Lithium-ion battery with improved electrolyte wetting

Abstract

The present invention discloses a lithium-ion battery with improved electrolyte infiltration, comprising a positive electrode, a negative electrode, a separator and an electrolyte injected into the battery, wherein the negative electrode active material is graphite; a wetting-promoting layer is coated on the side of the negative electrode surface facing the separator, which increases the contact between the electrolyte and the negative electrode material along the thickness direction of the negative electrode and introduces the electrolyte; the wetting-promoting layer contains a polymer; the polymer is chain-shaped and conductive; as the electrolyte contacts the polymer, the polymer swells along the wetting-promoting layer to form a polymer network to guide the electrolyte to be evenly distributed in the wetting-promoting layer; the present invention utilizes a contact structure between the electrolyte and the negative electrode structure constructed on the negative electrode surface to reduce the internal resistance of the battery at low temperatures and improve the cycle performance of the battery at low temperatures.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a lithium ion battery and a method for improving electrolyte infiltration. Background Art

[0002] In lithium-ion batteries, the electrolyte plays the role of conducting ions between the positive and negative electrodes and serving as a charging and discharging medium. Therefore, the degree of electrolyte penetration in the battery directly affects the battery's performance indicators such as energy density and life.

[0003] The main reason for the poor cycle performance of batteries at low temperatures is that the fluidity of the solvent is poor at low temperatures, the resistance to lithium ion transfer is large, and the internal impedance of the battery is large, resulting in serious capacity loss.

[0004] The prior art proposes a series of strategies to improve the electrolyte wettability problem. For example, CN109768223A discloses a method for preparing a wound lithium-ion battery negative electrode sheet. The main inventive concept of this invention is to use a laser etching method to open grooves in the width direction of the negative electrode sheet on the surface of the active material layer. This method is easy to form and provides more space and channels for electrolyte infiltration. However, the grooves on the battery negative electrode sheet are independent and not interconnected, which limits the improvement of the contact between the electrolyte and the electrode. Summary of the Invention

[0005] The purpose of the present invention is to provide a lithium-ion battery with improved electrolyte infiltration. The present invention utilizes a contact structure between the electrolyte and the negative electrode structure constructed on the negative electrode surface to reduce the internal resistance of the battery at low temperatures and improve the cycle performance of the battery at low temperatures.

[0006] To solve this technical problem, the present invention provides a technical solution: a lithium-ion battery with improved electrolyte infiltration, comprising a positive electrode, a negative electrode, a separator, and an electrolyte injected into the battery, wherein the negative electrode active material is graphite; a wetting-promoting layer is coated on the negative electrode surface facing the separator, which increases contact between the electrolyte and the negative electrode material and introduces the electrolyte along the thickness direction of the negative electrode;

[0007] The wetting-promoting layer includes the following substances in terms of mass fraction:

[0008]

[0009] As the electrolyte contacts the polymer, the polymer swells along the wetting-promoting layer to form a polymer network that guides the electrolyte to be evenly distributed in the wetting-promoting layer.

[0010] The thickness of the wetting-promoting layer is preferably 10 μm to 20 μm. In the present invention, the wetting-promoting layer is applied to the side of the negative electrode facing the diaphragm and has a thickness of 10 μm to 20 μm; if the thickness of the wetting-promoting layer is too thin, the contact area between the electrolyte and the graphite material is insufficient, the internal connectivity in the wetting layer is insufficient, and the wetting layer is not uniform enough; if the thickness of the wetting-promoting layer is too large, the proportion of graphite in the wetting layer is small, which affects the overall capacity of the negative electrode and does not meet the design requirements of lithium-ion batteries. In the present invention, the wetting-promoting layer and the negative electrode graphite layer are both solid layers, and there is no interface resistance between the two layers. The interface resistance between the electrolyte and the negative electrode and the wetting-promoting layer comes from the electrolyte and the wetting-promoting layer. The present invention utilizes a polymer conductive network uniformly distributed in the wetting-promoting layer to form a conductive conduit for the electrolyte, which promotes the formation of three-dimensional contact between the electrolyte organic solvent and the graphite material along the thickness direction of the negative electrode.

