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Bacteria Absorption-Based Mn2P2O7-Carbon @ Reduced Graphene Oxides for High Performance Lithium-Ion Battery Anodes Yuhua Yang, Bin Wang, Jingyi Zhu, Jun Zhou, Zhi Xu, Ling Fan, Jian Zhu, Ramakrishna Podila, Apparao M. Rao, and Bingan Lu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b02036 • Publication Date (Web): 03 May 2016 Downloaded from http://pubs.acs.org on May 3, 2016

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Bacteria absorption-based binder-free flexible anodes exhibit high capacities, long cycle-life and excellent rate performance. 90x39mm (300 x 300 DPI)

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Bacteria Absorption-Based Mn2P2O7-Carbon @ Reduced Graphene Oxides for High Performance Lithium-Ion Battery Anodes Yuhua Yang,1 Bin Wang,1,2 Jingyi Zhu,3 Jun Zhou,1 Zhi Xu, 1,4 Ling Fan,1 Jian Zhu,1 Ramakrishna Podila,3 Apparao M. Rao,3 Bingan Lu1,4* 1

School of Physics and Electronics, Hunan University, Changsha 410082, P. R. China

2

Physics and Electronic Engineering Department, Xinxiang University, Xinxiang 453003, P. R. China

3

Department of Physics and Astronomy, Clemson Nanomaterials Center and COMSET, Clemson University,

Clemson, SC 29634, USA 4

2D Material Technology Company Limited, Wing Lok Street, Sheung Wan, Hong Kong 999077, P. R. China

*Correspondence to (Lu B.) [email protected]

Abstract: Development of freestanding flexible electrodes with high capacity and long cycle-life is a central issue for lithium-ion batteries (LIBs). Here, we use bacteria absorption of metallic Mn2+ ions to in-situ synthesize natural

micro-yolk-shell-structure Mn2P2O7–carbon, followed with the use of vacuum filtration to obtain Mn2P2O7–carbon @ reduced graphene oxides (RGO) papers for LIBs anodes. The Mn2P2O7 particles are completely encapsulated within the carbon film which was obtained by carbonizing the bacterial wall. The resulting carbon microstructure

reduces the electrode–electrolyte contact area, yielding high Coulombic efficiency. In addition, the

yolk-shell-structure with its internal void spaces is ideal for sustaining volume expansion of Mn2P2O7 during charge/discharge processes, and the carbon shells act as an ideal barrier limiting most solid-electrolyte interphase

formation on the surface of the carbon films (instead of forming on individual particles). Notably, the RGO films

have high conductivity and robust mechanical flexibility. As a result of our combined strategies delineated in the

manuscript, our binder-free flexible anodes exhibit high capacities, long cycle-life and excellent rate performance.

Keywords: Bacteria, yolk-shell Mn2P2O7-carbon, reduced graphene oxides, binder-free flexible anode, lithium-ion batteries

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Flexible energy storage systems critically need components with high capacity, long-life, high coulombic efficiency, high safety and low cost.1-12 Among the various energy storage technologies, lithium-ion batteries

(LIBs) are ubiquitous in portable personal electronics and grid storage due to their relatively high discharge voltage and excellent energy density.13-19 However, the cycle stability of electrodes is still limited due to the

degradation of the electrode materials and the unstable solid-electrolyte interphase (SEI) on the electrode’s

surface- the latter instability is due to the large volume change which occurs during the charge / discharge processes.20-23 Furthermore, the breakdown of the SEI layer leads to the formation of a fresh SEI on the

electrodes surface in the subsequent cycle, which leads to poor Coulombic efficiency and poor Li ion transport due to the accumulated SEI.24, 25 Thus suitable electrode materials that satisfy the long term stability requirement with high capacities are important for realizing large-scale energy storage technologies. 26-29

Biological structures that have resulted from millions of years of natural selection, often serve as an effective guide for novel materials design.30, 31 Bacteria are constantly exposed to stressful situations and an ability to resist

those stresses is essential for their survival. Moreover, the ability of bacteria to grow in the presence of high

metal-concentrations and absorb metallic ions plays an important role in many technological applications, such as in the fields of biomineralization, nanoscience, and energy.14,32-34 As a cheap resource that is present all over the world,35 bacteria can serve as effective templates to synthesize materials (nanometer to micrometer dimensions) with interesting features, especially for advancing energy storage technologies.31 However, the template method

has its own drawbacks, viz., the need to remove the templates, or template’s inability to effectively protect the material when used as anodes for LIBs.36,

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To circumvent these drawbacks, and to develop flexible high

performance LIBs, we synthesized free-standing reduced graphene oxide (RGO)-based composite papers that are mechanically strong and electrically conducting for use as electrodes in flexible energy storage devices.13, 32, 38-41 In this article, we use bacteria to absorb Mn2+ ions, and then in-situ synthesize micro-yolk-shell-structure of

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Mn2P2O7–carbon. Next, using the vacuum filtration process, we prepare Mn2P2O7–carbon @ RGO composite paper as a freestanding binder-free flexible anode for LIBs. We find that the Mn2P2O7–carbon @ RGO paper when used as an anode material exhibits a high capacity, long cycle-life and excellent rate performance.

