JP2011171248A - Solid electrolyte and all-solid lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solid electrolyte with further increased lithium ion conductivity using lithium phosphorus oxynitride as a main component, and an all-solid lithium secondary battery. <P>SOLUTION: The all-solid lithium secondary battery 20 is provided with a positive electrode 24, a negative electrode 28, and a solid electrolyte 26 interposed between the positive electrode 24 and the negative electrode 28. The solid electrolyte 26 is made of lithium phosphorus oxynitride, with an atomic ratio (Li/P) of lithium atom to phosphorus atom 3.0 or more and 6.0 or less, and the peak of P<SB>2P</SB>coupling of the phosphorus atom of the lithium phosphorus oxynitride in XPS spectrum exists in a range of 134 eV or more and 135 eV or less. Furthermore, it is preferable that the solid electrolyte 26 be manufactured by a high-frequency sputtering method, by using a mixed powder of lithium phosphate and lithium oxide as a target. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a solid electrolyte and an all solid-state lithium secondary battery.

Conventionally, development of an all-solid battery using a solid electrolyte instead of a liquid electrolyte has been performed. Since all components of the battery are solid, not only the reliability of the battery is improved, but also the battery can be made smaller and thinner. Even in the case of a lithium secondary battery, development of an all-solid lithium secondary battery, which is an all-solid battery using a solid electrolyte composed of a noncombustible solid material, is desired. Among all solid lithium ion secondary batteries, the most important is the solid electrolyte. For this reason, oxide-based solid electrolyte materials have been developed from the standpoint of ease of thin film production and handling, reactivity with lithium, decomposition voltage, and the like. Among them, solid electrolyte materials based on Li ₃ PO ₄ have been actively developed. For example, Bates et al. Of Oak Ridge National Laboratory (ORNL) have reported an all-solid battery using LiPON as a solid electrolyte (see, for example, Patent Document 1). Here, LiPON is a lithium ion conductor in which nitrogen is introduced into Li ₃ PO ₄ .

US Pat. No. 5,597,660

However, lithium nitride phosphate, which is a solid electrolyte described in Patent Document 1, has an insufficient lithium ion conductivity of about 1 × 10 ⁻⁶ to 2 × 10 ⁻⁶ S / cm. When the lithium ion conductivity of the lithium ion conductor is low, for example, when it is used as a solid electrolyte of an all-solid lithium ion secondary battery, the internal resistance of the battery increases. For this reason, the voltage drop becomes large, and good large current discharge characteristics cannot be obtained.

  The present invention has been made in view of such a problem, and provides a solid electrolyte and an all-solid-state lithium secondary battery that can further increase lithium ion conductivity in a lithium nitride phosphate as a main component. The main purpose is to do.

  As a result of diligent research to achieve the above-mentioned object, the present inventors made high-frequency sputtering using a mixed powder of β-phase lithium phosphate and lithium oxide, and produced a solid electrolyte membrane of lithium nitride phosphate As a result, it has been found that the lithium ion conductivity can be further increased, and the present invention has been completed.

  That is, the solid electrolyte of the present invention is made of lithium nitride lithium having an atomic ratio (Li / P) of lithium atoms to phosphorus atoms of 3.0 or more and 6.0 or less.

The all solid-state lithium secondary battery of the present invention is
A positive electrode having a positive electrode active material;
A negative electrode having a negative electrode active material;
And the above-described solid electrolyte interposed between the positive electrode and the negative electrode.

The solid electrolyte of the present invention can further increase lithium ion conductivity. The reason why such an effect is obtained is not clear, but is presumed as follows. For example, in general, LiPON can be obtained by using a sintered body or hot-press fired lithium phosphate (Li ₃ PO ₄ ) as a target and sputtering it in nitrogen gas. In this firing process, lithium phosphate changes to a γ phase. According to this method, in the N _1s bond of the nitrogen atom of lithium nitride lithium phosphate in the XPS spectrum, more PN = P bonds are formed than the PNPP (-P) bond. On the other hand, in the solid electrolyte of the present invention, many P—N—P (—P) bonds are formed. That is, in the N _1s bond of the nitrogen atom, in the P— _N═P bond, two phosphorus atoms are bonded to one nitrogen atom, but in the P—N—P (—P) bond, one It is presumed that the lithium ion conductivity is improved because three phosphorus atoms are bonded to the nitrogen atom. Alternatively, the solid electrolyte of the present invention has a large amount of lithium and has a lithium crystal phase in the crystal. This is presumed to improve the lithium ion conductivity.

