JP2018146519A - Sample preparation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sample preparation method enabling observation with SEM simply and accurately, without generating coagulation of particles.SOLUTION: A preparation method for a sample to be observed by a scanning electron microscope comprises: a filtration step S1 of filtrating a sample contained in solvent with a filter paper; a washing step S2 of washing the sample filtrated in the filtration step S1 with water; an organic washing step S3 of washing the sample washed in the washing step S2 with water-soluble organic solvent; and a dry step S4 of drying the sample washed in the organic washing step S3 for less than 5 minutes at 120°C or less, wherein in the filtration step S1, a film thickness of the sample accumulated on the filter paper is controlled so as to be equal to or more than 1 mm.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for preparing a sample to be observed with a scanning electron microscope.

  In recent years, with the widespread use of portable electronic devices such as mobile phones and notebook computers, development of non-aqueous electrolyte secondary batteries having high energy density, small size and light weight is strongly desired. As a secondary battery satisfying such requirements, there is a lithium ion secondary battery. A lithium ion secondary battery includes a negative electrode, a positive electrode, an electrolytic solution, and the like, and a material capable of detaching and inserting lithium is used as an active material of the negative electrode and the positive electrode.

Lithium ion secondary batteries that have been proposed as positive electrode materials so far include lithium nickel composite oxide (LiCoO ₂ ) that is relatively easy to synthesize and nickel that is cheaper than cobalt. In addition to complex oxide (LiNiO ₂ ), lithium manganese complex oxide using manganese (LiMn ₂ O ₄ ), lithium nickel manganese complex oxide (LiNi _0.5 Mn _0.5 O ₂ ), lithium nickel cobalt manganese Examples include complex oxides (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ).

  In general, any positive electrode material produces a transition metal composite hydroxide (hereinafter also referred to as a precursor) as a precursor by a neutralization crystallization method (hereinafter also referred to as crystallization). There is known a method of obtaining a lithium transition metal composite oxide by mixing and baking with a lithium compound. Therefore, in order to improve the performance of the positive electrode material, it is necessary to manufacture the precursor as a base so as to have an appropriate particle size and particle size distribution.

  This precursor is obtained by a crystallization method that can be efficiently produced with a homogeneous composition. Usually, when a precursor is produced industrially by a crystallization method, a continuous crystallization method is often used. This method is a method capable of easily producing a large amount of precursors having the same composition. However, this continuous crystallization method has a problem that the particle size distribution of the obtained product tends to be a relatively wide normal distribution, and particles having an appropriate particle size and particle size distribution cannot always be obtained.

  In order to solve the above problem, in Patent Document 1, the nucleation reaction and the particle growth reaction do not proceed at the same time in the same tank as in the conventional continuous crystallization method, but mainly the nucleation reaction ( Production to obtain nickel cobalt manganese composite hydroxide particles with small particle size and high particle size uniformity by clearly separating the time when the nucleation step) occurs and the time when the particle growth reaction (particle growth step) mainly occurs The method etc. are described.

  Furthermore, in Patent Document 2, the alkalinity as an active material is obtained by using composite hydroxide particles having a multilayer structure with different Mn / Ni ratios as a composite hydroxide that is a raw material of the lithium nickel manganese composite oxide. And a production method for obtaining lithium nickel manganese composite oxide particles having high particle size uniformity. In order to obtain the particle size uniformity of the composite hydroxide particles, it is optimal to use a batch-type seed crystal method, and a production method based on the batch-type seed crystal method is described.

  Here, since the nucleation step in the precursor and the particle growth step for growing the generated nuclei are difficult to control conditions, confirming the particle morphology during crystallization controls the physical properties of the lithium transition metal composite oxide. Especially important above. Among various evaluation methods used to confirm the particle morphology, the method of directly observing a sample with a scanning electron microscope (hereinafter also referred to as SEM) is effective for knowing the morphology of the particle surface and cross section. It is.

