JP5962956B2 - Lithium secondary battery - Google Patents

Description

  The present invention relates to a lithium secondary battery including a gas generating additive that generates a gas during overcharge in an electrolyte.

In recent years, lithium secondary batteries (typically lithium ion batteries) that are lightweight and have a high energy density have become increasingly important as high-output power supplies for vehicles.
In one typical configuration of this type of secondary battery, an electrode material (electrode active material) including an electrode active material capable of reversibly occluding and releasing charge carriers (for example, lithium ions in the case of a lithium ion battery). The material layer is provided with an electrode held by a conductive member (electrode current collector), and this electrode (positive electrode and negative electrode) is housed in a battery case together with a nonaqueous electrolyte.

  Such lithium secondary batteries are generally used in a state where the voltage is controlled within a predetermined voltage range (for example, 3.0 V or more and 4.2 V or less). Even in this normal use, the electrolyte is slightly decomposed on the surface of the positive electrode. It is known that gas is generated. On the other hand, if a current exceeding the normal level is supplied to the battery due to malfunction or the like, the battery exceeds the predetermined voltage and falls into an overcharged state. When the battery is in an overcharged state, it is assumed that the decomposition of the electrolyte is promoted and the amount of gas generated in the battery case increases, or the temperature inside the battery rises due to heat generation of the active material. Therefore, as a safety mechanism to stop the progress of overcharge, the current cutoff valve operates when the pressure in the battery case exceeds a predetermined value due to the generation of the cracked gas, and the battery has a wide current cutoff mechanism that cuts off the charging current. It has been.

  In a battery having the above-described current interruption mechanism, it has been proposed that a compound having an oxidation potential (that is, a voltage at which an oxidative decomposition reaction starts) is lower than that of a nonaqueous solvent of an electrolyte in advance (for example, Patent Document 1). When the battery is overcharged, the compound is rapidly oxidized and decomposed on the surface of the positive electrode before the electrolyte undergoes a decomposition reaction, and as a result, a large amount of gas is generated. The generated gas causes an increase in the internal pressure of the battery, so that the current interruption mechanism is activated early (that is, in a safer state of the battery). Typical examples of this type of compound (that is, hereinafter referred to as gas generating additive) include hydrocarbon compounds such as cyclohexylbenzene (CHB) and biphenyl (BP).

JP 2003-257479 A

  By the way, about granular lithium transition metal complex oxide widely used as a positive electrode active material, durability, such as a capacity | capacitance maintenance factor, may be improved by containing elements, such as tungsten (W). Are known. In particular, when a lithium secondary battery is stored in a high charge state (high SOC) in a high-temperature environment, the battery can have excellent durability. This is considered to be because the decomposition of the electrolyte on the surface of the positive electrode, which is seen even under normal use environment, is suppressed by the presence of tungsten (W). However, according to the study by the present inventors, contrary to the effect of improving the durability, the cathode active material contains tungsten (W), so that the electrolyte and the gas generating additive are decomposed in an overcharged state. It has been found that there is a problem that has not been known so far, in which the amount of gas generated during overcharging is significantly reduced. Such a phenomenon may hinder normal operation such as delaying or not operating the current interrupting mechanism.

  The present invention has been created in view of such a conventional situation, and the object of the present invention is to include a gas generating additive in an electrolyte, and even when a positive electrode active material containing tungsten is used during overcharge. An object of the present invention is to provide a lithium secondary battery that can ensure a sufficient amount of gas generation and has both storage characteristics and overcharge safety.

  A lithium secondary battery provided by the present invention includes a positive electrode including a positive electrode active material and a conductive material, a negative electrode including a negative electrode active material, and a nonaqueous electrolyte including a gas generating additive composed of a hydrocarbon compound. Here, the positive electrode active material is a granular lithium transition metal composite oxide having a layered structure containing tungsten (hereinafter sometimes simply referred to as “W”), and the conductive material includes primary particles. In the form of aggregates. In such a secondary battery, the average particle size of the aggregate is controlled within a range of less than 9 μm, so that the gas generated due to the decomposition of the gas generating additive in the presence of the aggregate. The amount of generation is controlled.

The gas generating additive described above comes into contact with the surface of the positive electrode (typically, the surface of the positive electrode active material or conductive material) during overcharge (that is, when the battery voltage increases), and is oxidatively decomposed and polymerized. To generate hydrogen ions. The hydrogen ions diffuse into the negative electrode and receive electrons on the negative electrode to generate hydrogen as a reducing gas. For this reason, the amount of gas generated due to the decomposition reaction of the overcharge inhibitor can depend on the amount of the overcharge inhibitor present in the vicinity of the positive electrode and the contact area (reaction field) between the positive electrode and the overcharge inhibitor.
In the invention disclosed herein, the average particle diameter of the aggregate of the conductive material is 9 μm, compared with the average particle diameter of about 10 μm (typically, the average particle diameter of 9 μm or more) which has been generally used. By appropriately controlling in a range below (for example, in the range of 2 μm to 8 μm), the dispersibility of the conductive material in the positive electrode active material layer is increased and the reaction field of the gas generating additive is increased. That is, according to such a configuration, the decomposition reaction of the gas generating additive can be caused not only on the surface of the positive electrode active material but also on the surface of the conductive material expanded by appropriately controlling the average particle diameter. . Therefore, it is not necessary to change the amount of the gas generating additive and the amount of the conductive material in the positive electrode active material layer (that is, the composition ratio of the conductive material in the positive electrode active material layer). The required amount of gas can be generated in Thereby, a decrease in the amount of gas generated at the time of overcharging in the secondary battery having the above configuration is suppressed, and both safety and storage characteristics at the time of overcharging can be ensured.

In a preferred embodiment of the lithium secondary battery disclosed herein, the W is segregated on the surface of the lithium transition metal composite oxide, and the W concentration on the surface is 0.9 atomic% to 2.9 atomic%. And the average particle diameter of the said aggregate in the said electrically-conductive material is 2.5 micrometers-7.8 micrometers, It is characterized by the above-mentioned.
When W contained in the lithium transition metal composite oxide is present on the surface in a granular form (typically unevenly distributed on the surface), the storage characteristics (eg, output characteristics and cycle characteristics) are greatly improved. Can be. On the other hand, if W contained in the lithium transition metal composite oxide is present (unevenly distributed) on the surface, the degree of reduction in the amount of gas generated during overcharge also increases. Therefore, in the invention disclosed herein, it is preferable to define the W concentration segregated on the surface of the lithium transition metal composite oxide and the average particle diameter of the conductive material within the above ranges. As a result, for example, a storage characteristic with a capacity retention rate of 80% or more after storage for 60 days can be obtained, and a sufficient amount of gas generated to achieve an internal pressure that activates a current interruption mechanism or the like during overcharge can be secured in a balanced manner. can do.

Note that the “W concentration” on the surface of the lithium transition metal composite oxide in this specification refers to the lithium transition metal composite oxide measured based on X-ray photoelectron spectroscopy (XPS). It can be defined as the ratio (atomic%) of the W element in the elements constituting the outermost surface (depending on the measurement conditions, but about several nm, for example, about 0.1 to 10 nm). As the W concentration, for example, a value (atomic%) measured using an apparatus capable of composition analysis based on X-ray photoelectron spectroscopy called XPS, ESCA or the like can be adopted.
In addition, the “average particle diameter” in the present specification means a cumulative 50% particle diameter (median diameter: hereinafter referred to as D) in a particle size distribution measured by a particle size distribution measuring apparatus based on a laser diffraction scattering method, unless otherwise specified. ₅₀ may be abbreviated as ₅₀ ).

