A p2-type nacrfti layered oxide, a preparation method thereof and application thereof as an electrode material

Abstract

This invention discloses a P2-type NaCrFeTi layered oxide, which is obtained by high-temperature solid-state sintering using sodium carbonate, iron oxide, chromium oxide, and titanium dioxide as raw materials. 0.67 Cr 1 / 3 Fe 1 / 3 Ti 1 / 3O2 layered oxide NCFT-33 / 33; belonging to the P2 phase structure of the P63 / mmc space group; microstructure is a hexagonal prism structure with a particle size of 1-3 micrometers, exhibiting a single-crystal structure; the main phase content reaches over 95%. Its preparation method is a one-step sintering method, where raw materials are ball-milled and mixed according to the chemical formula ratio, followed by sintering and natural cooling. When used as a sodium-ion battery anode material, the reversible specific capacity is 60.00-80.00 mAh g. ‑1 When the number of cycles is 3000, the capacity retention rate is 91-93%, and the capacity loss rate per cycle is only 0.0023-0.0030%.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery anode material technology, specifically to a P2-type NaCrFeTi layered oxide, its preparation method, and its application as an electrode material. Background Technology

[0002] Sodium-ion batteries have the advantages of readily available raw materials and safety. Among the anode materials for sodium-ion batteries, the two most important types are hard carbon anode materials and titanium-based oxide materials.

[0003] Regarding the technical problems of hard carbon anode materials, as can be seen from existing literature 1 (Ultrafast Synthesis of HardCarbon Anodes for Sodium-ion Batteries: An Intense-Pulsed-Light-Assisted Approach to Photothermal Carbonization of Polymer / Carbon Nanotube Composite Films, Small Methods, 2025, 2401801. https: / / doi.org / 10.1002 / smtd.202401801), this technical solution uses agricultural waste olive shells as raw materials. By controlling the carbonization temperature and pre-carbonization strategy, a hard carbon anode material with excellent sodium storage performance was successfully prepared, achieving a current density of 1 A g. -1 Under certain conditions, the technical effect is that the capacity retention rate is 87% after 1000 cycles. However, this type of hard carbon anode material has the characteristic that more than half of the discharge capacity is distributed on an extremely low discharge plateau of 0-0.1V. According to existing literature 2 (Electrochemical NaInsertion and Solid Electrolyte Interphase for Hard-Carbon Electrodes and Application to Na-Ion Batteries, Adv. Funct. Mater., 2011.21:3859-3867. https: / / doi.org / 10.1002 / adfm.201100854), all organic electrolytes will undergo electrochemical decomposition near 0V, resulting in capacity decay. Furthermore, the heat released by the decomposition reaction can also lead to thermal runaway safety issues, referred to as voltage problems. As can be seen from existing literature 1 and literature 2, due to the characteristics of hard carbon anode materials, the above-mentioned voltage problem cannot be completely solved.

[0004] Regarding titanium-based oxides, existing literature 3 (An O3-type Oxide with Low Sodium Content as the Phase-Transition-Free Anode for Sodium-Ion Batteries, Angew. Chem. Int. Ed., 2018, 57, 7056. https: / / doi.org / 10.1002 / anie.201801923) uses the traditional solid-state sintering method to prepare O3-type Na with low Na content. 0.66 Mg 0.34 Ti 0.66 O2 oxide materials. Achieving a 98mAh / g capacity at 0.1C rate within a voltage range of 0.4-2.0V. -1 The technology achieves reversible specific capacitance and an average voltage of 0.83V. A comparison with existing literature shows that the technology based on titanium-based oxides (Ti) exhibits superior performance. 3+ / Ti 4+ The redox potential is 0.4-0.8V, meaning that the electrolyte decomposition problem caused by voltage issues mentioned above does not exist. However, this technical solution suffers from poor cycle stability. There are two main reasons for this poor cycle stability:

[0005] 1. The failure problem caused by gas generation due to trivalent titanium is as follows: trivalent titanium itself has poor stability. In this technical solution, the Ti element transforms between trivalent and tetravalent titanium. Therefore, after 128 cycles, the capacity retention drops to 94.2%. In the presence of trivalent titanium, it is easy to react with the electrolyte to generate gas, which will cause battery failure. This technical problem is referred to as the failure problem caused by gas generation due to trivalent titanium.

