Analysis of Limiting Processes of Power Performance Within Li-ion Batteries
Résumé
Li-ion batteries have become a necessity in human daily life as the most versatile, efficient and
performing energy storage & conversion system to power our nomad electronic, cars and buffer the
renewable intermittent energy sources.1 However, improvement of current battery systems is needed to
meet the requirements of the transport sector in terms of energy density, safety, and cycle life. Besides the
active materials, among the strategies to increase battery autonomy, one consists of optimizing the design
parameters of the electrode such as the formulation, loading, microstructure, and porosity. The idea is
simply to increase the ratio of active materials (negative and positive electrode thickness) to inactive
components (separator, current collector…). However, by doing so, the output power density becomes
strongly limited by the charge transport within the composite electrodes. Thus, our objective is ultimately
to optimize the battery design to find the best compromise between energy and power density in Li-based
batteries.2 In this work, we study Li-battery capacity as a function of the current density with respect to
electrode porosity, formulation, loading, microstructure as well as temperature. For this purpose, (LiFePO4)
LFP and (LiNi0.8Mn0.1Co0.1O2) NMC-811-based electrodes were formulated at different loadings (from 0.4
to 3.4 mAh.cm-2), compositions (Active material%, Carbon%, PVDF %), and calendered to reach different
porosities (from 20 to 50 %). The microstructure of electrodes is investigated using SEM, Granulometry
and BET to determine their microstructure and specific area. The electrode tortuosity is analysed through
impedance spectroscopy in a symmetric positive/positive electrode.3 Subsequently, the power performance
is fully captured and analyzed using a time-saving methodology.4 The limiting current density, Jlim, is
obtained through capacity vs discharge current curves, which allows us to determine an effective diffusion
coefficient of the limiting transport process (Deff) via Sand equation. We also analyse the diffusion
coefficient and charge transfer resistance (Rct) as a function of the state of charge (SOC) by coupling GITT
and EIS. Afterwards, the correlation between design parameters and the effective electrochemical
parameters such as Deff and Jlim, Rct is discussed