with Reduced Graphene Oxide as Anode Materials for Lithium Ion

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Composites of Layered M(HPO4)2 (M = Zr, Sn, and Ti) with Reduced Graphene Oxide as Anode Materials for Lithium Ion Batteries Mei Ma, Shouwu Guo, and Wenzhuo Shen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16797 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Applied Materials & Interfaces

Composites of Layered M(HPO4)2 (M = Zr, Sn, and Ti) with Reduced Graphene Oxide as Anode Materials for Lithium Ion Batteries Mei Ma, Shouwu Guo*, and Wenzhuo Shen* Department of Electronic Engineering, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China

Correspondence

should

be

addressed

to:

[email protected]

or

[email protected]

Keywords: Composite; Metal phosphates; Reduced graphene oxide; Electrochemical property; Anode Abstract Tetravalent metal phosphates (M(HPO4)2, M=Zr, Sn, and Ti) have robust layered structure with the interlayer d-spacing over 7.5 Å, but show poor electric conductivity. On the other hand, single-atomic-layered reduced graphene oxide (rGO) sheets exhibit a high electric conductivity. In this work, the combination of rGO and M(HPO4)2 is explored for their potential as anode materials for lithium ion batteries (LIBs).

Specifically,

rGO/M(HPO4)2 composites

are

prepared,

and

their

electrochemical performances are investigated systematically. In comparison with bare M(HPO4)2, the composites rGO/M(HPO4)2 not only exhibit larger specific capacity, higher rate capability, better cyclic stability, lower voltage for lithium ions insertion and extraction, but also show the improved first coulombic efficiency. We 1

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propose that the super electrochemical performances of the composites are contributed primarily by the large interlayer space of M(HPO4)2, and the rGO sheets cladded on the surfaces of layered M(HPO4)2. The attached rGO sheets bridge the layers together forming a network that is beneficial for the electron and ions diffusion within the composites, thus enhance discharge/charge rate capability of the composites. In addition, the attached rGO sheets provide extra anchoring sites for Li+, the specific capacity of the composites as anode materials is thus enhanced.

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1. Introduction Lithium ion batteries (LIBs) have been widely used in portable electronic devices, electric vehicles and stationary electricity storage systems,1-3 however, the electrochemical properties including specific capacity, rate capability, cycling stability, and safety of most conventional LIBs remain to be improved.4-6 In principle, those properties are

determined by cathode, anode, electrolyte, and separating

membrane,7-10 in which the cathode and anode materials are particularly crucial. So far, for anode materials, graphite, graphitized carbonaceous materials, and lithium titanate have been commercialized.11-14 But, the LIBs made of these commercialized anode materials are still far away from satisfaction, especially, for the LIBs that used in electric/hybrid vehicles and other devices that require high power and energy densities.15 Hence, the exploration of novel anode materials with high power and energy densities for LIBs has attracted substantial research and industrial interest during last decades.16,17 So far, nanosized metal oxides, carbonaceous nanomaterials, silicon and silicon based alloys, and metallic materials have been explored as potential candidates for anode materials.18-23 It is well known that graphite, the most commonly used anode material, has a unique layered structural motif in which the interlayer space plays a key role in the lithium ion/atom storage and transportation during the charge-discharge of LIBs.24 Similar to the graphite, tetravalent metal phosphates assume the layered structures as well, and the interlayer d-spacings of most tetravalent metal phosphates are over 7.5 Å, much larger than that of the graphite (3.4 Å).25,26 In fact, owing to their layered 3



structural feature, the tetravalent metal phosphates have been extensively utilized for ion exchange, proton transportation, catalyst supporter, and many others.27-30 Predictably, the large interlayer spacing of tetravalent metal phosphates would also allow efficient lithium ion/atom transportation and storage, and might be employed as anode materials for LIBs. Additionally, low production cost and easy preparation of tetravalent metal phosphates will definitely be beneficial for their practical applications. However, if used as anode materials, the poor electric conductivity of tetravalent metal phosphates may limit their charge-discharge rate capability.31 Recently, it was reported that the electrochemical performances of the electrode materials in LIBs could be improved when they were coated with graphene or its derivatives that have pronounced electric conductivity.16,32-37 Hence, in the work, composites of M(HPO4)2 (M= Zr, Sn, Ti) with reduced graphene oxide (rGO) are prepared and their electrochemical performances as anodes for LIBs are evaluated on coin cells. As will be shown, the as-prepared rGO/M(HPO4)2 composites exhibit higher capacity, excellent rate capability and cyclic stability, and the improved first coulombic efficiency compared to that of the corresponding M(HPO4)2 (M= Zr, Sn, Ti), reduced GO, and commercialized mesocarbon microbeads (MCMB), indicating that the rGO/M(HPO4)2 (M= Zr, Sn, Ti) composites are a novel category of anode materials for LIBs that are worth to pursue. 2. Experimental 2.1 Materials Zirconyl chloride octahydrate (ZrOCl2·8H2O) was purchased from Aladdin 4


