Negative electrode, lithium ion secondary battery, and method for manufacturing negative electrode

Abstract

To suppress influence of expansion and shrinkage in association with charge-discharge cycles and suppress reduction of a capacity maintenance rate.SOLUTION: A negative electrode includes a negative electrode collector and a negative electrode active material layer. The negative electrode active material layer includes a first active material, a second active material, an ionic conductive polymer, and a conductive material. The negative electrode active material layer has a gap around the first active material.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a negative electrode, a lithium-ion secondary battery, and a method for manufacturing a negative electrode.

Background Art

[0002] Japanese Unexamined Patent Application Publication No. 2021-48106 (Patent Document 1) discloses an active material for a secondary battery having sufficient capacity and good cycle characteristics by coating silicon, which is a negative electrode active material, with polyethylene oxide, which is a polar polymer, to form a negative electrode composite material containing a conductive material.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In Patent Document 1, the cycle characteristics are improved by coating silicon with polyethylene oxide, but there is still room for improvement.

[0005] An object of the present disclosure is to suppress the influence of expansion and contraction associated with charge and discharge cycles and to suppress a decrease in the capacity retention rate.

Means for Solving the Problems

[0006] [1] A negative electrode including a negative electrode current collector and a negative electrode active material layer, The negative electrode active material layer includes a first active material, a second active material, an ion-conductive polymer, and a conductive material, The negative electrode active material layer has a structure having voids around the first active material.

[0007] Since the first active material has a larger expansion and contraction during charge and discharge compared to the second active material, it is likely to cause electrode cracking and deteriorate the cycle characteristics. Therefore, the negative electrode active material layer has a structure with voids around the first active material so that the occurrence of electrode cracking is suppressed even when the first active material expands and contracts. As a result of reducing the influence of the expansion and contraction of the first active material, an improvement in cycle characteristics such as the capacity retention rate is expected.

[0008] [2] The negative electrode according to [1], wherein the ion-conductive polymer is polyethylene oxide.

[0009] [3] The ratio of the area of the voids in the negative electrode active material layer is 30% or less, The ratio of the area of the voids around the first active material to the area of the voids in the negative electrode active material layer is 40% or more, the negative electrode according to [1] or [2].

[0010] [4] A lithium-ion secondary battery including the negative electrode according to any one of [1] to [3].

[0011] [5] A step of forming first particles by mixing the first active material, an ion-conductive polymer, and a conductive material and drying them; A step of forming second particles by mixing the first particles and the second active material; A step of forming a negative electrode active material precursor layer by applying a liquid paint obtained by dissolving the second particles in a solvent to a negative electrode current collector; A step of forming a negative electrode active material layer by bringing the negative electrode active material precursor layer into contact with an organic solvent, including: The method for manufacturing a negative electrode, wherein the negative electrode active material layer has voids around the first active material.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

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Figure 5

Mode for Carrying Out the Invention

[0013] Hereinafter, embodiments of the present disclosure (hereinafter may be abbreviated as "the present embodiment"), and examples of the present disclosure (hereinafter may be abbreviated as "the present examples") will be described. However, the present embodiment and the present examples do not limit the technical scope of the present disclosure.

[0014] <Negative electrode> Figure 1 is a schematic diagram showing an example of the negative electrode of the present embodiment. The negative electrode 20 includes a negative electrode current collector 21 and a negative electrode active material layer 22. The negative electrode current collector 21 may include, for example, a copper (Cu) foil, a nickel (Ni) foil, or the like.

[0015] 《Negative electrode active material layer》 The negative electrode active material layer 22 includes a first active material 1, a second active material 2, an ion conductive polymer 3, and a conductive material 4. The negative electrode active material layer 22 has voids around the first active material 1.

[0016] The first active material 1 includes a single active material containing an element selected from the group consisting of silicon (Si), aluminum (Al), and tin (Sn), or particles such as an oxide. From the viewpoint of capacity, the first active material 1 preferably contains Si, and examples thereof include Si alone, silicon oxide, and silicon carbide composite.

[0017] The content of the first active material 1 in the negative electrode active material layer 22 is 5% by mass or more and 20% by mass or less.

