Low-temperature Electrochemical Performance Research Articles

Starch Gel Electrolyte and its Interaction with Trivalent Aluminum for Aqueous Aluminum-Ion Batteries: Enhanced Low Temperature Electrochemical Performance.

This study explores trivalent Al interaction with aqueous starch gel in the presence of two different anions through salting effect. Salting-out nature of Al2(SO4)3·18H2O with starch gel causes precipitation of starch; this happens due to competitive anion-water complex formation over starch-water interaction, thereby reducing polymer solubility. Salting-in effect of AlCl3 with starch gel happens through Al3+ cation interaction with hydroxyl group of starch and increases polymer solubility, making gel electrolyte viable for battery applications. Prepared gel electrolyte exhibits ionic conductivity of 1.59mS cm-1 and a high tAl 3+ value of 0.77. The gel electrolyte's performance is studied using two different cathodes, the Al|MoO3 cell employing starch gel electrolyte achieves discharge capacity of 193mA h g-1 and Al|MnO2 cell achieves discharge capacity of 140mA h g-1 @0.1 A g-1 for first cycle. The diffusion coefficient of both cells using starch gel electrolyte is calculated and found to be 2.1×10-11cm2 s-1 for Al|MoO3 and 3.1×10-11 cm2 s-1 for Al|MnO2 cells. The Al|MoO3 cell at lower temperature shows improved electrochemical performance with a specific capacity retention of ≈87.8% over 90 cycles. This kind of aqueous gel electrolyte operating at low temperature broadens the application for next generation sustainable batteries.

Lithium fluorosulfonate-induced low-resistance interphase boosting low-temperature performance of commercial graphite/LiFePO4 pouch batteries

The performance of Li-ion batteries (LIBs) at sub-ambient temperatures is limited by the resistive interphases due to electrolyte decomposition, particularly on the anode surface. In this study, lithium fluorosulfonate (LFS) was added to commercial electrolytes to enhance the low-temperature electrochemical performance of LiFePO4 (LFP)/graphite (Gr) pouch cells. The addition of LFS significantly reduced the charge transfer resistance of the anode, substantially extending the cycle life and discharge capacity of commercial LFP/Gr pouch cells at −10 and −30 °C. Compared with the capacity retention rate of the baseline electrolyte at −10 °C (80 % after 25cycles), the capacity retention rate of the LFS electrolyte after 100 cycles under 0.5 C/0.5 C was retained at 94 %. Further mechanistic studies showed that the LFS additive induced the formation of a solid electrolyte interphase (SEI) film comprising inorganic-rich LiF, Li2SO4, and additional organic fluorides and sulfides to maintain good stability at the Gr/electrolyte interface during low-temperature operation. LFS suppressed electrolyte decomposition by forming a robust and low-resistance SEI film on the anode. These results demonstrate that LFS is a promising electrolyte additive for low-temperature LFP/Gr pouch cells.

Achieving Highly Stable Zn Metal Anodes at Low Temperature via Regulating Electrolyte Solvation Structure.

Zinc metal is an attractive anode material for rechargeable aqueous Zn-ion batteries (ZIBs). However, the dendrite growth, water-induced parasitic reactions, and freezing problem of aqueous electrolyte at low temperatures are the major roadblocks that hinder the widely commercialization of ZIBs. Herein, tetrahydrofuran (THF) is proposed as the electrolyte additive to improve the reversibility and stability of Zn anode. Theoretical calculation and experimental results reveal that the introduction of THF into the aqueous electrolyte can optimize the solvation structure which can effectively alleviate the H2O-induced side reactions and protect the Zn anode from corrosion. Moreover, THF can act as a hydrogen bond acceptor to interact with H2O, which can greatly reduce the activity of free H2O in electrolytes and improve the low-temperature electrochemical performance of Zn anode. As a result, the Zn anodes demonstrate high cyclic stability for 2800h at 27°C and over 4000h at -10°C at 1.0mA cm-2 /1.0mAhcm-2. The full cell exhibits excellent cyclic stability and rate capability at 27 and -10°C. This work is expected to provide a new approach to regulate the aqueous electrolyte and Zn anode interface chemistry for highly stable and reversible Zn anodes.

