KR101283336B1 - Electrolyte Solution Membrane Containing Liquid Electrolyte Solution - Google Patents

Abstract

The present invention can form an insulating layer that electrically insulates the positive electrode and the negative electrode when applied between the positive electrode and the negative electrode, and comprises an electrolyte solution comprising a electrolyte solution, characterized in that from the separator solution containing the electrolyte solution An electrolyte comprising a separator including an electrolyte, an electrode including a separator layer including an electrolyte solution, wherein the separator solution including an electrolyte solution is directly applied to an electrode, and the separator solution including an electrolyte solution, or the separator including an electrolyte solution, or an electrode of the separator layer including an electrolyte solution. It provides an electrochemical cell, characterized in that.

Description

Electrolyte Separator Containing Liquid Electrolyte {Electrolyte Solution Membrane Containing Liquid Electrolyte Solution}

The present invention can form an insulating layer that electrically insulates the positive electrode and the negative electrode when applied between the positive electrode and the negative electrode, and comprises an electrolyte solution comprising a electrolyte solution, characterized in that from the separator solution containing the electrolyte solution An electrolyte comprising a separator including an electrolyte, an electrode including a separator layer including an electrolyte solution, wherein the separator solution including an electrolyte solution is directly applied to an electrode, and the separator solution including an electrolyte solution, or the separator including an electrolyte solution, or an electrode of the separator layer including an electrolyte solution. It relates to an electrochemical cell, characterized in that.

Problems caused by environmental pollution, global warming, and resource depletion due to the use of fossil fuels are emerging every day, and many researchers are concentrating on developing alternative energy as a way to solve them. Various methods such as solar, wind, tidal, geothermal and nuclear power are being sought, but the use of such a power generation system for portable and mobile is still difficult to commercialize, and the manufacturing cost is also very difficult to commercialize at present. In addition, for the development of such alternative energy, as well as a new power generation system, a battery that is a power storage system that can efficiently manage and use the obtained power is essential.

Lead-acid batteries are mainly used in some systems, but their volume is very large, there are environmental problems, and their lifetimes are also not at the desired level. In order to overcome these problems, Ni-MH batteries, lithium secondary batteries and the like are actively employed in portable systems and automotive battery systems. In particular, research on lithium secondary batteries having high energy density per volume and weight and excellent life characteristics has been actively conducted.

In addition, as technology development and demand for mobile devices increase, the demand for secondary batteries as energy sources is rapidly increasing, among them, high energy density and operating potential, long cycle life, and low self discharge rate Lithium secondary batteries have been commercialized and widely used.

Recently, with increasing interest in environmental issues, electric vehicles (EVs) and hybrid electric vehicles (HEVs), which can replace fossil fuel-based vehicles such as gasoline and diesel vehicles, which are one of the main causes of air pollution, There is a lot of research going on. Although a nickel metal hydride (Ni-MH) secondary battery is mainly used as a power source for such an electric vehicle (EV) and a hybrid electric vehicle (HEV), a lithium secondary battery having a high energy density, a high discharge voltage, Research is being actively carried out and it is in the stage of commercialization.

In particular, lithium secondary batteries used in electric vehicles have high energy density and high power output in a short time, and must be able to be used for 10 years or more under severe conditions. Lifespan characteristics are inevitably required. However, in the case of a lithium secondary battery, the energy density per volume and weight is high, so it is vulnerable to safety.

In general, a secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator. After winding or laminating together with a positive electrode, a negative electrode, and a separator, the secondary battery is inserted into a cylindrical or square can, a polymer pouch, etc. Therefore, the gas is removed and then assembled by sealing using welding or the like. After such an assembling process, an aging process of about 1 to 7 days is performed to impregnate the electrolyte solution and stabilize the battery. The aging process is a very important process in order for the battery to exhibit uniform performance and to improve cycle characteristics.

Due to the need for the electrolyte injection and aging process, it is very difficult to improve the process of the battery, and there is a problem such as a rapid increase in battery manufacturing time.

