Metal–Air Battery

A metal–air electrochemical cell's anode is made of pure metal, while the cathode is connected to an infinite supply of air. In the electrochemical process, only oxygen from the air is used. Because of its greater specific energy excluding oxygen (theoretically 11.14 kWh/kg), the lithium air battery is the most enticing of the several metal–air battery chemical couples [22]. Due to lithium's high reactivity to air and humidity, it can catch fire, creating a serious safety risk. Only a zinc–air battery with a theoretical specific energy of 1.35 kWh/kg (without oxygen) is theoretically practical at the moment. It is difficult to design rechargeable zinc–air cells since zinc precipitation from the water-based electrolyte must be properly handled. Although a viable, electrically rechargeable metal–air system could offer low material costs and high specific energy, none has yet attained marketability [22–26].

#### Sodium–Sulphur Battery (NaS)

In sodium–sulfur batteries, a solid beta-alumina ceramic electrolyte isolates the active constituents (molten sulfur at the anode and molten sodium at the cathode). NaS batteries have a discharge time of 6.0 to 7.2 h and a standard life cycle of around 4500. They are both effective and quick to respond (round-trip efficiency based on AC is around 75%) [23]. Over 200 places in Japan have tested the NaS battery technology, largely for peak shaving. Many countries employ NaS batteries as well. Although the lack of a heat source is a significant drawback, with correctly sized insulation, the heat developed in the battery may be managed in frequent use by its own reaction heat. These batteries are suited for high-frequency cycling applications [27,28]. The construction of typical NaS battery module is depicts in Figure 6.

**Figure 5.** Typical Li-ion battery module.

**Figure 6.** NaS battery system.

#### Sodium–Nickel Chloride Battery (NaNiCl)

The sodium–nickel chloride (NaNiCl) battery, also known as the ZEBRA battery is a high-temperature (HT) battery that, like the NaS battery, has been available on the market since approximately 1995 [24]. NaNiCl batteries outperform NaS batteries in terms of safety and cell voltage, and they can withstand limited overload and discharge. These batteries have been employed effectively in a variety of electric vehicle designs, and they are a viable alternative for fleet applications. Upgraded variants of the ZEBRA battery with greater power density values for hybrid electric vehicles, as well as high-energy versions for conserving renewable power for load-leveling and industrial purposes, are presently being developed.

### 3.2.2. Flow Batteries

NASA invented flow batteries in the early 1970s as an EES for long-term space flights [25]. They have the potential to store energy for hours or days and have a power of many megawatts. Flow batteries are of two types: redox flow batteries and hybrid flow batteries.

#### Redox Flow Battery (RFB)

The electrolytes present at the negative and positive electrodes of a redox flow battery are anolyte and catholyte. During discharge, electrodes are continually provided with dissolved active masses from the tanks; once converted, the product is returned to the tank. During the charge exchange, a current flows between the electrodes, which may be used by a battery-powered device. Redox flow batteries are being studied for use in electric vehicles; however, electrolyte energy density has proved too low thus far. An RFB may potentially be "refilled" in minutes by draining out the emptied electrolyte and replacing it with recharged electrolyte. In RFBs today, many redox couples, such as a Fe-Ti system or a poly S-Br system, have been investigated and tested (shown in Figure 7) [27,28].

**Figure 7.** Schematic of redox flow battery.

Hybrid Flow Battery (HFB)

One active mass in a hybrid flow battery (HFB) is kept within the electrochemical cell, while the other is kept externally. The benefits of classic secondary batteries and RFBs are combined in HFBs. HFBs include the Zn-Ce and Zn-Br systems. The anolyte is a Zn2+ ion-acid solution, and the electrodes are primarily carbon-plastic composites. Exxon pioneered the Zn-Br hybrid flow battery in the early 1970s, and it is now being commercialized by a variety of companies. In addition, 5 kW/20 kWh community energy storage devices are also being developed [22,28].

#### *3.3. Chemical Energy Storage*

A chemical energy storage system is the only idea that allows for the long-term storage of significant amounts of energy, up to TWh, even as periodic accumulation. SNG and hydrogen may be used in a range of industries, including commuting, movement, heating, and the chemical industry. They have lesser overall efficiency than PHS and Li-ion storage technologies, but are more cost efficient and effective than ordinary batteries [26].
