Structural stability and electrochemical properties of

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Structural stability and electrochemical properties of gadolinium-substituted LiGdx Mn2-x O4 spinel as cathode materials for Li-ion rechargeable batteries Kumaran Vediappan a,b,∗ , Kadirvelayutham Prasanna b , Swaminathan Shanmugan a , RM Gnanamuthu a , Chang Woo Lee b,∗ a

SRM Research Institute and Department of Chemistry, SRM University Kattankulathur-603203, Tamil Nadu, India Department of Chemical Engineering and Green Energy Centre, College of Engineering, Kyung Hee University, 1 Seochun, Gihung, Yongin 446-701, South korea b

a r t i c l e

i n f o

Article history: Received 18 July 2017 Received in revised form 10 October 2017 Accepted 30 October 2017 Available online xxx Keywords: Spinel LiMn2 O4 Gadolinium Co-precipitation Charge transfer process Lithium-ion batteries

a b s t r a c t Gadolinium-substituted spinel LiGdx Mn2-x O4 (x = 0.0 and 0.5) cathode crystals were synthesized by coprecipitation and dual calcination. Structural characterization using X-ray diffraction pattern revealed that gadolinium doping in LiMn2 O4 resulted in the highly ordered cubic spinel structure with only slight increase in the average d-spacing and lattice parameters. X-ray photoelectron spectroscopic analysis indicated an increase in the average oxidation state of manganese in the Gd-doped material in comparison to its pristine spinel counterpart. Morphological characterization using field emission scanning electron microscopy and transmission electron microscopy revealed that gadolinium doping in LiMn2 O4 resulted in a decrease of the average particle size. Electrochemical charge/discharge studies at various current rates showed that the LiGd0.5 Mn1.5 O4 spinel exhibited excellent and stable cycling stability in comparison to Gd-free LiMn2 O4 spinel. Gd-substitution in LiMn2 O4 brought structural stability via the expansion of LiO4 tetrahedra, contraction of MnO6 octahedra, and avoidance of the Jahn-Teller distortion effect, which translated in high-rate performance and less capacity fading. In addition Gd-doping was found via electrochemical AC impedance spectroscopy to lead to significant increase in electronic conductivity as evident by less charge transfer resistance than the pristine. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Nowadays rechargeable lithium-ion batteries have been extensively adopted as power source for consumer electrical and electronic devices in various application areas such as medicine, aerospace, and defense [1–3]. In particular, their suitability for applications like electric and plug-in hybrid vehicles has attracted a lot of interest [1,4] in this context an important consideration is structural stability upon cycling as is high performance. Towards this goal a main approach explored is reducing the structural instability by doping the electrode material with transition and non-transition elements, e.g., Ti [5], Cr [6], Mn [7], Ni [8], Fe, Zn, Mo, V [9], Cu [10], Bi, Zr, Sn [11], Ga, Si [12] etc., so that more and faster

∗ Corresponding authors at: Department of Chemical Engineering and Green Energy Centre, College of Engineering, Kyung Hee University, 1 Seochun, Gihung, Yongin 446-701, South korea. E-mail addresses: [email protected] (K. Vediappan), [email protected] (C.W. Lee).

Li can be extracted from the structure with minimum strain. Theoretical studies predict that doping with transition metals increases the capacity, whereas doping with non-transition metals leads to increased voltage. Liu et al. reported that the structural change during charge/discharge and the solution reactions of manganese can be significantly reduced by using transition metals such as zirconium and cobalt as dopants [13]. Recently, there has been a significant interest in lanthanide-based materials, because of their optical, magnetic, electronic and structural applications [14,15]. Among the lanthanide group of compounds, the oxide materials based on gadolinium possess high dielectric constant, better breakdown strength and have thermally stable structures. To the best of our knowledge, very few reports are available on the electrochemical influence of doping gadolinium in LiMn2 O4 based cathode active spinel materials. Although it is known that lanthanide elements are expensive, their study as dopants can provide valuable structural and electrochemical information that can help in our pursuit for identifying/developing structurally stable advanced cathode materials with increased safety and rate capability. Gd-doping with

https://doi.org/10.1016/j.apsusc.2017.10.223 0169-4332/© 2017 Elsevier B.V. All rights reserved.

