JP2015122159A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery arranged so that the occurrence of the unevenness in a salt concentration of a nonaqueous electrolyte can be suppressed.SOLUTION: A lithium ion secondary battery provided according to the present invention comprises: an electrode body including a positive electrode, a negative electrode and a separator; and a nonaqueous electrolyte. In the secondary battery, the negative electrode includes negative electrode active material particles. The negative electrode active material particles are composite particles formed by plating core particles with a metal. Further, the metal used for the metal plating is a metal which occludes no Li ion. In the composite particles, the percentage of metal plating is 11-35% based on a specific surface area.

Description

  The present invention relates to a lithium ion secondary battery.

  Lithium ion secondary batteries are preferably used as, for example, a high-output power source for driving a vehicle because they are lightweight and have a high energy density. The lithium ion secondary battery typically has a configuration including an electrode body including a positive electrode, a negative electrode, and a separator, and a non-aqueous electrolyte. As a document disclosing the prior art regarding this type of secondary battery, for example, Patent Document 1 is cited. Patent Document 1 is a technical document regarding a control device that controls the discharge so that the discharge power of the secondary battery does not exceed the upper limit value, and takes into account the uneven concentration of salt in the electrolyte solution, and high-rate charge / discharge It is disclosed that the deterioration due to the above (so-called high-rate deterioration) is suppressed.

JP 2013-225397 A

  As the cause of the high rate deterioration, for example, the following may be considered. That is, when high-rate charge / discharge is continuously performed, the electrolyte is discharged from the electrode body (particularly the negative electrode) due to the effects of the volume expansion of the electrolyte due to heat generation and the pump effect due to the expansion of the electrode body. As a result, the salt concentration in the electrolytic solution is uneven. It is considered that the resistance increases due to this salt concentration unevenness and high-rate deterioration occurs. That is, if the salt concentration unevenness can be suppressed, it can be an effective means for suppressing high rate deterioration. Therefore, the present inventor has intensively studied, and as a result, the occurrence of the salt concentration unevenness is caused by the self-heating of the battery (Joule heat). I knew that. Furthermore, as a result of various studies on means for reducing the electronic resistance in the negative electrode, an effective means capable of suppressing the occurrence of the salt concentration unevenness has been found, and the present invention has been completed.

  The objective of this invention is providing the lithium ion secondary battery which can suppress generation | occurrence | production of the salt concentration nonuniformity of a non-aqueous electrolyte.

In order to achieve the above object, the present invention provides a lithium ion secondary battery comprising an electrode body including a positive electrode, a negative electrode, and a separator, and a non-aqueous electrolyte. In the secondary battery, the negative electrode includes negative electrode active material particles. The negative electrode active material particles are composite particles formed by performing metal plating on the core particles. Furthermore, the metal used for the metal plating is a lithium ion non-occlusion metal. And the metal plating rate on the basis of the specific surface area in the said composite particle is 11%-35%. Here, the metal plating rate based on the specific surface area is defined as A [m ² / g], which is the specific surface area of the core particles before plating treatment (BET specific surface area; the same applies hereinafter), and the specific surface area of the composite particles after plating treatment. When B [m ² / g], it is a value obtained from the formula: metal plating rate (%) = (A−B) / A × 100;
According to such a configuration, by using negative electrode active material particles that have been subjected to metal plating treatment at a predetermined plating rate, the electron resistance of the negative electrode is significantly increased while ensuring a good reaction region with Li ions in the active material. To reduce. Due to this reduction in resistance, the self-heating of the battery is kept low, and the occurrence of uneven salt concentration in the non-aqueous electrolyte due to the self-heating is suppressed.

  Moreover, according to this specification, the negative electrode active material for lithium ion secondary batteries is provided. This active material is composite particles formed by performing metal plating on the core particles. The metal used for the metal plating is a Li ion non-occlusion metal. And the metal plating rate on the basis of the specific surface area in the said composite particle is 11%-35%.

  The lithium ion secondary battery disclosed herein (hereinafter sometimes abbreviated as “secondary battery”) suppresses the occurrence of uneven salt concentration in the non-aqueous electrolyte, and therefore is durable against charge / discharge at a high rate. Excellent (high-rate cycle durability). Therefore, taking advantage of this feature, it can be suitably used as a drive power source for vehicles such as hybrid vehicles (HV), plug-in hybrid vehicles (PHV), electric vehicles (EV) and the like. According to the present invention, there is provided a vehicle equipped with the secondary battery disclosed herein (which may be in the form of an assembled battery in which a plurality of batteries are connected).

It is a schematic cross section of a lithium ion secondary battery according to an embodiment.

