Sustainable Electrolytes: Design Principles and Recent Advances

Today, rechargeable batteries are omnipresent and essential for our existence. In order to improve the electrochemical performance of electric fields, the introduction of electrolytes with fluorine (F)-based inorganic elemental compositions is a direction of exploration. However, most fluorocarbons have a high global warming potential and ozone depletion potential, which do not meet the sustainability requirements of the battery industry. Therefore, developing sustainable electrolytes is a viable option for future battery development. Although researchers have made much progress in electrolyte optimization, little attention has been paid to developing low-toxic and safe electrolytes. This review aims to elucidate the design principles and recent advances in this direction for solvents and salts. It concludes with a summary and outlook on future research directions for the molecular design of green electrolytes for practical high-voltage rechargeable batteries.

Bio-inspired carbon electrodes for metal-ion batteries

Carbon has been widely used as an electrode material in commercial metal-ion batteries (MIBs) because of its desirable electrical, mechanical, and physical properties. Still, traditional carbon electrodes suffer from limited mechanical stability and electrochemical performance in MIBs. Drawing inspiration from biological species, the carbon allotropes, such as fullerenes, carbon nanotubes, and graphene, can be engineered into mechanically robust, highly conductive frameworks with enhanced ion storage and transport capabilities for MIBs. Here, we present an assortment of bio-inspired carbon electrodes that have enhanced the cycling stability, capacity retention, and overall performance of MIBs. In addition, mimicking the structure and functionality of biological systems has led to the development of flexible MIBs whose performance does not degrade even when stretched, bent, or twisted. Finite element analysis (FEA) is a useful guide in identifying such bio-inspired carbon frameworks because it can simulate and analyze potential failure scenarios, such as stress build-up or structural collapse in MIBs. This review highlights through several examples that there is much scope for improving carbon-based electrode materials through bio-inspired designs for practical high-performance MIBs.

Molecular modulation strategies for two-dimensional transition metal dichalcogenide-based high-performance electrodes for metal-ion batteries

In the past few decades, great efforts have been made to develop advanced transition metal dichalcogenide (TMD) materials as metal-ion battery electrodes. However, due to existing conversion reactions, they still suffer from structural aggregation and restacking, unsatisfactory cycling reversibility, and limited ion storage dynamics during electrochemical cycling. To address these issues, extensive research has focused on molecular modulation strategies to optimize the physical and chemical properties of TMDs, including phase engineering, defect engineering, interlayer spacing expansion, heteroatom doping, alloy engineering, and bond modulation. A timely summary of these strategies can help deepen the understanding of their basic mechanisms and serve as a reference for future research. This review provides a comprehensive summary of recent advances in molecular modulation strategies for TMDs. A series of challenges and opportunities in the research field are also outlined. The basic mechanisms of different modulation strategies and their specific influences on the electrochemical performance of TMDs are highlighted.

Building Stable Solid-State Potassium Metal Batteries

Solid-state potassium metal batteries (SPMBs) are promising candidates for the next generation of energy storage systems for their low cost, safety, and high energy density. However, full SPMBs have not yet been reported due to the K dendrites, interfacial incompatibility, and limited availability of suitable solid-state electrolytes. Here, we present stable SPMBs using a new iodinated solid polymer electrolyte (ISPE). The functional ions reconstruct ion transport channels, providing efficient potassium ion transport. ISPE shows a combination of high ionic conductivity, superior interfacial compatibility, and electrochemical stability. In-situ alloying and iodinated interlayer increase K metal compatibility for prolonged cycling with low polarization. Moreover, the ISPE enables SPMBs with Prussian blue cathode stable operation at a high voltage of 4.5 V, a superior rate capability, and long-term cycling over 3,000 cycles (4.2 V versus K+ /K) with an ultra-high coulombic efficiency of 99.94%. More importantly, a classic solid-state potassium metal pouch cell achieved 4.2 V stable cycling over 800 cycles with an high retention of 93.6%, presenting a new development strategy for secure and high-performance rechargeable solid-state potassium metal batteries. This article is protected by copyright. All rights reserved.

Green Ether Electrolytes for Sustainable High-voltage Potassium Ion Batteries

Ether-based electrolytes are promising for secondary batteries due to their good compatibility with alkali metal anodes and high ionic conductivity. However, they suffer from poor oxidative stability and high toxicity, leading to severe electrolyte decomposition at high voltage and biosafety/environmental concerns when electrolyte leakage occurs. Here, we report a green ether solvent through a rational design of carbon-chain regulation to elicit steric hindrance, such a structure significantly reducing the solvent's biotoxicity and tuning the solvation structure of electrolytes. Notably, our solvent design is versatile, and an anion-dominated solvation structure is favored, facilitating a stable interphase formation on both the anode and cathode in potassium-ion batteries. Remarkably, the green ether-based electrolyte demonstrates excellent compatibility with K metal and graphite anode and a 4.2 V high-voltage cathode (200 cycles with average Coulombic efficiency of 99.64 %). This work points to a promising path toward the molecular design of green ether-based electrolytes for practical high-voltage potassium-ion batteries and other rechargeable batteries.

