KR20100004025A - Improved stable lithium secondary battery - Google Patents

Abstract

PURPOSE: A positive electrode for a lithium secondary battery is provided to produce a lithium secondary battery with a high temperature storage and excellent safety in internal short while not degrading a charge and discharge cycle property. CONSTITUTION: A positive electrode for a lithium secondary battery comprises a positive electrode active material applied on a current collector and a phosphate solid electrolyte coating layer formed on the surface of the positive electrode active material. A method for preparing the positive electrode for a lithium secondary battery comprises the steps of preparing a semi-product positive electrode coated with a positive electrode material on a metal foil; melting a phosphate solid electrolyte in a solvent to obtain slurry in which phosphate solid electrolyte is dispersed; spraying and applying the slurry on the positive electrode active material through a dip-coating method; and incinerating the positive electrode coated with the phosphate solid electrolyte.

Description

Lithium secondary battery with improved safety {IMPROVED STABLE LITHIUM SECONDARY BATTERY}

The present invention provides a lithium secondary battery with improved safety, and relates to a positive electrode included in a lithium secondary battery, a manufacturing method thereof, and a lithium secondary battery including the same.

Among the secondary batteries currently applied, lithium secondary batteries developed in the early 1990s have a higher operating voltage and significantly higher energy density than conventional batteries such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries that use an aqueous electrolyte solution. I am in the spotlight. As technology development and demand for mobile devices such as mobile phones and laptops have increased, the demand for secondary batteries as an energy source is rapidly increasing, and the demand for lithium secondary batteries with high energy density and discharge voltage among these secondary batteries is increasing. It's getting worse.

However, such a lithium secondary battery has safety problems such as ignition and explosion caused by using an organic electrolyte, and has a disadvantage in that manufacturing is difficult. In particular, when the lithium secondary battery is short-circuited by the contact between the positive electrode and the negative electrode, there is a big problem in safety because it explodes with extreme heat generation. In fact, there are not many cases where a lithium secondary battery installed in a mobile device ignites and explodes.

There have been many efforts to solve the safety problem of such a lithium secondary battery and the related art is as follows.

First, a taping method can be mentioned.

On the current collector, the electrode active material is intermittently coated. In this case, the start portion of the active material coated is not uniformly applied as compared to the remaining portion, thereby causing thickness variation. This thickness deviation may cause problems such as poor adhesion and may cause internal short circuits. Therefore, by applying a tape to the above site to apply a method of suppressing internal short circuit.

KR 10-1996-0016155 (Document 1) discloses a group of wound electrode plates produced by a method of tapping the start of a positive electrode plate with an alkali resistant material. This winding electrode group prevents battery short-circuits and reduces the packing rate, resulting in improved battery capacity, lifetime and yield.

However, in the taping method, there is a problem that the capacity of the battery is inevitably caused due to the increase in the local thickness of the tapered portion, the problem that the portion cannot be moved to lithium, and become a kind of dead space. In addition, there is a problem that the capacity of the battery is reduced due to the non-coating portion of the electrode active material not coated with the electrode active material, and internal short circuit occurs due to contact between the non-coating portions.

As another method, a method of forming a coating layer containing an oxide on the surface of an electrode may be mentioned.

When the oxide coating layer is formed on the surface of the electrode, the metal oxide coating layer prevents direct contact between the electrolyte and the electrode, thereby suppressing the reaction between the electrode and the electrolyte.

JP 2003-221234 (Document 2), and a metal oxide, such as 0.05 to 1% by weight of _{MgO, TiO 2, ZnO, ZnO} 2 and LiCoO ₂ dry mixed and heat-treated to form a layer of the metal oxide on the surface of LiCoO ₂ A method is disclosed.

However, while these metal oxide layers have the effect of suppressing the reaction with the electrolyte, the metal oxide layers are electrically insulators and have very low lithium ion conductivity, which has a problem of lowering the speed performance of the battery. On the other hand, if the throughput of the oxide is reduced to overcome this problem, there is a problem that can not expect the effect of reducing the reaction of the positive electrode and the electrolyte generated during operation of the battery in high potential or high temperature storage.

