Material synthesis and characterization

A complex hydride lithium superionic conductor was synthesized by stabilizing the disordered high-T phase of a closo-type complex hydride containing closo-type (cage-like) complex anions (Supplementary Fig. 1 and Supplementary Note 1) at room temperature. In this study, Li(CB 9 H 10 ) was chosen as the host starting material for the stabilization of the high-T phase because of its low phase transition temperature (90 °C) and high lithium ion conductivity, approaching 10−1 S cm−1 for the high-T phase24. The stabilization of the high-T phase of Li(CB 9 H 10 ) was achieved by partially replacing (CB 9 H 10 )− complex anions with (CB 11 H 12 )− complex anions using a mechanical ball-milling technique (see the Methods section for details). (CB 11 H 12 )− complex anions were used as they have similar geometry and size and the same valence compared to those of (CB 9 H 10 )− complex anions. The phase transition temperatures and ionic conductivities of closo-type complex hydrides are summarized in Supplementary Table 1. Additional descriptions of the starting materials (Li(CB 9 H 10 ) and Li(CB 11 H 12 )), including the geometries of (CB 9 H 10 )− and (CB 11 H 12 )− complex anions (Supplementary Fig. 1), X-ray diffraction (XRD) patterns (Supplementary Fig. 2), field-emission scanning electron microscopy (FE-SEM) images (Supplementary Fig. 3), and differential thermal analysis (DTA) profiles (Supplementary Fig. 4), are provided in the Supplementary Information.

From the testing of various (CB 11 H 12 )− complex anion content levels, a 0.3 molar fraction of (CB 11 H 12 )− was chosen as the main composition (denoted as 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 )) for stabilizing the high-T phase of Li(CB 9 H 10 ), as lower content (0.1 molar fraction) resulted in the incomplete stabilization of the high-T phase and higher content (0.5 molar fraction) led to the formation of other impurity phases (Supplementary Fig. 5 and Supplementary Note 2). The XRD pattern of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) shows new diffraction peaks that are not assigned to the low-temperature (low-T) phases of the starting materials (Fig. 1a, Supplementary Figs. 6 and 7, and Supplementary Note 3). These peaks were indexed by the hexagonal unit cell, which is consistent with that (space group P31c (Z = 2)24) of the high-T phase of Li(CB 9 H 10 ). The estimated unit cell volume per formula unit (V/Z = 219 Å3) of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) is larger than that of Li(CB 9 H 10 ) (V/Z = 205 Å3) (Supplementary Table 2), revealing the formation of a solid-solution phase of Li(CB 9 H 10 ) and Li(CB 11 H 12 ), in which the centres of (CB 9 H 10 )− and (CB 11 H 12 )− occupy the same sites (2b = (1/3, 2/3, z)) in the crystal structure. The expanded lattice is consistent with the substitution of the larger (CB 11 H 12 )− for the smaller (CB 9 H 10 )−. These results verify that the high-T phase of Li(CB 9 H 10 ) is stabilized at room temperature by the partial substitution of complex anions. The lowered phase transition temperature can be possibly attributed to the increased entropy change due to the effects of the formation of the solid-solution phase, by which disordered distributions of both lithium ions and complex anions are thermodynamically preferred.

Fig. 1 Stabilization of high-T phase at room temperature. a XRD patterns of Li(CB 9 H 10 ) at 150 °C and 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) at room temperature. b DTA curves of Li (CB 9 H 10 ) and 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 1 2 ). c Raman spectra of Li(CB 9 H 10 ), Li(CB 11 H 12 ), and 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ). d FE-SEM images of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ). The magnified image (right) is of the yellow marked area (left). Scale bars, 20 μm for the left image in d and 3 μm for the right image in d Full size image

The phase transition between the low-T and high-T phases was investigated by DTA (Fig. 1b and Supplementary Table 1). The DTA profile of Li(CB 9 H 10 ) cycled between 25 and 200 °C exhibits endothermic and exothermic peaks at 90 and 80 °C upon heating–cooling cycling, which originate from the reversible transitions to and from the disordered high-T phase24. Importantly, 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) displays no such endothermic and exothermic peaks, reconfirming the stabilization of the high-T phase at room temperature. Additionally, the high thermal stability of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) was verified by XRD measurements after the heat-treatment at 473 K for 12 h (Supplementary Fig. 8).

