KR102328430B1 - Anode comprising porous cellulose and secondary battery having the same - Google Patents

Abstract

The present invention is to lower the diffusion resistance of lithium ions in a lithium secondary battery having a high-loading negative electrode and thereby improve the discharge capacity and cycle characteristics during charge/discharge. A porous polymer was added. Through this, there is an effect of lowering the diffusion resistance of lithium ions and thereby improving the discharge capacity and cycle characteristics during charge/discharge.

Description

Anode comprising porous cellulose and secondary battery having the same

The present invention relates to an electrode including a porous body and a secondary battery including the same, specifically, to a porous polymer, more specifically to an anode including a cellulose pore and a secondary battery including the same.

Secondary batteries include nickel cadmium, nickel hydrogen, lithium ion and lithium ion polymer batteries. Secondary batteries are not only small products such as digital cameras, mobile phones, wearable devices, power tools and electric bicycles, but also large products that require high output such as electric and hybrid vehicles, and are used to store surplus power or renewable energy. It is also used in power storage devices and power storage devices for backup.

Lithium secondary batteries are currently the most used secondary batteries and consist of a cathode, a separator, and an anode. It is selected in consideration of stability and the like.

As the demand for high-capacity and high-efficiency secondary batteries increases, various efforts are being made to increase the capacity and energy density of the batteries.

The most tried for a high capacity lithium secondary battery is to make a high loading electrode. The most basic method of disposing a high-loading electrode and an active material layer per unit area of the electrode is to make the active material thick. Although there is no difficulty in manufacturing the high loading method, there is a problem in that the mobility of lithium ions in the electrode is greatly reduced, so that the resistance is greatly increased.

In particular, as the loading increases, the movement resistance of the lithium ions moving for charging/discharging increases because the movement distance of the lithium ions increases and the number of obstacles increases. Ion movement and related resistance are mainly diffusion resistance. As the C-rate increases, the diffusion resistance increases.

In the case of a high-loading electrode, even at a 0.5C rate, the designed capacity is not properly expressed, and the cycle life also deteriorates rapidly, so there is no special advantage compared to a low-loading electrode.

Various studies have been attempted to solve these problems.

Patent Document 1 discloses that a negative electrode active material such as artificial graphite, carbon fiber, and a binder are mixed with water-soluble cellulose capable of forming pores to prepare a negative electrode mixture, and then the negative electrode is prepared in a sheet form, and the anode is immersed in water to remove cellulose By doing so, a negative electrode having pores of a specific volume is manufactured.

In Patent Document 1, since the negative electrode mixture is compressed with a hydraulic hot press after manufacturing the negative electrode mixture, there is a concern about the loss of many pores. In addition, the cellulose used in Patent Document 1 is dissolved and removed by water, and has completely different physical and chemical properties from the microcrystalline cellulose (hereinafter 'MCC') used in the present invention. Unlike the cellulose used in Patent Document 1, the MCC of the present invention does not dissolve in water at all. On the other hand, in Patent Document 1, in the case of high loading, the role of the pore body is not recognized at all.

Patent Document 2 contains cellulose beads on a positive electrode mixture containing manganese dioxide and graphite in order to increase the liquid absorption efficiency of an electrolyte in a positive electrode mixture for an alkaline battery capable of improving discharge performance.

Patent Document 2 is a patent using cellulose as a liquid absorbent rather than a porous body in an alkaline battery. Cellulose has the property of holding water and swelling in water. Therefore, in alkaline batteries and secondary batteries, cellulose cannot be used as a 'porous body', and in Patent Document 2, only a 'liquid absorber' is specified, and the absorption and release of the electrolyte are described.

On the other hand, in the case of the present invention, a non-aqueous electrolyte is used for application in a lithium secondary battery, and the MCC neither absorbs nor does swelling of the non-aqueous electrolyte.

