Method for preparing bulk phase-surface layer gradient doped niobium-titanium-oxygen negative electrode material by adopting fused salt electrochemical method and application of bulk phase-surface layer gradient doped niobium-titanium-oxygen negative electrode material

Abstract

The invention discloses a method for preparing a bulk phase-surface layer gradient doped niobium titanium oxygen negative electrode material by adopting a fused salt electrochemical method and application, and belongs to the field of negative electrode material preparation. The method comprises the following specific steps: preparing a niobium-titanium-oxygen sheet as a cathode; the method comprises the following steps: by taking a LiCl-KCl eutectic mixture as basic fused salt, sequentially adding a first group of dopants and a second group of dopants; in an inert atmosphere, electrolysis is firstly carried out at a relatively low voltage to reduce the first group of elements and diffuse to a bulk phase; electrolysis is carried out under high voltage, so that the second group of elements are reduced and enriched on the surface layer; and washing, drying and grinding the product to obtain the negative electrode material with component and functional gradients from a bulk phase to a surface layer. A continuous gradient structure with a bulk phase rich in high-conductivity elements and a surface layer rich in fast ion elements is constructed in a single particle. According to the structure, the intrinsic electron conductivity, the bulk phase ion diffusion rate and the interface ion migration kinetics of the material are synergistically improved, and the low-temperature fast charging performance, the rate capability and the cycling stability of the material serving as the negative electrode of the lithium ion battery are remarkably enhanced.

Description

Technical Field

[0001] This invention belongs to the field of secondary batteries, specifically relating to a method and application for preparing bulk-surface gradient-doped niobium titanium oxide anode materials using molten salt electrochemical method. Background Technology

[0002] With the increasing performance requirements of electrochemical energy storage devices in fields such as electric vehicles, large-scale energy storage, and extreme environment applications, the development of lithium-ion batteries that combine high energy density, high power output, and excellent low-temperature performance has become an urgent industry need. Traditional graphite anodes have lithium intercalation potentials close to the deposition potential of metallic lithium at low temperatures, posing a serious risk of lithium dendrite growth, leading to decreased safety and rapid capacity decay. Therefore, finding novel anode materials with higher and safer operating potentials is crucial. Among numerous candidate materials, niobium titanium oxide (represented by TiNb2O7) stands out due to its high theoretical specific capacity (approximately 387 mAh g⁻¹). -1 ), and a safe lithiation potential (approximately 1.65 V vs. Li). + With its excellent bulk structural stability, it is considered a highly promising high-power, high-safety anode material.

[0003] However, TiNb2O7's low intrinsic electronic conductivity severely limits its rate performance. More critically, its bulk lithium-ion diffusion barrier is high, and the ion desolvation process at the electrode / electrolyte interface becomes extremely slow at low temperatures. These factors collectively lead to severe polarization, insufficient capacity utilization, and limited fast-charging capability at high rates, especially at low temperatures. Current mainstream modification strategies, such as nano-sizing, carbon coating, surface modification, or bulk uniform doping, can improve individual performance indicators to some extent, but they are difficult to achieve a synergistic improvement in electronic conductivity, bulk ion diffusion, and interfacial ion dynamics. For example, while nano-sizing shortens the ion diffusion distance, it sacrifices the electrode's tap density and volumetric energy density; uniform doping cannot optimize the ion transport environment near the surface region while improving bulk conductivity; and simple surface coatings may introduce additional interfacial impedance and pose a risk of peeling during cycling.