The schematic diagram which shows an example of the all-solid-type lithium secondary battery 20 of this invention. FIG. The voltammogram obtained by the CV measurement of Example 1. The XPS spectrum of the _N1s state of Example 3 and the comparative example 3 and a _P2p state. The X-ray-diffraction measurement result of the solid electrolyte membrane of Example 1 and Comparative Examples 1 and 3.

The solid electrolyte of the present invention is a lithium lithium phosphate (hereinafter also referred to as LiPON) in which the atomic ratio (Li / P) of lithium atoms contained in the solid electrolyte to phosphorus atoms contained in the solid electrolyte is 3.0 or more and 6.0 or less. ). That is, this solid electrolyte can be represented by Li _x PO _y N _z (x is 3.0 or more and 6.0 or less, and y and z are arbitrary values). When the atomic ratio is 3.0 or more, the lithium ion conductivity is preferably more than 3 × 10 ⁻⁶ S / cm. In addition, when the atomic ratio is 6.0 or less, chemical stability can be further improved. This atomic ratio is more preferably 3.2 or more and 4.0 or less, and further preferably 3.3 or more and 3.8 or less.

The solid electrolyte of the present invention may contain lithium nitride phosphate having a lithium crystal phase. In this way, the lithium ion conductivity can be further increased due to the presence of the lithium crystal phase. In general, it is considered that lithium nitride phosphate films are phase-separated in the form of Li ₂ O—P ₂ O ₅ —PON (Non-patent Document 1: J. Electrochem. Soc., 144, 524 (1997)). The solid electrolyte of the present invention is presumed to exhibit high lithium ion conductivity due to the excess of Li ₂ O in lithium nitride lithium phosphate, which is present as a lithium crystal phase.

When the solid electrolyte of the present invention is measured using X-ray photoelectron spectroscopy (XPS), the peak of the P _2p bond of the phosphorous atom of lithium lithium phosphate in the XPS spectrum is in the range of 134 eV to 135 eV. If the P _2p bond peak of the phosphorus atom of lithium nitride lithium phosphate is in this range, the lithium ion conductivity is further improved. In addition, when the solid electrolyte of the present invention is measured by using X-ray photoelectron spectroscopy (XPS), the N _1s bond of the nitrogen atom of lithium nitride lithium in the XPS spectrum is compared with the P— _N═P bond. Thus, many P—N—P (—P) bonds are formed. That is, in the N _1s bond of the nitrogen atom, in the P— _N═P bond, two phosphorus atoms are bonded to one nitrogen atom, but in the P—N—P (—P) bond, one Three phosphorus atoms are bonded to the nitrogen atom, so that lithium ion conductivity is improved (see Non-Patent Document 2: Journal of Power Sources, 43-33 (1993) 103-110). It is presumed that the solid electrolyte of the present invention having many P—N—P (—P) bonds exhibits high lithium ion conductivity.

  The solid electrolyte of the present invention is not particularly limited, but is preferably formed in a film shape. The film thickness is preferably 0.1 μm or more and 100 μm or less, and more preferably 0.5 μm or more and 10 μm or less. When the thickness is 0.1 μm or more, short-circuiting of the electrode can be further prevented.

The solid electrolyte of the present invention may have a lithium ion conductivity exceeding 3.0 × 10 ⁻⁶ S / cm. The lithium ion conductivity of the solid electrolyte of the present invention is more preferably 3.5 × 10 ⁻⁶ S / cm or more, and more preferably 4.0 × 10 ⁻⁶ S / cm or more.

