Value-state-constrained sodium-ion battery positive electrode material, sodium-ion battery and preparation method of sodium-ion battery

Abstract

The invention discloses a valence-state-constrained sodium ion battery positive electrode material, a sodium ion battery and a preparation method of the valence-state-constrained sodium ion battery positive electrode material, the chemical general formula of the valence-state-constrained sodium ion battery positive electrode material is Naa (FexMnhVk) PO4, and a is greater than or equal to 0.95 and less than or equal to 1.05; 0.48 < = x < = 0.52; 0.32 < = h < = 0.38; 0.15 < = k < = 0.17; x + h + k = 1. The material is a synergistically doped Naa (FexMnhVk) PO4 material with triple constraints of component-performance-geometric structure, and high magnification, long service life and process reproducibility are taken into account by controlling the component range and limiting measurable performance indexes and geometric structure.

Description

Technical Field

[0001] This invention relates to the field of secondary battery cathode materials technology, specifically to a co-doped sodium-ion battery cathode material and its preparation method, as well as an electrochemical cell or battery containing the material. The general chemical formula of the cathode material is Na. a (Fe x Mn h V k PO4 belongs to the category of multi-element transition metal phosphate cathode materials. Background Technology

[0002] Sodium-ion battery cathode materials have advantages in terms of cost, resource reserves, and safety. While olivine-type NaFePO4 exhibits good stability, it is limited by its one-dimensional ion channels and single Fe atoms. 2+ / Fe 3+ The voltage plateau, energy density, and rate performance are all limited. Introducing multiple transition metal doping (such as Mn and V) can extend the operating voltage range and kinetic performance, but existing technologies suffer from the following problems:

[0003] (1) Significant high-potential side reactions: A distinct dQ / dV peak appears in the 3.80–4.05V region, leading to a decrease in coulombic efficiency.

[0004] (2)Mn 3+ Induced structural distortion: Mn 3+ Excessive amounts can cause strong Jahn-Teller distortion, accelerating capacity decay.

[0005] (3) It is difficult to balance the rate of increase and the cycle performance: shortening the diffusion path to increase the rate of increase often sacrifices the cycle life.

[0006] (4) Poor reproducibility of preparation: thermal process and atmospheric fluctuations can easily lead to inconsistent performance. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of existing technologies by providing a valence-constrained sodium-ion battery cathode material. This material is a synergistically doped Na material with triple constraints on composition, performance, and geometry. a (Fe x Mn h V k By controlling the composition range, limiting measurable performance indicators and geometric structure, PO4 materials can achieve a balance between high rate capability, long lifespan and process reproducibility.

[0008] The technical solution of this invention is achieved as follows: the chemical formula of the material of this invention is Na. a (Fe x Mn h V j PO4, satisfying the following conditions:

[0009] (1) Component range: 0.95≤a≤1.05, 0.48≤x≤0.52, 0.32≤h≤0.38, 0.15≤k≤0.17, and x+h+k=1.

[0010] (2) Performance indicators: 25℃, voltage window 2.5–4.05V, dQ / dV curve peak area ratio of 3.80–4.05V during 0.1C first charge ≤20%;

[0011] After the initial charge to 4.05V, XANES measured Mn 3+ It accounts for ≤25% of the total Mn; the amount of reversible sodium removal α = 0.58–0.65, and α ≤ x + k.

[0012] (3) Geometric structure: particle size D50 is 150–250 nm; carbon coating thickness is 3–4 nm (mass fraction 2.5–3.5%).

[0013] (4) Preparation method: The precursor is a citric acid-ethylene glycol sol-gel or wet ball milling mixture, which is pretreated (320-350℃, 2-3h) and calcined (660-700℃, 7-9h, under argon or argon / hydrogen protective atmosphere, with hydrogen gas integral ≤3%; temperature rise 2.0-2.5℃·min). -1 Temperature drop of 3.0–4.0℃·min -1 ) and secondary annealing (510–530℃, 1.5–2.5h).

