A positive electrode active material, a method for manufacturing the same, a positive electrode sheet, and a battery

Abstract

The application relates to the technical field of positive electrode active materials, in particular to a positive electrode active material, a preparation method thereof, a positive electrode sheet and a battery. The positive electrode active material comprises a substrate and a coating layer covering at least part of the surface of the substrate, the substrate comprises a layered oxide material, the positive electrode active material comprises lanthanum elements, the lanthanum elements are at least partially distributed in the coating layer, and the content of the lanthanum elements decreases along a first direction; the first direction refers to the direction in which the coating layer points to the substrate. The positive electrode active material is not easy to lose oxygen, the crystal structure is stable, and the cycle stability of the battery is improved.

Description

Technical Field

[0001] This invention relates to the field of positive electrode active material technology, specifically to a positive electrode active material and its preparation method, a positive electrode sheet, and a battery. Background Technology

[0002] Cathode active materials are a core component of energy storage devices such as batteries, and their performance directly affects the battery's energy density, cycle life, safety, and cost. Based on their chemical composition and structural characteristics, cathode active materials are mainly classified into the following categories: layered oxides, spinel structures, olivine, polyanionic materials, and sulfide cathode active materials.

[0003] Among them, layered oxide cathode active materials (such as lithium-rich manganese-based materials) have oxygen atoms located at the vertices of octahedrons (MO6) due to their layered crystal structure characteristics, forming strong MO bonds with transition metals. However, the coordination environment of interlayer oxygen is relatively open, and the interaction with other oxygen atoms is weak. This structure leads to: (1) high chemical activity of oxygen, and interlayer oxygen atoms are easily affected by the electrochemical environment. Under high voltage or overcharge conditions, the electron cloud density of oxygen decreases, the bonding with transition metals weakens, and oxygen is easily released, resulting in oxygen loss; (2) during charging, lithium ions escape from the layered structure and shrink, and transition metal ions migrate to the lithium layer, triggering an irreversible phase transition of layered-spinel-rock salt. During the phase transition, oxygen atoms escape from the lattice position to form oxygen, resulting in oxygen loss. In addition, the anion redox reaction under high voltage will also cause oxygen loss. These problems directly lead to structural collapse and capacity decay, thus affecting the cycle stability of the battery. Summary of the Invention

[0004] This invention provides a positive electrode active material that does not easily lose oxygen and has a stable crystal structure, which is beneficial to improving the cycle stability of the battery.

[0005] The method for preparing positive electrode active materials provided by this invention can produce positive electrode active materials that are less prone to oxygen loss and have a more stable structure.

[0006] The positive electrode sheet provided by this invention is beneficial for improving the cycle stability of the battery.

[0007] The battery provided by this invention has excellent cycle stability.

[0008] A first aspect of the present invention provides a positive electrode active material, comprising a matrix and a coating layer covering at least a portion of the surface of the matrix, the matrix comprising a layered oxide material, the positive electrode active material comprising lanthanum, the lanthanum being at least partially distributed in the coating layer, and the lanthanum content decreasing along a first direction; the first direction refers to the direction in which the coating layer points toward the matrix.

[0009] The positive electrode active material described above has a crystal structure of lanthanum that includes a crystal structure belonging to the P321 space group and a crystal structure belonging to the R-3m space group, with the crystal structure belonging to the P321 space group located on the side of the crystal structure belonging to the R-3m space group away from the matrix.

[0010] From the above-described positive electrode active material, the gradient factor G of the lanthanum element La Satisfy: 1.1≤G La <1.4;

[0011] in, ;

[0012] I0 La This represents the peak area of ​​La on the surface of the cathode active material before etching, as characterized by XPS; I30 La I60 La I90 La The values ​​represent the peak areas of the La element on the inner layer surface exposed after etching of the positive electrode active material for 30s, 60s, and 90s along the first direction, as characterized by XPS. The etching rate is 5 nm / min, with Ta2O5 material as the etching rate calibrator.

[0013] The thickness of the coating layer in the above-described positive electrode active material is 1nm-5nm.

[0014] The positive electrode active material described above further includes manganese, and the manganese satisfies I120. Mn <I150 Mn <I180 Mn <I210 Mn ;

[0015] Among them, I120 Mn I150 Mn I180 Mn I210 Mn The values ​​represent the peak areas of manganese on the inner layer surface exposed after etching of the positive electrode active material for 120s, 150s, 180s, and 210s along the first direction, as characterized by XPS. The etching rate is 5 nm / min, with Ta2O5 material as the etching rate calibrator.

[0016] From the above-described positive electrode active material, the gradient factor G of the manganese element Mn Satisfy: G Mn ≥1.2;

[0017] in, .

[0018] The positive electrode active material described above, wherein the layered oxide material is a lithium-rich manganese-based material.

[0019] The chemical structural formula of the layered oxide material described above is: Li 1+n [Ni x Co y M z Mn 1-x-y-z ] 1-n O 2-m R m Where 0 < x ≤ 0.4, 0 ≤ y ≤ 0.15, 0 ≤ z ≤ 0.05, 0 < n < 0.4, 0 ≤ m < 0.1, M includes at least one of Mg, Al, Ti, Sr, Y, Zr, Nb, Mo, In, Sb, Ta, W, P, Cr, and La, and R includes at least one of F, S, Se, Cl, and B.

[0020] The positive electrode active material described above has a D50 of 3μm-15μm.

[0021] A second aspect of the present invention provides a method for preparing the positive electrode active material described in the first aspect, comprising at least the following steps:

[0022] The positive electrode active material precursor was treated with acid to obtain the pretreated precursor material;

[0023] The pretreated precursor material was mixed evenly with a lithium source to obtain the first intermediate.

