JP2013157126A - Lithium ion secondary battery and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery which uses a cathode active material including LiMnOand whose capacity is less likely to decrease, and a method for manufacturing the same.SOLUTION: A method for manufacturing a lithium ion secondary battery comprises: a step S2 of injecting a nonaqueous electrolyte 140 into a battery 100 accommodating a cathode active material 153 including LiMnOand an anode active material 154 in a battery case 110; and a step S3 (conditioning step) of forming a SEI film on a surface of the anode active material 154 by initially charging and discharging the battery 100 after the injection step. This method further includes a step S4 of adding an oxygen radical trap agent (MFEC) into the nonaqueous electrolyte 140 after the conditioning step.

Description

  The present invention relates to a lithium ion secondary battery and a method for manufacturing the same.

  Lithium ion secondary batteries are attracting attention as power sources for portable devices and as power sources for electric vehicles and hybrid vehicles. At present, as the lithium ion secondary battery, a battery having a positive electrode active material made of a composite oxide containing lithium and a transition metal, a negative electrode active material made of a carbon material, and a non-aqueous electrolyte is mainly used ( For example, see Patent Document 1).

JP 2005-190874 A

By the way, by using a positive electrode active material containing Li ₂ MnO ₃ as the positive electrode active material, the capacity (initial capacity) of the battery can be increased.
However, when a positive electrode active material containing Li ₂ MnO ₃ is used, there is a possibility that the battery capacity is greatly reduced by repeated charge and discharge.

Specifically, in the lithium ion secondary battery using the positive electrode active material containing Li ₂ MnO _3, during charge and discharge, oxygen desorption occurs from Li ₂ MnO _3. The desorbed oxygen is reduced on the negative electrode surface to become oxygen radicals. This oxygen radical decomposes the SEI film formed on the surface of the negative electrode active material.

  In the lithium ion secondary battery, when a part of the SEI film on the surface of the negative electrode active material is decomposed (disappears), the SEI film is formed by Li salt or the like in the non-aqueous electrolyte solution. In other words, the Li salt in the non-aqueous electrolyte is consumed to replenish the SEI film that has disappeared due to decomposition. As a result, the battery capacity is reduced.

The present invention has been made in view of the present situation, and in a lithium ion secondary battery using a positive electrode active material containing Li ₂ MnO ₃ , a lithium ion secondary battery in which battery capacity is unlikely to decrease, and its manufacture It aims to provide a method.

One embodiment of the present invention includes a liquid injection step of injecting a non-aqueous electrolyte into a battery containing a positive electrode active material and a negative electrode active material containing Li ₂ MnO ₃ , and after the liquid injection step, A conditioning step of forming an SEI film on the surface of the negative electrode active material by performing initial charge and discharge, and a method of manufacturing a lithium ion secondary battery, wherein after the conditioning step, the non-ion in the battery This is a method for producing a lithium ion secondary battery in which an oxygen radical trapping agent is added to a water electrolyte.

  In the manufacturing method described above, an oxygen radical trapping agent is added to the non-aqueous electrolyte contained in the battery after the conditioning process. That is, after forming a SEI (Solid Electrolyte Interface) film on the surface of the negative electrode active material, an oxygen radical trapping agent is added to the non-aqueous electrolyte.

For this reason, in the lithium ion secondary battery manufactured by the above-described manufacturing method, oxygen radicals (reduced on the negative electrode surface) desorbed from Li ₂ MnO ₃ during the charge and discharge are reduced by oxygen radical trapping agents. Appropriate trapping is possible. Thereby, decomposition | disassembly of the SEI film by oxygen radical can be suppressed, As a result, the fall of battery capacity can be suppressed. This is because the “reaction for forming the SEI coating derived from the Li salt in the non-aqueous electrolyte for replenishing the SEI coating that has disappeared due to decomposition” is suppressed.

The oxygen radical trapping agent is a substance having a property of trapping (capturing) oxygen radicals, and specifically, a substance that reacts with oxygen radicals to stabilize it.
As the oxygen radical trapping agent, an organic compound having any of the following electron-withdrawing groups in the molecular structure can be used.
Electron withdrawing group; —CF ₃ , —NR ₂ , —CO ₂ R, —OR, —X
Here, R is an alkyl group or an alkylene group, and X is F, Cl, Br, or I.

  Further, the oxygen radical trapping agent is not contained in the battery (in the non-aqueous electrolyte) when performing the conditioning process. Therefore, in the liquid injection process, a nonaqueous electrolytic solution that does not contain an oxygen radical trapping agent is injected into the battery.

