The thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out to determine the ideal temperature for the decomposition of precursor gel of LFP and LFTP. Simultaneous TG/DTA characterizations, performed on LiFePO4 and LiFe0.95Ti0.05PO4 of dried precursor powder at a heating rate of 10 oC min− 1 in nitrogen from room temperature to 900 οC, are shown in Fig. 2 (a & b). As shown in Fig. 2 (a & b) the LFP and LFTP show nearly the similar type of thermal behavior. The thermo-grams of the samples revels the two-step weight losses. The initial one displayed the very slight weight loss within the temperature range room temperature to 185 °C. Secondly the major weight loss is found in the temperature range, 180 to 440 °C while in the LFTP is 208 to 490 °C. There is no further weight loss was noticed beyond 600 °C. The initial weight loss is owing to evaporation of residual water and the dehydration of hydrated water present in the sample. The second major weight loss is associated to the combustion of gel and excess citric acid . On the DTA curve, there were two exothermic DTA signals observed on decomposition processes. The initial exothermic process has been rise at temperature of 275 °C and the other signal appears between 340 and 470 °C. These exothermic signals demonstrate the auto-combustion process where the nitrate ions of reagents act as an oxidizing agent and citric acid as fuel. The lack of signal in the DTA curve after 600 °C also validate the findings of TG analysis and established the formation of LFP and LFTP at around 700 °C.
X-ray diffraction (XRD) patterns of LFP and LFTP calcined at 700 °C for 5 h are shown in Fig. 3. The XRD patterns are agree very well with that of phospho-olivine standard phase and indexed to an orthorhombic crystal structure (space group Pnma) . The samples displayed the narrow diffraction peaks demonstrating the good crystallinity degree and there is no Ti compound phase or other impurity phase was observed. In inset in Fig. 3 showed the magnified patterns in the 2θ range of 35.4–36° which displayed the shifting of the XRD peak towards lower 2θ which suggest the enhancement of lattice parameter as listed in Table 1. It is noted from the table that the remarkable increases in parameters a, b and c are observed in LFTP and confirms the Ti4+ ions successfully incorporated at Fe2+ sites in LiFePO4.
The lattice parameters and unit cell volume of the samples are calculated using cell software and values are shown in Table 1.
Scanning electron microscopy characterization was made to determine the morphology and grain size using scanning electron microscope (SEM). Figure 4 show the SEM images of LFP and LFTP polycrystalline material. As seen from the SEM images the morphology of the grains are irregular in shape in both materials. The LFP displayed the separate grains while in the case of LFTP showing the large grain with some agglomeration. The grain size of LFP and LFTP polycrystalline material found to be in the range of ⁓120 nm-0.4 µm and ⁓200 nm- 0.45 µm, respectively.
Figure 5 (a, b) displayed the energy dispersive X-ray (EDX) images of the LFP and LFTP polycrystalline material to ascertain the composition of the grains during the SEM measurement. As seen from the Fig. 5 (a, b) all the metal elements were detected in the grain region except Li because it has very low energy of characteristic radiation, not easy to detect in EDX analysis. It is confirmed that the existence of Fe, P and O in LFP and Fe, P, Ti and O metal ions in LFTP which confirms that Ti ion successfully enter into the crystal lattice of LiFePO4.
The LFP and LFTP polycrystalline material were further analyzed with High resolution transmission electron microscopic (HR-TEM) analysis as shown in Fig. 6. As displayed in Fig. 6a, the LFP crystallites have irregular shape in the size range ⁓200–300 nm while LFTP has larger crystallites size in the size range ⁓200 nm. It is also evidently observed from the TEM images that each crystallite of LFP and LFTP were coated by a carbon layer to form a carbon–shell structure generated by carbonization of the citric acid. As distinguished from Fig. 6 (b, d) the thickness of the carbon layer is about 2.4 nm and 7.5 nm for LFP and LFTP polycrystalline material, respectively.
Furthermore, the EDX spectra of LFP and LFTP polycrystalline material were observed with HR-TEM (Fig. 7a-d) showing the existence of Fe, P and O in LFP and Fe, P, Ti and O metal ions for LFP and LFTP polycrystalline material, respectively which are well matched with the EDX analysis (Fig. 5).
The distribution of elements is shown in the EDS mapping images (Fig. 8 (a) and (b)) for LFP and LFTP polycrystalline material. As can be seen, the Ti dopant ions is distributed homogeneously in the LiFePO4 crystal structure without the presence of any trace amounts of the impurities. A very homogeneous uniform distribution of the Fe, P, O and Fe, Ti, P, O elements is also detected in LFP and LFTP polycrystalline material because during the synthesis of LFP and LFTP polycrystalline material, the constituents Li+, Fe2+ and PO43− ions were dissolved in the water and mixed on the atomic scale. This investigation confirms the advantages of the citric acid-assisted semi-wet combustion synthesis.
X-ray photoelectron (XPS) spectra of LFP and LFTP polycrystalline material was carried out to ascertain the oxidation state of the metal ions and elemental composition as shown in Fig. 9 and Fig. 10.
The wide spectrum in Fig. 9 (a) and 10 (a) evidently shows the existence of Li 1 s, Fe 2p, P 2p, O 1 s and Li 1 s, Fe 2p, P 2p, Ti 2p, and O 1 s XPS signals of these elements, which verified the EDX observation and immaculacy of LFP and LFTP polycrystalline material with additional peak of carbon C 1 s located at ⁓284.8 eV [Figs. 9 (f) and 10 (f)] was used as a reference for binding energy calibration. From the Figs. 9 (b) and 10 (b) LFP and LFTP displayed the Li 1 s XPS spectrum located at 55.78 eV and 55.27 eV corresponding to the lithium in LFP and LFTP polycrystalline material . The positions of the Fe 2p region peaks [Figs. 9 (c) and 10 (c)] in the XPS spectra of the both material were corresponding to the Fe 2p3/2 main peak situated at 710.5 eV & 710.9 and Fe 2p1/2 main peak located at 724.3 eV & 724.4 for LFP and LFTP, respectively. These binding energy positions of Fe correspond to the characteristic peaks of the Fe2+, and the observations are consistent with earlier . XPS spectra, shown in [Figs. 9 (d) and 10 (d)], demonstrate that the only one P 2p doublet peaks with the binding energy values of 133.5 eV and 133.3 eV for LFP and LFTP, respectively. The binding energy positions of the P 2p affirm the existence of (PO4)3− phosphate group . The O 1 s [Figs. 9 (e) and 10 (e)] of the LFP and LFTP materials were similar at 531.4 eV with respect to the corresponding spectra for oxygen in LiFePO4 . For the dopant as Ti in LiFePO4 in Fig. 10e, the XPS peaks of Ti 2P are located at 458.4 and 464.3 eV for Ti 2p3/2 and Ti 2p1/2, respectively, which affirm the oxidation state of Ti ions is + 4 in good consistency with oxidation state of Ti [19, 20].