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GeO thin film deposition on graphene oxide by the hydrogen peroxide route: Evaluation for lithium ion battery anode Alexander G. Medvedev, Alexey A. Mikhaylov, Dmitry A. Grishanov, Denis Yau Wai Yu, Jenny Gun, Sergey Sladkevich, Ovadia Lev, and Petr V. Prikhodchenko ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16400 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 25, 2017

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ACS Applied Materials & Interfaces

GeO2 Thin Film Deposition on Graphene Oxide by the Hydrogen Peroxide Route: Evaluation for Lithium Ion Battery Anode

Alexander G. Medvedev,†,§ Alexey A. Mikhaylov, †,§ Dmitry A. Grishanov,§ Denis Y.W. Yu,‡,∥ Jenny Gun,† Sergey Sladkevich,† Ovadia Lev,*,† and Petr V. Prikhodchenko*,§ †

The Casali Center and the Institute of Chemistry and The Harvey M. Krueger Family Center for

Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Jerusalem 91904, Israel §

Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,

Leninskii prosp. 31, Moscow 119991, Russia ‡

TUM CREATE Centre for Electromobility, 1 CREATE Way, 10/F Create Tower, Singapore

138602, Singapore ∥School

of Energy and Environment, City University of Hong Kong, Tat Chee Avenue,

Kowloon, Hong Kong SAR 999077

ACS Paragon Plus Environment

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KEYWORDS: peroxogermanate, germanium oxide, germanium, hydrogen peroxide, reduced graphene oxide, lithium ion battery

ABSTRACT: A peroxogermanate thin film was deposited in high yield at room temperature on graphene oxide (GO) from peroxogermanate sols. The deposition of the peroxo-precursor onto graphene oxide and the transformations to amorphous GeO2, crystalline tetragonal GeO2 and then to cubic elemental germanium were followed by electron microscopy, XRD and XPS. All of these transformations are influenced by the graphene oxide support. The initial deposition is explained in view of the sol composition and the presence of graphene oxide, and the different thermal transformations are explained by reactions with the graphene support acting as a reducing agent. As a test case, the evaluation of the different materials as lithium ion battery anodes was carried out revealing that the best performance is obtained by amorphous germanium oxide@GO with > 1000 mAh g-1 at 250 mA g-1 (between 0-2.5 V vs. Li/Li+ cathode), despite the fact that the material contained only 51% wt germanium. This is the first demonstration of the peroxide route to produce peroxogermanate thin films and thereby supported germanium and germanium oxide coatings. The advantages of the process over alternative methodologies are discussed.

INTRODUCTION Germanium oxide exhibits a wide band gap (>5 eV), high visible and infra-red transparency and high refractive index (1.6-1.7). Therefore, GeO2 is widely used either alone or as mixed oxide in optical waveguides and fibers, and as a transparent hole transport layer in solar cells.1,2 More recently, germanium oxide composites have also attracted attention as high capacity lithium ion battery (LIB) anodes.3–21


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Germanium attracts much less scientific attention compared to its neighbors in the 14th column of the periodic table, silicon and tin.22 This is primarily due to its high cost and scarcity, but it also reflects its processing complexity. The alkoxide sol gel precursors of Si (e.g. Si(OC2H5)4) are widely used and provide high versatility in thin film formation for silica and silicates, but the tin and germanium analogues (e.g. Sn(OC2H5)4, Ge(OC2H5)4 and recently reported23 Ge(OtBu)4) hydrolyze more rapidly due to the facile coordination expansion of the larger atoms, and are thus less useful to control film formation.24 Coating of particulates is perhaps the most challenging form of film deposition, since polycondensation acceleration cannot be performed by simple solvent evaporation and concentration increase as in spin-, dip-, spray-, and spread-coating processes, though aerosol assisted chemical vapor deposition is an interesting recent alternative.25 The Pechini method for film formation is widely used for the coating of tin moieties,26–28 and it allows thickness control under acidic conditions, but we have not found any report on germanium oxide film formation by the Pechini method. The few reported protocols of the application of the Pechini method for germanium yield nanocrystalline germania particles,29 unless a supporting network of another oxide (e.g. bismuth) is added.30,31 Under basic conditions, germanium is in the form of hydroxogermanate complexes of low nuclearity, and addition of an antisolvent would give, in the best case, germanate salts rather than germania.32 Precipitation from ammonium hydroxogermanate solution yields relatively large (50-1000 nm) GeO2 crystals and does not lead to oligomerization and uniform coating.33–36 At basic pH, the conventional capping agents (e.g. carboxylates, acetyl acetonate) are less successful since they have lower affinity to germanium (and tin) compared to the hydroxo ligand, and therefore reduction of the pH leads to rapid polycondensation and particle