[0014] (5) Electrochemical cell / battery: comprising the aforementioned positive electrode material, conductive agent, binder, and 0.8–1.0 M NaPF6 carbonate electrolyte, with an areal density of 7–8 mg·cm³. -2 .

[0015] The beneficial effects of this invention are as follows:

[0016] (1) High potential side reaction suppression: peak area ≤20%, significantly lower than that of Example A (38–41%).

[0017] (2)Mn 3+ Content control: ≤25%, compared with Example B (approximately 33%), which effectively reduced Jahn-Teller distortion.

[0018] (3) Balance between scaling and cycling: 1C 500 cycles retention rate ≥92%, 5C retention rate >80%, compared to only 60-65% for comparative example C.

[0019] (4) High reproducibility: inter-batch performance fluctuation ≤ ±2 percentage points. Attached Figure Description

[0020] Figure 1 This invention is an XRD and Rietveld refinement overlay.

[0021] Figure 1 The XRD and Rietveld refined overlay images show the sample from Example 1. Test conditions: Cu Kα 2θ = 10–60°, step size approximately 0.02°, dwell time approximately 0.5 s / step, rotating stage on; Kα2 was removed using the Rachinger method. Solid black lines represent observed spectra, dashed black lines represent calculated curves, and dotted lines represent difference curves (shifted upwards by 5.0 counts for easier observation). The main peak's apex is labeled (hkl) as the olivine Pnma structure; the ■ at the bottom of the figure represents the Pnma scale, and ○ represents the NASICON reference ticks. As shown in the figure, all diffraction peaks are well fitted by the model, and no unassigned impurity peaks are observed; the refined statistics and cell parameters are shown in the text boxes in the figure. The NASICON scale is only used to indicate possible epitaxial / short-range ordered features, and its intensity is below the detection limit.

[0022] Figure 2 This is a comparison chart of the peak area of ​​the dQ / dV curve for the first charge at 0.1C according to the present invention. The peak area integral range and proportion in the 3.80–4.05V region are marked. Figure 2 The performance parameters of all embodiments are shown to meet the limits of the claims (≤20%), and the comparative examples are significantly inferior to the embodiments of the present invention in this respect.

[0023] Figure 3a These are graphs showing the cycle life of the embodiments and comparative example 1C. Figure 3b The graph shows the retention rate of the 3C / 5C@500 cycles in the examples and comparative examples.

[0024] Figure 4 This is the XANES valence state fitting diagram of Mn K-edge after the first charge of this invention, labeled Mn. 3+ / Total Mn.

[0025] Figure 5 This is a graph showing the relationship between the GITT / Nyquist curve and α–(x+k) of the present invention, with the α value and limit line (α=x+k) marked for each embodiment.

[0026] Figure 6 This is a schematic diagram of the δ definition in this invention, showing the distribution ratio and δ value of Fe / Mn / V in different valence states.

[0027] Figure 7 This is the flowchart of the δ design solution of the present invention, which is a step framework from target performance input to optimal ratio output.

[0028] Figure 8 This is a flowchart of the δ verification and acceptance process of the present invention, which marks the judgment conditions for comparing the measured δ with the design value. Detailed Implementation

[0029] I. Terminology and Measurement Methods

[0030] (1) dQ / dV peak area: the total area is the interval between 2.5–4.05V and the sub-interval is 3.80–4.05V; the integration step size is ≤2mV and the baseline subtraction adopts the third-order polynomial.

[0031] (2)Mn 3+ Content: XANES LCF analysis, standard reference spectrum is Mn 2+ / Mn 3+ / Mn 4+ The calibration is performed with the Mn metal foil as the zero point.

[0032] (3) Calculation of α value: The Na removal ratio is converted from the reversible capacity of the first charge, and α≤x+k is verified.

[0033] (4) Particle size and carbon layer thickness: Particle size D50 was determined by laser particle size analyzer + SEM, and carbon layer thickness was determined by TEM cross-section measurement of 50 points.

