Comparing the Electrochemical Performance of LiFePO 4 / C Modified by Mg Doping and MgO Coating

Supervalent cation doping and metal oxide coating are the most efficacious and popular methods to optimize the property of LiFePO 4 lithium battery material. Mg-doped and MgO-coated LiFePO 4 /C were synthesized to analyze their individual influence on the electrochemical performance of active material. The specific capacity and rate capability of LiFePO 4 /C are improved by both MgO coating and Mg doping, especially the Mg-doped sample—Li 0.985 Mg 0.015 FePO 4 /C, whose discharge capacity is up to 163mAh g, 145.5mAh g, 128.3mAh g, and 103.7mAh g at 1 C, 2 C, 5 C, and 10C, respectively. The cyclic life of electrode is obviously increased by MgO surface modification, and the discharge capacity retention rate of sample LiFePO 4 /C-MgO 2.5 is up to 104.2% after 100 cycles. Comparing samples modified by these two methods, Mg doping is more prominent on prompting the capacity and rate capability of LiFePO 4 , while MgO coating is superior in terms of improving cyclic performance.

Lots of work were carried out on supervalent cation doping since Chung et al. [7] claimed that they raised the electronic conductivity of bulk LiFePO 4 by 8 orders of magnitude to 10 −2 S cm −1 through low-level doping with some super-valent cations (Nb 5+ , Ti 4+ , and W 6+ ).Although the origin of the increased conductivity by doping is still under controversy, many groups have prepared LiFePO 4 positive material with excellent property through super-valence metal ion doping.Surface modification is also extensively applied to improve the electronic conductivity of LiFePO 4 , especially carbon coating.But the tap density of active material dramatically decreases after only a little amount of carbon coating [10,11].Hence, researchers expect to find an appropriate coating to wholly or partly substitute for carbon, such as metal oxide.CeO 2 [7], V 2 O 3 [12], MgO [13], SiO 2 [14], TiO 2 [15], CuO [16] and so forth have been employed to modify LiFePO 4 , and predominant performance was gained.
In conclusion, super-valent cation doping and metal oxide surface modification are two efficient ways to improve the electrochemical property of lithium cathode electrode.The mechanism of enhancing electrode performance is dissimilar for doping and coating as well as their promoted aspects.Up to now, the different influence on LiFePO 4 material by these two methods using the same metal element was not reported.
Therefore, we take magnesium modification, for example, to discuss the different effects of metal ion doping and metal oxide coating on electrode material.Mg doping or MgO coating has been carried out to ameliorate the electrochemical performance of LiCoO 2 [17], LiNiO 2 [18], LiNi 0.8 Co 0.2 O 2 [19], Li 3 V 2 (PO 4 ) 3 [20], and LiFePO 4 [13], and excellent rate capability or cyclic performance has been gained.It indicates that magnesium modification is an efficient and popular method to improve the property of lithium positive material.Mg-doped and MgO-coated LiFePO 4 /C were prepared in our paper, and their individual effect on electrode material performance was, respectively, discussed to analyze how Mg doping and MgO coating enhance the electrochemical property of LiFePO 4 and where their protrudent part is.

Experiment
2.1.Synthesis of Material.Iron oxide (Fe 2 O 3 ) and lithium dihydrogen phosphate (LiH 2 PO 4 ) with Li : Fe : P molar ratios of 1 : 1 : 1 were mixed with citric acid through a balling process for 5 h, followed by calcining at 700 ∘ C for 12 h under pure nitrogen to obtain LiFePO 4 powders.Different amounts of magnesia (with LiFePO 4 : MgO molar ratios of 1 : 0.025, 1 : 0.05) were dissolved in 20 mL citric acid solution to prepare magnesium citrate complex and mixed with LiFePO 4 powders through balling for 5 h after drying at 150 ∘ C. Finally, all of them were calcined at 700 ∘ C for 4 h under inert atmosphere to obtain MgO-coated LiFePO 4 /C, referred to as LFP/C-M  ( = 2.5, 5).

Characterization of the Samples.
The crystallographic structural characterization of samples was analyzed by X-ray diffraction (XRD, Rigaku D/max-2500/pc, CuK radiation).The particle morphology and surface texture of samples were observed with field-emission scanning electron microscope (FE-SEM, S-4800) and transmission electron microscope (TEM, JEOL, JEM-2010).

