Lithium metal has re-emerged as an exciting anode for high energy lithium-ion batteries due to its high specific capacity of 3860 mAh g–1 and lowest electrochemical potential of all known materials. ...However, lithium has been plagued by the issues of dendrite formation, high chemical reactivity with electrolyte, and infinite relative volume expansion during plating and stripping, which present safety hazards and low cycling efficiency in batteries with lithium metal electrodes. There have been a lot of recent studies on Li metal although little work has focused on the initial nucleation and growth behavior of Li metal, neglecting a critical fundamental scientific foundation of Li plating. Here, we study experimentally the morphology of lithium in the early stages of nucleation and growth on planar copper electrodes in liquid organic electrolyte. We elucidate the dependence of lithium nuclei size, shape, and areal density on current rate, consistent with classical nucleation and growth theory. We found that the nuclei size is proportional to the inverse of overpotential and the number density of nuclei is proportional to the cubic power of overpotential. Based on this understanding, we propose a strategy to increase the uniformity of electrodeposited lithium on the electrode surface.
The lithium–sulfur batteries are susceptible to the loss of sulfur as dissolved polysulfides in the electrolyte and their ensuing redox shutting effect. The acceleration of the conversion kinetics of ...dissolved polysulfides into the insoluble sulfur and lithium sulfide via electrocatalysis has the appeal of being a root‐cause solution. MoS2 is the most common electrocatalyst used for this purpose. It is demonstrated that how the effectiveness can be improved by simultaneous cobalt and phosphorus doping of MoS2 nanotubes (P‐Mo0.9Co0.1S2‐2, containing 1.81 at% of P). Cobalt doping induces the transformation of MoS2 from 2H phase to metallic 1T phase, which improves the electrical conductivity of the MoS2. The Co–P coordinated sites on the catalyst surface are highly active for the polysulfide conversion reactions. Consequently, a sulfur cathode with P‐Mo0.9Co0.1S2‐2 can decrease the capacity fade rate from 0.28% per cycle before modification (over 150 cycles at 0.5C rate) to 0.046% per cycle after modification (over 600 cycles at 1C rate). P‐Mo0.9Co0.1S2‐2 also enhances the high rate performance from a capacity of 338 to 931 mAh g−1 at 6C rate. The results of this study provide the first direct evidence of the beneficial effects of heteroatom codoping of polysulfide conversion catalysts.
Cobalt and phosphorus codoped MoS2 nanosheets grown on the CNT surface are evaluated as a catalyst for polysulfide conversion reactions. The improvement of catalyst effectiveness with this hierarchical structure is due to the lowering of the energy barriers in polysulfide conversion reactions resulted from the active Co–P coordinated sites and the metallic 1T‐MoS2.
Lithium metal has shown great promise as an anode material for high-energy storage systems, owing to its high theoretical specific capacity and low negative electrochemical potential. Unfortunately, ...uncontrolled dendritic and mossy lithium growth, as well as electrolyte decomposition inherent in lithium metal-based batteries, cause safety issues and low Coulombic efficiency. Here we demonstrate that the growth of lithium dendrites can be suppressed by exploiting the reaction between lithium and lithium polysulfide, which has long been considered as a critical flaw in lithium-sulfur batteries. We show that a stable and uniform solid electrolyte interphase layer is formed due to a synergetic effect of both lithium polysulfide and lithium nitrate as additives in ether-based electrolyte, preventing dendrite growth and minimizing electrolyte decomposition. Our findings allow for re-evaluation of the reactions regarding lithium polysulfide, lithium nitrate and lithium metal, and provide insights into solving the problems associated with lithium metal anodes.
