Silicon Anode SEI Layer Stability via Fluorinated Carbonate Additives
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Solution Overview
Problem
Lithium-ion rechargeable battery cells with silicon anodes face challenges due to significant volume changes during charging and discharging, leading to mechanical degradation and reduced cycle life, along with issues in forming a stable Solid Electrolyte Interface (SEI) layer, which affects charge/discharge efficiency and cycle life.
Innovation Solution
The use of structured silicon materials with specific additives such as vinylene carbonate (VC) and halogenated cyclic carbonates like fluoroethylene carbonate (FEC) in the electrolyte, which help in forming a stable and flexible SEI layer, and maintaining anode performance and longevity by controlling the SEI formation and expansion during lithiation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If silicon is used as anode material instead of graphite, then capacity is significantly increased, but volume change during charging/discharging causes mechanical degradation
Solution Approach 1:
A flexible coating layer comprising fluorinated cyclic carbonate is applied to the silicon-based anode material. This coating acts as a protective shell that accommodates the volume expansion and contraction of silicon during lithiation and delithiation, preventing mechanical degradation and maintaining structural integrity over multiple cycles.
Solution Approach 2:
The patent modifies the chemical composition and physical properties of the coating layer by using fluorinated cyclic carbonate with specific fluorine substitution patterns. This changes the coating's flexibility, adhesion, and stability parameters to match the dynamic volume changes of silicon, enabling it to withstand expansion/contraction cycles while maintaining protection.
2Quantity of substance
If silicon anode material is charged to full lithiation, then maximum capacity is achieved, but crystalline phase formation causes additional mechanical stresses
Solution Approach 1:
The flexible coating layer is formed on the silicon anode material before full lithiation occurs. This pre-formed coating prevents the formation of crystalline Li15Si4 phase by accommodating volume expansion at lower lithiation levels, thereby avoiding the mechanical stresses associated with crystalline-amorphous transitions while still achieving high capacity.
3Ease of operation
If conventional electrolyte is used with silicon anode, then basic battery operation is achieved, but SEI layer formation is unstable affecting efficiency
Solution Approach 1:
The patent changes the chemical parameters of the electrolyte by introducing fluorinated cyclic carbonate additives with specific molecular structures and fluorine substitution patterns. This modification alters the SEI layer formation process, creating a more stable and flexible interface that accommodates silicon volume changes while maintaining ionic conductivity for basic battery operation.
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 enhances the cycle performance and efficiency of lithium-ion cells by maintaining a stable SEI layer, reducing mechanical stress, and extending the cycle life of silicon anode materials, thereby improving the overall performance and longevity of the battery.
Implementation Method 1
the electrolyte including as an additive at least 5wt% of a halogenated cyclic carbonate... forming a stable and flexible SEI layer
Implementation Method 2
the volume of a fully lithiated Li-Si alloy can be 3-4 times larger than the unalloyed silicon volume... associated with the insertion and removal of lithium ions
Data Source
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AI summary
The present invention claims the addition of vinylene carbonate (VC) and optionally also fluoroethylene carbonate to the electrolyte of lithium ion cells having a structural silicon composite anode, i.e. an anode containing fibres or particles of silicon. The additive significantly improves the cycling performance of the cells. A VC content in the range 3.5 - 8 wt% based on the weight of the electrolyte has been found to be optimum.