Anna Douglas

Ultrafine Iron Pyrite (FeS2) Nanocrystals Improve Sodium–Sulfur and Lithium–Sulfur Conversion Reactions for Efficient Batteries

Nanocrystals with quantum-confined length scales are often considered impractical for metal-ion battery electrodes due to the dominance of solid-electrolyte interphase (SEI) layer effects on the measured storage properties. Here we demonstrate that ultrafine sizes (∼4.5 nm, average) of iron pyrite, or FeS2, nanoparticles are advantageous to sustain reversible conversion reactions in sodium ion and lithium ion batteries. This is attributed to a nanoparticle size comparable to or smaller than the diffusion length of Fe during cation exchange, yielding thermodynamically reversible nanodomains of converted Fe metal and NaxS or LixS conversion products. This is compared to bulk-like electrode materials, where kinetic and thermodynamic limitations of surface-nucleated conversion products inhibit successive conversion cycles. Reversible capacities over 500 and 600 mAh/g for sodium and lithium storage are observed for ultrafine nanoparticles, with improved cycling and rate capability. Unlike alloying or intercalation processes, where SEI effects limit the performance of ultrafine nanoparticles, our work highlights the benefit of quantum dot length-scale nanocrystal electrodes for nanoscale metal sulfide compounds that store energy through chemical conversion reactions.

Sulfur Vapor-Infiltrated 3D Carbon Nanotube Foam for Binder-Free High Areal Capacity Lithium–Sulfur Battery Composite Cathodes

Here, we demonstrate a strategy to produce high areal loading and areal capacity sulfur cathodes by using vapor-phase infiltration of low-density carbon nanotube (CNT) foams preformed by solution processing and freeze-drying. Vapor-phase capillary infiltration of sulfur into preformed and binder-free low-density CNT foams leads to a mass loading of ∼79 wt % arising from interior filling and coating of CNTs with sulfur while preserving conductive CNT-CNT junctions that sustain electrical accessibility through the thick foam. Sulfur cathodes are then produced by mechanically compressing these foams into dense composites (ρ > 0.2 g/cm3), revealing specific capacity of 1039 mAh/gS at 0.1 C, high sulfur areal loading of 19.1 mg/cm2, and high areal capacity of 19.3 mAh/cm2. This work highlights a technique broadly adaptable to a diverse group of nanostructured building blocks where preformed low-density materials can be vapor infiltrated with sulfur, mechanically compressed, and exhibit simultaneous high areal and gravimetric storage properties. This provides a route for scalable, low-cost, and high-energy density sulfur cathodes based on conventional solid electrode processing routes.

Carbon Nanotubes Produced from Ambient Carbon Dioxide for Environmentally Sustainable Lithium-Ion and Sodium-Ion Battery Anodes

The cost and practicality of greenhouse gas removal processes, which are critical for environmental sustainability, pivot on high-value secondary applications derived from carbon capture and conversion techniques. Using the solar thermal electrochemical process (STEP), ambient CO2 captured in molten lithiated carbonates leads to the production of carbon nanofibers (CNFs) and carbon nanotubes (CNTs) at high yield through electrolysis using inexpensive steel electrodes. These low-cost CO2-derived CNTs and CNFs are demonstrated as high performance energy storage materials in both lithium-ion and sodium-ion batteries. Owing to synthetic control of sp3 content in the synthesized nanostructures, optimized storage capacities are measured over 370 mAh g–1 (lithium) and 130 mAh g–1 (sodium) with no capacity fade under durability tests up to 200 and 600 cycles, respectively. This work demonstrates that ambient CO2, considered as an environmental pollutant, can be attributed economic value in grid-scale and portable energy storage systems with STEP scale-up practicality in the context of combined cycle natural gas electric power generation.

A Sugar-Derived Room-Temperature Sodium Sulfur Battery with Long Term Cycling Stability

We demonstrate a room-temperature sodium sulfur battery based on a confining microporous carbon template derived from sucrose that delivers a reversible capacity over 700 mAh/gS at 0.1C rates, maintaining 370 mAh/gS at 10 times higher rates of 1C. Cycling at 1C rates reveals retention of over 300 mAh/gS capacity across 1500 cycles with Coulombic efficiency >98% due to microporous sulfur confinement and stability of the sodium metal anode in a glyme-based electrolyte. We show sucrose to be an ideal platform to develop microporous carbon capable of mitigating electrode-electrolyte reactivity and loss of soluble intermediate discharge products. In a manner parallel to the low-cost materials of the traditional sodium beta battery, our work demonstrates the combination of table sugar, sulfur, and sodium, all of which are cheap and earth abundant, for a high-performance stable room-temperature sodium sulfur battery.

