Electrode Materials for Na-Ion Batteries:A New Route for Low-Cost Energy Storage

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

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. From these images, it is possible to observe that the Sample 2 shows the worst distribution of the particles size, it presents irregular shaped agglomerates. Probably, this phenomenon depends on the fact that the powders were grounded only by a mortar and hence with an inadequate energy. On the contrary, Sample 3 present the best distribution, it is characterized by uniformly distributed particles. Probably, it is a consequence of the synthetic method. In fact sol-gel synthesis permit to “dissolve” the compound in a liquid in order to bring it back as a solid in a controlled manner. The chemical characterization of synthesized powders was carried out by the Inductively Figure 3: SEM images of Sample 2 (solid state Coupled Plasma Mass Spectrometry analysis, reaction) powder at different magnifications (32840 X using an Agilent 7500 series spectrometer, on the left and 5000 X on the right). with high frequency 3MHz quadrupole. This analysis was conducted with the aim to 4 understand the stoichiometry of metal oxides In the Figure 6 we can identify the presence obtained with different synthetic routes. of Na2CO3 reagent impurities. The pattern Metal oxides synthetized result to be: correspond to the Na0.71CoO2 gamma phase Sample 1 Na CoO ; 0.83 2 (mix phase γ + α’). Sample 2 Na 0.65 CoO 2; Sample 3 Na 0.28 CoO 2. The structural characterization of synthesized powders was carried out by the X-ray diffraction technique (XRD) using a Philips

X-ray diffractometer with Cu Kα radiation. The diffraction patterns were obtained between 10° and 70°. In the Figure 2 we can observe the X-Ray diffraction patterns of the synthetized powders.

Figure 7: XRD pattern of Sample 2. In the Figure we can identify the same reagent impurities and diffraction pattern. On the contrary, in this case there are different synthetic conditions (800°C, 0.65:1=Na:Co), that lead to pure γ phase.

Figure 2:Comparison of XRD pattern obtained by the three different synthesis. Comparing these data with those of “JCPDS- International Centre for Diffraction Data” database, chemical impurities and phase were identified.

Figure 8: XRD pattern of Sample 3. In the Figure we can identify the presence of

CO3O4 impurities. The pattern correspond to the Na0.60CoO2 beta phase.

Figure 6:XRD pattern of Sample 1.

Electrochemical characterization techniques

The electrochemical measurement are performed employing T-shaped polypropylene type cells (Figure 9) equipped Table 1: Percentage composition of Co01, Co02 and with stainless steel current collectors. Disk of Co03 layers. high-purity sodium foil are used as counter A solution of PVdF 5% in NMP (0.95 ml) and reference electrodes. A glass fiber .has been prepared into a vial. NaxCoO2 and (Whatman GF/A) with a diameter of 14 mm SC65 have been mixed finely in a mortar to is used. obtain an homogeneous powder and then they have been poured into the same vial which contains the solution of binder. Then, 0.25 mL of NM2P have been added in order to obtain a suspension liquid enough. The slurry has been stirred overnight with a magnetic anchor. The mixture has been stratified on an Al foil, scratched by employing sandpaper, through Doctor Blade technique setting a thickness of 200 µm. The obtained layers has been dried at 50°C, under hood, in order to remove completely the solvent. Several circular electrodes with diameter of 9mm, Figure 3:Schematic representation of a T-cell. have been cut before (Co01B, Co02B and Co03B) and after (Co01A, Co02A and Co03A) the use of a roll press. The cells are assembled into dry-box. All the Three different layer, for each powder synthetized, were manufactured using Sodium electrochemical characterizations are Carboxymethyl Cellulose. These layers haves performed using a Galvanostatic/potentiostat been prepared by casting a slurry of NaxCoO2 VMP2/Z by Bio-Logic. (active material), Super C65 (conductive The electrodes based on this three type of carbon) and Na-CMC (binder) in ultrapure NaxCoO2 were prepared by using different H2O (solvent), whose composition is shown binders: PVdF, Na-CMC and PAA. At the in Table 2. same time two different electrolyte were tested: NaPF6 and NaClO4. Each binder presents a different preparation technique.

