Lithium iron phosphate battery

Abstract

The present invention provides a lithium iron phosphate battery, comprising a positive electrode, a negative electrode, and an electrolyte. The positive electrode comprises a positive electrode active material, and the electrolyte comprises a lithium salt, a non-aqueous organic solvent, and an additive. The positive electrode active material is a lithium iron phosphate material, and the additive comprises a compound A as shown in structural formula I, wherein R1, R2, and R3 are each independently selected from hydrogen, a halogen, a substituted or unsubstituted C1-C6 alkyl group, a substituted or unsubstituted C2-C6 alkenyl group, a substituted or unsubstituted silyl group, a C1-C6 cyano group, or a C1-C6 ester group. The present invention uses the compound as shown in structural formula I to form a dual interface film of an inorganic SEI film and an organic SEI film at the positive electrode-electrolyte interface, effectively preventing the dissolution of Fe ions in the lithium iron phosphate material, thereby improving the cycling and storage performance of the lithium iron phosphate battery.

Description

Technical Field

[0001] The present invention relates to the field of energy storage devices, and in particular to a lithium ion secondary battery, and in particular to a lithium iron phosphate battery. Background Art

[0002] At present, lithium-ion batteries have the advantages of high energy density, long life, and environmental friendliness. They have been widely used in electronic products such as mobile phones, laptops, digital cameras, etc., and are also gradually being widely used as power batteries in electric bicycles, model aircraft, and electric vehicles.

[0003] Among them, the positive electrode material, as one of the key materials of lithium-ion batteries, plays a decisive role in the capacity of the entire lithium-ion battery. It is also becoming the focus of research and development, industrialization and large-scale application in the field of new energy materials in various countries. In the current research on many positive electrode materials, lithium cobalt oxide and lithium nickel oxide have safety issues due to their unstable structure, and environmental problems are not suitable for large-scale application. Lithium manganese oxide materials have the problem of severe capacity decay in high-temperature cycles, which restricts their further application in the field of power batteries. Although the comprehensive performance of NCM ternary materials is relatively outstanding, there are still problems with toxic elements and strategic resource element Co. This material can only be used as a transitional product. As for lithium iron phosphate materials, due to their excellent cycle performance, high safety performance, environmental friendliness and moderate cost, they have become the positive electrode material with the most potential for large-scale application in the field of energy storage and power batteries in the future.

[0004] However, during the cycle of lithium iron phosphate batteries using lithium iron phosphate as the positive electrode material, Fe atoms in the lithium iron phosphate material will dissolve and deposit on the surface of the negative electrode, resulting in the formation of metallic lithium dendrites, which will eventually deteriorate the cycle performance of the lithium iron phosphate battery. Summary of the Invention

[0005] The object of the present invention is to provide a lithium iron phosphate battery, in which a specific additive is used to effectively prevent the dissolution of Fe atoms in the lithium iron phosphate material, thereby improving the electrochemical performance of the lithium iron phosphate battery.

[0006] To achieve the above object, the present invention provides a lithium iron phosphate battery, comprising a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode comprises a positive electrode active material, the electrolyte comprises a lithium salt, a non-aqueous organic solvent and an additive, the positive electrode active material is a lithium iron phosphate material, and the additive comprises a compound as shown in structural formula I.

[0007]

[0008] Wherein, R1, R2 and R3 are each independently selected from hydrogen, halogen, substituted or unsubstituted C1-C6 alkyl, substituted or unsubstituted C2-C6 alkenyl, substituted or unsubstituted silyl, C1-C6 cyano, C1-C6 ester.

[0009] The present invention uses compound A as shown in structural formula I. During the first charging process, the lone pairs of electrons on the nitrogen on both sides of the five-membered ring of compound A will bind to lithium to form a relatively dense interfacial film. This interfacial film is an inorganic SEI film. This inorganic SEI film will be deposited on one side of the electrode at the electrode / electrolyte interface. At the same time, the conjugated double bonds within the five-membered ring in its structure will polymerize to form a polymer with an NNN-double bond structure at the positive electrode-electrolyte interface. This is an organic SEI film. The formation of this film can optimize the positive electrode / electrolyte interface. Thus, during the first charging process of the battery, a dual interfacial film of an inorganic SEI film and an organic SEI film is formed, which can effectively prevent the dissolution of Fe ions in the lithium iron phosphate material, thereby improving the cycling and storage performance of the lithium iron phosphate battery.

