Bridging Redox Species-Coated Graphene Oxide Sheets to Electrode

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Bridging Redox Species-coated Graphene Oxide Sheets to Electrode for Extending Battery Life by Using Nanocomposite Electrolyte Yi Fu Huang, Wen Hong Ruan, Dongling Lin, and MingQiu Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13145 • Publication Date (Web): 16 Dec 2016 Downloaded from http://pubs.acs.org on December 18, 2016

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ACS Applied Materials & Interfaces

Bridging Redox Species-coated Graphene Oxide Sheets to Electrode for Extending Battery Life by Using Nanocomposite Electrolyte Yi Fu Huang, *1,2 Wen Hong Ruan, *1,2 Dong Ling Lin, 1,2 and Ming Qiu Zhang 1,2 1

Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education,

GD HPPC Laboratory, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China 2

Guangdong Provincial Key Laboratory for High Performance Polymer-based Composites,

Guangzhou 510275, China *

Corresponding authors: [email protected]; [email protected]

KEYWORDS: nanocomposite electrolyte, redox species, graphene oxide, battery life, bridging

ABSTRACT

To substitute conventional electrolyte for redox electrolyte has provided a new intriguing method for extending battery life. The efficiency of utilizing the contained redox species (RS) in the redox electrolyte can benefit from increasing the specific surface area of battery electrodes from electrode side of the electrode-electrolyte interface, but is not limited to that. Herein, a new strategy by using nanocomposite electrolyte is proposed to enlarge the interface with the aid of

ACS Paragon Plus Environment

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the nano inclusions from electrolyte side. To do this, graphene oxide (GO) sheets are firstly dispersed in the electrolyte solution of tungstosilicic salt / lithium sulfate / polyvinyl alcohol (SiWLi / Li2SO4 / PVA), and then the sheets are bridged to electrode, after casting and evaporating the solution on the electrode surface. By applying in-situ conductive atomic force microscope and Raman spectra, it is confirmed that the GO sheets doped with RS of SiWLi / Li2SO4 can be bridged and electrically reduced as extended electrode-electrolyte interface. As a result, the RS-coated GO sheets bridged to LiTi2(PO4)3 // LiMn2O4 battery electrodes are found to deliver an extra energy capacity (~ 30 mAh / g) with excellent electrochemical cycling stability, which successfully extends the battery life by over 50 %.

Introduction The demands on the high performance batteries for electric vehicles, smart grid, wearable electronic devices, smart phone and unmanned plane have been growing in last decades. However, the short battery life is still an unsatisfactory fact,

[4]

[1-3]

which is ascribed to low energy

capacity of the batteries when battery energy is typically generated only from electrode materials. To overcome this limitation, some strategies like dissolving redox species (RS)

[5-9]

into electrolyte have been conducted, since electrolyte can be treated as another energy supplier for improving the energy capacity. By using the electrolyte containing RS, or so-called “redox electrolyte”,

[10, 11]

additional redox reactions from the RS would occur in the electrolyte side of

the electrode-electrolyte interface, which is undoubtedly a new way to enhance the ability of extending battery life.

[12, 13]

Accordingly, a sufficient amount of RS would be preferred to be

stored into a given volume of electrolyte to further facilitate the reactions. Recently, some reports


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on utilizing redox electrolyte have been shown among flow redox battery, supercapattery and supercabattery, etc. [14-16] As for batteries assembled with redox electrolyte, in fact, there is a critical issue about maximizing the energy utilization of the contained RS in the redox electrolyte, [10] because only the RS attaching on the electrode-electrolyte interface is able to be converted. Considering that facilitating redox reaction process of RS benefits for energy conversion, a feasible solution can lie in enlarging the electrode-electrolyte interface, which would enhance the interaction between the RS and electrode surface. As the electrode-electrolyte interface is a result of the combination of battery electrode and electrolyte, electrode materials, [19]

[18]

[17]

such solution is often given by designing highly porous

depositing / growing conductive nanostructure on the electrode surface,

or arranging electrode structure,

[20, 21]

with the aim to increase the specific surface area of

electrode materials, but accordingly, it is limited by the implement only from electrode side. It is expected that redox electrolyte with the incorporation of the nanoparticles can have better interaction between the electrode-electrolyte interface and RS, as reports have showed that some nanoparticles dispersed in the electrolyte would participate in the formation of electrodeelectrolyte interface. [17, 22] In this case, the electrode-electrolyte interface could be also enlarged from the electrolyte side by the nanoparticles, even under a given highly porous electrode. Such nanoparticles can act as “bridges” to expand the interface because of the affinities to both electrode and electrolyte.

