Lithium intercalation compounds represent a class of technologically relevant nanoparticles that have revolutionized energy storage, namely, in their practical application as cathode materials used in rechargeable lithium ion batteries. An important class of such materials is the lithium metal oxides with the form LiMO2, where M represents Fe, Mn, Co, or Ni. The demand for these materials has led to a rapid increase in production volume in the last decade; however, there is very little infrastructure in place for disposal of these materials after use and very little is known about their transformations under environmental conditions. In this perspective, the authors highlight recent work investigating the surface properties of these materials to gain a fundamental molecular-level understanding of the transformations of complex metal oxides in experimental and computational studies.

Due to the unique physical and chemical properties of materials at the nanoscale, nanoparticles (with features <100 nm in size) have been utilized for centuries to impart desired properties that are not possible in the bulk. One well-known example is the incorporation of metallic nanoparticles that were used to add red and yellow colors to stained-glass windows in the 15th century.1 More recently, advances in scientific knowledge have led to the development of tools to study nanoparticles and synthesize them with atomic-level precision over composition, size, shape, and surface chemistry. In turn, these innovations have led to a rapid commercialization of nanoparticles that have become ubiquitous in many commercially available products ranging from cosmetics and clothing to pharmaceuticals, electronic devices, and vehicles.2,3 While utilization of these nanotechnologies are generally viewed to be of great societal benefit, much remains to be learned about nanoparticle interactions and their impact on the environment. This is especially true for nanoparticles being used in emerging nanotechnologies, which often have complex and/or chemically reactive chemical compositions. Prior work has focused on transformations of first-generation nanoparticle types intentionally used in industrial and consumer applications (e.g., zerovalent metals and metal oxides).4–6 Yet, there remains an increasingly prevalent knowledge gap in the chemical transformations of complex next-generation nanoparticle types.

One example of an emerging class of nanoparticles includes lithium intercalation metal oxides with the general formula LiMO2, where M represents Co, Mn, Fe, or Ni. These materials are of interest because lithium intercalation and deintercalation leads to high energy densities and lifetimes when these are incorporated as cathodes in lithium ion batteries. In particular, lithium cobalt oxide (LiCoO2, LCO) and related oxide materials such as lithium nickel manganese cobalt oxide [LixNiyMnzCo(1 − y − z)O2, NMC] are among the most commonly used materials in rechargeable lithium ion batteries.7 While there have been extensive research efforts to enhance the performance of these lithium metal oxide materials for their intended application, what remains unknown is the environmental impact and biological safety of these materials as they continue to be produced in large volumes. Lithium ion cathode manufacturing is currently estimated to be upward of 100 000 metric tons per year and rising.8,9 It has also been estimated that by 2030, total mass of used lithium ion batteries will reach 2 × 106 metric tons per year.10 While these materials continue to be made and used at a rapid rate, there is little, if any, industrial infrastructure currently in place to recycle or safely dispose of these materials in these large quantities.

One of the key challenges in assessing environmental impact of engineered nanoparticles is the complexity of the nanoparticle surface reactions and modifications that determine their fate.11 Conventionally, environmental impact is thought to be governed by molecular and classical thermodynamic and kinetics of the bulk. However, in the nanoscale regime, the fate of nanoparticles is predominately determined by interactions of the surface.12,13 Fundamental materials research studies have demonstrated that the transformations of nanoparticles under environmental conditions are dynamic and, therefore, difficult to control and predict.13 Metal oxide nanoparticles, such as TiO2 and ZnO, are among some of the first commercially used nanoparticles in applications such as cosmetics and sunscreen.14 Studies of these “first-generation” nanoparticles have previously been reviewed and show that transformations of these nanoparticle types are dictated by their intrinsic properties including core reactivity, crystallinity, size, shape, and surface composition. With that in mind, there are several approaches to studying these surface interactions and consequently their environmental impact including dissolution, morphology/structural changes, and aggregation or stabilization.

