An Investigation of Na1-XLi2x MnyNizOd Compounds for High Performance Sodium-Ion Batteries

Lithium ion batteries (LIBs) are the current energy storage technology of choice for numerous applications ranging from portable electronics to transportation and grid energy storage. The demand for LIBs production is predicted to considerably increase in the near future. The rising price of lithium and geopolitical problems associated with the lithium extraction urge us to look for alternative energy storage technologies.  Sodium ion batteries (SIBs) are an attractive choice due to the natural abundance and lower cost, as compared to lithium, of sodium. However, due to the higher atomic mass of Na as compared to Li, SIBs have a lower energy density than LIBs and therefore current research efforts are focused on the development of high performance, low-cost, easy to manufacture, safe and non-toxic SIBs.

Compounds of the type P2-Na_xMO₂ are intensively investigated due to their suitability for Na-ion intercalation [1]. However, a drawback of these compounds is the de-stabilization during cycling attributed to layer-gliding and phase transformations (often seen as steps and kinks in GCPL curves) [2]. Recently, C.S Johnson et al. has proposed that introduction of Li-ions in compounds of the type P2-NaLi_x Mn_yNi_zO_δ and Na_1-xLi_x Mn_yNi_zO_δ prvents layer-gliding during cycling, stabilizing the P2 phase (for the former compound) and therefore enabling an improved power and rate performance [2-4]. In the pristine compounds, the transition metals are in the Mn (4+) and Ni (2+) states (implying values of  δ > 2.0, i.e oxygen rich materials) [3]. The authors confirmed that during cycling, only the Ni (+2) provides redox activity whereas the manganese remains inactive providing stability to the compounds [2]. In the case of P2-NaLi_x Mn_yNi_zO_δ, Li was found to reside on both the alkali and the transition metal layer and the lack of layer gliding was attributed to the presence of a Li₂MnO₃-like phase where Li remained immobile [2].  In the case of Na_1-xLi_x Mn_yNi_zO_δ , the stability was attributed to the inter-growth of P2/O3 phases.

We are intrigued by the complex nature of these compounds and the associated stabilizing mechanism during Na-ion insertion/de-insertion processes. We are interested on exploring alternative stoichiometries, particularly those that perhaps will allow stability to a higher upper cut-off voltages enabling a higher energy density while maintaining a high power performance. We believe that besides adequate stoichiometry, the performance of these compounds can be improved by adequate electrolyte formulation. In this work, compounds of the type Na_1-xLi_2xMn_yNi_zO_δ  were synthesized using a facile sol-gel technique The electrochemical performance of the compounds was investigated in a two-electrode cell using the metal oxide as working electrode, metallic sodium as pseudo-reference and counter electrodes and a 1 M NaClO₄/EC/PC/additive as electrolyte. The main electrochemical techniques used were  GCPL and EIS.The morphology and crystal structure were investigated by XRD, SEM and TEM.

[1]          J. Xu, D. H. Lee, and Y. S. Meng, Functional Materials Letters 06, 1330001 (2013).

[2]          N. K. Karan, M. D. Slater, F. Dogan, D. Kim, C. S. Johnson, and M. Balasubramanian, Journal of The Electrochemical Society 161, A1107 (2014).

[3]          D. Kim, S.-H. Kang, M. Slater, S. Rood, J. T. Vaughey, N. Karan, M. Balasubramanian, and C. S. Johnson, Advanced Energy Materials 1, 333 (2011).

[4]          E. Lee et al., Advanced Energy Materials 4, 1400458 (2014).