3O2 into Nanosheets Using

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Exfoliation of LiNi Mn Co O into Nanosheets using Electrochemical Oxidation and Reassembly with Dialysis or Flocculation Qian Cheng, Ting Yang, Man Li, and Candace K. Chan Langmuir, Just Accepted Manuscript • Publication Date (Web): 26 Apr 2017 Downloaded from http://pubs.acs.org on April 27, 2017

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Exfoliation of LiNi1/3Mn1/3Co1/3O2 into Nanosheets using Electrochemical Oxidation and Reassembly with Dialysis or Flocculation

Qian Cheng, Ting Yang, Man Li, Candace K. Chan*

Materials Science and Engineering, School for Engineering of Matter, Transport and Energy, Arizona State University, 501 E. Tyler Mall, ECG 301, Tempe, AZ 85287 *[email protected]

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ABSTRACT

Two-dimensional (2D) materials such as nanosheets are increasingly attracting attention for applications in energy storage and conversion. Many conventional battery compounds have layered structures, which can facilitate the exfoliation of these materials into nanosheet morphologies. In this work, LiNi1/3Mn1/3Co1/3O2 (NMC) particles were exfoliated into nanosheets using an electrochemical oxidation method enabled by the intercalation of tetraethylammonium cations into the interlayer space. The exfoliated materials were monolayer or double-layer nanosheets with hexagonal shapes and sizes < 50 nm. Two different methods were used to reassemble the nanosheets into NMC particles: 1) a slow, dialysis-based approach, and 2) direct flocculation. Characterization of the NMC materials at different stages of the exfoliation and reassembly processes was performed using compositional analysis, X-ray diffraction, electron microscopy, and electrochemical methods. The dialysis reassembly method allowed for the restacking of the nanosheets into faceted, hexagonal-shaped nanoplatelets, while the flocculation approach only yielded ill-defined particles. The differences in the observed potential-dependent redox behavior and electrochemical cycling characteristics is attributed to the role of the reassembly method on the formation of phase segregated domains, with the particles reassembled using the dialysis approach displaying the best performance.


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Introduction Two-dimensional (2D) materials have attracted a great deal of attention for their unique electrical and magnetic properties1-3, but may also play important roles in energy storage applications4,5. The ability to assemble nanosheets into particles with desirable characteristics or integrate them into hybrid nanocomposites with other layered materials6-9 is a twist on the “bottom-up” approach to obtaining materials by design. For example, one may envision using nanosheets as “building blocks” to create novel materials with improved structural stability or rate capabilities, unique redox properties, or new crystal structures better suited for batteries beyond Li-ion (e.g., Na+, multivalent cations) in ways not yet possible from direct synthesis. While a number of chemical synthesis and templating approaches have been reported for the preparation of inorganic nanosheets4,10, the ability to create nanosheet building blocks using a general exfoliation methodology that can be applied to a number of bulk layered materials is attractive. Electrochemical or chemical reduction-based methods that enable intercalation of alkali cations into the interlayer space have been broadly applied to exfoliate materials into nanosheets, but these approaches are more effective for compounds with weak (van der Waals) interlayer bonding, such as graphene-analogs and metal chalcogenides2,11,12. Many conventional Li-ion battery cathode materials have layered structures, which can facilitate their preparation into nanosheet morphologies. For layered metal oxides with interlayer bonding dominated by electrostatic interactions, osmotic swelling approaches13,14 are more appropriate than reductive methods to obtain nanosheets by exfoliation. However, this usually requires an acid treatment to insert protons inside the interlayer space; these protons are subsequently exchanged with bulky organic cations to decrease the electrostatic interactions between layers, increase the interlayer spacing, and allow for exfoliation of the materials into


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nanosheets. Previous attempts applying osmotic swelling to cathode materials such as LiCoO2 have been largely unsuccessful for the preparation of electrochemically useful materials due to the fact that the introduced protons irreversibly bind to the negatively charged transition metal oxide layers and occupy Li+ sites, resulting in lower than expected specific capacities15. To address this problem, we recently developed an exfoliation approach that circumvents the acid treatment by taking advantage of the redox properties of the transition metal in the layered compound. Electrochemical oxidation is used to increase the interlayer distance and create Li+ vacancies ( ) in the interlayer space, which are subsequently filled with alkylammonium cations in a process that is coupled with the hydroxide oxidation reaction, reduction of the transition metal, and driving force for the material to maintain electroneutrality16. Our initial demonstration showed that this method was effective for exfoliating commercially available LiCoO2 into nanosheets 50 – 200 nm in size composed of single and double atomic layers. Furthermore, purification and reassembly processes to restack the LiCoO2 nanosheets back into bulk particles were studied. Compared to another report using the conventional osmotic swelling method to prepare LiCoO2, where only 74 mAh/g capacity was achieved15, materials prepared using our approach could display 131 mAh/g 16. In this work, we explore the generality of this approach and extend our exfoliation method to LiNi1/3Mn1/3Co1/3O2 (NMC) a promising cathode material for electric vehicle applications17-19. NMC, a solid solution of LiCoO2, LiMnO2, and LiNiO2, has the same structure as LiCoO2 (R3m) but improved thermal and structural stability20. Chemical synthesis methods have been reported for the preparation of NMC with nanoplate-like structures, but they usually require structure-directing agents or carefully controlled heat treatments21-24. Oh, et al. reported the preparation of NMC nanosheets by exfoliation of bulk particles using the conventional


