High Voltage Pyrophosphate Cathodes for Li- and Na-Ion Batteries: Atomic-Scale Insights into Defects and Diffusion

Major advances in rechargeable batteries for electronics, electric vehicles and grid storage requires the development of new materials. Recently, it has been proposed that the pyrophosphate framework may provide a new platform for important electrode research. Li₂FeP₂O₇ displays reversible reaction at 3.5 V vs Li/Li⁺ without nanosizing or carbon coating, which is the highest voltage for any Fe-based phosphate cathode [1]. There is growing interest in sodium-ion batteries largely due to the natural abundance of sodium and cost issues for grid-storage. The sodium pyrophosphate, Na₂FeP₂O₇, has been found to be electrochemically active (3V vs Na/Na⁺) [2], which is the first pyrophosphate-type of cathode for Na-ion batteries.

This presentation highlights recent studies of both Li₂FeP₂O₇ and Na₂FeP₂O₇ using atomistic simulation techniques [3] to provide detailed insights into defect, dopant and ion migration properties relevant to their electrochemical behaviour [4]. Li/Fe and Na/Fe antisites were found to be the most favourable intrinsic defects suggesting significant disorder, which would be sensitive to synthesis conditions. In contrast to 1D diffusion in LiFePO₄, fast Li⁺ transport in Li₂FeP₂O₇ is predicted to be through a 2D network. Interestingly, the Na₂FeP₂O₇ structure appears to support quasi-3D Na⁺ diffusion. Hence, a significant result is that alkali-ion diffusion within this framework is found to be either 2D or 3D with low energy barriers, which are important for good rates of charge/discharge.

References

[1] S. Nishimura, M. Nakamura, R. Natsui, A. Yamada, J. Am. Chem. Soc. 2010, 132, 13596.

[2] P. Barpanda, T. Ye, S. Nishimura, S.C. Chung, Y. Yamada, M. Okubo, H.S. Zhou, A. Yamada, Electrochem. Commun. 2012, 24, 116.

[3] M.S. Islam and C.A.J. Fisher, Chem. Soc. Rev. 2014, 43, 185.

[4] J.M. Clark, S.-I. Nishimura, A. Yamada, M.S. Islam, Angew. Chem. Int. Ed. 2012, 51, 13149.