Li-S Cells Research Articles

Novel Composite Polymer Electrolytes of PVdF-HFP Derived by Electrospinning with Enhanced Li-Ion Conductivities for Rechargeable Lithium–Sulfur Batteries

Composites of poly(vinylidene fluoride-co-hexafluoro propylene) (PVdF-HFP) incorporating 10 wt % bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) and 10 wt % particles of nanoparticulate silica (nm-SiO2), nanoparticulate titania (nm-TiO2), and fumed silica (f-SiO2) were prepared by electrospinning. These membranes served as host matrix for the preparation of composite polymer electrolytes (CPEs) following activation with lithium sulfur battery electrolyte comprising 50/50 (vol %) dioxolane/dimethoxyethane with 1 M LiTFSI and 0.1 M LiNO3. The membranes consist of layers of fibers with average fiber diameter of 0.1–0.2 μm. CPEs with f-SiO2 exhibited higher ionic conductivity with a maximum of 1.3 × 10–3 S cm–1 at 25 °C obtained with 10 wt % filler compositions. The optimum CPE based on PVdF-HFP with 10 wt % f-SiO2 exhibited enhanced charge–discharge performance in Li-S cells at room-temperature eliminating polysulfide migration, delivering initial specific capacity of 895 mAh g–1 at 0.1 C-rate and a v...

A Self-Repairing Cathode Material for Lithium-Selenium Batteries: Se-C Chemically Bonded Selenium-Graphene Composite.

Lithium-selenium batteries, employing selenium as a cathode material, exhibit some notable advantages, such as high discharge rates and good cycling performance, due to their high electrical conductivity, high output voltages, and high volumetric capacity density. However, an important problem, termed the "shuttle effect", can lead to capacity decay in Li-Se cells (and in Li-S cells), which arises from aggregation and the loss of Se or S from the cathode into the electrolyte. In this work, in order to solve this problem, a new self-repairing system has been devised, in which some Se atoms are chemically bonded to the carbon atoms of graphene and act as reclaiming points for dissociated Se atoms through the establishment of -Se-Se-Se- chains. Se-decorated graphene (Se-GE) was first constructed through a facile high-energy ball-milling process. Its formation was confirmed by XRD, SEM, HRTEM, XPS, and Raman analyses. As we anticipated, in examining cell properties, the as-prepared Se-GE composite underwent an initial capacity decay in the first 20 cycles (from 1050 mAh g-1 to 750 mAh g-1 , ca. 29 % loss), but the capacity then reverted to 970 mAh g-1 (ca. 92 % of the initial value). Other measurements were also consistent with the recapture of dissociated Se atoms.

Effect of Electrolyte on High Sulfur Loading Li-S Batteries

Practical lithium-sulfur batteries require high sulfur electrode loading and lean electrolyte designs, which entail more research efforts on the two cell-design parameters - sulfur loading and electrolyte/sulfur loading ratio (E/S). In this work, a systematic investigation is performed to understand the impact of these two variables over key Li-S cell performance parameters. It is demonstrated that Li-S cells’ power performance strongly depends on the E/S ratio, while both E/S ratio and sulfur loading significantly influence the cycle life of Li-S cells. Low E/S ratio and high sulfur loading both give rise to fast lithium anode corrosion, which induces fast capacity fade and Coulombic efficiency decay. Pre-passivation of the lithium anode with an ionic conductor Li3PO4 protection layer only improves the Coulombic efficiency retention at sulfur loading levels much lower than the practical threshold. Meanwhile, increasing the concentration of LiNO3 additive in the electrolyte is found effective in sustaining the cycling capacity and the Coulombic efficiency over a reasonable usage window (∼200 cycles). The role of LiNO3 is the protection of lithium anode during cycling.

