JP2018125216A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery which can suppress excessive heat generation during overcharge.SOLUTION: A lithium ion secondary battery includes a lithium transition metal complex oxide containing at least one transition metal element selected from the group consisting of nickel, cobalt, and manganese as a positive electrode active material. The positive electrode includes metal-containing particles that elute at a positive electrode potential lower than the positive electrode potential at which transition metal elution of the lithium transition metal complex oxide occurs.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a lithium ion secondary battery including a positive electrode active material made of a lithium transition metal composite oxide.

  Lithium ion secondary batteries that are lightweight and have a high energy density are preferably used as high-output power sources for mounting on vehicles or power sources for personal computers and portable terminals. In a typical configuration of this type of lithium ion secondary battery, a material (electrode active material) capable of reversibly occluding and releasing lithium ions is held in a conductive member (electrode current collector). With electrodes. For example, as one of the electrode active materials (positive electrode active materials) used for the positive electrode, lithium (Li) and at least one transition metal element of nickel (Ni), cobalt (Co), and manganese (Mn) are used. Oxides (lithium transition metal composite oxides) containing constituent metal elements are widely used. Patent document 1 is mentioned as a prior art regarding the positive electrode active material which consists of lithium nickel cobalt manganese complex oxide.

JP2015-133217A

  However, the positive electrode using the lithium transition metal composite oxide has a problem that the transition metal is eluted when exposed to a high potential during overcharge. When the transition metal is eluted, oxygen is released when the crystal structure of the lithium transition metal composite oxide collapses, and there is a possibility that a large exothermic reaction occurs due to the reaction between the released oxygen and the non-aqueous electrolyte. The present invention solves the above problems.

  The lithium ion secondary battery proposed here includes a positive electrode and a negative electrode having a lithium transition metal composite oxide containing at least one transition metal element selected from the group consisting of nickel, cobalt and manganese as a positive electrode active material. A separator and a non-aqueous electrolyte are provided. The positive electrode includes metal-containing particles that elute at a positive electrode potential lower than the positive electrode potential at which transition metal elution of the lithium transition metal composite oxide occurs. According to such a configuration, the elution of the metal-containing particles prior to the elution of the transition metal of the lithium transition metal composite oxide during overcharge suppresses the increase in battery potential, thereby reducing the lithium transition metal composite oxide. Structural collapse (and hence oxygen release) can be suppressed.

It is sectional drawing of the lithium ion secondary battery which concerns on one Embodiment. It is a figure for demonstrating the winding electrode body which concerns on one Embodiment. It is a figure for demonstrating the conventional positive electrode structure. It is a figure for demonstrating the positive electrode structure which concerns on one Embodiment. It is a graph which shows the relationship between content of a metal-containing particle, the elution amount of a metal-containing particle, and a capacity | capacitance maintenance factor. It is a graph which shows the change of the voltage at the time of overcharge. It is a graph which shows the change of the battery temperature at the time of overcharge.

  Hereinafter, a lithium ion secondary battery according to an embodiment of the present invention will be described. The embodiments described herein are, of course, not intended to limit the present invention in particular. In addition, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect actual dimensional relationships. Further, members / parts having the same action are denoted by the same reference numerals, and redundant description is omitted or simplified.

  In the following, an example of a lithium ion secondary battery in which a wound type electrode body (hereinafter referred to as “wound electrode body”) and a non-aqueous electrolyte are accommodated in a rectangular (here, rectangular box shape) case is shown as an example. To The battery structure is not limited to the illustrated example, and is not particularly limited to a prismatic battery.

  FIG. 1 shows a lithium ion secondary battery 100 according to an embodiment of the present invention. As shown in FIG. 1, the lithium ion secondary battery 100 includes a wound electrode body 20 and a battery case 30. FIG. 2 is a view showing the wound electrode body 20. As shown in FIGS. 1 and 2, a lithium ion secondary battery 100 according to an embodiment of the present invention includes a flat wound electrode body 20 having a flat rectangular shape together with a liquid electrolyte (electrolytic solution) (not shown). The battery case (that is, the exterior container) 30 is accommodated.

