Physical properties and compatibility with graphite and lithium metal anodes of non-flammable deep eutectic solvent as a safe electrolyte for high temperature Li-ion batteries

Perspective of material evolution Induced by sinusoidal reflex charging in lithium-ion batteries

Background

Lithium-ion batteries are globally prominent and extensively employed alternative energy sources with decisive applications. In depth understanding of influences of various charging and discharging cycles on electrode materials and life span of these batteries is critical as cycle-life and safety of lithium-ion batteries are closely related crystallinity of electrode materials. This study is a detailed investigation endeavor in observing the degree of damage to electrode materials under multiple charging and discharging cycles.

Method

ology: A constant current-sinusoidal reflex charging method (CC-Sinusoidal) was implemented to charge commercial cathode Lithium cobalt oxide (LiCoO2) electrodes and anode graphite electrodes in comparison to the conventional charging method of constant current-constant voltage (CC-CV). After 100, 300, and 500 cycles of charging and discharging, EIS, SEM, XRD, and Raman spectroscopies were used to compare the degree of electrode damage caused by different charging methods.

Significant outcomes

The structure of positive LiCoO2 electrode of the battery was observed to be stable, with no significant change in both the charging methods after 500 cycles. The use of CC-CV charging method had caused severe damages to graphite electrode with generation of solid electrolyte interface (SEI) films. The CC-Sinusoidal charging method had maintained the electrode material in a relatively ideal state.

Non-Flammable Sodium Asymmetric Imide Salt-Based Deep Eutectic Solvent for Supercapacitor Applications

This study reports two deep eutectic solvents (DESs) based on alkaline imide salts with asymmetric anions as functional electrolytes for supercapacitor (SC) application. The eutectic mixture of sodium (fluorosulfonyl) (trifluoromethanesulfonyl) imide (NaFTFSI) or sodium cyano-trifluoromethanesulfonyl imide (NaTFSICN) with ethylene carbonate (EC) delivers a non-flammable and stable liquid. The eutectic diagrams of the electrolytes directed to an optimal composition ( w salt = 0.25), hinging to that of conventional carbonate-based electrolytes, i.e., 1 mol L -1 . The volumetric properties of the DESs revealed a "stacking" effect, reflecting a strong coordination bond between the imide and EC anions without solvating the Na + cations. The DES transport properties (i.e., viscosity, conductivity, and ionicity) and temperature variations designate a high organization, similar to ionic liquids. The DESs, when coupled with activated carbon electrodes in a two-electrode symmetric configuration, yield specific capacities of 150 F g -1 at a normalized current density of 0.5 A g -1 (and 120 F g -1 at 2 A g -1 ). The SC maintained 80% of its initial capacity beyond 100 h of floating at an operating voltage of 2.4 V and showed a 150 mV per hour potential loss under self-discharge. The devised eutectic mixtures offer a promising new pathway for simple, safe, and effective electrolytes for SC applications.

Synthesis, Physical Properties and Electrochemical Applications of Two Ionic Liquids Containing the Asymmetric (Fluoromethylsulfonyl)(Trifluoromethylsulfonyl)imide Anion

Novel ionic liquid (IL) electrolytes based on the asymmetric (fluoromethylsulfonyl)(trifluoromethylsulfonyl)imide (FTFSI)− anion, combined with the N-trimethyl-N-butyl-ammonium (N1114)+ and N,N-diethyl-N-methyl-N(2-methoxyethyl)-ammonium (N122(2O1))+ cations, were successfully synthesized and investigated in terms of thermal, vibrational and electrochemical properties. Thermogravimetric measurements revealed that the ionic liquids are stable up to 300 °C (2% mass loss). Differential scanning calorimetry measurements evidenced no phase transition down to −90 °C, suggesting a transition towards a glass state at lower temperatures. Infrared spectroscopy measurements, for the first time performed on ILs containing FTFSI, could not detect any crystallization down to −140 °C. The frequency of the main absorption bands of the ILs are in good agreement with DFT calculations. The FTFSI ionic liquid electrolytes, containing 20% mol of LiTFSI, show no solid-liquid phase transition due to the asymmetry of the FTFSI− anion, increasing the −10 °C conductivity up to 10−4 S cm−1. These interesting ion transport properties remarkably extend the operative temperature range down to low temperatures. The FTFSI electrolytes exhibit remarkable electrochemical stability up to 4.8 V, this making them appealing for realizing safer and highly reliable lithium battery systems operating at high voltages.