Monoclinic sodium vanadium fluorophosphate, preparation and application thereof, and sodium ion battery positive electrode or negative electrode

Abstract

The invention discloses a preparation method of monoclinic sodium vanadium fluorophosphate, which comprises the following steps: mixing carbon-coated sodium vanadium phosphate as a crystal nucleus with a solid vanadium fluoride raw material at high temperature (gt; (800 DEG C) vanadium fluoride is converted into a gaseous state, and the gaseous state and carbon-coated sodium vanadium phosphate are subjected to a chemical vapor deposition reaction to obtain a reaction product containing the monoclinic sodium vanadium fluorophosphate material; the synthesis method disclosed by the invention is simple and convenient, and effectively solves the problem of generation of an impurity phase caused by extremely easy loss of fluorine elements in a high-temperature calcination stage in a traditional solid-phase reaction process; meanwhile, a reaction mechanism is simple, conditions are controllable, and the prepared monoclinic sodium vanadium fluorophosphate material is good in consistency and high in purity; in conclusion, the obtained material has relatively high specific discharge capacity and excellent rate capability, so that the preparation method of the high-performance monoclinic sodium vanadium fluorophosphate material has a good application prospect in preparation of a sodium-ion battery material.

Description

TECHNICAL FIELD

[0001] The application relates to the field of sodium ion batteries, and relates to a sodium ion battery electrode material, in particular to a preparation method of monoclinic sodium vanadium fluorophosphate, and a positive electrode material and / or a negative electrode material for a sodium ion battery. BACKGROUND

[0002] Promoting the development of new energy storage batteries such as lithium ion batteries and sodium ion batteries is a key measure to strengthen national energy reserves and promote the transformation of national energy structure, and is also one of the key technologies to achieve the carbon peak and carbon neutralization target. Breaking through the existing technical barriers of lithium ion batteries and sodium ion batteries and accelerating their application in the field of energy storage has become the focus of research and development in the field of energy storage at the present stage. Sodium and lithium are in the same group in the periodic table and have similar chemical properties. Compared with lithium ion batteries, sodium ion batteries have more advantages in resource abundance, raw material cost and safety performance. In addition, lithium ion batteries will form an aluminum-lithium alloy under low-voltage conditions, while sodium ion batteries do not have this alloying effect, so aluminum current collectors can be used to replace copper current collectors, thereby further reducing the cost of sodium ion batteries and improving their energy density. Based on this, sodium ion batteries will have more resource and price advantages in the future energy development field, and it is particularly important to deeply study and solve the problems in the development of sodium ion battery technology.

[0003] Currently reported sodium ion battery positive electrode materials mainly include three categories: layered oxides, prussian blue and vanadium-based polyanion compounds. Among them, polyanion compounds such as sodium vanadium phosphate, sodium vanadium fluorophosphate and composite sodium iron phosphate have unique advantages such as stable structure, fast sodium diffusion and high safety. Among them, sodium vanadium fluorophosphate (NaVPO4F) has become the first choice for high-capacity sodium ion batteries due to its high theoretical specific capacity (143 mAh / g). Sodium vanadium fluorophosphate material is divided into monoclinic and tetragonal two crystal phases. Tetragonal sodium vanadium fluorophosphate (NaVPO4F) is mainly prepared by low-temperature solvothermal method, and monoclinic sodium vanadium fluorophosphate (NaVPO4F) is mainly prepared by high-temperature calcination. Compared with the tetragonal phase, monoclinic sodium vanadium fluorophosphate (NaVPO4F) has more advantages in stability and rate performance. However, the fluorine element in monoclinic sodium vanadium fluorophosphate (NaVPO4F) is easy to volatilize and lose during high-temperature process, resulting in low material purity and the inability to effectively exert the electrochemical advantages. SUMMARY

[0004] To solve the problem of loss of fluorine element in high-temperature calcination stage in the preparation process of the above-mentioned monoclinic sodium vanadium fluorophosphate material, the application provides a preparation method of a monoclinic sodium vanadium fluorophosphate (NaVPO4F) material with controllable reaction conditions and high purity. First, a carbon-coated sodium vanadium phosphate (Na3V2(PO4)3) material is obtained by a sol-gel method, a spray drying method, a solid-phase calcination method and the like, and then the material is further converted into the monoclinic sodium vanadium fluorophosphate (NaVPO4F) by a chemical reaction under high temperature and pressure conditions

[0005] Na3V2(PO4)3+ VF3(g) = 3NaVPO4F (equation 1)

[0006] The preparation method of the monoclinic sodium vanadium fluorophosphate provided by the application comprises the following steps: 1) mixing the carbon-coated sodium vanadium phosphate with solid-state vanadium fluoride raw materials, converting the vanadium fluoride into a gaseous state under high temperature (>800 DEG C), and obtaining a reaction product containing the monoclinic sodium vanadium fluorophosphate by a chemical vapor deposition reaction of the carbon-coated sodium vanadium phosphate and the vanadium fluoride.

[0007] Specific technical solutions:

[0008] The preparation method of the monoclinic sodium vanadium fluorophosphate comprises the following steps:

[0009] Step (1), obtaining carbon-coated sodium vanadium phosphate, denoted as sodium vanadium phosphate@C, wherein the mass content of the coated carbon is 0.5wt%-15wt%, and preferably 6wt%-12wt%.

[0010] Step (2), mixing the sodium vanadium phosphate@C and a vanadium trifluoride powder VF3(s), and obtaining the monoclinic sodium vanadium fluorophosphate by a chemical vapor deposition reaction of the mixture; the metering ratio of the vanadium trifluoride powder VF3(s) to the sodium vanadium phosphate@C is 1.0-1.3:1.0, and preferably 1.15-1.2:1; the metering ratio is calculated based on the molar amount of the vanadium trifluoride powder VF3(s) and the sodium vanadium phosphate@C.

