Morphological and Chemical Tuning of High-Energy-Density Metal

Morphological and Chemical Tuning of HighEnergy-Density Metal Oxides for Lithium Ion Battery Electrode Applications Lei Wang,†,# Shiyu Yue,†,# Qing Zhang,‡ Yiman Zhang,† Yue Ru Li,† Crystal S. Lewis,† Kenneth J. Takeuchi,†,‡ Amy C. Marschilok,†,‡ Esther S. Takeuchi,†,‡,§ and Stanislaus S. Wong*,†,∥ †

Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794-3400, United States Department of Materials Science and Chemical Engineering, State University of New York at Stony Brook, Stony Brook, New York 11794-2275, United States § Energy Sciences Directorate, Brookhaven National Laboratory, Interdisciplinary Sciences Building, Building 734, Upton, New York 11973, United States ∥ Condensed Matter Physics and Materials Sciences Division, Brookhaven National Laboratory, Building 480, Upton, New York 11973, United States ‡

ABSTRACT: Metal oxides represent a set of promising materials for use as electrodes within lithium ion batteries, but unfortunately, these tend to suffer from limitations associated with poor ionic and electron conductivity as well as low cycling performance. Hence, to achieve the goal of creating economical, relatively less toxic, thermally stable, and simultaneously high-energy-density electrode materials, we have put forth a number of targeted strategies, aimed at rationally improving upon electrochemical performance. Specifically, in this Perspective, we discuss the precise roles and effects of controllably varying not only (i) morphology but also (ii) chemistry as a means of advancing, ameliorating, and fundamentally tuning the development and evolution of Fe3O4, Li4Ti5O12, TiO2, and LiV3O8 as viable and ubiquitous energy storage materials. the electrolyte, and brought back into the cathode.1 Figure 1A highlights a typical LIB configuration.6 The nature of the electrode is particularly critical in directly impacting upon the observed capacity and energy density and, ultimately, the performance of the battery. Graphite is a common and widely used anode material, in part because it maintains a high capacity and allows for reasonable conductivity. However, the ubiquitous use of graphite is limited in part by lithium deposition onto the anode surface, thereby leading to poor cycling stability. The interaction of lithium ions with the graphite matrix, which is a carbon-based material, can be described by the reaction shown in eq 1.7

Lithium Ion Batteries. The potential of lithium ion batteries (LIBs) as a viable and practical energy storage medium has engendered significant interest in perfecting and improving upon their functionality. Not surprisingly, especially over the past few decades, significant effort has been expended in addressing various scientific issues affecting the nature of the different components of the LIBs. Possessing a high rate, a superior durability, as well as a desirable high energy density, LIBs are widely used for a broad range of applications spanning from portable electronics to electric vehicles (EVs).1−4 The key components of conventional LIBs include (i) a graphite anode, (ii) a cathode formed by a lithium metal oxide (e.g., LiCoO2), and (iii) an electrolyte consisting of a solution of a lithium salt (such as LiPF6) either dispersed within a mixed organic solvent or embedded in a solid polymer electrolyte (such as poly(ethylene oxide) (PEO) or poly(methyl methacrylate) (PMMA)).5 In terms of ion flow and movement, the lithium ions are extracted from the cathode during charging, transported through the electrolyte, and finally deposited at the anode. During the discharge process, the process is reversed; lithium ions are extracted from the anode, conveyed through © XXXX American Chemical Society

Li+ + 6C + e− ⇌ LiC6

(1)

As for the cathode counterpart, a conventional and commonplace cathode material used, lithium cobalt oxide (LiCoO2), sheds lithium according to the reaction shown in eq 2.8 This particular material is associated with problems of overall Received: March 12, 2017 Accepted: May 5, 2017

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DOI: 10.1021/acsenergylett.7b00222 ACS Energy Lett. 2017, 2, 1465−1478

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toxicity, high cost, and low thermal stability, which limit its ubiquitous applicability.

LiCoO2 ⇌

1 + 1 − Li + e + Li 0.5CoO2 2 2

(2)

It is not surprising, therefore, that improvements in the future design of electrode materials have sought to simultaneously address issues of performance, toxicity, and cost. To this end, one key area of interest has focused on considering alternative structures and motifs, consisting of economical, thermally stable, relatively less toxic, high-energy-density electrode materials. Idea of High-Energy-Density Materials. Therefore, to deploy LIBs within the next generation of vehicles necessitates the development of electrodes with a consistently and reproducibly high energy density.9 The term energy density denotes the amount of energy stored within a given system per unit volume or mass, which reflects the effectiveness and efficaciousness of the material as a viable energy storage medium for batteries and analogous devices. Moreover, energy density can be described as a function of the cell’s voltage and capacity, which are highly dependent upon the nature of the electrode materials themselves. As such, electrode materials characterized by either a high capacity or a high operating potential could meet the requirements while delivering a high energy density. In theory, a gasoline engine can deliver an energy density of as high as 13 000 Wh/kg, but practically speaking, the observed energy density is much less, specifically, 1700 Wh/kg.10 Conventional LIBs possess a working voltage on the order of 4 V with a practical energy density ranging between 100 and 200 Wh/kg.11 Figure 1B illustrates the energy density of different energy storage devices in terms of both theoretical expectations as well as practical limitations. It has been proposed that an advanced LIB for vehicle applications needs to deliver at least an observed energy density of 300 Wh/kg.12 As such, this requirement can only be achieved by creating alternative and inherently better electrode materials with a higher capacity and a higher operating potential. One such class of materials that we have focused on, in particular, are transition metal-based oxides

Figure 1. (A) Conventional Li ion battery configuration. (B) Theoretical and practical energy density values of a number of different energy density storage devices. Use of energy density values was adapted from refs 8, 11, and 19.

