WO2024236864A1 - Coin-shaped lithium primary battery - Google Patents

Abstract

This coin-shaped lithium primary battery comprises a positive electrode, a negative electrode, a separator which is disposed between the positive electrode and the negative electrode, and an electrolyte solution. The positive electrode contains manganese dioxide, graphite, a binder, and a boron compound. The thickness T (mm) of the positive electrode and the average major axis length (mm) of the graphite satisfy formula (1) 0.02 ≤ T (mm) × DL (mm) ≤ 0.12.

Description

Coin-type lithium primary battery

This disclosure relates to coin-type lithium primary batteries.

Patent Document 1 proposes a non-aqueous battery including "an active metal negative electrode, a positive electrode, and an ionically conductive electrolyte solution containing a solute dissolved in a non-aqueous electrolyte, the solute being a salt of a first component which is a halide of an element selected from the group consisting of Al, Sb, Zr, and P, and a second component which is a halide, sulfide, sulfite, oxide, or carbonate of calcium or an alkali metal selected from the group consisting of Li, Na, and K, the active metal negative electrode having a surface layer of a boron-containing material."

Patent Document 2 proposes "a lithium primary battery comprising a positive electrode made of manganese dioxide, a negative electrode made of lithium or a lithium alloy, and an electrolyte, characterized in that the manganese dioxide is manganese dioxide to which boron has been added, and the electrolyte contains butylene carbonate as a solvent."

Patent Document 3 proposes "a nonaqueous electrolyte battery including a positive electrode, a negative electrode, and an electrolyte, in which the positive electrode comprises a positive electrode mixture including electrolytic manganese dioxide as an active material, expanded graphite as a conductive agent, and a binder, and the ratio of the median diameter of the expanded graphite to the median diameter of the electrolytic manganese dioxide is 1.5 to 3."

Japanese Patent Application Publication No. 59-117070 JP 2007-134270 A JP 2019-102152 A

Coin-type lithium primary batteries have a sealed structure, but there is a large increase in internal resistance (IR) during storage. The main cause of the increase in IR is the phenomenon in which moisture in the outside air that enters from the outside reacts with the negative electrode and forms an inactive film on the surface of the negative electrode. Coin-type batteries have a small internal volume, which makes it easy for the concentration of moisture that enters to increase, so an inactive film can form in a short period of time, causing a significant increase in IR.

In view of the above, one aspect of the present disclosure relates to a coin-type lithium primary battery comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, the positive electrode comprising manganese dioxide, graphite, a binder, and a boron compound, and the thickness T (mm) of the positive electrode and the average length DL (mm) of the major axis of the graphite satisfy the formula (1): 0.02≦T (mm)×DL (mm)≦0.12.

The coin-type lithium primary battery disclosed herein can suppress the increase in internal resistance caused by moisture penetration during storage.

FIG. 1 is a vertical cross-sectional view of a coin-type lithium primary battery according to an embodiment of the present disclosure.

Below, examples of embodiments of the present disclosure are described, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, etc. may be exemplified, but other numerical values, materials, etc. may be applied as long as the effects of the present disclosure are obtained. Note that publicly known secondary battery components may be applied to components other than the characteristic parts of the present disclosure. In this specification, when a "range between numerical value A and numerical value B" is mentioned, the range includes numerical value A and numerical value B. In the following description, when a lower limit and an upper limit are exemplified for numerical values of specific physical properties, conditions, etc., any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit.

The present disclosure encompasses a combination of the features of two or more claims arbitrarily selected from the multiple claims set forth in the appended claims. In other words, the features of two or more claims arbitrarily selected from the multiple claims set forth in the appended claims may be combined, provided that no technical contradiction arises.

The coin-type lithium primary battery according to the embodiment of the present disclosure includes, as power generating elements, a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte. The power generating elements are hermetically housed within a specified exterior body.

