ICMCTF2015 Session F6-1: Thin Films and Coatings for Fuel Cells & Batteries

We present an alternative approach to integrate ultrathin Al₂O₃ into stacked C/SiO_x layers for the stabilization of anode materials in Li-ion batteries. The heterogeneous C/SiO_x/Al₂O₃ thin films were constructed by utilizing plasma magnetron PVD techniques. The deposition of the anodes followed either C/SiO_x/C or C/SiO_x/C/SiO_x/C assemblies, 200nm on C substrates and 100 nm on SiO_x substrates with an additional 20 nm Al₂O₃ coating to immobilize the two established films. Raman spectra show the C/SiOx stacks consist of amorphous structures for both C and SiO_x layers. The multilayer thin films with Al₂O₃ coating exhibit a maximum initial discharge capacity of about 1000 mAh/g and maintained a reversible capacity of about 500 mAh/g over 100 cycles at a current density of 300 mA/g (~C/2) with a half-cell anode. The Al₂O₃ layers slightly improved the electrochemical performance of the stacked C/SiO_x anodes, and the capacity retention rate increased about 25% with good rate-capability in comparison to those not coated with Al₂O₃. The improved electrochemical performance of depositing C/SiO_x layers with Al₂O₃ coating can be attributed to the formation of an artificial solid electrolyte interphase layer stabilizing the interaction of C/SiO_x and the electrolyte. Furthermore, we examined a full-cell battery using a C/SiO_x/Al₂O₃ layered anode and a LiNiO_x cathode, and their electrochemical performance will be presented.

Several physical processes show surface roughening under moderate to high stress under diffusive processes either at high or at low temperatures. Although the length scales and the time scales observed in these phenomenon are vastly different; they are governed by the same set of equations. Starting from internal energy and entropy terms, governing equation is derived that gives the amplitude change of such surfaces as a function of time. A parametric study is then carried out using a diverse range of practically important cases such as thin films electrodes of Li-ion batteries and metal films in thermal barrier systems. It is shown that the characteristic surface roughening observed depends upon the stress as well as diffusivity. These results have an important bearing on the crack initiation at thin film surfaces and interfaces.

Tin oxide, a potential anode material for all-solid-state batteries has high capacity (~ 1490 mAh g^-1). Although it has high capacity, tin oxide shows volume expansion during electrochemical reaction and poor electrical conductivity. The volume expansion induces delamination of the active material from current collectors and then isolation from the electrical networks. Thus, modification of the active material structure is necessary to enhance the resistance of the volume expansion. In order to reduce the volume expansion and low conductivity, we have introduced a buffer and conducting layer of C60 thin film below SnOx layer which microstructure can be confirmed by scanning electron microscope (SEM) image and electric conductivity was measured by 4 point prove. After 50^th cycle of charge-discharge, it shows significant reduction of volume expansion. The SnO_x thin film layer was deposited on C60 thin layer using radio-frequency (RF) sputtering system under Ar gas atmosphere at different input power. Their electrochemical properties were evaluated with charge/discharge test and differential capacity plot. In these results, capacity, cycle life, and electrical conductivity of the SnO_x/C60 structure show higher stability than bare SnO_x thin film material.

In the planar design of a solid oxide fuel cell (SOFC) stack, the interconnect acts not only as electrical connection between the various cells but also as the mechanical support for the thin electroactive ceramic parts and as gas-proof separation of air and fuel gas. With the reduction of the SOFC operating temperatures to 600-800°C, chromia-forming ferritic steels are widely used as interconnect materials in the planar-type SOFC stacks currently under development. Two of the most serious problems for these ferritic steels such as Crofer 22 APU is (1) the Cr volatility and associated “poisoning” of the cathode under the operating environments of SOFC; and (2) the continuous oxidation of the alloy and subsequent thickening of the chromia scale and increase in scale electrical resistance. Reactive element-doped (Co,Mn)₃O₄ spinel is considered as the most promising coating system to protect the interconnect alloy. In this presentation, different approaches to adding reactive element into the spinel coating are reviewed. Two methods that have been developed in our research lab, i.e. electrolytic codeposition and environment-assisted reactive sintering, are compared and their advantages are outlined. The effect of the CeO₂-doped (Co,Mn)₃O₄ coating on the oxidation resistance and scale area-specific resistance of Crofer 22 APU are discussed .