Composite sodium-ion battery and preparation method thereof

Abstract

This invention discloses a composite sodium-ion battery and its preparation method. The positive electrode active material comprises a layered sodium-ion positive electrode material and a lithium iron phosphate positive electrode material; the working voltage range of the sodium-ion positive electrode material is a1~b1V, and the working voltage range of the lithium iron phosphate positive electrode material is a2~b2, where a1…<a2,b1> b2; The negative electrode active material includes hard carbon and graphite, and the mass percentage of hard carbon in the negative electrode active material is greater than 50 wt.%; the electrolyte salt includes sodium salt and lithium salt. This invention can improve the energy density of a sodium-ion battery while offering a significant cost advantage compared to lithium batteries. Furthermore, the manufacturing process is simple, identical to existing battery manufacturing processes, and can be mass-produced.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and specifically to a composite sodium-ion battery. Background Technology

[0002] In today's society, the application of lithium-ion batteries is becoming increasingly widespread, ranging from large-scale energy storage power stations to small-scale applications like e-cigarettes. However, lithium, an indispensable element in lithium-ion batteries, is largely sourced from South America and Australia. While my country's new energy technology is among the world's most advanced, it remains a lithium-poor nation, importing over 80% of its lithium resources annually. In the past year, the price of lithium carbonate has skyrocketed from 90,000 yuan / ton to 500,000 yuan / ton, leading to a continuous increase in the cost of lithium-ion batteries, and judging from current trends, it's unlikely to fall back to a lower level. Therefore, it is necessary to vigorously develop other new energy sources. Among them, sodium-ion batteries can replace lithium-ion batteries in some applications, alleviating my country's demand for lithium resources.

[0003] Sodium-ion batteries were designed with resource advantages in mind, using little or no precious metals to achieve a cost advantage. However, due to the inherent characteristics of the materials and technological limitations, sodium-ion batteries have a lower energy density compared to lithium-ion batteries, and can only be used as an alternative to lead-acid batteries in small, low-speed electric vehicles.

[0004] Sodium-ion batteries and lithium-ion batteries operate in the same way, both consisting of a positive electrode active material that provides sodium / lithium as a source, a negative electrode active material that receives sodium / lithium, and an electrolyte. The electrolyte is composed of a composite organic solvent such as EC, PC, DEC, DMC, EMC, VC, FEC, and PS, along with an electrolyte salt. In sodium-ion batteries, the commercially available positive electrode materials are Prussian series, layered oxide series, and polyanionic series, while the negative electrode material is hard carbon. In lithium-ion batteries, the commercially available positive electrode materials are mainly lithium manganese oxide, ternary lithium, and lithium iron phosphate, while the negative electrode materials are mainly graphite and silicon. Combining commercially available sodium-ion and lithium-ion materials to create sodium / lithium composite batteries with charge / discharge capabilities is not difficult; however, achieving high cost-effectiveness in sodium / lithium composite batteries presents a challenge.

[0005] Some studies have explored connecting lithium-ion and sodium-ion battery cells in series and parallel to form modules, with control implemented at the BMS (Battery Management System). However, due to the different voltage operating ranges of lithium-ion and sodium-ion batteries, forming battery modules through series and parallel connections requires complex BMS electronic circuit designs to precisely control the charge and discharge levels of each battery. This leads to a sharp increase in the size of the BMS, and more complex electronic circuits reduce the stability of electronic components, resulting in a higher overall BMS failure rate. Other research has addressed these issues by designing electrode structures; for example, CN113193166A separates the lithium-ion and sodium-ion active materials during battery electrode fabrication, but this coating process is complex and increases processing costs.

[0006] Therefore, providing a composite sodium-ion battery that balances energy density and cost to facilitate the application of sodium-ion batteries is of significant research importance. Summary of the Invention

[0007] The purpose of this invention is to provide a composite sodium-ion battery and its preparation method. This invention provides a composite sodium-ion battery that, by selecting suitable sodium-ion materials and lithium-ion materials for blending, and combining them with a specific composition of negative electrode active material and electrolyte, can increase the energy density by 15% to 20% compared to existing sodium-ion batteries, thereby broadening the application scenarios of sodium-ion batteries. Simultaneously, this invention can be manufactured using the same process as existing batteries, possessing the advantages of simple process and large-scale production capability, further alleviating dependence on lithium resources.

