Liquid Metal Batteries Research Articles

Risk assessment for Na-Zn liquid metal batteries

Background Na-Zn liquid metal batteries, which operate at 600 °C, have recently been proposed as inexpensive stationary energy storage devices. As with any other electrochemical cell, their fabrication and operation involves certain risks, which need to be well understood in order to be minimised. Methods A risk assessment according to ISO 12100 is performed at the cell level for operating Na-Zn cells in the laboratory environment. Hazard identification and risk evaluation are systematically addressed, including a thorough literature review, theoretical calculations and selected experiments. Results Cell overpressure is found to be one of the main risks – and might be caused either by mistakes in battery production (humidity) or operation (over-charge/discharge). In terms of cell housing, the weakest component is clearly the feedthrough. Its failure might lead to the release of hazardous aerosols to the environment. In this context, the candidate electrolyte components LiCl and BaCl2 are especially dangerous, and should therefore be reduced or avoided if possible. Conclusions Overall, Na-Zn cells are expected to reach a very high safety level, similar to state-of-the-art ZEBRA technology, as they are not prone to thermal runaway. However, considering the still low TRL level and open questions concerning the durability of certain parts of their housing, the batteries should preferably be operated under a fume hood.

Liquid Bi-Sb-Sn Electrodes with Synergistic Stabilization Mechanism for Long-Lifespan Sodium-Based Liquid Metal Batteries.

Sodium-based liquid metal batteries are well suited for stationary energy storage due to their long life, intrinsic safety, and ease of scale-up. However, the irreversible alloying reaction between the positive current collector (PCC) and the cathodes at high temperatures leads to severe capacity degradation of the battery, severely limiting its scale-up application. In this work, a Bi-Sb-Sn alloy cathode based on a synergistic stabilization mechanism was designed for the first time. Due to the density difference of Bi, Sb, and Sn and the compatibility difference of Bi and Sn with the PCC, a part of Bi and Sn is spontaneously distributed in the region close to the PCC. The protection of Sb is realized by blocking the contact of Sb with the PCC as well as removing the PCC material dissolved in the cathode to prevent the loss of active material. Based on such protection, the Na||Bi36Sb24Sn40 cell maintained 99% Coulombic efficiency for 450 cycles at a rate of 0.75 C, with a capacity retention of 99.56% and a capacity decay rate of 0.001% per cycle. In addition, the interaction of Bi, Sb, and Sn during discharge also promotes capacity release and energy efficiency. At 0.3 C, the Na||Bi36Sb24Sn40 cell achieved 89% capacity utilization and 82% energy efficiency. These results provide an idea for the design of other batteries based on liquid metal electrodes.

Tough and temperature-resistant mask-based separator realized by K-β”-Al2O3 protective layer for stabilizing NaK liquid metal batteries

The development of low-cost, high-temperature-resistant separators is crucial for optimizing the reaction interface and improving the safety performance of liquid NaK metal batteries. This paper presents a novel separator with a solid electrolyte layer, Mask@40 %β-Al2O3, primarily composed of K-β”-Al2O3 known for its exceptional ionic conductivity and high-temperature resistance. Specifically designed for room-temperature liquid NaK metal batteries, the Mask@40 %β-Al2O3 separator effectively transforms the unstable liquid–liquid interface between liquid NaK metal electrodes and traditional organic electrolytes into a stable solid–liquid interface. This transformation prevents short-circuiting caused by area collapse under high-temperature conditions. Moreover, the Bare Mask component can adsorb a conventional organic electrolyte to construct a stable solid–liquid interface on the cathode side, thereby reducing the typically observed high interfacial resistance associated with conventional solid electrolytes. The cost-effective nature and simple preparation process of the Mask@40 %β-Al2O3 separator address cost concerns while presenting an encouraging new direction for applying room-temperature liquid NaK metal batteries in extreme conditions.

