Recent advances in lithium-rich manganese-based cathodes for high energy density lithium-ion batteries

The development of society challenges the limit of lithium-ion batteries (LIBs) in terms of energy density and safety. Lithium-rich manganese oxide (LRMO) is regarded as one of the most promising cathode materials owing to its advantages of high voltage and specific capacity (more than 250 mA h g^-1) as well as low cost. However, the problems of fast voltage/capacity fading, poor rate performance and the low initial Coulombic efficiency severely hinder its practical application. In this paper, we review the latest research advances of LRMO cathode materials, including crystal structure, electrochemical reaction mechanism, existing problems and modification strategies. In this review, we pay more attention to recent progress in modification methods, including surface modification, doping, morphology and structure design, binder and electrolyte additives, and integration strategies. It not only includes widely studied strategies such as composition and process optimization, coating, defect engineering, and surface treatment, but also introduces many relatively novel modification methods, such as novel coatings, grain boundary coating, gradient design, single crystal, ion exchange method, solid-state batteries and entropy stabilization strategy. Finally, we summarize the existing problems in the development of LRMO and put forward some perspectives on the further research.

Highly Reversible Local Structural Transformation Enabled by Native Vacancies in O2-Type Li-Rich Layered Oxides with Anion Redox Activity

A novel O2-phase Li_1.033Ni_0.2[□_0.1Mn_0.5]O₂ cathode with native vacancies (denoted as "□") was delicately designed. By a combination of noninvasive ⁷Li pj-MATPASS NMR and electron paramagnetic resonance measurements, it is unequivocally shown that the reservation of native vacancies enables the fully reversible local structural transformation without the formation of Li in the Li layer (Li_tet) in Li_1.033Ni_0.2[□_0.1Mn_0.5]O₂ during the initial and subsequent cycling. In addition, the pernicious in-plane Mn migration that would result in the generation of trapped molecular O₂ is effectively mitigated in Li_1.033Ni_0.2[□_0.1Mn_0.5]O₂. As a result, the cycle stability of Li_1.033Ni_0.2[□_0.1Mn_0.5]O₂ is significantly enhanced compared to that of the vacancy-free Li_1.033Ni_0.2Mn_0.6O₂, showing an extraordinary capacity retention of 102.31% after 50 cycles at a rate of 0.1C (1C = 100 mA g^-1). This study defines an efficacious strategy for upgrading the structural stability of O2-type Li-rich layered oxide cathodes with reversible high-voltage anion redox activity.

Constructing Robust Cathode/Electrolyte Interphase for Ultrastable 4.6 V LiCoO2 under -25 °C

Improving the durability of cathode materials at low temperature is of great importance for the development nowadays of lithium ion batteries, since the practical capacity and cycling stability of the electrode are reduced significantly at low temperature. Herein, by amorphous Zr₃(PO₄)₄ surface engineering, we realize a stable high-voltage LiCoO₂ operation (4.6 V) at -25 °C. The highly amorphous surface layer can help to form a high-quality cathode-electrolyte interphase with strong stability and low interface resistance, especially at low temperature. Such a surface-engineered LiCoO₂ shows a capacity of 179.2 mAh g^-1 at 0.2C and an excellent cyclability with 91% capacity retention after 300 cycles (1C). As a comparison, bare LiCoO₂ shows only 161.6 mAh g^-1 and 1% capacity retention under the same circumstances. This work confirms that surface regulation and control engineering is an effective route to improve the high-voltage and low-temperature performance of LiCoO₂.

Effect of Al2O3 on structural and dielectric properties of PVP-CH3COONa based solid polymer electrolyte films for energy storage devices

Nanocomposite polymer (NCP) films were prepared by doping sodium acetate (CH3COONa) in polymer of polyvinyl pyrrolidone (PVP) by complete dispersion of aluminum oxide (Al2O3) with different wt% proportions using solution cast method. The acquired NCP films were systematically characterized. The crystalline structure of the prepared NCP films was confirmed by XRD. The little agglomeration and grain sizes involved in the films were analyzed by SEM. The chemical bond formation and interchange reaction between the host, dopant salt and the nanofiller were confirmed by FTIR and Raman. The lowest energy bandgap values were observed to be 3.0 eV for the synthesized film with wt% ratio of PVP + CH3COONa:Al2O3 (80:20:1%). The highest ionic conductivity was found to be 1.05 × 10-3 S/cm for the prepared film with wt% ratio of PVP + CH3COONa:Al2O3 (80:20:1%). From the charge discharge characteristics it was concluded that the film with wt% ratio of PVP + CH3COONa:Al2O3 (80:20:1%) possesses long durability when compared to the other prepared films.

Investigation on the Electrochemical Properties and Stabilized Surface/Interface of Nano-AlPO4-Coated Li1.15Ni0.17Co0.11Mn0.57O2 as the Cathode for Lithium-Ion Batteries

Being considered as one of the most potential cathode materials, Li_1.15Ni_0.17Co_0.11Mn_0.57O₂ draws plenty of attention towards its optimization on cycling and rate performance. The surface coating process provides a longer cycling life and better rate performance for the cathodes. A systematic investigation has been carried out on the nano-AlPO₄ coating layer for the Li_1.15Ni_0.17Co_0.11Mn_0.57O₂ cathode material through a facile in situ dispersion process. The 1% coated cathode material can hold about 90% capacity retention after 100 cycles. Besides, the surface coating enhances the rate ability of Li_1.15Ni_0.17Co_0.11Mn_0.57O₂, which holds a reversible capacity of 202.3 mAh g^-1 at the rate of 1C. Surface information is collected during cycling, which reveals that less side reactions occur on the electrode-electrolyte interface after the coating process for improved cycling and rate performance.

Facile Fabrication of Ethoxy-Functional Polysiloxane Wrapped LiNi0.6Co0.2Mn0.2O2 Cathode with Improved Cycling Performance for Rechargeable Li-Ion Battery

Dealing with the water molecule on the surface of LiNi0.6Co0.2Mn0.2O2 (NCM) cathode and hydrogen fluoride in the electrolyte is one of the most difficult challenges in Li-ion battery research. In this paper, the surface polymerization of tetraethyl orthosilicate (TEOS) on NCM to generate ethoxy-functional polysiloxane (EPS) wrapped NCM (E-NCM) cathode under mild conditions and without any additions is utilized to solve this intractable problem. The differential scanning calorimetry, transmission electron microscopy, and X-ray photoelectron spectroscopy results show that the formed amorphous coating can provide a protective shell to improve the NCM thermal stability, suppress the thickening of the solid electrolyte interphase (SEI) layer, and scavenge HF in the electrolyte. The E-NCM composite with 2 mol % EPS delivers a high discharge capacity retention of 84.9% after 100 cycles at a 1 C discharge rate in the 2.8-4.3 V potential range at 55 °C. Moreover, electrochemical impedance spectroscopy measurements reveal that the EPS coating could alleviate the impedance rise during cycling especially at an elevated temperature. Therefore, the fabricated E-NCM cathode with long-term cycling and thermal stability is a promising candidate for use in a high-energy Li-ion battery.

A study of the structure–activity relationship of the electrochemical performance and Li/Ni mixing of lithium-rich materials by neutron diffraction

4 mol% Mg-doped and 1 mol% Al-doped lithium-rich 0.3Li₂MnO₃·0.7LiNi_0.5Mn_0.5O₂ materials exhibit enhanced electrochemical performance due to reduced Li/Ni mixing.