High Energy Density Rechargeable Batteries Research Articles

High Energy Density Rechargeable Batteries Based on Li Metal Anodes. The Role of Unique Surface Chemistry Developed in Solutions Containing Fluorinated Organic Co-solvents.

To date, lithium ion batteries are considered as a leading energy storage and conversion technology, ensuring a combination of high energy and power densities and prolonged cycle life. A critical point for elaboration of high energy density secondary Li batteries is the use of high specific capacity positive and negative electrodes. Among anode materials, Li metal anodes are considerably superior due to having the highest theoretical specific capacity (3860 mAh g-1) and lowest negative redox potential (-3.040 V vs a standard hydrogen electrode). Combination of Li metal anodes with Li[NiCoM]O2-layered cathodes with a high stable specific capacity of about 200 up to 250 mAh g-1 is particularly attractive. The development of advanced electrolyte solutions which ensure effective passivation of the electrodes' surfaces is of critical importance. Considerable efforts have been focused on fluorinated organic co-solvents and specifically fluoroethylene carbonate (FEC) due to the formation of thin, flexible Li-ions-conducting surface films with excellent protective properties. However, in the FEC-based solutions, detrimental "cross talk" between the Li anodes and the Li[NiCoM]O2 cathodes leads to worsening of the passivation of Li metal anodes, consumption of the electrolyte solutions, and limited cycle life of full Li|Li[NiCoM]O2 cells cycled with a low amount of the electrolyte solution and practical cycling parameters. The addition of difluoroethylene carbonate (DFEC) co-solvent with lower LUMO energy leads to a significant improvement in the cycling behavior of full cells. Using fluorinated co-solvents possessing synergistic effects is very promising and paves the way for developing rechargeable batteries with the highest energy density.

Lithium Metal Deposition and Stripping in Porous Hosts Coupled with Viscous Polymer Electrolytes

High energy density rechargeable batteries are desired to meet global goals of increased use of renewable energy and vehicle electrification. Higher energy densities can be achieved by replacing graphite anodes with metal anodes. Among metal anode materials, lithium metal anodes are promising due to their high theoretical capacity and low reduction potential. However, obstacles such as dendrite growth and continuous electrolyte decomposition need to be addressed for successful commercialization of rechargeable batteries with lithium metal anodes. It is widely reported that porous 3D lithium metal hosts can mitigate dendrite growth by accommodating lithium deposition uniformly within the hosts during cycling. Continuous electrolyte decomposition can also be mitigated by replacing liquid electrolytes with solvent-free electrolytes. We incorporate both strategies in this work to address the critical issues described above by demonstrating lithium deposition/stripping cycling on 3D electrode hosts with viscous polymer electrolytes. First, lithium metal deposition/stripping cycling within non-conductive and conductive hosts were compared, where zinc oxide (ZnO) coated polyacrylonitrile (PAN) nanofibers was used as the non-conductive lithium metal host and ZnO coated copper nanowires were prepared as the conductive lithium metal host. The non-conductive host showed successful cycling of 5 mAh/cm2(25 µm thickness) of lithium metal without short circuiting. Further, deposition/stripping cycling with viscous star polymers (polystyrene-co-poly(polyethylene glycol) methyl ether acrylate (PS-co-PPEGMA)) and a linear oligomer polyethylene glycol dimethyl ether (PEGDME) were compared. The effects of the physical and electrochemical properties of the polymer electrolytes on lithium deposition and interfacial impedance will be discussed.

Lithium–copper alloy embedded in 3D porous copper foam with enhanced electrochemical performance toward lithium metal batteries

Suppressing dendrite growth and accommodating volume change, among others, are the main challenges for lithium (Li) metal anode to be used in rechargeable Li batteries. The commercial macroporous copper (Cu) foam current collector may only tackle these challenges to a little extent, and it is usually unable to provide sufficient Li nucleation sites, leading to rapidly increased polarization and unstable cycling performance. Herein, we report a three-dimensional composite anode comprising Li–Cu alloy melt-cast on a commercial Cu foam (CF) current collector (Li–Cu/CF), which can be converted to a unique architecture consisting of Li metal supported by an interconnected CuLix alloy nanowire network formed because of the phase separation, when the molten Li–Cu alloy cools down and gets solidified. Compared to the bare Li foil, the Li–Cu/CF anode shows a smaller polarization and better cycle stability in the carbonate electrolyte at various current densities ranging from 1 to 5 mA/cm2 and is free from dendrite growth upon repeated Li plating/stripping. This can be attributed to the low Li nucleation overpotential and high Coulombic efficiency (96%) during Li plating on and stripping from the thus-obtained hierarchically structured CF collector, as well as the higher proportion of Li2O relative to LiF in the solid-electrolyte interphase layer. Moreover, when assembled in a full cell paired with the LiFePO4 cathode, the Li–Cu/CF anode also exhibits much better rate capability and cycle performance than the bare Li foil. Our work provides a new convenient approach to construct a dendrite-free Li metal anode that can be potentially deployed in the next-generation high energy density rechargeable Li batteries.

