ELECTROCHEMICAL DEVICE HAVING AT LEAST ONE GELLED ELECTRODE CROSS-REFERENCE TO RELATED APPLICATION This application claims priority to European patent application No. 20151214.2 filed on January 10,2020, the whole content of this application being incorporated herein by reference for all purposes. Technical field The present invention relates to an electrochemical device comprising a) a positive electrode, b) a negative electrode, c) a separator, and d) a liquid electrolyte, wherein at least one of said positive electrode and said negative electrode is a gelled electrode comprising an electronic conductive substrate and directly adhered onto the electronic conductive substrate, at least one layer of a gelled electrode-forming composition, and wherein the d) liquid electrolyte comprises at least one organic carbonate and/or at least one ionic liquid, and at least one metal salt. The present invention also relates to a process for manufacturing an electrochemical device comprising at least one gelled electrode. Background of the invention For more than two decades, Lithium batteries have retained dominant position in the market of rechargeable energy storage devices due to their many benefits comprising light-weight, reasonable energy density and good cycle life. A liquid electrolyte is a substance which produces an electrically conducting solution when it is dissolved in a polar solvent. The dissolved electrolyte splits into cations and anions, which disperse through the solvent in a uniform manner. Such a solution is electrically neutral, and conducting ionically and electronically insulating. Basic requirements to be a suitable electrolyte for an electrochemical cell include high ionic conductivity, (electro)chemical stability, and safety. The conventional electrolyte, which is in liquid, has played an essential and dominant role in the field of electrochemical energy storage for several decades due to its high ionic conductivity and good interface with electrodes. However, such a liquid electrolyte has brought safety issues caused by its leakage and inherent explosive nature, e.g., combustion of the organic solvent, generating volatile gaseous species, which are flammable. That is, Li-ion batteries have suffered from poor safety and relatively low energy density with respect to the required energy density for high power applications such as electrical vehicles (EVs), hybrid electrical vehicles (HEVs), grid energy storage, etc. and the presence of liquid electrolyte is at the basis of such shortcomings. Accordingly, safety has been a prerequisite for batteries. Several protective mechanisms have been considered as measures to ensure battery safety. External protection relies on electronic devices such as temperature sensors and pressure vents, which eventually increase the volume/weight of the battery and are unreliable under thermal/pressure abuse conditions. Internal protection schemes focus on using intrinsically safe materials for battery components and are hence considered to be the more appropriate solution for battery safety. Subsequently, hybrid organic/inorganic polymer composites where inorganic materials on a nano-scale or molecular level are dispersed in organic polymers have raised a great deal of scientific, technological and also industrial interests, because of the unique properties they have. Hybridization of organic and inorganic compounds is an evolutionary manner to create a polymeric structure, notably to increase mechanical properties. In this regards, it is well known that a sol-gel process using metal alkoxides is the most useful and important approach, in elaborating hybrid organic/inorganic polymer composites. In particular, the hydrolysis and condensation of metal alkoxides in the presence of pre-formed organic polymers, starting from fluoropolymers, in particular from vinylidene fluoride (VDF) polymers can be properly controlled to obtain hybrid organic/inorganic polymer composites with improved properties in comparison with the original organic and inorganic compounds. The polymer as organic compound may enhance the toughness and processability of inorganic materials, i.e., metal alkoxides, which are brittle in general, wherein the inorganic network may enhance scratch resistance, mechanical properties and surface characteristics of the resulting hybrid organic/inorganic polymer composite. In particular, WO 2015/169834 (SOLVAY SA and COMMISSARIAT A L'ENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES) discloses a fluoropolymer hybrid organic/inorganic composite membrane obtainable by using sol-gel technique, which exhibits increased electrolyte retention ability, to be suitably used as a polymer electrolyte membrane in an electrochemical device. WO 2015/169835 (SOLVAY SA and COMMISSARIAT A