Rechargeable Calcium-Sulfur Batteries Enabled by an Efficient Borate-Based Electrolyte

Cathode materials for non-aqueous calcium rechargeable batteries

Calcium rechargeable batteries based on divalent charge carriers have the potential to meet the future demands for large-scale energy storage applications, due to the crustal abundance of Ca element and the high capacity and high safety of Ca metal anodes. The discernible progress in electrolyte and anode materials has put calcium battery technology a step closer to practice. However, the pursuit of high-voltage, high-capacity and stable cathode materials had been formidable because of the sluggish ion migration kinetics and the instability of host lattices during Ca2+ insertion and extraction. Unlocking the potential of Ca rechargeable batteries particularly hinges on the strategic identification of high-performance cathode materials. Herein, this review summarizes the representative strategies to develop novel cathode materials that allow reversible accommodation of Ca2+ ions for high energy output. The cathode materials can be classified into intercalation-type (layered structure, polyanionic compounds, and Prussian blue analogues) and conversion-type (organic materials, sulfur, and oxygen). The scrutinization of their performances and drawbacks sheds light on the current stage of cathode material advancement and provides informative suggestions for future studies to develop advanced calcium rechargeable batteries with competitive performance.

Building a High-Potential Silver-Sulfur Redox Reaction Based on the Hard-Soft Acid-Base Theory

Sulfur holds immense promise for battery applications owing to its abundant availability, low cost, and high capacity. Currently, sulfur is commonly combined with alkali or alkaline earth metals in metal-sulfur batteries. However, these batteries universally face challenges in cycling stability due to the inevitable issue of polysulfide dissolution and shuttling. Additionally, the inferior stability of metal sulfide discharge compounds results in low S0/S2- redox potentials (<-0.41 V vs SHE). Herein, we leverage the principle of the hard-soft acid-base theory to introduce a novel silver-sulfur (Ag-S) battery system, which operates on the reaction between the soft acid of Ag+ and the soft base of S2-. Due to their high reaction affinity, the discharge compound of silver sulfide (Ag2S) is intrinsically insoluble and fundamentally stable. This not only resolves the polysulfide dissolution issue but also leads to a predominantly high S0/S2- redox potential (+1.0 V vs. SHE). We thus exploit the Ag-S reaction for a primary zinc battery application, which exhibits a high capacity of ∼620 mAh g-1 and a high voltage of ∼1.45 V. This work offers valuable insights into the application of classic chemistry theories in the development of innovative energy storage devices.

Rechargeable Metal-Sulfur Batteries: Key Materials to Mechanisms

Rechargeable metal-sulfur batteries are considered promising candidates for energy storage due to their high energy density along with high natural abundance and low cost of raw materials. However, they could not yet be practically implemented due to several key challenges: (i) poor conductivity of sulfur and the discharge product metal sulfide, causing sluggish redox kinetics, (ii) polysulfide shuttling, and (iii) parasitic side reactions between the electrolyte and the metal anode. To overcome these obstacles, numerous strategies have been explored, including modifications to the cathode, anode, electrolyte, and binder. In this review, the fundamental principles and challenges of metal-sulfur batteries are first discussed. Second, the latest research on metal-sulfur batteries is presented and discussed, covering their material design, synthesis methods, and electrochemical performances. Third, emerging advanced characterization techniques that reveal the working mechanisms of metal-sulfur batteries are highlighted. Finally, the possible future research directions for the practical applications of metal-sulfur batteries are discussed. This comprehensive review aims to provide experimental strategies and theoretical guidance for designing and understanding the intricacies of metal-sulfur batteries; thus, it can illuminate promising pathways for progressing high-energy-density metal-sulfur battery systems.