  When the oxygen radical trap agent is contained in the battery (in the non-aqueous electrolyte) when performing the conditioning process (for example, when a non-aqueous electrolyte containing an oxygen radical trap agent is injected in the injection process) Causes the following problems.

  Specifically, in the conditioning process, an SEI film derived from an oxygen radical trapping agent is formed on the surface of the negative electrode active material. Since this SEI film is unstable compared to a SEI film derived from a non-aqueous electrolyte (solvent or Li salt), decomposition proceeds with use. As a result, since the SEI film disappeared by the decomposition is replenished, the Li salt in the non-aqueous electrolyte is consumed, thereby reducing the battery capacity.

In addition, since the oxygen radical trapping agent is consumed in the conditioning process, there is a possibility that oxygen-derived oxygen radicals desorbed from Li ₂ MnO ₃ at the time of charge / discharge of the battery cannot be sufficiently captured (trapped). . For this reason, decomposition of the SEI film by oxygen radicals also proceeds, and the battery capacity may be further reduced.

  By adding an oxygen radical trapping agent after the conditioning step, a SEI coating derived from a nonaqueous electrolyte (solvent or Li salt) can be formed in the conditioning step. Since the SEI coating derived from the non-aqueous electrolyte (solvent or Li salt) is more stable than the SEI coating derived from the oxygen radical trapping agent, it is possible to suppress a decrease in battery characteristics (battery capacity) over a long period of time. .

  Furthermore, in the above method for manufacturing a lithium ion secondary battery, the oxygen radical trapping agent may be a method for manufacturing a lithium ion secondary battery that is monofluoroethylene carbonate.

  In the manufacturing method described above, monofluoroethylene carbonate (hereinafter also referred to as MFEC) is used as the oxygen radical trapping agent. The MFEC can appropriately trap oxygen radicals in the lithium ion secondary battery. Therefore, by adding MFEC to the non-aqueous electrolyte after the conditioning process, a lithium ion secondary battery in which the battery capacity is hardly reduced can be obtained.

Another aspect of the present invention is a lithium ion secondary battery comprising a positive electrode active material containing Li ₂ MnO ₃ , a negative electrode active material, and a non-aqueous electrolyte, wherein any one of the lithium ion secondary batteries It is a lithium ion secondary battery manufactured by this manufacturing method.

The above-described lithium ion secondary battery is a battery manufactured by any one of the manufacturing methods described above. That is, it is a lithium ion secondary battery manufactured by adding an oxygen radical trapping agent to the non-aqueous electrolyte after the conditioning process. For this reason, in the above-described lithium ion secondary battery, oxygen radicals (reduced on the negative electrode surface) desorbed from Li ₂ MnO ₃ during charge / discharge are appropriately captured (trapped) by an oxygen radical trapping agent. )can do. Thereby, decomposition | disassembly of the SEI film by oxygen radical can be suppressed, As a result, the fall of battery capacity can be suppressed.

Further, another aspect of the present invention includes a positive electrode active material containing Li ₂ MnO ₃ , a negative electrode active material, a non-aqueous electrolyte, an SEI film formed on the surface of the negative electrode active material, and an oxygen radical trapping agent. The SEI film is a lithium ion secondary battery containing the component derived from the non-aqueous electrolyte without containing the component derived from the oxygen radical trapping agent. .

The above lithium ion secondary battery includes a positive electrode active material containing Li ₂ MnO ₃ , a negative electrode active material, a non-aqueous electrolyte, a SEI film formed on the surface of the negative electrode active material, and an oxygen radical trapping agent. Prepare. In such a lithium ion secondary battery, oxygen radicals (reduced on the negative electrode surface) desorbed from Li ₂ MnO ₃ during charge / discharge are appropriately captured (trapped) by an oxygen radical trapping agent. be able to. Thereby, decomposition | disassembly of the SEI film by oxygen radical can be suppressed, As a result, the fall of battery capacity can be suppressed.

  In addition, in the above-described lithium ion secondary battery, the SEI coating does not contain a component derived from an oxygen radical trapping agent but contains a component derived from a non-aqueous electrolyte (not including an oxygen radical trapping agent). That is, it has the SEI film derived from the non-aqueous electrolyte (solvent or Li salt) without having the SEI film derived from the oxygen radical trapping agent. Since the SEI coating derived from the non-aqueous electrolyte (solvent or Li salt) is more stable than the SEI coating derived from the oxygen radical trapping agent, the above-described lithium ion secondary battery has battery characteristics (battery capacity) over a long period of time. ) Can be suppressed.

The above-described lithium ion secondary battery can be manufactured, for example, by any of the manufacturing methods described above.
The component contained in the SEI film can be measured by, for example, known XPS (X-ray photoelectron spectroscopy) analysis.