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osmotic swelling approach25, but without any electrochemical investigation of the materials. Our studies not only demonstrate the generality of our oxidation-assisted exfoliation method, but also investigate the self-assembly mechanisms in the restacked NMC particles and the resulting effect on the electrochemical properties of the reassembled particles. The various NMC samples are labelled as shown in Scheme 1, with different numbers assigned to samples at different steps in the exfoliation and reassembly process.

Scheme 1. Flowchart of the NMC exfoliation and reassembly processes. Sample 1 is the asreceived NMC, which was electrochemically oxidized to form delithiated NMC, 2. Then, 2 was 5 ACS Paragon Plus Environment

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intercalated with tetraethylammonium cations, TEA+ (red spheres), to make 3. Sonication of 3 caused exfoliation to form the NMC nanosheet dispersion 4. At this stage, there are two different pathways for reassembling the NMC nanosheets into particles. In Route 1, dialysis was performed on 4 in 1 M LiNO3 to re-insert Li+ (yellow spheres) into the structure and make 5; then electrophoresis was used to remove the TEA+ to form 6. In Route 2, 4 was treated with electrophoresis directly to remove the TEA+ to make 7; 7 was then flocculated in 1 M LiOH to re-insert Li+ into the structure to form 8. Both 6 and 8 were then heat treated using microwave hydrothermal reaction and thermal annealing to improve the crystallinity prior to electrochemical testing.

Experimental Section Electrochemical oxidation of NMC NMC powder was obtained from Sigma-Aldrich (part no. 761001, < 0.5 µm particle size). The NMC was mixed in de-ionized (DI) water with 5 wt% carboxymethyl cellulose (CMC, Mw ~ 250k, Sigma Aldrich) serving as binder. Conducting carbon additive was not used in preparing the NMC electrodes to avoid introduction of impure products. The slurry was subsequently coated onto aluminum foil current collectors and the dried electrodes were assembled with lithium metal counter electrodes into pouch cells. The half-cells were then potentiostatically charged at a 4.3 V vs. Li/Li+ in 2 h intervals separated by 15 min relaxation periods at open circuit (Figure S1). This was repeated until the open circuit voltage after the rest period was higher than 4.29 V vs. Li/Li+. After electrochemical oxidation, the charged NMC electrodes were washed with DI water several times to thoroughly remove the electrolyte and CMC binder.


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Exfoliation of NMC The washed NMC (0.14 g) obtained after electrochemical oxidation was immersed in a tetraethylammonium hydroxide (TEA, ~40% in water, Sigma Aldrich) solution with a volume of 45 mL. The amount of TEA was varied with respect to the number of lithium vacancies ( ) in the oxidized NMC. After immersing the charged NMC particles into the TEA solutions, the mixture was sonicated using a tip probe ultrasonicator (CPX 600, 660 W) for 15 minutes to assist with the intercalation of TEA+ into the interlayer space of NMC. Afterwards, the mixture was sonicated using a bath sonicator (CPX 1800H, 70 W) at 60 oC for two days. The solution was then centrifuged at 5000 rpm for 15 minutes to remove any unexfoliated particles to obtain a suspension of NMC nanosheets. The unexfoliated particles found in the pellet after centrifugation were used for XRD characterization, while the nanosheet dispersions were taken from the decanted phase for all other (e.g., ICP-OES, TEM, SEM, AFM) characterization, dialysis, and electrophoresis. Dialysis of NMC nanosheets dispersion To remove the excess TEA from the nanosheet dispersions, dialysis was employed using a Slide-A-Lyzer Dialysis Cassette (30 mL capacity, Thermo Scientific) with pores that can allow molecules with molecular weight 20K or below to pass through. The nanosheet dispersions were placed inside the dialysis cassette, which was then placed in 1 L of DI water and allowed to sit with slow stirring until the pH of the water decreased to 7. To reassemble the NMC nanosheets using dialysis, the DI water in the 1 L beaker was replaced with a 1 M LiNO3 solution. As the Li+ from the LiNO3 solution diffused into the nanosheet dispersion inside the dialysis cassette, the nanosheets would slowly flocculate from the electrostatic interactions between the Li+ and the negatively charged nanosheets. 7 ACS Paragon Plus Environment

