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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al 4.5. SSEs for Na-ion batteries Chris I Thomas1,3, Peter Gross1,3, Marco A Amores2,3, Serena A Cussen1,2,3 and Edmund J Cussen1,2,3 1 Department of Materials Science and Engineering, The University of Sheffield, Sheffield S1 3JD, United Kingdom 2 Department of Chemical and Biological Engineering, University of Sheffield, Sheffield S1 3JD, United Kingdom 3 The Faraday Institution, Quad One, Harwell Campus, OX11 0RA, United Kingdom Status An ideal solid-state sodium electrolyte should deliver minimal ohmic losses and (electro)chemical stability with respect to both anode and cathode materials. Both of these parameters must be retained throughout the cycling processes and lifetime of the battery. Delivering such performance requires a combination of transport properties, chemical passivity, and processability, which represent a major challenge for materials science. Sodium electrolytes have been a long-standing challenge, and Na+ mobility is generally less facile than for Li+ analogues. Conduction through solid polymers, such as Na salts in polyethylene oxide [200], can realise around 10−4 S cm−1 at room temperature and the relatively low melting point of Na makes operation above ambient temperature more fraught than in analogous Li batteries. The increased size and mass of Na+ compared to Li+ are both implicated in the more sluggish kinetics of the heavier group 1 metal, but it should be recalled that the heavier Ag+ ion shows superionic conduction in a range of materials. Here, the polarisability of Ag+ offers an advantage over the harder Na+ cation. This points towards the potential of using polarisable anions, such as sulphide, in building a lattice for fast Na+ conduction. SSEs based on the material Na3PS4 have shown that the replacement of P5+ by Ge4+, Ti4+, or Sn4+ ions enables an increase in carrier concentration in the phases Na3+xMxP1−xS4. The introduction of dopants causes a tetragonal distortion away from the body-centred cubic parent phase, Na3PS4, leading to ordering of the Na ions. Unlike other ionic conducting systems, such as Li-stuffed garnets, this is not associated with a large decrease in Na+ conduction, and computational simulations indicate that vacancy generation can be realised in the tetragonal phase, and that the conductivity of Na3.1Sn0.1P0.9S4 is similar to that realised in solid salt:polymer composites [201]. The efficacy of the approach is further demonstrated by the room-temperature Na+ conductivity of 4 cm−1 afforded in the related phase, Na11Sn2PS12, where a higher concentration of SnS4 tetrahedral units delivers a complex three-dimensional pathway for Na+ conduction involving all five crystallographic Na positions, and additionally, two low-energy interstices [202]. Similar conductivities have been realised in the NASICON structural family of oxide-based materials shown in figure 25, exemplified by series such as Na1+xZr2SixP3−xO12, where the carrier concentration is adjusted by the classic approach of using an aliovalent chemical substitution, in this case, replacing P5+ with M4+ ions. In Na3Zr2Si2PO12 [203], the conductivity can be increased towards 4 mS cm−1 through control of the material’s microstructure and densification processes. Current and future challenges The deployment of these materials into battery technologies faces the great challenge of interface management. Addressing this requires a better understanding of the role of electrolytes in interfacial processes. The minimisation of interfacial resistance is vital to avoid ohmic losses, which both reduce the energy storage capacity and lead to potentially hazardous heating. The challenge is to develop an interface which shows low resistance whilst retaining (electro)chemical stability during the multiple processes of battery cycling involving a change in potential and the (de-)swelling of electrodes. Meeting these challenges requires moving beyond the scale of material manipulation via crystal chemistry. The NASICON phase has been explored via different synthetic routes, and these lead to variations in conductivity spanning an order of magnitude. An interesting development is the use of a ‘fluoride-assisted’ route to deliver a multi-phase sample with the composition Na3Zr2Si2PO12+x NaF [204]. The presence of NaF leads to an increased grain size, which reaches a maximum for x = 0.7. As the grain size increases, the conductivity is enhanced from 0.23 × 10−3 to 1.4 × 10−3 S cm−1 as shown in figure 26. This is accompanied by a negligible change in the activation energy, indicating that the increase in grain size and the associated reduction in grain boundary volume is the likely cause for this enhancement in conductivity. Similar enhancements of conductivity up to 10−3 S cm−1 have been achieved using liquid-phase sintering [205] to aid the densification of Na3Zr2Si2PO12 at 900 ◦C rather than the 1200 ◦C necessary for conventional ceramic synthesis. Many solid electrolytes have limited thermodynamic stability and react with metallic sodium. An examination of the interface between Na3Zr2Si2PO12 and a metallic Na anode has shown that the reduction of the electrolyte is kinetically limited; the initial reduction of Zr and Si is self-limiting and the electrolyte can 50PDF Image | 2021 roadmap for sodium-ion batteries
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