A sodium-sulfur battery created by engineers at The University of Texas at Austin solves one of the biggest hurdles that has held back the technology as a commercially viable alternative to the ubiquitous lithium-ion batteries that power everything from smartphones to electric vehicles.
Sodium and sulfur stand out as appealing materials for future battery production because they are cheaper and more widely available than materials such as lithium and cobalt, which also have environmental and human rights concerns. Because of this, researchers have worked for the past two decades to make room-temperature, sodium-based batteries viable.
“I call it a dream technology because sodium and sulfur are abundant, environmentally benign, and the lowest cost you think of,” said Arumugam Manthiram, director of UT’s Texas Materials Institute and professor in the Walker Department of Mechanical Engineering. “With expanded electrification and increased need for renewable energy storage going forward, cost and affordability will be the single dominant factor.”
In one of two recent sodium battery advances from UT Austin, the researchers tweaked the makeup of the electrolyte, the liquid that facilitates movement of ions back and forth between the cathode and anode to stimulate charging and discharging of the batteries.
They attacked the common problem in sodium batteries of the growth of needle-like structures, called dendrites, on the anode that can cause the battery to rapidly degrade, short circuit, and even catch fire or explode.
The researchers published their findings in a recent paper in the Journal of the American Chemical Society.
In previous electrolytes for sodium-sulfur batteries, the intermediate compounds formed from sulfur would dissolve in the liquid electrolyte and migrate between the two electrodes within the battery. This dynamic, known as shuttling, can lead to material loss, degradation of components, and dendrite formation.
The researchers created an electrolyte that prevents the sulfur from dissolving and thus solves the shuttling and dendrite problems. That enables a longer life cycle for the battery, showing a stable performance over 300 charge-discharge cycles.
“When you put a lot of sugar in water, it becomes syrupy. Not everything is dissolved away,” said Amruth Bhargav, a doctoral student in Manthiram’s lab. “Some things are half linked and half dissolved. In a battery, we want this in a half-dissolved state.”
The new battery electrolyte was designed in a similar vein by diluting a concentrated salt solution with an inert, nonparticipating solvent, which preserves the “half-dissolved” state. The researchers found that such an electrolyte prevents the unwanted reactions at the electrodes and thus prolongs the life of the battery.
The price of lithium has skyrocketed during the past year, underscoring the need for alternatives. Lithium mining has been criticized for its environmental impacts, including heavy groundwater use, soil and water pollution, and carbon emissions. By comparison, sodium is available in the ocean, cheaper, and more environmentally friendly.
Lithium-ion batteries typically also use cobalt, which is expensive and mined mostly in Africa’s Democratic Republic of the Congo, where it has significant impacts on human health and the environment. Last year, Manthiram demonstrated a cobalt-free lithium-ion battery.
The researchers plan to build on their breakthrough by testing it with larger batteries to see whether it can be applicable to technologies, such as electric vehicles and storage of renewable resources such as wind and solar.
Other authors on the paper include Texas Materials Institute postdoctoral fellows Jiarui He and Woochul Shin.
With the development of society and the depletion of natural resources, people have to start using renewable energy to develop low-cost and high-efficiency energy storage devices, such as secondary batteries. The ideal performance characteristics of energy storage devices are high energy density, high power density, long cycle life, low cost and high safety .
Among the existing secondary batteries, lithium-ion batteries have been industrialized, but their high cost, low practical energy density (100–200 Wh kg−1) and poor safety performance limit their application [2,3]. In order to meet the energy storage needs of current society, it is necessary to design and develop other batteries with lower cost, longer cycle life and higher energy density and power density.
Sodium is a low-cost alternative to lithium. The content of sodium in the Earth’s crust and water is 28,400 mg kg−1 and 1000 mg L−1, respectively, which far exceeds the content of lithium. The electrochemical reduction potential of sodium is −2.71 V, which is slightly higher than that of lithium (−3.02 V) , and is similar to the standard hydrogen electrode (SHE) potential .
When sodium is used as the anode, it can provide a battery voltage greater than 2 V when matched with an appropriate cathode. The high content, low cost and ability to provide high voltage make sodium an ideal choice for the anode materials of high-energy secondary batteries .
Sulfur has the advantages of strong oxidizing property, mature treatment technology, low cost, ready use , no toxicity and high capacity (when each atom transfers two electrons , the capacity of sulfur is 1.675 mAh g−1) , etc.
Sulfur has an attractive advantage over lithium as a battery cathode. Compared with lithium-sulfur batteries, sodium-sulfur batteries are a better choice from the perspective of sustainable development and economy, or from the perspective of battery preset performance .
The earliest sodium-sulfur battery was constructed in the laboratory of Ford Motor Company, and Kummer and Weber confirmed its feasibility . The battery uses sodium and sulfur as the active materials for the cathodes and anodes, and β-Al2O3 ceramics are used as both the electrolyte and the separator. In order to reduce the transmission resistance of sodium ions in the alumina solid electrolyte, it is necessary to ensure that the electrode material is in a molten state, so the working temperature is set at 250–300 °C.
