| Flexible batteries provide important technical support for the birth and development of new flexible electronic devices,and also trigger the emergence of new forms of electronic devices,which have revolutionized people’s daily lives.For example,foldable and flexible displays have changed interactive mode with people for pictures and movies,and made electronics such as mobile phones and televisions more novel and portable.Flexible sodium ion batteries have the characteristics of low cost,abundant raw material reserves,reaction potentials similar to flexible lithium ion batteries,and a wide operating temperature.They have become a hot spot in the field of flexible energy storage research,and are expected to be used in flexible electronic devices.It is widely used in fields such as portable medical equipment.However,the radius of sodium ions is larger than that of lithium ions,so the ion migration rate and chemical reaction kinetics of sodium ions are weaker than that of lithium ions during the charging and discharging of the battery,and the larger volume change of the electrode in the sodium ion battery is very easy to cause Electrode pulverized.Therefore,the rate performance(rapid charge and discharge performance)and long cycle life of flexible sodium ion batteries are poorer than that of traditional lithium ion batteries.In addition,flexible batteries are usually used in portable devices that are in close contact with the human body,such as flexible mobile phones,flexible watches,implantable medical devices,etc.Once electrolyte leak,thermally runaway,or even fire happened to the flexible sodium ion battery,it will severely threat to human life and lead to inestimable property damage.Thus the safety requirements for flexible sodium ion batteries are higher than that for conversional lithium-ion batteries.In the research work,several key scientific issues for flexible sodium ion batteries have been focused on:how to enhance the rate performance and long cycle life of flexible sodium ion battery electrode materials;at the same time,achieving the high safety of the flexible sodium ion battery under the premise of not damage the electrochemical performances.Therefore,it is very important to develop a method for preparing high-performance flexible sodium ion battery electrode materials and improving battery safety.In this paper,the transition metal chalcogenide(cobalt disulfide)was used as the active material for the electrode material of flexible sodium ion battery.Firstly,electrodes with high pseudocapacitance behavior was designed based on the rapider ion migration with the pseudocapacitor characteristics.Also carbon-coated structure resulted in higher rate performance and longer cycle life for flexible electrode material.Then to further enhance the rate and cycling performance of electrode,a flexible integrated design idea was proposed by using multi-channel carbon nanofibers as a conductive matrix and in-situ growing array-type cobalt disulfide nanoparticles on its surface.The self-supporting flexible sodium-ion battery elecreode material was finally obtained by self-assembly method.The flexible integrated design avoids the use of adhesives and conductive additives,thus resulting in the decrease of the internal resistance of the electrode material and the manufacturing cost.When applied in batteries,it maintains the excellent rate and long cycle performance.Further,based on the self-supporting flexible integrated design,a dual-morphology cobalt disulfide/porous carbon nanofiber flexible composite used as electrode was successfully prepared by a solvothermal method,which further improved the flexibility and long-cycle stability of the electrode material.Finally,using synchrotron radiation XANES and SVUV-PIMS spectroscopy techniques,the catalytic pyrolysis mechanism of transition metal sulfides to ether-based electrolytes and carbonate-based electrolytes for sodium ion batteries were carefully studied.In addition,a non-combustible high-salt concentration electrolyte was designed.On the premise of ensuring the performance of sodium ion batteries,a highly safe non-combustible electrolyte for flexible sodium ion batteries was obtained,which improved the safety performance of the batteries.The main research work is as follows:1.Polyhedral carbon-coated CoS2 flexible electrode with pseudocapactive behavior:It is reported that electrodes with pseudocapacitive behavior can achieve rapid and reversible transfer of charge within the electrode,thereby enhancing the rate performance of the electrode.Therefore,using the solvothermal method,a polyhedral carbon-coated CoS2 electrode material(NCP-CoS2)with both pseudocapacitance behavior and desorption sodium storage mechanism was designed and prepared.Then the active material was coated within between the flexible carbon cloth layers by flurry coating method to obtain the flexible sodium-ion battery electrode NCP-CoS2@CC.The pseudocapacitance contribution of the flexible electrode increases with the current density increasing.After 1000 cycles at a large current density of 1 A/g,the specific capacity fading is only 0.021%/cycle compared to the peak capacity.Besides,the electrolyte can run at an ultra-large current density of 10 A/g and charging can be completed in only 2.7 min.2.Flexible integrated double-mophlogy CoS2/hollow carbon nanofiber flexible electrode:In order to further improve the rate performance and cycle stability of flexible sodium ion batteries,the flexible integrated CoS2 electrodes(DM-CoS2@MCNFs)by growing CoS2 directly on the flexible carbon substrate was successfully prepared.On the one hand,the integrated electrode design avoids using traditional adhesives,current collectors,etc.,which greatly reduces the interface impedance,improves the charge transfer rate,and finally helps improve the rate performance of the battery.On the other hand,the nanostructure design is used to grow CoS2 nanoparticles in situ in the hollow channels and outer surfaces of the carbon fiber,thereby minimizing the seperation of the active CoS2 nanoparticles for the flexible electrode during the bending process and improving Charge and discharge stability under bending conditions.3.Catalytic thermal decomposition effect of CoS2 flexible electrode to electrolyte:Using synchrotron radiation XANES and SVUV-PIMS spectroscopy techniques,the catalytic pyrolysis of CoS2 electrode to the electrolyte was studied.It was found that with the participation of the electrode,a part of O·free radicals will be generated during the thermal decomposition of DEGDME,and most of these free radicals will electronically pair with H.free radicals in the environment to generate H2O.However,there is a certain probability that a small amount of O2 is generated by itself,which increases the thermal hazard of the battery.Further research and analysis of the catalytic thermal decomposition products dominated by temperature as carefully investigated.The critical temperature of thermal decomposition runaway was obtained,and the catalytic thermal decomposition mechanism was analyzed in detail.4.Safety optimization of flexible CoS2 sodium ion battry:In order to improve the safety of flexible sodium ion batteries and reduce the thermal hazards caused by electrolyte decomposition promoted by CoS2.Based on the design concept of high salt concentration electrolyte,a non-combustible safe sodium ion battery electrolyte was prepared.The flash point test,self-extinguish time measurement,TGA analysis and ignition experiment confirmed the high safety of the electrolyte itself.At the same time,the electrochemical performance test was used to investigate the performance of the electrolyte in the sodium ion battery.It was found that the flexible sodium ion battery maintained high rate performance and long cycle charge.The electrolyte makes it possible for combining superior electrochemical properties and high safety. |
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