Optimal design and discharge operation of lithium-ion whole-cell

Abstract

The optimal design of Li-ion whole-cells is presented for different discharge operating constraints: constant current (CC), constant power (CP), and constant resistance (CR). In addition, a new Optimal Theoretical Operation Policy (OTOP) is derived resorting to Variational Calculation. When applied to cell discharge, this policy relates equilibrium voltage, applied current, and discharge time. The considered design variables comprise electrode thicknesses, porosity volume fractions, total solid volume fractions, and cell mass. The objective of the optimal design formulation is to maximize the total specific energy delivered for a set of given discharge time values with the aim of relating delivered energy to delivered power. The simultaneous optimization of multiple design variables under different operating conditions is efficiently achieved by using a simple phenomenological mathematical model that predicts the main aspects of these systems. The importance of this procedure is reflected in remarkable improvements to total specific energy delivered. The optimized whole-cell designs gained average improvements of more than 40%, depending on time discharge, when compared to the initial design taken from the literature; and when the optimal designs obtained under the different operating conditions are compared, an additional specific energy is obtained (OTOP results in the best operating condition, followed by CR, CC, and CP, with average differences of 7, 8, and 15%, respectively).