Patent ID: 12249725

EMBODIMENTS FOR CARRYING OUT THE INVENTION

The all-solid-state battery according to an embodiment includes: an exterior can with a bottom; a seal can having a flat portion and facing the exterior can; and a power generation element contained between the exterior can and the seal can, the power generation element including a cathode member, an anode member and a solid electrolyte layer located between the cathode member and the anode member, where at least one of an inner surface of the bottom of the exterior can and an inner surface of the flat portion of the seal can may include a recess-protrusion structure. The battery may include a conductive sheet located between the at least one inner surface including the recess-protrusion structure and the power generation element, the conductive sheet having an ability to recover against a pressing force. A rate of recovery of the conductive sheet against a pressing force may be not lower than 7%.

As used herein, rate of recovery is defined by the expression indicated below, where t1 is the thickness of the conductive sheet, t2 is the thickness of the conductive sheet as compressed by a predetermined pressing force, and t3 is the thickness of the conductive sheet as found when the pressing force has been removed. Further, a conductive sheet is deemed to be able to recover if its rate of recovery is equal to or higher than a predetermined level.
(t3−t2)/(t1−r2)×100(%)

Rate of recovery can be measured by the method described in the Japanese Industry Standards (JIS), R3453 2001 (joint sheets).

The conductive sheet may be a graphite sheet.

Since a conductive sheet, such as a graphite sheet or conductive tape, is provided between that one of the inner surface of the bottom of the exterior can and the inner surface of the flat portion of the seal can which includes a recess-protrusion structure, on one hand, and the power generation element, on the other, this will prevent the power generation element from contacting the recess-protrusion structure and being damaged.

The conductive sheet, able to recover against a pressing force, has good flexibility as well as good conductivity. Thus, the conductive sheet can function as a current collector and, at the same time, absorb expansion and contraction resulting from charging and discharging of the power generation element or a pressing force during crimping of the exterior can over the seal can. Thus, the all-solid-state battery can reduce a decrease in battery performance resulting from damage to the power generation element or formation of a gap.

Further, the conductive sheet has an appropriate ability to recover, i.e., a rate of recovery not lower than 7%, against expansion of the power generation element due to charging or compression due to a pressing force during crimping of the exterior can over the seal can. Thus, in the all-solid-state battery, the conductive sheet presses the power generation element to an appropriate degree, thereby maintaining good conductivity between the inner surface of the bottom of the exterior can and the power generation element and/or maintaining good conductivity between the inner surface of the flat portion of the seal can and the power generation element, thus maintaining the battery's performance.

Because of the recess-protrusion structure formed on at least one of the inner surface of the bottom of the exterior can and the inner surface of the flat portion, the conductive sheet has an increased area of contact with the at least one inner surface having the recess-protrusion structure, i.e., an increased current collection area. This allows the all-solid-state battery to yet better maintain battery performance.

If the conductive sheet is a graphite sheet, the graphite sheet may preferably have an apparent density of 0.3 to 1.5 g/cms. This takes account of the fact that an excessively low apparent density reduces the conductivity of the graphite sheet and an excessively high apparent density reduces flexibility.

Preferably, the conductive sheet may have a thickness of 0.05 to 0.5 mm. This takes account of the fact that an excessively small thickness results in an insufficient ability of the conductive sheet to recover against compression, and an excessively large thickness means that the conductive sheet will occupy a significant portion of the interior space of the all-solid-state battery, necessitating a reduced capacity of the power generation element.

Embodiment 1

Now, an embodiment of the present disclosure with a conductive sheet constituted by a graphite sheet, Embodiment 1, will be specifically described with reference toFIG.1. First, as shown inFIG.1, an all-solid-state battery1generally includes an exterior can2, a seal can3, a power generation element4, a graphite sheet5located between the exterior can2and power generation element4, and another graphite sheet5located between the seal can3and power generation element4. In the present embodiment, the all-solid-state battery1is a flat battery. In other implementations, the conductive sheet may be a conductive tape.

