Graphite Electrode Layer Tortuosity for High-Density Rate Performance
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Solution Overview
Problem
Existing electrodes with high basis weight and density face a trade-off between maintaining bonding strength and improving rate characteristics, as high tortuosity reduces lithium ion mobility.
Innovation Solution
The electrode design incorporates two regions with different tortuosities, a first region closer to the base material with higher tortuosity and a second region closer to the surface with lower tortuosity, maintaining bonding strength while enhancing lithium ion mobility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the basis weight and density of the negative electrode active material layer are increased to enhance energy density, then the energy density is improved, but the tortuosity increases which reduces the rate characteristic
Solution Approach 1:
The negative electrode active material layer is divided into two regions with different tortuosities: a first region (deep portion) with lower tortuosity and a second region (shallow portion) with higher tortuosity. This segmentation allows different parts of the layer to serve different functions - the first region prioritizes Li-ion transport speed while the second region accommodates higher material density, thus resolving the contradiction between energy density and rate characteristic.
Solution Approach 2:
Different regions of the negative electrode active material layer are assigned different local properties regarding tortuosity. The first region has optimized tortuosity for fast Li-ion diffusion, while the second region has different tortuosity characteristics. This local differentiation allows the overall layer to achieve both high energy density and good rate characteristic by optimizing each region for its specific purpose.
2Speed
If the binder amount is reduced to decrease tortuosity and improve rate characteristic, then the rate characteristic is improved, but the bonding strength between the negative electrode active material layer and the base material becomes insufficient
Solution Approach 1:
The binder is segmented and distributed non-uniformly across the negative electrode active material layer. The binder concentration is optimized differently in the first region versus the second region, allowing the first region to maintain low tortuosity for fast Li-ion transport while ensuring sufficient bonding strength is achieved through appropriate binder distribution in both regions.
Solution Approach 2:
The binder amount is optimized locally in different regions. Rather than uniformly reducing binder throughout the layer, the binder distribution is tailored so that regions requiring strong bonding have appropriate binder content, while regions prioritizing ion transport maintain lower tortuosity. This local optimization resolves the contradiction between bonding strength and rate characteristic.
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
This design improves lithium ion mobility and maintains sufficient bonding strength, thereby enhancing the rate characteristic of the battery.
Implementation Method 1
During long-term discharge, lithium (Li) ions move from a shallow portion to a deep portion of the negative electrode active material layer. The tortuosity represents complexity of a Li-ion moving path (an air gap) in the thickness direction of the negative electrode active material layer.
Data Source
AI summary
An electrode for battery includes a base material and a negative electrode active material layer. The negative electrode active material layer is disposed on a surface of the base material. The negative electrode active material layer includes graphite and a binder. The negative electrode active material layer has a basis weight of 25 mg/cm2 or more and a density of from 1.2 g/cm3 to 1.6 g/cm3. An area fraction of the binder in an entire section parallel with a thickness direction of the negative electrode active material layer is 1.9% or more. The section includes a first region and a second region. The first region is disposed between the second region and the base material. Relationships of “1.0<(τ1/τ2)<4.4” and “τ2<2.1” are satisfied. Symbol “τ1” represents a tortuosity in the first region. Symbol “τ2” represents a tortuosity in the second region.