KR20080036254A - Anode material of excellent conductivity and high power secondary battery employed with the same - Google Patents

Abstract

A negative electrode material for an electrode active mass, its preparation method, and an electrochemical cell using the negative electrode material are provided to improve the electrical conductivity of a non-graphitizable carbon-based negative electrode active material, thereby enhancing output density without the deterioration of high temperature characteristics and capacity. A negative electrode material is such that 0.1-10 wt% of a conductive metal powder is uniformly contained in the inside and on the surface of a negative electrode active material based on the total weight of a negative electrode active material. Preferably the metal powder is at least one selected from the group consisting of Cu, Ni, Al, Zn, Au and Ag, and has a diameter of 1-500 nm. The negative electrode material is prepared by mixing a precursor of a non-graphitizable carbon-based negative electrode active material and a metal micropowder homogeneously in a solvent; drying the mixture; sintering the dried one; carbonizing the sintered one; and pulverizing the carbonized one.

Description

Anode Material of Excellent Conductivity and High Power Secondary Battery Employed with the Same}

The present invention relates to a negative electrode material having excellent electrical conductivity and a high output secondary battery including the same, and more particularly, a negative electrode material for electrode mixture consisting of uniformly distributed conductive metal powder on the inside and the surface of the negative electrode active material and the same It relates to a high output secondary battery comprising.

As technology development and demand for mobile devices increase, the demand for secondary batteries as energy sources is rapidly increasing, and lithium secondary batteries having high energy density and voltage are commercially used in such secondary batteries. When a secondary battery is used as a power source for a mobile phone or a laptop, a secondary battery that provides a stable output is required, while when used as a power source of a power tool such as an electric drill, a high output can be instantaneously provided. And, there is a need for a secondary battery that can be stable against external physical shocks such as vibration and dropping.

In addition, as interest in environmental problems grows, research on electric vehicles and hybrid electric vehicles that can replace vehicles using fossil fuel, such as gasoline and diesel vehicles, which are one of the main causes of air pollution, is being conducted. . As a power source of such electric vehicles and hybrid electric vehicles, nickel-metal hydride secondary batteries are mainly used, but researches using lithium secondary batteries with high energy density and discharge voltage have been actively conducted and some commercialization stages are in progress.

The lithium secondary battery has a structure in which a non-aqueous electrolyte containing lithium salt is impregnated in an electrode assembly having a porous separator interposed between a positive electrode and a negative electrode on which an active material is coated on a current collector. The positive electrode active material is mainly composed of lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium composite oxide and the like, and the negative electrode active material is mainly composed of a carbon-based material.

The carbon-based active material is composed of soft carbon having an almost graphitized graphene structure (a structure in which hexagonal honeycomb planes of carbon are arranged in layers), and these structures are mixed with amorphous portions. It is classified as hard carbon, which is classified as hard carbon, and is classified as graphite when the layered crystal structure is completely formed like natural graphite.

In particular, since the graphite-based material, which is a layered crystal, is mainly used as a negative electrode active material, the negative electrode made of such a graphite-based material has a theoretical maximum capacity of 372 mAh / g (844 mAh / cc) and has a limited capacity increase, thus rapidly changing the next generation. It is difficult to play a sufficient role as an energy source of mobile devices. In addition, lithium metal, which has been reviewed as a negative electrode material, can realize a high capacity due to its high energy density, but has a problem of safety due to dendrite growth and short cycle life during repeated charging and discharging. In addition, attempts have been made to use carbon nanotubes as negative electrode active materials, but problems such as low productivity, high price, and low initial efficiency of 50% or less have been pointed out.

As another cathode material, it has been known that silicon, tin, or alloys thereof can reversibly occlude and release a large amount of lithium through a compound formation reaction with lithium. Is going on. For example, silicon is promising as a high capacity cathode material because the theoretical maximum capacity is about 4020 mAh / g (9800 mAh / cc, specific gravity 2.23), which is much larger than graphite-based materials. However, the negative electrode material has a disadvantage that the volume change during charge and discharge is very large.

On the other hand, a technique for facilitating the movement of electrons by doping a conductive metal or the like on the surface of the negative electrode active material in order to improve the conventional output characteristics. For example, Korean Patent Application Publication No. 2002-062193 includes a battery active material having an average particle diameter of 0.1 to 100 μm and conductive powders such as nickel and aluminum having an average particle size of 10 nm to 10 μm attached around the active material. A carbon material for secondary batteries is disclosed. However, according to the above technique, the conductivity on the surface of the negative electrode active material may be partially improved. However, in the case of an active material having low conductivity such as an amorphous carbon compound, the conductivity inside the particle is difficult to be improved. have.

