Unveiling the Origin of Superior Electrochemical Performance in

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Unveiling the Origin of Superior Electrochemical Performance in Polycrystalline Dense SnO2 Nanospheres as Anodes for Lithium-ion Batteries Jun Young Cheong, Joon Ha Chang, Chanhoon Kim, Jiyoung Lee, Yoon-Su Shim, Seung Jo Yoo, Jong Min Yuk, and Il-Doo Kim ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02103 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 24, 2019

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ACS Applied Energy Materials

Unveiling the Origin of Superior Electrochemical Performance in Polycrystalline Dense SnO2 Nanospheres as Anodes for Lithium-ion Batteries Jun Young Cheong, §,† Joon Ha Chang, §,† Chanhoon Kim, ‡ Jiyoung Lee, § Yoon-Su Shim, § Seung Jo Yoo, §,∥ Jong Min Yuk, §,* and Il-Doo Kim, §,*

§Department

of Materials Science & Engineering, Korea Advanced Institute of Science & Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea, ‡Clean

Innovation Technology Group, Korea Institute of Industrial Technology, 102, Jejudaehak-ro, Jeju-si, Jeju-do, 63243, Republic of Korea, ∥ Electron

Microscopy Research Center, Korea Basic Science Institute, 169-148 Gwahak-ro, Yuseong-gu, Daejeon 34133, Republic of Korea †These authors contributed equally to this work. *E-mail: [email protected], [email protected]

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ABSTRACT. Development of feasible electrode materials is significant to realize high energy density Liion batteries (LIBs). Tin (IV) oxide, in particular, has a number of merits including higher theoretical capacity compared with graphite (1493 mAh g-1), low cost, and environmental friendliness. Nevertheless, the huge volume changes and subsequent pulverization usually resulted in poor capacity retention of SnO2, where various nanostructures have been adopted to overcome its intrinsic limitations. Here we introduce the new insights into employing polycrystalline dense SnO2 nanospheres (NSs), rather than its hollow structures, as high performance electrode for LIBs. Contrary to the previous notions, polycrystalline dense SnO2 NSs can exhibit highly stable cycle retention characteristics (1009.9 mAh g-1 after 300 cycles at 0.5 A g-1) as well as considerable rate capabilities (349 mAh g-1 at 5.0 A g-1), even superior to those of polycrystalline hollow SnO2 NSs. Based on the in situ TEM analyses and electrochemical/postmortem analyses, such improved electrochemical performance can be attributed to the (i) predominant isotropic volume changes of polycrystalline SnO2, (ii) formation of numerous nanograins within the NSs, and (iii) maintenance of structural integrity without pulverizations. This work sheds lights on the importance of using polycrystalline dense nanostructures to mitigate the effects of large volume changes and minimize pulverization, which can also be applied other electrode materials.

KEYWORDS: dense SnO2 nanosphere, electrode, in situ TEM, lithium, battery

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1. Introduction Since their first commercialization in the early 1990s, lithium-ion batteries (LIBs) have been applied to various electronics (e.g. laptops and mobile phones) as well as electric vehicles (EV)/hybrid electric vehicles (HEV).1-3 With increasing demands for energy supply and continuous population growth, the development of advanced LIBs with higher energy density are highly sought after.4,5 Among various candidates for next-generation electrodes, tin (IV) oxide (SnO2) has been intensively researched, due to its several advantages6-9: (1) it exhibits the theoretical capacity of 1493 mAh g-1 which far exceeds the theoretical capacity of graphite (372 mAh g-1). (2) It is chemically stable in the electrolytes. (3) It is a low cost material and abundant. (4) It can be easily synthesized, using various methods. Despite all its merits, however, huge volume changes and subsequent pulverization has mainly contributed to the formation of unstable solid electrolyte interphase (SEI) layer and rapid capacity fading of SnO2,10,11 which hampered its use as promising electrodes. Therefore, various ways to overcome such issues have been suggested. As one of the most suitable and practical approaches, various studies have been conducted to tune the morphology of SnO2 to better improve its electrochemical performance. In particular, various SnO2 nanostructures have been developed for improved performance.12 Among them, hollow SnO2 nanostructures have garnered particular interests, as they possess void space that alleviate the volume changes hence the structural integrity can be partially maintained.13 A number of hollow SnO2 nanostructures,14-16 ranging from SnO2 nanotubes to SnO2 hollow spheres, have been synthesized and tested for their electrochemical performances. On the other hand, most of dense SnO2 nanostructures have been reported to exhibit

