Structure and Stability of Lithium Superoxide Clusters and Relevance to Li–O₂ Batteries

The discharge mechanism of a Li–O₂ battery involves lithium superoxide (LiO₂) radicals. In this Letter, we have performed high-level quantum chemical calculations (G4MP2) to investigate the structure and stability of LiO₂ clusters. The clusters have planar ring-shaped structures, high spins, and are thermodynamically more stable than LiO₂ dimer. The computed energy barrier for disproportionation of the larger clusters is also significantly higher than the corresponding barrier in the LiO₂ dimer (1.0 eV vs 0.5 eV). This means that disproportionation rate should be much slower if the reaction involves LiO₂ clusters other than the dimer. As a result, the clusters may survive long enough to be incorporated into the growing discharge product. These results are discussed in terms of recent experimental studies of the electronic structure and morphology of the discharge products in Li–air batteries

Electronic Structure of Lithium Peroxide Clusters and Relevance to Lithium–Air Batteries

The prospect of Li–air­(oxygen) batteries has generated much interest because of the possibility of extending the range of electric vehicles due to their potentially high gravimetric density. The exact morphology of the lithium peroxide formed during discharge has not been determined yet, but the growth likely involves nanoparticles and possibly agglomerates of nanoparticles. In this article, we report on density functional calculations of stoichiometric lithium peroxide clusters that provide evidence for the stabilization of high spin states relative to the closed shell state in the clusters. The density functional calculations indicate that a triplet state is favored over a closed shell singlet state for a dimer, trimer, and tetramer of lithium peroxide, whereas in the lithium peroxide monomer, the closed shell singlet is strongly favored. Density functional calculations on a much larger cluster, (Li₂O₂)₁₆, also indicate that it similarly has a high spin state with four unpaired electrons located on the surface. These results have been confirmed by higher level G4 theory calculations that indicate that the singlet and triplet states of the dimer are nearly equal in energy and that the triplet state is more stable than the singlet for clusters larger than the dimer. The high spin states of the clusters are characterized by O–O moieties protruding from the surface, which have superoxide-like characteristics in terms of bond distances and spin. The existence of these superoxide-like surface structures on stoichiometric lithium peroxide clusters may have implications for the electrochemistry of formation and decomposition of lithium peroxide in Li–air batteries including electronic conductivity and charge overpotentials

Interactions of Dimethoxy Ethane with Li₂O₂ Clusters and Likely Decomposition Mechanisms for Li–O₂ Batteries

One of the major problems facing the successful development of Li–O₂ batteries is the decomposition of nonaqueous electrolytes, where the decomposition can be chemical or electrochemical during discharge or charge. In this paper, the decomposition pathways of dimethoxy ethane (DME) by the chemical reaction with the major discharge product, Li₂O₂, are investigated using theoretical methods. The computations were carried out using small Li₂O₂ clusters as models for potential sites on Li₂O₂ surfaces. Both hydrogen and proton abstraction mechanisms were considered. The computations suggest that the most favorable decomposition of ether solvents occurs on certain sites on the lithium peroxide surfaces involving hydrogen abstraction followed by reaction with oxygen, which leads to oxidized species such as aldehydes and carboxylates as well as LiOH on the surface of the lithium peroxide. The most favorable site is a Li–O–Li site that may be present on small nanoparticles or as a defect site on a surface. The decomposition route initiated by the proton abstraction from the secondary position of DME by the singlet cluster (O–O site) requires a much larger enthalpy of activation, and subsequent reactions may require the presence of oxygen or superoxide. Thus, pathways involving proton abstraction are less likely than that involving hydrogen abstraction. This type of electrolyte decomposition (electrolyte with hydrogen atoms) may influence the cell performance including the crystal growth, nanomorphologies of the discharge products, and charge overpotential

Understanding Side Reactions in K–O₂ Batteries for Improved Cycle Life

Superoxide based metal–air (or metal–oxygen) batteries, including potassium and sodium–oxygen batteries, have emerged as promising alternative chemistries in the metal–air battery family because of much improved round-trip efficiencies (>90%). In order to improve the cycle life of these batteries, it is crucial to understand and control the side reactions between the electrodes and the electrolyte. For potassium–oxygen batteries using ether-based electrolytes, the side reactions on the potassium anode have been identified as the main cause of battery failure. The composition of the side products formed on the anode, including some reaction intermediates, have been identified and quantified. Combined experimental studies and density functional theory (DFT) calculations show the side reactions are likely driven by the interaction of potassium with ether molecules and the crossover of oxygen from the cathode. To inhibit these side reactions, the incorporation of a polymeric potassium ion selective membrane (Nafion-K⁺) as a battery separator is demonstrated that significantly improves the battery cycle life. The K–O₂ battery with the Nafion-K⁺ separator can be discharged and charged for more than 40 cycles without increases in charging overpotential

A Mo₂C/Carbon Nanotube Composite Cathode for Lithium–Oxygen Batteries with High Energy Efficiency and Long Cycle Life

