No abstract

The impact of lithium bis(oxalate)borate (LiBOB) electrolyte additive on the performance of full lithium-ion cells pairing the high-voltage spinel cathode with the graphite anode was systematically investigated. Adding 1 wt % LiBOB to the electrolyte significantly improved the cycle life and Coulombic efficiency of the full-cells at 30 and 45 °C. As the LiBOB was preferentially oxidized and reduced compared with LiBOB-free electrolyte during cycling, their relative contributions to the improved capacity retention in full-cells was gauged by pairing fresh and LiBOB-treated electrodes with various combinations. The results indicated that a solid–electrolyte interphase (SEI) film on graphite produced by the reduction of the LiBOB additive is more robust and stable against Mn dissolution problem during cycling at 45 °C compared with the SEI formed by the reduction of the base (LiBOB-free) electrolyte. In addition, a 3 wt % LiBOB-added electrolyte showed reduced Mn dissolution compared with the base electrolyte after storing the fully charged Li1–x Ni0.42Fe0.08Mn1.5O4 (LNFMO) electrodes at 60 °C for one month. It is believed that LiBOB aids in stabilizing the electrolyte by trapping the PF5, i.e., sequestering the radical which tends to oxidize EC and DEC electrolyte solvents. Thus, oxidation is suppressed on the carbon black particles in the positive electrode, as evidenced by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR) analyses. As a result, HF generation is suppressed, which in turn results in less Mn dissolution from the spinel cathode.

In order to achieve consistent electrochemical properties essential for the commercialization of the high-voltage spinel cathode LiMn 1.5 Ni 0.5 O 4 , a deeper fundamental understanding of the factors contributing to capacity fade is required. Specifically, the relationship between cation ordering, impurity phases present, and particle morphology must be elucidated. We present here a comparison of stoichiometric LiMn 1.5 Ni 0.5 O 4 cathodes with a 3:1 Mn/Ni ratio prepared by different methods with varying morphologies and degrees of cation ordering. Careful structural, chemical, and electrochemical characterizations illuminate the relative influence of the various factors on the electrochemical cycling stability and high-rate performance. It is found that although an increase in the degree of cation ordering decreases the rate capability, the crystallographic planes in contact with the electrolyte have a dominant effect on the electrochemical properties.

High-voltage spinel cathodes LiMn 1.5 Ni 0.5 O 4 are promising candidates for large-scale energy-storage applications such as electric vehicles. However, the widespread adoption of this high-voltage spinel cathode is hampered by severe capacity fade, particularly at elevated temperatures, resulting from aggressive formation of a thick solid-electrolyte interphase (SEI) layer through side reactions with the electrolyte at the high operating voltage, cationic ordering between Mn 4+ and Ni 2+ ions in the crystal lattice, and formation of a rock salt Li x Ni 1Àx O impurity phase. While these issues have been explored, the wide variation in physical and electrochemical properties with different synthesis methods is not fully understood. In this investigation, we present how the synthesis conditions of the co-precipitation method influence the microstructure and morphology through nucleation and growth of crystals in solution. The samples were prepared by two similar wet-chemical routes and were characterized by microscopy and electrochemical methods to determine the role of microstructure and morphology in the electrochemical performance. Various factors such as the degree of cation ordering between Mn 4+ and Ni 2+ , Mn 3+ content, Ni-Mn ratio in the sample, change in lattice parameter with the state of charge, and surface crystal planes were examined to develop a better understanding of the factors influencing the electrochemical performance. It is found that the surface crystal planes, the arrangement of lithium ions near the surface, and the lithium diffusion mechanism have a dominant effect on the capacity retention and rate performance.

The electrocatalytic activity of the spinel oxide LiMn 1.5 Ni 0.5 O 4 with different morphologies (cubic, spherical, octahedral, and truncated octahedral) has been investigated for the oxygen evolution reaction (OER) in alkaline solutions that is of interest for metal−air batteries. The OER activity increases in the order truncated octahedral < cubic < spherical < octahedral, despite a larger surface area (2.9 m 2 g −1 ) for the spherical sample compared to nearly similar surface areas (0.3−0.7 m 2 g −1 ) for the other three samples. The high activity of the octahedral sample is attributed to the regular octahedral shape with low-energy {111} surface planes, whereas the lowest activity of the truncated octahedral sample is attributed to the high-energy {001} surface planes. The octahedral sample also exhibits the lowest Tafel slope of 70 mV dec −1 with the highest durability whereas the truncated octahedral sample exhibits the highest Tafel slope of 120 mV dec −1 with durability similar to the cubic and spherical samples. The study demonstrates that the catalytic activities of oxide catalysts could be tuned and optimized by controlling the surface morphologies/planes via novel synthesis approaches.