The vibrational modes of complex anions in 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) were examined by Raman spectroscopy measurements (Fig. 1c). In the low-Raman-shift region below 1200 cm−1, the Raman profile of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) exhibits various deformation vibration modes25 of both (CB 9 H 10 )− and (CB 11 H 12 )−. In addition, Raman peaks at 3110 and 3050 cm−1, which are ascribed to C–H stretching modes25 in (CB 9 H 10 )− and (CB 11 H 12 )−, respectively, were observed for 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ). These results indicate that (CB 9 H 10 )− and (CB 11 H 12 )− remain intact and coexist in the solid-solution phase.

FE-SEM images of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) depict that the prepared sample forms secondary particles with sizes of 10–20 μm consisting of primary particles of imperfect circular morphology with sizes of 1–3 μm (Fig. 1d and Supplementary Fig. 9). The primary particles were interconnected with very smooth edges. These morphologies reflect the softness and deformability of the prepared sample, which can allow close contact with electrode materials during cell preparation.

Lithium ion conductivity

The ionic conductivity of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) was assessed using electrochemical impedance spectroscopy (EIS) measurements with Au electrodes. For the measurements, the pelletized samples were prepared by cold-pressing at 153.6 MPa. To provide a reference for a quantitative comparison, the results for Li(CB 9 H 10 ) are also shown24. The impedance profile of Li(CB 9 H 10 ) at 25 °C (=298 K) exhibits one semicircle in the high-frequency region and one spike in the low-frequency region (Fig. 2a), which correspond to contributions from the bulk/grain boundary and the electrode, respectively. In contrast, 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) displays only one spike, which is ascribed to the electrode contribution. The impedance of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ), which includes bulk and grain boundary resistances, was determined from the intercept of the electrode spike on the Z’-axis. Of note, the impedance measured for 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) is orders of magnitude lower compared to that of Li(CB 9 H 10 ). The lithium ion conductivity (σ) at 25 °C of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) is 6.7 × 10−3 S cm−1, which is three orders of magnitude higher than that (σ = 3.6 × 10−6 S cm−1) of Li(CB 9 H 10 ).

Fig. 2 Lithium ion conductivity of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ). a Nyquist plots of Li(CB 9 H 10 ) and 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) at 25 °C (left). Magnified Nyquist plots of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) in the high-frequency region (right). b Arrhenius plots of the lithium ion conductivities of Li(CB 9 H 10 ) and 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ). c Arrhenius plots of the diffusion coefficients calculated from the impedance and NMR measurements Full size image

An Arrhenius plot of ionic conductivities shows that as the temperature increases from 25 to 90 °C, Li(CB 9 H 10 ) displays a drastic jump in ionic conductivity, which originates from a transition to the high-T phase (Fig. 2b, Supplementary Fig. 10a, and Supplementary Note 4). In contrast, for 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ), the Arrhenius plot shows no phase-transition-derived changes in conductivity and a linear increase in the logarithmic values (Fig. 2b and Supplementary Fig. 10b). Importantly, at above 90 °C, the conductivities of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) match well with those of Li(CB 9 H 10 ): the conductivities at 110 °C and activation energies (E a ) of Li(CB 9 H 10 ) and 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) are 8.1 × 10−2 S cm−1 and 28.9 kJ mol−1, and 8.5 × 10−2 S cm−1 and 28.4 kJ mol−1, respectively (Supplementary Table 1 and Supplementary Note 5). It has been reported that the fast ionic conduction in the high-T phase of complex hydrides results from their vacancy-rich disordered cation sublattices within networks of reorientationally disordered complex anions24,26,27,28,29.

7Li-pulsed field gradient nuclear magnetic resonance (PFG NMR) analyses indicate that the self-diffusion coefficient (D NMR ) of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) is lower than the conductivity diffusion coefficient (D σ ) calculated from the impedance measurements (Fig. 2c, Supplementary Fig. 11, and Supplementary Note 6). The Haven ratio, H R = D NMR /D σ , was ~0.17 in the temperature range of 25–60 °C, clarifying that the high ionic conductivity of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) is the result of a strong correlation between highly disordered lithium ions, which induces concerted ionic diffusion30,31,32.

