Electrolyte Salt Concentration Research Articles

The Role of the Anion in Concentrated Electrolytes for Lithium-Sulfur Batteries

Highly concentrated electrolytes show promise in enhancing lithium-sulfur (Li-S) battery performance by mitigating polysulfide (PS) solubility. The role of the salt anion for the performance improvement(s) is however not well understood. Here a systematic characterization using (concentrated) electrolytes based on three different salts: LiTFSI, LiTf, and LiTDI, in a common DOL:DME solvent mixture is reported for a wide range of physicochemical and electrochemical properties: ionic conductivity, density, viscosity, speciation, and PS solubility. While increased salt concentration in general improves Li-S battery performance, the role of the salt anion introduces complexity. The 2 m LiTDI-based electrolyte, with a slightly higher viscosity and lower PS solubility, outperforms the LiTFSI-based counterpart in terms of accessible reversible capacity. Conversely, the 2 m LiTf-based electrolyte exhibits subpar performance due to the formation of ionic aggregates that renders more free solvent and, therefore higher PS solubility, which, however can be improved by using a 5 m concentrated electrolyte. Hence, using electrolyte salt concentration as a rational design route demands an understanding of the local molecular structure, largely determined/affected by the choice of anion, as well as how it connects to the global properties and in the end improved Li-S battery performance.

Unlocking the role of electrolyte concentration for Na-O2 batteries

Na-O2 batteries have attracted great interest in recent years mainly due to their high energy density, in theory having prospects to outperform the commercialized lithium-ion batteries. In the quest for optimization, a recently explored approach is to use highly concentrated electrolytes (HCEs). The knowledge of molecular level of solvation as function of electrolyte concentration and its impact on Na-O2 battery performance is, however, still very limited. In this work, experimental and computational methods are used to characterize the cation solvation and when the emergence of anions into the cation first solvation shell occurs, which affects the de-solvation process and formation of discharge products. Furthermore, the solid electrolyte interphase (SEI) formed using HCEs demonstrates presence of anion fragments, with poorer protection of the Na metal anode. Moreover, the use of HCEs is also linked to lowered capacity, possibly due to a decrease in the size of the cubic-shaped discharge products as the electrolyte concentration increases, causing clogging of the pores of the air cathode. Thus, increasing the electrolyte salt concentration seems to have a detrimental effect on the cyclability of Na-O2 batteries. Instead, electrolytes with a lower than conventional salt concentration show the best performance, which highlights the importance of carefully tuning the cation solvation alongside overall physico-chemical properties to enhance battery performance.

Experimental study of the effect of different pulse charging patterns on lithium-ion battery charging

This study aims to experimentally investigate the impact of different pulse charging patterns on the charging time and performance of lithium-ion batteries at room temperature. Experimental results indicate that most pulse charging protocols have a positive impact on shortening the total charging time compared to the CC-CV charging method with the same input charge capacity. This effect is observed not only in LFP batteries but also in NMC batteries. Three pulse charging patterns are studied: constant current charge (C–C), charge rest (C–R), and charge discharge (C-D). The C-D mode results in the shortest charging time and the smallest cell internal resistance. Compared to the 1C CC-CV baseline, the pulse C-D mode reduces the charging time by approximately 6.8 %, decreases the average pulse internal resistance by about 17.5 %, and increases the remaining discharge capacity ratio by 3.76 % after 800 cycles. Additionally, a pseudo-2D physics model is used to study the internal effects on the battery using the pulse C-D mode. The results show that using the pulse charging method (C-D) results in a more even distribution of electrolyte salt concentration and electrode particle lithium concentration across the cell, thereby improving battery performance.

Role of Salt Concentration in Stabilizing Charged Ni-Rich Cathode Interfaces in Li-Ion Batteries.

