JP7707974B2 - Electrode mixture and non-aqueous lithium-ion secondary battery - Google Patents

Description

This disclosure relates to an electrode mixture and a non-aqueous lithium-ion secondary battery.

Patent document 1 discloses a composite comprising a silicon material, a carbon layer covering the silicon material, and a cationic polymer layer having a cationic polymer covering the carbon layer.

Patent Document 2 discloses an anode material for a lithium storage battery, comprising a core containing silicon, the surface of the core having O _y SiH _x groups (wherein 1<x<3, 1≦y≦3, and x>y), the material having a positive zeta potential in the pH range of 3.5 to 9.5.

JP 2016-48628 A International Publication No. 2013/087780

There is a need to suppress the decrease in charge/discharge capacity of batteries, such as lithium-ion batteries, that occurs with cycling.

The present disclosure aims to provide an electrode mixture and a nonaqueous lithium-ion secondary battery that can suppress the decrease in charge/discharge capacity that accompanies the charge/discharge cycles of the battery.

The present inventors have found that the above object can be achieved by the following means:
Aspect 1
1. An electrode composite comprising: a first silicon-based active material, the silicon-based particles of which are at least partially coated with a cationic polymer; and a second silicon-based active material, the silicon-based particles of which are at least partially coated with an anionic polymer.
Aspect 2
2. The electrode mixture of claim 1, wherein the silicon-based particles are Si particles.
Aspect 3
The electrode mixture according to aspect 1 or 2, wherein the content ratio of the first silicon-based active material to the second silicon-based active material is 25:75 to 75:25.
Aspect 4
The electrode mixture according to any one of aspects 1 to 3, wherein the cationic polymer is at least one selected from the group including poly(diallyl dimethyl chloride), polyethyleneimine, polypyrrole, and polyaniline.
Aspect 5
The anionic polymer is at least one selected from the group consisting of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, polyacrylic acid, and polystyrene sulfonic acid.
Aspect 6
The electrode mixture according to any one of aspects 1 to 5, wherein the first silicon-based active material has a zeta potential in water of +30.0 mV or more.
Aspect 7
The electrode mixture according to any one of aspects 1 to 6, wherein the second silicon-based active material has a zeta potential in water of −15.0 mV or less.
Aspect 8
A negative electrode body comprising the electrode mixture according to any one of aspects 1 to 7.
Aspect 9
A non-aqueous lithium ion secondary battery comprising a positive electrode body, a separator, and the negative electrode body according to aspect 8, housed in this order in a battery case filled with an electrolyte.

The present disclosure provides an electrode mixture and a nonaqueous lithium-ion secondary battery that can suppress the decrease in charge/discharge capacity that accompanies the charge/discharge cycles of the battery.

FIG. 1 is a non-aqueous lithium-ion secondary battery 1 according to a first embodiment of the present disclosure. FIG. 2 is a graph showing the relationship between the content (%) of silicon-based active material B and the capacity retention (%) after 50 cycles of charge and discharge in the nonaqueous lithium ion secondary batteries of Examples 1 to 3, Comparative Example 1, and Reference Example 1.

The following describes in detail the embodiments of the present disclosure. Note that the present disclosure is not limited to the following embodiments, and can be modified in various ways within the scope of the disclosure.

《Electrode composite material》
The electrode mixture of the present disclosure contains a first silicon-based active material in which silicon-based particles are at least partially coated with a cationic polymer, and a second silicon-based active material in which silicon-based particles are at least partially coated with an anionic polymer.

In non-aqueous lithium-ion secondary batteries, in order to suppress the formation of an SEI with the electrolyte, it is possible to use a silicon-based active material in which the surface of the silicon-based negative electrode active material is coated with a cationic polymer.

However, such silicon-based active materials have zeta potentials of the same sign and therefore do not attract each other. Therefore, it is not possible to suppress mechanical deterioration of the electrode body caused by the expansion and contraction of the silicon-based active material accompanying the charging and discharging of the non-aqueous lithium-ion secondary battery. As a result, a decrease in charge and discharge capacity as the charge and discharge cycles progress is inevitable.

In contrast, the electrode mixture of the present disclosure contains a first silicon-based active material in which silicon-based particles are at least partially coated with a cationic polymer, and a second silicon-based active material in which silicon-based particles are at least partially coated with an anionic polymer.

