Study On The Construction And Effectiveness Of Expansion Buffer Space For Electrode Materials

With the growinging demand of portable electronic devices and energy storage systems,rechargeable Lithium-ion batteries（LIBs）and Supercapacitors as the most promising energy storage devices have attracted widespread attention.In the operation of these devices,there are many factors that lead to the attenuation of their capacity such as phase change and volume change of electrode materials.Alloy anode silicon has become one of the most promising anode materials for lithium-ion batteries because of its higher theoretical specific capacity(4200 m Ah g^-1)than graphite anode(372 m Ah g^-1)and lower electrode potential;However,the volume expansion and other problems in the process of intercalation/de-intercalation will lead to the decrease of capacity and cycling life deterioration.The volume change of charging and discharging process also exists in other energy storage devices,such as conductive polymer-based pseudocapacitor materials.The mechanism of charging and discharging corresponds to the doping/dedoping of conductive polymer.This process involves the constant expansion of volume,thus affecting the long-term cycle stability and other electrochemical properties.Based on these problems,the purpose of this thesis is to design a rational volume expansion buffer space for this kind of electrode so as to maintain the volume integrity of the electrode in long time operation,and enhance its cycle life and performance retention rate.The specific content is as follows:1.In order to effectively alleviate the volume expansion of silicon during the process of charging and discharging,and improve the conductivity of the material,the method with combination of soft template and high temperature calcination was used to prepare a void-rich materials which is Si NP@Void@N,S doped C composites.In the composite,nano silicon was wrapped by calcium carbonate（Ca CO₃）template and carbon nanotubes were added as conductive additives.Then polypyrrole（PPy）was in-situ grown around the Si NP@Ca CO₃composite.The obtained material was carbonized at high temperature and then etched with dilute hydrochloric acid to obtain Si NP@Void@N,S doped C composites.The electrochemical study showed that the material exhibits a good cycle stability and rate performance.At a current density of 0.3 A g^-1,the coulombic efficiency for the first cycle was 63.3%,and the reversible capacity of 952.6 m Ah g^-1 remained after 90 cycles of charge and discharge.When the current density returned from 2 A g^-1 to 0.1 A g^-1,the material had a charging capacity of 1042m Ah g^-1,with the capacity retention rate of 92.6%compared to the initial charging capacity,which showed a good rate performance.The rich void with the carbon coating structure provides a buffer space for the volume expansion of Nano silicon in the process of cycling.On the other hand,the nitrogen and sulfur doped carbon layer effectively increases the electrical conductivity of the material.2.In order to further simplify the synthesis steps,a method combining the one-step synthesis and high-temperature calcination was adopted to prepare Si@Void@C composites with a void-rich structure:Sulfur as soft template agent was soluble in carbon disulfide,then mixed Sulfur with Nano silicon in above liquid,using ethylcellulose as carbon source in acetone to coat Si NP@S composites.Sulfur was evaporated at a high temperature calcination process and ethycellulose was carbonized which yielded Si@Void@C composite materials.The reversible capacity was 1663.5 m Ah g^-1and 1201.60 m Ah g^-1 at the current density of 0.2 A g^-1 and 1 A g^–1 after 100 cycles of charge and discharge.The charge-discharge curves indicated that the synthesized Si@Void@C material has excellent cycling stability and electrochemical performance.3.Focusing on the flexible supercapacitor electrode,we turned a conventional PPy film from a dense structure into a three-dimensional（3D）porous structure using a modified vapor phase polymerization method（VPP）with the use of the pore template（Ca CO₃）.The porous design provided the PPy film with an improved surface area and pore volume.It was revealed by the electrochemical investigation that more pseudocapacitive contribution than diffusion-controlled process contribution was observed in the total charge in the redox reaction.The galvanostatic charge/discharge（GCD）measurements showed that the optimized 3D porous PPy film electrode delivered a high capacitance of 313.6 F g^-1 and an areal capacitance of98.0 m F cm^-2 at 1.0 A g^-1 in a three-electrode configuration,which are nearly three times those of the dense counterpart electrode.A specific capacitance of 62.5 F g^-1 at 0.5 A g^-1and31.1 F g^-1 at 10 A g^-1 was obtained in a symmetric capacitor device.It turned out that a high capacitance retention up to 81.3%after 10,000 GCD cycles was obtained for the symmetric supercapacitor device with the 3D porous PPy electrode.Comparison of the SEM images of the electrode before and after the cycle showed,the porous PPy film electrode maintained a good morphology,while the dense PPy electrode ruptured and collapsed.In addition,the porous structure provided the PPy film attractive capability of accommodating the volume change during doping/de-doping process,the strategy and the insight analysis are expected to provide valuable guidance for the design and the synthesis of flexible and wearable film electrodes with high performance.