Fast Lithium-Ion Conduction in Atom-Deficient closo-Type Complex

Dec 20, 2017 - closo-type complex hydrides contain large cage-type complex polyanions in their crystal structures and thus can exhibit superior ion-co...
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Cite This: Chem. Mater. 2018, 30, 386−391

Fast Lithium-Ion Conduction in Atom-Deficient closo-Type Complex Hydride Solid Electrolytes Sangryun Kim,*,† Naoki Toyama,† Hiroyuki Oguchi,‡ Toyoto Sato,† Shigeyuki Takagi,† Tamio Ikeshoji,† and Shin-ichi Orimo†,‡ †

Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan

‡

S Supporting Information *

ABSTRACT: closo-type complex hydrides contain large cage-type complex polyanions in their crystal structures and thus can exhibit superior ion-conducting properties (e.g., Li and Na). However, the unique structures of complex polyanions have made it challenging to modify crystal structures, making systematic control of ion conductivity difficult. Here, we report an atom deficiency approach to enhance lithium-ion conductivity of complex hydrides. We find that lithium and hydrogen could be simultaneously extracted from Li2B12H12 by applying a small external energy, enabling the formation of atom deficiencies. These atom deficiencies lead to an increase in carrier concentration, improving lithium-ion conductivity by 3 orders of magnitude compared to that of a pristine material. An all-solid-state TiS2/Li battery employing atom-deficient Li2B12H12 as a solid electrolyte exhibits superior battery performance during repeated discharge−charge cycles. The current study suggests that the atom deficiency can be a useful strategy to develop high ion-conducting complex hydride solid electrolytes.

1. INTRODUCTION Recent intensive research effort has led to considerable progress in all-solid-state batteries, in which both the electrolyte and electrodes are in their solid states. The advantages of all-solidstate batteries lie in their ability to overcome the intrinsic drawbacks of conventional liquid-based batteries, such as electrolyte leakage, flammability, and limited voltage window, as solid-state electrolytes are usually nonexplosive, nonvolatile, and electrochemically stable, even up to ∼5.0 V vs Li+/Li.1,2 In particular, increasing safety concerns regarding the application of lithium batteries with liquid electrolytes in electric vehicles have stimulated research into all-solid-state batteries. Lithium-ion-conducting solid electrolytes are a key component of all-solid-state batteries because the stability and ionic conductivity of the solid electrolyte determine battery performance.3−8 Among the variety of materials reported to date, complex hydrides have recently attracted particular attention as a new class of solid electrolytes owing to their superior stability against lithium metal, which results from their high reducing ability, as well as their high ionic conductivity.9−11 Complex hydrides are generally denoted by M(M′xHy), where M is a metal cation and M′xHy is a complex anion. Initial interest in complex hydride ionic conductors has mainly focused on lithium borohydride (LiBH4) and related derivative materials.9 A series of closo-boranes containing complex polyanions such as [B12H12]2−, [CB11H12]−, and [CB9H10]− have recently been reported to exhibit high lithium-ion conductivities.12−14 © 2017 American Chemical Society

Despite their impressive ionic conductivities, the unique complex anion structure formed by strong covalent bonding between the boron and hydrogen atoms greatly complicates efforts to modify crystal structures, which are directly related to their ionic conductivities. For example, systematic cationic and/ or anionic substitutions, which have been widely adopted to control the crystal structures (and thus ionic conductivities) of various ionic conductors15,16 are largely inappropriate because the structural stabilities of complex anions are significantly lowered by such substitutions, leading to structural collapse. For these reasons, substitution of the complex anions themselves to form so-called mixed complex anions9,17,18 has been explored, but usable candidates are very limited. Therefore, a more general approach is required to broaden the structure and conduction property scopes of complex hydrides. Herein, in an effort to overcome the chronic drawbacks of complex hydrides and enhance their ionic conductivities, we report the unconventional approach of introducing atom deficiencies to lithium dodecahydro-closo-dodecaborate (Li2B12H12). Our experimental results reveal that simple ballmilling generates both lithium and hydrogen deficiencies and that the increased carrier concentration by these deficiencies increases lithium-ion conductivity significantly. Furthermore, an Received: September 19, 2017 Revised: December 19, 2017 Published: December 20, 2017 386

