Date of Award

January 2019

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Chemical Engineering

First Advisor

Michael Mann

Abstract

Lithium ion batteries present a promising solution for energy storage applications which can be utilized to make green energy generation from sources such as wind and solar more practical. Lithium iron phosphate is an attractive battery cathode material due to its long lifespan, safety and stability, and environmentally friendly chemistry. One shortcoming of lithium iron phosphate is its inherently low conductivity, which is commonly overcome through the addition of conductive carbon. Graphene is a two-dimensional nanomaterial comprised of a single layer of carbon atoms, which displays excellent electrical, thermal, and mechanical properties. Commercial incorporation of graphene into battery electrode materials is limited due to the high cost of graphene.

Humic acid is a naturally occurring substance which exhibits properties similar to those of graphene oxide, a common graphene precursor material. High molecular weight humic acid has been proven capable of reducing to graphene that is functionally identical to graphene synthesized from graphene oxide. Humic acid can be found naturally in soils, coals, and other decayed plant matter. North Dakota leonardite, a form of oxidized lignite coal, was used in this research due to its low cost and high humic acid content. This thesis details a study on the feasibility of obtaining a high-purity humic acid material from North Dakota leonardite which can be used as a graphene-precursor in the preparation of lithium-ion battery cathode materials.

Major goals of this research included: i) develop an extraction method to obtain humic acid at acceptable yields while minimizing iron, ash, and other impurities, ii) identify and develop additional purification steps necessary to reduce the level of impurities in the extracted humic acid to acceptable levels for highly technical applications such as lithium-ion battery components, iii) develop a method of synthesizing graphene-modified lithium iron phosphate cathode material using purified humic acid as a graphene precursor, and iv) verify the formation of graphene and increase in electrochemical performance of graphene-modified lithium iron phosphate cathode materials in comparison to reference lithium iron phosphate samples.

To meet these goals and prove or disprove the hypothesis, the research was broken down into five main areas: i) testing and developing an optimized humic acid extraction procedure, ii) testing various processes for reducing impurities in extracted humic acid, iii) testing various methods for synthesizing lithium iron phosphate which were conducive to using humic acid as a carbon source, allowing for adequate mixing and interaction between the humic acid and active material, iv) verifying the formation of graphene through a variety of materials characterization methods, and v) determining the improvement in electrochemical performance of coin cells prepared with graphene-modified lithium iron phosphate compared to reference cells.

The extraction process and purification regime developed in this research has resulted in a procedure for reliably obtaining high-purity humic acid from North Dakota leonardite materials. Ash and iron contents in purified humic acid samples have been less than 0.50% and 0.01%, respectively, on a dry mass basis. Lithium iron phosphate materials have been synthesized using humic acid which exhibit multiple characteristics of graphene-modification. Coin cells prepared with graphene-modified lithium iron phosphate materials exhibited a reversible specific capacity of 145 mAh/g at a charge/discharge rate of 0.1C, which is an approximately 25% increase when compared to coin cells prepared with reference lithium iron phosphate material. This research has displayed that humic acid is a viable, low-cost alternative to graphene oxide for the synthesis of graphene-modified battery cathode materials.

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