Letter pubs.acs.org/JPCL
Potassium Ions Promote Solution-Route Li2O2 Formation in the Positive Electrode Reaction of Li−O2 Batteries Shoichi Matsuda,† Yoshimi Kubo,† Kohei Uosaki,† and Shuji Nakanishi*,‡ †
Global Research Center for Environment and Energy based on Nanomaterials Science, National Institute of Material Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan S Supporting Information *
ABSTRACT: Lithium−oxygen system has attracted much attention as a battery with high energy density that could satisfy the demands for electric vehicles. However, because lithium peroxide (Li2O2) is formed as an insoluble and insulative discharge product at the positive electrode, Li−O2 batteries have poor energy capacities. Although Li2O2 deposition on the positive electrode can be avoided by inducing solution-route pathway using electrolytes composed of high donor number (DN) solvents, such systems generally have poor stability. Herein we report that potassium ions promote the solution-route formation of Li2O2. The present findings suggest that potassium or other monovalent ions have the potential to increase the volumetric energy density and life cycles of Li−O2 batteries.
T
here is a growing demand for energy-storage devices with high energy density for use in portable electronic devices and electric vehicles. Rechargeable aprotic lithium−oxygen (Li−O2) battery that has the potential to achieve higher energy densities than that of Li−ion battery is a candidate for nextgeneration energy-storage devices.1−6 A typical aprotic Li−O2 battery consists of Li metal and porous carbon as the negative and positive electrodes, respectively, and a Li ion-conducting nonaqueous electrolyte. In an ideal Li−O2 battery, atmospheric oxygen is reduced in pores of the positive electrode and then combines with Li ions to form solid lithium peroxide (Li2O2) during the discharge. Because the reverse reaction occurs during the charging process, the total reaction can be written as 2Li + O2 = Li2O2 (U0 = 2.96 V). The initial step of Li2O2 formation in Li−O2 batteries involves the reduction of oxygen, generating surface-bound superoxide anions, which then react with Li ions to form surface-bound LiO2, as denoted in the following reactions O2 + e− = *O2−
(1)
Li+ + *O2− = *LiO2
(2)
In contrast, the formation of Li2O2 proceeds mainly via the solution-route pathway in high DN solvents, in which *LiO2 dissolves and forms a stable solvated complex containing Li+ and O2− *LiO2 ⇌ solvated complex: [solvent]n [Li+][O2−]
The solvated complex is finally converted to Li2O2 via reduction or disproportionation reactions. The mechanism of Li2O2 formation strongly influences the performance of Li−O2 batteries, particularly with respect to energy capacity.7−10 During operation in low DN solvents, insulative Li2O2 deposits accumulate on the positive electrode surface, leading to a gradual reduction in electrically active area and a reduction in the discharge voltage at constant current densities.11,12 In contrast, Li2O2 formation in high DN solvents proceeds (reactions 3-1 and 3-2) at a distance from the site of the initial electrochemical reactions that generates surfacebound LiO2 (reactions 1 and 2). Thus the deposited Li2O2 does not suppress the successive discharge reactions (reaction 1), resulting in improved energy capacity. However, as solvents with high DN are generally unstable against nucleophilic reactions by superoxide,13 alternative approaches for inducing the solution-route pathway using low DN are required to be developed for realizing high energy density Li−O2 batteries. Recent evidence suggests that Li2O2 formation can be enhanced through the use of additives. For example, Aetukuri et al.8 reported that the addition of trace amounts of water to a
The symbol “*” in the above reactions denotes the surface species. The reaction pathway for Li2O2 formation is also influenced by the donor number (DN) of the solvents (7−10). In low DN solvents, LiO2 can be further reduced or disproportionated, forming Li2O2 as the final discharge product, as illustrated in the following reactions *LiO2 + Li+ + e− = 2Li 2O2
(3-1)
*2LiO2 = Li 2O2 + O2
(3-2) © XXXX American Chemical Society
(4)
Received: January 8, 2017 Accepted: February 23, 2017
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DOI: 10.1021/acs.jpclett.7b00049 J. Phys. Chem. Lett. 2017, 8, 1142−1146
Letter
The Journal of Physical Chemistry Letters low DN solvent promoted the solution-route of Li2 O2 formation and significantly improved the energy capacity of a Li−O2 cell. In this system, the high Lewis acidity of water was speculated to stabilize oxygen superoxides in the form of a solvated complex, resulting in the efficient dissolution of LiO2. Although water cannot be used as an additive for practical Li− O2 batteries due to the high potential for reactivity with Li2O2 and Li metal, these findings suggest that other materials may be suitable for constructing Li−O2 battery systems with high energy capacity and rechargeability. In the present study, we examined the potential of alkali and alkali earth metal ions to promote the solution-route Li2O2 formation when added to Li−O2 cells. These ions have several properties that are expected to promote the solution-route of Li2O2 formation, including low reactivity with Li2O2 and Li metal and high Lewis acidity. Thus alkali and alkali earth metal ions can potentially stabilize oxygen superoxide through the formation of solvated complexes. As a test system, coin-type Li−O2 cells consisting of lithium foil, a graphene-based gas-diffusion-electrode equipped with a glass fiber separator, and 1 M LiTfO in tetraglyme as the electrolyte were used. We confirmed by Karl Fischer titration that the water content of the electrolyte was
