Corey T. Love

Improved lithium capacity of defective V2O5 materials

We demonstrate that point defects may be introduced into a metal oxide to increase its Li-ion capacity by using various heat treatments to modify the defect structure of polycrystalline V2O5 and then comparing the Li capacity of the materials. The V2O5 that is heated under O2/H2O at 460 jC has a 23% higher Li capacity than the as-received material despite no change to its longrange structure. Other heating conditions lower the Li capacity of the V2O5. We infer that heating under O2/H2O introduces defects, such as cation vacancies associated with lithiated oxygen sites, which can electrochemically exchange Li ions and serve as additional charge-storage sites. This study may also explain how metal oxides synthesized from sol–gels, such as xerogels and aerogels, insert lithium ions without concomitant reduction of transition-metal-ion sites—high-surface-area metal oxides are likely to be nonstoichiometric and rich with surface point defects which can serve as additional charge-storage sites. D 2002 Published by Elsevier Science B.V.

Selective Vapor Deposition of Hydrous RuO2 Thin Films

Thin films of mixed-conducting hydrous ruthenium dioxide (RuO 2 .xH 2 O or RuO x H y ) were deposited via chemical vapor deposition at ambient pressure and temperature. The RuO 4 (g) precursor was generated in situ and reducedto RuO 2 .xH 2 O when it contacted specific functional groups, including alcohols, thiols, amines/amides, and clean metal surfaces. The RuO 4 was nonreactive when in contacted with surfaces of hydrocarbons, carboxylic acid groups, or metal oxides. Because of the selectivity of the deposition process, RuO 2 .xH 2 O lines and patterns can be formed on substrates that are decorated with specific areas of reactive and nonreactive functional groups. RuO 2 .xH 2 O films are conductive, catalytic for chloride evolution from brine, and may ultimately be useful as oxidation catalysts and methanol barrier layers in direct methanol fuel cells.

Enhanced Lithiation Cycle Stability of ALD-Coated Confined a-Si Microstructures Determined Using In Situ AFM

Microfabricated amorphous silicon (a-Si) pits ∼4 μm in diameter and 100 nm thick were fabricated to be partially confined in a nickel (Ni) current collector. Corresponding unconfined pillars were also fabricated. The samples were coated with 1.5, 3, or 6 nm of Al2O3 ALD. These samples were tested in electrolytes of 3:7 by weight ethylene carbonate:ethyl methyl carbonate (EC:EMC) with 1.2 M LiPF6 salt with and without 2% fluoroethylene carbonate (FEC) and in a pure FEC electrolyte with 10 wt % LiPF6. The samples were imaged with an atomic force microscope during electrochemical cycling to evaluate morphology evolution and solid electrolyte interphase (SEI) formation. The partially confined a-Si structures had superior cycle efficiency relative to the unconfined a-Si pillars. Additionally, samples with 3 nm of ALD achieved higher charge capacity and enhanced cycle life compared to samples without ALD, demonstrated thinner SEI formation, and after 10 cycles at a 1 C rate remained mostly intact and had actually decreased in diameter. Finally, the samples with 3 nm of ALD had better capacity retention in the baseline 3:7 EC:EMC than in either of the FEC containing electrolytes.

Characterization and Electrochemical Properties of Li2Cu0.5Ni0.4M0.1O2 Lithium-ion Battery Cathodes

Doped lithium copper-nickel mixed-metal oxides are prepared and studied as cathode materials for secondary Li-ion batteries. The compounds, Li2Cu0.5Ni0.4M0.1O2 (where M = Al, Ga), are made using solid state and micro-emulsion synthesis techniques. All compounds have a body-centered orthorhombic (Immm) crystal structure. The undoped compound has an initial discharge capacity of 270 mAh/g but exhibits poor capacity retention with cycling. The Aland Ga-doped compounds made by solid-state synthesis have a higher discharge capacity than those made by microemulsion. Micro-emulsion processed compounds exhibit better cycleability and capacity retention upon charge/discharge suggesting a more stable crystal structure. Gallium doping improved the structural stability resulting in lowered irreversible capacity loss. Two reversible reaction mechanisms, lithium intercalation/de-intercalation and a displacement reaction are suggested to occur in the Li2Cu0.5Ni0.4M0.1O2 system.

