The Yushin Group leads six main areas of interest for energy storage: conversion electrodes, selective metal dealloying, aqueous-ion storage devices, nanocomposite materials, multifunctional electrodes, and solid-state electrolytes/batteries. For a complete listing of publications, please visit Google Scholar.

Hierarchical Fabric Decorated with Carbon Nanowire/Metal Oxide Nanocomposites for 1.6 V Wearable Aqueous Supercapacitors


Advanced Energy Materials

W Fu, E Zhao, X Ren, A Magasinski, G Yushin

Aqueous asymmetric supercapacitors (ASCs) may offer comparable or higher energy density than electric double-layer capacitors (EDLCs) based on organic electrolytes. As such, ASCs may be more suitable for integration into smart textiles, where the use of flammable organic solvents is not acceptable. However, reported ASC devices typically suffer from poor rate capability and low areal loadings. This study demonstrates the development of nitrogen-doped carbon (N-C) nanowire/metal oxide (Fe2O3 and MnO2) nanocomposite electrodes directly produced on the internal surface of a conductive fabric for use as high-rate electrodes for solid-state ASCs. The N-C nanowires provide fast and efficient pathways for electrons, while short diffusion paths within nanosized metal oxides enable fast ion transport, leading to greatly enhanced performance at high rates. The porous structure of the fabric enables high areal capacitance loading in each electrode (≈150 mF cm−2). Both electrodes show high specific capacitance of ≈180 F g−1 (Fe2O3) and ≈250 F g−1 (MnO2) and excellent rate capability. Solid-state ASCs assembled by using an aqueous gel electrolyte operate at 1.6 V and deliver over 60 mF cm−2 during ≈50 s charging/discharging time and over 30 mF cm−2 for ≈5 s discharge. W Fu et al., Adv. Energy Mater. 2018, 1703454 [](

Protons Enhance Conductivities in Lithium Halide Hydroxide/Lithium Oxyhalide Solid Electrolytes by Forming Rotating Hydroxy Groups


Advanced Materials

A.-Y. Song, Y Xiao, K Turcheniuk, P Upadhya, A Ramanujapuram, J Benson, A Magasinski, M Olguin, L Meda, O Borodin, G Yushin

Li-halide hydroxides (Li2OHX) and Li-oxyhalides (Li3OX) have emerged as new classes of low-cost, lightweight solid state electrolytes (SSE) showing promising Li-ion conductivities. The similarity in the lattice parameters between them, careless synthesis, and insufficient rigor in characterization often lead to erroneous interpretations of their compositions. Finally, moisture remaining in the synthesis or cell assembling environment and variability in the equivalent circuit models additionally contribute to significant errors in their properties. Thus, there remains a controversy about the real values of Li-ion conductivities in such SSEs. Here an ultra-fast synthesis and comprehensive material characterization is utilized to report on the ionic conductivities of contaminant-free Li2+xOH1−xCl (x=0-0.7), and Li2OHBr not exceeding 10-4 S cm-1 at 110 °C. Using powerful combination of experimental and numerical approaches, it is demonstrated that the presence of H in these SSEs yields significantly higher Li+ -ionic conductivity. Born-Oppenheimer molecular dynamics simulations show excellent agreement with experimental results and reveal an unexpected mechanism for faster Li+ transport. It involves rotation of a short OH-group in SSEs, which opens lower-energy pathways for the formation of Frenkel defects and highly-correlated Li+ jumps. These findings will reduce the existing confusions and show new avenues for tuning SSE compositions for further improved Li-ion conductivities. A.Y Song et al., Advanced Energy Mat. 2018, [ ](

Conversion cathodes for rechargeable lithium and lithium-ion batteries


Energy & Environmental Science

F Wu, G Yushin

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. F Wu et al., Energy & Environmental Science 10 (2), 2017, 435-459 [](

Carbide-derived carbon aerogels with tunable pore structure as versatile electrode material in high power supercapacitors



