JP4174729B2 - Method for forming inorganic solid electrolyte - Google Patents

Description

  The present invention relates to a method for forming an inorganic solid electrolyte.

  Practical use of lithium secondary batteries using organic electrolyte is progressing. The feature is that the energy output per unit volume or unit weight is higher compared to other batteries, and the development of practical use has been promoted as a power source for mobile communications, notebook computers and electric vehicles. Yes.

  In order to improve the performance of the lithium secondary battery, there is an attempt to use lithium metal as a negative electrode, but dendritic lithium metal grows on the negative electrode during charging and discharging, causing an internal short circuit with the positive electrode, and finally There is a risk of explosion. As a technique for suppressing this risk, as described in Patent Document 1, formation of a sulfide-based inorganic solid electrolyte thin film on lithium metal has been studied.

  Moreover, as a solid electrolyte used for a lithium battery etc., nonpatent literature 1, nonpatent literature 2, nonpatent literature 3, nonpatent literature 4, patent literature 2, patent literature 3, patent literature 4, patent literature 5, patent literature 6 The technique described in is known.

JP 2000-340257 A JP 2001-250580 A Japanese Patent Publication No. 5-48582 JP-A-4-231346 U.S. Patent No. 6,025,094 U.S. Pat.No. 5,314,765 Rene Mercier, Jean-Pierre Malugani, Bernard Fahys, Guy Robert, "SUPERIONICCONDUCTION IN Li2S-P2S5-LiI-GLASSES", Solid State Ionics5, North-Holland Publishing Company, 1981, P663-666 Kazunori Takada, Shigeo Kondo, “Inorganic Solid Electrolyte”, Electrochemistry 65, No. 11, 1997, P914-919 “J.Am.Cream.Soc, 84 [2]” 2001, P477-479 Shigenori Tsuji, Satoshi Hayashi, Hideyuki Morimoto, Masahiro Isago, Tsutomu Minami, "Synthesis and High Lithium Ion Conductivity of Novel Li2S-P2S5 Glass Ceramics", Proceedings of the 26th Solid State Ionics Conference, Solid State Ionics Society , November 15-17, 2000, P174-175

However, when a sulfide-based inorganic solid electrolyte thin film containing silicon sulfide is brought into contact with lithium metal, the silicon of silicon sulfide (SiS ₂ ) is reduced by the lithium metal, and the inorganic solid electrolyte changes over time even at room temperature. There was found.

  In general, a layer having a low ion conductivity such as an oxide is formed on lithium metal. When this oxide layer is present, the reaction between the lithium metal and silicon sulfide tends to be suppressed. . However, it has been found that when the oxide layer is removed to improve the battery performance, the deterioration of the inorganic solid electrolyte over time associated with the reaction between lithium metal and silicon sulfide becomes obvious.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a method for forming a sulfide-based inorganic solid electrolyte in which the deterioration with time of the inorganic solid electrolyte accompanying the reduction reaction of silicon is suppressed even when contacted with lithium metal.

The present invention is based on the knowledge that lithium metal and silicon sulfide (SiS ₂ ) react at room temperature to cause deterioration of the inorganic solid electrolyte. The above object is achieved by not containing Si in the inorganic solid electrolyte. To do.

<Inorganic solid electrolyte>
First, the method for forming an inorganic solid electrolyte of the present invention is a method for forming an inorganic solid electrolyte thin film on a substrate, and the inorganic solid electrolyte formed on the substrate is substantially composed of Li, P and S. It is characterized by not containing Si. In particular, it is preferable that Li is contained in an amount of 20 atomic% or more and 60 atomic% or less, and the balance is substantially composed of P and S and contains no Si. More preferably, Li is 25 atomic% or more and 60 atomic% or less, and the balance is substantially composed of P and S and does not contain Si. In the present invention, the term “substantially” means that inevitable impurities are contained in addition to the elements that are significantly contained.