The solid electrolyte of the present invention may have a potential window that is stable against lithium metal and has a potential window wider than the range of +0 V or more and +5 V or less on the basis of Li / Li ⁺ . Such a lithium nitride phosphate is easy to use for an all-solid-state lithium secondary battery because the selection range of the active material is wider. This potential window is more preferably in the range of +0 V to +5.5 V on the basis of Li / Li ⁺ .

The solid electrolyte of the present invention can be produced by a thin film production method using a vacuum apparatus. Note that it may be manufactured by a method other than a manufacturing method using a vacuum apparatus, for example, a method in which a raw material paste having the above atomic ratio is manufactured and fired. As a manufacturing method using a vacuum apparatus, a method combining vapor deposition and ion beam irradiation for introducing nitrogen ions can be given. As this vapor deposition method, for example, a sputtering method in which a target is sputtered with nitrogen (N ₂ ) by means such as magnetron or high frequency, a resistance heating vapor deposition method in which the vapor deposition source is heated by vapor deposition, and a vapor deposition source is heated by an electron beam. And an electron beam vapor deposition method for vapor deposition, and a laser ablation method for vapor deposition by heating a vapor deposition source with a laser. Among these, what is produced by a high frequency sputtering method is preferable. In this high-frequency sputtering method, a mixture of β-phase lithium phosphate and lithium oxide is preferably used as a target, and a mixed powder is more preferably used as a target. When this mixture is used, for example, a solid electrolyte having a low energy and the above-mentioned atomic ratio can be produced relatively easily as compared with a lithium phosphate sintered body. Here, the mixture includes a mixed powder obtained by mixing a lithium phosphate powder and a lithium oxide powder, and a molded body obtained by molding the mixed powder. Alternatively, lithium phosphate or lithium oxide (not a sintered body) that has not been heat-treated may be used as a sputtering target. If it carries out like this, it will suppress that a lithium phosphate changes to a gamma phase, and it will be easy to utilize as a beta phase. For example, when lithium phosphate is fired at 500 ° C. or higher, lithium phosphate changes to a metastable γ phase, and even if cooled after firing, lithium phosphate exists in the γ phase instead of the β phase (Non-patent Document 3). : Chem. Mater., 20 5574-5854 (2008)). Compared to this γ-phase lithium phosphate, it is more preferable to use β-phase lithium phosphate as a raw material because the lithium ion conductivity is improved. For example, the lithium phosphate powder preferably has an average particle size of 0.1 μm to 500 μm, more preferably 1.0 μm to 100 μm, and still more preferably 10 μm to 50 μm. The lithium oxide powder preferably has an average particle size of, for example, 0.1 μm to 500 μm, more preferably 1.0 μm to 100 μm, and still more preferably 10 μm to 50 μm. As the average particle size of the mixed powder, it is preferable that the average particle size of lithium oxide is smaller than that of lithium phosphate because the lithium content of the solid electrolyte can be easily increased.

The solid electrolyte of the present invention is preferably produced using a mixture having a mixed molar ratio of lithium oxide to lithium phosphate (Li ₂ O / Li ₃ PO ₄ ) of 0.5 or more and 3.0 or less. . If it carries out like this, it will be easy to make Li / P ratio in a solid electrolyte into 3.0 or more and 6.0 or less. The mixing molar ratio is more preferably 1.0 or more and 2.0 or less.

  An all solid-state lithium secondary battery of the present invention includes a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and any of the solid electrolytes described above interposed between the positive electrode and the negative electrode. It is. The shape of the all solid-state lithium secondary battery of the present invention is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a rectangular type. FIG. 1 is a schematic view showing an example of an all solid-state lithium secondary battery 20 of the present invention. In this all solid-state lithium secondary battery 20, a conductive layer 22 is formed on a substrate 21, and a positive electrode 24, a solid electrolyte 26, and a negative electrode 28 are sequentially stacked on the conductive layer 22. A current collecting lead 23 is connected to the conductive layer 22, and a current collecting lead 29 is connected to the negative electrode 28. In this all solid-state lithium secondary battery 20, the solid electrolyte 26 is formed of lithium nitride phosphate having an atomic ratio (Li / P) of lithium to phosphorus of 3.0 or more and 6.0 or less. In the all solid-state lithium secondary battery 20 having such a configuration, for example, the substrate 21 may be a silicon substrate or a quartz plate, the conductive layer 22 may be a platinum layer, and the current collecting leads 23 and 29 are gold wires. It is good. As the positive electrode active material and the negative electrode active material, those generally used in all solid-state lithium secondary batteries can be used.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  Below, the example which produced the solid electrolyte of this invention concretely is demonstrated as an Example.

