Pocket power gets Nobel for chemistry

John Goodenough, M. Stanley Whittingham and Akira Yoshino have been awarded the Noble Prize in Chemistry for “creating the rechargeable world” by developing lithium-ion batteries. Interestingly, an Indian scientist Samar Basu, played a crucial role in the development of viable lithium-ion batteries. After retirement, he returned from the US to India and motivated research on Lithium batteries in Indian institutions.

Storing electricity

Otto von Guericke’s static electric generator and Michael Faraday’s dynamo showed how electricity can be generated. The generated electrical energy had to be transmitted through a wire and consumed as soon as it was produced. Until Alessandro Volta invented the battery, there was no way of storing or transporting it.

There are three essential elements in any battery - anode, the negative end of the battery; cathode the positive end of the cell; and electrolyte a gel-like substance with chemical energy. Apart from this, some have a fourth component, a separator that keeps the anode and cathode apart to prevent short circuit. The electrical power in the battery is stored in the form of chemical energy and released when the electrochemical reaction takes place.

The electrolyte liquid or paste-like substance contains electrically charged particles or ions. When in contact with the anode, the electrolyte undergoes oxidation reaction. Two or more ions combine with the anode to form a compound, and one or more electrons are released. Simultaneously, the cathode undergoes a reduction reaction with the electrolyte. Ions and free electrons combine with cathode and form compounds.

During the oxidation-reduction (redox) electrochemical reaction, free electrons congregate around the anode. As a result, the anode and cathode are negatively and positively charged, respectively. A potential difference between the two ends is generated. The electrons from the anode are itching to move towards the cathode. The separator keeps the electrons at bay, and the reaction is under stalemate.

However, once you place this battery, in a flashlight and flip the switch on, a new pathway between the positive and the negative terminal of the battery is established. The electrical charge moves through the wire, from one terminal to the other in the cell completing the circuit. On its way around, the current passes through the filament in the bulb. The resistance of the filament makes it heat up and radiate heat and light. Once the circuit is complete, the redox reaction continues to take place until the electrodes run out of reagents for their respective reactions. Once the stored chemical energy is used up, the electric current stops and the battery is ‘dead’.

Rechargeable batteries

Unlike ‘use and throw’ batteries used in a flashlight, typically the battery used in an automobile is rechargeable. The rechargeable batteries have unique materials as anode, cathode and electrolyte. When you plug such rechargeable battery into a power source, electrical current supplies electrons to the anode. Further, the electrons from the cathode are removed. The reverse chemical reaction restores the anode, cathode and the electrolyte to the near-original state, which we call as researching. Recharge is reverse of discharge of a battery.

One of the very widely used rechargeable batteries is lead-acid battery. In this battery the negative and positive plates are made of lead and lead dioxide respectively. The electrolyte, sulphuric acid, reacts with the plates to form lead sulfate. As more lead sulfate is produced, the charge in the battery goes down. When the battery connected to the power supply in the reverse direction, lead sulfate reverts to lead, lead dioxide and sulphuric acid and once again it is recharged.

Dynamos, dry cells, rechargeable lead-acid batteries were all adequate for the Industrial Revolution until the 1950s until the emergence of the semiconductor electronic devices. Electrical power was used typically in motor, electromagnet and vintage radio receivers made with bulky valves. The development of electronic devices required electrical power devices that are compact, potent and durable.

‘Use and throw’ zinc carbon battery will do for a flashlight. The rechargeable lead-acid batteries are excellent, to give a punch of energy at the turn of the ignition key to kick off the starter motor and crank the engine spring to life.

But think of a battery sitting inside a pacemaker, prodding the heart to tick. You don’t want that to stop forever. Nor the battery unit can be bulky. Consumer electronics devices such as electronic watches, toys, cameras, mobile phones and laptops also require robust, enduring batteries that pack more power in lightweight package.

Distinct chemistry of various types of batteries results in voltage output ranging from 1.0 to 3.6 V. By serially stacking cells, voltage can be multiplied and by parallel connection current can be increased.