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Industrial

Corporation

(Shanghai,

China).

Tin

(IV)

chloride

pentahydrate

(SnCl4·5H2O), tetrabutyl titanate (C16H36O4Ti), and phosphoric acid (H3PO4) were bought from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China), and all chemicals were used as received. 2.2 Preparation of rGO/M(HPO4)2 (M= Zr, Sn, Ti) composites Graphene oxide (GO) used in the work was prepared through a modified Hummers method that was described in our previous work.38,39 The morphology of GO sheets was characterized using AFM (Abstract Graphic). The layered tetravalent metal phosphate hydrates, Zr(HPO4)2·H2O, Sn(HPO4)2·H2O, and Ti(HPO4)2·H2O, were first prepared through the hydrothermal reactions of ZrOCl2·8H2O, SnCl4·5H2O, and C16H36O4Ti with H3PO4, respectively, following a literature procedure.26 To prepare the composites, in a typical experimental, aqueous dispersion of Sn(HPO4)2·H2O, 100 mL, 1.0 mg mL-1, was mixed with aqueous solution of GO, 25 mL, 1.0 mg mL-1 (with final GO to Sn(HPO4)2·H2O ratio of 2.5:100 in weight), and was stirred for 2 h at room temperature. Then, the water was removed by vacuum rotary evaporation, and the solid product was annealed at 225 °C for 4 h in argon atmosphere. During the heat treatment, Sn(HPO4)2·H2O was dehydrated, GO was reduced into rGO, and rGO/Sn(HPO4)2 composite was obtained finally. For comparison, the rGO/Sn(HPO4)2 composites with initial GO to Sn(HPO4)2·H2O ratios of 5:100 and 10:100 were prepared. The composites of rGO/Zr(HPO4)2, rGO/Ti(HPO4)2 were prepared under the same condition except for the different metal phosphates. 5



2.3 Characterizations X-ray powder diffraction (XRD) patterns of M(HPO4)2 (M= Zr, Sn, and Ti) and their composites with rGO were collected on a D8 ADVANCE diffractometer (Bruker, Germany) with Cu/Kα irradiation (λ = 1.55406 Å) at the scanning rate of 6°/min and the scanning range from 5 to 40°. Scanning electron microscopy (SEM) images were obtained using an Ultra 55 field-emission scanning electron microscope (Zeiss, Germany), and the Energy dispersive spectrometer (EDS) data were acquired on the same instrument. Atomic force microscopy (AFM) images of GO were acquired using a Multimode nanoscope V AFM (Veeco, USA). Raman spectra were acquired using a Senterra R200-L Dispersive Raman Microscope (Bruker, Germany). 2.4 Electrochemical property measurements The electrochemical properties of M(HPO4)2 (M= Zr, Sn, and Ti) and the corresponding composites with rGO were studied on coin cells in which the working electrodes were prepared with the metal phosphate or composite as active materials. Briefly, the active material, acetylene black and polyvinylidene fluoride (PVDF) were mixed with a ratio of 65:25:10 in weight in 1-methyl-2-pyrrolidone (99.0%) through manual grinding (~ 10 minutes) in an agate mortar. The slurry mixture was then coated on the copper foil forming a thin film with thickness of ~ 10 µm, followed by a pre-baking at 110 °C for 13 h in a vacuum oven. The surface density of active materials loaded onto the Cu foil was about 1 mg cm-2. The thin film was then cut into disks with a diameter of 12 mm, and the disks were used as the working electrode.

