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Lysozyme-templated Meso-Macroporous Hollow TiO2 for Lithium-ion Battery Anode Muhammad Saad Salman, A Reum Park, Min Jin Cha, Youngjin Choi, Sung Kyu Jang, Lihan Tan, Pil J. Yoo, and Woo Seok Choe ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00164 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Applied Nano Materials

Lysozyme-templated Meso-macroporous Hollow TiO2 for Lithium-ion Battery Anode Muhammad Saad Salman,†,# A Reum Park,†,# Min Jin Cha,‡ Youngjin Choi,† Sung Kyu Jang,‡ Lihan Tan,§,* Pil J. Yoo,†,‡,* and Woo-Seok Choe†,‡,* †

School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Republic

of Korea ‡

SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU),

Suwon 16419, Republic of Korea §

Bioprocessing Technology Institute, A*STAR, Downstream Processing Group, 20 Biopolis

Way, Centros #06-01, Singapore 138668, Singapore ABSTRACT The use of biomolecules as templates for biomineralization is widely viewed as a favorable green synthesis route to produce particles of tailored properties. To synthesize TiO2 particles suitable for lithium-ion battery (LIB) anodes, four different types of biotemplates (i.e. native and denatured lysozyme with or without phosphate) were used to direct TiO2 biomineralization. The different size, morphology and rigidity of the deliberately designed biotemplates enabled the controlled mineralization of TiO2 particles of versatile morphology, particle/grain/pore size, crystallinity, surface area and specific pore volume. This rendered differential electrochemical 1 ACS Paragon Plus Environment

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characteristics and potential of the biomineralized TiO2 as LIB anode. In particular, mesomacroporous hollow spherical TiO2 (biomineralized using denatured lysozyme in the presence of phosphate ions, followed by calcination at 500 oC) shows great potential to be used as an LIB anode for high power application with its average specific capacity of 103 mA h g-1 at a C-rate of 40 C over 400 cycles. KEYWORDS: lysozyme, biotemplate, heat denaturation, phosphate, hollow meso-macroporous TiO2, lithium-ion battery, anode INTRODUCTION Currently, fossil fuels are widely used as energy sources. However, major issues are known to arise from the use of fossil fuels as they are non-renewable and result in serious air pollution issues leading to global warming. In the search for efficient alternative energy sources, electrochemical storage and conversion devices come forth as promising alternatives for energy storage and renewable energy source.1 Among various electrochemical energy storage devices, LIBs are widely studied as they possess desirable functional properties such as lightweight,2 high energy3 and power density.4 However, despite the outstanding performance of LIBs, some safety concerns pertaining to their usage remain unresolved. For instance, graphite has been used as anode in LIBs for many years. However, its use has recently become a safety concern as the intercalation potential of graphite is usually less than 1.0 V vs Li+/Li, and the deposition of lithium directly on the anode might occur and subsequently leads to catastrophic failure.5 To address this issue, various potential graphite replacements including Co3O4,6 MnO2,7 Fe2O3,8 Fe3O49 and Si10 have been explored. Nevertheless, these materials suffer from rapid battery

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capacity reduction during usage, mainly due to their inherently poor electronic conductivity and large volume expansion during the cycling process.3 Recently, the use of TiO2-based anodes as graphite replacement have gained considerable attention because of their abundance, environment friendliness, low cost, structural stability, increased electrode potential difference (i.e. 1.5-1.8 V vs Li/Li+), negligible volume expansion (3-4%) during Li+ insertion/extraction and good cycling stability.3 Thanks to the nanomaterials synthesis, the performance of TiO2 as anode material can be tuned by tailoring its phase, crystallinity, porosity and size. Among the various polymorphs of TiO2, anatase phase has extensively been investigated because its crystal structure is believed to be most suitable for lithiation/delithiation.11 However, for anodes made of anatase TiO2, the relatively lower lithium ion diffusion, poor electronic conductivity, and increased resistance at the interface of electrodeelectrolyte at high current densities always hamper their practical high-power application.12-13 Beside anatase phase, TiO2 in amorphous phase has also been reported to be beneficial for achieving higher rate capability due to its higher Li+ diffusivity and reversibility than anatase.1415

To improve the performance of TiO2-based LIB anodes, various strategies have been conducted to optimize electronic conductivity and Li+ ion diffusion including, tuning particles size down to nanoscale and/or the morphology of the nanostructures.12, 16-19 Among various TiO2 nanostructures such as nanoparticles,20 nanosheets,21 nanowires,22-23 nanoribbons,24 nanotubes,25 and flakes,26 much attention has been focused on the fabrication of hollow spheres.27 The porous structure of hollow spheres offers high resistance to strain, and large capability of Li+ ion accommodation. This can effectively shorten the diffusion paths for Li+ ions and maximize the contact area between electrode and electrolyte.28 The fabrication of hollow TiO2 spheres is often 3 ACS Paragon Plus Environment


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achieved via conventional synthesis methods such as sol-gel or hydrothermal.27 However, these methods usually employ high temperature and/or pressure conditions, and have disadvantages such as difficulties in controlling critical particle characteristics (e.g. size, shape and crystallinity), and generation of hazardous waste. In this regard, the synthesis of nanostructures via environmentally benign biomineralization processes overcome problems arising from conventional approaches, while allowing precise control of particle characteristics and reducing environmental pollution issues. The synthesis of numerous promising nanomaterials for LIB anodes and cathodes via biomineralization has been reported.29 This includes carbon hollow spheres,30 Li3V2(PO4)3,31 Co3O4,32 LiFePO4,33 Fe2O3,34 FeS2,35 MnO36 and TiO2.37 In biomineralization, engineered biomolecules with specific recognition and clustering ability can enable synthesis of inorganic materials with exquisite structural controllability.38-39 In addition, biomineralization allows the in vitro biosynthesis of semiconducting materials (e.g. TiO2, WO3 and ZnO) which is rarely produced in vivo in nature. This is achieved via the use of diverse biomolecules including peptides (e.g. R5 peptide derived from Cylindrotheca fusiformis,40 and STB1 peptide identified by phage display technique),41 proteins (e.g. lysozyme),42-43 and bacteriophages (e.g. M13) as catalyst and/or template for biomineralization.44 Among the biomolecules, proteins are particularly attractive since they offer rigid and stable biotemplates in various conformations, which is conducive to controlling size, morphology, and crystallinity of inorganic material during biomineralization.45 One of the important proteins in this regard is lysozyme. Lysozyme has compact tertiary structure maintained by four disulfide bridges (Cys6-Cys127, Cys30-Cys115, Cys64-Cys80 and Cys76-Cys94) crosslinking the polypeptide chain.46 Recent reports indicate that basic proteins such as lysozyme (isoelectric 4 ACS Paragon Plus Environment