In the electrolyte, the first silicon-based active material has a positive zeta potential, whereas the second silicon-based active material has a negative zeta potential. Therefore, when the electrode composite of the present disclosure is applied to an electrode body of a non-aqueous lithium-ion secondary battery, the first silicon-based active material and the second silicon-based active material attract each other in the electrolyte, thereby suppressing mechanical deterioration of the electrode body caused by the expansion and contraction of the silicon-based active material accompanying the charging and discharging of the non-aqueous lithium-ion secondary battery. This suppresses the decrease in charge and discharge capacity as the charge and discharge cycles proceed.

In the electrode mixture of the present disclosure, the content ratio (A:B) of the first silicon-based active material A to the second silicon-based active material B may be 25:75 to 75:25. Here, the content ratio is the ratio of the content (mass) of the first silicon-based active material to the content (mass) of the second silicon-based active material.

A:B may be 25:75, 30:70, 50:50, 70:30, or 75:25.

<First Silicon-Based Active Material>
The first silicon-based active material is a first silicon-based active material in which silicon-based particles are at least partially coated with a cationic polymer.

The silicon-based particles may be, for example, Si alloy particles or Si particles. The Si alloy particles may be, for example, an alloy of Si and one or more metals selected from the group consisting of Sn, Ti, Fe, Ni, Cu, Co, and Al.

The cationic polymer may be any cationic polymer that has low reactivity with the electrolyte under the operating conditions of the nonaqueous lithium ion secondary battery and has a positive zeta potential in the electrolyte.

The cationic polymer may be at least one selected from the group including, for example, poly(diallyl dimethyl chloride), polyethyleneimine, polypyrrole, and polyaniline.

The first silicon-based active material preferably has a zeta potential in water of +30.0 mV or more. In water, this means in pure water.

The zeta potential of the first silicon-based active material in water may be +30.0 mV or more and +60.0 mV or less. The zeta potential may be +30.0 mV or more, +33.0 mV or more, +36.0 mV or more, or +39.0 mV or more, and may be +60.0 mV or less, +50.0 mV or less, +45.0 mV or less, or +40.0 mV or less.

The "zeta potential" is also called the electrokinetic potential, and refers to the portion of the potential difference at the interface between a solid and a liquid that effectively affects the electrokinetic phenomenon. In the present invention, this zeta potential is the zeta potential measured by the laser Doppler method, which is an electrophoretic light scattering method.

<Second Silicon-Based Active Material>
The second silicon-based active material is a second silicon-based active material in which silicon-based particles are at least partially coated with an anionic polymer.

Examples of silicon-based particles include those described for the first silicon-based active material.

The anionic polymer may be any anionic polymer that has low reactivity with the electrolyte under the operating conditions of the nonaqueous lithium ion secondary battery and has a negative zeta potential in the electrolyte.

The anionic polymer may be at least one selected from the group including poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, polyacrylic acid, and polystyrene sulfonic acid.

The second silicon-based active material preferably has a zeta potential in water of -15.0 mV or less. In water, we mean in pure water.

The zeta potential of the second silicon-based active material in water may be -15.0 mV or less and -30.0 mV or more. The zeta potential in water may be -15.0 mV or less, -16.0 mV or less, -17.0 mV or less, or -18.0 mV or less, and may be -30.0 mV or more, -28.0 mV or more, -25.0 mV or more, or -20.0 mV or more.

Negative electrode body
The negative electrode body has a negative electrode current collector and a negative electrode active material. The negative electrode body may have a shape in which the surface of the negative electrode body, such as a layer or a rod, is partially or entirely covered with a negative electrode active material layer. The negative electrode active material layer contains the electrode mixture of the present disclosure.

The negative electrode active material layer may contain the electrode mixture of the present disclosure, and optionally a binder and a conductive additive, etc.

The negative electrode active material layer may be formed, for example, by applying and drying a slurry in which the electrode mixture of the present disclosure and an optional binder and conductive assistant are dispersed on a substrate. The substrate may be made of a conductive material such as a metal, and may also serve as a negative electrode current collector.

