CN101009388A - Lithium-ion secondary battery with high current discharge capability - Google Patents

Abstract

The invention relates to a LiFePO using low conductivity₄A lithium ion secondary battery having a large current discharge capacity as a main component of a positive electrode active material. A lithium ion secondary battery comprises LiFePO₄As a main component of the positive electrode active material. In order to make the lithium ion secondary battery have large current discharge capability (the capacitance at the discharge rate of 10C is more than 80% of that at the discharge rate of 1C), the coating containing the positive electrode active material on a current collecting substrate is designed so that the ratio (A/t) of the coating area to the thickness thereof is more than 1.2 x 10⁶(mm) such that the coating has a reduced ionic resistance; and providing more than one conductive handle on the current collecting substrate of the positive electrode to reduce the electronic impedance of the current collecting substrate, so that the positive electrode has a reduced total impedance. The design of the invention can also be applied to other positive active materials with low conductivity.

Description

Lithium-ion secondary battery with high current discharge capability

technical field

The present invention relates to a lithium ion secondary battery with high current discharge capacity, in particular to a lithium ion secondary battery with high current discharge capacity using low-conductivity _LiFePO4 as the main component of the positive electrode active material.

Background technique

Lithium iron phosphate (LiFePO ₄ ) has an olivine (olive) crystal structure, and is one of the positive active materials for lithium-ion secondary batteries that has been studied in recent years. The biggest disadvantage of using LiFePO ₄ as a positive electrode active material for lithium-ion secondary batteries is its very low electrical conductivity (on the order of 10 ^-9 S/cm), which is far lower than the commonly used positive electrode active materials LiMn ₂ O ₄ or LiCoO ₂ (10 ^-3 ~ 10 ^-4 S/cm), causing lithium ions to encounter obstacles when moving into/out of the positive electrode, so that the high-current discharge capacity of lithium-ion secondary batteries using LiFePO ₄ as the positive electrode active material is affected. It is worse than those using LiMn ₂ O ₄ or LiCoO ₂ as positive electrode active materials. In general literature, in order to solve the problem of poor electrical conductivity of LiFePO ₄ , they all focus on the preparation of raw materials, which can be divided into the following three points:

^1. Coating carbon layer on the surface of LiFePO ₄ to improve its conductivity 22 October 1999; 2.N.Ravet, Y.Chouinard, JF Magnan, S.Besner, M.Gauthier, and M.Armand, J.power sources, 97～98, 503(2001); 3.PPProsini, D.Zane , M. Pasquali, Electrochim. Acta, 46, 3517 (2001); 4. H. Huang, S.-C. Yin, F. Nazar, Electrochem. Solid State Lett., 4, A170 (2001); 5.Z . Chen, JR Dahn, J. Electrochem. Soc., 149, A1189 (2002)];

2. Doping metals into LiFePO ₄ to increase its conductivity [1. SYChung, JT Bloking, and YM Chiang, Nat. Mater., 1, 123 (2002)];

3. Use nanoscale LiFePO ₄ to increase its high-current discharge capability [1.A.Yamada, SCHung, and K.Hinokuma, J.Electrochem.Soc., 148, A224 (2001); 2.PP Prosini, M . Carewska, S. Scaccia, P. Wisniewski, S. Passerini, M. Pasquali, J. Electrochem. Soc., 149, 886 (2002); 3. S. Franger, F. Le Cras, C. Bourbon, H. Rouault, Electrochem. Solid State Lett., 5, 231 (2002).].

At present, the general commercial LiFePO ₄ is coated with carbon, and its particle size is between submicron and micron; although LiFePO ₄ is coated with carbon, its conductivity can rise to 10 ^-2 to 10 ^-3 S/ cm, but the high-current discharge capability is still poor, and no specific electrode design method has been found to solve the problem of LiFePO ₄ high-current discharge.

U.S. Patent Publications 2005/0233219A1 and 2005/0233220A1 disclose a lithium-ion secondary battery with high current discharge capability using _LiFePO4 as the main component of the positive electrode active material, wherein the _LiFePO4 is further doped with Ti, Zr, Nb, Al or Mg, and the coating of the positive electrode active material on a current collector substrate has an area specific impedance (ASI) less than 20Ω-cm ² . The contents of these two US patents are incorporated into this case by reference.

