A series of LiFe0.7 (Mn1/3Co1/3Ni1/3)3x−yVyPO4 (x = 0.1, y = 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08 and 0.10) compositions doped with high concentration (30 mol%) of transition metals having higher redox potential are synthesized by sol-gel process. In the composition, assumption is made that equal contents of Mn, Co and Ni are replaced simultaneously by vanadium (V), and changes in structural and electrochemical characteristics are systematically investigated. The X-ray diffraction analysis confirms that V is doped successfully into host lattice with 0.0 ≤ y ≤ 0.04, whereas a secondary phase Li3V2(PO4)3 is observed for 0.05 ≤ y ≤ 0.10. The Rietveld refinement performed on XRD data shows continuous change in lattice parameters and cell volume with increasing y. X-ray photoelectron spectroscopy study confirms the oxidation state of Fe, Mn, Co and Ni in + 2, whereas V in + 4 state. The electrochemical characteristics show the positive contribution of Li3V2(PO4)3 in capacity as well as cycle life among doped samples.
Research Article
High redox potential transition metals incorporated olivine structure: LiFe0.7(Mn1/3Co1/3Ni1/3)3x-yVyPO4 (x = 0.1, 0.0 ≤ y ≤ 0.10) cathode for Li-ion battery
https://doi.org/10.21203/rs.3.rs-2758441/v1
This work is licensed under a CC BY 4.0 License
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Li-ion battery
multi-elements doping
LiFePO4
Rietveld refinement
electrochemical performance
Among the various cathode materials, olivine LiMPO4 (M = Mn, Fe, Co, Ni) is considered a potential candidate for Li-ion batteries. Particularly, LiFePO4 has attracted the most attention due to its low cost, nontoxicity, thermal stability and relatively high theoretical capacity (170 mAh g− 1). However, LiFePO4 suffers from poor electronic (~ 10− 9 S cm− 1) and ionic (< 10− 14 cm2 s− 1) conductivities, and relatively low redox potential of Fe+ 2/Fe+ 3 with respect to Li/Li+ (3.45 V). Several methods such as coating with conducting layers, improving kinetics by reducing defects and doping with other elements have been applied to overcome the limitations of LiFePO4, [1, 2].
However, it was reported that doping with transition metals having high redox potential Mn+ 2 (4.1 V), Co+ 2 (4.8 V) and Ni+ 2 (5.1 V) can increase the redox potential of the cathode material along with electronic and ionic conductivity of LiFePO4 [1, 3–6]. Unfortunately, these single transition metal phosphates have shown relatively lower electrochemical performance as compared to LiFePO4 [7]. LiMnPO4 shows very low capacity, poor cycle life due to large lattice distortion and also very poor electronic conductivity (~ 10− 14 S cm− 1) than LiFePO4 (~ 10− 9 S cm− 1) [8, 9]. However, LiCoPO4 has higher theoretical capacity (167 mAh g− 1) but it shows lower practical capacity and cyclability, because mostly electrolytes decompose at this high redox potential [5, 10]. Similar problems are faced with LiNiPO4, even it is very hard to determine electrochemical performance of LiNiPO4 due to instability of electrolyte at 5.1 V [1, 7].
Various studies have reported that single element Mn [11–14], Co [15–17] and Ni [18–21] or multi-element [22–24] doping on Fe position have improved the electronic conductivity and electrochemical performance of LiFePO4. The multi-element doping can modify the particles morphology, improves the electronic conductivity, decreases the charge transfer resistance and enhances the lithium ion diffusion in LiFePO4. In addition, vanadium (V) doping has also been attracted due to wide its range of oxidation states (V+ 2 to V+ 5) and V doping refines the morphology of LiFePO4, induce lattice distortion by decreasing the Li-O bonds strength thereby reducing the energy requirement for Li-ion diffusion and improve the electrochemical performance [2, 25–28]. However, synthesis method, selection of doping element, amount of doping and doping site are the important factors which control the electrochemical performance [25].