  It is desirable that Si is not included in the main skeleton of the glassy composition of the inorganic solid electrolyte. Conventionally, it was naturally considered that sulfide-based inorganic solid electrolyte thin films contain Si, but this Si is not known to adversely affect the performance of inorganic solid electrolytes. It was not done. Therefore, in the present invention, by not containing Si, the temporal change of the inorganic solid electrolyte accompanying the reaction between lithium metal and silicon sulfide is suppressed. Note that “not containing Si” means that the Si content is less than 1 atomic%.

  The lithium-phosphorus-sulfur inorganic solid electrolyte containing no silicon is superior to other sulfide inorganic solid electrolytes, particularly sulfide inorganic solid electrolytes containing silicon, for the following reasons.

1． High ionic conductivity.
2． Even if it is brought into contact with the lithium metal surface, it does not react with the lithium metal like the sulfide inorganic solid electrolyte containing silicon.
3． A lithium secondary battery using a non-aqueous organic electrolyte has the following characteristics when an inorganic solid electrolyte is formed on lithium metal to form a negative electrode.
• Excellent dendrite growth suppression effect.
・ Excellent chemical stability against electrolytes.
・ Strong mechanical strength against morphological changes during charge / discharge cycles.
Four. Although SiS ₂ is expensive, phosphorus sulfide (P ₂ S ₅ ) is suitable for industrial production because it is relatively inexpensive and easily available.

  The lithium element content in the inorganic solid electrolyte layer is 20 atomic% or more and 60 atomic% or less. If it is less than 20 atomic%, the ionic conductivity is lowered and the resistance is increased. Moreover, the adhesiveness of an inorganic solid electrolyte layer and a lithium metal layer falls. On the other hand, when the lithium element content exceeds 60 atomic%, the adhesion between the inorganic solid electrolyte layer and the lithium metal layer is improved, but the inorganic solid electrolyte layer becomes polycrystallized and porous, resulting in a dense inorganic solid electrolyte. It becomes difficult to form a continuous film. In addition, electronic conductivity is manifested, causing an internal short circuit when the battery is constructed, thereby reducing battery performance.

  Therefore, the electrolyte layer is preferably an amorphous body. In the present invention, the amorphous is substantially formed of amorphous (glass), and in addition to a halo pattern by X-ray diffraction, a slight amount due to starting materials and products in addition to the halo pattern. This includes cases where a peak is observed (when starting material or product crystal grains are partially precipitated) and when a peak of the substrate (when forming a thin film) is observed.

  The phosphorus element content in the inorganic solid electrolyte layer is preferably 3 atomic% to 20 atomic%. The sulfur element content is preferably 30 atomic% or more and 60 atomic% or less. If the phosphorus or sulfur content is low, defects are likely to occur. In addition, if there is a large amount of phosphorus or sulfur, phosphorus or sulfur alone as impurities increases in the solid electrolyte.

As a specific compound contained in the inorganic solid electrolyte in the present invention, a compound of lithium sulfide (Li ₂ S) and phosphorus sulfide (P ₂ S ₅ ) is preferable. In particular, when the composition ratio of lithium (Li) is X and the composition ratio of phosphorus (P) is Y, X / Y is preferably 1.0 or more and 19 or less.

  Furthermore, this inorganic solid electrolyte may contain at least one of oxygen and nitrogen. Inclusion of a trace amount of oxygen or nitrogen of about 5 atomic% or less makes it possible to exhibit even higher lithium ion conductivity. This is presumably because the inclusion of a trace amount of oxygen or nitrogen atoms has the effect of widening the gaps between the formed amorphous skeletons, reducing the hindrance to the movement of lithium ions. .

Examples of the compound containing oxygen include Li ₃ PO ₄ , Li ₂ SO ₄ , Li ₂ O, and P ₂ O ₅ . Examples of the compound containing nitrogen include Li ₃ PO _4-X N _{2X / 3} (0 <X <4).

