Self-Assembly of Hybrid Nanorods for Enhanced Volumetric

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Self-assembly of hybrid nanorods for enhanced volumetric performance of nanoparticles in Li-ion batteries Mohammad H Modarres, Simon Engelke, Changshin Jo, David Seveno, and Michael De Volder Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03741 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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Self-assembly of hybrid nanorods for enhanced volumetric performance of nanoparticles in Li-ion batteries Mohammad Hadi Modarres, Simon Engelke, Changshin Jo, David Seveno, Michael De Volder* M.H. Modarres, S. Engelke, C. Jo, M. De Volder Department of Engineering, University of Cambridge, 17 Charles Babbage Road, Cambridge, CB3 0FS, UK E-mail: [email protected] S. Engelke Cambridge Graphene Centre, University of Cambridge, 9 JJ Thomson Avenue, Cambridge, CB3 0FA, UK D. Seveno Department of Materials Engineering, KULeuven, Kasteelpark Arenberg 44 - bus 2450, B-3001 Heverlee, Belgium Abstract The benefits of nanosized active particles in Li-ion batteries are currently ambiguous. They are acclaimed for enhancing the cyclability of certain electrode materials and for improving rate performance. However, at the same time, nanoparticles are criticized for causing side reactions as well as for their low packing density and therefore poor volumetric battery performance. This paper demonstrates for the first time that selfassembly can be used to pack nanoparticles into dense battery electrodes with up to fourfold higher volumetric capacities. Further, despite the dense packing of the self-assembled electrodes, they retain a higher volumetric capacity than randomly dispersed nanoparticles up to rates of 5C. Finally, we did not observe any degradation in capacity after 1000 cycles and post-mortem analysis indicates that the self-assembled structures are maintained during cycling. Therefore, the proposed self-assembled electrodes profit from the advantages of nanostructured battery materials without compromising the volumetric performance. Keywords: Li-Ion Battery, Self-assembly, Alignment, Nanorods, Titanium dioxide

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Li-ion batteries (LIBs) are taking an increasingly prominent role in our everyday lives with applications ranging from pocket sized consumer electronic devices to electric vehicles.1–4 As these applications advance, so do their requirements in terms of energy and power density. One strategy, which is pursued to address these needs, is to nanostructure the active battery materials. On the one hand, nanostructuring shortens the Li-diffusion path in the active material and thus improves the rate performance and the gravimetric power density.4,5 On the other hand, nanostructuring allows for a reduction in mechanical stress during cycling, which is important for the stability of high energy density alloying and conversion battery materials such as silicon anodes.6 While nanomaterials have fostered impressive advances in the gravimetric energy and power density, this comes at the price of low volumetric capacity, which is often not reported in literature.7 The challenge here is that nanosized particles are difficult to pack densely (< 0.5 g cm-3) and therefore result in batteries that are too large for practical applications.7 Furthermore, the empty space between the nanoparticles needs to be filled by additional electrolyte, and therefore this low packing density also indirectly decreases the gravimetric performance.7 In this paper, we seek to address this challenge by self-assembling nanoparticles into densely packed electrodes. Here, self-assembly refers to a process in which nanoparticles spontaneously form highly ordered aggregates.8 Self-assembly has been used in batteries, for instance to selfassemble poly(methyl methacrylate) (PMMA) spheres into colloidal crystals to create porous inverse opal electrodes with controlled porosity,9,10 and surfactant and polymer directed selfassembly has been used to create mesoporous metal oxides11 and ordered stacks of metal oxidegraphene nanocomposites.12 Here, we do not use self-assembly to create controlled pore structures, but to increase the volumetric performance. As a model system, this paper investigates the self-assembly of hybrid reduced graphene oxide (rGO) - TiO2 nanorods. TiO2 is an abundant, low cost and environmentally benign material, which is structurally stable during cycling and has a theoretical capacity of 335 mAhg-1,13 2 ACS Paragon Plus Environment

