Direct Cathode Recycling - Battery Design

Selective Relithiation vs Pyro- and Hydrometallurgical Routes

Recycling Processes and Recycling Process Engineering both mention direct recycling in passing, alongside the pyrometallurgical and hydrometallurgical routes. This post stays with direct recycling specifically, and with the step that makes it viable at all: relithiation, putting the lithium a degraded cathode has lost back where it belongs without ever dissolving the cathode structure.

Repair, not resynthesis

A cell loses capacity in service partly because cyclable lithium gets consumed – trapped in a growing SEI layer, plated out on the anode, or stranded in cathode particles that have cracked away from the current-collecting network. Pyro- and hydrometallurgical recycling both deal with this by going all the way back to elemental or salt form: smelt or leach the black mass down to metals or metal salts, then resynthesize fresh cathode powder from scratch. Direct recycling skips that teardown. The bulk crystal structure of the cathode is treated as still basically sound, and the process just tops the lithium back up and anneals the lattice defects that built up in service, so the particle comes out the other side close to its original stoichiometry without ever being taken apart to the element.

That’s the appeal: skip dissolution and resynthesis and you skip most of the energy and reagent cost that go with them. It’s also the limit on how far direct recycling can go, because it only works on cathode material that’s degraded rather than contaminated or physically destroyed – feedstock has to be sorted by chemistry before relithiation, not after.

Relithiation routes

A few different ways of getting lithium back into the lattice are being worked on in parallel, each trading off simplicity against how well it copes with a real, degraded feedstock:

Solid-state (thermal) relithiation – the cathode powder is mixed with a lithium source and fired, similar to the calcination step used when the cathode was first made. It’s the most direct route conceptually, but a uniform anneal depends on getting the lithium salt evenly distributed across particles of mixed size and morphology, which gets harder as the feedstock gets messier.
Hydrothermal relithiation – the cathode is relithiated in a lithium-salt solution under moderate heat and pressure, which lets the structure re-order at a lower temperature than a solid-state anneal needs. Recent work has also shown this can run on the residual lithium already present in the black mass rather than needing an external lithium salt feed at all, which cuts a reagent cost out of the process entirely.
Electrochemical (redox-mediated) relithiation – the degraded cathode is put back in a cell and lithium is driven back in electrochemically, sometimes using a redox mediator dissolved in the electrolyte as a shuttle to carry lithium and electrons to cathode particles a direct electrical contact can’t reach efficiently. This is closest to just running the cell in reverse, and it’s better suited to correcting a genuine lithium deficit than it is to fixing structural disorder.
Molten-salt (ionothermal) relithiation – the cathode is relithiated in a molten lithium-salt eutectic, which wets particle surfaces more evenly than a solid mix can and has been reported to cut process energy sharply against conventional resynthesis in lab-scale trials, though that figure is specific to the process and feedstock tested and shouldn’t be read as a general number for the route.

None of these is a clean drop-in replacement for the others yet. Hydrothermal relithiation is generally reported as the best-performing route on product quality and cost, solid-state relithiation is the simplest to run but copes worst with feedstock variability, and the electrochemical and molten-salt routes sit somewhere in between depending on what the incoming cathode actually needs corrected.

Set against the metallurgical routes

The case for direct recycling is clearest on energy. One review modelling LFP battery recycling routes with Argonne National Laboratory’s EverBatt tool put life-cycle energy use at roughly 18.4 MJ per kg of cell for pyrometallurgical recycling and 30.6 MJ/kg for hydrometallurgical, against about 3.5 MJ/kg for direct recycling – the smelting step dominates the pyro number, and upstream production of the leaching chemicals dominates the hydro number, and direct recycling largely avoids both.

Hydrometallurgy still earns its place for anything direct recycling can’t touch: mixed or unidentified chemistry, physically damaged cells, and feedstock where the priority is pulling out cobalt and nickel as pure salts rather than preserving a crystal structure. Pyrometallurgy remains the simplest route for badly mixed or contaminated scrap precisely because it doesn’t care about feedstock purity going in. Direct recycling only makes sense once the feedstock has already been sorted to a single, known cathode chemistry – it’s a narrower tool, not a universal replacement.

Why this isn’t the default route yet

The gap between direct recycling’s lab results and a real recycling line is feedstock heterogeneity. A real waste stream has no standard cell chemistry, form factor, module design, or degradation history – two NMC622 cells that left the factory identical can arrive at the recycler with different lithium deficits depending on how they were used. The ReCell Center, the US DOE’s applied-research hub for direct recycling, has been explicit that this variability, not the relithiation chemistry itself, is the harder problem: getting a lithium salt evenly distributed across particles of inconsistent size and surface condition is what makes treating a genuinely mixed feedstock less effective than treating a clean, single-chemistry lab sample. Pilot-scale trials on realistic mixed feedstock have reported capacity retention meaningfully below what the same process achieves on optimised lab-prepared material.

That’s why sorting is doing as much work as chemistry here. Magnetic and other bulk-property separation of binary and ternary cathode mixtures is being developed specifically so a relithiation line can be fed a known, single chemistry rather than having to relithiate a blend and hope the anneal conditions suit every particle in it.

Moving from lab to line

The clearest sign direct recycling is moving past pilot scale is Princeton NuEnergy’s build-out in Chester County, Pennsylvania: a commercial-scale facility built around the company’s low-temperature plasma-assisted separation process, backed by a US DOE grant, aimed explicitly at what it calls a closed-loop “Cathode-to-Cathode” line – taking manufacturing scrap and end-of-life cathode material in and returning rejuvenated, battery-grade cathode active material to a cell maker in roughly a week. That’s the commercial bet behind all of the lab chemistry above: that sorted, single-chemistry cathode scrap can be relithiated and shipped back into cell production faster and cheaper than resynthesising it from ore or leached salts.

References

Yang, X. et al., Towards Greener Recycling: Direct Repair of Cathode Materials in Spent Lithium-Ion Batteries, Electrochemical Energy Reviews, 2023
Maximizing Lithium Recovery in Direct Recycling: Relithiation of Layered Oxide Cathodes Using Residual Lithium, 2025
A Comparison of Relithiation Methods for the Direct Recycling of End-of-Life NMC622 Cathode Materials, Recycling, MDPI, 2025
Recycling of spent lithium iron phosphate batteries – a review of processes, economics, and carbon footprint, Waste Management, ScienceDirect, 2025 (energy figures modelled using Argonne National Laboratory’s EverBatt tool)
Gaines, L., Direct Recycling R&D at the ReCell Center, Recycling, 2021
U.S. Department of Energy Selects Princeton NuEnergy for $50 Million Grant to Build Closed-Loop Cathode-to-Cathode Facility, PR Newswire, 2025