Structure and Charge Regulation Strategy Enabling Superior Cyclability for Ni-Rich Layered Cathode Materials

Molybdenum-Doped Cobalt-Free Cathode Realizing the Electrochemical Stability by Enhanced Covalent Bonding

The doping strategy effectively enhances the capacity and cycling stability of cobalt-free nickel-rich cathodes. Understanding the intrinsic contributions of dopants is of great importance to optimize the performances of cathodes. This study investigates the correlation between the structure modification and their performances of Mo-doped LiNi0.8Mn0.2O2 (NM82) cathode. The role of doped Mo's valence state has been proved functional in both lattice structural modification and electronic state adjustment. Although the high-valence of Mo at the cathode surface inevitably reduces Ni valence for electronic neutrality and thus causes ion mixing, the original Mo valence will influence its diffusion depth. Structural analyses reveal Mo doping leads to a mixed layer on the surface, where high-valence Mo forms a slender cation mixing layer, enhancing structural stability and Li-ion transport. In addition, it is found that the high-valence dopant of Mo6+ ions partially occupies the unfilled 4d orbitals, which may strengthen the Mo─O bond through increased covalency and therefore reduce the oxygen mobility. This results in an impressive capacity retention (90.0% after 200 cycles) for Mo-NM82 cathodes with a high Mo valence state. These findings underscore the valence effect of doping on layered oxide cathode performance, offering guidance for next-generation cathode development.

A Study on the Microstructure Regulation Effect of Niobium Doping on LiNi0.88Co0.05Mn0.07O2 and the Electrochemical Performance of the Composite Material under High Voltage

High-nickel ternary materials are currently the most promising lithium battery cathode materials due to their development and application potential. Nevertheless, these materials encounter challenges like cation mixing, lattice oxygen loss, interfacial reactions, and microcracks. These issues are exacerbated at high voltages, compromising their cyclic stability and safety. In this study, we successfully prepared Nb5+-doped high-nickel ternary cathode materials via a high-temperature solid-phase method. We investigated the impact of Nb5+ doping on the microstructure and electrochemical properties of LiNi0.88Co0.05Mn0.07O2 ternary cathode materials by varying the amount of Nb2O5 added. The experimental results suggest that Nb5+ doping does not alter the crystal structure but modifies the particle morphology, yielding radially distributed, elongated, rod-like structures. This morphology effectively mitigates the anisotropic volume changes during cycling, thereby bolstering the material's cyclic stability. The material exhibits a discharge capacity of 224.4 mAh g-1 at 0.1C and 200.3 mAh g-1 at 1C, within a voltage range of 2.7 V-4.5 V. Following 100 cycles at 1C, the capacity retention rate maintains a high level of 92.9%, highlighting the material's remarkable capacity retention and cyclic stability under high-voltage conditions. The enhancement of cyclic stability is primarily due to the synergistic effects caused by Nb5+ doping. Nb5+ modifies the particle morphology, thereby mitigating the formation of microcracks. The formation of high-energy Nb-O bonds prevents oxygen precipitation at high voltages, minimizes the irreversibility of the H2-H3 phase transition, and thereby enhances the stability of the composite material at high voltages.

Revealing the Nanoscopic Corrosive Degradation Mechanism of Nickel-Rich Layered Oxide Cathodes at Low State-of-Charge Levels: Corrosion Cracking and Pitting

Ni-rich layered oxides have received significant attention as promising cathode materials for Li-ion batteries due to their high reversible capacity. However, intergranular and intragranular cracks form at high state-of-charge (SOC) levels exceeding 4.2 V (vs. Li/Li+), representing a prominent failure mechanism of Ni-rich layered oxides. The nanoscale crack formation at high SOC levels is attributed to a significant volume change resulting from a phase transition between the H2 and H3 phases. Herein, in contrast to the electrochemical crack formation at high SOC levels, another mechanism of chemical crack and pit formation on a nanoscale is directly evidenced in fully lithiated Ni-rich layered oxides (low SOC levels). This mechanism is associated with intergranular stress corrosion cracking, driven by chemical corrosion at elevated temperatures. The nanoscopic chemical corrosion behavior of Ni-rich layered oxides during aging at elevated temperatures is investigated using high-resolution transmission electron microscopy, revealing that microcracks can develop through two distinct mechanisms: electrochemical cycling and chemical corrosion. Notably, chemical corrosion cracks can occur even in a fully discharged state (low SOC levels), whereas electrochemical cracks are observed only at high SOC levels. This finding provides a comprehensive understanding of the complex failure mechanisms of Ni-rich layered oxides and provides an opportunity to improve their electrochemical performance.