[23, 24]

The solid electrolyte system containing nanoparticles can have

some other advantages, e.g. to stably bridge nanoparticles to electrode surface without having fluidity, [10] or to suppress the shuttle effect of RS [25] due to the barrier of dispersed nanoparticles in the electrolyte. However, to our best knowledge, such concept by using nanocomposite


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electrolyte to improve energy utilization of the RS with the aid of nano inclusions as extended electrode-electrolyte interface has been still less involved. To address above idea, we put forward a facile route by firstly casting redox nanocomposite electrolyte solution on the electrode, and then after being solidfied, bridging some RS-coated nanoparticles from electrolyte to electrode as extended interface. The aim of solution casting method for forming solid electrolyte on the electrode is to ensure well electrode-electrolyte contact.

[26]

By adopting graphene oxide (GO) as nanoinclusion, the electrolyte solution also

consists of polyvinyl alcohol (PVA) / lithium sulphate (Li2SO4) solution blended with all-lithium salt of tungstosilicic acid (Li4SiW12O40, SiWLi) as RS.

[27]

The PVA is a polar polymer often

good for dispersing inorganic nanoparticles and salts as solid solvent, resulting solid electrolyte with excellent binding property.

[30]

[28, 29]

and endowing the

The reason why we consider

SiWLi as redox substance is that solid electrolyte mixed with heteropoly compounds can have an excellent ionic conductivity for practical application in solid electrochemical systems.

[31]

In

addition, the introduction of the Li2SO4 was aimed to increase the ion concentration of the electrolyte system. To introduce GO into electrolyte system, GO is firstly dispersed in PVA / Li2SO4 solution and then blended with SiWLi to form casting solution. Here, the GO is expected as an intriguing candidate among the nanoparticle types owing to its electrochemical conductivity property.

[32, 33]

More connections through electrical contact among GO sheets and

RS would be established, as the bridged GO on the electrode could be likely electrically reduced and further enrich postulated interface. The reason why not directly choose reduced-GO for the bridging structure is that GO has superior affinities to polar components (e.g. metal oxides, active polymers

[25]

or salt ions

[35]

[34]

), which can be used for anchoring RS and achieving a

molecule-level dispersion in PVA matrix. [36]


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In this work, to apply in-situ conductive atomic force microscope and Raman spectra for characterizing the structure of the bridged RS-coated GO sheets, the redox nanocomposite electrolyte film on an indium tin oxide (ITO) transparent electrode was formed, to construct an electrode-electrolyte interfacial model by bridging RS-coated GO sheets to the electrode. Furthermore, the RS-coated GO sheets was bridged to battery electrodes in a LiTi2(PO4)3 // LiMn2O4`system in which the performance on extending battery life was investigated. During the study, relevant mechanisms about the bridged interface with various content of RS-coated GO were discussed and analyzed to gain better performance of the battery`system. Results and discussion Characterization of RS-coated GO sheets bridged on an ITO transparent electrode A simplified interfacial model with bridged RS-coated GO sheets from electrolyte to electrode，was constructed by firstly spinning a small quantity of the redox nanocomposite electrolyte solution on a transparent electrode of indium tin oxide (ITO), and then drying it at ambient condition to form an ultrathin and smooth electrolyte film. Based on this model, the unbridged RS-coated GO sheets in the solid electrolyte also coexisted. Subsequently, atomic force microscopy (AFM) measurement was applied to image the ITO surface in the tunnelling AFM (TUNA) mode, by measuring a tunnelling current between a conductive AFM tip and electrolyte to study the morphological and electrical properties. In Figure 1a-f, the conductive AFM (C-AFM) images of the ITO surfaces coated with SiWLi / Li2SO4 / PVA containing GO were shown, and those without containing GO were given as comparisons. It was found that the sample without GO exhibited a relatively homogenous morphology (Figure 1a-b), and by contrast, the structure of RS-coated GO could be distinguished (Figure 1d-e), which showed that


