Oxidation state of cross-over manganese species on the graphite electrode of lithium-ion cells.

It is well known that Li-ion cells containing manganese oxide-based positive electrodes and graphite-based negative electrodes suffer accelerated capacity fade, which has been attributed to the deposition of dissolved manganese on the graphite electrodes during electrochemical cell cycling. However, the reasons for the accelerated capacity fade are still unclear. This stems, in part, from conflicting reports of the oxidation state of the manganese species in the negative electrode. In this communication, the oxidation state of manganese deposited on graphite electrodes has been probed by X-ray absorption near edge spectroscopy (XANES). The XANES features confirm, unequivocally, the presence of fully reduced manganese (Mn(0)) on the surface of graphite particles. The deposition of Mn(0) on the graphite negative electrode acts as a starting point to understand the consequent electrochemical behavior of these electrodes; possible reasons for the degradation of cell performance are proposed and discussed.

3D nanoporous nanowire current collectors for thin film microbatteries.

Conventional thin film batteries are fabricated based on planar current collector designs where the high contact resistance between the current collector and electrodes impedes overall battery performance. Hence, current collectors based on 3D architectures and nanoscale roughness has been proposed to dramatically increase the electrode-current collector surface contact areas and hence significantly reduce interfacial resistance. The nanorod-based current collector configuration is one of several 3D designs which has shown high potential for the development of high energy and high power microbatteries in this regard. Herein we fabricate a nanoporous nanorod based current collector, which provides increased surface area for electrode deposition arising from the porosity of each nanorods, yet keeping an ordered spacing between nanorods for the deposition of subsequent electrolyte and electrode layers. The new nanostructured 3D current collector is demonstrated with a polyaniline (PANI)-based electrode system and is shown to deliver improved rate capability characteristics compared to planar configurations. We have been able to achieve stable capacities of ~32 µAh/cm(2) up to 75 cycles of charge/discharge even at a current rate of ~0.04 mA/cm(2) and have observed good rate capability even at high current rates of ~0.8 mA/cm(2).

Three-dimensionally engineered porous silicon electrodes for Li ion batteries.

The ultimate goal of Li ion battery design should consist of fully accessible metallic current collectors, possibly of nanoscale dimensions, intimately in contact with high capacity stable electrode materials. Here we engineer three-dimensional porous nickel based current collector coated conformally with layers of silicon, which typically suffers from poor cycle life, to form high-capacity electrodes. These binder/conductive additive free silicon electrodes show excellent electrode adhesion resulting in superior cyclic stability and rate capability. The nickel current collector design also allows for an increase in silicon loading per unit area leading to high areal discharge capacities of up to 0.8 mAh/cm(2) without significant loss in rate capability. An excellent electrode utilization (∼85%) and improved cyclic stability for the metal/silicon system is attributed to reduced internal stresses/fracture upon electrode expansion during cycling and shorter ionic/electronic diffusion pathways that help in improving the rate capability of thicker silicon layers.

Building energy storage device on a single nanowire.

Hybrid electrochemical energy storage devices combine the advantages of battery and supercapacitors, resulting in systems of high energy and power density. Using LiPF(6) electrolyte, the Ni-Sn/PANI electrochemical system, free of Li-based electrodes, works on a hybrid mechanism based on Li intercalation at the anode and PF(6)(-) doping at the cathode. Here, we also demonstrate a composite nanostructure electrochemical device with the anode (Ni-Sn) and cathode (polyaniline, PANI) nanowires packaged within conformal polymer core-shell separator. Parallel array of these nanowire devices shows reversible areal capacity of ∼3 µAh/cm(2) at a current rate of 0.03 mA/cm(2). The work shows the ultimate miniaturization possible for energy storage devices where all essential components can be engineered on a single nanowire.

Conformal coating of thin polymer electrolyte layer on nanostructured electrode materials for three-dimensional battery applications.

Various three-dimensional (3D) battery architectures have been proposed to address effective power delivery in micro/nanoscale devices and for increasing the stored energy per electrode footprint area. One step toward obtaining 3D configurations in batteries is the formation of core-shell nanowires that combines electrode and electrolyte materials. One of the major challenges however in creating such architectures has been the coating of conformal thin nanolayers of polymer electrolytes around nanostructured electrodes. Here we show conformal coatings of 25-30 nm poly(methyl methacralate) electrolyte layers around individual Ni-Sn nanowires used as anodes for Li ion battery. This configuration shows high discharge capacity and excellent capacity retention even at high rates over extended cycling, allowing for scalable increase in areal capacity with electrode thickness. Our results demonstrate conformal nanoscale anode-electrolyte architectures for an efficient Li ion battery system.

