Structural Changes in Li2MnO3 Cathode Material for Li‐Ion Batteries

Structural changes in Li₂MnO₃ cathode material for rechargeable Li‐ion batteries are investigated during the first and 33^rd cycles. It is found that both the participation of oxygen anions in redox processes and Li⁺‐H⁺ exchange play an important role in the electrochemistry of Li₂MnO₃. During activation, oxygen removal from the material along with Li gives rise to the formation of a layered MnO₂‐type structure, while the presence of protons in the interslab region, as a result of electrolyte oxidation and Li⁺‐H⁺ exchange, alters the stacking sequence of oxygen layers. Li re‐insertion by exchanging already present protons reverts the stacking sequence of oxygen layers. The re‐lithiated structure closely resembles the parent Li₂MnO₃, except that it contains less Li and O. Mn⁴⁺ ions remain electrochemically inactive at all times. Irreversible oxygen release occurs only during activation of the material in the first cycle. During subsequent cycles, electrochemical processes seem to involve unusual redox processes of oxygen anions of active material along with the repetitive, irreversible oxidation of electrolyte species. The deteriorating electrochemical performance of Li₂MnO₃ upon cycling is attributed to the structural degradation caused by repetitive shearing of oxygen layers.     

Exploiting Lithium‐Depleted Cathode Materials for Solid‐State Li Metal Batteries

Solid‐state lithium metal batteries (SSLMBs) may become one of the high‐energy density storage devices for the next generation of electric vehicles. High safety and energy density can be achieved by utilizing solid electrolytes and Li metal anodes. Therefore, developing cathode materials which can match with Li metal anode efficiently is indispensable. In SSLMBs, Li metal anodes can afford the majority of active lithium ions, then lithium‐depleted cathode materials can be a competitive candidate to achieve high gravimetric energy density as well as save lithium resources. Li_0.33MnO₂ lithium‐depleted material is chosen, which also has the advantages of low synthesis temperature and low cost (cobalt‐free). Notably, solid‐state electrolyte can greatly alleviate the problem of manganese dissolution in the electrolyte, which is beneficial to improve the cycling stability of the battery. Thus, SSLMBs enable practical applications of lithium‐depleted cathode materials.     

Si‐Doping Mediated Phase Control from β‐ to γ‐Form Li3VO4 toward Smoothing Li Insertion/Extraction

The γ phase Li₃VO₄ which possesses higher ionic conductivity is more preferable for lithium ion batteries, but it is only stable at high temperature and would convert to low temperature β phase spontaneously when cooling down. Here, the phase control of Li₃VO₄ to stabilize its γ phase in room temperature is successfully mediated by introducing proper Si‐doping, and for the first time the electrochemical performances of γ‐Li₃VO₄ is investigated. It is found that pure γ‐Li₃VO₄ can be obtained in a doping ratio of x = 0.05–0.15 in Li_3+xV_1?xSi_xO₄ with addition of excess Li source in synthesis design. The doping mechanism and the energy changes are investigated in detail by using the first principle calculations, it reveals that an interstitial Li⁺ is formed with doping of Si⁴⁺ in Li₃VO₄ to balance the system charge. When served as an anode, the Si‐doped γ‐Li₃VO₄ shows much smoothed Li⁺ insertion/extraction and enhanced cycle stability with only a single pair of redox peaks, which behaves much different with the complex multicouples of redox peaks in typical β‐Li₃VO₄. These changes in electrochemical performances implies that γ‐Li₃VO₄ can effectively accommodate Li⁺ in an easier and more facile way and relieved structure stress during the charge/discharge process.     

Reversible Lithium‐Ion Uptake in Poly(methylmethacrylate) Thin‐Film via Lithiation/Delithiation at In Situ Formed Intramolecular Cyclopentanedione

Herein, it is proposed that poly(methylmethacrylate) (PMMA), a widely‐used thermoplastic in our daily life, can be used as an abundant, stable, and high‐performance anode material for rechargeable lithium‐ion batteries through a novel concept of lithium storage mechanism. The specially‐designed PMMA thin‐film electrode exhibits a high reversible capacity of 343 mA h g^?1 at C/25 and maintains a capacity retention of 82.6% of that obtained at C/25 when cycled at 1 C rate. Meanwhile, this pristine PMMA electrode without binder and conductive agents shows a high reversible capacity of 196.8 mA h g^?1 after 150 cycles at 0.2 C with a capacity retention of 73.5%. Additionally, PMMA‐based binder is found to enhance both the reversible capacity and rate capability of the graphite electrodes. Hence, this new type of organic electrode material may have a great opportunity to be utilized as the active material or rechargeable binder in flexible or transparent thin‐film batteries and all‐solid batteries. The present work also provides a new way of seeking more proper organic electrode materials which don't contain conjugated structures and atoms with lone pair electrons required in traditional organic electrode materials.     

