Substitution Chemistry of Li2MSiO4: A Highly Flexible Framework

Lithium transition metal silicates, Li₂MSiO₄(M=Fe, Mn, Co)[1], have attracted a lot of interest recently as positive electrodes for Li-ion batteries due to their potentially high theoretical capacities and to their rich crystal chemistry as they undergo complex polymorphism. One of the drawbacks of the silicates is their modest rate capability, particularly at room temperature.  The present work addressed this problem.

The polymorphs of Li₂MSiO₄can be classified into low (β) and high (γ) temperature forms, which differ in the ordering/distribution of cations within tetrahedral sites of an hcp based packing of oxygen.  β forms exhibit only corner sharing and γ polymorphs both corner and edge sharing. It has been shown by us and by others that  Li₂FeSiO₄ exists in at least 3 different polymorphic forms and that.  in the case of Li₂MnSiO₄two stable polymorphs are observed [1-3].

The silicates are analogous to the LISICON materials (Lithium SuperIonic Conductors), of the form Li_2+2xZn_1-xGeO₄, which adopt the γ structure with interstitial and highly mobile lithium.  Therefore,   stabilization of the γ polymorph to room temperature represents a potentially attractive route to enhancing the rate performance thus overcoming one of the major barriers to adoption of Li₂MSiO₄ cathodes. 

With appropriate substitutions we have been able to stabilise the γ polymorph for both Li₂FeSiO₄ and Li₂MnSiO₄systems.  We will report the effects on structure and electrochemistry of the various substitutions. In the iron system good cycling stability and improved rate capability are observed (fig. 2).

References

[1] a) A. Nytén, A. Abouimrane, M. Armand, T. Gustafsson and J.O. Thomas, Electrochem. Comm., 7, (2005) 156; b) M. S. Islam, R. Dominko, C. Masquelier, C. Sirisopanaporn, A. R. Armstrong and P. G. Bruce, J. Mater. Chem., 21 (2011) 9811.

[2] S. I. Nishimura, S. Hayase, R. Kanno, M. Yashima, N. Nakayama, A. Yamada, J. Am. Chem. Soc., 130 (2008) 13212.

[3] C. Sirisopanaporn, A. Boulineau, D. Hanzel, R. Dominko, B. Budic, A. R. Armstrong, P. G. Bruce, C. Masquelier, Inorg. Chem., 49 (2010) 7446.