JP6323873B2 - Electrode for measuring secondary battery operand using soft X-ray - Google Patents

Description

  The present invention relates to an electronic state analysis indispensable for elucidating a charge / discharge mechanism of a lithium ion secondary battery electrode. The present invention also relates to a lithium ion secondary battery electrode having a thin film structure.

Materials such as LiCoO ₂ , LiMn ₂ O ₄ , and LiFePO ₄ that are widely used as positive electrodes for lithium ion secondary batteries have insufficient performance such as charge / discharge capacity for large-scale applications such as in-vehicle use. Development and search for new high performance cathode materials are underway. In addition, carbon-based materials are generally used as negative electrode materials, but studies on 3d transition metal oxides such as Fe ₂ O ₃ and CoO are also being conducted in parallel.

In order to improve the performance of such transition metal compound-based electrode materials, it is important to elucidate the charge / discharge mechanism of existing materials, and it is necessary to track the oxidation-reduction reaction of transition metal elements during the charge / discharge reaction. The intended electronic state analysis is widely performed. As a method of electronic state analysis, synchrotron radiation hard X-ray absorption spectroscopy is the mainstream, but this method can elucidate the information of the transition metal 3d-oxygen 2p hybrid orbital, which is the most important for charge and discharge. I'm not good at it.
On the other hand, the application of synchrotron radiation soft X-ray absorption spectroscopy and soft X-ray emission spectroscopy capable of obtaining such information is also in progress. However, in this method, it is necessary to place a sample, that is, an electrode in a vacuum. Yes, measurement under the charge / discharge operation with the electrolyte (operand) is not possible as it is.

In recent years, by using a thin-film window material mainly composed of Si ₃ N ₄ that can transmit soft X-rays and can be separated from a vacuum chamber and an atmospheric pressure chamber, atmospheric pressure can be applied to samples such as fuel cell catalysts and liquid samples. The following technologies have been developed that enable the operand soft X-ray absorption / emission spectroscopy [Non-Patent Documents 1, 2, 3].
However, there is no example in which this method is applied to a lithium ion secondary battery with an electrolytic solution and its electrode material. This is because, in order for such a method to be possible, the sample to be analyzed needs to be in close contact with a soft X-ray transmissive window material, whereas the electrode material of a lithium ion secondary battery In the case of charging / discharging, the electrode active material is easily peeled off from the window material. This is considered to be related to the high reactivity of the organic electrolyte and metal Li, the wide range of applied voltage, etc., which are peculiar to lithium ion secondary batteries.
On the other hand, in a lithium ion secondary battery, various reaction phases are formed at the component member interface due to the high reactivity of the above-described secondary battery component member, which affects the secondary battery characteristics. In the state where the secondary battery is operated, it is essential to analyze the electronic state of the structural member, and a technique that enables operand analysis using soft X-rays has been awaited.

JP2008-261650

H. Niwa et al., Electrochem. Commun. 35 (2013) 57. T. Tokushima et al., Chem. Phys. Lett. 460 (2008) 387. J. Guo and Y. Luo, J. Electron Spectrosc. Relat. Phenom. 177 (2010) 181. J. Chen et al., Adv. Mater. 17 (2005) 582. C. T. Cherian et al., J. Mater. Chem. 22 (2012) 12198. W. Yang et al., Phys. Rev. B 80 (2009) 014508.

It is an object of the present invention to provide a thin film electrode using a Si ₃ N ₄ thin film that can realize electrochemical operand soft X-ray absorption / emission spectroscopy of a lithium ion secondary battery electrode.