Synthesis,Characterization, and Structural Modeling of High‐Capacity,Dual Functioning MnO2 Electrode/Electrocatalysts for Li‐O2 Cells

It has become clear that cycling lithium‐oxygen cells in carbonate electrolytes is impractical, as electrolyte decomposition, triggered by oxygen reduction products, dominates the cell chemistry. This research shows that employing an α‐MnO₂/ramsdellite‐MnO₂ electrode/electrocatalyst results in the formation of lithium‐oxide‐like discharge products in propylene carbonate, which has been reported to be extremely susceptible to decomposition. X‐ray photoelectron data have shown that what are likely lithium oxides (Li₂O₂ and Li₂O) appear to form and decompose on the air electrode surface, particularly at the MnO₂ surface, while Li₂CO₃ is also formed. By contrast, cells without α‐MnO₂/ramsdellite‐MnO₂ fail rapidly in electrochemical cycling, likely due to the differences in the discharge product. Relatively high electrode capacities, up to 5000 mAh/g (carbon + electrode/electrocatalyst), have been achieved with non‐optimized air electrodes. Insights into reversible insertion reactions of lithium, lithium peroxide (Li₂O₂) and lithium oxide (Li₂O) in the tunnels of α‐MnO₂, and the reaction of lithium with ramsdellite‐MnO₂, as determined by first principles density functional theory calculations, are used to provide a possible explanation for some of the observed results. It is speculated that a Li₂O‐stabilized and partially‐lithiated electrode component, 0.15Li₂O·α‐Li_xMnO₂, that has Mn^4+/3+ character may facilitate the Li₂O₂/Li₂O discharge/charge chemistries providing dual electrode/electrocatalyst functionality.     

Direct Growth of Flower‐Like δ‐MnO2 on Three‐Dimensional Graphene for High‐Performance Rechargeable Li‐O2 Batteries

A challenge still remains to develop high‐performance and cost‐effective air electrode for Li‐O₂ batteries with high capacity, enhanced rate capability and long cycle life (100 times or above) despite recent advances in this field. In this work, a new design of binder‐free air electrode composed of three‐dimensional (3D) graphene (G) and flower‐like δ‐MnO₂ (3D‐G‐MnO₂) has been proposed. In this design, graphene and δ‐MnO₂ grow directly on the skeleton of Ni foam that inherits the interconnected 3D scaffold of Ni foam. Li‐O₂ batteries with 3D‐G‐MnO₂ electrode can yield a high discharge capacity of 3660 mAh g^?1 at 0.083 mA cm^?2. The battery can sustain 132 cycles at a capacity of 492 mAh g^?1 (1000 mAh g_carbon ^?1) with low overpotentials under a high current density of 0.333 mA cm^?2. A high average energy density of 1350 Wh Kg^?1 is maintained over 110 cycles at this high current density. The excellent catalytic activity of 3D‐G‐MnO₂ makes it an attractive air electrode for high‐performance Li‐O₂ batteries.     

Dendrite‐Suppressed Lithium Plating from a Liquid Electrolyte via Wetting of Li3N

Lithium metal is an ultimate anode material to provide the highest energy density for a given cathode by providing a higher capacity and cell voltage. However, lithium is not used as the anode in commercial lithium‐ion batteries because electrochemical dendrite formation and growth during charge can induce a cell short circuit that ignites the flammable liquid electrolyte. Plating of lithium through a bed of Li₃N particles is shown to transform dendrite growth into a 3D lithium network formed by wetting the particle surfaces; plating through a Li₃N particle is without dendrite nucleation. The Li₃N particles create a higher overpotential during Li deposition than that with dendrite growth in galvanostatic charge/discharge tests. The characteristic overpotential increase is correlated with the morphological changes and a more isotropic growth behavior. The Li₃N‐modified Li electrode shows a stable cycling performance at 0.5 and 1.0 mA cm^?2 for more than 100 cycles. The origin of the bonding responsible for wetting of the Li₃N particles by lithium and for plating through a Li₃N particle is discussed.     

