JP5207206B2 - Manufacturing method of all-solid lithium secondary battery - Google Patents

Description

  The present invention relates to an all-solid lithium secondary battery having a solid electrolyte layer formed by a vapor phase method. In particular, the present invention relates to a thin-film all-solid lithium secondary battery having high safety and excellent charge / discharge cycle characteristics.

  Lithium secondary batteries have a long life, high efficiency, and high capacity, and are used as power sources for mobile phones, notebook computers, digital cameras, and the like.

  The lithium secondary battery is charged and discharged by exchanging lithium ions between the positive electrode and the negative electrode through the electrolyte layer. Recently, research and development of all-solid-state lithium secondary batteries using non-flammable solid electrolytes instead of organic solvent electrolytes has been actively conducted, and a thin film type in which a positive electrode, a negative electrode, and a solid electrolyte layer are formed by a vapor phase method. Have also been proposed (see, for example, Patent Documents 1 and 2).

  In Patent Document 3, a layer having low ion conductivity is formed between the positive electrode and the negative electrode by chemically reacting with metal lithium for the purpose of preventing internal short circuit of the battery due to the growth of dendrites of metal lithium. It has been proposed that a first solid electrolyte layer that can be formed and a second solid electrolyte layer different from the first solid electrolyte layer on the negative electrode side.

JP 59-31570 A Japanese Patent Laid-Open No. 10-284130 JP 2004-206942 A

  However, pinholes may be produced in the solid electrolyte layer formed by sputtering, vacuum deposition, CVD, or the like as described in Patent Documents 1 and 2. Therefore, when an all-solid lithium secondary battery is configured using such a solid electrolyte layer, in a structure in which a positive electrode, a solid electrolyte layer, and a negative electrode are stacked, the metal lithium passes through this pinhole with repeated charge and discharge. There is a possibility that the dendrite grows and an internal short circuit of the battery occurs. In particular, when metal lithium is used for the negative electrode, there is a tendency that dendrites are likely to occur and grow, and even when metal lithium is not used for the negative electrode, charging / discharging is repeated under high current density charging / discharging conditions. When this is done, dendrites are likely to be generated and grown.

  In addition, in the solid electrolyte layer having a two-layer structure described in Patent Document 3, since one of the solid electrolyte layers reacts with metallic lithium, the reaction with metallic lithium does not stop and the solid electrolyte is decomposed. There is.

  The present invention has been made in view of the above circumstances, and one of its purposes is to provide an all solid lithium secondary battery having high safety and excellent charge / discharge cycle characteristics.

  The all solid lithium secondary battery of the present invention has a structure in which a positive electrode, a solid electrolyte layer, and a negative electrode are laminated, and the solid electrolyte layer is formed by a vapor phase method. And the liquid substance which reacts with metallic lithium and inactivates metallic lithium exists in the pinhole of this solid electrolyte layer, It is characterized by the above-mentioned.

  According to this configuration, even if dendrites of metallic lithium grow through pinholes generated in the solid electrolyte layer, the metallic lithium reacts with the liquid substance, so that the metallic lithium can be deactivated. An internal short circuit can be reliably prevented. Therefore, it is possible to obtain an all-solid lithium secondary battery having high safety and excellent charge / discharge cycle characteristics.

  As said liquid substance, a well-known ionic liquid can be used, for example. An ionic liquid is a substance composed only of ionic molecules composed of cations and anions, and is a liquid at room temperature. In addition, the ionic liquid has high ionic conductivity, and the presence of the ionic liquid in the pinhole of the solid electrolyte layer improves the lithium ion conductivity in the thickness direction and reduces the internal resistance of the battery. This can contribute to higher battery output.

The cations that make up the ionic liquid are trimethylpropylammonium (TMPA), trimethylmethoxymethylammonium (TMMMA), trimethylphenylammonium (TMPhA), 1-ethyl-3-methylimidazolium (EMI), triethylsulfonium (TES) ) And butylpyridinium (BP) are selected from the group consisting of trifluoromethanesulfonylimide (TFSI), fluorosulfonylimide (FSI), and trifluorosulfonylacetylimide (TSAC). ), Chloroaluminate (AlCl ₄ ), tetrafluoroborate (BF ₄ ), fluoride (F), chloride (Cl), iodide (I) and bromide (Br). preferable. In particular, it is preferable to select an imidazolium type such as EMI as the cation and a halogen such as Cl as the anion.

