High-performance Na-doped LiFePO4 cathode material derived from acid-washed iron red for the simultaneous immobilization of multi-metals

To immobilizing hazardous elements of metallurgical waste and meet the demand for cathode materials in lithium-ion battery industries, LiFePO4/C cathode material was successfully prepared via a simple carbothermal reduction method using acid-pickled iron oxide red as raw material by orthogonal tests. To further improve electrochemical performance, Na-doped LiFePO4/C cathode material designed with the first-principles calculation verification was synthesized by solid-state method at the optimal experimental conditions including the ball-milling medium of 3 h, the reaction temperature at 750 °C in heating rate of 5 °C·min−1 for 4 h. The results reveal that Na doping can effectively change the band gap structures and microstructure, which reduced the size of the particle and increased the electronic conductivity. The Li0.75Na0.25FePO4/C electrode showed a discharge capacity of 139.2 mAh·g−1 at 0.5 C and an excellent capacity retention of 98.9% after 50 cycles. The synergy strategy was a sustainable solution for immobilizing hazardous heavy metal elements, which paves a novel facile and cost-effective way towards high-performance LiFePO4 cathodes and promising markets for lithium-ion battery industries.


Introduction
In the process of industrial production and mining and steelmaking, wastes such as waste water and slag will be generated [1,2].The heavy metals in it will be released into the environment with wastewater and eventually enter the food chain, causing harm to humans and nature [3].Therefore, it is urgent to solidify the heavy metals in wastewater.When iron and steel is rolled at high temperature, iron oxide scale will be formed on the surface.The usual way to deal with iron oxide is pickling.However, a large amount of acid will be consumed in the process of pickling, and waste acid and iron salt solution will be produced at the same time [4], which will pollute the environment if discharged directly.The iron salt obtained in the process of treating and recycling these wastes is "pickling iron red."At present, the use of pickling iron red is not very thorough, the utilization rate of resources is not very high, and most of them are used to make pigments.Therefore, it is of great significance to use pickled iron red as iron source to prepare battery cathode.
On account of its low cost, non-toxic, and high theoretical capacity, LiFePO 4 is the most anticipated cathode material in the next generation.However, in the process of practical production and application of LiFePO 4 batteries, its electronic and ion conductivity is very poor [5][6][7][8].People have taken a variety of methods to weaken or even overcome these shortcomings.The commonly used methods are to reduce the size of the particles [9], wrap the carbon layer on the surface of the material [10,11], and add ions [12].Chung [13] and his team doped multivalent ions (Al 3+ , Zr 4+ , etc.) with LiFePO 4 .The doping position is mainly in the Li 4a site.Subsequently, it was found that the electronic conductivity of the material was greatly increased by eight orders of magnitude.LiFePO 4 can also get good properties by replacing ions at Fe site [14][15][16][17][18][19][20].In addition, doping anions such as F on the O site can effectively increase the capacity of the battery [21,22].Meanwhile, a large number of theoretical studies on the electrical properties of pure LiFePO 4 have been carried out by using the first-principles method [23][24][25][26][27][28][29][30][31][32].First-principles calculations are helpful for understanding the electronic structure of doped LiFePO 4 .Shi's team [28] used first-principles calculations to study the energy and electronic structure of the LiFePO 4 system, and it is found that the electronic conductivity of LiFePO 4 can be improved when Cr 3+ replaces Li + .Zhou [24] used DFT + U method to calculate the electronic structure and band gap of LiFePO 4 .The ~ 3.8 eV band gap calculated by DFT + U is consistent with the experimental results [21].
In this paper, LiFePO 4 /C were compounded by carbothermal reduction method via citric acid, pickled iron red, lithium hydroxide (LiOH), and NH 4 H 2 PO 4 as raw materials, and the influence of several control parameters LiFePO 4 /C cathode materials was investigated by orthogonal test.Then, we doped LiFePO 4 with single ion and doped Na + with Li + site to improve the charge-discharge performance of LiFePO 4 .Finally, we studied the electronic structure of Na doped LiFePO 4 by first-principles calculation method.In addition, their electronic conductivity is also discussed.