Contents of the invention

The invention discloses a lithium ion secondary battery, which contains LiFePO ₄ as the main component of the positive electrode active material. In order to make the lithium-ion secondary battery have a large current discharge capability (the electric capacity at 10C discharge rate can be greater than 80% of the 1C discharge rate), the present invention will be located on a current collector substrate containing the coating of the positive electrode active material The layer is designed such that the ratio of its coating area to its thickness (A/t) is greater than 1.2×10 ⁶ (mm), so that the coating has a reduced ionic resistance; Based on a conductive handle, the electronic impedance of the current-collecting substrate is reduced, so that the positive electrode has a reduced total impedance. The design of the present invention is also applicable to other positive electrode active materials with low conductivity.

Through the design of the lithium-ion secondary battery, the present invention respectively discusses the influence of the electronic impedance and ionic impedance of the coating material on the positive plate and the positive plate on the discharge capacity of the battery, so as to overcome the current LiFePO ₄ that cannot be discharged at a large current due to poor conductivity. Disadvantages, the preparation of a lithium ion secondary battery with high current discharge capacity, for example, the capacity ratio (hereinafter referred to as 10C capacity) of the 10C discharge rate relative to the 1C discharge rate (hereinafter referred to as the 10C rate) is greater than 80%, which is suitable for high power Product application.

The preferred specific implementation of the present invention includes (but not limited to) the following items:

1. A lithium ion secondary battery comprising a positive pole; a negative pole; a separator which separates the positive pole and the negative pole; and an electrolyte which forms a lithium ion channel between the positive pole and the negative pole, wherein the positive pole comprises a A current collector substrate; a single or a plurality of conductive handles (tabs) connected to the current collector substrate; and a coating of a positive electrode material on a surface of the substrate, the positive electrode material comprising A positive electrode active material, conductive carbon, and a binder (binder) for bonding the positive electrode active material on the substrate; the feature includes that the positive electrode active material includes a main component LiFePO ₄ , and the surface area of the coating of the positive electrode material is The ratio of its thickness is greater than 1.2×10 ⁶ mm.

2. The lithium ion secondary battery as in item 1, wherein when the positive electrode only includes a single conductive handle, the farthest distance between the single conductive handle and the edge of the substrate is less than 1200mm; when the positive electrode includes multiple conductive handle, the distance between two adjacent conductive handles along the length direction of the substrate is less than 2400mm.

3. The lithium-ion secondary battery according to Item 1, wherein the actual 1KHz impedance (ACIR) of the full battery of the battery is ≤30mΩ.

4. The lithium ion secondary battery according to item 2, wherein the single-side coating of the positive electrode material has a thickness of 30-150 μm.

5. The lithium ion secondary battery according to item 2, wherein the ratio of the capacity at a discharge rate of 10C to the capacity at a discharge rate of 1C of the lithium ion secondary battery is greater than 80%.

6. The lithium ion secondary battery according to item 2, wherein the positive electrode active material further comprises LiMn ₂ O ₄ , LiCoO ₂ , Li[Ni, Co, Mn]O ₂ , or Li[Ni, Co, Al]O ₂ .

7. A lithium ion secondary battery comprising a positive pole; a negative pole; a separator which separates the positive pole and the negative pole; and an electrolyte which forms a lithium ion channel between the positive pole and the negative pole, wherein the positive pole comprises a A current collector substrate; a single or a plurality of conductive handles (tabs) connected to the current collector substrate; and a coating of a positive electrode material on a surface of the substrate, the positive electrode material comprising A positive electrode active material and a binder (binder) for binding the positive electrode active material on the substrate; the feature includes that the positive electrode active material includes a lithium compound as a main component, and the lithium compound has 10 ^-5 to 10 ^-10 S/ The electrical conductivity of the order of cm, and the ratio of the coating area to the thickness of the coating of the positive electrode material is greater than 1.2×10 ^6-- mm.

8. The lithium ion secondary battery according to item 7, wherein the lithium compound is a lithium transition metal phosphorus compound.