In addition, as a major cause of the deterioration of the current battery, safety problems, process defects, ignition and explosion in a harsh environment, in general, mismatch of the positive electrode, the negative electrode and the separator, and shrinkage of the separator The short circuit by this, or the space | interval generation between electrodes by gas generation | occurrence | production and an impurity production | generation etc. are mentioned. In addition, the difference in electrode resistance due to such spacing increases the problem exponentially, causing a larger problem.

Due to the above problems it is very difficult to change the structure of the battery or improve the process.

Accordingly, various methods have been attempted to solve these problems. For example, KR 2009-0031628 et al. Propose a method of using a polymer membrane with ion conductivity. However, even when the polymer has ion conductivity, when used with a solid active material of a positive electrode or a negative electrode, the resistance at the interface between the polymer and the solid rapidly increases. Due to this increasing problem, it is very difficult to use it as it is. In addition, KR 2001-0060192 discloses a gel-like membrane, but it merely serves as a separator. In addition, WO 98/053465, etc., relates to a polyolefin membrane for immobilizing an electrolyte solution, in which an electrolyte solution is impregnated with an electrolyte fixing agent such as a monomer to a polyolefin polymer membrane that is generally used. It impregnates the electrolyte solution and does not contain the electrolyte solution in the separator itself. In addition, it is proposed to use the immobilized liquid film conductor for the positive electrode or the negative electrode, which is a very complicated process for use in an actual battery, and it is difficult to exhibit particular advantages.

As such, many studies have been conducted, but it is true that there are many limitations for use in actual battery systems.

SUMMARY OF THE INVENTION Accordingly, the present invention has been made to solve the above-mentioned problems of the prior art and the technical problems required from the past.

After extensive research and various experiments, the inventors of the present application, as described later, apply a coating solution containing an electrolyte solution and an electrically insulating substrate as a solvent directly to the positive electrode and / or the negative electrode, or such a coating solution. Nevertheless, we developed a new concept of electrolyte-impregnated separator solution that can be used in the form of an insulating layer and a separator made from it. It was confirmed that can be solved at once, and came to complete the present invention.

Accordingly, the present invention can form an insulating layer that electrically insulates the positive electrode and the negative electrode when applied between the positive electrode and the negative electrode, and provides a separator solution containing an electrolyte, characterized in that consisting of a composition containing an electrolyte.

In addition, the present invention provides a separator containing an electrolyte, characterized in that made from the separator solution containing the electrolyte.

The present invention also provides an electrode, characterized in that the electrolyte solution-containing separator solution comprises a separator layer including an electrolyte applied directly to the electrode.

Based on this, the present invention provides an electrochemical cell comprising an electrode of the electrolyte solution-containing separator, or the electrolyte solution-containing separator, or the electrolyte-containing separator layer.

As described above, according to the present invention, a predetermined electrolyte solution-containing separator, a predetermined electrolyte-containing separator, or a predetermined electrolyte-containing separator layer is interposed between the positive electrode and the negative electrode, thereby providing desired excellent safety and operating performance. Can be exerted, and the manufacturing process can be greatly simplified.

Hereinafter, the present invention will be described in further detail with reference to the following examples. However, the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

In one preferred embodiment, the electrochemical cell is an insulating layer between the positive electrode and the negative electrode, the electrolyte solution containing the separator, or the electrolyte containing separator, or the electrolyte containing separator layer is interposed, the insulating layer is a solvent It can be formed by applying a coating solution containing an electrolyte and an electrically insulating substrate directly to the positive electrode and / or the negative electrode.

Therefore, the electrochemical cell of the present invention includes a predetermined insulating layer which enables the movement of ions while electrically insulating the positive electrode and the negative electrode, and a kind of solvent containing an electrolyte solution which is included as an essential element in the electrochemical cell. It is characterized in that the coating is formed on the electrode using.

Therefore, it is based on a special structure not found in the existing electrochemical cell, and its manufacturing process can be innovatively reduced.

Such electrochemical cells are not particularly limited as long as they can obtain electrical energy through chemical change, and preferably, secondary batteries, capacitors, fuel cells, and the like, more preferably secondary batteries Can be.

