ENERGY STORAGE MATERIALS: NEW TRANSITION METAL OXIDES, OXY-FLUORIDES AND PHOSPHATES

Abstract

Energy demands are increasing day by day due to rapid industrial and technological growth of developing nations along with a staggering growth of the world population. Considering the receding levels of fossil fuels, if the energy load is met mostly by fossil based fuels the world might have to see catastrophic consequences in addition to augmented adverse effects of global warming and climate change. Thus, increased use of clean and green energy with minimal environmental impact is a major challenge in the 21st century. To curtail the global warming and reduce our reliance on fossil fuel based energies, energy generation from renewable sources is imperative. But, the source and magnitude of renewable energy are intermittent in nature and variable in time; it varies during the span of a day or part of a year. Therefore, the electrical energy generated from such renewable sources needs to be stored using a suitable energy storage technology. While electromagnetic waves, electricity grid and chemical energy are among the major energy carriers, chemical energy storage in batteries is one of the most convenient forms of energy storage. Lithium-ion battery (LIB) technology has been proven to be one of the well-established technologies for energy storage. Moreover, lithium being the lightest metal in the periodic table, LIB offers highest energy densities among all other rechargeable battery technologies. Owing to the easy handling and portability, LIBs are used as power sources in portable electronics (e.g., cell phones and laptops), medical implants (e.g., pumps, pacemakers), power tools, defense, transportation and aerospace applications. Despite continuous efforts of the scientific community to improve lithium ion battery technology over the last four decades, current battery systems are still far behind to replace internal combustion engines in fully electric powered vehicles, because to compete with the driving distance per full gasoline tank for vehicles with internal combustion engines, nearly 5-fold increments in the energy density of the current batteries is necessary. A great deal of research effort has been devoted during the last two decades to develop high capacity and high energy density batteries, but the electrode materials essentially remained the same as far the LIB technology is concerned. Various strategies and methodologies are being adopted by several research groups across the globe to improve the different components of LIBs. A large body of research exists mostly involving the study of oxides and phosphates with various structural families such as, rock-salt, spinel, olivine etc. Among the different series of compounds, ii much attention has been paid to the transition metal oxides with the layered rock-salt and spinel structures and phosphates with the olivine type structure. For example, LiCoO2, LiNiO2, LiNi0.5Mn0.5O2, LiMnO2 and LiNi1-x-yCoxMnyO2 constitute the major class of materials that are largely being investigated as cathodes in Li-ion batteries. Similarly, in the spinel family, LiMn2O4 and LiNi0.5Mn1.5O4 are the main candidates that have attracted attention as cathodes. The olivine type phosphates, on the other hand, comprising LiFePO4, LiMnPO4, LiCoPO4 and LiNiPO4, are also being investigated as cathode materials. In spite of vast studies and exploration of cathode materials, LiCoO2 still remained the workhorse for the LIB technology. In addition, LiNi0.8Co0.15Al0.05O2, LiNi1/3Mn1/3Co1/3O2, LiMn2O4 or LiFePO4 are also being used in conventional LIBs. But, a number of problems exist with most of them pertaining to issues, such as, high cost, toxicity, phase/structural transition, capacity fading due to electrolyte instability and transition metal dissolution (when cycled at higher potential, e. g., for LiCoO2 above 4.2 V), to mention a few. Recently, LiNi1xyCoxMnyO2 (NCM), a layered oxide with rock-salt structure, has been considered as a viable cathode material for electrification of transport due to its high capacity and good rate capability, but, low thermal stability, Li/Ni mixing and high reactive surface are the main issues that limits its cathode performance. With respect to anodes of LIBs only limited class of materials, such as, graphite and various forms of carbon, TiO2 based oxides, Li4Ti5O12 and Li3VO4, which are mostly of insertion/de-insertion types, are being