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Conventional Li-ion cathodes store charge by reversible intercalation of Li coupled to metal cation redox. There has been increasing interest in new materials capable of accommodating more than one Li per transition-metal center, thereby yielding higher charge storage capacities. We demonstrate here that the lithium-rich layered iron sulfide Li₂FeS₂ as well as a new structural analogue, LiNaFeS₂, reversibly store ≥1.5 electrons per formula unit and support extended cycling. Ex situ and operando structural and spectroscopic data indicate that delithiation results in reversible oxidation of Fe²⁺ concurrent with an increase in the covalency of the Fe–S interactions, followed by reversible anion redox: 2 S²⁻/(S₂)²⁻. S K-edge spectroscopy unequivocally proves the contribution of the anions to the redox processes. The structural response to the oxidation processes is found to be different in Li₂FeS₂ in contrast to that in LiNaFeS₂, which we suggest is the cause for capacity fade in the early cycles of LiNaFeS₂. The materials presented here have the added benefit of avoiding resource-sensitive transition metals such as Co and Ni. In contrast to Li-rich oxide materials that have been the subject of so much recent study and that suffer capacity fade and electrolyte degradation issues, the materials presented here operate within the stable potential window of the electrolyte, permitting a clearer understanding of the underlying processes.

Mg-based batteries are an attractive next-generation energy storage chemistry due to the high natural abundance and inexpensive cost of Mg, along with the high theoretical energy density compared to that of conventional Li-ion chemistry. The greater energy density is predicated on a Mg metal anode, and pathways to achieving reversible Mg electrodeposition and stripping are reliant on the development of Mg electrolytes. Although Mg electrolyte chemistry has advanced significantly from the reactive Grignards of the 1920s to the carboranes of this decade, there remains significant challenges in correlating the Mg metal anode electrochemistry with the composition of the electrolyte salts as a result of the complicated interface of Mg metal and the electrolyte. To probe the effect of the interface on Mg electrodeposition, we turn to an electrolyte with a known solution-phase composition: the magnesium aluminum chloride complex (MACC) electrolyte. The MACC electrolyte requires electrolytic conditioning to support reversible Mg electrodeposition and stripping. Here, we show that a small concentration (2-5 mM) of Mg(HMDS)2 with respect to the MACC electrolyte salts suppresses Al3+ deposition and promotes reversible Mg electrodeposition and stripping in the first cycle. The significant effect of a small concentration of additive is attributed to changes to the electrode interface. The impact of the Mg interface on the observed electrochemical performance is discussed.

As Li-ion battery optimization approaches theoretical limits, interest has grown in designing next-generation batteries from low-cost earth-abundant materials. Mg–S batteries are promising candidates, exhibiting widespread abundance of elemental precursors and a relatively large theoretical energy density albeit at lower cell voltage. However, Mg–S batteries exhibit poor reversibility, in part due to interactions between dissolved polysulfides and the Mg anode. Herein, we employ electrochemical experiments using Ag₂S quasi-reference electrodes to probe the interactions between Mg anodes and dissolved polysulfides. We show that Mg²⁺ reduction (charging) is impeded in the presence of polysulfides, while Mg metal oxidation (discharging) remains facile. Large reduction overpotentials arise due to the formation of a passivation layer on the anode surface, likely composed primarily of MgS. The passivation layer is removed under oxidative conditions but quickly reforms during reduction. We discover that dissolved S8 influences the rate of MgS formation by shifting the polysulfide disproportionation equilibria. Shorter-chain polysulfides react more readily than longer-chain polysulfides at the Mg electrode, and thus, film formation is mediated by the electrochemical generation of shorter-chain polysulfide species.

Mg batteries are attractive next-generation energy storage systems due to their high natural abundance, inexpensive cost, and high theoretical capacity compared to conventional Li-ion based systems. The high energy density is achieved by electrodeposition and stripping of a Mg metal anode and requires the development of effective electrolytes enabled by a mechanistic understanding of the charge-transfer mechanism. The magnesium aluminum chloride complex (MACC) electrolyte is a good model system to study the mechanism as the solution phase speciation is known. Previously, we reported that minor addition of Mg(HMDS)2 to the MACC electrolyte causes significant improvement in the Mg deposition and stripping voltammetry resulting in good Coulombic efficiency on cycle one and, therefore, negating the need for electrochemical conditioning. To determine the cause of the improved electrochemistry, here we probe the speciation of the electrolyte after Mg(HMDS)2 addition using Raman spectroscopy, 27Al nuclear magnetic resonance spectroscopy, and 1H-29Si heteronuclear multiple bond correlation spectroscopy on MACC + Mg(HMDS)2 at various Mg(HMDS)2 concentrations. Mg(HMDS)2 scavenges trace H2O, but it also reacts with MACC complexes, namely, AlCl4-, to form free Cl-. We suggest that although both the removal of H2O and the formation of free Cl- improve electrochemistry by altering the speciation at the interface, the latter has a profound effect on electrodeposition and stripping of Mg.

