Liquid Na-K Alloys Research Articles

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.

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.

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.

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.

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.

Defect engineering in oxides by liquid Na-K alloy for oxygen evolution reaction

Defect engineering in oxides can effectively modulate their electric states and offers more active sites for electrocatalytic reactions. However, creating oxygen defects often require high processing temperatures or long-time consumption. Here, we exploited a simple and universal method to create defects in several metal oxides such as Co3O4, ZnO, SnO2, and CeO2 using highly active liquid sodium-potassium (Na-K) alloy as the reducing agent at room temperature. To verify the available potential of this synthetic strategy, we choose defective Co3O4 as an example and investigate its oxygen evolution reaction (OER) performance. The oxygen vacancies have a significant promotion effect on the OER activity and its starting potential is 1.54 V with a Tafel slope of 47.9 mV/dec, which is much lower than pristine Co3O4 nanoparticles. Our method of creating defects in oxides may be used for many other related applications.

A stable carbon host engineering surface defects for room-temperature liquid Na[sbnd]K anode

Alkali metal anode (Li, Na, K) are promising anodes for high-energy-density batteries because of high theoretical capacities and low redox potentials. While dendrite growth still restricts practical applications of alkali metal batteries. In contrast, liquid metal anode materials can avoid the problem of dendrite formation. But the poor wettability of liquid metal on most of hosts makes it difficult to assemble cell for the strong surface tension. Herein, we report a treated carbon cloth (denoted TCC) that can fast absorbs liquid NaK alloy at room temperature owing to the enhancing wettability by engineered surface defects of oxygenic groups and nanoscale cracks. When assembled in a symmetric cell with NaK alloy, it renders long cycle life over 1800 h and low voltage polarization below 300 mV compared with Na symmetric cell. When cycling with Na3V2(PO4)3 cathode as full cell, it also exhibits excellent capacity and capacity retention of 87.58% after 1000 cycles at 10 C.

New Insights into the Electrochemistry Superiority of Liquid Na-K Alloy in Metal Batteries.

The significant issues with alkali metal batteries arise from their poor electrochemical properties and safety problems, limiting their applications. Herein, TiO2 nanoparticles embedded into N-doped porous carbon truncated ocatahedra (TiO2 ⊂NPCTO) are engineered as a cathode material with different metal anodes, including solid Na or K and liquid Na-K alloy. Electrochemical performance and kinetics are systematically analyzed, with the aim to determine detailed electrochemistry. By using a galvanostatic intermittent titration technique, TiO2 ⊂NPCTO/NaK shows faster diffusion of metal ions in insertion and extraction processes than that of Na-ions and K-ions in solid Na and K. The lower reaction resistance of liquid Na-K alloy electrode is also examined. The higher b-value of TiO2 ⊂NPCTO/NaK confirms that the reaction kinetics are promoted by the surface-induced capacitive behavior, favorable for high rate performance. This superiority highly pertains to the distinct liquid-liquid junction between the electrolyte and electrode, and the prohibition of metal dendrite growth, substantiated by symmetric cell testing, which provides a robust and homogeneous interface more stable than the traditional solid-liquid one. Hence, the liquid Na-K alloy-based battery exhibits to better cyclablity with higher capacity, rate capability, and initial coulombic efficiency than solid Na and K batteries.

Exploring Self-Healing Liquid Na-K Alloy for Dendrite-Free Electrochemical Energy Storage.