Results/Discussion The overall synthetic process of the Mn2P2O7–carbon @ RGO paper is schematically illustrated in Figure 1. As shown in Figure 1 (a) and Supplementary Fig. 1 two main strategies were involved in this process. Firstly,

the Gram-positive Bacteria Bacillus Subtilis (GPBBS) bacteria absorb the metallic salt (manganese acetate). In this

process, the bacteria were cultured in deionized water with glucose for 16 hours to obtain the bacterial precursor

solution. In our experiment, when More enough Manganese acetate particles (5g in 200 ml of deionized water)

were added into the GPBBS solution after the GPBBS were cultured for 16 hours at room temperature, which

indicates the GPBBS will die or sleep when more enough Manganese acetate particles was added, and the GPBBS which absorbed Mn2+ ions subsequently annealed at 700 ° C for 2 h with a heating rate of 11.7 ° C min-1 in an argon filled tube furnace. the GPBBS will be carbonize, they should have no chance to oxidize the absorbed Mn2+ ions into Mn3+ by the polysaccharides, proteins or enzymes catalyzing. The manganese acetate salt was used as metallic precursor and dissolved in the bacterial precursor solution for 24 hours. The bacteria absorb the Mn2+ ions

via their bacterial walls and retain them in their interior (see schematic in Figure 1 (b) and Supplementary Fig. 2). Next, the Mn2+ absorbed bacteria were separated by centrifugation with different rotational speed, and added to a

GO solution to obtain freestanding binder-free flexible paper via the vacuum filtration method. Finally, the paper was annealed at 700 oC for 2 h with a heating rate of 10 oC min-1. During annealing, the bacterial wall and GO are changed into carbon and RGO, respectively, while the absorbed Mn2+ ions combine with the phosphorous organic

compounds which mainly composed of CHPO ingredients (for example, teichoic-acid or peptidoglycan) present in

the bacteria to form MnH2PO4 nanoparticles (Supplementary Fig. 3), and after annealing at 700 ° C for 2 h with a

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heating rate of 11.7 ° C min-1 in an argon filled tube furnace, the MnH2PO4 nanoparticles will transform into very

stable Mn2P2O7 nanoparticles. Hence, the free standing binder-free flexible Mn2P2O7–carbon @ RGO paper was obtained.

The storage of Li ions in Mn2P2O7 proceeds via Li intercalation into specific sites and subsequent relaxation of the unit cell as shown in Figure 1(c) and Supplementary Fig. 4. The present calculations were performed

based on the density functional theory (DFT) within the Cambridge Serial Total Energy Package (CASTEP) plane wave code.42-44 We first examine what are the typical positions for Li intercalation in Mn2P2O7 and study the nature of Li-Mn2P2O7 interaction. Four high-symmetry intercalation sites were considered for a single Li atom in

Mn2P2O7 (Supplementary Fig. 4). The Li intercalation energy (Ei) is defined as where

E Li + Mn P O

2 2 7

and

EMn P O

2 2 7

Ei = ELi+Mn2P2O7 − ELi − EMn2P2O7

are total energies of Li-intercalated and pristine Mn2P2O7, respectively, while ELi

,

is

the energy of isolated Li. By our definition ( Ei < 0 ) corresponds to an exothermic reaction and is the preferred interaction. Supplementary Table 1 summarizes the calculated intercalation energies. After geometrical

optimization, the volume of Mn2P2O7 with Li intercalation was found to expand by 21, 7, 18 and 5% for M1, T, M and B sites (see Supplementary Fig. 4) respectively. We found that the T site is the most energetically stable intercalation site which is due to the lowest intercalation energy. When the Li atom is located at the T site, it has 4

O nearest and 2 Mn neighbors with Li. All the neighboring Mn and O atoms move outwards from the Li atom after

relaxation, and the Mn-O bond is broken. When Li is at M1 site it has 4 O nearest neighbors, and the neighboring

Mn and O atoms move outwards from the Li atom, and the largest volume expansion is obtained for this

configuration. The M site is a less stable position for Li, since Li tends to moves towards the M1 site. The B site is

not a stable position, as it exhibits positive adsorption energy. As shown in Supplementary Table 1, while the

intercalation energies of Li for M1, T, M sites are negative (indicating a favorable Li- Mn2P2O7 interaction), Li intercalation is most favored at the T site. Thus, 8 Li atoms can intercalate into one unit bacterial of Mn2P2O7.