Laser direct writing of microbatteries for integrated power electronics

A novel laser-based process developed at the Naval Research Laboratory has been used to fabricate pseudocapacitors and microbatteries with tailored capacities for small electronic devices having size and/or weight restrictions. This process, called MAPLE DW (for matrix-assisted pulsed-laser evaporation direct write) can deposit rugged mesoscale (1 micrometers to 10 mm) electronic components over any type of substrate. A pulsed laser operating at 355 nm is used to forward transfer material from a tape-cast ribbon to a suitable substrate to form a precision design. With MAPLE DW, customized mesoscale electronic components can be produced, eliminating the need for multiple fabrication techniques and surface-mounted components. Direct write processing is especially attractive for the fabrication of micro-power sources and systems. The versatility of laser processing allows battery designs to be easily modified. Batteries and/or pseudocapacitors can be integrated with power management electronics to deliver a wide range of power outputs. By building power sources directly on electronic components, the weight of the power sources is decreased as the electronic substrate becomes part of the battery packaging and the lengths of interconnects are shortened, reducing conductor losses. RuOxHy pseudocapacitors deposited with MAPLE DW show good storage capacities. Pads of hydrous RuO2 having dimensions of 2.2 mm x 1.0 mm x 30 micrometers have been deposited in a planar configuration on gold current collectors. Rechargeable Zn/MnO2 alkaline microbatteries comprising of MnO2, Zn, an ethyl cellulose separator barrier layer and a KOH electrolyte have also been fabricated by MAPLE DW. The resulting structures with dimensions of 1.5 mm x 1.5 mm x 60 micrometers represent the first demonstration of a multilayer microbattery made by MAPLE DW. The performance of these prototypes are shown and the potential impact of MAPLE DW for the fabrication of novel microbattery systems for integrated power applications are discussed.

Internal Morphologies of Cycled Li-Metal Electrodes Investigated by Nano-Scale Resolution X-ray Computed Tomography

While some commercially available primary batteries have lithium metal anodes, there has yet to be a commercially viable secondary battery with this type of electrode. Research prototypes of these cells typically exhibit a limited cycle life before dendrites form and cause internal cell shorting, an occurrence that is more pronounced during high-rate cycling. To better understand the effects of high-rate cycling that can lead to cell failure, we use ex situ nanoscale-resolution X-ray computed tomography (nano-CT) with the aid of Zernike phase contrast to image the internal morphologies of lithium metal electrodes on copper wire current collectors that have been cycled at low and high current densities. The Li that is deposited on a Cu wire and then stripped and deposited at low current density appears uniform in morphology. Those cycled at high current density undergo short voltage transients to >3 V during Li-stripping from the electrode, during which electrolyte oxidation and Cu dissolution from the current collector may occur. The effect of temperature is also explored with separate cycling experiments performed at 5 and 33 °C. The resulting morphologies are nonuniform films filled with voids that are semispherical in shape with diameters ranging from hundreds of nanometers to tens of micrometers, where the void size distributions are temperature-dependent. Low-temperature cycling elicits a high proportion of submicrometer voids, while the higher-temperature sample morphology is dominated by voids larger than 2 μm. In evaluating these morphologies, we consider the importance of nonidealities during extreme charging, such as electrolyte decomposition. We conclude that nano-CT is an effective tool for resolving features and aggressive cycling-induced anomalies in Li films in the range of 100 nm to 100 μm.

Thermal Stability of Delithiated Al-substituted Li(Ni1/3Co1/3Mn1/3)O2 Cathodes

The thermal stability of a layered transition metal oxide with complete Al-substitution, Li1-xNi1/3Co1/3Al1/3O2 (NCA1/3), is studied in comparison to its better understood analogue, Li1-xNi1/3Co1/3Mn1/3O2 (NCM), to elucidate information critical to the safety of lithium-ion batteries. NCA1/3 and NCM have crystalline structures with the R-3m space group by high temperature synthesis from coprecipitate precursors. The specific capacities for NCA1/3 and NCM are 115 mAh/g and 180 mAh/g within 2.5-4.5 V. Thermogravimetric analysis coupled with mass spectrometry confirms the loss of oxygen at 271°C and 292°C for NCM and NCA1/3 delithiated to 4.4 V, respectively. The heat generation reaction of delithiated NCM is 91.7 J/g compared to 10.5 J/g for NCA1/3. The improved thermal stability offered by NCA1/3 is attributed to its low Li+ capacity due to the substitution of Al3+ for Mn4+ and its low heat of reaction due to the stability of the intermediate crystalline phases during delithiation and annealing.