inferior

electrochemical

performances

compared

with

hollow

SnO2

nanostructures,11,17 which were mostly suggested to be arisen from the poor structural 3



stability upon cycling. In view of volumetric capacity, however, dense SnO2 nanostructures are more suitable compared with hollow SnO2 nanostructures as the void spaces are minimized and more active sites are present for Li storage in a given volume resulting in higher tap density,18 but very little research has so far taken place to probe into the synthesis of rationally designed dense SnO2 nanostructures for high performance LIBs. In this work, we have provided a new insight that the polycrystalline dense SnO2 nanospheres (NSs) can also be utilized as high performance electrodes for LIBs. Unlike the previous literatures,11,14-17 it has been demonstrated that the suitable crystallinity and grain sizes of SnO2 within dense SnO2 NSs can lead to more superior electrochemical performance even compared with hollow SnO2 counterparts. Based on in situ transmission electron microscopy (TEM) analysis, polycrystalline SnO2 undergoes isotropic volume expansion, as different facet planes are present. Moreover, ex situ postmortem analyses further demonstrate that the dense SnO2 NSs are tightly agglomerated into secondary SnO2 particles (2 – 10 μm), without undergoing complete agglomeration, unlike hollow SnO2 NSs that undergo cracking upon cycling and subsequent pulverization/complete agglomeration. Attributed to these factors, dense SnO2 NSs exhibit excellent cycle retention characteristics (1009.9 mAh g-1 after 300 cycles at 0.5 A g-1) as well as considerable rate capabilities (349 mAh g-1 at 5.0 A g1).

Based on the results, polycrystalline dense SnO2 NSs are not only advantageous in their

theoretical volumetric capacity in comparison with hollow SnO2 NSs but also in their isotropic volume expansion compared with single crystalline SnO2 nanostructures. Finally, the suggested finding can also be broadly applied to fabricating dense nanostructures for other polycrystalline electrode materials (metal oxides and metals) that undergo conversion and/or alloying reactions, which are expected to yield much research in near future.

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2. Experimental 2.1. Fabrication of dense SnO2 NSs and hollow SnO2 NSs Both dense SnO2 NSs and hollow SnO2 NSs were synthesized by simple hydrothermal reaction by simply using four different chemical reagents, similar to the previously reported method:19 de-ionized (DI) water, tin chloride dehydrate (SnCl2·2H2O, Sigma Aldrich), ethanol (C2H5OH, Merck), and concentrated hydrochloric acid (HCl) solution (36.46%, Junsei Chemical). To start with, 0.5 g of HCl solution was added to the 30 g of solution containing DI water and C2H5OH (wt/wt = 1/9). Then, 0.3 g of SnCl2·2H2O was added to the solution and stirred for 10 min at 300 rpm. Then the solution was ultrasonicated for 20 min to make sure that the solution was well mixed and dispersed. The transparent solution was then transferred to the Teflon-lined stainless steel autoclave and was kept at 200 °C for 3 h for dense SnO2 NSs and 20 h for hollow SnO2 NSs with a ramping rate of 5 °C min-1. After the hydrothermal reaction, the solution from the autoclave was transferred to the falcon tube, and was washed and centrifuged at 3500 rpm for 10 min three times. Then the sample was dried under air at 50 °C for 12 h. 2.2. Cell Assembly To measure the electrochemical performance, 2032 coin-type half cells were assembled. The cells were assembled inside the glove box. They were consisted of active materials, separator, electrolyte, and counter electrode (Li metal), along with other cell components. For the active material, it was consisted of 80 wt% of SnO2 NSs (dense and hollow), 10 wt% of super P carbon black, and 10 wt% of binder consisting of poly(acrylic acid)/sodium carboxymethyl cellulose (PAA/CMC) (50/50 wt%/wt%, Sigma Aldrich). They were mixed altogether and later slurry casted on the Cu foil. The slurry casted active materials were then dried under vacuum at 150 °C for 2 h, to remove all the residual solvents. The loading amount of the 5



