Positive electrode active material and lithium ion battery containing same

Abstract

The invention relates to the technical field of lithium battery materials, and particularly discloses a positive electrode active material and a lithium ion battery containing the positive electrode active material, and an ion conductive composite coating containing lithium phosphate, lithium borate and lithiated metal oxide is constructed on the surface of a positive electrode particle. The lithium phosphate and the lithium borate form a Li < + >-containing transferable inorganic phase continuous channel in the coating, so that the migration resistance of lithium ions at an interface is reduced, and the reaction uniformity is improved; the lithiated metal oxide nanophase has high chemical stability and interface matching performance, can cover and passivate surface active sites prone to side reaction under high potential, and inhibits electrolyte oxygenolysis and CEI continuous thickening and component deterioration induced by the electrolyte oxygenolysis. According to the structure, the oxygenolysis tendency of the electrolyte at high temperature and high potential is effectively reduced, continuous thickening and component deterioration of CEI are slowed down, and dissolution of transition metal ions and chain type side reactions caused by the dissolution of transition metal ions can be inhibited through surface bonding.

Description

Technical Field

[0001] This invention relates to the field of lithium battery materials technology, specifically to a positive electrode active material and a lithium-ion battery containing the same. Background Technology

[0002] High-nickel layered ternary cathode materials have become a key cathode system for high-energy-density lithium-ion batteries due to their high specific capacity. However, under harsh conditions of high-temperature cycling and storage, these materials are prone to severe interfacial instability and kinetic degradation. Specifically, at high potential windows, the oxidative decomposition reaction of the electrolyte intensifies, leading to a continuous thickening and compositional instability of the electrode-electrolyte interface (CEI); simultaneously, transition metal ions (especially Ni)... 3+ / 4+ Lithium dissolves and migrates to the negative electrode surface, depositing and disrupting the solid electrolyte interphase (SEI), leading to the loss of active lithium. Furthermore, the increased surface lattice oxygen activity induces structural reconstruction and intergranular microcrack propagation, further exacerbating capacity decay and impedance increases, thus limiting the long-term cycling and storage reliability of high-nickel systems at high temperatures.

[0003] To alleviate the aforementioned problems, existing technologies mainly focus on two approaches: electrolyte additive modification and cathode surface coating. Electrolyte additives can form a protective film on the cathode surface, but this film formation is often time-sensitive; at high temperatures, the film layer is prone to dissolution and redeposition, leading to a decrease in protective effect and potentially exacerbating side reactions. On the other hand, while single inorganic oxide coatings (such as Al2O3, ZrO2, etc.) can, to some extent, isolate the electrode from direct contact with the electrolyte and reduce interfacial side reactions, these inert coatings typically have low ionic conductivity, limiting lithium-ion cross-interface transport and resulting in increased polarization and decreased rate performance. Using single lithium salt coatings (such as Li3PO4, LiAlO2, etc.) may face problems such as loose coating structure and insufficient coverage uniformity, making it difficult to effectively suppress the corrosion of acidic substances in the electrolyte and the dissolution of transition metals, thus having limited effect on improving interfacial chemical stability.

[0004] Especially in high-nickel cathode systems, the increased surface activity of Ni leads to more oxygen vacancies and enhanced catalytic activity, resulting in a more pronounced tendency for oxygen release and related chain side reactions. There is often a contradiction between the requirements for interfacial stability and ion transport efficiency: constructing excessively thick or dense protective layers to pursue chemical stability hinders lithium-ion migration and increases interfacial impedance; while thin-layer or discontinuous modifications that focus on maintaining high ion transport are difficult to withstand electrolyte erosion and structural degradation under high temperature and pressure. Therefore, developing an interfacial modification strategy that can balance high-temperature chemical stability and efficient ion transport, and possess structural adaptability or multifunctional synergistic effects, has become a key technological challenge for improving the high-temperature performance of high-nickel layered cathode materials. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the purpose of this invention is to provide a positive electrode active material and a lithium-ion battery containing the same.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A positive electrode active material, the positive electrode active material comprising positive electrode active substance particles, the surface of the positive electrode active substance particles being loaded with an ion-conducting composite coating.

[0007] Based on the above technical solutions, the ion-conducting composite coating comprises lithium phosphate, lithium borate, and lithium metal oxide.

[0008] Based on the above technical solutions, preferably, the average thickness of the ion-conducting composite coating is 10-20 nm.

[0009] Based on the above technical solutions, preferably, the molar ratio of boron to phosphorus in the ion-conducting composite coating is 0.3-0.6:1.

[0010] Based on the above technical solutions, preferably, the coverage of the ion-conducting composite coating on the surface of the positive electrode active material particles is ≥80%.

[0011] Based on the above technical solutions, preferably, the positive electrode active material particles are selected from one or more of layered oxide positive electrode materials, spinel positive electrode materials, and olivine positive electrode materials.