Why the Synthesis Affects Performance of Layered Transition Metal Oxide Cathode Materials for Li-Ion Batteries

The limited cyclability of high-specific-energy layered transition metal oxide (LiTMO2) cathode materials poses a significant challenge to the industrialization of batteries incorporating these materials. This limitation can be attributed to various factors, with the intrinsic behavior of the crystal structure during the cycle process being a key contributor. These factors include phase transition induced cracks, reduced Li active sites due to Li/Ni mixing, and slower Li+ migration. In addition, the presence of synthesis-induced heterogeneous phases and lattice defects cannot be disregarded as they also contribute to the degradation in performance. Therefore, gaining a profound understanding of the intricate relationship among material synthesis, structure, and performance is imperative for the development of LiTMO2. This paper highlights the pivotal role of structural play in LiTMO2 materials and provides a comprehensive overview of how various control factors influence the specific pathways of structural evolution during the synthesis process. In addition, it summarizes the scientific challenges associated with diverse modification approaches currently employed to address the cyclic failure of materials. The overarching goal is to provide readers with profound insights into the study of LiTMO2.

Structure and Charge Regulation Strategy Enabling Superior Cycling Stability of Ni-Rich Cathode Materials

Ni-rich layered oxides LiNixCoyMn1-x-yO2 (NCMs, x > 0.8) are the most promising cathode candidates for Li-ion batteries because of their superior specific capacity and cost affordability. Unfortunately, NCMs suffer from a series of formidable challenges such as structural instability and incompatibility with commonly used electrolytes, which seriously hamper their practical applications on a large scale. Herein, the Al/Ta codoping modification strategy is proposed to improve the performance of the LiNi0.83Co0.1Mn0.07O2 cathode, and the as-prepared Al/Ta-modified LiNi0.83Co0.1Mn0.07O2 delivers exceptional cycling stability with a capacity retention of 97.4% after 150 cycles at 1C and an excellent rate performance with a high capacity of 143.2 mAh g-1 even at 3C. Based on the experimental study, it is found that the structural stability of NCM is strengthened due to the regulated coordination of oxygen by introducing a robust Ta-O covalent bond, which prevents the layered structure from collapsing. Moreover, the reconstructed rock-salt-like surface is capable of effectively inhibiting interfacial side reactions as well as the overgrowth of the cathode-electrolyte interface. Theoretically, the energy of Li/Ni mixing is significantly increased with the introduction of Al and Ta elements in Al/Ta codoped NCM, leading to inhibited adverse phase transition during cycling. A feasible pathway for designing and developing advanced Ni-rich cathode materials for Li-ion batteries is provided in this work.

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.

How sintering temperature affects the electrochemical performance of ultra-high nickel (Ni > 0.9) cathode material

The burgeoning demand for electric vehicles with extended driving ranges has propelled ongoing development efforts for ultra-high nickel (Ni > 0.9) cathode materials. Despite significant ongoing research focused on Ni-rich cathode materials, a more comprehensive foundational understanding of ultra-high nickel cathode materials is essential. In our research, we employed LiNi0.94Co0.06O2 as a model ultra-high nickel cathode material to systematically delve into the interplay between sintering temperature, structural features, and electrochemical behavior. Within a sintering temperature spectrum of 660-720 °C, we discerned that specimens produced at diminished temperatures manifest a reduced initial discharge capacity yet excel in cycling endurance. In stark contrast, their counterparts produced at augmented temperatures behave inversely. Identifying a singular sintering temperature that achieves equilibrium between initial discharge capacity and cycling performance proves elusive. Through X-ray diffraction and high-resolution transmission electron microscopy, it became evident that samples synthesized at lower temperatures exhibit pronounced lithium-nickel mixing and develop a thicker NiO layer on the surface, leading to compromised initial discharge performance and capacity. Utilizing focused ion beam scanning electron microscopy, differential capacity analysis, and in-situ X-ray diffraction, we confirm that samples synthesized at lower temperatures possess smaller particle sizes, enabling them to withstand volumetric expansion stress during cycling, resulting in enhanced cycling performance. In the realm of ultra-high nickel cathode materials, elevating the sintering temperature is a conduit to superior initial discharge efficiency and capacity. Yet, the imperative of preserving diminutive particle dimensions, as a stratagem to bolster cycling performance, stands out as a pivotal research frontier.

