Composite-Structure Material Design for High-Energy Lithium Storage

WS2 nanosheets vertically grown on Ti3C2 as superior anodes for lithium-ion batteries

Tungsten disulfide (WS2) is considered as a promising anode material for high-performance lithium-ion batteries (LIBs) result from its inherent characteristics such as high theoretical capacity, large interlayer spacing and weak interlayer Van der Waals force. Nevertheless, WS2 has the drawbacks of easy agglomeration, severe volume expansion and high Li+ migration barrier, which lead to rapid capacity degradation and imperfect rate ability. In this work, a novel two-dimensional (2D) hierarchical composite (Ti3C2/WS2) consisting of WS2 nanosheets vertically grown on titanium carbide (Ti3C2) nanosheets is prepared. Thanks to this distinctive hierarchical structure and synergy between WS2 and Ti3C2, the Ti3C2/WS2 composite demonstrates exceptional electrochemical performance in LIBs. In addition, we investigate the effect of the mass proportion of WS2 in Ti3C2/WS2 composite on the electrochemical performance, and find that the optimal mass ratio of WS2 is 60%. As expected, the optimal electrode exhibits a high specific capacity (650 mAh/g at 0.1 A/g after 100 cycles) and ultra-long cycle stability (400 mAh/g at 1.0 A/g after 5000 cycles).

Adjusting Oxygen Redox Reaction and Structural Stability of Li- and Mn-Rich Cathodes by Zr-Ti Dual-Doping

Li- and Mn-rich cathodes (LMRs) with cationic and anionic redox reactions are considered as promising cathode materials for high-energy-density Li-ion batteries. However, the oxygen redox process leads to lattice oxygen loss and structure degradation, which would induce serious voltage fade and capacity loss and thus limit the practical application. High-valent and electrochemical inactive d⁰ element doping is an effective method to tune the crystal and electronic structures, which are the main factors for the electrochemical stability. Herein, noticeably inhibited oxygen loss, reduced voltage fade, enhanced rate performance, and improved structure stability and thermal stability of LMRs have been realized by Ti⁴⁺ and Zr⁴⁺ dual-doping. The underlying modulation mechanisms are unraveled by combining differential electrochemical mass spectrometry, soft X-ray absorption spectroscopies, in situ XRD measurements, etc. The dual-doping reduces the covalency of the TM-O bond, mitigates the irreversible oxygen release during the oxygen redox, and stabilizes the layered framework. The expanded lithium layer facilitates the lithium diffusion kinetics and structure stability. This study may result in the fundamental understanding of crystal and electronic structure evolution in LMRs and contribute to the development of high capacity cathodes.

Composite Nanostructure Construction on the Grain Surface of Li-Rich Layered Oxides

Li-rich layered oxides (LLOs) are fascinating high-energy cathodes for lithium-ion batteries (LIBs), but still suffer from critical drawbacks that retard their practical applications. Although surface modification is effective to protect LLOs from structural deterioration, the delicate design of structures on a grain surface with promising scalability for industrial application is still challenging. Herein, using the atomic layer deposition (ALD) technique, a composite nanostructure comprising a uniform LiTaO₃ coating layer (≈3 nm) and a spinel interlayer structure (≈1 nm) is constructed on the grain surface of industrial LLO (Li_1.13 Mn_0.517 Ni_0.256 Co_0.097 O₂ ) agglomerated spheres. The surface composite nanostructure can not only enhance the structural/interfacial stability of the LLO, but also facilitates Li⁺ diffusion, thereby significantly improving its cycle stability, rate performance, thermal stability, and voltage maintenance. Specifically, the LLO coated with 10 ALD cycles exhibits a small voltage decay rate of 0.9 mV per cycle, a reversible capacity of 272.8 mAh g^-1 at 0.1 C, and a capacity retention of 85% after 200 cycles at 1 C, suggesting the important role of surface composite nanostructure for improving the electrochemical performance. This work provides new insights into the composite nanostructure design on the grain surface of cathode materials for high-performance LIBs.

Thermodynamic analysis and kinetic optimization of high-energy batteries based on multi-electron reactions

Multi-electron reaction can be regarded as an effective way of building high-energy systems (>500 W h kg⁻¹). However, some confusions hinder the development of multi-electron mechanisms, such as clear concept, complex reaction, material design and electrolyte optimization and full-cell fabrication. Therefore, this review discusses the basic theories and application bottlenecks of multi-electron mechanisms from the view of thermodynamic and dynamic principles. In future, high-energy batteries, metal anodes and multi-electron cathodes are promising electrode materials with high theoretical capacity and high output voltage. While the primary issue for the multi-electron transfer process is sluggish kinetics, which may be caused by multiple ionic migration, large ionic radius, high reaction energy barrier, low electron conductivity, poor structural stability, etc., it is urgent that feasible and versatile modification methods are summarized and new inspiration proposed in order to break through kinetic constraints. Finally, the remaining challenges and future research directions are revealed in detail, involving the search for high-energy systems, compatibility of full cells, cost control, etc.

