Abstract
This chapter analyses the main advances made in the field of sodium–vanadium fluorophosphates as cathodes for Na-ion batteries and tries to clarify some discrepancies and common errors published about these compounds. The sodium–vanadium fluorophosphate family can be divided in two main members: Na3V2(PO4)2F3 (V+3 extreme phase) and Na3V2O2(PO4)2F (V4+ extreme phase). Na3V2O2x(PO4)2F3-2x, where 0 < x < 1 would correspond to intermediate V3+/4+ mixed valence phases. Among them, the V3+ extreme has demonstrated to be difficult to isolate, whereas the V4+ and mixed valence phases can be more easily prepared by different synthesis methods and from different vanadium sources. In terms of electrochemical performance, mixed valent compound provides good performance, with high specific capacity at moderate/high cycling rates, and long cycle life. The future perspectives for this family of compounds are discussed in terms of raw materials availability, price, and performance relative to other cathode systems for Na-ion batteries.
Keywords
- Na-ion batteries
- cathodes
- fluorophosphates
- vanadium
- High voltage
1. Introduction
Energy production is a key issue for the support of our society and way-of-life. Driven by the need to reduce the emissions of CO2 and increase energy security, policy makers have implemented different measures in order to shift to low-carbon energy resources. One of these policies is to promote the use of renewable energy sources at the expense of the carbon-based fuels [1,2]. However, the increasing use of these clean energy sources entails other problems, such as modulating time-variable energy production from renewable resources due to their dependence on the weather to integrate them into the grid. Thus, in order to solve the problem concerning the intermittency of these resources, additional energy storage devices are needed. This way, electrochemical energy storage is a strategic research area that will help to increase the weight of renewable energy sources in the energy mix.
In this field, batteries are one of the most promising systems to act as buffer to regulate the variable energy income that can be obtained from the wind, sun and from the ocean waves. For this purpose, one of the main technologies under development is Na-ion batteries. In the last 20 years, Li-ion batteries have attracted all the attention in energy storage, but recently, Na-based compounds have made a comeback because of controversial debates regarding the size of reserves and higher cost to obtain Li [3]. The use of Na instead of Li in rocking chair batteries could mitigate the feasible shortage of lithium in an economic way for many reasons. The first one, the unlimited sodium sources, being the sodium content in the earth’s crust and water of 28,400 mg·kg−1 and 11,000 mg·L−1 compared to 20 mg·kg−1 and 0.18 mg·L−1 for lithium [4]. Moreover, sodium presents lower price than lithium and it is easy to recover [5].
Although sodium (Na)-ion batteries are considered a promising alternative to lithium-ion systems, it is usually said that they possess limited electrochemical activity compared to Li due to two intrinsic reasons. First, Na has a lower ionization potential than Li (5.139eV vs. 5.392 eV) [6], leading to lower operating voltages and thus lower energy densities. Second, Na+ ions are heavier and larger than Li+ ions, leading to slow diffusion within a solid electrode during cycling and larger volume expansion of the electrode [7]. These two factors would induce lower gravimetric energy densities, so renewed research on these systems has been focused on their utility as buffering systems in the electrical grid, where battery volume is not so crucial due to the stationary nature of the practical use of these batteries.
However, and in spite of the great efforts devoted to the development of higher performance Li-ion batteries, if practical energy densities achieved up till now for both Li- and Na-based systems are compared, it can be seen that Na-ion batteries are catching up with the practical energy densities of Li-ion in a quite short time (Figure 1).
This great advance in gravimetric energy densities for Na-ion batteries is due to the extensive research that has been done in the last years to identify and optimise the best materials for these systems, so they can be soon commercially viable. As it has been said, the incorporation of this kind of systems to the electrical grid would lead to a significant increase of the proportion of renewable energies on the energy mix, with the positive environmental consequences that this fact would result in. Thus, it is a priority to continue searching for and optimising the best-performing materials to be used in the different components of a battery: cathode, electrolyte and anode.
In the case of the cathode, a good performing material should provide high energy density to the battery, that is, it should be able to charge and discharge the highest possible specific capacities with also high operating voltage and for many cycles (long cycle life). In this sense, sodium–vanadium fluorophosphates have demonstrated to fulfil all of these characteristics.