As can be seen from FIG. 1 , the MCC according to the present invention does not swell at all with respect to the non-aqueous electrolyte. Therefore, it can be seen that the porous material used in Patent Document 2 is a completely different technology from the MCC of the present invention as an electrolyte 'liquid absorbent'.

Patent Document 3 discloses that, in order to manufacture a negative electrode material with high capacity and excellent cycle characteristics and battery efficiency, a pore-forming agent such as methyl cellulose is coated in advance on a fine silicon powder, and then heat-fired to form pores in the negative electrode material. However, the structure is different in that the electrode does not include a porous material, but forms pores in the negative electrode material through firing. Patent Document 3 does not recognize any matters related to the movement of ions due to high loading, and simply uses a porous body to provide an extra space for absorbing the volume expansion of the silicon fine powder.

As described above, in a lithium secondary battery having a high-loading negative electrode, a technique for lowering the diffusion resistance of lithium ions and thereby improving the discharge capacity and cycle characteristics during charge/discharge has not been proposed yet.

Japanese Patent Laid-Open No. 2009-146581 Japanese Laid-Open Patent Publication No. 2002-042818 Japanese Patent Publication No. 5158460

The present invention is to solve the conventional problems as described above, in a lithium secondary battery having a high-loading anode, to lower the diffusion resistance of lithium ions and thereby improve the discharge capacity and cycle characteristics during charge/discharge are doing

A first aspect of the present invention for achieving this object is a lithium secondary battery prepared by applying an electrode slurry including an active material, a conductive agent, and a binder to a current collector at 350 mg/25 cm 2 or more, a non-aqueous electrolyte solution in the electrode slurry It provides an electrode further comprising a porous polymer that does not swell.
The amount of applying the electrode slurry to the current collector means the weight after drying the electrode slurry including the solvent is applied to the current collector.

A second aspect according to the present invention provides an electrode in which the porous polymer is porous cellulose.

A third aspect according to the present invention provides an electrode wherein the porous cellulose is microcrystalline cellulose.

A fourth aspect according to the present invention provides an electrode having a particle size D50 of the porous polymer of 1 to 50 μm.

A fifth aspect according to the present invention provides an electrode in which the content of the porous polymer in the electrode slurry is 0.01 to 5% based on the weight of the active material.

A sixth aspect according to the present invention provides an electrode in which the amount of the electrode slurry applied to the current collector is 350 mg/25 cm 2 or more and 1000 mg/25 cm 2 or less.

A seventh aspect according to the present invention provides an electrode, wherein the electrode is a negative electrode.

An eighth aspect according to the present invention provides a lithium secondary battery including the electrode.

An eighth aspect according to the present invention provides a battery pack including the lithium secondary battery.

In general, a lithium secondary battery is composed of a positive electrode, a negative electrode, a separator, and a lithium salt-containing non-aqueous electrolyte.

The negative electrode used in the present invention, for example, may be manufactured by applying a negative electrode active material composed of negative electrode active material particles to a negative electrode current collector, and a negative electrode mixture in which a conductive material and a binder are mixed, and if necessary, the negative electrode Additional fillers may be added to the mixture. In particular, the negative electrode used in the present invention contains a porous polymer that does not swell in the non-aqueous electrolyte.

A representative example of the porous polymer that does not swell in the non-aqueous electrolyte is microcrystalline cellulose (MCC), and other porous polymers that do not swell in the non-aqueous electrolyte may also be used.

The present invention greatly improves the diffusion resistance, which is a disadvantage of the conventional high-loading electrode, by adding MCC. MCC forms internal pores in the electrode to facilitate the movement of lithium ions, and since it is electrochemically stable, it does not affect the lifespan of the battery.

The preferred particle size D50 of MCC is 1 μm to 50 μm. When D50 is 1㎛ or less, the size is too small to block pores of graphite to increase diffusion resistance, and when D50 is 50㎛ or more, the required content in the electrode increases and high capacity characteristics may be deteriorated.

