Negative electrode for nonaqueous electrolyte secondary battery

Abstract

A negative electrode for a nonaqueous electrolyte secondary battery includes, in order, a current collector, a first active material layer, and a second active material layer. The negative electrode for a nonaqueous electrolyte secondary battery further includes carbon nanotubes. The carbon nanotubes are biased toward an interface between the first active material layer and the second active material layer.

Description

Technical Field

[0001] This invention relates to a negative electrode for a non-aqueous electrolyte secondary battery. Background Technology

[0002] U.S. Patent Application Publication No. 2021 / 0391570 discloses a negative electrode active material layer with a double-layer structure and each layer having a different composition. Summary of the Invention

[0003] For example, by stacking layers of different active materials, it is hoped that various properties can be improved. However, conversely, due to the different compositions of the layers, a mismatch in expansion and contraction may occur between the layers. This mismatch in expansion and contraction can lead to interlayer delamination.

[0004] The purpose of this invention is to prevent interlayer delamination in multilayer structures.

[0005] The technical structure and effects of the present invention will be described below. However, the mechanism of action includes speculation. The mechanism of action does not limit the technical scope of the present invention.

[0006] The negative electrode for a non-aqueous electrolyte secondary battery according to one aspect of the present invention comprises, in sequence, a current collector, a first active material layer, and a second active material layer.

[0007] Carbon nanotubes are also used as negative electrodes in non-aqueous electrolyte secondary batteries.

[0008] The carbon nanotubes are located at the interface between the first and second active material layers.

[0009] Hereinafter, "negative electrode for non-aqueous electrolyte secondary batteries" can be simply referred to as "negative electrode". Figure 1 This is a conceptual cross-sectional view showing the first layered structure in this embodiment. The negative electrode 20 sequentially includes a current collector 21, a first active material layer 22a, and a second active material layer 22b. To prevent interlayer delamination, for example, an adhesive layer 22d is considered to be introduced between the first active material layer 22a and the second active material layer 22b. However, the adhesive may hinder ion diffusion and electron conduction. By introducing the adhesive layer 22d, the resistance may increase.

[0010] Figure 2This is a conceptual cross-sectional view illustrating the second stacked structure in this embodiment. In the second stacked structure, carbon nanotubes (CNTs) are offset between the first active material layer 22a and the second active material layer 22b. The CNTs are fibrous. The CNTs can entangle with both the particles contained in the first active material layer 22a and the particles contained in the second active material layer 22b. Therefore, it is expected that the CNTs will hinder interlayer delamination. Moreover, the CNTs can form long-distance electron conduction paths. By forming electron conduction paths connecting the first active material layer 22a and the second active material layer 22b through the CNTs, it is also expected that the electrical resistance can be reduced.

[0011] The negative electrode for a non-aqueous electrolyte secondary battery described above may, for example, include the following configuration: A first active material layer contains a first negative electrode active material.

[0012] The second active material layer contains the second negative electrode active material.

[0013] The second negative electrode active material has a different composition than the first negative electrode active material.

[0014] For example, because the composition of the negative electrode active material differs between the first and second active material layers, mismatches may occur in the amount and timing of expansion and contraction during charging and discharging. For instance, the content of Si-based active materials may differ between the first and second active material layers.

[0015] The negative electrode for a non-aqueous electrolyte secondary battery described above may, for example, include the following configuration: A first active material layer contains a first negative electrode active material.

[0016] The second active material layer contains the second negative electrode active material.

[0017] The second negative electrode active material has a different orientation angle than the first negative electrode active material. The orientation angle represents the acute angle formed by the major axis diameter of the particle and the surface of the current collector.

[0018] The orientation angle is an indicator of orientation. It is believed that the closer the orientation angle is to 90°, the more the particles are oriented along the thickness direction of the active material layer. For example, because the orientation of the negative electrode active material differs between the first and second active material layers, mismatches may occur in the amount and direction of expansion and contraction during charging and discharging. For instance, the orientation of natural graphite and artificial graphite can differ between the first and second active material layers.

[0019] The negative electrode for a non-aqueous electrolyte secondary battery described above may, for example, include the following configuration: The second active material layer has a different porosity than the first active material layer.

