Positive electrode active material for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery

Abstract

The positive electrode active material for non-aqueous electrolyte secondary batteries contains the formula Li x Mn y Ni z Ge a M b O 2‑c F c (In the formula, M is at least one selected from Ti, Co, Si, Al, Nb, W, Mo, P, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Sr, Zr, Ru, K, and Bi, 1.0)

Description

Technical Field

[0001] This disclosure relates to a positive electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the positive electrode active material. Background Technology

[0002] In non-aqueous electrolyte secondary batteries such as lithium-ion batteries, the positive electrode active material has a significant impact on battery performance, including input / output characteristics, capacity, and cycle characteristics. Lithium transition metal composite oxides containing metal elements such as Ni, Co, Mn, and Al are commonly used as positive electrode active materials. The properties of lithium transition metal composite oxides vary considerably depending on their composition; therefore, extensive research has been conducted on the types and amounts of added elements.

[0003] For example, Patent Document 1 discloses a positive electrode active material for a non-aqueous electrolyte secondary battery, which uses the formula Li x Ni 1-y Co y-z M z O 2-a X b It is indicated that the lattice constant of the a-axis, measured by X-ray diffraction, is 2.81~2.91 Å, the lattice constant of the c-axis is 13.7~14.4 Å, and the ratio of the diffraction peak intensity of the (104) plane to the peak intensity of the (003) plane is 0.3~0.8.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent No. 4197002 Summary of the Invention

[0007] Lithium-excess composite oxides with a Li molar ratio to transition metal exceeding 1 were also proposed. These lithium-excess composite oxides are expected to serve as high-capacity next-generation cathode active materials, but issues such as the easy dissolution of transition metals exist. It is known that adding F to lithium-excess composite oxides can suppress transition metal dissolution and improve durability, but this results in a decrease in operating voltage with repeated charge-discharge cycles, a problem that cannot be solved by adding F.

[0008] As one aspect of this disclosure, the positive electrode active material for a non-aqueous electrolyte secondary battery comprises the formula Li x Mn y Ni z Ge a M b O 2-c F c(In the formula, M is at least one selected from Ti, Co, Si, Al, Nb, W, Mo, P, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Sr, Zr, Ru, K, and Bi; 1.0 < x ≤ 1.2, 0.4 ≤ y ≤ 0.8, 0 ≤ z ≤ 0.4, 0 < a < 0.01, 0 < b < 0.03, 0 < c < 0.1, and x + y + z + a + b ≤ 2) a lithium transition metal composite oxide represented by

[0009] A non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes: a positive electrode containing the above positive electrode active material; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte.

[0010] The lithium-excess positive electrode active material of the present disclosure has a high voltage retention rate and excellent cycle characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a cross-sectional view of a non-aqueous electrolyte secondary battery as an example of an embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0012] As described above, when F is added to the lithium-excess composite oxide, the dissolution of the transition metal is suppressed and the durability of the composite oxide is improved, but the effect is not sufficient, and further improvement is sought. As a result of the research by the present inventors, it was confirmed that the addition of Ge is beneficial to the improvement of durability, but the improvement effect is small only by adding Ge. It is considered that Ge suppresses the intercalation and deintercalation of oxygen and stabilizes the structure of the composite oxide.

[0013] The present inventors further conducted in-depth research and found that in a lithium-excess F-containing composite oxide containing at least Mn as a transition metal, the durability is greatly improved and the voltage retention rate during charge and discharge is improved by adding Ge and one or more specific elements.

[0014] Hereinafter, an example of an embodiment of a positive electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the positive electrode active material will be described in detail with reference to the drawings. It should be noted that it is initially contemplated to selectively combine a plurality of embodiments and modification examples described below.

[0015] Hereinafter, a cylindrical battery obtained by housing a wound electrode body 14 in a bottomed cylindrical outer can 16 is exemplified, but the outer casing is not limited to a cylindrical outer can. For example, it can be a square outer can (square battery), a coin-shaped outer can (coin-shaped battery), or an outer casing made of a laminated sheet including a metal layer and a resin layer (laminated battery). In addition, the electrode body may be a laminated electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated with a separator interposed therebetween.

