Lithium-ion battery

Abstract

The present invention relates to a lithium secondary battery with improved output characteristics and low-temperature characteristics. The lithium secondary battery comprises a positive electrode, a negative electrode, and an electrolyte comprising a lithium salt and a non-aqueous organic solvent, wherein the positive electrode is a positive electrode current collector, and an active material layer formed on the positive electrode current collector comprises an oxide-based solid electrolyte having a positive electrode active material, a conductive material, and a lithium ion supply source, wherein the positive electrode active material and the oxide-based solid electrolyte are dispersed in the active material layer in particulate form, and the average particle size D50 of the particles satisfies a certain ratio.

Description

Technical Field

[0001] [Cross - reference to Related Applications] This application claims the benefit of priority based on Korean Patent Application No. 10 - 2023 - 0058563 filed on May 4, 2023 and Korean Patent Application No. 10 - 2023 - 0131540 filed on October 4, 2023, and all the contents disclosed in the documents of the Korean patent applications are included as part of this specification.

[0002] The present invention relates to a lithium secondary battery with improved output characteristics, low - temperature characteristics, etc.

Background Art

[0003] Recently, as the application areas of lithium secondary batteries have rapidly expanded not only to power supply for electronic devices such as electric, electronic, communication, and computers but also to power storage supply for large - area devices such as automobiles and power storage devices, the demand for lithium secondary batteries with high capacity, high output, long life, and high stability has been increasing.

[0004] A lithium secondary battery generally includes a positive electrode, a negative electrode, a separator, a lithium salt, and an electrolyte containing an organic solvent, and lithium - ion movement occurs through the electrolyte during the charge - discharge process.

[0005] In the electrolyte, lithium ions exist in a solvated state surrounded by an organic solvent. Therefore, in order for such lithium ions in the electrolyte to move and be inserted into the electrode active material, a desolvation process in which the lithium ions are separated or detached from the organic solvent molecules occurs. Therefore, the smooth progress of such a desolvation process may affect the output, resistance, or stability of the lithium secondary battery.

[0006] On the other hand, with the recent demand for higher capacity and higher output lithium secondary batteries, the development of new electrolytes is being actively pursued. In particular, in order to accelerate the desolvation of lithium ions and increase the output of lithium secondary batteries, new electrolyte compositions have been proposed, such as those containing high concentrations of lithium salts or those that change the composition of the organic solvent to form localized high-concentration regions of lithium salts around the electrodes.

[0007] However, when applying the aforementioned new electrolyte composition containing high-concentration lithium salts, there is a possibility of problems arising from the decrease in fluidity due to the increase in the viscosity of the electrolyte. In particular, at low temperatures, the ionic conductivity of the electrolyte decreases, leading to increased resistance or a decrease in battery output, which is a disadvantage as it degrades the low-temperature characteristics of lithium secondary batteries.

[0008] Therefore, there is a continuing need for the development of technologies that can accelerate the desolvation of lithium ions within the electrolyte to improve the output characteristics of lithium secondary batteries while maintaining excellent low-temperature characteristics. [Overview of the project] [Problems that the invention aims to solve]

[0009] Therefore, the present invention aims to provide a lithium secondary battery that exhibits improved output characteristics by accelerating the desolvation of lithium ions in the electrolyte, while maintaining excellent low-temperature characteristics such as high output at low temperatures. [Means for solving the problem]

[0010] The present invention comprises a positive electrode, a negative electrode, and an electrolyte comprising a lithium salt and a non-aqueous organic solvent. The positive electrode includes a positive electrode current collector, and an active material layer formed on the positive electrode current collector, which includes a positive electrode active material, a conductive material, and an oxide-based solid electrolyte having a lithium ion supply source. The positive electrode active material and the oxide-based solid electrolyte are dispersed in the active material layer in particle form, and the average particle size D50 of the particles is such that the oxide-based solid electrolyte and the positive electrode active material have a ratio of 1:4 to 1:90. [Effects of the Invention]

[0011] In the lithium secondary battery of the present invention, the positive electrode contains oxide-based solid electrolyte particles dispersed within the active material layer, and such oxide-based solid electrolyte particles can have an average particle size D50 in a constant ratio with respect to the positive electrode active material particles. Such oxide-based solid electrolyte particles can be uniformly distributed around the positive electrode active material particles within the active material layer, and can be distributed in large numbers near the interface between the active material layer and the electrolyte.

[0012] Such oxide-based solid electrolyte particles can reduce the activation energy associated with the desolvation process of lithium ions in the electrolyte at the interface where the active material layer and the electrolyte come into contact, thereby accelerating the desolvation of lithium ions and improving the output characteristics of the lithium secondary battery.

[0013] Furthermore, since the output characteristics of lithium secondary batteries can be improved without applying new electrolyte compositions such as those containing high concentrations of lithium salts, problems such as increased electrolyte viscosity, decreased fluidity, and reduced ionic conductivity and battery output at low temperatures caused by the application of the aforementioned new electrolyte compositions can be solved, thereby achieving excellent low-temperature characteristics for lithium secondary batteries.

