The lithium-sulfur (Li-S) battery has received a lot of attention because it is characterized by high theoretical energy density (2,600 Wh/kg) and low cost. Though many works on the "shuttle effect" of polysulfide have been investigated, lithium metal anode is a more challenging problem, which leads to a short life, low coulombic efficiency, and safety issues related to dendrites. As a result, the amelioration of lithium metal anode is an important means to improve the performance of lithium sulfur battery. In this paper, improvement methods on lithium metal anode for lithium sulfur batteries, including adding electrolyte additives, using solid, and/or gel polymer electrolyte, modifying separators, applying a protective coating, and providing host materials for lithium deposition, are mainly reviewed. In addition, some challenging problems, and further promising directions are also pointed out for future research and development of lithium metal for Li-S batteries.

As one of the most promising electrode materials, lithium metal has excellent performance with its ultrahigh theoretic specific capacity (3860 mAh/g) and the lowest potential (−3.04 V versus standard hydrogen electrode). Nevertheless, the practical application of lithium metal batteries is hindered by the low coulombic efficiency and safety issues, which originate from the uneven Li deposition/dissolution process and the continuous growth of lithium dendrites. The composition, structure, and morphology of the solid–electrolyte interface (SEI) are key factors in regulating the lithium deposition behavior and suppressing dendrite growth. In this review, the failure mechanism of the lithium metal anode and structural and compositional properties of ideal and practical SEIs are briefly summarized, including models describing the ion transfer process and the evolution of uneven lithium deposition at the interface. There are three main strategies to obtain a high-performance lithium metal anode from the perspective of regulating the electrolyte/electrode interface. They are (i) liquid electrolyte regulation, (ii) solid electrolyte application and separator modification, and (iii) electrode interface coating and composite anode. Lastly, the remaining challenges to be further solved and possible directions for future development are discussed.

Lithium-sulfur batteries (LSBs) are regarded as an optimum candidate of energy storage technologies for intermittent natural resources (wind energy, solar energy, tide energy, etc.) storage, characterized by the high energy density (2600 kWh kg −1 ), non-toxicity and abundant sources of sulfur cathode material, etc. [4][5][6][7][8][9] Nonetheless, the practical application of LSBs is impeded by several drawbacks, including the poor electrical conductivity of sulfur and lithium sulfide (Li 2 S), volumetric fluctuation of sulfur cathode during cycling, shuttle effect triggered by lithium-polysulfides (LiPSs) dissolution in the electrolyte, and short circuit caused by lithium dendrite growth. [10,11] Among them, the shuttle effect of LiPSs is the most urgent issue in LSBs (Scheme 1), where the highly soluble LiPSs (Li 2 S n , n > 4) tend to diffuse from sulfur cathode to Li metal anode. Previous work has devoted considerable efforts to improving the electrical conductivity of sulfur cathode and suppressing the shuttle effect of LiPSs, but the consequences of the shuttle effect in terms of reduced reaction kinetics are overlooked. [12] 1) The shuttle effect leads to the high LiPSs concentration near the surface of the cathode and further increases the viscosity of the electrolyte, which impedes the lithium ions (Li + ) transport; 2) Regarding the traditional polar materials, it is difficult to simultaneously realize high Li + and electrons transfer for further LiPSs conversion since the limited conductivity; 3) The nucleation rate and growth behavior of Li 2 S strongly affect the Coulomb efficiency. Although the shuttle effect can be effectively suppressed, the irregular or slow Li 2 S nucleation will still reduce the sulfur utilization. [13][14][15][16] Previous approaches to alleviating LiPSs shuttling include sulfur host material development, [17][18][19] binder optimization, [20][21][22] solid-state electrolytes, [23][24][25][26] electrolyte additives, [27][28][29] separator modifications, [30,31] etc. Among them, separator modifications are one of the most commonly explored strategies for entrapping LiPSs as a simple and efficient technology that is ideal for large-scale manufacturing and outperforms other laborious procedures. As a core component in LSBs, the separator has the role of separating the cathode and anode to prevent short circuits and maintain the Li + ion diffusion, which is a superior platform for enhancing its properties withThe large-scale application of lithium-sulfur batteries (LSBs) has been impeded by the shuttle effect of lithium-polysulfides (LiPSs) and sluggish redox kinetics since which lead to irreversible capacity decay and low sulfur utilization. Herein, a hierarchical interlayer constructed by boroxine covalent organic frameworks (COFs) with high Li + conductivity is fabricated via an in situ polymerization method on carbon nanotubes (CNTs) (C@COF). The asprepared interlayer delivers a high Li + ionic conductivity (1.85 mS cm −1 ) and Li + transference number (0.78), which not only a...

