Negative plate of sodium metal battery without negative electrode and application of negative plate

Abstract

The invention belongs to the technical field of electrochemical energy storage, and particularly discloses a negative plate of a negative-electrode-free sodium metal battery and application of the negative plate. The negative electrode plate comprises a metal-based current collector and a sodium-philic functional layer, wherein the sodium-philic functional layer forms a solid solution structure by introducing one or more sodium-philic elements into the current collector. Compared with a traditional coating type or heterogeneous alloy type current collector, the solid solution type current collector can effectively inhibit sodium dendritic crystal growth and interface debonding in the circulation process, the cycle life of the battery is remarkably prolonged, and the safety and the rate capability of the battery are remarkably improved. Meanwhile, the structure simplifies the design of the negative electrode of the battery, improves the energy density, reduces the manufacturing cost, and has a good industrial application prospect. Besides, the negative electrode has no special requirements on electrolyte and a positive electrode material and shows relatively high universality, and the assembled sodium battery has the advantages of high energy density, long service life, safety, reliability and the like.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage and metal secondary battery technology, and in particular to a negative electrode sheet for a sodium metal battery without a negative electrode and a sodium metal battery without a negative electrode containing the negative electrode sheet. Background Technology

[0002] Sodium metal batteries, as a new generation of high-energy-density energy storage systems, have become an important research direction in large-scale energy storage, distributed energy, renewable energy consumption, and electric transportation due to their advantages such as abundant sodium resources, low cost, high theoretical specific capacity (1166 mAh / g), and suitable operating potential (-2.71 V vs SHE). In particular, in the anode-free sodium metal battery architecture, removing the pre-placed sodium metal can further improve energy density, simplify structural design, and enhance safety, representing a significant trend for future commercial development.

[0003] However, in practical applications, conventional current collectors such as copper foil and aluminum foil often face the following key technical bottlenecks when directly subjected to the initial nucleation and deposition of sodium ions without pre-existing sodium conditions:

[0004] 1. Sodium nucleation is uncontrollable, has a high nucleation energy barrier, and produces uneven deposition morphology, which easily induces dendrite growth and short-circuit failure;

[0005] 2. During the deposition / stripping process, the volume expansion is severe and the stress concentration leads to interface damage and debonding of the current collector / sodium layer;

[0006] 3. During long-term cycling, nucleation active sites are depleted, nucleation uniformity deteriorates, and cycle life is limited;

[0007] 4. Traditional surface coating or heterojunction modification methods generally suffer from problems such as coating crushing, peeling, and local stress accumulation, resulting in insufficient mechanical integrity and interfacial connectivity, and thus failing to achieve long-term stable and efficient cycling.

[0008] To overcome the above problems, some studies have attempted to introduce sodium-loving metal coatings (such as Sn, Zn, In, and Sb) onto the surface of current collectors, which to some extent reduces nucleation barriers and alleviates instability during the initial deposition process. However, coated materials typically have the following inherent defects: First, the interfacial bonding between the coating and the substrate is limited, and delamination and debonding are prone to occur during repeated alloying-dealloying stress cycles; second, the alloy layer lacks a continuous lattice support structure, and local stress concentration can easily lead to crack propagation and loss of nucleation active sites, making it difficult to maintain nucleation uniformity and interfacial integrity stably over a long period.

[0009] Therefore, there is an urgent need to develop a novel current collector modification structure with continuous lattice support capability, highly dispersed nucleation activity, excellent electronic conductivity and stress relief capability, so as to fundamentally solve the technical bottleneck problems of existing anode-free sodium metal batteries in terms of interface stability, long life cycle performance and rate performance. Summary of the Invention

[0010] The purpose of this invention is to provide a novel anode sheet for sodium metal batteries without anodes, its preparation method and application. This anode sheet has the characteristics of high structural stability, good nucleation uniformity and long cycle life. It can achieve efficient and stable operation in the application scenarios of sodium metal batteries without anodes, and overcome the problems of uneven nucleation, dendrite growth, coating debonding and cycle failure in the existing technology.

[0011] To achieve the above objectives, the present invention can be implemented through the following technical solutions:

[0012] This invention discloses a negative electrode sheet for a non-negative electrode sodium metal battery, comprising a metal-based current collector and a sodium-loving functional layer. The sodium-loving functional layer can be a solid solution structure formed by the metal-based current collector and one or more sodium-loving dopants, or a solid solution structure directly coated on the surface of the metal-based current collector. The solid solution structure achieves uniform composition distribution and synergistic crystal structure at the nanometer to micrometer scale, possessing excellent electronic conductivity, interface stability, and stress mitigation capabilities. It is adaptable to various types of non-negative electrode sodium metal battery systems, and is particularly suitable for applications requiring high current density, high areal capacity, and long cycle life.

