Nanoengineering Strategies for Metal–Insulator–Metal Electrostatic Nanocapacitors

Nanostructures can improve the performance of electrical energy storage devices. Recently, metal–insulator–metal (MIM) electrostatic capacitors fabricated in a three-dimensional cylindrical nanotemplate of anodized aluminum oxide (AAO) porous film have shown profound increase in device capacitance (100× or more) over planar structures. However, inherent asperities at the top of the nanostructure template cause locally high field strengths and lead to low breakdown voltage. This severely limits the usable voltage, the associated energy density (¹/₂CV²), and thus the operational charge–discharge window of the device. We describe an electrochemical technique, complementary to the self-assembled template pore formation process in the AAO film, that provides nanoengineered topographies with significantly reduced local electric field concentrations, enabling breakdown fields up to 2.5× higher (to >10 MV/cm) while reducing leakage current densities by 1 order of magnitude (to ∼10^–10 A/cm²). In addition, we consider and optimize the AAO template and nanopore dimensions, increasing the capacitance per planar unit area by another 20%. As a result, the MIM nanocapacitor devices achieve an energy density of ∼1.5 Wh/kgthe highest reported

<i>In Situ</i> Transmission Electron Microscopy Study of Electrochemical Lithiation and Delithiation Cycling of the Conversion Anode RuO₂

Conversion-type electrodes represent a broad class of materials with a new Li⁺ reactivity concept. Of these materials, RuO₂ can be considered a model material due to its metallic-like conductivity and its high theoretical capacity of 806 mAh/g. In this paper, we use <i>in situ</i> transmission electron microscopy to study the reaction between single-crystal RuO₂ nanowires and Li⁺. We show that a large volume expansion of 95% occurs after lithiation, 26% of which is irreversible after delithiation. Significant surface roughening and lithium embrittlement are also present. Furthermore, we show that the initial reaction from crystalline RuO₂ to the fully lithiated mixed phase of Ru/Li₂O is not fully reversible, passing through an intermediate phase of Li_<i>x</i>RuO₂. In subsequent cycles, the phase transitions are between amorphous RuO₂ in the delithiated state and a nanostructured network of Ru/Li₂O in the fully lithiated phase

<i>In Situ</i> Transmission Electron Microscopy Study of Electrochemical Lithiation and Delithiation Cycling of the Conversion Anode RuO₂

Conversion-type electrodes represent a broad class of materials with a new Li⁺ reactivity concept. Of these materials, RuO₂ can be considered a model material due to its metallic-like conductivity and its high theoretical capacity of 806 mAh/g. In this paper, we use <i>in situ</i> transmission electron microscopy to study the reaction between single-crystal RuO₂ nanowires and Li⁺. We show that a large volume expansion of 95% occurs after lithiation, 26% of which is irreversible after delithiation. Significant surface roughening and lithium embrittlement are also present. Furthermore, we show that the initial reaction from crystalline RuO₂ to the fully lithiated mixed phase of Ru/Li₂O is not fully reversible, passing through an intermediate phase of Li_<i>x</i>RuO₂. In subsequent cycles, the phase transitions are between amorphous RuO₂ in the delithiated state and a nanostructured network of Ru/Li₂O in the fully lithiated phase

<i>In Situ</i> Transmission Electron Microscopy Study of Electrochemical Lithiation and Delithiation Cycling of the Conversion Anode RuO₂

Conversion-type electrodes represent a broad class of materials with a new Li⁺ reactivity concept. Of these materials, RuO₂ can be considered a model material due to its metallic-like conductivity and its high theoretical capacity of 806 mAh/g. In this paper, we use <i>in situ</i> transmission electron microscopy to study the reaction between single-crystal RuO₂ nanowires and Li⁺. We show that a large volume expansion of 95% occurs after lithiation, 26% of which is irreversible after delithiation. Significant surface roughening and lithium embrittlement are also present. Furthermore, we show that the initial reaction from crystalline RuO₂ to the fully lithiated mixed phase of Ru/Li₂O is not fully reversible, passing through an intermediate phase of Li_<i>x</i>RuO₂. In subsequent cycles, the phase transitions are between amorphous RuO₂ in the delithiated state and a nanostructured network of Ru/Li₂O in the fully lithiated phase

Atomic Layer Deposition of the Solid Electrolyte LiPON

We demonstrate an atomic layer deposition (ALD) process for the solid electrolyte lithium phosphorousoxynitride (LiPON) using lithium <i>tert</i>-butoxide (LiO^tBu), H₂O, trimethylphosphate (TMP), and plasma N₂ (^PN₂) as precursors. We use in-situ spectroscopic ellipsometry to determine growth rates for process optimization to design a rational, quaternary precursor ALD process where only certain substrate–precursor chemical reactions are favorable. We demonstrate via in-situ XPS tunable nitrogen incorporation into the films by variation of the ^PN₂ dose and find that ALD films over approximately 4.5% nitrogen are amorphous, whereas LiPON ALD films with less than 4.5% nitrogen are polycrystalline. Finally, we characterize the ionic conductivity of the ALD films as a function of nitrogen content and demonstrate their functionality on a model battery electrodea Si anode on a Cu current collector