active materials was about 1.5 mg cm-2. For separator, Celgard 2325 separator was used. For an electrolyte, 1.3 M lithium hexafluorophosphate (LiPF6) in ethylene carbonate/diethylene carbonate (EC/DEC, 3/7 v/v) with 10 wt% fluoroethylene carbonate (FEC) (PANAX ETEC.) was used. After the cell assembly, the cells were later tested for their electrochemical performances. 2.3. Characterizations Morphological characteristics of dense SnO2 NSs and hollow SnO2 NSs were analyzed by Field Emission SEM (SU5000, Hitachi) operating at 10 kV. To analyze the morphology and diffraction patterns of dense SnO2 NSs and hollow SnO2 NSs, field emission transmission electron microscopy (FE-TEM, JEM-2100F, JEOL Ltd.) operating at 200 kV was used. The overall crystal structure of dense SnO2 NSs and hollow SnO2 NSs was further confirmed by a powder X-ray diffractometer (XRD, D/MAX-2500, Rigaku) with Cu Kα radiation (λ=1.54 Å) between 20° and 80° at a scan rate of 0.066 ° s-1. To confirm the chemical states of Sn and O for both dense SnO2 NSs and hollow SnO2 NSs, X-ray photoelectron spectroscopy (XPS) (Kalpha, Thermo VG Scientific) was employed. The assembled coin cells with active materials were cycled at a current density of 100-5000 mA g-1 between 0.01 and 3 V using battery testing device (Maccor Series 4000, KOREA THERMO-TECH). Cyclic voltammetry (CV) was conducted at 0.1 mV s-1 within the range of 0.01 to 3.0 V using battery testing device (Maccor Series 4000, KOREA THERMO-TECH). As for the impedance tests, they were examined by an AC impedance analyzer (ZIVE SP1, Wonatech). The slurry from disassembled coin cell was washed with ethanol and sonicated for 2 h. Ex situ TEM images and SAED patterns were taken with JEOL JEM 3010 under accelerating voltage of 300 kV with Gatan US4000SP. 2.4. In situ TEM observation 6


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In situ TEM experiment was conducted using STM-TEM holder (Nanofactory) with JEOL JEM 3010 under 300 kV and a homemade TEM holder for a high voltage electron microscope (HVEM, JEOL Ltd., JEM ARM 1300S) operating at 1250 kV.. The SnO2 NSs were deposited on Pt-coated W tip and Li metal was deposited on STM tip. Li metal was exposed to air before TEM holder is loaded to TEM, to form Li2O which works as a solid electrolyte. After SnO2 NSs and Li/Li2O were kept in contact, voltages of -5V/+5V were applied for lithiation/delithiation, using Keithley 2636B. The dynamic behaviors of the SnO2 NSs were recorded using Gatan Orius SC200. Electron energy loss spectroscopy (EELS) spectra (Gatan Inc., HV-GIF) for the Li K-edge were obtained with an energy dispersion of 0.2eV/ch and an accumulated acquisition time of 0.4s after lithiation of SnO2 NSs during the in situ experiment in the HVEM.