Representative examples of the secondary battery may include a lithium secondary battery. In this case, the electrolyte membrane may include a lithium salt in addition to the electrolyte and the electrically insulating substrate. The lithium salt is a lithium-containing material exhibiting a high dissociation degree in the electrolyte, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide have.

As described above, the electrochemical cell according to the present invention does not include a separate separator between the anode and the cathode, and the electrically insulating substrate in the insulating layer provides electrical insulation between the anode and the cathode.

Examples of such electrically insulating substrates include one or two or more selected from the group consisting of an electrically insulating polymer, an ion conductive polymer, cellulose, an electrically insulating inorganic material, and an ion conductive inorganic material. Especially, an ion conductive polymer is especially preferable.

In the present invention, any solvent may be used as long as the solvent is generally a substance which can be used as an electrolytic solution of an electrochemical cell. Specific examples include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butylo lactone, 1,2 Dimethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolon, formamide, dimethylformamide, dioxorone, acetonitrile, nitromethane, formic acid Methyl, methyl acetate, phosphate triester, trimethoxy methane, dioxorone derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, Aprotic organic solvents such as ether, methyl pyroionate and ethyl propionate can be used. Especially, the mixture of linear carbonate and cyclic carbonate is especially preferable.

In some cases, for the purpose of improving the charge and discharge characteristics, flame retardancy, etc. to the electrolyte solution, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate Triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol, aluminum trichloride It may also be added. In addition, in order to impart nonflammability, a halogen-containing solvent such as carbon tetrachloride and ethylene trifluoride may be further included, and carbon dioxide gas may be further included to improve high temperature storage characteristics, and may include FEC (Fluoro-Ethylene carbonate), PRS (Propene sultone), FEC (Fluoro-Ethlene carbonate) may be further included.

In the present invention, if too little of the electrically insulating substrate is included in the coating solution, it may not be able to properly perform the role of the separator in the electrochemical cell may cause a short (short) between the positive electrode and the negative electrode, it is difficult to apply a predetermined thickness On the contrary, if too many electrically insulating substrates are included, the content of the relatively solvent electrolyte is too small to achieve the expected electrochemical performance, and the viscosity of the coating solution may be too high, making it difficult to apply uniformly. Considering the point, it can be adjusted appropriately.

In the present invention, the thickness of the insulating layer is preferably 3 ㎛ to 200 ㎛. If the thickness is 3 μm or less, a short circuit may occur between the positive electrode and the negative electrode, and if the thickness is 200 μm or more, the volume of the cell may be larger than necessary to cause a decrease in capacity per volume, and the resistance may be high, thereby improving operating performance. Can be degraded. Therefore, for the same reason as above, the thickness of the electrolyte membrane is more preferably 10 µm to 50 µm.

In one preferred embodiment, the coating liquid may be applied to the positive electrode or the negative electrode, and the electrode of the opposite electrode may be placed on the coating layer to form a laminate, and then pressurized to the thickness to form an electrolyte membrane.

In the pressurization process as described above, a part of the electrolyte solution included in the electrolyte membrane is introduced into the positive electrode and / or the negative electrode from the electrolyte membrane to impregnate the positive electrode and / or the negative electrode.

In this process, as the coating liquid increases in viscosity, the hardness of the solid film form is further enhanced to increase mechanical strength. In addition, the binding force is enhanced for the positive electrode and the negative electrode to exhibit high structural stability.

In addition, as described above in the example of the secondary battery, in the manufacturing process of a general secondary battery, a aging period is required to inject the electrolyte separately and impregnate the electrode, but to form a film containing the electrolyte as described above Since the electrode may be impregnated with the electrolyte in the process, it is possible to exclude or minimize the aging process in the manufacturing process of the cell.

In addition, since the remaining electrolyte solution other than the electrolyte solution moved to the electrode remains in the insulating layer, an additional pouring process of the electrolyte solution can also be excluded if necessary. However, the electrolyte may be additionally infused in order to secure an appropriate electrolyte.

