There is intensive research activity targeted on developing new electrode materials for the next generation of rechargeable lithium batteries, particularly for large-scale use in future hybrid electric or pure electric vehicles and in stationary energy storage of solar/wind power [1-3]. Polyoxyanion compounds are receiving considerable interest as cathodes since the strong binding of the oxygen enhances stability and thus safety, compared with transition metal oxides. The electronic inductive effect shifts the d-states of the transition metal ions and hence the redox potentials, providing a means of tuning the latter. Phosphate materials, particularly LiFePO₄, have been extensively studied and continue to be important [2,3]. Recent work on oxalates by Tarascon and co-workers [4,5] has demonstrated the possible use of oxalate materials as a new class of polyoxyanion.  Here we report the synthesis and characterization of hitherto-unknown alkali metal iron compounds in the oxalate system with new compositions and structures. The three dimensional framework structures were determined by single crystal X-ray diffraction, with an example shown in Fig. (a).  All the materials displayed redox activity vs. either Li or Na, depending on the compound with stable cycling as shown in Fig. (b).

(1) B. L. Ellis, K. T. Lee and L. F. Nazar, Chem. Mater., 2010, 22, 691;

(2) J. B. Goodenough and Y. Kim, Chem. Mater., 2010, 22, 587.

(3) A. K. Padhi, K. S. Nanjundaswamy, and J. B. Goodenough, J. Electrochem. Soc. 1997, 144, 1188.

(4) H. Ahouari, G. Rousse, Y. Klein, J.N. Chotard, M.T. Sougrati, N. Recham, J.M. Tarascon. Solid State Sci. 2015, 42(6), 13.

(5) H. Ahouari, G. Rousse, J.R. Carvajal, M.T. Sougrati, M. Saubanere, M. Courty, N. Recham, J.M. Tarascon. Chem. Mater., 2015, 27, 1631.