Additive for positive electrode of lithium secondary battery, method for manufacturing the same, and lithium secondary battery including the same

Abstract

The present invention provides an additive for a positive electrode of a lithium secondary battery, and the additive includes a lithium transition metal oxide, Li3PO4, Li5AlO4, and Li3BO3. The lithium transition metal oxide has a form doped with aluminum. The additive including the lithium transition metal oxide, Li3PO4, Li5AlO4, and Li3BO3 has a function of improving battery stability when used in the positive electrode of a lithium secondary battery. Specifically, when used in the positive electrode of a lithium secondary battery, a general lithium transition metal oxide causes a side reaction with an electrolyte, thereby causing problems such as generating gas in the battery and reducing stability. However, Li3PO4, Li5AlO4, and Li3BO3 are uniformly mixed with the lithium transition metal oxide or some of them form a partial coating, or a part of aluminum is doped into the lithium transition metal oxide, thereby suppressing the side reaction of the lithium transition metal oxide with the electrolyte.

Description

Technical Field

[0001] The present invention relates to an additive for a positive electrode of a lithium secondary battery, a method for preparing the same, and a positive electrode of a lithium secondary battery including the same. In particular, the present invention relates to an additive for a positive electrode of a lithium secondary battery including an aluminum-doped lithium transition metal oxide, Li3PO4, Li5AlO4, and Li3BO3, a method for preparing the same, and a positive electrode of a lithium secondary battery including the same.

[0002] This application claims the benefit of priority based on Korean Patent Application No. 10-2020-0161400, filed on November 26, 2020, and Korean Patent Application No. 10-2021-0156630, filed on November 15, 2021, the entire contents of which are incorporated herein by reference. Background Art

[0003] With the technological development and increasing demand of mobile devices, the demand for secondary batteries as an energy source has increased rapidly. Among such secondary batteries, lithium secondary batteries having a high energy density and voltage, a long cycle life, and a low self-discharge rate have been commercialized and widely used.

[0004] A lithium transition metal oxide is used as a positive electrode active material of a lithium secondary battery, and among them, lithium cobalt oxide LiCoO2 having a high working voltage and excellent capacity performance is mainly used. However, LiCoO2 has very poor thermal properties due to the instability of the crystal structure caused by de-lithiation, and is also expensive, and thus is limited in mass use as a power source in fields such as electric vehicles. As materials to replace LiCoO2, lithium manganese oxides (LiMnO2, LiMn2O4, etc.), lithium iron phosphate compounds (LiFePO4, etc.), lithium nickel oxides (LiNiO2, etc.) have been developed. Among them, lithium nickel oxides capable of easily obtaining a large-capacity battery by having a high reversible capacity of about 200 mAh / g have been actively studied and developed.

[0005] In addition, regarding the negative electrode active material, the demand for using an Si-based negative electrode active material to obtain a high-capacity lithium secondary battery has increased, and since the irreversible capacity of the Si-based negative electrode active material is large, it is also necessary to balance the irreversible capacity in the positive electrode in order to effectively drive the battery. Therefore, studies have also been conducted on positive electrode additives having a large irreversible capacity, and in this process, various additives such as Li2NiO2, Li2CuO4, and Li6CoO4 have been used as positive electrode additives having a large irreversible capacity.

[0006] Li2NiO2 is synthesized by a solid-phase method using Li2O and NiO, and due to the low synthesis rate, Li2O and NiO basically remain unreacted substances. Such Li2O and NiO are difficult to achieve capacity, and Li2O may be converted into LiOH and Li2CO3, thereby causing problems such as gelation during the electrode preparation process and gas generation during charge and discharge and high-temperature storage.

[0007] To solve these problems, research on cathode additives for improving the performance of lithium secondary batteries has been continuously carried out in the art.

[0008] [Prior art documents]

[0009] [Patent documents]

[0010] (Patent Document 1) Korean Patent Application Publication No. 10-2014-0092739 Summary of the Invention

[0011] Technical Problem

[0012] The present invention provides an additive for a lithium secondary battery cathode, a method for preparing the same, and a lithium secondary battery cathode including the same. The additive for a lithium secondary battery cathode can improve battery stability when used in a lithium secondary battery by including a lithium transition metal oxide in which a part of the transition metal is doped with aluminum, Li3PO4, Li5AlO4, and Li3BO3.

[0013] Technical Solution

[0014] According to a first aspect of the present invention, there is provided an additive for a lithium secondary battery cathode, the additive including a lithium transition metal oxide, Li3PO4, Li5AlO4, and Li3BO3, wherein the lithium transition metal oxide has a form doped with aluminum.

[0015] In one embodiment of the present invention, the lithium transition metal oxide is represented by the following Chemical Formula 1:

[0016] [Chemical Formula 1]

[0017] Li2Ni 1-x Al x O2

[0018] Wherein, x is from 0.001 to 0.005.

