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Effect of bication (Cu-Cr) substitution on the structure and electrochemical performance of LiMn2O4 spinel cathodes at low and high current rates
Accepted Manuscript Original article Effect of bication (Cu-Cr) substitution on the structure and electrochemical performance of LiMn2O4 spinel cathodes at low and high current rates Azhar Iqbal, Yousaf Iqbal, Abdul Majeed Khan, Safeer Ahmed PII: DOI: Reference:
S1319-6103(17)30092-3 http://dx.doi.org/10.1016/j.jscs.2017.07.011 JSCS 900
To appear in:
Journal of Saudi Chemical Society
Received Date: Revised Date: Accepted Date:
15 May 2017 17 July 2017 31 July 2017
Please cite this article as: A. Iqbal, Y. Iqbal, A.M. Khan, S. Ahmed, Effect of bication (Cu-Cr) substitution on the structure and electrochemical performance of LiMn2O4 spinel cathodes at low and high current rates, Journal of Saudi Chemical Society (2017), doi: http://dx.doi.org/10.1016/j.jscs.2017.07.011
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Effect of bication (Cu-Cr) substitution on the structure and electrochemical performance of LiMn2O4 spinel cathodes at low and high current rates Azhar Iqbala,b, Yousaf Iqbalb, Abdul Majeed Khanc, Safeer Ahmeda*, aDepartment of Chemistry, Quaid-i-Azam University,45320, Islamabad, Pakistan bInstitute of Chemical Sciences, University of Peshawar, Pakistan c General Studies Department, Jubail Industrial College,P/O Box 10099,Jubail Industrial City 31961, Kingdom of Saudi Arabia *Email: [email protected] , Tel: +92-051-90642145, Fax: +92-051-90642241 Abstract This report describes the detailed structural and electrochemical characterization of a series of low content (0.01 to 0.05) Cu-Cr bi-metal doped LiMn2O4 cathode material synthesized by solgel method. The structural and morphological features were described using XRD, SEM, TEM, EDAX and FTIR techniques. The electron transfer and its feasibility were discussed through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. The charge-discharge studies were performed to evaluate the capacity fading and rate capability. It was found that the electrochemical performance is very much dependent on the amount of Cu-Cr bi-metal doping and interestingly decreased the capacity fading with high cycleability. The sample with the least amount of dopants (i.e., LiCu0.01Cr0.01Mn1.98O4) demonstrated much improved capacity, cycleabilit y and high rate capability. The LiCu0.01Cr0.01Mn1.98O4 cathode exhibited a discharge capacity of 112 mAhg-1 a t v e r y f i r s t c yc l e a n d retained 93 mAhg-1 after 100 cycles at a C rate of 0.3. Further, t h e s a m e m a t e r i a l at very high current density (5C) r e t a i n e d 83% of the initial discharge capacity. The Cu-Cr doping stabilized the
spinel structure by suppressing the Jahn-Teller distortion effect and Mn dissolution and the resultant material showed the workability of the cathodes for devices which work at substantially high C-rate of 5C. Key Words: LiMn2O4; Cu-Cr low content doping; high rate capability; electrochemical properties; impedance. 1. Introduction. Because of their high energy density, good cycling performance and high working voltage, lithium-ion batteries have been widely adopted as the most promising energy source for portable electronics such as laptop computers, cell phones and digital cameras [1]. Lithium-ion batteries are composed of intercalation compounds as positive electrode materials, graphite or carbon as the negative electrode and an organic electrolyte as an ionic conductor but electronic insulator. Among the cathode materials, LiMn2O4 friendliness
owing to its economic and environmental
is highly studied material compared with LiCoO2 [2-4].
Spinel LiMn2O4
crystallizes in the cubic Fd3m space group composed of a cubic close packing of O atoms in which the lithium ions occupy the tetrahedral 8a sites while manganese ions are present at the octahedral 16d sites. Electrochemical extraction of lithium ions (Li+) f r o m the spinel LiMn2O4 occurs in two stages at approximately 2.9 and 4 V [5,6]. The extraction/insertion of Li+ ions from/into tetrahedral sites corresponds to 4V which in turn is the redox process of Mn 3+/Mn4+. The problem lies with the continuous decrease of these Li+ as this adversely affects the working of lithium ion battery. Consequently, LiMn2O4
suffers from severe capacity fade during charge/ discharge
cycling and poor electrochemical performance at high current rates, which limits its practical applications as cathode material for lithium-ion batteries [7,8]. The following three aspects are believed to be responsible for the capacity fade of LiMn2O4 during cycling; (i) Jahn-Teller distortion of the cubic spinel structure; (ii) Manganese dissolution due to disproportionation
reaction; (iii) Electrolyte decomposition at higher potential [9]. approaches,
doping
with
cations
to
substitute
part
Among
the
various
of Mn3+ ions at 16d sites is an
effective way to suppress the Jahn-Teller distortion and to improve the cycling performance of LiMn2O4 [10]. Multiple cation-substituted LiMn2O4 has been well reported [11,12] and it has been pointed out that co-doping has a synergistic effect on the improvement of the cycle life. Among the various metal cations that can partially substitute Mn3+ ions, Cr3+, similar in size to Mn3+ has been reported to form the robust spinel framework via Cr3+-driven increased covalency of Li-O-Mn bond. Furthermore, it has also been shown that Cr3+ doping facilitates the presence of lithium that is pinned in the Td site near the octahedral Mn ions and hence prevents the formation of a series of cation-ordered phases during charging [13]. Cr doping for spinel LiMn2O4 has been studied both alone and along with other metals [13-15]. In one of our recent reports the synergetic effect of Cr along with Ni resulted into remarkable improvement in capacity retention [16]. Thus, Cr3+ has been chosen as one of the dopants in the present study. It has also been reported that the activation barrier for Li diffusion in spinel LiMn2O4 can be lowered when small amount of Cu is doped, so that better rate capability can be obtained [14]. The role of Cu as a dopant is well established, particularly due to its high conductivity and stability improvement in the manganese oxide structures [17]. It is reported that Cu decreases the initial discharge capacity [18] however, when doped with other metal ions there is a considerable decrease in capacity fade [18,19]. Considering the characteristic features of the two metal ions as mentioned above and our recently published work [16] the Cr-Cu co-doped LiMn2O4 was investigated in this very report. The major purpose was to stabilize the spinel LiMn2O4 framework for the repeated charge/discharge cycling. The work describes the structural and morphological patterns of the bication (Cu-Cr) doped material followed by detailed electrochemical investigation while correlating the performance with structural changes. 2. Experimental
All the chemicals used w e r e o f h i gh p u r i t y . These involved lithium acetate (Li(CH3COO); Aldrich, 99.95%), manganese acetate (Mn(CH3COO)2; Aldrich, 98%), copper(II)acetate (Cu(OOCCH3)2; Alfa Aesar, 99.999%), chromium nitrate nonahydrate (Cr(NO3)2·9H2O; Aldrich, 99.99%) and ammonium hydroxide (NH3·H2O, 28-30 wt%) . 2.1 Synthesis of pure and Cu-Cr bi-metal doped LiMn2O4 LiMn2O4 and its Cu-Cr doped analogues (Li[CuxCryMn2-x-y]O4) (where, x = y = 0.01-0.05) were synthesized by sol-gel method using citric acid as a chelating agent. In a typical synthesis for the LiCu0.01Cr0.01Mn1.98O4, stoichiometric amount of lithium acetate (1.0 mol), manganese acetate (1.98 mol) copper acetate (0.01 mol), chromium nitrate nonahydrate (0.01 mol) and citric acid (3.0 mol) were separately dissolved in 50 mL de-ionized water. These solutions were mixed together to get a final solution having a total volume of 250 mL. Citric acid to metal ions molar ratio was kept at 1. Ammonium hydroxide was slowly added to this solution with constant stirring to control the pH at 6.0. The resultant solution was evaporated at 80ºC while being mechanically stirred with a magnetic stirrer for 5 h until a gel was obtained. The gel precursor obtained was dried in an oven for overnight at 120ºC to remove moisture and thereby obtained the dry mass. The powder obtained was first heated at 400ºC for 5 h and then at 750ºC for 10 h at a heating rate of 5ºC per min to obtain fine black powder of LiCu0.01Cr0.01Mn1.98O4. All the doped samples were also synthesized by the same procedure except that the stoichiometric amount of the dopants was varied. The summary of synthetic procedure is shown below in the Scheme 1.
