This paper presents an overview of room temperature Sodium-Sulfur batteries. Efficiency, safety and lifespan are challenging problems that face current energy storage technology. Room temperature Sodium-Sulfur batteries are a viable, low-risk and high-density energy storage option which are also considered a low-cost alternative for on grid energy storage and other electrochemistry practices. This overview regarding room temperature Sodium-Sulfur batteries focuses on the construction and operation, applications and comparisons with other types of batteries. The paper discusses Sodium-Sulfur batteries at room temperature due to the risks associated with high temperature Sodium-Sulfur batteries. This overview also highlights safety aspects and current age tests and inspection methods that are conducted and applied to ensure battery safety. Room temperature Sodium-Sulfur batteries are currently gaining attention because they have proven to provide high specific capacity at room temperature. The regions and segments alongside chemical equations of the charge and discharge process are also discussed in this paper. Several applications and comparisons are also made in this paper.
Introduction
Fossil fuels and usage of other traditional energy sources contribute to alarming critical issues such as climate change and global warming. These issues alongside the demand to alleviate the dependency on Greenhouse Gases and Fuels raise the need to develop and evolve high quality energy storage systems and devices [1][2].
The necessity and demand to use electrical storage devices and appliances has accelerated the development of energy storage systems and devices in recent years [3]. The need for lightweight and affordable energy storage devices and has caused the search and development for new types of batteries with technologies that surpass the widely used Lithium-ion intercalation chemistry [2]. Different kinds of batteries and a simple comparison between current technologies. The concept of Battery Energy Storage (BESS) is being widely used for its various power system applications, while the power systems in this modern age are rapidly increasing, renewable engines are now more of an obligation rather than a simple target; BESS can pre-occupy the perturbations and adjust its annual load factor. BESS can support very sensitive small voltage disturbances. Liquid crystals & semi-conductors require provisions to keep a high-power quality; hence, why BESS is at high demand [4].
Recent feasibility studies of several projects indicate that Sodium-Sulfur batteries are optimal for use in large scale (BESS) due to it being maintenance free and having a long life-cycle that can hold up to 15 years. Its high efficiency advances the development of Sodium-Sulfur for numerous energy storage systems that work under diverse energy laws and power devices for power applications [5].
Sodium is a resource that exists in most regions worldwide and is relatively cheaper than Lithium. The contents of Sodium and Lithium in the earth shell are 28,400 mg/kg, 1000 mg/L and 20 mg/kg, 0.18 mg/L, respectively. Sodium gives an in-cell voltage at a quantity that is larger than 2 V. The amalgamation of high voltage with low mass increases the likelihood of using Sodium as an anode in rechargeable batteries for obtaining high specific energy [6][7]. A supply of high energy density (theoretical specific energy density of 760 Wh kg-1) can be obtained by pairing a Sodium anode with a Sulfur cathode [1].
Current status of Sodium-Sulfur batteries
Current research has selected certain indicators such as storage capacity, energy density, efficiency and cost. They are currently being used as a set of fundamentals to discuss and compare different kinds of batteries and their application. Storage capacity is one of the most popular indicators which is explained as the energy stored in original reference conditions or the amount of energy that can be retrieved without harming the battery. Energy density, which is essentially the amount of energy you can fit per volume in batteries or energy storing devices. It can also be called specific energy which is defined as the energy per unit mass. Efficiency, commonly called “round trip” which is a very simple definition due to the fact that it neglects certain losses and losses measured during the process of charging and discharging, is a percentage or a ration between the energy that we use and withdraw from the battery to the energy that we store or put in the battery. The cost indicator is evaluated differently based on the type of operation or system needed. The cost could simply be a capital cost which are the funds needed to build and operate a certain process or system, and it could also be explained as other types of cost such as maintenance costs, insurance costs or manufacturing costs [8].
