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Thought Leadership from Mewburn Ellis: Aviation battery systems and energy development

Words by Thomas Lons­dale, Patent Attor­ney and Helen East­mond, Patent Tech­ni­cal Assis­tant, Mew­burn Ellis

Bat­ter­ies are the most effi­cient way to pow­er vehi­cles using green ener­gy, so there is cur­rent­ly great inter­est in bat­tery-pow­ered air­craft for short-dis­tance flights. Nor­way, for exam­ple, has com­mit­ted to elec­tri­fy­ing all short-haul flights by 2040.

A key com­pa­ny work­ing on elec­tri­fy­ing flight is Rolls-Royce, who have devel­oped the world’s fastest all-elec­tric pro­peller air­craft. This small plane has achieved a speed of over 560 kph with a bat­tery con­tain­ing 6,480 cells, weigh­ing 700 kg and described as ‘the most pow­er- dense propul­sion bat­tery pack ever assem­bled in aero­space’. eVTOL air­craft envis­aged for short­er flights/urban mobil­i­ty often make use of pro­pri­etary bat­tery sys­tems.

One com­pa­ny work­ing in this area is Wisk, who have devel­oped a dri­ver­less eVTOL, with a range of 144 km and a speed of 225 kph. With six pro­pel­lors mount­ed on each wing, once aloft it flies like a small plane. A cen­tral issue for bat­tery pow­ered avi­a­tion is that bat­ter­ies are heavy, mean­ing they only store a frac­tion of the ener­gy per unit weight vs kerosene.

Typ­i­cal­ly, small air­craft oper­ate with an ener­gy den­si­ty of 250–270 Wh/kg; how­ev­er, experts pre­dict ener­gy den­si­ties of 350- 400 Wh/kg will be need­ed for the elec­tric avi­a­tion indus­try to real­ly take-off, with still high­er fig­ures required to make long jour­neys prac­ti­cal.

Which Bat­tery is Best?

Sev­er­al dif­fer­ent types of bat­tery are cur­rent­ly being explored for bat­tery-pow­ered avi­a­tion. Lithi­um-ion bat­ter­ies are the most wide­ly used bat­tery, e.g. the Tes­la Mod­el 3 uses a Li-ion bat­tery with an ener­gy den­si­ty of 260 Wh/kg. A num­ber of com­pa­nies are work­ing to use these in the sky.

Joby Avi­a­tion has designed a five-seat air taxi that can trav­el 240 km pow­ered by Li-ion cells that are already used in elec­tric cars. They hope to launch their eVTOLs in New York in 2025.

Li-ion bat­ter­ies tra­di­tion­al­ly have graphite and tran­si­tion met­al oxide elec­trodes, con­tribut­ing sig­nif­i­cant­ly to their weight. Recent­ly, the ener­gy den­si­ty of Li-ion cells has been improved with alter­na­tive elec­trode solu­tions, such as Amprius’ sil­i­co­nan­ode bat­tery which was mar­ket­ed with a record 500 Wh/kg this year. Fel­low man­u­fac­tur­er Ion­blox have also patent­ed sil­i­con-anode cells which are being used in eVTOLs.

Anoth­er bat­tery with big poten­tial is sol­id-state bat­ter­ies (SSBs). Using a sol­id elec­trolyte avoids the safe­ty issues, such as swelling and flam­ma­bil­i­ty, seen with Li-ion bat­ter­ies; as well as hav­ing the poten­tial to achieve high­er ener­gy den­si­ties.

NASA is devel­op­ing a sul­fur and sele­ni­um SSB which has achieved 500 Wh/kg in pro­to­type and can oper­ate in tem­per­a­tures twice that of Li-ion bat­ter­ies. Fur­ther work is required before SSBs move from basic research to com­mer­cial real­i­ty, but devel­op­ments in imag­ing and in situ char­ac­ter­i­sa­tion tech­niques are push­ing these bat­ter­ies for­ward.

The Fara­day Insti­tu­tion expects SSBs may make up over 10% of the glob­al mar­ket share for air­craft by 2040. Lithi­um-air bat­ter­ies could also find appli­ca­tion in pas­sen­ger avi­a­tion. Researchers at Japan’s Nation­al Insti­tute for Mate­ri­als Sci­ence have devel­oped Li-air bat­ter­ies with an ener­gy den­si­ty of more than 500 Wh/kg, which are also able to under­go repeat­ed charg­ing and dis­charg­ing cycles.

While Li-ion bat­ter­ies dom­i­nate for now, the avi­a­tion indus­try is often quick to take up new tech­nol­o­gy and in the longer term the indus­try is like­ly to shift to alter­na­tives such as SSBs or Li-air bat­ter­ies as they devel­op, unlock­ing high­er ener­gy den­si­ties and offer­ing oth­er advan­tages beyond the lim­its of Li-ion bat­ter­ies.

Com­pa­nies such as Ion­blox who have pro­tect­ed their intel­lec­tu­al prop­er­ty in this area are set to ben­e­fit as eVTOLs start fly­ing but there is still space for inno­va­tion as the indus­try turns to future tech­nolo­gies.

From L‑R: Helen East­mond, Patent Tech­ni­cal Assis­tant, and Thomas Lons­dale, Patent Attor­ney.

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Jason Pritchard

Jason Pritchard is the Editor of eVTOL Insights. He holds a BA from Leicester's De Montfort University and has worked in Journalism and Public Relations for more than a decade. Outside of work, Jason enjoys playing and watching football and golf. He also has a keen interest in Ancient Egypt.

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