Cathode material for sodium-ion battery with low-sodium phase, preparation method thereof, and sodium-ion battery

Abstract

The present invention provides a method for preparing a cathode material for a low-sodium-phase sodium-ion battery. By first sealing and heating a layered sodium metal oxide cathode material and XPF6, the gas decomposed from XPF6 is used to etch the surface of the cathode material to construct a protective film, and then the sodium-ion battery cathode material after surface modification is subjected to electrochemical sodiation removal treatment to control the sodium content, thereby forming a stable low-sodium-phase sodium-ion battery cathode material, and the cycle performance of the obtained low-sodium-phase sodium-ion battery cathode material is significantly improved. The present invention also provides a low-sodium-phase sodium-ion battery cathode material prepared by the above preparation method and a sodium-ion battery based on the low-sodium-phase sodium-ion battery cathode material.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium-ion battery materials, and specifically relates to the modification of a low-sodium-phase cathode material for sodium-ion batteries. Background Art

[0002] In the research of sodium-ion batteries, the stability and cycling performance of cathode materials have always been key issues. O3-type sodium-ion cathode materials have attracted much attention due to their high theoretical capacity and excellent sodium-ion conductivity. However, in practical applications, such materials are prone to phase transformation and structural collapse during charge-discharge cycles, resulting in rapid attenuation of battery capacity. It has been found that the sodium content in the cathode material has a significant impact on its structural stability and electrochemical performance. At a lower sodium content, the material is more likely to form a stable phase structure, thereby improving its cycle life and overall performance. Therefore, how to effectively control the sodium content in the cathode material of sodium-ion batteries and achieve the preparation of a low-sodium phase with high stability has become one of the key technologies to improve the performance of sodium-ion batteries.

[0003] Currently, the traditional method is mainly to regulate the sodium content by reducing the amount of sodium source used during the synthesis process. However, this method usually makes it difficult to precisely control the sodium content, resulting in poor electrochemical performance of the material. Summary of the Invention

[0004] In view of the above technical problems, the present application provides a low-sodium-phase cathode material for sodium-ion batteries, its preparation method, and a sodium-ion battery.

[0005] To achieve the above object, the present application proposes the following technical solutions:

[0006] In the first aspect, a preparation method of a low-sodium-phase cathode material for sodium-ion batteries is provided, including:

[0007] S1. Sealing a layered sodium metal oxide cathode material and XPF 6 in a reactor, and the layered sodium metal oxide cathode material and XPF 6 do not directly contact, then heating and maintaining the reactor at a constant temperature, cooling after the holding reaction, to obtain a cathode material for sodium-ion batteries with a protective film on the surface; where X is Na and / or Li;

[0008] S2. Preparing the cathode material for sodium-ion batteries with a protective film on the surface into a pole piece, soaking the pole piece in an organic electrolyte solution, using the pole piece as the positive electrode and an inert electrode as the negative electrode, and performing constant-current charging under a protective atmosphere, controlling the charging duration according to the target sodium removal amount;

[0009] S3. After the sodium removal treatment is completed, taking out the pole piece, washing and drying it to obtain a low-sodium-phase cathode material for sodium-ion batteries.

[0010] Further, the chemical formula of the layered sodium metal oxide cathode material is NaMn y Fe z Ni w O 2 , where 0 < y ≤ 0.5, 0 ≤ z ≤ 0.5, 0 < w ≤ 0.8, and y + z + w = 1.

[0011] Further, the low-sodium-phase sodium-ion battery cathode material includes a matrix and an interfacial layer on the surface of the matrix; the chemical formula of the matrix is Na x Mn y Fe z Ni w O 2 , where 0.65 ≤ x ≤ 0.9, 0 < y ≤ 0.5, 0 ≤ z ≤ 0.5, 0 < w ≤ 0.8, and y + z + w = 1.

[0012] Further, in step S1, the amount of the XPF 6 is 0.1 wt% - 1.5 wt% of the layered sodium metal oxide cathode material.

[0013] Further, in step S1, the temperature for heat preservation is 100 - 180 °C, and the time for heat preservation is 10 - 18 h.

[0014] Further, the organic electrolyte includes an organic solvent and a sodium salt; the organic solvent is a mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC); the sodium salt is NaPF 6 ; the concentration of the sodium salt in the organic electrolyte is 1 - 1.5 mol / L.

[0015] Further, in step S2, the current density for constant-current charging is 0.1 - 0.17 mA / g; the end voltage for constant-current charging is 2.65 - 3.0 V.

[0016] Further, in step S2, after the sodium-ion battery cathode material with a protective film on its surface is immersed in the organic electrolyte, the electrolytic cell containing the organic electrolyte is sealed, and nitrogen or an inert gas is continuously introduced into the electrolytic cell to maintain a protective atmosphere inside the electrolytic cell.

[0017] Further, in step S2, the electrolysis system further includes a reference electrode; the negative electrode is a platinum sheet or a carbon rod; the reference electrode is an Ag / AgCl electrode.

[0018] In a second aspect, a low-sodium-phase sodium-ion battery cathode material is provided, which is prepared by the foregoing preparation method.

