ELECTROCHEMICAL DEVICE COMPRISING CARBON QUANTUM DOT IONIC COMPOUND ELECTROLYTE

Abstract

The present invention relates to an electrochemical device, more particularly to an electrochemical device including a first electrode, a second electrode spaced apart from the first electrode and an electrolyte filled between the first electrode and the second electrode, wherein the electrolyte comprises a salt form of a carbon quantum dot anion and a metal cation having an average diameter in the range of 2 to 12 nanometers (nm) and a surface potential of 20 mV or less, the present invention provides an electrochemical device dramatically improving reliability, performance and durability by adopting an carbon quantum dot ion compound electrolyte having selective ion conductivity with a specific cation and suppressing side reactions caused by electrolyte as well as applicable in liquid, gel or solid phase.

Claims

1. An electrochemical device including a first electrode, a second electrode spaced apart from the first electrode and an electrolyte filled between the first electrode and the second electrode, wherein a reversible electrochemical redox reaction occurs in at least one of the first electrode and the second electrode and the electrolyte comprises a salt form of a carbon quantum dot anion and a metal cation having an average diameter in the range of 2 to 12 nanometers (nm) and a surface potential of 20 mV or less.

2. The electrochemical device according to claim 1, characterized in that the metal is an alkali metal, alkaline earth metal or transition metal.

3. The electrochemical device according to claim 2, characterized in that the metal is at least one selected from the group consisting of Li, Na, K, Mg and Zn.

4. The electrochemical device according to claim 1, characterized in that the electrochemical device is one selected from the group consisting of a secondary battery, a solar cell, an electrochromic device and an electroluminescent device.

5. The electrochemical device according to claim 1, characterized in that the secondary battery is a lithium ion battery or a lithium polymer battery.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a schematic diagram of an electrochromic device according to the present invention.

[0022] FIG. 2 (a) is the electron microscope image and a schematic diagram of the structure of the carbon quantum dot ionic compound, (b) is a graph showing the absorption and emission profiles of the carbon quantum dot ionic compound of an electrolyte for an electrochemical device according to the present invention.

[0023] FIG. 3 (a) shows a structure diagram of a three-electrode system composed of a first electrode (working electrode), a platinum (Pt) second electrode (relative electrode) and reference electrodes (Ag/AgCl) and containing a color change material in an aqueous solution comprising a carbon quantum dot ionic compound of the present invention, and (b) shows the results of cyclic voltammograms with 0.1 V/s scan rate using said three-electrode system.

[0024] FIG. 4(a) is the results of cyclic voltammogram with 0.02 V/s scan rate using the three-electrode system with Prussian blue coated working electrode, and (b) is the result of measuring current change with varying scan rate.

[0025] FIG. 5 is an electrochemical impedance spectroscopy of the electrochemical device of Example 1 and comparative example 1 measured by changing metal cations in carbon quantum dot ionic compounds using the three-electrode system.

[0026] FIG. 6 shows the results of transmittance at 700 nm and current change measurement with voltage switching between 0.16V(discoloring state) and +0.4V(coloring state) for the electrochromic device inducing 1.2V to 2.2V pulse with 10 sec interval to produce color change.

[0027] FIG. 7(a) shows the result of durability test for each electrolyte in the electrochromic device according to the present invention and (b) is the result of durability test for the carbon quantum anion-potassium cation electrolyte.

[0028] FIG. 8 shows the results of transmittance change measurement with voltage switching in an electrochromic device, and photographs of coloring/discoloring according to transmittance of the electrochromic device.

[0029] FIG. 9 shows a light emission intensity measurement result according to the carbon quantum dot ion compound concentration under the two-electrode system conditions for the electroluminescent device in one embodiment according to the present invention.

[0030] FIG. 10 shows charge/discharge test result of a lithium secondary battery according to the present invention applying a carbon quantum dot ion compound.