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Electrophoresis of NMC nanosheets and reassembled NMC particles Both NMC nanosheets (Sample 4) and reassembled NMC particles (obtained through dialysis in LiNO3 solution, i.e. Sample 5) were treated with electrophoresis for further purification. NMC nanosheets were collected from the TEA solution using centrifugation at 8500 rpm. The NMC nanosheets were not fully dried so that they could be easily re-dispersed in acetonitrile. The reassembled NMC particles were also collected using centrifugation and washed with DI water multiple times to remove any residual salts. Then, the washed particles were dispersed in acetonitrile using bath sonication for 15 minutes. Once the NMC materials were dispersed in acetonitrile, electrophoresis was performed using a 30 V applied voltage with a DC regulated power supply (Circuit Specialists, CSI 3003X5) to remove adsorbed TEA+ from the NMC surface. Stainless steel electrodes (cleaned sequentially in DI water, ethanol, then acetone) were used for both the cathode and anode during electrophoresis. Microwave hydrothermal treatment of NMC particles After electrophoresis, the reassembled NMC particles or NMC nanosheets were collected from the acetonitrile and re-dispersed by sonication in DI water containing 1 M LiOH to replenish the Li+ removed during electrophoresis. The suspension was then treated in a microwave hydrothermal reactor (CEM Corp., Discover-SP, 300 W, 160 oC, 300 psi) for 30 minutes to improve the crystallinity of the reassembled particles. Calcination of NMC particles The NMC reassembled particles obtained after the microwave treatment were recovered using vacuum filtration, then annealed at 500 oC for 6 hours in air using an oven (Lindberg M, Thermal Scientific) to remove solvent and any residual organics or carbon, as well as to improve the crystallinity of the sample. 8 ACS Paragon Plus Environment

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Materials characterization X-ray diffraction (XRD) characterization was performed with monochromatic Cu Kα radiation (λ = 1.5405 Å, Panalytical X’pert Pro). Field emission scanning electron microscopy (SEM) was used to examine the morphology of reassembled NMC particles (FEI XL30). Multiple-collector inductively coupled plasma optical emission spectrometry (ICP-OES, Thermo iCAP 6300) was used to analyze the composition of the NMC during the different steps of the exfoliation and reassembly. ICP-OES measurements of Samples 1, 2, 4, 5, 6, and 7 (Scheme 1) were obtained. To perform ICP-OES on Sample 4, the solution containing NMC suspended in TEA was centrifuged at 5000 rpm to remove the unexfoliated NMC particles, followed by vacuum-filtration through a 0.22 µm PVDF filter membrane (Sigma-Aldrich, Durapore) to obtain only the exfoliated nanosheets. All samples were digested with 70% nitric acid (trace metal grade) at 160 oC and 300 psi using a microwave hydrothermal reactor (Discover-SP, 909150). To prepare samples for transmission electron microscopy (TEM) characterization, the NMC nanosheets (Sample 4) were separated from the TEA solution using centrifugation at 14,000 rpm (Microfuge, I8) and collected from the decanted phase. The recovered NMC nanosheets were re-dispersed in isopropanol by sonicating with a tip probe sonicator (CPX 600, 660W) for 10 min and dropped onto a Cu grid for TEM characterization (JEOL 2010F, 200 kV). For atomic force microscopy (AFM) characterization, the NMC nanosheets dispersion in isopropanol was dropcast onto clean silicon substrates using spin coating at 600 rpm. The silicon substrate was sonicated in DI water, ethanol, and acetone for 30 minutes separately to clean the surface. AFM measurements were performed using an Asylum Research (MFP 3D, classic) microscope with tapping mode.


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Electrochemical characterization The NMC materials were dispersed in NMP by sonication, then mixed with graphite conducting additive and polyvinylidene fluoride (PVDF) binder at a weight ratio of 8:1:1. The particles were dropcast onto Al foil and dried at 120 oC overnight to be assembled in coin cells for electrochemical characterization. Galvanostatic cycling was performed using a 0.2 C rate (25 mA/g) from 3 to 4.2 V vs. Li/Li+ (MPG2, Biologic). Differential capacity analysis was performed using the EC-lab software from Biologic.