  Hereinafter, preferred embodiments according to the lithium ion secondary battery will be described. It should be noted that the dimensional relationships (length, width, thickness, etc.) in the figure do not reflect actual dimensional relationships. Further, 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 for 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.

  As shown in FIG. 1, the secondary battery 100 includes a battery case 10 and a wound electrode body 20 accommodated in the battery case 10. The battery case 10 has an opening 12 on the upper surface, and the opening 12 is sealed by the lid 14 after the wound electrode body 20 is accommodated in the battery case 10 from the opening 12. A non-aqueous electrolyte solution 25 is also accommodated in the battery case 10. The lid body 14 is provided with an external positive terminal 38 and an external negative terminal 48 for external connection, and a part of the terminals 38 and 48 protrudes to the surface side of the lid body 14. A part of the external positive terminal 38 is connected to the internal positive terminal 37 inside the battery case 10, and a part of the external negative terminal 48 is connected to the internal negative terminal 47 inside the battery case 10. These internal terminals 37 and 47 are connected to the positive electrode 30 and the negative electrode 40 constituting the wound electrode body 20, respectively.

  The wound electrode body 20 includes a long sheet-like positive electrode (positive electrode sheet) 30 and a long sheet-like negative electrode (negative electrode sheet) 40. The positive electrode sheet 30 includes a long positive electrode current collector 32 and a positive electrode active material layer 34 formed on at least one surface (typically both surfaces) thereof. The negative electrode sheet 40 includes a long negative electrode current collector 42 and a negative electrode active material layer 44 formed on at least one surface (typically both surfaces) thereof. The wound electrode body 20 also includes two long sheet-like separators (separator sheets) 50A and 50B. The positive electrode sheet 30 and the negative electrode sheet 40 are laminated via two separator sheets 50A and 50B. The laminated body is formed into a wound body by being wound in the longitudinal direction, and is further formed into a flat shape by crushing the rolled body from the side surface direction and causing it to be ablated. The electrode body is not limited to a wound electrode body. Depending on the shape of the battery and the purpose of use, an appropriate shape and configuration, such as a laminate mold, can be employed as appropriate.

  Next, each component constituting the secondary battery will be described. As the positive electrode current collector constituting the positive electrode of the secondary battery, for example, a foil-like material made of aluminum or an alloy containing aluminum as a main component can be used. In addition to the positive electrode active material, the positive electrode active material layer can contain additives such as a conductive material and a binder (binder) as necessary.

The positive electrode active material is not particularly limited, and one or more of the positive electrode active materials used for the lithium ion secondary battery can be used. For example, a lithium transition metal composite oxide having a spinel structure or a layered structure, a polyanion type (eg, olivine type) lithium transition metal compound, or the like can be used. In a preferred embodiment, a lithium transition metal composite oxide containing Li and at least one transition metal element (preferably at least one of Ni, Co, and Mn) is used as the positive electrode active material. Specific examples include LiNiO ₂ , LiCoO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O _{2 and the} like. The proportion of the positive electrode active material in the positive electrode active material layer is preferably about 70 to 97% by weight (for example, 75 to 95% by weight).

  As the conductive material, for example, carbon black such as acetylene black (AB) can be preferably used. Examples of the binder include various polymer materials. For example, one or more of a halogenated vinyl resin such as polyvinylidene fluoride (PVdF); a cellulose polymer such as carboxymethyl cellulose (CMC); a rubber such as styrene butadiene rubber (SBR); In addition, the said binder may be used as a thickener other additives of the composition for positive electrode active material layer forming.

The basis weight of the positive electrode active material layer on the positive electrode current collector is not particularly limited. From the viewpoint of securing a good conductive path, for example, about 3 to 45 mg / cm ² (for example, 12 to 12 per side) of the positive electrode current collector. 25 mg / cm ² ) is preferable.

  As the negative electrode current collector constituting the negative electrode, for example, a foil-like one made of copper or an alloy containing copper as a main component can be used. The shape of the negative electrode current collector is not particularly limited, and may be, for example, a sheet shape, a foil shape, a mesh shape, or the like.

  The negative electrode active material layer includes negative electrode active material particles. The negative electrode active material particles are composite particles formed by performing metal plating on the core particles. In other words, the composite particle is composed of a core particle and a metal plating part that covers a part of the surface of the core particle. In the composite particles, the metal plating rate based on the specific surface area is in the range of 11% to 35%. When the metal plating rate is 11% or more, the electronic resistance is significantly lowered, the self-heating of the battery is suppressed to a low level, and the occurrence of uneven salt concentration in the non-aqueous electrolyte due to the self-heating is suppressed. As a result, the high rate cycle durability is improved. Moreover, when the metal plating rate is 35% or less, a reaction region with Li ions in the active material can be secured satisfactorily. The metal plating rate is preferably 15% or more (for example, 18% or more, typically 25% or more), and preferably 34% or less (for example, 32% or less, typically 30% or less).

