GeO2 shell-encapsulated Nb2O5 nanoparticle assemblies for high-performance lithium-ion battery anodes

Electrochimica Acta 340 (2020) 135952

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Three-dimensional Ge/GeO2 shell-encapsulated Nb2O5 nanoparticle assemblies for high-performance lithium-ion battery anodes Kyungbae Kim a, b, Hyungeun Seo a, b, Han-Seul Kim a, b, Hyun Seung Lee b, Jae-Hun Kim a, b, * a b

School of Materials Science and Engineering, Kookmin University, Seoul, 02707, Republic of Korea Module System Smart Fashion Platform Research Center, Kookmin University, Seoul, 02707, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 October 2019 Received in revised form 11 February 2020 Accepted 23 February 2020 Available online 24 February 2020

We report an efficient nanostructure of three-dimensional germanium/germanium dioxide (Ge/GeO2) shell-encapsulated niobium pentoxide (Nb2O5) nanoparticle assemblies for high-performance Li storage anode materials. The Ge/GeO2-encapsulated Nb2O5 nanoassemblies are prepared via a simple solvothermal synthesis method. The Ge/GeO2 layer binds the surfaces and interfaces of the interconnected Nb2O5 nanoparticle assemblies, presenting as dense and uniform microspheres. The tightly bound Ge/ GeO2 layer interconnects the nanoassemblies to provide high capacity and a pathway for Liþ and electron transport. Via the Nb2O5 assemblies, fast Liþ transport kinetics are secured using a pseudocapacitive reaction mechanism during Li insertion and extraction, while Ge nanoparticles provide the high specific capacity. As a result, the Ge/GeO2-encapsulated Nb2O5 microspheres deliver a greatly enhanced reversible capacity of 800 mA h g1 and excellent high-rate capability. The superior high-performance in Ge/GeO2-encapsulated Nb2O5 microspheres arises from a synergetic effect between the interconnected microstructure and the highly reversible GeO2 phase. The encapsulated interconnection material design concept allows control of the Ge-based electrode materials composition making it ideal for fabrication of high-performance electrodes in Li-ion batteries. © 2020 Elsevier Ltd. All rights reserved.

Keywords: Germanium Germanium dioxide Niobium pentoxide Encapsulation Li storage

1. Introduction Li-ion batteries (LIBs) are the most widely used power sources for portable electronic devices and electric vehicles. The current LIBs are confronted with challenges to increase the energy density as the market demand grows rapidly. For example, the commercial graphite anode exhibits a lower specific capacity (372 mA h g1) compared to alternative Li-alloy-based materials [1]. More specifically, some materials such as group-IV elements (e.g., Si, Ge, Sn) show high specific capacities by using Li-alloy formation during electrochemical reactions (Si: 3580 mA h g1 for Li15Si4; Ge: 1390 mA h g1 for Li15Ge4; Sn: 990 mA h g1 for Li22Sn5) [1e4]. These materials have attracted great attention due to the high capacity values. Recently, Ge-based materials have been considered as promising anode materials because of the higher electronic conductivity and ionic diffusivity compared to other elements

* Corresponding author. School of Materials Science and Engineering, Kookmin University, Seoul, 02707, Republic of Korea. E-mail address: [email protected] (J.-H. Kim). https://doi.org/10.1016/j.electacta.2020.135952 0013-4686/© 2020 Elsevier Ltd. All rights reserved.

[5e11]. Ge shows ~15 times greater numerical Li diffusivity than Si (Ge: 2.14  107 cm2 s1 and Si: 1.47  108 cm2 s1) [12,13]. Although the high production cost of elemental Ge has acted as a hindrance to research efforts, Ge is one of the most abundant elements in nature. Representative Ge-based materials are germanium oxides (e.g., GeOx and GeO2). In practice, Ge surfaces minimally passivate in contact with atmospheric oxygen [14]. Theoretically, the principal oxide form (GeO2) can deliver a high specific capacity of 2200 mA h g1 during the first Li insertion based on a conversion reaction into Ge and Li2O and a subsequent alloying reaction of Ge with Li [7e9]. The reversibility of the first step to Ge and Li2O is a crucial factor to yield a high reversible capacity. To generate advanced electrochemical performance with enhanced reversibility and long cycle life of Ge-based anodes, a number of researchers have noted that several problems should be solved [5e11]: (i) a volume change during Li-alloying reaction with Ge pulverizes the electrodes and decreases the reversible capacity, (ii) the slow Li-alloying reaction in the bulk material limits the charge transfer for high rate performance, and (iii) formation of a thick solid electrolyte interphase

