JP2014130826A - Negative electrode active material for electricity storage device, and manufacturing method for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode active material for an electricity storage device that achieves a higher capacity as compared with conventional negative electrode active materials and is excellent in charging and discharging cycle characteristics and safety.SOLUTION: A negative electrode active material for an electricity storage device includes 70 to 95 mol% of SnO, and 5 to 30 mol% of PO(excluding 70 mol% of SnO and 30 mol% of PO).

Description

  The present invention relates to a negative electrode active material used for power storage devices such as non-aqueous secondary batteries represented by lithium ion secondary batteries used in portable electronic devices and electric vehicles, and a method for producing the same.

  In recent years, with the spread of portable personal computers and mobile phones, there is an increasing demand for higher capacity and smaller size of power storage devices such as lithium ion secondary batteries. If the capacity of the electricity storage device is increased, it will be easy to reduce the size of the battery material. Therefore, there is an urgent need to develop an electrode material for the electricity storage device with a higher capacity.

For example, high-potential type LiCoO ₂ , LiCo _1-x Ni _x O ₂ , LiNiO ₂ , LiMn ₂ O _{4 and the} like are widely used as positive electrode materials for lithium ion secondary batteries. On the other hand, a carbonaceous material is generally used as the negative electrode material. These materials function as electrode active materials that reversibly occlude and release lithium ions by charging and discharging, and constitute so-called rocking chair type secondary batteries that are electrochemically connected by a non-aqueous electrolyte or a solid electrolyte. .

  Carbonaceous materials used as active materials (negative electrode active materials) that can occlude or release lithium ions in the negative electrode include graphitic carbon materials, pitch coke, fibrous carbon, and high-capacity soft carbon that is fired at low temperatures. There is. However, since the carbon material has a relatively small lithium insertion capacity, there is a problem that the battery capacity is low. Specifically, even if a stoichiometric amount of lithium insertion capacity can be realized, the battery capacity of the carbon material is limited to about 372 mAh / g.

  Therefore, a negative electrode active material containing SnO has been proposed as a negative electrode active material capable of inserting and extracting lithium ions and having a high capacity density exceeding that of a carbon-based material (see, for example, Patent Document 1). . However, the negative electrode active material proposed in Patent Document 1 cannot relieve the volume change due to the insertion and extraction reactions of Li ions during charge and discharge, and the structure deterioration of the negative electrode active material is significantly cracked when repeatedly charged and discharged. Is likely to occur. As cracks progress, in some cases, cavities are formed in the negative electrode active material and may be pulverized. When a crack occurs in the negative electrode active material, the electron conduction path is interrupted, so that a reduction in discharge capacity (charge / discharge cycle characteristics) after repeated charge / discharge has been a problem.

  In order to solve the above problem, a negative electrode active material composed of an oxide mainly composed of tin oxide and a method for producing the negative electrode active material by a melting method have been proposed (for example, see Patent Document 2). . Furthermore, a manufacturing method by a sol-gel method has been proposed as a method for manufacturing a negative electrode active material made of an oxide containing tin oxide and silicon and having a large specific surface area (see, for example, Patent Document 3). However, the negative electrode active material manufactured by these manufacturing methods has a low ratio of the discharge capacity to the initial charge capacity (initial charge / discharge efficiency), and a decrease in the discharge capacity (cycle characteristics) after repeated charge / discharge is a problem. It was.

  In addition, by using an amorphous oxide mainly composed of tin oxide, a negative electrode active material for a non-aqueous secondary battery that can relieve the volume change associated with insertion and extraction of lithium ions and has an excellent charge / discharge cycle has been proposed. (For example, see Patent Documents 4 and 5). However, these negative electrode active materials contain a considerable amount of oxides other than tin oxide, which are not related to occlusion and release of lithium ions, so as to be amorphous oxides. For this reason, there is a problem that the tin oxide content per unit mass of the negative electrode active material is small and it is difficult to further increase the capacity.

Japanese Patent No. 2887632 Japanese Patent No. 3498380 Japanese Patent No. 3890671 Japanese Patent No. 3605866 Japanese Patent No. 3605875

  An object of the present invention is to provide a negative electrode active material for an electricity storage device that can be further increased in capacity as compared with a conventional negative electrode active material and is excellent in charge / discharge cycle characteristics and safety.

