Improved cathode materials for secondary lithium batteries

Abstract

The present invention provides an improved cathode material comprising a compound having the formula LiMPO4, where M is at least one of Fe, V, Mn, Co, and Ni, the compound comprising (i) secondary particles formed by aggregation of (ii) primary particles, and (iii) pores between the secondary particles and the primary particles, wherein the primary particles have a plate-like morphology and an average particle size d in the range of 20 to 150 nm. 50 The secondary particles have a spherical shape and an average particle size d in the range of 1 to 10 μm. 50 and

Description

[Technical Field]

[0001] The present invention relates to cathode materials for secondary (or rechargeable) alkaline batteries, particularly for lithium batteries, that exhibit exceptional specific capacity, especially exceptional capacity retention, even at higher discharge rates (e.g., 30 C or even higher) at ambient and extreme temperatures. [Background technology]

[0002] In recent years, the increasing demand for mobile devices has led to a significant increase in the demand for secondary batteries as energy sources. Among these secondary batteries, alkaline secondary batteries, especially lithium secondary batteries, are widely used due to their high energy density and voltage, long lifespan, and low self-discharge.

[0003] Lithium batteries generally use a carbon material as the anode and a lithium material as the cathode.

[0004] One of the best-known cathode materials for lithium secondary batteries is a compound of formula LiFePO4, a mineral of the olivine group. This compound was first identified as a functional cathode material for lithium-ion batteries in a patent filed in 1997 (Patent Document 1). At that time, the reversible extraction of lithium from LiFePO4 and insertion of lithium into FePO4 (delithiation-lithium intercalation) were demonstrated. This compound has attracted considerable market interest since the early 20th century due to its low cost, non-toxicity, natural abundance of iron, excellent thermal stability, safety profile, electrochemical performance, and specific capacity (170 mA.h / g or 610 C / g).

[0005] However, in order to fully enter the market, several improvements have been made to the LiFePO4 material (also commonly referred to as "LFP") to further improve its performance or mitigate some of its shortcomings.

[0006] However, LiFePO4 has limited practical applications due to the following drawbacks:

[0007] One of the main drawbacks of LiFePO4 as a cathode material is its inherently low electrical conductivity, which disadvantageously causes an increase in the internal resistance of the battery, which leads to an increase in polarization potential when the electrical circuit is closed, and therefore a decrease in battery capacity.

[0008] This problem was gradually overcome by reducing the particle size and / or by coating the LiFePO4 particles with a conductive material such as carbon.

[0009] Regarding particle size reduction, it has been disclosed that the particle size of LFP materials can be nanoscaled. Although this nanoscaled approach has been proven to approach the theoretical capacity of LFP materials, the significantly increased surface area of ​​LFP materials increases the electrochemical reaction products of batteries, thereby causing serious undesirable side reactions in batteries. At the same time, a large surface area also requires a large amount of binder, which can reduce process efficiency, cause poor electrode coating performance, or affect conductivity performance.

[0010] Studies on lithium iron phosphate materials have also shown that the material morphology has a non-negligible effect on the electrochemical properties / performance of lithium iron phosphate materials.

[0011] For example, Patent Document 2 discloses a specific method for preparing lithium iron phosphate having spherical primary particles (sized about 30 μm) made from an aggregate of primary particles (sized about 300 to 500 nm) with cavities between the primary particles.

[0012] Patent Document 3 discloses one-dimensional nanostructured lithium iron phosphate synthesized by a hydrothermal method by adjusting the pH of the reaction.

[0013] In the examples of Patent Document 4, the average particle diameter d 50 5-100 μm mean particle diameter d formed by aggregation of primary particles 50 The lithium iron phosphate is disclosed as being composed of secondary particles (or "aggregates") having a porosity of 22-28%. The secondary particles exhibit a spherical shape, while the primary particles have no particular shape or morphology.

[0014] Patent Document 5 discloses lithium iron phosphate having a specific crystal structure, i.e., when the Li diffusion direction is the direction

[0010] in the lattice structure of the crystal, the length of the direction

[0001] is greater than the length of the direction

[0010] , which means that the length of the direction perpendicular to the Li diffusion direction is greater than the length of the Li diffusion direction. In particular, the macroscopic morphology of the lithium iron phosphate primary particles in this document is rod-like. The secondary particles exhibit a size of about 300 microns (illustrated in Figure 1 of Patent Document 5).

