Battery System Research Articles

Two‐dimensional sp2‐carbon‐linked covalent organic framework for large‐capacity and long‐life Na metal batteries

AbstractNa metal batteries are regarded as an encouraging route for energy‐dense and low‐cost battery systems. However, the unstable and irreversible Na plating/stripping, caused by the uncontrolled dendritic Na growth, prevents their practical applications. Herein, a two‐dimensional sp2‐carbon‐linked covalent organic framework (cyano‐sp2c‐COF) is adopted as seeding/hosting coating layer for a highly stable interface with long cycling life, large capacity, and high Na utilization. Benefit from the features of a fully π‐conjugated structure and well‐defined cyano groups, cyano‐sp2c‐COF with superior sodiophilicity and small interface resistance can reduce the nucleation barrier, enable Na ion flux uniformity, and enhance interface stability. Ultimately, the system achieves a low nucleation overpotential of only 10 mV, a remarkable average Coulombic efficiency of 99.7% maintained over 500 cycles in half cells, and exceptional interfacial durability of 8500 h with a high accumulated capacity of 8.5 Ah cm−2 in symmetric cells. Furthermore, the symmetric cells also present a steady cycling, even increasing the depth of discharge up to 90%. As proof, full cells demonstrate a long lifespan enduring 2700 cycles with tiny capacity decay, providing valuable insights into the long‐life Na batteries.

Techno-economic feasibility of integrating hybrid battery-hydrogen energy storage system into an academic building

Green hydrogen production and storage are vital in mitigating carbon emissions and sustainable transition. However, the high investment cost and management requirements are the bottleneck of utilizing hybrid hydrogen-based systems in microgrids. Given the necessity of cost-effective and optimal design of these systems, the present study examines techno-economic feasibility of integrating hybrid hydrogen-based systems into an outdoor test facility. With this perspective, several solar-driven hybrid scenarios are introduced at two energy storage levels, namely the battery and hydrogen energy storage systems, including the high-pressure gaseous hydrogen and metal hydride storage tanks. Dynamic simulations are carried out to address subtle interactions in components of the hybrid system by establishing a TRNSYS model coupled to a Fortran code simulating the metal hydride storage system. The OpenStudio-EnergyPlus plugin is used to simulate the building load, validate against experimental data, according to the measured data and monitored operating conditions. Aimed at enabling efficient integration of energy storage systems, a techno-enviro-economic optimization algorithm is developed to simultaneously minimize the levelized cost of the electricity and maximize the CO2 mitigation in each proposed hybrid scenario. The results indicate that integrating the gaseous hydrogen and metal hydride storages into the photovoltaic-alone scenario enhances 22.6% and 14.4% of the annual renewable factor. Accordingly, the inclusion of battery system to these hybrid scenarios gives a 30.4% and 20.3 % boost to the renewable factor value, respectively. Although the inclusion of battery energy storage into the hybrid systems increases the renewable factor, the results imply that it reduces the hydrogen production rate via electrolysis. The optimized values of the levelized cost of electricity and CO2 emission for different scenarios vary in the range of 0.376–0.789 $/kWh and 6.57–9.75 ton, respectively. The multi-criteria optimizations improve the levelized cost of electricity and CO2 emission by up to 46.2% and 11.3%, with respect to their preliminary design.

Power hardware-in-loop emulation of a battery for charging systems and grid applications

Relevance. Batteries are playing an increasingly vital role in power systems due to their utilization in various applications including microgrids, electric vehicles, sustaining geographically isolated communities, and energization of automated devices. Since they are considered as the enabling technology for renewable energy integration, the absence of battery systems from islanded microgrids can result in decreased system reliability and compromised performance due to the intermittency of local sources. Nevertheless, the hazardousness associated with their charging mechanism has led to the urgent continuous development of charging technologies and battery management systems. Aim. To develop a safe testbed to examine the functionality of newly produced battery charging stations and battery managers without employing actual physical batteries to avoid the hazardous manipulation of batteries and increase flexibility during the design and validation stage. This is accomplished by modeling the electrochemical dynamics of the battery system and integrating the device-under-test to a DC converter, which reacts based on these modeled dynamics. Novelty. This work adapts one of the most successful Li-ion battery models available in the literature and utilizes it to interact with power electronic devices that exchange power signals. Unlike other work in this field, the design is based on power hardware-in-loop principles and has minimized power consumption characteristics due to its unique configuration. The constructed computer model can be easily reparametrized to describe the dynamics of various battery capacities. Methods. MATLAB-based simulations of the proposed testbed were conducted for high and low power capacity. A LabView-based program was interfaced with the testbed hardware using a NI-DAQ board to validate the proposed design practically. The testbed hardware components were entirely developed from scratch for experimentation purposes. Results. The proposed testbed successfully imitated the dynamics of the battery, while the practical results concurred the simulated ones.

