Recycling, second life and other energy storage

The end of a battery’s life and the wider storage picture on one page: pretreatment of spent batteries, hydrometallurgical and direct recycling, the second-life loop from the car to stationary storage, and the other ways to store energy, from pumped hydro, compressed air, flywheels and heat to hydrogen, supercapacitors, flow batteries and lithium-ion. Every caption and number is quoted from the cited papers.

1. Pretreatment: Waste batteries are deactivated (pre-discharged), dismounted and mechanically separated: pre-discharged by immersion in NaCl solution, or by high-speed shredding in cryogenic nitrogen, which avoids disassembly; dismounting still needs intensive manual work, while separation can be automated (Abe 2024), and emerging robotic solutions improve disassembly (Esmaeilian 2026). 2. Hydrometallurgy: Pyrometallurgy roasts, calcines and pyrolyses at high temperature into alloys of Co, Cu, Ni and Fe and slags of Li, Al, Mn, Si and Ca. Hydrometallurgy starts from the black mass, the pyrolysis residue: acids leach the metals as ions, collected by solvent extraction, chemical precipitation and electrolytic deposition, and sulfuric acid is common in industry (Abe 2024). Pyrometallurgy has robust throughput but is energy-intensive; hydrometallurgy allows selective recovery (Esmaeilian 2026). 3. Direct recycling: Direct recycling peels the cathode material from the aluminium foil, by sintering under inert conditions or immersion in an organic solvent such as N-methylpyrrolidone, and reactivates it with a lithium source such as Li₂CO₃ or LiOH, with fewer steps and a lower environmental burden (Abe 2024); it is a nondestructive repair of degraded electrode materials (Wu 2024) that supports closed-loop circularity but needs scale-up (Esmaeilian 2026). 4. Second life: Most automotive lithium-ion batteries are recycled, but they keep 70–80% of their capacity, which can be adequate for residential storage with rooftop PV, utility-level PV firming and peak-shaving (Kamath 2020). EV batteries are typically retired after 8–10 years; second-life systems need health-based sorting, grouping into homogeneous clusters and an advanced BMS (Lekbir 2026). Recycled EV batteries could be a valuable secondary source of materials (Harper 2019). 5. Mechanical and thermal: Pumped storage hydropower pumps water to an upper reservoir off-peak and releases it at peak demand; about 160 GW was installed worldwide in 2021 (Ge 2025). Adiabatic compressed-air storage pairs the air store with thermal storage; salt caverns are preferred for large, long-duration stores, and 300 MW-class demonstrations exceed 70% round-trip efficiency (Xu 2026). A flywheel rotor on magnetic bearings is charged and discharged through a motor–generator (Hashim 2026) and high-speed flywheels run in a vacuum (Düsterhaupt 2024); thermal storage holds heat as sensible, latent or thermochemical energy (Saher 2024). 6. Hydrogen and electrochemical: Surplus solar electricity makes hydrogen in an electrolyser, and the stored hydrogen feeds a fuel cell when sunlight is low (Li 2026). Supercapacitors store energy by ion adsorption or fast surface redox, for high power (Simon 2008); in redox flow batteries the electrolyte tanks sit outside the cell, so power and capacity can be designed separately (Jiménez-Blasco 2023); lithium-ion batteries are the most economical storage medium currently available for energy storage systems (McKinnon 2022).

Abe et al., “Cathode active materials using rare metals recovered from waste lithium-ion batteries: a review”, Heliyon 2024 (doi:10.1016/j.heliyon.2024.e28145); Esmaeilian et al., “Electric vehicle batteries recycling: a review”, Waste Manag. 2026 (doi:10.1016/j.wasman.2025.115302); Wu et al., “Toward circular energy: exploring direct regeneration for lithium-ion battery sustainability”, Adv. Mater. 2024 (doi:10.1002/adma.202403818); Kamath et al., “Evaluating the cost and carbon footprint of second-life electric vehicle batteries in residential and utility-level applications”, Waste Manag. 2020 (doi:10.1016/j.wasman.2020.05.034); Lekbir et al., “Second-life lithium-ion batteries for circular energy systems: a techno-economic and environmental pathway to affordable grid-connected renewable storage”, Glob. Chall. 2026 (doi:10.1002/gch2.70150); Harper et al., “Recycling lithium-ion batteries from electric vehicles”, Nature 2019 (doi:10.1038/s41586-019-1682-5); Ge et al., “Embodied carbon emissions of pumped storage hydropower infrastructure construction in China across temporal-spatial and driving factors dimensions”, Sci. Rep. 2025 (doi:10.1038/s41598-025-30019-z); Xu et al., “Recent advances in adiabatic compressed air energy storage systems with thermal energy storage: classification and control strategies”, ACS Omega 2026 (doi:10.1021/acsomega.6c07207); Hashim et al., “A model predictive control framework with fatigue life assessment for flywheel energy storage systems supported by active magnetic bearings”, Sci. Rep. 2026 (doi:10.1038/s41598-026-66926-y); Düsterhaupt et al., “Economic evaluation of kinetic energy storage systems as key technology of reliable power grids”, PLoS One 2024 (doi:10.1371/journal.pone.0311160); Saher et al., “Trimodal thermal energy storage material for renewable energy applications”, Nature 2024 (doi:10.1038/s41586-024-08214-1); Li et al., “AI-driven optimization of integrated solar systems for hydrogen production and building energy supply”, Sci. Rep. 2026 (doi:10.1038/s41598-026-49520-0); Simon & Gogotsi, “Materials for electrochemical capacitors”, Nat. Mater. 2008 (doi:10.1038/nmat2297); Jiménez-Blasco et al., “Recent advances in bromine complexing agents for zinc-bromine redox flow batteries”, Materials 2023 (doi:10.3390/ma16237482); McKinnon et al., “Full-scale walk-in containerized lithium-ion battery energy storage system fire test data”, Data Brief 2022 (doi:10.1016/j.dib.2022.108712).

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