Lithium-ion rechargeable battery

Abstract

The present invention relates to a lithium secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode contains a lithium iron phosphate compound represented by [Chemical Formula 1], and the lithium iron phosphate compound has an L value defined in formula (1) of 0.3926 to 0.3929, and a method for manufacturing the same.

Description

[Technical Field]

[0001] The present invention relates to a lithium secondary battery, and more specifically, to a lithium secondary battery containing a lithium iron phosphate compound as a positive electrode active material, and to a method for manufacturing the same. [Background technology]

[0002] Lithium secondary batteries are generally manufactured by forming an electrode assembly with a separator interposed between a positive electrode containing a positive electrode active material and a negative electrode containing a negative electrode active material, inserting the electrode assembly into a battery case, injecting a non-aqueous electrolyte that acts as a medium for transferring lithium ions, and then sealing it. The non-aqueous electrolyte generally consists of a lithium salt and an organic solvent that can dissolve the lithium salt.

[0003] Lithium-ion oxides, lithium manganese oxides, lithium iron phosphate compounds, lithium nickel cobalt manganese oxides, and lithium nickel cobalt aluminum oxides are used as positive electrode active materials in lithium-ion batteries. Among these, lithium iron phosphate compounds are widely used as positive electrode active materials in lithium-ion batteries because they have excellent thermal stability, resulting in superior lifespan and safety, as well as being inexpensive. However, lithium iron phosphate compounds have the problem of having a lower energy density and inferior capacity characteristics compared to other positive electrode active materials.

[0004] Furthermore, lithium iron phosphate compounds have varying stoichiometry and impurity content depending on their initial synthesis and storage conditions. This can lead to deviations in initial charge capacity and lifespan characteristics when applied to batteries, resulting in poor quality uniformity. [Overview of the project] [Problems that the invention aims to solve]

[0005] The present invention aims to solve these problems and to provide a lithium secondary battery that can improve the capacity characteristics and quality uniformity of the secondary battery by applying a lithium iron phosphate compound as the positive electrode active material, in which the crystal lattice constants a, b, and c measured by X-ray diffraction analysis (XRD) satisfy specific conditions and the Li molar ratio to iron and doping element (M) measured by ICP analysis satisfies a specific range. [Means for solving the problem]

[0006] According to one embodiment, the present invention provides a lithium secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode comprises a lithium iron phosphate compound represented by the following chemical formula 1, and the lithium iron phosphate compound has an L value defined by the following formula (1) of 0.3926 to 0.3929. [Chemical formula 1] Li 1-a [Fe 1-x M x ] 1-y PO 4-b A b In the above chemical formula 1, M is one or more selected from the group consisting of Mn, Ni, Co, Cu, Sc, Ti, Cr, V, and Zn, and A is one or more selected from the group consisting of S, Se, F, Cl, and I, -0.5 <a<0.5、0≦x<1、-0.5<y<0.5、0≦b≦0.1、1.07≦(1-a) / (1-y)≦1.09である。 Formula (1)

number

[0007] On the one hand, for the lithium iron phosphate compound, the molar ratio of Li to Fe and M, 1 / (1 - y), can be 1.02 to 1.10.

[0008] Also, the lithium iron phosphate compound can further include a conductive coating layer. According to another embodiment, the present invention includes the steps of measuring the lattice constants a, b, and c of the lithium iron phosphate compound by X-ray diffraction analysis, measuring the L value defined by the following formula (1), measuring the molar ratio of Li to Fe and M in the lithium iron phosphate compound by ICP analysis, selecting a lithium iron phosphate compound having an L value satisfying a preset range and a molar ratio of Li to Fe and M of 1.07 to 1.09 as a positive electrode active material, manufacturing a positive electrode including the selected positive electrode active material, manufacturing an electrode assembly including the positive electrode, a separator, and a negative electrode, and injecting an electrolyte after accommodating the electrode assembly in a battery case, thereby providing a method for manufacturing a lithium secondary battery. Formula (1)

Number

[0009] Preferably, the preset range can be 0.3926 to 0.3929.

Advantages of the Invention

[0010] The present invention measures the lattice constant values a, b, and c by XRD analysis and measures the molar ratio of Li to Fe and the doping element (M) in the lithium iron phosphate compound by ICP analysis, and uses a lithium iron phosphate compound having the lattice constant values a, b, and c and the molar ratio of Li to Fe and the doping element (M) satisfying specific conditions as a positive electrode active material, thereby improving the capacity characteristics of the LFP battery.

[0011] Further, in the method for manufacturing a lithium secondary battery of the present invention, after analyzing a lithium iron phosphate-based compound by XRD and ICP, based on the analysis results, a step of selecting a lithium iron phosphate-based compound satisfying specific conditions as a positive electrode active material is included, so that a secondary battery having uniform quality can be manufactured without going through a complicated process of manufacturing a cell and directly measuring its performance.

