V. Focused Fundamental Research - EERE - U.S. Department of ...

V.C.9 New Layered Nanolaminates for Use in Lithium Battery Anodes (Drexel U.) Gogotsi, Barsoum – Drexel U.
other MAX phases and varying the amounts of A and M
atoms extracted from the carbides or nitrides. In addition
we believe given the outstanding mechanical properties of
the MAX phases such issues as electrode degradation due
to solvent induced volume change shall be minimized.
Overall based on the known properties of MAX
phases one can expect the following improvements to be
obtained: (i) higher energy density; (ii) higher power
density; (iii) small irreversible capacity, and, (iv) longer
cycle life due to moderate expansion and much better
mechanical properties.
Approach
Since at this time the relationship between capacity
and MAX phase chemistry is unknown, a rapid screening
of as many MAX phases as possible shall be carried out to
find out the most promising chemistry, by testing their
performance in LIB. This process will be guided by ab
initio calculations. Reducing particle size, selective etching
of an A element from the MAX structure, and exfoliation
of these layered structure also will be investigated to
increase the Li uptake of these structures and increase the
charge density.
Results
Ab intio calculations- using density functional theory
(DFT) using the plane-wave pseudo-potential approach,
with ultrasoft pseudopotentials and Perdew Burke
Ernzerhof (PBE) exchange - Wu-Cohen (WC) correlation
functional, as implemented in the CASTEP code in
Material Studio software - showed possibility of Li–
intercalation inside MAX phases with volume expansion
around 30 %. The elechtrochemical measurments for Li
uptake, however, into different MAX phases (~20 μm
particle size) was however found to be low. Reduction of
particle size to around 1 µm resulted in doubling the
capacity, but it was still low (see Figure V - 106). In order to
increase the Li uptake, selective etching of the “A” layer
was carried out to introduce more space for Li in the
structure. Selective etching of the Al out of Ti 3 AlC 2 (a
typical MAX phase) structure using diluted hydrofluoric
acid at room temperature, followed by ultrasonication
resulted in the formation of the exfoliated Ti 3 C 2 layers
(that are calling “MXene” to emphasize its graphene-like
morphology). The exposed Ti surfaces appear to be
terminated by OH and/or F. Not only individual layers are
formed (Figure V - 104, a-d), but also conical scrolls and
nanotubes (Figure V - 105, a-d). The elastic modulus
(predicted by ab initio simulation) of a single, exfoliated
Ti 3 C 2 (OH) 2 layer, along the basal plane, is calculated to be
around 300 GPa, which is within the typical range of
transition metal carbides and significantly higher than most
oxides and clays. Ab initio calculations also predict that
MXene band gap can be tuned by varying the surface
terminations. When terminated with OH and F groups, the
band structure has a semiconducting character with a clear
separation between valence and conduction bands by 0.05
eV and 0.1 eV, respectively. The good conductivity and
ductility of the exfoliated powders suggest uses in Li-ion
batteries, pseudocapacitors and other electronic
applications. Assuming 2 Li atoms layers can be
accommodated inside the structure (viz. Ti 3 C 2 Li 2 ) a
theoretical capacity of 320 mAh g -1 - which is comparable
to the 372 mAh g -1 of graphite for (LiC 6 ) – is predicted.
There are over 60 currently known MAX phases and
our discovery opens the door for the synthesis of a large
number of 2-D M n+1 X n structures, including the carbides
and nitrides of Ti, V, Cr, Nb, Ta, Hf and Zr. The latter
could include 2-D structures of combination of M-atoms,
(e.g. (V 0.5 Cr 0.5 ) 3 C 2 ) and/or different carbo-nitrides (e.g.
Ti 3 (C 0.5 N 0.5 ) 2 ).
Figure V - 104: TEM images of exfoliated MXene nanosheets. (a) TEM
micrographs of exfoliated 2-D nanosheets of Ti-C-O-F. (b) Exfoliated 2-D
nanosheets; inset SAD shows hexagonal basal plane. (c) HRTEM image
showing the separation of individual sheets after ultra-sonic treatment. (d)
HRTEM image of bilayer Ti3C2(OH)xFy.
Energy Storage R &D 568 FY 2011 Annual Progress Report