In general, the negative electrode composition for preparing the slurry and the electrode is preferably negative active material: conductive material: binder (thickener and binder) = 94 ~ 99: 0 ~ 1.0: 1.0 ~ 5.0 wt%. The appropriate content of MCC is 0.01 to 5% of the weight of the active material.

In the case of double layer coating, it is possible to obtain a large effect by adding a small amount of MCC to the slurry composition at the bottom (foil side).

The negative electrode is manufactured by coating and drying the negative electrode active material on the negative electrode current collector, and if necessary, the above-described components may be optionally further included.

The negative electrode current collector is generally made to have a thickness of 3 to 500 micrometers. Such a negative current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface. Carbon, nickel, titanium, one surface-treated with silver, an aluminum-cadmium alloy, etc. may be used. In addition, like the positive electrode current collector, the bonding strength of the negative electrode active material may be strengthened by forming fine irregularities on the surface, and may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a non-woven body.

Examples of the negative electrode active material include carbon such as non-graphitizable carbon and graphitic 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 composite oxides such as Al, B, P, Si, elements of Groups 1, 2, and 3 of the periodic table, halogen; 0<x≤1;1≤y≤3;1≤z≤8); lithium metal; lithium alloy; 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 metal oxides such as _{Bi 2} O _{5 ;} conductive polymers such as polyacetylene; Li-Co-Ni-based materials and the like can be used.

A binder may be optionally further included together with the anode active material.

The binder acts as a paste of the negative active material, mutual adhesion between the active materials, adhesion between the active material and the current collector, and a buffering effect against expansion and contraction of the active material. Specifically, the binder includes polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, and ethylene oxide. Polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride (PVDF), polyethylene, polypropylene, styrene-butadiene rubber (SBR), acrylated styrene-butadiene rubber, epoxy resin, It may be nylon, etc., but is not limited thereto. The binder is added in an amount of 0.1 to 30% by weight based on the total weight of the mixture including the negative electrode active material.

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 0.01 to 10 μm, and the thickness is generally 5 to 300 μm. As such a separation membrane, For example, olefin polymers, such as chemical-resistant and hydrophobic polypropylene; A sheet or nonwoven fabric made of glass fiber or polyethylene is used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

The non-aqueous electrolyte solution containing lithium salt consists of a non-aqueous electrolyte solution and a lithium salt. The non-aqueous electrolyte includes, but is not limited to, a non-aqueous organic solvent, an organic solid electrolyte, an inorganic solid electrolyte, and the like.

Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, Gamma-butylolactone, 1,2-dimethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxo Run, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene car Aprotic organic solvents such as bonate derivatives, tetrahydrofuran derivatives, ethers, methyl pyropionate, and ethyl propionate can be used.

Examples of the organic solid electrolyte include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphoric acid ester polymers, poly agitation lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, A polymer containing an ionic dissociation group or 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, etc. 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 material readily soluble in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO4, 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, for the purpose of improving charge/discharge characteristics, flame retardancy, etc. in the non-aqueous electrolyte solution, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide , nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. may be added. may be In some cases, in order to impart incombustibility, 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 FEC (Fluoro-Ethylene) Carbonate), PRS (Propene Sultone), etc. may be further included.

The positive electrode may be manufactured by, for example, coating a positive electrode active material composed of positive electrode active material particles and a positive electrode mixture in which a conductive material and a binder are mixed on a positive electrode current collector, and if necessary, further adding a filler to the positive electrode mixture can do.

The positive electrode current collector is generally manufactured to have a thickness of 3 to 500 μm, and is not particularly limited as long as it has high conductivity without causing a chemical change in the battery, for example, stainless steel, aluminum, nickel, titanium , and carbon, nickel, titanium or silver surface-treated on the surface of aluminum or stainless steel may be used, and specifically aluminum may be used. The current collector may increase the adhesion of the positive electrode active material by forming fine irregularities on the surface thereof, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam body, and a nonwoven body are possible.