Entropy-Tuned Layered Oxide Cathodes for Potassium-Ion Batteries

The manganese-based layered oxides as a promising cathode material for potassium ion batteries (PIBs) have attracted considerable interest owing to their simple synthesis, high specific capacity, and low cost. However, due to the irreversible phase transition and the Jahn-Teller distortion of the Mn3+ , its application in potassium ion batteries is limited, leading to slow potassium ion kinetics and severe capacity attenuation. Here, entropy-tuning by changing the content of cathode material composition is proposed to address the above challenges. Compared to low and high entropy variants of K0.45 Mnx Co(1- x )/4 Mg(1- x )/4 Cu(1- x )/4 Ti(1- x )/4 O2 , where x = 0.8, 0.6, and 0.4, the medium entropy K0.45 Mn0.6 Co0.1 Mg0.1 Cu0.1 Ti0.1 O2 shows more balanced electrochemical properties in the PIBs. Benefiting from entropy-tuned suppression of the Jahn-Teller distortion of the Mn3+ , the K0.45 Mn0.6 Co0.1 Mg0.1 Cu0.1 Ti0.1 O2 can achieve a high K+ ion transport rate and alleviated volume variation while retaining high specific capacity. Accordingly, the medium entropy K0.45 Mn0.6 Co0.1 Mg0.1 Cu0.1 Ti0.1 O2 cathode in the full cell exhibits a high capacity of 100 mAh g-1 at 50 mA g-1 , delivers superior rate capability (65.8 mAh g-1 at 500 mA g-1 ) and cycling stability (67.8 mAh g-1 after 350 cycles at 100 mA g-1 ). The entropy-tuning strategy is expected to open new avenues in designing PIB cathode materials and beyond.

Trimming the Degrees of Freedom via a K+ Flux Rectifier for Safe and Long-Life Potassium-Ion Batteries

High degrees of freedom (DOF) for K+ movement in the electrolytes is desirable, because the resulting high ionic conductivity helps improve potassium-ion batteries, yet requiring support from highly free and flammable organic solvent molecules, seriously affecting battery safety. Here, we develop a K+ flux rectifier to trim K ion's DOF to 1 and improve electrochemical properties. Although the ionic conductivity is compromised in the K+ flux rectifier, the overall electrochemical performance of PIBs was improved. An oxidation stability improvement from 4.0 to 5.9 V was realized, and the formation of dendrites and the dissolution of organic cathodes were inhibited. Consequently, the K||K cells continuously cycled over 3,700 h; K||Cu cells operated stably over 800 cycles with the Coulombic efficiency exceeding 99%; and K||graphite cells exhibited high-capacity retention over 74.7% after 1,500 cycles. Moreover, the 3,4,9,10-perylenetetracarboxylic diimide organic cathodes operated for more than 2,100 cycles and reached year-scale-cycling time. We fabricated a 2.18 Ah pouch cell with no significant capacity fading observed after 100 cycles.

Ultrafast Carrier Drift Transport Dynamics in CsPbI3 Perovskite Nanocrystalline Thin Films

We study the early time carrier drift dynamics in CsPbI3 nanocrystal thin films with a sub 25 ps time resolution. Prior to trapping, carriers exhibit band-like transport characteristics, which is similar to those of traditional semiconductor solar absorbers including Si and GaAs due to optical phonon and carrier scattering at high temperatures. In contrast to the popular polaron scattering mechanism, the CsPbI3 nanocrystal thin film demonstrates the strongest optical phonon scattering mechanism among other inorganic-organic hybrid perovskites, Si, and GaAs. This ultrafast dynamics study establishes a foundation for understanding the fundamental carrier drift properties that drive perovskite nanocrystal optoelectronics.

Co-activation for enhanced K-ion storage in battery anodes

The relative natural abundance of potassium and potentially high energy density has established potassium-ion batteries as a promising technology for future large-scale global energy storage. However, the anodes' low capacity and high discharge platform lead to low energy density, which impedes their rapid development. Herein, we present a possible co-activation mechanism between bismuth (Bi) and tin (Sn) that enhances K-ion storage in battery anodes. The co-activated Bi-Sn anode delivered a high capacity of 634 mAh g^-1, with a discharge plateau as low as 0.35 V, and operated continuously for 500 cycles at a current density of 50 mA g^-1, with a high Coulombic efficiency of 99.2%. This possible co-activation strategy for high potassium storage may be extended to other Na/Zn/Ca/Mg/Al ion battery technologies, thus providing insights into how to improve their energy storage ability.

Electrochemical Performance of Polymer-Derived Silicon-Oxycarbide/Graphene Nanoplatelet Composites for High-Performance Li-Ion Batteries

Amorphous polymer-derived silicon-oxycarbide (SiOC) ceramics have a high theoretical capacity and good structural stability, making them suitable anode materials for lithium-ion batteries. However, SiOC has low electronic conductivity, poor transport properties, low initial Couloumbic efficiency, and limited rate capability. Therefore, there is an urgent need to explore an efficient SiOC-based anode material that could mitigate the abovementioned limitations. In this study, we synthesized carbon-rich SiOC (SiOC-I) and silicon-rich SiOC (SiOC-II) and evaluated their elemental and structural characteristics using a broad spectrum of characterization techniques. Li-ion cells were fabricated for the first time by pairing a buckypaper composed of carbon nanotubes with SiOC-I or SiOC-II as the anode. When mixed with graphene nanoplatelets, the SiOC-II/GNP composites exhibited improved electrochemical performance. High specific capacity (average specific capacity of 744 mAh/g at a 0.1C rate) was achieved with the composite anode (25 wt % SiOC-II and 75% GNP), which was much better than that of monolithic SiOC-I, SiOC-II, or GNPs. This composite also exhibited excellent cycling stability, achieving 344 mAh/g after 260 cycles at a 0.5C rate and high reversibility. The enhanced electrochemical performance is attributed to better electronic conductivity, lower charge-transfer resistance, and short ion diffusion length. Due to their superior electrochemical performance, SiOC/GNP composites with CNT buckypaper as a current collector can be considered a promising anode material for LiBs.

