Phase Behavior in Nasicon Electrolytes and Electrodes

Abstract

An avenue to create safer batteries is to replace the liquid electrolyte with a non-flammable solid electrolyte.1–3 A good candidate is the Natrium superionic conductor (NaSiCON) Na1+xZr2SixP3-xO12 (0 x 3) which displays high bulk ionic conductivity and good relative stability towards other NaSiCON-based electrodes.2 Despite the sizeable share of research on the Na1+xZr2SixP3-xO12 material, the structural properties of NaSiCON are still poorly understood and as a result the optimisation of this class of materials often follows chemical intuition.4–9 Here, we analyse the phase behaviour of the NaSiCON electrolyte by constructing the Na1+xZr2SixP3-xO12 phase diagram as a function of temperature and composition (0 x 3) for the high-temperature rhombohedral phase. This phase is also common in several Na-based positive electrodes, such as Na3Ti2(PO4)3, Na3V2(PO4)3 and Na3Cr2(PO4)3. Using a multi-scale approach, based on density functional theory, the cluster expansion formalism and Monte Carlo simulations, we elucidate: i) the entire phase-diagram of NaSiCON as a function of temperature and Na content, identifying the regions providing the highest Na+-ion conductivity; ii) previously, unreported phase-separation behaviour in a specific region of the phase diagram, iii) the relationship between the population of Na-sites and the relative ratio of Si:P in the structure. We reveal that kinetic effects hinder the expected mechanism of phase separation in Na1+xZr2SixP3-xO12. We then extended these principles between the competition of thermodynamic and kinetic driving forces derived on Na1+xZr2SixP3-xO12 to popular mono-transition metal NaSiCON electrodes, which all tend to phase separate upon Na extraction/insertion. These results are important for the development of inexpensive Na-ion batteries.(1) Janek, J.; Zeier, W. G. A Solid Future for Battery Development. Nature Energy 2016, 1 (9), 16141. https://doi.org/10.1038/nenergy.2016.141.(2) Famprikis, T.; Canepa, P.; Dawson, J. A.; Islam, M. S.; Masquelier, C. Fundamentals of Inorganic Solid-State Electrolytes for Batteries. Nat. Mater. 2019, 18, 1278–1291. https://doi.org/10.1038/s41563-019-0431-3.(3) Canepa, P.; Dawson, J. A.; Sai Gautam, G.; Statham, J. M.; Parker, S. C.; Islam, M. S. Particle Morphology and Lithium Segregation to Surfaces of the Li7La3Zr2O12 Solid Electrolyte. Chemistry of Materials 2018, 30 (9), 3019–3027. https://doi.org/10.1021/acs.chemmater.8b00649.(4) Hong, H. Y.-P. Crystal Structures and Crystal Chemistry in the System Na1+xZr2SixP3−xO12. Materials Research Bulletin 1976, 11 (2), 173–182. https://doi.org/10.1016/0025-5408(76)90073-8.(5) Boilot, J. P.; Collin, G. Relation Structure-Fast Ion Conduction in the NASICON Solid Solution. J. Solid State Chem. 1988, 73, 160–171.(6) Masquelier, C.; Croguennec, L. Polyanionic (Phosphates, Silicates, Sulfates) Frameworks as Electrode Materials for Rechargeable Li (or Na) Batteries. Chem. Rev. 2013, 113 (8), 6552–6591. https://doi.org/10.1021/cr3001862.(7) Ma, Q.; Guin, M.; Naqash, S.; Tsai, C.-L.; Tietz, F.; Guillon, O. Scandium-Substituted Na3Zr2(SiO4)2(PO4) Prepared by a Solution-Assisted Solid-State Reaction Method as Sodium-Ion Conductors. Chemistry of Materials 2016, 28 (13), 4821–4828. https://doi.org/10.1021/acs.chemmater.6b02059.(8) Deng, Y.; Eames, C.; Nguyen, L. H. B.; Pecher, O.; Griffith, K. J.; Courty, M.; Fleutot, B.; Chotard, J.-N.; Grey, C. P.; Islam, M. S.; Masquelier, C. Crystal Structures, Local Atomic Environments, and Ion Diffusion Mechanisms of Scandium-Substituted Sodium Superionic Conductor (NASICON) Solid Electrolytes. Chem. Mater. 2018, 30 (8), 2618–2630. https://doi.org/10.1021/acs.chemmater.7b05237.(9) Zhang, Z.; Zou, Z.; Kaup, K.; Xiao, R.; Shi, S.; Avdeev, M.; Hu, Y.-S.; Wang, D.; He, B.; Li, H.; Huang, X.; Nazar, L. F.; Chen, L. Correlated Migration Invokes Higher Na+-Ion Conductivity in NaSICON-Type Solid Electrolytes. Advanced Energy Materials 2019, 9 (42), 1902373. https://doi.org/10.1002/aenm.201902373.