Research Article www.acsami.org
High Performance Liquid Metal Battery with Environmentally Friendly Antimony−Tin Positive Electrode Haomiao Li, Kangli Wang,* Shijie Cheng, and Kai Jiang* State Key Laboratory of Advanced Electromagnetic Engineering and Technology, College of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, China 430074 S Supporting Information *
ABSTRACT: For the first time, Sb−Sn alloys are reported as environmentally friendly positive electrodes for high performance liquid metal batteries (LMBs). Meanwhile, the dominant role of Sb in setting the potential and the inert “solvent” role of Sn in lowering the melting point and decreasing the cell cost are clarified on the basis of electrochemical titration and ex situ analysis. The Li||Sb−Sn LMB exhibits superior rate performance (only 13% capacity loss from 100 mA cm−2 to 1 A cm−2 of current densities), low materials cost (73 $ kW h−1), and high energy density (200.4 W h kg−1) at reduced operating temperature. Most notably, after 3500 h of operation (more than 430 full charge− discharge cycles), a discharge capacity of 20.6 Ah is maintained with a capacity retention of 96.7%, corresponding to a fade rate of 0.0078% per cycle, which potentially meets the metrics of large-scale energy storage without environmental concerns. KEYWORDS: liquid metal batteries, tin−antimony alloys, cathode metals, environmentally friendly, energy storage negative electrode,11,12 bismuth (Bi), and Bi-based positive electrodes,13−15 have also been investigated. However, those candidates may be unable to be utilized in LMBs because of high melting point and high activity of Ca or high cost of Bi. Recently, Wang and Jiang et al. discovered alloying Sb with Pb can significantly decrease the melting point of the positive electrode as well as the cell operating temperature without an attendant drop in cell voltage.16 On the basis of this critical finding, operating at much lower temperature of 450 °C, the Li||Sb−Pb cell exhibits ultralong cyclic life and excellent rate performance with low materials cost, which potentially meets the metrics of stationary energy storage and shows great attractiveness for stationary applications. However, the underlying mechanism responsible for alloying Sb with Pb without a decrease in cell voltage is unclear and need further investigation. Moreover, the introduction of Pb into an electrode may bring potential environmental concerns for large-scale applications. Herein, for the first time, we report an environmentally friendly Sb−Sn positive electrode material for high performance Li||Sb−Sn LMBs with promising thermodynamic properties (>0.9 V of OCV) and excellent cycling performance (over 96.7% of capacity retention after 430 cycles) at low cost (73 $ kW h−1). More importantly, the dominant role of Sb for cell voltage and the inert “solvent” role of Sn in the Sb−Sn system are revealed.
1. INTRODUCTION Large-scale stationary energy storage is critical for improving the grid reliability and utilization and, more importantly, enhancing the integration of renewable energy sources, such as solar and wind. Battery systems are considered as promising solutions owing to their high round-trip efficiency, flexible power and energy characterization, low maintenance, and pollution-free operation.1−7 However, the high cost of existing battery technologies, combined with relatively short cycle life, is the biggest barrier to the grid energy storage.8 To address these problems, considerable effort has been devoted to exploring novel materials and/or designing new battery architectures. Recently, liquid metal battery (LMB), a three-liquid-layer electrochemical cell, has triggered active interest due to the inherent advantages of low cost, facile cell fabrication, and unprecedented cycle life benefiting from cheap materials and the amorphous liquidity of electrode and electrolyte.9 The Mg|| Sb electrode couple is the first chemistry of LMB reported by Bradwell et al.10 and exhibits 94% and 69% of Coulombic and energy efficiencies, respectively. The Mg||Sb cell successfully proves the concept of LMB. However, the high operating temperature of 700 °C, low discharge voltage of ca. 0.21 V (at 200 mA cm−1), and high electrode materials cost of 375 $ kW h−1 make this chemistry impractical for energy storage applications. Therefore, exploration of high voltage, low melting point, and low cost electrode materials is the major challenge to develop high performance LMB. To improve the cell voltage and lower the cost of LMBs, other possible electrode materials, such as calcium (Ca) © 2016 American Chemical Society
Received: March 1, 2016 Accepted: May 5, 2016 Published: May 5, 2016 12830
DOI: 10.1021/acsami.6b02576 ACS Appl. Mater. Interfaces 2016, 8, 12830−12835
Research Article
ACS Applied Materials & Interfaces
Figure 1. Electromotive force (EMF) of Li−Sb−Sn electrodes measured by coulometric titration at 500 °C. EMF as a function of Li concentration in Sb−Sn alloys (a), and as a function of Li concentration normalized with respect to Sb (b). Pure Sb and Sn data are from refs 21 and 22. (c) Voltage curve of Li||Sb−Sn cells as a function of capacity for different component alloys (50:50, 40:60, 30:70, 20:80 mol % Sb−Sn).