Li(CH3CO2) +Mn(CH3CO2)2 + Cu/Cr-salt
Citric acid (aqueous solution)
(aqueous solutions)
Uniform mixing with stirring
Mixed aqueous solution
Aqueous Ammonia, pH adjusted at 6.0
Homogeneous solution
Stirring and heating at o
80 C o
Gel precursors
heating at 120 C in vacuum oven for 5 h o
Annealing at 400 C for 5 h o
LiCuxCryMn2-x-yO4 powder
& 750 C for 10 h
Scheme I: Flow chart for the synthesis of samples.
2.2. Instrumentation used for morphological characterization The products were characterized by scanning electron microscopy (SEM, Hitachi S-4800), energy- dispersive X-ray spectroscopy (EDAX, Horiba EMAX 7593-H), Thermo gravimetric analysis (TGA/DTA, PerkinElmer Diamond), Transmission electron microscopy (TEM, Tecnai G20 S-TWIN), X-ray diffraction (XRD,
Panalytical
X'Pert-Pro
MPD)
and
Fourier
transform infrared spectroscopy (FT-IR, PE2000). Chemical composition of the pure and
doped LiMn2O4 was determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Perkin Elmer Optima 5300DV). 2.3. The procedure and instrumentation used for electrochemical characterization The electrochemical properties of the products were analyzed by making CR2032 coin-type cells with lithium metal as the negative electrode. The active material, acetylene black and polyvinylidene fluoride were mixed in a weight ratio of 80:10:10. N-methyl-2-pyrrolidone was used as a solvent to make the slurry. The blended slurry was then cast onto an aluminum foil current collector and dried at 120°C for 12 h in a vacuum oven. Then, circular cathode discs were punched from the aluminum foil. The punched cathodes were weighed to determine the amount of active materials before being loaded into coin-type cells. For all the fabricated cells, the average film thickness (38 µm) and loading of different casts (5.77mg) with 80% active material were kept the same. The coin cells were assembled in an argon-filled glove box. The electrolyte composed of 1 M LiPF6 dissolved in ethylene carbonate/dimethyl carbonate (1:1 in volume). Galvanostatic charge/discharge experiments were performed between 3.0 and 4.8 V using LAND battery testing system (CT2001A). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed on an electrochemical workstation (CHI 660C). 3. Results and Discussion The TGA/ DTA curves for the pure LiMn2O4 and the representative bi-metal doped sample (LiCu0.03Cr0.03Mn1.94O4 ) are shown in Fig.1. For the pure LiMn2O4 (Fig. 1a), the TGA curve depicts two weight loss regions. The initial weight loss of about 10% up to 180 °C can be attributed to the removal of moisture accompanied by decomposition of citric acid. The major weight loss region observed between 180 °C and 290 ºC corresponding to a weight loss of 62%, which is also accompanied by a sharp exothermic peak at 285 ºC in the DTA curve, is associated
with the decomposition of the acetate precursors of Li and Mn salts. It is apparent from the figure that no appreciable weight loss takes place after 300 °C and the TGA curve becomes flat depicting the formation of the pure LiMn2O4. The Cu-Cr bi-metal doped LiMn2O4 also showed the same thermal behavior except that the sharp exothermic peak appeared at 345 ºC in the DTA. From the thermal analysis, it can be concluded that the spinel LiMn2O4 starts to form at about 300 to 350ºC, but it has also been reported that at low temperature (250 and 450ºC) some other impurities such as α-Mn2O3 and LiMn2O3 are also present [1]. So, all the pure and doped samples were synthesized at 750ºC to get phase pure and well crystallized spinel compounds. The crystalline structure of the pure and Cu-Cr substituted LiMn2O4 was investigated by X-ray powder diffraction (Fig.2). XRD patterns of all the synthesized samples depicted well-defined peaks, pointing towards the formation of single-phase spinel structure. No extra reflections are observed in the XRD patterns of the Cu-Cr doped samples. The similar XRD patterns of spinal structure for the doped samples indicate that Cu and Cr have entered the lattice at 16d positions to substitute manganese cations. All the XRD patterns can be assigned to a well-crystallized spinel LiMn2O4 (JCPDS No. 35-0782). The Bragg peaks of the pure and doped LiMn2O4 are indexed to a cubic system with space group Fd3m in which the lithium ions occupy the tetrahedral (8a) sites and the Mn3+/ Mn4+ cations, as well as the doped metal ions reside at the octahedral (16d) sites [3]. The lattice constant and t h e cell volume of the pure and Cu-Cr doped LiMn2O4 calculated from the XRD data are shown in Table 1. Shrinkage of the unit cell is observed, however, with increasing dopants concentration the lattice parameters also increased. This may be due to the difference in the effective ionic radii of the doped metals and manganese cations. The concentrations of Li, Mn, Cr and Ni are determined using ICP-OES as shown in Table 2. The stoichiometric compositions of all the synthesized samples are in good agreement with the nominal compositions.
Table 1 and Table 2 FTIR spectroscopy has been proven to be a very useful tool to quantitatively resolve cation ordering in the spinel LiMn2O4 [20]. Fig. 3 shows the FTIR spectra of the pure and Cu-Cr doped LiMn2O4 in the wave number range of 400-4000 cm-1. The characteristic peaks observed in the range of 514-620 cm-1 are assigned to the stretching vibration of MO6 octahedral groups [21, 22]. It means that no structural change takes place for the low content doping in the spinel LiMn2O4. However, there is a slight frequency shift towards higher wave number pointing to the successful doping of the metal cations. The morphology of the as-synthesized nanostructures was evaluated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images shown in Fig. 4 confirm the low particles agglomeration for the Li[CuxCryMn2-xy]O4
samples compared to the pure LiMn2O4. This further indicated that crystallites
agglomeration to form bigger particles during sol-gel synthesis has not occurred for the doped samples. TEM images (Fig. 5) further show that unlike pure LiMn2O4, Cu-Cr doped samples are composed of more uniform and smaller particles having individual grain morphology with well separated grain boundaries. High-resolution TEM (HRTEM) image of a representative particle (LiCu0.01Cr0.01Mn1.98O4) demonstrates (inset of Fig. 5b) highly crystalline nature. The observed lattice spacing of 0.47 nm corresponds to the (111) plane of the cubic spinel LiMn2O4. The average particle size calculated was found varying in the range of 100 to 150 nm. Though the particles themselves are a bit larger to be exactly considered as nanoparticles or for that matter nanostructures but their surface topology bears few (one to five) nano-surfaces. T he energy dispersive X-ray spectroscopy (EDAX) profiles of the pure and Cu-Cr doped LiMn2O4 as given in Fig. S1 (Supplementary data) further confirmed the presence of all the metal cations in the synthesized samples. The electrochemical performances of the synthesized pure and Cu-Cr doped LiMn2O4 were evaluated as cathode materials for lithium-ion batteries. Coin cells fabricated with the synthesized
cathode materials and lithium metal anode were galvanostatically charged and discharged at a current density 43 mAg-1 (0.3 C) between 3.0 V and 4.8 V. The initial charge/discharge curves of the pure and doped samples are shown in Fig. 6 (A). In the given potential range, the charge/discharge curves of all the synthesized samples correspond to the characteristic voltage profile of the spinel LiMn2O4 structure in which lithium ions are located at the tetrahedral sites while the manganese ions occupied the octahedral sites [23-25]. The presence of two plateaus in the c h a r g e / discharge curves at around 4.0 V and 4.1 V vs. Li/ Li+ indicate two step oxidation/ reduction process for the extraction /insertion of lithium ions into the spinel structure. In other words, during discharge, lithium ions are inserted from anode to cathode whereas during charging lithium ions are extracted from anode. It has been reported that the higher voltage plateau at about 4.1 V corresponds to two phase transition of λ-MnO2/ Li0.5Mn2O4 vs. Li/ Li+, while the second plateau at lower potential (about 4.0 V) indicates single phase transition between Li0.5Mn2O4/ LiMn2O4. By controlling the formation of these unstable phases, the electrochemical performance of LiMn2O4 based cathode materials could be improved [26, 27]. Compared to the un-doped LiMn2O4, the boundaries of the two plateaus in the discharge curves of Cu-Cr doped cathode materials are not highly sharp. This may point towards the fact that the doping elements have stabilized the spinel structure by suppressing the Jahn-Teller effect which is considered to be one of the main causes of capacity fading in LiMn2O4 [27].
