The world has been eagerly awaiting the promised benefits of solid-state batteries: Lighter weight, longer range, faster charging, no flammable liquids or gels, nothing to freeze, etc. In December 2020 it looked like QuantumScape was out in front of a crowded field of would-be solid-state battery suppliers, but now Colorado-based Solid Power has announced it's launching a pilot production line, scaled to produce up to 15,000 cells per year for its OEM partners to use in development testing. Solid Power expects to be the first solid-state battery supplier to reach this critical commercialization milestone. Might that suggest its primary investors BMW and Ford could be first to market with an EV powered by this highly anticipated battery type?

The anode and electrolyte distinguish this battery concept from others. It still qualifies as a lithium-ion battery, and as such its cathode (positive electrode) consists of nickel-manganese-cobalt as found in most of today's EV batteries. But the anode is comprised of more than 50 percent silicon, rather than the more typical (and often heavier) carbon graphite. The rest of the anode consists of binders and conductive additives as well as some of the sulfide electrolyte.

Solid Power won't divulge the precise chemistry of its electrolyte, except to say that it is a sulfide (other solid-state battery research has involved ceramic and solid-polymer type electrolytes). Solid Power says its battery design can be manufactured using similar processes and equipment currently in use producing today's lithium-ion batteries.

Batteries proposed by QuantumScape and others that rely on a current-collector anode onto which metallic lithium simply plates as the battery discharges all expand as that reaction occurs. Silicon anodes can also expand during discharge, but the structure of Solid Power's silicon composite material limits this total expansion. Some compliance is still required to accommodate this expansion at the module level.

Solid Power claims its cells should deliver a cost savings at the pack level of 14-35 percent, coming in at roughly $85/kWh at the cell level. They're also more power dense in terms of both power-to-mass (estimated at 320 kW/kg, while Tesla's 2170 cells achieve 260 kW/kg) and power-per-volume. So switching to solid-state promises a choice of benefits—equivalent range at lower cost, with lighter batteries that also might permit lightening other components; or equivalent pricing for a pack with more smaller and cheaper cells that extend range.

The company plans to optimize its solid-state battery cells for capacities ranging from 60 to 100 amp-hours. Here each pouch type "cell" consists of 40 double-sided cathodes and 41 anodes. That works out to 24 to 40 kWh at 400 volts, 48 to 80 kWh at 800 volts. This flexibility is expected to meet most automotive partners' specifications while offering a breadth of performance options.

Solid Power is claiming a 10-to-90-percent charge time of less than 15 minutes at a "2C" charge rate (or double the rated amp-hours, so 48kW for the 60Ah cells at 400 volts to 160 kW for a 100 Ah cell at 800 volts). Here again, EV manufacturers will have knobs they can turn during development of their proprietary modules, packs, inverters, charging and battery-management systems, to tweak those charging times to whatever they want to claim (and provide warranty coverage for).

Lithium metal spikes, called dendrites, can form in any lithium-based cell under extreme conditions, and if these spikes penetrate the separator, they can short the cell. Solid Power works to prevent this by controlling lithium plating and creating a barrier to dendrite growth with a separator layer.

Solid Power has subjected its pouch cells to nail penetration, overcharge, and external short-circuit abuse and found them to suffer only "benign failures"—so no flames, venting, or loss of material, and no extreme temperatures (they hit 81 degrees F with the nail, 156 degrees after a 200-percent overcharge—that's warm, but not dangerously so).

Early test cells have shown themselves to retain 80 percent of their original capacity after 650 to 1,000 cycles, depending on conditions. The lower end of that range presumed a DC fast charge every fifth cycle, while the high end represented more normal charging at moderate temperatures. But that testing was conducted on early sample cells. Solid Power expects that fully developed production cells will last 400,000 miles in an EV. If that prediction pans out, the challenge may be ensuring that the rest of a BMW iX or Ford F-150 Lightning survives as long.