JP2009252421A - Negative electrode active material, manufacturing method therefor, and battery including negative electrode active material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode active material preventing the spontaneous formation of SEI caused by reduction decomposition reaction of an electrolyte and having high capacity and excellent temperature characteristics. <P>SOLUTION: The negative electrode active material 10 for a lithium ion secondary battery includes a carbon particle 12 capable of holding lithium ions and a surface film 14 made of lithium titanate covering the carbon particle 12. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a negative electrode active material for a lithium ion secondary battery and a method for producing the same.

  In recent years, lithium secondary batteries, nickel hydride batteries, and other secondary batteries have become increasingly important as power sources for mounting on vehicles or as power sources for personal computers and portable terminals. In particular, a lithium ion secondary battery that is lightweight and obtains a high energy density is expected to be preferably used as a high-output power source mounted on a vehicle. One typical configuration of this type of lithium ion secondary battery includes an electrode having a configuration in which an electrode active material capable of reversibly occluding and releasing lithium ions is formed on an electrode current collector. For example, a lithium ion secondary battery using a carbon-based material for the negative electrode is a typical example of such a battery.

In a lithium ion secondary battery using a carbon-based material such as graphite for the negative electrode, the surface of the carbon-based material (specifically, carbon-based particles) is generally reduced by a reductive decomposition reaction of the electrolyte during the initial charge. In addition, it is known that a thin film called SEI (Solid Electrolyte Interface) is formed. The presence of SEI can inactivate and stabilize the surface of carbon-based particles such as graphite particles so that lithium can be inserted. In addition, patent documents 1-3 etc. are mentioned as a prior art regarding the lithium ion secondary battery which used the carbon-type material for the negative electrode of the lithium ion secondary battery.
JP 2007-299728 A JP 2005-533373 A JP 2000-348725 A

  However, in the lithium ion secondary battery, the SEI produced by the decomposition reaction of the electrolytic solution (non-aqueous electrolytic solution) has poor conductivity at low temperatures with respect to lithium ions. It can be a factor that deteriorates characteristics. In addition, the SEI produced by the decomposition reaction of the electrolytic solution has relatively low heat resistance and easily decomposes when the battery temperature rises, so that the durability of the battery is deteriorated. Thus, although SEI produced by the decomposition reaction of the electrolytic solution has the effect of inactivating and stabilizing the surface of the carbon-based particles so that lithium can be inserted, it has poor stability as a coating, and the temperature of the battery It is also a factor that deteriorates characteristics (for example, high-rate characteristics at low temperatures).

  The present invention has been made in view of the above points, and its main purpose is to prevent spontaneous SEI formation due to reductive decomposition reaction of the electrolytic solution, and to have high capacity and excellent temperature characteristics. An object is to provide a negative electrode active material for a lithium ion secondary battery. Another object of the present invention is to provide a method for producing a negative electrode active material capable of stably producing a negative electrode active material having such performance. Another object is to provide a lithium ion secondary battery including such a negative electrode active material in the negative electrode.

  The negative electrode active material provided by the present invention is a negative electrode active material for a lithium ion secondary battery. The negative electrode active material includes carbon-based particles that can hold lithium ions and a surface coating composed of lithium titanate that covers the carbon-based particles. According to the configuration of the present invention, the surface coating made of lithium titanate having excellent conductivity with respect to lithium ions is used to inactivate and stabilize the surface of the carbon-based particles, and to reduce and decompose the electrolytic solution. As a result, it is possible to prevent the generation of SEI having the above-mentioned drawbacks that may occur naturally. Thereby, the influence (for example, the fall of the high-rate characteristic in low temperature and the fall of the durability accompanying SEI decomposition | disassembly at high temperature) resulting from the decomposition reaction of electrolyte solution can be avoided. That is, according to the negative electrode active material of the present invention, a lithium ion secondary battery having a high capacity and excellent temperature characteristics can be provided.