Cycling performance of the synthesized samples is shown in Fig. 6 (B). An improvement in cycling performance is observed for the doped samples. For the pure LiMn2O4, 72% of the initial discharge capacity was retained after 100 cycles. Factors such as the structural degradation due to co-operative John-Teller distortion effect and the dissolution of Mn into the electrolyte during charge/discharge cycling have been suggested to be the main causes for capacity fade [14]. For Cu-Cr doped samples, the initial discharge capacities for the increasing amount (0.01, 0.02,
0.03, 0.04 and 0.05) of Cu and Cr were 112, 112, 97, 56 and 68 mAhg-1, respectively. After 100 charge/ discharge cycles, the synthesized Cu-Cr doped LiMn2O4
cathode materials delivered
discharge capacities of 93, 82, 72, 47 and 60 mAhg-1 that correspond to the capacity retention of 83, 73, 74, 84 and 88% respectively, of the initial discharge capacities. It can be seen that compared to the pure LiMn2O4 that has 72% capacity retention after 100 cycles, the cycling performance of Cu-Cr doped samples is better. Although LiCu0.04Cr0.04Mn1.92O4 and LiCu0.05Cr0.05Mn1.90O4 showed higher capacity retention but their initial discharge capacities were q u i t e low. This may be due to the relatively higher amount of the doping elements. The sample with the lowest Cu-Cr contents i.e., LiCu0.01Cr0.01Mn1.98O4 showed the best electrochemical performance both in terms of the initial discharge capacity as well as the cycling stability. The large binding energy of CrO2 (1142 kJ/ mol) compared to MnO2 (946 kJ/ mol) has resulted in the stabilization of the octahedral sites and thus enhanced the cycling performance [15]. Furthermore, copper doping in the spinel oxide has been reported to
induce
higher
electronic conductivity and lower diffusion barrier for the intercalation/ de- intercalation of Li+ ions that resulted in higher rate capability [14]. Coulombic efficiency for all the samples is also given in the Fig. 6(B). Compared to the pure LiMn2O4, high initial coulombic efficiencies (90% for the pure LiMn2O4 and more than 93% for all the doped samples) were obtained for the doped samples. The irreversible capacity loss for both the pure and doped spinels can be seen from the first charge/discharge curves shown in Fig. 6(A). It is evident that doped spinels exhibit low irreversible capacity loss due to irreversible film formation reactions as compared to the pure spinel. Further, the irreversible capacity is present only during the first charge/discharge cycle, as nearly 100% coulombic efficiency was achieved on the subsequent cycles. Table 3 summarizes the electrochemical performances of the pure and Cu-Cr doped
LiMn2O4.
It is evident that, although the doped samples have low initial discharge capacities
than the pure LiMn2O4, but they exhibited better cycling performance compared to the pure LiMn2O4. Furthermore, the decrease in the initial discharge capacity with increasing concentration of the doping elements indicated that even for the doped materials, Mn3+ effectively contributed to the specific capacity during the electrochemical reaction. For all the doped series, sample with the lowest doping contents i.e., LiCu0.01Cr0.01Mn1.98O4 performed relatively better than the higher doping concentrations. This means that among the synthesized Cu-Cr doped LiMn2O4, the optimum dopant level in terms of capacity and cycling stability is approximately x = 0.01, y = 0.01. A close look on the experimental CV profiles (Fig.7 ) showed that the charge transfer redox peaks are sharper and with large currents for x = y= 0.01 mol dopants content than for the higher amounts. It reveals that, this very amount of the dopants kinetically facilitates the Li+ intercalation/ deintercalation process, while a bit higher amounts start decelerating the process. A second evidence comes from EIS data (discussed in following paragraphs) giving the smallest Rct values for 0.01 mol content of dopants and this value starts increasing for higher amounts of the dopants. Thus the overall picture supports the same conclusion as stated above for charge-discharge measurements. Table 3 The better cycling stability for the doped LiMn2O4 at the expense of specific capacity is mainly attributed to the inhibition of the Jahn-Teller effect on deep discharge of the various bi-metal doped LiMn2O4 cathode materials. Furthermore, unlike pure LiMn2O4 that upon complete extraction of Li from the spinel matrix results in an unstable λ-MnO2, all the Li cannot be extracted from the doped samples. The presence of this residual Li in the delithiated bi-metal doped spinel LiMn2O4 is considered to play a significant role in enhancing the structural stability to the repeated Li insertion/ extraction processes [28].
To evaluate the effect of Cu-Cr doping on the structural stabilization of LiMn2O4, cyclic voltammetric (CV) measurements were performed (Fig. 7). CV curves were recorded over the potential range of 3.4-4.8 V at a scan rate of 0.1 mV s-1.The oobtained anodic and cathodic peaks at around 4.0 V correspond to Mn3+/ Mn4+ redox couple [13]. The anodic and cathodic peaks observed in the CV curves represent the reversible redox reactions that correspond to Li ions removal and insertion from/ into the spinel framework. Compared to the pure sample, the redox peaks for the doped compounds are sharp and showed well-defined splitting; indicating the structural stabilization of the doped spinel LiMn2O4 materials [29]. From the CV curves, it can also be seen that for the doped samples, the oxidation and reduction peaks are much closer, the peak current is increased and the peak width is narrowed which means that the internal resistance for the doped LiMn2O4 cathodes is minimum and the diffusion rate of Li+ ions is the fastest compared to the pure LiMn2O4. These results substantiated that the polarization of LiMn2O4 has been reduced due to the improved kinetics of Li+ ions into the spinel LiMn2O4 framework. Thus, improved electrochemical performance for Cu-Cr doped LiMn2O4 cathodes is observed. Cyclic voltammetric results are in good agreement with the cycling performance. To further understand the advantage of Cu-Cr doped LiMn2O4 as the cathode material for lithium-ion batteries, electrochemical impedance spectroscopy (EIS) measurement were carried out (Fig. 8). In the Fig. 8A the relative impedance profile for pure and doped LiMn2O4 samples is shown. Unlike pure LiMn2O4, the lower charge transfer resistance (Rct) values for the Cu-Cr doped electrodes clearly indicate the improved electronic conductivity and the high Li+ transportation speed across the active electrode material and the electrolyte. Small impedance is favourable for the efficient extraction/ insertion of Li ions from/ into the spinel LiMn2O4 during charge/ discharge process. This resulted in the improved high rate cycling performance. Fig 8B describes EIS spectra of the pure and LiCu0.01Cr0.01Mn1.98O4 samples along with simulation
spectra. The impedance spectra were recorded after five charge/discharge cycles. The semicircle portion describes the charge transfer resistance (Rct) while the straight line in the low frequency region corresponds to Warburg impedance (W1) that represents the lithium ion diffusion in the bulk of the electrode. The circuit element CPE (constant phase element) is used to model the double layer capacitance. The intercept of the semicircle with Zreal axis at high frequency refers to the ohmic electrolyte resistance (R1). From the equivalent circuit the corresponding values obtained for R1 are 6.3 Ω and 2.4 Ω and that of Rct are 217 Ω and 121 Ω , for undoped and doped samples, respectively. The higher rate performance of LiCu0.01Cr0.01Mn1.98O4 can also be attributed to the smaller crystallite size than the pure sample. It is well established that for large particles size, it takes long time for Li+ ions to diffuse into the interior of the crystallite. The impedance analysis suggests that low content Cu and Cr substitution in LiMn2O4 may help to form a thinner surfaceelectrolyte interface (SEI) layer that possess lower ohmic resistance. From Figs. 7 & 8 it is obvious that the charge transfer process is facile having maximum current and with minimum charge transfer resistance, for LiCu0.01Cr0.01Mn1.98O4 among all the doped samples and thus corroborating the charge discharge results as conferred above. Another important aspect of the battery material is its working at high current rates which is usually not done and performance at low C-rates is reported. Here we also evaluated the rate performances, which present the electrochemical behavior under different charge-discharge rates or current densities. To test the rate and cycling performance we charged and discharged the pure spinel LiMn2O4 and LiCu0.01Cr0.01Mn1.98O4 cathodes in the voltage range of 3.0 V- 4.8 V at Crates from 0.1 C to 5 C (Fig. 9). As can be seen, LiCu0.01Cr0.01Mn1.98O4 cathode exhibited high capacity retention than the pure LiMn2O4. As a comparison, LiCu0.01Cr0.01Mn1.98O4 cathode material was able to deliver initial discharge capacity of 112 mAhg-1 at 0.1 C. However,
cycling at high current rate (5 C) provided 93 mAhg-1 discharge capacity, which is about 83% of the initial discharge capacity at 0.1 C. After deep cycling at high current rates, a discharge capacity of 106 mAhg-1 was recovered that is 93 % of the initial discharge capacity. This electrochemical performance was superior to that of the pure LiMn2O4 which showed only 41 % capacity retention at high current rate of 5C. The improved rate performance for LiCu0.01Cr0.01Mn1.98O4 may be due to the higher electronic and ionic conductivities as is clear from the EIS and CV results. It has been reported that although Cu doped spinel provides lower discharge capacity, it results in stable electrochemical cycling [14]. The higher octahedral stabilization energy of Cr3+ may also contribute in the stability of the LiMn2O4 structure [30]. The current work differs in the aspect that it evidently showed that the low content doping can be utilized to tune the properties of material. Further, the prepared material performs superbly at h i gh C rates. The higher conductivity, small particle size and good structural stability resulted in the improved electrochemical performance of Cu-Cr doped LiMn2O4. 4. Conclusions In this work, a series of Cu-Cr doped LiMn2O4 was synthesized by a facile and cost effective sol-gel process. As the cathode materials for lithium-ion batteries, the Cu-Cr doped LiMn2O4 structures showed significantly improved cycling performance compared to the pure sample. Among all the synthesized samples, LiCu0.01Cr0.01Mn1.98O4 exhibited excellent electrochemical performance. This includes a capacity of 112 mAhg-1 at 0.1 C, excellent cyclic stability (83 % capacity retention after 100 charge/ discharge cycles), and a good rate capability. Performance of the doped material at high current rate i.e. (5C) was even better than that of undoped LiMn2O4 material.