  As a result of various studies, the present inventors have found that the above problem can be solved by using a negative electrode active material for an electricity storage device containing a high proportion of tin oxide in the composition, and propose the present invention. is there. In this specification, the “electric storage device” includes a non-aqueous secondary battery, in particular, a lithium-ion non-aqueous secondary battery used in portable electronic devices such as notebook computers and mobile phones, electric vehicles, and the like. Hybrid capacitors such as ion capacitors are included.

That is, the present invention contains, in mol%, SnO 70 to 95% and P ₂ O ₅ 5 to 30% (not including SnO 70 mol% and P ₂ O ₅ 30 mol%). The present invention relates to a negative electrode active material for devices.

Since the negative electrode active material for an electricity storage device of the present invention contains SnO at a high ratio of 70 to 95% (not including 70 mol%), the content of tin oxide per unit mass of the negative electrode active material is large, and the further high Capacitance can be achieved. Incidentally, SnO ingredient content in the present invention, the tin oxide component other than SnO (SnO _2, etc.) also refers to that summed in terms of SnO.

  In the present invention, the negative electrode active material is preferably amorphous.

  With the above structure, a volume change accompanying insertion and extraction of lithium ions can be reduced, and a negative electrode active material for an electricity storage device having an excellent charge / discharge cycle can be obtained. Note that “substantially amorphous” means that the degree of crystallinity is substantially 0%, which is a 2θ value obtained by powder X-ray diffraction measurement using CuKα rays and is 10 to 60. A diffraction line profile having a broad diffraction line at 10 to 40 ° in a diffraction line profile of 0 ° and no diffraction peak being confirmed.

  The present invention also relates to a method for producing the above negative electrode active material for an electricity storage device, wherein the raw material powder is melted and vitrified in a reducing atmosphere or an inert atmosphere.

  According to this method, it is possible to obtain a negative electrode active material that can be a secondary battery having excellent initial charge / discharge efficiency (ratio of discharge capacity to initial charge capacity). The reason for this can be explained as follows.

  For example, as an example of a non-aqueous secondary battery, it is known that a lithium ion secondary battery undergoes the following reaction at the negative electrode during charging and discharging.

Sn ^{x +} + xe− → Sn (1)
Sn + yLi ⁺ + ye− ← → Li _y Sn (2)

First, during the first charge, a reaction in which Sn ^{x +} ions accept electrons and produce metal Sn occurs irreversibly (formula (1)). Subsequently, the produced metal Sn combines with Li ions that have moved from the positive electrode through the electrolytic solution and electrons supplied from the circuit, and a reaction occurs to form a Sn—Li alloy. This reaction occurs as a reversible reaction that proceeds rightward during charging and proceeds leftward during discharging (formula (2)).

Here, focusing on the reaction of the formula (1) that occurs during the first charge, the smaller the energy required for the reaction, the smaller the initial charge capacity, and as a result, the first charge / discharge efficiency is excellent. Here, the smaller the valence of Sn ^{x +} ions (reduced state), the fewer electrons required for the reduction, which is advantageous for improving the initial charge / discharge efficiency of the secondary battery. Therefore, by melting the raw material powder in a reducing atmosphere or an inert atmosphere and vitrifying it, Sn ^{x +} ions can be reduced effectively and the valence can be reduced, and a secondary battery with excellent initial charge efficiency can be obtained. Can be obtained.

  The raw material powder used in the above production method is preferably a composite oxide containing phosphorus and tin.

  By using a composite oxide containing phosphorus and tin as the starting raw material powder, a negative electrode active material excellent in homogeneity can be easily obtained. And the non-aqueous secondary battery with stable discharge capacity is obtained by using the negative electrode material containing the said negative electrode active material as a negative electrode.

The negative electrode active material for an electricity storage device according to the present invention contains SnO 70 to 95% and P ₂ O ₅ 5 to 30% (not including SnO 70% and P ₂ O ₅ 30%) in terms of mol%. The reason for limiting the composition in this way will be described below. In the following description, “%” means “mol%” unless otherwise specified.

  SnO is an active material component that serves as a site for occluding and releasing lithium ions in the negative electrode active material. The SnO content is preferably 70 to 95% (70% not included), 70.1 to 87%, 70.5 to 82%, and particularly 71 to 77%. When the SnO content is 70% or less, the discharge capacity per unit mass of the negative electrode active material becomes small. Moreover, the charging / discharging efficiency at the time of first charge / discharge becomes small. When the content of SnO is more than 95%, the amorphous component in the negative electrode active material is reduced, and the volume change due to insertion and extraction of lithium ions during charging and discharging cannot be sufficiently relaxed, and charging and discharging are repeated. There is a risk of rapid capacity loss.