As said core particle, the 1 type (s) or 2 or more types of the substance conventionally used as a negative electrode active material particle of a lithium ion secondary battery can be used. Examples of such particles include carbon materials (carbon particles) such as graphite carbon (graphite) and amorphous carbon. A particulate carbon material containing a graphite structure (layered structure) at least partially is preferably used. Of these, the use of a carbon material mainly composed of natural graphite is preferred. The natural graphite may be obtained by spheroidizing flaky graphite. Alternatively, a carbonaceous powder having a graphite surface coated with amorphous carbon may be used. In addition, as core particles, oxides such as lithium titanate, silicon materials, simple substances such as tin materials, alloys, compounds, and composite materials using the above materials in combination can be used. Although the specific surface area of the core particles is not particularly limited, from the viewpoint of the energy density and the non-aqueous electrolyte impregnation, etc., 1 to 10 m ² / g (e.g., 2 to 5 m ² / g, typically 2.5～4M ² / G). The particle size of the core particles is suitably about 1 to 30 μm (typically 3 to 20 μm), for example. In this specification, “particle size” means a particle size corresponding to 50% cumulative from the fine particle side in a volume-based particle size distribution measured by particle size distribution measurement based on a general laser diffraction / light scattering method. D ₅₀ particle diameter, also referred to as median diameter).

  A metal used for metal plating (in other words, a metal constituting the metal plating portion) is typically a Li ion non-occlusion conductive material. When a Li ion occlusion metal is used as the plating material, the metal plating portion expands during Li ion occlusion and there is a possibility that the detachment of the plating occurs. By using a Li ion non-occlusion metal as a plating material, detachment of the metal from the core particles is prevented. Moreover, since the said metal is an electroconductive substance, electronic resistance can be reduced favorably. Examples of Li ion non-occlusion metals include Ni, Cu, Mn, Fe, Co, Zn, Ag, Au, In, and metal elements belonging to Groups 4, 5, and 6 of the periodic table. Of these, Ni and Cu are preferable, and Ni is more preferable.

  The method of metal plating is not particularly limited, and either a wet plating method or a dry plating method can be employed. Examples of the wet plating method include an electroless plating method and an electroplating method. Examples of the dry plating method include a vacuum deposition method, a sputtering method, a CVD method, and a PVD method. Of these, the wet plating method is preferable, and the electroless plating method is more preferable. When carbon particles such as natural graphite are employed as the core particles, the electroless plating method is particularly suitable. The specific conditions of the above method may be set as appropriate based on common technical knowledge so that the metal plating rate falls within a predetermined range.

  As a preferred embodiment of the technology disclosed herein, nickel plating by an electroless plating method will be described. In this method, first, a plating solution containing a nickel salt such as nickel sulfate is prepared. The plating solution may typically contain a reducing agent such as sodium hypophosphite, tetrahydroborate, dimethylaminoborane. The particles to be plated (core particles) are charged into the plating solution and stirred. In order to form a good metal film, it is preferable that an activation treatment such as immersion in palladium chloride is performed on the core particles in advance. The plating conditions (Ni concentration, reducing agent concentration, temperature, stirring time, pH, etc. in the plating bath) may be appropriately set so as to obtain a desired metal plating rate. For example, the plating temperature is preferably in the range of about 60 to 100 ° C. By appropriately washing and drying after plating, composite particles subjected to metal plating can be obtained.

The specific surface area of the composite particles is 0.5 to 9 m ² / g (for example, 1 to 4 m ² / g, typically 1.5 to 3 m ² / g) from the viewpoint of reducing electronic resistance and securing the Li ion reaction region. g) is preferable.

  The negative electrode active material layer may contain one or two or more binders, thickeners, and other additives as needed in addition to the negative electrode active material particles. As the binder, for example, a material that can be contained in the positive electrode active material layer can be preferably used. The proportion of the negative electrode active material particles in the negative electrode active material layer is suitably about 90 to 99% by weight (typically 97 to 99% by weight). The proportion of the additive in the negative electrode active material layer is suitably about 0.8 to 10% by weight (typically 1 to 3% by weight).

