Feixiang Wu

Lithium Iodide as a Promising Electrolyte Additive for Lithium–Sulfur Batteries: Mechanisms of Performance Enhancement

Lithium Iodide (LiI) is reported as a promising electrolyte additive for lithium-sulfur batteries. It induces formation of Li-ion-permeable protective coatings on both positive and negative electrodes, which prevent the dissolution of polysulfides on the cathode and reduction of polysulfides on the anode. In addition to enhancing the cell cycle stability, LiI addition also decreases the cell overpotential and voltage hysteresis.

Conversion cathodes for rechargeable lithium and lithium-ion batteries

Commercial lithium-ion (Li-ion) batteries built with Ni- and Co-based intercalation-type cathodes suffer from low specific energy, high toxicity and high cost. A further increase in the energy storage characteristics of such cells is challenging because capacities of such intercalation compounds approach their theoretical values and a further increase in their maximum voltage induces serious safety concerns. The growing market for portable energy storage is undergoing a rapid expansion as new applications demand lighter, smaller, safer and lower cost batteries to enable broader use of plug-in hybrid and pure-electric vehicles (PHEVs and EVs), drones and renewable energy sources, such as solar and wind. Conversion-type cathode materials are some of the key candidates for the next-generation of rechargeable Li and Li-ion batteries. Continuous rapid progress in performance improvements of such cathodes is essential to utilize them in future applications. In this review we consider price, abundance and safety of the elements in the periodic table for their use in conversion cathodes. We further compare specific and volumetric capacities of a broad range of conversion materials. By offering a model for practically achievable volumetric energy density and specific energy of Li cells with graphite, silicon (Si) and lithium (Li) anodes, we observe the impact of cathode chemistry directly. This allows us to estimate potentials of different conversion cathodes for exceeding the energy characteristics of cells built with state of the art intercalation compounds. We additionally review the key challenges faced when using conversion-type active materials in cells and general strategies to overcome them. Finally, we discuss future trends and perspectives for cost reduction and performance enhancement.

Nanoporous Li2S and MWCNT-linked Li2S powder cathodes for lithium-sulfur and lithium-ion battery chemistries

In order to achieve high capacity utilization and high rate performance of lithium sulfide (Li2S) cathode materials, it is critical to identify scalable methods for low-cost preparation of nanostructured Li2S or Li2S-carbon composites. Here, we report on the preparation and characterization of nanoporous Li2S and multiwalled (MW) carbon nanotube (CNT) – linked Li2S powders, prepared for the first time via a versatile solution-based method. The addition of MWCNTs enhances electrical conductivity and structural stability of the Li2S-based cathodes and reduces polarization of cells operating at high current densities. The nanostructured Li2S-based cathodes containing 20 wt% MWCNT showed promising discharge capacities of up to ∼1050 mA h g−1S at a slow rate of C/20 and ∼800 mA h g−1S at a C/2 rate. Quite remarkably, without any electrolyte additives (such as polysulfides or lithium nitrate) MWCNT-linked Li2S cathodes demonstrated up to ∼90% capacity retention after 100 cycles in half cells (vs. Li foil) at a C/5 and C/10 rates.

Graphene–Li2S–Carbon Nanocomposite for Lithium–Sulfur Batteries

Lithium sulfide (Li2S) with a high theoretical specific capacity of 1166mAh g(-1) is a promising cathode material for next-generation Li-S batteries with high specific energy. However, low conductivity of Li2S and polysulfide dissolution during cycling are known to limit the rate performance and cycle life of these batteries. Here, we report on the successful development and application of a nanocomposite cathode comprising graphene covered by Li2S nanoparticles and protected from undesirable interactions with electrolytes. We used a modification of our previously reported low cost, scalable, and high-throughput solution-based method to deposit Li2S on graphene. A dropwise infiltration allowed us to keep the size of the heterogeneously nucleated Li2S particles smaller and more uniform than what we previously achieved. This, in turn, increased capacity utilization and contributed to improved rate performance and stability. The use of a highly conductive graphene backbone further increased cell rate performance. A synergetic combination of a protective layer vapor-deposited on the material during synthesis and in situ formed protective surface layer allowed us to retain ∼97% of the initial capacity of ∼1040 mAh gs(-1) at C/2 after over 700 cycles in the assembled cells. The achieved combination of high rate performance and ultrahigh stability is very promising.

A Hierarchical Particle–Shell Architecture for Long‐Term Cycle Stability of Li2S Cathodes

A hierarchical particle-shell architecture for long-term cycle stability of Li2S cathodes is described. Multiscale and multilevel protection prevents mechanical degradation and polysulfide dissolution in lithium-sulfur battery chemistries.

