Lithium secondary battery comprising amorphous positive electrode active material

Abstract

Provided is a secondary battery having excellent electrochemical stability and excellent reversibility in lithium ion intercalation reaction and conversion reaction. According to one aspect of the present invention, provided is a secondary battery comprising a transition metal-based lithium compound positive electrode, characterized in that there is no peak in the range of 3.8 V to 4.0 V in the differential capacity (dQ / dV)-voltage (V) curve of the secondary battery.

Description

Technical Field

[0001] The present invention relates to a lithium secondary battery including an amorphous positive electrode active material, and more particularly to a high-capacity lithium secondary battery having excellent reversibility of lithium ion insertion-conversion reaction. Background Art

[0002] In recent years, with the rapid development of the miniaturization of electronic devices, there has been an increasing demand for a secondary battery that is small, lightweight, and has a high energy density as a driving power source. At the same time, due to environmental protection considerations, the development and practical application of electric vehicles and hybrid electric vehicles have progressed, resulting in a significant increase in the demand for lithium ion secondary batteries for large-scale use with excellent storage characteristics. Against this background, lithium ion secondary batteries with a large charge-discharge capacity as a major advantage have attracted much attention.

[0003] Traditionally, positive electrode active materials suitable for 4V-class high-energy lithium ion secondary batteries include: LiMn2O4 having a spinel structure, LiMnO2 having a zigzag layered structure, LiCoO2 and LiNiO2 having a layered rock salt structure, etc. Among them, lithium ion secondary batteries using LiNiO2 have received particular attention due to their high charge-discharge capacity. However, these materials have poor thermal stability and cycle performance during charging, and there is an urgent need to further improve their characteristics. Summary of the Invention

[0004] An object of the present invention is to provide a lithium secondary battery including an amorphous positive electrode active material, which can stably achieve the reversibility of lithium ion insertion-conversion reaction.

[0005] Another object of the present invention is to provide a lithium secondary battery having excellent electrochemical stability.

[0006] Still another object of the present invention is to provide a lithium secondary battery having long-term cycle stability.

[0007] Another object of the present invention is to provide a lithium secondary battery including a high-capacity electrode.

[0008] The objects of the present invention are not limited to the above, and other objects and advantages not mentioned can be understood from the following description and will be more clearly defined by the embodiments of the present invention. In addition, the objects and advantages of the present invention can be achieved by the technical means and combinations thereof described in the claims.

[0009] To achieve the above object, the present invention provides a secondary battery, characterized in that it includes a positive electrode containing a transition metal-based lithium compound; and there is no peak in the differential capacity (dQ / dV)-voltage (V) curve of the secondary battery in the range of 3.8 to 4.0V.

[0010] In an embodiment of the present invention, the transition metal-based lithium compound may include an amorphous phase.

[0011] In an embodiment of the present invention, the differential capacity (dQ / dV)-voltage (V) curve of the secondary battery may be obtained through the first to fifth cycles under conditions of a rate of 0.01C to 0.5C and 25°C.

[0012] In an embodiment of the present invention, the transition metal-based lithium compound may include a compound represented by the following general formula 1:

[0013] [General formula 1]

[0014] Li x M 2-x SO4A

[0015] In general formula 1: M is one or more elements selected from Fe, Mg, Ni, Co, Cr, Ti, Cu, Ca, Zn, Zr, Nb, Mo, Sr, Sb, W, and Bi; A is a halogen atom; and 0.5 ≤ x ≤ 1.5.

[0016] In an embodiment of the present invention, the transition metal-based lithium compound may include LiFeSO4F.

[0017] In an embodiment of the present invention, the particle size (D 50 ) of the transition metal-based lithium compound may be 0.5 to 3.0 μm.

[0018] In an embodiment of the present invention, the transition metal-based lithium compound may include an amorphous matrix and crystal grains.

[0019] In an embodiment of the present invention, the crystal grains may include a plurality of crystal grains, and the average size of each crystal grain may be 4 to 8 nm.

[0020] In an embodiment of the present invention, the differential capacity (dQ / dV)-voltage (V) curve of the secondary battery may have a first peak in the range of 2.5 to 2.7V.

[0021] In an embodiment of the present invention, the differential capacity (dQ / dV)-voltage (V) curve of the secondary battery may have a second peak in the range of 2.1 to 2.3V.

[0022] In an embodiment of the present invention, a secondary battery may further be provided, which includes a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte.

[0023] It should be noted that the above technical solutions do not exhaust all the features of the present invention. The various features of the present invention and the advantages and effects brought by them can be understood in more detail through the following specific embodiments.

[0024] According to one aspect of the present invention, excellent cycling performance with electrochemical stability can be achieved without additional processing steps.

[0025] According to another aspect of the present invention, even in the case of significant bond reorganization expected during the conversion reaction, stable maintenance of the lithium-ion insertion reaction in the electrode can be achieved without significant performance degradation, thereby providing a secondary battery with stable reversibility.

[0026] In addition to the above effects, the specific effects of the present invention will be described in detail in the following embodiments in combination with specific technical solutions. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 X-ray diffraction (XRD) patterns of LiFeSO4F after ball milling in Synthesis Example 1, LiF-FeSO4 before ball milling in Synthesis Example 1, LiF, and FeSO4

[0028] Figure 2 Fe K-edge hard X-ray absorption spectroscopy (XAS) spectra of LiFeSO4F after ball milling in Synthesis Example 1 and FeSO4

[0029] Figure 3 Radial distribution plots around Fe of LiFeSO4F after ball milling in Synthesis Example 1 and FeSO4

[0030] Figure 4 F K-edge and FeL3 edg spectra of LiFeSO4F in Synthesis Example 1 obtained by soft X-ray absorption spectroscopy (SXAS) analysis

[0031] Figure 5 Electron diffraction patterns of LiFeSO4F in Synthesis Example 1 and various reference substances (Ref)

[0032] Figure 6 Transmission electron microscopy (TEM) images of LiFeSO4F in Synthesis Example 1

[0033] Figure 7 Charge-discharge curves of the half-cell in Preparation Example 2

[0034] Figure 8 Charge-discharge curves of the half-cell in Comparative Example 1 at 60 °C

[0035] Figure 9 Charge-discharge curves of the half-cell in Comparative Example 2 at 60 °C

[0036] Figure 10 Differential capacity curves of the half-cell in Preparation Example 2

[0037] Figure 11a are the charge-discharge curves of the half-cell of Example 1 over time in the high voltage region (2.2 - 4.7 V) and the low voltage region (1.5 - 2.2 V).

[0038] Figure 11b is the Fe K-edge XANES spectrum of the half-cell of Example 1 during the charging process in the high voltage region.

