Sodium Iron (II) Pyrosilicate: A Potential Cathode Material in the Na2O-FeO-SiO2 System

This article explores the potential of Sodium Iron (II) Pyrosilicate as a cathode material in the Na2O-FeO-SiO2 system. It discusses its crystal structure, electrochemical performance, and advantages over lithium batteries. The article also examines the morphology of the material and its impact on battery characteristics.

MATERIAL SCIENCE
[Author Name(s), First M. Last, Omit Titles and Degrees]
[Institutional Affiliation(s)]

SODIUM IRON (II) PYROSILICATE Na2Fe2Si2O7: A POTENTIAL CATHODE MATERIAL
IN THE NA2O-FEO-SIO2 SYSTEM
Background
Sodium Iron (II) Pyrosilicate may be used as a replacement for lithium in dry batteries due to its
abundance on the Earth’s crust. The layered system of NaxMO2 that is based on similar
intercalation chemistry as LixMO2 system is seemingly illustrating relatively more stable
operations as well as accommodation of a greater range of transition metals M in comparison
with the Li counterparts owing to the differences in the size of Na+ and Li+. Sodium ion batteries
work in the same manner as the lithium ion batteries (Abeysinghe, 2017).
When charging, there is flow of metal ion from the cathode which is mainly a compound that
contains sodium to the anode which is typically carbon via the electrolyte which is an organic
solvent containing salt of dissolved sodium. During discharge, the direction of flow of the metal
ions is in the reverse (Chen et al., 2019). Sodium metal anode have the paternal of storing to the
tune of four fold as much charge as carbon meaning the battery is capable of holding as well as
releasing more energy.
Figure 1: Sodium-ion batteries with electrolytes made of ceramic nanospheres kg (Deng, Zhang
& Zhao, 2016)

The solid electrolytes are meanwhile used in the prevention of explosion risks that may occur as
a result of the flammable and volatile solvent besides ensuring the batteries remain safer even at
high temperatures. Reports have indicated sodium ions are having a voltage of about 3.6 V and
are able to maintain 15 Ah/kg in every 50 cycles which is equivalent to a specific energy of the
cathode that is about 400 Wh/kg (Deng, Zhang & Zhao, 2016). The ability of the non-aqueous
sodium ion batteries to effectively compete with the commercial lithium ion cells has been
limited by inferior performance of cycling.
The design of the battery is such that energy in the chemical bonds is stored in the SIBs of the
cathode. Charging of battery results in de-intercalation of sodium ions from the cathode and
migration in the direction of the anode with the electrons involved in charge balancing pass
through the exterior circuit which contains the charger from the cathode and into the anode. The
reverse process occurs during the process of discharge (Karfa, Majhi & Madhuri, 2018). Upon
the completion of the circuit, the electrons pass back to the cathode from the anode with the
sodium ions finding their way back to the cathode.
Advantages of Na+ batteries Disadvantages of Na+ batteries
Cheap and readily available battery grade salts as compared to
the battery grade salts of lithium. This renders them affordable
more often in place where the energy density as well as weight
tend to be a minor concern for example storage of grid energy
for renewable sources of energy including solar power and the
wind energy
They take a relatively longer time
to charge
Excellent features of the electrochemical: These excellent
features are with regard to the reversibility, high capacity of the
specific discharge, reversibility ad well as the efficiency of the
coulomb.
They take a relatively longer time
to discharge
Cells can be stored and transported safely: It is possible to fully
drain the cells to attain a zero charge without having the active
materials damaged. Lithium-ion batteries have to retain
approximately 30% of the charge during the process of storage,
charge that is large enough to short-circuit and ignite fire during
the shipment process.
They discharge at a very slow rate
which does not offer enough
power density that can be adopted
in high power applications.

Table 1: Advantages and Disadvantages of Na-ion batteries
Crystal Structure
The composition of Na2F2Si2O7 was targeted by the as-synthesize powder that had a primary size
of between 200 and 500 nm and illustrated a dark grey color. The sample was revealed to be
containing traces of amounts of impurities including carnegiete-like phases that were unknown
as well as ∝−Fe even though a majority of the reflections would be identified as coming from a
hitherto unreported stage.
Figure 2: Crystals of Na2F2Si2O7
High resolution X-ray diffraction pattern was used in an attempt to resolve the structure and a
monoclinic lattice having symmetry of P121/nl used in indexing of the noted reflections. The
integrated intensities of each of the reflections were extracted using the Pawley method and the
charge-flipping method used in the obtaining of the initial set phase (Kubota et al., 2016).

