A pre-embedded sodium hard carbon negative electrode sheet, a preparation method and application thereof

Abstract

The application belongs to the technical field of electrochemical energy storage, and particularly relates to a pre-sodium-embedded hard carbon negative electrode sheet and a preparation method and application thereof. The pre-sodium-embedded hard carbon negative electrode sheet comprises the following raw materials in parts by weight: pre-sodium-embedded hard carbon 70-90 parts, conductive agent 5-15 parts, and binder 5-15 parts; and the pre-embedded amount of sodium in the pre-sodium-embedded hard carbon is 5%-30% of the theoretical sodium storage capacity of the hard carbon. The pre-sodium-embedded hard carbon negative electrode sheet has the following beneficial effects: when assembled into a secondary battery, sodium ions in the negative electrode sheet will preferentially react with an electrolyte to form a stable and dense SEI film mainly composed of sodium-containing compounds on the surface of the hard carbon. When assembled into a full battery for the first time, lithium ions are embedded into the negative electrode from the positive electrode, and a large amount of additional lithium is no longer needed to build the SEI film, thereby effectively improving the first coulomb efficiency of the full battery and improving the initial energy density of the battery.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage technology, specifically relating to a pre-embedded sodium hard carbon negative electrode sheet, its preparation method, and its application. Background Technology

[0002] With the rapid development of electric vehicles and large-scale energy storage markets, higher demands are being placed on the performance, cost, and resource sustainability of rechargeable batteries. Currently, lithium-ion batteries dominate due to their high energy density and mature industrial chain, but face challenges such as limited and unevenly distributed lithium resources and cost fluctuations. Sodium-ion batteries, due to abundant sodium resources and low cost, have become a highly promising supplementary or alternative technology, but their energy density and cycle life are generally lower than those of high-performance lithium-ion batteries.

[0003] To balance performance and cost, researchers have recently proposed the concept of lithium-sodium hybrid batteries (or "lithium-sodium hybrid ion batteries"). The core idea is to use lithium and sodium ions that can be simultaneously inserted / extracted within the same battery system. + Na + Electrode materials and electrolytes containing two salts, utilizing Li + and Na + This study aims to combine the advantages of both charge storage and transport. In existing technologies, hard carbon materials, due to their abundant microcrystalline structure, low cost, and good lithium / sodium storage capacity, have been widely studied as anode candidates for lithium-sodium composite batteries. However, directly using hard carbon in lithium-sodium composite batteries faces the following key technical bottlenecks: ① Low initial coulombic efficiency: Hard carbon materials have a large specific surface area and abundant defects. During the first charge and discharge process, the electrolyte decomposes on its surface to form a solid electrolyte interphase (SEI) film, irreversibly consuming a large amount of active Li. + / Na + This results in a low initial coulombic efficiency of the battery, severely reducing the overall energy density of the cell. ② Reaction kinetic mismatch: Na + The Stokes radius (4.0 Å) is greater than that of Li. + The Stokes radius (3.8 Å). In the disordered carbon layers and micropores of hard carbon, larger Na... + The diffusion barrier is higher, and the kinetics are slower. When the battery is charged and discharged at higher rates, the slow diffusion of sodium ions dominates and limits the reaction rate of the entire negative electrode, resulting in poor rate performance of the battery. ③ Cycle stability challenge: Under repeated Li + / Na + During the insertion / extraction process, the hard carbon structure may undergo uneven volume changes. This is because Li... + and Na +Differences in embedding behavior (potential plateau, embedding amount) make this volume change more complex, which may lead to repeated rupture and reconstruction of the SEI membrane, continuously consuming electrolyte and active ions, and accelerating capacity decay.

[0004] Therefore, developing a hard carbon anode that can simultaneously improve initial efficiency, rate performance, and cycle stability is key to advancing the practical application of high-performance lithium-sodium composite batteries. Summary of the Invention

[0005] This application provides a pre-intercalated sodium hard carbon anode sheet, its preparation method, and its application, aiming to solve the problems of low initial coulombic efficiency, poor overall battery reaction kinetics, poor rate performance, and poor stability when hard carbon is directly used as the anode of lithium-sodium composite batteries.