Results and Discussion Preparation of NMC nanosheets by exfoliation The first step of the oxidation-assisted exfoliation method to obtain nanosheets is to electrochemically oxidize the NMC. During oxidation, the NMC is delithiated and the Ni2+/Ni3+, Ni3+/Ni4+, and Co3+/Co4+ redox couples are activated at approximately 3.8, 4.0, and 4.6 V vs. Li/Li+, while the Mn4+ cations play a stabilizing role and do not react19. Previous studies showed that the c-axis parameter, which correlates to the interlayer distance, will steadily increase as Li1-xNi1/3Mn1/3Co1/3O2 is formed during Li+ extraction (charging) to reach a maximum at x = 2/3 19,26

. With the aim of obtaining the largest interlayer spacing, the pristine NMC particles were

charged potentiostatically at 4.3 V vs. Li/Li+ (Figure S1). Galvanostatic cycling results in previous studies showed that x ~ 2/3 when the charging voltage has reached this potential, and the nickel cations should be oxidized to the +4 valence state19,26-28. XRD analysis of the films after oxidation showed a shift in the (003) reflections to lower angles (Sample 2 in Figure 1A), confirming the extraction of Li+.


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Figure 1. XRD patterns of (A) different NMC samples during the exfoliation and reassembly processes (the samples are named as in Scheme 1); (B) unexfoliated NMC particles after oxidation and reaction in TEA-OH solutions with different concentrations (TEA+ : = 0.5, 1.5, 2, 3, 10). * refers to NMC intercalated with TEA+.

After electrochemical oxidation, the NMC particles were dispersed into solutions containing different concentrations of tetraethylammonium hydroxide (TEA-OH). During this reaction, the NMC becomes reduced while the hydroxide ions are oxidized to O2. Since the NMC was charged to 4.3 V vs. Li/Li+, the Ni species should have sufficient oxidation potential to oxidize hydroxide ions to O2. This is also consistent with reports showing that delithiated forms of NMC and related compounds exhibit good activity as electrocatalysts for the oxygen evolution reaction (OER)29,30. In order to maintain electroneutrality when the Ni is reduced, the


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TEA+ is inserted into the NMC interlayer space and results in layer expansion and eventually exfoliation. To verify the exfoliation behavior, the ratio of TEA+ to the number of in the oxidized NMC (assuming a composition of Li1/3Ni1/3Mn1/3Co1/3O2 was obtained after potentiostatic charging) was varied from 0.5 to 1, 1.5, 2, 5, and 10 (Figure 2A). After sonication and centrifugation to remove the unexfoliated NMC particles, a dark reddish solution was observed. The solution exhibited the Tyndall effect (Figure 2B) in the range of TEA+ : = 0.5 - 2, which is an indication of the successful exfoliation and dispersion of the NMC nanosheets. When the as-obtained NMC powder was exposed to the same sonication protocol without performing the oxidation step first, very little exfoliation was observed and most of the powder was collected in the pellet, indicating very little nanosheet formation (Figure S2). This confirms that the exfoliation mechanism is not solely caused by the mechanical agitation imparted by the sonication. As shown in Figure 2A, however, for the oxidized NMC immersed in solutions with TEA: higher than 5, exfoliation was barely observed. The explanation for the TEA concentration dependence on the exfoliation of NMC into nanosheets is similar to what we previously observed in LiCoO2 16. When the concentration of TEA is too high, the higher pH increases the hydroxide oxidation rate by decreasing the OH- oxidation redox potential16, which in turn also increases the NMC reduction rate. However, the solid state diffusion of the TEA+ ions in between the NMC layers is not affected by pH. As a consequence, in the solutions with high TEA concentrations, the NMC reduction could be faster than the intercalation rate of TEA+ into the NMC interlayer space, increasing the possibility of the Ni4+ cations in NMC becoming reduced to Ni2+. The Ni2+ ions can migrate to the Li+ sites due to similarities in the ionic radii


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(0.69 Å for Ni2+ and 0.76 Å for Li+, compared to 0.6 Å for Ni3+)31, which could impede the TEA+ intercalation.

Figure 2. (A) NMC Nanosheet dispersions obtained after exfoliation and centrifugation in solutions with TEA: of 0.5, 1, 1.5, 2, 5, and 10; (B) the Tyndall effect in dispersion with TEA: =1.