Experimental and optimization
Using the method of orthogonal experiment to explore the optimal preparation conditions.And we discussed the influence of heating rate, sintering time, calcination temperature, and ball milling time on the performance of LiFePO 4 /C.

Preparation of Na + doping LiFePO 4 /C
The Na + doping LiFePO 4 /C were prepared by a carbothermal reduction using Na 2 CO 3 , C 6 H 8 O 7 , LiOH, pickled iron oxide red, and NH 4 H 2 PO 4 as raw materials.Among then, citric acid as carbon source and Na 2 CO 3 as modified material were added to the mixture.Then, we use ethanol as the ball-milling medium for 3 h.Subsequently, the temperature was raised to 750 °C at a heating rate of 5 °C•min −1 in argon atmosphere for 4 h.In other words, the Li 1-x Na x FePO 4 /C (0, 0.25, 0.5, 0.75) cathode materials were obtained.
The results of composition analysis of pickled iron red in raw materials are shown in Table 2, in which the content of Fe 2 O 3 reaches 99.6%, the content of Fe reaches 69.72%, and other impurities are less.The purity of Fe 2 O 3 sample of Fuchen Chemical Reagent Co., Ltd., is 69.8 to 70.1%.There is little difference in purity between the two.Therefore, the effects of other impurity ions are not considered in this study.

Characterization and electrochemical measurements
Using a powder X-ray diffraction (XRD) to characterize the crystal structure of the materials.In order to characterize the morphology and structure of the material, scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to realize it.Using X-ray photoelectron spectroscopy (XPS) to analyze the element types and valence states.The calcination temperature of the sample was determined by thermogravimetric analysis (TG, HTG-1) in the temperature range of 25 ~900 °C.
The electrochemical performance of the sample was tested by using the CR2032 coin cell.First of all, put the raw materials into the glass mixing bottle.The mixture was stirred on a magnetic stirrer for about 3 h to make it evenly mixed.After that, the slurry obtained was coated on the aluminum foil and placed in the oven for drying.The oven temperature was set at 60 °C, and the time was 8 h.Cut the dried aluminum foil into a small round piece with a diameter of about 14 mm, which is the cathode material of the battery.Lithium sheet is the anode material.Finally, the electrochemical test of the button battery was carried out.

The first-principles calculation
All the calculations in this study were performed using the Vienna Ab initio Simulation Program (VASP) [32,33].The atomic configuration of the system and the corresponding electronic structure were calculated by density functional theory (DFT) and generalized gradient approximation (GGA) [34].In order to reduce the amount of calculation, the situation of electron spin is ignored.The electron exchange correlations energy was calculated the Perdew-Wang exchange correlation functional (PW91) [35].Energy cutoff for the plane waves is set to 340 eV.The Monkhorst-Pack [36] scheme with 3 × 4 × 5 k-point sets has been used for the integration in the irreducible Brillouin zone.The ultrasoft pseudopotential (USPP) was used to be the atomic pseudopotential function.

