Lithium sulphur cells set to change the battery landscape

4 mins read

Since the invention of the battery by Volta in the early days of the 19th Century, users have been looking for more performance. Volta's device was superseded in 1859 by the lead acid battery – a device still in widespread use today. By the end of the 19th Century, the nickel cadmium battery had been developed.

It then took eight decades for nickel metal hydride technology to appear and to start meeting the need for smaller rechargeable batteries for the ever smaller consumer devices that were appearing. Better performance came in the early 1990s with the lithium ion battery. But, except for some academic projects and developments, by companies such as Nexeon, of more sophisticated anode materials, that's as far as the battery has come (see fig 1).

Meanwhile, users remain, on the whole, dissatisfied. Decreasing device size means smaller volumes for the battery. Even though power consumption has decreased, the battery operating life between charges remains annoyingly short for many. Batteries can be classified according to their specific energy, measured in Whr/kg. The higher this figure, the more charge the device can deliver. And it is this figure which battery developer Oxis Energy is addressing as it looks to commercialise lithium sulphur technology. Mark Crittenden, Oxis' customer brand manager, said the technology is different to that of lithium ion. "It comprises lithium, sulphur, an electrolyte and a separator. The key reaction is lithium combining with the sulphur to produce lithium sulphide. Because you get a lot more electrons – or charge – from the reaction than you do from the reaction in a lithium ion cell, lithium sulphur cells have a much higher theoretical energy density." Crittenden contended that energy density is five times greater. The basic construction of a lithium sulphur cell is not radically different from that of competing technologies (see fig 2). A lithium anode is matched with a cathode made from sulphur, carbon and a polymer material deposited on aluminium foil. "The cathode also includes some 'smart materials'," Crittenden noted, "but we aren't saying what they are."

Sulphur is a low cost material sourced as a bi-product of the oil industry. But sulphur by itself has very low conductivity, so incorporation of carbon improves the cathode's performance. Electrons from the reaction are transported through an electrolyte which differs significantly from that in a lithium ion cell. "Our electrolyte has a very high flash point – around 180°C," Crittenden claimed. "Compare this to the lithium ion electrolyte, whose flash point is between -4 and 4°C." And this is a major claim on behalf of the technology. Where lithium ion cells have been known to catch fire if charged or operated incorrectly, lithium sulphur batteries are seen to be much safer. Oxis claims its technology overcomes the risk of using lithium metal in a battery. One problem is the formation of dendritic – or mossy – lithium. These thin lithium fibres can reduce the amount of charge the cell can hold and, in some instances, create a fire risk through short circuits. An important part of a lithium sulphur battery's life comes during the first charge/discharge cycle. "Cells go through a formation cycle," Crittenden explained. "This initial cycle results in a lithium sulphide passivation layer being formed on the lithium anode. This has a melting point of 938°C, so it's a strong protective layer. When you compare the technology with lithium ion, the passivation layer and the electrolyte make lithium sulphur batteries much safer." Other important differences between Li-ion and lithium sulphur include the fact the latter can be fully discharged without damage to the cell and the nominal output of 2.1V, compared to lithium ion's 3.7V. Importantly, lithium sulphur cells should be much lighter. This latter point has attracted the attention of the UK's Ministry of Defence, which has awarded a contract to Oxis and battery specialist Lincad to develop lithium sulphur cells for UK forces. The average patrol soldier carries around 8kg of batteries to power various devices and the MoD is looking to halve this. Because lithium sulphur cells have a theoretical energy capacity five times that of lithium ion cells, the potential is there to reduce this weight significantly. Quintin Moore, Lincad's project manager, said: "This is an exciting programme which brings together the major benefits of Oxis' cell technology with Lincad's expertise in producing high energy density military power solutions to produce a solution that is not only inherently safe but, more significantly, will also reduce the soldier's burden." A further benefit is safety – that's important in a range of applications, said Crittenden. Oxis claims the cells can withstand a range of abuse, including extreme temperatures, short circuits and nail penetration. It also says it has shown a cell can be penetrated by a bullet and still work. Oxis was formed in 2005 to commercialise earlier academic work on the technology. Since being formed, the company has been granted 47 patents and has a further 30 pending. Many of those relate to the fundamental technology, but some address how you make the cells – and that's something to which Oxis is now turning its attention. "Until a year ago, we were an R&D company," Crittenden claimed. "Now, we're moving to commercialisation and expect the first lithium sulphur cells to enter production at the end of 2013 or early in 2014." Oxis has decided to take a licensing approach to the technology, rather than manufacture batteries itself. "Our first licensee is GP Batteries of Singapore," said Crittenden. "But there will be some production in the UK; we believe this is important from the technology and IP angles." In fact, Oxis will be making the cathode active material – the mix of sulphur, carbon and polymer which is applied to aluminium foil. "This is simple to produce," Crittenden explained, "and cost will be important." Overall, making lithium sulphur cells is broadly the same as the lithium ion manufacturing process. "It's about 70% similar," said Crittenden. "Making anodes, for example, is just a matter of cutting lithium foil to the right lengths to make a pouch cell." And it's likely that pouch cells will be the initial products made. "But I can see production moving to address cylindrical cells, particularly the 18650 format," he claimed. Oxis has ambitions beyond small batteries. "There are many applications for big battery packs," Crittenden pointed out. Because lithium sulphur cells have a non linear charge/discharge curve, such packs will still need electronic management similar to that found in lithium ion systems. But if the construction and control hurdles can be overcome, lithium sulphur could have a major impact on electric vehicles. While the technology has a theoretical density of 2700Whr/kg – and one research site claims to have achieved 80% of this – Oxis has achieved 350Whr/kg. "In the next few years," Crittenden concluded, "we expect to have exceeded 500Whr/kg. If we can do that, it will have a huge impact on many applications, including allowing electric vehicles to have an operating range in excess of 400 miles."