59th Annual Report on Research 2014

One of the main goals of the project is to investigate new low temperature routes to earth abundant (transition) metal borates (TMBs) and to investigate them as cathode materials in Li-ion batteries. TMBs have been traditionally prepared using solid state reactions which require lengthy procedures involving firing, quenching, regrinding, refiring and calcination at high temperatures (>800 °C). These methods lead to the formation of the most thermodynamically stable, crystalline or polycrystalline phases and control over material composition and structural properties is limited. In contrast, it is anticipated that the low temperature synthetic routes we are investigating will enable us to cleanly prepare TMBs in high yield at or below room temperature and which have amorphous or polycrystalline morphologies. The key to our synthetic strategy involves clean, low temperature reactions of well-defined soluble transition metal organometallic species with boric acid (H3BO3). This methodology offers great synthetic flexibility in preparing a wide range of unique kinetically stable phases in a short timeframe. Our initial efforts have focused on the synthesis, evaluation and optimization of earth abundant Fe and Mn containing TMBs. 1. Synthesis and Electrochemical Studies. We have focused our studies on two materials produced by these methods. Firstly, we have investigated the reaction of Fe(N(SiMe3)2)3 with H3BO3 under a variety of different conditions using different solvents, reaction times and temperatures. Reactions in both THF and an Et2O/dioxane mixture at elevated temperatures (up to 100 ºC) produced amorphous FeBO3 in good yields (equation 1). Fe(N(SiMe3)2)3+H3BO3 > FeBO3+3HN(SiMe3)2 Eq. 1 Current studies show promising performance towards electrochemical lithiation/delithation with a gravimetric discharge capacity of around 260 mAh g-1 at 0.2C (45 mA/g) on the first cycle and achieving a constant capacity of ~206 mAh g-1 after five cycles which translates to 88% of the estimated theoretical capacity of FeBO3 (234 mAh g-1). The majority of the irreversible capacity loss on the first cycle is most likely associated with irreversible formation of a decomposition layer (SEI) layer and possible conversion induced reaction to form metallic Fe common for materials initially cycled between 3-0.5 V vs Li/Li+. It is interesting to note that the reversible gravimetric capacity of this material is significantly greater than that found for other nanostructured FeBO3 materials prepared via high temperature routes (123 to mAh g-1 at 50 mA/g). An incremental capacitance plot analogous to slow scan cyclic voltammetry (SSCV) shows that the amorphous FeBO3 possess a reversible lithiation potential centered around ~1.87 V vs Li/Li+. We are currently attempting to tune the mean charge/discharge potential of this material by substitution of Co2+, Ti3+ or Cr3+ in place of Fe3+ and by incorporation of P. We have also prepared samples of amorphous LiMnBO3 by similar reactions of Mn(N(SiMe3)2)3 with H3BO3 and have investigated the electrochemical properties of these materials. So far, these materials have gravimetric capacity values of approximately 30 mAh g-1 after the first cycle and tend to exhibit a large capacity fade upon subsequent cycles to approx. 5 mAh g-1, much lower than the theoretical value of 222 mAh g-1. Although this is not the ideal outcome we hope to improve this behavior by careful annealing at in vacuum at moderate temperatures (up to 400 ºC) in order to produce monoclinic phases which have shown good capacity retention of ca. 100 mAh g-1 over multiple cycles. Further improvement may be expected by surface coating and substitutional doping to increase ion and electronic conductivity. 2. Raman Spectroscopy We have initiated preliminary in situ spectroelectrochemical Raman experiments in order to gain high resolution mechanistic insight, in which differential electrochemical reactivity will be directly spatially, spectrally, and temporally imaged and correlated with high resolution structural characteristics. The key strength of this approach lies in the ability to map or image local chemistry as a function of applied potential without the need for extravagant sample preparation. Thus, we are performing Raman microprobe measurements on these cathode materials in order to collect spectral “finger-prints” of structural inhomogenieties and surface chemistries. Analysis of characteristic vibrational frequencies will provide information on the nature of structural order (e.g., molecular bonding environment, defect structure, surface impurities, and degree of crystallinity) and material composition (e.g., metal valency, elemental composition). The low frequency regions are very sensitive to the presence of defects, impurities, dopants, and structural distortions. The Stevenson group has demonstrated the power of this technique with situ electrochemical Raman studies of LiFePO4. Impact on Research Support from the ACS Petroleum Research Fund has been pivotal to the ongoing success of this project. Two graduate students have received meaningful levels of financial support. Several other graduate students and undergraduates have also worked on the project being supported by other funding. The multidisciplinary nature of the research has meant that all the students have been involved in state-of-the-art research in several important areas including inorganic and organometallic synthesis, electrochemical studies, electrochemical cell fabrication, materials characterization and Raman spectroscopic studies. There is no doubt that this highly multidisciplinary approach will be of considerable benefit to their future scientific development. The grant has helped the two PI’s maintain a high level of research in a highly competitive and rapidly emerging technological field.