Gel polymer electrolytes comprising electrolyte additive

Abstract

Systems and methods which provide an aqueous gel polymer electrolyte having one or more additive therein selected to configure the aqueous gel polymer electrolyte, and batteries formed therewith, for improved performance are described. Aqueous gel polymer electrolytes may, for example, have an additive compound including boron (e.g., a borate ion-containing salt) therein to configure batteries formed using the aqueous gel polymer electrolyte to increase the ionic conductivity of the gel polymer electrolyte. The addition of borax in Zinc-ion battery gel electrolytes of embodiments is configured to enhance the dissociation of zinc ions and anions, and subsequently release more mobile zinc ions. Furthermore, the interaction between borax and divalent transition metal (Zn) in electrolyte according to embodiments may enhance the transportation of mobile zinc ions.

Claims

1. An aqueous gel polymer electrolyte comprising: an aqueous electrolyte solvent including at least one of a divalent zinc salt having a concentration of 0.2-3 M or a bivalent manganous salt solution having a concentration of 0.05-0.5 M; a crosslinked polymer matrix, the crosslinked polymer matrix having a zinc salt solvent dispersed therein, wherein the aqueous electrolyte solvent is introduced into the crosslinked polymer matrix and forms an aqueous gel polymer; and an electrolyte additive compound configured to increase ionic conductivity of the aqueous gel polymer electrolyte, wherein the electrolyte additive compound comprises sodium tetraborate decahydrate (Na.sub.2B.sub.4O.sub.7.10H.sub.2O), wherein the electrolyte additive compound is introduced into the aqueous gel polymer and forms the aqueous gel polymer electrolyte.

2. The aqueous gel polymer electrolyte of claim 1, wherein the aqueous electrolyte solvent comprises at least one bivalent manganous salt, wherein the bivalent manganous salt solution has a concentration of 0.05-0.5 M.

3. The aqueous gel polymer electrolyte of claim 1, wherein the crosslinked polymer matrix comprises at least one polymer selected from the group consisting of: polyethylene oxide (PEO); polypropylene oxide (PPO); polyvinyl alcohol (PVA); polyacrylamide (PAM); polyacrylonitrile (PAN); polymethyl methacrylate (PMMA); polyacrylic acid (PAA); gelatin; and starch.

4. A rechargeable Zinc-ion battery comprising: a cathode; an anode; and an aqueous gel polymer electrolyte comprising a crosslinked polymer matrix comprising an aqueous gel polymer, an aqueous electrolyte solvent including at least one of a divalent zinc salt and a bivalent manganous salt dispersed in the aqueous gel polymer, and an electrolyte additive compound including boron, wherein the electrolyte additive compound comprises tetraborate decahydrate (Na.sub.2B.sub.4O.sub.7.10H.sub.2O) and the electrolyte additive compound is configured to increase ionic conductivity of the aqueous gel polymer electrolyte.

5. The rechargeable Zinc-ion battery of claim 4, wherein the rechargeable Zinc-ion battery is a rechargeable Zn—MnO.sub.x battery.

6. The rechargeable Zinc-ion battery of claim 5, wherein the cathode comprises a current collector coated with a manganese dioxide (MnO.sub.2) material and one or more binders.

7. The rechargeable Zinc-ion battery of claim 5, wherein the anode comprises a current collector coated with a zinc material.

8. The aqueous gel polymer electrolyte of claim 1, wherein the crosslinked polymer matrix comprises a gel monomer, crosslinking agent, and initiator mixed to form the crosslinked polymer matrix.

9. The aqueous gel polymer electrolyte of claim 1, wherein the crosslinked polymer matrix comprises gelatin, and wherein the electrolyte additive compound reduces a crystallinity of the gelatin.

10. The aqueous gel polymer electrolyte of claim 1, wherein the aqueous gel polymer electrolyte is formed via steps including: mixing gel monomer, crosslinking agent, and initiator with aqueous electrolyte solvent for forming an aqueous gel polymer electrolyte mixture; including an electrolyte additive compound including boron in the aqueous gel polymer electrolyte mixture to provide the aqueous gel polymer electrolyte having the electrolyte additive compound therein; and curing the aqueous gel polymer electrolyte having the electrolyte additive compound therein.

11. The aqueous gel polymer electrolyte of claim 10, wherein the gel monomer, crosslinking agent, and initiator form the crosslinked polymer matrix.

