Self-assembling solid cathode enables high-capacity, low-cost Ca-Sb battery

doi:10.21203/rs.3.rs-4356928/v1

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Self-assembling solid cathode enables high-capacity, low-cost Ca-Sb battery

https://doi.org/10.21203/rs.3.rs-4356928/v1

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For renewable energy technologies to decarbonize the power grid without compromising its reliability, low-cost grid-scale energy storage with resilient long-term performance is required. Here, we report a new type of high-temperature liquid metal battery (LMB) that achieves unprecedented capacity, low electrode costs, and strong cycling performance by replacing the traditional liquid LMB cathode with one based on solid particles. Through the combination of a liquid calcium (Ca) alloy anode, a cathode based on solid antimony (Sb) particles, and an all-chloride electrolyte, the Ca||Sb(s) system achieved 318% higher discharge capacity per unit mass of the cathode (715 mAh g^-1 vs. 171 mAh g^-1) and 76% lower electrode cost (15.5 $/kWh vs. 65 $/kWh) than the lowest cost LMB chemistry yet published. These Ca||Sb(s) batteries cycled with high Coulombic (>98.4%) and energy efficiencies (79–84%) at C-rates relevant for daily cycling applications. The remarkable increase in specific capacity is due to the self-assembly of Sb into a micro-structured, electronically connected cathode network, which enables nearly complete utilization of Sb. Despite using a solid cathode, the Ca||Sb(s) system retains the characteristic minimal capacity fade of LMBs, with no meaningful degradation observed over ~4000 full depth-of-discharge cycles. Additionally, the liquid Ca alloy anode mitigates the formation of solid Ca dendrites which would be detrimental to stable cycling performance.

Physical sciences/Energy science and technology/Energy storage/Batteries

Physical sciences/Materials science/Materials for energy and catalysis/Batteries

The urgent need to decarbonize the electric grid and integrate renewable energy technologies has created unprecedented growth in the demand for large-capacity energy storage. While storage markets are diverse and need solutions with varying characteristics, a common thread is that the cost of electricity storage must be low enough to displace more carbon-intensive methods of ensuring grid stability, like natural gas peaker plants^1–3. This cost constraint has thus featured prominently in the recent development of liquid metal batteries (LMBs) made from earth-abundant materials, which display reliable performance and high current capabilities due to fast liquid-liquid kinetics^4–7. For example, a Li||Sb-Pb(l) LMB achieved a specific capacity of 171 mAh g^-1 and an electrode cost of 65 $/kWh, while successfully demonstrating a long life span with negligible capacity fade at 500 °C⁵. Even as Li-ion battery (LIB) technology has advanced in recent years leading to lower electrode costs (70–250 $/kWh)^8–10, the low-cost floor of LMB chemistries suggests that they could be a cost-effective contributor to stationary energy storage markets.

Even among LMBs, those with calcium-based anodes stand out because low-cost, earth-abundant Ca can be paired with several viable cathode materials^2,4,11,12. Bradwell, and then Ouchi, et al. first demonstrated the feasibility of Ca-based LMBs using a Ca-Mg||Bi(l) cell at 550 °C. However, the use of costly Bi metal and the reported low capacity (90 mAh g^-1 Bi) resulted in a high energy cost of ~144 $/kWh^6,12. The Ca||Sb couple has been considered one of the most cost-effective electrode pairs since the inception of the LMB due to the high cell voltage (~1.0 V) and low cost of Ca and Sb metals^12,13. Unfortunately, the promise of a Ca||Sb cell has not previously been realized due to the constraint of a liquid cathode in traditional LMBs, which necessitates an operating temperature of ~700 °C based on the melting point of Sb (T_{m, Sb} = 631 °C). Furthermore, the limited solubility of Ca in liquid Sb (~25 at% Ca) results in a low specific capacity (136 mAh g^-1 Sb) and a relatively high projected electrode cost of 90 $/kWh¹³.

