Thin film batteries for integrated high energy density storage

Overview & Synopsis

Among disruptive technologies, the Internet of Things (IoT) is the most popular. The concept was born in 1999, and today it is still a topic worth discussing. The vision behind the word IoT is a hyper-connected environment where each inch of our world is populated by countless devices, smaller than a coin: a mist of intelligent droplets, engineered to catch even thesmallest piece of available data. The IPv6 protocol and the 5G technologies aim to provide the communication infrastructure to manage such a huge amount of data, while cloud computing and machine learning algorithms are developed to process the data lakes, resulting in the aggregation of millions of droplets. So far, so good, but what about the hardware? While advancements in semiconductor manufacturing have made it possible to compress a microcomputer and a meteorological station into a millimeter-square silicon die, the footprint of an energy storage devices is stuck in the 1990s. The new generation of all-solid-state thin-film batteries aims to provide the last functional block to the implementation of the IoT. The core element to attain the required energy density and minimize the size of the storage unit is the solid-state electrolyte. Thanks to its mechanical stability, it is possible to reduce the spacing between anode and cathode from hundreds of microns to hundreds of nanometers. Being mechanically rigid, solid electrolyte also prevents the growth of harmful lithium dendrites allowing lithiummetal batteries and improves the safety of the battery concerning short circuit and liquid leakage. In the present thesis work, we studied a thin-film solid-state micro-battery based on LiCo2 cathode and LiPON solid electrolyte in an anode-freeconfiguration. The device is realized in a 0.5 mm2 footprint, with an active thickness smaller than50 μm. The main objective of the thesis was the maximization of the volumetric energy density and the cycle life of the device. The first chapter deals with the study of the intrinsic capacity loss of LiCoO2 cathodes in thin-film configuration. In order to understand the physical nature of this reversible loss, a continuous 1D physical model based on the drift-diffusion equation and the Buttler-Volmer equation was realized. The model was solved by means of COMSOL Multiphysics and the capacity loss was attributed to the drop in lithium diffusivity at the cathode interface for low state-of-charge (SOC). The capacity loss dependency on the discharge current was also extracted, showing the possibility of recovering the theoretical capacity at anextremely small current density (