[0006] The first aspect of this application provides a pre-intercalated sodium hard carbon negative electrode sheet, comprising the following raw materials in parts by weight: 70-90 parts of pre-intercalated sodium hard carbon, 5-15 parts of conductive agent and 5-15 parts of binder; wherein the amount of sodium pre-intercalated in the pre-intercalated sodium hard carbon is 5%-30% of the theoretical sodium storage capacity of the hard carbon.

[0007] According to some embodiments of the pre-embedded sodium hard carbon negative electrode sheet described in this application, the hard carbon includes one or more of biomass-based hard carbon, resin-based hard carbon, pitch-based hard carbon, and sugar pyrolysis hard carbon.

[0008] According to some embodiments of the pre-embedded sodium hard carbon negative electrode sheet described in this application, the conductive agent includes one or more of Super P carbon black, Ketjen black, acetylene black, carbon nanotubes, and graphene.

[0009] According to some embodiments of the pre-embedded sodium hard carbon negative electrode sheet described in this application, the binder includes one or more of polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA).

[0010] The second aspect of this application provides a method for preparing the pre-sodium-intercalated hard carbon negative electrode sheet described in the first aspect of this application, comprising the following steps:

[0011] (1) Mix hard carbon, conductive agent, binder and solvent to obtain negative electrode slurry; coat the negative electrode slurry onto current collector to prepare hard carbon negative electrode sheet; (2) A button cell is assembled using a hard carbon negative electrode as the working electrode and a sodium metal sheet as the counter electrode / reference electrode. (3) Place the coin cell half-cell into a charge-discharge test device and discharge it at a current density of 0.02C-0.1C to 0.005-0.05V vs. Na. + / Na cutoff potential, recharge to remove sodium to 0.1-0.5V vs. Na +The / Na cutoff potential is used to obtain the pre-intercalated sodium hard carbon negative electrode.

[0012] According to some embodiments of the preparation method of the pre-sodium-embedded hard carbon negative electrode sheet described in this application, the solvent includes water and / or N-methylpyrrolidone.

[0013] According to some embodiments of the preparation method of the pre-embedded sodium hard carbon negative electrode sheet described in this application, in step (2), the separator of the button half cell is glass fiber, and the electrolyte is a 0.8-1.2 mol / L NaPF6 solution; preferably, the solvent of the electrolyte is a mixture of ethylene carbonate and diethyl carbonate with a volume ratio of 1:(0.8-1.2).

[0014] According to some embodiments of the preparation method of the pre-sodium-embedded hard carbon negative electrode sheet described in this application, the method further includes the step of disassembling the button half-cell that has been charged and desodiumed in step (3) and taking out the pre-sodium-embedded hard carbon negative electrode sheet for cleaning and drying.

[0015] The third aspect of this application provides a lithium-sodium composite secondary battery, including the pre-sodium-intercalated hard carbon negative electrode sheet described in the first aspect of this application or the pre-sodium-intercalated hard carbon negative electrode sheet obtained by the preparation method described in the second aspect of this application.

[0016] According to some embodiments of the lithium-sodium composite secondary battery described in this application, it also includes a positive electrode containing lithium-sodium dual ions and an electrolyte containing lithium-sodium dual ions.

[0017] According to some embodiments of the lithium-sodium composite secondary battery described in this application, the molar ratio of lithium ions to sodium ions in the positive electrode containing lithium-sodium dual ions is 1:4-4:1.

[0018] According to some embodiments of the lithium-sodium composite secondary battery described in this application, the molar ratio of lithium ions to sodium ions in the electrolyte containing lithium-sodium dual ions is (1-8):1.

[0019] According to some embodiments of the lithium-sodium composite secondary battery described in this application, the positive electrode includes a Na4Fe3(PO4)2P2O7 and LiFePO4 composite positive electrode, a Na3V2(PO4)3 and Li3V2(PO4)3 composite positive electrode, or a NaNi composite positive electrode. 1 / 3 Fe 1 / 3Mn 1 / 3 O2 and LiNi 0.6 Co 0.2 Mn 0.2 O2 composite positive electrode sheet; And / or, the electrolyte includes at least two of LiPF6, NaPF6, LiTFSI, LiFSI, NaTFSI, and NaFSI.