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(SEI) layer and irreversible Li2O phase from the chemical passivation layer of pure Ge and/or Ge oxide phases results in a low initial coulombic efficiency. To enhance the performance of Ge-based anode materials for meeting the standard of commercialization, all these factors should be addressed. With regard to those problems, some research efforts have been directed towards various nanomaterials such as nanoparticles, nanowires, nanotubes, and nanoporous structures [15e26]. The nanomaterials can minimize the volume changes by lessening mechanical stress; also the high surface areas lead to increased interfaces between electrode and electrolyte for fast reaction kinetics. However, nanostructured active materials exhibit a low mass density in the electrode, and additional side reactions such as production of new SEI layers during cycling occur, which are limiting factors for an actual battery application. In terms of cycling stability, Ge oxide materials have usually exhibited better performance than pure Ge-based materials because of the conversion reaction mechanisms mentioned above. However, the formation of Li2O is detrimental for the initial coulombic efficiency and electronic transport in the electrode materials. Therefore, a newly designed structure is required to meet the standards for long cycle life and fast reaction kinetics with minimized side reactions. In this work, the Ge-based electrode materials are designed as dense, uniform, and efficient nanostructures that simultaneously provide fast Li-ion diffusion with electron transport in the matrix, while yielding high reversible capacity and relief of mechanical stress during LieGe alloying/de-alloying reactions. We synthesized an encapsulated microsphere consisting of an efficient nanoarchitecture. A three-dimensional (3D) Ge/GeO2 layer bound the surfaces and interfaces of interconnected Nb2O5 nanoparticle assemblies in the dense microspheres. Nb2O5 was incorporated in the matrix because it is one of the most promising high-rate Li storage materials. It can promote fast Li-ion diffusion by its Li-ion intercalation mechanism, storing Liþ ions in the crystal structure with relatively small volume changes [27e35]. Thereby, the volume changes of the embedded Ge nanoparticles during LieGe alloying/ de-alloying reactions are ameliorated by the GeO2 phase and interconnected Nb2O5 nanostructure. The electrode exhibited a high reversible capacity of 800 mA h g1 with an initial Coulombic efficiency of 72.7%, and excellent cycle and rate performance. This electrochemical performance of the encapsulated microsphere could be attributed to its efficient nanostructure to facilitate fast electron transfer and Li-ion diffusion and to relieve the volume changes. We carried out various analyses to understand its Li insertion/extraction mechanism producing improved electrical conductivity and Li-ion storage kinetics. 2. Experimental 2.1. Materials synthesis 3D Ge/GeO2 shell-encapsulated Nb2O5 nanoparticle assembly microspheres were prepared using a solvothermal method as reported previously [36,37]. Ammonium niobate(V) oxalate hydrate (C4H4NNbO9$xH2O, 3.03 g, Sigma Aldrich) and Pluronic F-127 (0.8 g, Sigma Aldrich) surfactant were dissolved in a closed vial containing a mixed solution of ethyl alcohol and dimethylformamide (4:6 in volume ratio, total amount: 120 mL) under magnetic stirring for 30 min. Then, 2 mL of 2 M HCl and a polyvinylprrolidone (PVP, 0.2 g, Sigma Aldrich) solutions were added in 2 mL of tetrahydrofuran solvent under additional stirring for 10 min. After stirring, germanium(IV) chloride (GeCl4, 0.7 mL, Alfa Aesar) solution was slowly put into the mixed solution. The final solution was transferred into a Teflon-lined autoclave reactor (200 mL) and was heat-treated at 200  C for 24 h. After the solvothermal reaction, the precipitate was

