An ionic liquid modified high-loading dry-process fluorinated iron positive electrode and a preparation method and application thereof

Abstract

The application relates to the technical field of secondary batteries, in particular to a high-loading dry-process fluorinated iron positive electrode modified by an ionic liquid and a preparation method and application thereof. The preparation method comprises the following steps: uniformly mixing fluorinated iron and a conductive agent; then adding polytetrafluoroethylene adhesive and uniformly mixing, and fully fibrillating the polytetrafluoroethylene adhesive to obtain a positive electrode powder in which a three-dimensional fiber bonding network is formed among the fluorinated iron and the conductive agent; uniformly mixing the positive electrode powder and an ionic liquid; spreading and pressing into an FeF3 electrode film; hot-pressing the electrode film on the surface of a current collector; and post-processing to obtain the dry-process fluorinated iron positive electrode. The preparation method disclosed by the application modifies the high-loading dry-process fluorinated iron positive electrode by the ionic liquid, so that the problem that ion transmission in the positive electrode is insufficient, resulting in uneven infiltration and uneven conversion reaction, is solved.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, and in particular to an ionic liquid-modified high-load dry-process iron fluoride cathode, its preparation method, and its application. Background Technology

[0002] With the increasing demands for energy density from electric vehicles and energy storage systems, developing novel cathode systems that surpass the specific capacity limits of traditional intercalated cathode materials (such as lithium cobalt oxide and ternary materials) has become a research hotspot. Metal fluorides, due to their high theoretical specific capacity and relatively high operating potential, are considered important candidate material systems for constructing high-energy-density lithium batteries. Among them, iron fluoride (FeF3) can undergo multi-electron conversion reactions and possesses a significantly higher theoretical specific capacity than traditional cathode materials, showing great potential in improving battery energy density.

[0003] However, FeF3 materials and their electrodes face many challenges in practical applications. First, FeF3 has extremely low intrinsic electronic and ionic conductivity, and its conversion reactions during charge and discharge are accompanied by complex two- or multi-phase reconstruction and significant volume changes. These factors together lead to a continuous increase in interfacial impedance within the electrode and severe electrochemical polarization, which limits its capacity utilization and rate performance.

[0004] To address the aforementioned issues, current research focuses on increasing the proportion and areal loading of active materials in FeF3 cathodes to effectively enhance the overall energy density of the battery cell. In electrode fabrication technology, traditional wet coating processes rely heavily on organic solvents. When fabricating thick electrodes, the drying stage easily leads to uneven distribution of conductive agents and binders along the thickness direction, as well as surface cracking, severely impacting electrode consistency and mechanical properties. As an alternative, dry electrode technology utilizes the fibrillated network of polytetrafluoroethylene (PTFE) to achieve self-supporting bonding between powder particles, effectively avoiding the use of organic solvents. It holds promise for fabricating ultra-high areal loading thick electrodes and is considered one of the key fabrication routes for the practical application of FeF3 cathodes.

[0005] However, existing dry-process FeF3 thick electrode technology still has significant shortcomings. During high-pressure roll forming, the internal pore structure of the electrode is easily compacted, resulting in a significant reduction in porosity. Simultaneously, the PTFE binder inherently has extremely poor ionic conductivity, meaning the external liquid electrolyte can only slowly penetrate the electrode through subsequent wetting processes. With increased surface loading, Li₂ often appears around the deep active particles due to insufficient electrolyte wetting. + Insufficient supply leads to excessively long ion transport paths, severely uneven distribution of the reaction front, and a significant increase in electrode polarization, preventing deep-layer active materials from effectively participating in electrochemical reactions. Simply increasing the electrolyte injection volume to improve wetting will significantly sacrifice the cell's mass energy density, increase the risk of interfacial side reactions, and accelerate battery performance degradation. Summary of the Invention

[0006] To address the problems of uneven wetting and non-uniform coupling of conversion reactions caused by insufficient ion transport in high-load dry-process iron fluoride cathodes, this invention provides a method for preparing a high-load dry-process iron fluoride cathode modified with ionic liquid.

[0007] This invention provides a method for preparing a high-loading dry-process iron fluoride cathode modified with ionic liquid, comprising the following steps:

[0008] Iron fluoride and conductive agent are mixed evenly; then polytetrafluoroethylene binder is added and mixed evenly, and the polytetrafluoroethylene binder is fully fibrillated to obtain positive electrode powder in which a three-dimensional fiber bonding network is formed between iron fluoride and conductive agent. The positive electrode powder and ionic liquid are mixed evenly; spread and pressed into a FeF3 electrode film; The electrode film is hot-pressed onto the surface of the current collector, and then post-processed to obtain a dry-process iron fluoride cathode.

