This month, our columnist examines the architecture, advantages and limitations of the Lithium-ion battery. Energy storage in large quantities is important if we have to reduce our dependency on fossil fuels and adopt renewable power like solar and wind. Can the Lithium-ion battery power us into the future?
This year’s Nobel Prize for chemistry has put the spotlight on battery, a device that we have grown to take for granted. The “Baghdad Battery”, a 2200-year-old clay jar, is believed to be the oldest known electric battery. Since then, the battery has evolved remarkably to the present day Lithium-ion cell, a lightweight and rechargeable battery that is used to power everything from mobile phones to electric vehicles. Stanley Whittingham laid the foundation for the Lithium-ion battery during the oil crisis of the 1970s. John Goodenough made a substantial breakthrough on the battery’s cathode in 1980, based upon which Akira Yoshino made the first commercially viable lithium-ion battery in 1985. Whittingham, Goodenough and Yoshino were conferred with the Nobel Prize for chemistry, earlier this month, for their contributions to the development of lithium-ion battery.
Lithium Battery
Lithium-ion battery has numerous advantages compared to its predecessor, the lead-acid battery. They can charge and discharge at high rates and this offers the versatility required in many applications. Rapid charging minimises downtime. High discharge rate provides the perfect burst of power. Unlike lead-acid batteries, temperature fluctuations do not affect the power delivery of lithium-ion battery. Lead-acid batteries need regular water replacement to avoid structural damage and have a shortened lifespan if not maintained properly. In comparison, lithium-ion batteries do not need active maintenance. But the biggest advantage of lithium-ion batteries is their energy storage density. Compared to other battery chemistries, lithium-ion batteries provide same or more energy at less than half the weight and size. The biggest driver of the penetration of renewable energy will be a robust energy storage.
Battery Architecture
A typical battery uses cathode and anode, made out of metals or compounds having different chemical potentials. The flow of electrons from the anode to cathode through the external connection delivers the power. Electrons are produced inside the battery as a result of a chemical reaction. The electrolyte, which is a conducting fluid, is used to transfer the electrons internally within the battery from the cathode to the anode. In primary batteries, the chemical reaction that produces the flow of electrons cannot be reversed, whereas in rechargeable batteries the chemical reaction can be reversed.
“A battery by definition is a collection of cells. So the cell is a little can of chemicals. And the challenge is taking a very high-energy cell, and a large number of them, and combining them safely into a large battery.” – Elon Musk
Lithium Chemistry
Lithium-ion battery does not depend upon chemical reactions that break down the cathode and anode, but upon a flow of lithium ions back and forth between the anode and cathode. Whittingham’s battery used cathode made from titanium disulphide and anode from metallic lithium. The ions released from the metallic lithium were intercalated inside the molecular-sized spaces of the cathode. The high reactivity of metallic lithium however made this battery very vulnerable to fire and explosion. Goodenough discovered that cathodes made out of oxides of a transition metal like Cobalt or Nickel improved the battery’s performance substantially. The first commercial lithium-ion battery developed by Yoshino used anodes made from petroleum coke instead of the reactive metallic lithium and were able to intercalate the lithium ions.
Electrolyte Hazard
The most commonly used electrolytes for lithium-ion batteries are based on LiPF6(Lithium Hexafluorophosphate) and mixtures of carbonate solvents. The carbonates, such as dimethyl carbonate, ethyl methyl carbonate, or diethyl carbonate are required to keep the electrolyte viscosity low and the electrolyte conductivity high. Unfortunately, they are highly volatile and flammable and have flash points close to room temperature. One of the strategies to mitigate the fire hazard associated with the electrolyte is to use non-flammable electrolytes. Another option is to use flame-retardant additives. However, these additives affect the electrochemical performance of batteries negatively by deteriorating the cycling stability and/or the rate capability.
Solid Electrolyte
Researchers are exploring the possibility of replacing the flammable liquid electrolytes by solid electrolytes. The challenge is the low mobility of lithium ions through the solid electrolytes will limit the battery performance in terms of charge and discharge rates. Researchers at the Universite Catholique de Louvain, Belgium have achieved a breakthrough that could enhance the mobility of lithium ions. Their new compound is known as LiTi (PS4)3 or LTPS. It has a unique crystal structure with the highest lithium diffusion coefficient ever measured in a solid. LTPS holds the promise to be a new high performance and safe solid electrolyte in lithium-ion batteries. The discovery of LTPS opens up new avenues to search for similar materials with even better lithium diffusion coefficients thus paving the way to build safer solid-state batteries of the future.
Alternative
Lithium-ion batteries have already been in existence close to three decades now. And it has got better and cheaper. Manipulating the cathode material has increased the storage capacity of the battery. The battery cost has plummeted from $1160 / KWH in 2010 to $175 / KWH last year and is expected to further go down to $100 / KWH by 2024. While the fall in price is opening up new markets for the lithium-ion batteries, not everyone is convinced that the road to future is paved with lithium. Automobile manufacturers are hoping and praying for a breakthrough in battery technology that will enable their EVs to travel 800 kms on a charge. Bulk storage of renewable power also requires alternative battery technologies. At present lithium-ion batteries plugged into power grid can supply energy for just about four hours. This is woefully inadequate for integrating large quantities of renewable energy. Carbon dust and silicon dust have shown to boost the storage capacity of lithium-ion battery. But even as researchers are exploring alternate materials and examining radically different architectures for the battery, the worldwide manufacturing capacity for lithium-ion batteries has tripled in the last five years.
Epilogue
Catastrophic climate change has created a new urgency to accelerate adoption of green energy. Renewable energy like solar and wind are not only intermittent but also have a geographic diversity. As we move into a new energy regime with diminished dependency on fossil fuels, we will need bigger and powerful batteries. Robustness of batteries will be the key to a rapid and smooth switch to cyclic forms of energy like solar and wind.
Readers’ responses may be sent to k.sahasranaman@gmail.com or chemindigest@gmail.com