Results and discussion
The TG-DSC curve of the mixture after ball milling is shown in Fig. 1.It can be seen from the diagram that the pyrolysis of the precursor mixture is mainly divided into three stages, which are mainly corresponding to the reduction of ferric iron in Fe 2 O 3 and the formation of lithium iron phosphate crystals in the temperature range of 442.7~700.1°C.The weight loss in this stage is about 6.7 wt%.When the temperature exceeds 700 °C, the mass no longer changes obviously and tends to be stable, and the final weight is 69.1 wt%.According to the thermogravimetric analysis of the precursor mixture, the precursor mixture will be calcined in the temperature range of 650~750 °C [37].
Figure S1 shows the SEM diagram of LiFePO 4 /C synthesized under different conditions of orthogonal experiment.It is found that all the samples have no special morphology.The irregular block particles can be observed in the scanning diagram of S2, S3, S4, and S7, and the size is different.On the other hand, the scanning images of other samples show planes with holes, which may be caused by the corrosion of alkaline LiOH in the sample.Even a morphology similar to the gully shape was observed in the S5 sample.The R ct values of samples S1-S9 are 539, 368, 459, 475, 616, 499, 416, 503, and 647 Ω, respectively.Among them, the intersection point between the curve of S2 and S7 and the X axis is smaller, which means that the charge transfer polarization resistance of the two samples is smaller.
Figure 2 b shows the first circle charge and discharge curve of all samples under 0.5 C. All samples have a discharge plateau at about 3.4 V.The longer the platform length, the smaller the voltage difference between the charging platform and the discharging platform, the less obvious the polarization of the sample during the charging and discharging process, and the better the electrochemical performance.The capacity of S2 and S7 is obviously higher than that of other samples, and above 130 mAh•g −1 , they reach more than 76.5% of the standard capacity.Figure 2 c, d are comparison of all samples cycled for 50 cycles at 0.5 C charge-discharge rate.The discharge capacities of samples S1-S9 are 97, 132, 128, 112, 88, 98, 130, 106, and 96 mAh•g −1 , respectively.We notice that the discharge capacity of S2 sample increases after 50 cycles, reaching 136 mAh•g −1 , indicating that the battery will have a slight polarization phenomenon in the initial charge-discharge process, and the subsequent discharge capacity will increase after the completion of polarization.
To select the optimal preparation conditions, we drew the horizontal factor diagram of the orthogonal experiment, as shown in Fig. 3.The ordinate of the diagram represents the discharge capacity of the battery, and the abscissa represents different factors.According to the difference between the highest point and the lowest point of the corresponding capacity of each factor in Fig. 3, the order of influence of each factor on the performance of LiFePO 4 can be determined as follows: ball milling time > heating rate > calcination temperature > sintering time.Moreover, the highest point of each factor is the best.To sum up, the optimum ball milling time for preparing LiFePO 4 is 3 h, the heating rate is 5 °C•min -1 , calcination temperature is 750 °C, and the holding time is 4 h.
We try to improve the cyclic performance of the material by doping Na + into LiFePO 4 .Figure 5   were not observed in the XRD spectrum, indicating that the raw materials reacted completely and no impurities were introduced under these conditions.To explore the effect of different Na + doping content on cell parameters and cell volume, all samples were refined by Rietveld.The lattice parameters after refinement are shown in Table 3.More specific atomic position information after XRD refinement is shown in Table S2.As the amount of doping increases, the a, c, and v values of the samples decrease, which has also been confirmed by Liu et al. [16,38].The values of a, c, and v of Na-0.25 are the largest of the three samples.And the lithium ion channel of this sample is the widest.The decrease of lattice constant b is beneficial to the intercalation/delamination of Li + and shortens the diffusion distance of Li + .Generally speaking, the lattice constant changes little with the increase of Na content.When the doping amount is too high, LiFePO 4 with different structure may be formed.
Figure 4 a, b, c show the SEM images of three samples.There is no special appearance, and the samples are composed of different sizes and irregular blocks.In addition, there are also concave surfaces.LiFePO 4 /C showed different particle size morphologies under different Na + doping amounts.This is attributed to the fact that the radius of Na + is larger than that of Li + , and the lattice volume can expand after Na + replaces part of Li + , which can affect the degree of agglomeration of particles.To further explore the microstructure of the materials, Na-0.25 samples were scanned by TEM.The result is shown in Fig. 4d. Figure 4 e shows the Bragg lattice of the sample, and both of them can see the ordered bright spots corresponding to the (121) crystal plane of lithium iron phosphate crystal respectively.Figure 4 f, g are high-resolution transmission images of the sample.Na + doping LiFePO 4 /C particles have high crystallinity, the particle surface is covered with a uniform carbon layer, and carbon exists in amorphous form.Continuous and complete carbon coating can not only effectively restrain the growth of grains but also ensure the full contact between electrons to realize the rapid transfer of electrons [9,39,40].
