Effect of Building Block Connectivity and Ion Solvation on Electrochemical Stability and Ionic Conductivity in Novel Fluoroether Electrolytes

Novel electrolytes are required for the commercialization of batteries with high energy densities such as lithium metal batteries. Recently, fluoroether solvents have become promising electrolyte candidates because they yield appreciable ionic conductivities, high oxidative stability, and enable high Coulombic efficiencies for lithium metal cycling. However, reported fluoroether electrolytes have similar molecular structures, and the influence of ion solvation in modifying electrolyte properties has not been elucidated. In this work, we synthesize a group of fluoroether compounds with reversed building block connectivity where ether moieties are sandwiched by fluorinated end groups. These compounds can support ionic conductivities as high as 1.3 mS/cm (30 °C, 1 M salt concentration). Remarkably, we report that the oxidative stability of these electrolytes increases with decreasing fluorine content, a phenomenon not observed in other fluoroethers. Using Raman and other spectroscopic techniques, we show that lithium ion solvation is controlled by fluoroether molecular structure, and the oxidative stability correlates with the “free solvent” fraction. Finally, we show that these electrolytes can be cycled repeatedly with lithium metal and other battery chemistries. Understanding the impact of building block connectivity and ionic solvation structure on electrochemical phenomena will facilitate the development of novel electrolytes for next-generation batteries

Noninvasive In Situ NMR Study of "Dead Lithium" Formation and Lithium Corrosion in Full-Cell Lithium Metal Batteries.

Capacity retention in lithium metal batteries needs to be improved if they are to be commercially viable, the low cycling stability and Li corrosion during storage of lithium metal batteries being even more problematic when there is no excess lithium in the cell. Herein, we develop in situ NMR metrology to study "anode-free" lithium metal batteries where lithium is plated directly onto a bare copper current collector from a LiFePO4 cathode. The methodology allows inactive or "dead lithium" formation during plating and stripping of lithium in a full-cell lithium metal battery to be tracked: dead lithium and SEI formation can be quantified by NMR and their relative rates of formation are here compared in carbonate and ether-electrolytes. Little-to-no dead Li was observed when FEC is used as an additive. The bulk magnetic susceptibility effects arising from the paramagnetic lithium metal were used to distinguish between different surface coverages of lithium deposits. The amount of lithium metal was monitored during rest periods, and lithium metal dissolution (corrosion) was observed in all electrolytes, even during the periods when the battery is not in use, i.e., when no current is flowing, demonstrating that dissolution of lithium remains a critical issue for lithium metal batteries. The high rate of corrosion is attributed to SEI formation on both lithium metal and copper (and Cu+, Cu2+ reduction). Strategies to mitigate the corrosion are explored, the work demonstrating that both polymer coatings and the modification of the copper surface chemistry help to stabilize the lithium metal surface.A.B.G acknowledges the support from the Royal Society (RP/R1/180147) and EPSRC-EP/M009521/1. C.V.A acknowledges financial support from the TomKat Center Postdoctoral Fellowship in Sustainable Energy at Stanford, and a Visiting Fellowship from Corpus Christi College at the University of Cambridge. S.M thanks the Blavatnik Cambridge Fellowships. C.P.G thanks the EU/ERC for an Advanced Fellowship. A.B.G thanks the NanoDTC Cambridge for travel funding

Influence of Ammonium Salts on Discharge and Charge of Li–O₂ Batteries

Li–air (O₂) batteries are promising because of their high theoretical energy density. However, these batteries are plagued with numerous challenges, one of which involves modulating the battery discharge process between a solution or surface-driven formation of the desired lithium peroxide (Li₂O₂) discharge product, and the oxidation of Li₂O₂ below 4 V (vs Li/Li⁺). In this work, we show that tetrabutylammonium (TBA) salts dissolved in ether or dimethyl sulfoxide (with no lithium salt present) can be used as a Li–O₂ electrolyte with a lithium metal anode to support Li₂O₂ formation, lead to >500 mV reduction in charging overpotentials at low current rates as compared to that with lithium salt, and support the oxidation of Li₂O₂ during charge. Furthermore, on the basis of results from several spectroscopic techniques, we propose a mechanism that involves electrochemical-induced transformation of TBA to tributylamine at ∼3.55 V, and the formation of a tributylamine oxide intermediate in the presence of O₂ or Li₂O₂ that is responsible for Li₂O₂ oxidation during charging. This mechanism can also be translated to other ionic liquid-based Li–O₂ batteries where significantly low charging potentials are observed. This work showcases an additive that can be used for Li–O₂ batteries to allow for finer control of the discharge process, and the ability of amine oxides to oxidize Li₂O₂

