Conditions for High Rate, High Capacity Li-Ion Convection Cells through Dimensional Analysis

Abstract

Improved performance of Li-ion cells for applications, such as, electric vehicles, renewable energy storage, and grid load leveling, calls for both high energy and rate capability. An implied high capacity under high rate cell (dis)charge is presently very difficult or unachievable in closed Li-ion cells. A common limitation under such demands is electrolyte transport and frequently Li-ion salt bulk diffusion in particular. The limitation leads to salt concentration variation and depletion in the cathode. A convection cell with in-operation electrolyte flow can significantly mitigate this limitation under quantitatively defined ranges of conditions, as published recently.1 This finding emerged from the addition of convection to a DUALFOIL-based pseudo 2D Li-ion cell model to create LIONSIMBA+c and dimensional analysis of input electrolyte transport properties, dimensions, and operational parameters. With cell performance output expressed as %capacity accessed, inputs combined into dimensionless groups explain the balance of migration versus diffusion and convection and allow for collapse of myriad output data points into families of curves for analysis.Here, we elaborate on the LIONSIMBA+c result analysis including the dimensionless groups that describe Li-ion cell capacity limitations with and without flow. Full material working capacity is achievable with low current or fast electrolyte transport – diffusion or convection. Notably, flow serves as an operational throttle for obtaining desirable transport. With discharge rate increase or sluggish transport, deliverable capacity drops precipitously and asymptotically approaches reduced levels, particularly when there is low initial Li-ion salt amount. A material balance explains the reduced capacity values at high rate. The accessible discharge time and thus capacity is gated by the time that migration depletes Li-ion salt in the cathode. We also examine the corresponding common electrolyte concentration profile behavior under different parameter combinations with similar dimensionless group values. Reference Gao, M. J. Orella, T. J. Carney, Y Román-Leshkov, J. Drake, and F. R. Brushett, J. Electrochem. Soc. 167 140551 (2020).