Figure 2. Crystal structure of (A) spinel Fe3O4, (B) spinel Li4Ti5O12, (C) layered LiV3O8, (D) anatase phase TiO2, (E) brookite phase TiO2, and (F) rutile phase TiO2. 1466


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charge−discharge cycles,24 and (iii) reduced electron and Li ion diffusion lengths.25 In our studies, we have focused on designing and synthesizing (a) zero-dimensional (0D) nanoparticles and nanospheres, (b) one-dimensional (1D) nanowires and nanotubes, as well as (c) three-dimensional (3D) hierarchical architectures. Why these motifs? Generally speaking, 0D motifs maintain increased electrochemically active surface areas with smaller sizes, thereby providing for good contact between the electrode and the electrolyte along with shortened diffusion distances. 1D structures denote the smallest dimensional motifs that can be used for the efficient transport of electrons and Li ions.26 Finally, 3D nano/ microhierarchical structures combine the merits of nanosized blocks with those of micron/submicron-sized assemblies, the latter of which are often characterized by thermodynamic stability and a high tap density.27 The second strategy used to improve the inherent electrochemical performance of the metal oxides in question is through the use of chemically inspired protocols, such as either ion doping, surface modification, and/or incorporation of carbon. Ion doping denotes a general method for improving upon intrinsic electron and Li ion conductivity by either increasing electron density or creating lattice distortion of the metal oxide matrix.28 Surface modification induced by coating with either a conducting metal (e.g., Ag, Cu) or with a conductive carbonbased material can potentially further improve upon conductivity.29,30 As an example of the latter strategy, due to the anomalously high conductivity (i.e., 104 S/cm) of carbon nanotubes (CNTs)31 coupled with their intrinsically high specific surface area, the incorporation of conductive carbon species with metal oxides has led to notable improvements in conductivity.18 We will explore all of these complementary structural and chemical approaches to enhance conductivity with the goal of understanding the roles of (i) morphology and (ii) chemistry in terms of inducing the economical and viable formation of “high-energy-density” analogues of the four metal oxides that we are focused on in this Perspective. As such, the insights achieved herein will be useful and potentially generalizable to the behavior and rational design of additional high-energydensity metal oxides as promising electrode materials, thereby enabling and facilitating their incorporation into LIBs for a variety of demanding practical applications. Role of Morphology Control. Benef its of Dif ferent Morphologies. Nanomaterials for energy storage applications have been extensively researched for many years because of a number of beneficial attributes including but not limited to a high electrode and electrolyte contact area, short electron and ion transport distances, and, in certain cases, the ability to perform electrochemical reactions that are not otherwise possible with bulk materials. Moreover, the intriguing properties of nanomaterials can be tailored by varying their morphologies, which allows one to differentiate among various types of nanostructures by means of their dimensionality. In turn, the morphologies (i.e., their intrinsic holistic shape) of nanomaterials can be categorized in terms of their inherent dimensionality. For example, (i) nanospheres denote 0D nanomaterials, because all of their dimensions are confined to the nanoscale, (ii) anisotropic nanowires, nanorods, or nanotubes are 1D nanomaterials with at least one dimension outside of the nanometer range, and (iii) hierarchical nanoscale flower geometries are 3D nanomaterials with overall micron-scale dimensions but are composed of individual constituent nanometer-scale building blocks.

because they offer the potential of not only yielding a high energy density but also surmounting issues associated with the poor cycling performance of graphite and the undesirable toxicity of LiCoO2.13 Metal Oxides. As electrode materials for LIBs, transition metal-based oxides possess a number of desirable advantages, including good rate capability, decent cycling stability, and ease of formation using relatively nontoxic protocols. Our team has focused on the synthesis and characterization of four “highenergy-density” metal oxides in particular, which we highlight in this Perspective, namely, (i) spinel anodic Fe3O4, (ii) anodic TiO2, (iii) spinel anodic Li4Ti5O12, and (iv) layered cathodic LiV3O8. Figure 2 presents the individual crystal structures of each of these materials. Fe3O4 has long been regarded as a promising anode material for LIBs, owing to its high theoretical capacity of 924 mAh/g, low cost, and relative nontoxicity.14−17 Possessing the same spinel structure as Fe3O4, Li4Ti5O12 is considered to be a “zero strain” anode material and maintains a reasonably high capacity of 175 mAh/g at a flat potential of 1.55 V.18 TiO2 is characterized by a number of different crystallographic phases including anatase, rutile, and brookite, many of which are electrochemically active. Our team has specifically focused on the anatase phase of TiO2, which possesses a large surface area for electrochemical activity due to its overall 3D crystalline tetragonal structure created by the stacking of 1D zigzag chains, each of which consists of distorted edge-sharing TiO6 octahedra.19 Moreover, TiO2 has been touted as a viable anode material, delivering a capacity of ∼200 mAh/g18 and providing for a high energy density at low potential with a correspondingly low lithiation potential.20 LiV3O8 represents an interesting cathode material with a theoretical capacity of 360 mAh/g, a high specific energy, and a good rate capacity.21,22 To be considered as a prospective electrode material requires simultaneously excellent electronic and Li ionic conductivity (i.e., “mixed” conductivity). For example, graphite is a semiconductor with conductivity of 10−2−10−3 S/cm, and LiC6 is an excellent conductor with both a high electronic conductivity of 102−103 S/cm and a correspondingly good Li ion mobility of 10−8−10−10 cm2/s.23 Unfortunately, many metal oxides are characterized by a relatively low “mixed” conductivity metric, which thereby leads to an overall poor electrochemical performance. Moreover, the volume expansion and physical destruction of these materials induced by Li cation intercalation upon electrochemical cycling, for example, are important concerns.

The first approach to address limitations for electrode materials has been to design viable nanostructured motifs of the targeted transition metal oxides of interest. To address all of these limitations, we have employed two key and complementary strategies, often in concert. The first approach has been to design viable nanostructured motifs of the targeted transition metal oxides of interest. Indeed, nanoscale motifs of metal oxides possess a number of intrinsic advantages, including, most significantly, (i) a large surface area between the electrode and the electrolyte to enable improved Li ion transport, (ii) a high tolerance for volume expansion (and, hence, the preservation of structural integrity) over many 1467


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Figure 3. Electron microscopy images of various morphologies of (A) 0D TiO2 nanoparticles, (B) a 1D submicron LiV3O8 fiber motif, (C) 3D “flower-like” Li4Ti5O12 hierarchical composites, and (D) 3D sea urchin-like TiO2-based aggregates. Panel (C) is adapted with permission from ref 42, Copyright 2015, John Wiley and Sons.