filtered and cleaned with ethyl alcohol by centrifugation and then vacuum-dried at 65  C for 12 h. Finally, the as-prepared precipitate was heated to 600  C (heating rate: 10  C min1) for 2 h under Ar atmosphere. 2.2. Materials characterization The crystal structure was characterized by X-ray diffraction method (XRD, Rigaku D/MAX-2500V with Cu Ka radiation). The morphologies and microstructures of the synthesized materials were examined by field-emission scanning electron microscopy (FE-SEM, JEOL 7500) and high-resolution transmission electron microscopy (HR-TEM, Talos F200X) using an energy dispersive spectroscopy (EDS) detector and fast Fourier transform (FFT) pattern. The focused ion beam (FIB) microtome was used to prepare the cross-sections of encapsulated microspheres. The BrunauerEmmett-Teller (BET) surface area and pore size of the synthesized material were measured using the N2-physisorption instrument (Micromeritics TriStar II 3020) at 77 K. X-ray photoelectron spectroscopy (XPS, Thermo Scientific, Al Ka radiation) analysis was performed to characterize the chemical states from surface to bulk state of the encapsulated particles. Raman (Renishaw, Nd:YAG laser at a wavelength of 532 nm) and thermogravimetric analyses (TGA, TA instruments Q600 V20.9 Build 20) were applied to determine the amount of pyrolyzed carbon content in the synthesized materials. The thermal stability of the synthesized materials and other references was examined by differential scanning calorimetry (DSC, Mettler Toledo STARe system) after full lithiation of the electrodes to 0.001 V vs. Liþ/Li within the temperature range of 25e500  C (heating rate: 10  C min1). 2.3. Electrochemical measurements The active material (70%wt), a conducting agent (15 wt%, Super P, Sigma Aldrich), and a binder (15 wt%, polyacrylic acid, Sigma Aldrich) were dispersed in deionized water under magnetic stirring for a sufficient time. The prepared slurry was coated onto a copper foil as an electrode film and then dried in oven at 80  C for 3 h. The electrode was roll-pressed to 70% of its pristine film thickness. The pressed electrode was dried once again under vacuum at 80  C for 12 h and then cut into coin-type disks with a diameter of 10 mm. The average mass loading of the active materials was approximately 2.0 mg cm2. Half-cell tests were carried out using assembled CR2032 type cells comprised of a working electrode, a lithiummetal foil as counter/reference electrode, a porous polyethylene separator (PE, SETELA™, Toray Battery Separator Film Inc.), and 1.0 M LiPF6 in ethyl carbonate/diethyl carbonate (3:7 in volume ratio, Panax Etec) containing 10 wt% fluoroethylene carbonate as an electrolyte. Galvanostatic cycling tests were performed using a battery cycler (Basytec CTS-Lab) at several constant currents between 0.001 and 3.0 V (vs. Liþ/Li). Discharge and charge mean Li insertion and extraction, respectively. A galvanostatic intermittent titration technique (GITT) test for the first cycle was also carried out within the same potential window at a constant current density of 50 mA g1. During the GITT test, electrochemical impedance spectroscopy (EIS, Biologic VSP) measurements were also performed over the frequency range from 1 MHz to 10 mHz with an amplitude of 5 mV in each step. 3. Results and discussion Fig. 1 shows the general synthesis scheme as well as TEM images of the final products. The presence of Ge and GeO2 nanocrystallites in the microspheres is important for achieving high capacity as an anode material for Li-ion batteries. Interestingly, the

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Fig. 1. (a) Schematic illustration for synthesis, and (b) low- and (c) high-magnification TEM images (cross sections) of 3D Ge/GeO2-encapsulated Nb2O5 nanoassembly microspheres.

