A high-rate long-cycle lithium iron phosphate battery and a preparation method thereof

Abstract

This invention belongs to the field of lithium-ion battery technology, specifically disclosing a high-rate, long-cycle lithium iron phosphate battery and its preparation method. The preparation method of the high-rate, long-cycle lithium iron phosphate battery provided by this invention includes the following steps: preparing a positive electrode material with a hexagonal close-packed mesoporous array using a CTAB template agent, wherein the pore order index is ≥0.85 and the pore size is 10~20nm; forming a composition-porosity dual-gradient positive electrode sheet through multi-stage rolling, with the surface layer to the near-separator layer being Al... 3+ As the doping concentration increases, the porosity also increases. A composite electrolyte is prepared by adding LiFSI, FEC, and TEGDME to the base electrolyte. The positive electrode, composite electrolyte, negative electrode, and separator are then assembled into a lithium iron phosphate battery. This invention utilizes a triple technology—deep coupling of pore ordering, precise gradient parameter control, and electrolyte solvation engineering—to achieve a breakthrough in battery performance.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of lithium ion batteries, in particular to a high-rate long-cycle lithium iron phosphate battery and a preparation method thereof. BACKGROUND

[0002] With the rapid development of new energy vehicles and energy storage industries, lithium ion batteries have become a research hotspot in the field of energy storage batteries at present. Among them, lithium iron phosphate batteries are concerned due to their high safety, long cycle life, high charging and discharging efficiency and environmental protection. However, the current lithium iron phosphate battery faces ion transmission bottleneck in high-rate discharge, and the traditional disordered pore structure has problems such as long ion diffusion path (>100 nm) and insufficient electrolyte infiltration. The contradiction between mechanical stress concentration and mass transfer efficiency in the electrode structure leads to the crushing of active materials in the cycle process. The stability of the electrode / electrolyte interface is insufficient in a wide temperature range, especially the volume expansion (>300%) of the silicon-carbon negative electrode causes the repeated rupture of the SEI film. Although some related technologies mention the application of pore structure and LiFSI / FEC additives, it is still difficult to achieve a breakthrough in performance.

[0003] The lithium iron phosphate battery mainly faces the following problems: 1. Lithium ion transmission bottleneck in the positive electrode material: the intrinsic ion conductivity (about 10 -9 S / cm) and the electronic conductivity (about 10 -9 S / cm) are low and poor, respectively, which seriously restricts the high-rate performance. Although the traditional carbon-coated modification can improve the electronic conductivity, there are still problems of single pore structure and long ion diffusion path: 1) the lithium ion diffusion path inside the micron-sized particles is more than 100 nm, and the concentration polarization is significant during charging and discharging; 2) the electrolyte is difficult to fully infiltrate the dense structure, and the ion migration is further hindered at low temperature; 3) most of the related technologies prepare mesoporous materials through pore-forming agents, but the pore size distribution is uneven (10-200 nm) and the specific surface area is limited (<50 m 2 / g), which cannot balance high ion flux and structural stability. 2. Stress distribution imbalance in the electrode structure: the electrode is the carrier of ion / electron transmission, and its pore structure directly affects the kinetic performance of the battery. The conventional isoporous electrode design has the contradiction between mechanical stress concentration and mass transfer efficiency: the high compaction density layer (porosity <20%) improves the conductivity, but the volume change of the active material (about 6.8% for LFP) during the cycle causes stress accumulation, leading to particle crushing; the low compaction density layer (porosity >30%) is beneficial to ion diffusion, but reduces the volume energy density. Chinese invention patent CN114512648A proposes a porous substrate structure, but does not solve the problem of stress mutation between layers, and lacks a coordinated optimization design for each structural part. 3. Insufficient stability of the electrode / electrolyte interface: the commercial LiPF6 / carbonate electrolyte faces uncontrollable interface side reactions in high-voltage and low-temperature environments: positive electrode side: trace Fe 2+Dissolution and Mn impurities (from raw materials) cause the continuous thickening of the CEI film, increasing the interfacial impedance (>100 Omega*cm 2 ); negative side: volume expansion of silicon-carbon materials (>300%) leads to the rupture and regeneration of traditional EC-based SEI films, reducing the cycle coulombic efficiency; although additives such as FEC or LiFSI can partially improve the interface, they cannot simultaneously meet the differentiated stability requirements of the positive / negative electrode interface.

[0004] Therefore, it is urgent to carry out full-chain collaborative innovation on lithium iron phosphate batteries, and to systematically break through the high-rate and full-weather application limits of lithium iron phosphate batteries. SUMMARY

[0005] The present application aims to at least solve one of the technical problems existing in the prior art. To this end, the present application provides a preparation method of a high-rate long-cycle lithium iron phosphate battery, specifically through a multi-stage pore positive electrode material design-gradient compaction electrode sheet-solvation regulation composite electrolyte collaborative optimization scheme, by constructing hierarchical pores to accelerate ion transmission, gradient pores to relieve mechanical stress, and composite additives to strengthen the stability of the electrode / electrolyte interface, the high-rate performance and full-weather applicability of the lithium iron phosphate battery are systematically improved.