2. EXPERIMENTAL SECTION Pretreatment of Molten Salt Electrolyte. High purity (>99%), ultradry grade LiF, LiCl, LiBr, and KCl (all from Aladdin) were used for electrolytes in all the experiments. The pretreatment of molten salt electrolyte was reported in previous work.16 Typically, salt mixtures (LiCl−KCl 58.8−41.2 mol % or LiF−LiCl−LiBr 22−31−47 mol %) were dried under vacuum at 150 °C for 4 h to remove residual water, and then melted at 500 °C under Ar atmosphere. Electrochemical Titration. For the electrochemical titration, Li− Al alloys were used as counter and reference electrodes. The Li−Al alloys were prepared using a small furnace in a glovebox filled with Ar gas. Lithium and aluminum granules with different compositions (45− 55 mol % and 25−75 mol % of Li−Al) were placed into a diameter of 15 mm graphite crucible and premelted at 650 °C, which were used as counter and reference electrodes, respectively.17 High purity Sb and Sn (99.5% for Sn and 99.999% for Sb, both from Aladdin) were premelted in a small alumina crucible at 500 °C. Molybdenum wires (1.5 mm diameter, Aladdin) and tungsten wire (1 mm diameter, Aladdin) were immersed in the molten Li−Al and Sb−Sn alloys as electrical leads, respectively. The three electrodes and a blank molybdenum wire were immersed in an alumina cup (60 mm diameter, 80 mm depth) as shown in Figure S1. All the electrochemical measurements were performed under a high purity Ar atmosphere with an Autolab PGSTAT 302N potentiostat/galvanostat. Cell Construction and Test. For the cell construction, all the cells were assembled in an Ar atmosphere glovebox and tested with galvanostatic charge and discharge using an Arbin BT2000. The phase and elemental analysis of positive electrodes were characterized by Xray diffraction (XRD), scanning electron microscopy (SEM, FEI Nova NanoSEM 450), and energy dispersive spectrometer (EDS). The XRD patterns were obtained on the PANalytical X’Pert PRO (Cu K generator) in the range 20−80°.
Figure 2. Voltage curves during charge−discharge at different current densities (from 100 to 1000 mA cm−2) of a Li||Sb−Sn cell.