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point (pI) of ~11) can effectively induce biomineralization of TiO2 (pI of ~5.0) and SiO2 (pI of ~2.0) via electrostatic interactions.43,

47

Furthermore, the interaction of phosphate ions with

specific amino acid residues (i.e. Asn103 to reduce steric hindrance, and Arg14 via electrostatic interaction) in native lysozyme protein has also been reported.48 The rigidity and shape of biotemplate can also be controlled by using either thermal or chemical denaturation to induce conformational changes in lysozyme structure through disruption of protein tertiary structure and disulfide bond. Therefore, harnessing lysozyme as a model protein to produce distinctive biotemplates by tuning its state and/or rigidity for the synthesis of TiO2 is an attractive strategy. With this aim, herein, the rigidity and conformation of native lysozyme were first altered via addition of phosphate ion and/or heat denaturation. The various lysozyme forms were then used as templates for biomineralization with the use of anionic titanium bis(ammonium lactato) dihydroxide (TiBALDH) as titanium precursor at ambient condition. The TiO2 thus formed was calcined to obtain different phases with distinctive properties, and was investigated as potential LIB anodes. Particularly, electrochemical analyses show that anatase non-hollow structure (obtained from native biotemplate) has better cycling performance at low C-rates than amorphous hollow structure (obtained from denatured biotemplate in phosphate). Nevertheless, amorphous hollow structure exhibits superior rate capability and long-term cycling to anatase non-hollow phase. The superior rate capability of amorphous hollow structure can be attributed to its higher specific surface area and pore volume originating from its meso-macroporous structure which provides shorter diffusion lengths for Li+ transport, and high contact area between electrolyte and electrode.



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RESULTS AND DISCUSSION Preparation of the lysozyme biotemplates

Scheme 1. Illustration of the preparation processes of various lysozyme biotemplates: (a) N, (b) NP, (c) D and (d) DP.

The overview of conditions required for fabrication of various lysozyme biotemplates to support biomineralization is shown in Scheme 1. Native and denatured lysozyme biotemplates were prepared in the presence or absence of phosphate ions. Hereafter, N, NP, D and DP refer to lysozyme biotemplate that is native in water, native in PB (phosphate buffer), denatured in water and denatured in PB, respectively (Scheme 1). Denatured lysozyme was obtained via heat denaturation at 80 oC for 1 h (Schemes 1c and d). This condition is confirmed to be sufficient to ensure complete denaturation of lysozyme via reverse phase high-performance liquid chromatography (RP-HPLC) (Figure S1a). In the presence of phosphate ions, the phosphate ions bound to the both native and denatured lysozymes (via preferential and electrostatic interactions) increased the rigidity of lysozymes (Schemes 1b and d).48-49 Following their preparation, the sizes of lysozyme biotemplates were investigated using transmission electron microscope (TEM) and atomic force microscope (AFM) imaging (Figure 6 ACS Paragon Plus Environment

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1) and summarized in Table 1. Liquid tapping mode AFM was used for in situ observation of hydrated protein clusters or assemblies. Note that the sizes of lysozyme biotemplates indicated in Table 1 are for individual discernible clusters of biotemplates, which may be either individual lysozyme molecules or aggregates. For native lysozyme biotemplates (N and NP), individual sizes of ca. 15-30 nm were obtained in both the absence and presence of phosphate ions via TEM and/or AFM imaging (Figures 1a and b). For NP, it is not easy to distinguish its individual size from the TEM image. This is possibly due to the precipitation of uranyl formate, which is used to stain NP to increase the contrast during imaging, in the presence of phosphate ions. However, the size of NP at ca. 15-30 nm can be obtained via AFM imaging. TEM result also shows that soluble native lysozymes form discrete single particle with compact and stable structure (Figure 1a), whereas denatured lysozymes show varying degree of aggregation dependent on the denaturation conditions (Figures 1c and d). The heat denaturation of lysozymes triggers the exposure of buried hydrophobic amino acids, which leads to the deformation and aggregation of lysozymes. The presence of phosphate ions significantly increased the degree of aggregation of denatured lysozymes. In the absence of phosphate ions, soluble biotemplate D formed random aggregates comprising individual lysozyme particle of size ca. 20-30 nm (Figure 1c and Table 1). On the other hand, in the presence of phosphate ions, insoluble biotemplate DP formed welldefined dumbbell-shaped aggregate comprising two spheres of size ca. 550-800 nm (Figure 1d and Table 1). Each sphere is made up of individual lysozyme particles as illustrated in Scheme 1d. AFM observations are similar to the results obtained by TEM (Figure 1). In the native state, lysozyme biotemplates have sizes of ca. 15-30 nm, and are evenly distributed in the liquid (Figures 1a and b). In the denatured state, lysozyme aggregates were formed with enhanced aggregation observed in the presence of phosphate ions. DP with an average size of ca. 1000 nm 7 ACS Paragon Plus Environment


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for one of the spheres in the dumbbell-shaped aggregate (Figure 1d) is much larger than D with an average size of ca. 180 nm (Figure 1c).

