Adam Tornheim

Functionality Selection Principle for High Voltage Lithium-ion Battery Electrolyte Additives

A new class of electrolyte additives based on cyclic fluorinated phosphate esters was rationally designed and identified as being able to stabilize the surface of a LiNi0.5Mn0.3Co0.2O2 (NMC532) cathode when cycled at potentials higher than 4.6 V vs Li+/Li. Cyclic fluorinated phosphates were designed to incorporate functionalities of various existing additives to maximize their utilization. The synthesis and characterization of these new additives are described and their electrochemical performance in a NMC532/graphite cell cycled between 4.6 and 3.0 V are investigated. With 1.0 wt % 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphospholane 2-oxide (TFEOP) in the conventional electrolyte the NMC532/graphite cell exhibited much improved capacity retention compared to that without any additive. The additive is believed to form a passivation layer on the surface of the cathode via a sacrificial polymerization reaction as evidenced by X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonsance (NMR) analysis results. The rational pathway of a cathode-electrolyte-interface formation was proposed for this type of additive. Both experimental results and the mechanism hypothesis suggest the effectiveness of the additive stems from both the polymerizable cyclic ring and the electron-withdrawing fluorinated alkyl group in the phosphate molecular structure. The successful development of cyclic fluorinated phosphate additives demonstrated that this new functionality selection principle, by incorporating useful functionalities of various additives into one molecule, is an effective approach for the development of new additives.

Methodology for understanding interactions between electrolyte additives and cathodes: a case of the tris(2,2,2-trifluoroethyl)phosphite additive

Use of electrolyte additives is a promising route to address surface destabilization issues of lithium transition metal (TM)-oxide cathodes (for example, lithium nickel-manganese-cobalt oxides (NMCs)) that occur as they are charged to high voltages (>4.3 V vs. Li/Li+). Despite the successful discovery of several additives, their working mechanisms are often vaguely understood. In this work, we provide a methodology to comprehensively understand additive/cathode interactions in lithium-ion batteries. A case of the tris(2,2,2-trifluoroethyl)phosphite (TTFP) additive is presented where its decomposition behavior was investigated at 4.6 V vs. Li/Li+ in a Li4Ti5O12 (LTO)/Li1.03(Ni0.5Mn0.3Co0.2)0.97O2 (NMC532) cell. Overall, we found that while some of the additive does modify the surface film on the cathode and binds at the surface, it does not passivate the cathode surface towards electrolyte oxidation. Rather, the majority of the TTFP forms stable, free tris(2,2,2-trifluoroethyl)phosphate (TTFPa) molecules by removing O atoms from the charged NMC cathode surface, some of which then further react with the electrolyte solvents and stay in solution. Finally, we propose a stable configuration in which TTFP is bound to the cathode surface via a P–O–TM bond, with one of the –CH2CF3 side groups removed, leading to the formation of BTFPa (bis(2,2,2-trifluoroethyl)phosphate). We anticipate that these techniques and findings could be extended to other additives as well, especially phosphite-based additives, allowing the effective design of future additives.

Additives for Functional Electrolytes of Li-Ion Batteries

The electrolyte is an indispensable element of Li-ion batteries. In normal operation, the electrolyte does not participate in electrochemical reactions but rather conducts ions to enable the electrode reactions on the cathode and anode. The electrolyte is typically composed of a lithium salt as the solute for lithium ions and a solvent or mixed solvent as the medium for ionic conduction.