A magnetic/force coupling assisted lithium-oxygen battery based on magnetostriction and piezoelectric catalysis of CoFe2O4/BiFeO3 cathode

The high energy density of Li−O2 batteries surpasses all existing batteries, and it holds the potential to emerge as the most outstanding solution for energy storage in the future. However, the insulated, insoluble discharge product (Li2O2) has impeded the practical applications. Conventional catalyst design based on electronic structure and interfacial charge transfer descriptors are incapable of overcoming these limitations of Li2O2. Herein, a magnetic/force coupling assisted Li−O2 battery based on magnetostrictive and piezoelectric catalysis with CoFe2O4/BiFeO3 core-shell structure cathode was established for the first time. An external magnetic field is introduced to produce a magnetostrictive stress on CoFe2O4, contributing to the piezoelectric electron hole transport by the generated piezoelectric potential energy with a built-in electric field based on the piezoelectric catalytic mechanism, and thus promoting the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics, and reducing the overpotential during charge/discharge. The magnetic/force coupling assisted Li−O2 battery equipped with the unique CoFe2O4/BiFeO3 cathode deliver an ultra-low charging platform of 3.49 V and an ultra-high discharge platform of 2.83 V. This unique magnetic/force assisted strategy provides an important insight into solving the high overpotential in metal-air battery systems.

Synergistic Coupling of Sulfide Electrolyte and Integrated 3D FeS2 Electrode Toward Long‐Cycling All‐Solid‐State Lithium Batteries

FeS2 cathode is promising for all‐solid‐state lithium batteries due to its ultra‐high capacity, low cost, and environmental friendliness. However, the poor performances, induced by limited electrode‐electrolyte interface, severe volume expansion, and polysulfide shuttle, hinder the application of FeS2 in all‐solid‐state lithium batteries. Herein, an integrated 3D FeS2 electrode with full infiltration of Li6PS5Cl sulfide electrolytes is designed to address these challenges. Such a 3D integrated design not only achieves intimate and maximized interfacial contact between electrode and sulfide electrolytes, but also effectively buffers the inner volume change of FeS2 and completely eliminates the polysulfide shuttle through direct solid–solid conversion of Li2S/S. Besides, the vertical 3D arrays guarantee direct electron transport channels and horizontally shortened ion diffusion paths, endowing the integrated electrode with a remarkably reduced interfacial impedance and enhanced reaction kinetics. Benefiting from these synergies, the integrated all‐solid‐state lithium battery exhibits the largest reversible capacity (667 mAh g−1), best rate performance, and highest capacity retention of 82% over 500 cycles at 0.1 C compared to both a liquid battery and non‐integrated all‐solid‐state lithium battery. The cycling performance is among the best reported for FeS2‐based all‐solid‐state lithium batteries. This work presents an innovative synergistic strategy for designing long‐cycling high‐energy all‐solid‐state lithium batteries, which can be readily applied to other battery systems, such as lithium‐sulfur batteries.

Design And Fabrication of Solar Rechargeable Multipurpose Electric Cart for Small Scale Vegetable Vender

The solar-operated cycle cart for small-scale vegetable transportation is an impersishable and innovative solution designed to address the problems faced by local farmers and vendors. This project integrates renewable energy technology with a conventional cycle cart, providing an eco-friendly alternative to traditional manual methods or fossil fuelpowered carts. The system incorporates a photovoltaic panel mounted on the cart's roof, harnessing solar energy to charge a high-capacity battery. This stored energy powers an electric motor that assists the rider in propelling the cart, reducing the physical exertion required for transporting vegetables. The design includes a lightweight and durable frame to optimize energy efficiency and enhance the cart's load-carrying capacity. The inclusion of a user interface allows the operator to monitor battery levels and system performance. Additionally, safety features such as efficient braking systems and reflective materials enhance visibility, especially during low-light conditions. This solar-operated cycle cart not only promotes sustainable transportation but also contributes to minimize carbon emissions corelated with conventional modes of vegetable transport. By converting sunlight into electricity, the project aims to enhance the livelihoods of small-scale farmers and vendors, offering an affordable and environmentally friendly solution for their daily transportation needs