[0004] Therefore, existing technologies urgently need a novel material design and preparation method that can break through the limitations of traditional modification approaches and achieve controllable, gradient functional construction from the bulk phase to the surface layer within niobium titanium oxide particles. This would synergistically and specifically address the three interconnected yet spatially distinct bottleneck problems of poor electronic conductivity, slow bulk ion diffusion, and sluggish interfacial dynamics, ultimately leading to the development of next-generation niobium titanium oxide anode materials with excellent comprehensive performance, especially outstanding low-temperature fast-charging performance. Summary of the Invention

[0005] The purpose of this invention is to address the aforementioned problems in the background art by providing a method and application for preparing bulk-to-surface gradient-doped niobium-titanium oxide anode materials using molten salt electrochemistry. This method actively constructs a compositional and functional gradient from the bulk to the surface within the niobium-titanium oxide particles through programmed molten salt electrochemistry, thereby simultaneously and specifically enhancing its intrinsic electronic conductivity, bulk lithium-ion diffusion rate, and electrode-electrolyte interface reaction kinetics. Ultimately, this yields a novel anode material exhibiting high rate performance, excellent cycle stability, and superior low-temperature charging capability. This method offers strong process controllability, providing a new material design paradigm and feasible preparation path for developing next-generation lithium-ion batteries suitable for wide-temperature-range, high-power applications.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing bulk-surface gradient-doped niobium-titanium oxide anode materials using molten salt electrochemistry, the method comprising the following steps:

[0008] Step 1: Niobium titanium oxide (TiNb2O7) is prepared using the traditional high-temperature solid-state method and pressing sintering method: Titanium dioxide (TiO2) and niobium pentoxide (Nb2O5) powders are weighed in a molar ratio of 1:1. The powders are placed in a ball mill jar containing an appropriate amount of anhydrous ethanol and ground thoroughly to make the mixture uniform. The slurry is collected and dried in a forced-air drying oven. The dried precursor powder is pressed into a disc blank.

[0009] Step 2: Place the disc blank from Step 1 into an alumina crucible and calcine it in a tube furnace. After cooling to room temperature, obtain a dense, high-mechanical-strength grayish-white TiNb2O7 disc, which will be used as the cathode working electrode for subsequent molten salt electrolysis.

[0010] Step 3: Weigh lithium chloride (LiCl) and potassium chloride (KCl) according to the mass ratio, grind and mix thoroughly to obtain a LiCl-KCl eutectic mixture; add anhydrous stannous chloride (SnCl2), and optionally add sodium tungstate (Na2WO4) and / or ammonium molybdate ((NH4)6Mo7O 24 ·4H2O), mix all components thoroughly in a mortar to obtain the mixed molten salt A required for the first step of electrolysis; weigh anhydrous zirconium chloride (ZrCl4), and optionally add anhydrous aluminum chloride (AlCl3) as the mixed molten salt B required for the second step of electrolysis;

[0011] Step 4: Place the mixed molten salt A from Step 3 into a corundum crucible as the molten salt electrolyte; connect the TiNb2O7 disc prepared in Step 2 to the cathode lead via a molybdenum or nickel wire and completely immerse it in the mixed molten salt A; insert a high-purity graphite rod into the molten salt as the anode; place the crucible in the constant temperature zone of a tube furnace and seal the furnace; throughout the electrolysis process, always keep the exhaust port of the anode zone connected to two tail gas absorption bottles containing 1-2M sodium hydroxide solution to absorb the chlorine gas generated during electrolysis; introduce inert gas into the sealed tube furnace, then raise the temperature to a certain level and hold it to ensure the molten salt is completely melted and mixed evenly; after the temperature stabilizes, apply a constant DC voltage between the cathode and anode to dissolve the Sn... 2+ W 6+ Mo 6+ Plasma reduction;

[0012] Step 5: After the first step of electrolysis is completed, disconnect the power supply and, while continuously purging inert gas, lower the furnace temperature below the solidification point of the molten salt. Open the furnace body, and under the protection of the glove box, add the mixed molten salt B into the crucible, then reseal the furnace body. Purge inert gas again, raise the furnace temperature back to a certain level, and hold the temperature to allow the molten salt to mix evenly again. Then, apply a constant DC voltage and continue electrolysis to reduce Zr. 4+ And Al 3+ After the second electrolysis step is completed, the power supply is disconnected, and under the protection of inert gas, the TiNb2O7 disc is lifted above the molten salt surface and cooled to room temperature.

