Current density and state of charge inhomogeneities in Li-ion battery cells with LiFePO4 as cathode material due to temperature gradients

Abstract

Current density distributions and local state of charge (SoC) differences that are caused by temperature gradients inside actively cooled Li-ion battery cells are discussed and quantified. As an example, a cylindrical Li-ion cell with LiFePO4 as cathode material (LiFePO4-cell) is analyzed in detail both experimentally and by means of spatial electro-thermal co-simulations. The reason for current density inhomogeneities is found to be the local electrochemical impedance varying with temperature in different regions of the jelly roll. For the investigated cell, high power cycling and the resulting temperature gradient additionally cause SoC-gradients inside the jelly roll. The local SoCs inside one cell diverge firstly because of asymmetric current density distributions during charge and discharge inside the cell and secondly because of the temperature dependence of the local open circuit potential. Even after long relaxation periods, the SoC distribution in cycled LiFePO4-cells remains inhomogeneous across the jelly roll as a result of hysteresis in the open circuit voltage. The occurring thermal electrical inhomogeneities are expected to influence local aging differences and thus, global cell aging. Additionally the occurrence of inhomogeneous current flow and SoC-development inside non-uniformly cooled battery packs of parallel connected LiFePO4-cells is measured and discussed.