The positive electrode is formed by pressurizing the positive electrode mixture into a predetermined shape. The positive electrode mixture contains manganese dioxide, graphite, a binder, and a boron compound. The shape of the positive electrode is a pellet or a cylinder (disk) according to the shape of the coin-type lithium primary battery. The thickness T of the positive electrode is not particularly limited, but is, for example, 400 μm or more and 2000 μm or less.

The negative electrode is formed, for example, by processing a metal sheet or metal foil into a predetermined shape. At least one of lithium metal and lithium alloys can be used for the metal sheet. Examples of lithium alloys include lithium-aluminum alloys, lithium-tin alloys, lithium-silicon alloys, and lithium-lead alloys. The shape of the negative electrode is a disk that corresponds to the shape of a coin-type lithium primary battery. Lithium metal or lithium alloy foil may be punched into a circle and used as the negative electrode.

The exterior body includes, for example, a case having a bottom plate portion and a side portion rising from the periphery of the bottom plate portion and having a first terminal surface on its outer surface, a sealing plate having a top plate portion and a periphery portion extending from the top plate portion to the inside of the side portion of the case and having a second terminal surface on its outer surface, and a gasket compressed and interposed between the side portion and the periphery portion. The end of the side portion of the case is crimped to the sealing plate via the gasket.

Inside the exterior body, the following three reactions mainly take place competitively on the surface of the negative electrode: (A) Reaction between infiltrating moisture and the surface of the negative electrode to form a non-conductive film (A).
(B) A reaction of a boron-containing intermediate produced by contacting a boron compound with manganese dioxide with the negative electrode surface to form an intermediate film (B).
(C) A reaction of a boron-containing product produced from a boron-containing intermediate with the negative electrode surface to form a coating (C).

The boron-containing intermediate can exist in various forms, but is thought to be an ionic species generated by the decomposition of boron compounds by the catalytic action of manganese dioxide. The intermediate film (B) formed by the reaction of the boron-containing intermediate with the negative electrode surface has a lower resistance than the non-conductive film (A) derived from water that is generated by the reaction of moisture with the negative electrode surface, and can suppress the increase in internal resistance (IR), but this is not sufficient.

The boron-containing product may exist in various forms, but according to the analysis results, it is believed to exist in the electrolyte mainly as ionic species such as _{Li2BO2 cation} and _LiBO2 anion. Hereinafter, the boron-containing product is also referred to as "LBO ion".

The film (C) formed by the reaction of LBO ions with the negative electrode surface suppresses the increase in IR by sacrificially reacting with water. In other words, it is thought that film (C) is repeatedly formed and disappeared. Therefore, film (C) has little effect on the battery reaction and suppresses the formation of a non-conductive film derived from water. By containing a boron compound in the positive electrode, the LBO ions necessary for repairing film (C) are continuously supplied. This significantly suppresses the increase in internal resistance caused by moisture entering the battery during storage.

Here, the thickness T (mm) of the positive electrode and the average length DL (mm) of the major axis of the graphite are
Formula (1): 0.02≦T(mm)×DL(mm)≦0.12
Meet the following.

The product of T and DL in mm (T DL) is 0.02 or more, may be 0.04 or more, or may be 0.05 or more. The larger T DL is, the thicker the positive electrode is and the flatter the graphite particles are.

The boron compounds move through the electrolyte in the positive electrode while repeatedly coming into contact with manganese dioxide. The basal surfaces of the flattened graphite particles act as a barrier to the movement of the boron compounds. The boron compounds move around the graphite particles, so their path of movement in the positive electrode becomes longer. This increases the opportunities for contact between the boron compounds and manganese dioxide, and a larger amount of the boron compounds changes to boron-containing intermediates, which then change to LBO ions before reaching the negative electrode surface. As a result, the above (C) "the reaction of forming a good conductor film by the reaction between the boron-containing intermediates and the negative electrode surface, which produces a boron-containing product" becomes more dominant. In other words, the larger the T·DL, the more dominant the above reaction (C) becomes.

In addition, since there is an abundance of electrolyte in the separator, boron-containing products that reach the electrolyte in the separator can quickly reach the surface of the negative electrode.