To the best of our knowledge, the room-temperature conductivity (6.7 × 10−3 S cm−1 at 25 °C) of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) is the highest value reported to date for complex hydride lithium ion conductors (Fig. 3). This conductivity is comparable to those of oxide-based9,11 and sulfide-based2,4,7 solid electrolytes. Considering that organic liquid electrolytes33 have transport numbers of below 0.5, 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) has higher conductivity than organic liquid electrolytes.

Fig. 3 Arrhenius plots of the conductivities of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) and those of other complex hydride lithium ion conductors23,24,41,42,43,44,45,46. 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) exhibits a high lithium ion conductivity of 6.7 × 10−3 S cm−1 at 25 °C, which is the highest value reported for complex hydride lithium ion conductors Full size image

Stability against lithium metal anode

0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) shows markedly high stability against the lithium metal anode in terms of potential window, interfacial resistance, voltage polarization, and lithium plating/stripping cycling. Electrochemical stability was first evaluated by cyclic voltammetry (CV) with a Mo/0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 )/Li cell at 25 °C (Fig. 4a and Supplementary Fig. 12). The cell displays sharp and reversible cathodic and anodic currents at around 0 V, which correspond to lithium deposition (Li+ + e− \(\rightarrow\) Li) and dissolution (Li \(\rightarrow\) Li+ + e−), respectively, revealing the high reducing ability of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ). In addition, the lack of oxidation currents within the scanned voltage range (−0.1 to 5 V) demonstrates the wide potential window of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ).

Fig. 4 Stability of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) with lithium metal. a CV curve of a Mo/0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 )/Li cell at a scan rate of 0.5 mV s−1 and a scan range of −0.1 to 5.0 V (vs. Li+/Li). b Nyquist plot of a Li/0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 )/Li cell. Inset shows the magnified plot in the region of the semicircle indicating the 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 )/Li interfacial resistance. c FE-SEM image of the 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 )/Li interface. d 10th galvanostatic cycling profile of the Li/0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 )/Li cell. Inset shows the magnified profile. e Galvanostatic cycling profiles for prolonged cycles. 1st–100th cycles (top), 101st–200th cycles (middle), and 201st–300th cycles (bottom). All electrochemical measurements were conducted at 25 °C. Scale bar, 30 μm in c Full size image

The interfacial resistance between 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) and the lithium metal anode was investigated using the EIS measurement with a symmetric Li/0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 )/Li cell at 25 °C (Fig. 4b). The impedance profile of the Li/0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 )/Li cell displays one semicircle in the high-frequency region and one spike in the low-frequency region, which correspond to contributions from the 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 )/Li interface and the electrode, respectively (Supplementary Fig. 13, Supplementary Table 3, and Supplementary Note 7). The 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 )/Li interfacial resistance was calculated to be 0.78 Ω cm2. This interfacial resistance is quite noticeable, as it is far lower than those of other solid electrolyte/Li interfaces reported to date3,14,16,18,19,34,35,36,37. The interfacial resistances between the solid electrolyte and the lithium metal anode are summarized in Supplementary Table 4. An FE-SEM image confirms close physical contact at the interface between 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) and lithium metal (Fig. 4c). The remarkable interfacial compatibility of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) with the lithium metal anode is ascribed to its high chemical stability and high physical deformability.

0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) also shows extremely stable lithium ion transfer capability across the interface with the lithium metal anode. When galvanostatically cycled at a current density of 0.2 mA cm−2 in both directions for 30 min at 25 °C, the Li/0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 )/Li cell exhibited very small voltage polarization (6.0 mV) (Fig. 4d). More importantly, the Li/0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 )/Li cell shows superior voltage retention after repeated lithium plating and stripping cycles. At a current density of 0.2 mA cm−2, no voltage changes were observed during 300 cycles (Fig. 4e and Supplementary Fig. 14). The low and unvaried interfacial resistances were also verified by EIS measurements (Supplementary Fig. 15). The superior lithium ion transfer performance confirms the high electrochemical stability of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) against the lithium metal anode.