In a preferred aspect of the lithium secondary battery disclosed herein, an average particle diameter of the aggregate in the conductive material is 3.0 μm to 4.5 μm.
By keeping the average particle size of the aggregate within the above range, the reaction field with the gas generating additive can be more suitably secured. For this reason, for example, even when the W concentration on the surface of the lithium transition metal composite oxide is increased to, for example, about 2 atomic% to 2.9 atomic%, a sufficient gas generation amount can be ensured, resulting in better storage. Characteristics and overcharge safety can be achieved.

In the preferable one aspect | mode of the lithium secondary battery disclosed here, the said electrically conductive material is mix | blended in the ratio of 1 mass part-20 mass parts with respect to 100 mass parts of said positive electrode active materials.
In order to increase the reaction field of the gas generating additive, for example, it is conceivable to increase the amount of the conductive material in the positive electrode active material layer. However, too much conductive material is not preferable because it causes a decrease in the density of the positive electrode active material layer and a decrease in capacity per unit volume. In addition, too little conductive material is not preferable because it cannot increase the amount of gas generated and causes an increase in internal resistance and a decrease in durability. For example, the conductive material having the average particle size as defined above should be included in the ratio defined above with respect to the positive electrode active material when considering battery performance such as capacity, internal resistance, and durability. Is preferred.

In a preferred embodiment of the lithium secondary battery disclosed herein, the gas generating additive is blended in the electrolyte at a ratio of 0.01% by mass to 10% by mass.
Further, as a method for increasing the amount of gas generated during overcharge, for example, increasing the amount of overcharge inhibitor added can be considered. However, since the gas generating additive acts as a resistance component of the battery reaction, an excessive amount of the gas generating additive is not preferable because there is a concern that the battery performance may be deteriorated (for example, an increase in internal resistance or a decrease in durability). On the other hand, if the amount is too small, a sufficient gas generation amount cannot be secured during overcharge. From such a viewpoint, it is preferable that the gas generating additive is contained in the electrolyte at a ratio defined above.

In a preferred aspect of the lithium secondary battery disclosed herein, the gas generation amount is 20 ml / Ah or more (preferably 30 ml / Ah or more) as the gas generation amount per rated capacity of the battery. It is said.
By appropriately controlling the average particle size of the aggregate of the conductive material and increasing the reaction field of the gas generating additive, it becomes possible to generate the gas as described above during overcharge. According to such a configuration, a sufficient amount of gas can be generated for the rapid operation of the current interrupt mechanism, and a lithium secondary battery with higher safety is provided.

In a preferred embodiment of the lithium secondary battery disclosed herein, as at least a part of the lithium transition metal composite oxide, the general formula (1):
_{Li x Ni a Co b Mn c} M d W e O 2 (1)
(Here, M does not exist or is at least one selected from Zr, Nb, and Al, x satisfies 1.0 ≦ x ≦ 1.25, and a, b, c, d, e Satisfies a + b + c + d + e = 1 and 0.001 ≦ (d + e) ≦ 0.02, and at least one of a, b and c is greater than 0 and e>0);
The lithium transition metal complex oxide represented by these is included.
The lithium transition metal composite oxide having the composition represented by the general formula (1) can be a positive electrode active material that can simultaneously realize excellent output characteristics and cycle characteristics. On the other hand, the lithium transition metal composite oxide has essentially low electron conductivity (even in a composition not containing W) and cannot sufficiently provide a reaction field for decomposition of the gas generating additive. This tendency becomes more prominent in the lithium transition metal composite oxide having the composition containing W represented by the general formula (1). However, according to the lithium secondary battery having such a configuration, since the average particle size of the aggregate of the conductive material is appropriately controlled, the lithium transition metal composite oxide having the composition represented by the general formula (1) is not used. Even when used as a positive electrode active material, a sufficient amount of gas can be generated for the operation of the current interruption mechanism. As a result, a lithium secondary battery having superior storage characteristics such as output characteristics and cycle characteristics and overcharge safety is provided.

In a preferred aspect of the lithium secondary battery disclosed herein, the battery case further includes a battery case containing the positive electrode, the negative electrode, and the non-aqueous electrolyte. It has a current interruption mechanism that prevents the gas flow between the inside and the outside and deforms when the internal pressure in the case rises above a predetermined level and breaks the electrical connection between the positive electrode and the negative electrode. It is said.
The lithium secondary battery disclosed herein can generate a sufficient amount of gas during overcharge while using a lithium transition metal composite oxide containing W as a positive electrode active material. Therefore, the present invention can be suitably applied to a secondary battery having a configuration including a current interruption mechanism that operates when the internal pressure of the battery increases.
In order to operate the current interrupting mechanism with the generation of a small amount of gas, for example, it is possible to consider reducing the pressure value in the battery case that operates the current interrupting valve. However, such a design change may cause a malfunction due to a slight change in the surrounding environment, and is not a desirable measure. On the other hand, the invention disclosed herein solves a new problem that has not occurred in the past without greatly changing the basic design and configuration of the battery.

  As described above, the lithium secondary battery disclosed herein can realize, for example, an excellent capacity maintenance ratio simultaneously with high output characteristics and cycle characteristics. For this reason, the lithium secondary battery disclosed herein is preferably used as a secondary battery for a vehicle driving power source that is mounted on a vehicle such as an automobile that requires durability over a long period of time and high safety and reliability. be able to. Thus, the present invention can also suitably provide a vehicle equipped with the lithium secondary battery as a driving power source.

It is a cross-sectional schematic diagram which shows an example of the structure of the lithium secondary battery which concerns on one Embodiment of this invention. It is a schematic diagram which shows the winding electrode body of the lithium secondary battery which concerns on one Embodiment of this invention. It is a schematic diagram which shows the III-III cross section in FIG. It is a side view which shows typically the vehicle provided with the lithium secondary battery which concerns on one Embodiment. It is the figure which illustrated the relationship between the tungsten (W) density | concentration of the surface of lithium transition metal complex oxide, the gas generation amount at the time of overcharge, and a capacity | capacitance maintenance factor. Is a diagram showing the relationship between the average particle diameter (D ₅₀₎ and the amount of gas generated during overcharge of the conductive material according to one embodiment. It is the figure which extracted and showed a part of data of FIG.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

  In this specification, a “lithium secondary battery” refers to a secondary battery that uses lithium ions as a charge carrier and is charged and discharged by the movement of charges between positive and negative electrodes by the lithium ions. Such lithium secondary batteries can typically include secondary batteries such as lithium ion batteries (lithium ion secondary batteries) and lithium polymer batteries. In this specification, an “active material” can reversibly occlude and release (typically insertion and removal) chemical species (that is, lithium ions in this case) that serve as charge carriers in a secondary battery. A substance.

  The lithium secondary battery disclosed herein includes, as essential components, a non-aqueous electrolyte including a positive electrode including a positive electrode active material and a conductive material, a negative electrode including a negative electrode active material, and a gas generating additive composed of a hydrocarbon compound. And. Below, the structure of this lithium secondary battery is demonstrated in order.

The positive electrode typically supplies a positive electrode active material layer forming composition containing a particulate positive electrode active material, a conductive material, and a binder that binds them to the positive electrode current collector. It is prepared by forming an active material layer.
As the positive electrode current collector, a conductive member made of a highly conductive metal or resin can be used as in the case of a conventional electrode current collector for a lithium secondary battery. For example, a metal mainly composed of aluminum, nickel, titanium, iron or the like or an alloy thereof can be preferably used. More preferably, it is aluminum or an aluminum alloy. There is no restriction | limiting in particular about the shape of a positive electrode electrical power collector, Various things can be considered according to the shape etc. of a desired secondary battery. For example, there are various shapes such as a rod shape, a plate shape, a sheet shape, a foil shape, and a mesh shape, and typically an aluminum foil can be suitably used.

