Isothermal Sulfur Condensation into Carbon Scaffolds: Improved Loading, Performance, and Scalability for Lithium–Sulfur Battery Cathodes

Here we demonstrate an isothermal technique that enables rapid vapor infiltration of sulfur into carbon templates to overcome scalability and performance bottlenecks associated with common melt infiltration techniques. Building on straightforward thermodynamic principles of capillary condensation, self-limited sulfur loadings up to 82 wt % can be achieved in as little as 10 min at temperatures between 155 and 175 °C. We demonstrate a broad range of device performance criteria using a carbon black–single-walled carbon nanotube binder-free cathode framework, including a side-by-side comparison to melt infiltrated electrodes with 74 wt % loading that shows improved capacity (1015 mAh/g vs 768 mAh/g), ∼92% capacity retention after 200 cycles at 0.5 C, and ∼98% Coulombic efficiency as a result of enhanced uniformity and conductivity. Further, we demonstrate this technique over a range of different electrodes (1) electrodes with high sulfur loading (82 wt %) with high initial discharge capacity of 1340 mAh/g, (2) electrodes with high areal loading of 8 mg/cm² sulfur with >6.5 mAh/cm² areal capacity, and (3) electrodes based on carbons with microporous confining pores. Most importantly, this vapor infiltration approach requires over 5× less energy input and enables over 60× greater throughput than standard melt infiltration, enabling integration into roll-to-roll rapid processing schemes without compromising device performance. This technique liberates cost and manufacturing barriers to commercialization of Li–S batteries at larger scales while opening new avenues to infiltrate preformed cathode assemblies with sulfur for assessment at lab scales

Sustainable Capture and Conversion of Carbon Dioxide into Valuable Multiwalled Carbon Nanotubes Using Metal Scrap Materials

Increasing amounts of nondegradable waste and rising levels of atmospheric carbon dioxide (CO₂) similarly threaten a sustainable future, leaving routes to address these issues at the forefront of ongoing research efforts. Here, we demonstrate a route where electrochemical processing of scrap metals extracts catalytic species to the surface that actively convert CO₂ scavenged from the atmosphere to form multiwalled carbon nanotubes (MWCNTs). Our findings demonstrate two distinct pathways for this technique that is generalizable to a broad range of scrap metals. First is the case where the catalytic elements are the primary constituents of the material (e.g., Fe in stainless steel) and the reaction with CO₂ consumes the material. Second is the case where the catalytic elements are impurities (e.g., Fe in brass) where reaction with CO₂ leads to impurity consumption. Our results demonstrate facile growth of MWCNTs directly from irregular scraps, such as shavings and pipes. Overall, this study presents a route where input sources of atmospheric CO₂ and low-value metal scraps can be transformed to higher valued purified metals and high valued MWCNTs with the promise of an overall carbon negative capture and conversion strategy

Size-Dependent Phononic Properties of PdO Nanocrystals Probed by Nanoscale Optical Thermometry

With the advent of novel nanoscale devices, fast and reliable thermal mapping with high spatiotemporal resolution is imperative for probing the characteristics of phonons and evaluating the local temperature at the nanoscale. In this work, Raman spectroscopy is employed as a rapid and noncontact optical thermometry technique to investigate phononic properties of macroscopic assemblies of monodisperse palladium oxide (PdO) nanocrystals. PdO has been extensively employed in high temperature catalytic devices; however, the phonon behavior which determines the thermal stability of PdO remains unexplored thus far. Our study focuses on homogeneous, large-scale assemblies of monodisperse 4 and 10 nm nanocrystals synthesized using colloidal chemistry to understand size-dependent effects on the measured thermal properties. By monitoring the Raman peak shifts, peak broadening, and alterations in peak intensities as a function of laser power and particle concentration, a size-dependent trend is observed attributable to confinement of optical phonons within nanocrystal grain boundaries and laser-induced heating, both influenced by nanocrystal size. This study correlates size-dependent single-particle heating effects with size-dependent interparticle heat transfer under laser irradiation and is enabled by controlled nanocrystal synthesis

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

Role of Catalyst Oxidation State in the Growth of Vertically Aligned Carbon Nanotubes

The impact of gas-phase pretreatment of supported iron-oxide catalyst utilized in aligned carbon nanotube (CNT) growth is studied to understand the correlation between the catalyst oxidation state and the growth characteristics of the aligned CNT forests. By varying the pretreatment conditions from a reducing to an oxidizing environment, notable changes are observed in both the collective CNT array growth behavior and the individual CNT characteristics. Although the greatest catalytic activity was observed following a full reduction to the zerovalent (metallic) Fe catalyst, growth is also observed from a catalyst composed of both Fe₂O₃ and Fe₃O₄ particles. XPS core-level analysis, following pretreatment of the catalyst, emphasizes the critical nature of the combined catalyst–underlayer interaction to achieve optimal catalyst activity during growth and hence the most efficient catalyst reduction process. Additionally, CNT diameters during growth were strongly affected by the pretreatment process. Overall, this work gives a collective picture of how the catalyst oxidation state affects the CNT growth based on the catalyst pretreatment environment and the nature of the catalyst–underlayer interactions. Such concepts are critical for the rational design of alternative catalyst–underlayer systems for efficient CNT synthetic processes