As large-scale production of existing nanotechnologies continues and new nanoparticle compositions with reactive properties are discovered, a molecular-level understanding of reactive nanoparticles becomes essential for a sustainable nanoenabled future. This perspective aims to highlight some of the recent approaches taken by the Center for Sustainable Nanotechnology to study the transformations of technologically relevant metal oxide nanoparticles. We will highlight the study of two technologically relevant nanoparticle types including LCO and NMC studied experimentally and computationally. This work emphasizes a fundamental molecular-level understanding of the interactions of these nanoparticles that provide important insights into linking fundamental research with biological outcome. We summarize this manuscript with an outlook on the opportunities to fill remaining knowledge gaps ranging from fundamental to applied studies in this area.

The lithium ion battery was recently recognized as one of the greatest scientific achievements when Goodenough, Whittingham, and Yoshino won the Nobel Prize in Chemistry in 2019 for its development.15,16 This discovery was enabled by the use of LCO as the cathode material, which has a high energy density, is lightweight, and operates safely under typical battery operation conditions. For these reasons, LCO is still widely and commercially used in rechargeable lithium ion batteries in electronic devices. Although commercially available cathode materials, such as LCO and NMC, are not always intentionally manufactured at the nanoscale, it has been shown that they acquire nanosized features as a result of typical battery cycling (charge and discharge).17 Due to the high cost and limited worldwide supply of cobalt, cathode material chemistries have rapidly evolved in the last five years to include lithium complex metal oxide analogs, where cobalt is either replaced or substituted in the crystal lattice with varying ratios of Ni and Mn. These isostructural compositions of NMC are made with differing stoichiometric ratios of transition metal but are widely used in the equistoichiometric form: LiNi1/3Mn1/3Co1/3O2. NMC is often manufactured in battery applications in micrometer-sized particles consisting of sintered nanoparticles. The high surface area of the exposed nanoparticles enhance lithium transport and reduce mechanical stress due to charge/discharge cycles, providing fast charge/discharge rates necessary for automotive applications and good performance for a reduced cost.18 

It is well known that morphology and crystallinity are vital design considerations of reactive materials for their intended use. Synthesized LCO and NMC can be synthesized with varying crystal structures including layered delafossite ( R 3 ¯ m space group), cubic spinel (Fd3m space group), or a mix of both in varying ratios.18 The layered structure crystal geometry of the delafossites is ideal for lithium ions to migrate as the cathode material undergoes charge and discharge cycles and a common choice in battery cathode materials. Therefore, the nanoparticles studied in the work highlighted here are predominantly the layered R 3 ¯ m form as determined by x-ray diffraction (XRD). Powder XRD of these synthesized nanoparticles may show some irregular diffraction peaks aside from the R 3 ¯ m structure but can be associated with intermixing of spinel phases and incomplete lithiation (e.g., LixCoO2 where x < 1).19,20 Furthermore, it should also be noted that crystallinity of a nanoparticles with domain sizes below 5 nm in size can become difficult to analyze by XRD alone, as there is significant size-dependent peak broadening and low signal-to-noise ratios.21 

NMC nanoparticles adopt a similar crystal structure to that of LCO (Fig. 1). Variations in nanoparticle morphology, therefore, have different distributions of exposed crystal planes. For example, NMC and LCO nanosheets are dominated by the basal (001) plane, which are comprised of fully coordinated transition metal and oxygen atom surfaces.24,25 Consequently, the basal plane has the lowest surface energy. In contrast, the (104), (012), and ( 1 1 ¯ 0 ) crystal planes have varying degrees of undercoordination and have been shown to be more reactive.26 The distribution of exposed crystal faces can be controllably varied using synthetic strategies to make, for example, nanosheets or nanoblocks, which will be discussed further in Sec. III B.

FIG. 1.

Side view of the crystal structure of a delafossite ( R 3 ¯ m space group) lithium metal oxide, where M represents Ni, Mn, or Co. Drawing produced in vesta based on experimental lattice parameters (Refs. 22 and 23).

