High-Capacity Molecular Scale ConversionAnode Enabled by Hybridizing Cluster-TypeFramework of High Loading with Amino-Functionalized GrapheneJunjie Xie,† Ye Zhang,† Yanlin Han,† and Chilin Li*,†
†State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, ChineseAcademy of Sciences, 1295 Ding Xi Road, Shanghai 200050, China
*S Supporting Information
ABSTRACT: Exploring high-capacity anodes with multielectronreaction, sufficient charge/mass transfer, and suppressed volumeexpansion is highly desired. The open frameworks consisting ofindependent structure units, which possess conversion reactionpotentiality, can meet these demands and show advantages overroutine insertion-type open frameworks with at most one-electrontransfer or conversion materials with compact ligand linkage. Here,we report a class of electrochemically stable cluster-likepolyoxometalates (POMs) as such open framework anodes. Theirhigh loading and low solubility are enabled by Al- or Si-drivenpolymerization and hybridization with positively charged graphene,which immobilizes polyanions of POMs and improves their electriccontact. Al-based POM composite (NAM−EDAG) for Li-storageachieves a high reversible capacity above 1000 mAh g−1 andtolerates a long-term cycling with more than 1100 cycles and a current density up to 20 A g−1. A six-electron conversionreaction occurring at molecular scale and the consequent optimized distribution of products benefiting from original openframework are also responsible for the high electroactivity. POM-based open frameworks give inspiration for exploringadvanced, less soluble (or insoluble) framework materials made up of electroactive molecule or cluster moieties for Li- andNa-storage.
Recently, Li-ion batteries (LIBs) have displayed hugeprospect in hybrid electric vehicles (HEVs), electricvehicles (EVs) and smart grids, as a consequence of
growing public concerns about environmental issues.1 Pursuingnew, safe and low-cost materials as LIB anodes with highcapacity, rate and stable cycle performance has been becomingan important goal.2 Graphite, as the most commonlycommercial LIBs anode, has limited capacity (<300 mAh g−1)based on staging-type insertion reaction.3 Alternative anodematerials with higher capacity via alloying or conversionreaction, e.g. Si or FeOx, suffer from low conductivity andlarge volume expansion which usually results in electrodedisintegration during repeated bonding/debonding with Liatoms.4,5 To address these issues, one of the popularapproaches is to encapsulate or hybridize the nanosizedelectroactive species with conductive carbon (e.g., hollowcarbon, carbon nanotube, or graphene).6 The carbon matrix or
decoration layer not only functions as a structural buffer toeffectively accommodate volume change and extra strain, butalso serves as conductive porous network to allow favorableelectron/ion transfer. It leads to improved capacity and cyclingperformance, however, usually with trade-off of compositedensity due to extremely small grain size and loose graindistribution. Furthermore, for soluble electroactive species (e.g.,lithium polysulfides), general contact with undecorated carbonsurface does not take effect on trapping negatively charged(intermediate) products during cycling.7 Charged electroactivespecies would peel off and migrate into electrolyte, leading tomass loss, pollution, or passivation of counter electrode.Therefore, constructing a functionalized or charged carbon
Received: February 22, 2016Accepted: April 26, 2016Published: April 26, 2016
surface with the preservation of high surface area or porosity ishighly desired.Polyoxometalates (POMs) are a class of anionic polynuclear
metal oxides based on high-valent transition metals (e.g., V,Mo, W), and have been attracting considerable interests ontheir applications in catalysis, magnetism, electronics, andenergy conversion.8 In view of the characteristics of cluster-typeopen framework and multielectron redox center, POM isexpected to release high capacity and rate when lithiation.However, its electronic conductivity from molecular metal-oxostructure is low and furthermore its anion is prone to dissolvein electrolyte.8 POM is usually activated by linking toconductive carbon substrates, for example, carbon nanotubes,first explored by Awaga et al. when introducing the concept ofmolecular cluster batteries (MCBs).9 They developed hybridmaterial by dispersing [PMo12O40]
3− cluster nanoparticles assmall as its molecular size (1.5 nm) on single-wall carbonnanotubes (SWNTs). The MCB exhibited sufficient Li-storagecapacity (250−300 mAh g−1) but with quite poor Coulombicefficiency and evident capacity degradation. Afterward, Song etal. reported POMs anchored on SWNTs via π−π stacking orcovalent bonding in order to improve the affinity ofelectroactive species to conductive network, leading toimproved cycling stability.10,11 However, single- or few-molecule POM particles as small as several nanometers witha discrete distribution on the carbon surface resulted in a lowloading (∼30% in composite electrode) with compromise involume energy density of entire electrode. Recently, Sonoyamaet al. prepared nanocomposite consisting of Anderson-typePOM Na3[AlMo6O24H6] (NAM) and ketjen black (KB) byhigh-energy ball milling.12 The structure of NAM is thought tobe more stable in electrolyte than other common POMs byconnecting six MoO6 octahedra with central AlO6 octahedrathough strong covalent bonding to form MoO6−AlO6 structureunits (i.e., Al-driven polymerization). However, its capacityperformance (∼400 mAh g−1) was activated only whenremarkably increasing KB amount even close to 50 wt % ofcomposite.