The multi-element doping e.g. (V and Na) [29], (V and Ti) [30], or (V and Mn, Cr) [31] in LiFePO4 have been reported by the researchers. However, to the best of authors’ knowledge, no study has been reported so far on high concentration multi-element doping of V, Mn, Co, and Ni simultaneously at Fe sites in LiFePO4. Therefore, in the present work, incorporation of transition metals V, Mn, Co and Ni in LiFePO4 has been performed to determine the combined effect of high level of doping elements on structure, electrical conductivity and electrochemical performance of LiFePO4. For the study, composition LiFe0.7 (Mn1/3Co1/3Ni1/3)3x−yVyPO4 (x = 0.1, y = 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08 and 0.10) has been selected by considering V to replace Mn, Co and Ni simultaneously, and Fe was kept constant at 70 mol% for all the compositions.
The multi transition metals (Mn Co, Ni and V) doped LiFePO4 was synthesized by sol-gel process using C6H8O7.H2O (99%, Sigma Aldrich, Germany) as chelating agent as well as carbon source. Mn(CH3COO)2.2H2O (99%, Sigma Aldrich, Germany), Co(CH3COO)2.2H2O (98%, Sigma Aldrich, Germany), Ni(CH3COO)2.2H2O (98%, Sigma Aldrich, Germany), and NH4VO3 (> 99%, Sigma Aldrich, Germany) were used as a source of doping elements Mn, Co, Ni and V respectively. These compounds were dissolved in ethanol and deionized water separately, using magnetic stirrer. Similarly, Li(CH3COO).2H2O (99%, Sigma Aldrich, Germany), Fe(NO3)3.9H2O (99%, Sigma Aldrich, Germany), NH4H2PO4 (99%, HIMEDIA, India) and citric acid were dissolved as sources of Li, Fe, PO4 and carbon respectively. The detailed process followed for materials synthesis is given in the reported papers [32, 33, 34]. The precursor solutions as prepared separately were mixed together by adding at an interval of 5 min and the resultant solution mixture was stirred further for 30 min and dried in an oven at 100°C. The dried precursor was ground using mortar pestle and preheated at 350°C for 2 h in the presence of Ar gas flow. The preheated sample was further ground in mortar pestle and calcined at 700°C for 5 h in Ar gas atmosphere to get crystalline powder.
The quantities of the precursors were taken according to the estimated stoichiometric composition LiFe0.7 (Mn1/3Co1/3Ni1/3)3x−yVyPO4 (x = 0.1, y = 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08 and 0.10). In all the compositions, a constant 70 mol% Fe and doping elements up to 30 mol% are considered. In the compound formula, V doping is assumed to replace equal number of Mn, Co and Ni atoms simultaneously. The samples are designated according to the concentration of V-doping (0%, 1%, 2%, 3%, 4%, 5%, 6%, 8%, and 10%) as LFP-0V, LFP-1V, LFP-2V, LFP-3V, LFP-4V, LFP-5V, LFP-6V, LFP-8V and LFP-10V and non-doped pristine sample is named as LFP as given in Table 1.
Phase identification of prepared samples was done by X-ray diffractometer (XRD) (Smart Lab, Rigaku, Japan) using CuKα radiation (λ = 1.54 Å). XRD data were collected at a scanning rate of 0.5º/min with a step size of 0.02º in the range of 2θ = 15–80º. The Rietveld refinements were performed to determine lattice parameters using FullProf Suite with pseudo-Voigt profile function of Thompson et al. [35]. For the Rietveld refinements, initial cell parameters and atomic positions were taken from the reported work [36]. Morphology and particle size of the samples were determined by field emission scanning electron microscope (FESEM) (FEI Quanta 200 FEG, Netherland). The morphology and elemental mapping of doped LFP samples were determined by using transmission electron microscope (TEM) (JEOL FE, 3200FS HRTEM, Japan) with an acceleration voltage of 300 kV. The surface analysis for chemical composition and oxidation states of prepared samples was done by X-ray photoelectron spectrometer (PHI 5000 Versa Probe III, Japan) using Al-Kα (hν = 1486.6 eV) X-ray source. MULTIPAK software was used for peak analysis and de-convolution of the peak patterns and the correction of binding energies was done with reference to C 1s peak at 284.8 eV. The charge-transfer resistance and Li-ion diffusion of freshly prepared electrodes were determined using electrochemical impedance spectroscopy (EIS) (Gamry Instruments, INTERFACE 1000) over the frequency range of 0.1 Hz − 1 MHz and at voltage amplitude of ± 10 mV.
