The role of imide electrolytes on the interfacial reactions with electrodes for lithium ion batteries and lithium metal batteries

Abstract

This study investigates the availability of lithium (Li) imide salt based electrolytes for lithium ion batteries (LIBs) and lithium metal batteries (LMBs) and the role of imide electrolytes on the interfacial reactions with electrodes. Li imide salts such as lithium bis(flourosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) are the promising candidate for LIBs and LMBs as compared with the widely used Li salt, lithium hexafluorophosphate (LiPF6) due to its improved solubility, thermal stability, HF contamination resistivity and comparable ionic conductivity. I firstly study the suitability of Li imide electrolytes in LIBs which consist of two electrodes, Li metal oxide cathode and graphite anode. In terms of graphite/electrolyte, it has been reported that Li imide electrolyte presents slightly/far better electrochemical performances at ambient/elevated temperature, resulting from reduced internal charge transfer resistance and thermally stable solid electrolyte interphase (SEI) formation on graphite/electrolyte interface. However, a severe corrosion issue of aluminum (Al) cathode current collector is a bottleneck for the use of Li imide salt with cathodes. Through a systematic comparison of Al corrosion behavior in the electrolyte with Li imide salt and various borate Li salt additives, Al corrosion is largely suppressed by the addition of those additives and the inhibition ability is revealed to be in the following order: lithium difluoro(oxalato)borate (LiDFOB) > lithium tetrafluoroborate (LiBF4) ≈ LiPF6 > lithium bis(oxalato)borate. Particularly, the corrosion inhibition ability of Li imide electrolyte with LiDFOB additive is comparable to Al corrosion-free LiPF6 electrolyte. As evidenced by X-ray photoelectron spectroscopy (XPS), such superior inhibition ability is attributed to the formation of a passive layer which consists of Al-F, Al2O3, and B-O species on Al/electrolyte interface. A LiCoO2/graphite cell with 0.8 M LiFSI+0.2 M LiDFOB electrolyte exhibits a rate capability comparable to a cell with 1 M LiPF6 solution, whereas a cell with 0.8 M LiFSI solution without LiDFOB suffers from poor power performance resulting from severe Al corrosion. Li imide electrolytes feature comparable/better advantages in LIBs by prevention of Al corrosion. However, it is still insufficient to satisfy an urgent need in energy market demands. Thus, Li metal anode can possibly be a good candidate of graphite owing to its high theoretical capacity and low redox potential after success of addressing issues such as low Li coulombic efficiency (CE) and dendrite formation. Recently, Li imide electrolytes were introduced with a non-coordinating fluorinated ether. However, very little is known about the performance of LHCEs under harsh temperature conditions. I investigate glyme-based LHCE over a wide temperature range (5−60 °C) with a focus on the beneficial role of the fluorinated ether. Compared to 4 M LiFSI dimethoxyethane (gHCE), 1 M LiFSI dimethoxyethane/fluorinated ether (gLHCE) displays improved physicochemical properties, good wettability toward polyethylene separators, and non-flammability due to the advantageous character of the fluorinated ether. In-depth analysis confirms that in gLHCE, most FSI-anions exist as contact ion pairs and agglomerates, leading to an inorganic-rich solid electrolyte interphase on the Li anode that mitigates the side reactions with the electrolyte and facilitates the interfacial charge transport. The formidable advantages of gLHCE enable an excellent Li cycling behavior and long-term stability of FeS2/Li and anode-free LiFePO4/Cu cells over 5−60 °C. Lastly, the superior rate capability of HCEs and LHCEs is scrutinized in comparison with conventional diluted electrolyte (DE). In LiCoO2/Li rate test, LiFSI-DMC based DE, HCE, and LHCE are compared at various discharge rates from 0.2C to 20C. Although discharge capacity retentions of LiCoO2/Li cells with HCE and LHCE are higher than the capacity retention of DE up to 10C, the capacity retention of HCE is drastically decreased at 15C, and eventually the capacity retention of HCE becomes lower than DE. Meanwhile, LHCE features great capacity retention after 15C as compared with HCE. The electrochemical impedance spectroscopy (EIS) study reveals that HCE enables the lower charge transfer resistance (Rct) at LiCoO2/electrolyte interface. However, during ultrafast discharge, the ohmic resistance and ion transport resistance in the electrode pores (Rsol and Rion) and concentration overpotential (ηconc) affect the cell performance to a higher extent than the discharge process at low/moderate rates. Thus, LHCE is found to exhibit the highest rate capability due to the facile charge transfer (from lower activation energy for charge transfer and lower viscosity) and better mass transport (from better ionic conductivity and quite lower viscosity than HCE). This study indicates that the availability of electrolytes for batteries is not determined by only its physicochemical natures, but it should be considered comprehensively with the nature of electrodes and interphase properties at electrode/electrolyte interface which can affect ionic and charge transport.