Mark Bissett, lecturer in nanomaterials at The University of Manchester, poses for a photograph holding a model showing the hexagonal structure of graphene inside a laboratory at the National Graphene Institute facility, part of the The University of Manchester, in Manchester, U.K., on Thursday, April 12, 2018. Researchers are studying ways to use graphene in batteries, and the material has the potential to significantly boost performance in a much-needed technology.
Building a Better Lithium Battery
Last year I wrote about A Battery That Could Change The World, which addressed the development of a solid-state lithium battery that won’t catch fire if damaged. More recently, I wrote about a different approach to the problem of fires in lithium-ion batteries, which quickly dissipates the heat released from a fire in a cell before it can spread.
Development of better batteries is critical as more electric vehicles hit the roads, and as electric utilities seek better options for storing power from intermittent renewables. In addition, lithium-ion batteries have become ubiquitous in our lives through any number of consumer electronics.
Three key issues that companies are working to address are safety, energy density, and cost. Safety mainly concerns the possibility that lithium-ion batteries can catch fire if damaged. The two aforementioned stories are mostly focused on that aspect of the problem.
The problem of energy density concerns the ability to store energy in a specific volume (or weight). Batteries have low energy density compared to liquid fuels. Gasoline, for example, has about 100 times the volumetric energy density of a Li-ion battery pack. However, the greater efficiency of an electric motor versus a combustion engine substantially narrows the gap for usable energy.
Finally, batteries have historically been an expensive way to store energy. According to the Energy Information Administration, in recent years the cost to install large-scale battery storage systems was typically thousands of dollars per kilowatt (kW). In comparison, the capital costs of producing electricity by power plants can be under $1,000/kW. Importantly, lower battery storage costs would be a critical enabler for utilities seeking to incorporate higher levels of intermittent renewables into the power mix.
It isn’t surprising that companies are working to solve each of these battery challenges. But the solution to one problem can create another.
Consider energy density. Lithium-metal batteries allow for much higher energy density than lithium-ion batteries by using lithium-metal electrode instead of graphite electrode. But lithium deposits called dendrites can spontaneously grow from the lithium metal electrodes whenever the battery is being charged. If these dendrites bridge the gap between the anode and cathode (the two opposite electrodes in a battery), a short circuit results. This can cause the battery to fail, which may result in a fire or explosion.
The lithium-ion battery was a solution to this problem. The dendrite problem can be resolved by replacing the lithium-metal electrode with a carbon electrode that has a layered sheet structure (i.e., graphite), which hosts discrete tiny lithium ions between the layers. However, the result is a lower lithium storage capacity than a battery utilizing a solid, continuous lithium-metal electrode.
Solving the Dendrite Problem
An ideal battery would contain solid lithium electrodes while avoiding the dendrite problem. A company called Zeta Energy, using technology licensed from a top university, believes it has created just such a solution.
I recently spoke to Zeta Energy CEO Charles Maslin, who explained that Zeta is the sixth Greek letter and corresponds to carbon, the sixth element in the periodic table. Zeta potential is also a measure of the effective electric charge on a nanoparticle surface.
In an interview I conducted with Maslin, he provided a description of the new battery along with peer-reviewed data from the 10+ years of research and testing that led to its development.
The key innovation in the Zeta battery is a hybrid anode created from graphene and carbon nanotubes. The resulting three-dimensional carbon anode approaches the theoretical maximum for storage of lithium metal – about 10 times the lithium storage capacity of graphite used in lithium-ion batteries.
According to published research, the hybrid anode is one of the best-known conductors of electricity, and when the battery is charged, lithium metal is deposited on the sidewalls of the carbon nanotubes and in the pores between the nanotubes. These are chemically bonded to the surface of the graphene that is in turn chemically bonded to a copper substrate. The graphene-carbon nanotube-copper connections introduce no additional electrical resistance that is typically generated at the interface when electrode materials are coated on copper in batteries. This means no heat is generated at this interface.
The combination of a dendrite-free electrode with the seamless interface enables fast charging and discharging of the Zeta battery, unlike in a graphite-based lithium-ion battery where very fast charging can cause lithium dendrites to form on top of graphite. No resistance at the interface means electrons can travel to the electrode much more rapidly. The Zeta battery can thus be charged safely within a few minutes.
More Energy, Less Cost
Despite the breakthrough, building the ideal battery requires more than perfecting the anode. A cathode that matches the high capacity anode is required to unleash the boost in energy density, but current cathodes do not have enough capacity to match the lithium metal anode.
Zeta’s second key innovation is a hybrid cathode created from sulfur and carbon, which has 8 times the capacity of current cathodes based on metal oxide. Thus, Zeta’s anode-cathode combination means a packaged lithium-sulfur battery with three times the energy storage capacity of lithium-ion batteries.
Further, Zeta has eliminated the use of expensive metals such as cobalt in its battery. That translates to a significant reduction in battery cost. Though others have been trying to develop sulfur-based cathode for decades, they just could not get it to cycle well, and the battery dies after just a couple of hundred cycles at most. Maslin explained that Zeta has succeeded in stabilizing the sulfur cathode and pointed to test results that show it can be cycled with minimal capacity loss over thousands of cycles.
Furthermore, the Zeta lithium-sulfur battery does not self-discharge to any appreciable extent and holds charge for a significantly long time, thus boasting a superb shelf life. A major battery performance concern is self-discharge – you charge the battery then store it, but when you use it later, only a fraction of the stored battery capacity is left.
Based on Zeta’s measurements, it is estimated that more than 90% of its battery capacity will remain after 10 years of storage at full charge, unlike regular lithium-ion batteries that will have at best 10% capacity over the same period of storage. That means the Zeta battery is ready to work after virtually any period of storage.
In summary, the Zeta battery addresses both energy density and safety. Test results published in several scientific journals including Nature Magazine show that the Zeta battery has:
- Up to 3 times the energy storage capacity of lithium-ion batteries
- Faster charge time (minutes instead of hours)
- Lower battery temperature
- Little degradation over charge/recharge cycles
- Outstanding shelf life
- Significantly lighter than lithium-ion batteries
- Zero cobalt
- Significantly lower cost than lithium-ion batteries
If their research and test results are accurate, Zeta Energy may hold the new gold standard in energy storage. The technology appears to address multiple shortcomings of current lithium-ion batteries and has done so at a lower price point. These are exactly the kinds of breakthroughs that are needed if battery storage is to be adopted widely by utilities seeking to smooth out the intermittency of renewables like wind and solar power.