Vanadium doping of LiMnPO4 cathode material: Correlation between changes in the material lattice and the enhancement of the electrochemical performance

Electrochimica Acta 325 (2019) 134930

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Vanadium doping of LiMnPO4 cathode material: Correlation between changes in the material lattice and the enhancement of the electrochemical performance
squez, J.A. Caldero
n* F.A. Va
n, Innovacio
n y Desarrollo de Materiales e CIDEMAT, Universidad de Antioquia UdeA, Calle70 N
52 e 21, Medellín, Colombia Centro de Investigacio

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 June 2019 Received in revised form 20 September 2019 Accepted 21 September 2019 Available online 24 September 2019

Different LiMn1-xVxPO4 (x ¼ 0, 0.05, 1, and 0.15) samples of olivine-type structure were successfully synthesized by solvothermal method in ethylene glycol. The effect of vanadium incorporation on the performance of LiMnPO4 was systematically investigated using X-ray diffraction, Raman spectroscopy, Xray photoelectron spectroscopy, charge/discharge measurements, cyclic voltammetry and electrochemical impedance spectroscopy tests. The control of the vanadium insertion in 4c sites within octahedral sites, particle size, desired morphology and conductivity, was achieved by an adequate thermal treatment. TEM images showed particle sizes of 100 nm and carbon coating thickness of between 3 and 4 nm. The vanadium insertion increases the number of lithium ion diffusion pathways and free charges in the LiMnPO4 structure. For this reason, the doped materials exhibit superior ionic and electronic conductivity to undoped material. Consequently, the LiMn1-xVxPO4 cathode material can retain more than 60% capacity when tested at rates as high as 5C. Vanadium-doped olivine exhibited a capacity of 126 mAhg1 at 0.2C and a high cycling stability. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Lithium-ion battery Positive electrode Olivine Electrochemical performance Vanadium insertion

1. Introduction With a view to improving the performance of energy storage equipment, research has been carried out into increasing the energy density, stability and rate capability of lithium-ion batteries. Strategies such as morphology control, particle size reduction, structural modification, and partial substitution of the transition metal with dopant elements have been implemented, in order to obtain active materials with superior performance in aspects including ionical and electronically conductivity and stability [1,2]. Due to the high structural stability and energy density (170 mAhg1) of LiMPO4 (M ¼ Fe, Mn, Co), olivine-type materials have been studied extensively [3,4]. Currently, LiFePO4 with olivine-like structure is subject to a high level of attention due to its high theoretical capacity, low toxicity, superior stability, low cost and high coulombic efficiency [5]. Nevertheless, LiFePO4 presents some drawbacks such as unidirectional diffusion and low redox voltage (3.2e3.4 V vs. Li/Liþ) compared to other cathode materials such as the spinel type [5]. Active spinel materials present 3D

* Corresponding author.
n). E-mail address: [email protected] (J.A. Caldero https://doi.org/10.1016/j.electacta.2019.134930 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

diffusion and potentials of 4.0 and 4.7 V vs. Li/Liþ [4,6]. Consequently, studies have been conducted to control the particle size, morphology, and crystal orientation of the LiFePO4 olivine-type in order to reduce the diffusion distance and, in turn, the intercalation time of lithium ions [5,7]. The LiMnPO4 compound is the most promising olivine-type active material due to its superior redox voltage (4.0e4.1 V vs. Li/ Liþ) and similar theoretical capacity (171 mA h g1) to LiFePO4 [8]. Meanwhile, to control the particle size and growth direction of the LiFePO4, olivine-like structure, solvothermal synthesis has been developed, obtaining materials with high stability and C-rate at 10C [5]. Consequently, this kind of synthesis has also been implemented for LiMnPO4 materials to reduce the particle size and diffusion distance of lithium ions [8]. However, drawbacks remain despite the high discharge potential of LiMnPO4. These include high electrical resistance and low ionic conductivity, which restrict the performance of this active material, especially in terms of the C-rate [9]. Additionally, in manganese olivine active material the structural stability is strongly affected by shear deformation generated by the elongation of the bonding distance during the lithium-ion intercalation-deintercalation process [10]. In order to solve these disadvantages, the partial substitution of manganese with metal

