By Kent Griffith
July 27, 2020 | Lithium metal is ultimate battery anode material. The lightest metallic element, in its simplest form, cannot be beat when it comes to lithium batteries. Lithium metal is such an ideal choice in terms of energy density that it was used by Nobel Laureate Stan Whittingham in the first lithium batteries in the 1970s. So, why change now? Graphite, rather than lithium metal, is used as the negative electrode in state-of-the-art lithium batteries for two big reasons: safety and stability.
In its pure metallic form, lithium can form needles, or dendrites, that act as short-circuits and lead to rapid overheating and ignition of the flammable electrolyte solution used in today’s batteries. Even without this risk, lithium metal is extremely reactive and will degrade the cell components leading to capacity fade and premature cell failure. Despite these challenges, lithium metal is experiencing a renaissance owing to the theoretical gains in energy density that are highly coveted by battery makers and electric vehicle OEMs in particular. Global research initiatives are increasingly setting their sights on technologies such as solid-state batteries or innovative separators that could enable the use of the lithium metal anode.
Solid electrolytes are generally ceramics and polymers, and both face challenges. Ceramics, while hard and seemingly impenetrable, actually do not sufficiently inhibit the growth of lithium dendrites. Lithium is a soft, malleable metal that is easily manipulated. Nevertheless, it grows steadily along defects and grains within a ceramic solid and penetrates through the electrolyte from anode to cathode, creating a short-circuit that will still cause heating and cell failure, though perhaps no fire as the battery no longer has a liquid fuel source.
Another non-trivial challenge with ceramic electrolytes is the creation of good interfaces that allow lithium ions to travel unimpeded. Liquids can coat a surface with perfect contact, but the same connection for ceramics typically requires very high temperatures, which are incompatible with most lithium battery materials. Once the battery starts cycling, they may not retain good contact anyway.
Polymer-based solid electrolytes, on the other hand, are relatively soft and facilitate the formation of compatible interfaces with good contact but lack mechanical strength and may be more susceptible to chemical reactivity.
In an article in the July issue of Nature Materials, a team led by Brett Helms at Lawrence Berkeley National Laboratory reported an innovative hybrid polymer–ceramic–coated separator that effectively and stably suppresses lithium dendrites (DOI: 10.1038/s41563-020-0655-2). The new device is not a solid-state battery—it still relies on a conventional flammable liquid electrolyte—but it combines elements of both ceramic and polymer solid-state battery technology in a new way. The hybrid electrolyte technology falls in a sweet spot that the team identified in terms of the physical properties that enable stable lithium deposition. They describe their material as a soft polymer infiltrated with nanostructured hard ceramic particles. The ceramic compound in this study was lithium fluoride, with a diameter of about 12 nanometers.
To generate the hybrid, lithium fluoride nanoparticles were distributed throughout a porous polymer host via a reaction between a lithium salt and a fluoride salt within the polymer itself. Lithium fluoride is not one of the typical ceramic electrolytes used in solid-state batteries because in bulk it is not a good lithium ion conductor. However, in this nanoscale form, and in the polymer composite, lithium transport occurs via a surface diffusion mechanism. Lithium fluoride is also chemically stable with respect to both lithium metal and the common lithium cathode materials such as nickel manganese cobalt oxide. In terms of the manufacturing process, the ceramic–polymer hybrid can be deposited via an ink onto a conventional porous polymer separator used in conventional lithium-ion batteries. The authors believe this process will be attractive because it uses rather conventional techniques and materials.
The team tested a variety of ratios of ceramic and polymer before settling on one with 2.5% lithium fluoride loading, which provided the optimal combination of high lithium ion conductivity and low interfacial resistance. The conductivity of this hybrid component is considerably lower than typical solid electrolytes, about a factor of 1000 lower than the state-of-the-art sulfide ceramics for example. Batteries were assembled with lithium metal anodes and industry-standard NMC622 cathodes and the study demonstrates that a battery with the polymer–ceramic-coated separator was able to last at least 300 cycles while the polymer(only)-coated separator or uncoated separator both failed after about 150 cycles.
A more futuristic battery, representing a loftier energy density target, was assembled by paring the lithium metal anode with sulfur-based cathodes. This cell, with an initial cathode capacity of about 750 mAh/g, retains nearly 60% of its capacity after 300 cycles with the new hybrid polymer–ceramic-coated separator, but retains less than 40% capacity with a conventional separator. These two battery systems show that the lithium fluoride–porous polymer composite can increase the stability of lithium metal batteries. A patent has been filed covering these materials and their uses as PCT patent application 62/431,300.
Battery cycling and advanced characterization methods clearly showed suppression of the dendrites. However, the coated separator with its low lithium ion conductivity may hinder the rate performance of the battery, affecting its power output and recharging time. Furthermore, while lithium metal could improve the anode capacity by a factor of 10 if it was simply swapped in for graphite, this study, like most others, used a large excess of lithium metal. The negative-to-positive ratio, which describes the excess of the negative electrode, is typically 1.1 in graphite-based cells; here the value was 5, which roughly cuts the possible energy density benefits in half.
Though dendrite suppression lowers the probability of a short-circuit, the use of a flammable liquid electrolyte means that these batteries still possess the inherent risk of catastrophic failure. For the future, the authors suggest that another layer, one that operates by a pressure-driven mechanism to block dendrites, may be required to improve the safety characteristics of lithium metal batteries. Whether through solid-electrolytes, coated separators, multilayer systems, or complex intermediate or hybrid solutions, it is clear that the electrolyte region must be kept thin and a minimal amount of lithium metal should be used for the anode if actual gains in the energy density and practical charge and discharge rates are going to be realized in lithium metal batteries.