Electrolytes For Li Batteries Research Articles

Widening the Operational Temperature Range of Lithium Batteries Using Flame-Retardant Sulfone-Based Highly Concentrated Electrolytes

High-concentration Li salt/sulfone solutions have attracted attention as promising liquid electrolytes for Li batteries owing to their high oxidative stability, nonflammability, and high Li+ ion transference number (t Li+). Herein, we report the temperature-dependent electrolyte properties of a sulfone-based ternary mixture composed of LiN(SO2F)2, sulfolane, and dimethyl sulfone, which enables Li batteries to operate in a wide temperature range. At −20 °C, the rate capability of a Li/LiCoO2 cell with the sulfone-based electrolyte was comparable to that with a conventional carbonate-based electrolyte, even though the ionic conductivity of the electrolyte was significantly lower in the former case (0.11 versus 2.92 mS cm−1). This is because the former electrolyte has a higher t Li+ value, effectively suppressing the concentration overpotential during cell charging and discharging. Moreover, the vapor pressure was much lower for the sulfone-based electrolyte than for the carbonate-based one, and the Li/LiCoO2 cell with the former electrolyte was successfully operated at 60 °C. This study provides insights into the characteristics of high-concentration electrolytes that affect the temperature dependence of Li battery performance.

Solvation-Property Relationship of Lithium-Sulfur Battery Electrolytes Probed via Potentiometric Measurements

The Li-S battery is a promising next-generation battery chemistry that offers high energy density and low cost. Li-S battery has a unique chemistry with intermediate sulfur species readily solvated in electrolytes, and understanding their implications is important from both practical and fundamental perspectives. However, the nature of the solvated sulfur species, their interactions and their impact on the battery remains elusive, in part due to the paucity of techniques. Recently, our group developed a potentiometric technique to probe the solvation free energy of electrolytes. In this study, we utilize the technique to formulate solvation-property relationships in various electrolytes and investigate its impact on the solvated lithium polysulides (LiPS). We find that solvation free energy impacts Li-S battery voltage profile, LiPS solubility, Li-S battery cyclability and the Li metal anode, which are all critical to the battery's performance. Weaker solvation leads to a lower 1st plateau voltage, higher 2nd plateau voltage, lower LiPS solubility, and superior cyclability of Li-S full cell and Li metal anode. We explore the mechanisms behind these observed phenomena and discuss the implications for the design of high-performance electrolytes for Li-S batteries.

Correlation between Ionic Conductivity and Mechanical Properties of Solid-like PEO-based Polymer Electrolyte.

Poly(ethylene glycol) methyl ether methacrylate polymer networks (PEO-based networks), with or without anionic bis(trifluoromethanesulfonyl)imide (TFSI)-grafted groups, are promising electrolytes for Li-metal all solid-state batteries. Nevertheless, there is a need to enhance our current understanding of the physicochemical characteristics of these polymer networks to meet the mechanical and ionic conductivity property requirements for Li battery electrolyte materials. To address this challenge, our goal is to investigate the impact of the cross-linking density of the PEO-based network and the ethylene oxide/lithium ratio on mechanical properties (such as glass transition temperature and storage modulus) and ionic conductivity. We have synthesized a series of cross-linked PEO-based polymers (si-SPE for single ion solid polymer electrolyte) via solvent-free radical copolymerization. These polymers are synthesized by using commercially available lithium 3-[(trifluoromethane)sulfonamidosulfonyl]propyl methacrylate (LiMTFSI), poly(ethylene glycol)methyl ether methacrylate (PEGM), and [poly(ethylene glycol) dimethacrylate] (PEGDM). In addition, we have synthesized a series of cross-linked PEO-based polymers (SPE for solid polymer electrolyte) using LiTFSI as the ionic species. Most of the resulting polymer films are amorphous, self-standing, flexible, homogeneous, and thermally stable. Interestingly, our research has revealed a correlation between ionic conductivity and mechanical properties in both the SPE and si-SPE series. Ionic conductivity increases as glass transition temperature, α relaxation temperature, and storage modulus decrease, suggesting that Li+ transport is influenced by polymer chain flexibility and Li+/EO interaction.

