Li-rich Cathode Research Articles

Boosting Anionic Redox Reactions of Li-Rich Cathodes through Lattice Oxygen and Li-Ion Kinetics Modulation in Working All-Solid-State Batteries.

The use of lithium-rich manganese-based oxides (LRMOs) as the cathode in all-solid-state batteries (ASSBs) holds great potential for realizing high energy density over 600Wh kg-1. However, their implementation is significantly hindered by the sluggish kinetics and inferior reversibility of anionic redox reactions of oxygen in ASSBs. In this contribution, boron ions (B3+) doping and 3D Li2B4O7 (LBO) ionic networks construction are synchronously introduced into LRMO materials (LBO-LRMO) by mechanochemical and subsequent thermally driven diffusion method. Owing to the high binding energy of B─O and high-efficiency ionic networks of nanoscale LBO complex in cathode materials, the as-prepared LBO-LRMO displays highly reversible and activated anionic redox reactions in ASSBs. The designed LBO-LRMO interwoven structure enables robust phase and LBO-LRMO|solid electrolyte interface stability during cycling (over 80% capacity retention after 2000 cycles at 1.0 C with a voltage range of 2.2-4.7V vs Li/Li+). This contribution affords a fundamental understanding of the anionic redox reactions for LRMO in ASSBs and offers an effective strategy to realize highly activated and reversible oxygen redox reactions in LRMO-based ASSBs.

Towards High-Performance Li-Rich Cathode Materials: from Morphology Design to Electronic Structure Modulation.

Lithium-rich cathode materials (LRMs) have garnered significant interest owing to their high reversible discharge capacity (exceeding 250 mAh g⁻¹), which is attributed to the redox reactions of transition metal (TM) ions as well as the distinctive redox processes of oxygen anions. However, there are still many problems, such as their relatively poor rate performance and voltage fading and hysteresis, hindering their practical applications. Herein, the recent insights into the mechanisms and the latest advancements in the research of LRMs are discussed. Strategies to promote the performance of LRMs are discussed following a top-down approach from the morphology design to electronic structure modulation. Finally, the ongoing efforts in this area are also discussed to inspire more new ideas for the future development of LRMs.

Mitigating Li-Rich Layered Cathode Capacity Loss by Using a Siloxane Electrolyte Additive.

The instability of the electrode-electrolyte interface in high-voltage cathode materials significantly hinders the development of high-energy-density lithium-ion batteries (LIBs). In this study, 1,3-diphenyl-1,1,3,3-tetramethyldisiloxane (DTS) is employed as an electrolyte additive to enhance the cycling stability and capacity retention for Li||LLO (Li-rich layered oxide) batteries operating at 4.8 V. Theoretical calculations show that DTS can preferentially oxidize on the surface of the cathode. The oxidation forms a robust cathode electrolyte interface (CEI) on the LLO surface, significantly mitigating cracking, regeneration, and irreversible phase transitions of the LLO cathode. As anticipated, the Li||LLO batteries with the DTS electrolyte exhibit a capacity retention of 85.4% after 100 cycles at 4.8 V compared to the baseline electrolyte (45.2%). Furthermore, these batteries demonstrate superior capacity retention after 100 cycles at 4.8 V, even with the presence of 1000 ppm of H2O.

Simultaneous Regulating the Surface, Interface, and Bulk via Phosphating Modification for High-Performance Li-Rich Layered Oxides Cathodes.

Li-rich Mn-based layered oxides (LRMOs) are regarded as the leading cathode materials to overcome the bottleneck of higher energy density. Nevertheless, they encounter significant challenges, including voltage decay, poor cycle stability, and inferior rate performance, primarily due to irreversible oxygen release, transition metal dissolution, and sluggish transport kinetics. Moreover, traditionally single modification strategies do not adequately address these issues. Herein, an innovative "all-in-one" modification strategy is developed, simultaneously regulating the surface, interface, and bulk via an in-situ gas-solid interface phosphating reaction to create P-doped Li1.2Mn0.54Ni0.13Co0.13O2@Spinel@Li3PO4. Specifically, Li3PO4 surface coating layer shields particles from electrolyte corrosion and enhances Li+ diffusion; in-situ constructed spinel interfacial layer reduces phase distortion and suppresses the lattice strain; the strong P─O bond derived from P-doping stabilizes the lattice oxygen frame and inhibits the release of O2, thereby improving the reversibility of oxygen redox reaction. As a result, the phosphatized LRMO demonstrates an exceptional capacity retention of 82.1% at 1C after 300 cycles (compared to 50.8% for LRMO), an outstanding rate capability of 170.5 mAh g-1 at 5C (vs 98.9 mAh g-1 for LRMO), along with excellent voltage maintenance and thermostability. Clearly, this "all-in-one" modification strategy offers a novel approach for high-energy-density lithium-ion batteries.

