Flowing Electrolyte Metal Batteries

Author:

Parekh, Mihir

Graduate Program:

Mechanical Engineering

Degree:

Doctor of Philosophy

Document Type:

Dissertation

Date of Defense:

July 15, 2021

Committee Members:

Sukwon Choi, Major Field Member
Christopher Rahn, Chair & Dissertation Advisor
Chao-Yang Wang, Major Field Member
Daniel Connell Haworth, Program Head/Chair
Long-Qing Chen, Outside Unit, Field & Minor Member

Keywords:

Dendrite
Solid Electrolyte Interphase
Flowing Electrolyte
Metal anodes
Creeping flow

Abstract:

Instabilities during metal electrodeposition create dendrites on the plating surfaces. In high energy density lithium metal batteries (LMBs) dendrite growth causes safety issues and accelerated aging. In this thesis, analytical models predict that dendrite growth can be controlled and potentially eliminated by small advective flows normal to the surface of lithium metal electrode introducing a new class of batteries, namely, flowing electrolyte metal batteries (FEMBs). Electrolyte flow towards the Li metal electrode lowers the dendrite growth rate, overpotential, and impedance. Flow in the opposite direction, however, enhances the dendrite growth. For every current density, there exists a velocity above which dendrite growth can be totally eliminated. The critical velocity increases almost linearly with increasing current density. For typical current densities and inter-electrode separation, the critical velocity is of the order of μm/s, indicating the potential for practical application. The critical flow velocity scales linearly with charging rate, requiring 37 mL/Ah of flow volume. The effect of creeping electrolyte flow through perforated metal anodes on dendrite growth and energy density is further analyzed using a 2D COMSOL Multiphysics model. The flowing electrolyte enhances plating inside the slot (2D model of pore) and reduces plating on the part of electrode directly facing the counter-electrode. This reduces the chances of short circuit via dendrite growth. Larger slot separation, lower slot widths, and thicker electrodes alleviate dendrite growth but lower the specific charge density. Very narrow slots may get plugged due to plating inside the hole. Thus, slot width, slot separation, and electrode thickness should be optimized to ensure high specific charge density and non-dendritic plating in the inter-slot gap. We also derive analytical models for electrodeposition with creeping Poiseuille and Couette flows parallel to the two electrodes. The models predict that creeping electrolyte flow parallel to the surface of metal electrode increases the stability of lithium plating by reducing the dendrite growth rate. Moreover, parallel flow reduces the curvature of dendrites leading to flatter electrodeposits. For the same average flow rate, Poiseuille flow can be upto two times more stabilizing than Couette flow. It is also not possible to completely stabilize the metal electrode with creeping parallel flows. In LMBs, solid electrolyte interphase (SEI) growth reduces coulombic efficiency and cycle life. In this thesis, we develop a steady-state model that predicts that small advective electrolyte flow towards the lithium metal electrode at a fixed current density improves the coulombic efficiency and decreases SEI layer growth rate. Low flow rates (μm/s) can increase coulombic efficiency by up to 6%, extending capacity retention and cycle life by a factor of 7X. The sensitivity of the coulombic efficiency to plating and SEI layer reaction rate constant is also explored. Higher plating reaction rate constant and lower SEI formation rate constant leads to higher coulombic efficiency. Zinc metal batteries are a widely considered alternative to lithium metal batteries that also suffer from dendrite growth. We explore the effect of creeping normal electrolyte flow on dendrite growth in zinc metal batteries using a transient model that predicts concentration distribution evolution and a linear stability analysis that predicts dendrite growth. Dendrite growth on zinc metal anodes can occur due to surface instabilities and/or concentration depletion. Creeping normal flow with a flow rate greater than the critical flow rate ensures stable plating and prevents ion depletion near the negative electrode, thus eliminating both causes of dendrite growth. Unlike lithium, increasing the flow rate does not necessarily reduce the electrostatic potential difference between the two electrodes, thus indicating the importance of ion diffusivity ratio in the electrolyte impedance.