2. High-voltage cathode materials for Na-ion batteries
In the last decade, three dimensional frameworks built on transition metals (M) and polyanions (XO4)n- have become a hot topic in the research field of electroactive materials for lithium and Na-ion batteries. The smaller theoretical gravimetric capacity achieved due to the presence of polyanion groups is compensated by the positive features presented by these compounds, such as very stable frameworks and high inductive effect [9]. The “inductive effect” is a concept proposed by Goodenough
On one hand, sulphate-based cathodes offer a good combination of sustainable syntheses and high energy density owing to their high-voltage operation due to electronegative SO42− units [11]. Na2Fe2(SO4)3 has demonstrated to be the most promising compound being operable at 3.8 V
On the other hand, framework materials based on the phosphate polyanion have also been identified as promising electro-active materials for sodium metal and Na-ion battery applications. It is the strong inductive effect of the PO43− polyanion that moderates the energetic of the transition metal redox couple to generate relatively high operating potentials for these compounds [14]. NaMnPO4 is one of the most studied compounds in this field due to the high operating voltage provided by its lithium analogue: LiMnPO4 which works at 4.1 V
Na4Co3(PO4)2P2O7 pyrophosphate is another promising candidate due to its high working potential region (between 4.1 and 4.7 V
Among the framework materials fluorophosphates possess even higher operating voltages than phosphates, because the inductive effect of fluorine is added to the effect of phosphate. This latter feature makes them a key to solve the energy density issue of sodium-based batteries. Fe, Mn, and V have been the most investigated transition metals but the three of them present different structures: Whereas the sodium–iron fluorophosphate possesses a two-dimensional layered structure [19], the sodium–manganese fluorophosphate presents a three-dimensional tunnel structure [20]. However, manganese compound has demonstrated to be poorly electrochemically active so that more studies have been performed for the iron-based compound. For the moment, one of the best results achieved has been for carbon coated porous hollow spheres of Na2FePO4F phase. This nanostructured material contained about 6–8
Regarding sodium–vanadium fluorophosphates, three phases have been described in the literature: NaVPO4F, Na3V2O2(PO4)2F and Na3V2(PO4)2F3 with theoretical specific capacity values of 143, 130 and 128 mAh·g−1, respectively. Barker
A deep study of the bibliographic data related to these three sodium fluorophosphates leads to doubt about the real existence of these three different compounds. It is worth noting that the existence of the NaVPO4F phase has already been questioned by authors such as Sauvage
Electrochemical studies on the three mentioned tetragonal structure compounds have been performed under different conditions
3. Sodium–vanadium fluorophosphates as cathode materials for Na-ion batteries
The study of sodium–vanadium fluorophosphates is especially relevant because of the high operating voltages offered by these compounds that could lead the way to high energy Na-ion batteries.
The similarity of the X-ray diffraction patterns as well as the almost identical electrochemical data for Na3V2O2(PO4)2F and Na3V2(PO4)2F3 suggested that both materials could belong to the same family of compounds, where the fluorine content is modulated by the presence of V3+ and VO2+ (V4+) leading to the following general formula Na3V2O2x(PO4)2F3-2x which was proposed by our group for the first time [40]. The extreme members (x = 0 and x = 1) would correspond to the mentioned phases, Na3V2(PO4)2F3 for x = 0 (Figure 2a) and Na3V2O2(PO4)2F for x = 1 (Figure 2b), whereas intermediate compounds (0 < x < 1) would be V3+/V4+ mixed valence phases (Figure 3). The existence of a mixed valence family of compounds between Na3V2(PO4)2F3 and Na3V2O2(PO4)2F phases was also reported by Park
In order to understand the structure and properties of this family of compounds, a series of sodium–vanadium fluorophosphate samples were hydrothermally prepared, varying the type and amount of carbon used as reductive agent during the synthesis [40]. This way, the possible influence of that carbon on both the final properties of the sample and the attainment of different compositions in the family of general formula Na3V2O2x(PO4)2F3-2x (0 < x < 1) was analysed. Intermediate amounts of carbon present in the Na3V2O2x(PO4)2F3-2x final product (1 – 50
Regarding the V4+ phase, Na3V2O2(PO4)2F, the first electrochemical results were presented by Sauvage
Additional works showed that the reversible capacity of Na3V2O2(PO4)2F/graphene electrodes exceed 100 mAhg−1 after 200 cycles at a C/20 current rate [37]. Apart from that, a theoretical and experimental study on a solvothermally obtained Na3V2O2(PO4)2F phase was presented were structural and electrochemical characterisation of this material was carried out [18]. Moreover, computational studies revealed the possibility of extracting the third sodium of the mentioned phase at 5.3 V
Concerning the V+3 phase, Na3V2(PO4)2F3 , the electrochemical mechanism of the reversible Na extraction was described by Chihara
The crystal structure of the Na3V2(PO4)2F3 phase at high temperature (400 K) obtained from high resolution synchrotron radiation measurements in
Concerning
Concerning Na3V2O2(PO4)2F phase, different space group symmetries have been described in the literature to define it [28,29]. In terms of sodium insertion/extraction, the
Finally, different time-resolved
The electrochemical behaviour of the two electrode materials studied
To finish with, the structural evolution of V3+ Na3V2(PO4)2F3 phase has recently been analysed by using high angular and intensity resolution synchrotron radiation [34]. The two voltage domains (at 3.7 and 4.2 V
4. Conclusions
Polyanions (XO4)n- based on transition metals (M) are one of the most promising candidates for high-voltage Na-ion batteries. Among them, sulphate and phosphate based framework materials stand out. Regarding phosphate-based materials, and as it has been previously shown in this chapter, there has been some controversy about the structural and electrochemical features of sodium–vanadium fluorophosphates. This family of materials show very high energy density due to the high voltage provided by the adding inductive effect of fluorine. Moreover, these materials have demonstrated to be very stable and present outstanding electrochemical properties. V+3 Na3V2(PO4)2F3, V+3.8 Na3V2O1.6(PO4)2F1.4 and V+4 Na3V2O2(PO4)2F phases included in the family of compounds Na3V2O2x(PO4)2F3-2x have been structurally and electrochemically deeply analysed. Despite the similarity of all these materials, up to date studies show that the electrochemical behaviour of these electrode materials is clearly dependant not only on the sodium extraction/insertion mechanism, the occupancy and distribution of sodium and the electrochemical cycling history but also on the synthesis process employed for the obtaining of the starting material that determines its properties.
