Positive electrode and lithium-ion secondary battery

Abstract

Provided is a technique for effectively suppressing the elution of Mn and improving cycle characteristics. 【Solution means】 As the positive electrode of a lithium-ion secondary battery, it includes a first region facing the separator and a second region facing the current collector. The first region contains the first lithium manganese iron phosphate represented by the general formula (1), and the second region contains the second lithium manganese iron phosphate represented by the general formula (2), where a < c. Li α Mn a Fe b PO4(0.50 ≦ α ≦ 1.5, 0 < a < 1.0, 0 < b < 1.0, 0.9 ≦ a + b ≦ 1.1) Formula (1) Li β Mn c Fe d PO4(0.50 ≦ β ≦ 1.5, 0

Description

[Technical Field]

[0001] The technologies disclosed herein relate to cathodes and lithium-ion secondary batteries, etc. [Background technology]

[0002] Olivine-type compounds such as LiMnFePO4 (LMFP) are used as active material for the positive electrode of lithium-ion secondary batteries. Patent document 1 describes using granules of Mn-rich LMFP and Fe-rich LMFP as positive electrode active material particles for the purpose of improving energy density and cycle resistance. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2021-9838 [Overview of the project] [Problems that the invention aims to solve]

[0004] The method described in Patent Document 1 is intended to combine a Mn-rich LMFP, which contributes to energy density, with an Fe-rich LMFP, which contributes to cycle resistance. However, according to the inventors, there is still room for improvement in the cycle characteristics.

[0005] This specification provides cathodes and lithium-ion secondary batteries, etc., that effectively suppress Mn elution and improve cycle characteristics. [Means for solving the problem]

[0006] The technologies disclosed herein are embodied in the following cathodes and lithium-ion secondary batteries.

[0007] [1] The positive electrode of a lithium-ion secondary battery, It includes a first region facing the separator and a second region facing the current collector, The first region contains a first lithium iron manganese phosphate represented by formula (1) which is a general formula, and the second region contains a second lithium iron manganese phosphate represented by formula (2) which is a general formula, and a < c, a positive electrode. Li α Mn a Fe b PO4(0.50 ≤ α ≤ 1.5, 0 < a < 1.0, 0 < b < 1.0, 0.9 ≤ a + b ≤ 1.1 Formula (1) Li β Mn c Fe d PO4(0.50 ≤ β ≤ 1.5, 0 < c < 1.0, 0 < d < 1.0, 0.9 ≤ c + d ≤ 1.1) Formula (2) [2] In the formula (1), 0 < a < 0.80, 0.20 < b < 1.0, and in the formula (2), 0.70 ≤ c < 1.0, 0 < d ≤ 0.30, the positive electrode according to [1]. [3] In the formula (1), 0 < a ≤ 0.60, 0.40 ≤ b < 1.0, the positive electrode according to [2]. [4] In the formula (1), 0.55 ≤ a ≤ 0.65, 0.35 ≤ b ≤ 0.45, and in the formula (2), 0.75 ≤ c ≤ 0.85, 0.15 ≤ b ≤ 0.25, the positive electrode according to [3]. [5] A lithium ion secondary battery including the positive electrode according to any one of [1] to [4].

[0008] In the positive electrode, the Mn ratio a of LMFP in the first region on the separator side is smaller than the Mn ratio c of LMFP in the second region on the current collector side. Therefore, elution of Mn can be suppressed on the separator side in contact with the electrolyte, and as a result, it can contribute to excellent cycle characteristics. Further, since the secondary battery includes the positive electrode, it can exhibit excellent cycle characteristics.

Brief Description of the Drawings

[0009] [Figure 1] It is a diagram schematically showing an example of a cell of a lithium ion secondary battery. [Figure 2]It is a cross-sectional view schematically showing an example of the structure of the electrode disclosed in this specification. [Figure 3] It is a process chart schematically showing an example of the manufacturing method of the electrode sheet disclosed in this specification.

Embodiments for Carrying Out the Invention

[0010] The disclosure of this specification relates to a positive electrode for a lithium-ion secondary battery and a lithium-ion secondary battery, etc. According to the positive electrode disclosed in this specification, regarding LMFP, without using particles obtained by compositing LMFP having different element ratios, a simple structure is provided in which the separator side and the current collector side are provided with LMFP having different element ratios regarding Mn, and elution of Mn can be suppressed to obtain excellent cycle characteristics.

[0011] Also, since the Mn ratio a of LMFP in the first region on the separator side is low, it is possible to avoid the low electron conductivity, which is a demerit of Mn-rich LMFP, and reduce the resistance.