  The battery case 30 has a box-shaped (that is, bottomed rectangular parallelepiped) case body 32 having an opening at one end (corresponding to the upper end in a normal use state of the battery), and the opening attached to the opening. It is comprised from the sealing board (lid body) 34 which consists of a rectangular-shaped plate member which plugs up a part. The material of the battery case 30 is exemplified by aluminum, for example. As shown in FIG. 1, a positive terminal 42 and a negative terminal 44 for external connection are formed on the sealing plate 34. A thin safety valve 36 is formed between the terminals 42 and 44 of the sealing plate 34 so as to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. In the technique disclosed herein, as will be described later, the structure of the positive electrode 50 is maintained during overcharge and oxygen release is suppressed, so that the safety valve 36 can be appropriately operated.

  As shown in FIG. 2, the wound electrode body 20 includes a long sheet-like positive electrode (positive electrode sheet 50) and a long sheet-like negative electrode (negative electrode sheet 60) similar to the positive electrode sheet 50 in total. And a long sheet separator (separators 70 and 72). The positive electrode sheet 50, the negative electrode sheet 60, and the separators 70 and 72 are wound around the winding axis WL.

  The positive electrode sheet 50 includes a strip-shaped positive electrode current collector 52 and a positive electrode active material layer 54. For example, a strip-shaped aluminum foil is used for the positive electrode current collector 52. An uncoated portion 52 a is set along the edge on one side in the width direction of the positive electrode current collector 52. In the illustrated example, the positive electrode active material layer 54 is held on both surfaces of the positive electrode current collector 52 except for an uncoated portion 52 a set on the positive electrode current collector 52. The positive electrode active material layer 54 includes a positive electrode active material, metal-containing particles, a conductive material, and a binder.

As the positive electrode active material (typically in particulate form), a lithium transition metal composite oxide containing at least one transition metal element selected from the group consisting of nickel, cobalt, and manganese is used. For example, a lithium transition metal composite oxide having a layered structure or a spinel structure including lithium and the transition metal element as constituent metal elements can be used. Preferred examples include layered structure lithium nickel composite oxide such as LiNiO ₂ , layered structure lithium cobalt composite oxide such as LiCoO ₂ , and layered structure lithium nickel cobalt such as LiNi _1/3 Co _1/3 Mn _1/3 O _2. Examples include manganese composite oxides, spinel structure lithium manganese composite oxides such as LiMn ₂ O ₄ , and spinel structure lithium nickel manganese composite oxides such as LiNi _0.5 Mn _1.5 O ₄ .

The positive electrode active material layer 54 disclosed here contains metal-containing particles in addition to the positive electrode active material. The metal-containing particles can be eluted at a positive electrode potential lower than the positive electrode potential at which the transition metal (Ni, Co, Mn) is eluted in the lithium transition metal composite oxide. For example, the metal-containing particles are preferably a simple metal or a metal compound having an electrode potential of 4.8 V or less (typically 4.2 V to 4.8 V), preferably 4.6 V or less on the basis of metallic lithium. . Examples of such metal-containing particles include dichromic acid (H ₂ Cr ₂ O ₇ ) particles, gold (Au) particles, cerium (Ce) particles, and lead dioxide (PbO ₂ ) particles. The electrode potential of each metal species on the metal lithium basis is about 4.3V for dichromic acid, about 4.5V for gold, about 4.6V for cerium, and about 4.6V for lead dioxide.

  The metal-containing particles can suppress the release of oxygen from the lithium transition metal composite oxide by eluting before the lithium transition metal composite oxide during overcharge. That is, as shown in FIG. 3, when the positive electrode 50 including the above-described lithium transition metal composite oxide is exposed to a high potential during overcharge, the transition metal (Ni, Co, Mn) is eluted from the lithium transition metal composite oxide. Can be. When the transition metal is eluted, oxygen is released when the crystal structure of the lithium transition metal composite oxide collapses, and there is a possibility that a large exothermic reaction occurs due to the reaction between the released oxygen and the non-aqueous electrolyte. However, according to the technique disclosed herein, as shown in FIG. 4, by containing the metal-containing particles (Au particles in the illustrated example) in the positive electrode 50, the lithium transition metal composite oxide is overcharged. The metal-containing particles are eluted before the transition metal is eluted. As a result, the potential increase of the battery is suppressed, and the structure collapse of the positive electrode and thus the release of oxygen is suppressed. As a result, a highly reliable lithium ion secondary battery in which excessive heat generation due to the reaction between the released oxygen and the electrolytic solution is suppressed can be realized.