[0024] The first intermediate is immersed in a solution containing a lanthanum source and transition metal ions to obtain the second intermediate;

[0025] The positive electrode active material is obtained by sintering the second intermediate.

[0026] According to the preparation method described above, the concentration of lanthanum in the solution containing lanthanum source and transition metal ions is 3000ppm-12000ppm.

[0027] According to the preparation method described above, during acid treatment, the solid-liquid ratio of the positive electrode active material precursor to the acid solution is 1g:(20-40)mL, the concentration of the acid solution is 0.005mol / L-0.02mol / L (calculated as hydrogen ions), the acid treatment temperature is 15℃-30℃, and the time is 5 min-30 min.

[0028] According to the preparation method described above, the first intermediate is immersed in a solution containing lanthanum source and transition metal ions at 60℃-90℃ for 25 min-40 min to obtain a wet-processed product. The wet-processed product is then kept at 160℃-220℃ for ≥2 h to obtain the second intermediate.

[0029] A third aspect of the present invention provides a positive electrode sheet, comprising a current collector and an active layer located on at least one surface of the current collector, the active layer comprising the above-described positive electrode active material or the positive electrode active material prepared by the above-described preparation method.

[0030] A fourth aspect of the present invention provides a battery comprising the above-described positive electrode.

[0031] The positive electrode active material provided by this invention has low oxygen loss and a stable crystal structure, which is of great significance for improving the cycle stability of batteries. Attached Figure Description

[0032] Figure 1 This is a composite electron backscattering (COMPO) image of the positive electrode active material in Example 1 of the present invention;

[0033] Figure 2 This is an EPMA elemental distribution diagram of the positive electrode active material in Example 1 of the present invention;

[0034] Figure 3 This is a high-resolution XPS spectrum of La 3d on the surface of the positive electrode active material (unetched, t=0s) in Example 1 of the present invention.

[0035] Figure 4 This is a high-resolution XPS energy spectrum of La 3d on the surface of the positive electrode active material (etched, t=30s) in Example 1 of the present invention.

[0036] Figure 5 This is a high-resolution XPS spectrum of La 3d on the surface of the positive electrode active material (etched, t=60s) in Example 1 of the present invention.

[0037] Figure 6 This is a high-resolution XPS spectrum of La 3d on the surface of the positive electrode active material (etched, t=90s) in Example 1 of the present invention.

[0038] Figure 7 The XRD pattern of the positive electrode active material in Example 1 of this invention was obtained using CuKα rays. Detailed Implementation

[0039] To enable those skilled in the art to better understand the solutions of this invention, the following provides a further detailed description of this application. The specific embodiments listed below are merely descriptions of the principles and features of this invention; the examples are only for explaining the invention and are not intended to limit its scope. Based on the embodiments of this invention, other embodiments obtained by those skilled in the art without creative effort are all within the scope of protection of this invention.

[0040] Addressing the problem of poor battery cycle stability caused by easy oxygen loss and lattice instability in existing cathode active materials, this invention provides a cathode active material comprising a substrate and a coating layer covering at least a portion of the surface of the substrate. The substrate comprises a layered oxide-based cathode active material, and the cathode active material includes lanthanum. The lanthanum is at least partially distributed in the coating layer, and the lanthanum content decreases along a first direction, wherein the first direction refers to the direction from the coating layer to the substrate.

[0041] This positive electrode active material can effectively improve the cycle performance of the battery. The reason is that the La-containing coating layer on the substrate surface acts as a physicochemical barrier. On the one hand, it can inhibit oxygen leakage and improve the stability of the crystal structure; on the other hand, it can block the direct contact between the electrolyte and the positive electrode, thereby suppressing interfacial side reactions and improving cycle stability. Lanthanum has good chemical / electrochemical stability. The bond energy of the chemical bond (La-O) formed by lanthanum (La) and oxygen (O) is much higher than that of the bond (MO) formed by transition metals nickel (Ni), cobalt (Co), and manganese (Mn) and oxygen. The stronger the bond energy, the higher the energy required for breakage, and the more stable the structure. When lanthanum is doped into the crystal lattice of a material (typically partially replacing transition metal sites), these strong La-O bonds act like solid "anchors" or "pillars" throughout the layered structure. When the material becomes unstable at the end of charging (high lithium depletion state) or due to overheating, oxygen atoms in the lattice need energy to escape and precipitate as oxygen. The strong La-O bonds significantly increase the energy barrier for oxygen atom escape, effectively "anchoring" surrounding oxygen atoms in the lattice. This thermodynamically suppresses oxygen precipitation and irreversible phase transitions, further inhibiting battery performance degradation and improving cycle stability. Furthermore, the structure with decreasing lanthanum content along the first direction optimizes the interfacial bonding and stress buffering between the coating layer and the substrate, further improving cycle stability. Through these synergistic effects, the cycle performance of the battery is effectively improved.

[0042] In the above, the coating layer covering at least part of the surface of the substrate means that the coating layer can completely encapsulate the substrate within the coating layer, or the coating layer can only cover a part of the outer surface of the substrate, preferably the coating layer completely covers the substrate.

[0043] The form in which lanthanum exists is not restricted in the above description. However, due to its high reactivity, lanthanum usually exists in the form of compounds, such as lanthanum oxide and lanthanum manganate. Furthermore, lanthanum can exist as a doped form, as an undoped form merely acting as a coating, or as both forms.

[0044] In this invention, the lanthanum content can be analyzed by X-ray photoelectron spectroscopy (XPS). The specific testing steps are as follows:

[0045] (1) Apply double-sided conductive adhesive to one side of the copper foil, take a small amount of positive electrode active material powder to be characterized, use a clean brush to pick up the positive electrode active material powder and brush it onto the double-sided adhesive of the foil, brush the material multiple times to ensure that the material is completely adhered to the double-sided adhesive.

