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We report here extracting SiO2 as spirosiloxane [(CH3)2C(O)CH2CH(O)CH3]2Si from rice hull ash (RHA) to carefully control the SiO2 : C mole ratios, allowing direct carbothermal reduction to SiC, Si3N4, or Si2N2O without the need to add extra carbon and as a mechanism to preserve the original nanocomposite structure. We can adjust SiO2 : C ratios from 2 : 15 to 13 : 35 simply by reacting RHA with hexylene glycol (HG) with catalytic base to distillatively extract SiO2 to produce silica depleted RHA (SDRHA) with SiO2 contents of 40–65 wt% and corresponding carbon contents of 60–35 wt% with specific surface areas (SSAs) of >400 m2 g−1. On heating SDRHA40–65 at 1400–1500 °C in an Ar, N2, or N2–H2 atmosphere, XRD patterns reveal formation of SiC, Si3N4, or Si2N2O as the major phase with some residual hard carbon. SEM studies reveal mixtures of particles and whiskers in the products, which show BET specific surface areas >40 m2 g−1 after oxidative removal of excess carbon. Dilute acid and boiling water prewashing of RHA with milling eliminates typical product impurities compared to those found using conventional carbothermal reduction of agricultural wastes, which qualifies the resulting composites as components for electrochemical energy storage devices among other applications, to be reported elsewhere. 

Electrochemical energy storage is a cost-effective, sustainable method for storing and delivering energy gener- ated from renewable resources. Among electrochemical energy storage devices, the lithium-ion battery (LIB) has dominated due to its high energy and power density. The success of LIBs has generated increased interest in sodium-ion battery (NaB) technology amid concerns of the sustainability and cost of lithium resources. In recent years, numerous studies have shown that sodium-ion solid-state electrolytes (NaSEs) have considerable potential to enable new cell chemistries that can deliver superior electrochemical performance to liquid-electrolyte-based NaBs. However, their commercial implementation is hindered by slow ionic transport at ambient and chemical/ mechanical incompatibility at interfaces. In this review, various NaSEs are first characterized based on individual crystal structures and ionic conduction mechanisms. Subsequently, selected methods of modifying interfaces in sodium solid-state batteries (NaSSBs) are covered, including anode wetting, ionic liquid (IL) addition, and composite polymer electrolytes (CPEs). Finally, examples are provided of how these techniques improve cycle life and rate performance of different cathode materials including sulfur, oxide, hexacyanoferrate, and phosphate-type. A focus on interfacial modification and optimization is crucial for realizing next-generation batteries. Thus, the novel methods reviewed here could pave the way toward a NaSSB capable of with- standing the high current and cycle life demands of future applications. 

Biomass-derived materials offer low carbon approaches to energy storage. High surface area SiC w/wo 13 wt% hard carbon (SiC/HC, SiC/O), derived from carbothermal reduction of silica depleted rice hull ash (SDRHA), can function as Li+ battery anodes. Galvanostatic cycling of SiC/HC and SiC/O shows capacity increases eventually to >950 mA h g−1 (Li1.2–1.4SiC) and >740 mA h g−1 (Li1.1SiC), respectively, after 600 cycles. Post-mortem investigation via XRD and 29Si MAS NMR reveals partial phase transformation from 3C- to 6H-SiC, with no significant changes in unit cell size. SEMs show cycled electrodes maintain their integrity, implying almost no volume expansion on lithiation/delithiation, contrasting with >300% volume changes in Si anodes on lithiation. Significant void space is needed to compensate for these volume changes with Si in contrast to SiC anodes suggesting nearly competitive capacities. 6Li MAS NMR and XPS show no evidence of LixSi, with Li preferring all-C environments supported by computational modeling. Modeling also supports deviation from the 3C phase at high Li contents with minimal volume changes. 

