A Lithium Amide‐Borohydride Solid‐State Electrolyte with Lithium‐Ion Conductivities Comparable to Liquid Electrolytes

Abstract

storage materials. [4] As SSEs for lithiumion batteries, they offer multiple advantages including the natural abundance of their constituent elements, their light weight, negligible electronic conduction, and low grain boundary resistance. [5] The prototypical example is LiBH 4 , whose Li + conductivity increases abruptly to >1 × 10 −3 S cm −1 above 100 °C due to a structural phase transition. [6] This superionic high-temperature phase can be stabilized at room temperature by incorporation of lithium halides (LiBH 4 -LiX, X = Cl, Br, I). Among these materials, the solid solution with lithium iodide displays the highest conductivity at room temperature of >10 −5 S cm −1 . [7] Conductivities reaching 10 −4 S cm −1 near room temperature have also been reported for two compounds of the lithium amide-borohydride Li(BH 4 ) 1−x (NH 2 ) x system, namely, for the cubic α phase (x = 3/4) and for the trigonal β phase (x = 1/2). [4,8] Here, we report the discovery of a transition to even higher Li + conductivities of up to 6.4 × 10 −3 S cm −1 near room temperature (40 °C) in BH 4 -rich lithium amide-borohydride (x = 2/3). We discuss the conduction mechanism in light of latent heat absorbed/released during the transition upon heating/cooling, respectively. We further demonstrate an allsolid-state Li 4 Ti 5 O 12 -based half-cell (employing the BH 4 -rich electrolyte) with excellent rate capability and cycling stability, comparable to a reference cell with standard liquid electrolyte representing an important step toward an all-solid-state amideborohydride-based battery. Results and DiscussionLi(BH 4 ) 1−x (NH 2 ) x powders, employing LiBH 4 and LiNH 2 precursors in amounts equivalent to x = 2/3, were prepared via reactive ball milling for 45 min and subsequent heat treatment at 120 °C for 12 h (see the Experimental Section for details). A reference sample with x = 3/4 (cubic α phase) and intermediate compositions were also synthesized.Ionic conductivities were determined for pellets pressed from the powders via temperature-dependent impedance spectroscopy. For low to intermediate conductivities, Nyquist plots take the typical form composed of a single semicircle and the electrode polarization tail (see Figure S1 in the Supporting Information for exemplary Nyquist plots). Conductivities as a High ionic conductivity of up to 6.4 × 10 −3 S cm −1 near room temperature (40 °C) in lithium amide-borohydrides is reported, comparable to values of liquid organic electrolytes commonly employed in lithium-ion batteries. Density functional theory is applied coupled with X-ray diffraction, calorimetry, and nuclear magnetic resonance experiments to shed light on the conduction mechanism. A Li 4 Ti 5 O 12 half-cell battery incorporating the lithium amide-borohydride electrolyte exhibits good rate performance up to 3.5 mA cm −2 (5 C) and stable cycling over 400 cycles at 1 C at 40 °C, indicating high bulk and interfacial stability. The results demonstrate the potential of lithium amide-borohydrides as solid-state electrolytes for high-power...

Slurry‐Fabricable Li+‐Conductive Polymeric Binders for Practical All‐Solid‐State Lithium‐Ion Batteries Enabled by Solvate Ionic Liquids

“…batteries which exclude flammable organic liquid electrolytes are considered as one of the most promising solutions. [5][6][7][8][9][10][11][12] Despite the poor chemical stability (e.g., instability in atmospheric air and conventional polar solvents), sulfide solid electrolyte (SE) materials are highly promising to realize all-solid-state Li-ion or Li batteries (ASLBs) which may outperform conventional LIBs. [5,[13][14][15][16][17][18][19][20][21] Li + conductivities of several state-of-the-art sulfide Li + superionic conductors have reached ≈10 −2 S cm −1 at room temperature (Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 [13] : 2.5 × 10 −2 S cm −1 , Li 7 P 3 S 11 [22] : 1.7 × 10 −2 S cm −1 ), which is comparable to that of organic liquid electrolytes.…”

Complex Hydrides for Energy Storage, Conversion, and Utilization

“…Density functional theory (DFT) calculations combined with quasielastic neutron scattering (QENS) characterization indicated that more lithium defects were created upon doping with LiI, and high concentration of Frenkel pair could mainly contribute to the improved ionic conductivity . Partially replacing [BH 4 − ] with [NH 2 ] − or [NH] 2‐ in LiBH 4 will also enhance Li ion mobility. Thanks to the new occupation sites for Li + ions in the phases or metastable state of Li 2 (BH 4 )(NH 2 ), Li 4 (BH 4 )(NH 2 ) 2 and Li 4 (BH 4 )(NH 2 ) 3 ionic conductivities of four orders of magnitude higher than that of neat LiBH 4 can be achieved at room temperature .…”

“…Partially replacing [BH 4 − ] with [NH 2 ] − or [NH] 2‐ in LiBH 4 will also enhance Li ion mobility. Thanks to the new occupation sites for Li + ions in the phases or metastable state of Li 2 (BH 4 )(NH 2 ), Li 4 (BH 4 )(NH 2 ) 2 and Li 4 (BH 4 )(NH 2 ) 3 ionic conductivities of four orders of magnitude higher than that of neat LiBH 4 can be achieved at room temperature . DFT calculations revealed that the Li + hopping from the filled sublattice into empty sublattice followed by Li + shuttling between empty sublattice.…”

Borohydride‐Scaffolded Li/Na/Mg Fast Ionic Conductors for Promising Solid‐State Electrolytes

“…In addition, the Li(BH 4 ) 1− x (NH 2 ) x ( x = 2/3) compound presented a preserved cubic α phase with a slightly increased lattice parameter (10.670 Å) compared to the cubic Li(BH 4 ) 1− x (NH 2 ) x ( x = 3/4) (10.655 Å) reported in the literature . A small peak at 2θ ≈ 25°–30° (marked by a star) was observed in the x = 2/3 phase (Figure h), which could be ascribed to a previously reported metastable γ phase within the amide–borohydride phase diagram . These peaks were absent in the patterns of the x = 3/4 phase.…”

“…In the fcc lattices with only edge‐ and corner‐sharing tetrahedral sites, Li ions are found to migrate along the intermediate octahedral sites . After mapping the cubic α phase of lithium amide–borohydrides to a distorted fcc lattice, to accommodate the fast reorientation of highly polarized NH 2 − groups, a different, yet less‐energy‐demanding, migration within two edge‐sharing tetrahedral sites is adopted, which enables improved lithium‐ion hopping and high ionic conductivity of the lithium amide–borohydride compound . The sulfides generally exhibit faster Li + ‐ion diffusion .…”