LixSiON (x = 2, 4, 6): a novel solid electrolyte system derived from agricultural waste

Abstract

A set of Li_xSiON (x = 2, 4, 6) polymer precursors to a novel solid-state electrolyte system were synthesized starting from rice hull ash (RHA), an agricultural waste, providing a green route towards the assembly of all solid-state batteries (ASSBs). Silica, ∼90 wt% in RHA, can be catalytically (alkali base) dissolved (20–40 wt%) in hexylene glycol (HG) and distilled directly from the reaction mixture as the spirosiloxane [(C₆H₁₄O₂)₂Si, SP] at 200 °C. SP can be lithiated using controlled amounts of LiNH₂ to produce Li_xSiON oligomers/polymers with MWs up to ∼1.5 kDa as characterized by FTIR, MALDI-ToF, multinuclear NMR, TGA-DTA, XRD, XPS, SEM and EDX. XPS analyses show that Li contents depend solely on added LiNH₂ but found N contents are only ≤1 at%. NH₂ likely is removed as NH₃ during sample preparation (vacuum/overnight). In contrast, MALDI indicates N contents of ∼5–30 at% N with shorter drying times (vacuum/minutes). ⁷Li NMR positive chemical shifts suggest that precursor bound Li⁺ ions dissociate easily, beneficial for electrochemical applications. The ⁷Li shifts correlate to Li contents as well as Li⁺ conductivities. ¹H, ¹³C and ²⁹Si NMRs of the Li₆SiON precursor show fluxional behavior implying high Li⁺ mobility. Dense microstructures are observed for Li₄SiON and Li₆SiON pellets heated to 200 °C/2 h/N₂. Impedance studies suggest that ionic conductivities increase with Li content; the Li₆SiON precursor offers the highest ambient conductivity of 8.5 × 10⁻⁶ S cm⁻¹ after heating to 200 °C/2 h/N₂.