Composite Lithium-Salt Cathode Coating for Cycle Life and Output
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Solution Overview
Problem
Lithium ion secondary batteries experience deteriorated cycle characteristics due to insufficient lithium ion conductivity and increased reaction resistance caused by coating the positive electrode active material with a fluorine compound, leading to decreased output.
Innovation Solution
A positive electrode active material with a surface coating of a solid film comprising multiple lithium salts, including fluorine and phosphorus compounds, which improves stability and conductivity, and is formulated to have a specific molar ratio and thickness to prevent electrolyte penetration and maintain high discharge capacity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the surface of the positive electrode active material is coated with a fluorine compound, then cycle characteristics are improved, but lithium ion conductivity becomes insufficient and output decreases
Solution Approach 1:
The solid film is formed as a composite of multiple lithium salts (fluorine compound, phosphorus compound, and sulfur compound) rather than using a single fluorine compound coating. This composite structure provides both the stability needed for cycle characteristics and the conductivity required for output performance.
Solution Approach 2:
The invention specifies precise compositional parameters for the solid film, including the presence of multiple lithium salts with specific ratios, and controls the film thickness within 10-70 nm. These parameter optimizations balance the protective function for cycle life with the conductive function for output.
2Reliability
If the surface of the positive electrode active material is coated with a fluorine compound, then cycle characteristics are improved, but reaction resistance increases
Solution Approach 1:
The solid film combines fluorine compounds (for stability), phosphorus compounds (for conductivity), and sulfur compounds (for additional stability) to create a composite coating that simultaneously reduces reaction resistance while maintaining cycle characteristics.
Solution Approach 2:
The coating provides different functional properties at different aspects: fluorine compounds provide chemical stability, phosphorus compounds provide ionic conductivity, and sulfur compounds enhance overall film stability. This localized functional distribution resolves the contradiction between protection and conductivity.
3Reliability
If the solid film thickness is increased to improve stability, then cycle characteristics improve, but lithium ion conductivity decreases
Solution Approach 1:
The solid film thickness is optimized within a specific range of 10-70 nm. This parameter control ensures the film is thick enough to provide stability for cycle characteristics but thin enough to maintain lithium ion conductivity for output performance.
Solution Approach 2:
The composite structure of multiple lithium salts within the solid film enhances both stability and conductivity simultaneously, allowing the film to achieve adequate protective function at thinner thicknesses compared to single-component coatings, thereby maintaining lithium ion conductivity.
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 enhances the cycle characteristics and discharge capacity of lithium ion secondary batteries by improving lithium ion conductivity and stability, while suppressing reaction resistance and electrolyte decomposition.
Implementation Method 1
particles having a surface on which a solid film including a plurality of types of lithium salts is formed
Implementation Method 2
The positive electrode active material includes particles having a surface on which a solid film including a plurality of types of lithium salts is formed
Data Source
AI summary
To provide a positive electrode active material capable of improving cycle characteristics of a lithium ion secondary battery and achieving a desirable discharge capacity. A positive electrode active material that is an aggregate of lithium compounds each including a lithium-containing transition metal oxide, includes particles having a surface on which a solid film including a plurality of types of lithium salts is formed. The solid film preferably includes a fluorine compound and a phosphorus compound. The solid film preferably contains 70 mol % or more of fluorine atoms with respect to the total number of moles of the fluorine atoms and phosphorus atoms.