[0006] The present application also provides a lithium iron phosphate battery.

[0007] The present application also provides a positive electrode sheet and an electrolyte for a lithium battery.

[0008] In a first aspect of the present application, a preparation method of a high-rate long-cycle lithium iron phosphate battery is provided, comprising the following steps:

[0009] S1, dissolving FeSO4, LiOH and H3PO4 in water, adding a template agent, and hydrothermally reacting to obtain a mesoporous LiFePO4 precursor, etching to remove the template in the precursor, mixing the precursor, a carbon source, graphene oxide and nano-Al2O3 and ball milling, and calcining to obtain a multi-stage ordered pore LiFePO4@C / Graphene positive electrode material with an ordered pore index of the pore channel of 0.85 or more;

[0010] S2, mixing the positive electrode material with a conductive agent and a binder, adding a solvent to prepare a slurry, and ball milling to obtain a surface layer, an intermediate layer and a near separator layer slurry, respectively, coating the surface layer, the intermediate layer and the near separator layer slurry on a current collector to obtain a sandwich structure of the surface layer, the intermediate layer and the near separator layer, and using a multi-stage rolling process to compact the three layers, respectively, to obtain a composition-porosity double gradient electrode sheet;

[0011] The surface layer is doped with 1.0at% to 1.4at% Al 3+ , and the intermediate layer is doped with 0.6at% to 1.0at% Al 3+The near-membrane layer is doped with 0.4at%-0.6at% Al 3+ ;

[0012] The surface layer is rolled to a porosity of 10%-20%, the intermediate layer is rolled to a porosity of 20%-30%, and the near-membrane layer is rolled to a porosity of 30%-40%;

[0013] S3, adding LiPF6 in a solvent to form a basic electrolyte, adding LiFSI, FEC and TEGDME in the basic electrolyte, and dissolving to obtain a solvation-regulated composite electrolyte;

[0014] S4, assembling the battery by using the component-porosity double-gradient pole piece, the solvation-regulated composite electrolyte, a negative electrode and a separator, thereby obtaining the high-rate long-cycle lithium iron phosphate battery.

[0015] The current lithium iron phosphate battery has the performance defects in the aspects of material, structure and electrolyte, which are coupled with each other: the single optimization of the positive electrode channel cannot solve the stress cracking of the pole piece, and the interface modification also cannot play the effect due to the lack of an adaptive ion transmission network; the present application realizes the whole-chain collaborative innovation of accelerating ion transmission through multi-stage channels, relieving mechanical stress through gradient porosity and strengthening the interface stability through composite additives, thereby systematically breaking through the high-rate and all-weather application limit of the lithium iron phosphate battery.

[0016] According to some embodiments of the present application, the channel pore size range of the multi-stage ordered channel LiFePO4@C / Graphene positive electrode material is 8-25nm.

[0017] According to some embodiments of the present application, the channel pore size range of the multi-stage ordered channel LiFePO4@C / Graphene positive electrode material is 10-20nm.

[0018] According to some embodiments of the present application, an Al2O3 modification layer with a thickness of 1-3nm is obtained on the channel wall of the multi-stage ordered channel LiFePO4@C / Graphene positive electrode material through atomic layer deposition.

[0019] According to some embodiments of the present application, the molar ratio of FeSO4, LiOH and H3PO4 in step S1 is 1.05:0.9-1.1:0.9-1.1.

[0020] According to some embodiments of the present application, the template agent in step S1 is cetyltrimethylammonium bromide (CTAB).

[0021] According to some embodiments of the present application, the hydrothermal reaction in step S1 is specifically carried out in a 150-210℃ high-pressure reaction kettle for 8-16h.

[0022] According to some embodiments of the present application, the etching method in step S1 is specifically: etching for 3-5 hours using 3wt%-8wt% hydrofluoric acid.

[0023] According to some embodiments of the present application, the mass ratio of the precursor, the carbon source and the graphene oxide in step S1 is 1000:50-200:3-10.

[0024] According to some embodiments of the present application, the carbon source comprises at least one of glucose, fructose, sucrose, citric acid, oxalic acid or polyethylene glycol.

[0025] According to some embodiments of the present application, the calcination method in step S1 is specifically: calcining at 4-6℃ / min to 700-800℃ in an Ar / H2 mixed atmosphere for 5-7 hours.

[0026] In the multistage ordered pore LiFePO4@C / Graphene positive electrode material of the present application, a carbon coating layer is formed on the surface of the positive electrode material precursor by calcining and pyrolyzing the carbon source, and the graphene oxide is reduced to graphene and interconnected with the carbon layer, and is combined with the Al2O3 modification layer deposited by atomic layer deposition, thereby constructing a three-stage synergistic structure of “pore transmission-interface modification-electronic conduction” to realize high ionic conductivity of the positive electrode material.

[0027] According to some embodiments of the present application, the volume ratio of Ar in the Ar / H2 mixed atmosphere is 93%-97%.

[0028] According to some embodiments of the present application, the pore size distribution of the multistage ordered pore LiFePO4@C / Graphene positive electrode material is in the range of 2-50nm, the average pore size is 10-20nm, the specific surface area is 60-80m 2 / g, and the mesopore volume is 0.35cm 3 / g.