To investigate this, XRD was used to examine the particles that were not successfully exfoliated, which were collected from the nanosheet dispersions using centrifugation. The XRD patterns (Figure 1B) of the unexfoliated particles were very similar to that for the pristine NMC particles, which have an α-NaFeO2 type structure with R3m symmetry32. A new reflection at


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approximately 2θ = 12.5o was observed for TEA+ : > 0.5 which we attribute to NMC particles intercalated with TEA+. These results suggest that there was unsuccessful or incomplete exfoliation of NMC, either from insufficient TEA+ insertion into the structure or the formation of TEA+ intercalated phases that could not be exfoliated. It was also observed that the (006)/(012) and (018)/(110) reflections in the XRD patterns were not well separated, not only in the unexfoliated particles, but also in the pristine NMC. This is usually observed when the NMC has a poorly developed layered structure or lithium deficiency33,34. Additionally, the relative intensity of the (003) to (104) reflections was close to 1 as the TEA+ : increased, compared to 1.3 for the pristine NMC. The I(003)/I(104) close to 1 indicates a relatively high cation mixing between Li+ and Ni2+ in the Li layer31. Therefore, although the small reflection from the intercalated NMC phase indicated that some TEA+ insertion was occurring, the lack of well-formed layered structure and increased potential for Ni2+ to be blocking the pathways can explain the reason for the limited exfoliation of NMC as the TEA concentration increased. ICP-OES analysis was performed to further understand the exfoliation process and composition of the obtained NMC nanosheets (Table 1). The analysis showed that the pristine NMC (Sample 1) had a composition of Li0.8Ni0.3Mn0.37Co0.33O2, which indicates that the asreceived sample was lithium deficient. Nonetheless, the charged NMC particles prior to exfoliation (Sample 2) had a composition of Li0.43Ni0.3Mn0.36Co0.33O2, confirming that Li+ vacancies ( ) were formed during the electrochemical oxidation. The composition of the exfoliated NMC nanosheets (Sample 4) was determined to be Li0.41Ni0.33Mn0.21Co0.46O2. This composition indicates that while Li+ are still adsorbed on the surface of nanosheets, some of the Mn ions were lost from the nanosheets. The Mn leaching, although unexpected, is possible since


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minor amounts of Mn3+ can be present in NMC 27 and it is known that Mn3+ can disproportionate into Mn4+ and Mn2+, the latter which can dissolve into the electrolyte35-37.

Table 1. Composition of NMC samples in different steps from ICP-OES results. The composition can be written as LiaNibMncCodO2. The samples are named as in Scheme 1.

Process

Composition, LiaNibMncCodO2

Sample Description

a (Li)

b (Ni)

c (Mn) d (Co)

1

As-obtained bulk NMC

0.80

0.30

0.37

0.33

2

Oxidized NMC

0.43

0.30

0.36

0.33

4

Exfoliated NMC nanosheets

0.41

0.33

0.21

0.46

5

NMC particles after dialysis in LiNO3

0.95

0.36

0.21

0.43

0.71

0.33

0.24

0.43

After microwave/calcination 0.85

0.34

0.24

0.42

NMC nanosheets after electrophoresis

0

0.35

0.23

0.42

0.82

0.35

0.22

0.43

Nanosheet Preparation

Reassembly Route 1

6

7 Reassembly Route 2

8

NMC particles after dialysis and electrophoresis (as-prepared)

NMC particles after electrophoresis + flocculation, followed by microwave/calcination


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The exfoliated NMC nanosheet samples were further studied using TEM and AFM. Figure 3A-B shows the TEM image of the exfoliated nanosheets. From the image, it can be clearly seen that the NMC was exfoliated to form thin nanosheets typically < 50 nm in width. The hexagonal crystal lattice was verified in the electron diffraction pattern (Figure 3C). The integrity of the basal plane indicates that the exfoliation occurs along the layer stacking direction in the c-axis. The thickness of the nanosheets was determined using AFM (Figure 3D-E). The results showed that the height dimensions of the NMC nanosheets were from 0.7 to 2 nm, indicating that the NMC was exfoliated into monolayer or double layer nanosheets34, similar to the results we obtained using this method to exfoliate LiCoO2 into nanosheets16.

Figure 3. Exfoliated NMC nanosheets (A)-(B) TEM images, (C) electron diffraction pattern of a single nanosheet from (B); (D) AFM image and (E) corresponding height profile.