12. The aqueous gel polymer electrolyte of claim 1, wherein the aqueous gel polymer electrolyte comprises 0.2 w % to 1 wt % tetraborate decahydrate (Na.sub.2B.sub.4O.sub.7.10H.sub.2O).

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:

(2) FIG. 1 shows an exemplary Zn-ion battery implementation comprising gel polymer electrolyte with electrolyte additive according to embodiments of the present invention;

(3) FIG. 2 shows a method for forming a gel polymer electrolyte comprising electrolyte additive according to embodiments of the present invention;

(4) FIG. 3 shows the Nyquist plot of an exemplary gel polymer electrolyte with borax additive and without borax additive;

(5) FIGS. 4A and 4B showing the Nyquist plot of exemplary gel polymer electrolyte with borax additive and without borax additive;

(6) FIG. 5 shows a comparison of the zinc ion transference number for exemplary gel polymer electrolyte with borax additive and gel polymer electrolyte without borax additive;

(7) FIGS. 6A and 6B show the Raman spectra of exemplary gel polymer electrolyte with borax additive and without borax additive; and

(8) FIG. 7 shows the imaginary impedance of exemplary gel polymer electrolyte with borax additive and without borax additive.

DETAILED DESCRIPTION OF THE INVENTION

(9) Electrochemical rechargeable energy storage devices may provide a suitable source of energy in a number of emerging areas. For example, Zinc-ion (Zn-ion) rechargeable batteries, produced primarily from zinc (Zn) and manganese dioxide (MnO.sub.2), provide an attractive energy storage platform due to their safe nature resulting from their use of aqueous electrolyte and stable Zn metal anode material. Embodiments of the present invention provide gel polymer electrolytes comprising electrolyte additive, such as for use in the above mentioned rechargeable Zinc-ion batteries.

(10) FIG. 1 shows an exemplary Zn-ion battery implementation according to embodiments of the present invention. In particular, Zn-ion battery 100 of the embodiment illustrated in FIG. 1 comprises anode 110, cathode 120, and electrolyte 130 provided in a cooperative relationship operative to function as a rechargeable energy storage device.

(11) Anode 110 and cathode 120 may, for example, be comprised of various materials, such as flexible conductive yarns (e.g., stainless steel yarn, carbon nanotube (CNT), Nickel-Titanium-based alloy wire, etc.), in various shapes and sizes coated (e.g., using an electrodeposition process) with a suitable material to provide electrodes for a flexible Zn-ion battery configuration. For example, anode 110 may comprise one of the above mentioned conductive materials providing a current collector coated with a zinc material (e.g., zinc, zinc alloy, zinc composites, etc.). Correspondingly, cathode 120 may comprise one of the above mentioned conductive materials providing a current collector coated with a manganese oxide (MnO.sub.x) (e.g., manganese dioxide (MnO.sub.2), such as in the form of MnO.sub.2 nanocrystallines) which can store and release zinc ions (e.g., Zn.sup.2+ ions). Accordingly, embodiments of Zn-ion battery 100 may provide a rechargeable Zn—MnO.sub.x battery configuration. It should be appreciated that, although the zinc materials of anode 110 may provide very good conductivity, the MnO.sub.x based material of cathode 120 of embodiments suffers from relatively low conductivity, thus potentially resulting in poor energy storage performance. Thus, embodiments of the present invention introduce a conductive polymer, such as polypyrrole (PPy), as both a binder and conductive additive with respect to the aforementioned coating of cathode 120.

(12) Electrolyte 130 may be combined with the foregoing cathode and anode electrodes to provide a Zn-ion battery implementation in accordance with the concepts herein. For example, electrolyte 130 may comprise an aqueous electrolyte, such as a polymer gel electrolyte having an additive therein selected to configure the battery for improved performance. Electrolyte 130 of embodiments may comprise an aqueous gel polymer electrolyte having an electrolyte additive compound including boron, such as a borate ion-containing salt (e.g., borax (e.g., sodium tetraborate decahydrate (Na.sub.2B.sub.4O.sub.7.10H.sub.2O)), potassium tetraborate (K.sub.2B.sub.4O.sub.7), etc.) and/or boric acid (H.sub.3BO.sub.3), included therein as an additive to configure Zinc-ion battery 100 for improved electrochemical performance. The use of borax as the above mentioned borate ion-containing salt of the electrolyte additive in accordance with the concepts herein is very cost effective and is environmentally friendly and a readily reproducible natural resource.