In Situ Polymerization of Hydrogel Electrolyte on Electrodes Enabling the Flexible All-Hydrogel Supercapacitors with Low-Temperature Adaptability.

All-hydrogel supercapacitors are emerging as promising power sources for next-generation wearable electronics due to their intrinsic mechanical flexibility, eco-friendliness, and enhanced safety. However, the insufficient interfacial adhesion between the electrode and electrolyte and the frozen hydrogel matrices at subzero temperatures largely limit the practical applications of all-hydrogel supercapacitors. Here, an all-hydrogel supercapacitor is reported with robust interfacial contact and anti-freezing property, fabricated by in situ polymerizing hydrogel electrolyte onto hydrogel electrodes. The robust interfacial adhesion is developed by the synergistic effect of a tough hydrogel matrix and topological entanglements. Meanwhile, the incorporation of zinc chloride (ZnCl2) in the hydrogel electrolyte prevents the freezing of water solvents and endows the all-hydrogel supercapacitor with mechanical flexibility and fatigue resistance across a wide temperature range of 20°C to -60°C. Such all-hydrogel supercapacitor demonstrates satisfactory low-temperature electrochemical performance, delivering a high energy density of 11mWhcm-2 and excellent cycling stability with a capacitance retention of 90% over 10000 cycles at -40°C. Notably, the fabricated all-hydrogel supercapacitor can endure dynamic deformations and operate well under 2000 tension cycles even at -40°C, without experiencing delamination and electrochemical failure. This work offers a promising strategy for flexible energy storage devices with low-temperature adaptability.

Na3(VO)2(PO4)2F coated carbon nanotubes: A cathode material with high-specific capacity for aqueous zinc-ion batteries

Aqueous zinc-ion batteries (AZIBs) are one of the candidate technologies for large-scale energy storage systems due to their environmental friendliness, safety and resourcefulness. However, designing and synthesizing high-performance cathode materials with practical application value is currently a great challenge. Herein, we synthesize a carbon-nanotube-coated Na3(VO)2(PO4)2F (Na3(VO)2(PO4)2F@CNT) cathode material for AZIBs, which has a continuous and interconnected ion transport channel structure. A Zn2+/H+ co-intercalation mechanism is revealed by ex-situ X-ray diffraction (XRD) and ex-situ X-ray photoelectron spectroscopy (XPS) characterizations. The Na3(VO)2(PO4)2F@CNT cathode material exhibits a high specific capacity of 143 mAh‧g Wan et al. (2019). at 0.2 C in the electrolyte of 3 M Zn(OTf)2 aqueous solution. In addition, Na3(VO)2(PO4)2F@CNT shows a good low-temperature electrochemical performance in the electrolyte of 2 M Zn(OTf)2 deionized water/ethylene glycol (DIW/EG) mixed solution with a different ion storage mechanism. This study provides a new polyanionic cathode material for AZIBs and an idea to modulate the electrolyte polarity to enhance the zinc storage capacity of the cathode material.

The optimization of the electrolyte for low temperature LiFePO4-graphite battery

The design of electrolyte is crucial to improve the low temperature electrochemical performance of the LiFePO4-graphite battery. In this paper, by optimizing the solvent composition and introducing film-forming additives in the electrolyte, the low temperature performance of the battery is significantly improved. In detail, the EP (ethyl propionate) solvent with low melting point and low viscosity is added to the original EC (ethylene carbonate)-EMC (ethyl methyl carbonate)-DMC (dimethyl carbonate) solvent systems, which improved the ionic conductivity of the electrolyte, especially at low temperature (−40 °C). On the other hand, the introduction of FEC (fluoroethylene carbonate)/DTD (ethylene sulfate) film-forming agents promotes the formation of SEI (solid electrolyte interface) film, and the SEI is thin and uniform. As a result, the LiFePO4-graphite battery exhibits high capacity and long cycle life under low temperature conditions.