The slurry for electrode was prepared by mixing active material, carbon black and PVDF binder in the weight ratio of 80:10:10 using N-methyl-2 pyrrolidone (NMP) solvent. The prepared slurry was pasted onto aluminium foil and dried in air (80°C for 12h) and vacuum (120°C for 8h). For electrode preparation, electrodes in circular shape of diameter 10 mm were punched from the prepared electrode sheet. The electrolyte 1M LiPF6 + EC:EMC (50:50 by volume) (Sigma Aldrich, Germany) was used, whereas separator was made of polypropylene for cell fabrication in teflon cell of cylindrical geometry.
Arbin Cycler (BT 2000, Arbin Instruments, USA) was used for determining galvanostatic charging/discharging cycles in the voltage range of 2–4V and 2–4.5V at different current rates (assuming 1C = 170 mA g− 1). Cell fabrication was carried out inside an Ar gas-filled glove-box (MBRAUN, MB 200G Unilab II, Germany) which have oxygen and moisture level less than 0.1 ppm.
XRD patterns of all prepared samples are shown in Fig. 1(a). The pattern of non-doped sample (LFP) shows single olivine phase of LiFePO4 without secondary phase(s) as confirmed with JCPDS # 01-081-117). Sample LiFe0.70Mn0.10Co0.10Ni0.10PO4 doped with equal 10 mol% of each element (Mn, Co Ni) having high redox potential also shows similar XRD pattern as that of pristine LFP. It means Mn, Co and Ni have successfully been able to replace Fe atoms in the host structure of LiFePO4. In addition, XRD patterns of V doped samples according to composition LiFe0.7 (Mn1/3Co1/3Ni1/3)3x−yVyPO4 for (0.01 ≤ y ≤ 0.04) show pure phase without observable impurity peaks. However, for 0.05 ≤ y ≤ 0.10 composition, a small peak of Li3V2(PO4)3 secondary phase is observed as shown by arrow (↓) in the magnified view of XRD patterns in Fig. 1(b).
The Li3V2(PO4)3 is a common secondary phase in LiFePO4 with excess V-doping. However, Li3V2(PO4)3 is an electrochemically active phase and also a cathode material for Li-ion battery. Figure 1(c) shows the XRD patterns over a smaller range of 2θ to demonstrate the clear peaks shift to higher angles with increasing y in LiFe0.7 (Mn1/3Co1/3Ni1/3)3x−yVyPO4 composition.
|
Sample according to LiFe0.7 (Mn1/3Co1/3Ni1/3)3x−yVyPO4 |
Refined occupancies of doping transition metals |
Abbr. |
||||
|---|---|---|---|---|---|---|
|
Fe |
Mn |
Co |
Ni |
V |
||
|
LiFePO4 |
1.00 |
- |
- |
- |
- |
LFP |
|
LiFe0.70Mn0.10Co0.10Ni0.10PO4 |
0.74(2) |
0.111(2) |
0.099(4) |
0.099(7) |
- |
LFP-0V |
|
LiFe0.70(Mn1/3Co1/3Ni1/3)0.29V0.01PO4 |
0.68(3) |
0.096(2) |
0.096(1) |
0.096(2) |
0.009(3) |
LFP-1V |
|
LiFe0.70(Mn1/3Co1/3Ni1/3)0.28V0.02PO4 |
0.68(6) |
0.093(3) |
0.093(2) |
0.093(5) |
0.019(2) |
LFP-2V |
|
LiFe0.70(Mn1/3Co1/3Ni1/3)0.27V0.03PO4 |
0.69(1) |
0.089(3) |
0.090(6) |
0.090(4) |
0.029(1) |
LFP-3V |
|
LiFe0.70(Mn1/3Co1/3Ni1/3)0.26V0.04PO4 |
0.69(9) |
0.086(7) |
0.086(2) |
0.087(1) |
0.039(1) |
LFP-4V |
|
LiFe0.70(Mn1/3Co1/3Ni1/3)0.25V0.05PO4 |
