Sodium ion battery without sodium precipitation risk

Abstract

The sodium ion battery comprises a positive electrode and a negative electrode, an active material of the positive electrode comprises a polyanionic sodium storage material, an active material of the negative electrode is hard carbon, the ratio of the capacity of a slope region of the negative electrode to the capacity of the positive electrode is 1.1-1.2, and the working voltage of the negative electrode is 0.1-1.0 V. The NP ratio is limited, the working voltage of the negative electrode is limited in a slope region, sodium ions are mainly adsorbed on the surface, and sodium metallization in micropores is avoided; na < 3 > Ti < 2 > (PO4) < 3 > is added into a positive active material, a sodium source is provided for formation of an SEI film by utilizing a reversible decomposition reaction of Na < 3 > Ti < 2 > (PO4) < 3 > at 2.1 V or above, and a sodium pre-treatment process is not needed; the positive electrode and the negative electrode adopt gradient porosity design, so that the dynamic bottleneck of a high-load electrode is effectively solved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a sodium ion battery without the risk of sodium precipitation. Background Art

[0002] In the research and application of sodium-ion batteries, the selection and structural design of positive and negative electrode materials have a crucial impact on battery performance. Existing sodium-ion battery technologies utilize a wide variety of positive electrode materials, such as layered oxides and polyanionic compounds. However, these materials differ in sodium ion storage performance and cycling stability. Regarding negative electrode materials, hard carbon materials, due to their unique structure and properties, have become a research hotspot for sodium-ion battery anodes.

[0003] However, existing sodium-ion batteries based on hard carbon negative electrodes still face some challenges in practical applications. For example, during the charging process, the precipitation of sodium ions on the surface of the hard carbon negative electrode is relatively common, which not only affects the cycle life of the battery but also may pose a safety hazard. At the same time, the low first charge efficiency (first efficiency) is also one of the problems that the existing technology urgently needs to solve. This is mainly because during the first charge process, some sodium ions are irreversibly consumed, forming a stable solid electrolyte interface film (SEI film) on the negative electrode surface.

[0004] Researchers have tried to solve the sodium ion consumption problem through pre-sodiumization process. However, the traditional pre-sodiumization process relies on metal sodium foil or sodium alloy and needs to be operated in an anhydrous and oxygen-free environment. It not only increases the complexity of the manufacturing process, but also brings the risk of thermal runaway due to the high activity of metal sodium, which restricts the large-scale production of sodium ion batteries. At the same time, for high-load cathode materials (>15mg / cm 2 ), such as the polyanionic Na3Ti2(PO4)3, whose ion diffusion path increases exponentially with the increase of coating thickness, resulting in a capacity retention rate of less than 60% at a 3C rate, which is difficult to meet the high power density requirements of grid energy storage.

[0005] Therefore, in terms of the overall structural design of the battery, parameters such as the loading capacity of the positive and negative electrodes and the porosity also have a significant impact on the battery's performance. Currently, further research and exploration is still needed to determine how to optimize these structural parameters to achieve optimal electrochemical performance. Summary of the Invention

[0006] In response to the deficiencies in the prior art, the present invention provides a sodium ion battery with no risk of sodium precipitation. By limiting the negative electrode slope region capacity / positive electrode capacity to 1.1-1.2 and limiting the negative electrode operating voltage slope region to 0.1-1.0V, sodium ions are mainly adsorbed on the surface to avoid sodium metallization in the micropores. Na3Ti2(PO4)3(NTP) is added to the positive electrode active material, and its reversible decomposition reaction above 2.1V is utilized to provide a sodium source for the formation of the SEI film, without the need for a pre-sodium process. The positive and negative electrodes are designed with a gradient porosity to effectively solve the kinetic bottleneck of high-load electrodes.

[0007] In order to solve the above technical problems, the present invention provides a sodium ion battery with no sodium precipitation risk, comprising a positive electrode and a negative electrode, wherein the positive electrode active material comprises a polyanion sodium storage material, the negative electrode active material comprises hard carbon, the negative electrode slope region capacity / positive electrode capacity = 1.1-1.2, and the negative electrode operating voltage is 0.1-1.0V.