[0012] Based on the above technical solutions, preferably, the lithium phosphate is selected from one or more of lithium phosphate, lithium pyrophosphate, and lithium difluorophosphate; the lithium borate is selected from one or more of lithium borate, lithium metaborate, and lithium tetraborate; and the lithium metal oxide is selected from one or more of lithium lanthanum zirconium oxide, lithium aluminum titanium phosphorus oxide, lithium titanium oxide, lithium germanium phosphorus oxide, and lithium zirconium oxide.

[0013] Based on the above technical solutions, the preferred method for forming the ion-conducting composite coating is as follows: Phosphoric acid source and boric acid source are added to a solvent and stirred evenly. Then, lithium alkali is added to adjust the pH of the solution to 6.3-6.8. After aging, a sol precursor is obtained. Then, lithium metal oxide dispersion is added and dispersed evenly to obtain a coating slurry. The coating slurry is coated on the surface of the positive electrode active material particles. After drying and curing, an ion-conducting composite coating is formed on the surface of the positive electrode active material particles.

[0014] Based on the above technical solutions, the preferred solid content in the coating slurry is 0.5-1.5 wt%.

[0015] Based on the above technical solutions, preferably, the amount of solid deposition in the coating is 0.2-1.2 wt% of the mass of the positive electrode active material particles.

[0016] The present invention also provides a lithium-ion battery, wherein the lithium-ion battery comprises the above-mentioned positive electrode active material.

[0017] Compared with the prior art, the present invention has the following beneficial effects: This invention constructs an ion-conducting composite coating comprising lithium phosphate, lithium borate, and lithium metal oxide on the surface of cathode particles. The lithium phosphate and lithium borate form a continuous channel of migratable Li⁺ inorganic phase within the coating, which helps reduce lithium ion migration resistance at the interface and improves reaction uniformity. The lithium metal oxide nanophase possesses high chemical stability and interface compatibility, covering and passivating surface active sites prone to side reactions at high potentials, inhibiting electrolyte oxidative decomposition and its induced continuous CEI thickening and compositional degradation. This structure not only effectively reduces the tendency of electrolyte oxidative decomposition at high temperatures and high potentials, slowing down the continuous thickening of CEI and compositional deterioration, but also inhibits the dissolution of transition metal ions and their chain-like side reactions through surface bonding. Simultaneously, the interconnected and stable ion transport channels formed between the components in the coating ensure rapid diffusion of lithium ions at the interface. Therefore, while achieving interfacial chemical stability at high temperatures and high voltages, it effectively reduces interfacial impedance and electrochemical polarization, improving the cycle life and storage performance of the high-nickel cathode under harsh operating conditions. Detailed Implementation

[0018] The present invention will be further described in detail below through specific preferred embodiments, but the present invention is not limited to the following embodiments.

[0019] It should be noted that, unless otherwise specified, all chemical reagents involved in this invention were purchased through commercial channels.

[0020] The present invention provides a positive electrode active material, the positive electrode active material comprising positive electrode active material particles, the surface of the positive electrode active material particles being loaded with an ion-conducting composite coating, the ion-conducting composite coating comprising lithium phosphate, lithium borate and lithium metal oxide.

[0021] The average thickness of the ion-conductive composite coating directly determines the ability to cover and repair the surface defects, grain boundaries, and highly active sites of the positive electrode, and also determines the path length and equivalent tortuosity of ion transmembrane migration, thereby affecting the rate of interfacial side reactions and the impedance growth trend under high-temperature conditions. In the technical solution disclosed in the present invention, the average thickness of the ion-conductive composite coating is 10-20 nm. For example, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm, 20 nm can be selected, but it is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0022] Boron and phosphorus elements determine the relative proportion of boron-containing and phosphorus-containing phases in the coating, thereby affecting the acid inhibition and buffering ability, inorganic network densification characteristics, and composite stability with the lithiated metal oxide framework of the coating. In the technical solution disclosed in the present invention, the molar ratio of boron to phosphorus in the ion-conductive composite coating is 0.3-0.6:1. For example, 0.3:1, 0.35:1, 0.4:1, 0.45:1, 0.5:1, 0.55:1, 0.6:1 can be selected, but it is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0023] The coverage rate reflects the shielding degree and interface consistency of the coating for the highly active sites on the particle surface, and is a key structural parameter that determines whether local side reactions are amplified at high temperatures. When the coverage rate is reduced to 80% or lower, the exposed area on the particle surface increases. These areas are more likely to trigger the oxidation and decomposition of the electrolyte and form a locally thickened CEI under high potential and high temperature conditions, thereby causing uneven spatial distribution of the interfacial impedance, resulting in polarization concentration and intensified thermal-reaction coupling, and ultimately manifested as a decrease in capacity retention and an increase in DCIR growth. Therefore, in the technical solution disclosed in the present invention, the coverage rate of the ion-conductive composite coating on the surface of the positive electrode active material particles is ≥80%.