L'ENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES) further discloses a composite electrode exhibiting high adhesion to metal collectors and high cohesion within the electro-active materials while ensuring high ionic conductivity. Moreover, US 2018/0123167 (COMMISSARIAT A L'ENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES) proposes a Li-ion battery comprising a positive electrode, a negative electrode and an electrolyte comprising a Li salt, wherein the positive electrode, the negative electrode and the electrolyte all three appear in the form of gels. Accordingly, it is not obvious for a person skilled in the art to combine at least one gelled electrode with a liquid electrolyte, which requires the presence of a polymeric material as standard separator, to produce an electrochemical device. It was unexpectedly demonstrated by the present inventors that the association of at least one gelled electrode with a liquid electrolyte and a standard separator makes it possible to produce a flexible/foldable electrochemical device exhibiting high electric capacity. The process according to the present invention also has an advantage in view of the substantially reduced time necessary for filling the electrochemical device as assembled with the liquid electrolyte. Summary of the invention A first object of the present invention is an electrochemical device comprising a) a positive electrode, b) a negative electrode, c) a separator, and d) a liquid electrolyte, wherein at least one of said positive electrode and said negative electrode is a gelled electrode comprising an electronic conductive substrate and directly adhered onto the electronic conductive substrate, at least one layer of a gelled electrode-forming composition, and wherein the d) liquid electrolyte comprises at least one organic carbonate and/or at least one ionic liquid, and at least one metal salt. A second object of the present invention is to provide a process for manufacturing an electrochemical device comprising the steps of: (I) assembling at least a) a positive electrode; b) a negative electrode; and c) a separator interposed between said positive electrode and said negative electrode, wherein at least one electrode is a gelled electrode obtained by - providing an electronic conductive substrate; - providing a gelled electrode-forming composition - applying the gelled electrode-forming composition onto the electronic conductive substrate; - optionally drying the electronic conductive substrate coated with the gelled electrode-forming composition; and - calendaring the same into a film having thickness between 80 urn and 900 urn, preferably between 100 urn and 800 urn, and more preferably between 200 urn and 600 urn, and (II) filling the electrochemical device as assembled with a liquid medium (II) comprising at least one organic carbonate and/or at least one ionic liquid, and optionally at least one metal salt. In an aspect, the gelled electrode-forming composition according to the present invention comprises; i) at least one partially fluorinated fluoropolymer comprising - at least one first recurring unit derived from at least one ethylenically unsaturated fluorinated monomer, and - at least one second recurring unit derived from at least one hydrogenated monomer comprising at least one carboxylic group; ii) at least one electro-active compound; iii) a liquid medium (I); iv) optionally, at least one conductive additive; and v) optionally, at least an organic solvent (S) different from liquid medium (I). Brief description of drawings Figure 1 shows the picture of a prismatic cell of Example 1. Figure 2 shows the pictures of the anode (a) and the cathode (b) of the prismatic cell of Example 1 after removal from the assembly and und-wind. Figure 3 shows the pictures of the anode (a) and the cathode (b) of the prismatic cell of Comparative Example 1 after removal from the assembly and und-wind. Detailed Description of the Invention Throughout this specification, unless the context requires otherwise, the word "comprise" or "include", or variations such as "comprises", "comprising", "includes", including" will be understood to imply the inclusion of a stated element or method step or group of elements or method steps, but not the exclusion of any other element or method step or group of elements or method steps. According to preferred embodiments, the word "comprise" and "include", and their variations mean "consist exclusively of. As used in this specification, the singular forms "a", "an" and "the" include plural aspects unless the context clearly dictates otherwise. The term "and/or" includes the meanings "and", "or" and also all the other possible combinations of the elements connected to this term. The term "between" should be understood as being inclusive of the limits. As used