[0046] (2) Use a press to apply 5-10 MPa pressure to the adhesive foil, flatten the material on the double-sided tape, and then attach the foil sample to the XPS sample stage with double-sided tape for testing;

[0047] (3) Select tantalum pentoxide (Ta2O5) material as the calibration standard for etching rate, adjust the etching parameters to ensure that the etching rate is 5nm / min; collect the fine XPS spectra of La element after etching 0s (i.e. no etching), 30s, 60s and 90s.

[0048] (4) For the fine La element spectra obtained at different etching times (0s, 30s, 60s, 90s), the Shirley method was used for background subtraction in the 825-845eV range. The area of ​​the peaks in the spectra was recorded according to each etching time point and denoted as I0. La I30 La I60 La I90 La .

[0049] The lanthanum content in the coating layer decreases along the first direction, and when detected using the above method, it satisfies I0. La >I30 La >I60 La >I90 La .

[0050] In some embodiments, the crystal structure of lanthanum includes a crystal structure belonging to the P321 space group and a crystal structure belonging to the R-3m space group, and the crystal structure belonging to the P321 space group is located on the side of the crystal structure belonging to the R-3m space group away from the matrix.

[0051] Lanthanum crystals belonging to the P321 space group have a hexagonal crystal system, while lanthanum crystals belonging to the R-3m space group (such as certain lanthanum-based alloys or compounds) exhibit a trigonal crystal system with triple rotation axis and helical axis symmetry. Synergistic effects among multiple lanthanum crystal systems can enhance grain boundary mechanical stability and reduce stress accumulation caused by active ion insertion / extraction, thereby significantly improving cycle stability.

[0052] Further research shows that in positive electrode active materials, when the gradient factor G of lanthanum... la Satisfy: 1.1≤G LaWhen the la content is less than 1.4, the resulting gradient La element coating structure is more robust and stable, and its impact on the kinetics of the positive electrode active material is small, so it will not affect the material's capacity.

[0053] In the above, the gradient factor G La The calculation formula is:

[0054] .

[0055] Among them, I0 La This represents the peak area of ​​La on the surface of the cathode active material before etching, as characterized by XPS; I30 La I60 La I90 La The values ​​represent the peak areas of the La element on the inner layer surface exposed after etching of the positive electrode active material for 30s, 60s, and 90s along the first direction, as characterized by XPS; the etching rate is the same as above.

[0056] In some implementations, the thickness of the coating layer is 1 nm to 5 nm, specifically, it can be 1 nm, 2 nm, 3 nm, 4 nm, 4.5 nm, 5 nm, or any value between any two of the above ranges. When the coating layer thickness is within this range, the coating layer has a strong effect on enhancing the stability of the material surface and interface without sacrificing the material's dynamic properties, thus avoiding the phenomenon of low capacity or deteriorated rate performance.

[0057] In some embodiments of the present invention, the positive electrode active material includes manganese, and when detected using the same detection method as for lanthanum, its content satisfies I120. Mn <I150 Mn <I180 Mn <I210 Mn .

[0058] Among them, I120 Mn I150 Mn I180 Mn I210 Mn The values ​​represent the peak areas of manganese on the inner layer surface exposed after etching of the positive electrode active material for 120s, 150s, 180s, and 210s along the first direction, as characterized by XPS; the etching rate is the same as above.

[0059] This structure can ensure a more stable structure of the positive electrode active material, and is beneficial for relieving cyclic stress, inhibiting the initiation and propagation of microcracks, and working synergistically with the coating layer to stabilize the oxygen lattice. This synergistic effect can further ensure the overall structural integrity of the material from the bulk phase to the surface, and improve its stability under long-term high-pressure cycling.

[0060] Furthermore, when the gradient factor G of manganese Mn Satisfy G Mn When the value is ≥1.2, it is more conducive to improving the stability of the crystal lattice, reducing oxygen loss, and thus improving cycle stability.

[0061] In the above, the gradient factor G Mn The calculation formula is:

[0062] .

[0063] In some embodiments of the present invention, the layered oxide material is a lithium-rich manganese-based material, and the chemical structural formula of the lithium-rich manganese-based material is Li. 1+n [Ni x Co y M z Mn 1-x-y-z ] 1-n O 2-m R m Where 0 < x ≤ 0.4, 0 ≤ y ≤ 0.15, 0 ≤ z ≤ 0.05, 0 < n < 0.4, 0 ≤ m < 0.1, M includes at least one of Mg, Al, Ti, Sr, Y, Zr, Nb, Mo, In, Sb, Ta, W, P, Cr, and La, and R includes at least one of F, S, Se, Cl, and B.

[0064] The chemical formula of layered oxide materials can be determined by inductively coupled plasma atomic emission spectrometry (ICP-AES), and the content of each element is determined in accordance with the industry standard "YS / T 1006.2-2014 Chemical Analysis Method for Lithium Nickel Cobalt Manganese Oxide".

[0065] In this invention, the particle size of the positive electrode active material is not specifically limited. D50 is usually 3μm-15μm, specifically, it can be 3μm, 5μm, 8μm, 10μm, 12μm, 15μm, etc.

[0066] The present invention also provides a method for preparing the above-mentioned positive electrode active material, which includes at least the following steps:

[0067] The positive electrode active material precursor was treated with acid to obtain the pretreated precursor material;

[0068] The pretreated precursor material was mixed evenly with a lithium source to obtain the first intermediate.

[0069] The first intermediate is immersed in a solution containing a lanthanum source and transition metal ions to obtain the second intermediate;

[0070] The second intermediate is sintered to obtain the positive electrode active material.