[0011] The chemical vapor deposition reaction in step (2) needs to be carried out in a closed reaction kettle, the loading amount of the mixture in the reaction kettle is 45%-75% (volume ratio), the gas pressure in the reaction kettle is 0.5Mpa-5Mpa, the gas atmosphere in the reaction kettle is one or more than two of argon, nitrogen, helium and the like, the reaction temperature of the mixture in the reaction kettle is 850 DEG C-1200 DEG C, and preferably 1000 DEG C-1100 DEG C, and the reaction time is 0.5h-2h, and preferably 0.95-1.3h.

[0012] The mixing of the sodium vanadium phosphate@C and the vanadium trifluoride powder VF3(s) also needs to be uniformly mixed, and the mixing mode is one or both of ball milling and tank milling.

[0013] The time for cooling to room temperature after the heat treatment is 30s-10min, preferably 2min-3min.

[0014] The preparation method of the carbon-coated sodium vanadium phosphate in step (1) is one or two or more of sol-gel high-temperature calcination, spray drying-high temperature calcination, solid phase mixing-high temperature calcination, high temperature melting-high temperature calcination.

[0015] The monoclinic sodium vanadium fluorophosphate prepared by the preparation method.

[0016] The monoclinic sodium vanadium fluorophosphate can be used as a positive active material and / or a negative active material in a sodium ion battery positive electrode and / or a negative electrode.

[0017] A positive electrode or a negative electrode for a sodium ion battery, wherein the positive electrode or the negative electrode contains the monoclinic sodium vanadium fluorophosphate.

[0018] Preferably, the content of the monoclinic sodium vanadium fluorophosphate in the positive electrode material or the negative electrode material is 65-95wt%.

[0019] Preferably, the positive electrode material further contains a conductive agent and a binder, and the mass ratio of the monoclinic sodium vanadium fluorophosphate, the conductive agent and the binder is (65-95):(5-35):(5-35).

[0020] Preferably, the negative electrode material further contains a conductive agent and a binder, and the mass ratio of the monoclinic sodium vanadium fluorophosphate, the conductive agent and the binder is (65-95):(5-35):(5-35).

[0021] The synthesis method is simple and effectively solves the problem of the generation of impurity phases caused by the loss of fluorine elements in the high-temperature calcination stage of the traditional solid-phase reaction process; at the same time, the reaction mechanism is simple and the conditions are controllable, and the prepared monoclinic sodium vanadium fluorophosphate material has good consistency and high purity; the material obtained above has high discharge specific capacity and excellent rate performance, so the preparation method of this high-performance monoclinic sodium vanadium fluorophosphate material has good application prospect in the preparation of sodium ion battery materials.

[0022] Advantages

[0023] During the synthesis of monoclinic sodium vanadium fluorophosphate (NaVPO4F), fluorine elements are often carried out of the system in the form of gaseous products (such as VF3(g), HF) with the reaction gas flow at high temperature (650℃-850℃), and impurity phases of sodium vanadium phosphate (Na3V2(PO4)3) and vanadium oxide VO x, sodium vanadium phosphate (Na3V2(PO4)3) has a theoretical specific capacity of only 117 mAh / g, which is lower than the theoretical specific capacity of 143 mAh / g of monoclinic sodium vanadium fluorophosphate (NaVPO4F), and cannot achieve high specific energy. However, the present application utilizes the reverse reaction process, and under the condition of a closed and pressurized environment, no element loss occurs. Under heating conditions, solid vanadium trifluoride is converted into a gas, and the fluorination reaction of sodium vanadium phosphate is realized through a gas phase deposition process, thereby generating monoclinic sodium vanadium fluorophosphate. Monoclinic sodium vanadium fluorophosphate and sodium vanadium phosphate have similar space group structures and belong to the monoclinic system. After the fluorination of sodium vanadium phosphate by introducing vanadium fluoride gas in the high-temperature gas phase deposition reaction process, the conversion of sodium vanadium phosphate to monoclinic sodium vanadium fluorophosphate is more likely to occur. Carbon-coated sodium vanadium phosphate is prepared by high-temperature calcination, and has stable cell structure. Therefore, the monoclinic sodium vanadium fluorophosphate converted by the high-temperature gas phase deposition method has high crystal structure stability. In addition, the chemical vapor deposition reaction needs to be carried out in argon and / or nitrogen to prevent side reactions such as oxidation and decomposition of sodium vanadium phosphate or the generated monoclinic sodium vanadium fluorophosphate material.

[0024] During the high-temperature gas phase deposition reaction process, the vanadium fluoride gas further reacts with the internal crystal nucleus across the carbon coating layer. In addition, the pore structure formed during the diffusion of the vanadium fluoride gas to the internal crystal nucleus helps to increase the exposure of the active sites of the sodium vanadium fluorophosphate material and the contact with the electrolyte. In addition, the sodium vanadium phosphate has been carbon-coated, and carbon-coated monoclinic sodium vanadium fluorophosphate (NaVPO4F) is generated in situ, which helps to improve the carbon conductivity of the sodium vanadium fluorophosphate (NaVPO4F).

[0025] In addition, compared with the traditional synthesis process, in the closed environment of high temperature and high pressure, the problem of further decomposition of the generated monoclinic sodium vanadium fluorophosphate to produce element loss is effectively avoided. In addition, the reactants are single, the controllability and uniformity of the reaction are good, and the consistency of the prepared monoclinic sodium vanadium fluorophosphate (NaVPO4F) is high.

