A review of high energy density lithium–air battery technology
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- Rahman, M.A., Wang, X. & Wen, C. J Appl Electrochem (2014) 44: 5. doi:10.1007/s10800-013-0620-8
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Today’s lithium (Li)-ion batteries have been widely adopted as the power of choice for small electronic devices through to large power systems such as hybrid electric vehicles (HEVs) or electric vehicles (EVs). However, it falls short of meeting the demands of new markets in these areas of EVs or HEVs due to insufficient energy density. Therefore, new battery systems such as Li–air batteries with high theoretical specific energy are being intensively investigated, as this technology could potentially make long-range EVs widely affordable. So far, Li–air battery technology is still in its infancy and will require significant research efforts. This review provides a comprehensive overview of the fundamentals of Li–air batteries, with an emphasis on the recent progress of various elements, such as lithium metal anode, cathode, electrolytes, and catalysts. Firstly, it covers the various types of air cathode used, such as the air cathode based on carbon, the carbon nanotube-based cathode, and the graphene-based cathode. Secondly, different types of catalysts such as metal oxide- and composite-based catalysts, carbon- and graphene-based catalysts, and precious metal alloy-based catalysts are elaborated. The challenges and recent developments on electrolytes and lithium metal anode are then summarized. Finally, a summary of future research directions in the field of lithium air batteries is provided.
Energy is the indispensable part of modern society. Recently, global warming, diminishing fossil-fuel supplies, and environmental pollution have driven the increasing usage of renewable energy. However, the supply of renewable energy fluctuates with time and the season of the year. The development of novel energy storage and conversion system is required for effective utilization of renewable energy sources in future smart grids and power delivery systems. Numerous energy storage systems, such as mechanical, magnetic, chemical, and electrochemical, are currently being investigated . Among them, one of the most attractive systems is electrochemical storage process, which converts chemical energy into electrical energy by sharing a common carrier electron . Batteries and fuel cells are among the electrochemical storage devises that ensure such conversions to occur in a reversible or a single way.
The pathways followed on O2 reduction and evolution reactions are different. An in situ spectroscopic study of O2 reaction in aprotic electrolyte by Peng et al.  showed that LiO2 is an intermediate species during O2 reduction before the final product Li2O2. However, Li2O2 decomposes directly in one-step reaction to evolve O2 and does not produce LiO2 as an intermediate product. It has been demonstrated that the formation of the discharge products (Li2O2 or LiO2) strongly depends on the kinetic of ORR, which is affected by the presence of various catalyst  and as well as by the type of electrolytes and solvents  used in the Li–air batteries. In addition, in situ transmission electron microscopic observations of electrochemical oxidation of Li2O2 are reported by Zhong et al. . In this study, oxidation of electrochemically formed Li2O2 particles, supported on multiwall carbon nanotubes (MWCNTs), was found to occur preferentially at the MWCNTs/Li2O2 interface and suggested that electron transport in Li2O2 ultimately limits the oxidation kinetics at high overpotential. These are the few mechanisms of O2 reduction and evolution reaction for Li–air batteries. The field of Li–air batteries is currently undergoing exciting development, with increasing achievements. This review paper summarizes the recent developments of rechargeable Li–air batteries, including air cathodes, catalysts, electrolytes, and lithium metal anodes, with the aim of providing a better understanding of this technology.
2 Recent breakthroughs in lithium–air battery elements
A liquid organic electrolyte is used in aprotic/nonaqueous electrolytic type of Li–air batteries. Lithium salts such as LiPF6, LiAsF6, LiN(SO2CF3)2, and LiSO3CF3 in organic solvent such as organic carbonates, ethers, and esters are commonly used electrolytes . The configuration of the aqueous electrolytic type of Li–air battery is similar to that of aprotic type except that the electrolyte used is aqueous based. However, in aprotic electrolyte, the specific capacity of the aprotic-type Li–air battery is limited by the precipitation of lithium oxides within the pores of carbon cathode and solubility of O2 in the electrolyte solution, which affect the transport of O2 within the interior of the air cathode . In contrast, the aqueous Li–air battery avoids the issue of cathode clogging because the reaction products are water soluble, which allows aqueous Li–air batteries to maintain performance over time . However, lithium as an anode reacts violently with water, and thus, the aqueous Li–air battery requires an artificial solid electrolytic interface (SEI). Usually, a lithium ion-conducting glass or ceramic is used as SEI . Both aprotic- and aqueous-type electrolytes are used in the mixed type of Li–air battery to overcome limitations of the aqueous-type or the aprotic-type Li–air battery. A lithium metal anode is in contact with the aprotic side while the porous carbon cathode is in contact with the aqueous side. Usually, a lithium ion-conducting ceramic is used as the membrane joining the two electrolytes . Rechargeable solid-state Li–air battery is composed of solid electrolyte . However, the main drawback of this type of battery is lower ionic conductivity of the glass–ceramic electrolytes such as NASICON structure of Li1+xGe2−xAl(PO4)3 electrolyte compared to liquid electrolytes . The ultimate choice for the best configuration is still a technical issue because each configuration of these Li–air batteries has inimitable merits and shows definite scientific challenges. A preview of some of these issues is given bellow.
