Secondary batteries containing low-efficiency cathodes

Abstract

According to the present embodiment, there is provided a secondary battery including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode has a positive electrode mixture layer formed on at least one surface of a positive electrode current collector, the positive electrode mixture layer includes a positive electrode active material, the positive electrode active material is doped with a dopant, and includes a lithium transition metal oxide having a molar ratio of Ni of 88% or more based on the total moles of transition metals excluding Li, the negative electrode has a negative electrode mixture layer formed on at least one surface of a negative electrode current collector, the negative electrode mixture layer includes a negative electrode active material, the negative electrode active material includes a silicon-based active material, and the positive electrode has a ratio of initial discharge capacity to initial charge capacity (initial efficiency) of 85% to 89% in a battery using lithium as a counter electrode.

Description

[Technical Field]

[0001] [Cross-reference to related applications] This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0131693 dated October 5, 2021, Korean Patent Application No. 10-2021-0141818 dated October 22, 2021, Korean Patent Application No. 10-2021-0187600 and No. 10-2021-0187480 dated December 24, 2021, and Korean Patent Application No. 10-2022-0127292 dated October 5, 2022, and all contents disclosed in the documents of these Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to secondary batteries containing low-efficiency positive electrodes. [Background technology]

[0003] The rapid increase in fossil fuel use has led to a growing demand for alternative and clean energy, and one of the most actively researched areas is the field of electrochemical power generation and storage.

[0004] Currently, a typical example of an electrochemical element that uses such electrochemical energy is a secondary battery, and the range of its use is expanding day by day.

[0005] Recently, technological developments and increased demand for portable devices such as portable computers, mobile phones, and cameras have led to a rapid increase in demand for secondary batteries as energy sources. Among these secondary batteries, much research has been conducted on lithium secondary batteries, which have excellent charge / discharge characteristics and life characteristics, and are environmentally friendly. These batteries have also been commercialized and are widely used.

[0006] Generally, a lithium secondary battery is manufactured by impregnating a lithium non-aqueous electrolyte into an electrode assembly including a positive electrode, a negative electrode, and a porous separator.

[0007] The basic performance characteristics of such lithium secondary batteries are significantly affected by the anode material. To maximize battery performance, the anode active material must have an electrochemical reaction potential close to that of lithium metal, high reversibility in the reaction with lithium ions, and a fast diffusion rate of lithium ions within the active material. Graphite is widely used to meet these requirements. Considering the excellent adhesive strength of natural graphite and the excellent output and life characteristics of artificial graphite, a mixture of natural and artificial graphite has been used to improve the performance of secondary batteries in many ways.

[0008] However, recently, with the growth of devices requiring high-capacity batteries, such as electric vehicles and hybrid electric vehicles, the energy density level required for lithium secondary batteries has been continuously increasing. Therefore, attempts have been made to use a negative electrode containing Si, which has a high theoretical capacity, as a negative electrode active material.

[0009] In addition, when a negative electrode active material such as SiO containing Si is used, irreversible capacity occurs, and to compensate for this, a sacrificial anode material is added to the positive electrode to manufacture the battery.

[0010] If the sacrificial anode material is not used, a lower capacity than that of the existing positive electrode active material is realized. Therefore, in order to match the capacity, it is necessary to increase the positive electrode loading, which results in a problem of increased cost. Furthermore, if the sacrificial anode material is used, there are problems such as the high price of the sacrificial anode material, the inability to uniformly mix the positive electrode active material and the sacrificial anode material, which results in poor battery performance, and the generation of a large amount of lithium by-product from the sacrificial anode material, which results in increased gas generation during storage, which ultimately results in reduced battery performance.

[0011] Therefore, there is an urgent need to develop a secondary battery technology that can solve these problems, that does not reduce capacity or life characteristics, and that significantly reduces the amount of gas generated when stored at high temperatures. Summary of the Invention [Problem to be solved by the invention]

[0012] The present invention aims to solve the above-mentioned problems of the prior art and the technical problems that have been required in the past.

[0013] Specifically, the problem to be solved by the present invention is to provide a secondary battery that has capacity and life characteristics equivalent to those of a battery that includes a separate sacrificial anode material, without using such a separate sacrificial anode material, and that exhibits a significantly reduced amount of gas generation during high-temperature storage. [Means for solving the problem]

[0014] To achieve this object, according to one embodiment of the present invention, A secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, The positive electrode has a positive electrode mixture layer formed on at least one surface of a positive electrode current collector, the positive electrode mixture layer contains a positive electrode active material, The positive electrode active material includes a lithium transition metal oxide doped with a dopant, in which the molar ratio of Ni is 88% or more based on the total moles of transition metals excluding Li, The negative electrode has a negative electrode current collector on at least one surface of which a negative electrode mixture layer is formed, the negative electrode mixture layer contains a negative electrode active material, the negative electrode active material includes a silicon-based active material, The positive electrode provides a secondary battery in which the ratio of the initial discharge capacity to the initial charge capacity (initial efficiency) of a battery using lithium as the counter electrode is 85% to 89%.

[0015] The lithium transition metal oxide may be doped with a dopant in an amount of 4000 to 5000 ppm based on the total weight of the lithium transition metal oxide, and the dopant may be Zr.

[0016] Specifically, the lithium transition metal oxide is represented by the following chemical formula (1). Li 1+x Ni a Cob Mn c M 1-(a+b+c) O 2-y A y (1) In the chemical formula (1), M is at least one selected from the group consisting of Cu, Ti, Mg, Al, and Pt; A is an oxygen-substituted halogen; 0≦x≦0.5, 0.88≦a<1, 0≦b≦0.2, 0≦c≦0.2, 0.9≦a+b+c≦1, and 0≦y≦0.001.