We present a paradigm shift by pairing a Ca-based liquid metal anode with a solid Sb cathode to achieve unprecedented capacity and low energy cost in a LMB. While it might be assumed that the relative slowness of diffusion in a solid electrode could reduce battery performance compared to a liquid electrode, the features observed and described herein for the Ca||Sb(s) system circumvent this logic. This battery operates at ~520 °C in a eutectic CaCl₂-LiCl electrolyte (35-65 mol%, or 58.5-41.5 wt%, T_m = 485 °C)¹⁴ which was chosen to allow direct comparison of cycling performance with prior studies of Ca-based LMBs. Figure 1a displays the discharge potential of an Sb electrode (vs. Ca metal) starting from monolithic Sb under constant current (25 mA), showing that a high, useful potential is maintained to a specific capacity beyond 700 mAh g^‑1 Sb. Post-mortem analysis of Sb electrodes at various stages of discharge indicates that the initial bulk Sb loaded transforms into fine particles with cycling (Figure 1b). Chemical analysis of the products at 160 mAh g^-1 Sb suggests that the cathode is fragmenting due to volume expansion from the progressive formation of Ca-Sb compounds, including CaSb₂ and Ca₁₁Sb₁₀. We postulate that the high capacity achieved here is due to the fragmentation of Sb which increases surface area and generates short mass transport pathways which allow operation beyond the typical kinetic limitations of solids.

Furthermore, Figure 1c is a scanning electron micrograph (SEM) of the cathode of a Ca||Sb(s) cell stopped in a fully charged state after ~1100 cycles and ~12000 hours of run time. The morphology of the Sb cathode in this state of charge, consisting of a porous network of micron-scale particles, was preserved by repeatedly immersing the electrode in clean water to remove the electrolyte with minimal agitation. As the operating temperature of the battery is an appreciable fraction of the melting point of antimony, there is likely an ongoing balance between cathode particle fragmentation and sintering over the course of cycling that spontaneously produces this interconnected structure. The self-assembly of a networked Sb electrode structure is likely to increase the electronic connectivity of the electrode to help achieve practical high capacity operation.

[Figure 1]

The equilibrium potential (E_eq) of a particulate Sb electrode during discharge is displayed in Figure 1d as a function of capacity and mole fraction (x_Ca). These data are obtained from the steady open-circuit potential achieved following sequential coulometric titration at a constant current (20 mA). The particulate Sb electrode demonstrated a useful specific capacity as high as 738 mAh g^-1 Sb by sustaining a high potential (E_eq > 0.87 V) until relatively deep states of discharge (x_Ca = 0.63), far surpassing the solubility-limited capacity of the Ca||Sb(l) couple even at higher temperatures, i.e., 136 mAh g^-1 Sb at 700 °C¹³. The high capacity achieved using particulate Sb electrodes is corroborated by electromotive force (emf) measurements of binary Ca-Sb alloys in two-phase regions, as carried out in previous works using a solid CaF₂ electrolyte¹⁵ and in this work using a eutectic CaCl₂-LiCl liquid electrolyte (Figure 1e), which indicates a thermodynamically limited specific capacity of 733 mAh g^-1 Sb (or 62.5 at% Ca in Sb). Despite the close agreement of the specific capacities determined from the measurement of E_eq through coulometric titration and the emf of binary Ca-Sb alloys, there is a deviation between E_eq and the emf of the binary alloys above 320 mAh g^-1 Sb (Figure 1d), which can be explained by phase analysis of the electrode as summarized in Figure 2 and discussed further below.

[Figure 2]

To observe practical cycling performance compared to the emf measurements discussed above and provide larger cathode samples for detailed analysis, a 3-electrode electrochemical cell was constructed for the Ca||Sb(s) system. Figure 2a–2b displays cycling performance for a particulate Sb electrode held in a stainless steel (SS) crucible subjected to constant currents equivalent to C-rates of C/20–1C (based on a theoretical capacity of 733 mAh g^-1 Sb) and corresponding to current densities of 50–1000 mA cm^-2(based on the macroscopic area of the exposed cathode surface). This current density is far greater than that typically demonstrated in room temperature Li-ion batteries (<10 mA cm^-2)¹⁶, consistent with the generally facile kinetics in LMBs. The achieved discharge capacity was more than 659 mAh g^-1 Sb (>90% of theoretical capacity) at C/20–C/2 rates and was about 400 mAh g^‑1 Sb at the highest 1C rate, demonstrating a far greater specific capacity for Ca||Sb(s) at 520 °C than the Ca||Sb(l) couple at 700 °C (<136 mAh g^-1 Sb) or advanced LIBs (<250 mAh g^‑1)^13,17. The Sb electrode performance in this cell was reliable with negligible capacity loss over 50 cycles (>30 days) and high round-trip coulombic efficiencies (>99.4%) during steady state operation at 90–100% of the theoretical capacity. At practical daily cycle rates (C/8–C/10), the calculated energy efficiency is 79–84%, demonstrating utility for grid-scale energy storage when coupled with intermittent renewable energy technologies.

