Ionic liquid inspired zwitterions with high conductivity and transport number

#The present disclosure relates to electrolytes and methods of making the same.

#(Note: This application references a number of different references as indicated throughout the specification by one or more reference numbers in parentheses, e.g., (x). A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these references is incorporated by reference herein.)

#The efficient and safe storage of electrochemical energy is critical for emerging technologies such as electric vehicles and portable electronic devices. Practical requirements for next-generation secondary batteries include higher energy densities and charge-discharge rates, which hinge on new high-voltage cathode materials and on electrodes and electrolyte components with high ionic conductivities (a). The wide operating potential window of such cells exceeds the safe operation limits of current organic solvent-based electrolytes and necessitates the design of alternative electrolytes that are leak-proof, electrochemically stable and nonflammable. Solid Polymer Electrolytes (SPEs) have attracted significant interest in this area for their stability and mechanical robustness. However, due to the strong coupling of polymer segmental relaxation rates with ion mobilities, it remains challenging to attain sufficiently high ionic conductivities across the battery operating temperature range without compromising the mechanical properties. What is needed are solid electrolytes with improved ion conduction and mechanical properties. The present disclosure satisfies this need.

#Referring now to the drawings in which like reference numbers represent corresponding parts throughout:

#FIG. 1: A schematic representation of the path of an ion through (A) a typical solid polymer electrolyte, in which ion transport is coupled to the timescale of local rearrangements, and (B) an ordered solid with sufficient free volume to enable superionic transport. The atoms of a crystal are typically confined to specific lattice positions, allowing sufficiently small ions with a low charge (blue spheres) to diffuse though the matrix (red spheres) via successive discrete hops involving vacant lattice sites or interstitial sites, while the motion of larger or highly charged ions (green sphere) is excluded. (C) The zwitterionic polymer studied herein comprises IL-inspired ions tethered to the backbone. Mobile ions are doped in through the addition of a Li+TFSI− salt at select molar ratios. D) The DC ionic conductivity of salt-containing polymeric zwitterions (PZIs) shown as a function of temperature is comparable to that of PEO (a best-in-class SPE)(24). E) While standard SPEs have molar ionic conductivities coupled directly to the timescale of molecular rearrangements, (τg˜1/Λ), the salt-containing ZIs demonstrate decoupling of these properties. Amorphous poly(ethylene oxide)(PEO) and poly(propylene glycol) (PPG) serve as reference SPEs, and ((Ag)0.5—(AgPO3)0.5) systems are typical superionic inorganic solids (literature data on PEO, PPG, and Ag systems from Sokolov(1)).

#FIG. 2: Structural analysis of the PZI indicates the presence of two phases, one containing an ordered structure very similar to the small molecule on which the PZI was based and another amorphous phase. A) WAXS curves demonstrate the presence of amorphous and ordered structures within the PZI sample. The crystalline peaks of a small molecule analog for this material appear to match well with the Bragg peaks of the PZI, suggesting a structural similarity between the structure of the small molecule crystal and the polymer. B) Schematic representation of the conduction mechanism in the polymer suggested by NMR Diffusometry. The amorphous domains are proposed to promote motion of lithium and its counterion through vehicular motion, the ordered domains are suggested to have high selectivity for lithium, transported via a ballistic conduction mechanism.

#In the following description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

#The present disclosure describes a solid electrolyte comprising zwitterionic compounds each comprising one or more cations and one or more anions. The zwitterionic compounds comprise at least one of zwitterionic molecules or charge neutral polymers comprising zwitterion pendants. An electrolyte salt is distributed through the solid such that the solid conducts the alkali metal ions obtained from the salt and the zwitterionic compounds each include zero or more amorphous regions and one or more crystalline regions characterized by a presence of Bragg diffraction peaks in an X-ray diffraction measurement of the solid. In one or more examples, the solid electrolyte has an ion conductivity of at least 10−4 S/cm at a temperature of 50 degrees Celsius when a transport number for the alkali metal ions is at least 0.5, and a molar ratio r of the salt to the zwitterionic monomer units in the charge neutral polymers is 0.9 or less and the molar ratio r of the salt to the zwitterionic units in the zwitterionic molecules is less than 0.9.

#Various examples of this solid electrolyte are described in the following examples.