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Play it again Steve— How direct recycling can repair battery cathodes

Dr Steve SloopDr Steve Sloop, CEO of OnTo Technology, describes how direct recycling holds the key to the cost-effective re-purposing of cathodes by unlocking the potential of the original energy and materials used to first fabricate them. By repairing rather than breaking-up the metals used in cathode construction considerable energy and materials, that would otherwise be wasted, can be saved.

The lithium-ion battery industry was launched in 1991 with cells powering portable video recorders. The prototypical original cathode chemistry, lithium cobalt oxide (LCO), if recycled at all in the early years, was done so with an infrastructure for recycling elemental cobalt. This relied on standard pyrometallurgical methods, such as smelting, which was commercially available, to address spent industrial catalysts. Now it seems, lithium-ion applications are limited only by our imagination, as chemistries diversify and move away from cobalt. According to studies by Bloomberg, in 2025 LCO will represent 8% of the anticipated recycled tonnage of lithium-ion batteries, with the balance represented by cobalt-dilute and cobalt-free formulations. The commercial incentive for recycling the next generation of lithium-ion batteries will increasingly be challenged by the inevitable cobalt dilution in their cathode materials. 

Nanoscale engineering— SEM comparison of harvested, hydrothermally treated, and heat-treated NCM523Powered by innovation, the diversification of lithium-ion chemistry has been driven by the need to improve safety and performance, whilst also decreasing the manufacturing cost. Nanoscale engineering, fed by understanding of failure mechanisms, has led to tough, resilient structures that can last many thousands of charge-discharge cycles. Longevity features, in general, aid the ability for positive electrodes to be recycled in one piece, and be reused directly to manufacture new batteries. And that is the theme of direct recycling: to directly reutilise the parts taken from used batteries, thereby conserving the energy and materials that have already been invested in their manufacture, to supply components that have the required structures and purity, to make new lithium-ion cathodes.

The aim of the direct recycling approach is to minimise energy and material input (cost) in recycling. An ideal situation would be to completely recycle an electrode to manufacture of a new battery, without its decomposition and separation into constituent elements, or its purification to remove trace elements. Decomposition of cathode material lattices in recycling is consistent with inputs such as high-temperature treatment, or chemically intensive hydrometallurgy. The cost of these processes increases with their chemical and energetic intensity. The direct approach conserves and repairs the structural characteristics of electrodes, especially the lattice, the original production of which is energy intensive. Cathode-healing™ stands out among the direct approaches as it conserves the lattice and cleans it (removes trace metals), and achieves the high purity required for battery manufacturing. 

Synthetic strategies, which have been used to produce lithium transition-metal oxide electrode materials in the first place, can be applied to repair them. A non-exhaustive list includes classic solid-state, electrochemical, hydrothermal, non-aqueous molten salt, sol-gel and ion-exchange synthetic methods. These all can be applied as direct recycling approaches to reset or make a new cathode material. We can examine each of these methods in turn to ascertain their effectiveness.

Solid-state synthesis

As a direct recycling approach, solid-state methods top-up, reintroduce, or replace the lithium that goes missing from the active complement in the original manufacture of the cathode. This requires a stoichiometric addition of lithium using a salt, relative to the amount of lithium ‘missing’ from the spent lattice. Also, that process ‘resets’ transition metal oxidation numbers to those supplied in ‘original-manufacturing’, for example Co(III) in LCO, and Mn(IV,III) in spinel structures. Opportunities and variations can be applied: for example, sol-gel may be used to coat a repaired cathode as a value-added step. Solid-state synthesis may be performed with a non-lithium salt to replace missing ions in the cathode, followed by an ion exchange step using a lithium-containing solution to replace the newly injected ions with lithium ions.

