Na-K Alloy Research Articles

Experimental investigation on the thermal performance of high temperature two-phase loop thermosyphon

High-temperature two-phase loop thermosyphons (TPLTs) have the potential to be applied in high-temperature thermochemical conversions, concentrated solar energy, and nuclear energy. However, there has been limited investigation on the thermal performance of TPLTs so far. This work experimentally examines the thermal performance of TPLTs using alkali metals (NaK alloy and pure Na) as working fluids, aiming for internal energy recovery of a double-riser reactor. The effects of filling ratios (FRs), heating temperature, and altitude difference between the evaporator and condenser are investigated regarding start-up characteristics and steady-state performance. The experimental results show that the TPLT with NaK as the working fluid can successfully complete the start-up when the heating temperature is set to above 650 °C under the test conditions. However, the TPLT using pure Na as the working fluid can't successfully complete the start-up from a frozen state without a preheating operation. According to the variations in thermal resistance, an FR of 38% in the TPLT allows for the best heat-transfer performance under steady state when compared to the conditions that FRs are equal to 25% and 50%. Additionally, placing the condenser slightly higher than the evaporator is beneficial for improving the steady-state performance. However, if the condenser is lower than the evaporator, its steady-state performance is remarkably degraded.

Stabilizing electrode-electrolyte interface via crown ether-containing selective coating for dendrite-free Na-K anode-based solid-state sodium batteries

Sodium-potassium (Na-K) alloy is considered a promising anode material due to its liquid characteristics at room temperature. However, unstable interfaces and disastrous ion exchange between Na-K alloy anodes and solid-state Na electrolytes (SSNEs) have hindered further applications of solid-state Na batteries (SSNBs) based on Na-K anodes. In this work, a specially designed Na+/K+ selective gel coating with 15-crown 5-ether (15C5) is constructed on the beta-alumina solid electrolyte (BASE) to achieve a good match with Na-K anodes. The coordination of 15C5 with Na+ forms Na+-permeable complexes and affects the electrolyte solvation structure in the gel coating, resulting in the formation of solid electrolyte interfaces (SEIs) that lack K+ channels and are organically rich, which effectively inhibit the ion exchange and stabilize the interface. Benefitting from the selective coating, the cycling lifetime of the Na-K|Gel-BASE-Gel|Na-K symmetric cell reaches over 1000 h, and the Na-K|Gel-BASE-Gel|Na3V2(PO4)3 full cell exhibits great long-cycle stability and excellent rate performance at room temperature. This work extends the application of Na-K anodes and provides a suggestion for achieving high-performance SSNBs at room temperature.

Liquid Na-K alloy is not viable anode material for High-Performance Na-Ion batteries

Liquid sodium–potassium alloy NaxK100-x at. % (≈14 < x < 70 at room temperature) has been proposed as a dendrite-free liquid metal anode for Na-ion batteries. Nevertheless, controversy surrounds the use of NaxK100-x as a Na-ion battery anode due to the presence of the two competitive redox-active species, Na/Na+ and K/K+. To resolve this ongoing controversy, we use x-ray diffraction (XRD) and cryogenic-focused ion beam/scanning electron microscopy (cryo-FIB/SEM) to investigate liquid Na50K50 electrodes cycled in symmetric cells using Na-ion electrolytes. We find that the reaction overpotential abruptly increases after ≈ 40 cycles. Cryo-FIB/SEM reveals the precipitation of K salts, which confirms that K/K+ species are redox-active in Na-ion electrolytes. Such an effect progressively leads to irreversible leaching of K from the initial liquid Na50K50 composition, resulting in a loss of liquid metal properties. Consequently, our results suggest that NaxK100-x cannot sustainably function as a liquid metal anode in Na-ion batteries.

Experimental study on the heat transfer performance of liquid metal high-temperature oscillating heat pipes with diamond nanoparticles

In order to realize effcient thermal dissipation at high temperatures, diamond nanoparticles were added in the sodium–potassium (NaK) alloy liquid metal high-temperature oscillating heat pipes (LMHOHPs) to improve the heat transfer performance. The NaK alloy with different mass fractions (0 wt%, 0.25 wt%, 0.50 wt%, 0.75 wt%, 1.00 wt%) of diamond nanoparticles was used as the working fluids. Under the input power of 2000–3500 W and the inclination angle of 0°, 30°, 60°, and 90°, the start-up characteristics and heat transfer performance of LMHOHPs after adding diamond nanoparticles were measured and analyzed. The results showed that after adding diamond nanoparticles, the LMHOHP started more smoothly and could effectively promote the unidirectional circulation flow of the working fluid. The enhancement effects of diamond nanoparticles were more obvious at low input powers. When the inclination angle was 0°, nanoparticles generally presented an enhancement effect. In this study, there was an optimal nanoparticle concentration of 0.50 wt%, which enhanced the heat transfer performance under all the investigated conditions. When the nanoparticle concentration was 0.75 % and the inclination angle was 30°, the maximum enhancement rate of 15.41 % was reached at the input power was 2250 W.

