Lithium-sulfur cells

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Nitash Balsara, LBNL

Lithium-sulfur cells are attractive targets for energy storage applications as their theoretical specific energy of 2600 Wh/kg is much greater than the theoretical specific energy of current lithium-ion batteries.  Unfortunately, the cycle-life of lithium-sulfur cells is limited due to migration of species generated at the sulfur cathode. These species, collectively known as polysulfides, can transform spontaneously, depending on the environment, and it has thus proven difficult to determine the nature of redox reactions that occur at the sulfur electrode.

X-ray spectroscopy was used to fingerprint the polysulfides present in a solid polymer electrolyte film. Molecular simulations were used to determine the molecular underpinnings of the polysulfide spectra. The first simulated spectra of polysulfides are presented in the adjoining figure.

This works lays the foundation for rational design of sulfur cathodes with improved cycle life.

In-situ Studies of Ion-exchange Synthesis for Developing New Cathodes

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Patrick Looney and Feng Wang Group at BNL

Ion exchange is an important method for preparing new cathode materials with metastable structures that are generally inaccessible via direct chemical reactions, but can be obtained via Li+ exchange of Na+ in iso-structural Na-containing compounds. Looney and Wang Group at Brookhaven National Laboratory have developed a new in-situ reactor for real time probing of ion exchange reactions, enabling quantitative measure of intermediate phases and reaction kinetics. In studies of Li+ exchange of Na+ in NaVPO5F compound, it was found that the reaction proceeds via a complicated phase transformation process, towards Li(Na)VPO5F — a new high-energy cathode. This new in-situ technique may also be applied for studies of other type of synthesis reactions, such as hydrothermal, solvothermal and solid-state. Real-time, quantitative identification of structure and phases during synthesis using time-resolved synchrotron XRD provides a new avenue for rational design and preparation of battery materials of desired phases and properties.

Develop high-energy cathodes via hydrothermal ion exchange (A) schematic illustration of in-situ reactor specialized for studies of synthesis reactions, and (B) time-resolved XRD patterns from ion exchange synthesis of Li(Na)VPO5F.

Advanced In Situ Diagnostic Techniques for Battery Materials

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Xiao-Qing Yang and Kyung-Wan Nam, BNL

The high energy density Li-rich layered materials xLiMO2·(1-x)Li2MnO3 are promising candidate cathode materials for electric energy storage in plug-in hybrid electric vehicles (PHEVs) or electric vehicles (EVs). The relatively low rate capability is one of the major problems need to be resolved for these materials. In order to gain fundamental understanding to the key factors limiting the rate capability, in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) studies of the Li1.2Ni0.15Co0.1Mn0.55O2 cathode material has been carried out. Through these studies direct experimental evidence is obtained showing that Mn sites have a much poorer reaction kinetics both before and after initial “activation” of Li2MnO3, comparing with Ni and Co. These results indicate that the Li2MnO3 might be the key component limiting the rate capability of the Li-rich layered materials, providing valuable guidance in designing various Li-rich layered materials with desired balance of energy densities and rate capabilities for different applications.

YangResHighlightFig
EXAFS spectra of Li1.2Ni0.15Co0.1Mn0.55O2 during constant voltage charging at 5V. Ni, Co, Mn reacted simultaneously using time-resolved XAS technique.

Micro-four-line probe technology for evaluating conductivity of intact electrodes

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Dean Wheeler and Brian Mazzeo of Brigham Young University (Provo, UT) have developed a new surface probe that can accurately measure electronic conductivity of intact electrodes. Measuring conductivity of intact thin-film electrodes (still attached to current collector) is difficult, and prior methods have not been sufficiently accurate and robust. The new method uses four small parallel lines to contact the surface with controlled applied pressure. The method also allows simultaneous measurement of bulk film conductivity and contact resistance between the film and the current collector.
The probe device is fabricated using semiconductor clean room techniques. A computer model is used to interpret the experimental results and obtain local values of the two properties.
A computer-controlled fixture allows the probe to be scanned across the surface of the electrode sample, allowing a local conductivity map to be created. In effect, “hot” and “cold” conductivity spots can be identified so that electrode process quality can be improved. The technology has been validated with several commercially produced electrodes and is being adopted by A123 to improve their electrode production processes.

 

Micro Four Line Probe  Local_conductivity map
Photograph of fabricated device with inset of the micro-four-line probe region Local map of conductivity of cathode film attached to aluminum current collector

Manganese Migration in Delithiated Li2MnO3

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Kristin Persson,  LBNL

Li2MnO3 is a critical component in the family of the so-called ‘Li-excess’ materials, which are attracting attention as advanced cathode materials for Li-ion batteries. In this work, first-principles calculations are performed to investigate the electrochemical activity and structural stability of stoichiometric LixMnO3 (0 ≤ x ≤ 2) as a function of Li content. It is shown that the Li2MnO3 structure is electrochemically activated above 4.5 V on delithiation and that charge neutrality in the bulk of the material is mainly maintained by the oxidization of a portion of the oxygen ions: from O2− to O1−. While oxygen vacancy formation is found to be thermodynamically favorable for x < 1, the activation barriers for O2− and O1− migration remain high throughout the Li composition range, impeding oxygen release from the bulk of the compound. Furthermore, defect layered structures, where some Mn resides in the Li layer, become thermodynamically favorable at lower Li content (x < 1), indicating a strong tendency towards the spinel-like structure transformation. Concurrently, the calculated energy barriers for Mn migration from the Mn-layer into the Li-layer suggests a Li2MnO3 structural instability for x < 0.5. Based on the observations, a critical phase transformation path is suggested for forming nuclei of spinel-like domains within the matrix of the original layered structure. Furthermore, the formation of defect layered structures during the first charge manifests in a significant depression of the voltage profile on the first discharge, providing one possible explanation for the observed ‘voltage fade’ of the Li-excess materials.

PerssonResHighlightFig

Two Mn migration paths – Edge path (continuous line) and Dumbbell path (dashed line), which both show feasible migration energies for Mn4+, in particular the Edge path for highly charged particles.