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The CR 2016 coin cells were assembled in an argon-filled glove box using 1 M LiPF6 in a mixed solvent of ethylene carbonate/diethyl carbonate/ethyl methyl carbonate (4:3:3, in volume) as the electrolyte, a thin lithium plate was used as the counter/reference electrode, and a Celgard 2325 membrane as a separator. LAND-CT2001A electrochemical workstation (Wuhan, China) was used to assess the specific capacity, the galvanostatic charge-discharge capability and cycling stability of the coin cells at different current densities of 0.1 to 10 A g-1, and a voltage range of 0.01 to 3.0 V vs. Li/Li+. The cyclic voltammogram (CV) measurements were carried out using AUTOLAB electrochemical workstation (Metrohm, Switzerland) at a scanning rate of 0.5 mV s-1 from 0.01 V to 3 V. Electrochemical impedance spectroscopy was conducted over the frequency range of 0.01 to 100 kHz. 3. Results and discussion It has been reported that the graphene and rGO can enhance the electrochemical performances of electrode materials for LIBs.40 In the work, to explore the possibility of the layered M(HPO4)2 (M= Zr, Sn, and Ti) in anode materials, rGO/M(HPO4)2 composites having different rGO to M(HPO4)2 ratios are prepared. The FE-SEM images (Figure 1a-c) and XRD patterns (Figure S1) of as-prepared composites show that the thin flake morphology and the layered crystalline structure of the M(HPO4)2 (see Figure S1 and S2) are preserved well. The TEM images of rGO/M(HPO4)2, Figure 1e-g, show also clearly the ultrathin wrinkled rGO sheets on the surfaces of M(HPO4)2, which is further confirmed by the Raman spectra (Figure 1h) and the element mapping based on EDS (Figure 1d). Figure 1h depicts the Raman spectrum of 7



rGO/Sn(HPO4)2 composite, and the typical G and D bands from the rGO showed up clearly.38 The element map (Figure 1d) demonstrates that the Sn(HPO4)2 surfaces are coated uniformly with carbon that are from rGO. These results illustrated unambiguously the formation of the composites of rGO/M(HPO4)2. It can be found that the rGO sheets not only clad on the surfaces of M(HPO4)2 crystals, but also bridge them together forming a network structure within the composites.

Figure 1. (a, b, and c) FE-SEM and (e, f and g) TEM images of rGO/Zr(HPO4)2 (10:100, in weight), rGO/Sn(HPO4)2 (5:100, in weight), and rGO/Ti(HPO4)2 (10:100, in weight), respectively. (d) C element map acquired on rGO/Sn(HPO4)2 (5:100) composite. (h) Raman spectra of Sn(HPO4)2 and rGO/Sn(HPO4)2 composite with rGO to Sn(HPO4)2 ratio of 5:100 (in weight).

The electrochemical performances of rGO/M(HPO4)2 composites are evaluated on coin cells and compared with that of the corresponding M(HPO4)2. The galvanostatic charge-discharge data of the coin cells were measured at different current densities of 0.1 to 10 A g-1, each for 10 cycles, and are shown in Figure 2a-c. Comparing with those of M(HPO4)2, the specific capacities of all composites increased dramatically. For instance, the reversible specific capacities of the composites of rGO/Zr(HPO4)2 (10:100), rGO/Sn(HPO4)2 (5:100), and rGO/Ti(HPO4)2 (10:100) can reach to 298.3, 632.2, and 525.6 mA h g-1, respectively, at 0.1 A g-1. 8


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These values are almost two-fold of those of the corresponding Zr(HPO4)2 (118 mA h g-1), Sn(HPO4)2 (393 mA h g-1), and Ti(HPO4)2 (229 mA h g-1). The composite anodes also showed good rate capabilities. The reversible capacities of rGO/Sn(HPO4)2, rGO/Ti(HPO4)2, and rGO/Zr(HPO4)2 can be retained at 255.5, 161.2, and 55.6 mA h g-1, respectively, even when the current density is up to 10 A g-1. Furthermore, as revealed in Figure 3a-c, the first cycle charge/discharge coulombic efficiencies of rGO/Zr(HPO4)2, rGO/Sn(HPO4)2, and rGO/Ti(HPO4)2 anodes are 24.37%, 48.54%, and 58.05%, respectively, which are higher than that of the corresponding Zr(HPO4)2 (15.96%), Sn(HPO4)2 (35.32%), and Ti(HPO4)2 (31.16%). To understand the effect of rGO on the electrochemical properties of M(HPO4)2, the composites with varied rGO contents were studied. As illustrated in Figure 2b, for rGO/Sn(HPO4)2, with the increase of the rGO content, both their specific capacities and rate capabilities are increased, and reach to the maxima when the rGO-to-Sn(HPO4)2 ratio is of 5:100 (in weight). The results suggest that rGO enhances the electron and Li+ conductivities of Sn(HPO4)2 and thus results in the high charge-discharge rate capability as expected. Also, the higher specific capacity of the coin cells with the composites indicates that rGO may provide additional binding/tolerating sites for Li+. When the rGO-to-metal phosphate ratio is of above 5:100, both specific capacity and rate capability are decreased (blue lines in Figure 2b). Similar trends are also observed in the cells with rGO/Zr(HPO4)2 and rGO/Ti(HPO4)2 as anode materials, see Figures 2a and c. This result possibly is caused by the aggregation of rGO themselves when the rGO sheets are in high ratio, 9



