Secondary phases developed from layered lithium nickel cobalt manganese oxide (NCM) cathode material waste: environmental mineralogy implications for advancing NCM recycling methodologies

The demand for reliable, high-performance, rechargeable batteries commercialized lithium-ion batteries (LIB). Increasing trends in energy storage requirements and the need to decrease cobalt consumption for layered lithium cobalt oxide LiCoO₂ (LCO) cathodes for LIB, among other reasons, resulted in ongoing research for nickel-rich cathode alternatives, e.g., layered lithium nickel cobalt manganese oxides LiNi_1-x-yCo_xMn_yO₂ (NCM). Ni-rich cathode compounds, though with higher energy density, are not yet widely implemented due to safety and cycle stability concerns. However, with a forecast of nearly 40% of total cathode materials produced in 2025 to be attributed to NCM, there are research efforts on sustainable recycling and regeneration strategies, as well as targeted repurposing.

In the present contribution, waste NCM cathode material samples are subjected to normal atmospheric conditions. The samples are characterized by X-ray diffractometry coupled with Rietveld refinement, scanning electron microscopy and bulk chemistry via ICP optical emission spectroscopy. Detailed evaluation of the produced diffractograms shows secondary pure or mixed, either Li-free or Li-bearing, Ni-, Co- and Mn-oxide and hydroxide crystalline phases due to alteration of the initial NCM active material. Commonly evaluated secondary phases are manganosite MnO, lithium manganese oxide LiMn₃O₄, spinel lithium Mn-oxide LiMn₂O₄, mixed cobalt nickel oxides 5CoO·3NiO and 3CoO·5NiO, and nickel oxide hydroxide NiO(OH). The controlling conditions generally seem to favor low-valence metal oxyhydroxide products in the alteration reactions, however the mechanism is not well understood.

The presence of secondary transition-metal oxyhydroxide phases has a bipartite set of implications, as battery degradation in natural systems has the potential to be both environmentally harmful and cost-intensive, in general. Firstly, the secondary phases present have direct environmental importance especially in the case of uncontrolled waste disposal, e.g., in landfills. In surficial geochemical environments, where the physical and chemical conditions favor metal mobility with further oxidation and metal complexation of the respective oxyanions, the secondary phases could be sources of metal release in the environment, preferentially in surface water and soil.

Additionally, it is crucial in industrial planning in terms of worker health risks, due to the physical form of the cathode waste, elemental mobility and potential human bioavailability. Furthermore, it is of economic importance in implementing recycling methodologies in non-pristine material. Secondary phases can especially disrupt the hydrometallurgical solution chemistry needed for optimal recovery and could lower the quality of the final product regarding direct recycling.

Our data shows for the first time that the NCM battery cathode material degrades under ambient atmospheric conditions. The production of secondary crystalline phases, which defines the material alteration, could be alarming in cases of uncareful handling and disposal, both in an environmental and industrial context.