2.1 Air cathode
The design and synthesis of a novel porous carbon material with high conductivity. It should provide sufficient pores to store the discharge products and channels for O2 diffusion. It also should have good electrolyte wettability, ensuring an adequate and suitable three-phase interface for the charge/discharge process .
Prevention of corrosion of the catalyst, which is considered to be the result of the oxidation of carbon .
Prevention of the ingression of H2O and CO2 into the cell .
2.1.1 Air cathode based on carbon
The microstructure of a porous carbon cathode is a major factor in the performance and life cycle of Li–air batteries. Various carbon materials, such as carbon powders, mesoporous carbon, carbon nanotubes and fibers, and graphene, have been used as the air electrodes for Li–air batteries. It has been shown that the uniformity of the pore size plays an important role in determining the electrochemical performance [31, 32]. The large volume expansion in the Ketjen black (KB)-based electrode led to extra triphase regions, able to facilitate the reaction in the electrode, and extra volume to hold the reaction product. Consequently, a Li–air battery assembled using KB-based air electrode exhibited the highest specific capacity of 851 mAh g−1. Furthermore, carbons with ordered and interconnected mesopores can be used as an efficient catalyst support that provides inner pores for gas diffusion and outer surfaces for electrolyte access . In addition, for a Li–air battery, an interconnected dual pore system with a multiple time-release catalyst has been proposed in order to increase the materials utilization, to decrease diffusion limitation, and to supply high power for long periods of time . The specific capacity can be optimized by the use of a carbon microstructure and loading as well as the porosity or thickness of the electrodes and the amount of electrolyte . Therefore, large and thin electrodes tightly wound into cylindrical cells or folded into prismatic cells cannot be used, since they would lead to air starvation. Moisture and CO2 in atmospheric air should be separated from O2 since the lithium electrode needs to be protected from moisture in air. In these circumstances, the specific surface area has positive effects on a specific capacity . The specific capacity is proportional to the surface area of the air cathode because it facilitates an electrochemical reaction for the formation of lithium oxide. The high surface area results in the more specific capacity of the batteries . It has been shown that an appropriate pore size is very important in regard to high specific capacity. If the entrances of the pores are too small, such as with micropores, the pores can easily be blocked by lithium oxide dendrites and restrict the full use of pore surfaces. Compared to commercial carbon powder materials, mesostructured materials, such as mesocelluar carbon foam (MCF-C), have larger mesopores and are beneficial for accommodating discharge products . Similar results have also been reached according to other reports, using mesoporous carbon aerogels with tunable porosity through the polycondensation of resorcinol with formaldehyde . The discharge capacity of the porous carbon showed that appropriate pore volume and pore diameters are the key factors in achieving high capacity. Furthermore, comparing discharge characteristics of Li–air batteries with different carbon cathodes has been revealed that mesopore volume of carbon materials has large contribution to the capacity . Therefore, the employment of carbon nanostructures with optimized morphologies and textures may offer great opportunity to attain high-performance Li–air batteries. However, the oxidation of carbon is a serious problem, especially at high voltage during charge, for the long-term stability of the air cathode. Arai et al.  investigated the KB electrode surface area of 1,300 m2 g−1 for ORR and OER in aqueous 8 M KOH solution. It is reported that the electrode deterioration during OER claimed that the degradation resulted in a loss of the electrochemically active surface area of electrode, mainly due to carbon corrosion. Recently, Bruce et al.  also reported that carbon is more unstable upon charging above 3.5 V versus Li/Li+ in aprotic electrolyte, and upon discharge, there is little or no decomposition of hydrophobic carbon, whereas some decomposition does occur for hydrophilic carbon. In addition, it