JP2011-116580A JP2012-256435A

  However, when the sample has a small particle size, such as a precursor in the middle of crystallization, the surface activity is high, and the sample tends to agglomerate when the sample is dried. ,very difficult. Therefore, there has been a demand for the development of a sample preparation method that can reliably capture the particle form of a small particle size by SEM.

  Therefore, the present invention is proposed in view of the above-described conventional situation, and does not cause aggregation of particles even in a small particle size and high activity sample in the middle of crystallization, and is simple and highly accurate. It is an object to provide a method for preparing a sample that can be observed with an SEM.

  One embodiment of the present invention is a method for preparing a sample to be observed with a scanning electron microscope, the filtration step of filtering a sample contained in a solvent with a filter paper, and the water washing of the sample filtered in the filtration step An organic washing step of organically washing the sample washed in the water washing step with a water-soluble organic solvent, and drying for drying the sample washed organically in the organic washing step in 120 ° C. or less and less than 5 minutes And the layer thickness of the sample to be volumed on the filter paper in the filtration step is controlled to be 1 mm or more.

  In this way, even a sample having a small particle size and high activity in the middle of crystallization does not cause aggregation of particles, and can be observed with an SEM simply and with high accuracy.

  At this time, in one embodiment of the present invention, the sample may be a transition metal composite hydroxide.

  In this way, the particles of the transition metal composite hydroxide can be agglomerated with each other both in the small particle size highly active sample and in the small particle size high activity sample in the middle of crystallization. Observation with an SEM is possible with ease and high accuracy without causing it.

  At this time, in one embodiment of the present invention, the particles of the sample may have a median diameter D50 of 4 μm or less.

  In this way, even a sample having a median diameter D50 of 4 μm or less can be observed with an SEM simply and with high precision without causing aggregation of particles.

  At this time, in one embodiment of the present invention, the water-soluble organic solvent may have a boiling point of 80 ° C. or lower.

  If it does in this way, removal of the organic solvent in a drying process will become a short time, and also aggregation of particles can be prevented.

  At this time, in one embodiment of the present invention, the addition amount of the water-soluble organic solvent may be 10 times or more with respect to the dry weight of the sample.

  If it does in this way, a water | moisture content can fully be removed and aggregation of particle | grains can be prevented.

  At this time, in one embodiment of the present invention, the sample may be dried under atmospheric pressure or reduced pressure in the drying step.

  If it does in this way, removal of the organic solvent in a drying process will become a short time, and also aggregation of particles can be prevented.

  According to the present invention, a small particle size and high activity sample in the middle of crystallization can be easily and highly accurately observed with an SEM without causing aggregation of particles.

FIG. 1 is a process diagram showing an outline of a sample preparation method according to an embodiment of the present invention. FIG. 2 is a diagram showing an image obtained by observing the sample obtained in Example 1 with an SEM. FIG. 3 is a diagram showing an image obtained by observing the sample obtained in Comparative Example 1 with an SEM. FIG. 4 is a diagram showing an image obtained by observing the sample obtained in Comparative Example 2 with an SEM.

  Hereinafter, preferred embodiments of the present invention will be described in detail. In addition, this embodiment described below does not unduly limit the content of the present invention described in the claims, and can be modified without departing from the gist of the present invention. In addition, all the configurations described in the present embodiment are not necessarily indispensable as means for solving the present invention.