Lithium sulfur batteries have brought significant advancement to the current state-of-art battery technologies because of their high theoretical specific energy, but their wide-scale implementation ...has been impeded by a series of challenges, especially the dissolution of intermediate polysulfides species into the electrolyte. Conductive polymers in combination with nanostructured sulfur have attracted great interest as promising matrices for the confinement of lithium polysulfides. However, the roles of different conductive polymers on the electrochemical performances of sulfur electrode remain elusive and poorly understood due to the vastly different structural configurations of conductive polymer–sulfur composites employed in previous studies. In this work, we systematically investigate the influence of different conductive polymers on the sulfur cathode based on conductive polymer-coated hollow sulfur nanospheres with high uniformity. Three of the most well-known conductive polymers, polyaniline (PANI), polypyrrole (PPY), and poly(3,4-ethylenedioxythiophene) (PEDOT), were coated, respectively, onto monodisperse hollow sulfur nanopsheres through a facile, versatile, and scalable polymerization process. The sulfur cathodes made from these well-defined sulfur nanoparticles act as ideal platforms to study and compare how coating thickness, chemical bonding, and the conductivity of the polymers affected the sulfur cathode performances from both experimental observations and theoretical simulations. We found that the capability of these three polymers in improving long-term cycling stability and high-rate performance of the sulfur cathode decreased in the order of PEDOT > PPY > PANI. High specific capacities and excellent cycle life were demonstrated for sulfur cathodes made from these conductive polymer-coated hollow sulfur nanospheres.
The accelerated conversion of soluble lithium polysulfides (the intermediate products in the charging and discharging of lithium–sulfur batteries) to insoluble sulfur and lithium sulfide is an ...effective method to suppress the “polysulfide shuttle” that is detrimental to the battery performance. The results of this study show that oxygen‐deficient tungsten oxide (WO3−x) nanoplate is an effective bidirectional electrocatalyst for the polysulfide conversion reactions. Two different designs of asymmetric cells are used to elucidate the catalysis of the polysulfide conversion reactions and to confirm the role of oxygen deficiency on the conversion kinetics. The fast polysulfide conversion prevents the accumulation of polysulfides in the sulfur cathode and their ensuing loss to the electrolyte. Consequently, a sulfur cathode with WO3−x nanoplates is able to improve the cycle stability from a fade rate of 0.49% per cycle (over 100 cycles) to 0.13% per cycle (over 300 cycles) at the 0.5 C rate. In addition, the WO3−x nanoplates also improve the high rate performance of the sulfur cathode (at the 4 C rate) from a capacity of 444.1 to 693.2 mAh g−1. The results of this study provide the first direct experimental proof of the beneficial effects of oxygen deficiency in polysulfide conversion reactions.
Oxygen‐deficient WO3−x nanoplates synthesized by heating WO3 in H2 are found to be an efficient catalyst for the polysulfide conversion in lithium–sulfur batteries. A novel asymmetric cell design is used to provide the experimental evidence and new insights such as the asymmetry in the catalysis of the polysulfide oxidation and reduction reactions.
Sulfur has a high specific capacity of 1673 mAh/g as lithium battery cathodes, but its rapid capacity fading due to polysulfides dissolution presents a significant challenge for practical ...applications. Here we report a hollow carbon nanofiber-encapsulated sulfur cathode for effective trapping of polysulfides and demonstrate experimentally high specific capacity and excellent electrochemical cycling of the cells. The hollow carbon nanofiber arrays were fabricated using anodic aluminum oxide (AAO) templates, through thermal carbonization of polystyrene. The AAO template also facilitates sulfur infusion into the hollow fibers and prevents sulfur from coating onto the exterior carbon wall. The high aspect ratio of the carbon nanofibers provides an ideal structure for trapping polysulfides, and the thin carbon wall allows rapid transport of lithium ions. The small dimension of these nanofibers provides a large surface area per unit mass for Li2S deposition during cycling and reduces pulverization of electrode materials due to volumetric expansion. A high specific capacity of about 730 mAh/g was observed at C/5 rate after 150 cycles of charge/discharge. The introduction of LiNO3 additive to the electrolyte was shown to improve the Coulombic efficiency to over 99% at C/5. The results show that the hollow carbon nanofiber-encapsulated sulfur structure could be a promising cathode design for rechargeable Li/S batteries with high specific energy.
