Method for determining diffusion speed control steps of sodium ions in hard carbon and optimizing microstructure

Abstract

The invention discloses a method for determining diffusion speed control steps of sodium ions in hard carbon and optimizing a microstructure. According to the method, longitudinal relaxation time T1 values of 23Na are measured under different potentials (0.3, 0.1, 0.05 and 0 V), migration behaviors of adsorbed sodium and intercalated sodium (NaCx) are distinguished, a diffusion coefficient curve is obtained by combining GITT, and dominant steps of limiting overall diffusion are judged; and dynamic collaborative optimization under the conditions of low temperature and large current is realized by selecting a cross-linking agent and adjusting a carbonization system and pore engineering. The reversible capacity of the optimized hard carbon at 25 DEG C and 5 DEG C can be greater than or equal to 297 mAh g <-1 >, and the capacity retention rate at-40 DEG C and 30 mA g <-1 > is greater than or equal to 86%. The method is suitable for various carbon sources, and a closed-loop path of NMR dynamic recognition, speed control judgment, structural regulation and electrochemical verification can be formed.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage materials technology, specifically relating to a method for determining the rate-controlling steps of sodium ion diffusion in hard carbon and optimizing its microstructure. More specifically, it relates to a method based on solid-state nuclear magnetic resonance (ssNMR) technology to identify the multi-scale transport behavior of sodium ions in hard carbon materials and its rate-controlling steps. This method uses NMR to determine the rate-controlling steps of sodium ion diffusion in hard carbon to optimize the microstructure of hard carbon and improve the performance of low-temperature / high-current batteries. By combining NMR relaxation and diffusion coefficient analysis, the key microstructural factors affecting sodium ion migration are systematically revealed, such as crystallite spacing, disordered carbon domains, pore structure, and surface functional group distribution. Background Technology

[0002] Sodium-ion batteries are considered an important candidate system for next-generation large-scale energy storage and low-temperature power batteries due to their abundant resources, low cost, and similar working principle to lithium-ion batteries. Among them, hard carbon (HC) is currently the most promising anode material for sodium-ion batteries, and has become a research hotspot due to its high specific capacity, low sodium intercalation potential, and good cycle stability. However, the sodium storage process of hard carbon is very complex, involving multi-step ion transport in disordered interlayers, nanopores, and surface active sites. Its diffusion kinetics are extremely sensitive to temperature and current density, which is a key factor limiting its performance improvement under low-temperature and high-current (fast charging) conditions.

[0003] Existing studies mostly characterize sodium ion diffusion behavior through electrochemical tests (such as GITT and EIS) or theoretical simulations. However, these methods often struggle to distinguish the movement types of sodium ions in different microscopic environments and to identify the rate-controlling steps of electrochemical reactions. For example, at low temperatures, sodium ions may be restricted by multiple factors such as pores, interfacial charge transfer, or surface adsorption, resulting in a complex overall diffusion process. Furthermore, traditional electrochemical tests often only provide macroscopic average diffusion coefficients and cannot reflect the true diffusion paths at the microscopic level.

[0004] Nuclear magnetic resonance (NMR), as a non-destructive, in-situ traceable nuclear spin probe technique, can directly observe the diffusion behavior and local chemical environment of sodium ions in solid materials, offering advantages such as wide temporal coverage and high spatial resolution. Although some studies have used NMR to analyze the chemical environment of sodium ions in hard carbon (such as the distribution of high-field and low-field peaks), a systematic quantitative method is still lacking to identify the rate-controlling links of different transport steps and, based on this, guide the optimization of the microstructure of hard carbon.

[0005] Therefore, there is an urgent need for a method that can quantitatively identify the rate-controlling steps of sodium ion diffusion in hard carbon based on NMR spectroscopic parameters (such as self-diffusion coefficient, relaxation time, peak shift and linewidth variation), and combine this kinetic understanding to directionally regulate the pore structure, disorder and surface chemistry of hard carbon, thereby achieving rapid and reversible transport of sodium ions under low temperature and high current conditions. Summary of the Invention