Fast Room‐Temperature Mg‐Ion Conduction in Clay‐Like Halide Glassy Electrolytes

AbstractThe discovery of mechanically soft solid‐state materials with fast Mg‐ion conduction is crucial for the development of solid‐state magnesium batteries. In this paper, novel magnesium gallium halide compounds are reported that achieve high ionic conductivity of 0.47 mS cm−1 at room temperature. These Mg‐ion conductors obtained by ball milling Mg and Ga salts exhibit clay‐like mechanical properties, enabling intimate contact at the electrode–electrolyte interface during battery cycling. With a combination of experimental and computational analysis, this study identifies that the soft‐clay formation is induced by partial anion exchange during milling. This partial anion exchange creates undercoordinated magnesium ions in a chlorine‐rich environment, yielding fast Mg‐ion conduction. This work demonstrates the potential of clay‐like halide electrolytes for all‐solid‐state magnesium batteries, with possible further extension to other multivalent battery systems.

Gaining More Insights from Synchrotron-Based X-ray Spectroscopy for Alkali Ion Rechargeable Batteries.

Alkali ion rechargeable batteries play a significant part in portable electronic devices and electronic vehicles. The rapid development of renewable energy technology nowadays demands batteries with even higher energy density for grid storage. To fulfill such demand, extensive research efforts have been devoted to optimizing electrochemical properties as well as developing novel energy storage schemes and designing new systems. In the investigation process, synchrotron-based X-ray spectroscopy plays a vital role in investigating the detailed degradation mechanism and developing novel energy storage schemes. Herein, we critically review the applications of synchrotron-based X-ray spectroscopy in battery research in recent years. This review begins with a discussion of the different scientific issues in alkali ion rechargeable batteries within various time and space scales. Subsequently, the principle of synchrotron-based X-ray spectroscopy is introduced, and the characteristics of various characterization techniques are summarized and compared. Typical application cases of synchrotron-based X-ray spectroscopy are then introduced into battery investigations. The final part presents perspectives in the development direction of both alkali ion rechargeable battery systems and synchrotron-based X-ray spectroscopy in the future.

A highly sensitive stretchable constraint fiber mat sensor with tailored near-zero Poisson’s ratio for very early stage of battery over charge/overdischarge monitoring

Battery state monitoring is crucial for safe operation and efficient management of lithium-ion battery (LIB) systems, but early stage of timely and accurate diagnosis remains challenging. Herein, we propose to utilize a highly sensitive stretchable strain sensor to in-situ monitor the slight strain variation of cells during charge and discharge, through which the state of the battery can be accurately reflected. The high sensing capability is realized by tailoring the Poisson’s ratio of low-modulus conductive fiber mats through a facile constraint manner. The non-uniform strain distribution together with the near-zero Poisson’s ratio synergistically result in the superior sensing performance, including ultralow strain detection limit (0.006%), high strain sensitivity particularly under small strains (gauge factor of 116.1 between 0 and 2% strain), and high strain resolution under large deformations. When being practically utilized for LIB monitoring, the constraint fiber mat sensor can produce instant resistance response reflecting the cell capacity, and offer timely warning signal once the cell is overcharged or overdischarged. The developed stretchable strain sensor possesses comprehensive advantages of low mechanical constraint, tailorable small size, easy integration and conformality to cells, and little temperature interference, which would serve as a promising candidate for future battery state analysis and safety monitoring.