The exterior can2includes a circular bottom21and a cylindrical side wall22that extends contiguously from the periphery of the bottom21and is cylindrical in shape. As viewed in a longitudinal cross section, the cylindrical side wall22extends generally perpendicularly to the bottom21. The exterior can2is formed from a metallic material, such as stainless steel.

The inner surface of the bottom21of the exterior can2has a recess-protrusion structure. Recesses23are formed on the inner surface of the bottom21by knurling. The recesses23are located in the area of the inner surface of the bottom21that faces the lower surface of the power generation element4.FIG.2is a plan view of the inner surface of the bottom21of the exterior can2, illustrating its structure. As shown inFIG.2, the recesses are generally shaped as a grid, including a plurality of grooves extending vertically as viewed in the drawing and uniformly spaced apart from each other and a plurality of grooves extending horizontally as viewed in the drawing and uniformly spaced apart from each other. Protrusions24are provided adjacent to the recesses23. As the recess-protrusion structure is arranged in this manner, the graphite sheet5, discussed further below, has an increased area of contact with the inner surface of the bottom21, i.e., current collection area. It will be understood that the recesses23are not limited to an arrangement that is generally grid-shaped as viewed in a plan view. For example, as viewed in a plan view, the recesses23may be shaped as vertical stripes extending vertically and parallel to each other, or may be shaped as polka dots, where a plurality of circular or ring-shaped recesses23are arranged in a balanced manner, for example. Conversely, a plurality of circular or ring-shaped protrusions24may be arranged in a balanced manner to form polka dots, for example. Further, the recess-protrusion structure includes implementations in which recesses23are provided only in a limited area of the bottom21, and implementations in which protrusions24are provided only in a limited area of the bottom21. Since the inner surface of the bottom21of the exterior can2has a recess-protrusion structure, there is higher friction between the bottom21of the exterior can2and the graphite sheet5(seeFIG.1). This prevents positional displacement of the graphite sheet5upon receiving vibration or impact.

As shown inFIG.3, a recess23has a depth a of 0.01 mm, an opening width b of 0.06 mm, and a bottom width c of 0.05 mm. A protrusion24has a width d of 0.49 mm.

The depth a of the recess23is preferably not smaller than 0.005 mm, and more preferably not smaller than 0.007 mm; the depth a is preferably not larger than 0.02 mm, and more preferably not larger than 0.015 mm. The opening width b of the recess23is preferably not smaller than 0.03 mm, and more preferably not smaller than 0.04 mm; the width bis preferably not larger than 0.09 mm, and more preferably not larger than 0.08 mm. The bottom width c of the recess23is preferably not smaller than 0.02 mm, and more preferably not smaller than 0.03 mm; the width cis preferably not larger than 0.08 mm, and more preferably not larger than 0.07 mm. The width d of the protrusion24is preferably not smaller than 0.30 mm, and more preferably not smaller than 0.35 mm; the width dis preferably not larger than 0.70 mm, and more preferably not larger than 0.65 mm.

As the recess-protrusion structure is constructed in this manner, the graphite sheet5can easily contact the inner surface of the bottom21of the exterior can2. That is, if the depth a of the recesses23is too large, the graphite sheet5cannot easily contact the recesses23; if the depth a is too small, the area of contact with the graphite sheet decreases. Further, if the opening width b of the recesses23is too small, the graphite sheet5cannot easily enter recesses23; if the opening width b is too large, this means a small width d for the protrusions24. If the width d of the protrusions24is small, the manner in which the upper surfaces of the protrusions24receive the graphite sheet5is less surface-based. For example, if the recesses23are to be provided as a grid as described above, then, the smaller the width d of a protrusion24, the narrower the protrusion top i.e. the smaller the upper surface of the rectangle as viewed in a plan view. Since the graphite sheet5is pressed by the power generation element4toward the bottom21of the exterior can2, the sheet may be damaged by contact with protrusions. In view of this, the width d of the protrusions24is suitably larger than the opening width b of the recesses23. In other words, the graphite sheet5suitably contacts the upper surfaces of protrusions24that are relatively wide. Thus, the depth a and opening width c of the recesses23and the width d of the protrusions24are preferably decided to strike the right balance. The bottom width c of the recesses23are suitably smaller than the opening width b to facilitate contact with the graphite sheet5.

