NICKEL-RICH MIXED METAL LAYERED DOUBLE HYDROXIDES AND OXYFLUORIDES FOR ENERGY STORAGE APPLICATIONS

Abstract

Rapid development of many emerging technologies such as portable electronics, electrical vehicles (EVs), power electrical vehicles (PEVs) and microgrids increases the demand of electrical energy storage systems with high energy and power densities [1-4]. Among all electrical energy storage systems, batteries exhibit high energy density but suffer from low power density and unsatisfactory cycle life. In contrary, supercapacitors show relatively high-power density and long cycle life but suffer from low energy density [5-7]. To overcome these disadvantages, traditional energy storage systems could be replaced by hybrid energy storage systems (HESs) having merits of both batteries and supercapacitors [6-8]. In general, HESs has one electrode made up of battery type material, while the other is made up of supercapacitor type material and show the merits of both systems [8, 9], which are necessary for the commercialization of electrical vehicles, high power portable electronics and backup energy systems. To obtain best performance from HESs, battery type positive electrode materials should well match with the supercapacitor type negative electrode materials [7, 8]. So, battery type electroactive materials with high capacity, rate performance and cycle life are highly desirable. Currently, battery type positive electrode materials are mainly focused on transition metal-based oxides and hydroxides, such as NiO, MnO2, CoO, Ni(OH)2, Co(OH)2 and Mn (OH)2. Among all, nickel-based hydroxides [10] are promising as electrode material for energy storage due to their high theoretical capacity, favorable redox behavior, low cost and abundance. Numerous efforts have been devoted to improve the electrical conductivity, structural and electrochemical stability of Ni(OH)2 and suppress the volumetric changes by transition metal doping. Therefore, bimetallic or trimetallic analogues of Ni(OH)2, formed by the doping of various transition metals (Fe, Co, Mn, Cu, Mg and Al) are of great interest to researchers [11-12]. Based on crystal structure, transition metal hydroxide can be classified into α- and β- phases. The α-hydroxide phase shows larger interlayer separation and turbostratically crystallized structure leading to its high activity as compared to the β-phase. α-phase hydroxide, involving two or more transitions (Zn, Mn, Co etc.) or a main group metal (Al) shows better electrochemical performance because the add on metals provide extra stability to the structure and prevent the phase transformation during the charge discharge reactions [12]. However, it has been a great challenge to synthesize bulk amount of pure mixed metal hydroxides (NiCoMn, NiMn) that too be stabilized in the crystalline α-phase. In general, β, α+β or amorphous double hydroxide phases are obtained during their synthesis by various methods. To find out better bimetallic or multi-metallic double hydroxides, it is primarily essential to choose suitable transition metals and maximize the charge storage performance, along with large-scale synthesis. In most of the reports, the compositions after synthesis are not well defined and commonly named as NiCoMn LDHs or NiMn-LDHs. For practical applications, microstructure and mass loading of the electrode materials are important parameters. Different microstructures may result in different structural features such as surface area, pore size and its distribution, pore volume and connectivity. These features directly affect the electrolyte penetration to inner active surfaces, electrode-electrolyte interfaces, diffusion pathways, reaction kinetics and electrochemical stability which are responsible for better electrochemical performance. Besides, mass loading has significant effect on the electrochemical performance. The less mass loading (< 1 mg/cm2) or thin electrodes can result in extraordinary specific capacity, while a significant decrease in the specific capacity is observed with high mass loadings or thick electrodes. The thesis is devoted to the systematic exploration of NiCoMn and NiMn LDHs. Here, Co substitution envisaged to facilitate anion intercalation by compensating for the charge imbalance caused by trivalent metal cations and improve the conductivity of the -type Ni(OH)2, while small Mn substitution will improve structural stability, electrode-electrolyte interface and electrochemical stability. The effect of precursor salts, precipitating agents, reaction conditions, surfactants and mass loadings on the overall electrochemical