is reported that direct chemical reaction with Li2O2 is not primarily responsible for carbon corrosion, and as carbon decomposition is much less at or below 3.5 V, carbon may be a suitable electrode if charging of Li2O2 could be carried out below 3.5 V . More recently, the air electrode performance of various carbon materials, such as KB, acetylene black (AB and AB-S), Vulcan XC-72R (VX), and vapor grown carbon fiber (VGCF), has been investigated . In this study, the gas in the cells with the KB, VX, and AB-S electrodes was analyzed, and hydrogen and CO were observed for the OER. The molar ratio of hydrogen to CO was approximately 10. It has been claimed that the CO gas may come from the carbon electrode, because there was no carbon source in the cell, except the carbon in the electrode. However, the carbon electrode performance for OER was improved, and the amount of CO was significantly decreased by the addition of catalyst . Therefore, the oxidation of carbon is a serious problem for the long-term stability of the air cathode. A more stable catalyst for the OER and ORR, especially for the OER, should be developed for the air cathode with a lower overpotential at a high current density.
2.1.2 Air cathode-based carbon nanotube (CNT)
Nitrogen-doped carbon nanotubes showed a typical bamboo-like structure, which indicates that nitrogen atoms were introduced into the carbon structure. Accordingly, the diameter of N-CNTs increased to 50–60 nm compared to 40–50 nm before doping. The N-CNTs showed a lower average charging plateau voltage (4.22 V) and a higher capacity (630 mAh g−1 of Li2O2) than CNTs. These results revealed that the N-CNTs were more efficient for Li2O2 decomposition, indicating a high catalytic activity in the charge process. In addition, the N-CNTs electrode delivered an initial discharge capacity of 866 mAh g−1, which is about 1.5 times that of the CNTs electrode (596 mAh g−1).
2.1.3 Air cathode based on graphene
Nitrogen-doped graphene nanosheets (N-GNSs) and sulfur-doped graphene nanosheets (S-GNSs) as cathode material of Li–air battery have been investigated [56, 57]. It was found that the morphology of discharge product in S-GNSs cathode was significantly different from the pristine GNSs cathode with improved charging performance . The morphology and distribution of discharge products of Li2O2 is critical to further catalytic effects, affecting the performance of battery. It is important to design optimal growth of Li2O2 via substrate control and therefore improve the discharge and charge properties of the battery.
The lithium–air battery has been the subject of intense research interest for its high energy density capacity but the voltage gap between discharge and charge is usually higher than 1 V. Thus, the round-trip efficiency is significantly lower than that of the lithium–ion battery. Electrocatalysts can reduce the overpotential of discharge/charge reactions and thus increase the round-trip efficiency and improve the cycle performance . It is also reasonable to assume that electrocatalysts also play an important role in ORR and OER, and hence improve the kinetics and efficiency of the Li–air battery [58, 59]. Therefore, great effort has been devoted to designing and developing novel electrocatalysts. There are a number of categories of electrocatalysts reported in the literature.
2.2.1 Metal oxides and their composites
Discharge voltage and discharge capacities at cycles 1, 5, and 10 of cathodes with various catalysts (Adapted from )
Capacity of cycle 1/mAh g−1
Capacity of cycle 5/mAh g−1
Capacity of cycle 10/mAh g−1
In contrast, transition metal-N4 macrocycles such as iron and cobalt phthalocyanines (FePc, CoPc) have attracted considerable attention as alternative to the precious metal cathode catalysts, due to their activity and selectivity toward the ORR . Li–air batteries with CoPc on mesoporous carbon have been investigated . The cathode carbon/CoPc showed a larger capacity of 2,220 mAh g−1 at the discharge rate of 0.1 mA cm−2 and a slightly greater capacity of 2,430 mAh g−1 at the discharge rate of 0.01 mA cm−2. In contrast, aprotic Li–air cell with heat-treated FeCuPc complexes as the catalyst on Ketjenblack EC-600JD has been investigated for O2 reduction . The Li–air batteries with FeCu/C showed at least 0.2 V higher discharge voltage at 0.2 mA cm−2 than those with pristine carbon, and it is attributed to the reduced polarization by FeCu/C catalyst.
