Reference electrode, method of using the reference electrode, potential measuring device using the reference electrode, and secondary battery

Abstract

To provide a reference electrode occupying a small space in a reaction system and capable of measuring four electrodes of a working electrode and a counter electrode.SOLUTION: A reference electrode of the present invention includes a reference negative electrode having a negative electrode active material on a surface of a negative electrode current collector, a reference positive electrode having a positive electrode active material on a surface of a positive electrode current collector, and an insulating layer sandwiched between a back surface of the negative electrode current collector and the positive electrode current collector. Since each of the negative electrode active material and the positive electrode active material is an active material having a charge / discharge voltage characteristic with a flat portion, it is possible to provide a reference electrode that can perform measurement with four electrodes including a working electrode and a counter electrode, can determine the presence or absence of deviation of a potential with the reference electrode, and occupies a small space in a reaction system.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a reference electrode, a method for using the reference electrode, a potential measuring device using the reference electrode, and a secondary battery. [Background technology]

[0002] A reference electrode is essential for measuring the half-cell response of one electrode. As a reference electrode used for measuring the potential of a lithium-ion secondary battery, an electrode with a two-phase coexistence reaction system is often used, in which two phases of different compositions coexist and charge / discharge proceeds while changing the ratio of the two phases, and the potential is stable over a wide range of charge rates (for example, Non-Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Non-Patent Document 1] A lithium iron phosphate reference electrode for ionic liquid electrolytes Electrochemistry Communications,Volume 93, August 2018, Pages 148-151 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the three-electrode measurement method, which uses one reference electrode, makes it difficult to determine whether a potential deviation has occurred at the reference electrode. In addition, the four-electrode measurement method, which uses two reference electrodes, can measure the potential more accurately, but the reference electrodes occupy a large space in the reaction system.

[0005] The present invention has been made in view of the problems associated with the prior art, and an object of the present invention is to provide a reference electrode that occupies a small space in a reaction system and is capable of measuring four electrodes, including a working electrode and a counter electrode. [Means for solving the problem]

[0006] As a result of extensive research into achieving the above object, the present inventors have found that the above object can be achieved by integrating a reference electrode having a negative electrode active material and a reference electrode having a positive electrode active material via an insulating layer, and have thus completed the present invention.

[0007] That is, the reference electrode of the present invention comprises a reference negative electrode having a negative electrode active material on the surface of a negative electrode current collector; The battery includes a reference positive electrode having a positive electrode active material on the surface of a positive electrode current collector, a back surface of the negative electrode current collector, and an insulating layer sandwiched between the positive electrode current collector and the back surface. The negative electrode active material and the positive electrode active material are each characterized in that the charge / discharge voltage characteristics have a flat portion.

[0008] Furthermore, a method of using the reference electrode of the present invention is characterized in that the reference electrode is disposed between the working electrode and the counter electrode, with the negative electrode active material of the reference negative electrode facing the working electrode and the positive electrode active material of the reference positive electrode facing the counter electrode.

[0009] Furthermore, the potential measuring device of the present invention includes the above-mentioned reference electrode between the working electrode and the counter electrode. The negative electrode active material of the reference negative electrode faces the working electrode, and the positive electrode active material of the reference positive electrode faces the counter electrode.

[0010] In addition, the secondary battery of the present invention includes the above-mentioned reference electrode between the positive electrode and the negative electrode. The negative electrode active material of the reference negative electrode faces the positive electrode, and the positive electrode active material of the reference positive electrode faces the negative electrode. [Effects of the Invention]

[0011] According to the present invention, a reference electrode having a negative electrode active material and a reference electrode having a positive electrode active material are integrated via an insulating layer, thereby enabling measurement using four electrodes including the working electrode and counter electrode, making it possible to determine the presence or absence of a potential deviation using the reference electrode, and providing a reference electrode that occupies a small space in the reaction system. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram showing an example of the structure of a reference electrode of the present invention. [Figure 2] 1 is a graph showing an example of charge-discharge characteristics of an active material whose charge-discharge voltage characteristics have a flat portion. [Figure 3] FIG. 1 is a diagram showing a state in which a reference electrode is placed in a potential measuring device or a lithium ion secondary battery. [Figure 4] 2A and 2B are a front view and a side view showing the size and positional relationship between a reference electrode and an electrode of a lithium ion secondary battery. DETAILED DESCRIPTION OF THE INVENTION

