Waste Recycling

Recycling Bacteria for the Synthesis of LiMPO4 (M = Fe, Mn) Nanostructures for High-Power Lithium Batteries Yanping Zhou, Dan Yang, Yi Zeng, Yan Zhou, Wun Jern Ng, Qingyu Yan,* and Eileen Fong*

In

this work, a novel waste-to-resource strategy to convert waste bacteria into a useful class of cathode materials, lithium metal phosphate (LiMPO4; M = Fe, Mn), is presented. Escherichia coli (E. coli) bacteria used for removing phosphorus contamination from wastewater are harvested and used as precursors for the synthesis of LiMPO4. After annealing, LiFePO4 and LiMnPO4 nanoparticles with dimensions around 20 nm are obtained. These particles are found to be enveloped in a carbon layer with a thickness around 3–5 nm, generated through the decomposition of the organic matter from the bacterial cell cytoplasm. The battery performance for the LiFePO4 is evaluated. A high discharge capacity of 140 mAh g−1 at 0.1 C with a flat plateau located at around 3.5 V is obtained. In addition, the synthesized particles display excellent stability and rate capabilities. Even under a high C rate of 10 C, a stable discharge capacity of 75.4 mAh g−1 can still be achieved.

1. Introduction Bioremediation using living microorganisms has been recognized as an organic and environmental friendly strategy to remove contaminants from the environment or transform into innocuous substances.[1] Bioremediation for instance, is particularly attractive in phosphorous (P) recovery. Groups of bacteria are used to biologically extract P in municipal

Y. P. Zhou, D. Yang, Dr. Y. Zeng, Prof. Q. Yan, Prof. E. Fong School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue, Singapore 639798 E-mail: [email protected]; [email protected] Y. P. Zhou, Y. Zhou, W. J. Ng Advanced Environmental Biotechnology Centre Nanyang Environment and Water Research Institute Nanyang Technological University 1 CleanTech Loop, Singapore 637141 Prof. Q. Yan TUM CREATE, 1 CREATE Way, #10–02 CREATE Tower Singapore 138602 DOI: 10.1002/smll.201400568 small 2014, DOI: 10.1002/smll.201400568

wastewater treatment plants.[2] Subsequently, the bacteria are harvested and reused as fertilizers or landfills. It is estimated that 15–20% of world demand for phosphate rock could potentially be satisfied by recovering phosphorus from domestic waste streams.[3,4] Hence, there is huge potential for bacteria used in wastewater treatment plants to serve as an alternative P resource. Yet, there had been little effort devoted to convert bacteria into other useful substances, apart from using the waste bacteria directly as fertilizers. In this work, we demonstrate a novel biological method to process bacteria into nanomaterials with potential applications for energy storage. Particularly, lithium metal phosphate (LiMPO4) nanomaterials have attracted considerable interest as a promising class of cathode materials in lithium ion batteries (LIBs). LiMPO4 nanomaterials are advantageous due to their large surface areas, high stability, and exceptional rate-cycling properties, while other cathode materials such as lithium metal oxides suffer from low stability, or inabilitiy to obtain high circuit voltages in the case of vanadium oxides.[5,6] Traditionally, chemical routes are used for the synthesis of LiMPO4, but recently, bio-molecules including proteins, bacteria, and viruses have also been explored as templates to prepare such inorganic nanomaterials with