P ₂ O ₅ is a matrix component that includes SnO, which is a site for the insertion and release of lithium ions, and the effect of relaxing the volume change associated with the storage and release of lithium ions by SnO to improve the charge / discharge cycle characteristics. There is. Furthermore, P ₂ O ₅ is a network-forming oxide and functions as a solid electrolyte to which lithium ions can move. The content of P ₂ O ₅ is preferably 5 to 30% (not including 30%), 5 to 29.2%, and particularly preferably 8 to 29.5%. When the content of P ₂ O ₅ is less than 5%, the volume change associated with insertion and extraction of lithium ions during charge and discharge cannot be relaxed, and structural deterioration is likely to occur. Therefore, the cycle performance is very poor and there is a risk of causing a rapid capacity drop. When the content of P ₂ O ₅ is 30% or more, the discharge capacity per unit mass of the negative electrode active material tends to decrease. In addition, the water resistance is likely to deteriorate, and when exposed to high temperature and high humidity for a long time, undesired foreign crystals (for example, SnHPO ₄ etc.) are formed, and moisture is easily impregnated or adsorbed in the negative electrode active material. As a result, water is decomposed inside the non-aqueous secondary battery, and it is inferior in safety because it causes explosion due to release of oxygen and heat generation due to reaction between lithium and water.

The total amount of SnO and P ₂ O ₅ is preferably 80% or more, 85% or more, particularly 87% or more. If the total amount of SnO and P ₂ O ₅ is less than 80%, it becomes difficult to achieve both cycle characteristics and high capacity.

The molar ratio of SnO to P ₂ O ₅ (SnO / P ₂ O ₅ ) is preferably 2.3 to 19, 2.3 to 18, and particularly preferably 2.4 to 17. When SnO / P ₂ O ₅ is smaller than 2.3, the Sn atom in SnO tends to be affected by the coordination of P ₂ O ₅ , and the valence of Sn atom tends to increase, resulting in the initial charge efficiency. Tends to decrease. If SnO / P ₂ O ₅ is greater than 19, the reduction in discharge capacity during repeated charge / discharge tends to increase. This is because the P ₂ O ₅ coordinated to SnO in the negative electrode active material is reduced and the P ₂ O ₅ component cannot contain SnO, and as a result, the volume change of SnO associated with insertion and extraction of lithium ions is mitigated. This is considered to be because it becomes impossible to cause structural deterioration.

Further, various components can be added in addition to the above components within the range not impairing the effects of the present invention. Examples of such components include CuO, ZnO, B ₂ O ₃ , MgO, CaO, Al ₂ O ₃ , SiO ₂ , R ₂ O (R represents Li, Na, K, or Cs). The total content of the above components is preferably 0 to 20%, 0 to 15%, particularly preferably 0.1 to 13%.

  The negative electrode active material for an electricity storage device according to the present invention preferably has a crystallinity of 95% or less, 80% or less, 70% or less, 50% or less, particularly 30% or less, most preferably substantially amorphous. It is preferable that the material is of a quality (crystallinity is substantially 0%). In a negative electrode active material containing SnO in a high proportion, the smaller the crystallinity (the larger the proportion of the amorphous phase), the more the volume change during repeated charging / discharging can be reduced, which is advantageous from the viewpoint of suppressing the reduction in discharge capacity. is there.

  The degree of crystallinity of the negative electrode active material is a 2θ value obtained by powder X-ray diffraction measurement using CuKα rays, and in a diffraction line profile of 10 to 60 °, the crystal diffraction line and the amorphous halo are separated into peaks. Desired. Specifically, the integrated intensities obtained by peak-separating a broad diffraction line (amorphous halo) at 10 to 40 ° from the total scattering curve obtained by subtracting the background from the diffraction line profile are Ia, 10 When the sum of integrated intensities obtained by peak-separating each crystalline diffraction line detected at 60 ° is Ic, the degree of crystallinity Xc is obtained from the following equation.

    Xc = [Ic / (Ic + Ia)] × 100

  The negative electrode active material according to the present invention may contain a phase composed of a complex oxide of metal and oxide, or an alloy phase of metal and metal.

  In addition, after charging / discharging an electrical storage device such as a non-aqueous secondary battery using the negative electrode material containing the negative electrode active material according to the present invention, the negative electrode material contains lithium oxide, Sn-Li alloy, or metallic tin. There is a case.

