Al(III) incorporated li-ion based free-standing solid-GEL electrolyte and process for preparation thereof

Abstract

The present invention relates to novel transparent free-standing Al (III) incorporated Li-ion based solid-gel electrolyte for enhancement of optical contrast of electrochromic device. More particularly, the present invention relates to Al (III) incorporated Li-ion based free-standing solid-gel electrolyte for enhancement of optical contrast which comprises of a unique transparent, flexible, free-standing, solid-gel polymer electrolyte, its composition and process for preparation thereof. The unique Al(iii) incorporated Li-ion based free-standing solid-gel electrolyte reveals a high optical contrast, as well as faster switching kinetics from bleached state to colored state or vice versa on changing the external voltage. A significant increase in optical contrast from 50% to 82% is obtained with a faster switching kinetics.

Description

[0001] Al(III) INCORPORATED Li-ION BASED FREE-STANDING SOLID-GEL ELECTROLYTE AND PROCESS FOR PREPARATION THEREOF

[0002] FIELD OF INVENTION

[0003] The present invention relates to Al (III) incorporated Li-ion based free-standing solid-gel electrolyte for enhancement of optical contrast of electrochromic device. More particularly, the present invention relates to Al (III) incorporated Li-ion based free-standing solid-gel electrolyte for enhancement of optical contrast which comprises of a unique transparent, flexible, free-standing, solid-gel polymer electrolyte, its composition and process for preparation thereof. The unique Al(III) incorporated Li-ion based free-standing solid-gel electrolyte reveals a high optical contrast, as well as faster switching kinetics from bleached state to colored state or vice versa on changing the external voltage.

[0004] BACKGROUND OF INVENTION

[0005] The recent research reports have accepted that, worldwide, the buildings have contributed to about 20% of the total energy consumption and CO2 emission. The elevating rise of global warming demands for the development of smart glass windows which will not only reduce the energy consumption but also minimize the amount of heat loss to the outside during the winters, resist the heat to come from outside during the summers, thereby reducing the air-conditioner bills and protect the building from UV and IR radiation of sunlight

[0006] Now, development of smart windows is based on providing different types of glazing to the window glasses. These include, photochromic glazing, thermochromic glazing, PDLC devices etc. However, these types of glazing systems have their own limitations associated with them.

[0007] Photochromic glasses are known to have longer response time which increases during the colder season and t does not work in absence of light i.e., during night time.

[0008] Whereas, Thermochromic materials are capable of working only in presence of heat, thus in the winter time the thermochromic devices do not work efficiently.

[0009] For the PDLC glasses Film originally remains in opaque state, hence without application of appreciate amount of voltage visibility through the PDLC glasses is not possible. It requires a continuous supply of voltage in order to maintain the transparent state and does not block the IR light. Owing to their inherent drawbacks of these glazing systems, the electrochromic (EC) devices are now- a-days have navigated the modern research world towards its game changing features like low voltage, high contrast, long term memory effect etc. The first EC material based on WO3 was invented long back, and this has progressively spread over the smart world including the other metal oxides such as, V2O5, TiO2, Nb2O5 etc. Till date many modifications have been made regarding the tuning of properties of EC films. However, still the challenges remain in optical contrast, fast switching etc. In terms of the designing of EC device, it generally consists of a 5 -layered structure having, ITO (Indium tin oxide) coated glass / EC film / Electrolyte / Ion-storage layer / ITO. The high optical contrast will lead to change in color of the smart window from a nearly transparent to a much darker opaque state, which will not only maintain the privacy but also protect the interior from harmful UV of IR light. A novel electrolyte composition, can be a facile solution from the EC device fabrication perspective, and will also be retrofitted with the existing EC film, without any need of the replacement for the entire EC device. Therefore, the present invention has been focused on the development of an electrolyte product for voltage tunable electrochromic device in smart window applications.

[0010] Reference may be made to CN patent application CN103531840A, wherein an electrolyte used in Li- S battery, which comprises Li salt (LiClCL), Al salt, plasticizer (PMMA) and THF / propylene carbonate solvent. The drawbacks are the process of incorporation of Al ions in Li ions is not disclosed in the patent; also, the application area dictated here is for battery system and not for the EC devices.

[0011] Reference may be made to CN patent applicationCN 109721042A, wherein the concept of combination of Li- Al electrolyte is represented. The drawbacks are the source of Li, Al and synthesis method of electrolyte are different from the information disclosed in the present invention, and here also the application in an EC system is not revealed.

[0012] Reference may be made to CN patent application CN111584932A, wherein electrolyte with Li-Al combination salt as filler; LiCICL and PC as the main components. The drawbacks are, the preparation method mentioned thereof consisted of various complex layer-by-layer deposition techniques and also, the application area is limited to only solid-state lithium-ion batteries and the EC aspect of this has not explored.