As another method, a method of reducing the reactivity of the positive electrode and the electrolyte using an electrolyte solution or an electrolyte additive may be exemplified.

In KR 10-1999-0015926 (Document 3), a non-aqueous electrolyte containing a monomer of a conductive polymer forms a conductive polymer film on the surface of a positive electrode active material during charging to prevent a large current flow, thereby improving lithium battery safety. It discloses about the electrolyte solution for secondary batteries.

However, in the case of using the electrolyte solution or the electrolyte additive, although the battery safety characteristics can be improved, there is a problem that the ionic conductivity of the electrolyte solution is lowered by the additive and thus other characteristics of the battery are reduced, that is, there is a tradeoff effect. .

The literature regarding said prior art was put together below. The prior art as described above has a big difference in construction from the present invention including the phosphate solid electrolyte coating layer.

Document 1 KR 10-1996-0016155 A 1996.05.15.

[Document 2] JP 2003-221234 (特 開) 2003. 8. 5.

Document 3 KR 10-1999-0015926 A 1999.05.03.

The conventional technology for improving the safety of the lithium secondary battery has improved the safety of the battery, but there are the same problems as described above. That is, there is a problem inevitably deteriorating the performance of the battery in order to improve the safety of the battery.

Therefore, there is a high demand for a battery having improved safety while minimizing or minimizing the performance of the battery, and this demand is a problem to be solved by the present invention.

After extensive research and various experiments, the inventors of the present invention form a phosphate solid electrolyte coating layer on the surface of lithium transition metal oxide particles as a cathode active material, as described later. Not only can the reactivity be reduced, but there is no deterioration in speed performance, and the present invention has been completed.

The present invention has been made to solve the above problems,

It provides a lithium secondary battery positive electrode comprising a positive electrode active material applied to a current collector and a phosphate solid electrolyte coating layer formed on the surface of the positive electrode active material.

In addition, in the present invention, the phosphate solid electrolyte coating layer provides a lithium secondary battery positive electrode, characterized in that it comprises Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ .

In the present invention, the porosity of the phosphate solid electrolyte coating layer provides a lithium secondary battery positive electrode, characterized in that 45% or more and 70% or less.

In addition, preparing a semi-finished positive electrode coated with a positive electrode active material on a metal foil;

Dissolving a phosphate solid electrolyte in a solvent to prepare a slurry in which the phosphate solid electrolyte is dispersed;

Applying the slurry to the cathode active material by spray spraying or dip-coating in a slurry bath;

Calcining the positive electrode to which the phosphate solid electrolyte is applied

It provides a lithium secondary battery positive electrode manufacturing method comprising a.

In addition, in a lithium secondary battery comprising a positive electrode, a negative electrode, a separator and a lithium salt-containing nonaqueous electrolyte, the positive electrode provides a lithium secondary battery comprising the lithium secondary battery positive electrode of the present invention.

In addition, in a lithium secondary battery comprising a positive electrode, a negative electrode, a separator and a lithium salt-containing nonaqueous electrolyte, the positive electrode provides a lithium secondary battery comprising the lithium secondary battery positive electrode of the present invention.

In the positive electrode according to the present invention, since a phosphate solid electrolyte coating layer is formed on the surface of a lithium transition metal oxide, the lithium secondary battery has excellent safety even at high temperature storage characteristics and internal short circuits without degrading battery performance such as charge / discharge cycle characteristics. There is an effect that can provide.

The positive electrode according to the present invention is characterized by including a phosphate solid electrolyte coating layer on the surface of a lithium transition metal oxide capable of occluding and releasing lithium ions.