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the aggregates of RS in size of ~20 nm were densely attached on the GO sheets. In the TUNA images of Figure 1c and 1f, the dark colour was mainly ascribed to consecutive (fast) ionic channels of dissociated tungstosilicic / lithium-ion salts (RS) between the conductive tip and ITO electrode, and the light colour regions belonged to inconsecutive (slow) ionic channels. According to this, for the redox electrolyte without GO, the interface in electrolyte side (Figure 1c) exhibited a relatively low ionic transportation because the small ionic channels across the interface were scattered and correlated weakly. However, in the fixed size of AFM imaging (1µm × 1µm) (Figure 1f), GO sheets incorporated into the solid redox electrolyte had a great improvement on the number and the size of ionic channels. Since ionized GO was an excellent ionic conductor, [37] the scattered RS attached to the bridged GO sheets (Figure 1e-Zone I) on the electrode could be more correlated to widely extend the total area of ionic channels, and accordingly, the amount of absorbed RS in the electrode-electrolyte interface could be increased, while the unbridged GO sheets (Figure 1e-Zone II) on the electrode had no effect on improving consecutive ionic channels although also coated with RS. Therefore, with the aid of TUNA current image of Figure 1f, the positions of RS-coated GO sheets whether bridged (Figure 1eZone I) or unbridged (Figure 1e-Zone II) to the electrode could be determined. So the electrical reduction behaviors of bridged RS-coated GO could be further explored. The measurements of I-V curves at different zones were done, as was shown in Figure 1g. The applied voltage for the micro-capacitor of tip / electrolyte / ITO was set in the range from 500 mV to 500 mV, with current threshold of all the measurements at 120 pA and time interval of ~30 s when I-V cycles repeated, which could ensure the electrochemical reversibility of RS. [27]

. Corresponding to the TUNA current image (Figure 1g) which could be also referred to the

yellow dot square regions in Figure 1d-f , three levels of conductive regions (Zone-L: low


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conductivity; Zone-M: middle conductivity; Zone-H: high conductivity) were selected. Obviously, RS–coated GO in the Zone-L was unbridged and that in both Zone-M/H was bridged. The difference in the conductivities of Zone-M and Zone-H related to the distribution area of bridged RS-coated GO. It was shown that the conductivities of Zone-L and Zone-H were unvaried during the cycles, and on the contrary, Zone-M had a significant increase in the conductivity with the cycle number and gradually owned an I-V behavior of Zone-H. It was suggested that the unvaried conductivity of Zone-L was due to electrochemical stability of unbridged RS-coated GO, which had no obvious contribution on the charge conduction, while the unvaried one of Zone-H was ascribed to the current threshold. The change of the conductivity of Zone-M from middle to high was probably a result of the postulated electrical reduction process from the bridged GO to the bridged rGO during I-V cycles, implying forming more enriched interface from the electrode. To explain the electrical reduction mechanism of the bridged RS-coated GO on the ITO electrode, Raman spectra were used to trace the Raman signals of RS-coated GO in the electrode-electrolyte interface. To do this, an ITO capacitor was assembled with the redox nanocomposite electrolyte sandwiched with two ITO electrodes and a Para film as a spacer. During cyclic voltammetry (CV) test of the capacitor, with the aid of reflection optical microscope from laser micro-Raman spectrometer, Raman signals from the interface through the transparent electrode could be collected. It was shown that, during the cyclic voltammetry scan within 1.0 V, a pair of CV peaks (peak i / i') (see dot line in Figure 2a) could be observed. The CV peaks of the capacitor was mainly associated with the redox reaction of SiW12VIO404-. The brown interface (GO-rich) (Figure 2b-I) was in-situ observed under a reflection optical microscope. The Raman spectra of the interface had the characteristic peaks of GO sheets, e.g.


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disorder peak for sp3 carbon (D peak, 1300~1360 cm-1) and graphene peak for sp2 carbon (G peak, ~1600 cm-1),

[38]

and peaks from tungstosilicic salt (970 cm-1 and 995 cm-1). [39] However,

no perceived change of reflected light and Raman signals (Figure 2c) on the interface indicated the electrochemical stability of RS-coated GO during the testing. Such phenomenon was often due to very slow electrochemical reduction of GO reported in some literatures. [33] Considering the experimental conditions of I-V measurement in Zone-M (Figure 1g) where the quick reduction of RS-coated GO probably related with the high current density (often several tens of A cm-2), a higher current density through the electrodes was generated by setting the charged voltage at 1.8 V to facilitate a similar reducing process. The increased current was ascribed to H2 / O2 evolution over Vd (~1.5 V, referred to the inset of Figure 2a). After charging the capacitor at this voltage for 5 h, the interfacial colour changed to black interweaving with brown (Figure 2bII). Two pairs of peak ii / ii' and iii / iii' (see solid line in Figure 2a) emerged during the voltammetry scan within 2.0 V, and the area under CV curve within 1.0 V increased significantly. More redox peaks were probably due to formation of more reduced anions of SiW12VIO404-, as more electrons could pass across the interfacial rGO. Relevant redox reactions were as follows: [40]

(1)

(2) The increase of CV area indicated the utilization of more RS due to the extended redox interface from the bridged GO. Besides, the intensity ratio (IG / ID) of black regions increased to