Electrodes processing procedure: Table 2: Percentage composition of Co11, Co12 and Three different layer, for each powder Co13 layers. synthetized, were manufactured using Polyvinylidene fluoride. These layers have A solution of Na-CMC 5% in ultrapure H2O been prepared by casting a slurry of NaxCoO2 (0.95 ml) has been prepared into a vial. (active material), Super C65 (conductive NaxCoO2 and SC65 have been mixed finely carbon) and PVdF (binder) in NM2P in a mortar to obtain an homogeneous powder (solvent), whose composition is shown in and then they have been poured the same vial Table 1. which contains the solution of binder. Then,

0.55 mL of H2O ultrapure have been added in Co23B) and after (Co21A, Co22A and order to obtain a suspension liquid enough. Co23A) the use of a roll press. The slurry has been stirred for five hours with The capacity of each electrodes have been a magnetic anchor. The mixture has been computed considering a specific theoretical stratified on an Al foil, previously scratched capacity of: Sample 1 (Na0.83CoO2) 235 mAh/g by employing sandpaper, through Doctor Sample 2 (Na0.65CoO2)253 mAh/g Blade technique setting a thickness of 200 Sample 3 (Na0.28CoO2)275 mAh/g µm. The obtained layers has been dried at All obtained electrodes have been dried room temperature, in order to remove overnight at 120 °C, under vacuum, and then completely the solvent. Several circular put into dry-box. electrodes with diameter of 9mm, have been Electrochemical experiments: cut before (Co11B, Co12B and Co13B) and A preliminary test was conducted for Co01 after (Co11A, Co12A and Co13A) the use of (PVdF) layer, in order to characterize the a roll press. material, with galvanostatic charge/discharge Three different layer, for each powder cycles at different C-rates: 5 cycle at C/10, 5 synthetized, were manufactured using cycles at C/5, 5 cycles at C/2, 5 cycle 1C and Polyacrylic Acid. These layers have been 5 cycles at 2C. Charge/discharge cycles have prepared by casting a slurry of NaxCoO2 been carried out within the potential window (active material), Super C65 (conductive 2-3.8 V, comparing pressed and non-pressed carbon) and PAA (binder) in ethanol electrodes in NaPF6 electrolyte. All the (solvent), whose composition is shown in Table 3. potentials are given vs. Na+/Na.

100 C/10 C/5 Charge 80 Discharge

C/2 60 Table 3: Percentage composition of Co21, Co22 and

Co23 layers. 40 A solution of PAA 5% in Ethanol (0.4 ml) 80:10:10=Na CoO :PVdF:SC65

20 0.82 2 SpecificCapacity(mAh/g) .has been prepared into a vial. NaxCoO2 and UNCOMPRESSED 2C EC:DMC=1:1 NaPF 1M 6 SC65 have been mixed finely in a mortar to 0 0 10 20 obtain an homogeneous powder and then they Cycle Number have been poured the same vial which contains the solution of binder. Then, 0.4 ml 100 of ethanol have been added in order to obtain C/10 C/5 80 Charge a suspension enough liquid. The slurry has Discharge been stirred for five hours with a magnetic 60

stirrer. The mixture has been stratified on an C/2

Al foil, scratched by employing sandpaper, 40

1C through Doctor Blade technique setting a 20 80:10:10=Na CoO :PVdF:SC65 SpecificCapacity (mAh/g) 0.82 2 COMPRESSED thickness of 200 µm. The obtained layers has EC:DMC=1:1 NaPF 1M 6 2C 0 been dried at 60°C for two hours, in order to 0 10 20 remove completely the solvent. Several Cycle Number circular electrodes with diameter of 9mm, Figure 10: Comparison of specific capacity vs cycle have been cut before (Co21B, Co22B and number between Co01A (pressed) and Co01B (non- pressed). 7

The cells exhibit an Open Circuit Voltage of At this point an optimization of this type of 2.87 V. As we can see in the galvanostatic active material was conducted with three type curves Na deintercalation/ intercalation, of binders, comparing pressing conditions, cycled between 2 and 3.8 V, undergoes differents potential windows and two different complicated series of successive phase electrolyte. In the following figures are depict transitions. First sodium deintercalation curve the rate capabilities of Co01 (PVdF/Sample1) shows four voltage plateaus and for the initial layers in different electrochemical discharge profiles eight plateaus are shown in environments. Figure 11. For this reason, a differential analysis of galvanostatic cycles have been done, in order to have a better understanding of mechanism occurring during cycling.