[0013] Preferably, the metal-based current collector can be selected from copper, aluminum, magnesium, titanium, nickel, iron, stainless steel or their alloys.

[0014] In industrial applications, the metal-based current collector is selected from at least one of copper, aluminum, magnesium, titanium, nickel, iron, stainless steel or their alloys, and may be a dense foil, a three-dimensional porous skeleton structure, a porous foam metal, a mesh structure or a surface roughening modification structure.

[0015] Preferably, the sodium-loving functional layer comprises: (1) a solid solution containing a metal-based current collector and one or more sodium-loving doping elements, wherein the sodium-loving doping element includes, but is not limited to, M. The M is selected from at least one of tin (Sn), zinc (Zn), indium (In), aluminum (Al), silver (Ag), gallium (Ga), antimony (Sb), cadmium (Cd), etc., and the doping mode is substitutional, interstitial, or composite lattice doping to form a continuous and stable metal solid solution structure; or (2) a solid solution film layer directly coated on the surface of the metal-based current collector, wherein the solid solution may be a sodium-loving metal solid solution or other solid solution materials that can improve sodium deposition / stripping performance.

[0016] In industrial applications, the doping element can also be a small-radius interstitial doping element mixed with M, wherein the small-radius interstitial doping element is selected from at least one of lithium, boron, nitrogen, and hydrogen.

[0017] The sodium-loving functional layer is a solid solution layer formed by the metal-based current collector and one or more sodium-loving doping elements. The sodium-loving doping elements are uniformly doped in the metal-based current collector lattice in the form of substitution, interstitial, or composite doping to form a continuous and stable solid solution structure.

[0018] The solid solution layer induces sodium to uniformly nucleate and deposit on its surface during the first charging process, improving the reversibility of sodium deposition / stripping and interface stability, mitigating volume expansion stress, enhancing interface mechanical integrity, stabilizing nucleation active sites in the long term, and enhancing electronic conductivity, thereby achieving long-term high-efficiency cycling.

[0019] In industrial applications, the atomic percentage of doped elements in the sodium-loving functional layer is initially selected based on the solid solution limit in the corresponding alloy phase diagram.

[0020] In this invention, the solid solution structure significantly improves performance through the following mechanisms: First, unlike the traditional alloying-dealloying mechanism, the solid solution structure of this invention exhibits non-phase-separation, structurally adaptive sodium-alloying behavior during sodium deposition / stripping. The lattice remains continuous and stable throughout the cycle, without significant phase boundary movement or structural damage. This adaptive transformation significantly reduces interfacial stress accumulation and volume fluctuations, fundamentally suppressing uneven nucleation and dendrite growth. Second, the uniformly distributed dopant atoms in the solid solution lattice can mitigate the volume expansion stress during sodium deposition / stripping, avoiding localized stress concentration and structural instability. Third, the continuous lattice solid solution eliminates the weak bonding problem at the coating-substrate interface, making it less prone to coating cracking, peeling, and migration during long-term cycling. Furthermore, the dopant elements continuously provide stable active nucleation sites, preventing depletion of nucleation sites and maintaining uniform deposition. Finally, the continuous metal structure of the solid solution possesses excellent electronic conductivity, reducing polarization and improving coulombic efficiency. Through these synergistic effects, the safety, rate performance, and long-cycle stability of anode-free sodium metal batteries can be significantly improved.

[0021] Furthermore, the sodium-loving functional layer of the present invention can be prepared by processes such as, but not limited to, electrodeposition, physical vapor deposition (PVD), chemical vapor deposition (CVD), solution dip coating, sol-gel, in-situ reaction synthesis, self-propagating high-temperature synthesis, sputtering deposition, thermal diffusion infiltration, mechanical alloying, and high-energy ball milling. Combined with various solid solution formation mechanisms such as heat treatment, plasma treatment, and laser-assisted alloying, a high degree of uniformity in the distribution of sodium-loving dopant elements within the solid solution structure can be achieved. This process system is flexible and adjustable, applicable to various metal substrates, and possesses industrial-scale feasibility.

[0022] In a preferred embodiment, the present invention employs a combination of electrodeposition and thermal treatment to prepare a solid solution-type negative electrode. Copper foil is preferably used as the metal-based current collector. It is ultrasonically cleaned sequentially with deionized water, ethanol, and a dilute acid (such as HCl or HNO3, wherein the concentration of the dilute acid is 0.05~0.20 mol / L HCl or 0.02~0.10 mol / L HNO3) to remove oil, oxides, and impurities, and then stored under dry conditions. The weak acid treatment not only removes surface contaminants but also micro-etches the surface of the copper foil, improving the adhesion between the deposited layer and the substrate.

