JP5632781B2 - Method for manufacturing positive electrode active material for power storage device - Google Patents

Description

  The present invention relates to a method for manufacturing a positive electrode active material for a power storage device, a method for manufacturing a positive electrode for a power storage device, and a method for manufacturing a power storage device. In particular, the present invention relates to a method for manufacturing a positive electrode active material for a lithium ion battery (lithium ion secondary battery) or a lithium ion capacitor as a power storage device, and a method for manufacturing a positive electrode.

  With increasing interest in environmental issues, the development of power storage devices such as secondary batteries and electric double layer capacitors used for power sources for hybrid vehicles has been active. As candidates, lithium ion batteries and lithium ion capacitors with high energy performance are attracting attention. Lithium-ion batteries can store large amounts of electricity even when they are small, so they are already installed in portable information terminals such as mobile phones and laptop personal computers, and play a role in miniaturization of products.

  Secondary batteries and electric double layer capacitors have a configuration in which an electrolyte is interposed between a positive electrode and a negative electrode. Each of the positive electrode and the negative electrode is known to have a current collector and an active material provided on the current collector. For example, a lithium ion battery is configured by using a material capable of inserting and extracting lithium ions as an active material for a positive electrode and interposing an electrolyte therebetween.

As a positive electrode active material having excellent characteristics for a lithium ion battery, olivine type lithium iron phosphate (LiFePO ₄ ) attracts attention (see Patent Document 1 and Non-Patent Document 1). Patent Document 1 also describes the charge / discharge behavior of NASICON lithium iron phosphate (Li ₃ Fe ₂ (PO ₄ ) ₃ ) as an example of a NASICON compound.

  When olivine-type lithium iron phosphate is used as a positive electrode active material, iron contained in the active material is divalent, and thus is easily oxidized and cannot be said to be a stable material. Moreover, lithium iron phosphate containing divalent iron is expensive.

  As an active material, crystallized olivine type lithium iron phosphate is often used. However, in order to obtain highly crystalline lithium iron phosphate, firing for a long time or the like is required. Therefore, the manufacturing process is long and tends to be complicated.

International Publication No. 1997/040541

Edited by Zenpachi Okumi, “Lithium Secondary Battery”, Ohm Co., Ltd., March 20, 2008, first edition, first printing, pages 96 to 97

  An object of one embodiment of the present invention is to provide a manufacturing method in which a positive electrode active material, a positive electrode, or a power storage device can be obtained by a simple method.

  Another object of one embodiment of the present invention is to provide a manufacturing method in which a positive electrode active material, a positive electrode, or a power storage device having favorable battery characteristics can be obtained.

  Another object of one embodiment of the present invention is to provide a manufacturing method in which a novel positive electrode active material, a positive electrode, or a power storage device can be obtained.

  The present inventor has found that by using a sputtering method, lithium iron phosphate containing trivalent Fe can be provided as a positive electrode active material exhibiting good battery characteristics.

As a starting material, a target having a composition containing at least trivalent Fe, Li, and PO ₄ is used. Specific examples of such a target include NASICON-type lithium iron phosphate, and the composition thereof is represented by the general formula Li _x Fe _y (PO ₄ ) _z (x, y, and z are positive real numbers). it can. _{Furthermore, Li 3 Fe 2 (PO 4} ) 3 and _Fe 2 _{O 3} may be mixed target.

Using a target having a composition containing at least trivalent Fe, Li, and PO ₄ , sputtering the target with ions such as a rare gas (eg, argon), the lithium iron phosphate containing trivalent Fe Get a thin film. In one embodiment of the present invention, the obtained thin film is used as a positive electrode active material. Here, the thin film (active material) obtained by the sputtering method is preferably not crystallized. That is, the thin film (active material) obtained by the sputtering method is preferably left in an amorphous structure.

One embodiment of the present invention is a method for manufacturing a positive electrode active material in which an active material having an amorphous structure is formed by a sputtering method using a target including trivalent Fe, Li, and PO ₄ in a composition.

One embodiment of the present invention is a positive electrode active material in which an active material having an amorphous structure is formed by a sputtering method using a target represented by the general formula Li _x Fe _y (PO ₄ ) _z and containing trivalent Fe in the composition. This is a manufacturing method.

