Modified lithium silicate-sulfur composite lithium supplement type lithium iron phosphate cathode material and preparation method thereof

Abstract

The application discloses a modified lithium silicate sulfur composite lithium supplement type lithium iron phosphate positive electrode material and a preparation method thereof, and belongs to the technical field of lithium iron phosphate. The preparation method comprises the following steps: S1, ball-milling lithium carbonate, nano silicon dioxide and a conductive agent, and then sintering and activating to obtain conductive coated Li4SiO4; S2, melt-coating the conductive coated Li4SiO4, elemental sulfur and polyvinylidene fluoride powder at 110-140 DEG C to obtain Li4SiO4 / S lithium supplement agent; and S3, mixing and blending lithium iron phosphate, the Li4SiO4 / S lithium supplement agent, a silane type phosphate ester, a cyclic sulfate and a solvent into a positive electrode slurry, and drying and solidifying the positive electrode slurry to obtain the modified lithium silicate sulfur composite lithium supplement type lithium iron phosphate positive electrode material. The prepared positive electrode material realizes efficient and stable lithium supplement.

Description

Technical Field

[0001] This invention belongs to the field of lithium iron phosphate technology, specifically a modified lithium silicate-sulfur composite lithium iron phosphate cathode material and its preparation method. Background Technology

[0002] As a core energy storage device in new energy vehicles and large-scale energy storage, lithium-ion batteries directly determine their application boundaries based on their cycle life, rate performance, and wide temperature range adaptability. Lithium iron phosphate (LiFePO4, LFP) has become the mainstream cathode material due to its abundant raw materials, high safety, and low cost. However, problems such as low ionic / electronic conductivity caused by the olivine structure and irreversible loss of active lithium during cycling severely limit its application in long-cycle energy storage and high-rate power scenarios.

[0003] Currently, existing patents such as CN111224084A and CN119340532A improve the ionic conductivity of LFPs by generating a lithium silicate (Li4SiO3 / Li4SiO4) ion conductor network inside or on the surface of the LFP, thus improving rate performance to some extent. However, this type of solution only enhances ion conduction and does not have the function of replenishing active lithium. It cannot compensate for the irreversible loss of active lithium caused by SEI film formation and cycling side reactions, resulting in limited improvement in cycle life. Moreover, the lithium silicate is only physically composited or generated in situ, and its bonding force with the LFP interface is weak. Interface peeling is prone to occur during cycling, and the electrode impedance continues to increase. Another patent, CN120015811A, uses bismuth silicate to coat lithium iron manganese phosphate, which only improves high-temperature cycling performance, without a lithium replenishment mechanism. Furthermore, the interface barrier between the coating layer and the substrate is not eliminated, and the improvement in high-rate performance is not significant. Summary of the Invention

[0004] To overcome the aforementioned technical problems, this invention provides a modified lithium silicate-sulfur composite lithium iron phosphate cathode material and its preparation method. This invention utilizes a Li4SiO4 / S lithium supplementer and achieves chemical bonding between the lithium supplementer and lithium iron phosphate using silane-based phosphate esters; simultaneously, it introduces polyvinylidene fluoride micropowder to fix the sulfur phase, solving the technical problems of traditional lithium silicate-modified lithium iron phosphate lacking lithium supplementation function and having weak interfacial bonding. The resulting cathode material achieves efficient and stable lithium supplementation.

[0005] The present invention solves the above-mentioned technical problems through the following technical solutions.

[0006] This invention discloses a method for preparing a modified lithium silicate-sulfur composite lithium iron phosphate cathode material, comprising the following steps: S1. Lithium carbonate, nano-silica and conductive agent are ball-milled and then sintered and activated to obtain conductive coated Li4SiO4; S2. Conductive coated Li4SiO4, elemental sulfur and polyvinylidene fluoride micro powder are melt-coated at 110~140℃ to obtain Li4SiO4 / S lithium supplement; S3. Lithium iron phosphate, Li4SiO4 / S lithium supplementer, silane phosphate ester, cyclic sulfate ester and solvent are mixed and formulated into a positive electrode slurry. After the positive electrode slurry is dried and solidified, a modified lithium silicate sulfur composite lithium iron phosphate positive electrode material is obtained. The silane phosphate ester is tris(trimethylsilyl) phosphate (CAS No.: 10497-05-9) or tris(triethylsilyl) phosphate (CAS No.: 14579-57-8); the silane phosphate ester acts as an interface modifier, with its silane groups covalently bonded to the -OH bonds on the LFP surface, and the phosphate groups bonded to the Li4SiO4 surface. + Coordination can improve interfacial compatibility and reduce interfacial impedance, which is beneficial for constructing continuous Li. + Transmission channel; The cyclic sulfate is 1,3-propanesulfonate lactone (PS) or 1,4-butanesulfonate lactone (BS); the cyclic sulfate, as an electrolyte compatibilizer, can be compounded with silane phosphate esters to improve the interfacial stability between the electrode and the electrolyte.