[0023] The pretreated copper foil is used as the working electrode and placed in a container containing a sodium-loving dopant element (such as Sn). 2+ Zn 2+ Sb 3+ Electrodeposition is performed in a metal salt electrolyte (e.g., ethylene glycol, propylene glycol, glycerol, dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), etc.). The electroplating solution can be a mixture of organic solvent and water, with organic solvents including but not limited to ethylene glycol, propylene glycol, glycerol, dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), etc.; more preferably, an ethylene glycol / deionized water mixture (volume ratio, e.g., 1:1 to 3:1). This mixture can improve the viscosity and wettability of the deposition solution, improve the uniformity of metal ion migration on the electrode surface, and inhibit hydrogen evolution reaction; ethylene glycol molecules can also coordinate with metal ions, adjusting the local reduction potential to achieve mild and controllable nucleation; simultaneously, they can selectively adsorb onto specific crystal planes, inhibiting lateral disordered growth and promoting the formation of a stable and continuous solid solution structure. In the metal salt electrolyte used for electrodeposition, the concentration of sodium-loving dopant is 0.01–0.5 mol / L, preferably 0.05–0.2 mol / L.

[0024] Furthermore, a complexing agent (such as sodium citrate or sodium tartrate) can be added to the deposition solution to adjust the reduction potential of the metal ions by forming complexes with them, thereby reducing the instantaneous deposition rate of the metal ions and preventing uncontrolled local nucleation. If necessary, a conductive salt (such as Na₂SO₄ or KCl) can be added to increase conductivity and reduce ohmic polarization. In the metal salt electrolyte used for electrodeposition, the concentration of the complexing agent is 0.01–0.5 mol / L, preferably 0.05–0.2 mol / L, and the concentration of the conductive salt is 0.01–1 mol / L, preferably 0.05–0.5 mol / L.

[0025] Furthermore, during the electrodeposition process, a constant current or constant potential mode can be used, with a preferred current density of 5-20 mA / cm² and a deposition time of 50-600 s, to obtain a doped layer with uniform thickness, controllable composition, and good adhesion. Too low a current density can easily lead to a loose deposited layer, uneven surface, and increased dendrite risk; too high a current density can easily lead to a loose coating, decreased adhesion, and in severe cases, the formation of a rough or spongy deposited layer, resulting in deposition failure. During the electrodeposition process, the temperature is 20-80℃, preferably 25-60℃.

[0026] After deposition, a solid solution is obtained through solution treatment. The solid solution formation process can be carried out by any method that promotes homogenization of the solid solution, such as heat treatment, plasma-assisted diffusion, laser-assisted alloying, short-time rapid annealing, ultrafast heat treatment, field-assisted sintering (FAST / SPS).

[0027] Furthermore, the electrodeposited sample is placed in a tube furnace and heat-treated under an inert atmosphere (such as Ar or an Ar / H2 mixture) to allow the dopant element to diffuse within the metal matrix and form a continuous and homogeneous solid solution structure. The preferred heat treatment temperature is 150-550℃, more preferably 280-550℃, with a holding time of 30 minutes to 2 hours. Temperatures that are too low or too high may induce abnormal reactions between the dopant element and the base metal, generating other types of intermetallic compound structures. This would disrupt the continuous lattice characteristics of the solid solution, leading to solid solution preparation failure and reducing its performance advantages in electrochemical cycling.

[0028] When the metal-based current collector is made of copper and the doping element is tin, the heat treatment temperature is 300~500℃, and more preferably 390~420℃.

[0029] Furthermore, after heat treatment, the furnace is allowed to cool naturally in an inert atmosphere to avoid high-temperature oxidation. To verify the preparation effect, methods such as scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) can be used to detect structural uniformity and doping degree.

[0030] The sodium-loving functional layer surface obtained by this invention can maintain a micro-nano three-dimensional structure morphology, including flower-like, needle-like, array-like, sheet-like, mesh-like, porous honeycomb-like, or other three-dimensional morphologies that induce sodium nucleation and deposition in a directional manner.

[0031] The solid solution structure designed and prepared in this invention provides the following performance advantages:

[0032] 1. Volumetric expansion stress during uniform slow-release sodium deposition / stripping process;

[0033] 2. Maintaining the mechanical integrity of the interface and preventing the inactivation of nucleation sites during long-term cycling;

[0034] 3. Provides continuous and uniform nucleation induction, inhibiting dendrite formation;

[0035] 4. Maintain excellent electronic conductivity and ion nucleation coupling interface migration capability to improve coulombic efficiency.

[0036] The present invention also discloses a sodium metal battery without a negative electrode, comprising the above-mentioned solid solution negative electrode sheet. During assembly, the negative electrode side of the battery is not pre-placed with sodium metal. The sodium source is entirely released from the positive electrode material and uniformly nucleates and deposits on the surface of the solid solution to form a sodium metal negative electrode during the first charging process.