[0019] In one embodiment of the present invention, based on the total weight of the additive, the content of the lithium transition metal oxide is 75 wt% to 90 wt%.

[0020] In one embodiment of the present invention, based on the total weight of the additive, the content of Li3PO4 is 1 wt% to 10 wt%.

[0021] In one embodiment of the present invention, based on the total weight of the additive, the content of Li5AlO4 is 0.5 wt% to 5 wt%.

[0022] In one embodiment of the present invention, based on the total weight of the additive, the content of Li3BO3 is 0.1 wt% to 3 wt%.

[0023] In one embodiment of the present invention, a part of the Li3BO3 exists in a state of being coated on the lithium transition metal oxide.

[0024] In one embodiment of the present invention, the additive further contains NiO.

[0025] In one embodiment of the present invention, based on the total weight of the additive, the content of NiO is 5 wt% to 15 wt%.

[0026] According to the second aspect of the present invention, there is provided a method for preparing an additive for a positive electrode of a lithium secondary battery as described above, the method comprising the following steps: (1) preparing a mixture by mixing a transition metal source material, a lithium source material, a phosphorus source material, an aluminum source material, and a boron source material; and (2) heat-treating the mixture.

[0027] In one embodiment of the present invention, in step (1), the transition metal source material is NiO, the lithium source material is Li2O, LiOH or Li2CO3, the phosphorus source material is (NH4)2HPO4, the aluminum source material is Al(OH)3, and the boron source material is B(OH)3.

[0028] In one embodiment of the present invention, in step (2), the mixture is heat-treated at 300 °C to 700 °C for 10 hours to 32 hours.

[0029] According to the third aspect of the present invention, there is provided a positive electrode for a lithium secondary battery, the positive electrode comprising the additive according to claim 1 and a positive electrode active material, wherein, based on 100 parts by weight of the positive electrode active material, the content of the additive is 10 parts by weight to 40 parts by weight.

[0030] Beneficial effects

[0031] The additive for a positive electrode of a lithium secondary battery according to the present invention contains a lithium transition metal oxide, Li3PO4, Li5AlO4, and Li3BO3, and the lithium transition metal oxide is doped with aluminum.

[0032] When an additive containing a lithium transition metal oxide and Li3PO4, Li5AlO4, and Li3BO3 is used in the positive electrode of a lithium secondary battery, a function of improving battery stability is obtained. Specifically, general lithium transition metal oxides cause side reactions with the electrolyte when used in the positive electrode of a lithium secondary battery, which causes problems of reduced stability such as gas generation in the battery. However, Li3PO4, Li5AlO4, and Li3BO3 are mixed with the lithium transition metal oxide or some of them form a coating, or a part of aluminum is doped into the lithium transition metal oxide, thereby suppressing the side reactions of the lithium transition metal oxide with the electrolyte. Detailed Description of the Invention

[0033] All embodiments provided according to the present invention can be implemented through the following description. The following description should be understood as describing the preferred embodiments of the present invention, and it should be understood that the present invention is not necessarily limited thereto.

[0034] When the measurement conditions and methods are not specifically described for the described properties in this specification, the measurement conditions and methods commonly used by those skilled in the art are used to measure the properties.

[0035] Additive and positive electrode active material

[0036] The present invention provides an additive for a positive electrode of a lithium secondary battery, and the additive for a positive electrode of a lithium secondary battery contains a lithium transition metal oxide, Li3PO4, Li5AlO4, and Li3BO3.

[0037] The lithium transition metal oxide is mixed with the positive electrode active material, thereby serving to supplement the properties such as the electrode capacity of the positive electrode active material. Among the lithium transition metal oxides, the transition metal is selected from Co, Ni, Cu, Mn, Fe, and combinations thereof. Specifically, the lithium transition metal oxide can be selected from the following materials commonly used as positive electrode active materials: LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li(Ni a Co b Mn c )O2 (0 < a < 1, 0 < b < 1, 0 < c < 1, a + b + c = 1), LiNi 1-y Co y O2 (0 < y < 1), LiCo 1-y Mn y O2, LiNi 1-y Mn y O2 (0 < y < 1), Li(Ni a Co b Mn c )O4 (0 < a < 2, 0 < b < 2, 0 < c < 2, a + b + c = 2), LiMn 2-zNi z O4 (0 < z < 2), LiMn 2-z Co z O4 (0 < z < 2) and combinations thereof. However, irreversible lithium transition metal oxides containing excessive lithium, such as Li2NiO2, Li2CuO4 or Li6CoO4, can be used, such that they are used together with Si-based anode active materials having a large irreversible capacity. According to one embodiment of the present invention, the lithium transition metal oxide is Li2NiO2. Li2NiO2 is synthesized by a solid-phase method using Li2O and NiO, and due to the low synthesis rate, Li2O and NiO are substantially still unreacted substances. Such Li2O and NiO are difficult to achieve capacity, and Li2O may be converted into LiOH and Li2CO3, thereby causing problems of gelation during the electrode preparation process and gas generation during charge and discharge and high-temperature storage. According to one embodiment of the present invention, the lithium transition metal oxide has an aluminum-doped form. This is due to the fact that the preparation of the additive is affected by the aluminum source material, phosphorus source material, etc. added together. The aluminum-doped lithium transition metal oxide can be represented by the following Chemical Formula 1.

