ARTICLE https://doi.org/10.1038/s41467-020-17951-6 OPEN Low dimensional nanostructures of fast ion conducting lithium nitride Nuria Tapia-Ruiz 1,7,8, Alexandra G. Gordon2,8, Catherine M. Jewell1, Hannah K. Edwards2,3, Charles W. Dunnill 1, James M. Blackman4, Colin P. Snape 4, Paul D. Brown3, Ian MacLaren 5, ✉ Matteo Baldoni 2,6, Elena Besley 2, Jeremy J. Titman 2 & Duncan H. Gregory 1 As the only stable binary compound formed between an alkali metal and nitrogen, lithium 1234567890():,; nitride possesses remarkable properties and is a model material for energy applications involving the transport of lithium ions. Following a materials design principle drawn from broad structural analogies to hexagonal graphene and boron nitride, we demonstrate that such low dimensional structures can also be formed from an s-block element and nitrogen. Both one- and two-dimensional nanostructures of lithium nitride, Li3N, can be grown despite the absence of an equivalent van der Waals gap. Lithium-ion diffusion is enhanced compared to the bulk compound, yielding materials with exceptional ionic mobility. Li3N demonstrates the conceptual assembly of ionic inorganic nanostructures from monolayers without the requirement of a van der Waals gap. Computational studies reveal an electronic structure mediated by the number of Li-N layers, with a transition from a bulk narrow-bandgap semiconductor to a metal at the nanoscale. 1 WestCHEM, School of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK. 2 School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK. 3 Department of Mechanical, Materials and Manufacturing Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK. 4 Department of Chemical and Environmental Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK. 5 School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK. 6 Istituto per lo Studio dei Materiali Nanostrutturati (ISMN), Consiglio Nazionale delle Ricerche (CNR), Via P. Gobetti 101, 40129 Bologna, Italy. 7Present address: Department of Chemistry, Lancaster University, Lancaster LA1 4YB, UK. 8These ✉ authors contributed equally: Nuria Tapia-Ruiz, Alexandra G. Gordon. email: [email protected] NATURE COMMUNICATIONS | (2020) 11:4492 | https://doi.org/10.1038/s41467-020-17951-6 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17951-6 ithium nitride, Li3N, was originally proposed for use as an GaN nanowires from Ga droplets and gallium/nitrogen vapour Lelectrolyte in all solid-state Li+ ion batteries given its species has been observed previously9.ThecomparisontoGaNis exceptional ionic conductivity at room temperature apposite in that both binary nitrides are formed from low melting (ca. 10−3 Scm−1)1. Indeed, for several decades it remained the point metals. highest conducting crystalline Li+ ion conductor at ambient ! þ ð Þ conditions hampered chiefly by its low decomposition potential 2Li3NðsÞ 6LiðlÞ N2ðgÞ 1 despite many ongoing attempts to stabilise it. Doping with late transition metals, however, triggers electronic conductivity that Two types of single-crystalline straight fibres with different can be exploited in anodes with more than twice the charging growth orientations (I and II) form under varying reaction capacity of graphite2.LiN has also been proposed for a myriad of conditions (see Supplementary Tables 1 and 2): type I nanofibres 3 other applications, e.g. as a means of converting CO2 into useful (Fig. 2a, b) exhibit a growth direction of <1010>, whereas, products3, as the electron injection layer in organic light-emitting conversely, type II nanofibres exhibit a growth direction of <0001> diodes4 and as an unusual reducing agent in preparative organic (Fig. 2c, d). Typically, type II nanofibres were found together with 5 fi and organometallic chemistry . Further, in 2002 Li3N was afew bres and nanosheets with zig-zag morphology (Supple- fi revealed as a potential candidate for solid-state hydrogen storage mentary Figs. 4 and 5). Thus Li3N nano bres exist in which the 6 given its capability to accommodate up to 10.4 wt.% H2 . Slow hexagonal [Li2N] layers are stacked either parallel or perpendi- kinetics for H2 sorption and high (de)hydrogenation tempera- cular to the principal fibre axis. Carbon nanofibres can assemble in tures are the primary hurdles to overcome before the Li–N–H an analogous fashion, where graphene layers can be oriented system can be exploited commercially, however. By combining either perpendicular or parallel to the principal fibre axis10,11.Our experiment and calculation, we demonstrate how the changes in experimental evidence shows that the pressure within the reaction electronic structure and reduction of diffusion lengths brought vessel is likely to be the crucial parameter for the preferential fi about by chemically nanostructuring Li3N can lead to dramatic growth of type I and II nano bres, i.e. under identical