Recently fluoride ion batteries have been proposed as an alternative energy storage system to lithium ion batteries. However, poor cycle life of the FIBs with conversion-based electrode materials is a major drawback of the FIB systems that have been introduced so far. In this dissertation, the concept of intercalation-based mechanism has been used in order to develop cathode electrode materials for FIBs to address the issue of poor cycling performance. Therefore, several compounds with different structures including Ruddlesden-Popper-type compounds (LaSr(Mn/Fe/Co)O4 and La2(Co/Ni)O4+d) and Schafarzikite-type compounds
((Co/Mg)0.5Fe0.5Sb2O4) have been investigated within all-solid-state electrochemical systems.
In a lack of well-developed intercalation-based anode materials for FIBs, several metal/metal fluorides have been examined as counter electrodes on the anode side.
As a primary approach, Ruddlesden-Popper-type (K2NiF4) compound of LaSrMnO4 have been
galvanostatically charged against a composite of Pb+PbF2. The results showed a stepwise
fluorination of the compound to form LaSrMnO4F and LaSrMnO4F2-δ in the first and second
steps, respectively. The structural changes after an electrochemical fluorination (expansion of the cell along the c-axis together with contraction of the cell along the a/b axes) were in a good agreement with the structural changes over intercalation of fluoride ions into the vacant interlayers of LaSrMnO4 by chemical methods which was previously reported by other research
groups [48, 100]. This suggests a successful insertion of the fluoride ions into the vacant anion rocksalt interlayers of LaSrMnO4 via the electrochemical method. This also shows that
electrochemical fluorination can be a safer alternative method to fluorinate oxides. Even more important, the fluoride ions could be electrochemically removed (de-intercalated) from LaSrMnO4F2-δ host resulting in a good recovery of the initial oxide structure upon discharging
to negative potentials. This is the first evidence that the intercalation of fluoride ions is fully reversible.
In other K2NiF4-type cathode composite compounds (LaSr(Fe/Co)O4 and La2(Co/Ni)O4+d)
fluoride ions could be electrochemically inserted (intercalated) resulting in structural changes in line with what has been observed for LaSrMnO4, but without the intermediate stage which
corresponds to occupation of only one interlayer. For the Schafarzikite-type structures of (Co/Mg)0.5Fe0.5Sb2O4 the intercalation of F− ions resulted in structural changes in accordance
with the results that have been obtained by insertion of ~ 0.5 fluoride ions by chemical methods. It can be concluded that Ruddlesden-Popper-type structures can accommodate more ions per formula unit as compared to Schafarzikite-type structures. Furthermore, a higher degree of reversibility in cathode materials with Ruddlesden-Popper-type structures as compared to Schafarzikite-type structures may arise from the dimensionality of the fluoride ion sublattices: 1D (channels) of fluoride ions in Schafarzikite-type structure (MSb2O4) vs. 2D
(planes) of fluoride ions in Ruddlesden-Popper-type structure (A2BO4).
From the electrochemical performance point of view La2NiO4+d and La2CoO4+d appeared to be
the most promising candidates among all other compounds in terms of structural reversibility, discharge capacity, energy density and cycling stability. According to the TEM based analysis (ADT and EDX) on a fluorinated La2NiO4+dFy single crystal, a total number of 1.59 – 1.72
fluoride ions could be inserted within the interstitial layers. For La2CoO4+d, however, the
capacity in relation with the structural changes during the galvanostatic charging of the cell. For both compounds, galvanostatic discharging results in a good recovery of the initial structure. However, potential of La2NiO4+d is higher than La2CoO4+d. This is due to the fact that,
Ni2+ ⟶ Ni3+ has a higher potential as compared to Co2+ ⟶ Co3+ and when it comes to potential
of the redox couple, the oxidizing power of the transition metal within the oxide is a determining factor. Therefore, La2NiO4+d would be more interesting as a high energy density
cathode material.