Although lithium–oxygen batteries are attracting considerable attention because of the potential for an extremely high energy density, their practical use has been restricted owing to a low energy efficiency and poor cycle life compared to lithium-ion batteries. Here we present a nanostructured cathode based on molybdenum carbide nanoparticles (Mo₂C) dispersed on carbon nanotubes, which dramatically increase the electrical efficiency up to 88% with a cycle life of more than 100 cycles. We found that the Mo₂C nanoparticle catalysts contribute to the formation of well-dispersed lithium peroxide nanolayers (Li₂O₂) on the Mo₂C/carbon nanotubes with a large contact area during the oxygen reduction reaction (ORR). This Li₂O₂ structure can be decomposed at low potential upon the oxygen evolution reaction (OER) by avoiding the energy loss associated with the decomposition of the typical Li₂O₂ discharge products

Disproportionation in Li–O₂ Batteries Based on a Large Surface Area Carbon Cathode

In this paper we report on a kinetics study of the discharge process and its relationship to the charge overpotential in a Li–O₂ cell for large surface area cathode material. The kinetics study reveals evidence for a first-order disproportionation reaction during discharge from an oxygen-rich Li₂O₂ component with superoxide-like character to a Li₂O₂ component. The oxygen-rich superoxide-like component has a much smaller potential during charge (3.2–3.5 V) than the Li₂O₂ component (∼4.2 V). The formation of the superoxide-like component is likely due to the porosity of the activated carbon used in the Li–O₂ cell cathode that provides a good environment for growth during discharge. The discharge product containing these two components is characterized by toroids, which are assemblies of nanoparticles. The morphologic growth and decomposition process of the toroids during the reversible discharge/charge process was observed by scanning electron microscopy and is consistent with the presence of the two components in the discharge product. The results of this study provide new insight into how growth conditions control the nature of discharge product, which can be used to achieve improved performance in Li–O₂ cell

Exploring Stability of Nonaqueous Electrolytes for Potassium-Ion Batteries

Recently nonaqueous potassium-ion batteries (KIBs) have attracted tremendous attention, but a systematic study about the electrolytes remains lacking. Here, the stability of a commonly used electrolyte (KPF₆ in ethylene carbonate (EC) and diethyl carbonate (DEC)) at the anodes (e.g., graphite, solid K, and liquid Na–K alloy) was studied. Interesting results show that the linear DEC is unstable. Possibly attributed to stronger reducibility against the anodes for KIBs, the decomposition of DEC is initiated by the C­(H₂)–O bond breaking of the solvent molecule. This study shows that a systematic study to look for a more stable electrolyte is critically important for KIBs

Effect of Hydrofluoroether Cosolvent Addition on Li Solvation in Acetonitrile-Based Solvate Electrolytes and Its Influence on S Reduction in a Li–S Battery

Li–S batteries are a promising next-generation battery technology. Due to the formation of soluble polysulfides during cell operation, the electrolyte composition of the cell plays an active role in directing the formation and speciation of the soluble lithium polysulfides. Recently, new classes of electrolytes termed “solvates” that contain stoichiometric quantities of salt and solvent and form a liquid at room temperature have been explored due to their sparingly solvating properties with respect to polysulfides. The viscosity of the solvate electrolytes is understandably high limiting their viability; however, hydrofluoroether cosolvents, thought to be inert to the solvate structure itself, can be introduced to reduce viscosity and enhance diffusion. Nazar and co-workers previously reported that addition of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) to the LiTFSI in acetonitrile solvate, (MeCN)₂–LiTFSI, results in enhanced capacity retention compared to the neat solvate. Here, we evaluate the effect of TTE addition on both the electrochemical behavior of the Li–S cell and the solvation structure of the (MeCN)₂–LiTFSI electrolyte. Contrary to previous suggestions, Raman and NMR spectroscopy coupled with ab initio molecular dynamics simulations show that TTE coordinates to Li⁺ at the expense of MeCN coordination, thereby producing a higher content of free MeCN, a good polysulfide solvent, in the electrolyte. The electrolytes containing a higher free MeCN content facilitate faster polysulfide formation kinetics during the electrochemical reduction of S in a Li–S cell likely as a result of the solvation power of the free MeCN

Dendrite-Free Potassium–Oxygen Battery Based on a Liquid Alloy Anode

The safety issue caused by the dendrite growth is not only a key research problem in lithium-ion batteries but also a critical concern in alkali metal (i.e., Li, Na, and K)–oxygen batteries where a solid metal is usually used as the anode. Herein, we demonstrate the first dendrite-free K–O₂ battery at ambient temperature based on a liquid Na–K alloy anode. The unique liquid–liquid connection between the liquid alloy and the electrolyte in our alloy anode-based battery provides a homogeneous and robust anode–electrolyte interface. Meanwhile, we manage to show that the Na–K alloy is only compatible in K–O₂ batteries but not in Na–O₂ batteries, which is mainly attributed to the stronger reducibility of potassium and relatively more favorable thermodynamic formation of KO₂ over NaO₂ during the discharge process. It is observed that our K–O₂ battery based on a liquid alloy anode shows a long cycle life (over 620 h) and a low discharge–charge overpotential (about 0.05 V at initial cycles). Moreover, the mechanism investigation into the K–O₂ cell degradation shows that the O₂ crossover effect and the ether–electrolyte instability are the critical problems for K–O₂ batteries. In a word, this study provides a new route to solve the problems caused by the dendrite growth in alkali metal–oxygen batteries

Elucidating the Solvation Structure and Dynamics of Lithium Polysulfides Resulting from Competitive Salt and Solvent Interactions

Elucidating the Solvation Structure and Dynamics of Lithium Polysulfides Resulting from Competitive Salt and Solvent Interaction