[0018] The average particle diameter (D50) of the first active material 1 is, for example, 3 to 20 μm, preferably 5 to 15 μm. The average particle diameter D50 in this specification is the particle diameter at which the cumulative frequency from the smaller particle diameter in the volume-based particle size distribution reaches 50%. The volume-based particle size distribution can be measured by a laser diffraction particle size distribution measuring device.

[0019] The second active material 2 contains or is one or more particles selected from the group consisting of carbon (C) such as graphite, hard carbon, soft carbon, and amorphous coated graphite. The second active material 2 preferably contains graphite, and more preferably consists of graphite. The graphite may be natural graphite or artificial graphite.

[0020] The D50 of the second active material 2 is, for example, 5 to 15 μm, preferably 8 to 10 μm. The D50 of the second active material 2 is preferably smaller than the D50 of the first active material 1.

[0021] The content of the second active material 2 in the negative electrode active material layer 22 is 75% by mass or more and 95% by mass or less.

[0022] The ion-conductive polymer 3 is a polymer that conducts lithium ions. The ion-conductive polymer 3 is mainly supported on the first active material 1. The ion-conductive polymer 3 is not particularly limited as long as it is a polymer that conducts lithium ions. For example, it may be polyethylene oxide (PEO), polyvinyl alcohol (PVA), Nafion (registered trademark), vinylidene fluoride-hexafluoropropene copolymer (PVDF-HFP), polymethyl methacrylate (PMMA), etc. The ion-conductive polymer 3 preferably contains PEO, and more preferably is PEO.

[0023] PEO may have a functional group. When PEO has a functional group, the ionic conductivity is improved. Examples of the functional group include a carboxyl group, a sulfonic acid group, a sulfonamide group, etc. PEO may be modified with ethylene carbonate and may be complexed with a lithium salt such as lithium bis(oxalato)borate (LiBOB). Thereby, the plasticity of PEO is improved.

[0024] The molecular weight of PEO is not particularly limited, but is, for example, 20,000 or more and 1,000,000 or less. It is preferable that the molecular weight of PEO is relatively low.

[0025] The content of the ion-conductive polymer 3 in the negative electrode active material layer 22 is 1% by mass or more and 5% by mass or less.

[0026] The conductive material 4 forms a conductive path. Examples of the conductive material 4 include carbon black (CB) (acetylene black (AB), ketjen black (KB)), carbon nanotube (CNT), vapor grown carbon fiber (VGCF), etc.

[0027] The content of the conductive material 4 in the negative electrode active material layer 22 is 1% by mass or more and 5% by mass or less.

[0028] The negative electrode active material layer 22 has a structure having voids 5 (hereinafter, the voids around the first active material 1 are also referred to as "first voids") around the first active material 1. Since the negative electrode active material layer 22 has the first voids 5, the influence of the expansion and contraction of the first active material 1 is reduced, and as a result, an improvement in cycle characteristics such as a capacity retention rate is expected. Here, the structure having voids 5 around the first active material 1 is particularly a structure having voids between the first active material 1 and the second active material 2. Specifically, in a state where charging is not performed, the first active material 1 contracts and the first voids 5 exist. On the other hand, in a charged state, the first active material 1 expands and fills at least a part of the first voids 5.

[0029] Here, the "region around the first active material 1" refers to the region within a circle having a radius of a certain distance from the center of gravity of the first active material 1. Typically, it refers to the region within a circle having a radius of a distance twice the average particle diameter of the first active material 1 from the center of gravity of the first active material 1. For example, the region around the first active material 1 having an average particle diameter of 10 μm refers to the region within a circle having a radius of 20 μm from the center of gravity of the first active material 1.

[0030] The ion-conductive polymer 3 and the conductive material 4 may be present so as to bridge the first active material 1 and the second active material 2 in the first void 5. Further, the first void 5 may be a region where the ion-conductive polymer 3 does not exist in the region surrounded by the first active material 1 and the second active material 2.

