Bio-synthesized TiO2 nanoparticles and the aqueous binder-based anode derived thereof for lithium-ion cells

Titanium dioxide nanoparticles (TiO2-NPs) are a promising anode material for Lithium-ion batteries (LIBs) due to their good rate capability, low cost, non-toxicity, excellent structural stability, extended cycle life, and low volumetric change (∼4%) during the Li+ insertion/de-insertion process. In the present paper, anatase TiO2-NPs with an average particle size of ~ 12 nm were synthesized via a green synthesis route using Beta vulgaris (Beetroot) extract, and the synthesized TiO2-NPs were evaluated as anode material in LIBs. Furthermore, we employed an aqueous binder (1:1 mixture of carboxy methyl cellulose and styrene butadiene) for electrode processing, making the process cost-effective and environmentally friendly. The results revealed that the Li/TiO2 half-cells delivered an initial discharge capacity of 209.7 mAh g−1 and exhibited superior rate capability (149 mAh g−1 at 20 C) and cycling performances. Even at the 5C rate, the material retained a capacity of 82.2% at the end of 100 cycles. The synthesis route of TiO2-NPs and the aqueous binder-based electrode processing described in the present work are facile, green, and low-cost and are thus practically beneficial for producing low-cost and high-performance anodes for advanced LIBs.


Role of beetroot extract in the synthesis
The precise identification of the secondary metabolites found in the beetroot used in this study has already been documented in works that are already reported in the literature.Beta vulgaris contains a variety of biologically active phytochemicals, such as betalains, flavonoids, polyphenols, saponins, and inorganic nitrate.It is also a rich source of a variety of minerals, including potassium, sodium, phosphorous, calcium, magnesium, copper, iron, zinc, and manganese [32][33][34].Betanin (betanidin 5-O-β-D-glucoside, the major red beet pigment) is a betalains pigment and is used as a powerful antioxidant and coloring agent in the food industry.Due to its strong antioxidant and reducing activity, which is based on its capacity to capture free radicals, betanin may function as a reducing agent [35].Additionally, betanin can act as a natural capping agent to stabilize metal/metal oxide nanoparticles because of its capacity to coordinate metal ions.The green synthesis of Ag/TiO 2 nanocomposites in the presence of beetroot extract has been reported [35], where Betanin was suggested to act both as a reducing and a capping agent.

Reaction mechanism involved in the synthesis of TiO 2 -NPs
The formation of TiO 2 -NPs in the present study involves two steps: (i) hydrolysis and (ii) condensation.Titanium isopropoxide (Ti(C 3 H 7 O) 4 ), is used as the Ti 4+ ion precursor, which usually undergoes hydrolysis in the presence of H 2 O as reported in the previous studies.The mechanism for the formation of TiO 2 -NPs is shown below [36] The type and concentration of phytochemicals found in the extract, along with the concentration of metal alkoxide precursor, are the key factors influencing the nanoparticle growth conditions, in the current study.The chemical interaction between the surface of TiO 2 -NPs and the phenolic -OH present in the betanin may be responsible for controlling particle size and stability [36].The proposed capping mechanism of betanin on the TiO 2 -NPs is shown in Scheme 1.
The Raman spectrum was used to further confirm the phase and purity of the synthesized TiO 2 -NPs to provide a deeper understanding of the structure (Fig. 2).The trigonal anatase phase has a space group of I41/amd and its local symmetry is D2d [40].The characteristic peaks centered at 142 (E g ), 195 (E g ), 393 (B 1g ), 514 (A 1g ), and 637 cm −1 (E g ) demonstrate the anatase phase, without any band for rutile or brookite phase [41].The band located at 637 cm −1 (E g ) corresponds to the Ti-O stretching mode and the band that appeared at 397 cm −1 (B 1g ) refers to the O-Ti-O bending mode [42].Thus, there is good agreement between the Raman spectrum and XRD data, confirming the existence of the high-purity TiO 2 anatase phase.
Figure 3 displays the FTIR spectrum of anatase TiO 2 -NPs.The broad band centered at 500-600 cm −1 corresponds to the Ti-O-Ti bending vibration in the TiO 2 structure.The broad band centered at 3600-3400 cm −1 refers to the intermolecular D = 0.9 cos Figure 4a represents the N 2 adsorption-desorption isotherm of as-synthesized TiO 2 -NPs and is used to measure the surface area.The results revealed that at the relative pressure P/P 0 of 0.99, the anatase TiO 2 -NPs possess type IV isotherm with a hysteresis loop.The mesoporous nature of the TiO 2 -NPs is suggested by the characteristic loop, and their presence can be attributed to the breakdown of biomolecules that have been capped on the surface of TiO 2 and the release of CO 2 during calcination.The adsorption isotherm curve is used to calculate the BET surface area of TiO 2 -NPs and the obtained value is around 63.2 m 2 g −1 .The associated pore size distribution plot derived from the adsorption data using the Density Functional Theory (DFT) approach is shown in Fig. 4b.Results show that the sample has an average pore diameter of 5.43 nm and a pore volume of 0.257 cm 3 g −1 .
SEM analysis was used to investigate the surface morphology of TiO 2 -NPs, as shown in Fig. 5. SEM images indicate that the particles are agglomerated into a spherical sponge-like bunch of particles.The irregular primary particles form a loose and porous structure that makes it easy for lithium-ion diffusion and electrolyte infiltration into the bulk phase.Moreover, the smaller particles enable faster Li + ion insertion and de-insertion in the TiO 2 anode material, leading to improved charge and discharge process, especially at high C-rate cycling.
TEM was employed to examine the actual shape and size of TiO 2 -NPs.The TEM image indicates that the synthesized TiO 2 -NPs are spherical in shape.The TEM image further demonstrates that the biomolecules in the beta extract act as  6a and b).The average particle size of each grain is 12 nm, which is comparable with the estimated crystallite size from XRD.The diffused rings in the selected area electron diffraction (SAED) pattern (Fig. 6c) indicate the polycrystalline nature of the synthesized TiO 2 -NPs.Moreover, it confirms the findings from XRD and Raman studies that the final TiO 2 possesses an anatase phase.Furthermore, the SAED pattern strongly supports the XRD findings by showing the growth of nanoparticles along the (101), (004), and (200) planes.The HR-TEM images shown in Fig. 6d show lattice fringes with a d-spacing of 0.35 nm, corresponding to the (101) plane of highly crystalline tetragonal anatase TiO 2 .The Fig. 6e represents the particle size histogram of TiO 2 -NPs, which indicates that the average particle size is ~ 12 nm.

