A negative electrode surface film layer, a preparation method and application thereof

Abstract

The application relates to the technical field of magnesium primary batteries, and discloses a negative electrode surface film layer, which comprises a magnesium base body, a composite film layer is generated on the surface of the magnesium base body, the composite film layer comprises phytic acid molecules, Mg 2+ a small amount of magnesium phosphate framework, magnesium fluoride, magnesium oxide and magnesium hydroxide; a preparation method of the negative electrode surface film layer, specific steps are as follows: firstly, a fluorination solution is used to pre-fluorinate the magnesium base body; then, a phytic acid solution is prepared, and the pH value of the phytic acid solution is regulated; then, the magnesium base body after the pre-fluorination treatment is immersed in the phytic acid solution with the regulated pH value to carry out reaction; after the reaction is completed, the magnesium base body is washed and dried, and finally, the negative electrode surface film layer is prepared; and the application of the negative electrode surface film layer is applied to the fields of magnesium primary batteries and degradable medical magnesium alloys. The technical scheme can effectively solve the problems of magnesium negative electrode self-corrosion and battery performance deterioration caused by the passivation film, and has the advantages of simple and controllable preparation process and wide application range.

Description

Technical Field

[0001] This invention relates to the field of magnesium primary battery technology, specifically to a negative electrode surface film, its preparation method, and its application. Background Technology

[0002] Magnesium primary batteries have become a highly anticipated research direction in the battery field due to their significant advantages such as high theoretical capacity, strong safety, and environmental friendliness. However, the severe self-corrosion and passivation film formation problems of magnesium metal anodes in electrolytes have become key factors restricting their practical application. When the magnesium electrode reacts with conventional anions in the electrolyte (such as ClO₂)... 4- PF 6- and BF 4- When in contact with reducing agents (such as oxygen and water), a chemical reaction easily occurs. On the one hand, self-corrosion consumes a large amount of magnesium anode material, resulting in low anode utilization and significantly shortening battery life; on the other hand, the passivation film formed by the reaction cannot conduct Mg... 2+ Ions inhibit the redox reaction of the magnesium anode, slow down the kinetics of magnesium anode deposition, and seriously affect battery performance.

[0003] To address these issues, researchers have explored the use of metal-organic frameworks (MOFs) to modulate the anode interface, with the application of ZIF-8 coatings attracting particular attention. For example, Liu et al. designed a ZIF-8 coating on the surface of a magnesium anode, effectively controlling its corrosion resistance and interface stability. This shortened the voltage hysteresis time of the magnesium battery, increased the initial discharge potential and voltage plateau, while simultaneously inhibiting magnesium anode corrosion, slowing hydrogen evolution rate, and providing some protection for the magnesium anode. However, there are still many shortcomings in using MOF coatings to control the anode interface. Firstly, the MOF coating structure has poor stability, especially in aqueous environments and environments containing Cl. - Hydrolysis easily occurs in the electrolyte, leading to structural collapse; during battery discharge, stress may also cause the coating to peel off from the negative electrode interface. Secondly, existing MOF coating preparation processes, such as solvothermal methods or microwave-assisted synthesis, have stringent reaction conditions, are complex, and costly. Thirdly, the hydrophobicity of the ZIF-8 coating itself may hinder electrolyte penetration to the electrode interface, reducing the effective reaction area, exacerbating polarization, and thus affecting battery discharge stability. Summary of the Invention

[0004] The present invention aims to provide a negative electrode surface film layer, its preparation method and application, to overcome the defects of existing MOF coatings such as poor structural stability, complex and costly preparation process and impact on battery discharge stability.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: a negative electrode surface film layer, comprising a magnesium substrate, wherein a composite film layer is formed on the surface of the magnesium substrate, the composite film layer comprising phytic acid molecules, a phosphate backbone formed by Mg2+, magnesium fluoride, a small amount of magnesium oxide and magnesium hydroxide.

[0006] Preferably, the magnesium matrix is ​​any one of magnesium metal and magnesium alloy.

[0007] The present invention also provides another technical solution, a method for preparing a negative electrode surface film, the specific steps of which are as follows: first, a magnesium substrate is pre-fluorinated with a fluorination solution; then, a phytic acid solution is prepared and its pH value is adjusted; next, the pre-fluorinated magnesium substrate is immersed in the phytic acid solution with pH value adjusted for reaction; after the reaction is completed, the magnesium substrate is washed and dried to finally obtain the negative electrode surface film.

