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The high specific energy and power available from lithium-ion batteries and the possibility to charge / discharge them many times are the reasons of their key importance in electronic portable devices, and future development of hybrid and fully electric vehicles. Graphite and LiCoO2 are used as electro-active materials in standard commercially available lithium-ion batteries. Despite their utilization, the current active materials have some severe disadvantages. They are expensive, have slow kinetics of reaction for high-power applications, cannot deliver the total charge stored inside, and LiCoO2 is carcinogenic. Many challenges rise from the effort to increase the specific energy and power, and also to increase the cycle life of the battery. A greater understanding of the Li exchange mechanism will facilitate the development of new active materials with higher performances. The motivation for this Ph.D. work was deepening the insight on the mechanisms of the overall intercalation processes by using the electrochemical impedance spectroscopy technique (EIS) and correlating the electro-catalytic properties of the active materials with their structure by using the differential electrochemical mass spectrometry technique (DEMS). Differential Electrochemical Mass Spectrometry permits a qualitative and quantitative analysis of the electrochemically developed volatile compounds. It was used to determine the interactions between the active materials and the electrolyte. These interactions are followed by degradation of the active material, with loss of the electro-catalytic properties, or passivation of the surface of the active material, which kinetically blocks further interactions. Both these processes commonly involve gaseous products, which can be detected by a mass spectrometer. A new cell and a new experimental setup were built to provide an increased sensitivity of the experimental data. The new cell has given the possibility to implement a third electrode as reference electrode for the electrochemical measurements. The new experimental setup, based on two controlled argon fluxes, has reduced the influence of the background intensity on the data quality. An analytical tool to process the experimental results was developed, to subtract the background intensity from the mass signals. The DEMS study on graphite particles has shown the development of CO2 at 1 V (Li/Li) in the first reductive half-cycle. The development of CO2 was indirectly suggested but never experimentally proved before. It was possible thanks to the increased sensitivity of the DEMS setup and the sophisticated subtraction of the background intensity from the mass signals. The DEMS study on Li1+x(Mn1/3Ni1/3Co1/3)1-xO2 compounds, possible substitutes of LiCoO2, has proved the development of oxygen in the first cycle at potentials above 4.5 V (Li/Li) for the compound with x = 0.1, while no oxygen was detected for the compound with x = 0. The quantitative analysis of the DEMS data for the compound with x = 0.1 has confirmed that the irreversible specific charge observed during the galvanostatic cycling experiments for the x = 0.1 compound in the first cycle is due to the oxygen loss. Electrochemical Impedance Spectroscopy is an electrochemical tool which helps in the detection of the kinetic limitations in the system under investigation. It can reach high time resolution (10 s) and is particularly suitable to the study of reaction mechanisms (effects of the surface energetic), current density distribution (effects of electrode geometry), and transport of mass (diffusion and migration in the bulk). Several physico-chemical models describing surface and bulk processes was developed and the correlation between EIS data and kinetic properties of the system were obtained. A new electrochemical cell was designed, to bring the experimental setup nearer to the mathematical description of the models. An analytical tool to fit the experimental data with different models was successfully developed and used. It was found out that the only plausible electrochemical process describing the EIS data before the formation of the SEI for several types of graphite particles is an adsorption through surface states. It was also observed that the limitations due to the pore structure in electrodes based on graphite are dependent on the size of the particles; in particular, the smaller the graphite particle, the higher is the resistance of the pores. Beside the original PhD goals, a new cell for the study of the current density distribution was developed and tested on electrodes based on graphite particles. The new cell is called multiple-working-electrode (MWE) cell. Using the MWE cell, it was observed that the layer of graphite nearer to the counter electrode has a higher value of the irreversible specific charge with respect to the farer layers. A hypothesis for explaining this behavior of the graphite electrodes was formulated.

Characterization of electrodes for lithium-ion batteries through electrochemical impedance spectroscopy and mass spectrometry