Highly sulfiphilic zinc selenide/carbon regulators for high-capacity and long-lifespan Li-S batteries

Lithium-sulfur (Li-S) batteries represent an attractive energy storage technology for the employment in next-generation high-energy–density rechargeable batteries. The main challenge is to increase the electrochemical utilization and stabilization of sulfur cathodes by mitigating the undesirable soluble polysulfide shuttle problem. Herein, a zinc selenide/carbon hybrid (ZnSe/C) is constructed as a regulator of commercial polypropylene (PP) separator to modify the sulfiphilicity of polysulfide species. It is revealed that the polar ZnSe/C hybrid greatly inhibits the shuttling effect by strong chemisorption and thereafter fast electrocatalytic transformation of polysulfides. Accordingly, the ZnSe/C-modified PP separator could kinetically expedite the redox reaction and effectively promotes the utilization and confinement of the active sulfur in the cathode side. As a consequence, the ZnSe/C-PP-cell exhibits excellent electrochemical properties, including an impressive capacity delivery of 1458/605 mAh g−1 at 0.1 C/5 C and long lifetime with 608 mAh g−1 retained after 1000 cycles at 1 C. This study paves a rational pathway for designing and propelling transition-metal-selenide electrocatalysts toward the practical application of Li-S batteries.

Carbon-black-based self-standing porous electrode for 500 Wh/kg rechargeable lithium-oxygen batteries

While lithium-oxygen batteries exhibit high theoretical energy densities that by far exceed those of conventional lithium-ion batteries, there are a limited number of examples that actually demonstrate cells with high energy density. The limitation of the positive electrode with a capacity high enough to sustain repeated discharge/charge cycles under high areal capacity conditions hinders the implementation of the lithium-oxygen batteries with practically high energy density. In this study, we develop a carbon black powder-based self-standing membrane, which exhibits a discharge capacity of ~7,000 mAh/gelectrode even under high current density conditions (>0.4 mA/cm2). Using the developed self-standing carbon membrane as the positive electrode, a 500-Wh/kg class rechargeable lithium-oxygen battery was fabricated and a repeated discharge/charge cycle was demonstrated at 0.1 C-rate. The results obtained in this study provide new material research directions to realize high energy density rechargeable lithium-oxygen batteries and facilitate their future development.

Optimization of Magnesium‐Doped Lithium Metal Anode for High Performance Lithium Metal Batteries through Modeling and Experiment

AbstractLithium (Li)‐magnesium (Mg) alloy with limited Mg amount, which can also be called Mg‐doped Li (Li‐Mg), has been considered as a potential alternative anode for high energy density rechargeable Li metal batteries. However, the optimum doping‐content of Mg in Li‐Mg anode and the mechanism of the improved performance are not well understood. Herein, density functional theory (DFT) calculations are used to investigate the effect of Mg amount in Li‐Mg anode. The Li‐Mg with about 5 wt. % Mg (abbreviated as Li‐Mg5) has the lowest absorption energy of Li, thus all the surface area can be “controlled” by Mg atoms, leading to the smooth and continuous deposition of Li on the surface around the Mg center. A localized high concentration electrolyte enables Li‐Mg5 to exhibit the best cycling stability in Li metal batteries with high‐loading cathode and lean electrolyte under 4.4 V high‐voltage, which is approaching the demand of practical application. This electrolyte also helps generate an inorganic‐rich solid electrolyte interphase, which leads to smooth, compact and less corrosion layer on the Li‐Mg5 surface. Both theoretical simulations and experimental results prove that Li‐Mg5 has optimum Mg content and gives best battery cycling performance.

