Phase Behavior in Nasicon Electrolytes and Electrodes
Authors: Deng Zeyu , Gautam Gopalakrishnan Sai , Chotard Jean-Noël , Kolli Sanjeev Krishna , Cheetham Anthony , Masquelier Christian , Canepa Pieremanuele 
Published: December 2020

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.
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