[0071] In the above process, the precursor of the positive electrode active material is first pretreated with acid, which can leach out a small amount of transition metal Mn from the surface of the precursor. On the one hand, this can result in a precursor in which the transition metal Mn gradually decreases from the inside to the outside. On the other hand, it can also create vacancies in the precursor, which is beneficial for the subsequent construction of a coating layer structure with varying lanthanum element concentration.

[0072] In some embodiments, the concentration of lanthanum in the lanthanum source solution is 3000 ppm to 12000 ppm. At this concentration, the lanthanum concentration is moderate, which is conducive to the formation of a clear surface coating layer and a transition metal Mn content gradient structure. Furthermore, the lanthanum content in the coating layer will not change rapidly. In addition, the appropriate coating layer thickness can also take into account the specific capacity.

[0073] In some embodiments, during acid treatment, the solid-liquid ratio of the positive electrode active material precursor to the acid solution is 1g:(20-40)mL, the concentration of the acid solution is 0.005mol / L-0.02mol / L (calculated as hydrogen ions), the acid treatment temperature is 15℃-30℃, and the time is 5min-30min. At this time, it is more conducive to the formation of a clear surface coating layer and a transition metal Mn content gradient structure.

[0074] In some embodiments, the acid treatment method is as follows: oxalic acid (or other acids, such as hydrochloric acid, nitric acid, citric acid, etc.) and ascorbic acid (antioxidant) are dissolved in an organic solvent to obtain an acid solution. The positive electrode active material precursor is immersed in the acid solution and stirred for 5 min-30 min. The reaction is immediately quenched with 0°C ethanol. After centrifugation, it is vacuum dried.

[0075] When mixing the pretreated precursor material with the lithium source, additives containing doping elements can be added as needed. These doping elements can be selected from one or more of Mg, Al, Ti, Sr, Y, Zr, Nb, Mo, In, Sb, Ta, and W. The precursor material and lithium source can be mixed by ball milling, for example, at a speed of 300-600 rpm for 2-6 hours.

[0076] In some embodiments, the first intermediate is immersed in a solution containing lanthanum source and transition metal ions at 60°C-90°C to obtain a wet-processed product. This wet-processed product is then removed and heated to 160°C-220°C, held for ≥2 hours to obtain the second intermediate. The role of the transition metal ions in the solution is to assist lanthanum in fully filling the vacancies left by acid treatment. The transition metal ions are not specifically limited and can be manganese, nickel, cobalt, etc., with a typical content of 0.005 mol / L-0.1 mol / L. Under these conditions, it is more conducive to forming a clear surface coating layer and a transition metal Mn content gradient structure, and the lanthanum content in the coating layer does not change rapidly.

[0077] In some implementations, in order to ensure that lanthanum can be deposited quickly on the material surface, a dispersant, such as polyacrylic acid (PAA), is usually added to the solution containing lanthanum source and transition metal ions. The addition of the dispersant can prevent particle aggregation and precipitation, thereby facilitating the rapid and uniform deposition of lanthanum on the surface of the first intermediate.

[0078] In some embodiments, the sintering process for the second intermediate is as follows: holding at 400℃-600℃ for 3-6 hours, and then holding at 850℃-950℃ for 8-12 hours. The heating and cooling rates are not limited and can be any values ​​between 1℃ / min and 10℃ / min.

[0079] In some embodiments, the general chemical formula of the above-mentioned positive electrode active material precursor is: Ni x Co y M z Mn 1-x-y-z (OH)2, where M represents one or more of the doping elements Mg, Al, Ti, Sr, Y, Zr, Nb, Mo, In, Sb, Ta, W, P, and Cr. The lithium source mentioned above can be selected from at least one of lithium hydroxide (LiOH), lithium carbonate (Li2CO3), and lithium oxalate (C2H3LiO4), and the lanthanum source can be selected from at least one of lanthanum nitrate (La(NO3)3) and lanthanum oxalate (La2(C2O4)3).

[0080] The present invention also provides a positive electrode sheet, comprising a current collector and an active layer located on at least one surface of the current collector, wherein the active layer comprises the above-described positive electrode active material or the positive electrode active material prepared by the above-described preparation method.

[0081] In the specific preparation of the positive electrode sheet, for example, the positive electrode active material of the present invention can be dispersed with a conductive agent and a binder in an appropriate amount of N-methylpyrrolidone (NMP) solvent, and thoroughly stirred and mixed to form a uniform positive electrode slurry; the positive electrode slurry is uniformly coated on the positive electrode current collector, and after drying, rolling and slitting, the positive electrode sheet is obtained. In one specific embodiment, the positive electrode active layer comprises, by mass percentage, 70-99 wt% of positive electrode active material, 0.5-15 wt% of conductive agent, and 0.5-15 wt% of binder, and more specifically, 80-98 wt% of positive electrode active material, 1-10 wt% of conductive agent, and 1-10 wt% of binder.

[0082] The positive current collector can be made of at least one of aluminum foil or nickel foil; the conductive agent can be selected from at least one of carbon black, acetylene black, graphene, Ketjen black, and carbon fiber; and the binder can be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, and polyurethane.

[0083] The present invention also provides a battery comprising the above-described positive electrode.

[0084] The specific type of battery of this invention is not particularly limited. For example, from the perspective of shape, the battery includes, but is not limited to, prismatic batteries, pouch batteries, and cylindrical batteries. From the perspective of the core structure, the battery core can be a wound core (i.e., a core formed by stacking positive electrode sheets, negative electrode sheets, and separators and then winding them) or a stacked core (i.e., multiple positive electrode sheets, negative electrode sheets, and separators are stacked to form a core). The outer shell can be a hard shell (such as a steel shell, a hard plastic shell, etc.) or a soft shell (such as an aluminum-plastic film shell, a pouch-type soft shell, etc.).

