Lithium secondary battery

Abstract

The present invention relates to a lithium secondary battery and a method for manufacturing the same, in which the lithium secondary battery comprises a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, in which the positive electrode comprises a lithium iron phosphate-based compound having an amorphous content index (AI) of 0.28 or less, preferably 0.20 to 0.28, more preferably 0.20 to 0.27, as defined by mathematical formula (1).

Description

Technical Field

[0001] The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery comprising a lithium iron phosphate compound as a positive electrode active material. Background Art

[0002] Lithium secondary batteries are generally manufactured by interposing a separator between a positive electrode containing a positive electrode active material and a negative electrode containing a negative electrode active material to form an electrode assembly, inserting the electrode assembly into a battery case, and then injecting a non-aqueous electrolyte as a lithium ion transport medium into the battery case, followed by 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] As the positive electrode active material of lithium secondary batteries, lithium cobalt oxides, lithium manganese oxides, lithium iron phosphate compounds, lithium nickel cobalt manganese oxides, lithium nickel cobalt aluminum oxides, etc. are used. Among the above substances, lithium iron phosphate compounds have excellent thermal stability, thus excellent life characteristics and safety, and low price, so they are widely used as positive electrode active materials of lithium secondary batteries. However, the problem is that compared with other positive electrode active materials, lithium iron phosphate compounds have low energy density and therefore poor capacity characteristics.

[0004] In addition, there is a problem that the stoichiometric ratio or impurity content in the lithium iron phosphate compound varies with the initial synthesis state or storage state of the lithium iron phosphate compound. Therefore, when used in a battery, there are deviations in the initial charging capacity and life characteristics, resulting in poor quality uniformity.

[0005] In addition, there is a problem that the content of amorphous phase material in lithium iron phosphate is unpredictable and cannot be well controlled during the manufacturing process. For example, even batches of the same lithium iron phosphate material made by the same manufacturer using the same process may have different amorphous content of lithium iron phosphate. As a result, the characteristics and performance of the lithium iron phosphate may be inconsistent, making the material difficult to use in electrode or battery production processes where repeatability and uniform quality are critical. Summary of the invention

[0006] Technical issues

[0007] The present invention aims to solve the above problems and provide a lithium secondary battery having excellent capacity characteristics by using a lithium iron phosphate active material whose AI index representing the content ratio of an amorphous phase in a lithium iron phosphate compound satisfies a specific range.

[0008] In addition, the present invention provides a method for manufacturing a lithium secondary battery, which measures the amorphous content index of lithium iron phosphate active materials by a specific method, selects and applies lithium iron phosphate active materials whose amorphous content index meets a specific range, and can ensure the electrochemical characteristics and quality uniformity of the lithium secondary battery.

[0009] Technical Solution

[0010] According to one aspect of the present invention, there is provided a lithium secondary battery, the lithium secondary battery comprising:

[0011] A positive electrode, a negative electrode, a separator between the positive and negative electrodes, and an electrolyte.

[0012] The positive electrode comprises a lithium iron phosphate active material, wherein the amorphous content index (AI) of the lithium iron phosphate active material defined by the following mathematical formula (1) is less than 0.28, preferably 0.20 to 0.28, more preferably 0.20 to 0.27, or most preferably 0.22 to 0.25.

[0013] Mathematical formula (1):

[0014] In the above mathematical formula (1),

[0015] is the peak area appearing at 2θ=43°±0.5° in the X-ray diffraction analysis diagram of a sample in which a lithium iron phosphate-based active material and MgO are mixed in a weight ratio of 70:30, and

[0016] It is the sum of the peak areas in the range of 2θ=15° to 36° in the X-ray diffraction analysis pattern of the sample.

[0017] At this time, the lithium iron phosphate-based active material may include a lithium iron phosphate-based compound represented by the following [Chemical Formula 1].

[0018] [Chemical formula 1]

[0019] Li 1-a [Fe 1-x M x ] 1-y PO 4-b A b

[0020] In the above chemical formula 1,

[0021] M is at least one selected from the group consisting of Mn, Ni, Co, Cu, Sc, Ti, Cr, V and Zn,

[0022] A is any one or more selected from S, Se, F, Cl and I,

[0023] -0.5 <a<0.5,0≤x<1,-0.5<y<0.5,0≤b≤0.1。

[0024] In addition, the molar ratio of Li to Fe and M (Li / (Fe+M)) of the lithium iron phosphate compound can be 1.0 to 1.1, preferably 1.05 to 1.09, and more preferably 1.06 to 1.085. In addition, the molar ratio of P to Fe and M (P / (Fe+M)) of the lithium iron phosphate compound can be 1.01 to 1.04, preferably 1.02 to 1.04.

[0025] In addition, the lithium iron phosphate compound may also include a conductive coating.

[0026] According to another aspect of the present invention, there is provided a method for manufacturing a lithium secondary battery, the method comprising the following steps:

[0027] preparing a sample in which a lithium iron phosphate-based active material and MgO are mixed in a weight ratio of 70:30;

[0028] The AI ​​value defined by the following mathematical formula (1) is measured by X-ray diffraction analysis of the sample:

[0029] Selecting a lithium iron phosphate active material whose AI value meets a preset range as a positive electrode active material;

[0030] manufacturing a positive electrode comprising a selected positive electrode active material;

[0031] manufacturing an electrode assembly including a positive electrode, a separator, and a negative electrode; and

[0032] The electrode assembly is housed in a battery case, and then an electrolyte is injected into the battery case.

[0033] Mathematical formula (1):

[0034] In the above mathematical formula (1),

[0035] is the peak area appearing at 2θ=43°±0.5° in the X-ray diffraction analysis diagram of a sample in which a lithium iron phosphate-based active material and MgO are mixed in a weight ratio of 70:30, and

[0036] It is the sum of the peak areas in the range of 2θ=15° to 36° in the X-ray diffraction analysis pattern of the sample.

[0037] On the other hand, the preset range may be 0.28 or less, preferably 0.20 to 0.28, more preferably 0.2 to 0.27, and most preferably 0.22 to 0.25.

[0038] Beneficial Effects

[0039] The present invention uses a lithium iron phosphate compound whose amorphous content index AI defined by mathematical formula (1) satisfies a specific range as a positive electrode active material, thereby controlling the heterogeneity of the lithium iron phosphate active material and improving the capacity characteristics of the LFP battery.