[0037] The sodium metal battery without a negative electrode according to the present invention includes a positive electrode, a negative electrode sheet, an electrolyte, and an encapsulation structure.

[0038] The cathode material may include, but is not limited to, layered oxides (such as Na). X NiMnO2, Na X CoO2, NaNi X Co Y Mn Z O2, etc.), polyanionic compounds (such as Na3V2(PO4)3, Na3V2(PO4)2F3, Na X FePO4, Prussian blue and its analogues, chalcogenides, titanium-based / niobium-based oxides, sodium super ferroelectric cathode materials, sodium-rich phosphates and other sodium-ion battery composite cathode systems, etc.

[0039] The electrolyte used in the negative electrode-free sodium metal battery described in this invention includes all electrolytes used in existing sodium metal batteries. For example:

[0040] The electrolyte is a liquid, gel, solid, or composite sodium ion electrolyte system, specifically including:

[0041] Liquid systems: ethers, esters, fluorinated ethers or mixtures thereof, wherein the sodium salt is selected from any one or a combination of NaPF6, NaFSI, NaTFSI, NaClO4, NaBF4, NaOTf, etc.

[0042] Gel system: A gel electrolyte formed by encapsulating liquid electrolytes in a polymer matrix (such as PEO, PVDF-HFP, PMMA, PAN, PVA, etc.);

[0043] Solid-state systems: oxide type, sulfide type, halide type, nitride type, polymer type and their composite solid-state sodium ion electrolytes;

[0044] Composite electrolyte systems: any form including liquid-solid composite, polymer-ceramic composite, interface-reinforced composite electrolyte, etc.

[0045] This invention is adaptable to various liquid, gel, solid, and composite sodium-ion electrolyte systems, as well as diverse battery structures, possessing excellent technical versatility and promising prospects for industrial application. The battery structures include, but are not limited to: button cells, stacked cells, wound cells, pouch cells, cylindrical cells, prismatic hard-shell cells, containerized energy storage modules, microchip batteries, flexible wearable batteries, micro-implantable medical batteries, and other battery structures compatible with portable, wearable, and flexible electronic and energy storage systems.

[0046] The solid solution-type negative electrode proposed in this invention solves the long-standing technical bottlenecks of existing non-negative electrode sodium metal batteries in terms of initial nucleation, long-term cycle stability, safety and rate performance, and has strong technical advancement and wide application and promotion value.

[0047] The proposed solid solution-type sodium-loving functional layer solves the core problems faced by traditional surface-coated anode materials during long-term sodium deposition / stripping, such as interface pulverization, coating peeling, nucleation site deactivation, and localized stress concentration, by forming a highly uniform and continuous doped solid solution structure within a metal-based current collector lattice. The doped atoms within the solid solution can mitigate volume expansion stress during cycling, improve overall interface mechanical integrity, and maintain high nucleation-induced activity over the long term, significantly suppressing the risk of dendrite growth. Simultaneously, the continuous metal solid solution framework ensures synergistic optimization of electron conduction and nucleation uniformity, enabling anode-free sodium metal batteries to possess excellent coulombic efficiency, long cycle life, and rate performance. This solution combines broad material adaptability with industrial manufacturing compatibility, possessing extremely high practical application value. Attached Figure Description

[0048] Figure 1 The images show the TEM and EDS images of the Cu-Sn solid solution sample annealed at 400 °C in Example 2.

[0049] Figure 2 The first-cycle charge-discharge curves of the Na3V2(PO4)3 sodium metal full cell without negative electrode in Example 2 and Comparative Example 1 are shown.

[0050] Figure 3 The cycling performance curves of Na3V2(PO4)3 full cells without a negative electrode sodium metal are shown for Example 2 and Comparative Example 1. Detailed Implementation

[0051] To enable those skilled in the art to better understand the technical solution of the present invention, the product and its preparation method of the present invention are further described in detail below with reference to embodiments. These embodiments are merely illustrative examples and should not be construed as limiting the scope of the claims of the present invention. Other embodiments obtained by those skilled in the art without inventive effort are all within the scope of protection of the present invention. Unless otherwise stated, the raw materials, equipment, and testing methods used in the following embodiments are all common conditions or conventional methods well known to those skilled in the art.

[0052] This invention discloses a negative electrode sheet for a sodium metal battery without a negative electrode, comprising a metal-based current collector and a sodium-loving functional layer. The sodium-loving functional layer may be a solid solution structure formed by the metal-based current collector and one or more sodium-loving dopants, or a solid solution structure directly coated on the surface of the metal-based current collector.

[0053] Furthermore, the metal-based current collector can be selected from copper, aluminum, magnesium, titanium, nickel, iron, stainless steel or their alloys; the sodium-loving doping element includes, but is not limited to, tin (Sn), zinc (Zn), indium (In), aluminum (Al), silver (Ag), gallium (Ga), antimony (Sb), cadmium (Cd), etc., and the doping mode can be substitutional, interstitial or composite lattice doping to form a continuous and stable metal solid solution structure.

