Dans le cadre du développement de technologies innovantes dans le domaine des accumulateurs Li-ion à charge rapide, typiquement inférieure à 5 minutes, des séparateurs commerciaux ont été caractérisés par différentes méthodes physico-chimiques et électrochimiques afin de corréler leurs structures poreuses aux performances en charge rapide enregistrées. L'architecture d'électrode choisie utilise l'oxyde de titane Li4Ti5O12 à l'électrode négative et le spinelle LiMn2O4 à la positive.<br />Afin d'augmenter les capacités chargées par rapport aux séparateurs commerciaux, des membranes à squelette poly(fluorure de vinylidène) et poly(fluorure de vinylidène) co poly(hexafluoropropylène) ont été élaborées par inversion de phase en utilisant la méthodologie des plans d'expériences. Les processus de formation ont été explicités à partir de la thermodynamique des systèmes ternaires polymère/solvant/non-solvant. Les membranes obtenues ont permis de gagner 20% de capacité chargée en 3 minutes par rapport aux séparateurs commerciaux.<br />Enfin, les limitations en charge rapide dues aux séparateurs ont été étudiées et identifiées à l'aide d'un code de modélisation d'accumulateurs Li-ion.

Electrical double-layer formed at the interface between an electrode and an electrolyte has been a topic of innumerable studies. The electrical interface plays a crucial role in kinetics, mechanisms and applications in variety of electrochemical reactions. The electrical double-layer and electron-transfer reactions lead to many important applications of electrochemistry, which include energy storage devices, namely, batteries, fuel cells and supercapacitors. Electrochemical supercapacitors can withstand to higher power than batteries and deliver higher energy than the conventional electrostatic and electrolytic capacitors. A supercapacitor can be used as an auxiliary energy device along with a primary source such as a battery or a fuel cell for the purpose of power enhancement in short pulse applications. Among the various materials studied for electrochemical supercapacitors, carboneous materials, metal oxides and conducting polymers received attention. Among carboneous materials, various forms of carbon such as powders, woven cloths, felts, fibers, nanotubes etc., are frequently studied for electrochemical supercapacitors. Low cost, high porosity, higher surface area, high abundance and well established electrode fabrication technologies are the attractive features for using carboneous materials. However, specific capacitance (SC) of these materials is rather low. These electrodes store charge by electrostatic charge separation at the electrode/electrolyte interface. Electronically conducting polymers are interesting class of materials studied for supercapacitor application because of the following merits: high electronic conductivity, environmental friendliness, ease of preparation and fabrication, high stability, high capacitance and low cost. Polyaniline (PANI), polypyrrole and polythiophene are studied in this category. Transition metal oxides have attracted considerable attention as electrode materials for supercapacitors because of the following merits: variable oxidation state, good chemical and electrochemical stability, ease of preparation and handling. Hydrated RuO2 prepared by sol-gel process at low temperature has a specific capacitance as high as 720 F g-1 due to solid state pseudo faradaic reaction. However, high cost, low porosity and toxic nature limit commercialization of supercapacitors using this material. MnO2 is attractive as it is cheap, environmentally benign, its resources are abundant in nature and also it is widely used as a cathode material in batteries. An early study on capacitance behaviour of MnO2 was reported by Lee and Goodenough. Amorphous hydrous MnO2 synthesized by co-precipitation method exhibited rectangular cyclic voltammogram in various aqueous alkali salt solutions. A specific capacitance of 200 F g-1 was reported. Following this report, several reports appeared on capacitance characteristics of MnO2. According to the charge-storage mechanism reported, a specific capacitance of 1370 F g-1 is expected from MnO2. However, this value can be obtained in practice only when the mass of MnO2 is at the level of a few micrograms per cm2 area. At such a low thickness range, the utilization of the active material is high. As thin layers of MnO2 are uneconomical for practical capacitors, studies with a mass range of 0.4-0.5 mg cm-2 have been extensively