In-situ growth of Ag particles anchored Cu foam scaffold for dendrite-free lithium metal anode

Metallic lithium is considered to be the potential anode for high energy density rechargeable Li batteries. Yet the growth of lithium dendrites impedes the industrial production of lithium metal batteries. Herein, we fabricate a dendrite-suppressed composite Li metal anode via introducing Ag particles as lithiophilic layer on the 3D copper foam (Cu-Ag). The Li ion prefers deposit on the surface of Cu-Ag foam, which is beneficial to the better adsorption of the Cu-Ag hierarchical heterojunction structure by the first-principles calculations. The heterojunction structure can further reduce the nucleation overpotential and surface current density of the composite Cu-Ag-Li anode to realize the homogeneous Li distribution for stable Li deposited/stripped. Thus, the Cu-Ag-Li composite electrode (CAL) presents better electrochemical performance in the symmetric battery and excellent rate performance in the full cell with a greatly enhanced capacity retention of 83% after 500 cycles. This strategy presents a general approach to suppress the growth of lithium dendrites and regulate the volume changes for long-life span lithium metal batteries.

(Invited) Corrosion Effects on Electrochemically Deposited Lithium Metal Electrode

The lithium metal anode is essential for high energy density rechargeable battery due to its high specific capacity (3,860 mAh·g-1, or 2,061 mAh·cm-3)and low electrochemical potential (–3.04 V versus the standard hydrogen electrode). Although extensive studies have been performed to overcome dendrite growth and low coulombic efficiency problems associated with the practical use of lithium metal anode, there is a lack of comprehensive understanding of the storage properties and shelf life of lithium metal batteries, where chemical/electrochemical corrosion plays a critical role. We introduced the TGC (titration gas chromatography) method to quantify the corrosion rate of electrochemically deposited Li in liquid electrolyte system. It is demonstrated that lithium metal is mainly suffered from chemical corrosion by the liquid electrolyte instead of by electrochemical corrosion, that is, the direct charge transfer between lithium-electrolyte interphase. Besides, the corrosion rate of electrochemically deposited lithium in different electrolytes is further studied, where different electrolytes are found to introduce different corrosion rates. By taking into account the influence of different electrolytes on the SEI and Lithium morphology, we differentiate these two aspects, and the results in (1) The corrosion rate is mainly affected by the surface area of deposited lithium. The larger the surface area that the lithium has, the faster the corrosion rate will be; (2) The SEI components formed by different electrolytes on deposited lithium are similar, and different electrolytes will lead to the similar corrosion rate when the surface area is the same. Our findings provide valuable insights on the corrosion problem of electrodeposited lithium, where the surface area of deposited lithium turns out to dominate its storage properties.

Mechanism of Effect of Roughness on Dendrite Growth

Lithium metal is an ideal anode material for high energy density rechargeable batteries. However, the repeated use of the battery leads to the increase of the roughness of the electrode surface, which aggravates the growth of harmful dendrites. The harmful dendrites will lead to short circuit and cause safety hazard. In this paper, the aspects of overpotential, curvature and pit shape were studied to understand the mechanism of effect of roughness on dendrite growth. The results show that the growth of needle like and dendritic dendrites is mainly due to the local disturbance of surface current density. The disturbance of surface current density is caused by the relative asymmetry of overpotential, curvature and deposition rate. High overpotential and small curvature will aggravate the disturbance of local current density, resulting in asymmetric dendrite formation. We also found lithium deposition in the pit, which is consistent with the published experimental results. However, whether dendrite can grow in the pit depends on the size of the pit. These results provide meaningful information for optimizing battery performance and improving safety.

Investigation of Ionic Conductive Behavior of SL-Based Gel Polymer Electrolyte and Interfacial Property of Li / Gel Polymer Electrolyte

