Article pubs.acs.org/accounts
Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX
Potassium Superoxide: A Unique Alternative for Metal−Air Batteries Neng Xiao, Xiaodi Ren, William D. McCulloch, Gerald Gourdin, and Yiying Wu*
Acc. Chem. Res. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 09/04/18. For personal use only.
Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States CONSPECTUS: Lithium−oxygen (Li−O2) batteries have been envisaged and pursued as the long-term successor to Li-ion batteries, due to the highest theoretical energy density among all known battery chemistries. However, their practical application is hindered by low energy efficiency, sluggish kinetics, and a reliance on catalysts for the oxygen reduction and evolution reactions (ORR/OER). In a superoxide battery, oxygen is also used as the cathodic active medium but is reduced only to superoxide (O2•−), the anion formed by adding an electron to a diatomic oxygen molecule. Therefore, O2/O2•− is a unique single-electron ORR/OER process. Since the introduction of K−O2 batteries by our group in 2013, superoxide batteries based on potassium superoxide (KO2) have attracted increasing interest as promising energy storage devices due to their significantly lower overpotentials and costs. We have selected potassium for building the superoxide battery because it is the lightest alkali metal cation to form the thermodynamically stable superoxide (KO2) product. This allows the battery to operate through the proposed facile one-electron redox process of O2/KO2. This strategy provides an elegant solution to the long-lasting kinetic challenge of ORR/OER in metal−oxygen batteries without using any electrocatalysts. Over the past five years, we have been focused on understanding the electrolyte chemistry, especially at the electrode/electrolyte interphase, and the electrolyte’s stability in the presence of potassium metal and superoxide. In this Account, we examine our advances and understanding of the chemistry in superoxide batteries, with an emphasis on our systematic investigation of K−O2 batteries. We first introduce the K metal anode electrochemistry and its corrosion induced by electrolyte decomposition and oxygen crossover. Tuning the electrolyte composition to form a stable solid electrolyte interphase (SEI) is demonstrated to alleviate electrolyte decomposition and O2 cross-talk. We also analyze the nucleation and growth of KO2 in the oxygen electrode, as well its long-term stability. The electrochemical growth of KO2 on the cathode is correlated with the rate performance and capacity. Increasing the surface area and reducing the O2 diffusion pathway are identified as critical strategies to improve the rate performance and capacity. Li−O2 and Na−O2 batteries are further compared with the K− O2 chemistry regarding their pros and cons. Because only KO2 is thermodynamically stable at room temperature, K−O2 batteries offer reversible cathode reactions over the long-term while the counterparts undergo disproportionation. The parasitic reactions due to the reactivity of superoxide are discussed. With the trace side products quantified, the overall superoxide electrochemistry is highly reversible with an extended shelf life. Lastly, potential anode substitutes for K−O2 batteries are reviewed, including the K3Sb alloy and liquid Na−K alloy. We conclude with perspectives on the future development of the K metal anode interface, as well as the electrolyte and cathode materials to enable improved reversibility and maximized power capability. We hope this Account promotes further endeavors into the development of the K−O2 chemistry and related material technologies for superoxide battery research. and potentially low cost.1−3 However, their practical applications are hindered by the irreversible Li−O2 chemistry when oxidizing lithium peroxide into oxygen, which leads to a large charging overpotential, low energy efficiency, and severe electrolyte/electrode decomposition.4,5 Our interest in superoxide batteries began with the hypothesis that the oxygen/superoxide redox couple can enable a singleelectron reversible oxygen reduction and evolution reactions (ORR/OER), which could provide an elegant solution to the long-lasting kinetic challenge of ORR/OER in metal−air
1. INTRODUCTION Over the past decade, the demand for renewable energy has grown rapidly due to the depletion of traditional fossil fuels and associated environmental concerns. Nevertheless, storing and efficiently utilizing the intermittently harvested electricity remains challenging. Since their launch in the 1990s, lithiumion batteries (LIBs) have dominated the energy storage market for portable electronic devices. However, their limited energy density and high cost impede their feasibility as next-generation large-scale energy storage (e.g., electric vehicles and smart grids). Among the alternatives beyond LIBs, lithium−oxygen (Li−O2) batteries were deemed the long-term target due to their high theoretical energy density (3500 Wh/kg based on Li2O2) © XXXX American Chemical Society
Received: July 4, 2018
A
DOI: 10.1021/acs.accounts.8b00332 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research
Figure 1. (a) Cell structure of a rechargeable K−O2 battery. (b) Chemical principles of fuel cell/water splitting, Zn−air, Li−air, and K−air batteries. (c) Comparison of voltage profiles.