[0038] [Chemical Formula 1]

[0039] Li2Ni 1-x Al x O2

[0040] Wherein, x is from 0.001 to 0.005, specifically from 0.0015 to 0.004, and more specifically from 0.002 to 0.003.

[0041] In the present invention, Li3PO4, Li5AlO4 and Li3BO3, which are included together with the lithium transition metal oxide as additives for the positive electrode of the lithium secondary battery, are coated on the lithium transition metal oxide and are particularly effective in suppressing side reactions with the electrolyte. Therefore, when used together with Li2NiO2 in the lithium transition metal oxide, more superior effects of improving battery performance can be expected by adding Li3PO4, Li5AlO4 and Li3BO3.

[0042] According to one embodiment of the present invention, based on the total weight of the additive, it contains a lithium transition metal oxide in an amount of 75% by weight to 90% by weight, preferably 77% by weight to 87% by weight, more preferably 80% by weight to 85% by weight. The lithium transition metal oxide is an additive that functions to supplement the performance such as the electrode capacity of the positive electrode active material, and a content above a certain level is required to achieve the performance of the lithium secondary battery. However, problems with stability may occur, and it is necessary to appropriately adjust the ratio with the additive materials of Li3PO4, Li5AlO4, and Li3BO3. When the lithium transition metal oxide is contained in an amount less than 75% by weight based on the total weight of the additive, the basic performance of the lithium secondary battery may decline, and when the lithium transition metal oxide is contained in an amount greater than 90% by weight based on the total weight of the additive, there may be problems with the overall stability of the positive electrode.

[0043] The additive according to the present invention contains Li3PO4 by adding a phosphorus source material during the preparation process and reacting it with Li2O, etc. The phosphorus source material will be specifically described in the "method for preparing the additive" described later. Li3PO4 improves the stability of the lithium transition metal oxide, and in particular, can inhibit the generation of gas in the battery caused by the reaction of the lithium transition metal oxide with the electrolyte. In addition, containing the phosphorus source material at a level above a certain level during the additive preparation process helps to dope and locate a part of aluminum into the lithium transition metal oxide. According to one embodiment of the present invention, based on the total weight of the additive, it contains Li3PO4 in an amount of 1% by weight to 10% by weight, preferably 2% by weight to 8% by weight, more preferably 3% by weight to 6% by weight. When Li3PO4 is contained in an amount less than 1% by weight based on the total weight of the additive, the effect of improving the stability of the lithium secondary battery may not be significant, and it is not preferable in terms of the performance of the lithium secondary battery to contain Li3PO4 in an amount greater than 10% by weight based on the total weight of the additive because the content of other components decreases.

[0044] The additive according to the present invention contains Li5AlO4 by adding an aluminum source material during the preparation process and reacting with Li2O or the like. The aluminum source material will be specifically described in the "method for preparing the additive" described later. A part of the aluminum supplied by the aluminum source material can be doped into the lithium transition metal. When doped with aluminum, the lithium transition metal oxide can be as shown in Chemical Formula 1. When aluminum is doped on the surface related to the h-index, Ni dissolution can be reduced while peak splitting occurs. According to an embodiment of the present invention, based on the total weight of the additive, Li5AlO4 can be contained in an amount of 0.5 wt% to 5 wt%, preferably 1 wt% to 4 wt%, more preferably 1.5 wt% to 3 wt%. When Li5AlO4 is contained in an amount less than 0.5 wt% based on the total weight of the additive, the effect of improving the stability of the lithium secondary battery may not be significant, and when Li5AlO4 is contained in an amount greater than 5 wt% based on the total weight of the additive, it is not preferable in terms of the performance of the lithium secondary battery because the content of other components decreases.

[0045] The additive according to the present invention contains Li3BO3 by adding a boron source material during the preparation process and reacting with Li2O or the like. The boron source material will be specifically described in the "method for preparing the additive" described later. According to an embodiment of the present invention, a part of Li3BO3 exists in a state of being coated on the lithium transition metal oxide. This is due to the nature of boron atoms. Li3BO3 improves the stability of the lithium transition metal oxide, and in particular, can inhibit the generation of gas in the battery caused by the reaction of the lithium transition metal oxide with the electrolyte. According to an embodiment of the present invention, based on the total weight of the additive, Li3BO3 is contained in an amount of 0.1 wt% to 3 wt%, preferably 0.3 wt% to 2 wt%, more preferably 0.5 wt% to 1.5 wt%. Compared with other additive materials, even with a relatively small amount used, Li3BO3 is effective in improving the stability of the lithium secondary battery. When Li3BO3 is contained in an amount less than 0.1 wt% based on the total weight of the additive, the effect of improving the stability of the lithium secondary battery may not be significant, and when Li3BO3 is contained in an amount greater than 3 wt% based on the total weight of the additive, it is not preferable in terms of the performance of the lithium secondary battery because the content of other components decreases.