The development of high-performance dendrite-free liquid-metal anodes at room temperature is of great importance for the advancement of alkali metal batteries. Herein an intriguing self-healing liquid dendrite-free Na-K alloy, fabricated by a facile room-temperature alloying process, aiming for application in potassium-ion batteries is reported. Through extensive investigation, its self-healing characteristics are rooted upon a thin solid K2 O layer (KOL) coated on the liquid Na-K alloy. The KOL not only acts as a protective layer to prevent the Na-K alloy from making contact with the electrolyte, but also greatly improves the wetting capability and adhesion between the liquid alloy and the carbon matrix (e.g., carbon fiber cloth (CFC)) to form a stable interface. Consequently, the as-prepared CFC/KOL@Na-K alloy anode exhibits prominent electrochemical performance with smaller hysteresis (less than 0.3 V beyond 140 cycles at 0.4 mA cm-2 ), better capacity retention, and higher Coulombic efficiency than the CFC/bare Na-K alloy counterpart. When coupled with a potassium Prussian blue (PPB) cathode, the full cell manifests higher capability retention and improved cycling stability. This research deepens the understanding of self-healing Na-K alloys and opens a new way to achieve high-performance dendrite-free alkali metal anodes for application in rechargeable batteries.

Potassium Superoxide: A Unique Alternative for Metal-Air Batteries.

Lithium-oxygen (Li-O2) batteries have been envisaged and pursued as the long-term successor to Li-ion batteries, due to the highest theoretical energy density among all known battery chemistries. However, their practical application is hindered by low energy efficiency, sluggish kinetics, and a reliance on catalysts for the oxygen reduction and evolution reactions (ORR/OER). In a superoxide battery, oxygen is also used as the cathodic active medium but is reduced only to superoxide (O2•-), the anion formed by adding an electron to a diatomic oxygen molecule. Therefore, O2/O2•- is a unique single-electron ORR/OER process. Since the introduction of K-O2 batteries by our group in 2013, superoxide batteries based on potassium superoxide (KO2) have attracted increasing interest as promising energy storage devices due to their significantly lower overpotentials and costs. We have selected potassium for building the superoxide battery because it is the lightest alkali metal cation to form the thermodynamically stable superoxide (KO2) product. This allows the battery to operate through the proposed facile one-electron redox process of O2/KO2. This strategy provides an elegant solution to the long-lasting kinetic challenge of ORR/OER in metal-oxygen batteries without using any electrocatalysts. Over the past five years, we have been focused on understanding the electrolyte chemistry, especially at the electrode/electrolyte interphase, and the electrolyte's stability in the presence of potassium metal and superoxide. In this Account, we examine our advances and understanding of the chemistry in superoxide batteries, with an emphasis on our systematic investigation of K-O2 batteries. We first introduce the K metal anode electrochemistry and its corrosion induced by electrolyte decomposition and oxygen crossover. Tuning the electrolyte composition to form a stable solid electrolyte interphase (SEI) is demonstrated to alleviate electrolyte decomposition and O2 cross-talk. We also analyze the nucleation and growth of KO2 in the oxygen electrode, as well its long-term stability. The electrochemical growth of KO2 on the cathode is correlated with the rate performance and capacity. Increasing the surface area and reducing the O2 diffusion pathway are identified as critical strategies to improve the rate performance and capacity. Li-O2 and Na-O2 batteries are further compared with the K-O2 chemistry regarding their pros and cons. Because only KO2 is thermodynamically stable at room temperature, K-O2 batteries offer reversible cathode reactions over the long-term while the counterparts undergo disproportionation. The parasitic reactions due to the reactivity of superoxide are discussed. With the trace side products quantified, the overall superoxide electrochemistry is highly reversible with an extended shelf life. Lastly, potential anode substitutes for K-O2 batteries are reviewed, including the K3Sb alloy and liquid Na-K alloy. We conclude with perspectives on the future development of the K metal anode interface, as well as the electrolyte and cathode materials to enable improved reversibility and maximized power capability. We hope this Account promotes further endeavors into the development of the K-O2 chemistry and related material technologies for superoxide battery research.

Cathode Dependence of Liquid-Alloy Na-K Anodes.

