Sheng-Heng Chung

A core–shell electrode for dynamically and statically stable Li–S battery chemistry

Sulfur is an appealing cathode material for establishing advanced lithium batteries as it offers a high theoretical capacity of 1675 mA h g−1 at low material and operating costs. However, the lithium–sulfur (Li–S) electrochemical cells face several formidable challenges arising from both the materials chemistry (e.g., low electrochemical utilization of sulfur and severe polysulfide diffusion) and battery chemistry (e.g., dynamic and static instability and low sulfur loadings). Here, we present the design of a core–shell cathode with a pure sulfur core shielded within a conductive shell-shaped electrode. The new electrode configuration allows Li–S cells to load with a high amount of sulfur (sulfur loadings of up to 30 mg cm−2 and sulfur content approaching 70 wt%). The core–shell cathodes demonstrate a superior dynamic and static electrochemical stability in Li–S cells. The high-loading cathodes exhibit (i) a high sulfur utilization of up to 97% at C/20–C/2 rates and (ii) a low self-discharge during long-term cell storage for a three-month rest period and at different cell-storage conditions. Finally, a polysulfide-trap cell configuration is designed to evidence the eliminations of polysulfide diffusion and to investigate the relationship between the electrode configuration and electrochemical characteristics. The comprehensive analytical results based on the high-loading cathodes suggest that (i) the core–shell cathode is a promising solution for designing highly reversible Li–S cells and (ii) the polysulfide-trap cell configuration is a viable approach to qualitatively evaluating the presence or absence of polysulfide diffusion.

Nano-cellular carbon current collectors with stable cyclability for Li–S batteries

Li–S batteries have been investigated with a simple modification of the electrode configuration by applying a nano-cellular carbon current collector (NC current collector). This micro-meso-macro-porous electrode is composed of interwoven carbon fibers with the carbon nanofoam firmly attached to them. The nanofoam plate functions as a reservoir to store the active material and localize the dissolved polysulfides, stabilizing the electrochemical reaction within the cathode region. As a result, the NC current collector offers a considerably high discharge capacity and superior cycle stability.

Highly flexible, freestanding tandem sulfur cathodes for foldable Li–S batteries with a high areal capacity

Li–S batteries with a high theoretical capacity are considered as the most promising candidate to satisfy the increasing demand for batteries with a high areal capacity. However, the low sulfur loading (<2 mg cm−2) and poor flexibility of current Li–S batteries limit their application in establishing foldable Li–S batteries with a high areal capacity. To solve this problem, we employ here a free-standing flexible tandem sulfur cathode with a remarkably high sulfur loading to demonstrate foldable, high-areal-capacity Li–S batteries. The design of the tandem cathode readily increases the sulfur loading and effectively retards the migration of polysulfides. Therefore, the Li–S cell employing the tandem cathode exhibits a high initial areal capacity of 12.3 mA h cm−2 with stable cycling stability even with a high sulfur loading of up to 16 mg cm−2. These tandem cathodes are promising for foldable Li–S cells with a high areal capacity and energy density.

A trifunctional multi-walled carbon nanotubes/polyethylene glycol (MWCNT/PEG)-coated separator through a layer-by-layer coating strategy for high-energy Li–S batteries

A trifunctional separator fabricated by using a light-weight layer-by-layer multi-walled carbon nanotubes/polyethylene glycol (MWCNT/PEG) coating has been explored in lithium–sulfur (Li–S) batteries. The conductive MWCNT/PEG coating serves as (i) an upper current collector for accelerating the electron transport and benefiting the electrochemical reaction kinetics of the cell, (ii) a net-like filter for blocking and intercepting the migrating polysulfides through a synergistic effect including physical and chemical interactions, and (iii) a layered barrier for inhibiting the continuous diffusion and alleviating the volume change of the trapped active material by introducing a “buffer zone” in between the coated layers. The multi-layered MWCNT/PEG coating allows the use of the conventional pure sulfur cathode with a high sulfur content (78 wt%) and high sulfur loading (up to 6.5 mg cm−2) to achieve a high initial discharge capacity of 1206 mA h g−1 at C/5 rate, retaining a superior capacity of 630 mA h g−1 after 300 cycles. The MWCNT/PEG-coated separator optimized by the facile layer-by-layer coating method provides a promising and feasible option for advanced Li–S batteries with high energy density.

Rational Design of Statically and Dynamically Stable Lithium–Sulfur Batteries with High Sulfur Loading and Low Electrolyte/Sulfur Ratio

The primary challenge with lithium-sulfur battery research is the design of sulfur cathodes that exhibit high electrochemical efficiency and stability while keeping the sulfur content and loading high and the electrolyte/sulfur ratio low. With a systematic investigation, a novel graphene/cotton-carbon cathode is presented here that enables sulfur loading and content as high as 46 mg cm-2 and 70 wt% with an electrolyte/sulfur ratio of as low as only 5. The graphene/cotton-carbon cathodes deliver peak capacities of 926 and 765 mA h g-1 , respectively, at C/10 and C/5 rates, which translate into high areal, gravimetric, and volumetric capacities of, respectively, 43 and 35 mA h cm-2 , 648 and 536 mA h g-1 , and 1067 and 881 mA h cm-3 with a stable cyclability. They also exhibit superior cell-storage capability with 95% capacity-retention, a low self-discharge constant of just 0.0012 per day, and stable poststorage cyclability after storing over a long period of six months. This work demonstrates a viable approach to develop lithium-sulfur batteries with practical energy densities exceeding that of lithium-ion batteries.