0.695(4) |
0.083(5) |
0.083(7) |
0.083(2) |
0.050(3) |
LFP-5V |
|
LiFe0.70(Mn1/3Co1/3Ni1/3)0.24V0.06PO4 |
0.70(2) |
0.080(1) |
0.079(2) |
0.080(3) |
0.060(2) |
LFP-6V |
|
LiFe0.70(Mn1/3Co1/3Ni1/3)0.22V0.08PO4 |
0.69(7) |
0.073(4) |
0.073(9) |
0.073(2) |
0.080(3) |
LFP-8V |
|
LiFe0.70(Mn1/3Co1/3Ni1/3)0.20V0.10PO4 |
0.696(3) |
0.066(9) |
0.067(2) |
0.067(8) |
0.099(5) |
LFP-10V |
A full Rietveld refinement was carried out on all XRD patterns to determine the lattice parameters and cell volumes. The refinement graphs of four samples LFP, LFP-0V, LFP-4V and LFP-10V are shown in Fig. 2. The refinement results demonstrate that lattice parameters a and b, and cell volume (V) decrease while lattice parameter c increases with increasing concentration of V-doping as shown in Fig. 3. The decrease in the unit cell volume with increasing V-doping concertation is due to formation of cations vacancies to balance the higher charge (V+ 3 or V+ 4) of vanadium, similar results are also reported earlier [28, 37, 38]. Furthermore, the reduction in unit cell volume of doped samples is also due to smaller atomic radius of doping elements V+ 4 (0.72 Å), Mn+ 2 (0.70 Å), Co+ 2 (0.75 Å) and Ni+ 2 (0.69 Å) as compared to Fe+ 2 (0.78 Å) [1, 39]. In addition, the change in the lattice parameters and continuous shift in XRD peaks with increasing V-doping concentration also confirm the incorporation of V into host lattice [37, 38].
Along with the lattice parameters, the site occupancies of transitional metals were also refined to check the charge compensation during V-doping. The Li occupancy was fixed at 100% and refinements was carried out on Fe, Mn, Co, Ni and V sites only. Initially, occupancies of Fe, Mn, Co, Ni and V were fixed according to estimated formula LiFe0.7 (Mn1/3Co1/3Ni1/3)3x−yVyPO4 (x = 0.1, y = 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08 and 0.10), in that case, slightly higher values of RF and ꭓ2 were found. When the occupancies of the transition metals are allowed to refine one by one without constraint, the values of RF and ꭓ2 were found lower and better fitting was observed.
However, only minor deviation was observed in the estimated occupancies and refined occupancies for all transition metals (Fe, Mn, Co, Ni and V) as shown in Table 1. An attempt has also been made to refine all the transition metal sites simultaneously, but in that case, convergence was not reached. However, due to poor resolution between the vacancies and Li-ions in the XRD data, the vacancy formation at both Fe and Li sites cannot be totally excluded. The samples LFP-8V and LFP-10V have shown higher ꭓ2 and RF values due to higher intensity peaks of Li3V2(PO4)3 secondary phase.
The oxidation states of the transition metals Fe, Mn, Co, Ni and V are determined by XPS and results are shown in the Fig. 4. The XPS spectra of Mn, Co and Ni are recorded from the two samples LFP-0V and LFP-10V.