[0009] This invention improves upon the deep Li-phase formation caused by pre-inserting a low-volatility ionic liquid inside a dry-process FeF3 thick electrode, creating a continuous ion-conducting microphase between particles and PTFE fibers. + To address the problem of insufficient supply, reduce the transport resistance and concentration polarization in the thickness direction, and improve the uniformity of the reaction of active materials under high surface loading conditions.

[0010] Ionic liquids, confined at the interface between active particles, conductive agents, and PTFE, can maintain local wetting and ion contact during the FeF3 conversion reaction and Fe / LiF nanophase reconstruction, reducing "ion channel interruption" caused by volume changes. This is beneficial for improving the accessibility and cycle reversibility of the reversible reaction. Simultaneously, the low vapor pressure and low volatility of ionic liquids help reduce dependence on high electrolyte volumes and improve interfacial stability during thick electrode fabrication and service.

[0011] The core of this invention is not to replace all electrolytes with ionic liquids, but to use them as a small amount of position-controlled internal functional phases to form a synergistic structure of "electronic conductive network - ionic conductive microphase - mechanical support network" with dry PTFE fiber network. While maintaining the advantages of high loading and high proportion of active materials in dry process, it performs targeted modification on the ion transport and dynamic interface problems of ferric fluoride conversion cathode.

[0012] Furthermore, the conductive agent includes at least one of Ketjen black (KB), conductive carbon black (Super P), acetylene black, carbon nanotubes (CNTs), carbon nanofibers, and graphene.

[0013] Ferric fluoride includes FeF3, FeF3 / C composites, or FeF3-carbon composite powder obtained by mechanical ball milling.

[0014] Furthermore, in the positive electrode powder, the mass ratio of the iron fluoride, the conductive agent, and the polytetrafluoroethylene binder is 75-94:4-20:1-7.

[0015] Furthermore, the ionic liquid is selected from at least one of pyrrolidineonium, imidazolium, quaternary ammonium salts, or quaternary phosphine salts.

[0016] Furthermore, the anion of the ionic liquid is bis(fluorosulfonyl)imide (FSI). - ), bis(trifluoromethanesulfonyl)imide (TFSI) - ), tetrafluoroborate (BF4) - One of them.

[0017] Furthermore, the ionic liquid is selected from N-methyl-N-propylpyrrolidone-onium bisfluorosulfonylimide (Pyr 13 FSI) or N-butyl-N-methylpyrrolidone bis(trifluoromethanesulfonyl)imide (Pyr 14 At least one of TFSI).

[0018] Furthermore, when the positive electrode powder and the ionic liquid are mixed, the mass of the added ionic liquid is 1-15 wt% of the mass of the positive electrode powder, more preferably 1-6 wt%.

[0019] Control the amount of ionic liquid added to avoid the mixture forming a macroscopic liquid phase or slurry state.

[0020] This invention also discloses a high-loading dry-process iron fluoride cathode modified with ionic liquid, wherein the cathode is prepared by the preparation method described above; the surface loading of FeF3 in the cathode is 30-60 mg / cm³. 2 .

[0021] The present invention also discloses a lithium-ion battery, characterized in that it includes a positive electrode, an electrolyte, a separator, and a negative electrode as described above.

[0022] Compared with the prior art, the present invention provides a method for preparing a high-loading dry iron fluoride cathode modified with ionic liquid, which has the following advantages: 1. Significantly improves ion transport kinetics of thick electrodes and reduces electrochemical polarization.

[0023] This invention pre-constructs an "embedded ion-conducting phase" within the dry electrode, allowing ionic liquids to preferentially distribute on the surface of active particles and in the gaps between PTFE fibers, forming a continuous ion-transporting microphase that spans the electrode thickness. This design effectively shortens the Li... +The effective transport distance in the thick electrode provides sufficient local ion supply for the deep active material, thereby significantly alleviating the problems of uneven reaction front distribution and electrochemical polarization caused by insufficient electrolyte wetting, and greatly improving the utilization rate of active material in the thick electrode.

[0024] 2. Effectively stabilizes the dynamic interface, improving the reversibility and cycle stability of the conversion reaction.

[0025] To address the inherent characteristics of iron fluoride during charge and discharge, including volume changes and multiphase reconstruction, this invention utilizes an ionic liquid confined to the particle surface as a stable interfacial wetting phase. This ionic liquid maintains good interfacial contact between components within the electrode during the volume expansion and contraction of the active particles, effectively buffering localized detachment caused by structural reconstruction. This ensures the uniformity of the Fe / LiF to iron fluoride reversal process, significantly improving the electrode's long cycle life and capacity retention.

