The instability and the fading of high voltage cathode materials above 4.3 V remains a major challenge for the next generation of high energy density Li-ion batteries. Here, we present a facile, environmentally friendly, cost effective and scalable method to address this problem by uniformly fluorinating the surface of cathode materials with CHF3, a mild fluorinating agent but a potent greenhouse gas. CHF3 is successfully transformed into ~2 nm LiF homogenous layer covering the surface of layered-oxide cathode materials.
The urgent need for accelerating the decarbonization of human activity to achieve climate neutrality by midcentury has triggered a radical shift in the transport industry toward the electrification of the sector in the next decade. The importance of this technological turning point is adding further urgency to the current search for the next generation of Li-ion/Li-metal batteries with higher energy density and safety standards, while keeping a low environmental footprint. One of the limitations to overcome is to further increase the operating voltage of high voltage cathode materials (nickel-rich and low cobalt content) above 4.3 V vs. Li+/Li without compromising the cycling performance. At high operating voltages, these cathodes suffer from strong interaction with the electrolyte causing various structural and chemical degradation processes on the surface and in the bulk of the cathode particles, leading to a rapid cell resistance rise and to voltage and specific capacity fading upon cycling.
For this purpose, several approaches have been employed for stabilizing the surface and the bulk structure of different families of high voltage cathode materials, including core-shell synthesis, electrolyte additives, coating with chemically and electrochemically inactive inorganic oxides, fluorides, organic polymers, heavy-ion surface doping and bulk anion substitution. Independent of their ability to improve the cycling performance, those processes need also to be scalable, high throughput, cost effective and environmentally friendly.
In this work, we propose an innovative and convenient approach by utilizing a costume made and designed gas flow-type rector operating in a vertical configuration using a mild fluorinating agent (CHF3) to fluorinate the surface of high voltage cathode materials at a moderate temperature of 300 °C. Our procedure consists of converting the thin adventitious Li2CO3 layer covering the surface of the vast majority of the layered oxide cathode materials into a passivating nanometer thick LiF layer that conformably coats the surface of the primary and secondary particles. It is worth mentioning that trifluoromethane (CHF3) is a very potent greenhouse gas, 11700 times more efficient than CO2, however, it is a non-toxic, non-corrosive and non-flammable gas at room temperature produced on a kiloton scale (~20 kton/year) as a side-product in the manufacturing of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and foams. Hence, consuming a stock of potent greenhouse gas by converting it into environmentally friendly byproducts in the form of a uniform thin Li fluoride protective layer on the surface of high-voltage cathode material is an efficient solution to reuse and monetize CHF3 by making it part of a circular economy.
Our approach is validated with a systematic study on the commercially relevant LiNi0.8Co0.15Al0.05O2 (NCA) high voltage cathode material cycled at 4.5 V vs Li+/Li. We demonstrated improved electrochemical cycling performance of the LiF coated NCA when cycled up to 4.5 V where the impedance and overpotential decrease by 30% and 100 mV respectively after 100 cycles, with a capacity retention better than 94% and improved rate performance at high current density. Furthermore, the universality of the fluorination approach is validated further on Ni-rich LiNi0.85Co0.1Mn0.05O2 cathode material cycled up to 4.3 and 4.8 V vs Li+/Li.
Moreover, the fluorination mechanism and the surface evolution of the fluorinated cathodes upon early and long cycling stages have been meticulously examined by combining non-destructive surface and near surface sensitive synchrotron-based techniques, like X-ray absorption spectroscopy (XAS) in total electron yield (TEY) and total fluorescence yield modes (TFY), as well as X-ray photoemission electron microscopy (XPEEM) allowing chemical depth profiling and high-resolution surface elemental mapping respectively. Additionally, operando X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements were used to shed light on the fluorination impact on (i) the structure integrity of the particles and (ii) the morphology, homogeneity, and thickness respectively.
Contact
Dr. Mario El Kazzi
Group head of Battery Materials and Diagnostics
Address: Forschungsstrasse 111, 5232 Villigen PSI, Switzerland
Telephone: +41 56 310 51 49
E-mail: mario.el-kazzi@psi.ch
Original Publication
Title: Converting the CHF3 Greenhouse Gas into Nanometer-Thick LiF Coating for High-Voltage Cathode Li-ion Batteries Materials
Authors: Aleš Štefančič, Carlos Antonio Fernandes Vaz, Dominika Baster, Elisabeth Müller, Mario El Kazzi
Journal: ChemSusChem (Wiley)
Article DOI: 10.1002/cssc.202402057