  One embodiment of the present invention is a method for manufacturing a positive electrode active material in which an active material having an amorphous structure is formed by a sputtering method using a target containing NASICON lithium iron phosphate.

One embodiment of the present invention is a method for manufacturing a positive electrode active material in which an active material having an amorphous structure is formed by a sputtering method using a target in which Li ₃ Fe ₂ (PO ₄ ) ₃ and Fe ₂ O ₃ are mixed.

  In the above structure, the active material having an amorphous structure contains trivalent Fe.

  In the above structure, the positive electrode can be manufactured by forming the positive electrode active material over the current collector. Furthermore, a power storage device can be manufactured by forming a negative electrode facing the positive electrode and an electrolyte interposed between the positive electrode and the negative electrode.

  According to one embodiment of the present invention, a positive electrode active material, a positive electrode, or a power storage device can be provided by a simple method. Alternatively, according to one embodiment of the present invention, a positive electrode active material, a positive electrode, or a power storage device having favorable battery characteristics can be provided. Alternatively, according to one embodiment of the present invention, a novel positive electrode active material, positive electrode, or power storage device can be provided.

4A and 4B illustrate an example of a method for manufacturing a positive electrode for a power storage device. FIG. 9 illustrates an example of a structure of a power storage device. FIG. 14 illustrates an example of characteristics of a power storage device. FIG. 14 illustrates an example of characteristics of a power storage device. FIG. 14 illustrates an example of characteristics of a power storage device.

  Hereinafter, embodiments will be described in detail with reference to the drawings. However, the following embodiments can be implemented in many different modes, and it is easy for those skilled in the art to change the modes and details in various ways without departing from the spirit and scope thereof. Understood. Therefore, the present invention is not construed as being limited to the description of the embodiments below. Note that in all the drawings for describing the embodiments, the same portions or portions having similar functions are denoted by the same reference numerals, and repetitive description thereof is omitted.

(Embodiment 1)
In this embodiment, a positive electrode active material for a power storage device will be described.

  The positive electrode active material is made of a thin film of lithium iron phosphate containing trivalent Fe obtained by using a sputtering method. By utilizing the sputtering method, even lithium iron phosphate containing trivalent Fe can be provided as a positive electrode active material that exhibits good battery characteristics or novel battery characteristics.

As a sputtering target, a target having a composition containing at least trivalent Fe, Li, and PO ₄ is used. Examples of the material constituting such a target include lithium iron phosphate, preferably Nasicon type lithium iron phosphate, and the composition thereof is represented by the general formula Li _x Fe _y (PO ₄ ) _z (x, y, z Is a positive real number). Alternatively, a mixed material of lithium iron phosphate and other materials (for example, a mixed material of Li ₃ Fe ₂ (PO ₄ ) ₃ and Fe ₂ O ₃ or the like) can be used.

Using a target having a composition containing at least trivalent Fe, Li, and PO ₄ , a thin film of lithium iron phosphate containing trivalent Fe is formed by a sputtering method. The thin film of lithium iron phosphate containing trivalent Fe functions as a positive electrode active material. Here, the lithium iron phosphate thin film (positive electrode active material) containing trivalent Fe is preferably not crystallized after being formed by sputtering. That is, it is preferable that a thin film in an amorphous state of lithium iron phosphate containing trivalent Fe is used as a positive electrode active material as it is.

The lithium iron phosphate thin film containing trivalent Fe formed by sputtering as described above can basically preserve the composition of the material used for the target. Note that the composition and the like of the lithium iron phosphate thin film containing trivalent Fe can be adjusted depending on the conditions of the sputtering method and the like. The composition of lithium iron phosphate containing trivalent Fe can be expressed by, for example, the general formula Li _x Fe _y (PO ₄ ) _z (x, y, and z are positive real numbers). Specific examples _{include Li 3 Fe 2 (PO 4)} 3.

