JP2015068664A - Evaluation method and evaluation device of electrode material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve a problem that a distribution of mechanical and electrical characteristics, such as current, elasticity, and absorption power, cannot be imaged or visualized simultaneously with SEI of a sample, conductive assistant, and a shape of binder resin while an electrochemical treatment being performed under desired conditions.SOLUTION: An evaluation method of an electrode material includes: injecting electrolyte liquid into a sample consisting of an anode and performing an electrochemical treatment on the sample; discharging the electrolyte liquid; cleaning, and making a measurement with a scanning probe microscope. These steps are performed in the same equipment to prevent the sample from being exposed to the atmosphere. A constant voltage is applied between the sample and a conductive probe of the scanning probe microscope to measure a current value at any point, and a current image is created on a basis of height information and the current value measured within a certain area where the probe is moved.

Description

  The present invention relates to an electrode material evaluation method and an evaluation apparatus, and more particularly to an evaluation method and an evaluation apparatus for an electrode surface film formation state and a conductive auxiliary agent or binder dispersion state of a lithium ion secondary battery.

  In order to evaluate the formation state of the electrode surface film of a secondary battery such as a lithium ion secondary battery (LIB) and the dispersion state of a conductive additive or binder, a scanning probe microscope (SPM) is used. Scanning Probe Microscope) is used.

  For example, in Patent Document 1, while applying a constant voltage between a probe of a scanning probe microscope and a positive electrode core material, the surface of the positive electrode mixture layer is scanned with the probe, It has been proposed to create a current value map by measuring current values flowing between the two. In Patent Document 1, a conductive path area ratio is obtained from the current value map thus created, and it is evaluated whether or not a suitable conductive path is formed on the positive electrode plate of the secondary battery.

  Patent Document 2 relates to a method for evaluating a dispersion state of a specific organic substance in a constituent material of a secondary battery. It is described that after the negative electrode plate of a lithium ion secondary battery is cut out and dyed with bromine, the dispersion state of the binder in the negative electrode plate is evaluated by an electron beam microanalyzer.

JP2012-243415A JP 2003-279508 A

  In measuring the SEI current image on the surface of the battery electrode using In-situ SPM, a probe for an electrochemical microscope coated with an insulating coating except for the tip of the conductive probe is used. At this time, it is necessary to suppress the leakage current below the current between the probe and the sample in the electrolyte including the periphery of the probe holder. The current between the sample and the probe cannot be measured with high sensitivity.

  The measurement method described above has the following problems.

  That is, the first problem is that, while performing electrochemical treatment under desired conditions, the SEI of the sample, the conductive auxiliary agent, the shape of the binder resin, as well as the mechanical characteristics such as current image, elastic image, adsorption force, etc. The distribution of characteristics cannot be visualized or visualized at the same time. Here, SEI (Solid-Electrolyte Interphase) is a coating derived from a non-aqueous electrolyte formed on the surface of the negative electrode.

  This is because it is difficult to take out only the current between the sample and the probe while insulating all the wetted parts around the probe mounting part in the current image measurement in the electrolytic solution.

  The second problem is that an electrode active material coating film made of particles of several tens of microns such as a lithium ion secondary electrode has large irregularities, and it is difficult to obtain a current distribution image by current measurement. In Patent Document 1, a so-called contact mode is adopted as a measurement mode of the scanning probe microscope, but it is difficult to obtain a current image in the conductive atomic force microscope measurement (conductive AFM measurement) in the contact mode.

  Therefore, the object of the present invention is to perform the electrochemical treatment under the desired conditions, the SEI of the sample, the conductive auxiliary agent, the shape of the binder resin, as well as the mechanical characteristics and electrical characteristics such as current image, elastic image, and adsorption force. An object of the present invention is to provide an evaluation method and an evaluation apparatus for an electrode material which can solve the problem that the distribution cannot be simultaneously imaged and visualized and it is difficult to obtain a current distribution image of an electrode active coating film.

In order to achieve the above-described object, the electrode material evaluation method according to the present invention is such that an electrolytic solution is injected into a negative electrode sample, the sample is subjected to electrochemical treatment, the electrolytic solution is discharged, and measured by a scanning probe microscope after washing. And perform the above steps without exposing to the atmosphere in the same equipment,
Height measured by applying a constant voltage between the sample and the probe of the scanning probe microscope having conductivity, measuring the current value at an arbitrary point, and moving the probe in a certain area A current image is created from the information and the current value.