[0014] In addition, when an electrolyte composition containing the aforementioned high-concentration lithium salt is also applied, the application of the positive electrode facilitates the desolvation of lithium ions, improving lithium ion conductivity and reducing resistance. This further improves the output characteristics of lithium secondary batteries with high-concentration lithium salts and reduces the deterioration of low-temperature characteristics.

[0015] As a result, the present invention can significantly contribute to the development of next-generation lithium-ion batteries exhibiting improved output characteristics and low-temperature characteristics. [Brief explanation of the drawing]

[0016] [Figure 1] Figure 1 is a graph comparing the degree of voltage drop for lithium secondary batteries of Comparative Example 1 and Example 1, while discharging them from a state of charge (SOC) of 50% for 30 seconds at -10°C, for different discharge times. [Modes for carrying out the invention]

[0017] The terms and words used in this specification and in the claims should not be interpreted in a manner limited to their ordinary or dictionary meanings, but rather in a manner and concept consistent with the technical idea of ​​the present invention, based on the principle that inventors can appropriately define the concepts of terms in order to best describe their invention.

[0018] In this specification, terms such as “includes,” “equip,” or “have” are intended to specify the presence of implemented features, figures, steps, components, or combinations thereof, and should be understood not to preemptively exclude the existence or possibility of adding one or more other features, figures, steps, components, or combinations thereof.

[0019] According to one embodiment of the present invention, the present invention comprises a positive electrode, a negative electrode, and an electrolyte containing a lithium salt and a non-aqueous organic solvent. The positive electrode includes a positive electrode current collector, and an active material layer formed on the positive electrode current collector, which includes a positive electrode active material, a conductive material, and an oxide-based solid electrolyte having a lithium ion supply source. The positive electrode active material and the oxide-based solid electrolyte are dispersed in the active material layer in particle form, and the average particle size D50 of the particles is such that the oxide-based solid electrolyte and the positive electrode active material have a ratio of 1:4 to 1:90.

[0020] In such lithium secondary batteries, oxide-based solid electrolyte particles are dispersed together with positive electrode active material particles within the positive electrode active material layer, and these oxide-based solid electrolyte particles can satisfy a ratio of average particle size D50 to positive electrode active material particles of 1:4 to 1:90, or 1:5 to 1:80, or 1:10 to 1:60, or 1:15 to 1:40, or 1:16 to 1:35, or 1:20 to 1:25.

[0021] The inventors' experimental results show that when oxide-based solid electrolyte particles satisfying the D50 ratio are included in the active material layer of the positive electrode, such oxide-based solid electrolyte particles can be uniformly distributed around the positive electrode active material particles within the active material layer, and in particular, a large number can be distributed near the interface between the active material layer and the electrolyte. As a result, the oxide-based solid electrolyte particles can reduce the activation energy associated with the desolvation process of lithium ions in the electrolyte at the interface where the active material layer and the electrolyte come into contact, thereby accelerating desolvation. Furthermore, this can improve the lithium ion migration speed and lithium ion conductivity around the positive electrode active material particles.

[0022] Therefore, a lithium secondary battery according to one embodiment including such a positive electrode can accelerate the desolvation of lithium ions, improve the migration speed and conductivity of lithium ions in the active material layer, reduce the resistance of the lithium secondary battery, and improve its output characteristics, even without applying an electrolyte composition that includes a high concentration of lithium salt. As a result, it is possible to solve problems such as increased electrolyte viscosity, decreased fluidity, and decreased ionic conductivity of the electrolyte and battery output at low temperatures caused by the application of the high concentration of lithium salt, and to achieve excellent low-temperature characteristics for the lithium secondary battery.

[0023] The following describes a lithium secondary battery according to one embodiment in more detail.

[0024] In the lithium secondary battery according to the above-described embodiment, the positive electrode includes a positive electrode current collector. Such a positive electrode current collector is not particularly limited as long as it has conductivity without inducing a chemical change in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or a material obtained by surface treatment of aluminum or stainless steel with carbon, nickel, titanium, silver, etc. can be used.

[0025] In addition, the active material layer on the positive electrode current collector can include a positive electrode active material, a conductive material, and an oxide-based solid electrolyte having a lithium ion supply source, and can selectively further include a binder.

[0026] At this time, the positive electrode active material is a compound capable of reversible intercalation and deintercalation of lithium, and specifically, can include a lithium metal oxide containing one or more metals such as iron, cobalt, manganese, nickel, or aluminum and lithium.

