PL248340B1 - Lithium-ion hybrid capacitor - Google Patents

Abstract

The subject of the application is a hybrid lithium-ion capacitor consisting of negative and positive electrodes placed in an anhydrous organic electrolyte, separated by a separator, where the positive electrode is made of carbon with a highly developed specific surface area, and the negative electrode is made of graphite. The electrolyte contains lithium acetate of formula 1.

Description

Description of the invention

The subject of the invention is a hybrid lithium-ion capacitor for use as an energy conversion and storage device.

Electric Double-Layer Capacitors (EDLCs), also known as supercapacitors (SCs), consist of two electrodes, most often made of activated carbon (AC), separated by a separator and immersed in an electrolyte solution, as exemplified in patent application P.434065. EDLCs are very attractive solutions on the energy storage/conversion market due to their high power density, long cycle life, and relatively short charging time. However, their significant disadvantage is the relatively low energy density, which limits their use only to niche industries [K. Naoi, W. Naoi, S. Aoyagi, J. Miyamoto, T. Kamino, Acc. Chem. Res., 2012, 46, 1075-1083]. The energy of an electrochemical capacitor can be calculated using the following relationship:

E=;CU ² _: z.

where: C - capacitance of the electrochemical capacitor [F] U - operating voltage [V]

According to the above relationship, in order to increase the amount of energy stored in a capacitor, in addition to increasing capacitance, it seems particularly effective to develop systems operating at higher voltages due to the exponential dependence of the capacitor energy as a function of its operating voltage. Therefore, it is important to achieve higher electrochemical stability of the electrolyte, as described in patent description P.392742. Currently, the maximum voltage of a symmetrical EDLC, where both the positive and negative electrodes are made of activated carbon, is limited to 2.7 V. In practice, applying a voltage above 2.7 V causes – over time – a significant decrease in capacitance associated with increasing resistance, resulting from mechanical and chemical damage to the carbon electrodes. Undesirable Faradaic processes (side reactions, e.g., gas evolution) also lead to increased resistance in the system and, consequently, a decrease in the system's power and energy. Therefore, there is a voltage limit of 2.7 V, which certainly limits further increases in energy density [K. Naoi, S. Ishimoto, J. Miyamoto, W. Naoi, Energy Environ. Sci., 2012, 5, 9363],

Currently, significant efforts are being made to increase the energy density of EDLCs to a target of approximately 20–30 Wh-kg ^-1 . There are three main approaches to achieving this goal: the first is to change the electrode material to a carbon-containing material exhibiting higher capacitance values or exhibiting redox activity; the second approach is to change the electrolyte to a more electrochemically stable one; the third and most promising approach is the development of hybrid capacitors that combine the energy storage mechanism of EDLCs with the Faradaic energy storage mechanism (used, for example, in Li-ion cells). Much research has been undertaken, and currently the concept of a conventional symmetric electrochemical capacitor has been extended in practice to two different categories: EDLCs and hybrid lithium-ion systems [D. Cericola, P. Novak, A. Wokaun, R. Kotz, J. Power Sources, 2011, 196, 10305–10313].

Hybrid capacitors combining a lithium-ion battery with an electrical double layer system are an interesting alternative to EDLC. They exhibit higher energy density while maintaining acceptable power density and system lifetime [L. Dong, W. Yang, W. Yang, Y. Li, W. Wu, G. Wang. J. Mater. Chem. A. 2019, 7. 13810-13832]. The operation mechanism of a hybrid capacitor is based on the characteristic properties of each electrode material. Activated carbon with a developed specific surface area, constituting the positive electrode, enables non-Faradaic charge storage, and its potential changes linearly during charging and discharging. In the case of lithium-intercalated graphite, constituting the negative electrode, the potential is almost always constant during charging and discharging and enables Faradaic charge storage, as described in patent description P.413467. Initially, intercalating compounds such as Li ₄ Ti ₅ Oi 2 [B. Serosali, Electrochim. Acta, 2000, 45, 2461 -2466] or L 1 C 0 O 2 [A. Yoshino, T. Tsubala, M. Shimoyamada, H. Satake, Y. Okano, S. Mori, S. Yata, J. Electroch. Soc., 2004, 151, A2180-A2182] were used for hybrid capacitors in organic electrolyte.

