Lithium secondary 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 particularly to a lithium secondary battery containing a lithium iron phosphate compound as a positive electrode active material and a method for manufacturing the same. [Background technology]

[0002] Lithium secondary batteries are generally fabricated by forming an electrode assembly by interposing a separator between a positive electrode containing a positive active material and a negative electrode containing a negative active material, inserting the electrode assembly into a battery case, injecting a non-aqueous electrolyte, which serves as a medium for transferring lithium ions, and sealing the battery case. The non-aqueous electrolyte is generally composed of a lithium salt and an organic solvent capable of dissolving the lithium salt.

[0003] Positive electrode active materials used in lithium secondary batteries include lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate compound, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide. Among these, lithium iron phosphate compound is widely used as a positive electrode active material for lithium secondary batteries because of its excellent thermal stability, excellent life characteristics and safety, and low cost. However, lithium iron phosphate compound has problems such as low energy density and inferior capacity characteristics compared to other positive electrode active materials.

[0004] In addition, lithium iron phosphate compounds have different stoichiometry and impurity content depending on the initial synthesis state and storage state, which causes deviations in initial charge capacity and life characteristics when used in batteries, resulting in poor quality uniformity. Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention is intended 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 using, as a positive electrode active material, a lithium iron phosphate-based compound whose crystal lattice constants a, b, and c measured by X-ray diffraction analysis (XRD) satisfy specific conditions and whose molar ratio of Li to iron and doping element (M) measured by ICP analysis satisfies specific ranges. [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-based compound represented by the following [Chemical Formula 1], and the lithium iron phosphate-based compound has an L value defined in 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 formula 1, M is at least one selected from the group consisting of Mn, Ni, Co, Cu, Sc, Ti, Cr, V, and Zn; A is at least one selected from the group consisting of S, Se, F, Cl, and I; and <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] Meanwhile, the lithium iron phosphate compound may have a molar ratio 1 / (1-y) of Li to Fe and M of 1.02 to 1.10.

[0008] Furthermore, the lithium iron phosphate-based compound may further include a conductive coating layer. According to another embodiment, the present invention provides a method for manufacturing a lithium secondary battery, the method comprising the steps of: measuring lattice constants a, b, and c of the lithium iron phosphate-based compound by X-ray diffraction analysis to measure the L value defined by the following formula (1): measuring the molar ratio of Li to Fe and M of the lithium iron phosphate-based compound by ICP analysis; selecting a lithium iron phosphate-based compound having the L value within a predetermined range and a molar ratio of Li to Fe and M of 1.07 to 1.09 as a positive electrode active material; fabricating a positive electrode including the selected positive electrode active material; fabricating an electrode assembly including the positive electrode, a separator, and a negative electrode; and housing the electrode assembly in a battery case and then injecting an electrolyte. Formula (1)

number

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

[0010] The present invention measures the lattice constants a, b, and c values by XRD analysis, and measures the molar ratio of Li to Fe and the doping element (M) in a lithium iron phosphate compound by ICP analysis, and uses a lithium iron phosphate compound whose lattice constants a, b, and c values and the molar ratio of Li to Fe and the doping element (M) satisfy specific conditions as a positive electrode active material, thereby making it possible to improve the capacity characteristics of LFP batteries.

[0011] In addition, the method for manufacturing a lithium secondary battery of the present invention includes the steps of performing XRD and ICP analysis on a lithium iron phosphate compound, and then selecting a lithium iron phosphate compound that satisfies specific conditions as a positive electrode active material based on the analysis results. This makes it possible to manufacture secondary batteries with uniform quality without going through a complicated process of manufacturing cells and directly measuring their performance. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a graph showing the initial charge capacities of the lithium secondary batteries produced in Examples 1 to 8 and Comparative Examples 1 and 2. DETAILED DESCRIPTION OF THE INVENTION

[0013] The terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as having meanings and concepts that are 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 explain their inventions.

[0014] In the present invention, "primary particles" refers to particle units that do not appear to have grain boundaries when observed under a scanning electron microscope at a magnification of 5,000 to 20,000. "Average particle size of primary particles" refers to the arithmetic mean value calculated after measuring the particle sizes of primary particles observed under a scanning electron microscope image.

[0015] In the present invention, the "average particle size D 50 " means the particle size at 50% of the volume cumulative particle size distribution of the positive electrode active material powder. 50can be measured using a laser diffraction method. For example, after dispersing the positive electrode active material powder in a dispersion medium, the dispersion is introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000) and irradiated with ultrasonic waves of about 28 kHz at an output of 60 W. A volume cumulative particle size distribution graph is obtained, and the particle size corresponding to 50% of the volume cumulative amount is determined.