Calculations in Li-Ion Battery Materials

Density functional calculations, or first principles calculations, are emerging as a critical tool for the evaluation of new lithium-ion battery materials. Density functional theory (DFT ) is ideal for battery materials because it can be used to calculate critical materials properties, such as electronic and ionic conductivity, phase stability with lithium intercalation, and the roles of defects and dopants. The methods are illustrated herein by the evaluation of charge/discharge properties of two Li-ion battery cathode materials, \(\mathrm{LiNi}_{1/3}\mathrm{Co}_{1/3}\mathrm{Mn}_{1/3}\mathrm{O}_{2}\) (NCM ) and \(\mathrm{LiNi}_{1/3}\mathrm{Co}_{1/3}\mathrm{Al}_{1/3}\mathrm{O}_{2}\) (NCA\({}_{1/3}\) ) and their comparison to a LiCoO2 standard. We investigate the effect of substituting Al for Mn on the structural and electronic properties of the compounds at various levels of Li deintercalation and correlate these to performance properties observed in the laboratory. We find a calculated and observable upward shift in the voltage with Al substitution due to a shift in the oxidation levels of the electrochemically active ions during cycling. The results are corroborated by experimental results, in which we observe much lower specific capacity for NCA (despite its higher theoretical value) that can be attributed to a restricted voltage window during deintercalation. There is also a strong increase in resistive losses for NCA. A comparison of our density functional calculations and measured data indicates that this loss is due mainly to disruptive Ni/Li cation disorder. The partial density of states of the materials can be used to calculate their propensity to evolve O2 when overcharged. DFT gives key insights into changes occurring at the atomistic level and can be used toward physical insights into both new and traditional materials.

A Novel Microdevice for In Situ Study of Mechano-Electrochemical Behavior with Controlled Temperature

Nanostructured electrodes have shown great potential in the development of Li-ion batteries with higher energy and power densities and longer cycle life. A fundamental understanding of the mechano-electrochemical behavior during charging/discharging cycles is essential for optimal and reliable design. Previous work has utilized in situ experimental techniques in an electron microscope to directly visualize material response during the reaction cycles. Unfortunately, the present in situ test methods are limited to room temperature and, as a result, the effect of temperature on charging/discharging cycles is not well understood. These electrochemical processes are intrinsically temperature sensitive, particularly for nanostructured electrodes. Here we present a novel microdevice that allows high resolution in situ observation of mechano-electrochemical response of nanomaterials in a scanning electron microscope with controlled temperature. The microdevice consists of built-in microcircuits for concurrent heating and temperature measurement during in situ experiments. To demonstrate these unique capabilities, we present the design, microfabrication and thermal characterization of this new class of microdevice.

Utilization of heavy alkali dopants as a beacon to study the cathode electrolyte decomposition layer in lithium-ion batteries

We introduce low levels of CsClO4 and RbClO4 into the electrolyte of LiCoO2 electrochemical half-cells to probe the composition of the passivation film on the surface of the cathode, the electrolyte decomposition layer (EDL). The advantages of these heavy alkali dopants lie in their large ionic radii, which limit intercalation, yet their strong light scattering cross-section creates a beacon that highlights the formation of products near the cathode surface. Detailed surface analysis and depth profiling with X-ray photoelectron spectroscopy, and bulk analysis utilizing X-ray absorption spectroscopy, show evidence for the formation of Cs/Rb compounds, such as carbonates, halides, and perchlorates, similar to those formed by lithium in previous studies, but also reveal the significantly reduced mobility of the Cs/Rb relative to Li in the non-uniform EDL. This unique approach could open several presently untapped techniques to gather new information on the EDL in Li-ion batteries.