Electrochemical performance of TiO 2 -NPs as anode material
The electrochemical performance of TiO 2 -NPs was studied in CR2032 coin cells with Li metal as the counter electrode.At a scan rate of 100 µV s −1 , half-cells were first subjected to CV measurements in the voltage window of 0.5 to 3.5 V vs Li + /Li. Figure 7 shows the CV test results for the initial three cycles.To facilitate lithium-ion insertion into the TiO 2 crystal structure, the half-cell was initially discharged.Thus, a change in the valence state of titanium from Ti 4+ to Ti 3+ takes place.The Li-ion insertion (reduction peak) and extraction (oxidation peak), which occurred during the cathodic and anodic sweeps, respectively, are illustrated by the strong peaks at 1.72 and 2.12 V.In crystalline anatase electrodes, the separation of cathodic and anodic peaks is prevalent.These peaks show the exceptional reversibility of the anatase TiO 2 -NPs as an insertion host because during discharging Ti 4+ was converted to Ti 3+ and then oxidized to Ti +4 during charging.Sharp cathodic/anodic peaks during electrochemical Li + intercalation/de-intercalation demonstrate the two-phase reaction mechanism in accordance with the following reaction: TiO 2 + xLi + + xe − ↔ Li x TiO 2 [44].Nevertheless, a slight deviation in the peak position is seen in the succeeding cycles which may be due to the small stress developed in the TiO 2 lattice during Li + insertion/extraction process.
The galvanostatic charge-discharge curves of TiO 2 -NPs in the voltage window of 1.0-3.0V (C/10 rate) are shown in Fig. 8.In the first step, the half-cell was discharged to intercalate Li + ions into the TiO 2 lattice.Due to the loss of symmetry in the y direction during the lithium-ion intercalation, the unit cell symmetry of anatase undergoes a first-order phase transition from its initial tetragonal (I41/amd) structure to the orthorhombic (Pmn21) structure [30].This phase transition happens simultaneously with a spontaneous phase separation of the lithium-poor (Li 0.01 TiO 2 ) phase into the lithium-rich (Li 0.69 TiO 2 ) phase, which was previously observed by several investigations [45,46].Though anatase TiO 2 -NPs have a theoretical specific capacity of 335 mAh g −1 , the actual capacity that may be achieved is much lower due to the significant Li-Li repulsion in the Li x TiO 2 framework at a higher degree of insertion, i.e., x > 0.5.In the present work, the anatase TiO 2 -NPs delivered an initial discharge capacity of 209.7 mAh g −1 , which is equivalent to inserting 0.62 mol of lithium per formula unit.During the initial charging, the half-cell exhibited a capacity of 184 mAh g −1 (0.55 mol lithium per formula unit).The irreversible capacity during the initial discharge-charge process is ~ 25.7 mAh g −1 (corresponds to 0.076 mol Li), which is lower than other previously published works (Table 1).The charge-discharge curves show shortened plateaus at 1.78 V and 1.9 V.The plateau at 1.78 V refers to the Li-ion insertion (discharge) and the plateau at ~ 1.9 V refers to the Li-ion de-insertion (charge) [47].The existence of a plateau region in the discharge-charge process demonstrates that the Li insertion occurs via a two-phase reaction mechanism and is in good agreement with CV peaks.The cycling performance and rate capability of anatase TiO 2 -NPs are shown in Fig. 9.The rate capability study is employed to examine the stability and versatility of the Li/TiO 2 half-cell.Figure 9a shows the rate performance of Li/TiO 2 half-cells at different current rates.The cell delivered discharge capacities of about 209.7,203,200,196,192,183,173, and 149 mAh g −1 at various current densities of C/10, C/5, C/2, 1C, 2C, 5C, 10C, and 20C, respectively.Figure 9a indicates that as the rate capability tests are repeated and the electrode cycled back at a high C-rate (20C), the material still delivers a discharge capacity of 132 mAhg −1 .Thus, this gives evidence of the structural stability and electrochemical reversibility of the TiO 2 -NPs sample.Figure 9b demonstrates the cyclability of Li/TiO 2 half-cell at three different current rates (1C, 2C, and 5C) for 100 cycles.The results revealed that the cell exhibited capacity retentions of 88.9% at 1C, 88.1% at 2C, and 81.2% at 5C rate at the end of 100 cycles.Figure 10 represents the coulombic efficiency of Li/TiO 2 half-cell during cycling, and rate performances and the results indicate that during both studies the coulombic efficiency is ~ 100%.As compared with previously reported work, the present bio-mediated anatase TiO 2 -NPs exhibited better cycling and rate performances (Table 1).
Cyclic voltammetry at various scan rates (100-900 µV s −1 ) was used to investigate the lithium-ion diffusion coefficient, D Li in TiO 2 -NPs (Fig. 11a).As the scan rate increases, the reduction peaks shift toward lower voltage and the oxidation peaks gradually shift toward high voltage.Figure 11b depicts the relationship between the square root of the scan rate (ν 1/2 ) and peak current (I p ).The diffusion coefficient was calculated by Randles Sevcik equation [48][49][50],    where n is the number of electrons transferred, A is the surface area of the electrode, D is the lithium-ion diffusion coefficient, C Li is the concentration of Li + ion in the electrode, ν is the scan rate and I p is the peak current.The diffusion coefficient of Li + -ions calculated based on both anodic and cathodic peaks for the TiO 2 -NPs is 5.72 × 10 -11 cm 2 s −1 and 1.86 × 10 -10 cm 2 s −1 for de-lithiation and lithiation processes, respectively.Thus, the higher diffusion coefficient is responsible for the superior rate and cycling performance of the present anatase TiO 2 -NPs.