Optimization of two-dimensional solid-state electrolyte–anode interface by integrating zinc into composite anode with dual-conductive phases

Solid-state electrolytes (SSEs) are a solution to safety issues related to flammable organic electrolytes for Li batteries. Insufficient contact between the anode and SSE results in high interface resistance, thus causing the batteries to exhibit high charging and discharging overpotentials. Recently, we reduced the overpotential of Li stripping and plating by introducing a high proportion of dual-conductive phases into a composite anode. The current study investigates the interface resistance and stability of a composite electrode modified with Zn and a lower proportion of dual-conductive phases. Zn-cation-adsorbed Prussian blue is synthesized as an intermediate component for a Zn-modified composite electrode (Li-FeZnNC). The Li-FeZnNC symmetric cell presents a lower interface resistance and overpotential compared with Li-FeNC (without Zn modification) and Li-symmetric cells. The Li-FeZnNC symmetric cell shows high electrochemical stability during Li stripping and plating at different current densities and high stability for 200 h. Full batteries with a Li-FeZnNC composite anode, garnet-type SSE, and LiFePO4 cathode show low charging and discharging overpotentials, a capacity of 152 mAh g−1, and high stability for 200 cycles.

Solvation-property relationship of lithium-sulphur battery electrolytes

The Li-S battery is a promising next-generation battery chemistry that offers high energy density and low cost. The Li-S battery has a unique chemistry with intermediate sulphur species readily solvated in electrolytes, and understanding their implications is important from both practical and fundamental perspectives. In this study, we utilise the solvation free energy of electrolytes as a metric to formulate solvation-property relationships in various electrolytes and investigate their impact on the solvated lithium polysulphides. We find that solvation free energy influences Li-S battery voltage profile, lithium polysulphide solubility, Li-S battery cyclability and the Li metal anode; weaker solvation leads to lower 1st plateau voltage, higher 2nd plateau voltage, lower lithium polysulphide solubility, and superior cyclability of Li-S full cells and Li metal anodes. We believe that relationships delineated in this study can guide the design of high-performance electrolytes for Li-S batteries.

Highly disordered amorphous Li-battery electrolytes

Highly disordered amorphous Li-battery electrolytes

Linear ether-based highly concentrated electrolytes for Li-sulfur batteries.

Li-S batteries have attracted attention as next-generation rechargeable batteries owing to their high theoretical capacity and cost-effectiveness. Sparingly solvating electrolytes hold promise because they suppress the dissolution and shuttling of polysulfide intermediates to increase the coulombic efficiency and extend the cycle life. This study investigated the solubility of polysulfide (Li2S8) in a range of liquid electrolytes, including organic electrolytes, highly concentrated electrolytes, and ionic liquids. The Li2S8 solubility was well correlated with the donor number (DNNMR), estimated via23Na-NMR, and was lower than 100 mM_(elemental sulfur) in electrolytes with DNNMR < 14, regardless of the type of electrolyte. Highly concentrated electrolytes comprising lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and linear chain dialkyl ethers such as methyl propyl ether (MPE), n-butyl methyl ether (BME), and ethyl propyl ether (EPE) were studied as sparingly solvating electrolytes for Li-S batteries. Monomethyl ethers, such as BME, showed more pronounced Li-ion coordination and higher ionic conductivity, whereas the steric hindrance of the longer alkyl chains in EPE lowered the solvation number, enhanced ion association, and lowered the ionic conductivity despite the solvents having similar dielectric constants. The charge-discharge rate capabilities of Li-S cells with dialkyl ether-based electrolytes were more impressive than those of cells with a localized high-concentration electrolyte using sulfolane (SL) and hydrofluoroether (HFE), [Li(SL)2][TFSA]-2HFE. The higher rate performance was attributed to the superior Li-ion transport properties of the dialkyl ether-based electrolytes. A pouch-type cell using lightweight [Li(BME)3][TFSA] demonstrated an energy density exceeding 300 W h kg-1 under lean electrolyte conditions.

