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Path to Solid-State Batteries for Electric Vehicles

Image Source: kkssr/Shutterstock.com

By Adam Kimmel for Mouser Electronics

Published October 29, 2021

Electric vehicle sales continue to grow exponentially in all global markets despite a significant stunt in the automotive market due to the COVID-19 pandemic. Europe and China lead the global markets with 1.4MM and 1.1MM new registrations in 2020. Market share in the EU remains undaunted by the pandemic, with 10% of European automobiles either battery electric vehicles (BEV), plug-in hybrid electric vehicles (PHEV), or fuel cell electric vehicles (FCEV). Moreover, as the global demand and supply ramp up, costs are coming down.

Cost-limiting components will soon dictate the timing for widescale adoption. For FCEVs, reducing the cost of hydrogen from the current $6.50/kg to $1.50/kg is the light-off point. Battery costs will pace the adoption of BEV and PHEVs, with the battery pack consuming 30% of the total cost to consumers. Still, with vehicle demand exponentially increasing, the cost per kilowatt-hour (kWh) is exponentially decreasing, down to $137/kWh at the end of 2020. BloombergNEF estimates that at $100/kWh, electric cars will be at cost parity with gasoline-powered vehicles.

While the debate between hydrogen and battery vehicles is far from over, BEVs will likely dominate the market in the near term, especially for light-duty vehicles that would not require a battery big enough to power a Class VIII truck. But even within the battery segment, there is not a clear choice for the best battery technology. Lithium-Ion (Li-ion) dominates the market, but there are other viable—and some technically superior—battery chemistries in the market today.

With the costs of Li-ion projected to plateau soon, there is an opportunity for innovation with alternate chemistries to gain a share of the battery segment. One such chemistry, solid-state batteries, is poised to emerge as the leading chemistry for EV batteries. This article reviews solid-state battery technology and compares it against the other top battery technologies in the market today.

Li-Ion Batteries and the Leading Alternatives

Most EV batteries are Li-ion-based. A liquid-state technology, Li-ion works by employing lithium to carry the electric charge between the electrodes. To put the scale of EV batteries into perspective, they use 10,000 times the amount of lithium of mobile phones, ramping up the demand for lithium and driving up its commodity price. However, Li-ion is not without its challenges, leading battery manufacturers to develop alternatives such as nickel-metal hydride (NMH), lead-acid, ultracapacitors, and solid-state batteries.

Li-Ion

One of the biggest reasons electric vehicle manufacturers prefer lithium-ion battery technology is its high power-to-mass ratio. Heavy components are the enemy of range, as the battery uses too much energy starting and stopping a heavier vehicle instead of traveling a further distance per charge. In addition, Li-ion batteries have high energy density and better performance than their alternatives at elevated temperatures. Li-ion's use in the consumer electronics industry is partly responsible for the high energy density.

The decreasing size of electronics and the desire for longer operating hours per charge spurred innovation in this parameter. As a result, Li-ion's energy density is more than 2.5 times greater than both NMH and lead-acid batteries. In addition, Li-ion batteries are recyclable, making them a good choice for environmentally-focused consumers.

Lithium batteries are comprised of:

• A cathode (made of cobalt, nickel, or manganese) that determines the capacity and voltage of the battery
• An anode (made of graphite or silicon) that enables electric current to flow through an external circuit
• An electrolyte comprised of salts and other additives that transfer ions from the cathode to the anode. It is what gives the name to the battery type ("Li-ion" means lithium carries the electrons in the form of a negative ion)
• A separator to prevent direct contact between the anode and the cathode

Though Li-ion is a low-cost and technically advantaged solution, exploding demand for this kind of battery coupled with commodity inflation affecting its raw materials has buoyed the costs, stopping prices from continuing downward. In addition, it does hold the risk of swelling from excessive temperature change or sharp impact and, because it is a liquid, it may leak upon intense impact. Finally, lithium is an alkali metal, meaning it is highly reactive and flammable. This feature represents another critical safety hurdle to clear.