[0013] Reference may be made to article published in Polym. Adv. Technol, 22(2011)1753-1759 by P.C. Barbosa et al, wherein a solid polymer electrolyte is prepared involving poly(ethylene oxide-co- propylene oxide) and poly(methyl methacrylate) host matrices for electrochromic application. The drawbacks are, that the EC system fabricated with the as prepared electrolyte showed a moderate optical modulation however, the use of poly( ethylene oxide-co-propylene oxide), in the long term can damage the electrochemical performance owing to its oxygen trapping tendency.

[0014] Reference may be made to article published in Journal of Materials Science: Materials in Electronics, 3(2016)28 by M. Solis et al., wherein a solid-gel like electrolyte comprising of LiCICM, PMMA and PC is developed for the electrochromic SiCh / PANI based system, it results into efficient EC performance. The drawbacks are, the electrolyte made of solely lithium perchlorate, thereby taking into account all the pitfalls of Li-ion and also, the prepared electrolyte is tested against a polymeric EC film, thus there left a doubt whether this would also be effective for inorganic transition metal oxidebased EC systems.

[0015] Reference may be made to article published in Electrochimica Acta, 260(2018)157-167 by P. Pal et al., wherein a LiCICM and PMMA based polymer is fabricated and its charge transfer performance has been thoroughly studied, which has also revealed a good electrical conductivity. The drawbacks are, the optical performance especially the % transmittance has not been studied which is an essential parameter for the evaluation of an EC device.

[0016] Reference may be made to article published in Journal of Chinese Chemical Society 61(2014)563-570 by Chung-Wen Kuo et al., wherein a solid-gel electrolyte made up of LiCICL, PMMA and PC has been employed to a polymer-based EC system, which results into an excellent coloration efficiency. The drawbacks are the optical contrast, which is the main concern of an EC smart window has reported to be very poor in this case.

[0017] Reference may be made to article published in Materials Today Proceedings, 23(2020) 352-358 by A.V. Kadam, wherein a Lithium perchlorate-propylene carbonatepoly(methyl methacrylate)- acetonitrile based electrolyte has been developed for a WO3 based EC device. The drawbacks are a faster switching kinetics is obtained with a compromised optical modulation of ~ 52%, which facilitates a poorer coloration and restricts the utilization of the EC device in the smart window application.

[0018] The following discussion presents a review of the existing literature pertaining to the production of graphene from coal, rendering an overview of the prior art references in this field. These references serve as a testament to the extensive research conducted in this area and provide valuable insights into the techniques and methodologies employed in the production process. The above information disclosed is only for the enhancement of understanding of the background of the invention. Thus, keeping in view the drawbacks of the hitherto reported prior arts and at least the aforementioned issues, developing of cost-effective methods of preparation of free-standing solid-gel electrolyte is still a critical challenge for commercial applications. On the other hand, some synthesis methods involve electrolyte showed a moderate optical modulation however, the use of poly(ethylene oxide-co- propylene oxide), in the long term can damage the electrochemical performance owing to its oxygen trapping tendency.Developing more environmentally friendly and sustainable synthesis approaches is a decisive focus in this free-standing solid-gel electrolyte having good electrical conductivity and optical performance.

[0019] In view of the above and obviate the drawbacks of prior arts, present invention relates to Al (III) incorporated Li-ion based free-standing solid-gel electrolyte for enhancement of optical contrast of electrochromic device / assembly. More particularly, the present invention relates to Al (III) incorporated Li-ion based free-standing solid-gel electrolyte for enhancement of optical contrast which comprises of a unique transparent, flexible, free-standing, solid-gel polymer electrolyte, its composition and process for preparation thereof. The unique Al(iii) incorporated Li-ion based freestanding solid-gel electrolyte reveals a high optical contrast, as well as faster switching kinetics from bleached state to colored state or vice versa on changing the external voltage.

[0020] OBJECTIVES OF THE INVENTION

[0021] The primary object of the present invention is to provide arelates to Al (III) incorporated Li-ion based free-standing solid-gel electrolyte for enhancement of optical contrast of electrochromic device / assembly.Another objective of the present invention is to provide a process for preparation of transparent aluminium incorporated Li-ion based free-standing solid-gel electrolyte for enhancement of optical contrast of electrochromic (EC) device / assembly comprising varying concentrations of lithium perchlorate (LiCICL) and aluminium sec-butoxide (C12H27O3AI) in weight ratios between the range of 0-4 wt%.

[0022] Yet another objective of the present invention is to provide a process for the transparent aluminium (Al3+) incorporated Li-ion based free-standing solid-gel electrolyte for enhancement of optical contrast of electrochromic window glass sheets

[0023] Yet another objective of the present invention is to provide Li- Al based free-standing solid-gel electrolytes which helps in the reduction of structural deterioration, relates to electrochemical cyclic stability, and is operable at ambient temperature and pressure conditions. Still another objective of the present invention is to provideelectrochromic (EC) device / assembly comprising of, sandwiching a five layered structure for modulation of the optical contrast between coloured and bleached state on application of small amount of voltage.

[0024] A further object of the present invention is to provide electrochromic device / assembly by sandwiching a five layered structure, consisting of ITO (Indium tin oxide) coated glass / EC film / prepared electrolyte / Ion-storage layer / ITO coated glass for modulation of the optical contrast between colored and bleached state and vice versa on application of small amount of external voltage.