(13) In accordance with embodiments of the invention, the addition of borax in Zinc-ion battery gel electrolytes enhances the dissociation of Zn.sup.2+ and anions, and subsequently releases more mobile zinc ions. Furthermore, the interaction between borax and divalent transition metal (Zn) in electrolyte according to embodiments enhances the transportation of mobile zinc ions. Accordingly, the electrochemical performance of Zinc-ion battery 100 is improved at all levels based on the optimized material of electrolyte 130 including the borax additive in accordance with embodiments herein.

(14) Directing attention to FIG. 2, a method for forming a gel polymer electrolyte comprising electrolyte additive, such as may provide embodiments of electrolyte 130 above, is shown. In accordance with flow 200 illustrated in FIG. 2, electrolyte 130 of embodiments comprises an aqueous gel polymer electrolyte formed from an aqueous electrolyte solvent, a polymer matrix having dispersed therein a zinc salt solvent, and one or more electrolyte additive including a borate ion-containing salt.

(15) In accordance with the foregoing, at block 201 of the illustrated embodiment of flow 200 gel polymer material is provided for forming an aqueous gel polymer as a gel base for the gel polymer electrolyte. For example, a suitable gel polymer may be obtained by mixing gel monomer, crosslinking agent, and initiator to form a polymer matrix. The polymer matrix of an aqueous gel polymer of embodiments may, for example, comprise one or more of polyethylene oxide (PEO), polypropylene oxide (PPO), polyvinyl alcohol (PVA), polyacrylamide (PAM), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), polyacrylic acid (PAA), gelatin, and/or starch.

(16) An aqueous electrolyte solvent is introduced with respect to the gel polymer for forming an aqueous gel polymer electrolyte at block 202 illustrated in FIG. 2. The aqueous electrolyte solvent of embodiments may, for example, comprise at least one divalent zinc salt (e.g., a zinc salt solution having a concentration of 0.2-3 M). Additionally or alternatively, the aqueous electrolyte solvent may comprise at least one bivalent manganous salt (e.g., a manganous salt solution having a concentration of 0.05-0.5 M).

(17) An electrolyte additive compound including boron, such as a borate ion-containing salt and/or boric acid, is introduced with respect to the gel polymer for forming an aqueous gel polymer electrolyte having improved performance due to increased ionic conductivity of the gel polymer electrolyte at block 203 of the embodiments illustrated in FIG. 2. The electrolyte additive compound including boron may, for example, comprise a borate ion-containing salt electrolyte additive, such as sodium tetraborate decahydrate (Na.sub.2B.sub.4O.sub.7.10H.sub.2O). For example, sodium tetraborate decahydrate in an amount of 0.2 wt % to 1 wt % based on the total amount of the aqueous gel polymer electrolyte may be provided in accordance with embodiments of the invention.

(18) At block 204 of the illustrated embodiment, the gel polymer, aqueous electrolyte solvent, and electrolyte additive compound including boron are mixed to form electrolyte 130 of embodiments herein. For example, the gel polymer, aqueous electrolyte solvent, and electrolyte additive may be mixed until all ingredients are totally dissolved, as may be indicated by obtaining a clear solution. For some gel polymers which are hardly dissolved in cold water (e.g., gelatin), mixing may be performed under heating (e.g., 80° C. for gelatin).

(19) It should be appreciated that the functions of low 200 set forth with respect to blocks 201-204 of the illustrated embodiment may be combined and/or performed in an order different than that of the example. For example, the functions of block 201-204 may be implemented in a process for synthesis of gel polymer electrolyte wherein gelatin powders of a gel polymer are added to an aqueous electrolyte (e.g., 4 g gelatin powders added in 40 mL 1 M ZnSO.sub.4 solution), wherein an electrolyte additive compound including boron (e.g., 0.2 g borax powders) is additionally added into the aqueous electrolyte solution for forming a gel polymer electrolyte with electrolyte additive. Thereafter, the solution may be heated (e.g., to 80° C.) under stirring until a clear solution of gel polymer electrolyte with borax additive is obtained. As can be appreciated from the foregoing, using compounds including boron as electrolyte additive according to embodiments need not introduce any significant extra processes in production of gel polymer electrolyte (e.g., introduction of a borax additive may be accomplished by adding an appropriate amount of a suitable borax material with the gel polymer and aqueous electrolyte materials when forming a gel polymer electrolyte).