[0017] More specifically, the lithium transition metal oxide can be represented by the following chemical formula (2): Li 1+x Ni a Co b Mn c Al 1-(a+b+c) O 2-y A y (2) In the chemical formula (2), A is an oxygen-substituted halogen; 0≦x≦0.5, 0.88≦a<1, 0≦b≦0.15, 0≦c≦0.15, 0.9≦a+b+c≦1, and 0≦y≦0.001.

[0018] In this case, specifically, a can be 0.90≦a<1.

[0019] The lithium transition metal oxide may also be in the form of a single particle.

[0020] At this time, the lithium transition metal oxide may have an average diameter (D50) of 1 to 5 μm or an average diameter (D50) of 10 to 20 μm.

[0021] More specifically, such a positive electrode can have an initial efficiency (ratio of initial discharge capacity to initial charge capacity) of 86% to 87% in a battery using lithium as the counter electrode.

[0022] Meanwhile, the negative electrode active material of the negative electrode may be a mixture of a silicon-based active material and a carbon-based active material.

[0023] The silicon-based active material may be contained in an amount of 1 to 10 wt % based on the total weight of the negative electrode active material.

[0024] Such a negative electrode may have an initial efficiency (initial efficiency) of 85% to 89% of the ratio of the initial discharge capacity to the initial charge capacity in a battery using lithium as the counter electrode, more specifically, 86% to 87%.

[0025] In addition, the positive electrode mixture layer may further include a conductive material, and the conductive material may be composed of single-walled carbon nanotubes (SWCNTs) having an impurity content of 300 ppm to 5000 ppm, specifically, the impurity content may be 3000 ppm to 4000 ppm. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a graph showing the occurrence of irreversible capacity in Experimental Example 1. [Figure 2] 10 is an SEM photograph of the positive electrode active material of Production Example 2 in Experimental Example 2. [Figure 3] 10 is a SEM photograph of a mixture of MWCNT and a positive electrode active material in Experimental Example 3. [Figure 4] 10 is a SEM photograph of a mixture of SWCNT and a positive electrode active material in Experimental Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention will now be described in more detail to aid in understanding the invention.

[0028] The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is appropriate to the technical idea of ​​the present invention, in accordance with the principle that the inventor himself can appropriately define the concept of terms in order to best explain the invention.

[0029] The terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless the context clearly indicates otherwise.

[0030] In this specification, the terms "comprises," "comprises," or "has" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, and should be understood as not precluding the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0031] According to one embodiment of the present invention, A secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, The positive electrode has a positive electrode mixture layer formed on at least one surface of a positive electrode current collector, the positive electrode mixture layer contains a positive electrode active material, The positive electrode active material includes a lithium transition metal oxide doped with a dopant, in which the molar ratio of Ni is 88% or more based on the total moles of transition metals excluding Li, The negative electrode has a negative electrode current collector on at least one surface of which a negative electrode mixture layer is formed, the negative electrode mixture layer contains a negative electrode active material, the negative electrode active material includes a silicon-based active material, The positive electrode provides a secondary battery in which the ratio of the initial discharge capacity to the initial charge capacity (initial efficiency) of a battery using lithium as the counter electrode is 85% to 89%.

[0032] As mentioned above, in the case of a secondary battery containing more than 5% by weight of silicon-based active material as the negative electrode active material, the negative electrode irreversible capacity is about 3-4%, resulting in an initial efficiency of about 87%. However, in this case, the irreversible capacity differs from when conventional lithium transition metal oxides are used as the positive electrode active material, so a separate sacrificial anode material such as Li2NiO2 is used to compensate for this and match the irreversible capacity.

[0033] However, when using such a sacrificial anode material, there is a problem that lithium by-products are generated due to excess lithium, and a large amount of gas is generated during high-temperature storage, resulting in poor battery performance.

[0034] Furthermore, since the sacrificial anode material is mixed with the existing lithium transition metal oxide cathode active material, the uniformity of the mixture is lower than when only one material is used, which also affects battery performance.

[0035] However, according to the present invention, when a positive electrode active material that achieves an initial efficiency of 85% to 89% is included, no sacrificial anode material is included. Therefore, by using a single material, mixing uniformity can be improved and gas generation during high-temperature storage can be reduced.

[0036] Specifically, the lithium transition metal oxide as the positive electrode active material of the present invention is doped with a dopant in an amount of 4000 to 5000 ppm based on the total weight of the lithium transition metal oxide, and the dopant may be Zr.

[0037] If the content is too low outside the above range, the initial efficiency cannot be sufficiently reduced, and if the content is too high, the dopant acts as a resistor, generating irreversible lithium, which is undesirable as it reduces the life characteristics.

[0038] More specifically, the content of the dopant may be 4300 to 4700 ppm based on the total weight of the lithium transition metal oxide.

[0039] Here, the doping is a concept different from substitution, and does not replace part of the transition metal crystal lattice but is positioned between the crystal lattice, stabilizing the positive electrode structure and increasing the electrical conductivity and ionic conductivity during charge and discharge, thereby improving life characteristics.

[0040] Therefore, the dopant is not represented by the following chemical formula (1), and the lithium transition metal oxide is represented by the following chemical formula (1). Li1+x Ni a Co b Mn c M 1-(a+b+c) O 2-y A y (1) In the chemical formula (1), M is at least one selected from the group consisting of Cu, Ti, Mg, Al, and Pt; A is an oxygen-substituted halogen; 0≦x≦0.5, 0.88≦a<1, 0≦b≦0.2, 0≦c≦0.2, 0.9≦a+b+c≦1, and 0≦y≦0.001.