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

Lithium-rich layered oxides (LLOs) are concerned as promising cathode materials for next-generation lithium-ion batteries due to their high reversible capacities (larger than 250 mA h g^-1 ). However, LLOs suffer from critical drawbacks, such as irreversible oxygen release, structural degradation, and poor reaction kinetics, which hinder their commercialization. Herein, the local electronic structure is tuned to improve the capacity energy density retention and rate performance of LLOs via gradient Ta⁵⁺ doping. As a result, the capacity retention elevates from 73% to above 93%, and the energy density rises from 65% to above 87% for LLO with modification at 1 C after 200 cycles. Besides, the discharge capacity for the Ta⁵⁺ doped LLO at 5 C is 155 mA h g^-1 , while it is only 122 mA h g^-1 for bare LLO. Theoretical calculations reveal that Ta⁵⁺ doping can effectively increase oxygen vacancy formation energy, thus guaranteeing the structure stability during the electrochemical process, and the density of states results indicate that the electronic conductivity of the LLOs can be boosted significantly at the same time. This strategy of gradient doping provides a new avenue to improve the electrochemical performance of the LLOs by modulating the local structure at the surface.

Integrated Surface Modulation of Ultrahigh Ni Cathode Materials for Improved Battery Performance

Ni-rich layered cathodes with ultrahigh nickel content (≥90%), for example LiNi_0.9 Co_0.1 O₂ (NC0.9), are promising for next-generation high-energy Li-ion batteries (LIBs), but face stability issues related to structural degradation and side reactions during the electrochemical process. Here, surface modulation is demonstrated by integrating a Li⁺ -conductive nanocoating and gradient lattice doping to stabilize the active cathode efficiently for extended cycles. Briefly, a wet-chemistry process is developed to deposit uniform ZrO(OH)₂ nanoshells around Ni_0.905 Co_0.095 (OH)₂ (NC0.9-OH) hydroxide precursors, followed by high temperature lithiation to create reinforced products featuring Zr doping in the crust lattice decorated with Li₂ ZrO₃ nanoparticles on the surface. It is identified that the Zr⁴⁺ infiltration reconstructed the surface lattice into favorable characters such as Li⁺ deficiency and Ni³⁺ reduction, which are effective to combat side reactions and suppress phase degradation and crack formation. This surface control is able to achieve an optimized balance between surface stabilization and charge transfer, resulting in an extraordinary capacity retention of 96.6% after 100 cycles at 1 C and an excellent rate capability of 148.8 mA h g^-1 at 10 C. This study highlights the critical importance of integrated surface modulation for high stability of cathode materials in next-generation LIBs.

Regulating Surface Oxygen Activity by Perovskite-Coating-Stabilized Ultrahigh-Nickel Layered Oxide Cathodes

Ultrahigh-Ni layered oxides are proposed as promising cathodes to fulfill the range demand of electric vehicles; yet, they are still haunted by compromised cyclability and thermal robustness. State-of-the-art surface coating has been applied to solve the instability via blocking the physical contact between the electrolyte and the highly active Ni⁴⁺ ions on the cathode surface, but it falls short in handling the chemo-physical mobility of the oxidized lattice oxygen ions in the cathode. Herein, a direct regulation strategy is proposed to accommodate the highly active anionic redox within the solid phase. By leveraging the stable oxygen vacancies/interstitials in a lithium and oxygen dual-ion conductor (layered perovskite La₄ NiLiO₈ ) coating layer, the reactivity of the surface lattice oxygen ion is dramatically restrained. As a result, the oxygen release from the lattice is suppressed, as well as the undesired irreversible phase transition and intergranular mechanical cracking. Meanwhile, the introduced dual-ion conductor can also facilitate lithium-ion diffusion kinetics and electronic conductivity on the particle surface. This work demonstrates that accommodating the anionic redox chemistry by dual-ion conductors is an effective strategy for capacity versus stability juggling of the high-energy cathodes.

Strain Engineering of Ni-Rich Cathode Enables Exceptional Cyclability in Pouch-Type Full Cells

Ni-rich layered oxides are at the forefront of the development of high-energy Li-ion batteries, yet the extensive applications are retarded by the deteriorative capacity and thermal instability. Herein, an in situ co-precipitation strategy is implemented to achieve the novel super-dispersed Nb-doped Ni-rich cathode that consists of the elongated and radially aligned primary particles with increased oxygen stable {001} planes. The unique microstructure homogenizes the intragranular and intergranular strain distribution and stabilizes the spherical secondary particles, effectively inhibiting microcrack formation and propagation and surface degradation. The super-dispersed Nb doping prevents the Li/Ni disordering and lattice oxygen escape, thereby further strengthening the crystal structure and thermal stability. Accordingly, this cathode delivers a high reversible capacity of 229.0 mAh g^-1 at 0.1 C with much better retention at 55 °C and 5 C after 100 cycles than the conventional Nb-doped Ni-rich cathodes. In a pouch-type full cell, it exhibits exceptionally long life with a capacity retention of 91.9% at 1 C after 500 cycles and 80.5% at 5 C after 2000 cycles within 3.0-4.2 V, greatly prolonging the service period to cater to the lightweight and intelligence of electric vehicles.

