Regulating Li2S Deposition by Ostwald Ripening in Lithium-Sulfur Batteries

The lithium-sulfur (Li-S) batteries have attracted tremendous attention from both academia and industry for their high energy density and environmental benignity. However, the cell performance suffers from the passivation of the conductive matrix caused by uncontrolled lithium sulfide (Li₂S) deposition. Therefore, regulation of Li₂S deposition is essential to advanced Li-S batteries. In this work, the role of temperature in regulating Li₂S deposition is comprehensively investigated. At room temperature (25 °C), Li₂S exhibits a two-dimensional (2D) growth mode. The dense and insulating Li₂S film covers the conductive surface rapidly, inhibiting the charge transfer for subsequent polysulfide reduction. Consequently, the severe passivation of the conductive surface degrades the cell performance. In contrast, three-dimensional (3D) Li₂S is formed at a high temperature (60 °C) because of a faster Ostwald ripening rate at an elevated temperature. The passivation of the conductive matrix is mitigated effectively, and the cell performance is enhanced significantly, thanks to the formation of 3D Li₂S. Ostwald ripening is also valid for Li-S cells under rigorous conditions. The cell working at 60 °C achieves a high specific capacity of 1228 mA h g^-1 under the conditions of high S loading and a lean electrolyte (S loading = 3.6 mg cm^-2, electrolyte/sulfur ratio = 3 μL mg^-1), which is substantially higher than that at 25 °C. This work enriches the intrinsic understanding of Li₂S deposition in Li-S batteries and provides facile strategies for improving the cell performance under practical conditions.

The Dual Functions of Defect-Rich Carbon Nanotubes as Both Conductive Matrix and Efficient Mediator for LiS Batteries

LiS batteries are considered a promising energy storage system owing to the great abundance of sulfur and its high specific capacity. Polysulfide shuttling and sluggish reaction kinetics in sulfur cathodes significantly degrade the cycle life of LiS batteries. A modified method is employed to create defects in carbon nanotubes (CNTs), anchoring polysulfides, and accelerating electrochemical reactions. The defect-rich CNTs (D-CNT) enable dramatic improvement in both cycling and rate performance. A specific capacity of 600 mAh g^-1 with a current density of 0.5 C is achieved after 400 cycles, and even at a very high current density (5.0 C), a specific capacity of 434 mAh g^-1 is observed. Cycling stability up to 1000 cycles is also achieved under the conditions of high sulfur loading and lean electrolyte. Theoretical calculations revealed that the improvement is mainly attributable to the electronic structure of defect-rich carbon, which has higher binding energy with polysulfides because of the upshift of the p-band center. Furthermore, rotating disk electrode measurements demonstrate that the defect-rich carbon can accelerate the polysulfide conversion process. It is anticipated that this new design strategy can be the starting point for mediator-like carbon materials with good conductivity and high catalytic activity for LiS batteries.

N–Doped Porous Carbon Microspheres Derived from Yeast as Lithium Sulfide Hosts for Advanced Lithium-Ion Batteries

Lithium sulfide (Li2S) is considered to be the best potential substitution for sulfur-based cathodes due to its high theoretical specific capacity (1166 mAh g−1) and good compatibility with lithium metal-free anodes. However, the electrical insulation nature of Li2S and severe shuttling of lithium polysulfides lead to poor rate capability and cycling stability. Confining Li2S into polar conductive porous carbon is regarded as a promising strategy to solve these problems. In this work, N-doped porous carbon microspheres (NPCMs) derived from yeasts are designed and synthesized as a host to confine Li2S. Nano Li2S is successfully entered into the NPCMs’ pores to form N-doped porous carbon microspheres–Li2S composite (NPCMs–Li2S) by a typical liquid infiltration–evaporation method. NPCMs–Li2S not only delivers a high initial discharge capacity of 1077 mAh g−1 at 0.2 A g−1, but also displays good rate capability of 198 mAh g−1 at 5.0 A g−1 and long-term lifespan over 500 cycles. The improved cycling and high-rate performance of NPCMs–Li2S can be attributed to the NPCMs’ host, realizing the strong fixation of LiPSs and enhancing the electron and charge conduction of Li2S in NPCMs–Li2S cathodes.