  Such an inorganic solid electrolyte is preferably a thin film. The thickness of the thin film is desirably 0.01 μm or more and 10 μm or less, and more preferably 0.05 μm or more and 1 μm or less. This is because if the thickness is too thin, defects such as pinholes increase, and if it is too thick, the process time increases and the manufacturing cost increases.

<Base material>
In the method for forming an inorganic solid electrolyte of the present invention, it is desirable that the inorganic solid electrolyte thin film be formed on the surface of lithium metal or lithium alloy (lithium-containing material) so that it can be used as a battery member. Specific examples of the lithium alloy include alloys of Li and In, Ti, Zn, Bi, Sn, or the like.

  On the surface of the lithium-containing material, a metal thin film such as Al, In, Bi, Zn, Pb or the like, which forms an alloy or intermetallic compound with lithium, may be formed. By using a negative electrode composed of this metal thin film and a lithium-containing material, the movement of the lithium metal during charge / discharge becomes smooth, and the use thickness of the lithium metal increases. Further, the deformation of the negative electrode during charge / discharge becomes uniform, and the strain on the electrolyte layer can be reduced.

<Method for forming inorganic solid electrolyte>
The method for forming an inorganic solid electrolyte of the present invention is a method for forming an inorganic solid electrolyte thin film on a substrate, wherein the substrate is heated at 40 ° C. to 180 ° C. with Li, P and S. An inorganic solid electrolyte thin film that is substantially configured and does not contain Si is formed. Alternatively, after forming an inorganic solid electrolyte thin film similar to the above at a substrate temperature of less than 40 ° C., the substrate on which the thin film is formed is heated to 40 ° C. or more and 180 ° C. or less.

  In particular, it is preferable that Li is contained in an amount of 20 atomic% to 60 atomic%, with the balance being substantially composed of P and S. More preferably, Li is 25 atomic% or more and 60 atomic% or less, and the balance is substantially composed of P and S and does not contain Si.

  Thus, a thin film with high ionic conductivity is obtained by heating a base material when forming an inorganic solid electrolyte thin film, or heat-treating a thin film after forming at room temperature.

  By the way, when forming an inorganic solid electrolyte containing Si on the surface of a lithium-containing material, if an inorganic solid electrolyte is formed while heating the substrate, or if heat treatment is performed after forming the inorganic solid electrolyte, the reaction between Li and Si Is promoted. In contrast, when an inorganic solid electrolyte that does not contain Si is formed on the surface of a lithium-containing material as in the present invention, an inorganic solid electrolyte with good characteristics such as ionic conductivity is formed by performing the above heat treatment. It is extremely effective. Moreover, when forming an inorganic solid electrolyte thin film, heating a base material, the effect that the adhesiveness of a thin film and a base material improves is also expectable.

  The inorganic solid electrolyte thin film is preferably formed by any one of sputtering, vacuum deposition, laser ablation, and ion plating.

  The method for forming an inorganic solid electrolyte of the present invention includes a step of irradiating a substrate surface with inert gas ions to etch the substrate surface, and an inorganic solid electrolyte that is performed at least partially overlapping with the etching step. It is preferable to include a film forming step.

In particular, when the above lithium-containing material is used as a substrate, this lithium-containing material may be used as it is without any pretreatment when forming the electrolyte layer. In many cases, an oxide layer (Li ₂ O, etc.), a carbonate layer (Li ₂ CO ₃ ), a hydroxide layer (LiOH), etc. are formed thin on the surface. Since these lithium ion conductivities are low, it is desirable to remove them. Removal of the oxide layer or the like can be performed by irradiation with an inert gas ion such as an argon ion beam. The argon gas used at that time is preferably as pure as possible. For example, argon having a purity of 99.9999% is suitable.

  Thereby, an inorganic solid electrolyte thin film can be directly formed on a lithium containing material, and the impedance of a lithium containing material and an inorganic solid electrolyte layer can be reduced more.