[0031] FIG. 11 (a) to (c) show cyclic voltammetry measurements with varying the concentrations (0.125, 0.25 and 0.5M, respectively) of the carbon quantum dot anion-lithium cation ion compound electrolyte prepared according to the present invention.

[0032] FIG. 12 (a) shows the results of cycling test under the condition of the current density of 160 mA/g in one embodiment of the present invention and (b) the result of measuring the change of the current density by concentration.

[0033] FIG. 13 (a) shows voltage-capacitance measurement results measured at an electrode of both lithium ion batteries applying a carbon quantum dot ion compound electrolyte and a LiPF.sub.6 electrolyte respectively in Example 5 of the present invention and (b) is differential result with respect to voltage.

[0034] FIG. 14 shows the result of measuring I.sub.0 and I.sub.ss (top) and R.sub.0 and R.sub.ss (bottom) in order to calculate the charge transfer index of lithium ions in the lithium ion battery applying the electrolyte of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0035] Hereinafter, the present invention will be described in detail referring the accompanying drawings as below.

[0036] In the present specification, the term electrochemical device refers to a device comprising a first electrode, a second electrode spaced apart from the first electrode and forming an electrically opposite to the first electrode, and electrolyte filled between the first electrode and the second electrode, wherein an electrochemical reaction is performed and a reversible electrochemical redox reaction occurs at the one or both of the first electrode and the second electrode. And the term carbon quantum dot refers to a quantum dot in the form of graphite oxide having at least one oxygen functional group capable of becoming anions on the surface and/or the edge thereof and having an average diameter in the range of 2 to 12 nm and the surface potential of 20 mV or less; or quantum dots derivatives produced by reacting the quantum dot with polymerizable group. And the term carbon quantum dot anion refers to carbon quantum dot of the oxygen functional group anionized.

[0037] The electrochemical device according to the present invention comprises a first electrode; a second electrode spaced apart from the first electrode and forming an electrically opposite to the first electrode; and electrolyte filled between the first electrode and the second electrode, wherein an electrochemical reaction is performed and a reversible electrochemical redox reaction occurs at the one or both of the first electrode and the second electrode and the electrolyte comprises ion salt of carbon quantum dot anion having an average diameter in the range of 0.1 to 8 nm and the surface potential of 20 mV or less and a metal cation.

[0038] In the present invention. The first electrode may be a working electrode or an anode, and the second electrode forming an electrically opposite of the first electrode may be a counter electrode or a cathode. Reversible electrochemical redox reaction occurs at one or both of the first electrode and the second electrode.

[0039] The carbon quantum dot anion in the present invention has a form of a polyanionic (A.sup.n), aromatic ring structure inside and oxygen functional groups on the surface and edge. The carbon quantum dot anion is combined with a metal cation to form a salt type ionic compound. FIG. 2 (a) is the electron microscope image and a schematic diagram of the structure of the carbon quantum dot ionic compound, (b) is a graph showing the absorption and emission profiles of the carbon quantum dot ionic compound of an electrolyte of an electrochemical device according to the present invention. The carbon quantum dot ionic compound as shown in FIG. 2 is expected as follows; 1) Due to the negative surface charges, ionic bonds with various metal cations such as alkali metals, alkaline earth metals, and transition metals are possible. 2) Large size and multiple negative surface charges, which have a large polarity and low lattice energy. 3) Delocalization of the electron cloud is large due to resonance of the internal structure. In addition, 4) it is a macromolecular anion, so there is almost no mass transport in the solution, and 5) there is no side reaction at the electrode interface due to electrochemical/thermal stability, thereby improving device reliability. The carbon quantum dot ionic compound of the present invention is not only easy to disperse in aqueous solutions and non-aqueous solvents, but also relatively free of mixing with organic solvents having low viscosity, low volatility, and high permittivity. Through above mentioned characteristics, the electrolyte in the present invention is deemed to have a highly ionic conductivity.

