Study On The High-voltage Electrolytes And Interfacial Electrochemistry Of Li-ion Battery

Exploration of high voltage and high specific capacity cathode materials is a general approach to increase the energy density of lithium-ion batteries.However,the structure change,the transition metal ions dissolution,the oxidation decomposition of electrolyte and instability of the electrode/electrolyte interphase and soon,can lead to a dramatic deterioration of capacity retention.What's worse,the decomposition of electrolyte will produce large amounts of gas and heat,which may cause security problem.These issues restrict further development of lithium-ion batteries.Therefore,it is very essential and urgent to develop 5 V electrolytes,aiming to broaden the electrochemical windows of the electrolyte and stabilize the cathode/electrolyte interphase(CEI).Extensive efforts have been carried out on some new solvents and functional additives toward the next generation of high-voltage electrolyte systems.In this paper,we focus on the nitrile compounds and phosphazene compounds as the research objects,so as to control the structure,composition and stability of electrode/electrolyte interphase by optimizing the electrolyte recipes,thus achieving the cyclic stability and safety performance of lithium ion battery.Electrochemical behavior of Suberonitrile(SUN)as a high-potential electrolyte additive and co-solvent for Li[Li0.2Mn0.56Ni0.16Co0.08]O2 cathode material has been investigated.The electrochemical performance of cathode material in electrolyte with 1%added SUN is improved significantly,with a capacity retention of 89%after 200 cycles,comparing with 74%in base electrolyte.However,SUN as co-solvent has a detrimental effect on the cycle performance of materials,the 20%SUN system yields only 15.7%capacity retention.DFT calculation and LSV results indicate that SUN has a higher oxidation potential than EC and DMC,and a small amount of SUN can greatly improve the oxidation potential of mixed electrolyte.Therefore,it is suggested that SUN is not decomposed during the CEI formation process.EIS,SEM and XPS techniques were used to study the impedance,morphology and components of CEI.The different effects and acting modes of SUN as an additive and solvent have been discussed and proposed.Based on stabilizing electrode/electrolyte interphase by strong interaction between-CN and Co ions,SUN and lithium bis(oxalate)borate(LiBOB)were investigated as binary additives for the high-voltage LiCoO2 electrodes to solve the compatibility problem between the BOB-and cobalt-based cathode materials.In the electrolyte adding LiBOB+SUN,the electrochemical performance of LiCoO2 is enhanced significantly.The cell exhibits discharge capacity of 115 mAh g-1.The capacity retention ratio reaches 62%after 500 cycles,a strong contrast with 25%measured in LD120 and 9.6%for the LD120+LiBOB sample.To reveal the mechanisms of surface adsorption derived by the SUN,the XAS measurements were carried out.The structure and composition of the surface film were systematically analyzed by EIS,SEM,TEM,SS-NMR and XPS,and the synergistic mechanism between LiBOB and SUN was proposed.X-Photo Emission Electron Microscopy(X-PEEM)has been firstly applied to explore additive effects on CEI formed in high-voltage LiCoO2 to obtain correlation between morphology and CEI chemistry on various components at nanometer pixel size.In the pristine electrode,a small amount of Co2+ is found in certain regions which are also rich in acetylene PVdF/black(AB).The chemical images confirm that at least two types of divalent Co,CoF2 and Co2+-SUN(might be through chemical interaction between LiCoO2 and SUN)exist in LiCoO2 when cycled with additives.The enrichment of Co2+ is on PVdF/AB region.While the apparently lower Co-O valence in LiCoO2 cycled with additives shall benefit the electrode/electrolyte stabilization.In contrast,when materials are cycled in base electrolyte,much less Co2+ is found on the surface of large particle of LiCoO2 with little F and C species covering.The cyclophosphazenes,containing alternating phosphorus and nitrogen atoms in their skeletons,have super-stability and excellent flame retardancy,so two additives,(phenoxy)pentafluorocyclotriphosphazene(PFPN)and bis(phenoxy)tetrafluorocyclotriphosphazenes(DPFPN)which