The radical cations of [15N]bacteriochlorophyll a and of the primary donor P865 in 15N-labeled reaction centers of Rhodobacter sphaeroides R-26.1 were investigated in frozen solutions using two-dimensional ESEEM methods. Both three-pulse (stimulated echo) and four-pulse (HYSCORE) sequences were employed to avoid ambiguities in data analysis. Computer simulations of the experimental powder spectra were performed, yielding a complete set of nitrogen hyperfine coupling tensors for both systems. The obtained tensor values are compared with those from ENDOR measurements and from semiempirical INDO-type MO calculations. The results obtained from 2D stimulated echo ESEEM and HYSCORE for are interpreted in terms of an asymmetric spin density distribution over the halves of the bacteriochlorophyll dimer ('special pair') in the reaction center with a ratio of approximately 5: 1. This asymmetry is considerably larger at low temperatures in the frozen state than at room temperature. It is postulated that two different conformational states of the dimer exist at these temperatures with different spin density distributions.15N ESEEM of Bchl a*+ and P&

A series of substituted bacteriochlorophyll molecules, all used in reconstitution experiments of reaction centers of Rhodobacter sphaeroides (Struck et al. Biochim. Biophys. Acta 1991,1060,262-270), were characterized by EPR, electron-nuclear double (ENDOR), and electron-nuclear-nuclear triple (TRIPLE) resonance spectroscopy in their monomeric radical cation states. Effects of different substituents at position 3 in the porphyrin macrocycle were considered, especially for two "crosslinks" between plant and bacterial chlorophylls. These are 3-vinylbacteriochlorophyll where the "bacteria" acetyl group at position 3 was substituted by vinyl and 3-acetylchlorophyll where the "plant" vinyl group was substituted by acetyl. In addition, effects of substitutions at position 132 were studied. All major hyperfine coupling constants of proton and nitrogen nuclei were elucidated from the spectra and assigned to molecular positions by comparison with the parent radicals. The data were compared with those calculated by an INDO-type program, showing that INDO essentially models the effect of the different substituents correctly.

The paper presents post-mortem analysis of commercial LiFePO4 battery cells, which are aged at 55 °C and − 20 °C using dynamic current profiles and different depth of discharges (DOD). Post-mortem analysis focuses on the structure of the electrodes using atomic force microscopy (AFM) and scanning electron microscopy (SEM) and the chemical composition changes using energy dispersive X-ray spectroscopy (SEM-EDX) and X-ray photoelectron spectroscopy (XPS). The results show that ageing at lower DOD results in higher capacity fading compared to higher DOD cycling. The anode surface aged at 55 °C forms a dense cover on the graphite flakes, while at the anode surface aged at − 20 °C lithium plating and LiF crystals are observed. As expected, Fe dissolution from the cathode and deposition on the anode are observed for the ageing performed at 55 °C, while Fe dissolution and deposition are not observed at − 20 °C. Using atomic force microscopy (AFM), the surface conductivity is examined, which shows only minor degradation for the cathodes aged at − 20 °C. The cathodes aged at 55 °C exhibit micrometer size agglomerates of nanometer particles on the cathode surface. The results indicate that cycling at higher SOC ranges is more detrimental and low temperature cycling mainly affects the anode by the formation of plated Li. Graphic abstract

This paper uses several techniques to monitor the ageing of commercial LiFePO 4 cells, which are cycled at 55 °C and −20 °C at various depths of discharge. Ageing at lower depth of discharge leads to higher capacity fading, as compared to higher depth of discharge. The highest capacity fading is observed using 50% depth of discharge for cycling at 55 °C, while the lowest capacity fading is observed for the cells aged at 100% depth of discharge when cycled at −20 °C. Using incremental capacity analysis and differential voltage analysis the capacity fading is monitored and underlying ageing mechanisms are described. The loss of lithium inventory and the loss of active material, especially on the cathode side, are the major degradation mechanisms for the cells. The first incremental capacity analysis peak of the discharge process can be used in our case to predict remaining life and cell capacity.

Electrochemical strain microscopy (ESM) is a powerful atomic force microscopy (AFM) mode for the investigation of ion dynamics and activities in energy storage materials. Here we compare the changes in commercial LiFePO4 cathodes due to ageing and its influence on the measured ESM signal. Additionally, the ESM signal dynamics are analysed to generate characteristic time constants of the diffusion process, induced by a dc-voltage pulse, which changes the ionic concentration in the material volume under the AFM tip. The ageing of the cathode is found to be governed by a decrease of the electrochemical activity and the loss of available lithium for cycling, which can be stored in the cathode.