[0013] The reference electrode of the present invention will now be described in detail. As shown in FIG. 1, the reference electrode of the present invention comprises a reference negative electrode having a negative electrode active material on the surface of a negative electrode current collector, a reference positive electrode having a positive electrode active material on the surface of a positive electrode current collector, and an insulating layer, and the insulating layer is sandwiched between the back surface of the negative electrode current collector and the back surface of the positive electrode current collector, thereby insulating the reference negative electrode from the reference positive electrode.

[0014] As described above, the reference electrode of the present invention is an integrated reference negative electrode and reference positive electrode, and therefore, measurement using four electrodes including the working electrode and counter electrode is possible.

[0015] As shown in FIG. 1, the negative electrode active material and the positive electrode active material are disposed at opposing positions across the insulating layer and two current collectors, and are not misaligned with each other. This reduces the space occupied by the negative electrode active material in the reaction system, suppressing the influence on the battery reaction to be measured, and enabling accurate measurement.

[0016] Furthermore, the negative electrode active material of the reference negative electrode and the positive electrode active material of the reference positive electrode each have a flat portion in the charge / discharge voltage characteristics.

[0017] In the present invention, "the charge / discharge voltage characteristics have a flat portion" means that, as shown in Figure 2, the change in voltage (ΔV) when the charge rate changes between 20% and 80% (both during charging and discharging) is less than 50mV.

[0018] Furthermore, a negative electrode active material or a positive electrode active material having charge / discharge voltage characteristics in which the above ΔV is less than 10 mV can be preferably used because the potential of the reference electrode becomes more stable.

[0019] As an active material having a flat portion of the charge-discharge voltage characteristic that can be used for the reference electrode of a lithium ion secondary battery, for example, Li4Ti5O 12 (LTO) and Na3LiTi5O 12 The negative electrode should preferably have a flat voltage and a potential of 1 V or more relative to Li.

[0020] Furthermore, examples of the positive electrode active material include compounds represented by the following formula (1). LiMPO4...Equation (1) In the formula (1), M represents at least one selected from the group consisting of manganese (Mn), iron (Fe), and cobalt (Co).

[0021] The negative electrode active material of the reference negative electrode and the positive electrode active material of the reference positive electrode have a flat portion in the charge / discharge voltage characteristics and have a stable potential over a wide range of charge rates, making it easy to adjust the potentials of the reference negative electrode and reference positive electrode.

[0022] The reference electrode of the present invention can be used not only in potential measuring devices but also in lithium ion secondary batteries. By using the reference electrode of the present invention in a lithium ion secondary battery, the negative electrode potential of the lithium ion secondary battery can be monitored in real time during charging and discharging, thereby improving the safety and reliability of the lithium ion secondary battery.

[0023] The reference electrode of the present invention is preferably used in a potential measuring device or a lithium ion secondary battery by being disposed between the working electrode and counter electrode of the potential measuring device or between the positive electrode and negative electrode of the lithium ion secondary battery, as shown in FIG. 3, such that the negative electrode active material of the reference negative electrode faces the working electrode or positive electrode (hereinafter, sometimes simply referred to as the "positive electrode") and the positive electrode active material of the reference positive electrode faces the counter electrode or negative electrode (hereinafter, sometimes simply referred to as the "negative electrode").

[0024] By disposing the reference electrode in this manner, there is no need to externally adjust the potential of the reference electrode to be within the range of the flat portion in advance, and this can be done with the reference electrode disposed between the positive electrode and the negative electrode. Therefore, by adjusting the potential after assembling the lithium ion secondary battery, deterioration of the reference electrode can be prevented.

[0025] Specifically, by disposing the reference electrode as described above and charging between the reference negative electrode and the positive electrode, lithium can be inserted into the reference negative electrode, and by charging between the reference positive electrode and the negative electrode, lithium can be desorbed from the reference positive electrode, and the potential of the reference electrode can be adjusted to within the range of the above-mentioned flat portion.