[0027] Specifically, the lithium metal oxide is a lithium-manganese-based oxide (e.g., LiMnO2, LiMn2O4, etc.), a lithium-cobalt-based oxide (e.g., LiCoO2, etc.), a lithium-nickel-based oxide (e.g., LiNiO2, etc.), a lithium-nickel-manganese-based oxide (e.g., LiNi 1-Y Mn Y O2 (where 0 < Y < 1), LiMn 2-Z Ni Z O4 (where 0 < Z < 2, etc.), a lithium-nickel-cobalt-based oxide (e.g., LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1), etc.), a lithium-manganese-cobalt-based oxide (e.g., LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2-Z1 Co Z1 O4 (where 0 < Z1 < 2), etc.), a lithium-nickel-manganese-cobalt-based oxide (e.g., Li(Ni p Co q Mn r)O2 (where 0 < p < 1, 0 < q < 1, 0 < r < 1, p + q + r = 1) or Li(Ni p1 Co q1 Mn r1 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r1 < 2, p1 + q1 + r1 = 2), etc., lithium-nickel-cobalt-transition metal (M) oxide (e.g., Li(Ni p2 Co q2 Mn r2 M s2 )O2 (where M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p2, q2, r2, and s2 are atomic fractions of independent elements, respectively, 0 < p2 < 1, 0 < q2 < 1, 0 < r2 < 1, 0 < s2 < 1, and p2 + q2 + r2 + s2 = 1), etc., or lithium iron phosphate (e.g., Li 1+a Fe 1-x M x (PO 4-b )X b (where M is one or more selected from Al, Mg, and Ti, X is one or more selected from F, S, and N, -0.5 ≤ a ≤ 0.5, 0 ≤ x ≤ 0.5, 0 ≤ b ≤ 0.1), etc., and one or more of these compounds may be included.

[0028] Among these, the positive electrode active material includes lithium; and a lithium metal oxide containing two or more metals selected from the group consisting of nickel, manganese, cobalt, and aluminum, and the lithium metal oxide may contain 60 mol% or more, or 60 to 99 mol%, or 70 to 95 mol% of nickel based on the total metal content excluding lithium. Such a lithium metal oxide may be represented by, for example, the following Chemical Formula 1.

[0029] [Chemical Formula 1] Li x Ni a Co b M 1 c M 2 d O2 In Chemical Formula 1, the M1 M may be one or more selected from Mn and Al, or a combination thereof. 2 a may be one or more elements selected from the group consisting of Zr, B, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P, and S, and may be 0.90≦x≦1.1, or 0.95≦x≦1.08, or 1.0≦x≦1.08, or 0.60≦a<1.0, or 0.70≦a≦0.99, or 0.80≦a≦0.95.

[0030] By using a lithium metal oxide containing such a high nickel content as the positive electrode active material and combining it with an oxide-based solid electrolyte, the output and capacity characteristics of lithium secondary batteries can be further improved.

[0031] The aforementioned positive electrode active material may be included in an amount of 60 to 99% by weight, 70 to 99% by weight, or 80 to 98% by weight, based on the total weight of the active material layer.

[0032] Furthermore, as the oxide-based solid electrolyte, any solid electrolyte can be used that contains lithium in its structure and has a lithium ion supply source, and that is in the form of a lithium metal oxide or lithium metal phosphorus oxide.

[0033] Specific examples include one or more lithium metal oxides or lithium metal phosphoroxides selected from Nasicon-type solid electrolytes, Lisicon-type solid electrolytes, Garnet-type solid electrolytes, Perovskite-type solid electrolytes, and LiPON-type solid electrolytes. More specific examples include one or more compounds selected from the group consisting of LAGP (lithium aluminum germanium phosphate) compounds, LLZO (lithium lanthanum zirconium oxide) compounds, LATP (lithium aluminum titanium phosphate) compounds, LLZTO (lithium lanthanum zirconium tantalum oxide) compounds, LLTO (lithium lanthanum titanium oxide) compounds, LSTP (lithium silicon titanium phosphate) compounds, and LGPO (lithium germanium phosphate) compounds.

[0034] Among these, a NASICON-type solid electrolyte such as the LAGP-type compound or LATP-type compound can be appropriately used from the viewpoint of accelerating the desolvation of lithium ions at the interface between the active material layer and the electrolyte, thereby improving the ionic conductivity and output of the lithium secondary battery.

[0035] In the aforementioned positive electrode, the positive electrode active material and the oxide-based solid electrolyte can be dispersed in particle form within the active material layer, and by each having a predetermined average particle size D50, the aforementioned D50 ratio can be satisfied. In this case, the average particle size D50 of each particle can be calculated, for example, by measuring the particle size distribution of the positive electrode active material or the oxide-based solid electrolyte using laser diffraction and a laser diffraction particle size analyzer, and determining the particle size corresponding to 50% of the volume accumulation on the particle size distribution curve.

[0036] Specifically, the positive electrode active material may have an average particle size D50 of 5 to 15 μm, 7 to 13 μm, or 8 to 12 μm, and the oxide-based solid electrolyte may have an average particle size D50 of 100 nm to 1.2 μm, 120 nm to 1 μm, 150 to 800 nm, or 200 to 600 nm. By satisfying the aforementioned average particle size D50 while also satisfying the aforementioned D50 ratio, the lithium secondary battery of one embodiment can exhibit superior capacity and output characteristics, as well as lower resistance and low-temperature characteristics.

[0037] Furthermore, the oxide-based solid electrolyte may be included in an amount of 0.1 to 5 parts by weight, 0.5 to 4 parts by weight, or 1 to 3.5 parts by weight based on 100 parts by weight of the positive electrode active material, from the viewpoint of improving lithium ion mobility and conductivity by accelerating the desolvation of lithium ions at the interface between the active material layer and the electrolyte without hindering the insertion and desolvation processes of lithium ions by the positive electrode active material.