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ions has been proposed. This could improve the stability and conductivity of the active materials [11]. This strategy has been successfully implemented for the LiFePO4 olivine. For instance, Liu et al. [12] found that doping with a heteroatom like Ti, Zr, V, Nb, and W could enhance the performance of LiFePO4 at high current due to the enlarged lattice volume that provides more space for lithiumion transfer. Similar results were found by Zhang et al. [13] during the doping of LiFePO4 olivine with vanadium. Wu et al. [14] synthesized a multidoped LiFePO4/C with Mn, V, and Cr ions. They found that those materials exhibited better rate capability than undoped LiFePO4/C, due to the improvement of the electrode reactivity by multidoping, particularly at high rates. Jin et al. [1,5] and Ma et al. [15] also found that the introduction of vanadium into carbon-coated LiFePO4 particles led to significant improvement in rate capability, which was attributed to the formation of other vanadium phases like V2O3 or Li3V2(PO4)3 [15,16]. For the case of the manganese olivine material, few works report the partial substitution of manganese by vanadium in order to improve the electrochemical performance of the active material. Kellerman and co-workers [17,18] performed the insertion of the pentavalent vanadium ion inside the tetrahedral spaces, mainly substituting the phosphate anions. However, under pentavalent oxidation state, the vanadium substitutes the phosphate anion and the capacity of the active material decreases, given that no vanadium oxide-reduction reaction takes place during the lithium intercalation or deintercalation. Meanwhile, Qin et al. [19] demonstrated that lower quantities of vanadium with oxidation states of V4þ and V3þ could be inserted in the octahedral sites by following an adequate synthesis route. Su et al. [20] synthesized vanadium doped LiMn1-xVxPO4 olivine using a conventional solidstate reaction method. They found that vanadium doping significantly enhanced the electrochemical properties of LiMnPO4, and that the materials were in single phase when the vanadium content (x) was lower than 0.05. Where that content was higher than 0.1, the samples were shown to contain an additional conductive phase of Li3V2(PO4)3. The optimal doping level of vanadium was found to be 0.1. Gutierrez et al. [21] prepared vanadium-doped LiMnPO4 by microwave-assisted solvothermal (MW-ST) process. They found that V3þ substituted Mn2þ ions in the material lattice, enhancing the overall kinetics of the material by lowering the charge-transfer resistance and increasing the lithium-diffusion coefficient. Although that work demonstrated enhancement of the material capacity by vanadium incorporation, unfortunately it did not show the role of the vanadium doping in the cycling stability the rate capability of the vanadium-doped LiMnPO4. This is a general drawback of the information published on the electrochemical performance of vanadium-doped LiMnPO4 active material, since most of the works report electrochemical tests performed at very low C rates. For instance, Dai et al. [22] reported that vanadiumdoped LiMnPO4 active material can hold 80% of its capacity up to the 50th cycle at low C-rates (0.2C). However, when the material is tested at higher C-rates the capacity retention falls dramatically. Additionally, the published works regarding LiMnPO4 active material show no correlation between the changes in the material lattice induced by vanadium incorporation and the enhancement of the electrochemical performance. In the current work, insertion of vanadium ions into the LiMnPO4 olivine active material was performed with a view to obtaining partial substitution of the manganese ions present in the olivine structure. This strategy is proposed as a promising option for improving the electrochemical performance of the active material, given that the presence of vanadium increases the conductivity of the material and reduces the shear effort during the cycling process, without an appreciable impact on energy density. Additionally, solvothermal synthesis of LiMnPO4 allows particles to be

obtained with a low diffusion distance, and improves the vanadium insertion within the structure. Following this synthesis route a cathode active material for lithium ion batteries with superior Crate and stability can be obtained. Moreover, it is proposed that there is correlation between the changes in the material lattice induced by vanadium incorporation and enhancement of the electrochemical performance. 2. Experimental 2.1. Materials synthesis LiMn1-xVxPO4 (x ¼ 0.0, 0.05, 0.1, and 0.15) with olivine-type structure was synthesized by the solvothermal method. The reagents were analytical grade and water was double distilled. V2O5 (Sigma Aldrich) was used as a vanadium source and dissolved at 60
C in a solution of water-oxalic acid at a V2O5:oxalic acid ratio of (1:3) to obtain vanadium oxalate (blue solution) [23]. The synthesized products were dried in the vacuum oven at 80
C for 12 h. For the synthesis of 14.7 mM of LiMn1-xVxPO4, stoichiometric quantities of H3PO4 (Panreac Applichem, 85%) were added drop by drop to Li(CH3COO)$2H2O (Alfa Aesar) pre-dissolved in 50 ml of ethylene glycol (EG) to obtain Li3PO4. Subsequently, Mn(CH3COO)2$4H2O (Sigma Aldrich) and vanadium oxalate were sequentially dissolved in the solution. The solution was then stirred for 3 h and transferred to an autoclave preheated to180
C. After sealing and purging with nitrogen, the autoclave was kept at 180
C for 16 h. The powder synthesized was washed several times in absolute ethanol and distilled water. Finally, the particles were dried in the vacuum oven at 80
C for 12 h to obtain the as-synthesis materials. Although some additional active materials were synthesized in EG-water solvent, these are not presented in this work, given that poor performance of the synthetized material was obtained. 2.2. Thermal treatment In order to improve the electronic conductivity of the active cathode material, a carbon layer was generated on the surface of particles. For this, 0.2 g of pharmaceutical grade sucrose (Panreac Applichem) was dissolved in 1 ml of distilled water and 4 ml of acetone. After vigorous homogenization, 0.5 g of active synthesized material was added to the solution. The dispersion was stirred for 2 h until slurry was obtained. The slurry was then kept at 80
C for 2 h in a vacuum oven. Finally, the powder was heated at 650
C for 8 h under argon atmosphere with a heating rate of 5
C min1. Although other thermal treatment temperatures (600
C and 700
C) were tested, the results of these are not shown in this work as the best results were obtained in the material thermally treated at 650
C. 2.3. Characterization Olivine phase identification was carried out by Raman spectroscopy and X-ray diffraction. Raman spectroscopy was performed on the synthesized materials before and after thermal treatment in a Horiba Jobin Yvon (Labram HR) Nikon (BX41) microscope with a 50X objective, equipped with a laser wavelength of 632 nm and 0.3 D filter. The X-ray diffraction patterns (XRD) were performed on the active material after thermal treatment, using PANalytical equipment in a 2q range of 10e70
. The flush angle between the specimen holder and the detector was fit at 5
. The XRD spectrum was analyzed by the High Score Plus software. X-ray photoelectron spectroscopy (XPS) was performed to the cathode electrode using a (NAP-XPS)-Spects device equipped with a monochromatic source of Al-Ka(1486.7 eV, 13 kV, 100 W). The morphology, particle size,


