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

Thackeray – ANL
V.B.9 Layered Cathode Materials (ANL)
· Design effective surface structures to protect the
underlying metal oxide particles from the electrolyte
and to improve their rate capability when charged
(delithiated) at high potentials
· Explore alternative synthesis techniques to synthesize
advanced electrode materials and surface structures
and architectures: 1) autogenic reactions; 2)
sonication methods
· Use first principles modeling to aid the design of bulk
and surface cathode structures and to understand
electrochemical phenomena.
Results
Stabilization of Composite Bulk Structures.
Lithium- and manganese-rich mixed metal oxide
electrodes with integrated structures, such as ‘layeredlayered’
xLi 2 MnO 3 (1-x)LiMO 2 (M=Mn, Ni, Co) can
provide a very high rechargeable electrochemical capacity
of 240-250 mAh/g if they are initially charged to 4.6 V.
Over recent years, it has been established that if these
composite electrode structures are continually cycled
between 4.6 and 2.5 V, the cell voltage decays during
discharge, compromising energy efficiency, cycling
stability and cell performance. In FY2011, studies were
initiated to address this issue.
The electrochemical charge/discharge profiles of
Li/Li 2 MnO 3 •LiNi 0.44 Co 0.25 Mn 0.31 O 2 cells cycled between
4.6 and 2.0 V at a ~C/15 rate at room temperature (RT)
and at 55C are shown in Figure V - 48a and Figure V - 48b,
respectively; dQ/dV plots of the high temperature profiles
(Figure V - 48b) are provided in Figure V - 48c. It is clear from
these data that, at both room temperature and at 55C, the
discharge voltage is continuously suppressed on cycling,
leading to a reduction in cell energy and energy efficiency;
the voltage decay occurs more rapidly at 55C. In
particular, the dQ/dV data clearly indicate a gradual shift
of the discharge potentials at approximately 3.7 and 3.3 V,
characteristic of the original activated, layered electrode
structure, to approximately 2.9 V after 50 cycles.
Although the reasons for this behavior are not yet fully
understood, the change in the electrochemical profiles
strongly suggest that activation of the
Li 2 MnO 3 •LiNi 0.44 Co 0.25 Mn 0.31 O 2 electrode at 4.6 V and
continued cycling to this high charging potential are likely
accompanied by migration of manganese and nickel ions
from the transition metal layers to the lithium layers,
yielding a complex, composite structure in which spinellike
components become embedded within the layered
component of the parent structure.
5.0 5.0
(a)
4.5 4.5
(b)
0.004
(c)
ltag
age (V)
Cell Volt
4.0
3.5
3.0
2.5
1 st ,2 nd ,10 th ,20 th ,30 th ,50 th
Cell Voltage (V)
4.0
3.5
3.0
2.5
5 th , 10 th ,20 th ,30 th ,50 th
dQ/dV (Ah/V)
0.002
0.000
-0.002
2.0 2.0
0 100 200 300
Capa
paci
city
(mAh/g)
-0.004
0 100
200 300
2.0 2.5 3.0 3.5 4.0 4.5
Capacity (mAh/g
) Cell Voltag
age (V)
Figure V - 48: Plots of Li/Li2MnO3•LiNi0.44Co0.25Mn0.31O2 cells cycled between 4.6 and 2.0 V, ~C/15 rate, at (a) room temperature, and (b) 55C; (c) dQ/dV plots
of (b).
Recent work using Li 2 MnO 3 as a template to prepare
stabilized composite electrode structures has yielded very
promising results; this aspect of the work will be continued
as part of a new BATT cathode project in FY2012 (See
BATT report in section V.B.14)
Surface Treatment Using Aggressive Reactions.
Aggressive reactions, such as autogenic reactions, have
been used to prepare electrode materials and surfacecoated
(specifically carbon-coated) electrodes, such as
carbon-coated TiO 2 , with considerable success. These
studies were extended to explore other aggressive
reactions, such as sonochemical reactions, as a route to
fabricating surface layers to strengthen and protect
composite xLi 2 MnO 3 •(1-x)LiMO 2 electrode structures and
particles to electrochemical reactions.
For the initial studies, an Argonne standard electrode
material with the composition
0.5Li 2 MnO 3 •0.5LiNi 0.44 Co 0.25 Mn 0.31 O 2 (referred to as
NMC in Figure V - 48 and Figure V - 49) was used. The
electrode particles were coated with a 10-30 nm layer of
TiO 2 by pulsed sonication in an aqueous solution of
TiOSO 4 . The morphology of the parent electrode powder
as determined by high-resolution scanning electron
microscopy (HR-SEM) showed that 10-20 µm secondary
particles consist of an assemblage of smaller primary
nanoparticles. EDS spectra confirmed the expected
composition (Mn, Ni, Co and O) of the compound, lithium
being too low in atomic number to be detected by this
method.
Figure V - 49 shows the results of a comparative rate
study of Li 2 MnO 3 •LiNi 0.44 Co 0.25 Mn 0.31 O 2 electrodes (both
FY 2011 Annual Progress Report 509 Energy Storage R&D