Lithium Sulfide as Cathode Materials for Lithium-Ion Batteries: Advances and Challenges

Due to the ever-growing demand for high-density energy storage devices, lithium-ion batteries with a high-capacity cathode and anode are thought to be the next-generation batteries for their high energy density. Lithium sulfide (Li2S) is considered the promising cathode material for its high theoretical capacity, high melting point, affordable volume expansion, and lithium composition. This review summarizes the activation and lithium storage mechanism of Li2S cathodes. The design strategies in improving the electrochemical performance are highlighted. The application of the Li2S cathode in full cells of lithium-ion batteries is discussed. The challenges and new directions in commercial applications of Li2S cathodes are also pointed out.

A new high-capacity and safe energy storage system: lithium-ion sulfur batteries

Lithium-ion sulfur batteries as a new energy storage system with high capacity and enhanced safety have been emphasized, and their development has been summarized in this review. The lithium-ion sulfur battery applies elemental sulfur or lithium sulfide as the cathode and lithium-metal-free materials as the anode, which can be divided into two main types. One is anode-type, where elemental sulfur is applied as the cathode, and the anode provides lithium ions. The other one is cathode-type, where lithium sulfide as the cathode provides lithium ions, and lithium-metal-free materials (e.g., graphite, silicon/carbon) function as the anode. Recently, some new lithium-ion sulfur battery systems have also been proposed, and are discussed in this review as well. The lithium-ion sulfur batteries not only maintain the advantage of high energy density because of the high capacities of sulfur and lithium sulfide, but also exhibit the improved safety of the batteries due to a non-lithium-metal in the anode. This review paper aims to track the recent progress in the development of lithium-ion sulfur batteries and summarize the challenges and the approaches for improving their electrochemical performances, including the lithiation methods to prepare lithium-metal-free anodes in anode-type lithium-ion sulfur batteries and the lithium sulfide cathode modification approaches in cathode-type lithium-ion sulfur batteries. Furthermore, the challenges and perspectives for future research and commercial applications have also been enumerated.

Trace ethanol as an efficient electrolyte additive to reduce the activation voltage of the Li2S cathode in lithium-ion–sulfur batteries

Trace ethanol as a cheap and efficient electrolyte additive to reduce the activation voltage of the Li₂S cathode in lithium-ion–sulfur batteries by converting a solid–solid reaction into a solid–liquid reaction.

Biomimetic Root-like TiN/C@S Nanofiber as a Freestanding Cathode with High Sulfur Loading for Lithium-Sulfur Batteries

It is a tough issue to achieve high electrochemical performance and high sulfur loading simultaneously, which is of important significance for practical Li-S batteries applications. Inspired by the transportation system of the plant root in nature, a biomimetic root-like carbon/titanium nitride (TiN/C) composite nanofiber is designed as a freestanding current collector for the high sulfur loading cathode. Like the plant root which absorbs water and oxygen from soil and transfers them to the trunk and branches, the root-like TiN/C matrix provides high-efficiency polysulfide, electron, and electrolyte transfer for the redox reactions via its three-dimensional-porous interconnected structure. In the meantime, TiN can not only anchor the polysulfides via the polar Ti-S and N-S bond but also further facilitate the redox reaction because of its high catalytic effect. With 4 mg cm^-2 sulfur loading, the TiN/C@S cathode delivers a high initial discharge capacity of 983 mA h g^-1 at 0.2 C current density; after 300 charge/discharge cycles, the discharge capacity remains 685 mA h g^-1, corresponding to a capacity decay rate of ∼0.1%. Even when the sulfur loading is increased to 10.5 mg cm^-2, the cell still delivers a high capacity of 790 mA h g^-1 and a decent cycle life. We believe that this novel biomimetic root-like structure can provide some inspiration for the rational structure design of the high-energy lithium-sulfur batteries and other composite electrode materials.