Significantly, upgraded charge-discharge response could be
attributed to the adjustment of the appropriate amount of Cu and Cr into the matrix that resulted in the stabilization of spinel LiMn2O4 framework by inhibiting both the Mn dissolution
into the electrolyte and the Jahn-Teller distortion effect. The improved electrochemical performance of LiMn2O4 with very low content doping of Cu and Cr and to sustain the capacity even at very high C-rate is encouraging and have potential for further investigations. Acknowledgement Authors gratefully acknowledge financial support from the Higher Education Commission (HEC) Pakistan. Authors are also grateful to Prof. Z.Tang (National Canter for Nanoscience and Technology, No. 11, Beiyitiao, Zhongguancun, Beijing, China) for providing the lab facilities for this work. References [1]
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LiMn1.4Cr0.2Ni0.4O4as 5 V cathode material by citric-acid-assisted method , J. Phys. Chem. Solids 70 (2009) 153-157. [16] A. Iqbal, Y.Iqbal, A.M.Khan, S.Ahmed, Low content Ni and Cr co-doped LiMn2O4 with enhanced capacity retention, Ionics, DOI 10.1007/s11581-017-2062-5 [17] J. J. Xu, Z. J. Yang, G. Jain, Effect of Copper Doping on Intercalation Properties of Amorphous Manganese Oxides Prepared by Oxidation of Mn(II) Precursors, Electrochem. Solid-State Lett. 5 (2002), A223-A226. [18] H. Sahan, H. Göktepe, S. Patat, Synthesis and Cycling Performance of Double Metal Doped LiMn2O4 Cathode Materials for Rechargeable Lithium Ion Batteries, Inorganic Materials, 44 ( 2008) 420–425. [19] M/C. Yang, B. Xu, J.H. Cheng, C. J. Pan, B.J. Hwang, Y. S. Meng, Electronic, Structural, and Electrochemical Properties of LiNixCuyMn2_x_yO4 (0 < x < 0.5, 0 < y < 0.5) HighVoltage Spinel Materials, Chem. Mater. 23 (2011) 2832–2841. [20] R. Alcá, M. Jaraba, P. Lavela, J.L. Tirado, E. Zhecheva, R. Stoyanova, Changes in the Local Structure of LiMgyNi0.5-y Mn1.5O4 Electrode Materials during Lithium Extraction, Chem. Mater. 16 (2004) 1573-1579. [21] C. Wu, Z. Wang, F. Wu, L. Chen, X. Huang, Spectroscopic studies on cation-doped spinel LiMn2O4 for lithium ion batteries, Solid State Ionics 144 (2001) 277-285. [22] B. Ammundsen, G. R. Burns, M. S. Islam, H. Kanoh, and J. Roziere, Lattice Dynamics and Vibrational Spectra of Lithium Manganese Oxides: A Computer Simulation and Spectroscopic Study, J. Phys. Chem B 103(1999) 5175. [23] C. Julien, S. Ziolkiewicz, M. Lemal and M. Massot, J. Mater, Synthesis, structure and
electrochemistry of LiMn2 − yAlyO4 prepared by a wet-chemistry, Chem 11 (2001) 1837. [24] W. Xu, A. Yuan and Y. Wang, Electrochemical studies of LiCrxFexMn2−2xO4 in an aqueous electrolyte, J Solid State Electrochem16 (2012) 429. [25] Y. L. Ding, J. Xie, G. S. Cao, T. J. Zhu, H. M. Yu and X. B. Zhao, Enhanced ElevatedTemperature Performance of Al-Doped Single-Crystalline LiMn2O4 Nanotubes as Cathodes for Lithium Ion Batteries, J. Phys. Chem. C 115 (2011) 9821. [26] Y. L, Ding, X. B. Zhao, J. Xie, G. S. Cao, T. J. Zhu, H. M. Yu and C. Y. Sun, J. Mater, Double-shelled hollow microspheres of LiMn2O4 for high-performance lithium ion batteries, Chem 21(2011) 9475. [27] L. Xiong, Y. Xu, C. Zhang, Z. Zhang and J. Li, Electrochemical properties of tetravalent Tidoped spinel LiMn2O4 , J Solid State Electrochem 151 (2011) 263. [28] R. J. Gummow, A. de. Kock and M. M. Thackeray, Improved capacity retention in rechargeable 4 V lithium/lithium-manganese oxide (spinel) cells, Solid State Ionics 69 (1994) 59. [29] B. L. He, W. J. Zhou, Y. Y. Liang, S. J. Bao and H. L. Li, Synthesis and electrochemical properties of chemically substituted LiMn2O4 prepared by a solution-based gel method, J. Colloid Int. Sci 300 (2006) 633. [30] R. Thirunakaran,B. R. Babu, N. Kalaiselvi, P. Periasamy, T. P. Kumar,N. G. Renganathan, M. Raghavan, N. Muniyandi and Bull. Mater, Electrochemical behaviour of LiMyMn2−yO4 (M = Cu, Cr; 0 ≤y ≤ 0.4), Sci 24(2001) 51.
Figure Captions Fig.1 TGA/DTA analysis of (a) LiMn2O4 (b) LiCu0.03Cr0.03Mn1.94O4 Fig.2 XRD patterns of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4 Fig.3 FTIR Spectra of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4 Fig.4 SEM images of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4 Fig.5 TEM images of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4. The inset in panel (b) is the corresponding HRTEM image of LiCu0.01Cr0.01Mn1.98O4 Fig.6 (A) Initial Discharge curves and (B) Capacity and coulombic efficiency vs. cycle number plots for (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4 Fig.7 Cyclic voltammograms (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4 Fig.8(A) Nyquist plots for (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4 (B) Experimental and simulated Nyquist plots for (a) and (b) samples with equivalent circuit diagram. Fig.9 Cycling performance at various rates for the pristine LiMn2O4 and LiCu0.01Cr0.01Mn1.98O4
Supplementary Data:
Fig.S1. EDAX profiles of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4
Table 1 Lattice parameter ‘a’ and unit cell volume for spinel LiMn2O4 and LiCuxCryMn2-x-yO4 compounds. S. No
Sample
Lattice constant ‘a’
Unit cell volume
(Å)
(Å3)
1
LiMn2O4
8.2478
561.06
2
LiCu0.01Cr0.01Mn1.98O4
8.2372
558.90
3
LiCu0.02Cr0.02Mn1.96O4
8.2384
559.15
4
LiCu0.03Cr0.03Mn1.94O4
8.2460
560.69
5
LiCu0.04Cr0.04Mn1.92O4
8.2467
560.84
6
LiCu0.05Cr0.05Mn1.90O4
8.2557
562.68
Table 2 The ICP-OES results of the pure and Cu-Cr doped LiMn2O4.
