JP2007324079A - Positive electrode material for all-solid lithium battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a positive electrode active material and a positive electrode material in which thio-LISICON is used as a solid electrolyte, and which are for realizing an electrochemical capacity of 200 mAh/g or more, superior cycle characteristics, and a low cost. <P>SOLUTION: This is the positive electrode material for the all solid lithium ion secondary battery constituted of a nickel sulfide positive electrode active material, an electrolyte composed of a sulfide series super ion conductive crystalline body (thio-LISICON), and a mixture with a conductive agent. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a positive electrode material for an all-solid-state lithium battery using thio-LISICON as an electrolytic solution and nickel sulfide as a positive electrode active material.

  Lithium ion secondary batteries have a large electrochemical capacity, a high operating potential, and excellent charge / discharge cycle characteristics, and are currently used for portable small computers and mobile phones. By the way, in the field where the application of lithium ion secondary batteries is expected in the next generation, there are electric vehicles and power storage devices for power generators that have been developed in various fields in recent years. Therefore, it is essential to increase the size of the lithium ion secondary battery. However, at present, large-sized lithium ion secondary batteries have not been commercialized due to cost and safety issues. Although the cost problem can be solved by developing new electrode materials, the safety problem cannot be fundamentally solved as long as flammable organic solvent electrolytes are used. This is because a polymer electrolyte, which has been developed in recent years, cannot be expected to improve safety in a sol form, and a solid polymer film cannot obtain sufficient electrochemical characteristics at room temperature.

On the other hand, thio-LISICON, a sulphide-based superionic conducting crystal, is ~ 2 × 10 ^-3 S
Since it exhibits a high lithium ion conductivity of about / cm ⁻² and is a promising candidate as a non-combustible solid electrolyte, several researches and developments have been conducted so far and the results have been published (for example, Patent Document 1) ). In addition, lithium secondary batteries using Li _3.25 Ge _0.25 P _0.75 S ₄ as the thio-LISICON-based solid electrolyte, Mo ₆ S _{8 as} the positive electrode material, and Li-Al alloy as the negative electrode are about ~ 100 mAh / g at room temperature. It has succeeded in extracting a reversible electrochemical capacity (Non-patent Reference 1).

By the way, in the thio-LISICON all-solid-state battery, the state of the positive electrode-electrolyte interface has an influence on the electrochemical characteristics, and the development of a positive electrode material excellent in compatibility with the electrolyte has become an issue. For example, Patent Document 2 discloses a LiNi-based composite oxide and a positive electrode active material for a nonaqueous electrolyte containing aluminum hydroxide and aluminum oxide supported on the surface thereof. Patent Document 3 discloses a LiNi-based composite oxide and its surface. A positive electrode active material for a non-aqueous electrolyte containing an oxide or sulfide of an element selected from any of Ti, Sn, V, Nb, Mo, and W supported on the substrate is illustrated.

R. Kannno, M. Murayama, et al, Electrochemical and Solid-State Letters, 7 (12) A455 (2004)

Japanese Patent No. 3233345 JP 2005-322616 A JP 2003-173775 A

However, known positive electrode materials have not yet been able to extract the electrochemical capacity required at a practical level in high-rate discharge at room temperature. Accordingly, an object of the present invention is to provide a positive electrode active material and a positive electrode material for realizing an electrochemical capacity of 200 mAh / g or more, good cycle characteristics, and low cost by using thio-LISICON as a solid electrolyte.

Here, the present inventors have achieved this problem by using nickel sulfide as a positive electrode active material. More specifically, an all-solid-state lithium ion secondary battery comprising a mixture of a nickel sulfide positive electrode active material, an electrolyte composed of a sulfide-based superionic conductive crystal (thio-LISICON), and a conductive agent. The problem can be achieved by providing a positive electrode material.

The electrolyte comprising the above-described sulfide-based superionic conductive crystal is a compound composed of lithium, germanium, phosphorus and sulfur, and more specifically, thio-LISICON (Li _3.25 Ge _0.25 P _0.75 S ₄ ) is generally used. Further, the conductive agent is acetylene black.

Next, the mixture of the positive electrode active material, the electrolyte and the conductive agent is in a weight mixing ratio (%), and the positive electrode active material: electrolyte: conductive agent = (40-70%) :( 20-50%) :( 3-6% ). Here, the total of the positive electrode active material, the electrolyte, and the conductive agent is adjusted to be always 100%.
The mixture is obtained by uniformly mixing a predetermined ratio of positive electrode active material, electrolyte, and conductive agent with a ball mill or the like.

  According to the present invention, the electrochemical capacity can be increased, the cycle characteristics can be stabilized, and safety can be ensured. Therefore, practical use as a large-sized secondary battery such as an automobile battery or a storage battery can be expected.

