Electrode materials for Na-ion batteries:a new route for low-cost energy storage. Massaccesi Valentina Instituto Superior Técnico, Universidade Técnica de Lisboa Abstract. With the imminent exhaustion of fossil fuel resources and increasing environmental problems, a variety of renewable and clean energy sources, such as the wind and sun, are growing rapidly . The use of these discontinuous energy source requires a large-scale energy storage system (ESS) to shift electrical energy from peak to off-peak periods, with the aim to realize smart grid management. Among various energy storage technologies, room-temperature stationary sodium-ion batteries have attracted great attention particularly in large scale electric energy storage applications for renewable energy and smart grid because of the huge abundant sodium resources and low cost1. The research work presented in this thesis deals with the investigation of electrochemical properties of electrode materials for this tipe of batteries, in particular NaxCoO2 as cathodic compound. In the first part of this thesis, several synthetic routes have been studied. The active materials obtained have been investigated by XRD and ICP-MS analysis to evaluate the correlation between stechiometry and crystal structure. A morphological characterization was conduced using SEM. In the second part of this thesis, matherials have been tested electrochemically by GCPL, CV and PEIS. Finally an optimization of the system have been conduced evaluating the use of different elecrolytes and binders. Key words: Na-Ion Batteries, NaxCoO2 Cathode Material, X-Ray Powder Diffraction, Electrochemical Characterization, Electrochemical Impedance Spectroscopy. Introduction Lithium-ion batteries, the most common type of secondary cells found in almost all portable electronic devices, are a possible solution to Energy storage has become a growing global these larger global concerns1. Lithium based concern over the past decade as a result of electrochemistry offers several appealing increased energy demand, combined with attributes: lithium is the lightest metallic drastic increases in the price of refined fossil element and has a very low redox potential fuels and the environmental consequences of + (E°Li /Li=-3.04V versus standard hydrogen their use. This has increased the call for electrode), which enables cells with high environmentally responsible alternative voltage and high energy density. Furthermore, sources for both energy generation and Li+ has a small ionic radius which is storage. Although wind and solar generated beneficial for diffusion in solids. Coupled electricity is becoming increasingly popular in with its long cycle life and rate capability, several industrialized countries, these sources these properties have enabled Li-ion provide intermittent energy; thus energy technology to capture the portable electronics storage systems are required for load- market. The demand for lithium-ion batteries levelling. as a major power source in portable electronic 1 devices and vehicles is rapidly increasing. could be developed, it could have the With the likelihood of enormous demands on advantage of using electrolyte systems of available global lithium resources, concerns lower decomposition potential due to the over lithium supply, but mostly its cost, have higher half-reaction potential for sodium arisen2. Even if extensive battery recycling relative to lithium. This low voltage operation programs were established, it is possible that would make Na-ion cells cheaper, because recycling could not prevent this resource water-based electrolytes could be used instead depletion in time. While the debate over the of organic ones. It must be pointed out that feasibility and environmental impact of electrochemical Na-ion cells will always fall lithium carbonate production continues, short of meeting energy densities compared to sodium-based compounds are under Li-ion batteries. First, because equivalent consideration as options for large scale energy weight of Na is higher than Li, and second storage coupled to renewable energy sources, because the size of the alkali metal is bigger. for example. Thus, Na-based cells will have difficulties With sodium’s high abundance, low cost and competing with Li based cells in terms of + very suitable redox potential (E°Na /Na=-2.71 energy density. However, they can be V versus standard hydrogen electrode, only considered for use in applications where the 0.3 V above that of lithium), rechargeable weight and footprint requirement is less electrochemical cells based on sodium drastic, such as storage of off-peak and represent the most promising device for essentially fluctuating renewable energies, energy storage applications3. All such as wind and solar farms. In spite of these characteristic of this alkali holds to make this considerations, there exists growing interest element strategic in innovative research of on Na-ion technology5. energy storage systems4. The use of Na The research work presented in this thesis instead of Li in rocking chair batteries could deals with the investigation of