[0008] Preferably, the pre-fluorination process is as follows: the magnesium matrix is ​​immersed in a fluorination solution of 0.1-0.5 mol / L for 6-48 h, and then the magnesium matrix is ​​washed and dried to obtain the pre-fluorinated magnesium matrix.

[0009] Preferably, the concentration of the phytic acid solution is 0.1-0.5 g / L.

[0010] Preferably, the pH value of the phytic acid solution is 3-5.

[0011] Preferably, the reaction time of the pre-fluorinated magnesium matrix in phytic acid solution is 18-25 min.

[0012] Preferably, NaOH solution is used to adjust the phytic acid solution.

[0013] The present invention also provides another technical solution, an application of a negative electrode surface film layer, which is applied to the fields of magnesium primary batteries and biodegradable medical magnesium alloys.

[0014] Preferably, the negative electrode surface film is suitable for use in aqueous electrolytes and organic electrolytes in magnesium primary batteries.

[0015] Compared with existing technologies, the beneficial effects of this solution are as follows: (1) Composite membrane design for superior performance: The composite membrane in this technical solution consists of phytic acid molecules, Mg 2+ The resulting structure consists of a phosphate backbone, magnesium fluoride, a small amount of magnesium oxide, and magnesium hydroxide. This inorganic-organic composite structure overcomes the original single-layer problem of balancing physical shielding and dynamic protection through a dual mechanism of inhibiting electrolyte permeation and optimizing magnesium ion conduction pathways.

[0016] (2) Significantly improved corrosion resistance: In an aqueous electrolyte (1 mol / L Mg(ClO4)2), the corrosion current density of the AZ31B anode is 1.05 × 10⁻⁶.-4 A / cm 2 The corrosion current density of the FPP-A anode decreased to 1.30 × 10⁻⁶. -5 A / cm 2 The hydrogen evolution rate was significantly reduced, and the side reactions were significantly weakened, effectively solving the problem of severe self-corrosion of magnesium anode.

[0017] (3) High discharge specific capacity and good rate performance: In Mg / MnO2 full cells, at a current density of 50 mA / g, the discharge specific capacity of the FPP-A anode reaches 1997 mAh / g (based on the mass of manganese dioxide), which is much higher than the 799 mAh / g of the AZ31B anode; at a high current density of 250 mA / g, the FPP-A anode still has a discharge specific capacity of 1300 mAh / g, showing good rate performance. In Mg / SOCl2 full cells, at a current density of 50 mA / g, the discharge specific capacity of the FPP-A assembled battery is 18181 mAh / g, and the areal capacity is 27.34 mAh / cm². 2 It also significantly outperforms the AZ31B negative electrode assembly battery's 14176mAh / g and 19.19mAh / cm², and exhibits good rate performance at higher current densities.

[0018] (4) Enhanced discharge stability: Through intermittent discharge test (current density 100mA / g, discharge for 40min and then stand for 60min), it was found that the battery assembled with the modified negative electrode had more stable performance during intermittent discharge, while the battery assembled with the unmodified AZ31B negative electrode was prone to short circuit and other problems.

[0019] (5) Good storage performance: After being stored in air for 25 days, the capacity loss of the Mg / SOCl2 full cell assembled by FPP-A was only 13.33%, which was significantly lower than the 30.68% of the AZ31B negative electrode assembled cell. It has a low self-discharge rate and better storage performance.

[0020] (6) Simple process and low cost: The preparation method adopts a stepwise film formation process of prefluorination treatment and phytic acid solution reaction. It does not require complex solvothermal method or microwave-assisted synthesis and other harsh conditions. The operation is simple. The reagents used, such as potassium difluoride, have low toxicity, harmless by-products, low cost and easy to purchase, which greatly reduces the preparation cost.

[0021] (7) Mild conditions and easy to control: The temperature and time of prefluorination treatment, the concentration of phytic acid solution, pH value and other parameters can be flexibly controlled. For example, by adjusting the pH value of phytic acid solution to 3-5 by NaOH solution, the reaction rate can be slowed down, the self-assembly process can be effectively combined, and a uniform and dense film layer can be formed. The process is highly controllable.