Hereinafter, a sample preparation method according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a process diagram showing an outline of a sample preparation method according to an embodiment of the present invention. As shown in FIG. 1, the sample preparation method according to an embodiment of the present invention includes a filtration step S1, a water washing step S2, an organic washing step S3, and a drying step S4. By preparing an observation sample through the above-mentioned steps, it is possible to observe with a simple and high-precision SEM without causing aggregation of particles even with a small particle size and high activity sample during crystallization. It becomes. A sample preparation method according to an embodiment of the present invention will be described in detail in the following order.
1. Filtration step S1
2, water washing process S2
3. Organic cleaning process S3
4. Drying step S4

<1, Filtration step S1>
First, the filtration step S1 in the sample preparation method according to an embodiment of the present invention will be described. In the filtration step S1, the slurry-like neutralized crystallization liquid containing the sample is poured into the funnel portion of the suction filter and suction filtration is performed. At this time, the layer thickness of the sample deposited on the filter paper is processed to be 1 mm or more. The reason why the layer thickness of the sample is 1 mm or more is to prevent dry aggregation in the filtration process. More preferably, it is 2 mm or more.

  As a method for setting the layer thickness of the sample to 1 mm or more, for example, a mark such as a line is provided on the funnel, the sample is filtered while visually confirmed, and the layer thickness of the sample to be deposited is controlled. In addition, the layer thickness of the sample to be deposited is controlled by making it perpendicular to the funnel with a marked bar or the like, filtering while visually checking.

  At this time, the flow rate of the suction filter is preferably 20 to 40 L / min. If it is said range, the organic solvent mentioned later can be volatilized also by attraction | suction at the time of filtration, and the heat drying time in a drying process can be shortened. Here, when the flow rate of the suction filtration is more than 40 L / min, there is a possibility that dry agglomeration may occur due to suction by suction. On the other hand, when it is less than 20 L / min, there is a possibility of causing poor drying due to insufficient drying in the drying step described later. In addition, it takes time for the suction, and the preparation efficiency may decrease.

As the sample is not particularly limited, a higher voltage among the lithium secondary battery is obtained, the synthesis is relatively easy lithium cobalt composite oxide (LiCoO ₂₎ First, using an inexpensive nickel than cobalt Lithium nickel composite oxide (LiNiO ₂ ), lithium manganese composite oxide using manganese (LiMn ₂ O ₄ ), lithium nickel manganese composite oxide (LiNi _0.5 Mn _0.5 O ₂ ), lithium nickel Examples include cobalt manganese complex oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ).

  Furthermore, the transition metal composite hydroxide which is a precursor in the nucleation process and the nucleus growth process of the said substance obtained by crystallization is mentioned. In particular, since the precursor has a small particle size, it has a high surface activity and tends to cause aggregation. Therefore, according to the sample preparation method according to an embodiment of the present invention, observation with an SEM can be performed easily and with high accuracy without causing aggregation. In addition, since it is possible to observe with a SEM simply and with high accuracy, it is possible to manufacture prepared samples, particularly the above-mentioned precursors, with an appropriate particle size and particle size distribution, thereby improving the performance of the positive electrode material. Can be made. Furthermore, if observation with an SEM can be performed easily and with high accuracy at the above-mentioned precursor stage, particles can be prepared in the upstream process of a plurality of manufacturing processes in which the positive electrode active material is manufactured. Preparation becomes easy and the performance of the positive electrode material can be further improved.

  In addition, the sample particles may have a median diameter D50 (particulate particle size distribution of 50% in the volume-based cumulative particle size distribution is D50) of 4 μm or less, particularly for sample preparation according to an embodiment of the present invention. The effect of the method becomes remarkable. In other words, the sample preparation method according to an embodiment of the present invention makes it possible to easily and highly accurately observe particles having a median diameter D50 of 4 μm or less, in particular, particles that tend to aggregate with each other with high accuracy.

<2, water washing process S2>
Next, the water washing step S2 will be described. In the water washing step S2, pure water is added to the separated sample, and the adhering matter and impurities of the filtrate that has become a high salt concentration by neutralization are sufficiently washed. The standard for the amount of washing is preferably such that the pH becomes almost neutral (pH = 5 to 8) when pure water is added. Although not particularly limited, it is sufficient if the amount is 50 times or more the amount of the neutralized crystallization solution.