  The negative electrode active material according to the present invention is produced, for example, by heating and melting raw material powder to vitrify it. Here, it is preferable to melt the raw material powder in a reducing atmosphere or an inert atmosphere.

  An oxide containing Sn is likely to change the oxidation state of Sn atoms depending on the melting condition, and the bond energy of electrons is likely to change. By performing melting in a reducing atmosphere or an inert atmosphere, the change in the oxidation state of Sn atoms can be suppressed as described above, and a secondary battery excellent in initial charge / discharge efficiency can be obtained.

In order to melt in a reducing atmosphere, it is preferable to supply a reducing gas into the melting tank. As the reducing gas, it is preferable to use a mixed gas of N ₂ 90 to 99.5% and H ₂ 0.5 to 10%, particularly N ₂ 92 to 99% and H ₂ 1 to 8% in volume%. .

  When melting in an inert atmosphere, it is preferable to supply an inert gas into the melting tank. As the inert gas, it is preferable to use any of nitrogen, argon, and helium.

In the method for producing a negative electrode active material according to the present invention, it is preferable to use a composite oxide containing phosphorus and tin as a starting material powder. By using a composite oxide containing phosphorus and tin for the starting raw material powder, a negative electrode active material with few devitrified foreign substances and excellent uniformity is easily obtained. By using the negative electrode material containing the negative electrode active material as a negative electrode, a nonaqueous secondary battery with a stable discharge capacity can be obtained. Examples of the composite oxide containing phosphorus and tin include stannous pyrophosphate (Sn ₂ P ₂ O ₇ ).

  In addition, the negative electrode of electrical storage devices, such as a non-aqueous secondary battery, is formed using the negative electrode material containing the negative electrode active material demonstrated above. Specifically, the negative electrode material is obtained by adding a binder such as a thermosetting resin or a conductive auxiliary agent such as acetylene black, ketjen black, highly conductive carbon black, or graphite to the negative electrode active material. .

  Moreover, the negative electrode active material and the negative electrode material of the present invention are not limited to lithium ion secondary batteries, but other nonaqueous secondary batteries, and further, positive electrode materials and lithium ion secondary batteries for nonaqueous electric double layer capacitors It can also be applied to a hybrid capacitor combined with a negative electrode material.

  A lithium ion capacitor, which is a hybrid capacitor, is a type of asymmetric capacitor that has different charge / discharge principles for a positive electrode and a negative electrode. The lithium ion capacitor has a structure in which a negative electrode for a lithium ion secondary battery and a positive electrode for an electric double layer capacitor are combined. Here, the positive electrode forms an electric double layer on the surface and is charged / discharged by utilizing a physical action (electrostatic action), whereas the negative electrode has a lithium ion chemistry similar to the lithium ion secondary battery described above. Charge and discharge by reaction (occlusion and release).

  A positive electrode material made of a carbonaceous powder having a high specific surface area such as activated carbon, polyacene, or mesophase carbon is used for the positive electrode of the lithium ion capacitor. On the other hand, as the negative electrode, the negative electrode material of the present invention in which lithium ions and electrons are occluded can be used.

The means for occluding lithium ions and electrons in the negative electrode active material of the present invention is not particularly limited. For example, a metal lithium electrode as a supply source of lithium ions and electrons may be disposed in a capacitor cell, and may be brought into contact with the negative electrode containing the negative electrode active material of the present invention directly or through a conductor, or in another cell. The negative electrode active material may be preliminarily occluded with lithium ions and electrons and then incorporated into the capacitor cell.

  Hereinafter, although the negative electrode active material for electrical storage devices which concerns on this invention is demonstrated in detail using an Example, this invention is not limited to these Examples.

(1) Production of negative electrode active material for non-aqueous secondary battery Table 1 shows Examples 1 to 6 and Comparative Examples 1 to 3. Each negative electrode active material was produced as follows.

In order to obtain the composition shown in Table 1, stannous pyrophosphate (Sn ₂ P ₂ O ₇ ) is used as the main raw material, and the raw material powder is made of various oxides, phosphate raw materials, carbonate raw materials, metals, carbon raw materials, etc. Prepared. The raw material powder was put into an alumina crucible and melted at 950 ° C. for 40 minutes in a nitrogen atmosphere using an electric furnace to be vitrified.