herein, "alkyl" groups include saturated hydrocarbons having one or more carbon atoms, including straight-chain alkyl groups, such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, cyclic alkyl groups (or "cycloalkyl" or "alicyclic" or "carbocyclic" groups), such as cyclopropyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl, branched-chain alkyl groups, such as isopropyl, tert-butyl, sec-butyl, and isobutyl, and alkyl-substituted alkyl groups, such as alkyl-substituted cycloalkyl groups and cycloalkyl-substituted alkyl groups. As used herein, the terminology "(Cn-Cm)" in reference to an organic group, wherein n and m are integers, respectively, indicates that the group may contain from n carbon atoms to m carbon atoms per group. Ratios, concentrations, amounts, and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a temperature range of about 120°C to about 150°C should be interpreted to include not only the explicitly recited limits of about 120°C to about 150°C, but also to include sub-ranges, such as 125°Cto 145°C, 130°Cto 150°C, and so forth, as well as individual amounts, including fractional amounts, within the specified ranges, such as 122.2°C, 140.6°C, and 141.3°C, for example. Unless otherwise specified, in the context of the present invention the amount of a component in a composition is indicated as the ratio between the weight of the component and the total weight of the composition multiplied by 100 (i.e., % by weight orwt%). By the term "electrochemical device", it is hereby intended to denote an electrochemical cell/assembly comprising a positive electrode, a negative electrode and a liquid electrolyte, wherein a monolayer or multilayer separator is in contact to at least one surface of one of the said electrodes. Non-limitative examples of suitable electrochemical devices include, notably, secondary batteries, especially, alkaline or an alkaline-earth secondary batteries such as lithium ion batteries, lead-acid batteries, and capacitors, especially lithium ion-based capacitors and electric double-layer capacitors (supercapacitors). The constituents of the electrochemical device according to the present invention are described hereinafter in details. It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed. Accordingly, various changes and modifications described herein will be apparent to those skilled in the art. Moreover, descriptions of well-known functions and constructions may be omitted for clarity and conciseness. The present invention provides an electrochemical device comprising a) a positive electrode, b) a negative electrode, c) a separator, and d) a liquid electrolyte, wherein at least one of said positive electrode and said negative electrode is a gelled electrode comprising an electronic conductive substrate and directly adhered onto the electronic conductive substrate, at least one layer of a gelled electrode-forming composition, and wherein the d) liquid electrolyte comprises at least one organic carbonate and/or at least one ionic liquid, and at least one metal salt. According to the present invention, the combination of at least one of the a) positive electrode and the b) negative electrode, which is in the form of gel, together with a liquid electrolyte and a standard separator makes it possible to produce a flexible/foldable electrochemical device exhibiting highly loaded electrodes with an areal capacity between 1.0 mAh/cm2and 9.0 mAh/cm2, preferably between 4.0 mAh/cm2and 7.0 mAh/cm2. A gelled electrode presents higher flexibility than a classical electrode, notably with higher loading of electro-active materials without damage to the electrode structure. Moreover, in view of manufacturing process, the filling time of the electrochemical device once being assembled with a liquid electrolyte can be substantially reduced. In the present invention, the term "negative electrode" is intended to denote, in particular, the electrode of an electrochemical cell, where oxidation occurs during discharging. In the present invention, the term "positive electrode" is intended to denote, in particular, the electrode of an electrochemical cell, where reduction occurs during discharging. For the purpose of the present invention, the term "gelled electrode" is defined below. In an embodiment, at least one of said positive electrode and said negative electrode according to the present invention has thickness between 80 urn and 900 urn, preferably between 100 urn and 800 urn, and more preferably between 200 urn and 600 urn. The gelled electrodes used in the electrochemical device of the present invention can thus have a quite high thickness that allows high loading of the electrodes, while retaining homogeneous distribution of active material, partially fluorinated fluoropolymer and conductive substrate. The