investigated. While graphite is a low cost, high capacity and long life anode material, used in most of the commercialized Li-ion batteries, but slow Li-ion diffusion, structural collapse during cycling and dendritic Li-growth at low operating voltages limit its use in high power density applications. Low theoretical capacity, poor electrical conductivity and poor ionic diffusion are the main obstacles for TiO2 based anodes in LIBs. Although, titanium based Li4Ti5O12 has emerged as a feasible anode material for low power battery applications due to its high structural stability, minimum volume change upon insertion/de-insertion of Li-ions and flat voltage plateau, but low theoretical capacity (175 mAh g1) and low electronic conductivity prevents its use in high capacity battery applications. Recently, Li3VO4 has been considered as a promising anode material as a replacement to graphite in commercial LIB due to its high theoretical capacity (394 mAh g1) and suitable working potential. But, low ionic and electronic conductivities of Li3VO4 are the main drawbacks that essentially results in poor electrochemical iii performance and prevents its commercial use. Research efforts are underway to alleviate the problems of poor electrical and ionic conductivity of Li3VO4. In the backdrop of contemporary issues with the present day cathode and anode materials of LIBs, Chapter-1 gives a brief overview of them along with the working principle of LIBs and various active electrode materials. Chapter-2 describes the synthetic methodologies and the details of all the characterization techniques used in the present study. The compounds were synthesized by solid state reaction, sol-gel or hydrothermal method employing high purity simple metal carbonates / oxalates / oxides, alkali hydroxides, alkali fluorides, metal nitrates, phosphates, pyrophosphates and citric acid or ethane glycol as complexing agents / solvents. The progress of reactions and formation of final products were monitored by powder X-ray diffraction (P-XRD) and the morphological and compositional characterizations were carried out by Field Emission-Scanning Electron Microscopy (FE-SEM), Transmission-Electron Microcopy (TEM) and Energy Dispersive X-ray Spectroscopy (EDS) analysis. The thermal stability of the compounds and the amount of carbon in the as synthesized compounds were evaluated using Thermo-gravimetric (TG) analysis. X-Ray Photoelectron Spectroscopy (XPS) was used to ascertain the oxidation states of the redox active metals in the compounds. The charge transfer resistance (Rct) was estimated using Electrochemical Impedance Spectroscopy (EIS) data. Finally, Cyclic Voltammetry (CV) and Galvanostatic charge-discharge analysis were carried out in fabricated Teflon half-cells to evaluate the electrochemical performance of the materials presented here. In Chapter-3, we report the synthesis and characterization of a nickel, cobalt, manganese (NCM) based rock-salt layered oxy-fluoride, Li1.25Ni0.25Co0.25Mn0.25O1.5F0.5. The compound is synthesized using solid state reaction. P-XRD pattern simulation and Rietveld refinement studies confirm the ordered rock-salt structure of the oxy-fluoride without Li/Ni cation disorder in the Li-only layer. While the anodic and cathodic peaks at 3.95 and 3.72 V, respectively, in the CV trace ascertain the intercalation/de-intercalation of Li-ions into/out of the lattice, the charge-discharge curves show the intercalation/de-intercalation potential at a slightly higher voltage (~ 3.91 V) than its oxide analogs. The observed higher working voltage is attributed to the incorporation of fluoride ions into the oxide lattice. A high charge capacity of 216 mAh g1 and a discharge capacity of 148 mAh g1 at 0.1 C for the first cycle are observed for Li1.25Ni0.25Co0.25Mn0.25O1.5F0.5. A high charge-discharge capacity is obtained at slow cycling rate, but substantial capacity fading is observed iv when the cell is cycled at higher cycling rates. The present work is significant due to the fact that it demonstrates a large amount of fluoride doping in the oxide lattice of the Li-rich oxy-fluoride maintaining a completely ordered structure and avoiding any likely consequences of Li/Ni disorder in the Li-only layer. Chapter-4 deals with synthesis, characterization and electrochemical properties of Na3Fe(PO4)2, a layered phosphate based cathode material for Li-ion batteries. In search for new cathodes involving intercalation-deintercalation