Li3N as a Cathode Additive for High‐Energy‐Density Lithium‐Ion Batteries

The formation of a solid‐electrolyte interphase on the anode surface of an Li‐ion battery using an organic liquid electrolyte robs Li⁺ irreversibly form the cathode on the initial charge if the cells are fabricated in the discharged state. In order to increase the cathode capacity, the use of Li₃N as a sacrificial source of Li⁺ on the initial charge has been evaluated chemically and electrochemically as an additive to an LiCoO₂ cathode. Li₃N is shown to be chemically stable in a dry atmosphere as small particles with fresh surfaces and can increase the reversible capacities of a full cell without compromising the rate capability of the cells.     

Electrochemical and Structural Investigation of Calcium Substituted Monoclinic Li3V2(PO4)3 Anode Materials for Li‐Ion Batteries

In this work, the effect of Li⁺ substitution in Li₃V₂(PO₄)₃ with a large divalent ion (Ca²⁺) toward lithium insertion is studied. A series of materials, with formula Li_3?2xCa_xV₂(PO₄)₃/C (x = 0, 0.5, 1, and 1.5) is synthesized and studied in the potential region 3–0.01 V versus Li⁺/Li. Synchrotron diffraction demonstrates that Li₃V₂(PO₄)₃/C has a monoclinic structure (space group P2₁/n), while Ca_1.5V₂(PO₄)₃/C possesses a rhombohedral structure (space group R‐3c). The intermediate compounds, Li₂Ca_0.5V₂(PO₄)₃/C and LiCaV₂(PO₄)₃/C, are composed of two main phases, including monoclinic Li₃V₂(PO₄)₃/C and rhombohedral Ca_1.5V₂(PO₄)₃/C. Cyclic voltammetry reveals five reduction and oxidation peaks on Li₃V₂(PO₄)₃/C and Li₂Ca_0.5V₂(PO₄)₃/C electrodes. In contrast, LiCaV₂(PO₄)₃/C and Ca_1.5V₂(PO₄)₃/C have no obvious oxidation and reduction peaks but a box‐type voltammogram. This feature is the signature for capacitive‐like mechanism, which involves fast electron transfer on the surface of the electrode. Li₃V₂(PO₄)₃/C undergoes two solid‐solution and a short two‐phase reaction during lithiation and delithiation processes, whereas Ca_1.5V₂(PO₄)₃/C only goes through capacitive‐like mechanism. In operando X‐ray absorption spectroscopy confirms that, in both Li₃V₂(PO₄)₃/C and Ca_1.5V₂(PO₄)₃/C, V ions are reduced during the insertion of the first three Li ions. This study demonstrates that the electrochemical characteristic of polyanionic phosphates can be easily tuned by replacing Li⁺ with larger divalent cations.     

Hierarchically Porous Li4Ti5O12 Anode Materials for Li‐ and Na‐Ion Batteries: Effects of Nanoarchitectural Design and Temperature Dependence of the Rate Capability

Integrated design of both porous structure and crystalline morphology is expected to open up the way to a new class of materials. This report demonstrates new nanostructured Li₄Ti₅O₁₂ materials with hierarchically porous structures and flower‐like morphologies. Electrochemical studies of the electrodes of Li‐ion and Na‐ion batteries clearly reveal the advantage of nanoarchitectural design of active materials. In addition, the temperature dependence of Na⁺‐insertion/extraction capacity in relation to Li₄Ti₅O₁₂ electrodes is for the first time evaluated and it is found that elevation of the cell operating temperature effectively improves the rate capability of the Na‐ion batteries. Based on the new findings, it is suggested that specially designed Li₄Ti₅O₁₂ materials allow for high‐performance Na‐ion batteries that are available as large‐scale storage devices for applications such as automotive and stationary energy storage.     