An electrode material evaluation apparatus according to the present invention is a conductive plate on which a sample is mounted, a holder for sandwiching a sample to be evaluated between the plate, an opening, and an inner wall of the opening A holder in which an electrode serving as both a reference electrode and a counter electrode is formed along a first flow path for supplying an electrolyte and gas to the surface of the sandwiched sample, and an electrolyte and gas supplied to the surface of the sample. A second flow path for discharging, and an electrochemical cell having a fixture for fixing between the plate and the holder in a state where the sample is sandwiched between the plate and the holder;
A probe of a scanning probe microscope, which is arranged above the electrochemical cell and to which a constant voltage is applied between the sample and the sample sandwiched between the electrochemical cells and a current value at an arbitrary point is measured. Have.

  According to the present invention, without special treatment on the probe holder side, changes in the shape, mechanical properties, and electrical properties of the sample surface treated under desired electrochemical conditions and the distribution of the binder resin are measured with high spatial resolution and sensitivity. It is possible to obtain a current distribution image of the electrode active coating film.

It is a flowchart for demonstrating the evaluation method of the electrode material by one Embodiment of this invention. It is sectional drawing for demonstrating the evaluation apparatus of the electrode material by one Embodiment of this invention. It is a top view of the electrochemical cell of FIG. It is an image which shows the example of a measurement before electrochemical processing, (a) shows a shape image, (b) shows an adsorption power image, (c) shows an electric current image, respectively. It is an image which shows the example of a measurement after potential scanning in electrolyte solution presence, (a) shows a shape image, (b) shows an adsorption power image, (c) shows an electric current image, respectively. It is an image which shows the example of a measurement after an electric potential scan in the presence of an additive in electrolyte solution, (a) shows a shape image, (b) shows an adsorption power image, (c) shows an electric current image, respectively.

  Preferred embodiments of the present invention will be described in detail with reference to the drawings.

  Next, an electrode material evaluation method and evaluation apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings. In this embodiment, as an electrode material evaluation method, an electrode material evaluation method constituting a negative electrode of a secondary battery will be described as an example.

  FIG. 1 is a flowchart for explaining a method for evaluating a secondary battery according to an embodiment of the present invention. FIG. 2 is a cross-sectional view illustrating a secondary battery evaluation apparatus according to an embodiment of the present invention. FIG. 3 is a plan view of the electrochemical cell of the evaluation apparatus of FIG.

In the secondary battery evaluation method according to the present embodiment, the steps shown in FIG. 1 are sequentially performed in a glove box under O ₂ and H ₂ O free conditions. First, after disassembling a secondary battery such as a lithium ion secondary battery, the electrode constituting the negative electrode is taken out, cut into a size suitable for evaluation, for example, a square of 1 cm × 1 cm, and washed (step S1). . Next, a sample is fixed to the electrochemical cell shown in FIG. 2 or FIG. 3, and a liquid / gas flow path is connected (step S2). Next, the electrochemical cell on which the sample is fixed is set on the stage (SPM stage) of the scanning probe microscope, and SPM measurement in a blank state is performed (step S3). Next, an electrolytic solution is injected into the electrochemical cell (step S4). Next, the sample is connected to a potentio / galvanostat (P / G stat), and a desired electrochemical treatment is performed on the sample (step S5). Next, the electrolytic solution is discharged from the electrochemical cell (step S6). Next, it dries with an inert gas (step S7). Then, the SPM scanner is aligned and measurement is performed (step S8).

  For example, when using an SPM system equipped with Peak Force Mode such as Bruker Dimension Icon, the probe is moved up and down at each point on the sample surface, and height information at each point and the tunnel current spectrum, adsorption force, elasticity, etc. Record mechanical and electrical properties simultaneously. The probe can be repeatedly moved up and down and moved at high speed (several kHz to several tens of kHz) over the entire desired region, and the obtained physical property values can be imaged. Compared to the current image measurement in the contact mode of the background art, it is possible to measure a sample with large irregularities such as an electrode coating film of a lithium ion secondary battery, which is an aggregate of particles of several microns in diameter, with high spatial resolution and high sensitivity. it can.

  A specific example of an evaluation apparatus used for such an evaluation method will be described with reference to FIGS. As shown in FIG. 2, the electrochemical cell of this embodiment has a high chemical stability by sandwiching the sample between the plate 3 made of a highly conductive material and the plate 3 in order to keep the sample conductive. It has a holder 2 made of a resin material. The holder 2 has an opening, and a Li metal electrode (lithium metal electrode) serving as a reference electrode and a counter electrode 6 is formed along the inner wall of the opening. Further, the holder 2 is provided through the holder 2, and is provided through the holder 2 and the flow path 7 as an example of a first flow path for supplying an electrolyte or gas to the surface of the sandwiched sample. And has a flow path 8 as an example of a second flow path for discharging the electrolyte or gas supplied to the surface of the sample. Between the plate 3 and the holder 2, O-rings 5 a and 5 b disposed between the surface of the sandwiched sample and the holder 2, and tightening for fixing the plate 3 and the holder 2 by pressure while the sample is sandwiched It has a tool 9.