On the other hand, T·DL is 0.12 or less, and may be 0.10 or less, or 0.09 or less. If T·DL is too large, the positive electrode becomes excessively thick or the graphite particles become excessively flattened. If T·DL is too large, the path of movement of LBO ions in the positive electrode becomes too long, and the above (A) "reaction of forming a non-conductive film due to the reaction between the invading moisture and the negative electrode surface" becomes more dominant. In other words, if T·DL is too large, it is difficult to suppress the increase in the IR of the battery.

Graphite has a basal surface and an edge surface. The basal surface is the surface that determines the length of the long axis of the graphite. The edge surface is the end surface of the layered structure, and is much shorter than the basal surface. In other words, most graphite particles have a flat shape with a long axis and a short axis. The average length of the long axis (DL) and the average length of the short axis (DW) can be measured, for example, by the following procedure using a scanning electron microscope photograph (SEM image) of the cross section of the positive electrode. The aspect ratio of graphite is defined as the ratio of DL/DW.

(1) Randomly select 200 or more graphite particles from the SEM image.

(2) Determine the rectangle min with the smallest area that encloses each particle.

(3) Measure the length of the long side of the rectangle min as the length of the particle's long axis (dL).

(4) Measure the length of the short side of the rectangle min as the length of the particle's short axis (dW).

(5) The lengths (dL) of the long sides of the rectangles (min) of 200 or more particles are averaged to obtain the average length of the long axis (DL). DL is generally between 10 μm and 100 μm, and may be between 20 μm and 80 μm.

(6) The lengths of the short sides (dW) of the rectangles (min) of 200 or more particles are averaged to obtain the average length of the short axis (DW). DW is generally 0.1 μm to 5 μm, but may be 0.5 μm to 4 μm or 1 μm to 3 μm.

(7) Calculate the aspect ratio as the ratio of DL/DW. The aspect ratio is generally between 2 and 1000, but can also be between 5 and 800.

Graphite acts as a conductive additive. The positive electrode mixture contains graphite, ensuring sufficient electronic conduction paths within the positive electrode and ensuring sufficient output.

The graphite may include expanded graphite. The expanded graphite particles have a flat shape. The expanded graphite is prepared, for example, by inserting sulfuric acid between layers of the basal plane of graphite and then foaming. The expanded graphite has a large interplanar spacing in the c-axis direction perpendicular to the basal plane, and is prone to peeling and becoming flat. The interplanar spacing (d002) of the (002) plane of the expanded graphite obtained by powder X-ray diffraction using CuKα radiation may be, for example, 3.37 Å or more. The crystallite size Lc(002) in the c-axis direction may be, for example, 500 Å or less. The expanded graphite contains a trace amount of residual sulfate ions (SO ₃ ⁻ ).

The interplanar spacing (d002) is calculated using the Bragg formula (λ = 2d × sinθ) by X-ray diffraction.

λ: wavelength of CuKα radiation (= 0.15418 nm)
d: average interplanar spacing of (002) plane d002
θ: 1/2 angle (rad) of 2θ at the peak position determined by the centroid method
The crystallite size Lc(002) can be calculated from the half-width of the X-ray diffraction peak assigned to the (002) plane using Scherrer's formula (D(nm)=0.9×λ/(β×cos θ)).

λ = wavelength of CuKα β = half-width (rad) of the above peak
θ = 1/2 angle (rad) of 2θ of the peak position determined by the centroid method
The content of graphite contained in the positive electrode (i.e., positive electrode mixture) is preferably, for example, 1% by mass or more and 10% by mass or less, and more preferably 1% by mass or more and 5% by mass or less. If the content of graphite is 1% by mass or more, the movement path of the boron compound or the boron-containing intermediate becomes sufficiently long, and a sufficient amount of LBO ions can reach the negative electrode surface. If the content of graphite is 10% by mass or less, the movement path of the boron compound or the boron-containing intermediate does not become excessively long, and it is possible to avoid the formation of a non-conductor film derived from water becoming dominant. In addition, if the content of graphite is 1% by mass or more and 10% by mass or less, the function as a conductive assistant can be sufficiently secured while securing the positive electrode capacity. The content of expanded graphite in graphite is, for example, 80% by mass or more, may be 90% by mass or more, or may be 100%.