The seal can3includes a circular flat portion31and a cylindrical peripheral wall32extending contiguously from the periphery of the flat portion31. The opening of the seal can3faces the opening of the exterior can2. The seal can3is formed from a metallic material such as stainless steel.

The inner surface of the flat portion31of the seal can3is also provided with a recess-protrusion structure. The recesses33and protrusions34have the same constructions as the above-discussed recesses23and protrusions24of the exterior can2, and thus their description will be omitted. Further, as the inner surface of the flat portion31of the seal can3has a recess-protrusion structure, there is higher friction between the flat portion31of the seal can3and the graphite sheet5(seeFIG.1). This prevents positional displacement of the graphite sheet5of the all-solid-state battery1upon receiving vibration or impact.

After the power generation element4and graphite sheets5are placed within the interior space of the exterior can2and seal can3, a gasket6is placed between the cylindrical side wall22of the exterior can2and the peripheral wall32of the seal can3before crimping. Specifically the openings of the exterior can2and seal can3are positioned so as to face each other and the peripheral wall32of the seal can3is inserted into the cylindrical side wall22of the exterior can2; then, a gasket6is placed between the cylindrical side wall22and peripheral wall32and the seal can3is crimped over the exterior can3. Thus, the interior space defined by the exterior can2and seal can3is closed up tightly. It will be understood that the constructions of the exterior can2, seal can3and gasket6are similar to those for well-known flat batteries, and are not limited to any particular material or shape or any other feature.

The power generation element4includes a cathode member41, an anode member42, and a solid electrolyte layer43. The solid electrolyte layer43is located between the cathode and anode members41and42. In the power generation element4are stacked, in the direction away from the bottom21of the exterior can2(i.e., away from the bottom in the drawing): the cathode member41, solid electrolyte layer43and anode member42. The power generation element4is shaped as a circular column. The power generation element4is positioned adjacent to the inner surface of the bottom21of the exterior can2, with the graphite sheet5positioned in between. Thus, the exterior can2functions as a positive-electrode can. Further, the power generation element4is in contact with the inner surface of the flat portion31of the seal can3, with the graphite sheet5positioned in between. Thus, the seal can3functions as a negative-electrode can. It will be understood that the shape of the power generation element4is not limited to a circular column, and may be modified in various ways depending on the shape of the all-solid-state battery1: for example, the element may be shaped as a rectangular parallelepiped or a prism.

The cathode member41is made of a cathode active material that is used for lithium ion secondary batteries, constituted by 180 mg of a cathode mixture of LiNi0.6Co0.2Mn0.2O2particles of an average diameter of 3 μm, a sulfide solid electrolyte (Li6PS5Cl), and carbon nanotubes serving as a conductive aid in a mass ratio of 55:40:5, which has been formed in a mold with a diameter of 10 mm to be a columnar cathode pellet. It will be understood that the cathode member41can be made of any material that can function as a cathode member for the power generation element4, and may be made of, for example, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, a lithium-nickel-cobalt-manganese composite oxide, an olivine-type composite oxide or the like, or an appropriate mixture thereof. It will also be understood that the size and shape of the cathode member41are not limited to such a circular column, and may be modified in various ways depending on the size and shape of the all-solid-state battery1.

The anode member42is made of an anode active material that is used for lithium ion secondary batteries, constituted by 300 mg of an anode mixture of LTO (Li4Ti5O12; lithium titanate), a sulfide solid electrolyte (Li6PS5Cl), and carbon nanotubes in a weight ratio of 50:45:5, which has been formed to be a columnar anode pellet. It will be understood that the anode member42can be made of any material that can function as an anode member for the power generation element4, and may be made of, for example, metallic lithium, a lithium alloy, a carbon material such as graphite or carbon with low crystallinity, SiO, or LTO (Li4Ti5O12; lithium titanate), or an appropriate mixture thereof. It will also be understood that the size and shape of the anode member42are not limited to such a circular column, and may be modified in various ways depending on the size and shape of the all-solid-state battery1.

