by Doug Campbell
Lithium ion batteries are emerging as the chemistry of choice for many hybrid and nearly all longer range full electric vehicle (EV) models. With a considerable head start in the consumer electronics space, where low cost and energy density reign supreme and some mild safety or performance issues can be tolerated if not widespread and improved upon through next generation product offerings. As new cell designs and advancements in materials drive energy density higher, alongside industry-wide competition for lower cost and first-to-market products, we’re frequently seeing spectacular failure emerge in the hands of consumers, and outside the reinforced walls of battery safety test labs. The re-design of a laptop battery for use in an electric vehicle takes an impressive amount of engineering and creativity both to ensure that a chassis containing thousands of small batteries work in harmony to produce desirable performance and assure safety of the occupants inside the vehicle.
Looking toward the future of automotive batteries, most experts predict adoption of EVs to grow significantly over the next 10-20 years, driven largely by policy alongside infrastructure and economic development in China. Demand for lithium ion batteries basis is poised to grow 4-5x over this timeframe, even by the most conservative estimates. As manufacturers spin up huge battery factories across China, Eastern Europe, and the western US, we need to consider what new technologies can be positioned to replace traditional lithium ion batteries in EVs. Is it reasonable to conjecture that lithium ion batteries won’t develop the decades-long foothold on the automotive industry that lead acid batteries have?
Among the novel battery chemistries still vying for a seat on the EV growth-curve are inorganic solid state batteries—a market where sales of lithium-ion batteries for electric vehicles increased by 66% in 2016. While organic/polymer solid state systems have proven useful in controlled fleet-type applications, these polymer-based systems still have important barriers to overcome to reach traditional consumer applications. Conversely, inorganic solid state cell chemistries are often lithium-sulfide based (such as LPS and LGPS) or garnet-type electrolytes.
A lithium ion battery consists of two porous solids, electrochemically active layers (electrodes) separated by a polymer membrane, which serves as a separator to protect the cell from internal shorts. Further, this materials system is infused with a gel or liquid electrolyte to facilitate ion mobility between electrodes. However in a solid-state cell, the electrode layers are considerably denser and separated by a thin ceramic layer that serves as both separator and electrolyte. This solid-state cell configuration enables higher energy density than traditional lithium ion chemistries, as it can be paired with very high energy electrode materials, including lithium metal on the negative side. Additionally, solid state cells containing inorganic ceramic electrolytes are inherently safer throughout charge and operation, as they contain no flammable electrolyte and have shown to be stable through a much wider temperature range. This high-temperature stability can substantially reduce costs of battery thermal management systems crucial to today’s lithium ion EV batteries.
In manufacturing solid state cells some notable advantages can be realized. When using traditional materials (for example, NMC), each electrode can be produced using the same equipment currently running in lithium ion plants. Only the densification step will need to be tuned to meet the low porosity needed for a solid-state system. With a lithium metal system, a lithium foil or lithium deposited on a substrate such as copper can be used. Additionally, for the lithium-sulfide based electrolyte materials, a tape-casting process can be utilized in the same manner as the electrode materials. When comparing lithium-sulfide solid state materials, vs. the garnet-type, no ultra-high-temperature sintering steps are required to produce the electrolyte-separator layer. A notable characteristic of considerable importance in manufacturing plant design: because lithium sulfide based materials are moisture-sensitive, the whole cell production process must be contained within a dry-room environment. Today, dry rooms exist in lithium ion production plants for final cell assembly as the liquid electrolyte component is moisture sensitive, however it is inconsistent between manufacturers whether these controlled environments extend back through the coating steps.
For solid-state battery hardware, cell form factor and design can be opened up to new conformations and packaging arrangements. This is because the cell package does not serve to contain the liquid electrolyte. Additionally, the cell stack can enable a more efficient bi-polar configuration.
While lithium ion appears to be safe for the next round of EV models, novel solid-state chemistries hold considerable promise for EV applications through 2022 and beyond, as evidenced by the recent news and rumored developments from Toyota, BMW, and Hyundai.
Doug Campbell is a start-up business veteran with over 15 years of experience in transitioning early-stage research and development into new product solutions within the cleantech, aerospace and military markets. Mr. Campbell co-founded Solid Power in 2012 based on technology licensed from the University of Colorado Boulder and this IP portfolio was expanded upon in 2015 with an additional licensing agreement with Oak Ridge National Laboratories. The company is a leading innovator in bulk, all solid-state rechargeable batteries.