[0006] 2. Regarding the structural changes caused by the O3 phase transition, according to existing literature 4 (Practical Cathodes for Sodium-Ion Batteries: Who Will Take The Crown? Adv. Energy Mater., 2023, 13, 2301975. https: / / doi.org / 10.1002 / aenm.202301975), P2-type layered materials have a lower Na diffusion activation energy than O3-type layered materials. In O3-type materials, the migration path of sodium ions is from an octahedral position through a tetrahedral position and back to an octahedral position. The transition state at the tetrahedral position has a higher energy, resulting in a larger energy barrier. In P2-type materials, the migration path of sodium ions is from a hexagonal prism to a hexagonal prism and back to a hexagonal prism, resulting in a smaller migration energy barrier. On the other hand, O3-type layered materials often undergo an "O3-P3-O3" phase transition during the insertion and extraction of Na ions, which leads to a decrease in rate performance and cycling performance. In contrast, P2 materials tend to maintain the P2 structure during cycling, which can avoid the above problems and ensure that they still have performance at high rates. Summary of the Invention

[0007] The purpose of this invention is to provide a P2-type NaCrFeTi layered oxide, its preparation method, and its application as an electrode material.

[0008] To address the technical problems existing in the current technology, the following methods are adopted to solve the above problems:

[0009] 1. By adjusting the ratio of transition metal elements and the sodium content, a P2 phase structure NaCrFeTi layered oxide was prepared, which improved the cycle performance and phase purity.

[0010] 2. By adjusting the voltage range, the redox reaction of Ti and Cr is suppressed during the charging and discharging process. The high reversibility of Fe, i.e., the change of Fe valence, is used to achieve charge balance and improve long-cycle stability.

[0011] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0012] A P2-type NaCrFeTi layered oxide is obtained by high-temperature solid-state sintering using sodium carbonate, iron oxide, chromium oxide, and titanium dioxide as raw materials. 0.67 Cr 1 / 3 Fe 1 / 3 Ti 1 / 3O2 layered oxide NCFT-33 / 33; the NCFT-33 / 33 layered oxide belongs to the P2 phase structure of space group P63 / mmc; the microstructure of the NCFT-33 / 33 layered oxide is a hexagonal prism structure with a particle size of 1-3 micrometers, and it has a single crystal structure; the main phase of the NCFT-33 / 33 layered oxide is Na. 0.67 Fe 1 / 3 Cr 1 / 3Ti 1 / 3 O2 content reaches 95-98% or more.

[0013] A method for preparing P2 type NaCrFeTi layered oxide includes the following steps: A one-step sintering method is used. First, sodium carbonate, iron oxide, chromium oxide, and titanium dioxide are prepared as raw materials to meet certain chemical formulas, with an additional 5 wt.% loss on ignition for Na. Then, under certain conditions, the raw materials are ball-milled to obtain a mixture. Afterward, the mixture is centrifuged, filtered, and dried to obtain a precursor, which is then pressed into tablets. Finally, under certain conditions, the precursor is sintered. After sintering, it is naturally cooled to obtain NaCrFeTi layered oxide, abbreviated as NCFT.

[0014] Among them, the chemical formula is Na 0.67 Cr 1 / 3 Fe 1 / 3 Ti 1 / 3 O2, that is, the molar ratio of Na:Cr:Fe:Ti is 2:1:1:1, or simply the Cr / Fe ratio is 0.33:0.33;

[0015] The chromium oxide is a Cr source;

[0016] The conditions for ball milling are as follows: under air conditions, the ball milling speed is 300-500 r / min, the ball milling time is 3-6 h, and the ball milling medium is anhydrous ethanol.

[0017] The sintering conditions are as follows: sintering temperature of 800-1000℃ and sintering time of 10-15h under argon atmosphere.

[0018] When P2-type NaCrFeTi layered oxide is used as a negative electrode material for sodium-ion batteries, it performs well in the voltage range of 0.8-2.7V and at a current density of 20 mAg. -1 Under these conditions, the reversible specific capacity is 60.00-80.00 mAh g. -1 It achieves a theoretical specific capacity of 89.36 mAh g. -1 The reversible specific capacity was 67.1-89.53% under the following conditions: voltage range of 0.8-2.7V, current density of 10C, and 3000 cycles; 52.36-56.47 mAh g.-1 The capacity retention rate is 91-93%, and the capacity loss rate per cycle is only 0.0023-0.0030%.