Please cite this article in press as: K. Vediappan, et al., Structural stability and electrochemical properties of gadoliniumsubstituted LiGdx Mn2-x O4 spinel as cathode materials for Li-ion rechargeable batteries, Appl. Surf. Sci. (2017), https://doi.org/10.1016/j.apsusc.2017.10.223

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transition metal oxides would be a prominent influence on the cell dimension and atomic arrangements which are directly associated to the major improvement of electrochemical performances. Also, it was slight volume changes of LiO4 tetrahedron and MnO6 octahedrons by Gd-doping with metal oxides are responsible for the high rate performance and stable electrochemical cyclability [23]. In the present research work, we attempted to synthesize gadolinium-doped LiMn2 O4 spinel material by co-precipitation and dual calcination and subsequently characterized using various tools to understand its structure stability, surface morphology, charge transfer process and electrochemical properties. 2. Experimental 2.1. Synthesis of the spinel cathode active materials In this present work, we have adapted a co-precipitation method [16] for synthesis of spinel LiMn2 O4 and LiGdx Mn2-x O4 (x = 0.5) as cathode active materials using lithium acetate dihydrate, manganese (II) acetate tetrahydrate, and gadolinium acetate hydrate as starting chemicals. Stoichiometric amounts of Mn, and Gd acetates were dissolved in de-ionized water and then neutralized slowly by adding 20% NH4 OH solution to reach and maintain the pH at 9–11.0 while constantly stirring. The solution was continued to be stirred at 85 ◦ C overnight until a transparent precipitate was obtained. The precipitate was filtered and washed with distilled water followed by drying at 110 ◦ C for 12 h. Subsequently to that an excess of Li2 CO3 was mixed with the as-prepared precursor co-precipitate and subjected to pre-heating at 450 ◦ C for 5 h in air followed by calcination at 800 ◦ C for 12 h under high purity argon atmosphere followed by slow cooling to room temperature. For comparison, the Gdfree spinel LiMn2 O4 was prepared following the same procedure as with LiGd0.5 Mn1.5 O4 . The crystal structure, and oxidation state, and chemical composition of spinel LiMn2 O4 and LiGd0.1 Mn1.9 O4 cathode active materials were characterized with XRD (XRD, D8 Discover with GADDS, Bruker AXS) diffractometer in the 2␪ range from 10 to 80◦ with Cu K␣ radiation (␭ = 1.5406 Å) and XPS (Model: K-Alpha, Thermo Electron with a monochromatic Al K␣ radiation, 1486.6 eV). The surface morphology, polarity of charge transfer process and lattice image of the obtained powder was observed with a field emission-scanning electron microscope (FE-SEM, Leo Supra 55, Genesis 2000, Carl Zeiss), field emission-transmission electron microscope (FE-TEM, Jeol-JEM 2100F) and AC impedance analysis (Compactstat Electrochemical Interface, IVIUM Netherlands). A charge-discharge characterization was carried out between 4.5 and 3.0 V vs. Li/Li+ using an electrochemical cycler (Arbin instruments). 2.2. Cell fabrication To prepare the two kinds of cathode active materials loading, 90 wt% spinel LiMn2 O4 and LiGd0.1 Mn1.9 O4 powders, 5 wt.% Super P Black as a conductive agent, 5 wt.% polyvinylidene fluoride (PVDF, kureha KF100) binder and N-methyl-2-pyrrolidinone (NMP) were mixed to form a suspension. After 15 min of grinding with mortar to form viscous slurry, the slurry was deposited on aluminum foil and then dried in oven at 100 ◦ C for 5 h. The dried coated electrode was pressed under a 7 t load and then it was punched out with 14 mm diameter and used as the cathode. The punched cathode was additionally dried at 120 ◦ C for 5 h in vacuum oven. The thickness of the cathode film was about 44 ␮m. The 2032 coin-type cells (20 mm in diameter and 3.2 mm in thickness) were assembled in a glove box under high purity argon atmosphere. The cell consisted of a prepared cathode as described above, pure Li metal as an anode, microporous membrane (Celgard 3501) as a separator, and a nonaqueous electrolyte made of 1 M LiPF6 in ethylene carbonate (EC):