In addition, a technology for replacing a carbon-based material with a high rate capability as a negative electrode active material has been proposed, but these active materials have a problem in that high temperature characteristics and capacity reduction of a battery are caused.

Therefore, while minimizing the above problems in the secondary battery, there is an urgent need to develop a negative electrode material with improved output characteristics.

In this regard, the present invention proposes a negative electrode material composed of a conductive metal powder uniformly contained in a predetermined content in the inside and the surface of the non-graphitizing carbon-based negative electrode active material.

Some conventional techniques for mixing metal powders with graphite-based negative active materials exist. For example, Korean Patent Application Publication No. 2000-058145 discloses a ball mill treatment including oxide metal particles and / or amorphous carbon in a carbon precursor. The present invention discloses a method for producing a graphite negative electrode material by mechanically pulverizing and indenting by carbonization and carbonizing.

However, the technique includes the metal oxide particles in a carbon-based precursor in an amount of at least 30% by weight or more in order to facilitate the occlusion and release of lithium ions, thereby increasing the internal resistance to significantly reduce the electrical conductivity I have a problem. In addition, when the metal oxide particles are mixed with the carbon precursor by mechanical grinding and indentation, when the phenol resin powder, which is a raw material of the non-graphitizing carbon material, is mixed and calcined and pulverized, the metal particles in the final active material In order to distribute uniformly, it is necessary to use an active material precursor having a very small particle size and has a technical disadvantage of performing a long process.

In addition, Japanese Patent Application Laid-Open No. 1996-231273 discloses a graphite system having an average particle diameter of 1 to 20 µm and a carbon interlayer distance of 0.335 to 0.340 nm in order to increase the electric capacity of the graphite-based negative electrode active material. Disclosed is a method for producing a carbide by mixing 10 to 60% by weight of a carbonaceous material, 80 to 20% by weight of tar pitch, and 2 to 20% by weight of at least one metal compound selected from aluminum, zinc, tin, and silicon. Doing.

However, the technique includes a metal compound in a crystalline graphite carbonaceous material for high capacity, and the tar pitch, which is a kind of amorphous carbon, is added to improve adhesion between the graphite carbonaceous material and the metal compound during sintering. Since only substantially graphite carbon is included, there are serious problems such as lower discharge capacity and lower battery characteristics such as rate characteristics compared to amorphous carbon materials. As a result, it is clearly distinguished from the present invention including a metal compound in order to increase the electrical conductivity of non-graphitized carbon.

Accordingly, an object of the present invention is to solve the problems of the prior art as described above and the technical problems that have been requested from the past.

The inventors of the present application, after continuing in-depth research and various experiments, the secondary battery using the negative electrode material for electrode mixture consisting of a uniform distribution of the conductive metal powder on the inside and the surface of the negative electrode active material has a high output density and energy density It has been found to be improved to have excellent battery characteristics, and the present invention has been completed.

Accordingly, the negative electrode material for electrode mixture according to the present invention is based on the total weight of the negative electrode active material on the inside and the surface of the negative electrode active material in order to improve the high output characteristics by increasing the electrical conductivity of the hard carbon negative electrode active material. It is comprised by 0.1-10 weight% that a conductive metal powder is contained uniformly.

As described above, in general, the non-graphitizable carbon-based material has a high discharge capacity and excellent rate characteristics, but has a disadvantage of low bulk density and low electrical conductivity between particles. However, in the negative electrode material according to the present invention, since the conductive powder is uniformly distributed on the inside and the surface of the non-graphitizing carbonaceous negative electrode active material, it exhibits excellent conductivity in the inside thereof, thereby showing high electrical conductivity, thereby improving high output characteristics. It can improve the disadvantage of non-graphitizing carbon-based materials and maximize the advantages of high discharge capacity.

Therefore, the secondary battery including the same has excellent battery characteristics by improving the output density without degrading battery characteristics such as high temperature characteristics and battery capacity.

In the present invention, the non-graphitizing carbon-based negative active material is not particularly limited, and for example, an amorphous carbon-based pyrolyzed phenol resin or furan resin may be used. The non-graphitizable carbon-based compound has a relatively high discharge capacity of 280 to 450 mAh / g may contribute to the high output characteristics of the battery.