(20) The aqueous gel polymer electrolyte provided in accordance with techniques of flow 200 may be cured, such as at room temperature or high temperature, to form a solid state electrolyte at block 205 of the illustrated embodiment of flow 200. In accordance with embodiments of the invention, anode 110, cathode 120, and electrolyte 130 comprising a gel polymer electrolyte produced using the foregoing exemplary technique may be combined to produce a Zinc-ion battery implementation. For example, anode 110 may be encapsulated with a portion of the gel polymer electrolyte of electrolyte 130 and cathode 120 may likewise be encapsulated with a portion of the gel polymer electrolyte of electrolyte 130, wherein the encapsulated anode and encapsulated cathode may be disposed adjacent to each other to form an embodiment of Zn-ion battery 100. The combination of anode 110 and cathode 120 each encapsulated by electrolyte 130 acting as a separator may further be encapsulated with the gel polymer electrolyte of electrolyte 130 as a binder. Thereafter, electrolyte 130 may be cured as described above to form a solid-state implementation of Zn-ion battery 100.

(21) The use of an electrolyte additive compound including boron in an aqueous gel electrolyte according to embodiments of the invention increases the number of mobile zinc ions in the electrolyte and enhances the transportation of mobile zinc ions in electrolyte. The effects of the addition of borax enhancing the ionic conducting performance of gel polymer electrolytes may be seen in the graphs of FIG. 3. In particular, FIG. 3 shows the Nyquist plot of an exemplary gel polymer electrolyte with borax additive (graph 310) and without borax additive (graph 320), wherein the inset shows magnification of the range between 0-9Ω. In the example implementation from which the data of graphs 310 and 320 were derived, two pieces of stainless steel sheets were utilized as electrodes. It was observed that, in middle frequency range, the gel polymer electrolyte with borax exhibits faster charge transference than that of the gel polymer electrolyte without borax. This indicates that the addition of borax will enhance the mobility of zinc ions in the electrolyte. A comparison of graph 310 (gel polymer electrolyte with borax additive) and graph 320 (gel polymer electrolyte without borax additive) reveals that the ionic conductivity of the exemplary gel polymer electrolyte increases more than 27% after adding borax. In particular, the first intersection point with real axis of the gel polymer electrolyte with borax is 27% smaller than that of the gel polymer electrolyte without borax. Accordingly, considering the identical dimension and thickness of the electrolytes, the ionic conductivity of the gel polymer electrolyte with borax is 27% higher than that of the gel polymer electrolyte without borax. Accordingly, compared to the existing technology, the ionic conductivity of a gel polymer electrolyte increases more than 27% after adding borax in accordance with concepts herein, thus resulting in better electrochemical performances of Zinc-ion rechargeable batteries formed using such gel polymer electrolytes with borax additive.

(22) Analysis of ion transference with respect to Zn.sup.2+ ions (see Y. Y. Lu, M. Tikekar, R, Mohanty, K. Hendrickson, L. Ma, L. A. Archer, Stable Cycling of Lithium Metal Batteries Using High Transference Number Electrolytes, Adv. Energy Mater. 5, 1402073 (2015) and W. Liu, S. W, Lee, D. C. Lin, F. F. Shi, S. Wang, A. D, Sendek, Y. Cui, Enhancing ionic conductivity in composite polymer electrolytes with well-aligned ceramic nanowires. Nat. Energy 2, 17035 (2017), the disclosures of which are incorporated herein by reference) is helpful in understanding the enhanced ionic conductivity provided by embodiments of a borax additive according to embodiments. The Zn.sup.2+ transference number of gel polymer electrolyte with and without borax may be derived in accordance with the following equation:

(23) $\begin{array}{cc}{t}_{{\mathrm{Zn}}^{2+}}=\frac{1}{2}\frac{I^s}{I^0}\frac{\Delta V-I^U{R}_i^0}{\Delta V-I^S{R}_i^S}& \left(1\right)\end{array}$
where I.sup.s and I.sup.0 are the currents in the polarized and unpolarized states respectively, ΔV is the potential difference (50 mV) used in test, and R.sub.i is the interfacial resistance. R.sub.i may be derived from the impedance spectra shown in FIGS. 4A and 4B showing the Nyquist plot of exemplary gel polymer electrolyte with borax additive and without borax additive, wherein the exemplary gel polymer electrolyte is sandwiched by two zinc foils and the potential bias (50 mV) applied. Curves 401 (i.e., 401a in FIG. 4A and 401b in FIG. 4B)) are measured at initial state. Curves 402 (i.e., 402a in FIG. 4A and 402b in FIG. 4B) are measured after applying a small voltage bias (50 mV) on the cell for 1500 s. For both impedance spectra, the first intersection point with real axis is originated from the bulk resistance (R.sub.b). The second intersection point is the sum of the bulk and interfacial resistance (R.sub.b+R.sub.i). Therefore, the interfacial resistance (R.sub.i) is obtained by subtracting value of R.sub.b from R.sub.b+R.sub.i.

(24) FIG. 5 shows a comparison of the zinc ion transference number for the foregoing exemplary gel polymer electrolyte with borax additive and gel polymer electrolyte without borax additive. As can be seen from the graphs of FIG. 5, the Zn.sup.2+ transference number increases about 45% (from 0.33 to 0.48) after adding the borax into the gel polymer electrolyte.

(25) FIGS. 6A and 6B show the Raman spectra of the exemplary gel polymer electrolyte with borax additive (FIG. 6A) and without borax additive (FIG. 6B). As can be seen in FIGS. 6A and 6B, two bands centered at 983.6 and 987.1 cm.sup.−1 are assigned to free anions (SO.sub.4.sup.2−) that do not directly interact with Zn.sup.2+ and contact ion pairs of Zn.sup.2+SO.sub.4.sup.2−, respectively (see A. C. Hayes, P. Kruus, W. A. Adams, Raman-Spectroscopic Study of Aqueous (Nh4)2so4 and Znso4 Solutions. J. Solution Chem. 13, 61-75 (1984), the disclosure of which is incorporated herein by reference). This reveals that the addition of a borax additive to the gel polymer electrolyte results in the increase of free anions together with the decrease of the contact ion pairs, indicating that borax will enhance the dissociation of Zn.sup.2+ and SO.sub.4.sup.2−, and subsequently release more mobile zinc ions. The increased number of mobile zinc ions could be attributed to the reduction of crystallinity of gelatin by adding borax. Furthermore, the interaction between borax and divalent transition metal (Zn) in electrolyte could enhance the transportation of mobile zinc ions (see N. A. Choudhury, S. Sampath, A. K. Shukla, Hydrogel-polymer electrolytes for electrochemical capacitors: an overview. Energ. Environ, Sci. 2, 55-67 (2009), the disclosure of which is incorporated herein by reference).

(26) For further investigate the mechanism of zinc-ion transportation, FIG. 7 shows the imaginary impedance of the exemplary gel polymer electrolyte with borax additive and without borax additive. In particular, the imaginary impedance is plotted as a function of frequency (Debye plot) and fitted with a Lorentzian function in FIG. 7. As may be seen from the graphs of FIG. 7, the frequency of peak maxima, which is associated with the conductivity relaxation of the electrolyte, shifts toward higher frequencies on adding borax, implying a faster ion conduction introduced by borax (see W. Liu, D. C. Lin, Sun, G. M. Zhou, Y. Cui, Improved Lithium Ionic Conductivity in Composite Polymer Electrolytes with Oxide-Ion Conducting Nanowires. ACS Nana 10, 11407-11413 (2016), the disclosure of which is incorporated herein by reference).

(27) As can be appreciated from the foregoing, embodiments of the present invention provide a relatively inexpensive, environmentally friendly additive for high ion conductive polymer electrolyte. Application of embodiments of the present invention provide a gel polymer electrolyte configuration in which a borax additive is utilized to facilitate increased ionic conductivity of aqueous gel polymer electrolyte for Zinc-ion battery implementations, whereby the electrochemical performances of solid-state aqueous Zinc-ion rechargeable batteries are improved. Zinc-ion rechargeable battery configurations using borax as an electrolyte additive for aqueous gel polymer electrolyte according to the concepts herein remain stable for a long period of time, provide high energy density, low cost, and comprise a commercially viable energy storage solution.

(28) Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.