[0019] In a third aspect, a sodium-ion battery is provided, which includes the foregoing low-sodium-phase sodium-ion battery cathode material.

[0020] Compared with the prior art, one or more of the above technical solutions can achieve at least one of the following beneficial effects:

[0021] By firstly combining the sodium ion battery positive electrode material and XPF 6 Sealed Heating, XPF 6 The generated phosphorus pentafluoride (PF 5 ) gas is used to etch the surface of the positive electrode material to construct a uniform and dense fluorine-containing artificial solid electrolyte interface (SEI) protective film to enhance the structural stability of the positive electrode material. The surface-modified sodium-ion battery positive electrode material is then placed in an electrolytic cell to control the sodium content by controlling the electrochemical sodium removal treatment, thereby forming a stable low-sodium phase. The protective film pre-formed on the surface of the positive electrode material during the electrolysis process can effectively inhibit the damage of the electrolyte to the internal material during the subsequent charging process and the occurrence of structural phase changes in the positive electrode material. It can achieve the adjustment of the target sodium content without destroying the structure of the positive electrode material. The formed low-sodium phase structure has higher stability, which reduces the capacity attenuation caused by structural changes in the material during the charging and discharging process, thereby greatly improving the cycle life of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0023] Figure 1 This is the XRD pattern of the layered oxide positive electrode material prepared in step (2) of Example 1.

[0024] Figure 2 TEM image of the positive electrode material coated with the artificial SEI film prepared in step (3) of Example 1.

[0025] Figure 3 This is the XPS image of the positive electrode material coated with the artificial SEI film prepared in step (3) of Example 1.

[0026] Figure 4 This is a TEM image of the low-sodium layered oxide positive electrode material prepared in step (7) of Example 1.

[0027] Figure 5 This is a cycle curve diagram of a battery assembled with the positive electrode material of Example 1 at a current density of 170 mA / g.

[0028] Figure 6 This is a cycle curve diagram of a battery assembled with the positive electrode material of Comparative Example 1 at a current density of 170 mA / g.

[0029] Figure 7 Cycling curve of the battery assembled with the cathode material of Comparative Example 2 at a current density of 170 mA / g.

[0030] Figure 8 Cycling curve of the battery assembled with the cathode material of Example 2 at a current density of 170 mA / g.

[0031] Figure 9 Cycling curve of the battery assembled with the cathode material of Example 3 at a current density of 170 mA / g.

[0032] Figure 10 Cycling curve of the battery assembled with the cathode material of Example 4 at a current density of 170 mA / g. Detailed implementation mode

[0033] The present invention provides a method for preparing a cathode material for a low-sodium-phase sodium-ion battery, comprising:

[0034] S1. Sealing a layered sodium metal oxide cathode material and XPF 6 in a reactor, and the layered sodium metal oxide cathode material and XPF 6 do not directly contact, then heating and insulating the reactor, cooling after the holding reaction, to obtain a sodium-ion battery cathode material with a protective film on the surface; wherein X is Na and / or Li;

[0035] S2. Preparing the sodium-ion battery cathode material with a protective film on the surface into a pole piece, soaking the pole piece in an organic electrolyte, using the pole piece as the positive electrode and an inert electrode as the negative electrode, and performing constant-current charging under a protective atmosphere, controlling the charging duration according to the target sodium deintercalation amount;

[0036] S3. After the sodium deintercalation treatment is completed, taking out the pole piece, washing and drying, to obtain a low-sodium-phase sodium-ion battery cathode material.

[0037] In this technical solution, in step S1, a pre-surface etching treatment is carried out, by heating sodium hexafluorophosphate (NaPF 6 ) or lithium hexafluorophosphate (LiPF 6 ), the generated PF 5The gas is used to treat the material surface, and a uniform and dense artificial SEI film is constructed through a solid-gas reaction; then, the sodium stripping in step S2 is carried out. By the method of precisely regulating the sodium stripping process through electrolysis, the precise control of the sodium content can be achieved without damaging the material structure, thereby obtaining a stable low-sodium phase. The protective film pre-formed on the surface of the cathode material can effectively inhibit the damage of the electrolyte to the interior of the material and the occurrence of structural phase change of the cathode material during the charging process. Without damaging the structure of the cathode material, the adjustment of the target sodium content can be realized. The formed low-sodium phase structure has higher stability and can reduce the capacity attenuation caused by structural changes during charge and discharge of the material, thereby greatly improving the cycle life and overall performance of the battery. By combining etching and precise regulation of sodium stripping in the electrolytic cell, not only the surface properties of the cathode material are optimized, but also the effective preparation of a high-stability low-sodium phase is realized, providing a practical solution for the long-life and high-performance application of sodium-ion batteries.

[0038] In some preferred embodiments, the chemical formula of the layered sodium metal oxide cathode material is NaMn y Fe z Ni w O 2 , 0 < y ≤ 0.5, 0 ≤ z ≤ 0.5, 0 < w ≤ 0.8, y + z + w = 1.

