Novel Materials for Lithium-Ion Batteries

Transcription

1 Novel Materials for Lithium-Ion Batteries John Bradley May 18th 2012 Project Supervisors: Prof. West & Chaou Tan

2 Abstract The effect of carbon coating on two novel battery cathode materials LiMnP 2 O 7 and LiFeP 2 O 7 was evaluated through electrochemical and electrical characterisation. It was found through a series of battery cycling tests that the overall reversible capacity of LiFeP 2 O 7 is superior to that of LiMnP 2 O 7, and that the intermixing of sucrose into either pyrophosphate compound, followed by subsequent heating to 700 C is the method of carbon coating that yields the greatest improvement in its electrochemical properties. The discharge profile of LiFeP 2 O 7 + sucrose shows a faint voltage plateau at 3.5V, and the overall capacity of this composite was 43mAh/g, which is 40% of that expected for one lithium removal. The electrical properties of only one composite, LiMnP 2 O 7 (no C), could be determined in this mini-project due to time constraints, and it was found through the use of impedance spectroscopy, that the conducting species in the bulk of this material was ionic rather than electronic, with an activation energy value of 0.95eV. 1. Introduction In order to overcome our current dependence on fossil fuels, a new energy economy must be established that is based upon the availability of a cheap but sustainable energy supply. One of the most energy demanding human endeavours is transportation; and here battery devices can potentially provide a practical solution as they have the capability to store energy from developing sustainable technologies such as wind and solar power. Currently, Li-ion batteries are the most promising means of efficiently storing chemical energy for large scale applications, due to their higher energy density and operating voltages in comparison to that of other battery systems (such as nickel metal hydride, which at the moment is the most widely used commercial battery in electric and hybrid electric vehicles). The superior properties of Li-ion batteries has already established them as the predominant energy storage method for small scale consumer electronics, but, although there has been a dramatic increase in research and commercialisation of Li-ion batteries for large scale energy storage, many challenges still remain in making low cost, high performance and highly safe Li-ion batteries for vehicle applications. Among the components in lithium-ion batteries, the cathode material has received most attention because it has a significant impact on battery capacity, cycle life, and overall cost [1]. The most dominant cathode material used currently in small scale applications is lithium cobalt oxide (LiCoO 2 ), but cobalt has limited availability and is also highly toxic, and therefore it is not really viable for use in electric vehicles in the long term. In the past decade, three-dimensional (3D) framework cathodes with PO 4 polyanions (such as lithium iron phosphate, LiFePO 4 ) have attracted considerable interest for use in large scale applications, due to their formidable chemical and structural stability, high operating voltages (in comparison to those of oxides) and low cost. Unfortunately, these compounds also possess low ion diffusion rates and poor electronic conductivities, and although techniques of carbon coating and particle size reduction have succeeded in significantly improving these properties [2], more research needs to be performed before they can be seriously considered as commercially viable large scale cathode materials. The aim of this project was to evaluate the effect of carbon coating on the electrochemical and electrical performance of two recently discovered cathode materials, lithium manganese diphosphate, LiMnP 2 O 7, and lithium iron diphosphate, LiFeP 2 O 7. Similar to LiFePO 4, these two materials show great potential for large scale applications, but they both suffer from low electronic conductivity and poor ion diffusion rates [3]. Although, as previously mentioned, carbon coating of similar materials has been extensively studied, little to no data exists concerning the modification of the electrochemical and electronic properties of these materials when synthesised with carbonaceous compounds. This report summarises the results of the carbon coating of LiMnP 2 O 7 and LiFeP 2 O 7 on their electrochemical and electrical properties. The synthesis routes and structural, electronic and electrochemical characterisation of the pyrophosphate compounds are all presented along with the procedures employed for the carbon coating of each.