reported. At this mass range, a maximum specific capacitance of about 240 F g-1 has been obtained. With an increase in mass per unit area, the specific capacitance of MnO2 decreases. The problem associated with low values of specific capacitance of thick layers of MnO2 is the following. The MnO2 deposits or coatings generally do not possess high porosity and the electrolyte cannot permeate into the coating. Only the outer layer of the electrode is exposed to the electrolyte. Consequently, the electrochemical utilization of the material decreases with an increase in thickness. Nevertheless, utilization of thick layers of the active materials is preferable for obtaining capacitance as high as possible in a given volume and area of the electrodes. Indeed, it would be ideal if specific capacitance of MnO2 is improved from its presently reported value of 240 F g-1 to a value equivalent to that of RuO2.xH2O, namely, 720 F g-1. In view of this, attempts are made to enhance specific capacitance of MnO2 by electrochemical deposition in presence of surfactants. Nanostructured MnO2 synthesized by inverse microemulsion route is also studied for electrochemical supercapacitors. The effect of crystallographic structure of MnO2 on the capacitance properties, studies on electrochemical deposition of MnO2 in acidic and neutral medium using electrochemical quartz crystal microbalance and capacitance characteristics of MnO2-polyaniline composites are also described in the thesis. Chapter 1 briefly discusses the importance of electrochemistry in energy storage and conversion, basics of electrochemical power sources, importance of MnO2, different synthetic procedures for MnO2 and its applications in energy storage and conversion in particular for electrochemical supercapacitors. Chapter 2 provides the experimental procedures and methodologies used for the studies reported in the thesis. In chapter 3, the effect of surface active agents, namely, sodium dodecyl sulphate (SDS) and Triton X-100 added to the electrolyte during electrodeposition of MnO2 on Ni substrate on capacitance properties is presented. Electrocrystallization studies show that MnO2 nucleates instantaneously under diffusion control and grows in three dimensions. The potentiodynamically prepared oxide provides higher specific capacitance than the potentiostatically and galvanostatically prepared oxides. Specific capacitance values of 310 and 355 F g-1 obtained for MnO2 electrodeposited in the presence of 100 mM SDS and 10 mM Triton X-100 are higher than the oxide electrodeposited in the absence of surfactants. Surfactant molecules adsorbed at the electrode/electrolyte interface alters structure of double-layer and kinetics of electrodeposition. Smaller particle size, greater porosity, higher specific surface area and higher efficiency of material utilization are the factors responsible for obtaining higher specific capacitance. Extended cycle-life studies indicate that the superior performance of MnO2 due to surfactants is present throughout the cycle-life tested. Chapter 4 pertains to electrochemical supercapacitor studies on nanostructured α-MnO2 synthesized by inverse microemulsion method and the effect of annealing. As synthesized nanoparticles of MnO2 was found to be in α-crystallographic structure with particles less than 50 nm size. Nanoparticles exhibited rectangular cyclic voltammograms between 0 and 1 V vs. SCE in aqueous 0.1 M Na2SO4 at sweep rates up to 100 mV s-1 due to the short diffusion path length. On annealing at different temperatures, a mixture of nanoparticles and nanorods with varying dimension is noticed. Specific capacitance of 297 F g-1 obtained during initial cycling decreases gradually on extended cycling. The capacitance loss is attributed to the increase in the resistance for intercalation/deintercalation of alkali cations into/from MnO2 lattice. MnO2 crystallizes into several crystallographic structures, namely, α-, β-, γ-, δ- and λ-structures. As these structures differ in the way MnO6 octahedra are interlinked, they possess tunnels or inter-layers with gaps of different magnitudes. Because capacitance properties are due to intercalation/deintercalation of protons or cations in MnO2, only some crystallographic structures, which possess sufficient gap to accommodate these ions, are expected to be useful for capacitance studies. The effect of crystal structure of MnO2 on its electrochemical capacitance properties is also included in chapter 4. Specific capacitance of MnO2 is found to depend