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/g_S 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

Ultrafast Solvent-Assisted Sodium Ion Intercalation into Highly Crystalline Few-Layered Graphene

A maximum sodium capacity of ∼35 mAh/g has hampered the use of crystalline carbon nanostructures for sodium ion battery anodes. We demonstrate that a diglyme solvent shell encapsulating a sodium ion acts as a “nonstick” coating to facilitate rapid ion insertion into crystalline few-layer graphene and bypass slow desolvation kinetics. This yields storage capacities above 150 mAh/g, cycling performance with negligible capacity fade over 8000 cycles, and ∼100 mAh/g capacities maintained at currents of 30 A/g (∼12 s charge). Raman spectroscopy elucidates the ordered, but nondestructive cointercalation mechanism that differs from desolvated ion intercalation processes. In situ Raman measurements identify the Na⁺ staging sequence and isolates Fermi energies for the first and second stage ternary intercalation compounds at ∼0.8 eV and ∼1.2 eV

Roll-to-Roll Nanomanufacturing of Hybrid Nanostructures for Energy Storage Device Design

A key limitation to the practical incorporation of nanostructured materials into emerging applications is the challenge of achieving low-cost, high throughput, and highly replicable scalable nanomanufacturing techniques to produce functional materials. Here, we report a benchtop roll-to-roll technique that builds upon the use of binary solutions of nanomaterials and liquid electrophoretic assembly to rapidly construct hybrid materials for battery design applications. We demonstrate surfactant-free hybrid mixtures of carbon nanotubes, silicon nanoparticles, MoS₂ nanosheets, carbon nanohorns, and graphene nanoplatelets. Roll-to-roll electrophoretic assembly from these solutions enables the controlled fabrication of homogeneous coatings of these nanostructures that maintain chemical and physical properties defined by the synergistic combination of nanomaterials utilized without adverse effects of surfactants or impurities that typically limit liquid nanomanufacturing routes. To demonstrate the utility of this nanomanufacturing approach, we employed roll-to-roll electrophoretic processing to fabricate both positive and negative electrodes for lithium ion batteries in less than 30 s. The optimized full-cell battery, containing active materials of prelithiated silicon nanoparticles and MoS₂ nanosheets, was assessed to exhibit energy densities of 167 Wh/kgcell^–1 and power densities of 9.6 kW/kgcell^–1

Anode-Free Sodium Battery through in Situ Plating of Sodium Metal

Sodium-ion batteries (SIBs) have been pursued as a more cost-effective and more sustainable alternative to lithium-ion batteries (LIBs), but these advantages come at the expense of energy density. In this work, we demonstrate that the challenge of energy density for sodium chemistries can be overcome through an anode-free architecture enabled by the use of a nanocarbon nucleation layer formed on Al current collectors. Electrochemical studies show this configuration to provide highly stable and efficient plating and stripping of sodium metal over a range of currents up to 4 mA/cm², sodium loading up to 12 mAh/cm², and with long-term durability exceeding 1000 cycles at a current of 0.5 mA/cm². Building upon this anode-free architecture, we demonstrate a full cell using a presodiated pyrite cathode to achieve energy densities of ∼400 Wh/kg, far surpassing recent reports on SIBs and even the theoretical maximum for LIB technology (387 Wh/kg for LiCoO₂/graphite cells) while still relying on naturally abundant raw materials and cost-effective aqueous processing

Role of Nitrogen-Doped Graphene for Improved High-Capacity Potassium Ion Battery Anodes

Potassium is an earth abundant alternative to lithium for rechargeable batteries, but a critical limitation in potassium ion battery anodes is the low capacity of KC₈ graphite intercalation compounds in comparison to conventional LiC₆. Here we demonstrate that nitrogen doping of few-layered graphene can increase the storage capacity of potassium from a theoretical maximum of 278 mAh/g in graphite to over 350 mAh/g, competitive with anode capacity in commercial lithium ion batteries and the highest reported anode capacity so far for potassium ion batteries. Control studies distinguish the importance of nitrogen dopant sites as opposed to sp³ carbon defect sites to achieve the improved performance, which also enables >6× increase in rate performance of doped <i>vs</i> undoped materials. Finally, <i>in situ</i> Raman spectroscopy studies elucidate the staging sequence for doped and undoped materials and demonstrate the mechanism of the observed capacity enhancement to be correlated with distributed storage at local nitrogen sites in a staged KC₈ compound. This study demonstrates a pathway to overcome the limitations of graphitic carbons for anodes in potassium ion batteries by atomically precise engineering of nanomaterials