Most structural water in a-FePOa'nH,O is removed Super P(TIMCAL, SUPER P Li throughout the electrodes. It is known that in from the structure around 200oC(7). Surprisingly, and polytetrafluoroethylene(PTFE) corporation of well-dispersed materials with high the viral nanowires produced on Ag NP-loaded E4 mass ratio of 70: 25: 5. Details of the conductivity and high aspect ratio leads to efficient were anhydrous as synthesized, as shown by of components are given in the sup- percolating networks(22, 23). Carbon nanotubes thermogravimetric analysis (TGA)(Fig. IB and porting online material(16). The first discharge (CNTs) have been shown to meet these needs fig S2). Without Ag NPs, nanowires have about 10 capacity at a low discharge rate of C/10(18)was (23) thus, well-dispersed single-walled CNTs eight percent (wt % structural water, which 165 mAh/g(93% of the theoretical value) and (SWNTs) in water were used. However, conven- coresponds to n= l in a-FePO4'nH2O. X-ray that of ic discharge rate was 110 mAh/g(Fig. tional composite electrode fabrication processes powder diffraction of a-FePO4 nanowires on Ag IC)(19). The rate performance is also presented inevitably suffer from aggregation of carbon NPs-loaded E4 (fig. S3A)showed only peaks as a Ragone plot(Fig. ID). In most electrode particles, thereby diminishing contact with the indexed as silver chloride(AgCi). We speculate materials, specific energy decreases substantially active materials(23). To achieve better electrical that the dehydration of FePOanH20 is related to as one applies more power(high rates), drawing wiring to our biologically derived a-FePO4,we the chlorination of Ag NPs, which could occur more current from the electrodes(20). These rate engineered a specific affinity between the con- during the incubation with the iron chloride performance values are similar to the best ducting material and active material reduced to metallic Ag after electrochemical test temperature (21). Even with this one-gene serves only as a template for a-FePO4 nanowi (fig. S3B). The reduced metallic Ag could stem, the nanostructuring of a-FePOA nano- growth, additional genetic modification was nhance local electronic conductivity as Au wires by the virus enabled an enhanced per- required to engineer the e4 virus to have a bind- tured nanowires(ID). Although the exact mech- and capacity retention upon cycling of both the Ill protein(pllm. a Is. In this context, the gene nanoparticles could in Co3 O/Au heterostruc- formance. However, high-power performance ing affinity for SWN anism of dehydration is under investigation, biologically and traditionally synthesized electro- one end of the virus(Fig. 2A), is an ideal tool be- dehydration of structural water without thermal des are still inferior to commercially available c- cause gene Ill can be controlled independently treatment was accomplished by low-temperature LiFePOa cathodes of gene VIll to insert foreign DNA encoding pIll and environmentally benign chemistry. The de Because our particles were already 10 to 20 displayed peptides. Moreover, the peptide sequene ydrated structure increases the theoretical ca- nm in diameter, our strategy for improved per- identified through the phage display with a pill pacity to 178 mAh/g, making it a good cathode formance was to increase the electronic conduc- phage-display library can be directly inserted into material ivity in the cathode by achieving better electrical the E4 virus without losing functionality (12) The electrochemical performance of viral contact between the active materials. Although Therefore, phage-display experiments to search a-FePOA nanowires as a lithium-ion battery metallic Ag NPs can locally enhance the electronic for peptide sequences with a strong binding cathode was evaluated( Fig. 1, C and D). Positive conductivity, more important for improved high- affinity for SWNTs were done first, followed by electrodes were prepared by mixing viral a- power performance is a percolating network genetic engineering into the E4 virus to produce B 250n 250nm 250nm Fig.3. Morphology of the a-FePOa grown on theD multifunctional viruses/SWNT hybrid nanostruc- tures. TEM images. (A) a-FePOa nanowires tem- plated on EC#2 viruses (before interacting with SWNTs). EC#2 virus is a two-gene system virus with the strongest binding affinity to SWNTs. B)SWNTs only(before interacting with viral a-FePOa. (C to E) SWNTS a-FePOa grown on EC#2 attached to SWNTs.(O Low magnification (x10, 000).(D)Higher magnifi- cation (x30,000).