Triple modifications of Li-rich manganese-based cathode materials using LiMnPO4 one-step method

Li-rich manganese-based layered oxide (LMR) is one of the most promising cathode materials for lithium batteries owing to its high specific capacity and low cost. Unfortunately, its commercial application is hindered by voltage decay, capacity diminishing, and poor rate performance during cycling. Thus, a facile surface triple modification method for Li1.2Ni0.2Mn0.6O2 has been proposed. This method constructs a protective LiMnPO4 coating layered, a spinel interface, and introduces some oxygen vacancies. The construction of the coating layer and oxygen vacancies alleviates interface side reactions and irreversible loss of O, ultimately suppressing voltage decay and improving cycling performance. The spinel phase constructs a 3D Li+ diffusion pathway, which improves the rate performance. As expected, electrochemical tests indicate that the materials coated by 3 wt% LiMnPO4 exhibit a mitigated voltage attenuation from 2.204 mV to 1.626 mV, an enhanced capacity retention of 85.60 % at 1C after 100 cycles, and a discharge capacity of 171.6 mAh·g−1 at 5C, which is better than that of bare materials (163.9 mAh·g−1 at 5C). This one-step processing method provides a simple and efficient method for modifying LMR materials.

Optimizing Oxygen Redox Activity by Local Chemical Disorder toward Robust Co-Free Li-Rich Cathode with High Voltage Stability.

Despite extensive investigation on the lattice oxygen redox (LOR) in Li-rich cathodes, significant challenges remain in utilizing LOR activity without compromising structural and electrochemical stability. Related breakthroughs are hindered by the lack of understanding regarding how different LOR activity influences the structural evolution and electrochemical stability, and what is the optimal LOR activity. Herein, the degree of LOR activity is successfully regulated from 22% to 92% in Co-free Li-rich cathodes (Li1+xMn0.62Ni0.18O2) by controlling local chemical disorder, and the relationship between LOR activity and cycling stability is revealed. Conventional consensus is challenged by new findings that the over-suppressive LOR activity also undermines electrochemical/structural stability, and even causes more severe voltage fading compared to cases with excessive LOR. However, their failure mechanisms related to lattice strain present different characteristics. Based on the established understanding, the appropriate LOR activation is necessary to balance the maximum reversible LOR activity and good stability, and the optimal degree is identified as 86% in Li1.18Mn0.62Ni0.18O2 (LR-78). LR-78 exhibits remarkable voltage retention of 96% after 400 cycles at 1 C, and superior high-rate cyclability without capacity decay within 600 cycles at 5 C. These findings significantly broaden the understanding of LOR mechanisms and provide critical guidance for designing durable LOR-based cathodes.

Quantitative Identification of Dopant Occupation in Li-Rich Cathodes.

Elemental doping is widely used to improve the performance of cathode materials in lithium-ion batteries. However, macroscopic/statistical investigation on how doping sites are distributed in the material lattice, despite being a key prerequisite for understanding and manipulating the doping effect, has not been effectively established. Herein, to solve this predicament, a universal strategy is proposed to quantitatively identify the locations of Al and Mg dopants in lithium-rich layered oxides (LLOs). Solid evidence confirms that Al prefers to occupy the transition metal (TM) layer, while Mg evenly occupies both TM and Li layers. As a result, Mg significantly reduces the thickness of LiO2 slabs at room temperature, which will increase the energy barrier of oxygen activation and enhance the structure stability of LLOs. The suppressed oxygen activity in Mg-doped LLO can be kinetically unlocked at 55°C. The different characteristics of Al and Mg enlighten an Al/Mg co-doping strategy to optimize LLOs, which significantly improves the cycle performance while lifting the capacity. These insights from the quantitative identification of doping sites shed light on the manipulation of doping effects toward better cathodes.

