Assessing the Surface Degradation Process of Manganese-Based Li-Ion Battery Cathodes Using Scanning Electrochemical Microscopy
Authors: Kim Dong Ok , Mishra Abhiroop , Zorigt Michelle , Rodriguez Lopez Joaquin 
Published: December 2024

Li-ion batteries (LIBs) are the favored technology driving the market toward sustainable energy sources and electrified transportation. However, cathode materials in LIBs pose a key challenge, limiting both energy storage capacity and cost-effectiveness. Among the different classes of cathode materials, transition metal oxides are most utilized due to their ability to provide high voltage and the reversible Li-ion intercalation during battery operation. Specifically, manganese (Mn)-based oxide cathodes are quite attractive due to their low toxicity and cost1. However, in promising cathodes such as LiMn2O3, Mn3+ ions are Jahn-Teller (J-T) active, making them prone to leaching out in the electrolyte during LIB operation2. The dissolved Mn not only leads to the loss of cathode active material but also gets subsequently deposited on the anodes, leading to gradual capacity fading of the LIB. Therefore, elucidating and addressing this issue continues to be an ongoing endeavor with practical implications.

Inductively coupled plasma (ICP) and electron paramagnetic resonance (EPR) spectroscopy are the two most widely used techniques to study the Mn-dissolution phenomenon from LIB cathodes. However, these techniques are typically ex-situ in nature and have low temporal resolution. Furthermore, they offer no spatial information on the heterogeneity of the Mn dissolution from cathode materials, and their limited sensitivity makes it challenging to quantitatively investigate Mn dissolution within a reasonable time frame.

This work introduces scanning electrochemical microscopy (SECM) as an in-situ tool to tackle this challenge and enable real-time quantitative investigation of Mn dissolution from LIB cathodes. We utilize LiMn2O3 as a model system to develop the SECM-based method to study Mn dissolution. Mercury (Hg)-based SECM probes allow selective investigation of the Mn dissolving from the cathode with exceptional sensitivity using anodic stripping voltammetry. The close proximity of the SECM probe to the cathode surface (<15 μm) offers high temporal resolution of ~300 ms. Correlating SECM experimental results with COMSOL Multiphysics simulations helps in the quantification of the Mn lost from the cathodes as a function of different cathode operating conditions. In addition, rastering the SECM tip across the cathodes provides the opportunity to study spatial heterogeneity in the Mn dissolution behavior. Furthermore, this work correlates the lattice oxygen loss from LiMn2O3 with Mn dissolution by detecting both oxygen and Mn within the SECM experimental setup, utilizing our previously developed method to investigate lattice oxygen loss from oxide cathodes in LIB3. In summary, this work provides insight into the degradation process of transition-metal oxide-based LIB cathodes involving both lattice oxygen loss and transition metal dissolution.

(1) Liu, S.; Wang, B.; Zhang, X.; Zhao, S.; Zhang, Z.; Yu, H. Reviving the Lithium-Manganese-Based Layered Oxide Cathodes for Lithium-Ion Batteries. Matter
2021, 4 (5), 1511–1527. https://doi.org/10.1016/j.matt.2021.02.023.

(2) Asl, H. Y.; Manthiram, A. Reining in Dissolved Transition-Metal Ions. Science
2020, 369 (6500), 140–141. https://doi.org/10.1126/science.abc5454.

(3) Mishra, A.; Sarbapalli, D.; Hossain, Md. S.; Gossage, Z. T.; Li, Z.; Urban, A.; Rodríguez-López, J. Highly Sensitive Detection and Mapping of Incipient and Steady-State Oxygen Evolution from Operating Li-Ion Battery Cathodes via Scanning Electrochemical Microscopy. J. Electrochem. Soc.
2022, 169 (8), 086501. https://doi.org/10.1149/1945-7111/ac857e.