Composition Analysis of the Solid Electrolyte Interface of NaK Anodes in Na-Ion Batteries.

Sodium-ion batteries have emerged as a promising alternative to Li-ion batteries due to the abundance of sodium. However, anodes in Na-ion batteries face challenges such as dendrite formation and an unstable solid electrolyte interface layer. To address these challenges, NaK liquid metal alloy anodes have been proposed as an alternative because they do not form dendrites. In our study, we demonstrate that the NaK alloy anode interacts with the commonly used ethylene carbonate and dimethyl carbonate electrolyte, leading to a continuously growing unstable SEI layer, evidenced by cycling failures under 100 cycles and an increasing charge transfer resistance in electrochemical impedance spectroscopy studies. In situ surface-enhanced Raman spectroscopy and X-ray photoelectron spectroscopy reveal that over the course of cycling the surface of the NaK anode becomes increasingly sodium-rich. After 30 cycles, XPS analysis detects only trace amounts of potassium on the NaK anode surface. When the electrolyte is analyzed postcycling using inductively coupled plasma optical emission spectroscopy, there is a noticeable increase in potassium levels, suggesting that potassium metal dissolves into the electrolyte. The introduction of a 10 wt % fluoroethylene carbonate additive can mitigate this problem to some extent, enabling an enhanced cycling performance of up to 800 cycles at 1C. Nevertheless, the dissolution of K metal is still evident in the XPS results, albeit to a lesser degree. These discoveries provide valuable insights for designing a more robust SEI layer for the NaK anode.

NaK alloy as a versatile reagent for template-free synthesis of porous metal- and metalloid-based nanostructures.

Using the strong reduction potential of the liquid NaK-78 alloy, we present a new versatile template-free approach to the synthesis of porous metal- and metalloid-based nanomaterials. With this novel approach, NaK can be simultaneously used as an agent for reduction, structure directing, and pore formation without the use of additional reagents.

Antiferromagnetism and insulator-metal transition of alkali metal-loaded sodalite.

Alkali metal clusters with a single unpaired s-electron can be arranged three-dimensionally in a sodalite crystal by loading the guest alkali atoms. Na, K, and K-Rb alloy clusters are known to be Mott insulators and to exhibit antiferromagnetic ordering. The Néel temperature increases from about 50 K to about 100 K in this order. In this study, Li-Na alloy, Na-K alloy, and pure Rb samples were newly prepared and their magnetic, electrical conductivity, and optical properties were investigated, including those of previous samples. The Na-K alloy samples showed antiferromagnetic properties, which were intermediate between those of the Na and K samples. However, the Rb sample showed a non-magnetic metallic state. The shallower ionization potential in Rb is thought to cause an insulator-metal transition (Mott transition) due to weaker on-site Coulomb repulsion between electrons and larger electron transfer energy between neighboring clusters. On the other hand, the Li-Na alloy sample showed a non-magnetic insulating state. It is thought that the two electrons form a spin-singlet pair due to the strong electron-lattice interaction. In terms of electron correlations and polaron effects, the full picture of the element species dependence of the alkali metal-loaded sodalite is reviewed.

A Novel Sodium–Potassium Anode Supported by Fluorinated Aluminum Foam

Sodium–potassium (NaK) liquid alloy is a promising candidate for use as an anode material in sodium batteries because of its fluidity, which effectively suppresses the growth of sodium or potassium dendrites. However, the poor wettability of NaK alloy on conventional metal substrates is unfavorable for cell fabrication due to its strong surface tension. In this paper, low-density and low-cost fluorinated aluminum foam is used as a substrate support material for NaK liquid alloy. By combining low-surface-tension NaKC with fluorinated aluminum foam, we obtain a uniformly distributed and structurally stable electrode material. The composite electrode has a cycling stability of more than 3000 h in a symmetrical cell. Furthermore, when coupled with a sulfurized polyacrylonitrile cathode in carbonate electrolyte, it maintains excellent stability even after 800 cycles, with 72% of capacity retention.

MXene-Derived Na+ -Pillared Vanadate Cathodes for Dendrite-Free Potassium Metal Batteries.

