Polyanionic cathode materials and the inductive effect
Besides oxide based cathode materials, polyanionic materials consisting of a transition metal and a polyanionic group (XOn)m- (X = C, B, P, S, Si, Mo, W, etc.) have gained strong interest since the discovery of electrochemically active lithium iron phosphate LiFePO4 in 1997.63 Although additional weight is introduced by the presence of the polyanionic group leading to a reduced gravimetric capacity, polyanionic electrode materials have several advantages as follows. First, they provide a stable anionic framework essential for long term stable cycling and safety issues (oxygen release in the case of oxides is circumvented). Second the redox potential of the Mnn+/M(n-1)+ redox couple can be modified through the inductive effect of the polyanionic group (described in detail in the next paragraph) and leads to higher values compared to oxide compounds, and finally these class of compounds offer a very rich chemistry due to a variety of different crystal structures and possible substitutions.
In the following section an overview about different polyanionic materials is given, however since numerous compounds have been investigated, materials mainly based on the Fe3+/Fe2+ redox couple are presented hereinafter.
NASICON-type compounds AxM2(XO4)3 (A = Li, Na)
In the 1980s it was demonstrated for the first time possible electrochemical reversible intercalation of Li or Na into 3D framework NASICON (NA-SuperIonic CONductors) phases Fe2(MoO4)3 (Figure 14) and (Li, Na)Ti2(PO4)3.66–68 In general they can be expressed by formula AxMM’(XO4)3 and are built up on a framework of MO6 and M’O6 octahedra sharing all their corners with XO4 tetrahedras, with alkali metal ions sitting interstitial spaces, enabling fast ion conduction.
Borate based cathode materials for lithium- and sodium-ion batteries
Boron, the fifth element of the periodic table, doesn’t occur in its elemental form in nature, only as its oxide form. More than 200 borate minerals are found in earth’s crust, so called borates, but only a few of them are of commercial interest: borax, kernite, ulexite and colemanite (Table 4).
Table 4: The four main borate based minerals found in earth’s crust with their chemical formula and the main deposit location.
Starting in the 19th century, borates based glasses have been studied for their optical properties and later on for alkali ion conduction.128,129 Nevertheless crystalline borate based materials were mainly investigated because of its non-linear optical properties and as host structures for light emitting phosphors. Since the discovery of β-BaB2O4 and LiB3O5 in the mid 1980’s (which are up to now still the most frequently used borate based non-linear optical properties (NLO) materials) an increased interest in the scientific community has led to the discovery of a large variety of borate based compounds.,132 It is believed that these optical properties derive from the unique crystal- and electronic resulting from the small boron atoms inside an oxide matrix.
Search for new pyroborate B2O5 based compounds
It is nowadays established that by modifying the polyanion of a cathode material, elevated redox potentials versus Li/ Na can be achieved. Following this direction, we tried in this thesis to increase the potential of borate based materials, since for instance LiFeBO3 shows a voltage of 2.8 V which is rather low compared to other polyanionic cathode materials. To increase the redox potential condensed polyanions can be introduced, as previous recalled in switching from LiFePO4 to Li2FeP2O7.
We have followed this strategy and our results are reported in this chapter which first gives a general overview about the reactivity of borates and possible synthetic routes for crystalline borate materials and then focuses on the experimental description of our first synthetic approaches, theoretically leading to pyroborate cathode materials.
Structural and electrochemical properties of a lithium copper pyroborate Li6CuB4O10
The synthesis of Li6CuB4O10 was first reported in 2006 by Pan et al.,171 but already mentioned in Sparta’s PhD thesis in 2003. The synthesis of this phase was done by the authors through mixing stoichiometric amounts of Li2CO3, CuO and H3BO3 and annealing the mixture at 590°C for 2-3 days, denoted as α-Li6CuB4O10 To obtain a better crystallized product our synthesis consisted in mixing stoichiometric amounts of the same precursors with mortar and pestle, followed by a decomposition of H3BO3 at 450°C for 4 h and annealing the mixture for 48 h at 650°C. A royal blue powder of pure α-Li6CuB4O10 was obtained, showing a particle size from 20 to 50 μm with no signs of impurities (Figure 38a).