[0012] Hereinafter, the positive electrode, the lithium-ion secondary battery, etc. will be described with reference to the drawings as appropriate.

[0013] (Positive Electrode of Lithium-Ion Secondary Battery) Figure 1 schematically shows an example of a cell 2 of a lithium-ion secondary battery (hereinafter simply referred to as a secondary battery) 100. As shown in Figure 1, the cell 2 as a unit structure of the secondary battery 100 includes a positive electrode 4, a separator 6, and a negative electrode 8. The cell 2 further includes a positive electrode current collector 10 and a negative electrode current collector 12. The secondary battery 100 usually has a structure in which a large number of cells 2 are stacked. The separator 6 may hold, for example, a liquid or gel electrolyte. The separator 6 is made of a known material such as a polyolefin microporous membrane having fine pores formed therein. The separator 6 is impregnated with a liquid electrolyte containing a lithium salt such as lithium hexafluorophosphate with an organic solvent such as ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), etc. as a medium. Note that a solid electrolyte layer may be used instead of these. The negative electrode 8 has a negative electrode composite layer 30.

[0014] Figure 2 schematically shows an example of a cross-section of the positive electrode 4. The positive electrode 4 includes a positive electrode composite layer (hereinafter also simply referred to as a composite layer) 20. Note that the positive electrode 4 may be in a form including a current collector 10 integrated with the composite layer 20. The current collector 10 is not particularly limited, and examples include metal or alloy foils containing, for example, aluminum, nickel, etc.

[0015] As shown in Figure 2, the composite layer 20 includes a first region 22 facing the separator 6 and a second region 24 facing the current collector 10 in the secondary battery 100. The first region 22 and the second region 24 may be provided so as to be divided into two in the thickness direction of the composite layer 20, or a region containing a positive electrode active material with an arbitrary composition may be provided between the first region 22 and the second region 24.

[0016] Both the first region 22 and the second region 24 contain a positive electrode active material. Both the first region 22 and the second region 24 contain lithium manganese iron phosphate (LMFP) as the positive electrode active material. The first region 22 and the second region 24 each contain a first LMFP 26 represented by a general formula of formula (1) and a second LMFP 28 represented by a general formula of formula (2).

[0017] The first LMFP 26 is represented by formula (1), the second LMFP 28 is represented by formula (2), and further, a < c. <Formula (1)> Li α Mn a Fe b PO4 (0.50 ≦ α ≦ 1.5, 0 < a < 1.0, 0 < b < 1.0, 0.90 ≦ a + b ≦ 1.1) <Formula (2)> Li β Mn c Fe d PO4 (0.50 ≦ β ≦ 1.5, 0 < c < 1.0, 0 < d < 1.0, 0.90 ≦ c + d ≦ 1.1)

[0018] As is clear from the above general formula, the Mn ratio a, which is the ratio of Mn in formula (1), has the characteristic of being smaller than the Mn ratio c, which is the ratio of Mn in formula (2).

[0019] By having "Mn ratio a < Mn ratio c", the amount of Mn eluted from the positive electrode 4 can be effectively reduced. Although not limiting the disclosure herein, in the composite material layer 20, by making the Mn ratio of the LMFP on the separator side relatively low, Mn elution can be suppressed more than in the case where it is not, and Mn diffusing toward the negative electrode in the electrolyte can be suppressed. As a result, it is considered that the deterioration of the cycle characteristics is suppressed.

[0020] The Mn ratio a and the Mn ratio c can be appropriately set within the ranges of 0 < a < 1.0 and 0 < b < 1.0 so that a < c, respectively.

[0021] The Mn ratio a is, for example, 0 < a < 0.80. Considering the energy density in particular, the Mn ratio a is 0.40 or more, 0.45 or more, 0.50 or more, 0.55 or more, 0.60 or more. Also, from the perspective of suppressing the elution of Mn and from the perspectives of electron conductivity and ion conductivity, it is 0.75 or less, 0.70 or less, 0.65 or less, 0.60 or less, 0.55 or less. The range of the Mn ratio a can be appropriately selected and set from these lower and upper limits. From the perspectives of suppressing Mn elution, energy density, electron conductivity, etc., for example, it can be 0.40 or more and less than 0.80, 0.40 or more and 0.75 or less, 0.40 or more and 0.70 or less, 0.50 or more and 0.70 or less, 0.55 or more and 0.65 or less, etc.