DOI: 10.1021/acs.chemmater.7b03986 Chem. Mater. 2018, 30, 386−391

Article

Chemistry of Materials

First-Principles Molecular Dynamics Simulation. The total energy changes due to atom deficiencies were estimated by firstprinciples molecular dynamics (FPMD) simulation at 300 K. In the FPMD simulations, an electronic structure calculation code implemented in the Vienna ab initio simulation package (VASP) using density functional theory (DFT), was used with plane-wave basis sets and Projector-Augmented-Wave (PAW) pseudopotentials under periodic boundary conditions. A Perdew−Burke−Ernzerhof (PBE) functional21 was used for the exchange correlation with a generalized gradient approximation. The cutoff energy was 320 eV for wave functions, and k-point sampling was 1 × 1 × 1. The starting structure had 32 Li2B12H12 formula units (f.u.) (Z = 4, 2 × 2 × 2 supercell), and the formation energies were calculated from the time average total energy E(LiaBbHc) using the following equations:

all-solid-battery using atom-deficient Li2B12H12 as the solid electrolyte exhibits good cycling stability during repeated discharge−charge cycles. The present investigation demonstrates that the exploitation of atom deficiencies is a viable approach to improving the ionic conductivities of closo-type complex hydride solid electrolytes. The effects of ball-milling have been previously investigated for the formation of a disordered high-temperature (HT) phase, in which fast ionic conduction is enabled predominantly by the dynamics of the complex anions.14,19 Our careful characterizations reveal the distinctly different ionic conduction mechanism of atom-deficient Li2B12H12 compared to those present in previous reports.

E Li = E(Li63B384 H384) + E(Li) − E(Li64B384 H384)

2. EXPERIMENTAL SECTION

E H = E(Li64B384 H383) + E(H 2)/2 − E(Li64B384 H384)

Sample Synthesis. Pristine Li2B12H12. The starting material, hydrated Li2B12H12·4H2O, was purchased from Katchem. Li2B12H12· 4H2O was first ground using a mortar and pestle for 15 min. To obtain anhydrous samples, the powders were subsequently dried under vacuum ( 9.9 Å) and the ball-milled phase (a = 9.6283(8) Å) reveals a large difference in lattice parameters, implying that both phases represent different crystal structures. More precise differences can be verified by focusing on the (021) peak, the intensity of which is related to the lithium arrangement. In the case of the HT phase, a reduced (or invisible) (021) peak indicates a completely disordered lithium arrangement after phase transition (Table S1).26 In contrast, the ball-milled Li2B12H12 presents a preserved (021) peak, reconfirming the structural disagreement between the phases. More critically, a transition to the HT phase is detected for the ball-milled Li2B12H12. The DTA data for the ball-milled Li2B12H12 present clear endothermic and exothermic peaks at ∼310 and 275 °C upon heating−cooling cycling (Figure 4),

Figure 3. (a) Arrhenius plots of the conductivities of pristine and ballmilled Li2B12H12. (b) Discharge−charge profiles of an all-solid-state battery consisting of a TiS2 cathode, a ball-milled Li2B12H12 solid electrolyte, and a Li metal anode at 80 °C and 0.05 C.