[0031] The negative electrode active material layer 22 may have voids other than the first void 5. The proportion of the area of the voids in the negative electrode active material layer 22 is preferably 30% or less. Also, the proportion of the area of the first void 5 to the area of the voids in the negative electrode active material layer 22 is preferably 40% or more. When the proportion of the area of the voids in the negative electrode active material layer 22 is 30% or less, it is considered that the proportion of the voids in the negative electrode active material layer 22 is low. Also, when the proportion of the area of the first void 5 to the area of the voids in the negative electrode active material layer 22 is 40% or more, it is considered that the proportion of the first void 5 in the negative electrode active material layer 22 is high. Therefore, suppression of electrode cracking is expected while maintaining high capacity. The proportion of the area of the voids in the negative electrode active material layer 22 may be 25% or less, or may be 20% or less. The proportion of the area of the voids in the negative electrode active material layer 22 may be 10% or more, or may be 15% or more. The proportion of the area of the first void 5 to the area of the voids in the negative electrode active material layer 22 may be 50% or more, or may be 60% or more. The proportion of the area of the first void 5 to the area of the voids in the negative electrode active material layer 22 may be 90% or less, or may be 80% or less. The proportion of the area of the voids in the negative electrode active material layer 22 is determined by measuring the proportion of the area of the voids to the cross-sectional area when the cross-section of the negative electrode active material layer 22 is observed with a scanning electron microscope (SEM). The proportion of the area of the first void 5 in the negative electrode active material layer 22 is determined by measuring the proportion of the area of the void between the first active material 1 and the second active material 2 when the cross-section of the negative electrode active material layer 22 is observed with SEM.

[0032] The negative electrode active material layer 22 may further contain a binder, a thickener, etc.

[0033] The binder may contain, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), etc. The thickener may contain, for example, carboxymethyl cellulose (CMC), methyl cellulose (MC), etc.

[0034] The content rates of the binder and the thickener contained in the negative electrode active material layer 22 are, for example, 0.1% by mass or more and 5% by mass or less.

[0035] The loading amount of the active material in the negative electrode active material layer 22 is preferably 20 mg / cm 2 or more. When the loading amount of the active material is 20 mg / cm 2 or more, an improvement in cycle characteristics is more expected. Note that the "active material" includes both the first active material 1 and the second active material 2.

[0036] <Method for manufacturing negative electrode> Figure 2 is a schematic flowchart of the method for manufacturing a negative electrode in the present embodiment. Hereinafter, the "method for manufacturing a negative electrode in the present embodiment" may be abbreviated as the "present manufacturing method". The present manufacturing method includes at least (a) a first particle formation step, (b) a second particle formation step, (c) a negative electrode active material precursor layer formation step, and (d) a negative electrode active material layer formation step.

[0037] 《(a) First particle formation step》 In the first particle formation step, the first active material, the ion conductive polymer, and the conductive material are mixed with a solvent and dried to form first particles. The first particles include a coating layer coated with the ion conductive polymer and the conductive material around the first active material.

[0038] The first aqueous solution is prepared by dispersing the first active material, the ion conductive polymer, and the conductive material in a solvent at a predetermined ratio. The solvent is not particularly limited as long as it can disperse the first active material, the ion conductive polymer, and the conductive material, and examples thereof include water. The first aqueous solution is subjected to particle disintegration and stirring and mixing by mechanical mixing means such as a mortar, a mixer, and a planetary ball mill, and dried to obtain first particles. The mixing conditions are not particularly limited and are appropriately adjusted according to the desired thickness of the coating layer.

[0039] 《(b) Second particle formation step》 In the second particle formation step, second particles are formed by mixing the first particles obtained in the first particle formation step with a second active material. The second particles include the second active material around the first particles.

[0040] For example, when the first particles and the second active material are dispersed in a solvent (e.g., water), a second aqueous solution is prepared. After the second aqueous solution is stirred and mixed, the droplets are instantaneously dried by a method such as the spray drying method to obtain the second particles. In this step, it is preferable that the D50 of the second active material is smaller than the D50 of the first active material. Thereby, the second particles are more easily formed.

[0041] 《(c) Negative electrode active material precursor layer formation step》 In the negative electrode active material precursor layer formation step, a negative electrode active material precursor layer is formed by applying a liquid paint obtained by dispersing the second particles obtained in the second particle formation step in a solvent to a negative electrode current collector.