As a characteristic positive electrode active material in the lithium secondary battery disclosed herein, a granular lithium transition metal composite oxide containing tungsten (W) and having a layered structure (hereinafter simply referred to as a W-containing lithium transition metal composite oxide). In some cases). Such a lithium transition metal composite oxide has, for example, a structure similar to that of a lithium transition metal composite oxide that can occlude and release lithium ions conventionally used in lithium secondary batteries as its essential structure. Various materials containing tungsten (W) as a part of the structure can be used without particular limitation.
In the positive electrode active material, the presence of W suppresses the decomposition of the electrolyte generated on the surface in a normal battery use state. Therefore, in addition to the characteristics (for example, high capacity, etc.) inherent to the positive electrode active material, the positive electrode active material can be excellent in durability such as capacity retention rate.

  Specifically, the W-containing lithium transition metal composite oxide is, for example, a lithium transition metal composite oxide having a layered structure, which includes at least one transition metal element of Ni, Co, and Mn, What contains W can be illustrated as a more preferable aspect. Here, the W-containing lithium transition metal composite oxide is preferably a so-called ternary W-containing lithium transition metal composite oxide containing three kinds of transition metal elements composed of Ni, Co, and Mn, and / or Alternatively, it is also preferable that Li is a so-called lithium-rich W-containing lithium transition metal composite oxide at a transition metal site in a layered crystal structure.

Such a W-containing lithium transition metal composite oxide is, for example, the general formula (1):
_{Li x Ni a Co b Mn c} M d W e O 2 (I)
(Here, M does not exist or is at least one selected from Zr, Nb, and Al, x satisfies 1.0 ≦ x ≦ 1.25, and a, b, c, d, e Satisfies a + b + c + d + e = 1 and 0.001 ≦ (d + e) ≦ 0.02, and at least one of a, b and c is greater than 0 and e>0);
It can be shown as including at least a part of the lithium transition metal composite oxide represented by

Note that such a W-containing lithium transition metal composite oxide includes other elements (for example, Mo, Cr, Fe, V, Ti, Cu, Zn, Ga, In, in addition to the elements specifically exemplified above). , Sn, La, Ce, Ca, Mg and Na selected from two or more).
By using the W-containing lithium transition metal composite oxide having such a structure as a positive electrode active material, excellent output characteristics and durability not only at room temperature but also at low temperatures (for example, −10 ° C. or less) and relatively high temperatures (for example, about 60 ° C.) It is possible to realize a lithium secondary battery having characteristics (cycle characteristics, etc.).

  Further, in such a W-containing lithium transition metal composite oxide, W may be uniformly contained in the entire lithium transition metal composite oxide, but is more preferably present or unevenly distributed on the surface thereof. . In other words, the lithium transition metal oxide is generally in the form of secondary particles in which primary particles are gathered, but it is preferable that W be present (distributed) in a biased manner on the surface of the primary particles. When W is unevenly distributed on the surface, the decomposition of the electrolyte performed on the surface of the positive electrode active material can be effectively suppressed with a smaller amount of W. For example, durability such as the capacity retention rate can be obtained while suppressing excessive use of W.

  Even if the amount of W present (unevenly distributed) on the surface of such a lithium transition metal composite oxide is very small, the decomposition of the electrolyte can be suppressed and the effect of increasing the capacity retention rate can be obtained. However, on the other hand, W present (or unevenly distributed) on the surface of the lithium transition metal composite oxide also has a new problem of reducing the amount of gas generated at the time of overcharge described later. Therefore, the concentration of W among the elements present on the surface of the lithium transition metal composite oxide is 0.9 atomic% to achieve compatibility between durability such as capacity retention and gas generation during overcharge. It is preferably about 2.9 atomic%. More preferably, it is 1.5 atomic% to 2.1 atomic%.

  The lithium transition metal composite oxide in which W is unevenly distributed on the surface is obtained by, for example, using a hydroxide (precursor) prepared by a wet method as an appropriate lithium source (for example, lithium such as lithium carbonate or lithium hydroxide). Salt) and firing at a predetermined temperature. More specifically, for example, first, an aqueous solution A containing at least one transition metal element selected from the above-mentioned Ni, Co, and Mn, and at least one element selected from Zr, Nb, and Al, and W An aqueous solution B containing is prepared. And this aqueous solution A and aqueous solution B are wet-mixed on alkaline conditions (for example, maintaining pH 11-14), and the hydroxide (precursor) containing these metal elements is precipitated. Next, this hydroxide and a lithium compound (lithium salt) are mixed, and the mixture is fired, whereby a lithium transition metal composite oxide in which W is unevenly distributed on the surface can be prepared. Even when the lithium transition metal composite oxide in which W is unevenly distributed has a different composition, it can be formed in the same manner.

  Although there is no strict restriction | limiting about the shape of the above positive electrode active material, said positive electrode active material can be grind | pulverized, granulated, and classified by an appropriate means. For example, disclosed herein is a W-containing lithium transition metal oxide powder substantially composed of secondary particles having an average particle size in the range of approximately 1 μm to 25 μm (typically approximately 2 μm to 15 μm). It can preferably be employed as a positive electrode active material in the technology.

  “Unevenly distributed” on the primary particle surface with respect to W means that W is distributed (concentrated) more unevenly (concentrated) on the surface (grain boundary) of the primary particles than inside the primary particles. Therefore, it does not mean only an aspect in which W exists only at the grain boundary (in other words, does not exist at all inside the primary particle). The presence of W on the surface of the primary particles is not limited to the above XPS, but can be applied to the surface of the active material particles using, for example, energy dispersive X-ray spectroscopy (EDX: Energy Dispersive X-ray Spectroscopy). And the mapping result shows that W is concentrated and present at the grain boundary (the amount of W present per area at the grain boundary is larger than that inside the primary particle). it can. The position of the grain boundary (primary particle surface) can be grasped by, for example, observation of a cross section of the positive electrode active material particles by a transmission electron microscope (TEM). For this observation, a TEM equipped with EDX can be preferably used. At the same time, the positive electrode active material is preferably one in which no significant deviation (agglomerate or the like) is observed in the element distribution of W at the grain boundary (primary particle surface) by EDX image observation. Alternatively, it is preferable that, even when the surface of the positive electrode sheet taken out by disassembling the battery is subjected to EDX analysis, a remarkable deviation is not recognized in the W distribution.

  And as a conductive material disclosed here, it has the form of the aggregate which primary particles gathered, and the average particle diameter of this aggregate is suitably in the range of less than 9 micrometers (for example, the range of 2 micrometers-8 micrometers). The controlled one is used. By controlling the average particle size of the aggregate within the above range, the dispersion degree of the conductive material in the positive electrode active material layer is increased, and the reaction field between the positive electrode, the conductive material, and the gas generating additive in the electrolyte is increased as desired. It can be controlled.

  That is, when the average particle size of the aggregate is too large (for example, the average particle size is 8 μm or less), there are few contacts between the conductive material and the active material, and a good contact state cannot be obtained. The electron supply performance is inferior. Also, the surface area of the conductive material itself is reduced. These can in turn lead to a decrease in the oxidative decomposition reactivity of the gas generating additive. That is, the amount by which the gas generating additive is oxidatively decomposed on the surface of the positive electrode active material is reduced, and the reaction field between the gas generating additive and the conductive material is reduced. Therefore, gas is hardly generated even during overcharge.

