An Aluminum-Sulfur Battery with a Fast Kinetic Response.

The electrochemical performance of the aluminum-sulfur (Al-S) battery has very poor reversibility and a low charge/discharge current density owing to slow kinetic processes determined by an inevitable dissociation reaction from Al2 Cl7- to free Al3+ . Al2 Cl6 Br- was used instead of Al2 Cl7- as the dissociation reaction reagent. A 15-fold faster reaction rate of Al2 Cl6 Br- dissociation than that of Al2 Cl7- was confirmed by density function theory calculations and the Arrhenius equation. This accelerated dissociation reaction was experimentally verified by the increase of exchange current density during Al electro-deposition. Using Al2 Cl6 Br- instead of Al2 Cl7- , a kinetically accelerated Al-S battery has a sulfur utilization of more than 80 %, with at least four times the sulfur content and five times the current density than that of previous work.

Spectroscopic Identification of the Au-C Bond Formation upon Electroreduction of an Aryl Diazonium Salt on Gold.

Electroreduction of aryl diazonium salts on gold can produce organic films that are more robust than their analogous self-assembled monolayers formed from chemical adsorption of organic thiols on gold. However, whether the enhanced stability is due to the Au-C bond formation remains debated. In this work, we report the electroreduction of an aryl diazonium salt of 4,4'-disulfanediyldibenzenediazonium on gold forming a multilayer of Au-(Ar-S-S-Ar)n, which can be further degraded to a monolayer of Au-Ar-S- by electrochemical cleavage of the S-S moieties within the multilayer. By conducting an in situ surface-enhanced Raman spectroscopic study of both the multilayer formation/degradation and the monolayer reduction/oxidation processes, coupled to density functional theory calculations, we provide compelling evidence that an Au-C bond does form upon electroreduction of aryl diazonium salts on gold and that the enhanced stability of the electrografted organic films is due to the Au-C bond being intrinsically stronger than the Au-S bond for a given phenylthiolate compound by ca. 0.4 eV.

Direct mass spectrometric detection of trace explosives in soil samples.

The detection of explosives in soil is of great significance in public security programmes and environmental science. In the present work, a ppb-level method was established to directly detect the semi-volatile explosives, RDX and TNT, present in complex soil samples. The method used thermal sampling technique and a direct current atmospheric pressure glow discharge source mounted with a brass cylinder electrode (9 mm × 4.6 mm i.d./5.6 mm o.d.) to face the samples, requiring no sample pretreatment steps such as soil extraction (about ten hours). It was characterized by the merits of easy operation, high sensitivity and fast speed, and has been validated by real soil samples from various locations around a factory or firecracker releasing fields. It took only 5 min per sample, with the limit of detection down to 0.5 ppb (S/N = 3) trinitrohexahydro-1,3,5-triazine in soils heated at 170 °C. It is also extendable to the analysis of other volatile analytes.

Revealing the Sulfur Redox Paths in a Li-S Battery by an In Situ Hyphenated Technique of Electrochemistry and Mass Spectrometry.

The lithium-sulfur (Li-S) battery is one of the most promising next generation energy storage systems due to its high theoretical specific energy. However, the shuttle effect of soluble lithium polysulfides formed during cell operation is a crucial reason for the low cyclability suffered by current Li-S batteries. As a result, an in-depth mechanistic understanding of the sulfur cathode redox reactions is urgently required for further advancement of Li-S batteries. Herein, the direct observation of polysulfides in a Li-S battery is reported by an in situ hyphenated technique of electrochemistry and mass spectrometry. Several short-lived lithium polysulfide intermediates during sulfur redox have been identified. Furthermore, this method is applied to a mechanistic study of an electrocatalyst that has been observed to promote the polysulfides conversion in a Li-S cell. Through the abundance distributions of various polysulfides before and after adding the electrocatalyst, compelling experimental evidences of catalytic selectivity of cobalt phthalocyanine to those long-chain polysulfide intermediates are obtained. This work can provide guidance for the design of novel cathode to overcome the shuttle effect and facilitate the sulfur redox kinetics.

Tackling the Interfacial Issues of Spinel LiNi0.5Mn1.5O4 by Room-Temperature Spontaneous Dediazonation Reaction.

The detrimental interfacial side reactions, inducing electrolyte decomposition and transition-metal dissolution, are regarded as "arch-criminal" for the utilization of spinel LiNi0.5Mn1.5O4 (LNMO) in high-power Li-ion batteries (LIBs). To conquer this issue, herein, we construct a thin polyphenyl film onto the surface of LNMO via the spontaneous dediazonation of C6H5N2+BF4- at room temperature. This conductive film facilitates the Li+ transport within cathode and at LNMO|electrolyte interface while reinforcing the compatibility of LNMO against electrolyte by efficiently suppressing the electrolyte decomposition catalyzed by LNMO and even the transition-metal dissolution. Consequently, polyphenyl-grafted LNMO exhibits improved electrochemical performances, e.g., the considerable discharge capacity of 136.7 mAh g-1 at low current density (0.1C), excellent rate capability, and long-term cyclability with a reversible capacity of 107.4 mAh g-1 along with high capacity retention of ∼85% after cycling 500 times, that are superior to those of the pristine LNMO counterpart. All these results demonstrate that our strategy is instrumental in solving the interface issues with respect to the spinel LNMO cathode, impelling the development of LNMO-based batteries with high energy density.