Figure 1. TEM (left) and liquid tapping mode AFM (right) images of lysozyme biotemplates: (a) N, (b) NP, (c) D and (d) DP.


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Table 1. The properties of the lysozyme biotemplates and biomineralized TiO2 particles. Template

Size of Template (TEM)a (nm)

Size of Template (AFM)b (nm)

Mineralized TiO2

Size of Biomineralized TiO2 (before calcination, 25 oC) (FESEM)c (nm)

TiO2 Phase and Grain size (nm)d (after calcination) At 500 oC

700 oC

Anatase (24)Rutile (N/A) Anatase (10)

N

15-30

19-30

TN

40-160

Anatase (7)

NP

-

15-30

TNP

30-40

Amorphous (N/A)

D

20-30

60-200

TD

50

Anatase (8)

Anatase (28)Rutile (4)

DP

550-800

1000-1100

TDP

1100

Amorphous (N/A)

Anatase (7)

a,b,c

The sizes of templates (obtained by TEM and AFM) and biomineralized TiO2 (obtained by FESEM) are of individual

discernible clusters of biotemplates and TiO2 particles, respectively. For DP and TDP, the indicated size is for half a dumbbellshaped aggregate.

d

The grain size of anatase and rutile phases is calculated by Debye Scherrer equation using the highest

intensity peak of each phase in Figure 3. N/A indicates that grain size cannot be determined due to the very weak peak intensity.

Among the four lysozyme biotemplates, DP is of great interest due to its unique dumbbellshaped bispherical structure, and potential to act as a biotemplate for the synthesis of TiO2 hollow spheres known for their superior performance in LIBs. The formation process of DP is depicted in Scheme 1d. Lysozyme consists of 129 amino acids in which four intra-molecular disulfide bonds are present.46 The heat denaturation of lysozyme in the presence of DTT leads to reduction of disulfide bonds, conformational change of individual protein molecules with surface exposure of their hydrophobic residues, and subsequent aggregation of lysozymes in the aqueous media as shown in D and DP (Figures 1c and d). The lysozyme molecules with surface exposed hydrophobic residues are less prone to form hydrogen bonds with water molecules, and thus less likely to be surrounded by a cage of highly ordered water molecules with low system entropy. As all systems tend to evolve toward a state of higher entropy, the denatured lysozyme molecules with exposed hydrophobic residues tend to aggregate to minimize the water interface area. 9 ACS Paragon Plus Environment


Moreover, in PB at pH 8.0, lysozyme (pI of ~11) is positively charged and interacts with negatively charged phosphate ions via preferential and electrostatic interaction.47-48 The presence of phosphate ions stabilizes and increases the rigidity of the DP as compared to D, leading to the arrangement of individual lysozyme molecules into hierarchical spherical structure, and the subsequent aggregation of two spheres into a dumbbell-shaped aggregate. In addition, the reoxidation of cysteine thiol groups (-SH) in the lysozyme also induces the cross-linking between the lysozyme molecules via formation of inter-molecular disulfide bonds, which maintains the rigidity of the hierarchical spherical structure. Note that although DTT initially used to denature lysozyme is not deliberately removed, given that DTT has a half-life of ca. 5 min at 80 oC, it is practically deactivated after 1 h incubation at 80 oC. This allows the reoxidation of cysteine thiol groups over time.

Synthesis and structural characterization of TiO2 particles Subsequently, all four biotemplates including DP were used as templates to direct TiO2 biomineralization. Hereafter, biomineralized TiO2 using N, NP, D and DP as templates are designated as TN, TNP, TD and TDP, respectively, where the prefix “T” indicates TiO2. During the biomineralization process, the biotemplate solution was mixed with TiBALDH precursor, and this resulted in immediate formation of TiO2 precipitates. All the biomineralized TiO2 samples were freeze-dried and analyzed by field emission electron microscope (FESEM). For mineralized TiO2 using native biotemplates (N and NP), TN and TNP show a network comprising interconnected particles of diameters 40-160 nm and 30-40 nm, respectively (Figures 2a, b and Table 1). For mineralized TiO2 using denatured biotemplates (D and DP), TD shows a network comprising interconnected particles of diameters ca. 50 nm (Figure 2c and Table 1). The formation of necked particles is due to the formation of new Ti-O-Ti chemical bonds after 10 ACS Paragon Plus Environment

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particle collision, with free energy reduction accompanied by decrease of particle surface area being the driving force.41 Interestingly, only TDP shows hierarchical spherical structure of individual size ca. 1100 nm and largely dumbbell-shaped aggregate of size ca. 2100 nm (Figure 2d), which resembles DP biotemplate (Figure 1). The biomineralization of TiO2 based on stable and rigid DP biotemplate with spherical structure (Figure 1d) produces more discrete (or less interconnected) TDP of similar size and morphology (Figure 2d). For mineralized TiO2 (Figure 2), the size and morphology of the mineralized TiO2 closely resemble the size and morphology of their respective biotemplates (Figure 1), indicating that the biotemplates greatly influenced TiO2 structure formation. However, the mineralized TiO2 particles were slightly larger than the biotemplates due to the formation of TiO2 layer on the biotemplates (Figures 1 and 2 and Table 1).

Figure 2. FESEM images of biomineralized TiO2 particles (before calcination): (a) TN, (b) TNP, (c) TD and (d) TDP.



