**3. Impact of phosphorus doping on TiO2 as anode for Lithium-ion batteries**

Another study by our group have evaluated also the impact of phosphorus doping on the electrochemical performances of TiO2 as anode material for lithium ion batteries. The phosphorus doped TiO2 was synthesized using a simple and eco-friendly synthesis method, in which titanium tetra-isopropoxide was used as a titanium precursor and sodium alginate as a complexing agent. The effects of P-doping on the crystal structure, morphology and lithium insertion mechanism

were investigated and compared with the undoped TiO2. Moreover, the P-TiO2 was tested electrochemically as anode material.

Concerning the synthesis process, TiO2 and P-TiO2 materials were synthetized via a gelation of biopolymers, following the synthesis technic proposed by El Ouardi *et al.* and using phosphoric acid as the phosphorus precursor. To prepare the working electrode, black carbon, PVDF and an active material were mixed in a 7:2:1 wt. ratio. The active material of the first electrode contained pure TiO2 while the second contained TiO2 doped at 2% phosphorus.

To identify the crystal structures of TiO2 and P-TiO2, XRD was carried out and the results are shown in **Figure 10a**. As it can be seen in the diffractograms, the diffraction peaks are centered at 25.3o , 37.9o , 48.1o , 54.7o , 55.0o , 62.7o , 68.9o , 75.04o and 83.0o . These are attributed to the (101), (004), (200), (105), (211), (204), (116), (215) and (312) diffraction planes of anatase TiO2, respectively, indicating that the crystal phase of TiO2 remained after phosphorylation treatments [39–41]. At 2θ = 30.7° there is a small peak (\*) for the undoped sample, which can be attributed to the existence of the brookite phase, (121) formed during the synthesis [42]. No diffraction peaks that could be attributed to impurities are found in the XRD patterns of TiO2 and P-TiO2, suggesting that the sol–gel method can give highly purified anatase TiO2 products. For the Raman spectra (**Figure 10b**), the obtained bands at 198, 400, 518, and 641 cm−1 represent the Raman active modes of anatase TiO2. These results prove that the prepared nanoparticles have an anatase structure; the non-doped TiO2 sample contained particles with uniform sizes and homogeneous granular surface, while the P-TiO2 samples remained unchanged. The energy

#### **Figure 10.**

*(a) XRD patterns and (b) Raman spectra of non-doped TiO2(bleu) and P doped TiO2 (red) materials obtained at 450°C.*

#### **Figure 11.**

*Nitrogen adsorption–desorption isotherm curves and the pore size distribution curves of non-doped and P doped TiO2materials obtained at 450°C.*

*TiO2 Based Nanomaterials and Their Application as Anode for Rechargeable Lithium-Ion Batteries DOI: http://dx.doi.org/10.5772/intechopen.99252*

dispersive X-ray (EDX) spectroscopic data of the P-doped TiO2 demonstrate the uniform distribution of Ti, O and P with no other impurity elements.

The Brunauer–Emmett Teller (BET) method from N2 adsorption and desorption isotherms carried out at 77 K (**Figure 11**) showed that both materials presented typical IV adsorption/desorption isotherms with mesoporous structures. Besides, both materials exhibited very similar BET surface, pore size distribution and mesopore diameter. For the absorbance measurement, UV-V spectroscopy showed that the phosphorus doping extended the wavelength response range of TiO2 into the visible-light region (**Figure 12**). Moreover, the band gap of TiO2 and P-TiO2 was 2.90 and 2.87 eV, respectively. This result shows the effect of phosphorus doping to reduce the band gap and improve the electrotonic conductivity of TiO2 [43–45].

Concerning the electrochemical tests, the charge/discharge curves (**Figure 13**) show the presence of two plateaus at 1.9 V and 1.7 V for both materials representing

**Figure 12.** *UV-V spectra of non-doped TiO2 (bleu) and P doped TiO2 (red) materials obtained at 450°C.*

#### **Figure 13.**

*(a) First charge/discharge profiles of TiO2 (bleu curve) and P-TiO2 (red curve) electrodes calcined at 450°C cycled between 3.0 and 1.0 V versus Li/Li<sup>+</sup> at C/10 current rate, (b) cyclic voltammograms of the first cycle scanned at 0.02 mV s−1.*

cathodic and anodic peaks of anatase TiO2 nanoparticles, respectively. This charge/ discharge process has shown also a good irreversible capacity which does not exceed 10 mAh/g for both materials. For the polarization, P-TiO2 has shown improved characteristics compared to non-doped TiO2. This result could be attributed to the improved electronic conductivity.

