By Kent Griffith
April 11, 2023 | The International Battery Seminar and Exhibit celebrated its 40th anniversary this March with over 2100 delegates in Orlando, Florida. The record-breaking attendance was felt most strongly when everyone came together in one room to hear Jeff Dahn and Shirley Meng discuss the latest developments on long-life and high-energy batteries, respectively.
Many Millions
Dahn, Dalhousie University, has been talking about the ‘Million-mile battery’ at the International Battery Seminar for several years, but it is becoming something of a misnomer. At this point, the question should really be, ‘How many million?’ Dahn introduced himself by saying, “I am obsessed with developing Li-ion batteries with exceptional lifetime.” His passion stems from fundamental questions regarding the sustainability of the battery industry. Dahn shared a slide from Tesla’s investment day deck where they estimated that the world needs an incredible 240 TWh of energy storage. Simple math makes it clear that if batteries only last 10 years, then we will need to manufacture 24 TWh of batteries a year, whereas 40-year batteries would cut this down to 6 TWh, which is a far more manageable number based on present predictions for gigafactory growth to 2040. Even with a fully circular materials supply chain, recycling and manufacturing batteries will always require a substantial amount of energy, so increasing the lifetime of each battery will yield a net decrease in the energy inputs required by our industry.
Now, testing batteries for 40- or even 100-year lifetime is rather impractical, so Dahn’s group in Canada studies batteries under accelerated degradation conditions and looks for key indicators such as rising voltage polarization or gas generation that can point to, or be extrapolated to, long-term cell performance. As he says, “Testing at room temperature takes far too long for good cells.” Elevated temperature is the most common way to accelerate the (electro)chemical reactions that cause capacity fade and eventually lead to cell failure.
For context of why accelerated testing is required, Dahn’s team has had NMC532//graphite cells cycling at room-temperature for five years with “virtually no degradation” in terms of capacity fade, outgassing, or increase in voltage polarization. These cells have undergone 16,500 cycles, which correlates to about five million miles for a battery pack with 300 miles driving range. For daily grid storage of intermittent renewable power, 16,500 cycles is about 45 years. Remember, these cells have only lost a few percent of their initial capacity, so the end is not yet in sight. The long lifetime is achieved through a combination of electrolyte additives and controlled cycling conditions, i.e., extracting a limited amount of lithium by only charging to 4.1 V.
Dahn wants to go even further though. The ‘5-Million-mile’ cell above dies after about 4000 cycles at 55 °C. To develop extremely long-lifetime batteries, Dahn has been testing pouch cells at 85 °C for the past year and, with new equipment, has recently started cycle testing at 100 °C. Along these lines, their team has encountered a new electrolyte solvent, called DMOHC or dimethyl-2,5-dioxahexanedioate, that has doubled the cell lifetime of their best performing electrolyte combinations at 85 °C. DMOHC is too viscous to work well on its own at room-temperature but it can be used successfully in electrolyte blends. He also noted that the use of DMOHC has been patented by Tesla. We’ll check back in with Dahn in 2024 to see how his cells are doing, but the take-home message is that he believes cell lifetime on the order of 50 years is going to be possible.
Solid State Futures
Meng, University of Chicago and Argonne National Laboratory, is in the relentless pursuit of technological breakthroughs that would enable lithium metal and all-solid-state batteries. On the lithium metal side, the Battery500 Consortium that Meng was involved with accelerated electrolyte architectures, electrolytes, and cell design to prototype longer and longer cycling in high-energy commercial-format cells, recently reaching 600 cycles in a 350 Wh/kg pouch cell. However, Meng cautions that the safety of modern lithium metal cells is still largely unknown, and it is “Risky for startups to commercialize the technology at this point.”
Regarding all-solid-state batteries, Meng said that she hears those who do not believe they will ever happen, but she was quick to note that there are many talented and rebellious minds who are working toward creative solutions and like to disprove such statements of doubt. In her view, all-solid-state batteries are a platform technology. Advances such as thin solid-electrolyte layers, on the order of 30 microns, enable cells with competitive energy density. From that point, there are several routes to increase the energy density such as thick electrodes from dry manufacturing methods, 100% silicon anodes, lithium metal anodes, and anode-free cells that are built without lithium metal using only lithium stored in the cathode. These technologies can be combined with any of a variety of cathodes from high-energy nickel-rich NMC to nickel- and cobalt-free technologies such as lithium (manganese) iron phosphate to low-nickel, cobalt-free lithium nickel manganese oxide. Building off the lithium work, she also sees the platform extending quite naturally to solid-state sodium-based batteries with sodium metal or alloy anodes and sodium layered oxide cathodes. Meng challenged the audience to put more energy into sodium solid-state batteries because sodium is more sustainable but also because anode-free solid-state batteries have manufacturing advantages over handling metallic sodium. In her lab, solid-state batteries are truly solid, without a drop of liquid in the cell.
One specific technology of interest to Meng’s laboratory is LiPON, a metal-free solid electrolyte material with excellent electrochemical stability and mechanical properties. In the absence of air and moisture, thin-film LiPON is actually a flexible ceramic, although it becomes brittle in the presence of water vapor and carbon dioxide. Her lab works with the company Ensurge to commercialize anode-free micro batteries with LiPON.
Meng also identified some remaining challenges that must be addressed for solid-state batteries. For sulfide solid-state batteries, the price of a key precursor, Li2S, needs to come down by 80–90%. More generally, a high degree of solid electrolyte particle size control is required. External pressure requirements are still in most cases not well defined, but needs to be manageable at the pack level, ideally 5 MPa or below.