The cathode-electrolyte interphase (CEI) in Li-ion batteries plays a key role in suppressing undesired side reactions while facilitating Li-ion transport. Ni-rich layered cathode materials offer improved energy densities, but their high interfacial reactivities can negatively impact the cycle life and rate performance. Here we investigate the role of electrolyte salt concentration, specifically LiPF6 (0.5-5 m), in altering the interfacial reactivity of charged LiN0.8Mn0.1Co0.1O2 (NMC811) cathodes in standard carbonate-based electrolytes (EC/EMC vol %/vol % 3:7). Extended potential holds of NMC811/Li4Ti5O12 (LTO) cells reveal that the parasitic electrolyte oxidation currents observed are strongly dependent on the electrolyte salt concentration. X-ray photoelectron and absorption spectroscopy (XPS/XAS) reveal that a thicker LixPOyFz-/LiF-rich CEI is formed in the higher concentration electrolytes. This suppresses reactions with solvent molecules resulting in a thinner, or less-dense, reduced surface layer (RSL) with lower charge transfer resistance and lower oxidation currents at high potentials. The thicker CEI also limits access of acidic species to the RSL suppressing transition-metal dissolution into the electrolyte, as confirmed by nuclear magnetic resonance (NMR) spectroscopy and inductively coupled plasma optical emission spectroscopy (ICP-OES). This provides insight into the main degradation processes occurring at Ni-rich cathode interfaces in contact with carbonate-based electrolytes and how electrolyte formulation can help to mitigate these.

Reevaluate low-concentration ether-based electrolytes for lithium metal batteries

Ether is regarded as an exceptional solvent in liquid electrolytes for lithium (Li) metal batteries due to its outstanding compatibility with Li metal anode. Low salt concentration in ether-based electrolytes (LCEEs) is highly favorable to reducing the cost of future Li metal batteries. Nevertheless, LCEEs have been reported to exhibit poor anodic stability (as low as 4.0 V vs. Li/Li+), therefore considered unsuitable for practical batteries using LiNixMnyCozO2 as cathodes and charge up to 4.4 V. Here, using 1.0 M lithium bis(fluorosulfonyl)imide (LiFSI) in 1,2-dimethoxyethane (DME) as the baseline, we demonstrate the intrinsic anodic stability window of LCEE to be above 4.5 V. We revealed the true failure mechanism of LCEE to be various electrochemical reactions with different working electrodes (WEs). We further provide electrolyte and electrode design rules to prevent LCEE decomposition, as well as electrode and packaging materials corrosion at high voltage. This study highlights the importance of previously overlooked battery component compatibility in electrolyte development and offers valuable insights into battery engineering considerations toward high-energy and cost-effective Li metal batteries.

Interfacial Chemistry in Aqueous Lithium-Ion Batteries: A Case Study of V2O5 in Dilute Aqueous Electrolytes.

Aqueous lithium-ion batteries (ALIBs) are promising for large-scale energy storage systems because of the cost-effective, intrinsically safe, and environmentally friendly properties of aqueous electrolytes. Practical application is however impeded by interfacial side-reactions and the narrow electrochemical stability window (ESW) of aqueous electrolytes. Even though higher electrolyte salt concentrations (e.g., water-in-salt electrolyte) enhance performance by widening the ESW, the nature and extent of side-reaction processes are debated and more fundamental understanding thereof is needed. Herein, the interfacial chemistry of one of the most popular electrode materials, V2O5, for aqueous batteries is systematically explored by a unique set of operando analytical techniques. By monitoring electrode/electrolyte interphase deposition, electrolyte pH, and gas evolution, the highly dynamic formation/dissolution of V2O5/V2O4, Li2CO3 and LiF during dis-/charge is demonstrated and shown to be coupled with electrolyte decomposition and conductive carbon oxidation, regardless of electrolyte salt concentration. The study provides deeper understanding of interfacial chemistry of active materials under variable proton activity in aqueous electrolytes, hence guiding the design of more effective electrode/electrolyte interfaces for ALIBs and beyond.

The Impact of Concentration and Electric-Field Dependent Susceptibilities on Electrolyte Models

Electrolytes have a significant role in a range of scientific and technological fields such as fundamental electrochemistry, biochemical systems, semiconductors, and industrial devices. A widely used mathematical framework for describing electrolytes is based on Poisson-Nernst-Planck-type equations and their successors[1], but the Poisson equation assumes a constant dielectric susceptibility. However, recent research has developed a thermodynamically consistent model for the dielectric susceptibility that accounts for its dependence on both concentration and electric field [5,6]. This allows for the inclusion of the dependency of the susceptibility on salt concentration in liquid electrolytes, referred to as dielectric decrement[2,3,4], as well as the decline in susceptibility for large electric fields[8], known as dielectric saturation.The research findings highlight the importance of considering the variable dielectric susceptibility in electrolyte models, particularly in relation to the decrease in dielectric permittivity in front of a charged electrode surface[7]. The study demonstrates the impact of the concentration and electric-field-dependent susceptibility on the overall equation system for electrochemical double layers. By providing insights into the theoretical aspects of the approach, the researchers aim to contribute to the development of more accurate and comprehensive models of electrolytes that consider the complex interplay of various factors.

