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

V.C.3 Search for New Anode Materials (U. Texas)
Goodenough – U. Texas
/Fe(CN) 6 4- and a commercial Li + electrolyte with a σ Li ≈
10 -4 S cm -1 containing Ti 4+ and (PO 4 ) 3- ions, neither of
which is stable on contact with lithium.
It soon became apparent that a large capacity with this
strategy would require a flow-through cathode, so we built
the cell of Figure V - 81and showed it gave a V ≈ 3.5 V
with no capacity fade on cycling. However, our use of an
aqueous cathode requires an oxide separator. Weppener’s
group has reported a σ Li ≈ 10 -4 S cm -1 in a nominal
Li 7 La 3 Zr 2 O 12 , which has Li + in the interstitial space of the
garnet framework La 3 Zr 2 O 12 . The interstitial space
consists, per formula unit (f.u.), of three tetrahedral sites
bridged at each face by an octahedral site sharing two
opposite faces to give a total of 9 interstitial sites/f.u. It
was also known that nominal Li 7 La 3 Zr 2 O 12 contains
adventitious Al 3+ as a result of sintering above 1100°C in
an alumina crucible. First, we obtained Al-free
Li 7 La 3 Zr 2 O 12 and found it decomposes above 800°C. Then
we used neutron diffraction to locate the Al 3+ in interstitial
octahedral sites and, with our Al-free sample, to show that
the maximum possible Li content in the garnet framework
would be 7.5 Li/f.u., and then only if the Li vacancies were
ordered on half the interstitial tetrahedral sites. The
practical upper limit would be ≈ 7 Li/f.u., and since the
Al 3+ would be stabilizing the garnet framework perhaps by
lowering the Li concentration to less than 7 Li/f.u., we
undertook a study of the system Li 7-x La 3 Zr 2-x Ta x O 12 to
determine the x at which σ Li ia a maximum. With x = 0.6
and sintering in an alumina crucible, we obtain a σ Li ≈10 -3
S cm -1 in a narrow range at x as shown in Figure V - 82.
However, the samples had a tan color, indicative of oxygen
vacancies acting as color centers that appear to be
associated with the adventitious Al 3+ . Our white samples
have proven difficult to densify at the low temperatures
needed to retain the Li, indicating that the Al 3+ is also
acting as a sintering aid. We are concerned that Li 2 O
and/or Al 2 O 3 in the grain boundaries will be attacked by an
aqueous cathode, so we are in the process of examining
how to make a dense ceramic membrane without a
sintering aid.
Conclusions and Future Directions
The viability of a Li battery containing a Li + ­
electrolyte separator and a liquid cathode has been
demonstrated. An oxide Li + electrolyte stable on contact
with lithium and having a σ Li ≈10 -3 Scm -1 has been
identified, but the next challenge is fabrication of the
electrolyte into a thin, dense membrane that is
mechanically robust. We plan to construct cells that allow
testing in electrolyte liquids, including oxidative salts, the
Li + electrolyte is stable on cycling, whether thin
membranes can be made that block dendrite growth, and to
determine what charge/discharge rates are practical.
3.8
0.45 120
3.6
0.40 100
Potential (V)
3.4
3.2
V=0.35 V 0.35 80
Capacity (mA h)
0.30 60
0.25 40
3.0
0.20 20
0.01 M Fe(CN) 3­
6 Capacity
0.1 M Fe(CN) 3­
Coulombic Efficiency
6
2.8
0.15 0
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0 200 400 600 800 1000
Normalized capacity
Cycle number
Coulombic Efficiency (%)
Figure V - 81: Structure of Lithium/aqueous cathode cell and its charge/discharge behavior.
Energy Storage R &D 546 FY 2011 Annual Progress Report