  Furthermore, in the above lithium ion secondary battery, the oxygen radical trapping agent may be a lithium ion secondary battery that is monofluoroethylene carbonate.

  In the above-described lithium ion secondary battery, monofluoroethylene carbonate (MFEC) is used as an oxygen radical trapping agent. The MFEC can appropriately trap oxygen radicals in the lithium ion secondary battery. Therefore, the above-described lithium ion secondary battery is a lithium ion secondary battery whose battery capacity is unlikely to decrease.

Note that the MFEC-derived SEI coating component is fluorophosphoric acid (P _X F _Y O _Z ). Therefore, whether or not the MFEC-derived component is contained in the SEI coating (that is, whether or not the MFEC-derived SEI coating is present) is determined by whether or not fluorophosphate is detected by XPS analysis of the SEI coating. It can be judged by.

  Specifically, the binding energy of fluorophosphoric acid has a peak at 687.8 eV in the XPS spectrum. Therefore, in the XPS spectrum obtained by XPS analysis of the SEI film, when a peak appears at a position of 687.8 eV, it is understood that fluorophosphoric acid is contained as a component of the SEI film. In other words, in the XPS spectrum of the SEI film, when a peak does not appear at the position of 687.8 eV, it is understood that the component derived from MFEC (oxygen radical trapping agent) is not contained in the SEI film.

It is a top view of the lithium ion secondary battery concerning an Example. It is a figure explaining the manufacturing method of a lithium ion secondary battery. It is a figure which shows the structure of a positive electrode. It is a figure which shows the structure of a negative electrode. It is a flowchart which shows the flow of the manufacturing method of the lithium ion secondary battery concerning an Example. It is a figure which compares the battery capacity after cycle charging / discharging. It is an XPS spectrum of the SEI film concerning an example. It is an XPS spectrum of the SEI film concerning a comparative example.

Next, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a plan view of a lithium ion secondary battery 100 according to the present embodiment. As shown in FIG. 1, the lithium ion secondary battery 100 of the present embodiment includes a battery case 110 having a rectangular shape in plan view, a positive electrode terminal 120 extending from the inside of the battery case 110 to the outside, and the inside of the battery case 110. And a negative electrode terminal 130 extending to the outside.

  The battery case 110 includes an inner resin film 111 positioned on the innermost side of the battery case 110, a metal film 112 positioned adjacent to the outer side of the inner resin film 111, and an outer side positioned adjacent to the outer side of the metal film 112. It is formed of a laminate film 101 in which a resin film 113 is laminated (see FIG. 2). In the battery case 110, as shown in FIG. 2, the laminate film 101 in which the electrode body 150 is disposed in the accommodating portion 119 is folded back at a folding position 110g. As shown in FIG. Stop part 115 (peripheral part of battery case 110) is sealed by heat welding and formed into a rectangular shape in plan view.

  Further, as shown in FIG. 2, an electrode body 150 is accommodated inside the battery case 110. The electrode body 150 is an oblong cross section, and is a flat wound body obtained by winding a long sheet-like positive electrode 155, negative electrode 156, and separator 157 into a flat shape.

  As shown in FIG. 3, the positive electrode 155 has a strip shape extending in the longitudinal direction DA, and is disposed in a strip shape extending in the longitudinal direction DA on both surfaces of the positive electrode current collecting member 151 made of aluminum foil and the positive electrode current collecting member 151. And two positive electrode mixture layers 152. The positive electrode mixture layer 152 includes a positive electrode active material 153, a conductive material made of acetylene black, and PVDF (binder) in a ratio of 85: 10: 5 (weight ratio).

  A portion of the positive electrode 155 where the positive electrode mixture layer 152 is applied is referred to as a positive electrode mixture layer coating portion 155c. On the other hand, a portion made only of the positive electrode current collecting member 151 without having the positive electrode mixture layer 152 is referred to as a positive electrode mixture layer uncoated portion 155b. The positive electrode mixture layer uncoated portion 155 b extends in a band shape in the longitudinal direction DA of the positive electrode 155 along one long side of the positive electrode 155. The positive electrode mixture layer uncoated portion 155b is wound to form a spiral shape, and is positioned at one end (left end in FIG. 2) in the axial direction (left and right direction in FIG. 2) of the electrode body 150. The positive electrode terminal 120 is welded to the positive electrode mixture layer uncoated portion 155b.

In this example, a positive electrode active material containing Li ₂ MnO ₃ is used as the positive electrode active material 153. Specifically, a positive electrode active material in which Li ₂ MnO ₃ and LiNi _1/3 Co _1/3 Mn _1/3 O ₂ are mixed at a ratio of 50:50 (weight ratio) is used.