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Furthermore, with intercalated Li the Mn-O bond breaks and the volume of the unit bacterial expands (see

Supplementary Table 2). However, Li doesn’t bond with Mn or O to form Li-Mn or Li-O bond.

Figure 2 (a) shows a photograph of the as-peeled Mn2P2O7–carbon @ RGO paper after vacuum filtration. The addition of GO during the vacuum filtration and annealing process makes the Mn2P2O7–carbon @ RGO paper robust, which peels off as a flexible freestanding paper in which no binder was used. It remains robust without

noticeable changes (from structural and electrochemical performance standpoints) when bent to a large extent

implying that the Mn2P2O7–carbon @ RGO paper can be used as a suitable electrode in flexible energy storage devices. The micro structure (from top-view), shape of the GPBBS and paper thickness (cross-section view) are

shown in SEM images of Figure 2 (b, c, d), respectively. Lots of GPBBS are distributed uniformly within the

RGO paper with no impurity adhering the GPBBS surface as well as RGO (Figure 2 (b)), confirming that the (i) metallic Mn2+ ions outside the GPBBS and on the RGO surface were washed away completely, and (ii) Mn2P2O7 inside the GPBBS plays an active role during the charge/discharge cycles (Supplementary Fig. 5). After

annealing, the Mn2P2O7–carbon @ RGO paper retains its microporous structure and Figure 2 (c) shows an expanded view of the annealed GPBBS (size ~900 nanometers) without any cracks, which also implies that the

GPBBS retain its flexibility. Annealing causes the GPBBS to shrink resulting in its wrinkled surface. As

exemplified in Supplementary Figure 6, the GPBBS acquire different wrinkled surfaces but are uniform in size

and distribute uniformly in the RGO. From the cross-sectional SEM image (Figure 2 (d)), the thickness of

Mn2P2O7–carbon @ RGO paper is estimated to be ~1.8 to 2.5 µm. Importantly, the Mn2P2O7–carbon @ RGO paper is flexible (Figure 2 (a)), and dispersed within it are a lot of GPBBS as indicated by the red arrows in

Figure 2 (c). To further confirm whether the Mn2+ ions have been absorbed into the GPBBS, the distribution of the

Mn2P2O7 in the bacteria and the crystal lattice of Mn2P2O7, transmission electron microscopy (TEM) and

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high-resolution TEM (HRTEM) studies were performed. In Figure 2 (e), it is evident that the GPBBS are coated

with graphene sheets and are distributed uniformly. Their sizes range from ~800 nm to 1 µm, and reveal the

presence of a single micro-yolk-shell Mn2P2O7–carbon (Figure 2 (f)). The carbon bacteria wall is integrated without any fissures, implying that the bacteria limit the intake of Mn2P2O7 very well, and protect the Mn2P2O7 from egregiously assembling or expanding by their inherent structures. An obvious carbon wall (thickness ~10 - 15

nm) is evident in the expanded image (Figure 2 (g)) and Figure 2 (h) represents its crystal lattice image (deduced

from HRTEM) with a lattice constant of 0.32 nm, corresponding to the (111) plane. Lastly, the HRTEM images

confirm that no Mn or other foreign materials are present outside the bacteria, which again proves the purity of the

as-prepared materials.

The crystal structure and phase purity of the Mn2P2O7–carbon @ RGO paper can also be inferred from Figure 2 (i). The diffraction pattern of the as-prepared material is consistent with the standard diffraction peak of (111) crystallographic plane of Mn2P2O7 (PDF#: 29-0891), indicating that the Mn2+ ions have been completely transformed into Mn2P2O7. The elemental maps shown in Figure 2 (j) further confirmed the nature of the Mn2P2O7–carbon @ RGO paper: elements Mn, P, and O are distributed homogeneously inside the bacterial, and are present in higher

concentration at both ends of the bacteria.

Raman spectroscopy to confirmed the nature of the Mn2P2O7–carbon @ RGO composite paper (see Supplementary Fig. 5) with the pristine RGO paper exhibiting the disorder-induced D (~1350 cm-1) and the graphitic G (~1585 cm-1) bands, while the Raman spectrum of Mn2P2O7–carbon @ RGO paper exhibited additional peaks ~372, 526, 716, and 1030 cm−1, which correspond to Mn2P2O7. Comparing with Figure 2 (i) and Figure 2 (j), it should be Mn2P2O7 inside the GPBBS. The Raman peak(s) located at (i) ~372 and 526 cm−1 correspond to the Raman vibration induced by the swing of the metallic Mn2+ ions and chemical P-O bond, (ii)

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~716 cm−1 correspond to the symmetrical and anti- symmetrical stretching vibration of the P-O-P bridge of P2O74-, and (iii) ~1030 cm−1 is due to the stretching vibration of chemical P-O bond of the two PO3

2-

group in the P2O74-

group.45-46

To investigate the electrochemical characteristics of the Mn2P2O7–carbon @ RGO paper, a series of electrochemical measurements were carried out. The cyclic voltammetry (CV) of the Mn2P2O7–carbon @ RGO paper was conducted at a scan rate of 0.2 mV s-1 in the voltage range between 0.01 to 3.00 V (see Figure 3 (a)).

