Parameswara Rao Chinnam

Self-assembled Janus-like multi-ionic lithium salts form nano-structured solid polymer electrolytes with high ionic conductivity and Li+ ion transference number

A solid polymer electrolyte with high ambient temperature conductivity, 4 × 10−4 S cm−1, and transference number, tLi+ = 0.6, is formed from blends of polyethylene oxide (PEO) and a multi-ionic polyoctahedral silsesquioxane lithium salt, POSS-phenyl7(BF3Li)3, with Janus-like properties. A two-phase morphology is proposed in which the hydrophobic phenyl groups cluster and crystallize, and the three –BF3− form an anionic pocket, with the Li+ ions solvated by the PEO phase. The high ionic conductivity results from interfacial migration of Li+ ions loosely bonded to three –BF3− anions and the ether oxygens of PEO. Physical crosslinks formed between PEO/Li+ chains and the POSS clusters account for the solid structure of the amorphous PEO matrix. The solid polymer electrolyte has an electrochemical stability window of 4.6 V and excellent interfacial stability with lithium metal.

Lamellar, micro-phase separated blends of methyl cellulose and dendritic polyethylene glycol, POSS-PEG.

Blends of methyl cellulose (MC) and liquid pegylated polyoctahedralsilsesquioxane (POSS-PEG) were prepared from non-gelled, aqueous solutions at room temperature (RT), which was below their gel temperatures (Tm). Lamellar, fibrillated films (pure MC) and increasingly micro-porous morphologies with increasing POSS-PEG content were formed, which had RT moduli between 1 and 5GPa. Evidence of distinct micro-phase separated MC and POSS-PEG domains was indicated by the persistence of the MC and POSS-PEG (at 77K) crystal structures in the X-ray diffraction data, and scanning transmission electron images. Mixing of MC and POSS-PEG in the interface region was indicated by suppression of crystallinity in the POSS-PEG, and increases/decreases in the glass transition temperatures (Tg) of POSS-PEG/MC in the blends compared with the pure components. These interface interactions may serve as cross-link sites between the micro-phase separated domains that permit incorporation of high amounts of POSS-PEG in the blends, prevent macro-phase separation and result in rubbery material properties (at high POSS-PEG content). Above Tg/Tm of POSS-PEG, the moduli of the blends increase with MC content as expected. However, below Tg/Tm of POSS-PEG, the moduli are greater for blends with high POSS-PEG content, suggesting that it behaves like semi-crystalline polyethylene oxide reinforced with silica (SiO1.5).

Highly Durable, Self-Standing Solid-State Supercapacitor Based on an Ionic Liquid-Rich Ionogel and Porous Carbon Nanofiber Electrodes

A high-performance, self-standing solid-state supercapacitor is prepared by incorporating an ionic liquid (IL)-rich ionogel made with 95 wt % IL (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) and 5 wt % methyl cellulose, a polymer matrix, into highly interconnected 3-D activated carbon nanofiber (CNF) electrodes. The ionogel exhibits strong mechanical properties with a storage modulus of 5 MPa and a high ionic conductivity of 5.7 mS cm-1 at 25 °C. The high-surface-area CNF-based electrode (2282 m2 g-1), obtained via an electrospinning technique, exhibits hierarchical porosity generated both in situ during pyrolysis and ex situ via KOH activation. The porous architecture of the CNF electrodes facilitates the facile percolation of the soft but mechanically durable ionogel film, thereby enabling intimate contact between porous nanofibers and the gel electrolyte interface. The supercapacitor demonstrates promising capacitive characteristics, including a gravimetric capacitance of 153 F g-1, a high specific energy density of 65 W h kg-1, and high cycling stability, with a capacitance fade of only 4% after 20 000 charge-discharge cycles at 1 A g-1. Moreover, device-level areal capacitances for the gel IL cell of 122 and 151 mF cm-2 are observed at electrode mass loadings of 3.20 and 5.10 mg cm-2, respectively.