[0010] Preferably, the chemical formula of lithium iron phosphate materials is Li m Fe α M β PO4 or Li (1-x) A x Fe (1-y) B y PO4 / C, wherein 0.85≤m≤1.0, 0.8≤α≤1.0, 0.01≤β≤0.2, 0≤x≤0.15, 0≤y≤0.15, M is at least one of Ti, Mg, V, Mn, W, Al, Nb, Mo, Zr, Cr, Ni and Co, A and B are different and are at least one of Ag, Mg, Zn, Cu, Al, In, Ti, Nb, Mo, V, Zr, Mn, Cr, Ni and Co, and C accounts for the mass percentage of Li (1-x) A x Fe (1-y) B y The content of the PO4 / C compound is preferably 1-20%, and R2 and R3 are each independently selected from hydrogen, a substituted or unsubstituted C1-C6 alkyl group.

[0011] Preferably, compound A is selected from at least one of compounds 1 to 5,

[0012]

[0013] Preferably, the mass percentage of compound A in the electrolyte is 0.01-0.5%.

[0014] Preferably, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, lithium bisoxalatoborate, lithium difluorooxalatoborate, lithium difluorophosphate, lithium bistrifluoromethanesulfonyl imide, lithium difluorobisoxalatophosphate and lithium bisfluorosulfonyl imide.

[0015] Preferably, the non-aqueous organic solvent is at least one selected from the group consisting of chain carbonates, cyclic carbonates and carboxylates.

[0016] Preferably, the non-aqueous organic solvent is at least one of ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propylene carbonate, butyl acetate, γ-butyrolactone, propyl propionate, ethyl propionate and ethyl butyrate.

[0017] Preferably, the electrolyte further comprises an auxiliary agent, and the auxiliary agent is selected from at least one of fluoroethylene carbonate, vinylene carbonate, 1,3-propane sultone, vinyl sulfate, tris(trimethylsilyl) phosphate and fluoroether.

[0018] Preferably, the negative electrode includes a negative electrode active material, and the negative electrode active material is selected from at least one of artificial graphite, natural graphite, lithium titanate, silicon-carbon composite material and silicon oxide. DETAILED DESCRIPTION

[0019] The lithium iron phosphate battery of the present invention comprises a positive electrode, a negative electrode and an electrolyte.

[0020] The positive electrode includes a positive electrode active material, which is a lithium iron phosphate material with a general chemical formula of Li m Fe α M β PO4 or Li (1-x) A x Fe (1-y) B y PO4 / C, wherein 0.85≤m≤1.0, 0.8≤α≤1.0, 0.01≤β≤0.2, 0≤x≤0.15, 0≤y≤0.15, M is at least one of Ti, Mg, V, Mn, W, Al, Nb, Mo, Zr, Cr, Ni and Co, A and B are different and are at least one of Ag, Mg, Zn, Cu, Al, In, Ti, Nb, Mo, V, Zr, Mn, Cr, Ni and Co, and C accounts for the mass percentage of Li (1-x) A x Fe (1-y) B y The content of PO4 / C compound is 1-20%. (1-α) M α PO4 represents pure lithium iron phosphate or metal ion doped lithium iron phosphate. (1-x) A x Fe (1-y) B y PO4 / C represents pure lithium iron phosphate coated with a carbon layer or lithium iron phosphate coated with a carbon layer and doped with metal ions, wherein the carbon layer is obtained by carbonization of a carbon source, which may be glucose, cellulose, sucrose, etc.

[0021] The negative electrode includes a negative electrode active material, and the negative electrode active material is selected from at least one of artificial graphite, natural graphite, lithium titanate, silicon-carbon composite material and silicon oxide.

[0022] The separator is located between the positive electrode and the negative electrode and can be a polyethylene separator, a polypropylene separator, a polyethylene separator with a ceramic coating on the surface, or a polypropylene separator with a ceramic coating on the surface.

[0023] The electrolyte includes a lithium salt, a non-aqueous organic solvent and an additive, wherein the additive includes a compound A as shown in structural formula I.

[0024] The lithium salt concentration is 0.5 to 1.5 M and is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate (LiSO3CF3), lithium bis(oxalatoborate) (LiDFOB), lithium difluorooxalatoborate (LiODFB), lithium difluorophosphate (LiPO2F2), lithium bis(trifluoromethylsulfonyl imide) (LiTFSI), lithium difluorobis(oxalatophosphate) (LiDFOP) and lithium bis(fluorosulfonyl imide) (LiFSI).

[0025] The non-aqueous organic solvent is selected from at least one of linear carbonates, cyclic carbonates, and carboxylates. Further, the non-aqueous organic solvent is at least one of ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), propylene carbonate (PC), butyl acetate (n-Ba), γ-butyrolactone (γ-Bt), propyl propionate (n-Pp), ethyl propionate (EP), and ethyl butyrate (Eb).

[0026] The mass percentage of compound A in the electrolyte is 0.01-0.5%, preferably 0.05-0.2%. In structural formula I of compound A, R1, R2, and R3 are each independently selected from hydrogen, halogen, substituted or unsubstituted C1-C6 alkyl, substituted or unsubstituted C2-C6 alkenyl, substituted or unsubstituted silyl, C1-C6 cyano, or C1-C6 ester. Preferably, R2 and R3 are each independently selected from hydrogen, substituted or unsubstituted C1-C6 alkyl. More specifically, compound A is selected from at least one of compounds 1 to 5.