nanoassemblies are not simply connected to each other but also form a long-range network in the Ge-embedded GeO2 matrix. Consequently, the Ge/GeO2 layer has a mechanical role as an interconnection medium for Nb2O5 nanoassemblies, producing densely packed microspheres. The average size of microspheres was approximately 3 mm in diameter. The overall morphology change is shown in Fig. S1. This uniform particle size distribution was induced by the incorporation of PVP co-polymer, which enabled a highly ordered configuration of the nanostructure [38]. Fig. 1b and c clearly show the internal structure of the microspheres. These high-angle annular dark-field TEM images revealed that assemblies several tens nanometers across were interconnected with each other. This configuration was observed for the entire range of cross-section of microspheres. To further investigate the microstructure, we performed HRTEM with EDS measurements on the cross-sections of the microspheres. Fig. 2a shows the TEM image of a quarter of one microsphere. The outermost primary particles, marked by the arrows, have a slightly different morphology compared to the ones in the bulk. Specific parts of the surface (marked with a red arrow and the letter [b]) and bulk structure (marked with the letter [c]) were enlarged as high-magnification TEM images in Fig. 2b and c. A large angular shaped nanoassembly was observed at the surface where a high temperature phase (monoclinic, HeNb2O5) of Nb2O5 crystal structures was found with a d-spacing of 3.79 Å corresponding to (110) (Fig. 2b). The Ge/GeO2 encapsulated layer could be also observed on the surface of the outermost assembly. In the bulk, three different types of crystal structures were observed (Fig. 2c). Three different areas in Fig. 2c (marked with squares labeled with [d], [e], and [f]) were enlarged as detailed HR-TEM images with corresponding FFT patterns, as shown in Fig. 2def. Unlike the assemblies on the surface, a low temperature phase (orthorhombic, T-

Nb2O5) of Nb2O5, with a d-spacing of 3.93 Å corresponding to the (001) planes, was found within the microsphere (Fig. 2d). Among the T-Nb2O5 nanoassemblies, a GeO2 crystal structure was clearly detected with a d-spacing of 3.43 Å, attributable to the (011) planes (Fig. 2e). The nanocrystallites approximately 10 nm in size, marked with yellow circles, were determined to be a Ge crystal structure with a d-spacing of 3.27 Å corresponding to the (111) reflection (Fig. 2f). In this synthetic system, the use of Pluronic F-127 and PVP polymers, and the heat-treatment at 600  C were sufficient to produce a reductive atmosphere so that Ge nanocrystallites, which were embedded in the GeO2 phase, were formed. In the microspheres, the Ge/GeO2 layers with a thickness of approximately 5 nm and the Nb2O5 nanoassemblies were densely interconnected to form a rigid microsphere. The connections of Ge/GeO2 layers were clearly observed from EDS elemental mapping images, as shown in Fig. 2g. This internal microstructure was also confirmed with the additional TEM and EDS elemental mapping images at low- and high-magnifications (Figs. S2 and S3). Fig. 2h shows the XRD pattern of the encapsulated microspheres. According to the crystal structure analysis, diffraction peaks attributable to both Ge (JCPDS No. 04e0545) and GeO2 (JCPDS No. 36e1463) were clearly observed. In addition, highly ordered crystal structures of T-Nb2O5 (JCPDS No. 30e0873) and HeNb2O5 (JCPDS No. 74e0312) appeared. The XRD pattern of the as-prepared GeOx-NbOx microspheres (prior to the heat treatment) is given in Fig. S4 for comparison. The XRD results of the 3D Ge/ GeO2-encapsulated Nb2O5 nanoassembly microspheres agree well with the microscopy studies. Using Scherrer’s equation, the mean crystallite size of Ge was estimated to be approximately 4.6 nm, similar to that observed from the HR-TEM image (Fig. 2f). To obtain the ratio between materials in the microspheres, Rietveld refinement was used to determine the mole fraction of each component.

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Fig. 2. Cross-sectional (a) low-magnification TEM image, (b, c) high-magnification TEM images for areas marked with a red arrow and a red square in (a), (def) HR-TEM images and corresponding FFT patterns for selected areas in (c), (g) EDS elemental mapping results, (h) XRD pattern, and (i) N2 adsorption-desorption isotherm (inset: corresponding pore-size distribution) of 3D Ge/GeO2-encapsulated Nb2O5 nanoassembly microspheres. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

The molar ratio of Ge, GeO2, and T- and HeNb2O5 materials was estimated to be 11.9:23.4:47.4:17.3 (see Fig. S5). This ratio exhibited reasonable values based on the starting materials ratio and EDS elemental distribution results (see Table S1) in our synthetic processes. The surface area and porosity of the encapsulated microsphere were measured through nitrogen adsorption/desorption

analysis (Fig. 2i). A type-IV isotherm was observed, which could be ascribed to a mesoporous structure. The BET surface area was measured to be approximately 31 m2 g1 indicating that the encapsulated material was in the form of compact micrometersized particles with some pores and large angular shaped surfaces. The pore size distribution was obtained from the nitrogen

