Phase Behavior and Ion Dynamics of Nanoconfined LiBH₄ in Silica

Abstract

The increasing demand for high capacity yet safe storage of renewable energy calls for the development of all-solid-state batteries. A major hurdle in this development is the identification of new suitable types of solid-state electrolytes. Nanoconfined lithium borohydride is a solid-state electrolyte candidate due to its high lithium-ion mobility at ambient temperatures. The origin of the high lithium-ion mobility is not fully understood, however. We studied nanocomposites of lithium borohydride and nanoporous silica Santa Barbara Amorphous-15 (SBA-15) with different pore sizes, using 1H, 6,7Li, and 11B solid-state NMR at various temperatures, to get in-depth insights into the phase behavior and ion dynamics of lithium borohydride in the silica pores. The results allow us to formulate a detailed dynamic model for lithium borohydride confined in SBA-15; bulklike LiBH4 is separated from the pore walls by an amorphous, highly dynamic LiBH4 fraction displaying both Li+ and BH4 – diffusion even at ambient temperatures. As shown by 11B temperature-jump exchange NMR, this dynamic fraction increases as a function of temperature. Li+ exchange between the bulklike and “dynamic” LiBH4 fraction is slow at ambient temperatures, but at elevated temperatures (≥90 °C), above the phase transition of the bulklike fraction, lithium ions rapidly diffuse through both LiBH4 fractions and exchange between these confined fractions at rates approaching the megahertz time scale.

Anion and Cation Dynamics in Polyhydroborate Salts: NMR Studies

“…Generally, such a confinement leads to a suppression of the long-range order in the confined material and, in many cases, to an enhancement of the mobility of anions and/or cations. Most of the studies in this direction have been performed on lithium borohydride infiltrated into nanoporous carbon or SiO 2 hosts [ 24 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 ]. The characteristic feature of these nanocomposites is a coexistence of two distinct fractions of nanoconfined LiBH 4 .…”

“…Indeed, the narrow component of the 1 H NMR spectra has a width of about 1 kHz [ 24 , 92 , 94 ], and the reorientational motion alone cannot lead to such a narrowing (see Section 2 ). Experiments at various temperatures have revealed that the fraction of mobile spins increases with temperature [ 24 , 92 , 94 , 96 ]. This suggests that there is no fixed interface between the two fractions, which is consistent with the presence of broad continuous distributions of the jump rates [ 94 ].…”

“…It has been found that at T = 298 K and 371 K (where the 1 H relaxation rate is governed by Li + jumps via 1 H– 7 Li dipole–dipole interactions), is proportional to ω −1 ; this suggests the presence of a broad distribution of Li + jump rates. Furthermore, the exchange of the anions between the two fractions of LiBH 4 confined in nanoporous carbon has been studied by the selective inversion of the narrow component of the 1 H NMR spectrum [ 94 ]; the exchange time constant of the order of 5 ms appears to be nearly temperature-independent over the range of 280–420 K. On the other hand, for LiBH 4 confined in nanoporous SiO 2 (SBA-15), the fast exchange of Li + cations between the two fractions has been observed only above the order–disorder (orthorhombic-hexagonal) phase transition temperature (~370 K) of the bulk-like LiBH 4 fraction [ 96 ]. It should be kept in mind that the detailed microscopic picture of motions in nanoconfined LiBH 4 may also depend on the material of nanoporous scaffold (carbon or SiO 2 ) and on the shape of the pores.…”

Room-Temperature Solid-State Lithium-Ion Battery Using a LiBH4–MgO Composite Electrolyte

“… 18 − 22 In this case, the improved Li-ion conductivity relies on the formation of a conductive interface, described by a core–shell model. 20 , 23 The fraction of LiBH 4 (the core) in direct contact with the oxide (the shell) forms an interfacial layer, featuring a Li-ion conductivity enhancement. The presence of different dynamics, due to the occurrence of slow and fast diffusivity of Li ions, has been detected for binary composites by 7 Li solid-state NMR spectroscopy.…”

The effect of nanoscaffold porosity and surface chemistry on the Li-ion conductivity of LiBH4–LiNH2/metal oxide nanocomposites

“…In the second method, the lithium salt is intimately mixed with a high surface area non-conducting oxide scaffold, such as SiO 2 or Al 2 O 3 . [35][36][37][38][39][40][41][42][43][44] Close contact can be achieved through nanoconnement by melt inltration of the metal hydride in the nanopores of the oxide, thereby forming a nanocomposite. 45 Interestingly, this method was originally used to improve hydrogen sorption properties of metal hydrides.…”