density and larger battery pack, difficulty of battery management accompanied with safety issues also emerges, drawing great concerns of public to solid state batteries (SSBs). [2] By replacing the ignitable organic liquid electrolytes (LE) with solid state electrolytes (SSEs), flammability of devices is supposed to be largely eliminated. [3,4] Besides, bipolar-stacked design with lithium anode realized by SSEs with a high mechanical strength could further burst the energy density of SSBs. [5] However, prior to implementation of solid-state batteries, inadequate properties and complicated manufacture process of SSEs remain issues to overcome. [6] Other than ionic conductivity that inferior to liquid electrolytes, performances of SSEs are greatly restricted by undesirable electrode/electrolyte interfacial nature, in either chemistry or physics. [7] Numerous researchers have discussed poor stability of oxide-type SSEs against lithium metal anode. [8,9] Transition element containing SSEs like Li 3x La (2/3−x) TiO 3 (LLTO) [10] and Li 1+x Al x Ge 2−x (PO 4 ) 3 (LAGP) [11] could undergo a fast and constant reduction to form an inserting interphase consisting of ionic/electronic mixed conductive species, followed with short circuit or physical deform of SSEs causing cell failure. [12] Garnet-type Li 7 La 3 Zr 2 O 12 (LLZO) was found electrochemically stable against lithium metal, but suffers from contact losing with anode because its rigidity and lithiophobic surface. [13] Sulfide-based SSEs like thiophosphates (Li 2 S-P 2 S 5 ) mostly suffers from undesired performance at cathode side caused by chemical deterioration and the space charge effect. [14] As incompetent nature of SSE materials themselves, many efforts were made to build an ideal system with heterogeneous interface. Surface treatments by vapor deposition, magnetron sputtering, [15] chemical soaking [16] or physical coating [17] were conducted to form an artificial layer on SSEs. The interphase that consist of Al, [18] Ge, [19] liquid alloy, [20,21] graphite, [22] ZnO [23] or C 3 N 4 [24] could change the surface to lithiophilic and thus improve Li|SSE contact. The compounds including ionic liquids, [25] conductive polymers [26] and salts like LiF, [27] exhibiting thermodynamic stability against the electrodes, were considered ideal interphase to inhibit electrochemical decomposition of SSEs. Element doping coupled with hierarchy design also achieved visible enhancements with less side effect on ion transport. [28,29] In parallel with researches unveiling the nature and mechanism in solid state battery, numbers of investigations have been pursuing methods to stabilize their performance as well as to reduce the cost. Simple preparation and earth-abundant ingredients are preconditions for a solid state electrolyte to be suitable for scalable production. In this work, a commercial anode active material, spinel Li 4 Ti 5 O 12 , is introduced for the first time, which has high ionic conductivity to sustain high rate charge/discharge with considerable high perform...

Lithium-metal anodes with high theoretical capacity and ultralow redox potential are regarded as a "holy grail" of the nextgeneration energy-storage industry. Nevertheless, Li inevitably reacts with conventional liquid electrolytes, resulting in uneven electrodeposition, unstable solid electrolyte interphase, and Li dendrite formation that all together lead to a decrease in active lithium, poor battery performance, and catastrophic safety hazards. Here, we report a unique nonporous gel polymer electrolyte (NP-GPE) with a uniform and dense structure, exhibiting an excellent combination of mechanical strength, thermal stability, and high ionic conductivity. The nonporous structure contributed to a uniform distribution of lithium ions for dendrite-free lithium deposition, and Li/NP-GPE/Li symmetric cells can maintain an extremely low and stable polarization after cycling at a high current density of 10 mA cm −2 . This work provides an insight that the NP-GPE can be considered as a candidate for practical applications for lithium-metal anodes.

