Multistep Improvement of Pilot-Scale 21700 Cells for Increased Fast-Charging Capability: Combining Optimized Electrolyte, Cell Design and Fast-Charging Protocol

Abstract

In this publication, different cell- and charging parameters (advanced fast-charging protocol, 21700 tab design, electrolyte composition) are changed in a systematic step-by-step approach to reduce charging time while keeping the anode and cathode cell chemistry and electrodes (graphite—NMC 622 full cell) unchanged. Preliminary tests were carried out using 3-electrode full cells with a Li metal reference electrode to identify charging conditions that avoid Li metal deposition. In addition, the effects of the anode potential are investigated in 3-electrode full cells with a Li metal reference electrode. The optimized charging protocols from the 3-electrode full cells were then transferred to 2-electrode pilot-scale 21700 full cells. Two different tab designs (1 × 1 welded tabs and 120 × 125 foil tabs) were used in these cells. To improve the charging time further, an electrolyte with higher ionic conductivity was used under the best conditions from the previous tests. Cross-sectional in situ optical microscopy was used to visualize the transport effects within the anode. In the optimized 21700 cell (advanced fast-charging, 120 × 125 foil tabs, better Li+ transport in the electrolyte), the synergistic effects of the three different optimization steps reduced the charging time to 80% SOC by 46% compared to the baseline cell.

Efficient Workflows for Detecting Li Depositions in Lithium-Ion Batteries

“…[172][173][174][175][176] In situ optical microscopy (IOM) methods have been used to investigate Li deposition on Li metal anodes 143,144,177 and other anode materials. 68,72,142,[178][179][180] By choosing a cross-sectioned full cell optical setup, a near realistic cell configuration was reached by Hogrefe et al 68,178,181 Due to the cross-sectional view, lithiation gradients through the anode coating can be observed by the different colours of Li-graphite intercalation compounds (Li-GICs: LiC 18 : blue, 180,182,183 LiC 12 : red, 180,182,183 and LiC 6 : yellow/golden 180,182 ). 68,178 Additionally, the nucleation of Li depositions, growths of Li dendrites, and internal short circuits by these dendrites can be directly observed, which become evident by the appearance of shiny-silver, dendritic, and moss-like structures on the anode.…”

“…8,10,11,34,35,37,38,40,223,224 Li depositions can be avoided by keeping the anode potential positive vs Li/Li + . 8,10,34,40,45,57,86,162,181,[225][226][227] One difficulty is that many cell types (e.g. all commercial cells) do not contain a reference electrode, so that the anode potential cannot be measured.…”

“…34,45 One approach to increase the cycle life of commercial 2-electrode cells by avoiding Li depositions is to implement reference electrodes in 3-electrode prototype cells and use the results of the anode potential measurements later in 2-electrode cells without reference electrode. 181 Alternatively, electrodes from commercial 2-electrode cells can be reconstructed into 3-electrode full cells to measure anode potentials. 34,40,45 Although such approaches do not take aging into account, cycle-life was shown to increase significantly in the corresponding 2-electrode cells.…”

“…Typical variations are the ambient temperature, the end-of-charge voltage (or the SOC), and the charging C-rate. 8,11,[33][34][35][37][38][39]45,50,55,56,93,94,181 For the 3electrode cells, a few cycles are sufficient as the anode potential is measured at each charge, whereas for the aging experiments a significant amount of irreversibly deposited Li must be generated by longer cycling. The electrochemical data obtained during cyclic aging, can already give hints on whether Li depositions have occurred under the given conditions.…”