[0025] These objectives of the present invention, as well as other objectives related thereto, will be readily apparent post consideration of the description of the invention, together with reference to the contents of the Figures of the drawings. Additional objects, advantages and other novel features of the invention will appear as the description proceeds and in part will become apparent to those skilled in the art upon examination of the following.

[0026] SUMMARY OF THE INVENTION

[0027] Additional features and embodiments of the present disclosure will be better understood through the techniques and other aspects of the disclosure. Other embodiments of the invention are described in detail herein and are considered a part of the claimed disclosure.

[0028] The present invention, and in accordance with main aspect of the present invention, achieves a transparent aluminium incorporated Li-ion based free-standing solid-gel electrolyte for enhancement of optical contrast of electrochromic device / assembly comprising varying concentrations of lithium perchlorate (LiC104) and aluminium sec-butoxide (C12H27O3AI) in weight ratios between the range of 0-4 wt%.

[0029] In another aspect of the present invention provides Li-Al based free-standing solid-gel electrolytes, wherein the ratio of Aluminium sec-butoxide and Lithium perchlorate is 1 : 1 wt % as optimize ratio which relates to the optical modulation from 50 % to 82 %, colouring efficiency in the range of 600 cm2 / C to 700 cm2 / C with a faster switching kinetics of 2.5 seconds.

[0030] In one of the embodiment of the present invention discloses a process for the preparation of transparent aluminium (Al3+) incorporated Li-ion based free-standing solid-gel electrolyte comprising the steps of: (a) employing lithium perchlorate (LiCIC ) and aluminium sec-butoxide (C12H27O3AI) as the electrolyte ion precursors to obtain a precursor mixture;

[0031] (b) adding the precursor mixture as obtained in step (a) with 0.25 ml of propylene carbonate (PC) along with 1 g of polymethyl methacrylate and kept in a hot-air oven at 110°C for 24 h to obtain a polymer matrix;

[0032] (c) dissolving the polymer matrix as obtained in step (b) in 20 ml of aprotic solvent and stirred for 24 h to obtain a prepared mixture;

[0033] (d) casting the prepared mixture as obtained in step (c) on a flat surface petri dish followed by covering the dish with aluminium foils and kept for 48 h for air drying to obtain a transparent solid;

[0034] (e) peeling off the transparent solid prepared as obtained in step (d) from the petri dish and cutting transparent free-standing solid-gel electrolyte of dimension 10 cm X 10 cm sheets.

[0035] In another embodiment of the present invention discloses the polymer matrix is having an ion mobilizing agents selected from the group consisting of phthalates, phosphates, carboxylic acid esters, epoxidized fatty acid esters, polymeric polyesters, modified polymers; liquid rubbers, and plastics, nitrile butadiene rubber (NBR), chlorinated PE, EVA, polymethyl methacrylate, propylene carbonate and a mixture thereof.

[0036] In yet another embodiment of the present invention discloses the aprotic solvent medium used in step (c) is selected from acetone, dimethylformamide DMF, tetrahydrofuran THF, and dichloromethane CH2C12, acetonitrile CH3CN, dimethyl sulfoxide DMSO, dimethylpropyleneurea, ethyl acetate, pyridine, sulfolane, hexamethyl phosphoramide and a mixture thereof.

[0037] In yet other embodiment of the present invention discloses the solid-gel electrolyte comprising Li- A1 solid-gel electrolyte with a polymeric network structure wherein the polymeric network structure is having circular pores and a porous matrix, occupying Li+and Al3+into the pores.

[0038] In other embodiment of the present invention discloses an electrochromic device prepared by sandwiching a five layered structure wherein the electrochromic device comprising:

[0039] (a) a counter and working electrode;

[0040] (b) electrochromic film; (c) a layer of Li-Al based transparent free standing solid-gel electrolyte sheet;

[0041] (d) ion-storage layer;

[0042] In yet other embodiment of the present invention discloses the process of preparation of the electrochromic device comprising the steps of: a. coating of Li-Al based transparent free-standing solid-gel electrolyte sheet with the ionstorage layer to obtain a coated transparent electrolyte sheet; b. putting the coated transparent electrolyte sheet as obtained in step (a) on the conducting side of the counter electrode; c. coating of the working electrode with the electrochromic film of rGO-WCh; d. sandwiching the transparent Al-Li based solid-gel electrolyte sheet of step (b) with working electrode of step (c) without air gap between the two electrodes.

[0043] In another aspect of the present invention provides a transparent aluminium (Al3+) incorporated Li-ion based free-standing solid-gel electrolyte, wherein aluminium (Al3+) incorporated Li-ion based freestanding solid-gel electrolyte, a broad integrated curve under cyclic voltammetry which relates to ion intercalation / de-intercalation resulting in an optical contrast of 50-82% with rGO-WCh / ITO working electrode (3.24 cm2) between the potential range -1 V to +1 V, it

[0044] In another aspect of the present inventionprovides a transparent aluminium (Al3+) incorporated Li-ion based free-standing solid-gel electrolyte, wherein aluminium (Al3+) incorporated Li-ion based freestanding solid-gel electrolyte shows minimal charge transfer resistance providing 50-82% optical contrast with small radius of semi-circle obtained from Electrochemical Impedance Spectroscopy (between Z’ / -Z” 0-140 ohm).

