JP2010192373A - All-solid secondary battery - Google Patents

Abstract

Provided are a positive electrode for an all-solid-state secondary battery exhibiting excellent rate characteristics and cycle characteristics, and an all-solid-state secondary battery using the same.
A surface-treated positive electrode active material in which at least a part of the surface of a positive electrode active material capable of occluding and releasing lithium is coated with an oxide containing at least one of group 13 elements in the periodic table. I tried to do it.
[Selection figure] None

Description

  The present invention relates to a positive electrode for an all-solid-state secondary battery exhibiting excellent rate characteristics and cycle characteristics, and an all-solid-state secondary battery using the same.

  Lithium ion secondary batteries have large electrochemical capacity, high operating potential, and excellent charge / discharge cycle characteristics, so portable information terminals, portable electronic devices, small household power storage devices, motorcycles powered by motors, There is an increasing demand for applications such as electric vehicles and hybrid electric vehicles. With the spread of such applications, there is a demand for improved safety and higher performance of lithium ion secondary batteries. However, since a conventional lithium ion secondary battery using a nonaqueous electrolytic solution in which a lithium salt is dissolved in an organic solvent as an electrolyte easily ignites at about 150 ° C., there is a concern about its safety. For this reason, research on all-solid secondary batteries using a solid electrolyte made of an inorganic material, which is an incombustible material, has been actively conducted for the purpose of improving safety.

  Solid electrolytes for all-solid-state secondary batteries include sulfides and oxides. From the viewpoint of lithium ion conductivity, sulfide-based solid electrolytes are the most promising materials. However, when a sulfide solid electrolyte is used, there is a problem that a reaction occurs at the interface with the positive electrode active material or the negative electrode active material and a resistance component is generated. The generation of the resistance component is particularly remarkable when compounds containing different anions are in contact with each other.

Conventionally, for the purpose of improving lithium ion conductivity at the interface, attempts have been made to improve the conductivity by bringing different compounds such as LiI-Al ₂ O ₃ into contact with each other to form a space charge layer at the interface. In the case of a sulfide solid electrolyte, the resistance at the interface tends to increase more due to a change in the lithium ion concentration or reaction with the positive electrode active material. With respect to such problems, studies have been made to coat the surface of lithium metal oxide such as Li—CoMnNi—O with an oxide such as Li ₄ Ti ₅ O ₁₂ (Patent Document 1). It is desired to further reduce the resistance component and improve the output as a battery.

As an improvement in the low output characteristics when using a solid electrolyte, examination of thinning of the solid electrolyte (Patent Document 2) and use of a positive electrode active material (compound having the same anion) of the same system as the solid electrolyte (Patent Document 3) In addition, an active material coating treatment with SiO ₂ which is stable as an oxide has been studied. However, the thin solid electrolyte has a low capacity, and high power output and improvement in cycle characteristics have not been achieved by combining materials of the same type, and characteristics are not sufficient only by treatment of oxides such as SiO _2. is there.

JP2008-103280 JP2000-340257 JP2007-324079

  Then, this invention makes it a subject to provide the positive electrode for all-solid-state secondary batteries which shows the outstanding rate characteristic and cycling characteristics, and the all-solid-state secondary battery using the same in view of the said present condition.

  That is, in the positive electrode for an all-solid secondary battery according to the present invention, at least a part of the surface of the positive electrode active material capable of occluding and releasing lithium is covered with an oxide containing at least one of group 13 elements in the periodic table. The surface-treated positive electrode active material thus formed is contained.

  In such a case, the surface of the positive electrode active material is coated with a compound having a bond of X—O (X represents a group 13 element B, Al, Ga, In, Tl). Direct contact between the positive electrode active material and the solid electrolyte can be prevented, and therefore, reaction at the interface between the positive electrode active material and the solid electrolyte is suppressed, and generation of a resistance component can be suppressed.