Temperature is an important parameter when Sodium-Sulfur batteries are discussed. The operating temperature is the difference between high temperature Sodium-Sulfur batteries and Room temperature Sodium-Sulfur batteries. A temperature of 300 °C must be maintained in order for Na-S batteries to operate and for the electrode materials to be kept in molten conducting state. Nonetheless, in order for the Na-S battery to be maintained at 300 °C, it must consume some of the electricity it generates, which in turn reduces the quality of the battery and its efficiency. Blasts and explosions are the aftermath and results of the cathode touching the anode due to a solid electrolyte failure [1]. Room temperature Na-S batteries have resurfaced recently after gaining some attention in the electrochemical and energy storage field due to some reports proving a high specific energy characteristic in room temperature. Room temperature Sodium-Sulfur batteries have been considered as an inexpensive alternative for grid storage applications, as a theoretical capacity of 1675 mAh g-1 due to using low-cost electrode materials has been obtained because sulfur is to accept two electrons per atom, theoretically [9].
Construction and Operation
Conventional batteries are manufactured with solid electrolyte membrane as the medium in-between the anode and cathode in comparison with liquid-metal Batteries where the anode, the cathode and membrane are liquidized. The sodium-sulfur battery-cell is conventionally manufactured in a towering cylindrical arrangement and is also protected from internal corrosion by a steel shell that is protected, using materials such as chromium and molybdenum. This outer shell functions as the positive electrode, the liquid sodium functions as the negative electrode. The shell is closed at the top and sealed with an air free alumina cap. BASE (beta-alumina solid electrolyte) membrane is important to have, due to it conducting NA+ in an arbitrary bias. As the battery-cell size increases, it becomes more cost-effective. In commercialized manufactured products, the battery-cells are made in block order for the purpose of reducing heat loss through convection and are usually confined in a vacuum insulated casing [10].
Safety
Requiring high temperature during operating (300–350 °C) is very dangerous and often times it could lead to corrosion that degrades the safety if not blasts and fires. This specific problem was the reason that developments started to occur with liquid electrolytes or solid polymer electrolyte membranes to provide high quality Na+ conductivity at room temperature for sodium-sulfur batteries and advance Na-ion batteries even more [13].
Leakage of the sodium or sulfur in the batter-cell could lead to a short circuit occurring. To validate the safety of the battery-cell, two tests are often conducted: The overcharge test and the short circuit test. The overcharge test evaluates the safety of the battery-cell if the solid electrolyte were to be destroyed by overcharging. The short circuit test evaluates the safety of the battery-cell should a short circuit occur, the test examines the cell, and how charged and overcharged it is until the reaction is over [14].
Applications
The use of lithium-ion sulfur (Li-S) batteries due to their high specific energy capabilities and eco-friendly nature has been an imperative factor for electric-vehicle applications. However, they were short in grid-related energy storage applications. The cost and operating temperature of the sodium-sulfur (Na-S) batteries were optimal to use, and due to their safety in overcoming their corrosive nature, they were selected as promising candidates to replace the Li-S batteries ever since 1980 [15] [1].
The research and the development of the (Na-S) sodium-sulfur batteries has taken place in 1983 in Tokyo Electric Power Company (TEPCO) following the first detailed release in 1966 by Ford Motor Company [16] [15]. Having a goal set to construct a sufficient energy storage power system for commercial use that needs high load-leveling applications, NGK insulators ltd has cooperated with TEPCO and started working on sodium-sulfur batteries by introducing the technologies linked with beta-alumina ceramic solid tubes for units up to 600+Ah and for storage cells up to 50kW for fixed power station [16]. By the year of 1992, an industrial program has been organized to support the use of sodium-sulfur since the materials are bountiful. In 2004, exciting development has been reached as NGK have been joint by the Department of Energy (DOE) and New York State Energy Research and Development Authority (NYSERDA) to demonstrate a prototype of a 1MW Na-S battery. In World Expo 2005, NGK participated by operating a power-plant with the help of the New Energy and Industrial Technology Development Organization (NEDO), which stored both night-time fuel cells electricity and photovoltaic-produced electricity. As of now, a variety of indications, such as the wind smoothing and wind leveling by Xcel energy in Luverne, Minnesota in 2008, powered by a 1MW Na-S Battery still verifies the efficiency of the system. Due to low temperature levels Minnesota can reach, NGK has manufactured and optimized a battery to withstand such temperatures and still be able to maintain a high level of performance [16] [17].
Following the massive earthquake that hit Japan in 2014, thermal power stations have been damaged by the incident, especially in northern Japan. The power could not be sustained due to the outage caused by this incident. Tohoku Electric Power (TEPO) has progressed and began installing 80MW Na-S batteries. It is anticipated by 2030 that the power station would decrease in peak demands and would adjust the supplies in Tohoku after the earthquake event [17].