[0020] For example, due to the difference in porosity between the first and second active material layers, a mismatch may occur in the amount of expansion and contraction during charging and discharging. For example, the second active material layer (upper layer) may have a relatively higher porosity than the first active material layer (lower layer). By making the upper layer relatively more porous, it is expected that electrolyte diffusion can be promoted and resistance reduced.

[0021] The negative electrode for a non-aqueous electrolyte secondary battery according to one aspect of the present invention comprises, in sequence, a current collector, a first active material layer, and a second active material layer.

[0022] The negative electrode for non-aqueous electrolyte secondary batteries also includes carbon nanotubes and binders.

[0023] The carbon nanotubes are located at the interface between the first and second active material layers.

[0024] The second active material layer has a different porosity than the first active material layer.

[0025] The first active material layer contains the first negative electrode active material.

[0026] The second active material layer contains the second negative electrode active material.

[0027] The second negative electrode active material has a different composition than the first negative electrode active material.

[0028] The second negative electrode active material has a different orientation angle than the first negative electrode active material.

[0029] The orientation angle represents the acute angle formed by the major axis diameter of the particle and the surface of the collector.

[0030] Adhesives include styrene-butadiene rubber, carboxymethyl cellulose, and acrylic polymers.

[0031] Styrene-butadiene rubber is biased towards the first active material layer.

[0032] Carboxymethyl cellulose is located in the second active substance layer.

[0033] Acrylic polymers are located at the interface between the first and second active material layers.

[0034] Figure 3This is a conceptual cross-sectional view illustrating the third layered structure in this embodiment. For example, at the interface between the first active material layer 22a and the second active material layer 22b, in addition to CNTs, an acrylic polymer (adhesive) can be introduced. The CNTs and the acrylic polymer can form an interlayer 22c. Compared to styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC), the acrylic polymer can have higher ion permeability. Through the synergistic effect of the acrylic polymer and CNTs, it is expected that peel strength can be improved.

[0035] Hereinafter, one embodiment of the present invention (hereinafter referred to as "this embodiment") will be described. However, this embodiment does not limit the technical scope of the present invention. This embodiment is illustrative in all respects. This embodiment is not limited. The technical scope of the present invention includes all modifications within the meaning and scope equivalent to the claims. For example, it is also intended to extract arbitrary configurations from this embodiment and combine them arbitrarily. Attached Figure Description

[0036] Hereinafter, with reference to the accompanying drawings, the features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described, in which the same reference numerals denote the same elements, and wherein:

[0037] Figure 1 This is a conceptual cross-sectional view representing the first layer structure in this embodiment.

[0038] Figure 2 This is a conceptual cross-sectional view representing the second layer structure in this embodiment.

[0039] Figure 3 This is a conceptual cross-sectional view representing the third layer structure in this embodiment.

[0040] Figure 4 This is a cross-sectional view illustrating the concept of the orientation angle in this embodiment.

[0041] Figure 5 This is a conceptual diagram illustrating an example of a non-aqueous electrolyte secondary battery in this embodiment. Detailed Implementation

[0042] Terms and Terminology

[0043] The terms "possessing," "including," and "having," and their variations, are open-ended expressions. A composition expressed in an open-ended manner may include additional elements besides the essential ones, or it may not include additional elements. The statement "composed of..." is a closed-ended expression. However, even a composition expressed in a closed-ended manner may include commonly impurities or additional elements unrelated to the target technology. The statement "substantially composed of..." is a semi-closed-ended expression. In a semi-closed-ended expression, it is permissible to add elements that do not substantially affect the fundamental and novel characteristics of the target technology.

[0044] Geometric terms should not be interpreted in a strict sense. As geometric terms, they may represent, for example, "parallel," "perpendicular," and "orthogonal." For instance, directions, angles, and distances can be relative displacements within a range that achieves substantially the same or similar function. Geometric terms may also include tolerances and errors in design, operation, and manufacturing. Dimensional relationships in drawings may sometimes differ from actual dimensional relationships. To aid understanding, dimensional relationships in drawings may be altered. For example, length, width, and thickness may be changed. Sometimes, certain components may be omitted.

[0045] Unless otherwise specified, the singular form is not limited to a single (1) element. The singular form can also include multiple elements.