Brief Description of the Drawings

[0012] [Figure 1] It is a graph showing the initial charge capacity of the lithium secondary batteries manufactured according to Examples 1 to 8 and Comparative Examples 1 to 2.

Modes for Carrying Out the Invention

[0013] In the present specification and claims, terms and words should not be construed as being limited to their ordinary or dictionary meanings. The inventors should interpret them in accordance with the meaning and concept consistent with the technical idea of the present invention, in accordance with the principle that they can appropriately define the concept of terms in order to explain their invention in the best way.

[0014] In the present invention, "primary particle" means a particle unit in which no grain boundary is present in appearance when observed at a magnification of 5000 to 20000 times using a scanning electron microscope. "Average particle size of primary particles" means the arithmetic average value calculated after measuring the particle size of the primary particles observed in a scanning electron microscope image.

[0015] In the present invention, "average particle size D 50 " means the particle size at the 50% reference of the volume cumulative particle size distribution of the positive electrode active material powder. The average particle size D 50This can be measured using the laser diffraction method. For example, after dispersing the positive electrode active material powder in a dispersion medium, the particle size can be measured by introducing it into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000), irradiating it with ultrasound at approximately 28 kHz at an output of 60 W, obtaining a volume-cumulative particle size distribution graph, and then determining the particle size corresponding to 50% of the volume-cumulative amount.

[0016] The mobility of lithium ions during charging and discharging in lithium secondary batteries is influenced not only by the composition of the positive electrode active material but also by its lattice structure. Therefore, in order to maintain constant electrochemical properties such as energy density and lifetime characteristics, it is necessary to use a positive electrode active material with minimal deviation in its lattice structure. However, the lattice structure of lithium iron phosphate compounds varies not only depending on the synthesis conditions but also on the storage environment, so even when using lithium iron phosphate compounds manufactured by the same manufacturer, deviations in capacity characteristics occur. For this reason, conventionally, there has been the inconvenience of having to directly fabricate lithium secondary battery cells and then test their electrochemical properties for quality control.

[0017] The inventors conducted numerous experiments to develop a secondary battery (hereinafter referred to as "LFP cell") using a lithium iron phosphate compound with excellent capacity characteristics and quality uniformity. As a result, they developed a novel parameter L that correlates with the electrochemical performance of the LFP cell, and found that it is possible to manufacture an LFP cell with excellent initial capacity characteristics using this parameter, thus completing the present invention.

[0018] The parameter L can be defined by the following equation (1), where a, b, and c represent the lattice constants a, b, and c of the lithium iron phosphate compound measured by X-ray diffraction (XRD), respectively.

[0019] Formula (1)

number

[0020] According to our research, when a lithium iron phosphate compound is used as the positive electrode active material, in which the L value satisfies a specific range and the molar ratio of lithium to iron (Fe) and doping element (M) satisfies a specific range, the initial capacity characteristics of the LFP cell are significantly improved.

[0021] The L value and the molar ratio of lithium to iron (Fe) and doping element (M) are closely related to the initial capacity characteristics of LFP cells. By measuring the L value by XRD analysis of lithium iron phosphate compounds and measuring the Li / (Fe+M) molar ratio by ICP analysis, the initial capacity characteristics of LFP cells can be predicted without manufacturing the cells. Therefore, secondary batteries with excellent quality uniformity can be manufactured without going through the complex processes of cell manufacturing and performance measurement.

[0022] The present invention will be described in detail below.

[0023] <Lithium-ion secondary battery> First, let me explain the lithium secondary battery according to the present invention.

[0024] The lithium secondary battery of the present invention comprises a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode has an L value defined by the following formula (1) of 0.3926 to 0.3929 and contains a lithium iron phosphate compound represented by [Chemical Formula 1].

[0025] Formula (1):

number

[0026] In formula (1), a, b, and c are the lattice constant values ​​of the lithium iron phosphate compound measured by X-ray diffraction (XRD).

[0027] [Chemical Formula 1] Li 1-a [Fe 1-x M x 1-y PO 4-b A b

[0028] In Chemical Formula 1 above, M is any one or more selected from the group consisting of Mn, Ni, Co, Cu, Sc, Ti, Cr, V, and Zn; A is any one or more selected from the group consisting of S, Se, F, Cl, and I; -0.5 < a < 0.5, 0 ≤ x < 1, -0.5 < y < 0.5, 0 ≤ b ≤ 0.1, and 1.07 ≤ (1 - a) / (1 - y) ≤ 1.09.

[0029] (1) Positive Electrode The positive electrode according to the present invention contains a lithium iron phosphate-based compound as a positive electrode active material. Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and containing the lithium iron phosphate-based compound.

[0030] Here, the lithium iron phosphate-based compound can be represented by the following [Chemical Formula 1].