3. Results and discussion 3.1. Synthesis and Characterizations of dense and hollow SnO2 NSs Schematic illustration on the properties of hollow and dense SnO2 NSs, electrochemical cell testing and in situ analysis is shown in Figure 1a. Based on Figure 1a, the main difference between the hollow and dense SnO2 NS lies in the vacancy in the core: the dense SnO2 NSs possess number of nanograins in the core, whereas hollow SnO2 NSs have vacant sites in the core. SEM image of dense SnO2 NSs (Figure 1b) indicates an average size of 100-300 nm, similar to the TEM image of dense SnO2 NSs (Figure 1c). Selected area diffraction (SAED) pattern (Figure 1d) of dense SnO2 NSs further confirm the polycrystalline cassiterite, which is different from single crystalline SnO2 that exhibits minimal grain boundaries. The SEM image (Figure 1e) and TEM image (Figure 1f) of hollow SnO2 NSs, on the other hand, show vacant sites in the core, although overall morphologies are similar. The SAED pattern (Figure 7



1g) of hollow SnO2 NSs also indicate the polycrystalline cassiterite, where no significant difference in the crystal structures of SnO2 between dense and hollow SnO2 NSs is apparent. To confirm the crystal structure of dense and hollow SnO2 NSs along with their average grain sizes, XRD patterns were analyzed for dense SnO2 NSs (Figure 2a) and hollow SnO2 NSs (Figure 2b). Both dense SnO2 NSs and hollow SnO2 NSs exhibit cassiterite (JCPDS 41-1445) structure, and the grain sizes of SnO2 were calculated according to the Scherrer equation20: τ = Kλ / βcos(θ)

(1)

In the equation, τ refers to the crystallite size of SnO2, K a dimensionless shape factor, λ the X-ray wavelength, β the line broadening at half the maximum intensity, and θ the Bragg angle. Based on the equation, the average grain sizes of dense SnO2 NSs and hollow SnO2 NSs are 10.3 and 10.1 nm. It has been demonstrated that very small SnO2 nanograins are formed within the NSs, whether dense or hollow, which are expected to allow facile ionic and electronic transport – as exemplified in the previous work21 which demonstrated that the grain size has a critical effect on the rate capabilities and cycle retention of batteries. The chemical states of both dense SnO2 NSs and hollow SnO2 NSs were also further confirmed by XPS analysis. To trace the chemical bonding pertinent with the Sn and O, XPS spectra of Sn and O were analyzed for both dense SnO2 NSs and hollow SnO2 NSs (Figure 2c-2f). For dense SnO2 NSs, XPS peaks of Sn (3d3/2 and 3d5/2) are located at 486.5 eV and 495.0 eV, which refers to Sn4+ state, in accordance with the previous literature.22 The similar trends of XPS were also shown for hollow SnO2 NSs. XPS peaks of Sn (3d3/2 and 3d5/2) of hollow SnO2 NSs are 486.5 and 494.9 eV, which refers to the Sn4+ state. For dense SnO2 NSs, the XPS peaks of O (O 1s) are located at 530.2, 531.0, and 532.1 eV, which are assigned to O2-, O-, and O2-, which are characteristic ionized oxygen species on the surface of 8


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SnO2.23 The similar XPS peaks for O also exist for hollow SnO2 NSs, located at 530.2, 531.0, and 532.0 eV. Although the overall morphology of dense SnO2 NSs and hollow SnO2 NSs is different, the chemical state of both Sn and O is similar. SnO2-based materials have been widely researched as gas sensors,24,25 particularly because it exhibits excellent chemical stability 24, which excludes the possibility that the additional O2 gas is adsorbed from air after synthetic process. 3.2. Electrochemical performances of dense and hollow SnO2 NSs The electrochemical performances of both dense SnO2 NSs and hollow SnO2 NSs were further measured by various electrochemical cell testing. To examine the redox reactions of dense SnO2 NSs and hollow SnO2 NSs, cyclic voltammetry (CV) was conducted (Figure 3a and Figure S1). As can be seen below, the overall redox reactions of SnO2 during discharge process proceed as follows: SnO2 + 4Li+ + 4e- → Sn + 2Li2O

(2)

xLi+ + xe- + Sn → LixSn (0 ≤ x ≤ 4.4)

(3)