[0046] "Artificial graphite" refers to graphite synthesized through artificial heat treatment. Artificial graphite is also called synthetic graphite (SG). "Natural graphite" refers to graphite that occurs naturally. Natural graphite includes flake graphite. Flake graphite can be curled by bending the ends of the particles. "Si-based active materials" include Si (or Si compounds, Si alloys, etc.) capable of alloying with Li. Si-based active materials may include, for example, at least one selected from the group consisting of pure Si, Si-based alloys, SiO, and Si-C. "Si-C" refers to composite particles containing at least one of Si and SiO and carbon. For example, Si can be loaded onto carbon materials.

[0047] "Aspect ratio" refers to the ratio of the longer side to the shorter side of the minimum bounding rectangle (MBR). It is calculated using the arithmetic mean of the aspect ratios of 100 randomly selected particles.

[0048] Figure 4This is a cross-sectional view illustrating the concept of the orientation angle in this embodiment. The "orientation angle" is the acute angle "θ" formed by the major axis diameter "L" of a particle (e.g., natural graphite) and the surface of the current collector 21 in a cross-sectional scanning electron microscope (SEM) image of the negative electrode 20. The orientation angle can take values ​​from 0° to 90°. The major axis diameter represents the maximum Feret diameter of the particle. The arithmetic mean of the orientation angles of 100 randomly selected particles is used.

[0049] "Basis Weight" refers to the mass per unit area.

[0050] Porosity is determined using image analysis. The cross-sectional SEM images of each layer are binarized to distinguish between pores and solids. The porosity is calculated by dividing the total area of ​​the pores (total pixels) by the total area of ​​the layer.

[0051] The "active substance layer" is the collective term for the first and second active substance layers.

[0052] Non-aqueous electrolyte secondary battery

[0053] Figure 5 This is a conceptual diagram illustrating an example of a non-aqueous electrolyte secondary battery according to this embodiment. The non-aqueous electrolyte secondary battery 100 may include, for example, a power generation element 50 and a housing 60. The housing 60 houses the power generation element 50. The housing 60 may be, for example, a metal container, a bag made of aluminum laminate, etc. The power generation element 50 may include, for example, a positive electrode 10, a negative electrode 20, a separator 30, and a non-aqueous electrolyte (not shown). The power generation element 50 may be, for example, a wound type or a stacked type. The power generation element 50 may have a unipolar structure or a bipolar structure. The separator 30 electrically isolates the positive electrode 10 from the negative electrode 20. The separator 30 may, for example, include a porous membrane made of resin. The non-aqueous electrolyte is a Li ion conductor. The non-aqueous electrolyte may, for example, include Li salts and organic solvents. The non-aqueous electrolyte may, for example, be impregnated in the separator 30. The non-aqueous electrolyte may, for example, include a polymer electrolyte, a gel electrolyte, or a solid electrolyte. The solid electrolyte may, for example, include a sulfide solid electrolyte, an oxide solid electrolyte, a halide solid electrolyte, etc. The positive electrode 10 contains a positive electrode active material. This active material may include, for example, lithium transition metal composite oxides (Li[NiCoMn]O2), lithium phosphate compounds (Li[FeMn]PO4), etc. Furthermore, the description of [NiCoMn], [FeMn], etc., indicates that the total composition ratio of the components within [ ] is "1". As long as the total is 1, the composition ratio of each component is arbitrary. For example, the composition ratio of some components may be zero.

[0054] negative electrode

[0055] The negative electrode 20 can be, for example, in sheet form. The negative electrode 20 has a multilayer structure. In the thickness direction, the negative electrode 20 sequentially includes a current collector 21, a first active material layer 22a, and a second active material layer 22b. The active material layers can be disposed on a portion of the surface of the current collector 21, or on the entire surface. The active material layers can be disposed on only one side of the current collector 21, or on both sides of the current collector 21. The current collector 21 can, for example, include a metal foil (Cu foil, Ni foil, etc.), a conductive resin layer, etc. The current collector 21 can, for example, have a thickness of 1 μm to 100 μm. In addition to the first active material layer 22a and the second active material layer 22b, the negative electrode 20 may also include a third active material layer, a fourth active material layer (not shown), etc.