The EMF profiles of binary Li||Sb and Li||Sn couples from literature19,20 are also shown for comparison. It should be noticed that Sb−Sn alloys and Sn are liquid whereas pure Sb is solid at temperature of 500 °C. Li||Sb chemistry exhibits the highest EMF of 0.92 V, while the EMF of Li||Sn is much lower (about 0.55 V). It is very interesting that, even at ultrahigh Sn level (80 mol % Sn in Sb−Sn alloy), Li||Sb−Sn chemistry can still maintain EMF at near Li||Sb level. The normalized EMF profiles versus the concentration of Li relative to Sb are also shown in Figure 1b. As in Li−Sb−Pb system, all Li−Sb−Sn alloys exhibit EMF trends that are very similar to that of the Li−Sb electrode, a high EMF plateau at the beginning followed by a sharp drop at 75 mol % Li relative to Sb. These data suggest that the Li−Sb−Sn electrode potential is determined primarily by the Li−Sb reaction. Sn mainly acts as the inert “solvent” role aiming to lower the melting point, as well as to improve the utilization of the Sb positive electrode. Even at the high temperature of 650 °C, the solubility of Li in pure Sb is limited to 45−50 mol % to avoid the formation of a solid intermetallic and a sharp change in voltage. Alloying Sb with Sn
3. RESULTS AND DISCUSSION The thermodynamic properties of Li||Sb−Sn chemistry were determined by coulometric titration in LiCl−KCl (58.8−41.2 mol %, Tm = 352 °C18) electrolyte at 500 °C, as shown in Figure 1. Four compositions of Sb−Sn alloys (Sb:Sn = 50:50, 40:60, 30:70, 20:80 mol %) were systematically investigated. 12831
DOI: 10.1021/acsami.6b02576 ACS Appl. Mater. Interfaces 2016, 8, 12830−12835
Research Article
ACS Applied Materials & Interfaces
ultrafast electrode charge-transfer kinetics, the high conductivity of the molten salt electrolyte, and fast mass transport within the liquid metal electrode, which is highly desirable in power applications such as frequency regulation. A typical Li||Sb− Sn(50:50 mol %) cell with theoretical capacity of 1.5 Ah was tested upon charge and discharge at different current densities from 100 to 1000 mA cm−2, as shown in Figure 2. The Li||Sb− Sn cell exhibits excellent rate performance with no obvious capacity loss even at the highest current density of 1000 mA cm−2 (only 13% capacity loss as compared to that of 100 mA cm−2). The relatively low average discharge voltage of 0.35 V at 1000 mA cm−2 is due to the high ohmic resistance resulting from the thick molten salt electrolyte of ∼1.3 cm in this experiment, which can be optimized by adjusting the distance between positive and negative current collectors. Electrochemical impedance spectroscopies (EISs) at different charge states (Figure 3) were also collected. The charge-transfer resistance (Rct) is much smaller (Table 1) as compared to the battery systems with solid electrodes, such as lithium/sodium ion battery,22−25 indicating ultrafast charge-transfer kinetics of liquid−liquid interface between electrode and electrolyte. To reveal more details about the charge−discharge processes of the Li||Sb−Sn system, the post-mortem analysis was carried out after the cells cool down at different charge/discharge states. As shown in Figure 4a(2), at full discharge state, there are two distinct layers in the positive electrode side, including an upper layer of intermetallics and a bottom layer of metal, while, at the state of full charge, there is only one positive layer of Sn−Sb alloy. The compositions of positive electrode at full charge/discharge states are primarily characterized by EDS as shown in Figure 4b. Although the element of lithium cannot be detected directly by EDS, it is able to be reflected indirectly by the content of oxygen in alloys, since the samples are exposed in air and oxidized quickly on the surface during the preparation process. The XRD patterns in Figure 4d,e also indicate that, during the discharge process, the Sb−Sn turns into LixSb and Sn phases, which further confirm that Sb is the major electrochemically active component. The main functions of Sn in the Li||Sb−Sn system can be defined as (i) lowering the melting temperature of positive electrode, (ii) increasing the utilization of Sb positive electrode, and (iii) reducing the cell cost. It is worth noting that electrode alloying is generally accompanied by a decrease in cell voltage and rate capability as well as an increase in cell cost, as observed in the Mg||Sb−Sn26 and Na||Sb−Bi27 systems. The uncommon phenomenon in the Li||Sb−Sn system that a dramatic drop in melting point over 200 °C by alloying Sb with Sn without an attendant drop in cell voltage is possible because of the large difference of Gibbs free energy of formation between LixSb and LiySn. In order to demonstrate the scalability of the Li||Sb−Sn system, four different size cells with capacities of 1.4, 6.1, 21.3, and 47 Ah were assembled, respectively. Details about the cell dimensions, electrode weights, materials cost, and energy
Figure 3. (a) Typical discharge curve of a 6.5 Ah cell. (b) Nyquist plots of a 6.5 Ah cell at different state of charge (SOC) marked in the discharge curve, obtained by applying a sine wave with an amplitude of 10 mV in the frequency from 0.01 to 100 kHz. The inset figure shows the corresponding equivalent electrical circuits for fitting the data at the first quartile.