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precipitation rather than film formation (since condensation is enhanced by the presence of both protonated and deprotonated germanols). At moderate pH, the solubility of germanium oxide in water is in the few g/L range,32 which is not sufficient for film formation from aqueous solution. At low pH, sol gel chemistry, starting with germanium alkoxide, partial hydrolysis and stabilization can be used for film coating as well as particle formation.37–39 However, even by this rich chemistry it is not possible to achieve selective and controlled precipitation only on the particulate substrates. Addition of an antisolvent does not efficiently reject the hydrophobic organic germanate complexes from the solution. Addition of 20-28 fold excess of isopropanol antisolvent to an aqueous solution of germanium chloride yielded small, 2 nm nanoparticles of germanium oxide5 but with very low yield. We have recently reported peroxo bridged germanates in hydrogen peroxide aqueous solutions,40 and demonstrated that the solution is comprised of pentacoordinated peroxo and oxo bridged hexanuclear germanate. The peroxocoordination provides a delicate route for polycondensation without immediate crystallization/precipitation. Alcohol destabilization of the peroxogermanate sol in the presence of graphene oxide deposits the peroxogermanate on the graphene oxide platelets. The hydroperoxo ligands promote the interaction of the sol to oxygenated surfaces (including graphene oxide) due to the strong hydrogen bonding of terminal hydroperoxo groups.41 Hydrogen peroxide is a cheap commodity, and unlike most other capping agents, the stabilizing peroxoligands can be easily decomposed by mild heat treatment of the solids, which does not require organic solvent, high pressure or expensive equipment. The peroxide deposition route was successfully applied for tin and antimony oxide and sulfide thin film deposition,42–47 and therefore we anticipated that an aqueous peroxogermanate precursor40 could be used for


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germanium oxide thin film formation on particulates and nanoparticulates. Graphene oxide (GO) supported germanium oxide lithium ion battery anodes demonstrate feasibility and practical processing advantages. EXPERIMENTAL SECTION Preparation of ammonium germanate precursor. 0.5 mL of GeCl4 (4.38 mmol) was dissolved in 5 mL of deionized water (DIW) and neutralized with ammonia to pH 7. The precipitate was washed several times with DIW in order to eliminate chloride residuals. The germanium hydroxide precipitate was dissolved in 4.5 mL DIW under sonication at 90°C according to a described procedure.35 The dissolution process took approximately 1 h. 1.5 mL of aqueous ammonia were added to the germanium hydroxide solution under stirring. The resulting solution was used as ammonium germanate precursor (0.36 M) in the coating protocols. Preparation of graphene oxide. Graphene oxide (GO) was synthesized from exfoliated carbon by a modified Hummers' method.48,49 GO supported peroxogermanate (denoted here as GO-Ge-RT) and peroxogermanate powder. Typically, 8.5 g of aqueous GO dispersion (0.44% wt) were mixed with 10 mL of hydrogen peroxide (30% wt) with sonication. Then 5.0 mL of the peroxogermanate precursor solution (0.36 M) was added. Precipitation of peroxogermanate onto the GO surface was accomplished by addition of 50 mL of ethanol under vigorous stirring. The coated GO was washed three times with ethanol and once with diethyl ether and then dried under vacuum at room temperature. The coated material was stored in a refrigerator. Peroxogermanate powder was prepared by the same procedure without adding GO. Preparation of the active LIB anode materials (denoted as GO-Ge-80, GO-Ge-200, GO-Ge650, and GO-Ge-900). Heat treatment of the GO-Ge-RT powder was carried out in a tube