In order to improve the mechanical strength of POM-basedcomposite, suppress its solubility, and enhance its volumeenergy density, in this work, we report a rational design, that is,hybridizing larger-sized flake-shaped NAM with two-dimen-sional (2D) reduced graphene oxide (RGO), which is capableof adsorbing soluble POM-anions during cycling after amino-functionalization. The immobilization of POM is achieved by asimple electrostatic interaction between positively chargedamino groups and negatively charged polyanions of NAM,inspired by that between ethylenediamine (EDA) and lithiumpolysulfides to improve Li−S battery performance.13 Comparedwith quite small nanodots of P-based POM (e.g.,[PMo12O40]
3−),9 our composite has a much higher content ofNAM (∼70 wt %), which is the highest loading amount in allreported POM-C composites. This NAM−EDAG (whereEDAG denoting EDA-functionalized RGO) composite materialenables dense and solid contact between components,improving the charge transfer of NAM at interface.8 It canserve as a molecular scale conversion anode for LIBs.Compared with common conversion oxides with compactcoordination, cluster-type open framework consisting ofisolated MoO6-based clusters with sufficient arrangementfreedom benefits to charge/mass transport. Our POM-basedanode can achieve an extremely high reversible capacity (>1000mAh g−1 even based on the composite weight) as well asexcellent cycling (>1000 cycles) and rate performance.
RESULTS AND DISCUSSIONAs shown in Scheme 1, two facile steps are involved tosynthesize NAM−EDAG composite. First, amino moieties areimmobilized onto graphene oxide (GO) by the EDA-mediatednucleophilic ring-opening reaction of epoxy groups on thesurface of GO;14 meanwhile, GO is reduced to RGO as aconsequence of the removal of oxygen-contained groups onGO. The EDA functionalized RGO is denoted as “EDAG” inthe following discussion. The d-spacing is decreased from 0.72nm for GO to 0.39 nm for EDAG according to their X-raydiffraction patterns (XRD, Figure S1). This value for EDAG is
Scheme 1. Preparation Procedure of Composite Consisting of Polyoxometalate and Ethylenediamine-Decorated ReducedGraphene Oxide through Electrostatic Adsorptiona
a(1) Amino groups are first introduced onto GO nanosheets with simultaneous deoxygenation into RGO via a hydrothermal process; (2) Anderson-type NAM (for simplification only the basic units are present) is then attached on EDAG owing to electrostatic interaction between negativelycharged molecular cluster ion [AlMo6O24H6]
3− and positively charged (protonated) amino groups.
comparable with that of graphite (0.34 nm), confirming asubstantial deoxygenation or reduction from GO.15 Thefunctionalization of EDA is indicated from the X-ray photo-electron spectra (XPS, Figure S2) which show remarkable N 1ssignal consisting of pyrrolic N (399.7 eV) and N (401.5 eV) inNH2 or NH3
+, as well as CN signal (285.8 eV) in C1s.13,14 The concomitant deoxygenation process is furtherconfirmed by significant weakening of intensity of XPS peaksassociated with carbon−oxygen bonds. Then as-synthesizedNAM is added into and strongly coupled with EDAG solutionto form NAM−EDAG composite in view of the electrostaticinteraction between negatively charged molecular cluster ion([AlMo6O24H6]
3−) of NAM and positively charged (proto-nated) amino groups of EDAG. Detailed synthetic proceduresare shown in Experimental Section.The crystalline structure of our NAM is similar to a previous
report on dried sample by Sonoyama et al. as observed from thealmost same pronounced XRD peaks (Figure 1a).12 There is alittle deviation on peak intensity and position between NAM−EDAG and NAM, indicating the structural flexibility of cluster-like NAM framework, which is influenced, for example, byelectrostatic interaction with EDAG. The XRD deviation maybe also caused by loss of Na (likely serving as structurepromoter) so as to cause rearrangement of MoO6AlO6octahedra moieties in NAM−EDAG as discussed in theirXPS later. The NAM content in NAM−EDAG composite isestimated to be 67.5 wt % from thermogravimetric analysis(TGA, Figure 1b). Fourier-transform infrared spectroscopy(FTIR) was employed to elucidate the electrostatic adsorptionbetween NAM and EDAG (Figure 1c and d). The absorptionpeaks of EDAG at 1564 and 3300 cm−1 are assigned to
antisymmetric CN and NH stretching vibrations,respectively, indicating covalent linkage of amino moieties toRGO.13,16 The absorption peak of NAM at 964 cm−1 associatedwith double bond linking Mo and terminal O atom (MoOt)shifts down to 942 cm−1 in NAM−EDAG.17 Such an obviousbathochromic shift could be ascribed to hydrogen bondbetween MoOt and NH2 or NH3
+ on EDAG. The similarshift is also observed for the peak assigned to bridged oxygen oftwo octahedra sharing an edge (MoOeMo) from 792 to775 cm−1. Note that the peak denoting bridged oxygen sharinga corner (MoObMo) at 870 cm−1 almost disappears inNAM−EDAG, implying more remarkable isolation of MoO6AlO6 units due to electrostatic repulsion after Na extraction.Raman spectra show that the intensity ratio of D and G peaks(ID/IG) for EDAG (1.09) is similar to that for NAM−EDAG(1.07) (Figure S3), indicating that electrostatic interaction inNAM−EDAG does not significantly influence the graphitiza-tion degree of EDAG.XPS was used to compare the elemental composition and