The effects of shear planes in perovskite materials have been studied in order to identify their role in the electrochemical behavior of Li+ intercalation hosts. These planes modulate the structural stability and ionic transport pathways and therefore play an intimate role in the characteristics and performance of shear compounds. Herein, two Nb-based compounds, NbO2F and Nb3O7F, were chosen as representative perovskite and shear derivatives respectively to investigate the role of crystallographic shear. A series of operando measurements, including X-ray diffraction and X-ray absorption spectroscopy, in conjunction with structural analysis, Raman spectroscopy, and detailed electrochemical studies identified the effect of shear planes. It was found that shear planes led to increased structural stability during Li+ (de)intercalation with shear layers being maintained, while perovskite layers were seen to degrade rapidly. However, disordering in the shear plane stacking introduced during delithiation ultimately led to poor capacity retention despite structural maintenance as Li+ diffusion channels are disrupted.

While the icosahedral closo-[B12H12]2– cluster does not display reversible electrochemical behavior, perfunctionalization of this species via substitution of all twelve B–H vertices with alkoxy orbenzyloxy (OR) substituents engenders reversible redox chemistry, providing access to clusters in the dianionic,monoanionic, and neutral forms. Here, we evaluated the electrochemical behavior of the electron-rich B12(O-3-methylbutyl)12 (1) cluster and discovered that a new reversible redox event that gives rise to a fourth electronic state is accessible through one-electron oxidation of the neutral species. Chemical oxidation of 1 with [N(2,4-Br2C6H3)3]•+ afforded the isolable[1] •+ cluster, which is the first example of an open-shell cationic B12 cluster in which the unpaired electron is proposed to be delocalized throughout the boron cluster core. The oxidation of 1 is also chemically reversible, where treatment of [1]•+ with ferrocene resulted in its reduction back to 1. The identity of [1]•+ is supported by EPR, UV-vis, multinuclear NMR (1H, 11B), and X-ray photoelectron spectroscopic characterization.

Magnesium batteries offer an opportunity to use naturally abundant Mg and achieve large volumetric capacities reaching over four times that of conventional Li-based intercalation anodes. High volumetric capacity is enabled by the use of a Mg metal anode in which charge is stored via electrodeposition and stripping processes, however, electrolytes that support efficient Mg electrodeposition and stripping are few and are often prepared from highly reactive compounds. One interesting electrolyte solution that supports Mg deposition and stripping without the use of highly reactive reagents is the magnesium aluminum chloride complex (MACC) electrolyte. The MACC exhibits high Coulombic efficiencies and low deposition overpotentials following an electrolytic conditioning protocol that stabilizes species necessary for such behavior. Here, we discuss the effect of the MgCl_2 and AlCl_3 concentrations on the deposition overpotential, current density, and the conditioning process. Higher concentrations of MACC exhibit enhanced Mg electrodeposition current density and much faster conditioning. An increase in the salt concentrations causes a shift in the complex equilibria involving both cations. The conditioning process is strongly dependent on the concentration suggesting that the electrolyte is activated through a change in speciation of electrolyte complexes and is not simply due to the annihilation of electrolyte impurities. Additionally, the presence of the [Mg_2(μ-Cl)_3·6THF]^+ in the electrolyte solution is again confirmed through careful analysis of experimental Raman spectra coupled with simulation and direct observation of the complex in sonic spray ionization mass spectrometry. Importantly, we suggest that the ∼210 cm^(–1) mode commonly observed in the Raman spectra of many Mg electrolytes is indicative of the C_(3v) symmetric [Mg_2(μ-Cl)_3·6THF]^+. The 210 cm^(–1) mode is present in many electrolytes containing MgCl_2, so its assignment is of broad interest to the Mg electrolyte community.

