24M, a startup battery company founded as a spin-off from MIT, claims it has made a breakthrough in creating semi-solid lithium-ion battery cells with an energy density exceeding 350Wh per kg.
MIT professor yet Ming Chiang hit upon the idea of mixing active materials in electrolytes together before forming the cell, rather than ‘injecting’ the electrolyte into a slurry, thought to be a much more efficient process for creating cells.
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In other words, compositionally distinct catholytes and anolytes are created, kept apart by an ionically conductive, non-permeable separator.
“Unlike a conventional technology where they use a solvent and sort of deposit the material and then throw it all up and then inject the electrolyte at the end, we mix the electrolyte at the beginning,” Joe Adiletta, senior director of products for 24M, said.
“Of course we mix the anode and the cathode separately so we can put a different electrolyte in the anode, called the anolyte or the cathode – catholyte. Typically the separator not only allows the ions to pass but allows the electrolyte to pass. So if you use a traditional separator, eventually the materials will diffuse and you get a homogenous mixture, which is exactly what you don’t want in this design.”
Keeping the anolyte and catholytes separated, Adiletta said, can open up “huge options” on which electrolytes to choose, meaning individual batteries or systems can be tailored to “specific approaches in the battery”. This means the ‘dual electrolyte’ technology can be applied to LFP cells, more commonly used in stationary storage, or NMC cells, higher energy density and typically used for electric cars, to give two prominent examples.
24M will open up its first pilot production line towards the end of this year, which Adiletta stressed is small, at 100MWh, but is nonetheless the first high volume production of this type of technology. This way of making higher energy density versions of lithium batteries will also enable ways to “tune the electrolyte, improve the voltage, make fast charge, slow discharge technologies – there’s a huge number of options,” Adiletta said.
liquid-solid versus solid-on-solid
The first projects to emerge using the technology are likely to be pilot projects and demonstrations. At 24M’s small pilot production line in Cambridge, Massachusetts, the company has already made quantities of cells that have been through “thousands and thousands of cycles”, but of course has not yet had the opportunity to work directly on applied projects.
The semi-solid approach is distinctly different from solid state technology, which some researchers are now looking at. Some big investments have been made in solid state, which is being looked at in the same way that solid state hard drives revolutionised computing.
“We’re not solid state in the traditional sense. Solid state battery technology has been around for about 20 or so years. The concept is to use a solid electrolyte rather than liquid electrolyte. The key advancement that people are looking to accomplish with that is to enable the active material to be lithium metal in the anodes,” Adiletta said.
Solid state batteries have experienced difficulties in finding solid materials with enough electrical conductivity, as well as suffering issues with cathode-anode separators, which 24M claims to have solved – or to be in the process of solving.
“As the battery charges and discharges, the active material changes size and shape, so they expand and contract, through that charge-discharge. What has happened, and as far as we can tell continues to happen – which is why solid state batteries are five years off or more, at the most optimistic projections – is that you form an interface in that boundary, because you’ve got solid on solid. Over time you form a gap and that gap is sort of a huge issue, a big problem because there’s an interface resistance and you’ve kind of lost access to the active material. That’s been the bane of solid state batteries forever. The promise is a lithium metal battery that cycles,” Adiletta said.
Instead, 24M has developed a liquid-solid interface, hence ‘semi solid’. According to the company, 24M can pick and choose specific materials for anode and cathode sides of the battery cells suitable for each application. The company said the Dual Electrolyte solution could improve cycle life, safety and cost as a consequence. While admitting the commercialisation of this technology likely lies a few years off from today, 24M is particularly excited about the prospect of using the semi solid tech to service growing longer duration applications for energy storage, taking lithium-ion batteries comfortably beyond the typical 1-4 hours of energy storage it is commonly used for.