Electrolyte Regulation in Stabilizing the Interface of a Cobalt-Free Layered Cathode for 4.8 V High-Voltage Lithium-Ion Batteries

The cobalt-free layered oxide cathode of LiNi0.65Mn0.35O2 is promising for high-energy-density lithium-ion batteries (LIBs). However, under high-voltage conditions, severe side reactions between the Co-free cathode and electrolyte, as well as grain boundary cracks and pulverization of particles, hinder its practical applications. Herein, an electrolyte regulation strategy is proposed by adding fluoroethylene carbonate (FEC) and LiPO2F2 as electrolyte additives in carbonate-based electrolytes to address the above issues. As a result, a homogeneous and dense organic-inorganic hybrid cathode electrolyte interface (CEI) film is formed on the cathode surface. The CEI film consists of C-F, LiF, Li2CO3, and LixPOyFz species, which is proven to be highly conductive and effective in suppressing structure damage and alleviating the interfacial reactions between the cathode and electrolyte. With such a CEI film, the interfacial stability of the Co-free cathode and the high-voltage cycling performance of Li||LiNi0.65Mn0.35O2 are greatly improved. A reversible capacity of 155.1 mAh g-1 and a capacity retention of 81.3% over 150 cycles are attained at a 4.8 V charge cutoff voltage with the tamed electrolyte, whereas the cell without the additives only retains 76.1% capacity retention. Therefore, our work demonstrates the synergistic effect of FEC and LiPO2F2 in stabilizing the interface of Co-free cathode materials and provides an alternative strategy for the electrolyte design of high-voltage LIBs.

Bonding State and Thermal Expansion Coefficient of Mn-Doped Ba0.5Sr0.5FeO3-δ Perovskite Oxides for IT-SOFCs

The oxygen vacancy formation behavior and electrochemical and thermal properties of Ba0.5Sr0.5Fe1-xMnxO3-δ (BSFMnx, x = 0-0.15) cathode materials were investigated. For thermogravimetric analysis, the weight decreased from 1.98% (x = 0) to 1.81% (x = 0.15) in the 400-950 °C range, which was due to oxygen loss from the lattice. The average oxidation state of the B-site increased, the Oads/Olat ratio decreased, and the binding energy of the Olat peak increased with Mn doping. These results indicate that Mn doping increases the strength of the metal-oxygen bond and decreases the amount of oxygen vacancies in the lattice. The electrical conductivity of BSFMnx increased with the temperature due to the thermally activated small-polaron hopping mechanism showing a maximum value of 10.4 S cm-1 (x = 0.15) at 450 °C. The area-specific resistance of BSFMn0.15 was 0.14 Ω cm2 at 700 °C and the thermal expansion coefficient (TEC) gradually decreased to 12.7 × 10-6 K-1, which is similar to that of Ce0.8Sm0.2O2 (SDC) (12.2 × 10-6 K-1). Mn doping increased the metal-oxygen bonding energy, which reduced the oxygen reduction reaction activity but improved the electrical conductivity and thermal stability with SDC.

Tuning the Phase Transition of SrFeO3-δ by Mn toward Enhanced Catalytic Activity and CO2 Resistance for the Oxygen Reduction Reaction

Developing high-performance cathodes with sufficient stability against CO₂ rooting in ambient atmosphere is crucial to realizing the practical application of solid-oxide fuel cells. Herein, the Mn dopant is investigated to regulate the phase structure and cathode performance of SrFeO_3-δ perovskites through partially replacing the B-site Fe. Compared with parent SrFeO_3-δ, Mn-doped materials, SrFe_1-xMn_xO_3-δ (x = 0.05 and 0.1), show stabilized cubic perovskites at room temperature. Meanwhile, doping Mn accelerates the oxygen reduction reaction process, showing a reduced polarization resistance of 0.155 Ω·cm² at 700 °C for SrFe_0.95Mn_0.05O_3-δ, which is less than 30% of SrFeO_3-δ. In addition, the Mn dopant improves the chemical oxygen surface exchange and bulk diffusion coefficients. Furthermore, Mn enhances the tolerance toward CO₂ corrosion in various CO₂ atmospheres. Density functional theory calculations also reveal that Mn can strengthen the structural stability and increase the activity for the oxygen reduction reaction.

Synthesis of LiNiO2 at Moderate Oxygen Pressure and Long-Term Cyclability in Lithium-Ion Full Cells

The widespread adoption of electric vehicles necessitates higher-energy-density and longer-life cathode materials for Li-ion batteries. LiNiO₂ offers a higher energy density at a lower cost than other high-Ni-content cathodes containing additional transition-metal ions. However, detrimental phase transformations and impedance growth, resulting from structural defects formed during synthesis, lead to poor cyclability and limit the practical viability of LiNiO₂. Herein, we demonstrate a considerably improved cycle life for LiNiO₂ by synthesizing it under a pressurized oxygen environment. The capacity retention in pouch-type full cells with a graphite anode after 1000 cycles is increased from 59 to 76% by applying a mere 1.7 atm of oxygen pressure during the synthesis of LiNiO₂. With iodometric titration and inductively coupled plasma optical emission spectroscopy analysis, we provide clear evidence that oxygen pressure during synthesis reduces the occurrence of lattice oxygen vacancies and increases the content of Ni³⁺ in LiNiO₂, improving its structural integrity and cyclability. Post-mortem analysis of the cycled cathodes provides insights into the sources of degradation occurring during long-term cycling. This work demonstrates a practically viable, synthetic approach combined with doping and coating to achieve improved performance with high-Ni layered oxide materials. Furthermore, this work represents the first report of extended cycling of LiNiO₂ in pouch full cells with graphite anode and will, therefore, serves as an important benchmark for future research on LiNiO₂.

Nanointegrated, High-Performing Cobalt-Free Bismuth-Based Composite Cathode for Low-Temperature Solid Oxide Fuel Cells

Cost-effective cathodes that actively catalyze the oxygen reduction reaction (ORR) are one of the major challenges for the technological advancement of low-temperature solid oxide fuel cells (LT-SOFCs). In particular, cobalt has been an essential element in electrocatalysts for efficiently catalyzing the ORR; nevertheless, the cost, safety, and stability issues of cobalt in cathode materials remain a severe drawback for SOFC development. Here, we demonstrated that by appropriate nanoengineering, we can overcome the inherent electrocatalytic advantages of cobalt-based cathodes to achieve comparable performance with a cobalt-free electrocatalyst on a bismuth-based fast oxygen ion-conducting scaffold that simultaneously enhances the performance and stability of LT-SOFCs. Consequently, the peak power density of the SOFCs reaches 1.2 W/cm² at 600 °C, highest performance of a cobalt-free-based cathode that has been ever reported. In addition, by the surface-protecting effect of covered nanoelectrocatalysts, the evaporation of highly volatile bismuth is greatly suppressed, resulting in an 80% improvement in performance durability, the best among all reported bismuth-based fuel cells.