[0022] (8) Applicable to a variety of electrolytes and fields: This negative electrode surface film is not only applicable to aqueous electrolytes and organic electrolytes in magnesium primary batteries, but can also be applied to the field of biodegradable medical magnesium alloys, adapting to the needs of different scenarios and having a wide range of application prospects. Attached Figure Description

[0023] Figure 1 The images shown are SEM and EDS images of the phytic acid film layer generated on the surface of a magnesium substrate in Example 1 of this invention. Figure 2 The images show the SEM and EDS images of the phytic acid film layer generated on the magnesium substrate surface in Comparative Example 1 of this invention. Figure 3 The images shown are SEM and EDS images of the phytic acid film layer generated on the magnesium substrate surface in Example 2 of this invention. Figure 4 The images shown are SEM and EDS images of the phytic acid film layer generated on the magnesium substrate surface in Comparative Example 2 of this invention. Figure 5 The images shown are SEM and EDS images of the negative electrode surface film prepared in Example 3 of this invention. Figure 6 The surface element distribution mapping spectrum of the negative electrode surface film prepared in Example 3 of the present invention; Figure 7 The polarization characteristics of the negative electrode surface film prepared in Examples 1-3 of the present invention and the AZ31B negative electrode of Comparative Example 3 are shown in the diagram. Figure 8 The discharge specific capacity diagrams of Mg / MnO2 full cells assembled with the negative electrode surface films prepared in Examples 1-3 of the present invention and the AZ31B negative electrode of Comparative Example 3 at different current densities are shown. Figure 9 Mapping spectra of the surface morphology and elemental distribution of the negative electrode in Mg / MnO2 full cells assembled from the negative electrode surface films prepared in Examples 1-3 of this invention and the AZ31B negative electrode in Comparative Example 3, respectively, after discharge. Figure 10 Discharge curves of Mg / SOCl2 full cells assembled with the negative electrode surface film prepared in Examples 1-3 of the present invention and the AZ31B negative electrode of Comparative Example 3 at different current densities. Figure 11 GITT curves of Mg / SOCl2 full cells assembled from the negative electrode surface films prepared in Examples 1-3 of the present invention and the AZ31B negative electrode of Comparative Example 3, respectively. Figure 12 Discharge curves of Mg / SOCl2 full cells assembled from the negative electrode surface films prepared in Examples 1-3 of the present invention and the AZ31B negative electrode of Comparative Example 3, respectively, before and after storage in air for 25 days. Figure 13 SEM images of Mg / SOCl2 full cells assembled from the negative electrode surface films prepared in Examples 1-3 of the present invention and the AZ31B negative electrode of Comparative Example 3, respectively, after discharge. Figure 14 The images show the EDS diagrams of Mg / SOCl2 full cells assembled from the negative electrode surface films prepared in Examples 1-3 of this invention and the AZ31B negative electrode in Comparative Example 3, respectively, after discharge. Detailed Implementation

[0024] The following detailed description illustrates the specific implementation method: Example 1 A method for preparing a phytic acid film on the surface of a negative electrode involves preparing a phytic acid solution with a concentration of 0.1-0.5 g / L, immersing a magnesium substrate in the phytic acid solution, reacting for 18-25 min, and then alternately washing with anhydrous ethanol and ultrapure water. After washing, the substrate is dried in an oven at 80°C for 20 min to obtain a phytic acid film layer formed on the surface of the magnesium substrate, denoted as PA-A. In this embodiment, a 0.5 g / L phytic acid solution is used for a reaction time of 20 min. If the reaction time is too short, the film layer cannot completely cover the surface of the magnesium substrate; if the reaction time is too long, the film layer will thicken, increasing the diffusion resistance of magnesium ions, and simultaneously exacerbating the side reactions between the magnesium negative electrode and the solution, increasing the loss of active material in the negative electrode, and possibly causing partial dissolution of the phytic acid film layer, leading to structural collapse. The magnesium substrate is a wrought magnesium alloy AZ31B, which has advantages such as lower cost and stronger corrosion resistance compared to rolled pure magnesium.

