Scalable 3D Honeycombed Co3O4 Modified Separators as Polysulfides Barriers for High-Performance Li-S Batteries

Progresses and Perspectives of Carbon-Free Metal Compounds-Modified Separators for High-Performance Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) have the advantages of high theoretical specific capacity, excellent energy density, abundant elemental sulfur reserves. However, the LSBs is mainly limited by shuttling of lithium polysulfides (LiPSs), slow reaction kinetics of sulfur cathode. For solving the above problems, by developing high-performance battery separators, the reversible capacity, Coulombic efficiency (CE) and cycle life of LSBs can be effectively enhanced. Carbon-free based metal compounds are expected to be highly efficient separator modifiers for a new generation of high-performance LSBs by virtue of superior chemical adsorption capacity, strong catalytic properties and excellent lithophilicity to a certain extent. They can give play to the synergistic effect of their "adsorption-catalysis" sites to accelerate the redox kinetics of LiPSs, and their good lithophilicity can accelerate the Li+ transport kinetics, thus showing more remarkable electrochemical performances. However, a comprehensive summary of carbon-free metal compounds-modified separators for LSBs is still lacking. Here, this review systematically summarizes the researching progresses and performance characteristics of carbon-free-based metal compounds modified materials for separators of LSBs, and summarizes the corresponding mechanisms of using carbon-based separators to enhance the performance of LSBs. Finally, the review also looks forward to the prospects of LSBs using carbon-free metal compounds separators.

Influence of Nd-MOF/CNF-derived Nd2O3-C/CNF composite on lithium-sulfur batteries

The shuttle effect of soluble lithium polysulfide (LiPS) is a major obstacle to the practical application of lithium-sulfur (Li-S) batteries. In order to reduce the negative impact of the shuttle effect, Nd-MOF was combined with carbon nanofibers (CNFs) so that Nd-MOF was embedded in the CNFs and the Nd2O3-C/CNF composite was realized as a separation modification material. This embedded structure made the combination between Nd2O3-C and CNFs tighter, and it exhibited better synergistic effects to inhibit the shuttle effect of polysulfides while also enhancing the tensile strength of the separator and improving the safety performance of the battery. Based on these advantages, a lithium-sulfur coin cell with the Nd2O3-C/CNF-modified separator exhibited excellent electrochemical performance.

2D Ultrathin Titanium Nitride Nanosheets as Separator Coatings for Li-S Batteries

Transition metal nitrides (TMNs) are affirmed to be an appealing candidate for boosting the performance of lithium-sulfur (Li-S) batteries due to their excellent conductivity, strong interaction with sulfur species, and the effective catalytic ability for conversion of polysulfides. However, the traditional bulk TMNs are difficult to achieve large active surface area and fast transport channels for electrons/ions simultaneously. Here, a 2D ultrathin geometry of titanium nitride (TiN) is realized by a facile topochemical conversion strategy, which can not only serve as an interconnected conductive platform but also expose abundant catalytic active sites. The ultrathin TiN nanosheets are coated on a commercial separator, serving as a multifunctional interlayer in Li-S batteries for hindering the polysulfide shuttle effect by strong capture and fast conversion of polysulfides, achieving a high initial capacity of 1357 mAh g-1 at 0.1 C and demonstrating a low capacity decay of only 0.046% per cycle over 1000 cycles at 1 C.

Ni-CoSe2 heterojunction coated by N-doped carbon for modified separators of high-performance Li-sulfur batteries

Functional separators modified by transition metal compounds have been proven to be effective in suppressing the shuttle effect of polysulfides and accelerating sluggish electrode dynamics in lithium-sulfur batteries (LSBs). However, the behaviors of heterojunctions composed of transition metals and their compounds in LSBs are still rarely studied. Herein, we report a novel Ni-CoSe2 heterostructure coated with nitrogen-doped carbon. Compared to homogeneous cobalt diselenide, it exhibits much stronger adsorption and catalytic conversion abilities towards polysulfides. With the modified separators, the lithium-sulfur batteries exhibit significantly improved capacity retention and reduced polarization during cycling.