  On the other hand, when the average particle size of the aggregate is too small (for example, an average particle size of 2 μm or less), there is a connection between voids (gaps) in the positive electrode active material layer formed by the positive electrode active material particles and the conductive material particles. Since it worsens, it will be in the state which is easy to inhibit the penetration | invasion of the electrolyte to this space | gap, and the movement of electrolyte ion. This is because the supply of overcharge additive to the surface of the positive electrode active material and the conductive material, and the reaction product formed by oxidative decomposition of the overcharge additive from the positive electrode active material layer is delayed, and gas generation at the positive electrode This may hinder the progress of the oxidative decomposition reaction of the additive. Therefore, gas is hardly generated even during overcharge.

  By appropriately controlling the average particle size of the aggregate within the above range, it is possible to ensure the amount of gas generated that can activate the current interruption mechanism during overcharge. The average particle size of such an aggregate cannot be generally specified because it depends on the W concentration on the surface of the positive electrode active material. For example, specifically, the average particle size is in the range of 2.5 μm to 7.8 μm. Is more preferable, and more preferably in the range of 3.0 μm to 4.5 μm.

  As the conductive material, any material can be used without particular limitation as long as it exhibits good conductivity. For example, carbon powders such as various carbon blacks (for example, acetylene black, furnace black, ketjen black) and graphite powder can be suitably used. These may use 1 type and may use 2 or more types together. And although such a conductive material is not particularly limited, it is exemplified that it is blended at a rate of, for example, 1 to 20% by mass, preferably 5 to 15% by mass with respect to 100% by mass of the positive electrode active material. The

  The binder has a function of binding each particle of the positive electrode active material and the conductive material contained in the positive electrode active material layer, or binding these particles and the positive electrode current collector. As such a binder, one kind or two or more kinds of substances conventionally used in lithium secondary batteries can be used without any particular limitation. Typically, various polymer materials corresponding to the solvent used when forming the positive electrode active material layer can be suitably used.

  For example, when the positive electrode active material layer is formed using an aqueous solvent, a polymer material that is dissolved or dispersed in water can be preferably used. Examples of such polymer materials include cellulose polymers, fluorine resins, vinyl acetate copolymers, rubbers, and the like. More specifically, carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), styrene butadiene rubber (SBR), acrylic acid-modified SBR resin (SBR latex), and the like. In addition, when the positive electrode active material layer is formed using a solvent-based solvent (typically a solvent-based composition in which the main component of the dispersion medium is an organic solvent), a polymer material that is dispersed or dissolved in the organic solvent Can be preferably employed. Examples of the polymer material include polyvinylidene fluoride (PVdF), polyvinylidene chloride (PVdC), polyethylene oxide (PEO), and the like.

Although such a binder is not specifically limited, For example, 0.5-30 mass% with respect to 100 mass% of positive electrode active materials, Preferably it is 0.5-20 mass%, More preferably, it is 1-10 mass. It is exemplified to be blended at a ratio of%.
In addition to the function as a binder, the polymer material exemplified as the binder is a thickener for a positive electrode active material layer forming paste (hereinafter sometimes simply referred to as a paste) prepared to form a positive electrode active material layer. It may be used for the purpose of exhibiting the function as another additive. In addition, a gas generating additive (for example, a phosphate or a carbonate (typically a hydrocarbon compound)) or the like that generates gas at the time of overcharge described later may be included in the binder in advance. Also good.

  Moreover, as a solvent used for preparation of said positive electrode, both an aqueous solvent (aqueous solvent) and a nonaqueous solvent (solvent solvent) can be used. Examples of the aqueous solvent include water and a composition using a mixed solvent mainly composed of water. As a solvent other than water constituting the mixed solvent, one or two or more organic solvents (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water can be appropriately selected and used. Preferable examples of the non-aqueous solvent include N-methyl-2-pyrrolidone (NMP).

The negative electrode typically has a negative electrode active material layer formed by supplying a negative electrode active material layer forming composition containing a negative electrode active material, a binder, and, if necessary, a conductive material onto a negative electrode current collector. Prepared by forming.
As the negative electrode current collector, a conductive member made of a highly conductive metal or resin or the like can be used as in the conventional negative electrode current collector for lithium secondary batteries. For example, a rod-like body, a plate-like body, a foil-like body, a net-like body or the like mainly composed of copper, nickel, titanium, stainless steel, or the like can be used. In addition, for example, as the negative electrode current collector, a film material obtained by depositing copper on a polypropylene film can be used.

  As the negative electrode active material, one type or two or more types of materials conventionally used in lithium ion batteries can be used without any particular limitation. For example, a particulate carbon material (carbon particles) including a graphite structure (layered structure) at least in part. More specifically, the negative electrode active material is, for example, natural graphite, graphite (graphite), non-graphitizable carbon (hard carbon), graphitizable carbon (soft carbon), or a carbon material combining these Alternatively, at least a part of the surface of these carbon materials may be coated with an amorphous carbon material. Further, for example, a metal compound (preferably silicide or metal oxide) having Si, Ge, Sn, Pb, Al, Ga, In, As, Sb, Bi, or the like as a constituent metal element may be used. Moreover, LTO (lithium titanate) can also be used as the negative electrode active material particles. About the negative electrode active material which consists of a metal compound, the surface of a metal compound may be fully coat | covered with a carbon film, for example, and you may use as a granular material excellent in electroconductivity. In this case, the negative electrode active material layer may not contain a conductive material, and the content rate of the following conductive material may be reduced as compared with the conventional case. The additional aspect of these negative electrode active materials and forms, such as a particle size, can be suitably selected according to a desired characteristic.

As the negative electrode active material layer and the binder, solvent, and thickener used for the preparation thereof, various materials exemplified as the binder, solvent, and thickener in the positive electrode active material layer can be similarly used. Moreover, the usage-amount of a binder with respect to 100 mass parts of negative electrode active materials can be 0.5-10 mass parts, for example, Preferably it can be 1-5 mass parts.
Note that when a conductive material is used, the same conductive material as that used in the formation of the positive electrode active material layer can be used, but the shape, size, and the like are not particularly limited and may be set as appropriate. it can. In this case, the amount of the conductive material used is approximately 1 to 30 parts by mass, preferably approximately 2 to 20 parts by mass, for example 5 to 10 parts by mass with respect to 100 parts by mass of the negative electrode active material.

  After forming the electrode active material layer by supplying a predetermined amount of the composition for forming an active material layer to one or both sides of the current collector as described above, for example, it is dried and appropriately pressed (for example, a roll press method). In addition, various conventionally known pressing methods such as a flat plate pressing method can be employed.), And the thickness and density of the electrode active material layer can be adjusted.

  The lithium secondary battery disclosed herein is constructed by laminating the positive electrode and the negative electrode to produce an electrode body and housing it in an appropriate battery case together with an electrolyte containing a gas generating additive. In the typical configuration of the lithium secondary battery disclosed herein, a separator is interposed between the positive electrode and the negative electrode.