[0034] (5) Cyclic performance: 500 cycles under constant current conditions of 1C, voltage of 2.5–4.05V; rate performance was tested at 3C, 5C, and 10C.

[0035] (6) Definition of term (δ): “δ” in this application refers to the mechanism model parameter. Its definition and measurement procedure shall be based on the formulas and steps listed in “IV. Mechanism Analysis - (E) δ Model Analysis” of this specification. Unless otherwise stated, the test time, environment and data processing of δ shall also be based on the description in that section.

[0036] II. Raw Materials and Precursors

[0037] The metal source is a sulfate or nitrate (Fe, Mn, V, and optionally Ti or Ni), and the phosphorus source is NH4H2PO4; the complexed carbon source is citric acid:ethylene glycol in a molar ratio of 1:(3–4), with a solid content of 20–30 wt%. The total metal content is strictly proportioned according to the ratio of (x:h:k) and optional substitutions, and the Na source is CH3COONa or Na2CO3.

[0038] III. Forming and Heat Treatment

[0039] (1) Sol-gel route: Stir and age at 60–80℃ for 2–4 hours to obtain sol, and dry at 120℃; or use wet ball milling for 6–10 hours (ethanol / water mixed solvent).

[0040] (2) Pretreatment: 320–350℃ for 2–3h to remove organic matter and form precursor carbon skeleton. (3) Main firing: 660–700℃ for 7–9h; protective atmosphere Ar or Ar / H2 (≤3%), temperature rise 2.0–2.5℃·min. -1 Temperature drop of 3.0–4.0℃·min -1 .

[0041] (4) Secondary annealing: 510–530℃ for 1.5–2.5h, which is conducive to ordering and surface reconstruction and stabilizes carbon coating (3–4nm).

[0042] Particle size is controlled by the specific surface area of ​​the precursor and the thermal history, with a target D50 of 150–250 nm.

[0043] IV. Mechanism Analysis

[0044] (A) Valence and Coordination: V 5+ / V 4+ The introduction of Mn increases the anion framework polarizability, raises the reaction barrier in the high potential region and disperses the electron density, weakening the peak in the 3.8–4.05 V region; 2+ / Mn 3+ In Fe 2+ / Fe 3+ The surrounding heterogeneous local environment reduces the probability of cooperative distortion of adjacent octahedrons.

[0045] (B) Channel Dimension: The random-short-range order (SRO) of Fe / Mn / V at the M site suppresses the "gate effect" of the b-axis single channel. The trace oxygen vacancies and the surface reconstruction induced by the carbon interface (approximately 1–2 nm) form a "shell-core" gradient, effectively shortening the Na... + Surface diffusion path.

[0046] (C) Carbon coating geometric constraints: 3–4 nm continuous carbon layers take into account the stability of electron channels and ion interfaces; thicknesses greater than 4 nm tend to increase interface resistance, while thicknesses less than 3 nm make it difficult to suppress high-potential side reactions and electrolyte decomposition.

[0047] (D) The relationship between α and (x+k): x corresponds to the reversible pair of Fe sites; k corresponds to the reversible participation of V sites, which is higher than that of Mn sites; when α approaches or exceeds (x+k), the signal enhancement of over-desodium and irreversible side reactions will occur in the high potential region (dQ / dV sub-peaks rise), so α ≤ x+k is limited.

[0048] (E) NASICON epitaxy: When (PO4) is replaced by (SiO4) / (SO4) at a molar ratio of 0.05–0.15, and subjected to a heat treatment of 680–700℃ and a moderately reducing atmosphere, a three-dimensional channel phase (NASICON-like) dominance can be induced, resulting in reduced polarization and improved cycle stability at high rates; its composition can be normalized and mapped to Na. a (Fe x Mn h V k The "equivalent M-bit occupancy" of the PO4 framework.