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furnace at 10-5 Pa pressure. Each sample was heated to the indicated temperature (80, 200, 650 ad 900 °C) at a rate of 0.8 °C min-1, and the temperature was then maintained for 3 h. Gradual heating was employed to prevent loss of the products by carryover. Details of the instrumental characterization by HR-TEM, HR-SEM, STEM, X-ray Photoelectron Spectroscopy, Raman Spectroscopy, and X-ray powder diffraction are described in the Supporting Information (SI). The elemental composition of the different active materials is depicted in Table S1 based on EDX measurements. Electrochemical evaluation. Each of the different lithium intercalation materials was mixed with acetylene black and carboxymethylcellulose sodium salt (CMC, Sigma-Aldrich) in a weight ratio of 6:2:2 and with deionized water as the medium to form a slurry. The slurry was then coated on roughened copper foil as a current collector using a doctor blade. The electrode was then dried at 80 °C and pressed in a roll press. The electrodes were cut into 16 mm diameter discs and further dried at 110 °C for 4 h in vacuum before being introduced into an argon-filled glove box. The electrodes were assembled with Li metal as counter electrodes in a 2016 coin cell. 1 M lithium hexafluorophosphate (LiPF6) in fluorinated ethylene carbonate (FEC) – diethyl carbonate (DEC) 1:1 solution was used as electrolyte. The cells were then tested with a battery tester between 0 to 2.5 V vs. Li/Li+. The typical charge-discharge rate was 100 mA g-1 during the first five cycles and 250 mA g-1 for subsequent cycles. The current rate used was typically 2.5 times higher than that reported in the literature. Cyclic voltammetry (CV) profiles were taken at a scan rate of 0.1 mV s-1 between 0.005 V and 3 V vs. Li/Li+. Rate performance was measured by testing the cell at current rates of 100, 250, 500, 1000, 1500 and 2000 mA g-1.


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Electrochemical Impedance Spectroscopy was conducted at a frequency range from 100 000 to 0.01 Hz with a perturbation amplitude of ±5 mV using a BioLogic VMP3 potentiostat. RESULTS Studies of the peroxogermanate precursors.

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O NMR of the peroxogermanate solution and

X-ray and Raman studies of isolated crystals have been reported recently showing the predominance of hexanuclear germanate bridged by oxo and peroxo groups in the solution.40 Coating of the graphene oxide was achieved by mixing GO dispersion in aqueous hydrogen peroxide solution with peroxogermanate precursor solution. Precipitation of amorphous peroxogermanate onto the GO surface was accomplished by addition of ethanol antisolvent. The same procedure under the same conditions was used to produce peroxogermanate powder without adding GO, but, interestingly, the peroxogermanate precipitate was crystalline, whereas GO supported material was amorphous, without any crystallinity. It was also important to add the graphene oxide to the solution prior to the addition of the hydrogen peroxide, because otherwise rapid precipitation of crystalline ammonium peroxogermanate takes place. Figure S1 in the SI shows that even after two times dilution of the peroxogermanate precursor the addition of 20 µL of ethanol to 1.5 mL of solution yielded a sol of 30 nm particles that underwent precipitation of crystalline peroxogermanate with time. The X-ray diffractogram of the crystalline ammonium peroxogermanate that was obtained under identical conditions, but without GO addition, fits the diffractogram of previously reported potassium ammonium peroxogermanate. The comparison is presented in Figure S2 in the SI. The fit suggests isomorphism of the ammonium peroxogermanate presented here with the previously reported mixed ammonium potassium analogue.40


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This is the first of several transformations that were directed by the graphene oxide to yield entirely different germanium oxide end-products than those resulting from a process that was conducted without GO. The graphene oxide provides a large number of nucleation centers due to the plethora of oxygen containing functionalities,50 and these induce formation of an amorphous coating rather than slow crystal growth in the solution. Since the heterogeneous nucleation process is faster than the homogeneous nucleation, the presence of the graphene oxide prevents the formation of crystalline particles in the solution. Characterization and transformations of coated GO. STEM and TEM studies: Electron microscope imaging of the coated graphenes, GO-Ge-RT and GO-Ge-200 are depicted in Figure 1. The heat treatment distorted the flat graphene sheets, but other than that there is not much change induced by the thermal treatment and it is difficult to notice the coatings on the GO by SEM imaging. Close scrutiny of the high magnification TEM images in Figure 1 (frames b and d) reveals that the graphene oxide is uniformly coated. The bright lines in the STEM figures mark the edge and folding of the graphene, where the electron beam is scattered by the higher germanium content. The amorphous nature of the peroxogermate coating can be seen in the selected area electron diffraction, SAED insert.