The first cycle displays two pairs of obvious characteristic cathodic and anodic peaks at the potential of (0.34, 1.07

V) and (0.01, 0.17 V), respectively, which can be ascribed to the irreversible decomposition of Mn2P2O7 to metallic Mn (equation (1)). Subsequently the cathodic peak at 0.34 V transferred to peak 1.17V in the second and

the later cycles, which is probably due to the formation of the solid electrolyte interphase (SEI) membrane

(equation (2)) and the decrease in the decomposition of Mn2P2O7 to metallic Mn (equation (1)). Meanwhile, the anodic peak at 1.07 V becomes weaker in the subsequent cycles indicates partial reversibility of the reaction

(equation (1)). The peak at about 0.01 V is attributed to the alloying of LixMn, and the oxidation peak at 0.17 V in the anodic process corresponds the reversible dealloying of LixMn (equation (3)).47, 48 On the other hand, the peak around 0.17 V indicates the Li extraction from the carbon material (equation (4)) and dealloying of the LixMn (equation (3)), respectively. Mn2P2O7 + 2Li+ + 2e-→Li2MnP2O7 +Mn

(1)

Li2MnP2O7 + 2Li+ +2e-↔ Li4P2O7 +Mn

(2)

Mn + xLi+ + xe-↔ LixMn

(3)

C + xLi+ + xe-↔LixC

(4)

Figure 3 (b) depicts the charge/discharge curves of the Mn2P2O7–carbon @ RGO paper. It is characterized at a current density of 100 mA g-1 with 0.01 - 3 V voltage range. The voltage plateau at 0.2 - 0.75 V is observed in

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the first discharge process, implying the conversion reaction between Mn2P2O7 and metallic Li, which leads to the formation of metallic Mn and Li2MnP2O7. The following long slope profiles of the Mn2P2O7–carbon @ RGO paper indicates the formation of Li–Mn alloys and the insertion of Li+ into the Mn2P2O7–carbon @ RGO paper. Because most of the metallic Mn is formed in the first cycle, its discharge plateau at 0.2 - 0.75 V changes to 0.5 -

1.2 V from the second cycle. The charge-discharge curves of Mn2P2O7–carbon @ RGO paper, when compared with the corresponding curves of pure bacteria on RGO paper (Supplementary Fig. 8 (a)), the discharge curve in

the first cycle for the latter has a shorter and less remarkable voltage plateau. Moreover, no discharge voltage

plateau is observed beyond the second cycle in the case of the pure bacteria on RGO paper.

We also investigated the performances of the Mn2P2O7–carbon @ RGO paper at increasing current densities. As shown in Figure 3 (c), The Mn2P2O7–carbon @ RGO paper displays an excellent rate capability, and presents approximately a reversible capacity of 880.0, 820.0, 740.0, 690.0, 630.0, 585.0 and 400.0 mA h g-1 at different current densities of 100, 200, 400, 600, 800, 1000 and 5000 m Ag-1, respectively. Particularly, even at a high current density of 1000 mA g-1, a reversible capacity of 590.0 mA h g-1 was achieved for the Mn2P2O7–carbon @ RGO paper. Furthermore, upon reducing the current density back to 100 mA g-1, the electrode delivers a specific discharge capacity of about 850.0 mA h g-1, implying a retention of 96.6%. Supplementary Fig. 8 (b) displays the

rate performance of the pure bacteria on RGO paper, which also exhibits a high Coulombic efficiency and excellent cycle performance. When compared to Figure 3 (c), its average capacity in the 280 to 300 m A g-1 range

is much lower than the corresponding capacities exhibited by the Mn2P2O7–carbon @ RGO paper. When the discharge rates were raised to 100, 200, 400, 600, 800, and 1000 mA g-1, the reversible capacities of the pure bacteria on RGO paper attained only 280.0, 169.0, 130.0, 117.0, 105.0 and 96.0 mA h g-1, respectively. Clearly,

this result indicates that the Mn2P2O7–carbon @ RGO paper sustains a relatively much higher current density, besides exhibiting a good recovery performance, which are necessary electrodes characteristics for realizing high

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power LIBs.

