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

Direct synthesis of a lithium carboxymethyl cellulose binder using wood dissolving pulp for high-performance LiFePO4 cathodes in lithium-ion batteries

Lithium carboxymethyl cellulose (CMC-Li) is a promising novel water-based binder for lithium-ion batteries. The direct synthesis of CMC-Li was innovatively developed using abundant wood dissolving pulp materials from hardwood (HW) and softwood (SW). The resulting CMC-Li-HW and CMC-Li-SW binders possessed a suitable degree of substitutions and excellent molecular weight distributions with an appropriate quantity of long- and short-chain celluloses, which facilitated the construction of a reinforced concrete-like bonding system. When used as cathode binders in LiFePO4 batteries, they uniformly coated and dispersed the electrode materials, formed a compact and stable conductive network with high mechanical strength and showed sufficient lithium replenishment. The prepared LiFePO4 batteries exhibited good mechanical stability, low charge transfer impedance, high initial discharge capacity (∼180 mAh/g), high initial Coulombic efficiency (99 %), excellent cycling performance (<3% loss over 200 cycles) and good rate capability, thereby outperforming CMC-Na and the widely used cathode binder polyvinylidene fluoride.

Why the Synthesis Affects Performance of Layered Transition Metal Oxide Cathode Materials for Li-Ion Batteries

The limited cyclability of high-specific-energy layered transition metal oxide (LiTMO2) cathode materials poses a significant challenge to the industrialization of batteries incorporating these materials. This limitation can be attributed to various factors, with the intrinsic behavior of the crystal structure during the cycle process being a key contributor. These factors include phase transition induced cracks, reduced Li active sites due to Li/Ni mixing, and slower Li+ migration. In addition, the presence of synthesis-induced heterogeneous phases and lattice defects cannot be disregarded as they also contribute to the degradation in performance. Therefore, gaining a profound understanding of the intricate relationship among material synthesis, structure, and performance is imperative for the development of LiTMO2. This paper highlights the pivotal role of structural play in LiTMO2 materials and provides a comprehensive overview of how various control factors influence the specific pathways of structural evolution during the synthesis process. In addition, it summarizes the scientific challenges associated with diverse modification approaches currently employed to address the cyclic failure of materials. The overarching goal is to provide readers with profound insights into the study of LiTMO2.

High Voltage Cycling Stability of LiF-Coated NMC811 Electrode

The development of LiNi0.8Mn0.1Co0.1O2 (NMC811) as a cathode material for high-energy-density lithium-ion batteries (LIBs) intends to address the driving limitations of electric vehicles. However, the commercialization of this technology has been hindered by poor cycling stability at high cutoff voltages. The potential instability and drastic capacity fade stem from irreversible parasitic side reactions at the electrode-electrolyte interface. To address these issues, a stable nanoscale lithium fluoride (LiF) coating is deposited on the NMC811 electrode via atomic layer deposition. The nanoscale LiF coating diminishes the direct contact between NMC811 and the electrolyte, suppressing the detrimental parasitic reactions. LiF-NMC811 delivers cycling stability superior to uncoated NMC811 with high cutoff voltage for half-cell (3.0-4.6 V vs Li/Li+) and full-cell (2.8-4.5 V vs graphite) configurations. The structural, morphological, and chemical analyses of the electrodes after cycling show that capacity decline fundamentally arises from the electrode-electrolyte interface growth, irreversible phase transformation, transition metal dissolution and crossover, and particle cracking. Overall, this work demonstrates that LiF is an effective electrode coating for high-voltage cycling without compromising rate performance, even at high discharge rates. The findings of this work highlight the need to stabilize the electrode-electrolyte interface to fully utilize the high-capacity performance of NMC811.

Surface Lattice Modulation through Chemical Delithiation toward a Stable Nickel-Rich Layered Oxide Cathode

Nickel-rich layered oxides (NLOs) are considered as one of the most promising cathode materials for next-generation high-energy lithium-ion batteries (LIBs), yet their practical applications are currently challenged by the unsatisfactory cyclability and reliability owing to their inherent interfacial and structural instability. Herein, we demonstrate an approach to reverse the unstable nature of NLOs through surface solid reaction, by which the reconstructed surface lattice turns stable and robust against both side reactions and chemophysical breakdown, resulting in improved cycling performance. Specifically, conformal La(OH)₃ nanoshells are built with their thicknesses controlled at nanometer accuracy, which act as a Li⁺ capturer and induce controlled reaction with the NLO surface lattices, thereby transforming the particle crust into an epitaxial layer with localized Ni/Li disordering, where lithium deficiency and nickel stabilization are both achieved by transforming oxidative Ni³⁺ into stable Ni²⁺. An optimized balance between surface stabilization and charge transfer is demonstrated by a representative NLO material, namely, LiNi_0.83Co_0.07Mn_0.1O₂, whose surface engineering leads to a highly improved capacity retention and excellent rate capability with a strong capability to inhibit the crack of NLO particles. Our study highlights the importance of surface chemistry in determining chemical and structural behaviors and paves a research avenue in controlling the surface lattice for the stabilization of NLOs toward reliable high-energy LIBs.

