Phosphazene Electrolyte Additives for Silicon Anode SEI Stability
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Solution Overview
Problem
Lithium-ion batteries with silicon-based anodes and high-voltage cathodes face challenges such as unstable solid electrolyte interphase layers, oxidative instability of conventional electrolytes, and inferior cycle life due to volumetric expansion and electrolyte decomposition, leading to reduced energy density and safety concerns.
Innovation Solution
An electrolyte system comprising a phosphazene-based compound, a linear carbonate, and a cyclic carbonate with a Li-containing salt is used to stabilize the solid electrolyte interphase, reduce electrolyte reactions, and enhance thermal stability, thereby improving the electrochemical performance and safety of silicon-based anode batteries.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If silicon-based anode materials are used to increase energy density, then the specific capacity and volumetric capacity are improved, but the volumetric expansion during lithiation causes disintegration and reduces cycling stability
Solution Approach 1:
The patent employs a flexible polymer coating layer comprising phosphazene compounds that forms a stable solid electrolyte interphase (SEI) on the silicon anode surface. This flexible film accommodates the volumetric expansion of silicon during lithiation while maintaining structural integrity and preventing electrolyte decomposition, thereby resolving the contradiction between high capacity and cycling stability.
Solution Approach 2:
The patent modifies the chemical composition parameters of the electrolyte by introducing phosphazene-based additives with specific molecular structures (Formulae 1-3). These parameter changes enable the formation of a stable SEI layer with appropriate mechanical and chemical properties that can withstand silicon expansion while preventing harmful reactions, thus improving cycling stability without sacrificing capacity.
2Power
If conventional non-aqueous electrolytes are used, then the battery operates at high voltage, but oxidative instability occurs beyond 4.5 V leading to accelerated decay
Solution Approach 1:
The phosphazene-based compounds act as intermediary substances that form a protective SEI layer on the anode surface. This intermediary layer prevents direct contact between the conventional electrolyte and the silicon anode, blocking oxidative decomposition at high voltages while still allowing efficient lithium ion transport, thus maintaining high operating voltage without accelerated decay.
Solution Approach 2:
The phosphazene additives are consumed during initial cycles to form a stable, long-lasting SEI layer. These short-living additive molecules sacrifice themselves to create a durable protective interface that persists throughout the battery's operational life, enabling high-voltage stability without continuous electrolyte decomposition.
3Reliability
If the solid electrolyte interphase layer is allowed to form on silicon anode surfaces, then electrochemical stability is achieved, but the layer becomes unstable during cycling due to expansion and contraction
Solution Approach 1:
The phosphazene-based SEI layer forms a flexible, elastic film that can dynamically expand and contract with the silicon anode during cycling. This flexible structure maintains continuous coverage and electrochemical stability throughout volume changes, preventing layer rupture and exposure of fresh silicon surfaces that would trigger unstable SEI formation.
Solution Approach 2:
The SEI layer comprises a composite structure formed by phosphazene compounds with specific molecular architectures (Formulae 1-3). The composite nature of this layer, combining rigid aromatic rings with flexible alkyl chains, provides both structural stability and mechanical flexibility, enabling the layer to maintain integrity during silicon expansion and contraction cycles.
4Quantity of substance
If high-voltage cathodes such as Ni-rich NCM or NCA are paired with silicon anodes, then energy density is improved, but thermal stability and safety concerns arise
Solution Approach 1:
The phosphazene-based SEI layer serves as a thermal barrier and intermediary protective layer between the silicon anode and the high-voltage cathode. This stable interface prevents thermal runaway propagation and reduces exothermic reactions, enabling safe operation of high-energy-density batteries with Ni-rich cathodes while maintaining electrochemical performance.
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 proposed electrolyte system improves the cycling stability, rate capability, and thermal stability of silicon-based anode batteries, reducing capacity fade and gas generation, and enhancing the overall performance and safety of lithium-ion batteries.
Implementation Method 1
an unstable solid electrolyte interphase (SEI) layer can develop on the surface of the cycled anodes. As the active material expands and contracts during each charge-discharge cycle, unreacted Si surfaces in the active material can subsequently be exposed to the liquid electrolyte and form thicker SEI layers
Implementation Method 2
oxidative instability of the conventional non-aqueous electrolyte takes place at voltages beyond 4.5 V, which can lead to accelerated decay of cycling performance
Implementation Method 3
the electrolyte serves to facilitate ionic transport between the first electrode and the second electrode
Data Source
AI summary
Electrolytes and electrolyte additives for energy storage devices comprising phosphazene based compounds are disclosed. The energy storage device comprises a first electrode and a second electrode, wherein at least one of the first electrode and the second electrode is a Si-based electrode, a separator between the first electrode and the second electrode, an electrolyte comprising at least two electrolyte co-solvents, wherein at least one electrolyte co-solvent comprises a phosphazene based compound.