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Fig. 3. XPS core-level spectra of (a) Ge 3d, (b) Nb 3d, and (c) O 1s of Ge/GeO2-encapsulated Nb2O5 nanoassembly microspheres.

adsorption/desorption curve (inset of Fig. 2i). The pore size peak is mainly centered at 8.9 nm with an average pore size of 5.6 nm, corresponding to pores in the bulk structure and the indented surfaces in the outermost part of the encapsulated microspheres. For further verification on the chemical state of the prepared materials, we performed an XPS analysis from the surface to the bulk structure using Ar-ion etching on the encapsulated microsphere. Fig. 3a shows the Ge 3d core-level spectra with discernible sub-profiles. On the surface, the characteristic sub-profiles centered at 30.1. 32.0, and 33.5 eV corresponded to metallic Ge, GeO, and GeO2, respectively [39,40]. After Ar-ion etching for 600 s, only the intensity for the metallic Ge sub-profile increased, which indicated that the metallic Ge is primarily located inside of the GeO2 component, as also shown in the HR-TEM images [39]. The deconvoluted Nb 3d core-level spectra of the encapsulated microspheres are presented in Fig. 3b. On the surface, a doublet at 207.2 and 209.9 eV for 3d5/2 and 3d3/2 corresponds to Nb5þ, which can be attributed to HeNb2O5 [41,42]. The unpaired valence state induces an interaction exchange of high- and low-spin core electrons and electrostatic interactions and hence broad high-spin valence state peaks were observed with unresolved multiplet splitting [43]. With this phenomenon, the binding energies for Nb 3d5/2 on the surface were attributed to the valence states of Nb4þ (NbO2) and Nb2þ (NbO) at 205.9 and 203.9 eV, respectively as reported in the literature [41,42]. After Arþ ion etching for 600 s, the binding energies were shifted to the lower energy values, in accordance with the existence of T-Nb2O5 [41]. This shift of binding energy occurs due to the crystal structure difference between the surface and bulk as in the aforementioned microscopy and crystal structure studies. Angular particles of HeNb2O5 were present on the outermost part and the spherical T-Nb2O5 nanoparticles were present in the bulk structure of the microsphere. O 1s core-level spectra are presented in Fig. 3c. The sub-profiles for valence states of GeO2 and Nb2O5 appear at 532.4 and 531.2 eV, respectively with narrow full width half maximum values after Arþ etching [39,41]. This result implies a uniform materials distribution in the bulk material, comprised of the 3D Ge/GeO2-encapsulated Nb2O5 nanoassemblies. In the encapsulated microsphere, there was evidence of O bonded to C due to the carbonization of Pluronic F127 and PVP polymers during heat-treatment. The C 1s core-level spectra for the sample is shown in Fig. S6. To determine the carbon content in the microspheres, Raman and TGA analyses were carried out and the results are shown in Fig. S7. From the Raman spectra, the D and G bands centered at 1360 and 1600 cm1 were