Improving dual electrodes compatibility through tailoring solvation structures enabling high-performance and low-temperature Li||LiFePO4 batteries

Li||LiFePO4 (LFP) batteries have good stability and high energy density. However, they exhibit unsatisfactory low-temperature electrochemical performance. Due to the fragile interfacial passivation layers and sluggish kinetics, commercial electrolytes fail to simultaneously achieve acceptable stabilization with dual electrodes in low-temperature Li||LFP batteries. Herein, a novel localized high-concentration electrolyte (LHCE) with great dual-electrodes compatibility is proposed to match with the low-temperature Li||LFP batteries. With increasing local concentration, the FSI- sequentially replaces the solvent molecules and enters the first solvation sheath, forming the anion-dominated solvation structures. This effectively suppresses free solvents decomposition and constructs the anion-derived passivation layers with inorganic-rich components, further contributing to the rapid transport kinetics and endowing the LHCE with great dual electrodes compatibility. These dual-electrodes co-stabilization effects of the LHCE are originally clarified in the low-temperature Li||LFP batteries. The designed LHCE also delivers low freezing point (-99.8 ℃), high ionic conductivity (2.4 mS cm−1 at −40 ℃), and superior stability (>4.7 V vs. Li/Li+). Hence, the Li||LFP batteries with LHCE possess superb cyclic stability at low temperatures, delivering a high discharge capacity of 120 mAh g−1 over 300 cycles at −20 ℃. Moreover, compared to commercial electrolytes, LHCE endows the Li||LFP batteries with superior low-temperature performances under practical conditions, including limited Li anode (3 mAh cm−2) and a wide temperature range (30 ℃ to −40 ℃).

Aqueous Organic Batteries Using Proton as Charge Carrier.

Benefiting from the merits of low cost, non-flammability and high operational safety, aqueous rechargeable batteries have emerged as promising candidates for large-scale energy storage applications. Among various metal-ion/non-metallic charge carriers, proton (H+ ) as a charge carrier possesses numerous unique properties such as a fast proton diffusion dynamics, a low molar mass and a small hydrated ion radius, which endow aqueous proton batteries (APBs) with a salient rate capability, a long-term life span and an excellent low-temperature electrochemical performance. In addition, redox-active organic molecules, with the advantages of structural diversity, rich proton-storage sites and abundant resources, are considered as attractive electrode materials for APBs. However, as far as application is concerned, the charge storage and transport mechanisms of organic electrodes in APBs is still in infancy. Therefore, finding suitable electrode materials and uncovering the H+ storage mechanisms are significant for the applications of organic materials in APBs. Herein, the latest research progresses on organic materials such as small molecules and polymers for APBs are reviewed. Furthermore, a comprehensive summary and evaluation of APBs employing organic electrodes as anode and/or cathode is provided, especially on their low-temperature and high-power performances, along with systematic discussions for guiding the rational design and the construction of APBs based on organic electrodes. This article is protected by copyright. All rights reserved.

Unlocking Reversible Silicon Redox for High-Performing Chlorine Batteries.

Chlorine (Cl)-based batteries such as Li/Cl2 batteries are recognized as promising candidates for energy storage with low cost and high performance. However, the current use of Li metal anodes in Cl-based batteries has raised serious concerns regarding safety, cost, and production complexity. More importantly, the well-documented parasitic reactions between Li metal and Cl-based electrolytes require a large excess of Li metal, which inevitably sacrifices the electrochemical performance of the full cell. Therefore, it is crucial but challenging to establish new anode chemistry, particularly with electrochemical reversibility, for Cl-based batteries. Here we show, for the first time, reversible Si redox in Cl-based batteries through efficient electrolyte dilution and anode/electrolyte interface passivation using 1,2-dichloroethane and cyclized polyacrylonitrile as key mediators. Our Si anode chemistry enables significantly increased cycling stability and shelf lives compared with conventional Li metal anodes. It also avoids the use of a large excess of anode materials, thus enabling the first rechargeable Cl2 full battery with remarkable energy and power densities of 809 Wh kg-1 and 4,277 W kg-1 , respectively. The Si anode chemistry affords fast kinetics with remarkable rate capability and low-temperature electrochemical performance, indicating its great potential in practical applications.