The electron density distribution map contained all the positions of the atoms. The final results of
refinement for the diffraction pattern are as shown in the figure below with the accompanying
fitting parameters as shown in the table.
The newly found compound Na2F2Si2O7 is established to be isostructural with Na2F2Mn2O7 with
minimal different local environment for the ions of transition metals. Distorted FeO4 tetrahedral
and distorted FeO5 bipyramids are formed through the coordination of two distinct ions of iron in
Na Na2F2Si2O7, Fe2 and Fe1 in that order.
Both ions of manganese are in the tetrahedral MnO4 environment by contrast and the variations
in the coordination of MOx between Fe and M=Mn is as a react of the variations in the relative
positions of oxygen (Li et al., 2016). The Mossbauer spectroscopy further confirmed the feature
of Fe environment. The shifts of the isomer δIS of the two main components 0.864 (5) mms-1 and
1.079(5) mms-1 were found to be in the typical Fe (II) species range having high configurations
of spin.
The large variation in the splitting of the quadrupole ∆QS is a reflection of the substantial
difference in the coordination environments about Fe2 and Fe1 (FeO4 or FeO5). Going by the
properly established emperical trend of splitting of quadrupole against local symmetry, the
bigger FeIO5 bipyramidal site is assigned ∆QS while the smaller one has Fe2O4 assigned
tetrahedral site assigned to it.

The two polyhedral result in the formation of single dimensional chains which run along the
direction of [001], sharing edges as well as apexes. A pyro silicate, Si2O7 is formed through the
sharing of an apex between two SiO4 tetrahedral and thus interconnection of a chain of Fe
polyhedral resulting in the formation of a framework of three-dimensional Fe2Si2O7. Na+ ions are
found on two crystallographic sites in tetrahedral environments that are highly distorted within
the interstitial spaces of the framework of Fe2Si2O7.
Not just crystal structure
The electrochemical performance of the material is not only linked to the variations in the crystal
structure of the compounds making it up but also to the differences in the morphology of the
various samples.
The morphology of Na+ ion battery that is made through the co-precipitation method is a factor
of the presence of NaCl flux agent as well as the annealing temperature. SEM is used in viewing
the samples in a bid to gain an understanding of the impacts of the factors on the resulting
materials and its battery characteristics (Wei, Mortemard de Boisse, Oyama, Nishimura &
Yamada, 2016).
A 500⁰C annealing temperature, the sample seem to be having primary particles whose size are
about 200-300 nm regardless of the availability of NaCl during the process of annealing. The
sizes are also established to be more of spherical in shape. The particles are highly conjoined and
look to be deficient of any significant porosity.
An increase in the temperature to higher values for example 700⁰C make the lack or presence of
NaCl flux agent more conspicuous (Oyama, Nishimura, Suzuki, Okubo & Yamada, 015). The
samples ten to be composed of particles that are between 300 and 500nm at the temperature and

the particles sint to form 3D network of secondary particles which contain macropores.
Nevertheless, the sample bearing NaCl may be observed to link to a preferential direction
resulting in the creation of crystals that look like rods. It is the 3D porous network of small
particles of crystalline which demonstrate the greater reversible capacities as well as the reduced
hysterias that occurs between the discharge and charge in comparison with the one that occurs at
a temperature of 500⁰C (Panigrahi et al., 2017).
A precise dependence in the morphology of the particles is noticed at temperature that exceeds
900⁰C and the presence of NaCl may be noted. A sample that does not have NaCl is sintered
highly during anneal with big primary particles which have a diameter of about 1 μm. The
porosity between the particles has been eliminated and sintering of the primary particles is done
into large agglomerates which are that and do not possess any features of porosity (Watanabe et
al., 2019). On the other hand, a sample that has NaCl flux agent seems to be having a 3 μm
diameyer of the primary particles during the process of heat treatment and the particles bear
more of regular spherical shapes. The particles in such a sample do not seem to be having a large
amount of space in between them as a result of not exhibiting high conjointly leading to the
sample having large porosity.
The best morphology that may be used in the enhancement of further studies is presented by a
sample that is treated with heat at a temperature of 700⁰C using NaCl agent flux. The sample
illustrates the lowest levels of hysteresis producing a difference of 0.08 between the discharge
and charge on the second cycle (Renman et al., 2018). The result may be attributed to the small
size of the primary particle that leads to a reduction in the length of ionic path diffusion as well
as the macropores property of the material allowing good penetration of electrolytes between the
various particles. The sample is as well demonstrative of the good retention capacity of the