All of these morphologies are uniquely advantageous for LIB applications. Specifically, 0D metal oxides are popular in LIB design due to their high surface area coupled with shortened Li+ and electron transport pathways in all dimensions. 1D metal oxides possess a number of beneficial aspects that render them as attractive electrode materials, including high aspect ratios, fewer lattice boundaries, and a limited number of deleterious defects. Among the various nanoscale metal oxides, the 3D nanostructures hold many inherent and critical advantages for energy storage over electrodes comprised of either 0D nanoparticles or 1D nanowires of electroactive materials alone. In particular, 3D motifs are characterized by a number of favorable characteristics, including (i) a very high active surface area of the electrode material, which is extremely important for LIBs, owing to an enhancement in not only the potential accessibility for lithium ions but also the accompanying electrochemical kinetics between the metal oxide materials and the surrounding electrolyte; (ii) a tunable overall free volume, which can readily accommodate the expansion of metal oxides upon lithium insertion; and finally, (iii) shortened lithium ion diffusion distances, which imply shortened transport lengths for Li ion movement either through the lattice itself (for the example of

intercalation materials) or through regions of lithium oxide (in the case of either conversion or alloying reactions).32 Many studies have analyzed and compared electrochemical performance as a function of metal oxide size. However, few groups have studied the precise role of morphology and performed morphology-dependent studies on metal oxidebased electrode materials for LIBs. In this vein, our groups have recently focused on decoupling the intimate roles of morphology and surface area with electrochemical behavior and purposely concentrated on deducing which specific variations in nanoscale morphology functionally determine and give rise to the enhanced performance metrics observed. To this end, we have correlated a number of measured parameters such as specific capacities, rate capabilities, and stabilities with the 0D, 1D, and 3D motifs of several metal oxide nanomaterials, such as Li4Ti5O12, TiO2, LiV3O8, and Fe3O4, with the basic objective of fundamentally understanding the nature of the improved LIB performance observed. Zero-Dimensional TiO2 Nanoparticles. TiO2 is a promising candidate as a potential anode material due to its ample abundance, low toxicity, and overall safety of handling.18 Especially when compared with graphite, Fe3O4, and SnO2, 1468


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Figure 4. (A) Li/TiO2 electrochemical cells containing TiO2 materials synthesized with (a) 0D, (b) 1D, and (c) 3D morphologies as well as with (d) commercially available TiO2. (A1) Discharge capacity versus cycle number measured at 0.1 (cycles 1−5), for 1 (cycles 6−10), 10 (cycles 11−15), 20 (cycles 15−20), 0.1 (cycles 20−30), 1 (cycles 31−35), 10 (cycles 36−40), 20 (cycles 41−45), 0.1 (cycles 46−50), 50 (cycles 51−60), 0.1 (cycles 61−70), and 1 C (cycles 71−100) discharge rates with corresponding 0.1 (cycles 1−70) and 1 C (cycles 71− 100) charge rates. (A2) Voltage profiles obtained at cycles 5 and 20. (B) Li/LiV3O8 electrochemical cells containing LVO materials, prepared via (a) sol−gel and (b) hydrothermal methods. (B1) Discharge capacity versus cycle number measured at C/10 discharge and charge rates. (B2) Voltage profiles collected at cycles 1 and 100 (solid lines represent discharge and dashed lines represent charge). (C) Li/Li4Ti5O12 electrochemical cells prepared with (a) spherical (commercial) and (b) “flower-like” LTO motifs. (C1) Discharge capacity versus cycle number measured at 0.2 (cycles 1−10), 1 (cycles 11−20), 0.2 (cycles 21−30), 10 (cycles 31−40), 0.2 (cycles 41−50), 10 (cycles 51−55), 20 (cycles 56−60), 50 (cycles 61−65), 100 (cycles 66−70), and 0.2 C (cycles 71−100) discharge rates with corresponding 0.2 C (all cycles) charge rates. (C2) Voltage profiles acquired at cycles 50 and 65. Panels (C1) and (C2) are adapted with permission from ref 42, Copyright 2015, John Wiley and Sons.

under discharge rates from 0.1 up to 50 C (1 C = 168 mA/g), with an active material mass loading of 1.22 mg/cm2. The corresponding voltage profiles obtained at cycles 5 and 20 are displayed in Figure 4A2. Specifically, the 0D TiO2-containing cells delivered 224, 162, 121, 88, and 23 mAh/g under rates of 0.1, 1, 10, 20, and 50 C, respectively. In terms of the capacity retention, a galvanostatic cycling test was applied using 1 C discharge and charge rates. The 0D TiO2-based cell yielded ∼215 mAh/g at cycle 1 but only ∼109 mAh/g at cycle 100, indicative of 51% capacity retention. One-Dimensional LVO Submicron Fibers. Possessing a layered structure (space group symmetry P21/m), LiV3O8 denotes a particularly intriguing cathode material due to favorable attributes such as its high specific capacity and excellent rate capabilities. Performance-wise, LiV3O8 maintains a high reversible lithium storage capacity of 237 mAh/g after 200 cycles coupled

TiO2 maintains not only a higher lithiation potential (i.e., ∼1.6 V versus Li/Li+) but also a lower volume change during the Li insertion/deinsertion process (i.e., ∼3−4%), a finding that is conducive to enhancing cycle life and that also reinforces the overall promise of TiO2 as an efficient anode battery candidate material. Our team was one of the first to pursue a systematic correlation between (i) the morphologies of crystalline and welldefined 0D, 1D, and 3D motifs of anatase TiO2, prepared by a seedless, surfactant-free hydrothermal reaction, and (ii) the resulting electrochemical activity.20 We even compared our data with and found that our samples surpassed the performance of more polydisperse, commercially available 0D TiO2 nanoparticles. For the 0D TiO2 nanostructures, Figure 3A features representative, as-prepared TiO2 spherical nanoparticles, possessing an average diameter of 10 ± 2 nm. Figure 4A1 displays the average capacity versus cycle number data for all of the TiO2 cells tested 1469