[0040] In the carbon quantum dot ionic compound of the present invention, the metal cation may be an alkali metal, an alkaline earth metal or a transition metal cation, and examples thereof may be Li, Na, K, Mg, or Zn. The carbon quantum dot ionic compound can be used in liquid, gel, solid form, and it is possible to adjust the appropriate content thereof according to the concrete usage.

[0041] The electrolyte comprising carbon quantum dot ion compound adapted in the electrochemical device of the present invention has an average diameter in the range of 2 to 12 nm, more preferably in the range of 5 to 8 nm, and at least one oxygen functional group being capable of an anion on the surface and/or the edge thereof, and a surface potential of less than or equal to 20 mV. If the average diameter of the carbon quantum dot is less than 2 nm, the carbon quantum dot anion would move to the anode by the potential formed on the electrode of the electrochemical device, which decreases the t+(ion transport number), resulting in a decrease in the efficiency of the electrochromic device and it will reduce the lattice energy of the carbon quantum dot resulting in ionic conductivity reduction. On the other hand, when the average diameter of carbon quantum dots is 12 nm or more, the - interaction between the carbon quantum dots increases, which causes aggregation and crystallization of carbon quantum dots in the electrochemical device and degrade the reliability of the device.

[0042] The electrolyte comprising carbon quantum dot ion compound in the present invention can be used as liquid by dissolving in water-soluble solvents (methanol, ethanol), non-aqueous solvents (acetonitrile, dimethyl carbonate, ethylene carbonate), and an aqueous solution, or optionally dispersed in a suitable dispersion medium/matrix dispersed as a gel form. The applicant of the present invention also have filed a patent application No. 10-2017-0064227 before KIPO regarding an electrolyte for electrochemical device comprising carbon quantum dot ionic compound and preparation method thereof. No more detailed explanation about carbon quantum dot ion compound in the specification, because one can refer said patent application for detail about it.

[0043] FIG. 1 is a schematic diagram of an electrochromic device according to the present invention. As shown in FIG. 1, an electrochemical device according to the present invention comprises a first electrode; a second electrode spaced apart from the first electrode and forming an electrically opposite to the first electrode; and electrolyte filled between the first electrode and the second electrode, and may comprise reference electrode as well in accordance with the characteristic of the device. In an embodiment of the present invention, an electrochromic device is used as an example of an electrochemical device, but the electrochemical device of the present invention is not limited to an electrochromic device only, but can be an electrochemical light emitting device, a secondary battery, or a solar cell with reversible electrochemical redox reactions at the working electrode (material).

[0044] Due to the low diffusion coefficient and transport rate of the conventional electrolyte metal cation, the reliability and performance of the electrochemical device is degraded. For example, when the diffusion coefficient of the metal cation in the electrochromic device is lower than the cation constituting the ionic liquid, the cation cannot be inserted into the color change material. Therefore, the color change material is difficult to maintain an electrically neutral state, the color change efficiency is degraded or the decomposition of the material occurs, the electrochromic device reliability and performance is reduced. The electric field is formed by the voltage applied in the electrochromic device, which causes the electrolyte anions to move along the direction of the electric field. At this time, the negative ions cause chemical reaction with the discoloration material and the electrode, thereby reducing the reliability and performance of the electrochromic device. In the case of a two-electrode electrochromic device having a sandwich form, a material capable of an oxidation/reduction reaction should be included. Otherwise, charge imbalance occurs on both electrode interfaces, thereby degrading the reliability and performance of the electrochromic device. In the electrochemical device according to the present invention, among the electrochromic devices, by applying a carbon quantum dot ion compound as an electrolyte, the above-described side reactions can be suppressed to increase the reliability and durability of the electrochemical device, as well as the electrode and electrolyte (quality). By controlling the inter-charge imbalance, the efficiency of the electrochromic device can be improved by increasing the conversion efficiency between electrical energy and chemical energy.

[0045] The present invention will be described below in greater detail in connection with preferred embodiments of the present invention. It should be noted that the following embodiments are provided merely for better understanding of the invention and the scope of the present invention is not limited only to the embodiments.