[0008] The present invention limits the negative electrode operating voltage to a slope region of 0.1-1.0 V by limiting the negative electrode slope region capacity / positive electrode capacity to 1.1-1.2. Molecular dynamics simulation proves that sodium ions in this region are mainly adsorbed on the surface, which can avoid sodium metallization in micropores, adjust the sodium precipitation threshold to below 0.05 V, and theoretically calculate the first efficiency value to more than 92%.

[0009] Furthermore, the polyanionic sodium storage material is sodium iron pyrophosphate Na3Fe2(P2O7)(PO4) and / or sodium iron sulfate NaFe(SO4)2.

[0010] Furthermore, the positive electrode active material further includes an additive: sodium-rich sodium titanium phosphate Na3Ti2(PO4)3, and the mass of the additive is 3%-8% of the total mass of the positive electrode active material.

[0011] The positive active material of the present invention is added with Na3Ti2(PO4)3(NTP), which undergoes a reversible decomposition reaction above 2.1V: Na3Ti2(PO4)3→Na Ti2(PO4)3+2Na + The sodium ions released during the first charge can accurately compensate for the sodium source required for SEI film formation (about 5% initial sodium content), without the need for additional pre-sodiumization process, simplifying the operation and making the process safe.

[0012] Furthermore, the preparation method of Na3Ti2(PO4)3 is as follows: preparing Na Ti2(PO4)3 / carbon composite material by solid phase reaction method, mixing Na Ti2(PO4)3 / carbon composite material with binder and solvent, coating on current collector, placing it in sodium deoxygenated aqueous solution as working electrode after drying, and embedding Na by electrochemical method. + , obtaining Na3Ti2(PO4)3.

[0013] Furthermore, the positive electrode includes a dense inner layer and a porous surface layer, wherein the porosity of the dense inner layer is 20-30%, preferably 25-30%, the porosity of the porous surface layer is 30-50%, preferably 40-50%, and the thickness of the porous surface layer is 15-25% of the thickness of the positive electrode.

[0014] Furthermore, the preparation method of the positive electrode is:

[0015] S1, mixing a positive electrode active material, a conductive agent, a binder and a solvent to obtain a positive electrode slurry;

[0016] The conductive agent is carbon nanotubes and Ketjen black; the binder is polyvinylidene fluoride (PVDF);

[0017] S2. Apply the positive electrode slurry on the current collector and roll-press at 14-16 MPa to form a dense inner layer.

[0018] S3. Take the positive electrode slurry, add a pore-forming agent to form a surface slurry, apply it on the surface of the dense inner layer, and vacuum dry it to obtain a porous surface layer.

[0019] Furthermore, the pore-forming agent is NH4HCO3, and the content of the pore-forming agent is 1-5% of the mass of the surface slurry.

[0020] Furthermore, in S3, the drying temperature is 110-130°C.

[0021] Furthermore, the hard carbon is doped with 2-4 at% N and 1-2 at% P, and the surface is coated with an Al2O3 layer with a thickness of 1-2 nm, serving as a negative electrode active material.

[0022] Furthermore, the negative electrode active material is prepared by immersing a hard carbon precursor in a urea phosphate or polyphosphazene solution, heat-treating it to obtain a hard carbon doped with nitrogen and phosphorus. After depositing an Al2O3 layer by ALD, the Al2O3 layer is in situ annealed at 290-310°C to achieve continuous Al2O3 coating. In-situ annealing eliminates dangling bonds at the Al-OC interface through atomic thermal migration, transforming the Al2O3 layer from island growth to continuous coating.

[0023] Furthermore, the aluminum source of the ALD deposition is trimethylaluminum (TMA), and the oxygen source is water.

[0024] Furthermore, the preparation method of the negative electrode is:

[0025] (1) Mixing the negative electrode active material, conductive agent, binder and solvent and coating them on the current collector;

[0026] The conductive agent is graphene, and the binder is sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR);

[0027] (2) Pre-rolling at 4-6 MPa and 0.5-1.0 m / min, and then finishing rolling at 18-22 MPa and 0.2-0.5 m / min to obtain a laminated core layer and skin layer;

[0028] The porosity of the core layer is 35-45%, the porosity of the skin layer is 20-30%, and the thickness of the skin layer is 5-10% of the thickness of the negative electrode.