M Oschatz, S Boukhalfa, W Nickel, JP Hofmann, C Fischer, G Yushin, S Kaskel

Carbide-derived carbon (CDC) aerogels with hierarchical porosity are prepared from cross-linked polycarbosilane aerogels by pyrolysis and chlorine treatment at 700 and 1000 °C. The low-temperature sample is further activated with carbon dioxide to introduce additional micropores. The influence of the micropore structures resulting from the different synthesis conditions and the effect of the combination of high specific surface areas of more than 2400 m2/g with the aerogel-type texture on the electrochemical properties of the carbons are investigated. Electrical double-layer capacitors (EDLCs) are assembled with 1 M aqueous (H2SO4, Li2SO4, LiCl, HCl), organic (1 M tetraethylammoniumtetrafluoroborate in acetonitrile), and ionic liquid (1-ethyl-3-methylimidazoliumtetrafluoroborate) electrolytes. The larger micropores and higher surface area of the CDC aerogel prepared at 700 °C lead to higher capacitance compared to the material prepared at 1000 °C. Carbon dioxide activation leads to extremely high capacitance retentions at high current densities up to 60 A/g, which is critical for high power applications. High gravimetric specific capacitances with stable cycling and high capacitance retentions are achieved in all the studied electrolytes. This renders CDC aerogels versatile electrode materials for EDLCs with various electrolytes. M Oschatz et al., Carbon 113, 2017, 283-291 [](

Toward a Long-Chain Perfluoroalkyl Replacement: Water and Oil Repellency of Polyethylene Terephthalate (PET) Films Modified with Perfluoropolyether-Based Polyesters


ACS applied materials & interfaces

T Demir, L Wei, N Nitta, G Yushin, PJ Brown, I Luzinov

Original perfluoropolyethers (PFPE)-based oligomeric polyesters (FOPs) of different macromolecular architecture were synthesized via polycondensation as low surface energy additives to engineering thermoplastics. The oligomers do not contain long-chain perfluoroalkyl segments, which are known to yield environmentally unsafe perfluoroalkyl carboxylic acids. To improve the compatibility of the materials with polyethylene terephthalate (PET) we introduced isophthalate segments into the polyesters and targeted the synthesis of lower molecular weight oligomeric macromolecules. The surface properties such as morphology, composition, and wettability of PET/FOP films fabricated from solution were investigated using atomic force microscopy, X-ray photoelectron spectroscopy, and contact angle measurements. It was demonstrated that FOPs, when added to PET film, readily migrate to the film surface and bring significant water and oil repellency to the thermoplastic boundary. We have established that the wettability of PET/FOP films depends on three main parameters: (i) end-groups of fluorinated polyesters, (ii) the concentration of fluorinated polyesters in the films, and (iii) equilibration via annealing. The most effective water/oil repellency FOP has two C4F9–PFPE-tails. The addition of this oligomeric polyester to PET allows (even at relatively low concentrations) reaching a level of oil repellency and surface energy comparable to that of polytetrafluorethylene (PTFE/Teflon). Therefore, the materials can be considered suitable replacements for additives containing long-chain perfluoroalkyl substances. T Demir et al., ACS applied materials & interfaces 9 (28), 24318-24330 [](

Toward in-situ protected sulfur cathodes by using lithium bromide and pre-charge


Nano Energy

F Wu, S Thieme, A Ramanujapuram, E Zhao, C Weller, H Althues, S Kaskel, O Borodin, G Yushin

Lithium-sulfur (Li-S) batteries suffer from the dissolution of its intermediate charge products (polysulfides) in organic electrolytes, which limits the utilization, rate performance and cycling stability of S cathode materials. Formation of protective surface coatings on S cathodes may effectively overcome such a challenge. Here, we explored a simple, low cost, and widely applicable method that offers in-situ formation of a protective coating on the S-based cathode by using lithium bromide (LiBr) as a novel electrolyte additive. Quantum chemical (QC) studies suggested that pre-cycling a S cathode at high potentials is needed to oxidize the Br- and induce formation of DME(-H) radicals, which are involved in the formation of a polymerized protective layer of a solid electrolyte interphase (SEI) on a S cathode at high potentials. Experimental studies with a LiBr additive confirmed that 3 pre-cycles in a voltage range of 2.5–3.6 V are sufficient to achieve the formation of a robust Li ion permeable SEI on the cathode, effectively preventing the dissolution of polysulfides into electrolyte. As a result, almost no degradation was observed within 200 cycles, compared to more than 40% of capacity loss in the benchmark control cells without LiBr or the pre-cycles. Post-mortem analysis on both the cathode and anode sides of the LiBr-comprising cells further provided evidence for the in-situ SEI formation on the cathode and the lack of polysulfides’ re-precipitation. In addition, such studies showed smooth surface on the cycled Li metal anode, in contrast to the rough Li SEI with dendrites and polysulfides in the benchmark cells. F Wu et al., Nano Energy 40, 170-179 [](