functional group among NAM−EDAG, NAM, and EDAG(Figure 2). Note that pristine NAM presents a remarkable Na1s signal at 1072 eV, whereas the Na content of NAM−EDAGsignificantly decreases to an undetected extent. It is associatedwith the difference of precipitation processes for NAM−EDAGand NAM. The latter is precipitated out because of over-saturation when evaporating the clear solution containing NAMprecursors.12 However, in the case of NAM−EDAG, thepolyanion of NAM is precipitated by adsorption to positivelycharged EDAG, likely leaving Na+ in solution. We found thatthe precipitation could not be obtained if adding NAM to GOsolution after stirring overnight because NAM polyanions
Figure 1. (a) XRD patterns of dried NAM and NAM−EDAG composite, (b) TGA results of NAM−EDAG, NAM, and EDAG with weight lossestimated based on the mass at 700 °C as shown by vertical dotted line, (c) FT-IR spectra of NAM−EDAG, NAM, and EDAG, and (d)magnified FT-IR spectra of NAM−EDAG and NAM in a wavelength range of 600−1200 cm−1. Charge transfer or loss of Na filler should beresponsible for XRD deviation of NAM−EDAG from NAM. FTIR peaks assigned to MoOt and MoOeMo undergo a visiblebathochromic shift. FTIR peak assigned to MoObMo disappears in NAM−EDAG.
cannot be steady anchored on GO in view of electrostaticrepulsion between negatively charged oxygen-containinggroups and polyanions. This result emphasizes on the effectof amino groups of EDAG on promoting precipitation andloading content. In view of N 1s peak partially overlapping withMo 3p signal (Figure S4), it is hard to detect the exact variationof binding energy of N 1s when loading with NAM. We deducethat the bonding situation with N is not influenced afterhybridizing from the similar peaks assigned to CN at 285.8eV (Figure 2b). Mo 3d spectra disclose that Mo is at its highestvalence of +6 in NAM, whereas is partially reduced to Mo4+
(Mo 3d5/2 at 230 eV) in NAM−EDAG (Figure 2c).18 Chargetransfer induced by amino group grafting is responsible for thereduction of Mo. From the XPS result, we can also calculate theratio of electroactive species in NAM−EDAG, and the value isin good agreement with that by TGA.Scanning electron microscopy (SEM) in Figure 3a reveals
that the NAM−EDAG hybrid presents wrinkled sheet-likestructure with numerous NAM flakes covering on EDAGsurface. The shape of NAM in NAM−EDAG is almostunchanged compared with the pristine NAM (Figure S5)with several micrometer in width and 200−300 nm inthickness. The transmission electron microscopy (TEM)image (Figure 3b) more clearly displays that NAM flakes areattached on EDAG nanosheets, which have a thickness ofaround 5 nm as observed from the nanosheet edge in highresolution TEM (HRTEM, Figure 3c). Figure 3d magnifies thecontact edge between NAM and EDAG and exhibits thatEDAG nanosheet closely binds to a single NAM crystallinegrain. The electrostatic effect of NAM−EDAG composite may
offer an excellent interfacial contact, which is highly beneficialfor electron transfer. Furthermore, it is found that some bulkNAM grains are enclosed by outer EDAG sheets from thescanning transmission electron microscopy (STEM) images(Figure 3e and f). The NAM grains with dark contrast areclearly seen in the bright field image (Figure 3f). This desiredspatial distribution is confirmed by element mapping analysisthrough energy dispersive X-ray spectroscopy on the sameregion (Figure 3g−k), where Mo, Al, and O elements mainlyexist in the flake-sized grains and C and N elements are morehomogeneously mapped in the entire nanosheet with lightcontrast in both the dark- and bright-field images. The presenceof N signal also confirms the effective functionalization of EDAon RGO. Additionally, we utilized the same strategy to prepareother POM−EDAG composite, for example, based onphosphato-molybdic acid (H3PMo12O40, HPM) and sodium12-molybdosilicate (Na4SiMo12O40, NSM), where PO andSiO polyhedra are utilized to bonding surrounding MoO6octahedra so as to form their characteristic structure units.9,19
The structural modification between HPM (or NSM) and itscomposite with EDAG is also observed from the discrepancy oftheir XRD patterns (Figure S6). Pristine NSM is made up ofstick-like grains with several micrometer in length and 300−400nm in width (Figure S7). This unique morphology is preservedwhen hybridizing with EDAG. Detailed preparation proceduresof these P- and Si-based POMs and their composites withEDAG are described in the Experimental Section.The NAM−EDAG composite as LIB anode displays better
capacity performance than the hand-milled mixture of NAMand EDAG (designated as NAM+EDAG, which contains the
Figure 2. (a) Overview XPS spectra of NAM, EDAG and NAM−EDAG. Gaussian fitting and deconvoluted peak assignment of (b) C 1s and(c) Mo 3d spectra of NAM, EDAG, and NAM−EDAG. The Na 1s signal at 1072 eV almost disappears in NAM−EDAG. Similar C 1s spectraare observed for EDAG and NAM−EDAG, both indicating CN bonding at 285.8 eV. There is a partial reduction to Mo4+ in NAM−EDAG,likely caused by charge transfer after amino group grafting.