And it is desirable to carry out the process of irradiating the surface of the substrate with inert gas ions to etch the surface of the substrate and the step of forming the inorganic solid electrolyte membrane. Even with XPS analysis that can maintain the analysis chamber in an ultra-high vacuum of 1.33 × 10 ^-9 hPa (1 × 10 ^-9 Torr) or less by forming such a film, oxygen is separated at the boundary between the lithium-containing material and the inorganic solid electrolyte. It can be confirmed that the amount of the lithium battery member gradually changes from the inorganic solid electrolyte toward the lithium-containing material, and the oxide layer present on the surface of the lithium-containing material is almost removed. As an XPS (X-ray Photo-electronic Spectroscopy) that can maintain the analysis chamber in an ultra-high vacuum of 1.33 × 10 ^-9 hPa (1 × 10 ^-9 Torr) or less, 5400 manufactured by Phi A type is mentioned.

Conventionally, in the formation of an electrolyte film using an ordinary vacuum apparatus having a degree of vacuum of about 1.3 × 10 ⁻⁷ hPa (1 × 10 ⁻⁷ Torr), an electrolyte thin film forming step is performed after performing an etching step. An oxygen-containing layer (oxide layer, carbonate layer, hydroxide layer, etc.) is easily formed between the lithium-containing material and the inorganic solid electrolyte. In the present invention, by performing the etching process and the electrolyte thin film forming process in an overlapping manner, even an ordinary vacuum apparatus can be formed by substantially eliminating the oxygen-containing layer.

  The etching step and the electrolyte thin film forming step may completely overlap, but it is preferable to partially overlap the etching process end side and the electrolyte thin film forming step start side. The oxygen-containing layer on the surface of the substrate is almost removed by the etching step, and the formation of the electrolyte thin film is started in a state where the oxygen-containing layer cannot be formed following the etching of the surface of the substrate. At that time, the electrolyte thin film is formed while being slightly removed by the inert gas ions.

  The time for performing the etching step alone may be set to such an extent that the oxygen-containing layer on the surface portion of the substrate can be substantially removed. Further, the time for overlapping the etching process and the electrolyte thin film forming process may be such that the substrate surface is covered with the electrolyte thin film material.

On the other hand, the electrolyte thin film may be formed after forming the nitride layer such as lithium nitride (Li ₃ N) after substantially removing the oxygen-containing layer. Li ₃ N has an ionic conductivity of 1 × 10 ⁻³ S / cm or more at room temperature, and even if it exists between lithium metal and an inorganic solid electrolyte thin film, it does not reduce the current density that can be passed. In addition, the presence of the nitride layer can be expected to suppress generation of an oxide layer or the like.

  The nitride layer can be formed by a mixed ion beam treatment of argon and nitrogen. It is sufficient that the nitride layer is formed only on the very surface of the lithium-containing material, and a thickness of 1 nm or more is preferable so that non-nitrided portions are not scattered. However, if the nitride layer is too thick, problems such as a decrease in ionic conductivity due to polycrystallization, a reaction with the electrolytic solution, and a deterioration in withstand voltage occur. Therefore, the thickness of the nitride layer is desirably 0.1 μm (100 nm) or less, more preferably 10 nm or less.

<Lithium secondary battery>
The battery member in which an inorganic solid electrolyte thin film is formed on the surface of lithium metal or lithium alloy (lithium-containing material) as described above can be used as a negative electrode for a lithium secondary battery.

  For example, a lithium secondary battery can be constructed by laminating a positive electrode, a porous separator, and a negative electrode, impregnating with a nonaqueous organic electrolyte solution, and housing and sealing the laminate in a battery case.

  In more detail, first, the negative electrode current collector and the negative electrode are joined, and an inorganic solid electrolyte thin film not containing an organic electrolyte solution is formed on the lithium-containing material to be the negative electrode. Make it. Further, a positive electrode material containing an organic polymer is formed on a positive electrode current collector (for example, copper or aluminum foil) to obtain a positive electrode. The joined body and the positive electrode are combined via a separator to produce a lithium secondary battery.