[0016] Figure 1 This is a cross-sectional view of a non-aqueous electrolyte secondary battery 10, as an example of an embodiment. Figure 1 As shown, the non-aqueous electrolyte secondary battery 10 includes: a wound electrode body 14, a non-aqueous electrolyte, and a housing 16 for storing the electrode body 14 and the non-aqueous electrolyte. The electrode body 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape with the separator 13 in between. The housing 16 is a bottomed cylindrical metal container with an opening on one axial side, and the opening of the housing 16 is sealed by a sealing body 17. Hereinafter, for ease of explanation, the side with the sealing body 17 of the battery will be referred to as the top, and the bottom side of the housing 16 will be referred to as the bottom.

[0017] Non-aqueous electrolytes comprise: a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. The non-aqueous solvent may also contain halogen substitutes, formed by replacing at least a portion of the hydrogen atoms of these solvents with halogen atoms such as fluorine. Examples of electrolyte salts include lithium salts such as LiPF6. It should be noted that non-aqueous electrolytes are not limited to liquid electrolytes and can also be solid electrolytes.

[0018] The positive electrode 11, negative electrode 12, and spacer 13 constituting the electrode body 14 are all strip-shaped elongated bodies, and are spirally wound, thereby being alternately stacked along the radial direction of the electrode body 14. To prevent lithium deposition, the negative electrode 12 is formed to be one size larger than the positive electrode 11. That is, the negative electrode 12 is longer in both the length and width (lateral) directions compared to the positive electrode 11. Two spacers 13 are formed to be at least one size larger than the positive electrode 11, and are arranged, for example, to clamp the positive electrode 11. The electrode body 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0019] Insulating plates 18 and 19 are respectively arranged above and below the electrode body 14. Figure 1 In the example shown, the positive lead 20 extends towards the sealing body 17 through the through hole in the insulating plate 18, and the negative lead 21 extends towards the bottom of the outer casing 16 through the outer side of the insulating plate 19. The positive lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the top plate, i.e., the cover 27, of the sealing body 17, which is electrically connected to the internal terminal plate 23, becomes the positive terminal. The negative lead 21 is connected to the inner bottom surface of the outer casing 16 by welding or the like, and the outer casing 16 becomes the negative terminal.

[0020] A gasket 28 is provided between the outer casing 16 and the sealing body 17 to ensure the airtightness of the battery interior. A grooved portion 22 is formed on the outer casing 16, a portion of which protrudes inward to support the sealing body 17. The grooved portion 22 is preferably formed in a ring shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17. The sealing body 17 is fixed to the upper part of the outer casing 16 by the grooved portion 22 and the open end of the outer casing 16 to which the sealing body 17 is riveted.

[0021] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cover 27 are stacked sequentially from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a circular or annular shape, and all components except the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at their respective central portions, and the insulating member 25 is sandwiched between their respective peripheral portions. When the internal pressure of the battery rises due to abnormal heat dissipation, the lower valve body 24 deforms and breaks by pushing the upper valve body 26 towards the cover 27 side, thereby blocking the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the opening of the cover 27.

[0022] The following describes in detail the positive electrode 11, negative electrode 12, separator 13 constituting the electrode body 14, and especially the positive electrode active material constituting the positive electrode 11.

[0023] [positive electrode]

[0024] The positive electrode 11 comprises: a positive electrode core and a positive electrode composite material layer disposed on the surface of the positive electrode core. The positive electrode core can be a foil of a metal stable within the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a thin film of the metal disposed on its surface. The positive electrode composite material layer comprises a positive electrode active material, a conductive material, and a binder material, and is preferably disposed on both sides of the positive electrode core. The positive electrode 11 can be manufactured, for example, by coating a positive electrode composite material slurry comprising a positive electrode active material, a conductive material, and a binder material onto the positive electrode core, drying the coating, and then compressing it to form a positive electrode composite material layer on both sides of the positive electrode core.

