JP2008257962A - All solid lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide composition of an all solid lithium secondary battery capable of suppressing deterioration of battery characteristics in the all solid lithium secondary battery having a solid electrolyte. <P>SOLUTION: The all solid lithium secondary battery includes a positive electrode layer 101 containing a sulfide solid electrolyte and a sulfide positive active material; a negative electrode layer 102; a sulfide solid electrolyte layer 103 interposed between the positive electrode layer 101 and the negative electrode layer 102; a conductive layer 104 formed on the outer surface of the positive electrode layer 101 containing the sulfide solid electrolyte and carbon material; and a positive terminal 107 connected to the outer surface of the conductive layer 104 through a conductive adhesive 105. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an all solid lithium secondary battery.

  In recent years, lithium secondary batteries have been used as power sources for portable electronic devices such as mobile phones and portable personal computers. A liquid electrolyte is generally used as the electrolyte of the lithium secondary battery. This liquid electrolyte is, for example, an organic solvent electrolyte in which a lithium compound is dissolved as a solute in a non-aqueous solvent such as an organic solvent. Since the organic solvent used for the electrolyte is a flammable substance, there is a risk that the battery may ignite. There is also a problem that the liquid electrolyte leaks during long-term use or storage. Therefore, in order to ensure the safety of the lithium secondary battery, it has been proposed to use a nonflammable solid electrolyte as the electrolyte instead of the organic solvent electrolyte.

  For example, Japanese Patent Laid-Open No. 6-275247 (Patent Document 1) describes a configuration of an all-solid lithium secondary battery including a nonflammable solid electrolyte.

  FIG. 10 is a longitudinal sectional view of a conventional resin-encapsulated lithium battery disclosed in the above publication.

As shown in FIG. 10, a positive electrode 501 and a negative electrode 503 sandwich a solid electrolyte 502 made of lithium ion conductive glassy solid electrolyte (0.5Li ₂ S-0.5SiS ₂ ). Carbon pastes 504 and 505 as conductive adhesives are attached to each, and the positive terminal 506 and the negative terminal 507 are joined. The positive electrode 501 is formed by pressing a mixture of titanium disulfide (TiS ₂ ) and lithium ion conductive glassy solid electrolyte (0.5Li ₂ S-0.5SiS ₂ ) into a disk shape. Reference numeral 503 denotes a disk-shaped metal lithium sheet. The battery body configured as described above is sealed with a thermosetting epoxy resin 508, and the thermosetting epoxy resin 508 is cured by heat treatment, whereby the all-solid lithium secondary battery shown in FIG. 10 is obtained.
JP-A-6-275247

  In the all-solid lithium secondary battery configured as described above, there are problems that cycle characteristics are reduced and battery capacity is also reduced.

  Accordingly, an object of the present invention is to provide a configuration of an all-solid lithium secondary battery capable of suppressing deterioration of battery characteristics in an all-solid lithium secondary battery provided with a solid electrolyte.

  In order to solve the above-mentioned problems, the present inventor presumed that the reason that the battery characteristics deteriorated was due to the direct contact between the positive electrode and the conductive adhesive, and as a result of various studies, the all-solid secondary It has been found that when a conductive layer made of a predetermined material is interposed between a conductive adhesive for joining external electrodes and a positive electrode in a battery, deterioration of battery characteristics can be suppressed. The present invention has been made based on this finding.

  An all-solid lithium secondary battery according to the present invention includes a positive electrode layer including a sulfide solid electrolyte and a sulfide positive electrode active material, a negative electrode layer, and a sulfide solid electrolyte layer interposed between the positive electrode layer and the negative electrode layer. And a conductive layer formed on the outer surface of the positive electrode layer and containing a sulfide solid electrolyte and a carbon material, and an external electrode bonded to the outer surface of the conductive layer with a conductive adhesive interposed therebetween.