  Further, as shown in FIG. 4, the negative electrode 156 has a strip shape extending in the longitudinal direction DA. The negative electrode current collecting member 158 made of copper foil and strips extending in the longitudinal direction DA on both surfaces of the negative electrode current collecting member 158, respectively. It has two negative electrode mixture layers 159 arranged. The negative electrode mixture layer 159 includes the negative electrode active material 154 and PVDF (binder) in a ratio of 92.5: 7.5 (weight ratio).

  A portion of the negative electrode 156 where the negative electrode mixture layer 159 is applied is referred to as a negative electrode mixture layer coating portion 156c. On the other hand, a portion including only the negative electrode current collector 158 without having the negative electrode mixture layer 159 is referred to as a negative electrode mixture layer uncoated portion 156b. The negative electrode mixture layer uncoated portion 156 b extends in a strip shape in the longitudinal direction DA of the negative electrode 156 along one long side of the negative electrode 156. The negative electrode mixture layer uncoated portion 156b is wound to form a spiral shape, and is located at the other end portion in the axial direction of the electrode body 150 (the right end portion in FIG. 2). The negative electrode terminal 130 is welded to the negative electrode mixture layer uncoated portion 156b.

  In this example, graphite is used as the negative electrode active material 154. Although not shown, an SEI film is formed on the surface of the negative electrode active material 154.

  The separator 157 is a separator composed of three layers of PP (polypropylene) / PE (polyethylene) / PP (polypropylene). The separator 157 is interposed between the positive electrode 155 and the negative electrode 156 to separate them. The separator 157 is impregnated with the nonaqueous electrolytic solution 140.

In this embodiment, an EC (ethylene carbonate) -based electrolyte is used as the non-aqueous electrolyte 140. Specifically, in a non-aqueous solvent in which EC (ethylene carbonate), DMC (dimethyl carbonate), and EMC (ethyl methyl carbonate) are mixed at a ratio of 30:30:40 (volume ratio), six lithium salts are formed. A nonaqueous electrolytic solution in which lithium fluorophosphate (LiPF ₆ ) is dissolved is used. In addition, the molar concentration of LiPF ₆ in the nonaqueous electrolytic solution 140 is 1.0 mol / L.

Meanwhile, in the lithium ion secondary battery using the positive electrode active material containing Li ₂ MnO _3, during charge and discharge, oxygen desorption occurs from Li ₂ MnO _3. This oxygen is reduced on the negative electrode surface to become oxygen radicals. This oxygen radical decomposes the SEI film formed on the surface of the negative electrode active material.

  In the lithium ion secondary battery, when a part of the SEI film on the surface of the negative electrode active material is decomposed (disappears), the SEI film is formed by Li salt or the like in the non-aqueous electrolyte solution. In other words, the Li salt in the non-aqueous electrolyte is consumed to replenish the SEI film that has disappeared due to decomposition. As a result, the battery capacity is reduced.

  On the other hand, in the lithium ion secondary battery 100 of the present embodiment, an oxygen radical trapping agent is added to the nonaqueous electrolytic solution 140. Specifically, as will be described later, after the SEI film is formed on the surface of the negative electrode active material 154 by performing initial charge / discharge on the battery into which the nonaqueous electrolyte 140 has been injected (after the conditioning step), the nonaqueous electrolyte In 140, an oxygen radical trapping agent is added. In this embodiment, monofluoroethylene carbonate (MFEC) is used as the oxygen radical trapping agent. In this embodiment, MFEC is added to the non-aqueous electrolyte 140 by 1.9 wt%.

For this reason, in the lithium ion secondary battery 100 of the present embodiment, oxygen radicals (reduced on the negative electrode surface) desorbed from Li ₂ MnO ₃ at the time of charge / discharge are appropriately removed by an oxygen radical trapping agent. It can be trapped. Thereby, decomposition | disassembly of the SEI film by oxygen radical can be suppressed, As a result, the fall of battery capacity can be suppressed.

  Moreover, in the lithium ion secondary battery 100 of the present example, the SEI film does not contain a component derived from the oxygen radical trapping agent, but contains a component derived from the non-aqueous electrolyte 140 (not including the oxygen radical trapping agent). doing. That is, it has the SEI film derived from the nonaqueous electrolyte solution 140 (nonaqueous solvent or Li salt) without having the SEI film derived from the oxygen radical trapping agent. Since the SEI coating derived from the non-aqueous electrolyte 140 (non-aqueous solvent or Li salt) is more stable than the SEI coating derived from the oxygen radical trapping agent, the deterioration of battery characteristics (battery capacity) is suppressed over a long period of time. be able to.