8

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0.874 (Figure 2c), compared to 0.808 of brown regions. The increase of IG / ID was attributed to the transformation from interfacial GO to interfacial rGO, consistent with colour change of the interface during optical observation. [41] So the black regions could be denoted as rGO-rich, and the postulated electrical reduction process of the bridged GO to the bridged rGO on the electrode was confirmed. Figure 2d showed the schematics of enlarging the redox interface for enhancing its interaction with RS, with the aid of bridged RS-coated GO (RS / GO) on the ITO electrode. For the redox electrolyte without GO, the interfacial reaction came from the RS on electrode surface (Figure 2d-1). When GO was incorporated into the redox electrolyte, the RS attached to the sp2 regions of bridged GO sheets (denoted as RS (interface)) could contribute to interfacial reaction, so the redox interface could be extended. However, the redox species on sp3 regions (denoted as RS (electrolyte)) of bridged GO sheets poorly related with interfacial reactions (Figure 2d-2). After electrochemical reduction of GO, more sp2 regions in rGO sheets was a result of the change from RS (electrolyte) to RS (interface). As a result, the redox interface could be further extended (Figure 2d-3), and the ITO capacitor could exhibit a higher energy delivery. Battery electrodes with the bridged RS-coated GO sheets for extending battery life Based on the above studies, it had shown the feasibility of the strategy of bridging some RScoated GO to the electrode by using nanocomposite electrolyte, and in this section, we further investigated the case, whether the energy delivery from electrolyte could be used for extending battery life of a LiTi2(PO4)3 // LiMn2O4 system as an example of batteries. In this new assembly, the electrode / electrolyte interface with bridged RS-coated GO could bear a high current density


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without gas evolutions for electrical reduction of bridged GO sheets. (See Supplementary Materials for the explanation & Figure S-1) Before considering the effect of the bridged RS-coated GO sheets on the battery electrodes, the CV behaviors of a LiTi2(PO4)3 // LiMn2O4 battery with solid redox electrolyte without containing GO or with Li2SO4 liquid electrolyte were evaluated from 0 to 2 V at a scanning rate of 0.1 mV s-1, as exhibited in Figure 3a. Two pairs of the redox peaks at 1.04 V / 0.99 V (i / i') and 1.29 V / 1.22 V (ii / ii') of the battery using the redox electrolyte were ascribed to the interfacial reaction mechanism of RS (Equation 1 and 2), since these peaks were not existed in the case with liquid electrolyte. Compared with those in ITO capacitor, the higher potentials of the interfacial reactions of SiW12VIO404- in the LiTi2(PO4)3 // LiMn2O4 battery were due to “electrochemical polarization” effect of the electrode / electrolyte interface. For a battery assembled with liquid electrolyte, two characteristic peaks at 1.59 V (iii) and 1.72 V (iv) were observed during the charging scan, which were assigned to the oxidation of Mn (III) to Mn (IV) or the reduction of Ti (IV) to Ti (III). [42] Conversely, the corresponding peaks of iii' and iv' in the discharging scan were attributed to the oxidation of Ti (III) to Ti (IV) or the reduction of Mn (IV) to Mn (III). The oxidation process corresponded to the removal of lithium ions (Li+) and the reduction process indicated the oscillating insertion of Li+ in the battery electrodes. The positions of intercalation / deintercalation redox peaks shifted to higher potentials of 1.67 V and 1.81 V, respectively, during charging scan when using solid redox electrolyte, as the “concentration of polarization” and “electrochemical polarization” in the solid-state interface were more remarkable. Additionally, Figure 3b showed that a LiTi2(PO4)3 // LiMn2O4 battery with solid redox electrolyte had a stable cycling property, and the “rocking chair” mechanism in the lithium-ion intercalated compounds was not affected.


