The crystal structure and phase composition of the FLTO/C, PLTO and PLTO/C were analyzed by XRD, and the results are shown in Fig. 2(a). According to JCPDS 49–0207, the three LTO materials crystallized in the F__d3m space group. No peak of Fe-containing oxide is observed in the FLTO/C electrode material, demonstrating that Fe doping into the LTO lattice did not change the structure of LTO, or that the Fe content is low and undetected. Typically, when the ionic radius is close to the doped metal ion radius, the substitution reaction is more likely to occur or the dopant can enter the lattice gap to form an active center. The ionic radius of Fe2+ (0.78 Å) 25,26is similar to those of Li+ (0.76 Å) 27–29 and Ti4+ (0.68 Å) 30–32. In LTO structure, Li+ occupies the tetrahedral 8(a) position or octahedral 16(c) position; its occupying position can be reflected in the intensities of the (311) and (400) peaks. The relative strength ratio of the (311) and (400) peaks increases with the increase in Fe content, thus, reflecting the change in the Li+ ion position on Fe doping 33,34. The I(311)/I(400) strength ratio of the FLTO/C electrode material is not much different from that of the original PLTO, indicating that Fe2+ is not doped into the Li site. Figure 2(b) shows a magnification of the XRD patterns of the three electrode materials for the (111) crystal plane. The peak of FLTO/C shifts to a smaller angle, indicating that the Fe2+ dopant entered the Ti site, further indicating that the carbon-coated Fe-doped LTO material had been successfully prepared.
The influence of Fe doping on the LTO structure was further examined by implementing Rietveld refinement on the Fullprof software, to calculate the crystal size, as depicted in Fig. 3(a-b). The lattice parameters and other relevant information are summarized in Table 1. Results indicate that Fe substitution increased the lattice parameters from 8.35458 to 8.35504 Å. This shift can be attributed to the larger ionic radius of Fe2+ compared to Ti4+. The partial substitution of Ti4+ by Fe2+ increased the unit cell volume in the LTO structure. The accurate Fe concentration in the FLTO/C electrode material was obtained by ICP-OES analysis. The concentration of Fe in FLTO/C was approximately 0.27%, confirming the formation of Li4Ti4.9865Fe0.135O12/C.
Table 1
༎Cell parameters from the Rietveld refinement of PLTO and FLTO/C.
Sample | Lattice parameter (Å) | V (Å3) |
PLTO | 8.35458 | 583.141 |
FLTO/C | 8.35504 | 583.238 |
The SEM images of PLTO, PLTO/C and FLTO/C in Fig. 4 show LTO microspheres of about 4–5 µm diameter. While the surface of the PLTO electrode material is smooth, the addition of 15% glucose roughens the surface and forms small primary particles and pores on the surfaces of PLTO/C and FLTO/C. This can be attributed to the addition of glucose, resulting in the thermal decomposition of glucose during calcination.
The EDS elemental mapping images of FLTO/C indicate that Ti, C, S and Fe were uniformly distributed in the electrode, as shown in Fig. 5. Trace amounts of surface sulfur on FLTO/C can be attributed to the residual sulfur from industrial H2TiO3, which is difficult to remove during the washing process. Figure 5(d) shows that the distribution of Fe is consistent with the distribution of Ti, O and C, indicating that Fe is uniformly distributed in the LTO crystal structure. The Fe ions are adsorbed by H2TiO3 with a strong binding force and good stability, which lays a foundation for subsequent doping reactions and uniform distribution. Meanwhile, XRD diffraction does not find the peak of iron oxide, so on the basis of the above discussion, we synthesized Fe-doped LTO/C microspheres.
