Expanded Graphite Conductive Network for Lithium Iron Phosphate Durability
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Solution Overview
Problem
Nonaqueous electrolyte secondary batteries with lithium-nickel composite oxide and lithium iron phosphate mixed positive electrodes face limitations in output characteristic at low state of charge due to increased reaction resistance and durability issues, primarily caused by the expansion/contraction of lithium iron phosphate particles, which disrupts the electric conduction path and leads to capacity deterioration over cycles and high-temperature storage.
Innovation Solution
A nonaqueous electrolyte secondary battery design featuring a positive electrode collector core material with a sheet body composed of granulation bodies containing lithium-nickel composite oxide, lithium iron phosphate, and expanded graphite, where the expanded graphite acts as a conductive material to maintain a continuous electric conduction path and withstand the expansion/contraction of lithium iron phosphate, improving durability and output characteristic.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If lithium iron phosphate particles are used in the positive electrode to improve output at low SOC, then the output characteristic is improved, but the durability deteriorates due to expansion/contraction disrupting the electric conduction path
Solution Approach 1:
A conductive material layer is introduced as an intermediary between the lithium iron phosphate particles and the conductive material, wrapping around the particles to maintain continuous electric conduction paths even when the particles expand and contract during charge-discharge cycles. This mediator absorbs the mechanical stress of volume change while preserving electrical connectivity.
Solution Approach 2:
The positive electrode is designed as a composite structure combining lithium iron phosphate particles, conductive material, and an additional conductive material layer. This composite approach leverages the high capacity of lithium iron phosphate while the conductive components compensate for its poor intrinsic conductivity and mechanical instability during cycling.
2Quantity of substance
If lithium-nickel composite oxide is used as the positive electrode active material, then the energy density is improved, but the output characteristic at low SOC deteriorates due to increased reaction resistance
Solution Approach 1:
The patent combines two different positive electrode active materials - lithium-nickel composite oxide and lithium iron phosphate - into a single electrode structure. This merging allows the electrode to benefit from the high energy density of lithium-nickel composite oxide while simultaneously achieving improved low SOC output through the lithium iron phosphate component, which maintains better conductivity at lower states of charge.
3Quantity of substance
If the upper limit voltage is set at 4.0 V to 4.1 V to maximize capacity, then the energy density is improved, but the durability deteriorates due to excessive expansion/contraction of lithium iron phosphate
Solution Approach 1:
The conductive material layer is applied beforehand to wrap around lithium iron phosphate particles, creating a protective cushion that accommodates expansion and contraction during charge-discharge cycles. This pre-applied protective layer prevents direct mechanical stress on the particle structure and maintains continuous electrical contact even at high voltage conditions that maximize 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
The battery exhibits enhanced durability and output characteristic at low state of charge, with improved electric conduction network stability and reduced resistance, maintaining performance after endurance and high-temperature storage.
Implementation Method 1
expanded graphite, which serves as a conductive material and has elasticity, to compensate for the expansion/contraction
Implementation Method 2
in lithium-nickel composite oxide, insertion reaction of lithium ions (Li+) proceeds mainly at a high to intermediate SOC (about 3.8 to 3.5 V)
Data Source
AI summary
A nonaqueous electrolyte secondary battery includes: a positive electrode collector core material; and a sheet body including a plurality of granulation bodies. The sheet body is disposed on the positive electrode collector core material. The granulation bodies each contain a first positive electrode active material particle, a second positive electrode active material particle, and expanded graphite, the first positive electrode active material particle including lithium-nickel composite oxide, the second positive electrode active material particle including lithium iron phosphate.