The next generation of high-performance batteries should include alternative chemistries that are inherently safer to operate than nonaqueous lithium-based batteries. Aqueous zinc-based batteries can answer that challenge because monolithic zinc sponge anodes can be cycled in nickel-zinc alkaline cells hundreds to thousands of times without undergoing passivation or macroscale dendrite formation. We demonstrate that the three-dimensional (3D) zinc form-factor elevates the performance of nickel-zinc alkaline cells in three fields of use: (i) >90% theoretical depth of discharge (DOD) in primary (single-use) cells, (ii) >100 high-rate cycles at 40% DOD at lithium-ion-commensurate specific energy, and (iii) the tens of thousands of power-demanding duty cycles required for start-stop microhybrid vehicles.

Zinc-based replacements for Li-ion batteries are now feasible by using 3D-interconnected Zn sponges that suppress dendrite formation when cycled.

Transition metal oxides that mix electronic and ionic conductivity are essential active components of many electrochemical charge-storage devices, ranging from primary alkaline cells to more advanced rechargeable Li-ion batteries. In these devices, charge storage occurs via cation-insertion/deinsertion mechanisms in conjunction with the reduction/oxidation of metal sites in the oxide. Batteries that incorporate such metal oxides are typically designed for high specific energy, but not necessarily for high specific power. Electrochemical capacitors (ECs), which are typically composed of symmetric high-surface-area carbon electrodes that store charge via double-layer capacitance, deliver their energy in time scales of seconds, but at much lower specific energy than batteries. The fast, reversible faradaic reactions (typically described as "pseudocapacitance") of particular nanoscale metal oxides (e.g., ruthenium and manganese oxides) provide a strategy for bridging the power/energy performance gap between batteries and conventional ECs. These processes enhance charge-storage capacity to boost specific energy, while maintaining the few-second timescale of the charge-discharge response of carbon-based ECs. In this Account, we describe three examples of redox-based deposition of EC-relevant metal oxides (MnO2, FeOx, and RuO2) and discuss their potential deployment in next-generation ECs that use aqueous electrolytes. To extract the maximum pseudocapacitance functionality of metal oxides, one must carefully consider how they are synthesized and subsequently integrated into practical electrode structures. Expressing the metal oxide in a nanoscale form often enhances electrochemical utilization (maximizing specific capacitance) and facilitates high-rate operation for both charge and discharge. The "wiring" of the metal oxide, in terms of both electron and ion transport, when fabricated into a practical electrode architecture, is also a critical design parameter for achieving characteristic EC charge-discharge timescales. For example, conductive carbon must often be combined with the poorly conductive metal oxides to provide long-range electron pathways through the electrode. However, the ad hoc mixing of discrete carbon and oxide powders into composite electrodes may not support optimal utilization or rate performance. As an alternative, nanoscale metal oxides of interest for ECs can be synthesized directly on the surfaces of nanostructured carbons, with the carbon surface acting as a sacrificial reductant when exposed to a solution-phase, oxidizing precursor of the desired metal oxide (e.g., MnO4(-) for MnO2). These redox deposition methods can be applied to advanced carbon nanoarchitectures with well-designed pore structures. These architectures promote effective electrolyte infiltration and ion transport to the nanoscale metal oxide domains within the electrode architecture, which further enhances high-rate operation.