The issue of overlapping of carbon oxidation with the desired electrochemical reaction within the cathode composite material could be observed in all examined systems. The cathode material seems to have a catalytic effect towards oxidation of carbon within the electrode composite. The carbon side reaction occurs at low potential region but does not significantly influence the conductivity of the cell. However, at higher potential/capacity region the side reaction results in a massive destruction of the conductivity of the cell leading to a poor cycling performance. It is not clear whether carbon oxidation occurs in two steps with intermediate conductive products or the morphology of the (carbon) oxidized particles is point-like and establishes a network only at high potentials. Regardless of the kinetic of carbon oxidation within the electrode material, cyclic voltammetry experiments and impedance spectroscopy show that the cutoff potential criteria play a vital role in the development of the carbon side reaction and consequently in cycle life of the cells. By selecting suitable cutoff criteria (capacity/potential), cycle lives as high as 220 cycles with a Coulombic efficiency close to 100% (97.68% in the whole cycling range) can be obtained within the La2NiO4+d system. This is the
highest cycle life that has ever been reported for an FIB, suggesting that La2NiO4+d can be a
suitable candidate to be considered as a high capacity and high potential cathode material for FIBs. It should be emphasized that the reported cycle life is obtained within an all-solid-state system and yet it is far more stable than other FIB systems including those that benefit from liquid electrolytes.
Slow formation of a stable interface within the first cycles may explain lower Coulombic efficiencies in the early cycles as compared to that at higher cycles which is also accompanied by a difference in the structural changes during the very first cycles and higher cycles.
Finally, the effect of volume change within the conversion-based electrode materials needs to be considered when it comes to the cycling stability of the whole cell; counter electrode materials with higher volume change were shown to build up higher internal resistances leading to a higher overpotential which results in a significant deterioration of the cycle life. On the contrary, cycling performance can be further improved by using anode materials with lower volume changes during redox reactions.
6.2 Outlook
Without any doubt, the best obtained cycling performance within this study is far away from being commercialized. The efforts in this dissertation were only a preliminary work in order to show that the intercalation mechanism in FIBs can pave the way to enhance the cycling performance. However, further efforts are required to get closer to application demands. In this respect, several measures are suggested in order to improve the FIB systems.
First of all, engineering of the active cathode materials is needed: The effect of doping on the cycling performance and energy density of the promising active cathode materials (such as La2NiO4+d) needs to be investigated. This is particularly important since doping can change the
site energy of the vacant interstitial sites and consequently may improve the energy density and cycle life. The other issue is to find out the optimized cathode material composite. This requires studies on finding replacements for carbon which seems to be problematic within the electrode composite or engineering the carbon particles in such a way to minimize the detrimental impact of carbon oxidation. Core-shell structures [40] may help for a better ionic and electronic conductivity. Also, the optimized ratio of active cathode material:conductive particles:electrolyte should be found. It is worth noting that some investigations have already been started within our research group in this respect. Using binder and applying pressure are also other engineering techniques that may have an influence on the cell performance, especially within all-solid-state batteries.
To obtain a better electrochemical behavior, counter electrodes need to be improved. In this study, conversion-based electrodes were mainly used since no state of the art for intercalation- based anode materials were available. Some efforts [44] within our research group have been done in order to develop intercalation-based anode materials for FIBs, though, they are still not operational at this state. However, some strategies such as using thin films [1], infiltrating the active electrode material into porous carbon hosts or dispersing or wrapping them in conductive matrices may mitigate the negative effect of volume change within conversion-based electrode materials [140].
Finally, enhancement of the electrolyte materials plays a vital role to develop FIBs further, especially when it comes to all-solid-state batteries. Particle engineering of the electrolyte may have some influences: For instance, sintering of La0.9Ba0.1F2.9 may help to achieve a better
electrochemical performance, as it is reported that sintering of this electrolyte material significantly improves the ionic conductivity [35].