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Nanosheet Reassembly Route 1: Dialysis In this reassembly route, the NMC nanosheets dispersed in the TEA solutions (Sample 4) were first subjected to dialysis in DI water using a semi-permeable membrane to allow TEA+ ions to slowly diffuse out of the nanosheet dispersion. While the excess TEA+ in the solution could be removed using dialysis, galvanostatic cycling results of these samples showed that the nanosheets did not display Faradaic activity (Figure S3A). Rather, only a small double layer capacitance was observed due to the insulating TEA+ adsorbates blocking the charge transfer. Due to the negative surface charge of the NMC nanosheets24, the introduction of Li+ to the dispersion by performing dialysis in a solution of LiNO3 (rather than just DI water) will cause the charges to neutralize and decrease the repulsion between the nanosheets. As a result of the slow diffusion of Li+ into the nanosheet dispersion as the excess TEA+ diffuse out through the membrane, the nanosheets can restack and grow into particles under a slow rate. The nanosheet reassembly process is supported by the ICP-OES results in Table 1. The lithium content increased from 0.41 per formula unit (f.u.) in the as-exfoliated nanosheets (Sample 4) to 0.95 in the particles reassembled with dialysis in LiNO3 (Sample 5). The increased lithium content indicates that the reassembly process was driven by the electrostatic interactions between the negatively charged NMC nanosheets and Li+ ions in the solution. Similar to our previous observations when performing dialysis on LiCoO2 nanosheet suspensions, we found that the sample recovered after dialysis (Sample 5) still contained residual TEA+, as confirmed by the presence of the reflection associated with the TEA+-intercalated phase of NMC in the XRD pattern (Figure 1A). Moreover, this sample also did not display Faradaic activity under galvanostatic cycling (Figure S3B). This is attributed to the inability of dialysis to remove TEA+ that are strongly adsorbed to the surfaces of the NMC nanosheets. To


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remove the TEA+ ions, electrophoresis was employed. The decrease in lithium content to 0.71 per f.u. after electrophoresis (the as-prepared Sample 6 in Table 1) indicates that some of the Li+ were removed during electrophoresis along with the TEA+. To better understand the role of the dialysis and electrophoresis processes on the morphologies of the reassembled NMC particles, SEM imaging was performed. The as-obtained, pristine NMC consisted of aggregates of particles, with a secondary particle size as large as 15 microns, but primary particle sizes roughly ~200 nm in diameter (Figure S4), which is typical for NMC prepared by co-precipitation38. The reassembled NMC particles obtained after dialysis but prior to electrophoresis (Sample 5) are shown in Figure 4A-B. Some of the particles showed hexagonal morphologies, but others, particularly the larger ones, did not have well-defined shapes. In contrast, the reassembled NMC particles obtained after dialysis and electrophoresis (Sample 6), were smaller and displayed sharp, faceted edges (Figure 4C-D).


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Figure 4. SEM images of particles obtained by reassembling NMC nanosheets (A)-(B) Sample 5, NMC nanosheets after reassembly using dialysis; (C)-(D) Sample 6, NMC particles obtained by reassembling nanosheets using dialysis followed by electrophoresis; (E)-(F) Sample 8, NMC particles obtained by direct flocculation, after treatment in microwave hydrothermal reactor.

Figure 5A-B shows the TEM images of some small hexagonal NMC nanoplates observed in Sample 6. The diffraction patterns of the reassembled particles show good crystallinity, indicating that the nanosheets were highly oriented and reassembled in the same direction. The electron diffraction pattern in Figure 5C reveals a lattice spacing of 4.76 Å, which 19 ACS Paragon Plus Environment

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corresponds to the (003) spacing for NMC. The contrast in the TEM images is not homogenous, but the electron diffraction patterns show fairly well-ordered structures. It appears that the nanosheets first reassemble into smaller, nanoplatelet-like particles with hexagonal morphologies. Due to the size distribution of the exfoliated nanosheets (Figure 3A), we speculate that the larger nanosheets can serve as “scaffolds” onto which the smaller nanosheets can stack with fairly good crystallographic matching, as illustrated in the schematic in Figure 5D. As the smaller nanosheets fill in each layer, the particle can grow in size to form the hexagonal nanoplatelets shown in Figure 5A-B. Based on this mechanism, the sizes of the nanoplatelets would be limited by the dimensions of the scaffold nanosheets. The observation of these nanoplatelets in Sample 6 implies that the larger, ill-defined particles (Figure 4A-B) obtained immediately after dialysis (Sample 5) were agglomerates made up of multiple nanoplatelets that were weakly bound to each other, as illustrated in Scheme 1. The electrophoresis removed the cations (most likely TEA+) that played a role in binding these nanoplatelets together, leading to the appearance of the faceted hexagonal nanoplatelets, while the Li+ ions inserted in between NMC nanosheets within each nanoplatelet were less affected by the electrophoresis. This is supported by the ICP-OES results, which showed that the as-prepared reassembled NMC particles obtained after dialysis and electrophoresis (as-prepared Sample 6 in Table 1) still contained Li+ after electrophoresis.