[0006] The purpose of this invention is to address the problems of unclear sodium ion diffusion mechanisms, difficulty in identifying rate control steps, and severe performance degradation under low temperature and high current conditions in existing sodium-ion battery hard carbon anodes. This invention proposes a diffusion kinetics identification and microstructure optimization method based on solid-state nuclear magnetic resonance characterization. Based on this identification result, the sodium ion diffusion rate and interfacial reaction kinetics are synergistically optimized by controlling the carbon source precursor structure, pyrolysis temperature, and pore engineering. By quantitatively analyzing the migration behavior of sodium ions in hard carbon under different chemical environments (such as interlayer, pore, and surface adsorption sites), the rate control steps in the diffusion process are accurately identified, and corresponding structural optimization strategies are proposed. This invention aims to achieve efficient diffusion and reversible intercalation of sodium ions in hard carbon, improving the sodium storage kinetics performance of the material under low temperature (-20 °C and below) and high rate (=5 C) conditions. This method can significantly reduce electrode polarization, thereby improving the application performance and reliability of sodium-ion batteries in fast-charging energy storage, low-temperature starting of electric vehicles, and high-power energy storage systems.

[0007] To achieve the above objectives, this invention provides a method for determining the sodium ion diffusion rate control steps in hard carbon and optimizing its microstructure based on solid-state nuclear magnetic resonance (NMR), comprising the following steps: 1) Sample preparation: Select a carbon source with or without crosslinking agent (preferably a mixture of crosslinking agent and carbon source with a mass ratio of 0-0.01 as a precursor) as a precursor, and perform pre-oxidation at a temperature of 150-200℃ (preferably 200℃) and carbonization in an inert atmosphere to obtain hard carbon; 2) The coin cell was tested using galvanostatic intermittent titration (GITT) to obtain the sodium ion chemical diffusion coefficient curve (specific capacity - sodium ion diffusion coefficient (DNa)) in the 0-2.5V voltage range. + The voltage value A at the lowest point of the curve is determined, and the period from A+0.02 to A-0.02 is defined as the speed control influence stage. ssNMR characterization: The prepared hard carbon anode material was fabricated into electrode sheets and assembled into coin cells. After cycling the cells for one or two cycles or more, the cells were discharged to any voltage during the rate-controlled influence phase as the target voltage, and data were collected at this potential. 23 NaNMR, and the saturation recovery method was used to determine adsorbed sodium and intercalated sodium NaC. x T1 values ​​of different spectral peaks; 3) Rate control determination: If the T1 relaxation time of intercalated sodium is greater than that of adsorbed sodium, then intercalation diffusion is determined to be the rate control factor, and the interlayer spacing, graphite microcrystal width and thickness of the hard carbon material are adjusted accordingly during the preparation of hard carbon material; if the T1 relaxation time of adsorbed sodium is greater than that of intercalated sodium, then the interfacial adsorption / desorption process is determined to be the rate control factor, and the ratio of closed pores to open pores in the hard carbon material is adjusted accordingly during the preparation of hard carbon material.

[0008] Structural control: When intercalation diffusion is the rate-controlling factor, it is achieved by increasing the pre-oxidation temperature. Specifically, the hard carbon preparation process and conditions of step 1 are repeated, but the pre-oxidation temperature is increased. Hard carbon is prepared from 150-200℃ to 300-350℃ in a step size of 5-20℃ (preferably from 200 to 300℃ in a step size of 5-20℃, more preferably from 250-300℃ in a step size of 10℃). Hard carbon is obtained, and the interlayer spacing A and graphite layer thickness B of the hard carbon at different pre-oxidation temperatures are measured to obtain the A / B value at different pre-oxidation temperatures. At the same time, the pre-oxidation temperature C corresponding to the maximum A / B is obtained, and any temperature between C+10℃ and C-10℃ is selected as the optimized pre-oxidation temperature. Alternatively, when the adsorption / desorption process is a rate-controlling factor, it can be achieved by adding a crosslinking agent to the carbon source. Specifically, the hard carbon preparation process and conditions of step 1 are repeated, except that a crosslinking agent is added to the carbon source. A mixture of crosslinking agent and carbon source is used as a precursor. Hard carbon is prepared by different mass ratios of crosslinking agent to carbon source in the precursor. Hard carbon is prepared from 0 to 1 with a step size of 0.01-0.5 (preferably 0.1-0.3; more preferably 0.2-0.25) of the mass ratio of crosslinking agent to carbon source. The closed-cell and open-cell ratios of hard carbon with different mass ratios of crosslinking agent to carbon source are measured. The mass ratio D of crosslinking agent to carbon source corresponding to the maximum closed-cell and open-cell ratio is obtained. Any temperature between D+0.05 and C-0.05 (preferably between D+0.03 and C-0.03) is selected as the mass ratio of crosslinking agent to carbon source.