Alkali ions can be plated dendrite-free into a liquid alkali-metal anode. Commercialized Na-S battery technology operates above 300 °C. A low-cost Na-K alloy is liquid at 25 °C from 9.2 to 58.2 wt% of sodium; sodium and/or potassium can be plated dendrite-free in the liquid range at room temperature. The co-existence of two alkali metals in an anode raises a question: whether the liquid Na-K alloy acts as a Na or a K anode. Here we show the alkali-metal that is stripped from the liquid Na-K anode is dependent on the preference of the cathode host. It acts as the anode of a sodium rechargeable cell if the cathode host structure selectively accepts only Na+ ions; as the anode of a potassium rechargeable cell if the cathode accepts K+ ions in preference to Na+ ions. This dual-anode behavior means the liquid Na-K alkali-alloy can be applied as a dendrite-free anode in Na-metal batteries as well as K-metal batteries.

Dendrite-Free Potassium-Oxygen Battery Based on a Liquid Alloy Anode.

The safety issue caused by the dendrite growth is not only a key research problem in lithium-ion batteries but also a critical concern in alkali metal (i.e., Li, Na, and K)-oxygen batteries where a solid metal is usually used as the anode. Herein, we demonstrate the first dendrite-free K-O2 battery at ambient temperature based on a liquid Na-K alloy anode. The unique liquid-liquid connection between the liquid alloy and the electrolyte in our alloy anode-based battery provides a homogeneous and robust anode-electrolyte interface. Meanwhile, we manage to show that the Na-K alloy is only compatible in K-O2 batteries but not in Na-O2 batteries, which is mainly attributed to the stronger reducibility of potassium and relatively more favorable thermodynamic formation of KO2 over NaO2 during the discharge process. It is observed that our K-O2 battery based on a liquid alloy anode shows a long cycle life (over 620 h) and a low discharge-charge overpotential (about 0.05 V at initial cycles). Moreover, the mechanism investigation into the K-O2 cell degradation shows that the O2 crossover effect and the ether-electrolyte instability are the critical problems for K-O2 batteries. In a word, this study provides a new route to solve the problems caused by the dendrite growth in alkali metal-oxygen batteries.

ChemInform Abstract: Facile and Efficient Exfoliation of Inorganic Layered Materials Using Liquid Alkali Metal Alloys.

AbstractSingle‐ and few‐layer MoS2, WS2, and BN nanosheets are prepared through intercalation and exfoliation using liquid NaK or NaLi alkali metal alloys in DME at room temperature for 24—48 h.

Viscosity of liquid Na-K alloy

The present work reports a theoretical assessment of the composition dependence of mixing properties of liquid Na-K alloy at 384K at fixed pressure, most likely, at the atmospheric pressure. In this work we have estimated the interaction energy in the alloy at the mentioned temperature on the basis of quasi-chemical approximation for regular alloy and employed it to work out basic thermodynamic properties of mixing such as free energy of mixing, entropy of mixing and enthalpy of mixing as function of composition of the alloy. To understand the dynamic behaviour in the liquid Na-K alloy, we have carried out theoretical investigation of viscosity under consideration of Moelwyn-Hughes equation by using data for enthalpy of mixing obtained from quasi-chemical approximation. Due to scanty of experimental data on viscosity of Na-K alloy at 384K, we have performed computation of viscosity also from Kaptay equation for comparison. The calculations have shown that there is a good match between theoretically computed thermodynamic functions and the available corresponding experimental data. The sets of viscosity values obtained against composition from the two equations show deviations from the ideality and themselves are reasonably comparable to each other.DOI: http://dx.doi.org/10.3126/bibechana.v12i0.11797BIBECHANA 12 (2015) 128-137

Excess entropy of mixing for binary square-well fluid in the mean spherical approximation: Application to liquid alkali-metal alloys

The expression for the entropy of binary square-well (SW) mixture is derived in the framework of the semi-analytical approach [J. Non-Cryst. Solids 353 (2007) 1798] for the mean spherical approximation. This expression is applied to calculate the concentration dependencies of the excess entropy of mixing for liquid Na-K and Na-Cs alloys at T=373K. It is shown that the SW model allows to achieve a better agreement with experiment than the hard-sphere model with the same values of hard-core parameters.