In order to perform electrochemical operand soft X-ray absorption / emission spectroscopy in a lithium ion battery, it is necessary that an electrode active material such as a transition metal oxide is in close contact with a soft X-ray transparent window material as described above. . Specifically, a soft X-ray transparent window material for performing an electrochemical operand analysis of an electrode active material is generated by an electrochemical reaction of the electrode active material on the side in contact with the electrode active material of the Si ₃ N ₄ layer. It is necessary that the metal current collector layer for taking out the current is laminated and the electrode active material is in close contact with the metal current collector layer.
In order to make the electrode active material adhere to the metal current collector layer disposed on the Si ₃ N ₄ thin film window material, a slurry of the electrode active material is not applied and dried as in the prior art related to fuel cells. It is important to obtain an electrode active material such as a transition metal oxide directly on the metal current collector by film formation with heat treatment.
On the other hand, in the soft X-ray transmissive window member provided with the current collector, it is necessary to reduce the thickness of the current collector (metal thin film) and increase the transmittance in order to sufficiently transmit the soft X-rays. For example, a soft X-ray transparent window material sold as an off-the-shelf product as disclosed in Patent Document 1 is Si ₃ N ₄ (150 nm) / Cr or Ti (3 nm using a single layer of Cr or Ti as an adhesion layer. ) / Au (10 nm). However, such a conventional window material can reduce the adhesion of the Si ₃ N ₄ / Cr or Ti interface due to heat treatment, and as a result, Cr or Ti is oxidized when air enters the interface, Although Au itself does not oxidize, it is easily imagined that the electrical resistance of the Au layer increases greatly as a result of the diffusion and movement of Au atoms and the decrease in uniformity of the 10 nm thick Au layer, It is difficult to use as a current collector for an operand analysis electrode of a lithium ion battery that requires the above-described heat treatment. On the other hand, by forming a thick metal film such as Au as a current collector to a thickness of 100-200 nm, oxidation of Cr or Ti that may occur due to heat treatment, diffusion of Au near the Cr or Ti / Au interface, etc. Although it is considered that the increase in resistance due to is suppressed, the soft X-ray transmittance is greatly reduced and does not serve as a window for operand measurement.

In addition, an off-the-shelf soft X-ray transmissive window material as disclosed in Patent Document 1 has a thickness of only about 160 nm and can be easily damaged during the heat treatment. After the Si wafer / Si ₃ N ₄ / current collector / active material film was fabricated in a process including heat treatment, wet etching was performed from the Si surface so as not to damage the current collector / active material film, and Si ₃ N ₄ windows need to be made. Therefore, it is required to etch Si while protecting the current collector / active material film against a basic solution used for wet etching.

The present inventors sputter-deposited Al ₂ O ₃ having high adhesion with a metal layer on a Si wafer / Si ₃ N ₄ thin film, and Ti (10-20 nm) thereon using an electron beam evaporation method. ) Layer, and then an Au (10-20 nm) layer was found to suppress the increase in resistance of the metal current collector after the active material was produced by the heat treatment.

  In addition, the present inventors use a polymer sheet such as polyethylene having a strong base resistance and a chemically reactive adhesive between an epoxy resin and a modified silicone polymer for the electrode after heat treatment, thereby providing a current collector / By etching Si and processing the window while protecting the active material film side, we succeeded in creating a soft X-ray transmission window without damaging the current collector / active material film.

The present application is based on the above findings, and specifically provides the following inventions.
(1) A thin film current collector produced on a Si wafer / Si ₃ N ₄ thin film and having a three-layer structure of Al ₂ O ₃ / Ti / Au and exhibiting low resistance even after electrode active material firing.
(2) The method for producing a thin film current collector according to (1), wherein Al ₂ O ₃ , Ti, and Au are sequentially formed on a Si wafer / Si ₃ N ₄ thin film.
(3) A method for producing a thin-film electrode for a battery, wherein the electrode active material precursor is coated on the thin-film current collector according to (1) and then fired at a high temperature to provide an electrode active material layer.
(4) A thin film electrode for a battery prepared by the method of (3).
(5) The thin film electrode for a battery according to (4), wherein the electrode active material is an active material for a positive electrode of a lithium ion battery.
(6) The thin film electrode for a battery according to (4), wherein the electrode active material is an active material for a negative electrode of a lithium ion battery.
(7) An electrochemical operand comprising a soft X-ray transparent window formed by etching the thin film electrode Si wafer according to (4) until the Si ₃ N ₄ layer is exposed. Electrode for soft X-ray absorption / emission spectroscopy.
(8) By etching the Si wafer at the window forming portion until the Si ₃ N ₄ layer is exposed, while protecting the portion other than the window forming portion on the Si side surface of the thin film electrode described in (4) A method for producing an electrode for electrochemical operand soft X-ray absorption / emission spectroscopy, comprising forming a soft X-ray transparent window.

  The present invention enables electrochemical operand soft X-ray absorption / emission spectroscopy of lithium ion battery electrode materials. By using soft X-ray absorption / emission spectroscopy in an actual environment of electrochemical operand measurement, it has become possible to obtain much richer and more accurate information than conventional electronic state analysis. Thus, according to the present invention, it is possible to analyze operand electronic states of various electrode materials.