detected as reported in the literature [44]. The weight percentage for total carbon content was measured to be approximately 1.8 wt% via the TGA result. Galvanostatic charge-discharge cycling was performed to investigate the electrochemical properties of 3D Ge/GeO2-encapsulated Nb2O5 nanoassemblies. Fig. 4a shows the initial voltage profiles, which were obtained at a constant current density of 0.1 A g1. The first discharge and charge capacities were 1107 and 793 mA h g1, respectively, corresponding to a coulombic efficiency of 72.7%. The reversible capacity of ~800 mA h g1 was well maintained in the subsequent cycles. These curves exhibited gradual slopes with several plateaus between 0.001 and 2.0 V vs. Liþ/Li. To examine the detailed Li insertion-extraction mechanisms in the 3D Ge/GeO2-encapsulated Nb2O5 nanoassemblies, differential capacity plots (DCPs) were obtained, as shown in Fig. 4b. In the DCPs for the first cycle, several peaks indicate that there are distinct electrochemical reactions, leading to phase transitions. The potentials around the peaks were marked by letter D for discharge and letter C for charge. The only irreversible peak (S) was observed during the first Li insertion at a potential range of 1.0 and 0.75 V (vs. Liþ/Li), which could be ascribed to the SEI layer formation on the GeO2 and Ge [8,45]. This peak disappeared during subsequent cycles. To investigate the reaction mechanisms at the specific potentials marked as symbols (P, D, and C with numbers in Fig. 4b), ex situ XRD analyses were carried out and the diffraction patterns for intermediate phase transitions are presented in Fig. 4c. The XRD pattern of the as-prepared fresh electrode is the same pattern as that for the powder (Fig. 2h) and the diffraction peaks for Ge, GeO2, T-Nb2O5, and HeNb2O5 were still observed (Fig. 4c-P1). During the Liþ insertion, a broad curve was first observed with a small peak (D1) at 1.6 V vs. Liþ/Li in DCPs. New diffraction peaks appeared, which could be assigned to Li1.7Nb2O5 (JCPDS No. 37e0977). This phase transition is the result of Liþ intercalation in the Nb2O5 material via a fast pseudocapacitive reaction. When the electrode was further discharged to a potential range between 0.75 and 0.3 V vs. Liþ/Li, one distinct peak (D2) was observed. The diffraction peaks for GeO2 disappeared and peaks for LiGe (JCPDS No. 77e2492) appeared with a slight intensity decrease of Ge peaks (Fig. 4c-D2). Moreover, new peaks for tetragonal Ge (JCPDS No. 72e1425) were observed and the peak intensities for Li1.7Nb2O5 increased. This is evidence of the conversion reaction of GeO2 into new Ge and Li2O phases and further Liþ intercalation into the Nb2O5 nanoparticles [27,45]. The intensities of tetragonal Ge peaks increased further at

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Fig. 4. Electrochemical properties of the nanoassembly microsphere electrode: (a) voltage profiles, (b) DCPs, and (c) ex situ XRD patterns during the first cycle at various potential steps marked in the DCPs.

D3 (0.3 and 0.2 V vs. Liþ/Li) and the Li1.7Nb2O5 phase started to transform into Li1.9Nb2O5 (JCPDS No. 37e0976) in Fig. 4c-D3. At this point, the LiGe phase from Ge (cubic crystal structure) finally transformed into Li7Ge2 (JCPDS No. 80e0531) and Li15Ge4 (JCPDS No. 89e2584) phases. The peaks for Li1.9Nb2O5 were slightly shifted from pristine T- and HeNb2O5 phases because the unit cell volume increased with Liþ intercalation [27]. After the electrode was fully discharged to the D4 state, the diffraction peaks for tetragonal Ge eventually disappeared with prominent peaks for Li15Ge4 and the fully Li inserted phase of Li1.9Nb2O5 (Fig. 4c-D4). The Li de-alloying reactions occurred during the Liþ extraction processes. In the potential range between 0.001 and 0.4 V vs. Liþ/Li in DCPs, the first reverse peak (C1) appeared due to the Li dealloying reaction of LieGe phases into Ge [17,45]. The diffraction peaks for tetragonal Ge were observed in Fig. 4c-C1, and the intensities of Li15Ge4 peaks greatly decreased. The peak (C*) at 0.5 V is consistent with further reversible Li de-alloying reactions from Li15Ge4 into Li7Ge2 and Ge phases as seen by the XRD peaks (Fig. 4cC*). In the potential range between 0.75 and 1.5 V vs. Liþ/Li, a peak (C2) was observed, which was attributed to the reduction of Li2O phases with simultaneous oxidation of Ge into GeO2 [8,45]. This is evidenced by the appearance of GeO2 diffraction peaks as well as those of further Li de-alloying reactions of Li7Ge2 into LiGe and cubic Ge in Fig. 4c-C2. A broad DCPs peak (C3) between the potential range of 1.5 and 2.5 V vs. Liþ/Li was observed at the top of