same NAM content 67.25 wt %) as well as pure NAM withsimilar discharge/charge profiles in a voltage range of 0−3.0 V(Figure 4a). The specific capacity of composites is calculatedbased on the total mass of NAM and EDAG, and the capacity isbased on pure NAM when EDAG is absent. In other words, theuniform loading of NAM into EDAG framework significantlyimproves both the electroactivity and voltage polarization ofEDAG. The initial discharge and charge capacities of NAM−EDAG are 1835 and 1145 mAh g−1, respectively, at 100 mAg−1, corresponding to a Coulombic efficiency (CE) of 63%(Figure 4b). The extra discharge capacity may be attributed tothe formation of solid electrolyte interface (SEI) film asindicated by XPS of cycled samples discussed later. Thefollowing capacity is quickly stabilized after few cycles. As acomparison, NAM+EDAG delivers both the lower initialdischarge and charge capacities (1296 mAh g−1 and 870 mAh
g−1) at the same current density (Figure S8c). Its followingcapacity is difficult to be stabilized. The pure NAM and EDAGrelease much less reversible capacities (around or less than 400mAh g−1, Figure S8a and b). The cyclic voltammetry (CV)curves of NAM−EDAG at a scan rate of 0.05 mV s−1 are inaccordance with its charge/discharge profiles (Figure S9).NAM−EDAG also displays superior cycling and rate
performance (Figure 4c−e). The reversible capacity ofNAM−EDAG is preserved at as high as 1000 mAh g−1 for atleast 100 cycles with high Coulombic efficiency (∼100%) andgood capacity retention, whereas that of NAM+EDAGdecreases from 800 to 600 mAh g−1 within 100 cycles. Atmuch higher current densities of 1, 5, and 10 A g−1 NAM−EDAG still shows good capacity retention with a reversiblecapacity of 800, 600, and 400 mAh g−1 respectively. Anextraordinary long-term cycling is achievable for NAM−EDAG,which shows a reversible capacity of 400 mAh g−1 after 200cycles, of 300 mAh g−1 after 600 cycles, and of 240 mAh g−1
after 1000 cycles at a high current density of 5.0 A g−1. Thetotal duration (time) of cycling is about 145 h under this fastcycling rate (from the 6th to 1100th cycles). That theelectrochemical performance of NAM−EDAG composite issuperior to its single components as well as their mixture can beascribed to electrostatic adsorption to suppress the dissolutionof redox species, the charge transfer to enhance itsconductivity/electroactivity. Loss of Na is also anotherpotential factor for higher capacity of NAM−EDAG thanNAM+EDAG. Such a hybridization route is beneficial to theelectroactivity upgrade of HPM and NSM as well (Figure S10and S11). NSM and its composite with EDAG displaycomparable electrochemical properties as those of NAM-based electrodes. Hybridizing of NSM with EDAG leads to adoubled capacity than pure NSM (from ∼400 mAh g−1 forNSM to ∼800 mAh g−1 for NSM−EDAG at 100 mA g−1).There is even a 5-fold increase in capacity after HPM attachingto EDAG adsorbent (from ∼100 mAh g−1 for HPM to ∼600mAh g−1 for HPM−EDAG at 100 mA g−1). This moreremarkable performance upgrade indicates a higher solubilityfor pure P-based POM, which should be also responsible forthe availability of extremely tiny particles even in molecularsize.9,11 Al- or Si-driven polymerization yields more stablePOMs with less solubility and larger size in one or twodimensions, which can serve as better anodes with high loading.The XPS spectra of Mo 3d at different cycling stages
demonstrate that NAM can be electrochemically reduced toMo0 metal phase (Mo 3d5/2 at 226 eV) after deep dischargingto 0 V as shown in Figure 5.18 After recharging to 3 V, Mo isreoxidized to Mo6+. This result indicates that POM can serve asa conversion anode with 6-electron transfer. Its cluster-likeframework can promote Li-ion transfer between molecule-sizedunits and improve the conversion efficiency. Therefore, POMenables a conversion reaction with much higher capacity thannanostructured MoO3 with more compact MoO6 linkage. Forinstance, ball-milled MoO3/C nanocomposite (2−180 nm)delivered a moderate reversible capacity of 700 mAh g−1 onlyunder a small current density (0.2 C) and low loading (50 wt%) of active species;20 MoO3−x nanowires (90 nm in diameter)maintained a moderate capacity of 630 mAh g−1 at 50 mA g−1