In the first cycle, two major cathodic peaks (0.75 V and 0.1 V) appear for dense SnO2 NSs, which can be attributed to the conversion reaction of SnO2/formation of SEI layer and the alloying reaction of Li with Sn.16,26 From the 1st cycle to 3rd cycle, three anodic peaks exist for dense SnO2 NSs, located at 0.75 V, 1.25 V, and 2.0 V. These peaks are related to the dealloying reaction of LixSn to Sn, oxidation of Sn to SnO, and subsequent oxidation of SnO to SnO2,6,27 although the capacity arising from the oxidation of Sn to SnO2 is not significant, as the CV peaks corresponding to oxidation of Sn and SnO are relatively broader and smaller compared with that corresponding to de-alloying of LixSn to Sn. Such trend suggests that irreversible capacity is still present in the initial cycle, due to the partial reversibility between 9



Sn and SnO2. In the 2nd cycle, new cathodic peaks appeared at 1.1 V and 0.2 V, which was shifted to 0.8 V and 0.2 V in the 3rd cycle. Similar to the previous literature,6 these cathodic peaks were attributed to the conversion reaction of SnO2 and alloying reaction of Sn. As for hollow SnO2 NSs, most of the anodic and cathodic peaks followed similar patterns, but two small cathodic peaks existed in the voltage range of 0.01 – 0.5 V, which can be ascribed to the fact that the alloying reaction of Sn in the case of hollow SnO2 NSs more in a step-wise manner. The charge and discharge profile of dense SnO2 NSs and hollow SnO2 NSs in the formation cycle (current density: 50 mA g-1) is presented (Figure 3b), where the irreversible capacities along with initial coulombic efficiency (I.C.E) can be compared. The discharge capacity of dense SnO2 NSs and hollow SnO2 NSs is 1532.5 mAh g-1 and 1598.1 mAh g-1, where hollow SnO2 NSs exhibit slightly higher capacity. Nevertheless, the charge capacity of dense SnO2 NSs and hollow SnO2 NSs is 1013.8 mAh g-1 and 937.5 mAh g-1, which correspond to the I.C.E of 66.2 % and 58.7%. Such difference in I.C.E is attributed to more side reactions that occur in the formation cycle on hollow SnO2 NSs, attributed to their higher surface areas compared with dense SnO2 NSs. Here, it is interesting to highlight the reduced polarization of dense SnO2 NSs compared with hollow SnO2 NSs in the formation cycle. Quite unusual to the previous literature28 where it claimed that shorter Li ion diffusion length results in reduced polarization, SnO2 dense NSs show reduced polarization compared with SnO2 hollow NSs, which will be discussed in detail later. To understand the voltage profiles in later cycles, the charge and discharge profiles of dense SnO2 NSs (Figure 3c) and hollow SnO2 NSs (Figure 3d) are presented for the 2nd, 10th, 50th, and 100th cycle (at 500 mA g-1). As cycle proceeds, hollow SnO2 NSs exhibit much reduced reversible capacity compared with dense SnO2 NSs, with much different profiles. Further comparison on cycle retention 10


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characteristics (at 500 mA g-1) (Figure 3e) and rate capabilities (expressed in mA g-1) (Figure 3f) were conducted for dense SnO2 NSs and hollow SnO2 NSs. As can be exemplified in the charge and discharge profiles, dense SnO2 NSs exhibit much enhanced cycle retention characteristics compared with hollow SnO2 NSs, also with a stable C.E. Moreover, it is significant to note that the C.E. of hollow SnO2 NSs is not recovered to above 97% even after 50 cycles, which lead to the quick capacity fading of hollow SnO2 NSs. A subtle increase in capacity for dense SnO2 NSs was also observed for other metal oxides,6,29 which can be attributed to the build-up of gel-like polymeric interfacial film, leading to increase in capacities. Dense SnO2 NSs exhibit a reversible capacity of 1009.9 mAh g-1 after 300 cycles, unlike hollow SnO2 NSs which show a reversible capacity less than 50 mAh g-1. Such cycle retention characteristics of dense SnO2 NSs are comparable and/or superior to previously reported SnO2-based nanostructures (Table S1), showing its potential to be utilized as feasible electrode materials for LIBs. When cycle retention characteristics were conducted at higher current density (3000 mA g-1), however, dense SnO2 NSs also showed very unstable cycle retention characteristics (Figure S2a), and such performance is attributed to the quick structural degradation that leads to agglomeration (Figure S2b). To examine the Li storage characteristics at different current densities, rate capability tests (Figure 3f) of dense SnO2 NSs and hollow SnO2 NSs were also conducted. In all current densities (expressed in mA g1),