The metal powder is not particularly limited as long as it is a conductive metal powder or a metal nano powder without causing chemical change in the battery, and preferably Cu, Ni, Al, Zn, Au, and Ag having excellent electrical conductivity. It may be one or two or more selected from the group consisting of.

The content of the metal powder may be appropriately adjusted within a range capable of improving the electrical conductivity in consideration of the type of the negative electrode active material, as defined above, and preferably 0.1 to 10 weight based on the total weight of the negative electrode active material. May be included as a%. When the content of the metal powder is too small, it is difficult to exert the effect of the addition. On the contrary, when the content of the metal powder is too large, the proportion of the metal contained in the negative electrode material is high, which may cause deterioration of battery characteristics.

The particle diameter of the metal powder is not particularly limited as long as it can be uniformly distributed on the inside and the surface of the negative electrode active material, preferably 1 to 500 nm. If the particle size of the metal powder is too small, uniform distribution becomes difficult due to the cohesive force, so that it is difficult to exert a desired effect. On the contrary, if the particle size of the metal powder is too large, it is not preferable because it is difficult to distribute inside the amorphous carbon-based negative electrode active material.

The present invention also provides

(1) uniformly mixing the non-graphitizable carbon precursor and the fine metal powder in a solvent;

(2) drying, calcining and carbonizing the mixed mixture; And

(3) pulverizing the produced carbide

It provides a method for producing the negative electrode material comprising a.

In the present invention, the non-graphitizable carbon-based precursor and the fine metal powder are mixed by a wet mixing method performed under a predetermined solvent. This is because, as described above, in the dry mixing method, in order to uniformly distribute the fine metal particles, an anode active material precursor having a very small size of 5 μm or less is required or a long time process is required.

When the fine metal powder is mixed by the wet mixing method, alcohol is preferably used as the solvent, and methanol may be particularly preferably used. In some cases, a surfactant may be further added to the solvent to prevent the fine powder metal particles from agglomerating in the solvent. The surfactant may be, for example, polyoxyethylene and polyoxypropylene, or a copolymer thereof, but is not limited thereto.

Since the non-graphitizable carbon precursor is capable of mixing the metal particles in an amorphous carbon raw material state before drying, firing, and pulverization without a melting process, the non-graphitizing carbon precursor is a structure in which the metal particles are distributed within the particles by a simple method. It can manufacture.

The non-graphitizable carbon-based precursor is not particularly limited, and preferably may be any one selected from a phenol resin powder or a phenol resin solution.

As described above, the metal powder is preferably added in an amount of 0.1 to 10% by weight based on the total weight of the negative electrode material mixture in consideration of the conductivity and the kind of the active material.

The firing and carbonization process may be performed in a vacuum or inert atmosphere, preferably in a nitrogen or argon atmosphere to prevent oxidation of the precursor material.

In addition, the firing process may be performed at a sufficiently high temperature above the carbonization temperature of the precursor so that the fine metal powder may be uniformly introduced into the precursor to be uniformly distributed on the inside and the surface of the precursor. For example, it may be carried out at a temperature of 600 to 2000 ℃.

In the grinding process, it is preferable to grind the carbide mixed with the precursor and the fine metal powder to a size of 8 to 25 μm. When the average particle diameter of the carbide is too large, there is a problem that the bulk density of the negative electrode material is lowered. If the average particle diameter is too small, the irreversible capacity of the negative electrode material is high and it is not preferable because the desired discharge capacity is difficult to be expected.

The present invention also provides a negative electrode mixture comprising the negative electrode material, a binder and a conductive material.

The binder is a component that assists in mutual bonding of the negative electrode material and the conductive material and bonding to the current collector, and is generally added in an amount of 1 to 50 wt% based on the total weight of the mixture including the negative electrode material. Examples of such binders include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), cellulose, polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropyl cellulose Roses, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers And polymer high polymerized polyvinyl alcohol.

The conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery. Examples of the conductive material include graphite such as natural graphite and artificial graphite; Carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder 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. Specific examples of commercially available conductive materials include Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company, Ketjenblack and EC, which are acetylene black series. Family (Armak Company), Vulcan XC-72 (manufactured by Cabot Company) and Super P (manufactured by Timcal).

The negative electrode mixture may also optionally include a filler, an adhesion promoter, and the like.

