Elie Paillard, Dominic Bresser, Stefano Passerini
Institute of Physical Chemistry & MEET Battery Research Centre, University of Muenster,
Corrensstr. 28/30 & 46, 48149 Muenster, Germany
Conversion anodes were firstly introduced by Tarascon et al.[i] in 2000. Briefly, transition metal oxides (e.g. Co3O4, CoO, CuO, Fe3O4, Fe2O3, FeO, or NiO) are electrochemically reduced upon Li-ion battery charge, forming metallic nanoparticles finely dispersed in an amorphous Li2O matrix. Remarkably, this fine dispersion of the metallic nanoparticles enables the reversible formation of Li2O upon discharge, which has commonly been believed to be electrochemically inactive, leading in some cases to specific capacities of more than 1000 mAh g-1.
MOx + 2x Li+ + 2x e– ↔ M0 + x Li2O
(M= transition metal, e.g. Co, Ni, Fe, or Cu)
Previous results have shown that the particle size is of primary importance for the cycling stability of such conversion materials,[ii] as a partially reversible formation of a polymeric layer on the particles surface, induced by electrolyte decomposition, is contributing to the obtained specific capacities, while at the same time leading to an increasing inner resistance.[iii] Thus, within the ORION project, Muenster University developed simple processes[iv] in order to prepare conversion material-based electrodes, starting from primary particles with an average diameter of around 20 nm, which originally exhibited a rather poor cycling stability. In particular, the formation of transition metal oxide nanoparticles/carbon composites resulted in a significantly improved electrochemical performance as well as in a simplified electrode processing. Indeed, the carbon coating, while enhancing the electrical conductivity within the electrode, ensures the confinement of the composite (Li2O + M0) particles, hence avoiding active material pulverization upon cycling. Moreover, the carbonaceous surface enables the formation of an efficient solid electrolyte interphase (SEI), thus reducing the electrolyte reactivity.
Encouraged by the good results obtained with conversion material-based nanocomposites, hybrid conversion-alloying nanomaterials, as for instance ZnFe2O4, have been subsequently investigated,[v],[vi],[vii] leading to the best results reported up to now, showing a stable specific capacity of around 1000 mAh g-1 and an advanced high rate capability, with capacities of around 525 mAh g-1 and 310 mAh g-1 for an applied specific current of 3.89 A g-1 and 7.78 Ah g-1, respectively (Figure 1). The main advantage of these materials, compared to pure conversion materials, is that once the metals are reduced, Zn can further alloy with lithium at lower potentials forming LiZn, thus resulting in an increased energy density relatively to pure conversion reactions, which usually operate at rather high potentials.
ZnxM1-xOy + (2y+x) Li+ + (2y+x) e– ↔ x LiZn + (1-x) M0 + 2y Li2O (0<x<1)
Another advantage as compared to alloying materials in general, is that Li2O acts as a buffering matrix for the volume change induced by the alloying reaction, as it has been already reported for other alloying materials, as for instance SnO2 or SiOx (x < 2). Although the cycling stability is certainly improved for such materials rather than for the elemental alloying materials, the first reduction of the alloying material is commonly irreversible in the absence of initially formed transition metal nanoparticles, which enable the reversible formation of Li2O.
More recent developments consisted in increasing the relative amount of the alloying element, switching from ZnFe2O4 to hybrid conversion-alloying materials with an increased Zn content (transition metal doped ZnO, having the general formula M0.1Zn0.9O) to decrease further the average voltage, while still enabling the reversible formation of Li2O.[viii] Simple synthesis methods were developed, starting form aqueous solutions, enabling the preparation of single nanoparticles with an average diameter of around 20 nm.
[i] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J-M. Tarascon, Nature (2000) 407, 496-499.
[ii] S. Grugeon, S. Laruelle, R. Herrera-Urbina, L. Dupont, P. Poizot, J-M. Tarascon, J. Electrochem. Soc. (2001) 148, A285-A292.
[iii] J.-M. Tarascon, S. Grugeon , M. Morcrette , S. Laruelle , P. Rozier ,P. Poizot , Co. R. Chim. (2005) 8 , 9-15.
[iv] E. Paillard, D. Bresser, M. Winter, S. Passerini, DE-10-2011-057-015.2.
[v] D. Bresser, E. Paillard, M. Winter, S. Passerini, DE-10-2012-101-457.4.
[vi] D. Bresser, E. Paillard, R. Kloepsch, S. Krueger, M. Fiedler, R. Schmitz, D. Baither, M. Winter, S. Passerini, Adv. Energy Mater. (2012) DOI: 10.1002/aenm.201200735.
[vii] F. Mueller, D. Bresser, E. Paillard, M. Winter, S. Passerini, J. Power Sources (2013) DOI: 10.1016/j.jpowsour.2013.02.051.
[viii] D. Bresser, F. Mueller, E. Paillard, M. Winter, S. Passerini, DE-10-2012-107-199.3.