Polysulfide Anchoring Mechanism Revealed by Atomic Layer Deposition of V2O5 and Sulfur-Filled Carbon Nanotubes for Lithium–Sulfur Batteries

Despite the promise of surface engineering to address the challenge of polysulfide shuttling in sulfur-carbon composite cathodes, melt infiltration techniques limit mechanistic studies correlating engineered surfaces and polysulfide anchoring. Here, we present a controlled experimental demonstration of polysulfide anchoring using vapor phase isothermal processing to fill the interior of carbon nanotubes (CNTs) after assembly into binder-free electrodes and atomic layer deposition (ALD) coating of polar V2O5 anchoring layers on the CNT surfaces. The ultrathin submonolayer V2O5 coating on the CNT exterior surface balances the adverse effect of polysulfide shuttling with the necessity for high sulfur utilization due to binding sites near the conductive CNT surface. The sulfur loaded into the CNT interior provides a spatially separated control volume enabling high sulfur loading with direct sulfur-CNT electrical contact for efficient sulfur conversion. By controlling ALD coating thickness, high initial discharge capacity of 1209 mAh/gS at 0.1 C and exceptional cycling at 0.2 C with 87% capacity retention after 100 cycles and 73% at 450 cycles is achieved and correlated to an optimal V2O5 anchoring layer thickness. This provides experimental evidence that surface engineering approaches can be effective to overcome polysulfide shuttling by controlled design of molecular-scale building blocks for efficient binder free lithium sulfur battery cathodes.

Role of carbon defects in the reversible alloying states of red phosphorus composite anodes for efficient sodium ion batteries

Here we report the first mechanistic study investigating the effect of carbon defects on the evolution of different sodium–red phosphorus (red P) alloy states for stable high capacity sodium ion battery anodes. Using tunable sp2/sp3 carbon composites containing controlled single-walled carbon nanotube (SWCNT) and single-walled carbon nanohorn (SWCNH) compositions, we identify potentials over which both stable and unstable alloying of red P occurs with sodium. Examination of the stable alloy region includes both NaP and Na5P4 formation that occurs between 0.40 and 0.15 V where alloying is mostly independent of the carbon composite matrix chemistry. However, an unstable region corresponding to Na3P formation below 0.15 V results in capacity degradation that directly correlates with the density of carbon defects. In the unstable region, defects are observed to initiate deep alloying and poor reversibility due to the formation of irreversible Na3P products that form over the carbon surface. Our results present a mechanistic roadmap to guide the design of red P–carbon composite anodes to approach high theoretical sodium ion capacity (2596 mA h g−1) while simultaneously addressing chemical interactions that compromise performance stability.

Particulate-free porous silicon networks for efficient capacitive deionization water desalination

Energy efficient water desalination processes employing low-cost and earth-abundant materials is a critical step to sustainably manage future human needs for clean water resources. Here we demonstrate that porous silicon – a material harnessing earth abundance, cost, and environmental/biological compatibility is a candidate material for water desalination. With appropriate surface passivation of the porous silicon material to prevent surface corrosion in aqueous environments, we show that porous silicon templates can enable salt removal in capacitive deionization (CDI) ranging from 0.36% by mass at the onset from fresh to brackish water (10 mM, or 0.06% salinity) to 0.52% in ocean water salt concentrations (500 mM, or ~0.3% salinity). This is on par with reports of most carbon nanomaterial based CDI systems based on particulate electrodes and covers the full salinity range required of a CDI system with a total ocean-to-fresh water required energy input of ~1.45 Wh/L. The use of porous silicon for CDI enables new routes to directly couple water desalination technology with microfluidic systems and photovoltaics that natively use silicon materials, while mitigating adverse effects of water contamination occurring from nanoparticulate-based CDI electrodes.

Toward Small-Diameter Carbon Nanotubes Synthesized from Captured Carbon Dioxide: Critical Role of Catalyst Coarsening

Small-diameter carbon nanotubes (CNTs) often require increased sophistication and control in synthesis processes, but exhibit improved physical properties and greater economic value over their larger-diameter counterparts. Here, we study mechanisms controlling the electrochemical synthesis of CNTs from the capture and conversion of ambient CO2 in molten salts and leverage this understanding to achieve the smallest-diameter CNTs ever reported in the literature from sustainable electrochemical synthesis routes, including some few-walled CNTs. Here, Fe catalyst layers are deposited at different thicknesses onto stainless steel to produce cathodes, and atomic layer deposition of Al2O3 is performed on Ni to produce a corrosion-resistant anode. Our findings indicate a correlation between the CNT diameter and Fe metal layer thickness following electrochemical catalyst reduction at the cathode-molten salt interface. Further, catalyst coarsening during long duration synthesis experiments leads to a 2× increase in average diameters from 3 to 60 min durations, with CNTs produced after 3 min exhibiting a tight diameter distribution centered near ∼10 nm. Energy consumption analysis for the conversion of CO2 into CNTs demonstrates energy input costs much lower than the value of CNTs-a concept that strictly requires and motivates small-diameter CNTs-and is more favorable compared to other costly CO2 conversion techniques that produce lower-value materials and products.