[0039] Figure 11c is the Fe K-edge XANES spectrum of the half-cell of Example 1 during the discharging process in the high voltage region.

[0040] Figure 11d is the Fe K-edge XANES spectrum of the half-cell of Example 1 during the charging process in the low voltage region.

[0041] Figure 11e is the Fe K-edge XANES spectrum of the half-cell of Example 1 during the discharging process in the low voltage region.

[0042] Figure 11f are the Fe K-edge EXAFS spectra of Fe metal (Ref), in the original state, the half-cell of Example 1 discharged to 1.5 V and charged to 2.2 V, relative to the Fe metal standard.

[0043] Figure 11g is the TEM image of the Fe metal contained in the discharged-state lithium compound matrix.

[0044] Figure 11h are the F K-edge spectra of the half-cell of Example 1 in the fully charged, half-discharged, and fully discharged states.

[0045] Figure 11i are the infrared spectroscopy (IR) spectra of the half-cell of Example 1 in the fully charged, half-discharged, and fully discharged states.

[0046] Figure 12 are the charge-discharge curves of the two-stage reaction (intercalation and conversion reaction) of α-LiFeSO4F in the half-cell of Example 1.

[0047] Figure 13 is the evaluation result of the electrochemical stability of the half-cell of Example 1 during long-term charge-discharge cycling.

[0048] Figure 14a are the charge-discharge curves of the half-cell of Example 1 at current densities of 0.02 A / g, 0.05 A / g, 0.1 A / g, 0.2 A / g, 0.5 A / g, 1 A / g, and 2 A / g.

[0049] Figure 14bIt shows the relationship between the cycle number and discharge capacity of the half-cell of Example 1 at temperatures of 60 °C and current densities of 0.02 A / g, 0.05 A / g, 0.1 A / g, 0.2 A / g, 0.5 A / g, 1 A / g, and 2 A / g. Detailed implementation mode

[0050] In this specification, unless the context clearly indicates otherwise, a singular expression includes a plural meaning.

[0051] In this specification, the numerical range indicated by the term "to" includes the numerical values recited before and after this term as the lower limit value and the upper limit value. When the upper and lower limits of any numerical range disclose multiple numerical values respectively, the numerical range disclosed in this specification should be understood as: any numerical range formed by taking any one of the multiple lower limit values and any one of the multiple upper limit values as the lower limit value and the upper limit value respectively.

[0052] In this specification, the meaning of "no peak" in the differential capacity (dQ / dV)-voltage (V) curve, according to the conventional understanding of those skilled in the art, refers to the phenomenon that no peak is observed in the corresponding voltage range.

[0053] According to one aspect of the present invention, there is provided a secondary battery including a cathode containing a transition metal-based lithium compound, characterized in that: the differential capacity (dQ / dV)-voltage (V) curve of the secondary battery has no peak in the range of 3.8 V to 4.0 V. If the differential capacity curve of the secondary battery shows a peak in the range of 3.8 V to 4.0 V in this voltage range, it may cause the rechargeable battery to be difficult to exhibit electrochemical activity in the low voltage range, thereby reducing the overall capacity. In addition, traditional electrode materials based only on conversion reactions have the following problems: due to side effects such as significant volume changes, compositional inhomogeneity, transition metal dissolution, and the formation of a cathode electrolyte interface film (CEI) caused by electrolyte decomposition during the conversion reaction, it is difficult to achieve stable capacity retention. Although these problems can be partially alleviated by additional processes such as surface protective layers, three-dimensional anode structures, or advanced electrolyte systems, one aspect of the present invention is that: without additional processing processes, excellent electrochemical stability cycle performance can be achieved through an amorphous-phase cathode active material with a layered structure. Another aspect of the present invention is that: even during the conversion reaction, significant bond reorganization occurs, and the stability of the lithium-ion insertion reaction in the electrode can still be maintained without obvious performance degradation, thereby providing a secondary battery with stable reversibility.

[0054] The following details the composition of the present invention.

[0055] Positive Electrode and Secondary Battery Comprising the Same

[0056] The secondary battery described in the present invention includes a cathode. Specifically, the cathode contains a transition metal-based lithium compound.

[0057] The differential capacity (dQ / dV)-voltage (V) curve is a curve obtained from time-voltage data acquired through constant current testing, where the differential value of capacity with respect to voltage, dQ / dV, is plotted against voltage (V). The factors affecting this curve include: the composition of the positive electrode active material, temperature, discharge rate, charging method, charging time, magnitude of the charging current, etc.

[0058] The differential capacity (dQ / dV)-voltage (V) curve of the secondary battery of the present invention has no peak within the range of 3.8 V to 4.0 V (specifically 3.9 V).

[0059] The differential capacity (dQ / dV)-voltage (V) curve of the secondary battery can be obtained through 1 to 5 cycles of testing under the conditions of a rate of 0.01 C to 0.5 C and a temperature of 25°C.

[0060] The transition metal-based lithium compound related to the present invention is a compound in which the elements constituting the positive electrode active material are arranged in a layered structure, and a structure allowing lithium ions to be intercalated or deintercalated during charge and discharge can be formed between the layered structures. Specifically, the lithium ions can be deintercalated from the lattice during charging of the secondary battery and reintercalated into the lattice during the discharge process of the secondary battery.

[0061] According to an embodiment of the present invention, the transition metal-based lithium compound may include an amorphous phase. It should be noted that the amorphous phase refers to a phase without a crystallized structure. Specifically, by including the amorphous phase, the transition metal-based lithium compound can achieve excellent electrochemical stability in terms of cycle performance, and at the same time, even during a conversion reaction where significant bond rearrangement is expected, the lithium ion intercalation reaction of the electrode can remain stable without significant performance degradation.

[0062] Specifically, the amorphous characteristics of the transition metal-based lithium compound contribute to maintaining a good lithium ion intercalation reaction after the secondary battery undergoes repeated conversion reactions. If the transition metal-based lithium compound contains LiFeSO4F, due to the amorphous characteristics, the substitution / diffusion of Fe 2+ is more easily achieved compared to the crystalline phase. Therefore, the substitution / diffusion ability of Fe 2+ is improved, which can optimize the kinetic behavior of the electrode, thereby promoting the efficient progress of the reversible lithium ion intercalation reaction - conversion reaction.

[0063] For example, as a method for analyzing the crystal structure of the transition metal-based lithium compound, X-ray diffraction (XRD) analysis or X-ray absorption spectroscopy (XAS) analysis can be used.

[0064] According to another embodiment of the present invention, the transition metal-based lithium compound may include a compound represented by the following general formula 1:

[0065] [General formula 1]

[0066] Li x M 2-x SO4A, in the general formula 1, M is one or more elements selected from the group consisting of Fe, Mg, Ni, Co, Cr, Ti, Cu, Ca, Zn, Zr, Nb, Mo, Sr, Sb, W, and Bi; A is a halogen atom; and 0.5 ≤ x ≤ 1.5. Specifically, A is a halogen atom that can combine with a lithium ion to form a salt structure.