[0020] The beneficial effects of this application include: when the pre-intercalated sodium hard carbon negative electrode sheet described in this application is assembled into a secondary battery, the sodium ions in the negative electrode sheet will preferentially react with the electrolyte to form a stable and dense SEI film mainly composed of sodium compounds on the hard carbon surface. When the battery is assembled into a full battery for the first charge, the lithium ions extracted from the positive electrode are embedded in the negative electrode, eliminating the need to consume a large amount of additional lithium to construct the SEI film, thereby effectively improving the initial coulombic efficiency of the full battery and increasing the initial energy density of the battery.

[0021] The sodium ions pre-embedded in the pre-sodium hard carbon negative electrode sheet described in this application can expand the interlayer spacing and ion transport channels inside the hard carbon, thereby reducing the insertion and extraction of smaller lithium ions during normal battery operation, increasing their diffusion coefficient, and improving the rate performance and reaction kinetics of the battery.

[0022] The stable SEI film pre-formed on the pre-sodium-embedded hard carbon negative electrode described in this application reduces continuous side reactions during cycling. The insertion of sodium ions expands the transport channels, making the insertion or extraction of lithium ions more uniform, reducing local stress concentration and mechanical fatigue of the hard carbon structure, inhibiting the rupture of the SEI film, and enhancing cycle stability. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the pre-sodium intercalation mechanism of the pre-intercalated sodium hard carbon negative electrode sheet described in Embodiment 1 of this application. Detailed Implementation

[0024] The embodiments of the present invention are described in detail below. These embodiments are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0025] In this invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0026] This application provides a pre-intercalated sodium hard carbon anode sheet, comprising the following raw materials in parts by weight: 70-90 parts pre-intercalated sodium hard carbon, 5-15 parts conductive agent, and 5-15 parts binder; the amount of sodium pre-intercalated in the pre-intercalated sodium hard carbon is 5%-30% of the theoretical sodium storage capacity of the hard carbon. When the amount of pre-intercalated sodium is less than 5%, the pre-intercalation effect is not significant, making it difficult to effectively establish a stable SEI film and expand ion transport channels; when the amount of pre-intercalated sodium is greater than 30%, excessive sodium ions occupy the active lithium storage sites in the hard carbon, leading to a decrease in reversible capacity, and may also cause excessive expansion and irreversible damage to the hard carbon structure.

[0027] The sodium ions pre-embedded in the pre-sodium-intercalated hard carbon negative electrode sheet described in this application participate in the formation of a stable and dense SEI film, improving the initial efficiency of the battery. Secondly, the pre-embedded sodium ions with a larger radius can "expand" part of the graphite microcrystal spacing and micropore channels of the hard carbon, creating a more spacious "channel" for the subsequent rapid insertion / extraction of lithium ions (e.g., ...). Figure 1 As shown in the figure, this significantly improves the lithium reaction kinetics.

[0028] In some embodiments of this application, the hard carbon includes one or more of biomass-based hard carbon, resin-based hard carbon, pitch-based hard carbon, and sugar pyrolysis hard carbon. Preferably, the hard carbon is coconut shell-based hard carbon or phenolic resin-based hard carbon, with a carbonization temperature of 1200-1500℃ and a specific surface area of ​​5-20 m². 2 / g, average interlayer spacing d 002 It is 3.7-4.0 Å.

[0029] In some embodiments of this application, the conductive agent includes one or more of Super P carbon black, Ketjen black, acetylene black, carbon nanotubes, and graphene.

[0030] In some embodiments of this application, the adhesive includes one or more of polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA).

[0031] This application also provides a method for preparing the pre-sodium-intercalated hard carbon negative electrode sheet as described in the first aspect of this application, comprising the following steps: (1) Mix hard carbon, conductive agent, binder and solvent to obtain negative electrode slurry; coat the negative electrode slurry onto current collector to prepare hard carbon negative electrode sheet; (2) A button cell is assembled using a hard carbon negative electrode as the working electrode and a sodium metal sheet as the counter electrode / reference electrode. (3) Place the coin cell half-cell into a charge-discharge test device and discharge it at a current density of 0.02C-0.1C to 0.005-0.05V vs. Na. + / Na cutoff potential, recharge to remove sodium to 0.1-0.5V vs. Na + The / Na cutoff potential is used to obtain the pre-intercalated sodium hard carbon negative electrode.