Fig 1. X-ray diffraction patterns of spinel active cathode materials: (A) LiMn2 O4 ; (B) LiGd0.5 Mn1.5 O4 .

ethylene methlyene carbonate (EMC) (3/7 V/V) (StarLyte, Ukseung Chemical Co., Ltd.). 3. Results and discussion 3.1. X-ray diffraction characterization Fig. 1 shows the X-ray diffraction patterns of the LiGdx Mn2-x O4 (X = 0.0 and 0.5) powders prepared by co-precipitation and calcination at 800 ◦ C for 12 h in high purity argon atmosphere. Both the samples with and without gadolinium (LiMn2 O4 and LiGd0.5 Mn1.5 O4 ) were synthesized under the same processing conditions as described in the experimental section by manipulating the stoichiometric compositions of the starting materials. The diffraction peaks of both samples correspond to the single phase cubic spinel structure with the space group Fd3 m in which the lithium ions (Li+ ) occupy the tetrahedral (8a) sites with the manganese ions (Mn3+ , Mn4+ ) and the substituted gadolinium ions (Gd3+ ) reside in the octahedral (16d) sites of the spinel structure [17]. However, a slight shift in peak positions to lower 2␪ values (corresponding to an increase in average d-spacing and cubic cell parameters, A) on doping gadolinium in the spinel LiMn2 O4 structure was observed as show in Table 1. For instance, the 2␪ value of (111) diffraction peak shifted to 18.6◦ (LiGd0.5 Mn1.5 O4 ) from 18.66◦ (LiMn2 O4 ) (Fig. 1 (inset)) and the average d-spacing and cubic cell parameter increased (Table 1), which can be interpreted in terms of replacement of the smaller Mn3+ cation (ionic radius, 75.4 pm) by Gd3+ (ionic radius, 107.4 pm). The associated expansion of the crystal lattice could provide more space for lithium intercalation and de-intercalation to occur. During the de-intercalation process, the lattice would be prevented from shrinking by the doped Gd3+ due to the large volume of the crystals [2]. Nevertheless, the peak intensities of the diffraction peaks of both LiMn2 O4 and LiGd0.5 Mn1.5 O4 are similar revealing that the doping of gadolinium does not affect the crystalline nature of the spinel material. 3.2. Surface morphology characterization Particle size and surface morphology can influence the electrochemical properties of the lithium-ion active cathode materials. Fig. 2 shows the FE-SEM images of the pristine LiMn2 O4 (a) and gadolinium doped spinel material (LiGd0.5 Mn1.5 O4 , (b)) powders. It can be seen from Fig. 2 that the cathode active materials with and without gadolinium doping exhibit similar morphology and homo-


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Table 1 The lattice parameter and average d-spacing values for LiMn2 O4 and LiGd0.5 Mn1.5 O4. Diffraction Peaks

111 311 222 400 331 511 440 531 533 622

LiMn2 O4

LiGd0.5 Mn1.5 O4

´˚ d-spacing (A)

´˚ Lattice parameter (a, A)

´˚ d-spacing (A)

´˚ Lattice parameter (a, A)

4.7508 2.5509 2.3776 2.0591 1.8898 1.5856 1.4566 1.3926 1.2565 1.2419

8.2286 8.4602 8.2362 8.2364 8.2375 8.2387 8.2397 8.2387 8.2393 8.2378

4.7655 2.5576 2.3820 2.0630 1.8926 1.5869 1.4591 1.3939 1.2573 1.2429

8.2540 8.4827 8.2515 8.2518 8.2497 8.2455 8.2538 8.2461 8.2447 8.2445

Fig. 2. FE-SEM images of spinel active cathode materials: (A) LiMn2 O4 ; (B) LiGd0.5 Mn1.5 O4 .