The characterization of cathode products, following 10 cycles with various discharge depths at C/6, reveals the dynamic phase evolution of the Sb(s) electrode as summarized in Figure 2c. X-ray diffraction (XRD) analysis indicates the formation of [Sb + CaSb₂] compounds at 168 mAh g^-1 Sb and [CaSb₂ + Ca₁₁Sb₁₀] at 312 mAh g^-1 Sb, in agreement with the known equilibrium phase behavior of the binary Ca-Sb system¹⁵. Interestingly, at later stages of discharge, the presence of a ternary LiCaSb compound was evident: [Ca₁₁Sb₁₀ + LiCaSb] at 672 mAh g^-1 Sb and [LiCaSb + Ca₂Sb] at the full depth of discharge (734 mAh g^-1 Sb). The formation of the LiCaSb compound implies an electrode reaction in which Ca and Li are co-deposited without an associated decrease in electrode potential, particularly at later stages of discharge (>320 mAh g^-1 Sb). The formation of LiCaSb explains the deviation of E_eq from the emf of binary Ca-Sb alloys as shown in Figure 1d.

[Figure 3]

Figure 3a schematically illustrates a Ca | CaCl₂-LiCl | Sb(s) battery constructed to demonstrate practical implementation of this system. Pure Ca metal was pre-embedded in an iron foam current collector by wetting the foam in molten Ca. The cycling performance of the Ca||Sb(s) battery was evaluated at 520 °C using a C/2 current rate, shown in Figure 3b–3c. The Ca||Sb(s) battery consistently achieved a high average discharge capacity of 715 mAh g^-1 Sb (97% of the theoretical capacity) with no indication of capacity loss over 100 cycles (16 days) with high coulombic (98.4%) and energy (86%) efficiencies. Based on battery performance and the cost of electrode materials, the energy cost and energy density of the Ca||Sb(s) battery are estimated at 15.5 $/kWh and 620 Wh/kg, outperforming those of prior LMBs (65 $/kWh and 194 Wh/kg)^5,7,18,19 by fully utilizing the low-cost particulate Sb cathode up to the thermodynamic limit.

[Figure 4]

Historically, the use of Ca metal in molten salt electrolytes has presented unpleasant challenges due to its high reactivity and solubility in molten salts, which typically leads to poor coulombic efficiency (< 82%) and rapid cell degradation^20,21. Furthermore, cell operation at 520 °C, below the melting point of calcium, introduces the possibility of dendritic growth of solid Ca during charging which may result in a short circuit between electrodes or unstable voltage. We report that the excellent cycling performance and absence of erratic behavior observed for this Ca||Sb(s) battery can be attributed to spontaneous formation of a Ca-Li alloy at the anode. Figure 4a displays a post-mortem image of a fully charged anode after 100 cycles, showing sound retention of anode materials within the foam current collector. Chemical analysis by ICP-AES in Table 1 confirms that the anode material is a binary Ca-Li (~41 at%, or 11 wt% Li) alloy, corroborated by the presence of both Ca(s) and Li₂Ca phases from XRD analysis (Figure 4b) and a hypo-eutectic microstructure (L ® L + Ca ® Ca + Li₂Ca) as observed via SEM (Figure 4c)²². Notably, the formation of this binary Ca-Li alloy means that the anode is liquid during operation, which eliminates the chance of solid dendrite growth and reduces the chemical reactivity of Ca. Furthermore, the liquid Ca-Li alloy exhibited remarkably low overpotentials (<10 mV) owing to facile liquid-liquid interfacial reactions and rapid mass transport during cycling at various currents (15–150 mA), as displayed in Figure 4d.

[Figure 5]

As further evidence for the cycling stability achievable with the Ca||Sb(s) chemistry, Figure 5 shows discharge capacity data from a battery over ~4000 cycles and 9 months. This cell was primarily cycled across its full voltage range (0.6–1.2 V) at a nominal rate of 1C. After an initial set of conditioning cycles, the cell was run under quasi-steady state conditions. Notwithstanding some minor discontinuities in the test data due to an unexpected power outage (partial cool-down), no net capacity fade is observed even after a number of cycles equivalent to over a decade of daily cycling. The increase in steady-state capacity observed after the period of deeper discharge cycles (black square symbol), in which the cell voltage was floated at 0.6 V for 0.5 h in a fully discharged state, further supports the hypothesis that increasing cathode fragmentation increases accessible cell capacity.