Boosting electrochemical performance by regulating rigid-flexible microphase separation of multiblock copolymers

The growth of lithium dendrites and associated safety issues hinder the further development of liquid lithium metal batteries. A novel rigid-flexible multiblock copolymer PBC-mb-PBS was successfully synthesized based on molecular design. Incorporating crystalline rigid phase polybutylene succinate (PBS) into PBC-mb-PBS significantly enhances its tensile strength and low-temperature toughness through physical crosslinking. The flexible polydibutyl 2-(2-cyanoethyl)malonate (PBC) phase with side-chain cyano groups facilitate ionic conduction, forming a LiN-rich stabilized interfacial layer, thereby improving oxidative stability and high-voltage cathode compatibility. A series of multiblock copolymers with varying segment lengths were synthesized to balance mechanical properties and ionic conductivity by adjusting microphase separation. The quasi-solid-state polymer electrolyte (QSPE), derived from PBC-mb-PBS with the molar block ratio of 0.42, exhibits optimal electrochemical performance with ionic conductivity of 6.20 × 10−5 S cm−1 at 30 °C and 4.22 × 10−4 S cm−1 at 60 °C. Due to its bicontinuous microphase structure, it also shows excellent mechanical strength and promotes uniform lithium deposition at high current densities. Li//LiFePO4 and Li//LiNi0.83Co0.05Mn0.12O2 cells demonstrate high capacity retention, confirming the potential of this polymer electrolyte in high-voltage applications. This design strategy offers insights for developing quasi-solid-state lithium metal batteries with superior overall performance.

A Na-Li dual cation liquid metal battery with high electrode utilization and high cycling stability

Liquid metal batteries (LMBs) with Na anode exhibit the advantages of low-cost, high-safety, long-lifespan, and easy scale-up, making them promising for large-scale energy storage applications. However, the high solubility of Na and its sluggish kinetics in discharge products result in severe self-discharge and low cathode utilization, significantly hindering the development of Na-based LMBs. In this study, a stable Na-Li dual cation LMB with metal Sb cathode is designed based on the in-situ displacement reaction between Na and molten salts of lithium halides electrolyte. This design effectively suppresses the self-discharge of the battery, and the multistage discharge mechanism of the dual-cation system enhances cathode utilization. Moreover, the self-healing property of the Sb cathode protects the electrode against volume expansion and degradation, enhancing cycling performance. The Na-Li| LiCl-KCl-NaCl |Sb cell was constructed and operated at 485 °C. The battery retained 97.02 % of its capacity after 800 cycles at 0.6 C, with a decay rate of 0.00373 % per cycle. Furthermore, the battery achieves a cathode material utilization of 89 % at 0.6 C. These improvements effectively promote the development and application of LMBs.

Enhancing capacity utilization of Li|LiCl-KCl-CsCl|Bi (300 °C) liquid metal batteries through the application of external magnetic fields

Long-cycle-life, high-safety liquid metal batteries (LMBs) are considered competitive alternatives for large-scale energy storage applications. LMBs typically operate at high temperatures, and reducing their operating temperature has been a focus of research. In this study, we adopt a multi-cation molten salt LiCl-KCl-CsCl as the electrolyte for LMBs and achieve the stable operation of Li|LiCl-KCl-CsCl|Bi battery at 300 °C for the first time. However, the lower operating temperature significantly deteriorates the kinetic performance, resulting in a capacity utilization of only about 30 %. To address this issue, we apply a certain intensity of magnetic field to the battery using Helmholtz coils, which increases the capacity utilization to 100 %. This study combines the strategy of low-temperature LMBs with external magnetic field regulation, not only achieving a low operating temperature but also ensuring complete capacity release. Furthermore, this approach paves the way for the potential use of permanent magnets within battery modules to provide the magnetic field environment, without the risk of demagnetization.

