Chenxi Zu

Lithium–Sulfur Batteries: Progress and Prospects

Development of advanced energy-storage systems for portable devices, electric vehicles, and grid storage must fulfill several requirements: low-cost, long life, acceptable safety, high energy, high power, and environmental benignity. With these requirements, lithium-sulfur (Li-S) batteries promise great potential to be the next-generation high-energy system. However, the practicality of Li-S technology is hindered by technical obstacles, such as short shelf and cycle life and low sulfur content/loading, arising from the shuttling of polysulfide intermediates between the cathode and anode and the poor electronic conductivity of S and the discharge product Li2 S. Much progress has been made during the past five years to circumvent these problems by employing sulfur-carbon or sulfur-polymer composite cathodes, novel cell configurations, and lithium-metal anode stabilization. This Progress Report highlights recent developments with special attention toward innovation in sulfur-encapsulation techniques, development of novel materials, and cell-component design. The scientific understanding and engineering concerns are discussed at the end in every developmental stage. The critical research directions needed and the remaining challenges to be addressed are summarized in the Conclusion.

Thermodynamic analysis on energy densities of batteries

The average increase in the rate of the energy density of secondary batteries has been about 3% in the past 60 years. Obviously, a great breakthrough is needed in order to increase the energy density from the current 210 Wh kg−1 of Li-ion batteries to the ambitious target of 500–700 Wh kg−1 to satisfy application in electrical vehicles. A thermodynamic calculation on the theoretical energy densities of 1172 systems is performed and energy storage mechanisms are discussed, aiming to determine the theoretical and practical limits of storing chemical energy and to screen possible systems. Among all calculated systems, the Li/F2 battery processes the highest energy density and the Li/O2 battery ranks as the second highest, theoretically about ten times higher than current Li-ion batteries. In this paper, energy densities of Li-ion batteries and a comparison of Li, Na, Mg, Al, Zn-based batteries, Li-storage capacities of the electrode materials and conversion reactions for energy storage, in addition to resource and environmental concerns, are analyzed.

Mesoporous Titanium Nitride‐Enabled Highly Stable Lithium‐Sulfur Batteries

The TiN-S composite cathode exhibits superior performance because of higher electrical conductivity and the capture of the soluble intermediate species of the electrode reactions by 2-5 nm mesopores and strong N-S surface bonding.

In Situ -Formed Li2S in Lithiated Graphite Electrodes for Lithium-Sulfur Batteries

Rechargeable lithium-sulfur (Li-S) batteries have the potential to meet the high-energy demands of the next generation of batteries. However, the lack of lithium in the sulfur cathode requires the use of lithium metal anode, posing safety hazards. Use of Li2S as the cathode can eliminate this problem, but it is hampered by intrinsic challenges (e.g., high electrical resistivity and reactivity in air). We report here the use of a lithiated graphite electrode to chemically reduce in situ the polysulfide Li2S6 in liquid electrolyte to insoluble Li2S. The chemical reduction slowly draws lithium out of graphite, resulting in a reduction of the d002 spacing of graphite from 3.56 to 3.37 Å and an increase in the open-circuit voltage of cells from 60 mV to 2.10 V after stabilizing over 6 days. X-ray photoelectron spectroscopic analysis shows 48.4% of sulfur in the polysulfide was converted to Li2S. The formed amorphous Li2S shows good cyclability with low charge overpotential. The results demonstrate that lithiated graphite can serve as a lithium donor in lithium-deficient cathodes, which could enable lithium metal-free Li-S, Li-air, or Li-organic batteries.

Stabilized Lithium–Metal Surface in a Polysulfide-Rich Environment of Lithium–Sulfur Batteries

Lithium-metal anode degradation is one of the major challenges of lithium-sulfur (Li-S) batteries, hindering their practical utility as next-generation rechargeable battery chemistry. The polysulfide migration and shuttling associated with Li-S batteries can induce heterogeneities of the lithium-metal surface because it causes passivation by bulk insulating Li2S particles/electrolyte decomposition products on a lithium-metal surface. This promotes lithium dendrite formation and leads to poor lithium cycling efficiency with complicated lithium surface chemistry. Here, we show copper acetate as a surface stabilizer for lithium metal in a polysulfide-rich environment of Li-S batteries. The lithium surface is protected from parasitic reactions with the organic electrolyte and the migrating polysulfides by an in situ chemical formation of a passivation film consisting of mainly Li2S/Li2S2/CuS/Cu2S and electrolyte decomposition products. This passivation film also suppresses lithium dendrite formation by controlling the lithium deposition sites, leading to a stabilized lithium surface characterized by a dendrite-free morphology and improved surface chemistry.

Insight into lithium–metal anodes in lithium–sulfur batteries with a fluorinated ether electrolyte

High energy density Li–S batteries are a promising green battery chemistry, but polysulfide shuttling and lithium anode degradation hinder the practical use of Li–S batteries. Tremendous efforts have been made including confining sulfur in a closed cathode porous matrix and stabilizing lithium–metal anodes with additives; however, satisfactory confinement is challenging to achieve and electrolyte additives could be electrochemically unstable, deteriorating the long-term cyclability of Li–S batteries. Here, we demonstrate the control of polysulfide shuttling and stabilization of the lithium–metal anode with a fluorinated ether electrolyte without either cathode confinement or additives, which can be beneficial for both the efficient use of electrolytes and safe operation of Li–S batteries. Moreover, a solid-electrolyte interphase (SEI) layer with a hierarchical chemical composition of LiF and sulfate/sulfite/sulfide was identified on lithium anodes, which suppresses parasitic reactions and helps preserve the anode quality.

Activated Li2S as a High-Performance Cathode for Rechargeable Lithium–Sulfur Batteries

Lithium-sulfur (Li-S) batteries with a high theoretical energy density of ∼2500 Wh kg(-1) are considered as one promising rechargeable battery chemistry for next-generation energy storage. However, lithium-metal anode degradation remains a persistent problem causing safety concerns for Li-S batteries, hindering their practical utility. One possible strategy to circumvent the aforementioned problems is to use alternative, high-capacity, lithium-free anodes (e.g., Si, Sn, carbon) and a Li2S cathode. However, a large potential barrier was identified on the initial charge of insulating bulk Li2S particles, limiting the cell performance. In this work, the bulk Li2S particles were effectively activated with an electrolyte containing P2S5, resulting in a lowered initial charging voltage plateau. This permits the direct use of commercially available bulk Li2S particles as a high-capacity cathode for room-temperature, rechargeable Li-S batteries, significantly lowering the manufacturing cost of Li-S cells.

Understanding the Redox Obstacles in High Sulfur-Loading Li–S Batteries and Design of an Advanced Gel Cathode

Lithium-sulfur batteries with a high energy density are being considered a promising candidate for next-generation energy storage. However, realization of Li-S batteries is plagued by poor sulfur utilization due to the shuttle of intermediate lithiation products between electrodes and its dynamic redistribution. To optimize the sulfur utilization, an understanding of its redox behavior is essential. Herein, we report a gel cathode consisting of a polysulfide-impregnated O- and N-doped porous carbon and an independent, continuous, and highly conducting 3-dimensional graphite film as the charge-transfer network. This design decouples the function of electron conduction and polysulfide absorption, which is beneficial for understanding the sulfur redox behavior and identifying the dominant factors leading to cell failure when the cells have high sulfur content and insufficient electrolyte. This design also opens up new prospects of tuning the properties of Li-S batteries via separately designing the material functions of electron conduction and polysulfide absorption.