[0041] More specifically, the lithium transition metal oxide is represented by the following chemical formula (2). Li 1+x Ni a Co b Mn c Al 1-(a+b+c) O 2-y A y (2) In the chemical formula (2), A is an oxygen-substituted halogen; 0≦x≦0.5, 0.88≦a<1, 0≦b≦0.15, 0≦c≦0.15, 0.9≦a+b+c≦1, and 0≦y≦0.001.

[0042] Here, the a may specifically be 0.90≦a<1.

[0043] That is, the dopant is not represented by the formula (1) but is separately contained in an amount of 4000 to 5000 ppm based on the total weight of the transition metal oxide.

[0044] In addition, the lithium transition metal oxide contained as the positive electrode active material according to the present invention should contain Ni in an amount of 88 mol% or more, specifically 90 mol% or more, more specifically 92% to 95%, based on the transition metal.

[0045] If the above conditions are met, the active material exhibits the most similar initial efficiency to the anode, and is therefore most preferable in terms of capacity and lifespan characteristics. In other words, it is an active material that can play a role similar to that of a sacrificial anode material mixed therein.

[0046] The lithium transition metal oxide may also be in the form of a single particle.

[0047] The term "single particle" means that the primary particles are present individually or aggregated in a number of less than 10 particles.

[0048] As described above, single-particle lithium transition metal oxides are more preferable because they can increase loading due to less particle cracking during rolling after coating with a positive electrode active material, and minimize particle resistance during life evaluation, improving life or output.

[0049] The lithium transition metal oxide may be used in a unimodal form or a bimodal mixed form in the positive electrode mixture layer, and is not limited thereto. Specifically, the lithium transition metal oxide may be used in a bimodal mixed form to improve performance by increasing the packing density, thereby reducing the resistance inside the electrode, and to improve performance by maximizing the contact between the positive electrode active material and the electrolyte.

[0050] The lithium transition metal oxide may have an average diameter (D50) of 1 to 7 μm or an average diameter (D50) of 10 to 20 μm. That is, in the case of a unimodal structure, only lithium transition metal oxide having an average diameter (D50) of 1 to 7 μm or only lithium transition metal oxide having an average diameter (D50) of 10 to 20 μm may be used, and in the case of a bimodal structure, large particles having an average diameter (D50) of 10 to 20 μm and small particles having an average diameter (D50) of 1 to 7 μm may be mixed and used.

[0051] The "average diameter D50" refers to the particle size at 50% of the cumulative volume distribution of particles by particle size. The D50 can be measured using a laser diffraction method. Specifically, the powder to be measured is dispersed in a dispersion medium and then introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500). When the particles pass through a laser beam, the difference in the diffraction pattern due to particle size is measured to calculate the particle size distribution. The 50% D50 of the cumulative volume distribution of particles by particle size can be measured using the analyzer.

[0052] In addition, a positive electrode containing such a low-efficiency positive electrode active material may have an initial efficiency (initial efficiency) of 85% to 89%, specifically 86% to 87%, and more specifically 87%, as a ratio of the initial discharge capacity to the initial charge capacity in a battery using lithium as the counter electrode.

[0053] The initial efficiency refers to the ratio of the initial discharge capacity to the initial charge capacity when a half coin cell is manufactured using the positive electrode and lithium metal as a counter electrode and a carbonate-based electrolyte, and is charged and discharged at 0.2 C between 4.2 V and 2.5 V.

[0054] As described above, the positive electrode of the secondary battery according to the present invention exhibits lower efficiency than conventional positive electrode active materials, and therefore, when used with a negative electrode containing a silicon-based active material, it is more preferable in terms of capacity and lifespan characteristics, and can have capacity and lifespan characteristics at the same level as when using a sacrificial anode material. On the other hand, since it does not contain a separate lithium-excess material like conventional sacrificial anode materials, it also has the effect of reducing gas generation during high-temperature storage due to the generation of lithium by-products.

[0055] In addition to the lithium transition metal oxides, the positive electrode active material may also include layered compounds such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or compounds substituted with one or more transition metals; 1+x Mn 2-xO4 (where x is 0 to 0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiV3O4, V2O5, and Cu2V2O7; and the chemical formula LiNi 1-x M x Ni-site type lithium nickel oxide represented by the chemical formula LiMnO2 (where M=Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x=0.01 to 0.3) 2-x M x Lithium manganese composite oxides represented by Li2Mn3MO8 (where M=Co, Ni, Fe, Cr, Zn, or Ta, and x=0.01 to 0.1) or Li2Mn3MO8 (where M=Fe, Co, Ni, Cu, or Zn); LiNi x Mn 2-x Lithium manganese composite oxides with a spinel structure represented by O4; LiMn2O4 in which part of the Li in the chemical formula is replaced with an alkaline earth metal ion; lithium iron phosphate oxide represented by LiFePO4; disulfide compounds; Fe2(MoO4)3, etc.

[0056] In this case, the lithium transition metal oxide may be included in an amount of 80 wt % to 100 wt % based on the total weight of the positive electrode active material, specifically, 100 wt %.

[0057] In addition, a negative electrode suitable for use with such a positive electrode may contain a silicon-based active material as a negative electrode active material, specifically a mixture of a silicon-based active material and a carbon-based active material. In this case, the silicon-based active material may be contained in an amount of 1 to 10 wt %, specifically 5 to 10 wt %, based on the total weight of the negative electrode active material.

[0058] If the content is too low, i.e., outside the above range, a high-energy secondary battery cannot be obtained, whereas if the content is too high, the irreversible capacity increases and swelling due to charging and discharging becomes large, which is undesirable.