[0013] Step 6: Place the TiNb2O7 cathode electrode in a centrifuge tube, add anhydrous ethanol, sonicate, and then centrifuge to collect the precipitate; then add deionized water, sonicate and centrifuge; transfer the obtained solid product to a vacuum drying oven to dry; finally, grind the vacuum-dried solid thoroughly in a mortar and sieve to obtain Sn / W / Mo-Zr / Al gradient modified niobium titanium oxide powder (g-Sn / W / Mo-Zr / Al@TNO).

[0014] Further, in step one, the anhydrous ethanol is 25-40 mL, the rotation speed is 200-400 rpm, and the ball milling time is 6-12 h; the drying temperature is 60-90℃, and the time is 10-20 h; the pressure required to compress the precursor into tablets is 10-40 MPa, the tablet diameter is 0.8-1.6 cm, and the tablet thickness is 0.1-0.4 cm; preferably, the preform is a round tablet with a diameter of 1.2 cm and a thickness of 0.2 cm.

[0015] Furthermore, in step two, the calcination temperature is 1000~1500℃, the time is 20~32 h, and the heating rate is 3~10 ℃ / min.

[0016] Further, in step three, the mass ratio of LiCl / (LiCl and KCl) is 40%~42%, and the LiCl+KCl mixed molten salt, SnCl2, Na2WO4, and (NH4)6Mo7O are present. 24 The masses of ·4H2O, ZrCl4, and AlCl3 are 20~50 g, 0.3~0.5 g, 0~0.5 g, 0~0.8 g, 0.1~0.5 g, and 0~0.3 g, respectively; the mass of the LiCl+KCl mixed molten salt accounts for 95%~98% of the total mass, and the other salts account for 2%~5% of the total mass; the mixed molten salt is ground by manual grinding or ball milling for 30~60 min.

[0017] Further, in step four, the inert gas is one of nitrogen, argon, or a hydrogen-argon mixture (volume ratio of hydrogen to argon is 5:95), the heating rate is 3~10 ℃ / min, the holding time is 20~120 min, the specific temperature is 300~700℃, the electrolysis time is 0.5~6 h, and the voltage applied during the electrolysis process is 0.5~2.8 V.

[0018] Further, in step five, the inert gas is one of nitrogen, argon, or a hydrogen-argon mixture (volume ratio of hydrogen to argon is 5:95), the heating rate is 3~10 ℃ / min, the holding time is 20~120 min, the specific temperature is 300~700℃, the electrolysis time is 0.5~6 h, and the voltage applied during the electrolysis process is 0.5~2.8 V.

[0019] Furthermore, in step six, the centrifugation speed is 8000~10000 rpm, the number of centrifugations is 3~5 times, and the centrifugation time is 10~15 min each time; the vacuum drying temperature is 60~90℃, and the time is 10~20 h; the sieve size is 300~600 mesh, preferably, a 500 mesh sieve is used.

[0020] An application of the niobium-titanium oxide anode material prepared by the above preparation method, wherein the g-Sn / W / Mo-Zr / Al@TNO material is used as an anode active material in the anode of a low-temperature lithium-ion battery, wherein the operating temperature range of the low-temperature lithium-ion battery is -40℃ to 60℃.

[0021] Furthermore, the g-Sn / W / Mo-Zr / Al@TNO material is used as the negative electrode active material and mixed with a conductive agent and a binder in N-methylpyrrolidone (NMP) solvent to form a uniform slurry; the slurry is coated on copper foil, dried, rolled, and sliced ​​to obtain a negative electrode sheet, and then assembled into a button cell.

[0022] The conductive agent is Ketjen Black or Super P, and the binder is PVDF and NMP (wherein, PVDF accounts for 2~8% of the total mass of PVDF+NMP).

[0023] The mass ratio of the Sn / W / Mo-Zr / Al gradient modified niobium titanium oxide material, conductive agent, and binder is 86%~96%:7%~2%:7%~2%, and the thickness of the negative electrode coating is 100~180 μm.

