A systematic study of the effect of acid concentration (0.5-5 M H 2 SO 4 ) on the electrolysis of electrolytic manganese dioxide (EMD or γ-MnO 2 ) has been performed. All the samples were characterized for their phase and crystallinity, their structural water, the pyrolusitic-to-ramsdellite ratio, BET surface area, pore size range; and were examined for their electrochemical performance. Concentration of the electrolysis acid affects the structural water content and the pyrolusitic-to-ramsdellite ratio of the γ-MnO 2 phase as well as the surface area and pore size. The best cycling performance was achieved for the samples prepared with an electrolysis bath containing an acid concentration of about 2 M H 2 SO 4 . Electrochemical performance over 100 cycles of the EMD prepared in 2 M H 2 SO 4 has shown higher energy efficiency and improved longer term cycling performance compared to commercial EMD products. Post-mortem analysis of the cycled EMD samples show partial loss of the EMD phase, loss of water content, and growth of irreversible manganese dioxide phases.
Lithium-ion batteries represent an emerging field. The development of battery materials could benefit from quick techniques that enable atomic-level diagnostics. High-performance cathodes, such as high-voltage spinel, often require coatings to protect against the destructive electrochemical environments at the particle–electrolyte interface. The preparations of these coatings are still in the early phases of development, and their analytical inspection by high-resolution scanning and transmission electron microscopy (HR-S/TEM) techniques presents a significant challenge due to the microscale dimensions of the cathode particles. In this work, a high-throughput ultramicrotome technique was assessed for the characterization of the particle–coating interface. The ultramicrotome technique enabled the rapid preparation of cross sections with a thickness of 126 ± 66 nm as determined by electron energy loss spectroscopy (EELS) measurements. Cathode particles composed of high-voltage spinel, LiNi0.5Mn1.5O4 (LNMO), coated with lithium niobate (LiNbO3) were synthesized, and cross sections were inspected using HR-S/TEM techniques. These ultrathin cross sections enabled the ability to obtain nanoscale information regarding the composition and crystallinity of the particle–coating interface over lateral areas of >1 μm. Accessible correlations between the electrochemical performance of the LiNbO3-coated LNMO particles and the HR-S/TEM results were enabled by the high-throughput method. Discharge capacity measurements were acquired over a series of 100 electrochemical cycles for both the LiNbO3 coated and the as-prepared LNMO particles. The limitations of the ultramicrotome technique are also discussed herein with respect to the coating morphology and the procedure for guidance toward technique optimization. The rapid preparation of ultrathin cross sections can assist the advancement of protective coatings on the surfaces of cathode particles for an efficient characterization of bulk–surface interfaces.
An increasing demand for clean and renewable energy provides motivation for additional innovations to be sought in energy storage. Lithium-ion batteries (LIBs) have seen many advancements in terms of their cathode chemistries, including electrode coatings that are used to increase durability and capacity of cathode materials. These coatings are effective, but create additional interfaces in the battery system, that adds to complexity in characterization. To address many key issues of LIB failures, further insights are necessary that provide details on currently unknown and partially known intermediate states, side reactions, and/or mechanisms of degradation at electrode interfaces. In this work, a method is developed to enable a detailed surface analysis and additional characterization of coated cathode particles harvested from post-mortem LIBs. Procedures are outlined for the safe handling and preparation of post-mortem materials for high-quality single particle electron microscopy analyses, paired with ultramicrotome techniques for high-throughput cross-sectional imaging with elemental and structural analyses of the coatings themselves. Commonly seen in post-mortem results, the materials are kept intact as whole electrodes or large collections of particles while still held within binders and other cathode slurry components from cell fabrication. These samples are often briefly rinsed with electrolyte solvents [e.g., most commonly dimethyl carbonate (DMC)] to rinse away residual lithium salts.1 While important insights have been gained from such studies, details of surface defects are difficult to discern amidst the debris of additives and binders. To address this, we developed a non-destructive procedure for the separation of cathodes particles from their supports, followed by washing to remove binders, additives, and residual electrolyte with relatively safe and accessible solvents. This method enables high-quality single particle imaging by scanning and transmission electron microscopy (SEM and TEM) techniques. Nano-scale features on the particle surfaces can be observed with high-resolution on the post-mortem cathode particles. With this procedure, insights can be gained about the retention of coatings after battery cycling, in addition to observing visible surface defects that result from charge cycling. Typical cross-sectional analyses of interfacial systems involve expensive, time-consuming methods like focused ion beam (FIB) milling that require extensive training, and are low-throughput when FIB lift-out samples are prepared for TEM. An alternative high-throughput technique for the cross-sectional analysis of coated cathode particles by ultramicrotome was previously developed in our group.2 This technique involves embedding coated cathode particles in epoxy, from which ultra-thin slices (<100 nm) that can contain many [approx. 