Operando Analysis of Li Plating on Graphite Anodes During Fast Charging

Abstract

Driven by the rapidly growing market for electric vehicle (EV), there is a strong need for fast charging capability of lithium ion (Li-ion) batteries. However, when high-energy-density Li-ion batteries are charged at fast rates, Li plating on the anode is often triggered by the large anode overpotential. Among the degradation mechanisms that occur during fast charging, Li plating is one of the primary sources of capacity fade and safety hazards. Therefore, to improve the fast-charging performance of Li-ion batteries, there is a need for 1) an improved fundamental understanding of the Li plating mechanism; and 2) strategies to monitor Li plating in industrially relevant cell formats. The primary focus of this thesis is to provide a fundamental understanding of the electrochemical reactions on the graphite anode during fast charging. Specifically, this work has two primary thrusts: 1) operando characterization of the graphite anode to study the dynamic evolution of the local state of charge (SoC) and Li plating during fast charging; and 2) incremental capacity (IC) analysis to detect Li plating in graphite-lithium nickel manganese cobalt oxide (NMC) pouch cells. In thrust one, single-micron spatial resolution was achieved via operando video microscopy to capture the morphological evolution of individual graphite particles. Three-electrode voltage measurements were time synchronized with the videos during 6C fast charging as well as during the following open circuit voltage (OCV) rest and discharge steps. The microscopy results showed that Li preferentially nucleates on the graphite particles that lithiate the fastest during fast charging. Moreover, after fast charging, cross-sectional microscopy was applied to visualize the phase equilibration between graphite particles through the thickness of the calendared electrode. In thrust two, IC analysis was applied for operando detection of Li plating in graphite-NMC pouch cells during fast charging. During charging at C/2 to 4C rates, an IC peak at > 3.9V cell voltage was observed to correlate with Li plating. Post-mortem optical microscopy and scanning electron microscopy (SEM) analysis was performed to confirm the occurrence of Li plating in cells that exhibited the plating IC peak. Furthermore, three-electrode pouch cells were used to measure the anode potential during C/2 to 4C charging, and a local minimum of the anode potential was correlated with the plating IC peak. A mathematical framework was presented to rationalize this correlation. Additionally, the plating IC peak was tracked during extended fast-charge cycling, where a decrease in the peak magnitude was correlated with a decrease in the amount of Li plating within each cycle. In summary, this thesis furthered our understanding of Li plating during fast charging of Li-ion batteries, and applied this understanding to perform diagnoses in an industrially relevant cell format. The implications of this work could aid in the development novel charging strategies and the materials design for Li-ion batteries.