This study compares the environmental impacts of a lithium‐ion battery (LiB), utilizing a lithium iron phosphate cathode, with a solid‐state battery (SSB) based on a Li6.4La3Zr1.4Ta0.6O12 garnet‐structured electrolyte. It uses a hybrid life cycle assessment (LCA), according to two functional units, delivery of 50 MJ of electrical energy and kg of battery, to expand the system boundary. The results of the process LCA indicate that the environmental impact of LiBs is lower than SSBs across most environmental impact categories. Conversely, the input–output upstream global warming potential (GWP) of the LiBs, calculated by hybrid LCA, is higher than that of the SSBs. Sensitivity analysis shows that the SSB cycle life must increase from 100 to 2800 to achieve a GWP impact lower than that of LiBs and therefore outperform LiBs in this environmental impact category. The study, therefore, demonstrates that research into SSBs must be accelerated to achieve a functional and safe battery technology with a reduced impact on the environment.