Zirconium substitution as a strategy for increasing the electrochemical stability of bulk Na2Ti3O7 as sodium-ion battery negative electrode

Abstract

Sodium is abundant and inexpensive, and can therefore be a good alternative to lithium for the stationary storage of renewable energy.1,2 However, sodium-ion batteries (SIBs) typically have a lower energy density than lithium-ion batteries because of sodium’s higher weight and less reducing potential.2 To become a sustainable technology, the electrodes and electrolyte of SIBs should be safe, non-toxic, inexpensive and stable. Unfortunately, there is no ideal negative electrode candidate for viable SIBs, as many negative electrodes have insufficient energy density, operate at an unsafe potential, or have an inadequate cycle life, which limits their sustainability. Therefore, there is a strong motivation to find a truly sustainable SIB negative electrode, by developing a new material or by increasing the stability of an existing negative electrode. Initially reported in 2011,3 Na2Ti3O7 delivers a high energy density as its sodium insertion reaction relates to a theoretical capacity of 177 mAh g-1 at a sodium insertion potential of 0.3 V vs Na+/Na.4 Unfortunately, this material suffers from a low electrochemical stability because of the degradation of the crystal structure during cycling, and because of the redissolution of the solid electrolyte interphase (SEI) upon sodium deintercalation.5 The electrochemical stability of Na2Ti3O7 may be increased by the substitution of zirconium for titanium. This may cause an expansion of the crystal lattice, which could facilitate the diffusion of sodium ions. Furthermore, the incorporation of very stable Zr-O bonds is expected to increase the structural stability of Na2Ti3O7. This work shows that zirconium can be substituted for titanium in the crystal lattice of Na2Ti3O7 by making use of a convenient solution-gel synthesis method. As determined by Rietveld refinement, its lattice parameters increase with an increasing amount of zirconium substitution. Zirconium-substituted Na2Ti3O7 experiences a lower extent of structural degradation during electrochemical cycling (as evidenced by post-mortem powder X-ray diffraction) and experiences fewer side reactions (as proven by electrochemical impedance spectroscopy). The electrochemical characteristics of zirconium-substituted Na2Ti3O7 were further improved by using sodium alginate as binder. These optimizations increased Na2Ti3O7’s capacity retention to 77% after 250 discharge/charge cycles at 1C rate. References 1. Slater, M. D., Kim, D., Lee, E. & Johnson, C. S. Sodium-ion batteries. Adv. Funct. Mater. 23, 947–958 (2013). 2. Chen, L. et al. Readiness Level of Sodium-Ion Battery Technology: A Materials Review. Adv. Sustain. Syst. 2, 1700153 (2018). 3. Senguttuvan, P., Rousse, G., Seznec, V., Tarascon, J. M. & Palacín, M. R. Na2Ti3O7: Lowest voltage ever reported oxide insertion electrode for sodium ion batteries. Chem. Mater. 23, 4109–4111 (2011). 4. Rudola, A., Saravanan, K., Mason, C. W. & Balaya, P. Na2Ti3O7: an intercalation based anode for sodium-ion battery applications. J. Mater. Chem. A 1, 2653–2662 (2013). 5. Muñoz-Márquez, M. A. et al. Composition and evolution of the solid-electrolyte interphase in Na2Ti3O7 electrodes for Na-Ion batteries: XPS and auger parameter analysis. ACS Appl. Mater. Interfaces 7, 7801–7808 (2015).