  The oxide may be lithium oxide Li—X—O. Since the Li—X—O compound functions as a path through which lithium ions can move, when the oxide is a Li—X—O compound, lithium ions are easily diffused, which leads to an improvement in ion conductivity.

  As the group 13 element, boron, aluminum, or gallium is preferably used. The oxides of these elements are easy to synthesize, and are excellent in insulation between the positive electrode active material and the solid electrolyte and lithium ion diffusibility.

  At least a part of the oxide preferably has a tetracoordinate structure. Since the tetracoordinate oxide has excellent lithium ion diffusibility, the ion conductivity can be improved by coating the surface of the positive electrode active material with the tetracoordinate oxide.

  Such an all-solid secondary battery including the positive electrode according to the present invention is also one aspect of the present invention. That is, the all-solid-state secondary battery according to the present invention includes a positive electrode according to the present invention, a negative electrode containing a negative electrode active material capable of being alloyed with lithium, occluded or released, and inorganic containing sulfur and lithium. And a solid electrolyte layer containing a solid electrolyte.

  The all-solid-state secondary battery using a sulfide solid electrolyte has a large interface resistance between the positive electrode active material and the solid electrolyte, but according to the present invention, the surface of the positive electrode active material has a (Li-) X—O compound (X Represents a group 13 element B, Al, Ga, In, or Tl.), The coating layer can prevent direct contact between the solid electrolyte and the positive electrode active material. Resistance components are less likely to be generated at the interface. Further, when the surface of the positive electrode active material is coated with a Li—X—O compound, a decrease in the lithium ion concentration at the interface between the positive electrode active material and the solid electrolyte is suppressed, and a path through which lithium ions can move is provided. As a result, the resistance at the interface between the positive electrode active material and the solid electrolyte can be reduced. For this reason, according to this invention, the all-solid-state secondary battery excellent in the rate characteristic and cycling characteristics can be obtained.

It is a chart which shows the result of the solid 27Al-NMR spectrum measurement of aluminum oxide. It is a photograph which shows the result of having observed the cross section of the positive electrode active material by SEM / WDX method.

  Hereinafter, an all solid state secondary battery according to an embodiment of the present invention will be described.

  The all solid state secondary battery according to the present embodiment is formed by laminating a positive electrode, a negative electrode, and a solid electrolyte layer sandwiched between the positive electrode and the negative electrode.

  The positive electrode active material contained in the positive electrode is not particularly limited as long as it can reversibly occlude and release lithium ions. For example, lithium cobaltate, lithium nickelate, nickel lithium cobaltate, nickel Examples include lithium cobaltaluminate, nickel cobalt lithium manganate, lithium manganate, lithium iron phosphate, nickel sulfide, copper sulfide, sulfur, iron oxide, and vanadium oxide. These positive electrode active materials may be used independently and 2 or more types may be used together.

  The positive electrode active material is contained in the positive electrode as a surface-treated positive electrode active material in which at least a part of the surface thereof is covered with an oxide of a group 13 element in the periodic table. Since the group 13 element is hardly oxidized and reduced and is chemically stable, when the surface of the positive electrode active material is covered with the oxide of the group 13 element, contact between the positive electrode active material and the solid electrolyte can be prevented. For this reason, reaction at the interface between the positive electrode active material and the solid electrolyte is suppressed, and generation of a resistance component can be suppressed.

  The positive electrode active material only needs to have at least a part of its surface covered with the oxide, and the entire surface of the positive electrode active material may not be covered with the oxide. The surface may be partially coated with the oxide.

  The group 13 element is specifically B, Al, Ga, In, or Tl, and the oxide may include one kind or two or more kinds. The oxide preferably contains boron, aluminum, or gallium. The oxides of these elements are easy to synthesize, and when the positive electrode active material is coated with an oxide of these elements, a secondary battery having excellent characteristics can be obtained.

