High energy density electrode materials are in pressing needs for next generation Li-ion batteries. While the stellar, laboratory-wise, is intercalation-type layered oxides involving both cationic and anionic redox processes,¹ conversion-type fluorine-based materials are also demonstrating attractive performances owning to the multi-electron transfer and high electronegativity of fluorine. Compared to pure fluorides, metal oxyfluorides possess higher conductivity and better electrochemical activity.² However, their performances are overwhelmingly dependent on several parameters, such as nominal composition, synthesis conditions, particle size and morphology. Recently, Kang and co-workers proposed a “redox composite” model, where a redox center (transition metal oxide/fluoride, MO/MF) and a Li⁺ reservoir (LiF) were blended by ball milling and directly used as the cathode.³This allows in principle countless pairs of composite electrodes to be explored as high energy density positive electrodes.

Herein, we report that manganese monoxide (MnO), who has long been believed as a promising conversion anode material based on the Mn²⁺/Mn⁰ redox couple, can indeed turn into a high energy density cathode material delivering over 200 mAh g^-1 capacity with an average potential around 3 V (Fig. 1). A one-step high energy ball milling method is adopted to trigger the reactivity of nanosized MnO particles. This process does not require any sophisticated synthesis routes, and can be easily scaled up for volume production.

Interestingly, this encouraging result is achieved without any LiF addition, thus calling for further exploration and understanding of the reaction paths. Based on various characterization techniques, it is found that the electrochemical activity is deeply rooted in the electrode-electrolyte interfacial reactions, and is strongly correlated with the stability of LiPF₆-based organic electrolyte. This finding raises the practical utility of such electrochemically made high capacity electrodes.   

Reference:

1   M. Sathiya, G. Rousse, K. Ramesha, C. P. Laisa, H. Vezin, M. T. Sougrati, M. L. Doublet, D. Foix, D. Gonbeau, W. Walker, A. S. Prakash, M. Ben Hassine, L. Dupont and J. M. Tarascon, Nature Mater., 2013, 12, 827–835.

2   N. Pereira, F. Badway, M. Wartelsky, S. Gunn and G. G. Amatucci, J. Electrochem. Soc., 2009, 156, A407.

3   S.-W. Kim, K.-W. Nam, D.-H. Seo, J. Hong, H. Kim, H. Gwon and K. Kang, Nano Today, 2012, 7, 168–173.

Fig. 1 First three cycles of MnO electrode cycled in LiPF₆-based electrolyte.