reaction changes in electronic properties and ionic transport behaviour. times and temperature, type II fibres will be favoured over type I fibres at higher reaction pressures (Supplementary Tables 1 and 2). There are several examples in the literature that describe how the Results total pressure and/or precursor partial pressure can control the Synthesis and characterisation .Li3N nanocrystals grow follow- growth direction of fibres; these preferences can be both ing the heating and cooling of the bulk nitride powder under a thermodynamically and kinetically driven12. Indeed, in this reduced pressure of nitrogen (see “Methods”). Powder X-ray respect, our density functional theory (DFT) calculations suggest diffraction (PXD) data for the fine powder product matches to that type I nanofibres (−325.44 eV Li Nunit−1 see “Methods”) α = = 3 hexagonal -Li3N(P6/mmm; a 3.656(2) Å; c 3.868(4) Å) are more thermodynamically stable than the equivalent type II fi fi − −1 (Supplementary Fig. 1). In the nano bre syntheses, long struc- bres ( 324.59 eV Li3N unit ). The dependence of growth tures with diameters ranging from 200 nm to 2 μm and lengths in direction on seed concentration in layered BN and GaN fibres excess of 10 μm were produced (Fig. 1). Atomic force microscopic has been previously reported13,14. Moreover, it has been well (AFM) measurements confirmed these ranges of thicknesses in documented that nanowire growth direction is directly impacted the different fibres observed (Supplementary Fig. 2). The mor- by pressure across diverse systems of varying complexity (e.g. in 15–18 phology of the nanostructured material can be tailored by con- semiconductors such as Si, In2O3 and InN) . One might trolling the preparative conditions to yield different types of expect an increased Li seed concentration at elevated reaction fi “ ” nano bres (see Methods ). pressure. An increase in N2 partial pressure (concentration) The growth of the one-dimensional (1D) nanostructures can be and the N:Li ratio at the growth interface should lead to the rationalised in terms of a self-assisted vapour–liquid–solid (VLS) formation of Li-N planes and type II fibre propagation. Local mechanism. Under vacuum, Li3N will decompose at a temperature variation in pressure and N:Li stoichiometry might, therefore, below the ambient pressure decomposition temperature (815 °C; account for the formation of zig-zag (kinked) nanofibres and Δ = − + − −1 7,8 H(298K) 171.3 / 7.7 kJ mol ) (Eq. 1), forming droplets sheets observed in type II samples (Supplementary Figs. 4 and 5); of liquid Li, since the melting point of Li (T = 180.5 °C) lies well analogous behaviour has been noted for Si and InN nanowires, for below the applied reaction temperatures. These Li seeds would act example16,17. fi as nucleation sites for the N2(g) species, which supersaturate the Li Raman spectra from nanostructured Li3N( bres type I and type fi −1 droplets and eventually lead to anisotropic growth and Li3N bre II) and bulk Li3N showed one prominent peak at ca. 580 cm , formation. Energy-dispersive X-ray spectroscopy (EDX) data of attributable to the only Raman active mode of Li3N, i.e. E2g the fibrous material did not show any traces of other metals such as (Supplementary Fig. 6). This phonon mode corresponds to the Fe (from the wire used to support the reaction vessel), ruling out the displacement of the Li(2) and N atoms along the ab plane in 19 possibility of Fe acting as a catalyst or seed for the formation of the Li3N and is consistent with group theory predictions and the fi 20 bres described here (Supplementary Fig. 3). Self-catalytic growth of spectra of bulk Li3N . We found that the E2g mode band was slightly blue-shifted (by ca. 5 cm−1) with respect to the bulk for a b both type I and type II fibres. Raman spectra of microcrystals of structurally similar graphite and BN21,22 and of BN nanotubes23 also revealed that the E2g band broadens and shifts to a higher frequency as particle size decreases. Ab-initio calculations on BN single-walled nanotubes attribute the blue shift in the E2g mode (of 5cm−1) to a shortening of the sp2 bonds with respect to the bulk23. The additional bands observed at ca. 500 cm−1 and 650 cm−1 in both bulk and nanoscale Li3N samples were assigned to the acoustic (A) and optical phonon (O) modes 2 TA (z, K), 2 TO(A) Fig. 1 SEM micrographs of typical Li3N nanofibres (type I) at different and 2 LA(A) (T = transverse, L = longitudinal) by direct compar- fi 19,24 magni cations. Images are taken at: a ×500 and b ×5000. Scale bars ison with the second-order Raman spectra of Li3N .Bands correspond to 50 μm(a) and 5 μm(b). corresponding to the second-order phonon mode (2 LA (M, K)) 2 NATURE COMMUNICATIONS | (2020) 11:4492 | https://doi.org/10.1038/s41467-020-17951-6 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17951-6 ARTICLE ab c d Fig.