Solid Electrolytes for Lithium Batteries

The use of solid electrolytes to produce an all-solid-state lithium battery is regarded as one promising alterative for improved safety. In this work, the researchers synthesized cubic garnet-type ceramic electrolyte Li_6.75La₃Zr_1.75Ta_0.25O₁₂ (LLZTO) via the coprecipitation method. For the characterization, the phase content was determined using X-ray diffraction (XRD), morphology was obtained by using a scanning electron microscope (SEM), and conductivity was measured by AC impedance spectroscopy using a frequency response analyzer. The results showed that the room temperature total conductivity and activation energies of LLZTO were 7.2×10^-6 S/cm and 0.31-0.46 eV, respectively. Composite electrolytes were prepared by mixing LLZTO with a ceramic (Li_1.4Al_0.4Ti_1.6(PO₄)) (NASICON) or a polymer polyethylene oxide (PEO) with LiClO₄ salt. The LLZTO-NASICON composite showed a higher total conductivity of 1.2×10^-5 S/cm and activation energies of 0.23-0.58 eV. Compared to LLZTO, the LLZTO-PEO(LiClO₄) composite showed a similar room temperature total conductivity of 1.2×10^-6 S/cm, but a higher activation energy of 0.59-0.85 eV.

Garnet; Solid composite electrolyte; Solid electrolyte

Lithium-ion batteries are widely used for energy storage in a variety of applications (Nitta et al., 2015). However, the commonly used organic carbonate liquid electrolytes have poor chemical stability, display high flammability, and can leak from the battery, which can create safety issues (Varzi et al., 2016). Using solid electrolytes instead of liquid electrolytes to produce an all-solid-state lithium battery is regarded as one promising way to improve safety (Reisch, 2017).

Solid-state electrolytes used in lithium-ion batteries can be divided into two categories: polymers and ceramics (Liu et al., 2018). The first category, solid polymer electrolytes (SPE), are formed by dissolving a lithium salt, such as LiClO₄, LiBF₄, or LiN(CF₃SO₂)₂, in a polymer host (Agrawal & Pandey, 2008; Zhang et al., 2017). The advantages of solid-state polymer electrolytes are ease of processing and mechanical flexibility. However, they exhibit poor electrochemical stability, low lithium ionic conductivity, and low mechanical strength (Ren et al., 2015; Zhao et al., 2016). The second category, ceramic electrolytes, have high chemical and electrochemical stability as well as high mechanical strength, but they are non-flexible. Much effort has been made toward the development of solid-state ceramic electrolytes, such as  perovskite-type titanates (LLTO) (Kwon et al., 2017), NASICON-type (sodium super ionic conductor) oxide electrolytes (Morimoto et al., 2013), sulfide-type and garnet-type materials (Thangadurai et al., 2014; Manthiram et al., 2017). 

The focus of this work was the garnet-type ceramic electrolyte Li₇La₃Zr₂O₁₂ (LLZO), as it has the most promising properties for solid electrolyte, such as high chemical stability and a high ionic conductivity of 3×10^-4 S/cm at 25°C (Murugan et al., 2007). LLZO can form a tetragonal structure with  low lithium-ion conductivity, approximately 2×10^?6 S/cm (Awaka et al., 2009), so the elemental doping method was used to stabilize the more conductive cubic phase. In this work, tantalum doping was used to prepare the cubic phase LLZO. LLZO-based composites were also prepared in this work by combining a solid polymer electrolyte, PEO(LiClO₄), or a NASICON-type material, Li_1.4Al_0.4Ti_1.6(PO₄), with LLZO

Li_6.75La₃Zr_1.75Ta_0.25O₁₂ with the cubic phase garnet-type structure and porosity of about 20% was obtained by the coprecipitation method. The total conductivity at room temperature and the activation energies were estimated to be at 7.24×10^-6 S/cm and 0.31-0.46 eV, respectively. The conductivity of the LLZTO-NASICON composite, 1.18×10^-5 S/cm, was higher than that of LLZTO and the activation energies were estimated to be 0.23-0.58 eV. The LLZTO-PEO(LiClO₄) composite has a similar room temperature total conductivity of 1.04×10^-6 S/cm, but higher activation energies of 0.59-0.85 eV as compared to LLZO. This work shows the preparation and conductivity of LLZTO based composites of LLZTO-NASICON and LLZTO-PEO(LiClO₄), which promotes the development of LLZO solid electrolyte for high safety solid-state lithium batteries.

This work was supported primarily by NASA under award number NNX15AP44A, 2015.

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