FIG. 1.

Side view of the crystal structure of a delafossite ( R 3 ¯ m space group) lithium metal oxide, where M represents Ni, Mn, or Co. Drawing produced in vesta based on experimental lattice parameters (Refs. 22 and 23).

Close modal

In what follows are two case studies that highlight surface specific interactions of LCO and NMC nanoparticle types. These examples highlight the insights gained by probing molecular-level interactions at the surface of nanoparticles that can be correlated to a broader environmental and biological outcome.

Nanoparticle interactions are, at a minimum, dependent on the surface chemistry of the particle themselves and composition of the environmental matrices that they are exposed to. One approach to studying these interactions is through controlled model environmental interactions by studying the adsorption of known species from aqueous environments. These species include compounds such as small organic acids and oxyanions that are commonly found in the environment such as phosphate, oxalic acid, and adipic acid, which have previously been shown to adsorb to nanoparticle surfaces.27,28 While it is known that these molecules can complex metal ions, the influence of these small acids and related organic molecules (e.g., “natural organic material”) on the reactivity of nanoparticles remains relatively unexplored. A recent study by Laudadio et al. highlight the importance of small molecules adsorption in one model system. Phosphate ions present in aqueous media irreversibly adsorbs onto LCO nanoparticle sheets (diameter ∼ 25 nm, thickness ∼ 5 nm) at environmentally relevant pH (pH 7.3) and phosphate concentrations.29 This affect was further studied by surface sensitive techniques to identify phosphate surface coverage, binding configuration, and colloidal surface charge.

By using x-ray photoelectron spectroscopy (XPS) measurements of the P 2p and Co 2p photoelectron regions in exposed samples, the surface coverage of phosphorus was calculated using a surface coverage equation [Eq. (1)] dependent on the measured photoelectron peak area (A), elemental sensitivity factors (SF), inelastic mean free path (λ from NIST XPS database), density (ρ) of cobalt in LiCoO2 (30 atoms/nm2), and the angle of the analyzer to the surface normal (θ),30 
Coverage = A P 2 p A Co 2 p × S F Co 2 p S F P 2 p × s c a n s Co 2 p s c a n s P 2 p × ρ Co 2 p × λ Co 2 p × cos θ .
(1)

XPS analysis of LCO nanoparticles after aqueous phosphate exposure showed that stable coverages around two phosphorus atoms/nm2 are observed at exposure times between 24 and 72 h. At longer times, there is a slight decrease to ∼1.2 phosphorus atoms/nm2, which was attributed to changes in solution pH as a result of LCO dissolution over time. This study noted that upon addition of LCO nanoparticles to an aqueous solution of sodium phosphate, the pH of the bulk solution rises by 0.5 pH units. Previous computational work using density functional theory (DFT) showed that dissolution of Li+ from LCO (001) nanoparticle surfaces is thermodynamically favored in water and results in surface exchange for H+ from surrounding water.24 Taken together with the observed increase in pH of the bulk system, it is assumed that pH increase can be correlated with lithium-hydrogen exchange at the surface of the nanoparticles. The computational models further demonstrated that Li-H surface exchange leads to a change in nanoparticle surface reactivity and thereby impacting adsorption. For example, it was shown that H-terminated LCO surfaces are energetically favored for adsorption from phosphate, while Li-terminated surfaces are unfavored. These findings demonstrate the sensitivity of nanoparticle surface structure to changes in an exposed chemical environment.

In order to understand the phosphate binding configurations, LCO nanoparticles were studied experimentally by in situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy flow cell setup, where three potential binding configurations were inferred including monoprotonated bidentate, deprotonated bidentate, and outer-sphere adsorption of H2PO4 (Fig. 2).29 A recent follow-up study to this work employed a two-dimensional correlation spectroscopy approach that showed the time evolution of phosphate containing ATR-FTIR vibrational modes.31 These results revealed a potential reaction scheme for phosphate adsorption via a phosphoryl transfer mechanism, resulting in phosphate coordinated to LCO in a bidentate deprotonated configuration [Fig. 2(c)].

