Improved Cycling Stability and Fast Charge-Discharge Performance of Cobalt-Free Lithium-Rich Oxides by Magnesium-Doping

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.

Unveiling the Roles of Trace Fe and F Co-doped into High-Ni Li-Rich Layered Oxides in Performance Improvement

High-Ni Li-rich layered oxides (HNLOs) derived from Li-rich Mn-based layered oxides (LRMLOs) can effectively mitigate the voltage decay of LRMLOs but normally suffered from decreased capacity and cycling stability. Herein, an effective, simple, and up-scalable co-doping strategy of trace Fe and F ions via a facile expanded graphite template-sacrificed approach was proposed for improving the performance of HNLOs. The trace Fe and F co-doping can far more effectively improve both its rate capability and cycling stability in a synergistic manner compared to the introduction of individual Fe cations and F anions. The co-doping of Fe and F increased the Li-O bonds by a magnitude far larger than the summation of the increments by their individual doping, quite favorable for the performance. The trace Fe doping can escalate the capacity and enhance the rate capability significantly by increasing the components of lower valence transition metals to activate their redox reactions more effectively and improving both the electronic and ionic conduction. In contrast, trace F can improve the cycling stability remarkably by lowering the O 2p band top to suppress the lattice oxygen escape effectively which were revealed by density functional theory calculations. The co-doped cathode exhibited excellent cycling stability with a superior capacity retention of 90% after 200 cycles at 1 C, much higher than 64% for the pristine sample. This study offers an idea for synergistically improving the performance of Li-rich layered oxides by co-doping trace Fe cations and F anions simultaneously, which play a complementary role in performance improvement.

Understanding and Control of Activation Process of Lithium-Rich Cathode Materials

Lithium-rich materials (LRMs) are among the most promising cathode materials toward next-generation Li-ion batteries due to their extraordinary specific capacity of over 250 mAh g−1 and high energy density of over 1 000 Wh kg−1. The superior capacity of LRMs originates from the activation process of the key active component Li2MnO3. This process can trigger reversible oxygen redox, providing extra charge for more Li-ion extraction. However, such an activation process is kinetically slow with complex phase transformations. To address these issues, tremendous effort has been made to explore the mechanism and origin of activation, yet there are still many controversies. Despite considerable strategies that have been proposed to improve the performance of LRMs, in-depth understanding of the relationship between the LRMs’ preparation and their activation process is limited. To inspire further research on LRMs, this article firstly systematically reviews the progress in mechanism studies and performance improving attempts. Then, guidelines for activation controlling strategies, including composition adjustment, elemental substitution and chemical treatment, are provided for the future design of Li-rich cathode materials. Based on these investigations, recommendations on Li-rich materials with precisely controlled Mn/Ni/Co composition, multi-elemental substitution and oxygen vacancy engineering are proposed for designing high-performance Li-rich cathode materials with fast and stable activation processes.

Graphical abstract

The “Troika” of composition adjustment, elemental substitution, and chemical treatment can drive the Li-rich cathode towards stabilized and accelerated activation.

Dilute Electrolyte to Mitigate Capacity Decay and Voltage Fading of Co-Free Li-Rich Cathode for Next-Generation Li-Ion Batteries

Li-rich cathodes have potential for use in next-generation Li-ion batteries (LIBs) owing to their high specific capacity and low cost. However, their intrinsic cycling decay and voltage fading limit practical applications. In addition, these cathodes contain Co, which is nonrenewable, scarce, and expensive. This situation severely limits the rapid and sustainable development of low-cost LIBs. This paper introduces a novel dilute electrolyte to overcome these limitations based on the Co-free Li-rich Li_1.2Mn_0.54Ni_0.26O₂ (LMNO) cathode. An even and robust cathode-electrolyte interface (CEI) formed on the surface of LMNO further protects it from side reactions in the dilute electrolyte. This Co-free Li-rich cathode exhibits the best electrochemical performance reported to date among Li-rich cathodes in terms of outstanding cycling stability (capacity retention of 99.8% at 0.5 C) and dramatically suppressed voltage fading (only 0.3% after 100 cycles). This study demonstrates the potential of Co-free Li-rich cathodes for applications in next-generation LIBs.

