Mesoporous Co3O4 nanosheets (Co3 O4 -NS) and nitrogen-doped reduced graphene oxide (N-rGO) are synthesized by a facile hydrothermal approach, and the N-rGO/Co3O4 -NS composite is formulated through an infiltration procedure. Eventually, the obtained composites are subjected to various characterization techniques, such as XRD, Raman spectroscopy, surface area analysis, X-ray photoelectron spectroscopy (XPS), and TEM. The lithium-storage properties of N-rGO/Co3O4 -NS composites are evaluated in a half-cell assembly to ascertain their suitability as a negative electrode for lithium-ion battery applications. The 2D/2D nanostructured mesoporous N-rGO/Co3O4 -NS composite delivered a reversible capacity of about 1305 and 1501 mAh g(-1) at a current density of 80 mA g(-1) for the 1st and 50th cycles, respectively. Furthermore, excellent cyclability, rate capability, and capacity retention characteristics are noted for the N-rGO/Co3O4 -NS composite. This improved performance is mainly related to the existence of mesoporosity and a sheet-like 2D hierarchical morphology, which translates into extra space for lithium storage and a reduced electron pathway. Also, the presence of N-rGO and carbon shells in Co3O4 -NS should not be excluded from such exceptional performance, which serves as a reliable conductive channel for electrons and act as synergistically to accommodate volume expansion upon redox reactions. Ex-situ TEM, impedance spectroscopy, and XPS, are also conducted to corroborate the significance of the 2D morphology towards sustained lithium storage.
Lithium ion batteries have attracted considerable attention as important energy storage and conversion systems for applications including electric vehicles (EVs) and hybrid electric vehicles (HEVs), owing to their high energy and power densities, as well as a long cycle life. Recently, lithium transition metal phosphates, such as LiFePO4, LiMnPO4, and Li3V2(PO4)3, have been considered as potential cathode materials for lithium ion batteries. It is well known that the phosphates display much better electrochemical and thermal stability compared to conventional lithium metal oxides [1]. However, recently, fluorophosphates are being developed as advanced cathode materials display higher operating potential, corresponding to the redox reaction of transition metal, when compared to the respective metal oxides and phosphates [2]. In addition, the highly electronegative fluoride ion helps to improve the cycle stability, which fluorophosphates make attractive materials for high energy batteries. Our current work is focused on maximizing the deliverable discharge capacity of Li2CoPO4F cathode material and to reach theoretical capacity by achieving more than one electron intercalation. The redox couple Co2+/3+ and Co3+/4+ was closely followed during galvanostatic charge-discharge test by x-ray photoemission spectroscopy and x-ray diffraction (XRD) analysis. It was found that the incomplete reduction of cobalt ions during discharge initiated the irreversible capacity loss rather than the electrolyte decomposition being solely responsible. A novel approach was carried out to activate the transition metal ions which resulted in a discharge capacity as high as 230 mAh g-1 at a current rate of 20 mA g-1 for Li/Li2CoPO4F cell. A long plateau at 4.8 V has been observed in the charge cycle with an additional voltage plateau at ~5.0 V, which can be attributed to the two lithium intercalation reaction and has been validated using cyclic voltammetry studies. The detrimental cycle stability observed [3] in Li2CoPO4F has been replaced with a stable cycle performance of > 90 % capacity retention as shown in figure 1. A quasi single phase reaction was confirmed using gravimetric intermittent titration technique in addition to the results from ex-situXRD studies. A discussion based on the obtained results will be presented in detail. References: [1] S.K. Martha, J. Grinblat, O. Haik, E. Zinigrad, T. Drezen, J.H. Miners, I. Exnar, A. Kay, B. Markovsky, D. Aurbach, Angew. Chem. Int. Ed.48 (2009) 8559–8563. [2] A. Kraytsberg, Y. E. Eli, Adv. Energy Mater.2 (2012) 922–939. [3] X. Wu, Z. Gong, S. Tan, Y. Yang, J. Power Sources 220 (2012) 122–129.
Development of cathode materials for lithium battery application like electric vehicle (EV) and plug-in hybrid electric vehicle (PHEV) requires high capacity materials[1]. At the same time, cost, environmental concerns and safety issues cannot be excluded. Recently, lithium transition-metal silicates such as Li2MSiO4 (M=Fe, Mn and Co) have been extensively studied as cathode materials for lithium ion batteries, due to high theoretical capacity and better thermal stability than lithium transition metal oxides such as LiCoO2. Among them, Li2MnSiO4 cathode could be a better choice as a high capacity( ~330 mAh/g) cathode for lithium ion batteries than Li2FeSiO4 cathode material. Due to Mn2+/Mn3+ and Mn3+/Mn4+ redox couples, extraction of two lithium ions can be achieved from Li2MnSiO4. However, due to defects such as structure change after the first charge and low electronic conductivity(~ < 10-14S/cm), this material has poor cycle performance and lower capacity than the theoretical prediction [2-3]. In this regard, the current work is aimed on the improvement of capacity and cycle stability of Li2MnSiO4 cathode active material by applying Cu doping and poly-aniline composites. The Li2Mn0.95Cu0.05SiO4were prepared using the sol-gel synthesis. PANI was synthesized by oxidative polymerization. XRD patterns as shown in Figure 1 exhibit that the Cu doping and PANI composite does not contribute to structure change of Li2MnSiO4. From the cycle performances as shown in Figure 2, the Li2Mn0.95Cu0.05SiO4/PANI cell shows a high discharge capacity, as well as improved cycle retention compared with pristine Li2MnSiO4 material. Synthesis methodology of Li2Mn0.95Cu0.05SiO4/PANI materials along with their physical, morphological and electrochemical characteristics will be discussed in detail. References : [1] R.J. Gummow, N.Sharma, V.K.Peterson, Y.He, J. solid state chem., 188 (2012) 32–37. [2] R. Dominko, M. Bele, A. Kokalj, M. Gaberscek, J. Jamnik , J. Power Sources., 174 (2007) 457–461. [3] D. M. Kempaiah, D. Rangappa, I. Honma, Chem. Commun.,48 (2012) 2698–2700.
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