[0008] To achieve the above objectives, the present invention provides a composite sodium-ion battery, comprising a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode active material in the positive electrode comprises a layered sodium-ion positive electrode material and a lithium iron phosphate positive electrode material.

[0009] The working voltage range of the sodium ion cathode material is a1~b1 V, and the working voltage range of the lithium iron phosphate cathode material is a2~b2, a1<a2,b1> b2;

[0010] The negative electrode active material in the negative electrode includes hard carbon and graphite, and the mass percentage of hard carbon in the negative electrode active material is greater than 50 wt.%, for example, 51 wt.%, 52 wt.%, 55 wt.%, 57 wt.%, 60 wt.%, 62.5 wt.%, 65 wt.%, 70 wt.%, 75 wt.%, 80 wt.%, 85 wt.%, 90 wt.%, or 90 wt.%, etc.;

[0011] The electrolyte salts in the electrolyte include sodium salts and lithium salts.

[0012] Different positive electrode active materials have specific reaction potentials for sodium / lithium intercalation, thus determining that each material has its own unique operating voltage range. For example, the operating voltage range of conventional ternary lithium batteries is 3.0V to 4.2V, meaning that only within this range can the material's capacity be fully utilized and its safety ensured during use. When the operating voltage range is smaller than this range, 100% capacity cannot be utilized, resulting in waste; when the operating voltage range is larger than this range, it can cause material structure collapse, damage the battery, and even lead to serious safety accidents. For composite sodium-ion batteries, when sodium / lithium composite materials or positive electrode materials containing both lithium-ion and sodium-ion positive electrode materials are used to prepare the electrodes and assemble the battery, the battery ultimately has only one definite operating voltage range. This requires us to determine the final operating voltage range of the battery based on the individual characteristics of each material in the composite material, ensuring that the capacity of each material is fully utilized without causing other problems, especially safety issues.

[0013] In this invention, the operating voltages of the layered sodium-ion cathode material and the lithium iron phosphate cathode material satisfy a certain relationship. The LiFePO4 lithium-ion cathode material has an olivine-type structure and possesses excellent stability, with a cycle life exceeding 2000 cycles. Therefore, during charging, even if the charging cut-off voltage exceeds the maximum value of the conventional operating voltage (typically 3.65V), it will only slightly reduce the material's cycle life without causing other problems. During discharging, when the battery discharges to the minimum operating voltage (typically 2.0V), it will only slightly reduce the battery's cycle life without causing other problems. Simultaneously, the layered sodium-ion cathode material (e.g., NaNi...) 1 / 3 Fe 1 / 3 Mn 1 / 3 The charge and discharge voltage of O2 is a gradually changing slope without a clear plateau, while the voltage plateau of LiFePO4 material is 3.2V, which is within the working voltage range of layered sodium ion cathode materials. Therefore, choosing to combine the two will not result in voltage abrupt changes, which is more friendly to subsequent BMS management.

[0014] Meanwhile, to fully utilize the respective capacities of sodium-ion and lithium-ion cathode materials, this invention optimizes the negative electrode active material. The negative electrode active material is primarily hard carbon, supplemented by a certain amount of artificial graphite. The main function of graphite is to reduce the specific surface area of ​​the negative electrode sheet, improve its compaction density and processing performance, and enhance the battery's initial coulombic efficiency. Although graphite and hard carbon are both carbon materials, their sodium and lithium storage mechanisms differ. Graphite stores lithium through interlayer intercalation and does not possess sodium storage capacity. Hard carbon, on the other hand, possesses both sodium and lithium storage capabilities. For sodium storage, over 80% of the capacity is primarily due to adsorption, with the remainder being intercalation. For lithium storage, it is a result of both adsorption and intercalation. Therefore, during charging, due to the low sodium removal potential of the sodium-ion cathode material, sodium ions preferentially detach from the cathode and adsorb onto defects and pores on the surface of the hard carbon cathode; subsequently, lithium ions detach from the cathode and intercalate or adsorb into the cathode. If the ratio of hard carbon to graphite is not appropriate, excessive sodium ions will adsorb onto the surface of the hard carbon in the early stages, blocking the intercalation channels for lithium ions in the later stages, reducing lithium ion kinetics performance, and increasing the risk of lithium plating. If there is too much graphite, less sodium ion cathode material is needed, increasing material costs; if there is too much hard carbon, the battery energy density will decrease.