Highly Reversible Li-Se Batteries with Ultra-Lightweight N,S-Codoped Graphene Blocking Layer

The desire for practical utilization of rechargeable lithium batteries with high energy density has motivated attempts to develop new electrode materials and battery systems. Here, without additional binders we present a simple vacuum filtration method to synthesize nitrogen and sulfur codoped graphene (N,S-G) blocking layer, which is ultra-lightweight, conductive, and free standing. When the N,S-G membrane was inserted between the catholyte and separator, the lithium-selenium (Li-Se) batteries exhibited a high reversible discharge capacity of 330.7 mAh g^-1 at 1 C (1 C = 675 mA g^-1) after 500 cycles and high rate performance (over 310 mAh g^-1 at 4 C) even at an active material loading as high as ~ 5 mg cm^-2. This excellent performance can be ascribed to homogenous dispersion of the liquid active material in the electrode, good Li⁺-ion conductivity, fast electronic transport in the conductive graphene framework, and strong chemical confinement of polyselenides by nitrogen and sulfur atoms. More importantly, it is a promising strategy for enhancing the energy density of Li-Se batteries by using the catholyte with a lightweight heteroatom doping carbon matrix.

Enabling High-Areal-Capacity Lithium-Sulfur Batteries: Designing Anisotropic and Low-Tortuosity Porous Architectures

Lithium-sulfur (Li-S) batteries have attracted much attention due to their high theoretical energy density in comparison to conventional state-of-the-art lithium-ion batteries. However, low sulfur mass loading in the cathode results in low areal capacity and impedes the practical use of Li-S cells. Inspired by wood, a cathode architecture with natural, three-dimensionally (3D) aligned microchannels filled with reduced graphene oxide (RGO) were developed as an ideal structure for high sulfur mass loading. Compared with other carbon materials, the 3D porous carbon matrix has several advantages including low tortuosity, high electrical conductivity, and good structural stability, which make it an excellent 3D lightweight current collector. The Li-S battery assembled with the wood-based sulfur electrode can deliver a high areal capacity of 15.2 mAh cm^-2 with a sulfur mass loading of 21.3 mg cm^-2. This work provides a facile but effective strategy to develop 3D porous electrodes for Li-S batteries, which can also be applied to other cathode materials to achieve a high areal capacity with uncompromised rate and cycling performance.

Sulfur Vapor-Infiltrated 3D Carbon Nanotube Foam for Binder-Free High Areal Capacity Lithium-Sulfur Battery Composite Cathodes

Here, we demonstrate a strategy to produce high areal loading and areal capacity sulfur cathodes by using vapor-phase infiltration of low-density carbon nanotube (CNT) foams preformed by solution processing and freeze-drying. Vapor-phase capillary infiltration of sulfur into preformed and binder-free low-density CNT foams leads to a mass loading of ∼79 wt % arising from interior filling and coating of CNTs with sulfur while preserving conductive CNT-CNT junctions that sustain electrical accessibility through the thick foam. Sulfur cathodes are then produced by mechanically compressing these foams into dense composites (ρ > 0.2 g/cm³), revealing specific capacity of 1039 mAh/g_S at 0.1 C, high sulfur areal loading of 19.1 mg/cm², and high areal capacity of 19.3 mAh/cm². This work highlights a technique broadly adaptable to a diverse group of nanostructured building blocks where preformed low-density materials can be vapor infiltrated with sulfur, mechanically compressed, and exhibit simultaneous high areal and gravimetric storage properties. This provides a route for scalable, low-cost, and high-energy density sulfur cathodes based on conventional solid electrode processing routes.