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.
The promising prospects of the Li/S battery, due to its theoretical energy density of about 2500 Wh kg ─1 , are severely limited by two main weaknesses: the poor conductivity of S and the solubility of the polysulphides in the electrolyte. A combination of carbon and transition metal oxides is the best option for mitigating both of these shortcomings simultaneously. In this work, we use hydrothermally-tailored γ-MnO 2 nanorods combined with an activated biomass-derived carbon, which is an inexpensive material and easy to prepare. This strategy was also followed for a AC/MnO 2 /S composite, a preparation of which was made by grinding; this is the simplest method for practical applications. More complex procedures for the formation of in situ hydrothermal MnO 2 nanorods gave similar results to those obtained from grinding. Compared with the AC/S composite, the presence of MnO 2 markedly increased the delivered capacity and improved the cycling stability at both low (0.1 C) and high (1 C) currents. This behaviour results from a combination of two main effects: firstly, the MnO 2 nanorods increase the electrical conductivity of the electrode, and secondly, the small particle size of the oxide can enhance the chemisorption properties and facilitate a redox reaction with polysulphides, more efficiently blocking their dissolution in the electrolyte.
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