which are illustrated in Figure S4. In addition, the charge-discharge cycling stabilities of the anodes made of three composites are also much better than those of M(HPO4)2

Figure 2. Galvanostatic charge-discharge curves of M(HPO4)2 and rGO/M(HPO4)2 (M=Zr, Sn, Ti) composites with different GO to M(HPO4)2 ratios (in weight): (a) rGO/Zr(HPO4)2, (b) rGO/Sn(HPO4)2, (c) rGO/Ti(HPO4)2 composites, measured at current densities from 0.1 to 10 A g-1; (d) Cycling performances of rGO/M(HPO4)2 (M=Zr, Sn, Ti) composites and bare metal phosphates at 0.1 A g-1.

(Figure 2d), and are comparable to that of the bare rGO anode (Figure S5). This suggests also that the rGO in the composites can promote the formation of the stable SEI film that improves the cycling stability. The assumption is verified by the FE-SEM images of rGO/Sn(HPO4)2 anode obtained before and after 1000 cycles of charge/discharge at 1 and 5 A g-1, respectively, on which the SEI film is formed clearly on the surface of rGO/Sn(HPO4)2 after the charge/discharge cycles (Figure S3). Figure 3 depicts the discharge and charge profiles of the coin cells using the composites as anode materials. Obviously, the voltage platforms for lithium ions 10


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insertion and extraction of the rGO/M(HPO4)2 anodes are much lower than that of M(HPO4)2 alone, and even lower than that of most transition metal oxide anodes, but higher than that of commercial graphite anodes.7,41 The low voltage platform could prevent the deposition of metallic lithium on the anode during the rapid charge, and thus could afford high voltage and energy density for the LIBs.41,42

Figure 3. Charge-discharge voltage profiles of the rGO/M(HPO4)2 composites and M(HPO4)2 at first few cycles: (a) rGO/Zr(HPO4)2 (10:100) and Zr(HPO4)2, (b) rGO/Sn(HPO4)2 (5:100) and Sn(HPO4)2, (c) rGO/Ti(HPO4)2 (10:100) and Ti(HPO4)2, measured at a current density of 0.1 A g-1.

The detailed electrochemical reactions occurred within M(HPO4)2 and rGO/M(HPO4)2

during

discharge/charge

processes

are

monitored

by

CV

measurements. As depicted in Figure 4a, during the first discharge/charge cycle three cathodic peaks at 0.02, 0.52 and 0.79 V can be observed for Zr(HPO4)2. The peak at 0.02 V is assigned to the Li+ insertion into the interlayer space of Zr(HPO4)2, and also the Li+ anchoring onto the rGO sheets. The other two at 0.79 and 0.52 V should be related to, respectively, the formation of solid electrolyte interface (SEI) layer,33 and a side reaction. The anodic peak observed at 0.25 V is corresponded to the reversible extraction of Li+. In contrast, for the rGO/Zr(HPO4)2 composite, excepting the peak at 0.02 V, the two peaks at 0.79 and 0.52 V that appeared in Zr(HPO4)2 are suppressed, especially the one at 0.52 V. This reveals that rGO can block certain side reaction during the first cycle discharge, which should be partially the reason why the 11