The boron compound is a source of LBO ions. The boron compound may be, for example, at least one selected from the group consisting of boron oxide and lithium-containing boron oxide. It is believed that, regardless of the type of boron compound, LBO ions (ionic species such as Li ₂ BO ₂ cation and LiBO ₂ anion) are ultimately generated in the electrolyte as a boron-containing product. In particular, it is highly likely that ionic species having a LiBO ₂ structure are generated in the electrolyte in the positive electrode, and the LiBO ₂ structure is maintained in the coating on the negative electrode surface.

The presence of LBO ions can be investigated by, for example, inferring the state of boron elements in the film using time-of-flight secondary ion mass spectrometry (TOF-SIMS).

As the boron compound to be mixed in the positive electrode mixture, for example, at least one selected from the group consisting of B ₂ O ₃ , Li ₂ B ₄ O ₇ , Li ₃ BO ₃ and LiBO ₂ can be used. All of these are suitable as a source of LBO ions. Among them, at least one selected from the group consisting of B ₂ O ₃ and LiBO ₂ is currently considered to be most suitable as a source of LBO ions.

The content of the boron compound contained in the positive electrode is, for example, 0.5% by mass or more and 5% by mass or less, and may be 0.8% by mass or more and 3% by mass or less. If the content of the boron compound is 0.5% by mass or more, it is sufficient to continuously supply the LBO ions necessary for repairing the coating (C). This significantly suppresses the increase in internal resistance caused by moisture entering the battery during storage. If the content of the boron compound is 5% by mass or less, it is possible to maximize the effect of suppressing the increase in internal resistance while ensuring sufficient positive electrode capacity.

The ratio Ve/(Vp+Vs), based on the volume Ve of the electrolyte in the battery and the total volume Vt of the void volume Vp of the positive electrode and the void volume Vs of the separator, i.e., Ve/Vt, is preferably 0.9 to 1.5, and more preferably 1.0 to 1.2.

When the volume Ve of the electrolyte in the battery is large and the Ve/Vt ratio exceeds 1.5, a part of the electrolyte may be present outside the power generating element without being impregnated into the positive electrode, separator, and negative electrode. Then, the boron compound that has dissolved from the positive electrode into the electrolyte may move through the electrolyte present outside the power generating element and reach the negative electrode surface. Therefore, the boron compound or boron-containing intermediate before being converted into LBO ions may react on the negative electrode surface, and the formation of a film derived from LBO ions may be insufficient. On the other hand, when the volume Ve of the electrolyte in the battery is small and the Ve/Vt ratio is less than 0.9, the movement of LBO ions in the positive electrode may be slowed down, and the above-mentioned (A) reaction of forming a non-conductive film becomes dominant, making it difficult to suppress the increase in the IR of the battery.

The volume Ve of the electrolyte in the battery can be calculated from the mass of the electrolyte contained in the battery and the specific gravity of the electrolyte. The mass of the electrolyte can be calculated by disassembling the battery, drying the power generating elements, and calculating the mass difference between the mass of the battery before disassembly and the mass of the battery after disassembly and drying.

The void volume Vp of the positive electrode can be measured, for example, by the following procedure using a scanning electron microscope (SEM) image of the cross section of the positive electrode.

(1) Take an SEM image that includes a cross section of a sufficiently large positive electrode (positive electrode mixture).

(2) Using image processing, the area Se occupied by the electrolyte or voids in the cross-sectional area Sp of a given positive electrode (positive electrode mixture) is determined.

(3) Calculate the void volume of the positive electrode, Vp = Sp x Se/Sp.

The void volume Vs of the separator can be measured, for example, using a scanning electron microscope (SEM) image of the separator cross section by the following procedure.

(1) Take an SEM image that includes a cross section of a separator of sufficient size.

