ICMCTF2015 Session F6-2: Thin Films and Coatings for Fuel Cells & Batteries

Layered Li-rich transition metal oxides have theoretical charge and energy densities exceeding 250 Ah-kg^-1 and 900 Wh-kg^-1 when these materials are charged beyond 4.5V vs. Li⁺/Li. However, significant capacity loss and impedance rise is observed when these materials are repeatedly cycled or held at high voltages against graphitic negative electrodes in electrochemical full-cells. In this presentation, we investigate one particular system, containing Li_1.2Ni_0.15Mn_0.55Co_0.1O₂-based positive electrodes, and show how capacity loss and impedance rise can be reduced by modification of the positive electrode. This modification is done in several ways that include the following: i) coating alumina onto composite electrodes via atomic layer deposition (ALD); ii) by blending commercially available Al₂O₃ powder with the other electrode constituents; (iii) by preparing electrodes from oxide particles coated with alumina by a sonochemical process. Full-cells containing the alumina-coated particles and positive electrodes, cycled between 2.2-4.6V, show better capacity retention and lower impedance rise than the baseline electrodes. Electrochemical and physicochemical data will be presented to discuss mechanisms that lead to the observed improvement in electrode performance and cell life.

With the development of space science and technology, it is necessary to develop lithium ion batteries that can be applied to high radiation environment. However, lithium-ion batteries based on organic materials have issues with material degradation by gamma irradiation. To overcome these problems, the electrode is desired to exhibit high stability and energy density in high radiation environment.

In this study, we have investigated the cycle stability of Fe₂O₃ coated LiCoO₂ (LCO) as cathode material in radiation environment. The Fe₂O₃ thin film layer can act as radiation shield in order to prevent structured degradation of LCO. The LCO powder was coated with Fe₂O₃ thin film using the radio frequency magnetron sputtering and mechanical stirring at the same time. The Fe₂O₃-coated LCO was irradiated by gamma-ray from 10 to 50 kGy. The electrochemical properties of various gamma-ray irradiated Fe₂O₃-coated LCO were studied using a battery cycler system and electrochemical impedance spectroscopy analyzer to confirm ion-conductivity. Also, their crystal structures were investigated using X-ray diffraction and transmission electron microscopy. It is observed that crystal structure in irradiated Fe₂O₃-coated LCO was slightly modified. Moreover, micro crack of LCO particles was not observed by scanning electron microscope after 100^th cycles. Although electrical conductivity of the coated LCO active material was decreased, capacity retention of the Fe₂O₃-coated LCO was improved by Fe₂O₃ shielding layer.

A specific challenge in low temperature plasma science is the investigation of chemical reactions in solid surface layers as a response to external plasma parameters. The formation of stable Pd hydrides under vacuum conditions at temperatures of 250 °C is not yet described to the best of our knowledge. In 1993, Fukai and Ökuma [1] discovered that gradual lattice contraction took place when Pd specimens were placed under high H2 pressures and temperatures (5 Gpa, 800°C). The formation of defect structures with vacancy-hydrogen (vac-H) clusters is now recognized as one of the fundamental properties of M-H alloys. [2,3].

In our experiments 20 nm thick palladium films were exposed to argon-hydrogen microwave plasma using different negative substrate voltages to study the hydride formation. The palladium hydride films were investigated by grazing incidence x-ray diffractometry (GIXD), in-situ high temperature diffractometry (HT-GIXD), and x-ray reflectometry (XR) .

The effect of hydrogen plasma depends on the applied negative bias voltage. Up to -50 V we observe an increase of the fcc Pd unit cell volume. Hydrogen atoms occupy octahedral interstices to form PdH_0.55. However, bias voltages of -100 V and -150 V cause a shrinking of the fcc Pd unit cell to fcc Pd_vac . Subsequent reactions under long time plasma exposure form pure cubic PdH_1.33 [4]. HT-GIXD experiments confirm the existence of different palladium hydrides. PdH_0.55 lost its hydrogen at temperatures > 300 °C. In situ high temperature diffractometry measurements also confirm the existence of Pd_vac as a palladium hydride phase. Up to 700 °C hydrogen removal does not occur. We observe phase transformations from cubic PdH_1.33 to fcc Pd_vac in the temperature range 400 to 600 °C and the transformation back again of Pd_vac into PdH_1.33 at 700 °C. The formation mechanisms of palladium hydrides will be discussed.

In the past decade, tin based anodes have been extremely focused as an alternative to graphite anode, because of its high theoretical capacity of 990 mAh/g [1]. However, pulverization of electrode during lithiation and de-lithiation process, which caused from high volume expansion and consequent rapid capacity fading is one of the main disadvantages [2]. To improve electrochemical performance of pure Sn anodes, Sn–M intermetallic compounds have been designed to provide an inactive phase that acts as buffer matrix against volumetric changes and contribute conductivity reported as one effective solution. These matrix systems are Sn–Ni [3], Sn–Co [4], Sn–Cu [5] etc. Nickel substrates with foam structure have been used as current collectors to enhance the electrochemical performance of Sn electrode, since the porous structure of nickel foam will accommodate stresses and electrode pulverization because of the volume expansion and also maximize the utilization of active materials by ensuring that the electrolyte is easily diffused into the anode materials [6].

