This item is under embargo and not available online per the author's request. For access information, please visit http://libanswers.wustl.edu/faq/5640.
Date of Award
Doctor of Philosophy (PhD)
Energy storage in the 21st century has become one of the most critical requirements to maintain sustainable development and a growing global economy. Today, the advancement of lithium-ion batteries is being taken to the next level with targeted applications being electric vehicles (EVs) and grid storage. Current widespread application of EVs is primarily limited by their short range and high price, which are significantly driven by the cost of the battery pack. The cost of the battery pack is driven by the cost of the cathode material that empowers it.
The most common conventional synthesis method of cathode materials is co-precipitation, which includes long processing time and complex steps. Moreover, poor batch-to-batch uniformity due to differences in solubility and diffusivity of precursors further hinders large-scale implementation. To reduce energy consumption during production, and improve homogeneity of the product, we use spray pyrolysis for synthesizing multi-component metal oxide cathode materials. Spray pyrolysis, a promising development for larger scale synthesis in industry, requires shorter residence time in the reactor, eliminates washing and purification steps, and achieves excellent batch-to-batch reproducibility.
Lithium, manganese-rich layered cathode material (LMR-NMC) has been studied intensively in the past decades and is one of the most attractive cathode materials under development. Its ability to reach discharge capacity above 200 mAh g-1 and low cobalt content make it a promising candidate for cathode material of electric vehicles. 0.5Li2MnO3·0.5LiMn1/3Ni1/3Co1/3O2 (Li1.2Mn0.54Ni0.13Co0.13O2) is currently the most widely studied chemistry. Yet, as recently demonstrated, the materials suffer from an inherent layered-spinel phase change. This leads to capacity and voltage fade over extended cycling, and this shortcoming needs to be addressed before commercial implementation is feasible.
In the first part of the dissertation, voltage fade was addressed by trace elemental doping. Results demonstrated for the first time that by selectively doping the LMR-NMC materials, voltage fade can be reduced. The aluminum doped Li1.2Mn0.54Ni0.13Co0.13O2 demonstrated improved capacity retention of 99.4 % comparing to 91.5 % of the undoped material after 100 cycles. Furthermore, Atomic Layer Deposition (ALD) was used to modify the surface of Li1.2Mn0.54Ni0.13Co0.13O2 with thin layer CeO2, aiming to decrease voltage and capacity fade by increasing the substrate conductivity and setting a barrier for metal dissolution. The optimal CeO2 film thickness was 2.5 nm deposited by 50 cycles of CeO2 ALD. The cyclic stability improved to 60 % capacity retention after 400 cycles at C/1 and 55 °C. The CeO2 coating also reduced voltage fade.
In addition, with the rising interest in sodium-ion battery research, tunnel structure sodium manganese oxide cathode materials were synthesized via spray pyrolysis. The materials demonstrate rod-like morphology after annealing. Optimal electrochemical performance was obtained from the sample produced with a Na/Mn precursor ratio of 0.50, which yielded phase pure Na4Mn9O18 structure. A discharge capacity of 115 mAh g-1 is reached for this material in the first cycle and the material demonstrates good cycleability and rate performance. This demonstrates the versatility of spray pyrolysis and its ability to synthesize a wide range of material with different structure and morphology.
In later part of the work, a low temperature flame spray pyrolysis (LT-FSP) process is developed for the synthesis of Li1.2Mn0.54Ni0.13Co0.13O2. High water content ethanol was used as a fuel and a swirl-stabilized burner was used to achieve stable operation at the low reactor temperature, which is lower than can be attained via traditional FSP. The effects of reactor temperature, which is controlled via altering ethanol concentration, on the physical properties and the electrochemical performances of the synthesized materials were characterized. Li1.2Mn0.54Ni0.13Co0.13O2 synthesized with 25 wt% ethanol showed the best results and delivered a discharge capacity of 203 mAh g-1 after 100 cycles under C/3. It also achieved good rate capability showing 201 mAh g-1 and 169 mAh g-1 under C/2 and C/1, which are comparable to state-of-the-art performances. The production rate of LT-FSP also reaches 90 g h-1.
In addition, LT-FSP was used to investigate the seed loading density of slurry spray pyrolysis. Slurry spray pyrolysis is the only known solution to the hollow sphere issue that has challenged spray pyrolysis synthesis for decades, namely producing particles greater than 2 μm size with a solid (non-hollow) but porous interior morphology. Tap densities achieved 1.1 g cc-1 with 32 wt% of seed loading, which is half the amount of what was previously demonstrated. Li1.2Mn0.54Ni0.13Co0.13O2 produced by slurry spray pyrolysis reproduces the electrochemical performance of the conventional spray pyrolysis, meeting or exceeding the performance of materials produced by co-precipitation.
Richard L. Axelbaum
Richard L. Axelbaum, Pratim Biswas, Mark Meacham, Elijah Thimsen,
Available for download on Wednesday, December 15, 2117