[0015] This invention comprehensively considers the synergistic relationship between the positive electrode, negative electrode, and electrolyte, achieving complementary advantages to prepare a high-performance, cost-effective composite sodium-ion battery. The operating voltage range of this composite sodium-ion battery is a. 1～ b1 V.

[0016] As a preferred embodiment of the composite sodium-ion battery of the present invention, the hard carbon accounts for 78 wt.% to 84 wt.% of the mass of the negative electrode active material. If the amount of hard carbon is too low relative to the amount of graphite (hard carbon accounts for less than 78 wt.% of the total amount of negative electrode active material), it indicates that the capacity provided by the graphite negative electrode is greater than the actual demand, resulting in excessive waste of the negative electrode, reducing the energy density of the cell, and increasing the battery cost; if the amount of hard carbon is too high relative to the amount of graphite (hard carbon accounts for more than 84 wt.% of the total amount of negative electrode active material), it indicates that the capacity provided by the graphite negative electrode is less than the actual demand, and there is a risk of lithium plating.

[0017] Preferably, the layered sodium-ion cathode material comprises NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2.

[0018] Preferably, the sodium-ion layered cathode material accounts for 72 wt.% to 78 wt.% of the mass of the cathode active material, such as 72 wt.%, 73 wt.%, 74 wt.%, 75 wt.%, 76 wt.%, 77 wt.%, or 78 wt.%. Within this range, a better balance between high energy density and low cost can be achieved, while fully utilizing the capacity. If the content of the sodium-ion layered cathode material is too high, the battery energy density will decrease, and it will easily clog the lithium storage channels; if the LiFePO4 content is too high, the cost will increase.

[0019] Preferably, the ratio of the usable capacity of the hard carbon to that of the layered sodium ion cathode material is 1.15 to 1.2, for example, 1.15, 1.16, 1.17, 1.18, 1.19, or 1.2. This ratio is also the ratio of the sodium storage capacity of the negative electrode to the amount of sodium that can be released from the positive electrode during the first charge of the battery. During charging, sodium ions preferentially detach from the positive electrode active material and adsorb onto the hard carbon surface of the negative electrode. The inventors discovered that if the ratio of sodium storage capacity of the negative electrode to the amount of sodium detached from the positive electrode is low (below 1.15), although it can ensure that the negative electrode completely adsorbs (or embeds) the sodium ions detached from the positive electrode, it will reduce the kinetic performance of sodium ion embedding, reduce the rate capability and low-temperature performance of the cell, and at the same time, it is easy to cause a high density of sodium ions adsorbed on the hard carbon surface after preferentially entering the negative electrode, which will block the embedding channels of lithium ions in the later stage, reduce the kinetic performance of lithium ions, and cause the risk of lithium plating. If the ratio of sodium storage capacity of the negative electrode to the amount of sodium detached from the positive electrode is high (above 1.2), it will cause a serious excess of hard carbon, resulting in a high overall porosity of the negative electrode, reducing the electrode density, increasing the amount of electrolyte, and thus reducing the energy density of the cell.

[0020] Preferably, the sodium salt accounts for 70% to 75% of the total mass of the electrolyte salt, for example, 70%, 71%, 72%, 73%, 74%, or 75%.

[0021] Preferably, the concentration of the electrolyte salt in the electrolyte is 0.7 mol / L to 0.8 mol / L, for example, 0.7 mol / L, 0.72 mol / L, 0.74 mol / L, 0.76 mol / L, 0.78 mol / L or 0.8 mol / L.

[0022] Preferably, in the composite sodium-ion battery, the amount of electrolyte injected is 20% to 25% of the cell mass of the composite sodium-ion battery, such as 20%, 21%, 22%, 23%, 24%, or 25%.

[0023] Secondly, the present invention provides a method for preparing a composite sodium-ion battery, the method comprising the following steps:

[0024] (1) The positive electrode material and the negative electrode material are dispersed in a solvent to obtain a positive electrode slurry and a negative electrode slurry, wherein the positive electrode material includes a positive electrode active material and the negative electrode material includes a negative electrode active material;

[0025] (2) The positive electrode slurry and the negative electrode slurry are coated onto the positive electrode current collector and the negative electrode current collector, respectively, and dried to obtain the positive electrode and the negative electrode;

[0026] (3) The positive electrode, negative electrode and separator are assembled into a cell, and after being put into the casing, electrolyte is injected to obtain the composite sodium-ion battery.

