الفهرس 
Chapter I: IntroductionOnly 14 pages are availabe for public view
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Abstract
Summary and conclusion
For four decades, lithium-ion batteries (LIBs) have been amongst the most promising rechargeable batteries for power electronic devices such as cellular phones, laptop computers, digital cameras. Great research efforts impel improving Li-ion battery (LIB) technology, especially the performance of positive electrodes (cathodes) to meet the ever-increasing demands for energy storage to be used in new and high-powered applications like electric vehicles (EVs) and hybrid electric vehicles (HEVs).
Spinel LiMn2O4 with its theoretical specific capacity of 148 mAh/g is one of lithium manganese oxide-based materials that has been attracting much attention as an ideal cathode material. This ideal virtue comes from its low toxicity, low cost, easy fabrication, high natural abundance of Mn, high acceptability of environmental impact and good thermal stability. Besides its open three-dimensional (3D) crystal structure which facilitates and improves Li+ diffusion through vacant octahedral and tetrahedral interstitial sites. However, LiMn2O4 suffers from poor cycle stability due to dissolution of Mn and JT-distortion and insufficient rate capability due to its low electrical conductivity.
In this work high performance spinel LiMn2O4 with good crystalline structure, rate performance and cycle stability was synthesized in a simple one step precipitation method using oxalic acid as a precipitating agent. This facile method does not require expensive and sophisticated laboratory equipments and extraordinary experimental circumstances, therefore it is fairly financially savvy, as well as time consuming and easily scalable for mass production. The good morphological properties of as prepared LiMn2O4 produced from this precipitation method improved its electrochemical properties. This is because it significantly promotes fast insertion/ extraction kinetics of lithium ions and facilitates charge transfer across the electrode/electrolyte interface and hence reducing lithium ion diffusion length. The effect of the synthesis route (evaporation or filtration) at different calcination temperatures on the chemical and electrochemical properties of LiMn2O4 was also investigated and discussed in details. Besides LMO materials, pure other low manganese oxides such as Mn2O3 and Mn3O4 were obtained also from filtration technique. The aim was extended to study the effect of coating of LiMn2O4 by silver nanoparticles (AgNPs).
Different techniques were used to characterize the prepared samples e.g. X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Brunauer-Emmett-Teller (BET) specific surface, Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS).
The XRD patterns of pristine LiMn2O4 and Ag/LiMn2O4 composites reveal the presence of well crystalline pure spinel LiMn2O4. All the reflections for samples with or without surface treatment are indexed to the characteristic Bragg lines of the spinel LiMn2O4 (Fd 3m space group, JCPDS card No. 89-1026) without any residual impurities. It has been concluded from the strain values, the FWHM values of the (311) and (400) diffraction lines of LiMn2O4 and the intensity ratio value of the I(311)/I(400) peaks that the spinel LiMn2O4 sample calcined at 900°C might have higher crystallinity, lower lattice strain and better ordering of local structure and stability than the sample calcined at 750°C, so, it has been selected to be coated by silver nanoparticles. The XRD patterns of the filtrate and precipitate phases produced from the filtration technique supports our idea that the evaporation technique succeeded to trap soluble lithium reagent beside manganese reagent to yield the required LiMn2O4 phase.
The small amount of the Ag additives and the overlapping of reflection peaks with those of LiMn2O4 is behind the reason of absence of the characteristic peaks of Ag (i.e., cubic (fcc) phase, (111)-reflection at 2θ= 38.12°) and/or its oxides (i.e., cubic Ag2O phase, (111)-reflection at 2θ = 32.85°; monoclinic AgO phase, (111)-reflection at 2θ = 37.23°). from the peak intensity ratio of the (311)/(400) reflections and the value of the cubic lattice parameter for Ag/LMO composites which does not vary significantly than the pristine LiMn2O4, it can be concluded that Ag+ does not penetrate the spinel lattice but remained at the surface only.
