GM has released more details about its next-generation Ultium batteries, including plans for lithium-metal (Li-metal) technology to boost performance and energy density. The automaker announced that it has signed an agreement to work with SolidEnergy Systems (SES), an MIT spinoff developing prototype Li-metal batteries with nearly double the capacity of current lithium-ion cells. As a reminder, Li-metal batteries replace carbon anodes with lithium metal, allowing for lighter and more powerful cells. The challenge with the technology is increased resistance and "dendrite" filaments that tend to form on the anodes, making batteries short-circuit and heat up. Previous lithium-metal batteries would only work when heated up to 175 degrees F, but SolidEnergy developed an electrolyte coating for lithium metal foil that works at room temperature.