dense SnO2 NSs exhibit higher reversible capacity compared with hollow SnO2 NSs. The

rate retention % (the ratio of the capacity at 5000 mA g-1 to that at 100mA g-1) of dense SnO2 NSs was also higher (36.1%), compared with hollow SnO2 NSs (32.1%). This indicates that the dense SnO2 NSs exhibit relatively more improved capacity at higher current densities compared with hollow SnO2 NSs.

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To take into account the possible difference in electronic conductivity, impedance tests were carried out for dense SnO2 NSs and hollow SnO2 NSs after the 1st cycle (Figure 4a) and 200th cycle (Figure 4b). In both cases (after the 1st and 200th cycle), dense SnO2 NSs exhibit slightly higher charge transfer resistance (RCT) compared with hollow SnO2 NSs. It is attributed to the dense nanostructures that lead to the longer Li diffusion length, which results in increased resistance for Li ion transport, although the grain sizes of both dense and hollow SnO2 NSs are similar. On the other hand, hollow SnO2 NSs allow easy electrolyte uptake, which results in smaller RCT. Based on the impedance tests, it can be verified that electronic conductivity was not the major factor that resulted in the difference in cycle retention characteristics between dense and hollow SnO2 NSs. The dynamics of both dense and hollow SnO2 NSs in the initial cycle, together with their morphological evolutions upon cycling, are important aspects that need to be probed into. 3.3. In situ TEM study of dense and hollow SnO2 NSs In situ TEM analysis is a useful tool to visualize the dynamics of electrode materials upon lithiation and delithiation, in a real time scale. Especially, a great advantage that in situ TEM technique offers is the real time morphological and phase transitions of electrode materials upon initial lithiation and delithiation. As electronic conductivity was not a major factor that influenced the electrochemical performance, the dynamics of electrode materials, especially in their morphological characteristics, in the initial cycle need to be investigated. To understand the dynamics of both dense and hollow SnO2 NSs, in situ TEM analysis was carried out for lithiation and delithiation by applying a bias (Figure 5 and Figure S3). During the lithiation, dense SnO2 NSs underwent volume expansion, which can be attributed to the conversion reaction and subsequent alloying reaction with Li. Such reaction pathways can be further verified by SAED snapshots of dense SnO2 NSs in the initial lithiation, middle 12


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lithiation, and final lithiation (Figure S4), where the phase transition from SnO2 to Sn, and Sn to LixSn was clear. To further characterize the interfacial properties, TEM-energy dispersive x-ray spectroscopy (EDS) mapping was conducted to understand the elemental distribution (Figure S5). Based on TEM-EDS mapping, the presence of Sn is widespread, whereas the presence of O and F in particular is selective, and this indicates that the formation of Li2O and/or LiF took place, attributed to the decomposition of FEC and/or LiPF6 as well as the result of conversion reaction. EELS analysis (Figure S6) further confirms that the as-formed amorphous/interfacial layer is LixO, which is not only the by-product of conversion reaction (as shown in Eq. 1) but also the component of SEI layer.30-32 Similar to the dense SnO2 NSs, hollow SnO2 NSs also undergo isotropic volume changes and retain their structural integrities. During the delithiation, both the dense and hollow SnO2 NSs undergo volume extraction and form an amorphous layer on the surface, which is attributed to the excess Li2O that leads to initial irreversible capacity. Quantitative analysis on the volume changes of both dense and hollow SnO2 NSs is carried out during the lithiation (Figure 5c) and delithiation (Figure 5d). Several important characteristics need to be noted: (1) the overall morphology of SnO2 NSs is not completely returned to its original state due to the irreversible formation of Li2O. (2) Both in the lithiation and delithiation, the volume of SnO2 steadily increases and decreases and reaches a critical saturation point, where it reaches the optimal value. This can be attributed to the unusual isotropic volume change which can arise from the polycrystalline nature of both dense and hollow SnO2 NSs, where the volume expansion can be somewhat mitigated by the presence of different crystallographic planes within the nanograins. (3) The amorphous shell is formed on the surface of SnO2 NS, and by excluding the area of the amorphous shell, the overall size of the NS is comparable to the initial size of the NS. Through in situ analysis, the 13