[0026] The above-mentioned comprehensive effects make the material have high discharge specific capacity and excellent rate performance. Therefore, the preparation method of this high-performance monoclinic sodium vanadium fluorophosphate (NaVPO4F) vanadium sodium material has good application prospects in the preparation of sodium ion battery materials. DETAILED DESCRIPTION

[0027] Preparation of carbon-coated sodium vanadium phosphate (Na3V2(PO4)3) material:

[0028] According to the stoichiometric ratio, 13.64 g of vanadium pentoxide, 53.58 g of citric acid, 200 mL of deionized water were added to a beaker, and the reaction was carried out at 80°C in a water bath for 0.5 h. When the solution color no longer changed, 35.10 g of sodium dihydrogen phosphate dihydrate was added, and the solvent was removed by rotary evaporation at 75°C for 2 h. The obtained mixture was vacuum dried at 120°C for 10 h, and then ground into powder. After grinding, the solid powder was pre-sintered at 360°C for 5 h and high-temperature sintered at 700°C for 8 h under an inert atmosphere. After cooling, Na3V2(PO4)3@C was obtained. The mass fraction of carbon in the prepared Na3V2(PO4)3@C was 7wt% by TG test.

[0029] Example 1

[0030] 9.8065 g of Na3V2(PO4)3@C prepared above and 2.5920 g of VF3 were weighed and transferred to a 60 mL high-pressure reaction kettle. The bulk volume of the material was 30 mL, and 3 MPa of argon was introduced into the reaction kettle (argon was used to replace the air atmosphere therein) and the reaction kettle was sealed. Then the sealed reaction kettle was placed in a heat treatment at 1050°C for 1.15 h. After the end, it was cooled to room temperature within 2-3 min by liquid nitrogen, denoted as sample 1#.

[0031] TG, XRD, and XRF tests were performed on sample 1#. XRD showed that sample 1# was monoclinic sodium vanadium phosphate fluoride with a purity of 99.55%. TG test showed that the carbon content of sample 1# was 5.5wt%. XRF test showed that the stoichiometric ratio of Na / V / P / F atoms in sample 1# was 0.991:0.993:0.995:0.990.

[0032] Example 2

[0033] The conditions and processes were the same as in Example 1, except that the amount of VF3 added was 2.16 g, denoted as sample 2#.

[0034] TG, XRD, and XRF tests were performed on sample 2#. XRD showed that sample 2# was mainly monoclinic sodium vanadium phosphate fluoride with a purity of 94.52%, and there was 4.53% of unconverted carbon-coated sodium vanadium phosphate and 0.95wt% of vanadium trifluoride. TG test showed that the carbon content of sample 2# was 6.2wt%. XRF test showed that the stoichiometric ratio of Na / V / P / F atoms in sample 2# was 0.992:0.973:0.9895:0.935.

[0035] Example 3

[0036] The conditions and process are the same as example 1, with the difference that the amount of VF3 added is 2.808 g, noted as sample 3#;

[0037] TG, XRD, XRF tests were performed on sample 3#, XRD indicated that sample 3# is mainly monoclinic sodium vanadium fluorophosphate with a purity of 92.90%, there is also 5.50% of sodium vanadium trifluorophosphate and 1.6% of vanadium phosphate VPO4, TG test indicated that the carbon content of sample 3# is 5.2wt%, XRF test indicated that the stoichiometric ratio of Na / V / P / F atoms in sample 3# is 0.979:0.975:0.976:0.975.

[0038] Example 4

[0039] The conditions and process are the same as example 1, with the difference that the closed reactor is heat treated at 850°C, noted as sample 4#;

[0040] TG, XRD, XRF tests were performed on sample 4#, XRD indicated that sample 6# is mainly monoclinic sodium vanadium fluorophosphate with a purity of 90.00%, there is also 8.1% of unconverted sodium vanadium phosphate and 1.9% of vanadium fluoride VF3, TG test indicated that the carbon content of sample 4# is 6.5wt%, XRF test indicated that the stoichiometric ratio of Na / V / P / F atoms in sample 4# is 0.982:0.982:0.984:0.982.

[0041] Example 5

[0042] The conditions and process are the same as example 1, with the difference that the closed reactor is heat treated at 1200°C, noted as sample 5#;

[0043] TG, XRD, XRF tests were performed on sample 5#, XRD indicated that sample 5# is mainly monoclinic sodium vanadium fluorophosphate with a purity of 91.50%, there is also 6.3% of sodium vanadium trifluorophosphate and 2.2% of vanadium phosphate, TG test indicated that the carbon content of sample 5# is 5.2wt%, XRF test indicated that the stoichiometric ratio of Na / V / P / F atoms in sample 5# is 0.982:0.98:0.98:0.982.

[0044] Example 6

[0045] The conditions and process are the same as example 1, with the difference that the closed reactor is heat treated at 1050°C for 0.5h, noted as sample 6#;

[0046] The sample 6# is tested by TG, XRD and XRF. The XRD test shows that the sample 6# is mainly monoclinic sodium vanadium fluorophosphate with a purity of 90.00%, and 7.5% of sodium vanadium phosphate and 2.5% of unreacted vanadium fluoride VF3 exist. The TG test shows that the carbon content of the sample 6# is 6.3wt%, and the XRF test shows that the stoichiometric ratio of Na / V / P / F atoms in the sample 6# is 0.975:0.987:0.977:1.01.

[0047] Example 7

[0048] The conditions and processes are the same as those in Example 1, except that the closed reaction kettle is heat treated at 1050℃ for 2h, which is recorded as sample 7#.

[0049] The sample 7# is tested by TG, XRD and XRF. The XRD test shows that the sample 7# is mainly monoclinic sodium vanadium fluorophosphate with a purity of 92.50%, and 6% of sodium vanadium trifluorophosphate and 1.5% of vanadium phosphate exist. The TG test shows that the carbon content of the sample 7# is 6.6wt%, and the XRF test shows that the stoichiometric ratio of Na / V / P / F atoms in the sample 7# is 0.988:0.978:0.98:0.988.