[0029] In the preparation method of the present application, the multistage pore structure with specific pore size gradient distribution and controllable specific surface area is constructed in the positive electrode material by adjusting the hydrothermal reaction time and the calcination temperature, and the multistage ordered pore system helps to significantly improve the ion transmission efficiency.

[0030] According to some embodiments of the present application, the conductive agent comprises at least one of conductive carbon black, acetylene black, carbon nanotube or graphite.

[0031] According to some embodiments of the present application, the binder comprises at least one of PVDF, butadiene rubber (SBR), PAA (polyacrylic acid) or polytetrafluoroethylene (PTFE).

[0032] According to some embodiments of the present application, the mass ratio of the positive electrode material, the conductive agent and the binder in step S2 is 94:2-4:2-4.

[0033] According to some embodiments of the present application, the solid content in the slurry in step S2 is 40%-50%.

[0034] According to some embodiments of the present application, the ball milling in step S2 adopts a staged ball milling, and the particle size of the positive electrode material in the surface layer, the intermediate layer and the near-separator layer is controlled to 1.5-2.5 μm, 2.5-4 μm and 4-6 μm, respectively.

[0035] According to some embodiments of the present application, 4-6 wt% of graphene nanosheets with a lateral size of 5-10 μm are additionally added in the near-separator layer in step S2.

[0036] The present application additionally dopes graphene nanosheets in the near-separator layer, and on the basis of the whole positive electrode sheet with a channel-Al2O3 modification layer-graphene carbon layer interconnection coating layer, the composition structure of the near-separator layer is further optimized to form a three-dimensional conductive network, and the conductive performance of the positive electrode sheet is improved.

[0037] According to some embodiments of the present application, the thickness ratio of the surface layer, the intermediate layer and the near-separator layer in the sandwich structure in step S2 is 3-5:7-11:9-13.

[0038] According to some embodiments of the present application, the multi-stage rolling process in step S3 is specifically as follows: preheating treatment is performed on the electrode sheet before compaction, the preheating temperature is 50-60℃, the linear pressure and the roller temperature parameters of each layer are controlled during the compaction process, post-heating treatment is performed after the compaction, the post-heating treatment temperature is 70-80℃, and the time is 1-2 h.

[0039] According to some embodiments of the present application, the linear pressure of the surface layer rolling is 400-500 kN / m, and the roller temperature is 80-90℃; the linear pressure of the intermediate layer rolling is 300-400 kN / m, and the roller temperature is 70-80℃; and the linear pressure of the near-separator layer rolling is 200-300 kN / m, and the roller temperature is 60-70℃.

[0040] The present application eliminates the internal stress of the electrode sheet and improves the interface bonding strength through the multi-stage rolling process.

[0041] According to some embodiments of the present application, the solvent in step S3 is a mixed solvent composed of ethylene carbonate (EC), methyl ethyl carbonate (EMC) and dimethyl carbonate (DMC).

[0042] According to some embodiments of the present application, the concentration of LiPF6 in the base electrolyte in step S3 is 1.0-1.4 mol / L.

[0043] According to some embodiments of the present application, the mass ratio of the base electrolyte, LiFSI, FEC and TEGDME in step S3 is 80-120: 1-2: 2.5-3.5: 4-6.

[0044] According to some embodiments of the present application, the proportion of FEC in the solvation sheath in the solvation-regulated composite electrolyte in step S3 is 36%-40%.

[0045] According to some embodiments of the present application, the negative active material in step S4 includes a silicon-carbon composite material, wherein the mass proportion of silicon is 5%-15%, the carbon material is a composite of graphite and graphene, and the mass ratio of graphite to graphene is (7-9):(1-3). The use of the silicon-carbon composite material with the above ratio in the present application helps to improve the specific capacity and cycle stability of the negative electrode.

[0046] In a second aspect of the present application, a lithium iron phosphate battery is provided, which is prepared by the preparation method of the first aspect of the present application.

[0047] In a third aspect of the present application, a positive electrode sheet for a lithium battery is provided, which is prepared by the method of steps S1 and S2 of the first aspect of the present application.

[0048] In a fourth aspect of the present application, an electrolyte for a lithium battery is provided, which is prepared by the method of step S3 of the first aspect of the present application.

[0049] The present application has the following beneficial effects:

[0050] The present application realizes a breakthrough improvement in battery performance by systematic regulation of the order of the pores of the lithium iron phosphate battery, the composition gradient of the electrode sheet and the solvation structure of the electrolyte.

[0051] The present application utilizes the coordination of the positive electrode pore-electrode sheet gradient: the ordered mesoporous network (pore size 10-20 nm) shortens the ion diffusion path, the gradient electrode sheet optimizes the mass transfer gradient through the "highly compact surface layer (15% porosity)-high porosity near the separator layer (35%)", and Al 3+ The doping concentration gradient (0.5at%→1.2at%) synchronously improves the conductivity and structural stability.