A computational study of electron transport in dynamic tetrahydrofuran and ethylene carbonate solvents on a Ca metal anode

Calcium-ion batteries offer many advantages to the current lithium-ion technology in terms of cost, sourcing materials, and potential for higher energy density. However, calcium-ion batteries suffer from lack of a stable electrolyte due to reduction from the anode. Building off of our recent work investigating the stability of two representative electrolyte solvents, tetrahydrofuran (THF) and ethylene carbonate (EC), we now use ab initio molecular dynamics (AIMD) and the non-equilibrium Green's function technique in conjunction with density functional theory (NEGF-DFT) to investigate charge transport as the solvent molecules dynamically interact with the anode surface. THF maintained a relatively consistent conductance throughout the trajectory, although some jumps in the conductance were attributed to THF molecular rearrangement. EC exhibited a large amount of molecular decomposition, and a corresponding decrease in conductance of several orders of magnitude was noted. Through this analysis, we show that molecular decomposition and early-stage solid-electrolyte interphase (SEI) formation plays a major role in the robustness of charge transport as the system evolves in time and with temperature.

A rechargeable Ca/Cl2 battery

Rechargeable calcium (Ca) metal batteries are promising candidates for sustainable energy storage due to the abundance of Ca in Earth's crust and the advantageous theoretical capacity and voltage of these batteries. However, the development of practical Ca metal batteries has been severely hampered by the current cathode chemistries, which limit the available energy and power densities, as well as their insufficient capacity retention and low-temperature capability. Here, we describe the rechargeable Ca/Cl2 battery based on a reversible cathode redox reaction between CaCl2 and Cl2, which is enabled by the use of lithium difluoro(oxalate)borate as a key electrolyte mediator to facilitate the dissociation and distribution of Cl-based species and Ca2+. Our rechargeable Ca/Cl2 battery can deliver discharge voltages of 3 V and exhibits remarkable specific capacity (1000 mAh g-1) and rate capability (500 mA g-1). In addition, the excellent capacity retention (96.5% after 30 days) and low-temperature capability (down to 0 °C) allow us to overcome the long-standing bottleneck of rechargeable Ca metal batteries.

A novel calcium fluorinated alkoxyaluminate salt as a next step towards Ca metal anode rechargeable batteries

Ca metal anode rechargeable batteries are seen as a sustainable high-energy density and high-voltage alternative to the current Li-ion battery technology due to the low redox potential of Ca metal and abundance of Ca. Electrolytes are key enablers on the path towards next-generation battery systems. Within this work, we synthesize a new calcium tetrakis(hexafluoroisopropyloxy) aluminate salt, Ca[Al(hfip)₄]₂, and benchmark it versus the state-of-the-art boron analogue Ca[B(hfip)₄]₂. The newly developed aluminate-based electrolyte exhibits improved performance in terms of conductivity, Ca plating/stripping efficiency, and oxidative stability as well as Ca battery cell performance. A marked improvement of 0.5 V higher oxidative stability can pave the path towards high-voltage Ca batteries. A critical issue of solvent quality during salt synthesis is identified as well as solvent decomposition at the Ca metal/electrolyte interface, which leads to passivation of the Ca metal anode. However, the new aluminate salt with preferable electrochemical properties over the existing boron analogue opens up a new area for future Ca battery research based on aluminium compounds.

Aprotic Sulfur-Metal Batteries: Lithium and Beyond

Metal-sulfur batteries constitute an extraordinary research playground that ranges from fundamental science to applied technologies. However, besides the widely explored Li-S system, a remarkable lack of understanding hinders advancements and performance in all other metal-sulfur systems. In fact, similarities and differences make all generalizations highly inconsistent, thus unavoidably suggesting the need for extensive research explorations for each formulation. Here we review critically the most remarkable open challenges that still hinder the full development of metal-S battery formulations, starting from the lithium benchmark and addressing Na, K, Mg, and Ca metal systems. Our aim is to draw an updated picture of the recent efforts in the field and to shed light on the most promising innovation paths that can pave the way to breakthroughs in the fundamental comprehension of these systems or in battery performance.