[0049] (E) δ model analysis: δ characterizes the deviation of the average valence state of the doped system from the reference value (Fe). 23 / Mn 2c / V 43 The degree of ) is expressed by the formula:

[0050] δ=∑i∈{Fe,Mn,V}(valence state i×mole fraction i)-2.0

[0051] When δ approaches 0, the d-electron distribution in the system is balanced, which can effectively suppress Jahn–Teller distortion and reduce the energy barrier of high-potential phase transition.

[0052] When δ is too high (>0.1), the proportion of high-valence transition metals is too high, which can easily trigger high-potential side reactions and capacity decay.

[0053] Calculation example (E1 sample):

[0054] Fe 2+ 50 mol% → Valence state 2.0

[0055] Mn 2+ 33 mol% → Valence state 2.0

[0056] V 5+ 17 mol% → Valence state 5.0

[0057] δ=(2.0×0.50)+(2.0×0.33)+(5.0×0.17)-2.0=0.51

[0058] The δ value of E1 is within the optimal range (0.45–0.55), corresponding to a dQ / dV peak area of ​​14.5%, and Mn 3+ With a content of 18%, it exhibits the best performance.

[0059] δ calculation combined with DFT can predict high-performance formulations, with appendix. Figure 7 –8 provides the definition, solution process, and verification process.

[0060] V. Examples and Data

[0061] General Test Rules

[0062] Electrochemical tests for all examples and comparative examples were performed at 25°C and within a voltage window of 2.5–4.05 V.

[0063] (1) dQ / dV peak area (3.80–4.05V): The integral over the interval 2.5–4.05V is the denominator, and the integral over the sub-interval 3.80–4.05V is the numerator; the integration step size is ≤2mV, and the baseline subtraction uses a third-order polynomial.

[0064] (2)Mn 3+ Content: Samples were taken after initial charging to 4.05V and analyzed using XANES LCF. The reference spectrum was Mn. 2+ / Mn 3+ / Mn 4+ Energy calibration is performed with the Mn metal foil as the zero point.

[0065] (3) α value: The Na extraction ratio is calculated based on the reversible capacity of the first charge, and α≤x+k is verified.

[0066] (4) Judgment criteria: Mn 3+ ≤25% is compliant; dQ / dV(3.80–4.05V)≤20% is compliant; α≤x+k is compliant.

[0067] Table 1. Key Indicator Comparison Table (Example and Comparative Examples)

[0068]

[0069]

[0070] Summary: Example Mn 3+ Average value 18.67%, dQ / dV 19.6%, comparative Mn 3+ The average is 34%.

[0071] The preparation method of this invention includes:

[0072] (1) Weigh the precursors containing Na, Fe, Mn, and V and the precursor containing P according to the stoichiometric ratio, and add the carbon source;

[0073] (2) Mix and ball mill until homogeneous to obtain a mixture;

[0074] (3) The mixture is sintered once and annealed twice under an inert atmosphere to obtain the positive electrode material.

[0075] The first sintering temperature is 660–700℃, held for 7–9 hours, and the second annealing temperature is 510–530℃, held for 1.5–2.5 hours. The atmosphere is argon or argon / hydrogen protective atmosphere, and the hydrogen gas integral is ≤3%.

[0076] Example 1 (central proportion, target product Na1(Fe)) 0.5 Mn 0.33 V 0.17 )PO4)

[0077] a = 1.00, x = 0.50, h = 0.33, k = 0.17; D50 = 200 nm; carbon layer 3.3 nm (3.0 wt%). 0.1C initial charge dQ / dV: 3.80–4.05 V peak area percentage 14.5%; α = 0.62 (≤ x + k = 0.67).

[0078] XANES: Mn 3+ / Mn total = 18%.

[0079] 1C 500 cycles: retention rate 94.8%; 5C 82.3%; 10C 68.1%.

[0080] Example 2 (Low Na Boundary)

[0081] a = 0.95, x = 0.48, h = 0.37, k = 0.15; D50 = 180 nm; carbon layer 3.0 nm (2.6 wt%). Peak area percentage 17.8%; α = 0.59; Mn 3+ =22%; retention rate 93.2%.