[0045] In another aspect of the present invention provides a transparent aluminium (Al3+) incorporated Li-ion based free-standing solid-gel electrolyte, wherein aluminium (Al3+) incorporated Li-ion based freestanding solid-gel electrolyte consists of Al3+which helps in the reduction of structural deterioration, relates to electrochemical cyclic stability, and is operable at ambient temperature and pressure conditions.

[0046] BRIEF DESCRIPTIONS OF THE DRAWING

[0047] To complete the description and in order to provide for a better understanding of the present invention, a set of drawings is provided. The drawings form an integral part of the description and illustrate an embodiment of the present invention, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be carried out. The drawings comprise the following figures:

[0048] Fig. 1 represents the process flow sheet for the synthesis of Al3+incorporated Li-ion based freestanding solid-gel electrolyte and electrochromic (EC) device / assembly

[0049] Fig. 2(a-b) represents the transparent free-standing solid-gel electrolyte product (10cm x 10cm) and the FESEM morphology of Li-Al based solid-gel electrolyte.

[0050] Fig.3 represents the cyclic voltammetry curves of rGO-WCh EC films using Li and Li-Al solid-gel electrolytes.

[0051] Fig.4 represents the enhancement of (a) optical modulation of rGO-WCh EC device with Li-Al based free-standing solid-gel electrolytes as compared to that, (b) conventional Li electrolyte.

[0052] Fig.5 represents the comparison of Electrochemical Impedance Spectroscopy of rGO-WCh EC film in Li and Li-Al based free-standing solid-gel electrolytes.

[0053] Fig. 6 represents schematic diagrams for the charge transfer mechanism of the typical EC devices with (a) Li and (b) Li-Al based free-standing solid-gel electrolytes

[0054] DETAILED DESCRIPTION OF THE INVENTION

[0055] The following description is not to be taken in a limiting sense but is given solely for the purpose of describing the broad principles of the invention. Embodiments of the invention will be described by way of example, with reference to the above-mentioned drawings showing elements and results according to the present invention.

[0056] The foregoing detailed description of the disclosure is elaborated to provide a clear understanding to the person who is skilled in the art. Additional features, embodiments and advantages of the invention will be described hereinafter which form the subject of the claims of the disclosure, However, the set forth disclosure provide in the specification will best be understood in conjunction with the appended claims and figures as provide heretofore. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent processes do not depart from the spirit and scope of the disclosure as set forth in the appended claims. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in various configurations, all of which are explicitly contemplated and make part of this disclosure.

[0057] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made, and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.

[0058] Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of "a", "an", and "the" include plural references. The meaning of "in" includes "in" and "on." Referring to the figures, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.

[0059] The present invention relates totransparent aluminium incorporated Li-ion based free-standing solidgel electrolyte for enhancement of optical contrast of ele ctro chromi c devi ce / a s s embly comprising varying concentrations of lithium perchlorate (LiC104) and aluminium sec-butoxide (C12H27O3AI) in weight ratios between the range of 0-4 wt%.

[0060] In an embodiment of the present inventionprovides Li-Al based free-standing solid-gel electrolytes, wherein the ratio of Aluminium sec-butoxide and Lithium perchlorate is 1 : 1 wt % as optimize ratio which relates to the optical modulation from 50 % to 82 %, colouring efficiency in the range of 600 cm2 / C to 700 cm2 / C with a faster switching kinetics of 2.5 seconds.

[0061] In an embodiment of the present invention, the process forthe transparent aluminium (Al3+) incorporated Li-ion based free-standing solid-gel electrolyte for enhancement of optical contrast of electrochromic device / assembly comprising the steps of: a. employing Lithium perchlorate (LiCICL) and Aluminium sec-butoxide (C12H27O3AI) as the electrolyte ion precursors b. adding the precursor mixture step (a), 0.25 ml of Propylene carbonate (PC) along with 1 g of Polymethyl methacrylate (kept in a hot-air oven at 110°C for 24 h), c. dissolving mixture obtained in step (b) in 20 ml of organic solvent Tetrahydrofuran (THF) and stirred for 24 h in order for uniform mixing, d. Casting the as prepared mixture in step (c) on a flat surface Petri dish, covering the dish with aluminium foils and kept for 48 h for air drying, e. peeling off transparent solid prepared in steps a-d from the Petri dish and cutting transparent free-standing solid-gel electrolyte of dimension 10 cm X 10 cm sheets.

[0062] In an embodiment of the present invention, the process for the transparent aluminium (Al3+) incorporated Li-ion based free-standing solid-gel electrolyte, wherein the ion mobilizing agents used in polymer matrix of step (b) is selected from phthalates, phosphates, carboxylic acid esters, epoxidized fatty acid esters, polymeric polyesters, modified polymers; liquid rubbers, and plastics, Nitrile Butadiene Rubber (NBR), chlorinated PE, EVA, Polymethyl methacrylate, Propylene carbonate and a mixture thereof.