Fig. 3. FE-TEM images of spinel active cathode materials: (A) LiMn2 O4 ; (B) LiGd0.5 Mn1.5 O4 .

geneous particle size distribution. However, the average particle size decreases on doping gadolinium in the LiMn2 O4 active cathode material. This is further corroborated from the TEM images of pristine and gadolinium doped spinel LiMn2 O4 materials (Fig. 3). Pristine LiMn2 O4 consists of particles with sizes in the range of 50 nm to 150 nm. However, upon gadolinium doping in the spinel cathode material, the particle size reduces down to the range of 40–100 nm. The calculated grain size of main diffraction peaks in LiMn2 O4 and LiGd0.5 Mn1.5 O4 were shown in Table 2. It is conformed that the smaller grain size and controlled particle size distribution of LiGd0.5 Mn1.5 O4 increases the specific discharge capacity and better cycleability performance than compared to Gd-free spinel material.

Table 2 The calculated grain size of LiMn2 O4 and LiGd0.5 Mn1.5 O4. Diffraction Peaks

111 331 622

Grain Size (nm) LiMn2 O4

LiGd0.5 Mn1.5 O4

16.94 46.28 83.21

16.91 45.64 80.00

3.3. X-ray photoelectron spectroscopy characterization X-ray photoelectron spectroscopy has been used to investigate the oxidation state and chemical composition of LiGd0.5 Mn1.5 O4 spinel materials annealed at 800 ◦ C [18]. Fig. 4 shows high resolution XPS spectra of Li 1s, Gd 4d, O 1 s and Mn 2p in the


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Fig. 4. X-ray photoelectron spectra of LiGd0.5 Mn1.5 O4 : (a) Li 1s, (b) Gd 4d, (c) O 1s, (d) Mn 2p.

LiGd0.5 Mn1.5 O4 spinel material. The line of the Li 1 s core level has a low intensity with the binding energy located at 55 eV (Fig. 4(a)), which appears to be slightly higher than that of lithium metal (54.8 eV). Based on the binding energies for the Li 1 s core level in LiF and LiBr at 55.7 and 56.8 eV, respectively, it can be assumed that lithium exists as Li+ ions in the LiGd0.5 Mn1.5 O4 spinel material. The line of Gd 4d core level shows a sharp peak at 150.1 eV along a smaller peak at 153.4 eV indicating the presence of gadolinium in oxide form (Fig. 4(b)). The line shape of the O 1 s core level is Gaussian-like with a binding energy of 532 eV (Fig. 4(c)). This peak presents a slight asymmetry without appearance of any shoulder peak at higher binding energy as observed by Kumagai et al., [19] revealing the absence of additional oxygen containing compounds at the surface of the oxide particles. There is no significant change in the O 1 s peak position in comparison to LiMn2 O4 (Figure not shown) indicating that the O 1 s core level does not change upon gadolinium doping in the spinel material. The main change that

occurs on doping Gd in the LiMn2 O4 is the energy shift of the Mn 2p3/2 core level. Earlier reports reveal that the binding energies of the Mn 2p3/2 peak are located at 642.6 eV for Mn4+ in LiMn3+4+ 2 O4 and Mn4+ O2 (pyrolusite) and 641.6 eV for Mn3+ in LiMn3+4+ 2 O4 and Mn3+ 2 O3 (bixbyite) [20]. In the case of LiGd0.5 Mn1.5 O4 spinel materials, the two peaks of Mn 2p (Mn 2p3/2 and Mn 2p1/2 ) are located at 642 and 653.5 eV, respectively with an energy separation of 11.5 eV. The binding energy of the Mn 2p3/2 peak is intermediate between those of Mn4+ (642.6 eV) and Mn3+ , which is characteristic of the spinel structure. To determine the average oxidation state of Mn, the relative ratio of Mn3+ to Mn4+ in the LiGd0.5 Mn1.5 O4 has been calculated through non-linear curve fitting of the experimental results using a Gaussian model (Fig. 4(d)). Considering the relative concentration data of both Mn3+ (31.8%) and Mn4+ ions (68.2%), the average oxidation state of Mn is determined to be 3.68 for LiGd0.5 Mn1.5 O4 , which is higher than the average valence state of manganese (3.5) in the standard stoichiometric formula of




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Fig. 5. Charge/discharge characterization of spinel active cathode materials: (–) LiMn2 O4 ; (−) LiGd0.5 Mn1.5 O4 at C/10 current rate.

