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

V.F.1 Energy Frontier Research Center at ANL (ANL)
Thackeray – ANL
improve surface stability and rate capability of the
electrode, but little is known about surface structures, or
the mechanisms by which lithium-ion transport occurs at
the electrode surface. The knowledge gained from these
studies will be used to improve the composition and
structure of electrode surfaces and to advance the overall
performance of the electrodes to meet DOE’s 40-mile
PHEV battery requirements.
Approach
Analytical techniques for probing the structureelectrochemical
property relationships of lithium battery
electrode materials, notably at electrode surfaces, include
neutron scattering, x-ray absorption, scattering and
photoelectron spectroscopy, nuclear magnetic resonance,
Raman spectroscopy, Fourier transform infrared
spectroscopy, and electron microscopy. In this project,
analytical efforts are focused predominantly on x-ray
spectroscopic techniques, including in situ experiments,
and high-resolution electron microscopy. Major facilities
are available at Argonne to conduct these experiments,
notably at the Advanced Photon Source (APS) and the
Electron Microscopy Center (EMC).
Surface-protected ‘coated’ cathode materials to be
studied include those with integrated ‘composite’
structures in which the coating contains specific 3d/4d
transition metals which are not present in the core
structure. Coatings will be applied by various techniques,
for example, from solution by standard sol-gel methods or
by atomic layer deposition (ALD). In situ synchrotron
hard x-ray spectroscopic techniques including x-ray
absorption spectroscopy (XAS), resonant and non-resonant
x-ray emission spectroscopy (XES) and x-ray Raman
scattering (XRS) will be used to monitor the interfacial
reactions at the electrode-electrolyte interface. The
important traits of these spectroscopic techniques,
specifically the element specific nature and the sensitivity
to dilute constituents, will allow the monitoring of changes
in the electronic and atomic structures of the coatings
during charge-discharge cycling or during repeated
cycling. A comparison of the electrochemical properties
of uncoated and coated electrodes will be made. It is
envisioned that these studies will provide key information
at the molecular level on the structure of the coatings, the
mechanism of lithium-transport at the electrode-electrolyte
interface and further provide insights into degradation
mechanisms during repeated cycling. Another aspect that
will be investigated is the effect of the coating on the bulk
structure of the composite material itself, particularly on
deintercalation at high voltages, for example 4.6 V, during
the initial charge. Recent XAS studies of uncoated
composite materials (such as those developed under the
BATT program) have shown convincing evidence of
oxygen loss during first charge at these high voltages. In
coated samples, the exact oxygen loss mechanism and the
possible condensation of the bulk structure might be
significantly different and a detailed understanding of the
local structure of the bulk should provide key insights on
the structure-property relationship of the coated
composites. The knowledge gained from both the bulk
and interface using x-ray spectroscopic methods will feed
into the design of improved electrodes to meet the 40-mile
PHEV goals. In addition to studies of composite cathode
structures, studies of well defined electrode surfaces will
be undertaken using spectroscopic and microbeam
methods. Such studies utilize the property of total external
reflection of x-rays at small incident angles, which
minimizes the contribution from the bulk of the material
and provides interface sensitivity without sacrificing the in
situ capability of hard x-rays. These spectroscopic
investigations will complement the x-ray based scattering
approaches which are currently an integral part of
Argonne’s EFRC, Center for Electrical Energy Storage-
Tailored Interfaces (CEES) effort.
Results
Lithium-metal-oxides remain the most promising
cathode materials for high-energy-density lithium-ion
batteries for plug-in hybrid electric vehicles (PHEV) and
all-electric vehicles (EVs). To this end, much effort has
been invested to advance these materials. However, a
significant challenge that remains is the mitigation of
irreversible surface damage at high potentials, which
occurs with adverse consequences to the power delivery of
the cathodes (i.e., rate capability). To date, several surface
passivation techniques have been studied with the goal of
addressing this issue in a variety of cathodes by way of 1)
enhancing the conductive properties of the surface, 2)
modifying the electrode surface chemistry to improve
performance, and 3) providing a physical barrier which
impedes reactions of the surface with the electrolyte.
For this project, a dilute (5 mol%) surface treatment
of lithium-nickel-phosphate (Li:Ni:PO 4 ratio = 1:1:1) was
deposited on a 0.5Li 2 MnO 3 •0.5LiCoO 2 (Li 1.2 Mn 0.4 Co 0.4 O 2 )
cathode material, referred to as LNP-coated LCMO.
Similar treatments have previously been reported by Kang
and Thackeray to enhance electrochemical performance.
However, exact structural determinations could not be
made regarding the outcome of the surface treatment
because the host material was itself a nickel-containing
oxide, 0.5Li 2 MnO 3 •0.5LiNi 0.44 Co 0.25 Mn 0.31 O 2 ,
alternatively, (Li 1.2 Mn 0.52 Ni 0.18 Co 0.1 O 2 ). In the present case,
the host material was chosen to be free of nickel
(0.5Li 2 MnO 3 •0.5LiCoO 2 ) and, therefore, structural and
chemical information obtained from XAS measurements
provide direct information about electrochemical
enhancements due to the Li-Ni-PO 4 treatment itself
relative to untreated electrodes. The results have exciting
implications for tailoring the structures and/or surfaces of
high-capacity composite electrodes derived from a
Energy Storage R &D 662 FY 2011 Annual Progress Report