Preferably, the insulating layer may be a structure that is bonded to the positive electrode and the negative electrode. Since the insulating layer is bonded to the positive electrode and the negative electrode, it is possible to prevent the short circuit caused by the shrinkage of the separator, which is a conventional problem, thereby improving the safety of the electrochemical cell. In addition, when the electrolyte membrane, the positive electrode, and the negative electrode are integrated, the bobbin pressure is further increased, whereby the safety can be further improved. For the adhesion of the electrolyte membrane and the electrode, a material used for adhesion of the separator and the electrode in a general secondary battery may be used as it is, and they may be added in a coating liquid or applied to the electrode surface.

In the electrochemical cell according to the present invention, the conductivity of the positive electrode active material or the negative electrode active material and the electrolyte membrane interface may be preferably in the range of 10 ⁻⁴ s / cm to 10 ⁻² s / cm.

In general, the conductivity of the interface between the solid electrolyte and the active material is 10 ⁻⁶ s / cm to 10 ⁻⁸ s / cm, while the internal conductivity of the solid electrolyte and the conductivity of the liquid electrolyte are 10 ⁻³ s / cm. Therefore, in the case of a lithium polymer battery using a solid electrolyte such as an ion conductive polymer, the conductivity of the interface between the solid electrolyte and the active material is low and its use has been limited. On the other hand, in the case of the present invention, the electrolyte may be impregnated with the electrode to improve the conductivity at the interface with the active material.

In the present invention, the positive electrode active material may be, for example, a layered compound such as lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), or a compound substituted with one or more transition metals; Lithium manganese oxides such as Li _{1 + x} Mn _{2 -x} O ₄ (where x is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ and the like; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ and Cu ₂ V ₂ O ₇ ; A Ni-site type lithium nickel oxide expressed by the formula LiNi _1-x M _x O ₂ (where M = Co, Mn, Al, Cu, Fe, Mg, B or Ga and x = 0.01 to 0.3); Formula _{LiMn 2-x M x O 2} ( where, M = Co, Ni, Fe , Cr, and Zn, or Ta, x = 0.01 ~ 0.1 Im) or Li ₂ Mn ₃ MO ₈ (where, M = Fe, Co, Ni, Cu, or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with an alkaline earth metal ion; Lithium composite transition metal oxide; Disulfide compounds; Olivine-type lithium iron phosphate (LiFePO ₄ ); Fe ₂ (MoO ₄ ) ₃ and the like.

A positive electrode mixture is prepared by including a conductive material, a binder, a filler, and the like in the positive electrode active material.

The conductive material is usually added in an amount of 1 to 30% by weight based on the total weight of the mixture including the cathode active material. Such a conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, for example, graphite such as natural graphite or artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskeys such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The binder is added to the binder in an amount of 1 to 30% by weight, based on the total weight of the mixture containing the cathode active material, as a component that assists in bonding between the active material and the conductive agent and bonding to the current collector. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, and various copolymers.

The filler is optionally used as a component for suppressing expansion of the anode, and is not particularly limited as long as it is a fibrous material without causing a chemical change in the battery. Examples of the filler include olefin-based polymerizers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

A positive electrode may be manufactured by applying a slurry prepared by mixing the positive electrode mixture prepared above to a solvent such as NMP onto a positive electrode current collector, followed by drying and rolling.

The cathode current collector is generally made to have a thickness of 3 to 500 mu m. Such a positive electrode collector is not particularly limited as long as it has conductivity without causing chemical change to the battery, and may be formed on the surface of stainless steel, aluminum, nickel, titanium, sintered carbon or aluminum or stainless steel Carbon, nickel, titanium, silver, or the like may be used. The current collector may have fine irregularities on the surface thereof to increase the adhesive force of the cathode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

In the present invention, the negative electrode active material may include, for example, carbon and graphite materials such as natural graphite, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotube, fullerene, and activated carbon; Metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt and Ti which can be alloyed with lithium and compounds containing these elements; Complexes of metals and their compounds and carbon and graphite materials; Lithium-containing nitrides, and the like.