Corrosion of stainless steel and molybdenum used as PCC in Na//Sb-Bi liquid metal batteries under cycling conditions

Material compatibility is a major challenge in the development of liquid metal batteries. In this study, the corrosion behavior of two candidate positive current collector materials, austenitic stainless steel and molybdenum, in Na//SbBi9 liquid metal batteries is investigated. In-situ corrosion in operating cells is compared with static corrosion in SbBi9 (corresponding to the fully charged state) and Na0.30Sb0.07Bi0.63 (representing the fully discharged state) at 450 °C. Stainless steel shows a much better corrosion resistance in Na-Sb-Bi alloy (corrosion depth below 1 μm after 750 h static exposure) than in SbBi9 (7–9 μm corrosion layer) thanks to the formation of a thin oxide layer. The corrosion under cycling conditions shows similar characteristics (formation of solid Fe-Cr-Ni-Sb compounds), though accelerated, as static exposure to SbBi9. Molybdenum exhibits a very good corrosion resistance both in static and in cycling conditions, showing merely a minor dissolution into the adjacent alloy.

Ternary intermediate phase mediated high energy density and kinetically accelerated Sb-Cd electrode for liquid metal batteries

Liquid metal battery (LMB) is an attractive technology that can address the big challenge of efficient utilization of renewable energies, ascribed to its potential ultralong service lifetime, ideal cost-efficiency, and high safety. However, breaking through the energy density bottleneck remains a persistent challenge, due to the limited lithiation voltage and specific capacity of cathodes. Herein, we design a novel dual-active Sb-Cd cathode, which shows a new ternary alloying discharge mechanism with reversible formation of ternary intermediate phase LiCdSb, accompanied by a high discharge voltage of 1.0 V. Coupled with the excellent Li storage capability of Cd, the Sb80Cd20 cathode attains an attractive capacity (> 500 mAh g-1), outperforming all reported LMB cathodes. More importantly, upon deep lithiation, the conversion reaction of ternary LiCdSb to Li3Sb mediates the regeneration and dispersion of Cd melt in the cathode, which constructs in situ fast electron and lithium diffusion networks for the subsequent discharge, significantly accelerating the electrode reaction kinetics. Thus, the Sb80Cd20 electrode achieves an exceptional combination of high energy density (398.4 Wh kg-1 at 200 mA cm-2) and superior rate capability (542.5 W kg-1 at 2400 mA cm-2). Additionally, the Li||Sb-Cd battery exhibits an outstanding tolerance and self-healing ability towards thermal runaway.

Optimizing the design of parallel busbars to suppress the Taylor Instability in large-scale liquid metal batteries

Abstract In this study, we utilize the open-source software OpenFOAM to numerically simulate the Tayler instability in liquid metal batteries. The simulation depicts the dynamic evolution of the electrolyte layer, fluid flow, and magnetic field distribution in the battery at different operating times. Special attention is paid to the influence of optimizing the parallel busbar configuration on Tayler instability and battery capacity. It is found that the additional magnetic field generated by the busbars can effectively alter the magnetic field distribution inside the battery, thereby significantly reducing the Lorentz force and suppressing the Tayler instability within the battery. Therefore, by optimizing the parallel busbar configuration, the stable operating time of the battery is prolonged, and the battery capacity is enhanced. This study provides theoretical support and practical guidance for the application of liquid metal batteries in large-scale grid-level energy storage systems and offers new insights into the development of energy storage technology.

Thermoelectricity at a gallium–mercury liquid metal interface

We present experimental evidence of a thermoelectric effect at the interface between two liquid metals. Using superimposed layers of mercury and gallium in a cylindrical vessel operating at room temperature, we provide a direct measurement of the electric current generated by the presence of a thermal gradient along a liquid–liquid interface. At the interface between two liquids, temperature gradients induced by thermal convection lead to a complex geometry of electric currents, ultimately generating current densities near boundaries that are significantly higher than those observed in conventional solid-state thermoelectricity. When a magnetic field is applied to the experiment, an azimuthal shear flow, exhibiting opposite circulation in each layer, is generated. Depending on the value of the magnetic field, two different flow regimes are identified, in good agreement with a model based on the spatial distribution of thermoelectric currents, which has no equivalent in solid systems. Finally, we discuss various applications of this effect, such as the efficiency of liquid metal batteries.