The lithium transition metal oxide is composed of a material exhibiting a voltage of 3 V or more, and examples of such materials include layered compounds such as lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide (LiNiO ₂ ), or one or more transition metals. Substituted compounds; Li _{1 + x} Mn _{2 - x} O ₄ (Where x is 0 to 0.33), lithium manganese oxides such as 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 , Cu ₂ V ₂ O ₇ and the like; Ni-site-type lithium nickel oxide represented by the formula LiNi _{1 - x} MxO ₂ , wherein M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x = 0.01 to 0.3; Formula LiMn _{2 -x} MxO ₂ , wherein M = Co, Ni, Fe, Cr, Zn or Ta, and x = 0.01 to 0.1, or Li ₂ Mn ₃ MO ₈ , where M = Fe, Co, Ni, Lithium manganese composite oxide represented by Cu) or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with alkaline earth metal ions, etc. may be mentioned, but is not limited thereto.

Phosphate solid electrolyte for forming the coating layer is characterized by containing _{Li 1 .3 Al 0 .3 Ti 1} .7 (PO 4) 3. Wherein _{Li 1 .3 Al 0 .3 Ti 1} .7 (PO 4) 3 is _{Li 2 CO 3, Al 2 O} 3, Ti0 2 And (NH ₄ ) ₂ HPO ₄ in a stoichiometric mixture.

Since the coating layer contains Li, there is an effect of contributing to the increase of the capacity of the battery.

The coating layer is formed between the positive electrode active material and the electrolyte. Since the coating layer is positioned between the positive electrode active material and the electrolyte, the safety of the internal short circuit may be improved because the coating layer prevents direct contact with the electrolyte.

The thickness of the phosphate solid electrolyte coating layer is not particularly limited and may be manufactured in various thicknesses according to battery model-specific characteristics according to battery electrode design.

The porosity of the coating layer is preferably 45% or more and 70% or less. At a porosity of 45% or less, permeability of lithium ions may be lowered, which may affect battery performance. At porosity of 70% or more, contact with an electrolyte may occur, which may be undesirable in terms of battery safety. Here, the porosity means a value expressed as a percentage of the porosity of the total volume.

The present invention also provides a method for producing the positive electrode.

A positive electrode (semifinished product state) coated with a positive electrode active material on a metal foil is prepared. The process of coating the positive electrode active material on the metal foil may use a general coating method commonly used in the art, and non-limiting examples thereof include solvent evaporation, co-precipitation, precipitation, sol-gel method, and post-sorption filter method. , Sputter, chemical vapor deposition (CVD), and the like. On the other hand, the metal foil usually uses a lot of aluminum foil. Aluminum foil is preferred as a metal foil because of its high electrical conductivity and high workability.

In order to form the phosphate solid electrolyte coating layer on the positive electrode active material of the positive electrode (semi-finished product), a slurry in which the phosphate solid electrolyte is dispersed is prepared. The phosphate solid electrolyte dispersion slurry is prepared by dissolving a phosphate with _{Li 1 .3 Al 0 .3 Ti 1} .7 (PO 4) 3 in the composition in a solvent. The solvent is not particularly limited and may be a solvent commonly used in the manufacture of electrodes in the art.

The prepared slurry is preferably sprayed onto the cathode active material (spray method) or applied in the form of dip-coating in a slurry bath. Here, the term "application" is a concept that includes both the case of contacting the surface of the object, or part of the entire surface or part of the object for chemical or physical reaction to form the coating layer.

The positive electrode (semi-finished product) coated with the phosphate solid electrolyte was calcined for 5 hours in a vacuum oven at 100 to 150 ° C. to finally produce a positive electrode coated with the phosphate solid electrolyte. The term “calcination” herein refers to the operation of heating a substance to a high temperature to remove some or all of its volatiles, in particular removing volatiles below the sintering temperature.

In another aspect, the present invention provides a lithium secondary battery comprising the positive electrode.

Typically, a lithium secondary battery is composed of a positive electrode, a negative electrode, a separator, and a lithium salt-containing nonaqueous electrolyte.

The positive electrode is prepared by, for example, applying a mixture of a positive electrode active material, a conductive agent, and a binder onto a positive electrode current collector, followed by drying, and optionally, a filler may be further added to the mixture.

