| Electrolyte is an important part of sodium-ion batteries(SIBs).It is a major cause of the safety hazards,and has a great impact on the electrochemical performance of batteries.By constructing the organic electrolyte with a wide electrochemical window and high ionic conductivity as well as good interface compatibility with the electrode,the electrochemical performance of the SIBs can be improved simply and efficiently.In this paper,the components(salts,solvents,additives)and concentration of the organic electrolytes were optimized to construct electrolytes with good performance for SIBs.Its electrochemical performance and its compatibility with the hard carbon electrode were studied.Also,its impact on the performance of the SIBs was explored.(1)Research on the optimization of electrolyte solvent system for SIBs.Using NaClO4 as the sodium salt,two ether-based and carbonate-based electrolyte systems were prepared with diethylene glycol dimethyl ether(Diglyme)and ethylene carbonate/diethyl carbonate(EC/DEC),respectively.The effect of the electrolyte solvent on the capacity and cycle performance of the hard carbon electrode was studied.The results show that in the 1 M NaClO4+EC/DEC electrolyte,the hard carbon electrode has much better electrochemical cycle stability than that in the 1 M NaClO4+Diglyme electrolyte,which delivers a specific capacity of 209 mAh g-1 after 100 cycles at 0.2 C with a capacity retention of 83.3%.It is attributed to the wide electrochemical window of the NaClO4+EC/DEC electrolyte.With charge-discharge cycling in the NaClO4+EC/DEC electrolyte,the stable and complete SEI film was formed on the surface of the electrode,which ensured the cycle stability of the electrode.However,the NaClO4+Diglyme electrolyte was oxidized at about 3 V,leading to serious adverse reactions.Thus,the formed SEI film on the electrode surface was unstable,and the electrode capacity decayed rapidly.(2)Research on the optimization of sodium salt in the electrolyte of SIBs.Four kinds of sodium salts,i.e.NaClO4,NaOTf,NaFSI and NaPF6,were combined with EC/DEC solvents,respectively,to prepare four electrolytes.The effect of the sodium salt on the cycle performance of the hard carbon electrode was studied.The results show that the sodium salt in the electrolyte can be reduced during charge-discharge cycling of the battery,and the reduction products would participate in the formation of the SEI film on the electrode surface.The electrolyte containing NaPF6 resulted in the strong SEI film with a large amount of inorganic components,making the hard carbon electrode exhibit good cycle stability.It shows a capacity retention of 96.2%after 100 cycles at 0.2 C.The electrolyte system containing NaOTf leaded to the smooth,compact,uniform and thin SEI film with low impedance,which is conducive to the rate performance of the electrode(68.6 mAh g-1 at 2 C).(3)Research on the electrolyte additives of SIBs.With 1M NaClO4+EC/DEC as the main components,two additives(FEC and VC)were introduced,respectively,to construct two kinds of organic electrolytes.The effect of the electrolyte additives on the cycle performance of the hard carbon electrode was studied.The results show that in the electrolyte containing FEC,the capacity retention of the hard carbon electrode after 100 cycles at 0.2 C is 92.5%,which is higher than the system without additives(83.3%),while in the electrolyte containing VC,the capacity retention was reduced to 77.4%.The possible reason is that the SEI formed in the electrolyte containing FEC contains much NaF,which makes the SEI film have a relatively stable structure and high strength.Thus,the cycle stability of the electrode was improved.Nevertheless,the main reduction products of VC are carbonates and organics,resulting in the SEI film with high impedance,which has an adverse effect on the cycle performance of the electrode.(4)Research on the optimization of electrolyte concentration of SIBs.Using the low-cost NaOTf as sodium salt and Diglyme as the solvent,the NaOTf+Diglyme electrolyte systems with different concentrations(0.5,1,2 and 2.5 M)were constructed,and the influence of the concentration on the electrolyte properties was studied.Furthermore,the effect on the performance of SIBs was investigated.The results show that with the concentration increasing,the amount of Na+in the electrolyte system increases,so a large number of solvent molecules are bound in the solvent sheath of Na+,and the number of the free solvent molecules decreases.Then,the dissociated anions from sodium salt participate in the SEI film-forming reaction.As a result,as the concentration increases from 0.5 M to 2 M,the conductivity of the electrolyte increased from 1.36 mS cm-1 to 3.89 mS cm-1(25℃),and the oxidation potential increased from 3.25 V to 4 V.Moreover,the high salt concentration also reduced the flammability of the electrolyte and improved the safety.The electrolyte with high salt concentration also showed good compatibility with metallic sodium and hard carbon electrodes.The hard carbon electrode in the 2 M NaOTf+Diglyme electrolyte exhibited a capacity retention of 98.7%after 50 cycles at 0.1 C.In addition,it still had a specific capacity of 196.4 mAh g-l at a rate of 2 C,and maintained at 157 mAh g-1 after 500 long-term cycles with a capacity retention of 73.4%,indicating excellent cycling and rate performance.This is because in highly concentrated electrolyte,the dissociated anions from sodium salt,i.e.CF3SO3-,were reduced during charge-discharge cycling,and the resultant products like Na2S2O6 participated in the formation of the SEI film.Therefore,the formed SEI film had low impedance and high strength,which is beneficial to the cycle and rate performance of the electrode.However,when the salt concentration was further increased to 2.5 M,the sodium salt in the electrolyte system cannot be completely dissolved,and the precipitated sodium salt would be deposited on the electrode surface,which hindered the mass and charge transfer at the electrode/electrolyte interface,resulting in poor battery performance. |
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