[0046] Compared with the case of containing all three materials simultaneously, when one or two materials are contained in the same amount or even a larger amount, the additive materials of Li3PO4, Li5AlO4, and Li3BO3 contained together with the lithium transition metal oxide are also ineffective in terms of the stability of the lithium secondary battery. Each of the three additive materials has a separate function, and when used together, a synergistic effect is obtained in terms of the stability of the lithium secondary battery.

[0047] In addition to lithium transition metal oxides, Li3PO4, Li5AlO4, and Li3BO3, the additive according to the present invention may further contain NiO. NiO is one of the transition metal source materials added during the preparation of the additive and is a stable compound with low reactivity, and thus may exist in the additive in an unreacted state. Depending on the type of transition metal of the lithium transition metal oxide used in the additive, Ni in NiO may be replaced by materials such as Co, Cu, Mn, and Fe. According to one embodiment of the present invention, based on the total weight of the additive, NiO may be contained in an amount of 5 wt% to 15 wt%, preferably 7 wt% to 13 wt%, more preferably 8 wt% to 11 wt%.

[0048] The positive electrode active material serves as the actual positive electrode active material for exchanging electrons in the positive electrode of a lithium secondary battery, and materials commonly used in the art may be used. Specifically, the positive electrode active material may be selected from: LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li(Ni a Co b Mn c )O2 (0 < a < 1, 0 < b < 1, 0 < c < 1, a + b + c = 1), LiNi 1-y Co y O2 (0 < y < 1), LiCo 1- y Mn y O2, LiNi 1-y Mn y O2 (0 < y < 1), Li(Ni a Co b Mn c )O4 (0 < a < 2, 0 < b < 2, 0 < c < 2, a + b + c = 2), LiMn 2-z Ni z O4 (0 < z < 2), LiMn 2-z Co z O4 (0 < z < 2) and combinations thereof. The additive serves to supplement the electrode capacity of the positive electrode active material, and thus, its content may be appropriately adjusted in consideration of the relationship with the positive electrode active material. According to one embodiment of the present invention, based on 100 parts by weight of the positive electrode active material, the additive may be contained in an amount of 10 parts by weight to 40 parts by weight, preferably 10 parts by weight to 30 parts by weight, more preferably 10 parts by weight to 20 parts by weight. Introducing the additive within the above range is effective in improving the performance of the positive electrode active material.

[0049] Method for preparing additive

[0050] The present invention provides a method for preparing an additive for a positive electrode of the above lithium secondary battery. The preparation method includes the following steps: (1) preparing a mixture by mixing a transition metal source material, a lithium source material, a phosphorus source material, an aluminum source material, and a boron source material; and (2) heat-treating the mixture.

[0051] In step (1), the transition metal source material is a material that supplies a transition metal to a lithium transition metal oxide or the like contained in the final positive electrode active material, and can generally be a transition metal oxide. According to an embodiment of the present invention, the transition metal source material is NiO. The lithium source material is a material that supplies lithium to a lithium transition metal oxide, Li3PO4, Li5AlO4, Li3BO3, etc. contained in the final additive, and can generally be a lithium oxide. According to an embodiment of the present invention, the lithium source material is Li2O, LiOH, or Li2CO3. The phosphorus source material is a material that supplies phosphorus to Li3PO4 or the like contained in the final additive, and can generally be ammonium phosphate. According to an embodiment of the present invention, the phosphorus source material is (NH4)2HPO4. The aluminum source material is a material that supplies aluminum to Li5AlO4 or the like contained in the final additive, and can generally be aluminum hydroxide. According to an embodiment of the present invention, the aluminum source material is Al(OH)3. The boron source material is a material that supplies boron to Li3BO3 or the like contained in the final additive, and can generally be boric acid. According to an embodiment of the present invention, the boron source material is B(OH)3.