[0059] Like the positive electrode, the negative electrode having such a configuration can have an initial efficiency (initial efficiency) of 85% to 89% of the ratio of the initial discharge capacity to the initial charge capacity in a battery using lithium as the counter electrode, specifically, an initial efficiency (initial efficiency) of 86% to 87% of the ratio of the initial discharge capacity to the initial charge capacity in a battery using lithium as the counter electrode.

[0060] The initial efficiency is the same as that described for the positive electrode, except that in this case, lithium is used as the negative electrode and counter electrode.

[0061] The positive electrode mixture layer and the negative electrode mixture layer may further contain a conductive material and a binder, respectively.

[0062] The conductive material is not particularly limited as long as it does not induce chemical changes in the battery and is conductive, and examples thereof include graphite such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskey such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. Specifically, the carbon fiber may be a carbon nanotube.

[0063] The carbon nanotubes may be single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs).

[0064] However, when using multi-walled carbon nanotubes (MWCNTs), which have a large number of bonds forming the walls, there was a problem that their conductivity decreased as the cycle progressed.

[0065] Therefore, the inventors of the present application considered using single-walled carbon nanotubes, which can simultaneously exhibit metallic and semiconducting properties and have a nanorod shape, are small in number but relatively long, and exhibit excellent conductivity between active materials. However, single-walled carbon nanotubes have a problem because they contain a high level of Fe impurity, approximately 70,000 ppm.

[0066] Therefore, we have considered single-walled carbon nanotubes with reduced impurities and increased purity, and have confirmed that when used in the positive electrode, sufficient conductivity can be maintained even with repeated cycling, and there are no problems with capacity reduction or increased resistance due to impurities.

[0067] Therefore, in the present invention, most specifically, single-walled carbon nanotubes with reduced impurity content can be used as the conductive material, particularly for the positive electrode.

[0068] Specifically, the conductive material may be single-walled carbon nanotubes having an impurity content of 300 ppm to 5000 ppm, most specifically, an impurity content of 3000 ppm to 4000 ppm, and even more specifically, an impurity content of 3000 ppm to 3500 ppm.

[0069] If the impurity content is too high, outside the above range, it is undesirable because the impurities act as resistance and may reduce output characteristics, and if the content is lower than the above range, it is difficult to remove the impurities. The impurity content can be measured using inductively coupled plasma (ICP), specifically, 1 g of single-walled carbon nanotubes is dissolved in hydrochloric acid (HCl), diluted 100 times with distilled water (DI water), and filtered through a Teflon (PTFE) filter.

[0070] As described above, single-walled carbon nanotubes with reduced impurity content can be obtained by carrying out a demetallization process.

[0071] The demetallization process involves preparing single-walled carbon nanotubes (e.g., SWCNTs from OCSiAL), mixing them with a 35% hydrochloric acid (HCl) solution and distilled water (DI water), centrifuging them, and then mixing the settled single-walled carbon nanotubes with distilled water (DI water) several times until the pH reaches 7.

[0072] Single-walled carbon nanotubes that are normally produced contain more than 70,000 ppm of metal impurities, specifically iron impurities. However, this demetallization process can significantly reduce the impurity content of the single-walled carbon nanotubes. As a result, there are no problems with resistance or side reactions, and the conductivity of the positive electrode is improved, thereby improving the lifespan characteristics.

[0073] Meanwhile, the diameter of such single-walled carbon nanotubes may be 0.5 nm to 10 nm, specifically 0.5 nm to 5 nm.

[0074] If the average diameter is outside the above range and is too small, the dispersed carbon nanotubes will be embedded between the positive electrode active material particles, making it difficult to form sufficient pores. If the average diameter is too large, it is not preferable because an excellent effect of improving conductivity cannot be obtained.

[0075] Furthermore, the length of the single-walled carbon nanotubes is not particularly limited, but may be 5 μm to 200 μm, specifically 10 μm to 100 μm.

[0076] The longer the length of the single-walled carbon nanotubes, the more the conductivity of the positive electrode, strength, and storage stability of the electrolyte can be improved. However, if the length is too long outside the above range, dispersibility may decrease, so the above range is most preferable.

[0077] Here, the diameter and length can be measured by AFM (Atomic Force Microscopy).

[0078] Thus, the aspect ratio (length / diameter) of the single-walled carbon nanotube, which is defined as the ratio of the diameter to the length of the single-walled carbon nanotube, may be 100 to 30,000, specifically 200 to 20,000.

[0079] The single-walled carbon nanotubes may generally have a shape in which multiple such single-walled carbon nanotubes are bundled together, and depending on the shape, they may have a secondary shape in which they are entangled or aggregated into a bundle, but specifically, they may have a secondary shape in which they are aggregated into a bundle.

[0080] Specifically, unless otherwise specified, the term "bundle type" refers to a secondary shape in which multiple CNTs are arranged side by side or twisted spirally into a bundle or rope shape, and the term "entangled type" refers to a shape in which multiple CNTs are entangled without being limited to a specific orientation.

[0081] The above shapes can be manufactured by varying the temperature in order to manufacture carbon nanotubes of the desired shape by chemical vapor deposition. In this case, the entangled carbon nanotubes have a solid structure and resemble an intermediate form between the point-like conductive material and the bundle-like carbon nanotubes, so the formation of a network structure is disadvantageous. In contrast, the bundle-like structure has carbon atoms spaced apart by a predetermined distance from each other, making electron transfer easier. Therefore, when ensuring electrical conductivity according to the present invention, it is more preferable to have a bundle-like structure.

[0082] The specific surface area of ​​the single-walled carbon nanotubes having such a secondary shape can be 100 to 3,000, specifically 500 to 1,000.

[0083] If the specific surface area is too small and outside the above range, it is difficult to ensure sufficient conductivity, and if it is too large, dispersibility may decrease, which is undesirable.