[0019] The beneficial technical effects obtained by this invention have been tested and confirmed to be:

[0020] EDS analysis showed that the prepared NaCrFeTi layered oxide contained Na, Cr, Fe and Ti elements, and the elements were evenly distributed.

[0021] XRD testing and refinement revealed the presence of Na in the NaCrFeTi layered oxide. 0.67 Fe 1 / 3 Cr 1 / 3 Ti 1 / 3 O2 and Cr 0.2 Fe 1.8 The characteristic peaks of O3, and Na 0.67 Fe 1 / 3 Cr 1 / 3 Ti 1 / 3 The O2 content reaches 95-98%, and the impurity phase is Cr. 0.2 Fe 1.8 O3 is present, with a content of 2-5%; in addition, Na 0.67 Fe 1 / 3 Cr 1 / 3 Ti 1 / 3 O2 belongs to the P2 phase structure in the P63 / mmc space group, thus confirming that the material has a P2 phase structure. It is demonstrated that by adjusting the elemental ratios and changing the precursor, two substantially different structures, P2 and O3 phases, can be obtained. Changing the ratios can reduce the content of impurity phases and improve the phase purity of the material.

[0022] SEM and TEM tests showed that the NaCrFeTi layered oxide has a hexagonal prism structure, a smooth surface, and a particle size of 1-3 micrometers. Interlayer spacing is It has a single-crystal structure.

[0023] Charge-discharge performance tests showed that the NaCrFeTi layered oxide electrode material exhibited good performance in the voltage range of 0.8-2.7V and a current density of 20 mAg. -1 Under these conditions, 60.00-80.00mAh g -1 It achieves a theoretical specific capacity of 89.36 mAh g. -1 67.1%-89.53%.

[0024] Long-term cycling tests show that the NaCrFeTi layered oxide electrode material exhibits a reversible specific capacity of 52.36-56.47 mAh g⁻¹ under conditions of 0.8-2.7 V voltage and 10 C current density, after 3000 cycles. -1 The capacity retention rate is 91-93%, and the capacity loss rate per cycle is only 0.0023-0.0030%.

[0025] Therefore, the present invention has the following advantages over the prior art:

[0026] 1. Compared with existing technologies, the prepared P2-type NaCrFeTi layered oxide has a higher redox potential, which can avoid the electrochemical decomposition of organic electrolytes and improve safety performance.

[0027] 2. Compared with the existing technology, in the prepared P2 type NaCrFeTi layered oxide, Cr and Ti play a role in stabilizing the structure. The redox of Fe during charge and discharge has high reversibility and stability, which can improve the long-cycle stability of the material. Attached image description:

[0028] Figure 1 EDS plot of NCFT-33 / 33 in Example 1;

[0029] Figure 2 The image shown is the refined XRD pattern of NCFT-33 / 33 in Example 1.

[0030] Figure 3 The image shown is a SEM image of NCFT-33 / 33 in Example 1.

[0031] Figure 4 This is a TEM image of NCFT-33 / 33 in Example 1;

[0032] Figure 5 The charge-discharge specific capacity diagram of the electrode material NCFT-33 / 33 in Example 1 is shown.

[0033] Figure 6 This is a long-cycle diagram of the electrode material NCFT-33 / 33 in Comparative Example 1;

[0034] Figure 7 The image shows the refined XRD pattern of the NCFT-30 / 36 material in Comparative Example 1;

[0035] Figure 8 The charge-discharge curves of the NCFT-30 / 36 material in Comparative Example 2 are shown.

[0036] Figure 9 The image shows the refined XRD pattern of the NCFT-Ac material with chromium acetate as the precursor in Comparative Example 2.

[0037] Figure 10 The charge-discharge curves of the NCFT-Ac material with chromium acetate as the precursor in Comparative Example 2 are shown. Detailed Implementation

[0038] The present invention will be further described in detail through embodiments and with reference to the accompanying drawings, but this is not intended to limit the scope of the invention.