(2) Using image processing, the area Sse occupied by the electrolyte or voids in the cross-sectional area Ss of a given separator is determined.

(3) Calculate the void volume of the separator, Vs = Ss x Sse/Ss.

The electrolyte contains a non-aqueous solvent and a solute (salt). The solute concentration in the electrolyte is, for example, 0.3 mol/L to 2 mol/L. As the non-aqueous solvent, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, or the like can be used. These may be used alone or in combination of two or more. As the solute, LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN(CF ₃ SO ₂ ) ₂ , or the like can be used. These may be used alone or in combination of two or more.

The manganese dioxide may be electrolytic manganese dioxide. The electrolytic manganese dioxide may be neutralized with an alkali and then calcined. For example, the electrolytic manganese dioxide may be calcined in air or oxygen at 300-450°C for about 6-12 hours. Calcination causes water to evaporate, promotes crystallization, and also reduces the specific surface area. This improves the structural stability and water-resistant reactivity of the manganese dioxide. When using uncalcined electrolytic manganese dioxide, the degree of crystallinity may be increased and the specific surface area reduced depending on the conditions during electrolytic synthesis.

The volume-based median diameter (D50) of manganese dioxide measured by a laser diffraction scattering type measuring device is, for example, 20 μm to 50 μm, or may be 20 μm to 45 μm, or may be 20 μm to 35 μm. If the median diameter of manganese dioxide is 20 μm or more, the opportunities for contact between the graphite and the boron compound in the positive electrode increase, and the number of contact points between the active materials also increases. On the other hand, if the median diameter of manganese dioxide is 50 μm or less, the pressure required to form the positive electrode mixture is reduced, making it easier to increase the packing density.

Examples of binders that can be used include fluororesins, rubber particles, and acrylic resins. Examples of fluororesins that can be used include polytetrafluoroethylene (hereinafter also referred to as "PTFE"), tetrafluoroethylene-hexafluoropropylene copolymer, and polyvinylidene fluoride. Examples of rubber particles that can be used include styrene butadiene rubber (SBR) and modified acrylonitrile rubber. Examples of acrylic resins include ethylene-acrylic acid copolymers. One type of binder may be used alone, or two or more types may be used in combination.

Among the binders, fibrous fluororesin is preferred. Fibrous fluororesin does not excessively cover the surface of the manganese dioxide and does not significantly impede contact between the surface of the manganese dioxide and the electrolyte (and further the boron compound or boron-containing intermediate). The fibrous fluororesin may be a fluororesin that does not dissolve in N-methyl-2-pyrrolidone (NMP) and disperses as particles. The fibrous fluororesin may be a fluororesin that fibrillates due to the pressure applied to the positive electrode mixture when preparing the positive electrode.

Among the fibrous fluororesins, for example, PTFE is preferable. PTFE is a fluororesin containing tetrafluoroethylene units (-CF ₂ CF ₂ -) as a main component. The content of tetrafluoroethylene units in PTFE is, for example, 90 mol % or more, and may be 95 mol % or more. The weight average molecular weight Mw of PTFE may be, for example, 200,000 to 800,000.

The binder content in the positive electrode is, for example, 1% by mass or more and 10% by mass or less, and may be 1% by mass or more and 5% by mass or less. If the binder content is 1% by mass or more, it is possible to ensure sufficient strength of the positive electrode while ensuring sufficient positive electrode capacity. If the binder content in the positive electrode is 10% by mass or less, it is possible to maximize the effect of suppressing an increase in internal resistance while ensuring sufficient positive electrode capacity. The PTFE content in the binder is, for example, 80% by mass or more, and may be 90% by mass or more, or may be 100%.

The separator may be made of any material that can prevent short circuits between the positive and negative electrodes and retain the electrolyte. Examples include woven fabrics, nonwoven fabrics, and microporous films made of polyolefins, polyesters, etc. In particular, nonwoven fabrics made of polypropylene are preferred.