[0016] The mobility of lithium ions during charge and discharge in lithium secondary batteries is affected not only by the composition of the positive electrode active material but also by its lattice structure. Therefore, to maintain consistent electrochemical properties such as energy density and lifespan, it is necessary to use a positive electrode active material with minimal lattice structure variation. However, the lattice structure of lithium iron phosphate compounds varies depending on synthesis conditions and storage environments, resulting in variations in capacity characteristics even when using lithium iron phosphate compounds manufactured by the same manufacturer. Therefore, in the past, quality control required directly fabricating lithium secondary battery cells and then testing their electrochemical properties, which was inconvenient.

[0017] The present inventors conducted numerous experiments to develop a secondary battery (hereinafter referred to as "LFP cell") that uses a lithium iron phosphate compound with excellent capacity characteristics and quality uniformity. As a result, they developed a new parameter L that correlates with the electrochemical performance of LFP cells, and found that this parameter can be used to manufacture LFP cells with excellent initial capacity characteristics, which led to the completion of the present invention.

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

[0019] Formula (1)

number

[0020] According to the research of the present inventors, it has been found that when a lithium iron phosphate compound 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 is used as a positive electrode active material, the initial capacity characteristics of an 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 through XRD analysis of a lithium iron phosphate compound and measuring the Li / (Fe+M) molar ratio through ICP analysis, the initial capacity characteristics of LFP cells can be predicted without cell fabrication. Therefore, secondary batteries with excellent quality uniformity can be manufactured without undergoing the complex processes of cell fabrication and performance measurement.

[0022] The present invention will be specifically described below.

[0023] <Lithium secondary battery> First, the lithium secondary battery according to the present invention will be described.

[0024] The lithium secondary battery of the present invention includes 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):

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[0026] In the formula (1), a, b, and c are 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] 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, and 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 satisfy 0 ≦ x < 1, preferably 0 ≦ x ≦ 0.8, and more preferably 0 ≦ x ≦ 0.7.

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

[0036] Also, b can satisfy 0 ≦ b ≦ 0.1, preferably 0 ≦ b ≦ 0.08, and 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-based 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-based compound, 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-based compound, that is, in Chemical Formula 1, 1 / (1 - y) can be 1.02 to 1.10, preferably 1.02 to 1.08, and 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] Meanwhile, the contents (moles) of Li, Fe, M, and P in the lithium iron phosphate compound are values measured by ICP analysis. The ICP analysis method can be performed as follows.

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

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

[0045] According to the research of the present inventors, when a lithium iron phosphate compound having an L value that satisfies the above range is used as a positive electrode active material, excellent initial capacity characteristics can be achieved.

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

[0047] The particle shape of the lithium iron phosphate compound is not particularly limited, but may be spherical in consideration of tap density.

[0048] The lithium iron phosphate-based compound may be composed of a single primary particle or a secondary particle formed by agglomeration of a plurality of primary particles. Here, the primary particle may be uniform or non-uniform. In the present invention, the primary particle refers to a primary structure of a single particle, and the secondary particle refers to an agglomerate formed by agglomeration of primary particles through physical or chemical bonding between the primary particles, i.e., a secondary structure.

[0049] Meanwhile, the lithium iron phosphate-based compound may further include a carbon-based coating layer. Although the lithium iron phosphate-based compound is structurally very stable, it has the drawback of relatively low electrical conductivity. Therefore, it is preferable to improve electrical conductivity and resistance by coating the surface of the lithium iron phosphate-based compound with highly conductive carbon.

[0050] The lithium iron phosphate compound has an average particle size (D 50 ) 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 characteristics of the positive electrode may be deteriorated due to a decrease in dispersibility caused by aggregation between particles during the production of the positive electrode. In addition, the average particle size (D 50 If the particle size exceeds 20 μm, the mechanical strength and the specific surface area may decrease, or the porosity between the lithium iron phosphate compound particles may become too large, resulting in a decrease in tap density, or sedimentation may occur during the production of the positive electrode slurry.

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

[0052] Meanwhile, the lithium iron phosphate-based compound may further include a conductive coating layer on its surface. The conductive coating layer is intended to improve the conductivity of the lithium iron phosphate-based compound and may include one or a mixture of two or more selected from the group consisting of a carbon-based material, a metal, and a conductive polymer. In particular, when a conductive coating layer of a carbon-based material is included, the conductivity of the lithium iron phosphate-based compound can be effectively improved without significantly increasing the weight of the lithium iron phosphate-based compound.