Development of next generation batteries is predicated on the design and discovery of new, functional materials. Divalent cations are promising options that go beyond the canonical Li-based systems, but the development of new materials for divalent ion batteries is hindered due to difficulties in promoting divalent ion conduction. We have developed a family of cathode materials based on the divalent cation conductor ZnPS3. Substitution of V for Zn in the lattice concomitant with vacancy introduction yields isostructural but redox-active materials that can reversibly store Zn2 + in the vacancies. A range of voltammetry and galvanostatic cycling experiments along with x-ray photoelectron spectroscopy support that redox is indeed centered on V and that capacity is dependent on the V content. The voltage of the materials is limited by the irreversible decomposition of the [ P 2 S 6 ] 4 − polyanion above 1.4 V vs. Zn/Zn2 + . The reversible capacity before anion decomposition is limited to half the vacancies and is due to the relative ratios of oxidized and reduced V centers. Such observations provide useful design rules for cathode materials for divalent cation based battery technologies, and highlight the necessity for a holistic interpretation of physical and electronic structural changes upon cycling.

Mg metal is a promising alternative anode material that offers higher theoretical specific capacity than conventional Li-ion batteries (LIBs), and the high natural abundance of Mg makes it more economically appealing than other post-LIB anode materials. The greater energy density hinges on a Mg-metal anode, and reversible Mg electrodeposition and stripping require an effective Mg electrolyte. Various electrolyte systems that support reversible have been introduced, but many of them contain drawbacks, such as narrow electrochemical window, reactivity with electrode materials, or poor Coulombic efficiency. In addition, challenges remain in understanding the relationship between electrochemistry at the Mg anode and the electrolyte composition due to the complicated interface of Mg metal and the electrolyte. To probe the effect of the interface on Mg electrodeposition, an electrolyte with a known solution phase composition, the magnesium aluminum chloride complex (MACC) electrolyte, is explored. The MACC electrolyte undergoes electrolytic conditioning to support reversible Mg electrodeposition and stripping. During conditioning, free Cl- is released electrolytically while Al3+ is irreversibly deposited, and the concentration of the active species [Mg2(µ−Cl)3·6(THF)]+ increases, activating the MACC electrolyte. Here, we show that a small concentration of Mg(HMDS)2 to the MACC electrolyte suppresses Al3+ deposition and promotes reversible Mg electrodeposition and stripping on the first cycle. Such a drastic change from a small concentration suggests that changes are localized at the electrode-electrolyte interface. We show spectroscopically that Mg(HMDS)2 scavenges trace water in the solution. Spectroscopic results also suggest that Mg(HMDS)2 reacts with AlCl4 - in the MACC electrolyte to form free Cl-. Based on the spectroscopic and electrochemical results, we suggest that the formation of Cl- is the primary cause for improved behavior on cycle one, highlighting the role of Cl- in Mg deposition and stripping.

Mg batteries are an attractive alternative to Li-based energy storage due to the possibility of higher volumetric capacities with the added advantage of using sustainable materials. A promising emerging electrolyte for Mg batteries is the magnesium aluminum chloride complex (MACC) which shows high Mg electrodeposition and stripping efficiencies and relatively high anodic stabilities. As prepared, MACC is inactive with respect to Mg deposition; however, efficient Mg electrodeposition can be achieved following an electrolytic conditioning process. Through the use of Raman spectroscopy, surface enhanced Raman spectroscopy, ^(27)Al and ^(35)Cl nuclear magnetic resonance spectroscopy, and pair distribution function analysis, we explore the active vs inactive complexes in the MACC electrolyte and demonstrate the codependence of Al and Mg speciation. These techniques report on significant changes occurring in the bulk speciation of the conditioned electrolyte relative to the as-prepared solution. Analysis shows that the active Mg complex in conditioned MACC is very likely the [Mg_2(μ–Cl)_3·6THF]^+ complex that is observed in the solid state structure. Additionally, conditioning creates free Cl^– in the electrolyte solution, and we suggest the free Cl^– adsorbs at the electrode surface to enhance Mg electrodeposition.

We propose a nanostructured Mn-doped V2O5 lithium-ion battery cathode material that facilitates cathodic charge transport. The synthesis strategy uses a layered compound, vanadium(III) jarosite, as the precursor, in which the Mn2+ ions are doped uniformly between the vanadium oxide crystal layers. Through a two-step transformation, the vanadium jarosite was converted into Mn2+-doped V2O5. The resulting aliovalent doping of the larger Mn cations in the modified V2O5 structure increases the cell volume, which facilitates diffusion of Li+ ions, and introduces oxygen vacancies that improve the electronic conductivity. Comparison of the electrochemical performance in Li-ion batteries of undoped and the Mn2+-doped V2O5 hierarchical structure made from layered vanadium jarosite confirms that the Mn-doping improves ion transport to give a high cathodic columbic capacity (253 mAhg–1 at 1C, 86% of the theoretical value, 294 mAhg–1) and excellent cycling stability.