[0024] The drying temperature is 75~85 ℃, the drying time is 8~22 h, and the rolling time is 4~10 min; the slice diameter is 10 / 14 / 16 mm, and the negative electrode sheet is preferably 14 mm in diameter.

[0025] The advantages of this invention over the prior art are as follows:

[0026] (1) The innovative gradient structure design of this invention, characterized by "bulk phase rich in high-conductivity elements and surface layer rich in fast-ion elements," spatially decouples and synergizes the traditionally mutually restrictive requirements of improving electronic conductivity (mainly in the bulk phase) and accelerating interfacial ion transport (mainly in the surface layer). The doping of elements such as Sn, W, and Mo in the bulk phase constructs a highly efficient internal electronic conductivity network, while W... 6+ Mo 6+ High-valence doping-induced lattice distortion and controllable defects provide rapid bulk diffusion channels for lithium ions. Surface-enriched Zr and Al elements effectively modulate the interfacial electronic structure, significantly enhancing the material's adsorption capacity for solvated lithium ions, thereby greatly accelerating the rate-limiting desolvation step at low temperatures. This spatial functional partitioning enables the modified niobium-titanium oxide material to achieve high intrinsic electronic conductivity, fast bulk ion diffusion rate, and excellent interfacial reaction kinetics, thus realizing superior rate performance, high capacity, and long cycle life at low temperatures.

[0027] (2) The programmed molten salt electrolysis process proposed in this invention, which combines "stepwise feeding" and "stepwise voltage increase," achieves for the first time the active design and controllable preparation of the spatial distribution of multiple functional elements within electrode material particles through precise control of voltage (thermodynamic driving force) and time (kinetic diffusion process). This method is applicable not only to niobium-titanium oxide systems, but its core idea of ​​"time-sequence-voltage programmed doping" also provides a general new paradigm for the gradient and functional modification of other electrode materials, exhibiting high process adjustability and repeatability.

[0028] (3) This invention prepares a niobium titanium oxide material with gradient doping from the bulk phase to the surface layer. The continuous gradient change in its composition forms a natural "stress buffer layer," which effectively alleviates the lattice stress concentration caused by volume changes or differences in properties in different regions during lithium ion insertion / extraction, and avoids particle pulverization that may be caused by high-concentration uniform doping. At the same time, the doping achieved by molten salt electrochemical reduction is an in-situ, atomic-level chemical bond. The doped atoms are firmly bonded to the niobium titanium oxide bulk, which is much more stable than the modified layer of physical coating or simple adsorption, ensuring the structural integrity and interface durability of the material during long-term electrochemical cycling.

[0029] (4) The present invention uses the LiCl-KCl system with low eutectic point as the molten salt medium, which significantly reduces the synthesis temperature from the traditional high temperature (above 700℃) molten salt electrolysis to about 500℃, greatly reducing energy consumption and the extreme requirements for equipment to be resistant to high temperature and corrosion. On the other hand, it effectively suppresses the violent volatilization and side reactions of highly volatile chlorides at high temperature, greatly improving the safety and controllability of experimental operation, and is more conducive to the realization of large-scale production. Attached Figure Description

[0030] Figure 1 A schematic diagram of the two-step molten salt electrolysis for synthesizing Sn / W / Mo-Zr / Al gradient modified niobium titanium oxide powder;

[0031] Figure 2 The graph shows the rate performance of the battery assembled with the g-Sn / W / Mo-Zr / Al@TNO modified negative electrode in Example 1 at -40℃.