Enhancing electrochemical performance of LiFePO4 by vacuum-infiltration into expanded graphite for aqueous Li-ion capacitors


Electrochimica Acta

C Qin, Y Li, S Lv, J Xiang, C Wang, X Zhang, S Qiu, G Yushin

Olivine-type LiFePO4 (LFP) is one of the most widely utilized cathode materials for high power Li-ion batteries (LIBs). In spite of rapidly growing popularity of LIBs, the rate performance of the highest power LFP cells is still insufficiently high for some high-power applications. In this work we demonstrate that vacuum-infiltration of LFP precursors into pores of low-cost expanded graphite (EG), an in-situ sol-gel process, followed by calcination, allows formation of LFP/EG nanocomposites that demonstrate remarkable performance in higher power Li-ion capacitor (LIC) applications. Such composites comprise spherical LFP particles embedded into EG pores and additionally wrapped by EG films, forming a highly efficient and stable conducting network. Such a morphology greatly accelerates Li-ion diffusion and improves Li-ion exchange between LFP and electrolyte. As a result, compared to commercial LFP particles of comparable size, the optimized LFP/EG nanocomposite shows significantly higher rate performance, dramatically better stability and higher specific capacitance of up to about 1200 F g−1. The use of environmentally friendly, safe and low-cost aqueous electrolyte is particularly advantageous for LIC applications that are cost-sensitive and require enhanced safety. Our results demonstrate a great promise of our approach, which is additionally applicable for a broad range of other intercalation chemistries. C Qin et al., Electrochimica Acta 253, 2017, 413–42 [](

Enhancing Cycle Stability of Lithium Iron Phosphate in Aqueous Electrolytes by Increasing Electrolyte Molarity


Advanced Energy Materials

D Gordon, MY Wu, A Ramanujapuram, J Benson, JT Lee, A Magasinski, N Nitta, C Huang, G Yushin

Aqueous lithium ion batteries (ALIBs) exhibit great potential to reduce the cost and improve the safety of rechargeable energy storage technologies. Lithium iron phosphate (LFP) cathodes have become a material of choice for many conventional, high power LIBs. However, experimental studies on LFP in aqueous lithium (Li) ion electrolytes are limited. Here, results of systematic studies are shown where it is demonstrated that the Li salt concentration of the aqueous electrolyte can signifi cantly improve discharge capacity retention while minimally impacting rate capability, for electrodes made with a typical commercial sub-micron sized LFP powder. Based on the postmortem analysis and the results of electrochemical characterization it is proposed that unde-sirable side reactions of aqueous electrolytes with LFP induce electrochemical separation of individual particles within the electrode, leading to the observed capacity fading. Increasing the salt concentration in aqueous solutions effec-tively reduces the concentration of water molecules in the electrolyte, which are mostly responsible for these undesirable side reactions. Similar trends observed with other cathode materials suggest that the use of concentrated aqueous electrolyte solutions offers an effective route to improve stability of aqueous Li ion batteries. D Gordon et al., Advanced Energy Materials 6 (2), 2016 [](

Performance enhancement and side reactions in rechargeable nickel–iron batteries with nanostructured electrodes


ACS applied materials & interfaces

D Lei, DC Lee, A Magasinski, E Zhao, D Steingart, G Yushin

We report for the first time a solution-based synthesis of strongly coupled nanoFe/multiwalled carbon nanotube (MWCNT) and nanoNiO/MWCNT nanocomposite materials for use as anodes and cathodes in rechargeable alkaline Ni–Fe batteries. The produced aqueous batteries demonstrate very high discharge capacities (800 mAh gFe–1 at 200 mA g–1 current density), which exceed that of commercial Ni–Fe cells by nearly 1 order of magnitude at comparable current densities. These cells also showed the lack of any “activation”, typical in commercial batteries, where low initial capacity slowly increases during the initial 20–50 cycles. The use of a highly conductive MWCNT network allows for high-capacity utilization because of rapid and efficient electron transport to active metal nanoparticles in oxidized (such as Fe(OH)2 or Fe3O4) states. The flexible nature of MWCNTs accommodates significant volume changes taking place during phase transformation accompanying reduction–oxidation reactions in metal electrodes. At the same time, we report and discuss that high surface areas of active nanoparticles lead to multiple side reactions. Dissolution of Fe anodes leads to reprecipitation of significantly larger anode particles. Dissolution of Ni cathodes leads to precipitation of Ni metal on the anode, thus blocking transport of OH– anions. The electrolyte molarity and composition have a significant impact on the capacity utilization and cycling stability. D Lei et al., ACS applied materials & interfaces 8 (3), 2016, 2088-2096 [](