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comparable to commercial graphite anodes (372 mAhg-1).14 Further, TiO2 (B) allows for fast charging and discharging of the electrode15 due to a pseudocapacitive mechanism for lithium insertion.16 Finally, TiO2 can be synthesised into high aspect ratio nanowires17 and nanotubes18 which is important for our self-assembly process, as discussed further on. However, TiO2 suffers from poor electrical conductivity, and therefore requires special attention to the conductive additives used in the electrode.19 This is challenging here, since adding classic conductive additive powders to our electrodes would prevent the TiO2 nanorods from selfassembling into a close packed structure. Therefore, we have adopted a process whereby rGO is wrapped around TiO2 nanorods (see further).20 This rGO coating has a dual functionality. On one hand, it improves the electrical conductivity of the electrodes, and on the other hand, charges on the rGO surface provide repulsive forces that are key for a robust self-assembly process.8 Our self-assembled electrodes achieve up to 4.5 times higher volumetric capacity than randomly organised electrodes using the same material, Further, we show that calendaring the electrodes does not allow for the same amount of electrode compaction. Finally, at rates up to 5 C, the volumetric capacity of our self-assembled nanorods remains higher than that of randomly aligned films.

Figure 1 Schematic overview of the rGO-TiO2 nanorod synthesis (1-2) and electrode fabrication process (3-5).


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Material synthesis and characterisationIn what follows, the fabrication process depicted in Fig. 1 is discussed in more detail. First, titania nanoparticles are mixed with GO in a 10 M NaOH solution and are then heated up to 150 ºC for 18 hours in a microwave hydrothermal reactor. To elucidate this reaction, we arrested it at different time steps as shown in Fig. 2. After one hour, there is evidence of small needles with a diameter of approximately 10 nm and a length of approximately 200 nm forming on the surface of the TiO2 precursor, which grow to approximately 3 µm long after 2 hours (Fig. 2a). After 9 hours, the nanorods appear more homogeneous, and after 18 hours they reach lengths of up to 15 µm and diameters of 75 nm (See SI Fig. 1). The nanorods are then washed and dried in a vacuum oven before being resuspended in water. XRD and Raman spectra at each of these reaction stages indicate that the TiO2 precursor is transformed into sodium titanate (NaTixOy), resulting in the formation of rGO-NaTixOy nanorods (Fig. 2b,c), with further details provided in Fig. S1. Further, the Raman spectra shows a steady increase in D/G ratio from 1.0 to 1.15 indicating that the microwave process is reducing the GO flakes as expected21.


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Figure 2 Fabrication of rGO- NaTixOy nanorods (a) SEM images of the material at different reaction points, from the start to 9 hours. Scale bar top row: 500 nm, bottom row: 5 µm. (b) XRD and (c) Raman spectra at different reaction times (d) TEM image of synthesised nanorod.

Simulation of rGO wrapping TEM images suggest that during the above process, the rGO flakes might wrap around the nanorods (Fig. 2d), as reported previously.20,22 We confirmed the wrapping of rGO around the nanorods by molecular dynamics simulations shown in Fig. 3a. These suggest that a graphene layer would first fold around the nanorod due to Van der Waals interactions, with a first layer wrapped in 43 ps. The graphene layer then scrolls up further and this new arrangement is stabilized by π-π interactions between the graphene layers. In this simulation, the 2nd, 3rd, and finally 4th layers are formed after 83, 116, and 157 ps respectively. The graphene/graphene interaction energy is initially constant until the 1st layer is formed and then shows a progressive decrease while the 2nd, 3rd, and finally 4th layers form as shown in Fig. 3a.


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Figure 3 (a) Nanorod/graphene and graphene/graphene interaction energies. The dotted lines indicate when the 1st, 2nd, 3rd, and 4th layers are formed, and the molecular dynamics simulation images show the progress of the wrapping (b) Nanorods of different length were simulated on a graphene sheet of length 10 nm, all nanorods are of radius 1 nm. (c) Summary of wrapping results, showing the dependence of wrapping on ratio between the rod and graphene length.