The solid electrolyte layer is preferably composed of a sulfide-based solid electrolyte having a high lithium ion conductivity. In particular, a sulfide-based solid electrolyte containing Li ₂ S—P ₂ S ₅ can be suitably used. Such sulfide-based solid electrolyte, other Li ₂ S-P ₂ S ₅ based solid electrolyte mainly composed of Li ₂ S and P ₂ S _5, Li ₂ S-P ₂ S ₅ containing SiS ₂ -SiS ₂ based ones and, further Al ₂ S ₃ laden _{Li 2 S-P 2 S 5} -SiS 2 -Al 2 S 3 based ones, or P ₂ O containing ₅ Li ₂ S-P ₂ S Examples include _5- P ₂ O ₅ series. The sulfide-based solid electrolyte containing Li ₂ S—P ₂ S ₅ exhibits stable charge / discharge characteristics even when metallic lithium or a lithium alloy is used as the negative electrode material. In addition, for example, when an oxynitride type material represented by Li ₃ PO _4-α N _α or an oxide type material is used as the negative electrode material, lithium lanthanum titanate (La _0.55 Li _0.35 TiO ₃ ) Alternatively, a lithium titanium phosphate composite salt (Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ ) solid electrolyte having a NASICON crystal structure can be used.

  The solid electrolyte layer can be obtained by a physical vapor deposition (PVD) method such as a sputtering method, a vacuum vapor deposition method, and an ion plating method, or a vapor phase method such as a chemical vapor deposition (CVD) method. By forming the solid electrolyte layer into a thin film using such a film formation technique, the thickness of the solid electrolyte layer can be reduced to 10 μm or less. In the present invention, the thickness of the solid electrolyte layer is not particularly limited, but by setting the thickness to 10 μm or less, the solid electrolyte layer can be made flexible and the battery can be made thinner. .

  One means for allowing the liquid substance to exist in the pinholes of the solid electrolyte layer is to apply the liquid substance to the surface of the formed solid electrolyte layer. By applying the liquid material in this manner, the liquid material can enter the pinhole, and the pinhole of the solid electrolyte layer can be filled with the liquid material.

  The all solid lithium secondary battery of the present invention has a structure in which a positive electrode, a solid electrolyte layer, and a negative electrode are laminated. For example, after forming a solid electrolyte layer on one electrode of a positive electrode or a negative electrode, the other electrode is formed. It can obtain by arrange | positioning on a solid electrolyte layer. An example of a method for producing an all solid lithium secondary battery of the present invention will be described briefly. A step of forming one electrode of a positive electrode or a negative electrode, a step of forming a solid electrolyte layer on one electrode, A step of forming the other electrode thereon; In addition, after applying a liquid substance on the surface of a solid electrolyte layer, when forming an electrode on the solid electrolyte layer using a vapor phase method, the solid electrolyte layer that is a target (object to be processed) for forming a film is formed. It is preferable that heating is not performed, that is, the electrode is formed at room temperature or in a state where the object to be processed is cooled.

By the way, when an excess liquid substance remains on the surface of the solid electrolyte layer, in the subsequent process, for example, the process of forming the electrode on the solid electrolyte layer, the adhesion between the solid electrolyte layer and the electrode is reduced. May affect performance. Therefore, in the all solid lithium secondary battery of the present invention, it is preferable that a liquid substance is present only in the pinhole of the solid electrolyte layer, and the method for manufacturing the all solid lithium secondary battery of the present invention includes the following steps. Is preferred.
1. Electrolyte layer formation process to form solid electrolyte layer by vapor phase method
2. A coating process in which a liquid material that reacts with metallic lithium to inactivate metallic lithium is applied to the surface of the solid electrolyte layer, and the liquid material is present in the pinholes of the solid electrolyte layer
3. Removal process to remove excess liquid material on the surface of the solid electrolyte layer after coating

  In the solid electrolyte layer obtained through such a process, no excess liquid substance remains on the surface, and the liquid substance can be present only in the pinhole of the solid electrolyte layer by the above-described removal process. Further, in the above removal step, it is preferable to use an absorbent body such as a porous material composed of organic fibers when removing excess liquid material so that the solid electrolyte layer is not destroyed. Specifically, after wiping off an excess liquid substance on the surface of the solid electrolyte layer with the above-described absorber, the surface of the solid electrolyte layer is further wiped with an absorber impregnated with an organic solvent. In this way, it is possible to reliably remove excess liquid substance on the surface of the solid electrolyte layer without destroying the solid electrolyte layer. Absorbers include porous materials made of materials that are soft enough not to damage the surface of the solid electrolyte layer, specifically, cloth or sponge made of natural or synthetic fibers, wipers made of paper, commercially available porous films, and oil absorption. A sheet can be used.