isotropic volume change along with stable maintenance of morphology was confirmed for both dense and hollow SnO2 NSs, which can be ascribed to their polycrystalline nature. 3.4. Postmortem analyses of dense and hollow SnO2 NSs Finally, to understand the overall morphology of dense and hollow SnO2 NSs in the initial cycle and during cycling, post-mortem analysis was taken place. To investigate the seemingly different cycle retention characteristics of dense and hollow SnO2 NSs (Figure 6a), the ex situ SEM analyses were taken place after the 1st cycle (yellow box) and 200th cycle (green box). Ex situ SEM images of dense and hollow SnO2 NSs after the 1st cycle are shown (Figure 6b, 6c). Hollow SnO2 NSs maintain hollow spheres after the initial cycle, whereas dense SnO2 NSs were partially agglomerated. Interestingly, based on ex situ SEM analysis, it can be clearly seen that more pore sites exist for dense SnO2 NSs than hollow SnO2 NSs. Various macroporous sites are present between the electrode materials for dense SnO2 NSs, but for hollow SnO2 NSs they are compactly assembled without much macropores. Although hollow SnO2 NSs have shorter Li ion diffusion length in a microscopic scale, more micron-scale pores exist within the slurry casted dense SnO2 NSs, which facilitate the electrolyte uptake. As noted in the previous work,33 when the slurry casted electrode is porous macroscopically, it can alleviate the volume changes of active materials, which results in better structural stability. Upon cycling, the significant morphological transitions took place. Ex situ SEM images of dense and hollow SnO2 NSs after the 200th cycle (Figure 6d and 6e) indicate that hollow SnO2 NSs undergo complete agglomeration, whereas dense SnO2 NSs retain structural integrity without undergoing complete agglomeration. Ex situ SAED patterns (Figure S7) of dense and hollow SnO2 NSs after the 1st cycle and 200th cycle further demonstrate that the crystal structure of both dense and hollow SnO2 NSs after the 1st cycle after the 200th cycle is similar, which is amorphous. The amorphization of electrode 14


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materials after cycling is common, as observed also in previous literatures.34,35 To further delve into the particle morphology, ex situ TEM analysis was conducted for dense and hollow SnO2 NSs after cycling (Figure S8). Although dense SnO2 NSs retained their shapes even in the midst of SEI layer, hollow SnO2 NSs underwent severe pulverization and/or agglomeration, similar to the ex situ SEM analysis shown above. Based on the in situ and ex situ analyses, the overall morphological dynamics of dense and hollow SnO2 NSs can be schematically explained (Scheme 1). After the lithiation in the 1st cycle, both dense and hollow SnO2 NSs undergo isotropic volume expansion, based on the in situ TEM analysis. However, after the delithiation in the 1st cycle, dense SnO2 NSs are already partially agglomerated, based on the ex situ SEM analysis, unlike hollow SnO2 NSs that undergo minimal agglomeration. Nevertheless, significantly different morphological evolutions take place as the cycling proceeds. Since the hollow SnO2 NSs have void spaces in the core, upon constant Li insertion and de-insertion, many of the spheres undergo cracking, which are suggested to be later formed into large agglomerates. Nevertheless, the partial agglomeration into secondary particles for dense SnO2 NSs actually acts to maintain their structural integrity during the cycling in a long run, which retain excellent cycle retention characteristics, as also demonstrated previously in other works for secondary particles.36

