(Georgia Institute of Technology, 2023-04-25)
Sandoval, Stephanie E
Lithium-ion batteries (LIBs) have powered consumer electronics for decades, and continued improvements have enabled the rapid growth of the electric vehicles (EV) market. However, increased energy density and specific energy are necessary to facilitate further expansion of EV market share and for emerging technologies such as electric flight. A promising avenue to achieve this is by replacing conventional graphite anodes in LIBs with lithium (Li) metal anodes. The use of Li metal in Li metal batteries (LMBs) is of great interest due to its high theoretical capacity (3,860 mAh g-1) and low redox potential (-3.04 V vs SHE). However, the uncontrollable growth of Li throughout cycling limits the electrochemical performance of LMBs. A battery configuration in which Li metal is plated onto a bare current collector during the first charge, known as the “anode-free” architecture, is particularly promising in that this increases volumetric energy density by ~85% compared to conventional LIBs. The main challenge of the anode-free architecture arises from limited Li+ inventory in the cell and uncontrolled morphological evolution of Li. Nonetheless, these cells are potentially safer and easier to manufacture than LMBs since they can be assembled in their fully discharged state like LIBs. To enable anode-free batteries, it is important to gain a fundamental understanding of Li nucleation and growth mechanisms as well as subsequent stripping mechanisms on bare current collectors. This thesis work focuses on understanding the cyclic evolution of Li in anode-free batteries using both liquid and solid electrolytes, while exploring methods, such as the use of alloy interlayers, to spatially control Li deposition/stripping behavior.
The nucleation and growth of Li on bare current collectors is first explored in liquid electrolyte systems by combining electrochemical methods with operando optical microscopy. We find that silver interlayers enable improved Coulombic efficiency (CE) for Li cycling in multiple electrolyte systems compared to bare current collectors or other alloy layers. Operando optical microscopy reveals reduced growth of dendritic Li on silver-coated current collectors at high current densities compared to bare current collectors, as well as different dendrite growth and stripping dynamics.
Unlike liquid electrolyte systems, there is little work in understanding the cyclic evolution of Li in solid-state batteries (SSBs). I first focus on investigating Li evolution on bare current collectors using cryogenic focused ion beam (cryo-FIB) and X-ray computed tomography (CT) imaging paired with electrochemical methods. It is demonstrated that substantial amounts of Li can be deposited on bare current collectors at relatively high current density in SSBs, thus showing that deposition is not the limiting process in anode-free SSBs. Instead, we find that Li stripping from bare current collectors causes accelerated short circuiting that limits cycle life. The use of nanoscale alloy interlayers is then explored as a method to improve cell performance. Here, we investigate the cyclic evolution of Li on bare and alloy interfaces in SSEs by leveraging cryo-FIB and plasma FIB methods paired with in situ electrochemical impedance spectroscopy (EIS). These methods provide insight into the Li cycling dynamics that occur at the solid-solid interface. We find that Li deposits non-uniformly in bare current collectors and upon stripping, there are clear electrochemical signatures of contact loss across the solid-solid interface. In contrast to bare current collectors, alloy coated electrodes enable uniform deposition and stripping, thus improving overall cell performance. Finally, we explore operando Li deposition and stripping dynamics in anode-free batteries through X-ray CT. Together, these datasets provide evidence that cyclic deposition/stripping performance can be significantly improved through the use of thin alloy interlayers, and mechanisms are proposed describing the dynamic action of these interlayers on electrochemical behavior.