[0032] Figure 3 Comparison of cycle stability of batteries assembled with g-Sn / W / Mo-Zr / Al@TNO modified anode and unmodified TNO anode at -40℃ and 1 C. Detailed Implementation

[0033] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are only some embodiments of the invention, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0034] Example 1:

[0035] A method for preparing bulk-surface gradient-doped niobium titanium oxide anode materials using molten salt electrochemistry and its application, comprising the following steps:

[0036] Step 1: TiNb2O7 was prepared using the traditional high-temperature solid-state method and pressing sintering method; 0.23 g TiO2 and 0.77 g Nb2O5 powder were weighed and placed in a ball mill jar containing 35 mL anhydrous ethanol and ground at 300 rpm for 10 h; the thoroughly mixed slurry was collected and dried in a forced-air drying oven at 80℃ for 12 h; then the dried precursor powder was pressed into a circular blank with a diameter of 1.2 cm and a thickness of 0.25 cm under a uniaxial pressure of 20 MPa;

[0037] Step 2: Place the preform in an alumina crucible, put it in a tube furnace, heat it at 5 °C / min and calcine it at 1250 °C for 24 h. After cooling to room temperature, a dense, grayish-white TiNb2O7 material with high mechanical strength is obtained, which is used as the cathode working electrode for subsequent molten salt electrolysis.

[0038] Step 3: Weigh 12.45 g LiCl and 17.55 g KCl into a mortar, add 0.45 g SnCl2 and 0.30 g Na2WO4 to it in sequence, grind for 45 min to obtain the mixed molten salt A required for the first step of electrolysis; weigh 0.30 g ZrCl4 and 0.15 g AlCl3 as mixed molten salt B for the second step of electrolysis.

[0039] Step 4: Place the mixed molten salt A in a corundum crucible. Connect the TiNb2O7 sheet to the cathode lead using a molybdenum or nickel wire and immerse it completely in the mixed molten salt A. Insert a high-purity graphite rod into the molten salt as the anode. Place the crucible in the constant temperature zone of a tube furnace, seal the furnace, and purge with a mixture of hydrogen and argon gas (volume ratio of hydrogen to argon: 5:95) for 30 min. Then, heat the mixture to 500℃ at a rate of 8℃ / min and hold for 60 min to completely melt and homogenize the molten salt A. After the temperature stabilizes, apply a constant DC voltage of 1.5 V between the cathode and anode and continue electrolysis for 1 h to remove Sn. 2+ W 6+ Ion reduction was then performed; subsequently, the power was disconnected, and the furnace temperature was allowed to drop to 300℃. The furnace body was opened, and the mixed molten salt B was added to the crucible. The furnace body was resealed, and a mixture of hydrogen and argon gas (volume ratio of hydrogen to argon was 5:95) was introduced for 30 min. The temperature was then increased to 500℃ at a rate of 8℃ / min and held for 30 min. After the temperature stabilized, a constant DC voltage of 2.6 V was applied between the cathode and anode, and electrolysis was continued for 0.5 h to reduce Zr. 4+ And Al 3+ After electrolysis, turn off the power, raise the TiNb2O7 electrode above the molten salt surface, and cool it to room temperature; the above two steps of molten salt electrolysis are illustrated in the diagram below. Figure 1 As shown.

[0040] Step 5: Take the modified TiNb2O7 electrode out of the glove box, place it in a 50 mL centrifuge tube, add 35 mL of anhydrous ethanol, sonicate for 10 min, centrifuge at 9000 rpm for 10 min, and repeat the operation 3 times; then add 35 mL of deionized water and repeat the above operation; transfer the finally obtained solid product to a vacuum drying oven and dry at 80℃ for 12 h; finally, grind the vacuum-dried solid thoroughly in a mortar and sieve it through a 500-mesh sieve to finally obtain g-SnW-ZrAl@TNO.

[0041] The g-SnW-ZrAl@TNO material obtained in this embodiment is used in the negative electrode of a low-temperature lithium-ion battery to improve the cycle stability and rate performance of the battery at low temperatures.

[0042] Step 6: The prepared g-SnW-ZrAl@TNO material was used as the negative electrode active material and mixed with Super P conductive agent and 5% PVDF binder in NMP solvent at a ratio of 87%:6.5%:6.5% to form a uniform slurry. The slurry was coated on copper foil with a thickness of 140 mm, then dried at 85℃ for 12 h, rolled for 5 min, and cut into discs with a diameter of 14 mm to obtain the negative electrode sheet. Then, a button cell was assembled and its electrochemical performance was tested.

