60th Annual Report on Research 2015

The overall goals of the project were focused on the development of new materials for a new generation of battery materials including those that could serve as cathode materials in Li-ion batteries. One of the main aims was to explore new low temperature routes to earth abundant (transition) metal borates (TMBs). Materials containing boron polyoxoanions such as borates are just emerging as new cathode materials. The low atomic weight, high electronegativity of B, and the ability of boron to adopt a three- or four-fold coordination environment, afford a greater variety in redox potential adjustment and a predicted increased lithium ion and electronic conduction. For these reasons, a safer, lower cost cathode material, with sufficiently high capacity, lithium-ion diffusivity and stability is sought involving a new class of transition metal borate (TMBs). The utilization of new cathode materials requires the optimization of capacity, cycle life, electronic and ionic mobility, and electron-transfer kinetics in order to utilize transition metal borates as alternative cathodes for vehicle applications. The ability to quickly prepare and evaluate cathode materials and our lack of fundamental mechanistic understanding of their electrochemical performance are key barriers for the advancement of lower cost, safer, higher energy density cathode materials.

We initially focused on lithium transition metal borates (LiMBO3, where M = Fe, Mn, V, Ni), which are attractive given the low atomic weight and high electronegativity of B. These materials have been proposed as advanced low cost and safe cathode materials that exhibit high theoretical gravimetric and volumetric energy density and stability (low volume change (<2%)) compared to current LiMPO4 materials (c.f., 172 mAh/g for LiFePO4 vs. 220 mAh/g for LiFeBO3). Unfortunately, previous high temperature (HT) solid state synthesis routes (>750 oC) have yielded poorly performing cathodes with very small capacity and/or very large polarization without any plateau like voltage region, including some conversion-type reduction as a result of impurity phase formation, surface reactivity, and low electronic conductivity. We have explored kinetically stabilized phases of transition metal borates using low temperature (LT) synthesis routes recently devised in our laboratories. These methods involve room temperature reactions of boric acid (B(OH)3), or a derivative, with suitable transition metal alkyls or alkylate anions. For example: 4/3B(OH)3 + Li2MnMe4 -> LiMnBO3 + 3CH4

The LT synthesis route should also minimize the possibility of impurity phase formation and surface poisoning reactions upon exposure to moist air typically exacerbated by high temperature processing. The ability to synthesize new materials at low temperature (<200 oC) and low cost while concurrently controlling composition, structure and morphology will aid new material discovery and foster mechanistic understanding of cathode architectures which have higher energy density and are more chemically and thermally stable. Our initial investigations produced results, which although promising, suffered from reproducibility issues. For example, although our preliminary electrochemical studies of amorphous LiMnBO3 showed promising gravimetric capacity values in the order of ~30 mAh/g after the first cycle with a large capacity fade upon subsequent cycles to ~5 mAh/g they were much lower than the theoretical value of 222 mAh/g. The performance was improved by annealing in vacuum at moderate temperatures (400 oC) which produced monoclinic phases and which showed good capacity retention of ca. 100 mAh/g over multiple cycles. Although further improvement to increase ion and electronic conductivity was expected by surface coating and substitutional doping this was not achieved. In order to address this problem we devised an "in-situ" route to boric acid with reproducible properties via the hydrolysis of BCl3.BCl3 + 3H2O -> B(OH)3 + 3HCl

Investigations of the boric acid produced in this fashion to prepare cathode materials with reproducible properties are ongoing. We have also broadened our studies of battery materials in two areas. Firstly we have examined alternative cathode materials based on transition metal phosphates. These materials also have potential for use in large-scale applications in Li-ion batteries but have not been commercialized due to poorly understood failure mechanisms. The surface chemistry of alpha-Li3V2(PO4)3 composite electrodes in non-aqueous electrolytes was studied using X-ray photoelectric spectroscopy (XPS). It was found that the carbon additive formed a solid electrolyte interphase both spontaneously and electrochemically. Thus demonstrating that the carbon additive and its properties are crucial to the lithium intercalation performance.

In other battery related studies we investigated new redox flow batteries. The redox flow battery (RFB) has excellent potential for electrical grid energy storage. However, it has not yet been widely deployed often because of problems of stability and limited cycle life. We have introduced the first redox flow battery based on the coordination chemistry of iron and cobalt with amino-alcohol ligands in strong base. In this system the alkaline redox flow battery (a-RFB) was constructed by employing coordination compounds of cobalt with 1-[Bis(2-hydroxyethyl)amino]-2-propanol (mTEA), and iron with triethanolamine (TEA) in 5 M NaOH. The overall redox system has a cell voltage of 0.93 V in the charged state. Importantly, the coordination compounds are negatively charged and have limited transport through the cation exchange membrane (e.g., Nafion), minimizing the extent of redox species crossover during charge-discharge cycling. Fe-TEA is electrochemically reversible and soluble up to 0.8 M, whereas Co-mTEA presents quasireversible electron transfer kinetics and can be solubilized up to 0.7 M. Cyclability was tested and the flow battery was optimized to achieve stable cycling with 71% average energy efficiency in 30 cycles when passing 30 mA cm-2, and using a 50 micron thick Nafion membrane as the separator, at a concentration 0.5 M. Importantly, crossover of the redox species through the membrane was below 5% of the original concentration at the end of the 30th cycle, with no evolution of gases detected during cycling. This is a significant improvement over existing commercial technologies that are known to deactivate due to crossover (e.g. vanadium systems and Fe/Cr systems).