strongly on the crystallographic structure, and it decreases in the following order: α ≅ δ > γ > λ > β. A specific capacitance value of 240 F g-1 is obtained for α-MnO2, whereas it is 9 F g-1 for β-MnO2. A wide (~ 4.6 Å) tunnel size and large surface area of α-MnO2 are ascribed as favorable factors for its high specific capacitance. A large interlayer separation (~7 Å) also facilitates insertion of cations in δ-MnO2 resulting in SC close to 236 F g-1. A narrow tunnel size (1.89 Å) does not allow intercalation of cations into β-MnO2. As a result, it provides very small SC. In Chapter 5, capacitance characteristics of PANI synthesized using (NH4)2S2O8, nanostructured MnO2 (α- and γ-form) and also PANI-MnO2 composites are presented. Morphology of PANI synthesized resembles the morphology of the MnO2 used as the oxidant. Electrochemical capacitance properties of PANI and composites are studied in a mixed electrolyte of 0.1 M HClO4 and 0.3 M NaClO4 between 0 and 0.75 V vs. SCE. Specific capacitance of 394 F g-1 is obtained for PANI synthesized using γ-MnO2. Chapter 6 describes the electrocatalytic behaviour of Mn3[Fe(CN)6]2 synthesized by ion-exchange reaction between MnSO4 and K3[Fe(CN)6] and the effect of annealing on its electrochemical capacitance properties. As prepared Mn3[Fe(CN)6]2 and also the sample heated at 100 oC exhibit redox couple in 0.1 M Na2SO4 electrolyte, corresponding to Fe(CN)64-/Fe(CN)63- present in the matrix. Mn3[Fe(CN)6]2 samples annealed at 150 oC and above decompose to oxides of manganese and iron, and hence exhibit capacitance characteristics in 0.1 M Na2SO4 electrolyte. A maximum specific capacitance of 129 F g-1 is obtained for Mn3[Fe(CN)6]2 annealed at 300 oC. Electrochemical quartz crystal microbalance (EQCM) investigations of kinetics of electrodeposition of MnO2 in acidic and neutral media, and capacitance behaviour are presented in chapter 7. Oxidation of Mn2+ to MnO2 is characterized by an anodic cyclic voltammetric peak both in acidic and neutral media. During the reverse sweep, however, reduction of MnO2 into Mn2+ occurs in two steps in the acidic medium and in a single step in the neutral medium. From EQCM data of mass variation during cycling, it is observed that the rate of electrodeposition of MnO2 is higher in the neutral medium than in the acidic medium. Specific capacitance of MnO2 deposited from the neutral medium is higher than that deposited from acidic medium owing to different crystallographic structures. Reversible insertion/deinsertion of hydrogen in to the layers of δ-MnO2 is observed in hydrogen evolution region. Details of the above studies are described in the thesis.

Thèse en cotutelle (avec l'Université Toulouse 3 - Paul Sabatier) en Science des matériaux et Electrochimie / Avec le développement des systèmes électroniques embarqués se pose la question de la miniaturisation des dispositifs de stockage d’énergie. De nos jours, cette fonction est principalement assurée par des micro-batteries. Ces composants possèdent cependant une faible puissance disponible, une durée de vie limitée et un domaine de fonctionnement en température restreint. Les “micro-supercondensateurs” sur puce permettraient de s’affranchir de ces limitations, mais ils ne sont aujourd’hui qu’au stade de la recherche universitaire avec des densités d’énergie bien inférieures à celles des micro-batteries. L’énergie et la puissance stockées dans un supercondensateur sont proportionnelles au carré de la fenêtre de potentiel, qui dépend elle-même de la stabilité électrochimique de l’électrolyte utilisé. L’électrolyte joue ainsi un rôle prépondérant sur les propriétés des supercondensateurs (tension, gamme de température, courant de fuite, durée de vie…). Cette thèse vise à développer des liquides ioniques protiques et aprotiques dédiés aux micro-supercondensateurs pseudocapacitifs à base d'oxydes métalliques (RuO2, MnO2). Les électrolytes à base de liquides ioniques présentent des propriétés intéressantes, notamment une faible pression de vapeur saturante, une stabilité aux hautes températures, ainsi qu’une large fenêtre de potentiel. Ils contribuent ainsi à améliorer la densité d’énergie surfaciques, principal problème rencontré par les micro-supercondensateurs actuels. Les liquides ioniques étudiés ont été conçus sur la base de leurs structures et leurs propriétés physico-chimiques. Des caractérisations électrochimiques ont été réalisées avec des micro-supercondensateurs à base d’oxyde de ruthénium et d’oxyde de manganèse. De très bonnes performances