Rietveld analysis data confirmed the presence of surface coating by AgNPs. It shows that the Ag/LMO(v) sample (calcined in vacuum) contains 2.6% metallic Ag and 0.8% AgO phase. In contrast, the sample heat treated in air (Ag/LMO(a)) has high content of AgO (3.2%). This difference in the percent of morphology in the coating layer created a local deformation of the structure observed through the microstrain values of the samples as it is 1.6 × 10-3 rd for Ag/LMO(v) increases considerably to 2.2 × 10-3 rd for the Ag/LMO(a).
The EDX spectroscopy shows only the additional peak of Ag in Ag/LMO(v) with concentration 2.85 wt.% agree well with results from Rietveld refinements.
The thermal behavior of LMO precursor resulted from evaporation technique shows stepwise decomposition with subsequent weight losses till formation of stable LiMn2O4. The first weight loss is attributed to loss of both water of hydration and water of constitution from mixed lithium hydrogen oxalate hydrate and hydrated manganese oxalate. The second weight loss was due to decomposition of the organic constitute (oxalate) that used during preparation to form lithium carbonate and manganese oxides in the range 330C-460C as an intermediate stage. This mixture can associate and transform to pure stable spinel LiMn2O4 which still stable to more than 950C. These TGA results agreed well with XRD results that showed formation of MnO2, Mn2O3 and Li2CO3 phases at 450C. These oxides and Li2CO3 combined to form primary spinel LiMn2O4 at 750C and the crystalline one at 900C. The thermal behavior of the precipitate precursor resulted from filtration technique and proved to be manganese oxalate hydrate agrees with the XRD patterns appear at different temperatures 450C, 750C and 900C. As mixed phases from (MnO2 and Mn2O3) was formed in the stable region from 340 to 440ºC then reduce to pure Mn2O3 at about 470ºC that still stable to nearly 885ºC. Further reduction through loss of oxygen and formation of lower manganese oxide of Mn3O4 is expected to be after 885ºC.
SEM of the as prepared spinel LiMn2O4 samples at 750C exhibits a relatively distinct aggregated (packed) and massive homogenous particles with average particle size 100–200 nm. Upon increasing the calcination temperature to 900C, the particles become regular, more crystalline and more uniform with average size 200-500 nm. The morphology shows truncated surfaces at the vertices and edges of the cubic octahedral particles of LiMn2O4 calcined at 900C which confirms the complete transformation of the co-incorporated Li and Mn ions into spinel LiMn2O4.
TEM verifies the truncated octahedral characteristic to LiMn2O4 with face center cubic structure (fcc) framework with the dimensions in nanosize. Spinel calcined at 750C shows particles with size of 75-100 nm, while 200-300 nm size was observed for particles of the spinel calcined at 900C. TEM for the coated samples Ag/LMO shows that the coating layer has no effect on the spinel morphology. For Ag/LMO(v), small quasi-spherical spots of Ag nanoparticles with diameters 8-15 nm were found to be distributed on the surface of LMO particle and for Ag/LMO(a), the surface becomes rough with a thick layer (~15 nm) of AgO is formed.
The HRTEM shows the interplanar distance indexed to the distance between (111) planes of LiMn2O4, that plane which is prone to form stable SEI layers and suppresses Mn dissolution in the electrolyte. For Ag/LMO(v), the lattice fringes match with the characteristic (111) plane of metallic Ag0 and for Ag/LMO(a) match well with the (110) plane of AgO. The SAED pattern of the whole particle of LiMn2O4 shows well-defined spots and reveal the single crystalline nature of LMO calcined at 900C.
BET specific surface area for E750 is 7.722 m2g-1 which is almost larger by ten order of magnitude than that of E900 (0.68939 m2g-1) and for EI750 is 7.6893 m2g-1 which is larger than that of EI900 (2.1189 m2g-1). from the values of specific surface area, it can be concluded that increasing calcination temperature to 900ºC increases the particle size as shown in TEM and XRD sections and hence decreases the entire surface area. The BET for Ag/LMO(v) increased to 4.1 m2 g-1 compared to 2.1 m2g-1 for the pristine LMO and decreased to 0.6 m2g-1 for Ag/LMO(a). This increase in BET value of Ag/LMO(v) is attributed to the formation of spherical-like AgNPs on the surface of LiMn2O4 particles. The isotherm curves for all the spinel samples calcined at different temperatures and also for the coated samples belong to type IV indicating the hierarchical mesoporous structure of LMO nanopowders.