[0067] For example, the transition metal-based lithium compound may include LiFeSO4F. When LiFeSO4F is used as the transition metal-based lithium compound, even after the secondary battery undergoes multiple conversion reactions, a good lithium ion insertion reaction can be maintained through high reversibility and stability. For example, the lithium ion insertion reaction and the conversion reaction can proceed according to the following reaction formulas 1 and 2:

[0068] [Reaction formula 1]

[0069] a-FeSO4F + Li + + e - → a-LiFeSO4F (lithium ion insertion reaction)

[0070] [Reaction formula 2]

[0071] a-LiFeSO4F + 2Li + + 2e - → Fe + LiSO3F + Li2O (conversion reaction)

[0072] According to another embodiment of the present invention, the particle size (D 50 ) of the transition metal-based lithium compound may be 0.5 to 3.0 μm, 0.7 to 2.8 μm, 0.8 to 2.7 μm, 0.9 to 2.6 μm, or 1.0 to 2.5 μm. When the particle size of the transition metal-based lithium compound satisfies the above numerical range, the dispersibility of the positive electrode active material in the positive electrode slurry can be significantly improved.

[0073] According to another embodiment of the present invention, the transition metal-based lithium compound may include an amorphous matrix and crystal grains. Specifically, the amorphous matrix refers to a non-crystalline structure phase formed inside the structure of the transition metal-based lithium compound, and the crystal grains include particles having a crystalline structure.

[0074] According to another embodiment of the present invention, the crystal grains may include a plurality of particles. The average particle size of each particle may be 8 nm or less, 4 to 8 nm, 5 to 7 nm, or 6 to 7 nm. Specifically, when the average particle size of each particle satisfies the above numerical range, the progress of the reversible lithium ion insertion reaction-conversion reaction can be effectively promoted.

[0075] According to another embodiment of the present invention, the differential capacity (dQ / dV)-voltage (V) curve of the secondary battery may have a first peak in the range of 2.5 to 2.7 V, specifically, may have a first peak at 2.6 V. Here, the first peak refers to the peak during the charging process of the differential capacity (dQ / dV)-voltage (V) curve.

[0076] According to another embodiment of the present invention, the differential capacity (dQ / dV)-voltage (V) curve of the secondary battery may have a second peak in the range of 2.1 to 2.3 V, specifically, may have a second peak at 2.2 V. Here, the second peak refers to the peak during the discharging process of the differential capacity (dQ / dV)-voltage (V) curve.

[0077] According to another embodiment of the present invention, the reversible capacity of the secondary battery may be 360 mAh / g or more. Specifically, the reversible capacity refers to the value obtained by subtracting the irreversible capacity from the charging amount of the positive electrode active material.

[0078] According to another embodiment of the present invention, the capacity retention rate of the secondary battery can be maintained at 90% or more at 25 °C after 200 cycles; or 98% or more at 60 °C. In addition, the capacity retention rate of the secondary battery refers to the percentage value of the maximum capacity of the secondary battery relative to the designed capacity.

[0079] According to another embodiment of the present invention, a method for preparing the positive electrode active material can be provided.

[0080] The method for preparing the positive electrode active material according to the present invention may include:

[0081] (S1) Prepare a precursor containing a lithium salt and an anhydrous transition metal sulfate; and

[0082] (S2) Crush the precursor and then perform heat treatment.

[0083] If an aqueous transition metal sulfate is used instead of an anhydrous transition metal sulfate in step (S1), the finally prepared positive electrode active material may not be able to form an amorphous phase. This will lead to the problem that the reversible stability of the lithium ion insertion reaction - conversion reaction cannot be fully improved. According to another embodiment of the present invention, by using an anhydrous transition metal sulfate as a precursor, a positive electrode active material with an amorphous phase can be effectively formed.

[0084] For example, the lithium salt may include any one selected from the group consisting of LiF, LiCl, and combinations thereof, and specifically may include LiF.

[0085] For example, the transition metal in the anhydrous transition metal sulfate may include one or more selected from Fe, Mg, Ni, Co, Cr, Ti, Cu, Ca, Zn, Zr, Nb, Mo, Sr, Sb, W, and Bi.

[0086] According to requirements, the precursor may further include graphite.

[0087] In step (S2), various comminution devices commonly used in the art can be used to comminute the precursor, and specifically, a ball mill can be used. Step (S2) may include comminuting the precursor at a rotation speed of 300 to 500 rpm for 30 to 50 hours.

[0088] In step (S2), the step of heat - treating the comminuted material may include heating at a heating rate of 1 to 3 °C / min at 200 to 400 °C for 60 to 80 hours. Specifically, when the combination of the anhydrous transition metal sulfate and the heat - treatment conditions is satisfied, a positive electrode active material with an amorphous phase can be prepared.

[0089] Separator

[0090] The separator described in the present invention may include a porous substrate or a glass fiber filter paper.

[0091] The separator described in the present invention is a porous structure with high electrolyte resistance and fine pore diameter, which can isolate the negative electrode and the positive electrode in an electrically insulating state to prevent short - circuit, and at the same time provide a transmission path for lithium ions.

[0092] The constituent material of the porous substrate can be an organic material or an inorganic material with electrical insulation properties, and there is no particular limitation. For example, the porous substrate may include at least one selected from the group consisting of polyolefins, polyethylene terephthalate, polybutylene terephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene ether, polyphenylene sulfide, and polyethylene naphthalene, and specifically may include polyolefins. Polyolefins not only have excellent coating properties but also can increase the proportion of the electrode active material layer in the battery by reducing the thickness of the separator, thereby increasing the capacity per unit volume. Specifically, the weight average molecular weight (Mw) of the polyolefin may be 100,000 to 500,000 g / mol. If the weight average molecular weight is below this numerical range, it may be difficult to ensure sufficient mechanical properties; if it exceeds this range, it may cause the thermal shut-down function to fail or molding difficulties. The thermal shut-down function refers to the function that when the temperature of the secondary battery rises, the thermoplastic resin melts to seal the pores of the porous substrate, thereby blocking ion movement and preventing thermal runaway of the battery.

[0093] The thickness of the porous substrate may be, for example, 3 to 50 μm or 4 to 15 μm. If the thickness is below this numerical range, the conductive barrier function may not be fully realized; if it exceeds this range, the separator resistance may increase excessively.

[0094] The average diameter of the pores contained in the porous substrate may be, for example, 10 to 100 nm. These pores are arranged in a mutually connected network structure so that gas or liquid can penetrate from one side of the porous substrate to the other side.