Optimized Temperature Effect of Li‐Ion Diffusion with Layer Distance in Li(NixMnyCoz)O2 Cathode Materials for High Performance Li‐Ion Battery

Understanding and optimizing the temperature effects of Li‐ion diffusion by analyzing crystal structures of layered Li(Ni_xMn_yCo_z)O₂ (NMC) (x + y + z = 1) materials is important to develop advanced rechargeable Li‐ion batteries (LIBs) for multi‐temperature applications with high power density. Combined with experiments and ab initio calculations, the layer distances and kinetics of Li‐ion diffusion of LiNi_xMn_yCo_zO₂ (NMC) materials in different states of Li‐ion de‐intercalation and temperatures are investigated systematically. An improved model is also developed to reduce the system error of the “Galvanostatic Intermittent Titration Technique” with a correction of NMC particle size distribution. The Li‐ion diffusion coefficients of all the NMC materials are measured from ?25 to 50 °C. It is found that the Li‐ion diffusion coefficient of LiNi_0.6Mn_0.2Co_0.2O₂ is the largest with the minimum temperature effect. Ab initio calculations and XRD measurements indicate that the larger Li slab space benefits to Li‐ion diffusion with minimum temperature effect in layered NMC materials.     

Mesoporous MoS2 as a Transition Metal Dichalcogenide Exhibiting Pseudocapacitive Li and Na‐Ion Charge Storage

The ion insertion properties of MoS₂ continue to be of widespread interest for energy storage. While much of the current work on MoS₂ has been focused on the high capacity four‐electron reduction reaction, this process is prone to poor reversibility. Traditional ion intercalation reactions are highlighted and it is demonstrated that ordered mesoporous thin films of MoS₂ can be utilized as a pseudocapacitive energy storage material with a specific capacity of 173 mAh g^?1 for Li‐ions and 118 mAh g^?1 for Na‐ions at 1 mV s^?1. Utilizing synchrotron grazing incidence X‐ray diffraction techniques, fast electrochemical kinetics are correlated with the ordered porous structure and with an iso‐oriented crystal structure. When Li‐ions are utilized, the material can be charged and discharged in 20 seconds while still achieving a specific capacity of 140 mAh g^?1. Moreover, the nanoscale architecture of mesoporous MoS₂ retains this level of lithium capacity for 10 000 cycles. A detailed electrochemical kinetic analysis indicates that energy storage for both ions in MoS₂ is due to a pseudocapacitive mechanism.     

Operando Lithium Dynamics in the Li‐Rich Layered Oxide Cathode Material via Neutron Diffraction

Neutron diffraction under operando battery cycling is used to study the lithium and oxygen dynamics of high Li‐rich Li(Li_x/3Ni_(3/8‐3x/8)Co_(1/4‐x/4)Mn_(3/8+7x/24)O₂ (x = 0.6, HLR) and low Li‐rich Li(Li_x/3Ni_(1/3‐x/3)Co_(1/3‐x/3)Mn_(1/3+x/3)O₂ (x = 0.24, LLR) compounds that exhibit different degrees of oxygen activation at high voltage. The measured lattice parameter changes and oxygen position show largely contrasting changes for the two cathodes where the LLR exhibits larger movement of oxygen and lattice contractions in comparison to the HLR that maintains relatively constant lattice parameters and oxygen position during the high voltage plateau until the end of charge. Density functional theory calculations show the presence of oxygen vacancy during the high voltage plateau; changes in the lattice parameters and oxygen position are consistent with experimental observations. Lithium migration kinetics for the Li‐rich material is observed under operando conditions for the first time to reveal the rate of lithium extraction from the lithium layer, and transition metal layer is related to the different charge and discharge characteristics. At the beginning of charging, the lithium extraction predominately occurs within the lithium layer. Once the high voltage plateau is reached, the lithium extraction from the lithium layer slows down and extraction from the transition metal layer evolves at a faster rate.     

A Novel Surface Treatment Method and New Insight into Discharge Voltage Deterioration for High‐Performance 0.4Li2MnO3–0.6LiNi1/3Co1/3Mn1/3O2 Cathode Materials

The Li‐rich cathode materials have been considered as one of the most promising cathodes for high energy Li‐ion batteries. However, realization of these materials for use in Li‐ion batteries is currently limited by their intrinsic problems. To overcome this barrier, a new surface treatment concept is proposed in which a hybrid surface layer composed of a reduced graphene oxide (rGO) coating and a chemically activated layer is created. A few layers of GO are first coated on the surface of the Li‐rich cathode material, followed by a hydrazine treatment to produce the reducing agent of GO and the chemical activator of the Li₂MnO₃ phase. Compared to previous studies, this surface treatment provides substantially improved electrochemical performance in terms of initial Coulombic effiency and retention of discharge voltage. As a result, the surface‐treated 0.4Li­₂MnO_3–0.6LiNi_1/3Co_1/3Mn_1/3O₂ exhibits a high capacity efficiency of 99.5% during the first cycle a the discharge capacity of 250 mAh g^?1 (2.0–4.6 V under 0.1C), 94.6% discharge voltage retention during 100 cycles (1C) and the superior capacity retention of 60% at 12C at 24 °C.     