S. No
Nominal composition
Experimental composition
1
LiMn2O4
Li0.98Mn1.98O4
2
LiCu0.01Cr0.01Mn1.98O4
Li0.983Cu0.013Cr0.014Mn1.977O4
3
LiCu0.02Cr0.02Mn1.96O4
Li0.972Cu0.018Cr0.019Mn1.966O4
4
LiCu0.03Cr0.03Mn1.94O4
Li1.051Cu0.027Cr0.028Mn1.936O4
5
LiCu0.04Cr0.04Mn1.92O4
Li1.012Cu0.038Cr0.037Mn1.925O4
6
LiCu0.05Cr0.05Mn1.90O4
Li1.054Cu0.049Cr0.048Mn1.893O4
Table 3 Electrochemical Performance of the pure and Cu-Cr doped LiMn2O4. S.No
Sample
Specific capacity (mAhg-1) at 1st cycle
Specific capacity
Capacity
(mAhg-1) after 100
Retention
cycles
(%)
1
LiMn2O4
122
88
72
2
LiCu0.01Cr0.01Mn1.98O4
112
93
83
3
LiCu0.02Cr0.02Mn1.96O4
112
82
73
4
LiCu0.03Cr0.03Mn1.94O4
97
72
74
5
LiCu0.04Cr0.04Mn1.92O4
56
47
84
6
LiCu0.05Cr0.05Mn1.90O4
68
60
88
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
50nm
Figure 6
Figure 7
Figure 8
Figure 9
32
Graphical abstract 01
Effect of bication (Cu-Cr) substitution on the structure and electrochemical performance of LiMn2O4 spinel cathodes at low and high current rates Azhar Iqbala,b, Yousaf Iqbalb, Abdul Majeed Khanc, Safeer Ahmeda*,
33
S1319-6103(17)30092-3 http://dx.doi.org/10.1016/j.jscs.2017.07.011 JSCS 900
To appear in:
Journal of Saudi Chemical Society
Received Date: Revised Date: Accepted Date:
15 May 2017 17 July 2017 31 July 2017
Please cite this article as: A. Iqbal, Y. Iqbal, A.M. Khan, S. Ahmed, Effect of bication (Cu-Cr) substitution on the structure and electrochemical performance of LiMn2O4 spinel cathodes at low and high current rates, Journal of Saudi Chemical Society (2017), doi: http://dx.doi.org/10.1016/j.jscs.2017.07.011
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Effect of bication (Cu-Cr) substitution on the structure and electrochemical performance of LiMn2O4 spinel cathodes at low and high current rates Azhar Iqbala,b, Yousaf Iqbalb, Abdul Majeed Khanc, Safeer Ahmeda*, aDepartment of Chemistry, Quaid-i-Azam University,45320, Islamabad, Pakistan bInstitute of Chemical Sciences, University of Peshawar, Pakistan c General Studies Department, Jubail Industrial College,P/O Box 10099,Jubail Industrial City 31961, Kingdom of Saudi Arabia *Email: [email protected] , Tel: +92-051-90642145, Fax: +92-051-90642241 Abstract This report describes the detailed structural and electrochemical characterization of a series of low content (0.01 to 0.05) Cu-Cr bi-metal doped LiMn2O4 cathode material synthesized by solgel method. The structural and morphological features were described using XRD, SEM, TEM, EDAX and FTIR techniques. The electron transfer and its feasibility were discussed through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. The charge-discharge studies were performed to evaluate the capacity fading and rate capability. It was found that the electrochemical performance is very much dependent on the amount of Cu-Cr bi-metal doping and interestingly decreased the capacity fading with high cycleability. The sample with the least amount of dopants (i.e., LiCu0.01Cr0.01Mn1.98O4) demonstrated much improved capacity, cycleabilit y and high rate capability. The LiCu0.01Cr0.01Mn1.98O4 cathode exhibited a discharge capacity of 112 mAhg-1 a t v e r y f i r s t c yc l e a n d retained 93 mAhg-1 after 100 cycles at a C rate of 0.3. Further, t h e s a m e m a t e r i a l at very high current density (5C) r e t a i n e d 83% of the initial discharge capacity. The Cu-Cr doping stabilized the
spinel structure by suppressing the Jahn-Teller distortion effect and Mn dissolution and the resultant material showed the workability of the cathodes for devices which work at substantially high C-rate of 5C. Key Words: LiMn2O4; Cu-Cr low content doping; high rate capability; electrochemical properties; impedance. 1. Introduction. Because of their high energy density, good cycling performance and high working voltage, lithium-ion batteries have been widely adopted as the most promising energy source for portable electronics such as laptop computers, cell phones and digital cameras [1]. Lithium-ion batteries are composed of intercalation compounds as positive electrode materials, graphite or carbon as the negative electrode and an organic electrolyte as an ionic conductor but electronic insulator. Among the cathode materials, LiMn2O4 friendliness
owing to its economic and environmental
is highly studied material compared with LiCoO2 [2-4].
Spinel LiMn2O4
crystallizes in the cubic Fd3m space group composed of a cubic close packing of O atoms in which the lithium ions occupy the tetrahedral 8a sites while manganese ions are present at the octahedral 16d sites. Electrochemical extraction of lithium ions (Li+) f r o m the spinel LiMn2O4 occurs in two stages at approximately 2.9 and 4 V [5,6]. The extraction/insertion of Li+ ions from/into tetrahedral sites corresponds to 4V which in turn is the redox process of Mn 3+/Mn4+. The problem lies with the continuous decrease of these Li+ as this adversely affects the working of lithium ion battery. Consequently, LiMn2O4
suffers from severe capacity fade during charge/ discharge
cycling and poor electrochemical performance at high current rates, which limits its practical applications as cathode material for lithium-ion batteries [7,8]. The following three aspects are believed to be responsible for the capacity fade of LiMn2O4 during cycling; (i) Jahn-Teller distortion of the cubic spinel structure; (ii) Manganese dissolution due to disproportionation
reaction; (iii) Electrolyte decomposition at higher potential [9]. approaches,
doping
with
cations
to
substitute
part
Among
the
various
of Mn3+ ions at 16d sites is an
effective way to suppress the Jahn-Teller distortion and to improve the cycling performance of LiMn2O4 [10]. Multiple cation-substituted LiMn2O4 has been well reported [11,12] and it has been pointed out that co-doping has a synergistic effect on the improvement of the cycle life. Among the various metal cations that can partially substitute Mn3+ ions, Cr3+, similar in size to Mn3+ has been reported to form the robust spinel framework via Cr3+-driven increased covalency of Li-O-Mn bond. Furthermore, it has also been shown that Cr3+ doping facilitates the presence of lithium that is pinned in the Td site near the octahedral Mn ions and hence prevents the formation of a series of cation-ordered phases during charging [13]. Cr doping for spinel LiMn2O4 has been studied both alone and along with other metals [13-15]. In one of our recent reports the synergetic effect of Cr along with Ni resulted into remarkable improvement in capacity retention [16]. Thus, Cr3+ has been chosen as one of the dopants in the present study. It has also been reported that the activation barrier for Li diffusion in spinel LiMn2O4 can be lowered when small amount of Cu is doped, so that better rate capability can be obtained [14]. The role of Cu as a dopant is well established, particularly due to its high conductivity and stability improvement in the manganese oxide structures [17]. It is reported that Cu decreases the initial discharge capacity [18] however, when doped with other metal ions there is a considerable decrease in capacity fade [18,19]. Considering the characteristic features of the two metal ions as mentioned above and our recently published work [16] the Cr-Cu co-doped LiMn2O4 was investigated in this very report. The major purpose was to stabilize the spinel LiMn2O4 framework for the repeated charge/discharge cycling. The work describes the structural and morphological patterns of the bication (Cu-Cr) doped material followed by detailed electrochemical investigation while correlating the performance with structural changes. 2. Experimental
All the chemicals used w e r e o f h i gh p u r i t y . These involved lithium acetate (Li(CH3COO); Aldrich, 99.95%), manganese acetate (Mn(CH3COO)2; Aldrich, 98%), copper(II)acetate (Cu(OOCCH3)2; Alfa Aesar, 99.999%), chromium nitrate nonahydrate (Cr(NO3)2·9H2O; Aldrich, 99.99%) and ammonium hydroxide (NH3·H2O, 28-30 wt%) . 2.1 Synthesis of pure and Cu-Cr bi-metal doped LiMn2O4 LiMn2O4 and its Cu-Cr doped analogues (Li[CuxCryMn2-x-y]O4) (where, x = y = 0.01-0.05) were synthesized by sol-gel method using citric acid as a chelating agent. In a typical synthesis for the LiCu0.01Cr0.01Mn1.98O4, stoichiometric amount of lithium acetate (1.0 mol), manganese acetate (1.98 mol) copper acetate (0.01 mol), chromium nitrate nonahydrate (0.01 mol) and citric acid (3.0 mol) were separately dissolved in 50 mL de-ionized water. These solutions were mixed together to get a final solution having a total volume of 250 mL. Citric acid to metal ions molar ratio was kept at 1. Ammonium hydroxide was slowly added to this solution with constant stirring to control the pH at 6.0. The resultant solution was evaporated at 80ºC while being mechanically stirred with a magnetic stirrer for 5 h until a gel was obtained. The gel precursor obtained was dried in an oven for overnight at 120ºC to remove moisture and thereby obtained the dry mass. The powder obtained was first heated at 400ºC for 5 h and then at 750ºC for 10 h at a heating rate of 5ºC per min to obtain fine black powder of LiCu0.01Cr0.01Mn1.98O4. All the doped samples were also synthesized by the same procedure except that the stoichiometric amount of the dopants was varied. The summary of synthetic procedure is shown below in the Scheme 1.