[0063] In an embodiment of the present invention, the process for the transparent aluminium (Al3+) incorporated Li-ion based free-standing solid-gel electrolyte, wherein the aprotic solvent medium used in step (d) is selected from acetone, dimethylformamide DMF, tetrahydrofuran THF, and dichloromethane CH2C12, acetonitrile CH3CN, dimethyl sulfoxide DMSO, dimethylpropyleneurea, ethyl acetate, pyridine, sulfolane, hexamethylphosphoramide and a mixture thereof;

[0064] In yet another embodiment of the present inventionprovides a transparent aluminium (Al3+) incorporated Li-ion based free-standing solid-gel electrolyte, wherein aluminium (Al3+) incorporated Li-ion based free-standing solid-gel electrolyte, a broad integrated curve under cyclic voltammetry which relates to ion intercalation / de-intercalation resulting in an optical contrast of 50-82% with rGO- WO3 / ITO working electrode (3.24 cm2) between the potential range -1 V to +1 V, it

[0065] In yet another embodiment of the present inventionprovides a transparent aluminium (Al3+) incorporated Li-ion based free-standing solid-gel electrolyte, wherein aluminium (Al3+) incorporated Li-ion based free-standing solid-gel electrolyte shows minimal charge transfer resistance providing 50- 82% optical contrast with small radius of semi-circle obtained from Electrochemical Impedance Spectroscopy (between Z’ / -Z” 0-140 ohm).

[0066] In yet another embodiment of the present inventionprovides a transparent aluminium (Al3+) incorporated Li-ion based free-standing solid-gel electrolyte, wherein aluminium (Al3+) incorporated Li-ion based free-standing solid-gel electrolyte consists of Al3+which helps in the reduction of structural deterioration, relates to electrochemical cyclic stability, and is operable at ambient temperature and pressure conditions. In yet another embodiment of the present inventionprovides a transparent aluminium (Al3+) incorporated Li-ion based free-standing solid-gel electrolyte, wherein aluminium (Al3+) incorporated Li-ion based free-standing solid-gel electrolyte consisting of Li-Al solid-gel electrolyte with a polymeric network structure containing circular pores and a porous matrix, occupying Li+and Al3+into the pores, where the ionic radii of Li+facilitates lesser number of ion absorption within the same pore area as compared to that of Al3+ions consequential the more number of Al3+tungsten bronze complex than that of Li+, results into more intense blue colour relates to increased optical contrast of the EC assembly.

[0067] In yet another embodiment of the present inventionprovides a transparent aluminium (Al3+) incorporated Li-ion based free-standing solid-gel electrolyte, wherein An electrochromic (EC) device / assembly comprising of, sandwiching a five layered structure, consisting a layer of Li-Al based transparent free-standing solid-gel electrolyte sheet, a counter electrode, and working electrode between ITO glass on both sides for modulation of the optical contrast between coloured and bleached state on application of small amount of voltage.

[0068] Still in another embodiment of the present inventionprovides a process for preparation of electrochromic (EC) device / assembly comprising the step of a. coating of transparent electrolyte sheet is carefully put on the conducting side of the NiOx / ITO counter electrode, b. coating the electrochromic rGO-WOa on ITO working electrode, c. sandwiching the ALLi based solid-gel electrolyte sheet / NiOx / ITO counter electrode with rGO-WOs / ITO working electrode without air gap between the two electrodes.

[0069] Still in another embodiment of the present inventionprovides composition remodeled free-standing solid-gel electrolyte obtained by varying the concentration of Lithium perchlorate (LiC104), which has been initially kept at 110°C for 24 h in a vacuum oven and Aluminium sec-butoxide (Ci2H27O3Al)in the weight ratios of 1:0, 1: 1, 2:1, and 1:2. To the measured quantity of the precursor mixture, 0.25 ml of Propylene carbonate (PC) has been added along with 1 g of Polymethyl methacrylate (PMMA) (kept at 110°C for 24 h in a hot-air oven). This mixture is then dissolved in 20 ml of organic solvent Tetrahydrofuran (THF) and stirred for 24 h in order for uniform mixing. After 24 h the electrolyte solution has become crystal transparent and it is casted on flat surface Petri dish. These are covered with aluminium foils and kept for 48 h for air drying, with small perforations on it in order for the solvent (THF) to escape. This is kept undisturbed for 48 h. After 48 h, the liquid electrolyte is transformed into a transparent solid consistency which can be easily peeled off from the Petri dish (12 cm diameter). A transparent free-standing solid-gel electrolyte of dimension 10 cm X 10 cm is cut thereof. The product is found to be stable over a year at an ambient temperature and moisture condition. The morphology of the electrolyte film is analyzed by FESEM (Field Emission Scanning Electron Microscopy) and its thermal property are measured by TGA (Thermogravimetric Analysis) measurements.