A Self-Binding, Melt-Castable, Crystalline Organic Electrolyte for Sodium Ion Conduction

The preparation and characterization of the cocrystalline solid-organic sodium ion electrolyte NaClO4 (DMF)3 (DMF=dimethylformamide) is described. The crystal structure of NaClO4 (DMF)3 reveals parallel channels of Na+ and ClO4- ions. Pressed pellets of microcrystalline NaClO4 (DMF)3 exhibit a conductivity of 3×10-4  S cm-1 at room temperature with a low activation barrier to conduction of 25 kJ mol-1 . SEM revealed thin liquid interfacial contacts between crystalline grains, which promote conductivity. The material melts gradually between 55-65 °C, but does not decompose, and upon cooling, it resolidifies as solid NaClO4 (DMF)3 , permitting melt casting of the electrolyte into thin films and the fabrication of cells in the liquid state and ensuring penetration of the electrolyte between the electrode active particles.

Multi-ionic lithium salts increase lithium ion transference numbers in ionic liquid gel separators

Solid ion-gel separators for lithium or lithium ion batteries have been prepared with high lithium ion transference numbers (tLi+ = 0.36), high room temperature ionic conductivities (σ → 10−3 S cm−1), and moduli in the MPa range. These were formed from the room temperature ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PYR14TFSI), a polysilsesquioxane multi-ionic lithium salt that contains four phenyl groups and four pendent LiTFSI (4mer-(LiTFSI)4) on a SiO1.5 ring, and methyl cellulose (MC). After co-dissolution in dimethyl sulfoxide (DMSO) and evaporation of the DMSO, ion-gels were formed at different PYR14TFSI/MC ratios but at constant 0.20 M 4mer-(LiTFSI)4. Conductivity decreased but tLi+ increased with increased MC content. Differential scanning calorimetry, dynamical mechanical analysis and X-ray diffraction data indicated that the PYR14TFSI/MC/4mer-(LiTFSI)4 ion gels were micro-phase separated into a conductive PYR14TFSI/4mer-(LiTFSI)4 phase and one in which the MC was swollen with PYR14TFSI/4mer-(LiTFSI)4. The high tLi+ was attributed to the large size of the anion, its decreased ability as the result of its rigid structure to form ion aggregates, hydrophobic/hydrogen bonding interactions of the 4mer-(TFSI−)4 anion with MC, and participation of the MC hydroxyl groups in the solvation sphere of Li+, weakening its interaction with the 4mer-(TFSI−)4 and TFSI− anions. The high moduli were the result of the preserved semi-crystalline, high glass transition (Tg), fibrillar structure of MC in the ion gels.

An alternative route to single ion conductivity using multi-ionic salts

Multi-ionic lithium salts comprised of polyoligomeric silsesquioxanes (POSS) functionalized with eight – (LiNSO2CF3) groups, referred to as POSS-(LiNSO2CF3)8, can be dissolved at very high loadings into tetraglyme (G4), where they can be considered solvent-in-salt electrolytes. With increasing dilution, colloidal solutions are formed. Two systems were investigated, neat POSS-(LiNSO2CF3)8 in G4 and mixtures of POSS-(LiNSO2CF3)8 with LiPF6 or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). PFG-NMR indicates that Li can be un-dissociated, completely dissociated and surrounded by G4 molecules, or as contact ion pairs (in which there are 3–4 ether oxygen contacts and one contact with the oxygen from the anion). Equilibria exist between the species of POSS-(LiNSO2CF3)8 and if there is rapid equilibration between the Li states, and close enough proximity between the POSS-(LiNSO2CF3), then the Li+ ions can migrate by a Grotthus-type coordinated hopping mechanism, as well as by a purely diffusive motion. Unlike polymer single ion conductors, where the backbone flexibility permits cluster/aggregate formation, which inhibits escape and mobility of the Li+ ions, the rigid POSS cube and its colloidal structure in G4 prevents formation of POSS-(NSO2CF3−)⋯Li+⋯(−CF3NSO2)-POSS triplets. Instead, the solvated Li+ in POSS-(NSO2CF3−)⋯Li+⋯G4 can be more easily removed to form conductive G4⋯Li+−⋯G4. Good ionic conductivities (∼10−4 S cm−1) and lithium ion transference numbers of tPP+ = 0.65 can be achieved in these systems. Mixtures of 80 wt% LiTFSI and 20 wt% POSS-(LiNSO2CF3)8 in G4 at an O/Li ratio of 20/1, yield both high conductivity (σ = 3.3 × 10−3 S cm−1) and high (tPP+ = 0.65) transference number. Stable cycling between C/2 and 2C with high capacity retention was achieved using Li/[G4/80 wt% LiTFSI/20 wt% POSS-(LiNSO2CF3)8]/LiFePO4 half-cells.

