Surface modified hard carbon material as well as preparation method and application thereof

Abstract

The invention discloses a surface modified hard carbon material as well as a preparation method and application thereof, and relates to the technical field of sodium-ion battery electrode materials. The surface modified hard carbon material comprises hard carbon particles and a mixed ion / electronic conductor, and the content of the mixed ion / electronic conductor in the surface modified hard carbon material is 1-5 wt%; the mixed ion / electron conductor is multi-phosphate, the molecular formula is NaxMyPO4, and M is selected from at least one of Li, Mg, Ca, Sr, Ba, Zn, Al, La, Fe, Ti, Si, Sn, Zr, Ce, Ge, Ta, Nb, W or Mo; wherein 0.4 < = x / y < = 0.7, x + n1a + n2b = 3, n1 is the valence of M, and n2 is the valence of N. The material disclosed by the invention can be used for effectively inhibiting side reaction and inhibiting deposition of metal sodium on the surface of a hard carbon electrode, so that electrode polarization is reduced, the median voltage of hard carbon to sodium is improved, and the cycle performance of battery performance is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sodium ion battery electrode materials, in particular to a surface-modified hard carbon material and a preparation method and application thereof. BACKGROUND

[0002] At present, lithium ion batteries (LIBs) dominate in secondary batteries due to their high energy density and mature technology. However, the uneven geographical distribution of lithium resources and the extremely low crustal reserves seriously restrict the large-scale application of lithium in the energy storage field. With the rapid development of electric vehicles, low-altitude economy and robotics, the demand for large-scale secondary batteries is increasing rapidly, and it is urgent to develop non-lithium ion battery energy storage systems represented by efficient and low-cost sodium ion batteries (SIBs).

[0003] The working mechanism of sodium ion batteries is similar to that of lithium ion batteries, but due to the larger radius of sodium ions and their incompatibility with graphite layers, graphite widely used as the negative electrode of lithium ion batteries cannot be used as the negative electrode of sodium ion batteries. Therefore, developing suitable non-graphite negative electrodes is a key factor in the development of sodium ion batteries.

[0004] Unlike highly ordered graphite, hard carbon materials have long-range disorder and short-range order structure, and their large interlayer spacing and pores, especially closed pores, are suitable for sodium ion storage, making them the current mainstream negative electrode materials for sodium ion batteries. However, it is difficult for the surface of hard carbon materials to form a stable SEI film, and solvent co-intercalation and side reactions on the surface are prone to occur. In addition, the transport dynamics of sodium ions at the electrolyte / hard carbon interface is poor, resulting in similar sodium intercalation potential and sodium metal deposition potential of hard carbon electrodes, and the deposition of metal sodium on the surface during high-current charging, which further triggers side reactions and electrode polarization, thereby affecting the cycle life and safety performance of sodium ion batteries. Therefore, it is necessary to study the modification of the surface of hard carbon to inhibit the deposition of metal sodium on the surface of hard carbon electrodes, thereby reducing electrode polarization, improving the median voltage of hard carbon relative to sodium, and improving the cycle performance and safety performance of battery performance.

[0005] Patent CN118299547A proposes a fast-charging type sodium ion battery hard carbon negative electrode material, which coats an inorganic sodium ion conductor to obtain a surface-coated hard carbon material, and obtains a fast-charging type sodium ion battery hard carbon negative electrode material by fusion coating and low-temperature calcination; the hard carbon negative electrode material has the characteristics of excellent rate performance, good safety and high cycle stability. However, this coating material cannot effectively inhibit the deposition of metal sodium on the surface of the hard carbon electrode, and there is a high risk of sodium deposition, which cannot effectively improve the median voltage of hard carbon relative to sodium metal during charging. SUMMARY

[0006] The present application aims to overcome the defects and deficiencies of the existing sodium ion battery hard carbon material, and provide a surface modified hard carbon material that can effectively inhibit the occurrence of side reactions, while inhibiting the deposition of metallic sodium on the surface of the hard carbon electrode, thereby reducing electrode polarization, improving the median voltage of hard carbon for sodium, and improving battery performance and stability.

[0007] Another object of the present application is to provide a preparation method of the surface modified hard carbon material.

[0008] Another object of the present application is to provide an application of the above-mentioned surface modified hard carbon material in the preparation of a sodium ion battery.