[0026] In this case, it is preferable that the discharge capacity of the reference electrode, i.e., the amount of the negative electrode active material of the reference negative electrode and the positive electrode active material of the reference positive electrode, is larger than the discharge capacity of the portion of the positive electrode facing the negative electrode active material, and that the amount of the positive electrode active material of the reference positive electrode is smaller than the discharge capacity of the portion of the negative electrode facing the positive electrode active material.

[0027] The discharge capacity of the positive electrode to be compared with the discharge capacity of the reference negative electrode is not the discharge capacity of the entire working electrode, but the discharge capacity of a portion of the entire positive electrode having the same area as the portion of the reference negative electrode to which the negative electrode active material is applied, as indicated by the dotted line in Figure 3 , and the discharge capacity of the negative electrode to be compared with the discharge capacity of the reference positive electrode is similarly the discharge capacity of a portion of the entire negative electrode having the same area as the portion of the reference positive electrode to which the positive electrode active material is applied, as indicated by the dotted line in Figure 3 .

[0028] This makes it possible to prevent lithium from being deposited on the reference negative electrode or the negative electrode when adjusting the potential of the reference electrode.

[0029] As described above, the discharge capacity of a portion having the same area as the portion of the reference negative electrode or reference positive electrode to which the active material is applied is of concern, rather than the overall discharge capacity of the negative electrode or positive electrode. This is because lithium ions during charging and discharging move linearly from the reference negative electrode to the positive electrode and from the reference positive electrode to the negative electrode due to the influence of the electric field, and lithium ions from portions of the positive electrode or negative electrode that do not face the reference negative electrode or reference positive electrode do not cause precipitation.

[0030] It is preferable that the length of the reference electrode provided in the lithium ion secondary battery in the longitudinal direction is the same as the length of the positive electrode and the negative electrode of the lithium ion secondary battery in the longitudinal direction, and that the length of the reference electrode in the lateral direction is shorter than the length of the positive electrode and the negative electrode of the lithium ion secondary battery in the lateral direction.

[0031] In the present invention, the "length of the reference electrode" refers to the length of the portion where the active material is provided and which contributes to the reaction, and does not refer to the entire length including the portion of the current collector and the tab portion which do not contribute to the reaction.

[0032] By arranging such a reference electrode in a position where it overlaps only a portion of the widthwise direction of the positive electrode and the negative electrode, but over the entire lengthwise direction of the positive electrode and the negative electrode, as shown in Figure 4, it is possible to monitor the average lengthwise potential without significantly interfering with the battery reaction, even in a lithium-ion secondary battery in which the electrode shape has a large difference in length between the widthwise direction and the lengthwise direction, which makes it easy for temperature distribution to occur and makes it easy for the potential to vary partially, thereby improving safety and reliability. [Example]

[0033] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0034] (Preparation of reference electrode) The resin film was sandwiched between two current collecting foils to produce a current collector insulated between one surface and the other surface by the resin film.

[0035] A reference positive electrode material was prepared by mixing 80 parts by mass of lithium iron phosphate (LFP), 10 parts by mass of polyvinylidene fluoride (PVdF), and 10 parts by mass of conductive carbon. The reference positive electrode material was dispersed in N-methyl-2-pyrrolidone to form a slurry. The obtained slurry was applied to one side of the current collector with a margin of 1 to 2 cm, and after drying, the thickness was 25 μm (2.3 g / cm 3 ) and press it down to the reference positive electrode (specific capacity 150mAh / g, 0.67mAh / cm 2 ) was obtained.

[0036] Next, Li4Ti5O 12 80 parts by mass of the material powder, 10 parts by mass of PVdF, and 10 parts by mass of conductive carbon were mixed together to prepare a reference negative electrode material. This reference negative electrode material was dispersed in N-methyl-2-pyrrolidone to form a slurry. The obtained slurry was applied to the other surface of the current collector in the same area as the reference positive electrode material, and after drying, a thickness of 25 μm (1.8 g / cm 3 ) and press it down to the reference negative electrode (specific capacity 160mAh / g, 0.52mAh / cm 2 ) was formed to obtain a reference electrode.