  In the all solid lithium secondary battery of the present invention, a conductive layer containing a sulfide solid electrolyte and a carbon material is formed on the outer surface of the positive electrode layer, and a conductive adhesive is interposed on the outer surface of the conductive layer. And the external electrode is joined. Thereby, since the positive electrode layer does not directly contact the conductive adhesive, it is presumed that the reaction of the components in the conductive adhesive to the constituent material of the positive electrode layer is suppressed by the intervention of the conductive layer. As a result, it is presumed that the deterioration of battery characteristics, specifically, the reduction of battery capacity can be suppressed.

  As described above, according to the present invention, since the deterioration of battery characteristics can be suppressed as compared with the conventional all-solid lithium secondary battery, the battery capacity can be increased.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

  1 to 4 are schematic cross-sectional views sequentially showing the configuration of an all-solid lithium secondary battery according to a manufacturing process as one embodiment of the present invention.

As shown in FIG. 1, a sulfide solid electrolyte is disposed between a positive electrode layer 101 made of a mixture of a sulfide solid electrolyte and a sulfide positive electrode active material and a negative electrode layer 102 made of a mixture of a sulfide solid electrolyte and a carbon material. A conductive layer 104 containing a sulfide solid electrolyte and a carbon material is formed on the outer surface of the positive electrode layer 101. As an example, the positive electrode layer 101 includes Li ₂ FeS ₂ as a positive electrode active material and a Li ₂ S—P ₂ S ₅ composition as a solid electrolyte. The negative electrode layer 102 includes graphite, which is carbon as a negative electrode active material, and a Li ₂ S—P ₂ S ₅ -based composition as a solid electrolyte. Solid electrolyte layer 103 sandwiched between the positive electrode layer 101 and the negative electrode layer 102 is a _Li 2 _S-P 2 _{S 5} based composition. The conductive layer 104 includes a graphite carbon as the anode active material, and _Li 2 _S-P 2 _{S 5} based composition as a solid electrolyte.

  Next, as shown in FIG. 2, a conductive adhesive 105 is attached on the outer surface of the conductive layer 104 on the positive electrode side, and a positive electrode terminal 107 serving as an external electrode is joined via the conductive adhesive 105. Is done. On the other hand, a conductive adhesive 106 is attached on the outer surface of the negative electrode layer 102, and a negative electrode terminal 108 as an external electrode is joined through the conductive adhesive 106. The conductive adhesives 105 and 106 include a conductive component, a resin, and a solvent. For example, the conductive adhesives 105 and 106 are silver pastes that include a filler made of silver (Ag) as the conductive component.

  Further, as shown in FIG. 3, a sealing resin layer 109 is formed so as to cover all the laminates on the negative electrode terminal. As an example, the sealing resin layer 109 is formed of an epoxy resin.

  Finally, as shown in FIG. 4, the sealing resin layer 109 is removed so that the surface of the positive electrode terminal 107 is exposed. Thus, the all-solid lithium secondary battery 1 is manufactured as one embodiment of the present invention.

  In the all solid lithium secondary battery 1 according to one embodiment of the present invention configured as described above, a conductive layer 104 containing a sulfide solid electrolyte and a carbon material is formed on the outer surface of the positive electrode layer 101. The positive electrode terminal 107 is formed on the outer surface of the conductive layer 104 with a conductive adhesive 105 interposed therebetween. Thereby, since the positive electrode layer 101 does not directly contact the conductive adhesive 105, the reaction between the components in the conductive adhesive 105 and the constituent materials of the positive electrode layer 101 is hindered and suppressed by the intervention of the conductive layer 104. It is estimated to be. As a result, it is presumed that the deterioration of battery characteristics, specifically, the reduction of battery capacity can be suppressed. Thereby, since the deterioration of battery characteristics can be suppressed as compared with the conventional all solid lithium secondary battery, the battery capacity can be increased.

  An embodiment of the present invention will be described below.

  The following materials were used as constituent materials for the all-solid lithium secondary battery.