[0040] In addition, the method for manufacturing a lithium secondary battery of the present invention is characterized in that the amorphous content index AI of the lithium iron phosphate active material is measured by a specific method, and the lithium iron phosphate compound whose amorphous content index meets a specific range is selected and applied as the positive electrode active material. On the other hand, the amorphous content index AI according to the present invention is a value measured by mixing the lithium iron phosphate active material with MgO in a weight ratio of 70:30 to prepare a sample, and then comparing the peak intensity (area) of the graph obtained by X-ray diffraction analysis of the sample, wherein, unlike the conventional spike method for analyzing the amorphous content in the crystal structure, the deviation according to the sample or the number of tests is small, and the reproducibility and discrimination are excellent. Therefore, by using the amorphous content index AI defined in the present invention, a secondary battery with uniform and excellent quality can be manufactured without going through the cumbersome process of manufacturing the battery and directly measuring the battery performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 Graph showing the change in initial charge capacity of a lithium secondary battery with the amorphous content index AI. DETAILED DESCRIPTION

[0042] It should be understood that the words or terms used in the specification and claims of the present invention should not be construed as limited to the meanings defined in commonly used dictionaries. It should also be understood that the words or terms should be interpreted as having a meaning consistent with their meanings in the context of the relevant technical field and the technical concept of the present invention, on the basis that the inventor can appropriately define the meaning of the words or terms to best explain the principles of the present invention.

[0043] In the present invention, the term "lithium iron phosphate" refers to LiFePO4. On the other hand, the term "lithium iron phosphate compound" refers to a compound comprising lithium, iron and phosphate, and also potentially various other elements and / or dopants disclosed herein, such as the elements and / or dopants in Chemical Formula 1. The term "lithium iron phosphate active material" refers to a final active material comprising lithium iron phosphate and / or a lithium iron phosphate compound, which may potentially also include additional compositional or structural components, such as a coating.

[0044] In this specification, the term "primary particle" refers to a particle unit that appears to have no grain boundaries when observed in a field of view of 5000 to 20000 times using a scanning electron microscope. The term "average particle size of primary particles" refers to the arithmetic mean value calculated after measuring the particle size of primary particles observed from a scanning electron microscope image.

[0045] In this specification, the term "average particle size D 50 " refers to the particle size at which 50% of the volume cumulative particle size distribution of the positive electrode active material powder is obtained. The average particle size D 50 It can be measured by laser diffraction. For example, the positive electrode active material powder is dispersed in a dispersion medium and then introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT 3000) to irradiate with ultrasonic waves of about 28 kHz to an output of 60 W. Thereafter, the average particle size can be measured by obtaining a volume cumulative particle size distribution diagram and then obtaining a particle size corresponding to 50% of the volume cumulative amount.

[0046] Hereinafter, the present invention will be described in detail.

[0047] The amorphous content of the positive electrode active material affects the energy density and / or life characteristics of the lithium secondary battery. In addition, the amorphous content may be unpredictable even in materials of the same chemical composition manufactured in the same way. Therefore, it is very important to accurately identify the degree of amorphous content in the positive electrode active material and predict the electrochemical performance when designing lithium secondary battery cells.

[0048] Usually, in order to measure the amorphous content of lithium iron phosphate compounds, the spike method is mainly used, in which an internal standard is introduced into the lithium iron phosphate compound, an X-ray diffraction analysis is performed, and then a Rietveld refinement analysis is performed to estimate the amorphous content. However, in the case of spike analysis, the calculated value varies with the structural model and fitting parameters, and the deviation within the sample is large, making it difficult to obtain a reliable value. Therefore, usually, in order to perform quality management, lithium secondary battery cells are directly manufactured and then their electrochemical properties are tested, which is very cumbersome.

[0049] In order to develop a secondary battery (hereinafter referred to as "LFP battery") in which lithium iron phosphate active materials with excellent capacity characteristics and quality uniformity are applied, the inventors conducted a large number of experiments, and as a result, developed a new amorphous content index (AI) capable of representing the amorphous content of lithium iron phosphate active materials, and found that the amorphous content index can be used to manufacture LFP batteries with excellent initial capacity characteristics. The AI ​​parameter of the present technology is different from the crystalline content of the lithium iron phosphate itself. In contrast, it is a parameter characteristic of the final LFP active material, which can be affected by and represent other characteristics and structures of the active material, such as the presence and / or content of a coating on the LFP, and the presence and / or content of impurities.

[0050] The amorphous content index AI can be defined by the following mathematical formula (1).

[0051] Mathematical formula (1):

[0052] In the above mathematical formula (1),

[0053] is the peak area appearing at 2θ=43°±0.5° in the X-ray diffraction analysis diagram of a sample in which a lithium iron phosphate-based active material and MgO are mixed in a weight ratio of 70:30, and

[0054] It is the sum of the peak areas in the range of 2θ = 15° to 36° in the X-ray diffraction analysis diagram of the sample. In addition, MgO is a material that is close to 100% crystalline.

[0055] The amorphous content W in the sample amorphrous It can be calculated as follows.

[0056] W amorphrous =100-W crystal =100-(W MgO +W crystal,LFP )=100-(W MgO +W MgO (W crystal,LFP / W MgO ))

[0057] In the above mathematical formula, W crystal is the crystal content in the sample, W crystal,LFP is the crystal content in lithium iron phosphate compounds, W MgO On the other hand, MgO is 100% crystalline and its weight is fixed at 30% by weight. According to the above mathematical formula, W amorphrous With W crystal,LFP / W MgOThat is, the amorphous content W in the sample amorphrous With W crystal,LFP / W MgO Since MgO is 100% crystalline, the amorphous content in the sample corresponds to the amorphous content in the lithium iron phosphate compound.

[0058] On the other hand, when the crystal ratio in the lithium iron phosphate-based active material increases, the sum of the peak areas representing the crystallized lithium iron phosphate in the X-ray diffraction analysis diagram, that is, Increase, and when W MgO When increasing, Therefore, W crystal,LFP / W MgO and As mentioned above, W amorphrous With W crystal,LFP / W MgO is negatively correlated with There is a positive correlation.

[0059] Therefore, since the amorphous content in the lithium iron phosphate active material is related to the above mathematical formula (1), There is a positive correlation, so the content of amorphous phase in lithium iron phosphate active materials can be relatively compared by using AI as an indicator.