Cation-intercalated vanadates, which have considerable promise as the cathode for high-performance potassium metal batteries (PMBs), suffer from structural collapse upon K+ insertion and desertion. Exotic cations in the vanadate cathode may ease the collapse, yet their effect on the intrinsic cation remains speculative. Herein, a stable and dendrite-free PMB, composed of a Na+ and K+ co-intercalated vanadate (NKVO) cathode and a liquid NaK alloy anode, is presented. A series of NKVO with tuneable Na/K ratios are facilely prepared using MXene precursors, in which Na+ is testified to be immobilized upon cycling, functioning as a structural pillar. Due to stronger ionic bonding and lower Fermi level of Na+ compared to K+ , moderate Na+ intercalation could reduce K+ binding to the solvation sheath and favor K+ diffusion kinetics. As a result, the MXene-derived Na+ -pillared NKVO exhibits markedly improved specific capacities, rate performance, and cycle stability than the Na+ -free counterpart. Moreover, thermally-treated carbon paper, which imitates the microscopic structure of Chinese Xuan paper, allows high surface tension liquid NaK alloy to adhere readily, enabling dendrite-free metal anodes. By clarifying the role of foreign intercalating cations, this study may lead to a more rational design of stable and high-performance electrode materials.

Suitability of NaK alloy for the sodium or potassium metal batteries: Competition between cathode size effect and ion reaction priority effect

Sodium-potassium (NaK) alloys in a specific ratio can maintain their liquid state at room temperature, rendering them dendrite-free anodes for alkali metal batteries. Nevertheless, a controversy has arisen regarding the suitability of either sodium metal batteries (NaMBs) or potassium metal batteries (KMBs) due to the presence of dual active ions in NaK. Based on the current research, there is a contradiction regarding the feasibility of making NaK anodes suitable for NaMBs through the cathode size effect (e.g., Na3V2(PO4)3), and for KMBs due to the ion reaction priority effect of K+. In this study, Na3V2(PO4)3 (NVP) cathodes are assembled into batteries with various anodes (Na, Na-rich-NaK, NaK, K-rich-NaK, and K), to investigate the dominant role of active ions in the competing process between the cathode size effect and the ion reaction priority effect. Tests and characterization results demonstrate that the ion preference reaction spontaneously displaces Na+ with K+ in the presence of K, although the cathode size effect may restrict K+ embedding and significantly reduce battery capacity. In summary, the ion reaction priority effect dictates the reactions that render NaK anodes exclusively suitable for KMBs without altering the reaction priority of K+.

Cobalt/Nitrogen Co-Doped Carbon Materials Enhance the Reaction Rate of Sodium-Potassium Alloy Electrodes.

Sodium-potassium (NaK) alloy electrodes are ideal for next-generation dendrite-free alkali metal electrodes due to their dendrite-free nature. However, issues such as slow diffusion kinetics due to the large K+ radius and the loss of active potassium during the reaction severely limit its application. Here a novel cobalt/nitrogen-doped carbon material is designed and it is applied to the construction of a NaK alloy electrode. The experimental and theoretical results indicate that the confining effect of the nitrogen-doped graphitic carbon layer can protect the cobalt nanoparticles from corrosion leaching, while the presence of Co─Nx bonds and cobalt nanoparticles provides more active sites for the reaction, realizing the synergistic effect of adsorption-catalytic modulation, lowering the K+ diffusion energy barrier and promoting charge transfer and ion diffusion. The application of this electrode to a symmetrical battery can achieve more than 1800 stable cycles under a current density of 0.4mA cm-2 and a charge/discharge specific capacity of 122.64 mAh g-1 under a current of 0.5C in a full battery. This finding provides a new idea to realize a fast, stable, and efficient application of NaK alloy electrodes.

Single-Atom Metallophilic Sites for Liquid NaK Alloy Confinement toward Stable Alkali-Metal Anodes.

Room temperature liquid NaK alloy is a promising candidate for high performance metal batteries, due to its dendrite-free property and high energy density. However, its practical application is hindered by the high surface tension of liquid NaK, which causes difficulties in maintaining a stable contact with a current collector. Here, the authors demonstrate the extraordinary stable confinement of NaK alloy at room temperature by constructing a super-wetting substrate, which is based on highly dispersed cobalt-single-atom carbon nanoarrays. The developed liquid anode electrode prevented successfully the leakage of NaK alloy even in harsh stress (>5MPa) or sharp shock conditions. The symmetric cells achieved ultra-long reversible plating/stripping cycling life in both Na-ion (>1010hrs) and K-ion electrolytes (>4000hrs) at 10mA cm-2 /10mAh cm-2 . Upon fitting with Na3 V2 (PO4 )3 , the NaK assembled full battery provided high energy density (332.6kWh kg-1 ) and power density (11.05kW kg-1 ) with excellent stability after >21600cycles, which is the best value reported so far. The prepared pouch cell was able to drive a four-axis aircraft, demonstrating a great prospect in practical application. This work offers a new approach in the preparation of advanced dendrite-free liquid metal anodes with promising applications in electrochemical energy storage.