  Thereby, the contact resistance between the negative electrode and the positive electrode and the electrolyte layer can be reduced, and good charge / discharge characteristics can be obtained. In addition to the button type battery thus laminated, a negative electrode, an electrolyte layer, and a positive electrode may be laminated and wound into a cylindrical shape.

  As a material of the separator disposed between the positive electrode and the solid electrolyte layer, a separator having pores through which lithium ions can move and insoluble and stable in the organic electrolyte is used. For example, a nonwoven fabric or a porous material formed from polypropylene, polyethylene, fluorine resin, polyamide resin or the like can be used. In addition, a metal oxide film having pores may be used.

  As described above, according to the method for forming an inorganic solid electrolyte of the present invention, an inorganic solid electrolyte thin film not containing Si can be formed on a substrate, so that deterioration of the electrolyte membrane due to the reaction between lithium metal and silicon sulfide is prevented. Can be suppressed. In particular, the electrolyte membrane is formed directly on the lithium-containing metal, and the oxygen-containing layer existing on the lithium-containing metal surface between the lithium-containing metal and the electrolyte membrane is almost completely removed by etching. While reducing the resistance of the interface between lithium-containing metals, high ionic conductivity can be realized.

Embodiments of the present invention will be described below.
(Example 1)
A 10 μm thick lithium metal thin film having a thickness of 10 μm is formed on a 100 mm × 50 mm copper foil by a vacuum deposition method, and an inorganic solid electrolyte thin film is formed on the surface. Instead of the lithium metal thin film, an inorganic solid electrolyte thin film can be formed on the lithium metal foil by laminating a lithium metal foil having the same size as the copper foil and a thickness of 30 μm.

The base material on which the lithium metal foil film is formed is placed in a vacuum film forming apparatus. Ultimate vacuum of the vacuum apparatus is ^{5.3 × 10 -7 hPa (4 ×} 10 -7 Torr). First, argon gas (purity 99.9999%) is flowed so that the pressure becomes 2.7 × 10 ⁻⁴ hPa (2 × 10 ⁻⁴ Torr), and the sample surface is irradiated with an ion beam at 15 mA and 500 V for 30 seconds by an ion gun. Furthermore, the film formation of the inorganic solid electrolyte is started by laser ablation without interrupting the ion beam irradiation. That is, the ion beam irradiation is continuously performed for 40 seconds, but the second half of 10 seconds is performed overlapping with the film formation of the inorganic solid electrolyte. The first 30 seconds is for removing the oxide layer and carbonate layer on the lithium metal.

When the ion beam irradiation is interrupted and the film formation is started after a while, it remains in the vacuum vessel in a general vacuum apparatus having a vacuum degree of about 1.3 × 10 ⁻⁷ hPa (1 × 10 ⁻⁷ Torr) Oxygen tends to produce a very thin oxide layer on the lithium metal surface. Therefore, in the latter 10 seconds, ion beam irradiation and film formation are performed at the same time, so that almost no oxide layer is formed.

After stopping the ion beam, the film formation continued, and within 3 minutes, the argon gas pressure was increased to 2.7 × 10 ⁻³ hPa (2 × 10 ⁻³ Torr), and the temperature was raised from room temperature to 140 ° C. Under these conditions, an inorganic solid electrolyte was formed to a thickness of 0.5 μm. The laser used for film formation was a KrF excimer laser, and the laser oscillation frequency was 5 Hz.

  The ion beam irradiation conditions (current, time, and gas pressure) shown here are only examples, depending on the thickness of the oxide layer and carbonate layer formed on the lithium metal surface, the distance between the ion gun and the sample, etc. Need to be adjusted.

In addition, if an ultrahigh vacuum of 1.33 × 10 ^-9 hPa (1 × 10 ^-9 Torr) or less can be reached and the adsorption of oxygen and moisture inside the container is extremely small, after ion beam irradiation, Even if the film formation is started at an interval, it is considered that the inorganic solid electrolyte film can be formed so that an oxide layer or the like hardly exists on the lithium metal surface. However, in order to manufacture such a device, an extremely advanced technique is required, and even if possible, the cost of the device becomes very high. Therefore, the method described above was adopted.