[0031] [Chemical Formula 1] Li 1-a [Fe 1-x M x 1-y PO 4-b A b

[0032] (0) In Chemical Formula 1 above, M is any one or more selected from the group consisting of Mn, Ni, Co, Cu, Sc, Ti, Cr, V, and Zn; A can be any one or more selected from the group consisting of S, Se, F, Cl, and I.

[0033] Also, a can be -0.5 < a < 0.5, preferably -0.3 ≤ a ≤ 0.3, and more preferably -0.1 ≤ a ≤ 0.1.

[0034] ​​Also, x can be 0 ≦ x < 1, preferably 0 ≦ x ≦ 0.8, more preferably 0 ≦ x ≦ 0.7.

[0035] y can be -0.5 < y < 0.5, preferably -0.3 ≦ y ≦ 0.3, more preferably -0.1 ≦ y ≦ 0.1.

[0036] Also, b can be 0 ≦ b ≦ 0.1, preferably 0 ≦ b ≦ 0.08, more preferably 0 ≦ b ≦ 0.05.

[0037] Among them, considering the conductivity and the improvement effects on the rate characteristics and capacitance characteristics thereby, the lithium iron phosphate compound can be, for example, LiFePO4, Li(Fe, Mn)PO4, Li(Fe, Co)PO4, Li(Fe, Ni)PO4 or a mixture thereof, and preferably can be LiFePO4.

[0038] On the other hand, the molar ratio of Li to Fe and M in the lithium iron phosphate compound (Li / (Fe + M)), that is, in Chemical Formula 1, (1 - a) / (1 - y) can be 1.07 to 1.09, preferably 1.08 to 1.09. When the ratio of Li / Fe satisfies the above range, particularly excellent initial capacitance is exhibited.

[0039] Also, the molar ratio of P to Fe and M in the lithium iron phosphate compound (P / (Fe + M)), that is, in Chemical Formula 1, 1 / (1 - y) can be 1.02 to 1.10, preferably 1.02 to 1.08, more preferably 1.03 to 1.07. If the molar ratio of P to Fe and M is too small, there is not enough polyanion (Polyanion) PO4 in the lattice structure, and if the molar ratio of P to Fe and M is too large, Li in the Fe site increases to a Li-excess state and the capacitance characteristics may deteriorate.

[0040] On the other hand, the molar content of Li, Fe, M, and P in lithium iron phosphate compounds is the value measured by ICP analysis. ICP analysis can be performed using the following method.

[0041] First, approximately 10 mg of lithium iron phosphate cathode active material is dispensed into a vial and its weight is accurately measured. Then, 2 ml of hydrochloric acid and 1 ml of hydrogen peroxide are added to the vial, and the mixture is dissolved at 100°C for 3 hours. Next, 50 g of ultrapure water is added to the vial, and 0.5 ml of 1000 μg / ml scandium (internal standard) is accurately added to prepare the sample solution. After filtering the sample solution through a PVDF 0.45 μm filter, the concentrations of Li, Fe, M, and P components are measured using an ICP-OES system (Perkin Elmer, AVIO500). If necessary, the sample solution can be further diluted so that the measured concentrations of each component fall within the calibration range.

[0042] On the other hand, the lithium iron phosphate compound has an L value defined by the following formula (1) of 0.3926 to 0.3929, preferably 0.3926 to 0.3928, and more preferably 0.3926 to 0.39275.

[0043] Formula (1):

number

[0044] In formula (1), a, b, and c are the lattice constant values ​​of the lithium iron phosphate compound measured by X-ray diffraction (XRD).

[0045] According to our research, when a lithium iron phosphate compound whose L value satisfies the above range is applied as a positive electrode active material, excellent initial capacity characteristics can be achieved.

[0046] The aforementioned L value is related to the Li concentration in the lattice structure of the lithium iron phosphate compound. When Li leaves the lattice structure of the lithium iron phosphate compound, Fe 2+ The O6 octahedral structure is Fe 3+ The O6 octahedral structure reduces the average Fe-O bond length, which in turn reduces the lattice constants a and b. 2+ The O6 octahedral structure is Fe 3+ In the O6 octahedral structure, the apical Fe-O1 decreases, and the PO4 tetrahedral structure rotates around the b-axis, thereby increasing the lattice constant c. That is, when the Li concentration in the LFP lattice structure decreases, the L value increases, and when the Li concentration increases, the L value decreases. The initial capacity of lithium iron phosphate compounds is determined by various factors such as Li content and carbon coating amount, but according to the inventors' research, among various factors, the Li concentration in the lattice structure represented by the L value has the greatest correlation with the initial capacity, and in particular, it was confirmed that the best initial capacity characteristics are observed when the L value is between 0.3926 and 0.3929.

[0047] The particle shape of the lithium iron phosphate compound is not particularly limited, but considering the tap density, it can be spherical.