[0060] On the other hand, in the present invention, when calculating AI, only the peak areas appearing in the range of 2θ=15 to 36° among the XRD peaks of lithium iron phosphate, excluding the peaks in the area overlapping with the peaks of MgO, are added, and only the area of ​​the strongest (200) peak (2θ=43°) appearing in the MgO peak is used, thereby minimizing the error caused by peak duplication.

[0061] In addition, according to the research of the present inventors, it was found that when a sample was formed by mixing 100% crystalline MgO as an internal standard at a content of 30 wt%, the result value had the smallest deviation and was the most reliable. At this time, as MgO, for example, MgO with a purity of ABCR of 99.95% was ground, heat-treated at 1200°C, and then ground again for use, but the present invention is not limited thereto, and MgO with a crystallinity close to 100% can be used without limitation.

[0062] According to the research of the present inventors, it is found that when lithium iron phosphate compounds with an AI value of less than 0.28, preferably 0.20 to 0.28, more preferably 0.20 to 0.27, and most preferably 0.22 to 0.25 are used as positive electrode active materials, the initial capacity characteristics of the LFP battery are significantly improved.

[0063] Hereinafter, the present invention will be described in detail.

[0064] <Lithium Secondary Battery>

[0065] First, a lithium secondary battery according to the present invention will be described.

[0066] The lithium secondary battery of the present invention comprises:

[0067] A positive electrode, a negative electrode, a separator between the positive and negative electrodes, and an electrolyte.

[0068] The positive electrode comprises a lithium iron phosphate active material, wherein the lithium iron phosphate active material has an amorphous content index (AI) of less than 0.28 as defined by the following mathematical formula (1).

[0069] Mathematical formula (1):

[0070] In the above mathematical formula (1),

[0071] is the peak area appearing at 2θ=43°±0.5° in the X-ray diffraction analysis diagram of a sample in which a lithium iron phosphate-based active material and MgO are mixed in a weight ratio of 70:30, and

[0072] It is the sum of the peak areas in the range of 2θ=15° to 36° in the X-ray diffraction analysis pattern of the sample.

[0073] (1) Positive electrode

[0074] The positive electrode according to the present invention comprises a lithium iron phosphate-based active material as a positive electrode active material, wherein the amorphous content index (AI) of the lithium iron phosphate-based active material defined by the following mathematical formula (1) is 0.28 or less, preferably 0.20 to 0.28, more preferably 0.20 to 0.27, even more preferably 0.22 to 0.27, and most preferably 0.22 to 0.25. Specifically, the positive electrode comprises a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and comprising the lithium iron phosphate-based active material.

[0075] Mathematical formula (1):

[0076] In the above mathematical formula (1),

[0077] is the peak area appearing at 2θ=43°±0.5° in the X-ray diffraction analysis diagram of a sample in which a lithium iron phosphate-based active material and MgO are mixed in a weight ratio of 70:30, and

[0078] It is the sum of the peak areas in the range of 2θ=15° to 36° in the X-ray diffraction analysis pattern of the sample.

[0079] As mentioned above, the AI ​​value is a value corresponding to the amorphous content in the lithium iron phosphate active material. That is, it can be said that if the AI ​​value is large, the amorphous ratio in the lithium iron phosphate active material is high, and if the AI ​​value is small, the amorphous ratio in the lithium iron phosphate active material is low. On the other hand, because the amorphous phase does not participate in the battery reaction, when the proportion of the amorphous phase in the lithium iron phosphate active material increases, the capacity characteristics of the battery decrease. When the AI ​​value is small (below 0.28), the amorphous ratio in the lithium iron phosphate active material is small, so that excellent capacity characteristics can be achieved.

[0080] On the other hand, the AI ​​value is not only affected by the ratio of crystallinity and amorphousness of the lithium iron phosphate particles themselves, but also by the additional structure and amorphous content in the LFP-type active material (such as the content of the conductive carbon layer formed on the surface of the lithium iron phosphate particles or the content of impurities that may be formed or introduced unintentionally during the manufacturing process). That is, when a conductive carbon layer is formed on the surface of the lithium iron phosphate particles, the content of the conductive carbon layer may also affect the AI ​​value. Specifically, when the content of the conductive carbon layer increases, the AI ​​value also increases, and when the content of the conductive carbon layer decreases, the AI ​​value decreases. When the AI ​​value of the lithium iron phosphate-type active material is less than 0.2, the content of the conductive carbon layer is small, and in the case described, the conductivity of the lithium iron phosphate-type active material is insufficient, resulting in reduced electrochemical performance.

[0081] On the other hand, the lithium iron phosphate-based active material may include, for example, a lithium iron phosphate-based compound represented by the following [Chemical Formula 1].

[0082] [Chemical formula 1]

[0083] Li 1-a [Fe 1-x M x ] 1-y PO 4-b A b

[0084] In the above chemical formula 1,

[0085] M may be any one or more selected from the group consisting of Mn, Ni, Co, Cu, Sc, Ti, Cr, V and Zn, and

[0086] A may be any one or more selected from S, Se, F, Cl and I.

[0087] Furthermore, a may be -0.5≤a≤0.5, preferably -0.3≤a≤0.3, and more preferably -0.1≤a≤0.1.

[0088] Furthermore, x may be 0≤x<1, preferably 0≤x≤0.8, and more preferably 0≤x≤0.7.

[0089] y may be -0.5≤y≤0.5, preferably -0.3≤y≤0.3, and more preferably -0.1≤y≤0.1.

[0090] Furthermore, b may be 0≤b≤0.1, preferably 0≤b≤0.08, and more preferably 0≤b≤0.05.

[0091] Among the above, considering the effect of improving conductivity and subsequently improving rate characteristics and capacity characteristics, LFP can be more specifically LiFePO4, Li(Fe,Mn)PO4, Li(Fe,Co)PO4, Li(Fe,Ni)PO4 or a mixture thereof, more specifically LiFePO4.

[0092] On the other hand, the molar ratio of Li to Fe and M (Li / (Fe+M)) of the lithium iron phosphate compound, that is, (1-a) / (1-y) in Chemical Formula 1, may be 1.0 to 1.1, preferably 1.05 to 1.09, and more preferably 1.06 to 1.085. When the Li / Fe ratio exceeds the above range, the lithium mobility may decrease, and accordingly, the initial capacity may decrease.

