Electrolytes in Organic Batteries.

Organic batteries using redox-active polymers and small organic compounds have become promising candidates for next-generation energy storage devices due to the abundance, environmental benignity, and diverse nature of organic resources. To date, tremendous research efforts have been devoted to developing advanced organic electrode materials and understanding the material structure-performance correlation in organic batteries. In contrast, less attention was paid to the correlation between electrolyte structure and battery performance, despite the critical roles of electrolytes for the dissolution of organic electrode materials, the formation of the electrode-electrolyte interphase, and the solvation/desolvation of charge carriers. In this review, we discuss the prospects and challenges of organic batteries with an emphasis on electrolytes. The differences between organic and inorganic batteries in terms of electrolyte property requirements and charge storage mechanisms are elucidated. To provide a comprehensive and thorough overview of the electrolyte development in organic batteries, the electrolytes are divided into four categories including organic liquid electrolytes, aqueous electrolytes, inorganic solid electrolytes, and polymer-based electrolytes, to introduce different components, concentrations, additives, and applications in various organic batteries with different charge carriers, interphases, and separators. The perspectives and outlook for the future development of advanced electrolytes are also discussed to provide a guidance for the electrolyte design and optimization in organic batteries. We believe that this review will stimulate an in-depth study of electrolytes and accelerate the commercialization of organic batteries.

Colloidal Synthesis of Silicon-Carbon Composite Material for Lithium-Ion Batteries.

We report colloidal routes to synthesize silicon@carbon composites for the first time. Surface-functionalized Si nanoparticles (SiNPs) dissolved in styrene and hexadecane are used as the dispersed phase in oil-in-water emulsions, from which yolk-shell and dual-shell hollow SiNPs@C composites are produced via polymerization and subsequent carbonization. As anode materials for Li-ion batteries, the SiNPs@C composites demonstrate excellent cycling stability and rate performance, which is ascribed to the uniform distribution of SiNPs within the carbon hosts. The Li-ion anodes composed of 46 wt % of dual-shell SiNPs@C, 46 wt % of graphite, 5 wt % of acetylene black, and 3 wt % of carboxymethyl cellulose with an areal loading higher than 3 mg cm-2 achieve an overall specific capacity higher than 600 mAh g-1 , which is an improvement of more than 100 % compared to the pure graphite anode. These new colloidal routes present a promising general method to produce viable Si-C composites for Li-ion batteries.

Giant Raman Response to the Encapsulation of Sulfur in Narrow Diameter Single-Walled Carbon Nanotubes.

Encapsulation of sulfur in HiPCO-SWNTs leads to large changes in the Raman spectra with the appearance of new peaks at 319, 395, and 715 cm(-1) which originate from the sulfur species within the SWNTs, while the high frequency SWNT bands (ν > 1200 cm(-1)) are decreased in intensity. The encapsulated species also shifts the near-IR interband electronic transitions to lower energy by more than 10%. These effects seem to originate with the van der Waals interaction of the confined sulfur species with the walls of the SWNTs which are not expected to be significant in the case of the previously studied large diameter SWNTs. We suggest that sulfur in the small diameter SWNTs exists as a helical polymeric sulfur chain that enters the SWNT interior in the form of S2 ((3)Σ(g)(-)) molecules which undergo polymerization to linear diradicals.

Development of Electrolytes under Lean Condition in Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries stand out as one of the promising candidates for next-generation electrochemical energy storage technologies. A key requirement to realize high-specific-energy Li-S batteries is to implement low amount of electrolyte, often characterized by the electrolyte/sulfur (E/S) ratio. Low E/S ratio aggravates the known challenges for Li-S batteries and introduces new ones originated from the high concentration of polysulfides in limited electrolyte reservoir. In this review, the connections between the fundamental properties of electrolytes and the electrochemical/chemical reactions in Li-S batteries under lean electrolyte condition are elucidated. The emphasis is on how the solvating properties of the electrolyte affect the fate of polysulfides. Built upon the mechanistic analysis, different strategies to design lean electrolytes to improve the overall process of Li-S reactions and Li anode protection are discussed.