small Li+, LiO2 is chemically disfavored and acts as a reaction intermediate with short lifetime. Upon disproportionation (I) or electrochemical reduction (II), Li2O2 is formed as the final discharge product in conventional Li−O2 batteries.8
batteries. K−O2 and Na−O2 superoxide batteries were introduced by our group and Hartmann et al., respectively, in 2013.6,7 The facile one-electron redox process of M+ + O2 + e− ↔ MO2 (M = K or Na) solves the major challenges in Li−O2 batteries. In return, the potential gap between charge and discharge was reduced to less than 50 mV for K−O2 batteries without catalysts or redox mediators.7 Furthermore, being both thermodynamically and kinetically stable over the long-term emphasizes the advantage of KO2 over NaO2. The significantly improved energy efficiency (>90%) and earth-abundant K make superoxide batteries attractive for energy storage applications. In this Account, we summarize the emergent superoxide batteries with an emphasis on our systematic investigation and development of K−O2 batteries. First, we introduce the principles of the K−O2 chemistry. The most challenging component in superoxide batteries, the highly reactive K metal anode, is then addressed and the different approaches summarized. The mechanism and (electro)chemical reactions, including the nucleation and growth of KO2 crystals on the cathode, are discussed. Finally, the stability and chemical reactivity of the superoxide is analyzed regarding parasitic reactions. We hope that our analysis and perspective on the development of K−O2 batteries will stimulate further research toward commercialization.
O2− + Li+ → LiO2*
2LiO2* → Li 2O2 + O2 LiO2* + Li+ + e− → Li 2O2
(I) (II)
Consequently, the charging stage for Li−O2 battery becomes a two-electron process that oxidizes Li2O2 into O2. The large charging overpotential (1−1.5 V) resulting from the sluggish kinetics and asymmetric reaction mechanism not only decreases the round-trip energy efficiency (ca. 60%) but also challenges the electrochemical window of the electrolyte and electrode. In stark contrast, K+ is a heavier cation that preferentially forms thermodynamically stable KO2 instead of K2O2 because of the steric repulsion between cations in the crystal packing structure. O2− + K+ → KO2
Charging is the reverse process that oxidatively decomposes KO2 back to O2 and K+, which allows the battery to operate via the one-electron redox couple of O2/O2− with a potential gap below 50 mV at modest current densities (Figure 1c). The greatly reduced charging overpotential is attributed to the comparable O−O bond lengths in O2− (1.28−1.33 Å) and O2 (1.21 Å) compared to O22− (ca. 1.49 Å), which implies a small reorganization energy and lower energy barrier for the KO2 to O2 conversion based on Marcus theory. KO2 has been confirmed as the only discharge product in K−O2 batteries.
2. PRINCIPLE OF K−O2 CHEMISTRY The working principle of a rechargeable K−O2 battery can be perceived as combing alkali metal plating and stripping on the anode with oxygen reduction/evolution on the cathode. The cell design of a conventional K−O2 cell is similar to other nonaqueous metal−O2 batteries, as shown in Figure 1a. It consists of a potassium metal foil as the anode, a porous carbon cathode connected to an O2/air reservoir, and a separator filled with nonaqueous electrolyte. It should be noted that unlike the multielectron processes in fuel cell/water splitting, Zn−Air, or Li−O2 batteries, K−O2 batteries eliminate the need for highcost electrocatalysts as a result of the facile single-electron process (Figure 1b). The discharge in metal−O2 batteries starts with the electrochemical reduction of molecular oxygen on the conductive carbon electrode.
3. POTASSIUM METAL ANODE Establishing a stable electrolyte is the biggest challenge in developing K−O2 batteries with long cycle life. A solid electrolyte interphase (SEI) is crucial to passivate and stabilize the K metal anode, due to the strong reducing power of K toward organic electrolyte, while electrolyte components must be chemically stable with the superoxide radical anion. To date, knowledge of the formation and composition of the SEI on K metal is limited and thus became a focus of our research.
