20 November/December 2023 | E-Mobility Engineering “But more cathodes are coming out, like LFMP [lithium iron manganese phosphate] and NMCA [nickel manganese cobalt aluminium]. And across the other ingredients there are new things like silicon anodes, inorganic electrolytes and ceramic separators, which could all significantly change cell characteristics like voltages, amphours, and thermal behaviours.” Xerotech therefore avoids sticking too closely to one cell chemistry or supplier. That said, the company exclusively uses 21700 (or 21700) cylindrical cells, which are 70 mm long, 21 mm in diameter, weigh 70 g net and with a 5600 mAh maximum capacity. “21700s are generally the most widely available cell for different chemistries,” Dr Flannery says. “They have their limitations, in that some future components like solid-state electrolytes or ceramic separators might not always be machine-wound well enough for cylindrical formats. “Prismatic and pouch cells also generally have a longer cycle life, because they can use more viscous electrolytes as you don’t have to inject them into a tightly packed can. But cylindrical cells have the best robustness against thermal runaways thanks to their metal cell walls – prismatic and pouches just can’t match that.” NMC, LFP and NCA cylindrical cells are the most common in Hibernium modules for their technological maturity and commercial availability. Given LFP’s often-touted safety benefits for risk-averse heavy industry operators, it might be assumed that LFP is inherently the best of these for off-highway, but Dr Flannery cautions against this. “We’ve carried out a lot of safety testing to understand all the cells’ behaviours across all temperatures, SoH and SoC bands, all vibration and harshness conditions, and so on, and our BMS reacts accordingly with our thermal and electrical controls to give an equivalent level of safety, whether it’s a pack built on LFP cells or the highest energy density NCA cells,” he says. “Cells also have different failure modes. For instance, you generally get less flame in a thermal runaway with LFP than other cathode chemistries, but a lot more gas, which can be even worse if you’re underground with significant hydrogen build-up. There, it could be better if the vented material’s burning so workers aren’t trapped underground with a gas build-up, or the ATEX risk of a small hydrogen explosion.” On examining the use case, Xerotech has found that heavy-duty off-highway work needs the maximum energy possible, owing to the sheer sizes of the vehicles and the heavy lifting required, making NMC chemistries optimal for most customers. “Maybe that’s unique to us because of our thermal and safety technologies, but there’s a real lack of education in the industry as to what data sheet parameters mean in practice,” Dr Flannery remarks. “Should we supply someone with an LFP pack with 60008000 cycles, or a 40% more energydense NMC pack and then manage it for a smaller depth of discharge [DoD] window to achieve the same cycle life? “In our view, OEMs will want either an energy pack, a power pack or a long cycle-life pack. And with new automotive standards requiring that smoke from a battery pack cannot enter the passenger compartment within 15 minutes of a thermal runaway being detected – which might be longer in future – we’ll pick and foster whatever future cell innovations make sense for each of those requirements, along with alternatives for supply chain resilience. We think that’s how most of the market sees it as well.” Among upcoming cell technologies, Xerotech is looking with anticipation at inorganic electrolytes and sodiumion chemistries. The former does not flame (and Xerotech has announced a partnership with Innolith for its inorganic electrolyte cells), while the latter stands to bring down pack prices. It also expects to work with 4680 cylindrical cells, pending wider commercial availability and sufficient in-house testing to characterise their performance and safety. Active thermal management The Xerotherm system of ducts runs longitudinally between every other row of cells in the Hibernium modules. The ducts are made from ultra-thin plastic, and inflated with water-glycol according to the BMS’ commands to the coolant pump. Xerotech says a key advantage of this versus typical sidewall cooling is the open and flexible nature of the duct, which means that the coolant pressure drop Dossier | Xerotech battery system At the time of writing, Xerotech was offering 678 different pack sizes, from 10 kWh to 290 kWh, each with a data sheet detailing test data across temperature, SoC and SoH bands
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