In conclusion, we demonstrate that the unprecedented high capacity of the Ca||Sb(s) couple in a molten CaCl₂-LiCl electrolyte is enabled by spontaneous fragmentation of Sb during cycling, the self-assembly of a porous, electronically conductive cathode network, and the formation of a ternary LiCaSb compound without a corresponding loss in cell voltage. Furthermore, in this electrolyte, the formation of a liquid Ca-Li alloy anode allows for rapid electrode kinetics and stable cycling. While the high fraction of LiCl in the electrolyte, which is considerably more expensive than CaCl₂ and other common chloride salts, provides an opportunity to further reduce system costs by minimizing the use of LiCl, these new findings allowed the successful construction and operation of high-capacity Ca||Sb(s) batteries with high efficiency, excellent cycle life, and a strong potential to be cost-effective at scale. These results are a foundation for continued development of this system to advance its potential for implementation as an energy storage system as part of a decarbonized electrical grid.

Cell Components and Configuration. All cell components were prepared and assembled in an inert Ar-filled glovebox (O₂ < 0.5 ppm). Eutectic CaCl₂-LiCl (35–65 mol%) electrolyte was prepared by mixing appropriate weights of anhydrous LiCl (> 99.0%) and CaCl₂ (> 98.0%) in a quartz crucible and pre-melted under the following temperature profile; 8 h at 100°C and 270°C for vacuum-drying of salt, 8 h at 430°C during which time the chamber was purged with Ar gas, and 700°C for 3 h to pre-melt the salt. The pre-melted electrolyte was then crushed into fine powder for electrochemical measurements.

Electrode materials were prepared using pure Sb (99.95%) and Ca (99.5%) metals. Monolithic Sb electrodes (Fig. 1a) were prepared by induction melting (IH15A-2 T, Across International) in a BN crucible (8 mm ID, 12 mm OD, 12 mm height), while particulate Sb electrodes were prepared by grinding Sb with a mortar and pestle before loading into a SS crucible (8 mm ID, 12 mm OD, 12 mm height) covered with SS meshes (100×100). Porous metal structures, such as 95% porosity iron foam, were prepared as an electrode holder for any materials deposited to evaluate the anode performance (Fig. 4).

The potential of each Sb working electrode (WE) was measured in a 3-electrode cell comprised of a Ca-Bi alloy (x_Ca = 0.40) as the reference electrode (RE) and a Ca-Sb alloy (x_Ca = 0.40) as the counter electrode (CE). The Ca alloy electrodes were fabricated using an arc-melter (MAM-1, Edmund Bühler GmbH). The Ca-Bi RE (2 g) was placed in a BN crucible (8 mm ID, 12 mm OD, 25 mm height) with two capillary holes (1.5 mm) drilled just above the bottom of the crucible and its emf value, 0.801 V vs. Ca(s) at 520°C²³ was used to report the measured electrode potentials versus Ca(s). The Ca-Sb CE (15 g) was placed in a large surface area BN crucible (21 mm ID, 25 mm OD, 12 mm height). Both RE and CE alloys in BN crucibles were remelted using an induction heater, and tungsten electrical leads were inserted into the alloys while still molten. The components and configuration of the 3-electrode cells used in this study are summarized in Figure S1 and Table S1.

The equilibrium potential of particulate Sb (Fig. 1d) was determined by the coulometric titration technique. The Sb was discharged using a constant current of 20 mA for 30 min in each titration step, followed by an open-circuit potential measurement for 2 h to allow for stabilization of the Sb electrode (Figure S2). For emf measurements of binary Ca-Sb alloys (Fig. 1e), arc-melted alloys (x_Ca = 0.54–0.65) were equilibrated by annealing at 650°C for 72 h after which the equilibrium phases were confirmed by X-ray diffraction (XRD) (Figure S3). The annealed Ca-Sb alloys were placed in BN crucibles with the same dimension as the RE crucible described above, before their potentials were measured with respect to the Ca-Bi RE at 520°C for 10 h.

