A Gradient Doping Strategy toward Superior Electrochemical Performance for Li-Rich Mn-Based Cathode Materials

Remineralization constructs stable surfaces to enhance the cycling stability of Li-rich Mn-based cathode

Li-rich layered oxides (LRMs) cathode materials offer excellent initial energy density due to the contribution of cation/anion redox chemistry. However, during the cycling process, the irreversible phase transitions occurred in the materials lead to rapid degradation of voltage and capacity. To address this issue, we adopted a modification method by treating the surface of LRMs with carbon-fluorine surfactants, combined with high-temperature calcination to enhance the stability of the surface structure. The results indicate that the modified material exhibits improved cycle stability. After 300 cycles at 1C (1C = 250 mA g-1), the capacity retention rate reaches 91.7 %, and the voltage decay is effectively suppressed. The improved electrochemical performance is attributed to the formation of strong transition metal-fluorine (TM-F) bonds, which inhibit the TM migration and maintain the intact layered structure during cycling. This strategy provides a new path for efficient surface structure modification of high-energy-density cathodes.

Cation/Anion Co-Doping Enhances Oxygen Redox Reversibility and Structural Stability in Single-Crystal Li-Rich Mn-Based Cathodes for Wide-Temperature Performance

Li-rich Mn-based cathode materials (LRMs) are the most promising cathodes for the next-generation Lithium-ion batteries due to their high energy density. However, LRMs encounter formidable challenges such as voltage/capacity decay, mediocre rate capability, low cyclability, and substantial capacity loss at low temperatures. These challenges stem from irreversible oxygen release and subsequent structural deterioration. As energy storage devices are required to operate across a wide temperature range, enhancing the electrochemical performance of LRMs at both room and low temperatures is crucial. Herein, an approach of Al and F co-doping on novel single-crystal Li1.2Mn0.54Ni0.13Co0.13O2 is proposed to promote oxygen redox reversibility and enhance structural stability. Investigations into the oxygen redox couple and manganese electronic structure demonstrate that the Al and F co-doped electrode (LRMAF) retains a higher amount of lattice oxygen (O2⁻) and a greater amount of Mn⁴⁺ after cycling. As a result, LRMAF exhibits a high energy density of 1185 Wh kg-1, an initial discharge capacity of 329 mAh g⁻¹ at 0.1C, achieves a rate performance of 155 mAh g⁻¹ at 5.0C and delivers 88% capacity retention after 100 cycles. Additionally, LRMAF exhibits excellent electrochemical performance at -20 °C. This enhancement is attributed to the novel single-crystal morphology combined with cation/anion co-doping.

Mitigating Capacity and Voltage Decay in Li-Rich Cathode Via Dual-Phase Design

High-capacity O3-type lithium-rich manganese-based (LRM) materials exhibit significant structural instability and severe voltage decay, which limit their practical applications. In contrast, the O2-type LRM materials demonstrate remarkable structural stability despite offering lower capacity. In this study, a composite material, O3@O2-LRM is designed, by coating the main structure of O3-type LRM with a minor amount of O2-type LRM to combine the high capacity of the O3 phase with the superior stability of the O2 phase. Electrochemical tests demonstrate that O3@O2-LRM exhibits both high specific capacity and reduced voltage decay. Furthermore, a series of characterizations after different cycles confirm its enhanced structure stability compared to O3-LRM. This novel structure holds great promise for developing advanced cathode materials capable of meeting the demanding requirements of next-generation Li-ion batteries.

Fundamentals, Status and Promise of Li-Rich Layered Oxides for Energy-Dense Li-Ion Batteries

Li-ion batteries (LIBs) are the dominant electrochemical energy storage devices in the global society, in which cathode materials are the key components. As a requirement for higher energy-dense LIBs, Li-rich layered oxides (LLO) cathodes that can provide higher specific capacity are urgently needed. However, LLO still face several significant challenges before bringing these materials to market. In this Review, the fundamental understanding of LLO is described, with a focus on the physical structure-electrochemical property relationships. Specifically, the various strategies toward reversible anionic redox is discussed, highlighting the approaches that take the basic structure of the battery into account. In addition, the application for all-solid-state batteries and consider the prospects for LLO is assessed.