  The organic solvent is useful for dissolving a liquid substance such as an ionic liquid and enhancing the effect of removing the liquid substance. Here, when the solid electrolyte layer is a sulfide-based solid electrolyte, a carbonate-based organic solvent such as diethyl carbonate is preferably used as the organic solvent, and in the case of an oxynitride-based solid electrolyte, such as isopropyl alcohol. It is preferable to use an alcohol-based organic solvent or an acetone-based organic solvent such as acetone. The organic solvent is preferably dried to remove water, and the water content in the organic solvent is preferably 20 ppm or less. Drying of the organic solvent can be achieved, for example, by using a commercially available desiccant or by distillation. Commercially available anhydrous alcohol or anhydrous acetone may be used.

In addition, the positive electrode active material is selected from lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ), and olivine-type lithium iron phosphate (LiFePO ₄ ). One kind of lithium metal oxide, manganese oxide (MnO ₂ ), or a mixture thereof can be used. In addition, sulfur (S), ferric sulfide (FeS), iron disulfide (FeS ₂ ), one sulfide selected from lithium sulfide (Li ₂ S) and titanium sulfide (TiS ₂ ), or these A mixture of these may also be used. Among these, lithium metal oxides, particularly LiCoO _2, are excellent because of their excellent electron conductivity.

As the active material of the negative electrode, metallic lithium (Li metal alone) or lithium alloy (comprising Li and an additive element), carbon (C) such as graphite, silicon (Si), indium (In), or titanium An amorphous oxide such as lithium acid (Li ₄ Ti ₅ O ₁₂ ) can be used. Among these, materials containing lithium, such as metallic lithium and lithium alloys, particularly metallic lithium, are advantageous in terms of increasing the battery capacity and voltage, and are preferable. In addition, when metal lithium or a lithium alloy is used for the negative electrode material, a current collecting function can be provided, so that a current collector described later can be omitted. As the additive element of the lithium alloy, aluminum (Al), silicon (Si), tin (Sn), bismuth (Bi), zinc (Zn), indium (In), or the like can be used.

  A current collector may be provided as appropriate on the positive electrode or the negative electrode. It is preferable to use a metal that does not alloy with lithium for the current collector. As the current collector for the positive electrode, for example, aluminum (Al), nickel (Ni), alloys thereof, and stainless steel can be suitably used. As the current collector for the negative electrode, for example, copper (Cu), nickel (Ni), iron (Fe), chromium (Cr), and alloys thereof can be suitably used. In addition to using these metal rolled foils and electrolytic foils, the current collector may be formed by forming a vapor deposition film of these metals on a plastic substrate.

  The all-solid-state lithium secondary battery of the present invention reliably prevents an internal short circuit of the battery by the presence of a liquid material that reacts with the lithium metal and inactivates the lithium metal in the pinhole formed in the solid electrolyte layer. It is safe and has high charge / discharge cycle characteristics.

  Embodiments of the present invention will be described below.

  The basic structure of an all-solid lithium secondary battery is a structure in which a positive electrode, a solid electrolyte layer, and a negative electrode are sequentially stacked. Here, the most characteristic feature of the present invention is that a liquid substance that reacts with metallic lithium and inactivates metallic lithium is present in the pinhole of the solid electrolyte layer.

[Example 1]
An all-solid lithium secondary battery of the present invention having a structure in which a positive electrode, a solid electrolyte layer, and a negative electrode were laminated was produced, and a charge / discharge cycle test was performed. The following materials were used for the substrate, the positive electrode active material, the solid electrolyte, the liquid material, and the negative electrode active material, respectively.
Base material: Stainless steel foil (diameter 10mm x thickness 10μm)
Cathode active material: LiCoO ₂
Solid electrolyte: Li ₂ S-P ₂ S ₅ solid electrolyte (composition ratio of Li ₂ S: P ₂ S ₅ is 4: 1 in molar ratio)
Liquid material: EMI-Cl ionic liquid Negative electrode active material: Li metal

<Battery preparation procedure>
A 1 μm-thick positive electrode active material layer made of LiCoO ₂ was formed on the stainless steel foil by laser ablation. The stainless steel foil also serves as a positive electrode current collector, and these were used as positive electrodes.