[0027]

[0028] The electrolyte may further include an auxiliary agent accounting for 0.1 to 5% by mass of the electrolyte, wherein the auxiliary agent is selected from at least one of fluoroethylene carbonate (FEC), vinylene carbonate (VC), 1,3-propane sultone (PS), vinyl sulfate (DTD), tris(trimethylsilyl) phosphate (TMSP) and fluoroether (D2).

[0029] The following specific examples further illustrate the purpose, technical solutions and beneficial effects of the present invention, but do not constitute any limitation of the present invention. Where specific conditions are not specified in the examples, the experiments can be carried out according to conventional conditions or conditions recommended by the manufacturer. Where the manufacturer of the reagents or instruments is not specified, they are all conventional products available on the market.

[0030] Example 1

[0031] (1) Preparation of lithium-ion battery electrolyte: In a nitrogen-filled glove box (O2 <2ppm, H2O <3ppm), a mixture of dimethyl carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) was mixed as an organic solvent in a mass ratio of 1:1:1 to prepare 87.49g of a non-aqueous organic solvent, and 0.01g of compound 1 was added. The solution was sealed and packaged and placed in a quick freezer (-4°C) for 2h before being taken out. In a nitrogen-filled glove box (O2 <2ppm, H2O <3ppm), 12.5g of lithium hexafluorophosphate was slowly added to the mixed solution. After mixing evenly, a lithium-ion battery electrolyte was prepared.

[0032] (2) Preparation of positive electrode: LiFePO4, a lithium iron phosphate material, PVDF, and a conductive agent SuperP are mixed evenly in a mass ratio of 97:2:1 to prepare a lithium-ion battery positive electrode slurry with a certain viscosity. The mixed slurry is coated on both sides of an aluminum foil, dried, and rolled to obtain a positive electrode sheet.

[0033] (3) Preparation of negative electrode: Silicon-carbon negative electrode material (10 wt.% Si) is mixed with conductive agent SuperP, thickener CMC, and adhesive SBR (styrene-butadiene rubber latex) in a mass ratio of 95:1:2:2 to prepare a slurry, mix them evenly, apply the mixed slurry on both sides of the copper foil, dry and roll-press to obtain the negative electrode sheet.

[0034] (4) Preparation of lithium-ion batteries: The positive electrode, separator and negative electrode are stacked to form a square battery cell, which is then packaged with a polymer and filled with the lithium-ion battery electrolyte prepared above. After the formation and capacity separation processes, the lithium-ion battery is made.

[0035] The electrolyte formulations of Examples 2 to 13 and Comparative Examples 1 to 5 are shown in Table 1. The steps for preparing the electrolytes are the same as those of Example 1.

[0036] Table 1 Electrolyte components of various examples

[0037]

[0038]

[0039]

[0040] The lithium-ion batteries prepared in Examples 1 to 13 and Comparative Examples 1 to 5 were subjected to room temperature cycle performance, high temperature cycle performance, and high temperature storage tests, respectively. The specific test conditions are as follows. The performance test results are shown in Table 2.

[0041] Normal temperature cycle test: At room temperature (25°C), the lithium-ion battery is charged and discharged at 1.0C / 1.0C once (the battery discharge capacity is C0), with an upper limit voltage of 3.65V, and then charged and discharged at 1.0C / 1.0C for 5000 cycles at room temperature (the battery discharge capacity is C1).

[0042] Capacity retention rate = (C1 / C0)*100%.

[0043] High temperature cycle test: Under high temperature (45°C) conditions, the lithium-ion battery is charged and discharged at 1.0C / 1.0C once (the battery discharge capacity is C0) with an upper limit voltage of 3.65V, and then charged and discharged at 1.0C / 1.0C for 2000 cycles (the battery discharge capacity is C1) under high temperature (45°C) conditions.

[0044] Capacity retention rate = (C1 / C0)*100%

[0045] High-temperature storage performance test: At room temperature (25°C), the lithium-ion battery is charged and discharged at 1.0C / 1.0C (the battery discharge capacity is C0, and the battery thickness is D0), with an upper limit voltage of 3.65V. It is then placed at high temperature (60°C) for 30 days. After cooling the battery to 25°C, it is discharged (the discharge capacity is C1, and the battery thickness is D1). The battery is then charged and discharged at 1.0C / 1.0C again (the discharge capacity is C2).