[0009] In step 1), the carbonization temperature is 1000-1500 ℃ (preferably 1200-1400 ℃), and the holding time is 2-24 h (preferably 3-6 h), with argon and / or nitrogen as the inert atmosphere; the pre-oxidation temperature is 240-300 ℃, and the holding time is 1-5 h (preferably 2-4 h). In step 1), the pre-oxidation process is carried out in air; In step 4), the crosslinking agent is one or a combination of two of terephthalic acid (PTA); the molar ratio of carbon source to crosslinking agent is (2±0.2):1, and the mixture is reacted at 150-170 °C (preferably 155-165 °C) for 3-6 h (preferably 4-5 h) to obtain the precursor.

[0010] The hard carbon is a substance used as the negative electrode active material in sodium-ion batteries.

[0011] 5) Low-temperature and high-rate electrochemical performance verification: The optimized hard carbon material was assembled into a sodium-ion half-cell, and constant current charge-discharge test and rate test were carried out in the range of -40℃ to 25℃.

[0012] Further, in step 1), the precursor is one or more of glucose, starch, and lignin; Furthermore, in step 1), the pre-oxidation calcination is carried out in a muffle furnace; Furthermore, in step 1), the inert atmosphere is one or both of argon or nitrogen; Further, in step 2), the data is acquired using a Bruker Avance III (500 MHz) nuclear magnetic resonance spectrometer. 23 The Na NMR spectrum was acquired using a pulse length of 1 / 12π. 23 The chemical shifts of Na are referenced to NaCl (7.2 ppm); Furthermore, in step 3), the GITT curve is measured on the blue electric system; Furthermore, in step 4), the type of crosslinking agent and reaction conditions can be specifically optimized according to different rate control steps to regulate interlayer spacing, disorder, and pore distribution.

[0013] The present invention also provides a structurally optimized hard carbon material prepared according to the above method.

[0014] This invention also provides the application of this optimized hard carbon in sodium-ion battery anode materials.

[0015] Hard carbon materials, refined through solid-state NMR analysis and structural optimization, exhibit significant performance improvements under low-temperature and high-current conditions: the charge-discharge capacity at room temperature (25 °C) at 5 C is increased to 297.2 mAh g⁻¹. -1 At low temperatures (-40 °C), the capacity retention is above 86%, maintaining high capacity reversibility up to 295 mAh g⁻¹. -1 .

[0016] This invention uses solid-state nuclear magnetic resonance to accurately identify the rate-limiting steps of sodium ion diffusion in hard carbon, and based on this, achieves precise design of microstructure by controlling the type of crosslinking agent and carbonization process.

[0017] This method realizes a closed-loop optimization path from "diffusion kinetics identification - rate control determination - structural parameter regulation - electrochemical performance verification", providing a new design idea and technical route for rapid charging and discharging and low-temperature energy storage of sodium-ion battery anode materials.

[0018] Beneficial effects This invention utilizes solid-state NMR relaxation and diffusion parameters to quantitatively identify the rate-controlling steps of sodium ion transport in hard carbon, overcoming the limitation of traditional electrochemical methods in distinguishing multi-scale diffusion behavior. Furthermore, based on the results, a quantitative correlation between "structural parameters – diffusion rate – electrochemical performance" is established, proposing targeted microstructure optimization pathways to achieve controllable design. This method is applicable to various carbon sources and preparation routes, and can be extended to fields such as low-temperature energy storage of sodium-ion batteries, fast charging of electric vehicles, and high-power energy storage systems, demonstrating broad industrial application potential.

[0019] This invention discloses a method for quantitatively identifying the diffusion rate-controlling steps of sodium ions in a hard carbon anode using solid-state nuclear magnetic resonance (ssNMR) and thereby directionally optimizing the microstructure. This method involves measuring the diffusion rate control steps of sodium ions at different potentials (0.3, 0.1, 0.05, 0 V). 23 The longitudinal relaxation time T1 of Na can be used to distinguish between adsorbed sodium and intercalated sodium (NaC). x The migration behavior of the hard carbon was analyzed, and the diffusion coefficient curve obtained by GITT was used to determine the dominant steps limiting overall diffusion. Furthermore, kinetic optimization under low temperature and high current conditions was achieved through the selection of crosslinking agents, adjustment of the carbonization regime, and pore engineering. The optimized hard carbon exhibited a reversible capacity of ≥297 mAh g⁻¹ at 25 °C and 5 °C. -1 , at –40°C, 30 mA g -1 Capacity retention ≥86% (295 mAh g) -1 This method is applicable to a variety of carbon sources and can form a closed-loop pathway of "NMR kinetic identification - rate control determination - structural regulation - electrochemical verification". Attached Figure Description