[0029] The present invention adopts a gradient porosity design: the porosity of the porous surface layer of the positive electrode is increased to 30-50%, constructing a rapid sodium ion transmission channel, and the dense inner layer maintains a porosity of 20-30% to ensure electronic conductivity; the negative electrode forms a "dense skin layer-porous core layer" structure through two pressing processes, which increases the ion diffusion coefficient by about 40%; the gradient porosity design effectively solves the kinetic bottleneck of high-load electrodes.

[0030] Furthermore, compared with the 1C rate capacity, the sodium ion battery has a 10C rate capacity retention rate greater than 98%.

[0031] Beneficial effects of the present invention:

[0032] The present invention limits the negative electrode operating voltage to a slope region of 0.1-1.0 V by limiting the negative electrode slope region capacity / positive electrode capacity to 1.1-1.2. Molecular dynamics simulation proves that sodium ions in this region are mainly adsorbed on the surface, which can avoid sodium metallization in micropores, adjust the sodium precipitation threshold to below 0.05 V, and theoretically calculate the first efficiency value to more than 92%.

[0033] The positive active material of the present invention is added with Na3Ti2(PO4)3(NTP), which undergoes a reversible decomposition reaction above 2.1V to produce sodium ions Na + The sodium ions released during the first charge can accurately compensate for the sodium source required for SEI film formation, without the need for additional pre-sodiumization process, simplifying the operation and ensuring process safety.

[0034] The present invention adopts a gradient porosity design: the porosity of the porous surface layer of the positive electrode is increased to 30-50%, constructing a rapid sodium ion transmission channel, and the dense inner layer maintains a porosity of 20-30% to ensure electronic conductivity; the negative electrode forms a "dense skin layer-porous core layer" structure through two pressing processes, which increases the ion diffusion coefficient by about 40%; the gradient porosity design effectively solves the kinetic bottleneck of high-load electrodes. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0036] Figure 1 1 is the charge and discharge curve of the battery of Example 1 of the present invention. DETAILED DESCRIPTION

[0037] The following will provide a clear and complete description of the technical solutions of the present invention in conjunction with specific embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0038] The present embodiment relates to a sodium-ion battery with no sodium precipitation risk, comprising a positive electrode and a negative electrode, wherein the positive electrode active material comprises a polyanionic sodium storage material, the negative electrode active material comprises hard carbon, the negative electrode slope region capacity / positive electrode capacity = 1.1-1.2, and the negative electrode operating voltage is 0.1-1.0 V. This embodiment limits the negative electrode slope region capacity / positive electrode capacity to 1.1-1.2, and limits the negative electrode operating voltage to the slope region of 0.1-1.0 V. Molecular dynamics simulation is used to demonstrate that sodium ions in this region are primarily surface adsorbed, thus avoiding sodium metallization in micropores. The sodium precipitation threshold is adjusted to below 0.05 V, and the theoretically calculated first efficiency value can reach over 92%.

[0039] As a preferred embodiment, the polyanionic sodium storage material is Na3Fe2(P2O7)(PO4) and / or NaFe(SO4)2; the positive electrode active material also includes an additive: Na3Ti2(PO4)3, and the mass of the additive is 3%-8% of the total mass of the positive electrode active material. By adding Na3Ti2(PO4)3(NTP), the sodium ions released during the first charge can accurately compensate for the sodium source required for the formation of the SEI film, without the need for an additional pre-sodiumization process, simplifying the operation, and the process is safe. Among them, the preparation method of the Na3Ti2(PO4)3 is: preparing a Na Ti2(PO4)3 / carbon composite material by a solid phase reaction method, mixing the Na Ti2(PO4)3 / carbon composite material with a binder and a solvent, coating it on a current collector, and placing it in a sodium-containing deoxygenated aqueous solution as a working electrode after drying, and embedding Na by electrochemical method. + , obtaining Na3Ti2(PO4)3.