composite assume larger reversible specific capacity. Figure 4b shows the CV curves of the coin cells with Sn(HPO4)2 and rGO/Sn(HPO4)2 as anodes. There are two cathodic peaks arisen at the potential of 0.02 and 0.85 V and three anodic peaks appeared at 0.64, 1.15 and 2.20 V during the first discharge-charge cycle for both Sn(HPO4)2 and rGO/Sn(HPO4)2. Similarly, the cathodic peak at 0.02 V is assigned to the Li+ insertion into the interlayer spacing of Sn(HPO4)2 and anchoring onto rGO sheets, and peak at about 0.85 V is related most probably to the valence state conversion of tin from Sn4+ to Sn2+ in Sn(HPO4)2,43,44 and the formation of SEI film. The anodic peak at 0.64 V correlates to the reversible extraction of Li+, the peak at 1.15 V may be raised from the partial decomposition of SEI,45,46 and the peak at 2.20 V is corresponding to the reversible conversion of tin from Sn2+ to Sn4+ in Sn(HPO4)2.44 However, starting from the second discharge/charge cycle, the peak at 0.85 V disappears, and a new weak cathodic peak arises at 1.52 V due might to the formation of new SEI layer.45 It is worth pointing out that during the charge-discharge cycling, the CV curves of Sn(HPO4)2 anode do not show any cathodic peaks around 0.38, 0.42, 0.45, 0.53 and 0.66 V related to LixSn alloys formation, which is different from other tin compounds.20,47-49 These data indicate that the Li+ insertion and extraction in Sn(HPO4)2 anodes are dominated by their common layered crystalline structures. In comparison, the positions of the redox peaks of rGO/Sn(HPO4)2 are almost the same as those of the Sn(HPO4)2, but the peak currents are relatively higher. This reflects that the rGO can provide additional reversible Li+ storage capacity and enhance the conductivity of the Sn(HPO4)2, which are fully in agreement with the 12


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charge-discharge measurement results (Figures 2 and 3). Similarly, for Ti(HPO4)2 and rGO/Ti(HPO4)2 anodes, the CV curves (Figure 4c) illustrate that the rGO can suppress certain side reactions during the charge-discharge cycle, increase the conductivity and also possibly the reversible Li+ storage capacity. Alternatively, the similar CV curve profiles starting from the second charge-discharge cycle imply that rGO/Ti(HPO4)2 has the identical Li+ storage mechanism as the Ti(HPO4)2. We therefore conclude that the layer space of M(HPO4)2 plays a key role in the Li+ storage, rGO might increase the anchoring of Li+.

Figure 4. CV curves of the rGO/M(HPO4)2 composites and M(HPO4)2 at first few cycles: (a) rGO/Zr(HPO4)2 (10:100) and Zr(HPO4)2, (b) rGO/Sn(HPO4)2 (5:100) and Sn(HPO4)2, (c) rGO/Ti(HPO4)2 (10:100) and Ti(HPO4)2, measured at a scanning rate of 0.5 mV s-1, scanning range from 0.01 V to 3 V.

To study the electrochemical kinetic properties of the composites as anodes for LIBs, electrochemical impedance data of the anodes made of the rGO/M(HPO4)2 and compared with M(HPO4)2 alone in Figure 5. The charge transfer resistance at the electrode/electrolyte interface (Rct) was determined based on the Nyquist plots, the semicircle in the intermediate frequency range of the impedance spectra, and the equivalent circuits (Figure 5a and b), and summarized in Table 1. Notably, the Rct of the rGO/M(HPO4)2 anodes are smaller than that of the corresponding M(HPO4)2 anodes, which is consistent with the galvanostatic charge-discharge measurements 13



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(Figure 2a-c) that show rGO/M(HPO4)2 have improved rate capabilities. The reason can be assigned to the inherent charge carrier capability of rGO and the unique structure of the rGO/M(HPO4)2 (M=Zr, Sn, Ti) (Figure 1), in which the rGO sheets not only clad on the surfaces of M(HPO4)2 (M=Zr, Sn, Ti) crystals, but also bridge them together forming a conductive network structure that may benefit the electron transmission and ions diffusion. In principle, the inclined line in the low frequency range of the impedance spectra represents the diffusion process of lithium ions within the anodes that is associated with the Warburg impedance (W). However, due to some irreversible electrochemical reactions that taken place in M(HPO4)2 (M=Zr, Sn, Ti) anodes, the lines are inclined with a deviate of ~ π/4. Therefore, the Li+ diffusion coefficients (D) within rGO/M(HPO4)2 and M(HPO4)2 (M=Zr, Sn, Ti) anodes are determined using a potential-step method (chronoamperometry method). To do this, the curves of anodic current (natural logarithm) versus charging time for the anodes are acquired and depicted in Figure 5c. At the beginning, the logarithm values of the anodic currents [ln(i)] decrease rapidly. However, after about one thousand seconds, ln(i) is linearly dependent on the charging time that should be closely related to the Li+ diffusion within the anodes,50 assuming that the concentrations of Li+ at the surface and in the bulk of the anode are the same.34 The approximate average lateral size of M(HPO4)2 (M=Zr, Sn, Ti) crystalline sheets estimated from the corresponding FE-SEM images (Figures S2 d-f) is of 0.4, 3 and 2 µm, respectively. Thus, based on equation (1),51 lni(t)=ln(2nFAD(C∞-C0)/r)-π2Dt/4r2