[0015] However, despite all the developments / improvements brought to LFP cathode materials, there remains a clear need to further improve the performance of LFP cathode materials compared to those disclosed, especially those commercially available, due to the increasing market demand for secondary batteries as an energy source. [Prior art documents] [Patent documents]

[0016] [Patent Document 1] U.S. Patent No. 5,910,382 [Patent Document 2] European Patent Application Publication No. 2285740 [Patent Document 3] Chinese Patent Application Publication No. 101752564 [Patent Document 4] European Patent Application Publication No. 2360117 [Patent Document 5] European Patent Application Publication No. 2562856 Summary of the Invention [Problem to be solved by the invention]

[0017] Object of the invention It is an object of the present invention to provide improved LFP-based cathode materials compared to those available on the market or described in the prior art.

[0018] It is a further object of the present invention to provide an LFP-based cathode material that exhibits exceptional specific capacity and excellent capacity retention even at higher discharge rates (e.g., 30 C or even higher) at ambient and extreme temperatures.

[0019] It is a further object of the present invention to provide an LFP-based cathode material that can be easily manufactured even at high tonnage and can be used as a drop-in material. [Means for solving the problem]

[0020] Description of the Invention The present invention provides a cathode material comprising a compound having the formula LiMPO4, wherein M is at least one metal cation selected from the list consisting of Fe, V, Mn, Co, and Ni, the compound comprising (i) secondary particles formed by aggregation of (ii) primary particles, and (iii) pores between the secondary particles and the primary particles, the primary particles having a plate-like morphology and an average particle size d in the range of 20 to 150 nm. 50 The secondary particles have a spherical shape and an average particle size d in the range of 1 to 10 μm. 50 and

[0021] The present invention is therefore based on a novel and inventive approach. In particular, the inventors have found that porous LFP materials composed of secondary particles with specific morphologies and sizes, formed by aggregation of primary particles with specific morphologies and sizes, can reach very satisfactory specific capacitances and excellent capacity retention at high discharge rates.

[0022] Furthermore, the present invention also relates to a process for preparing the cathode material of the present invention, wherein M is Fe, said process comprising a hydrothermal reaction starting from lithium hydroxide and iron(II) sulfate as metallic starting materials.

[0023] The present invention also relates to a secondary battery comprising an anode, a cathode, a separator and an electrolyte, wherein the cathode comprises the cathode material according to the present invention.

[0024] Throughout this specification and claims, those skilled in the art will appreciate that the terms "a," "an," or "the," as used herein, mean "at least one" and should not be limited to "only one" unless expressly stated to the contrary. Also, when ranges are stated, both ends are included. Furthermore, all integral and subdomain values ​​within numerical ranges are expressly included as if expressly written.

[0025] Other characteristics and advantages of the invention will become more apparent from reading the following description of preferred embodiments and figures given as simple illustrative and non-limiting examples. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is an XRD pattern of a cathode material of Example 1 according to the present invention. [Figure 2] 1 is a FE-SEM image showing the cathode material of Example 1 according to the present invention. [Figure 3]1 is a TEM image showing the cathode material (primary particles) of Example 1 according to the present invention. [Figure 4] 1 is a PSD graph of the cathode material of Example 1 according to the present invention. [Figure 5] 1 is a graph showing calculated capacity retention (%) at room temperature with increasing cycle number for secondary batteries prepared using the cathode material of Example 1 according to the present invention and a commercially available LFP cathode material. [Figure 6] 1 is a FE-SEM image showing a commercially available LFP cathode material. [Figure 7] 1 is a graph showing calculated capacity retention (%) at different temperatures (other than room temperature) with increasing cycle number for secondary batteries prepared using the cathode material of Example 1 according to the present invention and a cathode material from the market. [Figure 8] 8(b) and 8(a) are two graphs showing the impedance of a secondary battery according to the present invention (FIG. 8(b)) and the impedance for a similarly fabricated secondary battery using a "Benchmark LFP" (FIG. 8(a)). [Figure 9] 1 is a FE-SEM image showing the cathode material of Example 2 according to the present invention. [Figure 10] 1 is a PSD graph of the cathode material of Example 2 according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] According to the present invention, the cathode material comprises a compound having the formula LiMPO4, where M is at least one metal cation selected from the list consisting of Fe, V, Mn, Co and Ni.