The cycle performance of the Mn2P2O7–carbon @ RGO paper as electrodes was characterized in 0.01 – 3.0 V voltage range at a current density of 100 mA g-1 over 125 cycles. As plotted in Figure 3 (d), in the first cycle, the specific charge and discharge capacity of the Mn2P2O7–carbon @ RGO paper are 891.6 and 1153.3 mA h g-1 respectively, corresponding to an initial Coulombic efficiency of 77.3%. The coin cells have a large and

irreversible capacity which is due to the formation of amorphous LixMn matrix, as well as the intense surface

reactions with the Li–Mn compounds and the electrolyte solution.

With the increasing number of cycles, the

Coulombic efficiency of the material increases quickly, and stabilizes after the fifth cycle to ~98.0% - 99.8%. The discharge capacities of the Mn2P2O7–carbon @ RGO paper are 859.1, 860.7, 881.5, 890.4 and 885.51 mA h g-1 in the 10th, 20th, 50th, 100th and 125th cycles respectively. Similarly, the cycle performance of the pure RGO paper anode and the pure bacteria carbon on RGO paper anode were characterized in voltage range of 0.01 - 3.0 V at a current density of 100 mA g-1 over 20 and 125 cycles, respectively. As shown in Figure 3 (d), the pure RGO paper anode only presents ~ 140 mA h g-1 which

indicates the pure RGO contributes little capacity in Mn2P2O7 –carbon @ RGO paper anode. From Figure 3 (d), it is evident that the pure bacteria carbon on RGO paper maintains a good cycle performance, however, with much lower capacity values. The capacity values are 293.7, 292.6, 303.4, 275.4, 277.5 mA h g-1 in the 10th, 20th, 50th, 100th and 125th cycles respectively, which is roughly about one third of the capacity exhibited by the

Mn2P2O7–carbon @ RGO paper. The main reason for the low capacities of the pure bacteria carbon on RGO paper is the absence of metallic ions in the bacteria, which enhances the capacities. Therefore, the pure bacteria carbon on RGO paper exhibits much lower capacity retention reaching only a reversible capacity of 277.5 mA h g-1 in the 125th cycle. As Mn2P2O7 –carbon @ RGO paper, the bacteria serve as carbon (shell) source, after annealing at 700 ° C for 2 h with a heating rate of 11.7 ° C min-1 in an argon filled tube furnace, the bacteria will be carbonized and

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form fine graphitizing bacteria carbon (assuring the conductivity between the Mn2P2O7 particles with RGO in charge/discharge process) and form a close contact with RGO. The framework of the graphitizing bacteria carbon

with RGO not only hinders the aggregation of Mn2P2O7 but also provides enough space to buffer the volume changes during the lithium insertion and extraction reactions in Mn2P2O7. Moreover, as a result of the close contact between the graphitizing bacteria carbon and the RGO framework, the Mn2P2O7 nanoparticles (yolk) can be prevented from expelling out of the graphitizing bacteria carbon (sheell) during the course of lithiation and

delithiation, which is good for achieving high cycling stability performance. Apart from these reasons, the

synergistic effect between the ultrafine Mn2P2O7 nanoparticles insertion in the bacteria and RGO matrix is an important reason for high capacities and high cycling stability performance. From all of the above, the

Mn2P2O7–carbon @ RGO paper anode electrodes exhibit consistently higher capacities than the pure bacteria on RGO paper electrodes. It is noteworthy that the electrode Mn2P2O7–carbon @ RGO paper exhibits a high capacity even with ultrahigh current density (see Figure 3 (e) and Supplementary Fig. 9). As shown in Figure 3 (e), it has specific charge/discharge capacity averages approximately in 400 mA h g-1 with a current density of 5000 mA g-1

and the Coulombic efficiency averages in 99.0%, which is ascribed to the superiority of bacterial-assisted

structure of the anodes described above.

To confirm the mass percentage of the Mn2P2O7–carbon @ RGO paper, the as-prepared materials were analyzed by TGA in air (see Supplementary Fig. 10). As shown in Supplementary Fig. 10 (a), the major mass

loss of the GO arises mainly in two stages: 180 -220 ºC and 520-620 ºC, which correspond to the loss of functional

group and carbon component respectively. The Supplementary Fig. 10 (b) displays that the major mass loss of

the carbon @ RGO paper arises mainly in 470-575 ºC which correspond to the loss of carbon component. As

shown in Supplementary Fig. 10(c), the weight loss decreases slowly in the beginning, following by a big mass

loss arises mainly between 430 and 550 ºC, which is due to the loss of carbon component. The mass fraction of the

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Mn2P2O7 particles was ~49.2%, and the mass fraction of carbon in the Mn2P2O7–carbon @ RGO paper was ~50.8%. Suppose that the capacity of the carbon fraction is 372 mA h g-1,49, 50 then the capacity of pure Mn2P2O7 is estimated to be ~1350 mA h g-1.

