Abstract
Sodium-ion batteries have the potential to be cheaper alternatives to lithium-ion batteries, mainly due to sourcing the alkali metals; the price of the mineral sources of lithium have increased by more than 15% between 2015-2016 and have continued to rise.[1] Studies of sodium-ion cathodes establish layered nickel based oxides as potential active materials,[2] for which the highest energy density to date has been demonstrated in a full cell configuration (320Wh/L) [3]. However, further investigations have shown a need to modify some of the existing manufacturing methods used for lithium and sodium-ion batteries, due to revealed material instabilities in air.[4]
Current layered nickel based oxides are unstable in air and can cause gelation of the binder material, PVDF.[5] It has been shown that the active materials can degrade to produce basic conditions,[6] which leads to a degradation of the PVDF, causing the electrode inks to gel. Therefore low temperatures and a dry atmosphere are required. However, the inks still exhibit poor time stability (see Fig. 1), and variations in the viscosity of the inks appear within minutes of formulation.[7] It is generally deemed that at least 4-5 hrs (or longer) at a stable viscosity is required to coat homogeneously;[8] additives are required to increase the window of stability of the electrode inks.
We have investigated and compared a collection of additives that successfully reduce the effects of the instabilities of materials in an electrode ink, in air. These additives reduce the need for drastic changes to the manufacturing methodologies used in lithium ion batteries. Low concentrations (0.1-5 wt%) of additives were added to sodium-ion cathode inks containing layered nickel based oxides as the active material, C65 carbon black as a conductive additive, PVDF as a binding material, and NMP as solvent.
A number of measurements were used to verify these interactions. Under controlled conditions, ink shear flow rheology over time was measured, viscosity and stability improvements against 'standard' (or non-additive-containing) electrode formulations were observed. Fourier transform infra-red spectroscopy of the electrode inks over time showed enhancements to the stability of the materials in air. Similar observations were noted in the analysis of the physical properties and homogeneity of the electrodes. Furthermore, in a sodium metal half-cell arrangement, the cells demonstrated comparable electrochemical performance to 'standard' coatings, with slight changes observable in the first cycles of each additive-containing electrode.
[1] M. Á. Muñoz-Márquez, D. Saurel, J. L. Gómez-Cámer, M. Casas-Cabanas, E. Castillo-Martínez, and T. Rojo, "Na-Ion Batteries for Large Scale Applications: A Review on Anode Materials and Solid Electrolyte Interphase Formation," Advanced Energy Materials, vol. 7, no. 20. 2017.
[2] E. Kendrick et al., "Tin Containing Compounds," WO 2015177568 A1, 2015.
[3] K. Smith, J. Treacher, D. Ledwoch, P. Adamson, and E. Kendrick, "Novel High Energy Density Sodium Layered Oxide Cathode Materials: From Material to Cells," ECS Trans., vol. 75, no. 22, pp. 13–24, 2017.
[4] M. H. Han, E. Gonzalo, G. Singh, and T. Rojo, "A comprehensive review of sodium layered oxides: powerful cathodes for Na-ion batteries," Energy Environ. Sci., vol. 8, no. 1, pp. 81–102, 2015.
[5] M. Biso, R. Colombo, M. Uddin, M. Stanga, and S. Cho, "A Rheological Behavior of Various Polyvinylidene Difluoride Binders for High Capacity LiNi0.6Mn0.2Co0.2O2," Polym. Eng. Sci., pp. 6–10, 2016.
[6] J. M. Paulsen, H. Park, and Y. H. Kwon, "Process of making cathode material containing Ni-based lithium transition metal oxide," US8574541B2, 2013.
[7] S. Roberts and E. Kendrick, "The re-emergence of sodium ion batteries: testing, processing, and manufacturability," Nanotechnol. Sci. Appl., vol. 11, pp. 23–33, 2018.
[8] H.-C. Chen et al., "Electrochemical Na+ storage properties of SnO 2/graphene anodes in carbonate-based and ionic liquid electrolytes," J. Mater. Chem. A, vol. 5, no. 26, pp. 13776–13784, 2017.
Figure 1