Conversion Cathodes: Lithium–Iron Fluoride Battery with In Situ Surface Protection


Advanced Functional Materials

W Gu, O Borodin, B Zdyrko, HT Lin, H Kim, N Nitta, J Huang, A Magasinski, Z Milicev, G Berdichevsky, G Yushin

Lithium–metal fluoride (MF) batteries offer the highest theoretical energy density, exceeding that of the sulfur–lithium cells. However, conversion-type MF cathodes suffer from high resistance, small capacity utilization at room temperature, irreversible structural changes, and rapid capacity fading with cycling. In this study, the successful application of the approach to overcome such limitations and dramatically enhance electrochemical performance of Li–MF cells is reported. By using iron fluoride (FeF2) as an example, Li–MF cells capable of achieving near-theoretical capacity utilization are shown when MF is infiltrated into the carbon mesopores. Most importantly, the ability of electrolytes based on the lithium bis(fluorosulfonyl)imide (LiFSI) salt is presented to successfully prevent the cathode dissolution and leaching via in situ formation of a Li ion permeable protective surface layer. This layer forms as a result of electrolyte reduction/oxidation reactions during the first cycle of the conversion reaction, thus minimizing the capacity losses during cycling. Postmortem analysis shows the absence of Li dendrites, which is important for safer use of Li metal anodes. As a result, Li–FeF2 cells demonstrate over 1000 stable cycles. Quantum chemistry calculations and postmortem analysis provide insights into the mechanisms of the passivation layer formation and the performance boost. W Gu et al., Advanced Functional Materials 26 (10), 2016, 1507-1516 [](

Revealing Rate Limitations in Nanocrystalline Li4Ti5O12 Anodes for High‐Power Lithium Ion Batteries


Advanced Materials Interfaces

J Wang, H Zhao, Z Li, Y Wen, Q Xia, Y Zhang, G Yushin

Li4Ti5O12 is a promising anode material for lithium ion batteries due to its high safety, excellent cycling stability, environmental friendliness, and low cost. Strategies of incorporation with a conductive component (such as carbon) and constructing nano-structure are frequently adopted to improve the rate-capability of Li4Ti5O12 by means of enhancing the electronic conductivity and promoting the lithium ion transport within electrodes, respectively. However, which charge carrier transport process is the limiting step for Li4Ti5O12 electrode reactions still remains unclear, and this limits the abilities to rationally design high performance Li4Ti5O12 materials. In this work, the nanosized Li4Ti5O12 and Li4Ti5O12/C materials are prepared with nearly identical particle size and morphology. The results demonstrate that the synthesized single phase Li4Ti5O12 delivers a higher specific capacity and superior rate-capability than Li4Ti5O12/C composite. As such, in contrast to a popular belief, it is lithium ion transport that restricts kinetics of the electrochemical reactions on Li4Ti5O12. The synthesized single phase Li4Ti5O12 shows a specific capacity of ≈160 mAh g−1 at 0.5 C and 130 mAhg−1 at 50 C rates, respectively. This rate-capability is the best reported for Li4Ti5O12 anodes. The single phase Li4Ti5O12 also demonstrated remarkable stability at high-temperature (50 °C), showing cycling life of over 4000 cycles at 1 C. J Wang et al., Advanced Materials Interfaces 3 (13), 2016 [](

