Are Solid-State Cells Safer?
Research & Development

Are Solid-State Cells Safer?

Author: Solid Power Admin
Date Posted: 12.08.21

Are Solid-State Cells Safer?

Catchy headlines about lithium-ion battery malfunctions — exploding phones and Chevy Bolts — have made their rounds in the press in recent years. Unsurprisingly, these stories have created significant public discourse and questioned the safety of lithium-ion batteries.

Though design, size, and use case varies from one Li-ion cell to the next, they all rely on one critical factor: the stability of the liquid electrolyte. Like Earth needs specific atmospheric conditions to support life, Li-ion batteries have their own “goldilocks zone” to ensure the best possible performance and lifecycle. If a cell is too cold, current flow is inhibited and energy efficiency drops. If it’s too hot, the volatile liquid electrolyte can catch fire. This risk increases when a cell ruptures and expels its contents.

All-solid-state batteries are cited as a solution to many of these problems. Unlike liquid electrolytes, sulfide solid electrolytes can withstand extreme temperatures. The risk of ignition is lower, too. Though various parts of an all-solid-state cell are flammable (like the pouch encasing the electrodes), it’s far more difficult for all-solid-state batteries using a solid electrolyte throughout the entirety of the cell to catch fire.

To determine if all-solid-state cells are safer than Li-ion cells under certain conditions, Solid Power recently submitted multiple 2 Ah prototypes for third-party testing. The third party subjected the cells to several standard tests: nail penetration, overcharge, and external short circuit.

Nail Penetration

During a vehicle collision, the force of impact and subsequent mechanical damage can rupture a car’s battery. Nail penetration tests are an effective way to replicate that damage in a controlled, safe environment. Though specific parameters vary, every nail penetration test includes three elements:

  • a charged battery cell
  • puncture with a conductive nail
  • full penetration of the cell (in one side, out the other)

Solid Power’s cells were first heated to 45° C for three hours. They were then charged with a continuous 0.2 A current, and once the voltage reached 4.2 V, the cells were monitored for one hour. During testing, each cell was fully penetrated with a conductive nail at 8 cm/s. Despite this complete rupture, none of the cells showed signs of fire after nail penetration  — just mild voltage loss. No smoke or fire was reported.

Overcharge

Overcharge is possible when a vehicle’s alternator fails, an EV battery is charged with an improper voltage, or it’s plugged in too long. The resulting heat can cause lasting damage to the battery cell, either killing it completely or causing a fire.  Overcharge tests determine how particular chemistries react in these conditions.

Like the nail penetration test, each of the sample cells was heated to 45° C for three hours, charged with a continuous 0.2 A current until the voltage reached 4.2 V, and monitored for one hour. The cells were then charged with a 1 C-rate until they reached 15 V or charging throughput reached 2 Ah. Three separate test cycles were conducted.

Across the three tests, the cells reached an average maximum temperature of 69.2° C . During the first two tests, the cells experienced a slight voltage increase relative to the starting voltage (~ 4 V). The cells in the third test experienced the highest voltage spike, increasing by up to 0.406 V. However, no venting, rupture, or fire was observed on any of the tested cells.

Short Circuit

Short circuits occur when a battery’s anode and cathode make contact.

Once again, each of the sample cells was heated to 45° C for three hours, charged with a continuous 0.2 A current until the voltage reached 4.2 V, and monitored for one hour. Testing began when each cell reached ambient conditions, approximately 23° C .

One set of cells was subjected to a short circuit of 100 mΩ and the other 500 mΩ. The cells tested against 100 mΩ reached an average maximum temperature of 112.7° C, and on average lost 1.27 V. Similarly, the cells tested against 500 mΩ reached an average maximum temperature of 43.9 °C and lost an average of 0.713 V. Like the overcharge test, no venting, rupture, or fire was observed in any of the cells.

Verifiable Safety

Because solid-state battery technology is still in development, it’s easy for manufacturers to safeguard their chemistry and make lofty promises. But most claims are just that — claims. Lab findings are one thing but external verification of internal results is another.

This early data is a promising step forward, showing that sulfide-based all-solid-state batteries can be more stable, less reactive, and safer than traditional lithium-ion.  As we work to scale our EV cell line and submit prototypes for automotive qualification, we will continue to test and verify the safety of our all-solid-state cells.