Figure 5 b, c, d and Fig. S2 show the XPS spectra of three samples, and further analyzes the surface element composition and valence information of LiFePO 4 /C cathode materials.The peaks of Na 1s, Fe 2p, O 1s, C 1s, and P 2p can be observed from Fig. S2.The XPS spectrum of Na is shown in Fig. 5b.The main peak of the three samples is approximately at 1071 eV, which is attributed to Na 1s.To investigate the influence of doping on the oxidation state of Fe, the XPS spectrum of Fe 2p was studied, as shown in Fig. 5c.All samples have two peaks with binding energies around 710.2 and 723.8 eV, corresponding to Fe 2p 1/2 and 2p 3/2 , respectively.This corresponds to Fe 2p in LiFePO 4 [41].The binding energy of the main peak and subsidiary peak of each sample has no obvious change with the different doping amount, indicating that the doping of Na + has no obvious effect on the chemical valence of Fe (II).The lattice distortion is usually caused by the doping of other ions.This analysis shows that the charge difference caused by Na + doping may be balanced by cation vacancies, which is helpful to improve the electronic conductivity.[42].
Figure 6 shows the electrochemical performance diagram of four samples.We carried out electrochemical impedance spectroscopy (EIS) tests to explore the effect of Na doping on the kinetic behavior on the samples.The optimal equivalent circuit model is given in the figure, as shown in Fig. 6a.The R ct of the three doped samples are 355, 407, and 482 Ω, respectively.It is generally believed that the charge transfer resistance is closely related to the electrode reaction kinetics.The smaller the charge transfer resistance, the better the kinetic performance of the electrode.It can be easily seen that Na-0.25 samples can provide better kinetic behavior, which is anastomose with the above electrochemical tests.The Li + diffusion coefficient is further calculated, as shown in Table S1.
To further understand the structure of carbon, the Raman spectra of three samples were analyzed as shown in Fig. 6b.Two prominent peaks can be seen from the diagram: one is the G band related to graphite (sp2), which is located in ~1600 cm −1 , and the other is that the D band related to disordered carbon (sp3) is located in ~1360 cm −1 [43].In addition, according to the Raman analysis, the ID/IG values of the three samples are 0.480, 0.413, and 0.403, respectively.It shows that the LiFePO 4 /C cathode material with 0.25 doping content has higher degree of graphitization and better electrical conductivity.
Figure 6 c shows the first charge-discharge curves of four samples at 0.5 C. The initial discharge specific capacities of the four samples are 135.9,142.1, 130.8, and 123 mAh g −1 .The initial discharge capacity of Na-0.25 is the highest.With the increase of the doping amount, the decrease of the first charge/discharge capacity may be due to the different insertion/deintercalation modes of Na + and Li + during the charge-discharge process.It will stay in position 4a [44,45].With the decrease of Li + intercalation/deintercalation, the first discharge capacity of the sample will decrease.This may be due to the fact that the activity of Na + doped Na + is lower than that of Li + .The Na + maintains the 4a position and supports the one-dimensional channel, which makes the crystal structure of the material stronger and the cycling performance improved [46][47][48].Figure S3 shows the rate performance of the four samples, and the Na-0.25 exhibits better capacity retention than other samples, which is consistent with the cycle performance test results.
We use the first-principles calculation method to verify the correctness of the experiment.The lattice constants and Fermi energy obtained from the optimized Li 1-x Na x FePO 4 system are shown in Table 4.We can see from the table that with the increase of Na + content, the lattice constant increases and the Fermi energy decreases.This is because the radius of Na + is larger than that of Li + .After Na + occupies the Li site, the lattice expands slightly and the volume increases.The band structures of Li 1-x Na x FePO 4 (x = 0, 0.25, 0.5, 0.75) are shown in Fig. 7a-d.It can be seen that the position of the conduction band gradually moves down with the increase of the doping amount x; the width of the band gap becomes narrower.It means that the shorter the path from the valence band to the conduction band is, the less energy is needed, which improves the electronic conductivity of the system to a certain extent.Figure 7 e-h show the density of states (DOS) for all samples.By comparison, it is found that the peak near the Fermi energy of the system becomes slightly sharp after doping.This means that the number of energy levels near the Fermi energy increases, which may increase the electronic conductivity of the doped LiFePO 4 .The energy band near the −45 eV range should be contributed by Li-2s orbital electrons.With the increase of the amount of Na + doping, the number of Li decreases, and the intensity of this peak decreases.At the same time, there is an energy band formed by the participation of Na-3s orbital electrons in −51 eV.Therefore, the gradual increase of the amount of doping will gradually reduce the band gap.But with too much doping, too many Na atoms will occupy more Li sites.In addition, its radius is larger than that of Li + , which leads to greater distortion of the lattice and decrease the electrochemical performance.It can be inferred that the Li + diffusion of Li 0.75 Na 0.25 PO 4 is least affected by the hindrance of Na, and the electrochemical performance should be relatively good.This is consistent with the experimental results.
Figure 8 shows the total density of states of Li 0.75 Na 0.25 FePO 4 and the partial density of states of each element.By comparing the PDOS diagram of LFP, it can be found that the energy levels near the Fermi energy before and after doping are still mainly contributed by Fe-3d electrons.However, the band gap decreases after doping, indicating that the doping of Na is not directly involved in the formation of the energy level near the Fermi level, but indirectly changes the band gap of the system by affecting the electrons of the Fe-3d orbital.We compared the average length of Li-O bond before and after doping.It is found that before doping, the length is 2.10 Å, and after doping, the length is slightly increased to 2.14 Å.The deformation of the atomic position of Fe may lead to the increase of the length of Li-O bond, which slightly broadens the channel of