[0082] Example 3 (High Na Boundary)

[0083] a = 1.05, x = 0.52, h = 0.32, k = 0.16; D50 = 230 nm; carbon layer 3.8 nm (3.4 wt%). Peak area percentage 19.6%; α = 0.60; Mn 3+ =21%; retention rate 92.5%.

[0084] Example 4 (Low-temperature primary firing and secondary cooling lower limit)

[0085] Main heat 660℃×7h; Second heat 510℃×1.5h.

[0086] D50 = 170 nm; carbon 3.1 nm. Peak area percentage 16.9%; α = 0.61; Mn 3+ =23.5%; 1C retention rate 93.9%.

[0087] Example 5 (High-Temperature Main Firing and Upper Limit of Secondary De-firing)

[0088] Main heat 700℃×9h; Second heat 530℃×2.5h.

[0089] D50 = 240 nm; carbon 3.9 nm. Peak area percentage 18.7%; α = 0.60; Mn3+ =24%; 1C retention rate 93.0%.

[0090] Example 6 (Ar / H2 3% reducing atmosphere)

[0091] The rest is the same as in Example 1. Peak area percentage: 13.8%; Mn 3+ =17%; EIS initial Rct decreased by 12%, 1C retention rate was 95.1%.

[0092] Example 7 (Ti epitaxial substitution 0.08, target product Na1(Fe)) 0.5 Mn 0.33 V 0.09 Ti 0.08 In the PO4 formulation, 0.08% of V at position k (based on total transition metals) was replaced with Ti; the rest was the same as in Example 1.

[0093] Peak area percentage: 12.9%; α = 0.60 (≤ 0.67); Mn 3+ =16%; 5C retention rate 85.4%.

[0094] Example 8 (Ni epitaxial substitution of 0.10, target product Na1(Fe) 0.5 Mn 0.33 V 0.07 Ni 0.1 PO4) is replaced with Ni instead of 0.10; the rest is the same as in Example 1.

[0095] Peak area percentage: 13.6%; α = 0.61; Mn 3+ =18%; Discharge retention rate at -10℃ is 72%.

[0096] Example 9 (NASICON epitaxy, (SiO4) substitution 0.10, target product Na1(Fe) 0.5 Mn 0.33 V 0.17 (PO4) 0.9 (SiO4) 0.1 )

[0097] The normalized ratio satisfies x = 0.50, h = 0.33, k = 0.17, which is equivalent to M-position occupancy; the principal phase is NASICON.

[0098] 3C retention rate 90.2%, 5C 86.0%; 1C 500 cycles retention rate 93.7%; peak area definition converted to equivalent 3.8–4.05V region cathode reaction proportion 15–18%; α = 0.59; Mn 3+ =21.8%.

[0099] Example 10 (D50 lower limit and carbon 3.0 nm)

[0100] D50 = 150 nm; carbon 3.0 nm (2.5 wt%). Peak area 15.9%; α = 0.60; Mn 3+ =22.5%; retention rate 94.1%.

[0101] Example 11 (D50 upper limit and carbon 4.0 nm)

[0102] D50 = 250 nm; carbon 4.0 nm (3.5 wt%). Peak area 19.2%; α = 0.58; Mn 3+ =23%; retention rate 92.4%.

[0103] Example 12 (Upper limit of areal density 8 mg·cm³) -2 )

[0104] At 8 mg·cm -2 Under these conditions, the 1C retention rate was 92.1%, the 5C retention rate was 80.0%, and the peak area was 16.8%; α = 0.63; Mn 3+ =22.7%.

[0105] Comparative Example A (uncontrolled ratio, high Mn, low V)

[0106] x = 0.46, h = 0.40, k = 0.14 (outside the boundaries of this invention, for comparison only).

[0107] Peak area ratio 38–41%; Mn 3+ ≈35%; 1C retention rate 84%.