[0039] In some preferred embodiments, the low-sodium phase sodium-ion battery cathode material includes a matrix and an interface layer on the surface of the matrix; the chemical formula of the matrix is Na x Mn y Fe z Ni w O 2 , where 0.65 ≤ x ≤ 0.9, 0 < y ≤ 0.5, 0 ≤ z ≤ 0.5, 0 < w ≤ 0.8, y + z + w = 1; the interface layer at least includes sodium fluoride.

[0040] The sodium-ion battery cathode material can be prepared by conventional preparation methods in the art, including co-precipitation method, sol-gel method, spray drying method, solid-phase method, etc.

[0041] In some preferred embodiments, in step S1, the amount of XPF 6 is 0.1 wt% - 1.5 wt% of the layered sodium metal oxide cathode material, such as 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, etc.

[0042] In some preferred embodiments, in step S1, the temperature for heat preservation is 100~180°C, such as 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, etc., and the time for heat preservation is 10~18h, such as 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, etc.

[0043] In some preferred embodiments, the organic electrolyte includes an organic solvent and a sodium salt; the organic solvent is a mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC); the sodium salt is NaPF 6 ; the concentration of the sodium salt in the organic electrolyte is 1~1.5 mol / L, such as 1 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, etc.

[0044] In some preferred embodiments, in step S2, the current density for constant current charging is 0.1~0.17 mA / g, such as 0.1 mA / g, 0.11 mA / g, 0.12 mA / g, 0.13 mA / g, 0.14 mA / g, 0.15 mA / g, 0.16 mA / g, 0.17 mA / g, etc.

[0045] In some preferred embodiments, in step S2, the end voltage for constant current charging is 2.65~3.0V, such as 2.7V, 2.75V, 2.8V, 2.85V, 2.9V, 2.95V, 3.0V, etc., and the end voltage is determined according to the amount of sodium desorption. The initial voltage for constant current charging can be set conventionally.

[0046] In some preferred embodiments, in step S2, after soaking the sodium-ion battery cathode material with a protective film on its surface in the organic electrolyte, the electrolytic cell containing the organic electrolyte is sealed, and nitrogen or an inert gas is continuously introduced into the electrolytic cell to maintain a protective atmosphere inside the electrolytic cell.

[0047] In some preferred embodiments, in step S2, the electrolysis system further includes a reference electrode; the negative electrode is a platinum sheet or a carbon rod; the reference electrode is an Ag / AgCl electrode.

[0048] The present invention also provides a low-sodium-phase sodium-ion battery cathode material, which is prepared by the aforementioned preparation method.

[0049] The present invention also provides a sodium-ion battery, including the aforementioned low-sodium-phase sodium-ion battery cathode material.

[0050] To facilitate the understanding of the present invention, the present invention will be described more comprehensively and meticulously below in conjunction with the accompanying drawings of the specification and preferred embodiments. However, the protection scope of the present invention is not limited to the following specific embodiments.

[0051] Example 1

[0052] A method for preparing a cathode material for a high-stability low-sodium-phase sodium-ion battery, comprising the following steps:

[0053] (1) According to the molar ratio of each metal element in NaNi 0.33 Fe 0.33 Mn 0.33 O 2 0.33 mmol of nickel oxide, 0.33 mmol of manganese dioxide, and 0.33 mmol of iron oxide were evenly ground in a mortar until there was no obvious granular feeling to obtain a mixed metal oxide powder. Weigh 1.3 times the molar amount of the mixed metal oxide powder of oxalic acid powder and place it in deionized water to obtain an oxalic acid solution, and then add the mixed metal oxide powder, stir at room temperature for 8 h to obtain a reddish-brown solution; then continuously stir at 80 °C until the solvent evaporates to obtain a sol, and dry the obtained sol in an oven at 120 °C for 12 h to obtain a reddish-brown precursor powder.

[0054] (2) The precursor powder obtained in step (1) and sodium in sodium carbonate are proportioned according to a molar ratio of 1:1.10, ball-milled and mixed for 3 h at a rotation speed of 400 rpm, and then the mixed powder is placed in a tube furnace and kept at 450 °C for 5 h, and then heated to 900 °C at a heating rate of 3 °C / min and kept at this temperature for 12 h. Oxygen is passed through during the whole sintering process to obtain a layered oxide cathode material. Its XRD pattern is as Figure 1 shown. It can be seen that the obtained is an O3-type layered oxide cathode material. Combining the raw material composition, its chemical formula is NaNi 0.33 Fe 0.33 Mn 0.33 O 2 .

[0055] (3) In the glove box, the O3-type layered oxide cathode material and NaPF 6 are placed in two 5 mL open polytetrafluoroethylene bottles according to a mass ratio of 1:0.005 to avoid contact between the two. Then, the two open polytetrafluoroethylene bottles containing the materials are placed in a 500 mL polytetrafluoroethylene bottle and sealed in a hydrothermal autoclave reactor. The hydrothermal autoclave reactor is placed in an oven and reacted at 100 °C for 10 h, and then naturally cooled to obtain a modified cathode material. Figure 2 is the TEM image of the modified cathode material. It can be seen from the figure that a dense and uniform film layer with a thickness of about 5 nm is coated on the surface of the cathode material. After analysis, the formation of this film layer is through heating sodium hexafluorophosphate (NaPF 6), the generated PF 5 The gas is used to treat the material surface and is constructed through a solid-gas reaction. This film layer is a uniform and dense artificial SEI film. XPS analysis is performed on the surface of the modified cathode material, and the results are as Figure 3 shown. From Figure 3 it can be seen that the obtained artificial SEI film is NaF.