Conclusion
Firstly, using the orthogonal experiment to optimize the preparation of LiFePO 4 /C optimum conditions, and then using a high-temperature solid state method to successfully synthesize Na + doping LiFePO 4 /C electrode materials.And the first-principles calculation is used to verify the correctness of the experiment.The results show that Li 0.75 Na 0.25 FePO 4 material has better cycle rate performance.This may be due to the expansion of the lattice by a small amount of Na + doping, which deforms the positions of Fe atoms and reduces the band gap.At the same time, the Li + migration channel was slightly widened, which improved various properties of the material.With the increase of doping amount, Na + will occupy too many positions of Li + , which will narrow the channel and reduce various properties of the materials.The first discharge capacity of Li 0.75 Na 0.25 FePO 4 at 0.5C is 142.1 mAh g −1 , and after 50 cycles, the discharge capacity is 139.35 mAh g −1 .

Figure 2 a
shows the electrochemical impedance spectra of LiFePO 4 /C synthetic materials.The illustration in the figure is a fitted equivalent circuit diagram.It can be seen that the curve is composed of semicircle and slope.The intercept of semicircle in high-frequency region represents the resistance R E of electrolyte; R C is the contact resistance at the collector/cathode interface.The intersection of the semicircle and the X axis represents the charge transfer resistance R ct .
a shows the XRD patterns of Na-0, Na-0.25, Na-0.5, and Na-0.75 samples.The peak shape of each sample is complete, and the position and intensity of each diffraction peak are consistent with the XRD pattern of LiFePO 4 (PDF#40-1499).The sharp peak indicates that all the samples have good crystallinity and Na doping does not change the structure.At the same time, the diffraction peaks of Na 2 CO 3 , LiOH, Fe 2 O 3 , and impurities

Fig. 2
Fig. 2 Electrochemical performance of LiFePO 4 /C cathode materials synthesized under different conditions: a EIS curves, b charge-discharge curves of the first cycle, and c, d cycle performance Figure 5 d is the XPS spectrum of C1s.Three peaks approximately

Fig. 3
Fig. 3 Analysis diagram of the results of orthogonal experiment

Figure 6 d
shows the cycle performance curve of four samples after 50 cycles at 0.5 C. It can be seen that the first discharge capacity of the sample will change with the increase of Na + doping amount, which is 135.25, 139.35, 130.19, and 122.8 mAh•g −1 .The discharge capacity after 50 cycles is 131.45,137.65, 129.2, and 118.1 mAh•g −1 .

Fig. 5 a
Fig. 5 a XRD patterns of the four samples.XPS of sample prepared with Li 2 CO 3 as lithium source: b Na 3d; c Fe 2p; (d) C 1s

Fig. 6 a
Fig. 6 a The EIS curves of four samples.b The Raman spectroscopy of Na + doping samples.c The first charge/discharge curves of four samples.d Cyclic performance of four samples

Table 1
The factors and levels of orthogonal experimental

Table 3
Lattice parameters of Li 1−x Na x FePO 4 /C located at 283.8 eV, 284.5 eV, and 287.2 eV were detected, corresponding to C-C, C-O, and O-C=O

Table 4
Lattice constants and Fermi energies obtained after optimization of Li 1-x Na x FePO 4 system