O2 + e− → O2−
3.1. Oxygen Crossover and Anode Corrosion
The generated superoxide anions combine with the alkali metal cations (M+) to form MO2, which undergo various pathways depending on the nature of M+. When M represents
Glymes are considered to be more stable in the presence of O2•− compared to other solvents (e.g., carbonates).9,10 Meanwhile, B
DOI: 10.1021/acs.accounts.8b00332 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research
Figure 2. (a) Proposed mechanism of DME decomposition on the K surface with O2 presented. A−F are the proposed reaction intermediates; inset shows the K anode corrosion. Cycling of K−O2 batteries (b) without and (c) with Nafion-K+ membrane. Adapted with permission from ref 11. Copyright 2014 American Chemical Society.
Figure 3. K metal in (a) pure tetraglyme and (b) 1 M KTFSI-tetraglyme. SEM and elemental mapping on the cross-section of cycled K anode with (c) KPF6−DME electrolyte and (d) KTFSI−DME electrolyte. (e) Cycling K−O2 batteries in 1 M KTFSI−DME electrolyte. (f) Reversibility of K plating and stripping with different electrolytes in K/Cu cell. (g) Schematic of artificial SEI strategy and (h, i) improved cycle life with preformed SEI. Images in panels a−d adapted with permission from ref 17. Copyright 2016 John Wiley and Sons. Images in panels e and g−i adapted with permission from ref 19. Copyright 2018 John Wiley and Sons. Image in panel f adapted with permission from ref 18. Copyright 2017 American Chemical Society.
batteries, KPF6−DME, a thick yellow surface layer was always present on the K metal surface after battery failure.11 The consumption of metallic K and continuous growth of the surface
dimethoxyethane (DME) exhibits better compatibility with K metal than long-chain ethers because of the chelating ability of oxygen atoms. With our first electrolyte selection for K−O2 C
DOI: 10.1021/acs.accounts.8b00332 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research
Figure 4. (a) Modeling of the KO2 nucleation and growth. SEM on KO2 crystals formed under (b) 0.1 mA, (c) 0.2 mA, (d) 0.5 mA, and (e) 1.0 mA. (f−g) Growth mechanisms of KO2 in K−O2 batteries. (h) Morphology of KO2 limited by kinetics and diffusion. Images in panels a−g adapted with permission from ref 23. Copyright 2017 American Chemical Society. Image in panel h adapted with permission from ref 26. Copyright 2018 John Wiley and Sons.
blue-colored solvated electrons, which induce solvent decomposition. In contrast, addition of 1 M KTFSI in tetraglyme effectively prevents contact between the K metal and the solvent. In Figure 3c,d, the surface layer formed in KPF6 electrolyte grows to 500 μm due to continuous K consumption, while only a thin layer (2−3 μm) was observed with the KTFSI-based electrolyte. Furthermore, improved cycling stability of over 90 cycles was achieved without an additional membrane (Figure 3e). Surface characterization on the cycled anode also suggests that the influence of O2 and O2− was largely inhibited. Nevertheless, three-electrode measurement confirmed that this compact surface layer has low K+ conductivity and significantly increased the overpotential, which diminished the advantageous energy efficiency of K−O2 batteries.17
layer are believed to cause rapid and spontaneous anode corrosion, which reduces both shelf life and cycle life. The main inorganic components were characterized as KO2, KPF6, KOH, and K2CO3 (>80 wt %). Organic fragments including formate, acetate, and methoxyacetate are formed by DME decomposition.11,12 In the mechanistic study, the surface layer originates from the inherent interaction between potassium metal and ether molecules. Immersing K metal into pure ether solvent renders the solvent a characteristic blue color due to the solvated electrons from the dissolution of K metal.13 Generation of solvated electrons and potassides (negatively charged potassium ions) is caused by the strong chelation between oxygen atoms in ether molecules and K+. The solvated electron and potasside species are highly reductive, capable of cleaving the C−O bonds in ether molecules. Reduction of shuttled O2 into O2− also leads to the formation of KO2 on the anode surface. Lau and Curtiss further used DFT calculations to probe the reaction mechanism (Figure 2a), which showed that the electron transfer process from negatively charged DME to O2 molecules is highly exothermic.11 Its coupling with the subsequent H-abstraction reaction by O2− promotes the decomposition of DME and the formation of side products. A Nafion-K+ membrane was further incorporated as the separator to suppress oxygen crossover from the cathode. In Figure 2b,c, the additional oxygen-blocking membrane increased the cycle life to over 40 cycles with a barely increased overpotential.
