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

V.E.6 Investigation of Critical Parameters in Li-ion Battery Electrodes (LBNL)
Jordi Cabana
Lawrence Berkeley National Laboratory
1 Cyclotron Rd. MS62R0203
Berkeley, CA 94720-8168
Phone: (+1) 510-486-7097
Fax: (+1) 510-486-8097
e-mail: jcabana@lbl.gov
Start Date: September 2009
Projected End Date: September 2011
Objectives
· Establish chemistry-structure-properties correlations
that aid in the design of better materials active at high
voltages (>4.5 V).
· Synthesize materials with controlled crystal-chemistry
and microstructure.
· Assess origins of in-cycle and cycling inefficiencies in
positive electrode materials reaction at high voltages.
· Develop methods to couple electrode performance
and transformations at multiple length scales.
Technical Barriers
· Low energy-density, poor cycle life, safety.
Technical Targets
· PHEV: 96 Wh/kg, 5000 cycles; EV: 200 Wh/kg; 1000
cycles.
Accomplishments
· LiNi 0.5 Mn 1.5 O 4 was found to crystallize in a variety of
schemes with different Ni/Mn ordering. Some
samples showing unit cell superstructures were still
found to have different levels of Ni/Mn mixing. NMR
and neutron diffraction (ND) have been established as
the best tools to characterize ordering in this material.
· Mn over-stoichiometry was found in all
LiNi 0.5 Mn 1.5 O 4 samples made, but no evidence of O
vacancies. Mn 3+ observed in the cycling profile is due
to a preferential segregation of Ni in a rock salt
impurity, which also contains Mn.
· Amounts of impurity and Mn 3+ increase with
synthesis temperature. Impurity is detrimental to
performance; needs to be minimized during material
preparation.
· Developed a -XAS method to evaluate charge
distribution with high spatial resolution. Analyzed
discharge inefficiency dependence on rate for
conversion model system.
Introduction
Finding Li-ion battery electrode materials that can
bring about increases in energy is a critical need if the
social impact of their use in electric vehicles is to meet
expectations. In order to fulfill this goal, the following
strategies can be envisaged: i) raising the voltage of
operation of the battery by using electrodes that react at
very high and very low potentials, respectively, and/or ii)
improving the storage capacity by switching to alternative
electrode materials that can exchange a larger amount of
electrons/Li + ions. Yet these changes cannot come with a
penalty in terms of device safety and cycle life of the
device, which implies that the mechanisms of their
reaction with lithium need to be well understood in order
to locate possible sources of failure.
Spinel-type LiNi 0.5 Mn 1.5 O 4 is a promising candidate
for the positive electrode because lithium is extracted at
very high potentials (around 4.7 V vs. Li + /Li 0 ),
concomitant to the oxidation of Ni 2+ to Ni 4+ . While very
high rate capability has been reported in several cases, it
has not been fully ascertained what the role is of the
crystal-chemistry of the compound, such as metal ordering
and the existence of impurities of Mn 3+ in the spinel and
segregated rocksalt particles. During FY2010, it was
determined that nanostructuring is not necessary to get
satisfactory performance with this material. During
FY2011, we proceeded to investigate the crystalchemistry-properties
correlation.
Some gaps in the knowledge of how batteries operate
still remain. One example of these gaps is the difficulty in
probing charge distribution within battery electrodes.
These electrodes are usually composites of the
electrochemically active material with carbon and a
polymer binder to form a flexible film that is several tens
of microns thick, in which homogeneity and porosity are
the key for good electrical contact, electrolyte wetting and
mechanical properties. Non-uniformities in the state of
charge may impact performance in a variety of ways,
including reduced energy and power, underutilization of
capacity, localized heat generation, and overcharge or
overdischarge. Since reactions at an electrode involve
redox phase transformations, the state of charge can easily
be correlated to composition. Several models exist that
FY 2011 Annual Progress Report 647 Energy Storage R&D