Pengfei Yan

Increasing Ni content in layered lithium transition-metal oxides can boost energy density and reduce the cost of lithium-ion batteries, but its drawbacks on cycling stability and air stability remain as critical challenges. Herein, the two issues are investigated based on a Ni-rich layered cathode, and air instability-induced cycling degradations are revealed by diverse characterization techniques. We show that both single-crystalline and polycrystalline LiNi_0.8Co_0.1Mn_0.1O₂ (PC-NCM811) suffer similar surface degradations during air storage, while additional intergranular modification is revealed for PC-NCM811. Air degradation-induced Li₂CO₃ impurity is formed not only on the particle surface but also at the intergranular space of PC-NCM811. Intergranular Li₂CO₃ shows a bonding effect, leading to increased initial mechanical strength measured by an in situ nanoindenter. However, Li₂CO₃ impurity is verified to be very detrimental to cycling stability, which not only incurs unwanted interfacial degradations but also intergranular cracks, contributing to capacity fade during cycling. This work highlights that air storage of polycrystalline layered cathode can cause severe mechanical cracking degradations during subsequent battery cycling, which offers new insights into the air degradation mechanisms. Therefore, improving the air storage stability of polycrystalline Ni-rich layered cathode materials requires greater attention to the stabilization of the intergranular sites.

Layered O3–NaNi_1/3Fe_1/3Mn_1/3O₂ (NMF) is a promising cathode material for commercialization of sodium-ion batteries (SIBs), but it still faces challenges due to its poor cycling stability and air instability. Herein, we demonstrate that through optimized synthesis conditions (sintering and composition), a single Ca-dopant additive can synergistically achieve bulk-doping and surface-modification effects. Microscopic characterizations reveal that a robust rock-salt structure is constructed on the non-(003) surface facets due to Ca surface enrichment, which is developed through the Ca dopant exsolution process during high-temperature sintering. Combining bulk-doping and surface-modification effects, the 3% Ca-doped NFM cathode demonstrates superb cycling stability and improved rate capability, boosting the capacity retention from 61.4% to 90.5% after 200 cycles at 2.0–4.0 V. Further microanalysis reveals that the surface Ca-enriched modification layer not only effectively suppresses the cycling-induced surface degradation but also significantly improves the air storage stability by acting as a physical barrier layer to prevent moisture degradation. This work offers a new pathway to achieve bulk and surface modifications by a simple doping strategy and reveals the dual effects of Ca-doping for enhanced performance of O3-type layered oxide cathodes.

At the operating temperature (∼750°C) of solid oxide cells (SOCs), Ni diffusion has been revealed to cause aging degradations on catalytic performance, electronic conductivity, and mechanical failures. This work discloses that Ni diffusion during the high‐temperature (∼1400°C) SOC fabrication process can severely decrease the oxide ion conductivity due to Ni segregation (up to ∼7 at.%) at the YSZ (yttria‐stabilized zirconia) grain boundaries (GBs). Combining electrochemical tests and advanced electron microscopy, we reveal that higher Ni enrichment leads to thicker space charge layer and higher space charge potential, which generates a significant GB blocking effect for oxide ion diffusion. We have quantitatively estimated the ionic conductivity drop induced by Ni segregation at the operation temperature range. Utilizing the ultrafast high‐temperature sintering technique, we successfully mitigate Ni segregation at GBs, which can double the ionic conductivity at 700°C. This work not only clarifies that Ni segregation at YSZ GBs can significantly plague the ionic conductivity but also demonstrates that mitigating Ni segregation at YSZ GBs is a new avenue to reduce the cell's ohmic resistance and boost the SOC performance.

Conventional disassembly of a lithium–ion battery leads to a cathode self‐discharge issue, which decreases the original state of charge (SOC) and significantly changes its original lattice structure and phase distribution. Herein, to prevent the self‐discharge issue, a cryo‐sampling method is introduced, which enables the obtaining of high SOC layered LiCoO ₂ (LCO) and to directly visualize the H1‐3 and O1 phases by aberration‐corrected transmission electron microscopy. The most critical discovery is that Co migration into the vacant Li‐layers is an energy‐favored process in highly delithiated H1‐3 and O1 phases, which is the root‐cause of interlayer mixing degradation of layered structure. It is first clarified that phase transitions between O3 and H1‐3/O1 phases not only involves (003) in‐plane gliding but also out‐of‐plane Co ion migration, and the latter one has poor reversibility, leading to inevitable degradation of the layered structure after repeated cycles. The atomistic process of microcrack nucleation and spinel domain formation are revealed based on Co migration, which well explains the fast performance decay of a commercial LCO after 4.65 V cycling. The unique crystal structures of H1‐3/O1 phases along with their atomistic phase transition mechanisms revealed by this work deeply refresh the fundamental understanding of the layer structured cathode materials.