[0027] In the method of the present invention, the types of positive current collector and negative current collector are not specifically limited. For example, the positive current collector can be aluminum foil and the negative current collector can be copper foil.

[0028] Optionally, the positive electrode slurry and the negative electrode slurry are coated onto the positive electrode current collector and the negative electrode current collector, respectively. After drying, the positive electrode sheet and the negative electrode sheet are rolled and cut to obtain the positive electrode sheet and the negative electrode sheet of the required size.

[0029] Preferably, the positive electrode material further includes a conductive agent and a binder. This invention does not specifically limit the types of conductive agents and binders; for example, the conductive agent can be SP, and the binder can be PVDF.

[0030] Preferably, the preparation method of the positive electrode slurry includes the following steps: after solid-phase mixing of the positive electrode raw materials, NMP solvent is added to make the solid content of the slurry 68% to 73% (e.g., 68%, 69%, 70%, 71%, 72% or 73%), kneading and stirring are performed, NMP solvent is added again, high-speed dispersion is performed, and the viscosity of the slurry is adjusted to be suitable for the coating process.

[0031] In the preferred embodiment described above, NMP solvent is added to bring the slurry's solid content to 68%–73%, followed by kneading and stirring until the slurry is in a paste-like state. During stirring, solid particles undergo a solid-phase grinding effect, dispersing large particles and improving the dispersion of solid particles. If the solid content is high at this step (>73%), the stirring resistance will be high, potentially damaging the equipment; if the solid content is low (<68%), the slurry is in a liquid-phase dispersed state, and agglomerated particles will rotate with the stirring paddle, resulting in poor dispersion.

[0032] Preferably, the negative electrode material further includes a conductive agent and a binder, and the binder preferably includes CMC and SBR.

[0033] Preferably, the preparation method of the positive electrode slurry includes the following steps: after mixing the negative electrode raw materials in a solid phase, water is added to make the solid content of the slurry 57% to 62% (e.g., 57%, 58%, 59%, 60%, 61%, or 62%), kneading and stirring are performed, water is added again, high-speed dispersion is performed, and the viscosity of the slurry is adjusted to be suitable for the coating process.

[0034] Preferably, the outer shell used in the shell insertion step is an aluminum-plastic film, and the method further includes formation, degassing, secondary sealing and capacity division after injecting electrolyte.

[0035] Compared with existing technologies, the present invention has the following beneficial effects:

[0036] This invention provides a composite sodium-ion battery that can improve the energy density of the battery based on sodium-ion batteries, while having a significant cost advantage compared with lithium batteries. Moreover, the manufacturing process is simple, the same as the existing battery manufacturing process, and can be mass-produced. Detailed Implementation

[0037] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0038] In this embodiment of the invention, NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 The operating voltage range of O2 is 2.0V to 3.9V, and the operating voltage range of LiFePO4 is 2.5V to 3.65V.

[0039] Example 1

[0040] This embodiment provides a composite sodium-ion battery, prepared according to the following method:

[0041] Positive electrode preparation:

[0042] (A) The positive electrode active material, SP, and PVDF are added to a double planetary mixer in a mass ratio of 95:2.5:2.5 for solid-phase mixing. The positive electrode active material is composed of NaNi... 1 / 3 Fe 1 / 3 Mn 1 / 3 The cathode active material is composed of O2 and LiFePO4, and NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 accounts for 75% wt of the total positive electrode active material;

[0043] (B) Add a certain amount of NMP solvent to make the solid content of the slurry 70.3%, and knead and stir.

[0044] (C) Add a certain amount of NMP solvent to make the solid content 65%, disperse at high speed, and then add NMP to adjust the viscosity of the slurry to 9000 mPa·s;

[0045] (D) The well-dispersed slurry is evenly coated onto aluminum foil, and then rolled and cut to obtain the positive electrode sheet;

[0046] Negative electrode preparation:

[0047] (a) The negative electrode active material, SP, CMC, and SBR were added to a double planetary mixer in a mass ratio of 95:1:1.5:3.5 for solid-phase mixing. The negative electrode active material consisted of hard carbon and graphite, with hard carbon accounting for 81% wt% of the total negative electrode active material; the hard carbon negative electrode material was mixed with NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 The available capacity ratio of O2 cathode material is 1.18;

[0048] (b) Add a certain amount of deionized water to make the solid content of the slurry 60.5%, and knead and stir.

