Author's School

School of Engineering & Applied Science

Author's Department/Program

Energy, Environmental and Chemical Engineering

Language

English (en)

Date of Award

Spring 4-14-2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Richard L Axelbaum

Abstract

Rapid advancements of techniques for the synthesis of Li-ion battery materials are critically needed to address the requirement of a clean and efficient transportation sector. The current research serves this goal by developing an approach to producing layered cathode materials with superior electrochemical performance for electric vehicles (EVs). Current widespread application of EVs is primarily limited by their short range and their high price, which is primarily 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.

Novel, high throughput and inexpensive synthesis methods delivering nanostructured materials are a key to meeting these requirements. The synthesis techniques need to be scalable, robust, and reproducible while producing high-density materials for lithium ion batteries. To this end we advance spray pyrolysis for the synthesis of the layered NMC composite materials, which are showing high promise as a cathode material. Spray pyrolysis produces high purity materials, and the limited number of process parameters allows for low cost and excellent control over product properties and outstanding batch-to-batch reproducibility.

Layered Li-excess composite materials show nearly twice the capacity of commercial LiCoO2 cells. The materials are inexpensive, have improved safety characteristics and long cycle life. Yet, as recently demonstrated, the materials suffer from an inherent layered-spinel phase change. This leads to a voltage fade over extended cycling, and this shortcoming needs to be addressed before commercial implementation is feasible.

In this work spherical-shape layered xLi2MnO3*(1-x)LiNi1/3Mn1/3Co1/3O2 composites were synthesized. The relationship between composition and material stability under different synthesis conditions (350 °C - 800°C reactor temperatures, 0.5 - 2.5 M concentration, 6.6 - 10.4 lpm flow rates) were explored. We found that from among the compositions corresponding to x = 0.3, 0.5 and 0.7, the composition for x = 0.3, or Li1.14Mn0.46Ni0.2Co0.2O2, provides improved stability and the least amount of voltage fade while displaying capacities around 190 mAhg-1 after 100 cycles at C/10 rate at room temperature. At the same time, for x = 0.5, or Li1.2Mn0.54Ni0.13Co0.13O2, the material delivers 205-210 mAhg-1 discharge capacities at C/3 rate at room temperature after 100 cycles, but displays more voltage fade over cycling.

This work demonstrated that the major process parameters (flow rate, reactor synthesis temperature and concentration) can be accurately controlled and the synthesis method is robust. The reproducibility of the process was evaluated using charge and discharge tests and the standard deviation for cycling tests was 4 mAhg-1 at C/3 rate based on 4 batches produced under identical conditions on different dates. This indicates excellent batch-to-batch reproducibility.

Post-synthesis annealing temperature optimization was performed for cobalt doped samples at 850 °C and 900 °C and we found that annealing for 900 °C for 2 hours improves the cycling stability of the samples. We evaluated the effect of lithium content between 3.3 wt% excess and 3.3 wt% deficiency and annealed the materials for 2, 5 and 20 hours at 900 °C. This helped develop a fundamental understanding between surface area and internal structural changes related to the Li2MnO3 structural component of the materials. Spray pyrolysis uniquely allows for the accurate control of stoichiometry and composition to trace contaminant level at these concentrations.

Furthermore, through a collaborative research between Argonne National Laboratory, X-Tend Energy, LLC and Washington University in St. Louis a novel, highly scalable patent-pending slurry spray pyrolysis process was developed, which allows the production of battery materials with excellent electrochemical performance and provides a general platform for oxide materials at greater than 50 gh-1 scale. This unique process 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 greater than 1.0 gcm-3 are achieved at greater than 50 gh-1 scale as compared to 0.4-0.6 gcm-3 at 2 gh-1 scale. Li1.2Mn0.54Ni0.13Co0.13O2 produced by this novel process delivered ~205 mAhg-1 discharge capacity after 100 cycles at C/3 rate at room temperature, reproducing the electrochemical performance of the laboratory scale synthesis process and meeting or exceeding the performing of materials produced by co-precipitation.

Voltage fade was addressed in the latter part of the work by varying the compositional ratio and using trace elemental doping. Results demonstrated for the first time that by selectively doping the xLi2MnO3*(1-x)LiNi1/3Mn1/3Co1/3O2 materials voltage fade can be reduced, as indicated by dQ/dV curves.

The spray pyrolysis process for xLi2MnO3*(1-x)LiNi1/3Mn1/3Co1/3O2 materials, in particular for layered Li1.2Mn0.54Ni0.13Co0.13O2 displayed the highest capacity (c.a. 205-210 mAhg-1 after 100 cycles at C/3 rate at room temperature) among all cathode materials synthesized via spray pyrolysis to date.

DOI

https://doi.org/10.7936/K7610XD0

Comments

This work is not available online per the author’s request. For access information, please contact digital@wumail.wustl.edu or visit http://digital.wustl.edu/publish/etd-search.html.

Permanent URL: http://dx.doi.org/10.7936/K7610XD0

Included in

Engineering Commons

Share

COinS