Nanostructured Li2Se cathodes for high performance lithium-selenium batteries


Nano energy

F Wu, JT Lee, Y Xiao, G Yushin

We report on a simple and fast route to prepare lithium selenide (Li2Se) nanoparticles and show a versatile solution-based method to form uniform nanostructured carbon (C)-Li2Se composites with and without additional carbon shell. We systematically compare electrochemical performance characteristics of 50–100 nm high purity Li2Se nanoparticles with that of the C-Li2Se nanocomposites for rechargeable Li battery applications. While Li2Se nanopowder show high initial capacity, it suffers from active material loss and shuttle of dissolved polyselenides, resulting in low cycling stability and resistance growth, additionally aggravated by mechanical cathode degradation induced by repetitive volume changes during cycling. By embedding Li2Se nanoparticles into a conductive carbon matrix, mechanical stability of electrodes was greatly enhanced. More importantly, the dissolution and shuttle of polyselenides was suppressed significantly even for smaller (~20 nm) and thus more reactive Li2Se nanoparticles. As a result, C-Li2Se nanocomposite cathodes showed high rate capability and promising cycling stability with carbon-shell protected C-Li2Se showing virtually no degradation in 100 cycles. When compared with somewhat similar lithium sulfide (Li2S) nanoparticles and C-Li2S electrodes, we observe lower over-potential at different C-rates in case of Li2Se and C-Li2Se materials, which is advantageous for battery applications. Based on the postmortem analysis, significant Li dendrite growth observed in Li2Se/Li cells did not take place in C-Li2Se/Li and C-Li2Se@C/Li cells, suggesting that polyselenide shuttle may affect Li plating morphology. Beyond the organic electrolyte-based Li-Se batteries, all-solid Li-Se batteries based on the produced C-Li2Se nanocomposite cathode were built for the first time using conventional Li2S-P2S5 solid state electrolyte. These solid state cells showed very promising cycling stability, a single flat plateau and very small voltage hysteresis in the range of 0.1–0.4 V when tested at 60 and 80 °C. F Wu et al., Nano energy 27, 2016, 238-246 [](

Charge storage at the nanoscale: understanding the trends from the molecular scale perspective


Journal of Materials Chemistry A

J Vatamanu, O Borodin, M Olguin, G Yushin, D Bedrov

Supercapacitors or electrical double layer (EDL) capacitors store charge via rearrangement of ions in electrolytes and their adsorption on electrode surfaces. They are actively researched for multiple applications requiring longer cycling life, broader operational temperature ranges, and higher power density compared to batteries. Recent developments in nanostructured carbon-based electrodes with a high specific surface area have demonstrated the potential to significantly increase the energy density of supercapacitors. Molecular modeling of electrolytes near charged electrode surfaces has provided key insights into the fundamental aspects of charge storage at the nanoscale, including an understanding of the mechanisms of ion adsorption and dynamics at flat surfaces and inside nanopores, and the influence of curvature, roughness, and electronic structure of electrode surfaces. Here we review these molecular modeling findings for EDL capacitors, dual ion batteries and pseudo-capacitors together with available experimental observations and put this analysis into the perspective of future developments in this field. Current research trends and future directions are discussed. J Vatamanu et al., Journal of Materials Chemistry A 5 (40), 2016, 21049-21076 [](

Influence of binders, carbons, and solvents on the stability of phosphorus anodes for Li-ion batteries


ACS applied materials & interfaces

N Nitta, D Lei, HR Jung, D Gordon, E Zhao, G Gresham, J Cai, I Luzinov, G Yushin

Phosphorus (P) is an abundant element that exhibits one of the highest gravimetric and volumetric capacities for Li storage, making it a potentially attractive anode material for high capacity Li-ion batteries. However, while phosphorus carbon composite anodes have been previously explored, the influence of the inactive materials on electrode cycle performance is still poorly understood. Here, we report and explain the significant impacts of polymer binder chemistry, carbon conductive additives, and an under-layer between the Al current collector and ball milled P electrodes on cell stability. We focused our study on the commonly used polyvinylidene fluoride (PVDF) and poly(acrylic acid) (PAA) binders as well as exfoliated graphite (ExG) and carbon nanotube (CNT) additives. The mechanical properties of the binders were found to change drastically because of interactions with both the slurry and electrolyte solvents, significantly effecting the electrochemical cycle stability of the electrodes. Binder adhesion was also found to be critical in achieving stable electrochemical cycling. The best anodes demonstrated ∼1400 mAh/g-P gravimetric capacity after 200 cycles at C/2 rates in Li half cells. N Nitta et al. ACS applied materials & interfaces 8 (39), 25991-26001 [](

Increasing capacitance of zeolite-templated carbons in electric double layer capacitors