In this study, Sn was electrodeposited onto the porous nickel foam substrate under pulse electrodeposition conditions. Pulse electrodeposition was carried out at three different peak current densities of 10, 20 and 40 mA/cm² for 5 minutes in a pyrophosphate bath containing 40 g/L SnCl₂.2H₂O, 164 g/L K₄P₂O₇ and 19 g/L Glycin. Surface morphology of Sn-Ni foam electrode was characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) was conducted to understand the elemental surface composition of composites. X-ray diffraction (XRD) analysis was carried out to investigate the structure of Sn-Ni foam electrode. The electrochemical performance of electrodes have been investigated by charge/discharge tests, cyclic voltammetric experiments and the ac impedance technique. The results yielded encouraging discharge capacities since Ni foam behaves as a stress bearing support and electronic conductivity component.

References

[1] R. Zhang, J.Y. Lee, Z.L. Liu, J. Power Sources 112 (2002) 596-605.

[2] J.W. Park, J.Y. Eom, H.S. Kwon, Electrochem. Commun. 11 (2009) 596–598.

[3] N.-R. Shin, Y.M. Kang, M.S. Song, D.Y. Kima, H.S. Kwon, J. Power Sources 186 (2009) 201–205.

[4] H.Gul, M. Uysal, T. Cetinkaya, M. O. Guler, A. Alp, H. Akbulut, International Journal of Hydrogen Energy, In Press, Accepted Manuscript

[5] M. Uysal, T. Cetinkaya, M. Kartal, A. Alp, H. Akbulut, Thin Solid Films In Press, Accepted Manuscript

[6] Y. Xu, L. Fei, E. Fu, B. Yuan, J. Hill, Y. Chen, S. Deng, P. Andersen, Y. Wang, H. Luo, of Power Sources 242 (2013) 604-609

A functionally graded CuSi thin film was deposited by a magnetron sputtering technique on Cu discs. Cu and Si distribution in the thin film was confirmed by glow discharge optical emission spectrometry. The electrochemical performance of this material was compared to a thin film that was produced by the co-deposition of Cu and Si.

The results showed that a composite CuSi thin film having 30%at. Cu exhibited 1100 mAh g^-1 discharge capacity with 80% Coulombic efficiency; but the cycle life of this film degraded very quickly, only 300 mAh g^-1 capacity remaining after 50^th cycles. On the other hand, functionally graded thin film electrode delivered 1400 mAh g^-1 with 78% coulombic efficiency in the first cycle and then 600 mAh g^-1 with 99% coulombic efficiency even after 50^th cycles. The results showed that Cu content and its distribution along the film thickness were the main contributors to the superior performance of the functionally graded CuSi film. We believe that Cu enhanced the electrochemical performance of Si anodes mainly because it is electrically more conductive and mechanically more ductile than Si. However, an optimization of Cu content in the film was needed to minimize the impact on capacity due to the inactive behavior of Cu. Therefore, it is believed that a gradual decrease in Cu content of the film from the bottom (30% at. Cu) to the top (0% at. Si) of the coating would result in electrode formation with higher electrochemical performance. Because the film rich in Cu at the bottom and rich in Si on the top increases adhesion of the deposit to the substrate and also relaxes the stresses along the thin film electrode during cycling without sacrifying the capacity delivered by the cell.

Thin films of Bismuth Titanium Oxides (BTO) were growth onto oriented silicon and stainless steel AISI 316L substrates in an unbalanced magnetron sputtering system with a target of Bi 99,999% pure. On the surface of the target used for sputtering processes assisted by a magnetron system is the high erosion zone called “race-track”. Through this zone squares of titanium of 7*7 mm and 3 mm of thickness were located in four different configurations 1, 5, 9 and 17 squares. It conduces to the variation of the elementary composition in the thin films. In addition to that the temperature of the substrates during the growth were changed with the aim of evaluate the influence of this on the structural properties of the thin films.

Bismuth oxide has four main structural phases but in terms of conductivity the delta phase has values two times higher than for instance YSZ, a preferred material in the solid oxide fuel cells research field. In that way the study of the electrical properties on the thin films produced is of high interest. Four points probe method was used to measure the electrical resistance; thus four electrodes are situated on the surface of the thin film; through both of them a current is applied and by the others the voltage produced by the material is measured. It is supposed that the higher the current the higher the voltage and their relation must be lineal, the slope of a curve current versus voltage (I-V) tell us about the resistivity of the film. Resistivity on thin films produced by the system were measured and the results show well slopes and linearity. Actually studies on X-Ray diffraction and Auger electron spectroscopy are been made in order to know the morphological and structural properties and to correlate this properties with those of resistivity.

Regarding to the morphological properties we will perform scanning electron microscopy (SEM), atomic force microscopy (AFM) and profiler analysis. The value of roughness over the surface of the film is quite important as well as the topography for the behavior of the resistivity.