  The electrochemical cell of the present embodiment has a bipolar cell configuration with an electrode serving as a reference electrode and a counter electrode 6 and a sample. However, the present embodiment is not limited to this two-electrode cell configuration, and may have a three-electrode cell configuration including a reference electrode, a counter electrode, and a sample.

  The plate 3 is made of a plate-like highly conductive material, and is made of, for example, stainless steel, carbon, metal oxide, or conductive resin. The plate 3 becomes a working electrode.

  All of the O-rings 5 a and 5 b are disposed in the groove on the lower surface of the holder 2 facing the plate 3. The O-ring 5a holds the sample by pressing the outer peripheral portion of the sample in a state where the plate 3 and the holder 2 are crimped and fixed with the fastener 9. Furthermore, it is better to provide an O-ring 5b between the plate 3 and the holder 2 outside the O-ring 5a to prepare for leakage of the supplied electrolyte.

  The flow paths 7 and 8 for introducing and discharging the electrolytic solution, the cleaning liquid and the gas may be installed separately for the gas, the electrolytic solution and the cleaning solution. It can also be used. The sample introduction channel 7 and the discharge channel 8 may each be one as shown in FIG. 2, or may be plural as shown in the plan view of FIG. FIG. 3 shows a case where three discharge channels 8 are provided for one introduction channel 7. In FIG. 3, the flow path 7 and the flow path 8 are arranged so as to form an angle of 90 degrees toward the center of the electrochemical cell. FIG. 3 shows an example of the arrangement and the number of the flow paths 7 and the flow paths 8, and the arrangement and the number are not limited thereto.

  The replacement of the electrolytic solution can be performed with a pump, a syringe, or the like. In order to remove the electrolyte so that no electrolyte remains in the cell, the hole surface of the discharge port must face the lower surface of the holder, and the groove of the O-ring 5a is not blocked when the cell is crimped and fixed. Adjust the depth. In the present embodiment, Li metal that serves as both the reference electrode and the counter electrode 6 is arranged in the peripheral portion of the liquid reservoir, and one end is led out of the cell to be conductive. Fix with sealing tape or heat-shrinkable tube to prevent liquid leakage from the gap between the cell and Li metal. You may connect with a copper wire etc. in the middle. A rib is provided at the bottom of the liquid reservoir of the holder so that the Li metal does not contact the sample.

  In the measurement apparatus of the present embodiment, such an electrochemical cell and the SPM scanner 1 are housed in the glove box 10. A probe for a scanning probe microscope is formed at the tip of the SPM scanner 1. Then, after fixing the sample 4 to the electrochemical cell in the glove box 10 which is shut off to the atmosphere, an electrolytic solution / cleaning solvent (for example, DEC: Diethyl Carbonate) is stored and an Ar gas pipe is connected. The sample and the counter / reference electrode are connected to a potentiostat and set as an open circuit. An initial surface SPM measurement that becomes a blank is performed in an inert gas atmosphere (step S3 in FIG. 1). At this time, the bias between the sample and the probe is controlled from the control device of the scanning probe microscope.

  Thereafter, the SPM scanner 1 is pulled up and the electrolyte is slowly injected into the cell reservoir to check the open circuit voltage (OCV) of the sample 4. At this time, the potential / current of the sample with respect to the reference electrode is controlled by the potentiostat.

  Therefore, the measurement of the present embodiment is a bi-potentiometer that controls the potential of the sample and the probe with respect to the reference electrode in the case of a scanning probe microscope apparatus with electrochemical scanning probe microscope software and a potentiostat. You don't need a stat. After performing an arbitrary electrochemical treatment (step S5 in FIG. 1), the electrolytic solution is immediately discharged from the liquid reservoir (step S6 in FIG. 1), and a cleaning solvent is introduced and discharged to remove excess electrolyte. Thereafter, Ar gas is flown, the sample surface is dried (step S7 in FIG. 1), and SPM measurement is performed (step S8 in FIG. 1).

  According to the present embodiment, an electrochemical cell that enables SPM measurement in an inert gas from electrochemical measurement in series in the glove box 10 is used. This makes it possible to measure changes in the shape, mechanical and electrical properties of the sample surface treated with the desired electrochemical conditions without special treatment on the probe holder side, and the distribution of the binder resin with high spatial resolution and sensitivity. .