Example 1 (Preparation of Electrochromic Device Applying Carbon Quantum Dot Electrolyte)

[0046] A color change material layer was formed on a conductive transparent substrate by immersing the substrate in an aqueous solution containing 0.05 M HCl, 0.05 M K.sub.3Fe(CN).sub.6, and 0.05 M FeCl.sub.3.6 H.sub.2O. The thickness of the color change material layer can be regulated by controlling the current and time using chronopotentiometry. In the present invention, the color change material layer formed on the conductive transparent substrate used as working electrode with 40 uA and 140 s. On the other hand, ZnO buffer layer was formed by immersing another conductive transparent substrate in 5 mM ZnCl.sub.2, 0.1 M KCl, and an oxygen-saturated aqueous solution for 1000 s while applying 1 V at room temperature. Subsequently, the ZnO buffer layered transparent electrode was immersed in 0.5 mM ZnCl.sub.2, 0.1 M KCl and oxygen saturated water solution at 80 C. for 1000 s while applying 1 V, and then ZnO NW(nanowire)s layer was formed thereon, which was used as a relative electrode. Respective 3 working electrodes and the 3 relative electrodes were attached on the inside of the 3 electrochromic devices in the form of a sandwich using a thermal tape with the distance of 60 um between the two electrodes. Subsequently, through the fine holes formed in the second electrode, 0.5M solutions of carbon quantum dot ionic compound of with respective Li, Na, and K were injected into the corresponding electrochromic device. The pH of the aqueous electrolyte solutions were adjusted to 4, respectively.

Comparative Example 1 (Manufacture of Electrochromic Device Including Potassium Chloride Electrolyte)

[0047] An electrochromic device was manufactured in the same manner as in Example 1, except that 0.5 M potassium chloride (KCl) was used as the electrolyte.

[0048] Electrochemical properties of the electrochromic devices prepared in Example 1 and Comparative Example 1 were compared. FIG. 3 (a) shows a structure diagram of a three-electrode system composed of a first electrode (working electrode), a platinum (Pt) second electrode (relative electrode) and reference electrodes (Ag/AgCl) and containing a color change material in an aqueous solution comprising a carbon quantum dot ionic compound of the present invention, and (b) shows the results of cyclic voltammograms with 0.1 V/s scan rate using said three-electrode system. FIG. 4(a) is the results of cyclic voltammogram with 0.02 V/s scan rate using the three-electrode system with Prussian blue coated working electrode, and (b) is the result of measuring current change with varying scan rate

[0049] And the characteristics of the electrochromic device were analyzed by applying voltage at 0.14/0.4 V and 10 s/10 s (50% duty cycle) using a chronoamperometry method. As shown in FIG. 4(b), in the electrochemical device of the present invention, it can be seen that the oxidation/reduction current of the color change material corresponds to the same even when the scanning rate is increased (the oxidation is a coloration reaction (PB) and the reductive current is a decoloration reaction (PW)). Table 1 below shows the diffusion rate according to the metal cations of the carbon quantum dot ionic compound under the three-electrode system conditions.

TABLE-US-00001 TABLE 1 0.5M KCl 0.5M K.sup.+-C-dots.sup. 0.5M Na.sup.+-C-dots.sup. 0.5M Li.sup.+-C-dots.sup. Diffusion rate PB.fwdarw.PW 2.7 10.sup.10 7.7 10.sup.11 2.0 10.sup.10 1.8 10.sup.12 D.sub.0 (cm.sup.2/s) PW.fwdarw.PB 7.8 10.sup.10 3.3 10.sup.10 6.8 10.sup.10 3.4 10.sup.10

[0050] FIG. 5 is an electrochemical impedance spectroscopy of the electrochemical devices measured by changing metal cations in carbon quantum dot ionic compounds using three electrode system, and Table 2 summarizes the measured impedance measurements with coloration reaction (PB) and decoloration reaction (PW) using the circulating current voltage method in the three-electrode system.