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 change in the battery, and carbon, nickel on the surface of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel Surface treated with titanium, silver, or the like may 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 conductive agent is typically added in an amount of 1 to 50 wt% based on the total weight of the mixture including the positive electrode active material. Such a conductive agent is not particularly limited as long as it has conductivity without causing chemical change in the battery. Examples of the conductive agent 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 oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used. In some cases, addition of the conductive agent may be omitted because another conductive coating layer is added to the cathode active material.

The binder is a component that assists in bonding the active material and the conductive agent 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 positive electrode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, Polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers, and the like.

The filler is optionally used as a component for inhibiting expansion of the positive electrode, and is not particularly limited as long as it is a fibrous material without causing chemical change in the battery, and includes olefinic polymers such as polyethylene and polypropylene; Fibrous materials, such as glass fiber and carbon fiber, are used.

The negative electrode is manufactured by applying and drying a negative electrode material on the negative electrode current collector, and if necessary, the components as described above may be further included.

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, and copper, stainless steel, aluminum, nickel, titanium, calcined carbon, carbon, nickel on the surface of copper or stainless steel Surface treated with titanium, silver, or the like, aluminum-cadmium alloy, or 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 negative electrode material may be, for example, carbon such as hardly graphitized carbon or graphite carbon; Li _x Fe ₂ O ₃ (0 ≦ _x ≦ 1), Li _x WO ₂ (0 ≦ _x ≦ 1), Sn _x Me _{1 - x} Me ' _y O _z (Me: Mn, Fe, Pb, Ge; Me' Metal complex oxides such as Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen, 0 <x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8); Lithium metal; Lithium alloys; Silicon-based alloys; Tin-based alloys; SnO, SnO ₂ , PbO, PbO ₂ , Pb ₂ O ₃ , Pb ₃ O ₄ , Sb ₂ O ₃ , Sb ₂ O ₄ , Sb ₂ O ₅ , GeO, GeO ₂ , Bi ₂ O ₃ , Bi ₂ O ₄ and Oxides such as Bi ₂ O ₅ ; Conductive polymers such as polyacetylene; Li-Co-Ni-based materials and the like can be used.

The separator is interposed between the cathode 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 nonwovens 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 consists of a nonaqueous electrolyte and lithium. As the nonaqueous electrolyte, a nonaqueous electrolyte, a solid electrolyte, an inorganic solid electrolyte, and the like are used.

As said non-aqueous electrolyte, N-methyl- 2-pyrrolidinone, a propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyl Low lactone, 1,2-dimethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolon, formamide, dimethylformamide, dioxorone, Acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxy methane, dioxorone derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate Aprotic organic solvents such as derivatives, tetrahydrofuran derivatives, ethers, methyl pyroionate and ethyl propionate can be used.

Examples of the organic solid electrolytes include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyedgetion lysine, polyester sulfides, polyvinyl alcohols, polyvinylidene fluorides, Polymers 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,} CH3SO3Li, CF 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide.

In addition, the non-aqueous electrolyte includes pyridine, triethylphosphite, triethanol amine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triacetate, for the purpose of improving charge and discharge characteristics, flame retardancy, and the like. Amide, 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 May be 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.

Hereinafter, embodiments of the present invention will be described, but the scope of rights is not limited thereto.

Comparative example  One

A cylindrical battery is manufactured by combining a positive electrode (LiCoO ₂ ) negative electrode (Graphite) produced by a conventional method (only a positive electrode active material coated on Al-foil), and an organic electrolyte.

Comparative example  2

Using the same electrode (anode, cathode) as in Comparative Example 1, to prepare a cylindrical battery by applying a conventional oxide (Al ₂ O ₃ ) on the surface of the positive electrode.

Example

Using the same electrode (anode, cathode) as in Comparative Example 1, to prepare a cylindrical battery by applying a phosphate (solid electrolyte) to the surface of the positive electrode.