Later in 2013, Kuratieva et al. isolated a crystal of another Li6CuB4O10 polymorph denoted as β-Li6CuB4O10, in exploring the Li2O-CuO-B2O3 ternary system.172 Through the course of this thesis we found that bulk samples of β-Li6CuB4O10 can be obtained from α-Li6CuB4O10 if the latter is annealed for at least 9 days at 500°C in air with a particle size distribution from 10 to 50 μm,.
Synthesis, structural and electrochemical properties of sodium transition metal pentaborates Na3MB5O10 (M = Fe, Co)
In the previous chapter we have demonstrated through the synthesis of Li6CuB4O10 the feasibility to obtain high potentials in borate compounds. Therefore at the same time we show the difficulty in making new pyroborates. This was an impetus to explore other borate anions, hence our interest for the family of sodium metal pentaborates Na3MB5O10. Solid state synthesis and structures have been reported to prepare these phases with M = Mg, Ca, Zn. Their preparation consists briefly in mixing stoichiometric amounts of Na2CO3, M(II)carbonate/nitrate and H3BO3 followed by annealing of the mixture at temperatures between 650-700°C in air for several days.191,192 Our first attempts by replacing the M(II)-carbonate/nitrate through Co/Fe(II)-oxalate and heating the mixture under argon to avoid oxidation of the transition metal were not successful. Using NaOH instead of Na2CO3, led to the formation of Na3FeB5O10, but was again not successful in the case of Co. Na3CoB5O10 could solely be obtained if Co(OH)2 was used as precursor. If the oxalate was used, we always end up with Co metal due to the easy reduction of Co2+ in Co-oxalate if heated under argon. Since the precursors are prone to react with ambient atmosphere, all steps of the synthesis were carried out in an argon filled glove box or under argon flow in a tube furnace. All chemicals were stored in argon, sodium hydroxide NaOH and boric acid H3BO3 were dried prior to be used at 200°C for 4 h in vacuum, and at 55°C for 24 h in air respectively.
In general, stoichiometric amounts of NaOH, Fe(C2O4)∙2H2O or Co(OH)2 and H3BO3 were mixed together for 15 min in argon using a SPEX milling apparatus. We observed that the mechanochemical process causes most likely a reaction between the precursors which solidified within the milling jar, yielding in an amorphous intermediate product. The resulting solid was then reground with a mortar and pestle, and heated with a rate of 10°C/min up to 700°C for 1 h under argon flow. Once the synthesis is finished, the formed product Na3FeB5O10 was immediately transferred to the glove box for further use, since we observed degradation in ambient atmosphere. At the opposite, Na3CoB5O10 is stable in air. Both Sodium transition metal pentaborate Na3MB5O10 compounds possess particle sizes ranging from 1 to 50 μm as show in Figure 62 with grey and blue color for M=Fe and Co respectively.
