Formation of the Solid Electrolyte Interphase on the Graphite Anode in Lithium-Ion Batteries – an Operando Neutron Depth Profiling Study
Authors: Linsenmann Fabian , Trunk Markus , Rapp Philip , Werner Lukas , Gernhäuser Roman , Révay Zsolt , Gilles Ralph , Märkisch Bastian , Gasteiger Hubert A. 
Published: February 2020

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/ggraphite. 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.

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Figure 1