In a preferred embodiment of the negative electrode active material disclosed herein, the lithium titanate constituting the surface coating is one selected from the group consisting of Li ₄ Ti ₅ O ₁₂ , LiTi ₂ O ₄ and Li ₂ TiO _3. Or a mixture of any two or more thereof. The surface coating made of lithium titanate has high lithium ion conductivity at a low temperature and is not easily decomposed even at a high temperature as compared with SEI that can be naturally generated due to the reductive decomposition reaction of the electrolytic solution. Therefore, it is particularly preferable as a surface coating suitable for the purpose of the present invention.

  In another preferred embodiment of the negative electrode active material disclosed herein, the surface coating is composed of spinel type or anatase type lithium titanate. Since the surface film made of lithium titanate having such a crystal structure has good lithium ion conductivity, it can be particularly preferably used as a surface film suitable for the purpose of the present invention.

  Moreover, this invention provides the manufacturing method of the negative electrode active material for lithium ion secondary batteries comprised from the carbon-type particle | grains which can hold | maintain lithium ion. Such a production method includes a step of preparing a sol from a dispersion containing the carbon-based particles, peroxotitanic acid, and lithium carbonate or lithium hydroxide, and firing the prepared sol, Forming a surface coating composed of lithium titanate on the surface.

  In the production method of the present invention, a sol in which carbon-based particles are stably dispersed (uniformly dispersed) is used as a raw material, and typically a solvent is volatilized from the sol to produce a particle-dispersed gel having no fluidity. A surface coating composed of lithium titanate is formed on the surface of the carbon-based particles by heating. Thereby, a lithium titanate film can be uniformly formed on the surface of the carbon-based particles. Therefore, according to the production method disclosed herein, a negative electrode active material suitable for the object of the present invention can be produced stably (with high quality stability).

  In a preferred embodiment of the method disclosed herein, the preparation of the sol is performed by autoclaving a dispersion containing the carbon-based particles, the peroxotitanic acid, and the lithium carbonate or lithium hydroxide. . According to such a method, a sol having fluidity suitable for the object of the present invention can be easily and stably produced.

In a preferred embodiment of the method disclosed herein, the surface coating is one compound selected from the group consisting of Li ₄ Ti ₅ O ₁₂ , LiTi ₂ O ₄ and Li ₂ TiO ₃ , or any two or more thereof. Form with mixture. In one preferred embodiment of the method disclosed herein, the baking is performed at a heating temperature at which a surface film composed of spinel type or anatase type lithium titanate can be formed.

  In a preferred embodiment of the method disclosed herein, the calcination is performed in an air atmosphere. According to this method, a by-product (for example, a by-product gas) at the time of lithium titanate generation can be easily removed. Therefore, the sol can be fired stably (with high quality stability).

  Embodiments according to the present invention will be described below with reference to the drawings. In the following drawings, members / parts having the same action are described with the same reference numerals. Note that the dimensional relationship (length, width, thickness, etc.) in each drawing does not reflect the actual dimensional relationship. Further, matters other than the matters specifically mentioned in the present specification and matters necessary for carrying out the present invention (for example, the configuration and manufacturing method of an electrode body including a positive electrode and a negative electrode, the configuration and manufacturing method of a separator and an electrolyte, General techniques relating to the construction of lithium secondary batteries and other batteries, etc.) can be understood as design matters for those skilled in the art based on the prior art in this field.

  The configuration of the negative electrode active material according to this embodiment will be described with reference to FIGS. 1 and 2. FIG. 1 is a schematic cross-sectional view schematically showing a cross section of the negative electrode, and FIG. 2 is a schematic cross-sectional view schematically showing a cross section of the negative electrode active material.

  As shown in FIG. 1, the negative electrode 100 according to this embodiment is formed by laminating a negative electrode mixture layer 20 including a negative electrode active material 10 on a negative electrode current collector 30. The negative electrode current collector 30 may be the same as that used for a typical lithium ion secondary battery, and is, for example, a copper foil. The negative electrode mixture layer 20 includes a negative electrode active material 10 and a negative electrode binder that adheres the negative electrode active material 10 onto the negative electrode current collector 30. The negative electrode binder is, for example, PVDF. The negative electrode composite material layer 20 may include other negative electrode composite material layer forming components used as necessary.

