As people’s energy demand increases, an increasing use of renewable sources is needed to mitigate the effects of global warming. These sources (such as solar and wind) are intermittent, thus energy storage is crucial for their widespread adoption. Although Li-ion batteries are dominating the market for portable electronic devices and electric vehicles because of their high energy density, their application in grid scale storage has been limited due to their high cost. Na, as an abundant and evenly distributed element on earth, is a low-cost alternative to Li which is more suitable for large-scale applications.

A wide range of cathode materials have been proposed for Na-ion batteries, ^[1] and layered oxides of the type NaMO₂ (M = transition metal) have attracted significant attention because of their high theoretical energy density and scalable synthesis method. Following the notation of Delmas,^[2] there are different structures depending on the different oxygen stacking ordering, such as P2 and O3, where the letters stand for the different environment of Na ion while the number indicates the stacking order of oxygen layers. P2 structured Na_xMO₂ (0.67<x<0.75) have Na ions occupying two prismatic sites between ABBA stacked oxygen layers, and our previous research has identified that it has better mobility in general as compared to O3 ^[3], therefore higher promise for large reversible capacity. Some P2 materials have already shown high capacity, but the use of high-valent elements (Mn4+, Ti4+, Sn5+, Te6+) often resulted in relatively low average voltage, limiting their energy density [4]. Therefore, high voltage transition metal redox couples need to be used. In this work, using first principles calculations (DFT) we investigated in depth the phase diagram of Na-M-O, with M = Co, Ni, Mn, with a particular attention towards the stability of P2 structures for Mn-poor compositions. Thanks to these results we were recently able to synthesize by solid state methods new P2 structured Na-ion cathodes which showed high specific capacities (~140 mAh/g) at a high average discharge voltage of 3 V or higher. In our contribution, we will detail the synthesis and characterization of such cathodes, both from an electrochemical and structural standpoint. We employed several experimental techniques to probe the pristine material, and we studied in detail its oxidation made via desodiation. Employed techniques include galvanostatic cycling, GITT, ex situ and operando diffraction, SEM, SQUID and XANES. These promising results highlight the synergetic effect of different transition metals, and we will discuss how a deeper understanding of such effect (especially including further cation mixing) will guide us towards high-performance P2-type Na-ion battery cathodes.

[1] Dipan Kundu, et al, Angew. Chem. Int. Ed. 2015, 54, 3431 – 3448

[2] R. Berthelot, D. Carlier, C. Delmas, Nat. Mater. 2011, 10, 74-80

[3] Yifei Mo, et al, Chem. Mater. 2014, 26, 5208−5214

[4] Clément R. et al., J. Electrochem. Soc., 2015, 162 (14) A2589-A2604