Li(CH3CO2) +Mn(CH3CO2)2 + Cu/Cr-salt
Citric acid (aqueous solution)
(aqueous solutions)
Uniform mixing with stirring
Mixed aqueous solution
Aqueous Ammonia, pH adjusted at 6.0
Homogeneous solution
Stirring and heating at o
80 C o
Gel precursors
heating at 120 C in vacuum oven for 5 h o
Annealing at 400 C for 5 h o
LiCuxCryMn2-x-yO4 powder
& 750 C for 10 h
Scheme I: Flow chart for the synthesis of samples.
2.2. Instrumentation used for morphological characterization The products were characterized by scanning electron microscopy (SEM, Hitachi S-4800), energy- dispersive X-ray spectroscopy (EDAX, Horiba EMAX 7593-H), Thermo gravimetric analysis (TGA/DTA, PerkinElmer Diamond), Transmission electron microscopy (TEM, Tecnai G20 S-TWIN), X-ray diffraction (XRD,
Panalytical
X'Pert-Pro
MPD)
and
Fourier
transform infrared spectroscopy (FT-IR, PE2000). Chemical composition of the pure and
doped LiMn2O4 was determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Perkin Elmer Optima 5300DV). 2.3. The procedure and instrumentation used for electrochemical characterization The electrochemical properties of the products were analyzed by making CR2032 coin-type cells with lithium metal as the negative electrode. The active material, acetylene black and polyvinylidene fluoride were mixed in a weight ratio of 80:10:10. N-methyl-2-pyrrolidone was used as a solvent to make the slurry. The blended slurry was then cast onto an aluminum foil current collector and dried at 120°C for 12 h in a vacuum oven. Then, circular cathode discs were punched from the aluminum foil. The punched cathodes were weighed to determine the amount of active materials before being loaded into coin-type cells. For all the fabricated cells, the average film thickness (38 µm) and loading of different casts (5.77mg) with 80% active material were kept the same. The coin cells were assembled in an argon-filled glove box. The electrolyte composed of 1 M LiPF6 dissolved in ethylene carbonate/dimethyl carbonate (1:1 in volume). Galvanostatic charge/discharge experiments were performed between 3.0 and 4.8 V using LAND battery testing system (CT2001A). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed on an electrochemical workstation (CHI 660C). 3. Results and Discussion The TGA/ DTA curves for the pure LiMn2O4 and the representative bi-metal doped sample (LiCu0.03Cr0.03Mn1.94O4 ) are shown in Fig.1. For the pure LiMn2O4 (Fig. 1a), the TGA curve depicts two weight loss regions. The initial weight loss of about 10% up to 180 °C can be attributed to the removal of moisture accompanied by decomposition of citric acid. The major weight loss region observed between 180 °C and 290 ºC corresponding to a weight loss of 62%, which is also accompanied by a sharp exothermic peak at 285 ºC in the DTA curve, is associated
with the decomposition of the acetate precursors of Li and Mn salts. It is apparent from the figure that no appreciable weight loss takes place after 300 °C and the TGA curve becomes flat depicting the formation of the pure LiMn2O4. The Cu-Cr bi-metal doped LiMn2O4 also showed the same thermal behavior except that the sharp exothermic peak appeared at 345 ºC in the DTA. From the thermal analysis, it can be concluded that the spinel LiMn2O4 starts to form at about 300 to 350ºC, but it has also been reported that at low temperature (250 and 450ºC) some other impurities such as α-Mn2O3 and LiMn2O3 are also present [1]. So, all the pure and doped samples were synthesized at 750ºC to get phase pure and well crystallized spinel compounds. The crystalline structure of the pure and Cu-Cr substituted LiMn2O4 was investigated by X-ray powder diffraction (Fig.2). XRD patterns of all the synthesized samples depicted well-defined peaks, pointing towards the formation of single-phase spinel structure. No extra reflections are observed in the XRD patterns of the Cu-Cr doped samples. The similar XRD patterns of spinal structure for the doped samples indicate that Cu and Cr have entered the lattice at 16d positions to substitute manganese cations. All the XRD patterns can be assigned to a well-crystallized spinel LiMn2O4 (JCPDS No. 35-0782). The Bragg peaks of the pure and doped LiMn2O4 are indexed to a cubic system with space group Fd3m in which the lithium ions occupy the tetrahedral (8a) sites and the Mn3+/ Mn4+ cations, as well as the doped metal ions reside at the octahedral (16d) sites [3]. The lattice constant and t h e cell volume of the pure and Cu-Cr doped LiMn2O4 calculated from the XRD data are shown in Table 1. Shrinkage of the unit cell is observed, however, with increasing dopants concentration the lattice parameters also increased. This may be due to the difference in the effective ionic radii of the doped metals and manganese cations. The concentrations of Li, Mn, Cr and Ni are determined using ICP-OES as shown in Table 2. The stoichiometric compositions of all the synthesized samples are in good agreement with the nominal compositions.