ont été obtenus en utilisant des collecteurs de courant poreux à grande surface spécifique. Les électrolytes liquides constituant cependant un verrou technologique à la réalisation de micro-supercondensateurs fonctionnels compatible avec les procédés de microfabrication, des ionogels composés d’une matrice solide dans laquelle a été confinée le liquide ionique ont également été réalisés. / The rising growth of smart and autonomous microelectronic devices in the IoT (Internet of Things) era urges the development of advanced microscale energy sources with tailor-made features and customized energy/power requirements. Micro-supercapacitors (MSCs) emerged as potential energy storage devices complementing micro-batteries to power ubiquitous sensor networks needed to foster the development of IoT. However, the low cell voltage and low energy density remain major bottleneck that prevents their application at a large scale in real devices. To mitigate this issue, several studies have been devoted to the engineering of MSC electrode materials and structural architecting of current collectors to enhance the surface area and areal energy density by considering the limited available footprint area. This, however, has associated challenges such as a complex synthesis route, poor interfacial and mechanical stability of the electrode, and electrolyte compatibility issues, among others. Another key challenge to solve for reaching high energy density values in MSCs is the limited electrochemical stability window (ESW) of the electrolytes used as energy stored is directly related to the square of the cell voltage. The electrolytes play a major role in deciding the ESW and liquid-state electrolytes commonly used are troublesome for the microfabrication process due to leakage, evaporation, and safety issues. Therefore, it’s imperative to develop alternative electrolytes including solid-state electrolytes reconcilable to the target application of MSCs. This thesis aims at developing novel ionic-liquid (IL)-based electrolytes (both protic and aprotic) suitable for pseudocapacitive metal oxide (e.g., RuO2, MnO2)-based micro-supercapacitors (MSCs). IL-based electrolytes exhibit key properties including low vapor pressure, high temperature stability, low melting point, etc. with a wide ESW and help improve energy density performance, overcoming the major bottleneck faced by current MSCs. During this project, ILs are rationally designed based on their physicochemical properties. The detailed structure-property and electrochemical characterization studies were done using RuO2 and MnO2-based MSCs. We demonstrate state-of-the-art performance by developing high surface area porous current collectors with enhanced mass loading and solid-state devices using ionogel electrolytes, enabling their feasible integration with microelectronics to power connected IoT sensor networks.

Rechargeable Li-ion batteries and supercapacitors are the most promising electrochemical energy storage devices in terms of energy density and power density, respectively. Recently, nanostructured materials have gained enormous interest in the field of energy technology as they have special properties compared to the bulk. Commercially available Li-ion batteries, which are the most advanced among the rechargeable batteries, utilize microcrystalline transition metal oxides as cathode materials which act as lithium insertion hosts. To explore better electrochemical performance the use of nanomaterials instead of conventional materials would be an excellent alternative. High Li-ion insertion at high discharge rates causes slow Li+ transport which in turn results in concentration polarization of lithium ions within the electrode material, causing a drop in cell voltage. This eventually, leads in termination of the discharge process before realizing the maximum capacity of the electrode material being used. This problem can be addressed by decreasing the average particle size which leads to an increase in surface area of the electrode material. Nanostructured materials, because of their high surface area and large surface to volume ratio, to some extent can overcome the problem of slow diffusion of ions. Supercapacitors are electrical energy storage devices which can deliver large energy in a short time. A supercapacitor can be used as an auxiliary energy device along with a primary source such as a battery or a fuel cell to achieve power enhancement in short pulse applications. Active materials for supercapacitors are