Numerical Analysis for Effects of Cell Formats on Cycle Life of Lithium Ion Batteries with Large Capacities

Large-format LIBs have been preferentially used for EVs and HEVs to achieve high power and high capacity. Although the technologies of LIBs have been successfully developed in commercial markets for small electronic devices, the scale-up processes of LIBs still have challenges because of the size effects. Large LIB cells usually show different responses to small-format LIBs due to the spatial non-uniformity of the electric potential, current density and temperature over the cell volumes. So, battery manufacturers have difficulties to predict responses of large cells when the cell formats are changed. The complex interactions among various physics, such as lithium ion transport, kinetic reactions, heat transport and etc. make it difficult to estimate the effects of the cell formats on performance and life of LIBs. In this study, we investigate the effects of cell formats on cycle life of LIBs using a numerical model, which resolves thermally, electrically, electrochemically coupled physics. To understand behaviors of large LIBs, a three dimensional model resolving multi-physics in various size scales is required. Relatively long electric current paths in metal current collectors of large LIBs, which are determined by the geometry of the cell, could considerably influence Ohmic heat generation as well as voltage drops. It also can lead the spatial non-uniformity of the electric potential, current density and temperature. Moreover, a ratio of cooling surfaces to cell volumes, which also depends on the cell designs, determines thermal responses of the LIB cell. Therefore, a multi-dimensional model considering the cell geometries is needed to understand the complex physics in a large-format LIBs. In this work, we developed a three dimensional multi-physics model including a degradation mechanism based on the multi-scale multi-dimensional(MSMD) model framework, which resolves various physics in designated length scales as shown in Figure 1 [1-2]. To consider the life fade of LIB cells, the present model calculates growth of the SEI layers at the anode particle surfaces as shown in Figure 2, which is a major cause of the capacity fade [3]. Three different cell formats, pouch, cylindrical and prismatic cells are modeled to investigate the effects of the cell design on cycle life as shown in Figure 3(a)-(c). The pouch cell is considered to be stacked unit cell sandwiches (including pair of electrode layers and current collectors), and it has the positive and negative tabs located on the same side as typical pouch cells in the commercial market. The cylindrical and prismatic cells have wound structures of the unit cell sandwiches and localized tabs as shown in Figure 3(d). All three types of cells have the same capacity, the same volume and the same electrode design for the unit cell sandwich such as electrode material, coating thickness, porosity, particle size and etc. We simulated a pouch cell, a cylindrical cell and a prismatic cell with 20Ah and LFP/Graphite operating same cycles. Figure 4 shows contour results of the current density at 10min during 3C discharge. It is shown that the cell format has significant influences on distributions of electrochemical reaction rates. The non-uniformity of current density in the cylindrical cell is observed to be most severe. This study will present the inhomogeneous degradation of each cell format during the repeated charge/discharge cycles. Reference [1] G.-H. Kim, K. Smith, K.-J. Lee, S. Santhanagopalan, A. Pesaran, J. Electrochem. Soc. 158 (8) (2011) A955eA969. [2] K.-J. Lee, K.Smith, A.Pesaran, G.-H. Kim, J.Power Sources 241 (2013) 20-32. [3] G. Ning, B.N. Popov, J. Electrochem. Soc. 151 (2004) A1584. Figure.1 The concept diagram of multi-scale multi-dimentional(MSMD) model framework Figure.2 Aging mechanism for SEI growth at the anode particle surface Figure 3.Geometry of 20Ah (a) pouch, (b)cylindrical and (c)prismatic cells, (d)wound structures and tab location of cylindrical and prismatic cell. Figure 4.Current density distribution after 10min at 3C discharge of 20Ah cell Figure 1