Today’s State of the Art Technology
The portable battery market today is dominated by Lithium Ion (Li-Ion) chemistry with over 316GWh of battery capacity being produced in 2019. And the demand for Li-ion batteries is projected to surge another 8x from 2019 to 2030, mostly due to the boom in electric vehicles.
There are a number of common Li-Ion chemistries in use today, with the main distinctions between being energy density/capacity, cycle life, safety, and cost. The most common chemistries are listed below and generally refer to the cathode materials.
- NMC: Lithium Nickel Manganese Cobalt Oxide
- LCO: Lithium Cobalt Oxide
- NCA: Lithium Nickel Cobalt Aluminum Oxide
- LFP: Lithium Iron Phosphate
- LMO: Lithium Manganese Oxide
- LTO: Lithium Titanate
Cathodes
Many of the advances in energy capacity are expected to come from advances in cathode materials, both composition and structure. The cathode is the largest driver of energy density, as this is where most of the actual chemical reaction takes place (pure metals forming into metal oxides and back again during charge and discharge).
It can be worthwhile watching for startup companies that are developing new cathode materials. However, as mentioned previously, they must be compatible with existing factory automation equipment, or be enabling at least 2x increases in energy density over the current state of the art cathode materials.
A key metric to watch for in the latter case is demonstrated traction with major battery producers. It is not enough to show a promising chemical component in the lab; there must be demand, and ability to produce the material at massive volume.
Cobalt
Cobalt is a common component in cathode materials, and contributes significantly to performance and longevity. However, cobalt supplies on earth are limited to a small number of highly-contested geographical regions; particularly the unstable Congo region of Africa which holds over 55% of the world’s supply of this metal.
One of the major pushes in Li-Ion research has been to reduce or eliminate cobalt content in favor of more common metals. The problem is that most alternatives lead to poor performance in Lithium battery chemistries, in terms of energy capacity, longevity/cycle life, power delivery, and cost. Lithium iron phosphate (LFP), lithium manganese oxide (LMO) and lithium titanate (LTO) batteries are cobalt-free, but as can be seen in the chart above their energy densities are lower than the cobalt-containing chemistries (denoted by the “C” in the material abbreviations). They may also have other limitations such as shorter cycle-life.
LFP is a noteworthy exception here, being cobalt-free while demonstrating 2000+ cycles, which makes it a promising chemistry for vehicles and mobility despite the lower energy density. Essentially, a device maker using this chemistry will require more batteries, but they will be cheaper and last longer.
Silicon Anodes
An advance that is getting a lot of attention is the incorporation of silicon(Si) into the battery anode (traditionally graphite). Silicon-based anode materials can hold as much as 10x more lithium ions than the graphite anodes used commercially today. This 10x sounds impressive, but the anode is only a small fraction of the overall battery assembly, so this technology could lead to overall cell energy capacity gains of 10–50%. However, there is a well-known problem with mechanical swelling of the silicon matrix, expanding (and contracting) to as much as 400% of the original volume as the lithium ions diffuse into and bind with the silicon during charging. This swelling creates a significant mechanical challenge for batteries constructed of many layers of material. There are technologies emerging to compensate for this expansion such as making porous or spongy anode materials that have room to expand within their original volume. Anodes with a small amount of silicon additive have made it into existing commercial products, though the major manufacturers are generally holding these additions as closely guarded trade secrets. Anodes with higher silicon content are in the mid-to-late stages of development, but may not reach the market before better alternatives emerge.
Graphene Anodes
While most anode material today is made from graphite, we are beginning to see the use of graphene, a more highly structured form of carbon that is more thermally stable and electrically conductive. This suggests batteries with graphene-composite anodes could be charged and discharged at much higher C-rates, and thus be more useful for high-power applications.
There are a number of companies exploring silicon-graphite or silicon-graphene composite electrodes, seeking to capture the benefits of both additives. Further, there is research to show that these composite anodes can be charged to higher voltages of 4.2–4.35V, which can yield a 10% increase in energy density over 3.7V cells with plain graphite anodes.
Separator Membrane
The primary role of the separator is to prevent short circuits between the positive and negative electrodes. Thus safety is the major consideration in separator development. Common trends are to make thinner separator films, which will make the layers thinner and thus the battery smaller in volume (and perhaps reduce weight). However, this must be accomplished without sacrificing the separator’s ability to keep the electrodes electrically isolated from one another, while still allowing ions to migrate between the electrodes. Some key innovations here are advanced fiber materials that are more thermally stable and able to withstand the tremendous current density and heat, while also being substantially thinner. Also under development are structured nano-materials that contain and release flame-retardant compounds into the electrolyte in the event of a short circuit, rupture, or high-heat event.
Electrolyte
Most electrolytes in use today are strong organic liquid solvents with high volatility and high flammability, which are necessary to dissolve the high concentrations of lithium ions that serve as the charge carriers within the battery. Much work today is dedicated to trying different solvent chemistries or additives to lessen the potential for a “venting with flame” event, which is the technical term for a ruptured and burning cell, or “rapid disassembly” which is the nice way of saying something exploded. Flame retardant additives are common, but cannot be added at sufficient concentrations to reliably stop all fires without the additives having detrimental effects on battery performance. There is ongoing research to use non-flammable inorganic liquids or ionic liquids as electrolyte, or to do away with the liquid electrolyte all together in favor of a solid-state crystalline electrolyte.
Conclusion
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 the final portion, we will discuss the most promising future technologies to watch for.
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.