However, in the case of the most promising system based on Li4TisOi2 and activated carbon, a significantly larger mass of the positive electrode relative to the negative electrode is required to balance the high negative energy density, and the voltage window is limited to the range of 1.2 to 3.2 V. Consequently, the energy density of the hybrid capacitor is only slightly higher than that of symmetric EDLCs [G.G. Amatucci, F. Badaway, A.D. Pasquier, T. Zheng, J. Electrochem. Soc., 2001, 148, A930-A939]. Another approach to increasing the operating voltage is to combine a graphite negative intercalation electrode and a positive activated carbon double layer electrode using a lithium salt in an organic electrolyte. In this system, the negative graphite electrode is prelithified using a lithium metal auxiliary electrode to form a stable intercalation compound [T. [Aida, K. Yamada, M. Morita, Electrochem. Solid-Stale Lett., 2006, 9, A534-A536]. After the intercalation process, the cell can be charged and discharged within a voltage range of 1.9 to 3.6 V. The system proposed by Aida uses activated carbon and non-graphitizable carbon as the positive and negative electrodes. Without pre-intercalation of the negative electrode, the capacity of the hybrid device decreases over several dozen charge/discharge cycles. In the Fuji system, the graphite must be pre-intercalated with lithium to obtain a hybrid capacitor with a satisfactory service life. However, experience from lithium-ion battery technology shows that the use of metallic lithium can lead to thermal runaway and system ignition, as mentioned in patent US2007/0002524A1. The next step in technology development would be to eliminate the auxiliary lithium electrode.

The literature describes concepts of lithium-ion capacitors in which lithium ions are derived from a highly concentrated electrolyte solution [C. Decaux, G. Lota, E. Raymundo-Pinero, E. Frackowiak, F. Beguin. Electrochim: Acta, 2012, 86, 282-286]. Because the electrolyte is an integral part of the entire system, the use of metallic lithium and all the safety issues associated with its use was avoided. The electrochemical cell was constructed with activated carbon as the positive electrode and graphite as the negative electrode. In contrast to the commonly used electrolyte (1-molar LiPF6 solution in EC:DMC), a 2-molar solution of lithium bis(trifluoromethane)sulfonimide (LiTFSI) in an EC:DMC solvent mixture was used. The choice of LiTFSI was dictated by the distinctive solubility of this salt compared to conventional lithium salts. During cell charging, the potential of the activated carbon electrode increases while the potential of the graphite electrode decreases due to the intercalation of lithium ions from the electrolyte solution. Because the maximum potential of the positive electrode must be limited to approximately 4.2 V relative to Li/Li+ (to avoid electrolyte oxidation), it is impossible to introduce all the necessary lithium ions in a single charge pulse at C/10 (where C is the theoretical capacity of graphite). Despite the presence of an excess of lithium ions in the electrolyte, the pre-intercalation process leads to a noticeable reduction in the concentration of Li+ ions in the electrolyte volume. Consequently, reducing the electrolyte concentration causes a decrease in conductivity, which unfortunately leads to a reduction in the system's power. Therefore, it is necessary to ensure that, despite the intercalation of the graphite electrode with lithium ions from the solution, enough salt remains in the electrolyte to maintain its high conductivity. Additionally, taking into account the balance of charge accumulated on the negative and positive electrodes, the concentration of the redox reagent should be appropriately selected so that the intercalation charge (negative electrode) is balanced by the redox reaction (positive electrode).

The previously presented solution yielded positive results, as described in patent application P.437689. This application utilized a lithium thiocyanate salt, but an unknown oxidation reaction of the thiocyanate causes deposits to form on the positive electrode. This results in poor capacitor cyclic performance. Therefore, the search continues for an electrolyte additive that, in addition to effective intercalation, will also ensure long-term lithium-ion capacitor life.

Lithium acetate is a substance known and often used in industrial chemistry, mainly as a buffer for DNA or RNA gel electrophoresis [J.R. Brody, E.S. Calhoun, E. Gallmeier, T. D. Creavalle, S. E. Kern. Biotechniques. 2018, 37, 4]. Lithium acetate is also used in materials chemistry to modify graphene-polyurethane composites [Y. Jiang, C. Lu, X. Liu, Y. Jiang, Y. Ding, Micro Nano Lett. 2020, 15, 213 - 217], as well as in electrochemistry as a hydrogel polymer electrolyte of an electrochemical capacitor operating in an aqueous environment (Pat.240381) or in the regeneration of a carbonate fuel cell electrolyte (Pat.234555). However, the use of lithium acetate as an additive to an organic, anhydrous electrolyte to carry out a controlled oxidation reaction and obtain a capacitance enabling one-step intercalation of the negative electrode in a lithium-ion capacitor is unknown so far.

The invention is based on a hybrid lithium-ion capacitor consisting of a negative and positive electrode separated by a separator. The entire assembly is immersed in an anhydrous, organic electrolyte solution containing lithium acetate (LiOAc) of formula 1.

Preferably, the mass of LiOAc in the electrolyte of the capacitor should be at least 0.7 times the mass of the active graphite electrode and at most 1.5 times the mass of the active graphite electrode, preferably this ratio is equal to 0.9 (graphite : LiOAc).

The optimal electrolyte variant provides for a mixture of lithium acetate (LiOAc) of formula 1 in a 1-molar solution of lithium hexafluorophosphate (LiPF6) of formula 2, as well as solvents: ethylene carbonate (EC) of formula 3 and dimethyl carbonate (DMC) of formula 4. The volume ratio of EC:DMC is 1:1.