[0046] Capacity retention rate = (C1 / C0)*100%

[0047] Capacity recovery rate = (C2 / C0)*100%

[0048] Thickness expansion rate = D1 / D0*100%

[0049] Table 2 Lithium-ion battery performance test results

[0050]

[0051]

[0052] From the results in Table 2, it can be seen that the use of the compound of the present invention as an additive can greatly improve the cycle performance and high-temperature storage performance of the battery, indicating that the additive can effectively prevent the dissolution of Fe ions in the lithium iron phosphate material.

[0053] Comparing Example 4 with Examples 6-9 shows that Compound 3 exhibits superior high-temperature cycling and storage performance. This is likely due to the Si-N bond binding to water molecules in the battery, inhibiting the decomposition of lithium salts; the trifluoromethyl group improving the SEI interface components, further enhancing the SEI's lithium ion transport performance; and the structure of the present invention forming a thermally stable double interface, inhibiting Fe dissolution. Therefore, Compound 3 exhibits superior cycling and storage performance.

[0054] Comparative Example 8, Examples 11 to 13, and Comparative Examples 2 to 4 show that when Compound A of the present invention is used with an auxiliary agent, the battery's cycle and storage performance are better, especially when it is used with VC and PS. This is because VC and PS form a sulfur-containing polymer interface at the electrode / electrolyte interface. The interface has good structural stability, but under high temperature conditions, it will still decompose to a certain extent, causing the electrode / electrolyte interface to rupture and even cause Fe dissolution. The double interface formed by the structure of the present invention further improves the thermal stability and structural stability of the electrode / electrolyte interface, so that the lithium-ion battery still has sufficient SEI interface under high temperature conditions, protects the electrode / electrolyte interface, inhibits the dissolution of Fe, and allows the lithium-ion battery to maintain good cycle and storage performance.

[0055] By comparing Example 4, Examples 6 to 9, and Comparative Example 5, it can be seen that although the compound used in Comparative Example 5 also contains an intracyclic double bond and a ring-like nitrogen atom, its improvement in battery cycle and storage performance is significantly lower than that of Example 4 and Examples 6 to 9. This is because the compound is a six-membered ring, the conjugated double bond within the six-membered ring is very stable, the polymerization reaction is more difficult, and only one nitrogen atom on the ring has a lone electron pair, so the interface film formed by it is relatively loose, so it cannot form a dual interface film similar to the inorganic SEI film and organic SEI film of the present invention, and therefore it is difficult to effectively prevent the dissolution of Fe ions in the lithium iron phosphate material.

[0056] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A lithium iron phosphate battery comprising a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode comprises a positive electrode active material, and the electrolyte comprises a lithium salt, a non-aqueous organic solvent and an additive, characterized in that: The positive electrode active material is a lithium iron phosphate material, and the additive includes compound A, which is selected from compound three. Compound 3.

2. The lithium iron phosphate battery according to claim 1, wherein The general chemical formula of the lithium iron phosphate material is Li m Fe α M β PO4 or Li (1-x) A x Fe (1-y) B y PO4 / C, wherein 0.85≤m≤1.0, 0.8≤α≤1.0, 0.01≤β≤0.2, 0≤x≤0.15, 0≤y≤0.15, M is at least one of Ti, Mg, V, Mn, W, Al, Nb, Mo, Zr, Cr, Ni and Co, A and B are different and are at least one of Ag, Mg, Zn, Cu, Al, In, Ti, Nb, Mo, V, Zr, Mn, Cr, Ni and Co, and C accounts for the mass percentage of Li (1-x) A x Fe (1-y) B y The content of PO4 / C compounds is 1~20%.

3. The lithium iron phosphate battery according to claim 1, wherein The mass percentage of the compound A in the electrolyte is 0.01-0.5%.

4. The lithium iron phosphate battery according to claim 1, wherein: The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, lithium bisoxalatoborate, lithium difluorooxalatoborate, lithium difluorophosphate, lithium bistrifluoromethanesulfonyl imide, lithium difluorobisoxalatophosphate and lithium bisfluorosulfonyl imide.

5. The lithium iron phosphate battery according to claim 1, wherein: The non-aqueous organic solvent is selected from at least one of chain carbonates, cyclic carbonates and carboxylates.

6. The lithium iron phosphate battery according to claim 5, characterized in that The non-aqueous organic solvent is at least one of ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propylene carbonate, butyl acetate, γ-butyrolactone, propyl propionate, ethyl propionate and ethyl butyrate.

7. The lithium iron phosphate battery according to claim 1, wherein: The electrolyte further includes an auxiliary agent, which is selected from at least one of fluoroethylene carbonate, vinylene carbonate, 1,3-propane sultone, vinyl sulfate, tris(trimethylsilyl) phosphate and fluoroether.

8. The lithium iron phosphate battery according to claim 1, wherein: The negative electrode includes a negative electrode active material, and the negative electrode active material is selected from at least one of artificial graphite, natural graphite, lithium titanate, a silicon-carbon composite material, and silicon oxide.

Images