Identifying the solid-electrolyte interphase formed on lithium metal electrodes using room temperature ionic liquid based electrolytes.

摘要

Lithium metal is attractive for secondary cells as it has the highest theoretical voltage and electrochemical equivalence, 3860 Ah kg-1. However, dendrite formation while Li0 is charged/discharged in batteries causes short circuits, overheating and explosions. To resolve this, several room temperature ionic liquids (RTIL) electrolytes are receiving considerable attention due to their advantageous safety benefits including high thermal stabilities and/or negligible vapour pressure. However, there is a lack of information regarding the solid-electrolyte interphase (SEI) which passivates the surface of the lithium metal when using RTIL electrolytes. Electrodeposition/electrodissolution of Ag onto glassy carbon via cyclic voltammetry (CV) was proven to be effective from two RTILs, 1 butyl 2 methylimidazolium tetrafluoroborate ([BMIm+][BF4 ]) and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([C4mPyr+][TFSI ]) following nucleation-growth kinetics with close to full reversibility without side reactions. Chronoamperometry (CA) data indicated that Ag electrodeposition follows an instantaneous nucleation and growth type mechanism at all reduction potentials in RTIL. Trace water induced a progressive nucleation and growth type mechanism in [C4mPyr+][TFSI ] which significantly altered the morphology of the resultant electrodeposit. Lithium electrodeposition/electrodissolution was undertaken in N propyl N methylpyrrolidinium bis(fluorosulfonyl)imide ([C3mPyr+][FSI ]) at Pt and Li metal electrodes. Salts added include lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium tetrafluoroborate and (LiBF4), lithium hexafluorophosphate and (LiPF6), and lithium hexafluoroarsenate (LiAsF6). At Pt, CV data showed quasi reversibility for the LiFSI system with &80% cuolombic efficiency. Chronoamperometric data indicated instantaneous nucleation and growth type mechanism for Li electrodeposition from both the LiTFSI and LiBF4 electrolytes. At Li, CV behaviour was complicated by rapid chemical reaction between electrode and electrolyte resulting in the electrode cycling with an order of stability as follows: LiBF4&LiFSI&LiAsF6&LiTFSI&LiPF6 according to current amplitude and signal-to-noise ratio. Instantaneous nucleation and growth type mechanism was evidenced from the LiTFSI and LiPF6 electrolytes. SEI formation in neat RTIL was a dynamic process initially smoothing the surface with adhered RTIL, roughening after 12 days, culminating in smoothened surfaces after 18 days of reaction. Both cation and anion are reduced providing SEI species (LiF, LiOH) and/or entrapped RTIL moieties (methylpyrrolidone, NSO2). Salt addition affected changes in resultant SEI morphology and composition. The chemically formed SEI increases resistance of the Li electrode and acts as a passivating film protecting thermodynamically unstable underlying bulk Li metal. Upon cycling an SEI formed chemically after 12 hours, 12 or 18 days in symmetrical cells it was determined that lithium electrode resistance can be reduced. Best cycling results were obtained when cycling a 12 day SEI with each electrolyte, successfully completing 2000 charge-discharge cycles at 1.0 mA cm 2. Stability of these cells was as follows: LiBF4&LiFSI&LiAsF6&LiPF6&LiTFSI. The 18 day SEI pre treatment was unable to achieve 50 cycles prior to cell failure. However, post mortem analysis confirmed that no dendrite formation was observed using [C3mPyr+][FSI ] electrolytes. The work herein supports that the N propyl N methylpyrrolidinium bis(fluorosulfonyl)imide RTIL with a variety of lithium salts can be used to construct and cycle cells effectively. In fact the properties of these cells appear to be superior to other RTIL based systems which is attributed partly to the SEI formed prior to, and during cycling.