[0095] The separator according to the present invention may further include a coating provided on at least one surface of the porous substrate.

[0096] The coating according to the present invention can improve the mechanical strength and heat resistance of the separator for secondary batteries and enhance the ionic conductivity in the secondary batteries.

[0097] The coating according to the present invention may include a binder polymer and inorganic particles.

[0098] The binder polymer according to the present invention can connect and stably fix inorganic particles. The binder polymer can be, for example, mixed and used with one or more selected from the following group: Polyvinylidene fluoride-co-hexafluoropropylene, Polyvinylidene fluoride-co-trichloroethylene, Polymethylmethacrylate, Polyacrylonitrile, Polyvinylpyrrolidone, Polyvinylacetate, Poly(ethylene-co-vinyl acetate), Polyethylene oxide, Cellulose acetate, Cellulose acetate butyrate, Cellulose acetate propionate, Cyanoethylpullulan, Cyanoethylpolyvinylalcohol, Cyanoethylcellulose, Cyanoethylsucrose, pullulan, Carboxyl methylcellulose, Acrylonitrile-styrene butadienecopolymer, Polyimide, and Styrene-butadienerubber.

[0099] The inorganic particles according to the present invention can help improve the mechanical strength and heat resistance of the separator for secondary batteries. Specifically, there is no particular limitation as long as the inorganic particles are electrochemically stable. In other words, the inorganic particles that can be used in the present invention have no particular limitation as long as they do not undergo oxidation and / or reduction reactions within the operating voltage range of the applicable secondary battery (for example, 0 to 5V based on Li / Li + ).

[0100] For example, when using inorganic particles with a high dielectric constant, it can increase the ionic conductivity of the electrolyte by promoting the dissociation of electrolyte salts (such as lithium salts) in the liquid electrolyte. For the above reasons, the inorganic particles can be inorganic particles with a dielectric constant ≥ 5, inorganic particles with lithium ion conduction ability, or a mixture thereof.

[0101] The inorganic particles with a dielectric constant ≥ 5 can be one or a mixture of two or more selected from the following group: Al2O3, SiO2, ZrO2, AlO(OH), Al(OH)3, Mg(OH)2, BaSO4, TiO2, BaTiO3, Pb(Zr x Ti 1-x )O3 (PZT, where 0 < x < 1), Pb 1-x La x Zr 1-y Ti y O3 (PZT, where 0 < x < 1), Pb(Mg 1 / 3 Nb 2 / 3 )O 3-x PbTiO3 (PMN-PT, where 0 < x < 1), HfO2, SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO and SiC.

[0102] The inorganic particles with lithium ion conduction ability can be one or a mixture of two or more selected from the following group: lithium phosphate (Li3PO4), lithium titanium phosphate (Li x Ti y (PO4)3, 0 < x < 2, 0 < y < 3), lithium aluminum titanium phosphate (Li x Al y Ti z (PO4)3, 0 < x < 2, 0 < y < 1, 0 < z < 3), (LiAlTiP) x O y -based glass (0 < x < 4, 0 < y < 13), lithium lanthanum titanate (Li x La y TiO3, 0 < x < 2, 0 < y < 3), lithium germanium thiophosphate (Li x Ge y P z S w , 0 < x < 4, 0 < y < 1, 0 < z < 1, 0 < w < 5), lithium nitride (Li x N y , 0 < x < 4, 0 < y < 2), SiS2-based glass (Li x Si y S z , 0 < x < 3, 0 < y < 2, 0 < z < 4) and P2S5-based glass (Lix P y S z , 0 < x < 3, 0 < y < 3, 0 < z < 7).

[0103] For example, the average particle size (D 50 ) of the inorganic particles may be from 1 nm to 10 μm to form a coating with a uniform thickness and ensure an appropriate porosity; specifically, it may be from 10 nm to 2 μm, and more specifically from 50 nm to 1 μm. The "average particle size (D 50 )" refers to the particle size corresponding to the 50% quantile in the cumulative particle number distribution based on the particle size. This average particle size can be measured by the laser diffraction method. Specifically, after dispersing the powder to be measured in a dispersion medium, it is introduced into a commercially available laser diffraction particle size analyzer (such as Microtrac S3500), and the particle size distribution is calculated by measuring the change in the diffraction pattern generated when the particles pass through the laser beam due to the difference in particle size.

[0104] According to another embodiment of the present invention, the weight ratio of the inorganic particles to the binder polymer (inorganic particles: binder polymer) may be from 50:50 to 99:1, specifically from 70:30 to 95:5. If the content ratio of the inorganic particles relative to the binder polymer is lower than this numerical range, due to the increase in the content of the binder polymer, the improvement in the thermal safety of the separator may decrease, and at the same time, the reduction in the voids formed between the inorganic particles will lead to a decrease in the pore size and porosity, thereby causing deterioration of the final battery performance; if it exceeds this range, the peel resistance of the coating may be weakened due to too little content of the binder polymer.

[0105] For example, the thickness of the coating may be from 0.1 to 10 μm, specifically from 1 to 3 μm, and more specifically from 1.4 to 1.6 μm. When the coating thickness meets this numerical range, it can not only improve the insulation and thermal stability of the separator but also enhance the battery energy density at the same time.

[0106] For example, the packing density of the coating may be from 0.1 to 20 g / cm 3 , specifically from 0.5 to 12 g / cm 3 , and more specifically from 1 to 3 g / cm 3 . When the packing density of the coating meets this numerical range, it is beneficial to the lithium ion permeability and can maintain the heat resistance of the separator at an appropriate level. It should be noted that the packing density refers to the density of the coating loaded within a height range of 1 μm on a unit area (m 2 ) of the porous substrate.

[0107] Negative Electrode

[0108] The negative electrode according to an embodiment of the present invention may include lithium metal.

[0109] The negative electrode according to another embodiment of the present invention may include a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector. The negative electrode active material layer may include a negative electrode active material, a conductive agent, and a negative electrode binder.

[0110] The negative electrode current collector is used to transfer external electrons to the negative electrode active material to initiate an electrochemical reaction, or as a channel for receiving electrons from the negative electrode active material and conducting them to the outside. For example, the negative electrode current collector may be made of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, or a material surface-treated with carbon, nickel, titanium, silver, etc. on the surface of aluminum or stainless steel. Specifically, transition metals with good adsorption to carbon such as copper and nickel may be used as the current collector. For example, the thickness of the negative electrode current collector may be 6 μm to 20 μm, but this thickness range is not a limiting condition.

[0111] The negative electrode active material generates electrical energy by storing or releasing lithium ions. For example, the negative electrode active material may include at least one of silicon-based active material particles and graphite-based active material particles.