To evaluate the crystallinity of the electrode materials after cycling, we examined its in situ x-ray diffraction

pattern. The transparent pouch cell was placed on a diffractometer, and connected to a computer controlled battery

testing system. As shown in Figures 4 (a) (b), the changes in the diffraction pattern mainly concern the (-131),

(131) and (221) lattice planes. While discharging the transparent pouch cells from the capacity of 0 to 201 mA h g-1, 201 to 405 mA h g-1, and till the end of 880 mA h g-1, the (-131), (131) and (221) diffraction peaks grow in

intensity and gradually shift to the right. This indicates that the Mn2P2O7–carbon @ RGO paper has absorbed lithium ions,51 and the corresponding crystal lattice d = 2.348, 2.171, 2.078 Å, respectively. As the (-131), (131)

and (221) are the most intense peaks (38.3, 41.6 and 43.5 degrees, respectively) we focused our data collection

around two regions from 35-40, and 41-46 degrees.

As metallic lithium reacts with Mn2P2O7 (Figure 4 (a) (b)),

we observe a sustained growth of peak slowly in the Mn2P2O7 Bragg peaks at (-131), (131) and (221), which implying insertion of lithium ions into the Mn2P2O7 nanoparticles. Juxtaposing the results from charge/discharge measurements, the discharge plateau at 0.5 to 1.2V arising at a 100 mA g-1 due to a decomposition /amorphization

of the Mn2P2O7–carbon @ RGO paper. With exactly four electrons are consumed in this reduction plateau (equation (1), equation (2)), we can again confirm that Mn2P2O7 reacts with lithium in light of the following reaction: Mn2P2O7 + 2Li+ + 2e-→Li2MnP2O7 +Mn, Li2MnP2O7 + 2Li+ +2e-↔Li4P2O7 +Mn, Mn + xLi+ + xe-↔ LixMn and C + xLi+ + xe-↔LixC.

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The occurrences of the three apparent diffraction peaks show the formation of metallic Mn and Li4P2O7 nanoparticles, which is presumed to be bigger than the x-ray coherence length. When the transparent pouch cells are charged from capacity of 0 to 402 mA h g-1, and 402 to 800 mA h g-1, the (-131), (131) and (221) diffraction

peaks diminish in intensity and gradually shift to their original positions as seen in Figure 4 (a) (b), which can be

ascribed to the change in the number of the metallic Mn and Li4P2O7 nanoparticles, and the extraction of lithium ions, respectively.

Schematic illustration of the charge/discharge of the Mn2P2O7–carbon @ RGO paper is shown in Figure 4 (c). Firstly the Mn2P2O7–carbon @ RGO paper micro-yolk-shell-structure is small relatively with no SEI membrane inside or outside the bacteria. During discharge, a SEI membrane forms on the outer surface of the

bacteria, metallic Li react with Mn2P2O7 nanoparticles and intercalate into Mn2P2O7, resulting in their expansion. Accordingly the bacteria shell is forced to expand but is inhibited from boundless expansion by the nature of its

structure.

During the charging process, the bacteria and materials inside the bacteria will contract to their original

size, but the SEI membrane does not decrease. As seen in Figure 4 (d), the Mn2P2O7–carbon @ RGO paper inside the stainless steel battery case remains intact even after 125 cycles. The electrical conductivity of the

Mn2P2O7–carbon @ RGO paper is entirely dependent on RGO (without the need for any metallic current collector), which proves its free-standing nature and flexibility.

The SEM, TEM and HRTEM were carried out on the Mn2P2O7–carbon @ RGO paper after 200 charge/discharge cycles. The paper was first cycled with increasing current densities of 100, 200, 400, 600, 800, 1000 and 5000 mA g-1 for each 10 cycles. The same cycling process was preformed once again, and then finally the paper was additionally charge/discharged for 60 cycles with 100 mA g-1. As shown in Figure 4 (e), a lot of

bacteria are still present in the RGO paper. Furthermore, though the bacteria are coated with a thick SEI membrane,

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they can undergo charge/discharge for 200 cycles, implying good circulation capacity of the material (Figure 4 (f)).

Lastly, a negligible change in the size of the bacteria is observed even after 200 charge/discharge cycles.