[0038] On the other hand, the conductive material contained in the active material layer is a component for further improving the conductivity of the positive electrode active material, and such a conductive material is not particularly limited as long as it is conductive without inducing a chemical change in the battery. For example, carbon powder such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powder such as natural graphite, artificial graphite, or graphite with a highly developed crystalline structure; conductive nanomaterials such as carbon nanofibers or carbon nanotubes; fluorinated carbon powder; conductive powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives can be used. Among these, the conductive material may include conductive nanomaterials such as carbon nanotubes or carbon nanofibers, which can further reduce the resistance of the lithium secondary battery in one embodiment and further improve the output characteristics.

[0039] Typically, the conductive material may be included in an amount of 1 to 20% by weight, 1 to 15% by weight, or 1 to 10% by weight, based on the total weight of the active material layer.

[0040] The binder selectively included in the active material layer is a component that assists in the bonding of the positive electrode active material to conductive materials and to the current collector. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene (PE), polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, nitrile rubber, styrene-butadiene rubber, or fluororubber, and a mixture or copolymer of two or more selected types from these can also be used.

[0041] Typically, the binder may be present in an amount of 1 to 20% by weight, 1 to 15% by weight, or 1 to 10% by weight, based on the total weight of the positive electrode active material layer.

[0042] Furthermore, a filler may be selectively added to the positive electrode as a component to suppress its expansion. Such a filler is not particularly limited as long as it can suppress the expansion of the electrode without inducing a chemical change in the battery, and for example, olefin polymers such as polyethylene and polypropylene; fibrous materials such as glass fibers and carbon fibers; etc. can be used.

[0043] The aforementioned positive electrode can be manufactured, for example, by dispersing and mixing the positive electrode active material, oxide-based solid electrolyte, binder, and conductive material in a dispersion medium (solvent) to create a slurry, applying this slurry to a positive electrode current collector, and then drying and rolling it. In this case, the dispersion medium can be, but is not limited to, NMP (N-methyl-2-pyrrolidone), DMF (Dimethyl formamide), DMSO (Dimethyl sulfoxide), ethanol, isopropanol, water, and mixtures thereof.

[0044] On the other hand, in one embodiment of a lithium secondary battery, the negative electrode may have a conventional configuration known in the art. For example, the negative electrode may include a negative electrode current collector and a negative electrode active material layer formed on such a negative electrode current collector, and such a negative electrode active material layer may include, for example, a negative electrode active material and, optionally, additives such as conductive materials, binders, and fillers. Furthermore, such a negative electrode may be formed by a common method of dispersing and mixing each component of the negative electrode active material layer in a dispersion medium (solvent) to make a slurry, applying this slurry onto the negative electrode current collector, and then drying and rolling it.

[0045] In this case, the negative electrode current collector may be made of platinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag), ruthenium (Ru), nickel (Ni), stainless steel (STS), copper (Cu), molybdenum (Mo), chromium (Cr), carbon (C), titanium (Ti), tungsten (W), ITO (In doped SnO2), FTO (F doped SnO2), and alloys thereof, as well as copper (Cu) or stainless steel with a surface treatment of carbon (C), nickel (Ni), titanium (Ti), or silver (Ag), but is not necessarily limited to these. The negative electrode current collector may take the form of foil, film, sheet, punched material, porous material, foam, etc.

[0046] Furthermore, as the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium can be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Sb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; metallic oxides that can be doped and dedoped with lithium, such as SiOβ (0<β<2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing the metallic compound and carbonaceous material, such as Si-C composites or Sn-C composites. One or more of these can be used. In addition, a metallic lithium thin film may be used as the negative electrode active material.

[0047] In addition, both low-crystalline carbon and high-crystalline carbon can be used as the carbonaceous material. Typical low-crystalline carbons include soft carbon and hard carbon, while typical high-crystalline carbons include amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived cokes.

[0048] The aforementioned negative electrode active material may be included in an amount of 60 to 99% by weight, 70 to 99% by weight, or 80 to 98% by weight, based on the total weight of the negative electrode active material layer.

[0049] In other embodiments, the negative electrode may include only a negative electrode current collector without a negative electrode active material layer. In such a negative electrode, lithium ions that have moved from the positive electrode during the charge-discharge process can be electrodeposited onto the negative electrode current collector to form a lithium metal layer, and this lithium metal layer can act as the negative electrode active material.

[0050] On the other hand, the binder and conductive material included in the negative electrode active material layer can be the same as those described for the positive electrode, so no further explanation is needed. Furthermore, similar to the positive electrode as described above, the negative electrode active material layer may further include an oxide-based solid electrolyte having a lithium ion supply source as described above, in which case the oxide-based solid electrolyte may satisfy the D50 ratio described above with respect to the negative electrode active material particles.

[0051] The lithium secondary battery described above further includes an electrolyte containing a lithium salt and a non-aqueous organic solvent. Such an electrolyte acts as a transfer medium for lithium ions between the positive and negative electrodes. These lithium ions exist in the electrolyte in a solvated state, while they can be inserted into the electrode active material through desolvation at the interface between the electrolyte and the electrodes. As mentioned above, in one embodiment of the secondary battery, such desolvation is accelerated, resulting in improved output characteristics and other features.