Impedance Characterization of the Transport Properties of Electrolytes Contained within Porous Electrodes and Separators Useful for Li-S Batteries

Impedance spectroscopy is used to characterize the key transport properties (effective conductivity, MacMullin number, porosity and tortuosity) of electrolyte solutions confined in porous separators and carbon-sulfur composite electrodes useful for application in Li-S batteries. Three relevant electrolyte concentrations, ranging between 1 and 5 molal, are studied. Impedance measurements are performed in symmetrical cells with two identical electrodes, which overcome complications associated with the contributions of the counter-reference electrode. The electrolyte-filled carbon-sulfur composite electrodes can be represented by an “open” Warburg element, modelling the finite-diffusion of ions through the pores coupled to the double-layer charging of the electrode-electrolyte interface. The carbon-sulfur composite electrodes are at a high enough potential (ca. 3 V vs. Li+/Li) so that charge-transfer reactions of sulfur reduction to polysulfide species are absent during the impedance measurements, and hence capacitive-like behavior is observed at low frequencies. The analysis of the results shows that the rate of transport of ions through porous structures is markedly dependent on the electrode's structure and composition as well as the electrolyte concentration. Synergistic effects, able to enhance the effective conductivity of the electrolyte inside porous composite electrodes, are observed for particular electrode/electrolyte combinations, which are correlated to enhanced performance in Li-S cells.

Building Organic/Inorganic Hybrid Interphases for Fast Interfacial Transport in Rechargeable Metal Batteries.

We report a facile in situ synthesis that utilizes readily accessible SiCl4 cross-linking chemistry to create durable hybrid solid-electrolyte interphases (SEIs) on metal anodes. Such hybrid SEIs composed of Si-interlinked OOCOR molecules that host LiCl salt exhibit fast charge-transfer kinetics and as much as five-times higher exchange current densities, in comparison to their spontaneously formed analogues. Electrochemical analysis and direct optical visualization of Li and Na deposition in symmetric Li/Li and Na/Na cells show that the hybrid SEI provides excellent morphological control at high current densities (3-5 mA cm-2 ) for Li and even for notoriously unstable Na metal anodes. The fast interfacial transport attributes of the SEI are also found to be beneficial for Li-S cells and stable electrochemical cycling was achieved in galvanostatic studies at rates as high as 2 C. Our work therefore provides a promising approach towards rational design of multifunctional, elastic SEIs that overcome the most serious limitations of spontaneously formed interphases on high-capacity metal anodes.

The Effect of Current Inhomogeneity on the Performance and Degradation of Li-S Batteries

The effect of thermal gradients on the performance and cycle life of Li-S batteries is studied using bespoke single-layer Li-S cells, with isothermal boundary conditions maintained by Peltier elements. A temperature difference is shown to cause significant current imbalance between parallel connected single-layer cells, causing the hotter cell to provide more charge and discharge capacities during cycling. During charge, significant shuttle is induced in the hotter Li-S cell, causing accelerated degradation of it. A bespoke multi-tab cell in which the inner layers are electrically connected to different tabs versus the outer layers, is used to demonstrate that noticeable current inhomogeneity occurs during the operation of practical multilayer Li-S pouch cells, which is expected to affect their performance and degradation. The observed thermal and current inhomogeneity should have a direct consequence on battery pack and thermal management system design for real world Li-S battery packs.

Controlling the Wettability between Freestanding Electrode and Electrolyte for High Energy Density Lithium-Sulfur Batteries

The present study demonstrates how to improve the electrochemical performance and energy density of lithium-sulfur batteries based on controlling the wettability between the freestanding electrode and the electrolyte. First, to improve the volumetric energy density, we reduced the thickness of freestanding electrode by the applying pressure. Second, in order to improve electrolyte conductivity and wettability for pressed freestanding electrode, we reduced the viscosity of electrolyte solution by reducing the LiTFSI salt concentration. Through these controls, the Li-S cell is constructed from a pressed freestanding carbon nanotube-sulfur cathode, 0.5 M LiTFSI in DME/DOL with 0.4 M LiNO3 as a low-viscosity electrolyte (0.72 cP), and Li-metal foil as an anode. As a result, we can obtain a high discharge capacity of 1,400 mAh g−1 at 0.1 C and excellent cycle retention of 95% after 80 cycles at 0.5 C in coin-type cell. In addition, scale-up (3 by 5 cm) pouch-type Li-S cell delivers a high discharge capacity of 500 mAh at 0.1 C with high gravimetric (954.4 Wh kg−1cathode and >170 Wh kg−1cell) and volumetric (1,150 Wh L−1cathode and 515 Wh L−1cell) energy densities.

