An Ultra‐Long‐Life Lithium‐Rich Li 1.2 Mn 0.6 Ni 0.2 O 2 Cathode by Three‐in‐One Surface Modification for Lithium‐Ion Batteries

Inhibition of Structural Transformation and Surface Lattice Oxygen Activity for Excellent Stability Li-Rich Mn-Based Layered Oxides

Li-rich Mn-based layered oxides (LLOs) are one of the most promising cathode materials, which have exceptional anionic redox activity and a capacity that surpasses 250 mA h/g. However, the change from a layered structure to a spinel structure and unstable anionic redox are accompanied by voltage attenuation, poor rate performance, and problematic capacity. The technique of stabilizing the crystal structure and reducing the surface oxygen activity is proposed in this paper. A coating layer and highly concentrated oxygen vacancies are developed on the material's surface, according to scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. In situ EIS shows that structural transformation and oxygen release are inhibited during the first charge and discharge. Optimized 3@LRMA has an average attenuation voltage of 0.55 mV per cycle (vs 1.7 mV) and a capacity retention rate of 93.4% after 200 cycles (vs 52.8%). Postmortem analysis indicates that the successful doping of Al ions into the crystal structure effectively inhibits the structural alteration of the cycling process. The addition of oxygen vacancies reduces the surface lattice's redox activity. Additionally, surface structure deterioration is successfully halted by N- and Cl-doped carbon coating. This finding highlights the significance of lowering the surface lattice oxygen activity and preventing structural alteration, and it offers a workable solution to increase the LLO stability.

Impact of Ni Content on the Electrochemical Performance of the Co-Free, Li and Mn-Rich Layered Cathode Materials

Li and Mn-rich layered cathode (LLC) materials show great potential as the next generation cathode materials because of their high, practical and achievable specific capacity of ~250 mAh/g, thermal stability and lower raw material cost. However, LLC materials suffer from degradation of specific capacity, voltage fading due to phase transformation upon cycling and transition-metal dissolution, which presents a significant barrier for commercialization. Here, we report the effects of Ni content on the electrochemical performance, structural and thermal stability of a series of Co-free, LLC materials (Li1.2NixMn0.8-xO2, x = 0.12, 0.18, 0.24, 0.30 and 0.36) synthesized via a sol-gel method. Our study shows that the structure of the material as well as the electrochemical and thermal stability properties of the LLC materials are strongly dependent on the Ni or Mn content. An increase in the Ni to Mn ratio results in an increase in the average discharge voltage and capacity, as well as improved structural stability but decreased thermal stability.

Multifunctional Surface Construction for Long-Term Cycling Stability of Li-Rich Mn-Based Layered Oxide Cathode for Li-Ion Batteries

Li-rich Mn-based layered oxides (LMLOs) are promising cathode material candidate for the next-generation Li-ion batteries (LIBs) of high energy density. However, the fast capacity fading and voltage decay as well as low Coulombic efficiency caused by irreversible oxygen release and phase transition during the electrochemical process hinder their practical application. To solve these problems, in the present study, a multifunctional surface construction involving a coating layer, spinel-layered heterostructure, and rich-in oxygen vacancies is successfully conducted by a facile thermal reduction of the LMLO particles with potassium borohydride (KBH₄ ) as the reducing agent. The multifunctional surface structure plays synergistic effects on suppressing the interface side reaction, reducing the dissolution of transition metal, increasing electron conductivity and lithium diffusion rate. As a result, electrochemical performances of the LMLO cathode are effectively enhanced. With optimization of the addition of KBH₄ , the electrode delivers a reversible capacity of 280 mAh g^-1 at 0.1 C, which maintains after 100 cycles. The capacity retention with respect to the initial capacity is as high as 98% at 1 C after 400 cycles. The present work provides insights into designing a highly effective functional surface structure of LMLO cathode materials for high-performance LIBs.

Scalable Nitrate Treatment for Constructing Integrated Surface Structures to Mitigate Capacity Fading and Voltage Decay of Li-Rich Layered Oxides

Capacity fading and voltage decay is one of the biggest obstacles for the practical application of Li-rich layered oxides due to the serious surface-related detrimental reactions. Herein, we develop a versatile and scalable method to construct a robust surface-integrated structure. All the designed samples deliver outstanding capacity and voltage stability, among which the Zn-treated sample possesses the best electrochemical performance. Its capacity retention is larger than 90 % after 400 cycles with a voltage decay ratio as small as 0.73 mV per cycle. What is more, the rules of surface-integrated structure with different cations in terms of capacity and voltage stability is further deciphered by combining with density function theory (DFT) calculations. It is found that to obtain advanced Li-rich layered oxide cathodes, cations in Li-sites should firstly ensure the binding energy of the surface-integrated structure in a lower level and then provide Bader charge for the nearest O atoms as small as possible.