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The presence of phosphate ions also alters the TiO2 nucleation process and kinetics. First, since the main driving force of biomineralization is the electrostatic interaction between the cationic biotemplates and the anionic TiBALDH precursor, the presence of anionic phosphate ions might interfere with the biotemplate-TiBALDH precursor interaction. It is apparent that the lysozyme templates complexed with phosphate ions (i.e. NP and DP) mediated differential nucleation reactions of TiO2 precursor molecules departing from those triggered by N and D. The morphology of each biomineralized TiO2 (Figure 2) closely resembles the morphology of its corresponding lysozyme biotemplate (Figure 1). Second, the presence of phosphate ions results in faster biomineralization kinetics, and smaller biomineralized TiO2 particles, i.e. more nucleation occurred simultaneously, leaving less TiBALDH precursor for particle growth.41 For instance, despite N and NP having similar size and morphology (Figure 1 and Table 1), the smaller size of TNP indicates that the presence of phosphate ions results in faster biomineralization kinetics. Even though TDP might appear to be larger than TD, a single spherical TDP particle (as defined in our work) is actually constructed from a network of interconnected small TiO2 nanoparticles. Hence, this also indicates that the presence of phosphate ions results in faster biomineralization kinetics for denatured biotemplates.41 Following the biomineralization of TiO2, the core biotemplate contents of TN, TNP, TD and TDP were analyzed by thermo-gravimetric analysis (TGA) (Figure S2). The weight loss which indicates the biotemplate content was 64, 38, 75 and 95% for TN, TNP, TD and TDP, respectively. More detailed explanation of the TGA results can be found in the Supporting Information. Moreover, a majority of biomineralized TiO2 is amorphous at room temperature.43 Hence, TN, TNP, TD and TDP were calcined at 500 and 700 oC to synthesize TiO2 in various phases. 12 ACS Paragon Plus Environment

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Hereafter, for the calcined TN, TNP, TD and TDP samples, the suffix ‘500’ or ‘700’ is added to indicate the calcination temperature. From the X-ray diffraction (XRD) patterns (Figure 3 and Table 1), TN and TD both formed anatase phase at 500 oC and anatase-rutile phase at 700 oC (which is reported to be the usual critical temperature of anatase to anatase-rutile phase transition for TiO2).50 In the presence of phosphate ions, TNP and TDP remained in amorphous phase at 500 oC and formed anatase phase only at 700 oC. This suppression in crystalline phase transition at high temperature is mainly due to the phosphate ions and/or the decomposed lysozyme residues incorporated in mineralized TiO2, which impedes the necessary rearrangement of atoms for TiO2 phase transformation.51 Hence, the phase of biomineralized TiO2 can be controlled by varying the protein structure (i.e. native vs denatured), phosphate concentration and calcination temperature. A similar phenomenon of protein-mediated phase control has also been achieved using extrapallial fluid (EPF) proteins, a composite protein derived from mussel, where the increase in EPF protein significantly reduces brookite phase formation in anatase-brookite TiO2.52



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Figure 3. XRD patterns of calcined TiO2 particles: (a) TN-500 and TN-700, (b) TNP-500 and TNP-700, (c) TD-500 and TD-700, and (d) TDP-500 and TDP-700.

Figure 4. TEM images of calcined TiO2 particles: (a) TNP-500, (b) TNP-700, and morphological changes in TDP upon calcination at different temperatures of (c) 500, (d) 600, (e) 700, (f) 800 and (g) 900 °C. For TNP-700, TDP-600, TDP-700, TDP-800 and TDP-900, the area indicated by the orange box is enlarged to show the grain size and lattice spacing in Figure S4 (Supporting Information). (h) Schematic illustrating the process of hollow TiO2 sphere formation with increasing calcination temperature (700-900 o C).

After calcination at 500 and 700 oC, the morphology of TN, TNP, TD and TDP were further characterized by FESEM (Figure S3). The size range of the individual particles for TN-500, TNP-500, TD-500 and TDP-500 is ca. 45-175 nm, 40 nm, 50-150 nm and 530-1100 nm, 14 ACS Paragon Plus Environment

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respectively. Whereas, the size range of the individual particles for TN-700, TNP-700, TD-700 and TDP-700 is ca. 38-50 nm, 17-25 nm, 30-185 nm and 250-450 nm, respectively. From the FESEM analysis, the effect of phosphate ions on the template-directed particle size can be observed even after calcination. For instance, the individual particle size of TNP-700 (17-25 nm) is smaller than TN-700 (38-50 nm) and TD-700 (30-185 nm), which corresponds to the faster reaction kinetics due to the increase in biotemplate rigidity in the presence of phosphate ions. Subsequently, for TDP, due to the formation of hollow spherical structure, the effect of calcination temperature (500-900 oC) on TDP hollow structure formation as compared to TNP was further studied by TEM (Figure 4). For TNP-500 (Figure 4a), less well-defined and distinctive TiO2 particles can be seen as compared to TNP-700 (Figures 4b and S4a), possibly due to the amorphous nature of TNP-500. Neither TNP-500 nor TNP-700 are hollow structure. On the other hand, the strong contrast between the inner region and outer shell unambiguously illustrates the hollow nature of TDP (Figures 4c-g). However, some structural collapse in TiO2 hollow structure was observed at 800 oC (Figure 4f), and severe aggregation leading to excessive grain growth was observed at 900 oC (Figure 4g). Although there was an overall shrinkage of TDP particle size as calcination temperature increased from 600 to 900 oC, the shell thickness was increased due to the simultaneous occurrence of grain growth (Figures 4d-g). The size of an individual grain is ca. 21, 5, 8, 16 and 35 nm for TNP-700, TDP-600, TDP-700, TDP-800 and TDP-900, respectively (Figures S4a-e), and the lattice spacing of ca. 0.352 nm corresponds to the TiO2 anatase phase (Figures S4a-e). For TDP-600, TDP-700, TDP-800 and TDP-900, the particle size and shell thickness were 480 and 35 nm, 460 and 43 nm, 430 and 85 nm, and 310 and 100 nm, respectively (Figures 4d-g). In Figure 4h, the process of hollow TiO2 sphere formation with the increase in calcination temperature from 700 to 900 oC is illustrated. It should 15 ACS Paragon Plus Environment