[0048] Furthermore, the lithium iron phosphate compound may consist of a single primary particle, or of secondary particles formed by the aggregation of multiple primary particles. Here, the primary particles may be uniform or non-uniform. In the present invention, primary particles refer to a primary structure of a single particle, and secondary particles refer to aggregates formed by the physical or chemical bonding between primary particles, i.e., secondary structures.

[0049] On the other hand, the lithium iron phosphate compound may further contain a carbon-based coating layer. Although lithium iron phosphate compounds are structurally very stable, they have the disadvantage of relatively low electrical conductivity. Therefore, it is preferable to improve the electrical conductivity and resistance by coating the surface of the lithium iron phosphate compound with highly conductive carbon.

[0050] Furthermore, the lithium iron phosphate compound has an average particle size (D 50 The average particle size (D) of the lithium iron phosphate compound can be 1 μm to 20 μm, preferably 2 μm to 20 μm, and more preferably 2 μm to 15 μm. If the average particle size of the lithium iron phosphate compound is less than 1 μm, the properties of the positive electrode may deteriorate during manufacturing due to a decrease in dispersibility caused by aggregation between particles. In addition, the lithium iron phosphate compound has an average particle size (D 50 If the particle size exceeds 20 μm, it may result in a decrease in mechanical strength and specific surface area, or the porosity between lithium iron phosphate compound particles may become too large, leading to a decrease in tap density, or sedimentation may occur during the manufacturing of the cathode slurry.

[0051] On the other hand, when the lithium iron phosphate compound is a secondary particle, the primary particles can have an average particle size of 100 nm to 2 μm, preferably 100 nm to 1 μm, under conditions that satisfy the average particle size range of the secondary particles. If the average particle size of the primary particles is less than 100 nm, the dispersibility decreases due to aggregation between particles, and if the average particle size exceeds 2 μm, the capacitance characteristics of the electrode may decrease due to a decrease in packing density.

[0052] On the other hand, the lithium iron phosphate compound may further include a conductive coating layer on its surface. The conductive coating layer is for improving the conductivity of the lithium iron phosphate compound and may include one or more mixtures selected from the group consisting of carbon-based materials, metals, and conductive polymers. In particular, when a conductive coating layer of a carbon-based material is included, the conductivity can be effectively improved without significantly increasing the weight of the lithium iron phosphate compound.

[0053] The conductive coating layer can be formed by a conventional method for forming a coating layer, and can be contained in an amount of 1% to 7% by weight, more specifically 1% to 5% by weight, relative to the total weight of the lithium iron phosphate compound. If the content of the conductive coating layer exceeds 7% by weight, there is a risk that the battery characteristics will deteriorate due to a relative decrease in the LFP content, and if it is less than 1% by weight, there may be little improvement in conductivity due to the formation of the conductive layer.

[0054] On the other hand, the positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with surface treatment using carbon, nickel, titanium, silver, etc., can be used. The positive electrode current collector can also typically have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesion of the positive electrode active material. For example, it can be used in various forms such as film, sheet, foil, mesh, porous material, foam, and nonwoven fabric.

[0055] On the other hand, the positive electrode active material layer may further contain conductive materials, binders, dispersants, etc., in addition to lithium iron phosphate compounds.

[0056] In this case, the conductive material is used to impart conductivity to the electrodes and can be used without particular limitations as long as it does not cause chemical changes in the battery and has electronic conductivity. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, and carbon nanotubes; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. One of these alone or a mixture of two or more can be used. The conductive material can be included in an amount of 0.4% to 10% by weight, preferably 0.4% to 7% by weight, and more preferably 0.4% to 5% by weight, relative to the total weight of the positive electrode active material layer. When the content of the conductive material satisfies the above range, excellent positive electrode conductivity and capacity can be achieved.

[0057] Furthermore, the binder plays a role in improving the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, of which one or more can be used.

[0058] The binder may be present in an amount of 1% to 5% by weight, preferably 1.5% to 5% by weight, more preferably 1.5% to 4% by weight, and even more preferably 2% to 4% by weight, relative to the total weight of the positive electrode active material layer. When the binder content satisfies the above range, the adhesion between the current collector and the positive electrode active material layer is high, and no additional layer (e.g., a primer layer) is required to improve adhesion, and the positive electrode loading amount is high (e.g., 400 mg / 25 cm). 2 In addition to the above, excellent positive electrode adhesion is maintained, and excellent capacity characteristics and lifespan characteristics can be achieved.

[0059] The aforementioned dispersant is for improving the dispersibility of lithium iron phosphate compounds, conductive materials, etc. For example, hydrogenated nitrile-butadiene rubber (H-NBR) can be used, but is not limited to this, and various dispersants that can improve the dispersibility of the positive electrode slurry can be used. The dispersant can be included in an amount of 2% by weight or less, preferably 0.1 to 2% by weight, and more preferably 0.1 to 1% by weight, based on the total weight of the positive electrode active material layer. If the dispersant content is too low, the effect of improving dispersion will be minimal, and if it is too high, it may have a negative impact on the performance of the battery.