Fig 1: Cathode-healing™ preserves structure and improves purity. Solid-state methods maintain the purity level of the feedstock material, and HF by-products will fluorinate the cathode.Solid-state heating has some glaring limitations when applied to direct recycling. These are: unintended reactivity of the spent cathode itself towards an unusable material, a lithium-ion host with an irreversible failure mechanism, and mixtures of different cathode materials and cell components, which can yield products other than a functional electrode material. A notable issue is contamination due to a commonly found impurity in recycle-harvested electrodes, that is, the polyvinyl difluoride polymer that binds cathode-carbon mixtures. When heated, it produces hydrogen fluoride that attacks the cathode (Fig 1). Also common are cathode failure mechanisms linked to transition metal dissolution. When transition metals go missing from the lattice, they cannot easily be reinstated. 

Electrochemical reintroduction of lithium to spent material

Even the simple, obvious task of discharging a battery is an act of direct recycling. The electrode particles inside the cell take in lithium while reducing the oxidation state of the transition metal oxide host. But this is limited due to the battery management system pre-empting a full discharge and also the fact that some lithium inevitably goes missing from the lattice. This irreversible loss of the lithium inventory occurs during the very first charge-discharge cycle in the life of the battery. This gradual loss continues during the life of the battery with inevitable side reactions that trap lithium in the lattices, precipitate them from solutions, or paste them on the surface of electrodes as ‘solid-electrolyte-interfaces’. Because of this loss, and in order to effect a complete direct recycle of the component, it is necessary to have an additional input of lithium. This ensures that the positive electrode material can undergo a total electrochemical reduction and thereby provide the ampere capacity for the cell. 

For spent electrode materials harvested from end-of-life cells, their electrochemical reduction is achieved through a cathodic reaction in an external, electrochemical reactor. The system has a working electrode that contacts the spent electrode material, an electrolyte (non-aqueous, aqueous, or even a molten salt) and a counter electrode to provide lithium ions. Electrochemical reintroduction of lithium is akin to the discharge reaction in a lithium-cell except with an excess supply of lithium. While electrochemical reintroduction of lithium can address some lithium inventory deficiencies in spent electrodes, it may not be able to address cation mixing of transition metal ions with lithium-ions, or transition metal dissolution from electrodes. 

Cathode-Healing 

Fig 2: For nickel-rich NCM, cathode-healing repairs the structural deficiencies that develop during the life and use of the battery, typically in the form of cation-mixing. The Ni2+ in the used cathode is mixed with Li+; cathode-healing oxidises it to Ni3+ and returns functionality to the repaired cathode.Cathode-healing is the conservation and repair of electrode particles through reintroduction of lithium and repair of lattice structural deficiencies (Fig 2). These two issues can be addressed stepwise or simultaneously. Typically, the reintroduction of lithium occurs through a self-limiting process, accomplished by exposing the spent electrode material to a concentrated lithium-containing solution, made up from any suitable solvent (aqueous/organic), ionic liquid, or molten salt. Performing a hydrothermal treatment in concentrated lithium salt is a very effective way to re-establish the lithium inventory in a deficient, lithium metal oxide electrode. The electrode material reduces in solution and reaches an end point consistent with the typical layered oxide transition metal valence (3+). For olivine substrates, the endpoint is iron (2+). Reduction is achieved through kinetic mass action in the lithium containing solution, and it can be assisted through chemical or electrochemical means. Final regeneration of structure-property relationships, if necessary, can be achieved through a brief calcination. 

The typical situation for the spent electrode of a prototypical electrode material, LCO, is 10-15% loss of lithium inventory, structural changes from layered to cubic nature, and electrolyte decomposition products that have coated the cathode active material particles. Cathode-healing with a concentrated lithium containing solution, followed by calcination, reverses these deficiencies. 

Another major cathode class of layered metal oxide includes lithium nickel cobalt manganese oxides (NCMs). Isostructural with LCO, NCM replaces cobalt with nickel and manganese. On the formula diagram from LCO to LMO to LNO, there are low cobalt NCM811 (Ni:Co:Mn::8:1:1) formulations that have safety, performance, and cost advantages. A hallmark failure mechanism for these materials is cation-mixing, or formation of cubic nickel structures. 