The solid electrolyte layer43is made of a sulfide solid electrolyte (Li6PS5Cl) formed to be a column. Alternatively although not limiting, the solid electrolyte layer43may be made of other sulfur-based solid electrolytes, such as argyrodite ones, to provide ion conductivity. If a sulfur-based solid electrolyte is used, the surface of the cathode active material is preferably covered with niobium oxide to prevent reaction with cathode active material. Further, the solid electrolyte layer43may be made of a hydride-based solid electrolyte or an oxide-based solid electrolyte, for example. It will also be understood that the size and shape of the solid electrolyte layer43is not limited to such a circular column, and may be modified in various ways depending on the size and shape of the all-solid-state battery1.

One graphite sheet5is provided between the bottom21of the exterior can2and the cathode member41of the power generation element4, and another one between the flat portion31of the seal can3and the anode member42of the power generation element4. That is, as shown inFIG.1, if both the inner surface of the bottom21of the exterior can2and the inner surface of the flat portion33of the seal can3are provided with a recess-protrusion structure, each of the upper and lower surfaces of the power generation element4is provided with a graphite sheet5(i.e., conductive sheet). Each graphite sheet5is formed by rolling expanded graphite. The shape of the graphite sheet5as viewed in a plan view is generally analogous to the shape of the interior space of the all-solid-state battery1as viewed in a plan view. Thus, the graphite sheet5is generally circular in shape as viewed in a plan view. The area of the upper surface of that graphite sheet5which is adjacent to the exterior can2may be equal to the area of the lower surface of the cathode member41of the power generation element4, or may be larger than the area of the lower surface of the cathode member41of the power generation element4. The area of the lower surface of that graphite sheet5which is adjacent to the seal can3may be equal to the area of the upper surface of the anode member42of the power generation element4, or may be larger than the area of the upper surface of the anode42of the power generation element4. That is, it suffices if the upper surface of the graphite sheet5adjacent of the exterior can2covers the lower surface of the cathode member41. It suffices if the lower surface of the graphite sheet5adjacent to the seal can3covers the upper surface of the anode member42. It will also be understood that the graphite sheets5are not limited to a generally circular shape as viewed in a plan view, and their shape may be modified in various ways depending on the shape of the all-solid-state battery1as viewed in a plan view: for example, the sheets may be elliptical or generally polygonal in shape as viewed in a plan view.

More specifically, a graphite sheet5is manufactured in the following manner: First, acidifying graphite particles, which have been prepared by acidifying natural graphite, are heated. This causes acids between the layers of the acidified graphite to vaporize and foam the graphite, causing it to expand. This expanded graphite is formed into felt and then rolled by a rolling mill to form a sheet. A disk cut out of this sheet of expanded graphite serves as a graphite sheet5. As discussed above, the expanded graphite is formed by acids vaporizing and foaming the acidified graphite. Thus, the graphite sheet5is porous. As such, the graphite sheet5has good flexibility due to its porosity, in addition to the conductivity intrinsic to graphite. Manufacture of the graphite sheet5is not limited to this method, and the graphite sheet5may be manufactured with any method.

The apparent density of the graphite sheets5is preferably not lower than 0.3 g/cm3, and more preferably not lower than 0.7 g/cm3; the apparent density is preferably not higher than 1.5 g/cm3, and more preferably not higher than 1.3 g/cm3. This takes account of the fact that an excessively low apparent density of a graphite sheet5means that the graphite sheet5can easily be damaged, and an excessively high apparent density reduces flexibility.

The thickness of the graphite sheets5(i.e., conductive sheets) is preferably not smaller than 0.05 mm, and more preferably not smaller than 0.07 mm; the thickness is preferably not larger than 0.5 mm, and more preferably not larger than 0.2 mm. This takes account of the fact that an excessively small thickness of a graphite sheet5means an insufficient ability to recover against compression of the graphite sheet5, and an excessively large thickness means that the graphite sheet5will occupy a significant portion of the interior space of the all-solid-state battery1, necessitating a reduced capacity of the power generation element4.