Spatially Selective Solvation Structure by Electronegative Micro-Arrays for Stable Lithium-Metal Anode Interface.

For electrolytes with conventional lithium salt concentration, it is not easy to generate sufficient anion-derived beneficial inorganic components to stabilize the electrolyte-lithium metal anode interface due to the repulsion of the free-state anions by the anode. In this study, the above issues are solved through the strong interaction between electronegative materials and lithium ions (Li+ ). A locally high Li+ concentration strategy is proposed by preparing micro-arrays of electronegative nano-hydroxyapatite (nHA) on the Cu foil. It is found that the oxygenatoms in the phosphate group (-PO4 ) of the nHA can strongly adsorb Li+ to form a locally Li+ -rich region, which increases the probability of anions interacting with Li+ . The formation of more Li+ -coordinated anions at the electrolyte-anode interface can reduce the Li+ de-solvation energy barrier, and enable the anions to completely decompose into lithium fluoride (LiF) and lithium nitride (Li3 N) on the Li metal anode. The interfacial transfer dynamics is accelerated and the Li dendrites are effectively suppressed. Under high current density, the anode exhibits a long lifespan with high Coulombic efficiency and small polarization voltage. The nHA micro-arrays achieve the targeted solvation structure at the electrolyte-anode interface while ensuring conventional lithium salt concentration in the bulk electrolyte.

Revealing the Equilibrium of Lithium Cations Across a Solid–Electrolyte Interface by T_1 NMR Relaxation

Battery fast charging is pivotal for broader acceptance of electric mobility. While demonstrated for lithium titanate (text {Li}_4text {Ti}_5text {O}_{12}, LTO) anodes, the underlying mechanisms are still poorly understood. Recently, NMR T_1 relaxation time constants of {}^7text {Li} in the bulk of LTO were found to change if the surrounding electrolyte was altered. It was explained by interdiffusion of mobile lithium ions between the two phases, facilitated by unpinning of polarons from surface defects and leading to a pseudocapacitive effect that potentially influences fast charging. This effect is explored further by systematically varying the lithium salt concentration in an aprotic electrolyte in contact with LTO. Spectrally resolved {}^7text {Li}T_1 NMR relaxation times were used as a measure for bulk concentration changes of paramagnetic polaronic charges in LTO. Correlation of electrolyte concentration and {}^7text {Li}T_1 showed qualitatively different behavior above and below a salt concentration of about 5 mM, leading to a relaxation dispersion maximum in the LTO bulk. At intermediate concentrations, relaxation was consistent with a {}^7text {Li} exchange equilibrium between LTO and electrolyte. Upon contact of the two phases, yet without insertion into an electrochemical cell or applying an external potential, lithium ions redistributed between LTO bulk and liquid electrolyte. The results can be understood analogously to the distribution of mobile lithium ions between two phases separated by a text {Li}^{+} permeable membrane. This is the first demonstration of such an equilibrium for non-faradaic lithium exchange at an interface between a solid ceramic electrode and a liquid electrolyte outside an electrochemical cell, substantiating our previous hypothesis of a polaron-supported mechanism. This study provides a basis for more quantitative (surface)-defect engineering, which is key to optimize battery fast-charging properties.