  The separator is a member that insulates the positive electrode and the negative electrode and allows the electrolyte to move between the two electrodes. As long as this requirement is satisfied, the material constituting the separator substrate is not essentially limited. And as such a separator, the separator similar to the past can be used. Typically, a porous body, a nonwoven fabric, a cloth, or the like having fine pores that can move lithium ions can be used. For example, a porous sheet made of resin (a microporous resin sheet) can be preferably used. As a constituent material of such a porous sheet, polyolefin resins such as polyethylene (PE), polypropylene (PP), and polystyrene are preferable. In particular, a porous sheet such as a PE sheet, a PP sheet, a two-layer structure sheet in which a PE layer and a PP layer are laminated, and a three-layer structure sheet in which one PE layer is sandwiched between two PP layers. A polyolefin sheet can be suitably used. Although it is not particularly limited, preferred porous sheets (typically porous resin sheets) used as the separator base material have an average pore diameter of about 0.001 μm to 30 μm and a thickness of 5 μm to 100 μm. Examples of the porous resin sheet are (more preferably 10 μm to 30 μm). The air efficiency (porosity) of the porous sheet can be, for example, about 20 to 90% by volume (preferably 30 to 80% by volume). Note that in a lithium secondary battery (lithium polymer battery) using a solid electrolyte, the electrolyte may also serve as a separator.

As the electrolyte used here, one kind or two or more kinds similar to the non-aqueous electrolyte used in the conventional lithium secondary battery can be used without any particular limitation. Such a non-aqueous electrolyte typically includes a lithium salt as a supporting salt in a non-aqueous solvent (typically an organic solvent). Examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li (CF ₃ SO ₂ ) ₂ N, LiCFSO, and the like. These lithium salts can be used alone or in combination of two or more. The electrolyte concentration is not particularly limited, but a nonaqueous electrolyte containing a lithium salt at a concentration of about 0.1 mol / L to 5 mol / L (preferably about 0.8 mol / L to 1.5 mol / L) is preferably used. be able to. Further, it may be a solid (gel) electrolyte in which a polymer is added to the liquid electrolyte.

As the non-aqueous solvent, aprotic solvents such as carbonates, esters, ethers, nitriles, sulfones, and lactones can be used. For example, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, Examples include 2-methyltetrahydrofuran, dioxane, 1,3-dioxolane, diethylene glycol dimethyl ether, ethylene glycol dimethyl ether, acetonitrile, propionitrile, nitromethane, N, N-dimethylformamide, dimethyl sulfoxide, sulfolane, and γ-butyrolactone. Examples of the electrolyte include LiPF ₆ , LiBF ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC (SO ₂ CF ₃ ) ₃ , LiClO _{4 and the} like are exemplified.

  In the thirium secondary battery disclosed herein, the electrolyte contains a gas generating additive made of a hydrocarbon compound. As such a gas generating additive, the oxidation potential is higher than the operating voltage of the lithium secondary battery (for example, 4.2 V or higher in the case of a lithium secondary battery that is fully charged at 4.2 V) and is oxidized. Any compound that generates a large amount of gas can be used without any particular limitation. However, if the oxidation potential is close to the operating voltage of the battery, the local operating voltage gradually increases due to a local voltage increase. There is a risk of disassembly. On the other hand, when the decomposition voltage is 4.9 V or more, there is a possibility that thermal runaway may occur due to the reaction of the main component of the nonaqueous electrolyte and the electrode material before gas generation due to oxidative decomposition of the additive. Accordingly, a lithium secondary battery that is fully charged at 4.2 V preferably has an oxidation reaction potential in the range of 4.6 V or more and 4.9 V or less. More preferably, the gas generating additive is an aromatic hydrocarbon compound. Examples of such compounds include biphenyl compounds, cycloalkylbenzene compounds, alkylbenzene compounds, organic phosphorus compounds, fluorine atom-substituted aromatic compounds, carbonate compounds, cyclic carbamate compounds, and alicyclic hydrocarbons. More specifically, biphenyl (BP), alkylbiphenyl, terphenyl, 2-fluorobiphenyl, 3-fluorobiphenyl, 4-fluorobiphenyl, 4,4′-difluorobiphenyl, cyclohexylbenzene (CHB), trans-butylcyclohexyl Benzene, cyclopentylbenzene, t-butylbenzene, t-aminobenzene, o-cyclohexylfluorobenzene, p-cyclohexylfluorobenzene, tris- (t-butylphenyl) phosphate, phenyl fluoride, 4-fluorophenyl acetate, diphenyl carbonate, Examples thereof include methyl phenyl carbonate, bicterary butyl phenyl carbonate, diphenyl ether, dibenzofuran and the like. In particular, cyclohexylbenzene (CHB), cyclohexylbenzene derivatives and biphenyl (BP) are exemplified. The usage-amount of the overcharge inhibitor with respect to 100 mass% of electrolytes to be used can be about 0.01-10 mass%, for example, Preferably it is about 0.1-5 mass%.

  As a battery case used here, the material and shape which are used for the conventional lithium secondary battery can be used. Examples of the material include relatively light metal materials such as aluminum and steel, and resin materials such as PPS and polyimide resin. The shape (outer shape of the container) is not particularly limited, and may be, for example, a cylindrical shape, a rectangular shape, a rectangular parallelepiped shape, a coin shape, a bag shape (typically a laminate cell), or the like. The battery case has a current interrupt mechanism that disconnects the electrical connection between the positive electrode and the negative electrode when the internal pressure in the case exceeds a predetermined level. Such a current interrupting mechanism is not particularly limited in its configuration. For example, during normal use, the gas flow between the inside and the outside of the case is prevented, and the internal pressure in the case becomes a predetermined level during overcharging. A structure that can be deformed when it exceeds the upper limit and that can cut off the electrical connection between the positive electrode and the negative electrode can be preferably employed.

  Hereinafter, an embodiment of a lithium secondary battery disclosed herein will be described with reference to the drawings. In this embodiment, a square-shaped lithium secondary battery is described, but the present invention is not intended to be limited to this embodiment. Further, matters other than the matters specifically mentioned in the present specification and matters necessary for carrying out the present invention (for example, the configuration and manufacturing method of an electrode body including a positive electrode and a negative electrode, the configuration and manufacturing of a separator and an electrolyte) General techniques related to the construction of a method, a lithium secondary battery, and other batteries) can be understood as design matters of a person skilled in the art based on the prior art in this field. In addition, in the following drawings, the same code | symbol is attached | subjected to the member and site | part which show | plays the same effect | action, and the overlapping description may be abbreviate | omitted or simplified. In addition, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect actual dimensional relationships.

FIG. 1 shows a lithium secondary battery 100. The lithium secondary battery 100 includes a wound electrode body 80 composed of a positive electrode 10 and a negative electrode 20, a nonaqueous electrolyte containing a gas generating additive composed of a hydrocarbon compound (not shown), and a battery case 50. FIG. 2 is a view showing a wound electrode body 80. FIG. 3 shows a III-III cross section in FIG.
The battery case 50 includes a flat cuboid case main body 52 having an open upper end, and a lid 54 that closes the opening. As a material constituting the battery case 50, a metal material such as aluminum or steel is preferably used (in this embodiment, aluminum). Or the battery case 50 formed by shape | molding resin materials, such as a polyphenylene sulfide (PPS) and a polyimide resin, may be sufficient. On the upper surface (that is, the lid 54) of the battery case 50, a positive electrode terminal 70 that is electrically connected to the positive electrode 10 of the wound electrode body 80, and a negative electrode terminal 72 that is electrically connected to the negative electrode 20 of the electrode body 80 Is provided.