The rate of recovery of the graphite sheets5(i.e., conductive sheets) obtained from the above-indicated expression is suitably not lower than 7%. If the graphite sheets5(i.e., conductive sheets) have such an appropriate ability to recover, each graphite sheet5(i.e., conductive sheet) presses the power generation element4to an appropriate degree. This will enable maintaining good conductivity between the inner surface of the bottom21of the exterior can2and the power generation element4and maintaining good conductivity between the inner surface of the flat portion31of the seal can3and the power generation element4. The rate of recovery is more preferably not lower than 10% to maintain good conductivity.

Thus, the apparent density or thickness of the graphite sheets5is preferably decided to strike the right balance, based on flexibility, ability to recover and effective use of the interior space.

As discussed above, the graphite sheets5have good conductivity and flexibility. Thus, each graphite sheet5can function as a current collector and, at the same time, absorb expansion and contraction due to charging and discharging of the power generation element4or a pressing force during crimping of the exterior can2over the seal can3. Thus, the all-solid-state battery1can reduce a decrease in battery performance resulting from damage to the power generation element4or formation of a gap.

Further, as discussed above, each graphite sheet5, which is flexible, has an appropriate ability to recover against expansion due to charging of the power generation element, or compression due to a pressing force during crimping of the exterior can2over the seal can3. This will enable the all-solid-state battery1to maintain good conductivity between the inner surface of the bottom21of the exterior can2and the power generation element4, and maintain good conductivity between the inner surface of the flat portion31of the seal can3and the power generation element4, thereby maintaining the battery's performance.

Further, because of the recess-protrusion structure formed on the inner surface of the bottom21and the recess-protrusion structure formed on the flat portion31, each conductive sheet5has an increased area of contact with the associated one of the bottom21and flat portion31, i.e., current collection area. This will enable the all-solid-state battery1to yet better maintain battery performance.

(Variations)

As shown inFIG.4, only the inner surface of the bottom21of the exterior can2may be provided with a recess-protrusion structure, and the inner surface of the flat portion31of the seal can3may be flat.

Alternatively, although not shown, only the inner surface of the flat portion31of the seal can3may be provided with a recess-protrusion structure, and the inner surface of the bottom21of the exterior can2may be flat. Such implementations will also produce the above-identified effects. Although inFIG.4a graphite sheet5is located adjacent to the inner surface of a seal can3that has no recess-protrusion structure, the graphite sheet5may be omitted and the power generation element4may be in direct contact with the inner surface of the flat portion31of the seal can3. In such implementations, too, the graphite sheet5presses the power generation element4to maintain good conductivity between the inner surface of the flat portion31of the seal can3and the power generation element4.

Although embodiments have been described, the present disclosure is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit thereof.

EXAMPLES

Examples of the all-solid-state battery1according to the present disclosure will now be described.

Inventive Example

<Fabrication of Cathode Member>

LiNi0.6Co0.2Mn0.2O2particles of an average diameter of 3 μm, a sulfide solid electrolyte (Li6PS5Cl), and carbon nanotubes (“VGCF” (trade name) from Showadenkosya.co.ltd.), to serve as a conductive aid, were mixed in a mass ratio of 55:40:5, and were thoroughly kneaded to prepare a cathode mixture. Then, 90 mg of the cathode mixture was put into a powder molding die with a diameter of 10 mm, and pressure formed by a press under a condition of 10 t/cm2to fabricate a cathode-mixture molding, which constituted a cathode member41.

<Formation of Solid Electrolyte Layer>

Next, 45 mg of a sulfide solid electrolyte (Li6PS5Cl) was put on top of the cathode member41still within the powder molding die, and pressure formed by a press to form a solid electrolyte layer43on the cathode member41.

<Fabrication of Anode Member>

Lithium titanate particles of an average diameter of 35 μm, a sulfide solid electrolyte (Li6PS6Cl), and graphite powder to serve as a conductive aid were mixed in a mass ratio of 50:40:5, and were thoroughly kneaded to prepare an anode mixture. Then, 150 mg of the anode mixture was put on top of the solid electrolyte layer43still within the powder molding die, and pressure formed by a press to form an anode-mixture molding on the solid electrolyte layer43, which constituted an anode member42. A power generation element4composed of the cathode member41, solid electrolyte layer43and anode member42stacked on top of each other was fabricated in this wav.