Intrinsic and cycling‐induced defects in cathode materials of lithium‐ion battery are revealed to play conflicting roles in either boosting or deteriorating the electrochemical performance. However, the impact of mechanical defects introduced during the battery manufacturing process is not systematically investigated, nor is their evolution upon electrochemical cycling. Herein, taking high‐energy LiCoO ₂ cathode material as an example, it is shown that mechanical defects can be easily generated during the industrial manufacturing process. Electron microscopy characterizations reveal five major mechanical defects: slip, dislocation, lattice distortion, kink boundary, and crack, which are proven detrimental to cycling stability and rate capability. Upon low‐voltage cycling, aggravated interfacial degradations are the major cause of the fast performance decay, which is primarily due to crack propagation and consequential cathode electrolyte interphase (CEI) growth, leading to high interfacial polarization and reduced interfacial reaction kinetics. Upon high voltage cycling, all kinds of mechanical defects evolve significantly, which together lead to severe structural degradations by forming a degraded spinel structure, voids, and microcracks. This work highlights that manufacturing process‐induced mechanical defects can significantly aggravate material degradation during subsequent electrochemical cycling, suggesting that minimizing pre‐cycling mechanical defects and enhancing mechanical strength can effectively improve cathode material performance.

Advancements in structural engineering are expected to enhance the wide-range commercial application of lithium-ion batteries by enabling the implementation of thicker cathode materials. Increasing the thickness of these cathodes can yield significant increasements in gravimetric energy density while concurrently lowering manufacturing costs. These improvements are pivotal to the successful commercial deployment of sustainable transport systems. However, several substantial barriers persist in the adoption of such microstructures, including performance degradation, manufacturing complexities, and scalability concerns, all of which remain open areas of investigation. This review delves into the obstacles associated with current modifying techniques in thick cathodes and explores the potential opportunities to develop more robust and thicker cathodes, while ensuring long-term performance and scalability. Finally, we provide suggestions on the future directions of thick cathodes to promote their large-scale application.

Cathode interfacial electron/Li-ion transportation is equally important to realize high-rate performance and high cycling stability of lithium-ion batteries; thus, cathode surface engineering and cathode electrolyte interphase (CEI) regulation have gained tremendous research interest. Herein, to balance and optimize the interfacial electron/Li-ion transfer, we propose a novel design strategy to construct the respective conduction pathway for interfacial electron transfer or Li-ion transfer, which is realized by anchoring inert and electron-conductive island particles on an LiCoO₂ particle surface. Such an interfacial design can realize stable interfacial electron conductivity through the CEI-free island particles, thereby significantly enhancing the overall interfacial charge transfer kinetics and cycling stability. Furthermore, combining the surface Ti-doping to form a thin and stable CEI layer for fast Li-ion transfer, this work demonstrates that surface doping and island particle anchoring can work synergistically to achieve high-rate and high-stability cycling performance. Full cell cycling stability reaches 97% at a rate of 2C after 200 cycles. The proposed interfacial design strategy and our successful demonstration together emphasize that maintaining high interfacial conductivity for electrons and Li-ions is equally important to realize a robust cathode interface.

The unstable electrolyte–electrode interface and the trace H₂O in commercial organic electrolytes critically limit the cycling life of batteries. Herein, a locking-chain sodium 4,4′-(1,4-phenylenebis(oxy))-bis(butane-1-sulfonate)−15-crown-5 (15PBS) is designed for phase-to-interface electrolyte optimization. In the electrolyte phase, the strong hydrophilic sulfonate groups and 15-crown-5 in 15PBS effectively transform H₂O from a reactive aggregated state (strong H-bond) into an inactive state (weak H-bond) through adsorption, effectively suppressing H₂O-induced electrolyte decomposition. At the electrolyte–electrode interface, 15PBS preferentially adsorbed onto hard carbon, displacing solvents within the electric double layer to form insoluble phenyl-rich sulfide solid electrolyte interphase with fast Na⁺ transport. Simultaneously, 15PBS facilitates the formation of stable cathode-electrolyte interphase on Na_0.72Ni_0.32Mn_0.68O₂, improving Na⁺ migration kinetics and cycling reversibility. The hard carbon | |Na_0.72Ni_0.32Mn_0.68O₂ full cell with high specific energy of 191.7 Wh kg⁻¹ (based on the total active-material mass) delivers long lifespan of 2000 cycles at 500 mA g⁻¹. Moreover, 15PBS is compatible with ester-based electrolytes in lithium-ion batteries, enabling stable cycling of commercial graphite and Si/C negative electrodes. This work provides an effective approach for durable electrolytes towards safe and high-performance batteries.

A transient passivation effect was observed in single-crystal high-nickel cathode materials upon short-term exposure to high temperature and high humidity, followed by rapid performance deterioration upon prolonged storage.