Elsevier

Electrochimica Acta

Volume 343, 20 May 2020, 136163
Electrochimica Acta

Enhanced high energy density hybrid lithium ion capacitor by garnet ceramic electrolyte for lithium anode protection

https://doi.org/10.1016/j.electacta.2020.136163Get rights and content

Highlights

  • Lithium ion capacitors consist by protected-lithium-anode and 21 m LiTFSI electrolyte.

  • Garnet-type solid state electrolyte is used in high-voltage lithium ion capacitor.

  • High concentration Li3BO3 is used to densify Garnet-type electrolyte.

  • Garnet-type electrolyte shows a good stability in 21 m LiTFSI.

Abstract

In this study, we report the fabrication of garnet-structure composited solid-state electrolyte with high compactness as a water-proof interlayer for multilayer water-proof protected-lithium-anode. It is revealed that with the addition of Li3BO3 into the sintering precursor, the cubic phase garnet Li6.75La3Zr1.75Nb0.25O12 with high lithium ion conductivity and compactness can be achieved at a low temperature of 1000 °C with a “binder-like” Li3BO3 glassy molten phase in the grain boundary, indicating Li3BO3 plays an important role on the densification of grain boundary and the improvement of the relative density. 50 mol % Li6.75La3Zr1.75Nb0.25O12–Li3BO3 pellet shows a high lithium ion conductivity of 2.08 × 10−4 S cm−1 and a good chemical stability in 21 m LiTFSI “water-in-salt” electrolyte at 25 °C. Hybrid lithium ion capacitor using this garnet-ceramic electrolyte as interlayer for water-proof protected-lithium-anode, 21 m LiTFSI electrolyte and commercial activated carbon as the cathode shows a high working voltage of 4.0 V and a high energy density of 228.9 Wh (kg-carbon)−1 at a power density of 1343.2 W (kg-carbon)−1.

Introduction

As the world moves toward increasing electric automation and intelligence, advanced energy storage technologies with high-energy density and high-power density as well as high-reliability become increasingly important for mobile electronics. Aqueous lithium ion capacitors are expected as one promising candidate to meet the requirements of high-performance, long-life and safety for mobile devices by integrating the advantageous such as the high-energy density of lithium ion batteries and the high-power density of electrochemical double layer capacitors as well as their non-flammability and environmental feasibility [1,2].

The specific energy (E) storage in an aqueous hybrid lithium ion capacitor is proportional to the square of voltage (U2) and the specific capacitance (C) basing on E = 1/2CU2. Thus enhancing the cell voltage for a lithium ion capacitor is given utmost priority, since the energy density is proportional to the U2. However, the narrow potential stability window is a disadvantage for aqueous electrolyte compared to nonaqueous electrolyte, which limits the directly application of low potential anodes such as metallic Li (0 V vs Li/Li+) in an aqueous lithium ion capacitor. Previously, aqueous lithium ion capacitor consisting of the water-proof protected-lithium-anode, activated carbon (AC) cathode and 21 m LiTFSI “water-in-salt” electrolyte was reported to have a high working voltage of 4.0 V [3]. The most important issue to achieve the high working voltage of this aqueous system is to develop a water-proof multilayer protected-lithium-anode, which is consisted of the metallic Li, composite gel polymer electrolyte and a NASCION-type water-proof lithium ion conductor (Li1+x+yTi2-xAlxSiyP3-yO12, LTAP, 150 μm, Ohara) solid-state separator film. However, LTAP is proved to be unstable when it contacts with metallic Li directly [4]. Therefore, hybrid lithium ion capacitor with NASCION-type water-proof protected-lithium-anode still suffers the problem of interface side reaction between metallic Li and LTAP after several charge/discharge cycles due to the lithium dendrite formation and growth in the protected-lithium-anode, resulting in directly contact and continuous chemical reaction between LTAP and Li, thus reducing its long-term stability [3]. As one of the promising inorganic solid-state electrolytes, the lithium-stuffed garnet-type oxide Li7La3Zr2O12 (LLZ) was reported to have a good chemical compatibility with metallic Li and attract considerable interest in lithium metal batteries due to its high ionic conductivity and a wide electrochemical stability window [[5], [6], [7], [8]]. Meanwhile, cubic phase LLZ was reported to be stable in saturated aqueous solution by Takeda et al. [9]. Ohta et al. have also reported that cubic phase Nb-doped LLZ (LLZN) with a high lithium ion conductivity of 8 × 10−4 S cm−1 and wide electrochemical window at room temperature [10]. Therefore, LLZ could be considered as the one of the best separator candidates for water-proof protected-lithium-anode to replace LTAP in the high-voltage hybrid supercapacitor due to its good chemical compatibility and high lithium ion conductivity. However, issues such as the phase transition from high conductivity cubic LLZ to low conductivity tetragonal LLZ during thermal treatment, the difficulty in preparing dense and pinhole-free LLZ thin film in order to against moisture to corrode the protected-lithium-anode, hamper the realistic application of LLZ as the solid-state electrolyte separator film for multilayer water-proof protected-lithium-anode [11].