Free Energy of Mixing, Heat of Mixing and Viscosity of Na-K Liquid Alloy at Different Temperatures

The Free energy of mixing, Heat of mixing and Viscosity of Na-K alloy are investigated theoretically at different temperatures using the Regular solution model. The interaction energy is temperature dependent. The theoretical values of interchange energy at different temperatures are obtained by best fit parameter approximation with the help of experimental values at 384K. The properties have been studied with the help of computed theoretical interchange energy at different temperatures using interchange energy and temperature relation. A comparison of theoretical and experimental values at 384K shows that they are in good agreement and using this basis we have computed the values at different temperatures.

Semi‐Solid CNT@NaK Anode for Potassium Metal Battery

AbstractThe CK bond plays a significant role in stabilizing the Na‐K (NaK) alloy electrodes due to the enhancive interfacial affinity. In this study, a method for constructing semi‐solid K metal electrodes with rich CK bonds by in situ replacement of N‐doped carbon nanotubes (CNT) and liquid NaK alloy is proposed. Based on the in situ infrared thermal imaging technique combined with heat calculation, X‐ray photoelectron spectroscopy elemental analysis, and reaction thermodynamic calculation, graphite‐N, which is widely distributed on the wall of CNT, offers plenty of replacement sites for forming CK bonds. Due to the rich bonds, the amount of CNT sharply reduces in dendrite‐free semi‐solid CNT@NaK electrodes and the activity of NaK alloy raises to ≈90%. This discovery provides a new idea for establishing dendrite‐free anodes for K metal batteries.

Amorphous hollow carbon film as a flexible host for liquid Na-K alloy anode

Compared with solid alkali metal anodes (Li, Na, K), liquid metal anodes (LMAs) could enable high-energy batteries due to their unique advantages, such as self-healing property and no dendrites. Among LMAs, liquid Na-K alloy anode has become a hotspot due to its high theoretical capacity, low redox potential and formation at room temperature (RT). However, it is challenging to utilize liquid Na-K alloy directly and independently as an electrode; and the high surface tension makes it more difficult to immerse into porous current collectors at RT. Herein, an amorphous hollow carbon film (AHCF) consisting of hollow spheres with significant surface defects has been designed to quickly infiltrate Na-K liquid alloy into the hollow carbon film at RT, forming a composite electrode (Na-K@AHCF). The symmetric cell with Na-K@AHCF could exhibit a cycle lifespan up to 400 h at 0.1 mA/cm2 and achieve stable stripping/deposition even at 5 mA/cm2. When matching with cathode material of sulfurized polyacrylonitrile (SPAN), the obtained K-S full cell exhibits good cycle stability and rate performance.

A Decade of Germananes: Four Approaches to Their Functionalization.

Since the first synthesis of germanane (GeH) reported in 2013, two-dimensional germanium-based materials have been intensively studied. Over the past decade, several methodologies for the functionalization of germanane have been introduced. The first approach utilized exfoliation of Zintl phase CaGe2 with alkyl halides. Liu's solvothermal method was used for the synthesis of methyl germanane. Another methodology utilized Ge-H activation with sodium naphthalenide and its subsequent alkylation. All of these methods provide functionalized germananes; thus, a comparison of these methods is needed. In this paper, such a comparison of current synthetic approaches towards alkyl germananes is reported, and additionally, a new method for Ge-H activation utilizing a NaK equimolar alloy is presented as a fourth approach. For this purpose, eight alkyl reagents were chosen representing reactive benzyl bromides as well as linear esters and nitriles because they contain easily trackable functional groups. The materials were characterized using Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis, and the data were compared. The comparison of all methods revealed not only some drawbacks for each method but also their advantages. The method utilizing sodium naphthalenide provided the lowest degree of surface coverage, whereas the solvothermal method seemed to provide materials with the highest degree of functionalization; unfortunately, the functionalization was also accompanied by a high degree of surface oxidation, i.e., (Ge-OH/Ge═O) formation. The highest degree of surface coverage accompanied by the lowest degree of surface oxidation was achieved employing Goldberger's phase transfer direct exfoliation of CaGe2 as well as Ge-H activation using the NaK alloy with subsequent alkylation.