[0056] (4) Coating the etched cathode material on the aluminum foil and immersing it in the electrolyte in a sealed electrolytic cell. The electrolyte is NaPF 6 dissolved in a mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volume ratio of 1:1 at a concentration of 1M.

[0057] (5) Configure a three-electrode system in the electrolytic cell, including a working electrode, a counter electrode, and a reference electrode. The working electrode is the cathode material, the counter electrode is a carbon rod, and the reference electrode is an Ag / AgCl electrode. And the electrolytic cell is connected to an external nitrogen device to maintain an anhydrous and anaerobic environment in the electrolytic cell by continuously introducing nitrogen.

[0058] (6) Connect the electrochemical workstation to the electrodes of the electrolytic cell, select the constant current mode, control the current density to be 0.1 mA / g, and perform charging. The charging duration is controlled according to the target sodium removal amount.

[0059] (7) When the charging reaches a voltage of 2.83 V (corresponding to a sodium removal amount of 10%), take out the electrode from the electrolyte, rinse the electrode surface with anhydrous DMC solvent to remove the residual electrolyte, and then dry the electrode sheet in a vacuum drying oven at 60 °C. The obtained material is the layered oxide cathode material with a low sodium phase, and its TEM image is as Figure 4 shown. From Figure 4 it can be seen that the layered oxide cathode material with a low sodium phase includes a matrix material and an interfacial layer on the surface of the matrix material. Since the interfacial layer exists before and after electrolytic sodium removal, and the interfacial layer before electrolytic sodium removal is NaF, which is an inert material in the electrolyte and can avoid the reaction between the matrix material and the electrolyte. Therefore, it is speculated that the interfacial layer at least includes NaF of the interfacial layer before sodium removal.

[0060] The layered oxide cathode material with a low sodium phase prepared in Example 1 is respectively made into positive electrode sheets, with metallic sodium as the negative electrode, and button cells are assembled respectively. The button cells are subjected to charge and discharge tests at 25 °C within a voltage range of 2 V to 4 V.

[0061] Figure 5 is the cycle curve graph of the battery assembled with the low sodium phase cathode material prepared in Example 1 at a current density of 170 mA / g. From Figure 5It can be seen that in the voltage range of 2V to 4V, after 50 cycles, the specific capacity is 93.5 mAh / g, and from Figure 5 It can also be seen that the first charge-discharge efficiency of the battery is higher than 100%, and the charge-discharge efficiency during subsequent cycles is also as high as 100%. After analysis, this may be because some sodium has been removed from the cathode material to form a low-sodium-phase cathode material. In the sodium-deficient state, the material may undergo structural adjustments, such as changes in the layer spacing or rearrangement of oxygen atoms, to adapt to the low-sodium content state. This structural change may release more active sites, enabling more sodium ions to be embedded additionally during the discharge process.

[0062] Comparative Example 1

[0063] (1) According to the molar ratio of each metal element in NaNi 0.33 Fe 0.33 Mn 0.33 O 2 0.33 mmol of nickel oxide, 0.33 mmol of manganese dioxide, and 0.33 mmol of iron oxide were evenly ground in a mortar until there was no obvious particle feeling to obtain a mixed metal oxide powder. 1.2 times the molar amount of oxalic acid powder of the mixed metal oxide powder was weighed and placed in deionized water to obtain an oxalic acid solution. Then, the mixed metal oxide powder was added, and stirring was carried out at room temperature for 10 h to obtain a reddish-brown solution; then continuous stirring was carried out at 80 °C until the solvent evaporated to obtain a sol. The obtained sol was dried in an oven at 120 °C for 12 h to obtain a reddish-brown precursor powder.

[0064] (2) The precursor powder obtained in step (1) was mixed with sodium in sodium carbonate at a molar ratio of 1:1.15, ball-milled for 4 h at a rotation speed of 450 rpm, and then the mixed powder was placed in a tubular furnace and kept at 400 °C for 3 h, and then heated to 900 °C at a heating rate of 3 °C / min and kept at this temperature for 12 h. Oxygen was passed through during the whole sintering process to obtain the O3-type layered oxide cathode material NaNi 0.33 Fe 0.33 Mn 0.33 O 2 .

[0065] (3) In the glove box, the O3-type layered oxide cathode material and NaPF 6 were placed in two 5 mL open polytetrafluoroethylene bottles at a mass ratio of 1:0.003 respectively to avoid contact between the two. Then, the two open polytetrafluoroethylene bottles containing the materials were placed in a 500 mL polytetrafluoroethylene bottle and sealed in a hydrothermal autoclave reactor. The hydrothermal autoclave reactor was placed in an oven and reacted at 120 °C for 10 h, and then naturally cooled to obtain a cathode material with an artificial SEI film.