[0024] In the technical solution disclosed in the present invention, the positive electrode active material particles are selected from one or more of layered oxide positive electrode materials, spinel positive electrode materials, and olivine positive electrode materials.

[0025] Preferably, the layered oxide positive electrode material is selected from nickel-cobalt-manganese ternary materials or nickel-cobalt-aluminum materials.

[0026] More preferably, the layered oxide positive electrode material is selected from LiNi x Co y Mn z O2, where 0.80 ≤ x ≤ 0.92, 0 < y ≤ 0.10, 0 < z ≤ 0.10, and x + y + z = 1.

[0027] In the technical solution disclosed in this invention, the lithium phosphate is selected from one or more of lithium phosphate Li3PO4, lithium pyrophosphate Li4P2O7, and lithium difluorophosphate LiPO2F2.

[0028] The lithium borate is selected from one or more of lithium borate (Li3BO3), lithium metaborate (LiBO2), and lithium tetraborate (Li2B4O7).

[0029] The lithium metal oxide is selected from one or more of lithium lanthanum zirconium oxide, lithium aluminum titanium phosphorus oxide, lithium titanium oxide, lithium germanium phosphorus oxide, and lithium zirconium oxide.

[0030] Preferably, the lithium metal oxide includes Li7La3Zr2O 12 Li 1.3 Al 0.3 Ti 1.7 (PO4)3, Li4Ti5O 12 Li 1+x Al x Ge 2-x One or more of (PO4)3 and Li2ZrO3.

[0031] In the technical solution disclosed in this invention, the formation process of the ion-conducting composite coating is as follows: Phosphoric acid source and boric acid source are added to deionized water and stirred evenly. Then, lithium alkali is added to adjust the pH of the solution to 6.3-6.8. After aging, a sol precursor is obtained. Then, lithium metal oxide dispersion is added and dispersed evenly to obtain a coating slurry. The coating slurry is coated on the surface of the positive electrode active material particles. After drying and curing, an ion-conducting composite coating is formed on the surface of the positive electrode active material particles.

[0032] In the technical solution disclosed in this invention, the phosphoric acid source forms a lithium phosphate precursor under the neutralization of lithium base: H3PO4 + LiOH → LiH2PO4 + H2O; Boric acid source forms lithium borate precursor under the action of lithium base: H3BO3 + LiOH → LiBO2 + 2H2O; Dehydration condensation and structural rearrangement during the curing process: 2LiH2PO4→ Li2H2P2O7+ H2O; During the curing process, Li2H2P2O7 and LiBO2 undergo further dehydration and structural rearrangement to form a Li-BPO inorganic network phase consisting of lithium phosphate phase, lithium borate phase and its composite, which together with LLZO nanoparticles constitute an ion-conducting composite coating.

[0033] Preferably, the phosphoric acid source is selected from one or more of phosphoric acid, dihydrogen phosphate, and hydrogen phosphate.

[0034] Preferably, the boric acid source is selected from one or more of boric acid and borate.

[0035] Preferably, the lithium alkali is selected from one or more of lithium hydroxide, lithium carbonate, and lithium acetate.

[0036] Preferably, the solid content in the coating slurry is 0.5-1.5 wt%, for example, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, but not limited to the listed values, other unlisted values ​​within the range are also applicable.

[0037] Preferably, the amount of solid deposition in the coating is 0.2-1.2 wt% of the mass of the positive electrode active material particles. For example, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.1 wt%, and 1.2 wt% can be selected, but it is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0038] Preferably, the drying temperature is 40-60℃.

[0039] Preferably, the curing is carried out in a nitrogen atmosphere or a mixture of nitrogen and trace amounts of oxygen, wherein the oxygen volume fraction is 0.1-1.0%.

[0040] Preferably, the curing temperature is 300-450℃, such as 300℃, 320℃, 350℃, 380℃, 400℃, 420℃, and 450℃; the curing time is 0.5-4h, such as 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, and 4h, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0041] The present invention will be further described below through specific embodiments. The positive electrode active material particles used in the embodiments of the present invention are NCM811 powder with a D50 of approximately 10 μm; the chemical formula of LLZO is Li7La3Zr2O. 12 The D50 of the nanoparticles is ≤200nm.

[0042] Example 1 A method for preparing a positive electrode active material includes the following steps: S1. Add 80g of deionized water to a mixing tank, turn on the stirrer at 400rpm, then add 2.19g of 85wt% phosphoric acid, stir for 10min, add 0.53g of boric acid, stir until completely dissolved, then add 120g of ethanol, stir for 10min to obtain an acid source mixed solution.

[0043] S2. Control the system temperature at 28℃, slowly add 1.0 mol / L LiOH solution using a titration pump, monitor the pH in real time, stop adding when the pH reaches 6.55, continue stirring and aging for 30 min to obtain the lithium boron phosphate sol precursor.