About the comparative example 1, the comparative example 2, and the Example, it carried out like the content of the following Table 1.

Anode surface coating Coating layer thickness Anode total thickness Comparative Example 1 none 0 μm 150 μm Comparative Example 2 Al ₂ O ₃ 5 μm 150 μm Example _{Li 1 .3 Al 0 .3 Ti 1} .7 (PO 4) 3 solid electrolyte 5 μm 150 μm

[Safety test]

1. Experiment Contents

Using the batteries prepared according to Comparative Example 1, Comparative Example 2 and Example were subjected to 0.2 C, 1.0 C discharge capacity, 75 ℃ high temperature storage, 150 ℃ hot box explosion test, room temperature nail penetration test.

The 75 ℃ high temperature storage and 150 ℃ hot box explosion test is a test for high temperature safety by measuring the internal gas generation and the time to ignite or explode the lithium secondary battery at the corresponding temperature.

The room temperature nail penetration test is a safety test that observes whether or not fire occurs after artificially causing an internal short circuit of a battery with a sharp tip such as a nail.

2. Experimental Results

When the oxide (Al ₂ O ₃ ) is applied to the surface of the positive electrode as a positive electrode active material of the thickness is excluded, the capacity of the battery is reduced, but the phosphate containing Li as in the embodiment of the present invention In this case, lithium ions (Li-ion) may be provided in the coating layer to contribute to battery capacity, and the coating layer may be a material capable of diffusing Li-ion, and thus, prior to high-rate discharge (1.0 C), the prior art (Comparative Example 2 ), The efficiency was maximized. In addition, by minimizing the direct contact area between the positive electrode active material and the organic electrolyte solution, the CID operation time can be extended and the ignition delay can be extended to 150 ° C when stored at a high temperature of 75 ° C as in the prior art. By minimizing the large current caused by the ignition rate was minimized. The results are summarized in Table 2 below.

Discharge Capacity (mAh) 75 ℃ high temperature storage CID operating time 150 ℃ hot box test Explosion delay time Nail penetration (9 m / min) 0.2 C 1.0 C Comparative Example 1  2520 2445 70 to 74 h 15 min 10 of 10 samples Comparative Example 2 2498 2348 73 to 82 h 23 min 6 of 10 samples Example 2515 2401 72 to 85 h 21 min 7 of 10 samples

Those skilled in the art to which the present invention pertains will be able to carry out the invention by variously applying and modifying the present invention using conventional techniques well known in the art based on the above contents, which is also the right of the present invention. Range belonging is self explanatory.

Claims (6)

A lithium secondary battery positive electrode comprising a positive electrode active material applied to a current collector and a phosphate solid electrolyte coating layer formed on the surface of the positive electrode active material. The method of claim 1, wherein the solid electrolyte is a phosphate coating layer _{Li 1 .3 Al 0 .3 Ti 1} .7 (PO 4) a lithium secondary battery positive electrode according to that characteristic, including _3. The cathode of claim 1 or 2, wherein the porosity of the phosphate solid electrolyte coating layer is 45% or more and 70% or less. Preparing a semi-finished cathode coated with a cathode active material on a metal foil; Dissolving a phosphate solid electrolyte in a solvent to prepare a slurry in which the phosphate solid electrolyte is dispersed; Applying the slurry to the cathode active material by spray spraying or dip-coating in a slurry bath; Calcining the positive electrode to which the phosphate solid electrolyte is applied Lithium secondary battery positive electrode manufacturing method comprising a. A lithium secondary battery comprising a positive electrode, a negative electrode, a separator, and a lithium salt-containing nonaqueous electrolyte, wherein the positive electrode comprises the lithium secondary battery positive electrode of any one of claims 1 to 2. A lithium secondary battery comprising a positive electrode, a negative electrode, a separator, and a lithium salt-containing nonaqueous electrolyte, wherein the positive electrode comprises the lithium secondary battery positive electrode of claim 3.