[Example 1]
An experimental cell equipped with a solid electrolyte thin film was prepared, and the ionic conductivity of the solid electrolyte of the present invention was examined. FIG. 2 is a schematic cross-sectional view of the experimental cell 30. As shown in FIG. 2, the experimental cell 30 was composed of a glass substrate 31, a current collector layer 32 made of platinum / titanium, a solid electrolyte layer 33, and a current collector layer 34 made of gold. First, as a first step, a metal mask having a window (2 mm × 50 mm) is put on a predetermined position of the glass substrate 31, a titanium layer having a thickness of 20 nm is formed by RF sputtering, and then a film having a thickness of 80 nm by DC sputtering. A platinum layer was formed to produce a current collector layer 32. As a second step, the metal mask size is changed to 10 mm × 30 mm, a solid electrolyte film is formed on the platinum / titanium current collector layer 32 by RF sputtering, and a solid electrolyte layer 33 having a thickness of 2 μm is produced. did. At this time, a mixed powder of lithium phosphate and lithium oxide not subjected to heat treatment (not fired) was used as the target. As a result of the X-ray diffraction measurement, lithium phosphate was in the β phase in the powder used. The average particle size of the powder was calculated from the SEM image, and was about 100 μm for lithium phosphate and about 30 μm for lithium oxide. The mixing ratio of the two powders was 1: 2 (molar ratio) of lithium phosphate: lithium oxide, and the mixture was mixed in a mortar to obtain a target. RF sputtering was performed using nitrogen as a sputtering gas, with a chamber internal pressure of 2.5 mTorr, and a gas introduction amount of 20 sccm. The temperature of the substrate during film formation was set to 50 ° C. or lower. Subsequently, as a third step, a metal mask having a 2 mm × 50 mm window is disposed on the solid electrolyte layer 33 so as not to protrude from the solid electrolyte layer 33, and a gold film is formed by DC sputtering. A current collector layer 34 made of gold having a thickness of 0.1 μm was formed. The obtained one was used as the experimental cell of Example 1. Table 1 summarizes the raw materials of Example 1. In addition, this Table 1 showed the measurement result of Li / P ratio and ionic conductivity, and also showed the contents of Examples 2 and 3 and Comparative Examples 1 to 3 described later.

[Examples 2 and 3]
The same treatment as in Example 1 can be performed except that a mixed powder obtained by mixing lithium phosphate powder and lithium oxide powder not subjected to heat treatment at a mixing ratio of 1: 1 (molar ratio) was used as a target. This was used as the experimental cell of Example 2. Moreover, the process similar to Example 1 except having used the mixed powder which mixed the lithium phosphate powder and the lithium oxide powder which were not heat-processed with the mixing ratio of 1: 0.5 (molar ratio) as a target. The experimental cell obtained in Example 3 was used as the experimental cell of Example 3.

[Comparative Examples 1 and 2]
An experimental cell of Comparative Example 1 was obtained by performing the same treatment as in Example 1 except that only the lithium phosphate powder not subjected to heat treatment was used as a target. Further, the same treatment as in Example 1 was performed except that a mixed powder obtained by mixing a lithium phosphate powder and a lithium oxide powder not subjected to heat treatment at a mixing ratio of 1: 4 (molar ratio) was used as a target. The obtained one was used as an experimental cell of Comparative Example 2.

[Comparative Example 3]
An experimental cell of Comparative Example 3 was obtained by performing the same treatment as in Example 1 except that a sintered plate-like lithium phosphate sintered at 700 ° C. was used as a target. As a result of X-ray diffraction measurement, lithium phosphate was in the γ phase in the used sintered plate.