XPS spectra of Fe are recorded for LFP, LFP-0V and LFP-10V. The XPS spectra of Fe 2p, Mn 2p, Co 2p and Ni 2p core level show two peaks of 2p3/2 and 2p1/2 due to spin-orbit coupling and for each element satellite peaks appearing along with main peaks. The main peaks of Fe centered at about 709.7 eV and 723.3 eV correspond to 2p3/2 and 2p1/2, respectively; while satellite peaks are observed at about 714.0 eV and 727.6 eV. The observed peaks are well matched with Fe+ 2 oxidation state of Fe [40]. Similarly, Mn spectra show two main peaks at about 641.8 eV (2p3/2) and 653.3 eV (2p1/2) corresponding to Mn+ 2 and two satellite peaks at 646.4 eV and 658.3 eV [41–43]. The spectra peaks of Co are observed at about 781.7 eV (2p3/2) and 797.9 eV (2p1/2) with two satellite peaks at about 787.5 eV and 802.4 eV corresponding to Co+ 2 [44, 45]. The XPS spectra of Ni show main peaks at about 855.0 eV (2p3/2), 872.4 eV (2p1/2), and one satellite peak at about 860.3 eV which match well with Ni+ 2 oxidation state of Ni [40, 46]. Therefore, it is found from the XPS results, there is no change in the oxidation states of Fe, Mn, Co and Ni. At the end, the oxidation states of V in the composition LiFe0.70(MnxCoxNix)3x−yVyPO4 were also studied. The XPS spectra of three samples LFP-1V, LFP-4V and LFP-10V were recorded to examine the effect of doping concentration on oxidation state of V. The spectra of V 2p core level show two main peaks at 516.6 eV and 723.9 eV corresponding to 2p3/2 and 2p1/2 spin-orbit coupling, respectively. The binding energy position of V matches well with that appeared in VO2 [47, 48], suggesting the oxidation state + 4 of V in doped samples which is independent of doping concentration. However, in the composition LiFe0.70(MnxCoxNix)3x−yVyPO4 for 0.05 ≤ y ≤ 0.10, a small trace of secondary phase Li3V2(PO4)3 is observed in the XRD pattern. But, the oxidation state V+ 3 for Li3V2(PO4)3 is not observed in the present study. Moreover, distinguishing between V+ 3 and V+ 4 peaks is difficult by XPS due to its surface sensitivity as data is collected from surface, charge compensation used during data collection for non-conducting samples like LiFePO4 and calibration of the data to C or O peak [26, 47]. In addition, the binding energy difference between V+ 3 and V+ 4 peaks are very less (0.1–0.5 eV), and comparatively large variations in the binding energy (~ 1.9 eV) of the peak V 2p3/2 for Li3V2(PO4)3 are reported [47, 49–51].
Morphology and particle size analysis were performed by FESEM and results are shown in Figs. 5(a)-5(i); whereas, histograms of particle size distribution are shown in Figs. 5(j)-5(r). There is no direct relation found between morphology and doping concentration from the FESEM images. Moreover, samples show morphology variation from circular, elongated to mixed shapes. However, one can notice from the micrographs and histograms that average particle size increases for V doped samples as compared to without V doped sample. The average particle size of LFP-0V was found 175.51 ± 5.1 nm as shown in Figs. 5(a) and 5(j), whereas, the particle size of LFP-8P was found maximum 1068.0 ± 77 nm. The results suggest that the multi elements doping can affect particle morphology including the particle size. The previous studies also have reported the increased particle size of LFP on Mn, Co, Ni and V-doping [23, 38].
The morphology and particle size were further confirmed by TEM analysis of samples LFP-0V and LFP-4V and results are shown in Fig. 6. The TEM-EDX elemental mapping of Fe, Mn, Ni, and V was also determined to show distribution of elements in the doped samples. The LFP-4V samples shows bigger particle size as compared to LFP-0V which supports the FESEM results. The lattice fringes of samples LFP-0V and LFP-4V in high resolution mode show lattice spacing of 3.34 Å and 2.12 Å correspond to (111) and (112) planes, respectively as shown in the Figs. 5(b) and 5(h), and SAED patterns are shown in inset of respective image. From the elemental mapping results, it can be seen that both samples show homogeneous distribution of Fe, Mn, Ni and Co elements. However, V mapping was performed only on LFP-4V sample.
The electrochemical analysis is performed to understand the redox couples and presence of multiphase in the multi-doped LiFePO4. The charge-discharge characteristics of all samples are shown in Fig. 7 in two different potential ranges 2V – 4V and 2V – 4.5V at current rate of 0.1C. The samples charged-discharged in the voltage range 2V – 4V show only one redox couple at about 3.45V/3.48V for Fe+ 2/Fe+ 3 vs. Li. The electrochemical study of non-doped pristine LFP already has been reported in our previous papers [32, 33], therefore, in this work, we have mainly focused on electrochemical characteristics of doped LFP. The Initial capacity was found 112 mAh g− 1 for sample LFP-0V sample which is much lower than the reported capacity 150 mAh g− 1 of LFP [32]. Since redox couples of Mn (4.1V), Co (4.8V) and Ni (5.1V) are higher than the experimental voltage range 2V − 4V, so these elements behave as inactive materials in electrochemical reaction. So, the capacity contribution would be only from Fe i.e. 70% (105 mAh g− 1) of capacity reported for pristine LFP (150 mAh g− 1); however, sample shows higher capacity 112 mAh g− 1 (which is about 75% of reported LFP capacity) than predicted (105 mAh g− 1). Therefore, the higher capacity can be attributed to the enhancement in the electronic and ionic conductivity of LFP due to multi doping as also reported earlier [22–24, 52]. A fall in the capacity was noticed on low concentration V doping and shown lowest capacity about 78 mAh g− 1 for samples LFP-1V and LFP-2V. However, improvement in the capacity was found with increasing V doping concentration and reached maximum capacity about 105 mAh g− 1 (70% of LFP) for sample LFP-10V. The improvement in the capacity with higher V concentration may relate to the V doping weaken the Li-O bond and enhanced the Li-diffusion [2, 53].