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of the electrolyte with the Li4Ti5O12 itself, thereby ultimately giving rise to both improved high rate performance and favorable charge/discharge dynamics. The resulting electrodes based on our as-synthesized materials exhibited excellent high rate capability and stable cycling performance. Specifically, with an active material mass loading of 1.73 mg/cm2, our flower-like materials (i) delivered capacities of 141, 137, 123, and 60 mAh g−1 under high discharge rates of 10, 20, 50, and 100 C (1 C = 175 mA/g) at cycles of 55, 60, 65, and 70, respectively; (ii) maintained a capacity retention of ∼97% from cycle 10 to cycle 100 and of ∼99% from cycle 20 to cycle 100 at a discharge rate of 0.2 C in addition to an impressive capacity retention of ∼87% using a more rigorous discharge rate of 20 C from cycles 101 to 300; (iii) at the highest discharge rate of 100 C, provided for ∼3× the discharge capacity normally observed with a conventional, commercially available Li4Ti5O12 control sample (Figure 4C1,C2); and (iv) finally, appeared to retain their inherent and intrinsic structural morphology, even after 300 rigorous cycles of testing. With respect to TiO2 analogues, our 3D TiO2 urchins, characterized by a roughened external surface morphology with an average diameter of 1 ± 0.7 μm (Figure 3D), yielded the highest tested reversible capacity measured thus far under conditions of high current density. In fact, the electrochemical analysis implied superior performance of hydrothermally derived 3D urchin-like motifs as compared with both as-prepared 0D and 1D samples as well as with commercial Degussa P25. In effect, our studies suggested the greater overall significance of morphology as opposed to surface area in dictating the efficiency of the Li ion diffusion process. Specifically, the 3D TiO2 urchins yielded consistently higher rate capabilities, delivering 214, 167, 120, 99, and 52 mAh/g under corresponding discharge rates of 0.1, 1, 10, 20, and 50 C, respectively (Figure 4A1,A2), as compared with all other samples tested. Moreover, these 3D motifs gave rise to a desirably stable cycling performance and exhibited a capacity retention of ∼90% in cycles 1−100 under a discharge rate of 1 C. Similar Studies of Morphology Dependence in the Literature. In terms of analogous efforts by other groups to develop novel metal oxide morphologies for battery applications, the following studies are representative of these efforts. The underlying theme is that morphology, often in the guise of creating complex architectural assemblies, can frequently be used as a structure-based tool to tailor and optimize electrochemical performance. With TiO2, 1D nanofibers possessing various aspect ratios have been synthesized, utilizing a simple hydrothermal method, with reported capacities in the range of 114 mAh/g obtained at a high rate of 30 C after 6000 cycles.34 Moreover, a relatively new polymorph, specifically, the bronze phase of TiO2 or TiO2B, has attracted significant attention due to its reasonably high theoretical capacity of 335 mAh/g.36 Hierarchical composites, composed of nanosheet morphologies of TiO2-B combined with nitrogen-doped graphene nanosheets, have yielded capacity values of 326.4 mAh/g collected at 1 C after 100 cycles.35 A similar “combinatorial” approach to synthesis has also worked with spinel Li4Ti5O12 as complex aggregate combinations of LTO−TiO2 micron-scale spheres were found to deliver excellent electrochemical performance as well. Specifically, a high capacity of 158 mAh/g was measured at 5 C after 100 cycles, a finding ascribed to the presence of not only preferentially exposed facets but also abundant, electrochemically active interfaces, all of which likely contributed to the observation of an improved overall conductivity.36 Apart from TiO2-based systems,

with an excellent cycling stability and a high rate capability as promising as 176 mAh/g at 0.9 A/g and 59 mAh/g at 6 A/g, respectively.21 In our collective efforts, we compared Li1.4V3O8 motifs created using two very different synthesis methods. In one methodology, a one-pot, solvothermal-based synthesis protocol was developed by our group to prepare a novel, crystalline 1D submicron-sized fiber motif of Li1.4V3O8 with a pure phase. In that reaction, V2O5 was utilized as the V source and LiOH served as the Li source with H2O2 used to tailor, modify, and sculpt the resulting morphology prior to heating. The isolated submicron-sized 1D product, shown in Figure 3B, possesses an average diameter in the range of 300 nm to 1 μm with corresponding lengths of up to 1 mm. In the second sol−gel-based protocol, LiOH·H2O and V2O5 powders were used as starting materials and stirred together in an aqueous solution at low temperature in an inert atmosphere for 24 h, followed by freezedrying, with the isolated intermediate subsequently processed at a higher temperature to obtain the final product. The chemical compositions of the as-prepared LVO materials derived from the solvothermal and sol−gel reactions were determined to be Li1.4V3O8 and Li1.1V3O8, respectively. Results from the electrochemical testing are presented in Figure 4B1,B2, wherein lithium-based cells, containing both sol−gel and solvothermal LVO materials (active mass loading of 3.55 mg/cm2), were cycled at C/10 (1 C = 360 mA/g). The sol−gel-derived LVO displayed a first and second cycle discharge capacity of 207 and 227 mAh/g (Figure 4B1), respectively. By contrast, the solvothermal-formed LVO gave rise to a lower initial discharge capacity relative to the sol−gel material, wherein the first and second cycle discharge capacities of the cells, incorporating the solvothermal material, for the first and second cycles were 150 and 178 mAh/g (Figure 4B1), respectively. Notably, however, the solvothermal-processed LVO yielded better capacity retention as compared with its sol−gel counterpart. Specifically, the former solvothermal sample maintained 98% of its initial capacity after 100 cycles, whereas the latter sol−gel material only retained 60% of its initial capacity, as highlighted in Figure 4B2. Hence, the solvothermal synthesis approach for LVO appears to provide for desirable capacity retention properties, which are worthy of additional investigation. Three-Dimensional Li4Ti5O12 and TiO2 Flowers. Li4Ti5O12 has also been extensively studied as an excellent alternative anode material due to several intrinsic advantages. These include (i) its superior structural stability due to its “zero-volume” change during electrochemical cycling, (ii) its high and stable potential plateau value (i.e., 1.55 V versus Li/Li+), which circumvents solid electrolyte interphase (SEI) formation and also minimizes the possibility of a deleterious short circuit, triggered by the formation of lithium dendrite deposition on the electrode surface, as well as (iii) its overall fast electrode kinetics, resulting from the possibility of viably accessing a large number of prospective 3D Li ion diffusion pathways within the spinel structure itself.18,33 One of our groups designed a unique “flower-like” nanostructured Li4Ti5O12 motif by (i) a facile and large-scale hydrothermal process with unique and recyclable precursors, (ii) followed by short, low-temperature calcination in air. The resulting flower-like Li4Ti5O12 microspheres consisted of thin nanosheet constituents, characterized by roughened surfaces that thereby provided for not only an enhanced surface area but also a reduced lithium ion diffusion distance (Figure 3C). We found these attributes to induce favorable interactions 1470


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Figure 5. (A) Schematic of the attachment process between the LTO and the MWNTs using the APTES linker. (B) Schematic of the attachment process between the LTO and the MWNTs using the 4-MBA linker. (C) Morphology of MWNT−MBA−LTO composites. (D) Elemental mappings of various elements within the Li4−xCaxTi5O12 sample (x = 0.1), collected using an energy dispersive spectrometer (EDS). Panels (A−C) are adapted with permission from ref 43, Copyright 2017, The Electrochemical Society.