[0052] The present application utilizes the coordination of the electrolyte-interface: LiFSI promotes the formation of a CEI film rich in LiF (suppresses Fe 2+ dissolution, Mn dissolution <0.5ppm) on the positive electrode, and FEC / TEGDME regulates the SEI film to be a 12nm thick LiF / Li2CO2 composite crystal phase (elastic modulus 2.1GPa) that adapts to the volume expansion of the silicon-carbon negative electrode.

[0053] The application also realizes full-system performance coupling: the triple technology makes the battery have a 5C discharge capacity retention rate of 94.3%, a -20 DEG C discharge capacity of 62.5%, and a capacity retention rate of > 80% after 8500 times of 1C cycle, and realizes multi-dimensional performance breakthrough compared with the prior art.

[0054] Other features and advantages of the present application will be set forth in the following description, and in part will become apparent to those skilled in the art from the description, or can be learned by practice of the application. BRIEF DESCRIPTION OF DRAWINGS

[0055] The application will be further described below in conjunction with the drawings and examples, in which:

[0056] Figure 1 A schematic diagram of the three-dimensional synergistic optimization of the multi-stage ordered pore positive electrode-gradient compaction electrode sheet-solvation regulated composite electrolyte of the lithium iron phosphate battery of Example 1 of the application. DETAILED DESCRIPTION

[0057] The concept and technical effects of the application will be described below in conjunction with examples, so as to fully understand the purpose, features and effects of the application. Obviously, the described examples are only a part of the examples of the application, but not all the examples. Based on the examples of the application, other examples obtained by those skilled in the art without creative labor are within the protection scope of the application.

[0058] The specific conditions not mentioned in the examples are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments not mentioned by the manufacturer are conventional products that can be obtained by market purchase.

[0059] Example 1

[0060] The present embodiment provides a high-rate long-cycle performance lithium iron phosphate battery and a preparation method thereof.

[0061] The specific preparation steps of the lithium iron phosphate battery of the present embodiment are as follows:

[0062] 1) Preparation of multi-stage ordered pore LiFePO4@C / Graphene positive electrode material:

[0063] A template-induced hydrothermal-spray drying combined technology was used. FeSO4, LiOH, and H3PO4 were dissolved in deionized water at a molar ratio of 1.05:1:1, and 0.1 mol / L CTAB template agent was added. The mixture was subjected to a hydrothermal reaction at 180℃ for 12 h to generate a mesoporous LiFePO4 precursor. The precursor was then etched with 5% hydrofluoric acid for 4 h to remove the template (hydrofluoric acid etching formed a hexagonal close-packed mesoporous array). 100 g of the precursor was then combined with 10 g of glucose, 0.5 g of graphene oxide, and 50 nm diameter A... Al₂O₃ was ball-milled at 400 rpm for 4 h, then calcined at 750 °C for 6 h in an Ar / H₂ (95:5) atmosphere at a rate of 5 °C / min. Glucose pyrolyzed to form a 5-8 nm carbon coating layer, and graphene oxide was reduced to graphene and interconnected with the carbon layer. Al₂O₃ was deposited on the pore walls via atomic layer deposition to form a 2 nm modification layer. Synchrotron radiation CT characterization confirmed the three-dimensional connectivity of the pores, with a pore tortuosity factor of 1.3 and an order index > 0.85. The final product had a pore order index of 0.87 and a specific surface area of ​​72 m². 2 / g, pore size 10-20nm, mesopore volume 0.35cm³ 3 / g of multi-level ordered porous cathode material.

[0064] 2) Preparation of composition-porosity dual-gradient electrodes:

[0065] The above-mentioned LiFePO4@C / Graphene active material was mixed with conductive carbon black and polyvinylidene fluoride (PVDF) at a mass ratio of 94:3:3. N-methylpyrrolidone (NMP) was added to prepare a slurry with a solid content of 45%. The particle size of the active material in the surface layer was controlled to be 2 μm, the particle size of the active material in the middle layer was 3 μm, and the particle size of the active material near the diaphragm layer was 5 μm by staged ball milling. 5 wt% of graphene nanosheets with a transverse size of 5-10 μm were additionally added to the slurry near the diaphragm layer. The slurry was then coated in layers on a 10 μm thick aluminum foil. The wet film thicknesses of the surface layer, middle layer, and near the diaphragm layer were 80 μm, 180 μm, and 220 μm, respectively, corresponding to an active material areal density of 12.5 mg / cm³. 2 10.2 mg / cm 2 5.8 mg / cm 2 The process employs a multi-stage rolling process. Before compaction, the electrode is preheated to 55°C. The surface layer is compacted to a porosity of 15% under a linear pressure of 450kN / m and a roller temperature of 85°C. The intermediate layer is adjusted to 25% porosity under a pressure of 350kN / m and a temperature of 75°C. The near-diaphragm layer is controlled to 35% porosity under a pressure of 250kN / m and a temperature of 65°C. Finally, the electrode is heat-treated at 75°C for 1.5 hours, resulting in an interlayer bonding strength of 5.8N / cm.