[0043] The g-SnW-ZrAl@TNO material prepared in this embodiment, when applied to the anode of a low-temperature lithium-ion battery, can significantly improve the rate performance and cycle stability of the battery under low-temperature conditions. Figure 2 As shown, the g-SnW-ZrAl@TNO modified battery exhibits excellent rate performance at -40℃. Furthermore, the cycle performance of batteries assembled with g-SnW-ZrAl@TNO and pure TNO anodes was tested at -40℃ and 1 C conditions, as shown in the figure. Figure 3 As shown, the battery assembled with g-SnW-ZrAl@TNO maintained a high discharge specific capacity of 97.66 mAh / g after 280 stable cycles, which is significantly higher than the cycle stability of the pure TNO-assembled battery (77.29 mAh / g after 280 cycles). These results indicate that the low-temperature performance of the g-Sn / W / Mo-Zr / Al@TNO modified battery is significantly better than that of the unmodified sample. This performance improvement is mainly attributed to the following mechanism: the Sn and W dopants in the bulk phase construct an efficient internal electronic conductivity network, while W... 6+ High-valence doping-induced lattice distortion and controllable defects in Li + It provides a rapid bulk diffusion channel. The Zr and Al elements enriched on the surface effectively modulate the interfacial electronic structure, significantly enhancing the material's resistance to solvated Li. +The adsorption capacity significantly accelerates the rate-limiting desolvation step at low temperatures. This spatial functional partitioning enables the modified niobium-titanium oxide material to achieve high intrinsic electronic conductivity, fast bulk ion diffusion rate, and excellent interfacial reaction kinetics, thereby realizing excellent rate performance, high capacity, and long cycle life at low temperatures.

[0044] Example 2:

[0045] The difference between the anode material for the low-temperature lithium-ion battery prepared in this embodiment and that in Example 1 is that the Na2WO4 in the mixed molten salt A in Example 1 is replaced with 0.51 g of (NH4)6Mo7O. 24 ·4H2O, the temperature of the first step of electrolysis of molten salt A was changed to 450℃, the electrolysis voltage was changed to 1.2 V and electrolysis was continued for 1.5 h, and the other parameters were the same as in Example 1.

[0046] Example 3:

[0047] The anode material for the low-temperature lithium-ion battery prepared in this embodiment differs from that in Example 1 in that: SnCl2 and Na2WO4 in mixed molten salt A in Example 1 are replaced with 0.5 g SnCl2, ZrCl4 and AlCl3 in mixed molten salt B are replaced with 0.3 g ZrCl4, the electrolysis voltage in the second electrolysis process is changed to 2.8 V, the electrolysis duration is changed to 1 h, and all other parameters are the same as in Example 1.

[0048] Example 4:

[0049] The anode material for the low-temperature lithium-ion battery prepared in this embodiment differs from that in Example 1 in that: the ball milling speed of the TNO precursor in Example 1 is changed to 400 rpm, the ball milling time is changed to 8 h, the pressing diameter is changed to 1 cm, and the thickness is changed to 0.3 cm; the calcination temperature of the TNO precursor is changed to 1350℃; the holding time for dissolving molten salt A in the tube furnace is changed to 90 min, and the holding time for dissolving molten salt B is changed to 60 min. All other parameters are the same as in Example 1.