Structure and Inter-Diffusion Coefficients of Liquid Na x K1−x Alloys

Structure and atomic transport properties of liquid Na-K alloys are reported. Inter-diffusion coefficients of liquid NaxK1−x, alloys are calculated using scaling law proposed by Samanta et al. following Dzugutov which express the possible relationship between the excess entropy and diffusion coefficient. The interatomic interactions are described from the individual version of the electron-ion potential proposed by Fiolhais et al. The partial pair distribution functions and structure factors are calculated from the solution of Ornstein-Zernike integral equation with Rogers-Young closure. The evaluation with the composition of static structure and inter-diffusion properties are discussed.

Injectable liquid alkali alloy based-tumor thermal ablation therapy

The alkali metal was recently found to be a very useful agent for inducing minimally invasive tumor hyperthermia therapy. However, the solid-like metal makes it somewhat inconvenient to perform the surgery. Here, to overcome this drawback, the NaK alloy in liquid state at room temperature was proposed as a highly efficient thermal ablative agent for tumor treatment. For illustration purposes, the functionalized liquid NaK alloy at a mass ratio 1:1 was obtained and an amount of 0.35ml was injected into in vitro pork. The sizes of the damage region and temperature response were measured. It was found that significant temperature increase by a magnitude of > 80°C can easily be obtained. This produced a large coagulation and necrotic area within selected areas for in vitro tests and the necrotic region volume is three times that of the NaK injection quantity. Furthermore, for the in vivo experiment, breast EMT6 tumor in mouse was subjected to treatment by NaK alloy. Tumor was harvested after the experiment to assess its viability. Histological section showed complete necrosis at the target site. These conceptual results demonstrate that using injectable liquid alkali alloy to ablate tumor is rather promising. This study also raised interesting issues waiting for clarification in future technical and animal studies aiming to assess efficacy, side effects and safety of the new therapy.

Molecular dynamics simulation study of the interdiffusion properties of liquid Na-K alloys

We use classical molecular dynamics simulations to study the inter-diffusion properties of liquid Na-K alloys versus concentration for several compositions ranging from 10 to 90 atomic percent of Na. The self- and inter-diffusion coefficients are deduced from the successive configurations. The results are compared with the predictions of Darken's law and the validity of this approximation is discussed.

Structures of Alkali Metals in Silica Gel Nanopores: New Materials for Chemical Reductions and Hydrogen Production

Alkali metals and their alloys can be protected from spontaneous reaction with dry air by intercalation (with subsequent heating) into the pores of silica gel (SG) at loadings up to 40 wt %. The resulting loose, black powders are convenient materials for chemical reduction of organic compounds and the production of clean hydrogen. The problem addressed in this paper is the nature of the reducing species present in these amorphous materials. The atomic pair distribution function (PDF), which considers both Bragg and diffuse scattering components, was used to examine their structures. Liquid Na-K alloys added to silica gel at room temperature (stage 0) or heated to 150 degrees C (stage I) as well as stage I Na-SG, retain the overall pattern of pure silica gel. Broad oscillations in the PDF show that added alkali metals remain in the pores as nanoscale metal clusters. 23Na MAS NMR studies confirm the presence of Na(0) and demonstrate that Na+ ions are formed as well. The relative amounts of Na(0) and Na(+) depend on both the overall metal loading and the average pore size. The results suggest that ionization occurs near or in the SiO2 walls, with neutral metal present in the larger cavities. The fate of the electrons released by ionization is uncertain, but they may add to the silica gel lattice, or form an "electride-like plasma" near the silica gel walls. A remaining mystery is why the stage I material does not show a melting endotherm of the encapsulated metal and does not react with dry oxygen. Na-SG when heated to 400 degrees C (stage II) yields a dual-phase reaction product that consists of Na(4)Si(4) and Na(2)SiO(3).