[0039] Example 1

[0040] A method for preparing P2-type NaCrFeTi layered oxide employs a one-step sintering method. First, using Na... 0.67 Cr 1 / 3 Fe 1 / 3 Ti 1 / 3 O2, i.e., the molar ratio of Na:Cr:Fe:Ti is 2:1:1:1, or simply Cr / Fe ratio 0.33:0.33. Sodium carbonate, iron oxide, chromium oxide, and titanium dioxide are prepared as raw materials, with an additional 5 wt.% loss on ignition for Na. Then, under air conditions, the raw materials are ball-milled at 300 r / min for 3 h using anhydrous ethanol as the milling medium to obtain a mixture. The mixture is then centrifuged, filtered, and dried to obtain a precursor, which is then pressed into tablets. Finally, under argon conditions, the precursor is sintered at 950℃ for 15 h. After sintering, it is allowed to cool naturally to obtain NaCrFeTi layered oxide, abbreviated as NCFT. Specifically, the NCFT obtained in Example 1 is Na... 0.67 Cr 1 / 3 Fe 1 / 3 Ti 1 / 3 O2 layered oxide, abbreviated as NCFT, specifically the NCFT obtained in Example 1 is abbreviated as NCFT-33 / 33 because the Cr / Fe ratio is 0.33:0.33.

[0041] To confirm the composition of NCFT-33 / 33, EDS testing was performed. The test results are as follows: Figure 1 As shown, NCFT-33 / 33 contains Na, Cr, Fe and Ti elements, and the elements are evenly distributed.

[0042] To verify the crystal structure of NCFT-33 / 33, XRD tests were performed and refined. The test results are as follows: Figure 2 As shown, NCFT-33 / 33 contains Na 0.67 Fe 1 / 3 Cr 1 / 3 Ti 1 / 3 O2 and Cr 0.2 Fe 1.8The characteristic peaks of O3, and

[0043] The main phase is Na 0.67 Fe 1 / 3 Cr 1 / 3 Ti 1 / 3 O2 content reached 97.3%, and the impurity phase was Cr. 0.2 Fe 1.8 O3 content is 2.7%; in addition, Na 0.67 Fe 1 / 3 Cr 1 / 3 Ti 1 / 3 O2 belongs to the P2 phase structure in the P63 / mmc space group.

[0044] To further verify the microstructure of NCFT-33 / 33, SEM testing was performed. The test results are as follows: Figure 3 As shown, the microstructure of NCFT-33 / 33 is a hexagonal prism structure with a smooth surface and a particle size of 2 micrometers.

[0045] To further verify the crystal structure and microstructure of NCFT-33 / 33, TEM tests were performed. The test results are as follows: Figure 4 As shown, the microstructure of NCFT-33 / 33 is a regular hexagonal structure, and... Interlayer spacing is In addition, according to Figure 4 The diffraction spots of c indicate that the material has a single-crystal structure. Test results show that NCFT-33 / 33 is of type P2, and the conclusions obtained from TEM testing are consistent with those from XRD testing.

[0046] To demonstrate the electrochemical performance of NCFT-33 / 33, electrochemical performance testing was conducted. The specific method for this testing involved using the test material as the working electrode, a Na sheet as the negative electrode, and a coin cell constructed with 1M NaFP6 sodium salt and DMC:EC = 1:1 Vol% as the electrolyte.

[0047] The charge / discharge performance test results of NCFT-33 / 33 are as follows: Figure 5 As shown, the voltage range is 0.8-2.7V, and the current density is 20mAg. -1 Under these conditions, the reversible specific capacity is 80.00 mAh g. -1 It achieves a theoretical specific capacity of 89.36 mAh g. -1 89.5%.

[0048] The long-cycle test results of NCFT-33 / 33 are as follows: Figure 6As shown, under the conditions of a voltage range of 0.8-2.7V, a current density of 10C, and 3000 cycles, the reversible specific capacity is 52.36 mAh g. -1 The capacity retention rate was 92.72%, and the capacity loss rate per lap was only 0.0024%.

[0049] To demonstrate the effect of the Cr / Fe ratio on the material, Comparative Example 1 is provided, with NaCrFeTi oxide having a Cr / Fe ratio of 0.30 / 0.36.