Figure 13: Comparison of specific capacity vs cycle number between Co01A (pressed), Co01B (non- pressed), using NaPF and NaClO . 6 4 Figure11: Galvanostatic curves of Co01B at different From the Figure 13 we can observe that, with rate. the same electrolyte, Co01 layer present better The cells exhibit an Open Circuit Voltage of cycling performance, when the electrodes are 2.87 V. As we can see in the galvanostatic pressed. In fact the pressed electrode shows a curves Na deintercalation/ intercalation, specific capacity of 89.9 mA/h (charge C/10) cycled between 2 and 3.8 V, undergoes and 87 mA/h (discharge C/10), with a complicated series of successive phase capacity retention of 105.5% at C/5 rate, transitions. First sodium deintercalation curve characterized by an excellent reversibility. shows four voltage plateaus and for the initial Paying attention to capacity retention, the discharge profiles eight plateaus are shown in high value could be due to wettability of the Figure 11. For this reason, a differential electrode and to the change of active material analysis of galvanostatic cycles have been structure during charge/discharge cycles. A done, in order to have a better understanding probable capacity loss is due to the presence of mechanism occurring during cycling. of Na2CO3 impurities. We can find comparable capacity value in literature

600 although the specific theoretical capacity is 1st cycle C/10 500 2nd cycle C/10 much higher (235 mAh/g). We must 400 3rd cycle C/10

300 emphasize the fact that this type of battery is

200 not able to cycle at 1C rate with acceptable

100

dQ/dE (mAh/gV) dQ/dE capacity values. In the third experiment, 0

-100 reported in Figure 13, an higher potential -200 window was used, observing an increase in -300 2,0 2,2 2,4 2,6 2,8 3,0 3,2 3,4 3,6 3,8 E (V vs Na+/Na) specific capacity value, accompanied by a loss of reversibility. Figure12: dQ/dE vs E curves of Co01A. In the Figure 14, rate capabilities of Co11 (Na-CMC/Sample 1) non pressed layers are 8 depicted. Charge/discharge cycles have been Charge/discharge cycles have been carried out carried out within the potential window 2- within two different potential window (2-4V

3.8V, comparing NaPF6 and NaClO4 and 2-4.2 V), comparing NaPF6 and NaClO4 electrolytes. The experiment was conducted electrolytes. The cells have been cycled with with two different type of protocol. The the same protocol, at 5 cycle at C/10, 5 cycles Co01B/NaPF6/Na cell has been cycled at five at C/5, 5 cycles at C/2, 5 cycles at 1C, 5 different rates: 5 cycle C/10, 5 cycle C/5, 5 cycles at 2C and then again 5 cycles at C/10, cycle C/2, 5 cycle 1C and 5 cycle 2C. In the 5 cycles at C/5, 5 cycles at 1C and finally other side, the Co01B/NaClO4/Na cell has again 5 cycles at C/5. All the potentials are been cycled at 5 cycle at C/10, 5 cycles at given vs. Na+/Na. The best electrochemical C/5, 5 cycles at C/2, 5 cycles at 1C, 5 cycles performance has been obtained for the at 2C and then again 5 cycles at C/10, 5 Co21A/NaPF6/Na cell using a 2-4V potential cycles at C/5, 5 cycles at 1C and finally again window, although it present a low 5 cycles at C/5. All the potentials are given reversibility. The electrode shows a specific vs. Na+/Na. From both experiment it emerges capacity of 128.9 mA/h (charge C/10) and that this type of active material does not work 103 mA/h (discharge C/10), with a capacity with Na-CMC as binder. The cells have too retention of 112.5% at C/5 rate. This type of low capacity, this type of binder probably result is probably due to the electrode loading. provide stronger interaction than the other, In fact this electrode present 1.80 mg as providing insulating properties to the layers. weight, compared to 3.17 mg and 2.98 mg of the others two. Electrodes, obtained from Sample 2 powder, have been subjected to test with different binder, electrolyte and pressing.