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interplanar distance and carbon layer thickness were analyzed by transmission electron microscopy (TEM), performed using Tecnai F20 Super Twin TMP equipment equipped with a field emission source and GATAN US 1000XP-P camera. TEM images were interpreted using Digital Micrograph software. The chemical composition was determined by energy-dispersive X-ray spectroscopy (EDS). Thermal analysis (TGA) was performed on the active material after thermal treatment in order to determine the carbon content present on the particle surface. TGA curves were obtained using a TA Instruments model Q500 device, heating between 25 and 800
C under air atmosphere, at a scan rate of 10
C min1. 2.4. Electrochemical characterization Cathode electrodes (working electrodes) were fabricated by the doctor blade method on aluminum foil with a slurry containing 2 mg of active material (80%, including the carbon layer), carbon super P® (10%) and poly (vinylidene fluoride) (10%) mixed in Nmethyl-pyrrolidone solvent. The coating was dried at 80
C in the vacuum oven for 12 h. The half cells were assembled in “T-cells” (1.13 cm2 of area) using lithium foil as reference and counter electrodes, and glass microfiber (Whatman GF/D) as separator. The electrolyte was prepared using 1 M LiPF6 dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) with a weight ratio of (1:1). All the half cells were assembled in a MBraun glove-box under Argon atmosphere with 1 ppm of oxygen and moisture. The electrochemical tests were carried out using an Autolab PGSTAT 302 N potentiostat and a Gamry 600 potentiostat. The cycling test was performed at 0.2C (30 mAg1), and electrochemical impedance spectroscopy (EIS) was carried out after the 15th cycle at charge states of 0, 50, and 100%, in the frequency range of 20 kHze100 mHz using a sinusoidal amplitude of 10 mV. The cyclic voltammetry curves (CV) were performed at scan rates of 0.1, 0.2, 0.5 and 1 mVs1 in a voltage range from 2.2 V to 5 V. Finally, the rate capability was performed at C-rates of 0.1, 0.2, 0.5, 1, 2, 5, and 10C. The EIS measurements were fitted using the Gamry Echem Analyst software.

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carbon reaction with oxygen. The quantified percentage of carbon present in the materials was 9.04 ± 0.69% w/w. The carbon content obtained in the active material is desirable in order to ensure high conductivity, given that the particle size of the manganese olivine particles is presumably low [24]. Fig. 1 shows Raman spectra for vanadium-doped manganese olivine. Fig. 1(a) and (b) and correspond to samples without and with carbon coating, respectively. The most important band at 948 cm1, present in all samples, is related to the symmetric stretching of PO3 anions [8]. The oscillation frequency of the 4 phosphate groups is practically identical in the different compounds, and there is no appreciable difference from that of the free tetrahedral AB4 molecule. This is because the chemical bond between oxygen and phosphorus is much stronger than that between other ions [17]. In Fig. 1(a), no additional signals or shoulders in the band at 948 cm1 were observed, indicating higher purity of the olivine material [25]. In the materials with carbon coating, Fig. 1(b), the bands at 1323 cm1 and 1596 cm1 are associated with disordered carbon (D band) and graphite (G band). There is a relationship between the electronic conductivity of the carbon coating and the ratio of D and G bands (ID/IG) [26,27]. It is expected that the sample with the carbon coating with the lowest ID/IG ratio will exhibit higher electronic conductivity due to the higher graphitized carbon content in the material [28]. Table 1 shows the D and G bands ratio calculated from the Raman spectra. As can be seen, the (ID/IG) ratio of the samples, in general terms, decreases with the increase in vanadium content, indicating that the degree of graphitization of the carbon coating is enhanced with the increase in vanadium content. Fig. 2 shows the XRD patterns of carbon coated olivines. As shown in the pattern, the material can be indexed to LiMnPO4 with orthorhombic structure and spatial groups Pnma (ICSD No. 97763), where the manganese and vanadium occupy the 4c sites in octahedral spaces [8]. Some additional peaks labeled with the symbol “*“, that do not correspond to the desired olivine structure, appear in the materials with vanadium insertion at 2q of 27.0 and 28.9
. Those peaks are related with the presence of Li3V2(PO4)3 phase (ICSD No. 96962.) [20]. Table 2 shows the lattice parameters of the

3. Results and discussions 3.1. Material characterization Carbon layer on the surface of the synthesized active materials was generated by sucrose addition and thermal treatment, as explained in the experimental section. This was done in order to improve electronic conductivity and protect the particles from electrolyte attack. TGA tests of the active materials, performed under oxidizing atmosphere, exhibited weight loss related to the

Table 1 D and G bands ratio calculated from the Raman spectra of the carbon coated samples, shown in Fig. 1(b). Vanadium (mole fraction)

ID/IG

SD

0.00 0.05 0.10 0.15

0.9904 0.9715 0.9278 0.9502

0.0033 0.0039 0.0331 0.0025

Fig. 1. Raman spectra of manganese olivine containing several vanadium molar fractions (0.00e0.15). (a) and (b) without and with carbon coating, respectively.