  As described above, the positive electrode active material formed by the sputtering method becomes a thin film of the active material. Therefore, the density of the active material can be increased as compared with a positive electrode active material obtained by a conventional wet method, and a larger amount of active material can be packed per unit volume. Increasing the density of the positive electrode active material leads to an increase in the amount of occlusion and release of lithium in the positive electrode, leading to an increase in capacity of the positive electrode and the power storage device. Therefore, by forming the positive electrode active material by a sputtering method as in this embodiment mode, the capacities of the positive electrode and the power storage device can be increased, and the battery characteristics can be improved.

  A material containing trivalent Fe in the composition is used as a target. Trivalent Fe is preferable because it is difficult to oxidize and is stable, so that it is not necessary to control the chamber atmosphere at the time of film formation in a complicated manner and can be easily used as a raw material. Further, trivalent Fe is cheaper as a raw material than divalent Fe, and is advantageous in terms of cost. Furthermore, since a thin film formed by sputtering using a raw material (target) containing trivalent Fe can function as a positive electrode active material as it is, a crystallization process by heat treatment can be omitted, and a manufacturing process is simple. become.

  In addition, the power storage device to which the positive electrode active material according to this embodiment is applied may have a characteristic that the charge / discharge curve does not have a plateau (potential flat portion) and the potential gently and continuously changes with charge / discharge. . By using a sputtering method to produce lithium iron phosphate containing trivalent Fe, it can be provided as a novel positive electrode active material having such characteristics. Needless to say, the power storage device to which the positive electrode active material according to this embodiment is applied can have characteristics other than those described above.

  Note that the plateau (potential flat portion) in this specification refers to a portion where the potential changes while maintaining the flatness with respect to the change in the capacity in the charge / discharge curve with the horizontal axis representing the capacity and the vertical axis representing the potential. .

  Hereinafter, a positive electrode active material and a method for manufacturing a positive electrode according to this embodiment will be described with reference to FIGS.

By sputtering the target 104 with ions of a rare gas or the like using the target 104 having a composition containing at least trivalent Fe, Li, and PO ₄ (see FIG. 1A), the current collector 103 A positive electrode active material 105 is formed thereover (see FIG. 1B).

  As the current collector 103, a highly conductive material such as titanium is used.

The composition of the material constituting the target 104 includes Li _x Fe _y (PO ₄ ) _z (x, y, z are positive real numbers) as a general formula, and specifically, Li ₃ Fe ₂ (PO _{4 3} ). Further, the material constituting the target 104 may be a mixed material, for example a mixture of _{Li 3 Fe 2 (PO 4)} 3 and _Fe 2 _{O 3} and the like. Preferably, by using Nasicon type lithium iron phosphate for the target 104, an inexpensive and stable target can be obtained, and the positive electrode active material can be manufactured at low cost by a simple method.

  As the sputtering method, an RF sputtering method using a high frequency power source, a DC sputtering method using a DC power source, a pulse DC sputtering method in which a DC bias is applied in a pulsed manner, or the like can be used.

  As a sputtering gas, a rare gas, oxygen, a mixed gas of rare gas and oxygen, or the like can be used. Argon etc. are mentioned as a noble gas.

The composition of the positive electrode active material 105 formed on the current collector 103 by sputtering is basically the same as the composition of the target material. As the positive electrode active material 105, a thin film of lithium iron phosphate containing trivalent Fe is formed, and a general formula thereof is Li _x Fe _y (PO ₄ ) _z (x, y, and z are positive real numbers). Specifically, Li ₃ Fe ₂ (PO ₄ ) ₃ may be mentioned. Here, the thin film of lithium iron phosphate containing trivalent Fe that is the positive electrode active material 105 has an amorphous structure.

  Thus, a thin film of lithium iron phosphate containing trivalent Fe is formed as the positive electrode active material 105 by a sputtering method. A stacked structure of the current collector 103 and the positive electrode active material 105 constitutes the positive electrode 101.

  By using the sputtering method, it is possible to provide a power storage device that exhibits favorable battery characteristics even when lithium iron phosphate containing trivalent Fe is used as a positive electrode active material. Further, by using the sputtering method, it is possible to realize a high-density positive electrode active material and a high positive electrode capacity, and to provide a power storage device that exhibits favorable battery characteristics. Further, by using the sputtering method, a positive electrode can be manufactured by a simple method, and a power storage device that exhibits favorable battery characteristics can be provided.