contains phenyl and phosphazene group,were synthesized.The density functional theory results show that PFPN and DPFPN have both higher HOMO values,indicating that PFPN and DPFPN are easily oxidized,which is well consistent with the LSV and voltage-step tests.When charged to 4.5 V and operated at 30?,the capacity retention of the LiCoO2 in base electrolyte,2%PFPN and 2%DPFPN added electrolytes are 67.4%,91.4%and 67.4%after 300 cycles,respectively.It is important to note that the use of PFPN yields positive effects on the LiCoO2 cycling performance while DPFPN cannot.In addition,PFPN can also improve the cycle stability of the LiCoO2 material when increasing the operating temperature to 45? and cutoff potential to 4.55 V.Based on the measurements of SEM,TEM and XPS,it is concluded that the PFPN-derived surface layer(CEI-PFPN)is thinner and more uniform,mainly composed of electro-polymerization species of PFPN,then effectively suppressing the decomposition of carbonate solvent and lithium salt.On contrast,the CEI-DPFPN is thinner but not uniform;the dominant component is still C and O species,suggesting a serious deconposition of the carbonate solvents.Detailed analysis of the forming process of PFPN via FTIR,UV-VIS and XPS spectra confirm that PFPN is oxidized and decomposed at low potential,and the formation of the polymer is accompaniedby the P-F bond broken.In addition,good cycling performance of AG/Li cells have been observed with 2%PFPN or 2%DPFPN addition.This is attributed to the reduction decomposition of PFPN or DPFPN,helping in forming a good SEI on artificial graphite electrode and further effectively inhibiting the continuous reduction of electrolyte.The diethyl amine-sub stituted cyclotriphosphazene was designed and synthesized,named as diethylaminepentafluorocyclotriphosphazene(PNDE).The density functional theory calculation shows that PNDE has poor oxidation and reduction stability,which is well consistent with the linear sweep voltammetry,voltage-step and CV electrochemical test.When LiCoO2 is cycled in 3 V-4.5 V,operated at 30? or 45?,the cycling behavior of LiCoO2 added PNDE is enhanced compared with that in base electrolyte.When the cutoff voltage increased to 4.55 V,the capacity fading is accelerated after 100 cycles in the PNDE-containing electrolyte.This is possibly due to the instability of CEI-PNDE layer and decomposing at high voltage.The CEI characteristic of LiCoO2 material after 300 cycles in 3 V-4.5 V,30? conditions has been analyzed.The results reveal that the CEI-PNDE is more compact and uniform,but the CEI component is rich of decomposition products of LiPF6 and carbonate solvents.Surprisingly,the AG/Li cell with the PNDE-containing electrolyte exhibits high capacity of370 mAh g-1 after 200 cycles and good rate capability of 342 mAhg-1 at 2 C,while the capacity of the cell without PNDE is 303mAhg-1 at 2C.This is mainly attributed to the more stable SEIPNDE film with lower Li+ migration barrier.The flammability of the additives and the flame retardancy of the electrolytes were tested by self-extinguishing time(SET)experiments.The results disclose that PFPN,DPFPN and PNDE are non-combustible,and help in reducing the flammability of electrolyte.The effect of the additives on the thermal stability of the electrolyte and the LixCoO2 material were evaluated by DSC.Results of thermal tests strongly suggest that the addition of PFPN,DPFPN and PNDE additives in electrolyte significantly enhance the thermal stability of electrolyte and charged LixCoO2.This is mainly attributed to two aspects:First is the additive-derived CEI layer;the other is the addition of additive in the electrolyte.The effects of additives on thermal stability of delithiated LixCoO2 and cell systems are as follows:the reduction of Co3O4 produced by thermal decomposition of LixCoO2 can be weekened by the phosphazene additives;the thermal decomposition of phosphazene additives generates a large amounts of phosphorus free radicals to hinder the combustion chain branching reactions by scavenging the H or OH radicals of gas phase,then terminating the chain combustion reactions;high concentration of phosphate compounds can effectively isolate the electrode materials and electrolyte and prevent further reaction between electrolyte and materials.