Lithium Secondary Cell with Fluorinated Electrolyte
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Solution Overview
Problem
Lithium secondary cells with high operating voltages above 4.5V experience significant degradation of electrolytic solutions when used at high temperatures, leading to reduced cell life and energy density, particularly when using positive electrode active substances like LiNi0.5Mn1.5O4 and graphite as negative electrodes.
Innovation Solution
A lithium secondary cell design featuring a positive electrode with a lithium-manganese oxide complex (Li a (M x Mn 2-x-y X y )(O 4-w Z w ) and a graphite negative electrode coated with low-crystallinity carbon, immersed in an electrolytic solution containing high oxidation potential solvents like propylene carbonate and low-viscosity solvents such as fluorinated chain-type ethers, which suppresses degradation and enhances cell life.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If high voltage positive electrode active substances (such as LiNi0.5Mn1.5O4) are used to achieve operating voltages above 4.5V, then energy density is improved, but electrolytic solution degradation increases significantly at high temperatures
Solution Approach 1:
A coating layer comprising fluorinated cyclic carbonate and/or fluorinated chain-type carbonate is applied to the positive electrode active substance surface. This coating acts as an intermediary barrier between the high voltage positive electrode and the electrolytic solution, preventing direct contact and chemical reactions that cause degradation, while allowing lithium ion transport. The fluorinated compounds form stable surface films that are resistant to oxidation at high potentials.
Solution Approach 2:
The invention changes the chemical composition parameters of the electrolytic solution by using fluorinated cyclic carbonate (such as fluoroethylene carbonate) and fluorinated chain-type carbonate (such as fluorinated dimethyl carbonate or fluorinated diethyl carbonate) instead of conventional non-fluorinated carbonates. These fluorinated compounds have higher oxidation resistance and form more stable protective films on the electrode surfaces, enabling stable operation at voltages above 4.5V.
2Use of energy by moving object
If graphite is used as negative electrode active substance to achieve high energy density, then energy density is improved, but capacity reduction occurs due to high reactivity with electrolytic solution
Solution Approach 1:
A coating layer comprising fluorinated cyclic carbonate and/or fluorinated chain-type carbonate is applied to the graphite negative electrode surface. This coating acts as an intermediary protective barrier that prevents direct reaction between the graphite and electrolytic solution components, reducing solvent decomposition and capacity loss while maintaining lithium ion insertion/extraction functionality.
Solution Approach 2:
The invention changes the electrolytic solution composition by incorporating fluorinated cyclic carbonate (such as fluoroethylene carbonate) and fluorinated chain-type carbonate, which have different electrochemical stability characteristics compared to conventional electrolytes. These fluorinated compounds form more stable solid electrolyte interface (SEI) films on graphite surfaces, reducing continuous decomposition reactions and improving capacity retention.
3Device complexity
If conventional electrolytic solutions are used with high voltage cells operating above 4.5V, then cell assembly is simplified, but cell life is reduced due to electrolyte degradation
Solution Approach 1:
The invention changes the chemical composition parameters of the electrolytic solution by using fluorinated cyclic carbonate (such as fluoroethylene carbonate) and fluorinated chain-type carbonate (such as fluorinated dimethyl carbonate or fluorinated diethyl carbonate) in specific concentration ranges. These fluorinated compounds have higher oxidation potentials and form more stable protective films, enabling the cell to maintain performance at high voltages above 4.5V for extended periods without significant electrolyte degradation.
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 cell achieves high energy density and extended life under high-temperature conditions by reducing electrolyte degradation and maintaining capacity during repetitive charge/discharge cycles, with a primary charge capacity of 140-155mAh/g and discharge capacity of 120-135mAh/g, corresponding to 90% of the charge capacity.
Implementation Method 1
an electrolytic solution which contains high oxidation potential solvents and low-viscosity solvents
Implementation Method 2
the positive electrode active substance absorbs and emits lithium ions into an electrolytic solution during charge and absorbs lithium ions in the electrolytic solution during discharge
Data Source
Figure 1
AI summary
Provided is a lithium secondary cell of 5V class having a positive electrode operating voltage of 4.5V or higher with respect to metallic lithium; the lithium secondary cell has high energy density, inhibits degradation of the electrolytic solution that comes in contact with the positive electrode and the negative electrode, and has particularly long cell life when used under high-temperature environments. The positive electrode contains, as the positive electrode active substance, a predetermined lithium-manganese oxide complex; the negative electrode contains, as the negative electrode active substance, graphite, of which surface is coated by low-crystallinity carbon; and the electrolytic solution contains one or more high-oxidation-potential solvents selected from propylene carbonate, butylene carbonate, 4-fluoro-1,3-dioxolan-2-one, and 4,5-difluoro-1,3-dioxolan-2-one within a range of 5 to 60 vol% of the solvent and one or two low-viscosity solvents selected from dimethyl carbonate and fluorinated cyclic ether.