[0066] The LiFePO4@C / Graphene active material of the surface layer, the intermediate layer and the near-membrane layer is respectively obtained by controlling the addition amount of 50 nm particle size Al2O3 in step 1) to obtain a surface layer active material doped with 1.2 at% Al 3+ , an intermediate layer active material doped with 0.8 at% Al 3+ , and a near-membrane layer active material doped with 0.5 at% Al 3+ (Al 3+ doping amount is determined by inductively coupled plasma optical emission spectrometry (ICP-OES), and the deviation is ensured to be ≤±0.05 at%).

[0067] 3) Preparation of solvent-controlled composite electrolyte:

[0068] In an argon glove box, a 1.2 mol / L basic electrolyte is prepared by mixing solvents of ethylene carbonate (EC):methyl ethyl carbonate (EMC):dimethyl carbonate (DMC)=3:4:3 by volume ratio and adding LiPF6. To 100 g of the basic electrolyte, 1.5 g of LiFSI, 3 g of FEC and 5 mL of tetraethylene glycol dimethyl ether (TEGDME) are sequentially added, and the mixture is stirred until completely dissolved and then filtered through a 0.22 μm filter membrane. It is confirmed by 1H NMR characterization that the FEC accounts for 38% in the solvent sheath layer, forming a "Li + -FEC-TEGDME" preferential solvation structure, reducing the lithium ion desolvation energy barrier to 18.7 kJ / mol; DSC test shows that the glass transition temperature of the electrolyte is reduced to -65°C, and the ion conductivity at -20°C reaches 1.2×10 -3 S / cm, and the SEI film formation potential is raised to 1.2 V (vs Li⁺ / Li).

[0069] 4) Negative electrode and separator:

[0070] Negative electrode: active slurry: silicon-carbon composite material (silicon content 10 wt%, graphite:graphene=8:2), conductive carbon black, CMC / SBR binder are mixed in a mass ratio of 92:4:2:2, and deionized water is used for slurry, with a solid content of 45%; coating: coated on an 8 μm copper foil, with a double-sided area density of 6.5 mg / cm²; after drying, roll-pressed to a compacted density of 1.65 g / cm 3 , and the thickness of the pole piece is 150 μm.

[0071] Separator: 12 μm wet PE ceramic coated separator (Al2O3 coating 2 μm), porosity 42%, air permeability 220 s / 100 mL.

[0072] 5) Cell assembly:

[0073] Lamination: 23 positive electrodes, 24 negative electrodes and 47 separators are alternately laminated with the same direction of the tab; the designed capacity is 50 Ah.

[0074] Welding: ultrasonic welding of aluminum tab (positive electrode) and nickel-plated copper tab (negative electrode).

[0075] Into the shell: aluminum shell 148 mm x 96 mm x 26 mm, shell pre-coated with insulating glue.

[0076] 6) Liquid injection and pre-sealing:

[0077] Liquid injection: inject electrolyte 110 g (about 3.5 g / Ah) in argon glove box; vacuum-standby cycle 3 times (-85 kPa, 10 min each time) to ensure infiltration.

[0078] Pre-sealing: laser welding pre-sealing, leaving a 2 mm exhaust hole.

[0079] 7) Formation and secondary sealing:

[0080] Formation: 0.1C constant current charging to 3.65V, 45℃ standing for 4h; 0.2C discharging to 2.50V, recording the first coulomb efficiency 89.8%.

[0081] Degassing: vacuum (-90 kPa) degassing for 5 min after formation.

[0082] Final sealing: laser welding complete sealing, argon arc welding sealing pin.

[0083] 8) Capacity test and test:

[0084] Capacity test: 0.5C charge and discharge for 3 weeks, capacity 50.2 Ah, energy density 165 Wh / kg (single cell).

[0085] Performance verification:

[0086] 5C discharge capacity retention rate 94.3% (25℃);

[0087] -20℃ 0.5C discharge capacity retention rate 62.5%;

[0088] 1C / 100% DoD cycle 8500 times capacity retention rate 81.4%.

[0089] 9) Safety packaging:

[0090] Battery pack insulation test, air tightness test (helium leak rate <1x10 -6 Pa·m 3 / s), paste code into the warehouse.

[0091] The lithium iron phosphate battery with high rate and long cycle performance is finally obtained, and a three-dimensional synergistic optimization schematic diagram of the multistage ordered pore positive electrode-gradient compaction electrode sheet-solvation regulated composite electrolyte is as shown in Figure 1

[0092] Comparative Example 1

[0093] The present comparative example provides a lithium iron phosphate battery and provides a preparation method thereof.

