Abstract
Concentrated solutions of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium nitrate (LiNO3) salts in either diethylene-glycol dimethyl-ether (DEGDME) or triethylene-glycol dimethyl-ether (TREGDME) are herein characterized in terms of chemical and electrochemical properties in view of possible applications as the electrolyte in lithium–oxygen batteries. X-ray photoelectron spectroscopy at the lithium metal surface upon prolonged storage in lithium cells reveals the complex composition and nature of the solid electrolyte interphase (SEI) formed through the reduction of the solutions, while thermogravimetric analysis shows a stability depending on the glyme chain length. The applicability of the solutions in the lithium metal cell is investigated by means of electrochemical impedance spectroscopy (EIS), chronoamperometry, galvanostatic cycling, and voltammetry, which reveal high conductivity and lithium transference number as well as a wide electrochemical stability window of both electrolytes. However, a challenging issue ascribed to the more pronounced evaporation of the electrolyte based on DEGDME with respect to TREGDME actually limits the application of the former in the Li/O2 battery. Hence, EIS measurements reveal a very fast increase in the impedance of cells using the DEGDME-based electrolyte upon prolonged exposure to the oxygen atmosphere, which leads to a performance decay of the corresponding Li/O2 battery. Instead, cells using the TREGDME-based electrolyte reveal remarkable interphase stability and much more enhanced response with specific capacity ranging from 500 to 1000 mA h g–1 referred to the carbon mass in the positive electrode, with an associated maximum practical energy density of 450 W h kg–1. These results suggest the glyme volatility as a determining factor for allowing the use of the electrolyte media in a Li/O2 cell. Therefore, electrolytes using a glyme with sufficiently high boiling point, such as TREGDME, which is further increased by the relevant presence of salts including a lithium protecting sacrificial one (LiNO3), can allow the application of the solutions in a safe and high-performance lithium–oxygen battery.
Highlights
Increasing demand for environmentally sustainable energies has triggered the rapid development of systems employing renewable sources, promoted the electrified mobility, and concomitantly fetched the research for efficient electrochemical storage devices such as the lithium-ion batteries.[1]
An interesting field of research for lithium metal batteries was represented by the use of electrolytes based on solutions with high concentrations of lithium salts, that is, solvent-in-salt configurations, which can lead to notable cycling efficiency and high specific capacity because of the formation of an improved and stable solid electrolyte interphase (SEI) layer, providing at the same time a suitable safety content.[40−43] We have investigated in this work the performances of lithium−oxygen batteries using electrolyte solutions consisting of diglyme [diethylene-glycol dimethylether (DEGDME)] and triglyme [triethylene-glycol dimethylether (TREGDME)] with a relevant amount of lithium salts [lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and LiNO3]
The electrochemical stability window of the electrolytes was evaluated through cyclic voltammetry (CV) in the cathodic region in the 0.01−2 V versus Li+/Li potential range and through linear sweep voltammetry (LSV) in the anodic region from the open-circuit voltage (OCV) condition to 5 V versus Li+/Li
Summary
Increasing demand for environmentally sustainable energies has triggered the rapid development of systems employing renewable sources, promoted the electrified mobility, and concomitantly fetched the research for efficient electrochemical storage devices such as the lithium-ion batteries.[1]. Where i0 and iss are the current values at the initial and steady state, respectively, ΔV is the applied voltage, R0 and Rss are the interphase resistance values before and after cell polarization, respectively, calculated from the impedance spectra Both the chronoamperometric and the EIS measurements were carried out by using a VersaSTAT MC Princeton Applied Research (PAR, AMETEK) instrument. The electrochemical stability window of the electrolytes was evaluated through cyclic voltammetry (CV) in the cathodic region in the 0.01−2 V versus Li+/Li potential range and through linear sweep voltammetry (LSV) in the anodic region from the open-circuit voltage (OCV) condition to 5 V versus Li+/Li. The tests were carried out in cells employing a lithium anode, either DEGDME_HCE or TREGDME_HCE, and carbon as the working electrode, which was coated on copper or an aluminum current collector to perform the measurement in the cathodic or anodic region, respectively. The measurements were carried out by applying a current of 100 mA g−1, referred to the SPC mass, in the 1.5−4.6 V voltage range with a step time of 5 or 10 h, in order to limit the delivered specific capacity to 500 and 1000 mA h g−1, respectively
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