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Journal of the American Chemical Society
We investigate the performance of a representative soft
cycles and observed that the seperator turns yellowish
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carbon-pyrolyzed PTCDA annealed at 900 °C.
This
temperature is high enough to remove most of non-carbon
elements in PTCDA but low enough not to crystalize the
carbon structure. Most importantly, this carbon exhibits a
low density of 1.6 g/cc vs. 2.3 g/cc of graphite. Compared to
graphite, the (002) XRD peak of soft carbon is much
broadened (Figure 3A), which shifts to a lower angle,
(Figure S8). In future work, electrolyte optimization and
pre-potassiation will be carried out to improve the cycling
performance and CEs.
In summary, we, for the first time, reveal that potassium
can be reversibly inserted into graphite with a high capacity
of 273 mAh/g in electrochemical cells. Revealed by ex situ
XRD, we discovered that upon potassiation, the stage-one
KC8 forms via stage-three KC36 and stage-two KC24 as
intermediate phases, where the phase transformations are
reversible in converting KC8 back to a less crystalline
graphite. Graphite in KIBs suffers fast capacity fading and
moderate rate capability, which may be due to the large
volume change over cycling. To further improve the
performance, we investigate a low-density soft carbon as a
KIB anode, which exhibits much improved cycling life and
very high rate capability. The new results fill up important
knowledge gaps on carbon electrodes in alkali metal ion
batteries and may open up a completely new paradigm for
energy storage solutions.
revealing a larger average d-spacing of 0.355 nm.
A
representative TEM image reveals the turbostratic structure
of this soft carbon (Figure S5). The corresponding selected
area electron diffraction (SAED) presents halo rings,
indicating its polycrystalline nature with a short-range order
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(
inset of Figure S5). Wide bands and high intensity ratio of
D-band over G-band in Raman spectrum corroborate the less
graphenic carbon structure (Figure S6).
ASSOCIATED CONTENT
Supporting Information
Experimental section, Raman, XRD, TEM, digital photo,
molecular
structure
of
PTCDA
and
potassiation/depotassiation potential profiles at various
rates. The Supporting Information is available free of charge
on the ACS Publications website at DOI: XXXXXXXX.
AUTHOR INFORMATION
Corresponding Author
Figure 3. (A) XRD pattern of soft carbon compared with
graphite. (B) GPD profiles of soft carbon for the initial two
cycles between 0.01 and 1.5 V at C/40. (C) Rate performance
of soft carbon; and (D) cycling performance of soft carbon at
2C.
Notes
The authors declare no competing financial interests.
Compared to graphite, soft carbon exhibits very different
GPD potential profiles, where only slopes exist instead of
plateaus, as shown in Figure 3B, which is also observed in
sodiation of soft carbon. The cause for sloping instead of
plateau is under investigation. This carbon also exhibits a
high depotassiation capacity of 273 mAh/g at C/40, an
interesting coincidence since it is the same as graphite. The
rate capability of soft carbon is very impressive (Figure 3C).
At 1C and 2C, soft carbon exhibits very high capacities of 210
and 185 mAh/g, respectively, compared with 264 mAh/g at
C/10. Even at 5C (1395 mA/g), the soft carbon still retains a
capacity of 140 mA/g. All GPD potential profiles at different
rates show the similar slope behavior (Figure S7). Such high-
ACKNOWLEDGMENT
We acknowledge the financial supports from Oregon State
University and Advanced Research Projects Agency-Energy
(
ARPA-E), Department of Energy of the United States, Award
number: DE-AR0000297TDD. We thank Mr. Clement Bom-
mier, Dr. Wenze Han and Dr. Yang Sun for their discussion.
We thank Professor Michael M. Lerner for providing the
graphite samples and discussion. The authors are grateful to
Professor Douglas A. Keszler for ex situ XRD measurements
and Professor Chih-Hung Chang for Raman measurements.
rate behavior is comparable to the best rate capability of
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