Highly reversible Li-ion full batteries using a Mg-doped Li-rich Li1.2Ni0.28Mn0.468Mg0.052O2 cathode and carbon-decorated Mn3O4 anode with hierarchical microsphere structures

Microsphere structured Mg-doped Li-rich Li1.2Ni0.28Mn0.468Mg0.052O2 cathode and carbon-decorated Mn3O4 anode materials were prepared for application to lithium-ion full batteries. As-assembled lithium-ion full batteries exhibited enhanced electrochemical performances like high charge/discharge capacity, and long-term capacity retention.

The use of a single-crystal nickel-rich layered NCM cathode for excellent cycle performance of lithium-ion batteries

With the continuous development and progress of new energy electric vehicles, high-capacity nickel-rich layered oxides are widely used in lithium-ion battery cathode materials, and their cycle performance and safety performance have also attracted more and more attention.

Surface Modification of the LiNi0.8Co0.1Mn0.1O2 Cathode Material by Coating with FePO4 with a Yolk-Shell Structure for Improved Electrochemical Performance

Coating with FePO₄ with the size of 20-30 nm on the surface of a LiNi_0.8Co_0.10Mn_0.1O₂ (NCM811) cathode produces an LFP3@NCM811 cathode via a sol-gel method, which markedly reduces secondary crystal cracking. A stable particle structure greatly improves the cycling stability of the LFP3@NCM811cathode, which retains 97% of its initial discharge capacity compared to NCM811 (78%) after 100 cycles at 2.7-4.5 V. Furthermore, it retains 86 and 63% of its initial discharge capacity after 400 cycles for LFP3@NCM811 and NCM811, respectively. The initial discharge capacity of the LFP3@NCM811 cathode is 218.8 mAh g^-1 at 0.1 C, and the discharge capacity of the LFP3@NCM811 cathode is achieved to be 151.4 mAh g^-1 at 5 C, which is 15 mAh g^-1 higher than that of the NCM811 cathode. These are due to the reduction of cation mixing for a certain amount of Fe²⁺/Fe³⁺ or PO₄^3- doped into the NCM811 surface, and the yolk-shell structure formed by coating with FePO₄ helps improve the electronic conductivity and accelerate the Li⁺ transport. The cycling stability is mainly due to the secondary cleavage inhibition, which maintains the structural integrity of the cathode particles during the long cycle process and protects the inside of the particle from harmful electrolytes.

Fluorophosphorus derivative forms a beneficial film on both electrodes of high voltage lithium-ion batteries

Layered lithium-rich oxides, as a series of highly promising cathode material for lithium-ion batteries, attract extensive attention due to their high specific capacity and high working potential (4.6 V vs Li/Li⁺). However, the poor interface stability of the cathode and electrolyte seriously restricts their practical application. In this article, theoretical calculations, linear sweep voltammetry and cyclic voltammetry results indicate that tris (pentafluorophenyl) phosphine (TPFPP) is a potential dual-functional electrolyte additive to solve interface problems. The TPFPP additive can decompose preferentially on the surface of both electrodes and form uniform and stable protective films, which effectively inhibit the continuous decomposition of the electrolyte and significantly alleviate the dissolution of transition metal ions during cycling. Owing to the above effects, the capacity retention and coulombic efficiency of Li_1.17Ni_0.25Mn_0.58O₂ (LLO)/graphite (Gr) cells are improved from 62.6% and 97.7% to 90.6% and 99.8% after 200 cycles at 0.3 C (1 C = 300 mA g^-1), respectively. This study provides a wide prospect for the application of lithium-rich materials in the future.

Integrated Surface Functionalization of Li-Rich Cathode Materials for Li-Ion Batteries

As candidates for high-energy density cathodes, lithium-rich (Li-rich) layered materials have attracted wide interest for next-generation Li-ion batteries. In this work, surface functionalization of a typical Li-rich material Li_1.2Mn_0.56Ni_0.17Co_0.07O₂ is optimized by fluorine (F)-doped Li₂SnO₃ coating layer and electrochemical performances are also enhanced accordingly. The results demonstrate that F-doped Li₂SnO₃-modified material exhibits the highest capacity retention (73% after 200 cycles), with approximately 1.2, 1.4, and 1.5 times of discharge capacity for Li₂SnO₃ surface-modified, F-doped, and pristine electrodes, respectively. To reveal the fundamental enhancement mechanism, intensive surface Li⁺ diffusion kinetics, postmortem structural characteristics, and aging tests are performed for four sample systems. The results show that the integrated coating layer plays an important role in addressing interface compatibility, not only limited in stabilizing the bulk structure and suppressing side reactions, synergistically contributing to the performance enhancement for the active electrodes. These findings not only pave the way to commercial application of the Li-rich material but also shed new light on surface modification in batteries and other energy storage fields.

