Anodic Behavior Of Al Current Collector In Lithium Bis(Fluorosulfonyl) Imide Based Electrolyte

Recently,the interface electrochemistry between Al current collector of positive electrode and electrolytes has attracted much attention with Lithium-ion batteries（LIBs）dominately applicated in electric vehicles.It is inevitable to overcharge causing corrosion of Al for some cells in a power battery pack when the differences among cells is accumulated during the long time working.It is amply demonstrated the anions of lithium salts have a great influence on corrosion of Al foil.Among various salts,lithium bis（fluorosulfonyl）imide（Li[（FSO₂）₂N],LiFSI）has been attracted growing attention as one of the most promising lithium salts to replace lithium hexafluorophosphate（LiPF₆）with superior stability facing high temperature and water,high ionic conductivities and good compatibility with electrode materials.However,there are some debates on its corrosion to Al foil,presumably due to the fact that trace impurities inevitably occur in the final LiFSI product.The trace impurities have significant impact on the corrosion to Al foil.The preparation of M[（FSO₂）₂N]（M=alkali metal）is generally carried out by acid-base neutralization of H[（FSO₂）₂N]superacid with M₂CO₃,and subsequently vacuum drying at elevated temperature.During these processes,the structure of FSI^-anion may be degraded by either water or base at elevated temperature.To improve our understanding on the corrosion behavior of LiFSI to Al foil about LiFSI,a decomposition mechanism of LiFSI during its preparation is firstly proposed in this paper,and then the decomposition products are prepared and their properties about Al foil corrosion are investigated.The paper consists of two parts as below:（1）The decomposition of FSI^-anions was investigated by aging KFSI in aqueous KOH solution.It is demonstrated that the decomposition products are NH₂SO₃K,FSO₃K and K₂SO₄,and then a decomposition mechanism of LiFSI is proposed on the base of experimental phenomena and analysis of ¹H NMR and ¹⁹F NMR.（2）NH₂SO₃Li and FSO₃Li were prepared.Then their physical and electrochemical behaviors on Al foil are comparatively investigated,including solubility in carbonate[EC/DEC（1:1,v/v）],pitting potential（E_p）of Al foil measured by Cyclic Voltammetry（CV）in the electrolyte based on NH₂SO₃Li,FSO₃Li,Li₂SO₄.Additionally,the potentiostatic anodic polarization on Al foil is further used to investigate the compatibility between electrolyte and Al foil during a long time interaction.Meanwhile,the impedance evolutions in the surface of Al foil are measured by Electrochemical Impedance Spectroscopy（EIS）to observe the change in the surface of Al foil.It is demonstrated that trace impurity（>100 pm）FSO₃Li of LiFSI is mainly responsible for the corrosion to Al foil.