  Inside the battery case 50, a current interrupting mechanism 30 that is activated by an increase in the internal pressure of the battery case is provided. When the internal pressure of the battery case 50 rises due to gas generation due to the gas generating additive, the current interrupting mechanism 30 interrupts the charging current by cutting a conductive path from at least one electrode terminal to the electrode body 80. Configured to get. In this embodiment, the current interruption mechanism 30 is provided between the positive electrode terminal 70 fixed to the lid body 54 and the electrode body 80, and reaches from the positive electrode terminal 70 to the electrode body 80 when the internal pressure of the battery case 50 rises. The conductive path is configured to be cut.

  The current interrupt mechanism 30 includes a conducting member composed of a first member 32 and a second member 34. When the internal pressure of the battery case 50 increases, at least one of the first member 32 and the second member 34 (here, the first member 32) is deformed and separated from the other, so that the conductive path is cut. It is configured. In this embodiment, the first member 32 is a deformed metal plate 32, and the second member 34 is a connection metal plate 34 joined to the deformed metal plate 32 at a joint point 36. The deformed metal plate (first member) 32 has an arch shape in which a central portion is curved downward, and a peripheral portion thereof is connected to the lower surface of the positive electrode terminal 70 via a current collecting lead terminal 35. Further, the tip of the curved portion 33 of the deformed metal plate 32 is joined to the upper surface of the connection metal plate 34. A positive electrode lead terminal 74 is joined to the lower surface (back surface) of the connection metal plate 34, and the positive electrode lead terminal 74 is connected to the positive electrode sheet 10 of the electrode body 80. In this way, a conductive path from the positive electrode terminal 70 to the electrode body 80 is formed.

  The current interrupt mechanism 30 includes an insulating case 38 made of plastic or the like. The insulating case 38 is provided so as to surround the deformed metal plate 32, and hermetically seals the upper surface of the deformed metal plate 32. The internal pressure of the battery case 50 does not act on the upper surface of the hermetically sealed curved portion 33. The insulating case 38 has an opening into which the curved portion 33 of the deformed metal plate 32 is fitted, and the lower surface of the curved portion 33 is exposed from the opening to the inside of the battery case 50. The internal pressure of the battery case 50 acts on the lower surface of the curved portion 33 exposed inside the battery case 50.

  In the current interrupt mechanism 30 having such a configuration, when the internal pressure of the battery case 50 increases, the internal pressure acts on the lower surface of the curved portion 33 of the deformed metal plate 32, and the curved portion 33 curved downward is pushed upward. The upward pushing force of the curved portion 33 increases as the internal pressure of the battery case 50 increases. When the internal pressure of the battery case 50 exceeds the set pressure, the curved portion 33 is turned upside down and deformed so as to bend upward. Due to the deformation of the curved portion 33, the joint point 36 between the deformed metal plate 32 and the connection metal plate 34 is cut. As a result, the conductive path from the positive electrode terminal 70 to the electrode body 80 is cut, and the charging current is cut off. In this embodiment, the conductive members 32 and 34 that are deformed when the internal pressure is increased are illustrated as being divided into the first member 32 and the second member 34, but the present invention is not limited to this. For example, the conducting member may be a single member. The current interrupt mechanism 30 is not limited to the positive terminal 70 side, and may be provided on the negative terminal 72 side. Moreover, the electric current interruption mechanism 30 is not limited to the mechanical cutting | disconnection accompanied with a deformation | transformation of the deformation | transformation metal plate 32 mentioned above. For example, an external circuit that detects the internal pressure of the battery case 50 with a sensor and cuts off the charging current when the internal pressure detected by the sensor exceeds a set pressure may be provided as the current cutoff mechanism.

  A flat wound electrode body 80 is accommodated in the battery case 50 together with a non-aqueous electrolyte (not shown). In the wound electrode body 80 according to the present embodiment, the positive electrode 10 is formed using the positive electrode active material composed of the W-containing lithium transition metal composite oxide as described above and the conductive material with the average particle diameter controlled together with the positive electrode active material. Except for this point, it is the same as the wound electrode body of a normal lithium secondary battery. As shown in FIG. 2, a long (strip-shaped) sheet is formed before the wound electrode body 80 is assembled. It has a structure (sheet-like electrode body) 82.

  In the present embodiment, the positive electrode sheet 10 has a structure in which a positive electrode active material layer 14 containing a positive electrode active material is held on both surfaces of a positive electrode current collector 12 made of a long metal foil. However, the positive electrode active material layer 14 is not attached to one side edge (the left side edge portion in the drawing) along the widthwise end of the positive electrode sheet 10, and the positive electrode current collector 12 is exposed with a certain width. The positive electrode active material layer non-formed part 16 is formed.

  Similarly to the positive electrode sheet 10, the negative electrode sheet 20 holds a negative electrode active material layer 24 containing a negative electrode active material on both sides of a long sheet-like foil-shaped negative electrode current collector (hereinafter referred to as “negative electrode current collector foil”) 22. Has a structured. However, the negative electrode active material layer 24 is not attached to one side edge (the right side edge portion in the drawing) along the widthwise edge of the negative electrode sheet 20, and the negative electrode current collector 22 is exposed with a certain width. The negative electrode active material layer non-forming part 26 is formed.

  When producing the wound electrode body 80, the positive electrode sheet 10 and the negative electrode sheet 20 are laminated via the separator sheets 40A and 40B. At this time, the positive electrode sheet 10 and the negative electrode active material layer non-formed portion 16 of the positive electrode sheet 10 and the negative electrode active material layer non-formed portion 26 of the negative electrode sheet 20 protrude from both sides in the width direction of the separator sheets 40A and 40B. The negative electrode sheet 20 is overlaid with a slight shift in the width direction. The laminated body thus stacked is wound, and then the obtained wound body is crushed from the side surface direction and ablated, whereby a flat wound electrode body 80 can be produced. Instead of rolling the sheet laminate (sheet-like electrode body) in this way and then flattening it, for example, the laminate is initially rolled into a flat shape (substantially oval, elliptical, etc.). Also good.

  In the central portion of the wound electrode body 80 in the winding axis direction, a portion in which the positive electrode active material layer 14 of the positive electrode sheet 10, the negative electrode active material layer 24 of the negative electrode sheet 20, and the separator sheets 40A and 40B are densely stacked ( A wound core portion) is formed. Further, at both ends in the winding axis direction of the wound electrode body 80, the electrode active material layer non-formed portions 16 and 26 of the positive electrode sheet 10 and the negative electrode sheet 20 respectively protrude outward from the wound core portion. Yes. A positive electrode lead terminal 74 and a negative electrode lead terminal 76 are attached to the protruding electrode active material layer non-forming portions 16 and 26, respectively, and are electrically connected to the positive electrode terminal 70 and the negative electrode terminal 72, respectively.

  According to the lithium secondary battery having such a configuration, since the lithium transition metal composite oxide containing W as the positive electrode active material (for example, W may be unevenly distributed on the surface) is used, output characteristics, cycle characteristics, etc. The storage characteristics of have been greatly improved. On the other hand, since the average particle diameter of the aggregate of the conductive material contained in the positive electrode active material layer is appropriately controlled within a range of less than 9 μm, the amount of the gas generating additive added and the conductivity in the positive electrode active material layer A required amount of gas can be generated in the battery case during overcharge without changing the amount of material (that is, the composition ratio of the conductive material in the positive electrode active material layer). Therefore, for example, excellent storage characteristics such that the capacity retention rate after storage for 60 days at a relatively high temperature is 80% or more can be obtained, and both safety and storage characteristics during overcharge can be ensured. .