classified into three categories: (i) carbonaceous materials, (ii) conducting polymers and (iii) metal oxides. Among the materials studied over the years, metal oxides have been considered as attractive electrode materials for supercapacitors due to the following merits: variable oxidation state, good chemical and electrochemical stability, ease of preparation and handling. The performance of supercapacitors can be enhanced by moving from bulk to nanostructured materials. The theme of the thesis is to explore novel routes to synthesize nanostructured materials for Li-ion batteries and supercapacitors, and to investigate their physical and electrochemical characteristics. Chapter I is an introduction of various types of electrochemical energy systems such as battery, fuel cell and supercapacitor. A brief review is made on electrode materials for Li-ion batteries and supercapacitors, and nanostructured materials. Chapter II deals with the study of nanostrip orthorhombic V2O5 synthesized by a two-step procedure, with the formation of a vanadyl ethylene glycolate precursor and post-calcination treatment. The precursor and the final product are characterized for phase and composition by powder X-ray diffraction (XRD), infrared (IR) spectroscopy, thermal analysis (TGA) and X-ray photoelectron spectroscopy (XPS). The morphological changes are investigated using field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HRTEM). It is found that the individual strips have the following dimensions, length: 1.3 μm, width: 332 nm and thickness: 45 nm. The electrochemical lithium intercalation and de-intercalation of nanostrip V2O5 is investigated by cyclic voltammetry (CV), galvanostatic charge-discharge cycling, galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy. Chapter III describes the synthesis of nanoparticels of LiMn2O4 by microwave assisted hydrothermal method. The phase and purity of spinel LiMn2O4 are confirmed by powder XRD analysis. The morphological studies are carried out using FE-SEM and HRTEM. The electrochemical performance of spinel LiMn2O4 is studied by using CV and galvanostatic charge-discharge cycling. The initial discharge capacity is found to be about 89 mAh g-1 at a current density of 21 mA g-1 with reasonably good cyclability. Chapter IV deals with synthesis of MoO2 nanoparticles through ethylene glycol medium and its electrochemical characterization. XRD data confirms the formation MoO2 on monoclinic phase, space group P21/c. Polygon shape of MoO2 is observed in HRTEM. MoO2 facilitates reversible insertion-extraction of Li+ ions between 0.25 to 3.0 V vs. Li/Li+. CV and galvanostatic charge-discharge cycling are conducted on this anode material to complement the electrochemical data. Chapter V reports the synthesis of nanostructured MnO2 at ambient conditions by reduction of potassium permanganate with aniline. Physical characterization is carried out to identify the phase and morphology. The as prepared MnO2 is amorphous and it contains particles of 5 to 10 nm in diameter. On annealing at a temperature > 400 °C, the amorphous MnO2 attains crystalline α-phase with a concomitant change in morphology. A gradual conversion of nanoparticles to nanorods (length 500-750 nm and diameter 50-100 nm) is evident from SEM and TEM studies. High resolution TEM images suggest that nanoparticles and nanorods grow in different crystallographic planes. The electrochemical lithium intercalation and de-intercalation of nanorods was performed by (CV) and galvanostatic charge-discharge cycling. The initial discharge capacity of nanorod α-MnO2 is found to be about 197 mAh g-1 at a current density of 13.0 mA g-1. Capacitance behavior of amorphous MnO2 is studied by CV and galvanostatic charge-discharge cycling in a potential range from -0.2 to 1.0 V vs. SCE in 0.1 M sodium sulphate solution. The effect of annealing on specific capacitance is also investigated. Specific capacitance of about 250 F g-1 is obtained for as prepared MnO2 at a current density of 0.5 mA cm-2 (0.8 A g-1). Chapter VI pertains to electrochemical supercapacitor studies on nanostructured MnO2 synthesized by polyol method. Although X-ray diffraction (XRD) pattern of the as synthesized nano-MnO2 shows poor crystallinity, it is found that it is locally arranged in δ-MnO2 type layered structure composed of edge-shared network of MnO6 octahedra by Mn K-edge X-ray Absorption Near Edge Structure (XANES) measurement. Annealed MnO2 shows high crystalline tunneled based α-MnO2 as confirmed by powder XRD