Stabilizing dual-cation liquid metal battery for large-scale energy storage: A comprehensive hybrid design approach

Liquid metal batteries (LMBs) hold immense promise for large-scale energy storage. However, normally LMBs are based on single type of cations (e.g., Ca2+, Li+, Na+), and as a result subject to inherent limitations associated with each type of single cation, such as the low energy density in Ca-based LMBs, the high energy cost in Li-based LMBs, and the short cycling lifespan in Na-based LMBs. Here we propose a dual-cation (Ca2+ and Li+) liquid metal battery, which allows access to, simultaneously, high energy density, prolonged cycling lifespan, reduced energy cost, and enhanced cycling stability. For this strategy to work, the main obstacle to overcome is the instability of the dual-cation system in electrochemical reactions. We have undertaken a hybrid design approach to resolve this issue, by integrating phase diagram design, first-principles calculations, and machine learning techniques. We discover that incorporating magnesium as an inert additive can effectively stabilize the cycling performance of dual-cation LMBs. Furthermore, our results reveal that specific ion ratios are required to release tailor dual-cation chemistry, to enable the designed multi-component alloy (Ca-Li-Mg in our case) electrode to function properly in LMBs. These findings are expected to have general implications for future developments of innovative LMBs with enhanced performance.

Corrosion of austenitic and ferritic steels, Kovar, and molybdenum in liquid Sb-Bi alloys with different compositions

Sb-Bi alloys may be applied as positive electrode in liquid metal batteries. In this work, the material compatibility of two steels (SS 316L and T91), Kovar 4J29, and molybdenum with SbBi9, Sb2Bi8, and Sb3Bi7 is investigated in static corrosion tests for 740 h at around 450 °C. Kovar shows severe dissolution attack (800 µm depth for SbBi9), which is mitigated at higher Sb contents due to the formation of Fe-Sb compounds. Both steels exhibit moderate corrosion depths (10–20 µm) for all Sb-Bi alloys thanks to the additional formation of stable Fe-Cr-Sb compounds. Good corrosion resistance is obtained for molybdenum.

Modelling and analysis of nuclear reactor system coupled with a liquid metal battery

AbstractTraditionally, nuclear power plants in the U.S. provide baseload power to the power grid because they have less flexibility for ramping their output power than natural gas peaking plants. However, achieving climate goals to reduce the consumption of fossil‐based natural gas places pressure on nuclear power plants and other power generators to ramp up their power output to balance grid generation with demand. This paper presents the modelling and performance analysis of a nuclear reactor system (NRS) coupled to a liquid‐metal battery (LMB) to improve its dynamic response and enable its black start capability. The NRS and LMB thermal behaviour are modelled in Dymola, while the electrical dynamics of the LMB and power grid are modelled in RTDS‐RSCAD. Both simulation platforms are coupled and share their thermal and electrical data using a Transmission Control Protocol/Internet Protocol (TCP/IP) communication protocol. The dynamic performance of the NRS‐LMB integration is tested on the IEEE 9 bus, which demonstrates its ability to respond and provide frequency and voltage regulation. The black start capability of the NRS‐LMB is also evaluated by simulating a grid outage and using the LMB to supply the auxiliary loads required to bring the NRS back online as soon as possible. The results show that coupling an NRS to an LMB improves the system dynamic performance and enables it to black start after being disconnected from the grid for several days.

CuGa2 transition phase anchored liquid GaSn achieves high-performance liquid metal battery cathode

Severe volume expansion will cause the gallium-based liquid metal cathode material to detach from the collector thus deteriorating performance. Here, we propose a strategy for preparing electrodes by anchoring the liquid metal with an intermetallic compound transition phase to avoid the problem of liquid metal shedding during cycling. The good reactive wetting between the CuGa2 transition phase and gallium can firmly anchor the liquid metal to prevent the electrode material exfoliating from the collector. On the other hand, the unique two-dimensional layer structure of CuGa2 can also limit the lithiation volume expansion of liquid gallium. Density functional theory (DFT) reveals the anchoring mechanism of the CuGa2 transition layer to liquid gallium and the reaction mechanism of Li with CuGa2 during the electrochemical reaction. As a result, O-GaSn/CF cathodes exhibit low resistance, fast ion diffusion, high reversible capacity, excellent rate performance and outstanding cycling stability.