[0009] The above-mentioned objects of the present application are achieved by the following technical solutions: The present application protects a surface modified hard carbon material, which comprises hard carbon particles and a mixed ion / electron conductor coated on the surface of the hard carbon particles, the content of the mixed ion / electron conductor in the surface modified hard carbon material being 1-5wt%; The mixed ion / electron conductor is a multinary phosphate with a molecular formula of Na x M a N b PO4, In the formula, the M is selected from at least one of Ti, Si, Sn, Zr, Ce, Ge, Ta, Nb, W or Mo; the N is selected from at least one of Li, Mg, Ca, Sr, Ba, Zn, Al, La or Fe; wherein 0.4≤x / (a+b)≤0.7, x+n1a+n2b=3, n1 is the valence of M, n2 is the valence of N, the molar ratio of M and N is z, and 1≤z≤3 is satisfied. The surface modified hard carbon material of the present application uses a specific amount of a specific mixed ion / electron conductor to coat the hard carbon material, which can play the role of an artificial SEI film, achieve desolvation effect, prevent co-intercalation of solvents, and effectively inhibit surface side reactions and the formation of an interface layer that is not conducive to sodium ion / electron transport.

[0010] Specifically, the coating of the mixed ion / electron conductor can promote the transport of sodium ions at the electrode / electrolyte interface on the one hand, and promote the transport of electrons on the other hand, which is conducive to the rapid storage of sodium in the carbon layer and pores of the hard carbon, so as to inhibit the deposition of metallic sodium on the surface of the hard carbon electrode, thereby reducing the polarization of the electrode, improving the cycle performance and safety performance of the battery. At the same time, the content of the mixed ion / electron conductor is an important influencing factor, and too low a content of the mixed ion / electron conductor is not conducive to the establishment of a complete coating layer on the surface of the hard carbon particles, and too high a content of the mixed ion / electron conductor is not conducive to the interface transport of sodium ions and the storage of sodium in the hard carbon.

[0011] By optimizing the z molar ratio, the optimization of the molar ratio x / (a+b) of the coordinated Na ions and M ions, a higher ionic conductivity and electronic conductivity can be obtained in the mixed ionic / electronic conductor, so as to realize fast sodium ion transmission and electrode reaction kinetics.

[0012] Preferably, the valence n1 of M is +3~+6; the valence n2 of N is +1~+3.

[0013] Preferably, 0.4≤x / (a+b)≤0.52, 1≤z≤2.6.

[0014] Preferably, the M is selected from at least one of Ti, Zr, Nb.

[0015] Preferably, the N is selected from at least one of Li, Mg or Al.

[0016] Optionally, 0.2≤x≤0.6, 0.3≤a≤0.7, 0.1≤b≤0.5.

[0017] The hard carbon particles are not particularly limited in the present application, and the conventional hard carbon materials in the art can achieve the purpose of the present application. Preferably, the D50 particle size of the hard carbon particles is 1-10 µm, and the specific surface area is 3-10 m² / g.

[0018] In some embodiments, the ionic conductivity of the mixed ionic / electronic conductor is greater than 1×10 ‒5 S / cm. The ionic conductivity is calculated by the AC impedance spectroscopy (EIS) test method after the powder is pressed into a thin sheet, and the calculation formula is σ = L / (R·A), σ represents the ionic conductivity, L is the thickness of the electrolyte, R is the resistance value measured by experiment, A is the effective contact area of the electrode.

[0019] In some embodiments, the mixed ionic / electronic conductor is synthesized by a solid phase method, and the synthesis method is: first, uniformly mix the element precursors (Na source, M metal source, N metal source and phosphoric acid source) according to the stoichiometric ratio by ball milling, wherein the ball milling time is 2-10 hours, calcine at 800-1000 ℃ in air for 5-20 hours, and finally crush to obtain the mixed ionic / electronic conductor.

[0020] In some embodiments, the surface-modified hard carbon material is made of hard carbon particles and a mixed ionic / electronic conductor, and the content of the hard carbon particles in the hard carbon material accounts for 95-99% by weight percentage, and the content of the mixed ionic / electronic conductor accounts for 1-5%.

[0021] The application protects a preparation method of a surface-modified hard carbon material, comprising the following steps: S1, sufficiently pulverizing a mixed ion / electron conductor; S2, uniformly coating the mixed ion / electron conductor after pulverization in step S1 on the surface of hard carbon particles to obtain a coated mixture; S3, heat-treating the coated mixture at 300-500°C under an inert atmosphere to obtain the surface-modified hard carbon material.