[0084] Such a specific surface area indicates a BET specific surface area, which is measured by the BET method. Specifically, it can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77 K) using a BELSORP-mino II manufactured by BEL Japan.

[0085] Such conductive material may be included in each of the positive electrode mixture layer and the negative electrode mixture layer at 0.1 to 30 wt %, specifically 0.1 to 10 wt %, more specifically 0.5 to 5 wt %, based on the total weight of each layer. Most specifically, when carbon nanotubes with reduced impurities are used, the content can be reduced to 0.05 to 0.2 wt %.

[0086] If the content is too high and out of the above range, the content of the active material is relatively reduced, resulting in a decrease in capacity, whereas if the content is too low, the conductivity and output characteristics may be reduced, which is undesirable.

[0087] Specific examples of the binder include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof.

[0088] The binder may be contained in each of the positive electrode mixture layer and the negative electrode mixture layer in an amount of 0.1 to 30 wt %, specifically 0.1 to 10 wt %, more specifically 0.5 to 5 wt %, based on the total weight of each layer.

[0089] If the binder content is too high and out of the above range, the content of the active material is reduced, resulting in a decrease in capacity, whereas if the binder content is too low, the adhesive strength is reduced, resulting in a decrease in life characteristics, which is undesirable.

[0090] Other components included in secondary batteries are well known in the art, so a detailed description thereof will be omitted in the present invention, and conventional configurations are included in the present invention.

[0091] The following describes preferred embodiments of the present invention, comparative examples for comparison, and experimental examples for evaluating them. However, the above examples are merely illustrative of the present description, and it will be apparent to those skilled in the art that various changes and modifications can be made within the scope and technical concept of the present description. It goes without saying that such changes and modifications are within the scope of the appended claims.

[0092] <Production Example 1> MWCNT (Multi-walled carbon nanotubes, LG Chem, MWCNT) was prepared and loaded into a quartz tube reactor. The reactor was then heated to 900°C under vacuum. After reaching the target temperature, gaseous chlorine gas (Cl2) was supplied into the reactor for 15 minutes to carry out the chlorination process. The temperature inside the reactor was then raised to 1200°C, and nitrogen gas was injected to remove metal chlorides from the carbon nanotubes. MWCNT (Conductive Material 1) with an impurity content of 20 ppm was prepared.

[0093] <Reference example> LiNi as the positive electrode active material 0.93 Co 0.02 Mn 0.03 Al 0.02 O2 (active materials with an average diameter D50:12 μm and an average diameter D50:5 μm were mixed in a volume ratio of 8:2), conductive material 1 (MWCNT) as a conductive material, and PVdF as a binder were mixed in an N-methylpyrrolidone solvent in a weight ratio of 98:0.8:1.2 to prepare a composition for forming a positive electrode, and this was loaded onto a 15 μm thick aluminum current collector at a loading amount of 20 mg / cm. 2 At this time, the ratio of the initial discharge capacity to the initial charge capacity (initial efficiency) of the positive electrode in a battery using lithium as the counter electrode was 90%.

[0094] A mixture of graphite and SiO in a weight ratio of 95:5 was used as the negative electrode active material, styrene-butadiene rubber (SBR) as a binder, sodium carboxymethyl cellulose (CMC) as a thickener, and carbon black as a conductive material was mixed in a weight ratio of 97:1:1:1, and then added to water as a solvent to prepare a composition for forming a negative electrode. This was then applied to a 10 μm copper current collector with a loading amount of 10 mg / cm. 2 At this time, the ratio of the initial discharge capacity to the initial charge capacity (initial efficiency) of the negative electrode in a battery using lithium as the counter electrode was 87%.

[0095] A polyethylene separator (thickness: 15 μm) was interposed between the positive electrode and the negative electrode, and then an electrolyte containing 1.3M LiPF6 (1M of the total electrolyte solution) containing a solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) mixed in a volume ratio of 20:10:70 was injected to prepare a secondary battery.

[0096] <Experimental Example 1> The secondary battery manufactured in the reference example was subjected to CC / CV charging and CC discharging once at 0.2C in the range of 4.2V-2.5V, and the capacity was measured. The results are shown in FIG. 1 below.

[0097] Referring to FIG. 1, when SiO is used as the negative electrode active material as in the Reference Example, it can be seen that the discharge capacity is reduced due to the irreversible capacity unless a sacrificial anode material sufficient to compensate for this is used as the positive electrode active material.

[0098] <Production Example 2> LiOH and Ni 0.93 Co 0.02 Mn 0.03 Al 0.02The (OH)2 precursor was mixed to a Li:(NiCoMnAl) molar ratio of 1.05:1, and then 4500 ppm of ZrO2 was added to prepare a mixed material. The mixed material was placed in an alumina crucible and fired in an oxygen (O2) atmosphere at 900°C for 15 hours to prepare a cathode active material with an average diameter D50 of 12 μm.

[0099] <Production Example 3> LiOH and Ni 0.93 Co 0.02 Mn 0.03 Al 0.02 The (OH)2 precursor was mixed to a Li:(NiCoMnAl) molar ratio of 1.05:1, and then 4500 ppm of ZrO2 was added to prepare a mixed material. The mixed material was placed in an alumina crucible and fired in an oxygen (O2) atmosphere at 700°C for 15 hours to prepare a cathode active material with an average diameter D50 of 5 μm.

[0100] <Experimental Example 2> An SEM photograph of the positive electrode active material prepared in Preparation Example 2 was taken and is shown in FIG.

[0101] Referring to FIG. 2, it can be seen that the lithium transition metal oxide was prepared as a single particle.

