Adapted from J. Mater. Chem. A, 2017,5, 11671-11681

Additionally, solid polymer electrolytes are often poor conductors of lithium ions at room temperature.[10] Commercial solid polymer-based batteries only work at elevated temperatures, such as 60-80 °C. It’s possible to work around this using a very thin layer of polymer on top of a conventional plastic separator filled with liquid electrolyte. Still, approaches like this suffer from issues that combine the worst of both worlds — they insufficiently address dendrite formation and add a layer of material that increases cell resistance.

Expensive lithium foil

Finally, the third problem with using solid polymer or liquid electrolytes with a lithium-metal anode: the lithium anode material is typically manufactured first and then built into a cell. For automotive applications, this lithium foil must be made very thin, and because lithium is extremely reactive and difficult to handle, manufacturing at large scales can be cost prohibitive. Adding a coating on the lithium foil further complicates manufacturing. It all adds up, making lithium-metal anode material up to 100x more expensive than typical battery anode materials, potentially doubling the cost of materials per kilowatt-hour and undercutting any potential commercial advantage.[11] And the slightest impurities in the foil accelerate dendrite formation,[12] multiplying the existing performance limitations and safety risks.

Compromised performance

Lithium-metal cells using liquids or solid polymers can appear to work when tested under a set of compromised conditions. The workarounds deviate dramatically from real-world conditions. For example:

1. Dendrites are less likely to form at very low charge rates,[13] such as a five-hour charge (i.e., 0.2C or C/5), followed by rapid discharge (1C). Low charge rates are unacceptable for EV drivers accustomed to charging 0–100% in an hour or less.
2. High pressure applied to a lithium-metal cell can delay the destructive effects of dendrites. However, applying high levels of pressure in an EV pack requires expensive and heavy mechanisms, which erode the potential energy density advantage of lithium-metal.
3. Lithium-metal cells with high resistance can be made to work when operated at high temperatures, such as 80 °C. But this is impractical for passenger EV applications because it would require a large, complex, and expensive thermal management system that would be bulky and expensive. (See our blog post on temperature in battery development for more on this.)
4. The problems with resistance growth in liquid electrolyte/lithium-metal systems have been well-known for decades. Resistance growth leads to voltage fade and can have dramatic negative effects on cycle life. But cycle life testing that only measures capacity (Ah) can hide the evidence of voltage fade. Any liquid or solid polymer-based lithium-metal battery with commercial aspirations will need to demonstrate good cycle life by measuring energy retention (Wh), not merely capacity.

The QuantumScape difference

Our key innovation is a ceramic solid-electrolyte separator, which has allowed us to overcome the fundamental issues that prevent liquid or solid polymer electrolytes from delivering acceptable automotive performance when used with lithium metal. These are decades-old problems: as far back as 2002, Aurbach et al. concluded that, when liquid electrolytes are used, “there is no way that rechargeable [lithium-metal] batteries can compete with lithium-ion batteries in any application that requires high charging rates.”[14]To date, we haven’t seen any data to suggest the problems discussed here have been resolved.

In contrast, we have demonstrated that our solid-state electrolyte can resist dendrite formation at high rates of charge under realistic, automotive-relevant test conditions and is very stable with lithium metal – providing a barrier between the pure lithium anode and our cathode and catholyte materials. Crucially, our separator material also enables the cell to be manufactured anode-free, with the pure lithium-metal anode created during the first charge; this means that our lithium anode material is pure electrochemically distilled lithium metal, created without the cost and complexity involved in handling lithium foil. This combination of properties has allowed us to build a next-generation battery technology that we believe can deliver on the true promise of lithium metal: excellent cycle life, improved energy density, faster-charging speeds, enhanced safety and lower costs.