The electrochemical performance of alternative anode materials for Li-ion batteries is often measured using composite electrodes consisting of active material and conductive carbon additives. Cycling of these composite electrodes at low voltages demonstrates charge storage at the operating potentials of viable anodes, however, the conductive carbon additive is also able to store charge in the low potential regime. The contribution of the conductive carbon additives to the observed capacity is often neglected when interpreting the electrochemical performance of electrodes. To provide a reference for the contribution of the carbons to the observed capacity, we report the charge storage behavior of two common conductive carbon additives Super P and Ketjenblack as a function of voltage, rate, and electrolyte composition. Both carbons exhibit substantial capacities after 100 cycles, up to 150 mAh g^(−1), when cycled to 10 mV. The capacity is dependent on the discharge cutoff voltage and cycling rate with some dependence on electrolyte composition. The first few cycles are dominated by the formation of the SEI followed by a fade to a steady, reversible capacity thereafter. Neglecting the capacity of the carbon additive can lead to significant errors in the estimation of charge storage capabilities of the active material.

Li–S batteries are a promising next-generation battery technology. Due to the formation of soluble polysulfides during cell operation, the electrolyte composition of the cell plays an active role in directing the formation and speciation of the soluble lithium polysulfides. Recently, new classes of electrolytes termed “solvates” that contain stoichiometric quantities of salt and solvent and form a liquid at room temperature have been explored due to their sparingly solvating properties with respect to polysulfides. The viscosity of the solvate electrolytes is understandably high limiting their viability; however, hydrofluoroether cosolvents, thought to be inert to the solvate structure itself, can be introduced to reduce viscosity and enhance diffusion. Nazar and co-workers previously reported that addition of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) to the LiTFSI in acetonitrile solvate, (MeCN)_2–LiTFSI, results in enhanced capacity retention compared to the neat solvate. Here, we evaluate the effect of TTE addition on both the electrochemical behavior of the Li–S cell and the solvation structure of the (MeCN)_2–LiTFSI electrolyte. Contrary to previous suggestions, Raman and NMR spectroscopy coupled with ab initio molecular dynamics simulations show that TTE coordinates to Li^+ at the expense of MeCN coordination, thereby producing a higher content of free MeCN, a good polysulfide solvent, in the electrolyte. The electrolytes containing a higher free MeCN content facilitate faster polysulfide formation kinetics during the electrochemical reduction of S in a Li–S cell likely as a result of the solvation power of the free MeCN.

Nanostructured uniformly Mn-doped V2O5 Li on battery cathode material is prepared via layered VIII jarosite as precursor, which is obtained by microwave-assisted solvothermal reaction of VOSO4, Mn(OAc)2, and glucose in a H2O/1-hexanol emulsion containing cetyltrimethylammoniumbromide (sealed vial, 195 °C, 1 h) followed by vacuum annealing (900 °C, 1 h) and firing in air (400 °C, 1 h) to convert V2O3 into V2O5.

Redox active electrode materials derived from organic precursors are of interest for use as alternative cathodes in Li batteries due to the potential for their sustainable production from renewable resources. Here, a series of organic networks that either contain triazine units or are derived from triazine-containing precursors are evaluated as cathodes versus Li metal anodes as possible active materials in Li batteries. The role of the molecular structure on the electrochemical performance is studied by comparing several materials prepared across a range of conditions allowing control over functionality and long-range order. Well-defined structures in which the triazine unit persists in the final material exhibit very low capacities at voltages relevant for cathode materials (

The Li-S system offers a tantalizing battery for electric vehicles and renewable energy storage due to its high theoretical capacity of 1675 mAh g-1and its employment of abundant and available materials. One major challenge in this system stems from the formation of soluble polysulfides during the reduction of S8, the active cathode material, during discharge. The ability to deploy this system hinges on the ability to control the behavior of these polysulfides by containing them in the cathode and allowing for further redox. Here, we exploit the high surface areas and good electrical conductivity of mesoporous carbons (MC) to achieve high sulfur utilization while functionalizing the MC with sulfur (S-MC) in order to modify the surface chemistry and attract polysulfides to the carbon material. S-MC materials show enhanced capacity and cyclability trending as a function of sulfur functionality, specifically a 50% enhancement in discharge capacity is observed at high cycles (60-100 cycles). Impedance spectroscopy suggests that the S-MC materials exhibit a lower charge-transfer resistance compared with MC materials which allows for more efficient electrochemistry with species in solution at the cathode. Isothermal titration calorimetry shows that the change in surface chemistry from unfunctionalized to S-functionalized carbons results in an increased affinity of the polysulfide intermediates for the S-MC materials, which is the likely cause for enhanced cyclability. © 2014 American Chemical Society.