[0032] The method for preparing the pre-sodium-intercalated hard carbon negative electrode sheet described in this application does not completely remove all sodium ions during the charging sodium removal process, but retains a portion of sodium ions in the hard carbon structure. By controlling the charging capacity, the content of pre-intercalated sodium in the hard carbon can be precisely controlled. This achieves the goal of improving the battery's initial efficiency, stability, and reaction kinetics.

[0033] In some embodiments of this application, the preparation method of the pre-intercalated sodium hard carbon negative electrode sheet further includes a chemical sodium intercalation treatment. Specifically, under argon protection, metallic sodium and naphthalene are dissolved in a dehydrated and deoxygenated tetrahydrofuran (THF) or dimethyl ethylene glycol (DME) solvent to form a deep blue sodium naphthalene (Na-C) sheet. 10 The prepared hard carbon anode sheet (or hard carbon powder) is immersed in the above sodium naphthalene solution (H8) and stirred at room temperature or low temperature for a period of time (e.g., 1-12 hours). As an electronic conductor, hard carbon spontaneously acquires sodium ions and electrons from sodium naphthalene, achieving sodium ion intercalation. After the reaction, the electrode sheet is washed multiple times with THF or DME solvent (or the powder is filtered and washed) to remove residual organic matter and sodium salts. Then, it is vacuum dried to obtain a pre-intercalated sodium hard carbon anode. Sodium sources can also be introduced using vapor deposition or solid-phase mixing reactions, but the electrochemical method offers the highest precision in controlling the introduction of sodium ions.

[0034] In some embodiments of this application, the solvent includes water and / or N-methylpyrrolidone.

[0035] In some embodiments of this application, in step (2), the separator of the coin cell is glass fiber, and the electrolyte is a 0.8-1.2 mol / L NaPF6 solution; preferably, the solvent of the electrolyte is a mixture of ethylene carbonate and diethyl carbonate in a volume ratio of 1:(0.8-1.2).

[0036] In some embodiments of this application, the method further includes the step of disassembling the coin cell that has been charged and desodiumed in step (3), and removing the pre-sodium-embedded hard carbon negative electrode sheet for cleaning and drying. The disassembly is carried out in a glove box. After disassembly, the residual electrolyte salt on the surface of the pre-sodium-embedded hard carbon negative electrode sheet is cleaned with DMC solvent, and then vacuum dried for later use.

[0037] This application also provides a lithium-sodium composite secondary battery, comprising the pre-intercalated sodium hard carbon anode sheet described in the first aspect of this application or the pre-intercalated sodium hard carbon anode sheet obtained by the preparation method described in the second aspect of this application. The lithium-sodium composite secondary battery described in this application has high initial coulombic efficiency, good rate performance, and good stability.

[0038] In some embodiments of this application, a positive electrode containing lithium-sodium dual ions and an electrolyte containing lithium-sodium dual ions are also included. The use of a positive electrode containing lithium-sodium dual ions and an electrolyte containing lithium-sodium dual ions provides a source of lithium and sodium ions for the battery system, forming a highly efficient synergy with the pre-intercalated sodium anode.

[0039] In some embodiments of this application, the electrolyte uses a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). This ensures that the initial sodium desodiuming potential of the pre-intercalated sodium hard carbon negative electrode is higher than the sodium salt decomposition potential in the electrolyte, thus guaranteeing the stability of the pre-intercalated sodium. In practical applications, an appropriate amount of film-forming additive, such as fluoroethylene carbonate (FEC), can be added to the electrolyte, with the addition amount controlled at 2-5 wt% of the electrolyte mass.

[0040] In some embodiments of this application, the molar ratio of lithium ions to sodium ions in the lithium-sodium dual-ion positive electrode is 1:4-4:1. Preferably, the molar ratio of lithium ions to sodium ions in the positive electrode is 1:2-2:1. When the lithium-sodium molar ratio in the positive electrode is within this range, the coordinated intercalation and deintercalation behavior of lithium-sodium dual ions during charging and discharging can be ensured to remain stable, maintaining a high discharge voltage platform while also taking into account the cost advantages brought by sodium storage.