[0052] In step (1), the transition metal source material, the lithium source material, the phosphorus source material, the aluminum source material, and the boron source material can be introduced in appropriate amounts that match the content of the components of the above additive. According to an embodiment of the present invention, when mixing the source materials, the content of NiO and Li2O can be adjusted so that the molar number of Li2O / molar number of NiO is 1.5 to 2.5, specifically 1.7 to 2.3, more specifically 1.9 to 2.1, and the content of B(OH)3 can be adjusted so that based on the total weight of NiO and Li2O, B is contained at 500 ppm to 7,000 ppm, specifically at 750 ppm to 4,500 ppm, more specifically at 1,000 ppm to 2,000 ppm. According to an embodiment of the present invention, the content of Al(OH)3 can be adjusted to be 0.005 mol% to 0.05 mol%, specifically 0.01 mol% to 0.04 mol%, more specifically 0.01 mol% to 0.03 mol% based on the molar number of NiO, and the content of (NH4)2HPO4 can be adjusted so that based on the total weight of NiO and Li2O, P is contained at 1 wt% to 7 wt%, specifically at 1.5 wt% to 6 wt%, more specifically at 2 wt% to 5.5 wt%.

[0053] The mixture prepared in step (1) is heat-treated to prepare an additive. The heat treatment is sufficient as long as the mixture is calcined to obtain the components of the above-mentioned additive. According to one embodiment of the present invention, the mixture in step (2) is heat-treated at 300 °C to 700 °C, preferably at 300 °C to 600 °C, more preferably at 300 °C to 500 °C for 10 hours to 32 hours, preferably 10 hours to 26 hours, more preferably 10 hours to 20 hours.

[0054] Lithium secondary battery

[0055] The present invention provides a lithium secondary battery, which includes a positive electrode, a negative electrode, a separator, and an electrolyte. In the lithium secondary battery, the positive electrode and the negative electrode are placed facing each other, and the separator is disposed between the positive electrode and the negative electrode. The electrode assembly of the positive electrode, the negative electrode, and the separator is stored in a battery container, and the battery container is filled with an electrolyte.

[0056] The positive electrode includes a positive electrode current collector and a positive electrode active material layer, which is formed on the positive electrode current collector and contains the above-mentioned additive and a positive electrode active material.

[0057] In the positive electrode, there is no particular limitation on the positive electrode current collector as long as it has conductivity and does not cause chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, silver, etc. can be used. In addition, the thickness of the positive electrode current collector can generally be 3 μm to 500 μm, and fine irregularities can be formed on the surface of the positive electrode current collector to improve the adhesion strength with the positive electrode active material. The positive electrode current collector can be used in various forms, such as a film, a sheet, a foil, a net, a porous body, a foam, and a non-woven fabric.

[0058] The positive electrode active material layer may contain the above-mentioned additive, a positive electrode active material, a conductive material, and a binder.

[0059] The conductive material is used to provide conductivity to the electrode and can be used without particular limitation in the formed battery as long as it has electron conductivity and does not cause chemical changes. Specific examples of the conductive material may include: graphite, such as natural graphite or artificial graphite; carbon materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, or carbon fiber; powders or metal fibers of metals such as copper, nickel, aluminum, or silver; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; conductive polymers, such as polyphenylene derivatives, etc. Among them, a single kind or a mixture of two or more kinds can be used. The conductive material is usually contained in an amount of 1% by weight to 30% by weight based on the total weight of the positive electrode active material layer.

[0060] The binder serves to improve the adhesion between the positive electrode active material particles and the adhesion strength between the positive electrode active material and the positive electrode current collector. Specific examples of the binder may 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 monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, various copolymers of the above materials, etc. Among them, a single kind or a mixture of two or more kinds can be used. Relative to the total weight of the positive electrode active material layer, the binder may be included in an amount of 1% by weight to 30% by weight.

[0061] Relative to the total weight of the positive electrode active material layer, the additive and the positive electrode active material may be included in an amount of 60% by weight to 95% by weight.

[0062] In addition to using the above additives and positive electrode active material, the positive electrode can be prepared using a conventional positive electrode preparation method. Specifically, a composition for forming a positive electrode active material layer, which contains the above additives and positive electrode active material and optionally a binder and a conductive material, is coated on a positive electrode current collector, and then dried and roll-pressed to prepare the positive electrode. Here, the types and contents of the additive, the positive electrode active material, the binder, and the conductive material are as described above.

[0063] As the solvent, solvents commonly used in the art can be used, and specific examples thereof may include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, water, etc. Among them, a single kind or a mixture of two or more kinds can be used. Considering the coating thickness of the slurry and the manufacturing yield, as long as the additive, the positive electrode active material, the conductive material, and the binder are dissolved or dispersed, and a viscosity sufficient to achieve excellent thickness uniformity is obtained when coating is performed thereafter to prepare the positive electrode, the amount of the solvent is sufficient.

[0064] As another method, the positive electrode can also be prepared by the following method: casting a composition for forming a positive electrode active material layer on a separate support, and laminating a film obtained by peeling from the support on a positive electrode current collector.

[0065] The negative electrode includes a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector.

[0066] The negative electrode active material layer contains a negative electrode active material and optionally a binder and a conductive material.