pattern and XANES. As synthesized MnO2 exhibits good cyclability as an electrode material for supercapacitor. In Chapter VII, capacitance behavior of nanostrip V2O5, TiO2 coated V2O5 and nanocomposites of PEDOT/V2O5 are presented. Structural and morphological studies are carried out by powder XRD, IR, TGA, SEM and TEM. Cyclic voltammogram of pristine V2O5 shows the regular rectangular shape indicating the ideal capacitance behavior in aqueous 0.1 M K2SO4. The SC value of pristine V2O5 is found to be about 100 F g-1. Nanostrip V2O5 is modified with TiO2 using titanium isobutoxide to enhance the capacitance retention upon cycling. Only 48 % of the initial capacitance remains in the case of pristine V2O5 after 100 cycles, while TiO2 coated V2O5 exhibits better cyclability with capacitance of 70 % of the initial capacitance. The capacitance retention is attributed to the presence of TiO2 on the surface of V2O5 which prevents the vanadium dissolution into the electrolyte. Microwave assisted hydrothermally synthesized PEDOT/V2O5 nanocomposites are utilized as capacitor materials. The initial SC of PEDOT/V2O5 (237 F g-1) is higher than that of either pristine V2O5 or PEDOT. The enhanced electrochemical performance is attributed to synergic effect and an enhanced bi-dimensionality. Details of the above studies are described in the thesis with a conclusion at the end of each Chapter.

Lithium-ion battery is attractive for various applications because of its high energy density. The performance of Li-ion battery is influenced by several properties of the electrode materials such as particle size, surface area, ionic and electronic conductivity, etc. Porosity is another important property of the electrode material, which influences the performance. Pores can allow the electrolyte to creep inside the particles and also facilitate volume expansion/contraction arising from intercalation/deintercalation of Li+ ions. Additionally, the rate capability and cycle-life can be enhanced. The following porous electrode materials are investigated. Poorly crystalline porous -MnO2 is synthesized by hydrothermal route from a neutral aqueous solution of KMnO4 at 180 oC and the reaction time of 24 h. On heating, there is a decrease in BET surface area and also a change in morphology from nanopetals to clusters of nanorods. As prepared MnO2 delivers a high discharge specific capacity of 275 mAh g-1 at a specific current of 40 mA g-1 (C/5 rate). Lithium rich manganese oxide (Li2MnO3) is prepared by reverse microemulsion method employing Pluronic acid (P123) as a soft template. It has a well crystalline structure with a broadly distributed mesoporosity but low surface area. However, the sample gains surface area with narrowly distributed mesoporosity and also electrochemical activity after treating in 4 M H2SO4. A discharge capacity of about 160 mAh g-1 is obtained at a discharge current of 30 mA g-1. When the acid-treated sample is heated at 300 °C, the resulting porous sample with a large surface area and dual porosity provides a discharge capacity of 240 mAh g-1 at a discharge current density of 30 mA g-1. Solid solutions of Li2MnO3 and LiMO2 (M=Mn, Ni, Co, Fe and their composites) are more attractive positive electrode materials because of its high capacity >200 mAh g-1.The solid solutions are prepared by microemulsion and polymer template route, which results in porous products. All the solid solution samples exhibit high discharge capacities with high rate capability. Porous flower-like α-Fe2O3 nanostructures is synthesized by ethylene glycol mediated iron alkoxide as an intermediate and heated at different temperatures from 300 to 700 oC. The α-Fe2O3 samples possess porosity with high surface area and deliver discharge capacity values of 1063, 1168, 1183, 1152 and 968 mAh g-1 at a specific current of 50 mA g-1 when prepared at 300, 400, 500, 600 and 700 oC, respectively. Partially exfoliated and reduced graphene oxide (PE-RGO) is prepared by thermal exfoliation of graphite oxide (GO) under normal air atmosphere at 200-500 oC. Discharge capacity values of 771, 832, 1074 and 823 mAh g -1 are obtained with current density of 30 mA g-1 at 1st cycle for PE-RGO samples prepared at 200, 300, 400 and 500 oC, respectively. The electrochemical performance improves on increasing of exfoliation temperature, which is attributed to an increase in surface area. The high rate capability is attributed to porous nature of the material. Results of these studies are presented and discussed in the thesis.