[0007] In S1, lithium carbonate and nano-silica are mixed at a Li:Si molar ratio of 4.0~4.2:1, preferably 4.05~4.15:1.

[0008] In S1, the conductive agent accounts for 0.5~3.0 wt% of the sum of the mass of lithium carbonate and nano-silica, preferably 1.0~2.5 wt%. In S1, the conductive agent is at least one of conductive carbon black or carbon nanotubes; preferably, the conductive agent is a mixture of conductive carbon black and carbon nanotubes in a mass ratio of 5 to 8:1.

[0009] In S1, the D50 of the nano-silica is 50~100nm.

[0010] In S1, the D50 of the lithium carbonate is 8~15μm.

[0011] In S1, the ball-to-material ratio of the ball mill is 10~15:1, and the rotation speed is 300~450 r / min; In S1, the ball milling time is 2-4 hours; In S1, the sintering activation is carried out by calcination at 700~800℃ for 4~7h in an inert atmosphere, preferably by calcination at 730~770℃ for 4.5~5.5h in an inert atmosphere.

[0012] In S2, conductive coated Li4SiO4 and elemental sulfur (S8) are mixed at a mass ratio of 8~12:1, preferably 9~11:1. Due to the extremely high Si-O bond energy in Li4SiO4, it exhibits almost no electrochemical activity within the operating window of the lithium iron phosphate battery and cannot release active lithium; it can only act as an ion conductor to enhance Li... + The transport efficiency is low, and it lacks lithium replenishment capability; however, elemental sulfur (S8) can undergo multiple reversible redox reactions within this operating window, with S reacting with Li. + The combination forms a series of Li2S / Li2Sx sulfides, which can gradually decompose and release active Li during battery charge-discharge cycles. + .

[0013] In S2, polyvinylidene fluoride (PVDF) micropowder accounts for 0.5~1.5 wt% of the sum of the mass of conductive coated Li4SiO4 and elemental sulfur, preferably 0.8~1.2 wt%. When the PVDF micropowder melts, it forms a bonding layer that fixes the elemental sulfur on the surface of Li4SiO4, limiting the agglomeration and loss of the sulfur phase.

[0014] In S2, the melt coating is carried out in an inert gas atmosphere at 120~140℃ for 1~3 hours, followed by cooling and then pulverizing the particles D50 to 1.0~2.0μm.

[0015] In S3, lithium iron phosphate and Li4SiO4 / S lithium supplement are mixed at a mass ratio of 90~98:2~10, preferably 93~97:3~7.

[0016] In S3, silane phosphate esters account for 0.3~1.0 wt% of the total mass of lithium iron phosphate and Li4SiO4 / S lithium supplementer, preferably 0.4~0.7 wt%.

[0017] In S3, the solvent is NMP ( N 2-methylpyrrolidone), NEP ( N (-Ethylpyrrolidone), DMF (dimethylformamide), or DMAc (dimethylacetamide).

[0018] In S3, the solid / liquid ratio after the solvent is added is 1 / 1 to 1.5.

[0019] In S3, the cyclic sulfate accounts for 0.1~0.5 wt% of the sum of the mass of lithium iron phosphate and Li4SiO4 / S lithium supplementer, preferably 0.1~0.4%. The cyclic sulfate can optimize the flexibility and compactness of the SEI film and reduce interfacial side reactions.

[0020] In S3, the curing is carried out at a low temperature of 180~220℃ in an inert atmosphere for 2~3 hours; the low temperature curing allows the interface modifier to react fully, while avoiding the high temperature decomposition of the lithium-supplemented phase and ensuring the structural integrity of the lithium-supplemented phase.

[0021] In S3, the drying process involves vacuum drying at 80-100°C for 6-8 hours. In S3, the solidified material is further pulverized through a 300-500 mesh sieve.

[0022] The present invention also discloses a modified lithium silicate sulfur composite lithium iron phosphate cathode material, which is prepared by the aforementioned preparation method.