possess an inherently low electron conductivity coupled with sluggish lithium ion diffusivity, thereby hindering the practical application of such materials within the context of LIB design.38 Nevertheless, a significant amount of research effort has been expended in terms of developing strategies to mitigate these issues of poor conductivity, transport, and diffusion, associated with metal oxide-based battery materials. The first approach has been to design unique motifs of nanostructured metal oxides

analogous types of improvements have also been noted, for example, with uniform hierarchical Fe3O4@polypyrrole nanocages, which were found to evince a favorable and stable discharge capacity of ∼652 mAh/g, even after 500 cycles at 2000 mA/g.37 Chemically Inspired Strategies to Enhance the Intrinsic Electrode Performance of Metal Oxides. The vast majority of metal oxides consist of either semiconductors or insulators, which often 1471


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Figure 6. (A) Li/LTO−MWNT electrochemical cells created with LTO active material−MWNT composites prepared using (a) covalent, (b) in situ, (c) physical sonication, and (d) π−π interaction attachment methods, at a 5% MWNT loading level along with (e) control samples prepared without MWNTs. (A1) Discharge capacity versus cycle number measured at C/2 (cycles 1−20, 36−55, 71−90), 20C (cycles 21−25, 56−60, 91−95), 50C (cycles 26−30, 61−65, 96−100), and 100C (cycles 31−35, 66−70, 101−105) discharge and charge rates. (A2) Voltage profiles collected at cycles 20 and 30. (B) Li/Fe3O4−MWNT electrochemical cells created with Fe3O4 active material−MWNT composites prepared using (a) π-attachment, (b) sonication, and (c) covalent attachment modalities, along with (d) a control sample without attachment. (B1) Discharge capacity versus cycle number measured at 93 (cycles 1−30), 200 (cycles 31−35), 400 (cycles 36−40), 800 (cycles 41−45), 100 (cycles 46−55),1200 (cycles 56−60), 1600 (cycles 61−65), 2000 (cycles 66−70), and 100 mA/g (cycles 71−80) discharge rates and 93 mA/g charge rates (all cycles). (B2) Voltage profiles acquired from cycles 2 and 70. (C) Li/CaxLi4−xTi5O12 electrochemical cells created with CaxLi4−xTi5O12 materials with calcium doping levels of x = (a) 0.0, (b), 0.09, (c) 0.14, and (d) 0.22. (C1) Discharge capacity versus cycle number measured at 0.2C (cycles 1−10), 1C (cycles 11−20), 10C (cycles 21−30), 0.2C (cycles 31−50), 20C (cycles 51−60), 40C (cycles 61− 70), 100C (cycles 71−80), and 0.2C (cycles 81−100) discharge rates along with a corresponding 0.2C (all cycles) charge rate. (C2) Voltage versus discharge capacity at 0.5C and 100C. Panels (A) and (B) are adapted with permission from refs 43 and 44, Copyright 2017, The Electrochemical Society.

favorable ballistic electron transport, originating from their anisotropic structure.40 As such, CNTs have often been utilized in battery configurations in order to enhance overall electronic conductivity and/or accommodate for the large volume change associated with certain metal oxides.41 Although extensive research has been conducted in terms of generating metal oxide−CNT composites to engender enhancement in electrode performance, very few contributions in the literature have focused on controlling and improving upon the nature of charge transport through systematic engineering of the

in order to enhance their electron and Li ion conductivity, an approach that has been discussed in the previous sections. The second strategy has been to enhance the inherent electronic and ionic conductivity of the metal oxide materials through targeted mediation and introduction of not only (i) conductive carbon additives but also (ii) ion doping.39 Use of Carbon Nanotubes as Conductive Additives. CNTs represent a particularly promising conductive additive for improving the electrode performance of metal oxide-based materials. CNTs maintain desirably superior mechanical strength and 1472


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terms of balancing the needs of both quantitative performance and stability. In particular, the LTO-MWNT composite, produced by π−π interactions (Figure 5C), exhibited a high rate capability (Figure 6A2) as well as a reliably solid cycling stability, delivering 132 mAh/g at 50 C (1 C = 175 mA/g), even after 100 discharge/charge cycles (including 40 cycles at a relatively high rate) (Figure 6A1). These values denote, to the best of our knowledge, clearly superior performance to those of any previously reported LTO−CNT composite materials created to date, especially under the relatively low (5%) loading conditions that we have used. Similar results were observed for the Fe3O4−CNT system, wherein the composite generated using the 4-MBA linker demonstrated the highest specific capacity measured at all rates (Figure 6B2) as well as the best cycling stability observed as compared with its analogues (Figure 6B1).44 It is evident that necessitating explicit bond formation to connect the CNTs with the metal oxides may not be optimal. Rather, an electron-rich environment, induced by a π−π interaction strategy, represents a better way forward. Indeed, our work highlights how deliberative processing protocols can be used to tune and control fundamental material properties. In addition, because of the rather simplistic attachment route proposed and the fairly economical production costs associated with fabricating our metal oxide−MWNT composites generated by the π−π interaction method using the aromatic 4-MBA linker, our cumulative results suggest the viability of utilizing this noncovalent attachment strategy within the context of commercially relevant designs for energy storage. • Effect of Different Linker Molecules. In the previous section, we described how two different linker molecules, that is, APTES and 4-MBA, can be used to facilitate the attachment of metal oxides with CNTs within composite heterostructures using covalent and noncovalent approaches, respectively. Indeed, very different electronic and ionic transport properties have been observed within the composites generated using these two distinct linkers. For instance, we measured a charge transfer impedance of 89 ohms and an effective Li ion diffusion coefficient of 5.9 × 10−8 cm2/s, respectively, for the LTO− APTES−MWNT composite, whereas for the LTO−MBA− MWNT analogue, the corresponding values were 24 ohms and 2.3 × 10−7 cm2/s, respectively. The differences are likely not solely attributed to the treatment protocol. Rather, these results clearly highlight the additional and equally significant effect of the physicochemical structure of the different linker molecules on the resulting electrochemical properties. In a separate yet relevant study on the optoelectronic properties of CNT−CdSe quantum dot (QD) heterostructures, one of our groups systematically probed the effect of using different intervening bridging linker molecules (i.e., p-phenylenediamine (PPD), 2-aminoethanethiol (AET), and 4-aminothiophenol (ATP) to probe the effects of varying (i) the identity of the terminal functional moiety as well as (ii) the presence or absence of a conjugated π system) upon the observed charge transfer behavior between the constituent CNTs and QDs. A compilation of both experimental and theoretical results showed that (i) the presence of a π-conjugated carbon framework within the ligands themselves and (ii) the electron affinity of the pendant groups collectively played important roles in the resulting charge transfer from QDs to the underlying CNTs.45 Hence, of relevance for our discussion herein, our preceding body of work on these CNT−QD-based systems reinforced the importance of modifying the structure and chemistry of