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varied nano-architectures.[7–11] For example, Kim and coworkers reported bacteria-directed porous Co3O4 hollow nanorods which showed comparable electrochemical performance for rechargeable Li-ion batteries to the chemically synthesized Co3O4 nanostructures.[9] Belcher and co-workers demonstrated that engineered viruses were capable of nucleating amorphous iron phosphate (FePO4) and the as-obtained FePO4/single-walled carbon Figure 1. Schematic of biological route for conversion of P-rich bacteria into carbon-coated nanotubes (SWCNTs) composites showed LiMPO4 nanocrystals. power performances comparable to that of crystallized LiFePO4.[11] To our knowledge, successful preparation of LiMPO4 nanomaterials using found in activated sludge of wastewater treatment plants.[14] biological routes has not been reported. Magnified TEM images showed that the polyP granules were Groups of bacteria are known to accumulate phosphorus highly porous and consisted mostly of elemental P (see EDX in the form of polyphosphate (polyP) granules in nature, mapping; Figure 2b). including Escherichia coli (E. coli), Acinetobacter and ActinoThe P-rich recombinant E. coli cells were further treated bacter and so on.[12,13] E. coli bacteria was chosen as a model with FeCl3 solution at pH 2.5. High-energy phosphoanhydride bacterial system in this work due to their simple culture con- bonds found between phosphate units in polyP moleculars ditions, as well as little variations in the morphologies and are known to be readily broken down under acidic condicompositions of the polyP granules accumulated in the cell tions, releasing phosphates via the reaction of HO[PO3H]nH+ bodies. In addition, the genetically engineered E. coli bacteria n H2O ↔ n H3PO4.[15,16] At higher temperatures (i.e., 50 °C), strain used in this work was previously shown to accumulate this reaction is accelerated, where the rate of phosphate synP efficiently from wastewater by Kato et al., further serving thesis is futher increased. Figure 2c shows the TEM image as an industrially relevant model system for proof-of-concept of cells treated with Fe3+ solution. We observed perforated demonstration of our synthesis strategy.[12] The same E. coli cell bodies and the absence of polyP granules, suggesting bacteria strain was cultured in the laboratory and allowed to that the polyP granules may have degraded. In addition, accumulate P from the P-rich media. small particles were found to be scattered all over the cell As outlined in Figure 1, harvested cells were mixed bodies (Figure 2d). Electron diffraction patterns of the area with metal and lithium ions sequentially in equimolar ratios enclosed by the white square (see the diffraction pattern and the mixture was directly annealed to obtain LiMPO4 inserted in Figure 2d) indicated that the small particles were nanocrystals with dimensions of about 20 nm. The as-obtained amorphous in nature. Energy-dispersive X-ray (EDX) mapLiFePO4 nanocrystals were fabricated as lithium ion battery ping images of P and Fe (Figure 2e, 2f) and corresponding (LIBs) cathodes and subjected to electrochemical testing. We EDX spectra (see Figure S2, Supporting Information) further observed an excellent discharge capacity of 140 mAh g−1 at confirmed that the small particles were primarily composed 0.1 C and 75.4 mAh g−1 at 10 C, demonstrating the successful of element P and Fe. It is likely that the small particulates conversion of P-rich bacteria into LiMPO4 nanomaterials, were FePO4 complexes derived from the released phosphate with promising applications in LIBs. from hydrolysis of polyphosphate reacting with in-situ Fe3+ ions through the reaction of H3PO4 + Fe3+ ↔ FePO4 + 3H+. Subsequently, lithium acetate (LiAc) solution was added to the mixture, dried and further annealed under Ar at 600 °C 2. Results and Discussion for 5 h. Figure 3a shows the XRD patterns obtained from the The recombinant E. coli strain (i.e., E. coli + pBC29) was as-annealed samples. Peaks characteristic of well-crystallized shown previously to accumulate excess phosphate through triphylite LiFePO4 (JCPDF no. 15–7065) were obtained. The overexpression of polyphosphate kinase (PPK) in the cell.[12] Al peak was from the sample holder. We also did not detect The PPK enzyme is known to catalyze excess phosphate into any other impurity phases, indicating that we had successpolyphosphate (polyP), while keeping the overall cytoplasmic fully obtained LiFePO4. Figure 3b shows a TEM image of phosphate concentration constant.[12] When the PPK enzymes the annealed sample, showing small nanoparticles embedded are present in excess, bacteria cells take up phosphate from within a carbon matrix. At higher magnifications, we found their environment, and accumulate them as polyP granules. that LiFePO4 nanoparticles were about 20 nm in diameter, Figure S1 (Supporting Information) shows the concentra- polycrystalline in nature (see Figure 3c), and encased by an tion of phosphate present in the media depleted by about amorphous carbon shell which was around 3 nm thick. The 40% in the presence of the recombinant (E. coli + pBC29) observed lattice spacing of 2.42 Å corresponded to (121) strain, compared to only 20% for the wild-type E. coli cells planes of triphylite LiFePO4. The overall carbon content control. Characteristic polyP granules of 200 nm in diameters derived from the bacteria was determined by thermal graviwere observed in the cell bodies of the recombinant E. coli metric analysis (TGA), and was found to be about 20 wt% (Figure 2a, indicated by white arrows), consistent with those (see Figure S3, Supporting Information). During annealing,

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Recycling Bacteria for the Synthesis of LiFePO4 and LiMnPO4 Nanostructures

Figure 2. a) TEM images of recombinant E. coli + pBC29 taken at 5.5th h of a phosphate uptake experiment. White arrows indicate polyP granules. Insets in (a) show enlarged images of polyphosphate granules; b) EDX mapping of recombinant E. coli + pBC29 sample in (a), showing the presence of elemental P. c) TEM image of recombinant E. coli + pBC29 after treatment with Fe3+ solution (pH 2.5). d) Magnified image of sample in (c) and the corresponding EDX mapping images showing presence of e) elemental P and f) elemental Fe.