[0044] S3. Mix 60g of ethanol and 40g of water evenly to obtain a mixed solvent. Add 3.12g of LLZO to the mixture and disperse it under high-speed shear for 10min to obtain an LLZO pre-dispersion. Then add the LLZO pre-dispersion to the lithium boron phosphate sol precursor, stir for 20min, and ultrasonically disperse for 15min. Then add ethanol / water mixed solvent (ethanol to water mass ratio of 60:40) to the slurry to make the total mass of the slurry reach 600g, corresponding to a solid content of 1.0wt%.

[0045] S4. Add 1000g of NCM811 to a fluidized bed and purge with nitrogen. Set the bed temperature to 50℃. Use a dual-fluid spray gun with nitrogen as the atomizing gas, atomization pressure of 0.25MPa, and a spray rate of 12g / min. Spray 600g of coating slurry onto NCM811 in 3 coats. After spraying, continue drying at 50℃ for 60min to obtain dried coated powder. Place the dried coated powder in a tube furnace and purge with nitrogen for 30min. Raise the temperature to 380℃ at a rate of 5℃ / min and hold for 2h in a nitrogen atmosphere. Cool the furnace to room temperature to obtain the positive electrode active material.

[0046] The obtained positive electrode active material was tested, as follows: Thickness: At least 30 particles were randomly selected for cross-sectional TEM measurements at multiple points, and the statistical average thickness was 15 nm. B / P molar ratio: After acid digestion of the sample, the B and P contents were determined by ICP-OES, and the calculated B / P molar ratio was 0.45:1; Coverage: The particle surface was scanned using EDS, and the coverage area was statistically analyzed using image segmentation. The coverage rate was calculated to be approximately 90%.

[0047] Example 2 The difference between this embodiment and Embodiment 1 is that in this embodiment, only the lithium metal oxide is replaced by LATP instead of LLZO; everything else is the same as in Embodiment 1.

[0048] Example 3 The difference between this embodiment and Embodiment 1 is that in this embodiment, only the lithium metal oxide is replaced by Li2ZrO3 instead of LLZO; otherwise, they are the same as in Embodiment 1.

[0049] Example 4 The difference between this embodiment and Embodiment 1 is that the coating slurry spraying time is shortened from 60 min in Embodiment 1 to 40 min in this embodiment, which reduces the amount of solid deposition in the coating from 0.60 wt% to about 0.40 wt% and the average coating thickness to 10 nm. Everything else is the same as in Embodiment 1.

[0050] Example 5 The difference between this embodiment and Embodiment 1 is that the coating slurry spraying time is extended from 60 min in Embodiment 1 to 80 min in this embodiment, thereby increasing the solid deposition amount of the coating from 0.60 wt% to about 0.80 wt% and the average coating thickness to 20 nm. Everything else is the same as in Embodiment 1.

[0051] Example 6 The difference between this embodiment and Embodiment 1 is that this embodiment only adjusts the sol composition by reducing the amount of boric acid source while keeping the amount of phosphoric acid source unchanged. Specifically, the amount of boric acid source H3BO3 is reduced from 0.53g to 0.35g, so that the molar ratio of B to P in the coating changes from 0.45 to 0.30. Everything else is the same as in Embodiment 1.

[0052] Example 7 The difference between this embodiment and Embodiment 1 is that this embodiment only adjusts the sol composition by increasing the amount of boric acid source while keeping the amount of phosphoric acid source unchanged. Specifically, the amount of boric acid source H3BO3 is increased from 0.53 g to 0.70 g, so that the molar ratio of B to P in the coating changes from 0.45 to 0.60. Everything else is the same as in Embodiment 1.

[0053] Example 8 The difference between this embodiment and Embodiment 1 is that this embodiment reduces the coverage rate only by reducing the spray atomization pressure and reducing the number of spray passes. Specifically, the atomization pressure is reduced from 0.25MPa to 0.18MPa, and the number of spray passes is reduced from 3 to 2, so that the coating coverage rate on the particle surface changes from 90% to 80%. Everything else is the same as in Embodiment 1.

[0054] Example 9 The difference between this embodiment and Embodiment 1 is that this embodiment improves the coverage rate only by increasing the spray atomization pressure and increasing the number of spray passes. Specifically, the atomization pressure is increased from 0.25 MPa to 0.30 MPa and the number of spray passes is increased from 3 to 4, so that the coverage rate of the coating on the particle surface changes from 90% to 95%. Everything else is the same as in Embodiment 1.

[0055] Example 10 The difference between this embodiment and Example 1 is that this embodiment only adjusts the pH at the lithium-alkali neutralization endpoint. Specifically, the pH of the sol preparation is adjusted from 6.55 to 6.30, so that the pH of the coated sol becomes 6.30. Everything else is the same as in Example 1.