[0112] The silicon-based active material particles may be selected from one or more of the following groups: Si, SiOx (0 < x ≤ 2), Si-C composites, and Si-Y alloys (Y is any element selected from alkali metals, alkaline earth metals, transition metals, Group 13 elements, Group 14 elements, and rare earth elements).

[0113] The graphite-based active material particles may include one or more of the following: artificial graphite, natural graphite, graphitized carbon fiber, and graphitized mesophase carbon microspheres.

[0114] The content of the negative electrode active material may be 80 to 97% by weight based on all solid components in the negative electrode active material layer.

[0115] The conductive agent is used to improve the conductivity between active material particles in the electrode or between the active material particles and the metal current collector, and to prevent the binder from acting as an insulator. For example, the conductive agent may be a mixture of one or two or more conductive materials selected from the following groups: graphite, carbon black, carbon fiber, metal fiber, metal powder, conductive whisker, conductive metal oxide, activated carbon, and polyphenylene derivatives; more specifically, it may be a mixture of one or two or more conductive materials selected from the following groups: natural graphite, artificial graphite, super-p, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, denka black, aluminum powder, nickel powder, zinc oxide, potassium titanate, and titanium oxide.

[0116] The negative electrode binder is used to inhibit the separation between negative electrode active material particles or between the negative electrode and the current collector. The negative electrode binder can adopt polymer materials commonly used in the preparation of electrodes in the art, including but not limited to: poly(vinylidene fluoride co-hexafluoropropylene), poly(vinylidene fluoride-co-trichloroethylene), poly(methylmethacrylate), poly(ethylhexylacrylate), poly(butylacrylate), poly(acrylonitrile), poly(vinylpyrrolidone), poly(vinyl acetate), poly(ethylene-co-vinyl acetate), poly(ethylene oxide), polyacrylate, celluloseacetate, cellulose acetate butyrate, celluloseacetate propionate, cyano ethyl pullulan, cyanoethyl poly(vinylalcohol), cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, etc., but not limited thereto.

[0117] Electrolyte

[0118] The electrolyte according to the present invention may contain a solvent and a lithium salt.

[0119] The solvent in the present invention can be one or a mixture of two or more selected from the following group: propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), γ-butyrolactone (GBL), fluoroethylene carbonate (FEC), methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, amyl acetate, methyl propionate, ethyl propionate, ethyl propionate and butyl propionate.

[0120] The lithium salt in the present invention can contain, for example, NO 3- , F - , Cl - , Br - , I - , PF6 - and other anions.

[0121] The electrolyte in the present invention can further contain additives, specifically, compounds containing fluorine atoms. For example, the additive can be one or more selected from the group consisting of LiBF4, LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) and LiFSI (lithium bis(fluorosulfonyl)imide). Among them, LiTFSI and LiFSI can form a passivation film rich in lithium fluoride (LiF) on the surface of lithium metal through corrosion, thereby improving the lithium ion conductivity.

[0122] Application

[0123] According to another embodiment of the present invention, there can be provided any one of a battery module including the secondary battery as a unit cell, a battery pack including the battery module, and a device powered by the battery pack. Specific examples of the device include, but are not limited to: power tools driven by an electric motor; electric vehicles including electric vehicles (EV), hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), etc.; electric two-wheel vehicles including electric bicycles (E-bike) and electric scooters (E-scooter); electric golf carts; power storage systems, etc.

[0124] The present invention will be described in detail below through examples so that those with ordinary knowledge in the technical field can easily implement the present invention. However, these examples are only for illustration, and the scope of the rights of the present invention is not limited to the following content.

[0125] [Synthesis Example: Synthesis of Cathode Active Material]

[0126] [Synthesis Example 1: Synthesis of α-LiFeSO4F]

[0127] FeSO4 was prepared by dehydrating FeSO4·7H2O (Sigma-Aldrich) at 300 °C for 12 hours under an argon atmosphere. Subsequently, LiF and FeSO4 mixed in a 1:1 molar ratio and 20 wt% graphite (Bay Carbon Inc.) were placed in the ball mill (Pulverise tte 5; Fritsch) ball mill jar and sealed inside a glove box filled with argon (H2O, O2 < 1 ppm). To prevent high-temperature side reactions, the powder in the ball mill jar was ball milled at 400 rpm for 48 hours, with a 5-minute break every 30 minutes. The ball milled particles were heated at a heating rate of 2 °C / min at 300 °C for 72 hours to finally synthesize amorphous LiFeSO4F (a-LiFeSO4F).

[0128] <Comparative Synthesis Example 1: Synthesis of Triclinic LiFeSO4F>

[0129] To prepare triclinic (Tavorite phase) LiFeSO4F, FeSO4·H2O was obtained by dehydrating FeSO4·7H2O (Sigma-Aldrich) in EMI-TFSI (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) ionic liquid at 110 °C for 2 - 3 hours. After ball milling FeSO4·H2O and LiF in a 1:1 molar ratio for 11 minutes, they were heated at a heating rate of 2 °C / min at 300 °C for 72 hours to finally synthesize triclinic LiFeSO4F.

[0130] <Comparative Synthesis Example 2: Synthesis of Monoclinic LiFeSO4F>

[0131] To prepare monoclinic (Triplite phase) LiFeSO4F, LiF and the above-mentioned FeSO4·H2O were ball milled in absolute ethanol in a 1.05:1 molar ratio for 2 hours and dried in vacuum at 60 °C for 12 hours. The dried particles were heated at 450 °C for 45 minutes and then continued to be heated for 90 minutes to finally synthesize monoclinic LiFeSO4F.

[0132] [Experimental Example 1: XRD Pattern Analysis]

[0133] Figure 1 The X-ray diffraction (XRD) patterns of LiFeSO4F in Synthesis Example 1 after ball milling, the LiF-FeSO4 mixture before ball milling in Synthesis Example 1, and individual LiF and FeSO4 are shown. Using D8 ADVANCE 2020 (Bruker) equipped with a Cu Kα radiation source Data was collected at a scan speed of 0.15 ° / min in the 2θ range of 15° - 70° with a step size of 0.005°.

[0134] Refer to Figure 1It can be seen that the mixture containing the crystalline LiF and FeSO4 precursor turns into an amorphous state after ball milling.

[0135] [Preparation Example 1: Preparation of the positive electrode]

[0136] <Example 1: Positive electrode containing the positive electrode active material of Synthesis Example 1>

[0137] Preparation of Positive Electrode Slurry:

[0138] The positive electrode active material (α-LiFeSO4F) of Synthesis Example 1, polyacrylonitrile (PAN), and conductive carbon (Super P; Timcal, Switzerland) were mixed at a weight ratio of 7:2:1, and N-methyl-2-pyrrolidone (NMP, 99.5%; Sigma Aldrich) was added to make a slurry.