The figure which shows the preparation process of the electrode for electrochemical operand soft X-ray absorption / emission spectroscopy. The figure which shows the structure of the electrode for the completed electrochemical operand soft X-ray absorption / emission spectroscopy. Before etching, schematic view of a state in which the protective Si wafer / Si ₃ N ₄ film / current collector / electrode active material with a polyethylene sheet and the adhesive. The schematic diagram of the completed electrode after an etching. Si wafer / Si ₃ N ₄ (150 nm) / Al ₂ O ₃ (5 nm) / Ti (20 nm) / Au (15 nm) First to third cycles of Fe ₂ O ₃ thin film formed on the Au layer of the substrate Cyclic voltammetry curve. Electrochemical operand soft X-ray emission spectroscopy performed on the completed Fe ₂ O ₃ electrode.

The soft X-ray transmission part (window part) in the thin film electrode of the present invention has a film structure represented by Si ₃ N ₄ / Al ₂ O ₃ / Ti / Au / electrode active material. The thickness of each layer was determined in consideration of an optimum value within a range where soft X-ray absorption / emission spectroscopy can be practically performed. For Si ₃ N ₄ , 150 nm or more equivalent to a commercial product is desirable in order to maintain the mechanical strength of the window. The thickness is preferably about 15-20 nm for the Ti layer and about 10-20 nm for the Au layer. The Al ₂ O ₃ layer that plays the role of improving the adhesion of the Ti layer is preferably as thin as possible within the range that the Al ₂ O ₃ layer can be maintained, and 5 nm was selected as the optimum value.

If soft X-ray absorption / emission spectroscopy at the Mn 2p absorption edge is performed, the photon energy is about 640 eV, and each film thickness of the current collector layer is Si ₃ N ₄ (150 nm) / Al ₂ O ₃ (5 nm ) / Ti (20 nm) / Au (10 nm) windows, 35% of soft X-rays are transmitted. Even when Au is 20 nm, the transmittance is about 27%. Even in the 2p absorption edge excitation of other 3d transition metal elements except Ti, there is a transmittance of around 30% when a similar window is used. With such a transmittance, soft X-ray absorption / emission spectroscopy can be performed within a practical range.

After the electrode active material is prepared by heat treatment, it is considered that a certain amount of atomic migration / diffusion occurs between the Al ₂ O ₃ / Ti / Au current collector layers. There is no significant increase in electrical resistance at the Au surface that would interfere with chemical measurements.

The electrode active material prepared in the present invention assumes a positive electrode active material typified by LiMn ₂ O ₄ and a negative electrode material such as Fe ₂ O ₃ . It can be applied to all materials that can be made by the sol-gel method including high temperature heat treatment.
Examples of such electrode active material precursors include metal alkoxides and metal organic acid salts, and corresponding electrode active materials include LiCoO ₂ , LiFePO ₄ , LiV ₃ O ₈ , Li ₂ FeSiO. ₄ and so on.

  The electrode active material prepared by such a method usually has a film thickness of about 50-100 nm, but the film thickness may be controlled according to the purpose.

After the electrode active material is prepared by heat treatment, the Si wafer in the window is removed by wet etching using a basic solution. At this time, wet etching is performed after the Si ₃ N ₄ layer also existing on the surface on the Si wafer side is scraped off.

  When performing the etching, the outer edge side of the electrode active material and current collector side and the window portion on the Si wafer side is protected with a polyethylene sheet and an adhesive.

  For the polyethylene sheet and the adhesive, those having strong base resistance are selected.

  When performing the electrochemical operand soft X-ray emission spectrometry using the thin film electrode prepared in the present invention, it is assumed that a known cell (Non-patent Document 1) is used. About an electrode, the well-known thing used for the evaluation test of a general lithium ion secondary battery can be used.

  In the present invention, firstly, an electrochemical operand soft X-ray emission spectroscopic measurement for a lithium ion secondary battery is considered, but a sodium ion battery electrode material that can be synthesized by a method similar to a lithium ion battery electrode material, It is also possible to apply to other devices utilizing reactions at the solid-liquid interface, such as devices with various catalytic reactions such as fuel cells.

  Further, the present invention can be used not only for a lithium ion secondary battery using an electrolytic solution but also for an all solid state lithium ion secondary battery using a solid electrolyte.

  Hereinafter, the present invention will be described in more detail with reference to examples. It goes without saying that it is possible.

Example 1. Production of electrode substrate < Production of Si wafer / Si ₃ N ₄ thin film / thin film current collector>
A Si wafer whose surface is coated with Si ₃ N ₄ (150 nm) is cut, and one side of the wafer is subjected to plasma ashing, then Al ₂ O ₃ is sputtered to 5 nm, and after plasma ashing, Ti is 20 nm, an electron beam. A current collector was produced by vapor deposition followed by 10-20 nm electron beam vapor deposition of Au (FIG. 1). The electrical resistance in the Au plane shows a sufficiently low resistance of about 0.1Ω to several Ω.