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Figure 5. TEM images of nanoplatelets obtained after reassembling NMC nanosheets using dialysis followed by electrophoresis (Sample 6).

The as-prepared Sample 6 was further treated in a 1 M LiOH solution using a microwave hydrothermal reactor to replenish Li+ back into structure. Then, the particles were calcined to increase the crystallinity of particles. The XRD pattern (Figure 1A) of Sample 6 obtained after electrophoresis, microwave hydrothermal treatment, and calcination showed that the position of the (003) reflection at 18.6o remained unchanged, but the intensity of the TEA+-intercalate peak decreased, indicating the TEA+ ions could be removed without causing damage to the restacked NMC nanosheets. The ICP-OES results also confirmed that the Li content was increased in the final product to 0.85 per f.u. (Table 1).

Nanosheet Reassembly Route 2: Flocculation A different nanosheet reassembly route was also used for comparison to the method based on dialysis. Rather than using a slow process to allow the nanosheets to restack in an ordered manner, a fast reassembly process was used via flocculation. In our previous work on LiCoO2 nanosheets, we found that flocculation could enable the formation of a metastable phase 21 ACS Paragon Plus Environment

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that normally could not be obtained through direct synthesis16. The NMC nanosheet dispersion (Sample 4) was purified with electrophoresis first in order to remove the adsorbed TEA+, which could screen the negative surface charges of the nanosheets. The ICP-OES results of this sample (Sample 7 in Table 1) showed the lithium content was negligible, indicating that performing electrophoresis on the NMC nanosheets from the TEA dispersion caused all of the Li+ to be removed from the surfaces of the nanosheets. Then, the nanosheets were dispersed into a LiOH solution and allowed to flocculate. Afterwards, the nanosheets with treated using microwave hydrothermal reaction at 160 oC for 30 minutes and then calcined to increase the crystallinity of particles. The ICP-OES results showed that the lithium content of the final product increased to 0.82 per f.u. SEM imaging showed that the NMC particles reassembled by the direct flocculation method displayed ill-defined shapes lacking facets, with roughly textured surfaces and particle sizes in the micron range (Figure 4E-F). Therefore, while both nanosheet reassembly routes resulted in NMC particles with similar Li content, there is a large different in the morphologies of the final samples.

Electrochemical Properties Typically, NMC compounds will display one main redox peak associated with the Ni2+/Ni4+ redox couple between 3.6 – 3.8 V vs. Li/Li+ 22,23,27,39-41. This is usually a good indication that the solid-solution is well formed. However, in the differential capacity curve for the pristine, as-obtained NMC (Figure 6A), two redox pairs were observed, one with a sharp anodic peak at 3.93 and sharp cathodic peak at 3.89 V vs. Li/Li+ that was present in all of the cycles, and a broader one with oxidation peak at 3.84 and less-defined reduction peak at around 3.8 V. Since the XRD analysis (Figure 1B) indicated that the layered structure for the as-


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obtained NMC was not well formed, it is possible that there could be phase segregation of individual domains of LiCoO2, LiNiO2, and LiMnO2. Without more detailed structural characterization of the as-obtained NMC to confirm the presence of this phase segregation, we can only speculate that the lower voltage peak could be attributed to the reaction of Ni2+/Ni4+ in NMC, while the higher one at ca. 3.9 V could be from Co3+/Co4+ in LiCoO2 domains42. Since the Co3+/Co4+ couple in NMC normally does not become active until ca. 4.5 V vs. Li/Li+ 19, this would indicate a poorly formed solid-solution in the pristine NMC. The differential capacity curves for pure-phase LiNiO2 also exhibit several (as much as 7) redox peaks between 3 – 4.2 V vs. Li/Li+ attributed to different phase transitions43-45, some of which also match the potentials in Figure 6A, so the features could also be due to the reaction of LiNiO2 domains. It worth noting that the redox couple at the lower voltage only appears in the first cycle, while the higher voltage one remains in subsequent cycles. Since LiNiO2 is known to have rather poor cyclability46, it is more likely that the persistent redox peak at ca 3.9 V is from Co3+/Co4+.


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Figure 6. Differential capacity plots of (A) as-obtained pristine NMC (Sample 1); NMC particles reassembled from nanosheets using (B) dialysis (Sample 6) or (C) flocculation (Sample 8). (D) Discharge capacity vs. cycle results using galvanostatic cycling at 0.2 C rate (25 mA/g) between 3 - 4.2 V vs. Li/Li+ of NMC samples in half-cells.

