Effects of aqueous electrolytes on the voltage behaviors of rechargeable Li-air batteries

Abstract

Aqueous Li-air batteries have attracted a great deal of attention due to their high theoretical energy capacities. However, while still in the early stages of research, the reported energy capacities of Li-air batteries are far from what has been theoretically predicted. In this research, we have designed a Li-air battery that has a Li ǀ organic liquid electrolyte ǀ Li⁺-conducting glass ceramic plate (LiGC plate) ǀ aqueous electrolyte ǀ Pt air electrode structure and studied the impacts of the compositions of the aqueous electrolyte on the battery performance. With lower concentrations of alkali aqueous electrolytes (≤0.05 M LiOH), a discharge voltage of approximately 3.5 V (at 0.05 mA cm⁻²) and a voltage efficiency up to 84% were observed. The addition of LiClO₄ into the aqueous solution slightly lowered the discharge voltage to 3.3 V but dramatically decreased the internal resistance of the battery to 35.4 Ω cm⁻². With a charge voltage plateau observed at 3.90 V at a current of 0.05 mA cm⁻², the Li ǀ organic liquid electrolyte ǀ LiGC ǀ 1 M LiClO₄ ǀ Pt air battery showed an 85% voltage efficiency at room temperature. Adding LiClO₄ into the aqueous electrolytes resulted in an impedance reduction and slowed the pH increase of the alkaline-based electrolyte due to the fast or long-term discharge of the air electrode in the Li-air battery. The discharge and charge voltage behaviors of the battery and the changes to the pH values of the aqueous electrolyte at different current rates were also recorded and are presented in this paper.

Introduction

The Li-ion rechargeable battery has been successfully used in small portable electronic devices because it has many advantages, including its high gravimetric energy density (120–150 Wh kg⁻¹), relatively short charging time, and long cycle life. However, it is crucial to increase the energy density of the battery to further develop portable electronic devices that can better meet today's needs. The ability to increase the energy density of the present Li-ion batteries is limited by the use of Li intercalation solid compounds, which have been employed as both negative and positive electrodes, such as Li_xC₆ and Li_1−xCoO₂ [1]. With air (O₂) as the positive electrode and Li metal as the negative electrode, a Li-air battery has been developed. The specific energy density of the Li-air battery ranges between 5789 and 11248 Wh kg⁻¹, which is more than ten times higher than that of Li-ion batteries [2]. Based on the nature of the electrolyte and its reaction products, Li-air batteries can be divided into two groups:

• Li/O₂ in non-aqueous electrolytes [2]

  Li + O₂ = Li₂O₂ (peroxide) E = 3.10 V

  4Li + O₂ = 2Li₂O E = 3.10 V

• and Li/O₂ in aqueous electrolytes [2]

  Basic electrolyte: 4Li + O₂ + 2H₂O = 4LiOH E = 3.45 V

  Acidic electrolyte: 4Li + O₂ + 4H⁺ = 2H₂O + 4Li⁺ E = 4.27 V

  Seawater (pH 8.2): 4Li + O₂ + 2H₂O = 4LiOH E = 3.79 V

In theory, Li-air batteries with non-aqueous electrolytes can deliver a specific energy density up to 11248 Wh kg⁻¹ [3]. The first non-aqueous electrolyte Li-air battery with a Li|organic liquid electrolyte|air electrode structure was reported in 1996 [2]. With this battery, energy capacities of 1600 mAh g⁻¹ and 1410 mAh g⁻¹ were achieved in air and pure oxygen atmospheres, respectively. These energy capacities were also characterized by the weight of the carbon catalyst since a decrease in the discharge voltage was often caused by the carbon electrode, which could be choked by the deposition of the reaction product (Li₂O₂) in its pores. Interestingly, a better capacity, 2120 mAh g⁻¹, was achieved by changing the mass of the carbon catalyst [4]. Moreover, modification of the air catalytic electrode also improved the capacity up to 2825 mAh g⁻¹ at 0.05 mA cm⁻² [5]. The highest capacity for a Li-air battery in non-aqueous electrolytes was reported to be 5360 mAh g⁻¹ (discharged at 0.01 mA cm⁻²) by Kuboki et al. [6]. Nanometer-scale catalysts, based on precious metals, have also been introduced into non-aqueous electrolyte Li-air batteries as catalysts for the air electrode, including Au [7], Pd [8], Pt [9] and PtAu [7]. Higher discharge voltages were reported from the use of these nanometer-scale catalysts compared with metal oxide catalysts [7], [8], [10].