The cathode active material may include, in addition to the cathode active material particles, _{a layered compound such as lithium nickel oxide (LiNiO 2} ) or a compound substituted with one or more transition metals; Lithium manganese oxides such as Formula Li _1+x Mn _2-x O ₄ (where x is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , and LiMnO _{2 ;} lithium copper oxide (Li ₂ CuO ₂ ); vanadium oxides such as LiV ₃ O ₈ , LiV ₃ O ₄ , V ₂ O ₅ , and Cu ₂ V ₂ O _{7 ;} Ni site-type lithium nickel oxide represented by the formula LiNi _1-x M _x O ₂ (wherein 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, Zn or Ta and 0.01≤x≤0.1) or Li ₂ Mn ₃ MO ₈ (where M = Fe, Co, lithium manganese composite oxide represented by Ni, Cu or Zn; disulfide compounds; Fe ₂ (MoO ₄ ) ₃ It may be configured, and the like, but is not limited thereto.

The conductive material is typically added in an amount of 0.1 to 30% by weight based on the total weight of the mixture including the positive electrode active material. Such a conductive material is not particularly limited as long as it has conductivity without causing a chemical change 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 fibers and metal fibers; 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; A conductive material such as a polyphenylene derivative may be used.

The binder included in the positive electrode is a component that assists in bonding between the active material and the conductive material and bonding to the current collector, and is typically added in an amount of 0.1 to 30% by weight based on the total weight of the mixture including the positive electrode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers.

The present invention also provides a lithium secondary battery comprising the positive electrode, the negative electrode, and an electrolyte. The type of the lithium secondary battery is not particularly limited, but as a specific example, it may be a lithium secondary battery such as a lithium ion battery or a lithium ion polymer battery having advantages such as high energy density and output stability.

As an example of an electrode assembly including the electrode, one side of one separator is coated with a positive electrode mixture and the other side is coated with a negative electrode mixture to form a mono-cell, and a positive electrode mixture is applied to one side of one separator. to prepare a positive electrode by coating, a negative electrode mixture is coated on one side of the other separator to prepare a negative electrode can

Alternatively, a structure in which a positive electrode is prepared by coating a positive electrode mixture on both sides of one separator and a negative electrode mixture is coated on both sides of the other separator to prepare a negative electrode with an additional separator interposed between the positive electrode and the negative electrode Alternatively, an electrode assembly having a structure in which two or more are stacked may be manufactured.

The present invention may also provide a secondary battery for assembling the electrode assembly, and a battery pack including the secondary battery.

Specifically, the battery pack may be used as a power source for devices requiring high temperature stability, long cycle characteristics, and high rate characteristics, and specific examples of such devices include a mobile device, a wearable device, and the like. ), a power tool powered by an omniscient motor; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and the like; electric two-wheeled vehicles including electric bicycles (E-bikes) and electric scooters (E-scooter); electric golf carts; and an energy storage system, but is not limited thereto.

Since the structures of these devices and their manufacturing methods are known in the art, detailed description thereof will be omitted herein.

As described above, the electrode according to the present invention has the effect of lowering the diffusion resistance of lithium ions in a lithium secondary battery having a high-loading negative electrode, thereby improving the discharge capacity and cycle characteristics during charge/discharge.

1 is a swelling result of a porous polymer used in the present invention.
2 is a result of voltage and capacity at 0.1C charging for Examples 1, 2, and Comparative Examples of the present invention.
3 is a result of the voltage and capacity during 0.1 C discharge for Examples 1, 2, and Comparative Examples of the present invention.
4 is a result of voltage and capacity at 0.5C charging for Examples 1, 2, and Comparative Examples of the present invention.
5 is a result of voltage and capacity at 0.5 C discharge for Examples 1, 2, and Comparative Examples of the present invention.
6 is a measurement result of discharge characteristics according to the C rate for Examples 1, 2, and Comparative Examples of the present invention.
7 is a result of SEM measurement of a cross-section of an electrode according to Example 2 of the present invention.