In search of widening the electrochemical window of solid electrolytes for Li-batteries: the La0.29Li0.12+xM1−xZrxO3 (M = Nb, Ta) perovskite-type systems

La0.29Li0.12+xM1−xZrxO3 (M = Nb, Ta) perovskites present remarkable properties as potential solid electrolytes. Nb oxides present a higher conductivity while Ta induces the widest electrochemical window.

Realizing a “solid to solid” process via in situ cathode electrolyte interface (CEI) by solvent-in-salt electrolyte for Li-S batteries

Realizing a “solid to solid” process via in situ cathode electrolyte interface (CEI) by solvent-in-salt electrolyte for Li-S batteries

High-concentration LiPF6/sulfone electrolytes: structure, transport properties, and battery application.

Non-flammable and oxidatively stable sulfones are promising electrolyte solvents for thermally stable high-voltage Li batteries. In addition, sulfolane-based high-concentration electrolytes (HCEs) show high Li+ ion transference numbers. However, LiPF6 has not yet been investigated as the main salt in sulfone-based HCEs for Li batteries. In this study, we investigated the phase behaviors, solvate structures, and transport properties of binary and ternary mixtures of LiPF6 and the following sulfone solvents: sulfolane (SL), dimethyl sulfone (DMS), ethyl methyl sulfone (EMS), and 3-methyl sulfolane (MSL). The stable crystalline solvates Li(SL)4PF6 and Li(DMS)2.5PF6 with high melting points were formed in the LiPF6/SL and LiPF6/DMS mixtures, respectively. In contrast, LiPF6/EMS, LiPF6/MSL, and LiPF6/SL/another sulfone mixtures remained liquids over a wide temperature range. Raman spectroscopy revealed that SL and another sulfone are competitively coordinated to Li+ ions to dissociate LiPF6 in the ternary mixtures. Although the ionic conductivity decreased with increasing LiPF6 concentration due to an increase in viscosity, Li+ ions diffused faster than PF6-via exchanging ligands in the HCE [LiPF6]/[SL]/[DMS] = 1/2/2, resulting in a higher Li ion transference number than that in conventional Li battery electrolytes.

Segmental molecular dynamics boosts Li-ion conduction in metal-organic solid electrolytes for Li-metal batteries

Metal-organic frameworks have emerged as a promising class of solid electrolytes for Li batteries. However, previous efforts to enhance Li-ion conductivity exclusively rely on the incorporation of solvents/salts and modifications of the rigid frameworks. In this study, we demonstrate metal-organic solid electrolytes with superionic conductivity by grafting hemilabile anionic chains with segmental motions in the pore channels of an Al-based metal-organic framework. High room-temperature ionic conductivity of 1.4×10–5 and 1.1×10−3 S cm–1 have been achieved under solvent-free and lean-solvent conditions, respectively. Solid-state nuclear magnetic resonance analysis reveals the localized molecular rotation and vibration of the anionic chains within in the rigid framework. Such segmental molecular dynamics would build Li transport highways and facilitate the Li-ion transport even without solvent. The metal-organic solid electrolyte shows excellent stability against Li metal anode, and the as-developed Li-metal batteries exhibit remarkable rate performance up to 4 C and long lifespan over 300 cycles. This work sheds light on the design principles of advanced metal-organic solid electrolytes for solid-state Li batteries.

Gradient Perovskite Ionic–Electronic Heterointerphases for Deep Cycling Mg Metal Anodes

AbstractDirect use of metals with low redox potential and high capacity as anodes can enable high energy density batteries. Mg metal has been considered as an ideal anode for its high critical current density and earth‐abundance, but its development has been impeded by the lacking of Mg‐compatible yet cost‐effective electrolyte. Artificial layers that prevent direct Mg‐electrolyte contact while permit Mg2+ to transport through, mitigate the anode–electrolyte incompatibility and allow industrial Li battery electrolyte analog to implement. However, charge transport through these artificial layers remains elusive, let alone to quantitatively design the layer thickness, components, and their distribution. Here it is shown that by using a gradient mixed ion‐electron conducting layer, which is prepared by a transferable perovskite film, the ion/electron transport path can be individually and continually tuned. It is found that a high Coulombic efficiency of 99.2% can be obtained for Mg anode in 0.5 M Mg(TFSI)2/DME electrolyte. It is also shown that long cycling full cells with a low N/P ratio (N/P ≈ 3.42 for Mg|Mo6S8, N/P ≈ 2.06 for Mg|Cu2‐xS) can be achieved. It is suggested that rational interfacial engineering will open new way to design practical Mg metal batteries.