[0020] Figure 1 XRD pattern of the negative electrode material prepared in Example 1; Figure 2 This is a graph showing the relationship between the pre-oxidation temperature and d / Lc for the material in Example 3; Figure 3 This is a graph showing the relationship between the crosslinking agent ratio and the closed-cell volume / open-cell volume of the material in Example 4; Figure 4The image shows the GITT test results for the material in Example 1. Figure 5 Example 1: Measurement of T1 plot using the recovery saturation method at 0.3 V; Figure 6 Example 1: Measurement of T1 plot using the recovery saturation method at 0.1 V; Figure 7 Example 1: Measurement of T1 plot using the recovery saturation method at 0.05 V; Figure 8 The T1 graph for Example 1 is measured using the recovery saturation method at 0 V. Figure 9 The figure shows the experimental results of Example 1; Figure 10 The figure shows the experimental results of Example 4. Detailed Implementation

[0021] The present invention will be described in detail below through embodiments, but the present invention is not limited to the embodiments.

[0022] Example 1 (Preparation and structural control of hard carbon samples) Hard carbon preparation: 10g of glucose was placed in a muffle furnace and heated to 200℃ at a rate of 10℃ / min under air atmosphere, and held for 3h. Then it was placed in a tube furnace and heated to 800℃ at a rate of 10℃ / min under argon atmosphere, and then heated to 1300℃ at a rate of 3℃ / min, held for 3h, and allowed to cool naturally to room temperature. The mixture was then ball-milled (zirconia, ball milling beads: 50g: 1g of raw material) for 5h at a speed of 500rpm to obtain the final glucose-based hard carbon material solid powder HC-Glc with a particle size of 5-10μm.

[0023] Property testing: The physicochemical properties of HC-Glc obtained in Example 1 were tested, including the specific surface area, average pore size (open and closed pore volume), and microcrystalline structure (La, Lc, interlayer spacing).

[0024] The electrode preparation conditions were as follows: hard carbon HC was used as the negative electrode active material, carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) were added as binders, and conductive carbon black was used as a conductive agent. The mass ratio of each substance was negative electrode material:CMC:SBR:conductive carbon black = 94:2:2:2. Deionized water was used as the solvent to prepare a uniform slurry, which was then coated onto copper foil. After drying, the negative electrode sheet was obtained. The dry matter loading on the aluminum foil was 2 mg / cm³. 2 .

[0025] Sodium metal sheets, glass fiber membranes, and a solution of 1M NaPF6 in 100% DIGLYME (diethylene glycol dimethyl ether) were used as the counter electrode, separator, and electrolyte, respectively. A coin cell (CR-2016 cell) was fabricated using the prepared negative electrode sheet. Tests were conducted at room temperature (25°C) and -40°C. The test conditions were: discharge at a constant current density of 30-1500 mA / g until the voltage reached 0V, allow to stand for 5 minutes, and then charge at a constant current density of 30-1500 mA / g until the voltage reached 2.5V.

[0026] Example 2 The CR-2016 battery assembled in Example 1 was tested (diffusion kinetics analysis and NMR characterization): Determination of GITT diffusion coefficient (Unlocking the Local Structure of Hard Carbon to Grasp Sodium-Ion Diffusion Behavior for Advanced Sodium-Ion Batteries. Energy & Environmental Science 2024, 17 (4), 1387–1396. DOI: 10.1039 / D3EE03347C.): Intermittent galvanostatic titration (GITT) was performed using the LAND battery testing system. Apparent Na + Method for calculating diffusion coefficient: Applying 30 mA g to the CR-2016 battery assembled in Example 1 -1 A pulsed current (the process of discharging the negative electrode) lasts for 20 minutes, with a 2-hour interval between pulses. The diffusion coefficient (DNa) + The following general formula, derived from Fick's second law, can be used for estimation.