Further, the simulations show that the length of the nanorods needs to be more than half the width of the graphene sheet to overcome the bending energy needed to roll the graphene sheet (Fig. 3b,c), which is in agreement with theoretical calculations (see supplementary information). This might explain why in SEM images we only observe the rGO sheets disappearing after two hours of reaction when sufficiently long NaTixOy nanorods are formed.

Nanorod self-assembly process As sketched in Fig. 1, the nanorod suspension is then drop casted onto a substrate, and selfassembled into densely packed aligned films as shown in Fig. 4a. This self-assembly process can be explained by Onsager’s theory, which describes the phase diagram of rod-shaped colloid 6 ACS Paragon Plus Environment

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particles of diameter d and length L as a function of the particle’s concentration.23,24 At low concentrations our nanorod dispersions are isotropic, and during drying, the concentration of the nanorods increases and transitions to a biphasic system (app. volume fraction of 3.29*d/L here 0.025 assuming L = 10 µm and d = 75 nm) where small domains of particles align. During further drying, these domains are expected to grow and form nematic liquid crystal (LC) phases (app. volume fraction of 4.19*d/L, here 0.032). The formation of LC phases in high aspect ratio rod-like suspensions was confirmed experimentally for carbon nanotubes at concentration of about 1 wt%,25 and evaporation of nanowire suspensions for orienting nanowires has been reported for a variety of nanostructures and nanorods.26–29 We believe that our rGO-TiO2 nanorods show a similar behaviour. For this self-assembly processes to work, repulsive forces between the nanoparticles are important,8 otherwise, the nanorods would clump together due to van der Waals interactions before achieving aligned LC phases. Here we believe this is achieved by the negative surface charges on the rGO coating. We verified this assumption by lowering the pH of the nanorod suspension, which resulted in a drop of the zeta potential (Fig. S2), and resulted in non-aligned nanorod films (Fig. S3). In addition to this bulk LC phase formation, we believe that some nanorods are trapped at the liquid-air interface and are aligned following a process reported previously.26 An important obervation here is that because of the nanorod’s ultra-high aspect ratios (length up to 15 µm and diameter of ~75 nm) and the repulsive forces from the rGO coating, large area uniform films are obtained reliably on arbitrary substrates as illustrated in Fig. 4a,b.


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Figure 4 Characterisation of rGO-TiO2 films (a,b) Large area uniform coating of nanorods in freestanding films (c) XRD and (d) Raman comparing the spectra obtained from the powder and the freestanding films

Battery electrode fabrication To test the above self-assembled films in batteries, the rGO- NaTixOy nanorods were modified to rGO-TiO2 using ion-exchange and annealing (see Fig. 1). For this process, the nanorods are self-assembled on a copper film, dried and immersed in a 0.2 M HCl solution for the ion exchange. This process transforms the NaTixOy to hydrogen titanate and detaches the film from the copper substrate. SEM, EDX, XRD and Raman of these rGO-hydrogen titanate films is shown in Fig. S4. Finally, the films are annealed at 450 ºC for 4 hours in helium to convert the hydrogen titanate to TiO2. Optimisation of the ion exchange process and annealing conditions are described in Fig. S5,6. The XRD spectra in Fig. 4c show there is a close match between our films and TiO2, with the packed films being less crystalline than the powder form.20 SEM characterisation of the rGO-TiO2 powder is shown in Fig. S7. The Raman spectra of rGO-TiO2 (Fig. 4d) shows characteristic vibration bands associated with TiO2(B) and anatase TiO2 and 8 ACS Paragon Plus Environment

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confirm that D/G ratio of the rGO has not been affected by the ion exchange and annealing step.30,31 Finally, to improve the electrical conductivity of the freestanding electrodes, some were coated with a carbon nanotube - polyvinylidene fluoride (CNT-PVDF) suspension in N-Methyl-2pyrrolidone (NMP) as illustrated in Fig. S8,9.32 In addition to improving the electrical conductivity, this coating makes the electrodes flexible, at the expense of increasing their thickness by approximately ~10 µm.