Lithium Difluorophosphate (LiPO2F2): An Electrolyte Additive to Help Boost Low-Temperature Behaviors for Lithium-Ion Batteries

A promising lithium salt of Li difluorophosphate (LiPO2F2) is introduced and added to the basic electrolyte (1 M LiPF6 + dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC)/propylene carbonate (PC)/fluoroethylene carbonate (FEC)) to enhance the electrochemical performance of lithium-ion batteries by changing its concentration at low temperatures. Experiments show that LiPO2F2 can obviously improve the electrolyte’s ionic conductivity, especially. Compared with the baseline (6.29, 5.14, 4.62, 3.77, and 3.15 mS cm–1) under the same conditions, the ionic conductivities of the 2 wt % LiPO2F2-containing electrolyte (2 LiPO2F2) are 7.18, 5.32, 5.23, 4.04, and 3.57 mS cm–1. From this, it is clear that the integrated low-temperature electrochemical performances have been significantly improved. The specific discharge capacities of the 2 wt % LiPO2F2-added electrolyte at 0.2C are 161.4, 157.2, 148.9, and 137.5 mAh g–1 in the range from 25 to −40 °C, which are significantly much higher compared with the basic electrolyte (145.3, 149.4, 135.1, and 107.0 mAh g–1, respectively). Additionally, the addition of LiPO2F2 can promote profoundly the composition and morphology characteristics of the cathode–electrolyte interface (CEI) film; namely, it is a uneven lithium alkyl carboxylate before the addition of LiPO2F2; after that, it becomes an even LiF film, and meanwhile, the new film will facilitate Li+ transport in the electrolyte, further boosting the eletrochemical performance of the cells under low temperatures. At −40 °C under the same current density, the specific discharge capacity is 81.97 mAh g–1 after 50 cycles, while at the same temperature, the specific discharge capacity of the basic electrolyte battery is only 33.37 mAh g–1; that is, the discharge capacity has a staggering 145.6% improvement at −40 °C. In conclusion, the lithium-ion batteries containing LiPO2F2 may meet the application requirements of ultra-low-temperature conditions.

Solid Phase Fusion: A Simple and Eco-Friendly Modification Strategy for Li1.2Ni0.2Mn0.6O2Li-Rich Cathode Materials

Li-rich cathode materials, with promising specific capacity, have attracted continued research attention. However, when charged to a high voltage, the materials experience rapid capacity decline and transition metal ion dissolution over the long cycle of the battery. In this work, fast ion conductor modified Li[Li0.2Ni0.2Mn0.6]O2(LLNM), as a Li-rich cathode material, has been effectively synthesized by the solid phase fusion technology (SFT). Typically, the structural transformation, metal ion dissolution and interface side reactions with the electrolyte are efficiently suppressed by the NaNbO3modified layer. The modified cathode materials exhibit optimal cycling stability with an initial discharge capacity of 186.8 mA h g−1and merely 0.22 V voltage fading after 200 cycles at 1 C. And this material shows better high and low temperature electrochemical performances. Moreover, SFT is an eco-friendly method, which is suitable for large scale synthesis, and shows good commercial perspective.

Extending the low-temperature operation of sodium metal batteries combining linear and cyclic ether-based electrolyte solutions