[0040] As a preferred embodiment, the hard carbon is doped with 2-4 at% N and 1-2 at% P, and the surface is coated with an Al2O3 layer with a thickness of 1-2 nm, as a negative electrode active material; the preparation method of the negative electrode active material is: immersing the hard carbon precursor in a urea phosphate or polyphosphazene solution, heat treating to obtain a hard carbon doped with N and P, depositing the Al2O3 layer by ALD, and then in-situ annealing at 290-310°C to achieve continuous Al2O3 coating;

[0041] The aluminum source is trimethylaluminum (TMA) and the oxygen source is water.

[0042] As a preferred embodiment, the positive electrode includes a dense inner layer and a porous surface layer, wherein the porosity of the dense inner layer is 20-30%, preferably 25-30%, the porosity of the porous surface layer is 30-50%, preferably 40-50%, and the thickness of the porous surface layer is 15-25% of the thickness of the positive electrode; the preparation method of the positive electrode is:

[0043] S1, mixing a positive electrode active material, a conductive agent, a binder and a solvent to obtain a positive electrode slurry;

[0044] The conductive agent is carbon nanotubes and Ketjen black; the binder is polyvinylidene fluoride (PVDF);

[0045] S2. Apply the positive electrode slurry on the current collector and roll-press at 14-16 MPa to form a dense inner layer.

[0046] S3. Take the positive electrode slurry, add a pore-forming agent to form a surface slurry, apply it on the surface of the dense inner layer, and vacuum dry it at 110-130°C to obtain a porous surface layer;

[0047] Wherein, the pore-forming agent is NH4HCO3, and the content of the pore-forming agent is 1-5% of the mass of the surface slurry.

[0048] As a preferred embodiment, the preparation method of the negative electrode is:

[0049] (1) Mixing the negative electrode active material, conductive agent, binder and solvent and coating them on the current collector;

[0050] The conductive agent is graphene, and the binder is sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR);

[0051] (2) Pre-rolling at 4-6 MPa and 0.5-1.0 m / min, and then finishing rolling at 18-22 MPa and 0.2-0.5 m / min to obtain a laminated core layer and skin layer;

[0052] The porosity of the core layer is 35-45%, the porosity of the skin layer is 20-30%, and the thickness of the skin layer is 5-10% of the thickness of the negative electrode.

[0053] This embodiment adopts a gradient porosity design: the porosity of the porous surface layer of the positive electrode is increased to 30-50%, constructing a rapid sodium ion transmission channel, and the dense inner layer maintains a porosity of 20-30% to ensure electronic conductivity; the negative electrode forms a "dense skin layer-porous core layer" structure through a two-step pressing process, which increases the ion diffusion coefficient by approximately 40%; the gradient porosity design effectively solves the kinetic bottleneck of high-load electrodes.

[0054] Example 1

[0055] This embodiment relates to a method for preparing a sodium ion battery without the risk of sodium precipitation, comprising the following steps:

[0056] (1) Preparation of Na3Ti2(PO4)3:

[0057] Na2CO3 is used as a Na source material, TiO2 is used as a Ti source material, NH4H2PO4 is used as a P source material, and acetylene black is used as a carbon source material; the molar ratio of the Na source material, the Ti source material, and the P source material is 1:2:3, and the mass ratio of NaTi2(PO4)3 to the carbon source material is 0.95:0.05; the Na source material, the Ti source material, the P source material, and the carbon source material are evenly mixed by ball milling, and then spray dried to obtain a precursor powder; the precursor powder is sintered in a nitrogen-protected sintering kiln at a heating rate of 1-5°C / min to a sintering temperature of 700°C, and the heat preservation time is 12 hours. The sintered powder is subjected to air flow pulverization to obtain a Na Ti2(PO4)3 / carbon composite material;

[0058] The prepared Na Ti2(PO4)3 / carbon composite material and polyvinylidene fluoride (binder) were evenly mixed in N-methylpyrrolidone (solvent). The mass ratio of Na Ti2(PO4)3 / carbon composite material to polyvinylidene fluoride was 98:2. The mixed slurry was coated on a stainless steel current collector and used as a working electrode after drying. Sodium sulfate was dissolved in deionized water to obtain a 1 mol / L sodium sulfate solution. The oxygen contained in the solution was removed by vacuuming to obtain a sodium-containing deoxygenated aqueous solution. A graphite electrode was used as the counter electrode. An electrochemical method was used to carry out a charging reaction at a current of 200 mA / g. After the end, the sample was rinsed with deoxygenated water under an inert atmosphere and dried under an oxygen-free environment to obtain Na3Ti2(PO4)3 material.

