The Role of Electrolyte Composition in Enabling Li Metal‐Iron Fluoride Full‐Cell Batteries

Abstract

FeF 3 conversion cathodes, paired with Li metal, are promising for use in next‐generation secondary batteries and offer a remarkable theoretical energy density of 1947 Wh kg −1 compared to 690 Wh kg −1 for LiNi 0.5 Mn 1.5 O 4 ; however, many successful studies on FeF 3 cathodes are performed in cells with a large (>90‐fold) excess of Li that disguises the effects of tested variables on the anode and decreases the practical energy density of the battery. Herein, it is demonstrated that for full‐cell compatibility, the electrolyte must produce both a protective solid‐electrolyte interphase and cathode‐electrolyte interphase and that an electrolyte composed of 1:1.3:3 (m/m) LiFSI, 1,2‐dimethoxyethane, and 1,1,2,2‐tetrafluoroethyl‐2,2,3,3‐tetrafluoropropyl ether fulfills both these requirements. This work demonstrates the importance of verifying electrode level solutions on the full‐cell level when developing new battery chemistries and represents the first full cell demonstration of a Li/FeF 3 cell, with both limited Li and high capacity FeF 3 utilization.

Room Temperature Pseudo-Solid State Iron Fluoride Conversion Battery with High Ionic Conductivity and Low Interfacial Resistance

“…Gradual CEI formation is consistent with our previous report of Li-FeF3 full cells cycled with unencapsulated LHCE, 19 where ~20-30 cycles were required to stabilize capacity fade at a rate of C/20 (equivalent to 0.05 mA cm -2 in the present report). Although we cannot completely rule out the influence of current density on the rate of CEI stabilization, Figure S10 suggests that this process is more dependent on the number of initial cycles than on the particular current density.…”

“…Briefly, the slightly greater than theoretical (712 mAh g -1 ) initial discharge capacities and steep capacity drop between the first and second cycles (~14-19%) are believed to be due to CEI formation in iron fluoride batteries. 19,20 Furthermore, the downward sloping trend in the initial five cycles is consistent with our previous finding that Li-FeF3 capacity fade requires ~20-30 cycles at C/20 (or 0.05 mA cm -2 in the present report) to stabilize in LHCE. 19 Figure 5 shows that cycle stability improves at higher current densities (0.10 to 1.0 mA cm -2 = C/10 to 1 C).…”

“…19,20 Furthermore, the downward sloping trend in the initial five cycles is consistent with our previous finding that Li-FeF3 capacity fade requires ~20-30 cycles at C/20 (or 0.05 mA cm -2 in the present report) to stabilize in LHCE. 19 Figure 5 shows that cycle stability improves at higher current densities (0.10 to 1.0 mA cm -2 = C/10 to 1 C). As discussed in the previous section, Figure S10 suggests that the number of initial cycles (rather than the particular current density) governs capacity fade stabilization.…”

“…Introduction, instabilities with both FeF3 1, 7 and Li metal 3,16,22 contribute to low coulombic efficiency (CE) in Li-FeF3 batteries. Following our recent demonstration that the LHCE is stable with both the Li metal anode and FeF3 cathode, 19 we used Li half-cells and FeF3 half cells to evaluate stability of these electrodes with gel-2.…”

“…8,16,17 Recently, our group showed that the particular LHCE used in the present report (vide infra) is stable with both Li metal and FeF3, enabling high CE and rate capability for Li-FeF3 batteries. 19 Despite the beneficial properties of FeF3 (vide supra), commercialization of rechargeable Li-FeF3 batteries has been impeded by low CE, 1,7 voltage hysteresis, 1,7,18 slow kinetics, 1 low electronic conductivity, 1,6,18 electrolyte decomposition, 18,20 and volumetric changes during the conversion reaction. 18,20 Use of an LHCE can address low CE and electrolyte decomposition.…”