A Review of Nonaqueous Electrolytes, Binders, and Separators for Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are the most important electrochemical energy storage devices due to their high energy density, long cycle life, and low cost. During the past decades, many review papers outlining the advantages of state-of-the-art LIBs have been published, and extensive efforts have been devoted to improving their specific energy density and cycle life performance. These papers are primarily focused on the design and development of various advanced cathode and anode electrode materials, with less attention given to the other important components of the battery. The “nonelectroconductive” components are of equal importance to electrode active materials and can significantly affect the performance of LIBs. They could directly impact the capacity, safety, charging time, and cycle life of batteries and thus affect their commercial application. This review summarizes the recent progress in the development of nonaqueous electrolytes, binders, and separators for LIBs and discusses their impact on the battery performance. In addition, the challenges and perspectives for future development of LIBs are discussed, and new avenues for state-of-the-art LIBs to reach their full potential for a wide range of practical applications are outlined.

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Advancing to 4.6 V Review and Prospect in Developing High-Energy-Density LiCoO2 Cathode for Lithium-Ion Batteries

Layered LiCoO₂ (LCO) is one of the most important cathodes for portable electronic products at present and in the foreseeable future. It becomes a continuous push to increase the cutoff voltage of LCO so that a higher capacity can be achieved, for example, a capacity of 220 mAh g^-1 at 4.6 V compared to 175 mAh g^-1 at 4.45 V, which is unfortunately accompanied by severe capacity degradation due to the much-aggravated side reactions and irreversible phase transitions. Accordingly, strict control on the LCO becomes essential to combat the inherent instability related to the high voltage challenge for their future applications. This review begins with a discussion on the relationship between the crystal structures and electrochemical properties of LCO as well as the failure mechanisms at 4.6 V. Then, recent advances in control strategies for 4.6 V LCO are summarized with focus on both bulk structure and surface properties. One closes this review by presenting the outlook for future efforts on LCO-based lithium ion batteries (LIBs). It is hoped that this work can draw a clear map on the research status of 4.6 V LCO, and also shed light on the future directions of materials design for high energy LIBs.

Precise surface control of cathode materials for stable lithium-ion batteries

The increasing demand for high-energy Li-ion batteries (LIBs) continues to push the development of electrode materials, particularly cathode materials, towards their capacity limits. Despite the enormous success, the stability and reliability of LIBs are becoming a serious concern due to the much-aggravated side reactions between electrode materials and organic electrolytes. How to stabilize the cathode/electrolyte interface is therefore an imperative and urgent task drawing considerable attention from both academia and industry. An active treatment on the surface of cathode materials, usually by introducing an inert protection layer, to diminish their side reaction with electrolytes turns out to be a reasonable and effective strategy. This Feature Article firstly outlines our synthesis efforts for the construction of a uniform surface nanocoating on various cathode materials. Different wet chemical routes have been designed to facilitate the control of growth kinetics of targeted coating species so that a precise surface coating could be achieved with nanometer accuracy. Furthermore, we showed the possibility to transform the outer coating layer into a surface doping effect through surface solid reaction at high temperature. A detailed discussion on the structure-performance relationship of these surface-controlled cathode materials is introduced to probe the stabilization mechanism. Finally, perspectives on the development tendency of high-energy cathodes for stable LIBs are provided.

