Designing Cathodes and Cathode Active Materials for Solid State Batteries

Solid state batteries SSBs currently attract great attention as a potentially safe electrochemical high energy storage concept. However, several issues still prevent SSBs from outperforming today s lithium ion batteries based on liquid electrolytes. One major challenge is related to the design of cathode active materials CAMs that are compatible with the superionic solid electrolytes SEs of interest. This perspective, gives a brief overview of the required properties and possible challenges for inorganic CAMs employed in SSBs, and describes state of the art solutions. In particular, the issue of tailoring CAMs is structured into challenges arising on the cathode , particle , and interface level, related to microstructural, chemo mechanical, and electro chemical interplay of CAMs with SEs, and finally guidelines for future CAM development for SSBs are propose

Stability and performance evaluation of high-brightness light-emitting diodes under DC and pulsed bias conditions

This paper presents an experimental analysis of high brightness light emitting diodes (HBLEDs) performance and stability under dc and pulsed current bias. Three different families of HBLEDs from three leading manufacturers have been considered. The analysis was carried out by means of current-voltage, integrated optical power and electroluminescence measurements, and failure analysis. After an initial characterization of the electrical, optical and thermal behavior of the devices, a set of ageing tests was carried out, both under dc and pulsed bias conditions. Identified degradation modes were efficiency decrease, series resistance increase, leakage current increase, and modifications of the emitted spectrum. Characterization of devices behavior during stress indicated (i) generation of non-radiative components, (ii) degradation of the anode contacts and bonding wires, (iii) degradation of the phosphorous layer conversion efficiency and (iv) of the plastic package as possible responsible of the electrical and optical degradation of the LEDs. Comparison between dc and pulsed stress carried out using the same average current level and different duty cycle values showed that the use of pulsed bias can reduce the degradation rate with respect to dc bias. However, for duty cycles lower than 20 %, fast degradation and abrupt ruptures can take place, due to the high peak current levels

High temperature electro-optical degradation of InGaN/GaN HBLEDs

This paper presents a study of the high temperature degradation of high brightness light emitting diodes (HBLEDs) on gallium nitride. Two different families of devices, from two leading manufacturers, have been submitted to thermal stress: during treatment, the optical and electrical characteristics of the devices have been analyzed. Degradation modes detected after stress have been (i) operating voltage increase, (ii) output power decrease, (iii) modifications of the spectral properties. The degradation of the electrical and optical characteristics of the devices were found to have different kinetics: this fact indicates that optical power (OP) loss is not strongly related to the degradation of the electrical parameters of the LEDs. On the other hand, spectral analysis indicated that OP loss is strongly related to the decrease of the phosphors-related yellow emission band. Microscopic analysis showed that this effect can be ascribed to the carbonization of the package and phosphorous material. A degradation of the transparency of the top-side ohmic contact has been also detected after stress: these mechanisms are thought to be responsible for the detected OP decrease. OP decay process has been found to be thermally activated, with activation energy equal to 1.5 eV

Thermal stability analysis of High Brightness LED during high temperature and electrical aging

In this paper we report the analysis of thermal stability of High Brightness Light Emitting Diode subjected to thermal and bias ageing. The degradation mechanisms of several families of commercial available devices were investigated. In the first part of the work we estimated thermal resistance and thermal behaviour under dc bias condition. After this thermal characterisation two different ageing tests were carried out on devices: thermal aging at high temperature levels without biasing the devices and accelerated dc stress at nominal current value (400mA). At each step a complete electrical and optical characterisation of aged devices was performed, in order to find a correlation between different aging and a better understanding of degradation mechanism. This characterisation included I-V measurements, optical power vs current characteristics and spectral analysis. During thermal stress we observed the increase of forward voltage at nominal current and the degradation of optical power with nearly exponential kinetics. We found that lifetimes were well correlated with stress temperature: therefore it was possible to find an activation energy of degradation mechanism of about 1.5eV. Moreover, modifications of spectral properties during electrical and thermal stress were found. Thus, a package level analysis was carried out in order to clarify the role of modification in optical properties of reflector cup and the efficiency of phosphors. Finally, evaluation of differential structure functions indicated that stress induces also the worsening of the properties of the chip-to-package thermal path: this phenomenon has been attributed to the partial detachment and degradation of the ohmic contacts