Combined Electrochemical, XPS, and STXM Study of Lithium Nitride as a Protective Coating for Lithium Metal and Lithium-Sulfur Batteries.

Li3N is an excellent protective coating material for lithium electrodes with very high lithium-ion conductivity and low electronic conductivity, but the formation of stable and homogeneous coatings is technically very difficult. Here, we show that protective Li3N coatings can be simply formed by the direct reaction of electrodeposited lithium electrodes with N2 gas, whereas using battery-grade lithium foil is problematic due to the presence of a native passivation layer that hampers that reaction. The protective Li3N coating is effective at preventing lithium dendrite formation, as found from unidirectional plating and plating-stripping measurements in Li-Li cells. The Li3N coating also efficiently suppresses the parasitic reactions of polysulfides and other electrolyte species with the lithium electrode, as demonstrated by scanning transmission X-ray microscopy, X-ray photoelectron spectroscopy, and optical microscopy. The protection of the lithium electrode against corrosion by polysulfides and other electrolyte species, as well as the promotion of smooth deposits without dendrites, makes the Li3N coating highly promising for applications in lithium metal batteries, such as lithium-sulfur batteries. The present findings show that the formation of Li3N can be achieved with lithium electrodes covered by a secondary electrolyte interface layer, which proves that the in situ formation of Li3N coatings inside the batteries is attainable.

Solvothermal synthesis of nanoscale BaTiO3 in benzyl alcohol-water mixtures and effects of manganese oxide coating to enhance the PTCR effect.

A solvothermal method using various benzyl alcohol/water solvent mixtures has been used to synthesise phase pure nanocrystalline BaTiO3 samples with varying particle sizes in the range of 11-139 nm. The crystallite/particle size of BaTiO3 shows an overall decrease as the benzyl alcohol percentage increases, especially at higher percentages (≥80%) of benzyl alcohol. The decrease in crystallite/particle size can be attributed to the increased viscosity of the solvent mixture when raising the percentage of benzyl alcohol. A manganese oxide coating applied to the BaTiO3 surface had a negligible impact on its microstructure and morphology, but significantly enhanced the observed positive temperature coefficient of resistance. This research has been carried out to allow the development of smaller BaTiO3 particles for use in new battery, capacitor and thermistor technologies, whilst maintaining the PTCR property of the material that is typically observed in larger particle sizes.

Effects of the reaction temperature and Ba/Ti precursor ratio on the crystallite size of BaTiO3 in hydrothermal synthesis.

Nanocrystalline BaTiO3 has been prepared by hydrothermal synthesis from titanium isopropoxide and barium hydroxide octahydrate. Reaction conditions including synthesis temperature and Ba/Ti precursor ratio have been explored with the aim of producing BaTiO3 with small crystallites and a low concentration of defects. It has been found that the crystallite/particle size and tetragonality of the BaTiO3 samples increase as the synthesis temperature increases; and the crystallite/particle size of BaTiO3 is also affected by the Ba/Ti precursor ratio. The BaTiO3 sample synthesised using a Ba/Ti precursor ratio of 2 : 1 at a reaction temperature of 120 °C exhibited homogeneous crystallites of the smallest size of 107 nm. Additionally, the Ba/Ti precursor ratio of 2 : 1 with synthesis temperature of 220 °C was found to produce a smaller concentration of defects in BaTiO3.

Two electrolyte decomposition pathways at nickel-rich cathode surfaces in lithium-ion batteries.

