Prospects and limits for quantitative Energy dispersive X-ray spectroscopy (EDS) applied Li Ion batteries

Summary

The most commonly used batteries at present and in the near future are Li-ion-based systems. The quantification of Li and limited spatial resolution of EDS analysis are bottlenecks to overcome. There the spectroscopy of low energy x-rays with the windowless Oxford extreme EDS detector open new prospects. We show spectra of Li metal directly recorded after the transfer from the glove box and after 9 days storage at room temperature in air. The significant changes of the Li peak content, shape and peak position document the sensitivity and reactivity of Li. One sensitive part in Li-Ion batteries is the interface between the anode and the separator with the solid electrolyte interface covering the anode surface and eventually with reaction products on top. We show such reaction products of a LiFePO₄ cathode and a carbon anode. The EDS spectra deliver a significant enrichment of Li for dark surface coated areas on top of massive anode encrustation and in a round shaped particle. This enables to determine precipitations, which reduce the amount of Li inventory and on this way the battery performance. Consistently upcoming questions are: How reliable are EDS studies for real life application under not ideal conditions, what are the influences of etc. sample preparation, storage and spectrum processing. Therefore a systematic study was carried out on Li(Ni_xMn_yCo_z)O₂
cathode material. The first topic was the determination of the stoichiometric composition of Ni, Mn and Co, where Ni, Mn and Co share one position, consequently x+y+z=1. With the different settings:

1. shuttle =&gt“;“ glove box =&gt“;“ shuttle =&gt“;“ SEM vacuum, top view, EDS standard based

2. air =&gt“;“ in air, top view, EDS standard based

3. Au =&gt“;“ in air and coated with gold ( 4 nm), top view, EDS standard based

4. C =&gt“;“ in air and coated with carbon (10 nm), top view, EDS standard based

5. Ar-Ion milling =&gt“;“ in air, cross section (with an argon ion mill), EDS standard based

6. section =&gt“;“ in air, cross section (mechanical polished), EDS standard based

7. normalized =&gt“;“ in air, top view, EDS normalized to 100% (neglecting Li content)

All performed examinations deliver a stoichiometric relationship close to NMC (811). This proves the robustness of EDS analysis. Additionally results obtained with ICP-OES (Inductively Coupled Plasma – Optical Emission Spectrometry) agree with the EDS results. A persistent limitation for chemical quantification of Li-Ion battery materials is that bonded Li cannot be detected directly by EDS. We determine the Li amount as the missing element fraction of not normalized quantitative EDS analysis. The results of the studies performed with the conditions shuttle, air and Ar-Ion milling agree nicely with the expectation of 6,4% for a 100% lithiated cathode material after the formation process, where the gold, carbon coating lead to a significant overestimation of the Li content and the mechanical polished section cannot deliver reliable results.