[0059] (2) Preparation of positive electrode sheet:

[0060] Sodium iron pyrophosphate (Na3Fe2(P2O7)(PO4)) and sodium supplement agent NTP are uniformly mixed in a mass ratio of 95:5 to obtain a positive electrode active material.

[0061] The positive electrode active material was mixed with the conductive agent carbon nanotube (CNT), Ketjen black (KB) and the binder polyvinylidene fluoride in a mass ratio of 88:2:4:6, and the solvent N-methylpyrrolidone was added and stirred evenly to form a positive electrode slurry. The slurry was coated on the surface of the current collector aluminum foil and rolled to a pressure of 15 MPa to form a dense inner layer with a porosity of 25% and a thickness of 220 μm.

[0062] The positive electrode active material and the pore-forming agent NH4HCO3 are mixed in a mass ratio of 100:3, and the solvent N-methylpyrrolidone is added and stirred evenly to form a surface positive electrode slurry. The surface positive electrode slurry is coated on the dense inner layer with a coating thickness of 60 μm. After vacuum drying at 120°C, a porous surface layer of the electrode with a thickness of 21.4% (60 / (60+220)=0.214) and a porosity of 45% is formed.

[0063] (3) Preparation of negative electrode active material:

[0064] Urea phosphate ((NH2)2CO·H3PO4) is selected as the N and P precursors, and NH3 (N source) and PO4 are released during high temperature cracking. x (for P source).

[0065] Hard carbon (Kuraray Type 2) was immersed in a 20wt% urea phosphate aqueous solution with a liquid-to-solid ratio of 5:1 and vacuum dried at 80℃ for 12h to form a uniform coating layer. Nitrogen / phosphorus co-doping was achieved by heat treatment, wherein the protective atmosphere was Ar / H2=95 / 5, and urea phosphate was decomposed from 5℃ / min to 300℃, and then heated to 800℃ at a rate of 10℃ / min for 2h to promote the embedding of N / P atoms into the carbon skeleton to obtain a nitrogen-phosphorus co-doped hard carbon matrix.

[0066] A nitrogen-phosphorus co-doped hard carbon substrate was placed in a hot-wall atomic layer deposition (ALD) reactor, and an Al2O3 layer was deposited using a sequential pulse method: trimethylaluminum (TMA) was used as the aluminum source, H2O was used as the oxygen source, and high-purity nitrogen (≥99.999%) was used as the purge gas. The single-cycle process parameters were: TMA pulse 0.1s → N2 purge 20s → H2O pulse 0.2s → N2 purge 20s, with 10-15 cycles (corresponding to a thickness of 1.2±0.3nm). The substrate temperature was controlled at 150°C to prevent thermal damage to the carbon material, and the chamber pressure was maintained at 1.0 Torr to ensure sufficient diffusion of the precursors. Immediately after deposition, an in-situ annealing at 300°C (N2 shielding, ramp rate 5°C / min) was performed. Atomic thermal migration eliminated dangling bonds at the Al-O-C interface, transforming the Al2O3 layer from island growth to a continuous coating, resulting in a N / P-doped hard carbon anode active material coated with an Al2O3 layer.