  In addition, since a material containing trivalent Fe is used as a sputtering target, an inexpensive and stable raw material can be used. Since the raw material is inexpensive and stable, a simple method and a low-cost manufacturing process can be provided.

  This embodiment can be implemented in combination with any of the other embodiments and examples as appropriate.

(Embodiment 2)
In this embodiment, an example of a power storage device using the positive electrode active material described in Embodiment 1 will be described.

  FIG. 2A illustrates part of the structure of the power storage device 200. The power storage device 200 includes a positive electrode 201 and a negative electrode 211 provided to face each other with the positive electrode 201 and the electrolyte 207 interposed therebetween.

  The positive electrode 201 includes a current collector 203 and a positive electrode active material 205 provided on the current collector 203. A manufacturing method of the positive electrode active material 205 may be based on Embodiment Mode 1. The current collector 103 can be used as the current collector 203. As the positive electrode active material 205, the thin film of lithium iron phosphate containing trivalent Fe described in Embodiment Mode 1 can be used. Further, the positive electrode 101 can be applied to the positive electrode 201.

  The negative electrode 211 includes a current collector 213 and a negative electrode active material 215 provided on the current collector 213. As the material of the current collector 213, for example, a highly conductive material such as platinum, copper, or titanium can be used. As the material of the negative electrode active material 215, a carbon material such as graphite, lithium metal, silicon, or the like can be used.

  The electrolyte 207 has a function of conducting lithium ions. As a material of the electrolyte 207, solid or liquid can be used.

When the material of the electrolyte 207 is solid, for example, Li ₃ PO ₄ , Li ₃ PO ₄ mixed with nitrogen and Li _x PO _y N _z (x, y, z are positive real numbers), Li ₂ S—SiS ₂ , _{Li 2 S-P 2 S 5} , Li 2 S-B 2 S 3 or the like can be used. Moreover, what doped LiI etc. to these can be used.

When the material of the electrolyte 207 is a liquid, it contains a solvent and a solute (salt) dissolved in the solvent. As the solvent, for example, a cyclic carbonate such as propylene carbonate or ethylene carbonate, or a chain carbonate such as dimethyl carbonate or diethyl carbonate can be used. The solute (salt), for _example, can be used to contain LiPF 6, LiBF _4, or LiTFSA etc., light metal salt (lithium salt, etc.) one or more.

  Note that a separator 209 is provided when the electrolyte 207 is liquid. The separator 209 has a function of preventing contact between the positive electrode 201 and the negative electrode 211 and allowing lithium ions to pass therethrough. Examples of the material of the separator 209 include paper, non-woven fabric, glass fiber, or synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol fiber, also referred to as vinylon), polypropylene, polyester, acrylic, polyolefin, polyurethane, and the like. Can also be used. However, it is necessary to select a material that does not dissolve in the electrolyte 207. In addition, even when a solid electrolyte is applied to the electrolyte 207, the separator 209 can be provided.

  Next, an example of charge / discharge when a lithium ion secondary battery is used as the power storage device will be described.

  Charging is performed by connecting a power source 221 between the positive electrode 201 and the negative electrode 211 as shown in FIG. When a voltage is applied from the power source 221, lithium in the positive electrode 201 is ionized and released as lithium ions 217 from the positive electrode 201, and electrons 219 are generated. The lithium ions 217 move to the negative electrode 211 through the electrolyte 207. The electrons 219 move to the negative electrode 211 via the power source 221. Then, the lithium ion 217 receives the electron 219 at the negative electrode 211 and is occluded in the negative electrode 211 as lithium.

  On the other hand, discharge is performed by connecting a load 223 between the positive electrode 201 and the negative electrode 211 as shown in FIG. Lithium in the negative electrode 211 is ionized and released from the negative electrode 211 as lithium ions 217, and electrons 219 are generated. The lithium ions 217 move to the positive electrode 201 through the electrolyte 207. The electrons 219 move to the positive electrode 201 through the load 223. Then, the lithium ions 217 receive the electrons 219 at the positive electrode 201 and are stored in the positive electrode 201 as lithium.