[0056] CNTs are located at the interface between the first active material layer 22a and the second active material layer 22b. As long as they are located at this interface, CNTs can, for example, be contained within both the first and second active material layers 22a and 22b. That is, in the thickness direction (Z-axis direction), the presence rate of CNTs is greatest at the interface between the first and second active material layers 22a and 22b. CNTs can extend from the interior of the first active material layer 22a to the interior of the second active material layer 22b. CNTs can be, for example, monolayer CNTs or multilayer CNTs. The average fiber diameter of CNTs can, for example, be 1 nm or more, 10 nm or more, 50 nm or more, 100 nm or more, 200 nm or more, or 300 nm or more. The average fiber diameter of CNTs can, for example, be less than 1000 nm, less than 500 nm, less than 300 nm, less than 200 nm, less than 100 nm, or less than 50 nm. The average fiber length of CNTs can, for example, be 1 μm or more, 5 μm or more, or 10 μm or more. The average fiber length of CNTs can be, for example, less than 100 μm, less than 50 μm, less than 30 μm, or less than 20 μm. The "average fiber diameter" and "average fiber length" are determined by microscopy. The arithmetic mean of the fiber diameters of 10 randomly selected CNTs is the average fiber diameter. The arithmetic mean of the fiber lengths of 10 randomly selected CNTs is the average fiber length.

[0057] The first active material layer 22a and the second active material layer 22b can be, for example, coating layers. A coating layer means a layer formed by applying a coating material. The coating materials can be applied substantially simultaneously or sequentially. For example, CNTs can be dispersed on the surface of the first active material layer 22a after drying. For example, CNTs can be dispersed on the surface of the first active material layer 22a (coating material) before drying. For example, a coating material containing CNTs can be applied to the surface of the first active material layer 22a after drying. For example, a coating material containing CNTs can be applied to the surface of the first active material layer 22a (coating material) before drying.

[0058] The first active material layer 22a is the lower layer. The first active material layer 22a can be directly formed on the surface of the current collector 21. The first active material layer 22a contains a first negative electrode active material. The first active material layer 22a may also contain conductive materials, binders, etc. For example, the first active material layer 22a may contain, by mass fraction, 0% to 10% conductive material, 0% to 10% binder, and the remainder of the first negative electrode active material. For example, the first active material layer 22a may contain, by mass fraction, 0.5% to 1.5% conductive material, 0.5% to 3% binder, and the remainder of the first negative electrode active material. The first negative electrode active material may include at least one selected from the group consisting of natural graphite, artificial graphite, and Si-based active materials.

[0059] The second active material layer 22b is the upper layer. The second active material layer 22b can form the outermost surface of the negative electrode 20. The second active material layer 22b contains the second negative electrode active material. The second active material layer 22b may also contain conductive materials, binders, etc. For example, the second active material layer 22b may contain, by mass fraction, 0% to 10% conductive material, 0% to 10% binder, and the remainder of the second negative electrode active material. For example, the second active material layer 22b may contain, by mass fraction, 0.5% to 1.5% conductive material, 0.5% to 3% binder, and the remainder of the second negative electrode active material.

[0060] In the first active material layer 22a and the second active material layer 22b, the conductive material may include at least one selected from the group consisting of carbon black, CNTs, vapor-grown carbon fibers, and graphene sheets. The active material layer also includes CNTs, whereby the CNTs within the active material layer are intertwined with the CNTs disposed between the layers, which is expected to improve peel strength, electronic conductivity, etc.

[0061] In the first active material layer 22a and the second active material layer 22b, the adhesive may contain at least one selected from the group consisting of SBR, CMC, polyvinylidene fluoride, polytetrafluoroethylene, and acrylic polymers. The adhesive may be in the form of a film or in the form of fibers.

[0062] At least one of the compositions and structures differs between the first active material layer 22a and the second active material layer 22b. For example, the second negative electrode active material may have a different composition than the first negative electrode active material. For example, the content of Si-based active material may differ between the second and first negative electrode active materials. The ratio "C2 / C1" of the content of Si-based active material in the second negative electrode active material to the content of Si-based active material in the first negative electrode active material may, for example, be 0.1 or more, 0.5 or more, or 2 or more. "C2 / C1" may, for example, be 10 or less, 2 or less, or 0.5 or less.