Nonaqueous sodium-based batteries are ideal candidates for the next generation of electrochemical energy storage devices. However, despite the promising performance at ambient temperature, their low-temperature (e.g., < 0 °C) operation is detrimentally affected by the increase in the electrolyte resistance and solid electrolyte interphase (SEI) instability. Here, to circumvent these issues, we propose specific electrolyte formulations comprising linear and cyclic ether-based solvents and sodium trifluoromethanesulfonate salt that are thermally stable down to −150 °C and enable the formation of a stable SEI at low temperatures. When tested in the Na||Na coin cell configuration, the low-temperature electrolytes enable long-term cycling down to −80 °C. Via ex situ physicochemical (e.g., X-ray photoelectron spectroscopy, cryogenic transmission electron microscopy and atomic force microscopy) electrode measurements and density functional theory calculations, we investigate the mechanisms responsible for efficient low-temperature electrochemical performance. We also report the assembly and testing between −20 °C and −60 °C of full Na||Na3V2(PO4)3 coin cells. The cell tested at −40 °C shows an initial discharge capacity of 68 mAh g−1 with a capacity retention of approximately 94% after 100 cycles at 22 mA g−1.

Hierarchically nitrogen-doped mesoporous carbon nanospheres with dual ion adsorption capability for superior rate and ultra-stable zinc ion hybrid supercapacitors

Although significant progress has been achieved in developing high energy aqueous zinc ion hybrid super-capacitors (ZHSCs), the sluggish diffusion of zinc ion (Zn2+) and unsatisfactory cathodes still hinder their energy density and cycling life span. This work demonstrates the use of nitrogen-doped mesoporous carbon nanospheres (NMCSs) with appropriately hierarchical pore distribution and enhanced zinc ion storage capability for efficient Zn2+ storage. The as-prepared aqueous ZHSC delivers a significant specific capacity of 157.8 mA h g−1, a maximum energy density of 126.2 W h kg−1 at 0.2 A g−1, and an ultra-high power density of 39.9 kW kg−1 with a quick charge time of 5.5 s. Furthermore, the ZHSC demonstrates an ultra-long cycling life span of 50,000 cycles with an exciting capacity retention of 96.2%. More interestingly, a new type of planar ZHSC is fabricated with outstanding low-temperature electrochemical performance, landmark volumetric energy density of 31.6 mW h cm−3, and excellent serial and parallel integration. Mechanism investigation verifies that the superior electrochemical capability is due to the synergistic effect of cation and anion adsorption, as well as the reversible chemical adsorption of NMCSs. This work provides not only an innovative strategy to construct and develop novel high-performance ZHSCs, but also a deeper understanding of the electrochemical characteristics of ZHSCs.

Utilization of surface-induced capacitive behavior: A promising strategy to improve low temperature performance of Li-ion batteries

Graphite anode suffers from tremendous electrochemical performance fading at low temperature. To investigate the reason of this fading at low temperature from Li+ storage mechanism perspective, three different carbon structures with various amounts of defect and interlayer spacing are employed in this work, including natural graphite, hard carbon (HC) and mesocarbon microbead (MCMB). The galvanostatic charge/discharge (GCD) tests indicate that amorphous HC electrode shows higher lithiation capacity than graphite and MCMB electrode at −20 °C. Cyclic voltammetry tests reveal that higher lithiation capacity of HC electrode mainly stems from the surface-induced capacitive storage mechanism. This work confirms that the utilization of surface-induced capacitive storage mechanism could be considered as a promising strategy to improve the low temperature electrochemical performance.

One-pot synthesis of anti-freezing carrageenan/polyacrylamide double-network hydrogel electrolyte for low-temperature flexible supercapacitors

Stretchable carrageenan (CG) based double-network (DN) hydrogel has great potential in flexible energy storage devices. To achieve high conductivity at sub-zero temperatures, one-pot synthesis method is adopted for the fabrication of CG/polyacrylamide DN hydrogel with high LiCl concentration (termed CG/PAAm-xLi). In this method, a high temperature above 90 °C is required to fully melt CG network in the presence of high-concentration LiCl and to add the monomers of the second polymer network. Furthermore, by incorporation of small amounts of KCl, the obtained CG/PAAm-7Li/K DN hydrogel shows strong tensile properties with tensile elongation of 567.7%, and fracture energy of 967.3 kJ/m2. In addition, the conductivity of the CG/PAAm-7Li/K DN hydrogel achieves 1.9 S/m at − 40 °C. The flexible supercapacitor prepared by using CG/PAAm-7Li/K hydrogel electrolyte exhibits excellent low-temperature electrochemical properties (specific capacitance of 73.4 F/g, and capacitance retention of 95.6% after 20,000 cycles at − 40 °C). This high-temperature strategy may be extended to construct other ionic polysaccharide-based hydrogel electrolytes with high mechanical property and excellent low-temperature electrochemical performance, showing a broad prospect in the field of flexible electronics.