[0022] In some embodiments, the particle size of the pulverized mixed ion / electron conductor in step S1 is 100-500 nm. Optionally, the pulverization method can be selected from sand milling, ball milling or air flow pulverization treatment; preferably, the pulverization method is sand milling treatment.

[0023] In some embodiments, the treatment method for uniformly coating the mixed ion / electron conductor on the surface of the hard carbon particles in step S2 is at least one of ball milling, high-speed mixing, magnetron sputtering or mechanical fusion. The ball milling refers to the uniform adhesion of the coating material on the surface of the substrate powder through the high-speed collision and friction of the grinding balls and the powder in the ball mill jar. The high-speed mixing refers to the use of the shear force and centrifugal force generated by the high-speed rotating stirring paddle (or rotor-stator structure) to rapidly mix and disperse the liquid or molten coating agent and the powder. The magnetron sputtering refers to the deposition of atoms / molecules on the powder surface to form a nanoscale film by high-energy ion bombardment of the target material in a vacuum environment. The mechanical fusion refers to the physical embedding or cold welding of the coating material and the substrate particles to form a uniform core-shell structure by high-speed rotation, impact or shear mechanical action.

[0024] In some embodiments, the heat treatment time in step S3 is 1-3 h.

[0025] The heat treatment of the application can make the mixed ion / electron conductor and the hard carbon particles closely contact, and the presence of the mixed ion / electron conductor can promote the formation of closed pores on the surface of the hard carbon, thereby facilitating the rapid storage of sodium ions.

[0026] In some embodiments, the inert gas of the inert atmosphere is selected from argon, nitrogen or helium.

[0027] The application protects the use of a surface-modified hard carbon material in the preparation of a sodium ion battery.

[0028] Compared with the prior art, the application has the following beneficial effects: The surface modified hard carbon material of the present application adopts a specific amount of specific mixed ion / electron conductor to coat the hard carbon material, which can effectively inhibit the occurrence of side reactions, and is beneficial to the rapid storage of sodium in the carbon layer and the pores of the hard carbon, inhibits the deposition of metallic sodium on the surface of the hard carbon electrode, thereby reducing the electrode polarization, improving the median voltage of the hard carbon to sodium, and improving the cycle performance and safety performance of the battery performance. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 SEM image of the surface modified hard carbon material prepared in Example 1 of the present application.

[0030] Figure 2 SEM image of the surface modified hard carbon material prepared in Example 2 of the present application.

[0031] Figure 3 SEM image of the surface modified hard carbon material prepared in Example 3 of the present application.

[0032] Figure 4 SEM image of the simple hard carbon of Comparative Example 1 of the present application.

[0033] Figure 5 Energy spectrum surface scanning image of the surface modified hard carbon material prepared in Example 1 of the present application.

[0034] Figure 6 TEM image of the hard carbon part of the surface modified hard carbon material prepared in Example 1 of the present application.

[0035] Figure 7 TEM image of the simple hard carbon in Comparative Example 1 of the present application.

[0036] Figure 8 Charge-discharge curve diagram of the hard carbon material prepared in Examples 1-3 and Comparative Examples 1-2 of the present application.

[0037] Figure 9 Enlarged view along the ordinate of the present application. Figure 8

[0038] Figure 10 Physical image of the electrode surface after high current sodium intercalation of the surface modified hard carbon material prepared in Example 1 of the present application.

[0039] Figure 11 Physical image of the electrode surface after high current sodium intercalation of the surface modified hard carbon material prepared in Example 2 of the present application.

[0040] Figure 12 Physical image of the electrode surface after high current sodium intercalation of the surface modified hard carbon material prepared in Example 3 of the present application.

[0041] Figure 13 ​A physical picture of the electrode surface after high current sodium intercalation of the simple hard carbon in Inventive Comparative Example 1. DETAILED DESCRIPTION

[0042] The application will be further described in conjunction with the specific embodiments, but the embodiments do not limit the application in any form.