9. The lithium ion secondary battery according to item 8, wherein the lithium transition metal phosphorus compound is LiMPO ₄ having an olive or modified olivine crystal structure, wherein M is a transition metal.

10. The lithium ion secondary battery according to item 9, wherein the LiMPO ₄ is LiFePO ₄ , metal-doped LiFePO ₄ , surface-modified or carbon-coated LiFePO ₄ .

11. The lithium ion secondary battery as in Item 7, wherein when the positive electrode only includes a single conductive handle, the farthest distance between the single conductive handle and the edge of the substrate is less than 1200 mm; when the positive electrode includes multiple conductive handles handle, the distance between two adjacent conductive handles along the length direction of the substrate is less than 2400mm.

12. The lithium ion secondary battery according to item 7, wherein the actual 1KHz impedance (ACIR) of the full battery of the battery is ≤30mΩ.

13. The lithium ion secondary battery according to item 11, wherein the coating of the positive electrode material has a thickness of 30-150 μm.

14. The lithium ion secondary battery according to item 11, wherein the ratio of the capacity at a discharge rate of 10C to the capacity at a discharge rate of 1C of the lithium ion secondary battery is greater than 80%.

15. The lithium ion secondary battery according to item 11, wherein the positive electrode active material further comprises LiMn ₂ O ₄ , LiCoO ₂ , Li[Ni, Co, Mn]O ₂ , or Li[Ni, Co, Al]O ₂ .

Implementation

In the design of battery plates, the source of impedance can be mainly divided into two parts: 1. Electronic impedance, 2. Ionic impedance. For positive plates using low conductivity _LiFePO4 as the main active material, the main sources of impedance include the electronic impedance of the current-collecting substrate and the ionic impedance of a coating on its surface.

The electronic impedance R _electron of the current-collecting substrate can be expressed by the following formula:

R _electron ＝1/σ·L/A

Where σ is the electronic conductivity of the current-collecting substrate, L is the length of the conduction path, and A is the conduction cross-sectional area.

The ionic impedance R _ion of the coating of the positive electrode material can be represented by the following formula:

R _ion ＝1/κ·t/A

Wherein κ is the ionic conductivity of the coating of the positive electrode material, t is the thickness of the coating on one side, and A is the coating area.

κ＝κ ₀ ε ⁿ

Among them, κ ₀ = conductivity of the material, ε = porosity of the plate, n = tortuosity of the plate pores

In order to understand the performance of the 10C rate of LiFePO ₄ under different cathode designs, as shown in Fig. 1A to Fig. 1J, 10 sets of experiments with different cathode lengths and conductive handle positions were designed to understand the influence of electron/ion impedance on the 10C rate. The influence of discharge capacity, where L represents the length of the current-collecting substrate of the positive electrode (1L=300mm), and the conductive handle is represented by a short column, wherein the surface of the current-collecting substrate of the positive electrode except for the conductive handle is Covered with positive electrode material coating.

Table 1 and Table 2 below list the design of the 10 experiments.

The details of cell fabrication and measurement of actual 1 KHz impedance (ACIR) and 10C rate of full cells will be described in the following examples.

Table 1

group 1 (Figure 1A) 2 (Fig. 1B) 3 (Fig. 1C) 4 (Fig. 1D) 5 (Fig. 1E) Plate Length(mm) 300 600 600 600 2400 plate width(mm) 38 38 38 38 38 Coating thickness on one side (t, μm) 49 49 49 49 49 Coating area(A)/t(mm) 2.33E+5 4.65E+5 4.65E+5 4.65E+5 1.86E+6 Number of conductive handles 1 1 1 2 2 Conductive handle position beside middle beside beside middle Unit length (mm) ^* 300 300 600 300 600 ACIR(m-ohm) ^** 64.62 35.71 46.86 33.52 16.53 1C capacitance (discharge current) 230.7mAh (270mA) 454.7mAh (500mA) 431.1mAh (500mA) 438.1mAh (500mA) 1855.1mAh (1950mA) 10C capacity 115.7mAh 333.3mAh 314.0mAh 2999.9mAh 1764.9mAh 10C capability 50.1% 73.3% 72.8% 68.5% 95.1%

^* The farthest distance between the conductive handle and the edge of the base material or 1/2 of the distance between two adjacent conductive handles.

