New Insights into the Kinetics of Na Insertion and Extraction into the FePO4/NaFePO4 System

Sodium-ion batteries, which started being studied in parallel with lithium-ion (Li-ion) batteries, are the subject of a renewed interest in the recent years as a potential lower cost alternative to the later.[1] Fundamental differences related to phase stability, volume expansion, ionic diffusivity or voltage between the insertion of lithium versus insertion of sodium in the same host compound have been observed in several materials, resulting in successive phase transitions that are less common in the lithium counterparts.[2]

A material that well illustrates these differences is olivine NaFePO₄. This (meta)stable polymorph can reversibly insert/extract Na ions with a theoretical capacity of 154 mAh/g at an average voltage of 2.9V,[3] making it an interesting candidate for Na-ion battery cathodes. Moreover, it presents intriguing differences with respect to its Li-ion counterpart. Indeed, contrary to LiFePO₄ which usually operates through a reversible biphasic transformation,[4-5] the phase transformation mechanism is not symmetric between Na insertion and extraction into NaFePO₄ and an intermediate phase of composition Na_0.67FePO₄ is formed in both cases.[6-8] Moreover, the cell polarization is significantly higher which suggests less favorable kinetics of Na diffusion compared  to Li, confirmed by a larger diffusion coefficient for lithium as deduced from impedance spectroscopy performed at x=0.9.[9] However the diffusion coefficient has not been determined for other intermediate sodium compositions and a discrepancy is observed in the theoretical predictions of activation energy of ionic diffusion in the end members.[10-11]

Using in-situ electrochemical impedance spectroscopy and X-ray diffraction we have been able to precisely follow the charge and discharge processes. Phase transformation mechanism and activation energies of ionic diffusion have been determined in the whole composition range for both lithium and sodium insertion/extraction into the same host material. The obtained results will be discussed in terms of physical and mechano-chemical aspects of the charge and discharge reactions, with emphasis on the fundamental differences between lithium and sodium.

References:

[1] Palomares et al., Energy Environ. Sci. 5, 5884 (2012); Yabuuchi et al., Chem. Rev. 114, 11636 (2014)

[2] Ma et al., J. Electrochem. Soc. 158, A1307 (2011) Berthelot et al., Nature Materials 10, 74 (2011)

[3] Moreau et al., Chem. Mater. 22, 4126 (2010)

[4] Padhi et al., J. Electrochem. Soc. 144, 1188 (1997)

[5] Malik et al., J. Electrochem Soc., 160 (5), A3179-A3197, (2013)

[6] Casas-Cabanas et al., J. Mater. Chem. 22, 17421 (2012)

[7] Galceran et al., Phys. Chem. Chem. Phys., 16, 8837-8842, (2014)

[8] Gaubicher et al., Electrochem. Comm., 38, 104-106, (2014)

[9] Zhu et al., Nanoscale, 5, 780, (2013)

[10] Tripathi et al., Energy Environ. Sci, 6, 2257 (2013)

[11] Ong et al., Energy Environ. Sci., 4, 3680 (2011)