[0027]

[0028] Where τ is the duration of the current pulse (seconds), m B V represents the mass (grams) of the active substance in the sample. M and M B These represent molar volume and molar mass (in cubic centimeters per mole and grams per mole, respectively), and S represents the active surface area of ​​the electrode (in square centimeters). Finally, E S It is the difference between the stable potentials. E τ This represents the instantaneous total voltage change of the constant current battery at time τ. E S and Eτ The value can be obtained from the galvanostatic intermittent titration (GITT) curve.

[0029] Based on the GITT test results ( Figure 4 The chemical diffusion coefficient curves of sodium ions in the voltage range of 0-2.5V were obtained (horizontal and vertical). The voltage value of the lowest point of the curve was determined to be A=0.062V. The period from A+0.02 to A-0.02 was defined as the speed control influence stage. solid state 23 NMR testing: The CR-2016 cell of Example 1 was tested at 30 mA g. -1 A charge-discharge cycle was performed at a current density of 1000 MHz, followed by discharge to the target voltage. The battery was then carefully disassembled in an argon-filled drying oven with H2O and O2 concentrations below 0.01 ppm. The electrode plates (hard carbon samples) were removed and gently cleaned with dimethyl carbonate (DME). For in-situ nuclear magnetic resonance (NMR) analysis, the cleaned hard carbon samples were sealed in a 1.6 mm Φ sample rotor. In-situ magic angle rotation (MAS) solid-state NMR experiments were performed on a Bruker 500 MHz (11.7 T) solid-state NMR spectrometer with a MAS frequency of 40 kHz using a commercially available 4 mm HXY MAS probe. All samples... 23 The Na MAS NMR spectra were obtained by accumulating 3200 scans. 23 The chemical shift of Na was set to 0 ppm with 1 M NaCl aqueous solution as a reference. The T1 value was measured using the saturation recovery method (NMR T1 relaxation time measurement and calculations with translational and rotational components for liquid electrolytes containing LiBF4 and Propylene Carbonate. The Journal of Chemical Physics 2013, 139(21), 214501. DOI: 10.1063 / 1.4832038.). (Measurement of Spin-Lattice Relaxation Times in Multiphase Polymer Systems. Journal of Magnetic Resonance 2023, 357, 107597. DOI: 10.1016 / j.jmr.2023.107597.) This study investigated the kinetics of sodium ion transport in hard carbon. The saturation recovery method first used a series of π / 2 pulses to completely eliminate the magnetization vector in the z-direction of the spin system. After time τ, the magnetization vector was transferred to the xy-plane using π / 2 pulses for signal acquisition, and the recovery time was gradually varied (500 μs, 1 ms, 5 ms, 10 ms, 30 ms, 50 ms, 70 ms, 100 ms, 200 ms, 300 ms, 400 ms, and 600 ms) to obtain 12 curves showing Mz as a function of τ. The T1 value was obtained by fitting the curve using the following formula.

[0030]

[0031] Where M0 is the magnetization in thermal equilibrium, and A is a fitting parameter used to correct for any errors in the pulse duration. When the pulse duration is set perfectly, A is 1, but the value of A may vary slightly, which will not affect the T1 value. Because a complete decay process is required to fit the T1 recovery curve well, multiple τ values ​​are used, which are determined based on the T1 value.

[0032] Figures 5-8 The relaxation time of sodium species in hard carbon at voltages of 0.3, 0.1, 0.05, and 0 V was measured using the solid-state NMR saturation recovery method to determine the T1 value. Figure 9 The results show that a larger T1 indicates slower kinetics of sodium species.

[0033] 0.05V was used as the target voltage in the rate control stage. The T1 relaxation time of the intercalated sodium was greater than that of the adsorbed sodium. Intercalation diffusion was determined to be the rate control factor. Based on this, the interlayer spacing, graphite microcrystal width and thickness of the hard carbon material were adjusted in the preparation process of the hard carbon material. See Example 3 for details. from Figure 9 It can be seen that the relaxation time of sodium intercalation in the material prepared in Example 1 is longer than that of adsorbed sodium, so the overall kinetics of sodium intercalation are slow. At 0.05V, the T1 of sodium intercalation is the largest compared to 0.3V, and at 0.1V compared to 0V. Combined with the above determination of the diffusion coefficient, this indicates that the intercalation process is a rate-controlled step. Since intercalation is related to the carbon layer structure, we need to adjust the hard carbon layer.