Due to the advantages of long service life, high charging efficiency and high energy density, high-temperature sodium-sulfur battery systems have been used in stationary energy storage systems . However, in order to maintain the molten conductive state of the two poles, a high operating temperature is required.
The high operating temperature not only causes a loss of electrical energy, but also may cause the failure of the solid electrolyte, which causes explosions and fires due to contact between the cathode and the anode. These problems limit the wide application of high-temperature sodium–sulfur batteries .
In order to obviate the above problems, research has been directed toward the development of room temperature sodium-sulfur batteries. The first room temperature sodium-sulfur battery developed showed a high initial discharge capacity of 489 mAh g−1 and two voltage platforms of 2.28 V and 1.28 V .
The sodium-sulfur battery has a theoretical specific energy of 954 Wh kg−1 at room temperature, which is much higher than that of a high-temperature sodium–sulfur battery. Although room temperature sodium-sulfur batteries solve the problems of explosion, energy consumption and corrosion of high-temperature sodium-sulfur batteries, their cycle life is much shorter than that associated with high-temperature sodium-sulfur batteries. For a wider range of applications, its cycle performance needs to be improved .
Room temperature sodium-sulfur batteries have the advantages of high safety performance, low cost, abundant resource and high energy density [15,16]. They not only solve the safety problem of high-temperature sodium-sulfur batteries, but also solve the problem of high cost of lithium-ion batteries, and have received widespread attention. Like the lithium-sulfur battery system, room temperature sodium-sulfur batteries also face many problems, such as:
(1) Low conductivity of sulfur (5 × 10−30 S·cm−1) and significant volume expansion (180%) ; (2) capacity attenuation caused by the dissolution of intermediate polysulfide in the electrolyte; (3) short circuit caused by sodium dendrites piercing the separator; (4) low utilization rate of the cathode; (5) poor reversibility, etc. .
This article will start with a description of the electrochemical reaction mechanism for the room temperature sodium-sulfur battery, and describe the development of room temperature sodium-sulfur battery in recent years in terms of its cathode, electrolyte, separator design and anode protection.
Electrochemical Reaction Mechanism
The sodium-sulfur battery realizes the conversion between chemical energy and electrical energy through the electrochemical reaction between metallic sodium and elemental sulfur . When discharging, sodium metal produces Na+ and electrons. Na+ moves with the electrolyte through the separator to the sulfur cathode. Elemental sulfur is reduced and combined with sodium to form sodium polysulfide . When a certain voltage is applied to the external circuit, the reverse reaction of the decomposition of sodium polysulfide into metallic sodium and sulfur will occur. Figure 1 is a typical room temperature sodium-sulfur battery charge/discharge curve, with two potential platforms of 2.20 V and 1.65 V during discharge, and two potential slope discharge regions within the potential range of 2.20–1.65 V and 1.60–1.20 V. There are two potential platforms of 1.75 V and 2.40 V when charging. The above redox process corresponds to the cyclic voltammetry curve of a sodium-sulfur battery. In Figure 1, the two reduction peaks at 2.20 V and 1.65 V correspond to two discharge platforms, and the two oxidation peaks at 1.75 V and 2.40 V correspond to two charging platforms .
(a) Charge-discharge and (b) cyclic voltammetry (CV) curves of typical room temperature sodium sulfur batteries.
Among the more than 30 solid allotropes of elemental sulfur, the ring-shaped crown-shaped sulfur eight molecule (S8) is the most common and stable . According to the above redox process, the total reaction of the sodium-sulfur battery is:2Na + n8S8 ↔ Na2Sn (1 ≤ n ≤ 8)(1)
The actual charging and discharging process of room temperature sodium–sulfur battery is far more complicated than the above reaction (Figure 2).
First, the battery reaction involves a multistep reaction, which will produce a variety of polysulfide intermediate products with different chain lengths.
Second, the shuttle effect makes the system more complex, and the initial formation of soluble long-chain polysulfide (Na2Sx (4 ≤ x ≤ 8)), as the electrolyte diffuses to the anode, it is reduced to produce insoluble short-chain polysulfides (Na2Sx (1 ≤ x ≤ 3)) .
Schematic diagram of working mechanism and shuttle effect of sodium sulfur battery.
The short-chain polysulfide on the cathode side will also diffuse to the anode due to the effect of the electric field and the concentration difference and be reoxidized to long-chain polysulfide. The polysulfide dissolved in the electrolyte moves back and forth between the cathodes and anodes with the electrolyte, which is the shuttle effect .
It not only consumes active materials and reacts with metallic sodium, but also generates insoluble short-chain polysulfides that are deposited on the surface of the anode, hindering the transmission of electrons, resulting in low coulombic efficiency and reversible capacity of room temperature sodium-sulfur batteries.