[0056] Example 11 The difference between this embodiment and Example 1 is that this embodiment only adjusts the pH at the lithium-alkali neutralization endpoint. Specifically, the pH of the sol preparation is adjusted from 6.55 to 6.80, so that the pH of the coated sol becomes 6.80. Everything else is the same as in Example 1.

[0057] Comparative Example 1 Compared with Example 1, this comparative example does not have an ion-conducting composite coating loaded on the surface of the positive electrode active material particles.

[0058] Comparative Example 2 The difference between this comparative example and Example 1 is that this comparative example only omits the addition of phosphate source in the coating slurry, specifically by not adding H3PO4, so that the coating does not contain lithium phosphate. Everything else is the same as in Example 1.

[0059] Comparative Example 3 The difference between this comparative example and Example 1 is that this comparative example only omits the boric acid source in the coating slurry, specifically by not adding H3BO3, so that the coating does not contain lithium borate. Everything else is the same as in Example 1.

[0060] Comparative Example 4 The difference between this comparative example and Example 1 is that the coating slurry spraying time in this comparative example is shortened from 60 min in Example 1 to 20 min, which reduces the amount of solid deposited coating from 0.60 wt% to about 0.20 wt% and the average coating thickness to 5 nm. Everything else is the same as in Example 1.

[0061] Comparative Example 5 The difference between this comparative example and Example 1 is that the coating slurry spraying time in this comparative example is increased from 60 min in Example 1 to 120 min, which increases the coating solid deposition amount from 0.60 wt% to about 1.20 wt% and the average coating thickness to 30 nm. Everything else is the same as in Example 1.

[0062] Comparative Example 6 The difference between this comparative example and Example 1 is that this comparative example only adjusts the sol composition by reducing the amount of boric acid source while keeping the amount of phosphoric acid source unchanged. Specifically, the amount of boric acid source H3BO3 is reduced from 0.53 g to 0.12 g, so that the molar ratio of B to P in the coating changes from 0.45 to 0.10. Everything else is the same as in Example 1.

[0063] Comparative Example 7 The difference between this comparative example and Example 1 is that this comparative example only adjusts the sol composition by adjusting the amount of boric acid source while keeping the amount of phosphoric acid source unchanged. Specifically, the amount of boric acid source H3BO3 is increased from 0.53 g to 0.94 g, so that the molar ratio of B to P in the coating changes from 0.45 to 0.80. Everything else is the same as in Example 1.

[0064] Comparative Example 8 The difference between this embodiment and Example 1 is that this embodiment only adjusts the pH at the lithium-alkali neutralization endpoint. Specifically, the pH of the sol preparation is adjusted from 6.55 to 6.0, so that the pH of the coated sol becomes 6.0. Everything else is the same as in Example 1.

[0065] Comparative Example 9 The difference between this embodiment and Example 1 is that this embodiment only adjusts the pH at the lithium-alkali neutralization endpoint. Specifically, the pH of the sol preparation is adjusted from 6.55 to 7.5, so that the pH of the coated sol becomes 7.5. Everything else is the same as in Example 1.

[0066] The main parameter differences between the embodiments and comparative examples provided by this invention are shown in Table 1.

[0067] Table 1. Variation of main parameters in different groups The positive electrode active materials prepared in the examples and comparative examples were assembled into batteries, and the specific steps are as follows: Method for manufacturing positive electrode plates: The positive electrode active material, conductive carbon black, carbon nanotubes and polyvinylidene fluoride (PVDF) prepared in the examples and comparative examples were taken and thoroughly mixed in an N-methylpyrrolidone solvent system at a mass ratio of 96:1:1:2 to obtain a positive electrode coating material. The positive electrode coating material was then coated on a 12.0 μm thick aluminum foil, dried and cold-pressed to obtain a positive electrode sheet. Electrolyte preparation: An electrolyte was prepared by mixing lithium hexafluorophosphate (LiPF6), ethylene carbonate (EC), dimethyl carbonate (DMC), fluoroethylene carbonate (FEC), ethylene sulfate (DTD), and vinylene carbonate (VC) in a mass percentage ratio of 10.0: 22.0: 53.0: 3.0: 7.0: 5.0.

[0068] Negative electrode manufacturing method: The negative electrode sheet includes a negative current collector copper foil and a negative electrode coating material coated on both sides of the copper foil. Calculated by mass percentage, the negative electrode coating material includes 10.0% deposited silicon carbon, 76.0% graphite, 0.75% of the aforementioned conductive agent, 0.75% conductive carbon black, 1.0% thickener sodium carboxymethyl cellulose (CMC), 0.75% binder polyacrylic acid (PAA), and 0.75% binder styrene-butadiene rubber (SBR). The above substances are added to deionized water and stirred to form the negative electrode coating material with a solid content of 42%. The negative electrode coating material is then coated on both sides of the negative current collector (copper foil), and after drying and cold pressing, a negative electrode sheet is formed. Assembly of lithium-ion batteries: After the positive and negative electrode sheets are rolled and slit, they are wound together with the separator according to a set process to form a 21700 cylindrical battery core. Subsequently, the battery core is fixed to a pre-made connecting piece by welding and then installed into a metal battery casing. After completing key processes such as electrolyte injection, sealing, and formation, the lithium-ion battery described in Example 1 is obtained. This lithium-ion battery uses a cylindrical casing with an external dimension of 21.0 mm in diameter and 70.0 mm in length, conforming to the 21700 standard specification.