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Fig. 2. XRD patterns of carbon coated manganese olivine doped with different molar ratios of vanadium (x ¼ 0.00e0.15).

Table 2 Lattice parameters of the carbon coated olivine samples, LiMn1-xVxPO4 (x ¼ 0.0, 0.05, 0.1, and 0.15), calculated from the refinement of XRD patterns shown in Fig. 2. Vanadium (molar fraction)

Crystallite size (nm) (020)

Lattice constant a (Å)

Lattice constant b (Å)

Lattice constant c (Å)

Lattice volume V (Å3)

Chi Square

Goodness of fit

0.00 0.05 0.10 0.15

25.4 29.9 26.6 24.1

10.432 10.431 10.434 10.434

6.094 6.100 6.100 6.100

4.736 4.744 4.744 4.740

301.080 301.850 301.860 301.640

4.08E7 9.78E7 1.35E6 1.40E6

0.85 0.78 0.65 0.97

carbon coated olivine samples calculated from the refinement of XRD patterns using the High Score Plus Software. As can be seen, when vanadium is added the lattice distances a, b, c are enlarged respect to pristine LiMnPO4. Furthermore, the zoom in XRD patterns for the planes (111) and (311) are shown in Fig. 2, which evidenced the change in the lattice parameters. The largest change was observed for the c lattice distance. However, at superior vanadium doping than 10% the c distance is slightly reduced probably due to the increment of the lattice stress and by the Li3V2(PO4)3 phase precipitation; while distances a and b remains almost constant. A similar effect for the V-doped LiMnPO4/C active material was obtained by Su et al. [20]. The changes in lattice parameters prove that V was doped into the host lattice. Moreover, no further changes take place in the cell volume with vanadium contains superior to 10%. As a result, a distortion of the lattice and an increase in the total volume are obtained with the insertion of vanadium. The lattice distortion in the doped material is probably due to the partial substitution of the Mn2þ ions by V2þ and V3þ. The ionic radius of the V2þ and V3þ ions are 0.93 Å and 0.78 Å respectively. When comparing these with the ionic radii of Mn2þ (0.81 Å), some changes in the normal lattice distances and in the crystal volume can occur due to vanadium doping [20,22]. Consequently, the incorporation of V2þ and V3þ can respectively induce enlarging and shrinking of the lattice parameters and an increase in the total volume of the crystal could be experimentally observed. According to those results, an easier intercalation process of lithium ions into the lattice is expected, and as a consequence the material may exhibit better electrochemical performance with increased doping with vanadium. It is important to note that no diffraction peaks related with carbon were detected, which indicates that the residual carbon is amorphous or the carbon layer on the particles is too thin. The crystallite size of the all synthesized olivine samples, determined using the Scherrer equation from peak (020), was between 17 and 33 nm. This value is in accordance with that expected for this synthesis route [29]. The XPS characterization was carried out in order to determine the valence state of manganese and vanadium in the olivine

structure. As a representative sample, active materials with 0.05 and 0.15 M fraction of vanadium were chosen for this analysis. Fig. 3a and b Show the High resolution XPS spectrum in the Mn2p region for manganese. This measurement involves the determination and fitting of binding energies for Mn2p3/2 located at 641.5 eV, and for Mn2p1/2 located at 653.5 eV [30]. As can be seen, the Mn2p peak matches satisfactorily with the manganese in the oxidation state of 2þ. A peak at 646.3 eV can be also observed, related with a satellite feature of Mn2þ [30,31]. Additionally, the presence of the only Mn2þ in the spinel structure is corroborated by the difference between the two multiplet split components of the Mn3s peak, calculated as DE ¼ 5.9 eV [31]. Fig. 3c and d shows the High resolution XPS spectrum in the region of the V2p peak. Similarly to manganese, this peak exhibits two multiplet split components, V2p3/2 and V2p1/2, localized at 516.6 eV and 524.3 eV, respectively [20,32]. The fit of the V2p peak matches adequately with the vanadium oxidation state of 3þ. It is consequently expected that, in accordance with the vanadium insertion as V3þ, vanadium substitutes the manganese ions in the octahedral spaces. This result differs from the report of Kellerman and co-workers [17], where the vanadium was inserted as a V5þ in the tetrahedral spaces, substituting the phosphate anions in the olivine structure [20,22]. Additionally, some dispersion at the binding energy values around 513 and 521 eV is observed in the spectra. This feature could be related with the presence of a small quantity of vanadium in oxidation state 2þ; which is in accordance with the results obtained by XRD. The residual STD values for the high-resolution refinement of 0.05 M fraction of vanadium were 3.03 and 0.7880 for manganese and vanadium, respectively; and in 0.15 M fraction of vanadium were 1.99 and 1.17 for manganese and vanadium, respectively. The molar fractions of manganese substituted by the vanadium calculated from XPS spectrum were 0.047 and 0.17 for materials with vanadium molar fractions of 0.05 and 0.15, respectively. This value agrees with the vanadium addition expected during the synthesis process. This is why more open lattice structure and superior unitary cell volume was observed in XRD analysis, given that V2þ has larger ionic radius (0.93 Å) than


