The present disclosure is in the field of composite solid-state electrolytes, and more particular in the field of composite solid-state electrolytes with improved ionic conductivity.
Solid-state lithium batteries are regarded as the future of energy storage due to their advantages in safety and energy density. The key to the success of solid-state batteries is the implementation of a highly conductive solid electrolyte. A polymer electrolyte is one of the top candidates for achieving this outcome. However, polymer electrolytes traditionally suffer from low ionic conductivity (<10−5 S/cm), especially at room temperature, since ion transport in a conventional polymer electrolyte depends on segmental motion of the polymer chain.
In one aspect, the present disclosure is directed to electrodes useful in electrochemical cells. The electrodes may include a polymer electron donor, an electron acceptor, a lithium salt, and a solvent in some embodiments. In select embodiments, the polymer electron donor may be polyphenylene sulfide (PPS), polymethylphenylsilane (PMPS), and/or poly(ethylene oxide) (PEO). In these and other embodiments, the electron acceptor may be Chloranil, Fluoranil, N,N′-bis(2-phosphonoethyl)-1,4,5,8-naphthalenediimide (PNDI), 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), and/or an oxidizing agent. The lithium salt may be Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), in some embodiments. The solvent may be one or more of the following: tetrahydrofuran (THF), SN, EC, BMP TFSI (IL), and/or G4. The components of the electrode may or may not form a charge-transfer complex (CTC).
In select embodiments, the electrolytes may include a charge-transfer complex polymer (CTCP) and one or more additives to achieve high local lithium concentration and endow fast lithium mobility. In some such embodiments, the CTCP enhances the high local lithium concentration due to an overlapping of a double electric layer. According to some implementations, the high local lithium concentration originating from the CTCP and the high lithium mobility originating from the addition of one or more additives provides high lithium-ion conductivity.
In another aspect, an improved polymer electrolyte that uses block copolymers composed of monomers is described in which one of the monomers contains electron-rich pi systems and the other of the monomers contains electron-poor pi systems. The block copolymers may be combined with a salt to form the polymer electrolyte. In some embodiments, one of the polymers containing electron-rich pi systems is combined and blended with another polymer containing electron-poor pi systems. The blended polymers are combined with a salt to create a polymer electrolyte. The disclosed mixtures of electron-poor pi groups and electron-rich pi grounds are capable of dissociating the salts more easily than either of the polymer blocks could on its own. According to another aspect of the present disclosure, the disclosed polymer/salt matrix can be maintained above the glass transition temperature to work well and have even further improved ionic conductivity. Compared to previously known techniques, embodiments of the present disclosure have the advantage of not requiring additional dissociating solvents such as carbonates, water or nitriles to provide sufficient ionic conductivity at room temperature.
The disclosed polymer electrolytes may have an ionic conductivity of at least 1×10−4 S/cm or at least 1×10−3 S/cm at room temperature (25° C.). In select embodiments, the polymer electrolyte may contain between 0.5 wt %-50 wt % solvent, such as between 0.5 wt %-5 wt %, 0.5 wt %-15 wt %, 5 wt %-25 wt %, or 10 wt %-30 wt % solvent.
The presently disclosed electrolytes can be prepared by any suitable technique. For example, in some embodiments, the electrolytes are prepared by speed mixing. In select embodiments, a high shear mixer is used to prepare the electrolytes.
These and other aspects, features, advantages, and objects will be further understood and appreciated by those skilled in the art upon consideration of the following specification and enclosed drawings.
The present disclosure includes a composite solid-state electrolyte with improved ionic conductivity. The solid-state electrolyte can achieve high local lithium concentration with high lithium mobility. In some embodiments, a charge-transfer complex polymer (CTCP) with additives that endow fast lithium mobility is used to form a polymer electrolyte. However, in other embodiments, the polymer electrolyte does not form a charge-transfer complex (CTC) and an oxidizer is used to cause charge delocalization to provide improved ionic conductivity. Numerous variations are possible and discussed in detail herein.
As a preliminary matter, charge separation between electron donors and electron acceptors can be used to form a charge transfer complex (CTC).
It is expected that the negative surface charges will therefore adsorb positive ion (Li+ in case of lithium salt) to counter-balance the negative surface charge. Some of the Li+ will be transiently physiosorbed to the surface forming a Stern layer while other Li+ ions will form an layer with rapid thermal motion, therefore forming a diffuse electric double layer (EDL), as shown in
The concentration of Li+(ρx) in the EDL (Boltzmann distribution) should be expressed as:
ρx=ρ∞e−φ
Where x is the distance from the surface; φx is the electrostatic potential at position x. As shown in
Therefore, when the polymer chains are close enough to each other, meaning that the EDL from the two surfaces starts to overlap, the concentration of Li+ between polymer chains would be expected to be greatly enhanced and the concentration of counter ions became lower than that in the bulk electrolyte (see
According to Goyu-Chapman model, the size of the EDL is inversely proportional to the concentration of lithium ions:
κ−1=(ϵ0ϵkT/2ρ∞e2)1/2
As a result, for Debye length to effectively overlap at practical concentration, the length between polymer chains need to be sub-nanometer.