FIG. 2.

Identified binding configurations of phosphate to the surface of an LiCoO2 nanoparticle, including (a) outer-sphere deprotonated, (b) monoprotonated bidentate, and (c) deprotonated bidentate. Adapted with permission from Laudadio et al., Environ. Sci. Technol. 52, 10186 (2018). Copyright 2018, American Chemical Society (Refs. 29 and 31).

FIG. 2.

Identified binding configurations of phosphate to the surface of an LiCoO2 nanoparticle, including (a) outer-sphere deprotonated, (b) monoprotonated bidentate, and (c) deprotonated bidentate. Adapted with permission from Laudadio et al., Environ. Sci. Technol. 52, 10186 (2018). Copyright 2018, American Chemical Society (Refs. 29 and 31).

Close modal

The identified binding motifs are expected to change the surface charge of the LCO material, which can be measured by zeta-potential measurements. After phosphate exposure in the flow-cell system, the LCO nanoparticles were rinsed with water, resulting in effectively no change in the ATR-FTIR spectrum, suggesting that desorption of phosphate is kinetically unfavored. The results of ATR-FTIR measurements taken together with the surface charge measurements demonstrate that adsorption of phosphate results in stabilization of LCO in aqueous systems. Importantly, it was determined that the adsorption of phosphate enhances the dispersibility of LCO in aqueous environments.

The knowledge gained from these studies show that surface adsorption at the surface of LCO nanoparticles can result in stabilization of the nanoparticle. Importantly, this points to the possibility of enhanced bioavailability of inorganic nanoparticles in water. These studies also highlight that complexity of these fundamental studies extend beyond this one example, but should be considered for other nanoparticle types, and include other real and model environmental systems outside of aqueous media.

One of the fundamental challenges in evaluating the impact of nanoparticles on the environment is correlating nanoparticle morphology with biological outcome. It has widely been demonstrated that synthetic strategies can be used to precisely create desired nanoparticle sizes, shapes, and crystalline phases, and in doing so, it is possible to control nanoparticle physical, optical, and electrical properties.32,33 Previous studies have shown that nanoparticle cellular internalization pathways can be dependent on nanomaterial shape and, in some instances, aspect ratio of the nanomaterial.34,35 Yet, quantifying a nanoparticle “dose” and therefore correctly identifying a toxic threshold for different nanoparticle types remains ambiguous because there is yet to be a widely accepted way to describe the “concentration” of nanoparticle exposures. In order to extend mechanistic understanding of the biological interactions of nanoparticles, quantifiable measurements of the surface must also be explored in a similar capacity to which size and shape are considered.

In this section, we feature the capability to synthesize the same nanoparticle composition with different morphologies and therefore surface area, providing an opportunity to gain mechanistic insight into the molecular transformations that are dependent on these physical attributes. In the example discussed here, the impact of shape-dependent surface properties of NMC were investigated using nanosheets [predominantly (001) surface] and nanoblocks [surface dominated by the (012) plane] to assess the transformations and impact of these different facets.36 It was shown that when nanosheets and nanoblocks are placed in biologically relevant solutions, in this case bacterial growth medium, the cationic metals are released incongruently from the material and there is preferential dissolution in the order of Li > Ni > Co > Mn. Furthermore, in comparing the two nanomaterial morphologies, the nanosheets release more metal cations compared to the nanoblocks under identical mass concentrations and time points. It was shown that after 72 h in solution, the nanosheets release 4× as much Ni cations as compared to the nanoblocks.

The incongruent dissolution trend of NMC has also been supported by computational studies. Bennett et al. combined a DFT and thermodynamic approach to compute the free energy of transition metal dissolution from stoichiometric NMC.25 Importantly, this work highlighted to what extent the chemical transformations of NMC materials is dependent on a range of surface terminations and pH. Upon immersion of NMC into aqueous media, it is expected, and thermodynamically favorable, for surface lithium to exchange with water to produce a hydrogen terminated (hydroxylated) basal (001) surface. In agreement with the experimental studies of NMC, it was computationally shown that Ni dissolution is favorable under almost all environmental conditions studied between pH 1 and 9.