[0050] Example 8

[0051] The conditions and processes are the same as those in Example 1, except that the sample is cooled to room temperature by liquid nitrogen within 30s-50s after the reaction is completed, which is recorded as sample 8#.

[0052] The sample 8# is tested by TG, XRD and XRF. The XRD test shows that the sample 8# is mainly monoclinic sodium vanadium fluorophosphate with a purity of 99.99%, and 0.01% of sodium vanadium phosphate exists. The TG test shows that the carbon content of the sample 8# is 5.8wt%, and the XRF test shows that the stoichiometric ratio of Na / V / P / F atoms in the sample 8# is 0.982:0.984:0.984:0.981.

[0053] Example 9

[0054] The conditions and processes are the same as those in Example 1, except that the sample is cooled to room temperature by liquid nitrogen within 9.5min-10min after the reaction is completed, which is recorded as sample 9#.

[0055] The sample 9# is tested by TG, XRD and XRF. The XRD test shows that the sample 9# is mainly monoclinic sodium vanadium fluorophosphate with a purity of 98%, and 1.5% of sodium vanadium phosphate and 0.5% of vanadium trifluoride exist. The TG test shows that the carbon content of the sample 9# is 5.1wt%, and the XRF test shows that the stoichiometric ratio of Na / V / P / F atoms in the sample 9# is 0.981:0.984:0.982:0.990.

[0056] Example 10

[0057] The conditions and process are the same as Example 1, except that the volume of the closed reactor is 66.7 mL, noted as sample 10#;

[0058] TG, XRD, XRF tests are performed on sample 10#, XRD shows that sample 10# is mainly monoclinic sodium vanadium fluorophosphate with purity of 97%, there are also 2.45% of unconverted sodium vanadium phosphate and 0.55% of vanadium trifluoride, TG test shows that the carbon content of sample 10# is 5.8wt%, XRF test shows that the stoichiometric ratio of Na / V / P / F atoms in sample 10# is 0.982:0.983:0.984:0.980.

[0059] Example 11

[0060] The conditions and process are the same as Example 1, except that the volume of the closed reactor is 40 mL, noted as sample 11#;

[0061] TG, XRD, XRF tests are performed on sample 11#, XRD shows that sample 11# is mainly monoclinic sodium vanadium fluorophosphate with purity of 98%, there are also 1.5% of sodium vanadium trifluorophosphate and 0.5% of vanadium phosphate, TG test shows that the carbon content of sample 11# is 5.4wt%, XRF test shows that the stoichiometric ratio of Na / V / P / F atoms in sample 11# is 0.981:0.983:0.982:0.980;

[0062] Example 12

[0063] The conditions and process are the same as Example 1, except that the pressure in the closed reactor is 0.5 Mpa, noted as sample 12#;

[0064] TG, XRD, XRF tests are performed on sample 12#, XRD shows that sample 12# is mainly monoclinic sodium vanadium fluorophosphate with purity of 95%, there are also 4.0% of unconverted sodium vanadium phosphate and 1.0% of vanadium trifluoride, TG test shows that the carbon content of sample 12# is 5.8wt%, XRF test shows that the stoichiometric ratio of Na / V / P / F atoms in sample 12# is 0.982:0.983:0.983:0.984;

[0065] Example 13

[0066] The conditions and process are the same as Example 1, except that the pressure in the closed reactor is 5 Mpa, noted as sample 13#;

[0067] The sample 13# is tested by TG, XRD and XRF. The XRD shows that the sample 13# is mainly monoclinic sodium vanadium fluorophosphate with a purity of 99.75%, and 0.25% of sodium vanadium phosphate. The TG test shows that the carbon content of the sample 13# is 5.8wt%. The XRF test shows that the stoichiometric ratio of Na / V / P / F atoms in the sample 13# is 0.983:0.9835:0.9843:0.98;

[0068] Comparative Example 1

[0069] According to the stoichiometric ratio of NaVPO4F, 11.70g of ammonium metavanadate, 35.72g of citric acid and water are added to a beaker, and the reaction is carried out at 80°C in a water bath for 0.5h. When the color of the solution no longer changes, 4.20g of sodium fluoride and 11.50g of ammonium dihydrogen phosphate are added. The precursor solution is rotary evaporated to remove the solvent, and the rotary evaporation temperature is 75°C for 4h. The obtained mixture precursor is vacuum dried, and the vacuum drying temperature is 120°C for 8h. After drying, the solid is ground, and the solid powder is pre-sintered at 360°C for 5h and high-temperature sintered at 700°C for 8h under the protection of inert atmosphere. After cooling, a carbon-coated powder material is obtained, which is recorded as sample A#.

[0070] The sample A# is tested by TG, XRD and XRF. The XRD shows that the sample A# is a mixture of sodium vanadium phosphate (35wt%), sodium vanadium trifluorophosphate (10wt%), monoclinic sodium vanadium fluorophosphate (40wt%), vanadium sesquioxide (10wt%) and vanadium dioxide (5wt%). The TG test shows that the carbon content of the sample A# is 7.3wt%. The XRF test shows that the stoichiometric ratio of Na / V / P / F atoms in the sample A# is 0.95:1.12:0.908:0.523.

[0071] Comparative Example 2

[0072] The conditions and processes are the same as those of Comparative Example 1, except that 4.20g of sodium fluoride is replaced by 16.80g of sodium hexafluorophosphate, which is recorded as sample B#.

[0073] The sample B# is tested by TG, XRD and XRF. The XRD shows that the sample B# is mainly a mixture of sodium vanadium phosphate (10wt%), sodium vanadium trifluorophosphate (80wt%) and vanadium sesquioxide (7wt%) and vanadium dioxide (3wt%). The TG test shows that the carbon content of the sample B# is 5.5wt%. The XRF test shows that the stoichiometric ratio of Na / V / P / F atoms in the sample B# is 1.18:1.02:0.829:1.06.