<Preparation of LiMn ₂ O ₄ , LiNi _0.5 Mn _1.5 O ₄ , Fe ₂ O ₃ solution for spin coating>
Below, the preparation methods of each solution are shown.
-LiMn ₂ O ₄ : Prepare a 0.1 mol / l solution with a formula weight of Li and 0.2 mol / l with a formula weight of Mn. Each powder of anhydrous lithium acetate and manganese acetate tetrahydrate is dissolved in 9 ml of a mixed solution of 2 methoxyethanol and 1 ml of monoethanolamine. After sufficiently stirring at room temperature, the mixture is further stirred at 80 ° C. for 2 hours.
LiNi _0.5 Mn _1.5 O ₄ : Prepare a solution of 0.1 mol / l in the formula amount of Li, 0.05 mol / l in the formula amount of Ni, and 0.15 mol / l in the formula amount of Mn. Dissolve each powder of anhydrous lithium acetate, nickel acetate tetrahydrate, and manganese acetate tetrahydrate in 9 ml of a mixed solution of 2 methoxyethanol and 1 ml of monoethanolamine. After sufficiently stirring at room temperature, the mixture is further stirred at 80 ° C. for 2 hours.
・ Fe ₂ O ₃ : Prepare a 0.2 mol / l solution with the formula weight of Fe. Dissolve the iron iron citrate powder in 2 ml of ion exchange water. To this solution, add 8 ml of 2 methoxyethanol and stir well at room temperature.

<Production of electrode active material by heat treatment>
Both ends are masked on the Au side of the prepared Si wafer / Si ₃ N ₄ thin film / thin film collector, and these solutions are spin-coated, dried, and heat-treated in air. LiMn ₂ O ₄ and LiNi _0.5 Mn _1.5 O ₄ were fired at 600 ° C. for 10 minutes. Fe ₂ O ₃ was baked at 550 ° C for 10 minutes. FIG. 2 shows a schematic view of a completed film structure after the baking and after-mentioned etching treatment. In order from the side to be etched, Si / Si ₃ N ₄ / current collector / electrode active material (Si ₃ N ₄ layer also exists on the Si side of the film structure). As a result of observing the film thickness using a scanning electron microscope, the active material layer has a thickness of about 50-100 nm.

The structure of the current collector before the heat treatment is a three-layer structure of Al ₂ O ₃ / Ti / Au, but it is considered that atomic migration / diffusion occurs between the layers to some extent after the heat treatment. However, even after the heat treatment, the electric resistance in the portion where the masked Au is exposed at the time of spin coating is about several Ω to 30 Ω, and a low resistance that does not hinder electrochemical measurement is maintained.

<Window patterning>
Patterning of the window (3 mm × 0.3 mm) is performed on the Si side of the film structure after the heat treatment (upper figure A in FIG. 3). Si ₃ N ₄ on the Si side surface is removed by photolithography or excavation using a diamond pen.

<Masking for etching process>
As shown in FIG. 3, the above-mentioned film structure subjected to the window patterning is made of a commercially available polyethylene sheet (manufactured by Nippon Shokubai Co., Ltd., Unipack, thickness 0.04 mm) and an epoxy resin (100%) A agent. And window side for etching on the electrode side (bottom of Fig. 3) and Si side using commercially available epoxy resin adhesive (EP001N, manufactured by Cemedine) consisting of B agent of modified silicone polymer (100%) The surrounding area is covered (upper view B in FIG. 3). The four sides of the polyethylene sheet are pressure-bonded and sealed by cutting with a sealer (upper figure C in FIG. 3). About the circumference | surroundings of the window part which etches, a membrane structure and a sheet | seat are adhere | attached by using the chemical reaction type adhesive agent of the epoxy resin and modified silicone polymer with strong base resistance.

<Etching treatment>
Etching is performed using a mixed solution of 80 ml tetramethylammonium hydroxide and 920 ml ion-exchanged water at 90 ° C. Although there are individual differences, the time required for removing Si is about 12 to 14 hours. After the etching is completed, the polyethylene sheet and the adhesive are removed and washed with water and acetone.

FIG. 4 shows a schematic diagram of the completed electrode substrate. In the central part, the Si layer is removed by etching, and a layer structure of Si ₃ N ₄ / current collector / electrode active material is obtained (upper figure A in FIG. 4). The etched portion is transparent to the extent that visible light can be confirmed (B in FIG. 4). Further, as shown in the lower diagram of FIG. 4, the left and right portions of the substrate are exposed portions of Au on which no electrode active material is laminated, and function as a current collector.