IntroductionLithium-Sulfur (Li-S) batteries are attracting attention as high energy density rechargeable battery. Elemental sulfur (S8) has high theoretical capacity (1,672 mAhg-1) as positive electrode active material. However, S8 electrode is degraded by redox shuttle effect of lithium-polysulfide (Li2S n ), which are generated during charge-discharge reactions. To solve this problem, many electrolyte designs were proposed. Sulfolane (SL) based high Li salt concentration electrolyte solution was reported as low solubility electrolyte of Li2S n [1]. The aim of this concept is chemically protection of Li2S n dissolution into electrolyte solution. Therefore, we proposed new polymer electrolyte, which protects chemically and physically dissolution and diffusion of Li2S n . In this study, we prepared SL-based solid gel polymer electrolytes (GPEs), ionic conductive property and time dependence of Li/GPEs interfacial resistance were evaluated.ExperimentalSulfolane (SL), LiN(SO2CF3)2 (LiTFSA), and polyether-based P(EO/PO) macromonomer (EO/PO=8/2) were used as electrolyte materials for GPEs. First, electrolyte solution samples were prepared by mixing LiTFSA and SL at each molar ratio of SL : LiTFSA = x : 1 (x=1, 1.5 and 2, respectively). Next, prepared electrolyte solution and P(EO/PO) macromonomer were mixed at weight ratio of electrolyte solution is 70, 80, 90 wt%, respectively, and added DMPA as photo initiator. After that, these materials were radical polymerized by UV irradiation. Temperature dependence of ionic conductivity of prepared GPEs were investigated by AC impedance measurements (353～217 K). Impressed voltages were changed from 100mV to 500 mV under 258 K. Also, [Li | GPE | Li] symmetrical cells were assembled, and impedance measurements of these cells were carried out to investigation of Li/GPE interfacial resistance at 333 K.Results & DiscussionFig.1 shows temperature dependence of ionic conductivity (s) of GPEs. Conventional polyether-based solid electrolyte ([Li]/[O]=0.1: black circle) was showed for comparison. s of all GPEs indicated higher value compared with [Li]/[O]=0.1. s increased with electrolyte solution composition or decrease of Li salt composition. These should be related with mobility of polymer chain or microscopic viscosity of GPEs, like a T gs. Also, s observed at around or less than T gs by changing impressed voltage. At this area, temperature dependence was changed inconsecutively. Therefore, ionic transport mechanism changes with segmental motion of polymer chain might be occurred below T g.Fig.2 shows time dependence of Li/GPE interfacial resistance (R Li) at 333 K. R Li moderately increased over time, which wasn’t stabilized over 1000 h. (Measurement was interrupted temporary at broken line.) R Li of general polyether based solid electrolyte reported as ca. 100 W cm2 [2],[3]. However, R Li indicated quite low values of 30～40 W cm2 at 1000 h. Therefore, favorable SEI formation might be occurred between Li metal and GPE. In presentation, temperature dependence of R Li and lithium cation transport number (t Li+) will be discussed.

A novel dual-protection interface based on gallium-lithium alloy enables dendrite-free lithium metal anodes

Lithium (Li) metal has been considered as an ideal anode for high-energy density rechargeable batteries. However, it faces huge obstacles toward practical application due to the growth of Li dendrites and the uncontrollable side reactions with electrolyte. Here, we demonstrate a novel gallium-lithium alloy based dual-protection interface layer for Li metal by a facile in-situ ion-exchange reaction that possesses long service life for effectively alleviating the extra consumption of active Li as well as homogenizing the Li deposition. Such dual-protected feature is attributed to its outstanding stability, excellent Li affinity and favorable charge transfer kinetics. By means of in-situ visualization electrodeposition studies, the modified Li metal (GaLi-Li) anode can effectively suppress the Li dendrite even under a high deposition capacity of 7 mAh cm−2. Moreover, the GaLi-Li based symmetric cell achieves excellent cycling stability for over 1500 cycles at a high current density of 5 mA cm−2. Pairing the GaLi-Li anodes with LiFePO4 and LiNi0.8Co0.1Mn0.1O2 cathodes, the cells also realize better long-term cycling stability with higher discharge capacity than that of the bare Li. Our strategy offers a practical way to realize highly stable and safety Li metal batteries.

High Energy Density Rechargeable Aqueous Lithium Batteries with an Aqueous Hydroquinone Sulfonic Acid and Benzoquinone Sulfonic Acid Redox Couple Cathode

The demand for high energy density rechargeable batteries beyond lithium-ion batteries has increased for electric vehicles. In the present study, a novel high energy density rechargeable aqueous lithium battery was proposed. The battery was composed of a lithium metal anode, a lithium-stable non-aqueous electrolyte, a water-stable lithium-ion conducting solid electrolyte of Li1.4Al0.4Ge0.2Ti1.4(PO4)3-epoxy-TiO2 separator, and a hydroquinone sulfonic acid (HQS)/benzoquinone sulfonic acid (BQS) redox couple in an aqueous acetic acid solution (HAc). An open-circuit voltage of 3.7 V at 25 °C was recorded, and the theoretical energy density of the battery based on the reaction 2Li + BQS + 2H2O = 2 LiOH + HQS was 833 Whkg-1, about two times higher than that of the lithium-ion battery. The battery was successfully cycled at 0.5 mA cm-2 and 25 °C with low polarization.