The RS and XPS analyses are among the surface analysis tools. The vibration properties obtained from Raman scattering (RS) spectroscopy for all the samples are identical with spinel LiMn2O4 considering the cubic Fd-3m structure (Oh7spectroscopic symmetry). The characteristic six Raman active modes represented by the species 2A1g+1Eg+3T2g within LMO lattice in the spectral range 100-800 cm-1 exist for the spinel LMO and Ag-modified LMO samples. Besides the six Raman active modes characteristic to the spinel, four extra peaks appear for Ag-modified LMO samples which are fingerprints to the presence of the monoclinic AgO phase (P21/c S.G., C2h5 G.F) on the LMO surface.
The XPS measurements confirmed the chemical composition of LMO and Ag-modified LMO samples. The spectra lines of Li 1s, Mn 2p1/2, Mn 2p3/2, and O 1s match well with the characteristic XPS patterns of the spinel LMO phase. The average valence state of Mn is found to be 3.504, 3.507 and 3.505 in pristine LiMn2O4, Ag/LMO(a) and Ag/LMO(v), respectively, which confirms the stoichiometry of the sample.
The XPS spectra of Ag-modified LMO samples show additional peaks attributed to the Ag 3s, Ag 3p and Ag 3d core levels. The position of Ag 3d5/2 peak at 367.8 eV corresponds to that of Ag0 in Ag/LMO(v) sample, while the shift of the Ag 3d peaks toward lower energies in the spectrum of Ag/LMO(a) indicate the presence of AgO.
The electrochemical behavior of LiMn2O4 shows that the strategy followed to prepare spinel LMO material with high phase purity and crystallinity without any sophisticated equipment is a promising method. LMO delivers an initial discharge capacity of 115.2 mAh/g at 15 mA g-1 and a reversible discharge capacity of 104.8 mAh/g at the same rate with capacity retention of 90.97% after applying various C-rates for more than 50th cycle. The charge and discharge capacities upon cycling and at high current rates are stable and reached to 100% coulombic efficiency. This could be due to reversible structural change during the lithium insertion/extraction processes as an indication of good rechargeability and reversibility. The electrochemical behavior investigates also the effect of Ag coating on the electrochemical performance of LMO. Although Ag/LMO(v) delivers an initial specific discharge capacity of 105 mAh g-1 at 15 mA g-1 lower than 115 mAh g-1 for pristine LMO and 118 mAh g-1 for Ag/LMO(a), its capacity retention is (82%) higher than that of pristine one (76.6%) and Ag/LMO(a) (58.2%). Also Ag/LMO(v) displays a better coulombic efficiency of 99.4% compared to 98.1% for LMO cycled over 60 cycles. It is also worth noting that the specific discharge capacity of the Ag/LMO(v) after 35 cycles is higher at 75 mA g-1 current density compared to that at 15 mA g-1 and the coulombic efficiency for the Ag/LMO(v) electrode remains at 99% after 80 cycles at 30 mA g-1. The outstanding properties of Ag/LMO(v) sample are also verified through the small value of charge transfer resistance (Rct) and area-specific impedance (ASI) and the high value of lithium diffusion coefficient (DLi) which is 6.3×10-12 cm2 s-1 twice that in pristine LMO.
The improved electrochemical performance of Ag/LMO(v) is due to (i) the enhanced specific surface area (4.1 m2 g-1), (ii) the increase of the mesoporosity, which could lead to easy diffusion pathways for Li+ ions to access to LMO particles, (iii) the presence of the superficial conductive layer increasing the LMO interparticle electrical contact and thin insulating AgO layer which prevents side reactions. In contrast, the presence of the relatively thick AgO insulating phase on the surface of Ag/LMO particles treated in air acts as a barrier for the Li+ ions motion. The Ag/LMO(v) electrode shows high capacity retention, reduced side reaction effect, good cyclability and capacity fade of 0.06% per cycle.