By suitable combination, we can get the desired output. The problem was to find a battery that is light in weight, yet gives more punch of energy per kilogram of mass. Whittingham, Goodenough and Yoshino found the way. They share this year’s Noble chemistry prize for this radical discovery that made the mobile revolution possible. Compared to the energy density of 0.13 of zinc copper flashlight batteries, and the 0.14 of lead acid batteries, the lithium-ion batteries have a density of 0.70 Mj/Kg. While the lead-acid batteries can be recharged typically 500 times, the lithium ion batteries can be cycled 500–1000 times.

Lithium magic

With just three electrons and three protons, lithium is the third lightest of all elements. With two of the three electrons making a pair, lithium happily lets the third one wander away as a free electron. What’s more, the electron peels off easily compared to other elements. The energy needed to knock off one electron of lithium is almost half that of Zinc or Cadmium, other typical anode metals. Lithium-ion can store about 10 times as much energy as lead-acid or 5 times as much as nickel-cadmium. It is an excellent material for battery, but for the fact that it is dangerously reactive. Pure lithium burst into flames when it comes in contact with water.

M. Stanley Whittingham began experimenting with lithium as an anode material during the 1970s. Along with lithium anode, he used titanium disulfide as the cathode. In the discharge phase, when the battery was connected to a device, the lithium atom released an electron to become an ion. The positive lithium-ion moved towards the cathode. Titanium disulfide has a lattice structure and the ions snuggled between the layers. The circuit was completed, and the battery produced a 2 volts current. When the battery was recharged, the lithium ions flew back across the electrolyte to their starting position at the anode. Cathode and anode returned to its original state.

But there were two challenges. As lithium reacted violently, the anode had to be isolated from water and air. The electrolyte had to be a non-aqueous solution. Whittingham was able to identify a suitable organic electrolyte from other researches to overcome this hurdle. But the second one was serious. As the battery discharged and recharged, lithium crystals grow into a wispy, needle-like structure known as dendrites connecting anode and cathode. This was disastrous. Once such a defect forms, the battery short-circuited and at times even exploded.

Meanwhile, John Goodenough at Oxford was studying properties of metal oxides. He realized that a metal oxide can soak up more electrons than metal sulfide. He found that the cobalt oxide and the titanium disulfide both had similar lattice structure. Goodenough figured that like the titanium disulfide, cobalt oxide can also capture lithium ions during battery discharge and release it during recharge. In addition, cobalt oxide could house more ions than titanium disulfide. Energy potential doubled with this swap. Goodenough's design generated 4 volts, double that of Whittingham. Yet the problem of naked lithium remained.

Meanwhile, Samar Basu at Bell Labs in the US showed that lithium ions could embed in graphite. He developed a new battery with niobium selenide as cathode host and graphite as the anode host. The electrolyte was salt of lithium dissolved in an organic solvent. Both the anode and cathode could implant lithium-ion. Once the external circuit was switched on, the lithium ions were drawn from the graphite towards the niobium selenide, and the free electrons moved in the reverse direction. During the charging, the electrons could push the lithium ions back to graphite host. This was the first lithium-ion rechargeable battery where the lithium ions swung back and forth between anode and cathode during discharge and charge. As there was no free lithium, the battery was safer.

The next big step came when Akira Yoshino tried to use petroleum coke, a by-product of oil production, as an anode. The layers of carbon in petroleum coke could soak up lithium ions efficiently when charged. Goodenough's metal oxide cathode, Yoshino's carbon layer anode were combined to produce yet another version of the lithium-ion batteries. In the absence of pure lithium, the concerns of safety and dendrites formation vanished. The voltage was still just 4, but the new cocktail was safe, durable, lightweight and rechargeable. It could withstand hundreds of cycling. The lithium-ion battery technology matured. The new batteries hit the market around 1991.

(India Science Wire)

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[Images ©Johan Jarnestad/The Royal Swedish Academy of Sciences]