[0060] On the other hand, the positive electrode according to the present invention has a loading amount of 350 mg / 25 cm³. 2 ~2000mg / 25cm 2 Preferably 400 mg / 25 cm 2 ~1700mg / 25cm 2 More preferably 450 mg / 25 cm 2 ~1000mg / 25cm 2 This can be the case. When the positive electrode loading amount satisfies the above range, higher capacity characteristics can be achieved compared to conventional LFP batteries. Here, the positive electrode loading amount is 25 cm². 2 This refers to the weight of lithium iron phosphate compounds contained within the area.

[0061] Furthermore, the positive electrode can have a porosity of 25% to 60%, preferably 28% to 55%, more preferably 28% to 40%, even more preferably 28% to 35%, and even more preferably 25% to 30%. When the porosity of the positive electrode is formed within the above range, both energy density and electrolyte impregnation can be maintained at an excellent level. In the case of LFP batteries, the particle size of the lithium iron phosphate compound, which is the positive electrode active material, is small, resulting in a small void size within the positive electrode. This leads to poor electrolyte impregnation, so it is preferable to form a higher porosity of the positive electrode compared to batteries using other positive electrode active materials. However, as the porosity of the positive electrode increases, the energy density decreases. Therefore, it is necessary to appropriately adjust the porosity of the positive electrode in order to maintain both energy density and electrolyte impregnation at an excellent level.

[0062] (2) Negative electrode In the present invention, the negative electrode may be a negative electrode commonly used in the art, and may include, for example, a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0063] The negative electrode can be manufactured, for example, by applying a negative electrode slurry containing a negative electrode active material and selectively a binder and conductive material onto a negative electrode current collector, drying it to form a negative electrode active material layer, and then rolling it, or by casting the negative electrode slurry onto another support, peeling it off this support, and laminating the resulting film onto the negative electrode current collector.

[0064] The negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel with surface treatment using carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy can be used. The negative electrode current collector can usually have a thickness of 3 μm to 500 μm, and, similar to the positive electrode current collector, fine irregularities can be formed on the surface of the current collector to strengthen the bonding force of the negative electrode active material. For example, it can be used in various forms such as film, sheet, foil, mesh, porous material, foam, and nonwoven fabric.

[0065] The negative electrode active material layer selectively includes a binder and a conductive material together with the negative electrode active material.

[0066] As the negative electrode active material, compounds 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, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; and SiO2. x(0 < x < 2), metal oxides such as SnO2, vanadium oxides, and lithium vanadium oxides that can be doped and undoped with lithium; or composites containing the above metal compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, etc. Any one or a mixture of two or more of these can be used. Also, a thin film of metallic lithium may be used as the negative electrode active material. Further, as the carbon material, both low-crystalline carbon and high-crystalline carbon can be used. Representative examples of low-crystalline carbon are soft carbon and hard carbon, and representative examples of high-crystalline carbon are amorphous, plate-like, flaky, spherical or fibrous natural graphite or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbons such as petroleum or coal tar pitch-derived cokes.

[0067] Also, the binder and the conductive material are as described above for the positive electrode.

[0068] (3) Separator In the present invention, the separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. It can be used without particular limitations as long as it is a separator commonly used in lithium secondary batteries, and is particularly preferred if it has low resistance to ion movement of the electrolyte and excellent electrolyte moisture absorption capacity. Specifically, porous polymer films, such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof, can be used. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. Furthermore, coated separators containing ceramic components or polymeric substances can be used to ensure heat resistance or mechanical strength, and can be selectively used in single-layer or multi-layer structures.

[0069] (4) Electrolyte Examples of electrolytes used in the present invention include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0070] For example, the electrolyte may include an organic solvent and a lithium salt.

[0071] The aforementioned organic solvent can be used without particular limitations, as long as it serves as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the organic solvents that can be used include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); and alcohol solvents such as ethanol and isopropyl alcohol. Among these, carbonate-based solvents are preferred, and more preferably are mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of batteries, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate).

[0072] The lithium salt can be any compound capable of providing lithium ions for use in a lithium secondary battery, without any particular limitations. Specifically, the lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, etc. The concentration of the lithium salt is preferably within the range of 0.1M to 2.0M. When the concentration of the lithium salt falls within this range, the electrolyte can exhibit excellent electrolyte performance due to having appropriate conductivity and viscosity, and lithium ions can move effectively.