[0093] In addition, the molar ratio of P to Fe and M (P / (Fe+M)) of the lithium iron phosphate compound, that is, 1 / (1-y) in Chemical Formula 1, can be 1.01 to 1.04, preferably 1.02 to 1.04, and more preferably 1.025 to 1.035. When the ratio of P / (Fe+M) is too small, the polyanion PO4 is lacking in the lattice structure, and when the ratio of P / (Fe+M) is too large, the capacity characteristics may be reduced because of the increased Li-rich state at the Fe and M sites.

[0094] On the other hand, the contents (moles) of Li, Fe and P in the lithium iron phosphate compound are values ​​measured by ICP analysis. ICP analysis can be performed by the following method.

[0095] First, about 10 mg of lithium iron phosphate positive electrode active material was divided into a vial and weighed accurately. 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 of scandium (internal standard) was accurately added to it to prepare the sample solution. The sample solution was filtered through a PVDF 0.45 μm filter and the concentrations of Li, Fe and P components were measured using an ICP-OES device (Perkin Elmer, AVIO500). If necessary, dilution can also be performed so that the measured concentration of the sample solution is within the calibration range of each component.

[0096] The particle shape of the lithium iron phosphate compound is not particularly limited, but may be spherical in consideration of tap density.

[0097] In addition, the lithium iron phosphate compound may be composed of a single particle of a primary particle, or may be composed of a secondary particle in which a plurality of primary particles are aggregated. In this case, the primary particles 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 aggregate in which the primary particles are aggregated by physical or chemical bonding between the primary particles, i.e., a secondary structure.

[0098] On the other hand, the lithium iron phosphate compound may also include a carbon coating. The lithium iron phosphate compound has a very stable structure, but has a disadvantage of relatively low conductivity. Therefore, it is expected to improve conductivity and resistance by coating the surface of the lithium iron phosphate compound with highly conductive carbon.

[0099] In addition, the 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. When the average particle size of the lithium iron phosphate compound is less than 1 μm, the characteristics of the positive electrode may be reduced because the dispersibility decreases with the agglomeration of particles during the manufacture of the positive electrode. In addition, when the average particle size (D 50 ) is greater than 20 μm, the mechanical strength may be reduced and the specific surface area may be reduced, or the porosity between the particles of the lithium iron phosphate compound may be excessively increased, thereby reducing the tap density, or sedimentation may occur when preparing the positive electrode slurry.

[0100] On the other hand, when the lithium iron phosphate compound is a secondary particle, the average particle size (D 50 ) range, the average particle size of the primary particles can be 100 nm to 2 μm, preferably 100 nm to 1 μm. When the average particle size of the primary particles is less than 100 nm, the dispersibility is reduced due to agglomeration between particles, and when it is greater than 2 μm, the capacity characteristics of the electrode may be reduced due to reduced packing density.

[0101] On the other hand, the lithium iron phosphate compound may further include a conductive coating on its surface. The conductive coating is used to improve the conductivity of the lithium iron phosphate compound, and may include any one or a mixture of two or more selected from carbon materials, metals and conductive polymers. Among the above substances, when a conductive coating of a carbon material is included, the conductivity can be effectively improved without significantly increasing the weight of the lithium iron phosphate compound.

[0102] The conductive coating layer may be formed by a conventional method for forming a coating layer, and the content relative to the total weight of the lithium iron phosphate compound may be 1 wt % to 7 wt %, more specifically 1 wt % to 5 wt %. When the content of the conductive coating layer is too high, greater than 7 wt %, there is a risk that the battery characteristics may be reduced due to the relative reduction in the LFP content, and when less than 1 wt %, the effect of improving the conductivity by forming the conductive layer may not be significant.

[0103] The content of the lithium iron phosphate compound may be 85 wt % to 98 wt %, preferably 90 wt % to 98 wt %, and more preferably 94 wt % to 98 wt % relative to the total weight of the positive electrode active material layer. When the content of the lithium iron phosphate compound satisfies the above range, excellent energy density can be achieved.

[0104] The lithium iron phosphate compound can be manufactured by methods known in the art for manufacturing lithium iron phosphate compounds, such as solid phase synthesis, sol-gel method, hydrothermal synthesis, spray pyrolysis, etc., but is not limited thereto.

[0105] For example, precursor materials of a lithium iron phosphate compound (eg, an iron precursor, a phosphoric acid precursor, and / or a lithium precursor) may be mixed in a solid state and then calcined to synthesize the lithium iron phosphate compound (solid phase synthesis method).

[0106] Alternatively, the precursor material of the lithium iron phosphate compound (e.g., an iron precursor, a phosphate precursor, and / or a lithium precursor) may be dissolved in a solvent, an additive such as an acid / base or a chelating agent may be added to perform a sol-gel reaction, and then calcined to synthesize the lithium iron phosphate compound (sol-gel method).

[0107] Alternatively, the precursor materials of the lithium iron phosphate compound (eg, an iron precursor, a phosphate precursor, and / or a lithium precursor) may be dissolved in water and then reacted at high temperature and high pressure to prepare the lithium iron phosphate compound (hydrothermal synthesis method).

[0108] Alternatively, the lithium iron phosphate compound can be prepared by spraying a solution in which a precursor material of the lithium iron phosphate compound (eg, an iron precursor, a phosphate precursor, and / or a lithium precursor) is dissolved into droplets, evaporating the solvent, and heating (spray pyrolysis method).

[0109] On the other hand, the iron precursor may be, for example, iron oxalate (FeC2O4·2H2O), iron acetate (Fe(CH3COO)2), FeSO4, FeCO3, FeO, but is not limited thereto. The phosphoric acid precursor may be, for example, ammonium phosphate, diammonium phosphate, lithium phosphate, iron phosphate, phosphoric acid, phosphorus oxide (P2O5, P4O 10), diammonium hydrogen phosphate, but not limited thereto. The lithium precursor may be, for example, lithium chloride, lithium carbonate, lithium hydroxide, lithium phosphate, lithium nitrate, lithium sulfate, lithium oxide, lithium aluminum oxide, but not limited thereto.

[0110] On the other hand, the lithium iron phosphate-based active material having a conductive coating layer can be prepared by mixing lithium iron phosphate with a carbon source and / or a hydrocarbon gas and then heating the mixture.