Revisiting the Roles of Foreign Magnesium Dopants in Governing the Properties of Li-Rich Mn-Based Oxide Cathodes for Li-ion Batteries

Heteroatoms doping would validly stabilize the structure of Li-rich material and elevate electrochemical performance. For the synthesized Li-rich material, the conventional ex situ doping strategy can only incorporate heteroatoms in the outer particles, diminishing benefits for the internal structure of secondary particles. Recently, in situ doping has been employed to influence topological lithiation and achieve well-distribution in the bulk structure. However, the mechanisms of in/ex situ doping on elevating electrochemical performance for Li-rich material are ambiguous. Herein, by introducing magnesium (Mg2+) ions on the precursor and synthesized Li-rich material, the in situ Mg doping (LRO-Mg1) and the ex situ Mg doping (LRO-Mg2) samples are successfully designed. Characterizations shows that Mg ion tend to incorporate into the Li-rich bulk structure for in situ doping, while they enrich in the surface for ex situ doping. Electrochemical measurement manifest that the in situ Mg doping Li-rich cathode exhibits more elevated high-rate performance, while the ex situ doping Li-rich cathode exhibits more elevated specific capacity, illuminating that enhances bulk structure of Li-rich cathode through in situ doping strategy is more effective than the surface modification (ex situ doping). Ex situ doping may be more effective for surface treatment, contributing to specific capacity improvement.

Stabilizing Layered Oxide Cathodes Based on Universal Surface Residual Alkali Conversion Chemistry for Rechargeable Secondary Batteries

Layered transition metal oxides (LTMOs) are attractive cathode candidates for rechargeable secondary batteries because of their high theoretical capacity. Unfortunately, LTMOs suffer from severe capacity attenuation, voltage decay, and sluggish kinetics, resulting from irreversible lattice oxygen evolution and unstable cathode-electrolyte interface. Besides, LTMOs accumulate surface residual alkali species, like hydroxides and carbonates, during synthesis, limiting their practical application. Herein, a universal strategy is suggested to in situ convert surface residual alkali into a stable polymer coating layer for LTMOs, thus turning wastes into treasure. The formation process of polymer coating involves NH4F treatment to consume residual alkali, then utilizing generated fluorides to induce the ring-opening polymerization of tetrahydrofuran. Implementing this strategy to Li-rich Mn-based cathode materials (LRM) results in a notable reduction in voltage hysteresis, along with enhanced kinetics and cycling stability in lithium-ion batteries. With this layer of encapsulation, surface lattice oxygen release and layered-to-spinel phase transition of LRM are significantly alleviated with minimal mechanical degradation and surface parasitic reactions. Such strategy can also be applied to air-sensitive sodium-rich LTMOs in sodium-ion batteries, which showcases superior universality. This work might provide a promising solution to overcome residual alkali and interfacial instability issues for LTMOs in practical application.

Lithium-Rich Layered Oxide Cathode Materials Modified for Lithium-Ion Batteries by CoS of a 3D Rock Salt Structure Assisted by PVP

The problem of rapid degradation of the operating voltage and discharge specific capacity of lithium-rich layered oxide (LRMs) cathode materials is a major constraint for their commercial application. In this paper, CoS coating with a 3D layered structure assisted by PVP is used to enhance the cycle life and rate performance of the LRMs material. The introduction of PVP has the following effects: (1) it reduces the solubility of CoS in the electrolyte solution and forms a stable CoS coating, and (2) it acts as a nitrogen-containing carbon matrix material and the heteroatomic dopant, and can provide more active sites to improve the conductivity of CoS. In addition, the CoS coating is capable of efficaciously reducing the direct contact area between electrolyte solution and the LRMs material and alleviating the occurrence of harmful interface reactions. The result of this study manifests that after the modification through CoS modification by PVP, the problem of the capacity decay is obviously solved. When the current density is 0.2 C, the highest specific capacity of 248.87 mAh g-1 can be provided. The capacity retention ratio of the LRMs@CoS material is 87.21% after 100 cycles, and the capacity decay is 0.3182 mAh g-1 (1.1534 mAh g-1 for the LRMs material) per cycle. When the current density is 1 C, the first discharge specific capacity of 220.91 mAh g-1 is achieved, which demonstrates outstanding electrochemical performance. This study has come up with a simple and practical mentality to realize the modification of cathode materials for high-performance lithium-ion batteries.

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 mAh g-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.

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.

