JP4953406B2 - All-solid lithium secondary battery - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an all solid lithium secondary battery using a carbon material or a material in which lithium ions are inserted between carbon material layers as a negative electrode active material.
[0002]
[Prior art]
In recent years, with the development of portable devices such as personal computers and mobile phones, the demand for batteries as a power source has become very large. In particular, lithium batteries are actively studied in various fields as batteries capable of obtaining a high energy density because lithium has a small atomic weight and a large ionization energy.
[0003]
On the other hand, as lithium batteries become more and more versatile, internal energy increases due to an increase in the amount of active material, and the content of organic solvents, which are flammable substances used in electrolytes, increases the risk of battery ignition. Interest has been highlighted in recent years. As a method for ensuring the safety of a lithium battery, it is extremely effective to use a solid electrolyte that is a nonflammable substance instead of an organic solvent electrolyte, and the development of an all-solid lithium battery with high safety has been developed. It is desired.
[0004]
As the lithium ion conductive solid electrolyte used in the all solid lithium battery, those having high ion conductivity are preferable. Such materials include sulfide glasses having an ion conductivity of 10 ⁻³ S / cm in the 1980s, that is, LiI—Li ₂ S—P ₂ S ₅ , LiI—Li ₂ S—B ₂ S ₃ , LiI. such as -Li ₂ S-SiS ₂ is found, in recent _years, it has been found well as _{Li 3 PO 4 -Li 2 S-} SiS 2, Li 4 SiO 4 -Li 2 S-SiS 2. In addition to these glassy substances, high ionic conductivity has recently been observed in crystalline sulfides, and Li ₂ S—SiS ₂ —Ga ₂ S ₃ , Li ₂ S—P ₂ S ₅ (crystallized glass). A lithium ion conductive solid electrolyte that can be expressed by the following composition has been reported.
[0005]
[Problems to be solved by the invention]
However, of these solid electrolytes, the selection of a suitable one for a specific electrode active material has not been mentioned so far. For example, in Japanese Patent Application Laid-Open No. 11-219722, an all solid lithium secondary battery is manufactured. in "as the lithium ion conductive solid electrolyte in the present invention, in order to the output of the battery is large, .Li 2 _S-SiS 2 it is preferable to use a high ionic conductivity, Li _{2 S-B} Sulfide-based amorphous (glassy) lithium ion conductive solid electrolytes such as ₂ S ₃ and Li ₂ S—P ₂ S ₅ are suitable because they have a high ion conductivity of 10 ⁻⁴ S / cm or more. As a suitable solid electrolyte selected from these lithium ion conductive solid electrolytes, “these solid electrolytes” are described. Generally by melting a mixture of starting materials at a high temperature, _{.Li 2} S-SiS 2, which is synthesized by quenching the vapor pressure of SiS ₂ is higher than the _B 2 _{S 3} and _P 2 _{S 5} Therefore, there is little transpiration of the starting material at the time of electrolyte synthesis, and it is most suitable for industrial mass synthesis. "
[0006]
Regarding an all-solid lithium secondary battery using a Li ₂ S—SiS ₂ -based solid electrolyte as an electrolyte, R.C. Komiya, A .; Hayashi, H .; Morimoto, M .; Tatsumisago, T .; Minami, Solid State Ionics, 140 , 83 (2001); Iwamoto, N .; Aotani, K .; Takada, S .; Kondo, Solid State Ionics, 79 , 288 (1995), Iwamoto Kazuya, Fujino Nobu, Takada Kazunori, Kondo Shigeo, Electrochemistry, 67 , 151 (1999), and the like. Among these solid electrolytes, Li ₂ It gives the impression that the S-SiS ₂ solid electrolyte is most suitable for use in an all-solid lithium secondary battery.
[0007]
In these reports, an indium-lithium alloy or Li _4/3 Ti _5/3 O ₄ is used as the negative electrode active material of the all-solid-state secondary battery. On the other hand, carbon materials typified by graphite intercalation compounds are currently used as negative electrode active materials for lithium secondary batteries. The graphite intercalation compound shows a theoretical capacity of 372 mAh / g and a very low potential of about 0.1 V, and indium-lithium alloy reported in the above-mentioned literature in order to increase the energy density of a lithium secondary battery, or Li It is a material superior to _4/3 Ti _5/3 O ₄ .