resulting devices have thus high capacity and are capable of delivering high energy In the present invention, the term "filling time" is hereby defined as the time needed to inject the liquid medium and ensure proper distribution of the liquid medium within an electrochemical device to completely wet the electrodes and the separator. In the present invention, the nature of the electronic conductive substrate depends on whether the electrode thereby provided is a positive electrode or a negative electrode. Should the electrode of the invention be a positive electrode, the electronic conductive substrate typically comprises, preferably consists of, carbon (C) or at least one metal selected from the group consisting of Aluminium (Al), Nickel (Ni), Titanium (Ti), and alloys thereof, preferably Al. Should the electrode of the invention be a negative electrode, the electronic conductive substrate typically comprises, preferably consists of, Carbon (C) or Silicon (Si) or at least one metal selected from the group consisting of Lithium (Li), Sodium (Na), Zinc (Zn), Magnesium (Mg), Copper (Cu) and alloys thereof, preferably Cu. By the term "separator", it is hereby intended to denote a monolayer or multilayer polymeric or ceramic material/film, which electrically and physically separates the electrodes of opposite polarities in an electrochemical device and is permeable to ions flowing between them. In the present invention, the separator can be any porous substrate commonly used for a separator in an electrochemical device. In one embodiment, the separator is a porous polymeric material comprising at least one material selected from the group consisting of polyester such as polyethylene terephthalate and polybutylene terephthalate, polyphenylene sulphide, polyacetal, polyamide, polycarbonate, polyimide, polyether sulfone, polyphenylene oxide, polyphenylene sulfide, polyethylene naphthalene, polyethylene oxide, polyacrylonitrile, polyolefin such as polyethylene and polypropylene, or mixtures thereof. In a particular embodiment, the separator is a porous polymeric material coated with PVDF or inorganic nanoparticles, for instance, SiC^, Ti02, AI2O3, Zr02, etc. In the present invention, the term "liquid medium" is intended to denote a medium comprising one or more substances in the liquid state at 20°C under atmospheric pressure. In the present invention, the term "liquid medium (I)" is intended to denote a liquid medium comprised within a gelled electrode-forming composition. In the present invention, the term "liquid medium (II)" is intended to denote a liquid medium which is added at the filling stage. The liquid medium (II) is then present and distributed in the whole electrochemical device. In the present invention, the term "liquid medium" is intended to correspond to either a liquid medium (I) or a liquid medium (II). In the present invention, a liquid electrolyte comprises a mixture of a liquid medium (I) and a liquid medium (II). In the present invention, the liquid medium (I) and the liquid medium (II) is identical or different. In the present invention, the liquid medium (I) and the liquid medium (II) respectively comprise at least one organic carbonate and/or at least one ionic liquid. In the present invention, at least one of the liquid medium (I) and the liquid medium (II) additionally comprise at least one metal salt. In one embodiment, a separator and a liquid medium (II) comprising at least one organic carbonate and/or at least one ionic liquid is placed between the a) positive electrode and the b) negative electrode. In the present invention, the choice of the organic carbonate or the ionic liquid is not particularly limited provided that it is suitable for solubilising the metal salt. In one embodiment, the metal salt is selected from the group consisting of: (a) Mel, Me(PF6)n, Me(BF4)n, Me(CI04)n, Me(bis(oxalato)borate)n (" Me(BOB)n " ), MeCF3S03, Me[N(S02F)2]n, Me[N(CF3S02)2]n, Me[N(C2F5S02)2]n, Me[N(CF3S02)(RFS02)]n, wherein RF is C2F5, C4F9 or CF3OCF2CF2, Me(AsF6)n, Me[C(CF3S02)3]n, Me2Sn, wherein Me is a metal, preferably a transition metal, an alkaline metal or an alkaline-earth metal, more preferably Me being Li, Na, K or Cs, even more preferably Me being Li, and n is the valence of said metal, typically n being 1 or 2; R'. NJC N wherein RV is selected from the group consisting of F, CF3, CHF2, CH2F, C2HF4, C2H2F3, C2H3F2, C2F5, C3F7, C3H2F5, C3H4F3, C4F9, C4H2F7, C4H4F5, C5F11, C3F5OCF3, C2F4OCF3, C2H2F2OCF3 and CF2OCF3; and (c) combinations thereof. In one embodiment, the organic carbonate is partially or fully fluorinated carbonate compound. The organic carbonate compound according to the present invention may be either cyclic carbonate or acyclic carbonate. Non-limiting examples of the organic carbonate compound