Research on the thermo-electrochemical behavior during the electrodeposition process of lithium metal battery

Lithium metal batteries (LMBs) are promising renewable energy storage device with higher energy density. During charging/discharging, the electrodeposition or dendrite growth on surface of Li electrode and temperature influence thermo-electrochemical behaviors in LMBs greatly. Coupling electrochemical reaction kinetic equations and thermal relations in electrode, as well as diffusion migrating equations of Li+ ion in electrolyte is employed to investigate the effects of temperature, charging current density, volume fraction of active solid and Li concentration in positive electrode, as well as salt concentration in electrolyte on the entropy production relating to power consumption and Joule heat, as well as cell voltage and overpotential in positive electrode and negative electrode with protrusion surface during charging. More polarization reactions together with larger heat generation take place, and cause more entropy generation in area of protrusion. The voltage efficiency is defined to evaluate thermo-electrochemical behaviors in LMBs, and the higher voltage efficiency occurs with the decrease in charging current density. The 93.6% voltage efficiency can be obtained in the case with solid-phase volume fraction of 0.518 in positive electrode under charging current density of 43.608A/m2. All results from investigation will be beneficial to obtain better thermo-electrochemical behavior in Li metal batteries.

Depletion of Electrolyte Salt during Calendaric Aging at High State-of-Charge in Li-Ion Batteries

Batteries using Ni-rich cathode active materials are the most common cells in the automotive sector. They offer a high energy density at competitive prices which makes them attractive in a wide-range of applications.[1,2] However, a higher Ni-content results in more challenges for battery lifetime in comparison to low-Ni materials. These challenges are aggravated at elevated temperatures and high SOCs, stemming from detrimental side-reaction, such as parasitic heat and oxygen release.[3–6] In this study, we investigated an aging effect which is relevant in a wide-range of cell formats ranging from small single layer pouch cells to automotive cells. Upon storing cells at a high SOC at elevated temperatures (30 days, 45 °C) a significant drop in rate-performance was visible afterwards. At the same time, no active Li-loss and no significant increase in 10 second DCIR resistance at 50%SOC was seen. We propose a mechanism of LiPF6-loss during storage as the main reason for the worsened rate-performance.By conducting ion chromatography (IC) and NMR analysis of electrolytes from aged cells, it was found that cells stored at an upper cut-off potential of 4.2 to 4.4 V lose a significant fraction of LiPF6 after a 30-day storage at 45 °C. In addition to these storage experiments, cells with fresh electrolyte, but lower electrolyte salt concentrations (1 M LiPF6 to 0.2 M LiPF6) were built, with the goal of using them as a calibration curve to map the worsened rate performance after storage onto a corresponding electrolyte salt concentration without the need of a destructive analysis of the cells. As seen in Figure 1, the rate performance of freshly built single layer pouch cell is highly dependent on the electrolyte salt concentration. While all cells show good discharge capacities at C/10, at higher discharge rates the lower concentration cells show a worsened performance. 1 M LP572 cells show excellent rate-capabilities up to a rate of 2C and a 0.2 M cell already has a significant drop in discharge capacity at C/2. Using OEMS, electrochemical impedance measurements and theoretical models we further elucidate the mechanism of LiPF6-loss at elevated temperatures and high SOCs.

Limitations of Fast Charging of High Energy NMC‐based Lithium‐Ion Batteries: A Numerical Study

AbstractThe aim of this work is to answer the question: how to realize high energy and high‐power lithium‐ion batteries. Lithium‐metal and graphite anodes with nickel manganese cobalt (NMC) cathodes of varying thickness are investigated with finite element modelling. The overpotential analysis is performed to pinpoint the source of losses and the possible ways to decrease them. The electrolyte overpotential, resulting from the salt concentration gradient and leading to saturation and depletion of lithium in parts of the cell is identified as the main factor causing poor specific capacity at high discharge/charge currents. The influence of various parameters, including concentration and transference number of lithium salt in the electrolyte, NMC particle size, electrolyte conductivity and the exchange current density, on the galvanostatic response of modelled battery cells is discussed. The increase of the transference number would improve the performance as this would decrease the electrolyte salt concentration gradient. Lithium depletion effect can be also minimized by elevating the initial electrolyte salt concentration, as well as by increasing the porosity of the cathode, particularly at the cathode/separator boundary.

