Graphite Negative Electrode Coating for Magnesium Deposition Control

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Solution Overview

Problem

Lithium-ion secondary cells experience increased resistance due to magnesium deposition on non-carbon-coated graphite, leading to deactivation and nonuniform stress in the negative electrode active material layer, especially when using lithium-manganese composite oxide partially substituted by magnesium as the positive electrode material.

Innovation Solution

Incorporating a negative electrode with graphite coated with amorphous carbon, a carbon black-based conductive aid, and a fluororesin-based binding agent to trap magnesium, preventing its deposition on contact points and maintaining low resistance over long-term use.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If non-carbon-coated graphite is used as negative electrode active material, then lithium ion transfer is facilitated through exposed parts between crystalline layers, but magnesium deposition readily clogs these exposed parts during long-term use causing deactivation and resistance increase

Engineering Contradiction:

Improvelithium ion transfer speedVSAvoidnegative electrode stability

Core Design Contradiction:

SpeedVSReliability

Solution Approach 1:

The patent uses a composite negative electrode structure combining non-carbon-coated graphite particles with carbon-coated graphite particles. The non-carbon-coated graphite provides exposed parts for lithium ion transfer, while the carbon-coated graphite prevents magnesium deposition by providing a protective carbon layer. This composite approach resolves the contradiction by integrating the advantages of both materials while mitigating their individual disadvantages.

Inventive Principle:

Principle #40Composite materials

Solution Approach 2:

The patent applies different surface treatments to different portions of the negative electrode active material. Specifically, some graphite particles are left non-carbon-coated to maintain lithium ion transfer pathways, while other graphite particles are carbon-coated to prevent magnesium deposition. This local differentiation of surface properties allows simultaneous optimization of both lithium ion transfer and magnesium deposition resistance.

Inventive Principle:

Principle #3Local quality

2Reliability

If carbon-coated graphite is used to prevent magnesium deposition, then magnesium deposition is suppressed, but adhesion breakdown and resistance increase occur due to nonuniform stress from expansion/contraction differences between coated and non-coated particles

Engineering Contradiction:

Improveresistance stabilityVSAvoidparticle adhesion

Core Design Contradiction:

ReliabilityVSStrength

Solution Approach 1:

The patent uses graphite particles with uniform carbon coating thickness and composition to ensure consistent expansion and contraction behavior during charge-discharge cycles. This homogeneity in material properties across all particles prevents nonuniform stress distribution and maintains particle adhesion, resolving the contradiction between preventing magnesium deposition and maintaining structural integrity.

Inventive Principle:

Principle #33Homogeneity

3Use of energy by moving object

If magnesium-substituted lithium-manganese composite oxide is used as positive electrode material, then cell energy density is improved, but magnesium is deposited on negative electrode causing deactivation and resistance increase

Engineering Contradiction:

Improvecell energy densityVSAvoidmagnesium deposition

Core Design Contradiction:

Use of energy by moving objectVSObject-generated harmful factors

Solution Approach 1:

The patent converts the harmful magnesium ions released by the magnesium-substituted lithium-manganese composite oxide into a beneficial effect by using them to preferentially deposit on carbon-coated graphite particles. The carbon coating acts as a sacrificial layer that captures magnesium ions, protecting the non-carbon-coated graphite particles from deactivation. This transforms the harmful magnesium deposition into a protective mechanism for the active lithium ion transfer pathways.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

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 effectively suppresses the formation of lithium-resistant films and reduces resistance increase by capturing magnesium on the amorphous carbon and carbon black-based conductive aid, ensuring stable lithium ion transfer and preventing adhesion breakdown in the negative electrode.

Implementation Method 1

graphite coated with amorphous carbon... a carbon black-based conductive aid... to trap magnesium, preventing its deposition on contact points

Methodology Applied

Scientific EffectAdsorption: Adsorption

Implementation Method 2

lithium-manganese composite oxide partially substituted by magnesium... difference in expansion/contraction rate between particles of the coated graphite and non-coated graphite by charge and discharge operation

Methodology Applied

Scientific EffectIon transfer: Ion Exchange

Data Source

PatentUS10020489B2Lithium-ion secondary cell

Publication Date: 2018.07.10 ENVISION AESC JAPAN LTD

AI summary

A lithium-ion secondary cell according to the present invention is provided with a positive electrode and a negative electrode. The positive electrode contains a lithium-manganese composite oxide partially substituted by magnesium as a positive electrode active material. The negative electrode contains a graphite coated with amorphous carbon as a negative electrode active material, a carbon black-based conductive aid and a fluororesin-based binding agent.