[0025] Example 2 A method for preparing a negative electrode surface film is optimized based on Example 2: First, the pH value of the phytic acid solution is adjusted using a 2 mol / L NaOH solution to adjust the pH value of the 0.5 g / L phytic acid solution prepared in Example 1 to 3-5. Then, the deformed magnesium alloy AZ31B is immersed in the solution and reacted for 20 min. Afterwards, it is washed alternately with anhydrous ethanol and ultrapure water. After washing, it is dried in an oven at 80°C for 20 min to obtain a phytic acid film layer formed on the magnesium substrate surface, denoted as PP-A. In this example, the pH value of the phytic acid solution is specifically adjusted to 5. If the reaction rate of the magnesium substrate in the phytic acid solution is too fast, a uniform and dense phytic acid film layer cannot be formed. By adjusting the pH of the phytic acid solution, the reaction rate can be controlled, resulting in a uniform and dense phytic acid film layer.

[0026] Example 3 A negative electrode surface film includes a magnesium substrate, on which a composite film is formed, comprising phytic acid molecules and Mg. 2+The formed phosphate skeleton, magnesium fluoride, a small amount of magnesium oxide and magnesium hydroxide, and the magnesium matrix can be any one of magnesium metal and magnesium alloy. In this embodiment, the magnesium matrix is ​​selected from wrought magnesium alloy AZ31B.

[0027] A method for preparing a negative electrode surface film is optimized based on Example 2: before forming a phytic acid film on the surface of wrought magnesium alloy AZ31B, it is pre-fluorinated. Specifically, the wrought magnesium alloy AZ31B is immersed in a fluorination solution with a concentration of 0.1-0.5 mol / L for 6-48 hours, then washed alternately with ultrapure water and anhydrous ethanol, and dried in an oven at 80°C for 15 minutes. The pre-fluorinated wrought magnesium alloy AZ31B is then immersed in a phytic acid solution with a concentration of 0.5 g / L and a pH of 5 prepared in Example 2, reacted for 20 minutes, and then removed. It is then washed alternately with anhydrous ethanol and ultrapure water, and dried in an oven at 80°C for 20 minutes to obtain the negative electrode surface film, denoted as FPP-A.

[0028] In this embodiment, a 0.1 mol / L KF·2H2O solution was selected as the fluorination solution for 12 hours of pre-fluorination treatment of the deformed magnesium alloy AZ31B. Potassium fluoride dihydrate was chosen primarily because of its high solubility in water, facilitating the preparation of high-concentration fluoride ion solutions; its low toxicity and harmless byproducts, combined with its low cost and ease of procurement, also contribute to this advantage. The pre-fluorination treatment time must be strictly controlled: if the time is too short, the reaction rate of magnesium in the potassium fluoride solution is slow, and the film layer cannot completely cover the magnesium substrate surface; if the time is too long, the film thickness will exceed the standard. While magnesium fluoride possesses strong magnesium ion conductivity, its electrical conductivity is weak, which can easily lead to a decrease in battery performance.

[0029] An application of a negative electrode surface film layer, applicable to magnesium primary batteries and biodegradable medical magnesium alloys, suitable for aqueous electrolyte and organic electrolyte systems of magnesium primary batteries.

[0030] Comparative Example 1 Unlike Example 1, a phytic acid solution with a concentration of 1 g / L was prepared, and the deformed magnesium alloy AZ31B was immersed in the phytic acid solution and reacted for 20 min. Then, it was washed alternately with anhydrous ethanol and ultrapure water. After washing, it was dried in an oven at 80°C for 20 min to obtain a phytic acid film layer formed on the surface of the magnesium substrate.

[0031] Comparative Example 2 Unlike Example 2, the pH of the phytic acid solution was first adjusted using a 2 mol / L NaOH solution to bring the pH of the 0.5 g / L phytic acid solution prepared in Example 1 to 7. The deformed magnesium alloy AZ31B was then immersed in the solution and reacted for 20 min. After that, it was washed alternately with anhydrous ethanol and ultrapure water. After washing, it was dried in an oven at 80°C for 20 min to obtain a phytic acid film layer formed on the surface of the magnesium substrate.

[0032] Comparative Example 3 Unlike Example 1, no treatment was performed on the deformed magnesium alloy AZ31B, which is referred to as AZ31B.

[0033] The performance of Examples 1-3 and Comparative Examples 1-3 was tested. (1) The morphology of the phytic acid film layer generated on the magnesium substrate in Example 1 and Comparative Example 1 was detected by scanning electron microscopy (SEM), and the elemental composition of the film layer surface was determined by EDS energy dispersive spectroscopy.