Band Structure Engineering Promotes Anionic Redox Reversibility of Cobalt-Free Li-Rich Layered Oxides Cathodes

Li-rich layered oxides cathodes (LLOs) have prevailed as the promising high-energy-density cathode materials due to their distinctive anionic redox chemistry. However, uncontrollable anionic redox process usually leads to structural deterioration and electrochemical degradation. Herein, a Mo/Cl co-doping strategy is proposed to regulate the relative position of energy band for modulating the anionic redox chemistry and strengthening the structural stability of Co-free Li1.16Mn0.56Ni0.28O2 cathodes. The incorporation of Mo with high d state orbit and Cl with low electronegativity can narrow the band energy gap between bonding and antibonding bands via increasing the filled lower-Hubbard band (LHB) and decreasing the non-bonding O 2p energy bands, promoting the anionic redox reversibility. In addition, strong covalent Mo─O and Mn─Cl bonding further increases the covalency of Mn─O band to further stabilize the O2 n- species and enhance the reversible distortion of MnO6 octahedron. The strengthening electronic conductivity, together with the epitaxial structure Li2MoO4 facilitates the fast Li+ kinetics. As a result, the dual doping material exhibits enhanced anionic redox reversibility and suppressed oxygen release with increased cyclic stability and excellent rate performance. This strategy provides some guidance to design high-energy-density LLOs with desirable anionic redox reversibility and stable crystal structure via band structure engineering.

In Situ Surface Reaction for the Preparation of High-Performance Li-Rich Mn-Based Cathode Materials with Integrated Surface Functionalization

Li-rich Mn-based cathode materials (LLOs) are often faced with problems such as low initial Coulombic efficiency (ICE), limited rate performance, voltage decay, and structural instability. Addressing these problems with a single approach is challenging. To overcome these limitations, we developed an LLO with surface functionalization using a simple fabrication method. This two-step process involved a liquid-stage NaBF4 treatment followed by an in situ chemical reaction during sintering. This reaction led to the creation of oxygen vacancies (OV), spinel structures, and doping with Na at the Li site, B at the tetrahedral interstitial spaces of O in both the transition-metal (TM) layer and Li layers as well as the octahedral interstices in the TM layer, and F at the O site. We have carried out a thorough study and employed density functional theory calculations to reveal the hidden mechanisms. The treatment not only increases the electrical conductivity but also changes the oxygen charge environment and inhibits lattice oxygen activity. Surprisingly, the B-O bond is so strong that it prevents the migration of TM within the tetrahedral interstitial spaces of O in both the TM and Li layers, hence stabilizing its structure. This bonding interaction strengthens the transition of the TM 3d and O 2p states to lower energy levels, thus causing an increase in the redox potentials. Hence, a rise in the operating voltage occurs. Of special importance, this therapy dramatically increases the ICE to 90.29% and keeps a specified capacity of 203.3 mAh/g after 100 cycles at 1C, which is an excellent capacity retention of 89.94%. This study introduces ideas and methods to tackle the challenges associated with LLOs in batteries. It also provides compelling evidence for the development of high-energy-density Li-ion batteries.

Oxygen vacancy chemistry in oxide cathodes

Secondary batteries are a core technology for clean energy storage and conversion systems, to reduce environmental pollution and alleviate the energy crisis. Oxide cathodes play a vital role in revolutionizing battery technology due to their high capacity and voltage for oxide-based batteries. However, oxygen vacancies (OVs) are an essential type of defect that exist predominantly in both the bulk and surface regions of transition metal (TM) oxide batteries, and have a crucial impact on battery performance. This paper reviews previous studies from the past few decades that have investigated the intrinsic and anionic redox-mediated OVs in the field of secondary batteries. We focus on discussing the formation and evolution of these OVs from both thermodynamic and kinetic perspectives, as well as their impact on the thermodynamic and kinetic properties of oxide cathodes. Finally, we offer insights into the utilization of OVs to enhance the energy density and lifespan of batteries. We expect that this review will advance our understanding of the role of OVs and subsequently boost the development of high-performance electrode materials for next-generation energy storage devices.

High-Performance Single-Crystal Lithium-Rich Layered Oxides Cathode Materials via Na2WO4-Assisted Sintering

Single-crystal lithium-rich layered oxides (LLOs) with excellent mechanical properties can enhance their crystal structure stability. However, the conventional methods for preparing single-crystal LLOs, require large amounts of molten salt additives, involve complicated washing steps, and increase the difficulty of large-scale production. In this study, a sodium tungstate (Na2WO4)-assisted sintering method is proposed to fabricate high-performance single-crystal LLOs cathode materials without large amounts of additives and additional washing steps. During the sintering process, Na2WO4 promotes particle growth and forms a protective coating on the surface of LLOs particles, effectively suppressing the side reactions at the cathode/electrolyte interface. Additionally, trace amounts of Na and W atoms are doped into the LLOs lattice via gradient doping. Experimental results and theoretical calculations indicate that Na and W doping stabilizes the crystal structure and enhances the Li+ ions diffusion rate. The prepared single-crystal LLOs exhibit outstanding capacity retention of 82.7% (compared to 65.0%, after 200 cycles at 1 C) and a low voltage decay rate of 0.76 mV per cycle (compared to 1.80 mV per cycle). This strategy provides a novel pathway for designing the next-generation high-performance cathode materials for Lithium-ion batteries (LIBs).