Self-standing carbon nanotube aerogels with amorphous carbon coating as stable host for lithium anodes

Lithium metal anodes have a high theoretic capacity and are considered the promising electrodes for high energy density rechargeable batteries. However, the uneven deposition of lithium and lithium dendrites hinders the practical usage of lithium metal anodes. In order to solve this problem, carbon nanotube aerogels coated with amorphous carbon (AC@CNT aerogels) are used as the host, and molten lithium infuses into the carbon aerogels to obtain composite lithium metal anodes. The self-standing, porous AC@CNT aerogels provide a solid structure for lithium deposition/stripping and effectively reduce the local current density, so that lithium dendrites are suppressed and better electrochemical performances are achieved. Compared with the bare lithium metal anodes, the AC@CNT aerogel/lithium metal (AC@CNT/Li) anodes exhibit a longer cycling life of 500 h at deposition/striping capacity of 1 mAh cm−2 on symmetric cells, better cycling performance of 152 mAh g−1 at 0.1C in lithium cobalt oxide/lithium (LCO/Li) cells, and 120.3 mAh g−1 at 1 C in lithium iron phosphate/Li (LFP/Li) cells, demonstrating the potential of the AC@CNT/Li electrodes in developing high-performance lithium-metal batteries.

A Green Route for Preparation of Nickel-Vanadate-Nanoparticles as Negative Electrode for Rechargeable Lithium-Ion Batteries

A low-cost for large-scale production, it is still a big challenge for high capacity and good cycling ability of anode materials. Herein, we approve a facile green route to synthesize nickel-vanadate (Na3V2O8) nanoparticles (NPs) using an inexpensive universal solvent, water. An intermediate precipitate of Ni-V-ZIF (Zeolitic Imidazolate Framework) was obtained, which transformed into phase pure Ni3V2O8 NPs by slow annealing process in air, without affecting the original morphology. As an anode material for Li-ion batteries (LIBs), the Ni3V2O8 NPs with uniform spherical shapes can provide an extraordinary retained capacity of 1024 mA h g-1 after 525 cycles, at the current densities of 2 and 0.5 A g-1, which were alternated for every 100 cycles. Additionally, the electrode showed stable specific capacities of 940 and 305 mA h g-1 at 1 and 5 A g-1, respectively after 400 and 1000 cycles. Thus, the fine Ni3V2O8 NPs-based anode can withstand high current densities, indicating that this facile green route revealed an economically applicable and efficient preparation for the large-scale production of nanostructured-electrode materials for high energy density rechargeable batteries.

Preparation and Evaluation of Polymer Electrolytes with Low-Solubility of Lithium-Polysulfide

BackgroundLithium-Sulfur (Li-S) batteries are attracting attention as high energy density rechargeable battery. Elemental sulfur (S8) have high theoretical capacity (1,672 mAhg-1) as positive electrode active material. However, S8 electrode is degraded by redox shuttle effect of lithium-polysulfide (Li2S n ), which are generated during charge-discharge reactions. To solve this problem, many electrolyte designs were proposed. Sulfolane (SL) based high Li salt concentration electrolyte solution was reported as low solubility electrolyte of Li2S n [1]. The aim of this concept is chemically protection of Li2S n dissolution into electrolyte solution. Therefore, we proposed new polymer electrolyte, which protects chemically and physically dissolution and diffusion of Li2S n . In this study, we prepared SL based solid gel polymer electrolytes (GPEs), and thermal and ionic conductive properties of the samples were evaluated. Furthermore, Li-S batteries containing GPEs were prepared and evaluated by charge-discharge measurements.ExperimentalSulfolane (SL), LiN(SO2CF3)2 (LiTFSA), and polyether-based P(EO/PO) macromonomer were used as electrolyte materials for GPEs. First, electrolyte solution samples were prepared by mixing LiTFSA and SL at each molar ratio of SL : LiTFSA = x : 1 (x=1, 1.5 and 2, respectively). Next, prepared electrolyte solution and P(EO/PO) macromonomer were mixed at each weight ratio of electrolyte solution : polymer = y : (10-y) (y=7, 8 and 9, respectively), and added DMPA as photo initiator. After that, these materials were radical polymerized by UV irradiation. In this research, GPEs are expressed as (x),yEl+(10-y)Po. Thermal property and ionic conductivity of prepared GPEs were investigated by TG-DTA, DSC and AC impedance measurements. Coin cells were assembled by Li metal negative electrode, GPE ((1.5),8El+2Po) and S8-based positive electrode. At the time, small amount of electrolyte solution (x=1.5) were dropped onto S8 based positive electrode. Assembled cells were evaluated by constant current charge-discharge tests at 303 K, and current density was set C/18.Results & DiscussionFig.1 shows ionic conductivity of prepared GPE samples. All GPEs exhibited high ionic conductivity (σ) compared withgenuine solid polymer electrolyte (black circle) at all measured temperature. From results, σ increased with decrease of Li salt concentration (inverse of x). Also, σ increased with electrolyte solution composition at low temperature region. Especially, lowest polymer composition (y=9) samples exhibited specific behavior. At x=2 and 1.5, lowest polymer composition samples exhibited higher σ compared with other samples at 303 K. On the other hands, samples were contrary tendency at x=1. Therefore, ionic transport mechanism should be affected by Li salt concentration and electrolyte solution amount into GPE.Fig.2 shows charge and discharge profiles of prepared cell until 100 cycles. In the case of discharge curves, 1st plateau and 2nd plateau were observed at between 2.2 to 2.3 V and 2.0 to 2.1 V respectively. Therefore, appropriate reaction between S8 and Li+ should be occurred. Initial discharge capacity indicated 1,146 mAhg-1. Charge and discharge capacities decreased until 20 cycles, and the capacities stabilized approximately 520 mAhg-1. Coulombic efficiency also exhibited remarkable behavior, which indicated 99.7 % at 100 cycles. Therefore, sulfolane based GPE may restrict dissolution of Li2S x and improve performance of Li-S batteries.Reference[1] A. Nakanishi et al., J. Phys. Chem. C, 123, 14229-14238 (2019). Figure 1