All-solid-state lithium metal battery with complex hydride solid electrolyte

0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) can potentially be used to realize a wide range of lithium-metal-based all-solid-state batteries. Based on the high ionic conductivity and high stability with lithium metal of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ), the electrochemical investigation was expanded to all-solid-state full cells using 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) as the solid electrolyte and lithium metal as the anode. As shown in Fig. 5a, a high-energy-density S electrode (theoretical capacity = 1672 mAh g−1; working voltage = 2.1 V vs. Li+/Li) was used as the cathode to take advantage of the high-energy density of the lithium metal anode. The cathode composite was prepared by mixing an S–carbon (C) composite and 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) solid electrolyte. The all-solid-state batteries were fabricated by cold-pressing at 153.6 MPa. Detailed cell preparation and measurement conditions are described in the Methods section. Fig. 5b shows the voltage profiles recorded during the first two cycles measured in the voltage range of 1.0–2.5 V (vs. Li+/Li) for a rate of 0.03 C (50.2 mA g−1) at 25 °C. The C-rate is determined as follows: a rate of nC means that the current will (dis)charge the full capacity in 1/n hours. In the first cycle, the discharge capacity and the charge capacity were 2013 and 1557 mAh g−1, respectively. The higher capacity in the first discharge is presumably attributed to the capacity contribution from the solid electrolyte38. After the first discharge, reversible charge–discharge profiles, which present a charge plateau at around 2.2 V and a discharge plateau at around 2.0 V, were observed. The discharge capacity in the second cycle was 1618 mAh g−1, which corresponds to 96.8% of the theoretical capacity.

Fig. 5 High-energy-density all-solid-state lithium metal batteries. a Schematic illustration of the prepared all-solid-state batteries. S, Li/0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ), and lithium metal were used as the cathodes, solid electrolytes, and anodes, respectively. b Voltage profiles for a rate of 0.03 C (50.2 mA g−1) at 25 °C during the first two cycles. c Discharge–charge profiles at 0.03, 0.05, 0.1, 0.3, and 1 C after an initial cycle at 25 °C. d Capacity retention as a function of current density. e, f Cycling performances of discharge capacity and coulombic efficiency (e) for a rate of 1 C at 25 °C and (f) for a discharging rate of 3 C and a charging rate of 1 C at 50 °C Full size image

After the first cycle, as the C-rate increased by 1.67, 3.33, 10, and 33.3 times from 0.03 C (1 C = 1672 mA g−1), the all-solid-state cell retained 96.5%, 90.0%, 81.4%, and 73.4% of its capacity (1618 mAh g−1) in the second cycle, respectively (Fig. 5c, d). The all-solid-state cells also showed good cycling stability. At 0.1 C (Supplementary Fig. 16 and Supplementary Note 8) and 1 C (Fig. 5e and Supplementary Fig. 17), 98.9% and 84.4% of the capacity (1431 and 1239 mAh g−1) in the second cycle was retained after 10 and 20 cycles, respectively. In addition, for a discharging rate of 3 C and a charging rate of 1 C at 50 °C, the discharge capacity was 1533 mAh g−1 in the second cycle, and slightly dropped to 1469 mAh g−1 after 20 cycles, leading to high-energy densities of 2578–2782 Wh kg−1 (Fig. 5f and Supplementary Fig. 18). During cycling, the coulombic efficiencies became saturated at ~100% (Fig. 5e, f, and Supplementary Fig. 16b). From the perspective of energy density, the reversible energy densities of over 2500 Wh kg−1 at high current densities of 1–3 C are remarkable, as they are better than those of previously reported Li–S22,38, Li–LiCoO 2 4,19,36, Li–LiNi 0.5 Mn 1.5 O 4 16, and Li–Li 2 FeMn 3 O 8 3 all-solid-state batteries.

0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) also presents the high stability during prolonged cycling. When cycled for a discharging rate of 5 C and a charging rate of 1 C at 60 °C, the discharge capacity was 1472 mAh g−1 in the second cycle, and the reversible capacity of 1017 mAh g−1 with the coulombic efficiency of ~100% was retained after 100 cycles (Fig. 6a and Supplementary Fig. 19). An FE-SEM image verifies the preserved intimate contact at the 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 )/Li interface after cycling (Fig. 6b). Furthermore, no dendrite growth was observed across the 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 )/Li interface. This physical stability was confirmed in multiple measurement regions (Fig. 6b and Supplementary Fig. 20). In addition, XRD (Fig. 6c) and Raman spectroscopy (Fig. 6d) measurements indicate that the characteristic peaks of 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) are maintained after cycling. These results demonstrate the practical feasibility of the 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ) solid electrolyte for all-solid-state batteries employing the lithium metal anode.