Promising Solid-state Electrolyte Research Articles

A Poly(ethylene carbonate)/Lithium Bis(fluorosulfonyl)imide/Titanium Oxide Composite Electrolyte Containing a Pyrrolidinium-based Ionic Liquid

: Solid polymer electrolytes (SPEs), formed by the combinations of flexible polymers and metal salts, can be battery electrolyte alternatives to the conventional liquid-based ones because they are not flammable and leakage-free. In the present work, poly(ethylene carbonate) (PEC) has been selected as a polymer matrix. Notably, poly(alkylene carbonate)s, such as PEC, are synthesized by alternating copolymerizations between carbon dioxide (CO2) and epoxides. Therefore, we initially focused on the CO2/epoxide copolymers as polymer hosts for the SPEs, being motivated by a concept of CO2 utilization as a raw carbon source [1,2]. We have already reported extraordinary ion-conductive behaviors of PEC, including high ionic conductivities at quite high Li salt concentration regions and extremely high Li transference number (t Li+) values [3,4]. For instance, an electrolyte containing 80wt% of LiTFSI showed a 120 times higher ionic conductivity than that of a sample containing 20wt% of the LiTFSI salt [3]. In addition, the t Li+ of a PEC-LiFSI (80wt%) electrolyte with TiO2 particles was estimated to exceed 0.7, according to an electrochemical measurement, as well as to a pulse-field-gradient NMR technique [4]. In the present work, we propose a quaternary PEC-LiTFSI mixture-based composite electrolyte membrane containing N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI) ionic liquid (IL) as a plasticizing additive and a submicron-sized electrospun SiO2 fiber (SiF) as an inorganic filler, in view of creating more feasible battery materials. We demonstrate that the resulting mechanically stable membrane reinforced by SiF shows a conductivity of the order of 10-7 S cm-1 at 40 oC combined with a favorable t Li+ value of 0.36, which both are enhanced by the simultaneous presence of the IL and SiF. Furthermore, we demonstrate that the electrolyte works as a promising solid-state electrolyte in a galvanostatic cycling for a prototype battery having a LiFePO4-based cathode at an elevated temperature (75 oC). Further advancements can be achieved by the use of lithium bis(fluorosulfonyl)imide (LiFSI) as a salt and N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (Pyr13FSI) as an IL. We have confirmed that a quaternary PEC-LiFSI-Pyr13FSI-TiO2 electrolyte leads an ionic conductivity of the order of 10-5 S cm-1 at 40 oC which is more than an order of magnitude greater than that of the LiTFSI-based analogous sample mentioned above. Further detailed properties of the PEC-LiFSI mixture-based composite electrolytes, which are currently under investigation in our laboratory, will be presented.

First-Principles Study of Chemical Stability of the Lithium Oxide Garnets Li7La3M2O12 (M = Zr, Sn, or Hf)

Li-oxide garnet-related structures are promising solid-state Li-ion electrolytes in Li-ion batteries. However, garnet-type structures are susceptible to carbonate and hydroxide formation in environments containing gaseous CO2 and H2O. Therefore, in considering garnets for Li-ion conducting applications, chemical stability is an important issue. We examine the chemical stability of Li7La3Zr2O12, Li7La3Sn2O12, and Li7La3Hf2O12 with respect to carbonate and hydroxide formation reactions using density functional theory (DFT) calculations. From these studies, we rank the chemical stability of Li-oxide garnet-related structures against CO2 and H2O. The ranking of these materials by their chemical stability with respect to carbonate and hydroxide formation changes at higher partial pressures of CO2 and H2O.

Influence of lithium oxide additives on densification and ionic conductivity of garnet-type Li6.75La3Zr1.75Ta0.25O12 solid electrolytes

Abstract The lithium garnet ceramic has been considered as one of promising solid-state electrolytes for secondary lithium batteries. However, improvement of its ionic conductivity is hindered by the high porosity related to the severe volatilization of lithium components during sintering. In order to find the solution for this problem, Li2O additives in concentration of 2–8 wt.% were introduced into the cubic Li6.75La3Zr1.75Ta0.25O12 (LLZTO) at the solid-state reaction processes. It is found that the Li2O additives lead to formation of glassy-like phases at the grain boundaries related to the liquid-phase sintering behavior, which eliminates the residual pores therein and increases the relative density from 91.5% to 97.3%. Measurement of conductivity indicates that 6 wt.% is the optimum concentration of Li2O which leads to an ionic conductivity of 6.4 × 10− 4 S cm− 1 at room temperature. This value is approximately 3 times larger than that of the ceramic without the additive.

Atomic Layer Deposition of Lithium Tantalate Solid-State Electrolytes

D all-solid-state microbatteries are promising onboard power systems for autonomous devices. The fabrication of 3D microbatteries needs deposition of active materials, especially solid-state electrolytes, as conformal and pinhole free thin films in 3D architectures, which has proven very difficult for conventional deposition techniques, such as chemical vapor deposition and physical vapor deposition. Herein, we report an alternative technique, atomic layer deposition (ALD), for achieving ideal solid-state electrolyte thin films for 3D microbatteries. Lithium tantalate solid-state electrolytes, with well-controlled film composition and film thickness, were grown by ALD at 225 °C using subcycle combination of 1 × Li2 O+ n × Ta2O5 (1 ≤ n ≤ 10). The film composition was tunable by varying Ta2O5 subcycles (n), whereas the film thickness displayed a linear relationship with ALD cycle number, due to the self-limiting nature of the ALD process. Furthermore, the lithium tantalate thin films showed excellent uniformity and conformity in 3D anodic aluminum oxide template. Moreover, impedance testing showed that the lithium tantalate thin film exhibited a lithium ion conductivity of 2 × 10 −8 S/cm at 299 K. The lithium tantalate thin films by ALD, featured with well-controlled film thickness and composition, excellent step coverage, and moderate ionic conductivity at room temperature, would be promising solid-state electrolytes for 3D microbatteries.

Complete elastic characterization of lithium phosphorous oxynitride films using picosecond ultrasonics

We report on a complete elastic characterization by picosecond ultrasonics technique of lithium phosphorous oxynitride (LiPON) thin films, which are nowadays, considered as promising solid-state electrolytes for microbatteries technology. Longitudinal sound velocity is deduced from Brillouin oscillation detection at variable incidence angles. The density is estimated through an interfacial energy analysis related to the echo amplitude decay. Transverse wave velocity is obtained across the dispersion curve of Rayleigh surface waves. Combining these different approaches, Young's modulus and Poisson's ratio of LiPON are finally found to be 79GPa and 0.27, respectively.

Bis-quaternary ammonium cation-based organic ionic plastic crystals: plastic crystal behaviour and ionic liquid properties above melting points

A new family of organic ionic plastic crystals (OIPCs) based on bis-quaternary ammonium cations was synthesized and characterized. The synthesized bis-quaternary ammonium salts with bis(trifluoromethanesulfonyl) imide (TFSI−) anions exhibit plastic crystal properties as determined by differential scanning calorimetry (DSC). The high ionic conductivity of the compounds in the solid-state indicated that this new family of OIPCs can be promising solid-state electrolytes for electrochemical devices. Cyclic voltammetry characterization of OIPCs in the liquid-state revealed their wide electrochemical windows and highly reversible lithium metal electrodeposition properties.