[0074] Comparative Example 3

[0075] 9.8065 g of the Na3V2(PO4)3@C prepared above and 1.728 g of VF3 were weighed out after grinding, transferred into a 60 mL reaction kettle, the bulk volume of the material was 30 mL, argon gas with a pressure of 3 MPa was introduced into the reaction kettle which was then sealed, and then the sealed reaction kettle was placed in a heat treatment furnace at 1050 °C for 1.15 h. After the end of the heat treatment, the reaction kettle was cooled to room temperature in 2-3 min by liquid nitrogen, and the sample was recorded as sample C#;

[0076] The sample C# was subjected to TG, XRD and XRF tests. The XRD test showed that the sample C# was a mixture of monoclinic sodium vanadium fluorophosphate (85 wt%) and sodium vanadium phosphate (15 wt%). The TG test showed that the carbon content of the sample C# was 6.2 wt%. The XRF test showed that the mass ratio of Na / V / P / F atoms in the sample C# was 1.017:0.957:1.019:0.836.

[0077] Comparative Example 4

[0078] The conditions and processes were the same as those in Comparative Example 3, except that the amount of VF3 added was 3.24 g, and the sample was recorded as sample D#.

[0079] The sample D# was subjected to TG, XRD and XRF tests. The XRD test showed that the sample D# was mainly monoclinic sodium vanadium fluorophosphate, with a purity of 81%, and 14% of sodium vanadium trifluorophosphate and 5% of vanadium phosphate VPO4 also existed. The TG test showed that the carbon content of the sample D# was 5.9 wt%. The XRF test showed that the stoichiometric ratio of Na / V / P / F atoms in the sample D# was 0.981:0.976:0.977:0.980.

[0080] Comparative Example 5

[0081] The conditions and processes were the same as those in Comparative Example 3, except that the amount of VF3 added was 2.5920 g, and the sealed reaction kettle was placed in a heat treatment furnace at 600 °C, and the sample was recorded as sample E#.

[0082] The sample E# was subjected to TG, XRD and XRF tests. The XRD test showed that the sample E# was mainly sodium vanadium phosphate, with a purity of 72%, and 10% of monoclinic sodium vanadium fluorophosphate and 18% of unreacted vanadium fluoride VF3 also existed. The TG test showed that the carbon content of the sample E# was 6.0 wt%. The XRF test showed that the stoichiometric ratio of Na / V / P / F atoms in the sample E# was 0.972:0.990:0.974:1.023.

[0083] Comparative Example 6

[0084] The conditions and processes were the same as those in Comparative Example 3, except that the amount of VF3 added was 2.5920 g, and the reaction kettle was placed in a heat treatment furnace at 1400 °C, and the sample was recorded as sample F#.

[0085] The sample F# was tested by TG, XRD and XRF. The XRD test showed that the sample F# was sodium vanadate phosphate with purity of 15%, and 65% monoclinic sodium vanadate fluoride phosphate, 15% sodium vanadate trifluorophosphate and 5% vanadate phosphate. The TG test showed that the carbon content of the sample F# was 4.5wt%. The XRF test showed that the stoichiometric ratio of Na / V / P / F atoms in the sample F# was 1.020:0.950:1.011:0.838;

[0086] Comparative Example 7

[0087] The conditions and process parameters were the same as those in Comparative Example 3, except that the amount of VF3 added was 2.5920g, and the closed reaction kettle was placed in a heat treatment at 1050°C for 10min, which was recorded as sample G#.

[0088] The sample G# was tested by TG, XRD and XRF. The XRD test showed that the sample G# was sodium vanadate phosphate with purity of 60%, and 25% monoclinic sodium vanadate fluoride phosphate and 15% unreacted vanadium fluoride VF3. The TG test showed that the carbon content of the sample G# was 6.4wt%. The XRF test showed that the stoichiometric ratio of Na / V / P / F atoms in the sample G# was 0.975:0.989:0.976:1.015.

[0089] Comparative Example 8

[0090] The conditions and process parameters were the same as those in Comparative Example 3, except that the amount of VF3 added was 2.5920g, and the closed reaction kettle was placed in a heat treatment at 1050°C for 5h, which was recorded as sample H#.

[0091] The sample H# was tested by TG, XRD and XRF. The XRD test showed that the sample H# was sodium vanadate phosphate with purity of 40%, and 10% monoclinic sodium vanadate fluoride phosphate, 40% sodium vanadate trifluorophosphate, 5% vanadate phosphate and 5% vanadium trifluoride VF3. The TG test showed that the carbon content of the sample H# was 6.05wt%. The XRF test showed that the stoichiometric ratio of Na / V / P / F atoms in the sample H# was 1.116:0.917:0.995:0.886.

[0092] Comparative Example 9

[0093] The conditions and process parameters were the same as those in Comparative Example 3, except that the amount of VF3 added was 2.5920g, and after the end of the reaction, the sample was cooled to room temperature in 1s-5s by liquid nitrogen, which was recorded as sample I#.