discharge of about 82% on 25 cycles. This acts as a benchmark for more refined improvements
through optimization of electrode formation, streamlining of the electrolyte to the substance and
using the material with a single anode apart from Na metal.
Conclusion
The crystal structures, electrochemistry as well as Na+ dispersion rates of the cathode material
Na2CoSiO4 have been explored utilizing a multi-procedure approach. Electrochemical
estimations utilizing sodium metal anode cells show a reversible explicit limit on release of 4100
mA h g1 for all examples integrated at 4500 1C, with a normal release voltage of 3.3 V versus
Na/Na+. The structure of Na2CoSiO4 does not transform from the main cycle to the second
cycle, interestingly with the lithium silicate analog. Materials orchestrated at 700 1C with the
transition specialist shown a lower hysteresis and great limit maintenance due to the 3D system
of macroporous particles. The new bits of knowledge into the structure, morphology and
electrochemistry exhibited here give a stage on which future enhancement of silicate cathodes
for Na-particle batteries can be based.

References
Abeysinghe, D. (2017). Materials Discovery of Reduced Early Transition Metal Compounds:
Crystal Growth and Characterization (Doctoral dissertation, University of South
Carolina)
Chen, M., Liu, Q., Wang, S. W., Wang, E., Guo, X., & Chou, S. L. (2019). High‐Abundance and
Low‐Cost Metal‐Based Cathode Materials for Sodium‐Ion Batteries: Problems, Progress,
and Key Technologies. Advanced Energy Materials, 1803609
Deng, C., Zhang, S., & Zhao, B. (2016). First exploration of ultrafine Na7V3 (P2O7) 4 as a high-
potential cathode material for sodium-ion battery. Energy Storage Materials, 4, 71-78
Karfa, P., Majhi, K. C., & Madhuri, R. (2018). Shape-Dependent Electrocatalytic Activity of
Iridium Oxide Decorated Erbium Pyrosilicate toward the Hydrogen Evolution Reaction
over the Entire pH Range. ACS Catalysis, 8(9), 8830-8843
Kubota, K., Asari, T., Yoshida, H., Yaabuuchi, N., Shiiba, H., Nakayama, M., & Komaba, S.
(2016). Understanding the Structural Evolution and Redox Mechanism of a NaFeO2–
NaCoO2 Solid Solution for Sodium‐Ion Batteries. Advanced Functional
Materials, 26(33), 6047-6059
Li, S., Guo, J., Ye, Z., Zhao, X., Wu, S., Mi, J. X., ... & Ho, K. M. (2016). Zero-strain
Na2FeSiO4 as novel cathode material for sodium-ion batteries. ACS applied materials &
interfaces, 8(27), 17233-17238

Oyama, G., Nishimura, S. I., Suzuki, Y., Okubo, M., & Yamada, A. (2015). Off‐Stoichiometry
in Alluaudite‐Type Sodium Iron Sulfate Na2+ 2xFe2− x (SO4) 3 as an Advanced Sodium
Battery Cathode Material. ChemElectroChem, 2(7), 1019-1023
Panigrahi, A., Nishimura, S. I., Shimada, T., Watanabe, E., Zhao, W., Oyama, G., & Yamada, A.
(2017). Sodium Iron (II) Pyrosilicate Na2Fe2Si2O7: A Potential Cathode Material in the
Na2O-FeO-SiO2 System. Chemistry of Materials, 29(10), 4361-4366
Renman, V., Valvo, M., Tai, C. W., Gómez, C. P., Edström, K., & Liivat, A. (2018). Manganese
pyrosilicates as novel positive electrode materials for Na-ion batteries. Sustainable
Energy & Fuels, 2(5), 941-945
Watanabe, E., Chung, S. C., Nishimura, S. I., Yamada, Y., Okubo, M., Sodeyama, K., ... &
Yamada, A. (2019). Combined Theoretical and Experimental Studies of Sodium Battery
Materials. The Chemical Record
Wei, S., Mortemard de Boisse, B., Oyama, G., Nishimura, S. I., & Yamada, A. (2016). Synthesis
and Electrochemistry of Na2. 5 (Fe1− yMny) 1.75 (SO4) 3 Solid Solutions for Na‐Ion
Batteries. ChemElectroChem, 3(2), 209-213