The filler is not particularly limited as long as it is a fibrous material that does not cause chemical change in the battery as a component that inhibits expansion of the negative electrode. Examples of the filler include olefinic polymers such as polyethylene and polypropylene; Fibrous materials, such as glass fiber and carbon fiber, are used.

The adhesion promoter is an auxiliary component added to improve the adhesion of the active material to the current collector, it may be added in less than 10% by weight compared to the binder, for example, oxalic acid (oxalic acid), adipic acid (adipic acid), Formic acid, acrylic acid derivatives, itaconic acid derivatives, and the like.

In addition, the negative electrode on which the negative electrode mixture is applied on the current collector, for example, may be prepared by coating the negative electrode mixture on the current collector. Specifically, the negative electrode mixture may be added to a predetermined solvent to prepare a slurry, and then, the slurry mixture may be coated on a current collector such as a metal foil, dried, and rolled to prepare a predetermined sheet-shaped electrode.

The invention also relates to an electrochemical cell comprising the negative electrode mixture comprising a negative electrode applied on a current collector.

Generally, the negative electrode is prepared by adding a negative electrode mixture to a solvent such as NMP to prepare a slurry, and then applying the same to a current collector to dry and press. Preferred examples of the solvent used in the preparation of the electrode slurry include dimethyl sulfoxide (DMSO), alcohol, N-methylpyrrolidone (NMP), acetone, and the like. Up to 400% by weight can be used and removed during drying.

The negative electrode current collector is generally made to a thickness of 3 to 500 ㎛. Such a negative electrode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery. For example, the surface of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel Surface-treated with carbon, nickel, titanium, silver, and the like, aluminum-cadmium alloy, and the like can be used. In addition, like the positive electrode current collector, fine concavities and convexities may be formed on the surface to enhance the bonding strength of the negative electrode active material, and may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The method of evenly applying the negative electrode mixture paste on the negative electrode current collector may be selected from known methods or performed by a new suitable method in consideration of the properties of the material. For example, it is preferable to disperse the paste onto the current collector and then to disperse the paste uniformly using a doctor blade or the like. In some cases, a method of distributing and dispersing in one process may be used. In addition, die casting, comma coating, screen printing, or the like may be used. Alternatively, a separate substrate may be formed and then joined with a current collector by pressing or lamination. You can also Drying of the paste applied on the current collector is preferably dried for 1 to 3 days in a vacuum oven at 50 to 200 ℃.

The electrochemical cell is a device for providing electricity through an electrochemical reaction, preferably a high output lithium secondary battery containing a lithium salt-containing nonaqueous electrolyte. The lithium secondary battery may have a structure in which a lithium salt-containing non-aqueous electrolyte is impregnated in an electrode assembly having a structure in which a separator is interposed between the negative electrode and the positive electrode.

Other components of the lithium secondary battery according to the present invention will be described in detail below.

The positive electrode is produced by, for example, applying a positive electrode mixture containing a positive electrode active material onto a positive electrode current collector and then drying it.

The positive electrode current collector is generally made to a thickness of 3 to 500 μm. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, the surface of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel Surface treated with carbon, nickel, titanium, silver or the like can be used. The current collector may form fine irregularities on its surface to increase the adhesion of the positive electrode active material, and may be in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The positive electrode active material may be 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; Li _{1 + y} Mn _{2 - y} O ₄ (Where y is 0 to 0.33), lithium manganese oxide such as LiMnO ₃ , LiMn ₂ O ₃ , LiMnO _2, etc .; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ , Cu ₂ V ₂ O ₇ and the like; Ni-site-type lithium nickel oxide represented by the formula LiNi _1-y M _y O ₂ , wherein M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and y = 0.01 to 0.3; Formula LiMn _2-y M _y O ₂ , wherein M = Co, Ni, Fe, Cr, Zn or Ta and y = 0.01 to 0.1, or Li ₂ Mn ₃ MO ₈ , where M = Fe, Co, Lithium manganese composite oxide represented by Ni, Cu or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with alkaline earth metal ions; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ and the like, but are not limited to these.

The separator is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally from 0.01 to 10 ㎛ ㎛, thickness is generally 5 ~ 300 ㎛. As such a separator, for example, olefin polymers such as chemical resistance and hydrophobic polypropylene; Sheets or non-woven fabrics made of glass fibers or polyethylene are used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

The lithium salt-containing non-aqueous electrolyte solution consists of an electrolyte solution and a lithium salt, and a non-aqueous organic solvent, an organic solid electrolyte, an inorganic solid electrolyte, and the like are used as the electrolyte solution.