Unravelling the structural and dynamical complexity of the equilibrium liquid grain-binding layer in highly conductive organic crystalline electrolytes

Recent developments in organic crystalline electrolytes for lithium and sodium ion conduction have demonstrated bulk conductivities in the range of 10−4 S cm−1 with negligible grain boundary resistance. Experimental results from EM, XRD, and DSC point to a liquid boundary layer at the crystalline surface in equilibrium with the bulk solid that conducts ions between the grains. In this report we examine this behavior in the electrolyte DMF·LiCl (DMF = N,N-dimethylformamide), which has a bulk conductivity of 1.6 × 10−4 S cm−1, but which decomposes between 360–380 K. Molecular dynamics simulations predict a number of quantitative parameters consistent with experimental observation, such as decomposition temperature (Td(theor) = 380 K, Td(obs) = 360 K), bulk conductivity (σheor = 7 × 10−4, σobs = 1.6 × 10−4) and density (dtheor = 1.209 g mL−1, dobs = 1.306 g mL−1). Further, a number of qualitative properties of the material are predicted by simulation, namely, the crystal packing arrangement, the mechanism of decomposition by expulsion of DMF from the LiCl lattice, the existence of a liquid-like grain boundary layer, and most importantly, negligible grain boundary resistance from increased mobility of ions in the boundary layer vs. the bulk. Finally, from quantum mechanical calculations, various interaction energies between fragmental components explain lattice stability and decomposition of the co-crystal, and highlight the contributions from various possible small aggregates. The theoretical calculations predict decomposition of smaller aggregates, such as those expected in the liquid-like surface, to be more facile than larger aggregates that are more likely to be found in the crystal interior.

Systematic Doping of Cobalt into Layered Manganese Oxide Sheets Substantially Enhances Water Oxidation Catalysis

The effect on the electrocatalytic oxygen evolution reaction (OER) of cobalt incorporation into the metal oxide sheets of the layered manganese oxide birnessite was investigated. Birnessite and cobalt-doped birnessite were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and conductivity measurements. A cobalt:manganese ratio of 1:2 resulted in the most active catalyst for the OER. In particular, the overpotential (η) for the OER was 420 mV, significantly lower than the η = 780 mV associated with birnessite in the absence of Co. Furthermore, the Tafel slope for Co/birnessite was 81 mV/dec, in comparison to a Tafel slope of greater than 200 mV/dec for birnessite. For chemical water oxidation catalysis, an 8-fold turnover number (TON) was achieved (h = 70 mmol of O2/mol of metal). Density functional theory (DFT) calculations predict that cobalt modification of birnessite resulted in a raising of the valence band edge and occupation of that edge by holes with enhanced mobility during catalysis. Inclusion of extra cobalt beyond the ideal 1:2 ratio was detrimental to catalysis due to disruption of the layered structure of the birnessite phase.

The polyoctahedral silsesquioxane (POSS) 1,3,5,7,9,11,13,15-octaphenylpentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane (octaphenyl-POSS)

Solvent-free single crystals of 1,3,5,7,9,11,13,15-octaphenylpentacyclo[9.5.1.1(3,9).1(5,15).1(7,13)]octasiloxane (abbreviated as octaphenyl-POSS), C48H40O12Si8, were obtained by dehydration/condensation of the tetrol Si4O4(Ph)4(OH)4. The powder pattern generated from the single-crystal data matches well with the experimentally measured powder pattern of commercial octaphenyl-POSS. The geometry of the centrosymmetric molecule in the crystal was compared with that in the gas phase, and had shorter Si-O bond lengths and a broader range of Si-O-Si bond angles. The average Si-O bond length [1.621 (3) Å], and Si-O-Si and O-Si-O bond angles [149 (5) and 109 (1)°, respectively] were within the same range measured previously for octaphenyl-POSS solvates.