  The content of the metal-containing particles contained in the positive electrode active material layer 54 is approximately 0, with the total of the metal-containing particles and the positive electrode active material being 100% by mass from the viewpoint of suppressing excessive heat generation during overcharge. The content is suitably 1% by mass or more, preferably 0.5% by mass or more, more preferably 1% by mass, still more preferably 1.5% by mass or more, and particularly preferably 2% by mass or more. Although the upper limit of the metal-containing particles is not particularly limited, it is appropriate that the upper limit is approximately 10% by mass or less, preferably 5% by mass or less, more preferably 3% by mass or less, and further preferably 2.5% by mass or less. . In the case of a battery having too much metal-containing particles, the effect of suppressing battery heat generation during overcharging is slowed, and the amount of the positive electrode active material is relatively reduced as the content of the metal-containing particles is increased. Therefore, it is not preferable. The technique disclosed herein is preferably implemented in an embodiment in which the total content of the metal-containing particles and the positive electrode active material is 100% by mass, and the content of the metal-containing particles is 1.5% by mass or more and 3% by mass or less. obtain.

  In addition to the positive electrode active material and the metal-containing particles, the positive electrode active material layer 54 can contain additives such as a conductive material and a binder (binder) as necessary. As the conductive material, a conductive powder material such as carbon powder or carbon fiber is preferably used. As the carbon powder, various carbon blacks such as acetylene black are preferable. As the binder, polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), polytetrafluoroethylene (PTFE) or the like is preferably used. A positive electrode active material layer composition (paste) can be prepared by dispersing and kneading a positive electrode active material, metal-containing particles, a conductive material and a binder in a suitable dispersion medium. The positive electrode active material layer 54 is formed by applying the composition for positive electrode active material layer to the positive electrode current collector 52, drying it, and rolling (pressing) it to a predetermined thickness.

  The ratio of the positive electrode active material to the entire positive electrode active material layer is preferably more than about 50% by mass, and preferably about 70 to 97% by mass (for example, 75 to 95% by mass). Moreover, it is preferable that the ratio of the electrically conductive material to the whole positive electrode active material layer is about 2-20 mass% (for example, 3-10 mass%). Moreover, it is preferable that the ratio of the binder to the whole positive electrode active material layer is about 0.5-10 mass% (for example, 1-5 mass%).

  As shown in FIG. 2, the negative electrode sheet 60 includes a strip-shaped negative electrode current collector 62 and a negative electrode active material layer 64. For the negative electrode current collector 62, for example, a strip-shaped copper foil is used. On one side in the width direction of the negative electrode current collector 62, an uncoated portion 62a is set along the edge. The negative electrode active material layer 64 is held on both surfaces of the negative electrode current collector 62 except for the uncoated portion 62 a set on the negative electrode current collector 62. The negative electrode active material layer 64 includes a negative electrode active material, a thickener, a binder, and the like. As the binder, styrene butadiene rubber (SBR) is used. An example of the thickener is carboxymethyl cellulose (CMC).

  As the negative electrode active material contained in the negative electrode active material layer 64, one or more of materials conventionally used in lithium ion secondary batteries can be used without particular limitation. Examples of negative electrode active materials include carbon materials such as graphite (graphite), hard carbon (non-graphitizable carbon), and soft carbon (graphitizable carbon); silicon oxide, titanium oxide, vanadium oxide, lithium titanium composite oxide (Lithium Metal oxide materials such as Titanium Composite Oxide (LTO); metal nitride materials such as lithium nitride, lithium cobalt composite nitride, and lithium nickel composite nitride; and the like. Among these, a graphite-based carbon material can be preferably used.

  The negative electrode active material layer 64 may contain additives such as a binder (binder) and a thickener as necessary in addition to the negative electrode active material. As the binder and the thickener used for the negative electrode active material layer 64, the same binder as described for the positive electrode active material layer 54 can be used.