As the lithium salt-containing non-aqueous electrolyte, a solid electrolyte, an inorganic solid electrolyte, or the like may be used.

Examples of the organic solid electrolyte include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyedgetion lysine, polyester sulfides, polyvinyl alcohols, polyvinylidene fluorides, Polymerizers containing ionic dissociating groups and the like can be used.

Examples of the inorganic solid electrolyte include 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.

The lithium salt is a good material to be dissolved 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 and imide have.

In addition, in the electrolyte solution, for the purpose of improving the charge and discharge characteristics, flame retardancy, etc., for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, nitro Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxy ethanol, aluminum trichloride and the like may be added. . In some cases, in order to impart nonflammability, halogen-containing solvents 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.

The lithium secondary battery according to the present invention may be preferably used as a unit cell for a high output large capacity battery or a battery pack, and in particular, it may be preferably used as a vehicle power source such as an electric vehicle, a hybrid electric vehicle, etc. that require high output even at low temperatures.

In general, graphite and digraphitized carbon mainly use ethylene carbonate (EC) as an electrolyte solvent due to the properties of the material in a lithium secondary battery, whereas nongraphitized carbon uses propylene carbonate (PC) as an electrolyte solvent. Can be. According to experiments conducted by the inventors of the present application, the PC has a characteristic of being lower than that of the EC, and thus, PC has been found to be preferable for a material of a battery that requires operation at low temperatures.

That is, the negative electrode mixture according to the present invention uses a non-graphitized carbonaceous material as the negative electrode active material, thereby making it possible to use PC having a high dielectric constant and relatively low viscosity at a low temperature as the main component of the electrolyte solvent. Therefore, since the output characteristics at low temperatures are excellent, it can be preferably used in medium and large battery systems that must be operated under harsh conditions such as electric vehicles, hybrid electric vehicles, and the like.

In some cases, mixed solvents containing linear carbonates such as diethyl carbonate (DEC) and dimethyl carbonate (DMC) may be used in addition to PCs to increase the conductivity of lithium ions and to ensure reaction stability. have.

As described above, since the electrical conductivity of the negative electrode active material is increased by using the negative electrode material for electrode mixture in which the conductive metal powder is uniformly contained in the inside and the surface of the non-graphitizing carbon-based negative electrode active material, a secondary battery comprising the same Including electrochemical cells have high power characteristics, and thus have excellent battery characteristics with improved power density and energy density.

Claims (14)

In order to improve the high output characteristics by increasing the electrical conductivity of the non-graphitizing carbon-based negative electrode active material, the conductive metal powder is uniformly contained in the inside and the surface of the negative electrode active material at 0.1 to 10% by weight based on the total weight of the negative electrode active material. An anode material for electrode mixture, characterized in that. The negative electrode material of claim 1, wherein the metal powder is one or two or more selected from the group consisting of Cu, Ni, Al, Zn, Au, and Ag. The negative electrode material for electrode mixture according to claim 1, wherein the metal powder has a particle diameter of 1 nm to 500 nm. Comprising uniformly mixing the precursor of the non-graphitizing carbonaceous negative active material and the fine metal powder in a solvent, drying, calcining and carbonizing such a mixture, and grinding the resulting carbide. The manufacturing method of the negative electrode material in any one of Claims 1-3. The method of claim 4, wherein the precursor of the non-graphitizable carbon-based negative active material is any one selected from phenol resin powder or phenol resin solution. The method of claim 4, wherein the metal powder is added in an amount of 0.1 to 10 wt% based on the total weight of the mixture. The method of claim 4, wherein the calcining and carbonization are performed in a nitrogen or argon atmosphere. The method of claim 4, wherein the firing is performed at 600 to 2000 ° C. 6. The method of claim 4, wherein the carbide is ground to a size of 8 to 25 ㎛ in the grinding process. A negative electrode mixture comprising the negative electrode material according to any one of claims 1 to 3, a binder and a conductive material. An electrochemical cell consisting of the negative electrode mixture according to claim 10 coated on a current collector. 12. The electrochemical cell of claim 11, wherein the electrochemical cell is a high power lithium secondary battery containing a lithium salt-containing nonaqueous electrolyte. 13. The electrochemical cell of claim 12, wherein the solvent of the electrolyte contains propylene carbonate as a main component. 12. The electrochemical cell of claim 11, wherein said cell is used as a vehicle power source that requires high power even at low temperatures.