An ionic liquid (IL) electrolyte with 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIFSI) is applied to a silicon (Si) composite anode for Lithium-ion batteries (LIB). Si is one of the most promising anode materials for LIBs and fluoroethylene carbonate (FEC) has been widely used as an electrolyte additive with Si anodes to enhance electrochemical performance. However, the effect of FEC only lasts for a limited number of cycles. To overcome this issue, a bis(fluorosulfonyl)imide (FSI)-based IL is studied as a potential electrolyte candidate for a Si composite anode. Its effects on the electrochemical performance and the corresponding solid electrolyte interphase (SEI) formation on the Si composite anode are not well understood. This work addresses the correlation between the electrochemical performance and SEI formation to probe the surface chemistry on the Si composite anode. We find that the FSI-based electrolyte provides a stable and reversible capacity in long term cycling tests. This electrolyte has excellent rate capability compared to that of carbonate-based electrolytes. The decomposition products of these electrolytes on Si anodes are investigated by X-ray photoelectron spectroscopy. These results show that the chemical composition on the surface of the Si anode is largely different when using the FSI-based electrolyte than it is when using carbonate type electrolytes. The decomposition products of the IL lead to a large number of inorganic species such as LiOH and Li2O, which yield superior rate capability for the IL electrolyte. The FSI-based IL offers promising applicability for a practical Si composite anode.

Among the several challenges to enable next-generation batteries is the development of an electrolyte which compatible with both lithium (Li) metal anode and high-voltage cathode at wide temperature range. Liquefied gas electrolytes with a new cosolvent and higher salt concentration show improved ionic conductivity of > 4 mS/cm at wide temperature range from -80 to +70 °C. With a new solvation structure, the liquefied gas electrolytes demonstrated high-temperature operation of Li-metal batteries at 55°C, which is the operation above the electrolytes’ critical point for the first time. The electrolytes enable improved Li metal stability and coulombic efficiency at aggressive current and capacity of 3 mA·cm-2 and 3 mAh·cm-2 with average coulombic efficiency of 99%. The use of liquefied gas electrolytes presents stable cycling of Li/NMC (4.4 V) cell at all-temperature range between -60 and 55°C. This study opens up a promising avenue toward the applications of all-temperature high energy density Li-metal batteries.

Electrochemical capacitors and lithium-ion batteries have seen little change in their electrolyte chemistry since their commercialization, which has limited improvements in device performance. Combining superior physical and chemical properties and a high dielectric-fluidity factor, the use of electrolytes based on solvent systems that exclusively use components that are typically gaseous under standard conditions show a wide potential window of stability and excellent performance over an extended temperature range. Electrochemical capacitors using difluoromethane show outstanding performance from -78° to +65°C, with an increased operation voltage. The use of fluoromethane shows a high coulombic efficiency of ~97% for cycling lithium metal anodes, together with good cyclability of a 4-volt lithium cobalt oxide cathode and operation as low as -60°C, with excellent capacity retention.

Development of safe electrolytes that are compatible with both lithium metal anodes and high-voltage cathodes that can operate in a wide-temperature range is a formidable, yet important challenge. Recently, a new class of electrolytes based on liquefied gas solvents has shown promise in addressing this issue. Concerns, however, have been raised on the pressure, flammability and low maximum operating temperature of these systems. Here, we endeavor to mitigate safety and practicality concerns by demonstrating an enhanced safety feature inherent in liquefied gas electrolytes and by showing the viability of using difluoromethane as a liquefied gas solvent which has lower pressure, lower flammability, and improved maximum operation temperature characteristics compared with fluoromethane. We create a custom-built setup to enable liquefied gas electrolyte characterization through Raman spectroscopy and supplement this with molecular dynamics (MD) simulations. The electrolyte shows good conductivity through a wide temperature range and compatibility with both the lithium metal anode and 4 V class cathodes. The demonstrated use of such alternative liquefied gas solvents opens a path towards the further development of high-energy and safe batteries that can operate in a wide-temperature range.

Among the several challenges to enable next-generation batteries is the development of an electrolyte that maintains a dendrite-free and high Coulombic efficiency lithium-metal anode over extended cell cycling. A new electrolyte solvation structure and transport mechanism is demonstrated in fluoromethane-based liquefied gas electrolytes with the introduction of additive amounts of tetrahydrofuran, which is shown to fully coordinate with the lithium cations and greatly enhance salt dissociation and transport. The resulting electrolyte shows a high conductivity and transference number (t+ > 0.79), which leads to a dramatic improvement of the cycling performance of the lithium-metal anode. Systems using the enhanced liquefied gas electrolytes demonstrate a long-term high average Coulombic efficiency of 99.6%, 99.4%, and 98.1% (± 0.3%) at capacities of 0.5, 1, and 3 mAh·cm−2, respectively, with dendrite-free morphology and remarkable rate capability. Both the rate and cycling performance are well maintained from +20°C to −60°C.

The momentum in developing next-generation high energy batteries calls for an electrolyte that is compatible with both lithium (Li) metal anodes and high-voltage cathodes, and is also capable of providing high power in a wide temperature range. Here, we present a fluoromethane-based liquefied gas electrolyte with acetonitrile cosolvent and a higher, yet practical, salt concentration. The unique solvation structure observed in molecular dynamics simulations and confirmed experimentally shows not only an improved ionic conductivity of 9.0 mS cm-1 at +20 °C but a high Li transference number (tLi+ = 0.72). Excellent conductivity (>4 mS cm-1) was observed from-78 to +75 °C, demonstrating operation above fluoromethane's critical point for the first time. The liquefied gas electrolyte also enables excellent Li metal stability with a high average coulombic efficiency of 99.4% over 200 cycles at the aggressive condition of 3 mA cm-2 and 3 mA h cm-2. Also, dense Li deposition with an ideal Li-substrate contact is seen in the liquefied gas electrolyte, even at-60 °C. Attributed to superior electrolyte properties and the stable interfaces on both cathode and anode, the performances of both Li metal anode and Li/NMC full cell (up to 4.5 V) are well maintained in a wide-temperature range from-60 to +55 °C. This study provides a pathway for wide-temperature electrolyte design to enable high energy density Li-metal battery operation between-60 to +55 °C.