59th Annual Report on Research 2014

Overview

High capacity and high rate lithium-ion batteries (LIBs) with low cost and improved safety characteristics constitute a major requirement for electric vehicles, portable electronics, and other energy storage applications. Year-to-year electrochemical performance improvements in LIBs are typically limited to 3-4%, with a major bottleneck being the lack of appropriate materials to satisfy the energy and power density requirements. From a technology perspective, the key bottleneck is mainly on the cathode side, whereas recent progress in nanostructured anodes has significantly improved the potential of the practically achievable capacity and rates from the anode side.  Therefore, there is a need for the discovery of new materials that can participate in higher capacity charge storage reactions.

Recently, there has been a great deal of interest in polyanion [(XO_n)^m-] materials  for cathodes due to their low cost, good stability, and safety. An attractive feature of polyanions is that their reaction potential can be tuned through the ionic character of the anion or inductive effect. Additionally, many polyanions exist naturally as minerals or are formed as industrial waste or corrosion products. Therefore, they may represent a low-cost and abundant source of energy storage materials provided they are fully characterized and evaluated. The goal of this research program is to synthesize and investigate the electrochemical properties of novel polyanion electrode materials, with an emphasis on evaluating the characteristics of naturally derived mineral compounds.

Thus far, we have studied the following materials: 1) the layered mineral brochantite, Cu₄(OH)₆SO₄, one of the constituents of the green patina on the State of Liberty; and  2) compounds based on jarosite, AB₃(OH)₆(SO₄)₂ , A = Na or K, B = Fe or V, which are widely used in the metallurgical industry to precipitate iron from acidic processing solutions.

Significant Results

(1)  Brochantite, Cu₄(OH)₆SO₄

Through the use of nanostructured shape control, brochantite nanoplates were shown to demonstrate the full theoretical discharge capacity of 474 mAh/g upon reaction with lithium, with the materials following a conversion reaction mechanism corresponding to the 2 electron reduction of copper. The brochantite structure was investigated using X-ray diffraction (XRD), electron microscopy, and X-ray photoelectron spectroscopy (XPS) to understand the structural transformations after electrochemical cycling. XRD characterization suggested that the discharge products consist of Cu nanoparticles too small to be detected by X-rays within an amorphous matrix. High Coulombic efficiencies indicate that the conversion reaction in brochantite nanoplates has high reversibility, unlike other Cu conversion materials such as CuF₂. Owing to the formation of small Cu nanoparticles during the discharge of brochantite nanoplates, good reversibility was observed, but long-term capacity retention was limited by Cu dissolution into the electrolyte during charging. The fundamental knowledge gained from this study can be applied to better understanding of the electrochemical properties other mixed anion materials and add to the existing knowledge base related to Cu-based conversion electrodes for lithium-ion batteries. The results indicate that copper hydroxysulfate materials such as brochantite may be promising electrode materials for lithium-ion batteries if this dissolution problem is addressed.

Figure 1. (left) Scanning electron microscopy images of brochantite nanoplates prepared using hydrothermal synthesis. (right) Galvanostatic cycling of brochantite nanoplates using C/20 rate. Inset shows the crystal structure of brochantite.

(2)  Jarosite compounds, AB₃(OH)₆(SO₄)₂ , A = Na or K, B = Fe or V

Jarosite compounds were prepared using microwave hydrothermal synthesis. Although lithiated jarosites could not be directly synthesized, reversible electrochemical lithiation was possible in the sodium and potassium containing compounds. For example, for NaV₃(OH)₆(SO₄)₂, a discharge capacity of 164 mAh/g was observed in the first cycle and for KFe₃(OH)₆(SO₄)₂, 200 mAh/g was observed. The preliminary electrochemical and structural characterization of the jarosites indicate that the structure of the vanadium compounds is maintained during lithiation, but that the iron compounds may undergo a phase transformation such as a conversion reaction.

Figure 2. (left) Scanning electron microscope image of sodium vanadium jarosite with structure NaV₃(OH)₆(SO₄)_2. (right) Galvanostatic cycling results for NaV₃(OH)₆(SO₄)₂ in a Li half cell using a C/20 .

Impact

These studies show the opportunity for using natural minerals in lithium-ion batteries and lay the groundwork for further investigation into other similar materials. Due to the diverse and rich variety of structures in the mineral world, we hope that this can lead to identification of other promising electrode materials for electrochemical energy storage applications.

This funding has been valuable for the PI as a young investigator and greatly helped her build up her lab by supporting travel and student training. The project has trained one graduate student and undergraduate in synthesis, structural characterization techniques, and electrochemical testing protocols. The research supported by ACS-PRF has resulted in one journal article so far in Nano Letters and several others in preparation, as well as two patent applications. The results have been disseminated at national conferences. Through this funding, we have also developed collaborations with synthetic inorganic chemists.