[0049] (c) Add a certain amount of NMP solvent to make the solid content 50%, and disperse at high speed; and adjust the viscosity of the slurry to 9000 mPa·s;

[0050] (d) The well-dispersed slurry is evenly coated onto copper foil, and then rolled and cut to obtain the negative electrode sheet;

[0051] Assemble the battery:

[0052] The positive electrode, separator, and negative electrode are stacked / wound into the desired cell size, sealed in an aluminum-plastic film, and a certain amount of electrolyte is injected. The electrolyte consists of electrolyte and solvent. The electrolyte includes NaPF6 and LiPF6, with NaPF6 accounting for 72% of the total electrolyte. The electrolyte concentration in the electrolyte is 0.75 mol / L. The solvent is composed of EC:PC:DEC in a volume ratio of 1:1:1. The amount of electrolyte injected is 24% of the cell mass.

[0053] After the assembled battery undergoes formation, degassing, secondary sealing, and capacity testing, a composite sodium-ion battery is obtained.

[0054] Example 2

[0055] This embodiment provides a composite sodium-ion battery, prepared according to the following method:

[0056] Positive electrode preparation:

[0057] (A) The positive electrode active material, SP, and PVDF are added to a double planetary mixer in a mass ratio of 95:2.5:2.5 for solid-phase mixing. The positive electrode active material is composed of NaNi... 1 / 3 Fe 1 / 3 Mn 1 / 3 The cathode active material is composed of O2 and LiFePO4, and NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 accounts for 73 wt.% of the total positive electrode active material;

[0058] (B) Add a certain amount of NMP solvent to make the solid content of the slurry 73%, and knead and stir.

[0059] (C) Add a certain amount of NMP solvent to make the solid content 65%, and disperse at high speed; and adjust the viscosity of the slurry to 9000 mPa·s;

[0060] (D) The well-dispersed slurry is evenly coated onto aluminum foil, and then rolled and cut to obtain the positive electrode sheet;

[0061] Negative electrode preparation:

[0062] (a) The negative electrode active material, SP, CMC, and SBR were added to a double planetary mixer in a mass ratio of 95:1:1.5:3.5 for solid-phase mixing. The negative electrode active material consisted of hard carbon and graphite, with hard carbon accounting for 81% wt% of the total negative electrode active material; the hard carbon negative electrode material was mixed with NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 The available capacity ratio of O2 cathode material is 1.15;

[0063] (b) Add a certain amount of deionized water to make the solid content of the slurry 58%, and knead and stir.

[0064] (c) Add a certain amount of NMP solvent to make the solid content 50%, and disperse at high speed; and adjust the viscosity of the slurry to 9000 mPa·s;

[0065] (d) The well-dispersed slurry is evenly coated onto copper foil, and then rolled and cut to obtain the negative electrode sheet;

[0066] Assemble the battery:

[0067] The positive electrode, separator, and negative electrode are stacked / wound into the desired cell size, sealed in an aluminum-plastic film, and a certain amount of electrolyte is injected. The electrolyte consists of electrolyte and solvent. The electrolyte includes NaPF6 and LiPF6, with NaPF6 accounting for 74% of the total electrolyte. The electrolyte concentration in the electrolyte is 0.73 mol / L. The solvent is composed of EC:PC:DEC in a volume ratio of 1:1:1. The amount of electrolyte injected is 22% of the cell mass.

[0068] After the assembled battery undergoes formation, degassing, secondary sealing, and capacity testing, a composite sodium-ion battery is obtained.

[0069] Example 3

[0070] This embodiment provides a composite sodium-ion battery, prepared according to the following method:

[0071] Positive electrode preparation:

[0072] (A) The positive electrode active material, SP, and PVDF are added to a double planetary mixer in a mass ratio of 95:2.5:2.5 for solid-phase mixing. The positive electrode active material is composed of NaNi... 1 / 3 Fe 1 / 3 Mn 1 / 3 The cathode active material is composed of O2 and LiFePO4, and NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 accounts for 77 wt.% of the total positive electrode active material;

[0073] (B) Add a certain amount of NMP solvent to make the solid content of the slurry 70%, and knead and stir.

