The cycle performance of Li//LNMO cells at 1 C rate. The red line represents the new All-Fluorinated Electrolyte (AFE), which maintains significantly higher capacity and stability over 250 cycles compared to the standard carbonate electrolyte (blue line), enabling long-lasting high-voltage operation. Credit: Energy Materials and Devices, Tsinghua University Press.
New research suggests an all-fluorinated electrolyte can help unlock the potential of high-voltage lithium metal batteries.
As the global demand for electric vehicles (EVs) accelerates, the race is on to develop batteries that offer higher energy density, lower costs and reduced reliance on scarce resources like cobalt.
Spinel LiNi0.5Mn1.5O4 (LNMO) has emerged as a top contender for next-generation cathodes due to its high operating voltage (4.7 V vs. Li+/Li) and cobalt-free composition. However, its commercial adoption has been stalled by a critical flaw: electrochemical instability.
Standard carbonate-based electrolytes break down under the high-voltage conditions required by LNMO, leading to rapid performance fading.
Now, a team of researchers led by Huolin Xin from University of California, Irvinehas developed a solution: an all-fluorinated electrolyte (AFE) that acts as a stabiliser for these high-voltage systems.
Peichao Zou, a former postdoctoral researcher in the research group, said: “To boost the energy density of batteries, we need to push the voltage limits.
“But conventional electrolytes are like using an oil that burns off when the engine gets too hot – they simply oxidise and decompose at voltages above 4.2V.
“Our goal was to design an electrolyte that could withstand the aggressive 4.7V environment of LNMO cathodes.”
Special chemical properties
The team’s solution involves replacing traditional solvents with fluorinated counterparts combined with a boron-containing additive (TMSB).
Fluorinated solvents have special chemical properties that make them resistant to oxidation.
In testing, the new AFE demonstrated an impressive stability window, enduring voltages up to 6.5 V without decomposing.
The secret to this enhanced performance lies in the interface. The study reveals that the AFE promotes the formation of a robust Cathode-Electrolyte Interphase (CEI) layer rich in fluorine and boron.
Zou said: “Think of the CEI layer as a protective skin that forms on the cathode.
“With standard electrolytes, this skin is weak and keeps breaking, consuming the battery’s fluid.
“Our fluorinated electrolyte builds a tough, stable armour that stops side reactions and prevents the metal structure of the cathode from dissolving.”
Significant
The results are significant for the future of battery longevity. In comparative tests, Li//LNMO cells using the new AFE retained 84.1% of their capacity after 250 cycles at a high cut-off voltage of 4.9V, significantly outperforming cells with standard electrolytes.
Furthermore, the battery showed remarkable resilience at elevated temperatures (50°C), a common condition in real-world EV usage, where standard batteries typically degrade fastest.
While the study marks a major step forward, the team acknowledges that challenges remain, particularly regarding operation in freezing conditions.
The fluorinated electrolyte currently exhibits higher viscosity, which can slow down ion movement at temperatures like -10°C.
Lulu Ren, another former postdoctoral researcher in the research group, said: “This study presents a facile and effective approach to promoting the commercialization of high-voltage LNMO cathodes.
“Our next steps involve optimising the formula to improve low-temperature conductivity and fast-charging capabilities, ensuring these batteries are ready for all-climate applications.”
The study is published in Energy Materials and Devices.