[0034] Depend on Figure 1 and Figure 2 It can be seen that in a 1 g / L phytic acid solution, due to the high concentration of phytic acid and the fast reaction rate, the surface film cannot be tightly assembled and exhibits a loose and porous structure. At a concentration of 0.5 g / L, the phytic acid film structure is relatively complete and uniform, and the surface film is mainly composed of Mg, O, P and C, among which the P element originates from the phosphate groups or hydrogen phosphate groups in the phytic acid solution.

[0035] (2) The morphology of the phytic acid film layer generated on the magnesium substrate in Example 2 and Comparative Example 2 was detected by scanning electron microscopy (SEM), and the elemental composition of the film layer surface was determined by EDS energy dispersive spectroscopy.

[0036] Depend on Figure 3 and Figure 4It can be seen that when the pH of the phytic acid solution is adjusted to 5 using a 2 mol / L NaOH solution, the thickness of the film formed by the reaction increases significantly, and the overall integrity and uniformity of the negative electrode surface are improved. This indicates that adjusting the pH of the phytic acid solution with NaOH solution can slow down the reaction rate, thereby promoting the effective bonding of the self-assembly process. The surface film is mainly composed of Mg, O, P, and C, with the P element originating from the phosphate or hydrogen phosphate groups in the phytic acid solution. After adjusting the pH of the reaction solution, the structure of the film becomes more stable, and the P element content on the negative electrode surface increases. Further increasing the pH of the phytic acid solution results in an uneven composition of the surface film formed by the reaction, and the P element content in the surface film is not abundant. This indicates that there is no positive feedback on the assembly process of the surface film under high pH conditions. This is because phytic acid molecules change the dissociation state of hydroxyl and phosphate groups under high pH conditions, leading to a decrease in their complexation ability with metal ions.

[0037] (3) The morphology of the negative electrode surface film prepared in Example 3 was detected by scanning electron microscopy (SEM), and the elemental distribution on the film surface was determined by EDS and mapping energy dispersive spectroscopy.

[0038] Depend on Figure 5 It can be seen that the negative electrode surface film prepared in Example 3 is mainly composed of phytic acid molecules and Mg 2+ The composite membrane consists of a phosphate backbone, metal fluoride (MgF2), and small amounts of magnesium oxide and magnesium hydroxide. The morphology of the composite membrane is more uniform, and the number of microcracks and the gaps between cracks are minimized, indicating a more stable structure. EDS analysis shows that the P and F content in the composite membrane is significant, indicating successful membrane coverage.

[0039] Figure 6 The mapping pattern shows that the P and F elements that make up the film are uniformly covered on the surface.

[0040] (4) The polarization characteristics of the negative electrode surface film prepared in Examples 1-3 and the AZ31B of Comparative Example 3 were tested in an aqueous electrolyte (1 mol / L Mg(ClO4)2).

[0041] Polarization characteristic test conditions: In a three-electrode electrolytic cell, PA-A, PP-A, FPP-A and AZ31B negative electrodes were used as working electrodes, saturated calomel electrode as reference electrode and graphite electrode as counter electrode. Tafel polarization test was performed on a CHI660F electrochemical workstation. The scan rate was 5mV / s and the potential was set to the open circuit voltage Eocp+300mV.

[0042] Depend on Figure 7It can be seen that in an aqueous electrolyte (1 mol / L Mg(ClO4)2), the corrosion current density of the AZ31B negative electrode is 1.05 × 10⁻⁶. -4 The corrosion current density of the FPP-A negative electrode is 1.30 × 10 A / cm². -5 With an A / cm², the hydrogen evolution rate is significantly reduced, and side reactions are weakened, while the corrosion current density of the PA-A anode is 8.06 × 10⁻⁶. -5 The corrosion current density of the PP-A negative electrode is 1.87 × 10 A / cm². -5 The lowest corrosion current density (A / cm²) of the composite film indicates its superior corrosion resistance.