[0067] (4) Preparation of negative electrode sheet:

[0068] The negative electrode active material is mixed with the graphene conductive agent and the binder sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) in a mass ratio of 90:2.5:2.5:5. The solvent N-methyl pyrrolidone is added and stirred evenly to form a negative electrode slurry. The negative electrode slurry is coated on the copper foil current collector and, after drying, pre-rolled with a first roll press at a pressure of 5 MPa and a speed of 0.70 m / min. The second roll press is performed at a pressure of 20 MPa and a speed of 0.3 m / min to fine-press the surface to form a skin layer, thereby obtaining a laminated core layer and skin layer; the final electrode density is 1.5-1.7 g / cm 3 The thickness of the skin layer accounts for 10% of the thickness of the negative electrode, the porosity of the core layer is more than 40%, and the porosity of the surface layer is about 25%.

[0069] (5) Battery assembly

[0070] The prepared positive and negative electrode sheets were stacked in sequence according to the NP ratio (negative electrode slope area capacity / positive electrode capacity) of 1.15 and the polypropylene separator, placed in the battery case, injected with electrolyte (1M NaPF6 in EC / DEC (3:7 vol%) + 2% FEC additive), and then sealed.

[0071] The assembled batteries were subjected to formation treatment, using a charge and discharge system that first charged at a constant current to 3.5V, then charged at a constant voltage to a current less than 0.05C, and then discharged at a constant current to 2.0V to complete the battery formation process.

[0072] Example 2

[0073] The difference between this embodiment and embodiment 1 is that the amount of the sodium supplement NTP added in step (2) is 3% of the total mass of the positive electrode active material, and the other steps and parameters remain unchanged.

[0074] Example 3

[0075] The difference between this embodiment and embodiment 1 is that the amount of the sodium supplement NTP added in step (2) is 8% of the total mass of the positive electrode active material, and the other steps and parameters remain unchanged.

[0076] Comparative Example 1

[0077] The difference between this comparative example and Example 1 is that the amount of the sodium supplement NTP added in step (2) is 0% of the total mass of the positive electrode active material, and the other steps and parameters remain unchanged.

[0078] Comparative Example 2

[0079] The difference between this comparative example and Example 1 is that the amount of the sodium supplement NTP added in step (2) is 9% of the total mass of the positive electrode active material, and the other steps and parameters remain unchanged.

[0080] Comparative Example 3

[0081] The difference between this comparative example and Example 1 is that the step of coating the Al2O3 layer is omitted in step (3), and the other steps and parameters remain unchanged.

[0082] Comparative Example 4

[0083] The difference between this comparative example and Example 1 is that in step (3), the hard carbon precursor is immersed in water, the hard carbon matrix is not doped with N / P, and the other steps and parameters remain unchanged.

[0084] Comparative Example 5

[0085] The difference between this comparative example and Example 1 is that in step (3), the hard carbon precursor is immersed in water, the hard carbon matrix is not doped with N / P, and is not coated with an Al2O3 layer, and the other steps and parameters remain unchanged.

[0086] Comparative Example 6

[0087] The difference between this comparative example and Example 1 is that the thickness of the dense inner layer in step (2) is adjusted to 280 μm, the step of preparing the porous surface layer by mixing and coating the positive electrode active material and the pore-forming agent is omitted, and the other steps and parameters remain unchanged.

[0088] Comparative Example 7

[0089] The difference between this comparative example and Example 1 is that the dense inner layer preparation step is omitted in step (2), and a porous surface layer with a thickness of 280 μm is prepared, and other steps and parameters remain unchanged.

[0090] Comparative Example 8

[0091] The difference between this comparative example and Example 1 is that in step (4), only the first rolling parameters are used for rolling, the second rolling is omitted, and the other steps and parameters remain unchanged.

[0092] Comparative Example 9

[0093] The difference between this comparative example and Example 1 is that the first rolling is omitted in step (4), and only the second rolling parameters are used for rolling, while the other steps and parameters remain unchanged.

[0094] Comparative Example 10

[0095] The difference between this comparative example and Example 1 is that, during the battery assembly in step (5), the negative electrode slope area capacity / positive electrode capacity = 1, and the other parameters and steps remain unchanged.

[0096] Test example:

[0097] (1) First coulombic efficiency (ICE) is the ratio of discharge capacity to charge capacity during the first charge and discharge process of a sodium ion battery, reflecting the quality of film formation at the electrode interface and the utilization rate of active materials. The assembled battery was placed in a thermostat and charged at a constant current of 0.5 C (C is the current corresponding to the theoretical capacity) to a cutoff voltage of 3.5 V. After standing for 30 minutes, it was discharged at the same current to 1.5 V, and the ICE data were recorded.

