Screening of the Electrode Materials

In general, several selection criteria including economical and technical considerations play a vital role, when it comes to selection of the electrode materials. From the technical point of view, a suitable electrode material offers a large reversible storage capacity at the desired electrochemical potential with a suitable power density. As economic considerations one can name natural abundance, low production cost, eco-friendly nature and ease of recycling [56, 92]. In this study, however, the focus is more on the technical aspects since the economical perspectives becomes important when it comes to the state of the art and industrial mass production, a state which has not been obtained for FIBs yet.

One of the most important technical aspects of batteries lies on the capacity of the electrochemical cell. In many cases the practical capacity is much less than the theoretical capacity. As an example, LiCoO2 possesses a theoretical capacity of 273 mAh/g (for insertion /

removal of one Li ions per formula unit), however, only a capacity of 140 mAh/g can be obtained since only 0.5 Li ions can reversibly participate in the lithiation / de-lithiation (due to low structural stability of the de-lithiatet state) [93, 94]. The other essential criterion for a suitable electrode material is the voltage of the battery. To avoid decomposition of the electrolyte, the potential difference between the anode and cathode should lie within the potential window of the electrolyte material [49].

5.2.1 Selection of the Cathode Materials

Before considering the electrochemical fluorination, it is important to ensure that the possible structural changes during the electrochemical fluorination (charge) arises from the oxidative (electrochemical) intercalation of the F− _{ions and not from a chemical reaction between the}

electrode and the electrolyte materials. Due to limitations in ionic conductivity of the current FIB electrolyte materials, the electrochemical cells must be heated up (mainly around 170 °C). Thus, the chemical stability of the active cathode materials at the working temperature is an essential criterion. In this respect, full cells based on different active cathode materials were made and heated at the conditions identical to charging condition; i.e. the cells were heated up to 170 °C at typical experimental conditions without applying any current. Examples of the

structural changes in the active cathode materials over the heating process are presented in

Figure 5-2.

As can be seen in Figure 5-2a, SrFeO2 has low to no chemical stability on the heating: SrFeO2

precursor has a tetragonal structure (P4/mmm, a = 3.9933(1) Å, b = 3.4837(2) Å). However, after heating the cell at 170 °C, SrFeO2 active cathode material undergoes an oxidation reaction

and a pseudocubic compound with high similarity to SrFeO2F (Imma, aps.cub. = 3.9541(2) Å

[95]) is obtained. The structural changes in terms of lattice parameter and space group of the oxyfluoride phase (over the chemical fluorination which is reported in literature [96]) is in a very well agreement with obtained structure after heating SrFeO2 within the electrochemical

cell configuration. It is worth noting that heating the SrFeO2 cathode composite inside a

glovebox (controlled atmosphere with an oxygen content <0.1 ppm) for 6 h already resulted in the formation of this phase (which would require oxidation). It is not clear how this oxidation takes place, since redox reaction with the electrolyte [36] and traces of oxygen can be considered of minor importance regarding that the compound even reacts in the glovebox under additional vacuum. Therefore, it seems that SrFeO2 can be only stabilized at the highly reductive

conditions in the presence of excess of CaH2 used during its synthesis [97].

Figure 5-2. XRD patterns of different active cathode materials before and after heating the full cell (pellet) at 170 °C for a duration of 96 h without applying any current; (a) SrFeO2; (b) LaSrMnO4; (c) La2CoO4+d; and (d)

LaSrMnO4 shows a good chemical stability at the operational temperature (Figure 5-2b). Two

additional tetragonal phases (Phase 1: P4/nmm, a = 3.8076(15) Å, c = 13.2941(92) Å, Phase 2: P4/nmm, a = 3.8059(19) Å, c = 13.7263 (97) Å) with relatively small weight fractions can be found after heating, though the majority of the initial LaSrMnO4 phase (tetragonal I4/mmm,

a = 3.8202(5) Å, c = 12.9582(21) Å) can still be found after the heating without significant structural changes (Figure 5-2b).

The XRD diffraction pattern of La2CoO4+d before and after heating up the cell within the

experimental conditions can be seen in Figure 5-2c. The obtained lattice parameters for the

initial La2CoO4+d phase (Abma, a = 5.5243(7) Å, b = 5.4762(7) Å, c = 12.6153(19) Å) reveals

that the oxidation state of cobalt is ~ +2.32 (corresponding to d = 0.16) [98]. After heating the cell, however, the initial phase can be determined together with a similar phase but with a higher degree of orthorhombic distortion (Phase 1: Abma, a = 5.5791(7) Å, b = 5.4347(7) Å, c = 12.6237(19) Å) which can be due to a slight increase in the oxidation state of Co [98]. Nevertheless, the structure is chemically stable enough to be considered for the electrochemical fluorination at the experimental temperature.

For Co0.5Fe0.5Sb2O4 with Schafarzikite-type structure, the heating process does not result in a

significant change in the structure (Figure 5-2d) which again shows that Fe2+ _{seems to be stable}

here (same results are obtained for Mg0.5Fe0.5Sb2O4).

Since SrFeO2 seems to have no chemical stability over the heating process (Figure 5-2a) further

investigation on this compound was disregarded. However, other compounds with K2NiF4-type

structures (LaSrMnO4, La2CoO4+d, etc.) and Schafarzikite-type structures (MSb2O4) were

considered further as potential candidates.

5.2.2 Selection of the Anode Materials

Unlike other Li/Na ion battery systems, no standard can be proposed as a counter electrode material for testing the FIB electrode materials since it is not possible to use fluorine gas as an electrode material while for the Li/Na ion batteries the respective metal element can be simply used as a reference. So far no other reference electrode material has been proposed for testing the new electrode materials for FIBs. Therefore, at the early stage of this study several active anode materials including CeF3, MgF2 and PbF2 were used as a counter electrode. However, the

best results obtained by using PbF2 as the active anode material. In fact, using a less strong

reducing agent (such as PbF2) ensures less side reactions in the anode material during the redox

reactions. Moreover, low melting point of Pb (~ 327 °C [57]) is in favor of a better reactivity within the solid-state reactions since the reactivity of a compound within a solid-state reaction is strongly enhanced the closer it is to the melting temperature of a compound [99]. Such a higher reactivity leads to a reduction in the overpotentials [87]. In the course of this thesis, it was realized that the anode composite materials, which consist of a mixture of metal fluoride plus the corresponding metal have a better electrochemical performance due to two reasons: 1- The metal particles ease the nucleation (as nucleation agents) of formation of metal particles during the electrochemical fluorination [87]; 2- During the cycling of the electrochemical cells some parts of the new formed metal (after reduction in the anode material during the electrochemical charging (fluorination)) may become inactive and does not take part in further redox reactions. Thus, additional metal particles may compensate for the inactivated particles. Such a strategy has also been proposed for FIBs by Fichtner et al. [27]. Furthermore, for improving the energy density of the electrochemical cells and improving the cycling

performance other metal plus metal fluoride anode composites such as Zn+ZnF2 and Mn+MnF2

have also been used.