  In the lithium ion secondary battery 100 of the present example, the SEI film contains a component derived from the non-aqueous electrolyte 140 (not including the oxygen radical trapping agent) without containing a component derived from the oxygen radical trapping agent. This will be described later based on the XPS analysis result.

Next, the manufacturing method of the lithium ion secondary battery 100 of a present Example is demonstrated.
FIG. 5 is a flowchart showing the flow of the manufacturing method of the lithium ion secondary battery according to this example.

As shown in FIG. 5, first, in step S1, the battery 100 is assembled. Specifically, as shown in FIG. 3, a positive electrode 155 is prepared in which a positive electrode mixture layer 152 is coated on the surface (both surfaces) of a strip-shaped positive electrode current collecting member 151. In this example, a positive electrode active material containing Li ₂ MnO ₃ is used as the positive electrode active material 153. Specifically, a positive electrode active material in which Li ₂ MnO ₃ and LiNi _1/3 Co _1/3 Mn _1/3 O ₂ are mixed at a ratio of 50:50 (weight ratio) is used.

Also, as shown in FIG. 4, a negative electrode 156 is prepared in which a negative electrode mixture layer 159 is coated on the surface (both sides) of a strip-shaped negative electrode current collector 158. In this example, graphite is used as the negative electrode active material 154.
Further, a separator 157 composed of three layers of PP (polypropylene) / PE (polyethylene) / PP (polypropylene) is prepared.

  Next, the negative electrode 156, the separator 157, and the positive electrode 155 are stacked and wound. Specifically, the positive electrode mixture layer uncoated portion 155b of the positive electrode 155 and the negative electrode mixture layer uncoated portion 156b of the negative electrode 156 are located on opposite sides in the width direction (left-right direction in FIGS. 2 to 4). Thus, the negative electrode 156, the separator 157, and the positive electrode 155 are wound into a flat shape to form the electrode body 150.

  Next, the positive electrode terminal 120 is connected to the positive electrode mixture layer uncoated portion 155 b of the electrode body 150. Specifically, for example, in a state where the positive electrode terminal 120 is crimped to the positive electrode mixture layer uncoated portion 155b, both are welded (for example, ultrasonic welding, spot welding), so that the positive electrode mixture layer is not applied. The positive terminal 120 is connected to the part 155b. Similarly, the negative electrode terminal 130 is connected to the negative electrode mixture layer uncoated portion 156 b of the electrode body 150.

  Separately, a laminate film 101 is prepared. Specifically, after laminating the inner resin film 111, the metal film 112, and the outer resin film 113, this is press-molded to obtain a laminate film 101 in which the housing portion 119 is recessed (see FIG. 2). Next, as illustrated in FIG. 2, the electrode body 150 in which the positive electrode terminal 120 and the negative electrode terminal 130 are welded is disposed in the accommodating portion 119 of the laminate film 101. Next, the laminate film 101 is folded at the folding position 110g, and the electrode body 150 is accommodated therein.

  Next, the welding sealing portion 115 is heated while being pressurized in the thickness direction, and the inner resin films 111 are thermally welded. Thereby, the electrode body 150 can be sealed inside while the positive electrode terminal 120 and the negative electrode terminal 130 are extended from the inside of the battery case 110 to the outside. The battery 100 is assembled as described above.

  Next, it progresses to step S2 (liquid injection process), and the nonaqueous electrolyte solution 140 is inject | poured in the battery 100 (inside the battery case 110) through the liquid inlet provided in the battery case 110. Next, after the liquid injection port is temporarily sealed, the process proceeds to step S3 (conditioning process), and the battery 100 is initially charged and discharged. Thereby, a SEI film (not shown) derived from the non-aqueous electrolyte 140 (non-aqueous solvent or Li salt) is formed on the surface of the negative electrode active material 154.

  In this embodiment, the initial charge / discharge of the battery 100 is performed in a temperature environment of 25 ° C. Specifically, first, charging is performed at a constant current value of C / 3 until the battery voltage value reaches 4.7V. Then, after resting for 10 minutes, discharging is performed at a constant current value of C / 3 until the battery voltage value reaches 3.0V. Then, rest for 10 minutes. The initial charge / discharge of the battery 100 is performed by performing this charge / discharge cycle three times.

  Note that 1C is a current value at which discharge is completed in 1 hour after a battery having a rated capacity value (nominal capacity value) is discharged at a constant current. For example, when the rated capacity (nominal capacity) of the lithium ion secondary battery is 3.0 Ah, 1C = 3.0A and C / 3 = 1.0A.

