Like almost all rechargeable consumer electronics, Samsung's Galaxy Note 7 uses a lithium-ion battery, where lithium ions are used to move charge between the battery's positive and negative electrodes. The ions are carried between the two in a liquid electrolyte, typically a lithium salt. In the event of a short circuit, that highly flammable liquid electrolyte can heat up and even catch fire.
While it's still too early to tell what went wrong in Samsung's case, the company blamed a rare manufacturing error that "placed pressure on plates contained within battery cells" and "brought negative and positive poles into contact".
Solid-state batteries work in a different way to lithium-ion batteries. Rather than using that volatile liquid electrolyte, they use a solid material that's stable over a far larger range of temperatures -- meaning the risk of overheating and fires pretty much goes away.
Research into solid state batteries has been going on for almost 20 years, but the electronics industry still relies on traditional lithium-ion batteries. That's in part because ions need to be able to move freely between the anode and cathode -- the negative and positive electrodes of the battery -- and liquids allow far more of that free movement than solids do.
"The frustration with the solid state is that you have to have very, very high ion mobility or you're only going be able to draw tiny current from the battery, so the question is how do you get the robustness, the mechanical stability, and the thermal stability of a solid without sacrificing the high ion mobility of a liquid?" Professor Donald Sadoway, professor of materials chemistry at the Massachusetts Institute of Technology (MIT), told ZDNet.
Researchers at MIT have been working on various aspects of the battery chemistry, creating a polymer that works not only as a solid electrolyte but also as a separator between the two electrodes, meaning they won't short if they're pressed against each other.
The aim, says Sadoway, was to "develop a material that has solid-like mechanical properties and liquid-like electrical properties". Not only does the polymer offer that, it remains stable in the presence of metallic lithium, so the graphite on the anion can be replaced with lithium metal. The end result? The electrode is smaller, so the battery itself can also be made smaller. That gives electronics companies the choice of either reducing the size of the battery (and therefore shrinking the hardware overall, given how much of device real estate is given over to the battery) or having a solid-state battery that's the same size as a traditional lithium-ion one but offers greater energy density.
How much more energy density solid-state batteries offer compared to their traditional lithium ion equivalents for a same-sized battery is a matter of discussion, but companies in the field have reported between 20 to 30 percent and 100 percent.
Increased energy density is not the only way that solid-state batteries can offer space savings compared to the current generation of battery tech: LiPON (short for lithium phosphorous oxynitride), another electrolyte popular among several solid-state battery companies, can be made in thin film form factors, so the batteries can be incorporated in the tiniest and slightest of devices.
Oak Ridge National Laboratories, the organisation behind the LiPON electrolyte, has also reported a solid-state battery that went through 10,000 cycles with a higher voltage than conventional lithium ion batteries can manage -- 5.1V compared to 4.3V -- losing only 10 percent of its capacity in the process.
"Such a battery has a cycling lifetime of more than 27 years with a daily charge/discharge cycle, exceeding the lifetime of most devices and even vehicles," Oak Ridge National Laboratories said.
It's no wonder that solid-state batteries have been described as "almost perfect" by MIT. Why then has this smaller, higher voltage, more energy dense, less volatile, less flammable perfection yet to reach the market?
For one simple reason: cost. "Solid state battery tech needs to be cost-competitive with what's out there. If I give you a solid state battery that costs 10 times what a conventional battery costs, it's not going to go anywhere. We have to invest with cost in mind: the material needs to be affordable and the manufacturing process needs to simple," MIT's Sadoway said.
The cost-benefit ratio may be swinging in the favour of solid-state batteries.
"We have to invest with cost in mind: the material needs to be affordable and the manufacturing process needs to simple. That's where some of the manufacturing problems occur with the lithium-ion. It's an interesting chemistry and it's served us well, but if you take a look at lithium-ion manufacturing plant, it looks like a semiconductor fabrication facility, it's very complicated," said Sadoway.
While the electrochemistry needed to make solid-state batteries a reality is pretty much here, and with costs coming down, where might we see them appear first?
For now, the expense related to solid-state batteries means it will be a niche proposition. One example of an industry where solid-state batteries could thrive is drilling. This is because they're able to operate over a far wider range of temperatures than conventional lithium-ion, solid-state batteries can help power equipment at higher temperatures without risking safety. For similar reasons, biomedical device manufacturers could adopt solid-state batteries for implantable hardware for use within the human body.
Beyond that, electric car makers are also thought to be investing in the technology -- as well as making electric vehicle batteries lighter, not having a flammable electrolyte has obvious advantages in the event of a crash. Researchers working in collaboration with Toyota recently published a paper on a solid-state battery that uses sulfide superionic conductors and can charge and discharge in three minutes, for example.
From there, solid-state batteries will begin to make their way into consumer electronics -- and yes, even phones.
"I suspect by the end of the decade we're going to see the first adoption of solid state battery," said Sadoway. "I think it's really moving from the lab bench to the manufacturing facility."