Lithium-Iron Composite Fluoride Cathode for High-Voltage Li-Ion Batteries
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Solution Overview
Problem
Existing lithium ion secondary batteries face challenges in achieving high energy density and operating at high voltages due to the limitations of materials like LiFePO4 and LiFeO2, which have low voltage and capacity, while Fe-based compounds like Fe4+ are unstable and difficult to use at high voltages.
Innovation Solution
A positive electrode active material composed of lithium-iron composite fluoride (LixFeF(3+x)) is developed, where x satisfies 0.4≤x<1.5, with specific X-ray diffraction patterns and diffraction intensity ratios, allowing for high average discharge voltage and stable operation at high voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If LiFePO4 is used as positive electrode active material, then resource risk is reduced and cycle characteristics are improved, but voltage and capacity are low resulting in small energy density
Solution Approach 1:
The patent changes the chemical composition parameters by introducing fluorine atoms into the LiFePO4 structure to form LiFePO3F and LiFePO2F2 compounds. This parameter change transforms the material properties, enabling operation at higher voltages (3.8-4.0V) while maintaining the low resource risk and good cycle characteristics of iron-based materials, thus resolving the contradiction between reliability and energy density
Solution Approach 2:
The patent creates composite fluoride materials by combining lithium, iron, phosphorus, and fluorine elements in specific ratios. The composite nature of these materials allows simultaneous achievement of high voltage operation (improving energy density) and maintenance of stable cycle characteristics, while avoiding the use of scarce nickel or cobalt resources
2Use of energy by moving object
If high-valent transition metal (Fe4+) is used to increase voltage, then voltage increase is expected, but Fe4+ is very unstable and becomes Fe3+ by side reaction
Solution Approach 1:
The patent introduces fluorine atoms as intermediary elements that stabilize the high-valent iron species. The fluorine acts as a mediator that enables Fe4+ to maintain its high oxidation state without undergoing spontaneous reduction to Fe3+, thus achieving both high voltage and compositional stability simultaneously
Solution Approach 2:
The patent modifies the chemical environment by incorporating fluorine, which changes the electronic structure and stability parameters of the iron centers. This parameter change allows the system to access and maintain higher oxidation states (Fe4+) that would otherwise be unstable, enabling high voltage operation with stable composition
Data Source
AI summary
There is provided a positive electrode active material containing a lithium-iron composite fluoride as a principal component, wherein the lithium-iron composite fluoride is represented by the following formula (1):LixFeF(3+x) (1)where, in formula (1), x is a number satisfying 0.4≤x<1.5.