[0066] The cathode materials prepared in Comparative Example 1 were respectively made into cathode sheets, with metallic sodium as the anode, and button cells were assembled respectively. The button cells were subjected to charge and discharge tests at 25 °C within a voltage range of 2 V to 4 V.

[0067] Figure 6 Fig. is the cycle curve of the battery assembled with the cathode material of Comparative Example 1 at a current density of 170 mA / g. From Figure 6 it can be seen that within the voltage range of 2 V to 4 V, after 50 cycles, the specific capacity is 75.6 mAh / g.

[0068] By comparing the cycle curves of the batteries assembled with the cathode materials prepared in Example 1 and Comparative Example 1, it was found that the cathode material prepared in Example 1 had better cycle performance, indicating that by controlling the electrochemical sodium deintercalation treatment to control the sodium content after forming the SEI film, the obtained low-sodium-phase cathode material had better cycle performance, so the obtained was a high-stability low-sodium-phase cathode material. After analysis, this may be because by first forming a dense film layer through a gas-solid reaction and then performing electrolytic sodium deintercalation, the adjustment of the target sodium content can be achieved without damaging the structure of the cathode material, thereby forming a stable low-sodium phase. The protective film pre-formed on the surface of the cathode material during the electrolysis process can effectively inhibit the damage of the electrolyte to the interior of the material and the occurrence of structural phase transformation of the cathode material during the subsequent charging process. The formed low-sodium-phase structure has higher stability, reducing the capacity attenuation caused by structural changes of the material during charge and discharge, thus greatly improving the cycle life of the battery.

[0069] Comparative Example 2

[0070] (1) According to the molar ratios of the respective metal elements in NaNi 0.33 Fe 0.33 Mn 0.33 O 2 0.33 mmol of nickel oxide, 0.33 mmol of manganese dioxide, and 0.33 mmol of iron oxide were uniformly ground in a mortar until there was no obvious granularity to obtain a mixed metal oxide powder. Weigh 1.2 times the molar amount of oxalic acid powder of the mixed metal oxide powder and place it in deionized water to obtain an oxalic acid solution, then add the mixed metal oxide powder, and stir at room temperature for 8 h to obtain a reddish-brown solution; then continuously stir at 80 °C until the solvent evaporates to obtain a sol, and dry the obtained sol in an oven at 120 °C for 12 h to obtain a reddish-brown precursor powder.

[0071] (2) Mix the precursor powder obtained in step (1) with sodium in sodium carbonate at a molar ratio of 1:1.10, ball-mill for 3 h at a rotation speed of 400 rpm, then place the mixed powder in a tubular furnace, keep it at 450 °C for 5 h, and then heat it to 900 °C at a heating rate of 3 °C / min and keep it for 12 h. Oxygen is passed through during the whole sintering process to obtain the O3-type layered oxide cathode material NaNi 0.33 Fe 0.33 Mn 0.33 O 2 .

[0072] (3) Coat the cathode material on aluminum foil and soak it in the electrolyte in a sealed electrolytic cell. The electrolyte is NaPF 6 dissolved in a mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 at a concentration of 1 M.

[0073] (4) Configure a three-electrode system in the electrolytic cell, including a working electrode, a counter electrode, and a reference electrode. The working electrode is the cathode material, the counter electrode is a carbon rod, and the reference electrode is an Ag / AgCl electrode. The electrolytic cell is connected to an external nitrogen device, and an anhydrous and anaerobic environment in the electrolytic cell is maintained by continuously introducing nitrogen.

[0074] (5) Connect an electrochemical workstation to the electrodes of the electrolytic cell, select the constant current mode, control the current density at 0.1 mA / g, and perform charging. The charging duration is controlled according to the target sodium deintercalation amount.

[0075] (6) When the charging voltage reaches 2.87 V (corresponding to a sodium deintercalation amount of 15%), take out the electrode from the electrolyte, rinse the electrode surface with anhydrous DMC solvent to remove the residual electrolyte, and then dry the electrode sheet in a vacuum drying oven at 60 °C. The obtained material is the low-sodium phase layered oxide cathode material.

[0076] Prepare the low-sodium phase layered oxide cathode material prepared in Comparative Example 2 into cathode sheets respectively, use metallic sodium as the anode, and assemble coin cells respectively. The coin cells are subjected to charge and discharge tests at 25 °C in the voltage range of 2 V to 4 V.

[0077] Figure 7 is the cyclic voltammetry curve of the cell assembled with the low-sodium phase cathode material prepared in Comparative Example 2 at a current density of 170 mA / g. From Figure 7 it can be seen that in the voltage range of 2 V to 4 V, after 50 cycles, the specific capacity is 59.9 mAh / g.