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Figure 5 rGO-TiO2 freestanding films (a) Schematic, SEM top view and SEM cross section of aligned selfassembled freestanding film compared to (b) a non-aligned random freestanding film. (c) Volume against mass of freestanding films before and after annealing for aligned and non-aligned films

Next, the influence of the self-assembly process on the electrode volume is analysed by measuring the thickness of electrodes with the same areal loading of rGO-TiO2 particles. Crosssection SEM images shows that aligned films are approximately only half as thick as non10 ACS Paragon Plus Environment

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aligned films (Fig. 5a). This reduction in thickness was also confirmed by micrometer measurements and X-ray tomography (see Fig. S10). The difference in volume measurements between aligned and non-aligned electrodes before and after annealing is shown in Fig. 5b. The difference in electrode density is even larger when compared to standard battery electrode formulations using a slurry of the active material (rGO-TiO2 powder) with binder (15 wt%) and conductive additive (15 wt%). In this case, the self-assembled films offer a 5-fold decrease in volume. Additional reference electrodes were fabricated, and as detailed in Table S2, these vary in active material mass and thickness. Finally, thick electrodes of non-aligned (100 µm thick, 4.473 mg) and self-assembled nanorods (50.5 µm thick, 4.823 mg) µm have been tested to investigate the influence of the mass loading on the battery performance.

Electrochemical characterisation Cyclic voltammetry (CV) measurements (Fig. 6a) show that there is no significant difference in electrochemical behavior between the aligned and non-aligned electrodes or different types of casted electrodes. Small differences in the CV curves reflect the different phases observed in XRD measurements discussed above. The electrochemistry of rGO-TiO2 has been reported previously,20 in summary, the peaks at 1.5/1.6 V and 1.6/1.7 V are due to a pseudo capacity lithium storage mechanism in TiO2(B) and the peaks at 1.7/2.0 V are due to lithium insertion in the anatase TiO2.


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Figure 6 (a) Cyclic voltammogram of the electrodes at 0.05 C. (b) Volumetric capacity of electrodes from 0.1 C to 10 C. The volume of the CNT or Cu backing is included in the volumetric capacity. (c) An aligned freestanding film with backing folded 100 times and cycled over 1000 times at 1C Inset: Post-mortem SEM of an aligned electrode after cycling.

The initial gravimetric capacity (Fig. S11, S12) of the casted electrodes is ~300 mAhg-1 which is close to the theoretical capacity for TiO2(B), however, for the freestanding films it is only ~220 mAhg-1. This may be due to an incomplete ion exchange resulting in some low capacity NaTixOy residue (see XRD spectra in Fig. 4b). Fig. 6b shows the volumetric capacity of the electrodes at rates increasing gradualy from 0.1 C to 10 C (the gravimetric capacity is shown in 12 ACS Paragon Plus Environment