One-pot synthesis and multifunctional surface modification of lithium-rich manganese-based cathode for enhanced structural stability and low-temperature performance

High-energy–density lithium-rich Li1.2Ni0.13Co0.13Mn0.54O2 is regarded as one of the most promising cathode materials for lithium-ion batteries. However, its practical application is restricted by critical kinetics drawbacks and poor low-temperature electrochemical performances. In this research, Li1.2Ni0.13Co0.13Mn0.54O2 submicron particles coated by a Li7La3Zr2O12 (LLZO) layer and co-doped by La/Zr cations has been fabricated via a facile one-pot sol–gel technique and subsequent heat treatment. The coating LLZO layer with a few nanometers is able to build a rapid lithium-ion transport channel for adjacent particles and suppress severe side reactions between active material and the electrolyte. Moreover, large-radius La/Zr cations co-doping can broaden the diffusion paths of lithium ions, hinder the detrimental structural transformation, and improve the electrochemical structure stability of the cathode during repeated cycles. Owing to numerous merits from this multifunctional surface modification strategy, the modified Li1.2Ni0.13Co0.13Mn0.54O2 composite exhibits the significantly decreased interface impedance, enhanced Li+ diffusion kinetics and mitigated phase transformation, as well as excellent low-temperature electrochemical performance. It can contribute ultrahigh capacities of 173.8 mAh g−1 at −10 ℃ and 134.1 mAh g−1 at −20 ℃, respectively, displaying great application prospects of Li-rich cathode materials.

Flexible Antifreeze Zn-Ion Hybrid Supercapacitor Based on Gel Electrolyte with Graphene Electrodes.

Zn-ion energy storage devices employing hydrogel electrolytes are considered as promising candidates for flexible and wearable electronics applications. This is because of their safe nature, low cost, and good mechanical characteristics. However, conventional hydrogel electrolytes face limitation at subzero temperatures. Herein, we report an antifreezing, safe, and nontoxic gel electrolyte based on the poly(vinyl alcohol) (PVA)/Zn/ethylene glycol system. The optimal gel electrolyte membrane exhibits a high ionic conductivity (15.03 mS cm-1 at room temperature) and promising antifreezing performance (9.05 mS cm-1 at -20 °C and 3.53 mS cm-1 at -40 °C). Moreover, the antifreezing gel electrolyte can suppress the growth of Zn dendrites to display a uniform Zn plating/stripping behavior. Also, a flexible antifreezing Zn-ion hybrid supercapacitor fabricated with the optimum antifreezing gel electrolyte membrane exhibits excellent electrochemical properties. The supercapacitor possesses a high specific capacity of 247.7 F g-1 at room temperature under a high working voltage of 2 V. It also displays an outstanding cyclic stability at room temperature. Moreover, the supercapacitor shows an extraordinary electrochemical behavior and cyclic stability over up to 30 000 cycles at -20 °C under a current load of 5 A g-1, demonstrating its outstanding low-temperature electrochemical performance. Besides, the antifreezing supercapacitor device also offers high flexibility under different deformation conditions. Therefore, it is believed that this work provides a simplistic method of realizing the application of flexible antifreezing Zn-ion energy storage devices in a subzero-temperature environment.