LiMn2 O4 . This increase in the valence state of Mn in LiGd0.5 Mn1.5 O4 is ascribed to the doping effect of gadolinium that suppresses the Jahn-Teller effect leading to significant improvement in the stability of the spinel frame, which in turn inhibits structural change during electrochemical cycling as well as suppresses the solution reaction of manganese. 3.4. Charge/discharge characterization The charge-discharge characteristics of spinel LiMn2 O4 and LiGd0.5 Mn1.5 O4 /LiPF6 (EC + EMC)/Li coin cells were carried out at room temperature in the range of 3.0 − 4.5 V at various charge/discharge rates, namely. C/10, C/5, C/3 and C/1. Fig. 5 displays the charge-discharge behavior of LiMn2 O4 and LiGd0.5 Mn1.5 O4 at a constant charge-discharge rate of C/10. From Fig. 5, it can be clearly seen that the charge/discharge curves of both LiMn2 O4 and LiGd0.5 Mn1.5 O4 materials have two voltage plateaus at approximately 4.0 and 4.1 eV, which is the typical feature of spinel LiMn2 O4 based active cathode materials. The two-stage plateaus specify that the insertion and extraction of lithium ions occur in two stages. The first voltage plateau, at about 4.0 V, is attributed to the removal/addition of lithium ions from half of the tetrahedral sites in which Li–Li interaction occurs [20]. The second voltage plateau observed at about 4.1 V is due to the removal/addition of lithium ions from the remaining tetrahedral sites in which lithium ions do not experience Li–Li interaction [21]. The specific discharge capacity was determined to be 123mAhg−1 for Gd-free spinel. Similar plateau zones are observed for the gadolinium-doped spinel material, with significant improvement in the specific discharge capacity at 110mAhg−1 with excellent cycling stability. The increased cycling stability of Gd doped spinel reflects the pronounced effect of Gd3+ doping on the electrochemical performance of the LiMn2 O4 based active spinel material. Also, the higher open circuit voltage and stable discharge capacity of LiGd0.5 Mn1.5 O4 is attributed to the greater intercalation/deintercalation of lithium (Li+ ) ions in comparison to its control counterpart, i.e. LiMn2 O4 . It is because of Gd doping that the inter-

layer space expands, as detected by X-ray diffraction, for improving circulation of the Li+ ions in the LiMn2 O4 . The diffusion paths of Gddoped LiMn2 O4 lead to faster Li+ intercalation than the undoped LiMn2 O4 counterpart. The potential vs. dQ/dV curves, shown in Fig. 6, clearly point out that there are two different processes at the high potential range for the Gd-doped and undoped LiMn2 O4 materials. The two electrochemical processes observed in Fig. 6, which correspond to the two-plateaus region of 3.9 − 4.1 V in Fig. 5, is a clear indication of the co-existence/formation of two phases during the cycling of the materials as described by the equations below [1,22]: charge

LiMn2 O4 → xLi+ + Li1−x Mn2 O4 + xe− yLi+ + ye− + Li1−x Mn2 O4

discharge

→

(1)

Li1+y−x Mn2 O4

charge

LiGd0.5 Mn1.5 O4 → xLi+ + Li1−x Gd0.5 Mn1.5 O4 + xe− yLi+ + ye− + Li1−x Gd0.5 Mn1.5 O4

discharge

→

Li1+y−x Gd0.5 Mn1.5 O4

(2) (3) (4)