[0034] Example 3 (Structural regulation) Structural control: When intercalation diffusion is the rate-controlling factor, it is achieved by increasing the pre-oxidation temperature. Specifically, the hard carbon preparation process and conditions in Example 1 are repeated, except that the pre-oxidation temperature is increased. Hard carbon is prepared from 200℃ to 300℃ in 20℃ increments to obtain hard carbon. The interlayer spacing A and graphite layer length B of the hard carbon are measured at different pre-oxidation temperatures (pre-oxidation temperatures are 220℃, 240℃, 260℃, 280℃, and 300℃, denoted as HC-Glc-X, where X is the pre-oxidation temperature). The A / B value of different pre-oxidation temperatures is obtained, such as... Figure 2 As shown, the interlayer spacing / graphite layer thickness value is maximized when the pre-oxidation temperature is increased to 260℃.

[0035] To verify that the carbon layer structure can be controlled simply by adjusting the pre-oxidation temperature, we added a crosslinking agent, terephthalic acid. Specifically, we repeated the preparation process of the HC-Glc-260 sample, except that we added a crosslinking agent (a mixture of them as a precursor) to the carbon source. The mass ratio of the crosslinking agent to the carbon source was 1:2, and the sample was named HC-GlcPTA-260.

[0036] Property testing: The process and conditions are the same as those for the property testing in Example 1; Table 1. Carbon layer information of materials in Examples 1 and 3.

[0037] Based on XRD tests, and using the Bragg equation and the Debye-Scherrer equation (Scherrer after Sixty Years: A Survey and Some New Results in the Determination of Crystallite Size. J Appl Crystallogr 1978, 11 (2), 102–113. DOI: 10.1107 / S0021889878012844.),

[0038] Where Lc is the thickness of the graphite grains, β is the full width at half maximum (FWHM) of the XRD diffraction peak, and λ is the wavelength of the X-rays used (0.154 nm). The k values ​​of the (100) and (002) diffraction peaks of the carbon material are 1.84 and 0.90, respectively. d is the interlayer spacing.

[0039] Table 1 shows that when intercalation diffusion is the rate-controlling factor, by increasing the pre-oxidation temperature, the interlayer spacing and graphite layer thickness of hard carbon at different pre-oxidation temperatures were measured. When the pre-oxidation temperature was increased to 260℃, the interlayer spacing / graphite layer length value was the largest. When the pre-oxidation temperature was selected as 260℃, the interlayer spacing and Lc of hard carbon after adding the crosslinking agent did not change much, and the d / Lc ratio also did not change.

[0040] Table 2 Performance of the negative electrode materials prepared in Examples 1 and 3

[0041] Table 2 shows that, compared with the control sample HC-Glc, the sample HC-Glc-260 with the largest d / Lc exhibits the highest specific capacity at high current density (1500 mA / g) and the highest specific capacity at -40℃ (30 mA / g) (mAh / g). This verifies that when intercalation is the rate-controlling step, increasing d / Lc enhances the diffusion kinetics of sodium ions, thereby significantly improving high-rate capacity and low-temperature charging capacity. The hard carbon HC-GlcPTA-200 with added crosslinking agent also has a large specific capacity, verifying that when intercalation is the rate-controlling step, its performance is not significantly affected without changing d / Lc. This indicates that when intercalation diffusion is the rate-controlling factor, microscopic control of the carbon layer can be achieved simply by adjusting the oxidation temperature. The introduction of crosslinking agent does not affect the changes in the microcrystalline carbon layer of hard carbon graphite, further verifying the accuracy of the present invention's method of rapidly and accurately controlling structural parameters through NMR kinetic identification and rate-controlling determination.

[0042] Example 4 The hard carbon preparation process and conditions in Example 1 were repeated, except that the carbon source glucose was changed to starch. The other processes and conditions were the same as in Example 1 to obtain HC-Sta. Property testing: The process and conditions are the same as those for the property testing in Example 1; Example 5 The process and conditions are the same as in Example 2, except that the CR-2016 battery assembled in Example 4 is tested to determine the voltage value of the lowest point of the curve, A=0.05 V, and the period from A+0.03 to A-0.03 is defined as the speed control effect stage. The relaxation time of sodium species in hard carbon at voltages of 0.3, 0.1, 0.05, and 0 V was measured using the solid-state NMR saturation recovery method to determine the T1 value. 0.05V was used as the target voltage in the rate control stage. The T1 relaxation time of adsorbed sodium was greater than that of intercalated sodium. Adsorption diffusion was determined to be the rate control factor. Based on this, the ratio of closed pores to open pores of hard carbon was adjusted in the preparation process of hard carbon materials. See Example 6 for details. from Figure 10It can be seen that the relaxation time of sodium adsorption in the material prepared in Example 4 is longer than that of sodium, so the overall kinetics of sodium adsorption are slow. At 0.05V, the adsorption T1 is the largest compared to 0.3V and 0.1V compared to 0V. Combined with the above determination of the diffusion coefficient, this indicates that the adsorption process is a rate-controlling step. Since adsorption is related to pore structure, we need to adjust the pore structure.