  Thus, charging / discharging is performed by moving lithium ions through the positive electrode 201 and the negative electrode 211. In the positive electrode 201 of the power storage device 200, by applying the positive electrode active material 105 of Embodiment 1 to the positive electrode active material 205, it is possible to increase the amount of occlusion and release of lithium by increasing the density of the active material. A high-capacity power storage device can be obtained. Further, even when lithium iron phosphate containing trivalent Fe is applied as a positive electrode active material, charging and discharging can be performed satisfactorily, and a power storage device exhibiting favorable battery characteristics can be provided.

  This embodiment can be implemented in combination with any of the other embodiments and examples as appropriate.

  In this example, battery characteristics of a sample to which the positive electrode active material according to one embodiment of the present invention is applied will be described.

First, the measured sample will be described. As a sample, a 2032 type coin-shaped battery having a positive electrode, a negative electrode, and a separator containing an electrolytic solution between the positive electrode and the negative electrode was formed. The negative electrode was formed using lithium. As the separator, polypropylene (PP) was used. As the electrolytic solution, a mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC) in which LiPF ₆ was dissolved was used.

The positive electrode had a structure in which an active material was formed on a current collector (titanium). As a sample in Condition A, a target having a mixture of Li ₃ Fe ₂ (PO ₄ ) ₃ and Fe ₂ O ₃ was used, and a film having a thickness of about 100 nm formed by a sputtering method was formed as an active material. As a sample in Condition B, a target having a mixture of Li ₃ Fe ₂ (PO ₄ ) ₃ and Fe ₂ O ₃ was used, and a film having a thickness of about 50 nm formed by a sputtering method was formed as an active material.

In both conditions A and B, the active material was formed by a sputtering method with an RF power supply of 700 W, a pressure of 0.1 Pa, and a gas composition of O ₂ : Ar = 2.5: 50 (sccm).

The active material in the present embodiment _{(Li 3 Fe 2 (PO 4} ) 3 and _Fe 2 _{O 3} using a target that has been mixed, film formed by a sputtering method), shows the results of considerations. First, the active material density was about 3.3 g / cm ³ .

Moreover, about the active material in a present Example, the measurement of X-ray photoelectron spectroscopy (XPS: X-ray Photoelectron Spectroscopy) and the measurement of inductively coupled plasma mass spectrometry (Inductively Coupled Plasma Mass Spectrometer: ICP-MS) were performed. From the results of XPS and ICP-MS, the composition ratio of the active material is Li: Fe: P: O: Na: F: S = 9.8: 11: 12: 65: 1.2: 0.6: 1 .3 (Na, F, and S are impurities). Here, the valence of Fe contained in the active material is formed by sputtering using a target prepared from a raw material containing trivalent Fe and mixed with Li ₃ Fe ₂ (PO ₄ ) ₃ and Fe ₂ O _3. Since it is formed by the law, it was presumed to be trivalent.

When an X-ray diffraction (XRD) measurement of a sample obtained by subjecting the active material formed under the same conditions as the above condition A to further heat treatment (550 ° C., 6 hours) was performed, a NASICON type A peak of Li ₃ Fe ₂ (PO ₄ ) ₃ was observed. From the results of XPS, ICP-MS, and XRD analysis, the active material (film formed by sputtering using a target in which Li ₃ Fe ₂ (PO ₄ ) ₃ and Fe ₂ O ₃ are mixed) is at least It was inferred to have lithium iron phosphate containing trivalent Fe.

  A charge / discharge test (using Toyo System Co., Ltd. charge / discharge test apparatus TOSCAT-3100) was performed for each of the sample of Condition A and the sample of Condition B. The measurement voltage is set in the range of 2.5 V to 4.2 V, the measurement current is a constant current of 0.001 mA, constant current charging → rest 2 hours → constant current discharge → pause 2 hours charge at a constant temperature of 25 ° C. A discharge test was conducted.