[0028] In particular, compounds having the formula LiMPO4 are tetrahedral polyanions (PO4) 3-It typically has an ordered olivine crystal structure with a plurality of planes defined by zigzag chains and straight chains, in which M atoms occupy octahedral zigzag chains and Li atoms occupy straight chains in alternating planes of octahedral sites. In such a compound, since the metal M cation (or combination of cations) is reduced / oxidized by charge-compensating electrons supplied / removed by the external circuit of the battery during the charge / discharge cycle, the alkali ion Li+ can be reversibly inserted / extracted from / to the interstitial space of the host MPO4 framework from / to the electrolyte of the battery.

[0029] According to the present invention, M is at least one metal cation selected from the list consisting of Fe, V, Mn, Co, and Ni. Examples of the formula for the compounds of the present invention are LiFePO4, LiVPO4, LiMnPO4, LiCoPO4, and LiNiPO4, but are not limited thereto. Preferably, the compound of the present invention has the formula LiFePO4 (M is Fe).

[0030] According to one embodiment of the present invention, in the compound of the formula LiMPO4, M is a combination of at least two metal cations selected from the list consisting of Fe, V, Mn, Co, and Ni. In particular, in the compound of the formula LiMPO4, M is a combination of two metal cations selected from the list consisting of Fe, V, Mn, Co, and Ni. Examples of the combinations according to this embodiment are Fe 1-x Mn x PO4, Fe 1-x V x PO4, Fe 1-x Ni x PO4, Fe 1-x Co x PO4 (where 0 < x < 1), but are not limited thereto.

[0031] According to the present invention, this compound contains secondary particles formed by the aggregation of primary particles with a smaller diameter.

[0032] Furthermore, according to the invention, the compound contains further pores between said secondary particles and primary particles.

[0033] According to the present invention, the primary particles have a plate-like morphology. "Plate-like morphology" means, in the present invention and as generally recognized in the field of crystal morphology, a three-dimensional shape in which one of the dimensions of the primary particle (hereinafter referred to as "thickness") is significantly smaller than the other two dimensions of the primary particle (the larger one is width and the smaller one is length). In particular, in the present invention, the primary particles may have a quadrilateral plate-like morphology in which the length of the plate is at least twice the width and thickness of the plate. "Quadrilateral plate-like morphology" means, in the present invention and as generally recognized in the field of crystal morphology, a three-dimensional shape in which one of the dimensions of the primary particle ("thickness") is significantly smaller than the other two approximate dimensions of the primary particle.

[0034] According to one embodiment of the present invention, the primary particles having a plate-like morphology have an average width in the range of 5 to 100 nm, preferably in the range of 10 to 100 nm, more preferably in the range of 10 to 80 nm.

[0035] According to the present invention, the primary particles have an average particle size d in the range of 20 to 150 nm. 50 Preferably, the primary particles have an average particle size d in the range of 20 to 100 nm. 50 It has.

[0036] Throughout this specification and in the claims, "average particle size d 50 " means the particle size (also called median particle diameter or median particle size) at which the cumulative percentage reaches 50%, as generally accepted in the art. 50 can be measured by any method known in the art.

[0037] According to the present invention, the secondary particles have a spherical morphology. For the sake of clarity, "spherical morphology" means that the shape of the secondary particles is essentially spherical, as is the case in the present invention and generally accepted in the field of crystal morphology.

[0038] According to the present invention, the secondary particles have an average particle size d in the range of 1 to 10 μm. 50 Preferably, the secondary particles have an average particle size d in the range of 1.5 to 10 μm. 50 Preferably, the secondary particles have an average particle size d in the range of 1 to 8 μm. 50 More preferably, the secondary particles have an average particle size d in the range of 1.5 to 8 μm or even 2 to 8 μm. 50 It has.

[0039] The pores in the secondary particles may be of the closed or open type, and preferably of small size. The size of the pores according to the present invention is preferably in the range of 200 to 800 nm, more preferably 300 to 600 nm.

[0040] According to one embodiment of the present invention, the compound has a porosity in the range of 5 to 30%. Preferably, the compound has a porosity in the range of 10 to 30%. Also preferably, the compound has a porosity in the range of 5 to 25%. More preferably, the compound has a porosity in the range of 10 to 25%.

[0041] Advantageously, the cathode material of the present invention may optionally further comprise a conductive material (to increase the conductivity of the cathode material), a binder and / or a filler (to help bind the active cathode material to the conductive material and current collector).

