Jay Whitacre

Fabrication and testing of all solid-state microscale lithium batteries for microspacecraft applications

A microfabrication process has been developed to prepare thin film solid-state lithium batteries as small as 50 μm × 50 μm. Individual cells operate nominally at 3.9 V with 10 μA h cm−2 for a 0.25 μm thick cathode film. The cells are easily fabricated in series and parallel arrangement to yield batteries with higher voltage and/or capacity. Multiple charge/ discharge cycles are possible, though an apparent reaction of the in situ plated Li film with water or oxygen decreases cycle life several orders of magnitude from expected results. Further optimization of an encapsulating film will likely extend the cell cyclability. These microbattery arrays will be useful for providing on-chip power for low current, high voltage applications for microspacecraft and other miniaturized systems.

Investigation of Direct Methanol Fuel Cell Electrocatalysts Using a Robust Combinatorial Technique

A combinatorial approach to batch fabricating and evaluating fuel cell catalyst surfaces is described. The well-known binary Pt/Ru alloy and two compositional regimes of a novel quaternary Ni/Zr/Pt/Ru system were examinedin detail. Catalyst films no thicker than 10 nm were deposited onto an array of 36 gold electrodes 0.5 cm 2 in area that were microfabricated on a 12.5 × 12.5 cm glass substrate. The catalyst films had identical bulk and surface compositions, a result of the atom-level mixing that occurred during the room-temperature cosputtering method used. A multichannel pseudopotentiostat was implemented for electrochemical screening. Compositions with promising and/or contrasting catalytic activities were also studied using X-ray diffraction. X-ray energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy. A low-Pt-content Ni 3 1 Zr 1 3 Pt 3 3 Ru 2 3 film was found to exhibit nominally the same activity (at 0.45 V vs a normal hydrogen electrode in 1 M H 2 SO 4 , 1 M CH 3 OH) as the best PtRu alloys studied. This material had a fundamentally different crystal and electronic structure than that observed in the Pt/Ru films and exhibited a significantly higher degree of Pt site utilization. These results were consistent with the existence of a catalytic reaction pathway different than that reported for Pt/Ru.

Crystallographically Oriented Thin-Film Nanocrystalline Cathode Layers Prepared Without Exceeding 300°C

The highest capacity rf sputtered cathode layers created for use in thin-film solid-state batteries have been found to require an annealing step with temperatures in excess of 700°C. Since this high-temperature process step is incompatible with silicon device technology and flexible polymer substrates, the development of a low-process temperature (less than or equal to 300°C) cathode layer has been undertaken. Thin-film cathode layers consisting of LiCoO 2 were deposited using planar magnetron rf sputtering and subsequently incorporated into thin-film solid-state cells comprised of a LiPON electrolyte and lithium metal anode. Film composition was examined using Rutherford backscattering spectrometry and inductively coupled plasma mass spectroscopy, while phase content and crystal structure were studied through X-ray diffraction experiments conducted at the Stanford Synchrotron Radiation Laboratory. Microstructure and morphology were examined using transmission and scanning electron microscopy. It was found that LiCoO 2 could be deposited at room temperature in a nanocrystalline state with a defined (104) out of plane texture and a high degree of lattice distortion. By heating these layers to 300°C, the average grain size was increased while lattice distortion was minimized. Electrochemical cycling data revealed that the low temperature annealing step increases cell capacity to near theoretical values while significantly improving both the rate capability and discharge voltage. Impedance analysis on test cells showed that the electronic resistance of the cells is decreased after heating to 300°C.

Radio Frequency Magnetron-Sputtered LiCoPO4 Cathodes for 4.8 V Thin-Film Batteries

Thin-film batteries employing sputtered LiCoPO 4 cathode layers, Li 3.3 PO 3.8 N 0.22 (LiPON) electrolyte, and Li anode films have been successfully fabricated and tested. The cells exhibit a high-voltage charge/discharge plateau of 4.8 V, with a lower plateau at 2.5 V corresponding to a much higher conductivity phase. Cells fabricated with as-deposited cathode layers are highly resistive and cannot be charged or discharged to appreciable capacities, even at very low rates of approximately C/100. Upon annealing, the films sputtered from the LiCoPO 4 target become discontinuous and take on a granular morphology. X-ray diffraction results show that these grains consist of untextured crystalline LiCoPO 4 on a Pt current collector layer, a finding further supported by the results from subsequent electrochemical analyses. Thin-film batteries fabricated with crystalline LiCoPO 4 cathodes are among the highest voltage thin-film batteries yet reported.