Magnetic field based finite element method for magneto-static problems with discontinuous electric potential distributions

We introduce two finite element formulations to approximate magneto-static problems with discontinuous electric potential based respectively on the electrical scalar potential and the magnetic field. This work is motivated by our interest in Liquid Metal Batteries (LMBs), a promising technology for storing intermittent renewable sources of energy in large scale energy storage devices. LMBs consist of three liquid layers stably stratified and immiscible, with a light liquid metal on top (negative electrode), a molten salt in the middle (electrolyte) and a heavier liquid metal on bottom (positive electrode). Energy is stored in electrical potential differences that can be modeled as jumps at each electrode-electrolyte interface. This paper focuses on introducing new finite element methods for computing current and potential distributions, which account for internal voltage jumps in liquid metal batteries. Two different formulations that use as primary unknowns the electrical potential and magnetic field, respectively, are presented. We validate them using various manufactured test cases, and discuss their applications for simulating the current distribution during the discharge phase in a liquid metal battery.

Interface behavior in negative current collectors for sodium‖zinc liquid metal batteries: Implications for enhanced energy storage performance

All-liquid structure and elevated operational temperatures of liquid metal batteries (LMBs) cause severe corrosion of the current collector, impacting cycling performance. This investigation explores the interaction behaviors between negative current collectors (Mo, W, Fe) and the negative active material (Na) within Na|NaCl-CaCl2|Zn LMB. Experiment results indicate that the Na|NaCl-CaCl2|Zn LMB with Fe current collector displays higher cycle stability and smaller polarization than that using Mo or W current collector. The capacity fading rates of 1.23 %, 1.03 %, and 0.23 % in 40 cycles for Mo, W, and Fe current collectors, respectively. Characterizations of materials indicate that Fe4Zn9, W-Zn solid solution and MoZn7 are formed on the Fe, W, and Mo interfaces. Theoretical calculations reveal that the adsorption energy of Na atom on Fe/Fe4Zn9 surpassed that on other current collectors, which indicates the superior charge-transfer kinetics of Na+ on the Fe electrode. The high performance of Fe current collector is attributed to the formation of a dense structure of Fe-Zn alloy and fast charge-transfer ability. Our study contributes to a deeper understanding of the intricate electrochemical processes at the negative electrode interface and provides a practical framework for optimizing the performance of Na‖Zn LMBs.

Assessment of mixed-cation molten salt electrolytes for Li-based liquid metal batteries

The implementation of Li-based liquid metal batteries (LMBs) for grid-scale energy storage has been limited by the high cost of materials, especially for fabrication of Li-halide based molten salt electrolytes (e.g., LiCl, LiBr, LiF, LiI). This study aims to determine the impact of incorporating lower cost alkali/alkaline earth halide salts with foreign cations (e.g., Sr2+, K+) into a Li-based molten salt electrolyte to build a foundation for continued LMB electrolyte design and development. The widely reported LiF–LiCl–LiBr (22-31-47 mol%, Tm = 430–443 °C) electrolyte is used within the Li || Sb–Sn LMB system as a baseline from which to compare the performance of similar systems with eutectic salts of LiCl–SrCl2 (64.3–35.7 mol%, Tm = 489 °C) and LiCl–KCl (59.2–40.8 mol%, Tm = 352 °C) with a specific focus on the positive Sb–Sn electrode in a 3-electrode cell. While good chemical reversibility was maintained for the Li || Sb–Sn systems with mixed-cation molten salts, a shift in the electroactive species from Li + to Sr2+ limited the capacity of the LMB system with a LiCl–SrCl2 electrolyte and reduced rate capability was observed for the LiCl–KCl containing system.

