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

A set of Li x SiON ( 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 [(C 6 H 14 O 2 ) 2 Si, SP] at 200 °C. SP can be lithiated using controlled amounts of LiNH 2 to produce Li x SiON 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 LiNH 2 but found N contents are only ≤1 at%. NH 2 likely is removed as NH 3 during sample preparation (vacuum/overnight). In contrast, MALDI indicates N contents of ∼5–30 at% N with shorter drying times (vacuum/minutes). 7 Li NMR positive chemical shifts suggest that precursor bound Li + ions dissociate easily, beneficial for electrochemical applications. The 7 Li shifts correlate to Li contents as well as Li + conductivities. 1 H, 13 C and 29 Si NMRs of the Li 6 SiON precursor show fluxional behavior implying high Li + mobility. Dense microstructures are observed for Li 4 SiON and Li 6 SiON pellets heated to 200 °C/2 h/N 2 . Impedance studies suggest that ionic conductivities increase with Li content; the Li 6 SiON precursor offers the highest ambient conductivity of 8.5 × 10 −6 S cm −1 after heating to 200 °C/2 h/N 2 .