Cathode active material: Li ₂ FeS ₂
Solid electrolyte: sulfide glass _{(Li 2 S-P 2 S} 5)
Negative electrode active material: Graphite Conductive adhesive: Silver paste Sealing resin agent: Epoxy resin The material for forming the positive electrode layer was a mixture of 500 mg of the positive electrode active material and 500 mg of the solid electrolyte for 1 hour. As a material for forming the negative electrode layer and the conductive layer, a material obtained by mixing 500 mg of the negative electrode active material and 500 mg of the solid electrolyte for 6 hours was used. As the conductive adhesive, a polyester resin silver paste containing an ethyl acetate solvent was used.

  5-7 is typical sectional drawing which shows the structure of the all-solid-state lithium secondary battery produced as an Example and comparative example of this invention in order according to a manufacturing process.

  As shown in FIG. 5, as a comparative example of an all-solid lithium secondary battery, the negative electrode layer 102, the solid electrolyte layer 103, and the positive electrode layer 101 are filled in this order into a mold having an inner diameter of 10 mm using the materials prepared above. After being laminated, the pellets were produced by sandwiching with a punch from above and below the die and pressurizing with a pressure of 300 MPa. The thickness of the pellet was about 600 μm. At this time, the positive electrode layer 101 was 25% by volume, the solid electrolyte layer 103 was 45% by volume, and the negative electrode layer 102 was 30% by volume with respect to the pellet.

  On the other hand, as shown in FIG. 6, as an example of an all-solid lithium secondary battery, the negative electrode layer 102, the solid electrolyte layer 103, the positive electrode layer 101, After the conductive layers 104 were filled and stacked in this order, the pellets were formed by sandwiching them with punches from above and below the die and pressurizing them at a pressure of 300 MPa. The pellet thickness after molding was about 600 μm. At this time, the total of the positive electrode layer 101 and the conductive layer 104 was 25% by volume, the solid electrolyte layer 103 was 45% by volume, and the negative electrode layer 102 was 30% by volume with respect to the pellet.

  Next, as shown in FIG. 7, the conductive adhesive 106 was adhered on the stainless steel external electrode 108a using the material prepared above. The pellets of the comparative example and the example obtained above were each pressed onto the conductive adhesive 106 on the external electrode 108a, and then pressed to join the external electrode 108a. A conductive adhesive 105 is placed on each of the positive electrode layer 101 positioned on the upper part of the pellet of the comparative example bonded to the external electrode 108a and the conductive layer 104 positioned on the upper part of the pellet of the example bonded to the external electrode 108a, The conductive adhesives 105 and 106 were cured by heating at a temperature of 150 ° C. for 30 minutes with the external electrode 107a attached. In this way, the pellet, the external electrode 108a, and the external electrode 107a were integrated.

  Finally, as shown in FIG. 7, the sealing resin layer 109 is coated with the sealing resin agent prepared above and heated at a temperature of 120 ° C. for 30 minutes, and then at a temperature of 150 ° C. for 2 hours. Cured. Thereafter, a part of the sealing resin layer 109 was removed so that the external electrodes 108a and 107a were exposed. Thus, the comparative example of an all-solid-state lithium secondary battery and the sample as an Example were produced. All the battery manufacturing processes described above were performed in an inert atmosphere.

Charge / discharge measurement was performed on the samples of the comparative examples and examples of the produced all-solid lithium secondary batteries. Charging was performed at 50 μA (63.7 μA / cm ² ) until the potential reached 2.8V. Discharging was performed until the electric potential became 1 V at 100 μA (127.4 μA / cm ² ). Hereinafter, the voltage at which charging and discharging ends is referred to as a cut-off voltage.

  Table 1 shows the measurement results of the charge / discharge characteristics of the samples of the comparative examples and examples of the obtained all-solid lithium secondary battery. The internal resistance was calculated by (charge end voltage−discharge start voltage) / (charge current + discharge current).