[0008]
In this study, the battery characteristics when graphite was used as the negative electrode active material differed greatly depending on the type of solid electrolyte. In order to produce an all-solid lithium secondary battery with excellent performance, lithium ion conductivity It became clear that the choice of solid electrolyte was very important.
The object of the present invention is to select the most suitable electrolyte as the electrolyte to be used in an all-solid lithium secondary battery using a graphite intercalation compound as a negative electrode active material, and to increase the energy density of an all-solid lithium secondary battery And
[0009]
[Means for Solving the Problems]
In an all-solid-state lithium ion secondary battery in which a negative electrode active material is a carbon material or a material in which lithium ions are inserted between carbon material layers, at least a lithium ion conductive solid electrolyte in contact with the negative electrode active material is lithium sulfide and phosphorus sulfide. And component X.
[0010]
Further, the component X is at least one selected from lithium iodide, boron sulfide, aluminum sulfide, lithium phosphate, lithium borate, and lithium oxide.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The sulfide-based lithium ion conductive solid electrolyte is synthesized from a sulfide that basically forms a stationary skeleton structure and a sulfide that serves as a movable lithium ion source. As the latter sulfide, lithium sulfide is used, and as the former sulfide, the above-mentioned phosphorus sulfide, boron sulfide, silicon sulfide, germanium sulfide, aluminum sulfide, gallium sulfide and the like are used. For example, in Li ₄ GeS ₄ having a LISICON-type crystal structure, it is considered that GeS ₄ tetrahedrons form a skeletal structure, and movable lithium ions diffuse in the structure.
[0012]
The present invention uses a solid electrolyte using silicon sulfide or germanium sulfide for forming a skeleton structure among these skeleton structure-forming sulfides as a solid electrolyte of an all-solid secondary battery using a carbon material as a negative electrode active material. In this case, it is based on the finding that a reduction reaction of silicon or germanium occurs as a side reaction in addition to a reaction in which lithium ions are inserted between layers of carbon materials during charging.
[0013]
That is, when a solid electrolyte containing silicon or germanium, such as Li ₂ S—SiS ₂ , Li ₂ S—GeS ₂ , is used, the current that flows during the charging of the battery is an insertion reaction of lithium ions into the carbon material. Used for reduction reaction of silicon or germanium. Among these reactions, the latter reaction is poor in reversibility, and therefore the amount of electricity consumed in the reduction reaction of silicon or germanium out of the charged amount of electricity cannot be taken out when the battery is discharged.
[0014]
The most important point in the present invention made in view of this problem is that, in an all-solid lithium secondary battery using a carbon material or a material in which lithium ions are inserted between carbon material layers as a negative electrode active material, at least the negative electrode active material The solid electrolyte in contact with is a substance that does not contain silicon and germanium. Further, when phosphorus sulfide is used as the skeleton structure-forming sulfide, phosphorus is an element that is particularly difficult to be reduced, and it is preferable to obtain a solid electrolyte exhibiting higher ion conductivity.
[0015]
The lithium ion solid electrolyte in the present invention is based on the composition of Li ₂ S—P ₂ S ₅ . As long as it has this composition as a basic composition, the solid electrolyte can be either crystalline or amorphous. Furthermore, in the lithium ion conductive solid electrolyte used in the present invention, Li ₂ S—P ₂ S ₅ − is added by adding other component (X) to lithium sulfide and phosphorus sulfide within a range not including silicon and germanium. It is good also as a composition of X.
[0016]
Examples of X added to make the ion conductivity higher than that of Li ₂ S—P ₂ S ₅ itself include lithium iodide (LiI), boron sulfide (B ₂ S ₃ ), and aluminum sulfide (Al ₂ S). ₃ ). When X is LiI, LiI forms a micro domain in the Li ₂ S—P ₂ S ₅ structure and enhances ion conductivity. When X is B ₂ S ₃ or Al ₂ S ₃ , the ion conductivity is also increased due to the mixed anion effect.