Reusable Cell Design for High-Temperature (600 °C) Liquid Metal Battery Cycling

This paper presents the cycling of a novel low-cost Na-Zn liquid metal battery. Its 600 °C operating temperature presents multiple challenges that must be overcome to achieve commercial viability, both from a structural and electrochemical perspective. To enable long-term cycling of the Na-Zn battery in a realistic environment, we have developed a reusable, hermetically sealed, high temperature and sufficiently corrosion resistant cell concept. The design as well as various approaches for assembling and filling the cell are presented. The factors considered when selecting specific components are documented and explained. The active volume of the cell design can be up to 40 ml, corresponding to a nominal capacity of 1 A h, while the entire cell body weighs around 800 g and costs approximately €200 ($215). The performance of the cell is demonstrated in terms of longevity (1000 h) and high discharge current density (100 mA cm-2). The manuscript not only presents the first long-term cycling performance of the novel Na-Zn chemistry achieving Coulombic efficiency of up to 80%, but also demonstrates the design’s versatility with in situ dynamic neutron radiography of the cell.

A novel array current collector design enabling high energy efficiency liquid metal batteries

Liquid metal battery (LMB) is one of the most competitive large-scale energy storage technologies due to its low-cost, long-lifespan, and high-safety. However, the low energy efficiency of the battery is currently one of the major challenges hindering its application process. The problem of poor wettability of the cathode liquid alloy is exacerbated by the design of the conventional planar structure of the positive collector. And the resulting interfacial mass transfer modes are inefficient. These lead to slow electrode reaction kinetics and large internal polarization. To address this issue, a novel array current collector suitable for LMB was designed here for the first time. The unique structure of the collector greatly increases the effective reaction area at the electrolyte/cathode and the contact area of the cathode/collector, providing more efficient nucleation and growth modes of the products as well as richer ionic mass-transfer channels, thus accelerating the electrode reaction kinetics. Benefiting from the application of the array current collector, the voltage efficiency of Li||Sb-Sn LMBs with a capacity of 20 Ah at 0.5C rate is increased by 4.8 %. Meanwhile, an average voltage efficiency of ∼ 94.7 % and an average energy efficiency of ∼ 92.2 % are achieved at 0.1C, which is the highest efficiency among the most promising LMB systems known to date. These encouraging results provide new directions for the design of high-performance LMBs and further promote the practical process of LMBs.

A comprehensive multiphysics approach to model solutal buoyancy, thermal convection, and electro-vortex flow phenomena for liquid metal batteries

To alleviate the effects of pollution and global warming, the transition to renewable energy sources is taking place at an accelerating rate. The intermittent nature of renewable energy production can be addressed by an energy storage solution that is low-cost, efficient, and based on earth-abundant materials, and a liquid metal battery (LMB) is coming up as one such solution. Liquid metal battery (LMB) performance, especially its discharge voltage, depends on the concentration profile of cathode which in turn is influenced by fluid flow. In this study, we examine three significant phenomena that impact fluid behavior: solutal buoyancy, internally heated convection, and electro-vortex flow (EVF). Though several attempts have been made to model these processes individually or in a combination of two, a model that simulates all three aspects for resultant fluid flow has not been reported. Therefore, this study develops a comprehensive multiphysics model to simulate the positive electrode that is validated with experimental results for Li||Bi cell operating at 450 °C by coupling all of the aforementioned phenomena. While charging with a current density of 1 A/cm2, our model predicts an average velocity of ∼6 mm/s in the cathode solely due to solutal buoyancy. In the case of discharging, both solutal and internal thermal effects lead to an increase in overpotential to as high as ∼200 mV. Moreover, during discharging, the EVF induced a poloidal fluid flow with a maximum velocity of ∼3–5.5 mm/s at 1 A/cm2, reducing mass transport overpotential by ∼10 mV, but it could not disrupt the lighter intermetallic stratification layer on top of the cathode. Additionally, we investigated the charging overpotential phenomenon due to unstable stratification. This all-inclusive model leveraging solutal, thermal, and EVF effects not only offers insights into the fluid motion, concentration gradient, and voltage profile for a lab-scale LMB but can accurately simulate te conditions prevalent in commercial large LMBs, thereby catalysing the path to commercialization of LMBs.