posted on 2017-08-18, 00:00authored byChi-Cheung Su, Meinan He, Cameron Peebles, Li Zeng, Adam Tornheim, Chen Liao, Lu Zhang, Jie Wang, Yan Wang, Zhengcheng Zhang
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.