The basis weight of the negative electrode active material layer on the negative electrode current collector is not particularly limited. From the viewpoint of securing a good conductive path, for example, ^{2 to} 40 mg / cm ² (for example, 5 to 14 mg) per one side of the negative electrode current collector. / Cm ² ). The density of the negative electrode active material layer is not particularly limited, and is preferably about 1.0 to 3.0 g / cm ³ (for example, 1.2 to 2.0 g / cm ³ ), for example.

  The separator disposed so as to separate the positive electrode and the negative electrode may be a member that insulates the positive electrode active material layer and the negative electrode active material layer and allows the electrolyte to move. Preferable examples of the separator include those made of a porous polyolefin resin (typically polyethylene (PE) or polypropylene (PP)). The separator may be provided with a heat-resistant layer composed of an inorganic filler such as alumina. When a solid (gel-like) electrolyte in which a polymer is added to the electrolytic solution, for example, is used instead of the electrolytic solution, the separator itself is not necessary because the electrolyte itself can function as a separator. It can happen.

The non-aqueous electrolyte contained in the secondary battery can include at least a non-aqueous solvent and a supporting salt. Examples of the non-aqueous solvent include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and the like. These can be used individually by 1 type or in mixture of 2 or more types. Examples of the supporting salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiI and the like. One or more of the lithium salts can be used. The concentration of the supporting salt is not particularly limited, but can be about 0.1 to 5 mol / L.

  Next, some examples relating to the present invention will be described, but the present invention is not intended to be limited to those shown in the examples. In the following description, “parts” and “%” are based on weight unless otherwise specified.

<Preparation of negative electrode active material particles>
(Negative electrode active material particles A to D)
Natural graphite powder having a specific surface area shown in Table 1 was used as negative electrode active material particles AD.

(Negative electrode active material particles A-NiP-1 to 5, B-NiP-1 to 5)
Negative electrode active material particles A or B were used as core particles. The active material core particles were immersed in a 1M palladium chloride aqueous solution for a whole day and night, and then collected by filtration. The core particles were subjected to an electroless nickel plating bath using sodium hypophosphite as a reducing agent by a conventional method, and stirred at 80 ° C. to obtain negative electrode active material particles subjected to Ni-P plating. . The negative electrode active material particles are composite particles obtained by applying Ni-P plating to natural graphite particles as core particles. In addition, the metal plating rate in each negative electrode active material particle was adjusted by changing the stirring time. Table 1 shows the specific surface area and metal plating rate of each negative electrode active material particle.

(Negative electrode active material particles A-Sn-1 to 4)
Negative electrode active material particles A were used as core particles. The core particles were immersed in a 1M palladium chloride aqueous solution for a whole day and night, and then collected by filtration. The core particles were subjected to an electroless Cu plating bath and stirred at 80 ° C. to perform Cu plating by a conventional method. Next, the recovered particles were immersed in a substitutional Sn plating bath by a conventional method to perform substitution from Cu to Sn to obtain negative electrode active material particles subjected to Sn plating. The negative electrode active material particles are composite particles obtained by performing Sn plating on natural graphite particles as core particles. In addition, the metal plating rate in each negative electrode active material particle was adjusted by changing the stirring time. Table 1 shows the specific surface area and metal plating rate of each negative electrode active material particle.

(Negative electrode active material A + NiP, B + NiP)
NiP particles having a Ni: P ratio of 93: 7 were prepared. By adding and mixing the NiP particles to the negative electrode active material particles A and B so as to have a ratio of 0.6%, a negative electrode active material to which NiP particles were added was obtained.

<Examples 1-20>
[Preparation of positive electrode sheet]
Lithium cobaltate as a positive electrode active material, AB as a conductive material, and PVdF as a binder are mixed with N-vinylpyrrolidone so that the weight ratio of these materials is 93: 4: 3, and pasty A composition was prepared. This composition was uniformly applied to both sides of a long sheet-like aluminum foil (thickness 20 μm), dried, and then compressed to prepare a positive electrode sheet. The basis weight per one surface (solid content basis) of the positive electrode active material layer in this positive electrode sheet was 15 mg / cm ² (solid content basis).

[Preparation of negative electrode sheet]
A paste prepared by mixing negative electrode active material particles shown in Table 1, SBR as a binder, and CMC as a thickener with ion-exchanged water so that the weight ratio of these materials is 98: 1: 1. A shaped composition was prepared. This composition was uniformly applied to both sides of a long sheet-like copper foil (thickness: 10 μm), dried, and then compressed to prepare negative electrode sheets according to each example. In this negative electrode sheet, the basis weight per unit surface (solid content basis) of the negative electrode active material layer was 8.5 mg / cm ² , and the density was 1.3 g / cm ³ .