Li-Ion Battery Active Material Impedance Analysis I: Comparison of Measured NCM 111 Kinetics with Butler-Volmer Equation Based Predictions

The expression for the exchange current density to describe the intercalation kinetics of Li-ion battery materials proposed by Newman and coworkers has been used extensively for battery modeling, however its applicability to existing battery materials should be validated. Here we show an electrochemical impedance spectroscopy (EIS) analysis of the kinetic behavior of NCM 111 as a function of electrolyte salt concentration and state-of-charge (SOC) and compare it to the proposed theory. An areal capacity dependent EIS analysis first gives insights into the feasibility of measuring kinetic and transport parameters, including the solid diffusion resistance of lithium, showing that low-areal capacity electrodes are required to predominantly probe the kinetics. We then show how the charge transfer kinetics follow a Butler-Volmer type concentration dependent behavior for lower concentrated electrolytes (≤1.5 M) but deviate from the proposed theory at higher salt concentrations. A further SOC dependent analysis shows how NCM 111 generally follows the proposed theory of U-shaped symmetric kinetics, but the limited oxidative stability window leads to practically asymmetric kinetics for charging and discharging. This asymmetry is visible in NCM 111 lithiation and delithiation rate tests, where upon lithiation the kinetics generally become slower for higher degrees of lithiation, limiting the performance.

Investigation of the impacts of simple electrolytes and hydrotrope on the interaction of ceftriaxone sodium with cetylpyridinium chloride at numerous study temperatures.

Herein, interactions between cetylpyridinium chloride (CPC) and ceftriaxone sodium (CTS) were investigated applying conductivity technique. Impacts of the nature of additives (e.g. electrolytes or hydrotrope (HDT)), change of temperatures (from 298.15 to 323.15K), and concentration variation of CTS/additives were assessed on the micellization of CPC + CTS mixture. The conductometric analysis of critical micelle concentration (CMC) with respect to the concentration reveals that the CMC values were increased with the increase in CTS concentration. In terms of using different mediums, CMC did not differ much with the increase in electrolyte salt (NaCl, Na2SO4) concentration, but increased significantly with the rise of HDT (NaBenz) amount. In the presence of electrolyte, CMC showed a gentle increment with temperature, while the HDT showed the opposite trend. Obtained result was further correlated with conventional thermodynamic relationship, where standard Gibb's free energy change , change of enthalpy , and change of entropy ) were utilized to investigate. The values were negative for all the mixed systems studied indicating that the micellization process was spontaneous. Finally, the stability of micellization was studied by estimating the intrinsic enthalpy gain () and compensation temperature (Tc). Here, CPC + CTS mixed system showed more stability in Na2SO4 medium than the NaCl, while in NaBenz exhibited the lowest stability.

Study on the behavior of single oxygen bubble regulated by salt concentration in photoelectrochemical water splitting

The increase of electrolyte resistance near the electrode due to bubble evolution is the main obstacle to significantly improve the efficiency of photoelectrochemical water splitting. The oxygen bubble evolution on the TiO2 nanorod photocatalytic electrode was observed in situ by micro high-speed camera, and its corresponding current curve was measured synchronously by electrochemical workstation. The effect of single oxygen bubble evolution on current under different Na2SO4 electrolyte salt concentrations (0.01 M–1.0 M) was studied. When the salt concentration increases from 0.01 M to 0.8 M, the current in the bubble growth stage increases by about 2.5 times, while the bubble period and detachment diameter decrease by about 78.7% and 28.6%, respectively. When the salt concentration further increases, the current and bubble detachment diameter and period remain almost unchanged. The bubble growth coefficient reaches the optimum at 0.5 M, while the gas production rate reaches the optimum at 0.6 M. As the increase of electrolyte salt concentration, the increase rate of photocurrent density is lower than that of mass transfer coefficient, resulting in the decrease of solutal Marangoni force and the smaller bubble detachment diameter. The results show that regulating the concentration of electrolyte is an effective method to remove bubbles on the photocatalytic electrode.

Cotton Fiber/PVA-Based Neutral Hydrogel with Al3+ as an Electrolyte Additive for High-Performance Supercapacitors

Electrolyte additives have been widely studied in the improvement of stability of aqueous electrolytes. Here, we report a crosslinking cotton fiber-/PVA-based membrane (CPM) with an excellent porosity of 73.82% and an ultrahigh liquid electrolyte uptake of 900 wt %, prepared by the solution casting method. Li2SO4 as an electrolyte salt and various concentrations of Al3+ as additives were used for fabricating the gel polymer electrolyte (GPE). On adding Al3+ into the Li2SO4 electrolyte, dissociative water and bound water in Li+ solvation interact with Al3+; this affects water molecule activity and reduces Li+ solvation. While the presence of Al3+ electrolyte exhibits better electrochemical stability under a high operating voltage of 1.6 V, the highest specific capacitance of a supercapacitor (SC) is 136 F g–1 at 1.0 A g–1, and it has 91% initial capacitance retention after 20 000 charge–discharge cycles, which is higher than that of the Li2SO4 electrolyte. This work provides a path for designing a high-output-voltage aqueous supercapacitor via electrolytes and the membrane strategy.