Biomass-derived porous carbon modified glass fiber separator as polysulfide reservoir for Li-S batteries

Biomass-derived porous carbon has been considered as a promising sulfur host material for lithium-sulfur batteries because of its high conductive nature and large porosity. The present study explored biomass-derived porous carbon as polysulfide reservoir to modify the surface of glass fiber (GF) separator. Two different carbons were prepared from Oak Tree fruit shells by carbonization with and without KOH activation. The KOH activated porous carbon (AC) provides a much higher surface area (796 m2 g−1) than pyrolized carbon (PC) (334 m2 g−1). The R factor value, calculated from the X-ray diffraction pattern, revealed that the activated porous carbon contains more single-layer sheets with a lower degree of graphitization. Raman spectra also confirmed the presence of sp3-hybridized carbon in the activated carbon structure. The COH functional group was identified through X-ray photoelectron spectroscopy for the polysulfide capture. Simple and straightforward coating of biomass-derived porous carbon onto the GF separator led to an improved electrochemical performance in Li-S cells. The Li-S cell assembled with porous carbon modified GF separator (ACGF) demonstrated an initial capacity of 1324 mAh g−1 at 0.2 C, which was 875 mAh g−1 for uncoated GF separator (calculated based on the 2nd cycle). Charge transfer resistance (Rct) values further confirmed the high ionic conductivity nature of porous carbon modified separators. Overall, the biomass-derived activated porous carbon can be considered as a promising alternative material for the polysulfide inhibition in Li–S batteries.

Self-balancing feature of Lithium-Sulfur batteries

The Li-S batteries are a prospective battery technology, which despite to its currently remaining drawbacks offers useable performance and interesting features. The polysulfide shuttle mechanism, a characteristic phenomenon for the Li-S batteries, causes a significant self-discharge at higher state-of-charge (SOC) levels, which leads to the energy dissipation of cells with higher charge. In an operation of series-connected Li-S cells, the shuttle mechanism results into a self-balancing effect which is studied here. A model for prediction of the self-balancing effect is proposed in this work and it is validated by experiments. Our results confirm the self-balancing feature of Li-S cells and illustrate their dependence on various conditions such as temperature, charging limits and idling time at high SOC.

Improved performance of lithium-sulfur battery by a functional separator design

A novel polyacrylamide/mixed carbon black-coated separator was fabricated for lithium-sulfur batteries. It is demonstrated that the coated separator greatly improves cell performance. Although a quite commonly elemental sulfur cathode was employed, Li-S cell loading the cathode and coated separator still delivers a reversible capacity of up to 688 mAh g−1 after 50 cycles, with a decay rate of only 0.0017% per cycle based on the second discharge. The coated separator restrains polysulfides by a combination of physical and chemical interaction, hence suppressing shuttle effect during cycling. Besides, the coating contributes a highly conductive and hydrophilic surface, increasing the utilization of active material and reducing charge-transfer resistance.

Designing solid-electrolyte interphases for lithium sulfur electrodes using ionic shields

Lithium metal is among the most sought-after anode chemistries for next-generation electrical energy storage due to its high theoretical capacity (3860mAhg−1) and low reduction potential (− 3.04V vs S.H.E.). To realize its promise, reactive Li anodes must be paired with high-energy conversion cathodes, such as sulfur or oxygen. Chemical and physical instability at both electrodes pose formidable challenges to development of practical lithium metal batteries. These instabilities are compounded by problems with active material loss and anode passivation when Li is paired with conversion cathodes, such as elemental sulfur. Here, we report on design principles and a process for creating artificial solid electrolyte interphases composed of ionic shields that are able to stabilize electrochemical processes at both the anode and cathode of Li-S electrochemical cells. We show that ASEI composed of negatively-charged nanoparticles on Li stabilize deposition of Li at the anode by multiple fundamental mechanisms. A similar concept is used to design interphases composed of positively charged conductive nanoparticles at the cathode and shown to be effective at intercepting dissolved polysulfide anions and for enhancing sulfur reutilization. We combine the two ASEI design strategies to create Li-S cells based on high-loading sulfur cathodes and demonstrate their long-term cycling stability.

A Design of Solid-State Li-S Cell with Evaporated Lithium Anode To Eliminate Shuttle Effects.