[0054] In a preferred embodiment, the negative electrode is prepared by a combination of electrodeposition and heat treatment. First, a commercially available copper foil with a thickness of 12 μm is selected as the metal-based current collector, and is ultrasonically cleaned for 10 min each with 0.1 mol / L dilute hydrochloric acid, deionized water, and ethanol to remove oil, oxides, and impurities, and then dried at 60°C for later use.

[0055] The pretreated copper foil was used as the working electrode and placed in a solution containing 0.01–0.5 mol / L (preferably 0.05–0.2 mol / L) of Sn. 4+ Electrodeposition is carried out in an electroplating solution. The electrolyte is an ethylene glycol / deionized water mixture (volume ratio 1:1), with 0.01–0.5 mol / L (preferably 0.05–0.2 mol / L) sodium citrate added as a complexing agent, and 0.01–1 mol / L (preferably 0.05–0.5 mol / L) Na₂SO₄ added as a conductive salt. The deposition conditions are constant current mode, current density 10 mA / cm², deposition time 200 s, and deposition temperature 20–80 °C (preferably 30–60 °C).

[0056] After electrodeposition, the sample is placed in a tube furnace and heat-treated under an Ar / H2 (95:5 volume ratio) protective atmosphere at a temperature controlled between 150 and 800 °C (preferably 300–500 °C) for 1 h, followed by natural cooling with the furnace. This process allows the dopant elements to diffuse fully into the copper matrix, forming a continuous and stable solid solution structure. It also avoids insufficient diffusion due to excessively low temperatures or grain coarsening, surface oxidation, and the formation of other intermetallic compounds due to excessively high temperatures, thus ensuring successful solid solution preparation.

[0057] Furthermore, the positive electrode sheet includes an aluminum foil current collector with a thickness of 16 μm and a positive electrode active material coated on its surface, wherein the positive electrode active material is Na3V2(PO4)3.

[0058] Furthermore, the non-negative electrode sodium metal battery of the present invention also includes an electrolyte, wherein the electrolyte is 1 mol / L NaPF6 dissolved in diethylene glycol dimethyl ether (DIGLYME) solvent.

[0059] During battery assembly, no metallic sodium is pre-placed on the negative electrode side. The sodium source is entirely released from the positive electrode during the first charge and uniformly nucleates and deposits on the surface of the solid solution to form a metallic sodium negative electrode.

[0060] After the battery was assembled and left to stand for 12 hours, it was directly subjected to charge and discharge tests. The constant current charge and discharge mode was used, with a current density of 1C (1C = 117 mAh / g) and a voltage range of 3.8 V-2.2 V.

[0061] Example 1

[0062] This embodiment provides a method for preparing a negative electrode sheet for a sodium metal battery without a negative electrode under low-temperature solid solution conditions.

[0063] Commercially available copper foil with a thickness of 8 μm was selected as the metal-based current collector. It was ultrasonically cleaned sequentially with 0.05 mol / L dilute hydrochloric acid, deionized water, and ethanol for 5 min each to remove oil, oxides, and impurities. The foil was then dried at 45 ℃ for later use. The pretreated copper foil was used as the working electrode and electrodeposited in a 1:1 mixture of ethylene glycol and deionized water containing 0.03 mol / L SnCl4. 0.05 mol / L sodium citrate was added to the electrolyte as a complexing agent, and 0.05 mol / L Na2SO4 was added as a conductive salt. The deposition conditions were constant current mode, current density 6 mA / cm², deposition time 90 s, and deposition temperature 30 ℃. The deposited sample was placed in a tube furnace under an Ar / H2 (95:5 volume ratio) protective atmosphere and held at 300 ℃ for 45 min, then allowed to cool naturally with the furnace to form a Cu-Sn solid solution structure.

[0064] The positive electrode consists of a 16 μm thick aluminum foil current collector and a Na3V2(PO4)3 positive electrode active material coated on its surface, with a mass loading of approximately 3.0 mg / cm². The electrolyte is 1 mol / L NaPF6 dissolved in diethylene glycol dimethyl ether (DIGLYME). CR2025 coin cells were assembled in an Ar glove box (O2 and H2O contents both less than 0.1 ppm). No metallic sodium was pre-placed on the negative electrode side; the sodium source was entirely from the release of sodium from the positive electrode during the first charge and its nucleation and deposition on the solid solution surface. A PP membrane was used.