Quasi-solid aqueous electrolytes for low-cost sustainable alkali metal batteries

Aqueous electrolytes are highly important for batteries due to their sustainability, greenness, and low cost. However, the free water molecules react violently with alkali metals, rendering the high-capacity of alkali metal anodes unusable. Here, we confined water molecules in a carcerand-like network to build quasi-solid aqueous electrolytes (QAEs) with reduced water molecules' freedom and matched them with the low-cost chloride salts. The formed QAEs possess substantially different properties than liquid water molecules, including stable operation with alkali metal anodes without gas evolution. Specifically, the alkali metal anodes could directly cycle in a water-based environment with suppressed dendrites growth, electrode dissolution, and the polysulfide shuttle. The Li metal symmetric cells achieved long-term cycling over 7,000 h (and over 5,000/4,000 h for Na/K symmetric cells), and all Cu-based alkali metal cells exhibited a Coulombic efficiency of over 99%. Full metal batteries, such as Li||S batteries, attained high Coulombic efficiency, long life (over 4,000 cycles), and unprecedented energy density among water-based rechargeable batteries. This article is protected by copyright. All rights reserved.

Selective Potassium Deposition Enables Dendrite-Resistant Anodes for Ultrastable Potassium-Metal Batteries

Instability at the solid electrolyte interface (SEI) and uncontrollable growth of potassium dendrites have been pressing issues for potassium-ion batteries. Herein, a self-supporting electrode composed of bismuth and nitrogen-doped reduced graphene oxide (Bi₈₀ /NrGO) is designed as an anode host for potassium-metal batteries. Following the molten potassium diffusion into Bi₈₀ /NrGO, the resulting K@Bi₈₀ /NrGO exhibits unique hollow pores that provide K⁺ -diffusion channels and deposition space to buffer volume expansion, thus maintaining the electrode structure and SEI stability. The K@Bi₈₀ /NrGO also provides a controlled electric field that promotes uniform K⁺ flux, abundant potassiophilic N sites, and Bi alloying active sites, collectively enabling precise nucleation and selective deposition of potassium to achieve dendrite-resistant anodes. With the K@Bi₈₀ /NrGO-based optimized electrodes, the assembled K@Bi₈₀ /NrGO symmetrical cells can sustain stable cycling over 3000 h at a current density of 0.2 mA cm^-2 . Full cells with a Prussian blue cathode and K@Bi₈₀ /NrGO anode exhibit high stability (with no degradation for 1960 cycles at 1000 mA g^-1 ) with 99% Coulombic efficiency. This work may lead to the design of anodes with the triple attributes of precise nucleation, smooth diffusion, and dendrite inhibition, ideal for developing stable potassium-metal anodes and beyond.

Cyclic-anion salt for high-voltage stable potassium metal batteries

Electrolyte anions are critical for achieving high-voltage stable potassium-metal batteries (PMBs). However, the common anions cannot simultaneously prevent the formation of ‘dead K’ and the corrosion of Al current collector, resulting in poor cycling stability. Here, we demonstrate cyclic anion of hexafluoropropane-1,3-disulfonimide-based electrolytes that can mitigate the ‘dead K’ and remarkably enhance the high-voltage stability of PMBs. Particularly, even using low salt concentration (0.8 M) and additive-free carbonate-based electrolytes, the PMBs with a high-voltage polyanion cathode (4.4 V) also exhibit excellent cycling stability of 200 cycles with a good capacity retention of 83%. This noticeable electrochemical performance is due to the highly efficient passivation ability of the cyclic anions on both anode and cathode surfaces. This cyclic-anion-based electrolyte design strategy is also suitable for lithium and sodium-metal battery technologies.

Generating and Capturing Secondary Hot Carriers in Monolayer Tungsten Dichalcogenides

It remains challenging to capture and investigate the drift dynamics of primary hot carriers because of their ultrashort lifetime (∼200 fs). Here we report a new mechanism for secondary hot carrier (∼25 ps) generation in monolayer transition metal dichalcogenides such as WS₂ and WSe₂, triggered by the Auger recombination of trions and biexcitons. Using ultrafast photocurrent spectroscopy, we measured and characterized the photocurrent stemming from the Auger recombination of trions and biexcitons in WS₂ and WSe₂. A mobility of 0.24 cm² V^-1 s^-1 and a drift length of ∼3.8 nm were found for the secondary hot carriers in WS₂. By leveraging interactions between exciton complexes, we envision a new mechanism for generating and controlling hot carriers, which could lead to efficient devices in photophysics, photochemistry, and photosynthesis.

Phonon anharmonicity in binary chalcogenides for efficient energy harvesting

Thermoelectric (TE) materials have received much attention due to their ability to harvest waste heat energy. TE materials must exhibit a low thermal conductivity (κ) and a high power factor (PF) for efficient conversion. Both factors define the figure of merit (ZT) of the TE material, which can be increased by suppressing κ without degrading the PF. Recently, binary chalcogenides such as SnSe, GeTe, and PbTe have emerged as attractive candidates for thermoelectric energy generation at moderately high temperatures. These materials possess simple crystal structures with low κ in their pristine forms, which can be further lowered through doping and other approaches. Here, we review the recent advances in the temperature-dependent behavior of phonons and their influence on the thermal transport properties of chalcogenide-based TE materials. Because phonon anharmonicity is one of the fundamental contributing factors for low thermal conductivity in SnSe, Sb-doped GeTe, and related chalcogenides, we discuss complementary experimental approaches such as temperature-dependent Raman spectroscopy, inelastic neutron scattering, and calorimetry to measure anharmonicity. We further show how data gathered using multiple techniques helps us understand and engineer better TE materials. Finally, we discuss the rise of machine learning-aided efforts to discover, design, and synthesize TE materials of the future.

High-Potential Cathodes with Nitrogen Active Centres for Quasi-Solid Proton-Ion Batteries

Although proton-ion batteries have received considerable attention owing to their reliability, safety, toxin-free nature, and low cost, their development remains in the early stages because of lacking proper electrolytes and cathodes for facilitating a high output voltage and stable cycle performance. We present a novel cathode based on active nitrogen centre, which provides a flat discharge plateau at 1 V with a capacity of 115 mAh g^-1 and excellent stability. Moreover, a quasi-solid electrolyte was developed to overcome the issue of corrosion, broaden the potential window of the electrolyte, and prevent the active material from dissolving. While using the unique as-developed electrolyte, the newly designed cathode retained 89.67 % of its original capacity after 2000 cycles. Finally, we demonstrated the excellent cycle performance of the as-developed metal-free, flexible, soft-packed battery. Notably, even when a portion of the battery was cut off, it continued to function normally.