Li-S batteries hold great potential for energy storage systems due to their high theoretical capacity and low cost. However, the shuttle of polysulfides in conventional Li-S batteries with organic electrolyte results in severe problems, such as low Coulombic efficiency and poor cycling stability. Here, we developed a solid-state Li-S battery using Li1.5Al0.5Ge1.5(PO4)3 (LAGP) solid ceramic electrolyte instead of liquid electrolyte. Lithium film was deposited on one side of LAGP by an evaporation method to improve the contact between anode and LAGP. Sulfur-coated carbon nanotubes cathode (S-MWCNT) composition was casted on the other side of LAGP as the cathode and a small amount of ionic liquid was dropped in order to improve the ionic conductivity. This battery exhibited a high capacity of 1510 mAh g-1 in the first cycle and retained 1400 mAh g-1 after 30 cycles. Meanwhile, the Coulombic efficiency of the battery was almost 100% for each cycle, which indicated the shuttle effect was circumvented completely.

The Behavior of the Interfacial Charge Transfer between Solid and Liquid Li+-Electrolytes

Li-O2 and Li-S cells can be further improved, if the anodic side is operated with a nonaqueous electrolyte and the cathodic side with an aqueous electrolyte. The separation of two different liquid electrolytes can be realized by using a solid Li+ electrolyte, because only Li+ ions are transported. Electronic and anionic conduction is blocked [1]. In this study the kinetic parameters of the interfacial charge transfer between a liquid electrolyte with various concentrations between 0.001 and 1 mol l-1 LiPF6 in ethylene carbonate/dimethyl carbonate 1:1 vol. % and a Li7La3Zr2O12 (LLZO) solid electrolyte is investigated. It has been shown that a multi electrode cell is a suitable experimental setup [2]. An improved symmetric liquid/solid/liquid cell with six potential probes and two lithium metal electrodes to draw a Li+ current through the setup has been developed to measure the electrochemical potential within the electrolytes. The extrapolation of the potential gradients leads to the potential drop ΔLi +/F at the solid/liquid electrolyte interface. The almost symmetric S-shaped functional course of the current density i vs. ΔLi +/F at LiPF6 concentrations higher than 0.01 mol l-1 can be described by a serial connection between a constant ohmic resistance RSLEI (Solid-Liquid Electrolyte Interphase SLEI) and a current dependent thermally activated ion transfer process. The constant ohmic resistance RSLEI of about 800 Ω cm² can be attributed to a surface layer on the solid electrolyte with a comparatively low conductivity as it was described by Busche et al. [3]. It may be formed by degradation reactions with the liquid electrolyte. The thermally activated ionic transfer process obeys a Butler-Volmer like behavior. The exchange current density i0 is dependent on the Li+ concentration in the liquid electrolyte and has a value of 37 µA cm-2 at a LiPF6 concentration of 1 mol l-1. For very low concentrated liquid electrolytes (cLi + < 0.01 mol l-1) the polarization curves show a strong asymmetry due to a depletion of Li+-ions at one of the surfaces because of the increasing diffusion zone and decreasing convection and an enlargement of the Li+ concentration at the other one. The polarization resistance Rp , which can be obtained by the slope of the polarization curves at i = 0, has a characteristic dependence on the electrolyte concentration and behaves according to a power law for concentrations below 0.1 mol l-1. For higher concentrations Rp reaches the limit given by the solid-liquid electrolyte interphase RSLEI . [1] Girishkumar, G., McCloskey, B., Luntz, A. C., Swanson, S. & Wilcke, W. Lithium−Air Battery: Promise and Challenges. J. Phys. Chem. Lett. 1, 2193–2203 (2010). [2] Abe, T., Sagane, F., Ohtsuka, M., Iriyama, Y. & Ogumi, Z. Lithium-Ion Transfer at the Interface Between Lithium-Ion Conductive Ceramic Electrolyte and Liquid Electrolyte-A Key to Enhancing the Rate Capability of Lithium-Ion Batteries. J. Electrochem. Soc. 152, A2151–A2154 (2005). [3] Busche, M.R., Drossel, T., Leichtweiss, T., Weber, D.A., Falk, M., Schneider, M., Reich, M.-L., Sommer, H., Adelhelm, P., Janek, J., 2016. Dynamic formation of a solid-liquid electrolyte interphase and its consequences for hybrid-battery concepts. Nat Chem advance online publication.