Similar to model molecular systems, there are also model organisms that are well suited to broadly represent toxicity. This includes Shewanella oneidensis MR-1, which is widely used as a model bacterium to study metal-induced toxicity in the environment.37,38 A common way to measure toxicity is through viability response mechanisms in living organisms. This is typically achieved with high-throughput assay methods that utilize optical absorbance or luminescence measurements to identify an organism’s ability to live and grow in the presence or absence of a compound of interest.39 In the same study by Hang et al., they showed that biological response was dependent on the total the influence of nanoparticle morphology on ion release and biological response from NMC nanoblocks, nanosheets, and commercial NMC material.36 When nanoparticles were exposed to S. oneidensis bacteria, these particle types with differently distributed crystal faces were shown to have a mass dose dependent viability response with increasing particle concentration. However, the response in bacteria viability was incongruent between particle shape/type. When the viability response was normalized based on surface area of the particle, it was shown that viability collapses onto a single dose-response curve! This suggests that in this case, viability response was independent of exposed crystal faces but entirely dependent on exposed surface area of NMC.

Most studies of nanoparticle toxicity are based on mass, but for reactive nanoparticle types, such as NMC, the critical chemical phenomena are surface processes. Therefore, the environmental and biological impact scales with surface area, not mass. While mass-based or molar-based concentration units are frequently used, if the factors controlling the biological impact are chemical reactions that are kinetically limited by surface reactions, such as release of transition metals from surfaces, then the best way to express normalized data and/or present comparative data would be to represent “concentration” based on the surface area. Therefore, understanding the microscopic chemical processes that control biological impact is important in order to properly design and carry out experiments that can be replicated and/or extended to nanoparticles of different sizes and shapes. Understanding the critical nanoparticle factors controlling reactivity provides a rational way to normalize disparate datasets, as shown in the examples discussed here, and predict the impact of nanomaterials based on well-defined chemical and physical properties—an important step toward the ability to predict the environmental and biological impact of new and emerging nanomaterials.

As the demand for lithium ion battery materials continues to accelerate, it is critical to understand their eventual impact on the environment in order to fully assess the degree to which different battery chemistries and material compositions achieve sustainability goals. This perspective highlights some of the approaches taken to investigate the environmental impact of technologically relevant nanomaterials and shows how the relevant transformations are often dominated by surface reactions. The work highlighted here was motivated by the large-scale use of reactive nanoparticle types in lithium ion batteries. However, there are several opportunities for further investigation of transformations at the surfaces of other relevant nanoparticles cathode type materials including lithium iron phosphate, lithium titanium oxide, and lithium nickel cobalt aluminum oxide, which are also of interest as energy storage materials. Naturally, this approach should also be expanded to other nanoparticle types of interest. Taken what was learned from these studies may inform recycling regulations, design rules for safer materials, and shed light upon the impact of these materials in our environment. Here, we point to two important areas of fundamental surface chemistry and materials research problems that should be assessed.

  • Identifying nanoparticle parameters that control transformations: In this perspective, we emphasized work that identified important interactions and transformations at the surface of crystalline LCO and NMC nanoparticles. By using techniques that include in situ measurement approaches coupled with computational studies, key insights into nanoparticle surface transformations were gained. In that regard, this approach could be applied to many other systems for a better understanding of the dynamic solid-liquid interfaces of nanoparticles. Furthermore, there remains much to be understood about transformations of nanoparticle surfaces under a higher degree of surface and environmental complexity. For example, crystallographically pristine materials are often chosen in research studies to gain a fundamental understanding in key reaction mechanisms, but pristine materials are not necessarily representative of materials that enter the environment. Therefore, characterizing surfaces of multiphase and chemically complex nanoparticles may have a broader impact in identifying a set of general nanoparticle parameters and their link to transformations of the surface. Additionally, just as more complex surfaces should be considered, increasingly complex environmental matrices must also be explored. For example, competitive reactions at the surfaces of nanoparticles remain elusive, particularly at the nano-bio interface.