Preventing the decomposition reactions of electrolyte solutions is essential for extending the lifetime of lithium-ion batteries. However, the exact mechanism(s) for electrolyte decomposition at the positive electrode, and particularly the soluble decomposition products that form and initiate further reactions at the negative electrode, are still largely unknown. In this work, a combination of operando gas measurements and solution NMR was used to study decomposition reactions of the electrolyte solution at NMC (LiNi x Mn y Co1-x-y O2) and LCO (LiCoO2) electrodes. A partially delithiated LFP (Li x FePO4) counter electrode was used to selectively identify the products formed through processes at the positive electrodes. Based on the detected soluble and gaseous products, two distinct routes with different onset potentials are proposed for the decomposition of the electrolyte solution at NMC electrodes. At low potentials (<80% state-of-charge, SOC), ethylene carbonate (EC) is dehydrogenated to form vinylene carbonate (VC) at the NMC surface, whereas at high potentials (>80% SOC), 1O2 released from the transition metal oxide chemically oxidises the electrolyte solvent (EC) to form CO2, CO and H2O. The formation of water via this mechanism was confirmed by reacting 17O-labelled 1O2 with EC and characterising the reaction products via 1H and 17O NMR spectroscopy. The water that is produced initiates secondary reactions, leading to the formation of the various products identified by NMR spectroscopy. Noticeably fewer decomposition products were detected in NMC/graphite cells compared to NMC/Li x FePO4 cells, which is ascribed to the consumption of water (from the reaction of 1O2 and EC) at the graphite electrode, preventing secondary decomposition reactions. The insights on electrolyte decomposition mechanisms at the positive electrode, and the consumption of decomposition products at the negative electrode contribute to understanding the origin of capacity loss in NMC/graphite cells, and are hoped to support the development of strategies to mitigate the degradation of NMC-based cells.

Negating the Interfacial Resistance between Solid and Liquid Electrolytes for Next-Generation Lithium Batteries.

The combination of solid and liquid electrolytes enables the development of safe and high-energy batteries where the solid electrolyte acts as a protective barrier for a high-energy lithium metal anode, while the liquid electrolyte maintains facile electrochemical reactions with the cathode. However, the contact region between the solid and liquid electrolytes is associated with a very high resistance, which severely limits the specific energy that can be practically delivered. In this work, we demonstrate a suitable approach to virtually suppress such interfacial resistance. Using a NASICON-type solid electrolyte in a variety of liquid electrolytes (ethers, DMSO, acetonitrile, ionic liquids, etc.), we show that the addition of water as electrolyte additive decreases the interfacial resistance from >100 Ω cm2 to a negligible value (<5 Ω cm2). XPS measurements reveal that the composition of the solid-liquid electrolyte interphase is very similar in wet and dry liquid electrolytes, and thus the suppression of the associated resistance is tentatively ascribed to a plasticizer or preferential ion solvation effect of water, or to a change in the interphase morphology or porosity caused by water. Our simple estimates show that the improvement in the solid-liquid electrolyte interphase resistance observed here could translate to an enhancement of 15-22% in the practical energy density of a Li-S or Li-O2 battery and improvements in the roundtrip efficiency of 21-28 percentage points.

LiFePO4 Battery Material for the Production of Lithium from Brines: Effect of Brine Composition and Benefits of Dilution.

Lithium battery materials can be advantageously used for the selective sequestration of lithium ions from natural resources, which contain other cations in high excess. However, for practical applications, this new approach for lithium production requires the battery host materials to be stable over many cycles while retaining the high lithium selectivity. Here, a nearly symmetrical cell design was employed to show that LiFePO4 shows good capacity retention with cycling in artificial lithium brines representative of brines from Chile, Bolivia and Argentina. A quantitative correlation was identified between brine viscosity and capacity degradation, and for the first time it was demonstrated that the dilution of viscous brines with water significantly enhanced capacity retention and rate capability. The electrochemical and X-ray diffraction characterisation of the cycled electrodes also showed that the high lithium selectivity was preserved with cycling. Raman spectra of the cycled electrodes showed no signs of degradation of the carbon coating of LiFePO4 , while scanning electron microscopy images showed signs of particle cracking, thus pointing towards interfacial reactions as the cause of capacity degradation.

Facilitating Charge Reactions in Al-S Batteries with Redox Mediators.

The Al-S battery is a promising next-generation battery candidate due to high abundance of both aluminium and sulfur. However, the sluggish kinetics of the Al-S battery reactions produces very high overpotentials. Here, for the first time, it was demonstrated that the incorporation of redox mediators could dramatically improve the kinetics of Al-S batteries. On the example of iodide redox mediators, it was shown that the charging voltage of Al-S batteries could be decreased by about 0.23 V with as little as 2.3 wt% of redox mediator added as electrolyte additive. Control electrochemical measurements, without prior discharge of the battery, demonstrated that >97 % of the charge capacity was due to the desired oxidation of Al2 S3 and polysulfides, and X-ray diffraction experiments confirmed the formation of sulfur as the final charge product. The beneficial role of redox mediators was demonstrated with cheap and environmentally friendly electrolytes made of urea and AlCl3 . This work showed that dramatic performance improvements could be achieved with low concentration of electrolyte additives, and therefore, much further performance improvements could be sought by combining multiple additives.