Charge-discharge characteristics were further observed for doped samples in larger potential window 2V – 4.5V at current rate of 0.1C. The capacity increment in the sample LFP-0V is recorded about 11 mAh g− 1 above 4V due to contribution of Mn which becomes redox active at 4.1V [22]. The capacity drop on low concentration of V doping is also observed here as found in voltage range 2V – 4V. However, apart from the main plateau at about 3.45V of LiFePO4, a second plateau is also observed at about 4.1V of Mn+ 2 in sample LFP-0V and V-doped samples for more than 4 mol% (0.05 ≤ y ≤ 0.10). The second plateau with higher V doping concertation is due to presence of secondary phase Li3V2(PO4)3 for 0.05 ≤ y ≤ 0.10 (as shown in the XRD pattern) which shows redox activity at 4.1V for second Li+ ion extraction (V+ 3/V+ 4) [54]. The Li3V2(PO4)3 phase also shows redox activity at about 3.5V and 3.8V for first Li+ ion extraction (V+ 3/V+ 4) [54], which is not observed in the present study may be due to coincident with LiFePO4 curves. However, it is difficult to separate the capacity contribution of Mn and V in V-doped samples, because both show plateaus at 4.1V. The improvement in the capacity is reached maximum for samples LFP-8V and LFP-10V which shows capacity higher than LFP-0V. The sample LFP-10V shows maximum charge and discharge capacities of 133 mAh g− 1 and 129 mAh g− 1, respectively; in which, charge and discharge capacities contribution above 4V are about 13 mAh g− 1 and 9 mAh g− 1, respectively as shown by dotted square in Fig. 7.
The CV characteristics of all samples are shown in Figs. 8(a) and 8(b) in voltage range 2V – 4V and 2V – 4.5V, respectively. The all doped samples show broad redox peaks. The CV curves recorded in the 2V – 4.5V potential window, show a secondary redox peak at 4.1V is due to Mn+ 2/Mn+ 3 or/and V+ 3/V+ 4 as described above.
The cycle life of all samples is shown in the Fig. 8(c). The sampleLFP-0V show highest capacity retention 94% after 300 cycles at 0.5C rate. The lowest capacity retention of 59% was observed for sample LFP-1V which improved with increasing V-doping concentration and reached 91% for sample LFP-10V. Therefore, based on the electrochemical results, one can infer that high concentration of multi-elements doping in LiFePO4 is not feasible with presently used carbonate-based electrolytes. At the high redox potential also, these doping elements (Co and Ni) are redox active, however, the available carbonate based LiPF6 electrolyte is decomposed at that high potential [7]. Therefore, a new electrolyte is required for doped LFP to see complete electrochemical characteristics.
The contribution of multi elements doping in electrochemical reaction was evaluated by EIS study through charge transfer resistance (Rct) and Li-ion diffusion (DLi+). A Nyquist plot exhibiting a semicircle in the high-frequency region and an inclined line in the low frequency range, represents charge-transfer resistance at the electrode/electrolyte interface and the diffusion resistance of lithium ions in the bulk electrode materials, respectively [55]. The corresponding equivalent circuit is shown in the inset of Fig. 9(a) which is composed of series resistance (Rs) of cell, a constant phase element (CPE), a charge-transfer resistance (Rct), and a Warburg impedance (Zw) [2]. The Warburg impedance,slope of the inclined line, is used to determine DLi+.