  • Linking mechanistic information to design strategies for sustainable nanoparticles: Insights gained from these fundamental studies should be applied to design strategies that make for a sustainable nanotechnology future. One approach to improving nanoparticle sustainability is the development of synthetically facile approaches for nanoparticle coatings. Controlled coating strategies have already been pursued by wet and gas-phase chemical strategies but typically are subject to specific requirements including uniformity and stability without sacrificing the desired nanoparticle properties.40,41 Finally, the pursuit of new nanoparticle compositions that are environmentally benign and have the same or improved properties for an intended application would be the ideal discovery. However, in this case, the challenge remains—the scientific community must investigate the best methods to fully ensure environmental safety. The opportunities here are extensive and will require expertise from materials scientists, engineers, chemists, physicists, biologists, and more.

Surface science is critically important to understanding the operation as well as the eventual environmental fate of nanomaterials. The ability of modern computer methods to predict the chemical properties of solid-liquid interfaces has great potential to enhance our understanding of important environmental processes through close integration of synthesis, experimental measurement, and modeling. Support of fundamental and applied research at surfaces/interfaces by professional organizations, such as the AVS, continues to be important to this field of research.

This work was supported by the National Science Foundation (NSF) under the Center for Sustainable Nanotechnology (No. CHE-1503408). The Center for Sustainable Nanotechnology is part of the Centers for Chemical Innovation Program. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF. J.H.O. thanks Michael P. Schwartz for many useful discussions in preparation of this manuscript.

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).

Jenny K. Hedlund Orbeck earned her B.S. in Chemistry at the University of Iowa in 2012. While at the University of Iowa, she was awarded an Iowa Center for Research by Undergraduates Fellowship and began her scientific research by studying synthesis of noble metal nanoparticles in Professor Amanda J. Haes’ lab. During her final year of undergraduate studies, she received an internship at the Argonne National Laboratory as a part of the summer research participant program where she worked with Dr. Tijana Rajh in the Center for Nanoscale Materials. After graduation, she returned to Argonne where she studied thin films, coatings, and catalyst materials prepared by atomic layer deposition in Dr. Jeffrey Elam's group in the Energy Systems Division. She went on to complete her Ph.D. in Chemistry at the University of Texas at Dallas under the supervision of Professor Amy V. Walker in 2019. In her graduate work, she studied semiconductor thin films prepared by chemical bath deposition and became skilled at the practical application of surface sensitive analytical techniques including XPS and ToF SIMS. Jenny is currently a postdoctoral research associate at the University of Wisconsin–Madison in Professor Robert J. Hamers’ lab where she primarily works for the Center for Sustainable Nanotechnology. She is currently interested in studying synthetic strategies to control technologically relevant nanoparticle types, and the link between their surface properties and environmental impact.

Her advice to her 16-year-old self would be: “Pursue your interests and goals even if they seem daunting. You will meet people who might discourage you from pursuing such goals, but you will also cross-paths with many people who will want to support you and help you succeed. Connecting with these individuals who lift you up is time well spent!”

Robert J. Hamers is currently the Steenbock Professor of Physical Science in the Department of Chemistry at the University of Wisconsin–Madison and the Director for the Center for Sustainable Nanotechnology. He received his B.S. in Chemistry from the University of Wisconsin–Madison and Ph.D. from Cornell University. From 1985 to 1990, he conducted research at the IBM T.J. Watson Research Center in Yorktown Heights, NY. Since 1990, he has been on the faculty at UW-Madison. His research interests focus on surfaces and interfaces of materials, with particular emphasis on connections between electronic structure and reactivity of semiconductors of nanomaterials.