[0139] Coating the positive electrode slurry on the current collector to prepare a positive electrode:

[0140] The slurry was cast on one side of an aluminum foil at a coating amount of 2.7 - 3.0 mg / cm 2 and dried in vacuum at 60°C to obtain a positive electrode with a total thickness of 32 μm.

[0141] <Comparative Example 1: Positive electrode containing the positive electrode active material (triclinic phase) of Comparative Synthesis Example 1>

[0142] The same method as in Example 1 was used, but the positive electrode active material of Comparative Synthesis Example 1 was used to replace the material of Synthesis Example 1.

[0143] <Comparative Example 2: Positive electrode containing the positive electrode active material (monoclinic phase) of Comparative Synthesis Example 2>

[0144] The same method as in Example 1 was used, but the positive electrode active material of Comparative Synthesis Example 2 was used to replace the material of Synthesis Example 1.

[0145] [Preparation Example 2: Preparation of the half-cell]

[0146] Using lithium metal as the negative electrode and a glass fiber filter membrane (GF / F) as the separator, the positive electrodes of Example 1, Comparative Example 1, and 2 were respectively used, and the electrolyte was a solution of 1 M LiPF6 dissolved in a mixed solvent of EC / DMC (1:1 v / v). A CR2032 type button cell (Hohsen Corp.) was assembled. Specific operation: Place the separator between the positive and negative electrodes in an argon glove box (H2O, O2 < 1 ppm), inject the electrolyte, and then seal it.

[0147] [Experimental Example 2: XAS spectrum analysis]

[0148] Figure 2These are the Fe K-edge hard X-ray absorption spectroscopy spectra of LiFeSO4F and FeSO4 in Synthesis Example 1 after ball milling. The button cells (CR2032) used for Fe K-edge XAS analysis were charged or discharged to a certain SOC (State of charge) state, and then the cells were disassembled in a glove box filled with argon. Subsequently, the positive electrodes were taken out from the disassembled cells, washed with dimethyl carbonate (DMC), and sealed with Kapton film. The Fe K-edge XAS analysis was carried out on the 7D XAFS, 8C nano-XAFS, and 10C wide-field XAFS beamlines of the Pohang Light Source-II (PLS-II). All Fe K-edge XAS spectra were measured in transmission mode, and the reference spectra of Fe were obtained simultaneously for energy calibration. The storage ring was operated in top-up mode with a ring current of 300 mA and an energy of 3.0 GeV.

[0149] Refer to Figure 2 It can be seen that α-LiFeSO4F and FeSO4 exhibit distinct Fe K-edge spectra in the range of 7127 - 7200 eV (reflecting the Fe structural characteristics).

[0150] Figure 3 This is the radial distribution function of the atoms around Fe in LiFeSO4F and FeSO4 in Synthesis Example 1 after ball milling.

[0151] According to Figure 3 , LiFeSO4F in Synthesis Example 1 shows different radial distribution characteristics from the initial FeSO4 phase. At approximately and , the adjacent bonding changes accompanied by additional shoulder peaks can be clearly observed, and the second-nearest neighbor bonding distance of α-LiFeSO4F is slightly shorter than that of the initial pure FeSO4 phase .

[0152] Figure 4Results of F K-edge and Fe L3-edge analyses of LiFeSO4F of Synthesis Example 1 by soft X-ray absorption spectroscopy (SXAS). For F K-edge and Fe L3-edge analyses, the CR2032 button cell of Preparation Example 2 was charged and discharged to a specific SOC and then disassembled in an argon-filled glove box. After removing the positive electrode, it was washed with DMC, and the positive electrode powder was scraped off, ultrasonically treated with DMC, and then dropped onto a TEM sample grid. STXM analysis was completed at the 10A2 HR-PES beamline of Pohang Light Source-II (PLS-II), and the storage ring was operated in a constant current mode with an energy of 3.0 GeV and a ring current of 300 mA.

[0153] Refer to Figure 4 It can be seen that a new fluorine binding peak appearing in the LiFeSO4F prepared in Synthesis Example 1 can be detected by the SXAS spectrum. Specifically, this spectrum shows the presence of a pre-edge peak at 683.5 eV after the ball milling process; since this peak is a characteristic peak derived from the F1s-TM 3d hybrid orbital, it can be inferred that a new Fe-F chemical bond is formed in the LiFeSO4F of Synthesis Example 1.

[0154] [Experimental Example 3: STXM Analysis and TEM Images]

[0155] Figure 5 Electron diffraction patterns of LiFeSO4F of Synthesis Example 1 and various reference materials (Ref).

[0156] According to Figure 5 , the LiFeSO4F of Synthesis Example 1 shows an electron diffraction pattern different from that of monoclinic (Triplite) and triclinic (Tavorite) LiFeSO4F, and has characteristic peaks different from those of other component positive electrode active materials.

[0157] Figure 6 TEM photograph of LiFeSO4F of Synthesis Example 1.

[0158] Refer to Figure 6 It can be seen that the LiFeSO4F prepared in Synthesis Example 1 is composed of particles of 0.5 to 3.0 μm formed by randomly distributed grains of 4 to 8 nm. Here, the LiFeSO4F prepared in Synthesis Example 1 may contain an amorphous matrix and some grains forming a crystalline phase.

[0159] [Experimental Example 4-1: Half-Cell Electrochemical Performance Evaluation - Charge and Discharge Curves]

[0160] Figure 7It is the charge-discharge curve of the half-cell corresponding to Preparation Example 2. Specifically, the electrochemical characteristics of the half-cell corresponding to Preparation Example 2 were measured at 25 °C and 60 °C under a current density of (40 mA / g) and a voltage condition of 1.5 - 4.7 V.

[0161] Refer to Figure 7 It can be seen that during the first charge process, the half-cell corresponding to Example 1 showed a capacity of 145 mAh / g. For the half-cell corresponding to Example 1, the first discharge reaction started from about 3.9 V and showed an average discharge voltage of about 3.59 V (vs. Li + / Li). In this voltage range, the reversible reaction can be speculated to be due to the oxidation / reduction reaction of Fe 2+ / 3+ during the deintercalation / insertion process of lithium ions in α-LiFeSO4F.

[0162] The discharge voltage of the half-cell of Example 1 is slightly lower than that of Comparative Example 1 containing triclinic LiFeSO4F (~3.60 V) and Comparative Example 2 containing monoclinic LiFeSO4F (~3.90 V).

[0163] In addition, due to the amorphous structure characteristics of the positive electrode active material, the half-cell corresponding to Example 1 showed a significantly more inclined voltage curve compared to the half-cells of Comparative Examples 1 and 2. Specifically, oxidation / reduction peaks were observed at about 3.60 V and about 3.90 V for the half-cells of Comparative Examples 1 and 2, respectively, while no obvious oxidation / reduction peaks were observed for the half-cell of Example 1 in the same voltage range.