[0050] Comparative Example 1

[0051] A NaCrFeTi oxide with a Cr / Fe ratio of 0.30 / 0.36 is prepared. Unless otherwise specified, the steps are the same as in Example 1, except that the chemical formula is Na... 0.67 Cr 0.30 Fe 0.36 Ti 0.33 O2, i.e., keeping the addition amounts of Na and Ti constant, changing the Cr / Fe ratio from 0.33 / 0.33 to 0.30 / 0.36, yields NCFT of Na. 0.67 Cr 0.30 Fe 0.36 Ti 0.33 The O2 layered oxide, specifically the NCFT obtained in Comparative Example 1 and Example 2, is referred to as NCFT-30 / 36 because the Cr / Fe ratio is 0.30:0.36.

[0052] The XRD test and refinement results of NCFT-30 / 36 are as follows: Figure 7 As shown, NCFT-30 / 36 contains Na 0.622 Fe 0.67 Mn 0.33 Characteristic peaks of O2, NaTiFeO4, Na2CrO4, Cr2O3 and Fe2O3, and,

[0053] The main phase is Na 0.622 Fe 0.67 Mn 0.33 O2 content was only 68.2%, with impurity phases being NaTiFeO4, Na2CrO4, Cr2O3, and Fe2O3, totaling 31.8%. Specifically, NaTiFeO4 accounted for 17.9%, Na2CrO4 for 8.6%, Cr2O3 for 3.7%, and Fe2O3 for 1.6%. Furthermore, Na... 0.622 Fe 0.67 Mn 0.33 O2 belongs to the O3 phase structure of the R-3m space group.

[0054] Compared with Example 1, the following two conclusions can be drawn:

[0055] 1. Changing the Cr / Fe ratio results in two substantial differences in the crystal structure of the obtained material: the P2 phase structure and the O3 phase structure.

[0056] 2. Changing the Cr / Fe ratio will significantly affect the purity of the main phase.

[0057] The charge / discharge performance test results of NCFT-30 / 36 are as follows: Figure 8 As shown, the voltage range is 0.8-2.7V, and the current density is 20mAg. -1 Under these conditions, the reversible specific capacity is 53.00 mAh g. -1 It only has a theoretical specific capacity of 89.36 mAh g. -1 The reversible specific capacity of NCFT-30 / 36 is 59.3%. Compared with Example 1, the reversible specific capacity of NCFT-30 / 36 is significantly lower than that of NCFT-33 / 33, at only 66.25%.

[0058] As can be seen from Example 1 and Comparative Example 1, adjusting the Cr / Fe ratio of NCFT can substantially change the crystal structure of NCFT. When the Cr / Fe ratio is 0.33:0.33, the crystal structure of NCFT changes from the O3 phase structure to the P2 phase structure, and the purity of the main phase is significantly improved, thereby significantly improving the charge and discharge performance of NCFT.

[0059] To demonstrate the effect of the Cr source on the material, Comparative Example 2 is provided, showing NaCrFeTi oxide with chromium acetate as the Cr source.

[0060] Comparative Example 2

[0061] A NaCrFeTi oxide with chromium acetate as the Cr source is prepared. Unless otherwise specified, the steps are the same as in Example 1, except that chromium acetate is used instead of chromium oxide as the Cr source, resulting in Na... 0.67 Cr 1 / 3 Fe 1 / 3 Ti 1 / 3 O2 layered oxide, specifically the NCFT obtained in Comparative Example 2, is referred to as NCFT-Ac because the Cr source is chromium acetate.

[0062] The XRD test and refinement results of NCFT-Ac are as follows: Figure 9 As shown, NCFT-Ac contains Na 0.62 Fe 0.67 Mn 0.33 O2, Na 0.64 Mn 0.33 Fe 0.67 O2 and Ti 1.03 Fe 1.94The characteristic peaks of O4, and the phase content of NCFT-Ac does not exceed 50%, meaning there is no obvious main phase, specifically Na 0.64 Mn 0.33 Fe 0.67 The O2 content is 46.8%, Na 0.62 Fe 0.67 Mn 0.33 The O2 content is 38.1%, Ti 1.03 Fe 1.94 The O4 content is 15.1%;

[0063] In addition, special attention should be paid to,

[0064] Na 0.64 Mn 0.33 Fe 0.67 O2 belongs to the P2 phase structure of P63 / mmc;

[0065] Na 0.62 Fe 0.67 Mn 0.33 O2 belongs to the O3 phase structure of the R-3m space group.