[0023] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0024] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention reduces battery polarization and maintains the stability of electrochemical performance during cycling by improving the lithium replenishment system. The improvement principle is as follows: 1) Using silane-based phosphate esters as interface modifiers, the silane groups covalently bond with the -OH bonds on the LFP surface, and the phosphate ester groups bond with the Li4SiO4 surface Li + Coordination allows the lithium-added phase to chemically coat the LFP surface, eliminating the interfacial barrier between the two phases and forming a continuous Li. + 1) Transmission channel. 2) When polyvinylidene fluoride (PVDF) micropowder is melt-coated with the sulfur phase, it enhances the interfacial bonding between the lithium-supplementing phase and the LFP matrix, avoiding interfacial delamination during cycling. 3) Cyclic sulfate esters and silane phosphate esters work synergistically to construct a dense and stable SEI film at the electrode and electrolyte interface, suppressing side reactions between the electrolyte and the electrode surface, and further improving the stability of the interfacial structure.

[0025] 2. The cathode material of this invention possesses efficient, stable, and controllable lithium replenishment and wide temperature range performance. The polyvinylidene fluoride micropowder coating effectively avoids sulfur agglomeration and loss, enabling the Li4SiO4 / S lithium replenishment phase to uniformly release active lithium during battery cycling, compensating for the irreversible loss of active lithium caused by SEI film formation and cycling side reactions, thus achieving efficient, stable, and controllable lithium replenishment. The addition of cyclic sulfate esters optimizes the film formation performance at the electrode and electrolyte interface, improves the high and low temperature stability of the SEI film, and suppresses side reactions such as SEI film thickening at low temperatures and electrolyte decomposition at high temperatures.

[0026] 3. The cathode material prepared by this invention possesses efficient and stable lithium replenishment and excellent wide-temperature adaptability. Test data shows that the battery using the scheme of this invention has an initial coulombic efficiency of ≥96%, significantly compensating for active lithium loss; low interface impedance of ≤90Ω ensures efficient charge transport; and especially at a low temperature of -20℃, the capacity retention rate can still reach ≥75%. Therefore, this material broadens the application boundaries of lithium iron phosphate batteries in new energy vehicles, large-scale energy storage, and special power supplies. Attached Figure Description

[0027] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to the accompanying drawings.

[0028] Figure 1 This is a SEM image of the modified lithium silicate-sulfur composite lithium iron phosphate cathode material from Example 1.

[0029] Figure 2 The image shows the SEM image of the modified lithium silicate-sulfur composite lithium iron phosphate cathode material of Comparative Example 1.

[0030] Figure 3 The image shows the SEM image of the modified lithium silicate-sulfur composite lithium iron phosphate cathode material of Comparative Example 3. Detailed Implementation

[0031] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0032] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0033] Unless otherwise specified, the terms "comprising" and "including" as used in this invention can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0034] Unless otherwise specified, the term "or" is inclusive in this invention. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0035] The raw material information used in the following examples is as follows: The D50 of nano-silica is 80 nm, and the D50 of lithium carbonate is 12 μm; The carbon nanotubes have an aspect ratio of 300:1 and a diameter of 15 nm. The lithium iron phosphate is carbon-coated, with a D50 of 2.8 μm and a carbon content of 2.0 wt%.

[0036] [Cathode Materials] Example 1 The preparation method of the modified lithium silicate-sulfur composite lithium iron phosphate cathode material in this embodiment is as follows: S1. Preparation of conductive coated Li4SiO4 Lithium carbonate and nano-silica were weighed according to a Li:Si molar ratio of 4.1:1, mixed evenly, and then a conductive agent (conductive carbon black and carbon nanotubes were compounded at a mass ratio of 5:1) accounting for 2.0 wt% of the total mass of the two was added. The mixture was placed in a planetary ball mill, and zirconium oxide beads were added as the ball milling medium. The ball-to-material ratio was 10:1. The ball milling speed was set to 380 r / min, and the mixture was ball milled at room temperature for 3.0 h to obtain a uniformly mixed powder. The mixed powder was transferred to an alumina crucible and placed in a muffle furnace. Under a nitrogen atmosphere, the temperature was increased to 750°C at a heating rate of 5°C / min and calcined at a constant temperature for 5.0 h. After calcination, the powder was cooled to room temperature with the furnace and ground through a 200-mesh sieve to obtain conductive coated Li4SiO4 powder.

[0037] Preparation of S2.Li4SiO4 / S lithium supplement Weigh conductive coated Li4SiO4 powder / elemental sulfur at a mass ratio of 10 / 1, mix them evenly, and then add 1.0 wt% of polyvinylidene fluoride micro powder. Place the mixture in a high-temperature stirred reactor. Purge the reactor with argon gas to replace the air and maintain an argon atmosphere. Heat the mixture to 130°C, set the stirring speed to 80 r / min, and maintain the temperature for 1.5 h for melting and coating. After melting and coating is completed, maintain the argon atmosphere and allow it to cool naturally to room temperature. Remove the material and place it in an air jet mill to grind it to a powder D50 of 1.5 μm to obtain Li4SiO4 / S lithium supplement. Store the mixture in a sealed container.