[0070] For electrochromic [EC } device preparation, the transparent electrolyte sheet is carefully put on the conducting side of the counter electrode, which is previously coated with an ion-storage layer of NiOxon it, which is finally sandwiched with the working electrode coated with the electrochromic film of rGO-WCh in such a way that no air gap remained in between the two electrodes.

[0071] Cyclic Voltammetry (CV) measurement of the rGO-WCh film is done with the fabricated Li-Al and conventional Li electrolytes in a three-electrode system. The optical performance of the typical EC devices is measured with respect to the above-mentioned solid electrolytes. The detailed scheme is expressed in the following flow chart (Fig. 1).

[0072] Themorphological characterization of the Al (III) incorporated Li-ion based electrolyte is done by Field Emission Scanning Electron Microscope (FESEM) and thermal behavior is carried out with Thermogravimetric Analysis (TGA). The electrochromic performance with respect to electrochemical and optical characterizations of the as-fabricated EC device consisting of ITO coated glass / EC film / Electrolyte / Ion-storage layer / ITO coated glass are carried out employing Li and Li-Al electrolytes.

[0073] Fig. 2a demonstrates an image of the as-prepared electrolyte of 10cm xlOcm. Fig. 2b showing the FESEM micrograph of Li-Al solid-gel electrolyte clearly reveals a polymeric network structure containing circular pores. Fig. 3 depicts the electrochemical performance of the Li and Li-Al electrolytes with respect to Cyclic Voltammetry (CV) The larger integrated curve area of CV reveals much better ion intercalation / de- intercalation in case of Li-Al electrolyte. The Electrochemical Impedance Spectroscopy (EIS) plots are demonstrated in Fig. 4. The significantly smaller radius of semi-circle in the EIS spectra of the invented electrolyte relates to the lower charge transfer resistance. Together these correlates with the outstanding optical contrast represented in Fig. 5, and the lower charge transfer resistance also connects with the faster color switching response of the EC system. Fig. 6 a and b depict the schemes of charge transfer mechanism in a typical rGO-WOa EC device using Li and Li-Al electrolytes respectively. The solid-gel electrolyte, consisting of a porous matrix, as also observed from the FESEM image (Fig.-2a) can occupy Li+and Al3+into the pores, where the larger ionic radii of Li+facilitates lesser number of ion absorption within the same pore area as compared to that of Al3+ions. Thus, for Al3+the number of tungsten bronze complex formed is greater than that of Li+, which results into more intense blue color and as a consequence, the optical contrast of the EC assembly is increased.

[0074] The unique product of free-standing solid-gel electrolyte and the preparation method thereof has been disclosed by the invention is simple, convenient and has the excellent potential to enhance the optical contrast of a typical electrochromic device. Present invention describes a novel method of rectification of the Li-ion based electrolyte composition by insertion of trivalent cation [Al (III)]to produce a transparent stable and flexible free-standing electrolyte. The optimized composition of (1: 1 wt % ratio) the Al3+incorporated Li-ion based electrolyte is able to change the optical modulation from 50 % to 82 %, with a faster switching kinetics of2.5 s. Moreover, the present invention includes the first indigenously developed approach for the improvement of EC property with the help of electrolyte composition modification.

[0075] The synthesis process for the fabrication of the novel solid-gel electrolyte described in the present invention consisted of unique steps involving simple precursor combinations. The precursor materials used over the invention, are all operable at ambient temperature and pressure conditions.

[0076] For an electrochromic device / assembly, optical contrast i.e. the difference between the optical transmittance of the colored and the bleached state, is the most important property to be attained. In the present invention, both Li+and Al3+ions can occupy the pores of the polymer matrix. However, the smaller ionic radii of Al3+facilitated more ion absorption within the same pore area as compared to that obtained for pure Li-ion based electrolyte. Thus, for Al3+, the number of tungsten bronze complex formed is greater than that of Li+, which results into more intense blue color and as a consequence, the optical contrast of the EC assembly increases.

[0077] Li+ion is most widely used for electrolyte system in electrochromic devices but it is least abundant and hence quite costly. Therefore, requirement of alternative electrolytes is of high demand. Al3+ion based precursors are more abundant and more cost effective to be used in the electrochromic devices. However, the larger electrostatic interaction of Al3+with the electrochromic layer made us to optimize the concentration not beyond 1: 1. The present invention reduce the cost of electrolyte system to about 50 %.

[0078] The larger ionic radii of Li+can sometimes results into the deformation of crystal structure of the electrochromic (EC) material during repeated ion insertion / de-insertion cycle. The smaller Al3+radii help in the reduction of structural deterioration, thereby also acts to enhance the electrochemical cyclic stability.

[0079] The larger radii of Li+ion restricts the use of this electrolyte up to bulk materials, and causes to disintegrate the electrochromic EC properties of a material having a few nanometers size. Al3+in this regard is an established candidate for resonating with the nanocrystals of such materials. Hence, the present electrolyte composition can be well applied for materials bearing quantum confinements owing to their nanoscale dimension.