Table 1 and Table 2 FTIR spectroscopy has been proven to be a very useful tool to quantitatively resolve cation ordering in the spinel LiMn2O4 [20]. Fig. 3 shows the FTIR spectra of the pure and Cu-Cr doped LiMn2O4 in the wave number range of 400-4000 cm-1. The characteristic peaks observed in the range of 514-620 cm-1 are assigned to the stretching vibration of MO6 octahedral groups [21, 22]. It means that no structural change takes place for the low content doping in the spinel LiMn2O4. However, there is a slight frequency shift towards higher wave number pointing to the successful doping of the metal cations. The morphology of the as-synthesized nanostructures was evaluated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images shown in Fig. 4 confirm the low particles agglomeration for the Li[CuxCryMn2-xy]O4
samples compared to the pure LiMn2O4. This further indicated that crystallites
agglomeration to form bigger particles during sol-gel synthesis has not occurred for the doped samples. TEM images (Fig. 5) further show that unlike pure LiMn2O4, Cu-Cr doped samples are composed of more uniform and smaller particles having individual grain morphology with well separated grain boundaries. High-resolution TEM (HRTEM) image of a representative particle (LiCu0.01Cr0.01Mn1.98O4) demonstrates (inset of Fig. 5b) highly crystalline nature. The observed lattice spacing of 0.47 nm corresponds to the (111) plane of the cubic spinel LiMn2O4. The average particle size calculated was found varying in the range of 100 to 150 nm. Though the particles themselves are a bit larger to be exactly considered as nanoparticles or for that matter nanostructures but their surface topology bears few (one to five) nano-surfaces. T he energy dispersive X-ray spectroscopy (EDAX) profiles of the pure and Cu-Cr doped LiMn2O4 as given in Fig. S1 (Supplementary data) further confirmed the presence of all the metal cations in the synthesized samples. The electrochemical performances of the synthesized pure and Cu-Cr doped LiMn2O4 were evaluated as cathode materials for lithium-ion batteries. Coin cells fabricated with the synthesized
cathode materials and lithium metal anode were galvanostatically charged and discharged at a current density 43 mAg-1 (0.3 C) between 3.0 V and 4.8 V. The initial charge/discharge curves of the pure and doped samples are shown in Fig. 6 (A). In the given potential range, the charge/discharge curves of all the synthesized samples correspond to the characteristic voltage profile of the spinel LiMn2O4 structure in which lithium ions are located at the tetrahedral sites while the manganese ions occupied the octahedral sites [23-25]. The presence of two plateaus in the c h a r g e / discharge curves at around 4.0 V and 4.1 V vs. Li/ Li+ indicate two step oxidation/ reduction process for the extraction /insertion of lithium ions into the spinel structure. In other words, during discharge, lithium ions are inserted from anode to cathode whereas during charging lithium ions are extracted from anode. It has been reported that the higher voltage plateau at about 4.1 V corresponds to two phase transition of λ-MnO2/ Li0.5Mn2O4 vs. Li/ Li+, while the second plateau at lower potential (about 4.0 V) indicates single phase transition between Li0.5Mn2O4/ LiMn2O4. By controlling the formation of these unstable phases, the electrochemical performance of LiMn2O4 based cathode materials could be improved [26, 27]. Compared to the un-doped LiMn2O4, the boundaries of the two plateaus in the discharge curves of Cu-Cr doped cathode materials are not highly sharp. This may point towards the fact that the doping elements have stabilized the spinel structure by suppressing the Jahn-Teller effect which is considered to be one of the main causes of capacity fading in LiMn2O4 [27].
Cycling performance of the synthesized samples is shown in Fig. 6 (B). An improvement in cycling performance is observed for the doped samples. For the pure LiMn2O4, 72% of the initial discharge capacity was retained after 100 cycles. Factors such as the structural degradation due to co-operative John-Teller distortion effect and the dissolution of Mn into the electrolyte during charge/discharge cycling have been suggested to be the main causes for capacity fade [14]. For Cu-Cr doped samples, the initial discharge capacities for the increasing amount (0.01, 0.02,
0.03, 0.04 and 0.05) of Cu and Cr were 112, 112, 97, 56 and 68 mAhg-1, respectively. After 100 charge/ discharge cycles, the synthesized Cu-Cr doped LiMn2O4
cathode materials delivered
discharge capacities of 93, 82, 72, 47 and 60 mAhg-1 that correspond to the capacity retention of 83, 73, 74, 84 and 88% respectively, of the initial discharge capacities. It can be seen that compared to the pure LiMn2O4 that has 72% capacity retention after 100 cycles, the cycling performance of Cu-Cr doped samples is better. Although LiCu0.04Cr0.04Mn1.92O4 and LiCu0.05Cr0.05Mn1.90O4 showed higher capacity retention but their initial discharge capacities were q u i t e low. This may be due to the relatively higher amount of the doping elements. The sample with the lowest Cu-Cr contents i.e., LiCu0.01Cr0.01Mn1.98O4 showed the best electrochemical performance both in terms of the initial discharge capacity as well as the cycling stability. The large binding energy of CrO2 (1142 kJ/ mol) compared to MnO2 (946 kJ/ mol) has resulted in the stabilization of the octahedral sites and thus enhanced the cycling performance [15]. Furthermore, copper doping in the spinel oxide has been reported to
induce
higher
electronic conductivity and lower diffusion barrier for the intercalation/ de- intercalation of Li+ ions that resulted in higher rate capability [14]. Coulombic efficiency for all the samples is also given in the Fig. 6(B). Compared to the pure LiMn2O4, high initial coulombic efficiencies (90% for the pure LiMn2O4 and more than 93% for all the doped samples) were obtained for the doped samples. The irreversible capacity loss for both the pure and doped spinels can be seen from the first charge/discharge curves shown in Fig. 6(A). It is evident that doped spinels exhibit low irreversible capacity loss due to irreversible film formation reactions as compared to the pure spinel. Further, the irreversible capacity is present only during the first charge/discharge cycle, as nearly 100% coulombic efficiency was achieved on the subsequent cycles. Table 3 summarizes the electrochemical performances of the pure and Cu-Cr doped
LiMn2O4.
It is evident that, although the doped samples have low initial discharge capacities
than the pure LiMn2O4, but they exhibited better cycling performance compared to the pure LiMn2O4. Furthermore, the decrease in the initial discharge capacity with increasing concentration of the doping elements indicated that even for the doped materials, Mn3+ effectively contributed to the specific capacity during the electrochemical reaction. For all the doped series, sample with the lowest doping contents i.e., LiCu0.01Cr0.01Mn1.98O4 performed relatively better than the higher doping concentrations. This means that among the synthesized Cu-Cr doped LiMn2O4, the optimum dopant level in terms of capacity and cycling stability is approximately x = 0.01, y = 0.01. A close look on the experimental CV profiles (Fig.7 ) showed that the charge transfer redox peaks are sharper and with large currents for x = y= 0.01 mol dopants content than for the higher amounts. It reveals that, this very amount of the dopants kinetically facilitates the Li+ intercalation/ deintercalation process, while a bit higher amounts start decelerating the process. A second evidence comes from EIS data (discussed in following paragraphs) giving the smallest Rct values for 0.01 mol content of dopants and this value starts increasing for higher amounts of the dopants. Thus the overall picture supports the same conclusion as stated above for charge-discharge measurements. Table 3 The better cycling stability for the doped LiMn2O4 at the expense of specific capacity is mainly attributed to the inhibition of the Jahn-Teller effect on deep discharge of the various bi-metal doped LiMn2O4 cathode materials. Furthermore, unlike pure LiMn2O4 that upon complete extraction of Li from the spinel matrix results in an unstable λ-MnO2, all the Li cannot be extracted from the doped samples. The presence of this residual Li in the delithiated bi-metal doped spinel LiMn2O4 is considered to play a significant role in enhancing the structural stability to the repeated Li insertion/ extraction processes [28].