the bacteria cell bodies were transformed into the carbon, providing a carbon coating as well as a conductive carbon matrix that dispersed the LiFePO4 nanoparticles, preventing them from aggregating into bigger particles at high temperatures. Both carbon coating and tailoring the particle size would help to get better electrochemical performace of lithium ion battery.[17,18] To demonstrate the generality of our synthesis process, we extended a similar scheme to the preparation of LiMnPO4. The TEM image and EDX mapping images of recombinant E. coli cells treated with MnCl2 at pH 2.5 were shown in Figure S4 (Supporting Information). Like in the case of LiFePO4, we obtained perforated cell bodies with the absence of polyP granules; likewise, dispersed small particles were observed. Figure 4 shows TEM images of the samples after mixing with LiAc and annealing. Similarly, nanocrystallites of

LiMnPO4 were obtained embedded inside the carbon matrix (see Figure 4b); the nanoparticles were subsequently confirmed to be lithiophilite LiMnPO4 using XRD (Figure 4a; JCPDF no. 15–7067). The observed lattice spacing of 2.72 Å and 1.98 Å corresponded to (301) and (230) planes of lithiophilite LiMnPO4, respectively. Finally, the LiFePO4 synthesized in this work were prepared as cathodes to evaluate their electrochemical properties. The initial galvanostatic charge–discharge voltage profiles of LiFePO4/C were tested at a rate of 0.1 C (Figure 5a, 1 C = 170 mAg−1) using Li half-cell configurations. The cells exhibited the typical voltage plateaus along 3.5 V, corresponding to the Li+ extraction/insertion processes via the reaction of LiFePO4–Li+–e− ↔ FePO4. The initial charge capacity was 148.6 mAh g−1 with Coulombic efficiency of 94.8%. The Coulombic efficiency for the third cycle increased up to 99.5%

Figure 3. a) X-ray diffraction pattern and b) TEM image of LiFePO4 synthesized in this work. Black dots in (b) are LiFePO4 nanocrystals. c) TEM image of a single LiFePO4 nanocrystal and d) measured the lattice spacings (0.242 nm) of the crystals corresponding to (121) plane of triphylite LiFePO4. small 2014, DOI: 10.1002/smll.201400568

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Figure 4. a) X-ray diffraction pattern and b) TEM image of LiMnPO4 synthesized in this work. Black dots in (b) are LiMnPO4 nanocrystals. c) TEM images of LiMnPO4 nanocrystals. Inset in (c) shows the lattice spacings of 0.273 nm and 0.198 nm, corresponding to (301) and (230) planes of lithiophilite LiMnPO4.

with a charge capacity of 144.2 mAh g−1. Moreover, the composites exhibited good cycling stabilities (Figure 5b,c). During the 65th cycle under a rate of 0.1 C, it retained reversible discharge capacities of 139.9 mAh g−1, corresponding to 94.1% of their initial discharge capacities (Figure 5b).

For comparison, the performance of commercial LiFePO4 without addition of any carbon has also been evaluated. At a low rate of 0.1 C, the reversible discharge capacity of LiFePO4 was 132.1 mAh g−1 at the end of the 65th cycle. When the cell was cycled at a relative high rate (1 C), a reversible discharge

Figure 5. a) Charge–discharge voltage profiles of the LiFePO4/C electrodes at a current rate of 0.1 C for the first three cycles. b,c) Cycling performance of the LiFePO4/C electrodes at 0.1 C and 1 C, respectively. The performance of LiFePO4 electrodes without addition of carbon has also been shown for comparison. d) Cycling performance of the LiFePO4/C and LiFePO4 electrodes at various current rates.

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Recycling Bacteria for the Synthesis of LiFePO4 and LiMnPO4 Nanostructures

capacity of 120 mAh g−1 could still be achieved for the LiFePO4/C composites at the 65th cycle, while the discharge capacity retained for LiFePO4 was only 81.8 mAh g−1 (Figure 5c). Another remarkable advantage of nanostructured LiFePO4/C nanocomposites is their excellent rate capability, which is highly desirable for high-power LIB applications such as hybrid electric vehicles (HEVs) and electric vehicles (EVs). The cycling responses of the LiFePO4/C nanocomposites at different C rates (each sustained for 10 cycles) were evaluated and the results were shown in Figure 5d. The discharge capacities of LiFePO4/C nanocomposite electrodes were 145.6, 130.6, 117.8, and 92.3 mAh g−1 at discharge rates of 0.1, 0.5, 1, and 5 C respectively. Stable cycling performances could be obtained for all rates, even at a high rate of 10 C, a high-performance discharge capacity of 75.4 mAh g−1 could still be achieved. Remarkably, after repeated cycling at high C-rates, the discharge capacity could still be quickly recovered back to 145.5 mAh g−1 even when the C-rate was lowered to 0.1 C. In comparison, the discharge capacities of LiFePO4 without carbon additives were only 139.2, 118.3, 102.5, 79.8 and 60.7 mAh g−1 at discharge rates of 0.1, 0.5, 1, 5, and 10 C, respectively. When the C-rate was lowered to 0.1 C, the discharge capacity recovered was only 131.2 mAh g−1. The above results indicate that the LiFePO4/C nanocomposites generated in this work exhibited excellent stabilities and rate capabilities, compared to commercially available LiFePO4, in the absence of carbon additives. The superior performance may be originated from the unique nanostructure and the carbon generated from the cell organic matter, which can facilitate the lithium ion and electron transport efficiently.

