Microscale Evaluation of the Lithium Content in Lithium-Ion Battery Cathode Materials in the Scanning Electron Microscope

Lithium (Li) ion-based electrochemical cells have been widely adopted due to their capacity for energy storage and low mass. However, due to a lack of characterization techniques that allow microscale analysis of the Li content, understanding of lithium plating and dendrite growth, and solid electrolyte interphase formation remain a major obstacle in the development of improved Li-based cells.<br/>Commonly, microanalysis by energy dispersive X-ray spectroscopy (EDS) in the scanning electron microscope (SEM) is used for detection and mapping of elemental distribution, however, this remains unfeasible in commercially important materials consisting of elements with low atomic number (Z &lt; 4), including Li [1, 2]. However, recently the composition-by-difference method (CDM) was shown to be a viable method for determining the Li content quantitatively at the microscale, with an accuracy of ~1 wt. % and was demonstrated in lightweight structural alloys [3] and metal oxides [4]. CDM uses quantitative analysis of the EDS and backscatter electron (BSE) signals to calculate the lithium content indirectly. In this study, we extend the CDM approach to Li-ion battery cathode materials including lithium nickel manganese cobalt oxide particles (NMC).<br/>High-grade NMC 811 particles of nominal composition LiNi_0.8Mn_0.1Co_0.1O₂ were analyzed (MSE Supplies). A sample of the NMC 811 was prepared by embedding the powder in epoxy before a cross-section was prepared by broad-beam argon milling (PECS™ II system, Gatan, Inc.). The sample was held at a temperature of 0 °C during the milling process and, subsequently, a 2.0 nm carbon coating was deposited (PECS™ II system, Gatan, Inc.). The sample was transferred under vacuum conditions to a conventional field emission SEM and compositional analysis was performed using the Cipher™ system (Gatan, Inc.). Quantitative BSE analysis was performed and evaluated according to the modified electron approach [5] with a screening factor of 0.7.<br/>Quantitative EDS analysis at select locations within NMC particles was performed revealing O, Ni, Mn, and Co with little-to-no variation within or between particles that were analyzed; no other elements were found to be present above the minimum detection level. The Ni:Mn:Co ratio was determined experimentally to be 8.07:1.00:1.01, consistent with the nominal 8:1:1 ratio of this NMC. The lithium content from 6 different NMC particles was determined using Cipher and the mean lithium concentration was determined to be 22.5 at. % (5.7 wt. %) within ~1.5 wt. % of the nominal composition value of 7.3 ± 0.3 wt. %.<br/>This is a significant step forward in the analysis of battery materials as, for the first time, the charge state of a cathode material is determined in a conventional SEM—where 25 at. % Li corresponds to an uncharged battery state with NMC 811 cathode.<br/>References:<br/>[1] P. Hovington et al., Scanning 38 (2016) 571<br/>[2] R. Gauvin and N. Brodusch, Microsc. Microanal. 28 (Suppl 1), 2022<br/>[3] J. A. Österreicher et al., Scripta Materialia 194 (2021) 113664<br/>[4] J. Lee et al., Microscopy and Microanalysis (2022) 113<br/>[5] J. J. Donovan et al., Microscopy and Microanalysis 9 (2003) 202