  FIG. 3 shows a charge / discharge curve obtained by conducting a charge / discharge test on the sample of Condition A. In addition, although the charging / discharging curve of 1st cycle and 8th cycle is shown in FIG. 3, the substantially same charging / discharging curve was obtained also in 2nd cycle-7th cycle. The discharge capacity in the sample of Condition A was 76 mAh / g. As shown in FIG. 3, it was found that the sample under the condition A has good cycle characteristics with almost no change in the charge / discharge curve even when the number of cycles is increased. In addition, as shown in FIG. 3, it was found that the sample in Condition A has a characteristic that the charge / discharge curve does not have a plateau (potential flat portion), and the potential changes gently and continuously with charge / discharge. .

  As a comparative example with respect to condition A, a film having a thickness of about 100 nm formed under the same conditions as the active material of condition A was heat-treated at 550 ° C. for 6 hours as an active material (Comparative Example) (A-1)). Further, as another comparative example for the condition A, a film having a thickness of about 100 nm formed under the same conditions as the active material of the condition A was heat-treated at 650 ° C. for 6 hours as an active material ( Comparative example (A-2)).

  A charge / discharge test was performed on the sample of Comparative Example (A-1) and the sample of Comparative Example (A-2) under the same conditions as the sample of Condition A. As a result, the discharge capacity in Comparative Example (A-1) was 37 mAh / g, and the discharge capacity in Comparative Example (A-2) was 7.8 mAh / g. From the results of the charge / discharge test, it was found that the sample in Condition A had a higher discharge capacity than the samples of Comparative Example (A-1) and Comparative Example (A-2) subjected to heat treatment. In the sample of Condition A, the active material is in an amorphous state, and it is presumed that the active materials of Comparative Example (A-1) and Comparative Example (A-2) subjected to heat treatment are in a crystalline state. Moreover, it is guessed from the heat processing conditions that the sample of the comparative example (A-2) has higher crystallinity than the sample of the comparative example (A-1). From these facts, the active material in this example is presumed to have better battery characteristics in the amorphous state than in the crystalline state.

  FIG. 4 shows a charge / discharge curve obtained by conducting a charge / discharge test on the sample of Condition B. FIG. 4 shows charge / discharge curves in the first and second cycles. The discharge capacity in the sample of Condition B was 60 mAh / g. As shown in FIG. 4, it was found that the sample in Condition B has a characteristic that the charge / discharge curve does not have a plateau and the potential changes gently and continuously with charge / discharge.

  Note that the discharge rate characteristics of the samples of Condition A and Condition B were measured.

  Here, the discharge rate can be expressed by “C rate”. The current value sufficient to discharge the entire capacity of the battery in 1 hour is defined as 1C rate, and the number of times the current value is discharged at 1C rate is represented by “C rate”. Therefore, the nC rate (n is a positive real number) represents discharging at a current value n times the 1C rate. For example, in a battery having a total capacity of 2.2 [A / h], 1C = 2.2 [A] and nC (n is a positive integer) = 2.2 n [A]. In this embodiment, since the discharge rate characteristic is measured, “C rate” represents the discharge rate, and the larger n is, the faster the discharge rate is. Here, the C rate represents a current value during charging / discharging.

  It was found that the sample of condition A and the sample of condition B had rate characteristics as shown in FIG.

  This example can be implemented in combination with any of the other embodiments as appropriate.

101 positive electrode 103 current collector 104 target 105 positive electrode active material 200 power storage device 201 positive electrode 203 current collector 205 positive electrode active material 207 electrolyte 209 separator 211 negative electrode 213 current collector 215 negative electrode active material 217 lithium ion 219 electron 223 load 221 power supply

Claims (3)

A method for manufacturing a positive electrode active material for a power storage device, wherein an active material having an amorphous structure is formed by a sputtering method using a target including trivalent Fe, Li, and PO ₄ in a composition. A method for manufacturing a positive electrode active material for a power storage device, wherein an active material having an amorphous structure is formed by a sputtering method using a target in which Li ₃ Fe ₂ (PO ₄ ) ₃ and Fe ₂ O ₃ are mixed. In any one of Claim 1 or Claim 2 ,
The method for manufacturing a positive electrode active material for a power storage device, wherein the active material having an amorphous structure contains trivalent Fe.

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