TABLE-US-00002 TABLE 2 0.5M KCl 0.5M K.sup.+-C-dots.sup. 0.5M Na.sup.+-C-dots.sup. 0.5M Li.sup.+-C-dots.sup. D.sup.0(cm.sup.2/s) PB.fwdarw.PW 2.7 10.sup.10 7.7 10.sup.11 2.0 10.sup.10 1.8 10.sup.12 PW.fwdarw.PB 7.8 10.sup.10 3.3 10.sup.10 6.8 10.sup.10 3.4 10.sup.10 PB.fwdarw.PW R.sub.s (W) 34 34 45 71 Intercalation R.sub.ct (W) 62 52 285 359 PW.fwdarw.PB R.sub.s (W) 33 35 46 72 Detercalation R.sub.ct (W) 80 53 1585 4949

[0051] The durability of the electrochromic devices prepared in Example 1 and Comparative Example 1 was tested. FIG. 7 shows the test result of electrolyte durability of an electrochromic device, which is an example of electrochemical devices manufactured in Examples and Comparative Examples according to the present invention. As can be seen in FIG. 7, it can be seen that color change efficiency maintained constant even after 1000 cycles in the electrochromic device using a electrolyte, (C-dot).sup.K.sup.+, according to the present invention, whereas the color change efficiency is reduced to less than the initial half level within 50 cycles in the electrochromic device using a conventional KCl electrolyte. This means that the electrochromic device using the (C-dot).sup.K.sup.+ electrolyte has excellent durability. Specifically, it can be concluded that (1) the (C-dot).sup.K.sup.+ ionic compound serves as an electrolyte, (2) the electrochemical durability of the (C-dot).sup.K.sup.+ electrolyte is excellent, and (3) less electrochemical side reactions are induced in device. Coloration efficiency (CE) is determined by the change in absorbance from the amount of charge required for a chromogenic or discolored state (OD ()=log T.sub.b/T.sub.c, T.sub.b and T.sub.c means transmittance at 700 nm). The discoloration efficiency values of 0.5 M KCl and (C-dot).sup.K.sup.+ electrolyte were 81.6 cm.sup.2/C and 103.0 cm.sup.2/C, respectively. Therefore, it can be concluded that electrochromic devices using a carbon quantum dot ionic compounds are relatively superior in electrochromic stability and discoloration efficiency than that using KCl electrolyte. Table 3 summarizes the changes in absorbance and so forth.

[0052] In order to apply to the actual electrochromic device system, the characteristics of the sandwich type electrochromic device were evaluated. In the device test, the change in device transmittance at 700 nm was monitored according to the applied voltage change. The device generates a discoloration reaction by applying a pulse voltage of 1.2 V (colored state) to 2.2 V (colored state) with a pulse width of 10 seconds(FIG. 6). Theoretically, compared to the electrochromic device of the three-electrode system, the sandwich type electrochromic device shows a relatively high voltage charge injection due to the voltage drop phenomenon.

TABLE-US-00003 TABLE 3 OD (cm.sup.2/C) T(%) T.sub.200 cycle T.sub.300 cylce log(Tb/Tc) (OD/Q) ( = 700 nm) t.sub.b(s) t.sub.c(c) (%) (%) 0.5M KCl 0.71 81.6 67 1.9 3.0 1.0M KCl 0.96 110.5 92 1.7 2.5 5 0.5M K.sup.+-C- 0.89 103.0 85 1.6 2.8 99 98 dots.sup. 0.5M Na.sup.+-C- 0.71 84.0 66 2.4 2.6 39 dots.sup. 0.5M Li.sup.+-C- 0.52 78.7 dots.sup.

[0053] FIG. 8 shows the results of transmittance change measurement with voltage switching in an electrochromic device, and inserted photographs of coloring/discoloring according to transmittance of the electrochromic device. As shown in FIG. 8, the electrochromic device comprising carbon quantum anion-metal cation ion compound electrolyte of the present invention shows excellent performance, and in particular, durability compared to that adopting a conventional electrolyte.