The inorganic solid electrolyte is composed of a sample A formed using 63Li ₂ S-36.5SiS ₂ -0.5Li ₃ PO ₄ containing Si (a numerical value is a molar ratio) and 78Li ₂ S-21.5P ₂ S ₅ − containing no Si. Two types of sample B were prepared with 0.5Li ₃ PO ₄ (the numerical value is a molar ratio) as a target.

When the sample was taken out after film formation and the sample was observed, the thin film of sample A targeted for 63Li ₂ S-36.5SiS ₂ -0.5Li ₃ PO ₄ containing Si was completely black. Usually, a good Li ₂ S—SiS ₂ thin film is colorless and transparent. In addition, the ionic conductivity was 1 × 10 ⁻⁴ S / cm or less at 25 ° C., and it was found that it deteriorated by reacting with Si by direct contact with lithium metal. The ionic conductivity was measured by a direct current measurement method in the direction perpendicular to the film surface.

On the other hand, the thin film of Sample B targeted at 78Li ₂ S-21.5P ₂ S ₅ -0.5Li ₃ PO ₄ is colorless and transparent, and the ionic conductivity measured is 1.3 × 10 ⁻³ S / cm at 25 ° C. It was. The ionic conductivity was measured in the same manner as Sample A. The Li composition ratio (atomic%) of Sample B was 41% as a result of EPMA (Electron Probe Micro Analyzer) analysis. The composition (atomic%) of the thin film is measured by the above-mentioned EPMA, XPS, ICP (Inductively Coupled Plasma) emission analysis or gas analysis by inert gas melting infrared absorption method. Further, a thin film formed on the glass substrate under the same film formation conditions as Sample B was analyzed by ICP to obtain a composition ratio X of Li and a composition ratio Y of P, and the calculated ratio X / Y was 3.6. . The ICP analyzer used at this time is an SPS1200VR type manufactured by Seiko Electronics. X-ray diffraction confirmed that Sample B was in an amorphous state with only a very small peak other than the peak from the substrate.

Furthermore, inorganic solids using an XPS (X-ray Photo-electronic Spectroscopy) that can maintain the analysis chamber in an ultra-high vacuum of 1.33 × 10 ^-9 hPa (1 × 10 ^-9 Torr) or less Composition analysis was performed on the lithium metal thin film from the electrolyte membrane. The apparatus used for analysis is model 5400 made by Phi.

  As a result, it was confirmed that the amount of oxygen gradually changed (decreased) from the inorganic solid electrolyte toward the lithium metal thin film at the boundary between the inorganic solid electrolyte film and the lithium metal thin film. It was also confirmed that the oxide layer present on the lithium metal surface was almost removed before forming the inorganic solid electrolyte membrane.

Using a base material on which an inorganic solid electrolyte thin film targeting 78Li ₂ S-21.5P ₂ S ₅ -0.5Li ₃ PO ₄ is formed on a lithium metal thin film as a negative electrode, a separator (porous polymer film), a positive electrode, an organic electrolyte A lithium secondary battery composed of the above or the like was produced. The battery production procedure and battery evaluation results are described below.

Heats a mixed solution of ethylene carbonate (EC) and propylene carbonate (PC), cools a polyacrylonitrile (PAN) dissolved at a high concentration, and contains a large amount of EC and PC in which LiPF ₆ is dissolved PAN to make. In this PAN, LiCoO ₂ particles serving as an active material and carbon particles imparting electron conductivity were mixed and applied to a 20 μm thick aluminum foil (positive electrode current collector) at a thickness of 300 μm to form a positive electrode.