In the proposed system, the negative electrode is composed of a graphite-containing material, and the positive electrode is composed of a carbon material with a high specific surface area. The system allows lithium intercalation into the negative electrode using lithium ions from the electrolyte. The hybrid lithium-ion capacitor according to the invention is suitable for use at temperatures ranging from -25 to 65°C, preferably 30°C, at a voltage of 1.8 to 4.5 V, preferably 2.2 to 4.2 V.

The use of the solution according to the invention allowed for the following technological and operational benefits:

• applicable operating voltage - 2.2 - 4.2 V, • no need for pre-intercalation of the negative electrode using metallic lithium, • high electrochemical stability of the electrolyte.

The subject of the invention in an example embodiment is shown in the drawing, in which Fig. 1 shows the intercalation curve of lithium ions into the graphite structure with the addition of lithium acetate to the electrolyte, Fig. 2 shows a voltammogram showing the redox activity of acetate ions in the electrolyte - the presented curve shows a system in which the positive electrode is activated carbon and the negative electrode is metallic lithium, Fig. 3 shows a galvanostatic charge/discharge curve showing the redox activity of acetate ions in the electrolyte - the presented curve shows a system in which the positive electrode is activated carbon and the negative electrode is metallic lithium.

The invention is a hybrid lithium-ion capacitor based on a mixture of lithium acetate salt with lithium hexafluorophosphate in ethylene carbonate and dimethyl carbonate, the method of preparation of which is presented in the following examples:

Example I

A 1 ^dm3 volumetric flask was filled with 0.1 mol of lithium acetate and topped up with a 1-molar solution of lithium hexafluorophosphate in ethylene carbonate and dimethyl carbonate in a 1:1 volume ratio. The components were mixed at room temperature until a homogeneous mixture, called the electrolyte, was obtained. The negative electrode of the hybrid lithium-ion capacitor was made of a copper foil coated with a layer of a mixture of: SLC1512P graphite (91%), polyvinylidene fluoride (PVdF) (8%), and C65 carbon black (1%). The positive electrode of the hybrid lithium-ion capacitor was made of an aluminum foil coated with a layer of: Kuraray YP80F (80%), polytetrafluoroethylene (PTFE) (5%), and C65 carbon black (15%). Both electrodes, each with a surface area of 2 cm2 ^, were separated using two GF/D separators. The reference electrode was made of lithium metal. This complex arrangement was placed in a three-electrode vessel and then soaked in an electrolyte solution.

The constructed system was subjected to galvanostatic charging/discharging at a temperature of 65°C: the current density was expressed in terms of the mass of the negative electrode (0.5 Ag ^-1 ; 2.2 - 4.5 V).

Example II

A 1 ^dm3 volumetric flask was filled with 0.2 mol of lithium acetate and topped up with a 1-molar solution of lithium hexafluorophosphate in ethylene carbonate and dimethyl carbonate in a 1:1 volume ratio. The components were mixed at room temperature until a homogeneous mixture, called the electrolyte, was obtained. The negative electrode of the hybrid lithium-ion capacitor was made of a copper foil coated with a layer of a mixture of: SLC1512P graphite (91%), polyvinylidene fluoride (PVdF) (8%), and C65 carbon black (1%). The positive electrode of the hybrid lithium-ion capacitor was made of an aluminum foil coated with a layer of: Kuraray YP80F (80%), polytetrafluoroethylene (PTFE) (5%), and C65 carbon black (15%). Both electrodes, each with a surface area of 2 cm2 ^, were separated using two GF/D separators. The reference electrode was made of lithium metal. This complex arrangement was placed in a three-electrode vessel and then soaked in an electrolyte solution.

The constructed system was subjected to galvanostatic charging/discharging at 0°C: the current density was expressed in terms of the mass of the negative electrode (0.3 Ag ^-1 ; 1.8 - 3.8 V).

Example III

A 1 ^dm3 volumetric flask was filled with 0.01 mol of lithium acetate and topped up with a 1-molar solution of lithium hexafluorophosphate in ethylene carbonate and dimethyl carbonate in a 1:1 volume ratio. The negative electrode of the lithium-ion hybrid capacitor was made of aluminum foil coated with a layer of a mixture of: SLC152P graphite (91%), polyvinylidene fluoride (PVdF) (8%), and C65 carbon black (1%). The positive electrode of the lithium-ion hybrid capacitor was made of aluminum foil coated with a layer of: Kuraray YP80F (80%), polytetrafluoroethylene (PTFE) (5%), and C65 carbon black (15%). Both electrodes, each with a surface area of 2 ^cm2, were separated using two GF/D separators. The reference electrode was made of metallic lithium. This complex system was placed in a 3-electrode vessel and then soaked in an electrolyte solution.