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be noted that certain amorphous regions are also observed along with the lattice fringes (Figures S4a-e). These regions can be ascribed to amorphous carbon produced after the decomposition of lysozyme biotemplate and organic moiety of TiBALDH precursor during calcination (presence of carbon was confirmed by X-ray photoelectron spectroscopy (XPS), data not shown).53 However, this carbon was not doped in to the TiO2 crystal structure since no diffraction peak for carbon was found in XRD (Figure 3) and it exists as a conductive matrix for TiO2 (Figures 4 and S4), consistent with the previous literature.53-54 In addition, energy dispersive X-ray spectroscopy (EDX) elemental mapping of TDP-500 and TDP-700 also confirms the presence of Ti, O, C and P elements, and indicates the presence of carbon as well as both TiO2 and titanium phosphate (Figure S4f). When denatured lysozyme is in PB, DP thus formed is a hierarchical spherical structure that contains phosphate ions. After the addition of TiBALDH, negatively charged TiBALDH interacts with positively charged residues on the surface of the DP to form TiO2 with titanium phosphate as contaminant. In addition, excess free TiBALDH will also react with phosphate in PB to form titanium phosphate. When TDP was calcined, the absence of titanium phosphate peak shows that titanium phosphate remains in amorphous phase even at 700 oC (Figure 3d). Amorphous titanium phosphate was only converted into crystalline titanium phosphate at 800 oC (data not shown). Furthermore, it is reported that the phosphate ions adsorbed on the crystalline TiO2 surface could limit the growth of the TiO2 grains, and their surface concentration gradually increases during calcination.55 This phenomenon led to the reduction in the grain growth and impeded the transformation of anatase to rutile phase.55 To further investigate the phosphorus incorporation in TNP and/or TDP (i.e. phosphorus chemical states and bonds), the samples were characterized by XPS (Figure S5).


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N2 adsorption-desorption isotherms from Brunauer-Emmett-Teller (BET) method (Figure 5) and pore size distribution from Barrett-Joyner-Halenda (BJH) method (calculated from desorption branch of the isotherm) (Figure 5, insets) for all the calcined TiO2 samples were obtained. This is to assess their potential utility as LIB anode materials based on their pore sizes, specific pore volumes and surface areas. All the samples exhibit one or two hysteresis loops in the adsorption-desorption isotherms (Figure 5). The loop at relative pressure (P/P0) of ca. 0.40-0.90 indicates the presence of mesopores,56-57 whereas the loop at P/P0 of ca. 0.90-1.00 indicates mesopores and/or macropores.57 For both TN-500 and TN-700 (Figures 5a and b), hysteresis loops at P/P0 of ca. 0.40-0.90 and ca. 0.90-1.00 were observed, respectively. For TN-500, the BJH pore size distribution curve (Figure 5a, inset) exhibits bimodal pore size distribution with pore sizes of ca. 27 nm (intraparticle mesopores) and ca. 110-150 nm (interparticle macropores). However, for TN-700, a narrower pore size distribution centered at ca. 31 nm was observed (Figure 5b, inset). This indicates that majority of the macropores in meso-macroporous network are converted to mesopores when the calcination temperature is increased to 700 oC. For TNP-500 (Figure 5c), two hysteresis loops at P/P0 of ca. 0.40-0.90 and ca. 0.90-1.00 were observed. The isotherm shows steep uptake of N2 with no sign of plateauing even at high P/P0 of ca. 1.00.58 This could be due to the presence of larger mesopores or macropores in TNP-500. The BJH curve (Figure 5c, inset) also shows peaks in the mesoporous (ca. 3.5 nm) as well as macroporous region (ca. 119-150 nm), which is a clear indication of bimodal pore size distribution in TNP-500. However, for TNP-700 (Figure 5d), a single hysteresis loop at P/P0 of ca. 0.85-1.00 was observed, and its BJH curve shows the presence of the mesopores of ca. 19-38 nm (Figure 5d, inset). The



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conversion of bimodal TNP-500 to mesoporous TNP-700 also occurs upon phase transformation from amorphous to anatase during calcination at 700 oC. For TD-500 and TD-700 (Figures 5e and f), hysteresis loops were observed at both P/P0 of ca. 0.40-0.90 and ca. 0.90-1.00, with pore size of ca. 30-40 nm (Figures 5e and f, insets). It should be noted that TD-500 and TD-700 do not show significant differences in their isotherms and pore size distributions, indicating the presence of stable mesoporous network. Next, for TDP-500, a well-defined wide hysteresis loop at the P/P0 of ca. 0.40-1.00 was observed. The BJH pore size distribution curve for TDP-500 (Figure 5g, inset) exhibits bimodal pore size distribution, with pore sizes of ca. 7 nm (intraparticle mesopores) and ca. 116 nm (interparticle macropores). On the other hand, TDP-700 (Figure 5h) shows the two hysteresis loops at P/P0 of ca. 0.40-0.85 and 0.85-1.00, respectively. Interestingly, TDP-700 (Figure 5h, inset) is the only sample to exhibit bimodal pore size distribution (ca. 23 nm for mesopores, ca. 120-160 nm for macropores) despite the higher calcination temperature of 700 oC. This is probably due to the rigid spherical structure that helps to maintain the pore size distribution during calcination. Table 2. BJH pore sizes, BET surface areas and specific pore volumes of calcined TiO2 particles. Pore Size a (nm)

Surface Area b (m2 g-1) after calcination at

Specific Pore Volume c (cm3 g-1) after calcination at

after calcination at 500 oC

Sample Meso

700 oC

Macro

Meso

Macro

500 oC

700 oC

500 oC

700 oC

TN

27

110, 150

31

-

25

28

0.59

0.31

TNP

3.5

119, 150

19, 38

-

44

17

0.40

0.14

TD

30, 40

32, 42

-

53

32

0.40

0.25

151

20

1.10

0.66

TDP a

7

116

23

120, 160

Calculated by the BJH method from desorption branch of the isotherms, b,c Obtained from the BET equipment.