[0164] When the half-cells corresponding to Example 1, Comparative Example 1, and Comparative Example 2 were discharged to 1.5 V (vs. Li+ / Li) respectively, different from Comparative Examples 1 and 2, the half-cell of Example 1 showed capacity at a discharge voltage of 2.2 V (or 2.6 V during charging) due to the positive electrode active material with an amorphous structure, achieving a cumulative discharge capacity of 360 mAh / g (or an energy density of 906 Wh / kg). On the other hand, the half-cells corresponding to Comparative Examples 1 and 2 showed almost no capacity below 3 V, and it was confirmed that their cumulative discharge capacities were extremely low.

[0165] Figure 8 It is the charge-discharge curve of the half-cell corresponding to Comparative Example 1 at 60 °C.

[0166] Figure 9 It is the charge-discharge curve of the half-cell corresponding to Comparative Example 2 at 60 °C.

[0167] Refer to Figure 8 and Figure 9It can be seen that even when the cut-off voltages of the half-cells corresponding to Comparative Examples 1 and 2 were reduced from 1.5 V to 1.2 V to induce a conversion reaction, neither of them showed a meaningful capacity in the voltage range below 3 V.

[0168] [Experimental Example 4-2: Evaluation of the Electrochemical Performance of Half-Cells - Differential Capacity Curve]

[0169] Figure 10 The differential capacity curve of the half-cell of Preparation Example 2 is shown. It should be noted that the differential capacity curve is a curve of the differential value of capacity with respect to voltage (dQ / dV) plotted based on the time-voltage results under constant current conditions.

[0170] Refer to Figure 10 and Table 1 below. It can be seen that different from Comparative Examples 1 and 2, the half-cell of Example 1 showed a peak in differential capacity (dQ / dV) below 3 V, and showed an almost constant differential capacity value (dQ / dV) at 4.5 V, and no characteristic peak corresponding to 4.5 V was observed.

[0171] Table 1

[0172] Classification Example 1 Comparative Example 1 Comparative Example 2 Phase Amorphous Triclinic Phase Monoclinic Phase Peak at 4.5V Not Detected Detected Detected First Peak at 2.6V (Charging) Detected Not Detected Not Detected Second Peak at 2.2V (Discharging) Detected Not Detected Not Detected

[0173] In addition, oxidation / reduction peaks were observed at 3.60 V and 3.90 V for the half-cells of Comparative Examples 1 and 2 respectively, while no obvious characteristic peaks appeared in the same voltage range for Example 1.

[0174] [Experimental Example 5: Ex-situ hard X-ray absorption spectroscopy (Ex-situ hard XAS) analysis]

[0175] Figure 11a The charge-discharge curves of the half-cell of Example 1 as a function of time in the high voltage range (2.2 - 4.7 V) and the low voltage range (1.5 - 2.2 V) are shown.

[0176] Figure 11b The Fe K-edge XANES spectrum during the charging process of the half-cell of Example 1 in the high voltage range is shown.

[0177] Figure 11c The Fe K-edge XANES spectrum during the discharging process of the half-cell of Example 1 in the high voltage range is shown.

[0178] Figure 11d The Fe K-edge XANES spectrum during the charging process of the half-cell of Example 1 in the low voltage range is shown.

[0179] Figure 11e The Fe K-edge XANES spectrum during the discharging process of the half-cell of Example 1 in the low voltage range is shown.

[0180] The testing method in Experimental Example 5 is the same as that described in Experimental Example 2.

[0181] According to Figures 11a to 11c It can be seen that the Fe K-edge absorption edge moves towards higher and lower energy values respectively during charge and discharge processes, indicating that the electrochemical activity in this interval stems from the redox reaction of Fe 2+ / 3+ during the lithium ion insertion and extraction process.

[0182] Combined with Figure 11a and 11d 11e, it can be known that when the half-cell of Example 1 is discharged to the low voltage range, the XAS spectrum of α-LiFeSO4F shows significant changes and exhibits the characteristics of the Fe 0 metal phase.

[0183] As Figure 11e shown, a reversible reaction was observed during the subsequent charging process. During this electrochemical reaction, an isosbestic point that coincides with the Fe metal standard spectrum was clearly detected at 7121.2 eV. It should be noted that the isosbestic point refers to a characteristic that may appear in the X-ray absorption near-edge structure (XANES) when the combined ratio of each component remains unchanged during a two-phase reaction and only the fractions of the products and reactants change reversibly.

[0184] Figure 11f Fe K-edge extended X-ray absorption fine structure (EXAFS) spectra (using Fe metal as the reference) of Fe metal (Ref), pristine state, the half-cell of Example 1 discharged to 1.5 V and charged to 2.2 V.

[0185] According to Figure 11f , the reversible conversion reaction of the pristine α-LiFeSO4F electrode was confirmed by EXAFS analysis. Specifically: The Fe-Fe bonding peak at indicates the appearance of the Fe metal phase when the α-LiFeSO4F electrode is discharged (about 1.5 V), and this peak weakens during recharging (about 2.2 V). The residual signal intensity of the pristine α-LiFeSO4F electrode at

[0186] Figure 11g may be speculated to originate from the unreacted α-LiFeSO4F after discharge.

[0187] According to Figure 11g , the Fe metal fragments contained in the discharged lithium compound matrix have a particle size of 5 - 8 nm, and it is confirmed that they have a body-centered cubic (BCC) crystal structure along the

[111] axis direction.

[0188] Figure 11h F K-edge spectra of the half-cell of Example 1 in fully charged, half-discharged, and fully discharged states.

[0189] According to Figure 11h , to clarify the conversion reaction occurring in the low voltage region, by studying the change in fluorine local bonding, it was found that: the pre-edge peak characterizing the F-Fe bond in the pristine electrode (triangle mark) did not change in the high voltage region (>2.2 V), but significantly weakened when discharged to 1.5 V, which is consistent with the dissociation of the F-Fe bond in the conversion reaction. Notably, although LiF is a common discharge product in the conversion reaction of fluorine-based compounds, no characteristic LiF signal (~700 eV) was observed in the fully discharged state.

[0190] Figure 11i Infrared spectra (IR) of the half-cell of Example 1 in fully charged, half-discharged, and fully discharged states.