5. Future perspectives for sodium–vanadium fluorophosphates as cathodes for Na-ion batteries
As it has been presented, sodium–vanadium fluorophosphates are more than promising cathodic materials for near future commercial Na-ion batteries. However, the appearance of real commercial Na-ion batteries involves the development of the whole battery, that includes advancing in finding the appropriate components for a Na-ion cell based on a sodium–vanadium fluorophosphate cathode. In the first place, an anode matching this cathode must be chosen. This anode material should present relatively low operating voltage in order to provide the battery with the maximum available voltage. In the second place, the specific capacity of this anode and its rate capability should be in balance with the cathode performance in order to make an easier mass balance to prepare the cells. These two conditions will be necessary to assure that the battery, and thus its components, present the best possible performance in terms of battery voltage and power.
On the other hand, a special effort must be done to search for a long-lasting and high-voltage working electrolyte for these systems. Since the recovery of Na-ion battery research, great efforts have been directed to the search of new electrode materials whereas studies dealing with the electrolyte are much scarcer. One of the reasons for this trend can be that it has been shown that the SEI formed on carbonaceous electrodes is markedly different for sodium- and lithium-based electrolytes even using the same solvent [57,58]. Non-aqueous Na electrolytes presently used are mainly based on NaClO4 or NaPF6 salts in propylene carbonate or other solvents and mixtures such as ethylene carbonate: dimethyl carbonate. In general, a good electrolyte should exhibit: i) good ionic conductivity, ii) a large electrochemical window (i.e., high and low onset potential for electrolyte decomposition through oxidation and reduction at high and low voltages, respectively), iii) no reactivity towards the cell components, iv) thermal stability (i.e. melting point and boiling point lower and higher than the standard temperatures for the cell utilization, respectively). Finally, it should be intrinsically safe, have as low toxicity as possible and meet cost requirements for the targeted applications. All these features are intrinsically dependent on the nature of the salt and the solvent(s) and the possible use of additives [59].
Another argument pointed out as a drawback for Na-ion batteries based on sodium–vanadium fluorophosphates is the use of a vanadium compound in them. It is said that vanadium is not a green element that needs careful handling after battery life cycle, but the same can be said from the electrolytes used in both sodium- and lithium-based systems. In any case, a battery will always need to be collected to be treated as a residue after its cycle life, as it is nowadays done. Apart from that, it is also commented that vanadium cost is another drawback for the development of these batteries. But it must be recalled that there exist commercial vanadium-rich energy storage systems, such as redox flow batteries, that comprise the use of high amounts of vanadium for their two electrodes [60]. Thus, a careful cost analysis should be done to confirm or discard this statement once the appropriate anode and electrolyte materials are chosen and a viable cell prototype is proposed.
Finally, and now focusing our attention on the commercial development of only sodium–vanadium fluorophosphates compounds, a last challenge that must be overcome is to develop a synthetic method that allows an easy and economical mass production of these materials. For this purpose, different parameters will have to be taken into account: the different possible synthetic methods (hydrothermal, microwave or ceramic processing); the different possible vanadium starting reactants; and the yields obtained by crossing these latter two parameters (Figure 7).
To sum up, sodium–vanadium fluorophosphates family is one of the main possible future cathodic materials for high-voltage Na-ion batteries. Their excellent electrochemical properties and performance have been perfectly well demonstrated by different research groups, as it has been told in this chapter. Now, the great defy for these materials lays on the industrial pace, that is, in the assembly and building of a real Na-ion full cell based on these materials and the development of a commercially viable process to produce these compounds in great amounts.
Acknowledgments
This work was financially supported by the Ministerio de Economía y Competitividad (MAT2013-41128-R) and the Gobierno Vasco/Eusko Jaurlaritza (IT570-13). University of the Basque Country (UPV/EHU) is acknowledged for funding under project UFI11/15.
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