[0017]
Further, when the lithium ion conductive solid electrolyte is amorphous (glass), by using an oxide or an oxyacid salt as X, the glass skeleton can be made more stable. Examples of X having such an effect include lithium phosphate, lithium borate, and lithium oxide. These X can be used alone or in combination.
[0018]
The necessity that the solid electrolyte in the present invention does not contain a transition metal element is well known. When the transition metal element is contained in the solid electrolyte, the transition metal element is reduced in contact with the negative electrode active material, and the substance used as the solid electrolyte exhibits electronic conductivity and does not function as an electrolyte. .
[0019]
The negative electrode active material in the present invention is a carbon material or a material in which lithium ions are inserted between carbon material layers. When a carbon material typified by graphite is used as the negative electrode active material of a lithium secondary battery, lithium ions are inserted between the carbon material layers in the charged state, and lithium between the layers is removed in the fully discharged state. Release to become a carbon material. Therefore, the form of the negative electrode active material of the all-solid-state lithium secondary battery in the present invention is a carbon material or a substance in which lithium ions are inserted between carbon material layers.
[0020]
As the positive electrode material of the all-solid lithium secondary battery in the present invention, known materials for the positive electrode of the lithium battery such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ can be used. As other positive electrode materials, materials such as MnO ₂ and V ₂ O ₅ can also be used. However, since the negative electrode active material in the present invention is a carbon material, and the carbon material is stable when lithium ions are not inserted, it is industrially constituted by using a carbon material that does not contain lithium. It is preferable to do. Therefore, it is preferable that the positive electrode active material used for it contains lithium among these compounds.
[0021]
These LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ exhibit a potential of 4 V on the basis of lithium as lithium ions are desorbed. Therefore, when LiI is used as X, iodide ions are oxidized on the contact surface with the positive electrode active material. Therefore, when these materials are used as the positive electrode active material, a lithium ion conductive solid electrolyte that does not contain LiI is used as X, or other types of lithium ions that do not contain LiI in the portion in contact with the positive electrode active material. It is necessary to use a conductive solid electrolyte.
[0022]
【Example】
Hereinafter, the present invention will be described in detail with reference to examples.
[0023]
[Example]
In this example, LiI—Li ₂ S—P ₂ O ₅ sulfide glass was used as the lithium ion conductive solid electrolyte, and the electrode characteristics of graphite as the carbon material were examined. The solid electrolyte was synthesized by the following method.
[0024]
First, lithium iodide (LiI), lithium sulfide (Li ₂ S), and phosphorus pentasulfide (P ₂ S ₅ ) were mixed at a molar ratio of 40:41:19. Next, the mixture was sealed under reduced pressure in a quartz jar whose inner surface was coated with carbon, and heated to 950 ° C. Furthermore, this quartz soot was poured into water, and the melt of the above mixture was quenched to obtain an amorphous lithium ion conductive solid electrolyte.
[0025]
As the carbon material, graphite (manufactured by TIMCAL, SFG-50) was used, and the electrode characteristics were examined by the following method.
[0026]
The lithium ion conductive solid electrolyte obtained above and graphite were mixed at a weight ratio of 1: 1 to obtain an electrode material. This electrode material (20 mg) and an alloy of indium and lithium were used as a counter electrode, and a three-layer pellet was formed between the electrode material and the solid electrolyte between them.
[0027]
This measurement cell was charged / discharged at a constant current of 10 μA to examine the electrode characteristics of graphite. The results are shown in FIG. In the figure, the left vertical axis represents the terminal voltage of the measurement cell, the right vertical axis represents the potential of the graphite electrode based on the lithium electrode calculated from the terminal voltage, the lower horizontal axis represents the capacity relative to 1 g of graphite, and the upper horizontal The axis indicates the amount of lithium ion insertion calculated from the capacity. As is apparent from this figure, a reduction capacity of 344 mAh / g was observed in the first reduction process, and the amount of electricity oxidized in the first oxidation process was 292 mAh / g.