Prospects of Electrode Materials and Electrolytes for Practical Potassium-Based Batteries

Potassium-ion batteries (PIBs) have attracted tremendous attention because of their high energy density and low-cost. As such, much effort has focused on developing electrode materials and electrolytes for PIBs at the material levels. This review begins with an overview of the high-performance electrode materials and electrolytes, and then evaluates their prospects and challenges for practical PIBs to penetrate the market. The current status of PIBs for safe operation, energy density, power density, cyclability, and sustainability is discussed and future studies for electrode materials, electrolytes, and electrode-electrolyte interfaces are identified. It is anticipated that this review will motivate research and development to fill existing gaps for practical potassium-based full batteries so that they may be commercialized in the near future.

Advances in Studies of Boron Nitride Nanosheets and Nanocomposites for Thermal Transport and Related Applications

Hexagonal boron nitride (h-BN) and exfoliated nanosheets (BNNs) not only resemble their carbon counterparts graphite and graphene nanosheets in structural configurations and many excellent materials characteristics, especially the ultra-high thermal conductivity, but also offer other unique properties such as being electrically insulating and extreme chemical stability and oxidation resistance even at elevated temperatures. In fact, BNNs as a special class of 2-D nanomaterials have been widely pursued for technological applications that are beyond the reach of their carbon counterparts. Highlighted in this article are significant recent advances in the development of more effective and efficient exfoliation techniques for high-quality BNNs, the understanding of their characteristic properties, and the use of BNNs in polymeric nanocomposites for thermally conductive yet electrically insulating materials and systems. Major challenges and opportunities for further advances in the relevant research field are also discussed.

Cell-like-carbon-micro-spheres for robust potassium anode

Large-scale low-cost synthesis methods for potassium ion battery (PIB) anodes with long cycle life and high capacity have remained challenging. Here, inspired by the structure of a biological cell, biomimetic carbon cells (BCCs) were synthesized and used as PIB anodes. The protruding carbon nanotubes across the BCC wall mimicked the ion-transporting channels present in the cell membrane, and enhanced the rate performance of PIBs. In addition, the robust carbon shell of the BCC could protect its overall structure, and the open space inside the BCC could accommodate the volume changes caused by K+ insertion, which greatly improved the stability of PIBs. For the first time, a stable solid electrolyte interphase layer is formed on the surface of amorphous carbon. Collectively, the unique structural characteristics of the BCCs resulted in PIBs that showed a high reversible capacity (302 mAh g-1 at 100 mA g-1 and 248 mAh g-1 at 500 mA g-1), excellent cycle stability (reversible capacity of 226 mAh g-1 after 2100 cycles and a continuous running time of more than 15 months at a current density of 100 mA g-1), and an excellent rate performance (160 mAh g-1 at 1 A g-1). This study represents a new strategy for boosting battery performance, and could pave the way for the next generation of battery-powered applications.

Regulating Solvent Molecule Coordination with KPF6 for Superstable Graphite Potassium Anodes

Graphite is one of the most attractive anode materials due to its low cost, environmental friendliness, and high energy density for potassium ion batteries (PIBs). However, the severe capacity fade of graphite anodes in traditional KPF₆-based electrolyte hinders its practical applications. Here, we demonstrate that the cycling stability of graphite anodes can be significantly improved by regulating the coordination of solvent molecules with KPF₆ via a high-temperature precycling step. In addition to the solvents being electrochemically stable against reduction, a stable and uniform organic-rich passivation layer also forms on the graphite anodes after high-temperature precycling. Consequently, the PIBs with graphite anodes could operate for more than 500 cycles at 50 mA g^-1 with a reversible capacity of about 220 mAh g^-1 and an average Coulombic efficiency greater than 99%. Furthermore, full batteries based on Prussian blue cathodes and high-temperature precycled graphite anodes also exhibit excellent performance. Molecular dynamics simulations were performed to explore the solvation chemistry of the electrolytes used in this study.

Artificial SEI for Superhigh-Performance K-Graphite Anode

Although graphite with its merits of low cost, abundance, and environmental friendliness is a potential anode material for potassium ion batteries (PIBs), it suffers from a limited cycle life due to a severe decomposition of the solid electrolyte interface (SEI) in organic electrolytes. Herein, a simple and viable method is demonstrated for the first time through which an ultra-thin, uniform, dense, and stable artificial inorganic SEI film can be prepared on commercial graphite anodes and used with traditional carbonate electrolytes to achieve PIBs with long-cycle stability and high initial Coulombic efficiency (ICE). Specifically, such commercial graphite anodes exhibit a long-term cycling stability for more than 1000 cycles at 100 mA g-1 (a reversible capacity of around 260 mAh g-1) and a high average CE (around 99.9%) in traditional carbonate electrolytes with no discernable decay in capacity. More importantly, the commercial graphite anodes with the artificial inorganic SEI film in traditional carbonate electrolytes can deliver a high ICE of 93% (the highest ICE ever reported for PIBs anodes until now), which improves the performance of the PIB full cell. Considering the high ICE and long cycle stability performance, this study can promote the rapid deployment of PIBs on a commercial scale.