2.2.2 Carbon matrix and graphene
2.2.3 Precious metals alloy
Nonprecious metals or less noble metal such as palladium (Pd), silver (Ag), and their composites have also been the subject of many investigations because of their modest activity and relatively higher abundance [71–73]. In particular, Ag shows sensibly activity and stability with a price only about 1 % of Pt, representing an attractive ORR catalyst . However, Ag is relatively less stable and less electrocatalytically active in acidic media compared to Pt. Furthermore, studies have shown that porous Ag membranes provide electrocatalytic function with high exchange current density, mechanical support, and a means of current collection in alkaline cathodes [72, 75]. In contrast, rechargeable Li–air batteries with the mixture of Pd and mesoporous α-MnO2 electrode have been investigated . This electrode showed high activity to oxidation and reduction of Li to form Li2O2 or Li2O. The ORR activity of various noble materials such as Pd, Pt, Ruthenium (Ru), and Au has been investigated and compared with GC . The Li+-ORR activity was found to be Pd > Pt > Ru ~ Au > GC on bulk surfaces. Such a trend of Pd/C, Pt/C, Ru/C, Au/C, and VC, the ORR activity of 100 mA gcarbon−1 can reach a potential of 2.95, 2.86, 2.84, 2.76, and 2.74 VLi, respectively. Thus, palladium- and silver-based catalysts are promising cathode catalyst materials with good balance between cost and performance.
The electrolyte is currently the biggest obstacle to progress in Li–air batteries, receiving intense research attention. The first Li–air battery revealed problems such as O2 solubility in electrolyte, lithium oxide disbanding, instability of the lithium anode in various electrolytes, especially in aqueous electrolyte, and lack of catalyst for rechargeable batteries. It is impractical to use a lithium anode directly in aqueous electrolytes due to the reactivity between lithium and water in the Li–air battery . A solution has been sought using a solid-state battery  and incorporating a mixed electrolyte system  to avoid the undesirable reactions between the lithium anode and an aqueous electrolyte. Several types of electrolytes have been investigated in search for a rechargeable Li–air battery.
2.3.1 Organic carbonate
Carbonate-based organic electrolytes such as propylene carbonate (PC) have been widely investigated in Li–air batteries, mostly because carbonate mixtures are the dominant electrolyte solvents in Li-ion batteries and PC has a wide liquid temperature range from −50 to 240 °C and low volatility . Various types of electrolytes have been investigated for use in lithium batteries by dissolving LiPF6 in various solvents , notably PC, λ-butyrolactone (λ-BL), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), 1,2-dimethoxyethane (DME), tetrahydrofuran (THF), and tetrahydropyran (THP) and their mixtures. It has also been suggested that the viscosity of electrolyte influences the O2 solubility in the electrolytes: the greater the O2 solubility the greater the discharge capacity. Lower viscosity ensures higher O2 solubility of the electrolyte, with a tendency to decomposition. It should be noted that PC:DEC (1:1) solvent exhibited the highest discharge capacity of 2,120 mAh g−1. In addition, O2 solubility, electrolyte viscosity, and O2 partial pressure have direct correlation to discharge capacity and rate capability. The O2 transport properties of several electrolytes have been investigated and the results suggested the dependence of cell performance on O2 transport in organic electrolyte . It has been reported that specific capacity can be increased from 85 to 500 mAh g−1 by increasing O2 concentration in the electrolyte, either by electrolyte reformulation or by increasing O2 partial pressure. Furthermore, by decreasing electrolyte viscosity, discharge capacity can be increased . However, it has been reported that such types of electrolytes decompose in Li–air batteries during discharge. Bruce et al.  reported that the discharge product in a nonaqueous rechargeable Li–air battery is a mixture of Li and PC (particularly Li2CO3), rather than electrochemically reversible Li2O2, which severely affects the rechargeability and life cycle of aprotic Li–air batteries.