Porosity‐Controlled TiNb2O7 Microspheres with Partial Nitridation as A Practical Negative Electrode for High‐Power Lithium‐Ion Batteries

Titanium niobium oxide (TiNb₂O₇) has been recognized as a promising anode material for lithium‐ion batteries (LIBs) in view of its potential to operate at high rates with improved safety and high theoretical capacity of 387 mAh g^?1. However, it suffers from poor Li⁺ ion diffusivity and low electronic conductivity originated from its wide band gap energy (E_g > 2 eV). Here, porous TiNb₂O₇ microspheres (PTNO MSs) are prepared via a facile solvothermal reaction. PTNO MSs have a particle size of ≈1.2 μm and controllable pore sizes in the range of 5–35 nm. Ammonia gas nitridation treatment is conducted on PTNO MSs to introduce conducting Ti_1?xNb_xN layer on the surface and form nitridated PTNO (NPTNO) MSs. The porous structure and conducting Ti_1?xNb_xN layer enhance the transport kinetics associated with Li⁺ ions and electrons, which leads to significant improvement in electrochemical performance. As a result, the NPTNO electrode shows a high discharge capacity of ≈265 mAh g^?1, remarkable rate capability (≈143 mAh g^?1 at 100 C) and durable long‐term cyclability (≈91% capacity retention over 1000 cycles at 5 C). These results demonstrate the great potential of TiNb₂O₇ as a practical high‐rate anode material for LIBs.     

Asymmetric Rate Behavior of Si Anodes for Lithium‐Ion Batteries: Ultrafast De‐Lithiation versus Sluggish Lithiation at High Current Densities

The combined effect of lithium‐ion diffusion, potential‐concentration gradient, and stress plays a critical role in the rate capability and cycle life of Si‐based anodes of lithium‐ion batteries. In this work, Si nanofilm anodes are shown to exhibit an asymmetric rate performance: around 72% of the total available capacity can be delivered during de‐lithiation under a high current density of 420 A g^‐1 (100C where C is the charge‐rate) in 22 s; in striking contrast, only 1% capacity can be delivered during lithiation. A mathematical model of single‐ion diffusion is established to elucidate the asymmetric rate performance, which can be mainly attributed to the potential‐concentration profile associated with the active material and the ohmic voltage shift under high currents; the difference in chemical diffusion coefficients during lithiation and de‐lithiation also plays a role. This clarifies that the charge and discharge rates of lithium‐ion‐battery electrodes should be evaluated separately due to the asymmetric effect in the electrochemical system.     

Conductive Copper Niobate: Superior Li+‐Storage Capability and Novel Li+‐Transport Mechanism

Niobates with shear ReO₃ crystal structures are remarkably promising anode materials for Li⁺ batteries due to their large capacities, inherent safety, and high cycling stability. However, they generally suffer from limited rate capabilities rooted in their insufficient electronic and Li⁺ conductivities. Here, micrometer‐sized copper niobate (Cu₂Nb₃₄O₈₇) bulk as a new anode material having a high electronic conductivity of 2.1 × 10^?5 S cm^?1 and an impressive average Li⁺ diffusion coefficient of ≈3.5 × 10^?13 cm² s^?1 is exploited, which synergistically leads to an excellent rate capability (184 mAh g^?1 at 10 C) while remaining a large reversible capacity and superior cycling stability. Moreover, the fast Li⁺ transport pathways of grain boundary (micrometer scale) → lattice deformation area (nanometer scale) → (010) crystallographic plane (angstrom scale) are demonstrated in Cu₂Nb₃₄O₈₇. Therefore, these results could pave the way for practical application of Cu₂Nb₃₄O₈₇ in high‐performance Li⁺ batteries.