[0111] On the other hand, there is no particular limitation on the positive electrode current collector as long as it has conductivity without causing chemical changes in the battery, and for example: stainless steel, aluminum, nickel, titanium, fired carbon; or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. can be used. In addition, the thickness of the positive electrode current collector can generally be 3 μm to 500 μm, and fine concavoconvexity can be formed on the surface of the current collector to improve the adhesion of the positive electrode active material. For example, the positive electrode current collector can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam body, and a non-woven fabric body.

[0112] On the other hand, the positive electrode active material layer may further contain a conductive material, a binder, a dispersant, and the like in addition to the lithium iron phosphate-based active material.

[0113] At this time, the conductive material is used to impart conductivity to the electrode, and any conductive material can be used without particular limitation as long as it has electronic conductivity without causing chemical changes in the constructed battery. Specific examples thereof may include: graphite such as natural graphite or artificial graphite; carbon materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, and carbon nanotube; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and any one thereof or a mixture of two or more thereof may be used.

[0114] Based on the total weight of the positive electrode active material layer, the content of the conductive material may be 0.4 wt % to 10 wt %, preferably 0.4 wt % to 7 wt %, and more preferably 0.4 wt % to 5 wt %. When the content of the conductive material satisfies the above range, excellent positive electrode conductivity and capacity can be achieved.

[0115] In addition, the binder is used to improve the bonding between the positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples thereof may include: polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber or various copolymers thereof, and any one thereof or a mixture of two or more thereof may be used.

[0116] Based on the total weight of the positive electrode active material layer, the content of the binder may be 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 %. When the binder content satisfies the above range, since the adhesion between the current collector and the positive electrode active material layer is high, a separate layer (e.g., a bottom layer) is not required to improve the adhesion, and when the positive electrode loading is high (e.g., 400 mg / 25 cm 2 When the positive electrode has excellent adhesion, excellent capacity characteristics and life characteristics can be achieved.

[0117] Dispersants are used to improve the dispersibility of lithium iron phosphate active materials, conductive materials, etc., and can use, for example, hydrogenated nitrile rubber (H-NBR), etc., but the present invention is not limited thereto, and various dispersants that can improve the dispersibility of the positive electrode slurry can be used. Relative to the total weight of the positive electrode active material layer, the content of the dispersant can be 2% by weight or less, preferably 0.1% by weight to 2% by weight, and more preferably 0.1% by weight to 1% by weight. When the dispersant content is too low, the effect of improving the dispersibility is not significant, and when it is too high, the battery performance may be adversely affected thereby.

[0118] On the other hand, the loading amount of the positive electrode according to the present invention can be 350 mg / 25 cm 2 Up to 2000 mg / 25 cm 2 , preferably 400 mg / 25 cm 2 Up to 1700 mg / 25 cm 2 , more preferably 450 mg / 25 cm 2 Up to 1000 mg / 25 cm 2 When the positive electrode loading meets the above range, higher capacity characteristics can be achieved than conventional LFP batteries. At this time, the positive electrode loading refers to 25 cm 2 The weight of lithium iron phosphate active material contained in the area.

[0119] In addition, the porosity of the positive electrode can be about 25% to 60%, preferably 28% to 55%, more preferably 28% to 40%, even more preferably 28% to 35%, even more preferably 25% to 30%. When the formed positive electrode porosity is within the above range, both the energy density and the electrolyte wettability can be kept excellent. In the case of LFP batteries, because the particle size of the lithium iron phosphate active material as the positive electrode active material is small, the pore size in the positive electrode is small, so the electrolyte wettability is reduced, and it is expected that the formed positive electrode porosity is higher than the positive electrode porosity of the battery using other positive electrode active materials, but as the positive electrode porosity increases, there is a problem of reduced energy density. Therefore, it is necessary to properly control the positive electrode porosity so as to keep both the energy density and the electrolyte wettability excellent.

[0120] (2) Negative electrode

[0121] In the present invention, the negative electrode may be a negative electrode commonly used in the art, and, for example, may include a negative electrode current collector and a negative electrode active material layer on the negative electrode current collector.

[0122] For example, the negative electrode can be manufactured by coating a negative electrode slurry containing a negative electrode active material and optionally a binder and a conductive material onto a negative electrode collector and drying to form a negative electrode active material layer, followed by calendaring, or by casting the negative electrode slurry on a separate support and then laminating the film peeled off from the support on the negative electrode collector.

[0123] There is no particular limitation on the negative electrode current collector as long as it has high conductivity without causing chemical changes in the battery, and for example: copper, stainless steel, aluminum, nickel, titanium, fired carbon; copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc.; aluminum-cadmium alloy, etc. can be used. In addition, the thickness of the negative electrode current collector can generally be 3 μm to 500 μm, and as in the case of the positive electrode current collector, fine concavoconvexities can be formed on the surface of the current collector to improve the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam body, and a non-woven fabric body.

[0124] The negative electrode active material layer selectively contains a binder and a conductive material in addition to the negative electrode active material.

[0125] As the negative electrode active material, a compound that can reversibly intercalate and deintercalate lithium can be used. Specific examples thereof may include: carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber and amorphous carbon; (semi)metallic materials that can form alloys with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy or Al alloy; (semi)metallic oxides that can be doped and dedoped with lithium such as SiO x(0 < x < 2), SnO2, vanadium oxides, and lithium vanadium oxides; or composite materials containing (semi) metallic materials and carbonaceous materials such as Si-C composite materials or Sn-C composite materials, and any one of them or a mixture of two or more of them can be used. In addition, a lithium metal thin film can be used as the negative electrode active material. In addition, low-crystalline carbon, high-crystalline carbon, etc. can all be used as carbon materials. Representative examples of low-crystalline carbon can include soft carbon and hard carbon, and representative examples of high-crystalline carbon can include irregular, planar, flaky, spherical, or fibrous natural graphite or artificial graphite, condensated graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesophase carbon microbeads, mesophase pitch, and high-temperature sintered carbon such as coke derived from petroleum or coal tar pitch.

[0126] In addition, the binder and the conductive material can be the same as those described in the above description of the positive electrode.