Next, a 1 μm-thick solid electrolyte layer made of a Li ₂ S—P ₂ S ₅ solid electrolyte was formed on the positive electrode active material layer by laser ablation. After the formation, the EMI-Cl ionic liquid is applied to the surface of the solid electrolyte layer in the glove box, and after application, the excess ionic liquid remaining on the surface of the solid electrolyte layer is removed from the commercially available paper wiper (specific example, Kimwipe ( In addition, the surface of the solid electrolyte layer was lightly wiped with a commercially available paper wiper soaked with sufficiently dried diethyl carbonate to completely remove the ionic liquid on the surface. Diethyl carbonate having a water content of 20 ppm or less was used. Further, when the surface of the solid electrolyte layer was observed with a scanning electron microscope (SEM), it was confirmed that an ionic liquid was present in the pinhole of the solid electrolyte layer.

  Next, a 1 μm-thick negative electrode active material layer made of Li metal was formed on the solid electrolyte layer by vacuum deposition, and this was used as the negative electrode. The negative electrode active material layer was formed without heating the substrate.

  Finally, the laminate was accommodated in a case to complete a coin-type all-solid lithium secondary battery. This battery was designated as sample 1 and 10 identical batteries were produced.

  For comparison, an all-solid lithium secondary battery was produced in the same manner as Sample 1 except that the liquid substance (ionic liquid) was not applied to the surface of the solid electrolyte layer. This battery was designated as Comparative Example 1, and 10 identical batteries were produced. In addition, when the surface of the solid electrolyte layer was observed with a scanning electron microscope (SEM) after the formation of the solid electrolyte layer, it was confirmed that pinholes existed in the solid electrolyte layer.

<Battery evaluation>
Lead terminals are attached to both the positive and negative electrodes of Sample 1 and Comparative Example 1, and charging and discharging are performed for Sample 1 and Comparative Example 1 under the conditions of a charging upper limit voltage: 4.2 V, a discharging lower limit voltage: 3 V, and a charging / discharging current: 0.02 mA. A charge / discharge cycle test was conducted with one discharge cycle.

  As a result, in Sample 1, all of 10 samples could be stably charged and discharged even after 100 cycles without causing a short circuit between the positive and negative electrodes. On the other hand, in Comparative Example 1, 8 out of 10 short circuits between the positive and negative electrodes occurred by 50 cycles, and the charge / discharge operation could not be performed.

[Example 2]
An all-solid lithium secondary battery was fabricated in the same manner as Sample 1, except that EMI-F ionic liquid was used as the liquid material. This battery was designated as sample 2 and 10 identical batteries were produced. Sample 2 was subjected to the above charge / discharge cycle test, and the battery was evaluated. As a result, in Sample 2, as in Sample 1, all 10 out of 100 cycles were not short-circuited between the positive and negative electrodes. Later, stable charge and discharge were possible.

  Thus, it was confirmed that the all solid lithium secondary battery of the present invention has high safety and excellent charge / discharge cycle characteristics.

  Note that the present invention is not limited to the above-described embodiment, and can be modified as appropriate without departing from the gist of the present invention. For example, the material constituting the solid electrolyte layer may be appropriately changed, or a material other than Li metal may be used as the negative electrode material.

  The all-solid lithium secondary battery of the present invention can be suitably used for a lithium battery that requires high safety and excellent charge / discharge cycle characteristics.

Claims (4)

An electrolyte layer forming step of forming a solid electrolyte layer by a vapor phase method;
Applying a liquid substance that reacts with metallic lithium to inactivate metallic lithium on the surface of the solid electrolyte layer, and causes the liquid substance to exist in pinholes of the solid electrolyte layer; and
After application, a removal step of removing excess liquid substance on the surface of the solid electrolyte layer,
Bei to give a,
An ionic liquid is used as the liquid substance, and the method for producing an all-solid lithium secondary battery. The cation constituting the ionic liquid is one or more selected from the group consisting of trimethylpropylammonium, trimethylmethoxymethylammonium, trimethylphenylammonium, 1-ethyl-3-methylimidazolium, triethylsulfonium, and butylpyridinium. Yes,
The anion constituting the ionic liquid is one or more selected from the group consisting of trifluoromethanesulfonylimide, fluorosulfonylimide, trifluorosulfonylacetylimide, chloroaluminate, tetrafluoroborate, fluoride, chloride, iodide and bromide. The manufacturing method of the all-solid-state lithium secondary battery of Claim 1 characterized by the above-mentioned. The solid electrolyte layer is Li ₂ SP ₂ S _Five The method for producing an all-solid lithium secondary battery according to claim 1, wherein the solid-state electrolyte is a sulfide-based solid electrolyte. In the said removal process, after wiping off the excess liquid substance on the surface of a solid electrolyte layer with an absorber, the solid electrolyte surface is further wiped with an absorber impregnated with an organic solvent. The manufacturing method of the all-solid-state lithium secondary battery as described in any one.