Next, it progresses to step S4, the temporary sealing of the injection hole (not shown) of the battery case 110 is open | released, and oxygen radical trap agent is inject | poured in the battery 100 (inside of the battery case 110) through an injection hole. . As a result, the oxygen radical trapping agent is added to the nonaqueous electrolytic solution 140. In this embodiment, monofluoroethylene carbonate (MFEC) is used as the oxygen radical trapping agent. In this embodiment, MFEC is added to the non-aqueous electrolyte 140 by 1.9 wt%.
Thereafter, the injection port is sealed to complete the battery 100.

Meanwhile, in the lithium ion secondary battery using the positive electrode active material containing Li ₂ MnO _3, during charge and discharge, oxygen desorption occurs from Li ₂ MnO _3. The desorbed oxygen is reduced on the negative electrode surface to become oxygen radicals. This oxygen radical decomposes the SEI film formed on the surface of the negative electrode active material.

  In the lithium ion secondary battery, when a part of the SEI film on the surface of the negative electrode active material is decomposed, the SEI film is formed by Li salt or the like in the nonaqueous electrolytic solution to compensate for this. In other words, the Li salt in the non-aqueous electrolyte is consumed to replenish the SEI film that has disappeared due to decomposition. As a result, the battery capacity is reduced.

  On the other hand, in the manufacturing method of the present embodiment, as described above, the oxygen radical trapping agent is added to the nonaqueous electrolytic solution 140 after the conditioning process of step S3. That is, after an SEI film is formed on the surface of the negative electrode active material 154, an oxygen radical trapping agent is added.

For this reason, in the lithium ion secondary battery 100 of the present embodiment, oxygen radicals (reduced on the negative electrode surface) desorbed from Li ₂ MnO ₃ at the time of charge / discharge are appropriately removed by an oxygen radical trapping agent. It can be trapped. Thereby, decomposition | disassembly of the SEI film by oxygen radical can be suppressed, As a result, the fall of battery capacity can be suppressed.

(Charge / discharge cycle test)
Next, a charge / discharge cycle test was performed on the lithium ion secondary battery 100 of the example under a temperature environment of 60 ° C. Specifically, first, charging is performed at a constant current value of 2C until the battery voltage value reaches 4.7V. Thereafter, after resting for 10 minutes, discharging is performed at a constant current value of 2C until the battery voltage value reaches 3.0V. Then, rest for 10 minutes. This charge / discharge cycle was performed 50 cycles.

  Further, as Comparative Example 1, a battery different from the lithium ion secondary battery 100 of the example only in that an oxygen radical trap agent (MFEC) was not added was produced. The battery of Comparative Example 1 was also subjected to a charge / discharge cycle test in the same manner as the battery 100 of the example.

Further, as Comparative Example 2, a battery having a different positive electrode active material and different in that no oxygen radical trapping agent (MFEC) was added as compared with the Example was produced. Specifically, in Comparative Example 2, a positive electrode active material having only LiNi _1/3 Co _1/3 Mn _1/3 O ₂ without using Li ₂ MnO ₃ is used as the positive electrode active material.

Further, as Comparative Example 3, a battery in which only the positive electrode active material was different from that of the example was produced. Specifically, in Comparative Example 3, as in Comparative Example 2, only LiNi _1/3 Co _1/3 Mn _1/3 O ₂ is used as the positive electrode active material.

  The batteries of Comparative Examples 2 and 3 were also subjected to a charge / discharge cycle test under a temperature environment of 60 ° C. Specifically, first, charging is performed at a constant current value of 2C until the battery voltage value reaches 4.1V. Thereafter, after resting for 10 minutes, discharging is performed at a constant current value of 2C until the battery voltage value reaches 2.5V. Then, rest for 10 minutes. This charge / discharge cycle was performed 50 cycles.

  In addition, the upper limit voltage value at the time of charge and the lower limit voltage value at the time of discharge are determined according to the kind of positive electrode active material. Since the batteries of Comparative Examples 2 and 3 and the battery 100 of the example have different positive electrode active materials, the upper limit voltage value and the lower limit voltage value in the charge / discharge cycle test are different.

About the battery of an Example and Comparative Examples 1-3, the battery capacity (henceforth a post-cycle capacity) after the above-mentioned cycle charge / discharge was compared. The result is shown in FIG.
In FIG. 6, for the battery 100 of the example and the battery of the comparative example 1, the ratio of the post-cycle capacity of the example is shown with the post-cycle capacity of the comparative example 1 as a reference (100%). In addition, the ratio of the post-cycle capacity of Comparative Example 3 is shown for the battery of Comparative Example 2 and the battery of Comparative Example 3 with the post-cycle capacity of Comparative Example 2 as a reference (100%).