[0127] (3) Separator

[0128] In the present invention, the separator is used to separate the negative electrode and the positive electrode and provide a movement path for lithium ions, and any separator can be used without particular limitation as long as it is a separator commonly used in secondary batteries. In particular, a separator with excellent moisture retention of the electrolyte and low resistance to ion movement in the electrolyte is preferred. Specifically, a porous polymer film can be used, such as a porous polymer film made of polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer; or a stacked structure having two or more layers thereof. In addition, typical porous non-woven fabrics can be used, such as non-woven fabrics formed of high-melting-point glass fibers, polyethylene terephthalate fibers, etc. In addition, a coated separator containing a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and it can be selectively used in a single-layer or multi-layer structure.

[0129] (4) Electrolyte

[0130] The electrolyte used in the present invention can be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten-type inorganic electrolyte, etc. that can be used to manufacture lithium secondary batteries, but is not limited thereto.

[0131] For example, the electrolyte can contain an organic solvent and a lithium salt.

[0132] As the organic solvent, any organic solvent can be used without particular limitation, as long as it can be used as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, as the organic solvent, 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), ethyl methyl carbonate (EMC), ethylene carbonate (EC) and propylene carbonate (PC); alcohol solvents such as ethanol and isopropanol, etc. Among the above substances, carbonate solvents are preferred, and more preferably a mixture of cyclic carbonates (such as ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge / discharge performance of the battery and low-viscosity linear carbonate compounds (such as ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) can be used.

[0133] As a lithium salt, any compound can be used without particular limitation, as long as it can provide the lithium ions used in the lithium secondary battery. Specifically, as a lithium salt, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. can be used. The lithium salt can be used in a concentration range of 0.1 M to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has suitable conductivity and viscosity, thereby showing excellent performance, and the lithium ions can be effectively moved.

[0134] In order to improve the life characteristics of the battery, suppress the decline in battery capacity, improve the discharge capacity of the battery, etc., in addition to the above-mentioned electrolyte components, the electrolyte may also contain additives. As additives, various electrolyte additives used in lithium secondary batteries can be used, for example, the additives may be: halogenated carbonate compounds such as fluoroethylene carbonate; nitrile compounds such as succinonitrile; sultone compounds such as 1,3-propane sultone and 1,3-propylene sultone; carbonate compounds such as vinylene carbonate or a combination thereof, but not limited thereto. At this time, the content of the additive may be 0.1 wt% to 10 wt%, preferably 0.1 wt% to 5 wt%, relative to the total weight of the electrolyte.

[0135] The lithium secondary battery of the present invention as described above has excellent charging capacity compared with the prior art. Specifically, based on the theoretical capacity of lithium iron phosphate (170 mAh / g), the first charging capacity measured after charging the battery to 3.7 V at 0.1C according to the lithium secondary battery of the present invention can be 93% to 100% of the theoretical capacity, preferably 93% to 98%, and more preferably 94% to 97%.

[0136] Specifically, based on the theoretical capacity of lithium iron phosphate (170 mAh / g), the first charge capacity measured after the lithium secondary battery according to the present invention is charged to 3.7 V at 0.1C can be 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.

[0137] <Method for Manufacturing Lithium Secondary Battery>

[0138] Next, a method of manufacturing a lithium secondary battery according to the present invention will be described.

[0139] The method for manufacturing a lithium secondary battery according to the present invention comprises the following steps:

[0140] (1) preparing a sample in which a lithium iron phosphate-based active material and MgO are mixed in a weight ratio of 70:30;

[0141] (2) measuring the AI ​​value defined by the following mathematical formula (1) by X-ray diffraction analysis of the sample;

[0142] (3) selecting a lithium iron phosphate active material whose AI value meets a preset range as the positive electrode active material;

[0143] (4) manufacturing a positive electrode comprising a selected positive electrode active material;

[0144] (5) manufacturing an electrode assembly including a positive electrode, a separator and a negative electrode; and

[0145] (6) The electrode assembly is housed in a battery case, and then the electrolyte is injected into the battery case.

[0146] Mathematical formula (1):

[0147] In the above mathematical formula (1),

[0148] is the peak area appearing at 2θ=43°±0.5° in the X-ray diffraction analysis diagram of a sample in which a lithium iron phosphate-based active material and MgO are mixed in a weight ratio of 70:30, and

[0149] It is the sum of the peak areas in the range of 2θ=15° to 36° in the X-ray diffraction analysis pattern of the sample.

[0150] (1) Sample preparation steps

[0151] First, a sample was prepared by mixing a lithium iron phosphate-based active material with MgO at a weight ratio of 70:30. When the sample had the composition as described above, the deviation of the measured value when AI was measured was minimal, and the reproducibility was excellent.

[0152] The sample is used for XRD analysis and is a mixture of an internal standard and a lithium iron phosphate-based active material as a measurement target material. In order to measure the amorphous content in the lithium iron phosphate-based active material, a 100% crystalline material should be used as the internal standard.

[0153] According to the study of the present inventors, it was found that when MgO was used as an internal standard substance, and when the internal standard substance was mixed at 30 wt % of the total weight of the sample, the deviation of the measured value was minimized and the reliability was excellent.

[0154] (2) Measuring AI steps

[0155] The samples prepared as above were subjected to X-ray diffraction analysis.

[0156] X-ray diffraction analysis can be performed using a bruker D8 Endeavor device. Specifically, depending on the amount of sample to be measured, the sample powder is placed in the central slot of the universal powder holder or the holder for a small amount of powder of the bruker D8 Endeavor device, the sample surface is prepared to be uniform by aligning the sample height with the edge of the holder with a slide, and then the fixed divergence slit is adjusted to 0.3 according to the sample size, and the 2θ area is measured every 0.016 degrees for 0.5 seconds. In addition, in order to compensate for the non-uniformity that may occur during sampling, the measurement is performed while rotating the holder at 15 RPM.

[0157] Then, the peak area appearing at 2θ=43°±0.5° of the X-ray diffraction pattern is obtained, that is, , and the sum of the peak areas appearing in the range of 2θ = 15° to 36°, that is, , and substitute it into formula (1) to calculate the AI ​​value.

[0158] Mathematical formula (1):

[0159] In the above mathematical formula (1),

[0160] is the peak area appearing at 2θ=43°±0.5° in the X-ray diffraction analysis diagram of a sample in which a lithium iron phosphate-based active material and MgO are mixed in a weight ratio of 70:30, and

[0161] It is the sum of the peak areas in the range of 2θ=15° to 36° in the X-ray diffraction analysis pattern of the sample.

