Solid-State Battery Aces Tests for EV Charge Times, Range

Test data for QuantumScape’s lithium-metal solid-state battery cells indicate the battery technology has exceeded energy retention performance goals for its A-sample development phase, according to Volkswagen Group (VW).

 

Solid-state batteries promise longer EV ranges. Image used courtesy of QuantumScape

 

Months-long testing performed by Volkswagen’s subsidiary battery company PowerCo showed QuantumScape’s prototype cells could successfully complete 1,000 charging cycles while maintaining 95% of their nominal energy storage capacity, equivalent to driving 500,000 km without any perceived reduction in electric vehicle (EV) range.

If successful, solid-state batteries offer the potential for significant improvements over conventional lithium-ion battery cells in terms of improved EV ranges, charge times, and safety.

 

Solid-State Lithium-Metal Batteries

According to QuantumScape, traditional lithium-ion battery designs are rapidly approaching the limit for energy density. To address this performance barrier, QuantumScape has introduced key battery design innovations encompassing a proprietary ceramic solid-state separator combined with lithium-metal anodes.

The ceramic solid-state separator replaces the polymer separators found in conventional lithium-ion batteries. Since the ceramic does not react with lithium, the graphite/silicon anodes used in conventional batteries can be replaced with higher-density lithium-metal anodes. 

 

Solid-state lithium-metal cell design. Image used courtesy of QuantumScape

 

In addition to higher energy density (meaning longer EV ranges), solid-state lithium-metal batteries can also charge faster since they eliminate the lithium diffusion process found in conventional batteries. The batteries are also safer since the separator and anolyte materials do not contain combustible organic materials.

Based on QuantumScape’s commercial energy density target of 800–1,000 Wh/L for its solid-state batteries, the batteries could extend EV ranges by over 40% when compared with the 700 Wh/L energy density of today’s best-performing lithium-ion cells.

 

Pairing Lithium-Metal Anodes with Cathodes

The two main cathode types used in conventional lithium-ion batteries are nickel manganese cobalt and lithium iron phosphate (LFP). Battery cells using CMD cathodes have high energy densities well suited to EV powertrain applications, but they can be expensive. LFP-based cells are a more affordable option with a better cycle life, but a 20% lower energy density reduces EV driving ranges.

Together, the two cathode technologies form a price-performance frontier based on cost and energy density. 

With the solid-state ceramic separator and lithium-metal anode, QuantumScape asserts that their battery technology shifts the energy/cost performance frontier, increasing battery energy densities while lowering the cost per unit of energy density for both CMD and LFP battery types. 

 

Battery performance frontier with lithium-metal anodes. Image used courtesy of QuantumScape

 

Energy Retention Test Results

For newly developed battery cell technologies, robustness (as measured in number of charging cycles) and capacity retention are considered the most important criteria for standardized battery testing. Industry targets for these criteria are typically 700 charge cycles with 80% capacity retention.

QuantumScape’s battery exceeded both testing targets, successfully completing 1,000 charging cycles with only marginal aging, maintaining 95% of its nominal rated energy storage capacity. 

 

24-layer prototype battery cell. Image used courtesy of QuantumScape

 

In practical terms, this result indicates that a typical EV using the new batteries could be driven up to 500,000 km without any significant loss in range between charges.

 

Next Steps for Solid-State Batteries

The biggest challenge to the adoption of solid-state batteries in EVs is the development of high-volume manufacturing processes. This is the next goal for both VW and QuantumScape. Conveniently, the battery cells tested in the most recent program consisted of 24 layers, the same number planned for the series production cells.