Dual-Layer Negative Electrode Void Density Control
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Solution Overview
Problem
Negative electrodes with large specific surface areas experience increased side reactions, leading to decreased capacity retention and battery resistance, while particle-form polymers used as void-forming aids can exceed the heat resistance of binders or current-collecting foils during sintering, causing adverse effects.
Innovation Solution
A negative electrode design featuring a dual-layer active material structure with a first layer having a low void density and a second layer with a higher void density, where the second layer includes thermoplastic resin degradable at elevated temperatures, to enhance capacity retention and reduce initial resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the specific surface area of negative electrode active material is increased to facilitate electrode reaction and decrease resistance, then battery resistance decreases, but side reaction increases leading to decreased capacity retention
Solution Approach 1:
The negative electrode active material layer is divided into two distinct layers: a first layer with lower specific surface area (0.5-2.0 m²/g) to minimize side reactions and maintain capacity retention, and a second layer with higher specific surface area (2.0-5.0 m²/g) to facilitate electrode reactions and decrease resistance. This segmentation allows each layer to optimize for its specific function, resolving the contradiction between capacity retention and resistance.
Solution Approach 2:
Different regions of the negative electrode are assigned different properties: the first layer (closer to current collector) has lower specific surface area for stability and low side reaction, while the second layer (surface layer) has higher specific surface area for high reactivity. This local differentiation of quality allows the electrode to simultaneously achieve low resistance and good capacity retention.
2Reliability
If particle size of negative electrode active material is decreased to increase reaction area and decrease battery resistance, then battery resistance decreases, but void size decreases inhibiting electrolyte solution diffusion and reducing input-output properties
Solution Approach 1:
The electrode structure is segmented into two layers with different particle size characteristics. The first layer contains larger particles that maintain sufficient void spaces for electrolyte diffusion, while the second layer contains smaller particles that provide large reaction area. This segmentation resolves the contradiction between low resistance and good input-output properties.
3Productivity
If particle-form polymer is used as void-forming aid for creating porous structure, then electrolyte diffusion is improved, but heat treatment temperature exceeds heat resistance of binder or current-collecting foil causing adverse impact
Solution Approach 1:
The invention changes the thermal parameter (degradation temperature) of the void-forming aid from high temperature (particle-form polymer) to low temperature (water-soluble polymer). This allows void formation at temperatures below the heat resistance of binders and current collectors, eliminating the adverse thermal impact while maintaining electrolyte diffusion benefits.
Solution Approach 2:
The water-soluble polymer serves as a temporary, disposable void-forming aid that is easily removed by water washing at low temperature. Its temporary presence during formation creates necessary voids for electrolyte diffusion, then it is completely removed without requiring high-temperature treatment that would damage other electrode components.
4Strength
If amount of surface coating of negative electrode active material is reduced to strengthen particles, then particle strength increases, but when layer is rolled to high density it breaks causing decreased endurance
Solution Approach 1:
The coating strategy is segmented: the first layer uses adequately coated particles for structural integrity during rolling, while the second layer uses lightly coated or uncoated particles for high reactivity. This segmentation allows the electrode to withstand rolling while maintaining particle strength in the reactive surface layer.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The dual-layer structure effectively decreases battery resistance and maintains capacity retention by facilitating electrolyte diffusion and preventing layer breakage during high-density rolling, while the degradable resin manages heat without damaging the electrode components.
Implementation Method 1
the negative electrode active material layer has a first active material layer and a second active material layer in this order from a side closer to the substrate... the second active material layer has a larger average specific surface area than the first active material layer
Implementation Method 2
the size of voids in the negative electrode active material layer decreases and, thereby, diffusion of electrolyte solution during charge and discharge may become inhibited
Data Source
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AI summary
The present disclosure relates to a negative electrode comprising a substrate (10) and a negative electrode active material layer (50), wherein the negative electrode active material layer (50) has a first active material layer (51) and a second active material layer (52) in this order from a side closer to the substrate (10); in a transverse cross section parallel to a thickness direction, the first active material layer (51) has a number density of voids (40) having a diameter of 3 µm or more per unit area of less than 200/mm2; and in a transverse cross section parallel to a thickness direction, the second active material layer (52) has a number density of voids (40) having a diameter of 3 µm or more per unit area of from 200/mm2 to 1500/mm2. By the present disclosure, a negative electrode having a decreased battery resistance and having a capacity retention that is less likely to decrease even after an endurance test is provided.