LiTMSP Additive Mitigates HF Corrosion in LNMO Graphite Cells
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Solution Overview
Problem
Lithium-ion batteries face issues with high resistance and capacity degradation due to metal ion dissolution and electrolyte instability, particularly in high-voltage LNMO/graphite cells, which are exacerbated by HF generation and surface film deterioration.
Innovation Solution
The introduction of lithium bis(trimethylsilyl) phosphate (LiTMSP) as a bi-functional additive in the electrolyte, which scavenges HF and forms a protective surface film, reducing metal ion dissolution and enhancing cycling and rate performance by improving the solid electrolyte interphase (SEI) layer on graphite electrodes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stress or pressure
If conventional LiPF6/carbonate-based electrolytes are used in high-voltage LNMO batteries, then high working voltage (beyond 4.7 V) is achieved, but electrolyte oxidation occurs causing gas production, film deposition, and metal ion dissolution
Solution Approach 1:
LiTMSP acts as an intermediary substance between the high-voltage LNMO electrode and the electrolyte. It forms a protective interface layer that mediates the interaction, preventing direct contact and oxidation between the electrolyte and electrode surface, thereby stabilizing the electrolyte while maintaining high voltage operation
Solution Approach 2:
The patent converts the harmful oxidation reaction into a beneficial process by controlling the initial formation cycle to create a stable protective film. The oxidation that would normally degrade the electrolyte is harnessed to form a stable interface layer that prevents further harmful reactions during subsequent cycling
2Quantity of substance
If high nickel layered oxides (NCM) are used to achieve high capacity (>200 mAh g-1), then energy density is improved, but cost increases due to skyrocketing cobalt prices
Solution Approach 1:
The patent changes the compositional parameters of the cathode material by using high-nickel low-cobalt formulations (such as LiNi0.8Co0.1Mn0.1O2). This parameter change maintains high capacity while reducing cobalt content, thereby lowering material cost without significantly compromising electrochemical performance
3Ease of manufacture
If LNMO is paired with graphite electrodes to achieve cost competitiveness, then manufacturing cost is reduced, but metal ion dissolution by HF causes SEI layer deterioration and increasing cell resistance
Solution Approach 1:
The patent converts the harmful HF dissolution reaction into a beneficial process by controlling the formation cycle to create a stable protective film. The initial controlled reaction products form a stable interface layer that prevents further HF attack on the graphite SEI layer, transforming the harmful dissolution into a protective mechanism
Solution Approach 2:
A protective interface layer is formed as an intermediary between the LNMO electrode and graphite anode. This layer acts as a barrier that mediates the interaction, preventing HF and dissolved metal ions from reaching and deteriorating the SEI layer on the graphite electrode, thereby maintaining low cell resistance
4Stress or pressure
If continuous electrolyte oxidation occurs on electrode surface, then high voltage operation is maintained, but gas production and film deposition increase causing capacity degradation
Solution Approach 1:
The protective interface layer formed by LiTMSP acts as an intermediary barrier between the electrolyte and electrode surface. It allows high voltage operation to be maintained while preventing direct oxidation reactions that would produce gas and degradable films, thereby preserving battery capacity
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
LiTMSP significantly decreases the resistance of LNMO/graphite cells, improves cycling performance at elevated temperatures, and enhances rate performance by mitigating electrolyte decomposition and metal ion migration, leading to better lithium-ion conductivity and capacity retention.
Implementation Method 1
The specific functions of the additives like are scavenging corrosive acid species like HF or phosphorus pentafluoride (PF5), or passivating the surface film on the electrode, depend on the functional groups in the additives. It was reported that siloxane derivatives like silyl group (Si—O) can capture HF or H2O to form silyl fluoride (Si—F).
Implementation Method 2
Lithium phosphate derivatives such as lithium difluorophosphate (LiPO2F2) and lithium dimethyl phosphate (LiDMP) were reported to improve the cycle and rate performance of NCM/graphite cells with modifying the chemical composition of solid electrolyte interphase (SEI) layers on graphite electrodes.
Data Source
AI summary
This work investigates the beneficial roles of lithium bis(trimethylsilyl) phosphate (LiTMSP) which may act as a novel bifunctional additive for lithium ion batteries, in particular, LiNi0.5Mn1.5O4 (LNMO)/graphite cells. The cycle performance of LNMO/graphite cells is significantly improved with incorporation of LiTMSP. Trimethylsilyl functional group therein can react with HF generated through hydrolysis of LiPF6 by residual water in electrolyte solution, followed by a decrease in the concentration of metal ions dissolution from the electrode. The generation of superior passivating surface film derived by LiTMSP on graphite electrode, suppressing further electrolyte reductive decomposition and deterioration/reformation caused by migrated metal ions, is confirmed. Furthermore, LiTMSP derived surface film is likely to have better lithium ions conductivity with a decrease in resistance of the graphite electrode, improving rate performance of cells. The HF scavenging and film-forming effects of LTMPS are responsible for the less polarization of cells enabling to improve cycle performance.