[0080] EXAMPLES

[0081] The following examples, which include preferred embodiments, will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purpose of illustrative discussion of preferred embodiments of the invention and therefore should not be construed to limit the scope of the present invention.

[0082] EXAMPLE-1

[0083] 0.1 g of Lithium perchlorate (LiClCL), 0.1 g of Aluminium sec-butoxide (C12H27O3AI), 1 g of Polymethyl methacrylate (PMMA), 0.25 ml of Propylene carbonate (PC) and 20 ml of Tetrahydrofuran (THF)were added into a 100 ml beaker. The mixture was stirred for 24 h in order for uniform mixing and forming a homogeneous transparent solution. After 24 h, the electrolyte solution was casted on a flat Petri-dish (12 cm diameter) and covered with Aluminium foil with small perforations on top of it. This was kept for 48 h for providing sufficient time to dry and take the form of transparent free-standing solid-gel electrolyte, which was carefully peeled out from the Petri dish.

[0084] ELECTROCHEMICAL CHARACTERIZATION

[0085] The redox performance of the EC films with the invented electrolyte composition was evaluated by Cyclic Voltammetry measurement in Electrochemical Workstation (Pine Instrument, USA, Wave Driver 100) using a three-electrode system. All the experiments were performed using a 3.24 cm2EC film area working electrode, saturated calomel electrode as the reference electrode and platinum as the counter electrode. The redox reaction was monitored by changing the potential from -1 V to +1 V. The acquired data was then analyzed by comparing the integrated hysteresis curve area of the rGO-WCh samples in presence of Li and Li-A electrolytes.

[0086] OPTICAL CHARACTERIZATION The optical transmittance spectra of the samples were measured simultaneously during the cyclic voltammetry experiment using an in-situ Spectrophotometer from Shimadzu coupled with the Pine Wave Driver. The transmittance changes occurred during coloration and bleaching cycles with respect to time revealed the change in optical transmittance i.e. the optical contrast of the rGO-WCh film in presence of both the conventional and invented electrolytes.

[0087] EXAMPLE-2

[0088] 0.133 g of Lithium perchlorate (LiClCL), 0.067 g of Aluminium sec-butoxide (C12H27O3AI), 1 g of Polymethyl methacrylate (PMMA), 0.25 ml of Propylene carbonate (PC) and 20 ml of Tetrahydrofuran (THF)were added into a 100 ml beaker. The mixture was stirred for 24 h in order for uniform mixing and forming a homogeneous transparent solution. After 24 h, the electrolyte solution was casted on a flat Petri-dish (12 cm diameter) and covered with Aluminium foil with small perforations on top of it. This was kept for 48 h for providing sufficient time to dry and take the form of transparent free-standing solid-gel electrolyte, which was carefully peeled out from the Petri dish.

[0089] ELECTROCHEMICAL CHARACTERIZATION

[0090] The redox performance of the EC films with the invented electrolyte composition was evaluated by Cyclic Voltammetry measurement in Electrochemical Workstation (Pine Instrument, USA, Wave Driver 100) using a three-electrode system. All the experiments were performed using a 3.24 cm2EC film area working electrode, saturated calomel electrode as the reference electrode and platinum as the counter electrode. The redox reaction was monitored by changing the potential from -1 V to +1 V. The acquired data was then analyzed by comparing the integrated hysteresis curve area of the rGO-WCh samples in presence of Li and Li-A electrolytes.

[0091] OPTICAL CHARACTERIZATION

[0092] The optical transmittance spectra of the samples were measured simultaneously during the cyclic voltammetry experiment using an in-situ Spectrophotometer from Shimadzu coupled with the Pine Wave Driver. The transmittance changes occurred during coloration and bleaching cycles with respect to time revealed the change in optical transmittance i.e. the optical contrast of the rGO-WCh film in presence of both the conventional and invented electrolytes.

[0093] EXAMPLE-3 0.067g of Lithium perchlorate (LiCICL), 0.133 g of Aluminium sec-butoxide (C12H27O3AI), 1 g of Polymethyl methacrylate (PMMA), 0.25 ml of Propylene carbonate (PC) and 20 ml of Tetrahydrofuran (THF)were added into a 100 ml beaker. The mixture was stirred for 24 h in order for uniform mixing and forming a homogeneous transparent solution. After 24 h, the electrolyte solution was casted on a flat Petri-dish (12 cm diameter) and covered with Aluminium foil with small perforations on top of it. This was kept for 48 h for providing sufficient time to dry and take the form of transparent free-standing solid-gel electrolyte, which was carefully peeled out from the Petri dish.

[0094] ELECTROCHEMICAL CHARACTERIZATION

[0095] The redox performance of the EC films with the invented electrolyte composition was evaluated by Cyclic Voltammetry measurement in Electrochemical Workstation (Pine Instrument, USA, Wave Driver 100) using a three-electrode system. All the experiments were performed using a 3.24 cm2EC film area working electrode, saturated calomel electrode as the reference electrode and platinum as the counter electrode. The redox reaction was monitored by changing the potential from -1 V to +1 V. The acquired data was then analyzed by comparing the integrated hysteresis curve area of the rGO-WCh samples in presence of Li and Li-Al electrolytes.