Deciphering the Enigma of Li2CO3 Oxidation Using a Solid-State Li-Air Battery Configuration.

Li2CO3 is a ubiquitous byproduct in Li-air (O2) batteries, and its accumulation on the cathode could be detrimental to the devices. As a result, much efforts have been devoted to investigating its formation and decomposition, in particular, upon cycling of Li-O2 batteries. At high voltages, Li2CO3 is expected to decompose into CO2 and O2. However, as recognized from the work of many authors, only CO2, and no O2, has been identified, and the underlying mechanism remains uncertain so far. Herein, a solid-state Li-O2 battery (Li|Li6.4La3Zr1.4Ta0.6O12|Au) has been designed to interrogate the Li2CO3 oxidation without interferences from the decomposition of other battery components (organic electrolyte, binder, and carbon cathode) widely applied in conventional Li-O2 batteries. It is revealed that Li2CO3 can indeed be oxidized to CO2 and O2 in a more stable solid-state Li-O2 battery configuration, highlighting the feasibility of reversible operation of Li-O2 batteries with ambient air as the feeding gas.

Relieving the "Sudden Death" of Li-O2 Batteries by Grafting an Antifouling Film on Cathode Surfaces.

The "sudden-death" phenomenon has been frequently encountered during discharging of Li-O2 batteries and has been ascribed to the growth of a blocking film of Li2O2 on the cathode surface. Recent fundamental study revealed that this dilemma could be addressed by discharging Li2O2 in the electrolyte solution rather than on the cathode surface. However, even for Li-O2 batteries operated under the conditions favorable for the solution growth of Li2O2, sudden death still persists and its origin remains incompletely understood. Herein, by using a combination of in situ spectroscopy and theoretical calculation, we reveal that sudden death of Li-O2 batteries operated under the conditions (e.g., low discharge current density and high donor number electrolyte solvent) favorable for discharging Li2O2 in the electrolyte solutions is caused by adventitious adsorption of a minor quantity of Li2O2, which triggers a rapid transition of Li2O2 growth mode from solution- to surface-mediated growth. Moreover, a cathode surface modification strategy has been developed to effectively retard the Li2O2 adsorption and therefore significantly alleviate the sudden death of Li-O2 batteries.

Interstitial Hydrogen Atom Modulation to Boost Hydrogen Evolution in Pd-Based Alloy Nanoparticles.

Rational control of the components of noble metal alloys is paramount for achieving satisfactory electrocatalytic performances. Though transition metals are commonly used to modify noble metals, many potential elements remain to be explored. Here, we interstitially modulate hydrogen atoms into RhPd nanoparticles to boost the alkaline hydrogen evolution reaction (HER). The obtained stable RhPd-H nanoparticles exhibit pronounced alkaline HER activity with a small overpotential of 36.6 mV at 10 mA cm-2 and a low Tafel slope of 35.3 mV dec-1. The surface electronic state, bond distance, and coordination number of the Rh and Pd atoms are significantly influenced by the presence of interstitial hydrogen atoms. These modifications give RhPd-H nanoparticles a desirable hydrogen adsorption free energy, thus accelerating the hydrogen gas production. We further demonstrate that the interstitial hydrogen atom modulation strategy to improve the HER activity is universal for other Pd-based alloy nanostructures. This work presents a powerful strategy for designing efficient electrocatalysts for the HER and beyond.

Promoting Solution Discharge of Li-O2 Batteries with Immobilized Redox Mediators.

For years, the aprotic Li-O2 battery suffered from a severe capacity-current trade-off that would be unacceptable for a beyond Li-ion battery. Recent fundamental study of Li-O2 electrochemistry revealed that this dilemma is caused by the growth of Li2O2 on the cathode surface and can be solved by discharging Li2O2 in the electrolyte solution. Among the strategies that can promote solution growth of Li2O2, redox mediators (i.e., soluble catalysts) demonstrate prominent performance. However, soluble redox mediators may shuttle from the cathode to the lithium anode and decompose thereon, causing severe deterioration of the lithium anode and degradation of the mediators' functionality. Here, we report that immobilized redox mediators (e.g., anthraquinone, AQ) in the form of a thin conductive polymer film (PAQ) on the cathode can effectively promote solution growth of Li2O2 even in weakly solvating electrolyte solutions that would otherwise lead to surface film growth and early cell death. The PAQ-catalyzed Li-O2 battery can deliver a discharge capacity that is up to ∼50 times what its pristine counterpart does at the same current densities and is comparable to the capacity realized by soluble AQ-catalyzed Li-O2 batteries. Most importantly, the adverse "cross-talk" between the lithium anode and the redox mediators immobilized on the cathode has been completely eliminated.