[0042] For example, when the second particles are dispersed in a solvent (e.g., water), a liquid paint is prepared. In addition to the second particles, a binder, a thickener, etc. may be mixed. Further, the second active material and a conductive material may be added. The liquid paint is applied to the surface of the negative electrode current collector. For the application, for example, a doctor blade or a die coater is used. When the liquid paint is dried, a negative electrode active material precursor layer is formed. After drying, the negative electrode active material precursor layer may be compressed.

[0043] 《(d) Negative electrode active material layer formation step》 In the negative electrode active material layer formation step, a negative electrode active material layer is formed by bringing the negative electrode active material precursor layer obtained in the negative electrode active material precursor layer formation step into contact with an organic solvent.

[0044] For example, by immersing the negative electrode active material precursor layer in an organic solvent, the ion-conductive polymer in the coating layer contained in the second particles is removed, and a first void is formed inside the second particles. As the organic solvent, one that can selectively dissolve the ion-conductive polymer is used. For example, ethanol, acetonitrile, toluene, acetone, dichloromethane, hexane, chloroform, etc. may be mentioned. All of the ion-conductive polymer contained in the second particles may be removed, or a part thereof may be removed. By appropriately adjusting the type of organic solvent, immersion time, etc., the ratio of the removed ion-conductive polymer is adjusted.

[0045] <Lithium ion secondary battery> FIG. 3 is a schematic diagram showing an example of the lithium ion secondary battery (hereinafter, also simply referred to as "battery") of the present embodiment. The battery 100 may include an exterior body (not shown). The exterior body may house the power generation element 50 and an electrolytic solution (not shown). The exterior body can have any form. The exterior body may be, for example, a metal case, or a pouch made of a metal foil laminate film, etc. The exterior body may contain, for example, Al or the like.

[0046] The battery 100 includes a power generation element 50. The power generation element 50 may also be referred to as an electrode body or an electrode group. The power generation element 50 includes a positive electrode 10, a separator 30, and a negative electrode 20. The power generation element 50 has an arbitrary structure. For example, the power generation element 50 may be of a wound type. The positive electrode 10, the separator 30, and the negative electrode 20 may all be strip-shaped sheets. The power generation element 50 may be formed, for example, by laminating the positive electrode 10, the separator 30 (the first sheet), the negative electrode 20, and the separator 30 (the second sheet) in this order. After winding, the power generation element 50 may be formed into a flat shape.

[0047] <Positive electrode> The positive electrode 10 may include a positive electrode current collector 11 and a positive electrode active material layer 12. The positive electrode current collector 11 may contain, for example, aluminum (Al) foil or the like. The positive electrode active material layer 12 contains a positive electrode active material. The positive electrode active material layer 12 may further contain, for example, a conductive material, a binder, etc.

[0048] The positive electrode active material may be, for example, particulate. The positive electrode active material may have a D50 of, for example, 1 to 30 μm. The positive electrode active material may contain, for example, at least one selected from the group consisting of LiCoO2, LiNiO2, LiMnO2, Li(NiCoMn)O2, and Li(NiCoAl)O2. For example, in "Li(NiCoMn)O2", "(NiCoMn)" indicates that the total of the composition ratios in the parentheses is 1. As long as the total is 1, the individual component amounts are arbitrary.

[0049] The conductive material may contain, for example, AB or the like. The binder may contain, for example, PVdF or the like. The conductive material and the binder may be, for example, 0.1% by mass or more and 10% by mass or less with respect to the positive electrode active material layer 12.

[0050] 《Separator》 The separator 30 is porous. The separator 30 can permeate the electrolytic solution. The separator 30 separates the positive electrode 10 and the negative electrode 20. The separator 30 is electrically insulating. The separator 30 may contain, for example, polyolefin resins such as polyethylene (PE) and polypropylene (PP). The separator 30 may have, for example, a single-layer structure or a multilayer structure. The separator 30 may consist essentially of a PE layer, for example, or may be formed by laminating a PP layer, a PE layer, and a PP layer in this order.

[0051] 《Electrolytic Solution》 The electrolytic solution contains a solvent and a Li salt. The solvent is aprotic. The solvent can contain any components. The solvent may contain, for example, at least one selected from the group consisting of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).