[0074] (C) Add a certain amount of NMP solvent to make the solid content 65%, and disperse at high speed; and adjust the viscosity of the slurry to 9000 mPa·s;

[0075] (D) The well-dispersed slurry is evenly coated onto aluminum foil, and then rolled and cut to obtain the positive electrode sheet;

[0076] Negative electrode preparation:

[0077] (a) The negative electrode active material, SP, CMC, and SBR were added to a double planetary mixer in a mass ratio of 95:1:1.5:3.5 for solid-phase mixing. The negative electrode active material consisted of hard carbon and graphite, with hard carbon accounting for 83.5% wt% of the total negative electrode active material. The usable capacity ratio of the hard carbon negative electrode material to the NaNi1 / 3Fe1 / 3Mn1 / 3O2 positive electrode material was 1.19.

[0078] (b) Add a certain amount of deionized water to make the solid content of the slurry 62%, and knead and stir.

[0079] (c) Add a certain amount of NMP solvent to make the solid content 50%, and disperse at high speed; and adjust the viscosity of the slurry to 9000 mPa·s;

[0080] (d) The well-dispersed slurry is evenly coated onto copper foil, and then rolled and cut to obtain the negative electrode sheet;

[0081] Assemble the battery:

[0082] The positive electrode, separator, and negative electrode are stacked / wound into the desired cell size, sealed in an aluminum-plastic film, and a certain amount of electrolyte is injected. The electrolyte consists of an electrolyte and a solvent. The electrolyte includes NaPF6 and LiPF6, with NaPF6 accounting for 71% of the total electrolyte. The electrolyte concentration is 0.8 mol / L. The solvent is composed of EC:PC:DEC in a volume ratio of 1:1:1. The amount of electrolyte injected is 25% of the cell mass.

[0083] After the assembled battery undergoes formation, degassing, secondary sealing, and capacity testing, a composite sodium-ion battery is obtained.

[0084] Example 4

[0085] The difference from Example 1 is that, in the preparation process of the positive electrode, NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 accounts for 70 wt.% of the total positive electrode active material.

[0086] Example 5

[0087] The difference from Example 1 is that, in the preparation process of the positive electrode, NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 accounts for 80 wt.% of the total positive electrode active material.

[0088] Example 6

[0089] The difference from Example 1 is that, in the preparation of the negative electrode sheet, hard carbon accounts for 75 wt.% of the total amount of the negative electrode active material.

[0090] Example 7

[0091] The difference from Example 1 is that, in the preparation of the negative electrode sheet, hard carbon accounts for 85 wt.% of the total amount of the negative electrode active material.

[0092] Example 8

[0093] The difference from Example 1 is that the hard carbon anode material and NaNi1 / 3 Fe 1 / 3 Mn 1 / 3 The available capacity ratio of O2 cathode material is 1.13.

[0094] Example 9

[0095] The difference from Example 1 is that the hard carbon anode material and NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 The available capacity ratio of O2 cathode material is 1.22.

[0096] Example 10

[0097] The difference from Example 1 is that, in the preparation of the positive electrode, the slurry solid content in step (B) is 65%.

[0098] Example 11

[0099] The difference from Example 1 is that, in the preparation of the positive electrode, the slurry solid content in step (B) is 75%.

[0100] Comparative Example 1

[0101] The difference from Example 1 is that hard carbon is replaced with graphite, and the mass of graphite in this comparative example is equal to the total mass of hard carbon and graphite in Example 1.

[0102] Comparative Example 2

[0103] The difference from Example 1 is that graphite is replaced with hard carbon, and the mass of graphite in this comparative example is equal to the total mass of hard carbon and graphite in Example 1.

[0104] Comparative Example 3

[0105] The difference from Example 1 is that, in the preparation of the negative electrode sheet, hard carbon accounts for 40 wt.% of the total amount of the negative electrode active material.

[0106] Comparative Example 4

[0107] The difference from Example 1 is that LiPF6 is replaced with NaPF6, and the mass of NaPF6 in this comparative example is equal to the total mass of LiPF6 replaced with NaPF6 in Example 1.

[0108] Performance tests were conducted on the battery cells of each embodiment and comparative example, and the results are shown in Table 1:

[0109] Initial charge / discharge efficiency CE: Records the charging current during the cell formation stage, denoted as C. f Then, after degassing and secondary sealing, the capacity testing phase begins, and the charging capacity C of the first step in the capacity testing phase is recorded. gc Record the discharge capacity C in the second step of the capacity-balancing stage.gd Then the initial charge / discharge efficiency CE = C gd / (C f +C gc Take 5 to 10 sets of cell data and calculate the average value.