<3, organic cleaning step S3>
Next, the organic cleaning step S3 will be described. In the organic washing step S3, a water-soluble organic solvent is added to the sample and washed. Here, since the organic solvent is not allowed to dissolve the sample when cleaning the sample, the organic solvent is not usually used for the preparation of the SEM observation sample. However, in the sample preparation method according to one embodiment of the present invention, the target sample is a transition metal composite hydroxide and is used because there is no possibility of dissolving in an organic solvent. This removes water adhering to the sample, and the sample does not dissolve in the organic solvent, and since the affinity between the sample and the organic solvent is low, the sample can be dispersed in the organic solvent. This is because the particles can be prevented from aggregating by drying in S4.

  The kind of the organic solvent to be used may be water-soluble, and may be a single solvent or a mixed solvent. Moreover, a boiling point of 120 ° C. or lower is preferable. This is because if the boiling point is higher than 120 ° C., the drying time in the drying step S4, which will be described later, becomes longer, and there is a higher possibility that the sample will be altered or aggregated. More preferably, it is 80 degrees C or less. By setting the temperature to 80 ° C. or lower, the drying time is further shortened, and the possibility that the sample is altered or aggregated is further reduced.

  Specific examples of the water-soluble organic solvent include ethylene glycol and acetic anhydride. Examples of the organic solvent having a boiling point of 120 ° C. or lower include acetic acid, propanol, isopropyl alcohol, isobutyl alcohol, butanol, pyridine, dioxane and the like. Can be mentioned. Specific examples of the organic solvent having a boiling point of 80 ° C. or lower include methanol, ethanol, acetone, tetrahydrofuran, and ethyl methyl ketone. The compounds are not limited to the compounds listed in this specific example.

  The amount of the organic solvent added is not particularly limited, but in order to completely remove the water adhering to the sample and disperse the sample with a margin, the weight of the organic solvent should be at least 10 times the dry weight of the sample. It is preferable to add, and it is more preferable to add 20 times the weight or more of the dry weight of the sample. In addition, the upper limit of the addition amount is preferably 50 times or less the dry weight of the sample in consideration of cost and the like.

<4, drying step S4>
Next, the drying step S4 will be described. In the drying step S4, the organic solvent adhering to the sample is dried to quickly remove it. The drying temperature is preferably set to 120 ° C. or less in consideration of sample alteration and particle aggregation. Also, the drying time is preferably set to less than 5 minutes, and more preferably set to 3 minutes or less.

  Moreover, in drying process S4, it is preferable to dry under atmospheric pressure or pressure reduction. By doing so, the removal of the organic solvent in the drying step S4 becomes a short time, and further, aggregation of particles can be prevented, and an efficient sample can be prepared.

  The sample preparation method according to an embodiment of the present invention includes an organic washing step using a water-soluble organic solvent, and the organic solvent can remove adhering water adhering to the particles and volatilize the organic solvent in a short time. Therefore, it is much shorter than the drying time when no organic solvent is used. Therefore, the sample can be observed efficiently.

  Next, a sample preparation method according to an embodiment of the present invention will be described in detail with reference to examples. The present invention is not limited to these examples.

  In addition, the particle size distribution measurement of the sample used for the Example and the comparative example is performed, and the median diameter D50 is 3.7 μm by Microtrac 3300 (made by Nikkiso Co., Ltd.) which is a laser diffraction / scattering particle size distribution device. It was confirmed.

Example 1
The slurry-like neutralized crystallization liquid containing the sample was poured into a funnel of a suction filter, and filtered so that the layer thickness of the sample deposited on the filter paper was 2 mm. Next, after adding pure water and washing the sample with water, 20 times ethanol (100 g) was added to the dry weight (about 5 g) of the sample to completely remove moisture in the deposited layer. Thereafter, the sample was dried under atmospheric pressure at 120 ° C. for 3 minutes to obtain an SEM observation sample. And the image which observed the obtained sample with SEM was shown in FIG.