physicochemical nature of the molecular junctions between the metal oxides and the CNTs themselves. • Effect of Different Attachment Modalities. As such, our group has focused on the key issue of inducing improvements in ionic and electronic transport through the strategic and targeted control of the “connection” between the constituent metal oxide motifs and the adjoining CNTs through judicious selection of relevant physicochemical attachment strategies. In particular, our collective efforts on (i) Li4Ti5O12 (LTO)−CNT composites as well as (ii) Fe3O4−CNT heterostructures have highlighted the significance of tailoring the nanoscale “attachment modality” in terms of modifying and governing the resulting electrochemical observations and trends.

The second strategy used to improve the inherent electrochemical performance of the metal oxides in question is through the use of chemically inspired protocols, such as ion doping, surface modification, and/or incorporation of carbon. Specifically, to test the importance of this parameter in the case of LTO−CNT systems, identical loading ratios of 5 wt % multiwalled carbon nanotubes (MWNTs) have been successfully anchored onto the external surfaces of a unique flowerlike LTO microscale sphere motif developed by one of our groups,42 using a number of different and distinctive preparative approaches.43 We were very much interested in (i) probing the differences between using a “chemical-based” versus a “physicalbased” strategy as well as (ii) differentiating between “covalent” and “noncovalent” approaches. In particular, we investigated the use of (i) a sonication method, (ii) an in situ direct-deposition approach, (iii) a covalent attachment protocol, as well as (iv) a π−π interaction strategy. What do these methods entail and what are the main differences? A physical sonication method links together the metal oxides and MWNTs within a discrete composite via van der Waals interactions; no formal bond forms in this case. In our in situ direct-deposition process, we mixed together and reacted precursors of the metal oxides together with the CNTs within the same reaction “pot” to enable seamless formation of these oxide species directly onto the external surfaces of the CNTs without any obvious connective agent. By contrast, a covalent attachment protocol involves the actual creation of amide bonds between the pendant carboxyl/carbonyl species on the functionalized MWNTs and the amine end groups associated with an amine-functionalized linker, 3-aminopropyl triethoxysilane (APTES), that had been attached onto the surfaces of the metal oxides (Figure 5A). Finally, a noncovalent π−π interaction strategy connects the CNTs and the metal oxides through the facilitative mediation of π-bonds between the electron-rich aromatic phenyl rings within the specifically chosen 4-mercaptobenzoic acid (4-MBA) linker and the underlying MWNT’s intrinsic π electron-conjugated network (Figure 5B). Electrode tests have suggested that the composites generated by π−π interactions outperformed all of the other three analogous heterostructures due to not only a smaller charge transfer resistance but also faster Li ion diffusion, implying that a chemical, noncovalent-based treatment is highly desirable in 1473


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partial transformation of Ti4+ into Ti3+, based upon charge compensation considerations. The net result of this process would be to increase the concentration of electrons within the empty 3d orbitals of Ti4+, leading to a presumed enhanced electron conductivity.18,49 Electrochemical results have suggested that the Ca-doped samples have demonstrated a lower charge transfer resistance as compared with that of pure LTO, thereby giving rise to improved electrochemical performance. Specifically, the sample, possessing a doping ratio of 0.2, delivered 151 and 143 mAh/g under discharge rates of 20 and 40 C (1 C = 175 mA/g) at cycles 60 and 70, respectively, with an active material mass loading of 2.75 mg/cm2 (Figure 6C1). In addition, a high cycling stability was observed with a capacity retention of 92% after 300 cycles at a very high discharge rate of 20 C. Significantly, the LTO sample with a Ca doping ratio of 0.2 yielded the highest capacity observed under both low rate (i.e., 0.5 C) and high rate (i.e., 100 C) conditions (Figure 6C2). Hence, the superior high rate performance coupled with the facile synthesis route associated with the generation of Ca-doped LTO is expected to be promising for future commercial applications. • Advantages Associated with Ion Doping of Cathodes. In addition to its potentially broad applicability to anode materials, the concept of cationic doping has also been proven to be an effective means of optimizing the operating voltage, charge transport, capacity retention, and structural stability of cathode materials. For example, with respect to Mn-based cathode materials in particular, better capacity retention and a higher discharge capacity of LiMnO2 were achieved through either Co, Cr, Zn, Fe, or Al doping. Moreover, improved cycling stability, lithium diffusivity, and electronic conductivity were noted after either Cr or Ru doping within Li2MnO3.50 For these Mn-based materials, the majority of the as-obtained experimental and theoretical results have suggested that the cationic dopants are localized not only within the Mn sites but also at the Li sites during specific cell charge/discharge processes. Hence, to generalize this idea, further studies are essential to shed light on the precise role and functionality of doping sites. In addition to the extensive research efforts associated with cationic dopants, the complementary use of anionic dopants, such as F, S, and Cl, may also yield several unique advantages. First, by forming oxyfluoride through F doping, cathode materials may show enhanced stability to dissolution in HF-related acids. Second, anion doping precludes the undesirable occupation of either Li or transition metal ion sites, which may often prevent the attainment of the expected theoretical energy density. Third, an anion doping strategy that can directly modify the relevant anionic redox processes would denote a promising direction for Li-rich oxide engineering.51 Summary and Outlook. In summary, metal oxides offer a host of opportunities as electrode materials for LIBs but may suffer from limitations associated with poor ionic and electron conductivity as well as low cycling performance. Hence, achieving the goal of creating economical, relatively less toxic, thermally stable, and simultaneous high-energy-density electrode materials requires deliberative strategies aimed at rationally improving the electrochemical performance. As such, in this Perspective, we have purposely focused on exploring the precise roles of (i) morphology and (ii) deliberative chemical strategies in terms of advancing, ameliorating, and fundamentally tuning the development of Fe3O4, Li4Ti5O12, TiO2, and LiV3O8 (chosen as representative examples of particularly promising metal oxides)