FIG. 1 is a longitudinal cross-sectional view of a coin-type lithium primary battery according to one embodiment of the present disclosure. The coin-type battery 11 has an exterior body composed of a case 3, a sealing plate 8, and a gasket 7. The case 3 is a cylindrical, shallow-bottomed battery can having a bottom plate portion 3a and a side portion 3b that rises from the periphery of the bottom plate portion 3a via a first bend portion 9. The sealing plate 8 has a top plate portion 8a and a peripheral portion 8b that extends from the top plate portion 8a to the inside of the side portion 3b of the case 3. The end portion 3t (opening end) of the side portion 3b of the case 3 is bent inward at a second bend portion 10 and crimped to the peripheral portion 8b of the sealing plate 8 via the gasket 7, thereby sealing the gap between the case 3 and the sealing plate 8.

The power generating element is housed inside the exterior body. The power generating element includes a positive electrode 4, a negative electrode 5, a separator 6, and an electrolyte. In the illustrated example, the positive electrode 4 is arranged to face the bottom plate portion 3a of the case 3. Therefore, the outer surface of the bottom plate portion 3a functions as the positive electrode terminal surface. On the other hand, the negative electrode 5 is arranged to face the top plate portion 8a of the sealing plate 8. Therefore, the outer surface of the top plate portion 8a functions as the negative electrode terminal surface.

The above describes an example of a coin-type lithium primary battery, but the shape of the battery is not particularly limited. There are also no particular limitations on the thickness and diameter of the battery. Coin-type lithium primary batteries also include button-type lithium primary batteries.

(Additional Note)
The above description discloses the following techniques.

(Technique 1)
A positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte solution;
the positive electrode comprises manganese dioxide, graphite, a binder, and a boron compound;
The thickness T (mm) of the positive electrode and the average length DL (mm) of the major axis of the graphite are expressed by the following formula:
0.02≦T (mm)×DL (mm)≦0.12
A coin-type lithium primary battery that meets the above requirements.

(Technique 2)
2. The coin-type lithium primary battery according to claim 1, wherein the graphite comprises expanded graphite.

(Technique 3)
3. The coin-type lithium primary battery according to claim 1, wherein the content of the graphite in the positive electrode is 1% by mass or more and 10% by mass or less.

(Technique 4)
The coin-type lithium primary battery according to any one of claims 1 to 3, wherein the boron compound is at least one selected from the group consisting of boron oxides and lithium-containing boron oxides.

(Technique 5)
The coin-type lithium primary battery according to any one of the first to fourth aspects, wherein the boron compound is at least one selected from the group consisting of B ₂ O ₃ , Li ₂ B ₄ O ₇ , Li ₃ BO ₃ and _LiBO 2.

(Technique 6)
The coin-type lithium primary battery according to any one of claims 1 to 5, wherein the content of the boron compound contained in the positive electrode is 0.5% by mass or more and 5% by mass or less.

(Technique 7)
The coin-type lithium primary battery according to any one of the first to sixth aspects, wherein the ratio of the volume of the electrolyte to the void volume of the positive electrode plus the void volume of the separator is 0.9 or more and 1.5 or less.

(Technique 8)
The coin-type lithium primary battery according to any one of claims 1 to 7, wherein the binder contains a fibrous fluororesin.

(Technique 9)
The coin-type lithium primary battery according to claim 8, wherein the fluororesin comprises polytetrafluoroethylene.

<Example>
The present disclosure will be specifically described below based on examples. However, the present invention is not limited to the following examples. In the examples, a coin-type lithium primary battery having a structure as shown in FIG. 1 was fabricated.

Example 1
(1) Positive electrode As a positive electrode active material, electrolytic manganese dioxide was calcined in air at 400 ° C. for 10 hours to prepare calcined electrolytic manganese dioxide. 95 parts by mass of calcined manganese dioxide, 2 parts by mass of expanded graphite (average length of major axis (DL): 0.06 mm obtained by disassembling the battery after preparation and the method described above) and 1 part by mass of LiBO ₂ were dry-mixed. Next, an aqueous dispersion containing 2 parts by mass of PTFE was added to the obtained mixed powder, and then wet-mixed. The obtained mixture was dried to obtain a positive electrode mixture. The positive electrode mixture was tableted into a cylindrical shape with a diameter of 15 mm and a thickness (T) of 0.35 mm (T · DL = 0.021) to prepare a positive electrode. The cylindrical positive electrode pellets were dried at 250 ° C. for 8 hours.

