Determination of Irreversible Lithium Loss and Pre-Lithiation Studies for High-Capacity Anode Materials

A major disadvantage of anode materials, in particular high-capacity materials like silicon, is the high irreversible lithium loss attributed to solid electrolyte interphase (SEI) formation triggered by electrolyte decomposition. For this reason, pre-lithiation of anode materials is getting more attention. However, therefore the exact amount of irreversible lithium loss is needed. Even if the SEI formation presents the major part of lithium loss, there are some other parasitic reactions which can lead to irreversible lithium loss, e.g., positive electrode degradation or the reaction of lithium ions with water. In addition, there are also some parasitic reactions which do not change the active lithium content of the cell but still possess an impact on the capacity or are able to transfer lithium ions out of the electrolyte into the electrodes. Typically, the irreversible lithium loss is set equal with the accumulated irreversible capacity (sum of the differences between charge and discharge capacity in each cycle). It can be shown that this approach possesses a relatively low accuracy and one can only get an acceptable estimation of the first cycles. With increasing cycle number, the determination of the irreversible lithium loss with help of the accumulated irreversible capacity becomes more inaccurate because the impact of SEI formation and regeneration becomes smaller in comparison to other parasitic reactions. In this work, a novel approach is investigated in order to determine the irreversible lithium loss with high accuracy. Therefore, “pseudo full cells” are built, while the term “pseudo full cell” refers to a cell with an oversized cathode (in terms of capacity). These cells are cycled like “real” full cells and afterwards the positive electrode is de-lithiated with help of the reference electrode in order to determine the remaining active lithium content of the cell. Finally, the exact amount of irreversible lithium loss can be determined as difference between initial and remaining active lithium content.