Towards Realization of an Innovative Li-Ion Battery: Materials Optimization and System Up-Scalable Solutions

The optimisation of existing chemistries by the introduction of environmentally friendly materials and the simplification of the device production process are intriguing challenges to promote the future widespread diffusion of LIBs. Moreover, the recent development of the next-generation electronic devices promoted a new research field for the modification of the current systems into light, flexible and/or micro-sized device. The enhancement of mechanical properties through the introduction of flexible electrodes will enable LIBs to be embedded into various functional systems in a wide range of innovative products such as smart cards, displays and implantable medical devices. Moreover, the optimisation of the electrolyte by moving towards an all-solid-state configuration will offer adaptability to various designs and stressful mechanical handling, as well as enhance cell safety and reliability. During the three years of the Ph.D. course, the attention was focused on the optimisation of innovative materials for Li-ion batteries as well as the development of easily up-scalable procedures for the production of electrodes and polymer electrolytes. The basic idea was to start from eco-friendly materials to develop simple, low-cost and easily adaptable processes in order to propose innovative solutions for LIBs with a wide range of possible applications. Moreover, during my experimental activities, I considered the performances and the cycling stability of Li-ion batteries, by studying the mechanisms related to the capacity fade of lab-scale batteries and also by analysing commercial Li-ion batteries for automotive application. The results of the research work are presented in this thesis (Chapters 4-7) following an introductory section that provides the general information needed to follow the discussions (Chapters 1-3). The experimental research work presented in Chapter IV was carried out in collaboration with the Laboratory of Pulp and Paper Science and Graphic Arts (LGP2) in Grenoble (France). A well-known natural material such as cellulose was exploited for the production of innovative low-cost and easily recyclable electrodes for Li-ion batteries. A simple aqueous filtration process, based on a well-known industrialised paper-making technology, was developed and the electrodes (graphite-based anodes and LiFePO4-based cathodes) produced and partly characterized in Grenoble by Dr. Lara Jabbour were electrochemically studied in our Labs in Politecnico di Torino. In particular, cellulose fibres (FBs) were used as natural binder for the production of paper-like electrodes obtained without addition of any synthetic binder and/or solvent and showing electrochemical performance comparable to those produced with the same active materials by a standard process. In Chapter V, results are reported regarding a newly developed procedure where a methacrylic-based polymer electrolyte is directly formed in situ at the interface with the electrodes. Exploiting the versatile nature of UV-induced free-radical photo-polymerisation, novel ready-to-use multiphase electrode/electrolyte composites (MEEC) were developed in which the electrode is conformally coated by the polymer electrolyte. This “one-shot” process was successfully applied to enhance the cycling performances of two nanostructured materials conceived for microbattery application, such as Cu2O (in collaboration with CSHR@Polito IIT research institute in Torino) and V2O5 (in collaboration with Prof. Mustarelli’s group in University of Pavia), prepared in the form of thin films and proposed respectively as anode and cathode. The proposed one-shot process, thanks to the intimate interfacial contact between electrodes surface and electrolyte obtained by in situ process, induced a huge effect of stabilization thus improving the cycling stability of both the nanostructures. All along Chapter VI, the problems related to the assembling of complete Li-ion cells, starting from two well performing electrodes, are progressively discussed and valuable solutions are proposed. A strong capacity fade was initially found, thus the possible causes were studied also considering the failure mechanisms proposed in the literature. Several measures were adopted to improve the cycling stability, considering the effect of all the different cell components as well as the effects of both charging protocol and cell apparatus. Moreover, due the knowhow progressively achieved on the intimate characteristics of complete Li-ion cells and their assembly, even thanks to a three months stage at ENEA Casaccia Research Centre of Rome, the installation of a 10 m2 dry room was personally followed at our Electrochemistry Research Group Labs in Politecnico di Torino and the results obtained are presented in the same Chapter VI. These results include the realisation of an all-paper Li-ion battery with the cellulose-based electrodes and paper hand-sheets as separator. Finally, the cycling stability and the failure prediction issue was studied for a 53 Ah commercial battery. The results obtained, by means of different standard reference tests, are reported in Chapter VII. The commercial battery was also disassembled in the controlled atmosphere of an Ar-filled dry box in order to study the system structure and characterise the various components. A testing protocol was personally developed and the results obtained allowed to evaluate the commercial battery based on the performances requested for HEV and EV application. In particular, an easy measure of the internal resistance was developed, by opportunely modulating the measured parameters, and the obtained results were found to be very useful in directly predicting the cell failure which is fundamental in practical application.