A negative electrode, a separator (porous polymer film), and a positive electrode on which a solid electrolyte thin film was formed were placed on top of each other in a stainless steel sealed container, and further 1 mol% LiPF ₆ as an electrolytic salt in a mixed solution of ethylene carbonate and propylene carbonate. A lithium secondary battery was produced in an argon gas atmosphere with a dew point of −60 ° C. or lower by dropping an organic electrolyte solution in which the above solution was dissolved.

  The charge / discharge characteristics of the produced battery were evaluated. As a result, each battery had a charging voltage of 4.2 V, and the capacity until the voltage decreased to 3.0 V by 100 mA discharge was 0.5 Ah (ampere hour). The energy density was 500 Wh (watt hour) / 1 (liter). Furthermore, cycle charge / discharge was performed under the same conditions. It was stable over 600 cycles, and even after 600 cycles, over 50% of the initial capacity was maintained.

(Example 2)
A lithium metal thin film having a thickness of 10 μm was formed on a 100 mm × 50 mm copper foil having a thickness of 10 μm by a vacuum deposition method. An inorganic solid electrolyte thin film having a thickness of 0.5 μm was formed on the lithium metal thin film. As in Example 1, the lithium metal foil may be bonded to the copper foil.

The base material on which the lithium metal thin film was formed was placed in a vacuum apparatus, and ion beam treatment was first performed. Flow a mixed gas of argon and nitrogen (Argon 75 vol.%, Nitrogen 25 vol.%) At a pressure of 2.7 x 10 ^-4 hPa (2 x 10 ^-4 Torr) and irradiate the sample surface with an ion beam at 15 mA and 500 V with an ion gun did. The nitrogen content of the mixed gas is preferably 10 vol.% Or more and 50 vol.% Or less. This is because when the content of nitrogen is small, the nitriding effect is small, and when the content is large, the filament of the ion gun is significantly deteriorated. As a result of XPS analysis, the thickness of the formed nitride layer was 1 nm.

  Here, etching and nitriding of the oxide layer and the like were performed simultaneously using an argon-nitrogen mixed gas. However, after performing etching treatment of the oxide layer and the like only with argon gas, a nitride thin film such as lithium nitride is used. Can also be formed by a vapor phase method or the like. It is also possible to start the formation of the nitride layer in the second half of the ion beam irradiation, and simultaneously perform the etching process using the ion beam and the formation of the nitride layer.

  Then, an inorganic solid electrolyte thin film was formed. The formation conditions are shown in Tables 1 and 2. In addition, when forming a film at room temperature without heating the thin film during the thin film formation, a heat treatment is performed for 15 minutes at a temperature shown in Table 2 in an Ar gas at atmospheric pressure. The composition ratio (atomic%) of Li of the obtained inorganic solid electrolyte thin film, the ionic conductivity at 25 ° C., the composition ratio X of Li and the composition ratio Y of P were calculated, and the ratio X / Y calculated from this (in the table, Li / Li) / P ratio) was also obtained. The composition ratio of Li was determined by EPMA analysis, and the ratio X / Y was determined by ICP analysis. ICP analysis analyzed thin films deposited on glass or sapphire substrates under the same deposition conditions. These are also shown in Tables 1 and 2. Except for No.2-2 and No.2-8, the composition ratio of P (atomic%) is 3 atomic% to 20 atomic%, and the composition ratio of S (atomic%) is 30 atomic% to 60 atomic%. It was in the range of less than%.

  A lithium secondary battery comprising a separator (porous polymer film), a positive electrode, an organic electrolyte, and the like was produced in the same manner as in Example 1, using the base material on which the inorganic solid electrolyte thin film was formed on the lithium metal thin film as the negative electrode. .

  As a result of evaluating the charge / discharge characteristics of the batteries produced, the charge voltage was 4.2 V, and the capacity until the voltage decreased to 3.0 V by 100 mA discharge was 0.5 Ah (ampere hour). The energy density was 450 to 550 Wh (watt hour) / 1 (liter).