Suppressing Voltage Decay of a Lithium-Rich Cathode Material by Surface Enrichment with Atomic Ruthenium

Lithium-rich layered oxides are promising cathode materials for high-energy-density lithium-ion batteries. However, the development of cathode materials based on these layered oxides has been limited by voltage fading, poor rate performance, and the low tap density of these materials. In this work, we prepared a material consisting of micrometer-scale spherical lithium-rich layered oxide particles with a three-dimensional conductivity network design and modified the surface of the primary particles with ruthenium. The as-obtained product with a maximum tap density of 2.1 g cm^-3 shows a superior high reversible capacity with 280 mA h·g^-1 at 0.1 C, a capacity retention of 98.1% after 100 cycles, and an outstanding rate capability. More importantly, enrichment of the primary particle surface with ruthenium can effectively suppress voltage decay. This cathode is feasible to construct high-energy and high-power lithium-ion batteries. This novel design may furthermore open the door to new material engineering applications for high-performance cathode materials.

Characterization and Control of Irreversible Reaction in Li-Rich Cathode during the Initial Charge Process

Li-rich layered oxide has been known to possess high specific capacity beyond the theoretical value from both charge compensation in transition metal and oxygen in the redox reaction. Although it could achieve higher reversible capacity due to the oxygen anion participating in electrochemical reaction, however, its use in energy storage systems has been limited. The reason is the irreversible oxygen reaction that occurs during the initial charge cycle, resulting in structural instability due to oxygen evolution and phase transition. To suppress the initial irreversible oxygen reaction, we introduced the surface-modified Li[Li_0.2Ni_0.16Mn_0.56Co_0.08]O₂ prepared by carbon coating (carbonization process), which was verified to have reduced oxygen reaction during the initial charge cycle. The electrochemical performance is improved by the synergic effects of the oxygen-deficient layer and carbon coating layer formed on the surface of particles. The sample with suitable carbon coating exhibited the highest structural stability, resulting in reduced capacity fading and voltage decay, which are attributed to the mitigated layered-to-spinel-like phase transition during prolonged cycling. The control over the oxygen reaction of Li₂MnO₃ by surface modification affects the activation reaction above 4.4 V in the initial charge cycle and structure changes during prolonged cycling. X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy analyses as well as electrochemical performance measurement were used to identify the correlation between reduced oxygen activity and structural changes.

Synthesis of Hierarchical Sisal-Like V2O5 with Exposed Stable {001} Facets as Long Life Cathode Materials for Advanced Lithium-Ion Batteries

Vanadium pentoxide (V₂O₅) is considered a promising cathode material for advanced lithium-ion batteries owing to its high specific capacity and low cost. However, the application of V₂O₅-based electrodes has been hindered because of their inferior conductivity, cycling stability, and power performance. Herein, hierarchical sisal-like V₂O₅ microstructures consisting of primary one-dimensional (1D) nanobelts with [001] facets orientation growth and rich oxygen vacancies are synthesized through a facile hydrothermal process using polyoxyethylene-20-cetyl-ether as the surface control agent, followed by calcination. The primary 1D nanobelt shortens the transfer path of electrons and ions, and the stable {001} facets could reduce the side reaction at the interface of electrode/electrolyte, simultaneously. Moreover, the formation of low valence state vanadium would generate the oxygen vacancies to facilitate lithium-ion diffusion. As a result, the sisal-like V₂O₅ manifests excellent electrochemical performances, including high specific capacity (297 mA h g^-1 at a current of 0.1 C) and robust cycling performance (capacity fading 0.06% per cycle). This work develops a controllable method to craft the hierarchical sisal-like V₂O₅ microstructures with excellent high rate and long-term cyclic stability.