[0162] (3) Positive electrode active material selection steps

[0163] Next, a lithium iron phosphate active material whose measured AI value satisfies a preset range is selected as the positive electrode active material. At this time, the preset range can be appropriately selected in consideration of the electrochemical performance of the LFP battery to be manufactured, and can be, for example, less than 0.28, preferably 0.20 to 0.28, more preferably 0.20 to 0.27, and most preferably 0.22 to 0.25. When a lithium iron phosphate compound satisfying the above AI value range is applied as the positive electrode active material, the initial capacity characteristics of the LFP battery are excellent.

[0164] (4) Positive electrode manufacturing steps

[0165] Next, a positive electrode including the selected positive electrode active material is manufactured.

[0166] At this time, in addition to applying a lithium iron phosphate-based active material whose AI value satisfies a preset range as a positive electrode active material, the positive electrode can be manufactured by a method for manufacturing a positive electrode generally known in the art. For example, the positive electrode can be manufactured by a method in which a positive electrode active material, a binder, and a conductive material are mixed to prepare a positive electrode slurry, and then the positive electrode slurry is coated on a positive electrode collector and dried to form a positive electrode active material layer, and then rolled.

[0167] On the other hand, specific types and specifications of the positive electrode active material, the binder, and the conductive material are the same as those described above, and thus a detailed description thereof is omitted.

[0168] (5) Electrode assembly manufacturing steps

[0169] Next, an electrode assembly including the positive electrode, separator, and negative electrode manufactured as described above is manufactured. The specific types and specifications of the negative electrode and separator are the same as described above, so a detailed description thereof is omitted.

[0170] The electrode assembly can be manufactured by stacking a positive electrode, a separator and a negative electrode in sequence, and there is no particular limitation on the shape of the electrode assembly, and the electrode assembly can be an electrode assembly generally known in the field of lithium secondary batteries such as a wound type, a stacked type and / or a stacked and folded electrode assembly.

[0171] (6) Secondary battery manufacturing steps

[0172] Next, a lithium secondary battery is manufactured by housing the electrode assembly in a battery case and then injecting an electrolyte therein.

[0173] At this time, as the battery case, a battery case generally known in the field of lithium secondary batteries such as a cylindrical, prismatic, or pouch-shaped battery case may be used without limitation, and the battery case is not particularly limited.

[0174] On the other hand, the specific type and specification of the electrolyte are the same as described above, and the injection of the electrolyte can be performed by an electrolyte injection method generally known in the field of lithium secondary batteries.

[0175] Preferred Embodiments

[0176] Hereinafter, the present invention will be described in more detail with reference to specific examples.

[0177] Experimental Example 1: Measurement of the Amorphous Content Index AI of Lithium Iron Phosphate Compounds

[0178] Four lithium iron phosphate active materials A to D obtained from different manufacturing batches of the same supplier were prepared. The lithium iron phosphate active material was LiFePO4 with an amorphous carbon coating. Each of the lithium iron phosphate active materials A to D was mixed with MgO at a weight ratio of 70:30 to prepare a sample. The sample was subjected to X-ray diffraction analysis to obtain an XRD pattern, and the AI ​​value of the following mathematical formula (1) was calculated using the obtained XRD pattern.

[0179] Mathematical formula (1):

[0180] at this time,

[0181] is the peak area appearing at 2θ=43°±0.5° in the X-ray diffraction analysis diagram of a sample in which a lithium iron phosphate-based active material and MgO are mixed in a weight ratio of 70:30, and

[0182] It is the sum of the peak areas in the range of 2θ=15° to 36° in the X-ray diffraction analysis pattern of the sample.

[0183] On the other hand, the XRD pattern was measured using D8 Endeavor of Bruker, and the measurement conditions were as follows.

[0184] Light source: Cu tube (Cu K α1, α2), wavelength = 1.5406 (α1), 1.5444 (α2)

[0185] X-ray generator power: 40 kV, 40 mA

[0186] Divergence slit: 0.5°

[0187] 2θ=10~90

[0188] Step size: 0.015°

[0189] Time / step: 1 second

[0190] Total measurement time: 90 minutes

[0191] The measurement results are shown in Table 1 below.

[0192]

[0193] Example 1

[0194] The positive electrode slurry was prepared by mixing 95 parts by weight of sample B 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 coated on an aluminum current collector with a thickness of 15 μm, dried, and then rolled to produce a 500 mg / 25 cm 2 And the positive electrode has a porosity of 29%.

[0195] On the other hand, a negative electrode slurry was prepared by adding 95 parts by weight of artificial graphite as a negative electrode active material, 3 parts by weight of SBR as a binder, 1 part by weight of CMC, and 1 part by weight of carbon black as a conductive material to distilled water. The negative electrode slurry was coated on a copper current collector having a thickness of 8 μm, dried, and then rolled to produce a 240 mg / 25 cm 2 And the porosity of the negative electrode is 29%.

[0196] The positive electrode and negative electrode manufactured as described above were stacked together with a polyethylene separator to manufacture an electrode assembly, and then the electrode assembly was placed in a battery case, followed by injection of an electrolyte (in which 1 M LiPF6 was dissolved in a solvent prepared by mixing ethylene carbonate: ethylmethyl carbonate: diethyl carbonate in a ratio of 1:1:1).

[0197] Example 2

[0198] A lithium secondary battery was manufactured in the same manner as in Example 1, except that Sample C was used instead of Sample B.

[0199] Example 3

[0200] A lithium secondary battery was manufactured in the same manner as in Example 1, except that Sample D was used instead of Sample B.

[0201] Comparative Example 1

[0202] A lithium secondary battery was manufactured in the same manner as in Example 1, except that Sample A was used instead of Sample B.

[0203] Experimental Example 2

[0204] Based on the theoretical capacity of lithium iron phosphate (170 mAh / g), the lithium secondary batteries manufactured in Examples 1 to 3 and Comparative Example 1 were charged to 3.7 V at 0.1 C to measure the first charge capacity. The measurement results are shown in the following table. Figure 1 and Table 2.

[0205]

[0206] like Figure 1 As shown in Table 2, the lithium secondary batteries of Examples 1 to 3 using lithium iron phosphate-based active materials B to D having an AI value of 0.28 or less exhibited capacity characteristics superior to the capacity characteristics of the lithium secondary battery having an AI value of greater than 0.28 of Comparative Example 1. Specifically, Examples 1 to 3 showed a theoretical capacity percentage at least 2% higher than that of Comparative Example 1.