4. Conclusions In this study, we have successfully conducted in-depth study on the unusually enhanced electrochemical performance of polycrystalline dense SnO2 NSs. Both polycrystalline hollow and dense SnO2 NSs are composed of numerous SnO2 nanograins, which lead to facile Li ion transport. In situ TEM analysis illustrates the predominantly isotropic volume expansion of SnO2, where different crystallographic planes are in contact with each other to prevent 15



particular facet-dependent growth. Upon cycling, dense SnO2 NSs are transformed into secondary SnO2 particles without undergoing severe cracking and subsequent pulverization, which resulted in superior cycle retention characteristics upon cycling. Furthermore, such dense SnO2 NSs also exhibit superior rate capabilities (349 mAh g-1 at 5.0 A g-1), which can be attributed to the maintenance of structural integrity. This work paves the milestone for utilizing suitable polycrystalline dense nanostructures as feasible electrodes for advanced LIBs, where a number of promising candidates (such as Si, Ge, P, and others) can also be applied.

ASSOCIATED CONTENT Supporting Information Available: CV analysis of hollow SnO2 NSs; Time-series snapshots of hollow SnO2 NSs in real time upon lithiation and delithiation; Stage-dependent SAED patterns; TEM-EDS mapping; EELS analysis of Li; Ex situ SAED patterns of dense and hollow SnO2 NSs after the 1st cycle and cycling. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] and [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

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This work was supported by the National Research Foundation of Korea (NRF), grant no. 2014R1A4A1003712 (BRL Program), the funding from Korea CCS R&D Center (KCRC) grant by the Korea government (Ministry of Science, ICT & Future Planning) (No. NRF2014M1A8A1049303), Wearable Platform Materials Technology Center (WMC) (NR2016R1A5A1009926), and NRF (National Research Foundation of Korea) Grant funded by Korean Government (NRF-2017H1A2A1042006-Global Ph.D. Fellowship Program), NanoMaterial Fundamental Technology Development (NRF-2018M3A7B4065625), Young Researcher Program (NRF-2018R1C1B6002624) and Korea Basic Science Institute under the R&D program (D38700) supervised by the Ministry of Science and ICT.

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Figure 1. (a) Schematic illustration on the electrochemical cell testing and in situ TEM analyses of dense and hollow SnO2 NSs for their electrochemical performances. (b) SEM image, (c) TEM image, (d) SAED pattern of dense SnO2 NSs. (e) SEM image, (f) TEM image, (g) SAED pattern of hollow SnO2 NSs.

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Figure 2. XRD patterns of (a) dense SnO2 NSs and (b) hollow SnO2 NSs. XPS spectrum (Sn) of (c) dense SnO2 NSs and (d) hollow SnO2 NSs. XPS spectrum (O) of (e) dense SnO2 NSs and (f) hollow SnO2 NSs.

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Figure 3. (a) CV curves of dense SnO2 NSs. (b) Charge and discharge profile of dense and hollow SnO2 NSs in the formation cycle (at 50 mA g-1). Charge and discharge profile of (c) dense SnO2 NSs and (d) hollow SnO2 NSs (at 500 mA g-1). (e) Cycle retention characteristics (at 500 mA g-1) and (f) rate capabilities (expressed in mA g-1) of dense and hollow SnO2 NSs.

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Figure 4. Nyquist plots of dense and hollow SnO2 NSs after the (a) 1st cycle and (b) 200th cycle.

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Figure 5. Time-series snapshots of dense SnO2 NSs during (a) lithiation and (b) delithiation. The quantitative analysis on the degree of volume changes for dense SnO2 NSs during (c) lithiation and (d) delithiation.

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Figure 6. (a) The cycle retention characteristics of dense and hollow SnO2 NSs with different stage of cycling (yellow box: initial cycle, green box: 200th cycle). Ex situ SEM images of (b) hollow SnO2 NSs and (c) dense SnO2 NSs after the 1st cycle and (d) hollow SnO2 NSs and (e) dense SnO2 NSs after the 200th cycle.

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Scheme 1. Schematic illustration on the morphological dynamics of (a) dense SnO2 NSs and (b) hollow SnO2 NSs after 1st cycle and cycling.

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Unveiling the Origin of Superior Electrochemical Performance in

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