Nontraditional Approaches To Enable High-Energy and Long-Life Lithium–Sulfur Batteries

ConspectusLithium–sulfur (Li–S) batteries are promising for automotive applications due to their high theoretical energy density (2600 Wh/kg). In addition, the natural abundance of sulfur could mitigate the global raw material supply chain challenge of commercial lithium-ion batteries that use critical elements, such as nickel and cobalt. However, due to persistent polysulfide shuttling and uncontrolled lithium dendrite growth, Li–S batteries using nonencapsulated sulfur cathodes and conventional ether-based electrolytes suffer from rapid cell degradation upon cycling. Despite significant improvements in recent decades, there is still a big gap between lab research and commercialization of the technology. To date, the reported cell energy densities and cycling life of practical Li–S pouch cells remain largely unsatisfactory.Traditional approaches to improving Li–S performance are primarily focused on confining polysulfides using electronically conductive hosts. However, these micro- and mesoporous hosts suffer from limited pore volume to accommodate high sulfur loading and the associated volume change during cycling. Moreover, they fail to balance adsorption–conversion of polysulfides during charge–discharge, leading to the formation of massive dead sulfur. Such hosts are themselves electrochemically inactive, which decreases the practical energy density. In contrast, a series of nontraditional approaches, paired with advances in multiscale mechanistic understanding, have recently demonstrated exciting performance outcomes not only in conventional coin cells but also in practical pouch cells.In this Account, we first introduce our novel cathode design strategies to overcome polysulfide shuttling and sluggish redox kinetics in thick S cathodes via selenium–sulfur chemistry and cathode host engineering. Next, we gain a mechanistic understanding of Li–S batteries in various types of electrolytes via a series of spectroscopic, nuclear magnetic resonance, and electrochemical methods. Meanwhile, a novel cathode solid electrolyte interphase encapsulation strategy via nonviscous highly fluorinated ether-based electrolyte is introduced. The established selection rule by investigating how solvating power retards the shuttle effect and induces robust cathode/solid-electrolyte interphase formation is also included. We then discuss how the synergistic interactions between rational cathode structures and electrolytes can be exploited to tailor the reaction pathways and kinetics of S cathodes under high mass loading and lean electrolyte conditions. In addition, a novel interlayer design to simultaneously overcome degradation processes (polysulfide shuttling and lithium dendrite formation) and accelerate redox reaction kinetics is presented. Finally, this Account concludes with an overview of the challenges and strategies to develop Li–S pouch cells with high practical energy density, long cycle life, and fast-charging capability

Electrode Surface Film Formation in Tris(ethylene glycol)-Substituted Trimethylsilane–Lithium Bis(oxalate)borate Electrolyte

One of the silicon-based electrolytes, tris(ethylene glycol)-substituted trimethylsilane (1NM3)–lithium bis(oxalate)borate (LiBOB), is studied as an electrolyte for the LiMn₂O₄ cathode and graphite anode cell. The solid electrolyte interface (SEI) characteristics and chemical components of both electrodes were investigated by X-ray photoelectron spectroscopy and X-ray diffraction. It was found that SEI components on the anode are similar to those using carbonate–LiBOB electrolyte, which consists of lithium oxalate, lithium borooxalate, and Li_<i>x</i>BO_<i>y</i>. Moreover, we demonstrated that 1NM3–LiPF₆ electrolyte, which lacks an SEI formation function, could not maintain the graphite structure during the electrochemical process. Therefore, it is evident that the 1NM3–LiBOB combination and its suitable SEI film formation capability are vital to the lithium ion battery with graphite as the anode

Mechanistic Study of Electrolyte Additives to Stabilize High-Voltage Cathode–Electrolyte Interface in Lithium-Ion Batteries