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The pore sizes, specific pore volumes and surface areas of calcined TiO2 samples are tabulated in Table 2. Among all, TDP-500 shows the highest surface area of ca. 151 m2 g-1, which may be due to its amorphous meso-macroporous hollow structure (Figures 4c and S3a). Notably, it also possesses the highest specific pore volume of ca. 1.10 cm3 g-1 (Table 2). However, for TDP-700 relatively lower surface area of ca. 20 m2 g-1 (Table 2) could result from the aggregation of TiO2 hollow spheres (Figure S3b) due to the higher calcination temperature of 700 oC.



Figure 5. N2 adsorption-desorption isotherms from BET, and

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pore size distribution (inset) from

desorption branch by BJH method of calcined TiO2 particles: (a) TN-500, (b) TN-700, (c) TNP-500, (d) TNP-700, (e) TD-500, (f) TD-700, (g) TDP-500 and (h) TDP-700.

Electrochemical characterizations of TiO2 particles as LIB anodes


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In order to analyze and highlight the effect of biotemplate state (native vs denatured) and morphology/structures (non-hollow vs hollow spherical hierarchical structures) on the electrochemical performances of the lysozyme-templated TiO2 samples (i.e. TN vs TD and TNP vs TDP) as LIB anodes, cycling performance, cyclic voltammetry (CV), galvanostatic chargedischarge profiles and rate capabilities were investigated. Comparison of TN and TD The cycling performance of TN-500, TN-700, TD-500 and TD-700 was analyzed for 100 cycles at 0.5 C (at an applied C-rate of 1 C = 168 mA g-1) (Figure S6a). At the 100th cycle, all the TN and TD samples showed Coulombic efficiency of nearly 100% (calculated from Li+ ion charge-discharge values from TiO2 anode at the 100th cycle). TN-500 exhibited a specific capacity of ca. 190 mA h g-1 at 0.5 C as compared to TD-500 which showed specific capacity of ca. 114 mA h g-1 at 0.5 C (Figure S6a). On the other hand, TN-700 and TD-700 showed specific capacities of ca. 99 mA h g-1 and ca. 89 mA h g-1 at 0.5 C, respectively (Figure S6a). Although TN and TD exhibited similar crystalline phases (i.e. anatase at 500 oC and anataserutile at 700 oC, Figure 3), the variation in their cycling performances (both at 500 and 700 oC) might be attributable to the difference in their morphology (Figure S3), which produced varying pore sizes, specific pore volumes and surface areas (Table 2). The better cycling performance of TN-500 as compared with TD-500 could be resulting from the meso-macroporous structure, higher specific pore volume and more distinctively interconnected particles of TN-500. These results indicate that TiO2 templated from native biotemplate (N) outperformed the TiO2 obtained using denatured biotemplate (D) as LIB anode. However, the LIB performance of TN-700 and TD-700 was comparable despite the structural difference in the biotemplates (i.e. native vs 21 ACS Paragon Plus Environment


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denatured lysozyme) used for TiO2 biomineralization. This is presumably due to their similar porous structures, crystalline phases, grain sizes and specific pore volumes following the high temperature calcination treatment (Figures 3 and 5, and Tables 1 and 2). More detailed discussion on the cyclic voltammetry (CV), galvanostatic charge-discharge profiles and rate capabilities of TN and TD samples is provided in the Supporting Information. Overall, TN-500 has the best cycling and rate performance among the TN and TD samples (Figure S6).


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Figure 6. Electrochemical performances of calcined TiO2 particles as anodes: (a) Cycling performance of TNP and TDP for 100 cycles at 0.5 C, Cyclic voltammetry of (b) TNP-500 and (c) TDP-500, Galvanostatic charge-discharge profiles of (d) TNP-500 and (e) TDP-500 in the potential range of 1.0-3.0 V (vs Li/Li+) at a rate of 0.5 C, (f) Rate capabilities of TNP and TDP at 0.5, 1, 2, 5, 10, 20 and 40 C, and (g) electrochemical impedance spectroscopy (EIS) Nyquist plots for TNP-500 and TDP-500.



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Comparison of TNP and TDP Subsequently, the cycling performance of the TNP-500, TNP-700, TDP-500 and TDP-700 was also analyzed and compared (Figure 6a). At the 100th cycle (at 0.5 C), all the TNP and TDP samples also showed Coulombic efficiency of nearly 100%. TDP-500 exhibited a higher specific capacity of ca. 147 mA h g-1 as compared with TNP-500 (specific capacity of ca. 59 mA h g-1) (Figure 6a). Similarly, TDP-700 also showed better cycling performance with a specific capacity of ca. 144 mA h g-1 than TNP-700 (specific capacity of ca. 62 mA h g-1) (Figure 6a). It should be noted that the capacity loss in the first cycle (Figure 6a) of all the samples is associated with the decomposition of the electrolyte induced by the trace amount of chemisorbed water.27 However, the capacity gain in TDP-500 is unusual and can be ascribed to the electrode activation process.15 Despite their similar phases (i.e. amorphous at 500 oC and anatase at 700 oC, Figure 3) and grain sizes (Table 1), TNP and TDP anodes showed different LIB performances. The enhanced performance of TDP-500 might be attributable to the higher specific surface area and pore volume of ca. 151 m2 g-1 and 1.10 cm3 g-1, respectively, as compared to TNP-500 (Table 2). The higher specific pore volume and surface area could be resulting from meso-macroporous hierarchical structures (with carbon as conductive matrix) that provide more active sites and shorten diffusion paths for the Li+ ions in TDP-500,59 while TNP-500 possesses particles with no hierarchical meso-macroporous structure (Figures 5c, d and S3). However, the relatively lower cycling performance of TNP-700 and TDP-700 as compared with TNP-500 and TDP-500, respectively, might be caused by the higher calcination temperature which destructed the porous networks, resulting in lower specific pore volumes and surface areas. In addition, from the 100th cycle (Figure 6a), the capacity retention (%) was also estimated. The capacity retention of TNP24 ACS Paragon Plus Environment