performance is studied. Considering the advantage of Ni-rich electrode materials, a similar approach is adopted with NCM based Li and Ni-rich oxyfluorides as cathode material for Li ion batteries. Further, a series of new Li and Ni-rich NCM compositions are synthesized and the effect of high Li and Ni content and fluoride ion doping on the operating potential window, specific capacity and electrochemical stability is studied. With a brief introduction to the background for undertaking the present investigation in given in Chapter- I, while the results of the investigation including synthesis protocol and brief details of characterization techniques are presented in subsequent chapters. Chapter- II provide the details of synthesis method and characterization techniques used in the present study. Co-precipitation and hydrothermal method are used for the synthesis of layered double hydroxides (LDHs), while solid-state reaction method is used for the synthesis of oxyfluorides using metal oxalates and oxides as precursor salts. The progress of each reaction step and synthesis of the desired materials are analysed by powder X-ray diffraction (P-XRD). For microstructure, elemental composition and elemental mapping, Field Emission - Scanning Electron Microscopy (FE-SEM), Energy Dispersive X-ray Spectroscopy (EDS) and Transmission Electron Microscopy (TEM) are used. Furthermore, Fourier transform Infrared spectroscopy (FTIR) is used to examine the vibrational modes of molecules, types of chemical bonds and functional groups present in the samples. The oxidation state of the present elements is studied by X-Ray Photoelectron Spectroscopy (XPS), while thermal stability and decomposition kinetics of the materials is assessed by Thermogravimetric Analysis (TGA). Brunauer-Emmett-Teller (BET) analysis is opted for the investigation of surface area, pore size, pore size distribution and pore volume of the powder samples. Electrochemical measurements such as cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) are performed to understand the underlying mechanisms that govern the performance of batteries and supercapacitors type energy storage systems. The details of P-XRD, FESEM, EDS, TEM, FTIR, TGA, BET, electrode preparation, cell assembly and electrochemical characterizations (CV, GCD and EIS) are discussed in this chapter. Chapter- III reports the synthesis, structural and electrochemical characterization of Ni0.7Co0.2Mn0.1(OH)2. These layered double hydroxides (LDHs) are synthesized via a simple co-precipitation method by rapid pH switching using chloride, nitrate, acetate and sulphate salts of Ni, Co and Mn. It is envisioned that the designed transition metal ratio will improve the electrochemical performance, while maintaining structure and electrochemical stability in the light of designing Ni-rich compositions with 20 % Co and 10 % Mn, away from equimolar NCM and in line with new nickel rich layered rock salt oxide cathodes. To the best of our knowledge, this is one of the first reports for the formation of mix-metal double hydroxides, Ni0.7Co0.2Mn0.1 LDH, in pure and crystalline α-Ni(OH)2 structure by one step co-precipitation method. Generally, amorphous phases, a mix of α- and β-phases or non-stoichiometric phases are encountered in most of the synthesis. In three electrode assembly, chloride precursor assisted LDH shows the highest specific capacity of 978 C g–1 at 1 A g–1. The observed capacity is much higher than all other double hydroxides studied here and those we have come across in the literature reports on similar metal hydroxides. The superiority of Ni0.7Co0.2Mn0.1 LDHCl is also confirmed by its low charge transfer resistance (Rct = 17.6 Ω), Warburg impedance (Zw = 35 Ω) and high-capacity retention of 90 % at 100 mV sec–1 and 99 % at 5 A g–1 for 5000CV and 1000 GCD cycles, respectively. The symmetric device using Ni0.7Co0.2Mn0.1 LDH-Cl as anode and cathode light up the LED bulb, which confirm its potential for the practical applications. Chapter- IV describes the hydrothermal synthesis and electrochemical performance of NiMn- LDHs and Ni-LDH. NiMn-LDHs are synthesized using HMTA and urea as precipitating agents, while for Ni-LDH, HMTA is used as precipitating agent. The effect of microstructures, pore volume, pore hierarchy and small amount of Mn substitution in α-Ni(OH)2 lattice is observed on the specific capacity and cyclability. In three electrode set-up the specific capacity (C g–1) of 719, 487 and 316 at 1 A g–1 is observed for NiMn-LDH@HMTA, NiMn-LDH@Urea and Ni-LDH@HMTA, respectively. The NiMn-LDH@HMTA also exhibit