for limited 20 cycles.21 Note that the actual capacity exceeds thetheoretical value (908 mAh g−1) of NAM. Therefore, theinterface mass storage or the reversible formation/decom-position of SEI cannot be ruled out.22,23 The latter is alsoimplied from the reversible intensity evolution of Mo 3d and Al
Figure 3. (a) SEM and (b) TEM images of NAM−EDAG, (c, d)enlarged HRTEM images of the selected areas marked in (b). (e)High-angle annular dark-field and (f) bright-field STEM images ofNAM−EDAG; EDX mapping images of (g) C, (h) N, (i) O, (j) Moand (k) Al elements in the area of (f). NAM flakes are found toattach to EDAG nanosheets, which are about 5 nm in thickness.NAM crystalline grain can also be closely coated by EDAG layer.The presence of N mapping signal confirms an effectivefunctionalization of EDA on RGO.
2p spectra (Figure S12). The formation of SEI is responsiblefor the intensity weakening of XPS signals at discharge state.Conversion reaction mechanism of POM is further clarified
by TEM of cycled samples (Figure 6). When NAM−EDAG isdischarged to 0.1 V, two sets of product distribution scenarioare found and characterized by different sized particleshomogeneously embedded in amorphous matrix. The particlesare either in cluster size (Figure 6a) or in nanosize (<50 nm formost, Figure 6b), likely depending on local current densitypassing through the corresponding region. These particles arequite unstable under intensive e-beam irradiation because thecluster-sized ones would disappear and the lattice fringes ofnanosized ones would become vague in HRTEM measurement(Figure 6c). The nanoparticles of single-crystal feature can beassigned to metal Mo from selected area electron diffraction(SAED, Figure 6d). The cluster-magnitude particles can also bededuced to be metal Mo, evolving from the original openframework consisting isolated cluster units. Larger local currentlikely promotes the merging of neighbor Mo clusters andcrystallites to grow into bigger monocrystal-Mo nanoparticles.
The amorphous matrix should be mainly made up of Li2O andAlOx, which can restrict further coarsing of Mo particles. Aftercharging to 3 V, Mo is electrochemically oxidized to numerousmonoclinic MoO3 nanoparticles of about 5 nm rather thanoriginal POM framework (Figure 6e), which are also indicatedfrom the corresponding SAED pattern denoting MoO3
polycrystal rings (Figure 6f). Note that SEI-like film atdischarged grain surface is thicker than that at charge state(Figure 6c and e), agreeing with the XPS result.For POM structure, its small cluster size is favorable for
cation exchange during conversion reaction. The similar effectwas also indicated in the case of ultrafine “quantum dot” FeS2nanoparticles as conversion anode.24 From the observation ofaggregation of Mo crystallites after discharge, the cluster size ofPOM may be comparable to or smaller than the diffusionlength of Mo. The proximity to RGO could increase chargetransfer and local current, possibly further promoting themerging of Mo crystallites. These may be responsible for thegeneration of moderate-sized nanoparticles as conversionproducts in some regions (Figure 6b). Note that the bigger
Figure 4. Galvanostatic charge−discharge curves of (a) NAM−EDAG, NAM+EDAG, NAM, and EDAG as LIB anodes during the 10th cycleand (b) NAM−EDAG anodes during the first ten cycles at 100 mA g−1 in a voltage range of 0−3 V. (c) Charge/discharge capacity andCoulombic efficiency of corresponding anodes as a function of cycle number at 100 mA g−1. (d) Rate performance of NAM−EDAG from 100mA g−1 to 20 A g−1. (e) Cycling performance of NAM−EDAG at 5.0 A g−1 after a few cycles at 100 mA g−1. NAM−EDAG shows much bettercapacity performance than NAM+EDAG, NAM, and EDAG, and also displays superior cycling and rate performance (e.g., more than 1000cycles at 5 A g−1).