  As shown in FIG. 6, the battery 100 of the example had a battery capacity after cycle charge / discharge of about 23% greater than that of the battery of Comparative Example 1. Thus, in the battery 100 of the example, it was possible to suppress a decrease in battery capacity as compared with the battery of Comparative Example 1.

The battery 100 of the example is common to the battery of Comparative Example 1 in that a positive electrode active material containing Li ₂ MnO ₃ is used, but after step S3 (conditioning process), an oxygen radical trap is used. The difference is that an agent (MFEC) is added.
Therefore, in the case of manufacturing a lithium ion secondary battery using a positive electrode active material containing Li ₂ MnO ₃ , an oxygen radical trap agent (MFEC) is added to the non-aqueous electrolyte after the conditioning step, thereby It can be said that it is possible to manufacture a battery whose capacity is hardly lowered.

  On the other hand, as shown in FIG. 6, the battery of Comparative Example 3 had the same battery capacity after cycle charge / discharge compared to the battery of Comparative Example 2. Comparative Example 3 differs from Comparative Example 2 in that an oxygen radical trapping agent (MFEC) is added after the conditioning process. However, the battery of Comparative Example 3 had a capacity after cycle equivalent to that of the battery of Comparative Example 2 to which no oxygen radical trap agent (MFEC) was added (in other words, the capacity was reduced to the same extent).

The reason is that in Comparative Examples 2 and 3, a positive electrode active material having only LiNi _1/3 Co _1/3 Mn _1/3 O ₂ without using Li ₂ MnO ₃ was used as the positive electrode active material. Because. Since it does not have Li ₂ MnO ₃ , oxygen desorption does not occur from the positive electrode active material during charge and discharge, and as a result, oxygen radicals are not generated and the SEI film is not decomposed by oxygen radicals. It is. For this reason, it is thought that the capacity | capacitance reduction by cycle charging / discharging will become comparable irrespective of the presence or absence of oxygen radical trap agent (MFEC).

From this result, it can be said that a lithium ion secondary battery that does not have Li ₂ MnO ₃ as the positive electrode active material cannot obtain the effect of suppressing the capacity decrease by adding the oxygen radical trapping agent (MFEC).

(XPS analysis of SEI coating)
Next, XPS (X-ray photoelectron spectroscopy) analysis was performed on the SEI film formed on the surface of the negative electrode active material 154 according to the battery 100 of the example.

  Further, as Comparative Example 4, a battery was manufactured by changing only the timing of adding the oxygen radical trapping agent (MFEC) as compared with the Example. Specifically, the battery of Comparative Example 4 is different from the Example in that it is manufactured by adding an oxygen radical trapping agent (MFEC) before the conditioning process. Specifically, in the liquid injection process, a nonaqueous electrolytic solution containing MFEC is injected into the battery. XPS (X-ray photoelectron spectroscopy) analysis was also performed on the SEI film formed on the surface of the negative electrode active material according to the battery of Comparative Example 4.

XPS analysis is performed under the following conditions.
Equipment: PHI-5700 from ULVAC-PHI
X-ray source: AlKα monochrome, 1 × 1.2mm ² , output = 350W
Neutralizing gun: Emission control value = 21.0, Electron energy value = 6.0%
Analysis software: ULVAC-PHI MultiPak
Measurement conditions (quantitative / state analysis): PE (path energy) = 47 eV, STEP (energy step) = 0.1 eV, Time / STEP = 50 ms
Fitting input conditions are as shown in Table 1 below.

By the way, the SEI film component derived from MFEC is fluorophosphoric acid (P _X F _Y O _Z ). Therefore, whether or not the MFEC-derived component is contained in the SEI coating (that is, whether or not the MFEC-derived SEI coating is present) is determined by whether XPS analysis of the SEI coating detects fluorofluoric acid. It can be judged by.

  Specifically, the binding energy of fluorophosphoric acid has a peak at 687.8 eV in the XPS spectrum (see Table 1). Therefore, in the XPS spectrum obtained by XPS analysis of the SEI film, when a peak appears at a position of 687.8 eV, it is understood that fluorophosphoric acid is contained as a component of the SEI film. In other words, in the XPS spectrum of the SEI film, when a peak does not appear at the position of 687.8 eV, it is understood that the component derived from MFEC (oxygen radical trapping agent) is not contained in the SEI film.

  Here, the XPS spectrum obtained about the SEI film concerning an Example is shown in FIG. Moreover, the XPS spectrum obtained about the SEI film concerning the comparative example 4 is shown in FIG.

