The Role of Cations on the Performance of Lithium Ion Batteries: A Quantitative Analytical Approach

Lithium ion batteries are nowadays the state-of-the-art power sources for portable electronic devices and the most promising candidate for energy storage in large-size batteries, e.g., pure and hybrid vehicles. However, the degradation of the cell components minimizes both storage and operation lifetime (calendar and cycle life), which is called aging. Due to the numerous different aging effects, in either the single constituents or their interactions with each other, many reports about methodologies and techniques, both electrochemical and analytical, can be found in the literature. However, quantitative data about the degradation effects were seldom stated. One important effect is the cation distribution and migration during operation. Metal dissolution and metal migration of the cathode and the corresponding deposition of these metals on the graphitic anode are known harmful degradation effects, especially for the formed solid electrolyte interphase on the surface of the anode. Depending on the applied cell chemistries and therefore the cathode material, different mechanisms were reported so far. For lithium manganese oxide based cells, the acidification of the electrolyte due to composition of the conduction salt is attributed as the main source of metal migration. Due to subsequent loss of manganese from the cathode, the overall performance of the cell is seriously impaired. Based on the obtained observations, this degradation mechanism was adapted to lithium nickel cobalt manganese based cells as main cause of the capacity fading. However, with the help a developed total X-ray fluorescence method and additional surface and electrolyte investigations, the proposed HF based mechanism was disproven. Instead, the migration was directly associated with material defects or mechanical spalling of the particles. Furthermore, with the obtained quantitative data of the migrated transition metals on the anode and separator, the contribution on the capacity fade was determined. It ranged only the ‰ region and could therefore be excluded as the main source of the capacity in these lithium ion batteries. Nevertheless, the oxidation state of the cations is hardly accessible; but would provide further information about the exact migrating mechanisms. In addition, lithium can be "lost" or immobilized during charge/discharge and is therefore no longer available as an electrochemically active cation. For example, the formation, reformation, and growth of the solid electrolyte interphase and cathode electrolyte interphase leads to an increased active lithium loss during cycling. The investigations on this topic are frequently reported in literature; however, quantitative data on the actual lithium distribution throughout the cell are relatively few. Furthermore, the exact amount of lost lithium in the in the respective interphases is so far not available. In order to determine quantitatively the lithium distribution within the cell, inductively coupled plasma-based method was applied. For laboratory test cells, the lithium that was lost to the housing of the cell was 32 times higher than that for pouch bag cells. Furthermore, the determined concentration of lithium in the interphases ranged only from 2 to 4%. However, the investigations need to be repeated with isotope labeled material (⁶Li) in order to obtain statements that are more precise.