[0043] (5) The negative electrode surface films prepared in Examples 1-3 and the AZ31B negative electrode of Comparative Example 3 were assembled into Mg / MnO2 full cells, respectively. The discharge performance under different current densities was tested, and the morphological characteristics and elemental distribution mapping of the negative electrode surface after discharge were analyzed. The assembly process of the Mg / MnO2 full cell is illustrated using Example 1 as an example. The assembly processes of Examples 2-3 and Comparative Example 3 are the same as those of Example 1 and will not be repeated. The specific assembly is as follows: PA-A is used as the negative electrode. Preparation of the positive electrode material: Commercial electrolytic manganese dioxide (MnO2, no additional treatment required) is taken and mixed evenly at a mass ratio of MnO2: conductive agent acetylene black: binder PVDF = 7:2:1. GF / D glass fiber is used as the separator, and 1 mol / L Mg(ClO4)2 solution is used as the electrolyte to complete the assembly of the full cell.

[0044] Tests were conducted at current densities of 50, 150, and 250 mA / g at room temperature. Figure 8 It can be seen that at a current density of 50 mA / g, the discharge specific capacity of the FPP-A anode reaches 1997 mAh / g (based on the mass of manganese dioxide), while the specific capacity of the AZ31B anode is only 799 mAh / g. At a high current density of 250 mA / g, the discharge specific capacity of the FPP-A anode is 1300 mAh / g, while the specific capacity of the AZ31B anode is 520 mAh / g.

[0045] Depend on Figure 9 It can be seen from the surface morphology and elemental analysis of different negative electrodes after discharge that, after discharge, AZ31B has a large number of cracks and discharge products on the surface due to side reactions and the damage caused by discharge. After the negative electrode is modified, the surface damage is less and there are fewer discharge products. The FPP-A negative electrode shows the best state, with the least surface damage after discharge, uniform surface element distribution and complete double-layer structure can be observed. The PA-A and PP-A negative electrodes are more damaged after discharge.

[0046] (6) The negative electrode surface films prepared in Examples 1-3 and the AZ31B negative electrode of Comparative Example 3 were assembled into Mg / SOCl2 full cells, and the following performance tests and characterizations were carried out on the system. a. Discharge performance at different current densities The assembly process of the Mg / SOCl2 full cell is illustrated using Example 1 as an example. The assembly processes of Examples 2-3 and Comparative Example 3 are the same as those of Example 1 and will not be repeated. The specific assembly is as follows: PA-A is used as the negative electrode. For the positive electrode material preparation: Ketjen black and PVDF binder are mixed at a mass ratio of 9:1, and ethanol is added to prepare a uniform dilute solution. The solution is then dropped onto nickel foam, dried in an oven, and the above operation is repeated until the Ketjen black loading reaches 2-3 mg / cm³. 2 The full cell was assembled using GF / D glass fiber as the separator and 2MAlCl3-0.8MMg(Cl)2 / SOCl2 solution as the electrolyte (denoted as AMS electrolyte).

[0047] Discharge performance was tested at current densities of 50, 100, 200, and 500 mA / g. Figure 10 It can be seen that the Mg / SOCl2 full cells assembled with PA-A, PP-A, and FPP-A electrodes exhibit excellent discharge specific capacity and rate performance. Specifically, at a current density of 50 mA / g, the Mg / SOCl2 full cell assembled with the AZ31B anode has a discharge specific capacity of 14176 mAh / g and an areal capacity of 19.19 mAh / cm³. 2 The FPP-A assembled Mg / SOCl2 full cell has a discharge specific capacity of 18181 mAh / g and an areal capacity of 27.34 mAh / cm³. 2 It also exhibits good rate performance at higher current densities.

[0048] b. Performance changes during intermittent discharge To investigate the discharge stability of the Mg / SOCl2 battery anode before and after modification, the performance changes of the battery during intermittent discharge were tested. Discharge and rest tests were performed on the electrode materials under constant current conditions: current density 100 mA / g, discharge for 40 min followed by rest for 60 min. The GITT test results are as follows: Figure 11 As shown, the battery with AZ31B negative electrode experienced a short circuit after approximately 120 hours of discharge. The battery with PA-A negative electrode had a stable intermittent discharge process exceeding 180 hours, but a micro-short circuit occurred during further discharge and eventually failed. The battery with PP-A negative electrode had an even longer stable intermittent discharge time, exceeding 270 hours, but it also experienced a micro-short circuit and eventually a short circuit. In contrast, the battery with FPP-A negative electrode showed no adverse conditions throughout the entire discharge process, demonstrating the good stability of the battery after negative electrode modification.