A Critical Evaluation of the Effect of Electrode Thickness and Side Reactions on Electrolytes for Aluminum-Sulfur Batteries.

The high abundance and low cost of aluminum and sulfur make the Al-S battery an attractive combination. However, significant improvements in performance are required, and increasing the thickness and sulfur content of the sulfur electrodes is critical for the development of batteries with competitive specific energies. This work concerns the development of sulfur electrodes with the highest sulfur content (60 wt %) reported to date for an Al-S battery system and a systematic study of the effect of the sulfur electrode thickness on battery performance. If low-cost electrolytes made from acetamide or urea are used, slow mass transport of the electrolyte species is identified as the main cause of the poor sulfur utilization when the electrode thickness is decreased, whereas complete sulfur utilization is achieved with a less viscous ionic liquid. In addition, the analysis of very thin electrodes reveals the occurrence of degradation reactions in the low-cost electrolytes. The new analysis method is ideal for evaluating the stability and mass transport limitations of novel electrolytes for Al-S batteries.

Current Challenges and Routes Forward for Nonaqueous Lithium-Air Batteries.

Nonaqueous lithium-air batteries have garnered considerable research interest over the past decade due to their extremely high theoretical energy densities and potentially low cost. Significant advances have been achieved both in the mechanistic understanding of the cell reactions and in the development of effective strategies to help realize a practical energy storage device. By drawing attention to reports published mainly within the past 8 years, this review provides an updated mechanistic picture of the lithium peroxide based cell reactions and highlights key remaining challenges, including those due to the parasitic processes occurring at the reaction product-electrolyte, product-cathode, electrolyte-cathode, and electrolyte-anode interfaces. We introduce the fundamental principles and critically evaluate the effectiveness of the different strategies that have been proposed to mitigate the various issues of this chemistry, which include the use of solid catalysts, redox mediators, solvating additives for oxygen reaction intermediates, gas separation membranes, etc. Recently established cell chemistries based on the superoxide, hydroxide, and oxide phases are also summarized and discussed.

Synthesis of Hard Carbon-TiN/TiC Composites by Reacting Cellulose with TiCl4 Followed by Carbothermal Nitridation/Reduction.

Titanium tetrachloride is reacted with hydroxide groups on cellulose (cotton wool) before firing to convert the cellulose to hard carbon. Hard carbon-nanocrystalline titanium nitride composites with a good distribution of the titanium across the fibrous hard carbon structure were obtained by firing the treated cellulose under nitrogen. Hard carbon-nanocrystalline titanium carbide composites were obtained by firing under argon. The composites were tested as anode materials for sodium ion batteries, and the HC-TiN composite delivers a better capacity retention than that of hard carbon over 50 cycles. The synthesis method demonstrated here provides an effective route to composites of metal nitrides and carbides with carbon that may be of interest for other energy technologies as well as for sodium batteries.

Solvothermal water-diethylene glycol synthesis of LiCoPO4 and effects of surface treatments on lithium battery performance.

Olivine-structured LiCoPO4 is prepared via a facile solvothermal synthesis, using various ratios of water/diethylene glycol co-solvent, followed by thermal treatment under Ar, air, 5%H2/N2 or NH3. The diethylene glycol plays an important role in tailoring the particle size of LiCoPO4. It is found that using a ratio of water/diethylene glycol of 1 : 6 (v/v), LiCoPO4 is obtained with a homogenous particle size of ∼150 nm. The bare LiCoPO4 prepared after heating in Ar exhibits high initial discharge capacity of 147 mA h g-1 at 0.1C with capacity retention of 70% after 40 cycles. This is attributed to the enhanced electronic conductivity of LiCoPO4 due to the presence of Co2P after firing under Ar. The effects of carbon, TiN and RuO2 coating are also examined. Contrary to other studies, it is found that the solvothermally synthesised LiCoPO4 samples produced here do not require conductive coatings to achieve good performance.