Chemical Heterointerface Engineering on Hybrid Electrode Materials for Electrochemical Energy Storage

The chemical heterointerfaces in hybrid electrode materials play an important role in overcoming the intrinsic drawbacks of individual materials and thus expedite the in-depth development of electrochemical energy storage. Benefiting from the three enhancement effects of accelerating charge transport, increasing the number of storage sites, and reinforcing structural stability, the chemical heterointerfaces have attracted extensive interest and the electrochemical performances of hybrid electrode materials have been significantly optimized. In this review, recent advances regarding chemical heterointerface engineering in hybrid electrode materials are systematically summarized. Especially, the intrinsic behaviors of chemical heterointerfaces on hybrid electrode materials are refined based on built-in electric field, van der Waals interaction, lattice mismatch and connection, electron cloud bias and chemical bond, and their combination. The strategies for introducing chemical heterointerfaces are classified into in situ local transformation, in situ growth, cosynthesis, and other strategy. The recent progress about the chemical heterointerfaces engineering specially focusing on metal-ion batteries, supercapacitors, and Li-S batteries are introduced in detail. Furthermore, the classification and characterization of chemical heterointerfaces are briefly described. Finally, the emerging challenges and perspectives about future directions of chemical heterointerface engineering are proposed.

Ultrathin Li-Si-O Coating Layer to Stabilize the Surface Structure and Prolong the Cycling Life of Single-Crystal LiNi0.6Co0.2Mn0.2O2 Cathode Materials at 4.5 V

Single-crystal LiNi_1-x-yCo_xMn_yO₂ cathode materials can effectively suppress intergranular cracks that usually is seen in commercial polycrystal LiNi_1-x-yCo_xMn_yO₂ cathode materials. However, the surface structure degradation for single-crystal LiNi_1-x-yCo_xMn_yO₂ cathode materials is still aggravated at a higher cutoff voltage (over 4.5 V). In this work, we prepare single-crystal LiNi_0.6Co_0.2Mn_0.2O₂ cathode materials via a solid-state method and then coat an ultrathin Li-Si-O layer on their surface by a wet coating method. The results show that the single-crystal LiNi_0.6Co_0.2Mn_0.2O₂ cathode materials with a Li-Si-O coating layer deliver excellent cycling performance even at a higher cutoff voltage of 4.5 V. The optimized Li-Si-O-modified sample displays a capacity retention of 90.6% after 100 cycles, whereas only 68.0% for unmodified single-crystal LiNi_0.6Co_0.2Mn_0.2O₂. Further analysis of the cycled electrodes reveals that the surface structure degradation is the main reason for the decrease of electrochemical performance of single-crystal LiNi_0.6Co_0.2Mn_0.2O₂ at a high voltage (4.5 V). In contrast, with Li-Si-O coating, this phenomenon can be suppressed effectively to maintain interfacial stability and prolong the cycling life.

Nanoengineered Organic Electrodes for Highly Durable and Ultrafast Cycling of Organic Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have become increasingly important as next-generation energy storage systems for application in large-scale energy storage. It is very crucial to develop an eco-friendly and green SIB technique with superior performance for sustainable future use. Replacing the conventional inorganic electrode materials with green and safe organic electrodes will be a promising approach. However, the poor electrochemical kinetics, unstable electrode-electrolyte interface, high solubility of the electrodes in the electrolyte, and large amount of conductive carbon present great challenges for organic SIBs. In this study, the issues of organic electrodes are addressed through atomic-level manipulation of these organic molecules using a series of ultrathin (Å-level) metal oxide coatings (Al₂ O₃ , ZnO, and TiO₂ ). Uniform and precise coatings on the perylene-3,4,9,10-tetracarboxylicacid dianhydride by gas-phase atomic layer deposition technique shows a stable interphase, enhanced electrochemical kinetics (71C, 10 A g^-1 ), and excellent stability (89%-500 cycles) compared to conventional organic electrode (70%-200 cycles). Further studies reveal that the chemical stability of the metal oxide coating layer plays a critical role in influencing the redox behavior, and improving kinetics of organic electrodes. This study opens a new avenue for developing high-energy organic SIBs with performance equivalent to inorganic counterparts.

Recent advances in cathode materials for rechargeable lithium-sulfur batteries

Lithium-sulfur batteries (Li-S) are regarded as a promising candidate for next-generation energy storage systems due to their high specific capacity (1675 mA h g^-1) and energy density (2600 W h kg^-1) as well as the abundance, safety and low cost of sulfur materials. However, many disadvantages hinder the further development of Li-S batteries, such as the insulating nature of the active materials, the dissolution of intermediate products, large volume expansion and safety concerns related to metal lithium anodes. During the past decade, tremendous efforts have been made in the design and synthesis of electrode materials. In this review, we briefly discuss the electrochemical mechanism of Li-S batteries and their practical problems. Then, we systematically summarize the current strategies for designing cathode materials with stable and long cycling performance, including sulfur cathodes and Li₂S cathodes; subsequently, the current development of solid-state electrolytes and protective strategies for lithium metal anodes are briefly discussed. Finally, the current challenges and future perspectives of Li-S batteries are presented.