(E) High-resolution TEM(HRTEM) nages (x800,000). For HRTEM imaging, surfac tants were removed by washing with acetone. Material-specific tethering of the viral a-FePOa to the SWNTs is visualized. The amorphous nature of 4 nm FePO, was also confirmed www.sciencemag.orgScieNceVol32422May2009 1053Most structural water in a-FePO4·nH2O is removed from the structure around 200°C (17). Surprisingly, the viral nanowires produced on Ag NP-loaded E4 were anhydrous as synthesized, as shown by thermogravimetric analysis (TGA) (Fig. 1B and fig. S2). Without Ag NPs, nanowires have about 10 weight percent (wt %) structural water, which corresponds to n = 1 in a-FePO4·nH2O. X-ray powder diffraction of a-FePO4 nanowires on Ag NPs-loaded E4 (fig. S3A) showed only peaks indexed as silver chloride (AgCl). We speculate that the dehydration of FePO4·nH2O is related to the chlorination of Ag NPs, which could occur during the incubation with the iron chloride precursor. Part of the chlorinated AgCl was reduced to metallic Ag after electrochemical test (fig. S3B). The reduced metallic Ag could enhance local electronic conductivity as Au nanoparticles could in Co3O4/Au heterostruc￾tured nanowires (11). Although the exact mech￾anism of dehydration is under investigation, dehydration of structural water without thermal treatment was accomplished by low-temperature and environmentally benign chemistry. The de￾hydrated structure increases the theoretical ca￾pacity to 178 mAh/g, making it a good cathode material. The electrochemical performance of viral a-FePO4 nanowires as a lithium-ion battery cathode was evaluated (Fig. 1, C and D). Positive electrodes were prepared by mixing viral a￾FePO4 with Super P (TIMCAL, SUPER P Li) carbon black and polytetrafluoroethylene (PTFE) binder in a mass ratio of 70:25:5. Details of the weight ratio of components are given in the sup￾porting online material (16). The first discharge capacity at a low discharge rate of C/10 (18) was 165 mAh/g (93% of the theoretical value) and that of 1C discharge rate was 110 mAh/g (Fig. 1C) (19). The rate performance is also presented as a Ragone plot (Fig. 1D). In most electrode materials, specific energy decreases substantially as one applies more power (high rates), drawing more current from the electrodes (20). These rate performance values are similar to the best reported values for a-FePO4 synthesized at high temperature (21). Even with this one-gene system, the nanostructuring of a-FePO4 nano￾wires by the virus enabled an enhanced per￾formance. However, high-power performance and capacity retention upon cycling of both the biologically and traditionally synthesized electro￾des are still inferior to commercially available c￾LiFePO4 cathodes. Because our particles were already 10 to 20 nm in diameter, our strategy for improved per￾formance was to increase the electronic conduc￾tivity in the cathode by achieving better electrical contact between the active materials. Although metallic Ag NPs can locally enhance the electronic conductivity, more important for improved high￾power performance is a percolating network throughout the electrodes. It is known that in￾corporation of well-dispersed materials with high conductivity and high aspect ratio leads to efficient percolating networks (22, 23). Carbon nanotubes (CNTs) have been shown to meet these needs (23); thus, well-dispersed single-walled CNTs (SWNTs) in water were used. However, conven￾tional composite electrode fabrication processes inevitably suffer from aggregation of carbon particles, thereby diminishing contact with the active materials (23). To achieve better electrical wiring to our biologically derived a-FePO4, we engineered a specific affinity between the con￾ducting material and active material. Because the major coat protein of the E4 virus serves only as a template for a-FePO4 nanowire growth, additional genetic modification was required to engineer the E4 virus to have a bind￾ing affinity for SWNTs. In this context, the gene III protein (pIII), a minor coat protein located at one end of the virus (Fig. 2A), is an ideal tool be￾cause gene III can be controlled independently of gene VIII to insert foreign DNA encoding pIII￾displayed peptides.Moreover, the peptide sequences identified through the phage display with a pIII phage-display library can be directly inserted into the E4 virus without losing functionality (12). Therefore, phage-display experiments to search for peptide sequences with a strong binding affinity for SWNTs were done first, followed by genetic engineering into the E4 virus to produce a Fig. 3. Morphology of the a-FePO4 grown on the multifunctional viruses/SWNT hybrid nanostruc￾tures. TEM images. (A) a-FePO4 nanowires tem￾plated on EC#2 viruses (before interacting with SWNTs). EC#2 virus is a two-gene system virus with the strongest binding affinity to SWNTs. (B) SWNTs only (before interacting with viral a-FePO4). (C to E) a-FePO4 grown on EC#2 attached to SWNTs. (C) Low magnification (×10,000). (D) Higher magnifi￾cation (×30,000). (E) High-resolution TEM (HRTEM) images (×800,000). For HRTEM imaging, surfac￾tants were removed by washing with acetone. Material-specific tethering of the viral a-FePO4 to the SWNTs is visualized. The amorphous nature of FePO4 was also confirmed. www.sciencemag.org SCIENCE VOL 324 22 MAY 2009 1053 REPORTS