In-situ observation of trapped carriers in organic metal halide perovskite films with ultra-fast temporal and ultra-high energetic resolutions

We in-situ observe the ultrafast dynamics of trapped carriers in organic methyl ammonium lead halide perovskite thin films by ultrafast photocurrent spectroscopy with a sub-25 picosecond time resolution. Upon ultrafast laser excitation, trapped carriers follow a phonon assisted tunneling mechanism and a hopping transport mechanism along ultra-shallow to shallow trap states ranging from 1.72-11.51 millielectronvolts and is demonstrated by time-dependent and independent activation energies. Using temperature as an energetic ruler, we map trap states with ultra-high energy resolution down to < 0.01 millielectronvolt. In addition to carrier mobility of ~4 cm²V^-1s^-1 and lifetime of ~1 nanosecond, we validate the above transport mechanisms by highlighting trap state dynamics, including trapping rates, de-trapping rates and trap properties, such as trap density, trap levels, and capture-cross sections. In this work we establish a foundation for trap dynamics in high defect-tolerant perovskites with ultra-fast temporal and ultra-high energetic resolution.

High zT and Its Origin in Sb-doped GeTe Single Crystals

A record high zT of 2.2 at 740 K is reported in Ge0.92Sb0.08Te single crystals, with an optimal hole carrier concentration ≈4 × 1020 cm-3 that simultaneously maximizes the power factor (PF) ≈56 µW cm-1 K-2 and minimizes the thermal conductivity ≈1.9 Wm-1 K-1. In addition to the presence of herringbone domains and stacking faults, the Ge0.92Sb0.08Te exhibits significant modification to phonon dispersion with an extra phonon excitation around ≈5-6 meV at Γ point of the Brillouin zone as confirmed through inelastic neutron scattering (INS) measurements. Density functional theory (DFT) confirmed this phonon excitation, and predicted another higher energy phonon excitation ≈12-13 meV at W point. These phonon excitations collectively increase the number of phonon decay channels leading to softening of phonon frequencies such that a three-phonon process is dominant in Ge0.92Sb0.08Te, in contrast to a dominant four-phonon process in pristine GeTe, highlighting the importance of phonon engineering approaches to improving thermoelectric (TE) performance.

The correlation between phase transition and photoluminescence properties of CsPbX3 (X= Cl, Br, I) perovskite nanocrystals

We report a correlation between the structural phase transition of CsPbX₃ (X=Cl, Br, I) nanocrystals (NCs) and their temperature dependent steady-state photoluminescence (PL) and time-resolved PL (TRPL). In constrast to CsPbBr₃ and CsPbI₃ NCs which exhibited a continuous blue shift in their bandgap with increasing temperature, the CsPbCl₃ exhibited a blue shift until ~193 K, followed by a red shift until room temperature. We attribute this change from a blue to a red shift to a structural phase transtion in CsPbCl₃, which also manifested in the temperature dependent TRPL. Notably, the exciton recombination lifetimes showed a similar reverse trend due to the phase transition in CsPbCl₃, which has not been reported previously.

Three-Dimensional Si Anodes with Fast Diffusion, High Capacity, High Rate Capability, and Long Cycle Life

There are many interfaces in conventional nanostructured silicon anodes for LIBs, including (1) the solid-electrolyte interface (SEI), (2) interfaces between Si nanoparticles (NPs) and binders, and (3) interface between the current collector and active materials (CCAMI). Interfacial layers (e.g., graphene, activated carbon) coated on conventional Cu foil current collectors are often used to improve charge transfer and reduce CCAMI resistance. Indeed, our detailed studies show that the introduction of interfacial graphene layers results in an ∼20-60% increase in capacity after 500 cycles at 0.1 C. While the capacity is enhanced by inclusion of interfacial layers or conductive additives, they do not resolve problems associated with the diffusion of Li⁺ ions in the anode. Such electrodes that cannot accommodate the fast diffusion of Li⁺ ions are prone to plating. Here, we show that the use of freestanding and scalably produced carbon nanotube (CNT) Bucky paper or Bucky sandwich electrodes containing Si NPs (diameter of ∼100 nm) exhibits up to ∼1200 and 1900% increases in the gravimetric capacity after 500 cycles at 0.1 C, respectively, when discharged to 0.1 V. Using detailed electrochemical impedance spectroscopy, we show that the diffusion time constants in the Bucky paper and Bucky sandwich electrodes are increased by 2 orders of magnitude compared to that in the bare Cu foil. Furthermore, we demonstrate that the Bucky paper and Bucky sandwich electrodes can withstand high rates up to 4 C and show long cycle life up to ∼500 cycles at 0.1 C. Finally, we show that the Bucky sandwich electrode architecture with smaller diameter Si NPs (∼30 nm) leads to capacities as high as ∼1490 mAh/g (∼1635 mAh/g) at 0.1 C up to 100 cycles when discharged to 0.1 V (0.01 V).

Manipulating Charge Transfer from Core to Shell in CdSe/CdS/Au Heterojunction Quantum Dots

The photophysics of charge-transfer and recombination mechanisms in a heterojunction structure of CdSe/CdS/Au quantum dots (QDs) are studied by temperature-dependent steady-state photoluminescence (PL) and time-resolved PL (TRPL). We manipulate the charge transfer from core to shell surface by varying the tunneling barrier height resulting from temperature variation and the barrier width resulting from shell thickness variation. The charge-transfer process, which can be described by a tunneling transmission model, is manifested by two competitive recombination processes, an intrinsic exciton emission and a trap emission in the near-infrared (NIR) range. Our study establishes the photophysics foundation for the core/shell/metal application in photocatalyst and optoelectronics.

Impact of oxygen plasma treatment on carrier transport and molecular adsorption in graphene

Impact of plasma treatment on graphene's transport properties and interaction with gas molecules has been investigated with Raman, X-ray photoelectron spectroscopy, and Hall measurements. Experimental results indicate the formation of nanocrystalline domains and the enhanced fraction of adsorbed oxygen following oxygen plasma treatment, which correlates with a significant reduction in carrier mobility and an increase in carrier density. The oxygen plasma treated graphene was found to exhibit much stronger sensitivity toward NH₃ molecules both in terms of magnitude and response rate, attributable to increased domain edges and oxygen adsorption related enhancement in p-type doping. The carrier mobility in plasma exposed graphene was modeled considering both ionized impurity and short-range scattering, which matched well with the experimentally observed mobility.