[0073] The electrolyte may further contain additives in addition to the constituent components of the electrolyte, for the purpose of improving the battery's lifespan characteristics, suppressing the decrease in battery capacity, and improving the battery's discharge capacity. Various electrolyte additives used in lithium secondary batteries can be used as the additives, and are not limited to, for example, halocarbonate compounds such as fluoroethylene carbonate; nitrile compounds such as succinonitrile, sulfone compounds such as 1,3-propanesultone and 1,3-propensultone; carbonate compounds such as vinylene carbonate; or combinations thereof. Here, the additive may be included in an amount of 0.1% to 10% by weight, preferably 0.1% to 5% by weight, relative to the total weight of the electrolyte.

[0074] The lithium secondary battery of the present invention, as described above, has a superior charging capacity compared to conventional batteries. Specifically, when the lithium secondary battery of the present invention is charged to 3.7V at 0.1C based on the theoretical capacity of lithium iron phosphate (170mAh / g), the initial charging capacity can be 93% to 100%, preferably 93% to 98%, and more preferably 94% to 97% of the theoretical capacity.

[0075] Specifically, the lithium secondary battery according to the present invention, when charged to 3.7V at 0.1C based on the theoretical capacity of lithium iron phosphate (170mAh / g), can have an initial charge capacity of 158mAh / g to 170mAh / g, preferably 158mAh / g to 167mAh / g, and more preferably 159mAh / g to 165mAh / g.

[0076] <Manufacturing method for lithium secondary batteries> Next, a method for manufacturing a lithium secondary battery according to the present invention will be described.

[0077] The present invention provides a method for manufacturing a lithium secondary battery, comprising the steps of: (1) measuring the lattice constants a, b, and c of a lithium iron phosphate compound by X-ray diffraction analysis and measuring the L value defined by the following formula (1); (2) measuring the molar ratio of Li to Fe and M of the lithium iron phosphate compound by ICP analysis; (3) selecting a lithium iron phosphate compound as a positive electrode active material in which the L value satisfies a preset range and the molar ratio of Li to Fe and M is 1.08 to 1.09; (4) manufacturing a positive electrode containing the selected positive electrode active material; (5) manufacturing an electrode assembly including the positive electrode, a separator, and a negative electrode; and (6) housing the electrode assembly in a battery case and then injecting an electrolyte.

[0078] Formula (1)

number

[0079] In formula (1), a, b, and c are the lattice constant values ​​of the lithium iron phosphate compound measured by X-ray diffraction (XRD).

[0080] (1) X-ray diffraction analysis step First, the lattice constants a, b, and c of the lithium iron phosphate compound are measured by X-ray diffraction analysis.

[0081] Here, the X-ray diffraction analysis was performed using the bruker D8 Endeavor equipment in the following manner.

[0082] First, depending on the amount of sample to be measured, the powder was placed in the central groove of either a general powder holder or a small powder holder. Using a glass slide, the sample height was adjusted to match the periphery of the holder and the sample surface was made uniform. Then, the fixed divergence slit was adjusted to 0.3 according to the size of the sample, and measurements were taken in the 2θ region at 0.016-degree intervals. Using a complete structure model, Rietveld refinement was performed on the phases present in the sample in the 10-120° region.

[0083] Next, the L value is measured by substituting the lattice constants a, b, and c of the lithium iron phosphate compound, which were measured by X-ray diffraction analysis, into equation (1).

[0084] Formula (1)

number

[0085] In formula (1), a, b, and c are the lattice constant values ​​of the lithium iron phosphate compound measured by X-ray diffraction (XRD).

[0086] As described above, the electrochemical performance of an LFP cell is closely correlated with the L value represented by equation (1). Therefore, by performing XRD analysis on lithium iron phosphate compounds to measure the lattice constants a, b, and c before manufacturing a lithium secondary battery, and then using these to measure the parameter L value represented by equation (1), it is possible to select a positive electrode active material that can achieve the desired performance of a lithium secondary battery cell without going through the complex process of manufacturing the cell and directly measuring its performance.

[0087] (2) ICP analysis step Furthermore, ICP analysis is used to measure the molar ratio of each component of the lithium iron phosphate compound. The ICP analysis method can be performed as follows.

[0088] First, approximately 10 mg of lithium iron phosphate cathode active material is dispensed into a vial and its weight is accurately measured. Then, 2 ml of hydrochloric acid and 1 ml of hydrogen peroxide are added to the vial, and the mixture is dissolved at 100°C for 3 hours. Next, 50 g of ultrapure water is added to the vial, and 0.5 ml of 1000 μg / ml scandium (internal standard) is accurately added to prepare the sample solution. After filtering the sample solution through a PVDF 0.45 μm filter, the concentrations of components Li, Fe, M, and P are measured using an ICP-OES system (Perkin Elmer, AVIO500). If necessary, further dilution can be performed so that the measured concentrations of each component are within the calibration range of the sample solution.