[0043] Example 1 A preparation method of a surface-modified hard carbon material, using Na 0.34 Ti 0.38 Al 0.38 PO4surface modification, comprising the following steps: S1, preparation and crushing of a mixed ion / electron conductor; Specifically, Na2CO3, TiO2, Al2O3, and NH4H2PO4 are used as raw materials, and are mixed uniformly in ethanol by ball milling, and then reacted at 900°C in an air atmosphere for 10 hours to obtain a mixed ion / electron conductor Na 0.34 Ti 0.38 Al 0.38 PO4, which is subjected to sanding treatment to reduce the particle size to 200 nm.

[0044] S2, uniformly coating the crushed mixed ion / electron conductor on the surface of hard carbon particles to obtain a coated mixed material; Specifically, the crushed Na 0.34 Ti 0.38 Al 0.38 PO4is mixed uniformly with hard carbon at a weight ratio of 2:100, and a mechanical fusion method is used to coat the Na 0.34 Ti 0.38 Al 0.38 PO4on the surface of the hard carbon particles to obtain a coated mixed material. The D50 particle size of the hard carbon is 5 µm, and the specific surface area is 6 m² / g.

[0045] The parameter conditions of the mechanical fusion method are as follows: the mixing step uses 400 rpm for 15 min, followed by 1200 rpm stirring for 3 h at a temperature of 45°C, and the particles are sieved through a 300 mesh sieve.

[0046] S3, the coated mixed material is heat treated at 400°C for 1 h in an argon atmosphere to obtain the surface-modified hard carbon material.

[0047] Example 2 A preparation method of a surface-modified hard carbon material, using Na 0.48 Ti 0.54 Mg 0.18 PO4surface modification, comprising the following steps: S1, preparation and crushing of a mixed ion / electron conductor; Specifically, Na2CO3, TiO2, MgO, and NH4H2PO4 are used as raw materials, and are mixed uniformly in ethanol by ball milling according to stoichiometric ratio, and then are reacted at 800 DEG C for 20 hours in air atmosphere to obtain mixed ionic / electronic conductor Na 0.48 Ti 0.54 Mg 0.18 PO4, and sand milling is performed to reduce the particle size of Na

[0048] S2, the mixed ionic / electronic conductor after crushing is uniformly coated on the surface of hard carbon particles to obtain a coated mixture; Specifically, the crushed Na 0.48 Ti 0.54 Mg 0.18 PO4 is mixed uniformly with commercial hard carbon at a weight ratio of 2:100, and a mechanical fusion method is used to coat Na 0.48 Ti 0.54 Mg 0.18 PO4 on the surface of the commercial hard carbon particles to obtain a coated mixture.

[0049] S3, the coated mixture is heat treated at 400 DEG C for 1 hour in an argon atmosphere to obtain the surface-modified hard carbon material.

[0050] Example 3 A preparation method of a surface-modified hard carbon material, using Na 0.33 Zr 0.55 Mg 0.19 Li 0.08 PO4 surface modification, comprising the following steps: S1, preparation and crushing of a mixed ionic / electronic conductor; Specifically, Na2CO3, ZrO2, MgO, LiOH, and NH4H2PO4 are used as raw materials, and are mixed uniformly in ethanol by ball milling according to stoichiometric ratio, and then are reacted at 1000 DEG C for 5 hours in air atmosphere to obtain mixed ionic / electronic conductor Na 0.33 Zr 0.55 Mg 0.19 Li 0.09 PO4, and sand milling is performed to reduce the particle size of Na

[0051] S2, the mixed ionic / electronic conductor after crushing is uniformly coated on the surface of hard carbon particles to obtain a coated mixture; Specifically, the crushed Na 0.33 Zr 0.55 Mg 0.19 Li 0.08 PO4 is mixed uniformly with commercial hard carbon at a weight ratio of 2:100, and a mechanical fusion method is used to coat Na0.33 Zr 0.55 Mg 0.19 Li 0.08 PO4coated on the surface of commercial hard carbon particles to obtain a coated mixture.

[0052] S3, heat treating the coated mixture at 400℃ for 1h under argon atmosphere to obtain the surface-modified hard carbon material.

[0053] Example 4 A method for preparing a surface-modified hard carbon material, using Na 0.50 Nb 0.03 Si 0.50 Mg 0.16 Li 0.03 PO4surface modification, comprising the following steps: S1, preparation and pulverization of the mixed ionic / electronic conductor; Specifically, using Na2CO3, Nb2O5, SiO2, MgO, LiOH, and NH4H2PO4 as raw materials, dosing according to stoichiometric ratio, uniformly mixing in ethanol through ball milling, and then reacting at 900℃ under air atmosphere for 15h to obtain the mixed ionic / electronic conductor Na 0.50 Nb 0.03 Si 0.50 Mg 0.16 Li 0.03 PO4, and performing sanding treatment to reduce the particle size to 200nm.