Combining Accurate O2 and Li2O2 Assays to Separate Discharge and Charge Stability Limitations in Nonaqueous Li-O2 Batteries.

Li-air batteries have generated enormous interest as potential high specific energy alternatives to existing energy storage devices. However, Li-air batteries suffer from poor rechargeability caused by the instability of organic electrolytes and carbon cathodes. To understand and address this poor rechargeability, it is essential to elucidate the efficiency in which O2 is converted to Li2O2 (the desired discharge product) during discharge and the efficiency in which Li2O2 is oxidized back to O2 during charge. In this Letter, we combine many quantitative techniques, including a newly developed peroxide titration, to assign and quantify decomposition pathways occurring in cells employing a variety of solvents and cathodes. We find that Li2O2-induced electrolyte solvent and salt instabilities account for nearly all efficiency losses upon discharge, whereas both cathode and electrolyte instabilities are observed upon charge at high potentials.

Hybrid nanostructures for energy storage applications.

Materials engineering plays a key role in the field of energy storage. In particular, engineering materials at the nanoscale offers unique properties resulting in high performance electrodes and electrolytes in various energy storage devices. Consequently, considerable efforts have been made in recent years to fulfill the future requirements of electrochemical energy storage using these advanced materials. Various multi-functional hybrid nanostructured materials are currently being studied to improve energy and power densities of next generation storage devices. This review describes some of the recent progress in the synthesis of different types of hybrid nanostructures using template assisted and non-template based methods. The potential applications and recent research efforts to utilize these hybrid nanostructures to enhance the electrochemical energy storage properties of Li-ion battery and supercapacitor are discussed. This review also briefly outlines some of the recent progress and new approaches being explored in the techniques of fabrication of 3D battery structures using hybrid nanoarchitectures.

Lithium storage mechanisms in purpurin based organic lithium ion battery electrodes.

Current lithium batteries operate on inorganic insertion compounds to power a diverse range of applications, but recently there is a surging demand to develop environmentally friendly green electrode materials. To develop sustainable and eco-friendly lithium ion batteries, we report reversible lithium ion storage properties of a naturally occurring and abundant organic compound purpurin, which is non-toxic and derived from the plant madder. The carbonyl/hydroxyl groups present in purpurin molecules act as redox centers and reacts electrochemically with Li-ions during the charge/discharge process. The mechanism of lithiation of purpurin is fully elucidated using NMR, UV and FTIR spectral studies. The formation of the most favored six membered binding core of lithium ion with carbonyl groups of purpurin and hydroxyl groups at C-1 and C-4 positions respectively facilitated lithiation process, whereas hydroxyl group at C-2 position remains unaltered.

Synthesis of nitrogen-doped graphene films for lithium battery application.

We demonstrate a controlled growth of nitrogen-doped graphene layers by liquid precursor based chemical vapor deposition (CVD) technique. Nitrogen-doped graphene was grown directly on Cu current collectors and studied for its reversible Li-ion intercalation properties. Reversible discharge capacity of N-doped graphene is almost double compared to pristine graphene due to the large number of surface defects induced due to N-doping. All the graphene films were characterized by Raman spectroscopy, transmission electron microscopy, and X-ray photoemission spectroscopy. Direct growth of active electrode material on current collector substrates makes this a feasible and efficient process for integration into current battery manufacture technology.

Coaxial MnO2/carbon nanotube array electrodes for high-performance lithium batteries.

Coaxial manganese oxide/carbon nanotube (CNT) arrays deposited inside porous alumina templates were used as cathodes in a lithium battery. Excellent cyclic stability and capacity of MnO2/CNT coaxial nanotube electrodes resulted from the hybrid nature of the electrodes with improved electronic conductivity and dual mechanism of lithium storage. The reversible capacity of the battery was increased by an order compared to template grown MnO2 nanotubes, making them suitable electrodes for advanced Li ion batteries.