^** ACIR is the actual 1KHz impedance of the full battery

From the 2nd, 3rd, and 4th group of experiments, the same substrate length is controlled, and different conductive mechanisms are changed. Because the coating area and the thickness of the coating on one side are the same, it can be considered that the ionic impedance on the coating is fixed, and the electronic impedance has different effects due to the different positions and numbers of the conductive handles. The actual impedance ACIR of the full battery is 35.71, 46.86, 33.52mΩ, and the 10C capability is 73.3%, 72.8%, and 68.5%, respectively. The results show that the electronic impedance difference in the current collection has no significant impact on the 10C capability.

From the first, second, and fifth groups of experiments, the electronic impedance is controlled, that is, the unit plate length (the length of the plate from the conductive handle) is the same, and the fifth group increases the unit plate length because of the number of conductive handles (parallel connection mode), so that the three The electronic impedance of the group is the same. Since the coating area is different, their ionic impedance is thus different. The actual impedance (ACIR) of the full battery is 64.62, 35.71, 16.53mΩ, and the 10C capability is 50.1%, 73.3%, and 95.1%, respectively. The results show that the difference in ACIR caused by ionic impedance has a significant impact on the 10C capability.

In addition, from groups 1 and 3, under the same number and position of conductive handles, the unit plate length of the third group is twice that of the first group, so the electronic impedance of the third group must be greater than that of the first group, but the third group The full battery ACIR of the group 46.9mΩ is lower than the 64.6mΩ of the first group, and the 10C capability of the third group is 72.8% better than the 50.1% of the first group.

From the above results, it can be found that the high current discharge capability of LiFePO ₄ is mainly affected by the ionic impedance on the plate.

Table 2

group 6 (Fig. 1F) 7 (Fig. 1G) 8 (Fig. 1H) 9 (Figure 11) 10 (Figure 1J) Plate Length(mm) 1200 1800 2400 2800 4200 plate width(mm) 38 38 38 38 77 Coating thickness on one side (t, μm) 49 49 49 49 49 Coating area(A)/t(mm) 9.31E+5 1.40E+6 1.86E+6 2.17E+6 6.60E+6 Number of conductive handles 1 1 1 2 5 Conductive handle position beside middle middle middle Unit length (mm) 1200 900 1200 700 420 ACIR(m-ohm) 48.02 23.43 25.25 14.12 5.0 1C capacity (mAh) (discharge current, mA) 1142.6 (1100) 1662.1 (1650) 2240.3 (2200) 2550.1(2600) 8157(8200) 10C Capacitance (mAh) 977.5 1530.6 1933.1 2194.7 7749 10C capability 85.6% 92.1% 86.3% 86.1% 95.0%

When the unit plate length is too long, the factor of LiFePO ₄ discharge capacity may be dominated by the ion impedance to the electronic impedance. In order to avoid this situation, it can be seen from the ninth group of experiments that the problem of high electronic impedance caused by increasing the length of the plate (the electronic conduction path is too long) can be solved by increasing the number and position of the conductive handle .

Because each group of experiments in Table 1 and Table 2 is similar, only the fifth group of experiments is represented in the following examples to be described in detail.

Description of drawings

1A to 1J show 10 different designs of the positive electrode of the present invention, including different lengths of the positive electrode and positions of the conductive handle, where L is the unit length.

Fig. 2 shows the ratio of coating area (A)/coating thickness (t) of 12 groups of different designs in Fig. 1A to Fig. 1J and the specific area impedance (area specific impedance, ASI) relationship.

Fig. 3 shows the relationship of the ratio of coating area (A)/coating thickness (t) of the 10 groups of different designs in Fig. 1A to Fig. 1J w head lithium-ion secondary battery's 10C capacity.

FIG. 4 shows the relationship between the actual 1 KHz impedance (ACIR) and the 10C capability of the lithium-ion secondary battery for 10 sets of full cells with different designs in FIGS. 1A to 1J .