  From the results shown in Table 1, it can be seen that the example has a higher charge / discharge capacity and lower internal resistance than the comparative example. When the internal resistance is high, the voltage value observed when a current is passed increases, and therefore the cut-off voltage is reached quickly. Therefore, it is thought that a charge / discharge capacity becomes low in the comparative example.

  In order to consider the results of the comparative example of the all-solid lithium secondary battery produced above and the sample of the example, the following verification experiment was performed.

  Using 50 mg of the material of the positive electrode layer prepared above, a pellet of only the positive electrode layer 101 was produced as shown in FIG.

  On the other hand, using 50 mg of the positive electrode material prepared above and 10 mg of the conductive layer (negative electrode layer) material, the conductive layer 104, the positive electrode layer 101, and the conductive layer 104 are stacked in this order as shown in FIG. 9. A pellet was prepared.

  The conductive adhesive prepared above was applied to both surfaces of the two pellets obtained, and cured in a state where the electrodes were drawn out from both surfaces with a conductive wire.

The electric resistance value was measured by passing a direct current of 50 μA (63.7 μA / cm ² ) through the conducting wire through the two samples thus produced.

  As a result, the electric resistance value of the sample of FIG. 8 was about 8030Ω, whereas the electric resistance value of the sample of FIG. 9 was about 16Ω. In the sample of FIG. 8 in which the material of the positive electrode layer and the conductive adhesive are in direct contact in this way, the electric resistance value is high, and the material of the conductive layer (negative electrode layer) and the conductive adhesive are in direct contact with each other. It can be seen that the electrical resistance value is lower in the sample. This is because the solid electrolyte and the positive electrode active material constituting the positive electrode layer are both sulfides, the negative electrode active material contained in the conductive layer (negative electrode layer) is graphite, and sulfide is more stable than graphite. It may be inferior. It is considered that the highly stable graphite is in contact with the conductive adhesive, so that the electron conductivity is ensured and the increase in internal resistance is suppressed.

  It should be considered that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is shown not by the above embodiments and examples but by the claims, and is intended to include all modifications and variations within the meaning and scope equivalent to the claims. .

It is typical sectional drawing which shows a 1st process as a manufacturing process of an all-solid-state lithium secondary battery as one embodiment of this invention. It is typical sectional drawing which shows a 2nd process as a manufacturing process of an all-solid-state lithium secondary battery as one embodiment of this invention. It is typical sectional drawing which shows a 3rd process as a manufacturing process of an all-solid-state lithium secondary battery as one embodiment of this invention. It is typical sectional drawing which shows a 4th process as a manufacturing process of an all-solid-state lithium secondary battery as one embodiment of this invention. It is typical sectional drawing which shows the manufacturing process of the sample of an all-solid-state lithium secondary battery as a comparative example of this invention. It is typical sectional drawing which shows the manufacturing process of the sample of an all-solid-state lithium secondary battery as an Example of this invention. It is typical sectional drawing which shows the sample of the all-solid-state lithium secondary battery produced as a comparative example and Example of this invention. It is typical sectional drawing which shows one pellet produced in the verification experiment of this invention. It is typical sectional drawing which shows another pellet produced in the verification experiment of this invention. It is a longitudinal cross-sectional view of the conventional resin-encapsulated lithium battery.

Explanation of symbols

  101: positive electrode layer, 102: negative electrode layer, 103: solid electrolyte layer, 104: conductive layer, 105, 106: conductive adhesive, 107: positive electrode terminal, 108: negative electrode terminal.

Claims (1)

A positive electrode layer comprising a sulfide solid electrolyte and a sulfide positive electrode active material;
A negative electrode layer;
A sulfide solid electrolyte layer interposed between the positive electrode layer and the negative electrode layer;
A conductive layer formed on the outer surface of the positive electrode layer and including a sulfide solid electrolyte and a carbon material;
An all-solid lithium secondary battery comprising: an external electrode joined via an electrically conductive adhesive on the outer surface of the conductive layer.

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