Study of a Li–air battery having an electrolyte solution formed by a mixture of an ether-based aprotic solvent and an ionic liquid

Abstract

Recent studies have clearly demonstrated that cyclic and linear carbonates are unstable when used in rechargeable Li–air batteries employing aprotic solvents mostly due to the cathodic formation of superoxide during the oxygen reduction reaction. In particular, it has been ascertained that nucleophilic attack by superoxide anion radical, O₂⁻, at O-alkyl carbon is a common mechanism of decomposition of organic carbonates. Moreover, theoretical calculations showed that ether chemical functionalities are stable against nucleophilic substitution induced by superoxide. Aim of this study is to report on a new electrolyte solution for Li–air battery formed by a mixture of an ether-based aprotic solvent with an ionic liquid (IL). The IL-based electrolyte was obtained by mixing the pure ionic liquid N-methyl-(n-butyl) pyrrolidinium bis(trifluoromethane sulfonyl) imide (here denoted as PYR₁₄TFSI) to a 0.91 M solution of lithium triflate (LiCF₃SO₃) in tetra ethylene glycol dimethyl etcher (TEGDME). We observed that the presence of IL beneficially affects the kinetics and the reversibility of the oxygen reactions involved at the cathode. The most significant result being a lower overvoltage for the charge reaction, compared to a Li/air cell containing the same electrolyte solution without IL.

Introduction

Lithium–air (oxygen) rechargeable batteries utilizing aprotic electrolytes have overcome all other electrochemical battery storage devices in terms of theoretical specific energy [1], [2], [3], [4], [5], [6], [7], [8], [9]. Consequently, these batteries have recently drawn considerable attention being an important candidate for energy storage devices for EVs (electric vehicles), HEVs (hybrid electric vehicles) and other high-energy storage applications. However, the practical exploitation of lithium–air systems is still very far due to the numerous scientific challenges that need to be addressed.

The use of a lithium anode combined with an O₂ cathode to obtain a high theoretical energy density was first suggested by Bouman and Littauer [10]. Subsequently Abraham and Jiang [1] demonstrated the utilization of nonaqueous electrolytes in the lithium–air system to avoid the major safety problem of H₂ evolution due to the reaction of lithium with water. The main overall reactions occurring during the discharge process in a lithium–air cell is the reduction of O₂ and oxidation of the metallic lithium to form Li₂O₂ and Li₂O as the reaction products.

Most studies revealed Li₂O₂ as the main reaction product after discharge [1], [3], although the formation of Li₂O₂ and Li₂O are both thermodynamically possible since their potentials are quite close. The theoretical potentials for the reactions: (I) 4 Li + O₂ → 2Li₂O and (II) 2Li + O₂ → Li₂O₂ are 2.91 and 2.96 V vs. Li⁺/Li°, respectively, as reported in Refs. [7], [11]. Moreover, it has been shown that the nature of the discharge products is mostly controlled by the kinetics of the oxygen reduction which in turn is affected by the presence of a catalyst [3], [12], but also by the nature of the electrolyte [13], [14], [15]. The main factors limiting the performances of a non-protic Li/air battery are the very high overpotentials for charging (∼1 to 1.5 V) compared to the modest overpotentials for discharging (∼0.3 V) [1], [5]. These overpotentials result in a large system irreversibility, which in turn results in very poor cycleability. At the microscopic level, the processes associated to the charging overpotential have been related to the poor O₂ solubility [3], [7] and to the precipitation of the discharge products in the nonaqueous organic electrolytes solutions [16]. In fact, it has been found that insulating discharge products are deposited on the surface of the electrodic active particles, either formed by carbon or by a carbon/catalyst mixture, which block further electron transfer to oxygen and also prevent oxygen from diffusing to the reaction sites [17]. It has also been recently reported in the literature that a key role is played by the porosity of the cathodic active particles as their pore volume consent the accommodation of the discharge products and their availability for the subsequent charging reaction [17], [18], [19]. It follows that acting on the morphology of the cathode supporting material as well as optimising the choice of catalysts for the Oxygen Reduction Reaction (ORR) and for the Oxygen Evolution Reaction (OER) are current important challenges [3], [13]. Indeed other important cell parameters, such as the electrolyte composition, could improve the cycle life and the cell efficiency. In fact the electrolyte composition could affect O₂ solubility, but also the nature and the solubility of the discharge reaction products by stabilising intermediary reduction species [21], [22] or by binding the oxygen anion in order to overcome the lattice energy of lithium oxides [23].

Recently some important properties of ionic liquids, such as their high conductivity, non-flammability, non-volatility and wide temperature range of operation, have attracted great interest in view of their application as electrolytes in rechargeable Li-ion batteries [24], [25]. These same properties could actually be promising for rechargeable Li/air batteries which generally present high overpotentials and poor cycle life as above mentioned. The present manuscript reports preliminary studies concerning the application of an ionic liquid based electrolyte to a lithium/air cell. Pure ionic liquids do not contain mobile Li⁺ ions which are necessary for the cell reactions which is the reason why it is necessary to add a lithium salt in order to obtain a lithium-ion conducting electrolyte. To accomplish this, lithium salts having a large, charge-delocalizing anion, are used [24], [26]. In our case, the ionic liquid based electrolyte was obtained by mixing the pure ionic liquid N-methyl-(n-butyl) pyrrolidinium bis(trifluoromethane sulfonyl) imide (here noted as PYR₁₄TFSI) to a 0.91 M solution of lithium triflate (LiCF₃SO₃) in tetraethylene glycol dimethyl ether (TEGDME). The volume ratio between the ionic liquid and the salt solution, in the ionic liquid based electrolyte, was kept 1:1 for all the experiments. Several experiments were also conducted in a Li/air cell containing a 0.91 M solution of lithium triflate (LiCF₃SO₃) in tetraethylene glycol dimethyl ether (TEGDME) as the electrolyte solution without the addition of the ionic liquid.