[0041] In some embodiments of this application, the molar ratio of lithium ions to sodium ions in the lithium-sodium dual-ion electrolyte is (1-8):1. Preferably, the molar ratio of lithium ions to sodium ions in the electrolyte is (2-6):1. A higher lithium salt concentration than a sodium salt concentration is beneficial for maintaining a higher lithium ion transport number, ensuring the rate performance and energy density of the battery; an appropriate amount of sodium salt supplements the sodium ion source for the system, maintaining the synergistic transport of lithium and sodium ions.

[0042] In some embodiments of this application, the positive electrode includes a composite positive electrode of Na4Fe3(PO4)2P2O7 (NFPP) and LiFePO4 (LFP), a composite positive electrode of Na3V2(PO4)3 and Li3V2(PO4)3, and a NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 and LiNi 0.6 Co 0.2 Mn 0.2 One type of O2 composite positive electrode. It employs a composite positive electrode containing both lithium and sodium active materials, capable of simultaneously providing Li... + and Na + Reversible insertion and extraction enables dual-ion synergistic energy storage.

[0043] In some embodiments of this application, the electrolyte includes at least two of LiPF6, NaPF6, LiTFSI, LiFSI, NaTFSI, and NaFSI. Preferably, the electrolyte comprises LiPF6 and NaPF6.

[0044] The technical solution of this application will be further described below with reference to specific embodiments.

[0045] Example 1 A method for preparing a lithium-sodium composite secondary battery includes the following steps: (1) Dissolve hard carbon (coconut shell-based hard carbon, carbonization temperature 1300℃), Super P conductive agent and PVDF binder in deionized water at a mass ratio of 8:1:1 to obtain a uniform slurry. Coat the slurry onto a copper current collector, dry it at 80℃ for 12 hours, roll it, and slice it to obtain a hard carbon negative electrode sheet. (2) In an argon-protected glove box, a hard carbon negative electrode sheet is used as the working electrode, a sodium metal sheet is used as the counter electrode / reference electrode, glass fiber is used as the diaphragm, and 1 mol / L NaPF6 is used as the electrolyte (the solvent is a mixture of ethylene carbonate and diethyl carbonate with a volume ratio of 1:1) to assemble a button-type CR2032 half cell. (3) Place the assembled half-cell in a constant temperature testing device at 25°C. First, discharge it at a current density of 0.05C until it reaches 0.01V vs. Na. + / Na cutoff potential, recharge to remove sodium to 0.2V vs. Na + / Na cutoff potential (at which point the amount of residual pre-intercalated sodium in the hard carbon is about 15% of the theoretical sodium storage capacity). Then, the half-cell with completed sodium pre-intercalation is disassembled in a glove box, the pre-intercalated hard carbon negative electrode is removed, the residual electrolyte salt on the surface of the negative electrode is cleaned with DMC solvent, and then vacuum dried for later use; (4) Dissolve the NFPP and LFP composite positive electrode active material, Super P conductive agent and PVDF binder in NMP solvent at a mass ratio of 8:1.5:0.5 to obtain a positive electrode slurry (wherein the molar ratio of NFPP to LFP is 1:1); coat the above positive electrode slurry onto an aluminum current collector, dry at 120°C, roll, and slice to obtain a positive electrode sheet; (5) Dissolve LiPF6 and NaPF6 in a mixed solvent (the mixed solvent includes EC:EMC:DEC in a volume ratio of 1:1:1) with a molar concentration of 0.8 mol / L for LiPF6 and a molar concentration of 0.2 mol / L for NaPF6 to obtain a double salt electrolyte. (6) In an argon glove box, the pre-sodium-embedded hard carbon negative electrode, positive electrode and polyolefin separator prepared above are stacked and injected into the dual salt electrolyte and packaged into a soft pack battery.

[0046] Example 2 The only difference between the preparation method of the lithium-sodium composite secondary battery in Example 2 and that in Example 1 is that the amount of sodium embedded in the pre-sodium hard carbon negative electrode sheet in the lithium-sodium composite secondary battery in Example 2 is different from that in Example 1.

[0047] The specific operation steps include: in step (3), discharging to 0.01V vs. Na at a current density of 0.05C. + After reaching the cutoff potential of / Na, recharge to remove sodium to 0.3V vs. Na. + / Na cutoff potential, at which point the amount of residual pre-intercalated sodium in the hard carbon is about 5% of the theoretical sodium storage capacity, and the remaining steps are the same as in Example 1.