[0053] The conductive coating layer may be formed by a conventional coating layer forming method and may be included in an amount of 1 wt % to 7 wt %, more specifically 1 wt % to 5 wt %, based on the total weight of the lithium iron phosphate-based compound. If the content of the conductive coating layer is too high, exceeding 7 wt %, the battery performance may be degraded due to a relative decrease in the LFP content, while if it is less than 1 wt %, the formation of the conductive layer may not have much effect in improving conductivity.

[0054] 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 surface-treated with carbon, nickel, titanium, silver, etc. can be used. The positive electrode current collector typically has a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesive strength of the positive electrode active material. It can be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0055] Meanwhile, the positive electrode active material layer may further include a conductive material, a binder, a dispersant, and the like in addition to the lithium iron phosphate-based compound.

[0056] The conductive material is used to impart conductivity to the electrode. Any conductive material can be used without particular limitations as long as it does not cause chemical changes in the resulting 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 powder or fiber, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials may be used alone or in combination. The conductive material may be included in an amount of 0.4 wt % to 10 wt %, preferably 0.4 wt % to 7 wt %, and more preferably 0.4 wt % to 5 wt %, based on the total weight of the positive electrode active material layer. When the conductive material content satisfies the above range, excellent positive electrode conductivity and capacity can be achieved.

[0057] The binder also serves to improve adhesion between positive electrode active material particles and between the positive electrode active material and the current collector. Specific examples of the binder include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These binders may be used alone or in combination.

[0058] The binder may be included in an amount of 1 wt % to 5 wt %, preferably 1.5 wt % to 5 wt %, more preferably 1.5 wt % to 4 wt %, and even more preferably 2 wt % to 4 wt %, based on the total weight of the positive electrode active material layer. When the binder content is within this range, the adhesion between the current collector and the positive electrode active material layer is high, and a separate layer (e.g., a primer layer) for improving the adhesion is not required. When the positive electrode loading is high (e.g., 400 mg / 25 cm), 2 Even in the above cases, excellent positive electrode adhesion is maintained, and excellent capacity and life characteristics can be achieved.

[0059] The dispersant is used to improve the dispersibility of the lithium iron phosphate compound, conductive material, etc., and may be, for example, hydrogenated nitrile-butadiene rubber (H-NBR), but is not limited thereto. Various dispersants that can improve the dispersibility of the positive electrode slurry may be used. The dispersant may be included in an amount of 2 wt % or less, preferably 0.1 to 2 wt %, and more preferably 0.1 to 1 wt %, based on the total weight of the positive electrode active material layer. If the amount of dispersant is too small, the effect of improving dispersion is insufficient, while if the amount is too large, it may adversely affect battery performance.

[0060] On the other hand, the positive electrode according to the present invention has a loading of 350 mg / 25 cm 2 ~2000mg / 25cm 2 , preferably 400 mg / 25 cm 2 ~1700mg / 25cm 2 , more preferably 450 mg / 25 cm 2 ~1000mg / 25cm 2 When the positive electrode loading amount satisfies the above range, a higher capacity characteristic can be achieved compared to a conventional LFP battery. Here, the positive electrode loading amount is 25 cm 2 This means the weight of the lithium iron phosphate compound contained in the area of

[0061] The positive electrode may 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 positive electrode porosity is within this range, both excellent energy density and electrolyte impregnation can be maintained. 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 small pores within the positive electrode, which in turn results in poor electrolyte impregnation. Therefore, it is preferable to increase the positive electrode porosity compared to batteries using other positive electrode active materials. However, the higher the positive electrode porosity, the lower the energy density. Therefore, it is necessary to appropriately adjust the positive electrode porosity to maintain both excellent energy density and electrolyte impregnation.

[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 disposed on the negative electrode current collector.

[0063] The negative electrode can be manufactured, for example, by coating a negative electrode slurry containing a negative electrode active material, and optionally a binder and a conductive material, on a negative electrode current collector, drying the coating, and then rolling the coating; or by casting the negative electrode slurry on a separate support, peeling the resulting film from the support, and laminating the resulting film on 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 surfaces treated with carbon, nickel, titanium, silver, or the like, and aluminum-cadmium alloys can be used. The negative electrode current collector typically has a thickness of 3 μm to 500 μm. As with the positive electrode current collector, the surface of the current collector can be formed with fine irregularities to enhance the binding strength of the negative electrode active material. The negative electrode current collector can be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0065] The negative electrode active material layer includes a negative electrode active material, and optionally a binder and a conductive material.