  The ratio of the negative electrode active material to the whole negative electrode active material layer is preferably more than about 50% by mass, and preferably about 80 to 99.5% by mass (for example, 90 to 99% by mass). Moreover, it is preferable that the ratio of the binder to the whole negative electrode active material layer is about 0.5-5 mass% (for example, 1-2 mass%). Moreover, it is preferable that the ratio of the thickener to the whole negative electrode active material layer is about 0.5-5 mass% (for example, 1-2 mass%).

  As illustrated in FIG. 2, the separators 70 and 72 are members that separate the positive electrode sheet 50 and the negative electrode sheet 60. In this example, the separators 70 and 72 are made of a strip-shaped sheet material having a predetermined width and having a plurality of minute holes. As the separators 70 and 72, for example, a single layer structure separator or a multilayer structure separator made of a porous polyolefin resin can be used. In this example, as shown in FIG. 2, the width b1 of the negative electrode active material layer 64 is wider than the width a1 of the positive electrode active material layer 54 (b1> a1). Further, the widths c1 and c2 of the separators 70 and 72 are wider than the width b1 of the negative electrode active material layer 64 (c1, c2> b1> a1). In the technique disclosed here, the temperature of the battery rises while the structure of the positive electrode 50 is maintained during overcharging, so that when the temperature reaches an appropriate temperature (for example, around 130 ° C.), the pores of the separators 70 and 72 are melted. A shut-down function that closes can be properly performed.

As the electrolytic solution (non-aqueous electrolytic solution), the same non-aqueous electrolytic solution conventionally used for lithium ion secondary batteries can be used without any particular limitation. Such a nonaqueous electrolytic solution typically has a composition in which a supporting salt is contained in a suitable nonaqueous solvent. Examples of the non-aqueous solvent include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and the like. Moreover, as said support salt, lithium salts, such as LiPF ₆ , can be used, for example.

  Several examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in the examples.

<Test Example 1>
In this example, a lithium ion secondary battery was constructed using gold particles as metal-containing particles, and an overcharge test was performed. A specific method will be described below.

<Construction of evaluation cell>
The positive electrode of the evaluation cell was produced as follows. First, lithium nickel cobalt manganese composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) powder as a positive electrode active material, gold powder as metal-containing particles, AB as a conductive material, and binder Of PVdF was mixed in NMP to prepare a positive electrode active material layer composition. This positive electrode active material layer composition was applied to a long sheet-like aluminum foil (positive electrode current collector) and dried to prepare a positive electrode in which a positive electrode active material layer was provided on the positive electrode current collector.

  The negative electrode of the evaluation cell was produced as follows. First, a negative electrode active material layer composition was prepared by dispersing graphite as a negative electrode active material, SBR as a binder, and CMC as a thickener in water. This negative electrode active material layer composition was applied to a long sheet-like copper foil (negative electrode current collector) to produce a negative electrode in which a negative electrode active material layer was provided on the negative electrode current collector.

  As a separator for the evaluation cell, a separator made of a base material of a microporous film (PP / PE / PP film) having a three-layer structure of polypropylene / polyethylene / polypropylene was prepared.

An evaluation cell was constructed using the positive electrode, negative electrode and separator prepared above. That is, the positive electrode and the negative electrode prepared above were laminated with a separator in between so that the active material layers of both electrodes were opposed to each other to produce an electrode body. Next, this electrode body was housed in a battery container together with a non-aqueous electrolyte and sealed. Thereafter, an evaluation cell was constructed by performing a predetermined conditioning process. As a non-aqueous electrolyte solution for the evaluation cell, a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) and LiPF ₆ as a supporting salt at a concentration of about 1 mol / liter. What was contained was used.

  In Examples 1 to 8, the content of the metal-containing particles is different. Specifically, the content of the metal-containing particles is 0% by mass in Example 1 and 0.5% by mass in Example 2 with respect to the total mass (100% by mass) of the metal-containing particles and the positive electrode active material. 3 is 1% by mass, Example 4 is 1.5% by mass, Example 5 is 2% by mass, Example 6 is 2.5% by mass, Example 7 is 3% by mass, and Example 8 is 3.5% by mass. Eight evaluation cells were produced.