3.3. Artificial SEI and State-of-the-Art
Besides KPF6 and KTFSI salts, we also investigated the bis(fluorosulfonyl)imide (KFSI)−DME electrolyte in potassium secondary batteries. Among the examined electrolytes in Figure 3f, FSI− anions demonstrated superior compatibility with K metal and enabled highly reversible, dendrite-free plating and stripping via buildup of a stable SEI with abundant fluoride species. An electrochemical window up to 5 V (vs K/K+) and compatibility with a high-voltage cathode in K-ion batteries have also been confirmed with concentrated KFSI−DME electrolyte.18 Nevertheless, FSI− was found to be chemically unstable with KO2 and KOH due to the nucleophilic attack on the S−F bond.19 An artificial SEI strategy was employed to circumvent this electrolyte dilemma (anode versus cathode compatibility). As illustrated in Figure 3g, cycling and electrodeposition in KFSI−DME electrolyte produced a KF-rich SEI on the plated K anode. It was then transferred to K−O2 cells with the KTFSI− DME electrolyte. The artificial SEI created by the KFSI-based electrolyte (Figure 3h,i) significantly reduces the overpotential compared to the pristine K anode and doubles the cycle life of K−O2 batteries (over 200 cycles, 800 h).19 By comparing the electrochemical performance of different electrolytes, we realized the fundamental challenge in designing
3.2. Surface Passivation by Impermeable Layer
Although different strategies have been demonstrated to alleviate alkali metal corrosion in metal−O2 batteries including membranes and concentrated electrolytes,11,14−16 a stable SEI is strongly desired to fundamentally eliminate K metal corrosion in the presence of electrolytes and reactive oxygen species (O2 and O2−). Among evaluated electrolyte formulations, potassium bis(trifluoromethanesulfonyl)imide (KTFSI) was found to generate a glyme- and O2-impermeable SEI.17 As shown in Figure 3a,b, K metal can dissolve in pure tetraglyme and form D
DOI: 10.1021/acs.accounts.8b00332 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research Table 1. Different Alkali Metal−O2 Batteries and Their Electrochemical Reactions battery chemistry
discharge product
overall reaction
ΔrG (kJ/mol)
electrons transferred
E° (V)
energy density (Wh/kg)
K−O2 battery Na−O2 battery
KO2 Na2O2 NaO2 Li2O2
K + O2 → KO2 2Na + O2 → Na2O2 Na + O2 → NaO2 2Li + O2 → Li2O2
−239.4 −449.7 −218.8 −570.8
1 2 1 2
2.48 2.33 2.27 2.96
935 1605 1108 3458
Li−O2 battery
a low discharge rate, the deposited KO2 crystals are large and thus passivate less surface area, while at higher rates, the KO2 crystals are small and densely packed onto the carbon electrode surface impeding the ORR. Furthermore, agglomeration of KO2 crystals near the oxygen-rich side could block oxygen diffusion and suffocate the battery through a severe oxygen gradient across the cathode.23 Therefore, the capacity of a K−O2 battery is inversely correlated with the discharge rate. In addition, the capacity is also highly dependent upon the surface area and pore size distribution of the electrode. An ideal cathode should not only supply enough surface area for the ORR and KO2 deposition but also ensure sufficient oxygen transport to maximize power. Two-dimensional materials such as reduced graphene oxide (rGO) offer large and accessible surface area owing to high aspect ratio and unrestricted pores, which benefits both rate performance and capacity.23 The growth of KO2 crystals after initial nucleation may involve two different mechanisms (Figure 4f,g): (1) direct oxygen reduction on KO2 surface and (2) solution transfer of superoxide anions.23 The first mechanism, where electrons transfer through the surface of KO2 crystals and react with dissolved O2 and K+, is governed by the electric conductivity of KO2. Bruce et al. verified that the surface route of the cathode discharge reaction can achieve large discharge capacity in K−O2 batteries, which is not limited by the growth of thin metal oxide layers (