Mo monocrystal particles are rather reoxidized to MoO3 withpolycrystal nanodomains rather than coarser monocrystals aftercharge (Figure 6e). The lacking of size memory effect appearsto be favorable for excellent rate performance.Because individual NAMs are precipitated and adsorbed to
well-dispersed EDAG frameworks during preparing NAM−EDAG, they have better electric contact with conductivesupporter than simply mixed NAM+EDAG. Larger-sizedPOMs with high loading cannot guarantee the insertion of
every POM into interlayer of EDAG or conformal coating onEDAG as done by tiny POM nanoparticles.9,11 Even if someelectroactive species are dissolved out of the larger-sized grainsduring cycling, they are still able to be easily seized bysurrounding positively charged graphene frameworks beforeescaping into electrolyte. The unique NAM open frameworkconsists of nearly independent MoO6−AlO6 building blocks,and enables favorable Li+/e propagation among these molecule-scale electroactive units. Charge transfer is likely acceleratedwhen amorphizing NAM after milling or cycling theelectrode.12 Flexibility or slidability of POM structural moietiesis responsible for the accessible amorphization. These explain ahighly stable electrochemical performance with high electro-activity. Relatively low electroactivity of pure POM is ascribedto original loss of soluble active material and/or poor electriccontact with Super-P additive in electrode. Electrochemicalproducts during cycling appear to be less soluble than pristinePOMs in electrolyte as observed from the good reversibilityduring the following cycles, in view of potentially irreversibledegradation of clusters into some other metal oxides (e.g.,MoOx, AlOy) or metal phase (e.g., Mo). Note that the initialopen framework properties benefit to an optimized distributionof these oxide or metal products in nanoscale or still inmolecule-scale (Figure 6). The presence of graphene networksand inactive Al (or Si, P)-based domains inhibits a coarsing ofelectrochemical products. For pure POM samples, a completedissolution in electrolyte cannot be achieved due to over-saturation even in the case of most soluble P-based POM,where a lower reversible capacity around 100 mAh/g is stillobservable (Figure S11). Note that the POMs have also beenattempted as moderate-voltage (2 V) cathodes.12 However, inour case a substantial capacity of NAM−EDAG in this voltage
Figure 5. Mo 3d XPS spectra of cycled NAM−EDAG electrodesafter 1st discharge to 0 V and 1st recharge to 3 V, as well as pristineNAM−EDAG and NAM. NAM in NAM−EDAG composite can beelectrochemically reduced to Mo0 (Mo 3d5/2 at 226 eV) after deepdischarge to 0 V. Mo is reoxidized to Mo6+ after recharging to 3 V.It indicates POM serving as a conversion anode with six-electrontransfer. The formation and decomposition of SEI are responsiblefor the intensity weakening and restrengthening of XPS signals atdischarge and recharge states, respectively.
Figure 6. (a,b) TEM images of discharged NAM−EDAG sample (to 0.1 V) in different regions, (c) HRTEM image and (d) SAED pattern ofthe corresponding particle in (b). (e) HRTEM image and (f) SAED pattern of charged NAM−EDAG sample (to 3 V). The particles either incluster size or in nanosize are homogeneously embedded in amorphous matrix in the discharged sample. The nanoparticles are assigned tomonocrystal Mo. In the charged sample, numerous MoO3 nanodomains of about 5 nm is observed. The SEI-like film at discharged grainsurface is thicker than that at charge state.
region cannot be observed, whereas the corresponding capacityfor NAM is evident during the first discharge (Figure S8a). Thisphenomenon is in accordance with a prior reduction of Mo6+ toMo4+ when hybridizing with graphene (Figure 2c), whichtherefore inhibits the occurrence of remarkable electrochemicalreduction at ∼2 V. The following lithiation at moderate voltageis also inactivated, agreeing with previous reports on NAM orMoO3.