from the atom deficiencies. The lithium deficiencies can allow the formation of additional lithium sites as well as a change in lithium arrangement, both of which correlate with the carrier concentration. The lowered intensity of the (021) peak after ball-milling, which is confirmed from the XRD analyses (Table S1), reflects the rearrangement of the carriers (lithium and vacancy) at multiple sites, increasing the carrier concentration. A previous structural study reported that the reduced intensity of the (021) peak indicates lithium arrangement at multiple sites (8c and 24d) with the lowered occupancies.26 Other lithium sites can also be generated by the hydrogen-deficient and/or distorted complex anions. Similar conductivity changes involved with the carrier concentration have been observed and are well explained for perovskite-type lithium ionic conductors.15,32,33 The ball-milled Li2B12H12 was examined as a solid electrolyte in an all-solid-state battery. The electrochemical stability was first evaluated from the CV of an SS/ball-milled Li2B12H12/Li cell at a scan rate of 5 mV s−1 and a scan range of −0.1 to 5 V (Figure S4). The cell displayed reversible cathodic and anodic currents around 0 V, which correspond to lithium deposition and dissolution, respectively. In addition, no oxidation currents were detected in the scanned voltage range, thus indicating the wide electrochemical stability of the ball-milled Li2B12H12. Figure 3b shows the discharge−charge curves of the all-solidstate battery, fabricated using a TiS2 cathode, a ball-milled Li2B12H12 solid electrolyte, and a lithium metal anode. When galvanostatically cycled in the voltage range of 1.6−2.7 V (vs Li+/Li) at 80 °C and 0.05 C (1 C = 239 mA g−1), the all solid-

Figure 4. DTA curves of pristine and ball-milled Li2B12H12 during a heating−cooling cycle.

which originate from the reversible transitions to and from the HT phase. These results, once again, indicate that the ballmilled Li2B12H12 is distinct from the HT phase. The slightly lower transition temperature for the ball-milled Li2B12H12 compared to that of the pristine material is presumably due to the effects of the increased entropy change. The fast ionic conduction in the atom-deficient complex hydrides observed in the present study is of importance because it can be utilized as a design principle for developing closo-type complex hydride solid electrolytes. Well-controlled atom deficiencies can further improve ionic conductivities, as previously demonstrated in various lithium ionic conduc389

DOI: 10.1021/acs.chemmater.7b03986 Chem. Mater. 2018, 30, 386−391

Chemistry of Materials

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tors.8,15,35 This approach can be applied to a variety of hydrides with cage-type complex polyanions. To achieve this, systematic routes to forming atom deficiencies need to be established. In addition, investigation of atom-deficient complex hydrides can be useful for understanding the ionic conduction in related materials of the same category. Since the HT phases of closotype complex hydrides exhibit extremely high ionic conductivities, a variety of approaches, such as mechanical ballmilling,14 the use of mixed complex anions,36 and atomic substitution in complex anions,13 have been attempted to stabilize the HT phase at lower temperatures. However, after the given treatments, the materials showed large disagreements in conductivities as compared to the pristine materials as well as nonlinear Arrhenius profiles, implying the possibility of other factors influencing the ionic conductivities. In this regard, the atom deficiencies might be a viable approach to revealing the unexplained conduction mechanisms of closo-type complex hydrides.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03986. Additional characterization data (SEM, ratios of XRD peaks, Raman, impedance, and CV) (PDF)

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REFERENCES

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4. CONCLUSIONS In summary, this investigation introduces a new strategy to enhance the ionic conductivity of closo-boranes, i.e., the formation of atom deficiencies. The atom deficiencies in Li2B12H12 result in high carrier concentration originating from multiple lithium sites and increased vacancies, thus improving lithium-ion conductivity. Highly important is the fact that the proposed high ionic conductivity of the atom-deficient phase can be utilized together with the effects of the HT phase. Thus, the introduction of atom deficiencies provides useful insight into strategies that can be applied to developing complex hydride solid electrolytes.

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Article

AUTHOR INFORMATION

Corresponding Author

*(S.K.) E-mail: [email protected]. ORCID

Sangryun Kim: 0000-0001-8617-3022 Toyoto Sato: 0000-0002-0527-1235 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Mr. K. Sato, Ms. H. Ohmiya, and Ms. N. Warifune for technical assistance and the use of SR16000 supercomputing resources at the Center for Computational Materials Science of the Institute for Materials Research, Tohoku University. This work was supported by JSPS KAKENHI (Grant Numbers 17H06519, 16K06766, 17K19168, 17K18972, and 25220911), Collaborative Research Center on Energy Materials in IMR (E-IMR), and Target Project 4 of WPI-AIMR, Tohoku University. 390

DOI: 10.1021/acs.chemmater.7b03986 Chem. Mater. 2018, 30, 386−391

Article

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DOI: 10.1021/acs.chemmater.7b03986 Chem. Mater. 2018, 30, 386−391