Safer Lithium-Metal Batteries through Lithium Protection and Thermal-Electrochemical Modelling

In the perspective of the energy transition, both market field and research world recognised the electric energy storage as the main medium for this purpose. Specifically speaking, lithium ion batteries are already one of the technologies of choice to accomplish the road-transport electrification. Indeed, this work is devoted to the study of a high energy density cell technology. The latter is the most important requirement for an Electric Vehicle, and it can be achieved thanks to a high voltage cathode combined with a very high capacity anode. In the light of this, in this work a Ni0.6Mn0.2Co0.2O2 electrode whose specific energy can reach 200 Wh/kg was chosen as cathode, while lithium metal foil has been exploited as anode since it shows the outstanding specific capacity of 3860 Ah/kg and the lowest electrochemical potential (-3.040 V vs SHE). To better investigate such electrochemical system, in this work are addressed two leading issues: the mathematical modelling and the lithium protection. The former is a smart and effective tool in the drive toward developing better, safer and long-life batteries. Herein, a thermal-electrochemical model was implemented in finite element package Comsol Multiphysics 5.5. This model takes advantage of the coupling of macro and microscale descriptions: the first one describes charges and species transport in solid and liquid phases of both porous electrodes and electrolyte. Lithium concentrations and electrochemical potentials are assumed to depend only on the spatial coordinate x along the cell thickness. The microscopic field deals with the lithium intercalation, which depends on the radial dimension of a spheric electrode particle sited at some spatial location along x. This pseudo dimension combined with the 1D, is called the pseudo two dimensional (P2D) model. Relying on the porous electrode theory, Li-ions and charges transport are described by Fick’s and Ohm’s law respectively. Governing equations for concentrations and potentials are the conservation of mass and charge which are coupled at the solid-liquid interface by the Butler-Volmer equation. Transport and intercalation phenomena generate heat, which is calculated in the P2D model and then inserted in a 3D thermal model where the average temperature of the cell is computed. The last one is thereby implemented in the first model to update thermodynamic and kinetic variables. A second part of the thesis focus on the protection of the lithium anode. Since it is thermodynamically unstable, as soon as it is immersed in the electrolyte, it reacts forming a heterogeneous film named Solid-Electrolyte Interphase (SEI). On the protrusion of this layer Li-ions reduce forming dendrites which can reach the cathode causing short circuits and thus thermal runaways. A solution proposed in this work to avoid this escalation is a methacrylate-based polymer separator in which zirconia nanoparticles are added. Inorganic material is added in order to reach a higher Young’s modulus in the resulting Composite Polymer Electrolyte (CPE) which can suppress the dendrite growth, along with avoiding the formation of crystalline phase which would hinder the ionic conductivity. The proposed membrane has been physically and electrochemically characterized, giving back encouraging results. Hence a full cell improved with the composite membrane has been further implAnalisi dei processi di degradazione nelle celle Li-metallico con validazione di modelli multi-physics.