12,20,21 The absence of substantial capacity at moderatevoltage is beneficial for POM-based composites as anode. Onecan find that the conversion process observed is near ∼1.5 V vsLi/Li+. Although the reaction voltage is somewhat high for ananode in full cell configuration, it is expected to enable a goodsuppression of lithium dendrite growth. This voltage region issimilar to that (1.5 V) of well-known Li4Ti5O12 anode;however, the capacity of NAM−EDAG is about one magnitudehigher than the latter.25 The loss of anode voltage can becompensated by resorting to high-voltage cathode (e.g.,LiNi0.5Mn1.5O4).We also attempted to use NAM−EDAG as conversion
anodes for Na-ion batteries (NIBs, Figure 7). Similar to thecase of Li-storage, NAM presents lower voltage polarizationthan EDAG during (de)sodiation. Hybridizing with EDAG isexpected to enable both the improved Na-storage capacity andcycling performance for NAM. The reversible capacities ofNAM−EDAG and EDAG are ∼150 mAh g−1 for at least 200cycles and less than 50 mAh g−1 after 150 cycles, respectively.In other words, the reversible Na-storage capacity based onNAM can reach to 200 mAh g−1 after at least 200 cycles.However, this value is still far lower than that for Li-storage.This unexpected result is likely associated with the formation ofNa2O passivation layer at the early stage, which blocks furthersodiation and leads to sluggish kinetics.26 Product micro-structure and intrinsic conductivity appear to be more
important than electroactive species size in Na-drivenconversion reaction.27
CONCLUSIONIn summary, we report a class of hybrids consisting of cluster-like POM and EDA-decorated RGO as high-capacity and long-life LIB anodes. Their electroactive upgrade is on the basis ofconstructing positively charged RGO surface to immobilizepolyanions of POMs with high loading. This electrostatic effectenables the suppressing of POM solubility and enhancement ofinterfacial charge transfer during cycling. For instance, NAM-EDGA releases a reversible capacity above 1000 mAh g−1 andtolerates a current density up to 20 A g−1. Such a compositeelectrode can last more than 1100 cycles. Six-electronconversion reaction characterized by favorable mass transferbetween isolated molecule-scale structure moieties is mainlyresponsible for the extraordinary energy storage performance aslong as their solubility and conductivity are addressed. POM-based open frameworks demonstrate advantages over commonconversion oxides with similar structural moieties but withmore compact linkage. These results inspire us to exploreadvanced, less soluble (or insoluble) framework materialsconsisting of electroactive molecule or cluster units for Li- andNa-storage.
EXPERIMENTAL SECTIONPreparation of NAM. The synthetic procedure was performed
based on previous report by Sonoyama et al.12 In a typical synthesis,0.85 g of AlCl3 was gradually dissolved into 25 of mL deionized waterunder stirring at room temperature. Next, 3.5 g of Na2MoO4·2H2Owas added to the above solution. Then, the pH value of solution wasadjusted to 1.80 by dropwise adding concentrated hydrochloric acid(35% HCl) under stirring to form clear solution. This solution washeated at 40 °C to slowly evaporate solvent under stirring. Theresultant white precipitate was collected through filtration and washed
Figure 7. Galvanostatic charge−discharge curves of (a) EDAG and (c) NAM−EDAG as NIB anodes during the first five cycles at 100 mA g−1
in a voltage range of 0−3 V. Charge/discharge capacity and Coulombic efficiency of (b) EDAG and (d) NAM−EDAG as a function of cyclenumber at 100 mA g−1. Hybridizing with EDAG is expected to improve both the Na-storage capacity and cycling performance of NAM (e.g., ∼150 mAh g−1 for at least 200 cycles based on NAM−EDAG).
by a mixture of acetone and deionized water (2:1, v/v) followed bypure acetone. The obtained white powder was dried at 80 °C invacuum for 1 day.Preparation of EDAG. GO was chemically exfoliated from natural
graphite flakes according to a modified Hummer’s method reportedelsewhere.28 The EDAG was synthesized by heating the mixture ofethylenediamine (EDA, 120 μL) and GO dispersion (90 mL, 1 mgmL−1) at 75 °C for 6 h in a 100 mL Teflon-lined autoclave. Aftercooling to room temperature, the product was collected by filteringand washing with deionized water, which was followed by freeze-drying.Preparation of NAM−EDAG. A total of 30 mg of EDAG was
dispersed into 60 mL of deionized water via ultrasonic dispersion for 1h. Then, the pH value of EDAG-dispersed solution was adjusted to 7.0by adding 0.1 M HCl solution. A mixture of 150 mg of NAM and 30mL of deionized water was added to the above EDAG dispersionunder vigorous stirring at room temperature and then wascontinuously stirred for 1 day. The product was collected by filtrationand washed by deionized water for 2−3 times. The obtained productwas NAM−EDAG, which was dried by way of freeze-drying.Preparation of NAM+EDAG. As a comparison with NAM−
EDAG by electrostatic adsorption, NAM+EDAG was prepared byphysical grinding. A mixture of 35 mg of EDAG and 65 mg of driedNAM were added into a mortar and then milled for 2 h by hand.Preparation of HPM−EDAG. A total of 30 mg of EDAG was