[0098] (2) 10C rate capacity retention rate: First, perform 1C charge and discharge for 3 cycles and record the average discharge capacity as C1. Then, perform 10C charge and discharge for 3 cycles and record the average discharge capacity as C10. The 10C capacity retention rate is R1 = C10 / C1*100%.

[0099] Figure 1 The charge and discharge data of Example 1 at a rate of 0.5C with a charge and discharge voltage range of 1.5-3.5V are shown in Table 1. The electrical properties of the assembled batteries of each embodiment and comparative example are shown in Table 1:

[0100] Table 1

[0101]

[0102]

[0103] From the comparison of Examples 1-3 and Comparative Examples 1-2, it can be seen that when there is no sodium supplement in Comparative Example 1, the first efficiency is only 80.4%. As the sodium supplement increases, the first efficiency increases. Due to the increase in the first efficiency, the volume space occupied by the sodium supplement in the battery cell is compensated, and the battery capacity is improved. However, there is an optimal value. When the sodium supplement is 5%, the battery capacity is the highest. When the sodium supplement continues to increase, the increase in the first efficiency is very small. Due to the increase in the proportion of the sodium supplement in the battery, the battery capacity decreases slightly (such as 8%). When it is greater than 8%, the battery capacity decreases significantly.

[0104] From the comparison between Example 1 and Comparative Examples 3-5, it can be seen that Al2O3, as a dense dielectric layer, can physically isolate the hard carbon from direct contact with the electrolyte, inhibit the continuous decomposition reaction of the electrolyte (especially EC solvent) in the slope area (0.1-1.0V), and reduce the repeated growth of the SEI film. When there is no Al2O3 layer coating, the first efficiency is reduced to 84.9%. At the same time, the Lewis acidic surface of Al2O3 can preferentially adsorb PF6 - Anions form a local high sodium ion concentration gradient at the interface, accelerating the Na + Desolvation process. When the Al2O3 layer is missing, the high rate capacity retention rate decreases, indicating that the coating significantly improves the ion migration kinetics at high rates. In addition, nitrogen doping (especially graphitic nitrogen) significantly increases sp 2The proportion of hybrid carbon increases by 2-3 orders of magnitude, and the electrode polarization is reduced; phosphorus doping expands the carbon layer spacing, combined with nitrogen-induced defect sites, so that Na + The diffusion coefficient was increased to 1.2×10 -10 cm 2 / s (undoped material is about 5×10 -12 cm 2 Therefore, nitrogen and phosphorus co-doping can improve the capacity in the slope region, which is beneficial to the improvement of battery capacity and the output of high-rate performance.

[0105] From the comparison between Example 1 and Comparative Example 6, it can be seen that when the positive electrode is a dense inner layer as a whole, the porosity is low and the battery rate performance is reduced; from Comparative Example 7, it can be seen that when the overall porosity of the positive electrode is too large, at the same thickness, the active material is reduced and the battery capacity is reduced. At the same time, due to the large porosity, ion transport is not restricted, but electronic conductivity is inhibited, so the 10C capacity retention rate is greatly reduced.

[0106] From the comparison between Example 1 and Comparative Examples 8-9, it can be seen that the hard carbon negative electrode structure with "dense skin and porous core" is formed by two differentiated rolling processes, which improves the ion diffusion kinetics. When only the first stage of the pressing process is used, the 10C capacity retention rate decreases due to the lack of a dense layer on the surface to reduce the interface resistance. Due to the large overall porosity, the capacity of the battery is also reduced. When only the second stage of the pressing process is used, the electrode porosity is low, the ion diffusion coefficient is affected, and the 10C capacity retention rate is seriously reduced.

