Lithium-ion whole-cell design and charging protocol optimization within safe operating conditions

Abstract

The main phenomena of battery degradation occur during the charge and, unlike the discharge, this process can be predefined and controlled. In this paper, the optimization of different charging protocols is proposed, considering three different objectives: charging time minimization, charge capacity maximization and consumed energy minimization. These optimization problems are subject to security constraints that define a feasible region, which offers protection against the main cell degradation phenomena. The study is posed by applying a phenomenological cell model in a mathematical programming environment. In addition, the whole-cell design optimization is presented within this framework and an analysis of internal energy losses is performed, allowing a holistic understanding of the system performance and the influence of each of its variables during the charging process. Therefore, the proposed approach allows the simultaneous optimization of the multiple design and operation variables. The characteristic variables for a particular chemistry and a given cell design are obtained: minimum full charge time, optimal constant charge current and the current and voltage values at the end of charge. Furthermore, an expression of an optimal theoretical charging protocol is obtained through the minimization of the consumed energy. Simulation results show considerable improvements in the objectives, such as a reduction of 30% in the total charging time for the traditional CC-CV protocol when multiple cell design variables are optimized.