By analyzing the X-ray photoelectron spectroscopy (Figure 3a,b) of the cathode after discharge, the final discharge product of the sodium-sulfur battery can be determined. The results show that after the battery was discharged, most of the sulfur was reduced. Combined with UV-visible light absorption spectroscopy (Figure 3c), it has proved the rationality that various forms of polysulfides can coexist in equilibrium .
Further through the calculation of the enthalpy of formation (DH), the thermodynamic stability of Na2S5, Na2S4, Na2S2 and Na2S and the metastability of Na2S3 were verified. Additionally, Na2S3 can be decomposed into Na2S2 and Na2S4 . Theoretical research on S, Na2S5, Na2S4, Na2S2 and Na2S through first-principles shows that these Na-S crystals are all potential products of room temperature sodium-sulfur battery charging and discharging.
The voltage curve of the Na concentration on the cathode of the sodium-sulfur battery (Figure 4a) was calculated by PBE-D2 (assuming that the Na concentration is the dependent variable of the discharge reaction), and the calculated main voltage regions were 2.09–2.11 V (Na2S5 and Na2S4), 1.79 V (Na2S2) and 1.68 V (Na2S) corresponding to the formation of the above-mentioned polysulfides [24,25,26].
Figure 4b shows the discharge curve of a room temperature sodium-sulfur battery . It can be seen that S8 has undergone a series of complex changes, completing the transformation from solid-phase simple substance to liquid-phase long-chain polysulfide and then to insoluble short-chain polysulfide.
From the analysis of thermodynamics and phase transition (the solid vertical line in the figure represents the theoretical capacity of each product), the discharge process can be divided into four parts according to the reaction steps.S8 (S) + 2Na+ + 2e− → Na2S8 (L)(2)
High-resolution S2p XPS spectra of the sulfur cathode being discharged to (a) 2.2 and (b) 1.2 V at a scan rate of 0.1 mVs−1. Insets are the voltammogram profiles of each electrode. (c) UV-Vis absorption spectra of the discharge product of the sulfur cathode that was discharged to 1.8 V.
(a) Voltages of a Na-S battery as a function of the Na concentration in the cathode. Voltages are calculated by PBED2 ((blue) open circles with dotted line) and experimental voltages ((red) broken line) for molten Na-S batteries measured at about 280–390 °C. (b) Discharge curve of room temperature sodium sulfur battery.
The solid-liquid transition from S8 to Na2S8 corresponds to the high voltage region of 2.20 V, and elemental sulfur is reduced to molten Na2S8.Na2S8 (L) + 2Na+ + 2e− → 2Na2S4 (L) (reactions that may be involved during the period: Na2S8 (L) + 2/3Na+ + 2/3e− → 4/3Na2S6 (L); Na2S6 (L) + 2/5Na+ + 2/5e− → 6/5Na2S5 (L); Na2S5 (L) + 1/2Na+ + 1/2e− → 5/4Na2S4 (L))(3)
Na2S8 is reduced to Na2Sx (4 ≤ x ≤ 5), and the most important product is Na2S4, which corresponds to the inclined area of 2.20–1.65 V. This area is most complicated by the chemical balance between various polysulfides in the solution.Na2S4 (L) + 2/3Na+ + 2/3e− → 4/3Na2S3 (S); Na2S4 (L) + 2Na+ + 2e− → 2Na2S2 (S); Na2S4 (L) + 6Na+ + 6e− → 4Na2S (S)(4)
Na2S3, Na2S2 and Na2S are generated from Na2S4, corresponding to a low voltage plateau of 1.65 V. The capacity and discharge voltage of this region depend on the competition among the three coexisting reactions .Na2S2 (S) + 2Na+ + 2e− → 2Na2S (S)(5)
The reduction process from Na2S2 to Na2S solid-solid two-phase, due to the insole ability and insulation of Na2S2 and Na2S, the kinetic reaction speed of this process is slow, and polarization may occur.
In general, the discharge process of room temperature sodium–sulfur batteries include the conversion of sulfur to long-chain soluble sodium polysulfide (Na2Sn, 4 ≤ n ≤ 8) and the conversion of long-chain sodium polysulfide to solid Na2S2 or Na2S. The reaction kinetics of the formation of solid polysulfides limits the discharge efficiency, resulting in irreversible capacity loss during cycling .
The actual discharge capacity (1050 mAh g−1) of the room temperature sodium–sulfur battery is between the theoretical capacities of Na2S2 and Na2S, and Na2S2 and Na2S are the least soluble compounds in organic solvents , so some researchers believe that their discharge products are Na2S2 and Na2S . Sometimes, it is also declared that the final discharge products are Na2S3 and Na2S2 .
reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7999928/
More information: Jiarui He et al, Stable Dendrite-Free Sodium-Sulfur Batteries Enabled by a Localized High-Concentration Electrolyte, Journal of the American Chemical Society (2021). DOI: 10.1021/jacs.1c08851