significantly increases the solubility of Li into positive electrode, and thus leads to a reduction of materials cost. The EMF results were further verified in 1.5 Ah theoretical capacity cells, composed of Sb−Sn alloys positive electrode, liquid lithium negative electrode, and LiF−LiCl−LiBr (22− 31−47 mol %, Tm = 430 °C21) molten salt electrolyte. The cells were assembled at a fully charged state in an Ar-filled glovebox, and tested in a vertical tube furnace under Ar atmosphere at 500 °C. At a current density of 100 mA cm−2, all Li||Sb−Sn cells can achieve ca. 0.8 V of discharge voltage as shown in Figure 1c, which is much higher than the previously reported Mg||Sb chemistry,10 and almost the same as Li||Sb−Pb chemistry.16 The cost of electrode materials is calculated to be in US$, 73 kWh−1 as shown in Table S1, which is one-fifth of the value of the Mg||Sb system (375 $ kW h−1).10,16 Moreover, there are no noticeable differences in the charge/discharge voltage plateau among the various compositions of Sb−Sn alloys, from the highest level of dilution (80 mol % Sn in Sb− Sn) to SbSn alloy (50 mol % Sn in Sb−Sn), which provide evidence for the predominant role of Sb in setting the potential and the inert “solvent” role of Sn in lowering the melting point and reducing the cost of materials and systems. One of the major advantages of liquid metal batteries is the ability to operate at high current densities benefiting from the
Table 1. Impedance Parameters of Li−Sb−Sn Cell at Different Charge States
a (full charge) b (discharge 2 h) c (discharge 4 h) d (discharge 6 h) e (full discharge)
Rs (mΩ)
Rct (mΩ)
CPEct (Mho)
W (Mho)
184 182 189 193 189
37.3 24.2 27.0 21.9 714
33.2 (N = 0.529) 82.6 (N = 0.532) 84.8 (N = 0.516) 91.8 (N = 0.532) 3.86 (N = 0.686)
259 358 454 493
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DOI: 10.1021/acsami.6b02576 ACS Appl. Mater. Interfaces 2016, 8, 12830−12835
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Figure 4. (a) Cross sections of Li||Sb−Sn cells cooled down at fully charged and discharged states. (b) SEM image and EDS of the positive electrode at discharge state. (c) Charge−discharge curve of one cycle at 100 mA cm−2 of current density, in which five states are marked as a, b, c, d, and e. (d and e) XRD patterns of positive electrodes at different charge/discharge states, with figure d for the metal part and figure e for the intermetallic part, respectively.
Table 2. Cell Materials Costs and Energy Densities for Four Different Sized Li−Sb−Sn Cells and Other Systems (Li−Sb−Pb, Li−Bi, and Mg−Sb) materials LMB system
diam (cm)
realized C (Ah)
discharge voltage (V)
Li−Sb−Sn
2.0 3.0 5.5 7.2 8.9 10 1.6
1.4 6.1 21.3 47 62 48.8 2.5
0.80 0.79 0.79 0.71 0.69 0.55 0.46
Li−Sb−Pb Li−Bi Mg−Sb
anode (g) electrolyte (g) 0.38 1.65 5.91 12.42 16 13.9 2.27
25.8 30 120 150 200 9.98
cathode (g)
energy density (W h kg‑1)
materials cost ($ kW h‑1)
3.2 (Sn), 2.2 (Sb) 13.3 (Sn), 9.1 (Sb) 50.9 (Sn), 34. 8 (Sb) 106.3 (Sn), 72.7 (Sb) 259.8 (Pb), 101.7 (Sb) 166.8 (Bi) 17.06 (Sb)
193.8 200.4 183.8 174.4 103.61 148.53 24.58
72.77 71.58 77.10 81.18 65.41 169 375.25
density are summarized in Table 2. For comparison, the performance of Mg||Sb, Li||Bi, and Li||Sb−Pb are also listed. Behaviors similar to the small scale (1.5 Ah) have been observed in scaled Li||Sb−Sn cells, as shown in Figure 5. The nominal discharge voltage of 47 Ah cell is measured to be 0.71 V at 100 mA cm−1. The average Coulombic efficiency and a round-trip energy efficiency are about 98% and 72%, respectively, which could be further optimized by adjusting the battery parameters, especially the thickness of molten salt electrolyte. On the basis of the cell performance, the electrode materials cost of Li||Sb−Sn system is estimated to be 72−81 $ kWh−1, which is very close to that of the Li||Sb−Pb system but much lower than that of Mg||Sb and Li||Bi systems. In addition, the calculated energy density of Li||Sb−Sn is about 174.4 Wh kg−1 at the large scale, which is the highest value among all the LMBs reported. This could be attributed to the high cell voltage of Sb coupled with Li negative electrode and relatively low density of Sn. Notably, the Li||Sb−Sn system demonstrates excellent cycle stability even at a large scale (21.3 Ah). After