Using polyoxometalates to enhance the capacity of lithium-oxygen batteries.

The Keggin-type polyoxometalate α-SiW12O404- increases the discharge capacity and potential of lithium-oxygen batteries, by facilitating the reduction of O2 to Li2O2, as confirmed by in situ electrochemical pressure measurements and XRD. Compared to organic redox mediators, polyoxometalates have higher chemical and structural stability, which could lead to longer cycling lithium-oxygen batteries.

The Effect of Water on Quinone Redox Mediators in Nonaqueous Li-O2 Batteries.

The parasitic reactions associated with reduced oxygen species and the difficulty in achieving the high theoretical capacity have been major issues plaguing development of practical nonaqueous Li-O2 batteries. We hereby address the above issues by exploring the synergistic effect of 2,5-di-tert-butyl-1,4-benzoquinone and H2O on the oxygen chemistry in a nonaqueous Li-O2 battery. Water stabilizes the quinone monoanion and dianion, shifting the reduction potentials of the quinone and monoanion to more positive values (vs Li/Li+). When water and the quinone are used together in a (largely) nonaqueous Li-O2 battery, the cell discharge operates via a two-electron oxygen reduction reaction to form Li2O2, with the battery discharge voltage, rate, and capacity all being considerably increased and fewer side reactions being detected. Li2O2 crystals can grow up to 30 µm, more than an order of magnitude larger than cases with the quinone alone or without any additives, suggesting that water is essential to promoting a solution dominated process with the quinone on discharging. The catalytic reduction of O2 by the quinone monoanion is predominantly responsible for the attractive features mentioned above. Water stabilizes the quinone monoanion via hydrogen-bond formation and by coordination of the Li+ ions, and it also helps increase the solvation, concentration, lifetime, and diffusion length of reduced oxygen species that dictate the discharge voltage, rate, and capacity of the battery. When a redox mediator is also used to aid the charging process, a high-power, high energy density, rechargeable Li-O2 battery is obtained.

Ion Speciation and Transport Properties of LiTFSI in 1,3-Dioxolane Solutions: A Case Study for Li-S Battery Applications.

Lithium-sulfur battery is considered to be one of the main candidates for "post-lithium-ion" battery generation because of its high theoretical specific capacity and inherently low cost. The role of electrolyte is particularly important in this system, and remarkable battery performances have been reported by tuning the amount of salt in the electrolyte. To further understand the reasons for such improvements, we chose the lithium bis(trifluoromethanesulfonyl)imide in 1,3-dioxolane electrolyte as a model salt-solvent system for a systematic study of conductivity and viscosity over a wide range of concentration from 10-5 up to 5 m. The experimental results, discussed and interpreted with reference to the theory of electrolyte conductance, lead to the conclusion that triple ion formation is responsible for the highest molal conductivity values before reaching the maximum at 1.25 m. At higher concentrations, the molal conductivity drops quickly because of a rapid increase in viscosity and the salt-solvent system can be treated as a diluted form of molten salt.

Improving Na-O2 batteries with redox mediators.

A new route to enhance the performance of Na-O2 cells is demonstrated. Redox mediators (such as ethyl viologen) are shown to facilitate the discharge reaction, producing an increased capacity (due to suppressed electrode passivation), higher discharge potential (due to faster kinetics) and stable cycling.

Understanding LiOH Chemistry in a Ruthenium-Catalyzed Li-O2 Battery.

Non-aqueous Li-O2 batteries are promising for next-generation energy storage. New battery chemistries based on LiOH, rather than Li2 O2 , have been recently reported in systems with added water, one using a soluble additive LiI and the other using solid Ru catalysts. Here, the focus is on the mechanism of Ru-catalyzed LiOH chemistry. Using nuclear magnetic resonance, operando electrochemical pressure measurements, and mass spectrometry, it is shown that on discharging LiOH forms via a 4 e- oxygen reduction reaction, the H in LiOH coming solely from added H2 O and the O from both O2 and H2 O. On charging, quantitative LiOH oxidation occurs at 3.1 V, with O being trapped in a form of dimethyl sulfone in the electrolyte. Compared to Li2 O2 , LiOH formation over Ru incurs few side reactions, a critical advantage for developing a long-lived battery. An optimized metal-catalyst-electrolyte couple needs to be sought that aids LiOH oxidation and is stable towards attack by hydroxyl radicals.