[Production of prismatic lithium-ion secondary battery]
The produced positive electrode sheet and the negative electrode sheet according to each of the above examples were laminated so that two separator sheets with a heat-resistant layer were arranged one by one between the positive electrode sheet and the negative electrode sheet, and this laminate was wound. Thus, a wound body was produced. Then, the wound electrode body was pressed from the side surface direction and ablated to produce a flat wound electrode body. The separator sheet was disposed so that the heat-resistant layer faced the positive electrode sheet. As the separator sheet, a long sheet-like single layer film (thickness: 16 μm) made of PE having an Al ₂ O ₃ heat-resistant layer (thickness: 3 μm) formed on one side thereof was used.
For the wound electrode body, electrode terminals were joined to the ends of the positive and negative electrode current collectors, respectively, and housed in an aluminum prismatic battery case together with a non-aqueous electrolyte, and then the battery case was sealed. As the non-aqueous electrolyte, a mixed solvent containing EC, EMC, and DEC at a volume ratio of 3: 5: 2 and containing LiPF ₆ as a supporting salt at a concentration of about 1 mol / L was used. . Thus, the prismatic lithium ion secondary battery (theoretical capacity: 25 Ah) according to each example was produced.

[High-rate cycle durability]
The high-rate cycle durability of the secondary battery according to each example was evaluated by counting the number of cycles when the resistance increased to 120% in the high-rate charge / discharge cycle. Specifically, the secondary battery according to each example was charged at 5C for 30 seconds under a temperature condition of 0 ° C., and adjusted to a SOC 40% charge state. Thereafter, the battery was discharged at 1C, the voltage 10 seconds after the start of discharge was measured, and the initial IV resistance value (mΩ) was determined. After the battery was adjusted to 40% SOC under the same conditions, the above charge / discharge was repeated 1000 cycles with the same charge / discharge as described above as one cycle. In each cycle, the IV resistance value (mΩ) is measured in the same manner as the measurement of the initial IV resistance value.
(IV resistance after cycle) / (initial IV resistance) × 100
The resistance increase rate (%) before and after the charge / discharge cycle was determined, and the number of cycles when the resistance increase rate (%) reached 120% was recorded. The results are shown in Table 1.

[Low temperature resistance]
An IV resistance test was performed on the secondary battery according to each example. Charging was performed in an environment at a temperature of 25 ° C. to adjust the SOC to 60%. Thereafter, pulse discharge was performed for 10 seconds at a current of 10 C at −15 ° C., and the IV resistance value (mΩ) was determined from the amount of voltage drop 10 seconds after the start of discharge. The obtained value was converted into a relative value with the IV resistance value (mΩ) of Example 2 as 100. In this relative value, it shows that resistance is so large that a value is large. The results are shown in Table 1.

  As shown in Table 1, negative electrode active material particles that were subjected to metal plating treatment using a Li ion non-occlusion metal (specifically, Ni) to a plating rate of 11% to 35% were used. In Examples 6 to 8 and Examples 11 to 13, Examples 1 to 4 using the negative electrode active material particles not subjected to the metal plating treatment, and negative electrode active material particles having a metal plating rate of less than 11% or more than 35% As compared with Examples 5, 9, 10, and 14 using the low temperature resistance, the low temperature resistance was significantly reduced and the high rate cycle durability was improved. From this result, it can be seen that by setting the metal plating rate in the range of 11% to 35% in the negative electrode active material particles, the electronic resistance was reduced and the salt concentration unevenness of the non-aqueous electrolyte was suppressed. On the other hand, in Examples 15 to 18 using the negative electrode active material particles subjected to metal plating with a Li ion storage metal (specifically, Sn), the low-temperature resistance tended to improve, but the high-rate cycle durability was observed. Sex declined. This is presumably because Sn was desorbed from the active material during charge / discharge at a high rate. In Examples 19 to 20 using the negative electrode active material to which a Li ion non-occlusion metal was added and mixed, the low temperature resistance and the high rate cycle durability could not be significantly improved. In this method, it is considered that the metal particles were not uniformly dispersed and the electronic resistance could not be reduced.

  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 Electrode body (winding electrode body)
25 Nonaqueous electrolyte 30 Positive electrode 40 Negative electrode 50A, 50B Separator 100 Lithium ion secondary battery

Claims (1)

An electrode body including a positive electrode, a negative electrode, and a separator, and a non-aqueous electrolyte,
The negative electrode includes negative electrode active material particles,
The negative electrode active material particles are composite particles formed by performing metal plating on the core particles,
The metal used for the metal plating is a lithium ion non-occlusion metal,
The lithium ion secondary battery in which the metal plating rate based on the specific surface area of the composite particles is 11% to 35%.

Images