Electrolyte Engineering with Carboranes for Next-Generation Mg Batteries.

To realize an energy storage transition beyond Li-ion competitive technologies, earth-abundant elements, such as Mg, are needed. Carborane anions are particularly well-suited to realizing magnesium-ion batteries (MIBs), as their inert and weakly coordinating properties beget excellent electrolyte performance. However, utilizing these materials in actual electrochemical cells has been hampered by the reliance on the Mg2+ salts of the commercially available [HCB11H11]- anion, which is not soluble in more weakly binding solvents apart from the higher glymes. Herein, we demonstrate it is possible to iteratively engineer the [HCB11H11]- anion surface synthetically to address previous solubility issues and yield a highly conductive (up to 7.33 mS cm-1) and electrochemically stable (up to +4.2 V vs Mg2+/0) magnesium electrolyte that surpasses the state of the art. This novel non-nucleophilic electrolyte exhibits highly dissociative behavior regardless of concentration and is tolerant of prolonged periods of cycling in symmetric cells at high current densities (up to 2.0 mA cm-2, 400 h). The hydrocarbon functionalized carborane electrolyte presented here demonstrates >96% Coulombic efficiency when paired with a Mo6S8 cathode. This approach realizes a needed candidate to discover next-generation cathode materials that can enable the design of practical and commercially viable Mg batteries.

Nonsolvating Fluoroaromatic Cosolvent Enabled Long-Term Cycling of High-Voltage Lithium-Ion Batteries with Organosulfur Electrolytes.

The structure-activity relationships of nonsolvating cosolvents for organosulfur-based electrolyte systems were revealed. The performance of nonsolvating dilutant fluorobenzene (FB) was compared to various fluorinated ether dilutants in high-voltage electrolytes containing a concentration of 1.2 M LiPF6 dissolved in fluoroethylene carbonate (FEC), ethyl methyl sulfone (EMS), and the dilutant. In a high-voltage and high-loading LiNi0.8Mn0.1Co0.1O2 (NMC811) full cell configuration, the organosulfur-based electrolyte containing FB dilutant enabled superior electrochemical performance compared to the electrolytes using other nonsolvating fluorinated ether formulations. Moreover, the FB-containing electrolyte exhibited the highest ionic conductivity and lowest viscosity among all organosulfur-based electrolytes containing nonsolvating dilutant. These improvements are attributed to the enhanced physical properties of electrolyte and lithium-ion mobility. Furthermore, by employing first-principles simulations, the observed suppression of side reactions at high voltage is linked to FB's lower reactivity toward singlet dioxygen, which is likely produced at the NMC interface. Overall, FB is considered an excellent diluent that does not impede cell operation by mass decomposition at the cathode.

Lithium-sulfur battery cathode enabled by lithium-nitrile interaction.

Lithium sulfide is a promising cathode material for high-energy lithium ion batteries because, unlike elemental sulfur, it obviates the need for metallic lithium anodes. Like elemental sulfur, however, a successful lithium sulfide cathode requires an inherent mechanism for preventing lithium polysulfide dissolution and shuttling during electrochemical cycling. A new scheme is proposed to create composites based on lithium sulfide uniformly dispersed in a carbon host, which serve to sequester polysulfides. The synthesis methodology makes use of interactions between lithium ions in solution and nitrile groups uniformly distributed along the chain backbone of a polymer precursor (e.g., polyacrylonitrile), to control the distribution of lithium sulfide in the host material. The Li(2)S-carbon composites obtained by carbonizing the precursor are evaluated as cathode materials in a half-cell lithium battery, and are shown to yield high galvanic charge/discharge capacities and excellent Coulombic efficiency, demonstrating the effectiveness of the architecture in homogeneously distributing Li(2)S and in sequestering lithium polysulfides.

