— Long live lithium!

The queen of recharge­able bat­ter­ies shall reign for the fore­see­able fu­ture.

Cosmos - - Contents - ALAN FINKEL is an elec­tri­cal engi­neer, neu­ro­sci­en­tist and the chief sci­en­tist of Aus­tralia.

WHEN IT COMES TO mak­ing com­puter cir­cuits, sil­i­con is king. Con­tenders for the throne in­clude op­ti­cal switches, DNA, pro­teins, ger­ma­nium and graphene. Each has le­git­i­mate grounds to be con­sid­ered but has strug­gled in devel­op­ment, while sil­i­con-based com­put­ers have re­lent­lessly im­proved, such that the per­for­mance gap be­tween sil­i­con and its po­ten­tial usurpers has widened.

A sim­i­lar widen­ing gap has oc­curred in recharge­able bat­ter­ies. If sil­i­con is the king of elec­tronic cir­cuits, lithium is the queen of bat­ter­ies. I find it sur­pris­ing that one el­e­ment so thor­oughly dom­i­nates bat­tery tech­nol­ogy but, like king sil­i­con for cir­cuits, queen lithium has prop­er­ties that make it su­pe­rior to all the al­ter­na­tives.

Bat­ter­ies com­prise three es­sen­tial com­po­nents: the neg­a­tive ter­mi­nal (also known as the an­ode), the pos­i­tive ter­mi­nal (the cath­ode) and the in­te­rior soup of ions called the electrolyte, usu­ally a liq­uid or gel. When the neg­a­tive and pos­i­tive ter­mi­nals are con­nected through an ex­ter­nal cir­cuit such as your flash­light, the bat­tery dis­charges by driv­ing elec­trons from the neg­a­tive ter­mi­nal to the pos­i­tive ter­mi­nal, pro­vid­ing the en­ergy to gen­er­ate light. In­side the bat­tery, the cir­cuit is com­pleted by the flow of pos­i­tive ions through the electrolyte. In a lithium-ion bat­tery, these pos­i­tive ions are lithium atoms that have been stripped of an elec­tron.

Most of the bil­lions of dol­lars spent each year to de­velop bet­ter lithi­u­mion bat­ter­ies are in­vested in im­prov­ing the ma­te­ri­als for the ter­mi­nals and the electrolyte. The neg­a­tive ter­mi­nal has to act like a sponge, ab­sorb­ing and stor­ing as many pos­i­tively charged lithium ions as pos­si­ble. Graphite is most com­monly used, but vari­a­tions such as graphene that can ab­sorb even more lithium ions with­out swelling are ac­tively be­ing sought. The pos­i­tive ter­mi­nal is of­ten made from lithium cobalt ox­ide, though there are many al­ter­na­tives in pro­duc­tion and in devel­op­ment. The electrolyte in­cludes lithium salts such as lithium hex­aflu­o­rophos­phate.

What makes lithium spe­cial? For starters, it is the light­est of all met­als and the third-light­est el­e­ment, sit­ting in the pe­ri­odic ta­ble im­me­di­ately af­ter hy­dro­gen and he­lium. Fur­ther, of the met­als com­monly used for bat­ter­ies, lithium has the high­est ‘work­ing volt­age’ – the volt­age dif­fer­ence be­tween the neg­a­tive ter­mi­nal and the pos­i­tive ter­mi­nal.

This com­bi­na­tion of a high work­ing volt­age (up to 3.6 volts) and light weight con­trib­utes to lithium bat­ter­ies hav­ing the high­est en­ergy stor­age per kilo­gram, mak­ing them ideal for mo­bile ap­pli­ca­tions. Thanks to lithium-ion (Li-ion) bat­ter­ies, a Tesla car can get away with a bat­tery weight of 600 kg, com­pared with 4,000 kg or more if it were to rely on con­ven­tional lead acid bat­ter­ies.

Un­like lead acid bat­ter­ies, in use for more than a cen­tury, lithium-ion bat­ter­ies can be dis­charged down to about 10% of their rated ca­pac­ity with­out fail­ure, and do so thou­sands of times. They do not carry the curse of the mem­ory ef­fect that re­duces the work­ing life­time in nickel-cad­mium (Nicd) and nickel-metal hy­dride (NIMH) bat­ter­ies un­less they are fully dis­charged be­fore recharg­ing. Fur­ther, lithium-ion bat­ter­ies left sit­ting on the shelf will lose charge at a much slower rate than other bat­tery chemistries.

Like any chem­i­cal at high con­cen­tra­tions, lithium is harm­ful to hu­mans if in­gested but is oth­er­wise very low on the tox­i­c­ity scale; in fact, it is so rel­a­tively harm­less that for more than 50 years lithium car­bon­ate salt has been rou­tinely used as a medicine to treat bipo­lar dis­or­der.

Won­der­ful as they are, lithium-ion bat­ter­ies do have some draw­backs. For ex­am­ple, they lose peak ca­pac­ity af­ter a few years of op­er­a­tion. Fur­ther, be­cause of safety con­cerns, bat­tery packs must be made with com­plex pro­tec­tion cir­cuits to limit over­heat­ing and max­i­mum cur­rents.

It is dif­fi­cult to pre­dict where the next big bat­tery break­through will come from. The com­pe­ti­tion is in­tense and ad­vances are an­nounced daily. My money is on re­plac­ing the liq­uid electrolyte with a solid-state electrolyte that is a kind of glass. If suc­cess­ful, the solid-state electrolyte will al­low faster charg­ing, in­creased safety, up to three times the en­ergy den­sity and longer life­times.

Sound too good to be true? The lat­est an­nounce­ment from Toy­ota Mo­tor Corporation con­fi­dently claims it will in­tro­duce solid-state lithium-ion bat­ter­ies in 2022. While it is early days, solid-state lithium-ion bat­ter­ies can pro­vide a step change in per­for­mance that will give us elec­tric cars able to go 1,000 km and smart­phones that can be used for sev­eral days be­tween charges.

What makes lithium spe­cial? For starters, it is the light­est metal.

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