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Then, the effect of the bridged RS-coated GO from the electrolyte to battery electrodes on extending battery life was considered. Figure 3c-d showed the initial charging / discharging curves of a LiTi2(PO4)3 // LiMn2O4 battery using the GO-incorporated redox electrolyte, compared with those using non-GO incorporated redox electrolyte or liquid electrolyte. According to the CV curves of Figure 3a, the charge capacity within ~1.5 V belonged to the energy delivery from the redox interface with bridged RS-coated GO sheets. So the energy supply from this part could be calculated conveniently from the charge capacity within ~1.5 V (Figure 3a). The results showed that such energy storage of the liquid battery could be neglected, and that of the battery using the solid redox electrolyte without GO was ~6.5 mAh g-1, which was ascribed to the storage from pure electrode surface interacting with RS. Surprisingly, such energy delivery of the battery with solid redox electrolyte containing 0.1 wt% GO increased to 24.1 mAh g-1. The significant enhancement on the energy storage was reasonably due to the redox reactions from the bridged RS-coated GO sheets on the battery electrodes. When the charging voltage increased up to over 1.5 V, the reduction of Ti (IV) to Ti (III) or the oxidation of Mn (III) to Mn (IV) happened, which could generate a high current density through the electrodes (Figure 3a). Notably the electrochemical reduction process from bridged GO to bridged rGO could be benefited from the charging process of the battery. However, the voltage drops (~0.13 V) in the charging curves at ~2 V could be seen for batteries assembled with solid redox electrolytes without or with GO (Figure 3c). Such decrease of charging voltage was not found in the battery assembled with Li2SO4 liquid electrolyte. It was also found that the XRD patterns of LiMnO4 and LiTi2(PO4)3 in the battery electrodes were not significantly affected after cycling (see Supplementary Materials Figure S-2). The voltage drops were probably ascribed to the solid electrolyte decomposition near the electrode-electrolyte interfaces,


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which resulted in the formation of both solid electrolyte interface (SEI) films [43] and the bridged rGO in the presence of RS. From the XPS survey on the elements of the interface coated with PVA / Li2SO4 / SiWLi solid electrolyte, it can be inferred that the SEI formation related with oxidization process on the electrolyte interface, when oxygen content increased remarkably from 24.9 % to 51.9 % after cycling (see Supplementary Materials Figure S-3). Besides, the incorporation of GO sheets seemed to inhibit the oxidation of RS-coated electrode surface, with oxygen content of 46.8 %. As a result, it could be seen that the energy storage of the battery within 1.5 V increased from ~6.3 mAh g-1 (without GO) to 28.1 mAh g-1 (with 0.1 wt % GO) during second charging (see the inset of Figure 3c). The higher energy storage of the battery with GO was the result of the extended interface from bridged RS-coated GO. In discharging curves of Figure 3d, it was found that the formation of both SEI films and bridged RS-coated rGO by using solid redox electrolyte could benefit for better cell performance. For the liquid battery, the discharge capacity was only 23 mAh g-1 owing to the instability of electrodes, less than the published results (40~55 mAh / g) of optimized LiTi2(PO4)3 // LiMn2O4 battery.[44] The capacity of the battery with solid redox electrolyte could be 2.6 times (without GO) and 3.5 times (0.1 wt% GO) of the case of liquid battery, and had been increased up to at least 50 % of the best obtained capacity (~55 mAh / g) by using carton-coated anode, [45] showing a great potential to extend battery life. Furthermore, the cyclic performance test showed excellent cycling stability of the battery using the solid redox electrolyte with 0.1 wt% GO (see Figure 3e & Supplementary Materials Figure S-4), compared with that with liquid electrolyte. Lastly, to maximize the extendibility of redox interface with bridged RS-coated GO sheets to achieve better utilization of RS in the electrolyte, electrochemical properties of the batteries with solid redox electrolyte containing various GO content were studied further. The conductivities


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including the ionic conductivity (σc) in the electrolyte and interfacial conductivity (σic) between electrode and electrolyte could be calculated according to the following formulas: ௅

ߪ௖ = ஺ோ

ߪ௜௖ =

(3) ್

௞ ோ೎೟

(4)

where L was the thickness of the electrolyte, A was the area of the electrode-electrolyte contact (1 cm × 1 cm), k was constant (using a maximum value of L / A) for a given battery, Rb was the bulk resistance and Rct was charge transfer resistance of interface. Rb and Rct could be obtained from electrochemical impedance spectrum curves. [46] (See Supplementary Materials Figure S-5) The σc and σic of the batteries assembled with solid redox electrolyte as a function of GO content were measured. As shown in Figure 4a, both conductivities increased sharply under low loading of GO (˂ 0.1wt %), then the increase of σc became slowly, but the σic showed decrease trend. It was shown that the unbridged GO sheets existing in the electrolyte could form “ion highways” to improve the ionic conduction of electrolyte, because ions attaching on the GO sheets could move faster (Figure 4b-I). [37] At higher current density of the battery, the higher σc could facilitate the interfacial reactions with higher capacity retention. (See Supplementary Materials Figure S-6) However, the steric hindrance of GO sheets at higher content would cause the elongation of ion pathways (Figure 4b-II). The promotion effect of GO on ion migration in the electrolyte was balanced out. Therefore, conductivity percolation phenomena of σc occurred. Compared to the case of the battery with non-GO incorporated redox electrolyte, the increase of σic with incorporated GO indicated that bridged GO in the electrolyte owned good adhesion with other components of electrolyte, resulting in new electrode-electrolyte solid interface