[0094] The preparation method of the lithium iron phosphate battery of the present comparative example is basically the same as that of Example 1, and the only difference is that the present comparative example controls the ordered degree index of the positive electrode material pore to be 0.65 by adjusting the hydrothermal reaction parameters, etching process parameters and calcination room temperature rate;

[0095] The preparation method of the multistage ordered pore LiFePO4@C / Graphene positive electrode material of the present comparative example which is different from Example 1 is as follows:

[0096] A template-induced hydrothermal-spray drying combined technology is adopted. FeSO4, LiOH and H3PO4 are dissolved in deionized water in a molar ratio of 1.05:1:1, and a CTAB template agent with a concentration of 0.1 mol / L is added. A mesoporous LiFePO4 precursor is generated by hydrothermal reaction in a 150℃ high-pressure reaction kettle for 12h. The precursor is etched with 3% hydrofluoric acid for 3h to remove the template. 100g of the precursor is mixed with 10g of glucose, 0.5g of graphene oxide and 50nm particle size of Al2O3 at a rotation speed of 400rpm for 4h, and then calcined at 750℃ in an Ar / H2 (95:5) atmosphere at a heating rate of 8℃ / min for 6h. The finally prepared positive electrode material is characterized by synchrotron radiation CT: the pore ordered degree index is 0.65, the pore bending degree factor is 1.8, and the mesopore volume is 0.22cm 3 / g.

[0097] Comparative Example 2

[0098] The present comparative example provides a lithium iron phosphate battery and provides a preparation method thereof.

[0099] The preparation method of the lithium iron phosphate battery of the present comparative example is basically the same as that of Example 1, and the only difference is that the present comparative example controls the positive electrode material pore size to be micronized to 2-5nm, while maintaining the ordered degree >0.85 (0.87).

[0100] Comparative Example 3

[0101] The present comparative example provides a lithium iron phosphate battery and provides a preparation method thereof.

[0102] ​The preparation method of the lithium iron phosphate battery of the present comparative example is basically the same as that of Example 1, except that the pore size of the positive electrode material of the present comparative example is enlarged to 30-50 nm, while maintaining the order degree > 0.85 (0.87).

[0103] Comparative Example 4

[0104] The present comparative example provides a lithium iron phosphate battery, and a preparation method thereof is provided.

[0105] The preparation method of the lithium iron phosphate battery of the present comparative example is basically the same as that of Example 1, except that the pore size of the positive electrode material of the present comparative example is enlarged to 30-50 nm, while maintaining the order degree > 0.85 (0.87). 3+ The doping gradient is controlled at 0.5at% uniform doping, while maintaining the three-layer 15%, 25%, 35% porosity gradient.

[0106] Comparative Example 5

[0107] The present comparative example provides a lithium iron phosphate battery, and a preparation method thereof is provided.

[0108] The preparation method of the lithium iron phosphate battery of the present comparative example is basically the same as that of Example 1, except that the pore size of the positive electrode material of the present comparative example is enlarged to 30-50 nm, while maintaining the order degree > 0.85 (0.87). 3+ The doping gradient is controlled at 0.5at% uniform doping, while maintaining the three-layer 15%, 25%, 35% porosity gradient.

[0109] Comparative Example 6

[0110] The present comparative example provides a lithium iron phosphate battery, and a preparation method thereof is provided.

[0111] The preparation method of the lithium iron phosphate battery of the present comparative example is basically the same as that of Example 1, except that the pore size of the positive electrode material of the present comparative example is enlarged to 30-50 nm, while maintaining the order degree > 0.85 (0.87). 3+ The doping gradient and the porosity gradient are controlled at 0.5at% uniform doping and 25% uniform porosity. 3+ The doping gradient and the porosity gradient are controlled at 0.5at% uniform doping and 25% uniform porosity.

[0112] Comparative Example 7

[0113] The present comparative example provides a lithium iron phosphate battery, and a preparation method thereof is provided.

[0114] The preparation method of the lithium iron phosphate battery of the present comparative example is basically the same as that of Example 1, except that the pore size of the positive electrode material of the present comparative example is enlarged to 30-50 nm, while maintaining the order degree > 0.85 (0.87).

[0115] Comparative Example 8

[0116] The present comparative example provides a lithium iron phosphate battery, and a preparation method thereof is provided.

[0117] The preparation method of the lithium iron phosphate battery of the present comparative example is basically the same as that of Example 1, and the only difference is that no FEC is added in the composite electrolyte of the present comparative example, and the components are unchanged.

[0118] Comparative Example 9

[0119] The present comparative example provides a lithium iron phosphate battery, and a preparation method thereof is provided.

[0120] The preparation method of the lithium iron phosphate battery of the present comparative example is basically the same as that of Example 1, and the only difference is that no TEGDME is added in the composite electrolyte of the present comparative example, and the components are unchanged.

[0121] Performance test:

[0122] 1. The performance of the multi-level ordered pore positive electrode material prepared in Example 1 of the present application and the commercial LiFePO4 material (CLFP-01 purchased from Hubei Ruishengxiang Technology Co., Ltd.) was tested, and the results are shown in Table 1 below:

[0123]

[0124] From the above test results, compared with the commercial LiFePO4 material, the positive electrode material prepared by the present application has significant advantages in many performance indicators. In terms of the compaction density, the material of Example 1 reaches 2.45 g / cm 3 , which is 6.5% higher than the commercial material of 2.30 g / cm 3 , and higher compaction density helps to improve the energy density of the battery; in terms of room temperature ionic conductivity, the material of Example 1 is 1.8 x 10 -4 S / cm, which is a significant improvement compared with the commercial material, and the significant improvement in ionic conductivity provides a guarantee for high-rate discharge; in terms of cycle performance, after 1C cycle for 1000 weeks, the capacity retention rate of the material of Example 1 is as high as 97.2%, which is much higher than the commercial material of 89.5%, and exhibits excellent cycle stability.

