Negative Electrode Slurry Stability via NMP-Water Solvent System
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Solution Overview
Problem
Current methods for manufacturing negative electrodes for nonaqueous electrolyte secondary cells face challenges in achieving high volume density and stable characteristics due to solubility issues with polyvinylidene fluoride and styrene-butadiene rubber in N-methylpyrrolidone systems, leading to defects like peeling and cracking, and limited availability of modified styrene-butadiene rubber powders.
Innovation Solution
A negative electrode composition including a polyvinylidene fluoride component, a styrene-butadiene component, a nonionic surfactant with a hydrophile-lipophile balance of 10 to 15, and N-methylpyrrolidone is used, forming a stable slurry that is coated onto a foil-shaped current collector and subjected to heat treatment, enhancing the volume density and stability of the negative electrode active material layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If polyvinylidene fluoride powder is mixed into an aqueous slurry containing carboxymethyl cellulose and styrene-butadiene rubber, then the binder composition can be formulated, but the polyvinylidene fluoride is not soluble in water making the slurry preparation difficult
Solution Approach 1:
The patent introduces N-methylpyrrolidone as an intermediary solvent that is miscible with water and capable of dissolving polyvinylidene fluoride. This intermediary enables the formation of a stable slurry by bridging the incompatibility between water-soluble carboxymethyl cellulose and water-insoluble polyvinylidene fluoride, allowing all binder components to be uniformly mixed in the N-methylpyrrolidone-water mixed solvent system.
2Ease of manufacture
If N-methylpyrrolidone is used as a solvent for polyvinylidene fluoride, then the slurry can be prepared, but styrene-butadiene rubber is not soluble in N-methylpyrrolidone creating formulation challenges
Solution Approach 1:
The patent changes the solvent system parameters by using a mixed solvent system consisting of N-methylpyrrolidone and water in specific proportions. This parameter change allows the slurry to simultaneously accommodate both polyvinylidene fluoride (soluble in N-methylpyrrolidone) and styrene-butadiene rubber (dispersible in the mixed system), achieving a stable and homogeneous binder composition.
3Quantity of substance
If the volume density of the negative electrode composition is increased to enhance capacity, then higher cell capacity is achieved, but defects like cutting of the electrode plate or peeling or cracking of the electrolyte layer occur
Solution Approach 1:
The patent employs a composite binder system combining polyvinylidene fluoride, styrene-butadiene rubber, and carboxymethyl cellulose in the N-methylpyrrolidone-water mixed solvent. This composite binder provides synergistic effects that maintain electrode flexibility and adhesion even at high volume densities (90-95% of theoretical density), preventing electrode plate cutting, peeling, and cracking while achieving high capacity.
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 approach allows for a higher volume density of the negative electrode composition layer close to its true density, improving charge-discharge characteristics and preventing defects like peeling and cracking, while maintaining flexibility and adhesive strength, thus enhancing the capacity and reliability of nonaqueous electrolyte secondary cells.
Implementation Method 1
polyvinylidene fluoride which is not soluble in water... a method in which a polyvinylidene fluoride powder is mixed into an aqueous slurry... a solution prepared by using N-methylpyrrolidone (which is widely used as an organic solvent) and containing polyvinylidene fluoride as a main ingredient
Implementation Method 2
a nonionic surfactant having an HLB of 10 to 15... preparation of a coating material (a slurry) and fabrication of an electrode plate may be impossible in an NMP system
Implementation Method 3
When the thus molded negative electrode plate is subjected to a heat treatment in a non-oxidizing atmosphere, new linkages including ether linkages based on cellulose ring opening or on side chains are formed
Implementation Method 4
coating with the dispersion and drying, the negative electrode is molded
Data Source
AI summary
A negative electrode for a nonaqueous electrolyte secondary cell, includes: a negative electrode active material layer containing a negative electrode active material, a polyvinylidene fluoride component including polyvinylidene fluoride and/or a derivative having polyvinylidene fluoride as a main chain, a styrene-butadiene component including a styrene-butadiene polymer and/or a derivative having a styrene-butadiene polymer as a main chain, a nonionic surfactant having an HLB of 10 to 15, and N-methylpyrrolidone; and a foil-shaped negative electrode current collector provided with the negative electrode active material layer on at least one principal surface of the collector.