[0085] The technical solution of this application will be further explained below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or as recommended by the manufacturer. Unless otherwise specified, all reagents used are commercially available or obtained through public channels.

[0086] Example 1

[0087] This embodiment provides a positive electrode active material and its preparation method. The specific preparation steps are as follows:

[0088] Step 1: Dissolve oxalic acid and antioxidant (specifically ascorbic acid) in ethanol to obtain an acid solution. The final concentration of oxalic acid in the acid solution (calculated as hydrogen ions) is 0.015 mol / L, and the final concentration of ascorbic acid is 0.1 mol / L.

[0089] Ni 0.35 Mn 0.65 The (OH)2 precursor was immersed in the above acid solution (solid-liquid ratio of 1g:40mL), stirred at room temperature for 10min to complete the acid treatment process, and then immediately quenched with anhydrous ethanol at 0℃, centrifuged to separate, collected the precipitate, and then dried under vacuum at 60℃ for 6 hours.

[0090] Step 2, pre-treated Ni 0.35 Mn 0.65The (OH)2 precursor material and the lithium source LiOH were mixed in a ratio of 1.36:1 (total molar amount of Li, Ni, and Mn). The mixture was then finely mixed by ball milling at 400 rpm for 5 h to obtain the first intermediate.

[0091] Step 3: Immerse the first intermediate in an ethylene glycol solution containing 3000 ppm La source La(NO3)3 (calculated as La), 0.01 mol / L manganese oxalate, and 0.5 wt% polyacrylic acid (PAA). First, stir at 80°C for 30 min to allow the La... 3+ It is initially adsorbed onto the surface of the material, and then heated to 190℃ and kept at that temperature for 4 hours to obtain the second intermediate.

[0092] Step 4: The second intermediate is placed in a muffle furnace for high-temperature sintering. The temperature is increased to 500°C at a heating rate of 2°C / min and held for 4 hours. Then, the temperature is increased to 930°C at a heating rate of 5°C / min and held for 10 hours. Subsequently, the temperature is reduced to room temperature at a cooling rate of 5°C / min. After crushing, washing, drying and sieving, the positive electrode active material is obtained.

[0093] The prepared positive electrode active material was analyzed by ICP-AE. The results showed that the mass percentage of the main constituent elements was: lithium (Li): 9.66 wt%, nickel (Ni): 21.03 wt%, manganese (Mn): 36.55 wt%, oxygen (O): 32.76 wt%, and the content of la was 2998 ppm, satisfying the chemical formula Li. 1+n [Ni x Co y M z Mn 1-x-y-z ] 1-n O 2-m R m .

[0094] Example 2

[0095] This embodiment provides a positive electrode active material and its preparation method, which differs from Example 1 in that: in step 3, the La source La(NO3)3, calculated as La, has a concentration of 12000 ppm.

[0096] Example 3

[0097] This embodiment provides a positive electrode active material and its preparation method, which differs from Example 1 in that: in step 1, the final concentration of oxalic acid in the acid solution (calculated as hydrogen ions) is 0.020 mol / L.

[0098] Example 4

[0099] This embodiment provides a positive electrode active material and its preparation method, which differs from Example 1 in that: in step 3, the La source La(NO3)3, calculated as La, has a concentration of 13000 ppm.

[0100] Example 5

[0101] This embodiment provides a positive electrode active material and its preparation method, which differs from that of Example 1 in that: in step 3, the La source La(NO3)3, calculated as La, has a concentration of 6000 ppm.

[0102] Example 6

[0103] This embodiment provides a positive electrode active material and its preparation method, which differs from Embodiment 1 in that: in step 1, the precursor is Ni. 1 / 6 Co 1 / 6 Mn 4 / 6 (OH)2;

[0104] In step 2, the pretreated Ni 1 / 6 Co 1 / 6 Mn 4 / 6 (OH)₂ precursor material and LiOH are mixed in a ratio of 1.23:1, where the total molar amount of Li is equal to that of Ni, Co, and Mn.

[0105] The prepared positive electrode active material was analyzed by ICP-AE. The results showed that the mass percentage of the main constituent elements was as follows: lithium (Li): 8.80 wt%, nickel (Ni): 10.10 wt%, cobalt (Co): 10.10 wt%, manganese (Mn): 37.80 wt%, oxygen (O): 33.10 wt%, and lanthanum (La) content was 2999 ppm, satisfying the chemical formula Li 1+n [Ni x Co y M z Mn 1-x-y-z ] 1-n O 2-m R m .

[0106] Example 7

[0107] This embodiment provides a positive electrode active material and its preparation method, which differs from Embodiment 1 in that: in step 1, the final concentration of oxalic acid in the acid solution (calculated as hydrogen ions) is 0.005 mol / L.

[0108] Example 8

[0109] This embodiment provides a positive electrode active material and its preparation method, which differs from Embodiment 1 in that: in step 4, by adjusting the sintering and crushing conditions, the final obtained positive electrode active material particles have a D50 of approximately 3 μm. The specific sintering steps are as follows:

[0110] The second intermediate was placed in a muffle furnace for high-temperature sintering, heated to 500 °C at a heating rate of 2 °C / min and held for 6 h, then heated to 900 °C at a heating rate of 5 °C / min and held for 8 h, and then cooled to room temperature at a cooling rate of 5 °C / min. After crushing, washing, drying and sieving, the positive electrode active material was obtained.

[0111] Example 9

[0112] This embodiment provides a positive electrode active material and its preparation method, which differs from Embodiment 1 in that: after step 4, the sintering and crushing conditions are adjusted to make the final positive electrode active material particles with a D50 of approximately 15 μm. The specific steps are as follows:

[0113] The second intermediate was placed in a muffle furnace for high-temperature sintering, heated to 550 °C at a heating rate of 5 °C / min and held for 3 h, then heated to 950 °C at a heating rate of 8 °C / min and held for 14 h, and then cooled to room temperature at a cooling rate of 3 °C / min. After crushing, washing, drying and sieving, the positive electrode active material was obtained.