of multiple Li-ions, Na3Fe(PO4)2 was identified as an interesting compound. The compound is prepared as phase pure by a sol-gel method within 24 h reaction time, which is much faster than that of the solid state method reported earlier (reaction time > 7 days). The phase purity, microstructure and composition of the synthesized compound are ascertained by P-XRD, FE-SEM and EDX studies, respectively. The presence of an anodic peak at 3.04 V in the CV trace suggests Na3Fe(PO4)2 as potential cathode material for Li-ion batteries. The charge-discharge studies carried out between 1.5  4.0 V at different C rates (C/50, C/20, C/10 and C/5) confirm the electrochemically active nature as cathode material for Li-ion batteries, although the compound showed much reduced capacity than the theoretical (85 mAh g1). But, the excellent capacity retention at C/20 up to 100 charge-discharge cycles is noteworthy suggesting its structural robustness during electrochemical insertion-extraction of Li. Chapter-5 describes the synthesis, characterization and electrochemical properties of nut-shaped hierarchical mesoporous Li3VO4 (HM-Li3VO4). A rapid template free hydrothermal method is developed for the synthesis of HM-Li3VO4. P-XRD analysis confirm the formation of single-phase Li3VO4 with an orthorhombic structure having lattice parameters, 6.3189(3), 5.4454(2) and 4.9468(2) Å. SEM images show formation of nut-shaped morphology that are hollow from inside and essentially composed of nano particles of Li3VO4 with sizes ranging from 50-100 nm. Combined HR-TEM and BET surface area analysis establish hierarchical mesoporous nature for the as prepared Li3VO4. The electrochemical charge-discharge studies employing a lithium metal half-cell with the bare HM-Li3VO4 as active anode show a discharge capacity of 615 mAh g-1 and a charge capacity of 384 mAh g-1 at 0.1 C rate for the first cycle. The discharge capacity of 332 mAh g-1 observed at the 2nd cycle for HM-Li3VO4 is superior to those reported in the literature for other Li3VO4 in its bare form. The improved anode performance of nut-shaped HM-Li3VO4 is attributed to the hierarchical mesoporous microstructure which facilitates faster Li+ v diffusion through the mesoporous channels and accommodates Li+ ions within mesopores during intercalation-deintercalation process. However, capacity fade at higher rates are evident in the HM-Li3VO4 due to its poor electrical conductivity. In an effort to improve both the ionic and electrical conductivity of Li3VO4, a simple, short and cheaper template free one-pot solvothermal method is developed to synthesize mesoporous Li3VO4 (M-Li3VO4) on graphene oxide (GO). Here again, growth of nut-shaped Li3VO4 on GO with nut sizes ranging from 2 m to sub-micrometer levels with a multimodal mesopore distribution are confirmed by FE-SEM, TEM and BET surface area studies. HR-TEM analysis confirm the growth of Li3VO4 on GO. The presence of graphene oxide (GO) is further ascertained by D and G band features in the Raman spectra, in addition to crystalline Li3VO4 due to the bands at 785 and 818 cm1. The EIS data clearly indicate enhanced electrical conductivity of Li3VO4-GO as compared to HM-Li3VO4. The electrochemical charge-discharge studies employing a lithium metal half-cell with Li3VO4-GO as active anode material show a discharge capacity of 814 mAh g-1 and a charge capacity of 559 mAh g-1 at 0.1 C rate when cycled in the potential range 0.2 – 3V for the first cycle. Moreover, a capacity of 414 mAh g-1 after the second discharge at 0.5 C is achieved in the same potential range. A discharge capacity of 374 mAh g-1, which is achieved at the fifth cycle at 0.5 C rate, is comparable or superior to those reported in the literature for other Li3VO4 samples with similar carbon contents. The enhanced anode performance with more than double reversible capacity at 0.5 C rate for M-Li3VO4-GO as compared to that of bare HM-Li3VO4 is due to superior ionic and electronic conductivity of the material. The results of these investigations are discussed in Chapter-6. Chapter-7 presents the overall conclusions and future prospects of our current investigation. The present work gives valuable insights in finding new layered rock-salt based oxy-fluorides and phosphates with alkali metal rich compositions. This will have tremendous potential for the development of next generation high capacity, high energy density and high rate capable electrodes for Li/Na-ion batteries.