Effect of Electrolyte Type upon the High-Temperature Resilience of Lithium-Ion Cells

The effect of electrolyte type upon the resilience of lithium-ion cells to high-temperature storage has been investigated in experimental mesocarbon microbead carbon-Li x Ni y Co 1 - y O 2 three-electrode cells. Specifically, electrolytes have been studied where the solvent mixtures have been varied, with the intention of determining the impact of ethylene carbonate (EC) content upon performance. In addition to determining the reversible and irreversible capacity losses sustained as a result of high-temperature storage (55 to 70°C), a number of electrochemical measurements (ac impedance, Tafel polarization, and linear polarization) have been performed to determine the impact of the high-temperature exposure upon the electrode kinetics and the nature of the electrode surface films. It was observed that cells containing electrolytes with high EC content (i.e., 70% EC by volume) displayed superior resilience to high-temperature storage, in contrast to cells containing low EC content electrolytes (i.e., 30% EC by volume), which displayed much larger irreversible capacity losses and poorer lithium intercalation/deintercalation kinetics after exposure to high temperatures. Solid-state 7 Li nuclear magnetic resonance measurements were used to determine quantitatively the fraction of Li in the irreversible solid electrolyte interphase (SEI) as compared to Li in the active electrode (both anode and cathode) material. In addition, the electrodes were characterized using a scanning electron microscope equipped with an X-ray energy-dispersive spectrometer to examine the film morphology and composition. The results indicate that the nature of the SEI formed on the anode in low EC content cells correlates with the poor electrochemical performance observed after being subjected to high temperatures.

A Combinatorial Study of Li y Mn x Ni2 − x O 4 Cathode Materials Using Microfabricated Solid-State Electrochemical Cells

A methodology for batch-fabricating hundreds of submillimeter thin-film solid-state batteries was used in conjunction with a combinatorial materials synthesis technique. This approach allowed for the simultaneous creation of many solid-state microbatteries that had Li v Mn x Ni 2-x O 4 cathodes, where x varied continuously from 0.2 to 1.8 and y varied from 2.7 to 3.7 in the as-deposited films. Sputtered lithium phosphorous oxynitride (LiPON) was used as an electrolyte, while evaporated, micropat-temed Li metal served as the anode layer. The composition, thickness, and microstructure of the cathodes were examined using X-ray energy-dispersive spectroscopy, synchrotron-based X-ray diffraction, Rutherford backscattering spectroscopy, stylus profilometry, inductively coupled plasma mass spectroscopy, and scanning electron microscopy. Electrochemical cycling data allowed for cell performance to be correlated to cathode composition and structure. Cathodes with a composition of LiMn 1.4 Ni 0.6 O 4 had both the highest specific capacity as well as the longest high-potential (4.7 V) discharge plateau, while cells with higher Mn content had a longer 4 V discharge plateau, a result that qualitatively agrees with similar studies on conventional bulk-fabricated cathodes of the same material.

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.

Lithium Ion Batteries for Space Applications

Interplanetary missions require rechargeable batteries with unique performance characteristics: high specific energy, wide operating temperatures, demonstrated reliability, and safety. Li-ion batteries are fast becoming the most common energy storage solution for these missions, as they are able to meet the more demanding technical specifications without being excessively massive. At JPL, we have undertaken materials development studies on both cathodes and electrolytes with the goal of further enhancing battery specific energy, discharge and charge capability, and functional temperature range. Results of these studies are described below.

Enhanced Low-Temperature Performance of Li–CFx Batteries

This work describes the continued examination of the low-temperature performance of the Li–CFx electrochemical couple in a −40°C or colder environment. Previously, the efficacy of subfluorinated CFx (SFCFx) cathode active materials was demonstrated; preliminary results indicated that the material was functional at rates up to C/10 at −40°C. However, there were often substantial voltage fluctuations during discharge, accompanied with inconsistent capacity yields and sometimes dramatic polarization events. In the research described herein, an investigation of various electrolyte and cathode compositions was conducted in an effort to optimize performance. In particular, several different electrolyte solvent formulations were examined using a salt content of either 1 or 0.5 M LiBF4. A further modification consisted of addition of an anion receptor to the electrolyte that was intended to be a LiF-solvating agent. The most promising electrolyte was tested with several different SFCFx compositions created using either graphitic or multiwall nanotube precursor materials. Electrochemical evaluation showed that the best SFCFx-based test cells were able to deliver specific capacity values up to five times greater than control cells (containing conventional CF1.08 powder) at −40°C under discharge currents as high as C/5 with composite electrodes thicker than 100 µm.

The Kinetics of Sub-Fluorinated Carbon Fluoride Cathodes for Lithium Batteries

Sub-fluorinated carbon materials (CFx, x<1) were prepared by direct fluorination of synthetic graphite (Timcal KS15) and multi-walled carbon nanotubes (MWNT, MER). The fluorination parameters (temperature, fluorine partial pressure, fluorine flow rate and time) were set to achieve a target composition ’x’ in CFx, according to the starting carbonaceous material. The fluorine composition was determined by weight uptake and SEM/XEDX cross analyses.