  Next, the molten glass was poured out between a pair of rotating rollers and molded into a film having a thickness of 0.1 to 2 mm while rapidly cooling to obtain a glass sample. The glass sample was pulverized with an alumina separator and then passed through a sieve having an opening of 20 μm to obtain a glass powder having a mean particle size of 5 μm (a negative electrode active material for a non-aqueous secondary battery).

The structure was identified by powder X-ray diffraction measurement for each sample. The negative electrode active materials of Examples 1 to 6 except Example 5 were amorphous, and no crystals were detected. In Example 5, fine crystal precipitation of SnO ₂ was confirmed in the amorphous material, and the crystallinity Xc was 4%. Since the negative electrode active material of Comparative Example 1 had deliquescence immediately after production, the precipitated crystals were not measurable. The negative electrode active materials of Comparative Examples 2 and 3 had a structure in which an amorphous part and a crystalline part were mixed.

(2) Production of Negative Electrode For the glass powders (negative electrode active materials) of Examples and Comparative Examples, polyimide resin as a binder, ketjen black as a conductive material, glass powder: binder: conductive material = 85: 10: 5 (Mass ratio) was weighed and these were dispersed in N-methylpyrrolidone (NMP), and then sufficiently stirred with a rotation / revolution mixer to obtain a slurry-like negative electrode material. Next, using a doctor blade with a gap of 150 μm, the obtained slurry was coated on a copper foil having a thickness of 20 μm as a negative electrode current collector, dried at 70 ° C. with a dryer, and then between a pair of rotating rollers. An electrode sheet was obtained by pressing through a sheet. The electrode sheet was punched to a diameter of 11 mm with an electrode punching machine and imidized while reducing pressure at 200 ° C. for 10 hours to obtain a circular working electrode.

(3) Preparation of test battery The above working electrode was placed on the lower lid of the coin cell with the copper foil surface facing downward, and dried on the polypropylene for 8 hours at 60 ° C. under reduced pressure for 16 hours. A separator comprising Cellguard # 2400 manufactured by Needs Co., Ltd. and metallic lithium as a counter electrode were laminated to prepare a test battery. As the electrolytic solution, 1M LiPF ₆ solution / EC (ethylene carbonate): DEC (diethyl carbonate) = 1: 1 was used. The test battery was assembled in an environment with a dew point temperature of −60 ° C. or lower.

(4) Charging / discharging test Charging (occlusion of lithium ions into the negative electrode active material) was performed by CC (constant current) charging from 2 V to 0 V at 0.2 mA. Next, discharge (release of lithium ions from the negative electrode active material) was discharged from 0 V to 2 V at a constant current of 0.2 mA. This charge / discharge cycle was repeated.

  Table 1 shows the results of the initial charge / discharge characteristics when the charge / discharge test was performed for each sample and the cycle characteristics when the charge / discharge was repeated.

  The initial discharge capacity of the batteries using the negative electrode active materials of Examples 1 to 6 was 680 mAh / g or more, and the discharge capacity at the 50th cycle was good at 382 mAh / g or more. On the other hand, the negative electrode active material of Comparative Example 1 had deliquescence immediately after production and could not be used as a non-aqueous secondary battery electrode. The battery using the negative electrode active material of Comparative Example 2 had a low initial discharge capacity of 392 mAh / g. In addition, the battery using the negative electrode active material of Comparative Example 3 had an initial discharge capacity of 901 mAh / g, but the discharge capacity at the 50th cycle was significantly reduced to 52 mAh / g.

  The negative electrode active material for an electricity storage device of the present invention is suitable for a portable electronic device such as a notebook computer or a mobile phone, a lithium ion non-aqueous secondary battery used for an electric vehicle, and a hybrid capacitor such as a lithium ion capacitor. is there.

Claims (5)

In _{mol%, SnO 70~95%, P 2} O 5 5~30% negative active material for a power storage device which is characterized by containing _{(SnO 70%, P 2 O} 5 30% is not included).   The negative electrode active material for an electricity storage device according to claim 1, wherein the negative electrode active material is amorphous.   The negative electrode material for electrical storage devices containing the negative electrode active material for electrical storage devices of Claim 1 or 2.   A method for producing a negative electrode active material for an electricity storage device according to claim 1 or 2, wherein the raw material powder is melted and vitrified in a reducing atmosphere or an inert atmosphere. .   The method for producing a negative electrode active material for an electricity storage device according to claim 4, wherein the raw material powder is a complex oxide containing phosphorus and tin.