CHEMICAL ENERGY STORAGE 1: THE BATTERY

A battery is a device that converts the chemical energy of its cell components into electrical energy. A battery consists of a number of cells assembled in a common container and connected together to function as a source of electrical power. A battery consists of these basic elements; the anode, cathode, separator and the electrolyte.

The anode is the negative electrode and is made up of a material that undergoes oxidation during the cell discharge. The cathode is the positive electrode and is made up of a material that undergoes reduction during cell discharge. The separator or semi permeable membrane separates the anode from the cathode. While the electrolyte is the medium of ion transfer and the medium is usually liquid but can be solid, through which ions move from one electrode to the other during cell discharge.

There are two types of batteries; the primary and secondary battery. The primary batteries are non-rechargeable and are discharged once and discarded. While secondary batteries are rechargeable, that means they can be recharged electrically after discharge to their original condition by passing current through them in the opposite direction to that of the discharge current.

Examples of primary batteries are alkaline or zinc manganese dioxide, Zinc-carbon and mercury oxide batteries. Examples of secondary batteries are lead-acid, Nickel-Cadmium, Nickel metal hydride, lithium-ion, lithium iron phosphate and so on.

Battery technologies can be evaluated by the comparing their vital characteristics; like power or energy density, round trip efficiency, life span, cost and safety of use. Energy or power density is defined as the amount of energy that can be stored in a single system per unit volume or per unit weight or volume (watt-hour per kg or watt-hour per liter). The round trip efficiency is the charge and discharge efficiency in percentage. The average lifespan measured in years. The average cost per kilowatt-hour in dollar terms and the safety condition of the batteries during operation, which is classified as low, moderate and high.

The energy density of the following battery technologies are listed thus: lithium-ion batteries are between 100 to 150 watt-hour per kg or 250 to 350 watt-hour per liter. Nickel cadmium batteries are between 40 to 60 watt-hour per kg or 100 to 150 watt-hour per liter. Nickel metal hydride batteries are between 60 to 120 watt-hour per kg or 150 to 300 watt-hour per liter. Sodium sulfur batteries are between 150 to 200 watt-hour per kg or 350 to 500 watt-hour per liter. Lead acid batteries are between 30 to 50 watt-hour per kg or 100 to 150 watt-hour per liter. Lithium iron phosphate batteries are between 90 to 120 watt-hour per kg or 220 to 300 watt-hour per liter.

The round trip efficiencies of the batteries are as follows: A lithium ion battery is between 90% and 95%. Nickel cadmium batteries are between 70% to 80%. Nickel metal hydride batteries is between 70% and

80%. A sodium sulfur battery is between 85% and 90%. A lead acid battery is between 70% and 80% and a lithium iron phosphate battery is between 90% and 95%.

The lifespan of the batteries are as follows: lithium ion battery is between 10 to 15 years. Nickel cadmium battery is 10 to 20 years. Nickel metal hydride battery is between 10 to 15 years. Sodium sulfur battery is 10 to 20 years. Lead acid battery is 5 to 10 years and lithium iron phosphate batteries are between 10 to 20years.

The cost implication of these batteries in dollar term is as follows: lithium batteries are between $150 and $300 per kilowatt-hour. Nickel cadmium batteries are between $50 and $150 per kilowatt-hour. Nickel metal hydride batteries are between $50 and $200 per kilowatt-hour. Sodium sulfur batteries are between $100 and $300 per kilowatt –hour. Lead acid batteries are between $50 and $150 per kilowatt-hour. Lithium iron phosphate batteries are between $150 and $400 per kilowatt-hour.

The safety of batteries is classified according to its risk level during operation, as low, moderate or high as follows: lithium batteries safety ratings are considered to have moderate. Nickel cadmium batteries are considered low. Nickel metal hydride, sodium sulfur and lead acid batteries safety rating are considered moderate. While lithium iron phosphate batteries safety ratings are considered high.

The trend in batteries design and development are drifting towards higher density and lower cost, higher efficiencies, longer lifespan and more importantly higher safety.

The benefit of batteries to the modern world cannot be overemphasized, they are indispensable in the following applications; renewable energy integration (by acting as energy buffers or storage medium), energy independence (by reducing grid reliance), peak demand reduction (hence reduced energy or utility bills), improved power quality (by stabilizing voltage and frequency fluctuations) and environmental benefits (by supporting the transition to clean energy).

The challenges are obvious and they are; high upfront and installation cost , limited energy density or storage capacity, limited life cycles and safety concerns as regards thermal runaway, explosions, recyclability and other environmental issues.

Despite all this drawbacks batteries still find numerous applications as primary batteries in the following:  As alkaline batteries used for flashlights, toys, and portable devices. As zinc carbon batteries used in inexpensive low drain devices. As mercury batteries used in thermometer. As silver oxide batteries used in watches, hearing aids and medical devices.

Secondary batteries find application in the following: As lead acid batteries used in automotive systems, backup power and renewable energy systems. As lithium ion batteries used in electric vehicles, portable electronics and renewable systems. As nickel cadmium batteries used in power tools, two way radios and navigation. As nickel metal hydride batteries used in hybrid vehicles, cordless power tools and renewable energy system. As zinc air batteries used in hearing aids, medical implants and long duration energy storage systems.

It is obvious that the trend for the near future for batteries will revolve around the following technologies; solid state batteries ( because of their improved energy density, charging speed and safety), lithium air batteries ( because of improved energy density), sodium ion batteries (because of their  lower cost compared with lithium ion with same energy density).

Other battery technologies in contention for the far future are lithium air, zinc air , grapheme based batteries, lithium ceramic, magnesium ion, zinc air and probably biodegradable or bio batteries for sustainable biodegradable energy storage  and so on.

 

SOURCES:

• Electrochemical energy storage and conversion by Y. Wang and X. Li
• Battery technology: A handbook for engineers and innovators by D. Linden and T. Reddy.
• Battery materials: fundamentals and applications by J. Zhang and X. Li.
• Solid state electrochemistry by V. Thangadurai and W. Weppner.
• Advanced battery materials by Y. Wang and J. Liu.