[0065] After the battery was left to rest for 12 hours, it was tested using a constant current charge-discharge mode with a current density of 1C (1C = 117 mAh / g) and a voltage range of 3.8-2.2 V. The test results showed that its coulombic efficiency in the first cycle was 89.2%, and the discharge specific capacity was 92.15 mAh / g. After 300 cycles, the capacity retention was approximately 70.5%, and polarization gradually increased during cycling. The overall performance was lower than that of the preferred embodiment 2.

[0066] Example 2

[0067] This embodiment provides a method for preparing a negative electrode sheet for a sodium metal battery without a negative electrode under medium-temperature solid solution conditions. A commercially available copper foil with a thickness of 12 μm was selected as the metal-based current collector. It was ultrasonically cleaned sequentially with 0.10 mol / L dilute hydrochloric acid, deionized water, and ethanol for 10 min each; subsequently, it was dried at 60 ℃ for later use. The pretreated copper foil was used as the working electrode and electrodeposited in a mixture of ethylene glycol / deionized water (volume ratio 2:1) containing 0.10 mol / L SnCl4. 0.10 mol / L sodium citrate was added to the electrolyte as a complexing agent, and 0.10 mol / L Na2SO4 was added as a conductive salt. The deposition conditions were constant current mode, current density 10 mA / cm², deposition time 200 s, and deposition temperature 45 ℃. The deposited sample was placed in a tube furnace under an Ar / H2 (volume ratio 95:5) protective atmosphere and held at 400 °C for 60 min. Then it was allowed to cool naturally with the furnace to form a dense and homogeneous Cu-Sn solid solution structure. Figure 1 The image shows the transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) distribution of Cu-Sn solid solution samples prepared under medium-temperature annealing at 400 °C. Figure 1 In the high-angle annular dark field (HAADF) image shown on the left, the sample exhibits a dense, aggregated granular structure with a uniform particle size distribution and an average particle size of approximately 50 nm, indicating that the electrodeposited layer formed a uniform and dense nanoscale structure after heat treatment. Figure 1(middle) is the energy spectrum distribution of copper (Cu) (orange), showing that copper is uniformly distributed throughout the particle region, exhibiting good continuity and consistency, indicating that no obvious element migration or phase separation occurred during the annealing process. Figure 1 (Right) shows the distribution of tin (Sn) (purple), which also indicates that Sn is highly dispersed in the particles and does not form isolated enrichment regions. The elemental distributions of Cu and Sn highly overlap, further indicating that the two elements achieved effective interdiffusion during the heat treatment process, successfully forming a uniform Cu-Sn solid solution structure, rather than a simple physical mixture or phase separation structure. This result verifies that the designed medium-temperature solution treatment process (400℃, holding for 60 min) can achieve sufficient mutual solubility of Cu and Sn without causing grain coarsening, obtaining a stable and uniform solid solution, which is beneficial for providing a uniform sodium-loving interface and stable deposition / stripping behavior in subsequent anode-free sodium metal batteries.

[0068] The positive electrode consists of a 16 μm thick aluminum foil current collector and a Na3V2(PO4)3 positive electrode active material coated on its surface, with a mass loading of approximately 10.0 mg / cm². The electrolyte is 1 mol / L NaPF6 dissolved in diethylene glycol dimethyl ether (DIGLYME). CR2025 coin cells are assembled in an Ar glove box. No metallic sodium is pre-placed on the negative electrode side; the sodium source is entirely from the release of sodium from the positive electrode during the first charge and its nucleation and deposition on the solid solution surface. A PP membrane is used as the separator.

[0069] After the battery was left to stand for 12 hours, it was tested using a constant current charge-discharge mode with a current density of 1C (1C = 117 mAh / g) and a voltage range of 3.8-2.2 V. The test results show that its charge-discharge curve for the first week is as follows: Figure 2 As shown, the coulombic efficiency in the first week was 94.1%, and the discharge specific capacity was 100.7 mAh / g. (From...) Figure 3 It can be seen that after 300 cycles of the test, the capacity retention rate was 73.4%, which showed better cycle stability than Example 1 and Example 3.

[0070] Example 3

[0071] This embodiment provides a method for preparing a negative electrode sheet for a sodium metal battery without a negative electrode under high-temperature solid solution conditions.

[0072] A commercially available copper foil with a thickness of 20 μm was selected as the metal-based current collector. It was ultrasonically cleaned sequentially with 0.20 mol / L dilute hydrochloric acid, deionized water, and ethanol for 15 min each; subsequently, it was dried at 80 ℃ for later use. The pretreated copper foil was used as the working electrode and electrodeposited in a mixture of ethylene glycol / deionized water (volume ratio 3:1) containing 0.30 mol / L SnCl4. 0.20 mol / L sodium citrate was added to the electrolyte as a complexing agent, and 0.50 mol / L Na2SO4 was added as a conductive salt. The deposition conditions were constant current mode, with a current density of 18 mA / cm², a deposition time of 420 s, and a deposition temperature of 60 ℃. The deposited sample was placed in a tube furnace under an Ar / H2 (volume ratio 95:5) protective atmosphere and held at 500 ℃ for 60 min, then allowed to cool naturally with the furnace to form a Cu-Sn solid solution structure.