[0011] Preferably, the electrolyte comprises an organic solvent, a lithium salt and an additive for binding lithium ions;

[0012] The organic solvent includes ethylene carbonate (EC), methyl formate (MF) and dimethyl carbonate (DMC), and the volume ratio of the three is 3:2:1 or 5:3:2.

[0013] In the electrolyte of the present invention, MF is a low-temperature solvent with a melting point of -99°C. It is a linear carbonate with an extremely low melting point. Its viscosity changes little at low temperatures and the internal resistance does not increase significantly. EC is a film-forming solvent that forms the SEI film on the surface of the negative electrode, protecting the negative electrode from co-embedding of the electrolyte and reducing the solvation of the negative electrode material.

[0014] The concentration of lithium salt in the electrolyte is preferably 1.5 mol / L;

[0015] Lithium salts include lithium hexafluorophosphate and lithium hexafluoroarsenate.

[0016] Preferably, the amount of lithium hexafluorophosphate is 5 to 7 times that of lithium hexafluoroarsenate.

[0017] Preferably, the electrolyte additive is an ether solvent, wherein the mass fraction of the ether solvent in the electrolyte is 2% to 5%. In the present invention, lithium hexafluoroarsenate and the ether organic solvent are used together to effectively improve the electrolyte conductivity, that is, reduce the electrolyte resistance.

[0018] Further preferably, the ether solvent is diethyl ether. The greater the electronegativity, the greater the attraction of the bonding electrons. Diethyl ether is an extremely electronegative organic substance with a strong attraction to positively charged lithium ions. It is easy to combine with lithium ions. The bound lithium ions seriously weaken the attraction of lithium ions as cations to anions in the system. Anions cannot enter the negative electrode surface, preventing anions from entering the negative electrode surface and combining with lithium ions on the negative electrode surface to form irreversible lithium ion loss, avoiding the collapse of the negative electrode structure, and DEE combined with lithium ions forms a barrier on the negative electrode surface; on the other hand, DEE has a similar polarity to the polymer in the wetting layer, DEE easily penetrates into the wetting layer, increases the spatial infiltration of the electrolyte in the thickness direction of the negative electrode, reduces the interface resistance of the electrolyte on the negative electrode surface, and improves the low-temperature cycle life of the battery at low temperatures.

[0019] More preferably, the polymer is polymethyl methacrylate. The polymer in the wetting-promoting layer of the present invention is PMMA. The swollen PMMA network has both the mechanical properties of a solid and the high transmission properties of a liquid. It can introduce electrolyte solvents such as ethylene carbonate (EC), methyl formate (MF), diethyl carbonate (DEC), and dimethyl carbonate (DMC), helping the electrolyte to infiltrate the negative electrode interface, thereby increasing the electrolyte infiltration effect and reducing the interface resistance between the negative electrode and the electrolyte.

[0020] Preferably, the active material used in the positive electrode is lithium iron phosphate, and the separator is a PE separator.

[0021] Preferably, the mass fraction of graphite in the negative electrode layer is greater than the mass fraction of graphite in the lubrication-promoting layer.

[0022] By adopting the above technical solution, the beneficial effects of the present invention are:

[0023] Generally, electrolyte infiltration is achieved by standing still. Since the degree of infiltration of the electrode and battery cell and the standing time are closely related to parameters such as the electrode material, compaction, electrode group thickness, and battery size, the standing time of different products varies from more than ten hours to dozens of hours, and it is difficult to ensure uniform electrolyte infiltration. To address the above problems, a wetting-promoting layer is provided on the side of the negative electrode facing the separator to promote the uniform distribution of chain polymers in the wetting layer. As the chain polymers come into contact with the electrolyte during the standing process, the polymers swell and gradually form uniformly distributed channels for conducting the electrolyte in the wetting-promoting layer, effectively promoting the contact between the negative electrode material and the electrolyte, while increasing the overall electrolyte retention capacity of the negative electrode.