Conclusions In conclusion, we have developed a micro-yolk-shell-structure of Mn2P2O7–carbon and method to prepare freestanding binder-free flexible Mn2P2O7–carbon @ RGO paper as high performance anodes for LIBs. In our strategy, we used bacteria encapsulated manganese acetate as a precursor to prepare the Mn2P2O7–carbon @ RGO paper via the vacuum filtration method. The TEM and SEM studies indicated that ultrafine Mn2P2O7 nanoparticles were absorbed into the bacteria, which were uniformly dispersed in RGO. As a result of micro-yolk-shell-structure

and the double protection of the carbon shell of the bacteria and the RGO, our Mn2P2O7–carbon @ RGO paper exhibited a highly reversible capacity, high cycling stability, long cycle-life and excellent rate performance with an average capacity of 880.0 mA h g-1 at a current density of 100 mA g-1. Such nanostructured electrodes also exhibited an extremely durable high-rate capability: a capacity of 585 mA h g-1 at a current density of 1000 mA g-1 over 200 cycles and 400 mA h g-1 at a current density of 5000 mA g-1 over 2000 cycles. We posit that this

synthetic strategy can be further used to assemble various other nanomaterials with promising applications in

energy storage.

Methods/Experimental Materials

Ethanol (CH3CH2OH, Tianjin Fuyu Fine Chemical Co. Ltd, China), Glucose (C6H12O6·H2O, Tianjin Hengxing Chemical Reagent Manufacturing Co. Ltd, China), Manganese acetate (Mn( CH3COO) 2·4H2O, Tianjin Guangfu Fine Chemical Institute, China), Graphene Oxides (GO, Nanjing Xianfeng nanomaterial technology Co.

Ltd, China) were analytical grade and used without further purification. GPBBS (Beihai Qunlin Bio-engineering

Co. Ltd, China) were purchased and cultured by ourselves. Preparation of the bacterial precursor solution 13


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Firstly, 10g of GPBBS powder and 4g of glucose were dissolved in 200 ml of deionized water in a culture

dish which had been washed. Then the solution was stirred slowly for five minutes until a uniform solution was

obtained. Finally, the solution is left standing for 16 hours. Preparation of the Manganese acetate @ bacteria

5g of Manganese acetate particles were dissolved in the bacterial precursor solution under magnetic stirring

for five minutes then let alone for 24 hours. Then the resulting solution was centrifuged at 1500 r/min for 2

minutes to get rid of dregs, a moderate suspended solution was then centrifuged at 4500 r/min for 5 minutes and

washed three times (Ethanol, deionized water, deionized water, respectively) for five minutes every time to obtain

Manganese acetate @ bacteria. Preparation of the Mn2P2O7–carbon @ RGO paper 0.15g of graphene oxides (GO) were dispersed in 10 g of ethanol with ultrasonic sound for two hours. Then

they were transferred into the as prepared Manganese acetate @ bacteria solutions (1:1, w/w) with vigorous stirred

for 30 minutes. Thus the mixture solutions were filtrated by vacuum using an ordinary filtration membrane.

Subsequently the mixture film was dried in vacuum drier for 2 hours, then the freestanding binder-free flexible

paper was peeled off from the filtration membrane, and was annealed at 700 ° C for 2 h with a heating rate of 11.7 °C min-1 in an argon filled tube furnace. Thus the free standing binder-free flexible micro-yolk-shell

Mn2P2O7–carbon @ RGO paper was prepared. Materials characterizations

The images of the as-prepared materials were acquired by field-emission scanning electron microscopy

(FE-SEM, Hitachi S-4800, 5kV). The transmission electron microscopy (TEM) and high-resolution TEM

(HRTEM) images were recorded on a FEI Tecnai G2 F-20 high-resolution transmission electron microscope. The

phase composition and crystallinity of the as-prepared materials were taken on X-ray diffraction (XRD, Rigaku

D/max-2500, Cu Ka, λ=0.154056 nm) over a 2θ range of 5-80 degrees. The Raman spectrum was recorded by

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Raman spectra meter (Witec alpha 300R, Germany). In addition, the thermal gravimetric analysis (TGA) was

performed on a Thermo-Gravimetric Analysis (TGA, PerkinElmer, Diamond TG/DTA) from room temperature to 1000 ° C at a heating rate of 6 ° C min-1 in air. Computational method

The present calculations were performed based on the DFT

42, 43

within the Cambridge Serial Total Energy

Package (CASTEP) plane wave code44. Ultrasoft pseudopotentials were used to describe the interaction of ionic core and valence electrons. Valence states were considered in this study corresponding to P 3s23p3, O 2s22p4, Mn 3d54s2 and Li 1s22s. The generalized gradient approximation (GGA) of Perdew–Burke–Ernzerh method parameterized by Perdew was used to calculate the exchange and correlation terms.52, 53 Brillouin-zone integrations were performed using Monkhorst and Pack k-point meshes.54 During the calculation, the 380 eV for cutoff

energies and 4×3×6 for the numbers of k-point can ensure the convergence for the total energy. All the calculations

were considered converged when the maximum force on the atom was below 0.01eVÅ−1, maximum stress was

below 0.01GPa, and the maximum displacement between cycles was below 0.001 Å. After geometry optimization,

it is found that the ground state of Mn2P2O7 should be antiferromagnetic (Supplementary material). Electrochemical measurements