3 2 John Bradley Experimental Work Synthesis and Structural Characterisation Both of the pyrophosphate compounds, LiMnP2 O7 and LiFeP2 O7, were synthesised through the conventional solid-state ceramic route. Firstly, the correct stoichiometric amounts of Li2 CO3, 2NH4 H2 PO4, MnCO3 and Fe(C2 O4 )2H2 O were mixed in a planetary ball mill for 12h, then pelletised and heated at 350 C for 8h. The heated mixtures were then crushed, and placed in the planetary ball mill once more for 12h to produce a finely ground powder that was again pelletised and heated at 700 C for 8h. To ensure minimal oxidation of the Fe containing compound, both heating cycles for this mixture were carried out within a N2 atmosphere. Powder purity and phase composition of the newly synthesised LiMnP2 O7 and LiFeP2 O7 compounds were examined via X-ray diffraction (XRD) using a Philips PW1830 X-ray diffractometer equipped with Cu Kα X-ray tube. The measurements were performed in the θ range with a 1 /min scan speed and a step size of Following characterisation, each quantity of the LiMnP2 O7 and LiFeP2 O7 powders was separated into three equally weighted batches. 5%wt of carbon was introduced into two of the batches through the addition of a carbonaceous material, with the remaining batch functioning as a control. Black carbon (Carbon Super P R ) and sucrose (C12 H22 O11 ) were mixed into separate batches through ball milling (ball-to-powder weight ratio 14:1) for 2h, with the sucrose/compound mixture then being heated to 700 C for 8h to sequester the carbon and optimise carbon-particle coverage. Finally, XRD measurements were performed on both of the carbon/compound composites ( θ range, 1 /min scan speed, 0.05 step size) to assess any possible changes in the original compound phase composition. 2.2 Electrochemical and Electrical Characterisation The electrochemical properties of each of the six sample batches were evaluated in Swagelok R constructed battery half cells (an example of which is displayed in figure 1), using a VMP multichannel potentiostat with lithium metal as the anode. The cathode was prepared by mixing each sample with both a 10%wt of black carbon and organic binder, with the resulting slurry spread by a doctor blade on a glass surface and cut using a metal hole cutter after drying. The electrolyte was 1M LiPF6 (lithium hexafluorophosphate) dissolved in a mixture solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1:1. The assembling of each of the Swagelok R cells was done in a glovebox filled with pure argon. Battery cycling tests were performed between 3.1V and 4.5V, with a current density of 0.02mA/cm2 at room temperature. Figure 1: Photograph of a Swagelok R battery cell constructed for battery cycling tests. Due to time constraints, the electrical properties of only one sample batch, LiMnP2 O7 (no C), could be determined using (a.c) impedance spectroscopy. Two separate samples were prepared by mixing a quantity of powder with a small amount of water, pelletising and heating to 700 C for 8h. Measurements were carried out using silver paste and sputtered gold film electrodes in the temperature range C, between 50Hz and 107 Hz. 3. Results and Discussion The XRD patterns of the different LiMnP2 O7 and LiFeP2 O7 composites are shown in figures 2a and 2b. As can be seen from each of the figures, peak positions match well with theoretical peak positions (calculated from cell parameters provided by [3] and [4]), indicating that a single pure phase was achieved for all of the composites. It is also observed that the intermixing of sucrose in each compound and subsequent heating of the mixture did not noticeably alter the overall structural properties. The electrochemical behaviour of each of the LiMnP2 O7 materials are compared in figure 3. Each composite exhibits very poor cycling properties, with no discernible plateau in the voltage during charge or discharge. The composite that possesses the best reversible capacity value is that which

4 Novel Materials for Lithium-Ion Batteries 3 Figure 2: XRD patterns of (a) LiMnP 2 O 7 and (b) LiFeP 2 O 7 without C (in blue) and with sucrose (in green). The red dashed lines in both figures denote theoretical peak positions. Additional peaks in (b) indicate the presence of a small amount of FeP 2 O 7 impurity (as indicated). contains sucrose, but this value, 0.6mAh/g, is only 0.3% of the theoretical capacity (assuming all lithium ions can be cycled). In comparison to the LiMnP 2 O 7 materials, the LiFeP 2 O 7 composites display much better electrochemical properties with a voltage plateau appearing at 3.5V in the discharge profile (see figure 4). Again, the best reversible capacity value of 43mAh/g is found for the composite containing sucrose, which is 40% of the theoretical capacity assuming that only one lithium can be cycled. The data presented in figures 3 and 4 show that the composites of LiFeP 2 O 7 display much better cycling characteristics than those of LiMnP 2 O 7, which, considering that Mn possesses more oxidation states at these energies, can perhaps be attributed to the small structural differences observed between each of the compounds [3]. The data also suggests that carbon coating does indeed seem to improve the electrochemical properties of both LiMnP 2 O 7 and LiFeP 2 O 7, with the sucrose method of coating (as detailed earlier) appearing to yield the best results. The electrical behaviour of the two LiMnP 2 O 7 (no C) pellets with gold film and silver paste electrodes are displayed in figures 5a and 5b respectively. In the case of each pellet, a single semicircle in the complex Z* plane is observed at all temperatures, indicating that this is the bulk response of the material [5]. This assumption is further validated by Figure 3: Electrochemical performance of LiMnP 2 O 7 composites cycle tested between 3.1V and 4.5V, at a current density of 0.02mA/cm 2. examining the capacitance values at all measured temperatures in figure 6, with the average capacitance value of 1pF being observed at mid-to-high frequencies. At lower frequencies, it is noticed that as the temperature increases, the capacitance dramatically increases, signifying the presence of a surface impedance at the sample-electrode interface. This feature is also observed in the complex Z* plot (figure 5b, inset) as a spike in the impedance and suggests that conduction in the bulk of the material is ionic rather than electronic. The activation energy value of this material, calculated from an Arrhenius plot of the data displayed in figure 5a, was 0.95eV (see figure 7). Such a high activation energy shows that this material is indeed a poor ionic conductor (as is declared in the literature) and helps to explain the substandard cycling capability of the family of manganese based batteries that were discussed earlier in this report. 4. Conclusion and Future Work The effect of carbon coating on two novel battery cathode materials LiMnP 2 O 7 and LiFeP 2 O 7 was evaluated through electrochemical and electri-