[0054] S2, uniformly coating the pulverized mixed ionic / electronic conductor on the surface of hard carbon particles to obtain a coated mixture; Specifically, uniformly mixing the pulverized Na 0.50 Nb 0.03 Si 0.50 Mg 0.16 Li 0.03 PO4with commercial hard carbon according to a weight ratio of 2:100, and using mechanical fusion method to coat the Na 0.50 Nb 0.03 Si 0.50 Mg 0.16 Li 0.03 PO4on the surface of commercial hard carbon particles to obtain a coated mixture.

[0055] S3, heat treating the coated mixture at 400℃ for 1h under argon atmosphere to obtain the surface-modified hard carbon material.

[0056] Example 5 A method for preparing a surface-modified hard carbon material, different from Example 1 in that the chemical formula of the mixed ionic / electronic conductor in this embodiment is Na 0.38Ti 0.43 Al 0.30 PO4.

[0057] Example 6 A method for preparing a surface-modified hard carbon material, which is different from Example 1 in that the chemical formula of the mixed ionic / electronic conductor in this example is Na 0.36 Ti 0.51 Al 0.20 PO4.

[0058] Example 7 A method for preparing a surface-modified hard carbon material, which is different from Example 1 in that the mixed ionic / electronic conductor Na 0.34 Ti 0.38 Al 0.38 PO4 in this example has a weight ratio of 1:100 to the hard carbon.

[0059] Example 8 A method for preparing a surface-modified hard carbon material, which is different from Example 1 in that the mixed ionic / electronic conductor Na 0.34 Ti 0.38 Al 0.38 PO4 in this example has a weight ratio of 5:100 to the hard carbon.

[0060] Example 9 A method for preparing a surface-modified hard carbon material, which is different from Example 1 in that the temperature of the heat treatment (in step S3) in this example is 300°C.

[0061] Example 10 A method for preparing a surface-modified hard carbon material, which is different from Example 1 in that the temperature of the heat treatment (in step S3) in this example is 500°C.

[0062] Example 11 A method for preparing a surface-modified hard carbon material, which is different from Example 1 in that the mechanical fusion method of step S2 in this example is replaced by a ball milling method, so that Na 0.34 Ti 0.38 Al 0.38 PO4 is coated on the surface of the commercial hard carbon particles.

[0063] The steps of the ball milling method are as follows: the two materials are stirred at 400 rpm for 15 min; a ball-to-material ratio of 2:1 is used, and the rotation speed is 450 rpm, and the treatment is performed for 5 h, and the post-treatment is performed by sieving through a 300-mesh sieve.

[0064] Example 12 A method for preparing a surface-modified hard carbon material, which is different from Example 1 in that the mixed ionic / electronic conductor Na0.34 Ti 0.38 Al 0.38 PO4of 500 nm.

[0065] Comparative Example 1 A method for producing a hard carbon material, which is different from Example 1 in that the hard carbon material of the present comparative example is not surface-modified with a mixed ionic / electronic conductor.

[0066] Comparative Example 2 A method for producing a surface-modified hard carbon material, which is different from Example 1 in that the mixed ionic / electronic conductor Na 0.34 Ti 0.38 Al 0.38 PO4is used in the present comparative example.

[0067] Comparative Example 3 A method for producing a surface-modified hard carbon material, which is different from Example 1 in that the mixed ionic / electronic conductor Na 0.34 Ti 0.38 Al 0.38 PO4is used in the present comparative example.

[0068] Comparative Example 4 A method for producing a surface-modified hard carbon material, which is different from Example 1 in that the mixed ionic / electronic conductor Na 0.33 Ti 0.33 Al 0.45 PO4is used in the present comparative example.

[0069] Comparative Example 5 A method for producing a surface-modified hard carbon material, which is different from Example 1 in that the mixed ionic / electronic conductor Na 0.28 Ti 0.50 Al 0.24 PO4is used in the present comparative example.