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500, TDP-500, TNP-700 and TDP-700 was ca. 53, 91, 76 and 85%, respectively. TDP-500 showed highest capacity retention (ca. 91%). The better cycling stability of TDP-500 vs TNP500 could be because of its amorphous phase, higher specific pore volume, higher surface area, presence of hollow meso-macroporous structure and relatively smaller grain size (Figures 3 and S3a, and Tables 1 and 2). Furthermore, the electrochemical characteristics of TNP-500, TDP-500, TNP-700 and TDP700 during Li+ insertion/extraction were also elucidated by CV (at a scan rate of 0.1 mV s-1) (Figures 6b and c) and galvanostatic charge-discharge profiles (first two cycles) in the voltage range between 1.0 and 3.0 V (vs Li+/Li) (Figures 6d and e). Amorphous TiO2 samples tend to show less distinct oxidation/reduction peak as compared to anatase or anatase-rutile TiO2 samples. For instance, amorphous TNP-500 and TDP-500 (Figures 6b and c) show CV curves with less distinct oxidation/reduction peaks as compared to anatase TNP-700 and TDP-700 (Figures S7c and g), respectively. Similarly, amorphous TNP-500 and TDP-500 also exhibit CV curves with less distinct oxidation/reduction peaks as compared to anatase TN-500 and TD-500 (Figures S6b and c). These results show that the distinctive CV characteristics are mainly due to the difference in their amorphous (for TNP-500 and TDP-500) and anatase (for TN-500, TD500, TNP-700 and TDP-700) phases which exhibit different Li+ insertion/extraction characteristics.15 Hence, a sloping curve without a voltage plateau during the insertion/extraction of Li+ was only observed for the amorphous TNP-500 and TDP-500 (Figures 6d and e). The absence of noticeable voltage plateau shows non-insertion mechanism of Li+ ions, especially, in the amorphous hollow structure (i.e. TDP-500), and the hollow structure of TDP-500 is endowed with high specific surface area and pore volume unambiguously leads to an enhanced interfacial Li-storage.27 25 ACS Paragon Plus Environment


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It is important to note that although the specific capacity (at 100th cycle, 0.5 C) of TN-500 at ca. 190 mA h g-1 is better than TDP-500 at ca. 147 mA h g-1, but severe deterioration in the specific capacity was observed in TN-700 (Figure S6a) as compared to TDP-700 (Figure 6a) after 30 cycles at 0.5 C. This could be due to the anatase-rutile mixed phases, bigger grain sizes, and lower specific pore volume for TN-700 as compared to TDP-700 (Tables 1 and 2). It is anticipated that the enhanced cycling performance of anatase TDP-700 as compared to anataserutile TN-700 (Figures 6a and S6a) might be caused by the presence of anatase phase with higher electronic conductivity than rutile phase14 and relatively smaller anatase grain size (Table 1). Overall, TDP samples showed good capacity retention (at both 500 and 700 oC) which results in excellent reversible capacity and cycling stability. In particular, TDP-500 provided excellent reversibility for the Li+ insertion/extraction, primarily because of the presence of Li+ diffusion pathways throughout its disordered structure and absence of internal stress due to phase transition from amorphous to crystalline phases.15 Hence, it is anticipated that amorphous phase (i.e. TDP-500) could outperform crystalline phases of TiO2 for long-term rate performance. High rate performance is one of the critical requirements of LIBs for high power applications such as in electric vehicles.60 In this regard, the rate capabilities of TNP and TDP were analyzed after cycling at C-rates ranging from 0.5 to 40 C (Figure 6f), and the specific capacities obtained for each sample are presented in Table 3. From the rate capability results, it can be concluded that TDP exhibits better rate capability than TNP at all C-rates (Figure 6f and Table 3). This is mainly due to the meso-macroporous hierarchical hollow spherical structure of TDP with higher specific pore volume. In particular, Figure 6f shows that TDP-500 exhibits the best rate capability. To unveil the reason behind the better rate capability of TDP-500 at all C-rates than TNP-500, both samples were analyzed by EIS (Figure 6g). Nyquist plots (Figure 6g) show that 26 ACS Paragon Plus Environment

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TDP-500 exhibits relatively lower charge-transfer resistance than TNP-500; thus, depresses electrode polarization and enhances Li+ diffusion.61-62 Taken together, the large difference in the rate-capability of TDP over TNP is ascribed to its higher surface, lower charge-transfer resistance, higher Li+ diffusion, larger meso-macroporous structure and larger specific pore volume. Table 3. The rate performances and the estimation of retention (%) in specific capacity of calcined TiO2 particles obtained at different C-rates. Sample

Specific Capacity (mA h g-1) of TiO2 particles At 0.5 C 1C 2C 5C 10 C 20 C 40 C

Retention (%) in Specific Capacitya At 1C 2C 5C 10 C 20 C 40 C

TN-500 200 186 173 150 126 100 72 93 86 75 63 50 36 TNP-500 69 57 42 27 18 10 7.0 83 61 39 26 15 10 TD-500 178 158 139 117 98 70 44 89 78 66 55 39 25 TDP-500 148 146 142 130 124 115 103 99 96 88 84 78 70 TN-700 150 135 120 91 69 48 34 90 80 61 46 32 23 TNP-700 51 39 29 16 9.5 4.7 2.5 76 57 31 19 9.2 5.0 TD-700 126 110 96 73 56 40 30 87 76 58 44 32 24 TDP-700 126 119 111 96 81 63 44 94 88 76 64 50 35 a retention (%) in specific capacity is the capacity retained at a particular C-rate, calculated by dividing the specific capacities at 1, 2, 5, 20 and 40 C by the specific capacity obtained at 0.5 C.