[0066] The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples thereof 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, and Al alloys; and SiO. x(0 < x < 2), metal oxides such as SnO2, vanadium oxides, lithium vanadium oxides that can be doped and undoped with lithium; or composites containing the metallic compound and a carbonaceous material such as Si-C composites or Sn-C composites, etc. may be mentioned, and any one or a mixture of two or more of these can be used. Further, a thin film of metallic lithium may be used as the negative electrode active material. Also, 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, scaly, 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] Further, 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 path for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without particular limitation. In particular, a separator that exhibits low resistance to electrolyte ion movement and excellent electrolyte humidification ability is preferred. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin polymer, such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material can also be used, and can be selectively used in a single-layer or multi-layer structure.

[0069] (4) Electrolyte Examples of the electrolyte used in the present invention include 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 production of lithium secondary batteries, but are not limited to these.

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

[0071] The organic solvent can be any organic solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent that can be used include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; carbonate-based solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); and alcohol-based solvents such as ethanol and isopropyl alcohol. Among these, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of the battery, and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred.

[0072] The lithium salt can be any compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the lithium salt may be LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(C, F, SO), LiN(C, F, SO), LiN(CF, SO), LiCl, LiI, or LiB(C, O) . The lithium salt is preferably used at a concentration in the range of 0.1 M to 2.0 M. When the lithium salt concentration is within this range, the electrolyte has appropriate conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0073] In addition to the electrolyte components, the electrolyte may further contain additives for purposes such as improving the battery's lifespan, suppressing battery capacity loss, and improving the battery's discharge capacity. The additives may include various electrolyte additives used in lithium secondary batteries, including, but not limited to, halocarbonate compounds such as fluoroethylene carbonate; nitrile compounds such as succinonitrile; sulfone compounds such as 1,3-propane sultone and 1,3-propene sultone; carbonate compounds such as vinylene carbonate; or combinations thereof. The additives may be present in an amount of 0.1 wt % to 10 wt %, preferably 0.1 wt % to 5 wt %, based on the total weight of the electrolyte.

[0074] The lithium secondary battery of the present invention as described above has a charge capacity superior to that of conventional batteries. Specifically, when the lithium secondary battery of the present invention is charged to 3.7 V at 0.1 C based on the theoretical capacity (170 mAh / g) of lithium iron phosphate, the initial charge 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 can have an initial charge capacity of 158 mAh / g to 170 mAh / g, preferably 158 mAh / g to 167 mAh / g, and more preferably 159 mAh / g to 165 mAh / g when charged to 3.7 V at 0.1 C based on the theoretical capacity (170 mAh / g) of lithium iron phosphate.

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

[0077] The method for manufacturing a lithium secondary battery according to the present invention includes 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, as a positive electrode active material, a lithium iron phosphate compound whose L value satisfies a predetermined range and whose molar ratio of Li to Fe and M is 1.08 to 1.09; (4) manufacturing a positive electrode including 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 the formula (1), a, b, and c are 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] The X-ray diffraction analysis was carried out using a Bruker D8 Endeavor instrument in the following manner.

[0082] First, depending on the amount of sample to be measured, powder was placed in the central groove of a general powder holder or a small-volume powder holder. A glass slide was used to ensure the sample height was aligned with the holder's periphery and the sample surface was uniform. The fixed divergence slit was then adjusted to 0.3° to accommodate the sample size, and the 2θ region was measured every 0.016°. Using a complete structure model, Rietveld refinement was performed to identify the phases present in the sample in the 10-120° region.

[0083] Next, the lattice constants a, b, and c of the lithium iron phosphate compound measured by X-ray diffraction analysis are substituted into formula (1) to measure the L value.

[0084] Formula (1)

number

[0085] In the formula (1), a, b, and c are 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 expressed in Equation 1. Therefore, prior to fabricating a lithium secondary battery, an XRD analysis is performed on a lithium iron phosphate compound to measure the lattice constants a, b, and c, and the parameter L value expressed in Equation 1 is measured using the lattice constants. This allows for the selection of a positive electrode active material that can achieve the desired lithium secondary battery cell performance without the need for a complex process of fabricating a cell and directly measuring its performance.