High specific capacity battery electrode materials have attracted great research attention. Phosphorus as a low-cost abundant material has a high theoretical specific capacity of 2596 mAh/g with most ...of its capacity at the discharge potential range of 0.4–1.2 V, suitable as anodes. Although numerous research progress have shown other high capacity anodes such as Si, Ge, Sn, and SnO2, there are only a few studies on phosphorus anodes despite its high theoretical capacity. Successful applications of phosphorus anodes have been impeded by rapid capacity fading, mainly caused by large volume change (around 300%) upon lithiation and thus loss of electrical contact. Using the conducting allotrope of phosphorus, “black phosphorus” as starting materials, here we fabricated composites of black phosphorus nanoparticle-graphite by mechanochemical reaction in a high energy mechanical milling process. This process produces phosphorus–carbon bonds, which are stable during lithium insertion/extraction, maintaining excellent electrical connection between phosphorus and carbon. We demonstrated high initial discharge capacity of 2786 mAh·g–1 at 0.2 C and an excellent cycle life of 100 cycles with 80% capacity retention. High specific discharge capacities are maintained at fast C rates (2270, 1750, 1500, and 1240 mAh·g–1 at C/5, 1, 2, and 4.5 C, respectively).
For future applications in portable electronics, electric vehicles and grid storage, batteries with higher energy storage density than existing lithium ion batteries need to be developed. Recent ...efforts in this direction have focused on high-capacity electrode materials such as lithium metal, silicon and tin as anodes, and sulphur and oxygen as cathodes. Lithium metal would be the optimal choice as an anode material, because it has the highest specific capacity (3,860 mAh g(-1)) and the lowest anode potential of all. However, the lithium anode forms dendritic and mossy metal deposits, leading to serious safety concerns and low Coulombic efficiency during charge/discharge cycles. Although advanced characterization techniques have helped shed light on the lithium growth process, effective strategies to improve lithium metal anode cycling remain elusive. Here, we show that coating the lithium metal anode with a monolayer of interconnected amorphous hollow carbon nanospheres helps isolate the lithium metal depositions and facilitates the formation of a stable solid electrolyte interphase. We show that lithium dendrites do not form up to a practical current density of 1 mA cm(-2). The Coulombic efficiency improves to ∼ 99% for more than 150 cycles. This is significantly better than the bare unmodified samples, which usually show rapid Coulombic efficiency decay in fewer than 100 cycles. Our results indicate that nanoscale interfacial engineering could be a promising strategy to tackle the intrinsic problems of lithium metal anodes.
Sulfur is an exciting cathode material with high specific capacity of 1,673 mAh/g, more than five times the theoretical limits of its transition metal oxides counterpart. However, successful ...applications of sulfur cathode have been impeded by rapid capacity fading caused by multiple mechanisms, including large volume expansion during lithiation, dissolution of intermediate polysulfides, and low ionic/electronic conductivity. Tackling the sulfur cathode problems requires a multifaceted approach, which can simultaneously address the challenges mentioned above. Herein, we present a scalable, room temperature, one-step, bottom-up approach to fabricate monodisperse polymer (polyvinylpyrrolidone)-encapsulated hollow sulfur nanospheres for sulfur cathode, allowing unprecedented control over electrode design from nanoscale to macroscale. We demonstrate high specific discharge capacities at different current rates (1,179, 1,018, and 990 mAh/g at C/10, C/5, and C/2, respectively) and excellent capacity retention of 77.6% (at C/5) and 73.4% (at C/2) after 300 and 500 cycles, respectively. Over a long-term cycling of 1,000 cycles at C/2, a capacity decay as low as 0.046% per cycle and an average coulombic efficiency of 98.5% was achieved. In addition, a simple modification on the sulfur nanosphere surface with a layer of conducting polymer, poly(3,4-ethylenedioxythiophene), allows the sulfur cathode to achieve excellent high-rate capability, showing a high reversible capacity of 849 and 610 mAh/g at 2C and 4C, respectively.
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