Rational Design of TiO2@g-C3N4/CNT Composite Separator for High Performance Lithium-Sulfur Batteries to Promote the Redox Kinetics of Polysulfide

Lithium-sulfur batteries (LSB) show excellent potential as future energy storage devices with high energy density, but their slow redox kinetics and the shuttle effect seriously hinder their commercial application. Herein, a 0D@2D composite was obtained by anchoring polar nano-TiO2 onto a 2D layered g-C3N4 surface in situ, and a functional separator was prepared using multi-walled carbon nanotubes as a conductive substrate. Due to their long-range conductivity, multi-walled carbon nanotubes make up for the low conductivity of TiO2@g-C3N4 to some extent. A lithium-sulfur battery prepared with a modified separator exhibited excellent long-term cycle performance, a good lithium ion diffusion rate, and rapid redox kinetics. The initial specific discharge capacity of the composite was 1316 mAh g-1 at 1 C, and a high specific discharge capacity of 569.9 mAh g-1 was maintained after 800 cycles (the capacity decay rate per cycle was only 0.07%). Even at the high current density of 5 C, a specific capacity of 784 mAh g-1 was achieved. After 60 cycles at 0.5 C, the modified separator retained the discharge capacity of 718 mAh g-1 under a sulfur load of 2.58 mg cm-2. In summary, the construction of a heterojunction significantly improved the overall cycle stability of the battery and the utilization rate of active substances. Therefore, this study provides a simple and effective strategy for further improving the overall performance and commercial application of lithium-sulfur batteries.

Li+ mobility powered by a crystal compound for fast Li-S chemistry

Placing blocking layers between electrodes has shown paramount prospects in suppressing the shuttle effect of Li-S batteries, but the associated ionic transport would be a concurrent obstacle. Herein, we present a Li-based crystal composited with carbon (LiPN2@C) by a one-step annealing of Li+ absorbed melamine polyphosphate, which simultaneously achieves alleviated polysulfide-shuttling and facilitated Li+ transport. As a homologous crystal, LiPN2 with abundant lithiophilic sites makes Li+ transport more efficient and sustainable. With a LiPN2@C-modified separator, the Li2S cathode exhibits a much-lower activation potential of 2.4 V and a high-rate capacity of 519 mA h g-1 at 2C. Impressively, the battery delivers a capacity of 726 mA h g-1 at 0.5C with a low decay rate of 0.25% per cycle during 100 continuous cycles.

Two-Dimensional Imide-Based Covalent Organic Frameworks with Tailored Pore Functionality as Separators for High-Performance Li-S Batteries

Modifying the separator of lithium-sulfur batteries (LSBs) is considered to be one of the most effective strategies for relieving the notorious polysulfide shuttle effect. Constructing a stable, lightweight, and effective LSB separator is still a big challenge but highly desirable. Herein, a stable and lightweight imide-based covalent organic framework (COF-TpPa) is facilely fabricated on reduced graphene oxide (rGO) through an oxygen-free solvothermal technique. With the directing effect of rGO and changing the side functional group of the monomer, the morphology and the pore tailoring of COF-TpPa can be simultaneously achieved and two-dimensional (2D) COF nanosheets with different functionalities (such as -SO₃H and -Cl) are successfully constructed on rGO films. The specific functional groups inside the COF's pore channels and the narrowed pore size result in efficient absorption and restriction of Li₂S_n for weakening the "shuttle effect". Meanwhile, the 2D COF nanosheets on the rGO is a favorable morphology for better exploiting pores inside the COF materials. As a result, the COF-SO₃H-modified separator, consisting of rGO and COF-TpPa-SO₃H, exhibits a high specific capacity (1163.4 mA h/g at 0.2 C) and a desirable cyclic performance (60.2% retention rate after 1000 cycles at 2.0 C) for LSBs. Our study provides a feasible strategy to rationally design functional COFs and boosts their applications in various energy storage systems.