With more capacity and faster charging, solid-state batteries could be the next big thing in energy. And good news: researchers may have pinned down one major reason these batteries still fail before they can reach widespread commercial use.<\/p>\n
A team at the Max Planck Institute for Sustainable Materials in D\u00fcsseldorf, Germany, published<\/a> its findings in Nature on Wednesday, saying it has identified a key mechanism behind cracking in the ceramic solid electrolytes that distinguish solid-state batteries from the liquid-electrolyte designs used in most modern electronics.<\/p>\n As mentioned above, solid-state batteries swap a liquid electrolyte for a solid one, which offers several potential advantages. Energy density can be higher, cells can be made smaller and lighter, and some designs may allow faster charging and longer operating life than conventional lithium-ion batteries. They are also generally considered safer because solid electrolytes are less flammable than liquid ones and cannot leak.<\/p>\n Unfortunately, the very same thing that makes them superior – the solid electrolyte – is also what makes SSBs prone to failure. Typically made of a ceramic material, these electrolytes can develop microscopic cracks that plated lithium from the metal anode may fill as it grows through the material, forming dendritic filaments inside the electrolyte. If those filaments continue to propagate, they can extend cracks and eventually short-circuit the cell.<\/p>\n The Max Planck team believes it has identified what drives that dendrite-induced fracture in its garnet electrolyte samples, and has also proposed a couple of possible ways to limit the damage in future solid-state battery designs.<\/p>\n As explained in the paper, there are two theories for how dendrites fracture the electrolyte: either internal stress within the lithium dendrite causes the ceramic to break, or electron leakage at grain boundaries promotes isolated lithium nuclei that later interconnect and contribute to short-circuiting.<\/p>\n By prepping a number of samples subjected to vacuum conditions at a cryogenic temperature to eliminate influences from outside forces, the Planck team said that it found proof that dendrite-induced cracking has everything to do with mechanical stress.\u00a0<\/p>\n Not only was there no lithium enrichment ahead of the tip of dendrites they studied, meaning the electron leak theory is out, but stress measurements made the mechanical failure theory seem likely to be true.\u00a0<\/p>\n “The soft lithium metal is able to penetrate the stiff ceramic electrolyte, like a continuous waterjet that penetrates a rock,” lead author Yuwei Zhang said<\/a> in a press release from the Planck Institute. “We calculated that hydrostatic stress in the dendrite leads to brittle fracture of the solid electrolyte in the end.”\u00a0<\/p>\n Not only did the team conclude that mechanical failures are the source of dendrite-induced short circuiting, but they also did initial research into how to stop the fractures, too.\u00a0<\/p>\n Zhang and his colleagues propose some possible solutions for electrolyte fractures: First, they suggest coming up with tougher solid electrolytes that’d simply resist cracks. Failing that, there’s the second option, which sees the team suggesting that SSB makers leave microscopic voids in the electrolyte to force dendrites to take a path that prevents fracturing and inhibits growth. That, or just coat the lithium anode with something that’d stop cracks from forming.\u00a0<\/p>\n It’s a pretty simple solution on its face, though there’s surely plenty of complex chemistry involved that’ll take researchers years to sort out – we asked the Planck team when its recommendations might be tested more broadly, but didn’t hear back.\u00a0<\/p>\n Don’t rule out chemical contributions yet<\/p>\n The Planck team’s answer to a question that’s plagued battery experts for decades comes just a couple of weeks after a team from the Massachusetts Institute of Technology similarly published research<\/a> in Nature saying they found pressure and mechanical forces alone aren’t causing ceramic electrolytes to shatter: There’s an electrochemical reason for the failure, too.\u00a0<\/p>\n “Normally you would expect that the faster a dendrite grows, the more stress it creates,” senior author and MIT materials science professor Yet-Ming Chiang said<\/a> of his team’s findings. “Instead, we observed exactly the opposite. The faster it grew, the lower the stress around it, meaning the solid electrolyte is breaking under a lower stress.”\u00a0<\/p>\n That, said Chiang, means mechanical force isn’t solely to blame, and is indicative of embrittling happening in the electrolyte.\u00a0<\/p>\n By turning to a cryogenic scanning transmission electron microscope, the MIT team was able to study the electrolyte at practically the atomic scale, which they said showed evidence of ionic current passing through the electrolyte and causing it to become brittle, contributing to fractures.\u00a0<\/p>\n Not only is there a chemical component causing the electrolyte breakdown in the eyes of the MIT team, but they also said that they spotted a concentrated flow of lithium ions at the tip of the dendrites they studied.\u00a0<\/p>\n Lead author and MIT materials science and engineering PhD candidate Cole Fincher told The Register in an email that he finds the Planck team’s discovery complementary to his.<\/p>\n