[0052] The lithium salt contained in the electrolyte is used as a medium for transferring ions within a lithium secondary battery. The lithium salt is, for example, Li as a cation. + Includes F - Cl - , Br - , I - NO3 - , N(CN)2 - BF4 - ClO4 - B 10 Cl 10 - AlCl4 - AlO2 - PF6 - CF3SO3- CH3CO2 - CF3CO2 - AsF6 - SbF6 - CH3SO3 - , (CF3CF2SO2)2N - (CF3SO2)2N - , (FSO2)2N - BF2C2O4 - BC4O8 - PF4C2O4 - PF2C4O8 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - , C4F9SO3 - CF3CF2SO3 - CF3CF2(CF3)2CO - (CF3SO2) 2CH - CF3(CF2)7SO3 - and SCN - It can also contain anions selected from the group consisting of the following.

[0053] Specifically, the lithium salts are LiCl, LiBr, LiI, LiBF4, LiClO4, and LiB 10 Cl 10 It may contain one or more selected from the group consisting of LiAlCl4, LiAlO2, LiPF6, LiCF3SO3, LiCH3CO2, LiCF3CO2, LiAsF6, LiSbF6, LiCH3SO3, LiFSI (Lithium bis(fluorosulfonyl)imide, LiN(SO2F)2), LiBETI (lithium bis(perfluoroethanesulfonyl)imide, LiN(SO2CF2CF3)2), and LiTFSI (lithium bis(trifluoromethanesulfonyl)imide, LiN(SO2CF3)2).

[0054] The concentration of the lithium salt can be appropriately changed within a range that is normally usable, and may be included in the electrolyte at a concentration of 0.5 M to 6 M, or 1 M to 5 M.

[0055] In a more specific embodiment, the electrolyte may contain lithium salt at a relatively low concentration of 0.5 M to less than 2 M, or 0.7 M to 1.5 M. Even in such cases, as the desolvation of lithium ions is accelerated by the action of the positive electrode described above, the lithium secondary battery of this embodiment can exhibit improved output characteristics. As a result, problems such as increased electrolyte viscosity, decreased fluidity, and decreased ionic conductivity of the electrolyte and battery output at low temperatures, which occur when using electrolyte compositions containing high concentrations of lithium salt, can be solved, and excellent low-temperature characteristics of the lithium secondary battery can be achieved.

[0056] On the other hand, in other embodiments of the present invention, the electrolyte may contain a lithium salt at a high concentration of 2 M to 6 M, or 2.5 M to 5.5 M. In such cases, the inclusion of a high concentration of lithium salt and the application of the positive electrode described above can further accelerate the desolvation of lithium ions, thereby further improving the output characteristics of the secondary battery.

[0057] Furthermore, by applying the aforementioned positive electrode, lithium-ion conductivity can be improved even at low temperatures, and resistance can be reduced. This reduces the deterioration of low-temperature power characteristics caused by high-concentration lithium salts, etc.

[0058] On the other hand, the type of non-aqueous organic solvent that may be included in the electrolyte is not particularly limited, and any organic solvent that has been known to be applicable to lithium-ion battery electrolytes can be used. Examples of such organic solvents include one or more selected from the group consisting of carbonate-based solvents, ether-based solvents, nitrile-based solvents, phosphate-based solvents, and sulfone-based solvents.

[0059] More specifically, the carbonate-based solvent can be dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, methyl propyl carbonate, ethyl methyl carbonate, ethyl propyl carbonate, or methyl (2,2,2-trifluoroethyl) carbonate, and the phosphate-based solvent can be trimethyl phosphate, triethyl phosphate, or 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphosphorane 2-oxide.

[0060] Furthermore, as the ether-based solvent, dibutyl ether, tetraglyme, diglyme, dimethoxyethane, or tetrahydrofuran derivatives such as 2-methyltetrahydrofuran can be used, and as the nitrile-based solvent, succinonitrile, adiponitrile, sebaconitrile, acetonitrile, or propionitrile can be used.

[0061] Furthermore, as the sulfone-based solvent, dimethyl sulfone, ethyl methyl sulfone, or sulfolane can be used.

[0062] On the other hand, the electrolyte may further contain, in addition to the lithium salt and non-aqueous organic solvent described above, a diluent that exhibits solubility in the lithium salt that is 10 times or more less than that of the non-aqueous organic solvent. Such a diluent may be an organic solvent that is substantially insoluble in the lithium salt and miscible with the non-aqueous organic solvent, for example, an ether solvent having a fluorine-substituted alkyl group, and more specifically, one or more selected from the group consisting of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), bis(2,2,2-trifluoroethyl) ether (BTFE), 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE), methoxynononafluorobutane (MOFB), and ethoxynononafluorobutane (EOFB).