[0073] The positive electrode consists of a 16 μm thick aluminum foil current collector and a Na3V2(PO4)3 positive electrode active material coated on its surface, with a mass loading of approximately 20.0 mg / cm². The electrolyte is 1 mol / L NaPF6 dissolved in diethylene glycol dimethyl ether (DIGLYME). CR2025 coin cells are assembled in an Ar glove box. No metallic sodium is pre-placed on the negative electrode side; the sodium source is entirely from the release of sodium from the positive electrode during the first charge and its nucleation and deposition on the solid solution surface. A PP membrane is used as the separator.

[0074] After the battery was left to rest for 12 hours, it was tested using a constant current charge-discharge mode with a current density of 1C (1C = 117 mAh / g) and a voltage range of 3.8-2.2 V. The test results showed that its coulombic efficiency in the first cycle was 83.1%, and its discharge specific capacity was 95.02 mAh / g. After 300 cycles, the capacity retention was approximately 71.8%, and the cycle stability was better than that of Example 1, but slightly lower than that of Example 2.

[0075] Comparative Example 1

[0076] Other conditions were the same as in Example 2, except that the solution treatment was omitted. After assembly, the battery was allowed to stand for 12 hours before being directly charged and discharged using a constant current charge-discharge mode with a current density of 1C (1C = 117 mAh / g) and a voltage range of 3.8-2.2 V. Cyclic test results showed that the coulombic efficiency in the first cycle was only 38.7%, and the discharge specific capacity was 38.7 mAh / g. After 300 cycles, the capacity retention was approximately 21.3%, indicating severe capacity decay and significantly inferior stability compared to Example 2 with solution treatment. This is mainly due to the lack of a solution treatment process, resulting in a significant phase separation characteristic in the deposited layer and the presence of a large amount of Sn-rich phase in some areas. This makes the material prone to alloying-dealloying phase transitions during cycling, consuming a large amount of active sodium ions and leading to rapid capacity decay.

[0077] Comparative Example 2

[0078] Other conditions were the same as in Example 3, except that the battery was heated to 600 °C for 60 min under an Ar / H2 (95:5) protective atmosphere and then cooled in the furnace. After assembly, the battery was allowed to stand for 12 h before being directly subjected to charge-discharge tests using a constant current charge-discharge mode at a current density of 1C (1C = 117 mAh / g) and a voltage range of 3.8-2.2 V. The test results showed that the coulombic efficiency was 70.5% in the first cycle and the discharge specific capacity was 85.13 mAh / g. After 300 cycles at a current density of 1C, the capacity retention was only 58.7%. The main reason for the accelerated capacity decay was that the excessively high sintering temperature reduced the solid solution stability of Sn in the Cu matrix, causing some Sn to precipitate from the solid solution during cycling and form Sn-rich phases or intermetallic compounds locally. These new phases exhibit significant lattice mismatch with the Cu matrix, which can easily lead to interfacial debonding and crack propagation. Meanwhile, Sn precipitation results in local compositional and structural inhomogeneity, forming unstable electrochemical reaction regions, increasing interfacial impedance and accelerating the loss of active sodium, ultimately leading to rapid capacity decay.

[0079] Comparative Example 3

[0080] This comparative example is essentially the same as Example 1, but the electrodeposition current density was set to 100 mA / cm². After assembly and 12 hours of rest, the battery was directly subjected to charge-discharge tests using a constant current charge-discharge mode at a current density of 1C (1C = 117 mAh / g) and a voltage range of 3.8–2.2 V. The test results showed that the coulombic efficiency was 62.8% in the first cycle, the discharge specific capacity was 80.26 mAh / g, and the capacity retention was less than 50.1% after 300 cycles at a current density of 1C. Due to the excessively high current density, the deposition process was diffusion-limited, leading to a sharp increase in local overpotential, a decrease in nucleation density, severe dendrite growth, and a significant increase in the surface roughness of the deposited layer. The surface of the deposited Cu-Sn solid solution contained a large number of high-energy defect regions, making it difficult to form a dense and uniform solid solution structure after subsequent heat treatment at 300 °C. During cycling, these structural defects became active areas for stress concentration and local Na deposition, promoting rapid dendrite growth, which in turn exacerbated interface peeling and loss of active area, resulting in rapid capacity decay.

[0081] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A negative electrode sheet for a sodium metal battery without a negative electrode, characterized in that: The negative electrode includes a metal-based current collector and a sodium-loving functional layer. The sodium-loving functional layer is a solid solution structure formed by the metal-based current collector and one or more sodium-loving dopants, or a solid solution structure directly coated on the surface of the metal-based current collector.