The differential capacity plots of the NMC particles reassembled from the nanosheets also exhibited multiple redox peaks that are atypical for NMC. The particles prepared through the dialysis reassembly route (Sample 6) had fewer redox couples with smaller peak separation (i.e. overpotential) (Figure 6B) compared to the ones prepared through flocculation (Sample 7, Figure 6C). This could be due to the better stacking of the nanosheets during the dialysis reassembly process, which would minimize the formation of isolated pure phase domains. For Sample 6, the redox peak of Ni2+/Ni4+ at 3.81 V in the first cycle can still be clearly observed


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even after 50th cycles, although it shifted to 3.72 V (Figure 6B). If we assume that the redox peak at 3.92 V is from the Co3+/Co4+ redox couple, it is interesting to see that the relative activity is smaller in Sample 6 compared to the as-obtained NMC, which would indicate the presence of fewer LiCoO2 domains in the NMC prepared from the reassembled nanosheets. For Sample 7, the anodic peak at 3.82 V could be from Ni2+/Ni4+ in NMC, but numerous other peaks were also observed that could be due to the reaction of phase pure domains or LiNiO2 transitions, as mentioned previously. Furthermore, the reversibility was poor, with the redox peaks largely disappearing as the cycle number increased. Due to the fast reassembly by direct flocculation, it is likely that the nanosheets did not have good crystallographic matching, which would make the formation of well-defined, interlayer Li+ sites less likely. The morphology and rough surface of the particles also could promote more contact with the electrolyte, facilitating the reaction of the isolated pure-phase domains. When using an upper cutoff voltage of 4.2 V, specific capacities around 130 mAhg/g are typically observed for NMC under galvanostatic cycling28,37. As shown in Figure 6D, the capacities of the as-obtained NMC and both types of NMC reassembled particles were lower than expected and also showed poor capacity retention. This is likely due to the poorly formed layered structures and phase-segregation, as discussed before. It is not clear to what extent the poor performance in the reassembled particles was due to the less-than ideal properties of the asobtained NMC starting materials. However, we can see from these results that the NMC reassembled from nanosheets using dialysis and electrophoresis (Sample 6) showed the best performance, with better capacity retention compared to the pristine NMC (Sample 1) and NMC particles obtained by direct flocculation of the nanosheets (Sample 8). This shows that this


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reassembly method could be a promising approach for building up layered compounds from nanosheets.

Conclusions The oxidation-assisted exfoliation method previously demonstrated for LiCoO2 was successfully used to obtain NMC nanosheets, showing the generality of this approach for Li-ion battery cathode materials. The as-obtained NMC particles could be exfoliated to monolayer or double-layer nanosheets with hexagonal shapes. NMC particles were obtained by reassembling the nanosheet dispersions either by dialysis or flocculation. In the dialysis method, the slow diffusion of Li+ into the NMC dispersion allowed for the formation of faceted hexagonal nanoplatelets built up from the stacking of smaller nanosheets onto larger nanosheets serving as scaffolds with fairly good crystallographic matching. Multiple nanoplates were likely held together through electrostatic interactions with residual TEA+ to form larger particles that were actually agglomerates of nanoplates. Electrophoresis was effective for breaking apart these agglomerated particles to obtain the individual faceted nanoplatelets. NMC particles prepared using this method displayed better electrochemical performance than the as-obtained NMC starting materials. On the other hand, the NMC particles obtained from flocculation of the nanosheets displayed ill-defined morphologies and displayed potential-dependent redox characteristics suggesting the presence of segregated metal oxide domains. The methodologies described herein can be applied to prepare functional nanosheets of Li-ion battery layered cathodes, control the morphologies and electrochemical properties of particles reassembled from these nanosheets, and offer a twist on the bottom-up approach to materials synthesis for the study of complicated NMC compounds and development of novel materials.


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ASSOCIATED CONTENT Supporting Information. Voltage vs. time plot showing the potentiostatic charging protocol used to oxidize the as-obtained NMC; photographs of exfoliation attempt on as-obtained NMC without oxidation step; galvanostatic cycling data of NMC nanosheets after dialysis in DI water or LiNO3 solution; SEM images of as-received NMC particles.

AUTHOR INFORMATION

Corresponding Author To whom correspondence should be addressed: *Candace K. Chan, [email protected]

ACKNOWLEDGMENT We gratefully acknowledge support from the Fulton Schools of Engineering and the use of facilities within the LeRoy Eyring Center for Solid State Science at ASU. We thank M. K. Benipal and R.J. Nemanich for assistance with AFM.


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3O2 into Nanosheets Using

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