[0108] Comparative Example B (Fe / Mn binary, V-free)

[0109] Peak area ratio 30–34%; Mn 3+ ≈33%; 1C retention rate 82–86%.

[0110] Comparative Example C (Fe / V binary, Mn-free)

[0111] The initial charging platform shifts, with enhanced side reactions in the 3.8–4.05V region; the 1C retention rate is 86–88%, but drops sharply to 60–65% at 5C.

[0112] VI. Industrial Applicability

[0113] This invention offers cost advantages and safety redundancy in scenarios such as energy storage power stations, two-wheeled and light commercial vehicles, and 48V industrial and commercial backup power. The material preparation window and electrode formulation are both industrially available, making it suitable for ton-scale scaling. The aforementioned performance indicators, when repeatedly tested under mass production conditions, show batch-to-batch variations of ≤±2%.

Claims

1. A valence-state-constrained sodium-ion battery cathode material with the general chemical formula Na. a (Fe x Mn h V k )PO4, where: 0.95≤a≤1.05; 0.48≤x≤0.52; 0.32≤h≤0.38; 0.15≤k≤0.17; and x+h+k=1.

2. The valence-constrained sodium-ion battery cathode material according to claim 1, characterized in that: Under the conditions of 25℃, charge / discharge voltage range of 2.50–4.05V, and rate of 0.1C, (i) the peak area of ​​dQ / dV in the voltage range of 3.80–4.05V accounts for ≤20% of the peak area of ​​the entire voltage range (2.50–4.05V); (ii) after the first charge, Mn 3+ (iii) The mole fraction is ≤25%; and the amount of reversible sodium removal α satisfies 0.58≤α≤0.65 and α≤x+k.

3. The valence-constrained sodium-ion battery cathode material according to claim 1, wherein the particle size D 50 It is 150–250nm.

4. The valence-constrained sodium-ion battery cathode material according to claim 1 or 2, wherein the carbon coating layer has a thickness of 3–4 nm.

5. The valence-constrained sodium-ion battery cathode material according to claim 1, characterized in that: The δ value of the positive electrode material of the battery is 0.45–0.55, where δ represents the deviation of the average valence state of the doped system from the reference value Fe. 2+ Mn 2+ V 4+ The degree of valence is calculated using the formula: δ=∑i∈{Fe,Mn,V}(valence state) i × mole fraction i -2.0; where valence state i is compared with the standard reference spectrum Mn by X-ray absorption near-edge structure spectrum XANES. 2+ / Mn 3+ / Mn 4+ Fe 2+ / Fe 3+ V 3+ / V 4+ / V 5+ The mole fraction was obtained by linear combination fitting of LCF. i The results were obtained by inductively coupled plasma emission spectroscopy (ICP-OES) under the following conditions: room temperature and sampling after initial charging to 4.05V.

6. A method for preparing a valence-state-constrained sodium-ion battery cathode material, comprising: (1) Weigh the precursors containing Na, Fe, Mn, and V and the precursor containing P according to the stoichiometric ratio, and add the carbon source; (2) Mix and ball mill until homogeneous to obtain a mixture; (3) The mixture is sintered once and annealed twice under an inert atmosphere to obtain the positive electrode material.

7. The method for preparing a valence-constrained sodium-ion battery cathode material according to claim 6, wherein the primary sintering temperature is 660–700℃ and the holding time is 7–9 hours, the secondary annealing temperature is 510–530℃ and the holding time is 1.5–2.5 hours, the atmosphere is argon or argon / hydrogen protective atmosphere, and the hydrogen gas integral is ≤3%.

8. A sodium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the active material of the positive electrode is a valence-constrained sodium-ion battery positive electrode material according to any one of claims 1-4, and the negative electrode is hard carbon or soft carbon; the electrolyte is an organic carbonate system containing NaPF6; and the areal density of the positive electrode is 8-12 mg·cm³. -2 .

9. The sodium-ion battery according to claim 8, wherein the capacity retention rate after 500 cycles at 1C is ≥92%.

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