Integrated Modification Strategy Enables Remarkable Cyclability and Thermal Stability of Ni-Rich Cathode Materials for Lithium-Ion Batteries

Structural degradation and surface chemical instability are dominant issues of Ni-rich layered cathodes, which trigger capacity fading and safety concerns, hindering the extensive application of Ni-rich cathodes toward high-energy, long-life lithium-ion batteries. Here, by combining trace Ta doping and an ultrathin Zr-Y mixed oxide coating, an integrated modification strategy significantly improves the cycling and thermal stability of Ni-rich LiNi_0.88Co_0.10Al_0.02O₂ (NCA) cathodes. The integrated modified Ni-rich cathode provides an unprecedented comprehensive performance with a high discharge capacity of 212.2 mA h g^-1 at 0.1 C, an 88.6% cycling retention after 500 cycles at 1 C, and a high exothermic peak temperature of 261 °C compared with the pristine NCA cathode (67.4% capacity retention for 500 cycles and 221 °C for the exothermic peak). Further mechanism studies illustrate that a dual-structural surface constructed of a rock salt surface induced by Ta doping and ultrathin Zr-Y mixed oxide coating jointly suppresses surface side reactions between cathodes and electrolytes. Moreover, trace Ta doping in the bulk stabilizes the bulk structure and prevents mechanical cracks. This study highlights the importance of comprehensive modification of the bulk and surface for improving the electrochemical performance and provides a potential optimizing strategy for the commercialization of high-capacity Ni-rich cathode materials.

Mitigating the Surface Reconstruction of Ni-Rich Cathode via P2-Type Mn-Rich Oxide Coating for Durable Lithium Ion Batteries

Ni-rich materials have received widespread attention as one of the mainstream cathodes in high-energy-density lithium-ion batteries for electric vehicles. However, Ni-rich cathodes suffer from severe surface reconstruction in a high delithiation state, constraining their rate capabilities and life span. Herein, a novel P2-type Na_xNi_0.33Mn_0.67O₂ (NNMO) is rationally selected as the surficial modification layer for LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathode, which undergoes a spontaneous Na⁺-Li⁺ exchange reaction to form an O2-type Li_xNi_0.33Mn_0.67O₂ (LNMO) layer revealed by combining X-ray diffraction and solid-state nuclear magnetic resonance techniques. Owing to the specific oxygen stacking sequence, O2-type LNMO significantly prevents the initial layered structure of NCM811 from transforming to the spinel or rock-salt phases during cycling, thus effectively maintaining the integral surficial structure and the Li⁺ diffusion channels of NCM811. Eventually, the NNMO@NCM811 electrode yields enhanced thermal stability, outstanding rate performance, and long cycling stability with 80% capacity retention after 294 cycles at 200 mA g^-1, and its life span is further extended to 531 cycles while enhancing the mechanical stability of the bulk material.

Unblocking Oxygen Charge Compensation for Stabilized High-Voltage Structure in P2-Type Sodium-Ion Cathode

Layered transition-metal (TM) oxides are ideal hosts for Li⁺ charge carriers largely due to the occurrence of oxygen charge compensation that stabilizes the layered structure at high voltage. Hence, enabling charge compensation in sodium layered oxides is a fascinating task for extending the cycle life of sodium-ion batteries. Herein a Ti/Mg co-doping strategy for a model P2-Na_2/3 Ni_1/3 Mn_2/3 O₂ cathode material is put forward to activate charge compensation through highly hybridized O_{2 p} TM_{3 d} covalent bonds. In this way, the interlayer OO electrostatic repulsion is weakened upon deeply charging, which strongly affects the systematic total energy that transforms the striking P2-O2 interlayer contraction into a moderate solid-solution-type evolution. Accordingly, the cycling stability of the codoped cathode material is improved superiorly over the pristine sample. This study starts a perspective way of optimizing the sodium layered cathodes by rational structural design coupling electrochemical reactions, which can be extended to widespread battery researches.