Natural Cellulose Fiber as Substrate for Supercapacitor

Cellulose fibers with porous structure and electrolyte absorption properties are considered to be a good potential substrate for the deposition of energy material for energy storage devices. Unlike traditional substrates, such as gold or stainless steel, paper prepared from cellulose fibers in this study not only functions as a substrate with large surface area but also acts as an interior electrolyte reservoir, where electrolyte can be absorbed much in the cellulose fibers and is ready to diffuse into an energy storage material. We demonstrated the value of this internal electrolyte reservoir by comparing a series of hierarchical hybrid supercapacitor electrodes based on homemade cellulose paper or polyester textile integrated with carbon nanotubes (CNTs) by simple solution dip and electrodeposited with MnO₂. Atomic layer deposition of Al₂O₃ onto the fiber surface was used to limit electrolyte absorption into the fibers for comparison. Configurations designed with different numbers of ion diffusion pathways were compared to show that cellulose fibers in paper can act as a good interior electrolyte reservoir and provide an effective pathway for ion transport facilitation. Further optimization using an additional CNT coating resulted in an electrode of paper/CNTs/MnO₂/CNTs, which has dual ion diffusion and electron transfer pathways and demonstrated superior supercapacitive performance. This paper highlights the merits of the mesoporous cellulose fibers as substrates for supercapacitor electrodes, in which the water-swelling effect of the cellulose fibers can absorb electrolyte, and the mesoporous internal structure of the fibers can provide channels for ions to diffuse to the electrochemical energy storage materials

Stabilization of Lithium Metal Anodes by Hybrid Artificial Solid Electrolyte Interphase

Li metal is among the most attractive anode materials for secondary batteries, with a theoretical specific capacity > 3800 mAh g^–1. However, its extremely low electrochemical potential is associated with high chemical reactivity that results in undesirable reduction of electrolyte species on the lithium surface, leading to spontaneous formation of a solid electrolyte interphase (SEI) with uncontrolled composition, morphology, and physicochemical properties. Here, we demonstrate a new approach to stabilize Li metal anodes using a hybrid organic/inorganic artificial solid electrolyte interphase (ASEI) deposited directly on the Li metal surface by self-healing electrochemical polymerization (EP) and atomic layer deposition (ALD). This hybrid protection layer is thin, flexible, ionically conductive, and electrically insulating. We show that Li metal protected by the hybrid protection layer gives rise to very stable cycling performance for over 300 cycles at current density 1 mA/cm² and over 110 cycles at current density 2 mA/cm², well above the threshold for dendrite growth at unprotected Li. Our strategy for protecting Li metal anodes by hybrid organic/inorganic ASEI represents a new approach to mitigating or eliminating dendrite formation at reactive metal anodesillustrated here for Liand may expedite the realization of a “beyond-Li-ion” battery technology employing Li metal anodes (e.g., Li–S)

MWCNT/V₂O₅ Core/Shell Sponge for High Areal Capacity and Power Density Li-Ion Cathodes

A multiwall carbon nanotube (MWCNT) sponge network, coated by ALD V₂O₅, presents the key characteristics needed to serve as a high-performance cathode in Li-ion batteries, exploiting (1) the highly electron-conductive nature of MWCNT, (2) unprecedented uniformity of ALD thin film coatings, and (3) high surface area and porosity of the MWCNT sponge material for ion transport. The core/shell MWCNT/V₂O₅ sponge delivers a stable high areal capacity of 816 μAh/cm² for 2 Li/V₂O₅ (voltage range 4.0–2.1 V) at 1C rate (1.1 mA/cm²), 450 times that of a planar V₂O₅ thin film cathode. At much higher current (50×), the areal capacity of 155 μAh/cm² provides a high power density of 21.7 mW/cm². The compressed sponge nanoarchitecture thus demonstrates exceptional robustness and energy-power characteristics for thin film cathode structures for electrochemical energy storage

Atomic Layer Deposition and <i>in Situ</i> Characterization of Ultraclean Lithium Oxide and Lithium Hydroxide

We demonstrate the ultraclean atomic layer deposition (ALD) of Li₂O and LiOH using lithium <i>tert</i>-butoxide (LiO^<i>t</i>Bu) precursor with H₂O and plasma O₂ as oxidants, along with conversion of Li₂O and LiOH products to Li₂CO₃ upon CO₂ dosing. Using LiO^<i>t</i>Bu and H₂O results in LiOH below 240 °C and Li₂O above 240 °C for otherwise identical process parameters. Substituting plasma O₂ as the oxidation precursor results in a combination of Li₂CO₃ and Li₂O products, indicating modification of the ALD reaction preventing volatilization of the C from the Li precursor. The chemistry of the films is definitively characterized for the first time with XPS utilizing an all-UHV transfer procedure from the ALD reactor. We use <i>in situ</i> UHV gas dosing to investigate the reaction mechanisms of ALD Li₂O and LiOH with H₂O and CO₂ to simulate reactions upon air exposure. Lastly, we employ <i>in situ</i> spectroscopic ellipsometry to determine the reaction kinetics of thermal LiOH decomposition, and we report an activation energy of 112.7 ± 0.6 kJ/mol

DMSO–Li₂O₂ Interface in the Rechargeable Li–O₂ Battery Cathode: Theoretical and Experimental Perspectives on Stability

One of the greatest obstacles for the realization of the nonaqueous Li–O₂ battery is finding a solvent that is chemically and electrochemically stable under cell operating conditions. Dimethyl sulfoxide (DMSO) is an attractive candidate for rechargeable Li–O₂ battery studies; however, there is still significant controversy regarding its stability on the Li–O₂ cathode surface. We performed multiple experiments (in situ XPS, FTIR, Raman, and XRD) which assess the stability of the DMSO–Li₂O₂ interface and report perspectives on previously published studies. Our electrochemical experiments show long-term stable cycling of a DMSO-based operating Li–O₂ cell with a platinum@carbon nanotube core–shell cathode fabricated via atomic layer deposition, specifically with >45 cycles of 40 h of discharge per cycle. This work is complemented by density functional theory calculations of DMSO degradation pathways on Li₂O₂. Both experimental and theoretical evidence strongly suggests that DMSO is chemically and electrochemically stable on the surface of Li₂O₂ under the reported operating conditions