[0107] From the comparison between Example 1 and Comparative Example 10, it can be seen that when the capacity of the negative electrode slope region / the positive electrode capacity = 1, the operating voltage of the hard carbon negative electrode will be lower than 0.1V, thereby entering the platform region, forming sodium metallization in the micropores, resulting in a decrease in the first efficiency. At the same time, due to the slower power performance of the platform region, the 10C capacity retention rate decreases.

[0108] The present invention has been described in detail above with reference to specific embodiments and exemplary examples. However, these descriptions should not be construed as limiting the present invention. Those skilled in the art will appreciate that various equivalent substitutions, modifications, or improvements may be made to the technical solutions and implementations of the present invention without departing from the spirit and scope of the present invention, all of which fall within the scope of the present invention. The scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A sodium ion battery without sodium precipitation risk, characterized in that: It includes a positive electrode and a negative electrode, the positive electrode active material includes a polyanion sodium storage material, the negative electrode active material includes hard carbon, the negative electrode slope area capacity / positive electrode capacity = 1.1-1.2, and the negative electrode operating voltage is 0.1-1.0V.

2. The sodium ion battery without sodium precipitation risk according to claim 1, characterized in that The polyanion sodium storage material is sodium ferric pyrophosphate and / or sodium ferric sulfate.

3. The sodium ion battery without sodium precipitation risk according to claim 1, characterized in that The positive electrode active material further comprises an additive: sodium-rich sodium titanium phosphate Na3Ti2(PO4)3, and the mass of the additive is 3%-8% of the total mass of the positive electrode active material.

4. The sodium ion battery without sodium precipitation risk according to claim 3, characterized in that The preparation method of Na3Ti2(PO4)3 is as follows: preparing Na Ti2(PO4)3 / carbon composite material by solid phase reaction method, mixing Na Ti2(PO4)3 / carbon composite material with binder and solvent, coating on current collector, placing it in sodium deoxygenated aqueous solution as working electrode after drying, and embedding Na by electrochemical method. + , obtaining Na3Ti2(PO4)3.

5. The sodium ion battery without sodium precipitation risk according to claim 1, characterized in that The positive electrode comprises a dense inner layer and a porous surface layer, wherein the porosity of the dense inner layer is 20-30%, the porosity of the porous surface layer is 30-50%, and the thickness of the porous surface layer is 15-25% of the thickness of the positive electrode.

6. The sodium ion battery without sodium precipitation risk according to claim 5, characterized in that The preparation method of the positive electrode is: S1, mixing a positive electrode active material, a conductive agent, a binder and a solvent to obtain a positive electrode slurry; S2. Apply the positive electrode slurry on the current collector and roll-press at 14-16 MPa to form a dense inner layer. S3. Take the positive electrode slurry, add a pore-forming agent, apply it on the surface of the dense inner layer, and vacuum dry it to obtain a porous surface layer.

7. The sodium ion battery without sodium precipitation risk according to claim 1, characterized in that The hard carbon is doped with 2-4 at% N and 1-2 at% P, and the surface is coated with an Al2O3 layer with a thickness of 1-2 nm, serving as a negative electrode active material.

8. The sodium ion battery without sodium precipitation risk according to claim 7, characterized in that The preparation method of the negative electrode active material comprises: impregnating a hard carbon precursor in a urea phosphate or polyphosphazene solution, heat-treating to obtain hard carbon doped with N and P, depositing an Al2O3 layer by ALD, and then in-situ annealing at 290-310°C to achieve continuous Al2O3 coating.

9. The sodium ion battery without sodium precipitation risk according to claim 1, characterized in that The preparation method of the negative electrode is: (1) Mixing the negative electrode active material, conductive agent, binder and solvent and coating them on the current collector; (2) Pre-rolling at 4-6 MPa and 0.5-1.0 m / min, and then finishing rolling at 18-22 MPa and 0.2-0.5 m / min to obtain a laminated core layer and skin layer; The porosity of the core layer is 35-45%, the porosity of the skin layer is 20-30%, and the thickness of the skin layer is 5-10% of the thickness of the negative electrode.

10. The sodium ion battery without sodium precipitation risk according to claim 1, characterized in that Compared with the 1C rate capacity, the sodium ion battery has a 10C rate capacity retention rate greater than 98%.

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