[0024] The uniform distribution of the electrolyte along the thickness direction of the negative electrode is conducive to the formation of a film of the organic solvent and graphite in the electrolyte on the surface of the active material in the thickness direction of the negative electrode to protect the negative electrode material. The electrolyte-philic substances are contained in the wetting layer, which reduces the interface resistance and makes the electrolyte easier to infiltrate. While promoting infiltration, the negative electrode solvation is avoided, and the interface resistance between the negative electrode and the electrolyte is reduced while ensuring the stability of the negative electrode, thereby improving the cycle performance of the battery, especially the cycle performance at low temperatures. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 20°C cycle life test curves of lithium-ion batteries obtained in Examples 1 to 4 and Comparative Examples 1 to 3 of the present invention;

[0026] Figure 2 25° C. cycle life test curves of lithium-ion batteries obtained in Examples 1 to 4 of the present invention and Comparative Examples 1 to 3. DETAILED DESCRIPTION

[0027] In order to further explain the technical solution of the present invention, the present invention is described in detail below through specific embodiments.

[0028] Example 1

[0029] This embodiment discloses a lithium-ion battery with improved electrolyte wetting, comprising a positive electrode, a negative electrode, a separator, and an electrolyte injected into the battery, wherein the negative electrode active material is graphite; a wetting-promoting layer is coated on the negative electrode surface facing the separator, which increases contact between the electrolyte and the negative electrode material along the thickness direction of the negative electrode and introduces the electrolyte;

[0030] The negative electrode includes the following substances by mass fraction:

[0031] 97% graphite, 1% conductive agent, 1% binder and 1% dispersant.

[0032] The thickness of the wetting-promoting layer is 10 μm, and the wetting-promoting layer includes the following substances in terms of mass fraction:

[0033] 94% graphite, 1% conductive agent, 1% binder, 1% dispersant and 3% PMMA;

[0034] The polymer is chain-shaped and conductive. As the electrolyte contacts the polymer, the polymer swells along the wetting-promoting layer to form a polymer network that guides the electrolyte to be evenly distributed in the wetting-promoting layer.

[0035] The composition of the electrolyte in this embodiment is as follows:

[0036] The organic solvents include EC, MF, and DMC, with a volume ratio of 3:2:1;

[0037] The lithium salt concentration is 1.5 mol / L, the lithium salt includes lithium hexafluorophosphate and lithium hexafluoroarsenate, and the molar ratio of lithium hexafluorophosphate to lithium hexafluoroarsenate is 7:1;

[0038] The additive is DEE with a mass fraction of 2% in the electrolyte;

[0039] The positive electrode is lithium iron phosphate, and the separator is PE separator. The selection of the positive electrode and the separator does not affect the results of the present invention.

[0040] The above negative electrode, positive electrode, electrolyte and separator are stirred, coated, rolled, sheeted, wound, injected and formed to obtain a 2.6Ah cylindrical battery.

[0041] Example 2

[0042] The main difference between this embodiment and embodiment 1 is:

[0043] The negative electrode includes the following substances by mass fraction:

[0044] 97% graphite, 1% conductive agent, 1% binder and 1% dispersant;

[0045] The thickness of the wetting-promoting layer is 13 μm, and the wetting-promoting layer includes the following substances in terms of mass fraction:

[0046] 91% graphite, 1.5% conductive agent, 1.5% binder, 2% dispersant and 4% PMMA;

[0047] The polymer is chain-like and conductive. As the electrolyte contacts the polymer, the polymer swells along the wetting-promoting layer to form a polymer network that guides the electrolyte to be evenly distributed in the wetting-promoting layer. The composition of the electrolyte in this embodiment is as follows:

[0048] The organic solvents include EC, MF, and DMC, with a volume ratio of 5:3:2;

[0049] The lithium salt concentration is 1.5 mol / L, and the lithium salt includes lithium hexafluorophosphate and lithium hexafluoroarsenate, and the molar ratio of lithium hexafluorophosphate to lithium hexafluoroarsenate is 6:1;

[0050] The additive is DEE accounting for 3% by mass of the electrolyte.

[0051] Example 3

[0052] The main difference between this embodiment and embodiment 1 is:

[0053] The negative electrode includes the following substances by mass fraction:

[0054] 96% graphite, 1.5% conductive agent, 1% binder and 1.5% dispersant;

[0055] The thickness of the wetting-promoting layer is 16 μm, and the wetting-promoting layer includes the following substances in terms of mass fraction:

[0056] 89% graphite, 2% conductive agent, 2% binder, 2% dispersant and 5% PMMA;

[0057] The polymer is chain-shaped and conductive. As the electrolyte contacts the polymer, the polymer swells along the wetting-promoting layer to form a polymer network that guides the electrolyte to be evenly distributed in the wetting-promoting layer.