<Measurement of initial capacity>
The initial capacity was measured by the following procedures 1 to 3 at a temperature of 25 ° C. and a voltage range of 3 V to 4.1 V for the evaluation cell after the conditioning process.
Procedure 1: Charge with a constant current of 1C, and after reaching 4.1V, continue charging while lowering the current for a while to reach a constant voltage of 4.1V, and charge when the current reaches 0.02C finish. After this charge, pause for 20 minutes.
Procedure 2: After Procedure 1, discharge until constant voltage of 1C is reached until 3V is reached.
Here, the discharge capacity in the constant current discharge in the procedure 2 was defined as the initial capacity (battery rated capacity). The results are shown in FIG. Here, it is shown in terms of a relative value (capacity maintenance ratio) when the initial capacity of the evaluation cell of Example 1 (the content of the metal-containing particles is 0% by mass) is 100%.

<Overcharge test>
The evaluation cell of each example was charged to 4.1V (SOC 100%) at 1C with a constant current and constant voltage method in a 25 ° C test tank, and further charged to 5V with a constant current of 1C and overcharged. It was. Thereafter, discharge treatment was performed up to 3 V with a constant current of 1 C. And the elution amount of the metal-containing particle | grains to a negative electrode was measured. The amount of the metal-containing particles eluted to the negative electrode was determined from the amount of metal-containing particles in the positive electrode active material layer before and after the overcharge test. The metal-containing particles eluted in the electrolyte have a positive charge and move to the negative electrode. The results are shown in FIG.

  As shown in FIG. 5, the elution amount of the metal-containing particles showed an increasing tendency as the content of the metal-containing particles increased. However, when the content of metal-containing particles exceeded 2% by mass, the amount of elution hardly changed. From the viewpoint of appropriately eluting the metal-containing particles during overcharge, the content of the metal-containing particles is preferably 1.5% by mass or more. Further, the initial capacity tended to decrease as the content of the metal-containing particles increased. From the viewpoint of the initial capacity, the content of the metal-containing particles is preferably 3% by mass or less.

<Test Example 2>
In this example, the voltage transition during overcharge was examined using the evaluation cells of Examples 1 and 5. Specifically, the change in voltage when each evaluation cell was charged to exceed 5.2 V was plotted against the time on the horizontal axis. The results are shown in FIG.

  As shown in FIG. 6, the evaluation cell of Example 1 using the positive electrode containing no metal-containing particles has a rapid voltage increase near 4.8 V, and thereafter, the transition metal is actively eluted. You can see that On the other hand, in the evaluation cell of Example 5 using the positive electrode containing metal-containing particles, the voltage increase in the region of 4.5 V to 4.8 V is moderate compared to Example 1. It is presumed that the metal-containing particles are eluted in the region of 4.5V to 4.8V, and the voltage rise is suppressed.

<Test Example 3>
In this example, the transition of the battery temperature during overcharge was examined using the evaluation cells of Examples 1 and 5. Specifically, the change in battery temperature when each evaluation cell was charged to exceed 5.2V was plotted against the time on the horizontal axis. The results are shown in FIG.

  As shown in FIG. 7, in the evaluation cell of Example 1 using the positive electrode containing no metal-containing particles, the battery temperature increased even after exceeding 130 ° C. In contrast, in the evaluation cell of Example 5 using the positive electrode containing metal-containing particles, the battery temperature showed a tendency to decay after reaching 130 ° C. In the evaluation cell of Example 5 using the positive electrode containing metal-containing particles, the structure of the positive electrode was maintained until the metal-containing particles eluted before the transition metal of the positive electrode active material and the shutdown function due to melting of the separator worked. Therefore, it is presumed that excessive heat generation of the battery was suppressed because the release of oxygen was suppressed.

  Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The invention disclosed herein may include various modifications and alterations of the specific examples described above.

20 Winding electrode body 30 Battery case 50 Positive electrode 60 Negative electrode 70, 72 Separator 100 Lithium ion secondary battery

Claims (1)

A positive electrode having a lithium transition metal composite oxide containing at least one transition metal element selected from the group consisting of nickel, cobalt and manganese as a positive electrode active material, a negative electrode, a separator, and a non-aqueous electrolyte A lithium ion secondary battery provided,
The lithium ion secondary battery, wherein the positive electrode includes metal-containing particles that elute at a positive electrode potential lower than a positive electrode potential at which transition metal elution of the lithium transition metal composite oxide occurs.

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