Enabling stable cobalt-free Li-rich cathodes through a one-step dual-modified strategy

Cobalt-free Lithium-rich layered oxides (LRLOs) are promising cathodes for low-cost and high-energy-density Li-ion batteries. However, their remarkable capacity comes with challenges including structural degradation, irreversible oxygen release and sluggish kinetics. Herein, we conduct a one-step dual-modified strategy by yttrium doping and Li3PO4 surface modification. Combining density-functional theory calculations with in-situ X-ray diffraction and in-situ differential electrochemical mass spectrometry, the Y3+ doping and Li3PO4 nano coating modified LRLOs is demonstrated has an excellent structural stability with enhanced Li⁺ diffusion kinetics and stabilized oxygen lattice. Excellent rate performance and thermal stability are achieved: high discharge specific capacity of 221 mAh·g−1 at room temperature (96.5 % at 1 C after 100 cycles) and incredible discharge specific capacity of 210 mAh·g−1 at 55 °C (91.8 % at 2 C after 100 cycles). This work resolves the safety and stability issues and provides a feasible strategy for Co-free LRLOs.

(Invited) Elucidating Redox Mechanisms in Battery Materials through Resonant Inelastic X-Ray Spectroscopy (RIXS)

Novel battery materials offer the promise of greatly increased energy densities for automative and other applications. However, their practical adoption is often limited by problems in electrochemical performance, such as poor rate capability, voltaic efficiency, and cyclability. Resonant Inelastic X-ray Spectroscopy (RIXS) has become a valuable tool for understanding the redox mechanisms in such materials, representing a first step towards solving problems in electrochemical performance. In many cases, however, the underlying origins of RIXS features are unclear. For example, redox mechanisms in Li-rich cathodes are associated with several absorption and emission features in oxygen RIXS measurements, but a clear picture of what redox mechanisms create these features remains elusive. As a step towards identifying redox mechanisms in Li-rich and related cathode materials, this presentation reviews existing theories of reaction mechanisms in these compounds, revisits experimental RIXS data on these materials and related oxides, and presents ab initio calculations evaluating possible origins of commonly observed features. Results suggest that some features could arise from molecular oxygen residing in low-symmetry environments. Additionally, prospects for using error-corrected quantum computation to more accurately model such electronic structure problems will be discussed.

Salt-Concentrated Electrolyte Constructing High Elasticity Modulus Interphase for Li-Rich Layered Oxide Cathode.

Stable electrolytes are urgently required for lithium-ion batteries based on lithium-rich layered oxides (LLOs), which generally suffer from fast capacity and voltage decay at high voltages up to 4.8 V. Herein, we report a salt-concentrated electrolyte consisting of 4 M lithium hexafluorophosphate (LiPF6) salt in ester solvents of fluoroethylene carbonate (FEC) and dimethyl carbonate (DMC) to alleviate the above challenges. The solvent structure in the 4 M electrolyte shows more volatile DMC integrated with Li+ and more free antioxidative FEC compared with a 1 M electrolyte, broadening the operation voltage. Simultaneously, this electrolyte endows a thin yet high elasticity modulus LiF-rich interphase on the LLOs surface, which can effectively prevent diverse side reactions and transition metal migration, consequently improving the electrochemical performance with a voltage decay of only 0.46 mV/cycle and capacity retention of 80.3% after 500 cycles. This simple and effective approach boosts the development of high-energy-density batteries using LLOs.

Toward High-Performance Li-Rich Mn-Based Layered Cathodes: A Review on Surface Modifications.

Lithium-rich manganese-based layered oxides (LRMOs) have received attention from both the academic and the industrial communities in recent years due to their high specific capacity (theoretical capacity ≥250 mAhg-1), low cost, and excellent processability. However, the large-scale applications of these materials still face unstable surface/interface structures, unsatisfactory cycling/rate performance, severe voltage decay, etc. Recently, solid evidence has shown that lattice oxygen in LRMOs easily moves and escapes from the particle surface, which inspires significant efforts on stabilizing the surface/interfacial structures of LRMOs. In this review, the main issues associated with the surface of LRMOs together with the recent advances in surface modifications are outlined. The critical role of outside-in surface decoration at both atomic and mesoscopic scales with an emphasis on surface coating, surface doping, surface structural reconstructions, and multiple-strategy co-modifications is discussed. Finally, the future development and commercialization of LRMOs are prospected. Looking forward, the optimal surface modifications of LRMOs may lead to a low-cost and sustainable next-generation high-performance battery technology.

Constructing Targeted Strong Nb4d-O2p-Li2s Configurations to Obtain Excellent Energy Density and Long Life for Li-Rich Cathodes.

