Nicole Leifer

A review of advanced and practical lithium battery materials

Presented herein is a discussion of the forefront in research and development of advanced electrode materials and electrolyte solutions for the next generation of lithium ion batteries. The main challenge of the field today is in meeting the demands necessary to make the electric vehicle fully commercially viable. This requires high energy and power densities with no compromise in safety. Three families of advanced cathode materials (the limiting factor for energy density in the Li battery systems) are discussed in detail: LiMn1.5Ni0.5O4 high voltage spinel compounds, Li2MnO3–LiMO2 high capacity composite layered compounds, and LiMPO4, where M = Fe, Mn. Graphite, Si, LixTOy, and MO (conversion reactions) are discussed as anode materials. The electrolyte is a key component that determines the ability to use high voltage cathodes and low voltage anodes in the same system. Electrode–solution interactions and passivation phenomena on both electrodes in Li-ion batteries also play significant roles in determining stability, cycle life and safety features. This presentation is aimed at providing an overall picture of the road map necessary for the future development of advanced high energy density Li-ion batteries for EV applications.

On the Surface Chemistry of LiMO2 Cathode Materials (M = [ MnNi ] and [MnNiCo]): Electrochemical, Spectroscopic, and Calorimetric Studies

This study examined the aging mechanisms of layered cathode materials for lithium batteries upon exposure to air and the influence of this aging on the thermal stability and electrochemical performance of these materials composed of solid solutions of LiMO 2 (M = [MnNi] or [MnNiCo]) in Li cells. A unique methodology for the quantitative characterization of surface carbonates on LiMO 2 compounds based on differential scanning calorimeter (DSC) measurements was developed. Correlations were made between the formation of Li 2 CO 3 and other carbonates on the surface of the lithiated metal oxide powders and the changes in the structure and electrochemical performance of the cathode materials. The techniques used included solid-state NMR, X-ray photelectron spectroscopy, Fourier transform IR, high resolution scanning electron microscopy, high resolution transmission electron microscopy and the thermal analysis, DSC, and accelerating rate calorimetry in conjunction with electrochemical measurements. .

Reversible Intercalation of Fluoride-Anion Receptor Complexes in Graphite

We have demonstrated a route to reversibly intercalate fluoride-anion receptor complexes in graphite via a nonaqueous electrochemical process. This approach may find application for a rechargeable lithium–fluoride dual-ion intercalating battery with high specific energy. The cell chemistry presented here uses graphite cathodes with LiF dissolved in a nonaqueous solvent through the aid of anion receptors. Cells have been demonstrated with reversible cathode specific capacity of approximately 80 mAh/g at discharge plateaus of upward of 4.8 V, with graphite staging of the intercalant observed via in situ synchrotron X-ray diffraction during charging. Electrochemical impedance spectroscopy and 11B nuclear magnetic resonance studies suggest that co-intercalation of the anion receptor with the fluoride occurs during charging, which likely limits the cathode specific capacity. The anion receptor type dictates the extent of graphite fluorination, and must be further optimized to realize high theoretical fluorination levels. To find these optimal anion receptors, we have designed an ab initio calculations-based scheme aimed at identifying receptors with favorable fluoride binding and release properties.

Nuclear Magnetic Resonance and X-Ray Absorption Spectroscopic Studies of Lithium Insertion in Silver Vanadium Oxide Cathodes

Structural studies have been carried out on Ag{sub 2}V{sub 4}O{sub 11} (silver vanadium oxide, SVO) and Li{sub x}Ag{sub 2}V{sub 4}O{sub 11}, lithiated SVO with x=0.72, 2.13, and 5.59 using nuclear magnetic resonance (NMR) and X-ray absorption spectroscopy (XAS). Lithium-7 NMR indicates the formation of a solid electrolyte interphase layer on the x=0.72 sample and lithium intercalation into both octahedral and tetrahedral sites in the SVO lattice, and that most but not all of the Ag (I) is reduced prior to initiation of V(V) reduction. Vanadium-51 NMR studies of SVO and lithiated SVO show decreased crystallinity with increased lithiation, as previously reported. Silver XAS studies indicate the formation of metallic silver crystallites in all the lithiated samples. A comparison of X-ray absorption near edge spectroscopy spectra for vanadium in these samples with those of reference compounds shows that some reduction of vanadium (V) occurs in the lithiated SVO with x=0.72 and increases with further lithiation leading to the formation of V(IV) and V(III) species. The results of this study indicate that vanadium(V) reduction occurs in parallel with silver (I) reduction during the initial stages of SVO lithiation, leading ultimately to the formation of vanadium (IV) and (III) species with further lithiation.

Solid-State NMR Studies of Chemically Lithiated CF.