To evaluate the effect of Cu-Cr doping on the structural stabilization of LiMn2O4, cyclic voltammetric (CV) measurements were performed (Fig. 7). CV curves were recorded over the potential range of 3.4-4.8 V at a scan rate of 0.1 mV s-1.The oobtained anodic and cathodic peaks at around 4.0 V correspond to Mn3+/ Mn4+ redox couple [13]. The anodic and cathodic peaks observed in the CV curves represent the reversible redox reactions that correspond to Li ions removal and insertion from/ into the spinel framework. Compared to the pure sample, the redox peaks for the doped compounds are sharp and showed well-defined splitting; indicating the structural stabilization of the doped spinel LiMn2O4 materials [29]. From the CV curves, it can also be seen that for the doped samples, the oxidation and reduction peaks are much closer, the peak current is increased and the peak width is narrowed which means that the internal resistance for the doped LiMn2O4 cathodes is minimum and the diffusion rate of Li+ ions is the fastest compared to the pure LiMn2O4. These results substantiated that the polarization of LiMn2O4 has been reduced due to the improved kinetics of Li+ ions into the spinel LiMn2O4 framework. Thus, improved electrochemical performance for Cu-Cr doped LiMn2O4 cathodes is observed. Cyclic voltammetric results are in good agreement with the cycling performance. To further understand the advantage of Cu-Cr doped LiMn2O4 as the cathode material for lithium-ion batteries, electrochemical impedance spectroscopy (EIS) measurement were carried out (Fig. 8). In the Fig. 8A the relative impedance profile for pure and doped LiMn2O4 samples is shown. Unlike pure LiMn2O4, the lower charge transfer resistance (Rct) values for the Cu-Cr doped electrodes clearly indicate the improved electronic conductivity and the high Li+ transportation speed across the active electrode material and the electrolyte. Small impedance is favourable for the efficient extraction/ insertion of Li ions from/ into the spinel LiMn2O4 during charge/ discharge process. This resulted in the improved high rate cycling performance. Fig 8B describes EIS spectra of the pure and LiCu0.01Cr0.01Mn1.98O4 samples along with simulation
spectra. The impedance spectra were recorded after five charge/discharge cycles. The semicircle portion describes the charge transfer resistance (Rct) while the straight line in the low frequency region corresponds to Warburg impedance (W1) that represents the lithium ion diffusion in the bulk of the electrode. The circuit element CPE (constant phase element) is used to model the double layer capacitance. The intercept of the semicircle with Zreal axis at high frequency refers to the ohmic electrolyte resistance (R1). From the equivalent circuit the corresponding values obtained for R1 are 6.3 Ω and 2.4 Ω and that of Rct are 217 Ω and 121 Ω , for undoped and doped samples, respectively. The higher rate performance of LiCu0.01Cr0.01Mn1.98O4 can also be attributed to the smaller crystallite size than the pure sample. It is well established that for large particles size, it takes long time for Li+ ions to diffuse into the interior of the crystallite. The impedance analysis suggests that low content Cu and Cr substitution in LiMn2O4 may help to form a thinner surfaceelectrolyte interface (SEI) layer that possess lower ohmic resistance. From Figs. 7 & 8 it is obvious that the charge transfer process is facile having maximum current and with minimum charge transfer resistance, for LiCu0.01Cr0.01Mn1.98O4 among all the doped samples and thus corroborating the charge discharge results as conferred above. Another important aspect of the battery material is its working at high current rates which is usually not done and performance at low C-rates is reported. Here we also evaluated the rate performances, which present the electrochemical behavior under different charge-discharge rates or current densities. To test the rate and cycling performance we charged and discharged the pure spinel LiMn2O4 and LiCu0.01Cr0.01Mn1.98O4 cathodes in the voltage range of 3.0 V- 4.8 V at Crates from 0.1 C to 5 C (Fig. 9). As can be seen, LiCu0.01Cr0.01Mn1.98O4 cathode exhibited high capacity retention than the pure LiMn2O4. As a comparison, LiCu0.01Cr0.01Mn1.98O4 cathode material was able to deliver initial discharge capacity of 112 mAhg-1 at 0.1 C. However,
cycling at high current rate (5 C) provided 93 mAhg-1 discharge capacity, which is about 83% of the initial discharge capacity at 0.1 C. After deep cycling at high current rates, a discharge capacity of 106 mAhg-1 was recovered that is 93 % of the initial discharge capacity. This electrochemical performance was superior to that of the pure LiMn2O4 which showed only 41 % capacity retention at high current rate of 5C. The improved rate performance for LiCu0.01Cr0.01Mn1.98O4 may be due to the higher electronic and ionic conductivities as is clear from the EIS and CV results. It has been reported that although Cu doped spinel provides lower discharge capacity, it results in stable electrochemical cycling [14]. The higher octahedral stabilization energy of Cr3+ may also contribute in the stability of the LiMn2O4 structure [30]. The current work differs in the aspect that it evidently showed that the low content doping can be utilized to tune the properties of material. Further, the prepared material performs superbly at h i gh C rates. The higher conductivity, small particle size and good structural stability resulted in the improved electrochemical performance of Cu-Cr doped LiMn2O4. 4. Conclusions In this work, a series of Cu-Cr doped LiMn2O4 was synthesized by a facile and cost effective sol-gel process. As the cathode materials for lithium-ion batteries, the Cu-Cr doped LiMn2O4 structures showed significantly improved cycling performance compared to the pure sample. Among all the synthesized samples, LiCu0.01Cr0.01Mn1.98O4 exhibited excellent electrochemical performance. This includes a capacity of 112 mAhg-1 at 0.1 C, excellent cyclic stability (83 % capacity retention after 100 charge/ discharge cycles), and a good rate capability. Performance of the doped material at high current rate i.e. (5C) was even better than that of undoped LiMn2O4 material.
Significantly, upgraded charge-discharge response could be
attributed to the adjustment of the appropriate amount of Cu and Cr into the matrix that resulted in the stabilization of spinel LiMn2O4 framework by inhibiting both the Mn dissolution
into the electrolyte and the Jahn-Teller distortion effect. The improved electrochemical performance of LiMn2O4 with very low content doping of Cu and Cr and to sustain the capacity even at very high C-rate is encouraging and have potential for further investigations. Acknowledgement Authors gratefully acknowledge financial support from the Higher Education Commission (HEC) Pakistan. Authors are also grateful to Prof. Z.Tang (National Canter for Nanoscience and Technology, No. 11, Beiyitiao, Zhongguancun, Beijing, China) for providing the lab facilities for this work. References [1]
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Figure Captions Fig.1 TGA/DTA analysis of (a) LiMn2O4 (b) LiCu0.03Cr0.03Mn1.94O4 Fig.2 XRD patterns of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4 Fig.3 FTIR Spectra of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4 Fig.4 SEM images of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4 Fig.5 TEM images of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4. The inset in panel (b) is the corresponding HRTEM image of LiCu0.01Cr0.01Mn1.98O4 Fig.6 (A) Initial Discharge curves and (B) Capacity and coulombic efficiency vs. cycle number plots for (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4 Fig.7 Cyclic voltammograms (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4 Fig.8(A) Nyquist plots for (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4 (B) Experimental and simulated Nyquist plots for (a) and (b) samples with equivalent circuit diagram. Fig.9 Cycling performance at various rates for the pristine LiMn2O4 and LiCu0.01Cr0.01Mn1.98O4
Supplementary Data:
Fig.S1. EDAX profiles of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4
Table 1 Lattice parameter ‘a’ and unit cell volume for spinel LiMn2O4 and LiCuxCryMn2-x-yO4 compounds. S. No
Sample
Lattice constant ‘a’
Unit cell volume
(Å)
(Å3)
1
LiMn2O4
8.2478
561.06
2
LiCu0.01Cr0.01Mn1.98O4
8.2372
558.90
3
LiCu0.02Cr0.02Mn1.96O4
8.2384
559.15
4
LiCu0.03Cr0.03Mn1.94O4
8.2460
560.69
5
LiCu0.04Cr0.04Mn1.92O4
8.2467
560.84
6
LiCu0.05Cr0.05Mn1.90O4
8.2557
562.68
Table 2 The ICP-OES results of the pure and Cu-Cr doped LiMn2O4.
S. No
Nominal composition
Experimental composition
1
LiMn2O4
Li0.98Mn1.98O4
2
LiCu0.01Cr0.01Mn1.98O4
Li0.983Cu0.013Cr0.014Mn1.977O4
3
LiCu0.02Cr0.02Mn1.96O4
Li0.972Cu0.018Cr0.019Mn1.966O4
4
LiCu0.03Cr0.03Mn1.94O4
Li1.051Cu0.027Cr0.028Mn1.936O4
5
LiCu0.04Cr0.04Mn1.92O4
Li1.012Cu0.038Cr0.037Mn1.925O4
6
LiCu0.05Cr0.05Mn1.90O4
Li1.054Cu0.049Cr0.048Mn1.893O4
Table 3 Electrochemical Performance of the pure and Cu-Cr doped LiMn2O4. S.No
Sample
Specific capacity (mAhg-1) at 1st cycle
Specific capacity
Capacity
(mAhg-1) after 100
Retention
cycles
(%)
1
LiMn2O4
122
88
72
2
LiCu0.01Cr0.01Mn1.98O4
112
93
83
3
LiCu0.02Cr0.02Mn1.96O4
112
82
73
4
LiCu0.03Cr0.03Mn1.94O4
97
72
74
5
LiCu0.04Cr0.04Mn1.92O4
56
47
84
6
LiCu0.05Cr0.05Mn1.90O4
68
60
88
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
50nm
Figure 6
Figure 7
Figure 8
Figure 9
32
Graphical abstract 01
Effect of bication (Cu-Cr) substitution on the structure and electrochemical performance of LiMn2O4 spinel cathodes at low and high current rates Azhar Iqbala,b, Yousaf Iqbalb, Abdul Majeed Khanc, Safeer Ahmeda*,
33