Example 3 (Electrochemical Light Emitting Device)

[0054] An electrochemical light emitting device was prepared ad follows;

[0055] (1) Forming a thin film of TiO2 particles on the surface of the cathode.

[0056] (2) Performing heat treatment on the TiO2 thin film coated cathode at 120 C. for 10 minutes in order to increase the conductivity and transmittance.

[0057] (3) Immersing the cathode in which the TiO2 thin film was formed in an emitting material solution for 55 C. for 6 hours.

[0058] (4) After 6 hours, washing the surface of the cathode with ethanol.

[0059] (5) Attaching the cathode and anode inside the device using a thermal tape respectively.

[0060] (6) Injecting solution containing the light-emitting material and the electrolyte through the hole formed in the anode.

[0061] (7) Sealing the hole.

[0062] FIG. 9 shows a light emission intensity measurement result according to the carbon quantum dot ion compound concentration under the two-electrode system conditions for the electroluminescent device in one embodiment according to the present invention. As can be seen in FIG. 9, as the carbon quantum point anion-metal cation ion compound concentration is increased, the ionic conductivity is improved, thereby reducing the resistance in the device and eventually increasing the luminescence intensity.

Example 4 (Lithium Secondary Battery)

[0063] FIG. 10 shows the results of measuring the specific capacitance with charging and discharging the lithium secondary battery applying the carbon quantum anion-lithium metal cation ion compound electrolyte of the present invention instead of LiPF.sub.6 electrolyte of the conventional secondary battery. The anode of the battery was constructed using Li.sub.4Ti.sub.5O.sub.12 (active material, LTO), 10 wt. % PVDF (binder) and NMP (Solvent), and the cathode was graphite. The concentration of the carbon quantum point anion-lithium metal cation ion compound electrolyte was 0.5M. As shown in FIG. 10, it was found that a stable charge/discharge cycle is observed in the secondary battery.

Example 5 (Lithium Secondary Battery Electrolyte Evaluation)

[0064] In order to check the characteristics of the electrolyte, various experiments were performed on the lithium ion battery of above Example 4. First, a cyclic voltammetry was performed with varying concentrations of an electrolyte applied to a lithium ion battery. FIGS. 11 (a) to 11 (c) show cyclic voltammetry measurements with varying the concentrations (0.125, 0.25 and 0.5M, respectively) of the carbon quantum dot anion-lithium cation ion compound electrolyte prepared according to the present invention. As shown in FIG. 11, the higher the electrolyte concentration (the higher the content) shows the same tendency as shown in the charge and discharge data, and it can be seen that the difference in electrochemical performance. Although that of 0.25M sample showed unstable in the anodic region from 2.5 V or higher, that of 0.5M sample, though it was slightly shifted from 1.6 V, which is the theoretical Li ion intercalation/deintercalation region of the LTO, shows distinct anodic/cathodic peaks. Table 4 below shows the results of measuring polarization according to each concentration.

TABLE-US-00004 TABLE 4 0.125 0.25 0.5 Polarization 0.715 V 0.542 V 0.43 V

[0065] In order to determine the rate characteristic of the electrolyte according to each concentration, charge and discharge cycling was performed at various current densities of 80 to 400 mA/g, and the results are shown in FIG. 12. FIG. 12 (a) is the result of the 0.5M concentration electrolyte, (b) is the result of measuring the cycle for each concentration. As can be seen in FIG. 15, the rate characteristic of the relatively high concentration of 0.5M sample was the best. When calculating the average capacity for each current density, that of 0.5M sample is the best at all current density. The results were summarized in Table 5.

TABLE-US-00005 TABLE 5 0.125 0.25 0.5 80 mA/g 193.47 194.28 194.33 160 mA/g 166.45 161.96 171.67 240 mA/g 148.38 151.61 162 320 mA/g 121.36 134.46 149.33 400 mA/g 78.91 104.28 134.33

[0066] As can be seen from Table 5, in particular, when the current density of 320, 400 mA/g, the performance of each sample shows the largest difference, which is due to the concentration of the electrolyte, that is, the difference in the content of Li ions.