Lithiation of an Agarose-Based Biopolymer Electrolyte for Li-S Batteries

Driven by the continually accelerating demand for energy storage, the development of batteries with high energy, high power, low cost, and high safety is an endless goal and eternally inspires researchers to pursue advanced energy storage technologies. Battery systems based on non-aqueous chemistry of lithium and sulfur have garnered overwhelming attention in the past decade. So far, many significant progresses have been made through rigorous research over the past decade. However, this battery technology still confronts considerably critical challenges. Due to their unique charge-discharge mechanism, electrochemical processes of a Li–S cell experience the formation of a sequence of soluble intermediate products existing in a variety form of polysulfides dissolved in the non-aqueous liquid electrolyte. Under the operating conditions of a Li-S cell, the solvated polysulfide species have a tendency to migrate from the positive electrode through the conventional porous separator to react with Li-metal anode. The shuttle of polysulfide can severely degrade the cell performance, lowers the cycling efficiency, and induces capacity fade during cycling. On the other hand, use of a lithium-metal anode in Li-S batteries would unavoidably lead to the additional persistent problem of Li dendrite with can easily penetrate a conventional porous separator and short the cell. These two problems — the polysulfide shuttle and lithium dendrites — are the most critical challenges for Li–S battery development. To overcome the above two issues, we present here a lithiated polymer membrane (as pictured in Figure 1a) which is developed from agarose (AG, with a unit molecular structure as displayed in Figure 1b) as a cation-selective electrolyte for Li–S batteries to suppress the polysulfide diffusion and to reduce the formation Li dendrite. Preliminary results of a Li ǁ AG ǁ Li symmetric cell showed that the membrane could sustain high-current-density operation with low overpotential (Figure 1c). The AG membrane exhibited amorphous structure (Figure 1d) and nonporous feature (Figure 1e). Our ongoing work includes the evaluation of dendrite-suppression capability and polysulfide-shuttle-inhibition function of the AG-based membrane, as well as the electrochemical performance of Li-S batteries with the AG membrane. In addition, ionic transport mechanism in the AG-based membrane will be presented as well. Figure 1

Designing High Entropy Amorphous Oxides for Li-Battery Electrolytes

Amorphous Li-oxides such as LiPON and amorphous Li garnet are considered as promising solid-state electrolytes for use in hybrid or all-solid-state oxide- and sulfide-based batteries as separators or protective layers.1-3 These materials possess several appealing characteristics, such as their intrinsic grain-boundary-free nature and relatively low manufacturing temperature (ranging from room temperature to 600 °C), which facilitates co-synthesis with Co-substituted or even Co-free cathodes that are unstable at standard electrolyte sintering temperatures. Alternatively, they can be applied as protective coatings toward Li anodes and other Li-free anode concepts, bridging the electrochemical stability voltage gap with liquid electrolytes or catholytes and preventing uneven interfacial reactions. The amorphous Li-oxides can be classified based on their structural entropy, i.e., the number and types of local bonding units (LBUs). As of today, ‘low entropy’ amorphous LiPON with only one type of LBU achieves the highest cycle number and battery lifetime.4 ‘High entropy’ amorphous Li-ion conductors, such as Li perovskites or Li garnets, exhibit an unusually high number of LBUs. local ordering and the Li-ion dynamics remain poorly understood, in part owing to the difficulty in characterizing their disordered states. This study employed a novel synthesis protocol to stabilize amorphous Al-doped Li garnets, representing so far the highest number of LBUs (≥ 4) in an amorphous Li-ion conductor.5 We resolved their phase evolution and local structures by a combination of spectroscopy, microscopy, and calorimetry techniques. A much wider (<680 °C) but processing-friendly temperature range was identified to stabilize various amorphous phases with edge- and face-sharing Zr, La, and Li LBUs that do not conform to the formation rules for Zachariasen’s glasses. These amorphous Li-ion conductors reveal an unusual setting in which Li and Zr act as network formers and La acts as a network modifier, with the highest Li-dynamics observed for smaller Li–O and Zr–O coordination among the amorphous phases. Our insight provides fundamental guidelines for the phase, local structure, and Li-transport modulation for amorphous Li garnets and pave the way for their integration in next-generation solid-state or hybrid battery designs with enhanced safety and lifetime. Acknowledgments Y.Z. acknowledges financial support provided by the MIT Energy Initiative fellowship offered by ExxonMobil. This research was supported by Samsung Electronics. This research was performed in part at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which was supported by the National Science Foundation under NSF award no. 1541959. CNS is a part of Harvard University. This research used resources of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, and was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357, and the Canadian Light Source and its funding partners.