Na-S batteries can encounter many problems such as operating in ambient temperatures, where they face serious challenges coming from both material and the power plant itself. Another major problem is the sulfur having low conductivity. Moreover, there is a non-correlated link between both sodium and the sulfur in the electrolyte. These problems would leave the plant running at a sluggish ratio and a presence of electroactivity [11].
NGK has developed a solution for inconsistent problems, such as the ones mentioned before. In 2008 – 2013, a 34MW (204 MWh) Na-S battery system connected to a windfarm in Rokkasho, Japan has been in constant work, where it focused on eliminating any inconsistencies and balancing the supply system by having a separator to block any shuttle effects that may occur and by confining sodium polysulfides [1]. It is said that the power station is still being developed to this day where they aim for extending its life cycle for up to 20 years (4,500+ cycles).
Na-S batteries are currently on the rise market wise, as they are trending in the U.S and will soon see the highest growth in utility-scale applications.
As things stand, both Japan and India are leading the race with over 100,000$ being used in projects. The current market is divided into four categories: The commercial being the highest with (29%), the electricity powered companies (26%), water treatment plants (23%) and industrial being the lowest (22%) [16][17].
Research works are taking place, with the Na-S battery’s overall condition, and the short installation time required, modules are being assembled for Electric Vehicles applications fueled by a 6kW battery. With the technology on the rise, stationary energy storage improvements are advancing [18].
Battery Comparison
The Sodium sulfur (Na-S) batteries are promising energy storage technologies and are considered a high factor in large-grid applications due to their potential as high-energy sources and their unique solutions such as: peak shaving, power outage and the quality to unneeded issues [19][20].
Many manufacturers are now considering using Na-S batteries as fuel cells for their projects, mainly because of their long-life cycles, low cost, and high energy density which stands 3 times higher than lead-acid batteries. The reason why lead-acid batteries are being ignored is for having both low energy-weight and energy-volume ratios, but due to their high surge current, they are known for their power-weight ratio, which makes them optimal for automobile starter motors [21][22]
A main problem with Na-S batteries is that they cannot be used in portable applications. Lithium ion (Li) batteries are applicable for such projects. That is because Li batteries give higher energy density outputs compared to Na-S batteries. However, they require two-times the size of Na-S batteries. Li batteries can be developed rapidly, and their battery management is getting improvements to optimize the cells to its low life cycle [19][18].
A main characteristic needed in this current technology, which can be found in Nickel-Hydrogen (Ni-H) batteries, is having a great specific energy (50-60) at a very long lifecycle which can reach more than 15 years and can withstand incidents such as overcharging and unwanted polarity reversals, improvements that are yet to be developed in NaS batteries [23].
A different kind of load balancing has been used is the Pumped Hydro System (PHS), which uses hydroelectric energy and stores them in a gravitational potential form. PHS plants have been in work for centuries, where the energy is pumped from a low altitude to a higher altitude. They are most applicable in intermittent sources (Solar, Wind), as well as excess sources (Coal, Nuclear, Geothermal) and many more. The PHS plants require a high level of response, frequency and adjustment to voltage regulations, thus, in recent technologies, Na-S and Li batteries have been integrated into the projects, due to their competitive electrochemical characteristics and for their quality in grid-optimization and stabilization. Moreover, the ability to move from the negative and to the positive electrode during the discharge has made the implementation of using the Na-S and Li batteries a must [24][25][26].All the research points in this current era is to fact a large energy storage system which would enable integration of a large amount of renewable energy to the existing grids. However, challenges and opportunities still remain for scientist and researchers in this field, due to its high complexity with the studies [25].
Conclusion
Room temperature Sodium-Sulfur batteries are a viable option for storing energy as a lost cost and efficient method. The applications of Sodium-Sulfur vary, and they are proven to be safer than high temperature Sodium-Sulfur batteries. The economical aspect of the batteries is not very clear to this day due to many reasons and due to the fact that other types of batteries lead the market, manufacturing and production wise. The safety aspect of the Sodium-Sulfur batteries is of importance as it can affect efficiency and other important characteristics. Room temperature Sodium-Sulfur batteries are still being researched and studied and showing optimistic results with various different kinds of technology.
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