By Kent Griffith
July 16, 2019 | Representatives from all sectors of the electric vehicle battery industry gathered on Coronado Island in San Diego, Calif. for the Advanced Automotive Battery Conference (AABC). The week opened with a series of tutorials covering materials chemistry all the way up to pack engineering and an overview of the entire rechargeable battery market. In parallel, the Renewable Energy Partnering & Investment Forum brought together private investors, government funding agencies, and start-up companies. Aron Newman, a contractor for Advanced Research Projects Agency – Energy (ARPA-E), discussed the importance of transformational research projects. ARPA-E usually focuses on device performance but the solid-state battery is so potentially impactful that the IONICS program was funded to develop a better solid electrolyte. He also projects a necessity for energy storage beyond batteries, e.g. for heavy-duty transport. Research into this area is being supported by the REFUEL program, focusing on carbon-neutral liquid fuels such as ammonia. The cleantech start-ups Enevate Corporation, Solid State Battery Inc., and NanoFlex Power Corporation discussed their respective strategies to bring a silicon anode, composite solid polymer electrolyte, and organic photovoltaics/cheaper gallium arsenide to market, which led to an interesting debate on whether it is better to focus on materials (Enevate), manufacturing (Solid State Battery), or partnerships with ‘champion customers’ to help drive performance focus from an early stage (NanoFlex).
The Joint Center for Energy Storage—a US Department of Energy (DOE) Energy Innovation Hub—was well-represented at AABC with talks from director George Crabtree and deputy director Venkat Srinivasan. Discoveries and lessons from the past five years (JCESR 1.0) were covered, leading into research directions for the future (JCESR 2.0) of this collaborative project between national laboratories, universities, and industry. Special attention was given to the symbiotic relationship between computational materials discovery and experimental synthetic approaches.
Silicon was a hot topic, with talks focusing on both elemental silicon electrode structures as well as silicon oxide (SiOx). Anthony Burrell of the National Renewable Energy Laboratory (NREL) in Colorado led a “deep-dive” into silicon. Burrell postulated that particle fracturing is mostly a solved problem thanks to nanostructures but that solid–electrolyte interface (SEI) stabilization is still a detrimental issue. Ask anyone who has ever studied electrode surfaces, understanding the SEI is non-trivial. A systematic study of surface oxidation revealed that insulating SiO2 forms natively on the surface of Si in air and leads to large overpotentials; lithiation was completely precluded with a 3.2 nm SiO2 surface layer. Furthermore, even with thinner oxide layers, the SEI continuously changes on cycling—it is not a stable component, even after formation cycles. On the commercial side, Zenlabs Energy is developing a SiOx anode based on micron particles and working on strategies for pre-lithiation to improve cycle life. Peter Lamp of the BMW group expressed interest in SiOx anodes but noted that there are challenges related to industrialization and cost, especially if pre-lithiation is required. Alternatively, several companies are working on non-oxide silicon anodes. Paraclete Energy presented their surface-modified silicon powder, which can be used in mixtures with graphite. Paraclete argues that the artificial SEI enables silicon air stability without SiO2 formation, aids with active particle attachment to carbon and binder, and permits cycling up to 3590 mAh/g silicon capacity. Rather than a silicon powder, Enevate has taken the approach of producing silicon-dominant electrodes and is working with automotive and battery customers to optimize performance and lower cost. Ultimately, silicon technologies need to compete with graphite in cost, capacity, rate, manufacturability, and safety. Graphite does not have a perfect record in all these areas, but it does have the momentum of nearly 30 years in commercial products.
Other next-generation battery technologies that were widely discussed at AABC include solid-state batteries and lithium metal batteries. Initially, solid-state and Li metal may be established individually; convergence is the grand, long-term goal. Jun Liu of Pacific Northwest National Laboratory (PNNL) gave an overview of the Battery500 program. Battery500 is doing research toward a lithium metal battery with energy density up to 500 Wh/kg, which requires a thin Li metal anode and a thick, high-capacity cathode such as nickel-rich nickel manganese cobalt oxide (Li[NixMnyCoz]O2). Battery500 is working on non-flammable and locally-concentrated electrolytes but not solid-electrolytes at present. Despite the name, SolidEnergy Systems is also in the business of thin lithium metal anode batteries with concentrated electrolytes to achieve high gravimetric and volumetric energy density when paired with nickel-rich NMC. Saft is interested in solid-state batteries and holds the view that these will be Li-ion first, and Li metal eventually. Saft is exploring different electrolyte options and, in particular, wet vs. dry processing methods. They also noted that PEO is of minimal interest because they want a room-temperature battery.
On the true solid-state side, Dee Strand of Wildcat Discovery Technology presented their systematic approach, in partnership with CBMM, on the optimization of Nb-doped garnet solid electrolytes. PolyPlus Battery is developing a hybrid technology with an ultrathin lithium anode, on the order of a few microns, a thin and flexible sulfide glass solid electrolyte, and a liquid electrolyte to wet the NMC cathode interface. Ionic Materials, in partnership with A123 Systems, is planning to produce 50–60 Ah cells this year based on their novel polymer electrolyte in combination with a conventional graphite anode and NMC cathode. While it seems that all-solid-state Li metal batteries with non-flammable solid electrolytes are a distant target, variations that incorporate Li metal or and/or electrolytes with increased safety properties are rapidly approaching commercialization.