Since the positive electrode material Ni ₃ S ₂ and the thio-LISICON solid electrolyte Li _3.25 Ge _0.25 P _0.75 S ₄ according to the present invention are both sulfides having a high vapor pressure, the synthesis method needs to be devised. In this invention, it synthesize | combined in argon stream using the airtight container shown in FIG. As shown in FIG. 1, the sealed container is composed of two quartz tubes, and the sample is heated in a carbon pot in an argon flow path. Compared with the conventional vacuum sealing method using a quartz tube, the vapor pressure of sulfide is high, making it difficult to control the composition, but the sealing operation is simpler and the quartz tube can be recycled.

By the way, Ni ₃ S ₂ can be synthesized by using Ni powder and sulfur as starting materials, mixing and molding in an argon atmosphere, and firing in an argon stream for about 700 ° C. × 12 hours. Also, in the synthesis of Li-Ge-P thio-LISICON, Li ₂ S, GeS ₂ and P ₂ S ₅ were used as starting materials, mixed and molded in an argon atmosphere, then 700 ° C x 2 hours in an argon stream It can be created by firing to a certain extent. Here, since the vapor pressure of sulfide during high-temperature firing is high and a lot of sulfur is lost during firing, it is necessary to mix more sulfide than the target composition in the initial mixed composition, for example, Ni ₃ In the synthesis of S ₂ , sulfur is mixed, and in the synthesis of thio-LISICON, P ₂ S ₅ is mixed in excess by about 10%.

FIG. 2 shows an example of a configuration diagram of an all-solid-state lithium ion secondary battery using each of the compounds obtained as described above. FIG. 2 shows an example in which Ni ₃ S ₂ is used as the positive electrode active material, Li _3.25 Ge _0.25 P _0.75 S ₄ is used as the electrolyte, and a Li / Al alloy sheet is used as the negative electrode. However, the negative electrode is limited to Li / Al. For example, a carbon / graphite-based material can also be used.

Li _3.25 Ge _0.25 P _0.75 S _{4 as} electrolyte layer
70 mg is molded into a cylinder with a diameter of 10 mm, and positive electrode composite on one side
10 mg was laminated in the order of Al and Li sheets on the other side, and then pressure molding was performed. For the positive electrode layer (positive electrode mixture), Ni ₃ S ₂ , Li _3.25 Ge _0.25 P _0.75 S ₄ , and acetylene black as a conductive agent were mixed in a ratio of 60: 40: 5, and mixed with a planetary ball mill. The integrated laminated battery (FIG. 2) was sealed in a sealed container under pressure and subjected to constant current charge / discharge measurement in the air at 0.1 to 0.2 mA and a final voltage of 300 to 2300 mV.

FIG. 3 shows X-ray diffraction _{patterns of} Ni ₃ S ₂ and Li _3.25 Ge _0.25 P _0.75 S ₄ obtained by synthesis. In the synthesis of Ni ₃ S _2, although containing some Ni as an impurity, it was possible to obtain a Ni ₃ S ₂ substantially in a single phase. The synthesis of Li _3.25 Ge _0.25 P _0.75 S ₄ was reported by Murayama et al. In vacuum encapsulated synthesis (R. Kannno and
M. Murayama, J. Electrochem, Soc., 148 (7), A742 (2001)) x = 0.65, region 2, monoclinic phase crystals could be obtained. The conductivity measurement by AC impedance measurement of this sample showed 2.39 × 10 ⁻³ S / cm ^−2, and it was confirmed that the sample had sufficient characteristics as an electrolyte material for an all-solid-state lithium ion secondary battery.

A mixture of Ni ₃ S ₂ and Li _3.25 Ge _0.25 P _0.75 S ₄ in a ratio of 6: 4 was used as the positive electrode, Li _3.25 Ge _0.25 P _0.75 S ₄ was used as the electrolyte, and a Li / Al sheet was used as the negative electrode. The result of performing the current charge / discharge is shown in FIG. A constant current of 100 μA was passed from the discharge direction, and then charging was performed at 100 μA. The discharge and charge end voltages were set to 0.3 V and 2.3 V, respectively. In the first discharge, the voltage gradually dropped from an initial potential of 1.3 V to 0.3 V. In subsequent charging, the voltage rose from around 0.5 V to around 1.5 V and then gradually changed to 0.23 V. Unlike the initial discharge behavior in the second and subsequent discharges, the voltage dropped from around 2.0 V to 1.0 V and then slowly reached 0.3 V. The obtained electrochemical capacity was as large as ˜300 mAh / g, and the capacity after 20 cycles was also kept as high as ˜280 mAh / g.