Study of the electrochemical driven reaction mechanism of bismuth oxyborate Bi4B2O9 versus lithium
In the previous chapters, we have shown through the synthesis of Li6CuB4O10 the feasibility to obtain high redox potentials for borate materials, however our further exploration for new pyroborate based compounds has encountered strong synthetic difficulties. Nevertheless, this had led us to the synthesis of two new sodium 3d metal pentaborates Na3MB5O10 (M = Fe, Co) with poor electrochemical performances. Therefore we considered another reaction mechanism, namely conversion type reactions. They differ from classical insertion/ deinsertion reactions in which Li+/Na+ is incorporated into a host structure while the metal M changes its oxidation state to account for charge balance. In a reversible conversion reaction, M is fully reduced upon lithiation and forms nanoparticles embedded in an amorphous matrix composed of LinX (X = O2-, S2-, F-, P3-, etc.) (equation 7). Upon charge this process is reversible to a certain extend. +−++↔0+ (7) An interesting tool to tune the redox potential of electrode materials relies on the inductive effect. In short the ionicity of the M─X bond influences the average redox potential of the conversion reaction with compounds having the highest ionicity showing the highest redox potentials. As fluorine is the most electronegative element, fluorides were widely studied in the past, however one has to note that highly ionic compounds are usually poor electronic conductors.123,208–212. Therefore in most cases these materials have to be “activated” through a prolonged high energy milling process with a large amount of carbon to achieve good electronic contact. Worthwhile mentioning that this addition of carbon diminishes drastically the volumetric energy density which is one problem regarding applications. Thus the quest for denser materials, with the most promising among them being bismuth fluoride BiF3 (5.23 g·cm-3). But again due to the insulating nature of this compound, a relatively large polarization was observed (~1 V), hence we decided to switch to Bi-borates. Owing to the weaker inductive effect of the borate BO3 group compared to fluorine, they should show a lower polarization while keeping the benefit of the high volumetric density (around 5 to 8 g·cm-3), comparable to BiF3. Among all reported Bi-borates, Bi4B2O9 presents the highest theoretical capacity correlated to the reduction of Bi3+ to Bi0 (~320 mAh/g) therefore, we chose this material to investigate its reaction mechanism versus Li. So far this materials was solely investigated for its structural and photocatalytical properties.
Although the theoretical capacity is rather low for conversion type electrode materials (usually > 600 mAh/g), we strongly emphasize the fact that this material presents a very high volumetric density ~8.2 g·cm-3, hence being favorable from the viewpoint of volumetric energy density. Bi4B2O9 was obtained by mixing stoichiometric amounts of bismuth oxide Bi2O3 and boric acid H3BO3 with a mortar and pestle and annealing the mixture at 600°C for 36 h in air with one intermediate regrinding. After the synthesis a pale white powder was recovered, consisting of particles with sizes ranging from 5 to 30 μm (Figure 74, inset).
Table of contents :
Table of content
Table of content
1.1 Batteries for electrochemical energy storage
1.2 Lithium- and sodium ion batteries
1.2.1 Lithium ion batteries
1.2.2 Sodium ion batteries
1.3 Cathode materials for lithium- and sodium –ion batteries
1.3.1 Lithium- and sodium transition metal oxides
1.3.2 Polyanionic cathode materials and the inductive effect
1.3.3 Conversion type cathode materials
1.4 Borate based cathode materials for lithium- and sodium-ion batteries
1.4.1 Why borate based materials?
1.4.2 Lithium transition metal borates
1.5 Motivation and aim of the thesis
2 Search for new pyroborate B2O5 based compounds
2.1 A few synthesis considerations
2.2 Ceramic synthesis
2.3 Low temperature synthesis
3 Structural and electrochemical properties of a lithium copper pyroborate Li6CuB4O10
3.1 Structure and polymorphism
3.2 Electrochemical characterization
3.2.1 Activity versus lithium
3.2.2 Ionic conductivity
4 Synthesis, structural and electrochemical properties of sodium transition metal pentaborates Na3MB5O10 (M = Fe, Co)
4.1 Synthesis and structure
4.2 Electrochemical characterization
4.2.1 Activity versus sodium
4.2.2 Ionic conductivity
5 Study of the electrochemical driven reaction mechanism of bismuth oxyborate Bi4B2O9 versus lithium
5.1 Synthesis and structure
5.2 Electrochemical characterization
6 General conclusion
7.1 Electrochemical characterization
7.1.1 Galvanostatic techniques
7.1.2 Potentiostatic techniques
7.1.3 Electrochemical impedance spectroscopy
7.1.4 Direct current polarization measurements
7.2 Structural characterization
7.2.1 Laboratory XRD
7.2.2 Synchrotron XRD
7.3 Other physical characterization
7.3.1 Thermal analysis
7.3.2 EPR spectroscopy
7.3.3 Mössbauer spectroscopy
7.3.4 Scanning- and transmission electron microscopy
7.3.5 Density functional theory calculations
7.3.6 Bond valence energy landscape calculations