[0078] By comparing the cyclic voltammograms of the batteries assembled with the cathode materials prepared in Example 1 and Comparative Example 2, it was found that the cathode material prepared in Example 1 had better cycling performance, indicating that the cathode material obtained by electrochemical sodium deintercalation after forming the SEI film was more structurally stable than that obtained by direct electrochemical sodium deintercalation. After analysis, this may be because by first forming a dense film layer through a gas-solid reaction and then performing electrolytic sodium deintercalation, it is possible to adjust the target sodium content without destroying the structure of the cathode material, thereby forming a stable low-sodium-phase cathode material. In contrast, direct electrolytic sodium deintercalation results in the erosion of the surface and internal structure of the cathode material by the electrolyte during sodium deintercalation, causing damage to the structure of the cathode material, and thus the obtained low-sodium-phase layered oxide cathode material has poor cycling performance.

[0079] Example 2

[0080] (1) According to the molar ratios of the various metal elements in NaNi 0.40 Fe 0.20 Mn 0.40 O 2 0.40 mmol of nickel oxide, 0.40 mmol of manganese dioxide, and 0.20 mmol of iron oxide were evenly ground in a mortar until there was no obvious granularity, obtaining a mixed metal oxide powder. 1.1 times the molar amount of oxalic acid powder of the mixed metal oxide powder was weighed and placed in deionized water to obtain an oxalic acid solution, and then the mixed metal oxide powder was added. Stirring was carried out at room temperature for 12 h to obtain a reddish-brown solution; continuous stirring was then carried out at 80 °C until the solvent evaporated to obtain a sol. The obtained sol was dried in an oven at 120 °C for 12 h to obtain a reddish-brown precursor powder.

[0081] (2) The precursor powder obtained in step (1) was mixed with sodium in sodium carbonate at a molar ratio of 1:1.15 and ball-milled for 3 h at a rotation speed of 400 rpm. Then the mixed powder was placed in a tubular furnace and kept at 450 °C for 5 h, and then heated to 950 °C at a heating rate of 3 °C / min and kept at this temperature for 12 h. Oxygen was passed through during the entire sintering process, thus obtaining the O3-type layered oxide cathode material NaNi 0.40 Fe 0.20 Mn 0.40 O 2 .

[0082] (3) In a glove box, the O3-type layered oxide cathode material and NaPF 6 were respectively placed in two 5 mL open polytetrafluoroethylene bottles at a mass ratio of 1:0.001 to avoid contact between them. Then the two open polytetrafluoroethylene bottles containing the materials were placed in a 500 mL polytetrafluoroethylene bottle and sealed in a hydrothermal autoclave reactor. The hydrothermal autoclave reactor was placed in an oven and reacted at 180 °C for 12 h, and then cooled naturally to obtain a cathode material with an artificial SEI film.

[0083] (4) Coating the etched cathode material on the aluminum foil and immersing it in the electrolyte in a sealed electrolytic cell. The electrolyte is NaPF 6 Dissolved in a mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 at a concentration of 1M.

[0084] (5) Configuring a three-electrode system in the electrolytic cell, including a working electrode, a counter electrode, and a reference electrode. The working electrode is the cathode material, the counter electrode is a carbon rod, and the reference electrode is an Ag / AgCl electrode. And the electrolytic cell is connected to an external nitrogen device to maintain an anhydrous and anaerobic environment in the electrolytic cell by continuously introducing nitrogen.

[0085] (6) Connecting an electrochemical workstation to the electrodes of the electrolytic cell, selecting the constant current mode with a current density of 0.1 mA / g, and performing constant current charging. The charging duration is controlled according to the target sodium deintercalation amount.

[0086] (7) When the charging voltage reaches 2.89 V (corresponding to a sodium deintercalation amount of 20%), taking out the electrode from the electrolyte, rinsing the electrode surface with anhydrous DMC solvent to remove the residual electrolyte, and then drying the electrode sheet in a vacuum drying oven at 60 °C. What is obtained is the layered oxide cathode material with a low sodium phase.

[0087] The cathode materials prepared in Example 2 were respectively made into cathode sheets, with metallic sodium as the anode, and button cells were assembled respectively. The button cells were subjected to charge and discharge tests at 25 °C in the voltage range of 2 V to 4 V.

[0088] Figure 8 It is the cyclic voltammogram of the battery assembled with the cathode material of Example 2 at a current density of 170 mA / g. From Figure 8 It can be seen that in the voltage range of 2 V to 4 V, after 50 cycles, it has a specific capacity of 101.5 mAh / g, indicating that the prepared layered oxide cathode material with a low sodium phase has the characteristic of high structural stability.

[0089] Example 3

[0090] (1) According to NaNi 0.40 Fe 0.20 Mn 0.40 O 2The molar ratios of the various metal elements. 0.40 mmol of nickel oxide, 0.40 mmol of manganese dioxide, and 0.20 mmol of iron oxide were uniformly ground in a mortar until there was no obvious sense of particles, obtaining a mixed metal oxide powder. Weighing oxalic acid powder with a molar ratio of 1.4:1 to the mixed metal oxide powder and placing it in deionized water to obtain an oxalic acid solution, then adding the mixed metal oxide powder, and stirring at room temperature for 10 h to obtain a reddish-brown solution; then continuously stirring at 80 °C until the solvent evaporated to obtain a sol, and drying the obtained sol in an oven at 120 °C for 12 h to obtain a reddish-brown precursor powder.