  Furthermore, cycle charge / discharge was performed under the same conditions. Except for No. 2-2 with a large amount of Li and No. 2-8 with a small amount of Li, the cycle was stable 600 times or more, and 50% of the initial capacity was maintained even after 600 cycles.

(Example 3)
A solid electrolyte thin film was prepared on lithium metal in the same manner as in Example 2 No. 2-5, and the production evaluation of a lithium secondary battery was further performed. In this example, evaluation was performed by changing the thickness of the nitride layer. The results are shown in Table 3.

  As apparent from Table 3, the thickness of the nitride layer is preferably 1 nm or more and 100 nm or less, and more preferably 1 nm or more and 10 nm or less.

  No. 3-1 corresponds to the case where the film was formed after the oxide or carbonate layer was removed with an argon ion beam in Example 1.

(Example 4)
An inorganic solid electrolyte thin film was produced on the same lithium metal thin film as in Example 2, and the production evaluation of a lithium secondary battery was further performed. In this example, an etching treatment was performed using an argon gas having a purity of 99.9999% instead of the argon-nitrogen mixed gas used in Example 2, and a solid electrolyte film was formed after the etching, and a nitride layer was not formed. . The composition ratio (atomic%) of Li of the obtained inorganic solid electrolyte thin film, the ionic conductivity at 25 ° C., the composition ratio X of Li and the composition ratio Y of P were obtained, and the ratio X / Y calculated from this was also obtained. Tables 4 and 5 show the formation conditions, ionic conductivity, and ratio X / Y (Li / P ratio in the table).

As shown in Table 5, all samples had an ionic conductivity of 1 × 10 ⁻⁴ S / cm or more. In addition, the same XPS as in Example 1 was used to analyze the composition of the lithium metal thin film from the inorganic solid electrolyte membrane. As a result, it was confirmed that the oxygen amount gradually changed from the inorganic solid electrolyte toward the lithium metal thin film at the boundary between the inorganic solid electrolyte film and the lithium metal thin film. It was confirmed that the oxide layer present on the surface of the lithium metal was almost removed before forming the inorganic solid electrolyte membrane.

  In addition, cyclic discharge was performed under the same conditions as in Example 2. As a result, as shown in Table 5, it was stable at 500 cycles or more, and even after 500 cycles, 50% or more of the initial capacity was maintained.

  The method for forming an inorganic solid electrolyte of the present invention is suitable for use in forming a negative electrode for a lithium secondary battery.

Claims (5)

An inorganic solid electrolyte forming method for forming an inorganic solid electrolyte thin film on a substrate,
Formation of an inorganic solid electrolyte characterized by forming an inorganic solid electrolyte thin film substantially free from Si and substantially composed of Li, P, and S while heating the substrate to 40 ° C or higher and 180 ° C or lower Method. An inorganic solid electrolyte forming method for forming an inorganic solid electrolyte thin film on a substrate,
After forming an inorganic solid electrolyte thin film that is substantially composed of Li, P, and S and does not contain Si at a substrate temperature of less than 40 ° C, the substrate on which the thin film is formed is 40 ° C to 180 ° C. A method for forming an inorganic solid electrolyte, characterized by heating to a temperature. 3. The method for forming an inorganic solid electrolyte according to claim 1, wherein Li is contained in an amount of 20 atomic% to 60 atomic%, and the balance is substantially composed of P and S. 4. The method for forming an inorganic solid electrolyte according to any one of claims 1 to 3, wherein the inorganic solid electrolyte thin film is formed by any one of a sputtering method, a vacuum deposition method, a laser ablation method, and an ion plating method. An inorganic solid electrolyte forming method for forming an inorganic solid electrolyte membrane on a substrate,
Irradiating the substrate surface with inert gas ions to etch the substrate surface;
A step of forming an inorganic solid electrolyte membrane substantially composed of Li, P and S and not containing Si ,
A method of forming an inorganic solid electrolyte, wherein the etching step and the electrolyte membrane forming step are performed so that at least a part thereof overlaps .