[0022] When the Mn ratio a is within the above range, the Fe ratio b is, for example, 0.20 < b < 1.0. Also, for example, the lower limit of b is 0.25 or more, 0.30 or more, 0.35 or more, 0.40 or more, 0.45 or more. The upper limit of b is 0.60 or less, 0.55 or less, 0.50 or less, 0.45 or less, 0.40 or less. Therefore, b is, for example, more than 0.20 and 0.60 or less, 0.25 or more and 0.60 or less, 0.30 or more and 0.60 or less, 0.30 or more and 0.50 or less, 0.35 or more and 0.45 or less.

[0023] From the perspective of energy density, the Mn ratio c is, for example, 0.70 ≤ c < 1.0. Also, for example, c is 0.75 or more, 0.80 or more, 0.85 or more. Also, from the perspective of suppressing the elution of Mn and from the perspective of electron conductivity, etc., for example, it is 0.90 or less, 0.85 or less, 0.80 or less, 0.75 or less. The range of the Mn ratio c can be appropriately selected and set from these lower and upper limits. For example, it can be 0.70 or more and 0.90 or less, 0.70 or more and 0.85 or less, 0.75 or more and 0.85 or less, etc.

[0024] When the Mn ratio c is within the above range, the Fe ratio d is, for example, 0 < b ≤ 0.30. Also, for example, the lower limit of d is 0.10 or more, 0.15 or more, 0.20 or more, 0.25 or more. The range of d can be appropriately selected and set from these lower and upper limits, and can be 0.10 or more and 0.30 or less, 0.15 or more and 0.30 or less, 0.15 or more and 0.25 or less, etc.

[0025] Regarding further other variables, etc. in formulas (1) and (2), they can be as follows.

[0026] Regarding α and β, they are 0.50 or more and 1.5 or less. Exceeding this range will cause the purity of LMFP to drop too much and it will be difficult to ensure the energy density. α and β may preferably be, for example, 0.90 or more and 1.1 or less. The purity of LMFP is sufficiently ensured and the energy density is ensured. Also, regarding a + b and c + d, both are 0.90 or more and 1.1 or less. When within this range, the purity of LMFP is ensured and the energy density is ensured.

[0027] The LMFP disclosed in this specification is Li α Mn a Fe b PO4, and Li [[ID=IS=19]] β Mn c Fe d O4 is a compound represented by, but even when elements other than the above are added to LMFP within the range of 0.1% by mass or more and 10% by mass or less as doping elements, they are also included in the LMFP in the present invention.

[0028] The composition of LMFP can be specified by atomic absorption spectrometry for lithium and ICP emission spectrometry for manganese, iron, and phosphorus. For the above formulas α, β, a - d, two significant figures are adopted. Also, when the raw material charging ratio during the production of LMFP is known, the composition can also be determined from that charging ratio.

[0029] The average particle diameters of the first LMFP26 and the second LMFP28 are not particularly limited.

[0030] The first LMFP26 and the second LMFP28 may be provided with a carbonaceous coating to improve electronic conductivity. The carbonaceous coating may include, for example, a carbonaceous material such as graphite or semigraphite. The thickness of the carbonaceous coating is not particularly limited. For example, it may be between 0.1 nm and 10 nm, or between 0.5 nm and 3 nm.

[0031] In the first region 22 and the second region 24, the positive electrode active material particles consist of the first LMFP26 and the second LMFP28, respectively, in amounts of, for example, 70% or more by mass, 80% or more by mass, 85% or more by mass, 95% or more by mass, 98% or more by mass, and 100% by mass, relative to the total mass of the positive electrode active material particles. The first region 22 and the second region 24 may also contain other active material particles besides the first LMFP26 and the second LMFP28. For example, particles containing other known olivine-type positive electrode materials can be used. Examples of olivine-type positive electrode materials include lithium iron phosphate (LiFePO4, LFP) and lithium manganese phosphate (LiMnPO4, LMP). In addition, known layered rock salt type oxides such as NCM can also be included.

[0032] The content (total amount) of positive electrode active material particles in the composite layer 20 is not particularly limited, but for example, it may be 80% or more by mass, 90% or more by mass, 95% or more by mass, 96% or more by mass, or 97% or more by mass of the total mass of the composite layer 20.

