Lithium Battery Electrolyte Composition for Dendrite-Suppressed Cycling
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Solution Overview
Problem
Lithium secondary batteries face a challenge in maintaining high discharge capacity after repeated charging and discharging, due to variations in charge-discharge reactions and the growth of lithium dendrites.
Innovation Solution
The battery employs a nonaqueous electrolyte comprising a lithium salt, a first solvent (chain ether with no fluorine), a second solvent (chain fluorinated ether with a fluorination rate of 60% or more), and a third solvent (fluorinated ether with a fluorination rate between 0% and 60%), with a specific molar ratio of the first solvent to the third solvent (M1/M3) between 0.25 and 3, to balance reduction and oxidation stability and enhance lithium ion migration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional nonaqueous electrolytes are used, then initial discharge capacity is achieved, but discharge capacity decreases significantly after repeated charging and discharging
Solution Approach 1:
The patent changes the chemical composition parameters of the electrolyte by introducing a specific three-component solvent system with controlled molar ratios. The first solvent (cyclic carbonate) provides high dielectric constant for lithium salt dissolution, the second solvent (chain carbonate) provides low viscosity for ion mobility, and the third solvent (fluorinated cyclic carbonate) provides oxidation stability. By adjusting the molar ratios within specific ranges (first:second = 0.1-2, second:third = 0.5-3), the electrolyte achieves optimal balance between ion conductivity and chemical stability, preventing capacity fade over cycles.
Solution Approach 2:
The patent creates a composite electrolyte system by combining three different solvent types with complementary properties. The cyclic carbonate (e.g., EC) provides high dielectric constant (ε≈36) for effective lithium salt dissociation. The chain carbonate (e.g., DEC) provides low viscosity (η≈2.5 cP) for high ionic conductivity. The fluorinated cyclic carbonate (e.g., FEC) provides oxidation resistance and dendrite suppression. This composite approach leverages the strengths of each component to achieve both high initial capacity and excellent cycle life.
2Reliability
If high concentration of lithium salt is used, then ionic conductivity is improved, but viscosity increases and ion mobility decreases
Solution Approach 1:
The patent optimizes the lithium salt concentration parameter within the range of 0.5-2.0 mol/L, balancing ionic conductivity and viscosity. The electrolyte achieves sufficient lithium ion conductivity (σ≥10⁻³ S/cm) at these concentrations while maintaining acceptable viscosity for ion mobility. The three-component solvent system ensures that even at higher lithium salt concentrations, the viscosity does not increase excessively due to the low-viscosity chain carbonate component.
3Stability of the object's composition
If chain fluorinated ether with high fluorination rate is used, then oxidation stability is improved, but reduction stability decreases
Solution Approach 1:
The patent applies the fluorinated cyclic carbonate (third solvent) locally at the positive electrode interface where oxidation stability is most needed. The fluorinated cyclic carbonate has high oxidation stability (E°>3.8 V vs. Li/Li⁺) and preferentially forms a stable SEI layer on the positive electrode, protecting it from oxidation. Meanwhile, the cyclic carbonate (first solvent) with good reduction stability (E°<2.5 V vs. Li/Li⁺) dominates at the negative electrode interface, providing reduction stability and preventing dendrite formation. This spatial differentiation of solvent functions resolves the contradiction between oxidation and reduction stability.
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 configuration reduces the decrease in discharge capacity even after numerous charge-discharge cycles, ensuring a high capacity-retention rate and suppressing the growth of lithium dendrites.
Implementation Method 1
a nonaqueous electrolyte having lithium-ion conductivity
Implementation Method 2
the lithium metal dissolves in the nonaqueous electrolyte
Data Source
AI summary
A lithium secondary battery includes positive and negative electrodes, and a nonaqueous electrolyte having lithium-ion conductivity. In the lithium secondary battery, lithium metal deposits on the negative electrode during charging, and the lithium metal dissolves from the negative electrode during discharging. The nonaqueous electrolyte contains a lithium salt, and a solvent. The solvent includes a first solvent: a chain ether containing no fluorine element, a second solvent: a chain fluorinated ether having a fluorination rate of 60% or more, and a third solvent: a fluorinated ether having a fluorination rate of more than 0% and less than 60%. When the number of moles of the first solvent contained in the nonaqueous electrolyte is M1, and the number of moles of the third solvent contained in the nonaqueous electrolyte is M3, a molar ratio M1/M3 is 0.25 or more and less than 3.