Silicon Negative Electrode Binder for Battery Cycle Life

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Solution Overview

Problem

Non-aqueous electrolyte secondary batteries with Si as the negative electrode active material face challenges in maintaining binding properties and preventing electrode decay due to expansion and contraction during charge/discharge, leading to decreased conductivity and charge/discharge cycle characteristics.

Innovation Solution

A non-aqueous electrolyte secondary battery with a negative electrode formed as a porous molded article, using a binder of non-crosslinked polyacrylic acid with a weight-average molecular weight of 300,000 to 3,000,000, which enhances the strength and retention of the electrode's shape, preventing decay and maintaining conductivity.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Use of energy by moving object

If Si is used as negative electrode active material to achieve high energy density, then energy density is improved, but electrode decay occurs due to expansion and contraction during charge/discharge

Engineering Contradiction:

Improveenergy densityVSAvoidcharge/discharge cycle characteristics

Core Design Contradiction:

Use of energy by moving objectVSReliability

Solution Approach 1:

The patent uses a flexible polymer binder (polyacrylic acid or its salts) that can accommodate the expansion and contraction of Si particles during charge/discharge cycles. The binder forms a flexible matrix that maintains electrode integrity while allowing volume changes, thus preventing electrode decay and maintaining charge/discharge cycle characteristics.

Inventive Principle:

Principle #30Flexible shells and thin films

Solution Approach 2:

The patent creates a composite structure where Si particles are bound within a polymer matrix. This composite material combines the high energy density of Si with the flexibility and binding properties of the polymer, allowing the electrode to withstand expansion/contraction while maintaining structural integrity and conductivity.

Inventive Principle:

Principle #40Composite materials

2Ease of manufacture

If polyacrylic acid with low weight-average molecular weight is used as binder to reduce viscosity, then ease of manufacture is improved, but binding property decreases leading to electrode decay

Engineering Contradiction:

ImproveviscosityVSAvoidbinding property

Core Design Contradiction:

Ease of manufactureVSStrength

Solution Approach 1:

The patent optimizes the weight-average molecular weight parameter of polyacrylic acid to a specific range (10,000 to 1,000,000). This parameter change balances the competing requirements: low enough molecular weight to maintain acceptable viscosity for manufacturing, but high enough to provide sufficient binding strength to prevent electrode decay during Si expansion/contraction.

Inventive Principle:

Principle #35Parameter changes

3Strength

If cross-linked polyacrylic acid is used as binder to improve binding property, then strength is improved, but hygroscopicity increases making it unsuitable for battery use

Engineering Contradiction:

Improvebinding propertyVSAvoidhygroscopicity

Core Design Contradiction:

StrengthVSObject-affected harmful factors

Solution Approach 1:

The patent extracts or removes the cross-linking component from the binder system. Instead of using cross-linked polyacrylic acid, the patent employs non-cross-linked polyacrylic acid or its salts, thereby eliminating the hygroscopicity problem associated with cross-linked structures while maintaining adequate binding properties through optimized molecular weight and polymer-chain interactions.

Inventive Principle:

Principle #2Taking out (Extraction)

4Device complexity

If molded article without current collector is used to simplify structure, then device complexity is reduced, but strength decreases due to expansion and contraction leading to electrode decay

Engineering Contradiction:

ImprovestructureVSAvoidstrength of molded article

Core Design Contradiction:

Device complexityVSStrength

Solution Approach 1:

The patent employs a flexible polymer binder system that acts as a binding matrix holding the active material particles together in molded articles without current collectors. This flexible binder network provides the necessary strength and structural integrity to withstand expansion and contraction during charge/discharge, enabling simplified electrode structures without sacrificing mechanical strength.

Inventive Principle:

Principle #30Flexible shells and thin films

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 solution results in a battery with high energy density and excellent charge/discharge cycle characteristics, as the non-crosslinked polyacrylic acid effectively absorbs and reduces the expansion and contraction of the Si active material, preventing electrode decay and maintaining conductivity.

Implementation Method 1

Si, which expands and contracts during charge/discharge

Methodology Applied

Scientific EffectExpansion and contraction: Thermal Expansion

Data Source

PatentUS7297444B2Non-aqueous electrolyte secondary battery

Publication Date: 2007.11.20 PANASONIC HOLDINGS CORP

AI summary

In a non-aqueous electrolyte secondary battery having a negative electrode containing Si as a negative electrode active material, a binder containing a non-crosslinked polyacrylic acid having a weight-average molecular weight of 300,000 to 3,000,000 is incorporated into a negative electrode molded article that constitutes the negative electrode, so as to prevent electrode decay resulting from expansion and contraction during charge/discharge, as well as to achieve high energy density and improved charge/discharge cycle characteristics.