dispersed into 60 mL of deionized water via ultrasonic dispersion for 1h. Then, the pH value of EDAG-dispersed solution was adjusted to 7.0by adding 0.1 M HCl solution. A mixture of 150 mg ofmolybdo(VI)phosphoric acid hydrate (TCI Reagent Co.) and 30mL of deionized water was added to the above EDAG dispersionunder vigorous stirring at room temperature and then wascontinuously stirred for 1 day. The product was collected by filtrationand washed by deionized water for 2−3 times. The obtained productwas HPM−EDAG, which was dried through freeze-drying.Preparation of NSM. Sodium 12-molybdosilicate (NSM) was
synthesized according to a reported method elsewhere.19 First, 2.0 g ofNaOH (0.100 mol) was added to 100 mL of deionized water, andstirring continued until all solids had dissolved. Then, 7.10 g ofNa2SiO3·9H2O (0.0250 mol, Shanghai Sinopharm Chemical ReagentCo.) was added into the above NaOH solution, and stirring continueduntil all solids had dissolved. Afterward, 43.2 g of MoO3 (0.30 mol,Alfa Aesar Reagent Co.) was added with continuous stirring thesolution, which was brought to a boil in 15 min. Boiling to 100 °C wascontinued for 2 h. The green color disappeared by adding a few dropsof bromine water. After cooling, the desired salt was obtained byfiltration and washed with deionized water for 2−3 times. The whitepowder was dried at 80 °C in vacuum for 1 day.Preparation of NSM−EDAG. The synthetic procedure is same as
that for NAM−EDAG.Physical Characterization. The morphology and component of
the (cycled) samples were observed through transmission electronmicroscope (TEM, JEOL JSM-6700F, operated at 200 kV) as well asscanning electron microscope (SEM, Magellan 400L, FEI), which wasalso used for scanning TEM (STEM) element mapping. The structureand crystallinity of the samples were analyzed by a X-ray diffractometer(XRD, D8 Discover, Bruker) in a 2θ range of 10°∼80° at a scanningrate of 1.0°/min using Cu Kα radiation. Raman spectroscope wasrecorded on DXR Raman Microscope (Thermal Scientific Corpo-ration) with 532 nm excitation length. X-ray photoelectron spectros-copy measurements (XPS, ESCAlab-250) with an Al anode sourcewere performed to characterize surface components and bondings ofpristine and cycled electrodes. Thermogravimetric analysis (TGA) wasacquired by using a DSC 800 from PerkinElmer under N2 flowing, andthe heating rate was 10 °C/min. Fourier-transform infrared (FT-IR)spectra were collected with Bruker Tensor 27 in a wavelength range of400−4000 cm−1, and the pellets for FT-IR measurement wereprepared by grinding and pressing samples with KBr powder.Electrochemical Characterizations. Two-electrode coin-2025
cells were assembled with POM-based composite, single POM, orEDAG samples as working electrode and lithium foil (or sodium foil)
as counter electrode. To prepare working electrode, a mixture ofPOM−EDAG (or POM, EDAG, POM+EDAG), Super-P carbon andpoly(vinylidene fluoride) (PVDF) with a weight ratio of 8:1:1 waspasted on pure Cu foil and followed by drying in vacuum at 80 °C for20 h. The loading mass of electroactive materials in electrode slurry is∼2 mg cm−2. Glass fiber (GF/B) from Whatman was employed as theseparator. The electrolyte involves 1 M LiPF6 in a nonaqueous mixtureof ethylene carbonate (EC) and diethyl carbonate (DEC) with avolume ratio of 1:1 (ShenZhen Industries Ltd.). For Na-ion batteries,the electrolyte was composed by dissolving 1 M NaClO4 in anonaqueous mixture of EC, propylene carbonate (PC) and fluoro-ethylene carbonate (FEC) with a weight ratio of 9.5:9.5:1. The cellswere assembled in an Ar-filled glovebox (<0.1 ppm of water andoxygen). Cyclic voltammetry (CV) was carried out on an AutolabPG302N electrochemical workstation at a scan rate of 0.5 mV s−1.Galvanostatic charge−discharge measurements of electrode materialswere performed at different rates from 100 mA g−1 to 20 A g−1 using aLAND-CT2001A test system at room temperature.
ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsnano.6b01321.
XRD and XPS of GO and EDAG, Raman spectra ofEDAG and NAM−EDAG, N 1s and Mo 3p XPS ofNAM and NAM−EDAG, SEM of NAM, XRD of HPM−EDAG and HPM, XRD and SEM of NSM−EDAG andNSM, STEM and EDX mapping of NSM−EDAG,charge−discharge curves of NAM, EDAG and NAM+EDAG, CV curves of NAM−EDAG, charge−dischargecurves and cycling performance of NSM, NSM−EDAG,HPM and HPM−EDAG, Al 2p XPS of cycled NAM−EDAG electrodes. (PDF)
ACKNOWLEDGMENTSThis work was supported by the National Natural ScienceFoundation of China under Grant No. 51372263, by the ChinaPostdoctoral Science Foundation under Grant No.2014M561527 and by the Key Research Program of ChineseAcademy of Sciences under Grant No. KGZD-EW-T06. C.L.L.would like to thank the supports from the “Hundred Talents”program of the Chinese Academy of Sciences and the ScienceFoundation for Young Researchers of State Key Laboratory ofHigh Performance Ceramics and Superfine Microstructures.The authors thank Prof. M. H. Yang for helpful discussion.
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