Design And System Optimization Of High Energy/-Power Density Lithium Storage Anode Materials

With the rapid development of portable electronic products and electric vehicles,the difficulties are increasing to make lithium-ion batteries(LIBs)and supercapacitors based on graphite,which can meet the demand for both high energy density and power density.Therefore,it is extremely important to develop an anode with high specific energy-rate and optimize the existing LIBs system.Lithium-ion hybrid supercapacitors(Li-HSC),as a new energy storage device,have become a potential new electrochemical energy storage system for research and industrial development due to their high energy density,high power density,and excellent long-cycle performance.Current researches on Li-HSC anode materials are mainly focused on inorganic transition metal compounds.However,larger volume changes and poor conductivity cause it to have the low reversible capacity and poor rate performance.In addition,inorganic materials generally have the problems of tremendous production energy consumption,severe pollution,and low natural abundance.As a result,it is of great significance to develop organic materials applied to energy storage systems.Organic electrode materials possess more appealing virtues,including environmental protection,molecular design,and abundant natural reserves have attracted much attention in decades.However,the lack of electrical conductivity and difficulty in nano-crystallization seriously restricts the organic electrode to display both high reversible capacity and excellent rate performance.As the most mature anode material for LIBs,graphite endows fascinating features,such as stable structure,low lithium insertion potential,long cycle life,high conductivity and low price.However,the graphite anode faces some defects,low theoretical specific capacity(372 mAh g-1),the continuous growth of the SEI film on the graphite surface,and difficulty in rapid charge and discharge included.While,silicon(Si)possesses ultra-high theoretical specific capacity(3579 mAh g-1),a low electrochemical reaction platform,and abundant resources.In recent years,it has attracted widespread attention from researchers and new energy companies as a LIBs anode.However,the Si electrode exposes the problem of large volume changes,low electronic conductivity,and dynamic growth of the solid electrolyte interface(SEI)film on the electrode surface,which greatly hinders its further commercial application.Aiming at the challenges,including low electronic conductivity and difficulty in nano-crystallization of organic electrode materials,a highly conductive MA@PVDF composite material composed of 10 nm maleic acid(MA)nanocrystals was designed and prepared by dissolution and recrystallization in this project to achieve superior electrochemical behaviors.The result shows that the MA@PVDF electrode has a reversible capacity far exceeding the theoretical specific capacity,excellent cycling stability and rate performance(the capacity of 994 mAh g-1 after 560 cycles at 3 A g-1,and 501.3 mAh g-1 at 30 A g-1).Further kinetic analysis reveals that MA@PVDF has a large surface capacitance contribution.Combining the theoretical analysis of surface capacitance contribution and diffusion control can explain the reason for the ultra-high reversible capacity of MA@PVDF applied in LIBs.Based on high reversible capacity and large surface capacitance contribution,the assembled MA@PVDF//AC Li-HSC exhibits high energy density(energy density of 158.4 Wh/kg at a power density of 107.5 W/kg),high power density(energy density of 70.9 Wh/kg at 10750 W/kg)and exceptional cycle stability.The reversible capacity exceeding the theoretical capacity has been interpreted as the contribution of surface capacitance through kinetic analysis.It was also the first time that small organic molecules were applied to Li-HSCs and achieved high energy density and power density.Aiming at the low electronic conductivity and slow reaction kinetics of 2,2'-bipyridine-4,4'-dicarboxylic acid(BPDCA),a highly conductive BPDCA electrode was also synthesized by the method of dissolution and recrystallization and applied in LIBs and Li-HSCs.The BPDCA electrode consists of 2-8 nm BPDCA nanocrystals,conductive carbon Super-P and PVDF composite.In addition,the BPDCA molecule has a lattice spacing of 0.461 nm.Both are beneficial for BPDCA electrodes to speed up reaction kinetics and store more Li+ion.Experimental results and theoretical calculations reveal that the lithium storage mechanism of BPDCA molecules can be divided into diffusion-controlled Faraday reactions(-C=O and-C=N groups)and intercalation pseudocapacitive behaviors,of which intercalation pseudocapacitance dominate.Due to the intercalation pseudocapacitance and the Faraday reaction