Nickel manganese cobalt oxide (NMC) is a class of lithium intercalation compounds with the composition Li_xNi_yMn_zCo_1-y-zO₂ (0 < x,y,z < 1). NMC compounds provide high performance at reduced cost, and are thus widely considered for large-scale implementation in electric vehicles. Further, nanoparticles yield enhanced lithium transport, better electrical conduction, and reduced fragmentation from mechanical stresses during lithium intercalation and de-intercalation.

Rapid, large-scale commercialization of NMC and related lithium intercalation materials in nanoparticle form increases the potential for environmental release and exposure during manufacture, use, and disposal. A single, modest electric vehicle with a typical ~24 kWh battery pack using NMC (specific capacity = 165 mAh/g at 3.8 V potential) contains >38 kg of nanoscale cathode material. With estimates of 20 million electric vehicles on the road by the year 2020, nanoscale metal oxides represent an emerging potential environmental contaminant.

In contrast to lead-acid batteries, little infrastructure exists for recycling Li-ion batteries, due in part to a lower economic incentive for recycling. Understanding the environmental behavior of the materials that comprise batteries can provide important insights into an comprehensive assessment of how to optimally use new materials to reduce energy usage and make more effective use of renewable sources.

—Hang et al.

For the study, the team used Li_xNi_⅓Mn_⅓Co_⅓O₂ (with x=1 corresponding to fully lithiated materials) due to its widespread use.

The genus Shewanella comprises Gram-negative bacteria that are distributed globally; Shewanella oneidensis MR-1 plays an important role in the cycling of metals in the environment and is a model system for environmental studies.

The study characterized the influence of NMC nanoparticles on S. oneidensis population growth and respiration, and linked these with corresponding changes in solution composition and NMC surface composition via X-ray photoelectron spectroscopy.

Subjected to the particles released by degrading NMC, the bacterium exhibited inhibited growth and respiration.

The researchers found that NMC nanoparticles in aqueous media under partial incongruent dissolution preferentially released Li⁺ and the transition metals Ni²⁺ and Co²⁺ into solution and left behind chemically transformed nanoparticles that are depleted in Ni and enriched in Mn. They demonstrated that the toxicity of NMC arises from the release of the transition metal ions in solution rather than the remaining transformed nanoparticles.

Shewanella oneidensis thrives on metal ions, converting them to metals such as iron that serve as nutrients for other microbes. The bacterium was shown to be harmed by NMC, which is produced in nanoparticle form and is poised to become the dominant material in the lithium-ion batteries that will power portable electronics and electric vehicles. Illustration: Ella Marushchenko/University Of Minnesota. Click to enlarge.

As far as we know, this is the first study that’s looked at the environmental impact of these materials.

—UW–Madison chemistry Professor Robert J. Hamers

Hamers collaborated with the laboratories of University of Minnesota chemist Christy Haynes and UW–Madison soil scientist Joel Pedersen to perform the new work.

Haynes noted that “it is not reasonable to generalize the results from one bacterial strain to an entire ecosystem, but this may be the first ‘red flag’ that leads us to consider this more broadly.”

According to Hamers, one big challenge will be keeping old lithium-ion batteries out of landfills, where they will ultimately break down and may release their constituent materials into the environment.

Our results suggest that NMC entering aqueous environments (e.g., resulting from battery disposal into landfills) may act as a source of dissolved nickel and cobalt, potential bacterial toxicants, as well as other ions such as Mn and Li. This work provides additional motivation for efforts to develop and implement effective recycling strategies for lithium ion batteries. We suggest that by reducing dissolution of metals from NMC, its toxicity to bacteria and other organisms in natural environments can be reduced.

Ultra-thin (~1 nm thickness) surface coatings of Al₂O₃ and other stable oxides have been shown to reduce the reactivity of NMC cathodes and thereby improve the performance of NMC- containing lithium-ion batteries. Such coatings of water-stable oxides might also play an important role in mitigating the potential for environmental impact of NMC and related complex oxides. Data for Al₂O₃ dissolution suggests that at pH ~6 a 1 nm thick coating would require on the order of one year to dissolve. This suggests that surface coatings may also have an important role in the environmental impact of NMC and other complex oxides.

—Hang et al.

The group, which conducted the study under the auspices of the National Science Foundation-funded Center for Sustainable Nanotechnology at UW–Madison, also plans to study the effects of NMC on higher organisms.

Resources

• Mimi N. Hang, Ian L. Gunsolus, Hunter Wayland, Eric S Melby, Arielle C. Mensch, Katie R Hurley, Joel A. Pedersen, Christy L. Haynes, and Robert J Hamers (2016) “Impact of Nanoscale Lithium Nickel Manganese Cobalt Oxide (NMC) on the Bacterium Shewanella oneidensis MR-1” Chemistry of Materials doi: 10.1021/acs.chemmater.5b04505