[0049] c. Storage performance testing The Mg / SOCl2 full cells assembled with the negative electrode surface films prepared in Examples 1-3 and the AZ31B negative electrode of Comparative Example 3 were stored in air for 25 days to test their capacity changes to reflect their self-discharge rate. The results are as follows: Figure 12 As shown, the self-discharge rate of the AZ31B negative electrode after battery storage is approximately 30.68%, while the self-discharge rates of PA-A and PP-A are approximately 21.30% and 17.02%, respectively. The self-discharge rate of FPP-A is the lowest at only 13.33%. The capacity decay caused by self-discharge has a positive feedback effect: product accumulation → increased polarization → increased side reactions → more product generation. The more stable the structure of the film layer, the more it can slow down the side reactions of the negative electrode and reduce self-discharge.

[0050] d. Surface morphology observation and EDS elemental analysis after battery discharge. The surface morphology of Mg / SOCl2 full cells assembled with the negative electrode surface films prepared in Examples 1-3 and the AZ31B negative electrode of Comparative Example 3 after discharge was observed, as follows: Figure 13 As shown, the negative electrode with the coating layer has a more uniform surface after discharge, with less accumulation of surface products and less surface damage. PA-A and PP-A are almost stripped off during discharge, while FPP-A still has a coating layer after discharge, showing good stability. In contrast, the surface of AZ31B is almost covered by discharge products, which greatly degrades the interface stability of the negative electrode. The surface elemental distribution of Mg / SOCl2 full cells assembled from the negative electrode surface films prepared in Examples 1-3 and the AZ31B negative electrode of Comparative Example 3 after discharge was analyzed, as follows: Figure 14 As shown, a large amount of O, S, Cl and other elements were detected on the surface of the AZ31B negative electrode after discharge. This indicates that the discharge products are composed of MgCl2, MgS and Mg(OH)2. These products covering the surface will hinder ion transport and weaken the battery performance. In contrast, fewer O, S and Cl elements were detected on the PA-A and PP-A negative electrodes after discharge, indicating that the film layer can effectively prevent the accumulation of discharge products. However, the P element content decreased significantly, indicating that the film layer was largely damaged. On the other hand, obvious P and F elements were detected on the surface of FPP-A, indicating the integrity of the surface film layer.

[0051] The above descriptions are merely embodiments of the present invention, and common knowledge such as specific technical solutions and / or characteristics are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for preparing a negative electrode surface film, characterized in that: A negative electrode surface film includes a magnesium substrate, on the surface of which a composite film is formed, the composite film including phytic acid molecules and Mg 2+ The resulting structure consists of a phosphate backbone, magnesium fluoride, a small amount of magnesium oxide, and magnesium hydroxide. The specific steps are as follows: First, the magnesium substrate is pre-fluorinated with a fluorination solution for 6-48 hours. Then, a phytic acid solution is prepared with a concentration of 0.1-0.5 g / L, and its pH value is adjusted to 3-5. Next, the pre-fluorinated magnesium substrate is immersed in the pH-adjusted phytic acid solution for reaction. After the reaction, the magnesium substrate is washed and dried to finally obtain the negative electrode surface film.

2. The method for preparing a negative electrode surface film according to claim 1, characterized in that: The magnesium matrix can be any of magnesium metal and magnesium alloy.

3. The method for preparing a negative electrode surface film according to claim 1, characterized in that: The pre-fluorination process is as follows: the magnesium matrix is ​​immersed in a fluorination solution of 0.1-0.5 mol / L, and then the magnesium matrix is ​​washed and dried to obtain the pre-fluorinated magnesium matrix.

4. The method for preparing a negative electrode surface film according to claim 3, characterized in that: The reaction time of the pre-fluorinated magnesium matrix in phytic acid solution is 18-25 min.

5. The method for preparing a negative electrode surface film according to claim 4, characterized in that: NaOH solution was used to adjust the phytic acid solution.

6. An application of a negative electrode surface film layer, characterized in that: The negative electrode surface film prepared according to any one of claims 1-5 is applied in the field of magnesium primary batteries.

7. The application of the negative electrode surface film layer according to claim 6, characterized in that: The negative electrode surface film is suitable for use in aqueous and organic electrolytes in magnesium primary batteries.

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