<Assembly of Battery>

An exterior can2and a seal can3, both made of stainless steel, were prepared that were together to serve as a metal container for containing the power generation element4. As shown inFIGS.1to3, recess-protrusion structures (recesses23,33and protrusions24,34) were formed on the inner surface of the bottom21of the exterior can2and the inner surface of the flat portion31of the seal can3by knurling.

Next, two flexible graphite sheets5were prepared. Each graphite sheet5had a thickness of 0.1 mm, an apparent density of 1.0 g/cm3and a rate of recovery of 15%, and was a circular cutout with a diameter of 10 mm. Each graphite sheet5was used as a current collector.

A gasket6made of polyphenylene sulfide was mounted on the seal can3and, with the opening of the seal can3facing upward (i.e., the flat portion31located lower), one of the two graphite sheets5was placed on the inner surface of the flat portion31of the seal can3, and the power generation element4was overlaid on top thereof such that the anode member42faces the graphite sheet5. Thereafter, the other graphite sheet5was mounted on the side of the power generation element4adjacent to the cathode member41. The exterior can2was then placed thereon to cover it and the exterior can2was crimped over the seal can3to provide a seal, resulting in a coin-shaped all-solid-state battery1.

Comparative Example 1

An all-solid-state battery of Comparative Example 1 had a basic construction similar to that of the all-solid-state battery1of the Inventive Example, and was different in that the graphite sheets5were replaced by a nonwoven fabric made of carbon fiber to be used as a current collector. The nonwoven fabric of carbon fiber had a thickness of 0.2 mm and a rate of recovery of 4%.

Comparative Example 2

An all-solid-state battery of Comparative Example 2 had a basic construction similar to that of the all-solid-state battery1described in the Examples, and was different in that the inner surface of the bottom of the exterior can and the inner surface of the flat portion of the seal can had no recess-protrusion structure created by knurling.

<Comparison Testing>

The all-solid-state battery1of the Inventive Example, the all-solid-state battery of Comparative Example 1 and the all-solid-state battery of Comparative Example 2 were tested under the following conditions, and their capacity retention rates were calculated and compared.

Each of these all-solid-state batteries was charged with a constant current of 0.2 C until the voltage of 3.1 V was reached; then, it was charged with a constant voltage of 3.1 V until the current of 0.02 C was reached; thereafter, it was discharged with a constant current of 0.2 C until the voltage of 1.2 V was reached. This charge/discharge cycle was repeated 300 times, and the ratio of the discharge capacity for the 300th cycle to the discharge capacity for the second cycle (i.e., capacity retention rate) was calculated.

The results show that the all-solid-state battery1of the Inventive Example had a capacity retention rate of 98%, which means that, even at the 300th cycle, it maintained substantially the same discharge capacity as directly after the initiation of charge/discharge cycles. The all-solid-state battery of Comparative Example 1 had a capacity retention rate of 72%, showing that, at the 300th cycle, the discharge capacity had decreased by as much as about 30% from that directly after the initiation of charge/discharge cycles.

This shows that the all-solid-state battery1of the Inventive Example had an appropriate ability to recover by virtue of the flexible graphite sheets5serving as current collectors and thus was capable of better maintaining battery performance than the all-solid-state battery of Comparative Example 1, i.e., all-solid-state battery using a nonwoven fabric of carbon fiber.

The all-solid-state battery of Comparative Example 2 had a capacity retention rate of 90%, which means that the capacity retention rate had decreased somewhat more than that of the Inventive Example. This shows that the all-solid-state battery1of the Inventive Example better maintained battery performance by virtue of the increased current collection area created by the recess-protrusion structures on the inner surface of the bottom21of the exterior can2and the inner surface of the flat portion31of the seal can3(recesses23,33and protrusions24,34).

EXPLANATION OF CHARACTERS

1: all-solid-state battery2: exterior can,21: bottom,22: cylindrical side wall,23: recesses,24: protrusions3: seal can,31: flat portion,32: peripheral wall,33recesses,34: protrusions4: power generation element,41: cathode member,42: anode member,43: solid electrolyte layer5: graphite sheets (conductive sheets)6: gasket