Lithium Battery Electrode Capacity Ratio Optimization
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Solution Overview
Problem
Lithium secondary batteries face challenges in achieving a balance between increased capacity and long cycle life, particularly due to the low operating potential and poor stability of nickel-type lithium-containing complex oxides, which limit their energy density and charge-discharge cycle characteristics.
Innovation Solution
A lithium secondary battery design incorporating a nickel-type lithium-containing complex oxide as the positive-electrode active material, with a specific ratio of irreversible capacities between the positive and negative electrodes, and a graphite-type material with high reversible capacity, optimized to control the discharge end potential within a range of 2.7 to 3.4 V on a lithium metal basis, enhancing charge-discharge cycle characteristics while maintaining capacity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If nickel-type lithium-containing complex oxide is used as positive-electrode active material to increase capacity, then reversible capacity increases, but operating potential decreases and charge-discharge cycle life shortens
Solution Approach 1:
The patent applies parameter changes by precisely controlling the discharge end potential within a specific voltage range (2.7-3.4V vs Li/Li+) and managing the irreversible capacity ratio between electrodes. This parameter optimization allows the nickel-type lithium-containing complex oxide to achieve high reversible capacity while maintaining stable crystal structure and extending charge-discharge cycle life.
Solution Approach 2:
The patent employs composite material strategy by combining nickel-type lithium-containing complex oxide with graphite-type negative electrode material having specific irreversible capacity characteristics. This composite electrode system balances the irreversible capacity between positive and negative electrodes, resolving the contradiction between capacity and cycle life.
2Quantity of substance
If nickel-type lithium-containing complex oxide is used to increase capacity, then reversible capacity increases, but energy density decreases due to low operating potential
Solution Approach 1:
The patent resolves this contradiction by changing the operating potential parameters - specifically controlling the discharge end potential to be no lower than 2.7V vs Li/Li+. This parameter control ensures that while high capacity is achieved through nickel-type material, the operating potential remains sufficiently high to maintain good energy density.
3Productivity
If charge-discharge is performed with nickel-type lithium-containing complex oxide, then capacity is utilized, but crystal structure becomes unstable and cycle life shortens
Solution Approach 1:
The patent applies preliminary action by pre-establishing the discharge end potential limit (2.7-3.4V vs Li/Li+) before charge-discharge cycling begins. This preliminary parameter setting prevents crystal structure degradation from occurring, allowing full capacity utilization while maintaining structural stability throughout the battery's operational life.
4Reliability
If irreversible capacity of positive electrode is reduced to improve reversibility, then charge-discharge cycle characteristics improve, but capacity decreases
Solution Approach 1:
The patent resolves this contradiction through parameter changes in the irreversible capacity ratio (Qc/Qa) between positive and negative electrodes. By optimizing this ratio parameter within a specific range, the patent achieves both good charge-discharge cycle characteristics and high capacity, eliminating the need to sacrifice capacity for reversibility.
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 improves the charge-discharge cycle characteristics and suppresses capacity decreases by controlling the discharge end potential, reducing reaction resistance and crystal structure changes in the positive electrode, thereby extending the battery's lifespan and maintaining a high reversible capacity.
Implementation Method 1
lithium-containing complex oxides are known, such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2) having a layer structure
Implementation Method 2
a positive electrode having a positive-electrode active material capable of occluding and releasing lithium ions
Implementation Method 3
a negative electrode having a negative-electrode active material capable of occluding and releasing lithium ions
Implementation Method 4
graphite-type material having a reversible capacity of 350 mAh/g or more
Implementation Method 5
an electrolyte having lithium-ion conductivity
Implementation Method 6
a separator interposed between the positive electrode and the negative electrode
Data Source
AI summary
A lithium secondary battery according to the present invention includes: a positive electrode having a positive-electrode active material capable of occluding and releasing lithium ions; a negative electrode having a negative-electrode active material capable of occluding and releasing lithium ions; a separator interposed between the positive electrode and the negative electrode; and an electrolyte having lithium-ion conductivity. The positive-electrode active material contains a nickel-type lithium-containing complex oxide. The negative-electrode active material contains a graphite-type material having a reversible capacity of 350 mAh/g or more and an irreversible capacity of 30 mAh/g or less. A ratio Qc/Qa between an irreversible capacity Qc per unit area in a portion of the positive electrode that opposes the negative electrode and an irreversible capacity Qa per unit area in a portion of the negative electrode that opposes the positive electrode is equal to or greater than 0.50 but less than 1. As a result, the charge-discharge cycle characteristics can be improved while ensuring a high capacity.