[0094] TG, XRD, XRF tests were performed on sample I#, XRD indicated that sample I# was mainly monoclinic sodium vanadium fluorophosphate with purity of 99.99%, TG test indicated that the carbon content of sample I# was 4.8wt%, XRF test indicated that the stoichiometric ratio of Na / V / P / F atoms in sample I# was 0.997:0.998:1.00:0.997; Comparative Example 10

[0095] The conditions and process parameters were the same as Comparative Example 3, except that the amount of VF3 added was 2.5920g, and after the end, it was cooled to room temperature by liquid nitrogen within 25-30min, which was recorded as sample J#;

[0096] TG, XRD, XRF tests were performed on sample J#, XRD indicated that sample J# was mainly monoclinic sodium vanadium fluorophosphate with purity of 88.5%, and there was also 11.5% sodium vanadium phosphate, TG test indicated that the carbon content of sample J# was 4.2wt%, XRF test indicated that the stoichiometric ratio of Na / V / P / F atoms in sample J# was 1.01:0.962:1.011:0.871;

[0097] Comparative Example 11

[0098] The conditions and process were the same as Comparative Example 3, except that the amount of VF3 added was 2.5920g, and after the end, it was naturally cooled to room temperature, which was recorded as sample K#;

[0099] TG, XRD, XRF tests were performed on sample K#, XRD indicated that sample K# was mainly monoclinic sodium vanadium fluorophosphate with purity of 55%, and there was also 35% sodium vanadium phosphate and 10% vanadium trifluoride, TG test indicated that the carbon content of sample K# was 4.0wt%, XRF test indicated that the stoichiometric ratio of Na / V / P / F atoms in sample K# was 0.965:0.996:0.967:1.055;

[0100] Comparative Example 12

[0101] The conditions and process were the same as Comparative Example 3, except that the volume of the closed reaction kettle was 90mL, which was recorded as sample L#;

[0102] TG, XRD, XRF tests were performed on sample L#, XRD indicated that sample L# was mainly monoclinic sodium vanadium fluorophosphate with purity of 50%, and there was also 37% unconverted sodium vanadium phosphate and 13% vanadium trifluoride, TG test indicated that the carbon content of sample L# was 5.8wt%, XRF test indicated that the stoichiometric ratio of Na / V / P / F atoms in sample L# was 0.940:1.012:0.942:1.155;

[0103] Comparative Example 13

[0104] The conditions and process are the same as those of Comparative Example 3, except that the volume of the closed reactor is 35 mL, and is denoted as sample M#;

[0105] TG, XRD, and XRF tests were performed on sample M#. XRD indicated that sample M# is mainly monoclinic sodium vanadium fluorophosphate with a purity of 65%, and 25% sodium vanadium trifluorophosphate and 10% vanadium phosphate also exist. TG tests indicated that the carbon content of sample M# is 4.2 wt%, and XRF tests indicated that the stoichiometric ratio of Na / V / P / F atoms in sample M# is 0.971:0.972:0.973:0.970;

[0106] Comparative Example 14

[0107] The conditions and process are the same as those of Comparative Example 3, except that a pressure of 0.1 MPa is applied in the closed reactor, and is denoted as sample N#;

[0108] TG, XRD, and XRF tests were performed on sample N#. XRD indicated that sample N# is 10% monoclinic sodium vanadium fluorophosphate, and 70% unconverted sodium vanadium phosphate and 20% vanadium trifluoride also exist. TG tests indicated that the carbon content of sample N# is 6.2 wt%, and XRF tests indicated that the stoichiometric ratio of Na / V / P / F atoms in sample N# is 0.950:1.007:0.950:1.124;

[0109] TG, XRD, and XRF tests were performed on sample N#. XRD indicated that sample N# is 10% monoclinic sodium vanadium fluorophosphate, and 70% unconverted sodium vanadium phosphate and 20% vanadium trifluoride also exist. TG tests indicated that the carbon content of sample N# is 6.2 wt%, and XRF tests indicated that the stoichiometric ratio of Na / V / P / F atoms in sample N# is 0.950:1.007:0.950:1.124;

[0110] Comparative Example 15

[0111] The conditions and process are the same as those of Comparative Example 3, except that a pressure of 10 MPa is applied in the closed reactor, and is denoted as sample O#;

[0112] TG, XRD, and XRF tests were performed on sample O#. XRD indicated that sample O# is 25% monoclinic sodium vanadium fluorophosphate, and 55% sodium vanadium trifluorophosphate and 20% vanadium phosphate also exist. TG tests indicated that the carbon content of sample O# is 6.2 wt%, and XRF tests indicated that the stoichiometric ratio of Na / V / P / F atoms in sample O# is 0.973:0.954:0.957:0.976;

[0113] Comparative Example 16

[0114] According to the stoichiometric ratio of NaVPO4F, 11.70 g of ammonium metavanadate, 35.72 g of citric acid, 4.20 g of sodium fluoride and 11.50 g of ammonium dihydrogen phosphate solids were mixed and transferred to a muffle furnace, reacted at 1000°C for 1 h to form a melt, then the melt was poured into liquid nitrogen to cool, reduced to room temperature in 2-3 min, the block in the crucible was taken out, broken and pre-sintered at 360°C for 5 h, high-temperature sintered at 750°C for 8 h under the protection of inert atmosphere of nitrogen, and after cooling, a carbon-coated powder material was obtained, denoted as sample P#;

[0115] The sample P# was tested by TG, XRD and XRF. XRD showed that the sample P# was a mixture of monoclinic sodium vanadium phosphate fluoride (99.0wt%) and sodium vanadium phosphate (1wt%), TG test showed that the carbon content of the sample P# was 6.2wt%, and XRF test showed that the stoichiometric ratio of Na / V / P / F atoms in the sample P# was 0.98:0.985:0.99:0.98;

[0116] Test Example 1 was implemented

[0117] The sample 1# prepared in Example 1 was used as a sodium ion battery positive active material, mixed with conductive agent acetylene black, binder polyvinylidene fluoride in a mass ratio of 8.35:0.82:0.83, and then mixed with solvent N-methyl pyrrolidone to form a paste, which was coated on an aluminum current collector, dried, and then cut into a circular piece with a diameter of 12.8 mm, and the active material area density was 8-12 mg / cm 2 (9.63 mg / cm 2 herein), which was used as a battery positive electrode. The negative electrode was selected as a metal sodium sheet, the electrolyte was selected as a 1M NaClO4 mixed solution of EC and DEC (V / V of EC / DEC = 1:1), and the separator was selected as a glass fiber membrane. The assembled battery was tested for charge and discharge, the charge cut-off voltage was 4.5V, the discharge cut-off voltage was 2.0V, the specific capacity was tested at 0.2C / 1.0C / 5.0C, and the ICE and discharge mid-voltage under 0.2C charge and discharge conditions were recorded, and the test data were recorded in Table 1.

