Study On The Effect Of Functional Additives On The Performance Of High Energy Density Ternary Li-ion Battery

At present,lithium-ion batteries mostly use carbonate-based organic electrolytes.Such conventional carbonate-based electrolytes cannot be safely used under high pressure,because the electrolyte will oxidize and decompose under high voltage,accompanied by gas generation.The cycle performance and service life of lithium-ion batteries have a greater impact,therefore,greatly limiting the development of power batteries.Based on this situation,in order to improve the performance of lithium-ion batteries at high operating voltages,this thesis studed the effects of three additives on the performance of high-energy-density ternary lithium batteries from the perspective of functional additives.The main findings are as follows:1)As an electrolyte additive,lithium bis(fluorosulfonyl)imide(LiFSI)can broaden the operating temperature range of LiNi0.5Co0.2Mn0.3O2/graphite battery and improve the electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode at high voltage.Linear sweep voltammetry(LSV)results indicated that LiFSI can broaden the electrochemical window of the electrolyte.According to the high voltage cycle test,it found that,at room temperature,the battery containing 5 wt%LiFSI was cycled for 120 times with capacity retention rate of 85.15%in the end.In addition,the battery cycled 100 times at high-temperature and high-voltage with capacity retention rate of 80.0%in the end.It was found by electrochemical impedance spectroscopy(EIS)that the addition of LiFSI decreased the impedance.Combined with scanning electron microscope(SEM),transmission electron microscope(TEM),X-ray diffraction analysis(XRD),and X-ray photoelectron spectroscopy(XPS)analysis,it was confirmed that LiFSI forms a more stable and uniform interface film on the electrode surface.At the same time,the decomposition of the electrolyte and the destruction of the electrode structure during high-voltage cycling are effectively suppressed,thereby improving the electrochemical performance.2)As a functional additive,succinonitrile(SN)can be used in LiNi0.5Co0.2Mn0.3O2/graphite lithium-ion batteries to broaden the oxidation electrochemical window of the electrolyte and significantly improve its rate performance and high voltage cycle performance.LSV showed that the ethylene carbonate/ethyl carbonate(EC/EMC)based electrolyte with SN had a higher oxidation potential(>6.1 V vs.Li/Li+).The capacity retention of full cell with 0.5 wt%SN added to the electrolyte and 120 cycles between 2.75 V and 4.40 V was significantly increased from 67.96%to 84.0%.It is indicated that the LiNi0.5Co0.2Mn0.3O2(NCM523)battery containing 0.5 wt%SN-based electrolyte has better cycleability and capacity retention at high cut-off voltage.In addition,EIS,SEM,XPS of the full cell were used to characterize the effect of SN on the battery.It is proved that the SN participates in the interfacial reaction between the electrode and the electrolyte to form a stable solid electrolyte interphase(SEI)layer,thereby effectively suppressing the increase of the charge transfer resistance and reducing the elution of the transition metal cations.These results indicated that SN can be used as a functional additive for high-voltage lithium-ion batteries.3)Adiponitrile(ADN)with a functional nitrile(-CN)group is used as a film-forming additive.Linear sweep voltammetry(LSV)tests have shown that EC/EMC based electrolytes with ADN have higher oxidation potentials(>6.2 V vs.Li/Li+).The LiNi0.5Co0.2Mn0.3O2/graphite full cells containing 0.5 wt%ADN showed significant high voltage cycling performance with a capacity retention of 85.2%,while the blank electrolyte after 120 high voltage cycles was 67.96%.This is attributed to the protective effect of SEI film prevents the further decomposition of electrolyte and transition metal ion dissolution,thus stabilizes the electrode-electrolyte interface.The SEM and XPS results also showed that there are fewer lithium compounds and side reactions on the ADN-containing cathode.The research in this thesis showed that LiFSI,SN,and ADN used as electrolyte additives in LiNi0.5Co0.2Mn0.3O2/graphite full batteries,can significantly improve the high voltage performance of the batteries.And effectively widened the battery's operating temperature range.This provides an effective and economical solution for the development of high-voltage lithium-ion batteries.At the same time,it provides data reference for the composition of the new high-voltage electrolyte system.