Study On The Solid Electrolyte Interphase Formation Mechanism By Propylene Carbonate Reduction

Organic carbonates are popular aprotic solvents for electrolytes used in lithium ion(Li+)batteries,sodium ion(Na+)batteries and also lithium(Li)metal batteries.Among all the Organic carbonates,propylene carbonate(PC)attracted the researchers’attention at the very early time.Propylene carbonate(PC)was ever a star solvent in the initial stage of Li+ ion batteries development due to its unique advantages,such as wide liquid range,high dielectric constant,superior ionic conductivityand so on.Although it was found that PC solvent is not adequate to graphite anode material,it is still expected that PC can be compatible to Li metal anode.In this study,LiClO4 is selected as the electrolyte,and we compared the reduction mechanism of PC with and without the Lithium salt.The main contents of this paper are divided into the following aspects:1.Study on the multi-step characteristic and oxidation performance of the PC in the open-circuit potential to OV potential interval:gradually lower the limit of the reduction potential,compare differences among the reduction process of PC in different potential interval.Then further compare the cyclic voltammetry characteristic and the oxidation of the reduced product is examined as well.The results show that the reduction products of both PC and trace impurities are oxidizable.2.Study the reduction process and the formation of sediment characteristics of PC in the 1V to 0.5V potential interval:the reaction occurs in the 1V to 0.5V range is the decomposition of PC itself.Due tothe high sensitivity and real-time response characteristics of the probe,we set the probe at a constant potential where the reduced productscan be oxidized.When the reduction reaction occurs on the substrate,if the oxidation current signal can be detectedby the probe,it means the reduced product on the substrate can dissolve into the electrolyte and then external diffuse.Otherwise,the reduction product on the substrate is insoluble.The results show that the PC reduction process can be divided into two stages by potential.The deposit formed by the reduction products of PC with Li+ ions are analyzed with Scanning Electron Microscopy(SEM)and Fourier Transform Infrared Spectroscopy(FTIR).The reduction product in the first stage should be CHs-CH-CH-OCO2-free radical anion defined by potential region of higher than 0.85V,which is soluble in LiClO4/PC electrolyte and can be re-oxidized.The PC reduction product in the second stage defined by potential region of 0.85V to 0.55V should be CH=CH-CH-OCO2Li and Li2CO3 which form solid electrolyte interphase(SEI)film.The characteristics of the SEI film strongly depend on the way of PC reduction,as PC is reduced by low constant current,the corresponding SEI film is more dense and uniform,and it is more efficient to prevent the PC from further reduction and gives a better plating/stripping cycling features of Li metal electrode.3.Try to apply SECM to studying the reaction of organic solvent and SEI film formation characteristics,provingits effectiveness and popularization and application value.