Comprehensive investigation of the electro-thermal performance and heat transfer mechanism of battery system under forced flow immersion cooling

Efficient cooling during rapid battery charging/discharging necessitates forced circulating flow in immersion cooling systems. However, under forced flow immersion cooling (FFIC), the comprehensive impact on the electrical and thermal performance of battery modules remains inadequately explored. This study constructs an immersion-cooled battery module test platform for experimental research on the evolution of electrical and thermal characteristics. The results show that under FFIC, when the depth of discharge (DOD) during 2C and 3C discharges is below 85 %, the voltage deviation of module (δU,t) remains stable within 1 % and 2 %, respectively. At DOD of 100 %, the maximum δU,t for 2C and 3C are 10 % and 24.5 %, respectively. Furthermore, the analysis of Pearson correlation coefficient under FFIC reveals that the δU,t exhibits a very strong positive correlation with temperature difference of battery module and cell, with correlation coefficients of +0.94 and + 0.87, respectively. Higher flow rates accelerate the recovery of module temperature and facilitate voltage distribution equalization after discharge. Additionally, to elucidate the influence of flow rate on FFIC, module-scale heat transfer characteristics during battery discharge are theoretically analyzed, establishing a fitting relationship between C-rates, flow rates, and the Nusselt number (Nu). This study provides a comprehensive understanding of integrated electro-thermal performance and external heat transfer capability for flow regulation in immersion cooling.

EV BMS With Charge Monitor and Fire Detection

Electric vehicles (EVs) are undoubtedly the way of the future. However, as of 2023, EV technology has not reached its full potential in terms of efficiency and safety. The cause of majority of the electric vehicle fire events is a battery explosion or fire. This paper presents an integrated approach to manage EV battery systems, which combines a Battery Management System (BMS) with charge monitoring and fire detection. The system is built to continuously monitor the battery's voltage, current, and temperature and to immediately turn off the battery's input or output if any unexpected behaviour is noticed. Keywords: EVs, BMS, Temperature Sensor

Mechanical Measurement Approach to Characterize Venting Behavior during Thermal Runaway of 18650 Format Lithium-Ion Batteries

The propagation of thermal runaway in a battery system is safety-critical in almost every application, such as electric vehicles or home storage. Abuse models can help to undestand propagation mechanisms and assist in designing safe battery systems, but need to be well-parametrized. Most of the heat during thermal runaway is released by venting that is why the characteristic of the vent flow plays an important part in the safety assessment. During venting, the cell generates a recoil force like a rocket, which depends on the flow speed and flow rate of the gas. This principle is used in this work to measure the velocity and mass flow rate of the vent gas. High-power and high-energy 18650 format lithium-ion batteries were overheated and the recoil and weight forces were measured to determine the venting parameter during thermal runaway. Our results show, that the linearized gas flow rate for the high-power and high-energy cell is 22.15gs−1 and 27.92gs−1, respectively. The progress of the gas velocity differs between the two cell types and in case of the high-energy cell, it follows a single peak asymmetrical pattern with a peak of 398.5ms−1, while the high-power cell shows a bumpy pattern with a maximum gas velocity of 260.9ms−1. The developed test bench and gained results can contribute insights in the venting behavior, characterize venting, support safety assessments, simulations and pack design studies.

Study on the electrochemical process of Li+/Na+ mixed organic dual-cation battery system

Organic materials have garnered significant attention due to abundant resources, recyclability, and ease of design. However, they are hampered in practical applications by poor conductivity and high solubility. Herein, a lithium metal battery is developed in this work, utilizing the cathode material polyanthraquinonyl sulfide (PAQS) doped with a trace of NaCl. Additionally, the ionic liquid lithium bis-trifluoromethanesulfonimide and lithium metal as the anode. Physicochemical characterizations utilizing Raman spectroscopy and X-photoelectron spectroscopy indicate that both Na+ and Li+ migrate within the cell, forming a dual-cation battery. In addition, the battery displays exceptional rate performance with a capacity of 86 mAh·g−1 at 20 C, which is more than triples the capacity without NaCl, and has excellent cycling performance. Moreover, it retains 98 % capacity after 1000 cycles at 2 C with no significant degradation. This research offers an innovative strategy for developing a high-performance double-cation battery.

