The Effect Of Electrolyte System On The Electrochemical Performance Of Graphite,Hard Carbon And Si/C Negative Electrode

Environmental pollution and energy shortage are becoming more and more serious,so new energy is urgently concerned.The search for a high-performance lithium-ion battery is imminent.Graphite is a anode material commonly used in lithium-ion batteries,which has poor performance and easy to be embedded with solvents.It is difficult to improve the theoretical capacity of graphite(374 m Ah/g).The following are the contents of this study: The properties of graphite electrodes in different electrolytic liquid systems were investigated.The effect of electrolytic liquid system on the electrochemical performance of hard carbon anode in lithium/sodium ion battery were studied and the influence of electrolytic liquid system on the electrochemical performance of Si/C anode in lithium ion battery was explored.The experimental results obtained by means of EIS,CV,charge/discharge,SEM,TEM are as follows:(1)It is known by the conductivity test that the ionic conductivity of 1 mol/L Li PF6-EC:EMC(1:1)(system 1)was bigger than 1 mol/L Li PF6-EC:EMC(3:7)(system 2).1 mol/L Li PF6-EC:DEC:EMC(3:4:3)(system 4)had smaller ionic conductivity than 1 mol/L Li PF6-EC: DMC:EMC(1:1:1)(system 3).For 1 mol/L Li PF6-EC:DEC:DMC:EMC(1.5:1:1:1.5)(system 5),its conductivity was bigger than that of 1 mol/L Li PF6-EC:DEC:DMC:EMC(2:1:1:2)(system 6).Through CV test,the graphite electrode had a consistent oxidation-reduction decomposition process under six electrolytic liquids.According to the results of charging/discharging test,the capacity in system 1,system 3 and system 4 were higher than that of system 2,system 4 and system 6.Through the SEM test,in the enlarged figure,the ribbed in system 1 of the graphite anode surface after the electrochemical cycle were more than system 2.The SEI film produced under system 1 was thinner.The SEI film generated of the graphite surface in system 3 was thinner than in system 4,and the SEI film in system 5 was thinner than the system 6.From the EIS test the RSEI resistance value of the graphite electrode in system 1 was less than the system 2.The RSEI resistance in system 3 was less than the system 4,and the RSEI resistance in system 5 was less than the system 6.(2)The results showed that in sodium ion batteries the electrochemical performance of the hard carbon electrode in 1 mol/L Na Cl O4-EC+ DEC+PC was better than that in 1 mol/L Na Cl O4-EC+DEC.The SEM results showed that the hard carbon still kept the hard carbon electrode in a block-like structure after electrochemical scanning in 1 mol/L Na Cl O4-EC+DEC+PC.But in 1 mol/L Na Cl O4-EC+DEC,the SEI film generated on the surface of the hard carbon electrode was so thick that it had obscured the block structure of the hard carbon.The EIS results showed that the RSEI resistance was smaller in 1 mol/L Na Cl O4-EC+DEC+PC.In lithium ion battery,the electrochemical properties of hard carbon electrode were improved most by adding 1 vol%FEC.The results of CV showed that FEC had certain effect on the decmposition of electrolyte.When without FEC,there were three reduction peaks in the hard carbon electrode.When 1 vol%FEC was added,the reduction peak of 0.2 V disappeared.When the amount of FEC was added more,the peak of 0.2 V appeared again.It is known from the SEM results,the most thin SEI film generated after the addition of 1 vol%FEC,and due to the impedance spectrogram that the resistance RSEI value was the minimum after adding 1 vol%FEC.(3)The results showed that the electrochemical properties of Si/C electrode in lithium ion battery were improved most by adding 1 vol%FEC.CV results showed that the addition of FEC had certain effect on the decomposition of Si/C electrode electrolyte.The film forming peak at the first week in the blank electrolyte was about 0.5 V.When 0.5 vol%FEC was added,the film peak of the first week shifted to the high potential about 1.0 V.When 1 vol%FEC was added,the SEI film was also generated around 1.0 V,and under this system,the first week had the least irreversible capacity.The content of FEC was further increased,although the first cycle of the film peaks toward the high potential,capacity loss had increased.SEM results displayed that the SEI film was most uniform with the addition of 1 vol%FEC.When FEC was added as high as 10 vol%FEC,after electrochemical scanning,there was a crack in the SEI film generated on the Si/C anode surface.Furthermore,the resistance value of the RSEI after adding 1 vol%FEC was the minimum.It can be explained that after adding 1 vol%FEC,the SEI film on the Si/C anode surface was the most thin.