Current developments of electrolyte additives to stabilize electrode–electrolyte interface in lithium-ion batteries highly rely on a trial-and-error search, which involves repetitive testing and intensive amount of resources. The lack of understandings on the fundamental protection mechanisms of the additives significantly increases the difficulty for the transformational development of new additives. In this study, we investigated two types of individual protection routes to build a robust cathode–electrolyte interphase at high potentials: (i) a direct reduction in the catalytic decomposition of the electrolyte solvent; and (ii) formation of a “corrosion inhibitor film” that prevents severely attack and passivation from protons that generated from the solvent oxidation, even the decomposition of solvent cannot be mitigated. Effect of two exemplary electrolyte additives, lithium difluoro­(oxalato)­borate (LiDFOB) and 3-hexylthiophene (3HT), on LiNi_0.6Mn_0.2Co_0.2O₂ (NMC 622) cathode were investigated to validate our hypothesis. It is demonstrated that understandings of both electrolyte additives and solvent are essential and careful balance between the cathode protection mechanism of additives and their side effects is critical to obtain optimum results. More importantly, this study opens up new directions of rational design of functional electrolyte additives for the next-generation high-energy-density lithium-ion chemistries

Stable Nanostructured Cathode with Polycrystalline Li-Deficient Li_0.28Co_0.29Ni_0.30Mn_0.20O₂ for Lithium-Ion Batteries

The lithium-ion battery, a major renewable power source, has been widely applied in portable electronic devices and extended to hybrid electric vehicles and all-electric vehicles. One of the main issues for the transportation application is the need to develop high-performance cathode materials. Here we report a novel nanostructured cathode material based on air-stable polycrystalline Li_0.28Co_0.29Ni_0.30Mn_0.20O₂ thin film with lithium deficiency for high-energy density lithium-ion batteries. This film is prepared via a method combining radio frequency magnetron sputtering and annealing using a crystalline and stoichiometric LiCo_1/3Ni_1/3Mn_1/3O₂ target. This lithium-deficient Li_0.28Co_0.29Ni_0.30Mn_0.20O₂ thin film has a polycrystalline nanostructure, high tap density, and higher energy and power density compared to the initial stoichiometric LiCo_1/3Ni_1/3Mn_1/3O₂. Such a material is a promising cathode candidate for high-energy lithium-ion batteries, especially thin-film batteries

Interactions of Dimethoxy Ethane with Li₂O₂ Clusters and Likely Decomposition Mechanisms for Li–O₂ Batteries

One of the major problems facing the successful development of Li–O₂ batteries is the decomposition of nonaqueous electrolytes, where the decomposition can be chemical or electrochemical during discharge or charge. In this paper, the decomposition pathways of dimethoxy ethane (DME) by the chemical reaction with the major discharge product, Li₂O₂, are investigated using theoretical methods. The computations were carried out using small Li₂O₂ clusters as models for potential sites on Li₂O₂ surfaces. Both hydrogen and proton abstraction mechanisms were considered. The computations suggest that the most favorable decomposition of ether solvents occurs on certain sites on the lithium peroxide surfaces involving hydrogen abstraction followed by reaction with oxygen, which leads to oxidized species such as aldehydes and carboxylates as well as LiOH on the surface of the lithium peroxide. The most favorable site is a Li–O–Li site that may be present on small nanoparticles or as a defect site on a surface. The decomposition route initiated by the proton abstraction from the secondary position of DME by the singlet cluster (O–O site) requires a much larger enthalpy of activation, and subsequent reactions may require the presence of oxygen or superoxide. Thus, pathways involving proton abstraction are less likely than that involving hydrogen abstraction. This type of electrolyte decomposition (electrolyte with hydrogen atoms) may influence the cell performance including the crystal growth, nanomorphologies of the discharge products, and charge overpotential

Cathode Material with Nanorod StructureAn Application for Advanced High-Energy and Safe Lithium Batteries

We have developed a novel cathode material based on lithium–nickel–manganese–cobalt oxide, where the manganese concentration remains constant throughout the particle, while the nickel concentration decreases linearly and the cobalt concentration increases from the center to the outer surface of the particle. This full concentration gradient material with a fixed manganese composition (FCG–Mn-F) has an average composition of Li­[Ni_0.60Co_0.15Mn_0.25]­O₂ and is composed of rod-shaped primary particles whose length reaches 2.5 μm, growing in the radial direction. In cell tests, the FCG–Mn-F material delivered a high capacity of 206 mAh g^–1 with excellent capacity retention of 70.3% after 1000 cycles at 55 °C. This cathode material also exhibited outstanding rate capability, good low-temperature performance, and excellent safety, compared to a conventional cathode having the same composition (Li­[Ni_0.60Co_0.15Mn_0.25]­O₂), where the concentration of the metals is constant across the particles