However, the use of a non-aqueous electrolyte Li-air battery as a rechargeable battery has resulted in two critical problems. The first problem is that the non-aqueous electrolyte allows the moisture of the air to travel into the Li metal anode, contaminating the Li metal and resulting in a poor cycle life for the battery. To minimize the effects of Li corrosion, the use of dry air and pure oxygen in place of atmospheric air was attempted [2], [4], [5], [6], [10], [11], [12], [13], [14], [15], but this approach is not a cost-effective solution. The second problem is that the discharge products, Li₂O₂ and Li₂O, are insoluble in the non-aqueous liquid electrolyte, and thus, the product particles gradually clog the porous air electrodes [2], [4], [5], [6], [10], [11], [12], [13], [14], [15], which eventually yield poor cycle lives.

To overcome the challenges faced by Li-air batteries with non-aqueous electrolytes, the use of aqueous electrolytes with a Li-air battery have been recently studied [7], [16], [17], [18], [19]. To protect the Li anode from being exposed to the aqueous electrolytes, a Li⁺-ion-conducting glass ceramic (LiGC) plate was used to separate the anode, which contains the Li metal in a non-aqueous electrolyte and the cathode, which, in turn, is composed of an oxygen reduction catalyst in an aqueous solution. The theoretical specific energy densities of the Li-air batteries in the aqueous solutions are approximately 5789 Wh kg⁻¹ based on the weight of the battery, where the weight of oxygen (O₂) is excluded because it is freely available from the atmosphere and, therefore, does not need to be stored in the battery or in the cell. Compared with a non-aqueous Li-air battery, better voltage efficiency and a longer cycle life should be possible in an aqueous Li-air battery because the discharged product (LiOH) can be dissolved in the aqueous electrolyte solutions, which would yield good reversible reactions. However, the solubility of LiOH in water is also a limitation for aqueous Li-air batteries to reach high capacities [18].

Recently, aqueous Li-air batteries that use a LiGC solid electrolyte have been reported including Li ǀ PEO₁₈LiTFSI ǀ LiGC ǀ 1 M LiCl ǀ Pt [13], Li–Al ǀ Li_3−xPO_4−yN_y ǀ LiGC ǀ 1 M LiCl ǀ Pt [17], Li ǀ 1 M LiClO₄ in EC:DMC ǀ LiGC ǀ1 M KOHǀ Mn₃O₄ [18] and Li ǀ 1 M LiPF₆ in EC:DMC ǀ LiGC ǀ1 M LiOHǀ CoMn₂O₄ graphene composites [20]. Wang et al. [19] also proposed a new Li–air fuel cell using metallic copper as the catalyst for the O₂ electrochemical reduction. The achieved results from testing of this Li–air fuel cell demonstrate that the cycle between Cu and Cu₂O can be used to catalyze the O₂ electrochemical reduction based on the copper corrosion mechanism. However, further study is needed to verify this system. Interestingly, a large capacity of 50,000 mAh g⁻¹ (based on total mass of catalytic electrode) was obtained after discharging Li ǀ 1 M LiClO₄ in EC:DMC ǀ LiGC ǀ1 M KOHǀ Mn₃O₄ for 500 h [18], but the charge performances, including the voltage efficiencies, were not acceptable for use as rechargeable batteries. The Li ǀ PEO₁₈ LiTFSI ǀ LiGC ǀ 1 M LiCl ǀ Pt air cell [16] showed a favorable rechargeable performance at the increased temperature of 60 °C, but it was reported to discharge and charge for only one hour. As for the LiGC solid electrolyte, it was found to not be stable in the aqueous Li-air batteries for extended use when strong alkaline solutions were used [16], [17], [18].

To the best of our knowledge, the LiGC electrolyte is the only ceramic solid electrolyte that is currently commercially available for use in aqueous Li-air batteries. While there are worldwide efforts to develop Li-ion conducting ceramics that are stable in strong alkaline or acid solutions, an alternative approach is to optimize the aqueous electrolyte and to alleviate the high pH by tuning the chemical compositions of the aqueous solutions.

In this study, weak alkaline aqueous solutions (≤0.05 M) were first used as electrolytes for a Li-air battery of the following structure: Li ǀ organic liquid electrolyte ǀ LiGC ǀ aqueous electrolyte ǀ Pt catalytic electrode. Platinum was selected as the electrocatalyst for the air electrode in this study because Pt has been considered to be the bench-mark active and stable catalyst for the oxygen reduction reaction via a direct 4-electron pathway [21]. The use of 1 M LiClO₄ was also attempted to reduce the impedance and alleviate the problem of the chemical instability of the LiGC plate in strong alkaline solutions. The effects of the compositions of the aqueous electrolytes on the electrochemical performance were studied in detail.