A Multiscale Hollow Spherical LATP Active Filler Improves Conductivity and Mechanical Strength in Composite Solid Electrolytes for Li Batteries

A Multiscale Hollow Spherical LATP Active Filler Improves Conductivity and Mechanical Strength in Composite Solid Electrolytes for Li Batteries

Suite of High-Throughput Experiments for Screening Solid Electrolytes for Li Batteries

All-solid lithium batteries are an important technology to achieve safer batteries with potentially longer life. Efforts over the past decade have generated a vast list of candidate solid electrolytes. High-throughput methods have already been useful in this context, but studies have been limited to room temperature ionic conductivities. Although a high ionic conductivity is necessary, this single property is insufficient to ensure function in a solid battery. Herein, a suite of high-throughput methods is introduced where 64 samples are synthesized simultaneously. Herein, we demonstrate for the first time the high-throughput capability of obtaining: (1) ionic conductivities at and above room temperature to extract activation energies, (2) electronic conductivities to evaluate the risk of dendrite growth within the electrolytes, (3) electrochemical stability window, and (4) chemical stability against lithium. Importantly, the stability window is obtained by testing the electrolyte in a composite electrode with conductive carbon, thereby avoiding the overestimations of stability that are rampant in the literature. Each method was validated using two reference materials chosen as they show high contrast for all properties. The results systematically show excellent reproducibility and good agreement with the literature. This suite of techniques provides meaningful properties necessary to evaluate candidate solid electrolytes.

Li+ Dynamics of Liquid Electrolytes Nanoconfined in Metal-Organic Frameworks.

Metal–organic frameworks (MOFs) are excellent platforms to design hybrid electrolytes for Li batteries with liquid-like transport and stability against lithium dendrites. We report on Li+ dynamics in quasi-solid electrolytes consisting in Mg-MOF-74 soaked with LiClO4–propylene carbonate (PC) and LiClO4–ethylene carbonate (EC)/dimethyl carbonate (DMC) solutions by combining studies of ion conductivity, nuclear magnetic resonance (NMR) characterization, and spin relaxometry. We investigate nanoconfinement of liquid inside MOFs to characterize the adsorption/solvation mechanism at the basis of Li+ migration in these materials. NMR supports that the liquid is nanoconfined in framework micropores, strongly interacting with their walls and that the nature of the solvent affects Li+ migration in MOFs. Contrary to the “free’’ liquid electrolytes, faster ion dynamics and higher Li+ mobility take place in LiClO4–PC electrolytes when nanoconfined in MOFs demonstrating superionic conductor behavior (conductivity σrt > 0.1 mS cm–1, transport number tLi+ > 0.7). Such properties, including a more stable Li electrodeposition, make MOF-hybrid electrolytes promising for high-power and safer lithium-ion batteries.

Molecular Level Differences in Ionic Solvation and Transport Behavior in Ethylene Oxide-Based Homopolymer and Block Copolymer Electrolytes.