[0118] Test Examples 2-14 were implemented

[0119] The samples obtained in Examples 2#-14# were tested according to the test method of Test Example 1, and the test data of 0.2C / ICE / 1.0C / 5.0C / mid-voltage were recorded in Table 1.

[0120] Comparative Test Example 1

[0121] The sample A# prepared in Comparative Example 1 was mixed with a conductive agent acetylene black and a binder polyvinylidene fluoride at a mass ratio of 8.35:0.82:0.83, and a proper amount of a solvent N-methyl pyrrolidone was added to form a paste, which was coated on an aluminum current collector, dried, and cut into a circular sheet with a diameter of 12.8 mm, and the active material surface density was 8-12 mg / cm 2 (Herein, 9.51 mg / cm 2 ), which was used as a positive electrode of a battery. A metal sodium sheet was selected as a negative electrode, 1M NaClO4 / EC / DEC (V / V=1:1 of EC / DEC) was selected as an electrolyte, and a glass fiber membrane was selected as a separator. The assembled battery was tested for charge and discharge, the charge cutoff voltage was 4.5 V, the discharge cutoff voltage was 1.5 V, the gram capacity at 0.2C / 1.0C / 5.0C was tested, and the ICE and the discharge midpoint voltage under the condition of 0.2C charge and discharge were recorded, and the test data of 0.2C / ICE / 1.0C / 5.0C / midpoint voltage were recorded in Table 2.

[0122] Comparative Test Example 2-16

[0123] The sample B#-P# obtained in Comparative Test Example 2-16 was tested according to the test method of Comparative Test Example 1, and the test data of 0.2C / ICE / 1.0C / 5.0C / midpoint voltage were recorded in Table 2.

[0124] Table 1 Test data recorded in the test of the implementation test example

[0125]

[0126] Table 2 Test data recorded in the test of the comparative test example

[0127]

[0128] Conclusion

[0129] The patent application aims at the problem that fluorine element is easy to be lost in the high-temperature calcination process of monoclinic sodium vanadium fluorophosphate material. The monoclinic sodium vanadium fluorophosphate is prepared by taking carbon-coated sodium vanadium phosphate as a crystal nucleus and vanadium fluoride raw material in a gaseous state at a high temperature, and depositing and reacting with sodium vanadium phosphate. The material has good consistency and high purity, effectively solves the problem of impurity phase caused by the loss of fluorine element in the traditional solid-phase reaction high-temperature calcination process, and has high discharge specific capacity and excellent rate performance by optimizing the reaction conditions. The gram capacity at 0.2C is 141 mAh / g, the ICE is 93%, the discharge midpoint voltage is 3.37 V, the gram capacity at 1.0C is 126 mAh / g, and the gram capacity at 5.0C is 117 mAh / g (Example 1).

[0130] It can be seen from the data of examples and comparative examples that the reaction conditions such as high-temperature vapor deposition temperature, amount of filler in a sealed reaction kettle, gas pressure range, time, cooling mode, and metering ratio of reaction raw materials all have an influence on the electrochemical performance of the powder material as an electrode material.

[0131] As in Examples 1-3, when the metering ratio of vanadium trifluoride VF3(s) to carbon-coated sodium vanadium phosphate is 1.0-1.3:1.0, preferably 1.15-1.2:1, the battery performance of the prepared material is relatively excellent, the gram capacity at 0.2C is 128-141 mAh / g, the ICE is 88%-93%, the discharge mid-voltage is 3.31-3.40 V, the gram capacity at 1.0C is 117-126 mAh / g, and the gram capacity at 5.0C is 104-117 mAh / g; when the amount of vanadium trifluoride VF3(s) added is low, the carbon-coated sodium vanadium phosphate is not completely converted, and when the amount added is too much, impurities such as sodium vanadium trifluorophosphate are likely to occur, as in Comparative Examples 3-4, the gram capacity at 0.2C is only 114-120 mAh / g, the ICE is only 79%-81%, the discharge mid-voltage is only 3.25-3.40 V, the gram capacity at 1.0C is only 99-108 mAh / g, and the gram capacity at 5.0C is only 87-90 mAh / g;

[0132] As in Examples 1, 4-9, the high-temperature vapor deposition temperature, time, and cooling mode of the reaction of vanadium trifluoride VF3(s) with carbon-coated sodium vanadium phosphate are also particularly important, and it is found through experiments that the temperature is preferably 850-1200°C (preferably 1000-1100°C), the heat treatment time is preferably 0.5-2h (preferably 0.95-1.3h), and the time for cooling to room temperature after the heat treatment is preferably 30s-10min (preferably 2-3min), and the battery performance of the prepared material is relatively excellent, the gram capacity at 0.2C is 120-141 mAh / g, the ICE is 85%-94%, the discharge mid-voltage is 3.25-3.45 V, the gram capacity at 1.0C is 108-128 mAh / g, and the gram capacity at 5.0C is 98-120 mAh / g; when the high-temperature vapor deposition temperature is low or the time is short, the prepared material also contains unreacted vanadium trifluoride VF3 and carbon-coated sodium vanadium phosphate, and when the high-temperature vapor deposition temperature is high or the time is long, impurities such as sodium vanadium trifluorophosphate are likely to be generated, as in Comparative Examples 5-8, the gram capacity at 0.2C is only 100-110 mAh / g, the ICE is only 76%-83%, the discharge mid-voltage is only 3.02-3.45 V, the gram capacity at 1.0C is only 81-100 mAh / g, and the gram capacity at 5.0C is only 70-88 mAh / g;