Fluorination promotes lithium salt dissolution in borate esters for lithium metal batteries

Lithium metal batteries promise higher energy densities than current lithium-ion batteries but require novel electrolytes to extend their cycle life. Fluorinated solvents help stabilize the solid electrolyte interphase (SEI) with lithium metal, but are believed to have weaker solvation ability compared to their nonfluorinated counterparts and are deemed ‘poorer electrolytes’. In this work, we synthesize tris(2-fluoroethyl) borate (TFEB) as a new fluorinated borate ester solvent and show that TFEB unexpectedly has higher lithium salt solubility than its nonfluorinated counterpart (triethyl borate). Through experiments and simulations, we show that the partially fluorinated –CH2F group acts as the primary coordination site that promotes lithium salt dissolution. TFEB electrolyte has a higher lithium transference number and better rate capability compared to methoxy polyethyleneglycol borate esters reported in the literature. In addition, TFEB supports compact lithium deposition morphology, high lithium metal Coulombic efficiency, and stable cycling of lithium metal/LiFePO4 cells. This work ushers in a new electrolyte design paradigm where partially fluorinated moieties enable salt dissolution and can serve as primary ion coordination sites for next-generation electrolytes

Instability of Poly(ethylene oxide) upon Oxidation in Lithium–Air Batteries

The instability of aprotic and polymer electrolytes in Li–air batteries limits the development of these batteries for practical use. Here, we investigate the stability of an electrolyte based on poly­(ethylene oxide) (PEO), which has been used extensively for polymer Li-ion batteries, during discharge and charge of Li–O₂ batteries. We show that applying potentials greater than open circuit voltage (OCV, ∼3 V_Li), which is typically required for Li–O₂ battery charging, increases the rate of PEO auto-oxidation in an oxygenated environment, with and without prior discharge. Analysis on the rate of reaction, extent of oxidation, and the oxidation products allows us to propose that rate of spontaneous radical formation in PEO is accelerated at applied potentials greater than OCV. We also suggest that the phenomena described here will still occur in ether-based electrolytes at room temperature, albeit at a slower rate, and that this will prevent the use of such electrolytes for practical long-lived Li–air batteries. Therefore, PEO-based electrolytes are unsuitable for use in Li–air batteries

Understanding the Chemical Stability of Polymers for Lithium–Air Batteries

Recent studies have shown that many aprotic electrolytes used in lithium–air batteries are not stable against superoxide and peroxide species formed upon discharge and charge. However, the stability of polymers often used as binders and as electrolytes is poorly understood. In this work, we select a number of polymers heavily used in the Li–air/Li-ion battery literature, and examine their stability, and the changes in molecular structure in the presence of commercial Li₂O₂. Of the polymers studied, poly­(acrylonitrile) (PAN), poly­(vinyl chloride) (PVC), poly­(vinylidene fluoride) (PVDF), poly­(vinylidene fluoride-<i>co</i>-hexafluoropropylene) (PVDF-HFP), and poly­(vinylpyrrolidone) (PVP) are reactive and unstable in the presence of Li₂O₂. The presence of the electrophilic nitrile group in PAN allows for nucleophilic attack by Li₂O₂ at the nitrile carbon, before further degradation of the polymer backbone. For the halogenated polymers, the presence of the electron-withdrawing halogens and adjacent α and β hydrogen atoms that become electron-deficient due to hyperconjugation makes PVC, PVDF, and PVDF-HFP undergo dehydrohalogenation reactions with Li₂O₂. PVP is also reactive, but with much slower kinetics. On the other hand, the polymers poly­(tetrafluoroethylene) (PTFE), Nafion, and poly­(methyl methacrylate) (PMMA) appear stable against nucleophilic Li₂O₂ attack. The lack of labile hydrogen atoms and the poor leaving nature of the fluoride group allow for the stability of PTFE and Nafion, while the methyl and methoxy functionalities in PMMA reduce the number of potential reaction pathways for Li₂O₂ attack in PMMA. Poly­(ethylene oxide) (PEO) appears relatively stable, but may undergo some cross-linking in the presence of Li₂O₂. Knowledge gained from this work will be essential in selecting and developing new polymers as stable binders and solid or gel electrolytes for lithium–air batteries

Phase Morphology Dependence of Ionic Conductivity and Oxidative Stability in Fluorinated Ether Solid-State Electrolytes

