs Li+/Li), a higher capacity of200mAh/g can be released with improved cycling property. The PMDA-ODA membrane is successfully obtained, and further work on PMDA-ODA/conductive network composite in membrane form is in process. The development of polymer membrane electrode will further widen the application of polymer material and supply more options to fabricate flexible, small battery.