[0070] Comparative Example 6 A method for producing a surface-modified hard carbon material, which is different from Example 1 in that the mixed ionic / electronic conductor Na 0.25 Ti 0.56 Al 0.17 PO4is used in the present comparative example.

[0071] Comparative Example 7 A method for producing a surface-modified hard carbon material, which is different from Example 1 in that the mixed ionic / electronic conductor Na 0.34 Ti 0.67 PO4is used in the present comparative example.

[0072] Comparative Example 8 A method for preparing a surface-modified hard carbon material, which is different from Example 1 in that the mixed ion / electron conductor in this comparative example is Na 0.34 Ti 0.33 Si 0.33 PO4.

[0073] Comparative Example 9 A method for preparing a surface-modified hard carbon material, which is different from Example 1 in that the mixed ion / electron conductor in this comparative example is Na 0.34 Li 0.38 Al 0.76 PO4.

[0074] Comparative Example 10 A method for preparing a surface-modified hard carbon material, which is different from Example 1 in that the heat treatment (in step S3) temperature in this comparative example is 200℃.

[0075] Comparative Example 11 A method for preparing a surface-modified hard carbon material, which is different from Example 1 in that the heat treatment (in step S3) temperature in this comparative example is 700℃.

[0076] Performance test The surface-modified hard carbon materials of each of the above examples and comparative examples were subjected to the following performance tests: 1. Scanning electron microscope (SEM) characterization As shown in FIG. 1, the surface-modified hard carbon materials prepared in Examples 1-3 were analyzed by SEM, which showed that the mixed ion / electron conductors were uniformly, completely and conformally coated on the surface of the hard carbon particles. The hard carbon material of Comparative Example 1 was analyzed by SEM (FIG. 2), which showed that the surface was flat and had no coating. Figures 1-3 Figure 4

[0077] 2. Energy dispersive spectroscopy (EDS) surface scanning characterization As shown in FIG. 3, the EDS surface scanning analysis showed that the elements in Example 1 were uniformly distributed, proving that the coating layer had high uniformity. Figure 5

[0078] 3. Transmission electron microscope (TEM) characterization As shown in FIG. 4, the TEM analysis of the hard carbon part of the surface-modified hard carbon material of Example 1 showed that the hard carbon surface was rich in closed pore structures, proving that the hard carbon treated by the preparation method of the present application could promote the formation of closed pores on the surface of the hard carbon, thereby facilitating the rapid storage of sodium ions. The surface of the hard carbon material of Comparative Example 1 had no closed pore structure (FIG. 5). Figure 6 Figure 7

[0079] 4. Electrochemical performance characterization ​​​​​The hard carbon material prepared in each of the above examples and comparative examples was taken as a working electrode, sodium metal was taken as a counter electrode, glass fiber was taken as a separator, and a 1 mol / L NaPF6 EC (ethylene carbonate) / DEC (diethyl carbonate) solution was taken as an electrolyte, EC:DEC = 1:1 (v / v) to assemble a button-type half cell, and then the following tests were performed, and the results are shown in Table 1.

[0080] 4.1 Charge-discharge test In a voltage range of 0-2.0 V, first, 0.1C constant current discharge was performed to 0 V, then 0 V constant voltage discharge was performed to a cutoff current of 0.01 mA, and finally, 0.1C constant current charging was performed to 2 V; the obtained charge-discharge curve is shown in Figures 8-9 , and the reversible capacity was determined.

[0081] As can be seen from Figure 8 and Figure 9 , the samples corresponding to examples 1-3 of the present application have higher voltage values and better charge-discharge performance than the samples of comparative examples 1-2 at the same charge-discharge depth.

[0082] 4.2 High-current sodium intercalation test Sodium aluminum phosphate was taken as a positive electrode, and the hard carbon obtained in the above examples and comparative examples was taken as a negative electrode to assemble a soft-pack full cell, and in a voltage range of 1.5-3.6 V, the battery was charged and discharged at a current of 1C, and after 10 cycles, the full cell was disassembled in an inert atmosphere, and it was observed that there was no metal sodium deposition on the surface of the hard carbon electrode, and the results are shown in Figures 10-13 .

[0083] As can be seen from Figures 10 to 12 , after the high-current charge-discharge experiment, the battery samples made of the hard carbon materials of examples 1-3 were disassembled, and it was observed that there was no metal sodium deposition on the surface of the hard carbon electrode; while after the high-current sodium intercalation test, the hard carbon electrode of comparative example 1 had obvious metal sodium deposition. Figure 13 .