Piezoresistive Graphene/P(VDF-TrFE) Heterostructure Based Highly Sensitive and Flexible Pressure Sensor

We report on a novel graphene/P(VDF-TrFE) heterostructure based highly sensitive, flexible, and biocompatible pressure/strain sensor developed through a facile and low-cost fabrication technique. The high piezoelectric coefficient of P(VDF-TrFE) coupled with outstanding electrical properties of graphene makes the sensor device highly sensitive, with an average sensitivity of 0.76 kPa^-1, a gauge factor of 445, and signal-to-noise ratio of 60.8 dB in the range of pressure up to 45 mmHg. A model was proposed to explain the sensor operation, based on carrier density and mobility changes induced by the piezoelectric charge generated in response to strain, which was supported by Hall measurements and Raman spectroscopy. Potential applications in wearable sensing for human activity monitoring were also demonstrated.

Thermoelectric Figure-of-Merit of Fully Dense Single-Crystalline SnSe

Single-crystalline SnSe has attracted much attention because of its record high figure-of-merit ZT ≈ 2.6; however, this high ZT has been associated with the low mass density of samples which leaves the intrinsic ZT of fully dense pristine SnSe in question. To this end, we prepared high-quality fully dense SnSe single crystals and performed detailed structural, electrical, and thermal transport measurements over a wide temperature range along the major crystallographic directions. Our single crystals were fully dense and of high purity as confirmed via high statistics 119Sn Mössbauer spectroscopy that revealed <0.35 at. % Sn(IV) in pristine SnSe. The temperature-dependent heat capacity (C p) provided evidence for the displacive second-order phase transition from Pnma to Cmcm phase at T c ≈ 800 K and a small but finite Sommerfeld coefficient γ0 which implied the presence of a finite Fermi surface. Interestingly, despite its strongly temperature-dependent band gap inferred from density functional theory calculations, SnSe behaves like a low-carrier-concentration multiband metal below 600 K, above which it exhibits a semiconducting behavior. Notably, our high-quality single-crystalline SnSe exhibits a thermoelectric figure-of-merit ZT ∼1.0, ∼0.8, and ∼0.25 at 850 K along the b, c, and a directions, respectively.

Role of Nanocomposite Support Stiffness on TFC Membrane Water Permeance

This paper discusses the role played by the mechanical stiffness of porous nanocomposite supports on thin-film composite (TFC) membrane water permeance. Helically coiled and multiwall carbon nanotubes (CNTs) were studied as additives in the nanocomposite supports. Mechanical stiffness was evaluated using tensile tests and penetration tests. While a low loading of CNTs caused macrovoids that decreased the structural integrity, adding higher loads of CNTs compensated for this effect, and this resulted in a net increase in structural stiffness. It was found that the Young's modulus of the nanocomposite supports increased by 30% upon addition of CNTs at 2 wt %. Results were similar for both types of CNTs. An empirical model for porous composite materials described the Young's modulus results. The nanocomposite supports were subsequently used to create TFC membranes. TFC membranes with stiffer supports were more effective at preventing declines in water permeance during compression. These findings support the idea that increasing the mechanical stiffness of TFC membrane nanocomposite supports is an effective strategy for enhancing water production in desalination operations.

Simultaneous Suppression of the Dendrite Formation and Shuttle Effect in a Lithium-Sulfur Battery by Bilateral Solid Electrolyte Interface

Although the reversible and inexpensive energy storage characteristics of the lithium-sulfur (Li-S) battery have made it a promising candidate for electrical energy storage, the dendrite growth (anode) and shuttle effect (cathode) hinder its practical application. Here, it is shown that new electrolytes for Li-S batteries promote the simultaneous formation of bilateral solid electrolyte interfaces on the sulfur-host cathode and lithium anode, thus effectively suppressing the shuttle effect and dendrite growth. These high-capacity Li-S batteries with new electrolytes exhibit a long-term cycling stability, ultrafast-charge/slow-discharge rates, super-low self-discharge performance, and a capacity retention of 94.9% even after a 130 d long storage. Importantly, the long cycle stability of these industrial grade high-capacity Li-S pouch cells with new electrolytes will provide the basis for creating robust energy dense Li-S batteries with an extensive life cycle.

A Versatile Carbon Nanotube-Based Scalable Approach for Improving Interfaces in Li-Ion Battery Electrodes

Resistive interfaces within the electrodes limit the energy and power densities of a battery, for example, a Li-ion battery (LIB). Typically, active materials are mixed with conductive additives in organic solvents to form a slurry, which is then coated on current collectors (e.g., bare or carbon-coated Al foils) to reduce the inherent resistance of the active material. Although many approaches using nanomaterials to either replace Al foils or improve conductivity within the active materials have been previously demonstrated, the resistance at the current collector active material interface (CCAMI), a key factor for enhancing the energy and power densities, remains unaddressed. We show that carbon nanotubes (CNTs), either directly grown or spray-coated on Al foils, are highly effective in reducing the CCAMI resistance of traditional LIB cathode materials (LiFePO₄ or LFP and LiNi_0.33Co_0.33Mn_0.33O₂ or NMC). Moreover, the CNT coatings displace the need for currently used toxic organic solvents (e.g., N-methyl-2-pyrrolidone) by providing capillary channels, which improve the wetting of aqueous dispersions containing active materials. The vertically aligned CNT-coated electrodes exhibited energy densities as high as (1) ∼500 W h kg^-1 at ∼170 W kg^-1 for LFP and (2) ∼760 W h kg^-1 at ∼570 W kg^-1 for NMC. The LIBs with CCAMI-engineered electrodes withstood discharge rates as high as 600 mA g^-1 for 500 cycles in the case of LFP, where commercial electrodes failed. The CNT-based CCAMI engineering approach is versatile with wide applicability to improve the performance of even textured active materials for both cathodes and anodes.