Figure 7. A comparison of cycling and rate performances of TN-500 and TDP-500 at 0.5 (for 100 cycles) and 40 C (for 400 cycles, the vertical dotted-line indicates electrode activation process for the first 10 cycles at 0.5 C).



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Comparison of TN-500 and TDP-500

TN-500 and TDP-500 which showed the best rate performances and retention (%) in specific capacities among all TiO2 samples tested (Table 3), respectively, were selected for long-term cycling at high C-rates (i.e. 400 cycles at 40 C) (Figure 7). This is compared with their performances for short-term cycling at low C-rates (i.e. 100 cycles at 0.5 C) (Figure 7). At 0.5 C, the cycling performance of TN-500 (specific capacity of ca. 190 mA h g-1) is better than the TDP-500 (specific capacity of ca. 147 mA h g-1) throughout the 100 cycles. However, at 40 C, the cycling performance of TDP-500 is better than TN-500 throughout the 400 cycles. TN-500 exhibited specific capacity of ca. 50 mA h g-1 across the 400 cycles. In contrast, TDP-500 exhibited initial specific capacity of ca. 70 mA h g-1 which increased to ca. 113 mA h g-1 after 200 cycles. The increasing specific capacity from ca. 70 to 113 mA h g-1 during the 200 cycles could be resulting from the gradual activation of electrode surface which was covered by adsorbed electrolyte.15 Importantly, the larger surface area and highly porous structure (originating from mesomacroporous hollow structure) of TDP-500 can also greatly reduce the specific current density of the active materials. The low specific current density can effectively stabilize the electrode and keep a high capacity (during cycling) at higher C-rate (i.e. 40 C).63-64 However, from the 200th to 400th cycle, the specific capacity of TDP-500 was reduced from ca. 113 to 93 mA h g-1 which might be attributable to the electrode deactivation process and/or changes in the morphology of hollow structure due to repeated cycling at 40 C. Nevertheless, TDP-500 exhibited better longterm cycling performance than TN-500 at 40 C. Hence, TDP-500 is better suited for high power applications such as in electric vehicles.


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Our results are also comparable and/or superior to the performances of previously reported TiO2-based anodes (Table S1). However, a comparison of the cycling performance between TN500 and TDP-500 (at 0.5 C) shows that TN-500 performs better than TDP-500 despite the presence of hollow structure and higher specific pore volume in TDP-500 (Figures 4 and 7, and Table 2). The relatively lower cycling performance of the TDP-500 as compared to TN-500 and the other published works (Table S1) might be caused by the presence of the titanium phosphate complexes in TDP-500. The phosphate remaining in DP or free phosphate could have reacted with TiBALDH precursor, formed titanium phosphate complexes with poor electrochemical properties, and thus reducing the cycling performance.65

CONCLUSIONS In summary, four different lysozyme biotemplates of various size, morphology and rigidity were prepared via heat denaturation in the presence or absence of phosphate ions. Notably, excessive aggregation of denatured lysozyme molecules in the presence of phosphate ions led to the formation of a stable insoluble biotemplate with hierarchical spherical structure. These lysozyme biotemplates were used to direct TiO2 biomineralization and found to critically affect the size and morphology of the mineralized TiO2. Using the different characteristics of the biotemplates and high temperature calcination to induce phase transition, TiO2 particles of versatile morphology, particle/grain/pore size, crystallinity, surface area and specific pore volume were produced. The differences in TiO2 properties led to particles of varying electrochemical performance and potential as LIB anode. The effect of biotemplate state (native vs denatured) and morphology (non-hollow vs hollow spherical hierarchical structures) on the electrochemical characteristics of TiO2 particles was investigated. TN-500 showed the best 29 ACS Paragon Plus Environment


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cycling and rate performance among TN and TD samples. More distinctively interconnected TN500 particles provided better electrochemical characteristics than highly fused TD-500 particles. TDP-500 with hollow spherical structure showed the best cycling and rate performance among TNP and TDP samples. TN-500 and TDP-500 which showed the best rate performances and retention (%), respectively, were selected for further investigation. At low C-rate of 0.5 C, TN500 exhibited better cycling performance and higher specific capacity than TDP-500. However, at high C-rate of 40 C, TDP-500 exhibited better cycling performance and higher specific capacity than TN-500. The hierarchical hollow spherical structure of TDP-500 allowed better Li+ ion transfer and structure stability, which led to its high performance at high C-rate despite the presence of titanium phosphate.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications Website at DOI: Experimental details, reverse-phase HPLC and UV-Vis of biotemplates, TGA of TN, TNP, TD and TDP, , HRTEM images of calcined TNP and TDP, EDX mapping of calcined TDP, FESEM and XPS analyses of TN, TNP, TD and TDP, cycling performance, rate capabilities, CV and galvanostatic charge-discharge profiles of TN and TD. CV analysis and charge-discharge profiles of TN-700, TNP-700, TD-700 and TDP700, EIS plots for TNP-500 and TDP-500, and comparison of the TiO2 LIB anode performance (ours vs those reported in the literature) are available in the Supporting Information. (PDF) 30 ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (Lihan Tan) * Email: [email protected] (Pil J.Yoo) * Email: [email protected] (Woo-Seok Choe) Author Contributions # These authors contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the research grants of the NRF (2014M3A7B4052200, 2017R1A2B2011341) and Basic Science Research Program (2017R1A5A1070259) funded by the National Research Foundation under the Ministry of Science, ICT & Future, Korea. This work was also supported by Industrial Technology Innovation Program. No. 10062694 funded by the Ministry of Trade, Industry & Energy (MOTIE), Korea. REFERENCES 1.

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