[0066] Test results show that although changing the Cr source can also yield a P2 phase structure, an O3 phase structure still exists, meaning both P2 and O3 phase structures coexist. Furthermore, the P2 phase content does not exceed 50%, thus it cannot become the main phase. Compared with Example 1, changing the Cr source can completely transform the O3 phase structure into a P2 phase structure, significantly increasing the P2 phase content by 207.9%.

[0067] The charge / discharge performance test results of NCFT-Ac are as follows: Figure 10 As shown, the voltage range is 0.8-2.7V, and the current density is 20mAg. -1 Under these conditions, the reversible specific capacity is 48.00 mAh g. -1 It only has a theoretical specific capacity of 89.36 mAh g. -1 The reversible specific capacity of NCFT-Ac is 53.72%. Compared with Example 1, the reversible specific capacity of NCFT-Ac is significantly lower than that of NCFT-33 / 33, at only 60.00%.

[0068] As can be seen from Example 1 and Comparative Example 12, adjusting the Cr source can substantially change the crystal structure of NCFT. When the Cr source is chromium oxide, the crystal structure of NCFT is substantially changed from O3 phase structure to P2 phase structure, and the purity of the main phase is significantly improved, thereby significantly improving the charge and discharge performance of NCFT.

Claims

1. A P2 type NaCrFeTi layered oxide, characterized in that: Na is obtained by high-temperature solid-state sintering using sodium carbonate, iron oxide, chromium oxide, and titanium dioxide as raw materials. 0.67 Cr 1 / 3 Fe 1 / 3 Ti 1 / 3 O2 layered oxide NCFT-33 / 33, wherein the NCFT-33 / 33 layered oxide belongs to the P2 phase structure of space group P63 / mmc; The microstructure of the NCFT-33 / 33 layered oxide is a hexagonal prism structure with a particle size of 1-3 micrometers and a single crystal structure. The main phase of the NCFT-33 / 33 layered oxide is Na. 0.67 Fe 1 / 3 Cr 1 / 3 Ti 1 / 3 O2 content reaches over 95%.

2. A method for preparing a P2 type NaCrFeTi layered oxide, characterized in that... Includes the following steps: The one-step sintering method first uses sodium carbonate, iron oxide, chromium oxide and titanium dioxide as raw materials to meet certain chemical formulas. In addition, 5 wt.% of Na source is added to compensate for high temperature burn-off. The raw materials are ball-milled to obtain a mixture. Then, the mixture is centrifuged, filtered and dried to obtain a precursor, which is then pressed into tablets. Finally, the precursor is sintered under certain conditions. After sintering, it is naturally cooled to obtain NaCrFeTi layered oxide, abbreviated as NCFT. Among them, the chemical formula is Na 0.67 Cr 1 / 3 Fe 1 / 3 Ti 1 / 3 O2, that is, the molar ratio of Na:Cr:Fe:Ti is 2:1:1:1, or simply the Cr / Fe ratio is 0.33:0.33; The chromium oxide is a Cr source; The conditions for ball milling are as follows: under air conditions, the ball milling speed is 300-500 r / min, the ball milling time is 3-6 h, and the ball milling medium is anhydrous ethanol. The sintering conditions are as follows: sintering temperature of 800-1000 ℃ and sintering time of 10-15 h under argon atmosphere.

3. The application of the P2-type NaCrFeTi layered oxide as a negative electrode material for sodium-ion batteries according to claim 1, characterized in that: The voltage range is 0.8–2.7 V, and the current density is 20 mA g. -1 Under these conditions, the reversible specific capacity is 60.00-80.00 mAh g. -1 It achieves a theoretical specific capacity of 89.36 mAh g. -1 67.1-89.53%.

4. The application of the P2-type NaCrFeTi layered oxide as a negative electrode material for sodium-ion batteries according to claim 1, characterized in that: Under conditions of voltage range of 0.8–2.7 V, current density of 10 C, and 3000 cycles, the reversible specific capacity is 52.36–56.47 mAh g⁻¹. -1 The capacity retention rate is 91-93%, and the capacity loss rate per cycle is only 0.0023-0.0030%.

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