Verifying the Rechargeability of Li-CO2 Batteries on Working Cathodes of Ni Nanoparticles Highly Dispersed on N-Doped Graphene.

Li-CO2 batteries could skillfully combine the reduction of "greenhouse effect" with energy storage systems. However, Li-CO2 batteries still suffer from unsatisfactory electrochemical performances and their rechargeability is challenged. Here, it is reported that a composite of Ni nanoparticles highly dispersed on N-doped graphene (Ni-NG) with 3D porous structure, exhibits a superior discharge capacity of 17 625 mA h g-1, as the air cathode for Li-CO2 batteries. The batteries with these highly efficient cathodes could sustain 100 cycles at a cutoff capacity of 1000 mA h g-1 with low overpotentials at the current density of 100 mA g-1. Particularly, the Ni-NG cathodes allow to observe the appearance/disappearance of agglomerated Li2CO3 particles and carbon thin films directly upon discharge/charge processes. In addition, the recycle of CO2 is detected through in situ differential electrochemical mass spectrometry. This is a critical step to verify the electrochemical rechargeability of Li-CO2 batteries. Also, first-principles computations further prove that Ni nanoparticles are active sites for the reaction of Li and CO2, which could guide to design more advantageous catalysts for rechargeable Li-CO2 batteries.

A High-Performance Li-O2 Battery with a Strongly Solvating Hexamethylphosphoramide Electrolyte and a LiPON-Protected Lithium Anode.

The aprotic Li-O2 battery has attracted a great deal of interest because theoretically it can store more energy than today's Li-ion batteries. However, current Li-O2 batteries suffer from passivation/clogging of the cathode by discharged Li2 O2 , high charging voltage for its subsequent oxidation, and accumulation of side reaction products (particularly Li2 CO3 and LiOH) upon cycling. Here, an advanced Li-O2 battery with a hexamethylphosphoramide (HMPA) electrolyte is reported that can dissolve Li2 O2 , Li2 CO3 , and LiOH up to 0.35, 0.36, and 1.11 × 10-3 m, respectively, and a LiPON-protected lithium anode that can be reversibly cycled in the HMPA electrolyte. Compared to the benchmark of ether-based Li-O2 batteries, improved capacity, rate capability, voltaic efficiency, and cycle life are achieved for the HMPA-based Li-O2 cells. More importantly, a combination of advanced research techniques provide compelling evidence that operation of the HMPA-based Li-O2 battery is backed by nearly reversible formation/decomposition of Li2 O2 with negligible side reactions.

Monodispersed Ru Nanoparticles Functionalized Graphene Nanosheets as Efficient Cathode Catalysts for O2-Assisted Li-CO2 Battery.

In Li-CO2 battery, due to the highly insulating nature of the discharge product of Li2CO3, the battery needs to be charged at a high charge overpotential, leading to severe cathode and electrolyte instability and hence poor battery cycle performance. Developing efficient cathode catalysts to effectively reduce the charge overpotential represents one of key challenges to realize practical Li-CO2 batteries. Here, we report the use of monodispersed Ru nanoparticles functionalized graphene nanosheets as cathode catalysts in Li-CO2 battery to significantly lower the charge overpotential for the electrochemical decomposition of Li2CO3. In our battery, a low charge voltage of 4.02 V, a high Coulomb efficiency of 89.2%, and a good cycle stability (67 cycles at a 500 mA h/g limited capacity) are achieved. It is also found that O2 plays an essential role in the discharge process of the rechargeable Li-CO2 battery. Under the pure CO2 environment, Li-CO2 battery exhibits negligible discharge capacity; however, after introducing 2% O2 (volume ratio) into CO2, the O2-assisted Li-CO2 battery can deliver a high capacity of 4742 mA h/g. Through an in situ quantitative differential electrochemical mass spectrometry investigation, the final discharge product Li2CO3 is proposed to form via the reaction 4Li+ + 2CO2 + O2 + 4e- → 2Li2CO3. Our results validate the essential role of O2 and can help deepen the understanding of the discharge and charge reaction mechanisms of the Li-CO2 battery.

Unlocking the energy capabilities of micron-sized LiFePO4.

Utilization of LiFePO4 as a cathode material for Li-ion batteries often requires size nanonization coupled with calcination-based carbon coating to improve its electrochemical performance, which, however, is usually at the expense of tap density and may be environmentally problematic. Here we report the utilization of micron-sized LiFePO4, which has a higher tap density than its nano-sized siblings, by forming a conducting polymer coating on its surface with a greener diazonium chemistry. Specifically, micron-sized LiFePO4 particles have been uniformly coated with a thin polyphenylene film via the spontaneous reaction between LiFePO4 and an aromatic diazonium salt of benzenediazonium tetrafluoroborate. The coated micron-sized LiFePO4, compared with its pristine counterpart, has shown improved electrical conductivity, high rate capability and excellent cyclability when used as a 'carbon additive free' cathode material for rechargeable Li-ion batteries. The bonding mechanism of polyphenylene to LiFePO4/FePO4 has been understood with density functional theory calculations.