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Keywords: Natural Sciences
Chemistry & allied sciences
Chemical sciences
Issue Date: 2013
Abstract: Spinel LiMn 2 O 4 is one of the most attractive positive electrode materials for Li-ion rechargeable batteries. In the present study, six series of low content bi-metal doped LiMn 2 O 4 with nominal compositions of LiNi x Cr y Mn 2-x-y O 4 , LiLa x Zn y Mn 2-x-y O 4 , LiCu x Cr y Mn 2-x-y O 4 , LiCu x Zn y Mn 2-x-y O 4 , LiNi x Cu y Mn 2-x-y O 4 and LiNi x Zn y Mn 2-x-y O 4 (where x = y = 0.01-0.05) were prepared by the sol-gel method. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) confirmed the formation of the pure as well as the doped spinel LiMn 2 O 4 between 285 o C and 350 o C. However, well crystallized spinel phase verified from the X-ray diffraction studies was obtained at 750 o C. XRD measurements further confirmed that all the synthesized compounds crystallized as single phase products in the cubic spinel Fd3m space group. The results showed that doping LiMn 2 O 4 with such small amount of metals has not affected the original spinel structure. Inductively coupled plasma optical emission spectrometry (ICP-OES) findings agreed the used nominal compositions. Energy dispersive X-ray analysis (EDX) also confirmed the purity of all the synthesized samples. SEM and TEM images showed that unlike the pure LiMn 2 O 4 , all the doped samples exhibited uniform size with smooth faceted polyhedral particles. The average particle size ranges from about 42 nm to 250 nm. High resolution TEM images also demonstrated the highly crystalline nature of all the six doped series. Cyclic voltammetric studies indicated that all the synthesized samples showed two pairs of well-defined anodic and cathodic peaks at around 4.0 V that corresponded to the redox couple of Mn 3+ / Mn 4+ . However, for the doped samples, the oxidation and reduction peaks were much closer to each other. The peak current was increased and the peak width was narrowed, indicating the reduced polarization of the bi-metal doped LiMn 2 O 4 , resulting from the faster iiinsertion/ extraction of Li + ions into the spinel matrix. Electrochemical impedance spectroscopy (EIS) was employed to have an insight about the synergetic effect of the bi-metal doping on the electrochemical performance of spinel LiMn 2 O 4 . The Nyquist plots showed that the charge transfer resistance (R ct ) decreases upon doping with Ni-Cr, La-Zn, Cu-Cr, Cu-Zn, Ni-Cu and Ni- Zn. The observed faster kinetics of Li + ions is attributed to the enhanced conductivities of the doped samples. Galvanostatic charge/ discharge measurements performed between 3.0 and 4.8 V for all the samples showed two plateaus around 4.0 V and 4.1 V vs. Li/ Li + that clearly demonstrated that insertion/ extraction of Li + ions takes place in two steps. The improved cycling performance of all the doped samples over the investigated 100 charge/ discharge cycles indicated that low content bi-metal doping has stabilized the spinel LiMn 2 O 4 structure by suppressing Jahn- Teller distortion. Rate capability of the pure and doped samples was also evaluated. The cells for each material were charged to 4.8 V at constant low current rate (0.1 C) and discharged to 3.0 V at 0.1 C, 0.3 C, 0.5 C, 1 C, 2 C, and 5 C, respectively. Compared to the pure LiMn 2 O 4 which retained only 41% of the initial discharge capacity when cycled at high current rate of 5 C, the capacity retention at 5 C for the Ni-Cr, La-Zn, Cu-Cr, Cu-Zn, Ni-Cu and Ni-Zn doped samples (x = y = 0.01) was 82%, 78%, 81%, 67%, 62%, and 58%, respectively. Among the various synthesized bi-metal doped series, samples with the lowest doping metal contents LiM 0.01 M' 0.01 Mn 1.98 O 4 (where M and M' are the various doping metal cations used in this study) appeared to be the best composition both in terms of the initial discharge capacity as well as the rate capability.
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