The lithium removal (charge) and lithium insertion (discharge) into and out the LiMn2 O4 and LiGd0.5 Mn1.5 O4 spinel structure involves typically one lithium-ion equivalent per mole. Fig. 6 shows that the charge process exhibits at the high potential range two oxidation peaks one at 4.03 V and the other at 4.18 V (LiMn2 O4 ), 4.02 V and 4.19 V (LiGd0.5 Mn1.5 O4 ) vs. Li, respectively. The process is reversed in discharge in two reduction peaks at 3.95 V and 4.09 V (LiMn2 O4 ), 3.97 V and 4.10 V (LiGd0.5 Mn1.5 O4 ), vs. Li, respectively. These peaks are related to the Mn3+ /Mn4+ and Mn4+ /Mn3+ redox couples. The oxidation and reductive potentials of Gd-doped LiMn2 O4 were slightly shifted to the higher potentials but in addition one reduction peak appeared at 4.0 V. This peak it may relate to the reduction reaction of Gd3+ /Gd2+ in the Gd-doped spinel LiMn2 O4 structure, which apparently inhibits the dissolution of Mn2+ and enhances electrochemical cycling stability. Furthermore, we note that initially in terms of spinel LiMn2 O4 there was 0.898 mol of Li+ inserted and 0.836 mol of Li+ extracted as opposed to the theoretical 1 mol. By contrast the respective num-


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Fig. 6. Differential capacity of spinel active cathode materials (–) LiMn2 O4 ; (–) LiGd0.5 Mn1.5 O4 .

Fig. 7. Life cycling performance of spinel active cathode materials: (䊐) LiMn2 O4 (䊏) LiGd0.5 Mn1.5 O4 at C/10.

bers for the Gd-doped LiMn2 O4 spinel were 0.99 (inserted) and 0.95 (extracted), i.e. closer to the theoretical capacity of Gd-doped spinel. This may be due to the expansion of the crystal lattice providing as result more spaces for lithium insertion and extraction. At the same time, during the discharge process, the lattice would be prevented from shrinking by the doped Gd3+ due to the expanded crystal volume. Also, Gd-doping brings changes to the unit cell parameter and atomic arrangement of the spinel, which associated with the changes of bond length make the LiO4 tetrahedron to expand facilitating fast electrochemical action while the MnO6 octahedron shrinks providing structural integrity [23]. In combination these factors lead to increased structural stability with high-rate performance and less capacity fading, due to their larger crystal volume associated with less ionic dissolution during Li+ insertion/extractions. Also they may have low electrolyte decomposition on the electrode surface when compared to the pristine.

Fig. 7 shows the cycling performance of both LiMn2 O4 and LiGd0.5 Mn1.5 O4 active spinel materials. It can be noted that the average discharge capacities of LiMn2 O4 and LiGd0.5 Mn1.5 O4 spinel materials with and without gadolinium doping are 85mAhg−1 (LiMn2 O4 ) and 115mAhg−1 (LiGd0.5 Mn1.5 O4 ) respectively after 50 cycles that correspond to capacity retention of 62% (LiMn2 O4 ) and >100% (LiGd0.5 Mn1.5 O4 ), i.e. it’s showing almost theoretical capacity of Gd-doped spinel and stable electrochemical performance. Fig. 8 shows the charge-discharge behavior of LiMn2 O4 and LiGd0.5 Mn1.5 O4 at various charge-discharge rates, namely C/10, C/5, C/3 and C/1. As expected, the specific charge capacity value decreases with increasing current rate. The third cycle of specific discharge capacity at various charge-discharge rates is observed to be: 114 (C/10), 104 (C/5), 97 (C/3) and 88 mAhg−1 (C/1) for spinel LiMn2 O4 materials and 121 (C/10), 65 (C/5), 52 (C/3), and 41 mAhg−1 (C/1) for LiGd0.5 Mn1.5 O4 , respectively. Stable values of




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Fig. 8. Potential profile at different rate capability of spinel LiGd0.5 Mn1.5 O4 active cathode for Li-ion batteries.