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Fig. 3. (a), (b) High resolution XPS spectra of the cathode electrode LiMn1-xVxPO4 for Manganese Mn2p peak region with x fraction of 0.05 and 0.15, respectively. (c), (d) High resolution XPS spectra of the cathode electrode LiMn1-xVxPO4 for Vanadium V2p peak region with x fraction of 0.05 and 0.15, respectively.

Mn2þ (0.81 Å) in the hosting lattice. Transmission electron microscopy (TEM) was used to assess morphology, particle size and carbon layer thickness in the active materials. TEM images are shown in Fig. 4. Nanoplate morphology with particle length in the nanometer range and a plane orientation of (100) was observed, features expected for olivine-type particles synthesized by solvothermal methods [5]. Table 3 shows the main data extracted from TEM analysis. As can be seen, the particle length was from 60 to 70 nm for the vanadium doped manganese olivine and between 90 and 110 nm for the manganese olivine without vanadium. This result indicates that vanadium incorporation induces a reduction in the particle size of the active cathode material. The low particle size exhibited could compensate for the poor ionic diffusivity of lithium ions within manganese olivine. That because a low diffusion distance reduces the intercalation time of lithium ions in the structure, in accordance with the equation tdiff ¼ l2/D [33,34]. A homogeneous carbon layer with a thickness between 3 and 4 nm in intimate contact with the particles is also observed in the TEM images. Further analysis of high resolution TEM images (Fig. 4(b, d, f, j)) show that the olivine nanoplates grow preferable on the a and c directions. The d-spacing in the planes (311) was 2.609 and 2.618, 2.619, 2.618 Å for the materials with 0.0, 0.05, 0.1, 0.15 M fraction of vanadium, respectively. In accordance with XRD results, the vanadium insertion enlarges some parameters lattice and increases the total volume of the crystal, and no further changes take place in the lattice parameters for vanadium doping higher than 10%. These results are consistent with literature reports concerning doped olivine materials [35e37]. In order to obtain a semi-qualitative determination of the vanadium doping in the manganese olivine materials, the chemical composition of the particles was evaluated by EDS. In these active materials the atomic percentage of vanadium found was 0.55, 0.74

and 1.07 for the active materials with 0.5, 0.10 and 0.15 M fractions of vanadium, respectively, see Table 3. The result reasonably agrees with the theoretical content of vanadium for the synthesis. Superior vanadium insertion in the active materials was achieved for the materials prepared with high vanadium addition during the synthesis process.

3.2. Electrochemical performance of active materials The general electrochemical behavior of the samples is shown in Fig. 5, which exhibits the charge-discharge profiles and the cyclability of the synthesized active materials. Fig. 5(a) shows the first (line) and second (dash) charge profile for 0.15 mol fraction of vanadium and the first discharge profile for manganese olivine active materials without and with several mole fraction of vanadium, performed at 0.2C. As can be seen, the charge curve initially exhibits two small plateaus at 3.6 V vs. Li/Liþ and 4.1 V vs. Li/Liþ, related with the two vanadium oxidation processes V2þ/V3þ andV3þ/V4þ [20,21]. These electrochemical results corroborate the existence of vanadium at low oxidation state (V2þ) in the active doped materials, previously suggested by XRD and XPS analysis. The main plateau observed in the charge curve at 4.2 V vs. Li/Liþ corresponds to manganese oxidation Mn2þ/Mn3þ. The discharge curves exhibited a main plateau at approximately 4.0 V vs. Li/Liþ, corresponding to manganese reduction Mn3þ/Mn2þ. However, the vanadium reduction processes were not clearly observed (except for some inflexions detected below 3.6 V vs. Li/Liþ) due to these processes taking place in the same potential range as the manganese ions reduction [21,36]. Moreover, the capacity of the material increases as the doping with vanadium increases. For instance, the discharge capacity of the olivine materials with vanadium additions of 0, 0.05, 0.1, 0.15 M fractions were 91.5, 119.5, 126 and 123 mAhg1, respectively. These results certainly show that the


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Fig. 5. Electrochemical behavior of the vanadium-doped manganese olivine samples. (a) First (line) and second (dash) charge profile for 0.15 M fraction of vanadium and the first discharge profile for manganese olivine active materials without and with several molar ratio of vanadium, performed at 0.2C. (b) Cyclability of active material LiMn1xVxPO4 (x ¼ 0.0, 0.05, 0.1, and 0.15) performed at 0.2C. The Inset shows coulombic efficiencies of the charge-discharge profiles.