Electrochemical Measurements.
The electrochemical performance of magnesium modified LiFePO 4 /C and pristine cathode materials was evaluated by using columnar cells.Active material powder (80 wt%) was mixed with acetylene black (10 wt%) and poly(vinylidene fluoride) binder (10 wt%) in N-methy1-2-pyrrolidone (NMP) to obtain slurry.The slurry was coated onto an aluminum foil and dried under vacuum at 120 ∘ C for 12 h.Finally, the laminate was cut into round dics (1.0 cm in diameter) to be used as working electrode.The electrolyte was 1 mol L −1 LiPF 6 dissolved in the mixture of ethylene carbonate (EC) and diethyl carbonate (DMC) with volume ratio of 1 : 1. Histogram cells were assembled in the glove box filled with argon gas.Charge-discharge tests were conducted on a battery test system (CT2001A, LAND, China) with cutoff voltages of 2.4 V and 4.2 V (versus  Li/Li + ) at different current rates at room temperature.Cyclic voltammetry (CV) measurements were performed on an electrochemical working station (LK2005, LANLIKE, China) at a slow scanning rate of 0.1 mV s −1 within a voltage range of 2.4-4.2V.And electrochemical impedance spectroscopy (EIS) was employed to characterize the interfacial resistance of cathode using a Chenhua CHI660A electrochemical workstation over the frequency range from 1 MHz to 0.01 Hz with amplitude of 10 mV ms −1 at room temperature.

Results and Discussion
3.1.Structural Analyses.XRD patterns of selected samples are shown in Figure 1.Obviously, the crystal phases of all the samples are to be an ordered olivine structure indexed orthorhombic Pnmb, and no extra reflection peak from impurity is observed, indicating that a small amount of magnesia or magnesium does not destruct the lattice structure of LiFePO 4 .Table 1 shows the corresponding lattice parameters of the pristine and Mg-doped LiFePO 4 /C.The lattice parameters and unit cell volume slightly increase after doping Mg.This result is attributed to the larger ionic radii of Mg 2+ compared with those of Li + .SEM images of LiFePO 4 /C and modified LiFePO 4 /C are shown in Figure 2. The LiFePO 4 /C particles are spherical or elliptical, and the size ranges from nanometer to micrometer, which is beneficial to high tap density.Low level magnesium doping and coating do not make apparent changes on the morphology, as shown in Figures 2(b) and 2(d).But the morphology changes a lot when the magnesium content rises up to a high level (Figures 2(c) and 2(e)).Small particles aggregate with each other to decrease surface energy in sample LFP/C-M 5 , which prevents electrolyte from penetrating through electrode and encumbers the Li-ion diffusion.
And it is shown that high level magnesium doping destroys the spherical structure, and the shape of particles becomes irregular in Figure 2(e).Mg steps into LiFePO 4 lattice and substitutes Li site, which is expected to change its morphology more or less.Figure 3 shows TEM images of sample LFP/C-M 2.5 and L 0.985 M 0.015 FP/C, both presenting a typical core-shell structure with an amorphous carbon wrapping and connecting out of particles.As shown in Figure 3(b), several dark grains with well-crystallized structure are observed around the LiFePO 4 particles, and the gap between every two parallel fringes is measured to be 0.242 nm, which corresponds to the interplanar spacing distance of (111) planes of MgO crystals.MgO comes from the decomposition of magnesium citrate complex, distributing on the surface of LiFePO 4 particles in magnesium-coated samples.As shown in Figure 3(d), the thickness of the carbon layer is about 2-3 nm in sample L 0.985 M 0.015 FP/C, which is favorable to enhance the conductivity but does not affect intercalation/deintercalation of Li + because Li + can readily penetrate through the thin carbon layer.