[0043] Example 6 (Structural regulation) When the adsorption / desorption process is a rate-controlling factor, it is achieved by adding a crosslinking agent to the carbon source. Specifically, the hard carbon preparation process and conditions in Example 4 are repeated, except that a crosslinking agent is added to the carbon source. A mixture of crosslinking agent and carbon source is used as a precursor. Hard carbon is prepared by varying the mass ratio of crosslinking agent to carbon source in the precursor, from 0 to 1 with a step size of 0.25. The closed-cell and open-cell ratios of the hard carbon with different crosslinking agent to carbon source mass ratios (0.25, 0.5, 0.75, and 1, denoted as HC-1 / 4StaPTA, HC-1 / 2StaPTA, HC-3 / 4StaPTA, and HC-1 / 1StaPTA) are measured. Figure 3 As shown, when the ratio of closed-cell to open-cell is at its maximum, the corresponding mass ratio of crosslinking agent to carbon source is 0.5.

[0044] Meanwhile, the hard carbon preparation process and conditions in Example 4 were repeated, except that a crosslinking agent was added to the carbon source, and a mixture of crosslinking agent and carbon source at a mass ratio of 0.5 was used as a precursor. Hard carbon was prepared at a pre-oxidation temperature of 260°C to obtain hard carbon HC-1 / 2StaPTA-260. Property testing: The process and conditions are the same as those for the property testing in Example 1; Table 3. Pore size distribution of the materials prepared in Examples 3 and 6

[0045] The orifice volume refers to the orifice volume into which N2 gas can enter, and the test method is BET. Closed-pore volume refers to the pore volume in which N2 gas cannot enter but He gas can. The test method is the He displacement true density test. For details, please refer to the reference (Revealing the Closed Pore Formation of WasteWood-Derived Hard Carbon for Advanced Sodium-Ion Battery). Nature Communications 2023 , 14,1,6024).

[0046] As can be seen, simply adding a crosslinking agent to the carbon source and using a mixture of crosslinking agent and carbon source as a precursor, different mass ratios of crosslinking agent to carbon source in the precursor were used to prepare hard carbon. When the ratio of closed-cell to open-cell structures was maximized, the corresponding mass ratio of crosslinking agent to carbon source was 0.5. When the mass ratio of crosslinking agent to carbon source was selected as 0.5, changing the pre-oxidation temperature did not change the ratio of closed-cell to open-cell structures in the hard carbon, indicating that changing the pre-oxidation temperature did not affect the change in the ratio of closed-cell to open-cell structures in the hard carbon.

[0047] Table 4. Performance of the negative electrode materials prepared in Examples 3 and 6

[0048] Table 4 shows that, compared with the control sample HC-Sta, the sample HC-1 / 2StaPTA, with the largest closed-pore to open-pore ratio, exhibits the highest charging specific capacity at 1500 mA / g under high current density and the highest charging specific capacity at 30 mA / g at -40℃. This verifies that when the adsorption / desorption process is the rate-controlling factor, adjusting the closed-pore to open-pore ratio can improve the diffusion kinetics of sodium ions, thereby significantly enhancing high-rate capacity and low-temperature charging capacity. In the sample HC-1 / 2StaPTA, with the largest closed-pore to open-pore ratio, the charging specific capacity is 291.1 mAh / g when the pre-oxidation temperature is set to 260℃ (HC-1 / 2StaPTA-260), showing no significant change compared to the sample with a pre-oxidation temperature of 200℃ (HC-1 / 2StaPTA). This indicates that when the adsorption / desorption process is the rate-controlling step, performance optimization can be achieved simply by changing the closed-pore to open-pore ratio, with little influence from the pre-oxidation temperature.