mitigate the feasible shortage of lithium in an electrochemical properties of electrode economic way, due to the unlimited sodium materials for this sodium-ion batteries, in sources, the ease to recover it and its lower particular NaxCoO2 as cathode. price. In the first part of this thesis, several synthetic routes have been studied. The active materials obtained have been investigated by X-ray and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analysis to evaluate the correlation between stoichiometry and crystal structure. A morphological characterization was carried out using Figure 1:Main characteristics of Na and Li materials Scanning Electron Microscope (SEM). In the second part of this thesis, materials have been Moreover, for positive electrode materials tested electrochemically by Galvanostatic sodium intercalation chemistry is very similar Cycling with Potential Limitation (GCPL), to Li, thus making it possible to use very Cyclic Voltammetry (CV) and similar compounds for both kinds of systems. Electrochemical Impedance Spectroscopy Furthermore, if a rechargeable sodium-ion (EIS). Finally an optimization of the system battery with good performance characteristics 2 has been done evaluating the use of different mechanical milling in an agate bowl with electrolytes and binders. agate balls at 200 rpm for one hour. Then the powder is fired in a preheated furnace at 6 Experimental Section T=800°C for 12 hours in air . The sol-gel method can overcome some Synthetic techniques: disadvantages of conventional solid state method thanks to its low processing NaxCoO2 has been synthetized by solid state temperature, high homogeneity, possibility of method, ball milling accompanied by post firing controlling size and morphology of the and sol-gel method. particles7. Molecular precursors are converted The first is the most common method in to nanometre-sized particles, to form a which stoichiometric mixture of starting colloidal suspension, or sol. Usually, materials is ground together and the resultant stoichiometric amounts of sodium acetate ( mixture is heat–treated in furnace. In the case CH3COONa ) and Cobalt(II) acetate of NaxCoO2, appropriate amount of starting tetrahydrate ( Co(CH3COO)2(H2O)4 ) are materials, Na2Co3 and CoO4, are thoroughly dissolved in an appropriate quantity of mixed in the ratio of 1:1. Subsequently, the distilled water at room temperature. The mixture is ground to ensure complete solution is stirred at T=50° C. Then calculated reaction. After drying, the powder is calcined amount of citric acid is added as a in a preheated furnace at 800° C to form the complexing agent in the polymeric matrix, in precursor. Initially the reaction is carried out order to form the sol. The amount of the citric for 12 hours. The product was again subjected acid and acetates is maintained at 3:1 molar to solid state reaction for 12 hours, under ratio. The temperature of the solution is raised oxygen flow, after intermediate grinding . to T=100°C for about 5 hours and continued The purity of the material depends on the stirring till the solution turned into high- choice of the ratio of starting materials, viscous pink gel. Subsequently, ethylene calcination temperature and time. glycol is added to the solution as gelling The Ball Mill and post firing method involve agent. This solution is further heated at T=80° the use of a particular grinder characterized C in order to get a precursor. The product by a hollow cylindrical bowl rotating about its results to be crystalline and purple. It is finely axis, partially filled with balls (grinding ground and calcined (at T=250° C for 10 h media). This is an alternative way to use the and at 700° for 10 h) to obtain the final classical solid state synthetic method. product. Finally, the black colored calcined Reagents are mixed together in a different product is ground, dried under vacuum and way, using a ball mill. It is used wherever the collected. highest degree of fineness is required and it works on the principle of impact and attrition, Chemical, structural and morphological size reduction is done by impact as the balls characterization techniques: drop from near the top of the bowl. In this case sodium cobalt oxide is synthesized from The chemical, structural and morphological Na2CO3 and CoCO3 powders in a 1:2 mol characterization of synthesized powders has ratio through the alternative approach, which been carried out by using several techniques, employs ball milling and subsequent firing. such as X-Ray Powder Diffraction (XRD), For this, the mixture is subjected to Scanning Electron Microscopy (SEM) and 3 Iductively Coupled Plasma Mass Spectrometry (ICP-MS). Figure 4: SEM images of Sample 3 (sol-gel method) powder at different magnifications (33070 X on the left and 5000 X on the right). Figure 2: SEM images of Sample 1(Ball-Milling and post firing) powder at different magnifications (32100 The morphology of the powders was probed X on the left and 5060 X on the right) by Scanning electrode microscopy (SEM). Images were obtained with JEOL Model JSM-5400 equipped with a Shimadzu 800HS EDX detector.