[0058] The composition of the electrolyte in this embodiment is as follows:

[0059] The organic solvents include EC, MF, and DMC, with a volume ratio of 5:3:2;

[0060] The lithium salt concentration is 1.5 mol / L, and the lithium salt includes lithium hexafluorophosphate and lithium hexafluoroarsenate, and the molar ratio of lithium hexafluorophosphate to lithium hexafluoroarsenate is 5:1;

[0061] The additive is DEE accounting for 4% by mass of the electrolyte.

[0062] Example 4

[0063] The main difference between this embodiment and embodiment 1 is:

[0064] The negative electrode includes the following substances by mass fraction:

[0065] 94% graphite, 2% conductive agent, 2% binder and 2% dispersant;

[0066] The thickness of the wetting-promoting layer is 20 μm, and the wetting-promoting layer includes the following substances in terms of mass fraction:

[0067] 87% graphite, 2% conductive agent, 3% binder, 3% dispersant and 5% PMMA;

[0068] The polymer is chain-shaped and conductive. As the electrolyte contacts the polymer, the polymer swells along the wetting-promoting layer to form a polymer network that guides the electrolyte to be evenly distributed in the wetting-promoting layer.

[0069] The composition of the electrolyte in this embodiment is as follows:

[0070] The organic solvents include EC, MF, and DMC, with a volume ratio of 5:3:2;

[0071] The lithium salt concentration is 1.5 mol / L, and the lithium salt includes lithium hexafluorophosphate + lithium hexafluoroarsenate, and the molar ratio of lithium hexafluorophosphate to lithium hexafluoroarsenate is 5:1;

[0072] The additive is DEE accounting for 5% by mass of the electrolyte.

[0073] Comparative Example 1 The main difference between this comparative example and Example 1 is:

[0074] No wetting-promoting layer is provided on the negative electrode surface;

[0075] The organic solvents in the electrolyte are EC, DMC and PC, and the volume ratio of the three is 1:1:1;

[0076] The lithium salt is 1.5 mol / L lithium hexafluorophosphate.

[0077] Comparative Example 2

[0078] The main difference between this comparative example and Example 1 is:

[0079] A wetting-promoting layer is provided on the surface of the negative electrode;

[0080] The thickness of the wetting-promoting layer is 10 μm, and the wetting-promoting layer includes the following by mass fraction:

[0081] 94% graphite, 1% conductive agent, 1% binder, 1% dispersant and 3% PMMA;

[0082] The composition of the electrolyte is EC:DMC:PC=1:1:1 (volume ratio), lithium hexafluorophosphate 1.5 mol / L.

[0083] Comparative Example 3

[0084] The main difference between this comparative example and Example 1 is:

[0085] No wetting-promoting layer is provided on the negative electrode surface;

[0086] The composition of the electrolyte is: EC:MF:DMC=3:2:1 (volume ratio);

[0087] The amount of lithium salt added is 1.5 mol / L, including lithium hexafluorophosphate + lithium hexafluoroarsenate, and the molar ratio of lithium hexafluorophosphate to lithium hexafluoroarsenate is 7:1;

[0088] The additive is DEE accounting for 2% by mass of the electrolyte.

[0089] The lithium-ion batteries obtained in Examples 1 to 4 and Comparative Examples 1 to 3 were subjected to internal resistance tests and cycle life tests at room temperature and -20°C. Specific data are shown in Tables 1 and Figure 1 shown.

[0090] Use an internal resistance meter to test the resistance of the battery at room temperature and -20°C;

[0091] -20℃ cycle life test: 0.5C charge, 0.5C discharge, record capacity change as Figure 1 and Table 2;

[0092] 25℃ cycle life test: 0.5C charge, 0.5C discharge, record capacity change as follows Figure 2 and Table 3;

[0093] The remaining capacity is calculated as follows: Remaining capacity = this week's capacity / first week's capacity * 100%.