Journal of The Electrochemical Society

JS Moon, H Kim, DC Lee, JT Lee, G Yushin

Enhancement of the specific capacitance in electrochemical double layer capacitors (EDLCs) is of high interest due to the ever increasing demand for high power density energy storage devices. Zeolite templated carbon (ZTC) is a promising EDLC electrode material with large specific surface area and straight, ordered well-defined micropores. In this study, ZTC samples were synthesized using a low pressure chemical vapor deposition (LP CVD) of carbon on sacrificial zeolite Y powder using acetylene gas as a precursor. We demonstrate for the first time how various post-treatments of the produced samples can affect the ZTC microstructure and porosity and how such modifications may significantly improve electrochemical performance characteristics of the ZTC-based EDLC electrodes. The effects of CO2 activation, ball milling and high temperature annealing process were systematically studied. The best performing samples achieved very large capacitance of over 240 Fg−1 at 1 mVs−1 in 1 M solution of tetraethylammonium tetrafluoroborate in acetonitrile and stable performance in symmetric EDLC devices with no noticeable degradation for over 20,000 cycles at a very high current density of 20 Ag−1. JS Moon et al., Journal of The Electrochemical Society 162 (5), 2015, A5070-A5076 [](

Carbon nanotube–CoF2 multifunctional cathode for lithium ion batteries: Effect of electrolyte on cycle stability



X Wang, W Gu, JT Lee, N Nitta, J Benson, A Magasinski, MW Schauer, G Yushin

Transition metal fluorides (MFx) offer remarkably high theoretical energy density. However, the low cycling stability, low electrical and ionic conductivity of metal fluorides have severely limited their applications as conversion-type cathode materials for lithium ion batteries. Here, a scalable and low-cost strategy is reported on the fabrication of multifunctional cobalt fluoride/carbon nanotube nonwoven fabric nanocomposite, which demonstrates a combination of high capacity (near-theoretical, math formula) and excellent mechanical properties. Its strength and modulus of toughness exceed that of many aluminum alloys, cast iron, and other structural materials, fulfilling the use of MFx-based materials in batteries with load-bearing capabilities. In the course of this study, cathode dissolution in conventional electrolytes has been discovered as the main reason that leads to the rapid growth of the solid electrolyte interphase layer and attributes to rapid cell degradation. And such largely overlooked degradation mechanism is overcome by utilizing electrolyte comprising a fluorinated solvent, which forms a protective ionically conductive layer on the cathode and anode surfaces. With this approach, 93% capacity retention is achieved after 200 cycles at the current density of 100 mA g−1 and over 50% after 10 000 cycles at the current density of 1000 mA g−1. X Wang et al., Small 11 (38), 2015, 5164-5173 [](

In situ TEM observation of electrochemical lithiation of sulfur confined within inner cylindrical pores of carbon nanotubes


Advanced Energy Materials

H Kim, JT Lee, A Magasinski, K Zhao, Y Liu, G Yushin

Lithium insertion into sulfur confined within 200 nm cylindrical inner pores of individual carbon nanotubes (CNTs) was monitored in situ in a transmission electron microscope (TEM). This electrochemical reaction was initiated at one end of the S-filled CNTs. The material expansion during lithiation was accommodated by the expansion into the remaining empty pore volume and no fracture of the CNT walls was detected. A sharp interface between the initial and lithiated S was observed. The reaction front was flat, oriented perpendicular to the confined S cylinder, and propagated along the cylinder length. Lithiation of S in the proximity of conductive carbon proceeded at the same rate as the one in the center of the pore, suggesting the presence of electron pathways at the Li2S/S interface. Density of states calculations further confirmed this hypothesis. In situ electron diffraction showed a direct phase transformation of S into nanocrystalline Li2S without detectable formation of any intermediates, such as polysulfides and LiS. These important insights may elucidate some of the reaction mechanisms and guide the improvements in the design of C–S nanocomposites for high specific energy Li–S batteries. The proposed use of conductive CNTs with tunable pore diameter as cylindrical reaction vessels for in situ TEM studies of electrochemical reactions proved to be highly advantageous and may help to resolve the ongoing problems in battery technology. H Kim et al., Advanced Energy Materials 5 (24), 2015 [](

Graphene–Li2S–carbon nanocomposite for lithium–sulfur batteries


ACS nano

F Wu, JT Lee, E Zhao, B Zhang, G Yushin

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. ACS nano 10 (1), 2015, 1333-1340 [](