Energy Density Limitation of Lithium-Sulfur Battery By Lithium Polysulfide Solubility in Electrolyte

Li-S batteries are among the most promising candidates for the next generation rechargeable batteries due to their high energy density, low raw material cost and environmental friendliness [1]. Despite this great potential, the complicated electrochemical process introduces many challenges for Li-S batteries, including low conductivity of both elemental sulfur and end discharge product Li2S, large volume expansion of 80% upon discharge, and unfavorable redox shuttle effect due to soluble nature of intermediate lithium polysulfide LiPS [2,3]. Although Li-S batteries possess a high theoretical cathode specific capacity of 1,672 mAh g-1, the energy density of practical Li-S batteries is much smaller and depends on electrolyte/sulfur (E/S in mL g-1) ratio. From previous works, successful operation of Li-S batteries under lean electrolyte conditions can be challenging, especially in the case when the solubility of LiPS sets an upper bound for polysulfide dissolution. Very recently, we have demonstrated that the E/S ratio of Li-S cells has a significant effect on both performance and theoretical energy density of Li-S batteries. Since the lower-bound for E/S ratio is restricted by the solubility of LiPS in the organic electrolyte, the theoretical energy density of Li-S batteries is significantly reduced. Experimentally, it was approved that when the LiPS concentration reached to the solubility limitation in the electrolyte, the reaction rate of reducing sulfur to LiPS in the cathode will reduce significantly [4]. In this talk, we will discuss the relationship between theoretical specific energy and the solubility of LiPS in the electrolyte. The theoretical specific energy of a Li-S battery can be expressed as: e=CLVcell/(MC+MA+Mele) (1) where, CL is the lower capacity in amount of cathode and anode, Vcell is the cell voltage (e.g. 2.1 V), MC, MA, Mele are weights of sulfur (cathode), lithium (anode), and electrolyte, respectively. In an ideal case, capacities of cathode and anode are equal and to be MA=2×6.94MC/32.065, the atomic weights of lithium and sulfur are 6.94 and 32.065 g/mol, respectively, the capacity is CL=cSMC where cS=1,672 mAh/g is the theoretical specific capacity of sulfur. The minimum weight ratio of electrolyte to sulfur is: Mele/MC=rele/32.065S (2) where, rele is the mass density of electrolyte (e.g. 1.2 g/cm3) and S is the solubility of LiPS in unit of M[S]/L; therefore, the theoretical specific energy for a Li-S battery is: e=3511/(1.4327+37.42/S[M/L])[Wh/kg] (3) Fig. 1 shows the theoretical specific energy of Li-S batteries when the LiPS solubility is considered as an intrinsic limitation for Li-S battery capacity. Here only the weight of active electrode material, such as sulfur and lithium, as well as electrolyte weight is included in calculation. From Fig. 1, it can be seen that for commonly used electrolytes with LiPS solubility less than 8 M[S]/L, the theoretical energy is less than 575 Wh/g. Fig. 2 further presents a series of specific energy calculation for Li-S battery with different E/S ratios. From Fig. 2(a), it can be seen that the electrolyte weight would dominate in a Li-S battery even in relatively low E/S conditions. Fig. 2(b) plots the effective theoretical specific capacity of a Li-S battery. Fig. 2(c) that the reduced effective specific capacity introduced by LiPS solubility limit would offset electrolyte weight loss under low E/S conditions, making the theoretical specific energy curve relatively flat under low E/S region, while without considering LiPS solubility the specific energy will increase sharply when E/S ratio is decreased. The above results imply that reducing E/S ratio would become pointless when LiPS solubility is reached. For further boosting capacity of Li-S batteries, electrolyte with even higher LiPS solubility is favorable. Fig. 2(d) plots the specific energy of an ideal Li-S cell under different LiPS solubility. Reference: [1] A. Manthiram, Y. Fu, S. Chung, C. Zu, Y. Su, Rechargeable Lithium–Sulfur Batteries, Chem. Rev. 114 (2014) 11751–11787. [2] Z.W. Seh, Y. Sun, Q. Zhang, Y. Cui, Designing high-energy lithium–sulfur batteries, Chem. Soc. Rev. (2016). [3] W. Kang, N. Deng, J. Ju, Q. Li, D. Wu, X. Ma, et al., A review of recent developments in rechargeable lithium-sulfur batteries, Nanoscale. 1 (2016) 16541–16588. [4] C. Shen, J. Xie, M. Zhang, J.P. Zheng, M. Hendrickson, and E.J. Plichta, J. Electrochem. Soc., 164 (2017) A1220-A1222. Figure 1