Detailed ways

Example: Group 5

Preparation of positive electrode:

After dissolving 8g polyvinylidene fluoride (PVDF) in 150g N-methyl-2-pyrrolidone (N-methyl-2-pyrrolidone; NMP), add 7g conductive carbon (Super P 1g and KS6 6g of Timcal Company) and 85g LiFePO ₄ (from Phostech company, this LiFePO ₄ has the average particle size of 5 μm and is coated with its carbon of 1.2 weight percents), after mixing homogeneously, make positive electrode slurry, be evenly coated on the two surfaces of aluminum foil base material again Above, the total thickness (including 20 μm substrate) and capacitance of the electrode plate with double-sided coating obtained after drying the solvent and rolling are 118 μm and 2.72 mAh/cm ² , respectively.

Preparation of negative electrode:

After dissolving 10g of PVDF in 88.7g of NMP, add 88g of mesophase microbead synthetic graphitic carbon (Osaka Gas Co., Ltd., code name MCMB10-28), mix well and make negative electrode slurry material, and then uniformly coated on both surfaces of the copper foil base material, the total thickness (including 14 μm base material) and the capacitance of the electrode plate with double-sided coating obtained after drying the solvent and rolling are 90 μm and 3.0 μm respectively. mAh/cm ² .

Assembly of the battery:

Take the positive and negative plates of appropriate length, use PE material (DSM, 14P01E) with a thickness of 25 μm as the separator between them, and add an appropriate amount of 1.1M LiPF ₆ EC/EMC electrolyte after winding to form a cylindrical battery ( EC is the abbreviation of ethylene carbonate; EMC is the abbreviation of ethyl methyl carbonate).

Measurement of full cell impedance:

AC Impedance Impedance measured at 1KHz.

1C capacitance measurement:

After the battery is fully charged (1C CCCV, 3.65v, I-Cut: 0.01C), it is the capacitance measured by discharging it to 2.3V with a current of 1C.

10C capacitance measurement:

After the battery is fully charged (1C CCCV, 3.65V, I-Cut: 0.01C), it is the capacity measured by discharging it to 2.3V with a current of 10C.

In order to prove that the design of the present invention is different from the design of specific area impedance (area specific impedance) used in U.S. Patent Publication 2005/0233219A1 and 2005/0233220A1, the following table 3 lists the A/t and Specific area impedance (ASI) value, Figure 2 is the relationship between ASI and A/t.

table 3

group A/t Specific area impedance (ASI, Ω-cm ² ) 1 2.33E+5 7.37 2 4.65E+5 8.14 3 4.65E+5 10.68 4 4.65E+5 7.64 5 1.86E+6 15.07 6 9.31E+5 21.90 7 1.40E+6 16.02 8 1.86E+6 23.03 9 2.17E+6 15.02 10 6.60E+6 330.00

It can be seen from Table 3 that when the design of the present invention requires the coating area (A)/t to be greater than 1.2*10 ⁶ , the ASI of some batteries is greater than 20Ω-cm ² ; and when A/t is less than 1.2*10 ⁶ , some The specific area impedance of the battery is less than 20Ω-cm ² . It can be seen from Figure 2 that there is no correlation between A/t value and ASI. In addition, for example, the ASI of the 10th battery group is > 20, but the 10C/1C capacity ratio (10C capacity) is only 2.5%; on the contrary, the ASI of the 5th battery group is < 20, but the 10C capacity is as high as 95.1%, so the battery’s 10C capacitance has no correlation with ASI.

3 shows the relationship between the 10C capability and the coating area (A)/coating thickness (t) of the lithium ion secondary batteries of groups 1 to 10 in Tables 1 to 2. FIG. It can be seen from FIG. 3 that when the design requirement of the present invention is that A/t is greater than 1.2*10 ⁶ , the 10C capacity of the battery can be greater than 80%.

FIG. 4 shows the relationship between 10C capability and ACIR of lithium ion secondary batteries of groups 1 to 10 in Tables 1 to 2. FIG. It can be seen from Figure 4 that the 10C discharge capacity of the battery is correlated with ACIR, and when the ACIR is less than 30mΩ, the 10C capacity of the battery can be greater than 80%.