The reaction in the positive electrode that proceeds by charging and discharging is as shown in the following formula, and it can be estimated that the reaction from left to right is progressing by discharging and the reverse reaction is proceeding by charging.
Ni ₃ S ₂ + 4Li⇔3Ni + 2Li ₂ S
The change in the discharge behavior after the second time was due to the further oxidation of Ni ₃ S ₂ shown in the following formula by applying a voltage exceeding the initial open circuit potential of Ni ₃ S ₂ // Li of 1.3 V. Conceivable.
Ni ₃ S ₂ + Li ₂ S⇔3NiS + 2Li
In the charging of Ni ₃ S ₂ , the charging potential tended to become unstable especially at a potential exceeding 2.0 V. If this behavior was remarkable, the cycle characteristics were greatly deteriorated. The reaction proceeding at this potential is the formation of NiS corresponding to the above formula, and when oxidation from Ni ₃ S ₂ to NiS is not performed smoothly, the secondary material such as isolation of the active material and decomposition of the electrolyte It may cause reaction and lead to capacity degradation.

Ni ₃ S ₂ / in positive electrode mixture
In order to investigate the relationship between the thio-LISICON ratio and electrochemical characteristics, we prepared a positive electrode mixture in which the Ni ₃ S ₂ / thio-LISICON ratio was mixed at 5: 5, 6: 4, and 7: 3. Fabrication and constant current charge / discharge were performed. FIG. 5 shows the results of charging / discharging at 0.3 V to 2.3 V cut-off, 0.1 mA from discharging. As the Ni ₃ S ₂ ratio increases to 50%, 60% and 70%, the capacity per gram of Ni ₃ S ₂ increases from 250 mAh / g to 450 mAh / g, while the initial irreversible capacity also increases There was a tendency to increase. Reduction reaction of Ni ₃ S ₂ by electric discharge
^{_{Ni 3 S 2 +4 Li + +}} 4e - → 3 Ni + 2 Li 2 S
The capacity per 1 g of Ni ₃ S ₂ derived from is 446.3 mAh / g, and in Ni ₃ S ₂ , 70%, the above reaction is almost completely completed. Ni ₃ S ₂ is considered to remain.

In order to confirm the possibility of high-speed charge / discharge, the results of a charge / discharge test conducted at a current of 0.5 mA, which is five times the current level, are shown in FIG. Test cell is Ni ₃ S ₂ : LISICON
= 6: 4 positive electrode mixture was used. Reversible charge / discharge was possible, and a capacity of 120 mAh / g was obtained. Compared to charging / discharging at 0.1 mA, the capacity was less than half, but no significant capacity degradation was observed with the cycle up to 20 cycles.

It is a schematic diagram which shows the reaction furnace for synthesize | combining a nickel sulfide type positive electrode material. It is a schematic diagram which shows the structure of an all-solid-state Li secondary battery. It is a figure which shows the X-ray-diffraction figure of positive electrode compound material (Ni3S2 / Li3.25Ge0.25P0.75S4). The results of constant current discharge for all solid state batteries using Ni3S2: Li3.25Ge0.25P0.75S4 = 6: 4 as the positive electrode mixture, Li3.25Ge0.25P0.75S4 as the electrolyte, and Li / Al as the negative electrode. FIG. It is a figure which shows the result of having performed constant current charging / discharging (0.1 mA) about the positive electrode compound material which mixed Ni3S2: Li3.25Ge0.25P0.75S4 by 5: 5, 6: 4, 7: 3, respectively. It is a figure which shows the result of having performed high-speed charging / discharging with the electric current of 0.5 mA about the all-solid-state Li secondary battery of Ni3S2: Li3.25Ge0.25P0.75S4 = 6: 4.

Claims (5)

  A positive electrode material for an all solid lithium ion secondary battery, characterized in that nickel sulfide is used as a positive electrode active material. A positive electrode material for an all-solid-state lithium ion secondary battery, comprising a mixture of the nickel sulfide positive electrode active material, an electrolyte comprising a sulfide-based superionic conductive crystal, and a conductive agent.   The positive electrode material for an all-solid-state lithium ion secondary battery according to claim 2, wherein the electrolyte composed of the sulfide-based superionic conductive crystal is a compound composed of lithium, germanium, phosphorus, and sulfur.   The positive electrode material for an all solid lithium ion secondary battery according to claim 2, wherein the conductive agent is acetylene black. The mixture according to any one of claims 2 to 4, in a weight mixing ratio (%), positive electrode active material: electrolyte: conductive agent = (40-70%) :( 20-50%) :( 3-6% The positive electrode material for an all-solid-state lithium ion secondary battery according to any one of claims 2 to 4, wherein

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