The thickness of the carbon layer and lattice distance of FLTO/C were assessed using HRTEM (Fig. 6). The FLTO/C electrode material shows a reasonably uniform carbon layer with a thickness of approximately 1.5 nm. The (111) crystal plane of FLTO/C corresponds to a crystal plane spacing of 0.488 nm, while that of pure phase LTO is about 0.483 nm 35. These results confirm that introducing Fe2+ into the LTO lattice increases the LTO lattice constant, aligning with the XRD results. An expanded crystal face spacing promotes the diffusion of Li+ culminating in better electrochemical performance of LTO.
The carbon content in PLTO/C and FLTO/C was measured as 2.09% and 2.11%, respectively, by a carbon-sulfur elemental analyzer. XPS was used to analyze the chemical state of Ti, C and Fe in FLTO/C, as shown in Fig. 7. The Ti 2p peaks can be assigned to Ti 2p1/2 (464.25 eV) and Ti 2p3/2 (458.5 eV) 36,37, corresponding to Ti4+ (Fig. 7a). Figure 7(b) shows the Fe 2p1/2 and Fe 2p3/2 peaks at 722.44 eV and 710.51 eV, respectively, indicating the presence of Fe2 + 38,39. Figure 7(c-d) reveals the results of FLTO/C and PLTO/C after C1s partial peak fitting. The entire spectrum can be divided into three peaks, i.e., C-C and C = C near 284.8 eV, C-O at 285.9 eV, and C = O at 289.3 eV 40–42. It is suggested that, when Fe is doped into the LTO lattice, the valence state and composition of C do not change.
Half-cells were assembled using dual-modified FLTO/C composites as the active material and PLTO and PLTO/C as the control samples. Figure 8(a) shows the rate performance of PLTO, PLTO/C and FLTO/C, at current densities ranging from 1 C to 10 C. At 0.2 C, 0.5 C, 1 C, 2 C, 5 C and 10 C, FLTO/C showed discharge capacities of 172.15, 168.21, 166.02, 164.27, 159.50, and 153.79 mAh g− 1, respectively, significantly exceeding those of PLTO and PLTO/C as summarized in Table S1. These results show that FLTO/C has a higher specific capacity than 150 mAh g− 1 over 2–10 C, significantly surpassing the limitations of higher rate performance of PLTO and PLTO/C.
The cycling performance of PLTO, PLTO/C and FLTO/C were tested at a current density of 1 C, as shown in Fig. 8(b). After 400 cycles, the capacity retention rates of PLTO, PLTO/C and FLTO/C were 70.42%, 95.97% and 95.97%, respectively. Figure 8(c) illustrates the charge and discharge cycles at 5 C for PLTO, PLTO/C, and FLTO/C after 1000 cycles. The long-term cycling stability of PLTO and PLTO/C shows initial discharge capacities of 94.29 and 139.27 mAh g− 1, respectively. After 1000 cycles, the capacity retention rates of PLTO and PLTO/C decrease to 35.99% and 85.91%, respectively, while their discharge capacities drop to 33.94 and 119.65 mAh g− 1, respectively. After 1000 cycles at 5 C, FLTO/C exhibits an initial and residual specific capacity of 156.62 and 145.02 mAh g− 1, respectively, with a corresponding capacity retention rate of 92.56% and an ultra-low capacity decay rate of 0.007% per cycle.
The above results show that due to the dual modification by Fe doping and carbon coating, the specific capacity and stability of LTO show dramatic improvement. This behavior can be explained as follows; when a certain amount of carbon is added, the surface of the prepared FLTO/C microspheres becomes coarser, leading to a larger specific surface area, and the surface holes become more obvious. The increase of these holes can further facilitate Li+ to pass through quickly. Moreover, after Fe doping, the cell parameters and cell volume of FLTO/C increase, which is conducive to the diffusion of Li+ and further improves the electronic conductivity 35. Therefore, the combined modifications result in excellent rate performance and cycling stability, particularly in FLTO/C.
FLTO/C also demonstrates superior rate and cycling performance compared to control samples PLTO and PLTO/C, and other LTO composite materials and graphite materials, as shown in Table 2.