[0038] S3. Preparation of modified lithium silicate-sulfur composite lithium iron phosphate cathode material Weigh lithium iron phosphate and Li4SiO4 / S lithium supplementer obtained from S2 at a mass ratio of 95:5, place them in a high-speed shear mixer, and mix them evenly; add 0.5wt% of tris(trimethylsilyl) phosphate and 0.38wt% of cyclic sulfate (1,3-propanesulfonate lactone) according to the total mass of the two, and then add NMP solvent, controlling the solid / liquid ratio to 1:1.2; start high-speed shear mixing, set the speed to 1000 r / min, and stir at room temperature for 1.5 h to obtain a uniform positive electrode slurry without particles or agglomeration; The cathode slurry was transferred to a vacuum drying oven, set at 90℃ and a vacuum of -0.095MPa, and vacuum dried for 7.0h to remove the solvent and obtain the dry material. The dry material was then transferred to a tube furnace, where nitrogen was introduced to replace the air and maintain a nitrogen atmosphere. The temperature was raised to 205℃ and cured at a low temperature for 2.5h. After curing, the material was cooled to room temperature in the furnace, removed, pulverized, and passed through a 325-mesh standard sieve to obtain the final product: modified lithium silicate-sulfur composite lithium iron phosphate cathode material. The SEM image of this cathode material is shown below. Figure 1 .

[0039] Example 2 The difference between this embodiment and Embodiment 1 is as follows: The conductive agent added to S1 accounts for 4.5 wt% of the total mass of lithium carbonate and nano-silica; The other raw materials, steps and parameters are the same as in Example 1.

[0040] Example 3 The difference between this embodiment and Embodiment 1 is as follows: The conductive agent in S1 contains only conductive carbon black and no carbon nanotubes; The other raw materials, steps and parameters are the same as in Example 1.

[0041] Example 4 The difference between this embodiment and Embodiment 1 is as follows: S3 contains tris(triethylsilyl) phosphate at a total mass of 0.65 wt% of lithium carbonate and nano-silica. The other raw materials, steps and parameters are the same as in Example 1.

[0042] Example 5 The difference between this embodiment and Embodiment 1 is as follows: S3 does not contain cyclic sulfates; The other raw materials, steps and parameters are the same as in Example 1.

[0043] Comparative Example 1 The difference between this comparative example and Example 1 is as follows: S2 does not contain polyvinylidene fluoride powder; Other raw materials, steps, and parameters are the same as in Example 1; the SEM image of the modified lithium silicate-sulfur composite lithium iron phosphate cathode material of this comparative example can be found in [reference needed]. Figure 2 The surface of the material particles showed obvious sulfur agglomeration, forming blocky sulfur aggregates of uneven size.

[0044] Comparative Example 2 The difference between this comparative example and Example 1 is as follows: S2 only melt-coats conductive coated Li4SiO4 powder and polyvinylidene fluoride micro powder without adding elemental sulfur. Therefore, S2 produces Li4SiO4 lithium supplementer. The positive electrode slurry of S3 is also prepared using this Li4SiO4 lithium supplementer as raw material to obtain modified lithium silicate sulfur composite lithium iron phosphate positive electrode material. The other raw materials, steps and parameters are the same as in Example 1.

[0045] Comparative Example 3 The difference between this comparative example and Example 1 is as follows: S3 does not contain silane phosphate esters; Other raw materials, steps, and parameters are the same as in Example 1. The SEM image of the modified lithium silicate-sulfur composite lithium iron phosphate cathode material of this comparative example can be found in [reference needed]. Figure 3 The lithium iron phosphate particles and the Li4SiO4 / S lithium supplement exhibit a physically mixed two-phase separation state. The lithium supplement attaches to the lithium iron phosphate surface in the form of large particle clusters, without a clear interface bonding layer.

[0046]

Battery

[0047] The button half-cells of each embodiment and comparative example were subjected to the following tests, and the test results are shown in Tables 1 to 3.

[0048] Test Example 1 Using a Newway battery tester, the battery was charged at a constant current of 0.1C to 3.8V, then charged at a constant voltage until the current was less than 0.05C, and then discharged at a constant current of 0.1C to 2.0V. The initial charge capacity and discharge capacity were recorded. The initial coulombic efficiency was calculated using the following formula: Initial coulombic efficiency (%) = Initial discharge capacity / Initial charge capacity × 100%.