2.3.2 Ethers and glymes
2.3.3 Solid-state electrolytes
Current thin-film solid-state Li–air batteries based on lithium phosphorous oxynitride (LIPON) electrolytes have been shown to have reasonable life cycles (more than 1,000 times) but, unfortunately, LIPON has low ionic conductivity (10−6 S cm−1 at 25 °C) . Low ionic conductivity, particularly at low temperatures, means that electrolytes need a very thick lithium electrode. It should be noted that a thick electrode is more suitable for an aqueous rather than a nonaqueous Li–air cell because of the higher conductivity of aqueous electrolyte solutions. However, a thick electrode is necessary in order to achieve high energy density, although thick electrode implies a poor power density .
2.3.4 Ionic liquids
Taking into consideration the limitation of solid-state-type electrolytes, a high ionic conductive, extremely low electronic conductive, and chemically or electrochemically stable solid-state electrolyte lithium superionic conductor (LiSICON) have been proposed [90, 91]. This has been demonstrated to be the highest ionic conductivity among any solid-state ionic conductive electrolytes (>10−3 S cm−1 at 25 °C). In contrast, room temperature ionic liquids (e.g., 1-alkyl-3-methylimidazolium ) have attracted much attention due to their special properties such as low flammability, their hydrophobic nature, low vapor pressure, their wide potential window, and high thermal stability [64, 92]. It exhibited a discharge capacity >5,000 mAh g−1 of carbon at the very low discharge current of 0.01 mA cm−2 with hydrophobicity and negligible vapor pressure. Therefore, ionic liquid Hg/1-ethyl-3-methyl imidazolium imide is considered to be a promising candidate as an electrolyte for a Li–air battery . However, the pyrrolidium-based ionic liquids were found to be more stable than the imidazolium-based ionic liquids, although lower discharge capacities were found in the pyrrolidium-based ionic liquid electrolytes. In addition, the chemical reaction of lithium metal with an aqueous electrolyte and moisturizing nonaqueous electrolyte from air is a great concern. Dual electrolyte lithium air cells, employing NASICON-type solid electrolytes (Li1+x+y AlxTi2−xSiyP3−yO12 or LTAP), have been presented: a lithium metal electrode in nonaqueous electrolyte and O2 electrode operates in an aqueous electrolyte . This cell exhibited a typical three-stage discharge capacity of 740 mAh g−1 that was 90.2 % of the theoretical capacity of buffer solution containing H3PO4 (820 mAh g−1), and cell voltage on average 3.3 V with energy density of 2,442 Wh kg−1. Recently, Peng et al.  reported a design that employed dimethyl sulfoxide as the electrolyte and gold nanoparticles as the cathode, with 100 charge cycles and only a 5 % capacity loss. This research highlights the importance of developing a stable electrolyte in order to achieve high-performance Li–air battery.
Summary of electrolytes used in lithium–air batteries
LiPF6 is dissolved in propylene carbonate (PC), λ-butyrolactone (λ-BL), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), 1,2-dimethoxyethane (DME), tetrahydrofuran (THF), and tetrahydropyran (THP)
Low viscous which ensure high oxygen solubility
Compatible with lithium metal
High discharge capacity and electrolytes can decompose
Ethers and glymes
LiPF6 in tetraethylene glycol dimethyl ether, and tetraglyme
High stable with lithium anode.
High oxidation potential of 4.5 V
More stable than carbonate base electrolytes
Easily wet the low polarity carbon cathode
1-Alkyl-3-methylimidazolium, Hg/1-ethyl-3-methyl imidazolium imide
Low flammability, hydrophobic nature, low vapor pressure, wide potential window, and high thermal stability.
High viscous and low ionic conductivity
Lithium phosphorous oxynitride (LIPON), lithium superionic conductor (LiSICON), NASICON (LTAP), glass–ceramic, and polymer ceramic
Thermal stability, Chemical stability and rechargeability
Act as a separator
Low ionic conductivity, particularly at low temperatures
2.4 Lithium metal anode
Choosing an electrolyte of liquid or polymer that is less reactive with lithium metal .
Forming a SEI layer by adding CO2, HF, or Sx2− to the electrolytes so that it can react with the lithium surface to form a SEI layer. This then acts as a SEI inner layer to subdue dendrite formation. The addition of a small amount of HF in the electrolytes forms LiF on the lithium surface and gives a smooth surface.
Using hydrocarbon or quaternary ammonium salt as an active agent on the surface of the lithium metal .