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(Figure 4b-I). The lower value of the σic than that of the σc was probably due to the low ionic conductivity of bridged rGO since rGO sheets had few functional groups for promoting ion transportation. At the same time, with more rGO in the interface, the ion and electron transport pathways in the interface were more roundabout (Figure 4b-II). [28] This limited the extendibility of the ion / electron redox interface by using the bridged rGO sheets. So the σic decreased with the increase of GO content, while the safety of avoiding short circuit when extending the interface in the electrolyte could be ensured. Meanwhile in according with the trend of σc, energy delivery from the RS in the nanocomposite electrolyte balanced at ~30 mAh g-1 after ~0.1 wt% GO loading. However, when GO content was so high, the intercalation and deintercalation of Liions into the battery electrodes were greatly affected due the barrier effect of GO sheets, and the battery could not be charged (Figure S-7). Through extending the electrode-electrolyte interface by bridged RS-coated GO in the electrolyte side, the highest energy supply from RS was increased up to ~4.6 times, with more efficient utilization of electrolyte energy for extending battery life. Conclusions In summary, a new strategy by bridging RS-coated GO sheets to electrode by using nanocomposite electrolyte for enlarging the electrode-electrolyte interface, has been put forward to achieve better utilization of the redox species in the electrolyte for high energy delivery. The RS-coated GO sheets bridged and electrically reduced on the ITO electrode was characterized by in-situ conductive atomic force microscope and Raman spectra, which confirmed the feasibility of the strategy. Then, the bridged RS-coated GO sheets on the battery electrodes of the LiTi2(PO4)3 // LiMn2O4 system was found to extend battery life by over ~50%, with a highest energy supply (~30 mAh / g) from electrolyte and better electrochemical cycling stability.


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Therefore, it is believed that the strategy of using nanocomposite redox electrolyte for delivering energy can give a hope for developing new electrochemical storage technologies with longer battery life. Experimental Section Synthesis of Electrode materials: Lithium titanium phosphate (LiTi2 (PO4)3) was synthesised by a sol-gel method.[44, 45] Firstly, the mixture containing the lithium carbonate (AR, Li2CO3, Aladdin reagent) (1.53 g), ammonium phosphate monobasic (AR, NH4H2PO4, Aladdin reagent) (2.16 g), and the nano-titanium oxide (nano-TiO2, P25, specific area 50 m2 / g, average size 21 nm, Degussa) (1 g) was blended with 12 mL of 2 wt% poly (vinyl alcohol) (PVA, alcoholysis degree 99.8 ~ 100%, Mw ~88,000, Aladdin reagent) solution under magnetic stirring at 90 °C to homogeneity and formed a white solid. Then, the solid was calcined at 900 °C (with the heating rate of 10 °C min–1) for 12 h under continuous N2 flow in a tube furnace to yield the desired product. Lithium manganite (LiMn2O4, AR) was supplied by Tianci High-tech Material Co., LTD., in Guangzhou, China. Electrode preparation: The electrodes were prepared as follows: 0.5 g of electrode material (LiTi2(PO4)3 or LiMn2O4), 60 mg of acetylene black (AB), and 50 mg of polytetrafluoroethylene adhesion (PTFE emulsion, 56 wt%) were mixed and milled into a solid-like slurry with an appropriate amount of ethanol. The solid slurry was pressed into a thin film with a thickness of approximately 100 µm. The film was cut into regular shapes (1 cm × 1 cm) and pressed together with a stainless steel mesh (200 mesh) under 18 MPa to produce an electrode. The mass loading of the film on the electrode was 1.8 ~ 2.3 mg. Then, the electrode was placed in a vacuum oven at 100 °C, dried for 12 h and weighed. The weight ratio of the LiTi2 (PO4)3 to LiMn2O4 in the electrodes was modulated to be 1:1.