[0063] When such a diluent is further included, locally high concentrations of lithium salt may be present in the non-aqueous organic solvent within the electrolyte, while lithium salt may be substantially absent in the diluent. In this way, the presence of locally high concentrations of lithium salt in a solvated form within the electrolyte can further improve the output characteristics of the lithium secondary battery and reduce the increase in viscosity and decrease in fluidity of the electrolyte. In addition, when such a locally high-concentration salt electrolyte is applied together with the aforementioned positive electrode, lithium ion conductivity may be improved and resistance reduced even at low temperatures, thereby reducing the decrease in low-temperature output characteristics caused by high-concentration lithium salts.

[0064] The amount of the diluent used can be adjusted depending on the type of non-aqueous organic solvent and lithium salt, and the overall concentration of the lithium salt. For example, the diluent may be included in the electrolyte in a quantity such that the molar ratio of the diluent to the non-aqueous organic solvent is 1:0.2 to 1:5, or 1:0.5 to 1:2.

[0065] On the other hand, the lithium secondary battery of the above-described embodiment may further include a porous separation membrane interposed between the positive electrode and the negative electrode.

[0066] Such porous separation membranes can be made from olefin polymers such as polyethylene and polypropylene, glass fibers, etc., in the form of sheets, multilayer membranes, fine porous films, woven fabrics, and nonwoven fabrics, but are not necessarily limited to these forms. However, it may be preferable to use porous polyethylene or porous glass fiber nonwoven fabric (glass filter) as the separation membrane, and it may be more preferable to use porous glass fiber nonwoven fabric (glass filter) as the separation membrane. The separation membrane may be a thin insulating film with high ion permeability and mechanical strength, and the pore diameter of the separation membrane may generally be in the range of 0.01 to 10 μm, and the thickness may generally be in the range of 5 to 300 μm, but is not limited thereto.

[0067] Furthermore, in another example of the lithium secondary battery, the separation membrane may not be interposed, and the aforementioned electrolyte may be interposed between the positive and negative electrodes in the form of an electrolyte membrane or electrolyte film. In this case, the electrolyte membrane or electrolyte film may be in a form in which the aforementioned lithium salt and non-aqueous organic solvent are contained within a polymer matrix, and as the polymer matrix, a well-known polymer-based solid electrolyte or a crosslinked polymer obtained by crosslinking polymerization of polyfunctional acrylate monomers can be used. In such a case, the lithium secondary battery of the above embodiment may be a semi-solid battery that uses both a liquid electrolyte and a solid electrolyte.

[0068] On the other hand, the lithium secondary battery of the above embodiment may be manufactured by conventional methods in the art. For example, it may be manufactured by housing an electrode assembly including a positive electrode, a negative electrode, and a separator membrane (or electrolyte membrane) in a case and injecting and impregnating it with the aforementioned electrolyte.

[0069] Such lithium secondary batteries can be applied not only to battery cells used as power sources for small devices, but are also particularly suitable for use as unit batteries in battery modules that power medium- and large-sized devices.

[0070] The invention will be described in more detail below through specific examples. However, the following examples are merely illustrative to aid in understanding the present invention and do not limit the scope of the invention.

[0071] Example 1: Manufacturing of a lithium secondary battery The positive electrode active material is lithium nickel cobalt manganese composite oxide (NCM811; D50: 10 μm), which contains 80 mol% nickel as the total transition metal. The conductive material is carbon nanotubes, the binder is polyvinylidene fluoride (PVdF), and the oxide-based solid electrolyte is LiAl with a D50 of 300 nm. 0.3 Ti 1.7 (PO4)3 (LATP-based) compound particles were used (ratio of oxide-based solid electrolyte to D50 of the electrode active material: 1:33.3).

[0072] The positive electrode active material, conductive material, binder, and oxide-based solid electrolyte were mixed in a weight ratio of 94:1.5:2:2.5 and dispersed in NMP solvent at 2500 to 3000 rpm to produce a slurry. This slurry was then coated onto 25 μm thick aluminum foil (Al foil) to a uniform thickness using a blade-type coating machine, a Mathis coater (Labdryer / coater type LTE, manufactured by Werner Mathis AG). The mixture was dried in a vacuum oven at 110 to 130°C for 12 to 24 hours and then rolled in a roll press machine to produce a positive electrode for a lithium secondary battery.

[0073] Using graphite as the negative electrode active material and without using an oxide-based solid electrolyte, the negative electrode was manufactured by mixing the negative electrode active material, conductive material, and binder in a weight ratio of 96.5:1.5:2, and then performing the same process as for the positive electrode.

[0074] After punching out the positive and negative electrodes, an electrode assembly was manufactured by laminating them with a separation membrane made of porous polyethylene (PE) film interposed between them.

[0075] After positioning the manufactured electrode assembly inside the battery case, an electrolyte solution was injected into the case to produce a lithium secondary battery. The electrolyte solution was prepared by dissolving 1.0 M LiFSI in an organic solvent consisting of ethylene carbonate / ethyl methyl carbonate (EC / EMC mixed volume ratio of 3 / 7).

[0076] Example 2: Manufacturing of a lithium secondary battery In the above Example 1, the oxide-based solid electrolyte is LiAl with a D50 of 600 nm. 0.3 Ti 1.7 Except for using (PO4)3 (LATP-based) compound particles (ratio of oxide-based solid electrolyte to D50 of the electrode active material: 1:16.7), a positive electrode and a lithium secondary battery were manufactured in the same manner as in Example 1.