[Composition: Li / P ratio]
The compositions of the films of Examples 1 to 3 and Comparative Examples 1 to 3 were measured for the atomic ratio of Li / P using an inductively coupled plasma emission spectrometer (CIROS-120EOP manufactured by RIGAKU). As shown in Table 1, in Examples 1 to 3, it was found that the theoretical ratio of LiPON was larger than 3.0 and the lithium content was large. Moreover, in Comparative Examples 1-3, Li / P ratio was less than 3.0.

[Measurement of lithium ion conductivity]
About the experimental cell of Examples 1-3 and Comparative Examples 1-3, alternating current impedance measurement was performed and the time-dependent change of ion conductivity was investigated. AC impedance measurement was performed using an equilibrium voltage of zero, an amplitude of ± 50 mV, and a frequency region from 10 ⁶ Hz to 1 Hz (see Table 1). As a result, the lithium nitride phosphate (Examples 1 to 3) of the present invention has an ionic conductivity of 3.0 × 10 ⁻⁶ S / cm or more, and 2.3 × of conventional LiPON (Comparative Example 3). It was confirmed that the ion conductivity was higher than 10 ⁻⁶ S / cm. On the other hand, the ionic conductivities of Comparative Examples 1 and 2 were 1.7 × 10 ⁻⁶ S / cm and 2.1 × 10 ⁻⁶ , respectively, which were inferior to conventional LiPON (Comparative Example 3). Therefore, in the present invention, the Li / P ratio in the solid electrolyte membrane is preferably 3.0 or more and 6.0 or less (the mixed molar ratio of Li ₂ O / Li ₃ PO ₄ is 0.5 or more and 3.0 or less). all right.

[Examination of potential window]
Measurement was carried out using a cyclic voltammetry method (CV method), and the stability of the produced solid electrolyte against lithium metal was evaluated. The electrode for CV measurement was produced by the following procedure. First, a titanium layer having a thickness of 20 nm was formed on a glass substrate by RF sputtering, and then a platinum layer having a thickness of 80 nm was formed by DC sputtering to form a current collector layer. A solid electrolyte membrane was formed thereon. In the formation of the solid electrolyte membrane, the solid electrolyte membrane of Example 1 for CV measurement was produced through the same steps as in Example 1 except that the film thickness was 1 μm. Measurement was carried out by the CV method using the produced electrode as a working electrode. The counter electrode and the reference electrode were lithium foil (thickness: about 0.5 mm), and a mixed solution of ethylene carbonate and diethylene carbonate in which LiPF ₆ was dissolved at 1 mol / L in a weight ratio of 3: 7 was used as the electrolyte. The potential of the working electrode is changed from a natural potential (about +0.4 V vs. Li ^/ Li ⁺ ) to −0.5 V (vs. Li ^/ Li ⁺ ), and then from +7.5 V (vs. Li ^/ Li ⁺ ) to the natural potential. The current was measured while changing the potential.

FIG. 3 is a voltammogram obtained by the CV measurement of Example 1. A large current flows at points a and b shown in FIG. 3, which is considered to correspond to lithium deposition and dissolution on the electrode. Also, the current increases rapidly at point c, which corresponds to the decomposition of the solid electrolyte. In addition, no signal that could correspond to the degradation of the solid electrolyte was observed. From this, the solid electrolyte membrane according to the present invention does not decompose even at a voltage (0 V vs. Li ^/ Li ⁺ ) at which lithium metal starts to be deposited, and does not decompose until it exceeds +6 V, and is stable. all right. From the above results, it was found that the solid electrolyte membrane of Example 1 was stable against lithium metal and that the potential window was wider than +0 V to +5 V (vs. Li ^/ Li ⁺ ).