(Invited) Direct Observation of the Sulfur-Based Chemical Species in Li-S Batteries By Spatially-Resolved X-Ray Fluorescence Microscopy and X-Ray Absorption Spectroscopy

The demands on low cost and high energy density rechargeable batteries for both transportation and large-scale stationary energy storage are attracting more and more research attention toward new battery systems such as metal, metal-sulfur, metal-air, and multivalent batteries. Since sulfur is an earth-abundant material with low cost and has a high theoretical capacity, Li-S battery chemistry has attracted significant interest during the past decade.The Li-S battery utilizes electrochemical conversion of sulfur (S8) to lithium sulfide (Li2S), going through multiple electron transfer processes associated with long- and short-chain polysulfide (Li2Sx) intermediates. It is well known that the long-chain polysulfides can be dissolved into electrolyte with aprotic organic solvents and migrated to the Li anode side. This so-called “shuttle effect” is considered as the main reason for the capacity loss and low coulombic efficiency of the Li-S system. A lot of efforts have been made on suppressing the polysulfide dissolution through new sulfur-based material and electrolyte, as well as cell engineering. Although the Li-S battery performance has been improved over the past decades, the long‐term cycling stability is still one of the major barriers for the practical application of Li–S batteries. More in‐depth studies on the fundamental understanding of the sulfur reaction mechanism and interactions among the different polysulfide species, the electrolyte, and the electrodes are still greatly needed.In this study, we utilize the spatially-resolved X-ray fluorescence (XRF) microscopy combined with X-ray absorption spectroscopy (XAS) to directly probe the inhomogeneous electrochemical/chemical reactions of the sulfur cathode, and chemical reactions of sulfur contained chemical species. The morphology changes and the redistribution of sulfur and polysulfides in both the S cathode and lithium metal anode were monitored through the XRF images, while the oxidation state of sulfur and changes of the sulfur-containing interfacial layer (i.e., SEI) were characterized through the XAS spectra. Our characterization studies on the sulfur-based cathode materials in Li-S batteries using spatially resolved XRF/XAS techniques provides critical information enabling us to understand the reaction processes and their correlation to cell cycling behavior and failure mechanisms. More details of the result will be discussed at the meeting. Acknowledgement The work done at Brookhaven National Laboratory was supported by Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the U.S. DOE, through the Advanced Battery Materials Research (BMR) Program, including Battery500 Consortium under contract DE-SC0012704. This research used beamline 8-BM (TES) of the National Synchrotron Light Source II, U.S. DOE Office of Science User Facilities operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.

