We demonstrate the reversible lithiation of SiO2 up to 2/3 Li per Si, and propose a mechanism for it based on molecular dynamics and density functional theory simulations. Our calculations show that neither interstitial Li (no reduction), nor the formation of Li2O clusters and Si–Si bonds (full reduction) are energetically favorable. Rather, two Li effectively break a Si–O bond and become stabilized by oxygen, thus partially reducing the SiO2 anode: this leads to increased anode capacity when the reduction occurs at the Si/SiO2 interface. The resulting LixSiO2 (x<2/3) compounds have band-gaps in the range of 2.0–3.4 eV.
Using physical insights and advanced first-principles calculations, we suggest that corundum (α-Al2O3) is an ideal gate dielectric material for graphene transistors. Clean interface exists between graphene and Al-terminated (or hydroxylated) Al2O3 and the valence band offsets for these systems are large enough to create injection barrier. Remarkably, a band gap of ∼ 180 meV can be induced in graphene layer adsorbed on Al-terminated surface with an electron effective mass of ∼ 8 × 10 −3 me. Moreover, the band gaps of graphene/Al2O3 system could be tuned by an external electric field for practical applications.
We explore the stability, electronic
properties, and quantum capacitance
of doped/co-doped graphene with B, N, P, and S atoms based on first-principles
methods. B, N, P, and S atoms are strongly bonded with graphene, and
all of the relaxed systems exhibit metallic behavior. While graphene
with high surface area can enhance the double-layer capacitance, its
low quantum capacitance limits its application in supercapacitors.
This is a direct result of the limited density of states near the
Dirac point in pristine graphene. We find that the triple N and S
doping with single vacancy exhibits a relatively stable structure
and high quantum capacitance. It is proposed that they could be used
as ideal electrode materials for symmetry supercapacitors. The advantages
of some co-doped graphene systems have been demonstrated by calculating
quantum capacitance. We find that the N/S and N/P co-doped graphene
with single vacancy is suitable for asymmetric supercapacitors. The
enhanced quantum capacitance contributes to the formation of localized
states near the Dirac point and/or Fermi-level shifts by introducing
the dopant and vacancy complex.
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