Investigations of Regenerative Braking and Vibration Energy Conservation for Efficient Charging of Lithium-Ion Battery and Supercapacitor Hybrid System in Electric Vehicle Applications

This study aims to determine new methods of making electric vehicles more energy efficient by focusing on regenerative braking and vibration energy conservation in a hybrid energy storage system integrating lithium-ion batteries and supercapacitors. The first part of the research identifies the regenerative braking system, evaluates its effectiveness, and proposes enhancements for capturing and storing braking energy. Additionally, an investigation is made into whether it would be possible to conserve vibrating energies through creative integration with energetic harvesting technologies. The purpose is to capture and convert vibrational power produced when a car moves into an electrical charge using piezoelectric device for hybrid power storage. More energy efficiency is achieved through this method while also trying to overcome the shortcomings of regenerative braking in some driving situations. This experiments involve Modeling, Simulation and development and testing of prototype systems under different driving and vibrating conditions as well as real-world performance evaluation. It will help to determine how effective the hybrid system that has been suggested enhances energy recovery and storage efficiency in electric vehicles.

Complementary Weaknesses: A Win-Win Approach for rGO/CdS to Improve the Energy Conversion Performance of Integrated Photorechargeable Li-S Batteries.

Integrating solar energy into rechargeable battery systems represents a significant advancement towards sustainable energy storage solutions. Herein, we propose a win-win solution to reduce the shuttle effect of polysulfide and improve the photocorrosion stability of CdS, thereby enhancing the energy conversion efficiency of rGO/CdS-based photorechargeable integrated lithium-sulfur batteries (PRLSBs). Experimental results show that CdS can effectively anchor polysulfide under sunlight irradiation for 20 minutes. Under a high current density (1 C), the discharge-specific capacity of the PRLSBs increased to 971.30 mAh g-1, which is 113.3 % enhancement compared to that of under dark condition (857.49 mAh g-1). Remarkably, without an electrical power supply, the PRLSBs can maintain a 21 hours discharge process following merely 1.5 hours of light irradiation, achieving a breakthrough solar-to-electrical energy conversion efficiency of up to 5.04 %. Ex situ X-ray photoelectron spectroscopy (XPS) and in situ Raman analysis corroborate the effectiveness of this complementary weakness approach in bolstering redox kinetics and curtailing polysulfide dissolution in PRLSBs. This work showcases a feasible strategy to develop PRLSBs with potential dual-functional metal sulfide photoelectrodes, which will be of great interest in future-oriented off-grid photocell systems.

Complementary Weaknesses: A Win‐Win Approach for rGO/CdS to Improve the Energy Conversion Performance of Integrated Photorechargeable Li−S Batteries

AbstractIntegrating solar energy into rechargeable battery systems represents a significant advancement towards sustainable energy storage solutions. Herein, we propose a win‐win solution to reduce the shuttle effect of polysulfide and improve the photocorrosion stability of CdS, thereby enhancing the energy conversion efficiency of rGO/CdS‐based photorechargeable integrated lithium‐sulfur batteries (PRLSBs). Experimental results show that CdS can effectively anchor polysulfide under sunlight irradiation for 20 minutes. Under a high current density (1 C), the discharge‐specific capacity of the PRLSBs increased to 971.30 mAh g−1, which is 113.3 % enhancement compared to that of under dark condition (857.49 mAh g−1). Remarkably, without an electrical power supply, the PRLSBs can maintain a 21 hours discharge process following merely 1.5 hours of light irradiation, achieving a breakthrough solar‐to‐electrical energy conversion efficiency of up to 5.04 %. Ex situ X‐ray photoelectron spectroscopy (XPS) and in situ Raman analysis corroborate the effectiveness of this complementary weakness approach in bolstering redox kinetics and curtailing polysulfide dissolution in PRLSBs. This work showcases a feasible strategy to develop PRLSBs with potential dual‐functional metal sulfide photoelectrodes, which will be of great interest in future‐oriented off‐grid photocell systems.

Constructing the charge channel with CFx using a well-dispersed carbon nanotube arrays to boost high-rate primary battery

Pursuing ultra-high specific energy density for lithium fluorinated carbon (Li/CFx) battery requires a high fluorine to carbon ratio, resulting in low-conductivity and surface-polarization issues, which restricts battery of high-rate performance. Herein, the rich charge transport channels among CFx were constructed using a well-dispersed and highly-oriented carbon nanotube arrays (CNTA) to enhance high-rate performance in Li/CFx battery. The advantage of high-orientation lies in maintaining the appropriate van der Waals forces of this CNTA prepared by catalytic chemical vapor deposition (CCVD), which helps to avoid carbon nanotubes (CNT) entanglement and achieves good dispersion. Compared with conductive additives of commercial carbon nanotubes (CCNT) and super-P (CSP), CNTA effectively releases CFx surface-polarization favouring to Li+ diffusion. Therefore, battery performance was enhanced with reduced voltage delay and low impedance. At a discharge rate of 5 C, the Li/CFx battery using CNTA had a specific capacity of 697.32 mAh/g, while these batteries using CCNT and CSP lost discharge-performance. This highly-oriented CNTA enriches the conductive additives system of lithium primary batteries and provides a novel pathway for suppressing CFx polarization.

