Research On The Formulation Of Electrolyte Additives For Power Lithium Ion Batteries

In recent years,energy crisis and environmental pollution have become two important issues of concern to the whole society.With the improvement of China’s energy structure,electric vehicles have gradually entered the field of our view,and the booming development of electric vehicles has put forward higher requirements on the performance of lithium-ion batteries.As an important part of lithium ion battery,the role of electrolyte is to transfer ions between positive electrode and negative electrode of battery,and it also has a crucial effect on capacity,cycle efficiency,safety and so on.However,the thermal stability of electrolyte in electric cars faces a severe challenge especially under extreme conditions.Therefore,the development of high security lithium ion power battery electrolyte and additive has become a current hot research problem in the field of lithium ion batteries.In this paper,dimethyl methyl phosphonic acid(DMMP),N,N-dimethyl formamide(DMF)and ethoxy(pentafluoro)cyclotriphosphazene(PFN)were investigated as the research objects,which were added into the based electrolyte(LiPF6/EC+DEC+DMC)to explore battery’s electrical performance including capacities,cycle life,resistance,capacity retention rates,high rate discharge characteristics and coulombic efficiency using lithium iron phosphate and high energy density lithium nickel manganate as positive electrode.We have studied the electrode sufance elements and composition changes before and after battery cycles,meanwhile we have analyzed the electrolyte composition and content before and after cycles,so as to reveal the mechanism of the different additives on batteries.And related important conclusions are listed as follows:1.LiFePO4 is considered as an extremely potential cathode material for large-scale application in lithium ion battery,however the iron dissolution for lithium iron phosphate under high temperature limits its application.The main reason is that traditional high temperature electrolyte are easier to produce decomposition by-products at high temperature,and HF as one of the by-products in the charging and discharging process makes the electrode materials easily dissolved.DMF as a new electrolyte additive is investigated to improve lithium ion battery performance under 55 ℃ of Li/LiFePO4 cell in this paper.Under 0.5C current density,the specific capacities at 50th and 100th cylces with t 1 wt.%DMF are 137.1 mAh·g-1 and 125.8 mAh·g-1,and at the same conditions,the specific capacities without additive are 131.2 mAh·g-1 and 125.8 mAh·g-1.Charge-discharge tests illustrate that 1%DMF can increase the capacity retention from 60.5%to 85.9%.The chemical changes of additives in battery between charge and discharge were analyzed through cyclic voltammetry(CV)and electrochemical impedance(EIS)tests.The XRD,SEM and XPS results indicate that DMF as a lewis base can capture lewis acidic PF5 from the decomposition of LiPF6 as well as block the chain reaction of LiFePO4 with HF,which alleviate the electrolyte decomposition and electrode dissolution under charge and discharge.2.In this chapter,DMMP was used to analyze the effect of electrochemical performance in the LiFePO4 battery.Under 0.5 C,the specific capacities at 50th and 100th cylces with 0.5 vol.%DMMP are 151.3 mAh·g-1 and 150.3 mAh·g-1,and at the same conditions,the specific capacities without additive are 148.7 mAh ·g-1 and 148.1 mAh·g-1.We found with the addition of 0.5%DMMP the battery coulombic efficiency was 99.73%,while only 97.11%for battery with no additive.The chemical changes of additives in battery between charge and discharge were analyzed through cyclic voltammetry(CV)and electrochemical impedance(EIS)tests.SEM and XPS characterization of electrode materials,revealed that the addition of DMMP can generate a layer of thin and uniform membrane on the electrode surface,the membrane formation effectively inhibit the further broken down between the electrolyte and electrode materials,thereby reducing the electrode and the electrolyte decomposition and making the battery performance improved.3.LiNi0.5Mn1.5O4 is a kind of positive electrode material which can provide high voltage and high energy density.The performance of LiNi0.5Mn1.5O4 battery was investigated by using PFN as an additive.After the combustion test,it was found that the 5 wt.%PFN could make the electrolyte completely flame retardant and make a great breakthrough in safety.Under 0.2C,the specific capacities at 50th and 100th cylces with the 5%PFN are 124.4 mAh·g-1 and 122.1 mAh·g-1,and at the same conditions,the specific capacities without additive are 117.7 mAh·g-1 and 105.8 mAh·g-1.The retention rate was 96.4%after 100 cycles,nevertheless the battery without additive was 86.1%,so we found that the battery’s circulation retention rate was high and there was no capacity attenuation after adding the additive.The CV and EIS test analysis on the battery of different electrolytes found that the PFN is reactive during charging and discharging,which is preferred in the battery to the electrolyte decomposition reaction,thereby reducing the decomposition of the electrolyte.The XRD,SEM and XPS results indicate the electrolyte can be more easily oxidized and decomposed with the addition of 5 wt.%PFN,thus producing some linear polymers,multi-ring polymers;LiNO3,RONO2Li(RONO2:nitrate ester functional group,with R standing for any organic residue),Li3PO4,and ROPO3Li(ROPO3:monoester phosphate)simultaneously.These as-generated materials form dense,uniform and thin protective layer on the surface of the cathode material,which suppresses the decomposition of electrolyte and electrode corrosion,correspondingly protecting the LiNi0.5Mn1.5O4 from structural destruction.