[0050] Comparative Example 1:

[0051] The anode material for the low-temperature lithium-ion battery prepared in this comparative example differs from that in Example 1 in that: TiNb2O7 (denoted as TNO), prepared in Example 1 using the traditional high-temperature solid-state method and pressing and sintering method, is directly used as the anode active material for the low-temperature lithium-ion battery. The pressing pressure is changed to 30 MPa, and the sintering temperature is changed to 1300℃; the mass ratio of TNO to Super P and 5% PVDF is changed to 90%:5%:5%; the drying conditions are changed to drying at 80℃ for 10 h, rolling for 5 min, and cutting into 14 mm diameter discs. All other parameters are the same as in Example 1. The cycle performance of the button cell assembled with TNO as the anode at -40℃ and 1 C is tested as follows: Figure 3 As shown, the battery assembled with pure TNO exhibits relatively poor cycle stability, with a capacity of only 77.29 mAh / g after 280 cycles, which is far lower than the battery assembled with g-SnW-ZrAl@TNO anode under the same test conditions (which still maintains a high discharge specific capacity of 97.66 mAh / g after 280 cycles).

[0052] The above results show that the low-temperature performance of the g-Sn / W / Mo-Zr / Al@TNO modified battery is significantly better than that of the unmodified sample.

[0053] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing bulk-surface gradient-doped niobium-titanium oxide anode materials using molten salt electrochemical method, characterized in that: The method includes the following steps: Step 1: Niobium titanium oxide (TiNb2O7) is prepared using the traditional high-temperature solid-state method and pressing sintering method: Titanium dioxide (TiO2) and niobium pentoxide (Nb2O5) powders are weighed in a molar ratio of 1:

1. The powders are placed in a ball mill jar containing an appropriate amount of anhydrous ethanol and ground thoroughly to make the mixture uniform. The slurry is collected and dried in a forced-air drying oven. The dried precursor powder is pressed into a disc blank. Step 2: Place the wafer blank from Step 1 into an alumina crucible and calcine it in a tube furnace. After cooling to room temperature, TiNb2O7 wafers are obtained. Step 3: Weigh lithium chloride (LiCl) and potassium chloride (KCl) according to the mass ratio, grind and mix thoroughly to obtain a LiCl-KCl eutectic mixture; add anhydrous stannous chloride (SnCl2), and optionally add sodium tungstate (Na2WO4) and / or ammonium molybdate ((NH4)6Mo7O 24 ·4H2O), mix all components thoroughly in a mortar to obtain the mixed molten salt A required for the first step of electrolysis; weigh anhydrous zirconium chloride (ZrCl4), and optionally add anhydrous aluminum chloride (AlCl3) as the mixed molten salt B required for the second step of electrolysis; Step 4: Place the mixed molten salt A from Step 3 into a corundum crucible as the molten salt electrolyte; connect the TiNb2O7 disc prepared in Step 2 to the cathode lead via a molybdenum or nickel wire and completely immerse it in the mixed molten salt A; insert a high-purity graphite rod into the molten salt as the anode; place the crucible in the constant temperature zone of a tube furnace and seal the furnace; throughout the electrolysis process, always keep the exhaust port of the anode zone connected to two tail gas absorption bottles containing 1-2M sodium hydroxide solution to absorb the chlorine gas generated during electrolysis; introduce inert gas into the sealed tube furnace, then raise the temperature to a certain level and hold it to ensure the molten salt is completely melted and mixed evenly; after the temperature stabilizes, apply a constant DC voltage between the cathode and anode to dissolve the Sn... 2+ W 6+ Mo 6+ Plasma reduction; Step 5: After the first step of electrolysis is completed, disconnect the power supply and, while continuously passing inert gas, lower the furnace temperature below the solidification point of the molten salt. Open the furnace body, and under the protection of the glove box, add the mixed molten salt B into the crucible, then reseal the furnace body; purge with inert gas again, raise the furnace temperature back to a certain level, and hold at that temperature to allow the molten salt to mix evenly again. Then apply a constant DC voltage and continue electrolysis to reduce Zr. 4+ And Al 3+ After the second electrolysis step is completed, the power supply is disconnected, and under the protection of inert gas, the TiNb2O7 disc is lifted above the molten salt surface and cooled to room temperature. Step 6: Place the TiNb2O7 cathode electrode in a centrifuge tube, add anhydrous ethanol, sonicate, and then centrifuge to collect the precipitate; then add deionized water, sonicate and centrifuge; transfer the obtained solid product to a vacuum drying oven to dry; finally, grind the vacuum-dried solid thoroughly in a mortar and sieve to obtain Sn / W / Mo-Zr / Al gradient modified niobium titanium oxide powder (g-Sn / W / Mo-Zr / Al@TNO).

