BU-210a: Why does Sodium-sulfur need to be heated

Learn about the battery that only operates when heated

Sodium-sulfur batteries, also known as sodium beta-alumina battery (NBB), molten salt or high temperature ceramic batteries, come in secondary versions only. The battery is characterized by a ceramic solid electrolyte and liquid electrodes: sodium in the anode and sulfur at the cathode. High conductivity is obtained by heating the stack to a temperature of 270–360°C (518–680°F).

Electrodes are solid are room temperature and therefore inactive when cold and the battery can be stored for more than 50 years in that state. The battery must be activated with a heat source and the electrodes become liquid. Good ionic conductivity of the molten salt facilitates high power and long life.

The rechargeable sodium-sulfur (NaS) gained worldwide attention during the 1970s and 1980s. Today this battery is successfully deployed in many applications, mainly to distribute supplementary energy during high usage periods or to provide renewable energy when necessary.

The Molten Sodium Ceramic battery comes in two chemistries: NaS and ZEBRA. ZEBRA originally stood for Zeolite Battery Research Africa but is now known as Zero Emission Battery Research Activities. Both batteries are based on similar technologies with these characteristics:
  1. The electrolyte is a solid  beta-alumina ceramic
  2. The anode is composed of pure sodium
  3. The ions are Na+ travelling through the beta-alumina 
  4. Battery performances are optimal around 300°C.
The NaS battery is different in that the cathode is sulfur; ZEBRA is sodium-metal halide, which is commonly nickel chloride NiCl2. A secondary liquid electrolyte improves the performances of ZEBRA batteries, generally molten sodium tetrachloroaluminate NaAlCl4.

Molten Sodium Ceramic Battery

Figure 1: Molten Sodium Ceramic battery
Source: NaS Energy Storage System

The cathode material governs the cell voltages: NaS has a nominal voltage of 2V; ZEBRA delivers 2.58V. The specific energy is 200–250Wh/kg, a level that is comparable with Li-ion. Manufacturers of modern NaS batteries claim a cycle life of 4,500 and a life time of 15–20 years. The C-rate loading is relatively low between 1/4 and 1/8 (0.12 to 0.25C). NaS and ZEBRA batteries can be fast charged, are non-toxic and the raw materials are abundant and low-cost. These batteries come in sizes of 10kWh or higher; typical applications of ZEBRA are forklifts, railways, ships, submarines and electric cars. NaS batteries typical come in 200kWh modules that allow stacking. 

In 1967, Ford Motors fitted a car with the Molten Sodium Ceramic battery. The Think City car was available with a ZEBRA or a Li-ion battery. Molten Sodium Ceramic batteries have an advantage over regular batteries when operating in a hot climate and when the battery is in continuous use, such as in taxis and delivery vans. However, the main application is in load leveling, also known as grid storage and Battery Energy Storage Systems (BESS).

In 1989, NGK in Japan added a thin Beta Alumina ceramic electrolyte membrane between the molten materials. The ceramic membrane only allows positively charged sodium ions to pass through during discharge from the negative molten sodium to the positive molten sulfur. Charging reverses the process. In 2002, NGK went into production with a commercial NaS stationary battery for grid support of renewable energy. Figure 2 illustrates such an installation.

Large grid-scale sodium sulfur (NaS) batteries
Figure 2: NGK developed large grid-scale sodium sulfur (NaS) batteries around the world. NaS represents about 3% of the total electrochemical grid storage.
Source: NGK

The Molten Sodium Ceramic battery must be heated to 270–350°C (518–662°F). Even with special insulation that minimizes heat loss, heating consumes 14 percent of the battery’s energy per day if not used. But with one cycle per day, ohmic losses are enough to maintain the system in operational range, no heating is then necessary. On-the-field experience shows that system’s heating corresponds to about 2% of losses for about 300 cycles per year. A cool down takes 3 to 4 days; depending on SoC, reheating is about 2 days.

Common failures are electrical shorts due to corrosion of the insulators, which then become conductive, as well as growth of dendrites, which increases self-discharge. ZEBRA batteries are safer than sodium-sulfur, and an electrical short does not cause a complete failure of the battery.

According to RWTH, Aachen, Germany (2018), the cost of NaS battery system is about $525 per kWh. Fraunhofer IKTS is developing a new ZEBRA battery with a target price of €100 per kWh for the cells, with material costs below $30 per kWh. The NaS battery has a favorable cost of ownership for power durations of 4 to 8 hours and longer at full capacity. Li-ion, the strong contender, is wining market on 1h to 2h of discharge. Lead acid batteries are normally used for standby operations that require minimal cycling.

Last updated 2018-12-13

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Comments (1)

On September 22, 2016 at 8:06am
Preethi wrote:

can we use this zebra battery for energy storage in power system applications