Portable Batteries: Part 4
In parts one, two and three of this piece, we have introduced the basics of battery technology, how batteries are manufactured and today’s start of the art technology. In this final section, we will discuss the most promising future technologies to watch for.
Lithium Metal Batteries
A development push that we are very optimistic about is the commercialization of Lithium-metal batteries. A lithium-metal battery uses a layer of pure lithium metal foil as the anode, where today’s lithium-ion batteries use a carbon graphite anode to bind and store lithium atoms (in a process called intercalation). When a lithium metal battery is charged, lithium ions simply plate out of solution onto the surface of the metal anode; growing in thickness slightly, but much less than the swelling expansion that today’s carbon-based anodes experience as they become lithiated.
This essentially means that the anode can hold a vast abundance of accessible lithium atoms. This would then make the cathode the only limiter on battery capacity. Further, the lithium metal foil is significantly thinner and lighter than graphite, leading to immediate decreases in weight and theoretical improvements of 80% or more in overall gravimetric energy density (already approaching 500Wh/kg in test cells).
This drastic leap forward in energy density could single-handedly enable the nascent electric aviation industry, which today is held back by the weight of heavy battery packs.
However, this approach has some significant challenges that must be addressed by any company putting forth a lithium metal anode technology. The primary factor that has hindered the commercialization of rechargeable lithium-metal batteries in the past is the uncontrolled deposition of small needle-like crystals of lithium metal (called dendrites) that grow on the surface of the metal anode during charging. These dendrites can project outward from the anode like a conductive needle, piercing the separator membrane and creating the potential for a short circuit condition and the fiery demise of the battery pack. Much work today is going into surface coatings and electrolyte chemistry to cause a flat, uniform plating of lithium atoms on the surfaces, to prevent the growth of these high aspect-ratio dendrites.
Another reason why we have not seen widespread commercialization of lithium metal anode technology is that the lithium metal itself is highly reactive if exposed to air or water. This means it must be handled carefully in production facilities, contributing to infrastructure and cost. And then even after assembly the lithium can heat up and burn if the battery cell is ruptured. This can be solved by modifying the battery chemistry to passivate the surface of the metal through the formation of a Solid Electrolyte Interphase (SEI) layer so it won’t react if exposed.
Long View — The Future
Solid-state Batteries
When considering the next 3–5 years of electronic devices there are a number of promising battery technologies to consider. Solid-state-batteries are getting a lot of press and attention from researchers, and are definitely a technology to watch. The basic premise with these lithium-based cells is that the electrolyte layer consists of a solid material instead of the liquid or polymer-gel electrolytes in use today. This solid layer would allow the use of the lithium metal anode by providing a barrier to problematic dendrite growth, as discussed above.
The solid electrolyte material is also generally non-flammable, whereas almost all of the liquid or gel electrolytes in use today contain chemicals that are flammable if exposed to air. So the big push for solid-state batteries is to enable the theoretical 2.5x increase in energy density over today’s Li-ion batteries that can be achieved by incorporating a lithium-metal anode, all while improving the safety. Additionally, the separator layer needed to keep the electrodes separate can be much thinner, (3–4um, vs the 20–30um films in use today), so volumetric energy density should improve. Further, these batteries are expected to have much greater cycle life (theoretically tens of thousands of cycles), thermal stability, and calendar life than liquid electrolyte batteries. This is especially interesting for electric vehicles, where the holy grail is to produce a battery pack that can undergo 5,000–10,000 cycles and last 20+ years to match the life of the car frame itself.
Solid-state technologies face many technical challenges that should be considered before making an investment. So far the proposed technologies are very expensive to produce and are generally not compatible with existing production methods. The current methods to produce these solid layers require complex vapor deposition processes, and produce a solid sheet that is not compatible with the roll-to-roll processes favored by the battery industry today. For this reason, we do not regard them as commercially viable within the next 3 years, though many of the large vehicle manufacturers are investing heavily to try to beat this. Further, these solid layers are often formed from metal oxides and ceramics, which are dense materials and thus lead to heavy battery packs. So this may inherently limit the adoption of this technology for electric aviation and other weight-sensitive applications.
Lithium-Air Batteries
On a longer time horizon, we consider Lithium-oxygen batteries an intriguing possibility. This class of battery derives energy by oxidizing pure lithium metal with a source of oxygen, traditionally in the form of ambient air. Reacting pure lithium with ambient oxygen can result in an electrochemical cell with the highest possible energy density of any metal, yielding theoretical capacities of 11,000 Wh/kg (not counting the weight of the reacted oxygen). This is noteworthy when Li-ion is today topping out at 250 Wh/kg, and Li-metal will theoretically top out around 3,000 Wh/kg. And especially interesting when you consider that liquid gasoline has a maximum energy density of 13,000 Wh/kg, with only 1,700 Wh/kg delivered to the wheels after losses. But a lithium-air battery in this basic configuration is not rechargeable. And significant technological challenges remain before any appreciable cycle-life is expected from batteries built with this technology. These lithium-air or lithium-oxygen batteries are at least 5–10 years away from commercialization, but could disrupt the market with a 10x step-change in energy density, rivaling liquid gasoline in terms of raw energy density.
Conclusion
In part one, we introduced the basics of battery technology and the benefits of lithium ion chemistry. In part two, we explored the battery manufacturing process and the importance of new technologies working within the existing infrastructure. In part three, we looked at the current state of the art cathode, anode, and electrolyte technology. In this final section, we discussed the most promising future technologies that will dominate the battery industry in the coming years.
It is worth mentioning that there are many different use-cases for battery technology, each of them with specific requirements that guide selection. Lithium-based batteries as outlined in this paper will dominate most of the portable-electronics applications. But lithium is a relatively unstable compound, so it may not be the chemistry of choice for applications that require ultra-reliability such as powering medical devices or spacecraft.
And then there is a massive and ever-growing need for stationary energy storage solutions to enable the future of renewable power generation. Many different mechanical and chemical means are being explored to address this market segment. Lithium-based batteries have historically been too expensive per kWh to be competitive in this market. But with continually-falling battery production costs this trend could soon reverse, opening opportunities for investment in grid-scale lithium battery storage systems. Stay posted for a future Prime Movers Lab Technology briefer on stationary energy storage.
Prime Movers Lab invests in breakthrough scientific startups founded by Prime Movers, the inventors who transform billions of lives. We invest in seed-stage companies reinventing energy, transportation, infrastructure, manufacturing, human augmentation and computing.