  As schematically shown in FIG. 2, when the carbon-based material constituting the negative electrode active material 10 is viewed microscopically, the negative electrode active material 10 includes carbon-based particles 12 that can hold lithium ions, and carbon-based particles 12. And a surface coating film 14 made of lithium titanate. The carbon-based particles 12 are made of a carbon-based material that can hold lithium ions (can store and release lithium ions electrochemically). The carbon-based material constituting the carbon-based particles 12 is not particularly limited as long as it is the same as that used for a typical lithium ion secondary battery, but a material having a large battery capacity is preferable. Examples of such a carbon-based material include natural graphite and its improved products, artificial graphite (for example, MCMB), low graphitized material, non-graphitizable carbon material, and the like. The particle size of the carbon-based particles 12 is not particularly limited, but for example, those having an average particle size in the range of about 2 μm to 20 μm are preferable. The carbon-based particles 12 having such a particle diameter can be easily formed by a conventionally known method, for example, mechanically pulverizing.

  The carbon-based particles 12 are covered with a surface coating 14 (hereinafter also referred to as “lithium titanate coating”) made of lithium titanate. The lithium titanate coating 14 is conductive to lithium ions, while having substantially no electronic conductivity. Therefore, the surface coating 14 made of lithium titanate can inactivate and stabilize the surface of the carbon-based particles 12 so that lithium ions can be inserted and removed. In addition, since the lithium titanate coating 14 has better lithium ion conductivity than the carbon-based particles, SEI due to the reductive decomposition reaction of the electrolytic solution is not generated during the initial charge.

Examples of such lithium titanate include one compound selected from the group consisting of Li ₄ Ti ₅ O ₁₂ , LiTi ₂ O ₄ and Li ₂ TiO _3, or a mixture of any two or more thereof. The surface coating 14 made of lithium titanate has high lithium ion conductivity at a low temperature and easy even at a high temperature as compared with SEI that naturally occurs in a battery (cell) due to a reductive decomposition reaction of an electrolytic solution. Is not disassembled. Therefore, it can be particularly preferably used as a surface coating suitable for the purpose of the present invention.

  The surface coating 14 is preferably composed of spinel type or anatase type lithium titanate. A surface film made of spinel type or anatase type lithium titanate has good lithium ion conductivity and can be particularly preferably used as a surface film suitable for the purpose of the present invention. In addition, not only a spinel type or an anatase type but a preferable structure can be suitably adopted according to the material (typically composition formula) of lithium titanate. For example, an anatase type, a rutile type, a spinel type, or a structure in which they are mixed may be used. Furthermore, the surface coating 14 may contain a material other than lithium titanate (for example, titanium oxide).

  According to the configuration of the negative electrode active material 10 according to the present embodiment, the surface coating 14 made of lithium titanate inactivates and stabilizes the surface of the carbon-based particles 12 and also results from the reductive decomposition reaction of the electrolytic solution. Thus, it is possible to prevent the generation of SEI having various defects that may occur naturally. Thereby, the bad influence by presence of SEI produced | generated by decomposition | disassembly of electrolyte solution can be avoided.

  In a negative electrode active material composed of typical carbon-based particles, SEI is generally generated on the surface of the carbon-based particles by the reductive decomposition reaction of the electrolytic solution during the initial charge. SEI naturally produced by the decomposition of the electrolytic solution has poor lithium ion conductivity at a low temperature, and can cause deterioration of the high rate characteristics as the battery temperature decreases. In addition, SEI generated by the decomposition of the electrolytic solution is easily decomposed when the battery temperature rises, which may be a factor that deteriorates the durability of the battery. On the other hand, according to the configuration of the present embodiment, the surface coating 14 made of lithium titanate can prevent the generation of SEI due to the reductive decomposition reaction of the electrolytic solution. Further, it is possible to avoid a decrease in durability at high temperatures. Therefore, according to the configuration of the negative electrode active material 10 of the present embodiment, a lithium ion secondary battery having a high capacity and excellent temperature characteristics can be provided.