2. The preparation method according to claim 1, characterized in that: In step one, the rotation speed is 200~400 rpm, the ball milling time is 6~12 h, the drying temperature is 60~90℃, and the time is 10~20 h. The pressure required to compress the precursor into tablets is 10~40MPa, the tablet diameter is 0.8~1.6 cm, and the tablet thickness is 0.1~0.4 cm.

3. The preparation method according to claim 1, characterized in that: In step two, the calcination temperature is 1000~1500℃, the time is 20~32 h, and the heating rate is 3~10 ℃ / min.

4. The preparation method according to claim 1, characterized in that: In step three, the mass ratio of LiCl / (LiCl and KCl) is 40%~42%, and the mixed molten salt of LiCl+KCl, SnCl2, Na2WO4, and (NH4)6Mo7O are present. 24 The masses of ·4H2O, ZrCl4, and AlCl3 are 20~50 g, 0.3~0.5 g, 0~0.5 g, 0~0.8 g, 0.1~0.5 g, and 0~0.3 g, respectively; the mass of the LiCl+KCl mixed molten salt accounts for 95%~98% of the total mass, and the other salts account for 2%~5% of the total mass; the mixed molten salt is ground by manual grinding or ball milling for 30~60 min.

5. The preparation method according to claim 1, characterized in that: In step four, the inert gas is one of nitrogen, argon, or a hydrogen-argon mixture (volume ratio of hydrogen to argon is 5:95), the heating rate is 3~10 ℃ / min, the holding time is 20~120 min, the specific temperature is 300~700℃, the electrolysis time is 0.5~6 h, and the voltage applied during the electrolysis process is 0.5~2.8 V.

6. The preparation method according to claim 1, characterized in that: In step five, the inert gas is one of nitrogen, argon, or a hydrogen-argon mixture (volume ratio of hydrogen to argon is 5:95), the heating rate is 3~10 ℃ / min, the holding time is 20~120 min, the specific temperature is 300~700℃, the electrolysis time is 0.5~6 h, and the voltage applied during the electrolysis process is 0.5~2.8 V.

7. The preparation method according to claim 1, characterized in that: In step six, the centrifugation speed is 8000~10000 rpm, the number of centrifugations is 3~5 times, and the centrifugation time is 10~15 min each time; the vacuum drying temperature is 60~90℃, and the time is 10~20 h; the sieve size is 300~600 mesh.

8. The application of a niobium-titanium-oxygen anode material prepared by the preparation method according to any one of claims 1 to 7, characterized in that: The g-Sn / W / Mo-Zr / Al@TNO material is used as the negative electrode active material in the negative electrode of a low-temperature lithium-ion battery, the operating temperature range of which is -40℃ to 60℃.

9. The application according to claim 8, characterized in that: The g-Sn / W / Mo-Zr / Al@TNO material is used as the negative electrode active material and mixed with a conductive agent and a binder in N-methylpyrrolidone (NMP) solvent to form a uniform slurry; the slurry is coated on copper foil, dried, rolled, and sliced ​​to obtain the negative electrode sheet, and then assembled into a button cell. The conductive agent is Ketjen Black or Super P, and the binder is PVDF and NMP (wherein, PVDF accounts for 2~8% of the total mass of PVDF+NMP). The mass ratio of the Sn / W / Mo-Zr / Al gradient modified niobium titanium oxide material, conductive agent, and binder is 86%~96%:7%~2%:7%~2%, and the thickness of the negative electrode coating is 100~180 μm. The drying temperature is 75~85 ℃, the drying time is 8~22 h, and the rolling time is 4~10 min; the slice diameter is 10 / 14 / 16 mm, and the negative electrode sheet is preferably 14 mm in diameter.

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