Development of Lithium-Stuffed Garnet-Type Oxide Solid Electrolytes with High Ionic Conductivity for Application to All-Solid-State Batteries

Abstract

All-solid-state lithium-ion batteries are expected to be one of the next generations of energy storage devices because of their high energy density, high safety, and excellent cycle stability. Although oxide-based solid electrolyte (SE) materials have rather lower conductivity and poor deformability than sulfide-based ones, they have other advantages, such as their chemical stability and ease of handling. Among the various oxide-based SEs, lithium-stuffed garnet-type oxide, with the formula of Li7La3Zr2O12 (LLZ), has been widely studied because of its high conductivity above 10 −4 S cm −1 at room temperature, excellent thermal performance, and stability against Li metal anode. Here, we present our recent progress for the development of garnet-type SEs with high conductivity by simultaneous substitution of Ta 5+ into the Zr 4+ site and Ba 2+ into the La 3+ site in LLZ. Li + concentration was fixed to 6.5 per chemical formulae, so that the formula of our Li garnet-type oxide is expressed as Li6.5La3−xBaxZr1.5−xTa0.5+xO12 (LLBZT) and Ba contents x are changed from 0 to 0.3. As a result, all LLBZT samples have a cubic garnet structure without containing any secondary phases. The lattice parameters of LLBZT decrease with increasing Ba 2+ contents x ≤ 0.10 while increase with x from 0.10 to 0.30, possibly due to the simultaneous change of Ba 2+ and Ta 5+ substitution levels. The relative densities of LLBZT are in a range between 89 and 93% and are not influenced in any significant way by the compositions. From the AC impedance spectroscopy measurements, the total (bulk + grain) conductivity at 27°C of LLBZT shows its maximum value of 8.34 × 10 −4 S cm −1 at x = 0.10, which is slightly higher than the conductivity (= 7.94 × 10 −4 S cm ) of LLZT without substituting Ba (x = 0). The activation energy of the conductivity tends to become lower by Ba substation, while excess Ba substitution degrades the conductivity in LLBZT. LLBZT has a wide electrochemical potential window of 0-6 V vs. Li + /Li, and Li + insertion and extraction reactions of TiNb2O7 film electrode formed on LLBZT by aerosol deposition are demonstrated at 60°C. The results indicate that LLBZT can potentially be used as a SE in all-solid-state batteries.Keywords: garnet-type oxide, solid electrolyte, ionic conductivity, all-solid-state battery, aerosol deposition inTrODUcTiOn Lithium-ion batteries (LIBs) consist of a graphite negative electrode, organic liquid electrolyte, and lithium transition-metal oxide. They were first commercialized in 1991, and since then such batteries have been widely distributed globally as a power source for mobile electronic devices, such as cell phones and laptop computers. Nowadays, large-scale LIBs have been developed for application to automotive propulsion and stationary loadleveling for intermittent power generation from solar or wind energy (Tarascon and Armand, 2001;Armand and Tarascon, 2008;Scrosati and Garche, 2010;Goodenough and Kim, 2011). However, increasing battery size creates more serious safety issues for LIBs;...

Formation and Stability of Interface between Garnet-Type Ta-doped Li7La3Zr2O12 Solid Electrolyte and Lithium Metal Electrode

“…In addition, the temperature dependence of σ was well-expressed by the Arrhenius equation as σ = σ0 exp(−Ea/(kBT)), where σ0 is constant, Ea is the activation energy of conductivity, and kB is the Boltzmann constant (1.381 × 10 −23 J K −1 ). From the slope data of σT in Figure 3b, Ea of LLZT was calculated as 0.40 eV, which is nearly the same as the Al-free LLZT reported in the literature [19,21,22].…”

Properties of Lithium Trivanadate Film Electrodes Formed on Garnet-Type Oxide Solid Electrolyte by Aerosol Deposition

“…LLZT pellets were prepared using a conventional solid-state reaction process reported in our previous work [19,35]. All starting materials were obtained from the Kojundo Chemical Laboratory (Saitama, Japan): stoichiometric amounts of LiOH·H 2 O (99%, 10% excess was added to account for the loss of Li at high temperatures), La(OH) 3 (99.99%), ZrO 2 (98%) and Ta 2 O 5 (99.9%), which were then pulverized and mixed by planetary ball-milling with zirconia balls (5 mm in diameter) and ethanol for 3 h in a zirconia pot, and then calcined at 900 °C for 6 h in air using a Pt-5% Au alloy crucible.…”

“…Deposition was carried out for 5–10 min and during the deposition process, the stage was moved uni-axially with a back-and-forth motion length of 50 mm and a speed of 10 mm s −1 . According to our previous works [33,35,38], the distance between a nozzle tip and a substrate and a flow rate of N 2 gas was set to 10 mm and 20 L min −1 .…”

“…Several works for the application of AD to battery materials have been already reported. The electrochemical properties of film-shaped electrodes of LiMn 2 O 4 [28], Si alloy or composite [29], LiFePO 4 [30], Li 4 Ti 5 O 12 [31], LiNi 1/3 Co 1/3 Mn 1/3 O 2 [32,33], Fe 2 O 3 [34], TiNb 2 O 7 [35] and LiNi 0.5 Mn 1.5 O 4 [36] formed on a metal and a SE substrate are investigated to verify the feasibility of AD. In addition, as-deposited oxide-based SE films with NASICON (Na Superionic Conductor) [37,38], perovskite [39] and garnet-type structure [40,41] show moderate Li + conductivity around 10 −7 –10 −5 S cm −1 at room temperature.…”

Lattice-geometry effects in garnet solid electrolytes: a lattice-gas Monte Carlo simulation study

“…The garnet lattice provides a model for diffusion pathways in the ‘lithium-garnets’, ${\mathrm{Li}}_{\mathrm{x}}{M}_3{M}_2^{\mathrm{\prime }}{\mathrm{normalO}}_{12}$ [29,30]. This family of solid lithium-ion electrolytes has attracted significant attention as candidate electrolytes for all-solid-state lithium-ion batteries [1,31–33]. The garnet crystal structure has an unusual three-dimensional network of lithium diffusion pathways, consisting of interlocking rings [34].…”