  Examples of the oxide include an X—O compound composed only of a group 13 element X and oxygen, and lithium oxide Li—X—O containing lithium. Although the surface of the positive electrode active material may be coated with only one of the X—O compound and the Li—X—O compound, it is preferable that the X—O compound and the Li—X—O compound coexist. The X—O compound is excellent in the ability to suppress the interfacial reaction between the positive electrode active material and the solid electrolyte, and the Li—X—O compound is excellent in lithium ion diffusibility by forming a path through which lithium ions can move. Therefore, the coexistence of the X—O compound and the Li—X—O compound makes it possible to achieve a good balance between the suppression of the interfacial reaction between the positive electrode active material and the solid electrolyte and the diffusion of lithium ions, Both safety and high output of the secondary battery can be achieved.

  The main coordination structure of the Group 13 element oxide is tetracoordinate and hexacoordinate, but at least a part of the oxide covering the surface of the positive electrode active material is a tetracoordinate oxide. It is preferable. Tetracoordinate oxide is excellent in lithium ion diffusibility, so that the surface of the positive electrode active material is coated with the tetracoordinate oxide to improve ion conductivity and increase output. Can do. Although all of the oxide covering the surface of the positive electrode active material may be a tetracoordinate oxide, it is more preferable that a tetracoordinate oxide and a hexacoordinate oxide coexist. Although the 6-coordinate oxide does not have sufficient lithium ion diffusibility, it is chemically stable and has excellent ability to suppress the interfacial reaction between the positive electrode active material and the solid electrolyte. When a 6-coordinate oxide coexists, it is possible to achieve a good balance between the suppression of the interfacial reaction between the positive electrode active material and the solid electrolyte and the diffusion of lithium ions. Can be achieved together.

  FIG. 1 is a chart showing the results of measuring a solid 27Al-NMR spectrum for aluminum oxide using a nuclear magnetic resonance apparatus (Varian NMR System 400WB) under the conditions of an observation frequency of 104.35 MHz and a probe of 2.5 mmφ. . Among these, when the 4-coordinate oxide is excessive (a), the lithium ion diffusion action is not sufficient, and when the 6-coordinate oxide is excessive (b), the interface between the positive electrode active material and the solid electrolyte is used. The inhibitory action of the reaction is not sufficient. On the other hand, when the tetracoordinate oxide and the hexacoordinate oxide are present in a ratio of approximately 1: 1 (c), the interfacial reaction between the positive electrode active material and the solid electrolyte is suppressed, and the lithium ion Good balance with diffusion.

  The method of coating the surface of the positive electrode active material with the oxide is not particularly limited. For example, a method of immersing the positive electrode active material particles in the oxide precursor solution and then heat-treating the precursor, the oxide precursor Examples include a method of spraying the body solution onto the positive electrode active material particles and then performing a heat treatment.

  Examples of the precursor of the oxide include an alkoxide of a group 13 element, which is dissolved in an appropriately selected organic solvent and used as the precursor solution.

  FIG. 2 shows a cross section of positive electrode active material particles before coating treatment (a) and a cross section of surface treated positive electrode active material particles after coating treatment (b) when aluminum isopropoxide is used as the oxide precursor. Is a photograph showing the result of observation by wavelength dispersive X-ray spectroscopy (WDX) using a scanning electron microscope (SEM). As shown in FIG. 2B, it can be seen that the surface of the surface-treated positive electrode active material particles is coated with aluminum oxide (fluorescent portion).

The negative electrode active material contained in the negative electrode is not particularly limited as long as it can be alloyed with lithium, or reversibly occluded and released from lithium. For example, lithium, indium, tin, aluminum, silicon etc. of metals and their _{alloys: Li 4/3 Ti 5/3 O 4,} SnO or the like transition metal oxides: artificial graphite, graphite carbon fibers, resin fired carbon, pyrolytic vapor grown carbon, coke, mesocarbon Examples thereof include carbon materials such as beads (MCMB), furfuryl alcohol resin calcined carbon, polyacene, pitch-based carbon fiber, vapor-grown carbon fiber, natural graphite, and non-graphitizable carbon. These negative electrode active materials may be used independently and 2 or more types may be used together.