[0049]

[0050] Test Example 2 The AC impedance of a fully charged battery was tested at 25°C using a Chenhua CHI660E electrochemical workstation, with a frequency range of 0.01~100kHz and a disturbance amplitude of 5mV.

[0051]

[0052] Test Example 3 The button cell was placed in a -20℃ low-temperature chamber for 3 hours and discharged at a rate of 0.1C. The retention rate of the discharge capacity was recorded. The low-temperature capacity retention rate (%) is calculated as -20℃ discharge capacity / 25℃ discharge capacity × 100%.

[0053]

[0054] Unless otherwise specified, all raw materials, reagents, instruments, and equipment used in this invention can be purchased commercially or prepared using existing methods. The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of this invention. It should be understood that the above descriptions are merely specific embodiments of this invention and are not intended to limit the invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A method for preparing a modified lithium silicate-sulfur composite lithium iron phosphate cathode material, characterized in that, Includes the following steps: S1. Lithium carbonate, nano-silica and conductive agent are ball-milled and then sintered and activated to obtain conductive coated Li4SiO4; S2. Conductive coated Li4SiO4, elemental sulfur and polyvinylidene fluoride micro powder are melt-coated at 110~140℃ to obtain Li4SiO4 / S lithium supplement; S3. Lithium iron phosphate, Li4SiO4 / S lithium supplementer, silane phosphate ester, cyclic sulfate ester and solvent are mixed and formulated into a positive electrode slurry. After the positive electrode slurry is dried and solidified, a modified lithium silicate sulfur composite lithium iron phosphate positive electrode material is obtained. The silane phosphate ester is tris(trimethylsilyl) phosphate or tris(triethylsilyl) phosphate; The cyclic sulfate ester is 1,3-propanesulfonate lactone or 1,4-butanesulfonate lactone.

2. The preparation method of the modified lithium silicate-sulfur composite lithium iron phosphate cathode material as described in claim 1, characterized in that, Lithium carbonate and nano-silica were mixed at a Li:Si molar ratio of 4.0 to 4.2:

1. And / or, the conductive agent accounts for 0.5 to 3.0 wt% of the sum of the mass of lithium carbonate and nano silica.

3. The preparation method of the modified lithium silicate-sulfur composite lithium iron phosphate cathode material as described in claim 2, characterized in that, The conductive agent is at least one of conductive carbon black or carbon nanotubes. And / or, the D50 of the nano-silica is 50~100nm; And / or, the D50 of the lithium carbonate is 8~15μm.

4. The preparation method of the modified lithium silicate-sulfur composite lithium iron phosphate cathode material as described in claim 1, characterized in that, The sintering activation is carried out by calcination at 700~800℃ for 4~7 hours under an inert atmosphere; And / or, the ball milling time is 2-4 hours.

5. The preparation method of the modified lithium silicate-sulfur composite lithium iron phosphate cathode material as described in claim 1, characterized in that, Conductive coated Li4SiO4 and elemental sulfur are mixed at a mass ratio of 8~12:1; And / or, the polyvinylidene fluoride micropowder accounts for 0.5 to 1.5 wt% of the sum of the mass of conductive coated Li4SiO4 and elemental sulfur.

6. The preparation method of the modified lithium silicate-sulfur composite lithium iron phosphate cathode material as described in claim 5, characterized in that, The melt coating is carried out in an inert gas atmosphere at 120~140℃ for 1~3 hours, followed by cooling and then pulverizing the particles to a D50 of 1.0~2.0μm.

7. The preparation method of the modified lithium silicate-sulfur composite lithium iron phosphate cathode material as described in claim 1, characterized in that, Lithium iron phosphate and Li4SiO4 / S lithium supplement are mixed at a mass ratio of 90~98:2~10; And / or, silane phosphate esters account for 0.3~1.0 wt% of the sum of the mass of lithium iron phosphate and Li4SiO4 / S lithium supplementer; And / or, the solvent is NMP, NEP, DMF or DMAc.

8. The preparation method of the modified lithium silicate-sulfur composite lithium iron phosphate cathode material as described in claim 7, characterized in that, The cyclic sulfate ester accounts for 0.1 to 0.5 wt% of the sum of the mass of lithium iron phosphate and Li4SiO4 / S lithium supplementer.

9. The preparation method of the modified lithium silicate-sulfur composite lithium iron phosphate cathode material as described in claim 1, characterized in that, The curing process involves curing at 180~220℃ in an inert atmosphere for 2~3 hours. And / or, the drying is performed under vacuum at 80~100℃ for 6~8 hours; The solidified material is then pulverized through a 300-500 mesh sieve.

10. A modified lithium silicate-sulfur composite lithium iron phosphate cathode material, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 9.

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