functional linker molecules as a plausible means of enhancing charge transfer within analogous metal oxide−CNT composite electrodes. • Other Types of Carbon Additives. Apart from the broad application of CNTs within metal oxide-based electrodes, extensive research effort has been expended with the use of other types of conductive carbon additives. For example, in view of their good electrical conductivity and excellent mechanical/ electrochemical stability, carbon black, carbon fibers, porous carbon, and graphene have also been coupled with metal oxide nanostructures for battery applications.46 Specifically, these carbon materials have often been attached onto the metal oxides in two distinctive and complementary ways in order to form the desired hybrid composites. First, carbon layers can be coated onto the oxide surface through either a sonication or a milling process, thereby inducing the formation of this carbon “covering” through a physisorption process. Second, the carbon coating can be created by means of an in situ deposition process, occurring via the carbonization of organic capping agent precursors. Again, very few studies have focused on the precise engineering of the surface functionalities in order to tailor the junction between these carbon additives and the underlying metal oxides. Therefore, rational variations of the linker group chemistry used to attach these different and distinctive carbon materials onto the underlying metal oxides represent a promising future direction for manipulating and improving on the performance metrics of metal oxide−carbon additive composites. Enhancement of Electrode Performance through Ion Doping. The presence of a carbon coating usually enhances electron conduction between the metal oxides and the current collector. However, to effectively modify the intrinsic electronic conductivity and/or Li ion diffusion coefficients within the metal oxide structures themselves, the idea of chemical doping in particular needs to be considered. • Advantages Associated with Ion Doping of Anodes. Our team has recently focused on the ion doping of a motif that one of our groups recently developed and synthesized, specifically, as-prepared flower-like LTO anode materials.42 Within the context of doping of LTO, there has been extensive prior research focused on the doping of either the [Li]8a, [Li]16d, [Ti]16d, or [O]32e sites, with metal ions such as Ni2+, Mg2+, Zn2+, Al3+, Pr3+, Cr3+, Zr4+, Nb5+, V5+, Ta5+, and F−.28,47 We have restricted our selections to Ca2+ (substituting for Li+ at the 16d site), Nb5+ (substituting for Ti4+ at the 16d site), and F− (substituting for O2− at the 32e site), denoting choices that take into consideration the reaction compatibility of the doping process with the synthesis procedures of our as-prepared LTO morphologies. In the specific illustrative example of the use of Ca, we found that this dopant could be readily and uniformly distributed within the underlying LTO motif (Figure 5D). The advantages associated with the ion doping of LTO are several-fold. If we consider Ca as a representative example of a desirable dopant species, we note the following. First, Ca is highly abundant and relatively inexpensive. Second, it possesses a Pauling’s crystal radius of 114 pm within the 6-coordination environment, which is larger than that of Li+ (i.e., 90 pm), and moreover, it localizes within the 16d site. Therefore, doping with Ca2+ can induce an optimal lattice expansion, which reduces the amount of geometric obstruction to lithium ion diffusion and, as such, enhances the ion conductivity of LTO, thereby contributing to a better rate performance.47,48 Third, the higher oxidation state of Ca2+ might potentially drive the 1474


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as viable and ubiquitous energy storage materials. Our studies have yielded the following. (1) The structural design of very specific nanostructure motifs can significantly and favorably influence the observed electrochemical performance, as defined by the measured rate and cycling performance, by not only accommodating for additional lattice strain with the underlying existing crystal structure through subtle lattice modifications but also increasing conductivity by shortening overall diffusion lengths for lithium ions. (2) Different morphologies, including 0D, 1D, and 3D motifs, of a number of our metal oxides have been systematically synthesized, and their individual electrochemical behaviors have been subsequently compared. It is fair to state that it is not conclusively obvious whether any particular morphology is intrinsically better than any of the others for each of the materials that we have analyzed. However, in general, complex 3D hierarchical assemblies were found to notably outperform their 0D conventional, commercial counterparts of identical composition with respect to a number of metrics. For example, as compared with these 0D materials, Li4Ti5O12 possessing a 3D flower-like morphology maintained a markedly higher capacity of 141 mAhg−1 under a 10 C rate at 55 cycles. In addition, to highlight the desirability of these more complex motifs, the anatase phase of TiO2 incorporating a 3D “sea urchin” morphology not only yielded a reasonable capacity of 214 mAh/g at 0.1 C but also showed a better durability, as evidenced by the 90% retention value measured after 100 cycles. (3) Rational chemically inspired protocols have been employed to improve both ionic and electric conductivity of metal oxides either (i) through the attachment of conductive carbon additives or (ii) by means of ion doping. For example, with respect to controlling and improving on the nature of charge transport through systematically engineering the nature of the molecular junctions between conductive CNTs and metal oxides, our cumulative results suggested the importance and optimal feasibility of fabricating high-performing metal oxide−MWNT composites using a noncovalent, π−π interaction method, mediated by an aromatic, electron-rich, conjugated linker molecule. In terms of doping, our preliminary data and prior reports have suggested this strategy to be an effective means of optimizing the operating voltage, charge transport, capacity retention, and structural stability of electrode materials, presumably by inducing a combination of greater stability to dissolution, favorable lattice distortions, and the incorporation of higher electron densities. A number of global scientific issues still remain though and will likely become the focus of future studies. As was implied earlier, there is no direct and obvious connection and correlation between morphology and electrochemical parameters, such as rate capability. Furthermore, the degree of ion and electron conductivity improvement induced by chemical functionalization is often uneven and, at times, ill-understood. Moreover, additional disparate and equally significant factors such as but not limited to (i) the intrinsic crystal structure of the metal oxide, (ii) the nature of the exposed electrochemically active crystallographic facet, as well as (iii) the exact mechanism associated with the Li ion intercalation and deintercalation