  Furthermore, in the electrochemical cell of this embodiment, the sample drift during measurement can be minimized by fixing the sample 4 from above and below by the action of the fastener 9 and the O-ring 5a. In addition, the measurement waiting time, the damage caused by the probe contacting the sample, and the improvement of the measurement data quality (spatial resolution and high sensitivity) can be expected. Moreover, since it is a simple system, handling in the glove box 10 is easy.

In order to explain the effects of the present invention more specifically, examples will be described.
Example 1
Using an SPM system with Peak Force Mode such as Bruker's Dimension Icon, the probe is moved up and down at each point on the sample surface, height information at each point, mechanical properties such as tunnel current spectrum, adsorption force, elasticity, etc. Record electrical characteristics at the same time. The probe is moved up and down and moved at high speed (several kHz to several tens of kHz) over the entire desired area, and the obtained physical property values are imaged. This makes it possible to measure samples with large irregularities, such as electrode coatings of lithium ion secondary batteries, which are aggregates of particles with a diameter of several microns, with high spatial resolution and high sensitivity compared to current image measurement using the contact mode of the background art. can do.

  FIG. 4 shows a measurement example before electrochemical treatment (FIG. 4A shows a shape image, FIG. 4B shows an adsorption force image, FIG. 4C shows a current image, and the sample is 15 × 15 μm. Region image is shown.) From the adsorption force image shown in FIG. 4B, it can be understood that binder particles having a diameter of about 300 nm partially exist on the surface of the carbon negative electrode (white portion in the figure, adsorption force: large). From the adsorption force image of FIG. 4B, it can be understood that the adsorption force is large at a portion where the binder particles are partially present. Further, the current image in FIG. 4C is white in that portion. That is, it became clear that the current was small and the resistance was high in that portion.

Since the appearance of the binder particles is not obtained in the shape image and the elastic image, it is considered that the binder has been thinned to such an extent that the height / elasticity information cannot be distinguished during drying. In addition, since most micelle particles exist on the surface in a single particle layer without overlapping on the surface, the binder particles self-assemble on the carbon surface at the time of the paste before drying, and are regularly adsorbed in the single layer. It is thought that the structure was kept dry. On the other hand, the fine particles on the surface, which are slightly present, are presumed to be a lump of conductive additive because the current value is very high and the adsorption power and elasticity are the same as carbon. The ratio of the region having a current value of 0.2 nA or less was 51.7%. Similarly, the coverage of the binder particles can be obtained from the adsorption force image.
(Example 2)
FIG. 5 shows a measurement example after potential scanning from open circuit voltage (OCV) to 1.0 V (vs. Li) in the presence of 1.6 wt% additive in the electrolyte (FIG. 5 (a) is a shape image, FIG. 5B shows an adsorption force image, and FIG. 5C shows a current image, showing an image of a 15 × 15 μm region of the sample. Compared with the measurement example before the electrochemical treatment shown in FIG. 4, no significant change was observed in the shape image and the adsorption force image, but the current image was entirely high in resistance (white). The ratio of the region having a current value of 0.2 nA or less was 91.0%, and it became clear that the initial film was formed by the additive by the potential scan up to 1.0 V where the reduction of the electrolyte did not start. At this time, the contrast derived from the binder particles is obtained in the adsorption force image, indicating that it is possible to discriminate the binder distribution and the film formation at the time of initial SEI formation.
(Example 3)
FIG. 6 shows a measurement example after three cycles between an open circuit voltage (OCV) and 25 mV (vs. Li) in the presence of an additive of 1.6 wt% in the electrolytic solution (FIG. 6A shows a shape image, FIG. 6B shows an adsorption force image, and FIG. 6C shows a current image, showing an image of a 15 × 15 μm region of the sample. Compared with FIG. 4 and FIG. 5, the current image is a white image on the entire surface. Thereby, it can be understood that the entire surface has high resistance. The ratio of the region having a current value of 0.2 nA or less is 97.1%, that the formation of SEI has progressed, that the conductive auxiliary agent remains conductive, and that the initial SEI coating is formed on the binder particles. Therefore, it can be determined with high sensitivity that the strong attraction force is still exhibited.

  As mentioned above, although preferable embodiment and the Example of this invention were described, this invention is not limited to this. For example, you may install a temperature control apparatus in the back surface of the plate 3 of the lower part of an electrochemical cell. Thereby, it is possible to observe and evaluate the SEI formation state / stability / electric property distribution / change at high temperatures, the presence / absence of Li precipitation at extremely low temperatures, the SEI formation state, and the electrical property distribution. For this purpose, a metal plate (for example, stainless steel or the like) having good thermal conductivity is preferable.