Cyclic voltammetry technique was used to study the insertion/extraction properties of lithium ions from the prepared electrodes in the 1.0 and 3.0 V potential window with a scanning speed of 0.02 mV s−1. As it can be seen in **Figure 13**, there are two cathodic (reduction peak I < 0) / anodic (oxidation peak I > 0) peaks at 1.7 and 1.9 V, respectively, attributed to the insertion / extraction of lithium ions in the TiO2 nanoparticles. This result is with agreement with the galvanostatic discharge/ charge profiles.

#### **Figure 14.**

*Galvanostatic discharge/charge curves vs. Li/Li+ of (a) P doped TiO2 and (c) non-doped TiO2 cycled at a rate of 0.1 C. cycling performance and coulombic efficiency of (b) P doped TiO2 and (d)non-doped TiO2 electrodes cycled between 3.0 and 1.0 V versus Li/Li+ at 0.1 C current rate.*

#### **Figure 15.**

*Rate capability of (a) non doped TiO2 and (b) P doped TiO2 electrodes at variant current rates from 0.1 C to 20 C (1C = 336 mA g−1).*

*TiO2 Based Nanomaterials and Their Application as Anode for Rechargeable Lithium-Ion Batteries DOI: http://dx.doi.org/10.5772/intechopen.99252*

For the cycling stability, **Figure 14** prove the excellent coulombic efficiency (about 100%) for both TiO2 and P-TiO2 electrodes. Besides, these electrodes showed a capacity retention of 78% after 70 cycles and 83% after 90 cycles, respectively. The reason behind these improved electrochemical properties for P-TiO2 can be the smaller TiO2 particle size which permits fast lithium insertion / disinsertion process.

**Figure 15** present the rate capabilities of TiO2 and P-TiO2 evaluated at different current rates at 1.0–3.0 V voltage range. The electrodes were discharged down to 1.0 V and recharged up to 3.0 V at different constant current density from 0.1 to 20 C (1 C = 336 mA g−1). It is clearly observed that the reversible capacity declined gradually with the increase of the current for both materials, but it still exceeds 80 mAh g−1 for non-doped TiO2 and 98 mAh g−1 for P doped TiO2 even at a rate of 5 C. The capacity at 0.1C rate after 70 cycles was recovered to about 185 mAh g−1 for non-doped TiO2 and 213 mAh/g for P doped TiO2 after 90 cycles. Thus, indicating the high stability of the anatase TiO2 nanoparticles and confirming the better performance of P-TiO2 compared to TiO2.

### **4. Conclusion**

In summary, this chapter show the huge interest in the development and improvement of TiO2 as anode for high performance rechargeable lithium ion batteries. Several strategies have been developed to improve the conductivity, the capacity, cycling stability, and rate capability of this material, such as designing different nanostructured (1D, 2D and 3D), Coating or combining TiO2 with carbonaceous materials**,** and Selective doping with mono and heteroatoms.

Biopolymer gelation is a simple and economically favorable approach for the preparation of TiO2 nanoparticles. This method that consists of using the Sol–Gel method with a sodium alginate biopolymer as a templating agent showed enhanced performances in comparison with other synthesis techniques. In fact, the prepared TiO2 electrodes displayed a high specific capacity above 275 mAh g−1 and excellent cycling stability with over 85% capacity retention after 100 cycles. Besides, combining phosphorus doping with this synthesis strategy demonstrated an important discharge capacity of 200 mAh g−1 after 90 cycles under C/10 current rate and has an excellent rate performance. The improved electrochemical performance can be explained based on the P-TiO2 particles size and band gap modifications upon doping proved by UV-V measurement.

Finally, from this chapter, we can conclude that the use of TiO2-based materials as anode for commercial lithium ion batteries requires more efforts to overcome the problems encountered, especially the low electrical conductivity, the low energy density, the poor cycling life and the low efficiency.

### **Acknowledgements**

The authors wish to acknowledge Office Chérifien des Phosphates (OCP S.A.) for financial support.

*Titanium Dioxide - Advances and Applications*
