Reversible process (thermodynamics)

In thermodynamics, a reversible process is a process, involving a system and its surroundings, whose direction can be reversed by infinitesimal changes in some properties of the surroundings, such as pressure or temperature. Throughout an entire reversible process, the system is in thermodynamic equilibrium, both physical and chemical, and nearly in pressure and temperature equilibrium with its surroundings. This prevents unbalanced forces and acceleration of moving system boundaries, which in turn avoids friction and other dissipation. To maintain equilibrium, reversible processes are extremely slow (quasistatic). The process must occur slowly enough that after some small change in a thermodynamic parameter, the physical processes in the system have enough time for the other parameters to self-adjust to match the new, changed parameter value. For example, if a container of water has sat in a room l

Mesoscopic manganese based cathode materials for high voltage lithium ion batteries

Nam Hee Kwon

Alternative cathode materials were investigated for application to high voltage lithium ion batteries. Manganese based materials were considered due to the low cost and environmentally sustainable impact. Layer-structured oxide and phospho-olivine materials were prepared by a sol-gel method to obtain mesoscopic particles. The particle size of the layered Li1.15Mn0.545Ni0.245Al0.06O2 material was 200 nm. A ball-milling process reduced the particle size to 60 nm. Doping with Ni and Al in the layered Li1.15Mn0.545Ni0.245Al0.06O2 material provided a redox potential at 4.1 V vs. Li+/Li. The specific capacities of 60 and 200 nm particle sized materials were 150 and 125 mAh/g, respectively at C/10. The 60 nm sized material showed a higher lithium intercalation rate than the 200 nm sized material. Particularly, at high charge-discharge rates (1C and 5C), 127 and 85 mAh/g, respectively compared to 95 and 10 mAh/g at 1C and 5C, respectively of 200 nm material. However, the 60 nm sized material degraded faster upon cycling after 80 cycles. The increased impedance indicates a large resistance associated with the interface reaction in the cycled 60 nm particlels. Two types of electrolytes were investigated in the same cathode material. 3-methoxypropionitrile (MPN) with 1M LiN(SO2CF3)2 electrolyte showed faster Li intercalation than ethylene carbonate/dimethyl carbonate (EC/DMC) with 1M LiPF6 at 1C and 5C in complete cells. LiMnPO4 was synthesized by a sol-gel method using glycolic acid. The decomposition temperature was lower in the dried gel prepared under strong acid conditions. Finally, highly ordered crystallites were formed between 520 and 570°C with particle sizes in the range of 140 to 160 nm. Subsequent dry ballmilling reduced the diameter to 130 ± 10 nm and also improved the carbon coating on the active material. LiMnPO4 material provided a redox potential of 4.1 V vs. Li+/Li. Reversible capacities of 156 mAh/g at C/100 and 134 mAh/g at C/10 were measured. At 92 and 79 % of the theoretical value, respectively, these are the highest values reported to date for this material. At faster charging rates, the electrochemical performance was found to be improved when smaller LiMnPO4 particles were used. The LiMnPO4 material showed an excellent capacity retention upon cycling as phosphate framwork is chemically and thermodynamically stable.

EPFL2006

Luminescent and electronic properties of perovskite solar cells

Brian Irving Carlsen

Perovskite solar cells show great promise to serve as an alternative for traditional silicon-based solar cells, however several problems present obstacles for their commercialization. Standard perovskite cells contain lead, which is toxic to humans and harmful to the environment. Alternative perovskite compositions offer the opportunity to replace lead with non-toxic metals. Lead can be directly replaced by substituting it for tin. Unfortunately, tin-based perovskite solar cells do not show the same efficiency or stability as their lead-based counterparts. Using numerical modeling, a study of a current optimized, tin-based solar cell was performed to understand its efficiency limitations and where the most gains can be acquired. While the short-circuit current of the device is high, its limiting factors arise from those already known, primarily the alignment of the electron transport layer with the perovskite conduction band.Another option for replacing lead is by using a double perovskite. This class of perovskites replaces lead with two different metals that maintain charge neutrality. While providing more compositional freedom compared to the direct replacement of lead, double perovskites offer their own challenges. To better understand transport phenomenon between the transport layers and the perovskite, an investigation into the temperature dependent photoluminescence of Cs2AgBiBr6 with different hole and electron transport layers was performed. No clear trends appear allowing a comparison of the transport layers, but some speculations are provided to explain the results common amongst all the samples.Lead is not the only roadblock for perovskites though, as they also show instability across multiple domains. Two of the most prominent ways instability shows up are closely linked -- ionic motion and reversible degradation. A continuation of an insightful paper by Domanski \emph{et. al.} was performed to further elucidate the effects of ionic movement on reversible degradation processes. We show that these processes can be controlled by the application of a bias voltage and that the enhancement of these processes by illumination is not significant. We continued the examination of these reversible processes by studying their effects under real world conditions. Several perovskite solar cells were subjected to real world conditions in a controlled environment during which we tracked their performance. By comparing the measured data to those calculated if the cells had remained pristine we are able to quantify and separate the reversible from irreversible processes.Finally, a more fundamental investigation was made into the internal mechanisms of the perovskite cells. A temperature dependent study correlating photoluminescence spectra and current-voltage curves was performed revealing different two regimes operation. The potential mechanisms underlying each regime are explored.

EPFL2022

Mesoscopic manganese based cathode materials for high voltage lithium ion batteries

Nam Hee Kwon

EPFL2006

Luminescent and electronic properties of perovskite solar cells

Brian Irving Carlsen

EPFL2022