Gradient Negative Electrode Layer for Battery Capacity Retention
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Solution Overview
Problem
The volume change of the negative electrode in nonaqueous electrolyte secondary batteries during charging and discharging can lead to the cutoff of conductive paths in the negative electrode mixture layer, resulting in a decrease in battery capacity over cycles.
Innovation Solution
A negative electrode design with a mixture layer comprising graphite, single-walled fibrous carbon, and multiwalled fibrous carbon, where the BET specific surface area of graphite varies across the layer thickness, with more multiwalled fibrous carbon on the surface and single-walled fibrous carbon on the core body side, enhancing conductivity and reducing reactivity with the electrolyte.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a uniform negative electrode mixture layer is used, then the structure is simple and easy to manufacture, but conductive paths are cut off during charging and discharging leading to capacity decrease
Solution Approach 1:
The negative electrode mixture layer is divided into a first region (40% from the surface) and a second region (40% from the core body interface), with each region having different compositions. This segmentation allows optimization of each region's function: the first region focuses on conductivity maintenance while the second region focuses on lithium ion occlusion, thereby preventing conductive path cutoff and capacity degradation without excessive complexity
Solution Approach 2:
Different regions of the mixture layer are assigned different material compositions tailored to their specific functions. The first region contains materials optimized for maintaining conductive paths during volume change, while the second region contains materials optimized for lithium ion occlusion. This local quality differentiation ensures that each region performs its specific function effectively, maintaining overall battery reliability
2Quantity of substance
If graphite with high BET specific surface area is used throughout the layer, then lithium ion occlusion is improved, but conductive paths are more easily cut off due to volume expansion
Solution Approach 1:
Graphite with high BET specific surface area is concentrated in the second region (40% from the core body interface) where lithium ion occlusion is prioritized, while the first region (40% from the surface) uses graphite with lower BET specific surface area to maintain structural stability and conductive path continuity. This local quality differentiation allows high capacity where needed without compromising conductive path integrity
Solution Approach 2:
The mixture layer uses composite materials with different properties in different regions: graphite particles with varying BET specific surface areas are combined with conductive agents in specific ratios for each region. This composite approach allows the second region to maximize lithium ion occlusion while the first region maintains conductive path continuity, resolving the contradiction between capacity and reliability
3Reliability
If single wall fibrous carbon is used throughout the layer, then conductivity is maximized, but reactivity with electrolyte increases leading to capacity loss
Solution Approach 1:
Single wall fibrous carbon is concentrated in the first region (40% from the surface) where its high conductivity is most beneficial for maintaining conductive paths, while multiwalled fibrous carbon is concentrated in the second region (40% from the core body interface) where its lower electrolyte reactivity is advantageous. This local quality differentiation ensures high conductivity where needed while minimizing harmful reactivity
Solution Approach 2:
The mixture layer uses a composite of single wall and multiwalled fibrous carbon in region-specific ratios. The first region uses a higher proportion of single wall fibrous carbon to maximize conductivity, while the second region uses a higher proportion of multiwalled fibrous carbon to reduce electrolyte reactivity. This composite material strategy resolves the contradiction between conductivity and reactivity by optimizing the material composition for each region's specific requirements
Data Source
AI summary
A negative electrode for a nonaqueous electrolyte secondary battery comprises a negative electrode core body and a negative electrode mixture layer provided. When the range of 40% of the thickness of the negative electrode mixture layer from the surface of the negative electrode mixture layer is defined as a first region and the range of 40% of the thickness of the negative electrode mixture layer from the surface of the negative electrode core body is defined as a second region, the BET specific surface area of the graphite included in the first region is smaller than that of the graphite included in the second region. The first region includes the multiwalled fibrous carbon more than the single wall fibrous carbon in terms of mass and the second region includes the single wall fibrous carbon more than the multiwalled fibrous carbon in terms of mass.