Advanced and functional materials for sodium secondary batteries

Over the past decades, the relevance of clean and efficient energy production and storage has enormously grown worldwide, primarily driven by concerns over global warming, dwindling fossil-fuel reserves and increasing demand for portable electronics, electric mobility and grid storage systems. Modern energy economy is at a serious risk due to a series of factors, including the continuous increase in the demand for oil, the depletion of non-renewable resources, the dependency on politically unstable oil producing countries and the related CO2 emissions, which results in global temperature rise with associated dramatic climate changes. Near-future implementation of renewable energy will demand a sudden growth of inexpensive, safe and efficient energy storage systems, thus the extension of batteries to large-scale storage will become essential in addressing the global challenge of clean and sustainable energy. In such a scenario, particularly for large-scale stationary electric energy storage systems, is crucial to find a valid alternative to lithium, in order to develop battery prototypes with similar characteristics in terms of energy densities and performances, but cheaper than the existing ones, and with a better look at the sustainability of the all of the components of the cells. Amongst the post-LIBs technologies under development, sodium ion batteries (NIBs) appears to be the most appealing and ready-to-use system. Indeed, sodium mineral resources are “unlimited”, attainable at low cost, and equally geodistributed. Clearly, technological advances, particularly in materials’ science viewpoint, must be effectively implemented: the electrode materials need to have high capacity and durability, while the electrolyte should be a solid membrane capable of high ionic conductivity even at ambient temperature, with good mechanical and interfacial properties and stable performances. In all cases, the materials must be inexpensive, ecologically friendly and safe. Taking into account the abovementioned challenges and expectations particularly for large-scale energy storage devices in the near future, the target of this PhD Thesis was the study and the development of novel polymer electrolytes and organic electrodes to fabricate high energy, safe and ecofriendly sodium-ion batteries and the assessment of their physico-chemical characteristics and electrochemical behaviour. The outcome of the work contains six chapters, comprising introductory section and concluding remarks. Chapter 1 deals with an introductory overview on the global energy scenario, present energy storage systems and future alternatives. Chapter 2 highlights the basic concepts and fundamentals of batteries and elucidates the different components of sodium-based batteries, comprising a concise explanation of the materials and components relevant to the Na-ion battery technology. Chapter 3 reviews in details the different sodium ion conducting polymer electrolytes developed by the scientific community during recent years, the state of the art of polymer-based electrolytes is illustrated along with the next future prospective in this field. The experimental part of this thesis deals with the research work carried out on the development of highly performing electrolyte materials for Na-ion cells. In Chapter 4 the development of cellulose-based hybrid polymer electrolytes for green and efficient Na-ion batteries is presented. This is the first ever report where the useful characteristics of carboxymethylcellulose sodium salt as additive in a Na+-ion conducting polymer electrolyte are explored. The same Na-CMC is also used as binder for the active electrode material particles, which enables the overall process including the electrodes and electrolyte preparation to be carried out through very simple, cheap and absolutely eco-friendly water based procedures. The preliminary results of lab scale cell testing in terms of galvanostatic charge/discharge cycling strongly recommend the use of such hybrid solid polymer electrolyte for the development of safe and sustainable Na-ion polymer cells. Chapter 5 details the experimental results regarding the UV-induced photopolymerization strategies adopted to produce crosslinked quasi-solid polymer membranes, highly suitable as electrolyte separators in Na-ion cells. In particular, the first section demonstrates the possibility of preparing gel-polymer electrolyte membranes using reactive methacrylic-based oligomers, together with different reactive diluents and some organic plasticisers. The prepared polymer membranes are activated by a swelling process to incorporate the sodium salt (source of Na+ ions) and, finally, fully characterized in both physical and electrochemical point of view. Very high values of ionic conductivity are obtained even at ambient temperature, results almost comparable to the values obtained for liquid electrolytes. These swelled methacrylic-based polymer membranes can provide efficient cycling behaviour as electrolyte separators in Na-ion cells. The second section deals with an innovative approach, in which a rapid one-step process is proposed to prepare quasi-solid polymer electrolyte membranes in a facile and versatile way, so that the maximum advantage of UV-induced photopolymerisation can be exploited. The poly(ethylene oxide) – PEO based polymer electrolytes are prepared by directly incorporating the Na-X electrolyte solution into the reactive mixtures during preparation. This approach is highly advantageous because it avoids the time consuming swelling step. In fact, in less than 15 minutes, a ready-to-use truly quasi-solid polymer electrolyte membrane can be prepared, showing promising characteristics to be used as a highly ionic conducting, safe separator in sodium based cells. The battery assembled with the crosslinked PEO-based electrolyte can provide a stable specific capacity of 250 mAh g-1 at ambient temperature and demonstrates to be very stable upon prolonged galvanostatic cycling at 0.1 mA cm-2 for more than 6 months of continuous operation. This rapid technique gives the opportunity to prepare polymer electrolytes in a fast way, which could give easy scale up features if considered for bulk industrial production. The final part of the PhD thesis work is focused on the development and electrochemical characterization of a carboxylate organic electrode for NIBs, the disodium benzenediacrylate (Na2BDA). The material was synthesized and optimized during a 6 months’ stage of research at the Ångströmlaboratoriet (Ångström Laboratory) of Prof. Daniel Brandell at the Uppsala University, Sweden. This is thoroughly discussed in Chapter 6. The main target is to find the most suited liquid electrolyte to exploit the full potential of the material. Such a study is fundamental particularly for organic electrodes, due to their high solubility in liquid electrolyte media, which always leads to rapid decay in electrochemical performances. The use of different (aqueous Na-CMC and non-aqueous PVdF) binders is also investigated, along with the use of mechanical calendering during electrode preparation in order to understand if the cell performances are affected to the composite electrode formulation. The electrochemical study is extended to severe rate capability test at ambient temperature as well as very long-term constant current cycling up to 5C current rate, which confirms the very long-term stability of the newly developed organic electrode as demonstrated by the very stable capacity output in Na metal cells upon prolonged constant current discharge/charge cycling for over 3700 cycles at ambient temperature.