(Invited) Material, Fabrication and Cell Design for High Loading Li-S Batteries Towards Commercialization

Development of novel energy storage systems with improved efficiency and reduced cost is highly desired due to the emerging storage applications, such as grid storage system. Lithium-sulfur battery (Li-S) has been recognized as one promising system beyond conventional Li-ion system due to its high gravimetric energy density and low material cost, which is superior to most of current Li-ion batteries. Unfortunately, the further commercialization was prohibited by some major issues of Li-S system, such as shuttling effect, which are still not fully surpassed after years’ development. In addition, conflict also exists between the overall electrochemical performance and active material loading. Other issues such as electrolyte amount are also critical. Both chemistry and engineering research and development are necessary to address all those issues that delay the large-scale industrial manufacturing and commercialization of the Li-S batteries. In order to eventually push for further industrial manufacturing and commercialization, we have spent years working on the Li-S chemistry for balanced solution. Here we’d like to share some results from our works on the Li-S system. A well-engineered carbon framework will be introduced and discussed for high sulfur loading electrodes fabrication. Corresponding critical parameters, as well as the performance evaluation of the obtained high loading electrode will be also discussed. The high loading of S on both material (> 75 wt%) and electrode level (> 70 wt %, > 3 mAh/cm2) enables a much improved electrochemical performance of the large size pouch format Li-S cell. A protected Li anode also helps improving Coulombic Efficiency and preventing shuttling effect. In addition, influence of polymer binder and electrolyte on the optimized electrodes will be discussed. Besides the cell performance, we’ll also share the principle on material screening, cell fabrication and cell design.

Predicting Lithium Polysulfide Solubility and Reaction Pathway Using a Unique Donicity Scale for Li-S Electrolytes

Sparingly Solvating Electrolytes for Li-S Cells: Use of electrolytes with low lithium polysulfide (Li2Sn) solubility limits has attracted considerable interest in recent literature.1 This “sparingly solvating” strategy has been adopted by some as a means for significantly reducing redox shuttling of Li2Sn between the cathode and anode. Interestingly, some sparingly solvating electrolyte compositions, employing significantly high concentrations of LiTFSI, have not only shown promising cycling performance, but have also shown a unique discharge voltage profile—lacking the typically observed two plateau response produced by sequential soluble long chain Li2Sn and short chain Li2Sn formation. This unique voltage profile is thought to be the result of significantly suppressed concentrations of longer chain Li2Sn (n > 4) leading to initial sulfur reduction through a solid state pathway followed by Li2Snreduction mediated by a low solubility intermediate. Predicting Li2Sn Solubility and Reaction Pathway: What remains to be elucidated is a more quantitative assessment for the solubility regimes in which the alternative sulfur reduction pathway occurs. Furthermore, a deeper understanding of the electrolyte characteristics that dictate Li2Sn solubility is lacking. In this talk, a unique 23Na NMR-derived donor number (DN) scale is presented, similar to that described by Schmeisser et al.2 Evaluation of donicity in this manner allows for a direct comparison between electrolytes utilizing different solvating species; see Figure 1 for THF and acetonitrile (AcN)-based electrolytes. It will be demonstrated that the 23Na-DN assignments are predictive indicators for solubility of Li2Sn (Figure 2). Finally, it will be shown that solubility can be used to predict the thermodynamic redox potentials for Li-S cells as a function of discharge capacity. Figure 3 shows representative galvanostatic intermittent titration technique (GITT) experiments performed on two different AcNxLiTFSI electrolytes, with drastically different Li2Sn solubility limits; the black traces follow the open circuit potentials at given capacities. Clearly, AcN6LiTFSI shows a classic two plateau response, in which the second plateau (later discharge) is at a lower potential than the early discharge plateau—consistent with higher Li2Sn solubility. Alternatively, AcN2LiTFSI shows a mid-discharge plateau higher than that of the potential at early discharge—consistent with a mechanism driven by a lack of solution phase Li2Sn (n > 4) species. Further details describing how these voltage profiles change as a function of Li2Snsolubility will be provided.

