The urgent need to reduce carbon emissions is driving a rapid move towards electrifying transport and expanding the deployment of solar and wind power on the grid. If these trends escalate as expected, the need for better methods of storing electrical energy will intensify.
We need all the strategies we can get to address the threat of climate change, says Dr Elsa Olivetti, an associate professor of materials science and engineering at Esther and Harold E. Edgerton. Clearly, the development of grid-based mass storage technologies is crucial. But for mobile applications - especially transportation - much research is focused on adapting today's lithium-ion batteries to be safer, smaller and able to store more energy for their size and weight.
Conventional lithium-ion batteries continue to improve, but their limitations remain, partly due to their structure. Lithium-ion batteries consist of two electrodes, one positive and one negative, sandwiched in an organic (carbon-containing) liquid. When the battery is charged and discharged, charged lithium particles (or ions) are passed from one electrode to the other through the liquid electrolyte.
One problem with this design is that at certain voltages and temperatures, the liquid electrolyte can become volatile and catch fire. The batteries are generally safe under normal use, but the risk remains, says Dr Kevin Huang Ph.D.'15, a research scientist in Olivetti's group.
Another problem is that lithium-ion batteries are not suitable for use in cars. Large, heavy battery packs take up space, increase the overall weight of the vehicle and reduce fuel efficiency. But it is proving difficult to make today's lithium-ion batteries smaller and lighter while maintaining their energy density - the amount of energy stored per gram of weight.
To solve these problems, researchers are changing the key features of lithium-ion batteries to create an all-solid, or solid-state, version. They are replacing the liquid electrolyte in the middle with a thin solid electrolyte that is stable over a wide range of voltages and temperatures. With this solid electrolyte, they used a high-capacity positive electrode and a high-capacity lithium metal negative electrode that was far less thick than the usual porous carbon layer. These changes allow for a much smaller overall cell while maintaining its energy storage capacity, resulting in a higher energy density.
These features - enhanced safety and greater energy density - are probably the two most commonly touted benefits of potential solid-state batteries, yet all of these things are forward-looking and hoped for, and not necessarily achievable. Nevertheless, this possibility has many researchers scrambling to find the materials and designs that will deliver on this promise.
Thinking beyond the laboratory
Researchers have come up with a number of intriguing scenarios that look promising in the laboratory. But Olivetti and Huang believe that given the urgency of the climate change challenge, additional practical considerations may be important. We researchers always have metrics in the lab to evaluate possible materials and processes, says Olivetti. Examples might include energy storage capacity and charge/discharge rates. But if the aim is implementation, we suggest adding metrics that specifically address the potential for rapid scaling.
Materials and availability
In the world of solid inorganic electrolytes, there are two main types of material - oxides containing oxygen and sulphides containing sulphur. Tantalum is produced as a by-product of the mining of tin and niobium. Historical data show that the production of tantalum is closer to the potential maximum than that of germanium during the mining of tin and niobium. The availability of tantalum is therefore a greater concern for the possible scaling up of LLZO-based cells.
However, knowing the availability of an element in the ground does not solve the steps required to get it into the hands of manufacturers. The researchers therefore investigated a follow-on question about the supply chain of key elements - mining, processing, refining, transporting, etc. Assuming there is an abundant supply, can the supply chain for delivering these materials be expanded quickly enough to meet the growing demand for batteries?
In a sample analysis, they looked at how much the supply chain for germanium and tantalum would need to grow year on year to provide batteries for the projected 2030 fleet of electric vehicles. As an example, a fleet of electric vehicles, often cited as a target for 2030, would need to produce enough batteries to provide a total of 100 gigawatt hours of energy. To achieve this goal, using only LGPS batteries, the germanium supply chain would need to grow by 50% year on year - a stretch, as the maximum growth rate has been around 7% in the past. Using only LLZO cells, the supply chain for tantalum would need to grow by around 30% - a growth rate well above the historical maximum of around 10%.
These examples show the importance of considering material availability and the supply chain when assessing the scaling-up potential of different solid electrolytes, says Huang: Even if the quantity of a material is not an issue, as in the case of germanium, scaling up all the steps in the supply chain to match the production of future electric vehicles may require a growth rate that is virtually unprecedented.
Materials and processing
Another factor to consider when assessing the scalability potential of a battery design is the difficulty of the manufacturing process and the impact it may have on cost. There are inevitably many steps involved in the manufacture of a solid-state battery, and the failure of any step increases the cost of each successfully produced cell.
As a proxy for manufacturing difficulty, Olivetti, Ceder and Huang explored the impact of the failure rate on the total cost of selected solid-state battery designs in their database. In one example, they focused on the oxide LLZO. LLZO is very brittle and large sheets thin enough to be used in high performance solid state batteries are likely to crack or warp at the high temperatures involved in the manufacturing process.
To determine the cost implications of such failures, they simulated the four key processing steps involved in assembling LLZO cells. At each step, they calculated the cost based on an assumed yield, i.e. the proportion of total cells that were successfully processed without failure. For LLZO, the yield was much lower than for the other designs they studied; moreover, as the yield decreased, the cost per kilowatt-hour (kWh) of cell energy increased significantly. For example, when 5% more cells were added to the final cathode heating step, the cost increased by about $30/kWh - a negligible change considering that the generally accepted target cost for such cells is $100/kWh. Clearly, manufacturing difficulties can have a profound impact on the feasibility of large-scale adoption of the design.
Post time: Sep-09-2022