(1) 14


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where i is anode current, t is testing time, F is Faraday constant, A is the area of the electrode, r is the half of the lateral size of M(HPO4)2 (M=Zr, Sn, Ti) crystalline sheet, C∞ and C0 are the surface concentration and initial concentration of Li+ in the bulk of the anode, respectively, the D values are derived and listed in Table 1. Obviously, the Li+ diffusion coefficients (D) of the rGO/M(HPO4)2 (M=Zr, Sn, Ti) are much larger than those of M(HPO4)2 (M=Zr, Sn, Ti). This can be attributed to the as-coated rGO sheets that provide extra channels and short-cuts for Li+ diffusion.

Figure 5. Electrochemical Impedance Spectroscopy (EIS) of coin cells (the Nyquist plots) of (a) M(HPO4)2 (M=Zr, Sn, Ti) anodes, (b) rGO/Zr(HPO4)2 (10:100), rGO/Sn(HPO4)2 (5:100), and rGo/Ti(HPO4)2 (10:100) anodes, as well as the corresponding equivalent circuit. (c) Chronoamperometry data (the natural logarithm of anodic current versus charge time) of M(HPO4)2 and rGO/M(HPO4)2 (M=Zr, Sn, Ti) anodes, acquired at step potential of + 0.5 V for 1000 s. Table 1. Rct and D of M(HPO4)2 (M=Zr, Sn, Ti) and rGO/Zr(HPO4)2 (10:100), rGO/Sn(HPO4)2 (5:100), and rGO/Ti(HPO4)2 (10:100) anodes.

samples Zr(HPO4)2 rGO/Zr(HPO4)2 Sn(HPO4)2 rGO/Sn(HPO4)2 Ti(HPO4)2 rGO/Ti(HPO4)2

Rct (Ω) 226 154 301 122 283 102

D (cm2·s-1) 1.29 ×10-13 1.61 ×10-13 5.97 ×10-12 8.73 ×10-12 2.50 ×10-12 3.14 ×10-12

4. Conclusions In summary, rGO/M(HPO4)2 (M=Zr, Sn, Ti) composites with varied 15



rGO-to-M(HPO4)2 ratios in weight were prepared and characterized. It was found that within the composites the M(HPO4)2 (M=Zr, Sn, Ti) crystals were cladded and bridged by rGO sheets forming a network structure. The electrochemical property measurements showed that, in comparison with bare M(HPO4)2, the rGO/M(HPO4)2 (M=Zr, Sn, Ti) composites have larger specific capacity, higher rate capability, better cyclic stability, lower voltage for lithium ions insertion and extraction, as well as the improved first coulombic efficiency as anode materials for LIBs. We believe that the inherent large interlayer spaces of M(HPO4)2 (M=Zr, Sn, Ti) and the network motif of rGO are key factors dominate the overall electrochemical performances of the composites as anodes for LIBs. Consequently, it can be envisaged that the rGO/M(HPO4)2 (M=Zr, Sn, Ti) composites should be potential anode materials for LIBs. Supporting Information XRD patterns of rGO/M(HPO4)2 and M(HPO4)2 (M=Zr, Sn, and Ti), FE-SEM images of M(HPO4)2·H2O, M(HPO4)2, and rGO/M(HPO4)2 (M=Zr, Sn, and Ti) materials, FE-SEM images of rGO/Sn(HPO4)2 electrodes at different charge-discharge states, The electrochemical performances of rGO anode for lithium ion battery. (PDF) Corresponding Author *Email: [email protected]; [email protected]. Notes The authors declare no competing financial interest. Acknowledgements 16


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The work was financially supported by the National “973 Program” of China (No.2014CB260411 and 2015CB931801), the National Science foundation of China (No.11374205).

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Abstract Graphic

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with Reduced Graphene Oxide as Anode Materials for Lithium Ion

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