[0025] Examples of conductive materials included in the positive electrode composite layer include carbon materials such as carbon black, acetylene black, Ketjen black, and graphite. Examples of binder materials included in the positive electrode composite layer include fluoropolymers such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. These resins can also be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, and polyethylene oxide (PEO).

[0026] The positive electrode active material contains the formula Li x Mny Ni z Ge a M b O 2-c F c (In the formula, M is at least one selected from Ti, Co, Si, Al, Nb, W, Mo, P, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Sr, Zr, Ru, K, Bi; 1.0 < x ≤ 1.2, 0.4 ≤ y ≤ 0.8, 0 ≤ z ≤ 0.4, 0 < a < 0.01, 0 < b < 0.03, 0 < c < 0.1, x + y + z + a + b ≤ 2) a lithium transition metal composite oxide. This composite oxide is a Li-excess system material in which the molar ratio of Li to the transition metal exceeds 1, and a predetermined amount of fluoride ions is introduced, and a part of O is replaced by F.

[0027] The positive electrode active material uses the composite oxide represented by the above compositional formula as the main component. Here, the main component means the component with the highest mass ratio among the constituent components of the composite oxide. In the positive electrode 11, as the positive electrode active material, a composite oxide other than the composite oxide represented by the above compositional formula (for example, a composite oxide that is not a Li-excess system or a composite compound that does not contain fluoride ions) may be used in combination. The content of the above composite oxide is preferably 50% by mass or more, and may be substantially 100% by mass. It should be noted that the composition of the composite oxide can be measured using an ICP emission spectroscopic analyzer (iCAP6300 manufactured by ThermoFisher Scientific).

[0028] The lithium transition metal composite oxide represented by the above compositional formula may contain Ni in addition to Li, Mn, and Ge. Two or more elements selected from Ti, Co, Si, Al, Nb, W, Mo, P, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Sr, Zr, Ru, K, Bi are also contained as essential components. Among them, Ti, Co, Nb, Sr, Mg, Al, Si, and W are preferred.

[0029] In the above compositional formula, M is particularly preferably at least one selected from Co and Al. That is, M is any one of (1) Co, (2) Al, and (3) Co and Al. In addition, the molar ratio (b) of M is preferably 0 < b < 0.02, more preferably 0.001 ≤ b ≤ 0.015, and particularly preferably 0.0002 ≤ b ≤ 0.010. When the element M is a combination selected from the above (1) to (3), the improvement effect of the voltage retention rate is more significantly exhibited.

[0030] In the above compositional formula, the molar ratio (x) of Li is 1.0 < x ≤ 1.2, preferably 1.1 ≤ x ≤ 1.2. The molar ratio (y) of Mn is 0.4 ≤ y ≤ 0.8, preferably 0.45 ≤ y ≤ 0.6. The molar ratio (a) of Ge is 0 < a < 0.01, preferably 0.001 ≤ a ≤ 0.007, more preferably 0.002 ≤ a ≤ 0.005. When the molar ratios of Li, Mn, and Ge are within this range, the improvement effect of the voltage retention rate is more significantly demonstrated. Ni is an optional component, but for example, it is preferably contained in an amount of 0.05 ≤ z ≤ 0.3.

[0031] In the lithium transition metal composite oxide represented by the above compositional formula, the total molar amount (x + y + z + a + b) of Li, Mn, Ni, Ge, and M is 2 or less, preferably 2. That is, this composite oxide is preferably a Li-excess type composite oxide and not a cation-excess type composite oxide. In addition, the molar ratio (c) of F is 0 < c ≤ 0.1, preferably 0.05 ≤ x ≤ 0.085. By adding a specified amount of F, the dissolution of the transition metal is suppressed and the durability is improved.

