In order to reduce the amount of greenhouse gas emissions, researchers are currently focusing on the development of renewable energy systems such as solar panels or wind mills. For example, in Payerne (Switzerland), a solar installation covering 38000 m² is providing the electricity needs for 1500 homes [1]. For overcoming the issue of the time delay between the energy production and its usage, stationary energy storage devices are becoming a crucial element in such installations. Na-ion batteries are considered as a promising technology since they are expected to be cheaper than the well-known Li-ion batteries. Recently, a first Na-ion prototype demonstrated 90 Wh/kg for over 2000 cycles [2], higher than the first commercialized Li-ion batteries in 1991.

Different families of sodiated transition metal layered oxides were tested as positive electrode and good electrochemical performances were reached with the P2 structure type. Chagas et al. [3] reported a specific charge of 106 mAh/g after 200 cycles for the P2-type Na_x(Ni_0.22Co_0.11Mn_0.66)O₂ whereas 100 mAh/g was obtained for the P3-type. Here, we focus on Na_0.67(Mn_0.5Fe_0.25Co_0.25)O₂, a new P2-type material for which a specific charge higher than 120 mAh/g after 30 cycles was recently reported [4]. This material was designed based on the idea that rate performance should increase if number of phase transitions and Na long range order are suppressed. As the use of Co could increase the overall price of the battery, we decided to investigate the Na_0.67(Mn_xFe_yCo_z)O₂ compounds, where the ratio of the transition metals was changed keeping the z values lower than 0.25. The different compositions were tested as cathode materials and their electrochemical performances were assessed.

Na_0.67(Mn_0.5Fe_0.25Co_0.25)O₂ (NaMFC-s) and Na_x(Ni_0.22Co_0.11Mn_0.66)O₂ (NaNCM-s) were synthesized in similar experimental conditions by using different precursors than the one described in the literature [4]. The obtained electrochemical performances of both NaMFC-s and NaNCM-s are compared in Figure 1. Similar specific charges ca. 120 mAh/g are obtained along the first cycle for both systems, but a better stability is clearly reached for NaMFC-s with 87 mAh/g after 100 cycles compared to 75 mAh/g for NaNCM-s. Those results show that this new compound NaMFC-s is superior to the state-of-the-art reference sample NaNCM-s in terms of cycling stability if similar synthesis conditions are applied. The results obtained for the Na_0.67(Mn_xFe_yCo_z)O₂ compounds with different transition metals ratios, synthesized in similar conditions, will be presented and compared to the reference material NaMFC-s. Cyclic voltammetry and characterization of the compounds by X-ray diffraction (XRD), X-ray absorption (XAS) and DSC will be presented. Operando XRD coupled with ex situ XAS measurements will be shown for the first cycle and long-term cycling in order to understand experimentally the impact of transition metal proportion onto the electrochemical performances. Hints will be given about the aging mechanisms and structure stability.

References:

¹ “How many solar panels does it take to power a whole town“, Le news - local swiss news in english 2015, 22^th October.

² J-M. Tarascon, C. Masquelier, L. Croguennec, S. Patoux, CNRS press “a promising new prototype of battery”, 27 Nov. 2015.

³ L. Chagas, D. Buchholz, C. Vaalma, L. Wu, S. Passerini, J. Mater. Chem. A  2014, 2, 20263.

⁴ L. Liu, X. Li, S. Bo, Y. Wang, H. Chen, N. Twu, D. Wu, G. Ceder, Adv. Energy Mater. 2015, 1500944.

Acknowledgment:

This work was performed within the Swiss Competence Center of Energy Research Heat and Storage (SCCER) framework.