[0207] In the battery field, such improvements are considered very significant. For example, when a battery product is included in an electric vehicle ("EV"), a 2% difference in capacity can result in a difference of at least 10 miles in the EV's driving range, which represents a significant practical advantage.

[0208] Experimental Example 3

[0209] Sample 1 was prepared by mixing the lithium iron phosphate active materials A and B of Experimental Example 1 with ZnO at a weight ratio of 70:30, and X-ray diffraction analysis was performed on Sample 1. Then, LiFePO4 (space group: Pnma, 62) and ZnO (space group: P6 3mc , 186), and the phases present in the sample in the region of 10 to 120° were subjected to Rietveld refinement analysis to measure the amorphous content in the lithium iron phosphate active material. At this time, each sample was analyzed three times, and the amorphous content in the lithium iron phosphate active material was calculated by fixing the ZnO content at 30 wt % and then using the relative ratio of the lithium iron phosphate active material content to it.

[0210] Sample 2 was prepared by mixing the lithium iron phosphate active materials A and B of Experimental Example 1 with MgO at a weight ratio of 70:30, and X-ray diffraction analysis was performed on Sample 2, and then Rietveld refinement analysis was performed on the phases present in the 10 to 120° region in the sample using the complete structural model of LiFePO4 (space group: Pnma, 62) and MgO (space group: Fm-3m, No, 225) to measure the amorphous content in the lithium iron phosphate active material. At this time, each sample was analyzed three times, and the amorphous content in the lithium iron phosphate active material was calculated by fixing the MgO content to 30% by weight and then using the relative ratio of the content of the lithium iron phosphate active material to it.

[0211] The AI ​​value of mathematical formula (1) was measured using an X-ray diffraction pattern obtained by performing X-ray diffraction analysis on Sample 2.

[0212] The measurement results are shown in the following [Table 3].

[0213]

[0214] As shown in [Table 3] above, the present invention shows that the deviation of AI with the number of measurements is very small and the reproducibility is excellent, but when the amorphous content is measured by the Rietveld refinement method, the deviation with the number of tests is very large, making it impossible to obtain a reliable value.

Claims

1. A lithium secondary battery, comprising: a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte, The positive electrode comprises a lithium iron phosphate active material, and the amorphous content index (AI) of the lithium iron phosphate active material defined by the following mathematical formula (1) is less than 0.28: Mathematical formula (1): In the above mathematical formula (1), is the peak area appearing at 2θ=43°±0.5° in the X-ray diffraction analysis diagram of a sample in which the lithium iron phosphate-based active material and MgO are mixed in a weight ratio of 70:30, and It is the sum of the peak areas within the range of 2θ=15° to 36° in the X-ray diffraction analysis diagram of the sample. 2 . The lithium secondary battery according to claim 1 , wherein the positive electrode comprises a lithium iron phosphate-based active material having an amorphous content index (AI) of 0.20 to 0.

27.

3. The lithium secondary battery according to claim 1, wherein the lithium iron phosphate-based active material comprises a lithium iron phosphate-based compound represented by the following [Chemical Formula 1]: [Chemical formula 1] Li 1-a [Fe 1-x M x ] 1-y NIGHT 4-b A b Wherein in the above 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 any one or more selected from S, Se, F, Cl and I, -0.5 <a<0.5,0≤x<1,-0.5<y<0.5,0≤b≤0.1。 4 . The lithium secondary battery according to claim 3 , wherein a molar ratio of Li to Fe and M (Li / (Fe+M)) of the lithium iron phosphate-based compound is 1.0 to 1.

1. 5 . The lithium secondary battery according to claim 3 , wherein a molar ratio of P to Fe and M (P / (Fe+M)) of the lithium iron phosphate compound is 1.01 to 1.

04. 6 . The lithium secondary battery according to claim 1 , wherein the lithium iron phosphate-based active material further comprises a conductive coating.

7. The lithium secondary battery according to claim 1, wherein the loading amount of the positive electrode is 350 mg / 25 cm 2 Up to 2000mg / 25 cm 2 . 8 . The lithium secondary battery according to claim 1 , wherein the positive electrode has a porosity of 25% to 60%.

9. The lithium secondary battery according to claim 1, wherein: The charge capacity measured after charging the lithium secondary battery to 3.7 V at 0.1 C is 93% to 100% of the theoretical capacity of the lithium iron phosphate-based active material.

10. A method for manufacturing a lithium secondary battery, the method comprising the following steps: preparing a sample in which a lithium iron phosphate-based active material and MgO are mixed in a weight ratio of 70:30; The AI ​​value represented by the following mathematical formula (1) is measured by X-ray diffraction analysis of the sample; Selecting a lithium iron phosphate active material whose AI value meets a preset range as a positive electrode active material; manufacturing a positive electrode comprising a selected positive electrode active material; manufacturing an electrode assembly including the positive electrode, a separator and a negative electrode; and The electrode assembly is housed in a battery case, and then an electrolyte is injected into the battery case: Mathematical formula (1): In the above mathematical formula (1), is the peak area appearing at 2θ=43°±0.5° in the X-ray diffraction analysis diagram of a sample in which the lithium iron phosphate-based active material and MgO are mixed in a weight ratio of 70:30, and It is the sum of the peak areas within the range of 2θ=15° to 36° in the X-ray diffraction analysis diagram of the sample. The method according to claim 10 , wherein the preset range is less than 0.

28. The method according to claim 10 , wherein the preset range is 0.20 to 0.

27. 13 . The lithium secondary battery according to claim 3 , wherein the lithium iron phosphate compound is LiFePO 4 provided with a conductive coating.

14. A positive electrode, comprising a lithium iron phosphate active material, wherein the lithium iron phosphate active material has an amorphous content index (AI) of 0.28 or less as defined by the following mathematical formula (1): Mathematical formula (1): In the above mathematical formula (1), is the peak area appearing at 2θ=43°±0.5° in the X-ray diffraction analysis diagram of a sample in which the lithium iron phosphate-based active material and MgO are mixed in a weight ratio of 70:30, and It is the sum of the peak areas within the range of 2θ=15° to 36° in the X-ray diffraction analysis diagram of the sample.

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