Similarly to the positive electrode, a negative electrode mixture further comprising a conductive material, a binder, a filler, and the like is prepared, and a slurry prepared by mixing the negative electrode mixture with a solvent such as NMP is coated on a negative electrode current collector, followed by drying and rolling to form a negative electrode. Can be prepared.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and examples thereof include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel Surface-treated with carbon, nickel, titanium, silver or the like, aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

In the present invention, when additionally injecting a lithium salt-containing non-aqueous electrolyte, the lithium salt-containing non-aqueous electrolyte is composed of an electrolyte and a lithium salt, the electrolyte is a non-aqueous organic solvent, an organic solid electrolyte, Inorganic solid electrolytes and the like are used.

Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma -Butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane , Acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate Nonionic organic solvents such as tetrahydrofuran derivatives, ethers, methyl pyrophosphate, ethyl propionate and the like can be used.

Examples of the organic solid electrolyte include a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, A polymer containing an ionic dissociation group and the like may be used.

As the inorganic solid electrolyte, for example, Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides, sulfates, and the like of Li, such as Li ₄ SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ , and the like, may be used.

As described above, the lithium salt is a material soluble in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF _{3 CO 2, LiAsF 6, LiSbF} 6, LiAlCl 4, CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate, already And the like can be used.

The present invention also provides a method for producing the electrochemical cell. Specifically, the manufacturing method,

(a) preparing a positive electrode to which the positive electrode active material is applied to the current collector, and a negative electrode to which the negative electrode active material is applied to the current collector;

(b) preparing a coating solution containing an electrically insulating substrate in an electrolyte solution as a solvent;

(c) applying the coating liquid of the step (b) to at least one surface of at least one of the positive electrode and the negative electrode of the step (a);

(d) placing an electrode of the opposite electrode on the electrode of step (c) to form a laminate, and then pressing to form an electrode assembly; And

(e) inserting the electrode assembly of step (d) into the cell case and then sealing the cell assembly.

In the pressing step of the step (d), the electrolyte is a solvent in the coating solution applied to the electrode is impregnated by moving to the adjacent electrode, the coating solution is converted to the electrolyte membrane by increasing the viscosity, it is not necessary to use a separate membrane.

Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims (12)

A separator layer including an electrolyte solution is interposed between the anode and the cathode, and the insulation layer is formed by directly applying a coating solution containing an electrolyte solution and an electrically insulating substrate as a solvent to the cathode, or the cathode, or the anode and the cathode. The insulating substrate is an electrochemical, characterized in that one or more selected from the group consisting of ion conductive polymer, cellulose, electrically insulating inorganic material, and ion conductive inorganic material. delete delete delete delete delete The electrochemical cell of claim 1, wherein the electrically insulating substrate is an ion conductive polymer. The electrochemical cell of claim 1, wherein the solvent is a mixture of a cyclic carbonate compound and a linear carbonate compound. The electrochemical cell of claim 1, wherein the insulating layer has a thickness of 3 µm to 200 µm. The method of claim 1, wherein the insulating layer is formed by applying a coating solution to an anode or a cathode, placing an electrode of the opposite electrode on the coating layer to form a laminate, and then pressing the layer to a thickness of 3 µm to 200 µm. An electrochemical cell characterized by the above. The electrochemical cell of claim 1, wherein the insulating layer is adhered to an anode and a cathode. 12. A method of manufacturing an electrochemical cell according to any one of claims 1 and 7,
(a) preparing a positive electrode to which the positive electrode active material is applied to the current collector, and a negative electrode to which the negative electrode active material is applied to the current collector;
(b) preparing a coating solution containing an electrically insulating substrate in the electrolyte as a solvent:
(c) applying a coating solution of step (b) to at least one surface of an electrode of the positive electrode and the negative electrode of step (a);
(d) placing an electrode of the opposite electrode on the electrode of step (c) to form a laminate, and then pressing to form an electrode assembly; And
(e) inserting the electrode assembly of step (d) into the cell case and then sealing it;
Manufacturing method characterized in that it comprises a.