The thickness of the lithium titanate coating 14 is preferably in the range of 100 nm to 2000 nm. When the thickness of the lithium titanate coating 14 is less than 100 nm, it becomes difficult to coat the surface of the carbon-based particles 12 uniformly (preferably with a uniform film thickness), and this results from the reductive decomposition reaction of the electrolytic solution. There may be cases where the generation of SEI cannot be prevented. On the other hand, when the thickness of the lithium titanate film 14 exceeds 2000 nm, the film thickness of lithium titanate (for example, 175 mAh / g in Li ₄ Ti ₅ O ₁ ) having a smaller battery theoretical capacity than the carbon-based particles becomes too thick. Therefore, the capacity density may be reduced. Therefore, by setting the thickness of the lithium titanate coating 14 in the range of 100 nm to 2000 nm (for example, 200 nm to 500 nm), it is possible to prevent the SEI formation caused by the decomposition reaction of the electrolytic solution and to use the carbon-based particles. It is possible to achieve both the benefits of high capacity.

  Next, a method for manufacturing the above-described negative electrode active material 10 will be described with reference to FIG. FIG. 3 is a diagram showing a manufacturing flow of the negative electrode active material 10. Briefly describing this production method, a step of preparing a sol from a dispersion containing the carbon-based particles, peroxotitanic acid, lithium carbonate or lithium hydroxide (step S10), and firing the prepared sol This includes a step of forming a surface coating composed of lithium titanate on the surface of the carbon-based particles (step S20). This will be described below.

  First, in step S12, a dispersion containing carbon-based particles, peroxotitanic acid, and lithium carbonate or lithium hydroxide is prepared.

Here, peroxotitanic acid is titanium peroxide in which a peroxo group (—O—O—) and titanium are bonded, and an aqueous solution thereof generally has a binuclear complex anion Ti ₂ O ₅ having the following basic structure. (OH) _x ^{(2-x) −} (x> 2) and its polyanion (Ti ₂ O ₅ ) _q (OH) _y ^{(y-2q) −} (2 <q / y) are included. As the peroxotitanic acid aqueous solution, those prepared by a conventionally known method can be used. For example, a hydrogen peroxide solution (H ₂ O ₂ ) can be used in an aqueous solution of a titanium compound or a suspension of hydrated titanium oxide. It can be easily produced by acting.

  In this embodiment, the dispersion is prepared by adding and mixing carbon-based particles and lithium carbonate to the peroxotitanic acid aqueous solution. At this time, it is desirable to weigh so that lithium carbonate and peroxotitanic acid have a desired Li / Ti ratio in the peroxotitanic acid aqueous solution.

  Next, in step S14, a dispersion containing the carbon-based particles, the peroxotitanic acid, and the lithium carbonate is heat-treated. Thereby, a sol can be prepared from the dispersion.

  In this embodiment, the sol is prepared by autoclaving a dispersion containing the carbon-based particles, the peroxotitanic acid, and the lithium carbonate. Specifically, the sol is prepared by placing a peroxotitanic acid aqueous solution containing the carbon-based particles and the lithium carbonate in an autoclave (a heat-resistant pressure-resistant sealed container) and heating at, for example, about 250 ° C. for about 5 hours. Done. Thus, by preparing a sol by autoclaving, a sol having fluidity suitable for the purpose of the present invention can be easily and stably produced.

  The autoclave treatment may be performed while stirring a dispersion containing the carbon-based particles, the peroxotitanic acid, and the lithium carbonate. Thereby, the sol suitable for the object of the present invention can be obtained more stably. Note that the heating temperature and heating time of the autoclave can be appropriately changed according to desired sol characteristics (for example, sol fluidity). In this way, the sol preparation process (step S10) is completed through steps S12 and S14.

  Next, in step S20, the prepared sol is baked to form a surface coating composed of lithium titanate on the surface of the carbon-based particles. That is, a sol in which carbon-based particles are stably dispersed (uniformly dispersed) is used as a raw material. Typically, a solvent is volatilized from the sol to produce a non-fluid particle dispersed state gel, and the gel is further heated. A surface coating composed of lithium titanate is formed on the surface of the carbon-based particles.