Enhancing the Stability of 4.6 V LiCoO2 Cathode Material via Gradient Doping

LiCoO2 (LCO) can deliver ultrahigh discharge capacities as a cathode material for Li-ion batteries when the charging voltage reaches 4.6 V. However, establishing a stable LCO cathode at a high cut-off voltage is a challenge in terms of bulk and surface structural transformation. O2 release, irreversible structural transformation, and interfacial side reactions cause LCO to experience severe capacity degradation and safety problems. To solve these issues, a strategy of gradient Ta doping is proposed to stabilize LCO against structural degradation. Additionally, Ta1-LCO that was tuned with 1.0 mol% Ta doping demonstrated outstanding cycling stability and rate performance. This effect was explained by the strong Ta-O bonds maintaining the lattice oxygen and the increased interlayer spacing enhancing Li+ conductivity. This work offers a practical method for high-energy Li-ion battery cathode material stabilization through the gradient doping of high-valence elements.

Dual Strategies with Anion/Cation Co-Doping and Lithium Carbonate Coating to Enhance the Electrochemical Performance of Lithium-Rich Layered Oxides

Lithium-rich layered oxides (LLOs, Li1.2 Mn0.54 Ni0.13 Co0.13 O2 ) are widely used as cathode materials for lithium-ion batteries due to its high specific capacity, high operating voltage and low cost. However, the LLOs are faced with rapid decay of charge/discharge capacity and voltage, as well as interface side reactions, which limit its electrochemical performance. Herein, the dual strategies of sulfite/sodium ion co-doping and lithium carbonate coating were used to improve it. It founds that modified LLOs achieve 88.74 % initial coulomb efficiency, 295.3 mAh g-1 first turn discharge capacity, in addition to 216.9 mAh g-1 at 1 C, and 87.23 % capacity retention after 100 cycles. Mechanism research indicated that the excellent electrochemical performance benefits from the doping of both Na+ and SO3 2- , and it could significantly reduce the migration energy barrier of Li+ and promote Li+ migration. Meanwhile, anion and cation are co-doped greatly reduces the band gap of LLOs and increase its electrical conductivity, and its binding effect on Li+ is weakened, making it easier for Li+ to shuttle through the material. In addition, the lithium carbonate coating significantly inhibits the occurrence of interfacial side reactions of LLOs. This work provides a theoretical basis and practical guidance for the further development of LLOs with higher electrochemical performance.

Atomic Manufacturing in Electrode Materials for High-Performance Batteries

The advancement of electrode materials plays a pivotal role in enhancing the performance of energy storage devices, thereby meeting the escalating need for energy storage and aligning with the imperative of sustainable development. Atomic manufacturing enables the precise manipulation of the crystal structure at the atomic level, thereby facilitating the development of electrode materials with customized physicochemical properties and enhancing their performance. In this Perspective, we elaborate on how atomic manufacturing enhances the important properties of electrode materials. Finally, we anticipate the prospect of materials and fabrication methods for atomic manufacturing in the future. This Perspective provides a comprehensive understanding for atomic manufacturing in electrode materials.

Improved Electrochemical Performance of Li-Rich Cathode Materials via Spinel Li2MoO4 Coating

Li-rich manganese-based cathode materials (LRMs) are considered one of the most promising cathode materials for the next generation of lithium-ion batteries (LIBs) because of their high energy density. However, there are problems such as a capacity decay, poor rate performance, and continuous voltage drop, which seriously limit their large-scale commercial applications. In this work, Li1.2Mn0.54Co0.13Ni0.13O2 coated with Li2MoO4 with a unique spinel structure was prepared with the wet chemistry method and the subsequent calcination process. The Li2MoO4 coating layer with a spinel structure could provide a 3D Li+ transport channel, which is beneficial for improving rate performance, while protecting LRMs from electrolyte corrosion, suppressing interface side reactions, and improving cycling stability. The capacity retention rate of LRMs coated with 3 wt% Li2MoO4 increased from 69.25% to 81.85% after 100 cycles at 1 C, and the voltage attenuation decreased from 7.06 to 4.98 mV per cycle. The lower Rct also exhibited an improved rate performance. The results indicate that the Li2MoO4 coating effectively improves the cyclic stability and electrochemical performance of LRMs.