High Capacity O3-Type Na[Li_0.05(Ni_0.25Fe_0.25Mn_0.5)_0.95]O₂ Cathode for Sodium Ion Batteries

In this work we report Na­[Li_0.05(Ni_0.25Fe_0.25Mn_0.5)_0.95]­O₂ layered cathode materials that were synthesized via a coprecipitation method. The Na­[Li_0.05(Ni_0.25Fe_0.25Mn_0.5)_0.95]­O₂ electrode exhibited an exceptionally high capacity (180.1 mA h g^–1 at 0.1 C-rate) as well as excellent capacity retentions (0.2 C-rate: 89.6%, 0.5 C-rate: 92.1%) and rate capabilities at various C-rates (0.1 C-rate: 180.1 mA h g^–1, 1 C-rate: 130.9 mA h g^–1, 5 C-rate: 96.2 mA h g^–1), which were achieved due to the Li supporting structural stabilization by introduction into the transition metal layer. By contrast, the electrode performance of the lithium-free Na­[Ni_0.25Fe_0.25Mn_0.5]­O₂ cathode was inferior because of structural disintegration presumably resulting from Fe³⁺ migration from the transition metal layer to the Na layer during cycling. The long-term cycling using a full cell consisting of a Na­[Li_0.05(Ni_0.25Fe_0.25Mn_0.5)_0.95]­O₂ cathode was coupled with a hard carbon anode which exhibited promising cycling data including a 76% capacity retention over 200 cycles

High-Capacity Sodium Peroxide Based Na–O₂ Batteries with Low Charge Overpotential via a Nanostructured Catalytic Cathode

The superoxide based Na–O₂ battery has circumvented the issue of large charge overpotential in Li–O₂ batteries; however, the one-electron process leads to limited capacity. Herein, a sodium peroxide based low-overpotential (∼0.5 V) Na–O₂ battery with a capacity as high as 7.5 mAh/cm² is developed with Pd nanoparticles as catalysts on the cathode

Solid-State Li-Ion Batteries Using Fast, Stable, Glassy Nanocomposite Electrolytes for Good Safety and Long Cycle-Life

The development of safe, stable, and long-life Li-ion batteries is being intensively pursued to enable the electrification of transportation and intelligent grid applications. Here, we report a new solid-state Li-ion battery technology, using a solid nanocomposite electrolyte composed of porous silica matrices with in situ immobilizing Li⁺-conducting ionic liquid, anode material of MCMB, and cathode material of LiCoO₂, LiNi_1/3Co_1/3Mn_1/3O₂, or LiFePO₄. An injection printing method is used for the electrode/electrolyte preparation. Solid nanocomposite electrolytes exhibit superior performance to the conventional organic electrolytes with regard to safety and cycle-life. They also have a transparent glassy structure with high ionic conductivity and good mechanical strength. Solid-state full cells tested with the various cathodes exhibited high specific capacities, long cycling stability, and excellent high temperature performance. This solid-state battery technology will provide new avenues for the rational engineering of advanced Li-ion batteries and other electrochemical devices

A Mo₂C/Carbon Nanotube Composite Cathode for Lithium–Oxygen Batteries with High Energy Efficiency and Long Cycle Life

Although lithium–oxygen batteries are attracting considerable attention because of the potential for an extremely high energy density, their practical use has been restricted owing to a low energy efficiency and poor cycle life compared to lithium-ion batteries. Here we present a nanostructured cathode based on molybdenum carbide nanoparticles (Mo₂C) dispersed on carbon nanotubes, which dramatically increase the electrical efficiency up to 88% with a cycle life of more than 100 cycles. We found that the Mo₂C nanoparticle catalysts contribute to the formation of well-dispersed lithium peroxide nanolayers (Li₂O₂) on the Mo₂C/carbon nanotubes with a large contact area during the oxygen reduction reaction (ORR). This Li₂O₂ structure can be decomposed at low potential upon the oxygen evolution reaction (OER) by avoiding the energy loss associated with the decomposition of the typical Li₂O₂ discharge products