  For the positive electrode and the negative electrode, additives such as, for example, a conductive agent, a binder, an electrolyte, a filler, a dispersant, and an ionic conductive agent may be appropriately selected and blended with the powder made of the above-described active material.

  Examples of the conductive agent include graphite, carbon black, acetylene black, ketjen black, carbon fiber, and metal powder. Examples of the binder include polytetrafluoroethylene, polyvinylidene fluoride, and polyethylene. Is mentioned. Examples of the electrolyte include a sulfide solid electrolyte described later.

  In order to produce the positive electrode or the negative electrode, for example, a mixture of the above active material and various additives is added to a solvent such as water or an organic solvent to form a slurry or paste, and the obtained slurry or paste is used as a doctor blade. It is applied to a current collector using a method or the like, dried, and consolidated with a rolling roll or the like to obtain a positive electrode or a negative electrode.

  Examples of the current collector include plates and foils made of indium, copper, magnesium, stainless steel, titanium, iron, cobalt, nickel, zinc, aluminum, germanium, lithium, or alloys thereof. Is mentioned.

  In addition, it is good also as a positive electrode or a negative electrode by carrying out the consolidation shaping | molding to a pellet form, without using a collector. Moreover, when using a metal or its alloy as a negative electrode active material, you may use a metal sheet (foil) as it is.

The solid electrolyte layer contains a sulfide solid electrolyte. The sulfide solid electrolyte is not particularly limited as long as it is an inorganic solid electrolyte containing sulfur and lithium. For example, Li ₂ S alone, Li ₂ S—P ₂ S ₅ , Li ₂ S—SiS ₂ , Li _{2 S-GeS 2, Li 2 S} -B 2 S 5, Li 2 S-Al 2 S complex compounds such as _5, and the like.

  The solid electrolyte layer can be obtained in the form of a pellet by pressing the sulfide solid electrolyte.

  The all-solid-state secondary battery according to the present embodiment can be manufactured by laminating and pressing these positive electrode, solid electrolyte layer, and negative electrode.

  The present invention will be described in more detail with reference to the following examples. However, the present invention is not limited to these examples.

Example 1
An In foil (thickness 0.05 mm) used as the negative electrode was punched out with φ13 (mm) and set in a cell container. On top of that, 80 mg of a solid electrolyte Li ₂ S—P ₂ S ₅ (80-20 mol%) (SE) mechanically milled (MM treated) was laminated, and the surface was lightly adjusted with a molding machine. Further, lithium cobaltate (LiCoO ₂ ) particles as a positive electrode active material are dispersed in ethanol, and aluminum isopropoxide is dissolved therein so that the amount of Al added is 0.05 wt%, followed by heat treatment, thereby producing Li-Al. A surface-treated positive electrode active material coated with lithium aluminum oxide having a —O bond was obtained. And what mixed the surface treatment positive electrode active material, SE, and the vapor growth carbon fiber (VGCF) which is a electrically conductive agent in the ratio of 60/35/5 wt% was laminated | stacked on SE as positive mix. . In that state, a pressure was applied at a pressure of 3 t / cm ² to produce a pellet to obtain a test cell.

  The obtained test cell was charged to an upper limit voltage of 4 V at a constant current of 0.02 C at 25 ° C., measured for initial capacity, discharged to 0.1 C to a final discharge voltage of 1 V, and charged and discharged in the same manner. Was repeated. The capacity retention rate with respect to the initial capacity after the end of 50 cycles was measured, and the cycle characteristics of the test cell were evaluated.

  Further, the obtained test cell was initially charged with a constant current of 0.02 C to an upper limit voltage of 4 V and discharged at 0.02 C. The second charge was performed under the same conditions as the first time, and discharged at a constant current of 0.1C. And the ratio (%) of the capacity | capacitance of the 2nd time with respect to initial stage capacity | capacitance was measured, and the rate characteristic of the said test cell was evaluated.