[0067] As the negative electrode active material, a compound capable of reversibly inserting or extracting lithium can be used. Specific examples of the negative electrode active material may include: carbon materials such as artificial graphite, natural graphite, graphitized carbon fiber or amorphous carbon; (semi) metallic materials capable of forming an alloy with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy or Al alloy; (semi) metal oxides capable of doping and undoping lithium such as SiO β (0 < β < 2), SnO2, vanadium oxide or lithium vanadium oxide; or a composite material containing a (semi) metallic material and a carbon material such as Si-C composite material or Sn-C composite material, etc., and any one or a mixture of two or more thereof can be used. In addition, a thin film of metallic lithium can also be used as the negative electrode active material. In addition, as the carbon material, low-crystalline carbon, high-crystalline carbon, etc. can all be used. Representative examples of low-crystalline carbon may include soft carbon and hard carbon, and representative examples of high-crystalline carbon may include high-temperature calcined carbon such as irregular, plate-shaped, flaky, spherical or fibrous natural graphite or artificial graphite, condensed graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase carbon microbead, mesophase pitch, and coke derived from petroleum or coal tar pitch.

[0068] The binder, conductive material and negative electrode current collector can be selected with reference to the constitution in the above-mentioned positive electrode, but are not limited thereto. In addition, the method of forming the negative electrode active material layer on the negative electrode current collector may include known coating methods such as those in the positive electrode and is not particularly limited.

[0069] In the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a migration path for lithium ions, and a separator generally used in lithium secondary batteries can be used without particular limitation, and a separator having low resistance to ion migration of the electrolyte and excellent electrolyte moisture retention ability is particularly preferred. Specifically, a porous polymer film such as a porous polymer film prepared from an olefin polymer (such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylic acid copolymer) can be used; or a laminated structure of two or more layers of the above porous polymer films. In addition, ordinary porous non-woven fabrics can also be used, for example, non-woven fabrics made of glass fibers with high melting points, polyethylene terephthalate fibers, etc. In addition, a coated separator containing a ceramic component or a polymer material can also be used to ensure heat resistance or mechanical strength, and a single-layer or multi-layer structure can be selectively used.

[0070] As the electrolyte used in the present invention, it may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc. that can be used when manufacturing lithium secondary batteries, however, the electrolyte is not limited thereto.

[0071] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0072] The organic solvent can be used without particular limitation as long as it can function as a medium through which ions participating in the electrochemical reaction of the battery migrate. Specifically, as the organic solvent, the following can be used: ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone or ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic solvents such as benzene or fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC) or propylene carbonate (PC); alcohol solvents such as ethanol or isopropanol; nitriles such as R-CN (R is a hydrocarbon group with a straight-chain, branched-chain or cyclic structure from C2 to C20 and may contain double bonds, aromatic rings or ether bonds); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; sulfolane and the like. Among them, carbonate solvents are preferred, and more preferably a mixture of a cyclic carbonate having high ionic conductivity and high dielectric constant (such as ethylene carbonate, propylene carbonate, etc.) that can improve the charge and discharge performance of the battery and a low-viscosity linear carbonate compound (such as ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, etc.). In this case, when the cyclic carbonate and the linear carbonate are mixed in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte is excellent.

[0073] There is no particular limitation on the lithium salt as long as it is a compound that can provide lithium ions used in lithium secondary batteries. Specifically, as the lithium salt, LiN(FSO2)2, LiSCN, LiN(CN)2, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiPF6, LiF, LiCl, LiBr, LiI, LiNO3, LiClO4, LiAlO4, LiAlCl4, LiSbF6, LiAsF6, LiBF2C2O4, LiBC4O8, Li(CF3)2PF4, Li(CF3)3PF3, Li(CF3)4PF2, Li(CF3)5PF, Li(CF3)6P, LiCF3SO3, LiC4F9SO3, LiCF3CF2SO3, LiCF3CF2(CF3)2CO, Li(CF3SO2)2CH, LiCF3(CF2)7SO3, LiCF3CO2, LiCH3CO2 and combinations thereof can be used. The concentration of the lithium salt is preferably in the range of 0.1 M to 2.0 M. When the lithium salt concentration is included in the above range, the electrolyte has appropriate conductivity and viscosity, resulting in excellent electrolyte performance, and lithium ions can migrate effectively.

[0074] For the purpose of improving the life performance of the battery, suppressing the reduction of the battery capacity, increasing the discharge capacity of the battery, etc., in addition to the electrolyte components, the electrolyte may further contain one or more additives, such as: haloalkyl carbonate compounds (such as ethyl difluorocarbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (poly)ethylene glycol dimethyl ethers, hexamethylphosphoramide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol or aluminum trichloride. Here, the additive may be contained in an amount of 0.1% by weight to 5% by weight based on the total weight of the electrolyte.

[0075] As described above, the lithium secondary battery including the positive electrode active material according to the present invention stably exhibits excellent discharge capacity, output performance, and capacity retention rate, and thus is useful in the fields of portable devices such as mobile phones, laptop computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEV).