[0133] In addition, the cooling time after the high-temperature vapor deposition reaction is short (1-5 s), which produces a large amount of energy consumption, but the electrochemical performance of the material is not obviously improved compared with Example 1 (Comparative Example 9). When the cooling time is too long, thermal decomposition of the formed sodium vanadium fluorophosphate may occur during the cooling stage, producing sodium vanadium trifluorophosphate and sodium vanadium phosphate, resulting in low material performance. For example, in Comparative Examples 10-11, the specific capacity at 0.2 C is only 109-120 mAh / g, the ICE is only 76%-83%, the discharge midpoint voltage is only 3.21-3.27 V, the specific capacity at 1.0 C is only 80-108 mAh / g, and the specific capacity at 5.0 C is only 70-95 mAh / g;

[0134] At the same time, the high-temperature vapor deposition reaction process is carried out in a sealed reaction kettle to avoid loss of elements. At this time, the filling amount and gas pressure of the sealed reaction kettle also affect the above process. When the filling amount of the sealed reaction kettle is 45%-75% (volume ratio) and the gas pressure is 0.5-5 MPa, the prepared material has excellent performance. For example, in Examples 1, 10-13, the specific capacity at 0.2 C is 100-110 mAh / g, the ICE is only 90%-93%, the discharge midpoint voltage is only 3.30-3.38 V, the specific capacity at 1.0 C is only 115-126 mAh / g, and the specific capacity at 5.0 C is only 100-117 mAh / g. When the filling amount and applied pressure are low, the reaction of vanadium trifluoride VF3 and carbon-coated sodium vanadium phosphate is not complete. When the filling amount and applied pressure are high, impurities such as sodium vanadium trifluorophosphate are easily present in the product. For example, in Comparative Examples 12-15, the specific capacity at 0.2 C is only 105-113 mAh / g, the ICE is only 78%-84%, the discharge midpoint voltage is only 3.22-3.42 V, the specific capacity at 1.0 C is only 84-89 mAh / g, and the specific capacity at 5.0 C is only 70-80 mAh / g.

[0135] In addition, the traditional solid-phase calcination method (Comparative Example 1) cannot obtain pure-phase monoclinic sodium vanadium fluorophosphate material. When an excess of fluorine source is used (Comparative Example 2), impurities such as sodium vanadium trifluorophosphate are easily generated, and the performance is slightly better than that of the existing high-temperature molten monoclinic sodium vanadium fluorophosphate (Comparative Example 16).

Claims

1. A process for producing a monoclinic sodium vanadium fluorophosphate, characterized by, The method comprises: Step (1), obtaining carbon-coated sodium vanadium phosphate, denoted as sodium vanadium phosphate@C, the mass content of the coated carbon being 0.5wt%-15wt%, preferably 6wt%-12wt%; Step (2), mixing sodium vanadium phosphate@C and vanadium trifluoride powder VF3(s), and obtaining the monoclinic sodium vanadium fluorophosphate through chemical vapor deposition reaction of the mixture; the metering ratio of the vanadium trifluoride powder VF3(s) to the sodium vanadium phosphate@C being 1.0-1.3:1.0, preferably 1.15-1.2:1; the metering ratio being based on the molar amount of the vanadium trifluoride powder VF3(s) and the sodium vanadium phosphate@C; The chemical vapor deposition reaction in step (2) needs to be carried out in a closed reaction kettle, the loading amount of the mixture in the reaction kettle being 45%-75% (volume ratio), the gas pressure in the reaction kettle being 0.5Mpa-5Mpa, the gas atmosphere in the reaction kettle being one or more than two of argon, nitrogen, helium and other inert gases; the reaction temperature of the mixture in the reaction kettle being 850°C-1200°C, preferably 1000°C-1100°C, and the reaction time being 0.5h-2h, preferably 0.95-1.3h.

2. The preparation method according to claim 1, characterized in that: The mixing of the sodium vanadium phosphate@C and the vanadium trifluoride powder VF3(s) also needs to be uniformly mixed, and the mixing mode is one or both of ball milling and tank milling; The cooling time to room temperature after the heat treatment is 30s-10min, preferably 2min-3min.

3. The preparation method according to claim 1, characterized in that, The preparation method of the carbon-coated sodium vanadium phosphate in step (1) is one or more than two of sol-gel high-temperature calcination, spray drying-high temperature calcination, solid phase mixing-high temperature calcination, and high-temperature melting-high temperature calcination.

4. Monoclinic sodium vanadium fluorophosphate prepared by the preparation method of any one of claims 1-3.

5. The use of the monoclinic sodium vanadium fluorophosphate according to claim 4, characterized in that The monoclinic sodium vanadium fluorophosphate can be used as a positive electrode active material and / or a negative electrode active material in a sodium ion battery positive electrode and / or a negative electrode.

6. A positive electrode or a negative electrode for a sodium ion battery, characterized in that, The positive electrode or the negative electrode contains the monoclinic sodium vanadium fluorophosphate according to claim 4; Preferably, the content of the monoclinic sodium vanadium fluorophosphate in the positive electrode material or the negative electrode material is 65-95wt%; Preferably, the positive electrode material further contains a conductive agent and a binder, and the mass ratio of the monoclinic sodium vanadium fluorophosphate, the conductive agent and the binder is (65-95):(5-35):(5-35); Preferably, the negative electrode material further contains a conductive agent and a binder, and the mass ratio of the monoclinic sodium vanadium fluorophosphate, the conductive agent and the binder is (65-95):(5-35):(5-35).

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