Table 2
Electrochemical performance comparison of FLTO/C with other LTO composites materials and graphite materials.
Materials | Current Rate (C) | Rate capacity (mAh g− 1) | Current Rate for cycling (C) | Cycle number | Capacity retention (%) | Reference |
MFLTO | 10 | 76.0 | 5 | 150 | 99.6 | [43] |
Li4Ti4.95Te0.05O12 | 10 | 120.3 | 5 | 100 | 98.2 | [44] |
B0.3-C@Li4Ti5O12 | 1 | 165.0 | 1 | 200 | 90.0 | [45] |
LTO-Ce | 10 | 61.5 | 2 | 400 | 81.7 | [46] |
LTO-Ti | 10 | 131.0 | 2 | 200 | 97.8 | [47] |
SGG | 10 | 390.0 | 0.8 | 600 | 92.0 | [48] |
GGCC | 10 | 478.8 | 0.5 | 325 | 86.9 | [49] |
FLTO/C | 10 | 153.8 | 5 | 1000 | 92.6 | Our work |
CV was used to further study the kinetic behavior of the electrodes during the lithiation/de-lithiation process. Figure 8(d) shows the CV curves of PLTO, PLTO/C and FLTO/C at a sweep rate of 0.5 mV s− 1 after 100 cycles, at 1 C. The proximity of the redox peak of FLTO, to that of PLTO, and the lack of heterodox peaks indicates that Fe doping does not alter the electrochemical reaction of the LTO substrate material. The peak separation values of PLTO, PLTO/C and electrode were 0.24 V and 0.23 V respectively, while the peak separation value of FLTO/C anode was the narrowest, only 0.19 V, so FLTO/C showed the smallest polarizability. Figure 8(e) displays the charge-discharge plateaus of the three electrode materials at 1 C. Compared with PLTO and PLTO/C, FLTO/C showed a slightly higher discharge voltage, the largest specific capacity and the smallest charge-discharge voltage difference (42.1 mV), which is consistent with the CV results.
The electrochemical behavior of the electrodes was studied through EIS and fitted to equivalent circuit models using the Z-view software, as shown in Fig. 8(f). Rs represents the total resistance of the electrolyte, separator, and fluid collector, Rct represents the charge transfer resistance, CPE represents a constant phase element, and Zw represents the Warburg impedance, which correlates to the Li+ diffusion process. Fig. S1 shows the impedance velocity (Z' diagonal) of ω−1/2 for PLTO, PLTO/C, and FLTO/C. The fitting results (Table S2) and calculations for the Li+ diffusion coefficient (Equations S1 and S2) indicate that the total resistance (Rs) values for PLTO, PLTO/C, and FLTO/C are 3.765, 3.323, and 3.175 Ω, respectively. In addition, the charge transfer resistance (Rct) for PLTO, PLTO/C, and FLTO/C are 35.34, 12.72, and 10.90 Ω, respectively. The calculated Li+ diffusion coefficients for PLTO, PLTO/C, and FLTO/C are 6.41×10− 15, 9.45×10− 14 and 5.95×10− 13 cm2 s− 1, respectively. Compared with the PLTO and PLTO/C electrode materials, the Li+ diffusion coefficient of FLTO/C increased by 1 ~ 2 orders of magnitude. Thus, the dual modification by Fe doping and carbon coating substantially enhanced the FLTO/C Li+ diffusion coefficients.
A four-point probe resistivity tester showed that the electronic conductivities of PLTO/C and FLTO/C were 2.15×10− 4 and 8.50×10− 3 S cm− 1, respectively. Compared with the intrinsic conductivity of LTO, the electronic conductivities of the two electrode materials had significantly improved. These results demonstrate that the dual-modified LTO shows a considerably enhanced electrical conductivity. This improvement can be attributed mainly to the carbon coating on the surface of LTO. Moreover, the addition of Fe results in an increase in the unit cell size, which promotes the migration of electrons.