An online state-of-health estimation method for lithium-ion battery based on linear parameter-varying modeling framework

The accurate estimation of state-of-health (SOH) is crucial for ensuring the safe and reliable operation of lithium-ion battery systems. However, the intimate coupling between SOH and state-of-charge (SOC) is often overlooked in existing estimation methods, leading to inaccurate estimates. To address this, we propose a linear parameter-varying (LPV) battery model that captures both gradual capacity degradation and rapid dynamic changes. This model integrates traditional linear models with emerging nonlinear models, providing a comprehensive online SOH estimation framework that effectively separates the effects of SOC in the LPV model structure. The model parameters are identified using a subspace algorithm with accelerated aging data. The proposed method is validated by accelerated aging experiments on two sets of battery samples, one for model development and another for model validation. The experimental data show that the LPV battery model can achieve high SOH estimation accuracy, with an average error of 2.85 % and 5.51 % for SOH, and 0.63 % and 1.20 % for capacity, respectively. The method also shows the advantages of being easy to implement and highly generalizable, making it suitable for different battery types and application scenarios.

Fluorinated carbon as high-performance cathode for aqueous zinc primary batteries.

Fluorinated carbon (CFx) has been extensively served as promising positive electrode material for lithium primary batteries due to its high energy density. However, there are comparatively far less reports about the use of CFx on other battery systems, let alone on the research of aqueous batteries. Herein in this study, we employed CFx as the cathode active for aqueous zinc batteries for the first time and systematically investigated its electrochemical behavior under a series of aqueous zinc-ion electrolytes. As is discovered that the F/C ratio (the x value in CFx) of CFx have significant effects on the electrochemical performance of aqueous Zn/CFx batteries. Specifically, CF0.85 exhibits excellent electrochemical property with delivering a remarkable discharge capacity of 503 mA h g-1 and energy density of 388 W h kg-1 (at a current rate of 30 mA g-1 under temperature of 25 °C), much better than several other CFx electrode with F/C ratio of 0.70, 0.95, and 1.10, respectively. Besides, it also exhibits decent temperature performance with discharge capacities of 550 mA h g-1 at 50 °C and 460 mA h g-1 at 0 °C under current density of 30 mA g-1. Furthermore, the electrochemical discharge mechanism based on conversion reaction was further uncovered by applying XPS, XRD, SEM and EDS elemental analysis characterization techniques. In conclusion, these results demonstrate the potential application value of CFx in aqueous zinc primary batteries.

Tuning the solubility of polysulfides for constructing practical lithium-sulfur battery

Li-S batteries are regarded as one of the most promising candidates for next-generation battery systems with high energy density and low cost. However, the dissolution-precipitation reaction mechanism of the sulfur (S) cathode enhances the kinetics of the redox processes of the insulating sulfur, which also arouses the notorious shuttle effect, leading to serious loss of S species and corrosion of Li anode. To get a balance between the shuttle restraining and the kinetic property, a combined strategy of electrolyte regulation and cathode modification is proposed via introducing 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoroprpyl ether (HFE) instead of 1,2-dimethoxyethane (DME), and SeS7 instead of S8. The introduction of HFE tunes the solvation structure of the LiTFSI and the dissolution of intermediate polysulfides with Se doping (LiPSSes), and optimize the interface stability of the Li anode simultaneously. The minor Se substitution compensates the decrease in kinetic due to the decreased solubility of LiPSs. In this way, the Li-SeS7 batteries deliver a reversible capacity of 1062 and 1037 mAh g−1 with 2.0 and 5.5 mg SeS7 cm−2 loading condition, respectively. Besides, an electrolyte–electrode loading model is established to explain the relationship between the optimal electrolyte and cathode loading. It makes more sense to guide the electrolyte design for practical Li-S batteries.