[0076] Therefore, another embodiment of the present invention provides a battery module including the lithium secondary battery as a unit cell and a battery pack including the same.

[0077] The battery module or the battery pack may be used as a power source for medium and large-sized devices for any one or more of the following: power tools; electric vehicle types, including electric vehicles (EV), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEV); or power storage systems.

[0078] Hereinafter, preferred embodiments will be provided to clarify the present invention. However, the following embodiments are provided for easier understanding of the present invention, and the present invention is not limited thereto.

[0079] Preferred Embodiments

[0080] Example

[0081] Example 1

[0082] Mix NiO, Li₂O, B(OH)₃, Al(OH)₃, and (NH₄)₂HPO₄, and heat-treat the prepared mixture at 300 °C for 10 hours to prepare an additive. When mixing, adjust the content of NiO and the content of Li₂O such that the mole number of Li₂O / the mole number of NiO is 2.03, and adjust the content of B(OH)₃ such that B is contained at 1,500 ppm based on the total weight of NiO and Li₂O. Additionally, adjust the content of Al(OH)₃ to be 0.03 mol% based on the mole number of NiO, and adjust the content of (NH₄)₂HPO₄ such that P is contained at 5.5 wt% based on the total weight of NiO and Li₂O.

[0083] Comparative Example 1

[0084] Different from Example 1, do not use B(OH)₃, and mix NiO, Li₂O, Al(OH)₃, and (NH₄)₂HPO₄ and heat-treat the prepared mixture at 700 °C for 10 hours to prepare an additive. When mixing, adjust the content of each of NiO, Li₂O, Al(OH)₃, and (NH₄)₂HPO₄ to be the same as in Example 1.

[0085] Comparative Example 2

[0086] As an additive, prepare a product containing Li₂NiO₂ and Li₅AlO₄ (manufacturer: POSCO Chemical Co., Ltd., product name: DN20).

[0087] Comparative Example 3

[0088] As an additive, prepare a product containing Li₂NiO₂ and Li₃PO₄ (manufacturer: POSCO Chemical Co., Ltd., product name: DN40).

[0089] Experimental example

[0090] Experimental Example 1: Component Analysis of Additives

[0091] Use an X-ray diffractometer device (manufacturer: BRUKER NANO Co., Ltd., product name: Bruker D8 Advance) to analyze the components of the additives of Example 1 and Comparative Examples 1 to 3 by X-ray diffraction analysis, and the results are shown in Table 1 below.

[0092] Table 1

[0093]

[0094] Manufacture of Lithium Secondary Battery

[0095] A lithium secondary battery was fabricated to confirm the performance of the additives prepared according to Example 1 and Comparative Examples 1 to 3. Specifically, each of the additives of Example 1 and Comparative Examples 1 to 3 was mixed with the positive electrode active material LiNi 0.83 Co 0.11 Mn 0.06 O2 at a weight ratio of 9:1 (positive electrode active material: additive), and mixed with a carbon black conductive material and a PVDF binder at a weight ratio of 85:10:5 (additive + positive electrode active material: conductive material: binder) in an N-methylpyrrolidone solvent to prepare a positive electrode active material slurry. It was coated on one surface of an aluminum current collector (20 μm) (loading amount: 0.2 mg to 0.3 mg / 25 cm 2 ), dried at 130 °C for more than 20 minutes, and then roll-pressed once or twice to obtain a porosity of 26% to prepare a positive electrode.

[0096] As the negative electrode, an electrode in which natural graphite and artificial graphite were mixed at 5:5 was used, and a porous polyethylene separator was disposed between the positive electrode and the negative electrode to prepare an electrode assembly. After placing the electrode assembly in a battery case, an electrolyte was injected into the case to fabricate a lithium secondary battery. Here, an electrolytic solution was prepared by dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 0.7 M and lithium bis(fluorosulfonyl)imide (LiFSI) at a concentration of 0.3 M in an organic solvent formed by ethylene carbonate / ethyl methyl carbonate (mixing volume ratio of EC / EMC = 3 / 7).

[0097] The fabricated lithium secondary batteries were used for the performance evaluation of Experimental Examples 2 and 3.

[0098] Experimental Example 2: Analysis of the gas generation amount according to the cycle of a lithium secondary battery

[0099] Each of the fabricated lithium secondary batteries was stored at 45 °C for 10 hours, then charged and discharged at 0.1 C (C rate) in the range of 2.5 V to 4.2 V for formation, and the initial formed gas generation amount was measured in the SOC 0 state. Thereafter, using Archimedes' principle, the change in the respective gas generation amounts at the 10th, 30th, and 50th cycles was measured volumetrically at 0.33 C (C rate). The measurement results are shown in Table 2 below.