Electrochemical Performances.
The first discharge curves of LiFePO 4 /C and magnesium modified LiFePO 4 /C electrodes at 1 C-rate are shown in Figure 4.According to results, proper magnesium modification efficiently enhances the specific capacity.Among all samples, sample L 0.985 M 0.015 FP/C shows the best discharge property, whose first special capacity is up to 163 mAh g −1 much higher than the pristine one, only 126.5 mAh g −1 .And the superfluous electrons, supplied by Mg ion, increase the amount of electrons in electrode material and advance the electronic conductivity in a further  The discharge capacity is also enhanced by a certain amount of MgO coating, and it reaches 152.1 mAh g −1 at 1 C when MgO content is 2.5 mol%.At the same time, MgO, distributing on the surface of LiFePO 4 particles, prevents electrolyte from corrupting active material and suppresses electrolyte-induced thermal decomposition [18] to improve the cyclic life of electrode, as shown in Figure 5.The discharge capacity of sample LFP/C-M 2.5 does not fade after 100 cycles at 1 C, whose retention ratio is up to 104.2%, while the retention ratios of pristine and sample L 0.985 M 0.015 FP/C are only 96.8% and 96.7%, respectively.But overmuch MgO coating also shows disadvantageous effect on electrochemistry performance.The first discharge capacity is 122.7 mAh g −1 when MgO content reaches 5 mol%.It is conjectured that redundant MgO should decrease Li + diffusion rate because Li + has to get across the magnesia coating layer when it drills through the interface of electrode and electrolyte.Transport of electrons from the particle surface to current collector is the critical step, particularly at high current rate [22].Therefore, the promotion effect by magnesium modification is obvious at high current.As Figure 6 shows, the average discharge capacity of pristine sample at 2 C, 5 C, and 10 C is only 115.5 mAh g −1 , 82.5 mAh g −1 , and 32.4 mAh g −1 , respectively.Both Mg doping and MgO coating improve the rate capability of the electrode material.The discharge capacity of sample LFP/C-M 2.5 is 145.2 mAh g −1 , 124.6 mAh g −1 , and 96.8 mAh g −1 at 2 C, 5 C, and 10 C, respectively, while L 0.985 M 0.015 FP/C is 145.5 mAh g −1 , 128.3 mAh g −1 , and 103.7 mAh g −1 .It indicates that Mg doping has Cycle voltammetry profiles reflect not only the electrochemical properties of active material, but also the activity of the entire electrode.CV curves of samples LFP/C, LFP/C-M 2.5 , and L 0.985 M 0.015 FP/C at a scan rate from 0.25 to 1 mV s −1 are shown in Figures 7(a), 7(b), and 7(c), respectively.The intensity and area of reduced and oxide peak increase with the scan rate.The linear relationship of the peak current as a function of square root of scan rate is illustrated in Figure 7(d).Thus, the apparent diffusion coefficient can be derived according to the following equation: where   is the peak current (),  is the charge transfer number which is one for the electrode reaction,  is the contact area between active material and electrolyte (approximate to the surface area of electrode, 0.785 cm 2 ), and V is the scan rate (V s −1 ).
Figure 8 shows the electrochemical impedance spectra (EIS) of the LiFePO 4 /C and magnesium modified LiFePO 4 /C electrode material after the 5th cycle.The EIS curves are composed of a depressed semicircle in high-frequency region and a straight line in low-frequency region.An intercept at the  re axis in the high-frequency region corresponds to the ohmic resistance of the electrolyte, followed by a semicircle in the middle-frequency range, indicating the charge transfer resistance, and a straight line in the low-frequency region, related to the Warburg impedance due to the diffusion of the lithium ion in the bulk of the electrode material.The impedance spectra can be described by the equivalent circuit presented in the inset picture, where   represents ohmic resistance,  ct represents the charge transfer resistance,   represents the Warburg impedance, and the constant phase element CPE is placed to represent the double-layer capacitance and passivation film capacitance [23]  resistance is related to complex reaction of charge transfer process between the electrolyte and the active materials [24].Compared with the pristine,  ct is obviously decreased by MgO coating and Mg doping.The smaller the charge transfer resistance is, the more feasible it is for lithium-ion and electron transformation.It indicates that magnesium modification is beneficial for active material to overcome the restriction of kinetics in the charge/discharge process and improve electrochemical activity.In addition, the promotion effect from Mg doping is more effectual than MgO coating.
The exchange current density ( 0 ) is an important parameter of kinetics for an electrochemical reaction and can measure the catalytic activity of electrodes.It is calculated using the following formula: where  is the gas constant (8.314J mol −1 K −1 ),  is the temperature (298 K),  is the charge transfer number per molecule during the intercalation which is 1 for LiFePO

Conclusion
Mg-doped and MgO-coated LiFePO 4 /C compounds were synthesized through a simple solid-state method, and the differences between super-valent cation doping and metal oxide coating on enhancing the electrochemical of LiFePO 4 /C material were analyzed.MgO particles, distributing on the surface of LiFePO 4 , improve the cyclic performance of electrode, and the discharge capacity of sample LFP/C-M 2.5 does not fade after 100 cycles.The specific capacity and rate capability of LiFePO 4 are dramatically increased by Mg doping, and the discharge capacity of sample L 0.985 M 0.015 FP/C is 163 mAh g −1 , 145.2 mAh g −1 , 124.6 mAh g −1 , and 96.8 mAh g −1 at 1 C, 2 C, 5 C, and 10 C, respectively.Thus, Mg doping is more prominent at prompting the capacity and rate capability of LiFePO 4 , while MgO coating has an advantage at improving cyclic performance.CV and EIS results demonstrate that apparent diffusion coefficients, catalytic activity, and the reversibility of LiFePO 4 are improved by both Mg doping and MgO coating.

Figure 8 :
Figure 8: The electrochemical impedance spectra profile of samples and equivalent circuit (inset).

Table 2 :
Impedance parameters of samples.Mg doping is more prominent at prompting the capacity and rate capability of LiFePO 4 , while MgO coating is good at improving cyclic performance.
. Fitting results were analyzed by Zview-Impedance 2.80 software, and  ct values are listed in Table 2.The charge transfer