To date, commercial LIB cells are mostly based on graphite as anode material. During the first inter­calation of lithium into graphite, the electrolyte gets reduced at the anode, forming a nm-thick surface layer, the so-called solid electrolyte interphase (SEI). The SEI stops further electrolyte reduction, but as it consumes lithium during its formation, there is a loss of active lithium inventory, which causes the irreversible capacity loss during the first cycle of an LIB.^[1] Neutron depth profiling (NDP) is a non-destructive technique and a suitable tool to measure lithium concentrations as a function of depth.^[2,3] When irradiating the sample with a cold neutron beam, ⁶Li nuclides emit charged particles after neutron capture. The residual energy and the signal rate of the emitted ³H particles are correlated to depth and amount of lithium, respectively. Thus, SEI growth and lithium (de‑)intercalation in graphite anodes can be studied up to a depth of ca. 30 µm. So far, in‑situ NDP measurements in LIBs have been limited to thin solid-state cells or to cells with sub-mm thick anodes.^[4,5] Here, we present operando NDP data for the first charge/discharge cycle of a 14 mm thick graphite anode (4 mm particles) vs. a capacitively over­sized LiFePO₄ cathode, using a custom-designed coin-cell casing with 0.5 mm diameter holes which are sealed with a 7.5 µm-thick Kapton^® window. Fig. 1 shows the voltage profile during cycling, demonstrating that the operando cell yields the expected first-cycle discharge capacity of 350 mAh/g_graphite. The higher irreversible capacity of ca. 98 mAh/g (ca. 22%) compared to conventional graphite used in commercial cells (ca. 10%) is due to the very high BET area of ca. 14 m²/g of the 4 µm sized graphite particles. Fig. 2 shows the NDP signals at three different states-of-charge (SOC): i) for the pristine graphite anode (0 h, black line), reflecting the lithium-containing electrolyte (1 M LiPF₆ in EC/EMC 3:7 v:v) within the electrode pores; ii) for 100% SOC at the end of the first charge (16 h, red line); and, iii) at the end of the first discharge at again 0% SOC (32 h, blue line). The difference between the red and the blue line indicates that ca. ¼ of the active lithium has been consumed irreversibly for the SEI formation, in good agreement with the electrochemical data.

References

[1] E. Peled and S. Menkin, J. Electrochem. Soc. 164 (7), A1703-A1719, 2017.

[2] M. Trunk, M. Wetjen, L. Werner, R. Gernhäuser, B. Märkisch, Z. Revay, H. A. Gasteiger, and R. Gilles, J. Mat. Char. 146, 127-134, 2018.

[3] L. Werner, M. Trunk, R. Gernhäuser, R. Gilles, B. Märkisch, and Z. Revay, J. Nucl. Instr. Meth. A, 911, 30-36, 2018.

[4] J. F. M. Oudenhoven, F. Labohm, M. Mulder, R. A. H. Niessen, F. M. Mulder, and P. H. L. Notten, Adv. Mat. 23, 4103–4106, 2011.

[5] D. X. Liu, L. R. Cao, and A. C. Co, Chem. Mater. 28, 556−563, 2016.