Promotion of the oxidation of carbon monoxide at stepped platinum single-crystal electrodes in alkaline media by lithium and beryllium cations.

The role of alkali cations (Li(+), Na(+), K(+), Cs(+), and Be(2+)) on the blank voltammetric response and the oxidative stripping of carbon monoxide from stepped Pt single-crystal electrodes in alkaline media has been investigated by cyclic voltammetry. A strong influence of the nature of the cation on both the blank voltammetric profile and the CO oxidation is observed and related to the influence of the cation on the specific adsorption of OH on the platinum surface. Especially Li(+) and Be(2+) cations markedly affect the adsorption of OH and thereby have a significant promoting effect on CO(ads) oxidation. The voltammetric experiments suggest that, on Pt(111), the influence of Li(+) (and Be(2+)) is primarily through a weakening of the repulsive interactions between the OH in the OH adlayer, whereas in the presence of steps also, the onset of OH adsorption is at a lower potential, both on steps and on terraces.

Thermodynamic evidence for K(+)-SO4(2-) ion pair formation on Pt(111). New insight into cation specific adsorption.

This work contributes to the understanding of cation specific effects on platinum electrochemistry by means of a thorough thermodynamic analysis of potassium adsorption on Pt(111) in sulfuric acid solutions. It is concluded that potassium specific adsorption is better described as the adsorption of the K(+)-SO ion pair. From the evaluation of the potassium sulfate concentration, it is found that potassium specific adsorption only takes place in the presence of coadsorbed sulfate species. Within the main sulfate adsorption state, for ∼0.3 V < E < ∼0.4 V (vs. SHE), the extent of potassium specific adsorption is small, reaching ∼0.1 × 10(14) species per cm(2) for c(K(+)) > 0.1 M. Then, at higher potentials, E > 0.55 V (vs. SHE), a second potassium adsorption process takes place, concomitant with the second sulfate adsorption process (associated to the small voltammetric feature called "the hump"). This last process involves the adsorption of an equal amount of potassium and sulfate species, leading to the adsorption of ∼0.5 × 10(14) ion pair species per cm(2) (∼0.03 ion pair species per platinum surface atom). Furthermore, the results of the formal partial charge numbers corroborate that potassium adsorption involves sulfate cooperative coadsorption, in such a way that the effective adsorbing species is anionic, rather than cationic. In conclusion, this work evidences that cation specific effects may originate from the formation of surface ion pairs, which is probably related to the presence of ion pairs in solution.

Elucidation of the chemical nature of adsorbed species for Pt(111) in H2SO4 solutions by thermodynamic analysis.

The nature of the adsorbed species for Pt(111) in sulfuric acid solutions has been elucidated by a careful thermodynamic analysis of the effect of pH on charge density data. This analysis takes advantage of the fact that, for solutions of constant total sulfate + bisulfate concentration, an increase of pH would increase the sulfate concentration, at the expense of decreasing the bisulfate concentration. As a result, sulfate adsorption would be shifted toward lower potentials, while bisulfate adsorption would follow the opposite trend. In the present work, coulostatic data for Pt(111) in (0.2 - x) M Me(2)SO(4) + x M H(2)SO(4) (Me: Li, Na; x: 10(-4) - 0.2) and (0.1 - x) M KClO(4) + x M HClO(4) + 10(-3) M K(2)SO(4) (x: 10(-4) - 0.1) solutions are carefully analyzed. It is concluded that sulfate rather than bisulfate adsorption takes place at potentials higher than the potential of zero charge. This result agrees with the fact that similar FTIRRAS bands for adsorbed sulfate species are observed for pH 0.8-3.5 in (0.2 - x) M K(2)SO(4) + x M H(2)SO(4) solutions.