[0110] Cell energy density ED: Connect the prepared battery to a charge / discharge test device for capacity testing. The test voltage range is 2.0V to 3.9V, the charging current is 0.2C, and the discharging current is 0.2C. Record the discharge energy E and weigh the battery M. The battery energy density ED = E / M, with the unit being Wh / kg. Take 5 to 10 sets of cell data and calculate the average value.

[0111] Table 1 Cell Performance

[0112] Examples / Comparative Examples First charge / discharge efficiency (%) Battery cell energy density (Wh / kg) Example 1 86 155 Example 2 88 157 Example 3 85 153 Example 4 79 145 Example 5 77 140 Example 6 78 141 Example 7 76 140 Example 8 77 142 Example 9 74 138 Example 10 84 145 Example 11 82 143 Comparative Example 1 43 58 Comparative Example 2 68 128 Comparative Example 3 66 89 Comparative Example 4 77 111

[0113] As shown in the table above, the method of the present invention can fully utilize the capacity of sodium-ion active materials and lithium-ion active materials, enabling the battery cell to achieve both high initial efficiency and energy density while maintaining low cost.

[0114] A comparison of Examples 1 and 4-5 shows that a suitable ratio of sodium-ion positive electrode active material to lithium-ion positive electrode active material is necessary. Exceeding this ratio will reduce the energy density of the battery cell to varying degrees. If the proportion of sodium-ion positive electrode active material is too high, its already low initial efficiency and energy density will directly reduce the final initial efficiency and energy density of the battery cell. If the proportion of sodium-ion positive electrode active material is too low, the relatively excessive hard carbon in the negative electrode will store lithium ions. Since lithium ions are also present between the hard carbon and the carbon layers, the abundant pores of the hard carbon material will be exposed to the electrolyte environment, thus consuming more lithium ions to form an interface film, thereby reducing the initial efficiency and energy density of the battery cell.

[0115] A comparison of Examples 1 and 6-7 shows that the mixing ratio of hard carbon anode and graphite anode also needs to be within a certain range. If there is too much hard carbon, the excessive hard carbon will store lithium ions, and the exposed pores will consume lithium ions to form a surface friction, reducing the initial efficiency and energy density of the cell. If there is too little hard carbon, some sodium-ion battery capacity will not be utilized, which will also directly reduce the energy density of the cell.

[0116] A comparison between Example 1 and Examples 8-9 shows that the hard carbon anode material and NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3Both excessively low and excessively high available capacity ratios of O2 cathode materials will reduce the initial efficiency and energy density of the battery cell to varying degrees. If the ratio is too low, sodium ions will easily precipitate on the negative electrode surface, forming "dead" sodium that can no longer contribute capacity; if the ratio is too high, it will consume more sodium ions to form an interface film, thereby reducing the initial efficiency and energy density of the battery cell.

[0117] A comparison of Examples 1 and 10-11 reveals that the homogenization process of the positive and negative electrodes is crucial to cell performance during cell fabrication, especially the kneading solids content during the homogenization stage. Slurry dispersion involves the dispersion of various micron- or even nano-sized materials. Kneading aims to ensure sufficient friction between material particles, achieving dispersion. If the kneading solids content is too high, the material will clump together and lose its fluidity, failing to achieve dispersion and severely damaging the dispersion equipment. Conversely, if the kneading solids content is too low, insufficient contact and friction between particles will prevent the effective dispersing of agglomerated particles. Both situations result in poor material dispersion, affecting the capacity performance of the active material and ultimately reducing the cell's initial efficiency and energy density.

[0118] A comparison of Example 1 and Comparative Example 1 shows that using pure graphite as the negative electrode significantly reduces the energy density of the battery cell. This is mainly because graphite has a very low sodium storage capacity; sodium ions released from the positive electrode are deposited as metallic sodium at the negative electrode and lose their activity. Essentially, only lithium ions exhibit corresponding activity in the entire battery cell.

[0119] A comparison between Example 1 and Comparative Example 2 shows that using a pure hard carbon anode also reduces the initial efficiency and energy density of the battery cell. This is because hard carbon materials contain abundant pores, which, when exposed to the electrolyte, consume active sodium ions and active lithium ions, with the consumption of active lithium ions being particularly severe.