mechanism,the BPDCA electrode as a LIBs anode delivers a high reversible capacity of 1205.7 mAh g-1 at 0.5 A g-1,which is very close to the theoretical capacity calculation of BPDCA electrode based on 10 lithium ion storage.In addition,the BPDCA electrode exhibits a high rate capacity of 478.9 mAh g-1 at 20 A g-1.The BPDCA electrode was further applied as an anode to Li-HSC,which exhibited a high energy density of 178.7 Wh/kg and a high power density of 20.43 kW/kg.In this paper,it is the first time through experiments and theories to prove that the electrochemical behavior of BPDCA is the result of the combined intereaction between Faraday reaction and intercalation pseudocapacitance,and it is confirmed that the theoretical capacity exceeded is intercalation pseudocapacitance.Cyclohexanone(C6O6)electrode with high theoretical specific capacity was also synthesized by the technique of dissolution and recrystallization,which used as an anode in Li-HSC.The C6O6 nanoparticles have the characteristics of large surface capacitance and wide crystal plane spacing.As an anode in LIBs,it exhibits high reversible capacity,excellent cycle stability and rate performance(the reversible capacity of 1202.9 mAh g-1 after 167 cycles at 0.1 A g-1;the reversible capacity of 767.9 mAh g-1 at 5 A g-1).According to high reversible capacity and excellent surface capacitance characteristics,it was used as a negative electrode to assemble C6O6//AC Li-HSC.The voltage window in C6O6//AC Li-HSC is 0-4.3 V,the maximum energy density and power density are 160.1 Wh/kg and 10750 W/kg,respectively.After 2405 cycles,the energy density can still retain 79.4%of the original energy density.In order to effectively solve the problems of low reversible capacity and poor rate performance of graphite anodes,in this paper,we utilized cis-Muconic acid(MCA)that has a high reversible capacity and excellent surface capacitance characteristics and graphite to build MCA@Graphite composite anode with high reversible capacity and fast charge and discharge capability.It was found that MCA has nanocrystals that are similar to MA and BPDCA and excellent conductive network structure,which is beneficial to rapid reaction kinetics of MCA.Based on the above characteristics,MCA as the anode electrode exhibits a reversible capacity of 668.2 mAh g-1 after 600 cycles at a current density of 0.5 A g-1,and it also demonstrates excellent rate capacity(188.8 mAh g-1)even at ultra-high current density(10 A g-1).According to excellent surface capacitance contribution and high reversible capacity,the prepared MCA@Graphite composite anode has both high reversible capacity and excellent rate performance(at 0.1 C,capacity can reach 766.2 mAh g-1,at 20 C After 2600 cycles,the capacity is 166.1 mAh g-1).Kinetic analysis demonstrates that the MCA introduced to the MCA@Graphite electrode can significantly improve the reaction kinetics,compared with the pure graphite anodes.More importantly,MCA can significantly ease the decomposition of the electrolyte on the graphite surface forming SEI film and grow dynamically,promoting graphite to still have high capacity and excellent cycling performance under large currents.The method about compounding small organic molecules based on surface capacitance with graphite provides a new idea for improving the electrochemical performance of inorganic anode materials.In order to effectively ameliorate the long-term cycle performance and rate capabilities of Si and Si/C composite anodes,a ftnctional fluorinated solvent(trifluoropropylene carbonate,TFPC)was proposed as a co-solvent among Si-based anodes.As a result of-CF3 group in TFPC with electron absorbing ability,it has lower LUMO energy(-0.28 eV)and high reduction potential(2.05 and 1.89 V)than traditional ethylene carbonate and diethyl carbonate.It is beneficial for TFPC to preferentially decompose on the Si surface to form a continuous SEI film and effectively alleviate the further the remaining electrolyte decomposition.The results reveal that the introduction of 10 wt.5 TFPC can form a SEI film composed of polyolefin and LiF in a suitable ratio on the Si surface,which enhancs the mechanical strength and ionic conductivity,significantly improves the recyclability and maintains the structural integrity for the Si electrode.As a result,both the Si electrode and the Si/C composite electrode show excellent cycle stability and extrardinary rate performance in a 10 wt.%TFPC-based electrolyte system.Compared to conventional electrolytes,the electrochemical properties of the electrodes have been markedly greatly improved.By selecting a suitable fluorinated solvent and lithium fluoride salt to optimize the composition of the SEI film on the Si-based anode surface,it is expected to realize the commercial application of the Si-based anode.