*3.1. Electrode Materials*

Amatucci and Pereira note in their review on metal fluoride based electrode materials that "*The use of fluorides stems from the intrinsic stability of fluorinated materials and their ability to generate high electrochemical energy as electrodes*" [11]. Indeed, metal fluoride cathodes generally produce higher potentials than the corresponding oxides of the same redox-couple, which can lead to higher energy densities [11]. Thus, fluoride materials could be used in high voltage batteries, where the stability of active materials is especially important. Fluorides can be used as cathodes either as pure fluorides or as doped materials, such as oxyfluorides, fluorosulphates or fluorophosphates [11,35]. Fluoride doping has been reported to improve capacity retention of intercalation cathodes such as lithium nickel oxide and lithium nickel cobalt oxide [11]. This could be related to a slower dissolution of transition metals into liquid electrolytes from the oxyfluorides [11]. For fluorophosphate cathodes, such as Li2CoPO4F, high potentials of over 5 V are obtainable, accompanied again by a slower dissolution of the transition

metal [11,35]. However, the performance of these cathodes is limited due to poor ionic and electronic conductivity and instability of liquid electrolytes at such high potentials [35].

As already mentioned in the Introduction, pure metal fluorides gained interest decades ago as electrodes for primary batteries because of their high capacities (Figure 5). With the increased interest in high capacity alloying and conversion anodes such as Si and SnO2, fluoride conversion cathodes could also resurface as interesting materials for secondary lithium-ion batteries [11,25,29,36–39]. Similar to alloying anodes, fluoride conversion cathodes suffer from large volume changes and subsequent pulverization during cycling. In addition, fluorides are very poor electron conductors due to their high band gaps and often show high overpotentials, which can make their integration as reversible electrodes challenging [25,29]. In the early years materials such as CuF2 and HgF2 were studied but with little success. Recently, interesting progress has been made in this research area. For example, BiF3, FeF2 and FeF3 have been studied extensively as cathodes using nanocomposites of the metal fluorides and conductive carbon [38,40]. Using nanocomposites with carbon can not only help with the volume change but also with the inherently low electrical conductivity of fluoride materials.

**Figure 5.** Theoretical (black), first discharge (dark grey) and charge (light grey) specific capacities of conversion fluoride cathode materials. Adapted with permission from [36]. Copyright 2010 Wiley-VCH.

Due to the work on primary batteries pure fluorides are generally considered only as conversion cathodes but some reports on intercalation fluorides have been published [41–43]. For example, Li3FeF6 has been reported to show intercalation of 0.7–1 Li+ ions per fluoride unit in a carbon nanocomposite form, resulting in a reversible capacity of 100–140 mAh/g [41,42]. The capacity depends on the size of the Li3FeF6 particles, with smaller particles resulting in a higher capacity [42]. A deeper discharge of the material was reported to lead to LiF formation, indicating a conversion reaction at low potentials. Similarly, a nanocomposite of Li3VF6 was reported to reversibly intercalate up to one Li+ per fluoride unit [43]. Calculations predict that fluorides such as LiCaCoF6 could provide very high intercalation voltages [44].

In addition to their potential use as electrode materials, fluorides can also be utilized as solid-electrolyte-interface layers deposited on the more conventional electrode materials to protect them from reactions with the organic liquid electrolytes. AlF3 has been studied extensively in this regard, on both cathodes [45–49] and anodes [50]. AlF3 is suitable for electrode protection because it is rather inert and the Al3+-ion cannot be reduced or oxidized in battery conditions [20]. The material has been reported to decrease the irreversible capacity losses of electrodes and improve cycling stability, [45,47] and increase the thermal stability of electrodes [45,46,49]. Figure 6 illustrates how a layer of AlF3 can increase the capacity retention in a lithium cobalt nickel manganese oxide cathode. Using too much AlF3, however, decreases the capacity considerably.

**Figure 6.** The effect of an AlF3 coating on the discharge capacity of a lithium cobalt nickel manganese oxide cathode as a function of the number of charge/discharge cycles. Reprinted with permission from [47]. Copyright 2012 Wiley-VCH.