[0042] The conductive material is generally added in an amount of 1 to 30 wt % based on the total weight of the cathode material. Examples of conductive materials that can be used in the present invention include carbon, graphite (natural or artificial), carbon fiber, precious metals, metals, conductive polymers, and combinations thereof. Advantageously, cathode materials containing carbon can efficiently improve conductivity without significantly increasing preparation costs and weight.

[0043] The binder is generally added in an amount of 1 to 30% by weight based on the total weight of the anode material. Examples of binders include polyvinylidene, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber, and fluororubber.

[0044] Fillers are used to suppress expansion of the electrodes. Examples of fillers are olefin polymers such as polyethylene and polypropylene, and fibrous materials such as glass fiber and carbon fiber.

[0045] Also advantageously, the cathode material of the present invention has a sulfate content of less than or equal to 100 ppm, preferably less than or equal to 80 ppm, or even less than or equal to 50 ppm.

[0046] Also advantageously, the cathode material of the present invention has a hydrogen content of less than or equal to 1000 ppm, preferably less than or equal to 800 ppm, or even less than or equal to 500 ppm.

[0047] The cathode material according to the present invention can be prepared by any method, so long as the method allows for the formation of the claimed characteristics of said cathode material.

[0048] Advantageously, the cathode material of the present invention may be prepared by hydrothermal / solvothermal synthesis.

[0049] For example, a method for preparing a cathode material of the present invention may include the following steps, in order: (a) first mixing a solution of starting material in a solvent in a stoichiometric amount relevant to the material of interest; (b) conducting the hydrothermal / solvothermal synthesis in a hydrothermal / solvothermal reactor / vessel; (c) Separating the precipitate / solids, for example by centrifugation.

[0050] Following steps (a)-(c), the method for preparing the cathode material of the present invention further comprises, after step (c), and optionally (d) washing the precipitate; (d) wet ball milling (optionally adding a conductive material, e.g., carbon); (e) drying (e.g., under vacuum or Ar or N), and (f) calcination (e.g., under Ar or N2 at 600°C to 1200°C) It may further include:

[0051] The starting material for lithium can be Li2CO3, Li(OH), Li(OH).H2O, LiNO3, etc. Preferably, the starting material for lithium in the present invention is lithium hydroxide, such as LiOH.H2O or Li(OH).

[0052] The starting material for iron can be FeSO or a hydrate of FeSO, FeC0.2H0, or FeCl. Preferably, the starting material for iron in the present invention is iron (II) sulfate, such as FeSO or a hydrate of FeSO (e.g., FeSO.7H0).

[0053] The starting material for phosphorus (P) can be H3PO4, NH4H2PO4, (NH4)2HPO4 or P2O5. Preferably, the starting material for phosphorus in the present invention is H3PO4.

[0054] In step (a), an example of the solvent is ethylene glycol or a mixture of ethylene glycol and water (for example, in a 1:1 ratio).

[0055] In step (b), the temperature may be in the range of 200 to 700° C. and the pressure may be in the range of 180 to 550 bar.

[0056] The present invention also relates to a method for preparing a cathode material according to the invention, wherein M is Fe, said method comprising a hydrothermal reaction starting from lithium hydroxide and iron(II) sulfate as metallic starting materials, which makes it possible to obtain a cathode material comprising the LiFePO4 compound.

[0057] Finally, the present invention also relates to a secondary battery comprising an anode, a cathode, a separator and an electrolyte, said cathode comprising the cathode material according to the present invention.

[0058] A cathode according to the present invention may include a cathode material and a current collector.

[0059] The anode of the present invention can include an anode material (optionally including a conductive material, a binder and / or a filler) and a current collector.

[0060] Examples of current collectors for the cathode and / or anode include stainless steel, aluminum, nickel, titanium, sintered carbon, nickel, titanium, or silver. The current collectors may have various forms such as films, sheets, foils, nets, porous structures, foams, and nonwoven fabrics, and generally have a thickness of 3 to 500 μm.

[0061] The separator is sandwiched between the cathode and anode. A thin insulating film with high ion permeability and mechanical strength can be used as the separator. The separator generally has a pore diameter of 0.01 to 10 μm and a thickness of 5 to 300 μm. Examples of separators include sheets or nonwoven fabrics made from olefin polymers such as polypropylene and / or glass fiber or polyethylene.