Solid-state polymer electrolytes can enable the safe operation of high energy density lithium metal batteries; unfortunately, they have low ionic conductivity and poor redox stability at electrode interfaces. Fluorinated ether polymer electrolytes are a promising approach because the ether units can solvate and conduct ions, while the fluorinated moieties can increase oxidative stability. However, current perfluoropolyether (PFPE) electrolytes exhibit deficient lithium-ion coordination and ion transport. Here, we incorporate cross-linked poly(ethylene glycol) (PEG) units within the PFPE matrix and increase the polymer blend electrolyte conductivity by 6 orders of magnitude as compared to pure PFPE at 60 °C from 1.55 × 10–11 to 2.26 × 10–5 S/cm. Blending varying ratios of PEG and PFPE induces microscale phase separation, and we show the impact of morphology on ion solvation and dynamics in the electrolyte. Spectroscopy and simulations show weak ion–PFPE interactions, which promote salt phase segregation into─and ion transport within─the PEG domain. These polymer electrolytes show promise for use in high-voltage lithium metal batteries with improved Li|Li cycling due to enhanced mechanical properties and high-voltage stability beyond 6 V versus Li/Li+. Our work provides insights into transport and stability in fluorinated polymer electrolytes for next-generation batteries

Evaluation and Stability of PEDOT Polymer Electrodes for Li–O₂ Batteries

Lithium–air (O₂) batteries have shown great promise because of their high gravimetric energy density—an order of magnitude greater than Li-ion—but challenges such as electrolyte and electrode instability have led to poor capacity retention and low cycle life. Positive electrodes such as carbon and inorganic metal oxides have been heavily explored, but the degradation of carbon and the limited surface area of the metal oxides limit their practical use. In this work, we study the electron-conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and show it can support oxygen reduction to form Li₂O₂ in a nonaqueous environment. We also propose a degradation mechanism and show that the formation of sulfone functionalities on the PEDOT surface and cleavage of the polymer repeat unit impairs electron conductivity and leads to poor cycling. Our findings are important in the search for new Li–O₂ electrodes, and the physical insights provided are significant and timely

Fluorinated Aryl Sulfonimide Tagged (FAST) salts: modular synthesis and structure–property relationships for battery applications

Solid-state electrolytes are attracting great interest for their applications in potentially safe and stable high-capacity energy storage technologies. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is widely used as a lithium ion source, especially in solid-state polymer electrolytes, due to its solubility and excellent chemical and electrochemical stability. Unfortunately, chemically inert LiTFSI cannot be easily modified to optimize its properties or allow for conjugation to other molecules, polymers, or substrates to prepare single-ion conducting polymer electrolytes. Chemical modifications of TFSI often Erode its advantageous properties. Herein, we introduce Fluorinated Aryl Sulfonimide Tagged (FAST) salts, which are derived from successive nucleophilic aromatic substitution (SNAr) reactions. Experimental studies and density functional theory calculations were used to assess the electrochemical oxidative stabilities, chemical stabilities, and degrees of ion dissociation of FAST salts as a function of their structures. FAST salts offer a platform for accessing functional sulfonimides without sacrificing many of the advantageous properties of TFSI

Probing Electrolyte Influence on CO₂ Reduction in Aprotic Solvents

Selective CO2 capture and electrochemical conversion are important tools in the fight against climate change. Industrially, CO2 is captured using a variety of aprotic solvents due to their high CO2 solubility. However, most research efforts on electrochemical CO2 conversion use aqueous media and are plagued by competing hydrogen evolution reaction (HER) from water breakdown. Fortunately, aprotic solvents can circumvent HER, making it important to develop strategies that enable integrated CO2 capture and conversion. However, the influence of ion solvation and solvent selection within nonaqueous electrolytes for efficient and selective CO2 reduction is unclear. In this work, we show that the bulk solvation behavior within the nonaqueous electrolyte can control the CO2 reduction reaction and product distribution occurring at the catalyst–electrolyte interface. We study different tetrabutylammonium (TBA) salts in two electrolyte systems with glyme ethers (e.g., 1,2 dimethoxyethane or DME) and dimethyl sulfoxide (DMSO) as a low and high dielectric constant medium, respectively. Using spectroscopic tools, we quantify the fraction of ion pairs that forms within the electrolyte. Also, we show how ion pair formation is prevalent in DME and is dependent on the anion type. More importantly, we show that as ion pair formation decreases within the electrolyte, CO2 current densities increase, and a higher CO Faradaic efficiency is observed at low overpotentials. Meanwhile, in an electrolyte medium where the ion pair fraction does not change with the anion type (such as in DMSO), a smaller influence of solvation is observed on CO2 current densities and product distribution. By directly coupling bulk solvation to interfacial reactions and product distribution, we showcase the importance and utility of controlling the reaction microenvironment in tuning the electrocatalytic reaction pathways. Insights gained from this work will enable novel electrolyte designs for efficient and selective CO2 conversion to desired fuels and chemicals