Figure 5. Charge−discharge curves of different capacity cells (1.5, 6, 23, and 47 Ah) of Li−Sb−Sn. 12833
DOI: 10.1021/acsami.6b02576 ACS Appl. Mater. Interfaces 2016, 8, 12830−12835
Research Article
ACS Applied Materials & Interfaces
Figure 6. Performance of a Li||Sb−Sn cell tested at 300 mA cm−2 (100 mA cm−2 for the initial 4 cycles). (a) Profiles of voltage and current during charge−discharge (the fifth cycle). (b) Charge and discharge capacity, Coulombic efficiency, and energy efficiency as a function of cycle numbers.
Figure 7. (a) Performance test of temperature fluctuation in a Li||Sb−Sn cell, which was cooled down to room temperature after 25th and reheated to operating temperature. (b) Voltage performance of a Li||Sb−Sn cell cycled at 300 mA cm−2, charge and discharge voltages as a function of capacity at the 10th, 20th, 50th, and 100th cycles.
decreasing the melting point of positive electrode, while maintaining relatively high discharge voltage of ca. 0.8 V at 100 mA cm−2. The voltage profiles are elucidated by electrochemical titration and ex situ electrode materials analysis, which reveal the dominant role of Sb for cell voltage and an inert “solvent” role of Sn in Sb−Sn system. The cells of Li|LiF− LiCl−LiBr|Sb−Sn reported herein exhibit excellent cyclic performance, high rate capability, and unprecedented thermal robustness, which benefit from the all-liquid-structure of liquid metal electrodes and molten salt electrolyte. These important findings provide an attractive lead-free LMBs material system with low cost and high performance for large-scale applications.
430 cycles, full charge−discharge (over 3500 h), a discharge capacity of 20.6 Ah was maintained with a capacity retention of 96.7% as shown in Figure 6, corresponding to a small fade rate of 0.0078% per cycle. The Coulombic efficiency and round-trip energy efficiency are over 98.5% and 68.5%, respectively, at 300 mA cm−2 of current density. The thermal robustness of the Li|| Sb−Sn system at a large scale is also evaluated, aiming to demonstrate the reliability of this system in the applications such as stationary energy storage. Cells were cooled down to room temperature during cycling (at the point of fully charged state after 25 cycles), and then reheated to the operating temperature. From the cycling performance shown in Figure 7a, the LMB cell resumed running at exactly the same discharge capacity and Coulombic and energy efficiencies as those before cool-down. When the charge−discharge voltage curves as a function of cell capacity are plotted together (Figure 7b), for the 10th, 20th, 50th, and 100th cycles, there are no obvious changes for all the voltage profiles. It can be clearly seen that large temperature fluctuation, which commonly causes the damage of solid ceramics and brings severe safety issues in alumina electrolyte-based high temperature batteries,28 does not bring any negative effect on the subsequent long cycling.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02576. Experimental details and results of electrochemical titration (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected].
4. CONCLUSION In summary, as an environmentally friendly metal, tin is alloyed with antimony to lower the LMB operating temperature by
Notes
The authors declare no competing financial interest. 12834
DOI: 10.1021/acsami.6b02576 ACS Appl. Mater. Interfaces 2016, 8, 12830−12835
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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (Grant 51307069), 973 Program (2015CB258400), and the National Thousand Talents Program of China.
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REFERENCES
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DOI: 10.1021/acsami.6b02576 ACS Appl. Mater. Interfaces 2016, 8, 12830−12835