The Li-rich Mn-based cathode materials (LMRs) deliver excellent energy density and exhibit low cost, which are considered as the most promising cathode materials for the next generation lithium-ion batteries. However, the irreversible redox reaction of the oxygen atoms directly leads to release oxygen and intensifies phase transformation. Besides, the local stress and strain will be generated due to the unit-cell volume difference between R-3m and C2/m phases, which continuously aggravates the collapse of secondary particles. Herein, the strong Nb4d-O2p-Li2s configurations at the Li1 sites of the TM-layer in the C2/m phase and secondary particles with the radial arrangement of refined primary particles are designed to inhibit oxygen release and relieve lattice stress by Nb2O5 treatment. Meanwhile, the preferential growth of the active {010} planes is presented to obtain an excellent transmission rate of Li+. As a result, the designed LMR delivers remarkable electrochemical properties with high discharge capacity and initial coulomb efficiency of 276 mAh g-1 and 85 % at 0.1 C, outstanding cycling retention rate of 81 % after 300 cycles. This novel crystal structure combining oxygen coordination regulation and micro-nano scale design provides inspiration for the design of high-performance LMRs.

Amorphous AlOCl Compounds Enabling Nanocrystalline LiCl with Abnormally High Ionic Conductivity.

LiCl is a promising solid electrolyte, providing it possesses high ionic conductivity. Numerous efforts have been made to enhance its ionic conductivity through aliovalent doping. However, aliovalent substitution changes the intrinsic structure of LiCl, compromising its cost-effectiveness and electrochemical stability. Here, we report nanocrystalline LiCl embedded in amorphous AlOCl compounds with a heterogeneous structure to enhance its ionic conductivity. Nanocrystallization enlarges the LiCl unit cell, while amorphization facilitates interfacial ion transport. As a result, the amorphous AlOCl-modified LiCl nanocrystal (AlOCl-nanoLiCl) demonstrates a high ionic conductivity of 1.02 mS cm-1, which is 5 orders of magnitude higher than that of LiCl. Additionally, it exhibits high oxidative stability, low cost ($19.87 US kg-1), and low Young's modulus (2-3 GPa). When AlOCl-nanoLiCl is coupled with Li-rich cathodes (Li1.17Mn0.55Ni0.24Co0.05O2, 4.8 V vs Li+/Li), all-solid-state batteries exhibit remarkable long-term cycling stability (>1000 cycles). This work presents a novel strategy to enhance the ionic conductivity of alkaline chlorides without compromising their intrinsic advantages.

Bifunctional modulation of Li-rich layered oxide cathode material via W-modification for improved electrochemical performance

Lithium-rich layered oxides (LRLOs) have raised the greatest attention to the high-energy-density Li-ion batteries. Nevertheless, the undesirable interfacial side reactions and the structure transformation cause fast voltage fading and poor capacity retention of LRLO cathodes, which extremely prevent their practical application. Surface coating or lattice doping has been an effective strategy to improve its electrochemical properties. However, the general performance of the material can only be limitedly promoted. Herein, we proposed a bifunctional modulation of LRLO cathodes coupled by bulk-phase W-doping and surficial Li2WO4 nanocoating of LRLOs using a facile synchronous lithiation strategy under a high-temperature solid-state treatment. The modified material (2 mol% W6+ modified) showed a distinguished discharge capability of 260.5 mAh/g at 0.1 C after 50 cycles. Meanwhile, a competitive capacity retention of 95.38% at 1 C after 100 cycles was achieved. The splendid electrochemical performance can be the result of the increased interlayer spacing for enhancing Li+ diffusion kinetics and the stronger W–O bond for alleviating oxygen release after W-doping and Li2WO4 nanocoating, which not only accelerates the Li+ transfer rate as its fast Li+ transfer channels but also act as a transitional interface layer to restrain the interfacial adverse reactions.

Enhancing the Reaction Kinetics and Stability of Co-Free Li-Rich Cathode Materials via a Multifunctional Strategy.