[0191] According to Figure 11i , the IR analysis results indicate that the discharge products are composed of compounds containing SO3F - polyanions. Specifically, when the a-LiFeSO4F electrode was discharged to 1.5 V, new peaks appeared at 559 cm -1 , 831 cm -1 , and 1,311 cm -1 (triangle mark), and these peaks weakened reversibly during recharging. These peak positions are highly consistent with the O-S-O deformation vibration (550 - 590 cm - ), S-F stretching vibration (~800 cm -1 ), and SO3 stretching vibration (~1300 cm -1 ) modes of typical compounds containing SO3F -1 polyanions. Combining the formation of Fe metal with the stoichiometry of LiFeSO4F, it can be inferred that the lithium-containing discharge product in the conversion reaction is LiSO3F, and it is speculated that the residual reaction product after discharge is Li2O.

[0192] [Experimental Example 6: Electrochemical Performance Evaluation of the Half-Cell of Example 1]

[0193] Figure 12 Charge-discharge curves of the two-stage reaction (insertion and conversion reactions) of a-LiFeSO4F in the corresponding half-cell of Example 1. Specifically, the same measurement method as in Experimental Example 6, i.e., the measurement method of Experimental Example 4-1, was used for determination.

[0194] Refer to Figure 12It can be seen that the insertion reaction of lithium ions proceeds according to Reaction Formula 1 below, while the conversion reaction can proceed according to Reaction Formula 2 below.

[0195] [Reaction Formula 1]

[0196] a-FeSO4F + Li + + e - → a-LiFeSO4F (lithium ion insertion reaction at 3.59 V)

[0197] [Reaction Formula 2]

[0198] a-LiFeSO4F + 2Li + + 2e - → Fe + LiSO3F + Li2O (conversion reaction at 2.18 V)

[0199] Specifically, the first redox reaction starts with the insertion reaction involving Fe 2+ / 3+ redox between a-LiFe 2+ SO4F and Fe 3+ SO4F. The subsequent redox reactions are achieved through the conversion reaction involving Fe 0 / 2+ redox, accompanied by the reversible formation of Fe 0 , LiSO3F, Li2O and a-LiFe 2+ SO4F.

[0200] Figure 13 This is the evaluation result of the electrochemical stability of the long-cycle charge and discharge of the half-cell in Example 1. This evaluation was carried out by comparing the capacity retention rate and Coulomb efficiency with the number of cycles under the conditions of a rate of 0.01C to 0.5C and a temperature (25°C or 60°C).

[0201] Referring to Figure 13 , at room temperature, the half-cell in Example 1 showed a capacity of more than 300 mAh / g in both reactions, and still maintained a capacity retention rate of 90% after 200 cycles, confirming that it can stably induce lithium ion insertion and conversion reactions for a long time. Different from the significant cycle attenuation shown by the traditional cathode materials FeF2 and FeF3, the electrochemical curve and average voltage of a-LiFeSO4F showed no obvious attenuation during 200 cycles.

[0202] In addition, due to the use of a-LiFeSO4F as the cathode, the half-cell in Example 1 showed a higher capacity of about 360 mAh / g at 60°C, and still maintained 98.6% of the initial capacity after 200 cycles.

[0203] It should be noted that traditional electrodes based solely on conversion reactions often struggle to achieve stable capacity retention due to accompanying side effects (such as significant volume changes, compositional inhomogeneity, transition metal dissolution, and the formation of a cathode electrolyte interface (CEI) due to electrolyte decomposition). Although reversibility can be improved through solutions such as surface protective layers, three-dimensional cathode structures, or advanced electrolyte systems, additional process treatments are required. In contrast, the amorphous a-LiFeSO4F in the present invention exhibits excellent electrochemical stability cycling performance without additional treatment. Even in the presence of significant bond reorganization during the conversion reaction, the lithium-ion insertion reaction of the a-LiFeSO4F electrode can be stably maintained without significant performance degradation.

[0204] Figure 14a Charge-discharge curves of the half-cell of Example 1 at current densities of 0.02 A / g, 0.05 A / g, 0.1 A / g, 0.2 A / g, 0.5 A / g, 1 A / g, and 2 A / g.

[0205] Figure 14b Relationship between cycle number and discharge capacity of the half-cell of Example 1 at 60 °C and current densities of 0.02 A / g, 0.05 A / g, 0.1 A / g, 0.2 A / g, 0.5 A / g, 1 A / g, and 2 A / g.

[0206] Refer to Figure 14a and 14b , the half-cell of Example 1 showed a decreasing trend in specific capacity as the current density increased from 0.02 A / g to 5 mA / g, but still maintained a specific capacity of over 250 mAh / g at a current density of 1 A / g, indicating its good rate performance.

[0207] In addition, since repeated conversion reactions can trigger morphological changes in the electrode structure reconstruction, it can be speculated that the sharp increase in the discharge capacity under the condition of 0.02 A / g in the last cycle originated from the reactivation of the initial inactivation stage.

[0208] Although the preferred embodiments of the present invention have been described in detail above, the scope of the rights of the present invention is not limited thereto. Based on the basic concept of the present invention defined in the claims, various modifications and improvements made by those skilled in the art also fall within the scope of the rights of the present invention.

[0209] This research was supported by the Samsung Future Technology Cultivation Project (Project No.: SRFC-TA1403-53).

Claims

1. A secondary battery, comprising: a positive electrode containing a transition metal-based lithium compound; characterized in that, In the differential capacity (dQ / dV)-voltage (V) curve of the secondary battery: there is no peak between 3.8 V and 4.0 V, there is a first peak in the range of 2.5 V to 2.7 V, and there is a second peak in the range of 2.1 V to 2.3 V.

2. The secondary battery according to claim 1, characterized in that, The transition metal-based lithium compound contains an amorphous phase.

3. The secondary battery according to claim 1, wherein The differential capacity (dQ / dV)-voltage (V) curve is obtained through the 1st to 5th cycles under conditions of a rate of 0.01 C to 0.5 C and 25 °C.

4. The secondary battery according to claim 1, characterized in that, The transition metal-based lithium compound contains a compound represented by the following general formula 1: [General formula 1] Li x M 2-x SO4A In general formula 1: M is one or more elements selected from Fe, Mg, Ni, Co, Cr, Ti, Cu, Ca, Zn, Zr, Nb, Mo, Sr, Sb, W, and Bi; A is a halogen atom; and 0.5 ≤ x ≤ 1.

5.

5. The secondary battery according to claim 1, wherein The transition metal-based lithium compound contains LiFeSO4F.

6. The secondary battery according to claim 1, wherein The particle size (D 50 ) of the transition metal-based lithium compound is 0.5 to 3.0 μm.

7. The secondary battery according to claim 1, characterized in that, The transition metal-based lithium compound contains: an amorphous matrix and crystal grains.

8. The secondary battery according to claim 7, wherein The crystal grains contain a plurality of crystal grains, and the average size of each crystal grain is 4 to 8 nm.

9. The secondary battery according to claim 1, wherein It further includes: a negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte.

Images