Nickel Metal Hydride (NMH)

While Li-ion is the standard for all-electric vehicles (AEV), NMH is better suited for hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV). While NMH has a better life cycle than Li-ion or lead-acid, the chemistry brings its share of downsides. Nickel metal hydride batteries are less expensive compared with Li-ion, but they experience higher self-discharge rates when not in use. NMH also generates substantial heat at the hotter end of the operating range. This excess heat generation leads to reduced range and shorter life cycles. This technology also carries the risk of undesired hydrogen loss that manufacturers and consumers must monitor and control.

Lead Acid

Though the closest technical performer to Li-ion among liquid-state options, lead-acid batteries are essentially relegated to research or stop-gap status. They exhibit poor cold temperature performance, challenging their use in broad global climates, and they have a short lifespan. While consumers are not likely to warm to replacing 30% of the EV cost at a higher rate than expected, this technology supplements prime-path chemistries with a safe, inexpensive, proficient [short-term] intermediate solution.

Ultracapacitors

Like lead-acid battery chemistry, ultracapacitors are also a secondary option for energy storage. However, their primary differentiator is that they help the battery level its load, taking excess or giving energy when needed. This flexibility is critical in a secondary energy storage source.

Li-Ion vs. Solid-State Electrolytes for BEV

Given the above comparison, the clear winner for liquid-state electrolytes remains Li-ion. But that realization is not the end of the story; as discussed above, Li-ion is susceptible to higher costs due to skyrocketing demand. In addition, there is an opportunity to improve the top safety concern of flammable liquid catching fire. Safety is one of the most critical success criteria for EVs to inspire public confidence, so the industry and safety regulators would likely welcome an opportunity to step up the safety features.

Solid-state batteries (comprised of lithium metal) address the most pressing safety challenges of Li-ion. They are more stable, have a higher energy density than the already high Li-ion, come from readily available materials, and offer lower flammability, faster charging, and more extended range. In addition, the solid electrolyte material improves and extends the performance range of EV batteries, charges more quickly, and uses readily-available materials.

Barriers to Wide-Scale Adoption

Solid-state is the future of EV batteries. Now, however, there are a few barriers to widescale adoption that battery developers must solve. Solid-state batteries will carry a higher development cost due to a lack of capital to produce mass quantities. It is crucial to get this cost down to encourage consumers to buy solid-state electric vehicles.

There are gaps in the solid Li electrolyte material that degrades battery performance and service life when implemented into BEVs. In addition, solid-state batteries are prone to cracking, and it is best to charge them at 140 degrees Fahrenheit for optimal performance.

As with any development material, it will be critical for the manufacturing process to be efficient. There has not been a mass-produced solid-state battery for electric vehicles to date. As a result, manufacturing challenges through the lack of experience with solid electrolyte materials would substantially delay wide-scale adoption. A manufacturing issue could also cause the EV battery plant to shut down, delaying the majority of the public's first interaction with the new battery and affecting consumer confidence.

From the funding standpoint, securing current investment through the automakers themselves will enable higher adoption and consumers to enter the market. In addition, companies are investing hundreds of millions of dollars into the technology to find a better solution to Li-ion and to decrease solid-state manufacturing costs.

Conclusion

Given solid-state batteries' benefits, the major automakers are pivoting their strategies to that electrolyte chemistry. As solid-state batteries will become the standard electrolyte technology of the future for EVs, Toyota announced it will invest $13.5B by 2030 into that chemistry. In addition, a group including VW, Ford, and BMW are among the OEMs committing to solid-state batteries in the coming years for electric vehicles.

Not without its challenges, solid-state batteries address flammability safety concerns with liquid-state batteries, charge faster, and provide a more extended driving range. Getting the capital equipment in place and ramping up the supply of the batteries will create the glide path to transition the market from liquid- to solid-state. The industry and consumers will all benefit from the shift.

Adam Kimmel has nearly 20 years as a practicing engineer, R&D manager, and engineering content writer. He creates white papers, website copy, case studies, and blog posts in vertical markets including automotive, industrial/manufacturing, technology, and electronics. Adam has degrees in Chemical and Mechanical Engineering and is the founder and Principal at ASK Consulting Solutions, LLC, an engineering and technology content writing firm.