[0048] Example 3 The only difference between the preparation method of the lithium-sodium composite secondary battery in Example 3 and that in Example 1 is that the amount of sodium intercalated in the pre-intercalated sodium hard carbon negative electrode in the lithium-sodium composite secondary battery in Example 3 is different from that in Example 1.

[0049] The specific operation steps include: in step (3), discharging to 0.01V vs. Na at a current density of 0.05C. + After reaching the cutoff potential of / Na, recharge to remove sodium to 0.15V vs. Na. + At the / Na cutoff potential, the amount of residual pre-intercalated sodium in the hard carbon is approximately 20% of the theoretical sodium storage capacity. The remaining steps are the same as in Example 1. Example 4 The only difference between the preparation method of the lithium-sodium composite secondary battery in Example 4 and that in Example 1 is that the amount of sodium embedded in the pre-sodium hard carbon negative electrode in the lithium-sodium composite secondary battery in Example 4 is different from that in Example 1.

[0050] The specific operation steps include: in step (3), discharging to 0.01V vs. Na at a current density of 0.05C. + After reaching the cutoff potential of / Na, recharge to remove sodium to 0.1V vs. Na. + At the / Na cutoff potential, the amount of residual pre-intercalated sodium in the hard carbon is approximately 30% of the theoretical sodium storage capacity. The remaining steps are the same as in Example 1. Example 5 The only difference between the preparation method of the lithium-sodium composite secondary battery in Example 5 and that in Example 1 is that the molar ratio of NFPP to LFP in the positive electrode of the lithium-sodium composite secondary battery in Example 5 is 1:3.

[0051] Example 6 The only difference between the preparation method of the lithium-sodium composite secondary battery in Example 6 and that in Example 1 is that the molar ratio of NFPP to LFP in the positive electrode of the lithium-sodium composite secondary battery in Example 6 is 1:2.

[0052] Example 7 The only difference between the preparation method of the lithium-sodium composite secondary battery in Example 7 and that in Example 1 is that the molar ratio of NFPP to LFP in the positive electrode of the lithium-sodium composite secondary battery in Example 7 is 3:1.

[0053] Example 8 The difference between the preparation method of the lithium-sodium composite secondary battery in Example 8 and that in Example 1 is that the molar ratio of LiPF6 to NaPF6 in the electrolyte of the lithium-sodium composite secondary battery in Example 8 is 2:1, wherein the molar concentration of LiPF6 is 0.67 mol / L and the molar concentration of NaPF6 is 0.33 mol / L. Example 9 The difference between the preparation method of the lithium-sodium composite secondary battery in Example 9 and that in Example 1 is that the molar ratio of LiPF6 to NaPF6 in the electrolyte of the lithium-sodium composite secondary battery in Example 9 is 6:1, wherein the molar concentration of LiPF6 is 0.86 mol / L and the molar concentration of NaPF6 is 0.14 mol / L. Example 10 The only difference between the preparation method of the lithium-sodium composite secondary battery in Example 10 and that in Example 1 is that the molar ratio of LiPF6 to NaPF6 in the electrolyte of the lithium-sodium composite secondary battery in Example 10 is 8:1, wherein the molar concentration of LiPF6 is 0.89 mol / L and the molar concentration of NaPF6 is 0.11 mol / L.

[0054] Comparative Example 1 The only difference between the preparation method of the lithium-sodium composite secondary battery in Comparative Example 1 and Example 1 is that NFPP positive electrode active material is used instead of the NFPP and LFP composite positive electrode active material in the preparation process of the lithium-sodium composite secondary battery in Comparative Example 1. That is, the positive electrode sheet contains only NFPP as the positive electrode active material and does not contain LFP.

[0055] Comparative Example 2 The only difference between the preparation method of the lithium-sodium composite secondary battery in Comparative Example 2 and that in Example 1 is that, in the preparation process of the lithium-sodium composite secondary battery in Comparative Example 2, LiPF6 with a molar concentration of 1.0 mol / L is used instead of the LiPF6 and NaPF6 dual-salt electrolyte. That is, the electrolyte contains only a single lithium salt, LiPF6, and does not contain NaPF6.