[0052] The Li salt is a supporting electrolyte. The Li salt is dissolved in the solvent. The Li salt may contain at least one selected from the group consisting of, for example, LiPF6, LiTFSI, and LiBF4. The Li salt may have a molar concentration of, for example, 0.5 mol / L or more and 2.0 mol / L or less.

[0053] The electrolytic solution may further contain an optional additive. The electrolytic solution may contain, for example, 0.01% by mass or more and 5% by mass or less of the additive. The additive may contain at least one selected from the group consisting of, for example, vinylene carbonate (VC) and vinyl ethylene carbonate (VEC).

Examples

[0054] 《No.1》 As the material of the negative electrode, Si (D50: 10 μm) (5% by mass) as the first active material, artificial graphite (D50: 6 μm) (91.5% by mass) as the second active material, SBR (1% by mass) as the binder, CMC (1% by mass) as the thickener, and AB (1.5% by mass) as the conductive material were prepared. These materials were mixed in distilled water and kneaded using a kneader to obtain a negative electrode paste.

[0055] The obtained negative electrode paste was applied to the surface of a Cu foil, which is a negative electrode current collector, using a doctor blade. After application, the negative electrode paste was dried at 100 °C for 15 minutes and compressed by a roll press to obtain the No. 1 negative electrode. The loading of the active material in the negative electrode active material layer was 20 mg / cm 2 and the density of the negative electrode active material layer was 1.4 g / cm 3 was.

[0056] As the material of the positive electrode, LiNi 0.6 Co 0.2 Mn 0.2 O2 (95% by mass), AB (2.5% by mass) as the conductive material, and PVdF (2.5% by mass) as the binder were prepared. These materials were mixed in N-methyl-2-pyrrolidone (NMP) and kneaded using a kneader to obtain a positive electrode paste.

[0057] The obtained positive electrode paste was applied onto the surface of an Al foil serving as a positive electrode current collector using a doctor blade. After application, the positive electrode paste was dried at 80°C for 15 minutes and then compressed by a roll press to obtain a positive electrode. The loading amount of the active material in the positive electrode active material layer was 38 mg / cm 2 and the density of the positive electrode active material layer was 3.2 g / cm 3 .

[0058] PE was prepared as a separator. A power generation element was formed by alternately laminating the positive electrode and the negative electrode with the separator interposed therebetween. The power generation element included four positive electrodes and five negative electrodes.

[0059] As an exterior body, a pouch made of a laminate film was prepared. The power generation element was housed in the exterior body. As an electrolytic solution, a mixture obtained by dissolving a supporting salt (LiPF6) at a concentration of 1.2 mol / L in a mixed solvent containing EC, DMC, and EMC was prepared. The electrolytic solution was injected into the exterior body. After injection of the electrolytic solution, the exterior body was sealed under a vacuum of -80 kPa. Thus, Battery No.1 was fabricated.

[0060] 《No.2》 As a material for the negative electrode, PEO as an ion conductive polymer and the same materials as those of No.1 were prepared except for this. Si, AB, and PEO were dissolved in distilled water maintained at 60°C and stirred for 12 hours by a planetary ball mill, and then the drying process was repeated twice to obtain first particles. In the obtained first particles, a coating layer (thickness: 2.0 μm) composed of AB and PEO was formed around Si.

[0061] The obtained first particles and artificial graphite were mixed and instantaneously dried by a spray dryer to obtain second particles.

[0062] The obtained second particles, artificial graphite, SBR, and CMC were mixed in distilled water and kneaded using a kneader to obtain a negative electrode paste. The content ratio (mass %) of each material in the negative electrode paste was Si:AB:PEO:artificial graphite:SBR:CMC = 5:1.25:0.25:91.5:1:1.

[0063] The obtained negative electrode paste was applied to the surface of a Cu foil, which is a negative electrode current collector, using a doctor blade. After application, the negative electrode paste was dried at 100 °C for 15 minutes and compressed by a roll press to obtain a negative electrode active material precursor layer.

[0064] The obtained negative electrode active material precursor layer was immersed in ethanol maintained at 30 °C for 15 minutes and then washed to remove PEO, forming voids around Si. Thus, a No. 2 negative electrode was obtained. The loading amount of the active material in the negative electrode active material layer was 20 mg / cm 2 and the density of the negative electrode active material layer was 1.3 g / cm 3 .

