Free-Standing, Robust, and Stable Li⁺ Conductive Li(Sr,Zr)₂(PO₄)₃/PEO Composite Electrolytes for Solid-State Batteries

Abstract

We developed promising soft–rigid and free-standing composite electrolytes by mixing a Sr-doped LiZr2(PO4)3 compound with a poly­(ethylene oxide) (PEO)-based polymer for Li+-ion conductors. Using crystal formation modeling, we synthesized a thermodynamically stable Li­(Sr,Zr)2(PO4)3 compound with the formation of composite electrolytes via PEO-based in situ radical polymerization, which possessed a good ionic conductivity (5.75 × 10–4 S cm–1). The cell with Li/composite electrolyte/LiFePO4 showed a capacity retention rate of 80% after 100 cycles at room temperature. The corresponding composite electrolyte film demonstrated superior electrochemical behavior even when assembling pouch-type cells without any interfacial control during the cell assembly, that is, without post-treatment such as soaking liquid electrolytes or over-coated interlayers. This suggested that the optimal phosphate composition followed by the formation of a PEO-based polymer electrolyte will be suitable for Li+-ion transporting films and/or the Li metal protection layer for solid-state Li+-ion rechargeable batteries operating at room temperature.

Na superionic conductor-type LiZr2(PO4)3 as a promising solid electrolyte for use in all-solid-state Li metal batteries

“…42 Another example is that the difference between the bulk Li-ion conductivity synthesized by sol-gel reaction is B100 times as high as the total conductivity according to the literature. 93 As the GB conductivity is strongly dependent on the process of synthesis, various preparation methods have been applied to reduce the GB and/or interface resistance for LZP and related compounds 38,94 and the other NASICON-type oxides. [95][96][97] The relationships among the composition, crystal structure, presence of the impurity phase, relative density of the sintered body, and ionic conductivity are quite complex.…”

“…12 Also, control of sintering degree and stabilization of a-phase, i.e. process optimization 38,[94][95][96][97] would be effective to improve ionic conductivity as well as composition optimization.…”

“…28 As in the case of LATP, doping metal cations to stoichiometric LZP significantly improves the Li-ion conductivity. For example, LZP doped with SrO, 27,29,38 CaO, 31,32 Y 2 O 3 , 33,39 Al 2 O 3 , 34 and As 2 O 5 has been reported to show improved Li-ion conductivity up to 100 times that of non-doped LZP. The common feature in these systems with a significant increase in ionic conductivity is when the molar ratio of Li ions to the formula unit (LiZr 2 (PO 4 ) 3 ) exceeds 1, indicating that the introduction of Li ions at the interstitial site by doping with aliovalent ions is effective.…”