<Negative electrode current collector>
The material used for the negative electrode current collector may be stainless steel (SUS), aluminum, copper, nickel, iron, titanium, gold, platinum, zinc, carbon, etc. The negative electrode current collector may be made of any of these materials with nickel, chromium, carbon, etc. plated or vapor-deposited on the surface. The negative electrode current collector is preferably made of copper or a copper alloy, or a material having nickel, chromium, carbon, etc. plated or vapor-deposited on the surface of the copper or copper alloy.

The shape of the negative electrode current collector is not particularly limited and may be, for example, rod-shaped or layer-shaped.

<Binder>
The binder may be, for example, but is not limited to, a material such as a polyacrylic acid-based binder, polyvinylidene fluoride (PVdF), carboxymethyl cellulose (CMC), butadiene rubber (BR), or styrene butadiene rubber (SBR), or a combination thereof.

<Conductive assistant>
The conductive assistant may be a known one, for example, a carbon material, metal particles, etc. The carbon material may be at least one selected from the group consisting of carbon black such as Ketjen black, acetylene black, furnace black, vapor grown carbon fiber (VGCF), carbon nanotubes, and carbon nanofibers, and from the viewpoint of electronic conductivity, may be at least one selected from the group consisting of VGCF, carbon nanotubes, and carbon nanofibers. The metal particles may be nickel, copper, iron, stainless steel, etc.

<Non-aqueous lithium-ion secondary battery>
The nonaqueous lithium ion secondary battery of the present disclosure is a nonaqueous lithium ion secondary battery in which a positive electrode body, a separator, and a negative electrode body of the present disclosure are housed in this order in a battery case filled with an electrolyte.

Figure 1 is a schematic diagram of a nonaqueous lithium ion secondary battery 1 according to a first embodiment of the present disclosure. Note that Figure 1 is not intended to limit the nonaqueous lithium ion secondary battery of the present disclosure.

As shown in FIG. 1, the nonaqueous lithium ion secondary battery 1 according to the first embodiment of the present disclosure may have a structure in which a positive electrode layer 10 having a positive electrode collector layer 11 and a positive electrode active material layer 12 arranged on the positive electrode collector layer 11, a separator layer 20, and a negative electrode layer 30 having a negative electrode collector layer 31 and a negative electrode active material layer 32 arranged on the negative electrode collector layer 31 are housed in an exterior body 100 filled with an electrolyte 40. Here, the negative electrode layer 30 is the negative electrode body of the present disclosure. Note that FIG. 1 is not intended to limit the present disclosure.

<Positive electrode body>
The positive electrode body has a positive electrode current collector and a positive electrode active material. The positive electrode body may have a shape in which the surface of a layered or rod-shaped positive electrode body is partially or entirely covered with a layer containing the positive electrode active material, i.e., a positive electrode active material layer.

The material used for the positive electrode current collector may be any of the materials described for the negative electrode current collector. In particular, it is preferable that the material for the positive electrode current collector is aluminum.

The shape of the positive electrode current collector layer may also be the shape described for the negative electrode current collector.

The positive electrode active material layer may contain any positive electrode active material that can be used in lithium ion batteries.

Such a positive electrode active material may be, for example, lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMn ₂ O ₄ ), LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ , or a different element-substituted Li-Mn spinel having a composition represented by Li _1+x Mn _2-x-y M _y O ₄ (M is one or more metal elements selected from Al, Mg, Co, Fe, Ni, and Zn), but is not limited thereto.

In addition, such a positive electrode active material may be, for example, a sulfur-based active material. Here, the sulfur-based active material is an active material containing at least an S element. The sulfur-based active material may or may not contain an Li element. Examples of the sulfur-based active material include elemental sulfur, lithium sulfide (Li ₂ S), and lithium polysulfide (Li ₂ S _x , 2≦x≦8).

<Separator>
The separator may be a layer of a porous resin, such as a layer of polypropylene or polyethylene.

As the separator, a porous polymer membrane such as a porous polyethylene membrane (PE), a porous polypropylene membrane (PP), a porous polyolefin membrane, or a porous polyvinyl chloride membrane can be used. In addition, a lithium ion conductive polymer electrolyte membrane can also be used as the separator. These separators can be used alone or in combination. From the viewpoint of increasing the battery output, it is preferable to use a three-layer coated separator in which a porous polyethylene membrane (PE) is sandwiched between two layers of porous polypropylene membrane (PP) on the top and bottom.