[0063] For example, the mixing ratio of natural graphite and artificial graphite can be different between the first negative electrode active material and the second negative electrode active material. In the first active material layer 22a, for example, the mass ratio can satisfy a relationship such as "natural graphite / artificial graphite = 10 / 0 to 6 / 4". In the second active material layer 22b, for example, the mass ratio can satisfy a relationship such as "natural graphite / artificial graphite = 0 / 10 to 4 / 6".

[0064] For example, the second negative electrode active material may have an orientation angle different from that of the first negative electrode active material. For example, the orientation angle can be adjusted by applying a magnetic field to the coating layer before drying. For example, the orientation angle of the first negative electrode active material may be greater than that of the second negative electrode active material. For example, the orientation angle of the second negative electrode active material may be greater than that of the first negative electrode active material. The orientation angles of the first and second negative electrode active materials may each be independently, for example, 1° or more, 5° or more, 15° or more, 30° or more, 45° or more, 60° or more, or 75° or more. The orientation angles of the first and second negative electrode active materials may each be independently, for example, 90° or less, 75° or less, 60° or less, 45° or less, 30° or less, or 15° or less.

[0065] For example, the particle shapes of the first and second negative electrode active materials can be different. For example, the first negative electrode active material can be spherical or ellipsoidal, and the second negative electrode active material can be scaly. For example, the second negative electrode active material can be spherical or ellipsoidal, and the first negative electrode active material can be scaly. For example, the second negative electrode active material can have a different aspect ratio than the first negative electrode active material. For example, the aspect ratio of the first negative electrode active material can be greater than that of the second negative electrode active material. For example, the aspect ratio of the second negative electrode active material can be greater than that of the first negative electrode active material. The aspect ratios of the first and second negative electrode active materials can each be independently, for example, 1 or more, 1.2 or more, 1.5 or more, 2 or more, 3 or more, 4 or more, or 5 or more. The aspect ratio of the first negative electrode active material and the aspect ratio of the second negative electrode active material can be independently, for example, 6 or less, 5 or less, 4 or less, 3 or less, 2 or less, 1.5 or less, or 1.2 or less.

[0066] For example, the second active material layer 22b may have a different porosity than the first active material layer 22a. For example, the porosity of the first active material layer 22a may be greater than the porosity of the second active material layer 22b. For example, the porosity of the second active material layer 22b may be greater than the porosity of the first active material layer 22a. The porosity of the first active material layer 22a and the porosity of the second active material layer 22b may each be independently, for example, 1% or more, 5% or more, 10% or more, 20% or more, 30% or more, or 40% or more. The porosity of the first active material layer 22a and the porosity of the second active material layer 22b may each be independently, for example, 50% or less, 40% or less, 30% or less, 20% or less, 10% or less, or 5% or less.

[0067] Between the first active material layer 22a and the second active material layer 22b, for example, the composition of the adhesive can be different. For example, SBR can be biased towards the first active material layer 22a. For example, CMC can be biased towards the second active material layer 22b. For example, in the first active material layer 22a, the mass ratio can satisfy relationships such as "SBR / CMC = 1.1 / 0.9 to 1.9 / 0.1". For example, in the first active material layer 22a, the mass ratio can satisfy relationships such as "SBR / CMC = 1.2 / 0.8 to 1.8 / 0.2" and "SBR / CMC = 1.5 / 0.5 to 1.8 / 0.2". For example, in the second active material layer 22b, the mass ratio can satisfy relationships such as "SBR / CMC = 0.9 / 1.1 to 0.1 / 1.9" and "SBR / CMC = 0.8 / 1.2 to 0.2 / 1.8". For example, in the second active material layer 22b, the mass ratio can satisfy the relationship such as "SBR / CMC = 0.5 / 1.5 to 0.2 / 1.8".

[0068] An intermediate layer 22c may be formed between the first active material layer 22a and the second active material layer 22b. The intermediate layer 22c includes CNTs and an acrylic polymer. For example, the intermediate layer 22c may be formed by coating the surface of the first active material layer 22a with a coating comprising CNTs and an acrylic polymer. That is, the acrylic polymer may be biased at the interface between the first active material layer 22a and the second active material layer 22b.