Microstructural Limitations in Lithium-Sulfur Battery Performance

Lithium-sulfur batteries present a lucrative option in the beyond lithium-ion chemistry category due to higher theoretical specific capacity (e.g. 1675 mAh/g of sulfur) compared to Li-ion batteries. However, experimental evidence suggests that typical Li-S cells exhibit reduced capacity even for the first discharge over a wide range of operating rates. The electrochemical operation of Li-S batteries is complex having its roots in both materials as well as electrode microstructural limitations. Transport in the liquid electrolyte progressively evolves during operation with disparate reduction speciation. The precipitating discharge products, such as Li2S, are electronically insulating, giving rise to local reaction shutdown and may even lead to partial pore blockage. Precipitation in the cathode may also lead to leakage of the electrolyte and equivalent loss of the sulfur active material. This work discusses the implications of the underlying microstructure-transport-electrochemical evolution coupling in the sulfur cathode and resulting influence on the cell performance.

In operando infrared spectroscopy of lithium polysulfides using a novel spectro-electrochemical cell

A new in operando spectro-electrochemical Li-S cell has been demonstrated. The novel design allows investigations of the liquid electrolyte phase, in a commercial coin cell geometry, at C rates much higher than conventional in situ cells. We use ATR FT-IR spectroscopy, coupled with a previously developed polysulfide diagnostic to quantify the evolution of lithium polysulfides during the discharge and charge cycles of a Li-S cell. The trends observed in the polysulfide order and concentration with respect to state of charge are consistent with prevailing understanding of the electrochemical mechanisms of Li-S battery operation. During discharge, we observe the reduction of elemental sulfur to dissolved Li2S8 polysulfides, and their cascading conversion to smaller polysulfides until insoluble species (Li2S2 and Li2S) are formed. During cell charging, we observe the oxidation of insoluble polysulfides to larger, soluble polysulfides (Li2Sn, n>3), and infer an eventual recovery of crystalline sulfur, from changes in polysulfides. Long-term evolution of polysulfides is observed over 7 discharge/charge cycles. Capacity fading is evident in the decay of polysulfide order and concentration at the same state of charge between cycles. Sulfur is not recovered by charging the cell in the latter cycles, and the active material is lost as solid Li2S.

Lithium metal protection enabled by in-situ olefin polymerization for high-performance secondary lithium sulfur batteries

Lithium metal is considered to be the optimal choice of next-generation anode materials due to its ultrahigh theoretical capacity and the lowest redox potential. However, the growth of dendritic and mossy lithium for rechargeable Li metal batteries lead to the possible short circuiting and subsequently serious safety issues during charge/discharge cycles. For the further practical applications of Li anode, here we report a facile method for fabricating robust interfacial layer via in-situ olefin polymerization. The resulting polymer layer effectively suppresses the formation of Li dendrites and enables the long-term operation of Li metal batteries. Using Li-S cells as a test system, we also demonstrate an improved capacity retention with the protection of tetramethylethylene-polymer. Our results indicate that this method could be a promising strategy to tackle the intrinsic problems of lithium metal anodes and promote the development of Li metal batteries.

Variations on Li3N protective coating using ex-situ and in-situ techniques for Li° in sulphur batteries

Lithium sulphur batteries are promising candidates for upcoming/future energy storage systems due to their theoretical specific energy (2500Whkg−1). Poor cycle life and low capacity retention are main issues restraining their commercialization. To achieve high energy density, metallic lithium or concentrated alloy negatives are required wherein both the challenge of high reactivity leading to degradation, and safety issues have to be dealt with. LiNO3 has been reported as an electrolyte additive to produce passivation layer on the metallic lithium anode. However, the passivation layer thickness cannot be controlled. In order to have similar passivation effect with controlled thickness and to avoid undesirable reaction on the surface of the lithium metal, Li3N protective layers has been investigated. This study demonstrates the effect of coating techniques and feasibility of Li3N protective layers for Li metal anodes in Li-S cells using just a standard sulphur/ carbon composite electrode, thereby independent from the effects of binders or any porous cathode architecture. A first approach to study the surface morphology of Li with Li3N layers in Li-S batteries has been made. Improved cycling efficiency and good lithium plating/stripping characteristics were observed with a smooth surface morphology of Li metal. A viable in-situ approach of depositing Li3N layer on the Li metal anode using ([(CH3)3SiN3]) as electrolyte additive is proposed.