5 4 John Bradley Figure 4: Electrochemical performance of LiFeP2 O7 composites cycle tested between 3.1V and 4.5V, at a current density of 0.02mA/cm2. Elongation of the discharge profile for the LiFeP2 O7 + black carbon composite is due to its lower discharge current of 0.005mA/cm2, which indicates that there is a problem with this battery. cal characterisation. It was found through a series of battery cycling tests that the overall reversible capacity of LiFeP2 O7 is far superior to that of LiMnP2 O7, and that the intermixing of sucrose into either pyrophosphate compound, followed by subsequent heating to 700 C is the method of carbon coating that yields the greatest improvement in its electrochemical properties. The discharge profile of LiFeP2 O7 + sucrose showed a faint voltage plateau at 3.5V, and the overall capacity of this composite was 43mAh/g, which is 40% of that expected for one lithium removal. Due to time constraints, the electrical properties of only one composite, LiMnP2 O7 (no C), could be determined in this mini-project. It was found through the use of impedance spectroscopy, that the conducting species in the bulk of this material was ionic rather that electronic, with an activation energy value of 0.95eV signifying that, as reported in the literature, LiMnP2 O7 is indeed a poor ionic conductor. In terms of future work, it will be useful to Figure 5: Impedance data for LiMnP2 O7 (no C) with (a) gold film and (b) silver paste electrodes, measured in the frequency range Hz. Inset in (b) emphasises presence of a surface impedance at low frequencies for temperatures higher that 350 C. Figure 6: Calculated capacitances of LiMnP2 O7 (no C) from the impedance data displayed in figure 5, over the temperature range C. At high temperatures, a surface capacitance manifests itself as sharp increase in capacitance at low frequencies. carry out impedance measurements on the composite powders containing carbon black and sucrose so that a proper evaluation of the how the electrical properties of LiMnP2 O7 and LiFeP2 O7 are transformed by carbon coating. It would also be beneficial to understand how the weight percentage of car-

6 Novel Materials for Lithium-Ion Batteries 5 Figure 7: Arrhenius plot of data displayed in figure 5a, demonstrating the temperature dependence of the bulk conductivity for LiMnP 2 O 7. bon introduced into a compound affects its overall electrochemical and electrical properties, and to try and calculate an optimum value based on battery cycling tests and impedance measurements. Finally, it would be useful to note the effect on the electrochemical and electrical properties of each compound when carbon, instead of being introduced in the manner described in this report, is added at the precursor stage of synthesis. Acknowledgements I would like to thank my both of my supervisors Prof. West and Chaou Tan of Faradion for allowing me to work on this project and for their continued support and direction. I would also like to thank the other members of the group, including Ruth Sayers and Kuang-Che (Shine) Hsiao that gave up their time to assist me over the duration of the project. References [1] X. Zhi, et al., J. Power Sources 189, 779 (2009). [2] J. Wang, X. Sun, Energy Environ. Sci. 5, 5163 (2012). [3] H. Zhou, et al., Chem. Mater. 23, 293 (2011). [4] S. Nishimura, et al., J. Am. Chem. Soc. 132, (2010). [5] D. C. Sinclar, F. D. Morrison, A. R. West, International Ceramics 2, 33 (2000).