3. Conclusion In this work, we demonstrated a generalized biological route to convert P-rich waste bacteria to high-performance LiMPO4 nanomaterials. When applied as a cathode for lithium ion batteries, the electrochemical performance of the as-obtained LiFePO4 nanomaterials were comparable to those derived from inorganic P precursors reported in the literature. This work highlights a novel waste-to-resource strategy, capable of converting “waste” microorganisms used for bioremediation into high-performance nanomaterials for energy storage.

determined by ascorbic acid method.[20] After 5.5 h, E. coli cells were harvested by centrifugation. Synthesis of LiMPO4 from P-Rich E. coli Cells: The harvested cells were washed with deionized (DI) water to remove residual salts and fixed with 4% paraformaldehyde water solution for 10 min. The cells were washed twice to remove the residual paraformaldehyde. Washed cells were subjected to further treatment as outlined in the Figure 1. Briefly, P-rich E. coli cells were rinsed in MCln solution (equimolar amounts of MCln solution were added to the calculated molarity of phosphate taken up by cells from the media after 5.5 h), and left at 50 °C for 2 days with a stirring rate of 150 rpm. Lithium acetate (in equimolar ratio) was added and mixed homogenously, dried at 80 °C overnight, and annealed at 600 °C for 5 h. Characterization: Powder X-ray diffraction (XRD, Shimadzu Powder) using Cu Kαa radiation was employed to identify the crystalline phase of the synthesized materials. The particle size and morphology were characterized using transmission electron microscope (TEM). TEM images were acquired using JEOL 2010F TEM equipped with energy dispersive X-ray spectroscope (EDX) operating at an accelerating voltage of 200 kV. Thermogravimetric analysis (TGA, Q500) was carried out from room temperature to 800 °C at a heating rate of 10 K min−1 in air. Electrochemical Measurements: The coin-type cells were assembled in an argon-filled glovebox, where both moisture and oxygen levels were less than 1 ppm. The electrodes were fabricated by mixing 70 wt% of our synthesized LiFePO4/C composites, multiwalled carbon nanotubes (20 wt%) and polyvinylidene fluoride (PVDF, 10 wt%) in N-methyl-2-pyrrolidone (NMP) solvent. The mixture was then applied onto aluminum foils (Ø = 14 mm). The mass loading was around 2.0 mg. Lithium foils were used as anodes and 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1/1, w/w) was used as the electrolyte solution. The cells were tested on a NEWARE multichannel battery test system with galvanostatic charge and discharge in the voltage ranges of 2.5−4.3 V. The specific capacity values in this paper were calculated based on pure LiFePO4.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

4. Experimental Section Materials: The plasmid pBC29 containing the poly-phosphate kinase (ppk) gene and promoter was a generous gift from H. Ohtake (Osaka University, Japan).[19] T-minimum media (T media) containing phosphate was purchased from Invitrogen (Life Technologies, Carlsbad, CA). All the other chemicals and reagents were purchased from Sigma-Aldrich (Seelze, Germany). Preparation of P-Rich E. coli: The pBC29 plasmid was transformed into E. coli BL21 cells (Stratagene) using heat shock (named E. coli + pBC29). Phosphate uptake experiments were performed using T media as reported by Ohtake et al.[12] Samples were taken at intervals for phosphate measurement, which were small 2014, DOI: 10.1002/smll.201400568

Acknowledgements The authors acknowledge funding from Singapore Ministry of Education AcRF Tier 1 (RG 61/14), A*STAR SERC grant 1021700144, Singapore MPA 23/04.15.03 grant, Singapore National Research Foundation under CREATE program: EMobility in Megacities and Nanyang Technological University (Start-up grant). Y.P.Z. was supported by the IGS-NEWRI scholarship. The authors thank Professor Hisao Ohtake from Osaka University for generously providing the pBC29 plasmid. The TEM and EDX mapping work were performed at the Facility for Analysis, Characterization, Testing and Simulation

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in Nanyang Technological University. Y.P.Z. and D.Y. contributed equally to this work.

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Received: March 3, 2014 Revised: May 10, 2014 Published online:

small 2014, DOI: 10.1002/smll.201400568

Recycling bacteria for the synthesis of LiMPO4 (M = Fe, Mn) nanostructures for high-power lithium batteries.

In this work, a novel waste-to-resource strategy to convert waste bacteria into a useful class of cathode materials, lithium metal phosphate (LiMPO4; ...
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