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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al 5. Testing protocols for Na-ion batteries Juan D Forero-Saboya and Alexandre Ponrouch Institut de Ciéncia de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Catalonia, Spain Status For all battery chemistries, a reliable electrochemical setup is essential. Half-cell configurations with typically two or three electrodes, using lithium metal as the counter electrode (CE) and reference electrode (RE), are standard setups for Li cells. However, several requirements need to be fulfilled to achieve reliable results (figure 27) [210, 211]. The CE must have high capacity, fast reaction kinetics, and should not contaminate the electrolyte solution with reaction byproducts. Meanwhile, the RE should ideally be nonpolarisable (a current flow should not affect its potential) and should have a reliable and stable potential. Although these conditions are satisfactorily met in Li half-cells, several studies point to various issues associated with the use of Na-metal CEs and REs [196, 212, 213]. Most of these issues are rooted in the low stability (and high solubility) of the SEI formed on the Na metal electrode (see the ‘electrolyte/interface, SEI layer’ section, by Reza Younesi). Considering the difficulty of developing new battery chemistries, in which both electrodes and electrolytes must be studied in parallel, the use of reliable electrochemical protocols is crucial in order not to discard potentially interesting material candidates or to make erroneous interpretations of the results. In this section, the most common issues arising from the use of Na metal CEs and REs are described, and recent strategies to overcome them are introduced. Current and future challenges The instability of the Na metal–electrolyte interface has several consequences for any electrochemical test involving Na metal as both the CE and the RE. First, the constant SEI dissolution/reformation results in the contamination of the electrolyte with soluble/gaseous species. When alkyl carbonate-based solutions are in contact with Na metal, a growing amount of CO, CO2, methane (CH4) and ethylene (C2H4) is generated (figure 28(a)). The presence of soluble compounds, such as dimethyl 2,5-dioxahexane dicarboxylate (DMDOHC), has been observed in the solution, even after 5 d [213]. While the nature/amount of such contaminants is dependent on the electrolyte formulation, their impact on the working electrode interface and electrochemistry is clear. For instance, the impedances of hard carbon (HC)||HC (2-electrode) and HC||Na (3-electrode) cells left at OCV for 10 d indicate that the HC interface becomes more and more resistive when Na metal is present [196]. When comparing cyclic voltammograms of 1 M NaClO4 in PC, Lee et al concluded that electrolyte decomposition products (formed on Na surface) diffuse to the working electrode (WE), where they are oxidised. Such parasitic reactions can obscure true measurements of the electrochemical performances of a given material [214]. Pfeifer et al observed that carbonate electrolytes (e.g., DMC, EC, PC) containing either NaClO4 or NaPF6 react readily in contact with Na, producing coloration or cloudiness of the electrolyte due to side reactions [302]. Sodium-ion intercalation into Li4Ti5O12 was investigated using Na metal and activated carbon (AC) as CEs, with a significant cyclability improvement in the latter case. This was ascribed to a different passivation layer being formed, depending on the CE used, and possibly to more severe PVDF binder decomposition when Na is present. The highly inhomogeneous plating of Na, together with an increase in the Na metal impedance as a result of time and cycling, could also result in significant limitations on the cyclability of half cells (figures 28(b)) and (c)). Thus, when tests are performed on two-electrode half cells, the real performance of the active materials being tested as WEs can be significantly underestimated. Finally, as the coulombic efficiency of Na plating/stripping is usually very low, the stripping of Na from the CE commonly involves two types of metallic Na: the freshly deposited one (Naplated) and the Na bulk originally present as a disk or a foil. This can lead to an artificial voltage step that arises during the reduction of the active material used as the WE (figure 28(d)). Such a step can only be seen in two-electrode cells and was ascribed to the higher overpotential for Nabulk when compared with Naplated stripping [212, 213]. Once again, such an electrochemical response is only associated with the CE, and could be misleading. Advances in science and technology to meet challenges Several strategies have been developed to circumvent the problems associated with Na metal. One of the main approaches has been to control the nature and properties of passivation layers. Artificial passivation layers using freestanding composite layers consisting of Al2O3 particles and liquid electrolyte-swollen poly(vinylidene fluoride-co-hexafluoropropylene) polymers were developed, leading to an improved cycle life of Na||Na symmetric cells. However, evidence of dendrite formation and/or a cell impedance increase 53PDF Image | roadmap for sodium-ion batteries
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