Study On Preparation And Properties Of Piperidine-based Ionic Liquid Electrolytes For Sodium Ion Batteries

The global energy consumption pattern is gradually changing to clean energy such as wind and solar energy.To rationalize the use of clean energy and improve energy utilization,energy storage technology is generally used to store energy,i.e.,the energy generated by clean energy is stored in energy storage devices and released when needed.As one of the main forms of electrochemical energy storage devices,sodium-ion batteries have a very broad application prospect in the field of energy storage.The electrolyte,as an important part of the battery,affects the specific capacity and cycling stability of the battery.Carbonate organic electrolytes are widely used because of their wide electrochemical window and low cost,but they are less compatible with the carbon-containing anode,especially hard carbon（HC）,in sodium-ion batteries,and show poor cycling stability and poor rate performance during cycling.As a room-temperature molten salt,ionic liquid has improved the compatibility of HC with carbonate electrolyte due to its high conductivity and good electrochemical stability and has become a hot research topic as an electrolyte additive in recent years.In this thesis,the piperidine-based ionic liquid was used to investigate the mechanism of the effect of alkenyl butyl piperidine ionic liquid on the solid electrolyte interfacial（SEI）at the hard carbon electrode/electrolyte interface in ester electrolytes.The electrochemical performance of HC electrodes in different electrolytes at room temperature and the high temperature was tested to investigate the transport pattern of Na⁺in HC and electrolyte and to reveal its effect on the surface of HC.The mechanism of the action of SEI was investigated.The main contents are as follows:（1）Study on the preparation method of piperidine-based ionic liquids.Functionalized piperidine-based ionic liquids were prepared by nucleophilic reaction using difluoro sulfonamide ion（FSI^-）as the anion and N-methyl piperidine（pp）as the reaction substrate.Based on the premise that double bond introduction can reduce the chemical hardness of ionic liquids,increase the reduction potential,and short carbon chains are easy to react with,brominated olefins were selected as the nucleophilic reaction monomers.The effects of monomer type,reaction temperature,reaction atmosphere,solvent system,reaction time,and other factors on the purity and yield of piperidine-based ionic liquids were investigated to obtain optimized parameters for the preparation of piperidine-based ionic liquids.The results showed that the reaction of bromopropene with piperidine in ethyl acetate solvent at 20℃for 26 h（nitrogen atmosphere）gave the highest yield of the brominated intermediate,and then the colorless allyl piperidine ionic liquid was obtained by FSI^-and Br-ion exchange reaction（pp13）;the reaction of 4-bromo-1-butene with piperidine in ethyl acetate solvent at 65℃for 36 h（nitrogen atmosphere）gave the highest yield of the brominated The highest yield of the intermediate was obtained after the FSI^-and Br^-ion exchange reaction to obtain a pale yellow alkenyl butyl piperidine ionic liquid（pp14）.The structure of the product was characterized by ¹H NMR spectroscopy,and the thermal analysis and flammability test proved that the synthesized ionic liquids have good thermal stability and non-flammability.Theoretical calculations showed that the alkenyl butyl piperidine ionic liquid has high reduction potential and low chemical hardness,and is easy to form SEI preferentially,while the allyl piperidine ionic liquid has low reduction potential and high chemical hardness,and is difficult to reduce to film before carbonate solvents.The viscosity of alkenyl butyl piperidine ionic liquid(123.57 m Pa s^-1,25℃)is larger than that of carbonate solvent,which may be unfavorable to the migration and diffusion of Na⁺in the electrolyte.（2）Electrolyte configuration and electrochemical performance study.A series of electrolytes were formulated with pp14 in 1 M Na FSI（EC:DEC=1:1wt%）at different volume ratios to assemble HC half-cells for electrochemical performance testing to systematically investigate the effect of piperidine-based ionic liquid content on the electrochemical performance and to obtain suitable ratios of piperidine-based ionic liquids.The cycling test results showed that the cycling stability of HC was good in the electrolyte containing 5%（v/v）pp14（5pp14）under the 5 C(1C=300 m A g^-1)current condition,and the reversible specific capacity was 80.90 m Ah g^-1 after 300 cycles,with a capacity retention rate of 82.16%.In contrast,in the electrolyte without pp14（BE）,the reversible capacity was only 50.40 m Ah g^-1 after 300cycles,with a capacity retention rate of 59.80%.The rate performance test showed that the HC rate performance in the 5pp14 electrolyte was good and showed a slightly higher reversible specific capacity than that in the initial test when the small current（0.1 C）cycle was tested again.The cyclic voltammetric scan results showed that the introduction of pp14 did not affect the electrochemical reaction process of the ester electrolyte,but too much addition would reduce the reversible specific capacity.The results of the impedance test,constant potential polarization method,and embedding activation energy calculation show that 5pp14 has the advantages of the low impedance,high Na⁺migration number,and low embedding activation energy compared with BE electrolyte,which is favorable to the migration and diffusion of Na⁺in the electrolyte and electrode.It can be inferred from the SEM results that pp14 is involved in the reaction on the HC surface to form a stable SEI and protect the integrity of the HC material.（3）Investigation of electrolyte high temperature performance and SEI film formation mechanismThe electrochemical performance of the five electrolytes assembled with HC electrode in half cells was tested at 60°C.The results showed that HC in electrolytes containing pp14 exhibited good high-temperature long-cycle performance with higher capacity retention than BE.15pp14 exhibited good high-temperature rate performance and high reversible specific capacity,indicating that pp14 has potential for high-temperature applications.The physical and structural analysis of HC at the end of the cycling process was performed to investigate the relationship between pp14 and SEI and to explain the influence of functional groups or functional groups on SEI formation.X-ray photoelectron spectroscopy（XPS）results showed that pp14 promotes sodium salt dissociation and participates in the generation of inorganic SEI inner layers.Transmission electron microscopy（TEM）results showed that the HC surface in 5pp14formed a uniform SEI with a thickness of 5 nm-10 nm and a graphitized"pseudo-SEI"layer between the SEI and the body,which enhanced the performance of the HC;the SEI of the HC in BE was irregular,with a thickness ranging from 30 nm-60 nm.The thickness of the SEI of the HC in BE varies from 30 nm to 60 nm.The results of Young’s modulus test by atomic force microscopy（AFM）showed that the mechanical strength of HC surface SEI in 5pp14 was stronger than that in BE,and the average Young’s modulus was about two orders of magnitude higher than that of HC surface SEI in BE.DFT calculations showed that pp14 was preferentially reduced on the HC surface during the cycling process.Based on the above results,the mechanism of pp14 involved in the reaction on the HC surface to form stable SEI was revealed.