The lithium–sulfur (Li–S) system is an attractive candidate to replace the current state-of-the-art lithium-ion battery due to the promising theoretical charge capacity of 1675 mA h/g and energy density of 2500 Wh/kg; however, the dissolution of intermediate polysulfides into the organic liquid electrolyte during cycling hinders its practical realization. We report the synthesis of mesoporous graphene–silica composite (m-GS) as a supporting material of sulfur for Li–S batteries. The ordered porous silica structure was synthesized parallel to functionalized graphene sheets (FGSs) through the ternary cooperative assembly of the graphene, silica, and block copolymer precursors. The well-defined, unique mesoporous structure integrates the electronic conductivity of graphene and the dual functions of silica as a structure building block and in situ polysulfide ab-/ad-sorbing agent to give a Li–S battery that has both good retention ability of polysulfides and good rate capability.

Nonprecious transition metal nitrides have attracted considerable attention as an alternative catalyst to noble metals for electrochemical reactions. Titanium nitride (TiN), in particular, is an interesting material that exhibits excellent thermal, chemical, and mechanical stability, electrical conductivity, and in particular electrocatalytic activity. TiN is often prepared by the nitridation of titanium oxide using gaseous ammonia or hydrazine as a nitrogen source. This high temperature synthesis, however, gives a material with low surface area, which is unfavorable for its application in heterogeneous electrocatalysis. The shape and size of the assembly, and thus the resulting g-CN materials, can be tailored simply by using different precipitation temperatures, solvents, and comonomers. Furthermore, the assembly develops unique nanostructures during the polycondensation without losing the pristine macrostructures.

Conversion reaction cells afford the ability to explore new energy storage paradigms that transcend the dogma of small, low‐charge cations essential to intercalative processes. Here we report the use of earth‐abundant and green calcium and sulfur in unprecedented conversion reaction Ca–S primary cells. Using S‐infiltrated mesoporous carbon (abbreviated S@meso‐C) cathodes, we achieve discharge capacities as high as 600 mAh g^(−1) (S basis) within the geometry Ca|Ca(ClO_4)_2/CH_3CN|S@meso‐C, at a discharge rate of C/3.5. The electrolyte system in the Ca–S battery is of paramount importance as the solid electrolyte interface (SEI) formed on the Ca anode limits the capacity and stability of the cell. We determine that 0.5 M Ca(ClO_4)_2 in CH_3CN forms an SEI that advantageously breaks down under anodic bias to allow oxidation of the anode. This same SEI, however, exhibits high impedance which increases over time at open circuit limiting the shelf life of the cell.

Here, we propose thiol-based electrolyte additives (biphenyl-4,4’-dithiol (BPD)) to enhance the capacity retention of lithium-sulfur batteries. In-situ Raman and UV-Vis spectroscopy are used to investigate the effect of the additive on the sulfur reduction mechanism. Raman spectroscopy shows that long chain polysulfides (S8 2-) are formed via S8 ring opening in the first reduction process at ~2.4 V vs Li/Li+ and short chain polysulfides such as S4 2-, S4 -, S3 ․ - and S2O4 2- are observed with continued discharge at ~2.0 V vs Li/Li+ in the second reduction process. In the Kinetic study, rate constants obtained for the appearance and disappearance polysulfide species show that short chain polysulfides are directly formed from S8 decomposition. The polysulfide oxidation and reduction is quasi-reversible. With the BPD additive, an additional reduction process is observed at ~2.1 V vs Li/Li+. This reduction is associated with the formation of BPD-polysulfide complexes. The BPD-polysulfide complexes form at ~2.1 V followed by the formation of short chain polysulfides upon further discharge. The reduction of these complexes is reversible during the charge process. The BPD additive inhibits the formation rate of short chain polysulfides, suggesting that there is a strong interaction between BPD and short chain polysulfides. In-situ UV-Vis spectroscopy shows that the polysulfide solubility decreases in the presence of the BPD additive and forms thiol-polysulfide complexes during the cycling. The (-)ESI-MS shows the formation of BPD-S and BPD-S2 and BPD-S3, suggesting that BPD interacts with short chain polysulfides. The decomposition of the thiol-based additive was found during the battery cycling and resulted in a dense and smooth SEI film on the Li anode. References: (1) Zhang, S.S. et al J. Power Sources 2013, 231, 153-162. (2) Barchasz, C. et al., Anal. Chem. 2012, 84, 3973. (3) Cuisinier, M. et al., J. Phys. Chem. Lett. 2013, 4, 3227. (4) Hagen, M. et al., J. Electrochem. Soc. 2013, 160, A1205.