In this embodiment, the sol is baked by heating the prepared sol in an electric furnace. At this time, the baking of the sol is preferably performed in an air atmosphere. Thereby, the by-product (for example, by-product gas etc.) at the time of lithium titanate production can be removed easily. Therefore, the sol can be fired stably (with high quality stability). The sol is preferably baked at a heating temperature and a heating time at which a surface film composed of spinel type or anatase type lithium titanate can be formed. For example, when it is desired to synthesize spinel-type Li ₄ Ti ₅ O ₁₂ , the prepared sol may be baked at 600 to 1600 ° C. for about 7 hours to 12 hours, for example, at 750 ° C. for 10 hours. In addition, when it is desired to synthesize anatase-type Li ₄ Ti ₅ O ₁₂ , the prepared sol may be baked at 400 to 550 ° C. for about 6 hours to 20 hours, for example, at 450 ° C. for 10 hours.

  The negative electrode active material obtained in this way is in a lump state in which carbon particles are aggregated with each other by the firing. Next, in step S30, the lump state of the aggregated carbon-based particles is pulverized and divided. Thus, the negative electrode active material comprised from the carbon-type particle | grains coat | covered with the surface film which consists of lithium titanates can be obtained.

  In the production method of the present embodiment, a sol in which carbon-based particles are stably dispersed (uniformly dispersed) is used as a raw material, and typically a solvent is volatilized from the gel to produce a particle-dispersed gel having no fluidity. Further, a surface coating composed of lithium titanate is formed on the surface of the carbon-based particles by heating. Thereby, a lithium titanate film can be uniformly formed on the surface of the carbon-based particles. Therefore, according to the production method disclosed herein, a negative electrode active material suitable for the object of the present invention can be produced stably (with high quality stability).

  Note that the negative electrode active material provided by the method of the present embodiment has a high capacity and excellent temperature characteristics (for example, high rate characteristics at low temperatures), and therefore is incorporated in various types of battery components or in the battery. It can be preferably used as a component of the negative electrode of the electrode body.

  For example, a negative electrode including the negative electrode active material 10 manufactured by any of the methods disclosed herein, a positive electrode (which may be a positive electrode manufactured by a conventionally known manufacturing method), and the positive and negative electrodes are disposed. It can be preferably used as a component of a lithium ion secondary battery comprising an electrolyte (electrolytic solution, particularly non-aqueous electrolytic solution) and a separator that typically separates the positive and negative electrodes. Structure (for example, metal casing or laminate film structure) and size of an outer container constituting such a battery, or structure of an electrode body (for example, a wound structure or a laminated structure) having a positive / negative electrode current collector as a main component There is no particular restriction on the etc.

In order to confirm that the production of SEI on the surface of the carbon-based particles can be prevented by constructing a lithium ion secondary battery using the negative electrode active material according to the present invention, the following experiment was conducted as an example. In this example, Li ₄ Ti ₅ O ₁₂ (spinel type) was prepared as the lithium titanate coating. Details will be described below.

First, lithium hydroxide (LiOH) powder was added to 300 mL of a 50% by mass peroxotitanic acid aqueous solution to prepare a predetermined Li / Ti ratio (4: 5 in this example). A predetermined amount (500 g in this case) of graphite powder as carbon-based particles was added and dispersed therein. Next, a sol was prepared by autoclaving the mixed solution at 250 ° C. for 5 hours while stirring. Next, the prepared sol is calcined at 750 ° C. for 10 hours in an air atmosphere, whereby Li ₄ Ti _{5 is} formed on the surface of a spherical graphite powder having a particle size distribution of approximately 10 μm to 100 μm (average particle diameter is approximately 20 μm to 50 μm). A lithium titanate film composed of O ₁₂ (mainly spinel type) was formed. In this way, a negative electrode active material according to this example was obtained. In addition, each material used for the said synthesis | combination is marketed.