Figure 14: Comparison of specific capacity vs cycle number of Co11B/NaPF6/Na and Co11B/NaClO4/Na cells. In the Figure 15 the rate capabilities of Co21A (PAA/Sample1) layers in different electrochemical environments are depicted. Figure 16: Comparison of specific capacity vs cycle number of Co02A/B layers with NaPF6 and NaClO4. Galvanostatic charge/discharge cycles, differential analysis of galvanostatic cycles, cyclic voltammetry and Electrochemical Impedance Spectroscopy have been conducted. In the Figure 16 the rate capabilities of Co02B (PVdF-Sample2) layers Figure 15: Comparison of specific capacity vs cycle in different electrochemical environment are number of Co21A/NaPF6/Na and Co21A/NaClO4/Na cells. depicted. Charge/discharge cycles have been carried out within the potential window 2-3.8 9

+ V, comparing NaPF6 and NaClO4 electrolytes, vs. Na /Na. Also in this case, from both for pressed and non-pressed electrodes. The experiment it emerges that this type of active experiment was conducted with two different material does not work with Na-CMC as type of protocol. The non-pressed electrodes binder. The cells have too low capacity. has been cycled at six different rates: 5 cycle In the Figure 18 the rate capabilities of Co22 C/10, 5 cycle C/5, 5 cycle C/2, 5 cycle 1C, 5 (PAA-Sample2) layers in different cycle 2C and finally again C/5 for 30 cycles. electrochemical environments are depicted. On the other side, the pressed electrodes has been cycled at 5 cycle at C/10, 5 cycles at C/5, 5 cycles at C/2, 5 cycles at 1C, 5 cycles at 2C and then again 5 cycles at C/10, 5 cycles at C/5, 5 cycles at 1C and finally again 5 cycles at C/5. All the potentials are given vs. Na+/Na. The best electrochemical behaviour is shown by the unpressed Figure 18: Comparison of specific capacity vs cycle electrode, using NaClO4 as electrolyte, that number of Co22A with NaPF6 and NaClO4. presents a specific capacity of 98 mA/h (charge C/10) and 94 mA/h (discharge C/10), Charge/discharge cycles have been carried out with a capacity retention of 101% at C/5 rate. within 2-4V potential window, comparing In the Figure 17 the rate capabilities of Co12 NaPF6 and NaClO4 electrolytes. The cells have been cycled with the same protocol, at 5 (Na-CMC-Sample 2) layers in different cycle at C/10, 5 cycles at C/5, 5 cycles at C/2, electrochemical environments are depicted. 5 cycles at 1C, 5 cycles at 2C and then again 5 cycles at C/10, 5 cycles at C/5, 5 cycles at 1C and finally again cycled at C/5. All the potentials are given vs. Na+/Na. The best electrochemical performance has been

acquired for the Co22A/NaPF6/Na cell, although it presents a low reversibility. The electrode shows a specific capacity of 123 mA/h (charge C/10) and 104 mA/h (discharge C/10), with a capacity retention of 96% at C/5 Figure 17: Specific capacity vs cycle number of rate. Co12B/NaPF6/Na. Electrodes, obtained from Sample 3 powder, The experiment was conducted with two have been subjected to test only with NaPF6 different type of protocol. The first cell has electrolyte, using PVdF as binder. been cycled at five different rates: 5 cycle Galvanostatic charge/discharge cycles have C/10, 5 cycle C/5, 5 cycle C/2, 5 cycle 1C and been conducted, obtaining very low specific 5 cycle 2C. In the other side, the other cell has capacity values. In the Figure 19 the rate been cycled at 5 cycle at C/10, 5 cycles at capabilities of Co03 (PVdF-Sample 3) layer C/5, 5 cycles at C/2, 5 cycles at 1C, 5 cycles are depicted. The cell has been cycled at five at 2C and then again 5 cycles at C/10, 5 different rates: 5 cycle C/10, 5 cycle C/5, 5 cycles at C/5, 5 cycles at 1C and finally again cycle C/2, 5 cycle 1C and 5 cycle 2C. All the 5 cycles at C/5. All the potentials are given potentials are given vs. Na+/Na. Only one

experiment has been conducted with this During the first cycle four cathodic peaks sample because it shows too low capacity were found at 2.9V, 2.95V, 3.3V and 3.7V. In values. It could be due to the different phase the second cycle, there are eight distinct presented by this type of stoichiometry, and cathodic peaks at 2.35 V, 2.43 V, 2.55 V, 2.6 by the low amount of sodium in the starting V, 2.7 V , 2.9 V, 3.25 V and 3.7 V while we active material of the electrode. can observe eight corresponding anodic

peaks. The strange shape of voltagram shown

50 in Figure 21, in the 4-4.2V region, could be Charge Discharge 40 C/10 due to an overpotential. From further studies