Highly Reversible Chevrel Phase Mo6Se8 Cathode with Low Voltage Hysteresis for Rechargeable Aluminum Batteries.

Rechargeable aluminum (Al) batteries have attracted considerable interest as potential large-scale energy storage technologies due to the abundance, high theoretical capacity, and high safety of Al. We report here a highly reversible Al-Mo6Se8 prototype cell with low discharge-charge hysteresis (approximately 50 mV under 30 mA g-1 at 50 °C), ultra-flat discharge plateau, and exceptional cycle stability: the reversible capacity retaining at a steady 77 mA h g-1 after more than 1800 cycles. The Al intercalation-extraction mechanism is probed with ex situ and operando XRD techniques, revealing the reversible intercalation reaction from Mo6Se8 to Al4/3Mo6Se8. The stable electrochemical performance and unambiguous intercalation mechanism of the Al-Mo6Se8 system provide an alternative for beyond-lithium battery technologies.

Synergistic Transition-Metal Selenide Heterostructure as a High-Performance Cathode for Rechargeable Aluminum Batteries.

We synthesize and characterize a rechargeable aluminum battery cathode material composed of heterostructured Co3Se4/ZnSe embedded in a hollow carbon matrix. This heterostructure is synthesized from a metal-organic framework composite, in which ZIF-8 is grown on the surface of ZIF-67 cube. Both experimental and theoretical studies indicate that the internal electric field across the heterostructure interface between Co3Se4 and ZnSe promotes the fast transport of electron and Al-ion diffusion. As a result, the heterostructured Co3Se4/ZnSe demonstrates superior specific capacity and cycle stability compared to the single-phase Co3Se4 and ZnSe cathode materials.

Sulfur-impregnated disordered carbon nanotubes cathode for lithium-sulfur batteries.

The commercialization of lithium-sulfur batteries is hindered by low cycle stability and low efficiency, which are induced by sulfur active material loss and polysulfide shuttle reaction through dissolution into electrolyte. In this study, sulfur-impregnated disordered carbon nanotubes are synthesized as cathode material for the lithium-sulfur battery. The obtained sulfur-carbon tube cathodes demonstrate superior cyclability and Coulombic efficiency. More importantly, the electrochemical characterization indicates a new stabilization mechanism of sulfur in carbon induced by heat treatment.

Correction: Cesium carbonate mediated C-H functionalization of perhalogenated 12-vertex carborane anions.

Correction for 'Cesium carbonate mediated C-H functionalization of perhalogenated 12-vertex carborane anions' by Sergio O. Lovera et al., Chem. Commun., 2022, 58, 4060-4062, DOI: https://doi.org/10.1039/D2CC00173J.

Cesium carbonate mediated C-H functionalization of perhalogenated 12-vertex carborane anions.

C-H functionalization of undecahalogenated carborane anions, [HCB11X11-] (X = Cl, Br, I), is performed with Cs2CO3 in acetonitrile. We show that the requisite Cl, Br and I carborane dianions can all be efficiently accessed with Cs2CO3. The utilization of Cs2CO3 eliminates the complications associated with competing E2 elimination reactions providing an efficient, more functional group tolerant, and broader scope than previously reported. The ensuing functionalized cages provide potential synthons for constructing advanced materials and other molecular architectures for various applications.

Performance Leap of Lithium Metal Batteries in LiPF6 Carbonate Electrolyte by a Phosphorus Pentoxide Acid Scavenger.