[0033] (Conductive additive) The first region 22 and the second region 24 may each contain a conductive additive. The conductive additive is an additive for improving the electronic conductivity in the composite layer 20. The conductive additive is not particularly limited, and various known conductive additives can be used. Examples of conductive additives include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; conductive fibers such as carbon fibers and metal fibers; carbon materials such as graphene and carbon nanotubes; carbon fluoride; metal powders such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. As for carbon nanotubes, for example, single-wall carbon nanotubes (SW-CNTs) and multi-wall carbon nanotubes (MW-CNTs) including double-wall carbon nanotubes may be preferred. One or more of these conductive additives can be used.

[0034] The content (total amount) of the conductive additive in the composite layer 20 is not particularly limited, but for example, it is 0.01% by mass or more and 5% by mass or less of the total mass of the composite layer 20.

[0035] (Binder) The composite layer 20 may contain a binder. The binder is not particularly limited, and any known binder used in the positive electrode can be used as appropriate. Examples include polyvinylidene fluoride and styrene-butadiene rubber. One or more of these can be used. The binder content in the composite layer 20 is not particularly limited, but for example, it is 0.3% by mass or more and 10% by mass or less.

[0036] The thickness of the composite layer 20 is not particularly limited, but for example, it has a thickness of about 10 μm to 500 μm. Similarly, the thicknesses of the first region 22 and the second region 24 are not particularly limited, but for example, they may be thicknesses that divide the thickness of the composite layer 20 into two equal parts.

[0037] According to this specification, a secondary battery 100 is also provided, which is a stack of cells 2, each comprising such a positive electrode 4, a separator 6, a negative electrode composite layer 30, and a negative electrode current collector 12.

[0038] Next, the method for manufacturing these positive electrodes 4 will be explained with reference to Figure 3 as appropriate.

[0039] In this manufacturing method, a mixture for the positive electrode composite layer corresponding to the second region 24 is applied to the current collector 10 by a known method such as a doctor blade, and dried as necessary to prepare the precursor layer 24' of the second region 24. Furthermore, a composition for the positive electrode composite layer corresponding to the first region 22 is applied on top of the precursor layer 24' to prepare the precursor layer 22' of the first region 22, which is then dried and pressed. In this way, a positive electrode 4 can be obtained in which the composite layer 20 is integrated with the current collector 10.

[0040] The mixture for the positive electrode composite layer corresponding to the second region 24 and the mixture for the positive electrode composite layer corresponding to the first region 22 are to which the various embodiments described above for the second region 24 and the first region 22 are applied, respectively.

[0041] According to this manufacturing method, by laminating LMFPs with different Mn ratios in the composite layer 20, it is possible to easily suppress the amount of Mn leaching and obtain a secondary battery 100 with excellent cycle characteristics, etc. The secondary battery 100 using the positive electrode 4 can be manufactured by referring to known methods as appropriate. [Examples]

[0042] The following describes embodiments that embody the disclosures of this specification, but these embodiments are for illustrative purposes only and are not limiting. [Examples]

[0043] In this example, lithium-ion secondary batteries were fabricated using the following method, and their cycle characteristics and Mn elution amount were measured under predetermined conditions. The composition of the sample and comparative example sample is shown in Table 1.

[0044] (1) Preparation of the positive electrode The positive electrode for Sample 1 was prepared as follows. Specifically, LMFP II (Mn 80 mol%, Fe 20 mol%) and LMFP I (Mn 60 mol%, Fe 40 mol%) were used as the positive electrode active material, and composite slurries were prepared with LMFP:conductive additive:binder in a ratio of 98:0.1:1.9 (mass%). These slurries were then mixed with a basis weight of 20 mg / cm² to form the electrode structure shown in Table 1, with a thickness approximately equivalent to that of the current collector (30 μm thickness). 2 The sample was coated using a doctor blade to achieve the desired consistency. Afterward, it was dried at 100°C for 10 minutes, and then pressed to a density of 2.0 g / cc to produce the positive electrode for sample 1.

[0045] For comparative examples 2 to 4, the cathodes were prepared in the same manner as for sample 1, except that the composite slurry was prepared and applied to create the electrode structure shown in Table 1.

[0046] (2) Fabrication of the negative electrode Using graphite as the negative electrode active material, a slurry was prepared with the ratio of active material:conductive additive:CMC:SBR = 97:0.1:0.4:2.5 (mass%), and a single-sided basis of 13 mg / cm² was applied to an 8 μm Cu foil. 2 After coating with a doctor blade so that the ratio of positive electrode capacity to negative electrode capacity was 1.1, the material was dried at 100°C for 10 minutes and then pressed to a density of 1.3 g / cc to produce the negative electrode.