Fig. 9. Rate capability of () spinel LiMn2 O4 ; () LiGd0.5 Mn1.5 O4 active cathode materials at different currents for Li-ion batteries.

specific discharge capacity over the range of accelerated current rates signify an improvement in the intercalation/de-intercalation process of Li+ ions brought by gadolinium doping in the spinel LiMn2 O4 cathode active material. Fig. 9 shows the life cycling performance at different rate capability of spinel LiMn2 O4 and LiGd0.5 Mn1.5 O4 active cathode materials. The gadolinium doped spinel material is seen to exhibit excellent and stable discharge capacity over the range of cycles at all current rates signifying once more higher Li+ intercalation/de-intercalation in comparison to its control counterpart (LiMn2 O4 ). Also, the strong octahedral structure has minimized contraction of the unit cell after repeated cycles with good capacity retention. This reduction in capacity fading of LiGd0.5 Mn1.5 O4 can be related to a smaller dimensional variation between charged and discharged states [23]. The improved longterm cyclability brought by Gd doping is attributed to avoidance of the Jahn-Teller distortion effect. In particular the spinal lat-

tice is prevented from shrinking upon doping with Gd3+ , due to their high amount of lithium insertion/extractions on the larger crystal volume of electrode surface. Also, In this better cycling stability and less capacity fading is associated with their higher amount of lithium insertion/extractions to the larger crystal volume with less ionic dissolution on the electrode surface which as low electrolyte decomposition when compared to the pristine. Therefore The LiGd0.5 Mn1.5 O4 compound shows significantly better reversible capacities and electrochemical performance than the pristine LiMn2 O4 compound. 3.5. Electrochemical AC impedance spectroscopy characterization Fig. 10 displays a typical Nyquist plot of the ac-impedance measured after complete discharge at 50 cycles for pure LiMn2 O4 and LiGd0.5 Mn1.5 O4 . The impedance spectra were obtained within the




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Fig. 10. AC impedance spectra of spinel active cathode materials: (O) LiMn2 O4 ; (䊉) LiGd0.5 Mn1.5 O4 , simulated curves using the equivalent circuit in inset.

frequency range of 10−2 Hz to 105 Hz. In general, the impedance spectra of LiMn2 O4 and LiGd0.5 Mn1.5 O4 consists of a depressed arc followed by a straight tail line inclined at a constant angle. Earlier proposed models suggest that the distorted arc at the high frequency region consists of two semicircles coincidence on the inclined straight line at low frequencies [24]. The first semicircle at the high frequency region is ascribed to the SEI film and/or contact resistance, while the second semicircle at the medium frequency region is attributed to the charge-transfer impedance on the electrode/electrolyte interface, and the inclined line at an approximate 45◦ angle to the real axis corresponds to the lithium-ion diffusion kinetics in the electrolyte towards the electrodes [25]. The obtained ac impedance spectra of LiMn2 O4 and LiGd0.5 Mn1.5 O4 were fitted with Randles equivalent circuit models, based on the models the charge transfer resistance (Qct) were calculated to 81.5 and 50.2 , respectively. From Fig. 10, it is clearly visible that the diameter of the semicircle at the medium frequency region is drastically reduced in the case of LiGd0.5 Mn1.5 O4 , when compared to its counterpart LiMn2 O4 . This reveals that the LiGd0.5 Mn1.5 O4 material has lower charge-transfer impedance that results in dramatic improvement in terms of electronic conductivity and cycling stability even at higher current rates, and hence significant improvement in terms of cycling performance in comparison to Gd-free LiMn2 O4 . 4. Conclusions LiMn2 O4 and LiGd0.5 Mn1.5 O4 have been successfully synthesized by co-precipitation method followed by dual calcination first at 450 ◦ C for 5 h under air and after at 800 ◦ C for 12 h under Argon. XRD data revealed that the synthesized materials have cubic spinel structure (Fd3m) with slight increase in the average d-spacing and cubic cell parameters of the Gd doped member in comparison to pristine LiMn2 O4 spinel material. XPS studies showed that the valence state of Mn is higher in gadoliniumdoped spinel signifying a higher structural stability in comparison to LiMn2 O4 . FE-SEM and FE-TEM studies show that the average particle size of LiGd0.5 Mn1.5 O4 is lower than that of LiMn2 O4 . The charge-discharge characteristics at various C rates showed the stable behavior of Gd-doped LiGd0.5 Mn1.5 O4 cathode material, even at higher current rate, indicating that this material offers high rate capability when compared with spinel LiMn2 O4 mate-