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Figure 1. STEM and TEM images of GO-Ge-RT (a and b, respectively) and GO-Ge-200 (c and d, respectively). SAED is depicted in the insert of frame b. Heat treatment at elevated temperature (650 and 900°C) further distorted the graphene oxide support and also induced some morphological changes that are depicted in Figure 2. The upper row in Figure 2 depicts the STEM (a) and TEM (b,c) images of GO-Ge-650. For the most part the graphene oxide is still covered by uniform coating (as shown in Figure 2a). The amorphous nature of the coating in these regions can be observed in the corresponding SAED that is depicted as insert in Figure 2b, which shows only the diffraction rings of the graphene oxide with d-spacings of about 2.1 Å and 1.2 Å. The (002) peak of graphite with d-spacing of approximately 3.5 Å is absent altogether in the TEM images and there is no pattern corresponding to a diffraction from the germanium oxide in this frame. However, closer scrutiny of the GO-Ge-650 sample reveals some dark spots (Figure 2c), which represent fairly large (50-150 nm) crystals


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that are not necessarily connected to the GO support. SAED of these particles is depicted as insert in Figure 2c showing d-spacing of 1.4, 2.2 and 4.3 Å corresponding to the hexagonal GeO2 phase. More detailed figures, the EDX spectrum and local atom composition at the different sections of GO-Ge-650 detailed in Figure S3 and Table S1, further demonstrate the conclusions that were drawn here based on the SAED studies. After heat treatment at elevated temperature (900°C), the large unbound crystalline material does not appear any more (Figure 2d,e), and instead only supported particles of up to 100 nm located exclusively on the rGO itself were obtained. The SAED (insert in Figure 2f) corresponds to the cubic phase of elemental germanium with d-spacing of about 3.3, 2.0 and 1.7 Å, respectively, and graphene oxide (dspacing of about 2.1 Å and 1.2 Å).


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Figure 2. STEM (a, d) and TEM (b,c,e,f) images of GO-Ge-650 (upper row) and GO-Ge-900 (lower row), inserts - selected area diffractions of the corresponding areas. A large GeO2 crystal is denoted by a circle. Permanganometry and Raman Studies: Permanganometric titration of GO-Ge-RT, GO-Ge-80, and GO-Ge-200 revealed that peroxide content of GO-Ge-RT is 6.8% wt. No peroxide remained after heat treatment at 80°C. We were not able to elucidate the characteristic O-O vibrations of the GO coated peroxogermanate due to light sensitivity of the peroxide and the very thin coating. The Raman spectroscopy of GO-Ge-RT, GO-Ge-200 and GO-Ge-650 reveals that the ratio between the intensities of D and G bands of the graphene were somewhat increased from 0.89 at room temperature to 0.92 and 0.99 at 200 and 650°C, respectively, and that the broad band corresponding to the 2D region appears at 2500-3000 cm-1 at T>200°C (Figure S4). The increased I(D)/I(G) ratio of rGO after reduction has been amply reported in the literature.51,52


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Therefore, this trend agrees well with the anticipated reduction of the graphene oxide support at high temperature. X-ray studies: The x-ray diffraction patterns in Figure 3 agree well with the electron microscopy observations. At up to 80°C, the dominant patterns in the x-ray diffractograms represent the broad reflection patterns of graphene oxide with a marked peak at 2θ = 10-12°. After heat treatment at 200°C the GO is already reduced, which culminates in the disappearance of the 10-12 degrees peak and its possible shift to 2θ = 23-27°. This peak - shift was noticed and explained in a similar way by others.53,54 However, even at very low temperature (RT and 80°C) we could observe two additional broad peaks with maxima at 2θ = 26 and 37° which most likely can be attributed to some contribution of the amorphous GeO2. The diffractogram of GO-Ge-650 reflects the structure of hexagonal GeO2. The crystallite size calculated by the Scherrer equation is 43.6 nm which agrees well with the TEM observations and reflects the aforementioned large GeO2 crystallites that are not necessarily supported on the graphene oxide. After treatment at 900°C the only observed XRD pattern belongs to cubic germanium.