Formation of LiF-rich Cathode-Electrolyte Interphase by Electrolyte Reduction

The capacity of transition metal oxide cathode for Li-ion batteries can be further enhanced by increasing the charging potential. However, these high voltage cathodes suffer from fast capacity decay because the large volume change of cathode breaks the active materials and cathode-electrolyte interphase (CEI), resulting in electrolyte penetration into broken active materials and continuous side reactions between cathode and electrolytes. Herein, a robust LiF-rich CEI was formed by potentiostatic reduction of fluorinated electrolyte at a low potential of 1.7 V. By taking LiCoO₂ as a model cathode, we demonstrate that the LiF-rich CEI maintains the structural integrity and suppresses electrolyte penetration at a high cut-off potential of 4.6 V. The LiCoO₂ with LiF-rich CEI exhibited a capacity of 198 mAh g^-1 at 0.5C and an enhanced capacity retention of 63.5 % over 400 cycles as compared to the LiF-free LiCoO₂ with only 17.4 % of capacity retention.

Effects of Ru doping on the structural stability and electrochemical properties of Li2MoO3 cathode materials for Li-ion batteries

The Li₂MoO₃ (LMO) material is one promising cathode material for lithium-ion batteries due to its high specific capacity and absence of oxygen release. However, its surface instability in air and poor conductivity have limited its application. To solve these problems, the Ru element has been successfully introduced into the LMO lattice with the aid of the molten salt method. XRD and TEM characterization showed that the introduction of Ru does not change the crystal structure but expands the crystal plane spacing of the {001} facets, which is further evidenced by density functional theory (DFT) calculations. XPS and EDS tests indicated that the introduction of Ru inhibits the oxidation of Mo species and leads to a more uniform distribution of the material. In addition, DFT calculations revealed that covalent interactions are formed between Mo_4d/Ru_4d and O_2p orbitals, leading to a significant reduction of the band gap. Therefore, Ru-doped samples exhibit good electrochemical performances. The initial discharge capacity of an LMRO-2 sample reaches 299.1 mA h g^-1 at a 1C rate, and the capacity remains at 125.2 mA h g^-1 after 100 cycles. In comparison, the initial discharge capacity of pure phase sample LMO is only 250.5 mA h g^-1 under the same conditions, and the capacity remains only at 76.5 mA h g^-1 after 100 cycles. The present results confirmed that Ru doping is an effective strategy to improve the performance of the LMO cathode material.

Boosting Electrochemical Performance of Lithium-Rich Manganese-Based Cathode Materials through a Dual Modification Strategy with Defect Designing and Interface Engineering

Low Coulombic efficiency, severe capacity fading and voltage attenuation, and poor rate performance are currently great obstacles for the industrial application of lithium-rich manganese-based cathode materials (LRMCs) in lithium-ion batteries (LIBs). Herein, a dual modification strategy combining defect designing with interface engineering is reported to solve the above problems synchronously. Oxygen vacancies, a carbon nitride protective layer, and a fast ion conductor are simultaneously introduced in the LRMCs. It has been found that oxygen vacancies can suppress the release of irreversible oxygen, which is in favor of improving the initial Coulombic efficiency, the carbon nitride protective layer can improve the structural stability and alleviate the attenuation of capacity and voltage, and the fast ion conductor can promote the diffusion rate of Li⁺ and electron conductivity and thus enhance the rate capability. The modified material exhibits significantly enhanced electrochemical performances, including a favorable capacity retention rate of 94.2% over 120 cycles at 1C (1C = 200 mAh g^-1) and excellent rate capabilities of 173.1 and 136.9 mAh g^-1 can be maintained at 5 and 10C after 100 cycles, respectively. Hence, the well-designed dual modification strategy with defect design and interface engineering provides significant exploration for the development and industrialization of LRMCs with high performance.

A Simple Gas-Solid Treatment for Surface Modification of Li-Rich Oxides Cathodes

Li-rich layered oxides with high capacity are expected to be the next generation of cathode materials. However, the irreversible and sluggish anionic redox reaction leads to the O₂ loss in the surface as well as the capacity and voltage fading. In the present study, a simple gas-solid treatment with ferrous oxalate has been proposed to uniformly coat a thin spinel phase layer with oxygen vacancy and simultaneously realize Fe-ion substitution in the surface. The integration of oxygen vacancy and spinel phase suppresses irreversible O₂ release, prevents electrolyte corrosion, and promotes Li-ion diffusion. In addition, the surface doping of Fe-ion can further stabilize the structure. Accordingly, the treated Feox-2 % cathode exhibits superior capacity retention of 86.4 % and 85.5 % at 1 C and 2 C to that (75.3 % and 75.0 %) of the pristine sample after 300 cycles, respectively. Then, the voltage fading is significantly suppressed to 0.0011 V per cycle at 2 C especially. The encouraging results may play a significant role in paving the practical application of Li-rich layered oxides cathode.