[0077] Example 3: Manufacturing of a lithium secondary battery The positive electrode and the negative electrode were manufactured using the same method as in Example 1.

[0078] On the other hand, to 100 parts by weight of an electrolyte prepared by dissolving 1.0 M LiFSI in an organic solvent consisting of ethylene carbonate / ethyl methyl carbonate (EC / EMC mixed volume ratio of 3 / 7), 4 parts by weight of trimethylolpropane triacrylate (ETPTA) and 0.6 parts by weight of AIBN as a thermal initiator were added. Next, the mixture was heat-treated at 60°C for 3 hours under an oxygen-barrier atmosphere to carry out a crosslinking reaction, thereby producing a gel electrolyte containing the electrolyte on a crosslinked polymer of ETPTA.

[0079] After punching out the positive and negative electrodes, an electrode assembly was manufactured by laminating them with the gel electrolyte interposed between them. The manufactured electrode assembly was then placed inside a battery case to produce a lithium secondary battery.

[0080] Comparative Example 1: Manufacturing of a positive electrode and a lithium secondary battery A positive electrode and a lithium secondary battery containing the same were manufactured in the same manner as in Example 1, except that oxide-based solid electrolyte particles were not used during the manufacturing of the positive electrode.

[0081] Comparative Example 2: Manufacturing of Cathode and Lithium Secondary Battery A positive electrode and a lithium secondary battery containing the same were manufactured in the same manner as in Example 3, except that oxide-based solid electrolyte particles were not used during the manufacturing of the positive electrode.

[0082] Experimental Example 1 - Evaluation of Battery Resistance at Different State of Charge (SOC) for Small Cells First, the lithium secondary batteries of Examples 1 and 2 and Comparative Example 1 were manufactured using small cells with a capacity of 60 mAh. Charge-discharge tests were repeatedly performed on these small-cell lithium secondary batteries of Examples 1 and 2 and Comparative Example 1 at room temperature (approximately 25°C) under the conditions of 1) 0.2C-0.33C charging (cut-off: 4.2V, 0.05C) and 2) 0.2C-0.33C discharging (cut-off: 2.5V), and the resistance was measured for each state of charge (SOC).

[0083] Specifically, the state of charge (SOC) was set to 10% to 90%, and the aforementioned charge-discharge tests were repeated. After each charge-discharge, the DC resistance (2.5C discharge (cut-off: 10s or 2V)) was measured.

[0084] The resistance measurement results for different charge states, specifically at SOC 10%, 50%, and 90%, are shown together in Table 1 below.

[0085] [Table 1]

[0086] Referring to Table 1 above, it was confirmed that the lithium secondary batteries of Examples 1 and 2 exhibited lower resistance and higher ionic conductivity compared to Comparative Example 1, depending on the charge state.

[0087] Experimental Example 2 - Evaluation of Battery Resistance at Different State of Charge (SOC) for Large Cells The lithium secondary batteries of Examples 1 and 3, and Comparative Examples 1 and 2 were manufactured as large 40Ah cells to confirm the characteristics of different electrolyte configurations. Charge and discharge tests were repeatedly performed at room temperature (approximately 25°C) on these large cell lithium secondary batteries of Examples 1 and 3, and Comparative Examples 1 and 2, under the conditions of 1) 0.2C-0.33C charging (cut-off: 4.2V, 0.05C) and 2) 0.2C-0.33C discharging (cut-off: 2.5V), and the resistance was measured for each state of charge (SOC).

[0088] Specifically, the state of charge (SOC) was set to 10% to 90%, and the aforementioned charge-discharge tests were repeated. After each charge-discharge, the DC resistance (2.5C discharge (cut-off: 10s or 2V)) was measured.

[0089] The resistance measurement results for different charge states, specifically at SOC 10%, 50%, and 90%, are shown together in Table 2 below.

[0090] [Table 2]

[0091] Referring to Table 2 above, it can be seen that the batteries of Comparative Example 2 and Example 2, which use a gel electrolyte, generally show increased resistance compared to the batteries of Comparative Example 1 and Example 1, which use an electrolyte and a separation membrane, due to the difference in electrolyte form. This is thought to be due to the relatively low ionic conductivity of the gel electrolyte.

[0092] However, regardless of the electrolyte form, it was confirmed that the lithium secondary batteries of Examples 1 and 2, in which an oxide-based solid electrolyte was added to the positive electrode as in the examples, exhibited lower resistance and higher ionic conductivity in each charge state compared to Comparative Examples 1 and 2, respectively.

[0093] Experimental Example 2 - Evaluation of Low-Temperature Characteristics A discharge test was performed on the lithium secondary batteries of Example 1 and Comparative Example 1 at a temperature of -10°C, starting from a state of charge (SOC) of 50% and lasting for 30 seconds.