[0037] (Fabrication of lithium-ion secondary batteries) The above-mentioned reference electrode was inserted between the positive electrode and the negative electrode of the lithium ion secondary battery in a direction such that the reference positive electrode faced the negative electrode and the reference negative electrode faced the positive electrode, a tab was welded to the margin of the current collector, an electrolyte was injected, and the cell was sealed.

[0038] (Charging rate adjustment) Connect the negative tab of the reference electrode to the positive tab, and measure 0.52 mAh / cm 2 The battery was charged for 30 minutes at a current density of 0.67 mAh / cm. The reference negative electrode tab of the reference electrode was connected to the negative electrode tab. 2 The battery was charged at the current density for 30 minutes.

[0039] A lithium ion secondary battery using the above reference electrode was charged and discharged, and the potential of the negative electrode was measured during charging and discharging. It was confirmed that the potential of the negative electrode of the above lithium ion secondary battery can be monitored in real time during charging and discharging. [Explanation of symbols]

[0040] 1 Reference negative pole 11 Negative electrode active material 12 Negative electrode current collector 2 Reference positive electrode 21 Cathode active material 22 Positive electrode current collector 3. Insulation layer 4 Working electrode (positive electrode) 5 Counter electrode (negative electrode)

Claims

1. a reference negative electrode having a negative electrode active material on the surface of a negative electrode current collector; a reference positive electrode having a positive electrode active material on the surface of a positive electrode current collector; a back surface of the negative electrode current collector and an insulating layer sandwiched between the positive electrode current collector and the back surface, The reference electrode is characterized in that the negative electrode active material and the positive electrode active material are active materials whose charge / discharge voltage characteristics have a flat portion.

2. The negative electrode active material is Li ４ Ti ５ O １２ and 2. The reference electrode according to claim 1, wherein the positive electrode active material is a compound represented by the following formula (1): LiMPO ４ ... Formula (1) In the formula (1), M represents at least one element selected from the group consisting of manganese (Mn), iron (Fe), and cobalt (Co).

3. 2. The reference electrode according to claim 1, wherein the negative electrode active material and the positive electrode active material are disposed at positions facing each other with the insulating layer, the negative electrode current collector, and the positive electrode current collector sandwiched therebetween.

4. A method for using the reference electrode according to any one of claims 1 to 3, comprising: The reference electrode is Between the working electrode and the counter electrode, A method for using a reference electrode, comprising: arranging the negative electrode active material of the reference negative electrode so as to face the working electrode; and arranging the positive electrode active material of the reference positive electrode so as to face the counter electrode.

5. 5. The method for using a reference electrode according to claim 4, wherein charging is performed between the working electrode and the reference negative electrode, and charging is performed between the counter electrode and the reference positive electrode, thereby adjusting the potential of the reference electrode.

6. The discharge capacity of the reference negative electrode is set to be larger than the discharge capacity of the portion of the working electrode facing the negative electrode active material of the reference negative electrode, and, 6. The method for using a reference electrode according to claim 5, wherein the discharge capacity of the reference positive electrode is set to be smaller than the discharge capacity of the portion of the counter electrode facing the positive electrode active material of the reference positive electrode.

7. A potential measuring device including a reference electrode between a working electrode and a counter electrode, The reference electrode is the reference electrode according to any one of claims 1 to 3, a negative electrode active material of the reference negative electrode facing the working electrode, and a positive electrode active material of the reference positive electrode facing the counter electrode;

8. A secondary battery comprising a reference electrode between a positive electrode and a negative electrode, The reference electrode is the reference electrode according to any one of claims 1 to 3, A secondary battery, characterized in that the negative electrode active material of the reference negative electrode faces the positive electrode, and the positive electrode active material of the reference positive electrode faces the negative electrode.

9. the reference electrode has a lateral length shorter than the lateral lengths of the positive electrode and the negative electrode, and a longitudinal length equal to the longitudinal lengths of the positive electrode and the negative electrode; 9. The secondary battery according to claim 8, wherein the reference electrode is disposed in a position overlapping the positive electrode and the negative electrode over the entire longitudinal area thereof.

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