Sodium Mesoxalate as PRE-Sodiation Agent in Sodium-ION Capacitors
Authors: Canal Rodríguez María , Arnaiz María , Correa Bruno , Ajuria Jon 
Published: February 2024

Lithium devices are the ones that reached the market to prompt the sustainable use of renewable sources, the alternative promise to leave behind fossil fuels dependency. However, lithium reservoirs seem not to be enough to supply the expected increase in future market demands. On account of that, the appearance of sodium-energy devices as an alternative to lithium is rising interest due to its abundance and easy availability. Those market demands are increasingly focused on delivering high energy and power density in a single device. These requirements lead to the appearance of Sodium-Ion Capacitors (SICs), which consist of a supercapacitor like cathode confronted with a battery like anode, extracting the main advantages of each technology, i.e, high energy stored, and power supplied as well as long cycle-life. Despite their potential, still some challenges need to be addressed to widespread their commercialization. The low coulombic efficiency of the first cycle together with the absence of a sodium source apart from that of the electrolyte, is the major challenge to ensure high energy densities without penalizing cycle life. A possible strategy is the addition of a pre-sodiation agent to the system [1]. Different agents have been tested so far, however an ideal solution has not been developed yet. In the present study sodium mesoxalate is suggested to be employed as pre-sodiation agent as it is a commercial, green and nontoxic compound [2].
Herein, sodium mesoxalate was added directly to the positive electrode. The carbons C65 an Ketjen black (KB) were explored as conductive additives. Two different procedures of positive electrode fabrication were studied and in depth analyzed with the aim of extracting the best performance of the sacrificial salt. From this study it was found that the KB mixed with the monohydrated salt followed by a joint dehydration seems to extract the best performance of the sodium mesoxalate.
To complete electrode manufacturing, YP50F was added as active material . The CV (Figure 1a) shows a single peak during the first cycle at around 4.25 V vs. Na+/Na, related to the decomposition of the Na2C3O5. This value is low enough to avoid electrolyte decomposition. In the second cycle, a little hump is also appreciated at ca. 3.6 V vs. Na+/Na probably due to a little remanent salt unable to be decomposed during the first cycle. In the following cycles, the voltammetries show a rectangular shape, attributed to the capacitance recorded by the electrode without the already decomposed salt, and mainly ascribed to the YP50F. Moreover, from the GA charge/discharge (Figure 1b) it can be seen that during the first charge, the sodium mesoxalate is able to provide a specific capacity of 405 mAh g-1 which is higher than the theoretical one (331 mAh g-1).
Consequently, the results shown herein evidence the potential of the sodium mesoxalate as pre-sodiation agent in SICs as long as the proper conductive additive and obtention procedure are employed.
[1] M. Arnaiz, D. Shanmukaraj, D. Carriazo, D. Bhattacharjya, A. Villaverde, M. Armand, J. Ajuria, A transversal low-cost pre-metallation strategy enabling ultrafast and stable metal ion capacitor technologies, Energy Environ. Sci. 13 (2020) 2441–2449. https://doi.org/10.1039/D0EE00351D.
[2] A.J. Fernández-Ropero, M. Zarrabeitia, G. Baraldi, M. Echeverria, T. Rojo, M. Armand, D. Shanmukaraj, Improved Sodiation Additive and Its Nuances in the Performance Enhancement of Sodium-Ion Batteries, ACS Appl. Mater. Interfaces. 13 (2021) 11814–11821. https://doi.org/10.1021/acsami.0c20542.

Figure 1