[0089] (3) Positive electrode active material selection step Next, a lithium iron phosphate compound is selected as the positive electrode active material, wherein the L value measured in (1) above satisfies a predetermined range, and the molar ratio of Li to Fe and the doping element (M) measured in (2) above is 1.08 to 1.09.

[0090] Here, the predetermined range can be appropriately selected considering the electrochemical performance of the LFP cell to be ultimately manufactured, and can be, for example, 0.3926 to 0.3929, preferably 0.3926 to 0.39285. When a lithium iron phosphate compound whose L value and the molar ratio of Li to Fe and the doping element (M) satisfy the above range is applied as the positive electrode active material, the initial capacity characteristics of the LFP cell are shown to be excellent.

[0091] (4) Cathode manufacturing step Next, a positive electrode containing the selected positive electrode active material is manufactured.

[0092] Here, the positive electrode can be manufactured by a general positive electrode manufacturing method known in the art, except that a lithium iron phosphate compound is used as the positive electrode active material, satisfying a predetermined range for L value and having a Li / (Fe+M) molar ratio of 1.08 to 1.09. For example, the positive electrode can be manufactured by mixing a positive electrode active material, a binder, and a conductive material to produce a positive electrode slurry, then applying the positive electrode slurry onto a positive electrode current collector and drying it to form a positive electrode active material layer, followed by rolling.

[0093] On the other hand, the specific types and specifications of the positive electrode active material, binder, and conductive material are as described above, and a detailed explanation will be omitted.

[0094] (5) Electrode assembly manufacturing step Next, an electrode assembly including the positive electrode manufactured as described above, a separator, and a negative electrode is manufactured. The specific types and specifications of the negative electrode and separator are as described above, and a detailed explanation is omitted.

[0095] The electrode assembly can be manufactured by stacking a positive electrode, a separator, and a negative electrode in that order, and the form of the electrode assembly is not particularly limited and can be a general electrode assembly well known in the field of lithium secondary batteries, such as a winding type, a stacked type, and / or a stack-and-folding type electrode assembly.

[0096] (6) Secondary battery manufacturing step Next, the electrode assembly is placed in a battery case, and then an electrolyte is injected to manufacture a lithium secondary battery.

[0097] Here, the battery case can be any common battery case well known in the field of lithium secondary batteries, such as cylindrical, rectangular, or pouch-type battery cases, and is not particularly limited.

[0098] On the other hand, the specific type and specifications of the electrolyte are as described above, and the electrolyte can be injected by a general electrolyte injection method well known in the field of lithium secondary batteries.

[0099] The present invention will be described in more detail below with reference to specific examples.

[0100] Experimental Example 1: Measurement of the physical properties of lithium iron phosphate compounds Samples of 11 lithium iron phosphate compound powders A to J were collected and analyzed by XRD to determine their L values. Additionally, the Li / Fe molar ratio and P / Fe molar ratio were determined by ICP analysis of the lithium iron phosphate compound powders A to J. The XRD and ICP analyses were performed using the methods described above.

[0101] The measurement results are shown in Table 1 below.

[0102] [Table 1]

[0103] Example 1 A positive electrode slurry was prepared by mixing 95 parts by weight of the aforementioned sample C as the positive electrode active material, 2 parts by weight of carbon black as the conductive material, and 3 parts by weight of PVDF as the binder in an N-methylpyrrolidone solvent. The positive electrode slurry was applied to an aluminum current collector with a thickness of 15 μm, dried, and then rolled to achieve a loading amount of 500 mg / 25 cm. 2 A positive electrode with a porosity of 29% was manufactured.

[0104] Meanwhile, a negative electrode slurry was prepared by adding 95 parts by weight of artificial graphite as the negative electrode active material, 3 parts by weight of SBR and 1 part by weight of CMC as binders, and 1 part by weight of carbon black as a conductive material to distilled water. The negative electrode slurry was applied to a copper current collector with a thickness of 8 μm, dried, and then rolled to obtain a loading amount of 240 mg / 25 cm. 2 A negative electrode with a porosity of 29% was manufactured.

[0105] After manufacturing an electrode assembly by stacking the positive and negative electrodes produced above together with a polyethylene separator, this assembly was placed in a battery case, and an electrolyte solution in which 1M LiPF6 was dissolved in a solvent of ethylene carbonate:ethyl methyl carbonate:diethyl carbonate mixed in a 1:1:1 ratio was injected to manufacture a lithium secondary battery.

[0106] Example 2 A lithium secondary battery was manufactured using the same method as in Example 1, except that sample D was used instead of sample C as the positive electrode active material.

[0107] Example 3 A lithium secondary battery was manufactured in the same manner as in Example 1, except that sample E was used instead of sample C as the positive electrode active material.

[0108] Example 4 A lithium secondary battery was manufactured in the same manner as in Example 1, except that sample F was used instead of sample C as the positive electrode active material.