Placing an ultra-thin plasma polymer layer on the surface of the lithium metal. The lithium metal surface is covered by a solid polymer that is prepared via plasma polymerization of 1, 1-difluoroethene (C2H4F2) and inhibits dendrite formation . In addition, polyethylene glycol dimethacrylate is solid and thin, allowing the electrons to pass to the electrolytes and cover the surface of the anode, keeping it smooth .
Formation of a stable metal alloy (LiI) by incorporating tin(II) iodide (SnI2) or aluminum iodide AlI3, SnI2, and AlI3 have been investigated and found as promising additives for improving cyclic efficiency for the lithium electrode . It is postulated that the thin layers of the lithium alloys at the lithium electrode surface result in an improvement of Coulombic efficiency (especially after 10th cycle) during lithium deposition and dissolution.
Formation of an ultra-thin polymer electrolyte layer based on ultraviolet (UV) irradiation polymerization or plasma polymerization [107, 108]. The formation of semi-interpenetrating network (semi-IPN) structure protective layer on lithium electrodes was attempted to make the lithium deposition morphology less dendritic. The UV-curable mixed solution consists of linear polymer (Kynar 2801), crosslinking agent (1,6-hexanediol diacrylate), liquid electrolyte (EC/PC/1 M LiClO4), oligo(ethylene glycol) borate (OEGB) anion receptor, and photoinitiator (methyl benzoylformate). This curable mixed solution was directly coated on the lithium metal surface, and it was UV-irradiated by UV light for 2 min after drying. Eventually, a protective layer based on semi-IPN structure was formed on the lithium metal surface .
Uniform lithium deposition by means of pressure and temperature [109–111]. In situ investigation of dendritic lithium growth during cycling has been carried out . Mechanical pressure has shown to have a crucial effect on lithium morphology, lithium cyclability, and dendritic lithium growth. This is because lithium was deposited densely and uniformly on the lithium electrode surfaces under pressure, and the isolation of deposited lithium from the electrode during the lithium stripping process was reduced. In addition, low temperature conditions (from −10 to 0 °C) could improve the C–D cycling efficiency of lithium metal of nickel substrate .
A cathode reaction delivers most of the energy because most of the cell voltage drop occurs at the air cathode. It is thought that nonaqueous Li–air energy falls far too short of the theoretical values because the discharge terminates well before all the pores in the air electrode (porous carbon) are filled with lithium oxide or peroxides. Therefore, it is essential to optimize the porous structure of a carbon electrode.
The catalyst has a significant effect on the cathode reaction. Several catalysts have been proposed for charging and discharging and some are still in progress. MnO2 and PtAu nanoparticles have shown good stability against OER and ORR. However, the cost of Au- and Pt-based catalysts is still prohibitive for commercialization. Therefore, more efforts should be given to the development of a cost-effective Li–air battery.
Currently, most aprotic Li–air batteries operate with pure O2. Environmental O2 is combined with moisture, which could degrade the nonaqueous electrolyte and lithium metal anode, and therefore, the battery has a poor life cycle compared with current lithium–ion batteries. O2 separation from air for the Li–air is a problem as teflonized layer membrane can only slow down H2O ingress. Therefore, intensive investigation is necessary to better understand the O2 solubility and diffusivity in electrolytes (both aqueous and nonaqueous), and the viscosity and polarity factors of the electrolytes.
The development of novel electrolytes with the properties of low viscous, low volatility, compatible with electrodes, hydrophobic, high O2 solubility or diffusivity, thermal and chemically stable, and less decomposing electrolytes, which will not release CO2 during charging, is the biggest challenges. Therefore, modifications of existing electrolytes and the quest for new electrolyte systems present new challenges for progress in the lithium–air battery.
The main problem associated with lithium metal as the anode is dendrite formation. This dendrite formation is heterogeneous, which can lead to dangerous battery short-circuiting and poor cycle performance. However, this can be suppressed by the formation of a stable SEI film on the surface of the lithium anode. Therefore, in order to combat this problem, a novel solid polymer electrolyte has been introduced to protect the lithium metal and conducts lithium ions, thereby preventing dendrite formation.
The authors acknowledge financial support for this research through the Australia–India Strategic Research Fund (AISRF, ST 060048).