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Figure 3. X-ray diffractograms of GO supported peroxogermanate heat treated in vacuum at different temperatures. The reduction trends that could be concluded based on the XRD results are confirmed also by the XPS (Figure 4) and Raman studies (Figure S4). The XPS spectrum of 1s C level reveals that most of the decrease of the high binding energy peak area attributed to the oxidized carbon takes place at 200°C, though clearly the reduction process continues at elevated temperature and may reflect faster CO2 evolution from oxygenated locations on the graphene oxide (Table S2). The 3d-Ge XPS studies show that at up to 650°C the only oxidation state of germanium is +4 and only after heat treatment at 900°C the elemental Ge(0) state appears. We postulate that the fact that the XPS studies of GO-Ge-900 still reveal the Ge(IV) state reflects the sensitivity of the XPS studies to the surface of the examined material, which is the last to be reduced by the underlying


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GO support. A similar observation was reported for GO supported antimony and antimony oxide nanoparticles,47 where XRD shows only elemental antimony structure, whereas XPS studies reveal the presence of both antimony and antimony oxide.

Figure 4. XPS signal of germanium (3d) and carbon (1s) in GO-Ge-RT (a), GO-Ge-200 (b), GOGe-650 (c) and GO-Ge-900 (d). Peak parameters are tabulated in Table S2 in the ESI. Electrochemical evaluations were carried out with the GO-Ge-80, GO-Ge-200 and GO-Ge650 and GO-Ge-900 samples. The cyclic voltammogram of GO-Ge-650 is shown in Figure 5a. The electrode shows stable charge-discharge reactions with oxidation peaks at about 0.48 V and 1.11 V vs. Li/Li+. The lower potential is attributed to alloying of Ge with Li and the higher potential is from reaction of Li2O.55 The discharge peak at 0.48V is somewhat higher than the corresponding lithiation peaks of tin (around 0.2 V),43,56 but lower than the corresponding peak for antimony lithiation (0.6-0.8 V).57,58 The charging peaks take place at 0.55 and 1.1 V. Figure


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5b shows the first and second charge-discharge cycle of GO-Ge-650 at 100 mA g-1. Initial discharge and charge capacity is about 2067 and 1424 mAh g-1, respectively, giving a first cycle efficiency of about 69%, which is attributed to the sluggish reduction of Li2O. Charging arrest are observed at 0.5 and 1.1 V in agreement with the cyclic voltammetry of this electrode.

Figure 5. Cyclic voltammetric and galvanostatic studies of GO-Ge-650 cells: a) Cyclic voltammograms at a scan rate of 0.1 mV s-1; b) 1st and 2nd charge-discharge curves. Charge capacity is found to be a function of treatment temperature: Higher charge capacity is observed for material treated at higher temperature up to 650°C (Figure 6). However, further increase of the temperature to 900°C deteriorated the electrode performance. This is attributed to better electrical conductivity of the graphene based composite at higher temperature, though the large crystalline germanium grains formed by the 900°C treatment deteriorated the electrode performance. The cycle performance of the material is good, with 87% capacity retention after 50 cycles at 250 mA g-1 for the GO-Ge-650 electrode (Figure 6a). The rate performance of the active material is very good, with a capacity of about 740 mAh g-1 at a current rate of 2000 mA g-1 (Figure 6b). This refers to a charge-discharge time of 20 min.


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The material can provide high capacity at high current rate due to the intimate contact between the graphene and the germanium moieties.