[XPS measurement]
X-ray photoelectron spectroscopy measurement (XPS measurement) was performed to examine the bonding state of atoms in the solid electrolyte of the present invention. The XPS measurement was performed using an XPS measurement apparatus (PHI-5500MC manufactured by ULVAC-PHI) and using MgKα as an X-ray source. The thickness of the film was set to 100 nm on the silicon substrate, and the solid electrolyte of Example 3 for XPS measurement was produced as a measurement sample through the same steps as in Example 3 described above. A solid electrolyte membrane of Comparative Example 3 for XPS measurement was produced as a measurement sample through the same process as Comparative Example 3 except that the thickness of the film was changed to 100 nm on the silicon substrate. FIG. 4 is an XPS spectrum of the N _1s state and the P _2p state in the solid electrolyte membranes of Example 3 and Comparative Example 3. It can be seen that Example 3 and Comparative Example 3 are significantly different. There are two types of N _1s states in the LiPON film: PN = P bond and P—N—P (—P) bond. Like the normal LiPON film, Comparative Example 3 has a low energy side (about 398 eV). The P—N═P bond was dominant (see Non-Patent Document 2). On the other hand, the solid electrolyte membrane of Example 3 was found to have a dominant PNP (-P) bond. The P _2p bond of the phosphorus atom in the solid electrolyte membrane of Comparative Example 3 has a peak at 133.5 eV, while the P _2p bond of the phosphorus atom in the solid electrolyte membrane of Example 3 has a peak at 134.25 eV. It was. That is, in Example 3, it was found that the peak of the P _2p binding of phosphorus atoms of lithium phosphorus oxynitride acid is present in 135eV following range of 134EV.

[X-ray diffraction measurement]
X-ray diffraction measurement was performed to examine the crystalline state in the produced solid electrolyte. X-ray diffraction measurement was performed using an X-ray diffractometer (RINT2200, manufactured by Rigaku Corporation) and using CuKα ray (λ = 1.5418) as an X-ray source. The thickness of the film was set to 2 μm on the silicon substrate, and the solid electrolyte of Example 1 for X-ray diffraction measurement was prepared as a measurement sample through the same steps as in Example 1 described above. Further, the thickness of the film is set to 2 μm on the silicon substrate, and the solid electrolytes of Comparative Example 1 and Comparative Example 3 for X-ray diffraction measurement are used as measurement samples through the same steps as Comparative Example 1 and Comparative Example 3 described above. Produced. FIG. 5 shows X-ray diffraction measurement results of the solid electrolyte membranes of Example 1 and Comparative Examples 1 and 3. In Comparative Examples 1 and 3, since no crystal peak other than the peak derived from the silicon substrate was confirmed, these thin films were amorphous. On the other hand, in Example 1, the peak derived from a lithium metal crystal was confirmed. This is presumed that lithium oxide excessively mixed in the target became lithium oxide with many oxygen defects by sputtering, and this was phase-separated into a lithium oxide phase and a lithium crystal phase.

  Thus, the solid electrolyte composed of lithium nitride phosphate of the present invention prepared by sputtering using a powder mixture of unsintered lithium phosphate and lithium oxide has an atomic ratio of lithium to phosphorus (Li / P ) Is 3.0 or more and 6.0 or less, and the lithium ion conductivity is higher, the potential window is wide, and it is clear that it is preferably used for an all-solid-state lithium secondary battery.

  20 Lithium secondary battery, 21 substrate, 22 conductive layer, 23, 29 current collector lead, 24 positive electrode, 26 solid electrolyte, 28 negative electrode, 30 experimental cell, 31 glass substrate, 32, 34 current collector layer, 33 Solid electrolyte layer.

Claims (4)

  A solid electrolyte comprising a lithium lithium phosphate having an atomic ratio (Li / P) of a lithium atom to a phosphorus atom contained of 3.0 or more and 6.0 or less.   The solid electrolyte according to claim 1, wherein the solid electrolyte is produced by a high-frequency sputtering method using a mixture of β-phase lithium phosphate and lithium oxide as a target.   The solid electrolyte of Claim 2 produced using the said mixture whose mixing molar ratio of lithium oxide with respect to lithium phosphate is 0.5 or more and 3.0 or less. A positive electrode having a positive electrode active material;
A negative electrode having a negative electrode active material;
The solid electrolyte according to any one of claims 1 to 3, which is interposed between the positive electrode and the negative electrode,
All-solid-state lithium secondary battery.

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