By
dispersing Li6.25Ga0.25La3Zr2O12 (Ga-LLZO) nanoparticles in poly(ethylene oxide)
(PEO) matrix, PEO:Ga-LLZO composite polymer electrolytes are synthesized.
The PEO: Ga-LLZO composite with 16 vol % Ga-LLZO nanoparticles shows
a conductivity of 7.2 × 10–5 S cm–1 at 30 °C, about 4 orders of magnitude higher than the conductivity
of PEO. The enhancement of the ionic conductivity is closely related
to the space charge region (∼3 nm) formed at the interface
between the PEO matrix and the Ga-LLZO nanoparticles. The space charge
region is observed by transmission electron microscope (TEM) and corroborated
by the phase-field simulation. Using the random resistor model, the
lithium-ion transport in the composite polymer electrolyte is simulated
by the Monte Carlo simulation, demonstrating that the enhanced ionic
conductivity can be ascribed to the ionic conduction in the space
charge regions and the percolation of the space charge regions.
Owing to their high conductivity, crystalline Li 7-3x Ga x La 3 Zr 2 O 12 garnets are promising electrolytes for allsolid-state lithium-ion batteries. Herein, the influence of Ga doping on the phase, lithium-ion distribution, and conductivity of Li 7-3x Ga x La 3 Zr 2 O 12 garnets is investigated, with the determined concentration and mobility of lithium ions shedding light on the origin of the high conductivity of Li 7-3x Ga x La 3 Zr 2 O 12 . When the Ga concentration exceeds 0.20 Ga per formula unit, the garnet-type material is found to assume a cubic structure, but lower Ga concentrations result in the coexistence of cubic and tetragonal phases. Most lithium within Li 7-3x Ga x La 3 Zr 2 O 12 is found to reside at the octahedral 96h site, away from the central octahedral 48g site, while the remaining lithium resides at the tetrahedral 24d site. Such kind of lithium distribution leads to high lithium-ion mobility, which is the origin of the high conductivity; the highest lithium-ion conductivity of 1.46 mS/cm at 25 °C is found to be achieved for Li 7-3x Ga x La 3 Zr 2 O 12 at x = 0.25. Additionally, there are two lithium-ion migration pathways in the Li 7-3x Ga x La 3 Zr 2 O 12 garnets: 96h-96h and 24d-96h-24d, but the lithium ions transporting through the 96h-96h pathway determine the overall conductivity.
Disciplines
Engineering | Physical Sciences and Mathematics
Introduction
of inorganic solid electrolytes is believed to be an ultimate strategy
to dismiss dendritic Li in high-energy Li-metal batteries (LMBs),
and garnet-type Li7La3Zr2O12 (LLZO) electrolytes are impressive candidates. However, the current
density for stable Li plating/stripping in LLZO is still quite limited.
Here, we create in situ formed Li-deficient shields by the high-temperature
calcination at 900 °C. By this novel process, the formation of
Li2CO3 on LLZO is restrained, and then we successfully
obtain Li2CO3-free LLZO after removing the Li-deficient
compounds. Without any surface modification, Li2CO3-free LLZO shows an intrinsic “lithiophilicity”
characteristic. The contact angles of metallic Li on LLZO garnets
are assessed by the first-principle calculation to confirm the lithiophilicity
characteristic of LLZO electrolytes. The wetting of metallic Li on
the Li2CO3-free LLZO surface leads to a continuous
and tight Li/LLZO interface, resulting in an ultralow interfacial
resistance of 49 Ω cm2 and a homogeneous current
distribution in the charge/discharge processes of LMBs. Consequently,
the current density for the stable Li plating/stripping in LLZO increases
to 900 μA cm–2 at 60 °C, one of the highest
current density for LMBs based on garnet-type LLZO electrolytes. Our
findings not only offer insight into the lithiophilicity characteristics
of LLZO electrolytes to suppress dendritic Li at high current densities
but also expand the avenue toward high-performance, safe, and long-life
energy-storage systems.
All-solid-state Li-ion batteries with metallic Li anodes and solid electrolytes could offer superior energy density and safety over conventional Li-ion batteries. However, compared with organic liquid electrolytes, the low conductivity of solid electrolytes and large electrolyte/electrode interfacial resistance impede their practical application. Garnet-type Li-ion conducting oxides are among the most promising electrolytes for all-solid-state Li-ion batteries. In this work, the large-radius Rb is doped at the La site of cubic LiGaLaZrO to enhance the Li-ion conductivity for the first time. The LiGaLaRbZrO electrolyte exhibits a Li-ion conductivity of 1.62 mS cm at room temperature, which is the highest conductivity reported until now. All-solid-state Li-ion batteries are constructed from the electrolyte, metallic Li anode, and LiFePO active cathode. The addition of Li(CFSO)N electrolytic salt in the cathode effectively reduces the interfacial resistance, allowing for a high initial discharge capacity of 152 mAh g and good cycling stability with 110 mAh g retained after 20 cycles at a charge/discharge rate of 0.05 C at 60 °C.
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