energy density. Owing to the ultrahigh theoretical specific capacity (3860 mAh g −1 ) and the lowest electrochemical potential (−3.04 V vs standard hydrogen electrode), lithium metal has become the most promising anode material to support the development of advanced energy storage system. [1] However, the low coulombic efficiency, short cycling lifespan and potential safety risks caused by the unstable electrode interface hinder its practical application. [2] Under practical condition, owing to the side reactions between high reactive lithium metal and electrolyte, a solid electrolyte interface (SEI) forms on lithium metal surface spontaneously, which could passivate the lithium metal to prevent further troublesome side reactions and endure sharp potential drop at the electrode interface. [3] However, the actual SEI is characterized by inhomogeneous composition and low mechanical strength, which results in uneven Li + ions distribution at the electrode interface, leading to non-uniform lithium deposition/stripping behavior, irreversible consumption of active lithium and electrolyte and the growth of lithium dendrites. [4] Therefore, stabilizing the electrode interface to ensure the uniform lithium deposition is of great importance for the practical application of lithium metal anode. [5] Many approaches have been developed in attempts to solve the risky interface problems related to lithium metal anode, which can be divided into 4 aspects: (1) Constructing stable SEI by modifying liquid electrolyte, which mainly includes the application of novel solvent, lithium salt and concentrated electrolyte. [6] Nevertheless, the mechanical strength of the SEI formed in situ is not strong enough to inhibit the lithium dendrite owing to the unaltered heterogeneous structure. Furthermore, under continuous irreversible consumption, the limited additives are not enough to sustain a long stable cycling lifespan. (2) Solid electrolyte is characterized by high mechanical strength and could reduce the safety risk of lithium metal anode greatly by eliminating flammable liquid electrolyte. [7] However, the application of solid electrolyte is still challenged by their low ionic conductivity and poor interface contact with electrode. (3) Employing composite electrode by using conductive materials or three-dimensional current collector has received much attention. It can reduce the local current density via ultrahigh electrode specific surface area to suppress lithium Lithium metal is regarded as the most promising electrode material for the next generation of energy storage devices. However, low coulombic efficiency and short cycle lifespan caused by the unstable electrode interface have hindered its practical application. Constructing artificial coatings on the anode is one of the most effective approaches for remedying this, but the practical effects are still limited due to their poor regulation of Li + ions transport and complex construction engineering. Herein, an "inverse concentration gradient" concept is put forward to im...

In recent years, the extensive application of lithium ion batteries (LIBs) based on LiFePO4 (LFP) cathode in electric vehicles will lead to a large amount of scraped LFP in the...

Lithium-sulfur batteries (LSBs) with extremely-high theoretical energy density (2600 Wh kg −1 ) are deemed to be the most likely energy storage system to be commercialized. However, the polysulfides shuttling and lithium (Li) metal anode failure in LSBs limit its further commercialization. Herein, a versatile asymmetric separator and a Li-rich lithium-magnesium (Li-Mg) alloy anode are applied in LSBs. The asymmetric separator is consisted of lithiated-sulfonated porous organic polymer (SPOP-Li) and Li 6.75 La 3 Zr 1.75 Nb 0.25 O 12 (LLZNO) layers toward the cathode and anode, respectively. SPOP-Li serves as a polysulfides barrier and Li-ion conductor, while the LLZNO functions as an "ion redistributor". Combining with a stable Li-Mg alloy anode, the symmetric cell achieves 5300 h of Li stripping/plating and the modified LSBs exhibit a long lifespan with an ultralow fading rate of 0.03% per cycle for over 1000 cycles at 5 C. Impressively, even under a high-sulfur-loading (6.1 mg cm −2 ), an area capacity of 4.34 mAh cm −2 after 100 cycles can still be maintained, demonstrating high potential for practical application.