  As described above, such a lithium secondary battery 100 has both safety and storage characteristics at the time of overcharging, and is used, for example, as a power source for driving the motor in an automobile equipped with a driving motor. The characteristics can be fully exhibited. For example, as shown in FIG. 4, a lithium secondary battery 100 disclosed herein includes a driving power source for a driving motor (typically, a driving power source for a hybrid vehicle) that requires high-rate input / output characteristics. The vehicle (for example, automobile) 1 provided with the lithium secondary battery 100 (which may be in the form of an assembled battery) disclosed herein is preferably provided.

  Hereinafter, some examples of the present invention will be shown to further explain the present invention. However, it goes without saying that the present invention is not limited to these examples.

[Positive electrode]
<Sample 1>
A lithium manganese composite oxide having a composition represented by the general formula: Li _1.15 Mn _0.33 Co _0.33 Ni _0.33 O ₂ was prepared as a positive electrode active material. Namely, pure water is put into a beaker, lithium acetate dihydrate [Li (CH ₃ COO) · 2H ₂ O] as a lithium source, and manganese (II) acetate tetrahydrate as a manganese source [Mn (CH ₃ COO) ₂ .4H ₂ O], nickel acetate (II) tetrahydrate as a nickel source [Ni (CH ₃ COO) ₂ .4H ₂ O], and cobalt acetate (II ) Tetrahydrate [Co (CH ₃ COO) ₂ .4H ₂ O] and manganese acetate (II) tetrahydrate as a manganese source [Mn (CH ₃ COO) ₂ .4H ₂ O], zirconium Zirconium acetate [Zr (CH3CO2) n] as a source and glycolic acid were added at a predetermined ratio suitable for forming the above composition and mixed by dissolution. This was then adjusted to pH 1.8 with acetic acid and the water was evaporated overnight in an 80 ° C. oil bath. Then, after baking at 600 degreeC for 5 hours and grind | pulverizing in a mortar, it further baked at 900 degreeC for 5 hours.

<Samples 2-6>
As a positive electrode active material, a lithium-containing composite oxide having W segregated on the particle surface was prepared.
That is, first, an appropriate amount of 3.25% aqueous sodium hydroxide and 25% aqueous ammonia was added at 40 ° C. under a nitrogen stream, the pH at the liquid temperature of 25 ° C. was 12.0, and the ammonia concentration in the liquid phase was 20 g / The basic aqueous solution was obtained by adjusting to L. The oxygen concentration in the reaction vessel was about 2.0%.

Next, nickel sulfate, cobalt sulfate, manganese sulfate, and zirconium sulfate have an element molar ratio Ni: Co: Mn of 0.33: 0.33: 0.33, and the total concentration of these transition metals is 1.8 mol. / L was dissolved in a basic aqueous solution to prepare a NiCoMn aqueous solution.
Further, ammonium paratungstate was dissolved in water at 40 ° C. under a nitrogen stream to prepare a W aqueous solution having a tungsten (W) concentration of 0.05 mol / L.

  The thus prepared NiCoMnZr aqueous solution and W aqueous solution were added to and mixed with the above basic aqueous solution while maintaining the pH at 12.0, and a hydroxide (precursor) containing Ni, Co, Mn, and W as constituent elements. ) The ratio of the NiCoMnZr aqueous solution and the W aqueous solution was changed in five stages. The hydroxide particles were heated in an air atmosphere at a temperature of 150 ° C. for 12 hours.

When the total number of moles of all transition metals (ie, Ni, Co, Mn, W) in the hydroxide is M, the molar ratio of lithium to M (Li / M) is 1.15. Lithium carbonate was weighed and mixed with the hydroxide particles after the heat treatment. The obtained mixture was baked at 760 ° C. for 4 hours in air of 21% by volume of oxygen, and then baked at 950 ° C. for 10 hours to obtain a general formula: Li _1.15 Mn _0.33X Co _0.33X Ni A granular lithium-containing composite oxide having a composition represented by _0.33X W _(1-0.99X) O ₂ was obtained.

  The lithium-containing composite oxide prepared in five ways was subjected to surface elemental analysis using an X-ray photoelectron spectrometer (XPS), and the surface W concentration was 0.4 atom. %, 0.9 atomic%, 1.5 atomic%, 2.1 atomic%, and 2.9 atomic%.

As the conductive material, acetylene black (AB) is used. The AB aggregate (secondary particles) has a cumulative 50% particle diameter (D50) of 2.5 μm, 4.0 μm, 7.8 μm, and 8. Five types of 9 μm and 10.3 μm were prepared.
As the binder, polyvinylidene fluoride (PVDF) was used.

When the positive electrode active material (samples 1 to 6), the conductive material (five types), and the binder prepared as described above are expressed in terms of positive electrode active material: conductive material: binder, the mass ratio of these materials is 93: 4: 3. The mixture was mixed with N-methyl-2-pyrrolidone (NMP) to prepare a slurry composition. This composition was applied to both surfaces of a 15 μm-thick aluminum foil as a current collector so that the total basis weight of both surfaces was about 30 mg / cm ² (based on solid content). This was dried and then pressed by a rolling press to produce a positive electrode sheet having a positive electrode active material layer density of 2.8 g / cm ³ .

[Negative electrode]
Natural graphite is used as the negative electrode active material, and styrene butadiene rubber (SBR) as the binder and carboxymethyl cellulose (CMC) as the thickener are blended so that the mass ratio of these materials is 98: 1: 1. And mixed with ion-exchanged water to prepare a slurry-like composition. This composition was applied to both sides of a 10 μm-thick copper foil as a current collector so that the total basis weight of both sides was about 18.0 mg / cm ² (based on solid content). This was dried and then pressed by a rolling press to produce a negative electrode sheet having a negative electrode active material layer density of 1.4 g / cm ³ .

[Production of evaluation cell]
Next, 1 mol / L of LiPF ₆ as a supporting salt was added to a non-aqueous solvent in which ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 3: 4: 3. In addition, cyclohexylbenzene (CHB) and biphenyl (BP) as gas generating additives were added so that each concentration was 2% by mass to prepare a nonaqueous electrolyte.

  The positive electrode sheet and the negative electrode sheet are laminated via two separators (a microporous sheet having a three-layer structure of PP / PE / PP and having a thickness of 20 μm), and wound in the longitudinal direction to form an electrode body. Produced. This electrode body was housed in a laminate-type container together with the non-aqueous electrolyte to construct a lithium ion secondary battery as an evaluation cell. The rated capacity of this evaluation cell is 50 mAh.

[conditioning]
The evaluation cell constructed as described above was allowed to stand for about 10 hours after the injection of the electrolyte, and was subjected to conditioning (initial charging) after the battery voltage became 2.0 V or higher. Here, conditioning was performed by repeating the operations of the following procedure 1 and procedure 2 three times.
[Procedure 1] Voltage between terminals at a constant current of 1 C (1 C means a current value that discharges a fully charged battery to a discharge end voltage in 1 hour, sometimes referred to as a discharge time rate). Is charged (CC charge) until it reaches 4.1V, and then rests for 5 minutes.
[Procedure 2] After Procedure 1, charge at a constant voltage for 1.5 hours (CV charge) and rest for 5 minutes.