A number of global scientific issues still remain though and will likely become the focus of future studies. As was implied earlier, there is no direct and obvious connection and correlation between morphology and electrochemical parameters, such as rate capability. Furthermore, the degree of ion and electron conductivity improvement induced by chemical functionalization is often uneven and, at times, illunderstood. process still need to be more thoroughly explored. Significantly, it is worth noting as a matter of particular concern that theory has not advanced to the point wherein sufficiently meaningful insights into the relative contributions of each of these various structural parameters can be reliably assessed in terms of the observed performance. Furthermore, all of these variables are inherently intertwined with the unique and individualized synthetic growth mechanisms associated with these various metal oxides and their corresponding motifs, which are themselves dependent on the precise synthetic protocol used. For example, factors such as reaction temperature, time, solvent viscosity, as well as the presence of organic ligands, to name simply a few, play important roles in determining the morphologies of the observed products. These are certainly complex issues. Moreover, it is still experimentally nontrivial to either (i) probe the growth and structural evolution of these structures kinetically or (ii) monitor their associated dynamic electrochemical processes in situ. Recent and complementary advances in both in situ microscopy imaging and spectroscopy techniques have certainly helped to address all of these issues.52−53 Nonetheless, work and corresponding progress in all of these key directions remain important challenges, moving forward. Apart from concerns associated with the synthesis and growth of these metal oxides, additional logistical obstacles need to be overcome with respect to the practical incorporation of these metal oxides within actual commercial Li ion battery applications. We highlight a few illustrative examples. First, despite the superior safety and durability advantages with respect to their conventional counterparts, LTO-based anodes are particularly prone to associated gas (e.g., CO2, H2, and CO) evolution not only during charge-and-discharge cycles but also, more generally, under operating conditions at elevated temperatures, all of which can lead to serious swelling and, hence, unpredictability in terms of long-term performance. Hence, the development of effective gas suppression methods is especially beneficial as a means of enabling the large-scale applicability of LTO-based power storage batteries. Second, the widespread applicability of these metal oxide-based electrodes within EVs requires pronounced enhancement in energy density, a parameter that is directly related to not only the practical specific capacities of the individual electrode materials but also the potential difference between the cathode and anode. Therefore, future studies need to focus on overcoming the inherently low energy density of the metal oxides discussed herein through a multipronged and complementary approach aimed at 1475


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(i) enhancing their specific capacity, (ii) judiciously selecting appropriate and mutually compatible combinations of cathodes, anodes, and electrolytes so as to achieve more favorable potential differences, (iii) eliminating nonactive material by generating “binder-free” electrodes, and (iv) ultimately, designing compact and high-density electrode architectures.52 Third, insights into the development of a facile and economical synthesis method that can be used for the continuous “scaled-up” production of chemically tuned (e.g., carbon-coated or ion-doped) metal oxide electrode materials are urgently needed for their incorporation within the generation of commercial Li ion battery configurations.

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Professor Takeuchi holds a B.A in Chemistry from the University of Cincinnati and a Ph.D. in Chemistry from The Ohio State University. His current scientific research involves the synthesis and utility of inorganic materials toward energy storage. Dr. Amy Marschilok is a Research Professor in the Departments of Materials Science and Chemical Engineering and Chemistry at Stony Brook University. She received her B.A. in Chemistry (English Minor) and Ph.D. in Chemistry from the University at Buffalo. Her research involves the synthesis, characterization, and electrochemistry of novel materials and systems for energy storage. Dr. Esther S. Takeuchi is a SUNY Distinguished Professor in the Departments of Materials Science and Chemical Engineering and Chemistry at Stony Brook University and Chief Scientist at Brookhaven National Laboratory. She received a B.A. in Chemistry and History from the University of Pennsylvania and a Ph.D. in Chemistry at The Ohio State University. Her research focus is the study of materials and mechanisms in battery systems.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. ORCID

Amy C. Marschilok: 0000-0001-9174-0474 Esther S. Takeuchi: 0000-0001-8518-1047 Stanislaus S. Wong: 0000-0001-7351-0739

Dr. Stanislaus S. Wong is Professor of Chemistry at SUNY Stony Brook with a joint appointment at Brookhaven National Laboratory, and he is an Associate Editor for ACS Applied Materials and Interfaces. He earned his Ph.D. in Chemistry from Harvard University under the tutelage of Professor Charles M. Lieber. He and his group are fundamentally interested in synthesizing novel nanomaterials possessing unique composition and morphology as components of batteries, fuel cells, and solar cells.

Author Contributions #

L.W. and S.Y. contributed equally to this work.

Notes

The authors declare no competing financial interest. Biographies

■

Dr. Lei Wang is a postdoctoral research associate at Stony Brook University. She graduated from Stony Brook University and earned her Ph.D. in Chemistry in 2016. Her current research focuses on the synthesis and characterization of nanostructured metal oxides and metal oxide−carbon nanotube composites with applications in energy storage and conversion.

ACKNOWLEDGMENTS All of the work described in these studies was funded as part of the Center for Mesoscale Transport Properties, an Energy Frontier Research Center supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award #DE-SC0012673. Research characterization was carried out in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC0012704.

Ms. Shiyu Yue received her B.Sc. degree in chemistry from Zhejiang University in 2014. She is currently a Chemistry Ph.D. student at Stony Brook University under the direction of Professor Stanislaus S. Wong. Her research interests involve the synthesis and characterization of metal oxides for solar cell and battery applications.

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Ms. Qing Zhang graduated from Tianjin University with a B.Eng. degree in Materials Science and Engineering in 2012. She is currently pursuing her Ph.D. in Materials Science and Engineering at Stony Brook University. Her current research focuses on understanding the synthesis, structure, and basic properties of materials and their batteryrelevant electrochemistry.

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Ms. Yiman Zhang studied Chemistry at Henan Normal University and obtained her M.S. in Chemistry from the University of Science and Technology of China (USTC) in 2012 on the synthesis and phosphorescent properties of organic radicals. She is pursuing her Ph.D. in Chemistry at Stony Brook University, studying active materials for Li ion batteries. Mr. Yue Ru Li graduated from The City College of New York, NY, with a B.S. degree in Chemistry in 2011. He is currently pursuing his Ph.D. in Chemistry at Stony Brook University. His research interest involves electrochemically active materials, such as metal oxides and metal phosphorus oxides for lithium primary and secondary batteries. Dr. Crystal S. Lewis received her B.Sc. degree in Biochemistry in 2011 from Oakwood University. She earned her Ph.D. in Chemistry from Stony Brook University in 2016 in the group of Stanislaus S. Wong. She is currently an Adjunct Instructor at Suffolk County Community College. Dr. Kenneth J. Takeuchi is a SUNY Distinguished Teaching Professor in the Department of Chemistry at Stony Brook University. 1476


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