50-200] particles at a time are cut with a diamond knife. The resultant sheets of epoxy are deposited onto TEM grids so the cross-sections can be imaged by TEM. These sections also enable the particle coatings to be characterized separately from the bulk cathode by electron dispersive X-ray spectroscopy (EDS) and selected area electron diffraction (SAED). This technique was demonstrated on as-synthesized (pre-electrochemistry) samples but was adapted here for the prepared post-mortem samples to assess degradation of the particles and their coatings. Combining the newly developed techniques for obtaining high-quality images of single whole-particles and particle cross-sections this work provides a safe, relatively inexpensive, and high-throughput methodology for the post-mortem characterization of LIB materials. This data can be correlated to as-synthesized materials and electrochemical cycling data to create a detailed profile of cathode aging and degradation as it relates to LIB failures. We are expanding this method to a variety of standard and coated cathode systems to provide detailed information needed for the next steps towards developing new innovations in electrode designs aimed at achieving more durable LIBs. References: 1) Xiong, R.; Pan, Y.; Shen, W.; Li, H.; Sun, F. Lithium-ion battery aging mechanisms and diagnosis method for automotive applications: Recent advances and perspectives. Renewable and Sustainable Energy Reviews. 2020, 131, 110048 2) Taylor, A. K.; Nesvaderani, F.; Ovens, J. S.; Campbell, S.; Gates, B. D. Enabling a High-Throughput Characterization of Microscale Interfaces within Coated Cathode Particles. ACS Appl. Energy Mater. 2021, 4, 9731−9741
In the present investigation, three composite coatings for high voltage spinels were prepared from Li3PO4:ZrO2 (with varying loadings of zirconia, ZrO2, and lithium phosphate, Li3PO4) using ball milling techniques. The coatings were prepared on high voltage spinel LNMO (LiNi0.5Mn1.5O4) using mechanical mixing, and the samples were heated at 600 °C to achieve a uniform, crystalline coating. A coating referred to as LZ75 (75 wt % zirconia and 25 wt % Li3PO4) exhibited the highest relative weight stability and a lower heat of reaction than other ratios of ZrO2 and Li3PO4 prepared for these custom coatings on LNMO. X-ray photoelectron spectroscopy indicated the presence of zirconia and phosphorous on the LNMO surfaces. Band gap studies indicated a decrease in the direct and indirect band gaps for the coatings with a higher ZrO2 content, which could enhance conductivity in the material. In contrast to this, the Urbach energy (E u) yielded a reverse trend and followed the order (LNMO) < (LZ25-coated LNMO) < (LZ50-coated LNMO) < (LZ75-coated LNMO). Electron microscopy studies indicated a change in morphology for the coated LNMO, especially for the LZ75-coated LNMO. Additional studies evaluated bare and coated LNMO for their stability in the presence of LiPF6 electrolyte. Scanning electron microscopy and X-ray diffraction analyses of the LNMO/LiPF6 interfaces for the coated and bare cathode materials indicated that the coating tends to suppress Mn dissolution in comparison to the uncoated LNMO. Additionally, more phases were seen in the LNMO/LiPF6 interface than for the coated LNMO materials. Electrochemical performance depicted an enhanced capacity retention of up to 88.5% for the LZ75-coated LNMO, compared to 69.2% for pristine LNMO after 100 cycles at 1 C (3–5 V vs Li/Li+). The lithium-ion diffusion coefficient (D Li+ ) also exhibited a significant increase in the coated cathodes along with a decrease in the charge transfer resistance and surface film resistance.
A systematic study of the effect of electrolysis acid concentration on the physical and electrochemical properties of electrolytic manganese dioxide (EMD) was conducted. High acid concentrations (> 2 M H2SO4), lead to multiphase production of manganese dioxide with poor electrochemical behavior; however, in low to mid-range acidic environments, superior EMD (compared to a commercial product) was obtained. With an electrolysis bath containing 1 M MnSO4 and 2 M H2SO4, the EMD produced had approximately twice the surface area (86.4 m 2 gEMD -1 ) and structural water content (4.4 %) of that of a commercial sample. The aforementioned synthesized EMD furthermore had an improved energy efficiency and an average specific capacity of approximately 65 mAh gEMD -1 after 100 cycles showing about 25 mAh gEMD -1 higher capacity compared to the commercial sample.
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