Fig. 4. TEM images of active material. (a) TEM and (b) HRTEM images of the material without vanadium addition. (c) TEM and (d) HRTEM images of the material with 0.05 M ratio. (e) TEM and (f) HRTEM images of the material with 0.1 M ratio. (i) TEM and (j) HRTEM images of the material with 0.15 M ratio.

capacity increases with vanadium addition, although there is no increase in the capacity when the vanadium addition rises from 0.1

to 0.15. This could be related with the existence of a vanadium substitution limit in the material, since other phases could precipitate during the synthesis process for superior vanadium additions [20]. The increment observed in the capacity of the active material with vanadium addition is related to both the enhancement of the electronic conductivity of LiMnPO4, and to the faster Liþ migration into the more open three dimensional (3D) framework found in LiMnPO4 when vanadium is inserted into the structure [13,20,21]. Fig. 5(b) shows the cycling performance of the active materials made at 0.2C. In the literature, cycling of the manganese olivine material is reported at very low rates, such as 0.05C [9,36,38]. Nevertheless, in the current work the discharge tests were performed at more demanding conditions and the half cells were evaluated at high C-rates such as 0.2C. This was done in order to assess the cathode material performance under more real conditions of use. For this reason, the charge retention decay rapidly during charge/discharge cycles, particularly for the un-doped olivine material. Additionally, our intention is just to compare the cycling performance of the un-doped vs. doped material. Since the tests have been done under the same conditions for all materials tested the comparison is valid. The synthesized vanadium-doped manganese olivine materials exhibited higher discharge capacity than un-doped material. The capacity decreases strongly during the initial cycles for the active material without vanadium, i.e. 50% of charge retention at 18th cycle. On the other hand, when vanadium is added to the active material, it retains the capacity above of that value, showing that the vanadium improves stability of the structure. For instance, the sample with 0.15 of vanadium retains up to 74.4% of the initial capacity after the 50th cycle. The coulombic efficiencies calculated from the charge/discharge profiles are shown in the inset of Fig. 5(b). Despite of the low coulombic efficiency observed in the first cycle (approx. 50%), due to the formation of the passive film on the active materials [39], the coulombic efficiency raised and became almost stable after the 5th cycle. Similar coulombic efficiency was achieved for undoped and vanadium doped materials (approx. 94%). The degradation of LiMnPO4 olivine during the charge-

Table 3 Particle size and carbon layer thickness of the active materials. Data extracted from TEM images shown in Fig. 4. Atomic percentage of vanadium (V) and fraction of vanadium doping in the molecular formula LiMn1-xVxPO4 calculated by EDS. V (fraction added)

V, at.%

X fraction

d-Spacing (311) (Å)

Particle length (nm)

Particle thickness (nm)

Thickness carbon coat (nm)

0.00 0.05 0.10 0.15

e 0.55 0.74 1.07

e 0.036 0.045 0.070

2.6085 ± 0.0035 2.6180 ± 0.0020 2.6187 ± 0.0033 2.6180 ± 0.0020

107.88 ± 10.61 66.90 ± 5.21 70.15 ± 18.90 54.37 ± 10.30

11.16 ± 0.78 8.60 ± 1.60 10.13 ± 1.63 e

3.13 ± 0.55 3.42 ± 0.57 3.28 ± 0.36 3.32 ± 0.56


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discharge process is due mainly to the high anisotropic deformation in the Manganese oxygen bonds (around 19%) generating shear efforts [10]. Consequently, bond recombination can take place and for this reason, phase transformation can be observed and manganese dissolution could occurs more easily [36,40]. The fast decay of the charge observed in the all materials tested in the current work could be related to the relative high C-rate used during the tests, looking for assessing the performance of active material under more critical conditions. In this case, at high C-rates (0.2C) test the shear efforts increases due to the manganese-oxygen bond anisotropy deformation, more phase transition and manganese dissolution takes place, losing the active places for the lithium insertion or de-insertion. Nevertheless, the results of the current work show that the vanadium insertion improve the cycling stability and the coulombic efficiency of the spinel, due to the improvement of the structural stability achieved when the manganese was partially substituted by the vanadium in the structure. The participation of manganese dissolution and the electrolyte degradation during the cycling as responsible, to a minor extent, for the decay of charge cannot be discarded. These issues have been reported in several works [41e44]. However, in the current work those processes would occur in a similar manner for all samples, because all samples were tested under the same conditions. Aurbach and co-workers investigated the possible dissolution of Li, Mn, and P from CeLiMnPO4 powder during aging in EC-DMC 1:2/ LiPF61.5 M, the total amount of dissolved ions Li, Mn, and P after 7 weeks is lower than 0.015%. They found that the reactivity of CeLiMnPO4 is much lower than the LiFePO4 active material [41,42]. They also found little amount of LiF and PF5 which are commonly

Fig. 6. (a) Rate-capability of vanadium-doped manganese olivines LiMn1-xVxPO4 (x ¼ 0.05, 0.1, 0.15) synthesized by solvothermal method. (b) and (c) discharge profiles performed of the vanadium-doped manganese olivine samples with vanadium content of x ¼ 0.1 and x ¼ 0.15, respectively, at several C-rates.