[0047] (3) Cell creation Laminate cells were fabricated by stacking the positive electrodes of the sample and comparative examples, a separator (a 3-layer laminate of PP / PE / PP, 16 μm), and a negative electrode. The electrolyte used was a 1.4 M LiPF6 / FSI mixture with EC:DMC:EMC = 30:40:30 (volume %). The cell confinement pressure was set to 100 kPa.

[0048] (4) Activation and evaluation of volume retention rate at 40°C and measurement of Mn elution amount The initial charging was performed using a constant current-constant voltage method. Constant current charging was performed at a current value of 0.05C up to 4.3V, then constant voltage charging was performed until the current value became 0.02C. Discharge was performed using the constant current method at a current value of 0.05C up to 3.0V to activate the battery. After 200 cycles, charging was performed using the constant current-low voltage method at a current value of 0.1C up to 4.3V, and discharge was performed using the constant current method at a current value of 0.1C up to 3.0V to calculate the capacity retention rate relative to the rated capacity. The results are also shown in Table 1.

[0049] The amount of manganese leached after 200 cycles was evaluated according to a predetermined method. The results are shown in Table 1.

[0050] [Table 1]

[0051] As shown in Table 1, when comparing Sample 1 with Comparative Example 1, placing the LMFP with a lower Mn ratio in the first region (separator side) improved the capacity retention rate and reduced the amount of Mn eluted. This indicates that when fabricating a stacked cathode structure with varying Mn ratios, it is useful to use an LMFP with a lower Mn ratio on the separator side.

[0052] Furthermore, comparing Sample 1 with Comparative Examples 2-4, it was found that placing LMFP with a lower Mn ratio on the separator side resulted in superior volume retention and lower Mn elution compared to a monolayer of LMFP with a higher Mn ratio. Moreover, it was found that placing LMFP with a lower Mn ratio on the separator side resulted in superior volume retention and lower Mn elution compared to a monolayer simply mixed with LMFP with a higher and lower Mn ratio.

[0053] From the above, it was found that by adopting a laminated structure in the positive electrode composite layer that uses LMFP with a lower Mn ratio on the separator side and LMFP with a higher Mn ratio on the current collector side, the amount of Mn leaching can be easily and efficiently suppressed, and the cycle characteristics can be maintained / improved.

[0054] The specific examples of the technology disclosed in this specification have been described in detail above, but these are merely illustrative and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes to the specific examples described above. The technical elements described in this specification or in the drawings exhibit technical usefulness individually or in various combinations, and are not limited to the combinations described in the claims at the time of filing. The technology illustrated in this specification or in the drawings can achieve multiple objectives simultaneously, and achieving even one of these objectives itself constitutes technical usefulness. [Explanation of symbols]

[0055] 2 cells, 4 positive electrodes, 6 separators, 8 negative electrodes, 10 positive electrode current collectors, 12 negative electrode current collectors, 20 positive electrode composite layer, 22 first region, 24 second region, 26 first LMFP, 28 second LMFP, 30 negative electrode composite layer, 100 lithium-ion secondary battery

Claims

1. The positive electrode of a lithium-ion secondary battery, It comprises a first region facing the separator and a second region facing the current collector, The positive electrode comprises a first region containing a first lithium manganese iron phosphate represented by the general formula (1), and a second region containing a second lithium manganese iron phosphate represented by the general formula (2), wherein a < c. Li α Mn ａ Fe ｂ PO ４ (0.50 ≤ α ≤ 1.5, 0 < a < 1.0, 0 < b < 1.0, 0.9 ≤ a + b ≤ 1.1) Formula (1) Li β Mn ｃ Fe ｄ PO ４ (0.50 ≤ β ≤ 1.5, 0 < c < 1.0, 0 < d < 1.0, 0.9 ≤ c + d ≤ 1.1) Formula (2)

2. The positive electrode according to claim 1, wherein in formula (1), 0 < a < 0.80 and 0.20 < b < 1.0, and in formula (2), 0.70 ≤ c < 1.0 and 0 < d ≤ 0.

30.

3. The positive electrode according to claim 2, wherein in formula (1), 0 < a ≤ 0.60 and 0.40 ≤ b < 1.

0.

4. The positive electrode according to claim 3, wherein in formula (1), 0.55 ≤ a ≤ 0.65 and 0.35 ≤ b ≤ 0.45, and in formula (2), 0.75 ≤ c ≤ 0.85 and 0.15 ≤ b ≤ 0.

25.

5. A lithium-ion secondary battery comprising the positive electrode described in any one of claims 1 to 4.

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