[0094] Specifically, first, the lithium secondary battery was charged to 50% of its state of charge (SOC) under conditions of 0.1C-0.33C at a temperature of 25°C. Then, the lithium secondary battery with 50% SOC was stored in a -10°C chamber for 3 to 5 hours to cool its temperature to -10°C. Next, it was discharged under conditions of 1C with a cut-off of 30 seconds or 2.5V, and the degree of voltage drop for different discharge times was measured.

[0095] Next, under conditions of -10°C and 0.1C, the battery was recharged for the amount of discharged as described above. The discharge test was then repeated while changing the discharge rate limiting condition from 1C to 2C, 3C, 3.5C, 4.5C, 6C, and 8C. Throughout this process, the voltage drop for each discharge time was measured and is shown in Figure 1. The low-temperature power characteristics calculated based on these measurements are shown in Table 3 below.

[0096] [Table 3]

[0097] Referring to Figure 1 and Table 3, it was confirmed that the lithium secondary battery of Example 1 exhibited superior output characteristics, with a lower voltage drop during the initial stages of discharge compared to Comparative Example 1, even at a low temperature of -10°C.

Claims

1. The system comprises a positive electrode, a negative electrode, and an electrolyte containing a lithium salt and a non-aqueous organic solvent. The positive electrode includes a positive electrode current collector, and an active material layer formed on the positive electrode current collector, which includes a positive electrode active material, a conductive material, and an oxide-based solid electrolyte having a lithium ion supply source. The positive electrode active material and the oxide-based solid electrolyte are dispersed in the active material layer in particle form, and the average particle size D50 of the particles is such that the oxide-based solid electrolyte and the positive electrode active material have a ratio of 1:4 to 1:90, in a lithium secondary battery.

2. The positive electrode active material includes lithium; and a lithium metal oxide containing two or more metals selected from the group consisting of nickel, manganese, cobalt, and aluminum. The lithium secondary battery according to claim 1, wherein the lithium metal oxide contains 60 mol% or more nickel relative to the total metal content excluding lithium.

3. The lithium secondary battery according to claim 1, wherein the oxide-based solid electrolyte comprises one or more lithium metal oxides or lithium metal phosphorus oxides selected from Nasicon-type solid electrolytes, Lisicon-type solid electrolytes, Garnet-type solid electrolytes, Perovskite-type solid electrolytes, and LiPON-type solid electrolytes.

4. The lithium secondary battery according to claim 1, wherein the oxide-based solid electrolyte comprises one or more compounds selected from the group consisting of LAGP (lithium aluminum germanium phosphate) compounds, LLZO (lithium lanthanum zirconium oxide) compounds, LATP (lithium aluminum titanium phosphate) compounds, LLZTO (lithium lanthanum zirconium tantalum oxide) compounds, LLTO (lithium lanthanum titanium oxide) compounds, LSTP (lithium silicon titanium phosphate) compounds, and LGPO (lithium germanium phosphate) compounds.

5. The positive electrode active material has an average particle size D50 of 5 to 15 μm. The lithium secondary battery according to claim 1, wherein the oxide-based solid electrolyte has an average particle size D50 of 100 nm to 1.2 μm.

6. The lithium secondary battery according to claim 1, wherein the oxide-based solid electrolyte is included in an amount of 0.1 to 5 parts by weight based on 100 parts by weight of the positive electrode active material.

7. The lithium secondary battery according to claim 1, wherein the conductive material comprises carbon nanotubes or carbon nanofibers, and the active material layer further comprises a binder.

8. The lithium salt is LiCl, LiBr, LiI, LiBF ４ , LiClO ４ , LiB １０ Cl １０ , LiAlCl ４ , LiAlO ２ , LiPF ６ , LiCF ３ SO ３ , LiCH ３ CO ２ , LiCF ３ CO ２ , LiAsF ６ , LiSbF ６ , LiCH ３ SO ３ , LiFSI (Lithium bis(fluorosulfonyl)imide, LiN(SO ２ F) ２ ), LiBETI (lithium bis(perfluoroethanesulfonyl)imide, LiN(SO ２ CF ２ CF ３ ) ２ ) and LiTFSI (lithium bis(trifluoromethanesulfonyl)imide, LiN(SO ２ CF ３ ) ２ ), and the lithium secondary battery according to claim 1, comprising one or more selected from the group consisting of

9. The lithium secondary battery according to claim 1, wherein the lithium salt is contained in the electrolyte at a concentration of 0.5 M to 6 M.

10. The lithium secondary battery according to any one of claims 1 to 9, wherein the electrolyte further comprises a diluent that exhibits solubility in the lithium salt at least 10 times lower than that of the non-aqueous organic solvent.

11. The lithium secondary battery according to claim 10, wherein the diluent comprises one or more selected from the group consisting of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), bis(2,2,2-trifluoroethyl) ether (BTFE), 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE), methoxynononafluorobutane (MOFB), and ethoxynononafluorobutane (EOFB).

12. The separation membrane further comprises an interposed membrane between the positive electrode and the negative electrode, The lithium secondary battery according to any one of claims 1 to 9, wherein the electrolyte has the form of an electrolyte membrane or electrolyte film containing the lithium salt and the non-aqueous organic solvent within a polymer matrix, and is interposed between the positive electrode and the negative electrode.

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