[0065] A No. 2 battery was fabricated using the same materials and method as No. 1, except that the No. 2 negative electrode was used as the negative electrode.

[0066] <Evaluation> 《Voids》 In the No. 2 negative electrode, the ratio of the area of voids in the negative electrode active material layer and the ratio of the area of the first void to the area of voids in the negative electrode active material layer were measured. Specifically, a cross-section of the No. 2 negative electrode was processed using a Cross Section Polisher (registered trademark). A cross-sectional SEM image was taken by SEM. By performing binarization processing on the cross-sectional SEM image, voids were identified, and the ratio of the area of each void was measured. As a result, the ratio of the area of voids in the negative electrode active material layer was 30%, and the ratio of the area of the first void to the area of voids in the negative electrode active material layer was 40%. In the No. 1 negative electrode, although voids existed in the negative electrode active material layer, voids were not unevenly distributed around Si.

[0067] 《Charge and Discharge Evaluation》 The batteries of each No. were placed in a constant temperature bath at 40°C. The batteries were charged to 4.2 V at 0.5C. With a 10-minute rest in between, the batteries were discharged to 3.0 V at 0.5C. The discharge capacity at this time is regarded as the "initial capacity". Note that "C" is a symbol indicating the time rate. A 1C current discharges the rated capacity of the battery in 1 hour.

[0068] The above charging, resting, and discharging cycle is regarded as one cycle, and for the batteries of each No., charge and discharge were performed 100 cycles. By dividing the discharge capacity at the 100th cycle by the initial capacity, the capacity retention rate was obtained. The results are shown in Fig. 4.

[0069] <Swelling ratio> A displacement meter was placed directly above the restraint jig (metal plate). The change in the thickness of the battery was measured due to the up and down movement accompanying the charge and discharge of the battery. The results are shown in Fig. 5. The swelling ratio at each time point is represented by T2 / T1. T1 is the initial thickness at the time of battery assembly. T2 is the thickness of the battery at that time point.

[0070] <Results> As shown in Fig. 4, it can be seen that in No. 2, the capacity retention rate is higher compared to No. 1. Also, as shown in Fig. 5, it can be seen that in No. 2, the increase in the swelling ratio is reduced compared to No. 1. From the above, it is considered that in No. 2, the capacity retention rate is high and the influence of the expansion and contraction of the negative electrode is reduced.

[0071] This embodiment and these examples are illustrative in all respects. This embodiment and these examples are not restrictive. The technical scope of the present disclosure includes all modifications within the meaning and scope equivalent to the description of the claims. For example, any configurations are extracted from this embodiment and these examples, and their arbitrary combinations are also initially planned.

Explanation of reference numerals

[0072] 1 First active material, 2 Second active material, 3 Ion-conductive polymer, 4 Conductive material, 5 Void, 10 Positive electrode, 11 Positive electrode current collector, 12 Positive electrode active material layer, 20 Negative electrode, 21 Negative electrode current collector, 22 Negative electrode active material layer, 30 Separator, 50 Power generation element, 100 Lithium-ion secondary battery.

Claims

1. A negative electrode including a negative electrode current collector and a negative electrode active material layer, wherein the negative electrode active material layer includes a first active material, a second active material, an ion-conductive polymer, and a conductive material, and the negative electrode active material layer has a structure having voids around the first active material.

2. The negative electrode according to Claim 1, wherein the ion-conductive polymer is polyethylene oxide.

3. The negative electrode according to Claim 1, wherein the ratio of the area of the voids in the negative electrode active material layer is 30% or less, and the ratio of the area of the voids around the first active material to the area of the voids in the negative electrode active material layer is 40% or more.

4. A lithium-ion secondary battery including the negative electrode according to any one of Claims 1 to 3.

5. A method for manufacturing a negative electrode, comprising: forming first particles by mixing a first active material, an ion-conductive polymer, and a conductive material and drying them; forming second particles by mixing the first particles and a second active material; forming a negative electrode active material precursor layer by applying a liquid paint obtained by dissolving the second particles in a solvent to a negative electrode current collector; and forming a negative electrode active material layer by bringing the negative electrode active material precursor layer into contact with an organic solvent, wherein the negative electrode active material layer has voids around the first active material.

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