The lithium/carbon monofluoride (Li/CFx) battery was one of the first lithium/solid cathode systems to be used commercially.1 Its theoretical specific energy is about 2180 Wh/kg and is among the highest for the solid cathode systems. The open-circuit voltage is 3.2 V with an operating voltage of 2.5–2.9 V. Its practical specific energy and energy density range from 250 Wh/kg and 635 Wh/L for smaller cells to 590 Wh/kg and 1050 Wh/L for larger sizes. Because of the relatively high cost of CFx compared to other solid cathodes such as manganese dioxide, its use is currently restricted to specialized applications such as biomedical and military, where its superior technical characteristics are required. The active components of the cell are lithium for the anode and carbon monofluoride (CFx) for the cathode where x is typically in the range 0.9–1.1 for commercial products. CFx is synthesized by the reaction of fluorine gas with carbon powder at a high temperature (HT). CFx is electrochemically active and stable up to 400°C, producing a cathode that resists self-discharge, resulting in a long shelf life for the Li/CFx cell. Typically, the electrolyte consists of lithium tetrafluoroborate (LiBF4) in gamma-butyrolactone or lithium hexafluoroarsenate (LiAsF6) in a mixture of propylene carbonate and dimethoxyethane. The simplified version of the discharge process is shown by the following reactions Anode:xLi=Li++xe−Cathode:CFx+xe−=xC+xF−Overallreaction:CFx+xLi=xLiF+xC The carbon monofluoride is converted into carbon, which is more conductive than CFx, thereby lowering the cell’s internal resistance, improving the voltage regulation and cell efficiency while the LiF precipitates in the cathode structure. Recent studies2,3 have shown that subfluorinated CFx (0.33< x < 0.66) materials exhibit a higher rate capability up to 25°C and an improved low temperature performance down to −40°C compared to a commercial CFx prepared from coke with x = 1.08. In practice, Li/CFx is a primary battery system in which lithium metal serves as the anode against a fluorinated graphite cathode in the presence of an electrolyte. Upon discharge, lithium is oxidized while fluorine is reduced, producing elemental carbon and LiF, which precipitates on the remaining CFx structure. Certain facts about the mechanism of discharge of this system are known, such as the increase in electrical conductivity, attributable to the formation of conductive graphite from CFx that occurs as the battery is discharged.4 However, the structure of the CFx cathode during and after discharge, the mechanism of the defluorination process, and the exact location and form of the LiF remain unresolved. The primary objective of this study is to investigate the three types of fluorinated graphite materials to determine any chemical and structural distinctions between the starting materials, and as they undergo lithiation. Electrochemical studies on these different types of CFx have indicated significant differences in electrochemical performance. Ultimately, the aim is to correlate these findings with electrochemical data collected on the same materials to better understand the discharge mechanisms. The three starting CF materials were CFx F, which is fiber based; CFx G, which is graphite based, and CFx C, which is petroleum coke based. These three compounds are of nominally the same composition, i.e., (CF)x, x ~ 1, but result from different preparation routes. Each starting compound was subjected to a chemical reduction in n-butyllithium (nBL) under identical conditions to achieve several levels of lithiation followed by 19F and 13C NMR analyses. Both the solid powders and the liquid filtrates were studied. Though it is appreciated that chemical lithiation is only a relatively crude model for electrochemical reduction, in part because of the very rapid, highly exothermic, and far-from-equilibrium nature of the reaction compared to relatively slow discharge rates characteristic of CFx cells, it does provide a useful means to characterize the samples in a timely manner. Therefore, although the spectroscopic details may somewhat differ between chemically and electrochemically reduced samples, it is expected that the main reaction species and the trends observed in their formation would be similar. The reactants were added very slowly, in a titration-like fashion, to allow the reaction to proceed uniformly, and the chemical reaction was closely monitored for drastic changes in temperature.

Pushing the limit of layered transition metal oxide cathodes for high-energy density rechargeable Li ion batteries

Development of advanced high energy density lithium ion batteries is important for promoting electromobility. Making electric vehicles attractive and competitive compared to conventional automobiles depends on the availability of reliable, safe, high power, and highly energetic batteries whose components are abundant and cost effective. Nickel rich Li[NixCoyMn1−x−y]O2 layered cathode materials (x > 0.5) are of interest because they can provide very high specific capacity without pushing charging potentials to levels that oxidize the electrolyte solutions. However, these cathode materials suffer from stability problems. We discovered that doping these materials with tungsten (1 mol%) remarkably increases their stability due to a partial layered to cubic (rock salt) phase transition. We demonstrate herein highly stable Li ion battery prototypes consisting of tungsten-stabilized Ni rich cathode materials (x > 0.9) with specific capacities >220 mA h g-1. This development can increase the energy density of Li ion batteries more than 30% above the state of the art without compromising durability.

Angular dependence of spin-wave resonance and relaxation in half-metallic Sr2FeMoO6 films

We investigated the magnetic anisotropic parameters and spin-wave relaxation of thin films of the ferromagnetic half-metallic Sr2FeMoO6 by ferromagnetic resonance technique. The resonance field and linewidth were recorded as a function of relative angle between applied magnetic field and crystallographic axes of the sample. The resonance field varies sinusoidally and considerable linewidth broadening occurs when the applied field is rotated parallel to the sample plane. The results are described using higher order components of anisotropy fields. We obtain the values of the cubic anisotropic field, 2K4∥∕M=0.09611T, the effective demagnetization field, 4πM−2K2⊥∕M=0.1216T, and the planar anisotropic field, 2K4⊥∕M=−0.108T. Moreover, we estimated the spin relaxation time (damping factor) from the analysis of the angular dependence of peak-to-peak linewidth, leading to an intrinsic value of α∼0.00025 (Gilbert damping).