[0125] 2. The performance of the full battery of Example 1 of the present application was compared with the typical value of the commercial lithium iron phosphate battery, as shown in Table 2.

[0126]

[0127] From the above test results, the energy density of the full battery of Example 1 is about 10%-18% higher than the commercial typical value, and it is speculated that the significant improvement in performance is due to the high compaction density (2.45 g / cm 3 vs 2.30 g / cm 3) and a unique multi-level ordered pore structure design. The 5C discharge capacity retention rate of Example 1 is about 9%-14% higher than the commercial typical value, and it is speculated that the significant improvement in performance is due to the multi-level pores of the positive electrode material used, which shortens the ion diffusion path (ion conductivity 1.8 x 10 -4 S / cm vs 3.2 x 10 -6 S / cm). The low-temperature discharge capacity of Example 1 at -20℃ is about 13%-25% higher than the commercial typical value, mainly because the composite electrolyte used in Example 1 can effectively improve the low-temperature ion migration, and the SEI film thickness of the full battery negative electrode of Example 1 is only 12nm (after the first formation, the SEI film thickness is measured by high-resolution transmission electron microscopy, and the traditional value is about 25nm).

[0128] In the 1C / 100% DoD cycle life test results, the cycle number of the full battery of Example 1 is increased by 70%-183%, and the gradient electrode sheet used in Example 1 can effectively alleviate stress, and combined with the effect of the composite electrolyte to inhibit the interface side reaction, the cycle life of the battery is synergistically improved. In the 1C cycle 1000 times capacity retention rate test results, the full battery of Example 1 achieves a capacity retention rate improvement of 1.5%-6.5%, the interlayer bonding force is 5.8N / cm (increased by 50% compared with the commercial homogeneous electrode sheet), and the electrode sheet delamination crack is reduced.

[0129] Through full battery testing, it is verified that the multi-dimensional synergistic optimization of the present application to the lithium iron phosphate battery achieves a significant improvement in the comprehensive performance of the battery (such as energy density 165Wh / kg, 5C discharge capacity retention rate 94.3% and -20℃ discharge capacity 62.5%), which embodies the engineering value of the lithium iron phosphate battery technical solution of the present application.

[0130] 3、The performance test results of the full battery of Example 1 of the present application and the full batteries of each comparative example are compared, as shown in Table 3:

[0131]

[0132] From the above performance test results, it can be seen that the lithium iron phosphate battery prepared by Example 1 using the technical solution of the present application has the best discharge capacity retention rate and long cycle performance compared with the batteries of each comparative example, and has the best battery capacity retention rate in the extreme case of -20℃.

[0133] The positive electrode material channel order degree used by the lithium iron phosphate battery of Comparative Example 1 is low, which leads to a significant decline in battery performance, especially long cycle capacity retention rate. It is speculated that the channel order degree parameter in the multi-level ordered channel LiFePO4@C / Graphene positive electrode material technical solution used in the application is more critical to the battery performance. The channel order degree of the positive electrode material is insufficient, which can significantly weaken the ion transmission efficiency, and further significantly reduce the electrochemical performance of the battery. The pore size of the multi-level ordered channel LiFePO4@C / Graphene positive electrode material of Comparative Example 2 and Comparative Example 3 is too small and too large, respectively. Even if a high channel order degree is maintained, too small pore size can also limit ion diffusion and reduce transmission efficiency, while too large pore size can easily lead to a decrease in specific surface area utilization and deterioration of structural stability, ultimately resulting in a significant decline in battery performance. Therefore, the multi-level ordered channel positive electrode material of the application needs to control a high channel order degree and a suitable pore size range in order to balance the high rate and long cycle life performance of the battery.

[0134] Comparative Example 4 does not use the component-porosity double gradient pole piece of the technical solution of the application. The Al 3+ doping is uniform, which ultimately leads to a significant decline in the rate performance and cycle life of the battery. It is speculated that the Al 3+ doping gradient in the pole piece weakens the surface structure stability; the pole piece used in Comparative Example 5 lacks the porosity gradient feature, and the rate performance and cycle life of the battery decrease more obviously. It is speculated that the porosity gradient in the pole piece is more critical for stress relief. The pole piece with a conventional uniform porosity gradient is difficult to better relieve stress. The Al 3+ doping and porosity of the three-layer structure of the pole piece used in Comparative Example 6 are uniform, which leads to a significant decline in the performance of the battery. This test result further highlights the necessity and advantages of the component-porosity double gradient pole piece design in the technical solution of the application.

[0135] The composite electrolyte used in Comparative Examples 7, 8 and 9 respectively lacks LiFSI, FEC and TEGDME components compared with Example 1, which further causes the 5C discharge capacity retention rate and -20℃ capacity retention rate of the battery to decrease to different degrees. This test result shows that each key component in the composite electrolyte technical solution adopted in the application is indispensable. LiFSI may be indispensable for the construction of LiF-rich CEI film, and FEC is crucial for the toughness of the low-temperature SEI film. The ternary composite electrolyte cooperatively constructs a flexible SEI film with a thickness of only 12 nm and a Li3PO4-rich CEI interface layer through solvation structure regulation, which is crucial for improving the high rate performance and stability of the battery in low temperature environment.