(Example 2)
A test cell was prepared in the same manner as in Example 1 except that an Al addition amount of 0.1 wt% was used as the surface-treated positive electrode active material, and the battery characteristics were evaluated.

(Example 3)
A test cell was prepared in the same manner as in Example 1 except that an Al addition amount of 0.2 wt% was used as the surface-treated positive electrode active material, and battery characteristics were evaluated.

Example 4
A test cell was prepared in the same manner as in Example 1 except that an Al addition amount of 0.5 wt% was used as the surface-treated positive electrode active material, and battery characteristics were evaluated.

(Example 5)
A test cell was prepared in the same manner as in Example 1 except that boron propoxide was used instead of aluminum isopropoxide, and the surface-treated positive electrode active material used had a B addition amount of 0.1 wt%. The battery characteristics were evaluated.

(Example 6)
A test cell was prepared in the same manner as in Example 1 except that gallium isopropoxide was used instead of aluminum isopropoxide, and the surface-treated positive electrode active material was a material with a Ga addition amount of 0.1 wt%. Fabricated and battery characteristics were evaluated.

(Comparative Example 1)
A test cell was prepared in the same manner as in Example 1 except that the positive electrode active material was not subjected to surface treatment, and battery characteristics were evaluated.

(Comparative Example 2)
A test cell was prepared in the same manner as in Example 1 except that aluminum oxide was mixed at the time of preparing the positive electrode so that the amount of Al added was 0.05 wt% without performing the surface treatment of the positive electrode active material. Evaluated.

(Comparative Example 3)
A test cell was prepared in the same manner as in Example 1 except that aluminum oxide was mixed at the time of preparing the positive electrode so that the amount of Al added was 0.5 wt% without performing the surface treatment of the positive electrode active material. Evaluated.

(Comparative Example 4)
A test cell was prepared in the same manner as in Example 1 except that boron oxide was mixed at the time of preparing the positive electrode so that the B addition amount was 0.5 wt% without performing the surface treatment of the positive electrode active material. Evaluated.

(Comparative Example 5)
A test cell was prepared in the same manner as in Example 1 except that gallium oxide was mixed at the time of preparing the positive electrode so that the Ga addition amount was 0.5 wt% without performing the surface treatment of the positive electrode active material. Evaluated.

  The results obtained in Examples 1 to 6 and Comparative Examples 1 to 5 are shown in Table 1 below.

  From the results shown in Table 1, when the surface of the positive electrode active material was coated with an oxide of a group 13 element, a high capacity was obtained and the rate characteristics and cycle characteristics were excellent. On the other hand, when the oxide of the group 13 element is simply mixed at the time of preparing the positive electrode mixture, the surface of the positive electrode active material is not covered with the oxide of the group 13 element, the capacity remains low, rate characteristics and It became clear that the cycle characteristics were not sufficient.

  According to the present invention, an all-solid secondary battery with high safety and high output can be provided.

Claims (5)

  It is characterized in that at least part of the surface of the positive electrode active material capable of occluding and releasing lithium contains a surface-treated positive electrode active material coated with an oxide containing at least one of group 13 elements in the periodic table. A positive electrode for an all-solid-state secondary battery.   The positive electrode for an all-solid-state secondary battery according to claim 1, wherein the oxide is a lithium oxide.   The positive electrode for an all-solid-state secondary battery according to claim 1, wherein the group 13 element is at least one selected from the group consisting of boron, aluminum, and gallium.   The positive electrode for an all-solid-state secondary battery according to claim 1, wherein at least a part of the oxide has a tetracoordinate structure. The positive electrode according to claim 1, 2, 3, or 4,
A negative electrode containing a negative electrode active material capable of alloying with lithium, or occluding and releasing lithium, and
And a solid electrolyte layer containing an inorganic solid electrolyte containing sulfur and lithium.