[0102] Example 1 A secondary battery was fabricated in the same manner as in the Reference Example, except that the positive electrode active materials prepared in Preparation Examples 2 and 3 were mixed in a volume ratio of 8:2. The ratio of the initial discharge capacity to the initial charge capacity (initial efficiency) of the positive electrode in a battery using lithium as the counter electrode was 87%.

[0103] <Example 2> The positive electrode active material is LiNi doped with Zr at 4000 ppm based on the total weight. 0.93 Co 0.02 Mn 0.03 Al 0.02Secondary batteries were fabricated in the same manner as in the Reference Example, except that ZrO2 was prepared (only the amount of ZrO2 was adjusted in Preparation Examples 2 and 3) and mixed together (an active material with an average diameter D50 of 12 μm and an active material with an average diameter D50 of 5 μm were mixed at a volume ratio of 8:2). The ratio of the initial discharge capacity to the initial charge capacity of the positive electrode in a battery using lithium as the counter electrode (initial efficiency) was 88%.

[0104] Example 3 The positive electrode active material is LiNi doped with Zr at 5000 ppm based on the total weight. 0.93 Co 0.02 Mn 0.03 Al 0.02 Secondary batteries were fabricated in the same manner as in the Reference Example, except that ZrO2 was prepared (only the amount of ZrO2 was adjusted in Preparation Examples 2 and 3) and mixed (active material with an average diameter D50 of 12 μm and active material with an average diameter D50 of 5 μm were mixed at a volume ratio of 8:2). The ratio of the initial discharge capacity to the initial charge capacity of the positive electrode in a battery using lithium as the counter electrode (initial efficiency) was 86%.

[0105] Example 4 The positive electrode active material is LiNi doped with Zr at 3000 ppm based on the total weight. 0.93 Co 0.02 Mn 0.03 Al 0.02 Secondary batteries were fabricated in the same manner as in the Reference Example, except that ZrO2 was prepared (only the amount of ZrO2 was adjusted in Preparation Examples 2 and 3) and mixed together (an active material with an average diameter D50 of 12 μm and an active material with an average diameter D50 of 5 μm were mixed at a volume ratio of 8:2). The ratio of the initial discharge capacity to the initial charge capacity of the positive electrode in a battery using lithium as the counter electrode (initial efficiency) was 88%.

[0106] <Example 5> The positive electrode active material is LiNi doped with Zr at 6000 ppm based on the total weight. 0.93 Co 0.02 Mn 0.03 Al 0.02Secondary batteries were fabricated in the same manner as in the Reference Example, except that ZrO2 was prepared in each case (only the amount of ZrO2 was adjusted in Preparation Examples 2 and 3) and mixed together (an active material with an average diameter D50 of 12 μm and an active material with an average diameter D50 of 5 μm were mixed in a volume ratio of 8:2). The ratio of the initial discharge capacity to the initial charge capacity of the positive electrode in a battery using lithium as the counter electrode (initial efficiency) was 85%.

[0107] Example 6 The positive electrode active material is LiNi doped with Zr at 1500 ppm based on the total weight. 0.88 Co 0.07 Mn 0.04 Al 0.01 Secondary batteries were fabricated in the same manner as in the Reference Example, except that ZrO2 was prepared (precursors in which the ratio of Ni, Co, and Mn was adjusted in Preparation Examples 2 and 3 were used, and the amount of ZrO2 was adjusted) and mixed (active material with an average diameter D50 of 12 μm and active material with an average diameter D50 of 5 μm were mixed at a volume ratio of 8:2). The ratio of the initial discharge capacity to the initial charge capacity of the positive electrode in a battery using lithium as the counter electrode (initial efficiency) was 85%.

[0108] <Production Example 4> SWCNT (single-walled carbon nanotubes, OCSiAl, SWCNT) was prepared. 5 g of SWCNT was mixed with 200 ml of 35% hydrochloric acid (HCl) solution and 30 ml of distilled water (DI water), and then centrifuged. The settled SWCNT was mixed with distilled water (DI water) and repeated several times until the pH reached 7, to prepare SWCNT (conductive material 2) with an impurity content of 3200 ppm.

[0109] At this time, the content of impurities was measured by ICP measurement. Specifically, 1 g of single-walled carbon nanotubes was dissolved in hydrochloric acid (HCl), diluted 100 times with distilled water (DI water), and filtered using a Teflon (registered trademark) (PTFE) filter to measure the solution.

[0110] <Production Example 5> SWCNT (single-walled carbon nanotubes, OCSiAl, SWCNT) was prepared. 5 g of SWCNT was mixed with 200 ml of 35% hydrochloric acid (HCl) solution and 50 ml of distilled water (DI water), and then centrifuged. The settled SWCNT was mixed with distilled water (DI water) and this process was repeated several times until the pH reached 7, to prepare SWCNT (Conductive Material 3) with an impurity content of 4000 ppm.

[0111] <Production Example 6> SWCNT (single-walled carbon nanotubes, OCSiAl, SWCNT) was prepared. 5 g of SWCNT was mixed with 200 ml of 35% hydrochloric acid (HCl) solution and 500 ml of distilled water (DI water), and then centrifuged. The settled SWCNT was mixed with distilled water (DI water) and repeated several times until the pH reached 7, to prepare SWCNT (Conductive Material 4) with an impurity content of 10,000 ppm.

[0112] Example 7 A secondary battery was fabricated in the same manner as in the Reference Example, except that the positive electrode active materials prepared in Preparation Examples 2 and 3 were mixed in a volume ratio of 8:2 and Conductive Material 2 of Preparation Example 4 was used as the conductive material instead of Conductive Material 1. The positive electrode had an initial efficiency (initial efficiency) of 86% relative to the initial discharge capacity of a battery using lithium as the counter electrode.