The coefficient DLi+ is calculated by following equation [56]:
$${D}_{{Li}^{+}}=\frac{{R}^{2}{T}^{2}}{{2A}^{2}{n}^{4}{F}^{4}{C}^{2}{{\sigma }^{2}}_{\omega }}$$
1
where R, T, A, n, F and c represent the gas constant (8.314 J mol− 1 K− 1), absolute temperature (300 K), area of electrode, number of electrons per molecule during oxidation (for LiFePO4, n = 1), Faraday constant (96485 Coulomb mol− 1), and Li-ion concentration in LiFePO4, respectively.
The Warburg coefficient (σω) is determined by the following equation [57]:
The σω is calculated from the slope of Zreal vs. ω−1/2 plot in the low frequency region as shown in Fig. 9(b), where, ω is the angular frequency of the applied alternating current.
The charge transfer resistance of pristine LFP was found lowest 158.7 Ω and highest DLi+ (1.72 × 10− 12 cm2 s− 1), may be due to lower particles size which helps in easy charge transfer. The values of Rct and DLi+ are found 455.8 Ω and 1.47 × 10− 12 cm2 s− 1, respectively for sample LVP-0V. The Rct value is higher while DLi+ is lower of LFP-0V as compared to reported values of pristine LFP [32]. This could be due to high concentration of inactive elements (Co and Ni) and very low electronic and ionic conductivity of olivine phase LiMnPO4 as compared to LiFePO4 [58, 59]. The Rct further increases with low V doping concentration and is found highest 470.9 Ω for LFP-1V while DLi+ found 9.97 × 10− 13 cm2 s− 1. The Rct decreases and DLi+ increases with increasing V-doping concentration due to improved kinetics of LiFePO4 by V doping [2, 53]. The Rct and DLi+ of 4 mol% V doped sample (LFP-4V) shows 303.8 Ω and 1.37 × 10− 12 cm2 s− 1, respectively. The Kinetics of the cell further increases with higher doping and found Rct (280.1 Ω) and DLi+ (1.57 × 10− 12 cm2 s− 1) for LFP-10V. This may be due to contribution of electrochemically active secondary phase Li3V2(PO4)3 because DLi+ of Li3V2(PO4)3 phase was reported 10− 9 to 10− 10 cm2 s− 1 which is higher than DLi+ of LiFePO4 [60, 61].
In the present study, a series of multi-elements doped LiFePO4 samples were synthesized according to the composition LiFe0.7 (Mn1/3Co1/3Ni1/3)3x−yVyPO4 (x = 0.1, y = 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08 and 0.10). In the composition, iron (Fe) concentration was kept fixed at 70 mol% and assumption was made that equal amount of Mn, Co and Ni elements have replaced simultaneously by V-doping. The effect of simultaneous doping of V, Mn, Co and Ni on the performance of LiFePO4 has systematically been investigated with X-ray diffraction, X-ray photoelectron spectroscopy, electrochemical measurements, and electrochemical impedance spectroscopy. The XRD analysis confirms that maximum 4 mol% V can be doped into the Fe sites, and a linear change in lattice parameters and cell volume on doping was also observed by Rietveld refinement. The oxidation states of Fe, Mn, Co and Ni were found in + 2 and V found in + 4 as confirmed by XPS. The capacity of the doped samples was found lower in the voltage range 2–4V due to high content of inactive dopant elements. However, the increased capacity was found when cells were charged/discharged in the larger voltage window 2–4.5V due to additional electrochemical activity of Mn and V in the doped compounds.
Ethical Approval
Not applicable
Competing interests
There are no conflicts to declare.
Authors' contributions
Author 1 conducted experiments, analysis and wrote the initial manuscript, author 2 conducted TEM analysis, author 3 helped in Rietveld refinement and XPS analysis, and author 4 reviewed the manuscript and all experimental data.
Funding
The authors acknowledge Ministry of Human Resources and Development (MHRD), Government of India (Grant No. MHR02-13-115-302) for financial support in the form of fellowship to Hari Raj.
Availability of data and materials
Additional information and data is available on request.
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No competing interests reported.
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Graphical Abstract “Incoroporation of multi transition metals in olivine stracture”