According to
the conductivity is positively correlated to the local concentration of lithium ions (c) and the diffusivity. With minor amounts of solvent/additives to guarantee diffusivity, the presence of the CTC gives rise to increased conductivity.
According to the Jorne model, the transference number (ti) is expressed in the following equation:
where F is the Faraday constant, ui is the ion mobility, ciavg is the average ion concentration, q2 is the constant surface charge density, λ is the Deby length, kavg is the average conductivity, μ is the viscosity, ro is the radius of the pore, I0, I1, I2 are the modified Bessel functions of the first kind of the order zero, one and two. When the surface charge is negative and the pore size and the Debye screening length is about the same order (ro/λ˜I), the transference number of the cation will approach 1.
Based on this information, for a charge-transfer co-polymer electrolyte (CTCP) to exhibit enhanced conductivity, three factors are advantageous:
According to the implementations provided by the present disclosure, various embodiments of CTCP co-electrolytes can be prepared by adding certain amount of succinonitrile, tetracyanoethelated pentaerythritol and BMP-TFSI (ionic liquid) respectively to a charge-transfer complex polymer (CTCP) comprising poly(dimethyl substituted phenylene sulfide), chloranil and LiTFSI. The CTCP increases local dielectric constant and local lithium concentrations while the additives increased the lithium mobility. As a result, the CTCP co-electrolytes show >10−4 S/cm conductivity at room temperature.
Historically, conventional polymer electrolytes rely solely on segmental motion of the polymer chains, and therefore, the conductivity is limited by the nature of polymer and is generally <10−5 S/cm at RT. In contrast, the lithium transport of the present disclosure is decoupled from segmental motion of the backbone. The CTCP serves as a local lithium-ion concentration enhancer and the interface between the CTCP and additives (e.g., solvent) serves as a pathway for lithium ion with high mobility. As used herein, the term “CTCP” refers to a polymer having both an electron donor and an electron acceptor. One or both of the electron donor and electron acceptor may be polymers. In select embodiments, a CTCP may include a polymeric electron donor and a small molecule electron acceptor. As a result, the CTCP has the potential to reach higher ionic conductivity and transference numbers than conventional polymer electrolyte while maintaining solid-state form.
In some embodiments, the polymer electrolyte comprises, consists of, or consists essentially of: a polymer electron donor, an electron acceptor, a lithium salt, and a solvent.
In select embodiments, the polymer electron donor may be polyphenylene sulfide (PPS), polymethylphenylsilane (PMPS), and/or poly(ethylene oxide) (PEO). In these and other embodiments, the electron acceptor may be Chloranil, Fluoranil, N,N′-bis(2-phosphonoethyl)-1,4,5,8-naphthalenediimide (PNDI), 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), and/or an oxidizing agent. Any type of oxidizing agent that can act as an electron acceptor may be used. For example, in some embodiments, the oxidizing agent may be iodine, 1,4-Benzoquinone (BQ), chloral, Tetracyanoquinodimethane (TCNQ), DDQ, chloranilic acid, and/or any polymeric version of these organic electron acceptors. The lithium salt may be Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), in some embodiments. The solvent may be one or more of the following: tetrahydrofuran (THF), SN, EC, BMP TFSI (IL), and/or G4.
In some embodiments, the polymer electrolyte comprises one or more block copolymers composed of monomers in which one of the monomers contains electron-rich pi systems and the other of the monomers contains electron-poor pi systems. Monomers with electron-rich pi systems include vinyl Imidazole and N-Vinyl Carbazole. Monomers with electron-poor pi systems include methylene glutaronitrile, cinnamonitrile, butyl methacrylate, thiazolo[5,4-d]thiazole, benzo[1,2-d:4,5-d′]bisthiazole, naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole, and thieno[3,2-b]thiophene-2,5-dione.
The block copolymers may be combined with a salt (e.g., a lithium-ion salt) to form the polymer electrolyte. The salt may be LiTFSI, if desired. In some embodiments, one of the polymers containing electron-rich pi systems is combined and blended with another polymer containing electron-poor pi systems. The blended polymers are combined with a salt to create a polymer electrolyte.
The disclosed mixtures of electron-poor pi groups and electron-rich pi grounds are capable of dissociating the salts more easily than either of the polymer blocks could on its own. In some embodiments, the polymer/salt matrix can be maintained above the glass transition temperature to work well and have even further improved ionic conductivity.
The disclosed polymer electrolytes may have an ionic conductivity of at least 1×10−4 S/cm or at least 1×10−3 S/cm at room temperature (25° C.). In select embodiments, the polymer electrolyte may contain between 0.5 wt %-50 wt % solvent, such as between 0.5 wt %-5 wt %, 0.5 wt %-15 wt %, 5 wt %-25 wt %, or 10 wt %-30 wt % solvent.