Facile Synthesis of Metastable Mg-Mn Spinel Nanoparticles for Magnesium-Ion Batteries

High energy and high density rechargeable batteries are indispensable for widespread portable electronic devices and the spreading of electric vehicles. Rechargeable Mg-ion batteries have gained much attention as promising alternatives to Li-ion batteries due to the high natural abundance, high volumetric energy density, and no dendrite formation of the Mg metal anode. While Mg-ion batteries are nowadays progressing rapidly, their practical use is still hampered by problems related to both cathode materials and electrolytes. One of the crucial problems is the very low rate capability at the cathodes due to the slow diffusion of Mg2+ ions in solids, therefore Mg-ion batteries work only at low current densities or at high temperature. Among various oxide cathode candidates for Mg-ion batteries, spinel oxides have a high redox potential and a relatively high ion-diffusivity. Especially in the common electrochemical window of electrolytes, MgMn2O4 spinel can exhibit a high theoretical capacity of 540 mAh g–1 using both Mn3+/Mn4+ and Mn3+/Mn2+ redox reactions (λ-MnO2 formation and rock-salt Mg2Mn2O4 formation reactions, respectively). However, these reactions should exhibit large polarizations because of the less reversible tetragonal-cubic phase transitions; MgMn2O4 has a tetragonal spinel structure due to the Jahn–Teller effect of Mn3+ ions, while λ-MnO2 and Mg2Mn2O4 are cubic phase. A suppression of the lattice distortion of MgMn2O4 is necessary for improving its cathode performance. Although a cubic MgMn2O4 phase is known as a metastable phase, this phase is only obtained at high-temperature or high-pressure in bulk. In this study, we aimed at synthesizing metastable cubic MgMn2O4 spinel nanoparticles under ambient conditions and applying them as cathodes for Magnesium-ion batteries.Mg-Mn spinel nanoparticles were synthesized by alcohol-reduction method[1] and hot-injection method. For the alcohol-reduction method, n-Bu4NMnO4 was reduced in anhydrous MgCl2 ethanol solution at room temperature to form fine particles. For the hot-injection method, MgCl2 and Mn(acac)3 were dissolved in anhydrous xylene solution with stearic acid and oleylamine, followed by water injection at 80ºC to form monodisperse nanoparticles. Charge/discharge tests between –1.0~1.0 V vs. Ag+/Ag was carried out using a three-electrode cell with an activated carbon anode and a 0.5 M Mg(ClO4)2/CH3CN electrolyte.The Rietveld refinements of XRD patterns suggested that the obtained samples were assigned to cubic phase Mg1–x Mn2O4, and a transformation to tetragonal phase was observed after annealing at 600ºC. Since the both methods utilize rapid nucleation reactions occurred at moderate temperature, once a meta-stable phase formed, the phase transition to more stable tetragonal phase hardly proceed. The cubic phase is obtained probably due to the rapid nucleation of cubic MgMn2O4 spinel nanoparticles at moderate temperature. According to TEM images, the primary particle size of both samples was below 10 nm, suggesting the rapid nucleation without crystal growth. However, the particles were aggregated to form large secondary particles. The electrochemical performances strongly depended on the morphology of secondary particles, therefore suppression of the aggregation is important for improving the performance. Reference: [1] H. Kobayashi, K. Yamaguchi, I. Honma, RSC Adv. 9 (2019) 36434–36439.

Effect of KOH Concentration on Mn(III) Dissolution in Rechargeable γ- MnO2 Cathodes

The low cost, high theoretical specific capacity (617 mAh g-1) and environmentally benign nature of MnO2 makes it a very attractive material for a host of electrical energy storage applications such as aqueous based Zn/MnO2 batteries or non-aqueous Li/MnO2 batteries and even supercapacitors. Realizing higher energy and power densities in γ- MnO2 cathodes is of crucial interest for both primary as well as secondary battery applications. But achieving a long lasting high energy density rechargeable MnO2 battery suffers from many setbacks. Prime among these setbacks is the inability of γ- MnO2 polymorph to retain its structural integrity when cycled to deeper depths of discharge (DOD). The most widely used model for γ- MnO2 discharge consists of a solid state proton-electron intercalation process upto the 1st electron forming MnOOH and a dissolution-precipitation process during the 2nd electron discharge forming Mn(OH)2. In this paper, we investigate the cycle life degradation mechanism of the γ- MnO2 polymorph in alkaline environment in the 1st electron region of discharge. We conduct three-electrode experiments on 80 wt. % loading γ- MnO2 cathodes to determine the failure mechanism in various KOH concentrations and at various depths of discharge. At 50 % DOD of the 1st electron, we find a significant effect of KOH concentration with upto 70 % retention of discharge capacity over 100 charge-discharge cycles in 10 wt. % KOH compared to only 20 cycles in 37 wt. % KOH. At 40 % DOD of the 1st electron of MnO2, we observe an astounding 177 % improvement in cycle life in 10 wt. % KOH in comparison to poor recyclability observed in 37 % KOH. Post mortem X-ray diffraction analysis revealed the presence of birnessite, δ- MnO2, in the cathode with nanoflower, nanosheet and nanowire morphologies of birnessite observed in 37 wt. %, 25 wt. % and 10 wt. % KOH respectively. To probe into the failure mechanism we conducted four-electrode Pt- wire experiments to detect Mn (III) soluble species during discharge, in-situ. The experiments indicate significant influence of KOH concentration with Mn(III) dissolution, as determined by the presence of δ-MnO2 deposited on the Pt wire, being detected in 37 wt. % KOH at just ~34 % DOD during the 1st electron discharge. We propose, in this paper, that the capacity fade of γ- MnO2 at high DOD of 1st electron is critically related to the amount of birnessite, which is formed during the charging of Mn (III) ions. Since birnessite discharges at a much lower potential than cycled to in our experiments, its contribution to discharge capacity in the 1st electron region of γ- MnO2 is negligible and hence, builds up as an inactive phase, ultimately resulting in capacity fade. Figure 1