<Negative electrode body>
The negative electrode body is the negative electrode body of the present disclosure.

<Electrolyte>
The electrolyte may be a composition in which a supporting salt is contained in a non-aqueous solvent. The non-aqueous solvent may be a material selected from the group consisting of an organic electrolyte, a fluorine-based solvent, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and a combination of two or more of these.

As the non-aqueous solvent, a fluorine-based solvent, for example, a fluorinated carbonate ester, is preferred. Specific examples of the fluorinated carbonate ester include methyl 2,2,2-trifluoroethyl carbonate (MFEC; carbonic acid, methyl 2,2,2-trifluoroethyl ester; CAS 156783-95-8) and/or difluorodimethyl carbonate (DFDMC), and a mixture of these in a volume ratio of 50:50 is particularly preferred.

Examples of the supporting salt include _materials selected from the group _{consisting of} LiPF6, _LiBF4 , LiClO4 _{, LiAsF6} , _LiCF3SO3 , _LiC4F9SO3 , LiN( _CF3SO2 ) ₂ , LiC( _CF3SO2 ) ₃ , lithium compounds ( _{lithium salts} ) of LiI, and combinations of _two or more of these. From the viewpoint of improving the battery voltage and durability, _LiPF6 is preferred as the supporting salt.

<Battery case>
The battery case can be made of a material that is inactive to the non-aqueous electrolyte, such as a resin.

Examples 1 to 3, Comparative Example 1, and Reference Example 1
Example 1
(Preparation of Silicon-Based Active Material A)
1 g of Si particles was dispersed in 1 L of a solution containing 0.5 wt % of cationic polymer poly(diallyldimethylchloride) (Sigma-Aldrich) using a homogenizer and stirred for 1 hour. The stirred solution was filtered and washed three times with 100 mL of water to remove excess polymer. The obtained powder was further vacuum dried at 60° C. for 24 hours to obtain silicon-based active material A.

The zeta potential of silicon-based active material A in water was +39.5 mV.

(Preparation of Silicon-Based Active Material B)
1 g of Si particles was dispersed in a 1 L solution containing 0.05 wt% of anionic polymer PEDOT:PSS (Sigma-Aldrich) using a homogenizer and stirred for 1 hour. The stirred solution was filtered and washed three times with 100 mL of water to remove excess polymer. The obtained powder was further vacuum dried at 60°C for 24 hours to obtain silicon-based active material B.

The zeta potential of silicon-based active material B in water was -19.5 mV.

(Adjustment of Positive Electrode Body)
NMP, a 5 wt% butyl butyrate solution of a PVdF-based binder, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ with an average particle size of 6 m as a positive electrode active material, and VGCF as a conductive additive were added to a screw tube bottle, and the mixture was stirred for 10 minutes with a Thinky Mixer (ARE-310, manufactured by Thinky Corporation). The mixture was applied onto an Al foil (manufactured by Showa Denko K.K.) by the blade method using an applicator, and dried on a hot plate at 80°C for 60 minutes.

(Adjustment of negative electrode body)
Water, a polyacrylic acid binder, silicon active materials A and B as negative electrode active materials, and VGCF and KB as conductive assistants were added to a screw tube bottle, and the mixture was stirred for 10 minutes with a mixer (ARE-310, manufactured by Thinky Corporation). The mixture was applied onto a Cu foil (manufactured by Furukawa Electric Co., Ltd.) by the blade method using an applicator, and dried on a hot plate at 80°C for 60 minutes.

The ratio of silicon-based active materials A and B was 75:25.

(Preparation of non-aqueous lithium-ion secondary battery)
The positive electrode body and the negative electrode body were punched out to have diameters of 14 mm and 16 mm, respectively, and then set in a roll press machine and pressed at 20 kN/cm. Thereafter, the positive electrode body was vacuum dried at 120° C. and the negative electrode body was vacuum dried at 60° C. for 24 hours.

The positive electrode body and the negative electrode body were moved to a glove box, ₁ ml of an electrolyte (1.2 M LiPF6 solution; solvent is FEC:EC:EMC:DMC (1:2:4:3 vol%)) was dropped onto the negative electrode body, a separator (made of polypropylene) was laminated, 1 ml of the electrolyte was again dropped, and then the positive electrode body was laminated, and a nonaqueous lithium ion secondary battery in the shape of a coin cell was produced using an automatic coin cell crimping machine (Hosen Co., Ltd.).