[0084] The hard carbon materials prepared in each of the examples and comparative examples were subjected to charge-discharge test and high-current sodium intercalation test, and the reversible capacity, median voltage data and high-current sodium intercalation phenomenon were summarized, and the results are shown in Table 1.

[0085] Table 1

[0086] As can be seen from Table 1, the reversible capacity of the surface-modified hard carbon material of the present application reaches 298.0 mAh·g -1 , the median voltage reaches 0.0522 V or more, and there is no metal sodium deposition phenomenon.

[0087] Compared with Example 1, the median voltage of Comparative Examples 1-3, which are not surface-modified with the mixed ionic / electronic conductor or are surface-modified with the mixed ionic / electronic conductor at too low or too high a content, decreases significantly, and there is a phenomenon of deposition of metallic sodium.

[0088] The median voltage of Comparative Examples 4-9, which are surface-modified with different mixed ionic / electronic conductors, decreases, and there is a phenomenon of deposition of metallic sodium. Some of the Comparative Examples also result in a decrease in reversible capacity.

[0089] When the heat treatment temperature of Comparative Examples 10-11 is too high or too low, the median voltage decreases, and there is a phenomenon of deposition of metallic sodium.

[0090] The above examples of the present application are merely examples for clearly illustrating the present application, and are not intended to limit the embodiments of the present application. Based on the above description, other different forms of changes or variations can be made by those of ordinary skill in the art. It is not necessary or possible to exhaust all embodiments. Any modification, equivalent replacement, and improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the claims of the present application.

Claims

1. A surface-modified hard carbon material, characterized by, The surface modified hard carbon material comprises hard carbon particles and a mixed ionic / electronic conductor coated on the surface of the hard carbon particles, wherein the content of the mixed ionic / electronic conductor is 1-5 wt%. The mixed ionic / electronic conductor is a multinary phosphate with the formula Na x M a N b PO4; In the formula, M is at least one selected from Ti, Si, Sn, Zr, Ce, Ge, Ta, Nb, W or Mo; N is at least one selected from Li, Mg, Ca, Sr, Ba, Zn, Al, La or Fe; wherein 0.4≤x / (a+b)≤0.7, x+n1a+n2b=3, n1 is the valence of M, n2 is the valence of N; the molar ratio of M to N is z, and 1≤z≤3.

2. The surface-modified hard carbon material of claim 1, wherein, M is at least one selected from Ti, Zr and Nb, and N is at least one selected from Li, Mg and Al.

3. The surface-modified hard carbon material of claim 1, wherein, 0.4≤x / (a+b)≤0.52, and 1≤z≤2.

6.

4. The surface-modified hard carbon material of claim 1, wherein, The mixed ionic / electronic conductor has an ionic conductivity greater than 1 x 10 ‒5 S / cm.

5. The surface-modified hard carbon material of claim 1, wherein, The preparation method of the mixed ionic / electronic conductor comprises the following steps: uniformly mixing element precursors in stoichiometric ratio by ball milling, and calcining at 800-1000℃ in air for more than 5h to obtain the mixed ionic / electronic conductor.

6. A method of producing the surface-modified hard carbon material according to any one of claims 1 to 5, characterized by, The method comprises the following steps: S1, sufficiently crushing the mixed ionic / electronic conductor; S2, uniformly coating the mixed ionic / electronic conductor on the surface of the hard carbon particles after crushing in step S1 to obtain a coated mixture; S3, heat treating the coated mixture at 300-500℃ in an inert atmosphere to obtain the surface modified hard carbon material.

7. The method for preparing the surface-modified hard carbon material according to claim 6, characterized in that, In step S1, the particle size of the crushed mixed ionic / electronic conductor is 100-500nm.

8. The method for preparing the surface-modified hard carbon material according to claim 6, characterized in that, In step S2, the treatment method for uniformly coating the mixed ionic / electronic conductor on the surface of the hard carbon particles is at least one of ball milling, high-speed mixing, magnetron sputtering or mechanical fusion.

9. The method for preparing the surface-modified hard carbon material according to claim 6, characterized in that, In step S3, the heat treatment time is 1-3h.

10. Use of the surface modified hard carbon material of any one of claims 1-5 in the preparation of a sodium ion battery.

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