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Figure S12 and the areal capacity in figure S13). The self-assembled films have a volumetric capacity which is 60% higher than films made from the same nanorods without self-assembly at 0.1 C. This difference increases to a factor of 4.5 when compared to classic electrodes which are cast with CB and PVDF on copper films. Up to relatively high rates of 5 C, the selfassembled nanorods outperform the non-aligned films in volumetric capacity. Only then, the better Li diffusion through highly porous non-aligned films become beneficial. Freestanding films with a conductive CNT backing perform better at higher rates relative to their counterparts without backing due to the lower ohmic losses. The difference in conductivity between these electrodes is analysed by the EIS measurements in Fig. S14. We also observed that the freestanding films with CNT-PVDF backing can be folded numerous times without a clear adverse effect on their performance. For instance, a film that was folded 100 times to a radius of 300 µm (Fig. 6c and S15) still showed stable cycling performance with a drop of capacity of only 6% over 1000 cycles (Fig. 6c). We also analysed the volumetric and areal capacity of thick freestanding electrodes (100 µm thick non-aligned and 50.5 µm thick aligned self-assembled electrodes). These behave similar to the electrodes discussed above, but show a steep drop in performance above 1C for both self-assembled and random electrodes (see figure S16). Further, we have compacted the slurry casted reference electrodes by calendaring. Crosssection SEM of electrodes before and after calendaring show a 50% decrease in thickness (see image S17), but even then, the highest measured volumetric capacity was 90 mAh/cm3 (or 60 mAh/cm3 including the Cu current collector) compared to 120 mAh/cm3 for self assembled electrodes (see Figure S18). Finally, port-mortem SEM analysis was performed on a freestanding electrode that was cycled over 90 cycles (cycle rates 0.1 C – 1 C), showing that nanorods retain their self-assembled structure during cycling (see inset Fig. 6c).

Conclusions


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Nanostructuring of battery materials is appealing for improving the power density of electrodes and for the adoption of new Li-Ion battery materials. However, an important limitation of nanoparticles for battery applications is their low packing density, which results in poor volumetric performance. This paper presents a new approach for densely packing nanoparticles. This is achieved by a large area self-assembly process using ultra high aspect ratio rGO-TiO2 nanorods. These self-assembled electrodes show up to 4.5 times higher volumetric capacity compared to traditional casted electrodes on copper and 60% relative to freestanding nonaligned films when cycled at 0.1 C. The self-assembled film showed the best volumetric performance up to rates as high as 5 C, only then the more porous non-aligned films start showing a better volumetric capacity. Finally, the self-assembled electrodes also outperform the volumetric capacity of calendared electrodes using the same material.

Experimental Section Synthesis of GO: GO was synthesized using the Tour method33. In a typical synthesis a solution of 360 ml of concentrated sulphuric acid and 40 ml phosphoric acid was added to 6g of graphite and 18 of potassium permanganate. The mixture was heated to 50 ºC for 12 hours. After the reaction had cooled down to room temperature, the solution was added to 400 ml of DI-water ice and stirred. 4 ml of 30 wt% hydrogen peroxide was added turning the solution bright yellow. The solution was subsequently passed through a 250 µm sieve. The filtrate is centrifuged 9 times in DI-water at 10,000 rpm for 1 hour until the pH of the supernatant decanted away is neutral. A final centrifugation step is performed in ethanol. The filtrate is then dried on a hot plate at 50 ºC leaving the brown powder as product.

Synthesis of rGO- NaTixOy nanorods:


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A large batch of rGO- NaTixOy nanorods was produced in a hydrothermal synthesis using the Anton Paar Masterwave BTR microwave reactor. In the synthesis, modified from Li et al

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350mg of graphene oxide (GO) was added to 100 ml of DI-water and sonicated for 1 hour. 5g of anatase TiO2 nanoparticle powder was added to the GO dispersion and stirred for 4 hours. The suspension was added to a 10 M sodium hydroxide (NaOH) solution which was prepared by dissolving 60 g of NaOH pellets to 150ml of DI-water. The reaction mixture was heated at 150 ºC for 18 hours (two runs of 9 hour each, since the maximum run time for the reactor was 9 hours). The reaction product was vacuum filtered and washed with DI-water numerous times until the pH of the filtrate was neutral. The resulting material was dried on a hot plate at 60 ºC.

Preparation of freestanding rGO-TiO2 powder: rGO- NaTixOy powder was dispersed in 0.1M HCl solution. The mixture was stirred for 48 hours. The mixture was vacuum filtered and washed with DI-water until the pH of the filtrate was neutral. The resulting powder was dried overnight at 60 ºC. To form rGO-TiO2 scrolls the powder was annealed in helium at 400 ºC for 4 hours.

