The flammability of conventional alkyl carbonate electrolytes hinders the integration of large-scale lithium-ion batteries in transportation and grid storage applications. In this study, we have prepared a unique nonflammable electrolyte composed of low molecular weight perfluoropolyethers and bis(trifluoromethane)sulfonimide lithium salt. These electrolytes exhibit thermal stability beyond 200°C and a remarkably high transference number of at least 0.91 (more than double that of conventional electrolytes). Li/LiNi 1/3 Co 1/3 Mn 1/3 O 2 cells made with this electrolyte show good performance in galvanostatic cycling, confirming their potential as rechargeable lithium batteries with enhanced safety and longevity.fluorinated polymers | nonflammable electrolytes L arge-scale rechargeable batteries are expected to play a key role in today's emerging sustainable energy landscape (1, 2). State-of-the-art lithium (Li) batteries not only are used to power zero-emission electric vehicles, but they currently are gaining traction as backup power in aircraft and smart grid applications (3, 4). The electrolyte used in these batteries, however, hinders their use in large-scale applications: it contains a flammable mixture of alkyl carbonate solvents that frequently leads to safety issues. Dimethyl carbonate (DMC), an important component in commercial Li-ion battery electrolytes, has an HMIS (Hazardous Materials Identification System) flammability rating of 3 on a scale of 0-4, indicating a high risk of ignition under most operating conditions. The intrinsic instability of carbonate-based solvents worsens at higher temperatures, at which exothermic electrolyte breakdown often leads to thermal runaway (5, 6), resulting in catastrophic failure of the battery. Although this failure rate stands at about one in ten million systems, it is intolerable for large-scale applications in which cost and user safety might be heavily compromised. This necessitates the development of radically new electrolytes with improved safety.Desirable electrolyte properties include a large window of phase stability (no vaporization or crystallization), complete nonflammability, a wide electrochemical stability window, and suitable ionic transport for the targeted application. There are many approaches to synthesizing materials with these properties, e.g., ionic liquids (7, 8), gel-polymer matrices (9, 10), and small molecule additives (11-13). Systems using poly(ethylene oxide) (PEO) also are well studied (14,15). PEO can solvate high concentrations of lithium salts and is considered nonflammable. Unfortunately, practical conductivity often is limited within a high temperature range (14), and it is well known that in these systems, the motion of the Li ion carries only a small fraction of the overall current (also known as the Li-ion transference number, t + ). PEO-based electrolytes typically exhibit t + values between 0.1 and 0.5 (16-20), leading to strong salt concentration gradients across the electrolytes that limit power density. Recently, we repor...
Particle replication in nonwetting templates (PRINT) is a continuous, roll-to-roll, high-resolution molding technology which allows the design and synthesis of precisely defined micro- and nanoparticles. This technology adapts the lithographic techniques from the microelectronics industry and marries these with the roll-to-roll processes from the photographic film industry to enable researchers to have unprecedented control over particle size, shape, chemical composition, cargo, modulus, and surface properties. In addition, PRINT is a GMP-compliant (GMP = good manufacturing practice) platform amenable for particle fabrication on a large scale. Herein, we describe some of our most recent work involving the PRINT technology for application in the biomedical and material sciences.
Electrolytes consisting of low molecular weight perfluoropolyether (PFPE), poly(ethylene glycol) (PEG), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) blends were prepared and systematically studied for salt concentration and stoichiometry effects on the materials' thermal and electrochemical properties. Herein we report that the tunable ratios of PFPE and PEG allow for precise control of crystalline melting and glass transition temperature properties. These blended liquid polymer electrolytes are inherently nonflammable and remain stable in the amorphous phase from approximately 150°C down to −85°C. The ionic conductivity of the electrolytes are on the order of 10 −4 S/cm at 30°C, which makes them suitable for rechargeable lithium batteries.
Despite high ionic conductivities, current inorganic solid electrolytes cannot be used in lithium batteries because of a lack of compliance and adhesion to active particles in battery electrodes as they are discharged and charged. We have successfully developed a compliant, nonflammable, hybrid single ion-conducting electrolyte comprising inorganic sulfide glass particles covalently bonded to a perfluoropolyether polymer. The hybrid with 23 wt% perfluoropolyether exhibits low shear modulus relative to neat glass electrolytes, ionic conductivity of 10 −4 S/cm at room temperature, a cation transference number close to unity, and an electrochemical stability window up to 5 V relative to Li + /Li. X-ray absorption spectroscopy indicates that the hybrid electrolyte limits lithium polysulfide dissolution and is, thus, ideally suited for Li-S cells. Our work opens a previously unidentified route for developing compliant solid electrolytes that will address the challenges of lithium batteries.hybrid electrolytes | inorganic sulfide glasses | fluorinated polymers | lithium batteries | lithium-sulfur batteries E lectrolytes used in lithium ion batteries that power personal electronic devices and electric vehicles comprise lithium salts dissolved in flammable organic liquids. Catastrophic battery failure often begins with the electrolyte decomposition and combustion. In addition, side reactions between the electrolyte and anode particles result in steady capacity fade. Some of the byproducts of side reactions can dissolve in the electrolyte and migrate from one electrode to the other. This effect is minimized in the case of solid electrolytes because of limited solubility and slow diffusion (1). Mixtures of liquids and salts have additional limitations. The passage of current results in an accumulation of salt in the vicinity of one electrode and depletion close to the other electrode, because only the cation participates in the electrochemical reactions. Both overconcentrated and depleted electrolytes have lower conductivity, which accentuates cell polarization and reduces power capability. Concentration polarization is absent in single-ion conductors, wherein the anions are immobilized (2). Nonflammable, single ionconducting solid electrolytes have the potential to dramatically improve safety and performance of lithium batteries (3-6).Solid electrolytes, such as inorganic sulfide glasses (Li 2 S-P 2 S 5 ), are single-ion conductors with high shear moduli (18-25 GPa) and high ionic conductivity (over 10 −4 S/cm) at room temperature (7,8). However, these materials, on their own, cannot serve as efficient electrolytes, because they cannot adhere to moving boundaries of the active particles in the battery electrode as they are charged and discharged. Hayashi et al. (9) prepared hybrid electrolytes by mixing sulfide glasses and poly(ethylene oxide) (PEO) polymers. Although the addition of PEO improves mechanical flexibility, there is a dramatic decrease in ionic conductivity because of the insulative nature of PEO. For exampl...
We prepared nonflammable liquid polymer electrolytes for lithium-ion batteries by mixing ethoxylated perfluoropolyethers (PFPEs) with LiN(SO 2 CF 3) 2 salt. Interestingly, we identified the presence of chain coupling in the PFPE polymers and their functionalized derivatives, resulting in a mixture of PFPEs with varying molecular weights. The distribution of molecular weights, along with PFPE's multiple functionalities, allows systematic manipulation of structure to enhance electrochemical and physical properties. Furthermore, the electrolytes exhibited a wide thermal stability window (5% degradation temperature > 180°C). Despite substantial increases in viscosity upon loading the PFPEs with lithium salt, the conductivity (σ≈5x10-5 S cm-1 at 28°C) of the novel electrolytes was about an order of magnitude higher than that of our previously reported PFPE electrolytes. Ethoxylated derivatives of PFPE electrolytes exhibit elevated conductivity compared to non-ethoxylated derivatives, demonstrating our capability to enhance the conductive properties of the PFPE platform by attaching various functional groups to the polymer backbone.
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