[0113] Example 8 A secondary battery was fabricated in the same manner as in the Reference Example, except that the positive electrode active materials prepared in Preparation Examples 2 and 3 were mixed in a volume ratio of 8:2 and Conductive Material 3 of Preparation Example 5 was used instead of Conductive Material 1. The ratio of the initial discharge capacity to the initial charge capacity (initial efficiency) of the positive electrode in a battery using lithium as the counter electrode was 85%.

[0114] Example 9 A secondary battery was fabricated in the same manner as in the Reference Example, except that the positive electrode active materials prepared in Preparation Examples 2 and 3 were mixed in a volume ratio of 8:2 and Conductive Material 4 of Preparation Example 6 was used instead of Conductive Material 1. The ratio of the initial discharge capacity to the initial charge capacity (initial efficiency) of the positive electrode in a battery using lithium as the counter electrode was 86%.

[0115] Example 10 A secondary battery was fabricated in the same manner as in the Reference Example, except that the positive electrode active materials prepared in Preparation Examples 2 and 3 were mixed in a volume ratio of 8:2 and the positive electrode active material:conductive material:binder was mixed in a weight ratio of 97.6:1.2:1.2. The ratio of the initial discharge capacity to the initial charge capacity (initial efficiency) of the positive electrode in a battery using lithium as the counter electrode was 88%.

[0116] <Comparative Example 1> LiNi as the positive electrode active material 0.88 Co 0.07 Mn 0.04 Al 0.01 A secondary battery was fabricated in the same manner as in Reference Example 1, except that a positive electrode composition was prepared by mixing LiNiO2 (a mixture of an active material with an average diameter D50 of 10 μm and an active material with an average diameter D50 of 5 μm in a volume ratio of 8:2, without Zr doping), a sacrificial anode material in a weight ratio of 95:5 with LiNiO2, a conductive material (MWCNT), and a binder in an N-methylpyrrolidone solvent in a weight ratio of 98:0.8:1.2 with PVdF. The positive electrode had an initial discharge capacity to initial charge capacity ratio (initial efficiency) of 91% in a battery using lithium as the counter electrode.

[0117] <Comparative Example 2> LiNi without Zr doping as the positive electrode active material 0.93 Co 0.02 Mn 0.03 Al 0.02A secondary battery was fabricated in the same manner as in the Reference Example, except that a mixture of O2 and an active material with an average diameter D50 of 12 μm was used (a mixture of an active material with an average diameter D50 of 5 μm at a volume ratio of 8:2). In this case, the ratio of the initial discharge capacity to the initial charge capacity (initial efficiency) of the battery using a lithium counter electrode was 90%.

[0118] <Comparative Example 3> LiNi doped with 4500 ppm of Zr as the positive electrode active material 0.5 Co 0.2 Mn 0.3 A secondary battery was fabricated in the same manner as in the Reference Example, except that a mixture of active materials with an average diameter D50 of 12 μm and active materials with an average diameter D50 of 5 μm was used (only the composition ratio of Ni, Co, and Mn in the precursors was adjusted in Preparation Examples 2 and 3). The ratio of the initial discharge capacity to the initial charge capacity of the positive electrode in a battery using lithium as the counter electrode (initial efficiency) was 91%.

[0119] <Experimental Example 3> The MWCNTs and SWCNTs prepared in Preparation Examples 1 and 4 were mixed with the cathode active material prepared in Preparation Example 2, respectively, and the results were photographed by SEM and are shown in FIGS.

[0120] Referring to Figures 3 and 4, in the case of MWCNTs, multiple layers of carbon nanotubes are entangled and form connections between the positive electrode active materials but do not surround the surface. In contrast, in the case of SWCNTs, a small number of bundled single-walled carbon nanotubes surround the positive electrode active materials and connect them to each other.

[0121] <Experimental Example 4> Life characteristics The secondary batteries prepared in Examples 1 to 10 and Comparative Examples 1 to 3 were subjected to CC / CV charging at 0.3 C between 4.2 V and 2.85 V at room temperature and CC discharging at 0.5 C for 200 cycles to evaluate their lifespan characteristics, and the results are shown in Table 1 below.

[0122] Specifically, the initial discharge energy was set to 100% and the reduction rate of discharge energy due to cycles was shown.

[0123] [Table 1]

[0124] Referring to Table 1, it can be seen that Examples 1 to 9 according to the present invention exhibited better life characteristics than Comparative Example 1, which used a sacrificial anode material. In Example 4, where the Zr doping amount was small, the life characteristics did not decrease significantly, but due to insufficient compensation for irreversible capacity, the life characteristics decreased slightly compared to Examples 1 to 3. Furthermore, it can be seen that in Example 5, where the Zr doping amount was excessively high, the Zr acted as a resistor, resulting in a slight decrease in the life characteristics.

[0125] On the other hand, compared to Example 1 using MWCNT, Examples 7 and 8 using SWCNT with reduced impurity content exhibit better life characteristics, and also exhibit better life characteristics than Example 10, which has an increased content of conductive material. However, it can be confirmed that Example 9, which uses SWCNT with a high impurity content, exhibits worse life characteristics than Example 1 due to the influence of the impurities.

[0126] <Experimental Example 4> High temperature storage characteristics The secondary batteries prepared in Examples 1 to 10 and Comparative Example 3 were CC / CV charged at 4.2 V and 0.3 C, and the amount of gas was immediately analyzed at 100% SOC. The cells charged to 100% SOC were stored in a high-temperature chamber at 72°C, and the amount of gas generated (μl) was analyzed after 4 weeks. The results are shown in Table 2 below.