Enhanced Activity and Reversibility of Anionic Redox by Tuning Lithium Vacancies in Li-Rich Cathode Materials

Li-rich Mn-based layered oxide cathodes (LLOs) are considered to be the most promising cathode candidates for lithium-ion batteries owing to their high-voltage platform and ultrahigh specific capacity originating from anionic redox. However, anionic redox results in many problems including irreversible oxygen release, voltage hysteresis, and so on. Although many efforts have been made to regulate anionic redox, a fundamental issue, the effect of lithium vacancies on anionic redox, is still unclear. Herein, we synthesized a series of LLO materials with different lithium vacancy contents by controlling the amount of lithium salt. Specifically, lithium-vacancy-type LLOs Li_1.11Ni_0.18Co_0.18Mn_0.53O₂ with a pompon morphology exhibit an ultrahigh specific capacity (293.9 mA h g^-1 at 0.1 C), an outstanding long-term cycling stability (173.5 mA h g^-1 after 300 cycles at 1 C), and an excellent rate performance (106 mA h g^-1 at 10 C). It reveals that lithium vacancy is a key factor to enhance anionic redox activity and reversibility. Lithium vacancies exhibit different inductive effects on the structure of the surface and bulk. Abundant surface oxygen vacancies and a surface spinel phase layer induced by lithium vacancies suppress irreversible oxygen release, while the bulk phase transformation and cation disorder combined with sufficient lithium vacancies in the bulk stabilize structure and improve anionic redox kinetic. The findings offer a significant theoretical guidance for the practical application of LLO materials.

Demystifying the Lattice Oxygen Redox in Layered Oxide Cathode Materials of Lithium-Ion Batteries

The practical application of lithium-ion batteries suffers from low energy density and the struggle to satisfy the ever-growing requirements of the energy-storage Internet. Therefore, developing next-generation electrode materials with high energy density is of the utmost significance. There are high expectations with respect to the development of lattice oxygen redox (LOR)-a promising strategy for developing cathode materials as it renders nearly a doubling of the specific capacity. However, challenges have been put forward toward the deep-seated origins of the LOR reaction and if its whole potential could be effectively realized in practical application. In the following Review, the intrinsic science that induces the LOR activity and crystal structure evolution are extensively discussed. Moreover, a variety of characterization techniques for investigating these behaviors are presented. Furthermore, we have highlighted the practical restrictions and outlined the probable approaches of Li-based layered oxide cathodes for improving such materials to meet the practical applications.

Deciphering the effects of hexagonal and monoclinic structure distribution on the properties of Li-rich layered oxides

Uniform distribution of Li₂MnO₃ and LiMO₂ components in a Co-free Li-rich layered oxide is achieved by treating precursors with NH₃·H₂O, which expands the lattice parameter and promotes the activation of Li₂MnO₃, resulting in excellent electrochemical performance. What's more, it also contributes to the storage stability of Li-rich layered oxides.

Function and Application of Defect Chemistry in High-Capacity Electrode Materials for Li-Based Batteries

Current commercial Li-based batteries are approaching their energy density limitation, yet still cannot satisfy the energy density demand of the high-end devices. Hence, it is critical to developing advanced electrode materials with high specific capacity. However, these electrode materials are facing challenges of severe structural degradation and fast capacity fading. Among various strategies, constructing defects in electrode materials holds great promise in addressing these issues. Herein, we summarize a series of significant defect engineering in the high-capacity electrode materials for Li-based batteries. The detailed retrospective on defects specification, function mechanism, and corresponding application achievements on these electrodes are discussed from the view of point, line, planar, volume defects. Defect engineering can not only stabilize the structure and enhance electric/ionic conductivity, but also act as active sites to improve the ionic storage and bonding ability of electrode materials to Li metal. We hope this review can spark more perspectives on evaluating high-energy-density Li-based batteries.

A general route of fluoride coating on the cyclability regularity of high-voltage NCM cathodes

The effect of fluoride coatings, an effective approach to suppress HF corrosion, on the cycling stability of NCM523 cathodes is systematically studied. It is the first study to reveal that fluoride coatings significantly restrain interfacial reactions when, simultaneously, the fluoride suspension possesses an appropriate pH (near 4.0) and there is a small cation ionic radius.