Saturable Absorption in 2D Ti3 C2 MXene Thin Films for Passive Photonic Diodes

MXenes comprise a new class of 2D transition metal carbides, nitrides, and carbonitrides that exhibit unique light-matter interactions. Recently, 2D Ti₃ CNT_x (T_x represents functional groups such as OH and F) was found to exhibit nonlinear saturable absorption (SA) or increased transmittance at higher light fluences, which is useful for mode locking in fiber-based femtosecond lasers. However, the fundamental origin and thickness dependence of SA behavior in MXenes remain to be understood. 2D Ti₃ C₂ T_x thin films of different thicknesses are fabricated using an interfacial film formation technique to systematically study their nonlinear optical properties. Using the open aperture Z-scan method, it is found that the SA behavior in Ti₃ C₂ T_x MXene arises from plasmon-induced increase in the ground state absorption at photon energies above the threshold for free carrier oscillations. The saturation fluence and modulation depth of Ti₃ C₂ T_x MXene is observed to be dependent on the film thickness. Unlike other 2D materials, Ti₃ C₂ T_x is found to show higher threshold for light-induced damage with up to 50% increase in nonlinear transmittance. Lastly, building on the SA behavior of Ti₃ C₂ T_x MXenes, a Ti₃ C₂ T_x MXene-based photonic diode that breaks time-reversal symmetry to achieve nonreciprocal transmission of nanosecond laser pulses is demonstrated.

Polymer-Derived Silicon Oxycarbide Ceramics as Promising Next-Generation Sustainable Thermoelectrics

We demonstrate the potential of polymer-derived ceramics (PDC) as next-generation sustainable thermoelectrics. Thermoelectric behavior of polymer-derived silicon oxycarbide (SiOC) ceramics (containing hexagonal boron nitride (h-BN) as filler) was studied as a function of measurement temperature. SiOC, sintered at 1300 °C exhibited invariant low thermal conductivity (∼1.5 W/(m·K)) over 30-600 °C, coupled with a small increase in both Seebeck coefficient and electrical conductivity, with increase in measurement temperature (30-150 °C). SiOC ceramics containing 1 wt % h-BN showed the highest Seebeck coefficient (-33 μV/K) for any PDC thus far.

A micro-Raman study of exfoliated few-layered n-type Bi2 Te2.7Se0.3

Previously we showed that the thermoelectric (TE) performance of bulk n-type Bi2Te2.7Se0.3 can be enhanced by subjecting it to a combined process of chemical or mechanical exfoliation (C/ME) followed by a rapid densification and restacking of the exfoliated layers via the spark-plasma-sintering technique (SPS). Here, we present a systematic micro-Raman study of two-dimensional flakes of n-type Bi2Te2.7Se0.3 produced by the C/ME process, as a function of the flake thickness. We found Raman evidence for flakes with: (i) integer number of quintuples which exhibited a strong electron-phonon coupling, and (ii) non-integer number of quintuples, or sub-quintuples which exhibited the forbidden IR active mode due to symmetry lowering. Detailed atomic force microscopy was used to confirm the number of quintuples in all flakes examined in this study. The restacking and densification of these flakes by SPS promoted the formation of charged grain boundaries, which led to the enhanced TE properties via the energy filtering process.

Influence of dopants on the impermeability of graphene

Graphene has attracted much attention as an impermeable membrane and a protective coating against oxidation. While many theoretical studies have shown that defect-free graphene is impermeable, in reality graphene inevitably has defects in the form of grain boundaries and vacancies. Here, we study the effects of N-dopants on the impermeability of few-layered graphene (FLG) grown on copper using chemical vapor deposition. The grain boundaries in FLG have minimal impact on their permeability to oxygen as they do not provide a continuous channel for gas transport due to high tortuosity. However, we experimentally show that the N-dopants in FLG display multiple configurations that create structural imperfections to selectively allow gas molecules to permeate. We used a comprehensive array of tools including Raman spectroscopy, X-ray photoelectron spectroscopy, optically stimulated electron emission measurements, and density functional theory of N-doped graphene on copper to elucidate the effects of dopant configuration on the impermeability of graphene. Our results clearly show that oxygen can permeate through graphene with non-graphitic nitrogen dopants that create pores in graphene and oxidize the underlying Cu substrate while graphitic nitrogen dopants do not show any changes compared to the pristine form. Furthermore, we observed that the work function of graphene can be tuned effectively by changing the dopant configuration.

Impact absorption properties of carbon fiber reinforced bucky sponges

We describe the super compressible and highly recoverable response of bucky sponges as they are struck by a heavy flat-punch striker. The bucky sponges studied here are structurally stable, self-assembled mixtures of multiwalled carbon nanotubes (MWCNTs) and carbon fibers (CFs). We engineered the microstructure of the sponges by controlling their porosity using different CF contents. Their mechanical properties and energy dissipation characteristics during impact loading are presented as a function of their composition. The inclusion of CFs improves the impact force damping by up to 50% and the specific damping capacity by up to 7% compared to bucky sponges without CFs. The sponges also exhibit significantly better stress mitigation characteristics compared to vertically aligned CNT foams of similar densities. We show that delamination occurs at the MWCNT-CF interfaces during unloading, and it arises from the heterogeneous fibrous microstructure of the bucky sponges.

Defect-Engineered Graphene for High-Energy- and High-Power-Density Supercapacitor Devices

Defects are often written off as performance limiters. Contrary to this notion, it is shown that controlling the defect configuration in graphene is critical to overcome a fundamental limitation posed by quantum capacitance and opens new channels for ion diffusion. Defect-engineered graphene flexible pouch capacitors with energy densities of 500% higher than the state-of-the-art supercapacitors are demonstrated.