[0069] This invention also provides a method for detecting the B / P molar ratio in the ion-conducting composite coating of an assembled lithium battery, the specific steps of which are as follows: S0, Battery Preparation Discharge the battery to a safe voltage of 2.5V and leave it to stand for ≥12 hours.

[0070] Disassembly was performed in an Ar glove box (H2O, O2 ≤ 1 ppm) to avoid introducing additional P signal through LiPF6 hydrolysis.

[0071] S1. Disassemble and remove the positive electrode plate. Open the casing, remove the core, and separate the positive electrode sheet.

[0072] Gently rinse the positive electrode three times (30-60 seconds each time) with anhydrous DMC / EMC to remove free electrolyte and soluble salts.

[0073] Vacuum drying at 40-60 ℃ for ≥12 h.

[0074] S2. Obtain "positive electrode active material particles" from the electrode sheet. Cut the positive electrode into small pieces (5×5 mm).

[0075] Soak in NMP (50-60 ℃) and gently sonicate / shake, then mechanically scrape off the active layer powder.

[0076] The powder was collected by filtration, washed twice with NMP, and then washed once with anhydrous DMC; then vacuum dried.

[0077] S3, Particle cross-section sample preparation The dried powder was dispersed in ethanol solvent, added dropwise to epoxy resin, and cured in a glove box.

[0078] Polish the solidified block with sandpaper in stages to expose the cross-section of the particles.

[0079] The surface is coated with gold.

[0080] S4. Preparation of TEM thin films Select particles from FIB-SEM (simultaneously take positive electrode powder from different locations: the side near the separator, the side near the current collector, and different regions).

[0081] S5, STEM imaging for locating "coating segments" Particle boundaries observed under STEM-HAADF: CEI layers are typically extremely thin and have low contrast. Inorganic coatings typically form continuous annular layers across the cross-section; The Ni / Co / Mn signal is significantly enhanced after entering the bulk.

[0082] The presence of B and P enrichment bands in the coating area was confirmed by STEM-EDS rapid surface scanning (for localization only, not as the final quantitative basis).

[0083] S6, EELS quantitative acquisition (B / P) STEM-EELS line scanning was used, with a step size of 0.5-1.0 nm, spanning "surface → coating → bulk".

[0084] Capture and extract the background: B: B K-edge (approximately 188 eV) P:P L-edge (approximately 132 eV) Element quantification is performed in the software: the atomic concentration (atomic %) is calculated for each pixel / dot.

[0085] S7. Define the "Coating ROI" and remove the CEI. The “significant rise point of Ni / Co / Mn signal” is defined as the body boundary.

[0086] The outermost 1-3 nm (or until the F / C / FP decomposition components decrease significantly) is defined as the CEI / residual layer and is not included in the B / P calculation.

[0087] The stable B and P bands "after CEI and before the body" are defined as the coating ROI.

[0088] Averaging the atomic concentrations of B and P within the ROI yields: B / P = nB / nP, where nB and nP are the average molar amounts (number of atoms) of B and P within the ROI.

[0089] The batteries prepared in the examples and comparative examples were subjected to performance tests, as detailed below: Place the battery in a 45℃ constant temperature chamber for 6 hours and test it according to the following steps: (1) First round of constant current and constant voltage charging: charge at a constant current of 0.1C to 4.25V, then switch to constant voltage charging until the current drops to 0.01C.

[0090] (2) Let it stand for 30 minutes after charging is complete.

[0091] (3) Perform constant current discharge at a rate of 0.33C to 2.5V and record the reference capacity C0.33.

[0092] (4) Let it stand for another 30 minutes.

[0093] P1: Capacity retention rate during 45℃ high-temperature cycling The battery cells obtained in step (2) were placed in a 45°C oven and left to stand for 6 hours. Charge and discharge at 1C for 500 cycles (2.5-4.2V), and retest the discharge capacity at 0.33C every 50 cycles at 25℃; Calculation: Retention rate = Capacity retested at 500th lap / C0.33 × 100%.

[0094] P2: Storage capacity retention rate at 60℃ After completing step (2), the battery was placed in an environment of 60°C for 7 days, then left to stand at 25°C for 2 hours, and then discharged at 0.33C to 2.5V. The capacity C60 was then measured. P2 = C60 / C0.33 100%.