charge. The diffraction peaks for GeO2 and Li1.7Nb2O5 phases became dominant in Fig. 4c-C3. This behavior was also found in the charged state of P2 (2.5 and 3 V vs. Liþ/Li). Some diffraction peaks for disordered Nb2O5 and Li1.7Nb2O5 were detected in Fig. 4c-P2, suggesting that interstitial Li sites in Nb2O5 are still occupied even after Liþ extraction [27]. The reaction mechanism was also investigated using ex situ XPS (see Fig. S8) and the results were consistent with the XRD analysis results. To compare the Li storage mechanisms, the voltage profiles and DCPs for each individual material (Ge, GeO2, and Nb2O5) are presented in Fig. S9. All the reaction peak positions of each component for the microsphere were empirically verified by the individual profiles. For investigation of fast Li-ion storage kinetics, GITT was carried out at a constant current density of 50 mA g1. EIS measurements were also performed between GITT intervals to determine the resistance. Fig. 5a and b shows the GITT profiles with reaction resistance during the first Li insertion and extraction processes. The measurement of interval-connected open circuit voltages (OCVs) indicated that variable overpotentials (h) occurred in each potential region. During the Liþ insertion, considerable overpotentials were observed within the regions for the Li intercalation reaction into Nb2O5 and the conversion reaction of GeO2. In comparison, a decrease of overpotential in the region for Li-alloying reaction of Ge was detected due to the fast Li diffusivity and high electrical conductivity of Ge. The reaction resistances during GITT intervals

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Fig. 5. GITT profiles and reaction resistance during the first cycle: (a) Li insertion and (b) Li extraction; (c) EIS spectra (Nyquist plots) measured at fully lithiated and delithiated states for the first and 100th cycles.

include the electrolyte resistance, charge transfer resistance, and resistance of Li-ion diffusion in bulk materials. During the Lialloying reaction with Ge, the resistance values were also relatively lower, which could be attributed to the fast kinetics in Ge. Fig. 5c shows the Nyquist plots for fully discharged and charged states of the 3D Ge/GeO2-encapsulated Nb2O5 nanoassembly electrodes after the first and 100th cycles (inset: equivalent circuit model). The profiles were fitted with dashed lines. During Li

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insertion and extraction, two quasi-semicircles at high to medium frequencies and a straight sloping line at low frequency were observed. The high-frequency intercept of the semicircles is typically related to the electrolyte resistance. The medium frequency intercept of the semicircles is related to the interfacial resistance at the electrode, which encompasses both the SEI layer and chargetransfer resistances. The sloping line region is related to the Liion diffusion process (Warburg impedance). Both Li inserted and extracted electrodes showed very small values for electrolyte resistance at the high frequency intercept, indicating the ionic conductivity is sufficient. The fully Li inserted electrode at the first cycle showed higher resistance values with larger semicircles than the same electrode at the 100th cycle. This indicated that the SEI and charge transfer resistances in the Li inserted state decreased after 100 cycles. It is evident that the 3D Ge/GeO2-encapsulated Nb2O5 nanoassembly structure is beneficial for the fast Li transfer kinetics on the electrode surfaces even after many cycles. In the Li extracted electrodes at the first and 100 cycles, the interfacial resistances were even lower, which is also advantageous for fast kinetics. Fig. 6a shows the cycle performance of the 3D Ge/GeO2encapsulated Nb2O5 nanoassembly electrodes at several constant current densities of 0.1, 0.5, and 5.0 A g1 (inset image). Reversible capacities at the first cycles were stably retained up to 100 cycles without capacity fade. The rate capability test results are given in Fig. 6b. Even at a high current density of 5.0 A g1, a capacity of approximately 400 mA h g1 was obtained. These stable cycle and high rate performances could be attributed to the interconnected nanostructure of the encapsulated microsphere. For comparison, the voltage profiles and cycle performance of a physical mixture are shown in Fig. S10. The electrode made up a mixture of Ge, GeO2, TNb2O5, and HeNb2O5 was prepared to have the same ratios of components as that of the nanoassembly electrode calculated from Rietveld refinement analysis (Fig. S5). The mixture showed a high first discharge capacity of 795 mA h g1 and however, but a low reversible charge capacity of 434 mA h g1 with a coulombic efficiency of 54.6%. In addition, the cycle performance was inferior. This result indicated that the interconnected microstructure of 3D Ge/GeO2-encapsulated Nb2O5 nanoassemblies is beneficial for cycle and rate performances due to the buffering effect and fast electronic/ionic conduction. Additionally, we compared the electrochemical performance of 3D Ge/GeO2-encapsulated Nb2O5 nanoassembly microsphere electrodes with other Ge- and Nb2O5based 3D microstructure electrodes for LIBs (Table S2). It can be seen that the performance is comparable to those of references even without carbon incorporation. The advantage of the encapsulated interconnection structure can be found in the dimensional stability after long-term cycling. Fig. 6c and d shows cross-sectional FE-SEM images of the 3D Ge/ GeO2-encapsulated Nb2O5 nanoassembly electrodes before cycling and after 100 cycles. The overall shape of the microspheres was well maintained even after 100 cycles and the thickness of the electrode was increased by only 8.8%, which was a low expansion value compared to Ge-based electrodes in the previous reports [46e49]. This can be attributed to the well-designed 3D microstructure where Nb2O5 nanoassemblies were surrounded by thin Ge/GeO2 layers forming a 3D network. The absolute volume changes of Ge/GeO2 layers were small during Li insertion/extraction reactions and moreover, the volume expansion could be suppressed by the Nb2O5 nanoassemblies. To examine the thermal stability of the encapsulated microsphere, DSC analysis was carried out in comparison with individual component of Ge, GeO2, and Nb2O5 and the results are presented in Fig. 7. The electrodes were fully discharged to 0.001 V vs. Liþ/Li. The encapsulated microsphere electrode showed much smaller heat flow and accumulated heat