[0032] Specific examples of the ideal lithium transition metal composite oxide are Li-excess type F-containing composite oxides containing Mn, Ni, Ge, and at least one selected from Co and Al. This composite oxide substantially does not contain elements other than, for example, Mn, Ni, Ge, Co, Al, Li, O, and F. The respective molar ratios of Co and Al are preferably 0.01 or less, more preferably 0.001 to 0.007, and particularly preferably 0.002 to 0.005, for example, less than the molar ratio of Ge.

[0033] The lithium transition metal composite oxide of the present embodiment can be synthesized, for example, by mixing a carbonate containing Mn and Ni, a compound containing Ge, Co, Al, etc. (such as germanium oxide, cobalt sulfate, aluminum hydroxide, etc.), and lithium fluoride (LiF), and roasting the mixture. An example of the roasting conditions is 700 to 900 °C for 10 to 30 hours.

[0034] [Negative electrode]

[0035] The negative electrode 12 has: a negative electrode core body, and a negative electrode composite material layer provided on the surface of the negative electrode core body. The negative electrode core body can use a foil of a metal stable within the potential range of the negative electrode 12 such as copper, or a thin film of this metal disposed on the surface layer. The negative electrode composite material layer contains a negative electrode active material and a binder material, and it is preferably provided on both sides of the negative electrode core body. The negative electrode 12 can be produced, for example, as follows: A negative electrode composite material slurry containing a negative electrode active material, a conductive material, and a binder material, etc. is coated on the surface of the negative electrode core body, the coating film is dried and then compressed, and a negative electrode composite material layer is formed on both sides of the negative electrode core body, thereby producing it.

[0036] The negative electrode composite material layer includes, for example, a carbon-based active material that reversibly absorbs and releases lithium ions as the negative electrode active material. Ideal carbon-based active materials include natural graphite such as flake graphite, block graphite, and amorphous graphite, as well as artificial graphite such as blocky graphite (MAG) and graphitized mesophase carbon microspheres (MCMB). Alternatively, the negative electrode active material can be a Si-based active material composed of at least one of Si and Si-containing compounds, or a combination of carbon-based and Si-based active materials can be used.

[0037] Similar to the case of the positive electrode 11, carbon materials such as carbon black, acetylene black, Ketjen black, and graphite can be used as the conductive material contained in the negative electrode composite layer. Similarly to the case of the positive electrode 11, fluoropolymers, PAN, polyimide, acrylic resins, and polyolefins can also be used as the binder material contained in the negative electrode composite layer, but styrene-butadiene rubber (SBR) is preferred. Furthermore, the negative electrode composite layer preferably also contains CMC or its salts, polyacrylic acid (PAA) or its salts, and polyvinyl alcohol (PVA). Ideally, SBR should be used in combination with CMC or its salts, and PAA or its salts.

[0038] [Separator]

[0039] The separator 13 is a porous sheet with ion permeability and insulation. Specific examples of porous sheets include microporous films, woven fabrics, and nonwoven fabrics. Ideal materials for the separator 13 include polyethylene, polypropylene, copolymers of ethylene and α-olefins, and cellulose. The separator 13 can be a single-layer structure or a multilayer structure. A heat-resistant layer containing inorganic particles, an aromatic polyamide resin, polyimide, polyamide-imide, or other resins with high heat resistance can be formed on the surface of the separator 13.

[0040] <Example>

[0041] The present disclosure will be further described below with reference to embodiments, but the present disclosure is not limited to these embodiments.

[0042] <Example 1>

[0043] Synthesis of Lithium Transition Metal Composite Oxides

[0044] A carbonate containing Mn and Ni in a 2:1 molar ratio was mixed with germanium oxide, aluminum hydroxide, and lithium fluoride. The mixture was then calcined at 800°C under an oxygen flow for 20 hours to obtain Li. 1.167 Mn 0.55 Ni 0.275 Ge 0.0025 Al 0.0025 O 1.9 2F 0.08The lithium transition metal composite oxide shown is shown.