[0095] P3: DCIR growth rate The cell from step (2) was discharged at 0.1C for 5 hours at 25°C. After standing for 30 minutes, the voltage V1 was recorded. Then, it was pulsed at 3C (for a discharge current of ΔI) for 10 seconds, and the voltage V2 was recorded. DCIR (1) = V0 - V1 / ΔI. After completing the P2 test, the battery cell was charged to 4.2V at 0.1C at 25℃, then discharged at 0.1C for 5h, and after standing for 30min, the voltage V3 was recorded. Then, it was pulsed discharged at 3C rate (for discharge current ΔI) for 10s, and the voltage V4 was recorded at this time. DCIR(2) = V3-V4 / ΔI. Growth rate = DCIR(2) / DCIR(1) 100%.

[0096] The experimental results are shown in Table 2.

[0097] Table 2 Performance test results for different groups Variable V1 Lithium metal oxide Comparing Examples 1-3, it is evident that replacing the lithium metal oxide alters the connectivity of the inorganic ion channel network within the coating, the interface lattice and chemical compatibility, and the ability to suppress electrolyte oxidation side reactions, thus shifting the stability and kinetic balance during high-temperature cycling and storage. In Example 1, using LLZO, the lithium-ion conductivity path is more stable and forms a more continuous inorganic conductive framework after curing. This framework, together with lithium phosphate and lithium borate, constructs a composite phase, which helps reduce electrolyte oxidation and decomposition on the cathode surface, slows transition metal dissolution, and inhibits continuous CEI thickening. In Example 2, using LATP, although it also possesses ion conductivity, the matching relationship between its surface chemical environment and the composite phase within the coating differs. The interface structure formed after curing is more prone to localized conduction bottlenecks or an increase in phase interfaces, leading to a slight increase in charge transfer resistance and intrafilm mass transfer resistance. In Example 3, using Li₂ZrO₃, the synergy between ion conductivity and the interface is further reduced. The coating's main contribution lies in shielding and suppressing side reactions rather than enhancing transport; therefore, its performance shows a more significant slight decrease compared to Example 1. In Comparative Example 1, after the coating was removed, the positive electrode surface was in direct contact with the electrolyte, and the oxidation decomposition, metal dissolution, and non-uniform thickening of CEI were significantly aggravated at high temperature.

[0098] Variable V2 coating thickness Comparing Examples 1, 4, 5, 4, and 5, it is evident that the average coating thickness directly determines the ability to cover and repair defects, grain boundaries, and highly active sites on the cathode surface. It also determines the path length and equivalent tortuosity of ion transmembrane migration, thus affecting the rate of interfacial side reactions and the impedance growth trend under high-temperature conditions. When the thickness is reduced to 10 nm or further to 5 nm, the continuity of the composite coating becomes more difficult to guarantee. Channels for direct electrolyte contact with the cathode surface still exist in localized areas. During high-temperature storage, electrolyte oxidation and decomposition, as well as the generation of acidic species, are more likely to occur at these sites, inducing continuous CEI reconstruction and leading to accelerated impedance growth. Increasing the thickness to 30 nm enhances coverage and shielding capabilities, more effectively suppressing electrolyte oxidation and transition metal dissolution. However, the thicker film increases the resistance to ion transmembrane migration and increases interfacial polarization. In high-rate or high-temperature cycling, charge transfer limitation and concentration polarization accumulation are more likely to occur, thus partially offsetting the advantages of kinetics and low impedance.

[0099] The molar ratio of variable V3 B / P Comparing Examples 1, 6, 7, 6, and 7, it is evident that the boron-to-phosphorus (B / P) ratio determines the relative proportion of boron-containing and phosphorus-containing phases in the coating, thereby affecting the coating's acid-suppressing buffering capacity, inorganic network densification characteristics, and composite stability with the lithium metal oxide framework. When the B / P ratio decreases to 0.30 or further to 0.10, insufficient boron-containing phase reduces the coating's buffering capacity against acidic species and side reaction chains. At high temperatures, electrolyte oxidation byproducts are more likely to erode and dissolve / redeposit at the interface, leading to faster CEI thickening accompanied by more significant impedance growth. When the B / P ratio increases to 0.80, while the increased proportion of boron-containing phase enhances acid suppression and stabilization, it may also increase the tendency for borate-related phases to dissolve / redeposit and undergo local structural rearrangement in the electrolyte environment. This affects the coating's microscopic uniformity and ion channel continuity, manifesting as slightly suppressed kinetics and a marginal increase in impedance growth. In Example 1, when B / P = 0.45, the boron-containing and phosphorus-containing phases exhibit more complete structural synergy, providing a stable Li–B–P–O inorganic network to resist high-temperature corrosion without significantly sacrificing ion migration and interfacial charge transfer processes.