Next, in order to evaluate the obtained negative electrode active material, a lithium ion secondary battery was produced using the negative electrode active material of this example. Specifically, 89% by mass of lithium nickelate as a positive electrode active material, 5% by mass of acetylene black as a positive electrode conductive agent, 5% by mass of PTFE (polytetrafluoroethylene) as a positive electrode binder, and a thickener CMC (Carboxymethylcellulose) as 1% by weight is dispersed in an appropriate solvent to prepare a positive electrode material paste, and this paste is applied to a positive electrode current collector (aluminum foil). A positive electrode sheet provided with a material layer was produced. On the other hand, 98% by mass of graphite coated with the lithium titanate film of the above example was added to a suitable solvent together with 1% by mass of SBR (styrene butadiene rubber) as a negative electrode binder and 1% by mass of CMC as a thickener. A negative electrode material paste was prepared by dispersing, and this paste was applied to a negative electrode current collector (copper foil) to prepare a negative electrode sheet in which negative electrode material layers were provided on both sides of the negative electrode current collector. Then, these positive electrode sheet and negative electrode sheet are wound through two separator sheets (porous polypropylene), and the wound electrode body is flattened by crushing the wound electrode body from the side surface direction. Was made. Subsequently, the wound electrode body obtained in this way was accommodated in a battery case. Then, the test battery (Example 1) was constructed | assembled by inject | pouring electrolyte solution from the injection hole of a battery case, and sealing this injection hole. Note that one of the electrolyte solution ethylene carbonate and diethyl carbonate: 1 those obtained by dissolving LiPF ₆ of about 1M to (mass ratio) mixed solvent was used 50 mL.

  The initial charge was performed on the test battery produced as described above. Then, the amount of gas generated as a by-product when SEI was generated by the reductive decomposition reaction of the electrolytic solution was measured. That is, it means that the smaller the amount of gas generated during initial charging, the more suppressed the generation of SEI due to the decomposition of the electrolytic solution. As a comparative example, a battery was manufactured using graphite without a lithium titanate coating as a negative electrode active material, and the same experiment was performed. In addition, it experimented on the conditions similar to an Example except having no surface film.

  As a result, in the battery produced using the negative electrode active material of the example, it was found that the amount of gas generated at the first charge can be reduced by about 13% compared to the comparative example. From this, it was confirmed that the formation of SEI due to the reductive decomposition reaction of the electrolytic solution can be prevented by covering the surface of the carbon-based particles with a surface coating composed of lithium titanate.

  As mentioned above, although this invention was demonstrated by suitable embodiment, such description is not a limitation matter and of course various modifications are possible.

The cross-sectional schematic diagram of the negative electrode which concerns on this embodiment. The cross-sectional schematic diagram of the negative electrode active material which concerns on this embodiment. The manufacturing flowchart of the negative electrode active material which concerns on this embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Negative electrode active material 12 Carbonaceous particle 14 Lithium titanate coating 20 Negative electrode compound material layer 30 Negative electrode collector

Claims (10)

A negative electrode active material for a lithium ion secondary battery,
Carbon-based particles capable of holding lithium ions;
A negative electrode active material for a battery, comprising: a surface coating composed of lithium titanate that coats the carbon-based particles. The lithium titanate constituting the surface coating is one compound selected from the group consisting of Li ₄ Ti ₅ O ₁₂ , LiTi ₂ O ₄ and Li ₂ TiO _3, or a mixture of any two or more thereof. Item 6. The negative electrode active material according to Item 1.   The negative electrode active material according to claim 1, wherein the surface film is made of spinel type or anatase type lithium titanate. A method for producing a negative electrode active material for a lithium ion secondary battery composed of carbon-based particles capable of holding lithium ions,
Preparing a sol from a dispersion containing the carbon-based particles, peroxotitanic acid, and lithium carbonate or lithium hydroxide;
Forming a surface coating composed of lithium titanate on the surface of the carbon-based particles by firing the prepared sol.   The production method according to claim 4, wherein the adjustment of the sol is performed by autoclaving a dispersion liquid containing the carbon-based particles, peroxotitanic acid, and lithium carbonate or lithium hydroxide. The said surface film is formed of one compound selected from the group consisting of Li ₄ Ti ₅ O ₁₂ , LiTi ₂ O ₄ and Li ₂ TiO _3, or a mixture of any two or more thereof. Manufacturing method.   The manufacturing method according to claim 4, wherein the firing is performed in an air atmosphere.   The said baking is a manufacturing method as described in any one of Claim 4 to 7 performed at the heating temperature in which the surface film comprised with a spinel type or anatase type lithium titanate can be formed.   The lithium ion secondary battery using the negative electrode active material as described in any one of Claim 1 to 3 for the negative electrode.   A vehicle comprising the lithium ion secondary battery according to claim 9.

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