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formed by electrolyte decomposition. Consequently, they conclude that CeLiMnPO4 particles are remarkably less reactive toward solution species in the electrolyte composed by alkyl carbonate solvents/LiPF6 [43]. Fig. 6 shows the rate-capability of vanadium-doped and undoped manganese olivine active materials. As can be observed in Fig. 6(a), the increase in vanadium content clearly enhances the rate capability of the active material. The materials with 0.1 and 0.15 vanadium exhibit superior capacity at high C-rates, retaining around 80% capacity at 2C. The rate capability of the materials is directly proportional to the molar ratio of vanadium in the olivine. The 0.15 vanadium-doped material exhibits the highest rate capability performance. The enhancement of the rate capability of the vanadium-doped materials is directly related with vanadium insertion in the olivine structure, which induces better electronic conductivity, low particle size of the active material and enlargement of the lattice parameters, in turn facilitating Liþ intercalation, as previously demonstrated by TEM and XRD analysis. Furthermore, the 0.15 vanadium-doped olivine evaluated at high C-rate showed superior discharge potential to the 0.1 vanadium-doped sample (see Fig. 6(b) and (c)), which seems to indicate that lower energy is required to intercalate and deintercalated the Liþ ions [5]. It was found that although the 10% doping sample has the best capacity, that sample did not exhibit the best cycling stability. On the other hand, superior electrochemical performance (cycling stability and charge retention) was obtained for the 15% doping active material, although some loss of initial capacity was observed. Superior vanadium addition to 15% is not recommended, given that no further increasing of the lattice parameters is achieved and the capacity decreases due to the Li3V2(PO4)3 phase precipitation. In order to consider previous works and papers published concerning vanadium-doped manganese olivine material, comparative data on the performance of the active cathode material obtained in the current work and others reported in the literature is presented in Table 4. As can be seen, better electrochemical performance in terms of energy capacity, cycling stability and rate capability was obtained by the active materials developed in the current work. This is a consequence of the correct application of the sequence of solvothermal synthesis, and adequate thermal treatment for the control of the phase purity, stoichiometry and particle size of the active material. For a better understanding of the kinetics involved in the intercalation and deintercalation process of lithium ions in the vanadium-doped manganese olivines, cyclic voltammetry (CV) tests and electrochemical impedance spectroscopy (EIS) were carried out after the 15th cycle. CV curves of the active materials performed at a scan rate of 1 mVs1 are presented in Fig. 7(a). Broad oxidation and reduction peaks respectively located at around 4.2e4.5 V vs. Li/Liþ and 4.0e3.8 V vs. Li/Liþ are observed. The broad oxidation and reduction peaks comprise both redox processes for manganese and vanadium ions in the active material. It was not possible distinguish or separate the vanadium redox and manganese for reasons including the low vanadium doping level, the proximity of the redox potential of both processes and the high

Table 4 Comparative performance of the vanadium-doped manganese olivine active materials developed in this work and others reported in the literature. Material

Synthesis route

Capacity (0.2C) mAhg1

Stability (50th Cycle %)

Retention %Qmax. (C-rate)

Reference

LiMn0.9V0.1PO4 LiMn0.85V0.15PO4 LiMnPO4 LiMn0.975V0.025PO4 LiMn0.9V0.1PO4 LiMn0.70V0.30PO4

Solvothermal Solvothermal Solvothermal Solid-state Hydrothermal Solvothermal

126.2 122.64 110 110 46 120 (C/20)

59.3 74.4 92.8 82.7 e e

59.0 68.4 25.8 41.2 45.5 e

This work This work [36] [22] [45] [21]

(5C) (5C) (1C) (5C) (5C)


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Fig. 7. (a) Cyclic voltammograms of vanadium-doped manganese olivines LiMn1(x ¼ 0.05, 0.1, 0.15) performed at 1 mV s1 after the 15th cycle. (b) Linear relationship (Randles-Sevcik plot) of anodic current peak vs. the square root of scan rate.

xVxPO4

speed of the scan (0.1 V). Fig. 7(b) shows a Randles-Sevcik plot for the samples tested in the CV. There is a linear relationship between anodic peak currents and the square root of the scan rates. The slope extracted from the lines could be related to the apparent diffusion coefficient of lithium ions, as described by equation (1) [46].

1=2

Ip ¼ 2:69  105 n3=2 CLi ADLi v1=2

(1)