include, notably, ethylene carbonate (1,3-dioxolan-2-one), propylene carbonate, vinylene carbonate (1,3-dioxol-2-one), 4-methylene-1,3-dioxolan-2-one, 4,5-dimethylene-1,3-dioxolan-2-one, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, methyl propyl carbonate, methyl butyl carbonate, ethyl butyl carbonate, propyl butyl carbonate, dibutyl carbonate, di-tert-butyl carbonate and butylene carbonate. The fluorinated carbonate compound may be mono-fluorinated or polyfluorinated. Suitable examples of the fluorinated carbonate compound comprises, but not limited to, mono-fluorinated ethylene carbonate (4-fluoro-1,3-dioxolan-2-one) and difluorinated ethylene carbonate, mono-and difluorinated propylene carbonate, mono- and difluorinated butylene carbonate, 3,3,3-trifluoropropylene carbonate, fluorinated dimethyl carbonate, fluorinated diethyl carbonate, fluorinated ethyl methyl carbonate, fluorinated dipropyl carbonate, fluorinated dibutyl carbonate, fluorinated methyl propyl carbonate, and fluorinated ethyl propyl carbonate. In a preferred embodiment, the organic carbonates chosen are mixture of ethylene carbonate and propylene carbonate. In another preferred embodiment, the organic carbonates chosen are a mixture of ethylene carbonate, propylene carbonate and vinylene carbonate. In another embodiment, the liquid medium further comprises at least one sulfone compound in addition to the organic carbonate. The sulfone compound according to the present invention may be either cyclic sulfone or acyclic sulfone. Non-limiting examples of the sulfone compound include, notably, tetramethylene sulfone (sulfolane), butadiene sulfone (sulfolene), pentamethylene sulfone, hexamethylene sulfone, thiazolidine 1,1-dioxide, thiomorpholine 1,1-dioxide, dimethyl sulfone, diethyl sulfone, ethyl methyl sulfone, and mixtures thereof. In a preferred embodiment, the liquid medium comprises a mixture of ethylene carbonate, propylene carbonate, vinylene carbonate and sulfolane. In a preferred embodiment, the liquid medium (II) is a mixture of organic carbonate compounds which may wet optimally the separator. In a more preferred embodiment, the mixture of organic carbonate compounds comprises cyclic carbonate and/or acyclic carbonate. Non-limiting examples of the organic carbonate compounds include, notably, ethylene carbonate (1,3-dioxolan-2-one), propylene carbonate, vinylene carbonate (1,3-dioxol-2-one), 4-methylene-1,3-dioxolan-2-one, 4,5-dimethylene-1,3-dioxolan-2-one, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, methyl propyl carbonate, methyl butyl carbonate, ethyl butyl carbonate, propyl butyl carbonate, dibutyl carbonate, di-tert-butyl carbonate and butylene carbonate. The term "ionic liquid" as used herein refers to a compound comprising a positively charged cation and a negatively charged anion, which is in the liquid state at temperature of 100°C or less under atmospheric pressure. While ordinary liquids such as water are predominantly made of electrically neutral molecules, ionic liquids are largely made of ions and short-lived ion pairs. As used herein, the term "ionic liquid" indicates a compound free from solvent. The term "cationic atom" as used herein refers to at least one non-metal atom which carries the positive charge. The term "onium cation" as used herein refers to a positively charged ion having at least part of its charge localized on at least one non-metal atom such as O, N, S, or P. In the present invention, the ionic liquid has a general formula of An~ Ql+(n/i), wherein - An- represents an anion; - Ql+(n/i) represents a cation; - n and I, independently selected between 1 and 5, represent respectively the charges of the anion An~ and of the cation Ql+(n/i). The cation(s) may be selected, independently of one another, from metal cations and organic cations. The cation(s) may be mono-charged cations or polycharged cations. As metal cation, mention may preferably be made of alkali metal cations, alkaline-earth metal cations and cations of d-block elements. In the present invention, Ql+(n/i)imay represent an onium cation. Onium cations are cations formed by the elements of Groups VB and VIB (as defined by the old European IUPAC system according to the Periodic Table of the Elements) with three or four hydrocarbon chains. The Group VB comprises the N, P, As, Sb and Bi atoms. The Group VIB comprises the O, S, Se, Te and Po atoms. The onium cation can in particular be a cation formed by an atom selected from the group consisting of N, P, O and S, more preferably N and P, with three or four hydrocarbon chains. The onium cation Ql+(n/i) can be selected from: - heterocyclic onium cations; in particular those selected from the group consisting of: R R. ^ „R R R R R R. -R Rv Mt R N+ R N ^N+ 4> fsl R R H" R BT \ / ~"~R R R R R R R R R -N+ >=N + -N, 4)3 wherein 0