[0067] In addition, the specific capacity change of the Li metal(anode)/electrolyte/graphite (cathode) battery system was observed while changing the voltage after constructing the half cell system in the lithium ion battery of Example 4. For comparison, a lithium ion battery employing LiPF.sub.6 as an electrolyte was prepared and the same observation was performed. FIG. 13 (a) shows voltage-capacitance measurement results measured at an electrode of both lithium ion batteries applying a carbon quantum dot ion compound electrolyte and a LiPF.sub.6 electrolyte respectively in Example 5 of the present invention and (b) is differential result with respect to voltage. As shown in FIG. 13, when LiPF.sub.6 was used as an electrolyte, a reaction estimated to form SEI was observed at 0.75 V, whereas when a carbon-dots electrolyte is used, a reaction at 0.75 V was not observed, but observed at 0.5 V estimated by reaction of graphite and Li.

[0068] The ion mobility of lithium ions was measured also. Equation 1 below is a formula for obtaining the ion mobility index of the cation. The equation is represented by a number from 0 to 1, and the closer to 1, the higher the contribution of charge transfer by cation.

[00001]$\begin{array}{cc}{t}_C=\frac{I_C}{I_C+I_A}& \left[\mathrm{Equation}.Math..Math.1\right]\end{array}$

[0069] Where t.sub.C=Cation transference number, I.sub.C=Current carried by cations, I.sub.A=Current carried by anions

[0070] Equation 1 may be expressed as Equation 2 below to measure the ion mobility index of Li ions.

[00002]$\begin{array}{cc}{t}_{\mathrm{Li}}=\frac{I_{\mathrm{SS}}\left(V-I_O.Math.R_O\right)}{I_O\left(V-I_{\mathrm{SS}}.Math.R_{\mathrm{SS}}\right)}& \left[\mathrm{Equation}.Math..Math.2\right]\end{array}$

[0071] In equation 2, t.sub.Li=Lithium transference number, V=Applied potential, R.sub.O=Initial resistance of the passivation layer, R.sub.SS=Resistance of the passivation layer, I.sub.O=Initial current, I.sub.SS=Steady state current.

[0072] A symmetrical cell of Li metal/electrolyte/Li metal was prepared, and initial impedance R.sub.O was measured(frequency range: 100 kHz-0.1 Hz), DC polarization experiment was performed at 0.05 mV to measure I.sub.SS, I.sub.O, and the impedance was measured again to determine R.sub.SS. And the result of that using LiPF.sub.6 as the electrolyte in the same manner was measured as a comparative example. FIG. 14 and Table 6 summarize the results.

TABLE-US-00006 TABLE 6 (Carbon-dots).sup.Li.sub.x.sup.+ LiPF.sub.6 t.sub.Li 0.77 0.1 0.44

[0073] As can be seen in FIG. 14 and Table 6, the carbon quantum dot anion-lithium cation ionic compound electrolyte of the present invention was found to be 1.5 to 2 times higher charge transfer index by the cation than LiPF.sub.6. Considering that a smaller t.sub.Li increases the overall resistance of the cell due to concentration polarization of anions in the electrolyte and the cation yield may be affected by the temperature, the concentration of salt in the electrolyte and the radius of the ions, the high t.sub.h of the electrolyte of the present invention is deemed to be caused by the large anion radius of the carbon dot.

[0074] Although the invention has been described with reference to specific exemplary embodiments, it is apparent for a person skilled in the art that various changes can be made and equivalents can be used as a replacement without departing from the scope of the invention. The invention should consequently not be restricted to the disclosed exemplary embodiments, but rather should enclose all the exemplary embodiments which fall into the scope of the enclosed claims. In particular, the invention also claims protection for the subject matter and the features of the subordinate claims independently of the claims referred to.