where, Ip is the anodic peak current value (A), n is the number of electrons transferred during oxidation reaction (1); A is the effective working electrode area (1.13 cm2); CLi is the concentration of lithium within the active material (0.02196 mol cm3); n is the scan rate (V s1); and D is the apparent diffusivity (cm2 s1) [36,47,48]. The apparent diffusivity coefficients calculated from the slope of the lines observed in Fig. 7(b) were 1.27 E12, 6.57 E12, and 9.81 E12 cm2 s1, for the samples with 0.05, 0.10 and 0.15 mol of vanadium, respectively. As can be seen, diffusivity increases as vanadium doping increases. Superior diffusivity was obtained for the active material with 0.15 mol of vanadium. This result is expected for the material with the highest C-rate performance, and is related to the improved ionic and electronic conductivity conferred by superior vanadium insertion (x ¼ 0.15). Additionally, the low particle size of the synthesized materials also contributes to the diminished diffusion time and enhanced diffusivity of the Liþ ions [5]. Fig. 8 shows the Nyquist plots of the EIS measurements for the vanadium-doped olivine after the 15th cycle (50% charge condition). In the EIS plot, the resistance at high frequencies is associated with electrolyte resistance (Re). In the EIS diagrams, three capacitive loops can be observed. Initially, two partially coupled semicircles were observed at high and intermediate frequencies. The loop at high frequencies is related with the SEI capacitance (4SEI) in parallel with solid-electrolyte interface resistance (RSEI). The loop at intermediate frequencies is related with the charge transfer resistance (Rct) in parallel with the double layer capacitance (4dl) of the active material particles. The semicircle at low frequencies is related with the lithium diffusion within the olivine structure (ZD). In accordance with previous works in lithium cells, the experimental EIS data was fitted using the equivalent-electrical circuit model (inserted in Fig. 8) [34]. The ZD was fitted with a constant phase element (CPE) in parallel with diffusion resistance (RD), according to the model described by Bisquert and co-workers [34]. The diffusivity coefficients (cm2 s1) were calculated using equation (2), and the effective capacitances (Ceff) according to the method proposed by Bisquert et al. [49]. Diffusivity ¼ L2/ƬDiff

(2)

Fig. 8. Electrochemical impedance spectroscopy in the half-cell performed at 50% charge, after the 15th cycle. Experimental values (dots), fitted values (lines).

Where L is the half distance of lithium ion diffusion, taking into account the crystal size of the active material particles listed in Table 2, and ƬDiff is the diffusion time listed in Table 3 [50]. Table 5 shows the parameters calculated from the fitting procedure of the experimental EIS plots using the equivalent-electrical circuit model. The electrical parameter values are directly related to the rate capability performance of the cathode materials. As can be seen, the charge transfer resistance (Rct) decreases with vanadium addition. These results demonstrate that vanadium doping improves the electronic conductivity of the manganese olivine material. Additionally, the presence of vanadium diminishes the diffusion resistance (RD) but enhances the diffusivity of the lithium ion into the manganese olivine material, which indicates that ionic conductivity is also enhanced by the presence of vanadium. Consistently with the previous electrochemical tests, superior energy capacity, cycling stability and rate capability can be expected for the material with a high level of vanadium incorporation [51]. 4. Conclusion In this work, successful synthesis of vanadium-doped olivine Table 5 Electrical parameters calculated by fitting of the experimental EIS results of the active cathode materials at 50% charge, using the equivalent-electrical circuit model inserted in Fig. 8. Parameter

V ¼ 0.05

V ¼ 0.10

V ¼ 0.15

Re (ohm g) Ceff. SEI (mF g1) R SEI (ohm g) Ƭ SEI (s) Ceff.dl (mF g1) Rct (ohm g) Ƭ Rct (s) RD (ohm g) Ceff.ZD_(mF g1) Ƭ ZD (s) Diffusivity (cm2 s-1) Goodness of Fit

3.45E3 2.74Eþ3 3.63E2 9.94E5 1.04Eþ4 1.69E1 1.76E3 3.92 1.13Eþ6 4.44 5.35E13 2.46E04

4.09E3 1.49Eþ3 3.44E2 5.11E5 9.62Eþ3 9.07E2 8.73E4 2.44 1.34Eþ6 3.28 5.40E13 5.04E05

3.85E3 1.83Eþ3 7.31E3 1.34E5 3.95Eþ3 6.20E2 2.45E4 1.20 1.47Eþ6 1.76 8.25E13 1.16E4


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material used for the positive electrode in lithium ion batteries is reported. XRD and Raman analysis showed that high purity of the active materials can be achieved by solvothermal synthesis in EG media. High-resolution XSP and XRD techniques showed that the vanadium substitutes the manganese in the 4c sites inside the octahedral spaces, while lattice distortion and an increase in the total volume are obtained with the insertion of vanadium in oxidation states V3þ and V2þ. The initial charge of LiMn0.9V0.1PO4 exhibited oxidation plateaus at 3.6 V and 4.1 V vs. Li/Liþ, associated with vanadium oxidation from V2þ to V3þ and from V3þ to V4þ, respectively. It was found that vanadium insertion enhances the energy capacity, cycling stability and rate-capability of the active material. The rate capability of the materials is directly proportional to the molar ratio of vanadium in the olivine. The materials with 0.1 and 0.15 of vanadium exhibit superior capacity at high C-rates, retaining around 80% capacity at 2C, while the 0.15 vanadiumdoped material retains more than 60% capacity at 5C. The enhancement of the electrochemical performance of the vanadium-doped materials is directly related with vanadium insertion in the olivine structure, which induces better electronic conductivity, low active material particle size, and enlargement of the lattice parameters, facilitating Liþ intercalation. Acknowledgments Authors thank the “Departamento Administrativo de Ciencia,
n e COLCIENCIAS”, TRONEX S.A.S and the Tecnología e Innovacio University of Antioquia for their support of project 1115-745-58653.
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