Temperature Dependent X-Ray Diffraction in K2FeP2O7 Pyrophosphate: Polymorphism and Preliminary Electrochemistry Towards Li, Na, and K Ion Intercalation
Authors: Gond Ritambhara , Pralong Valerie , Barpanda Prabeer 
Published: February 2020

Phosphate-based polyanionic materials have changed the era of commercial secondary batteries after the discovery of triphylite LiFePO4 in 1997. Such materials are encouraged due to its open and robust framework, chemical/ thermal stability, tunable redox potential and desirable electrochemical performance. Several phosphate-based electrode materials have been reported for lithium (Li+) and sodium (Na+) insertion to develop lithium-ion batteries (LIB) and sodium-ion batteries (SIB). The fact that Na+ can't be intercalated into graphite (which is widely used negative electrode for battery application) has diverted researcher’s attention toward potassium-ion batteries (KIB).[1] Also regarding redox center, polyanion chemistry has only created a basis for iron-based compounds to be used as cathode in LIB. The elemental abundance and non-toxic nature has encouraged battery community to explore Fe-based battery insertion materials.

The progressive studies on pyrophosphate polyanionic cathodes has developed them as potential compounds for next generation batteries. Promising electrochemical properties of Li2FeP2O7 and Na2FeP2O7 has motivated us to study potassium-based analogues K2FeP2O7. [2] K2FeP2O7 was synthesized using solid state and combustion method. In particular, at first this phospho-anionic compound was prepared by solution combustion synthesis using low cost Fe(III) based precursors [Fe(NO3)3.9H2O]. Fe(NO3)3 was dissolved in distilled water followed by the addition of ascorbic acid to convert all Fe (III) into Fe(II). Then stoichiometric amount of KH2PO4 and combustion fuel like citric acid or urea were added. These solutions were heated at 110 ºC to trigger an exothermic reaction developing an intermediate complex. Upon annealing this complex at high temperature (500, 600 and 700 ºC), similar unknown phase members at all three temperature were obtained inferred from X-ray diffraction technique (XRD). To validate the unknown phase, stoichiometric amounts of FeC2O4.2H2O and KH2PO4 were added and ball milled. Similarly, the ball milled intermediate were annealed at high temperature (500, 600 and 700 ºC). The synthesized phases were analyzed using XRD, 500 and 600 ºC compounds are same as obtained by solution combustion synthesis. Interestingly, 700 ºC synthesized phase was found to crystalize into the structure type of Ca2MgSi2O7 (previously reported by Keates et al. K2FeP2O7).[3] Such simple observation of change in phase observed from XRD profile gave us a clue of polymorphism phenomenon which was further cross-validated by temperature dependent XRD in K2FeP2O7 obtained at 500 ºC. High temperature XRD results clearly shows the existence of two stable phases (Figure 1. in the middle), low temperature (500 ºC) one can be assigned as β-K2FeP2O7 and high temperature (700 º C) can be assigned as α-K2FeP2O7. Structure determination of β-K2FeP2O7 is in its almost final stage with monoclinic (P21/c) crystal structure. α-K2FeP2O7 crystalizes into tetragonal crystal system of P-421
m space group. Pyrophosphate compounds offer a robust three-dimensional P2O7
4--framework with multiple sites for alkali ions and existence of polymorphism in such compounds is clear in the present case. Polymorphism arises due to the different angular orientation between the two phosphate units.

Here, we report the electrochemistry of the β-K2FeP2O7 and α-K2FeP2O7 with respect to Li+, Na+ and K+. Galvanostatic charge-discharge profile of β-K2FeP2O7 and α-K2FeP2O7 is compared and shown in the left and right side of the figure respectively. Both materials have low insertion voltages which is lesser than 3 V. This low working voltage is possibly coming due the existence of FeO4 tetrahedral environment in the crystal structure which leads to the weak inductive effects as compared to FeO6 octahedral environment during Fe2+/Fe3+ redox mechanism.[4] Seeing these unpromising intercalation chemistry both polymorphs were subjected to study the ac impedance conductivity by varying the temperature from room temperature to 500 ºC in neutral atmosphere. β-K2FeP2O7 polymorph was found to have conductivity ~10-6 S cm-1 at 300 ºC and 10-3 S cm-1 at 400 ºC. However, α-K2FeP2O7 was found to have very poor conductivity experimentally which is in sync with BVSE energy barrier of 1.08 eV. Thus the current work will summarize recent findings in ‘pyrophosphate’ class of battery materials capable of reversible insertion of Li+ into β-K2FeP2O7 along with high temperature conductivity measurements using AC-impedance analyzer.

References:

Komaba et al., K. Kubota, Electrochem. Commun. 2015, 60, 172.

Barpanda et al., S. Nishimura, A. Yamada, Adv Energy Mater 2012 , 2, 841.

Adam C. Keates et al., Mark T. Weller, of Sol. State Chem. 2014, 210, 10.

a) A. Nyte´n et al., J. O. Thomas, Electrochem. Commun. 2005, 7, 156.; b) Z. Gong et al., Y. Yang, Energy Environ. Sci. 2011, 4, 3223.

Figure1. Galvanostatic charge-discharge profile of β-K2FeP2O7 (left), temperature dependent X-ray diffraction β-K2FeP2O7 (middle) and galvanostatic charge-discharge profile of α-K2FeP2O7 (right).

Figure 1