The presently disclosed electrolytes can be prepared by any suitable technique. For example, in some embodiments, the electrolytes are prepared by speed mixing. In select embodiments, a high shear mixer may be used to prepare the electrolytes. The electrolytes may be prepared by ultrasonic mixing and heat melting/mixing, if desired. In these and other embodiments, the electrolytes may be formed by wet spray coating, drop casting, and/or dip coating.
Exemplary charge transfer complexes (CTCs) were formed containing a lithium-ion salt and tested against comparative examples without a lithium-ion source present. In particular, mixtures containing PMPS/Chloranil in THE with and without LiTFSI were created and UV/Vis spectra were obtained for each mixture using THF. The UV/Vis spectra are shown in
Compared to PMPS alone and chloranil alone, the spectrum of PMPS mixed with chloranil showed a characteristic absorbance signal at 600 nm at a molar ratio of 4/1 and as the ratio of chloranil increased to 4/1, the intensity of the absorbance signal also increased, which is indicative of the formation of charge transfer complex in solution. However, when LiTFSI are added, the characteristic peaks disappeared. This could be due to adsorption of Li+ to the PMPS backbone and TFSI− to chloranil, which disrupts the formation of CTC.
The observations suggest that PMPS and chloranil should have potential to form charge-transfer complex at solid state. And due to the limited diffusion rate of lithium salt at dryer state, even with LiTFSI, charge-transfer complex could still form.
The Zeta potentials for each mixture were also calculated, and the values are shown below in Table 1.
In all samples shown in Table 1, a PMPS concentration of 3 μg/mL was used.
When chloranil was added with or without LiTFSI, the Zeta potential reversed either from negative to positive or from positive to negative. Especially when there is only PMPS and LiTFSI dispersed in ethanol, the surface of the particles exhibited negative charges, indicating that the diffuse layer of PMPS is TFSI− dominant. When chloranil is added, the surface of particles become negatively charged and the vicinity of the surface (diffuse layer) become Li+ dominant. This suggests that the addition of chloranil enabled charge separation at the polymer surface, and thus a lithium-dominant surface.
In this experimental example, the effect of solvent amount and type was studied. Various polymer electrolyte mixtures were prepared by speed mixing. First, LiTFSI and solvent were speed-mixed at rpm of 2750 for 10 min to form a homogeneous liquid. Then polymer powder and chloranil powder were added to the LiTFSI/solvent solution, and speed mixed again at 2750 rpm for 10 min. The as-prepared mixture was then kept at 80° C. overnight to facilitate formation of a charge-transfer complex. The ionic conductivity of the resulting mixture was then measured with different solvent amounts.
In both cases of PPS and PMPS, at a molar ratio of sulfur/chloranil/LiTFSI=4/1/1.4, the more solvent that was added, the higher the ionic conductivity that resulted. At 20 wt % of G4, both PMPS/chloranil/LiTFSI (
In this experimental example, the ionic conductivity of different types of solvents (G4, EC, IL at 10 wt %) were evaluated.
In this experimental example, the effect of amount of salt was evaluated.
In this experimental example, the effects of acceptor quantity and type were considered.
In
In this experimental example, vinyl Imidazole (an electron-rich π-donor group), methylene glutaronitrile (an electron-poor π-acceptor group) and LiTFSI (a salt) are dissolved in a solution of THF. An initiator, such as AIBN is added, and the mixture heated at 65C until polymerized. In solution, the MGN and Vim pair up to form a charge transfer complex. On removal of the solvent, a homogenous orange plastic remains, and the salt is dissociated. The resulting polymer has ionic conductivity of 5×10−4 S/cm.
In this experimental example, N-Vinyl Carbazole (an electron-rich π-donor group), cinnamonitrile (an electron-poor π-acceptor group) and Zinc Triflate (a salt) are dissolved in a solution of THF. An initiator, such as AIBN is added, and the mixture heated at 65C until polymerized. In solution, the VCz and CNN pair up to form a charge transfer complex and the color changes from colorless to purple. On removal of the solvent, a homogenous purple plastic remains. The resulting polymer has ionic conductivity of 6×10−6 S/cm.
In this experimental example, N-Vinyl Carbazole (an electron-rich π-donor group) and butyl methacrylate (an electron-poor π-acceptor group) are dispersed in a solution of THF. An initiator such as AIBN is added and the mixture heated at 65C until polymerized. On removal of the solvent, a homogenous white plastic remains. 98% sulfuric acid is added and mixed into the polymer forming a green solid, which is then dried at 120° C. overnight. The resulting polymer has ionic conductivity of 1.5×10−4 S/cm.
This application claims the benefit of U.S. Provisional Patent Application No. 63/330,940, titled CHARGE-TRANSFER POLYMER CO-ELECTROLYTES, filed Apr. 14, 2022, and U.S. Provisional Patent Application No. 63/443,538, titled POLYMER ELECTROLYTE COMBINING ELECTRON POOR AND ELECTRON RICH PI GROUPS, filed Feb. 6, 2023, the disclosures of which are herein incorporated by reference in their entireties.
Number | Date | Country | |
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63330940 | Apr 2022 | US | |
63443538 | Feb 2023 | US |