Long-Cycle-Life Calcium Battery with a High-Capacity Conversion Cathode Enabled by a Ca2+/Li+ Hybrid Electrolyte

Calcium (Ca) batteries represent an attractive option for electrochemical energy storage due to physicochemical and economic reasons. The standard reduction potential of Ca (-2.87 V) is close to Li and promises a wide voltage window for Ca full batteries, while the high abundance of Ca in the earth's crust implicates low material costs. However, the development of Ca batteries is currently hindered by technical issues such as the lack of compatible electrolytes for reversible Ca²⁺ plating/stripping and high-capacity cathodes with fast kinetics. Herein, we employed FeS₂ as a conversion cathode material and combined it with a Li⁺/Ca²⁺ hybrid electrolyte for Ca batteries. We demonstrate that Li⁺ ions ensured reversible Ca²⁺ plating/stripping on the Ca metal anode with a small overpotential. At the same time, they enable the conversion of FeS₂, offering high discharge capacity. As a result, the Ca/FeS₂ cell demonstrated an excellent long-term cycling performance with a high discharge capacity of 303 mAh g^-1 over 200 cycles. Even though the practical application of such an approach is questionable due to the high quantity of electrolytes, we believe that our scientific findings still provide new directions for studying Ca batteries with long-term cycling.

Catalytic Effects of Electrodes and Electrolytes in Metal-Sulfur Batteries: Progress and Prospective

Metal-sulfur (M-S) batteries are promising energy-storage devices due to their advantages such as large energy density and the low cost of the raw materials. However, M-S batteries suffer from many drawbacks. Endowing the electrodes and electrolytes with the proper catalytic activity is crucial to improve the electrochemical properties of M-S batteries. With regard to the S cathodes, advanced electrode materials with enhanced electrocatalytic effects can capture polysulfides and accelerate electrochemical conversion and, as for the metal anodes, the proper electrode materials can provide active sites for metal deposition to reduce the deposition potential barrier and control the electroplating or stripping process. Moreover, an advanced electrolyte with desirable design can catalyze electrochemical reactions on the cathode and anode in high-performance M-S batteries. In this review, recent progress pertaining to the design of advanced electrode materials and electrolytes with the proper catalytic effects is summarized. The current progress of S cathodes and metal anodes in different types of M-S batteries are discussed and future development directions are described. The objective is to provide a comprehensive review on the current state-of-the-art S cathodes and metal anodes in M-S batteries and research guidance for future development of this important class of batteries.

Sodium Borates: Expanding the Electrolyte Selection for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are a promising grid-level storage technology due to the abundance and low cost of sodium. The development of new electrolytes for SIBs is imperative since it impacts battery life and capacity. Currently, sodium hexafluorophosphate (NaPF6 ) is used as the benchmark salt, but is highly hygroscopic and generates toxic HF. This work describes the synthesis of a series of sodium borate salts, with electrochemical studies revealing that Na[B(hfip)4 ]⋅DME (hfip=hexafluoroisopropyloxy, Oi PrF ) and Na[B(pp)2 ] (pp=perfluorinated pinacolato, O2 C2 (CF3 )4 ) have excellent electrochemical performance. The [B(pp)2 ]- anion also exhibits a high tolerance to air and water. Both electrolytes give more stable electrode-electrolyte interfaces than conventionally used NaPF6 , as demonstrated by impedance spectroscopy and cyclic voltammetry. Furthermore, they give greater cycling stability and comparable capacity to NaPF6 for SIBs, as shown in commercial pouch cells.

Monocarborane cluster as a stable fluorine-free calcium battery electrolyte

High-energy-density and low-cost calcium (Ca) batteries have been proposed as ‘beyond-Li-ion’ electrochemical energy storage devices. However, they have seen limited progress due to challenges associated with developing electrolytes showing reductive/oxidative stabilities and high ionic conductivities. This paper describes a calcium monocarborane cluster salt in a mixed solvent as a Ca-battery electrolyte with high anodic stability (up to 4 V vs. Ca²⁺/Ca), high ionic conductivity (4 mS cm⁻¹), and high Coulombic efficiency for Ca plating/stripping at room temperature. The developed electrolyte is a promising candidate for use in room-temperature rechargeable Ca batteries.