The Mn2P2O7–carbon @ RGO paper was used directly as the working anode, metallic lithium sheet as counter electrode. The electrolyte was 1M LiPF6 in ethylene carbonate and diethyl carbonate (1:1 v/v), the microporous polypropylene played the separator between metallic lithium sheet and our working anode

respectively. The cells were assembled in a glove box filled with pure argon in which both the moisture and

oxygen contents were controlled to be less than 0.5 ppm. The coin cells (pouch cells used in in-situ measurement)

were allowed to soak for 24 hours and measured on a computer controlled battery tester system (Neware

BTS-CT-3008-TC 5.X, Shenzhen, China).The cells were charged and discharged galvanostatically in the voltage

range between 0.01 V to 3 V at various current density. Cyclic voltammetry (CV) measurements were carried out 15


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on a computer controlled battery tester system (Arbin BT-2000) at a scan rate of 0.2 mVs-1 in the potential range of

0.01-3.0 V. Electrochemical impedance spectroscopy (EIS) measurements were carried out using an

electrochemical work station (Chenhua CHI 660E, Shanghai, China)in a frequency range from 0.01Hz to 100 kHz

at room temperature.

Conflict of Interest:

The authors declare no competing financial interest.

Supporting Information

The Supporting Information is available free of charge on the Internet at http://pubs.acs.org.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (no. 21303046 and 214

73052), the Natural Science Foundation of Hunan Provinces (No. 201324), the Research Fund for the Doctoral

Program of Higher Education (no. 20130161120014) and Hunan University Young Scientists fund

(531107040668). JZ and AMR acknowledge the financial support through the National Science Foundation SNM

Grant #1246800.

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Figure 1: Growth mechanism and calculations. (a) Schematic illustration of the Gram-positive Bacteria Bacillus Subtilis (GPBBS) absorption to the Mn2P2O7–carbon @ RGO paper. (b) Schematic illustration of the GPBBS absorbing Mn2+ ions. (c) Schematic illustration of the Li+ ions intercalate into Mn2P2O7.

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Figure 2: Morphology and crystalline structure. (a) Photograph of a Mn2P2O7–carbon @ RGO paper after annealing. (b, c, d) SEM images of the Mn2P2O7–carbon @ RGO paper after annealing. (b, c) the top view of the paper, ( d) cross-section view of the paper, the red arrows point to parts of those carbonized bacteria structures. (e, f, g, h) TEM images of the Mn2P2O7–carbon @ RGO paper. (i) XRD pattern of the Mn2P2O7–carbon @ RGO paper, and standard x-ray diffraction pattern. (j) Elemental mapping of the Mn2P2O7–carbon @ RGO paper.

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Figure 3:

Electrochemical performances. (a) Cyclic Voltammetry (CV) curves of the Mn2P2O7–carbon

@ RGO paper. (b) Charge/discharge curves of the Mn2P2O7–carbon @ RGO paper at a current density of 100 mA g-1 at the 1st, 2nd, 5th, and 10th cycles. (c) Capacity of

the measured Mn2P2O7–carbon @ RGO paper at increasing

current density, and coulombic efficiency plotted as a function of cycle number. (d, e) Capacity and coulombic efficiency plotted as a function of cycle number of Mn2P2O7–carbon @ RGO paper, pure RGO paper and pure bacteria carbon on RGO paper, for a current density of

(d) 100 mA g-1, and (e) 5000 mA g-1.

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Figure 4: Reaction mechanisms and morphology and structure changes. (a) (b) In-situ XRD patterns of the Mn2P2O7–carbon @ RGO paper charged/discharged to different charging capacity. (a) Enhanced intensity of the (-131) diffraction peak in the 35-40 degree region. (b) Enhanced intensities of the (131) and (221) diffraction peaks in 41-46 degree region. (c)

Schematic illustration of the Mn2P2O7–carbon @ RGO paper discharge/charge

processes. (d) Photograph of the Mn2P2O7–carbon @ RGO paper in stainless steel battery case after 125 cycles measurement. (e)

SEM images of the Mn2P2O7–carbon @ RGO paper after 200 cycles. (f) TEM images of the

Mn2P2O7–carbon @ RGO paper after 200 cycles.

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Carbon@Reduced Graphene - ACS Publications - American

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