Hereinafter, although described with reference to the drawings according to the embodiment of the present invention, this is for easier understanding of the present invention, and the scope of the present invention is not limited thereto.

1 is a swelling result of a porous polymer used in the present invention. As a result of swelling for MCC, it can be seen that swelling of MCC hardly occurs with respect to electrolyte.

Preparation of Example 1 and Comparative Example

In order to compare the performance of the electrode according to the present invention, coin half cells according to Example 1 and Comparative Example were prepared.

The composition ratio for the negative electrode of Example 1 and Comparative Example is as follows.

In Example 1, the size of the MCC was 50 μm, and in Example 2, the same composition as in Example was used, and only the size of the MCC was 10 μm. The loadings of Examples and Comparative Examples were 520mg/25cm2 and 510mg/25cm2, which were designed and manufactured to express the same dose.

As a result of measuring the porosity for Example 1, it was found to be 30%. The results of charging and discharging at 0.1C and 0.5C rates, respectively, for Examples 1, 2, and Comparative Examples for the coin half cell are shown in FIGS. 2, 3, 4, and 5 . The table below shows the charge/discharge efficiency according to each rate. When charging/discharging was carried out at a rate of 0.1C, there was no significant difference between Examples and Comparative Examples. However, it is inefficient to use a high-capacity battery at an actual low rate, and both Examples 1 and 2 according to the present invention showed much superior charge/discharge efficiency and capacity than the comparative example at 0.5C close to the actual environment. The effect of Example 2 using 10 μm MCC was the best. By using small MCC, more MCC particles were used compared to the same weight ratio, and the diffusion resistance was improved due to the increase in the number of pores.

6 is a result of examining the discharge characteristics of Examples 1, 2, and Comparative Example according to the C rate. In this result, both Examples 1 and 2 showed superior performance than the comparative example. In particular, it can be seen that at a high C rate suitable for the use of a high-capacity battery, the Example according to the present invention exhibits superior effects than the Comparative Example.

The table below digitizes the graph of FIG. 6 as a table. 6 shows the charging/discharging efficiency and capacity when discharging under the conditions shown in the table below after charging at 0.1C.

7 is a result of SEM measurement of a cross-section of an electrode according to Example 2 of the present invention. The cross section of the cathode of Example 2 was measured by SEM, and it was confirmed that the MCC formed internal pores.

8 is a 0.5C/0.5C charging/discharging result of Comparative Example and Example 2. 10 cycle test was conducted, Comparative Example and Example 2 showed a difference from the initial dose, and Comparative Example and Example 2 were evaluated to have a capacity retention rate of 24.5% and 53.6%, respectively, during 10 cycles. It can be seen that the comparative example shows a much higher stability, whereas the capacity of the comparative example is rapidly reduced according to the charge/discharge cycle.

Those of ordinary skill in the art to which the present invention pertains will be able to perform various applications and modifications within the scope of the present invention based on the above contents.

Claims (9)

In the electrode manufactured by applying the electrode slurry including the active material, the conductive agent, and the binder to the current collector in an amount of 350 mg/25 cm 2 or more,
Further comprising a porous polymer that does not swell in the non-aqueous electrolyte in the electrode slurry,
The porous polymer is microcrystalline cellulose,
An electrode having a particle size D50 of 1 to 50 μm of the porous polymer. delete delete delete According to claim 1,
An electrode wherein the content of the porous polymer in the electrode slurry is 0.01 to 5% based on the weight of the active material. According to claim 1,
An amount of the electrode slurry applied to the current collector is 350 mg/25 cm 2 or more and 1000 mg/25 cm 2 or less. According to claim 1,
The electrode is a negative electrode. A lithium secondary battery comprising the electrode according to any one of claims 1 to 7. A battery pack comprising the lithium secondary battery of claim 8.

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