[0026] 3. Optimize the dry electrode fabrication process to balance high energy density and high loading capacity.

[0027] This invention employs a specific process sequence of "first fibrillation, then introduction of ionic liquid," avoiding interference from the ionic liquid on the PTFE fibrillation process and ensuring the self-supporting mechanical strength and high compaction density of the dry-process electrode. This allows the electrode to maintain a high proportion of active material (>90%) and a density of 30-60 mg / cm³. 2 While achieving ultra-high areal loading, it also achieves excellent ion conduction performance, fundamentally solving the contradiction between the "mechanical properties" and "electrochemical properties" of dry thick electrodes.

[0028] 4. Avoid using excessive electrolyte to improve the actual energy density of the battery cell.

[0029] This invention replaces the traditional method of relying on external wetting with a large amount of liquid electrolyte by pre-fixing a low-volatility ionic liquid inside the electrode. This not only avoids the cell mass energy density loss and exacerbation of interfacial side reactions caused by simply increasing the amount of electrolyte injected, but also helps to improve the safety of the battery at high temperatures and high voltages due to the wide electrochemical window and high thermal stability of the ionic liquid itself. Attached Figure Description

[0030] Figure 1 The image shows the first charge-discharge curve of the battery assembled from the positive electrode prepared in Example 1 of this invention at 28°C and 0.05C. Figure 2 The image shows the first charge-discharge curve of the battery assembled from the positive electrode prepared in Example 2 of this invention at 28°C and 0.05C. Figure 3The image shows the first charge-discharge curve of the battery assembled from the positive electrode prepared in Example 3 of this invention at 28°C and 0.05C. Figure 4 The image shows the first charge-discharge curve of the battery assembled from the positive electrode prepared in Example 4 of this invention at 28°C and 0.05C. Figure 5 The image shows the first charge-discharge curve of the battery assembled from the positive electrode prepared in Example 5 of this invention at 28°C and 0.05C. Figure 6 The image shows the first charge-discharge curve of the battery assembled from the positive electrode prepared in Example 6 of this invention at 28°C and 0.05C. Figure 7 This is a graph showing the first charge-discharge curve of the battery assembled from the positive electrode prepared in Comparative Example 1 of this invention at 28°C and 0.05C. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0032] The embodiments of the present invention are described in detail below. The examples of the embodiments are only used to explain the present invention and should not be construed as limiting the present invention.

[0033] Example 1 Preparation of high-loading dry-process iron fluoride cathode modified with ionic liquid: Anhydrous FeF3 was selected as the positive electrode active component, and Ketjen Black (KB) was used as the conductive agent. FeF3 and the conductive agent were weighed and mixed evenly at a mass ratio of 91.6:6.2. PTFE powder was then added to the uniformly mixed powder, making the mass ratio of FeF3, conductive agent, and PTFE 91.6:6.2:2.2. High-speed mixing or mechanical shearing was used to fully fibrillate the PTFE, forming a three-dimensional fiber bonding network between the positive electrode active component and the conductive agent particles, thus obtaining the positive electrode powder.

[0034] N-methyl-N-propylpyrrolidone onium bis(fluorosulfonyl)imide (Pyr) was selected. 13 FSI is an ionic liquid.

[0035] An ionic liquid is added to the cathode powder at an amount of 10 wt% of the cathode powder mass. Low-speed mixing or low-shear kneading is used to uniformly adsorb the ionic liquid onto the surface of the cathode active component particles, the particle contact interface, and the gaps between PTFE fibers, resulting in an ionic liquid-modified composite powder suitable for dry film formation.

[0036] The ionic liquid-modified composite powder was uniformly spread and subjected to multi-stage rolling at 50-100℃ with a rolling pressure of 5-30 MPa. By adjusting the rolling gap, a self-supporting dry FeF3 electrode film was obtained, in which the surface loading of FeF3 in the electrode film was 45 mg / cm³. 2 .

[0037] The prepared self-supporting dry FeF3 electrode film is hot-pressed onto the surface of carbon-coated aluminum foil or other corrosion-resistant current collectors, and then dried under vacuum conditions at 60-100℃ to remove trace amounts of moisture and volatile impurities while retaining the ionic liquid functional phase, thus obtaining an ionic liquid-intercalated modified dry FeF3 cathode.