Example 2 Cyclic voltammetry (CV) test of electrode substrate Preparation of Fe ₂ O ₃ film in a bipolar electrochemical cell using lithium metal as the counter electrode and ethylene carbonate / diethyl carbonate solution with 1M LiClO ₄ dissolved in the electrolyte A cyclic voltammetry (CV) test was performed at a voltage sweep rate of 0.5 mV / s and a cutoff voltage of 3.2 V to 0.6 V.

FIG. 5 shows the result of the CV test of the Fe ₂ O ₃ film. Peak occurring downwardly 0.78V is, of Fe ₂ O _{3 ^{"Fe 2 O 3 + 6Li + +}} 6e - → 3Li 2 O + 2Fe " alloying reaction, the upper and lower peak around 1.3-2.3V is Li Corresponds to the ⁺ intercalation reaction. In addition, the peak of the alloying reaction decreased in the second and third cycles compared to the first cycle. These results are equivalent to the results of a standard charge / discharge test for Fe ₂ O ₃ prepared by a general solid phase method or the like (Non-Patent Documents 4 and 5).
Therefore, it was confirmed from the shape of the CV curve and the reaction potential of the alloying reaction and intercalation reaction that Fe ₂ O _{3 as} an active material was formed. At the same time, it was confirmed that the electrical resistance of the thin film current collector was low enough not to hinder battery operation.

Example 3 FIG. Electrochemical Operand Soft X-ray Emission Spectrometry Test of Electrode Substrate
Electrochemical operand soft X-ray emission spectrometry was actually performed on the Fe ₂ O ₃ film. The experiment was conducted with the soft X-ray beam line at SPring-8, a large synchrotron radiation facility at the Research Center for High-Intensity Optical Science. Charging / discharging was performed by CV measurement similar to the above.

FIG. 6 shows an electrochemical operand soft X-ray emission spectrum of the Fe ₂ O ₃ film. This spectrum is an emission spectrum due to the 2p → 3d → 2p transition when excited by a photon (709.8 eV) of the Fe 2p absorption edge energy, and reflects the Fe 3d occupied orbit. At the open end potential, the region corresponding to the electron transition between 3d orbitals (706-709eV) has a high intensity, suggesting the coexistence of Fe ²⁺ and Fe ³⁺ having this transition. On the other hand, at the time of charging (0.1 V), the intensity of 706-709 eV decreases and changes to Fe ⁰⁺ , that is, a metallic Fe shape (Non-patent Document 6).
Thus, for the electrode material of the battery in operation, the electronic state change as expected from the above CV measurement was also observed in the soft X-ray emission spectrum. Furthermore, this soft X-ray emission spectrum can be obtained by hard X-ray absorption spectroscopy, etc. by performing fitting by theoretical calculation that incorporates 3d orbital crystal field splitting and charge transfer effects with adjacent atoms as parameters. It is possible to obtain more detailed information about 3d orbits that cannot be performed.

  The present invention makes it possible to analyze the electrochemical operand electronic state of the secondary battery electrode in the soft X-ray region, which was impossible with the prior art, and to elucidate the charge / discharge mechanism in the electrode material, leading to the development of high-performance materials. Knowledge is expected to be obtained. The application of the present invention is expected to many electrode materials that can be synthesized by high-temperature heat treatment. Applications to devices using reactions at solid-liquid interfaces other than secondary battery electrodes can also be expected.

Claims (6)

A thin film current collector produced on a Si wafer / Si ₃ N ₄ thin film and having a three-layer structure of Al ₂ O ₃ / Ti / Au and exhibiting low resistance even after electrode active material firing. The method for producing a thin film current collector according to claim 1, wherein Al ₂ O ₃ , Ti, and Au are sequentially formed on the Si wafer / Si ₃ N ₄ thin film.   A method for producing a thin film electrode for a battery, comprising: coating a thin film current collector according to claim 1 with an electrode active material precursor; The manufacturing method of the thin film electrode for batteries of Claim 3 whose electrode active material is an active material for positive electrodes of a lithium ion battery. The manufacturing method of the thin film electrode for batteries of Claim 3 whose electrode active material is an active material for negative electrodes of a lithium ion battery. A thin film electrode is produced by coating an electrode active material precursor on the thin film current collector of claim 1 and then firing at a high temperature to provide an electrode active material layer, and a window on the Si side surface of the thin film electrode. It is characterized by forming a soft X-ray transmissive window by etching the Si wafer of the window forming part while protecting the part excluding the forming part until the Si ₃ N ₄ layer is exposed. Electrochemical operand: A method for producing electrodes for soft X-ray absorption / emission spectroscopy.

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