Figure 6. Comparison of electrode performance: (a) Charge capacities of GO-Ge-80, GO-Ge200 GO-Ge-650 and GO-Ge-900 upon repeated cycling. The cycles were conducted at a rate of 250 mA g-1 between 0 and 2.5 V vs. Li/Li+ (except for GO-Ge-900 which was conducted only at 100 mA/g). (b) Charge capacities of GO-Ge-80 and GO-Ge-650 at different rates. (c) The charge-discharge curves of GO-Ge-650 at different current rates. (d) Nyquist plots of GO-Ge-80, GO-Ge-650 and GO-Ge-900. Nyquist plots of the EIS studies of GO-Ge-80, GO-Ge-650 and GO-Ge-900 are plotted in Figure 6d. The Nyquist plots of all three electrodes manifested skewed semicircles starting at the electrolyte (and wiring) resistance (normalized to 20 Ω), but before the semicircle is complete, it


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is interrupted by almost linear curves. The interpretation of the EIS spectrum follows the Thomas Bruce and Goodenough59 and Aurbach60 surface layer model. The semicircle is attributed to lithium ion migration through the surface electrolyte interface and to charge transfer resistance, which in this case take place at the similar intermediate frequencies. At the low frequencies the straight line (with slopes larger than 45 degrees) manifest a combination of electrolyte diffusion (the Warburg impedance) and slow changes in the active film due to the lithiation. The interfacial processes (ion migration and charge transfer) are smallest for the GO-Ge-650 electrodes which is in-line with the higher charge capacity of these electrodes. DISCUSSION Coating of dispersions is more complicated and challenging than spin and dip coatings due to the competition between homogeneous nucleation and particle growth in the solution and heterogeneous deposition. On the one hand, the driving force for coating of dispersed particles, which is usually the degree of supersaturation, should be sufficiently low to retain the rather small energetic advantage of heterogeneous nucleation over homogeneous nucleation and film growth,61 but for practical purposes it should be high enough to allow high deposition yield and minimal reactor volume. Thus, although it should be possible to coat particulates with a germanium oxide film at near neutral and acidic conditions by addition of antisolvent to the germanium chloride aqueous solution, the yield would be too low for practical applications. Raising the pH on the other hand would lead to uncontrolled homogeneous nucleation and particle growth. Therefore, the conventional methods for obtaining germanium oxide decorated graphene oxide and other particulates rely on physical means, vacuum deposition techniques or hydrothermal processes under elevated temperature conditions. All of these techniques are energy extensive. In addition, hydrothermal and solvothermal processes that can be easily


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accomplished under laboratory conditions require glass or Teflon lined pressure tanks due to germanium salt (e.g. GeCl4) acidity and corrosivity under elevated temperatures. Moreover, since buffer addition is impossible, the hydrothermal process should be carried out with dilute solutions, which further increase the volume of the lined pressure reactors and their cost.62 Note that heat treatment of the dry active material is relatively inexpensive since a pressure reactor is not needed and the volumes involved are small compared to hydrothermal processing. What is needed is a strong and hydrophilic complexing agent that can stabilize the germanate moieties in the solution and still allow their rejection by the addition of a hydrophobic antisolvent. This is provided by hydrogen peroxide chemistry. The sol state: The coating methodology relies on the recently obtained and characterized crystalline hexanuclear peroxogermanates.40 Not only were the peroxogermanates isolated from aqueous hydrogen peroxide solution, but

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O NMR studies have shown the existence of the

peroxo coordinated germanium in the solution, and the same anion was found in cesium, potassium and ammonium crystals isolated from the corresponding solutions. The isolated hexanuclear peroxide anion [Ge6(µ-OO)6(µ-O)6(OH)6]6− has six peroxo and oxo bridges and six terminal hydroxo groups. It is customary to define a Brownian particle as having at least ten times the dimension of the solvent molecule.63 The size of the oligomeric peroxogermanate anion is 1.1 nm which is in itself sufficient to define the solution as colloidal, though it is very likely that clusters and larger oligomers exist in the solution. Thus the deposition process can be regarded as a sol-gel process. The substitution of hydroxo ligands by hydroperoxo groups in aqueous hydrogen peroxide solution was previously reported for hydroxostannate.41,64 Gas phase DFT studies revealed that the hydroxo ligands can be easily replaced by hydroperoxo ligands in peroxogermanate anions. The oligonuclearity and the hydroperoxo peripheral groups of the sol