Unpressed 80:10:10=Na CoO :PVdF:SC65 about structure change conducted by J.J.Ding 30 0.28 2 EC:DMC=1:1 NaPF 1M C/5 6 Errore. Il segnalibro non è definito. et Al. , we can 20

SpecificCapacity understand that sodium 10 C/2,1C,2C intercalation/deintercalation into/from 0 lamellar structure should be responsible for 0 5 10 15 20 25 Cycle Number the changes of c-lattice parameter. The c-

lattice parameter become larger with Figure 19: Specific capacity vs cycle number of decreasing sodium content. It is believe that Co03B/NaPF6/Na cell. the expansion in the c-axis direction should be Observing that non-conclusive results have attributed to increase in electrostatic repulsion been obtained with the differential analysis, from the negatively charged oxygen cyclic voltammetries have been done. Cyclic interactions of [CoO2] layers with the removal voltagram curves of Na/NaPF6/Co01B and of sodium. It can be estimated that Na

Na/NaPF6/Co02B cells at the scan rate of intercalation and deintercalation leads to 4% 20μV/s are shown in Figure 20 and 21. contraction and expansion along c- axis. The variation of the interatomic distance is closely related to the change in the valence state of the transition metal. In this direction we can ipotize a phase change during charge/discharge cycles of cell.

Figure 20:Cyclic voltagram for Co01B/NaPF6/Na cell between 2-3.8 V.

To better understand the mechanism of the processes during cycling, changing the potential as a linear function of time, every 20 mV impedance spectra have been acquired in the frequency region range from 101kHz to 4.9 mHz. This type of analysis has been conducted with PVdF layers of sample 1 and 2, using NaPF6 1M in EC:DMC=1:1 as

electrolyte. For every cell the same protocol Figure 21 Cyclic voltagram for Co02B/NaPF6/Na cell has been used: OCV (10000s), Linear Sweep between 2-4.2 V.

Voltammetry, Staircase Potentio high electronic resistance covers the diffusive Electrochemical Impedance Spectroscopy region, preventing us to recognize it. (2V4V) and Staircase Potentio Electrochemical Impedance Spectroscopy (4V2V); Figure 22 and 23 shows Nyquist plots of Co01 and Co02 layers at T=25°C. The aim of the experiment is to know how the variations of the electronic resistance are related to the quantity of sodium in the lattice. The preliminary electrochemical impedance spectroscopy study of these samples clearly showed a strong dependence of the shape of their EIS response spectra upon the working electrode potential, suggesting the existence of a potential region in which the electronic conductivity of the material is the limiting factor in controlling the electrochemical process. In the figures, it appears that the evolution of the spectra may be described as associated with all the physical phenomena that typically characterize a charge transfer at passivated cathode materials , that is: Figure 22: Nyquist plots of Co01A oxidation and (i) a high-frequency region (>1 kHz) reduction. characteristic of a SEI passivating layer; (ii) an intermediate-frequency region (between 10 Hz and 1 kHz) characteristic of a charge transfer process; (iii) a low-frequency region associated with the electronic properties of the material; (iv) the very low frequency region of the ionic diffusion. The data obtained from these experiment show different tendencies. In the high frequency region we can recognize the increase of resistance related to the formation of passivation layer and the gradual decomposition of electrolyte. In the intermediate frequency region, there is a semicircle related to the charge transfer between electrode and electrolyte resistance. Finally there is a large arc that could be associated to two different factors. In fact, the