[0120] This reduces the reversible activity of sodium / lithium ions, resulting in a decrease in the cell's initial efficiency and energy density.

[0121] A comparison between Example 1 and Comparative Example 3 shows that, similar to Comparative Example 1, the proportion of hard carbon material in the negative electrode is too low, causing the negative electrode to lose its sodium storage energy, thus resulting in a decrease in energy density.

[0122] A comparison between Example 1 and Comparative Example 4 shows that using pure NaPF6 as the electrolyte salt significantly affects the initial efficiency and energy density of the battery cell. This is because lithium ion migration occurs in this system; if the electrolyte does not contain lithium salt, lithium ion migration is hindered, and the lithium ion diffusion rate decreases. This leads to a significant increase in electrochemical polarization of the battery cell, preventing the effective utilization of the material's capacity and ultimately resulting in a decrease in the battery cell's energy density.

[0123] The applicant declares that the detailed method of the present invention is illustrated by the above embodiments, but the present invention is not limited to the above detailed method, that is, it does not mean that the present invention must rely on the above detailed method to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials of the product of the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A composite sodium-ion battery comprising a positive electrode, a negative electrode, and an electrolyte, characterized in that, The positive electrode active material in the positive electrode includes layered sodium ion positive electrode material and lithium iron phosphate positive electrode material; The working voltage range of the sodium ion cathode material is a1~b1 V, and the working voltage range of the lithium iron phosphate cathode material is a2~b2, where a1<a2, b1>b2; The negative electrode active material includes hard carbon and graphite, and the mass percentage of hard carbon in the negative electrode active material is 78 wt.% ~ 84 wt.%. The layered sodium ion cathode material accounts for 72 wt.% to 78 wt.% of the total mass of the cathode active material. The usable capacity ratio of the hard carbon to the layered sodium ion cathode material is 1.15~1.2; The electrolyte salts in the electrolyte include sodium salts and lithium salts.

2. The composite sodium-ion battery of claim 1, wherein, The layered sodium-ion cathode material comprises NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2.

3. The composite sodium-ion battery according to claim 1, characterized in that, The sodium salt accounts for 70% to 75% of the total mass of the electrolyte salt.

4. The composite sodium-ion battery according to claim 1, characterized in that, The concentration of electrolyte salts in the electrolyte solution is 0.7 mol / L ~ 0.8 mol / L.

5. The composite sodium-ion battery according to claim 1, characterized in that, In the aforementioned composite sodium-ion battery, the amount of electrolyte injected is 20% to 25% of the cell mass of the composite sodium-ion battery.

6. A method for preparing a composite sodium-ion battery as described in any one of claims 1-5, characterized in that, The method includes the following steps: (1) The positive electrode material and the negative electrode material are dispersed in a solvent to obtain a positive electrode slurry and a negative electrode slurry, wherein the positive electrode material includes a positive electrode active material and the negative electrode material includes a negative electrode active material; (2) The positive electrode slurry and the negative electrode slurry are coated onto the positive electrode current collector and the negative electrode current collector respectively, and then dried to obtain the positive electrode and the negative electrode; (3) The positive electrode, negative electrode and separator are assembled into a cell, and electrolyte is injected after being put into the casing to obtain the composite sodium-ion battery.

7. The method according to claim 6, characterized in that, The positive electrode material also includes a conductive agent and a binder.

8. The preparation method according to claim 6, characterized in that, The preparation method of the positive electrode slurry includes the following steps: after solid-phase mixing of the positive electrode raw materials, NMP solvent is added to make the solid content of the slurry 68% to 73%, kneading and stirring are performed, NMP solvent is added again, high-speed dispersion is performed, and the viscosity of the slurry is adjusted to be suitable for the coating process.

9. The method according to claim 6, characterized in that, The negative electrode material also includes a conductive agent and a binder, wherein the binder includes CMC and SBR.

10. The method according to claim 6, characterized in that, The preparation method of the negative electrode slurry includes the following steps: after mixing the negative electrode raw materials in a solid phase, water is added to make the solid content of the slurry 57% to 62%, kneading and stirring are performed, water is added again, high-speed dispersion is performed, and the viscosity of the slurry is adjusted to be suitable for the coating process.

11. The method according to claim 6, characterized in that, The shell used in the encapsulation step is an aluminum-plastic film. The method also includes formation, degassing, secondary sealing and volume division after injecting electrolyte.