[0114] Comparative Example 1

[0115] This comparative example provides a positive electrode active material and its preparation method, which differs from Example 1 in that the precursor material does not undergo the treatment in step 1. The specific preparation steps are as follows:

[0116] Step 1, Ni 0.35 Mn 0.65 (OH)2 precursor material and LiOH are mixed in a ratio of 1.36:1 of the total molar amount of Li to Ni and Mn. The mixture is then finely mixed by ball milling at 400 rpm for 5 h to obtain the first intermediate.

[0117] Step 2: Immerse the first intermediate in an ethylene glycol solution containing 3000 ppm La source La(NO3)3 (calculated as La), 0.01 mol / L manganese oxalate, and 0.5 wt% polyacrylic acid (PAA). First, stir at 80°C for 30 min to allow the La... 3+ It is initially adsorbed onto the surface of the material, and then heated to 190℃ and kept at that temperature for 4 hours to obtain the second intermediate.

[0118] Step 3: The second intermediate is placed in a muffle furnace for high-temperature sintering. The temperature is increased to 500°C at a heating rate of 2°C / min and held for 4 hours. Then, the temperature is increased to 930°C at a heating rate of 5°C / min and held for 10 hours. Subsequently, the temperature is reduced to room temperature at a cooling rate of 5°C / min. After crushing, washing, drying and sieving, the positive electrode active material is obtained.

[0119] The prepared positive electrode active material was analyzed by ICP-AE. The results showed that the mass percentage of the main constituent elements was: lithium (Li): 9.66 wt%, nickel (Ni): 21.03 wt%, manganese (Mn): 36.55 wt%, oxygen (O): 32.76 wt%, and lanthanum (La) content was 2998 ppm, satisfying the chemical formula Li 1+n [Ni x Co y M z Mn 1-x-y-z ] 1-n O 2-m R m .

[0120] Comparative Example 2

[0121] This comparative example provides a positive electrode active material and its preparation method. The difference between this and Example 6 is that the precursor material does not undergo the treatment in step 1. The specific method is the same as that in Comparative Example 1.

[0122] Performance testing:

[0123] (1) Electron microscopy observation results

[0124] The cathode materials prepared in the above embodiments and comparative examples were observed using electron probe X-ray microscopy (EPMA). The observation results of the cathode active material in Example 1 are as follows: Figure 1 , 2 As shown.

[0125] Figure 1 This is a composite electron backscattering (COMPO) image of the positive electrode active material in Example 1. Figure 2 This is the corresponding EPMA element surface distribution diagram. Figure 1 , 2 It is known that the positive electrode active material includes a coating layer containing lanthanum.

[0126] Further observation was performed using transmission electron microscopy (TEM). The coating layer was identified by combining HRTEM lattice images with elemental EDS-mapping. The layer rich in La was determined as the coating layer, and its thickness was measured. The results are as follows: Figure 1 , 2 And as shown in Table 1 below:

[0127] Table 1

[0128]

[0129] As can be seen from the table above, in Example 4, due to the excessively high concentration of lanthanum, the thickness of the coating layer was significantly increased.

[0130] (2) Analysis of lanthanum and manganese content:

[0131] The lanthanum and manganese content of the positive electrode active materials obtained in each example and comparative example was analyzed by X-ray photoelectron spectroscopy (XPS). The specific steps are as follows:

[0132] Double-sided tape is attached to one side of the copper foil. A small amount of positive electrode active material powder to be characterized is taken and brushed onto the double-sided tape with a clean brush. The material is brushed multiple times to ensure that the material is completely adhered to the double-sided tape.

[0133] A press was used to apply 8 MPa pressure to the adhesive foil to flatten the material on the double-sided adhesive, and the foil sample was then adhered to the XPS sample stage with double-sided adhesive for testing.

[0134] Ta2O5 material was selected as the calibration standard for etching rate. The etching parameters were adjusted to ensure that the etching rate was 5 nm / min. The fine XPS spectra of La element were collected for the sample after etching for 0 s (i.e. no etching), 30 s, 60 s and 90 s.

[0135] For the fine La element spectra obtained at different etching times (0s, 30s, 60s, 90s), the Shirley method was used for background subtraction in the 825-845 eV range. The peak areas representing La in the spectra were recorded at each etching time point and denoted as I0. La I30 La I60 La I90 La The results are shown in Table 2 below.

[0136] For the fine manganese element spectra obtained at different etching times (120s, 150s, 180s, and 210s), background subtraction was performed using the Shirley method in the 825-845 eV range. The peak areas representing manganese in the spectra were recorded at each etching time point and denoted as I120. Mn I150 Mn I180 Mn I210 Mn The result is as follows Figure 3 And as shown in Table 2 below:

[0137] Table 2

[0138]

[0139] In this context, "-" indicates that the element has no concentration gradient.

[0140] The above results show that the preparation conditions of this invention have a significant impact on the formation of the gradient coating layer. In Examples 1, 2, 5, 6, 8, and 9, effective lanthanum-manganese dual-gradient structures were formed, with gradient factors G... La Located within the preferred range of 1.1-1.4, while G Mn ≥1.2. In contrast, in Example 3, the acid concentration was as high as 0.02 mol / L, and the acid dissolution was too vigorous, destroying the layered structure of the matrix surface, leading to the disordered dissolution and rearrangement of Mn ions, G Mn Too low (0.73), not reaching G. Mn ≥1.2, and compared to Examples 1 and 7, it shows that lanthanum is distributed more internally, G La The value reached 1.83, exceeding the preferred gradient factor range; in Example 7, the acid concentration was only 0.005 mol / L, resulting in weak acid dissolution and insufficient channels for La ions to permeate inwards. La With a value of only 0.69, it does not reach the optimal gradient factor range, and manganese is enriched inside the material. Mn The value was 2.37; in Example 4, due to the excessively high concentration of lanthanum, a thick coating layer was formed, and although a manganese gradient was formed, the gradient factor G was 2.37. Mn <1.2.