[0045] [The production of the positive electrode]

[0046] The aforementioned lithium transition metal composite oxide was used as the positive electrode active material. The positive electrode active material was mixed with acetylene black and polyvinylidene fluoride at a solid content mass ratio of 7:2:1, and N-methyl-2-pyrrolidone (NMP) was used as the dispersion medium to prepare a positive electrode composite slurry. Next, the positive electrode composite slurry was coated onto a positive electrode core formed from aluminum foil. After the coating was dried and compressed, it was cut into the specified electrode size to obtain the positive electrode.

[0047] [Preparation of non-aqueous electrolytes]

[0048] Ethyl carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) are mixed in a specified volume ratio. LiPF6 is then added to this mixed solvent to obtain a non-aqueous electrolyte.

[0049] [Fabrication of the Experimental Battery Cell]

[0050] The positive electrode and the negative electrode formed of lithium metal foil are arranged opposite each other through a separator to form an electrode body, which is then housed in a coin-shaped outer casing. After injecting the non-aqueous electrolyte into the outer casing, the casing is sealed to obtain a coin-shaped experimental battery unit (non-aqueous electrolyte secondary battery).

[0051] For the test battery cells, the voltage retention rate was evaluated using the following method, and the evaluation results are shown in Table 1 together with the composition of the positive electrode active material.

[0052] [Evaluation of voltage maintenance rate]

[0053] The test battery cell was charged and discharged under the following conditions. The voltage maintenance rate was calculated using the average operating voltage (V20) of the 20th cycle and the average operating voltage (V1) of the 1st cycle, according to the following formula.

[0054] Voltage maintenance rate = (V20 / V1) × 100

[0055] Charge / discharge conditions: Charge the battery at a constant current of 0.05C until the battery voltage reaches 5.2V, then pause for 20 minutes. Discharge the battery at a constant current of 0.05C until the battery voltage reaches 2.5V. Repeat this charge / discharge cycle 20 times.

[0056] <Example 2, Comparative Example 1>

[0057] In the synthesis of lithium transition metal composite oxide, the types and mixing ratios of raw materials were changed to obtain the composition shown in Table 1. Except for the fact that the content of Ni and Mn was the same as in Example 1, test battery cells were made in the same manner as in Example 1, and the voltage retention rate was evaluated.

[0058] [Table 1]

[0059]

[0060] As shown in Table 1, compared with the test cell of the comparative examples, the test cell of the examples using a positive electrode active material made by adding at least one element selected from Co and Al to a lithium-excess type F-containing composite oxide containing Mn, Ni, and Ge as a positive electrode active material has a higher voltage retention rate. In particular, the positive electrode active material of Example 2 containing Ge, Al, and Co significantly improves the voltage retention rate of the test cell.

[0061] As mentioned above, adding at least one specific element selected from Co, Al, etc., together with Ge to a lithium-excess F-containing composite oxide containing at least Mn as a transition metal can significantly improve the voltage maintenance rate.

[0062] Explanation of reference numerals in the attached figures

[0063] 10 Non-aqueous electrolyte secondary batteries

[0064] 11 Positive electrode

[0065] 12 Negative electrode

[0066] 13. Separators

[0067] 14 Electrode bodies

[0068] 16 Outer shell cans

[0069] 17 Sealing body

[0070] Insulation boards 18 and 19

[0071] 20 Positive lead

[0072] 21 Negative lead

[0073] 22 Grooved section

[0074] 23 Internal terminal block

[0075] 24 Lower valve body

[0076] 25 Insulating components

[0077] 26 Upper valve body

[0078] 27 Cover

[0079] 28 gaskets

Claims

1. A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising the formula Li x Mn y Ni z Ge a M b O 2-c F c The lithium transition metal composite oxide shown has the formula where M is at least one selected from Co and Al, and 1.0 <x≤1.2、0.4≤y≤0.8、0≤z≤0.4、0.002≤a≤0.005、0.001≤b≤0.015、0<c<0.1、x+y+z+a+b≤2。 2. A non-aqueous electrolyte secondary battery, comprising: a positive electrode containing the positive electrode active material as described in claim 1; a negative electrode; a separator sandwiched between the positive electrode and the negative electrode; and a non-aqueous electrolyte.

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