Co-free Li-rich layered oxides (CFLLOs) with anionic redox activity are among the most promising cathode materials for high-energy-density and low-cost lithium-ion batteries (LIBs). However, irreversible oxygen release often causes severe structural deterioration, electrolyte decomposition, and the formation of unstable cathode-electrolyte interface (CEI) film with high impedance. Additionally, the elimination of cobalt elements further deteriorates the reaction kinetics, leading to reduced capacity and poor rate performance. Here, a multifunctional strategy is proposed, incorporating Li2MnO3 phase content regulation, micro-nano structure design, and heteroatom substitution. The increased content of Li2MnO3 phase enhances the capacity through oxygen redox. The smaller nanoscale primary particles induce greater tensile strain and introduce more grain boundaries, thereby improving the reaction kinetics and reactivity, while the larger micron-sized secondary particles help to reduce interfacial side reactions. Furthermore, Na⁺ doping modulates the local coordination environment of oxygen, stabilizing both the anion framework and the crystal structure. As a result, the designed cathode exhibits enhanced rate performance, delivering a capacity of 158 mAh g⁻¹ at 5.0 C and improved cyclic stability, with a high capacity retention of 99% after 400 cycles at 1.0 C. This multifunctional strategy holds great promise for advancing the practical application of CFLLOs in next-generation LIBs.

Local electronic structure modulation via S substitution enables fast-discharging capability for Li-rich Mn-based oxides cathodes

Anionic and cationic redox reaction contribute to the ultrahigh specific capacities of Li-rich Mn-based oxides cathodes (LMNC). However, the irreversible oxygen evolution and sluggish kinetics result in the continuous capacity decay and poor rate performance, which limited its application as cathode material in lithium-ion batteries. Herein, the local electronic structure of LMNC is appropriately modulated via S2− doping to alleviate oxygen release, stabilize the crystal structure stability, and facilitate Li+ diffusion via facile surface defect engineering. The effect of S2− doping site on the systematically energy is investigated by DFT. Under the guidance of this result, LNMC-S is prepared by co-precipitation and high-temperature solid-phase reaction method. The effect of doping concentration on electrochemical property of LMNC was systematically investigated. The results exhibit that S2− doping can effectively suppress the electronic insulation of Li2MnO3, enhance the transport dynamics of lithium ion and inhibit the phase transition of LMNC sample during charge-discharge process. Thereby, the modified sample exhibits superior electrochemical property, such as the 200th discharge capacity is 184.3 mAh·g−1 under 1C, with a capacity retention rate of 87.6 %. This paper is beneficial for enriching the existing theoretical findings and discoveries, providing theoretical guidance for the preparation of high-performance material.

Bulk/Interfacial Structure Design of Li-Rich Mn-Based Cathodes for All-Solid-State Lithium Batteries.

Li-rich Mn-based cathode materials (LRMO) are promising for enhancing energy density of all-solid-state batteries (ASSBs). Nonetheless, the development of efficient Li+/e- pathways is hindered by the poor electrical conductivity of LRMO cathodes and their incompatible interfaces with solid electrolytes (SEs). Herein, we propose a strategy of in-situ bulk/interfacial structure design to construct fast and stable Li+/e- pathways by introducing Li2WO4, which reduces the energy barrier for Li+ migration and enhances the stability of the surface oxygen structure. The reversibility of oxygen redox was improved, and the voltage decay of the LRMO cathode was addressed significantly. As a result, the bulk structure of the LRMO cathodes and the high-voltage solid-solid interfacial stability are improved. Therefore, the ASSBs achieve a high areal capacity (∼3.15 mAh/cm2) and outstanding cycle stability of ≥1200 cycles with 84.1% capacity retention at 1 C at 25 °C. This study offers new insights into LRMO cathode design strategies for ASSBs, focusing on ultrastable high-voltage interfaces and high-loading composite electrodes.

High Energy Sulfide-Based All-Solid-State Lithium Batteries Enabled by Single-Crystal Li-Rich Cathodes

High Energy Sulfide-Based All-Solid-State Lithium Batteries Enabled by Single-Crystal Li-Rich Cathodes

Topological Protection of Oxygen Redox in Li-Rich Cathodes.

Lithium-rich layered oxides (LRLOs) are regarded as promising candidates for next-generation cathode materials because of their high energy density derived from anionic redox activity. Recent years have seen increasing efforts in promoting the cyclability of LRLO cathodes, at the core of which is the suppression of irreversible internal structural evolution during cycling. The present article aims to provide an informative perspective on the materials design strategies related to the issue of oxygen release. Emphasis is placed on the underlying chemistry of oxygen redox in LRLOs and the strategies based on material topology that can mitigate oxygen migration to the cathode surface. We speculate that these insights could guide researchers in developing high-capacity cathodes with intrinsically high reversibility of oxygen redox.

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