[0094] Table 1 Internal resistance data of the batteries obtained in Comparative Examples 1 to 3 and Examples 1 to 4 at room temperature and -20°C

[0095]

[0096]

[0097] It can be seen from the internal resistance data at different temperatures in Table 1 that the present invention uses a surface-coated promoting wetting layer in combination with an electrolyte to effectively reduce the interface resistance. At the same time, DEE can form a protective barrier on the surface of the negative electrode, protect the stability of the negative electrode structure, and extend the cycle life; the electrolyte uses a low-temperature solvent combination to improve the conductivity of the electrolyte at low temperatures. At the same time, the ether solvent is combined with lithium hexafluoroarsenate to further improve the low-temperature conductivity of the electrolyte. The comprehensive effect is that under low-temperature conditions, the interface resistance is reduced and the electrolyte conductivity is not affected by low temperature, which can greatly reduce the internal resistance of the battery. It can be seen from the internal resistance data in Table 1 that compared with comparative examples 1 to 3, the internal resistance of embodiments 1 to 4 at room temperature and -20°C is significantly reduced. The internal resistance of embodiment 4 is the smallest, and its combination effect is the best.

[0098] Table 2 Residual capacity of lithium-ion batteries obtained in Examples 1 to 4 and Comparative Examples 1 to 3 after 100 cycles at -20°C

[0099]

[0100] Table 3 Residual capacity of lithium-ion batteries obtained in Examples 1 to 4 and Comparative Examples 1 to 3 after 200 cycles at 25°C

[0101]

[0102] contrast Figure 1 and Figure 2 As can be seen from the low-temperature and room-temperature cycle data in Tables 2 and 3, the capacity of Comparative Example 1 decays to 80% after 70 cycles at -20°C. After the electrolyte proposed in the present invention is used in combination with the wetting-promoting layer coated on the negative electrode surface, the low-temperature capacity is greatly improved. Among them, Example 4 has a remaining capacity of 86% after 100 cycles at -20°C, which is a battery system with good low-temperature performance. This proves that the electrolyte combined with the negative electrode active layer scheme proposed in the present invention can effectively reduce the internal resistance of the battery and improve the low-temperature cycle life of the battery.

Claims

1. A lithium-ion battery with improved electrolyte infiltration, comprising a positive electrode, a negative electrode, a separator, and an electrolyte injected into the battery, wherein the negative electrode comprises a negative electrode layer, the negative electrode layer comprises a negative electrode active material, and the negative electrode active material is graphite; characterized in that: The negative electrode surface is coated on the side facing the separator with a wetting-promoting layer that increases the contact between the electrolyte and the negative electrode material along the thickness direction of the negative electrode and introduces the electrolyte; The wetting-promoting layer includes the following substances in terms of mass fraction: Graphite 87% to 94%; Conductive agent 1% to 2%; Binder 1% to 3%; Dispersant 1% to 3%; polymer 3% to 5%; The polymer is chain-shaped, and the conductive agent in the wetting-promoting layer enables the polymer network to have conductivity; As the electrolyte contacts the polymer, the polymer swells along the wetting-promoting layer to form a polymer network that guides the electrolyte to be evenly distributed in the wetting-promoting layer. The electrolyte includes an organic solvent, a lithium salt and an additive for binding lithium ions; The additive in the electrolyte is an ether solvent, wherein the mass fraction of the ether solvent in the electrolyte is 2% to 5%; The ether solvent is diethyl ether; The polymer is polymethyl methacrylate; The mass fraction of graphite in the negative electrode layer is greater than the mass fraction of graphite in the wetting-promoting layer; Lithium salts include lithium hexafluorophosphate and lithium hexafluoroarsenate.

2. The lithium-ion battery according to claim 1, wherein: The thickness of the wetting-promoting layer is 10 μm to 20 μm.

3. The lithium-ion battery according to claim 1, wherein: The organic solvent includes ethylene carbonate, methyl formate and dimethyl carbonate, and the volume ratio of the three is 3:2:1 or 5:3:

2.

4. The lithium-ion battery according to claim 3, wherein: The concentration of lithium salt in the electrolyte is 1.5 mol / L.

5. The lithium-ion battery according to claim 4, wherein: The amount of lithium hexafluorophosphate is 5 to 7 times that of lithium hexafluoroarsenate.

6. The lithium-ion battery according to any one of claims 1 to 5, wherein: The active material used in the positive electrode is lithium iron phosphate, and the separator is a PE separator.

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