[0141] In Comparative Examples 1 and 2, since acid etching pretreatment was not used, no structure with gradually changing Mn concentration and no coating layer with gradually changing lanthanum element concentration were formed.

[0142] Figure 3 This is a high-resolution XPS spectrum of La 3d on the surface of the positive electrode active material in Example 1 (unetched, t=0s). Figure 4 This is a high-resolution XPS spectrum of La 3d on the surface of the positive electrode active material in Example 1 (after etching t=30s). Figure 5 This is a high-resolution XPS spectrum of La 3d on the surface of the positive electrode active material in Example 1 (after etching t=60s). Figure 6 The image shows the La 3d high-resolution XPS spectrum of the surface of the positive electrode active material in Example 1 (etched at t=90s). The orange line represents the background baseline, the gray line represents the original data, and the blue line represents the fitted curve. Figure 3 It can be seen that the surface of the positive electrode active material contains La element, and the content of La element decreases along the direction of the coating layer towards the substrate.

[0143] (3) Crystal structure (XRD) detection and analysis

[0144] The phase and crystal structure of the positive electrode active material prepared in Example 1 were analyzed using X-ray diffraction (XRD). The specific steps are as follows:

[0145] Take an appropriate amount of the positive electrode active material powder to be tested and grind it thoroughly in an agate mortar to obtain a uniform fine powder. Press the ground powder flat into the sample trough, ensuring that the sample surface is flush with the reference plane of the sample stage to reduce diffraction angle error. Place the sample stage with the sample loaded in an X-ray diffractometer, using CuKα rays (wavelength λ=1.5406 Å) as the radiation source. Set the instrument's scanning parameters to a scanning range (2θ) of 10° to 120° and a scanning rate of 5° / min to acquire a high signal-to-noise ratio diffraction pattern. Perform phase analysis on the acquired diffraction pattern, and determine the crystal phases present in the sample and their corresponding space groups by comparing and analyzing them with a standard crystallography database.

[0146] Figure 7 This is the X-ray diffraction (XRD) pattern of the positive electrode active material prepared in Example 1 of this invention. Figure 7 The diffraction peaks of the material are sharp and clear overall, indicating good crystallinity. The strongest characteristic peaks, located at diffraction angles of 18.3°, 36.5°, and 44.7°, belong to a lanthanum-containing crystal phase with space group R-3m. Crucially, in addition to the diffraction peaks of the R-3m main phase, a series of additional diffraction peaks were clearly observed. After careful identification and calibration, the characteristic diffraction peaks at 31.7°, 34.3°, and 47.5° belong to a lanthanum-containing crystal phase with space group P321. These results demonstrate that the crystal structure of lanthanum includes both those belonging to space group P321 and those belonging to space group R-3m.

[0147] (4) Confirmation of the spatial distribution of crystal structure

[0148] The positive electrode active material particles in Example 1 were processed by focused ion beam (FIB) to obtain particle cross-section samples, which were then observed using high-resolution transmission electron microscopy (HRTEM). Micro-area analysis was performed on the edge region. The diffraction spots obtained by fast Fourier transform (FFT) of the high-resolution lattice image and the selected area electron diffraction (SAED) pattern of the region could be successfully identified as P321 space group crystal structure and R-3m space group crystal structure. The crystal structure belonging to the P321 space group was located on the side of the crystal structure belonging to the R-3m space group away from the matrix.

[0149] In summary, the microstructure characterization results of XRD and TEM corroborate each other, confirming that the crystal structure of lanthanum includes crystal structures belonging to the P321 space group and crystal structures belonging to the R-3m space group, and that the crystal structure belonging to the P321 space group is located on the side of the crystal structure belonging to the R-3m space group away from the matrix.

[0150] (5) Battery performance test

[0151] Preparation of coin cell batteries: The positive electrode active materials from the examples and comparative examples were mixed with conductive carbon black and PVDF at a weight ratio of 90%:5%:5%, respectively, and dispersed to obtain a positive electrode slurry. This slurry was coated onto an aluminum foil current collector, with a positive electrode compaction density of 2.6 g / cm³. 3 The positive electrode sheet is prepared by rolling.

[0152] A lithium-ion coin cell is assembled from a positive electrode, a lithium metal negative electrode, and a separator (PP), and then injected with a non-aqueous electrolyte. The electrolyte is prepared by mixing ethylene carbonate (EC): diethyl carbonate (DEC): propylene carbonate (PC) in a mass ratio of 2:5:3, followed by the addition of 5% fluoroethylene carbonate (FEC) and 13% lithium hexafluorophosphate (LiPF6) by mass.

[0153] Specific capacity: Tested at 25 °C using an electrochemical testing system. First, charge at a constant current of 0.1C (Note: 1C = 180mA / g) to 4.60 V, then charge at a constant voltage of 0.05C until the current drops to 1 / 20 of the initial current, and then discharge at a constant current of 0.1C to 2.50 V. Repeat the above charge-discharge process three times, and take the discharge capacity of the third cycle as the specific capacity (unit: mAh·g⁻¹).

[0154] Battery cycle performance: At 25℃, the battery was charged at a constant current rate of 1C to 4.60V, then charged at a constant voltage rate of 0.05C to 4.60V, and then discharged at a discharge rate of 1C to 2.5V. This charge-discharge cycle was repeated 300 times. The discharge capacity Q1 at the first cycle and the discharge capacity Q300 at the 300th cycle were measured. The capacity retention rate Q = Q300 / Q1*100%, and the results are shown in Table 3 below.

