Modeling Improved Li-Ion Cell Capacity through Suppression of Temperature with Internal Electrolyte Flow

Abstract

Lithium-ion cells are ubiquitous in portable electronic applications and are growing in popularity for electric vehicles including passenger cars. Yet, a dramatic reduction in global emissions to combat climate change calls for significantly broader adoption of carbon neutral technologies in electric vehicles and in stationary energy storage to complement variable renewable sources and growing electricity demand.1 For lithium-ion batteries, a critical barrier to such broader adoption is the inability to sustain high rate while also accessing the full capacity of cells. In addition to mass transport effects, overheating commonly limits the duration of high-rate discharge or charge due to excessive heat generation and inadequate heat removal.Here, we provide a brief illustrative tutorial of these coupled effects in a pseudo two-dimensional electrochemical model and build on our previous modeling and dimensional analyses2-4 of how internal electrolyte flow across Li-ion cells reduces both mass and thermal transport limitations.First, we describe the intentional and sudden onset of internal electrolyte convection to curb or to avert further temperature rise in a Li-ion cell during high-rate discharge. We examine two modes for introducing electrolyte flow: temperature-triggered (“reactive”) and discharge-rate triggered (“proactive”) modes. In the reactive case, flow is introduced in the simulation when a cell under high-rate discharge reaches a high safety cutoff temperature. In the proactive case, flow begins as the discharge rate is step changed from midrate to high rate. A practical application of proactive mode is that a battery thermal management system could be programmed to initiate electrolyte flow in anticipation of greater heat generation at a higher rate.Next, using the aforementioned continuum modeling with circulation through an external tank, we compare and contrast Li-ion cell thermal responses to the onset of flow in reactive mode versus in proactive mode. In both cases, the threshold flow rate (~μm/s) to suppress temperature rise agrees in order of magnitude with dimensionless group estimates reported in our previous work.4 Namely, temperature rise is suppressed when the heat generation rate becomes lower than the heat removal rate. At the highest electrolyte flow rates simulated (>>μm /s), the heat removal by internal electrolyte flow dominates, and the cell achieves nearly full discharge capacity without prematurely crossing temperature or voltage cutoffs. For low flow rates (<μm /s), the proactive and reactive cases show quantitatively different transient temperature trajectories due to the temperature dependences of heat generation and removal. Heat generation rate decreases with elevated cell temperature due to facile mass transfer and resultant concentration uniformity, optimal electrolyte conductivity, and ease of charge transfer kinetics. Also, heat removal using internal electrolyte flow becomes more effective with high cell temperature relative to ambient inlet temperature. Hence, the flow rate needed to cool a cell that has reached a high safety cutoff temperature (in reactive mode) is lower than the flow rate required to curb temperature rise in a cell just above ambient temperature (in proactive mode) as high-rate discharge starts.