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

Chen – LBNL
V.B.11 Crystal Studies on High-energy Density Cathodes (LBNL)
spectroscopic, spectromicroscopic, scanning 5.0
calorimetry and electron microscopic techniques.
1 st cycle
· Optimize synthesis and processing conditions,
improve performance and safety of the cathode
materials based on the structural and mechanistic
4.0 1 st cycle a
understandings.
Results
3.0
Layered Oxides. Through systematic studies on
micron-sized, plate-shaped Li 1+x (Ni 0.33 Mn 0.33 Co 0.33 ) 1-x O 2
(x=0 for stoichiometric and 0.14 for overlithiated) single
crystals, the team has previously reported that excess Li
increases the average oxidation state of the transition metal
ions, facilitates the formation of an in-plane 3a hex
3a hex superstructure that converts R-3m to P3 1 12 space
group, improves the O3 phase stability and decreases the
unit cell volume change upon chemical delithiation. The
electrochemical performance of the oxides is further
compared in Figure V - 57. At C/20 rate, an irreversible
voltage plateau associated with O 2 release was observed at
4.4 V on the overlithiated oxide (Figure V - 57a). The
cathode delivered a discharge capacity of 175 mAh/g, as
compared to 140 mAh/g for the stoichiometric electrode.
On the dQ/dV plot (Figure V - 57b), a second discharge
peak at 3.3 V was observed, and the charging peak at 3.9 V
shifted to 3.6 V after charging through the activation
plateau, indicating the participation of lower voltage
components thereafter. When cycled between 2.5 to 4.8 V,
the overlithiated oxide showed improved capacity
retention and rate capability (Figure V - 57c), likely a result of
the enhanced phase stability upon deep Li extraction at
high voltages.
Spinel LiNi x Mn 2-x O 4 . Well-formed spinel crystals
were synthesized by a molten salt method. In the presence
of a flux, phase-pure LiNi 0.5 Mn 1.5 O 4 single crystals formed
at 550°C, well below the temperature typically used for
solid state synthesis. A rock-salt type impurity phase
appeared above 600°C when MnO 2 and Ni(OH) 2 were
used as precursors (Figure V - 58), but the temperature
increased to 700°C when Mn(NO 3 ) 2 and Ni(NO 3 ) 2 were
used. In both cases, increasing synthesis temperature led
to lattice expansion of the spinel phase but contraction of
the rock-salt phase, suggesting changes in chemical
compositions in both phases.
Voltage (V)
dQ/dV
Discha harge capacity (mAh/g)
4.5
3.5
2.5
2.0
0 50 100 150 200 250 300 350 400
Capacity (mAh/g)
1400
1200
1000 1 st cycle
800 1 st cycle
600
400
200
0
-200
-400
2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6
Voltage (V)
200
160
120
80
40 9 18 36 90 180
360
mA/g
0
0 2 4 6 8 10 12 14 16 18
Cycle number
Figure V - 57: a) Charge-discharge profiles, b) dQ/dV plots for the first two
cycles. Filled symbols: first cycle; open symbols: second cycle, and c) rate
comparison of the oxides at the indicated current densities. Data for x=0
and 0.14 are shown in black and red, respectively.
b
c
FY 2011 Annual Progress Report 519 Energy Storage R&D