[0056] Electrical performance study of the lithium-sodium composite secondary batteries described in Examples 1-10 of this application The lithium-sodium composite secondary batteries prepared in Examples 1-10 and Comparative Examples 1-2 were subjected to initial coulombic efficiency tests, rate performance tests (1C discharge specific capacity), and cycle performance tests (capacity retention after 200 cycles at 0.5C). Test conditions: voltage window of 1.5-4.0V, temperature of 25℃. The results are shown in Table 1.

[0057] Table 1

[0058] As can be seen from Table 1: (1) Comparing Examples 1-4, it can be seen that as the amount of pre-intercalated sodium increases from 5% to 30%, the initial coulombic efficiency gradually increases (from 86.8% to 93.0%). This verifies that pre-intercalated sodium can pre-form a stable SEI film on the hard carbon surface, reducing the irreversible consumption of positive electrode active lithium during the first charge. However, when the amount of pre-intercalated sodium reaches 30% (Example 4), although the initial efficiency is the highest, the 1C discharge specific capacity (118.6 mAh / g) and the capacity retention rate after 200 cycles (88.7%) decrease. This is because excessive residual sodium ions occupy the active sites in the hard carbon that can be used for lithium ion intercalation, reducing the reversible capacity. At the same time, the stability of the over-expanded carbon layer decreases during long cycles. In summary, when the amount of pre-intercalated sodium is 15%~20% (Examples 1 and 3), the battery achieves the best balance in terms of initial efficiency, rate performance, and cycle stability.

[0059] (2) Comparing Examples 1 and 5-7, it can be seen that the molar ratio of NFPP to LFP in the cathode has a significant impact on battery performance. When the LFP ratio is high (Example 5, NFPP:LFP=1:3), the 1C discharge specific capacity is the highest (132.8 mAh / g), which is due to the excellent rate performance and high conductivity of LFP. When the NFPP ratio is high (Example 7, NFPP:LFP=3:1), the specific capacity decreases to 119.4 mAh / g, and the cycle retention rate also decreases, which is related to the relatively low rate performance of NFPP itself. When the molar ratio of NFPP to LFP is 1:1-1:2, it has both good dual-ion synergistic effect and comprehensive electrochemical performance.

[0060] (3) Comparing Example 1 and Examples 8-10, the effect of the lithium-sodium salt molar ratio in the electrolyte on performance is reflected in the following: when the lithium-sodium salt molar ratio is 2:1 (Example 8), due to the excessively high sodium salt concentration, Na + The increased competition for insertion on the positive electrode side leads to lower overall specific capacity and cycle stability; when the lithium-sodium salt molar ratio is increased to 4:1-6:1 (Examples 1 and 9), Li+ As the primary charge carrier participating in efficient charge transport, a small amount of Na + Maintaining the stability of the negative electrode SEI film and the expansion effect of the carbon layer spacing results in optimal battery performance; when the lithium-sodium salt molar ratio is further increased to 8:1 (Example 10), Na + The concentration is too low, which weakens the ability to maintain the "expanding" effect of the negative electrode structure, and the performance is slightly reduced but still better than the comparative ratio.

[0061] (4) Comparing Example 1 and Comparative Example 1, it can be seen that when only NFPP single sodium-based cathode is used (Comparative Example 1), the 1C discharge specific capacity of the battery is only 105.2 mAh / g, and the capacity retention rate after 200 cycles is only 83.6%, which is much lower than Example 1 (128.5 mAh / g, 92.3%) using NFPP / LFP composite cathode. This shows that the introduction of LFP effectively improves the lithium-ion insertion / extraction activity and structural stability of the cathode, making the dual-ion synergistic energy storage mechanism more efficient.

[0062] (5) Comparing Example 1 and Comparative Example 2, it can be seen that when only LiPF6 single-salt electrolyte (Comparative Example 2) is used, the initial coulombic efficiency drops to 85.1%, and the capacity retention rate after 200 cycles is only 80.2%. This is because of the lack of Na in the electrolyte. + In terms of replenishment, the pre-intercalated sodium anode has insufficient SEI film repair capacity during long-term cycling. The lack of sodium salt also causes sodium ions in the anode to gradually be lost, and the carbon interlayer spacing cannot maintain an expanded state, ultimately leading to kinetic deterioration and rapid capacity decay.