Claims

1. A method for determining the rate-controlling steps of sodium ion diffusion in hard carbon and optimizing its microstructure, characterized in that, Includes the following steps: 1) Sample preparation: Select a carbon source with or without crosslinking agent as a precursor, perform pre-oxidation at 150-200℃, and then carbonize in an inert atmosphere to obtain hard carbon; 2) The coin cell was tested by constant current intermittent titration (GITT) to obtain the sodium ion chemical diffusion coefficient curve in the voltage range of 0-2.5V. The voltage value A corresponding to the minimum specific capacity of the curve was determined, and the range from A+0.02V to A-0.02V was defined as the rate control effect stage. ssNMR characterization: The prepared hard carbon anode material was fabricated into electrode sheets and assembled into coin cells. After cycling the cells for one or two cycles or more, the cells were discharged to any voltage during the rate-controlled influence phase as the target voltage, and data were collected at this potential. 23 Na NMR, and the adsorbed sodium and intercalated sodium NaC were determined by saturation recovery method. x T1 values ​​of different spectral peaks; 3) Speed ​​control determination: If the T1 relaxation time of intercalated sodium is greater than that of adsorbed sodium, then intercalation diffusion is determined to be the rate-controlling factor, and the interlayer spacing and the thickness of the graphite microcrystal layer of hard carbon material are adjusted accordingly during the preparation of hard carbon material. If the T1 relaxation time of adsorbed sodium is greater than that of intercalated sodium, the interfacial adsorption / desorption process is determined to be the rate-controlling factor, and the ratio of closed pores to open pores in the hard carbon material is adjusted accordingly during the preparation of hard carbon materials.

2. The method for determining the rate-controlling step of sodium ion diffusion in hard carbon and optimizing its microstructure according to claim 1, characterized in that: Structural control: When intercalation diffusion is the rate-controlling factor, it is achieved by increasing the pre-oxidation temperature. Specifically, the hard carbon preparation process and conditions of step 1 are repeated, but the pre-oxidation temperature is increased. Hard carbon is prepared from 150-200℃ to 300-350℃ in step size of 5-20℃ to obtain hard carbon. The interlayer spacing A and graphite layer thickness B of hard carbon at different pre-oxidation temperatures are measured to obtain the A / B value of different pre-oxidation temperatures. At the same time, the pre-oxidation temperature C corresponding to the maximum A / B is obtained. Any temperature between C+10℃ and C-10℃ is selected as the optimized pre-oxidation temperature. Alternatively, when the adsorption / desorption process is a rate-controlling factor, it can be achieved by adding a crosslinking agent to the carbon source. Specifically, the hard carbon preparation process and conditions of step 1 are repeated, except that a crosslinking agent is added to the carbon source. A mixture of crosslinking agent and carbon source is used as a precursor. Hard carbon is prepared by different mass ratios of crosslinking agent to carbon source in the precursor. Hard carbon is prepared from 0 to 1 with a step size of 0.1-0.3 for the mass ratio of crosslinking agent to carbon source. The closed-cell and open-cell ratios of hard carbon with different mass ratios of crosslinking agent to carbon source are measured. The mass ratio D of crosslinking agent to carbon source corresponding to the maximum closed-cell and open-cell ratio is obtained. Any temperature between D+0.05 and C-0.05 is selected as the mass ratio of crosslinking agent to carbon source.

3. The method for determining the rate-controlling step of sodium ion diffusion in hard carbon and optimizing its microstructure according to claim 1, characterized in that, In step 1), the carbonization temperature is 1000-1500 ℃, the holding time is 2-24 h, and the inert atmosphere is argon and / or nitrogen; the pre-oxidation holding time is 1-5 h. In step 1), the pre-oxidation process is carried out in air.

4. The method for determining the rate-controlling step of sodium ion diffusion in hard carbon and optimizing its microstructure according to claim 2, characterized in that, The crosslinking agent is one or a combination of two of IPA or PTA; The precursor is obtained by mixing the crosslinking agent and carbon source at 150-170 °C for 3-6 h.

5. The method for determining the rate-controlling step of sodium ion diffusion in hard carbon and optimizing its microstructure according to claim 1, characterized in that, Hard carbon is a material used as the negative electrode active material in sodium-ion batteries.

6. The method for determining the rate-controlling step of sodium ion diffusion in hard carbon and optimizing its microstructure according to claim 1, characterized in that, The carbon source is one or more of glucose, starch, and lignin.

Images