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

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

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

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

[0091] (4) Positive electrode 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 common method known in the art, except that a lithium iron phosphate-based compound having an L value satisfying a predetermined range and a Li / (Fe+M) molar ratio of 1.08 to 1.09 is used as the positive electrode active material. For example, the positive electrode can be manufactured by mixing a positive electrode active material, a binder, and a conductive material to prepare a positive electrode slurry, applying the positive electrode slurry to a positive electrode current collector, drying it to form a positive electrode active material layer, and then rolling it.

[0093] Meanwhile, the specific types and specifications of the positive electrode active material, binder, and conductive material are as described above, and a detailed description thereof will be omitted.

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

[0095] The electrode assembly may be manufactured by sequentially stacking a positive electrode, a separator, and a negative electrode. The form of the electrode assembly is not particularly limited and may be a common electrode assembly well known in the field of lithium secondary batteries, such as a wound type, a stacked type, and / or a stack-and-folded type electrode assembly.

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

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

[0098] Meanwhile, 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 kinds of lithium iron phosphate compound powders A to J were collected and analyzed by XRD to measure the L value. Furthermore, the lithium iron phosphate compound powders A to J were analyzed by ICP to measure the Li / Fe molar ratio and the P / Fe molar ratio. The XRD analysis and ICP analysis were carried out by 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 Sample C as a positive electrode active material, 2 parts by weight of carbon black as a conductive material, and 3 parts by weight of PVDF as a 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 a loading amount of 500 mg / 25 cm. 2 A positive electrode having a porosity of 29% was produced.

[0104] Meanwhile, 95 parts by weight of artificial graphite as a 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 were added to distilled water to prepare a negative electrode slurry. 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 having a porosity of 29% was produced.

[0105] The prepared positive and negative electrodes were stacked together with a polyethylene separator to prepare an electrode assembly, which was then placed in a battery case. An electrolyte solution prepared by dissolving 1M LiPF6 in a solvent containing a 1:1:1 mixture of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate was then injected to prepare a lithium secondary battery.

[0106] Example 2 A lithium secondary battery was produced in the same manner 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 produced 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 produced 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 produced in the same manner 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 produced in the same manner 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 produced 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 produced 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 produced in the same manner 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 produced in the same manner 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 prepared in Examples 1 to 8 and Comparative Examples 1 and 2 were charged to 3.7 V at 0.1 C based on the theoretical capacity of lithium iron phosphate (170 mAh / g), and then discharged to 2.5 V at 0.1 C. The lithium secondary batteries were then charged to 3.7 V at 0.1 C to measure the initial charge capacity. The measurement results are shown in Figure 1.

[0116] 1, the lithium secondary batteries of Examples 1 to 8, which employed lithium iron phosphate compounds C to J having an L value of 0.3926 to 0.3929 and a Li / Fe molar ratio of 1.07 to 1.09, exhibited a high initial capacity of 159 mAh / g, whereas the lithium secondary battery of Comparative Example 1, which employed lithium iron phosphate compound A having an L value of more than 0.3929 and a Li / Fe molar ratio of less than 1.07, and the lithium secondary battery of Comparative Example 2, which employed lithium iron phosphate compound B having an L value of less than 0.3926 and a Li / Fe molar ratio of less than 1.07, exhibited poor 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 includes a lithium iron phosphate compound represented by the following [Chemical Formula 1]: 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 2O ４－ｂ A ｂ In Chemical Formula 1, M is at least one selected from the group consisting of Mn, Ni, Co, Cu, Sc, Ti, Cr, V, and Zn; A is at least one selected from the group consisting of S, Se, F, Cl, and I; and −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) [Equation 1] In the formula (1), a, b, and c are lattice constant values of the lithium iron phosphate compound measured by X-ray diffraction (XRD).

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

10.

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

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

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

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

7. 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); measuring the molar ratio of Li to Fe and doping element (M) of the lithium iron phosphate compound by ICP analysis; selecting a lithium iron phosphate compound as a positive electrode active material, the L value of which satisfies a predetermined range and the molar ratio of Li to Fe and the doping element (M) is 1.07 to 1.09; fabricating a positive electrode comprising the selected positive electrode active material; manufacturing an electrode assembly including the positive electrode, a separator, and a negative electrode; and injecting an electrolyte into the electrode assembly after the electrode assembly is housed in a battery case. Formula (1) [Equation 2] In the formula (1), a, b, and c are lattice constant values of the lithium iron phosphate compound measured by X-ray diffraction (XRD).

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

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