Block copolymer electrolytes (BCE) such as polystyrene-block-poly(ethylene oxide) (SEO) blended with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and composed of mechanically robust insulating and rubbery conducting nanodomains are promising solid-state electrolytes for Li batteries. Here, we compare ionic solvation, association, distribution, and conductivity in SEO-LiTFSI BCEs and their homopolymer PEO-LiTFSI analogs toward a fundamental understanding of the maximum in conductivity and transport mechanisms as a function of salt concentration. Ionic conductivity measurements reveal that SEO-LiTFSI and PEO-LiTFSI exhibit similar behaviors up to a Li/EO ratio of 1/12, where roughly half of the available solvation sites in the system are filled, and conductivity is maximized. As the Li/EO ratios increase to 1/5 the conductivity, of the PEO-LiTFSI drops nearly 3-fold, while the conductivity of SEO-LiTFSI remains constant. FTIR spectroscopy reveals that additional Li cations in the homopolymer electrolyte are complexed by additional EO units when the Li/EO ratio exceeds 1/12, while in the BCE, the proportion of complexed and uncomplexed EO units remains constant; Raman spectroscopy data at the same concentrations show that Li cations in the SEO-LiTFSI samples tend to coordinate more to their counteranions. Atomistic-scale molecular dynamics simulations corroborate these results and further show that associated ions tend to segregate to the SEO-LiTFSI domain interfaces. The opportunity for "excess" salt to be sequestered at BCE interfaces results in the retention of an optimum ratio of uncompleted and complexed PEO solvation sites in the middle of the conductive nanodomains of the BCE and maximized conductivity over a broad range of salt concentrations.

High-voltage liquid electrolytes for Li batteries: progress and perspectives.

Since the advent of the Li ion batteries (LIBs), the energy density has been tripled, mainly attributed to the increase of the electrode capacities. Now, the capacity of transition metal oxide cathodes is approaching the limit due to the stability limitation of the electrolytes. To further promote the energy density of LIBs, the most promising strategies are to enhance the cut-off voltage of the prevailing cathodes or explore novel high-capacity and high-voltage cathode materials, and also replacing the graphite anode with Si/Si-C or Li metal. However, the commercial ethylene carbonate (EC)-based electrolytes with relatively low anodic stability of ∼4.3 V vs. Li+/Li cannot sustain high-voltage cathodes. The bottleneck restricting the electrochemical performance in Li batteries has veered towards new electrolyte compositions catering for aggressive next-generation cathodes and Si/Si-C or Li metal anodes, since the oxidation-resistance of the electrolytes and the in situ formed cathode electrolyte interphase (CEI) layers at the high-voltage cathodes and solid electrolyte interphase (SEI) layers on anodes critically control the electrochemical performance of these high-voltage Li batteries. In this review, we present a comprehensive and in-depth overview on the recent advances, fundamental mechanisms, scientific challenges, and design strategies for the novel high-voltage electrolyte systems, especially focused on stability issues of the electrolytes, the compatibility and interactions between the electrolytes and the electrodes, and reaction mechanisms. Finally, novel insights, promising directions and potential solutions for high voltage electrolytes associated with effective SEI/CEI layers are proposed to motivate revolutionary next-generation high-voltage Li battery chemistries.

(Invited) 3D Printing of Polymer Composite Electrolytes for Li Batteries

Polymer-based batteries offer new opportunities to transform the traditional manufacturing of Li ion batteries. This presentation provides an overview on efforts in the PI's lab to manufacture thermally safe and high energy density batteries using 3D printing techniques. In the first part of this presentation, I will showcase our 3D printing work for PVDF-based polymer electrolytes reinforced with TiO2 nanoparticles. We utilized elevated temperature printing technique to avoid the use of solvents that are often detrimental to the final printed structures due to the need for their evaporation resulting in shrinkage and structural collapse. We controlled the mechanical properties and shear properties of our electrolytes by controlling the concentration of TiO2 and use of ionic liquids to achieve 3D printing. The printed electrolyte make strong adhesion with the electrode material evidenced by low interfacial resistance across the electrode-electrolyte interface. The printed full batteries demonstrated successful cycling indicating of their potential for high cycle applications. In addition, we developed PEO-based polymer electrolytes reinforced with 2D materials in order to control the rheological properties of the ink electrolyte. The printed structures showed better thermal management indicating that the alignment of 2D materials can be an effective approach to control the printing of polymer batteries.