Claims (15)

1. A lithium ion secondary battery comprising a positive pole; a negative pole; a separator which separates the positive pole and the negative pole; and an electrolyte which forms a lithium ion channel between the positive pole and the negative pole, wherein the positive pole comprises a A current-collecting substrate; a single or a plurality of conductive handles connected to the current-collecting substrate; and a coating of a positive electrode material on a surface of the substrate, the positive electrode material comprising a positive electrode active material, conductive carbon and a binder for binding the positive electrode active material on the substrate; the feature is that the positive electrode active material contains a main component LiFePO ₄ , and the ratio of the surface area to the thickness of the coating of the positive electrode material is greater than 1.2×10 ⁶ mm. 2. The lithium ion secondary battery as claimed in claim 1, wherein when the positive pole only comprises a single conductive handle, the farthest distance between the single conductive handle and the edge of the substrate is less than 1200mm; when the positive pole comprises a plurality of In the case of conductive handles, the distance between two adjacent conductive handles along the length direction of the base material is less than 2400mm. 3. The lithium ion secondary battery as claimed in claim 1 or 2, wherein the actual 1 KHz impedance (ACIR) of the full cell of the battery is < 30 mΩ. 4. The lithium-ion secondary battery as claimed in claim 2, wherein the single-side coating of the positive electrode material has a thickness between 30-150 μm. 5. The lithium ion secondary battery as claimed in claim 2, wherein the ratio of the capacity of the lithium ion secondary battery at a discharge rate of 10C to the capacity at a discharge rate of 1C of the lithium ion secondary battery is greater than 80%. 6. The lithium ion secondary battery as claimed in claim 2, wherein the positive electrode active material further comprises LiMn ₂ O ₄ , LiCoO ₂ , Li[Ni, Co, Mn]O ₂ , or Li[Ni, Co, Al] O ₂ . 7. A lithium ion secondary battery comprising a positive pole; a negative pole; a separator which separates the positive pole and the negative pole; and an electrolyte which forms a lithium ion channel between the positive pole and the negative pole, wherein the positive pole comprises a A current-collecting substrate; a single or a plurality of conductive handles connected to the current-collecting substrate; and a coating of a positive electrode material on a surface of the substrate, the positive electrode material comprising a positive electrode active material and the A binder for bonding positive electrode active materials on the substrate; the feature is that the positive electrode active material contains a lithium compound as a main component, and the lithium compound has a conductivity of 10 ^-5 to 10 ^-10 S/cm order, and the The ratio of the coating area to the thickness of the coating of the positive electrode material is greater than 1.2×10 ^6-- mm. 8. The lithium ion secondary battery as claimed in claim 7, wherein the lithium compound is a lithium transition metal phosphorus compound. 9. The lithium ion secondary battery as claimed in claim 8, wherein the lithium transition metal phosphorus compound is LiMPO ₄ having an olivine or modified olivine crystal structure, wherein M is a transition metal. 10. The lithium ion secondary battery as claimed in claim 9, wherein the LiMPO ₄ is LiFePO ₄ , metal-doped LiFePO ₄ , surface-modified or carbon-coated LiFePO ₄ . 11. The lithium ion secondary battery as claimed in claim 7, wherein when the positive pole only comprises a single conductive handle, the farthest distance between the single conductive handle and the edge of the base material is less than 1200mm; when the positive pole comprises a plurality of In the case of conductive handles, the distance between two adjacent conductive handles along the length direction of the base material is less than 2400mm. 12. The lithium ion secondary battery as claimed in claim 7 or 11, wherein the actual 1 KHz impedance (ACIR) of the full cell of the battery is < 30 mΩ. 13. The lithium ion secondary battery as claimed in claim 11, wherein the coating of the positive electrode material has a thickness between 30-150 μm. 14. The lithium ion secondary battery as claimed in claim 11, wherein the ratio of the capacity of the lithium ion secondary battery at a discharge rate of 10C to the capacity at a discharge rate of 1C of the lithium ion secondary battery is greater than 80%. 15. The lithium ion secondary battery as claimed in claim 11, wherein the positive electrode active material further comprises LiMn ₂ O ₄ , LiCoO ₂ , Li[Ni, Co, Mn]O ₂ , or Li[Ni, Co, Al] O ₂ .

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