(Comparative Example 1)
The slurry-like neutralized crystallization liquid containing the sample was poured into a funnel of a suction filter, and filtration was performed so that the layer thickness of the sample deposited on the filter paper was 0.5 mm, which was thinner than that of the example. Next, after adding pure water and washing the sample with water, 20 times ethanol (40 g) was added to the dry weight (about 2 g) of the sample to completely remove moisture in the deposited layer. Thereafter, the sample was dried under atmospheric pressure at 120 ° C. for 5 minutes to obtain an SEM observation sample. And the image which observed the obtained sample with SEM was shown in FIG.

(Comparative Example 2)
The slurry-like neutralized crystallization liquid containing the sample was poured into a funnel of a suction filter, and filtration was performed so that the layer thickness of the sample deposited on the filter paper was 2 mm as in Example 1. Next, after adding pure water and washing the sample with water, 20 times ethanol (100 g) was added to the dry weight (about 5 g) of the sample to completely remove moisture in the deposited layer. Thereafter, the sample was dried under atmospheric pressure at 120 ° C. and for 10 minutes longer than the examples, and used as an SEM observation sample. And the image which observed the obtained sample with SEM was shown in FIG.

  In Example 1, detailed and clear information could be obtained about the shape as an aggregate of particles and the surface condition of each particle, as shown by the images at 1000 and 30000 times in FIG.

  On the other hand, in Comparative Example 1 in which the layer thickness is thinner than that in Example 1, as shown in the image with an observation magnification of 1000 times in FIG. 3, multilayering occurs due to particle aggregation and deposition, and the particle shape can be confirmed. There wasn't.

  Further, in Comparative Example 2 in which the drying time was longer than that of the example, as shown in FIG. 4, although the particles were not noticeably aggregated and multilayered, the aggregation itself was progressing, and the original particle The surface condition could not be captured reliably.

  As described above, according to the sample preparation method according to an embodiment of the present invention, even in a small particle size and high activity sample in the middle of crystallization, aggregation of particles does not occur, and the SEM is simple and highly accurate. Observing with is possible. Furthermore, since the above observation becomes possible, the performance of the positive electrode material can be further improved.

  Although the embodiments and examples of the present invention have been described in detail as described above, it will be understood by those skilled in the art that many modifications can be made without departing from the novel matters and effects of the present invention. It will be easy to understand. Therefore, all such modifications are included in the scope of the present invention.

  For example, a term described with a different term having a broader meaning or the same meaning at least once in the specification or the drawings can be replaced with the different term in any part of the specification or the drawings. The configuration and operation of the sample preparation method are not limited to those described in the embodiments and examples of the present invention, and various modifications can be made.

  S1 filtration step, S2 water washing step, S3 organic washing step, S4 drying step

Claims (6)

A method for preparing a sample to be observed by a scanning electron microscope,
A filtration step of filtering the sample contained in the solvent with a filter paper;
A water washing step of washing the sample filtered in the filtration step;
An organic washing step of washing the sample washed in the washing step with a water-soluble organic solvent;
A drying step of drying the sample that has been organically washed in the organic washing step in 120 ° C. or less and less than 5 minutes;
The sample preparation method, wherein a layer thickness of the sample to be volumed on the filter paper in the filtration step is controlled to 1 mm or more.   The sample preparation method according to claim 1, wherein the sample is a transition metal composite hydroxide.   The sample preparation method according to claim 1, wherein the particles of the sample have a median diameter D50 of 4 μm or less.   The sample preparation method according to claim 1, wherein the water-soluble organic solvent has a boiling point of 80 ° C. or less.   The sample preparation method according to any one of claims 1 to 4, wherein the addition amount of the water-soluble organic solvent is 10 times or more with respect to the dry weight of the sample.   The sample preparation method according to any one of claims 1 to 5, wherein the sample is dried under atmospheric pressure or reduced pressure in the drying step.