Benchmarking state-of-the-art sodium-ion battery cells – modeling energy density and carbon footprint at the gigafactory-scale

Abstract

Sodium-ion batteries (SIBs) are on the verge of mass adoption, with several players striving for gigafactory-scale production. Uncertainty remains regarding their competitiveness with lithium-ion batteries (LIBs). This study addresses this concern by quantifying the energy density and carbon footprint (CF) of commercially pursued SIB cell chemistries through comprehensive modeling. Multiple material and production scenarios are analyzed, with LiFePO₄ (LFP)-based LIB cells serving as an industry benchmark. Based on a screening of commercially relevant SIB active materials, the volumetric and gravimetric energy content is modeled for large-format pouch and prismatic cells, as well as varying cathode coating thickness. Additionally, a GWh year⁻¹ cell production is simulated using primary machine data and current production equipment. The outputs from cell and production modeling feed into a life cycle assessment to determine the CF by calculating the cradle-to-gate global warming potential, relative to the cell energy [kg CO₂-eq. kWh⁻¹]. Additionally, new life cycle inventory data are presented for the industrial-scale synthesis of hard carbon (HC) and SIB-cathode active materials (CAMs). Current SIB cells show notably lower energy content compared to the LFP benchmark, particularly in terms of volumetric energy density (Δ = 17–49%). However, this gap could be narrowed and even closed for specific SIB cell chemistries through optimization of HC, representing a near-future scenario. While HC is the bottleneck towards higher energy density, the opposite is true for the CF. Among the seven evaluated SIB cell chemistries, four demonstrate CFs that are highly competitive with the LFP benchmark (Δ = 1–8%), despite exhibiting lower energy densities. This is primarily due to the substantially lower CF of HC (3.2 kg CO₂-eq. kg⁻¹) compared to synthetic graphite (25.1 kg CO₂-eq. kg⁻¹). In contrast, LFP-CAM (7.6 kg CO₂-eq. kg⁻¹) causes fewer emissions than six of the seven analyzed SIB-CAMs (5.8 kg CO₂-eq. kg⁻¹ to 22.0 kg CO₂-eq. kg⁻¹). The key factors for further CF reduction in SIBs are the application of low-impact CAMs and increasing energy density. In contrast, transitioning to aqueous cathode processing was found to only have a small impact on the cell-level CF, considering state-of-the-art N-methyl-2-pyrrolidone recovery techniques.