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The market quest for fast-charging, safe, long-lasting and performant batteries drives the exploration of new energy storage materials, but also promotes fundamental investigations of materials already widely used. Presently, revamped interest in anode materials is observed -- primarily graphite electrodes for lithium-ion batteries. Here, we focus on the upper limit of lithium intercalation in the morphologically quasi-ideal highly oriented pyrolytic graphite (HOPG), with a LiC$_6$ stoichiometry corresponding to 100\% state of charge (SOC). We prepared a sample by immersion in liquid lithium at ambient pressure and investigated it by static $^7$Li nuclear magnetic resonance (NMR). We resolved unexpected signatures of superdense intercalation compounds, LiC$_{6-x}$. These have been ruled out for decades, since the highest geometrically accessible composition, LiC$_2$, can only be prepared under high pressure. We thus challenge the widespread notion that any additional intercalation beyond LiC$_6$ is not possible under ambient conditions. We monitored the sample upon calendaric aging and employed ab initio calculations to rationalise the NMR results. The computed relative stabilities of different superdense configurations reveal that non-negligible overintercalation does proceed spontaneously beyond the currently accepted capacity limit.

The defect chemistry of perovskite compounds is directly related to the stoichiometry and to the valence states of the transition-metal ions. Defect engineering has become increasingly popular as it offers the possibility to influence the catalytic properties of perovskites for applications in energy storage and conversion devices such as solid-oxide fuel- and electrolyzer cells. LaFeO$_3$ (LFO) can be regarded as a base compound of the family of catalytically active perovskites La$_{1-x}$A$_x$Fe$_{1-y}$B$_y$O$_{3-\delta}$, for which the defect chemistry as well as the electronic and ionic conductivity can be tuned by substitution on cationic sites. Combining theoretical and experimental approaches, we explore the suitability for A-site vacancy engineering, namely the feasibility of actively manipulating the valence state of Fe and the concentration of point defects by synthesizing La-deficient LFO. Formation energies and concentrations of point defects were determined as a function of processing conditions by first-principles DFT+U calculations. Based on the results, significant compositional deviations from stoichiometric LFO cannot be expected by providing rich or poor conditions of the oxidic precursor compounds (Fe$_2$O$_3$ and La$_2$O$_3$) in a solid-state processing route. In the experimental part, LFO was synthesized with a targeted La-site deficiency. We analyze the resulting phases by X-ray diffraction and scanning electron microscopy, (scanning) transmission electron microscopy in combination with energy-dispersive X-ray spectroscopy, and electron energy-loss spectrometry. Instead of a variation of the La/Fe ratio, a mixture of the two phases Fe$_2$O$_3$ and LFO was observed, resulting in an invariant charge state of Fe, in line with the theoretical results. We discuss our findings with respect to partly differing assumptions made in previous studies on this material system., Comment: 30 pages, 9 figures, preliminary reference: Chem. Mater. 2021, XXXX, XXX, XXX-XXX

We present a sensitive characterization method to image the microstructure of lithium deposits in lithium-ion battery components by Conduction Electron Paramagnetic Resonance Imaging (CEPRI). The versatility of the method is demonstrated for both, imaging surface-patterns of thick lithium metal anodes, as well as obtaining high-resolution images of lithium dendrites formed inside a separator with several micrometre pixel size. The determined spatial distribution of dendrites may then serve as an indicator of the current density distribution and, therefore, yields most valuable information for battery cell design. Accordingly, this method shows its capabilities in a research area where electron paramagnetic resonance imaging (EPRI) has not been utilized so far., Comment: 9 pages, 3 figures

Nuclear magnetic resonance (NMR) investigations of electrochemical systems require gas-tight and non-metallic cell housings. This contribution reports on the development and evaluation of a cylindrical battery container in combination with a numerically optimized saddle coil that is suitable for NMR investigations of battery cells over hundreds of charge-discharge cycles. The reliability of the new cell container design and its long-time gas-tight sealing are shown by rate capability comparisons to standard housings with LiCoO$_{2}$ (LCO) vs. Li-metal electrodes as well as a charge-discharge experiment of a LCO vs. graphite batteries over more than 2000 hours. To demonstrate the performance of the entire NMR setup, long-run in-operando measurements on a Li-metal vs. graphite cell are presented. The NMR data reveal the formation and evolution of mossy and dendritic Li microstructures over a period of 1000 h. Analyzing the measured rate of microstructure growth could help to identify dendrite mitigation strategies, such as enhanced cell pressure or additives, and could enable a method for battery lifetime prediction., Comment: 20 pages, 9 figures