[0091] (2) Mixing the precursor powder obtained in step (1) with sodium in sodium carbonate in a molar ratio of 1:1.20, ball-milling and mixing for 3 h at a rotation speed of 400 rpm, then placing the mixed powder in a tubular furnace, keeping it at 450 °C for 5 h, and then heating it to 800 °C at a heating rate of 3 °C / min and keeping it for 18 h. Oxygen was passed through during the whole sintering process, and thus the O3-type layered oxide cathode material NaNi 0.40 Fe 0.20 Mn 0.40 O 2 .

[0092] (3) In a glove box, the O3-type layered oxide cathode material and NaPF 6 were respectively placed into two 5 mL open polytetrafluoroethylene bottles in a mass ratio of 1:0.010 to avoid their contact. Then, the two open polytetrafluoroethylene bottles containing the materials were placed into a 500 mL polytetrafluoroethylene bottle and sealed in a hydrothermal autoclave reactor. The hydrothermal autoclave reactor was placed in an oven and reacted at 150 °C for 15 h, and then naturally cooled to obtain a cathode material with an artificial SEI film.

[0093] (4) Coating the etched cathode material on an aluminum foil and soaking it in an electrolyte in a sealed electrolytic cell. The electrolyte was NaPF 6 dissolved in a mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 at a concentration of 1 M.

[0094] (5) Configuring a three-electrode system in the electrolytic cell, including a working electrode, a counter electrode, and a reference electrode. The working electrode was the cathode material, the counter electrode was a carbon rod, and the reference electrode was an Ag / AgCl electrode. And the electrolytic cell was connected to an external nitrogen device, and an anhydrous and anaerobic environment in the electrolytic cell was maintained by continuously introducing nitrogen.

[0095] (6) Connecting an electrochemical workstation to the electrodes of the electrolytic cell, selecting the constant current mode, with a current density of 0.1 mA / g, and performing constant current charging. The charging duration was controlled according to the target sodium deintercalation amount.

[0096] (7) When charged to a voltage of 2.93 V (corresponding to a Na extraction amount of 30%), the electrode was taken out of the electrolyte, and the surface of the electrode was rinsed with anhydrous DMC solvent to remove the residual electrolyte, and then the electrode sheet was dried at 60 °C in a vacuum drying oven. The obtained product was the layered oxide cathode material with high stability and low sodium phase.

[0097] The cathode materials prepared in Example 3 were respectively made into cathode sheets, and using metallic sodium as the anode, coin cells were assembled respectively, and the coin cells were subjected to charge and discharge tests at 25 °C in the voltage range of 2 V to 4 V.

[0098] Figure 9 Figure 10 is the cycle curve diagram of the battery assembled with the cathode material of Example 3 at a current density of 170 mA / g. From Figure 9 it can be seen that in the voltage range of 2 V to 4 V, after 50 cycles, the specific capacity is 87 mAh / g.

[0099] Example 4

[0100] (1) According to the molar ratio of each metal element in NaNi 0.40 Fe 0.20 Mn 0.40 O 2 0.40 mmol of nickel oxide, 0.40 mmol of manganese dioxide, and 0.20 mmol of iron oxide were uniformly ground in a mortar until there was no obvious particle feeling, and a mixed metal oxide powder was obtained. Weigh out oxalic acid powder with a molar ratio of 1.1:1 to the mixed metal oxide powder and place it in deionized water to obtain an oxalic acid solution, and then add the mixed metal oxide powder and stir at room temperature for 12 h to obtain a reddish-brown solution; then continuously stir at 80 °C until the solvent evaporates to obtain a sol, and the obtained sol was dried in an oven at 120 °C for 12 h to obtain a reddish-brown precursor powder.

[0101] (2) The precursor powder obtained in (1) was mixed with sodium in sodium carbonate according to a molar ratio of 1:1.20, ball-milled for 3 h at a rotation speed of 400 rpm, and then the mixed powder was placed in a tube furnace and kept at 500 °C for 4 h, and then heated to 850 °C at a heating rate of 3 °C / min and kept for 15 h, and oxygen was passed through during the whole sintering process, and the O3-type layered oxide cathode material NaNi 0.40 Fe 0.20 Mn 0.40 O 2 .

[0102] (3) In the glove box, the O3-type layered oxide cathode material and NaPF 6Put them into two 5 mL open polytetrafluoroethylene bottles respectively according to the mass ratio of 1:0.008, avoiding contact between the two. Then, put the two open polytetrafluoroethylene bottles containing the materials into a 500 mL polytetrafluoroethylene bottle and seal it in a hydrothermal autoclave reactor. Place the hydrothermal autoclave reactor in an oven and react at 150 °C for 15 h, and then cool it naturally to obtain the cathode material with an artificial SEI film.

[0103] (4)Coat the etched cathode material on the aluminum foil and soak it in the electrolyte in a sealed electrolytic cell. The electrolyte is NaPF 6 Dissolve it in a mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 according to a concentration of 1 M.