[0028]
[Comparative Example 1]
In this comparative example, the electrode characteristics of graphite were examined in the same manner as in the example except that a Li ₂ S—GeS ₂ —P ₂ S ₅ solid electrolyte was used as the lithium ion conductive solid electrolyte. The solid electrolyte was synthesized by the following method.
[0029]
First, lithium sulfide (Li ₂ S), germanium sulfide (GeS ₂ ), and diphosphorus pentasulfide (P ₂ S ₅ ) were mixed at a molar ratio of 13: 2: 3. Next, this mixture was sealed under reduced pressure in a quartz jar whose inner surface was coated with carbon, and heated to 700 ° C. After heating for 8 hours, the mixture was cooled to obtain a lithium ion conductive solid electrolyte.
[0030]
A measurement cell was prepared in the same manner as in Example except that the solid electrolyte was used, and the electrode behavior of graphite was examined. As a result, as shown in FIG. 2, a reduction electric energy as large as 1385 mAh / g was observed in the first reduction process. In contrast, the amount of electricity oxidized in the first oxidation process was 241 mAh / g.
[0031]
[Comparative Example 2]
In this comparative example was examined electrode characteristics of the graphite in the same manner as in Example except for using _{Li 3 PO 4 -Li 2 S-} SiS 2 system solid electrolyte as lithium ion conductive solid electrolyte.
[0032]
As the solid electrolyte, lithium phosphate (Li ₃ PO ₄ ), lithium sulfide (Li ₂ S), and silicon disulfide (SiS ₂ ) were mixed at a molar ratio of 1:63:36, and the mixture was mixed at 950 ° C. in an argon stream. Then, the melt was rapidly cooled with a twin roller and synthesized.
[0033]
A measurement cell was prepared in the same manner as in Example except that the solid electrolyte was used, and the electrode behavior of graphite was examined. As a result, a large amount of reducing electricity of 1278 mAh / g was observed in the first reduction process. On the other hand, the amount of electricity oxidized in the first oxidation process was 401 mAh / g.
[0034]
As is apparent from the comparison of the examples and the comparative examples, when a solid electrolyte containing silicon or germanium is used, the theoretical capacity (372 mAh / g) of the lithium ion insertion reaction into graphite in the first reduction process. The amount of reduced electricity far exceeding) was observed, and the amount of electricity in the first oxidation process was extremely small compared to the amount of electricity in the reduction process. This result suggests that, in the first reduction process, the reduction reaction of silicon or germanium in the solid electrolyte occurs simultaneously with the lithium ion insertion reaction into graphite, and this reaction is poor in reversibility. It was.
[0035]
On the other hand, when a solid electrolyte containing no silicon or germanium is used, the first reduction electric charge is very close to the theoretical capacity of the lithium insertion reaction into graphite, and its reversibility is also high. It seemed that the electrolyte reduction reaction did not occur.
[0036]
In order to clarify the effect of the present invention in a clear form, only the unipolar behavior of the negative electrode was examined to show the effect of the present invention. In constructing an all-solid lithium secondary battery, as a positive electrode used for this, an all-solid lithium secondary battery is produced by using a known technique such as LiCoO ₂ or LiNiO _2. It is clear that this is possible.
[0037]
【Effect of the invention】
According to the present invention, an all-solid lithium secondary battery using a carbon material or a material in which lithium ions are inserted between carbon material layers as a negative electrode active material can be manufactured.
[Brief description of the drawings]
FIG. 1 is a graph showing electrode characteristics (charge / discharge characteristics) of a graphite material in one example of the present invention.
FIG. 2 is a diagram showing electrode characteristics (charge / discharge characteristics) of a graphite material in one comparative example of the present invention.

Claims (1)

The negative electrode active material is a carbon material or a material in which lithium ions are inserted between carbon material layers, and at least a lithium ion conductive solid electrolyte in contact with the negative electrode active material is composed of lithium sulfide, phosphorus sulfide, and component X. An all-solid lithium secondary battery, wherein X is at least one selected from lithium iodide, boron sulfide, aluminum sulfide, lithium phosphate, lithium borate, and lithium oxide.

Images