Repurposing retired batteries for cost-efficient carbon-neutral power system

Rapid expansion of variable renewable generation is straining national storage capacity, while electric vehicle (EV) battery packs with substantial residual life are slated for premature recycling. Repurposing these packs as second-life batteries (SLBs) could bridge the storage gap, yet their role at the grid level remains unclear. Here, we develop a techno-economic model to evaluate how SLB integrations shape China's power system over the next 25 years, with an emphasis on SLB chemistry type, replacement frequency, and geographic allocation. The results show system costs declined by 903.4 billion CNY (2.49%) until 2050, with SLBs meeting 67.09% of the total storage demand. SLBs substitute for pumped hydro storage and new batteries, with their installed capacities falling by 94.55% and 29.48%, respectively. This shift also reduces high-cost, carbon-intensive generations by 48.10%. Scenarios reveal substantial regional variation in SLB capacity and chemistry, especially in supply limited regions. This study provides actionable guidance for policymakers and industry stakeholders.