Correlation between lithium-ion accessibility to the electrolyte–active material interface and low-temperature electrochemical performance

With continuous growth in the demand for energy storage, batteries are increasingly required to operate under extreme environmental conditions, including low temperatures. In this study, we systematically investigate the correlation between lithium-ion accessibility to the electrolyte–active material interface and the low-temperature electrochemical properties of electrode materials. Holey graphene with different number densities of in-plane defects on the graphene basal plane is synthesized and employed for the synthesis of Li4Ti5O12/holey graphene composites. The electrochemical performances of the Li4Ti5O12/holey graphene composites are evaluated at operating temperatures of –25 to 50 °C. As the number density of in-plane defects in the holey graphene increased, the polarization decreased and the apparent lithium-ion diffusion coefficient increased at operating temperatures of –25, 0, and 25 °C, resulting in improved low-temperature electrochemical performance. Based on the comparative analysis of the electrochemical properties of the Li4Ti5O12/holey graphene composites, the introduction of in-plane defects in the carbon layer is an effective strategy for improving the low-temperature electrochemical performance of electrode materials. The results suggest that the lithium-ion accessibility to the electrolyte–active material interface becomes more important to the electrochemical properties at low operating temperatures.

Implementing Low Temperature Strategies to Advance “Really Cool” Molten Sodium Batteries

Though batteries for portable electronics and electric vehicles continue to advance at an impressive rate, the needs for safe, low-cost, reliable batteries suitable for large-scale implementation remain unsatisfied. Here, we describe advances toward a promising new class of low temperature molten sodium-halide batteries, capable of safe, reliable operation near 100oC, a dramatic reduction from the ~300oC operating temperatures common to traditional molten sodium batteries. While realizing this low temperature goal promises reduced system costs and improved material lifetimes, it introduces a host of new material performance challenges including molten catholyte phase control and electrochemistry, solid state separator performance, and charge transfer at critical solid-liquid interfaces. The new battery design described here relies on highly conductive NaSICON-based ion-conducting membranes to separate a molten sodium anode and, critically, a molten inorganic sodium iodide (NaI)-based halide salt catholyte. Through engineering of novel molten salt catholyte compositions, we have identified several chemistries capable of producing a high voltage (>3V) while avoiding problems of thermal runaway and electrolyte flammability. Our studies further show that catholyte composition not only impacts low temperature electrochemical performance, but also the accessible capacity and even techno-economic viability of the system. Implementing these catholytes in lab-scale prototypes with charge-transfer interfaces engineered for low-temperature operation, we have demonstrated 5 months of stable electrochemical cycling at 110oC. Continued refinement of these material-enabled strategies promises a new class of grid-scale energy storage technologies based on molten sodium.Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Improve the low-temperature electrochemical performance of Li4Ti5O12 anode materials by ion doping

We adopt the strategy of doping ions of Mg2+, Cr3+, and F− into Li4Ti5O12 (LTO) to substitute Li, Ti, and O, respectively (called the corresponding sample Mg-LTO, Cr-LTO, and F-LTO, respectively), and investigated its influences on the low-temperature electrochemical performance of LTO. After doping, the electrical conductivity of Mg-LTO, Cr-LTO, and F-LTO increased from less than < 10− 13 S cm− 1 to 3.07 × 10− 7 S cm− 1, 5.57 × 10− 7 S cm− 1, and 7.04 × 10− 7 S cm− 1, respectively. Structural refinement shows that doping has little effect on the radius of the crystal diffusion sites. Further research shows that the main reason for the improvement of low-temperature electrochemical performance is that doping affects the electrical conductivity, micromorphology, and phase composition of LTO. At −20 °C/10 C (1C corresponding to 175 mAh g− 1), the discharge capacities of Mg-LTO, Cr-LTO and, F-LTO are 113 mAh g− 1, 123 mAh g− 1, and 128 mAh g− 1, respectively. As a contrast, there is no discharge capacity for Pure LTO at the same conditions. After 600 cycles at −20 °C/5C, the discharge capacities of the sample of Pure LTO, Mg-LTO, Cr-LTO, and F-LTO are 69.7 mAh g− 1, 107.5 mAh g− 1, 142.3 mAh g− 1, and 133.2 mAh g− 1, respectively. Mg-LTO, Cr-LTO, and F-LTO exhibit excellent low-temperature rate performance and cycling stability. The related electrochemical factors and materials structure mechanisms involved were discussed in detail.