[0069] Intermediate layer 22c may be a layer that substantially does not contain any negative electrode active material. Intermediate layer 22c may, for example, contain 1% to 99% acrylic polymer and the remainder CNTs by mass fraction. The mass fraction of the acrylic polymer may, for example, be 5% or more, 10% or more, 20% or more, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more, 90% or more, or 95% or more. Alternatively, the mass fraction of the acrylic polymer may, for example, be less than 95%, less than 90%, less than 80%, less than 70%, less than 60%, less than 50%, less than 40%, less than 30%, less than 20%, less than 10%, or less than 5%.

[0070] The thickness "T3" of the intermediate layer 22c can, for example, be less than the thickness "T1" of the first active material layer 22a and the thickness "T2" of the second active material layer 22b. The thickness "T3" of the intermediate layer 22c can, for example, be less than 10 μm, less than 5 μm, or less than 3 μm. The thickness "T3" of the intermediate layer 22c can, for example, be more than 0.5 μm, more than 1 μm, or more than 3 μm. The area weight "W3" of the intermediate layer 22c can, for example, be less than the area weight "W1" of the first active material layer 22a and the area weight "W2" of the second active material layer 22b. The area weight "W3" of the intermediate layer 22c can, for example, be 5 mg / cm³. 2 Below, 3mg / cm 2 Below or 1 mg / cm 2 The following is an example of the weight per unit area "W3" of interlayer 22c: 0.1 mg / cm³. 2 Above or 0.5 mg / cm 2 above.

[0071] Acrylic polymers can be homopolymers or copolymers. At least a portion of the constituent units of acrylic polymers is derived from acrylic monomers. Acrylic monomers may, for example, have at least one of an acrylate group and a methacrylate group. Acrylic monomers may, for example, include at least one selected from the group consisting of acrylic acid, acrylates, methacrylic acid, and methacrylates. Acrylic monomers may, for example, include at least one selected from the group consisting of ethyl acrylate, methyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate. Acrylic monomers may, for example, include at least one selected from the group consisting of isononyl acrylate, hydroxyethyl acrylate, methyl methacrylate, glycidyl methacrylate, and acrylonitrile. Acrylic monomers may, for example, include at least one selected from the group consisting of acrylamide and methacrylamide. A portion of the constituent units of acrylic polymers may be derived from monomers other than acrylic monomers. For example, a portion of the constituent units may be derived from styrene.

Claims

1. A negative electrode for a non-aqueous electrolyte secondary battery, characterized in that, It consists of a current collector, a first active material layer, and a second active material layer, in sequence. It also includes carbon nanotubes, and The carbon nanotubes are located at the interface between the first active material layer and the second active material layer.

2. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, characterized in that, The first active material layer contains a first negative electrode active material. The second active material layer contains a second negative electrode active material, and The second negative electrode active material has a different composition from the first negative electrode active material.

3. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, characterized in that, The first active material layer contains a first negative electrode active material. The second active material layer contains a second negative electrode active material. The second negative electrode active material has a different orientation angle than the first negative electrode active material, and The orientation angle refers to the acute angle formed by the major axis diameter of the particles of the first or second negative electrode active material and the surface of the current collector.

4. The negative electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, characterized in that, The second active material layer has a different porosity than the first active material layer.

5. A negative electrode for a non-aqueous electrolyte secondary battery, characterized in that, It consists of a current collector, a first active material layer, and a second active material layer, in sequence. It also includes carbon nanotubes and adhesives. The carbon nanotubes are located at the interface between the first active material layer and the second active material layer. The second active material layer has a different porosity than the first active material layer. The first active material layer contains a first negative electrode active material. The second active material layer contains a second negative electrode active material. The second negative electrode active material has a different composition from the first negative electrode active material. The second negative electrode active material has a different orientation angle than the first negative electrode active material. The orientation angle refers to the acute angle formed by the major axis diameter of the particles of the first or second negative electrode active material and the surface of the current collector. The adhesive includes styrene-butadiene rubber, carboxymethyl cellulose, and acrylic polymers. The styrene-butadiene rubber is biased towards the first active material layer. The carboxymethyl cellulose is biased in the second active material layer, and The acrylic polymer is biased at the interface between the first active material layer and the second active material layer.

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