[0096] OPTICAL CHARACTERIZATION

[0097] The optical transmittance spectra of the samples were measured simultaneously during the cyclic voltammetry experiment using an in-situ Spectrophotometer from Shimadzu coupled with the Pine Wave Driver. The transmittance changes occurred during coloration and bleaching cycles with respect to time revealed the change in optical transmittance i.e. the optical contrast of the rGO-WCh film in presence of both the conventional and invented electrolytes [ Table-1].

[0098] Table: - 1 Optical transmittance spectral studies

[0099] ADVANTAGES a. Present invention employs Li+and Al3+ions which can occupy the pores of the polymer matrix resulted into more intense blue color and as a consequence, the optical contrast of the Electrochemical assembly increases. b. The process for the preparation of the novel solid-gel electrolyte in the present invention consisting unique facile process and operable at ambient temperature and pressure conditions. c. The process for the preparation of the novel Li-ion based free-standing solid-gel electrolyte in the present invention is cost effective and industrial viable. d. The smaller Al3+radii help in the reduction of structural deterioration in novel Li-ion based freestanding solid-gel electrolyte, thereby also acts to enhance the electrochemical cyclic stability. e. Present novel Li-ion based free-standing solid-gel electrolyte composition can be applied for materials bearing quantum confinements owing to their nanoscale dimension.

Claims

We claim:

1. A transparent aluminium incorporated Li-ion based free-standing solid-gel electrolyte for enhancement of optical contrast of electrochromic device comprising concentrations of lithium perchlorate (LiC104) and aluminium sec-butoxide (C12H27O3AI) in weight ratios between the range of 0-4 wt%.

2. The Li-Al based free-standing solid-gel electrolytes as in claimed in claim 1 , wherein the ratio of aluminium sec-butoxide and lithium perchlorate is 1 : 1 wt % and having the optical modulation from 50 % to 82 %, colouring efficiency in the range of colouring efficiency in the range of 600 cm2 / C to 700 cm2 / C with a switching kinetics of 2.5 seconds.

3. A process for the preparation of transparent aluminium (Al3+) incorporated Li-ion based freestanding solid-gel electrolyte as claimed in claim 1, comprising the steps of:(f) employing lithium perchlorate (LiCICL) and aluminium sec-butoxide (C12H27O3AI) as the electrolyte ion precursors to obtain a precursor mixture;(g) adding the precursor mixture as obtained in step (a) with 0.25 ml of propylene carbonate (PC) along with 1 g of polymethyl methacrylate and kept in a hot-air oven at 110°C for 24 h to obtain a polymer matrix;(h) dissolving the polymer matrix as obtained in step (b) in 20 ml of aprotic solvent and stirred for 24 h to obtain a prepared mixture;(i) casting the prepared mixture as obtained in step (c) on a flat surface petri dish followed by covering the dish with aluminium foils and kept for 48 h for air drying to obtain a transparent solid;(j) peeling off the transparent solid prepared as obtained in step (d) from the petri dish and cutting transparent free-standing solid-gel electrolyte of dimension 10 cm X 10 cm sheets.

4. The process as claimed in claim 3, wherein the polymer matrix is having an ion mobilizing agents selected from the group consisting of phthalates, phosphates, carboxylic acid esters, epoxidized fatty acid esters, polymeric polyesters, modified polymers; liquid rubbers, and plastics, nitrile butadiene rubber (NBR), chlorinated PE, EVA, polymethyl methacrylate, propylene carbonate and a mixture thereof.

5. The process as claimed in claim 3, wherein the aprotic solvent medium used in step (c) is selected from acetone, dimethylformamide DMF, tetrahydrofuran THF, and dichloromethane CH2C12, acetonitrile CH3CN, dimethyl sulfoxide DMSO, dimethylpropyleneurea, ethyl acetate, pyridine, sulfolane, hexamethylphosphoramide and a mixture thereof.

6. The Li-Al based free-standing solid-gel electrolytes as claimed in claim 1, wherein the solid-gel electrolyte comprising Li-Al solid-gel electrolyte with a polymeric network structure wherein the polymeric network structure is having circular pores and a porous matrix, occupying Li+and Al3+into the pores.

7. An electrochromic device prepared by sandwiching a five layered structure wherein the electrochromic device comprising:(a) a counter and working electrode;(b) electrochromic film;(c) a layer of Li-Al based transparent free standing solid-gel electrolyte sheet of claim 1 ;(d) ion-storage layer;8. The process of preparation of the electrochromic device as claimed in claim 7 comprising the steps of: e. coating of Li-Al based transparent free-standing solid-gel electrolyte sheet of claim 1, with the ion-storage layer to obtain a coated transparent electrolyte sheet; f. putting the coated transparent electrolyte sheet as obtained in step (a) on the conducting side of the counter electrode; g. coating of the working electrode with the electrochromic film of rGO-WCh; h. sandwiching the transparent ALLi based solid-gel electrolyte sheet of step (b) with working electrode of step (c) without air gap between the two electrodes.

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