  As shown in FIG. 7, in the XPS spectrum of the SEI film according to the example, no peak appears at the position of 687.8 eV. Therefore, it can be seen that the SEI coating according to the example does not contain a component derived from MFEC (oxygen radical trapping agent).

Moreover, as shown in FIG. 7, in the XPS spectrum of the SEI film concerning an Example, a peak appears in the position of 687.1 eV and 685 eV. The binding energy of LiPF ₆ has a peak at 687.1 eV in the XPS spectrum (see Table 1). The binding energy of LiF has a peak at 685 ± 0.2 eV in the XPS spectrum. LiPF ₆ and LiF are coating components derived from a non-aqueous electrolyte (Li salt). Therefore, it turns out that the component derived from a non-aqueous electrolyte (Li salt) is contained in the SEI film concerning an Example. Although not shown in FIG. 7, the SEI coating according to the example also contained a component derived from the solvent of the nonaqueous electrolytic solution.

  On the other hand, as shown in FIG. 8, in the XPS spectrum of the SEI film according to Comparative Example 4, a peak appears at a position of 687.8 eV. Therefore, it can be seen that the SEI coating according to Comparative Example 4 contains a component derived from MFEC (oxygen radical trapping agent).

Moreover, based on the XPS spectrum of FIG. 7, the peak area ratio (existence ratio) of fluorophosphoric acid (P _X F _Y O _Z ), LiPF ₆ and LiF was determined. The peak area ratio is calculated to be 100% with fluorophosphoric acid (P _X F _Y O _Z ), LiPF ₆ and LiF. The results are shown in Table 2.

  From the above results, in the lithium ion secondary battery 100 of the example, the SEI coating does not contain a component derived from MFEC (oxygen radical trapping agent) and is derived from a non-aqueous electrolyte (not including oxygen radical trapping agent). It was found to contain the following components. That is, it was found that the SEI film derived from the non-aqueous electrolyte (solvent or Li salt) was obtained without the SEI film derived from the oxygen radical trapping agent. Since the SEI coating derived from the non-aqueous electrolyte (solvent or Li salt) is more stable than the SEI coating derived from the oxygen radical trapping agent, it is possible to suppress a decrease in battery characteristics (battery capacity) over a long period of time. .

  On the other hand, in the lithium ion secondary battery of Comparative Example 4, it was found that the SEI film contained a lot of components derived from MFEC (oxygen radical trapping agent). The reason is that since the conditioning process is performed in the presence of MFEC (oxygen radical trapping agent), an MFEC-derived SEI film is formed in the conditioning process. Since the MFEC-derived SEI coating is unstable, the battery of Comparative Example 4 is likely to have lower battery characteristics (battery capacity) than the battery 100 of the example.

  In the above, the present invention has been described with reference to the embodiments. However, the present invention is not limited to the above embodiments, and it is needless to say that the present invention can be appropriately modified and applied without departing from the gist thereof.

100 Lithium ion secondary battery 110 Battery case 140 Non-aqueous electrolyte 150 Electrode body 153 Positive electrode active material 154 Negative electrode active material 155 Positive electrode 156 Negative electrode

Claims (5)

A liquid injection step of injecting a non-aqueous electrolyte into a battery containing a positive electrode active material and a negative electrode active material containing Li ₂ MnO ₃ ;
And a conditioning step of forming an SEI film on the surface of the negative electrode active material by performing initial charge and discharge of the battery after the liquid injection step, and a method for producing a lithium ion secondary battery,
A method for producing a lithium ion secondary battery, wherein an oxygen radical trapping agent is added to the non-aqueous electrolyte in the battery after the conditioning step. It is a manufacturing method of the lithium ion secondary battery according to claim 1,
The method for producing a lithium ion secondary battery, wherein the oxygen radical trapping agent is monofluoroethylene carbonate. A lithium ion secondary battery comprising a positive electrode active material containing Li ₂ MnO ₃ , a negative electrode active material, and a non-aqueous electrolyte,
The lithium ion secondary battery manufactured by the manufacturing method of the lithium ion secondary battery of Claim 1 or Claim 2. A lithium ion secondary battery comprising a positive electrode active material containing Li ₂ MnO ₃ , a negative electrode active material, a non-aqueous electrolyte, an SEI film formed on the surface of the negative electrode active material, and an oxygen radical trapping agent There,
The said SEI film is a lithium ion secondary battery containing the component derived from the said non-aqueous electrolyte, without containing the component derived from the said oxygen radical trap agent. The lithium ion secondary battery according to claim 4,
The oxygen radical trapping agent is a lithium ion secondary battery that is monofluoroethylene carbonate.

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