Preparation of freestanding rGO-TiO2 films: To form aligned films, 300 μl of a suspension of rGO- NaTixOy of concentration 5 mg/ml was dropcasted on circular discs of copper foil and left to dry on a hot plate at 80 ºC. To form non-aligned films concentrated HCl was added to the suspension to adjust the overall molarity to 0.1M HCl. Subsequently the same procedure was used to form non-aligned films on copper foil. After the films were dry, they were lowered into a 0.2M HCl solution. After brief shaking of the solution, the aligned films detached from the copper due to the underetching of the copper substrate, resulting in freestanding films. However, the non-aligned films became freestanding after remaining in the acid solution for 24 hours or more. 15 ACS Paragon Plus Environment

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The copper discs were removed from the acid solution and discarded. The freestanding films were left in the acid solution for 48 hours. Subsequently they were transferred to a large beaker of DI-water and gently shaken to remove any acid on the films. They were then gently picked up and heated on a hot plate at 60 ºC to dry the films. The films were annealed in helium at 450 ºC for 4 hours to form freestanding films of rGO-TiO2. The thickness of films was measured by using a micrometer and x-ray tomography.

Transfer of film as backing: A suspension of CNT (1 wt%), PVDF (1.5 wt%) was formed in NMP solution, and sonicated for 5 mins, followed by 1 hour of shaking. To deposit the backing, the freestanding rGO-TiO2 film was placed on a glass slide, with the bottom side (which had been in contact with the copper in the initial step) now facing up. 50 µl of the CNT-PVDF suspension was dropcasted on the freestanding film, and spread over the surface. The film was left to dry at 60 ºC for 12 hours.

Battery fabrication: The electrochemical properties of the compounds were evaluated in 2032 coin cells containing metallic lithium foils as cathodes (half-cells). For the reference electrodes, a slurry was prepared containing 70 wt % rGO-TiO2, 15 wt% activated carbon and 15 wt% polyvinylidene difluoride (PVDF) binder, or 85 wt % rGO-TiO2, 15 wt % PVDF, in N-methyl-2-pyrrolidinone (NMP). The slurry was cast onto etched copper. The electrodes were dried first in air and then under vacuum at 60 °C for 12 h before being cut to size and weighed. The freestanding electrodes were dried in a vacuum oven for 12h before assembly as well. The coin cells were assembled in a nitrogen-filled glovebox with 1.0 M LiPF6 EC/EMC=50/50 (v/v) electrolyte and Celgard separators.


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Galvanostatic cycling experiments at room temperature were carried out with Biologic VMP3 and for the reference electrodes with Lanhe potentiostats/galvanostats. Cyclic voltammetry measurements were taken with the Biologic VMP3 with a current of 0.02mA. The cells were cycled in between 1-3V and C-Rates from 0.05 to 10C. Supporting Information Supporting Information available: Characterisation of rGO-NaTixOy, Molecular dynamics simulation parameters and results, Zeta potential measurements on rGO-NaTixOy, SEM of non-aligned film, Characterisation of rGOhydrogen titanate, ion exchange and annealing of freestanding films, SEM of drop-casted rGOTiO2, synthesis routes for electrodes, SEM comparison of freestanding films, X-ray tomography of aligned and non-aligned films, details on fabricated anode electrodes, electrochemical performance of electrodes (charge discharge, gravimetric capacity, EIS), SEM of folded electrode and calendared electrodes.

Acknowledgements The authors acknowledge Anthony Dennis for X-ray tomography measurements, and Dr Caterina Ducati and Felix Utama Kosasih for TEM measurements. M.H.M acknowledges the support from EPSRC Cambridge NanoDTC, EP/G037221/1. S.E acknowledges funding from EPSRC grant EP/L016087/1. MDV acknowledges the ERC starting grant HIENA.

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