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Fig. 6. (a) Cycle performance at different current densities of 0.1e5.0 A g1 (inset: measured at 5.0 A g1), (b) rate performances, and cross-sectional FE-SEM images of the electrode for (c) before cycle and (d) after 100 cycles (Li inserted state).

Fig. 7. (a) DSC curves and (b) corresponding accumulated heat curves of Ge/GeO2-encapsulated Nb2O5 microspheres, Ge, GeO2, and Nb2O5 electrode materials at fully lithiated states.

than the reference electrodes. This effect could be used to minimize the thermal decomposition in such nanostructures. The 3D interconnected nanostructure has high interfacial areas between the

Nb2O5 nanoassemblies and the adhesive Ge/GeO2 layer inside of the spherical microsphere and it probably restricts the thermal motion, resulting in the enhanced thermal stability of the

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microsphere [50]. This indicates that the encapsulated nanostructure effectively reduces the heat flow and suppresses the exothermic reactions with the electrolyte in the cell. 3D Ge/GeO2encapsulated Nb2O5 nanoassembly microspheres have several advantages for high-performance Li storage materials. The dense microsphere particles effectively suppress the side reactions with electrolyte. Fast Li-ion diffusion in Nb2O5 and GeO2 can be enabled through the interconnected microstructure. The high reversible capacity and cycling stability can be obtained due to the Geembedded structure. It should be noted that this material design concept would be beneficial to improve the dimensional and thermal reliability of the anode. 4. Conclusions We successfully demonstrated a 3D Ge/GeO2 shell-encapsulated Nb2O5 nanoparticle assemblies by using a simple solvothermal method. FE-SEM and TEM characterization studies showed that the interconnected nanoassemblies of Nb2O5 were densely bound up with Ge/GeO2 layer. The encapsulated microsphere electrode exhibited a high reversible capacity of ~800 mA h g1 with longterm cycle stability. The dimensional and thermal stabilities were much better than those of each component alone. These results can be explained by the synergetic effect between interconnected microstructure and high reversibility of GeO2 phase. In this microstructure, the Nb2O5 nanoparticles provide the fast Liþ diffusion during Li-alloying reactions of Ge-based materials and the GeO2 layer tightly bound the nanoassemblies to facilitate Liþ and electron transport using its interconnected interface. The encapsulated interconnection material design concept can be applied to the development of electrode materials as high performance LIBs. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Kyungbae Kim: Conceptualization, Investigation, Data curation, Validation, Writing - original draft, Funding acquisition. Hyungeun Seo: Investigation, Data curation. Han-Seul Kim: Investigation, Data curation. Hyun Seung Lee: Data curation, Writing - review & editing, Project administration, Funding acquisition. Jae-Hun Kim: Conceptualization, Data curation, Writing - review & editing, Project administration, Funding acquisition, Supervision. Acknowledgements The authors are thankful to Dr. Marca M Doeff in Lawrence Berkeley National Laboratory for her helpful discussion and advice for this paper. This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (2015R1A5A7037615, 2019R1F1A1062835, and 2019R1A6A3A01094741). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2020.135952. References [1] C.-M. Park, J.-H. Kim, H. Kim, H.-J. Sohn, Li-alloy based anode materials for Li

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