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gel precursors are very different from aqueous germanium oxide solutions, which are dominated by uncharged, mononuclear Ge(OH)4, and these differences are responsible for the success of the particulate coating protocol. The peroxogermanate coating stage: A prerequisite for successful thin film coating of particulates is the preference of heterogeneous deposition over homogeneous nucleation and crystal growth. This is accomplished by the stabilizing negatively charged groups on the peroxogermanate moieties and by the affinity of the hydroperoxo terminal groups on the peroxogermanate particles to surface oxygen groups. Peroxogermanate particles are of lower charge and lower dispersion stability due to lower dissociation of the peripheral hydroxo compared to hydroperoxo groups, simply because the acid dissociation constant of hydrogen peroxide is over two orders of magnitude larger than water. Indeed, the precipitation of germanium oxide requires a large excess of antisolvent and results in small precipitation yield, whereas peroxogermanate moieties are precipitated with close to 100% yield by 50% volumetric addition of ethanol. Unlike germanium alkoxides and germanium hydroxide, peroxogermanate sols are readily destabilized by alcohol. Additionally, hydroperoxo groups are good hydrogen donors, much better than hydroxo groups and readily bond to epoxy, carbonyl and other oxygen containing surfaces.65-67 The graphene oxide is far from being a mere inert support in the deposition and thermal transformation processes. Carrying out the deposition process under identical conditions without adding graphene results in crystalline ammonium peroxogermanate, whereas in the presence of graphene oxide, a thin and amorphous film is formed. The graphene oxide basal surface contains a plethora of epoxy and carbonyl groups, which participate readily with the hydroperoxo groups


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in the formation of hydrogen bonds. The graphene oxide also participates actively in the thermal processing. High temperature transformations: Germanium dioxide does not evaporate. Rather, it decomposes to give germanium monoxide and oxygen at elevated temperature.62 At 650°C germanium dioxide reacts with elemental germanium to give GeO68 whose vapor pressure is highly dependent on the oxygen tension. For example, the vapor pressure of GeO at 650°C under congruent vaporization is 10 mPa.69 Thus, slow disintegration and vaporization of GeO at high temperature is possible, resulting in gradual deposition of the GeO and its disproportionation to give large particles of germanium dioxide at colder, active locations. The appearance of the germanium phase (at 900°C) is attributed to high temperature reduction of the germanium oxide. The Ellingham diagram shows that the reduction of GeO2 by graphite C(s) + GeO2(s) → CO2(g) + Ge(s)

(1)

becomes possible already at 687°C. For this calculation we took the thermodynamic constants of the graphene oxide to be equal to graphite, which can be regarded as a rough approximation (the intersection with the y-axis at 25°C should be somewhat higher than graphite but the slope representing the entropy loss is somewhat lower than horizontal since graphene oxide is partially oxidized). The relevant constants were taken from Bard and Jordan Handbook of Standard Potentials in Aqueous Solutions.70 Germanium is a relatively light element and the theoretical charging capacity of GeO2, 960 mAh g-1 (based on Ge4Li15 formation) exceeds that of graphite (372 mAh g-1), tin oxide (783), and antimony oxide (552). While it is inferior to silicon oxide (1570) and aluminum oxide (1580), the latter two have lower conductivity. Table 1 compares the charging performance and processing of the germanium oxide anodes reported to date. Performance-wise, the table shows


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that better performance germanium oxide anodes have been reported, reaching at times 1650 mAh g-1 17 which far exceeds the theoretical capacity of germanium oxide, and is attributed to the catalytic reduction of Li2O by germanium. However, recalling that our best performing electrode has only 51 %wt of germanium our charging capacity based on the germanium component alone exceeds the theoretical charging capacity of elemental germanium (2100 vs 1380 mAh g-1) and therefore there is large room for performance optimization – which, however, is outside the scope of this new processing demonstration. The large C to Ge ratio in the electrodes cannot explain the high charge capacities observed in our studies, since identical electrodes that were prepared from graphene oxide alone gave a charge capacity of only 275 mAh g-1 at 100 mA g-1.