[0063] In summary, this application has successfully constructed a lithium-sodium composite secondary battery system with high initial efficiency, high rate capability, and high stability through the synergistic design of a pre-embedded sodium hard carbon negative electrode, a composite positive electrode containing lithium and sodium dual active materials, and a lithium-sodium dual salt electrolyte.

[0064] Although the above embodiments have been shown and described, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Any changes, modifications, substitutions and variations made to the above embodiments by those skilled in the art are within the protection scope of the present invention.

Claims

1. A pre-embedded sodium hard carbon negative electrode sheet, characterized in that, The raw materials include the following parts by weight: 70-90 parts of pre-sodium-embedded hard carbon, 5-15 parts of conductive agent and 5-15 parts of binder; the amount of sodium pre-embedded in the pre-sodium-embedded hard carbon is 5%-30% of the theoretical sodium storage capacity of the hard carbon.

2. The pre-embedded sodium hard carbon negative electrode sheet according to claim 1, characterized in that, The hard carbon includes one or more of biomass-based hard carbon, resin-based hard carbon, pitch-based hard carbon, and sugar pyrolysis hard carbon; And / or, the conductive agent includes one or more of Super P carbon black, Ketjen black, acetylene black, carbon nanotubes, and graphene; And / or, the adhesive includes one or more of polyvinylidene fluoride, sodium carboxymethyl cellulose, styrene-butadiene rubber, and polyacrylic acid.

3. The method for preparing the pre-sodium-intercalated hard carbon negative electrode sheet according to any one of claims 1-2, characterized in that, Includes the following steps: (1) Mix hard carbon, conductive agent, binder and solvent to obtain negative electrode slurry; coat the negative electrode slurry onto current collector to prepare hard carbon negative electrode sheet; (2) A button cell is assembled using a hard carbon negative electrode as the working electrode and a sodium metal sheet as the counter electrode / reference electrode. (3) Place the coin cell half-cell into a charge-discharge test device and discharge it at a current density of 0.02C-0.1C to 0.005-0.05V vs. Na. + / Na cutoff potential, recharge to remove sodium to 0.1-0.5V vs. Na + The / Na cutoff potential is used to obtain the pre-intercalated sodium hard carbon negative electrode.

4. The method for preparing the pre-sodium-embedded hard carbon negative electrode sheet according to claim 3, characterized in that, The solvent includes water and / or N-methylpyrrolidone; And / or, in step (2), the separator of the coin cell is glass fiber, and the electrolyte is a 0.8-1.2 mol / L NaPF6 solution; preferably, the solvent of the electrolyte is a mixture of ethylene carbonate and diethyl carbonate in a volume ratio of 1:(0.8-1.2).

5. The method for preparing the pre-sodium-intercalated hard carbon negative electrode sheet according to claim 3, characterized in that, It also includes the step of disassembling the coin cell that was charged and desodiumed in step (3), and taking out the pre-sodium-embedded hard carbon negative electrode sheet for cleaning and drying.

6. A lithium-sodium composite secondary battery, characterized in that, Includes the pre-sodium-intercalated hard carbon negative electrode sheet according to any one of claims 1-2 or the pre-sodium-intercalated hard carbon negative electrode sheet obtained by the preparation method according to any one of claims 3-5.

7. The lithium-sodium composite secondary battery according to claim 6, characterized in that, It also includes positive electrode plates containing lithium-sodium dual ions and electrolytes containing lithium-sodium dual ions.

8. The lithium-sodium composite secondary battery according to claim 7, characterized in that, The molar ratio of lithium ions to sodium ions in the positive electrode containing lithium and sodium dual ions is 1:4-4:

1.

9. The lithium-sodium composite secondary battery according to claim 7, characterized in that, The molar ratio of lithium ions to sodium ions in the electrolyte containing lithium and sodium dual ions is (1-8):

1.

10. The lithium-sodium composite secondary battery according to claim 7, characterized in that, The positive electrode includes a composite positive electrode of Na4Fe3(PO4)2P2O7 and LiFePO4, a composite positive electrode of Na3V2(PO4)3 and Li3V2(PO4)3, or NaNi 1 / 3 Fe 1 / 3Mn 1 / 3 O2 and LiNi 0.6 Co 0.2 Mn 0.2 O2 composite positive electrode sheet; And / or, the electrolyte includes at least two of LiPF6, NaPF6, LiTFSI, LiFSI, NaTFSI, and NaFSI.

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