[0127] The gas amount was analyzed using a Binary Gas Analyzer (BGA-08) and the analytical method was AMT-5535-0k. In the results below, the gas amount is calculated based on 25 degrees and 1 atm.

[0128] [Table 2]

[0129] Referring to Table 2, it can be seen that the secondary batteries of Examples 1 to 10 did not use a sacrificial anode material, and therefore gas generation after high-temperature storage was reduced compared to Comparative Example 1.

[0130] On the other hand, it can be seen that the smaller the impurity content of the conductive material, the less gas is generated.

[0131] Furthermore, when a cathode material not doped with Zr is used (Comparative Example 2), it can be confirmed that a large amount of gas is generated due to a side reaction on the surface of the cathode.

[0132] Those skilled in the art will appreciate that various applications and modifications within the scope of the present invention can be made based on the above content. [Industrial Applicability]

[0133] As described above, the secondary battery according to an embodiment of the present invention includes a low-efficiency cathode active material similar to that of the anode, and therefore does not require a separate sacrificial anode material. This significantly reduces the amount of gas generated during high-temperature storage, and provides capacity and life characteristics comparable to those of a battery that includes a sacrificial anode material.

[0134] In addition, according to another embodiment of the present invention, the positive electrode of the present invention contains single-walled carbon nanotubes (SWCNTs) with a low impurity content as a conductive material, thereby ensuring excellent conductivity even as the cycles progress, thereby improving the life characteristics of a secondary battery containing the same.

Claims

1. A secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, The positive electrode has a positive electrode mixture layer formed on at least one surface of a positive electrode current collector, the positive electrode mixture layer contains a positive electrode active material, The positive electrode active material includes a lithium transition metal oxide doped with a dopant, in which the molar ratio of Ni is 88% or more based on the total moles of all transition metals excluding Li, The negative electrode has a negative electrode current collector on at least one surface of which a negative electrode mixture layer is formed, the negative electrode mixture layer contains a negative electrode active material, the negative electrode active material includes a silicon-based active material, The positive electrode has an initial discharge capacity ratio (initial efficiency) of 85% to 89% relative to the initial charge capacity in a battery using lithium as a counter electrode, The lithium transition metal oxide is doped with a dopant in an amount of 4000 to 5000 ppm based on the total weight of the lithium transition metal oxide, The secondary battery, wherein the dopant is Zr.

2. A secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, The positive electrode has a positive electrode mixture layer formed on at least one surface of a positive electrode current collector, the positive electrode mixture layer contains a positive electrode active material, The positive electrode active material includes a lithium transition metal oxide doped with a dopant, in which the molar ratio of Ni is 88% or more based on the total moles of all transition metals excluding Li, The negative electrode has a negative electrode current collector on at least one surface of which a negative electrode mixture layer is formed, the negative electrode mixture layer contains a negative electrode active material, the negative electrode active material includes a silicon-based active material, The positive electrode has an initial discharge capacity ratio (initial efficiency) of 85% to 89% relative to the initial charge capacity in a battery using lithium as a counter electrode, the lithium transition metal oxide is a single particle; The secondary battery, wherein the dopant is Zr.

3. The secondary battery according to claim 1 or 2, wherein the lithium transition metal oxide is represented by the following chemical formula (1): Li １＋ｘ Ni ａ Co ｂ Mn ｃ M １－（ａ＋ｂ＋ｃ） O ２－ｙ A ｙ (1) In the chemical formula (1), M is at least one selected from the group consisting of Cu, Ti, Mg, Al, and Pt; A is an oxygen-substituted halogen; 0≦x≦0.5, 0.88≦a<1, 0≦b≦0.2, 0≦c≦0.2, 0.9≦a+b+c≦1, and 0≦y≦0.

001.

4. The secondary battery according to claim 3, wherein the lithium transition metal oxide is represented by the following chemical formula (2): Li １＋ｘ Ni ａ Co ｂ Mn ｃ Al １－（ａ＋ｂ＋ｃ） O ２－ｙ A ｙ (2) In the chemical formula (2), A is an oxygen-substituted halogen; 0≦x≦0.5, 0.88≦a<1, 0≦b≦0.15, 0≦c≦0.15, 0.9≦a+b+c≦1, and 0≦y≦0.

001.

5. 4. The secondary battery according to claim 3, wherein a satisfies the condition 0.90≦a<1.

6. 3. The secondary battery according to claim 1, wherein the lithium transition metal oxide has an average diameter (D50) of 1 to 5 μm or an average diameter (D50) of 10 to 20 μm.

7. 3. The secondary battery according to claim 1, wherein the positive electrode has a ratio (initial efficiency) of an initial discharge capacity to an initial charge capacity in a battery using lithium as a counter electrode of 86% to 87%.

8. 3. The secondary battery according to claim 1, wherein the negative electrode active material is a mixture of a silicon-based active material and a carbon-based active material.

9. 3. The secondary battery of claim 1, wherein the silicon-based active material is contained in an amount of 1 to 10 wt % based on the total weight of the negative electrode active material.

10. 3. The secondary battery according to claim 1, wherein the negative electrode has a ratio of initial discharge capacity to initial charge capacity (initial efficiency) of 85% to 89% in a battery using lithium as a counter electrode.

11. The secondary battery according to claim 10, wherein the negative electrode has a ratio (initial efficiency) of an initial discharge capacity to an initial charge capacity in a battery using lithium as a counter electrode of 86% to 87%.

12. The positive electrode mixture layer further includes a conductive material, and the conductive material is a single-walled carbon nanotube (SWCNT) having an impurity content of 300 ppm to 5000 ppm. The secondary battery according to claim 1 or 2.

13. 13. The secondary battery according to claim 12, wherein the conductive material has an impurity content of 3000 ppm to 4000 ppm.

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