[0104] (5)Configure a three-electrode system in the electrolytic cell, including a working electrode, a counter electrode, and a reference electrode. The working electrode is the cathode material, the counter electrode is a carbon rod, and the reference electrode is an Ag / AgCl electrode. And the electrolytic cell is connected to an external nitrogen device to maintain an anhydrous and anaerobic environment in the electrolytic cell by continuously introducing nitrogen.

[0105] (6)Connect the electrochemical workstation to the electrodes of the electrolytic cell, select the constant current mode, with a current density of 0.1 mA / g, and perform constant current charging. The charging duration is controlled according to the target sodium deintercalation amount.

[0106] (7)The initial charging voltage is 2.70 V. When the voltage reaches 2.96 V (corresponding to a sodium deintercalation amount of 35%), take the electrode out of the electrolyte, rinse the electrode surface with anhydrous DMC solvent to remove the residual electrolyte, and then dry the electrode sheet in a vacuum drying oven at 60 °C. What is obtained is the layered oxide cathode material with high stability and low sodium phase.

[0107] Prepare the cathode materials prepared in Example 4 into cathode sheets respectively, use metallic sodium as the anode, and assemble them into coin cells respectively. The coin cells are subjected to charge and discharge tests at 25 °C in the voltage range of 2 V to 4 V.

[0108] Figure 10 It is the cycle curve graph of the battery assembled with the cathode material of Example 4 at a current density of 170 mA / g. From Figure 10 It can be seen that in the voltage range of 2 V to 4 V, after 50 cycles, the specific capacity is 82.9 mAh / g.

[0109] By comparing the cycle curve graphs of the coin cells assembled with the cathode materials of each example, it can be seen that the sodium deintercalation amount will affect the cycle performance of the battery.

[0110] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing a low-sodium phase sodium ion battery positive electrode material, characterized in that: include: S1. Sealing a layered sodium metal oxide positive electrode material and XPF6 in a reactor without direct contact between the layered sodium metal oxide positive electrode material and XPF6, then heating and keeping the reactor warm, cooling after keeping the reactor warm to obtain a sodium ion battery positive electrode material having a protective film on the surface; wherein X is Na and / or Li; S2. Prepare a sodium ion battery positive electrode material with a protective film on the surface into a pole piece, immerse the pole piece in an organic electrolyte, use the pole piece as the positive electrode and the inert electrode as the negative electrode, perform constant current charging under a protective atmosphere, and control the charging time according to the target sodium removal amount; S3. After the sodium removal treatment is completed, the electrode sheet is taken out, washed and dried to obtain a low-sodium phase sodium ion battery positive electrode material.

2. The method for preparing a low-sodium phase sodium ion battery positive electrode material according to claim 1, characterized in that: The chemical formula of the layered sodium metal oxide positive electrode material is NaMn y Fe z Ni w O2, 0<y≤0.5, 0≤z≤0.5, 0<w≤0.8, y+z+w=1; The low-sodium phase sodium ion battery positive electrode material comprises a substrate and an interface layer on the surface of the substrate; the chemical formula of the substrate is Na x Mn y Fe z Ni w O2, where 0.65≤x≤0.9, 0<y≤0.5, 0≤z≤0.5, 0<w≤0.8, y+z+w=1.

3. The method for preparing a low-sodium phase sodium ion battery positive electrode material according to claim 1, characterized in that: In step S1, the amount of XPF6 is 0.1wt%~1.5wt% of the layered sodium metal oxide positive electrode material.

4. The method for preparing a low-sodium phase sodium ion battery positive electrode material according to claim 1, characterized in that: In step S1, the insulation temperature is 100-180° C., and the insulation time is 10-18 hours.

5. The method for preparing a low sodium phase sodium ion battery positive electrode material according to any one of claims 1 to 4, characterized in that: The organic electrolyte includes an organic solvent and a sodium salt; the organic solvent is a mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC); the sodium salt is NaPF6; and the concentration of the sodium salt in the organic electrolyte is 1-1.5 mol / L.

6. The method for preparing a low-sodium phase sodium ion battery positive electrode material according to any one of claims 1 to 4, characterized in that: In step S2, the current density of the constant current charging is 0.1-0.17 mA / g; the endpoint voltage of the constant current charging is 2.65-3.0 V.

7. The method for preparing a low-sodium phase sodium ion battery positive electrode material according to any one of claims 1 to 4, characterized in that: In step S2, after the sodium ion battery positive electrode material having a protective film on the surface is immersed in an organic electrolyte, the electrolytic cell containing the organic electrolyte is sealed, and nitrogen or an inert gas is continuously introduced into the electrolytic cell to maintain a protective atmosphere in the electrolytic cell.

8. The method for preparing a low-sodium phase sodium ion battery positive electrode material according to any one of claims 1 to 4, characterized in that: In step S2, the electrolysis system further includes a reference electrode; the negative electrode is a platinum sheet or a carbon rod; and the reference electrode is an Ag / AgCl electrode.

9. A low sodium phase sodium ion battery cathode material, characterized in that: The method is prepared according to any one of claims 1 to 8.

10. A sodium ion battery, characterized in that Comprising the low-sodium phase sodium ion battery positive electrode material as described in claim 9.

Images