[0100] Variable V4 Coating coverage Comparing Examples 1, 8, and 9 with Comparative Example 1, it can be seen that the coverage reflects the degree of shielding of highly active sites on the particle surface and the interface consistency of the coating, which is a key structural parameter determining whether local side reactions are amplified at high temperatures. When the coverage decreases to 80% or lower, the exposed area on the particle surface increases. These areas are more likely to trigger electrolyte oxidation and decomposition under high potential and high temperature conditions, forming locally thickened CEI, which in turn leads to uneven spatial distribution of interface impedance, resulting in polarization concentration and intensified thermal-reaction coupling, ultimately manifesting as decreased capacity retention and increased DCIR growth. Increasing the coverage to 95% can significantly reduce exposed sites and defect channels, making CEI growth more uniform and reducing the probability of interface side reactions induced by metal dissolution. High-temperature storage and cycling stability are further improved. However, excessively high coverage is usually accompanied by a more continuous inorganic phase network and a higher degree of local densification. The improvement in kinetics has a marginal effect. Therefore, the 90% coverage of Example 1 still achieves the best overall balance. In Comparative Example 1, the coverage is practically zero without a coating, and the high-temperature side reactions are the most significant.

[0101] Variable V5 sol preparation pH Comparing Examples 1, 10, 11, Comparative Example 8, and Comparative Example 9, it can be seen that the pH of sol preparation determines the dissociation state, condensation, and complexation behavior of the phosphate and boric acid sources, thereby affecting the sol particle size distribution, adsorption and anchoring efficiency on the cathode surface, and uniformity of spray deposition, ultimately reflected in the coating defect rate, coverage, and microstructure thickness uniformity. When the pH decreases to 6.0, the degree of sol condensation and the charge state of the particle surface change, making it more difficult for the sol to form a stable and uniform colloidal structure. During spraying, local insufficient deposition or film defects are more likely to occur. After curing, the continuity and densification of the Li-BPO network in the coating decrease, resulting in a weakened ability to suppress electrolyte oxidation side reactions at high temperatures and a faster increase in impedance. When the pH increases to 7.5, sol particles are more likely to aggregate and condense, reducing the microstructure uniformity of the spray deposition. Local thickness unevenness and increased phase interfaces lead to discontinuous ion channels and increased charge transfer polarization, resulting in a weakening of the kinetics and low impedance advantages.

[0102] Finally, it should be noted that the above embodiments do not limit the present invention in any way. Those skilled in the art can make modifications and improvements based on the present invention. Therefore, any modifications or improvements made without departing from the spirit of the present invention are within the scope of protection claimed by the present invention.

Claims

1. A positive electrode active material, said positive electrode active material comprising positive electrode active substance particles, characterized in that, The surface of the positive electrode active material particles is loaded with an ion-conducting composite coating, wherein the molar ratio of boron to phosphorus in the ion-conducting composite coating is 0.3-0.6:

1.

2. The positive electrode active material according to claim 1, characterized in that, The average thickness of the ion-conducting composite coating is 10-20 nm.

3. The positive electrode active material according to claim 1, characterized in that, The ion-conducting composite coating comprises lithium phosphate, lithium borate, and lithium metal oxide.

4. The positive electrode active material according to claim 1, characterized in that, The ion-conducting composite coating has a coverage rate of ≥80% on the surface of the positive electrode active material particles.

5. The positive electrode active material according to claim 1, characterized in that, The positive electrode active material particles are selected from one or more of layered oxide positive electrode materials, spinel positive electrode materials, and olivine positive electrode materials.

6. The positive electrode active material according to claim 1, characterized in that, The lithium phosphate is selected from one or more of lithium phosphate, lithium pyrophosphate, and lithium difluorophosphate; the lithium borate is selected from one or more of lithium borate, lithium metaborate, and lithium tetraborate; and the lithium metal oxide is selected from one or more of lithium lanthanum zirconium oxide, lithium aluminum titanium phosphorus oxide, lithium titanium oxide, lithium germanium phosphorus oxide, and lithium zirconium oxide.

7. The positive electrode active material according to claim 1, characterized in that, The formation process of the ion-conducting composite coating is as follows: Phosphoric acid source and boric acid source are added to a solvent and stirred evenly. Then, lithium alkali is added to adjust the pH of the solution to 6.3-6.

8. After aging, a sol precursor is obtained. Then, lithium metal oxide dispersion is added and dispersed evenly to obtain a coating slurry. The coating slurry is coated on the surface of the positive electrode active material particles. After drying and curing, an ion-conducting composite coating is formed on the surface of the positive electrode active material particles.

8. The positive electrode active material according to claim 7, characterized in that, The solid content in the coating slurry is 0.5-1.5 wt%.

9. The positive electrode active material according to claim 7, characterized in that, The amount of solid deposit in the coating is 0.2-1.2 wt% of the mass of the positive electrode active material particles.

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes the positive electrode active material as described in any one of claims 1-9.

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