Phosphorus pentoxide (P2O5) is investigated as an acid scavenger to remove the acidic impurities in a commercial lithium hexafluorophosphate (LiPF6) carbonate electrolyte to improve the electrochemical properties of Li metal batteries. Nuclear magnetic resonance (NMR) measurements reveal the detailed reaction mechanisms of P2O5 with the LiPF6 electrolyte and its impurities, which removes hydrogen fluoride (HF) and difluorophosphoric acid (HPO2F2) and produces phosphorus oxyfluoride (POF3), OF2P-O-PF5- anions, and ethyl difluorophosphate (C2H5OPOF2) as new electrolyte species. The P2O5-modified LiPF6 electrolyte is chemically compatible with a Li metal anode and LiNi0.6Mn0.2Co0.2O2 (NMC622) cathode, generating a POxFy-rich solid electrolyte interphase (SEI) that leads to highly reversible Li electrodeposition, while eliminating transition metal dissolution and cathode particle cracking. The excellent electrochemical properties of the P2O5-modified LiPF6 electrolytes are demonstrated on Li||NMC622 pouch cells with 0.4 Ah capacity, 50 µm Li anode, 3 mAh cm-2 NMC622 cathode, and 3 g Ah-1 electrolyte/capacity ratio. The pouch cells can be galvanostatically cycled at C/3 for 230 cycles with 87.7% retention.

Cathode-Electrolyte Interfacial Processes in Lithium∥Sulfur Batteries under Lean Electrolyte Conditions.

The implementation of a low electrolyte/sulfur (E/S) ratio is essential to achieving high specific energy for lithium∥sulfur (Li∥S) batteries. In reality, however, the lean electrolyte condition results in low achievable capacity and inferior cyclability. In this study, we probe the interfacial processes on the sulfur cathode under the lean electrolyte condition using operando electrochemical impedance spectroscopy (EIS) and a galvanostatic intermittent titration technique (GITT). The operando EIS reveals a significant and rapid increase in the charge-transfer resistance during the transition from high-order polysulfides to low-order ones at a low E/S ratio, which is induced by a kinetic bottleneck at the interphase due to Li-ion mass transfer limitation. The GITT results confirm the kinetic bottleneck by revealing a large discharge overpotential during the transition phase. We further demonstrate that improving the adsorption of dissolved high-order polysulfides, a key step in the interfacial processes, can alleviate the kinetic limitation, thus enhancing the achievable capacity under the lean electrolyte condition.

Stability of Calcium Ion Battery Electrolytes: Predictions from Ab Initio Molecular Dynamics Simulations.

Multivalent batteries, such as magnesium-ion, calcium-ion, and zinc-ion batteries, have attracted significant attention as next-generation electrochemical energy storage devices to complement conventional lithium-ion batteries (LIBs). Among them, calcium-ion batteries (CIBs) are the least explored due to difficult reversible Ca deposition-dissolution. In this work, we examined the stability of four different Ca salts with weakly coordinating anions and three different solvents commonly employed in existing battery technologies to identify suitable candidates for CIBs. By employing Born-Oppenheimer molecular dynamics (BOMD) simulations on salt-Ca and solvent-Ca interfaces, we find that the tetraglyme solvent and carborane salt are promising candidates for CIBs. Due to the strong reducing nature of the calcium surface, the other salts and solvents readily decompose. We explain the microscopic mechanisms of salt/solvent decomposition on the Ca surface using time-dependent projected density of states, time-dependent charge-transfer plots, and climbing-image nudged elastic band calculations. Collectively, this work presents the first mechanistic assessment of the dynamical stability of candidate salts and solvents on a Ca surface using BOMD simulations, and provides a predictive path toward designing stable electrolytes for CIBs.

Enabling Magnesium Anodes by Tuning the Electrode/Electrolyte Interfacial Structure.