Figure 23: Nyquist plots of Co02A oxidation and research was focused on the optimization of reduction. the system, monitoring the effect of sodium This phenomenon could be explained by carboxymethyl cellulose (Na-CMC) and semi-conductor behaviour of the material, and Polyacrylic acid (PAA) as binder, and in the the change with E of the diameter of the low- same time the influence of different frequency arc may be ascribed to the complex electrolytes (NaPF6 and NaClO4). structural changes which occur in the From the data obtained, Na-CMC has shown structure of the active material as Na is negative effect in all experiment. Testing (de)intercalated. For the ‘similar’ Li electrodes with galvanostatic intercalation cathodes, this behaviour has charge/discharge cycles at different C-rate, been explained by a band electronic theory 8, CMC presented very low specific capacity, in which stated that AxMX2 compounds are the range between 8-25 mAh/g. These values divided in two categories in base on the RM-M result to be unacceptable for a battery device. distance in the lattice. When RM-M is higher For what concern PAA as binder, it result to than a critical value, the system assume an be a green valid substitute of PVdF. In fact it insulating behaviour, or vice versa a present good performances in particular conductor behaviour. Increasing the RM-M, conditions (low values of mass loading). For wavefunction overlap decrease, producing these reasons, further studies should be done band separation and electron localization. In to test the effect of the layer thickness. An the extreme case of Mott Insulators with optimization of electrode preparation could be localized electrons, the conduction occurs done, monitoring the capacity increase as a only through an hopping mechanisms of few function of decrease in mass loading. electrons that can jump when subjected to On the other hand, no significant differences thermic excitation or statistic deviation. We were observed, using NaClO4 or NaPF6 as can recognize this type of semi-conductor the electrolyte salt. The electrochemical behaviour in our Na-based samples, even if performance, probably, depend on the the dependency of resistance on the changes electrolyte solvent used. Further studies of potential and structure is hard to rationale should be done using EC:PC as electrolyte because of the complexity of phases involved solvent. in NaxCoO2 (de)intercalation processes. In regard on the active materials synthetized, Further studies could be done by in situ X-ray it was possible to observe a strong diffraction studies of the correlation between dependence of charge/discharge behaviour on the electronic resistance (i.e., charge the different stoichiometry obtained. This is transport) and the structural properties (i.e., related to different phases assumed by the cell parameters) during the stages of sodium layered oxide, influencing the deintercalation from NaCoO2, using an intercalation/deintercalation process and electrochemical cell that permits in situ strongly affecting the reversibility of measurements of ac-impedance dispersions charge/discharge process. The Sample 1(mix and X-ray diffraction spectra. phase γ + α’) obtained by “Ball-Milling and post firing”, result electrochemically active, Conclusion and future developments but a loss of specific capacity must be In this research work, a preliminary study of emphasized. It is probably due by the presence of impurities and two different NaxCoO2 , as cathodic material for sodium- ion batteries , has been conducted. The phases. Only one experiment has been

13 conducted with Sample 3 (β phase), obtained by sol-gel method, because it shows too low capacity values. It could be due to the not suitable phase presented by this type of stoichiometry, and to the low amount of sodium in the starting active material. Further studies could be done for the optimization of the synthetic condition of sol gel method, with the aim to obtain higher sodium content. The Sample 2 (pure γ phase), obtained by simple solid state reaction, result to be the best choice. It present high stability, reversibility and good specific capacities, so it represents a good starting point for future material and electrode optimization.

References

1. Tarascon JM, Armand M., Nature, 414 (2001) 359-367.

2. Yaksic A, Tilton JE., Resource Policy, 34 (2009);185.

3 Ellis B.L., Nazar L.F. , Science, 16 (2012) 168–169.

4 Palomares V., Casas-Cabanas M., Castillo- Martınez E., Han M H, Rojo T., Energy Environ. Sci., 6 (2013) 2312.

5 Palomares V., Serras P., Villaluenga I., Hueso K.B., Carretero-Gonzalez J., Rojo T., Energy Enviro, Sci.,5 (2012) 5884.

6 Amrtha Bhide, K. Hariharan, Solid State Ionics,192(2011)369.

7 B.J. Hwang, R. Santhanam, D.G. Liu, Journal of Power Sources,102(2001) 326.

8 J Molenda, A Stokłosa, T Ba̧k , Solid State Ionics, 36-1/2, (1989)53-58.