[0100] Table 2

[0101]

[0102] According to Table 2, it was confirmed that by doping a part of the transition metal with aluminum in the lithium transition metal oxide and by including Li3PO4, Li5AlO4, and Li3BO3 in the additive according to the present invention, as the cycling progresses, the increase in gas generation gradually decreases. This reduction in gas generation can contribute to improving the long-term life of the battery that undergoes 50 or more cycles.

[0103] Experimental Example 3: Analysis of Gas Storage Amount of Lithium Secondary Battery

[0104] Each of the fabricated lithium secondary batteries was stored at 45°C for 10 hours, and then charged and discharged at 0.1C (C-rate) in the range of 2.5V to 4.2V to set the SOC to 100. While the battery was stored in a high-temperature chamber at 60°C for 4 weeks, the change in gas generation amount was measured weekly using Archimedes' principle. The measurement results are shown in Table 3 below.

[0105] Table 3

[0106]

[0107] According to Table 3, it was confirmed that by doping a part of the transition metal with aluminum in the lithium transition metal oxide and by including Li3PO4, Li5AlO4, and Li3BO3 in the additive according to the present invention, even after a period of time, the gas generation amount remains low. With respect to the passage of time and the progress of cycling, the gas generation amount of the additive according to the present invention is small, and thus it can contribute to improving the long-term life of the battery.

[0108] Experimental Example 4: Analysis of Initial Charge and Discharge Capacities of Lithium Secondary Batteries According to Additive Content

[0109] For the lithium secondary battery using the additive of Example 1, when fabricating the lithium secondary battery, the mixing weight ratio of the positive electrode active material and the additive was adjusted to each of 100:0, 90:10, 80:20, and 0:100. Each of the fabricated lithium secondary batteries was stored at 45°C for 10 hours, and then charged and discharged at 0.1C (C-rate) in the range of 2.5V to 4.2V, and the initial charge capacity and discharge capacity were measured. The results are shown in Table 4 below.

[0110] Table 4

[0111]

[0112] According to Table 4, the additive prepared in Example 1 using Li2NiO2 (in a state with a small amount of lithium doping) contains an excessive amount of irreversible lithium, and it was confirmed that the initial charge capacity using the irreversible lithium increases with the increase in the additive content. In particular, when the additive content is 90:10 or 80:20, the initial charge capacity can be significantly increased without significantly reducing the initial discharge capacity. Therefore, the additive can be used by effectively combining with the irreversible negative electrode active material.

[0113] Simple variations and modifications of the present invention all fall within the scope of the present invention, and the specific scope of protection of the present invention will become apparent from the appended claims.

Claims

1. An additive for a positive electrode of a lithium secondary battery, the additive comprising: A lithium transition metal oxide; Li3PO4; Li5AlO4; and Li3BO3, Among them, The lithium transition metal oxide has a form doped with aluminum, Wherein, based on the total weight of the additive, the content of the lithium transition metal oxide is 75 wt% to 90 wt%, Wherein, based on the total weight of the additive, the content of Li3PO4 is 1 wt% to 10 wt%, Wherein, based on the total weight of the additive, the content of Li5AlO4 is 0.5 wt% to 5 wt%, Wherein, based on the total weight of the additive, the content of Li3BO3 is 0.1 wt% to 3 wt%.

2. The additive for a positive electrode of a lithium secondary battery according to claim 1, wherein The lithium transition metal oxide is represented by the following Chemical Formula 1: [Chemical Formula 1] Li2Ni 1-x Al x O2 Wherein, x is 0.001 to 0.

005.

3. The additive for a positive electrode of a lithium secondary battery according to claim 1, wherein, A part of the Li3BO3 exists in a state of being coated on the lithium transition metal oxide.

4. The additive for the positive electrode of a lithium secondary battery according to claim 1, wherein, The additive further comprises NiO.

5. The additive for a positive electrode of a lithium secondary battery according to claim 4, wherein, Based on the total weight of the additive, the content of NiO is 5 wt% to 15 wt%.

6. A method for preparing the additive for a positive electrode of a lithium secondary battery according to claim 1, the method comprising the following steps: (1) Preparing a mixture by mixing a transition metal source material, a lithium source material, a phosphorus source material, an aluminum source material, and a boron source material; and (2) Heat-treating the mixture.

7. The method for preparing an additive for a positive electrode of a lithium secondary battery according to claim 6, wherein, In the step (1), The transition metal source material is NiO, The lithium source material is Li2O, LiOH or Li2CO3, The phosphorus source material is (NH4)2HPO4, The aluminum source material is Al(OH)3, and The boron source material is B(OH)3.

8. The method for preparing an additive for a positive electrode of a lithium secondary battery according to claim 6, wherein, In the step (2), the mixture is heat-treated at 300 °C to 700 °C for 10 hours to 32 hours.

9. A positive electrode of a lithium secondary battery, the positive electrode comprising: The additive according to claim 1; and A positive electrode active material, Among them, Based on 100 parts by weight of the positive electrode active material, the content of the additive is 10 parts by weight to 40 parts by weight.

Images