A High-Capacity Ammonium Vanadate Cathode for Zinc-Ion Battery

Given the advantages of being abundant in resources, environmental benign and highly safe, rechargeable zinc-ion batteries (ZIBs) enter the global spotlight for their potential utilization in large-scale energy storage. Despite their preliminary success, zinc-ion storage that is able to deliver capacity > 400 mAh g-1 remains a great challenge. Here, we demonstrate the viability of NH4V4O10 (NVO) as high-capacity cathode that breaks through the bottleneck of ZIBs in limited capacity. The first-principles calculations reveal that layered NVO is a good host to provide fast Zn2+ ions diffusion channel along its [010] direction in the interlayer space. On the other hand, to further enhance Zn2+ ion intercalation kinetics and long-term cycling stability, a three-dimensional (3D) flower-like architecture that is self-assembled by NVO nanobelts (3D-NVO) is rationally designed and fabricated through a microwave-assisted hydrothermal method. As a result, such 3D-NVO cathode possesses high capacity (485 mAh g-1) and superior long-term cycling performance (3000 times) at 10 A g-1 (~ 50 s to full discharge/charge). Additionally, based on the excellent 3D-NVO cathode, a quasi-solid-state ZIB with capacity of 378 mAh g-1 is developed.

Crystalline Domain Battery Materials

The development of next-generation energy storage materials for secondary batteries relies more and more on the delicate design and tailoring of their local structures and properties. Crystalline domain battery materials (CDBMs) are defined as a family of materials that are hierarchically engineered primarily by bonding selective atoms in certain space groups with short-range order to form nanoscale crystal domains as fundamental constructive and functional units, secondarily by integrating these interactive crystal domains under certain configurations into grains to implement electrochemical synergy, and finally by optimizing grains through nanoengineering toward advanced electrode materials. In CDBMs, adjacent crystal domains can undergo structural co-transformations with noticeable interrelationships, and the overall electrochemical performance is determined not only by the intrinsic structure of each crystal domain (element, bonding, valence, stacking, orientation, etc.) but also by the configuration of crystal domains (size, ratio, interface, distribution, interaction, etc.). Pioneering studies have shown significant enhancement of electrochemical performance by controlling crystal domains, suggesting the prospect of developing novel electrode materials through crystal-domain engineering. However, fundamental understanding and delicate fabrication of this material family, in terms of structural identification, electrochemical structure evolution, reaction mechanism, design and adjustment, and structure-performance relationship, among others, still face great challenges to meet the compelling requirements of high-performance electrode materials for secondary batteries. This Account systematically introduces the structure and electrochemistry of CDBMs. The efficient structural identification of crystal domains, which is still challenging due to their structural complexity, is demonstrated using prototype materials by advanced characterization techniques such as high-energy X-ray diffraction combined with Rietveld refinement and spherical aberration-corrected transmission electron microscopy. Investigations on the structural evolution of CDBMs in electrochemical reactions by ex-situ and in-situ techniques provide insights into reaction scenarios such as how ions migrate in and across crystal domains and how these crystal domains transform synergistically. A crystal-domain reaction mechanism is thus proposed to explain the electrochemistry of these materials. Design principles and adjustment strategies for designated crystal-domain structures including their components, ratios, distributions, and interfaces are deduced from the structural identification, evolution and reaction mechanism. The relationship between crystal-domain structures and electrochemical performance can further be elucidated, inspiring us to explore efficient strategies for optimizing the electrochemical performance, as validated by examples of high-performance batteries using materials with controlled crystal-domain structures. Based on these systematic studies, the trends in the rapid enrichment, deep investigation, and practical application of CDBMs are envisioned to promote continuous studies on this nascent energy storage material family.