[0062] The electrolyte is ideally a lithium salt-containing non-aqueous electrolyte composed of a non-aqueous electrolyte and a lithium salt. A non-aqueous electrolytic solution, a solid electrolyte, or an inorganic solid electrolyte can be used as the non-aqueous electrolyte.

[0063] The secondary battery according to the present invention advantageously has a capacity of 120 to 200 mAh·g -1 The specific capacity ranges from .

[0064] Furthermore, the secondary battery according to the present invention also advantageously exhibits a capacity retention in the range of 80-90% at a discharge rate of 10C.

[0065] It should be understood that the present invention is not limited to the described embodiments, and that variations are possible without departing from the scope of the claims. Modifications and variations are possible within the scope of the appended claims. It should further be noted that the present invention relates to all possible combinations of the features and preferred features described in this specification and recited in the claims.

[0066] The following examples are provided for illustrative purposes and are not intended to limit the scope of the present invention. [Example]

[0067] Example 1: A cathode material according to the present invention was prepared by the following steps in order: - preparing a solution of LiOH.H2O, FeSO4.7H2O and H3PO4 (purchased from Merck co, Ltd Germany, analytical grade, without purification) in ethylene glycol and water (1:1) as solvent, - mixing the three solutions in stoichiometric amounts (relative to the target compound LiFePO4) under stirring; - transferring the mixture obtained in the previous step into a hydrothermal vessel and sealing said vessel under inert gas, - performing a hydrothermal treatment at 180°C for 6 hours under constant stirring without applying external pressure; - centrifuging to separate the precipitate; - washing the precipitate with distilled water; - drying under vacuum at 60°C for 12 hours; and - Calcination at 600°C for 6 hours under argon to reach the final material.

[0068] FIG. 1 shows the XRD powder diffraction spectrum of the final material obtained, along with the peaks indexation of olivine LiFePO4.

[0069] The FE-SEM image (Field Emission Scanning Electron Microscopy) of the final material obtained is shown in Figure 2 (the conditions for the measurement are listed below). As can be seen from Figure 2, the secondary particles are formed from an aggregate of primary particles and have a spherical morphology. Furthermore, pores exist between the secondary particles and the primary particles.

[0070] A TEM image of the primary particles (previously separated from the secondary particles before analysis) of the final material obtained is shown in Figure 3 (the conditions and scale for the measurement are given at the bottom). As can be seen from Figure 3, the observed primary particles exhibit a plate-like morphology.

[0071] The sizes of secondary and primary particles were determined in images of at least 300 particles per sample (SEM for secondary particles and TEM for primary particles). Figure 4 shows the PSD graph for the final material obtained ((a): primary particles, (b) secondary particles).

[0072] The following values ​​were identified: - d of primary particles 50 :141nm - d of secondary particles 50 :2.5 microns.

[0073] The porosity of the particles was also estimated to be 12.8%.

[0074] Ten identical multilayer pouch cells were fabricated (0.8 Ah capacity) using a cathode prepared from the final cathode material, an anode, a separator sandwiched between the cathode and anode, and an electrolyte. The cathode was prepared as follows: 95 percent of the resulting cathode material was ground with 3 wt. % polyvinylidene fluoride binder and 2 wt. % conductive carbon powder dispersed in NMP (N-methyl-2-pyrrolidone). The resulting slurry was poured onto a current collector (aluminum foil), and the resulting cathode was dried and used as the cathode in the secondary battery.

[0075] 5 is a graph showing calculated percent capacity retention at room temperature with increasing cycle number for the secondary battery according to the present invention (IPL3, IPL8, and IPL10 refer to cell numbers among 10 identical pouch cells fabricated) and for secondary batteries similarly fabricated using commercially available LFP cathode materials ("Benchmark LFP" from Aleees; REF_D, E, and F refer to cell numbers among 10 identical pouch cells fabricated using "Benchmark LFP"). FIG. 5 shows that the secondary battery according to the present invention has superior performance compared to existing secondary batteries on the market.

[0076] "Benchmark LFP" is a composite of LFP particles with an average particle size d of approximately 100 nm that associate with each other to form secondary particles with an unclear / random morphology and an average particle size of approximately 1 micron. 50The primary particles are nanospherical (non-uniform spheres with a wide particle size distribution ranging from 100 to 500 nm). Furthermore, the particles have a porosity value of approximately 30%. Figure 6 shows an FE-SEM image of the "benchmark LFP" excerpted from the paper "Aging of a Lithium-Metal / LFP Cell: Predictive Model and Experimental Validation" (Dessantis, D.; Di Prima, P.; Versaci, D.; Amici, J.; Francia, C.; Bodoardo, S.; Santarelli, M. in Batteries 2023, 9, 146).