Generally, depending on the sintering temperature, LLZ can be either tetragonal or cubic phase structure during the thermal treatment process. Among them, only cubic phase LLZ is responsible for producing high lithium ion conductivity. However, cubic phase LLZ usually requires a sintering temperature above 1100 °C via traditional solid-state reaction, the high sintering temperature could lead to lithium volatilization and phase transition. Thus, it is desirable to keep LLZ in cubic phase after densification at a lower temperature below 1100 °C, and to prepare dense LLZ solid-state electrolyte to achieve tightly interfacial contact and efficient Li+ diffusion in order to maintain high lithium ion conductivity. Meanwhile, grain boundary must be well tuned to prepare a pore-free water-proof lithium ion solid-state electrolyte film for the protected-lithium-anode. Recently years, many efforts have been exploited to increase the lithium ion conductivity and relative density, and to reduce the sintering temperature of LLZ. Partial substitution, elements doping, and low melting temperature second-phase sintering additive are the most useful technologies [[12], [13], [14], [15], [16]]. More recently, low concentration (10 wt%) Li3BO3 (LBO), Li3PO4 as additives have been reported to exhibit a positive effect to reduce the sintering temperature and maintain high lithium ion conductivity for LLZ, the additive presents a lower melting point than that of LLZ crystal. Therefore, during the sintering process, low melting point additive shows a highly effective for LLZ densification at a lower temperature by providing a liquid phase sintering route for the reaction system [[17], [18], [19]].

Considering above results, almost all of the previous literature report the weight percentage of additive (such as LBO, Li3PO4) in LLZ is less than 10 wt %, thus the LLZ composited electrolyte still cannot prevent the water from permeating through the electrolyte film due to the low relative density. Therefore, in this study, we prepare a highly compacted lithium ion conductive composited solid-state electrolyte of cubic phase garnet-type Li6.75La3Zr1.75Nb0.25O12–Li3BO3 (LLZN-LBO) due to the large amount of LBO additive (50 mol %) and use it as a novel electrolyte separator film for the multilayer water-proof protected-lithium-anode. During the sintering of the mixture precursors of LLZN and LBO, the amorphous LBO acts as a linkage for interface reaction between grain bulks and grain boundaries as well as the grain growth. In the obtained composited solid-state electrolyte, LBO would be existed as a glassy molten phase at the LLZ grain boundaries to increase the contact between individual LLZN crystals, and thus the resultant LLZN-LBO composite could achieve an excellent compactness. Meanwhile, the LLZN-LBO composited electrolyte film shows a good chemical stability in 21 m LiTFSI “water-in-salt” electrolyte. The hybrid lithium ion capacitor consisting of LLZN-LBO as the separator film for protected-lithium-anode, AC cathode and “water-in-salt” electrolyte can deliver a high working voltage of 4.0 V with a wide voltage window of 1.8 V, a high specific energy of 228.9 Wh (kg-carbon)−1 at a specific power of 1343.2 W (kg-carbon)−1 can be achieved by this advanced hybrid lithium ion capacitor.

Section snippets

Synthesis of garnet-type LLZN-LBO electrolyte

Cubic LLZN-LBO pellets were synthesized by a traditional solid-state reaction. First, LBO powder was prepared by sintering a mixture of Li2CO3 (99.5%) and B2O3 (99%) at 600 °C for 10 h. LLZN power was prepared by using the starting materials Li2O, La2O3, ZrO2, Nb2O5 powders (Aladdin, 99%) with a molar ratio of Li: La: Zr: Nb to be 7 : 3: 1.75 : 0.25. Different amount of Li3BO3 was added to above LLZN solute as a flux to stabilize the cubic structure and reduce its synthesis temperature. The

Results and discussion

The X-ray diffraction (XRD) patterns of 25, 50 and 75 mol % LLZN-LBO pellets are shown in Fig. 1, which are all indexed into typical cubic phase LLZ structure with an Ia-3d space group. All the thin pellets with different LLZN and LBO concentration sintered at 1000 °C show only a single phase without any imparity such as La2Zr2O7 or LBO [17]. Meanwhile, there is no any peak of LBO crystal in the XRD patterns even with 75 mol% LBO content in the sintering precursor, indicating that LBO acts as

Conclusions

Cubic garnet-type LLZN-LBO composited solid electrolyte could be successfully prepared at a low temperature of 1000 °C with the assistance of high concentration Li3BO3 additive. Amorphous Li3BO3 acts as a sintering aid to modify the grain boundary of LLZN and improve its relative density. The total lithium ion conductivity is estimated to be 2.08 × 10−4 S cm−1 for the sample of 50 mol % LLZN-LBO pellet at 25 °C and 6.06 × 10−4 S cm−1 at 60 °C. Hybrid lithium ion capacitor using this

CRediT authorship contribution statement

Chenyang Zhan: Data curation, Methodology, Writing - original draft. Kai Zhang: Methodology, Validation. Yaoting Li: Resources. Ming Zhang: Supervision, Writing - review & editing, Funding acquisition, Conceptualization. Zhongrong Shen: Supervision, Writing - review & editing, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by Xiamen Municipal Bureau of Science and Technology (No. 3502Z20182023), the State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (No. 20190016) and the National Natural Science Foundation of China (No. 21905282).

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