Sodium-Potassium Alloy Heat Pipe under Geyser Boiling Experimental Study: Heat Transfer Analysis

In the geyser boiling mode, the working fluid state is divided into a boiling process and a quiet process, and the sodium-potassium (Na-K) alloy heat pipe can discontinuously transfer heat at each boiling. The overheating of the liquid working fluid at the bottom causes short-term boiling and forms slug bubble, the strong condensing ability quickly conducts heat from the evaporator section. And geyser boiling can occur before the working fluid forms continuous flow, so it transfers more heat at lower temperatures than natural convection cooling. In this study, the heat transfer process of a Na-K alloy heat pipe with forced convection cooling under different heating power was experimental studied. The geyser boiling mode can make the Na-K alloy heat pipe work below 650 °C and reduce the start-up time. In the process of geyser boiling, the heat transfer quantity was increased by the boiling frequency and the amount of vapor produced in a single boiling. The boiling temperature had no obvious change with the increased of heating power, and the condenser section temperature increased with the heating power.

Liquid Alloying Na-K for Sodium Metal Anodes.

The prospects of sodium (Na) metal batteries have been fatally plagued by interfacial Na dendrites, mainly affected by preferred nucleation on the metal anode and the steep gradient of Na ions in the electrolyte, leading to limited Coulombic efficiency and short lifespans. Herein, an electrochemically inert potassium-based Na-K alloy demonstrates a liquid alloying diffusion mechanism that enables dendrite-free Na anodes. The extremely small Na fluctuation and flexible Na-K bonds in the liquid alloy phase bring isotropic nucleation of Na upon electroplating/stripping, which is directly observed by in situ optical imaging. Spontaneously, serving as (de)sodiation buffer with faster electron/mass transportation, the liquid inertia also provides attenuated concentration distribution of Na. Significantly, a record capacity retention of approximately 100% is rendered when coupled with Na3V2(PO4)3 cathodes (ca. 2 mg cm-2) over 500 cycles at 10C, advancing the possibility of using liquid alloy for stable metal anodes beyond Na storage systems.

Expounding the Initial Alloying Behavior of Na-K Liquid Alloy Electrodes.

Primary electrodeposition is an accepted strategy to elucidate the nucleation and growth kinetics of metal electrodes. Nevertheless, when confronted with the phase transition process caused by bi-active metals such as NaK liquid alloys, the research process becomes complex and elusive. Herein, we have reduced the intricate issues to relatively simple initial alloying behaviors. Two exchange diffusion mechanisms of the Na atom embedded in K crystals and K atom embedded in Na crystals are investigated by first-principles density functional theory (DFT) calculation and mechanical simulation. As a result, the process of embedding the Na atom in K crystals shows a better thermodynamic stability and lower activation barrier and structural stress than those of the other. The abovementioned conclusions are further proved by stepwise Na and K electrodeposition experiments, and the prepared NaK alloy electrode displays excellent electrochemical performance. Our findings correlate the original alloying mechanism model specification with electrodeposition experimental verification and provide strategies to achieve controllable NaK electrode construction.

An Interphase-enhanced Liquid Na-K Anode for Dendrite-free Alkali Metal Batteries Enabled by SiCl4 Electrolyte Additive

Thanks to the high theoretical capacity and low redox potential, alkali metals (Li, Na, K) show great superiority as anodes for high-energy-density rechargeable batteries. However, practical application of alkali metal battery (AMB) is limited due to its formidable dendrite growth. Liquid Na-K alloy is a promising choice to fulfill long-term cyclability of AMB due to its liquid characteristics. Nevertheless, the instability of electrolyte/liquid Na-K interface is still a challenge for the dendrite-free deposition/stripping. In this work, a stable organic/inorganic hybrid interface beneficial to improving the charge-transfer kinetics is formed on the liquid alloy with the assistance of SiCl4 electrolyte additive. The symmetric cell using interface-modified Na-K alloy electrode maintains an overpotential below 200 mV over 2000 h at 1mA cm−2 with 1mAh cm−2 for each cycle. The Na3V2(PO4)3 cathode coupled with interface-modified Na-K alloy shows a capacity of 91.5 mAh g−1 at 30 C, and remains 103.6 mAhg−1 after 1000 cycles at 2 C with nearly no capacity loss per-cycle. This work presents a facile approach for developing dendrite-free liquid alloy anodes and possesses potential application in liquid metal (LM)-related battery system with high energy density.