Operation of Li-ion
batteries below −20 °C is hindered
by low electrolyte conductivity and sluggish solid-state diffusion
in electrodes. Li metal anodes show promise for low-temperature operation,
but few electrolyte compositions exhibit high conductivity at reduced
temperature while also allowing Li electrodeposition/stripping with
high Coulombic efficiency. Here, we show that the Coulombic efficiency
of Li metal anodes can be substantially improved at low temperatures
(−60 °C) by tailoring the solid-electrolyte interphase
(SEI) structure through the use of two classes of electrolyte solvents:
cyclic carbonates and ethers. Cryogenic transmission electron microscopy
and other methods show that fluoroethylene carbonate (FEC) induces
temperature-dependent changes in the chemistry and structure of the
SEI to be abundant with LiF and Li2CO3, while 17O nuclear magnetic resonance and molecular dynamics calculations
show that FEC affects the solvation behavior and SEI formation process
in this new electrolyte system. Our results demonstrate the promise
of rechargeable Li-metal batteries to enable energy storage over a
broad temperature range.
We
report the fabrication and electrochemical performance of metal-foil
free Li4Ti5O12 (LTO) and LiNi1/3Co1/3Mn1/3O2 (NCM) electrodes
supported on conductive and porous reduced graphene oxide/poly(acrylic
acid) (rGO-PAA) aerogels. The highly porous rGO-PAA (∼6 mg
cm–3) enables slurry infiltration of LTO and NCM
to form composite electrodes with tunable mass loadings (∼3–30
mg cm–2), and the resultant composites can withstand
100-fold compression (from 3.2 mm to ∼30–130 μm)
to achieve electrode densities of 2–3 g cm–3. The adequate compressibility of the rGO-PAA coupled with removal
of the conventional metal-foil weight and volume provides high volumetric
energy densities of 1723 Wh L–1 for NCM and 625
Wh L–1 for LTO at low power density, representing
a 25% increase in energy density over similar electrodes built with
metal-foil current collectors. These metrics demonstrate the utility
of the rGO-PAA current collector to reduce the weight and volume of
lithium-ion electrodes without sacrificing energy density.
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