Table 1. Processing and performance of germanium peroxide LIB anodes

Process Dip-coating

Precursor GeCl4

GeCl4 Hydrothermal GeO2

Freeze-drying Electron-beam evaporation

Solvothermal

Ge(OC2H5)4 GeO2 (NH4)2GeO3 GeO2

GeCl4

Ge(OEt)4

Process T, °C Material time, h additives compositiona GeO2/rGO/Ni small 25 foam CNT-GeO2 10 110 GeO2; 25 GeO2/rGO GeO2/G 10 110 GeO2/C 25 SnO2/(GeO2)0.13/G 25 GeO2/C 4 150 GeO2/C 10 90 GeO2/rGO* 48 GeO2/rGO GeO2/G rGO/GeO2/PANI Ge/GeO2/C GeO2 (for Ge/C) GeO2/Ge/C As-prepared GeO2 + acetylene +Ar Ge/GeO2/C

8 8 3 0.5

Rate mAg-1

Capacity, Ref mAh g-1 #

1000

1159

3

200

814

4

100

816

5

200 1000 100 1500 100 100

999 914 942 697 1385 330 1200

6 7 8 9 10 11 12

100

919

13

220 70 25, H2 25 520

100 100 320

749 1018 896

14 15 16

2100

1650

17

100

500

1241

18


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Milling

Ge powder GeO2

GeO2/N dopp.C Ge/GeO2/C GeO2/SnCoC

12

80 25 25

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C/2 200 300

905 915 800

19 20 21

Hydrogen Peroxogerma this peroxide GeO2/rGO 20 min 25 250 1050 work nate sol process Graphite Theoretical 372 a C – carbon; N dopp. C –nitrogen dopped carbon; G – graphene; rGO – reduced graphene oxide; *material tested with Na cathode. From a processing point of view, the table illuminates the advantages of the hydrogen peroxide processing approach. Each of the processing methods in this table suffers from a major drawback. Dip coating of foams relies on a heavy substrate, ball milling is energy intensive and requires a large processing time, vacuum processing requires large infrastructure, and hydrothermal processing requires large reactor volumes and lined pressure tanks, which are too pricy even for battery processing. Moreover, hydrothermal and solvothermal processes require either excessively large processing times (which in practice translates to large reactor volumes) or high temperature which translates to pricy reactors or to low yield if this is compromised. Indeed, several publications demonstrated impressive performance even by room temperature hydrothermal processing, but while serving as a good feasibility demonstration, they all suffer from low germanium oxide deposition yields. On the down side, one has to take into account that GO is still relatively expensive compared to other forms of carbon.

Concluding Remarks A new sol-gel processing of germanium precursors is introduced and demonstrated to provide a fast, high yield, low temperature way to coat nanoparticles by hydroperoxogermanate films. The mechanistic role of the graphene oxide in the deposition process and the thermal transformations were unraveled, showing that the graphene oxide promotes the deposition


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process and shifts the product from crystalline moieties to an amorphous film. The graphene oxide continues to play active role as a reducing reagent in the thermal transformations of the germanate moieties. The advantages of the coated particulates are demonstrated for LIB anodes, which show high specific charging capacity (based on the expensive germanium ingredient) that can be obtained at lower costs compared to all previously reported germanium oxide LIB anodes.

ASSOCIATED CONTENT Supporting Information. Details of the instrumental characterization by HR-TEM, HR-SEM, STEM, X-ray Photoelectron Spectroscopy, Raman Spectroscopy and X-ray powder diffraction, peroxide content determination, additional figures (Figures S1-S6), including particle size distribution in ammonium peroxogermanate solution, X-ray diffractogram of crystalline ammonium peroxogermanate, Raman spectra, thermal and elemental analysis, cyclic voltammogramms, additional tables (Table S1,S2), including elemental composition and XPS binding energies. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This research was conducted by NTU-HUJ-BGU Nanomaterials for Energy and Water Management Programme under the Campus for Research Excellence and Technological Enterprise (CREATE), that is supported by the National Research Foundation, Prime Minister’s Office, Singapore. The financial support of Israel Research Foundation and the Ministry of Science is thankfully acknowledged. We thank the Harvey M. Krueger Family Center for Nanoscience and Nanotechnology of the Hebrew University of Jerusalem and the Israel Science Foundation. We thank the Russian Foundation for Basic Research (grant 14-29-04074), the Council on Grants of the President of the Russian Federation (MK-5796.2016.3, SP-995.2015.1) and the Presidium of the Russian Academy of Sciences (grant Arctic I.32П3). ABBREVIATIONS GO, graphene oxide rGO, reduced graphene oxide RT, room temperature FEC, fluorinated ethylene carbonate DEC, diethyl carbonate CMC, carboxymethylcellulose sodium salt REFERENCES (1)

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