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Preparation of redox nanocomposite electrolyte: Firstly, a certain amount of tungstosilicic acid hydrate (SiWA, AR, Aladdin reagent) was diluted into a quantity of PVA or GO / PVA aqueous solution with the PVA concentration of 0.1 g / mL. The procedure to prepare well-dispersed GO / PVA solution has been published in our previous work, [29] and then some lithium sulphate monohydrate (Li2SO4, AR, Aladdin reagent) was further mixed, and the mixture were stirred to a clear liquid state. After that, a small amount of the LiOH saturated solution was dropped very slowly and stirred continuously. The pH value of the precursor solution was adjusted to be ~7 so that the SiWA in the solution can be converted to lithium salt of tungstosilicic acid (SiWLi). So the redox-active electrolyte solution was obtained. Then, the as-prepared redox-active electrolyte solution was cast on the electrodes and evaporated at ambient conditions and the humility of 50 ~ 60% for 20 h, forming the sandwich structure of redox-active electrolyte / electrode material / stainless steel. The thickness of redox-active electrolyte film was about 0.15 ~ 0.2 mm. Here the sandwich structures containing LiTi2 (PO4)3 / AB / PTFE or LiMn2O4 / AB / PTFE active material films were marked with Ti-electrode or Mn-electrode, respectively. Besides, structural properties of redox-active electrolyte film were characterized as bellows. Water content of the composite redox-active electrolytes was determined by a thermogravimetric analyser (TGA, TA/ Q50) in nitrogen with a heating rate of 10 °C min-1. The water content of all the redox-active electrolytes with various GO content was ~15 wt%, calculated from the weight loss under the temperature range bellow 200 °C. (See Supplementary Materials Figure S-8) A wide angle X-ray diffractometer (WXRD, Rigaku / D-MAX 2200 VPC) was used to characterize the crystallization properties of samples. The scanning range of the Bragg 2θ angle ranged from 5o to 55o under a scanning rate of 5 °C min-1, with Cu Ka radiation (30 kV, 30 mA). The XRD


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pattern of the GO-contained solid redox-active electrolyte verified a full exfoliation of GO sheets and amorphous state of the sample. (See Supplementary Materials Figure S-9) Assembly of LiMn2O4 // LiTi2 (PO4)3 batteries: To assembly a solid LiMn2O4 // LiTi2 (PO4)3 battery, a Ti-electrode, a polyester spacer and a Mn-electrode were pressed in sequences to form the structure of Ti-electrode / Mn-electrode in which two redox-active electrolyte films on the electrodes combined. (See Supplementary Materials Figure S-10) The polyester spacer with a thickness of 0.13 mm and a cut shape (1 cm × 1 cm) in the centre was used to limit the distance of Ti-electrode and Mn-electrode. To avoid the influence of the air environment, the Ti-electrode / Mn-electrode was further assembled into a 2032 button battery. By contrast, to assembly a liquid LiMn2O4 // LiTi2 (PO4)3 battery, a Ti-electrode, a paper separator (~0.02mm) and a Mnelectrode were firstly pressed in sequences, and then assembled into a 2032 button battery filled with 1M Li2SO4 liquid electrolyte. Other characterizations: Atomic force microscopy (AFM, Multimode 8, Bruker) in the tunnelling AFM (TUNA) mode was used to do the morphological and current imaging of the electrode / electrolyte interface. The conductive AFM tip (PIC probe, Bruker) was platinumcoated with the tip radium of ~10 nm, which could perform as a traveling micro-electrode during the contact imaging of interface samples on the ITO electrode. As the tip / electrolyte / ITO integrated a changeable micro-capacitor, the tunnelling-AFM (TUNA) current between the tip and interface was recorded in real time. Therefore, the conductive properties across the interface could be characterized besides the morphological signals. Raman scattering signals of the sample were detected by using Raman spectroscopy (Raman, Renishaw inVia). The excitation wavelength was 514 nm with a scanning range 500–2500 cm-1. Reflection optical microscope of a VHX 5000 digital microscope system (Keyence) was used to image the electrode / electrolyte


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interface. Electrochemical workstation (IM6e, Zahner Zennium) was used to measure the cyclic voltammetries of an ITO capacitor and LiMn2O4 // LiTi2 (PO4)3 lithium-ion batteries. The electrochemical impedance spectroscopy (frequency scanning range: 10 mHz ~ 100 kHz, disturbed voltage: 5 mV) of cells in a two-electrode system was investigated. Galvanostatic charge-discharge curves and the cycling property of cell systems were measured by an Arbin Battery test system. X-ray photoelectron spectroscopy (XPS, Thermo / ESCALAB 250) was utilized to analyse the elements on the electrode-electrolyte interface.


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Figure 1. Conductive atomic force microscope (C-AFM) images of solid redox electrolyte coated on the indium tin oxide (ITO) electrode. For redox electrolyte without graphene oxide (GO) sheets: (a) height image, (b) deflection error image, and (c) TUNA current image of the same area; For redox electrolyte with GO sheets: (d) height image, (e) deflection error image, and (f) TUNA current image of the same area. Zone I and Zone II belonged to bridged RS-coated


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GO and unbridged RS-coated GO, respectively. (g) Current-Voltage (I-V) measurements at different conductive zones (marked as Zone-L: low conductivity; Zone-M: middle conductivity; Zone-H: high conductivity) corresponding to TUNA current image which could be also referred to the yellow dot square regions in (d) – (f). N was the cycle number of I-V measurements. I was determined by the following formulas: I = C dV / dt, where C was tip / sample capacitance, and dV / dt was time rate of change of voltage across the capacitor.