2. The negative electrode sheet of a sodium metal battery without a negative electrode according to claim 1, characterized in that: The negative electrode current collector is a metal-based current collector, selected from copper, aluminum, magnesium, titanium, nickel, iron, stainless steel, or their alloys. The sodium-loving functional layer is a solid solution layer formed by the metal-based current collector and one or more sodium-loving doping elements. The sodium-loving doping elements are uniformly doped in the metal-based current collector lattice in the form of substitution, interstitial or composite doping to form a continuous and stable solid solution structure. The sodium-loving dopant element includes at least one of tin, zinc, indium, aluminum, silver, gallium, cadmium, antimony, tellurium, lead, and selenium.

3. The negative electrode sheet of a sodium metal battery without a negative electrode according to claim 1, characterized in that: Sodium-loving functional layers can be prepared by processes such as, but not limited to, electrodeposition, physical vapor deposition, chemical vapor deposition, solution dipping, sol-gel, in-situ reaction synthesis, self-propagating high-temperature synthesis, sputtering deposition, thermal diffusion infiltration, mechanical alloying, and high-energy ball milling. They can also be combined with heat treatment, plasma treatment, and laser-assisted alloying to promote the formation and homogenization of solid solution structures.

4. The negative electrode sheet of a sodium metal battery without a negative electrode according to claim 3, characterized in that: When copper foil is selected as the metal-based current collector, it is ultrasonically cleaned sequentially with deionized water, ethanol and dilute acid to remove oil, oxides and impurities, and then stored under dry conditions. The concentration of the dilute acid is 0.05~0.20 mol / L HCl or 0.02~0.10 mol / L HNO3.

5. The negative electrode sheet of a sodium metal battery without a negative electrode according to claim 4, characterized in that: The pretreated copper foil is used as the working electrode and electrodeposited in a metal salt electrolyte containing a sodium-loving dopant. The electroplating solution can be a mixture of an organic solvent and water, including but not limited to ethylene glycol, propylene glycol, glycerol, dimethyl sulfoxide, or N-methylpyrrolidone; more preferably, it is an ethylene glycol / deionized water mixture (volume ratio, for example, 1:1 to 3:1); the concentration of the metal salt in the electrolyte is 0.01 to 0.5 mol / L, preferably 0.05 to 0.20 mol / L.

6. The negative electrode sheet of a sodium metal battery without a negative electrode according to claim 5, characterized in that: A complexing agent can be added to the deposition solution to adjust the reduction potential of metal ions by forming a complex with them, thereby reducing the instantaneous deposition rate of metal ions and preventing uncontrolled local nucleation. If necessary, a conductive salt can be added to improve conductivity and reduce ohmic polarization. The concentration of the complexing agent is 0.01–0.5 mol / L, preferably 0.05–0.2 mol / L, and the complexing agent is selected from at least one of sodium citrate and sodium tartrate. The concentration of the conductive salt is 0.01–1.00 mol / L, preferably 0.05–0.5 mol / L, and the conductive salt is selected from at least one of Na₂SO₄ and KCl.

7. The negative electrode sheet of a sodium metal battery without a negative electrode according to claim 5, characterized in that: The electrodeposition process employs a constant current or constant potential mode, with a current density of 5~20 mA / cm² and a deposition time of 50~600 s; the deposition temperature is 20~80℃, preferably 25~60℃.

8. The negative electrode sheet of a sodium metal battery without a negative electrode according to claim 5, characterized in that: The sample after electrodeposition is subjected to solid solution treatment to obtain a solid solution; the solid solution formation methods include heat treatment, plasma-assisted diffusion, laser-assisted alloying, short-time rapid annealing, ultrafast heat treatment or field-assisted sintering.

9. The negative electrode sheet of a sodium metal battery without a negative electrode according to claim 5, characterized in that: The electrodeposited sample is subjected to heat treatment under an inert atmosphere to allow the dopant element to diffuse in the metal matrix and form a continuous and uniform solid solution structure. The heat treatment temperature is 150~550 ℃, preferably 280~500 ℃, and the holding time is 30 minutes to 2 hours. When the metal matrix current collector is copper and the dopant element is tin, the heat treatment temperature is 300~500 ℃, more preferably 390~420 ℃.

10. The application of the negative electrode sheet of a sodium metal battery without a negative electrode according to any one of claims 1-9, characterized in that: The negative electrode is used to assemble a sodium metal battery. During battery assembly, no sodium metal is pre-placed on the negative electrode side. The sodium source is mainly released from the positive electrode material and uniformly nucleates and deposits on the surface of the solid solution to form a sodium metal negative electrode during the first charging process. The sodium metal battery includes a positive electrode, a negative electrode, an electrolyte, and an encapsulation structure.

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