Example 2
A nonaqueous lithium ion secondary battery of Example 2 was fabricated in the same manner as in Example 1, except that the ratio of silicon-based active materials A and B was 50:50.

Example 3
A nonaqueous lithium ion secondary battery of Example 3 was fabricated in the same manner as in Example 1, except that the ratio of the silicon-based active materials A and B was set to 25:75.

Comparative Example 1
A nonaqueous lithium ion secondary battery of Comparative Example 1 was produced in the same manner as in Example 1, except that the silicon-based active material B was not used, that is, the ratio A:B of the silicon-based active materials A and B was 100:0.

<Reference example 1>
A nonaqueous lithium ion secondary battery of Reference Example 1 was produced in the same manner as in Example 1, except that the silicon-based active material A was not used, that is, the ratio A:B of the silicon-based active materials A and B was 0:100.

Charge/discharge cycle test
For the nonaqueous lithium ion secondary battery of each example, a charge-discharge test was performed 50 times in which a battery capacity when the battery was charged at a constant current of 0.1 C rate to 4.2 V and then discharged at a rate of 0.1 C to 2.5 V was defined as an initial capacity, and the battery capacity was determined by repeating a charge-discharge test in which the battery was charged at a constant current of 1 C rate to 4.2 V and then discharged at a constant current of 1 C, and the discharge capacity retention rate after 50 cycles relative to the initial capacity was calculated.

<result>
The results are shown in Table 1 and FIG.

As shown in Table 1 and FIG. 2, the nonaqueous lithium ion secondary batteries of Examples 1 to 3 using silicon-based active materials A and B had discharge capacity retention rates of 79.1%, 82.5%, and 83.0%, respectively, after 50 cycles. On the other hand, the nonaqueous lithium ion secondary battery of Comparative Example 1, which did not use silicon-based active material B, had a discharge capacity retention rate of 77.6% after 50 cycles. In other words, the nonaqueous lithium ion secondary batteries of Examples 1 to 3 using silicon-based active materials A and B had a higher discharge capacity retention rate after 50 cycles than the nonaqueous lithium ion secondary battery of Comparative Example 1, which did not use silicon-based active material B.

In addition, in Reference Example 1, which did not use silicon-based active material A, the discharge capacity retention rate after 50 cycles was 81.8%.

REFERENCE SIGNS LIST 1 non-aqueous lithium ion secondary battery 10 positive electrode layer 11 positive electrode current collector layer 12 positive electrode active material layer 20 separator layer 30 negative electrode layer 31 negative electrode current collector layer 32 negative electrode active material layer 40 electrolyte

Claims (10)

The present invention comprises a first silicon-based active material, the silicon-based particles of which are at least partially coated with a cationic polymer, and a second silicon-based active material, the silicon-based particles of which are at least partially coated with an anionic polymer,
The first silicon-based active material and the second silicon-based active material are different silicon-based active materials.
Electrode composite material. The electrode mixture according to claim 1, wherein the silicon-based particles are Si particles. The electrode mixture according to claim 1 or 2, wherein the content ratio of the first silicon-based active material to the second silicon-based active material is 25:75 to 75:25. The electrode mixture according to any one of claims 1 to 3, wherein the cationic polymer is at least one selected from the group including poly(diallyl dimethyl chloride), polyethyleneimine, polypyrrole, and polyaniline. The electrode mixture according to any one of claims 1 to 4, wherein the anionic polymer is at least one selected from the group including poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, polyacrylic acid, and polystyrene sulfonic acid. The electrode mixture according to any one of claims 1 to 5, wherein the first silicon-based active material has a zeta potential in water of +30.0 mV or more. The electrode mixture according to any one of claims 1 to 6, wherein the second silicon-based active material has a zeta potential in water of -15.0 mV or less. A negative electrode body containing the electrode mixture according to any one of claims 1 to 7. A non-aqueous lithium ion secondary battery in which a positive electrode body, a separator, and the negative electrode body according to claim 8 are housed in this order in a battery case filled with an electrolyte. The first silicon-based active material has a positive zeta potential in pure water, and the second silicon-based active material has a negative zeta potential in pure water. The electrode mixture according to any one of claims 1 to 7.