[0038] Example 2 Preparation of high-loading dry-process iron fluoride cathode modified with ionic liquid: Anhydrous FeF3 was selected as the positive electrode active component. The conductive agent is Ketjen Black (KB). FeF3 and the conductive agent are weighed and mixed evenly at a mass ratio of 91.6:6.2. PTFE powder is then added to the uniformly mixed powder, making the mass ratio of FeF3, conductive agent, and PTFE 91.6:6.2:2.2. High-speed mixing or mechanical shearing is used to fully fibrillate the PTFE, forming a three-dimensional fiber bonding network between the positive electrode active component and the conductive agent particles, thus obtaining the positive electrode powder.

[0039] N-Butyl-N-methylpyrrolidone-onium bis(trifluoromethanesulfonyl)imide (Pyr 14 TFSI is an ionic liquid.

[0040] An ionic liquid is added to the cathode powder at an amount of 10 wt% of the cathode powder mass. Low-speed mixing or low-shear kneading is used to uniformly adsorb the ionic liquid onto the surface of the cathode active component particles, the particle contact interface, and the gaps between PTFE fibers, resulting in an ionic liquid-modified composite powder suitable for dry film formation.

[0041] The ionic liquid-modified composite powder was uniformly spread and subjected to multi-stage rolling at 50-100℃ with a rolling pressure of 5-30 MPa. By adjusting the rolling gap, a self-supporting dry FeF3 electrode film was obtained, in which the surface loading of FeF3 in the electrode film was 45 mg / cm³. 2 .

[0042] The prepared self-supporting dry FeF3 electrode film is hot-pressed onto the surface of carbon-coated aluminum foil or other corrosion-resistant current collectors, and then dried under vacuum conditions at 60-100℃ to remove trace amounts of moisture and volatile impurities while retaining the ionic liquid functional phase, thus obtaining an ionic liquid-intercalated modified dry FeF3 cathode.

[0043] Example 3 The difference between this embodiment and Embodiment 1 is that in step 2, the amount of ionic liquid added is 5 wt% of the mass of the positive electrode powder, and the surface loading of FeF3 in the electrode film is 30 mg / cm³. 2 .

[0044] Example 4 The difference between this embodiment and Embodiment 1 is that in step 2, the amount of ionic liquid added is 15 wt% of the mass of the positive electrode powder, and the surface loading of FeF3 in the electrode film is 30 mg / cm³. 2 .

[0045] Example 5 The difference between this embodiment and Embodiment 2 is that in step 2, the amount of ionic liquid added is 5 wt% of the mass of the positive electrode powder, and the surface loading of FeF3 in the electrode film is 30 mg / cm³. 2 .

[0046] Example 6 The difference between this embodiment and Embodiment 2 is that in step 2, the amount of ionic liquid added is 15 wt% of the mass of the positive electrode powder, and the surface loading of FeF3 in the electrode film is 30 mg / cm³. 2 Comparative Example 1 No ionic liquids are added.

[0047] Anhydrous FeF3 was selected as the positive electrode active component, and Ketjen Black (KB) was used as the conductive agent. FeF3 and the conductive agent were weighed and mixed evenly at a mass ratio of 91.6:6.2. PTFE powder was then added to the uniformly mixed powder, making the mass ratio of FeF3, conductive agent, and PTFE 91.6:6.2:2.2. High-speed mixing or mechanical shearing was used to fully fibrillate the PTFE, forming a three-dimensional fiber bonding network between the positive electrode active component and the conductive agent particles, thus obtaining the positive electrode powder.

[0048] The positive electrode powder is uniformly spread and subjected to multi-stage rolling at 50-100℃ with a rolling pressure of 5-30 MPa. The FeF3 electrode film is obtained by adjusting the rolling gap, and the surface loading of FeF3 in the electrode film is 45 mg / cm³. 2 .

[0049] The prepared dry FeF3 electrode film is hot-pressed onto the surface of carbon-coated aluminum foil or other corrosion-resistant current collectors, and then dried under vacuum conditions at 60-100℃ to remove trace amounts of moisture and volatile impurities while retaining the ionic liquid functional phase, thus obtaining an ionic liquid-intercalated modified dry FeF3 cathode.

[0050] Button battery assembly: The battery was assembled in a glove box where the water and oxygen content were both below 0.1 ppm. The negative electrode was made of lithium metal, the separator was made of glass fiber or other separators suitable for lithium metal batteries, and the electrolyte was a lithium salt electrolyte matched with the metal fluoride positive electrode, which was 1 M LiTFSI dissolved in 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) at a volume ratio of 1:1. After the positive electrode, negative electrode, separator and electrolyte prepared in the examples and comparative examples were assembled, they were allowed to stand to allow the electrolyte to fully impregnate them.