[0136] In summary, the lithium iron phosphate battery of the application is based on the deep coupling of three technologies of multi-stage ordered pore LiFePO4@C / Graphene positive electrode material, component-porosity double gradient pole piece and solvation regulation composite electrolyte, and the synergistic effect realizes the performance breakthrough of the battery, and systematically improves the high-rate performance, long cycle life and full-weather applicability of the lithium iron phosphate battery.

[0137] The above has described the embodiments of the application in detail, but the application is not limited to the above embodiments, and various changes can be made within the knowledge range of the ordinary skill in the art without departing from the purpose of the application. In addition, the embodiments of the application and the features in the embodiments can be combined with each other without conflict.

Claims

1. A method for preparing a high rate long cycle lithium iron phosphate battery, characterized in that, The method comprises the following steps: S1, dissolving FeSO4, LiOH and H3PO4 in water, adding a template agent, and obtaining a mesoporous LiFePO4 precursor by hydrothermal reaction, etching to remove the template in the precursor, mixing the precursor, a carbon source, graphene oxide and nano-Al2O3 for ball milling, and calcining to obtain a multi-level ordered pore LiFePO4@C / Graphene positive electrode material with an ordered pore index ≥0.85; S2, mixing the positive electrode material with a conductive agent and a binder, adding a solvent to prepare a slurry, and ball milling to obtain a surface layer, an intermediate layer and a separator proximity layer slurry, coating the surface layer, the intermediate layer and the separator proximity layer slurry on a current collector to obtain a sandwich structure of the surface layer, the intermediate layer and the separator proximity layer, and compacting the three layers by a multi-level rolling process to obtain a composition-porosity double gradient electrode piece; By controlling the amount of nano-Al2O3 in the positive electrode material, the surface layer is doped with 1.0at%-1.4at% Al 3+ , the middle layer is doped with 0.6at%-1.0at% Al 3+ , and the near-septum layer is doped with 0.4at%-0.6at% Al 3+ ; The surface layer is rolled to a porosity of 10% to 20%, the intermediate layer is rolled to a porosity of 20% to 30%, and the separator proximity layer is rolled to a porosity of 30% to 40%; S3, adding LiPF6 in a solvent to prepare a basic electrolyte, and adding LiFSI, FEC and TEGDME in the basic electrolyte to obtain a solvation-regulated composite electrolyte after dissolution; S4, assembling the composition-porosity double gradient electrode piece and the solvation-regulated composite electrolyte with a negative electrode and a separator to obtain a battery, thereby obtaining the high-rate long-cycle lithium iron phosphate battery.

2. The process for the preparation of high rate long cycle lithium iron phosphate battery as claimed in claim 1 wherein, The pore size of the multi-level ordered pore LiFePO4@C / Graphene positive electrode material ranges from 8 to 25 nm.

3. The process for the preparation of high rate long cycle lithium iron phosphate battery as claimed in claim 1 wherein, The template agent in step S1 is CTAB; the hydrothermal reaction is specifically carried out in a high-pressure reaction kettle at 150 to 210°C for 8 to 16 hours; and the etching is carried out by using 3wt% to 8wt% hydrofluoric acid for 3 to 5 hours.

4. The process for the preparation of high rate long cycle lithium iron phosphate battery as claimed in claim 1 wherein, The mass ratio of the precursor, the carbon source and the graphene oxide in step S1 is 1000:50 to 200:3 to 10; and the calcination is carried out by heating to 700 to 800°C at a rate of 4 to 6°C / min in an Ar / H2 mixed atmosphere for 5 to 7 hours.

5. The process for the preparation of high rate long cycle lithium iron phosphate battery as claimed in claim 1 wherein, The ball milling in step S2 adopts a grading ball milling, and the particle size of the positive electrode material in the surface layer, the intermediate layer and the separator proximity layer is controlled to 1.5 to 2.5μm, 2.5 to 4μm and 4 to 6μm, respectively.

6. The process for the preparation of high rate long cycle lithium iron phosphate battery as claimed in claim 1 wherein, The separator proximity layer in step S2 additionally contains 4 to 6wt% graphene nanosheets with a lateral size of 5 to 10μm.

7. The process for the preparation of high rate long cycle lithium iron phosphate battery as claimed in claim 1 wherein, The mass ratio of the basic electrolyte, LiFSI, FEC and TEGDME in step S3 is 80 to 120:1 to 2:2.5 to 3.5:4 to 6. ​ 8. A lithium iron phosphate battery, characterized in that, The lithium iron phosphate battery is prepared by the method of claim 1.

9. A positive electrode sheet for a lithium battery, characterized in that, The positive electrode piece is prepared by the method of claim 1.

10. An electrolyte solution for a lithium battery, characterized by comprising The electrolyte is prepared by the method of claim 1.

Images