[0109] Example 5 A lithium secondary battery was manufactured using the same method as in Example 1, except that sample G was used instead of sample C as the positive electrode active material.

[0110] Example 6 A lithium secondary battery was manufactured using the same method as in Example 1, except that sample H was used instead of sample C as the positive electrode active material.

[0111] Example 7 A lithium secondary battery was manufactured in the same manner as in Example 1, except that Sample I was used instead of Sample C as the positive electrode active material.

[0112] Example 8 A lithium secondary battery was manufactured in the same manner as in Example 1, except that sample J was used instead of sample C as the positive electrode active material.

[0113] Comparative Example 1 A lithium secondary battery was manufactured using the same method as in Example 1, except that Sample A was used instead of Sample C as the positive electrode active material.

[0114] Comparative Example 2 A lithium secondary battery was manufactured using the same method as in Example 1, except that Sample B was used instead of Sample C as the positive electrode active material.

[0115] Experimental Example 2 The lithium secondary batteries produced according to Examples 1-8 and Comparative Examples 1-2 were charged to 3.7V at 0.1C, based on the theoretical capacity of lithium iron phosphate (170mAh / g), then discharged to 2.5V at 0.1C. After that, the lithium secondary batteries were charged to 3.7V at 0.1C, and the initial charge capacity was measured. The measurement results are shown in Figure 1.

[0116] As shown in Figure 1, the lithium secondary batteries of Examples 1 to 8, which used lithium iron phosphate compounds C to J with an L value of 0.3926 to 0.3929 and a Li / Fe molar ratio of 1.07 to 1.09, showed a high initial capacity of 159 mAh / g. In contrast, the lithium secondary battery of Comparative Example 1, which used lithium iron phosphate compound A with an L value greater than 0.3929 and a Li / Fe molar ratio of less than 1.07, and the lithium secondary battery of Comparative Example 2, which used lithium iron phosphate compound B with an L value less than 0.3926 and a Li / Fe molar ratio of less than 1.07, exhibited inferior initial capacity characteristics.

Claims

1. A lithium secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, The positive electrode contains a lithium iron phosphate compound represented by the following [Chemical Formula 1], The lithium iron phosphate compound is a lithium secondary battery having an L value of 0.3926 to 0.3929 as defined by the following formula (1): [Chemical formula 1] Li １－ａ [Fe １－ｘ M ｘ ] １－ｙ 2O ４－ｂ A ｂ In the above chemical formula 1, M is one or more selected from the group consisting of Mn, Ni, Co, Cu, Sc, Ti, Cr, V, and Zn, and A is one or more selected from the group consisting of S, Se, F, Cl, and I, with -0.5 < a < 0.5, 0 ≤ x < 1, -0.5 < y < 0.5, 0 ≤ b ≤ 0.1, and 1.07 ≤ (1 - a) / (1 - y) ≤ 1.

09. Formula (1) [Math 1] In formula (1) above, a, b, and c are the lattice constant values ​​of the lithium iron phosphate compound measured by X-ray diffraction (XRD).

2. The lithium secondary battery according to claim 1, wherein the lithium iron phosphate compound has a molar ratio of P to Fe and M of 1 / (1-y) of 1.02 to 1.

10.

3. The lithium secondary battery according to claim 1, further comprising a conductive coating layer, the lithium iron phosphate compound.

4. The positive electrode has a loading amount of 350 mg / 25 cm². ２ ~2000mg / 25cm ２ The lithium secondary battery according to claim 1.

5. The lithium secondary battery according to claim 1, wherein the positive electrode has a porosity of 25% to 60%.

6. The lithium secondary battery according to claim 1, wherein, based on the theoretical capacity of lithium iron phosphate (170 mAh / g), when charged to 3.7 V at 0.1 C, the initial charge capacity is 93% to 100% of the theoretical capacity.

7. The steps include measuring the lattice constants a, b, and c of the lithium iron phosphate compound by X-ray diffraction analysis and measuring the L value defined by the following formula (1), The steps include: measuring the molar ratio of Li to Fe and doping element (M) of the lithium iron phosphate compound by ICP analysis; The steps include selecting a lithium iron phosphate compound as the positive electrode active material, wherein the L value satisfies a predetermined range and the molar ratio of Li to Fe and the doping element (M) is 1.07 to 1.09, A step of manufacturing a positive electrode containing the selected positive electrode active material, The steps include manufacturing an electrode assembly including the positive electrode, a separator, and a negative electrode, A method for manufacturing a lithium secondary battery, comprising the steps of housing the electrode assembly in a battery case and then injecting an electrolyte: Formula (1) [Math 2] In formula (1) above, a, b, and c are the lattice constant values ​​of the lithium iron phosphate compound measured by X-ray diffraction (XRD).

8. The method for manufacturing a lithium secondary battery according to claim 7, wherein the preset range is 0.3926 to 0.3929.

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