[0051] Performance testing: like Figure 1-7 Table 1 shows the discharge capacity of the assembled batteries at 0.1C, tested at 25℃. Modification of the high-load dry-process FeF3 cathode with ionic liquid significantly improved the discharge specific capacity of all electrodes. The discharge specific capacity of Comparative Example 1, without ionic liquid, was only 200.6 mAh g⁻¹. -1 The discharge specific capacity of Examples 1-6 was increased to 443.0-691.1 mAh·g. -1 Among them, the loading on the FeF3 surface was 30 mg cm. -2 Add 5 wt% Pyr 13 Example 3 of FSI and the addition of 5wt% Pyr 14 Example 5 of TFSI yielded 691.1 and 690.7 mAh g, respectively. -1 The discharge specific capacity of Example 4 and Example 6 was the highest among all examples, close to the theoretical specific capacity of FeF3. When the amount of ionic liquid added was increased to 15 wt%, the capacities of Example 4 and Example 6 decreased to 606.4 and 637.9 mAh g, respectively. -1 This indicates that higher amounts of ionic liquid are not necessarily more beneficial; excessive ionic liquid may occupy the internal pores of the electrode or affect electron contact between particles, thus reducing capacity. Furthermore, at higher concentrations (45 mg·cm⁻¹),... -2 Under area load conditions, Examples 1 and 2 still maintained 570.1 and 443.0 mAh g, respectively. -1The discharge specific capacity of the sample was significantly higher than that of the control group without ionic liquid, indicating that the ionic liquid modification method has a significant effect on improving the electrochemical utilization of the high-loading dry FeF3 cathode. Overall, under the experimental conditions, an addition of approximately 5 wt% ionic liquid showed a superior capacity enhancement effect, while Pyr... 13 The FSI system as a whole exhibits good adaptability to high loads.

[0052] Table 1 Test Results

[0053] Finally, it should be noted that any modification or equivalent substitution of some or all of the technical features made based on the technical solutions of this invention and the embodiments thereof, without departing from the corresponding technical solutions of this invention, shall fall within the patent scope of this invention and the embodiments thereof.

Claims

1. A method for preparing a high-loading dry-process iron fluoride cathode modified with ionic liquid, characterized in that, include: Mix the ferric fluoride with the conductive agent until homogeneous; Then add polytetrafluoroethylene binder and mix evenly, and fully fibrillate the polytetrafluoroethylene binder to obtain positive electrode powder that forms a three-dimensional fiber bonding network between iron fluoride and conductive agent; The positive electrode powder and ionic liquid are mixed evenly; spread and pressed into a FeF3 electrode film; The electrode film is hot-pressed onto the surface of the current collector, and then post-processed to obtain a dry-process iron fluoride cathode.

2. The preparation method according to claim 1, characterized in that, The conductive agent includes at least one of Ketjen black, conductive carbon black, acetylene black, carbon nanotubes, carbon nanofibers, and graphene.

3. The preparation method according to claim 1, characterized in that, In the positive electrode powder, the mass ratio of the iron fluoride, the conductive agent, and the polytetrafluoroethylene binder is 75-94:4-20:1-7.

4. The preparation method according to claim 1, characterized in that, The ionic liquid is selected from at least one of pyrrolidineonium, imidazolium, quaternary ammonium salts, or quaternary phosphine salts.

5. The preparation method according to claim 4, characterized in that, The anion of the ionic liquid is one of bis(fluorosulfonyl)imide, bis(trifluoromethanesulfonyl)imide, or tetrafluoroborate.

6. The preparation method according to claim 5, characterized in that, The ionic liquid is selected from at least one of N-methyl-N-propylpyrrolidone bis(fluorosulfonyl)imide or N-butyl-N-methylpyrrolidone bis(trifluoromethanesulfonyl)imide.

7. The preparation method according to claim 1, characterized in that, When the positive electrode powder and the ionic liquid are mixed, the mass of the added ionic liquid is 1-15 wt% of the mass of the positive electrode powder.

8. A high-loading dry-process iron fluoride cathode modified with ionic liquid, characterized in that, The positive electrode is prepared by the preparation method according to any one of claims 1-7; the surface loading of FeF3 in the positive electrode is 30-60 mg / cm³. 2 .

9. A lithium-ion battery, characterized in that, It includes the high-load dry-process iron fluoride positive electrode, electrolyte, separator, and negative electrode as described in claim 8.

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