posted on 2021-11-04, 20:04authored byAndrew
W. Ells, Richard May, Lauren E. Marbella
While
Li-ion is the prevailing commercial battery chemistry, the
development of batteries that use earth-abundant alkali metals (e.g.,
Na and K) alleviates reliance on Li with potentially cheaper technologies.
Electrolyte engineering has been a major thrust of Li-ion battery
(LIB) research, and it is unclear if the same electrolyte design principles
apply to K-ion batteries (KIBs). Fluoroethylene carbonate (FEC) is
a well-known additive used in Li-ion electrolytes because the products
of its sacrificial decomposition aid in forming a stable solid electrolyte
interphase (SEI) on the anode surface. Here, we show that FEC addition
to KIBs containing hard carbon anodes results in a dramatic decrease
in capacity and cell failure in only two cycles, whereas capacity
retention remains high (> 90% over 100 cycles at C/10 for both
KPF6 and KFSI) for electrolytes that do not contain FEC.
Using
a combination of 19F solid-state nuclear magnetic resonance
(SSNMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and
electrochemical impedance spectroscopy (EIS), we show that FEC decomposes
during galvanostatic cycling to form insoluble KF and K2CO3 on the anode surface, which correlates with increased
interfacial resistance in the cell. Our results strongly suggest that
KIB performance is sensitive to the accumulation of an inorganic SEI,
likely due to poor K transport in these compounds. This mechanism
of FEC decomposition was confirmed in two separate electrolyte formulations
using KPF6 or KFSI. Interestingly, the salt anions do not
decompose themselves, unlike their Li analogues. Insight from these
results indicates that electrolyte decomposition pathways and favorable
SEI components are significantly different in KIBs and LIBs, suggesting
that entirely new approaches to KIB electrolyte engineering are needed.