  The present invention can be directly applied to electrochemical AFM measurement (shape, mechanical properties) in an electrolytic solution as well as current image measurement of an electrochemically treated electrode surface. The reference electrode and the counter electrode 6 may be installed inside the cell liquid reservoir or outside the cell through the flow path. At this time, the resistance increases under conditions where a relatively large current flows. The size of the liquid reservoir is a balance with the sample diameter: scanner diameter, but if you want to increase only the liquid volume, expand the liquid reservoir diameter and install a lid that matches the scanner diameter to prevent solvent evaporation. You can also.

  In addition, the present invention can measure not only the electrode of a lithium ion secondary battery but also the reaction on the surface of a model sample with a controlled structure such as graphene or highly oriented pyrolite graphite (HOPG).

  Furthermore, this invention can measure not only a lithium ion secondary battery but the electrode surface of a lithium ion capacitor. In addition, the plate at the bottom of the cell is made of carbon and a humidified gas (hydrogen, oxygen, air) is allowed to flow, so that the surface of the membrane / electrode assembly (MEA surface) of the temperature-controlled fuel cell has shape, mechanical and electrical characteristics. The distribution can also be measured.

  As examples of utilization of the present invention, in addition to evaluation of lithium ion batteries, screening of electrode materials for electric double layer capacitors and fuel cells, confirmation of quality of processes / compositions, and durability / degradation evaluation methods are conceivable.

DESCRIPTION OF SYMBOLS 1 SPM scanner 2 Holder 3 Plate 4 Sample 5a, 5b O-ring 6 Reference electrode and counter electrode 7, 8 Channel 9 Fastener 10 Glove box

Claims (10)

Injecting an electrolyte into a sample composed of a negative electrode, electrochemically processing the sample, discharging the electrolyte, measuring with a scanning probe microscope after washing, and performing the above steps without exposing to the atmosphere in the same apparatus,
A constant voltage is applied between the sample and the probe of the scanning probe microscope having conductivity, the current value at an arbitrary point is measured, and the height measured in a certain region by moving the probe An electrode material evaluation method for creating a current image from information and the current value.   The method for evaluating an electrode material according to claim 1, wherein the surface shape and the resistance due to the formation of a coating derived from the non-aqueous electrolyte formed on the negative electrode surface are simultaneously evaluated from the current image.   The evaluation of the electrode material according to claim 1 or 2, wherein the distribution of the binder resin, which is a constituent material of the negative electrode, is simultaneously evaluated from the current image and the simultaneously measured adsorption force image between the probe and the sample. Method.   The distribution of the conductive auxiliary agent, which is a constituent material of the negative electrode, is simultaneously evaluated from the current image, and an adsorption force image and an elastic image between the probe and the sample measured simultaneously. Evaluation method of electrode material. A conductive plate on which a sample is mounted, a holder for holding the sample to be evaluated between the plate, an electrode having an opening, and serving as a reference electrode and a counter electrode along the inner wall of the opening A first channel for supplying an electrolyte and gas to the surface of the sample to be sandwiched, a second channel for discharging the electrolyte and gas supplied to the surface of the sample, and the plate An electrochemical cell having a fixture for fixing between the plate and the holder in a state where the sample is sandwiched between the holder and the holder;
A probe of a scanning probe microscope, which is arranged above the electrochemical cell, a constant voltage is applied between the sample and the sample sandwiched between the electrochemical cells, and a current value at an arbitrary point is measured. The evaluation apparatus of the electrode material which has.   The electrode material evaluation apparatus according to claim 5, further comprising a glove box in which the electrochemical cell and the probe of the scanning probe microscope are accommodated.   Applying a constant voltage between the sample and the probe of the scanning probe microscope, measuring a current value at an arbitrary point, moving the probe and measuring height information measured in a certain region The electrode material evaluation apparatus according to claim 5 or 6, wherein a current image is created from the current value.   The apparatus for evaluating an electrode material according to claim 7, wherein the surface shape and the resistance due to the formation of a coating derived from the non-aqueous electrolyte formed on the negative electrode surface are simultaneously evaluated from the current image.   The evaluation of the electrode material according to claim 7 or 8, wherein the distribution of the binder resin, which is a constituent material of the negative electrode, is simultaneously evaluated from the current image and the simultaneously measured adsorption force image between the probe and the sample. apparatus.   The distribution of the conductive auxiliary agent, which is a constituent material of the negative electrode, is simultaneously evaluated from the current image and the simultaneously measured adsorption force image and elasticity image between the probe and the sample. Evaluation equipment for electrode materials.