(2) Negative Electrode A negative electrode was prepared by punching out a lithium metal foil having a thickness of 0.15 mm into a circle having a diameter of 16 mm.

(3) Electrolyte An electrolyte was obtained by dissolving lithium perchlorate (LiClO ₄ ) as a solute at a concentration of 1.0 mol/L in a non-aqueous solvent prepared by mixing propylene carbonate and 1,2-dimethoxyethane in a volume ratio of 2:1.

(4) Case A case having a bottom plate diameter of 20 mm and a side portion 1b height of 1.1 mm was fabricated by drawing SUS430 (thickness 250 μm) having a nickel plating layer of 3 μm on its surface.

(5) Sealing Plate A sealing plate 8 having a top plate portion 8a with a diameter of 17 mm was produced by pressing SUS430 (thickness 250 μm) having a nickel plating layer with a thickness of 3 μm on its surface.

(6) Assembly of a coin-shaped battery A polypropylene gasket was placed on the sealing plate. A negative electrode was attached to the inside of the top plate of the sealing plate. Next, a 300 μm thick polypropylene nonwoven fabric was placed on the negative electrode as a separator. Then, a positive electrode was placed on the separator. Then, an electrolyte was poured into the sealing plate. A sealant made of blown asphalt and mineral oil was applied to the inside of the side of the case in advance, and the case was placed on the sealing plate. The end of the side of the case was curved inward and crimped to the peripheral edge of the sealing plate via the gasket to prepare a coin-shaped battery (battery E1).

The thickness of the assembled battery is 1.2 mm. The assembled battery (approximately 3.5 V) was pre-discharged for a specified amount of electrical capacity so that the battery voltage was 3.2 V. The battery capacity after pre-discharge was 230 mAh when the discharge capacity was measured up to 2 V after constant resistance discharge (15 kΩ).

The ratio of the volume of the electrolyte in the battery (Ve) to the void volume of the positive electrode (Vp) + the void volume of the separator (Vs) was Ve/(Vp+Vs) = Ve/Vt = 1.2.

Examples 2 to 8
Batteries E2 to E8 were produced in the same manner as in Example 1, except that the battery diameter, battery thickness, positive electrode thickness (T), average length of the major axis of the expanded graphite (DL), and T·DL were changed as shown in Table 1A.

Comparative Examples 1 to 5
Batteries R1 to R5 were produced in the same manner as in Example 1, except that the battery diameter, battery thickness, positive electrode thickness (T), average length of the major axis of the expanded graphite (DL), and T·DL were changed as shown in Table 1B.

[Battery evaluation]
Ten of each of the produced batteries were stored for 100 days in an environment of 60° C. and 90% relative humidity. After storage, the AC internal resistance was measured at room temperature under conditions of a voltage amplitude of 0.2 V and a frequency of 100 Hz.

Long-term storage of coin batteries at room temperature can be accelerated by storing them in an environment of 60°C and 90% relative humidity. Storage in this environment for 100 days is considered to be equivalent to storage at room temperature for 10 years. The average values calculated for 10 batteries are shown in Tables 1A and 1B.

Table 1A shows that the internal resistance remains small when 0.02≦T·DL≦0.12 is satisfied. This is thought to be the result of more boron compounds being converted to LBO ions before reaching the negative electrode surface, rather than moisture reaching the negative electrode surface and reacting with them.

Table 1B shows that the internal resistance increases significantly when T·DL<0.02 or 0.12<T·DL. This is thought to be because the reaction between moisture and the negative electrode surface is more dominant than the reaction between LBO ions and the negative electrode surface.