Photoresponse of a Single Y-Junction Carbon Nanotube

We report investigation of optical response in a single strand of a branched carbon nanotube (CNT), a Y-junction CNT composed of multiwalled CNTs. The experiment was performed by connecting a pair of branches while grounding the remaining one. Of the three branch combinations, only one combination is optically active which also shows a nonlinear semiconductor-like I-V curve, while the other two branch combinations are optically inactive and show linear ohmic I-V curves. The photoresponse includes a zero-bias photocurrent from the active branch combination. Responsivity of ≈1.6 mA/W has been observed from a single Y-CNT at a moderate bias of 150 mV with an illumination of wavelength 488 nm. The photoresponse experiment allows us to understand the nature of internal connections in the Y-CNT. Analysis of data locates the region of photoactivity at the junction of only two branches and only the combination of these two branches (and not individual branches) exhibits photoresponse upon illumination. A model calculation based on back-to-back Schottky-type junctions at the branch connection explains the I-V data in the dark and shows that under illumination the barriers at the contacts become lowered due to the presence of photogenerated carriers.

Flexible Ag-C60 nano-biosensors based on surface plasmon coupled emission for clinical and forensic applications

The relatively low sensitivity of fluorescence detection schemes, which are mainly limited by the isotropic nature of fluorophore emission, can be overcome by utilizing surface plasmon coupled emission (SPCE). In this study, we demonstrate directional emission from fluorophores on flexible Ag-C60 SPCE sensor platforms for point-of-care sensing, in healthcare and forensic sensing scenarios, with at least 10 times higher sensitivity than traditional fluorescence sensing schemes. Adopting the highly sensitive Ag-C60 SPCE platform based on glass and novel low-cost flexible substrates, we report the unambiguous detection of acid-fast Mycobacterium tuberculosis (Mtb) bacteria at densities as low as 20 Mtb mm(-2); from non-acid-fast bacteria (e.g., E. coli and S. aureus), and the specific on-site detection of acid-fast sperm cells in human semen samples. In combination with the directional emission and high-sensitivity of SPCE platforms, we also demonstrate the utility of smartphones that can replace expensive and cumbersome detectors to enable rapid hand-held detection of analytes in resource-limited settings; a much needed critical advance to biosensors, for developing countries.

Biomolecular Interactions and Biological Responses of Emerging Two-Dimensional Materials and Aromatic Amino Acid Complexes

The present work experimentally investigates the interaction of aromatic amino acids viz., tyrosine, tryptophan, and phenylalnine with novel two-dimensional (2D) materials including graphene, graphene oxide (GO), and boron nitride (BN). Photoluminescence, micro-Raman spectroscopy, and cyclic voltammetry were employed to investigate the nature of interactions and possible charge transfer between 2D materials and amino acids. Graphene and GO were found to interact strongly with aromatic amino acids through π-π stacking, charge transfer, and H-bonding. Particularly, it was observed that both physi and chemisorption are prominent in the interactions of GO/graphene with phenylalanine and tryptophan while tyrosine exhibited strong chemisorption on graphene and GO. In contrast, BN exhibited little or no interactions, which could be attributed to localized π-electron clouds around N atoms in BN lattice. Lastly, the adsorption of amino acids on 2D materials was observed to considerably change their biological response in terms of reactive oxygen species generation. More importantly, these changes in the biological response followed the same trends observed in the physi and chemisorption measurements.

Bacteria Absorption-Based Mn2P2O7-Carbon@Reduced Graphene Oxides for High-Performance Lithium-Ion Battery Anodes

The development of freestanding flexible electrodes with high capacity and long cycle-life is a central issue for lithium-ion batteries (LIBs). Here, we use bacteria absorption of metallic Mn(2+) ions to in situ synthesize natural micro-yolk-shell-structure Mn2P2O7-carbon, followed by the use of vacuum filtration to obtain Mn2P2O7-carbon@reduced graphene oxides (RGO) papers for LIBs anodes. The Mn2P2O7 particles are completely encapsulated within the carbon film, which was obtained by carbonizing the bacterial wall. The resulting carbon microstructure reduces the electrode-electrolyte contact area, yielding high Coulombic efficiency. In addition, the yolk-shell structure with its internal void spaces is ideal for sustaining volume expansion of Mn2P2O7 during charge/discharge processes, and the carbon shells act as an ideal barrier, limiting most solid-electrolyte interphase formation on the surface of the carbon films (instead of forming on individual particles). Notably, the RGO films have high conductivity and robust mechanical flexibility. As a result of our combined strategies delineated in this article, our binder-free flexible anodes exhibit high capacities, long cycle-life, and excellent rate performance.

C60 as an active smart spacer material on silver thin film substrates for enhanced surface plasmon coupled emission

In this study, we present the use of C60 as an active spacer material on a silver (Ag) based surface plasmon coupled emission (SPCE) platform. In addition to its primary role of protecting the Ag thin film from oxidation, the incorporation of C60 facilitated the achievement of a 30-fold enhancement in the emission intensity of rhodamine B (RhB) fluorophore. The high signal yield was attributed to the unique π-π interactions between C60 thin films and RhB, which enabled efficient transfer of energy of RhB emission to Ag plasmon modes. Furthermore, minor variations in the C60 film thickness yielded large changes in the enhancement and angularity properties of the SPCE signal, which can be exploited for sensing applications. Finally, the low-cost fabrication process of the Ag-C60 thin film stacks render C60 based SPCE substrates ideal, for the economic and simplistic detection of analytes.

Tuning the electronic structure of graphene through nitrogen doping: experiment and theory

Tuning the electronic properties of graphene by doping atoms into its lattice makes it more applicable for electronic devices.