Conclusions
In summary, anatase TiO 2 -NPs were successfully synthesized through a green-mediated process using Beta vulgaris (Beetroot) extract.The material was characterized by X-ray diffraction (XRD), Raman spectroscopy, infra-red (IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) analysis, and evaluated as anode in lithium-ion cells.Furthermore, an aqueous binder (a combination of CMC and SBR) is employed for electrode processing.
The electrochemical performance of the Li/TiO 2 half-cell was evaluated in the potential window of 1-3 V at C/10 rate.The cycling stability and rate capability studies demonstrate that the material exhibits superior performances than the previously reported TiO 2 -NPs material.Even at 20 C rate, the material delivered a discharge capacity of 149 mAhg −1 .Hence, the present work highlights a completely greener approach for both material synthesis and electrode processing.This work will be highly beneficial for producing anodes for high-power LIBs in a cost-effective and environmentally friendly route.

Fig. 5
Fig. 5 SEM images of TiO 2 -NPs at different magnifications, a at 25.0 kX and b at 10.0 kX

Fig. 7 1 Fig. 8
Fig. 7 CV plots of TiO 2 -NPs in the voltage window of 0.5-3.5 V at a scan rate of 100 µV s −1

Table 1
A comparison of electrochemical performances of anatase TiO 2 samples prepared in this work with previously reported materials 1/2 vs.I p