[0077] Figure 7 is a graph showing calculated capacity retention (%) at different temperatures (other than room temperature) with increasing cycle number for the same secondary battery as in Figure 5. The graph notably shows that the secondary battery according to the present invention has superior (nearly three times more) capacity retention at a 10C discharge rate and -10°C compared to the benchmark LFP product.

[0078] FIG. 8 shows two graphs of the impedance of a secondary battery according to the present invention (FIG. 8(b)) and the impedance for a similarly made secondary battery using a “Benchmark LFP” (FIG. 8(a)), again demonstrating that the secondary battery of the present invention is better.

[0079] Example 2: A cathode material according to the present invention was prepared by the following steps in order: - preparing a solution of LiOH.H2O, FeSO4.7H2O, MnSO4 and H3PO4 (purchased from Merck co, Ltd Germany, analytical grade, without purification) in ethylene glycol and water (1:1) as solvent, - Three solutions were prepared in stoichiometric amounts (target compound LiFe 0.5 Mn 0.5 PO4) with stirring, - transferring the mixture obtained in the previous step into a hydrothermal vessel and sealing said vessel under inert gas, - performing a hydrothermal treatment at 180°C for 6 hours under constant stirring without applying external pressure; - centrifuging to separate the precipitate; - washing the precipitate with distilled water; - drying under vacuum at 60°C for 12 hours; and - Calcination at 600°C for 6 hours under argon to reach the final material.

[0080] FE-SEM images of the final material obtained are shown in Figure 9 (the measurement conditions are listed at the bottom, (a)-(b): two different scales). As can be seen, the secondary particles are formed from the aggregation of primary particles and have a spherical morphology. Furthermore, pores exist between the secondary particles and the primary particles.

[0081] The sizes of secondary and primary particles were determined in images of at least 300 particles per sample (SEM for secondary particles and TEM for primary particles). Figure 10 shows the PSD graph for the final material obtained ((a): primary particles, (b) secondary particles).

[0082] The following values ​​were identified: - d of primary particles 50 :110nm - d of secondary particles 50 :1.96 microns.

Claims

1. LiMPO ４ A cathode material comprising a compound having (wherein M is at least one metal cation selected from the list consisting of Fe, V, Mn, Co and Ni), wherein the compound comprises (ii) secondary particles formed by an aggregate of primary particles, and (iii) pores between the secondary particles and the primary particles, The primary particles have a plate-like shape and an average particle size d in the range of 20 to 150 nm. ５０ The secondary particles have a spherical shape and an average particle size d in the range of 1 to 10 μm. ５０ and Cathode material.

2. The aforementioned compound is of the formula LiFePO ４ A cathode material according to claim 1, having the following characteristics.

3. The cathode material according to claim 1, wherein M is a combination of at least two metal cations selected from the list consisting of Fe, V, Mn, Co, and Ni.

4. The cathode material according to claim 1, wherein the compound has a porosity in the range of 5 to 30%.

5. The cathode material according to claim 1, which is prepared by hydrothermal or solvothermal synthesis.

6. The cathode material according to claim 1, wherein the primary particles have a rectangular plate-like shape in which the length of the plate is at least twice the width and thickness of the plate.

7. The cathode material according to claim 1, wherein the primary particles have an average width in the range of 5 to 100 nm.

8. The aforementioned secondary particles have an average particle size d in the range of 1.5 to 10 μm. ５０ A cathode material according to claim 1, having the following characteristics.

9. The cathode material according to claim 1, having a sulfate content of 100 ppm or less.

10. The cathode material according to claim 1, having a hydrogen content of 1000 ppm or less.

11. A method for preparing a cathode material according to any one of claims 1 or 2 and 4 to 12, wherein M is Fe, and the method comprises a hydrothermal reaction starting from lithium hydroxide and iron(II) sulfate as a metal starting material.

12. A secondary battery comprising an anode, a cathode, a separator, and an electrolyte, wherein the cathode comprises a cathode material according to any one of claims 1 to 10.

13. The secondary battery according to claim 12, exhibiting a capacity retention rate in the range of 80-90% at a discharge rate of 10C.

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