[Capacity maintenance rate after 60 days storage]
The capacity maintenance ratio (capacity maintenance ratio after storage) was evaluated based on an evaluation cell adjusted to a predetermined charge state after conditioning. That is, the initial capacity of the evaluation cell adjusted to a predetermined charge state and the capacity after the evaluation cell adjusted to the predetermined charge state is stored for a predetermined time in a predetermined environment (hereinafter referred to as “post-storage capacity” as appropriate) From the ratio (capacity after storage) / (initial capacity), the capacity retention rate after storage is calculated by the following formula.
“Capacity maintenance ratio after storage (%)” = (capacity after storage) / (initial capacity) × 100

  Here, the “initial capacity” is a discharge capacity measured based on an evaluation cell adjusted to 4.1 V (SOC 100%) at 25 ° C. The “capacity after storage” is a discharge capacity measured based on an evaluation cell stored at 60 ° C. for 60 days after being adjusted to 4.1 V (SOC 100%) at 25 ° C. The “discharge capacity” is measured when discharging at a constant current of 1 C from 4.1 V to 3.0 V at 25 ° C., and then discharging at a constant voltage until the total discharge time is 2 hours. Integrated capacity (discharge capacity).

[Overcharge gas generation amount]
For the evaluation cell after conditioning (50 mAh), measure the amount of gas generated after CC charging to SOC145% at 1C, and calculate the amount of gas generated per cell capacity (ml / Ah). The amount of gas generated was evaluated. Specifically, after conditioning, the volume of the cell is measured by the Archimedes method, the volume of the cell is measured again by the same method after overcharging, and the volume of the cell after conditioning is subtracted from the volume of the cell after overcharging. The gas generation amount was calculated. The Archimedes method is a method in which a measurement object (in this example, a laminate-type lithium secondary battery) is immersed in a liquid medium (for example, distilled water or alcohol) to measure the buoyancy that the measurement object receives. Thus, the volume of the measurement object is obtained.

[Evaluation]
An evaluation cell using a positive electrode active material having a surface W concentration of 0 atomic% using a conductive material having a D50 of 10.3 μm, which is a level generally used conventionally, and a surface W concentration of 2.9 atoms The results of measuring the “capacity maintenance ratio after 60 days storage” and “overcharged gas generation amount” of the cell for evaluation using the positive electrode active material of% were extracted and shown in FIG.
From FIG. 5, it can be seen that the presence of W on the surface of the positive electrode active material greatly improves the capacity retention rate after 60 days storage of the cell. However, on the other hand, the amount of gas generated at the time of overcharging is greatly reduced, and there is a concern that the current interruption mechanism does not operate normally at the time of overcharging.
The measurement results of “capacity maintenance ratio after 60 days storage” for each evaluation cell are shown in Table 1, and the measurement results of “overcharge gas generation amount” are shown in Table 2, FIG. 6 and FIG.

  As shown in Table 1, the capacity retention rate after 60 days storage increases as the W concentration on the surface of the positive electrode active material increases. For example, when the W concentration is 0.9 atomic% or more, the capacity after storage It was confirmed that a maintenance rate of 80% or more can be achieved. It was also confirmed that the capacity retention rate after storage had no correlation with the average particle diameter (D50) of the conductive material.

On the other hand, as shown in Table 2 and FIG. 6, the amount of gas generated at the time of overcharge decreases rapidly due to the presence of W on the surface of the positive electrode active material, and the amount of gas generated as the W concentration increases. It was confirmed that there was less. Moreover, it has also confirmed that the gas generation amount at the time of overcharge was related to the average particle diameter (D50) of a electrically conductive material.
In more detail, FIG. 7 shows the amount of overcharge gas generated in a cell using a positive electrode active material having a surface W concentration of 0.9 atomic% or more that can achieve a capacity retention ratio of 80% or more after storage.
In order to operate the current interruption mechanism at the time of overcharge, it is preferable that a gas of at least about 20 ml / Ah, preferably about 30 ml / Ah can be generated. From such a viewpoint, the W concentration on the surface of the positive electrode active material can be about 0.9 atomic% to 2.9 atomic%, and should be about 0.9 atomic% to 2.1 atomic%. It was confirmed that it was more preferable. In addition, it was confirmed that D50 of the conductive material can be about 2.5 μm to 7.8 μm, and more preferably about 4.0 μm to 7.8 μm.

  As mentioned above, although this invention was demonstrated by suitable embodiment, such description is not a limitation matter and of course various modifications are possible.

  According to the technology disclosed herein, even when a positive electrode active material containing tungsten and a gas generating additive is used in the electrolyte, a sufficient gas generation amount can be ensured during overcharge, and storage characteristics. A lithium secondary battery having overcharge safety can be obtained.

1 Vehicle 10 Positive electrode sheet (positive electrode)
12 Positive Electrode Current Collector 14 Positive Electrode Active Material Layer 16 Positive Electrode Active Material Layer Non-Forming Part 20 Negative Electrode Sheet (Negative Electrode)
22 Negative electrode current collector 24 Negative electrode active material layer 26 Negative electrode active material layer non-formed portion 30 Current interruption mechanism 32 Deformed metal plate (first member)
33 Curved portion 34 Connection metal plate (second member)
35 Current collecting lead terminal 36 Junction point 38 Insulating case 40A, 40B Separator 50 Battery case 52 Case body 54 Lid 70 Positive electrode terminal 72 Negative electrode terminal 74 Positive electrode lead terminal 76 Negative electrode lead terminal 80 Winding electrode body 100 Lithium secondary battery

Claims (8)

A lithium secondary battery comprising a positive electrode including a positive electrode active material and a conductive material, a negative electrode including a negative electrode active material, and a non-aqueous electrolyte including a gas generating additive composed of a hydrocarbon compound,
The positive electrode active material is a granular lithium transition metal composite oxide containing tungsten (W) and having a layered structure,
The conductive material has a form of an aggregate in which primary particles are gathered,
The average particle size of the aggregate is in the range of less than 9 μm, and the amount of gas generated due to the decomposition of the gas generating additive in the presence of the aggregate is a gas per rated capacity of the battery. The generation amount is controlled to 20 ml / Ah or more ,
Here, the W is segregated on the surface of the lithium transition metal composite oxide, and the W concentration on the surface is 0.9 atomic% to 2.9 atomic%.   The lithium secondary battery according to claim 1, wherein an average particle diameter of the aggregate in the conductive material is 2.5 μm to 7.8 μm.   The lithium secondary battery according to claim 2, wherein an average particle diameter of the aggregate in the conductive material is 3.0 μm to 4.5 μm.   The lithium secondary battery according to any one of claims 1 to 3, wherein the conductive material is blended at a ratio of 1 part by mass to 20 parts by mass with respect to 100 parts by mass of the positive electrode active material.   The lithium secondary battery according to any one of claims 1 to 4, wherein the gas generating additive is blended in the electrolyte at a ratio of 0.01% by mass to 10% by mass. As at least a part of the lithium transition metal composite oxide, the general formula (1):
_{Li x Ni a Co b Mn c} M d W e O 2 (1)
(Here, M does not exist or is at least one selected from Zr, Nb, and Al, x satisfies 1.0 ≦ x ≦ 1.25, and a, b, c, d, e Satisfies a + b + c + d + e = 1 and 0.001 ≦ (d + e) ≦ 0.02, and at least one of a, b and c is greater than 0 and e>0);
The lithium secondary battery of any one of Claims 1-5 containing the lithium transition metal complex oxide represented by these. And a battery case that houses the positive electrode, the negative electrode, and the non-aqueous electrolyte,
The battery case prevents gas flow between the inside and outside of the case during normal use, and is deformed when the internal pressure in the case rises above a predetermined level, and the battery case is electrically connected between the positive electrode and the negative electrode. The lithium secondary battery according to any one of claims 1 to 6 , further comprising a current interrupting mechanism that disconnects the electrical connection. The lithium secondary battery according to any one of claims 1-7 comprising as a power source for driving the vehicle.

Images