Examples 9 to 12
Batteries E9 to E12 were produced and evaluated in the same manner as in Example 5, except that the total mass of the expanded graphite and manganese dioxide was kept constant and the content of the expanded graphite was changed as shown in Table 2. The results are shown in Table 2.

Table 2 shows that the content of expanded graphite in the positive electrode (positive electrode mixture) is preferably 1% by mass to 10% by mass or less.

Examples 13 to 15
Batteries E13 to E15 were produced and evaluated in the same manner as in Example 5, except that the type of boron compound was changed as shown in Table 3. The results are shown in Table 2.

Table 3 shows that the effect of suppressing an increase in internal resistance can be obtained regardless of the type of boron compound contained in the positive electrode (positive electrode mixture), and that among boron compounds, _LiBO2 is highly effective.

Examples 16 to 17
Batteries E16 to E17 were produced and evaluated in the same manner as in Example 5, except that the type of graphite was changed to flake graphite or graphene having a major axis length DL of 0.04 mm as shown in Table 4. The results are shown in Table 4.

Table 4 shows that expanded graphite is highly effective as graphite to be included in the positive electrode (positive electrode mixture).

Examples 18 to 21
Batteries E18 to E21 were produced and evaluated in the same manner as in Example 5, except that the Ve/Vt ratio was changed as shown in Table 5. The results are shown in Table 5.

Table 5 shows that the Ve/Vt ratio is preferably in the range of 0.9 to 1.5.

Example 22
A battery E22 was produced and evaluated in the same manner as in Example 5, except that the binder was changed to a tetrafluoroethylene-hexafluoropropylene copolymer (FEP) as shown in Table 6. The results are shown in Table 6.

Table 6 shows that it is preferable to use fibrous PTFE as the binder.

As described above, the coin-type lithium primary battery disclosed herein suppresses an increase in internal resistance even after long-term storage at room temperature in an unused state. This makes it possible to extend the recommended usage period of the battery, and is of great industrial value. In addition, the coin-type lithium primary battery is suitable for use as a power source for small devices, memory backup, etc.

Reference Signs List 3: Case 3a; Bottom plate portion 3b; Side portion 3t; End portion 4: Positive electrode 5: Negative electrode 6: Separator 7: Gasket 8: Sealing plate 8a: Top plate portion 8b: Peripheral portion 9: First curved portion 10: Second curved portion 11: Coin-type lithium primary battery

Claims (9)

A positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte solution;
the positive electrode comprises manganese dioxide, graphite, a binder, and a boron compound;
The thickness T (mm) of the positive electrode and the average length DL (mm) of the major axis of the graphite are expressed by the following formula:
0.02≦T (mm)×DL (mm)≦0.12
A coin-type lithium primary battery that meets the above requirements. The coin-type lithium primary battery of claim 1, wherein the graphite includes expanded graphite. The coin-type lithium primary battery according to claim 1, wherein the graphite content in the positive electrode is 1% by mass or more and 10% by mass or less. The coin-type lithium primary battery according to claim 1, wherein the boron compound is at least one selected from the group consisting of boron oxides and lithium-containing boron oxides. 5. The coin-type lithium primary battery according to claim 1, wherein the boron compound is at least one selected from the group consisting of B ₂ O ₃ , Li ₂ B ₄ O ₇ , Li ₃ BO ₃ and LiBO ₂ . The coin-type lithium primary battery according to any one of claims 1 to 4, wherein the content of the boron compound contained in the positive electrode is 0.5% by mass or more and 5% by mass or less. The coin-type lithium primary battery according to any one of claims 1 to 4, wherein the ratio (Ve/(Vp+Vs) based on the volume Ve of the electrolyte, the void volume Vp of the positive electrode, and the void volume Vs of the separator is 0.9 or more and 1.5 or less. The coin-type lithium primary battery according to any one of claims 1 to 4, wherein the binder contains a fibrous fluororesin. The coin-type lithium primary battery according to claim 8, wherein the fluororesin includes polytetrafluoroethylene.

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