Homogeneous, nanocrystalline powders of yttria-stabilized zirconia (YSZ) were prepared using a non-alkoxide sol−gel method. Monolithic gels, free of precipitation, were prepared by addition of propylene oxide to aqueous solutions of Zr4+ and Y3+ chlorides at room temperature. The gels were dried with supercritical CO2(l), resulting in amorphous aerogels that crystallized into stabilized ZrO2 following calcination at 500 °C. The aerogels and resulting crystalline products were characterized using in situ temperature profile X-ray diffraction, Raman spectroscopy, thermal analysis, transmission electron microscopy (TEM), scanning electron microscopy (SEM), nitrogen adsorption/desorption analysis, and elemental analysis by inductively coupled plasma-atomic emission spectroscopy. TEM and N2 adsorption/desorption analysis of an aerogel prepared by this method indicated a porous network structure with a high surface area (409 m2/g). The crystallized YSZ maintained high surface area (159 m2/g) upon formation of homogeneous, nanoparticles (∼10 nm). Ionic conductivity at 1000 °C of sintered YSZ (1500 °C, 3 h) was 0.13 ± 0.02 Ω-1 cm-1. Activation energies for the conduction processes from 1000 to 550 °C and 550−400 °C were 0.95 ± 0.09 and 1.12 ± 0.05 eV, respectively.

Subambient thermal decomposition of ruthenium tetroxide from nonaqueous solution onto porous SiO(2) substrates creates 2-3 nm thick coatings of RuO(2) that cover the convex silica walls comprising the open, porous structure. The physical properties of the resultant self-wired nanoscale ruthenia significantly differ depending on the nature of the porous support. Previously reported RuO(2)-modified SiO(2) aerogels display electron conductivity of 5 x 10(-4) S cm(-1) (as normalized to the geometric factor of the insulating substrate, not the conducting ruthenia phase), whereas RuO(2)-modified silica filter paper at approximately 5 wt % RuO(2) exhibits approximately 0.5 S cm(-1). Electron conduction through the ruthenia phase as examined from -160 to 260 degrees C requires minimal activation energy, only 8 meV, from 20 to 260 degrees C. The RuO(2)(SiO(2)) fiber membranes are electrically addressable, capable of supporting fast electron-transfer reactions, express an electrochemical surface area of approximately 90 m(2) g(-1) RuO(2), and exhibit energy storage in which 90% of the total electron-proton charge is stored at the outer surface of the ruthenia phase. The electrochemical capacitive response indicates that the nanocrystalline RuO(2) coating can be considered to be a single-unit-thick layer of the conductive oxide, as physically stabilized by the supporting silica fiber.

We fabricate three-dimensional zinc electrodes from emulsion-cast sponges of Zn powder that are thermally treated to produce rugged monoliths. This highly conductive, 3D-wired aperiodic scaffold achieves 740 mA h gZn(-1) when discharged in primary Zn-air cells (>90% of theoretical Zn capacity). We use scanning electron microscopy and X-ray diffraction to monitor the microstructural evolution of a series of Zn sponges when oxidized in Zn-air cells to specific depths-of-discharge (20, 40, 60, 80% DOD) at a technologically relevant rate (C/40; 4-6 mA cm(-2)). The Zn sponges maintain their 3D-monolithic form factor at all DOD. The cell resistance remains low under all test conditions, indicating that an inner core of metallic Zn persists that 3D-electrically wires the electrode, even to deep DOD.

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The historically poor electrochemical rechargeability of Zn in alkaline electrolyte has hindered the commercial viability of Ni-Zn batteries, a system otherwise of interest because of high specific energy (up to 140 Wh kg -1 ). We have redesigned the Zn anode as a three-dimensional (3D), monolithic porous architecture ("sponge") that exhibits unprecedented Zn specific capacity and dendrite-free cycling. Maintaining the integrity of the 3D Zn sponge architecture throughout charge-discharge is required to ultimately achieve technologically relevant performance in terms of cycle life and capacity. En route to this goal, we systematically evaluated a series of electrolyte and electrode additives used in conjunction with our Zn sponge anode in order to down-select formulations that minimize electrode shape change with cycling in prototype Ni-3D Zn cells. The classes of additives chosen for this study include those that either inhibit ZnO passivation during discharge (Type I: LiOH, K 2 SiO 3 ) or promote it (Type II/III: KF, K 2 CO 3 , ZnO, Ca(OH) 2 ), as well as combinations thereof. We find that the second class of additives effectively retains the cycled Zn sponge in its pre-cycled condition.