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Figure 2. Electrochemical characterization accompanied with the laser micro-Raman spectrometer on bridged redox species-coated GO (RS / GO) on the ITO electrode. (a) Cyclic voltammograms (CVs) of the ITO electrode with bridged RS / GO, under scanning voltage ranges of 0 ~ 1 V and 0 ~ 2 V, respectively, at a scanning rate of 5 mV s-1. Before conducting the voltage scan of 0 ~ 2 V, the capacitor was pre-charged at 1.8 V for 5 h. In the inset of (a), the decomposition voltage (Vd) of the redox electrolyte in the ITO capacitor was shown. (b) Photographs of the electrode / electrolyte solid interface with bridged GO by using the reflection optical microscope imaging during cyclic voltammetry scans of (I) 0 ~ 1.0 V and (II) 0 ~ 2.0 V, respecitively. (c) Raman spectra of brown (GO-rich) regions and black (rGO-rich) regions in electrode side of the electrode-electrolyte interface respectively, excitation wavelength: 514 nm. (d) Schematic illustration on the bridged RS / GO or RS / reduced-GO (rGO) perfoming as extended redox interface for enhancing energy delivery.


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Figure 3. (a) The CVs of LiMn2O4// LiTi2(PO4)3 batteries assembled with PVA / Li2SO4 / SiWLi solid electrolyte and with 1 mol / L Li2SO4 liquid electrolyte at a scanning rate of 0.1 mV s-1, respectively. (b) Comparison of the three CV cycles of a LiMn2O4 // LiTi2(PO4)3 battery using solid redox electrolyte. (c) The initial charging and (d) discharging behaviors of the LiTi2(PO4)3 // LiMn2O4 battery systems using the solid redox electrolyte with 0.1 wt% GO, compared with those without GO or using Li2SO4 liquid electrolyte, at 0.2 A g-1 between 0 and 2.0 V. The energy storage from redox electrolyte in the initial charging (E) and second charging (E’) was calculated from the charge capacity within ~1.5 V. (e) Cycling stability of a battery assembled with solid redox electrolyte containing 0.1 wt% GO at a 0.2 A g-1 between 0 and 1.85 V.


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Figure 4. (a) Conductivity (σc) of the electrolyte, conductivity (σic) of the electrode-electrolyte interface and interfacial energy storage capacity plotted against GO content, respectively; (b) Schematics of RS-coated reduced GO (RS / rGO) sheets as extended interface for extending


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battery life with increasing GO content in the solid electrolyte, as well as unbridged RS / GO sheets for faciltating ionic transporation.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Additional information and figures: Cyclic voltammograms of battery electrodes in a three-electrode system, Electrochemical impedance spectrum of solid redox electrolyte, Capacity retention of the cells, Water content of solid electrolyte, Wide Angle X-ray diffraction patterns, schematic of assembly process of a solid battery

AUTHOR INFORMATION Corresponding Author * Corresponding author. Tel.: +86-20-84112715; Fax: +86-20-84114008; E-mail: [email protected]; [email protected] Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT The authors are grateful for the support of the National Natural Science Foundation of China (Grant: 51473186, U1201243), the Natural Science Foundation of Guangdong, China (Grants: 2012B0901000065, 2013B010135001 and 2015B090925002) and the Science and Technology Program of Guangzhou, China (Grant: 201508010052, 201604010105), and the Fundamental Research Funds for the Central Universities (Grant: 161gpy17).

ABBREVIATIONS RS, redox species; GO, graphene oxide; PVA, polyvinyl alcohol; SiWLi, Li4SiW12O40; ITO, indium tin oxide; AFM, atomic force microscopy; TUNA, tunneling AFM; C-AFM, conductive AFM; CV, cyclic voltammetry; rGO, reduced GO; SEI, solid electrolyte interface

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A new strategy, including casting redox nanocomposite electrolyte solution on the electrode and immobilizing redox species-coated graphene oxide (RS / GO) sheets in a solid phase after solidification, is proposed for extending battery life with the aid of RS / GO sheets bridged to the electrodes

32 ACS Paragon Plus Environment

Bridging Redox Species-Coated Graphene Oxide Sheets to Electrode

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