The electrochemical performance of LixSiON (x = 2, 4, and 6) polymer electrolytes derived from the agricultural waste, rice hull ash (RHA, 80−90 wt % SiO2), is reported. Silica can be extracted from RHA by base-catalyzed reaction with hexylene glycol forming the spirosiloxane [(C6H12O2)2Si, SP] that distills from the reaction solution. LixSiON polymer electrolytes form on reacting SP with xLiNH2, offering a low-cost, low- temperature, and green synthesis route. The effect of N and Li+ concentrations in the polymer electrolytes are correlated with ionic and electrical conductivity. X-ray photoelectron spectroscopy studies confirm that N and Li contents increase with increasing LiNH2 content. The amorphous nature and high Li+ contents of the Li6SiON electrolyte provide an optimal ionic conductivity (6.5 × 10−6) at ambient temperature when coated on Celgard. Furthermore, the LixSiON polymer electrolytes offer high Li+ transference numbers (∼0.75−1), enabling assembly of Li symmetric cells with high critical current densities (3.75 mA cm−2). Finally, Li-SPAN (sulfurized, carbonized polyacrylonitrile) half-cells with Li6SiON polymer electrolytes deliver discharge capacities of ∼765 and 725 mAh/g at 0.25 and 0.5 C rates over 50 cycles. 

A set of LixSiON (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 LixSiON 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, ben- eficial 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. 

Li4Ti5O12 (LTO) has received considerable interest as an alternate anode material for high power density batteries for large scale applications. However, LTO suffers from poor Li+ diffusivity and poor electronic conductivity, resulting in capacity loss and poor rate performance. Here we demonstrate a facile synthesis of LTO NPs using liquid-feed flame spray pyrolysis (LF-FSP) which provides high surface area (∼38 m2/g) spinel structure LTO NPs with average particle sizes (APSs) of 45 ± 0.3 nm. Pristine LTO-Li half-cells exhibit reversible capacity of 70 mAh/g at 10 C. In this study, we show that mixing LiAlO2 NPs (5 wt %) and Li6SiON polymer precursor (10 wt %) with pristine LTO via ball-milling and ultrasonication followed by tape casting enhances the LTO rate performance providing reversible capacity of ∼217 mAh/g at 5 C over 500 cycles. The Li6SiON polymer electrolyte is synthesized from rice hull ash (RHA), an agricultural waste, providing a green synthetic approach to electrode coating materials. CV and EIS studies indicate that adding the solid and polymer electrolytes reduces charge-transfer resistance and electrode polarization, enhancing both reversibility and the LTO Li+ diffusion coefficient from 4.6 × 10−14 to 2.7 × 10−12 cm2/s. 

We report here efforts to synthesize free-standing, dry polymer electrolytes that exhibit superior ionic conductivities at ambient for Li−S batteries. Co-dissolution of poly(ethylene oxide) (PEO) (Mn 900k) with LixPON and LixSiPON polymer systems at a ratio of approximately 3:2 followed by casting provides transparent, solid-solution films 25−50 μm thick, lowering PEO crystallinity, and providing measured impedance values of 0.1−2.8 × 10−3 S/cm at ambient. These values are much higher than simple PEO/Li+ salt systems. These solid-solution polymer electrolytes (PEs) are (1) thermally stable to 100 °C; (2) offer activation energies of 0.2−0.5 eV; (3) suppress dendrite formation; and (4) enable the use of lithium anodes at current densities as high as 3.5 mAh/cm2. Galvanostatic cycling of SPAN/PEs/Li cell (SPAN = sulfurized, carbonized polyacrylonitrile) shows discharge capacities of 1000 mAh/gsulfur at 0.25C and 800 mAh/gsulfur at 1C with high coulumbic efficiency over 100 cycles. 

Efforts to develop polymer precursor electrolytes that offer properties anticipated to be similar or superior to (lithium phosphorus oxynitride, LiPON) glasses are reported. Such precursors offer the potential to be used to process LiPON-like thin glass/ceramic coatings for use in all solid state batteries, ASBs. Here, LiPON glasses provide a design basis for the synthesis of sets of oligomers/polymers by lithiation of OP(NH2)3−x(NH)x [from OP(NH)3],OP-(NH2)3‑x(NHSiMe3)x and [PN]3(NHSiMe3)6−x(NH)x. The resulting systems have degrees of polymerization of 5−20. Treatment with selected amounts of LiNH2 provides varying degrees of lithiation and Li+ conducting properties commensurate with Li+ content. Polymer electrolytes impregnated in/on Celgard exhibit Li+ conductivities up to ∼1 × 10−5S cm−1 at room temperature and are thermally stable to ∼150 °C. A Li−S battery assembled using a Li6SiPON composition polymer electrolyte exhibits an initial reversible capacity of 1500 mAhgsulfur −1 and excellent cycle performance at 0.25 and 0.5 C rate over 120 cycles at room temperature. 

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