[0155] Table 3

[0156]

[0157] The results above show that the batteries prepared with the positive electrode active materials in Examples 1-5 and 7-9 have better specific capacity and capacity retention than Comparative Example 1, and the batteries prepared with the positive electrode active materials in Example 6 have better specific capacity and capacity retention than Comparative Example 2. This indicates that by setting a coating layer containing lanthanum on the surface of the layered oxide positive electrode active material matrix and ensuring that the lanthanum content in the coating layer decreases along the first direction, the capacity and cycle performance of the battery can be improved.

[0158] The results above also show that the batteries prepared from the positive electrode active materials in Examples 1, 2, 5, 6, 8, and 9 have better specific capacity and capacity retention, indicating that when the gradient factor G of La element... La Satisfy: 1.1≤G La <1.4 and the gradient factor G of manganese Mn Satisfy G Mn When the gradient factor G of the La element is ≥1.2, the overall performance of the battery can be further improved; compared with Example 3, Examples 4 and 7 have better capacity retention, indicating that when the gradient factor G of the La element is ≥1.2, the overall performance of the battery can be further improved; La Satisfy: 1.1≤G La <1.4, or the gradient factor G of the Mn element Mn Satisfy G Mn When the value is ≥1.2, the cycle performance of the battery can be further improved.

[0159] Finally, it should be noted that other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein, and is not limited to what has been described above. Various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A positive electrode active material, characterized in that, The material includes a substrate and a coating layer covering at least a portion of the surface of the substrate. The substrate comprises a layered oxide material, and the positive electrode active material includes lanthanum. The lanthanum is at least partially distributed in the coating layer, and the lanthanum content decreases along a first direction. The first direction refers to the direction in which the coating layer points towards the substrate.

2. The positive electrode active material according to claim 1, characterized in that, The crystal structure of the lanthanum includes a crystal structure belonging to the P321 space group and a crystal structure belonging to the R-3m space group, with the P321 space group crystal structure located on the side of the R-3m space group crystal structure away from the matrix; and / or The gradient factor G of the lanthanum element La Satisfy: 1.1≤G La <1.4; in, ; I0 La This represents the peak area of ​​La on the surface of the cathode active material before etching, as characterized by XPS; I30 La I60 La I90 La The values ​​represent the peak areas of the La element on the inner layer surface exposed after etching of the positive electrode active material for 30s, 60s, and 90s along the first direction, as characterized by XPS. The etching rate is 5nm / min, with Ta2O5 material as the etching rate calibrator.

3. The positive electrode active material according to claim 1 or 2, characterized in that, The thickness of the coating layer is 1nm-5nm.

4. The positive electrode active material according to any one of claims 1-3, characterized in that, The positive electrode active material also includes manganese, and the manganese satisfies I120. Mn <I150 Mn <I180 Mn <I210 Mn ; Among them, I120 Mn I150 Mn I180 Mn I210 Mn The values ​​represent the peak areas of manganese on the inner layer surface exposed after etching of the positive electrode active material for 120s, 150s, 180s, and 210s along the first direction, as characterized by XPS. The etching rate is 5 nm / min, with Ta2O5 material as the etching rate calibrator.

5. The positive electrode active material according to claim 4, characterized in that, The gradient factor G of manganese Mn Satisfy: G Mn ≥1.2; in, .

6. The positive electrode active material according to any one of claims 1-5, characterized in that, The layered oxide material is a lithium-rich manganese-based material; and / or The chemical structural formula of the layered oxide material is: Li 1+n [Ni x Co y M z Mn 1-x-y-z ] 1-n O 2-m R m Where 0 < x ≤ 0.4, 0 ≤ y ≤ 0.15, 0 ≤ z ≤ 0.05, 0 < n < 0.4, 0 ≤ m < 0.1, M includes at least one of Mg, Al, Ti, Sr, Y, Zr, Nb, Mo, In, Sb, Ta, W, P, Cr, and La, and R includes at least one of F, S, Se, Cl, and B; and / or The D50 of the positive electrode active material is 3μm-15μm.

7. A method for preparing the positive electrode active material according to any one of claims 1-6, characterized in that, At least the following steps are included: The positive electrode active material precursor was treated with acid to obtain the pretreated precursor material; The pretreated precursor material was mixed evenly with a lithium source to obtain the first intermediate. The first intermediate is immersed in a solution containing a lanthanum source and transition metal ions to obtain the second intermediate; The positive electrode active material is obtained by sintering the second intermediate.

8. The method for preparing the positive electrode active material according to claim 7, characterized in that, The concentration of lanthanum in the solution containing lanthanum source and transition metal ions is 3000 ppm - 12000 ppm; and / or During acid treatment, the solid-liquid ratio of the positive electrode active material precursor to the acid solution is 1 g:(20-40) mL, the concentration of the acid solution (calculated as hydrogen ions) is 0.005 mol / L-0.02 mol / L, the acid treatment temperature is 15℃-30℃, and the treatment time is 5 min-30 min; and / or The first intermediate is immersed in a solution containing lanthanum source and transition metal ions at 60℃-90℃ for 25 min-40 min to obtain a wet-processed product. The wet-processed product is then kept at 160℃-220℃ for ≥2 h to obtain the second intermediate.

9. A positive electrode plate, characterized in that, It includes a current collector and an active layer located on at least one surface of the current collector, the active layer comprising the positive electrode active material according to any one of claims 1-6 or the positive electrode active material prepared by the preparation method according to claim 7 or 8.

10. A battery, characterized in that, Includes the positive electrode sheet as described in claim 9.

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