The constructed system was subjected to galvanostatic charging/discharging at a temperature of 30°C: the current density was expressed in terms of the mass of the negative electrode (0.2 Ag ^-1 ; 1.8 - 3.6 V).

Example IV

A 1 ^dm3 volumetric flask was filled with 0.05 mol of lithium acetate and topped up with a 1-molar solution of lithium hexafluorophosphate in ethylene carbonate and dimethyl carbonate in a 1:1 volume ratio. The components were mixed at room temperature until a homogeneous mixture, called the electrolyte, was obtained. The negative electrode of the hybrid lithium-ion capacitor was made of a copper foil coated with a layer of a mixture of SLC1512P graphite (91%), polyvinylidene fluoride (PVdF) (8%), and C65 carbon black (1%). The positive electrode of the hybrid lithium-ion capacitor was made of an aluminum foil coated with a layer of Kuraray YP80F (80%), polytetrafluoroethylene (PTFE) (5%), and C65 carbon black (15%). Both electrodes, each with a surface area of 2 cm2 ^, were separated using two GF/D separators. The reference electrode was made of lithium metal. This complex arrangement was placed in a three-electrode vessel and then soaked in an electrolyte solution.

The constructed system was subjected to galvanostatic charging/discharging at a temperature of 50°C: the current density was expressed in terms of the mass of the negative electrode (0.1 Ag ^-1 ; 2.0 - 4.0 V).

Example V

A 1 ^dm3 volumetric flask was filled with 1 mole of lithium acetate and topped up with a 1-molar solution of lithium hexafluorophosphate in ethylene carbonate and dimethyl carbonate in a 1:1 volume ratio. The components were mixed at room temperature until a homogeneous mixture, called the electrolyte, was obtained. The negative electrode of the hybrid lithium-ion capacitor was made of a copper foil coated with a layer of a mixture of: SLC1512P graphite (91%), polyvinylidene fluoride (PVdF) (8%), and C65 carbon black (1%). The positive electrode of the hybrid lithium-ion capacitor was made of an aluminum foil coated with a layer of: Kuraray YP80F (80%), polytetrafluoroethylene (PTFE) (5%), and C65 carbon black (15%). Both electrodes, each with a surface area of 2 cm2 ^, were separated using two GF/D separators. The reference electrode was made of lithium metal. This complex arrangement was placed in a three-electrode vessel and then soaked in an electrolyte solution.

The constructed system was subjected to galvanostatic charging/discharging at room temperature: the current density was expressed in terms of the mass of the negative electrode (1 Ag ^-1 ; 2.0 - 4.0 V).

Example VI

A 1 ^dm3 volumetric flask was filled with 0.5 mol of lithium acetate and topped up with a 1-molar solution of lithium hexafluorophosphate in ethylene carbonate and dimethyl carbonate in a 1:1 volume ratio. The components were mixed at room temperature until a homogeneous mixture, called the electrolyte, was obtained. The negative electrode of the hybrid lithium-ion capacitor was made of a copper foil coated with a layer of a mixture of SLC1512P graphite (91%), polyvinylidene fluoride (PVdF) (8%), and C65 carbon black (1%). The positive electrode of the hybrid lithium-ion capacitor was made of an aluminum foil coated with a layer of Kuraray YP80F (80%), polytetrafluoroethylene (PTFE) (5%), and C65 carbon black (15%). Both electrodes, each with a surface area of 2 cm2 ^, were separated using two GF/D separators. The reference electrode was made of lithium metal. This complex arrangement was placed in a three-electrode vessel and then soaked in an electrolyte solution.

The constructed system was subjected to galvanostatic charging/discharging at room temperature: the current density was expressed in terms of the mass of the negative electrode (0.4 Ag ^-1 ; 2.4 - 4.4 V).

Claims (3)

1. A hybrid lithium-ion capacitor consisting of a negative and a positive electrode placed in an anhydrous organic electrolyte and separated by a separator, wherein the positive electrode is made of carbon with a highly developed specific surface area and the negative electrode is made of graphite, characterized in that the electrolyte contains lithium acetate of formula 1. 2. A hybrid lithium-ion capacitor according to claim 1, characterized in that the amount of lithium acetate additive in the electrolyte is in proportion to the mass of the active negative electrode in a graphite: lithium acetate mass ratio in the range of 0.7 - 1.5, preferably 0.9. 3. A hybrid lithium-ion capacitor according to claim 1 or 2, characterized in that the electrolyte is a mixture of lithium acetate of formula 1 in a 1-molar solution of lithium hexafluorophosphate of formula 2, and solvents: ethylene carbonate (of formula 3) and dimethyl carbonate (of formula 4), wherein the volume ratio of ethylene carbonate: dimethyl carbonate is 1:1.