rials with obvious practical advantages. The improved stability brought by Gd-substitution is largely attributed to avoidance of the Jahn-Teller distortion effect. After charge-discharge electrochemical cycling, the Gd-doped LiGd0.5 Mn1.5 O4 active cathode material demonstrated electrochemically stable interfacial properties due to apparently lower charge transfer resistance between the electrode and electrolyte. Therefore the LiGd0.5 Mn1.5 O4 compound or other similar to Gd doped spinel can provide better reversible capacities and electrochemical performance than the pristine LiMn2 O4 compound. Acknowledgement This work supported by the Department of Science and Technology (DST) − Science and Engineering Research Board (SERB) funded project by the Government of India (ECR/2017/000095). References [1] C. Jiang, Z. Tang, S. Deng, Y. Hong, S. Wang, Z. Zhang, RSC Adv. 7 (2017) 3746. [2] D. Arumugam, G. Paruthimal Kalaignan, K. Vediappan, C.W. Lee, Electrochim. Acta 55 (2010) 8439–8444. [3] L. Duan, X. Zhang, K. Yue, Y. Wu, J. Zhuang, W. Lü, Nanoscale Res. Lett. 12 (2017) 109. [4] H. Xia, Q. Xia, B. Lin, J. Zhu, J. Seo, Y.S. Meng, Nano Energy 22 (2016) 475–482. [5] S. Gopukumar, Y. Jeong, K.-B. Kim, Solid State Ionics 159 (2003) 223–232. [6] C. Julien, M.A. Camacho-Lopez, T. Mohan, S. Chitra, P. Kalyani, S. Gopukumar, Solid State Ionics 135 (2000) 241–248. [7] H. Kobayashi, S. Shigemura, M. Tabuchi, H. Sakaebe, K. Ado, H. Kageyama, A. Hirano, R. Kanno, M. Wakita, S. Morimotoand Nasu, J. Electrochem. Soc. 147 (2000) 960–966. [8] M. Zou, M. Yoshio, S. Gopukumar, J.-I. Yamaki, Chem. Mater. 17 (2005) 1284–1288. [9] J. Cho, Y.J. Kim, T.-J. Kim, B. Park, Chem. Mater. 12 (2000) 3788–3794. [10] W.T. Fu, D.J.W. Ijdo, J. Solid State Chem. 177 (2004) (2973-2973). [11] Y. Zhydachevskii, A. Durygin, A. Suchocki, A. Matkovskii, D. Sugak, G.B. Loutts, M.A. Noginov, J. Lumin. 109 (2004) 39–45. ˜ V. Palomares, I. Gil de Muro, L. Lezama, T. Rojo, J. [12] A. Iturrondobeitia, A. Goni, Power Sources 216 (2012) 482–488. [13] X. Liu, J. Wang, J. Zhang, S. Yang, J Mater Sci. Mater. Electron. 17 (2006) 865–870. [14] H.-H. Ko, L.-B. Chang, M. Jeng, P.-Y. Kuei, K.-Y. Horng, Jpn. Soc. Appl. Phys. 44 (2005) 3205–3209. [15] M. Marezio, P.D. Dernier, J.P. Remeika, J. Solid State Chem. 4 (1972) 11–17. [16] K. Vediappan, S.-J. Park, H.-S. Kim, C.W. Lee, J. Nanosci. Nanotechnol. 11 (2011) 865–869. [17] H. Yan, X. Huang, Z. Lu, H. Huang, R. Xue, L. Chen, J. Power Sources 68 (1997) 530–536.




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