A new deposition mechanism is presented in this study to achieve highly reversible plating and stripping of magnesium (Mg) anodes for Mg-ion batteries. It is known that the reduction of electrolyte anions such as bis(trifluoromethanesulfonyl)imide (TFSI-) causes Mg surface passivation, resulting in poor electrochemical performance for Mg-ion batteries. We reveal that the addition of sodium cations (Na+) in Mg-ion electrolytes can fundamentally alter the interfacial chemistry and structure at the Mg anode surface. The molecular dynamics simulation suggests that Na+ cations contribute to a significant population in the interfacial double layer so that TFSI- anions are excluded from the immediate interface adjacent to the Mg anode. As a result, the TFSI- decomposition is largely suppressed so does the formation of passivation layers at the Mg surface. This mechanism is supported by our electrochemical, microscopic, and spectroscopic analyses. The resultant Mg deposition demonstrates smooth surface morphology and lowered overpotential compared to the pure Mg(TFSI)2 electrolyte.

Synthesis and Electrochemical Properties of Aluminum Hexafluorophosphate.

We report the first synthesis of aluminum hexafluorophosphate (Al(PF6)3) and its electrochemical properties in dimethyl sulfoxide (DMSO). The single crystal structure of the synthesized Al(PF6)3 is revealed as [Al(DMSO)6](PF6)3, and 0.25 M Al(PF6)3 in DMSO with high ionic conductivity is obtained. The purity of this electrolyte was further confirmed with nuclear magnetic resonance spectroscopy and electrospray ionization mass spectrometry. We then demonstrated the reversibility of Al deposition-stripping in this electrolyte using scanning electron microscopy and an X-ray photoelectron spectroscopy depth profiling study. The parasitic reaction involving DMSO decomposition during Al deposition is also identified via gas chromatography/electron ionization mass spectrometry.

Origin of the Giant Enhanced Raman Scattering by Sulfur Chains Encapsulated inside Single-Wall Carbon Nanotubes.

In this work, we explain the origin and the mechanism responsible for the strong enhancement of the Raman signal of sulfur chains encapsulated by single-wall carbon nanotubes by running resonance Raman measurements in a wide range of excitation energies for two nanotube samples with different diameter distributions. The Raman signal associated with the vibrational modes of the sulfur chain is observed when it is confined by small-diameter metallic nanotubes. Moreover, a strong enhancement of the Raman signal is observed for excitation energies corresponding to the formation of excited nanotube-chain-hybrid electronic states. Our hypothesis was further tested by high pressure Raman measurements and confirmed by density functional theory calculations of the electronic density of states of hybrid systems formed by sulfur chains encapsulated by different types of single-wall carbon nanotubes.

Interfaces in rechargeable magnesium batteries.

This minireview provides a concise overview on the development of electrolytes for rechargeable magnesium (Mg) batteries. It elucidates the intrinsic driving force of the evolution from Grignard-based electrolytes to electrolytes based on simple Mg salts. Additional discussion includes the key electrochemical processes at the interfaces in Mg electrolytes, with a focus on unaddressed issues and future research directions.

Properties of Thin Lithium Metal Electrodes in Carbonate Electrolytes with Realistic Parameters.

To understand the baseline performance of lithium (Li) anode in liquid electrolytes, the electrochemical and physical properties of the Li anode are studied with realistic parameters, including thin thickness (50 µm), practical areal capacity (1-4 mA h cm-2), practical areal current (0.5-2 mA cm-2), and low electrolyte/capacity ratio. Two different Li salts, lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI), are used to probe the effects of the electrolyte chemistry and concentration. The cycling of Li/Li symmetric cells, combined with the scanning electron microscopic investigation, demonstrates that the soft-short of Li/Li cells is induced by the continuous volume expansion of Li electrodes during cycling instead of dendrites. The volume change of a Li electrode is dictated by the depth of deposition and stripping (i.e., areal capacity) and the electrolyte/capacity ratio, with no strong correlation with the type of Li salt and concentration. On the other hand, the average Coulombic efficiency (CE) measurement demonstrates inherent correlation with the type of Li salt and its concentration in the electrolyte. Li electrode surface chemical analysis indicates that the fluoride-rich surface layer formed in the LiPF6 electrolyte can be detrimental to both CE and Li deposition-stripping overpotential.