(Invited) Insight into Interfacial Processes between Lithium-Rich Layered Oxide Cathode and High-Voltage Electrolyte

Lithium-rich layered oxide, xLi2MnO3·(1-x)LiMO2 (M = Mn/Ni/Co), is a promising high-capacity cathode active material for high-energy density rechargeable lithium batteries. While the capacity of lithium-rich layered oxide cathode can increase by increasing the charge cut-off voltage toward 5 V, it t relies on the anodic stability of electrolyte and the interface stability of charged cathode. We have been developing and suggesting fluorinated carbonates as effective high-voltage additives and solvents of new designed electrolytes, which provide a significant improvement in the stability of cathode-electrolyte interface, solid electrolyte interphase (SEI) layer and cycling performance, and longer life span of various high-capacity cathodes, with respect to commercial electrolyte system. Herein, we present our recent progress on the exploration of the limit of anodic stability of fluorinated carbonate-assisted electrolyte toward 5 V level and the capacity of a Li-rich cathode based full-cell, and the enhancement of battery safety. The mechanistic studies utilizing systematic spectroscopic analyses on the correlation among interfacial processes, the SEI formation and stabilization, and performance improvement would be discussed in the meeting. Acknowledgements This research was supported by the Ministry of Trade, Industry and Energy (A0022-00725) and the National Research Foundation grant (2019R1A2C1084024) funded by the Ministry of Science and ICT of Korea.

Porous Silicon-Cobalt Silicide-Carbon Composites for Reversible Lithium Storage

With increasing use of portable electronic devices and electric vehicles, demand for development of high-energy density rechargeable batteries is rising. To meet the market demand for long-lasting rechargeable batteries, energy density of Li-ion batteries (LIBs) should be increased. The energy density of batteries is determined by the capacities of electrodes. Carbonaceous materials have been used as negative electrodes (anodes) in commercial LIBs because of their low operating potential and reasonable cycle life. However, the theoretical capacity of graphite anodes is relatively low, which is 372 mAh g-1 for the fully intercalated LiC6 phase. To increase the capacity, different types of materials using alloying or conversion reaction should be developed. Li-alloy-based materials can store several Li ions with an atom by electrochemical alloying reactions and therefore, high capacity can be obtained. In particular, Si has a high theoretical capacity of 3580 mAh g-1 for Li15Si4 and low operating potential. However, serious capacity fading usually occurs because of huge volume changes during cycling and their low electric conductivity. To address this problem, many strategies have been developed through changes in materials design. For example, incorporation of carbon is the most famous method to solve the capacity fading of Li-alloy-based materials upon cycling. The existence of carbon in Si matrix can buffer the volume expansion of Si materials with increased electric conductivity, thus simultaneously improving both cycling and rate performance. Porous structure can be another strategy to relieve the volume expansion by using the internal pores. However, the process is generally time and energy consuming. In this study, we report a simple preparation process of porous Si-CoSi2-C composites as anode materials for LIBs. The porous composite was synthesized through cheap starting materials and cost-effective methods. Si, magnesium silicide, and cobalt oxide were mixed using high energy mechanical milling (HEMM). Then, a carbon material was incorporated by HEMM. To produce porous structure, MgO formed by HEMM was removed by a wet etching process. As a result, porous Si-CoSi2-C composites were prepared. These composites exhibited a reversible capacity of 1100 mAh g-1 and good capacity retention after 100 cycles. The porous structure and the amorphous carbon matrix alleviated the volume change to allow the enhanced electrochemical performance.