Recent Progress on the Performance of Zn-Ion Battery Using Various Electrolyte Salt and Solvent Concentrations

Recent Progress on the Performance of Zn-Ion Battery Using Various Electrolyte Salt and Solvent Concentrations

Electrolyte Development for Fast Charging of High Energy Density Li-Ion Cells

Different lithium salt concentrations in carbonate electrolyte were studied on the fast charging performance of LiNi0.6Mn0.2Co0.2O2 (NMC622)/graphite pouch cells. The cells with electrolyte concentration from 0.75 M to 1.50 M showed similar fast charging capabilities. Further increase of the concentration to 1.75 M and 2.00 M decreased the attainable capacity in fast charging. In the long-term cycling test, the capacity retention after 200 fast charging cycles increased with the increase of salt concentration in electrolyte. Cells with 1.5 M electrolyte showed the best overall performance in fast charging capacity and long-term cycling. Li plating were observed in the cells with 0.75 M, 1.00 M and 1.25 M electrolyte. It was improved with greatly reduced Li plating area in 1.50 M, and no Li plating at all in 1.75 M and 2.00 M electrolyte. Post-mortem analysis such as neutron powder diffraction (NPD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the electrode after cycling. It suggests that electrolyte concentration needs to be optimized for a given cell configuration with specific electrode and loading. Figure 1

Physics-Based Simulation of Electrochemical Impedance Spectroscopy of Complex Electrode Microstructures

Electrochemical impedance spectroscopy (EIS) is a widely used technique to measure macroscopic properties of electrodes. However, the underlying connections between the obtained macroscopic properties and electrode microstructures are not well understood because of the complexity of coupled electrochemical mechanisms and microstructure morphology. In this work, we present a smoothed-boundary-method (SBM) electrochemical simulation framework to directly simulate the electrochemical dynamics with explicit consideration of electrode microstructures and extract EIS curves from the electrochemical simulations. This method uses continuous domain parameters to define the complex geometries of electrode particles and electrolyte phase in the interparticle space, thus circumventing the requirement of mesh conforming to the complex electrode microstructures in the simulations. The SBM is also utilized to calculate the double-layer capacitance that is required in the physics-based EIS simulations. The effects of the state of charge, salt concentration in electrolyte, and particle size distribution in electrode on the resulting EIS curves are examined using the SBM simulations. This simulation tool allows us to accessibly reveal the underlying connections between intrinsic material properties, microstructures, and macroscopic EIS measurements of battery electrodes.

Physics-based simulation of electrochemical impedance spectroscopy of complex electrode microstructures using smoothed boundary method

Electrochemical impedance spectroscopy (EIS) is a widely used technique to measure macroscopic properties of electrodes. However, the underlying connections between the obtained macroscopic properties and electrode microstructures are not well understood because of the complexity of coupled electrochemical mechanisms and microstructure morphologies. In this work, we present a smoothed-boundary-method (SBM) electrochemical simulation framework that allows us to directly simulate the electrochemical dynamics with parameterized material properties and explicit consideration of electrode microstructures, and to extract EIS curves from the electrochemical simulations. This method uses continuous domain parameters to define the complex geometries of electrode particles and electrolyte phase in the interparticle space, thus circumventing the requirement of computational mesh conforming to the complex electrode microstructures in the simulations. The SBM is also utilized to calculate the double-layer capacitance that is required in the physics-based EIS simulations. The effects of the state of charge, salt concentration in electrolyte, and particle size distribution in electrode on the resulting EIS curves are examined using the SBM simulations. This simulation tool allows us to reveal the underlying connections between intrinsic material properties, microstructures, and macroscopic EIS behavior of battery electrodes.