For the NCM class of electrodes, cathode-healing can perform the not-so-obvious task of reintroduction of lithium (a reduction) and reformation of the electrode’s layered oxide character (an oxidation). As the battery industry utilises more NCM in many applications, for reasons of cobalt thrifting and performance, they may accidentally be designing for direct recycle on a fundamental, materials basis.

Harvesting materials

Once scrap cells are safely deactivated, removing and separating the active electrode materials from the aluminum and copper foil current collectors is accomplished via mechanical and/or physical-chemical means. Battery packs are disassembled to modules of cells, or individual cells that may be shredded and pulverised or soaked to delaminate electrodes from current collectors. The harvested material (or black mass) is composed of lithium metal oxide, graphite (and/or silicon), carbon, bits of current collector, binder, and electrolyte residues. Cathode-healing can directly accept such harvested materials. 

As noted above the initial step reintroduces lithium and repairs lattice features of the cathode. Along with that, trace metal impurities are removed, and electrode binders are decomposed without producing hydrogen fluoride. Binders effectively glue cathode particles together to prevent their physical separation.

One feature in the initial steps in cathode-healing is liberation of the electrode particles, which allows for facile separation of graphite and metal oxides. Methods such as flotation rely on solvent-surface interactions, which is difficult with different particle types bound together or coated with polymer. Liberated particles from the process can be separated using surfactants that preferentially adhere to a particle-surface-type and assist in floating that material in a bath.

Direct precursors

The cathode particles are very pure materials, even in scrap materials. Maintaining that purity is one of the opportunities for direct recycling. Inevitably, scrap electrode materials may not be suitable for repair and reuse in manufacture. Materials suffer from irreversible decomposition, mixtures may be completely incompatible for treatment, and a cathode may be out of demand. These can still be used, in many cases, as precursor materials for new cathodes. For example, LCO may well be able to be repaired, however, it may not have a suitable market for use. In such a case, the clean, low-cost LCO from cathode-healing can be used as a direct precursor for another cathode such as NCM for less cost than traditional cobalt sulfate.

Materials that have been through the cathode-healing process are clean, well-characterised, and therefore useful as source material for new cathodes. While going from precursor to new particle doesn’t conserve the energy which has been input into synthesis of the original (first-life) cathode lattice, it does conserve the energy input from original material ingredients purification.

For example, the product from cathode-healing is a low-cost precursor for cathode manufacturing simply through incorporation into their feedstocks. The cathode-healing™ intellectual property, know-how, and business will provide a synergistic link to the existing cathode-manufacturing intellectual property, know-how, and business community without being new competition. 

Cost and safety

No matter how lithium-ion batteries are recycled, at end-of-life they are Class 9 hazardous materials due to flammability of the electrolyte and lithium components. For this reason, the transportation cost contributes about one half of the overall recycling cost. Transforming lithium-ion into a class of non-hazardous material reduces that to about 2% of the overall cost. 

OnTo Technology has demonstrated improved cost and safety through:

  1. Deactivation processes to eliminate flammability, a step that minimises the cost of transportation, storage, and processing of lithium-ion and other batteries. 
  2. Cathode-healing methods to efficiently recover high-performance, high-value lithium-ion cathodes for a fraction of the energy used in original manufacturing, at costs below $10/kg. 

Fig 3: Direct Recycling comparison of OnTo’s deactivation and Cathode-Healing™ with a solid-state approach. OnTo eliminates hazards, repairs structure and improves purity of the recycled product. It is a scalable, low-cost, safe strategy to address recycling of lithium-ion batteries.

The combination of novel deactivation and cathode-healing offers, for the first time, a cost-positive strategy for vertical integration of recycling into the lithium-ion battery value chain (Fig 3)

 
Year of publication: 
2020
Wordcount: 
1,918