Silicon Negative Electrode Composition for High-Rate Capacity Retention
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Solution Overview
Problem
Conventional energy storage devices using silicon-based negative active materials face challenges with capacity retention ratio and high-rate discharge performance due to the expansion and contraction of silicon-based materials during charge-discharge cycles, leading to impaired bonding and isolation, which is exacerbated by high silicon content.
Innovation Solution
A negative electrode composition with a silicon-based negative active material layer containing 68 mass % or more silicon, a rubber-based binder at 3.0 mass % or more, and a carbon nanotube content of 0.4×(n2+4n)/(2n+3) mass % or less, where n is the number of graphene layers, enhances bonding and contact with conductive agents, suppressing capacity retention ratio decline and maintaining high-rate discharge performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the content of silicon-based negative active material is increased to increase discharge capacity, then the discharge capacity increases, but the capacity retention ratio deteriorates due to expansion and contraction during charge-discharge cycles
Solution Approach 1:
The negative active material layer is segmented into multiple layers (first, second, and third negative active material layers) with different silicon-based material contents. The first layer has high silicon content (68 mass % or more) for high capacity, the second layer has intermediate content, and the third layer has low content. This segmentation allows the high-capacity region to expand and contract while the low-silicon regions provide structural stability, preventing overall isolation of silicon particles.
Solution Approach 2:
The patent uses a composite structure combining silicon-based negative active material with carbon nanotubes and rubber-based binder. The carbon nanotubes form a conductive network that maintains electrical contact during volume changes, while the rubber-based binder provides flexible bonding that accommodates expansion and contraction. This composite material approach allows high silicon content while maintaining capacity retention.
2Quantity of substance
If the content of silicon-based negative active material is increased to increase discharge capacity, then the discharge capacity increases, but the high-rate discharge performance deteriorates due to impaired bonding
Solution Approach 1:
By dividing the negative active material layer into multiple layers with different silicon contents, the patent creates a gradient structure where high-silicon regions provide capacity while low-silicon regions maintain bonding integrity. This segmentation ensures that even at high overall silicon content, the bonding network remains intact for efficient ion transport at high rates.
Solution Approach 2:
Carbon nanotubes serve as an intermediary conductive network between silicon-based active material particles. These nanotubes maintain continuous electrical pathways during charge-discharge cycles, enabling efficient electron transport even when silicon particles expand and contract, thus preserving high-rate discharge performance.
3Strength
If the content of rubber-based binder is increased to improve bonding between silicon-based active materials, then the bonding is improved, but the discharge capacity decreases
Solution Approach 1:
The patent segments the negative active material layer into multiple layers with different silicon-based material contents. This segmentation allows the use of rubber-based binder in specific regions (particularly in the second and third layers with lower silicon content) to provide necessary bonding without significantly reducing the overall discharge capacity, as the high-capacity first layer maintains most of the active material volume.
Solution Approach 2:
Different regions of the negative active material layer have different compositions optimized for their specific functions. The first layer has high silicon content for maximum capacity, while subsequent layers have progressively lower silicon content and higher binder content, providing localized bonding support where it is most needed to prevent isolation during volume changes.
4Reliability
If the content of carbon nanotube is increased to improve conductivity and contact, then the bonding and contact are improved, but the discharge capacity decreases
Solution Approach 1:
The patent optimizes the carbon nanotube content within a specific range (0.01 mass % to 0.5 mass %) to achieve the minimum necessary conductivity and contact improvement without excessive capacity loss. This parameter optimization ensures that carbon nanotubes provide sufficient conductive network formation while minimizing their proportion in the negative active material layer, thereby maximizing discharge capacity.
Data Source
AI summary
A negative electrode includes a negative active material layer containing a silicon-based negative active material, a rubber-based binder, and a carbon nanotube, wherein a content of the silicon-based negative active material in the negative active material layer is 68 mass % or more, a content of the rubber-based binder in the negative active material layer is 3.0 mass % or more, and a content of the carbon nanotube in the negative active material layer is 0.4×(n2+4n)/(2n+3) mass % or less where n is the number of layers of graphene forming the carbon nanotube.