Stabilization of selenium cathodes via in situ formation of protective solid electrolyte layer

The lithium/selenium (Li/Se) rechargeable battery chemistry offers a higher energy density than traditional Li ion battery cells. However, high solubility of polyselenides in suitable electrolytes causes Se loss during electrochemical cycling, and leads to poor cycle stability. This study presents a simple technique to form a protective, solid electrolyte layer on the cathode surface. This protective layer remains permeable to Li ions, but prevents transport of polyselenides, thus dramatically enhancing cell cycle stability. The greatly reduced reactivity of polyselenides with fluorinated carbonates (such as fluoroethylene carbonates [FEC]) permits using their in situ reduction for low-cost formation of protective coatings on Se cathodes.

A stable lithiated silicon–chalcogen battery via synergetic chemical coupling between silicon and selenium

Li-ion batteries dominate portable energy storage due to their exceptional power and energy characteristics. Yet, various consumer devices and electric vehicles demand higher specific energy and power with longer cycle life. Here we report a full-cell battery that contains a lithiated Si/graphene anode paired with a selenium disulfide (SeS2) cathode with high capacity and long-term stability. Selenium, which dissolves from the SeS2 cathode, was found to become a component of the anode solid electrolyte interphase (SEI), leading to a significant increase of the SEI conductivity and stability. Moreover, the replacement of lithium metal anode impedes unwanted side reactions between the dissolved intermediate products from the SeS2 cathode and lithium metal and eliminates lithium dendrite formation. As a result, the capacity retention of the lithiated silicon/graphene—SeS2 full cell is 81% after 1,500 cycles at 268 mA gSeS2−1. The achieved cathode capacity is 403 mAh gSeS2−1 (1,209 mAh cmSeS2−3).

Layered LiTiO2 for the protection of Li2S cathodes against dissolution: mechanisms of the remarkable performance boost

Lithium sulfide (Li2S) cathodes have been viewed as very promising candidates for next-generation lightweight Li and Li-ion batteries. Prior work on the deposition of carbon shells around Li2S particles showed reduced dissolution of polysulfides and improved cathode stability. However, due to the substantial volume changes during cycling and the low chemical binding energy between carbon and sulfides, defects almost inevitably forming in the carbon shell during battery operation commonly lead to premature cell failure. In this study, we show that conformal coatings of layered LiTiO2 may offer better protection against polysulfide dissolution and the shuttle effects. Density functional theory (DFT) calculations revealed that LiTiO2 exhibits a strong affinity for sulfur species (Li2Sx) and, most importantly, induces a rapid conversion of longer (highly soluble) polysulfides to short polysulfides, which exhibit minimum solubility in electrolytes. Quite remarkably, even the mere presence of the electronically conductive layered oxides (LiMO2, M = metal) such as LiTiO2 in the cathodes (e.g., as a component of the mix with Li2S) enhanced the cell rate and cycling stability dramatically. Advanced material characterization in combination with quantum chemistry calculations provided unique insights into the mechanisms of the incredible performance boost, such as interactions between Li2Sx and the LiTiO2 surface, leading to breakage of S–S bonds.

Nanostructured composites for high energy batteries and supercapacitors

High power energy storage devices, such as Li-ion batteries and supercapacitors, are critical for the development of zero-emission electric vehicles, large scale smart grid, energy efficient ships and locomotives, wearable devices and portable electronics. This review will focus on our progress with the developments of nanocomposite electrodes capable to improve both the energy and power storage characteristics of the state of the art devices. We review recent advancements in ultra-high capacity conversion-type anodes and cathodes for Li ion batteries as well as carbon-metal oxide and carbon-conductive polymer (nano)composite electrodes for supercapacitors. Various routes to overcome existing challenges will be discussed, including various solution deposition techniques, atomic layer deposition (ALD), chemical vapor deposition (CVD) and electro-deposition. Several designs and implementations of multi-functional electrodes will also be presented.

Lithium Sulfide Cathodes: A Hierarchical Particle–Shell Architecture for Long-Term Cycle Stability of Li2S Cathodes (Adv. Mater. 37/2015)

Hierarchical Li2 S-carbon-nanocomposite particles are synthesized by G. Yushin and co-workers, as described on page 5579, using a simple solution-based method followed by vapor deposition. The multiscale and multilevel protection enabled by the proposed architecture prevents mechanical degradation and polysulfide dissolution in lithium-sulfur batteries. The proposed hierarchical particle-shell design can be effectively utilized for a variety of other conversion-type cathode materials.