All-Solid-State Lithium-Ion Batteries Using the Li₃AlF₆-Based Composite as the Solid Electrolyte

Abstract

All-solid-state lithium-ion batteries (LIBs) are considered promising energy storage devices owing to their high energy density and safety. The development of solid electrolytes with high Li+ conductivity, wide electrochemical stability window, air stability, and favorable mechanical properties directly leads to the realization of high-performance all-solid-state LIBs. Fluoride-based materials are potential candidates that meet these requirements. In the present study, Li+ conductivity and the electrochemical performance of β-Li3AlF6-based composites have been reported. The amorphous Li3AlF6–Li2SO4 composites were fabricated via planetary ball milling. The conductivity of Li3AlF6–Li2SO4 (Li2SO4: 50 mol %), when ball-milled for 70 h, was 6 × 10–4 S/cm at 150 °C. The conductivity did not degrade when Li3AlF6–Li2SO4 was kept at 150 °C for 12 h. In the cyclic voltammetry measurements, the Li plating/stripping current was clearly observed at 0 V (vs Li+/Li), and the continuous anodic decomposition of Li3AlF6–Li2SO4 was not confirmed up to 5 V (vs Li+/Li). Using Li3AlF6–Li2SO4 as the solid electrolyte, the all-solid-state LIB of graphite/Li (Ni0.3Co0.6Mn0.1)O2 was fabricated by uniaxial pressing without sintering. The battery was fabricated and its performance was evaluated at 120 °C in a dry room, where the dew point was maintained at −40 °C. A discharge capacity of 99 mAh/g was obtained at 0.1 C, which was maintained at 88 mAh/g at 1 C. The charge–discharge cycles were stable, and drastic capacity fading was not observed for 47 cycles at 1 C. The results indicate that fluoride-based materials are promising candidates for solid electrolytes in all-solid-state LIBs.

Enhancement of the Li+ Conductivity of Li3AlF6 for Stable All-Solid-State Lithium-Ion Batteries

“…Li 3 AlF 6 was fabricated following a previously reported method. , LiF and AlF 3 (Kojundo Chemical Lab. Co., Ltd.) were mixed in a molar ratio of 3:1 and fused at 900 °C for 15 min in the electric furnace set in a glovebox.…”

“…The Arrhenius plot of the conductivity of 4Li 3 AlF 6 •Li 2 SiF 6 is shown in Figure 3a together with previously reported results for ball-milled Li 3 AlF 6 and Li 3 AlF 6 • Li 2 SO 4 . 33 The conductivities of 4Li 3 AlF 6 •Li 2 SiF 6 were higher than those of Li 3 AlF 6 •Li 2 SO 4 , which were further enhanced after the ball-milling time. As shown in Figure 1, Li 3 AlF 6 − Li 2 SiF 6 maintained its crystalline state although the crystallite size was decreased to approximately 9 nm (Figure S3), which is different compared with previous results on amorphous Li 3 AlF 6 −Li 2 SO 4 .…”

“…33 The battery was constructed by uniaxial pressing in a dry room, indicating an excellent powder pressing ability and a chemical stability under an oxygen atmosphere. 33 Although the cycle performances were moderately stable, the battery operation was limited to 120 °C due to the lowering of the conductivity at room temperature. 33 Hence, enhancement of the conductivity of Li 3 AlF 6 affords nontoxic high performance all-solid-state LIBs with air stability and high pressing ability, which are distinctive features of fluoride materials.…”

“…33 Although the cycle performances were moderately stable, the battery operation was limited to 120 °C due to the lowering of the conductivity at room temperature. 33 Hence, enhancement of the conductivity of Li 3 AlF 6 affords nontoxic high performance all-solid-state LIBs with air stability and high pressing ability, which are distinctive features of fluoride materials. In this study, Li 2 SiF 6 was doped to Li 3 AlF 6 to introduce Li + vacancies, and variation of the crystalline phase of Li 3 AlF 6 was investigated.…”

“…As shown in Figure 1, Li 3 AlF 6 − Li 2 SiF 6 maintained its crystalline state although the crystallite size was decreased to approximately 9 nm (Figure S3), which is different compared with previous results on amorphous Li 3 AlF 6 −Li 2 SO 4 . 33 Hence, one possible explanation for the conductivity enhancement is the increase in the Li + vacancies in the Li 3 AlF 6 crystal lattice. When one Si 4+ ion replaces the Al 3+ ion in the AlF 6…”