In the two-electrode Ca||Sb(s) battery cell of Fig. 3, the Ca anode was fabricated by immersing iron foam (95% porosity) in molten Ca while the particulate Sb cathode was prepared in a SS crucible with a SS mesh (40×40) spot welded to the top. Both electrodes were then assembled in a SS crucible with an interelectrode distance of 0.5 cm. Once assembled, pre-melted electrolyte powder (120 g) was poured into the crucible. Detailed descriptions of cell components are summarized in Table S2. The Ca | CaCl₂-LiCl | Sb(s) battery was placed in an air-tight SS test chamber under an argon atmosphere and cyclically discharged and charged at 520°C. Electrochemical measurements were carried out using a potentiostat (Interface 5000E, Gamry Instruments) and cell temperature was monitored using a data acquisition system (NI 9211, National Instruments).

For the characterization of cycle life data (Fig. 5), a two-electrode Ca||Sb(s) battery cell was prepared and cycled using an Arbin Instruments battery tester for both electrochemical control and data logging. This cell consisted of a SS housing sealed with welds and a brazed ceramic to ensure electrical isolation between the positive and negative terminals. Internally, the liquid anode was held within a stack of SS meshes (35 mesh) with a macroscopic area of ~ 19.7 cm². The cathode was contained in a SS crucible with an opening of equal area covered with SS mesh (80×700 Dutch weave). The electrode holding components were assembled facing each other in a prismatic configuration at a nominal interelectrode spacing of 1.3 cm. Molten salt electrolyte was then poured into the housing around the electrode assemblies as a liquid and allowed to freeze before the cell was sealed. Further construction details for this cell are summarized in Table S2.

Characterization/Analysis. Upon completion of electrochemical measurements, the electrodes were raised above the electrolyte and the cell was cooled to room temperature. Each electrode was separated from the cell assembly, immersed in dimethyl sulfoxide (99.9%) for 3 days to remove the entrained salt layer, mounted in epoxy resin, and cross sectioned for post-mortem characterization. For characterization of Sb electrodes in a fully charged state (Fig. 1c), multiple changes of deionized water were used over several days to remove salt and reveal the electrode morphology. One half of the cross sectioned electrode was polished for microstructural characterization using scanning electron microscopy (SEM, FEI Quanta 200) coupled with energy dispersive spectroscopy (EDS). The second half of the electrode was used for structural analysis by XRD (PANalytical Empyrean, Cu Kα-radiation) or chemical analysis by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Thermo iCAP 7400) after removing the electrode materials from the current collectors/holders.

The electrode materials cost per unit energy, C, was calculated using $C={\sum }_{i}{P}_{i}{m}_{i}/{E}_{D}$ , where ${P}_{i}$ is the commodity bulk price of each electrode material in $/kg, ${m}_{i}$ is the mass in g, and ${E}_{D}$ is the average discharge energy in kWh. A detailed cost calculation for the Ca||Sb(s) battery is presented in Table S3 based on electrochemical performance (Fig. 3). This approach for electrode cost calculation is consistent with the previously reported values for various LMB cells and the estimated values are compared in Table S4.

Acknowledgements

This work was supported by Ambri Incorporated. The authors also wish to acknowledge Prof. Donald Sadoway for his mentorship, support, and leadership in advancing the research and development of Liquid Metal Batteries.

Author contributions.

Sanghyeok Im: Conceptualization, Methodology, Investigation, Writing–original draft, Visualization. Peyman Asghari-Rad: Methodology, Visualization, Writing – review & editing. Kelly Elizabeth Varnell: Methodology, Investigation, Writing – review & editing. Alex T. Vai: Validation, Writing – review & editing. Jianyi Cui: Validation, Writing – review & editing. Rachael Howland: Methodology development. David Bradwell: Validation, Writing – review & editing, Project administration. Hojong Kim: Writing – review & editing, Project administration, Supervision.

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Table 1. ICP-AES chemical composition analysis of the fully charged anode after 100 charge/discharge cycles (Figure 3).

Sample	Concentration
	Ca	Li	Sb	Cr	Fe	Ni
	(at %)			(ppm)
#1	57.7	41.3	0.9	246	570	1,332
#2	57.8	41.5	0.6	216	403	2,091

There is NO Competing Interest.

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Self-assembling solid cathode enables high-capacity, low-cost Ca-Sb battery

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