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Contributed by William A. Goddard III, May 20, 2015 (sent for review December 9, 2014)
Significance
The combined study of experiments and molecular dynamics simulations demonstrates that metal oxide nanocrystals on graphene can be rescaled into atomic clusters. It is notable that the capacitance of 3,023 F per the mass of NiO, matching the measured capacitance of 2,231 per the total electrode mass, exceeds the theoretical gravimetric capacitance of 2,618 F available via ion-to-atom redox reactions. This approach thus provides a new pathway to realize full capacitance via ion-to-atom Faradaic redox reactions. Furthermore, assembly with a rescaled metal oxide positive electrode shows that further development of high-capacity negative counter electrode materials can pave a new route to address challenging energy storage issues.
Abstract
Nanocrystals are promising structures, but they are too large for achieving maximum energy storage performance. We show that rescaling 3-nm particles through lithiation followed by delithiation leads to high-performance energy storage by realizing high capacitance close to the theoretical capacitance available via ion-to-atom redox reactions. Reactive force-field (ReaxFF) molecular dynamics simulations support the conclusion that Li atoms react with nickel oxide nanocrystals (NiO-n) to form lithiated core–shell structures (Ni:Li2O), whereas subsequent delithiation causes Ni:Li2O to form atomic clusters of NiO-a. This is consistent with in situ X-ray photoelectron and optical spectroscopy results showing that Ni2+ of the nanocrystal changes during lithiation–delithiation through Ni0 and back to Ni2+. These processes are also demonstrated to provide a generic route to rescale another metal oxide. Furthermore, assembling NiO-a into the positive electrode of an asymmetric device enables extraction of full capacitance for a counter negative electrode, giving high energy density in addition to robust capacitance retention over 100,000 cycles.
- rescaled atomic clusters
- metal oxide nanocrystals
- energy storage
- molecular dynamic simulation
- in situ electrochemical spectroscopy
Footnotes
- ↵1To whom correspondence may be addressed. Email: wag{at}wag.caltech.edu or jeung{at}kaist.ac.kr.
Author contributions: H.M.J. and J.K.K. designed research; H.M.J., K.M.C., R.Z., I.W.O., and J.K.K. performed research; D.K.L. and I.W.O. contributed new reagents/analytic tools; T.C. and W.A.G. analyzed data; and H.M.J., K.M.C., T.C., D.J.M., W.A.G., and J.K.K. wrote the paper.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1503546112/-/DCSupplemental.
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