the highest capacity retention of 82.6 % and 86 % for 10,000 CV and 5000 GCD cycles, respectively. It is believed that the introduction of Mn in the -Ni(OH)2 creates partial disorder in the Ni-lattice, which improves the electrochemical reactions involving both Ni and Mn species. Moreover, the presence of higher valent Mn3+/Mn4+ allows the intercalation of charge compensating anionic species in the interlayer galleries and hinders complete phase transformation into the - Ni(OH)2 phase, thereby enhances the capacity and cyclability of the bimetallic NiMn-LDHs. The large pore volume (0.237 cc g–1) and hierarchal mesoporosity of NiMn-LDH@HMTA further supports its improved performance compare to other studied LDHs. The mass loading study revealed 1.9 mg cm−2 as the optimum mass loading for maximizing both the specific and areal capacity at graphite foil. Moreover, the supercapattery device assembled using NiMn- LDH@HMTA as cathode and activated carbon as anode exhibit the high-capacity retention of 80.1 % after 5000 GCD cycles along with an energy density of 50 and 34 Wh kg–1 at power densities of 800 and 4000 W kg–1, respectively. To further enhance the electrochemical performance of NiMn-LDHs in Chapter- V comprehensive research has been done on the effect of different surfactants and structure directing agents or modifiers including cationic (CTAB, DTAB, EDTA), anionic (SDS) and nonionic (PVP, PEG and PEG-PVP-PEG) substances on the electrochemical performance. Among studied samples, NiMn LDH-SDS shows the highest interlayer spacing of 14.4 Å, which will help to improve the electrolyte-electrode interfaces and ion diffusion, essential for the better electrochemical performance. The relatively high surface area (m2 g–1) and pore volume (cc g–1) of 70.2 and 0.076 is observed for NiMn LDH-SDS compare to (36.3 and 0.021), (76.3, 0.054), (36.7 and 0.022), (56.6 and 0.032), (38.9 and 0.016) and (55.9 and 0.034)for NiMn LDHs synthesized using CTAB, DTAB, PEG, PVP, PEG-PVP-PEG, respectively. The GCD measurements confirm the high specific capacity of 802 C g–1 at 1 A g–1 with excellent capacity retention of 98 and 88.2 % for 4000 GCD and 5000 CV cycles, respectively for NiMn-LDH@SDS. The above results show its superiority over other LDHs, which is corresponding to its high interlayer spacing, surface area and pore volume. In line with Ni-rich compositions for supercapacitors, Chapter- VI represents a series of new Li and Ni-rich oxyfluorides as cathode materials for Li-ion batteries. The oxyfluorides, Li1.2Ni0.4Co0.2Ti0.2O1.4F0.6 (LNCTOF-1), Li1.2Ni0.4Co0.2Mn0.1Ti0.1O1.4F0.6 (LNCTMOF-2), Li1.2Ni0.4Co0.2Mn0.2O1.4F0.6 (LNCMOF-3), Li1.2Ni0.4Co0.15Mn0.25O1.45F0.55 (LNCMOF-4), Li1.2Ni0.4Co0.15Mn0.25O1.45F0.5 (LNCMOF-5) and Li1.25Ni0.25Co0.25Mn0.25O1.5F0.5 (LNCMOF-6) are synthesized by solid-state reaction method. All compounds are phase pure and having particle like microstructure. Powder cell simulations confirm completely ordered structure for LNCMOF-3 and LNCMOF-6, while disordered structure for LNCMOF-2, 4 and 5, respectively. The Rietveld refinements are performed at the ordered structures to obtained the refined lattice parameters. The electrochemical measurements confirm better Li intercalationdeintercalation and high operating potentials for Mn substituted compounds compare to LNCTOF-1. CV curves of LNCMOF-5 and LNCMOF-6 shows their high oxidation potential of 4.3 and 4.1 V, respectively, compare to reported oxide lattice and electrochemical structure stability. The GCD results of LNCMOF-6 represents a high charge and discharge capacity of 216 mAh g−1 and 151 mAh g−1, respectively for the first cycle, when cycled between 3.0 − 4.49 V at 0.1 C with a capacity retention of 91 % at 0.8 C after 70 cycles. This work exhibits the large amount of fluoride incorporation into the oxide lattice and improved voltage and capacities of oxyfluorides as compare to its oxide analogues. Chapter- VII represents the conclusions and future prospects of the present investigation. The present work gives valuable insights for the improvement of the electrochemical performance of existing materials by varying simple reaction parameters such as precursor salts, precipitating agents and use of surfactants or structure directing agents during the synthesis. In-line with Ni-rich LDHs for supercapcitors, the new Li and Ni-rich rock-salt based oxyfluorides are studied for Li-ion batteries. This will help tremendously to develop supercapacitor or battery cathode materials, which can exhibit high specific capacity, energy and power density, rate capability and cyclability.