Precise Surface Engineering of Cathode Materials for Improved Stability of Lithium-Ion Batteries

As lithium-ion batteries continue to climb to even higher energy density, they meanwhile cause serious concerns on their stability and reliability during operation. To make sure the electrode materials, particularly cathode materials, are stable upon extended cycles, surface modification becomes indispensable to minimize the undesirable side reaction at the electrolyte-cathode interface, which is known as a critical factor to jeopardizing the electrode performance. This Review is targeted at a precise surface control of cathode materials with focus on the synthetic strategies suitable for a maximized surface protection ensured by a uniform and conformal surface coating. Detailed discussions are taken on the formation mechanism of the designated surface species achieved by either wet-chemistry routes or instrumental ones, with attention to the optimized electrochemical performance as a result of the surface control, accordingly drawing a clear image to describe the synthesis-structure-performance relationship to facilitate further understanding of functional electrode materials. Finally, perspectives regarding the most promising and/or most urgent developments for the surface control of high-energy cathode materials are provided.

A Novel Bifunctional Self-Stabilized Strategy Enabling 4.6 V LiCoO2 with Excellent Long-Term Cyclability and High-Rate Capability

Although the theoretical specific capacity of LiCoO2 is as high as 274 mAh g-1, the superior electrochemical performances of LiCoO2 can be barely achieved due to the issues of severe structure destruction and LiCoO2/electrolyte interface side reactions when the upper cutoff voltage exceeds 4.5 V. Here, a bifunctional self-stabilized strategy involving Al+Ti bulk codoping and gradient surface Mg doping is first proposed to synchronously enhance the high-voltage (4.6 V) performances of LiCoO2. The comodified LiCoO2 (CMLCO) shows an initial discharge capacity of 224.9 mAh g-1 and 78% capacity retention after 200 cycles between 3.0 and 4.6 V. Excitingly, the CMLCO also exhibits a specific capacity of up to 142 mAh g-1 even at 10 C. Moreover, the long-term cyclability of CMLCO/mesocarbon microbeads full cells is also enhanced significantly even at high temperature of 60 °C. The synergistic effects of this bifunctional self-stabilized strategy on structural reversibility and interfacial stability are demonstrated by investigating the phase transitions and interface characteristics of cycled LiCoO2. This work will be a milestone breakthrough in the development of high-voltage LiCoO2. It will also present an instructive contribution for resolving the big structural and interfacial challenges in other high-energy-density rechargeable batteries.

The Effects of Trace Yb Doping on the Electrochemical Performance of Li-Rich Layered Oxides

Layered lithium-rich cathode materials are one of the most promising cathode materials owing to their higher mass energy density than the commercial counterparts. A series of trace Yb-doped lithium-rich cathode materials Li_1.2 Mn_0.54 Ni_0.13 Co_0.13-x Yb_x O₂ (0≤x≤0.050) were synthesized and the effects were investigated by XRD, X-ray photoelectron spectroscopy, and high-resolution TEM. The participation of Yb ions in electrochemical reactions and the larger binding energy of Yb-O than M-O (M=Mn, Ni, Co), which expands the lithium layer spacing and stabilizes the oxygen stacking, resulted in excellent performance of materials doped with a limited Yb content (x≤0.005). However, higher doping amounts (x>0.005) significantly increased the charge-transfer impedance and led to a sharp deterioration in electrochemical performance. The reason lies in the large difference in ionic radius between the transition metals (Mn, Co, and Ni) and Yb. There is an upper limit to the amount of Yb ions in the lattice. If the amount of Yb is higher than the limit, excess Yb ions enter the Li layers instead of staying in the transition-metal layers or even segregate on the surface and form electrochemically inert oxides.

Hierarchical nanostructures derived from cellulose for lithium-ion batteries

Recent advances in natural cellulose substance derived hierarchical nanomaterials applied as anodic materials for lithium-ion batteries are summarized.

Hybrid Architectures based on 2D MXenes and Low-Dimensional Inorganic Nanostructures: Methods, Synergies, and Energy-Related Applications

Being conductive and flexible, MXenes, including transition metal carbides and nitrides, are expected to compete with, or even outperform graphene as 2D substrates serving in versatile applications. On the other hand, the extraordinary electrochemical activities of MXenes make them promising candidates as electrode materials in rechargeable batteries and supercapacitors, or as electrocatalysts in water splitting. However, MXenes are inclined to self-restack due to hydrogen bonding or van der Waals interactions, which may lead to substantial loss of electroactive area as well as inaccessibility of ions and electrolytes. In this sense, hybridizing 2D MXenes and low-dimensional inorganic nanostructures in elaborately designed architectures is of utmost significance, and provides a chance to integrate their unique properties in a complementary way. As such, this review is dedicated to highlighting recent progress in this regime, putting emphasis on the methods, structural and functional synergies, and energy-related applications. Moreover, the present challenges and the future development directions are also discussed in depth.