A nickel-foam@carbon-shell with a pie-like architecture as an efficient polysulfide trap for high-energy Li–S batteries

A high-loading sulfur cathode is critical for establishing rechargeable lithium–sulfur (Li–S) batteries with the anticipated high energy density. However, its fabrication as well as realizing high electrochemical utilization and stability with high-loading sulfur cathodes is a daunting challenge. We present here a new pie-like electrode that consists of an electrocatalytic nickel-foam as a “filling” to adsorb and store polysulfide catholytes and an outer carbon shell as a “crust” for facilitating high-loading sulfur cathodes with superior electrochemical and structural stabilities. The inner electrocatalytic nickel-foam is configured to adsorb polysulfides and facilitate their redox reactions. The intertwined carbon shell assists to shield the polysulfides within the cathode region of the cell. Both the nickel-foam and the carbon shell have high conductivity and porous space, which serve simultaneously as interconnected current collectors to enhance the redox kinetics and as polysulfide reservoirs to confine the active material. The effectiveness of such a pie-like structure in improving the electrochemical efficiency enables the cathode to host an ultrahigh sulfur loading of 40 mg cm−2 and attain a high areal capacity of over 40 mA h cm−2 at a low electrolyte/sulfur (E/S) ratio of 7. The enhanced cyclability is reflected in a high reversible areal capacity approaching 30 mA h cm−2 at C/5 rate after 100 cycles and excellent rate capability up to 2C rate.

A three-dimensional self-assembled SnS2-nano-dots@graphene hybrid aerogel as an efficient polysulfide reservoir for high-performance lithium–sulfur batteries

Reliable sulfur cathodes hold the key to realizing high-performance lithium–sulfur (Li–S) batteries, yet the electrochemical inefficiency and instability arising from the poor conductivity of sulfur and lithium sulfide together with polysulfide diffusion present challenges. We present here a new three-dimensional graphene aerogel embedded with in situ grown SnS2 nano-dots (SnS2-ND@G) as an efficient sulfur host. First, benefiting from a highly conductive, hierarchically porous, and mechanically self-supported architecture, the SnS2-ND@G aerogel enables the cathode to hold high sulfur content (75 wt%) and loading (up to 10 mg cm−2). Both values exceed most of the reported metal-compound-related cathode work (<60 wt% sulfur content and <3 mg cm−2 sulfur loading) in the literature. Second, this work takes advantage of a facile one-pot self-assembly fabrication, effectively guaranteeing a homogeneous deposition of SnS2 nano-dots in the graphene aerogel with a small amount of SnS2 (16 wt%). It greatly overcomes the shortcomings of physical incorporation methods to make metal-compound/carbon substrates reported in previous studies. More importantly, by rationally combining the physical entrapment from graphene and chemical adsorptivity from SnS2 nano-dots towards polysulfides, the SnS2-ND@G aerogel demonstrates remarkably improved polysulfide-trapping capability and electrochemical stability. As a result, a high peak capacity of 1234 mA h g−1, a high reversible capacity of 1016 mA h g−1 after 300 cycles, exceptional rate capability (C/10 – 3C rates), and impressive areal capacity (up to 11 mA h cm−2) are achieved. This work provides a viable path to integrate a conductive graphene network and nano-sized SnS2 as a promising cathode substrate for developing advanced Li–S batteries.

Transforming waste newspapers into nitrogen-doped conducting interlayers for advanced Li–S batteries

Nitrogen-doped conducting (NC) interlayers, derived from waste newspapers, are inserted between a sulfur cathode and a separator to enhance the electrochemical performances of lithium–sulfur (Li–S) batteries. A simple strategy transforms wastes into valuable polysulfide trappers that improve the active-material utilization to 77% and accomplish good cycling stability in Li–S batteries.

Oligoanilines as a suppressor of polysulfide shuttling in lithium–sulfur batteries

The migration of small polysulfide (LiPS) chains through the porous polymeric separators seriously jeopardizes the cycle life and energy density of lithium–sulfur (Li–S) batteries. Herein, we present a new concept in which an organic oligoaniline, amine-capped aniline trimer (ACAT), serves as an effective suppressor of the LiPS migration in Li–S cells. The strong interaction between LiPS and ACAT facilitates the formation of bulky ACAT–LiPS complexes (organoLiPS complexes), which are then size-selectively sieved by the porous polymeric separators employed in Li–S cells. Thus, the addition of ACAT significantly ameliorates the electrochemical performances of Li–S batteries due to suppressed LiPS migration. This new concept offers a viable strategy to achieve practically viable Li–S batteries.

Quantitative Analysis of Electrochemical and Electrode Stability with Low Self-Discharge Lithium-Sulfur Batteries

The viability of employing high-capacity sulfur cathodes in building high-energy-density lithium-sulfur batteries is limited by rapid self-discharge, short shelf life, and severe structural degradation during cell resting (static instability). Unfortunately, the static instability has largely been ignored in the literature. We present in this letter a long-term self-discharge study by quantitatively analyzing the control lithium-sulfur batteries with a conventional cathode configuration, which provides meaningful insights into the cathode failure mechanisms during resting. Utilizing the understanding obtained with the control cells, we design and present low self-discharge (LSD) lithium-sulfur batteries for investigating the long-term self-discharge effect and electrode stability.