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International audienceDeviation from ideal stoichiometry of LiFePO4 has been investigated. Any attempt to increase the Li concentration of samples prepared either by the precursor precipitation route or by the continuous aqueous precursor synthesis route results in the formation of lithium phosphate impurity, in addition to stoichiometric LiFePO4 free of any Li vacancy. On the other hand, Li-deficient homogeneous solid solutions of composition Li1−2xFexFePO4 could be obtained. For x ≥ 0.06, however, a sarcopside impurity phase is formed. Investigations of structural properties allow us to define the defect responsible for the solid solution as Fe•Li + V′Li in the Krger−Vink notation. Because the chemical formula of the sarcopside is obtained by writing x = 1/2 in the chemical formula of the solid solution, this impurity phase can be viewed as a condensation of the Fe•Li + V′Li defects. Magnetic measurements show that isolated lithium vacancies V′Li are also diluted in the Li1−2xFexFePO4 matrix. The negative charge of the isolated V′Li is compensated by the valence change Fe2+ → Fe3+ of an iron ion in its vicinity, forming a small magnetic polaron that is detected by magnetic measurements. The concentration of such polarons, however, remains very small as it saturates to a concentration of 0.2−0.3 mol %, much smaller than the concentration x in V′Li bound to Fe•Li. The electrochemical features are significantly damaged by the Fe•Li defects that block the diffusion of lithium along the corresponding channel, while the Li3PO4 only acts as an inert mass

The study observes thermal behavior of six Lithium-ion batteries with different cell designs. In operando temperature measurements are conducted using a thermographic camera as well as internal and external temperature sensors. The investigated cell designs include pouch cells as well as two high-power and two high-energy 18650 battery types. It is found for all cells that maximum temperatures of the cells correlate linearly with the discharge current. This trend is explained by basic physical laws. High-power and high-energy cells show significant differences, which correlate with electrode thickness and cell resistance. Furthermore, the influence of heat transport at different ambient conditions (climate chambers and use of heat sink) is discussed. A general difference is found for the temperature gradients inside pouch and cylindrical cells.

Durability and performance of Li-ion cells are impaired by undesirable side reactions, observed as capacity decreases and resistance increases during their usage. This degradation is caused by aging mechanisms on the material level including surface film formation, especially in the case of graphite-based anodes. The present study evaluates the applicability of glow discharge optical emission spectroscopy (GD-OES) as a powerful tool to study aging-induced film formation on graphite anodes of Li-ion cells, including deposition of metallic Li. The technique provides depthresolved information on the elemental distribution in the samples from the anode surface to the current collector (through-plane resolution). Additionally, conducting GD-OES depth profiling at different positions of an aged graphite anode reveals differences in surface film growth across the anode plane (in-plane resolution). After verification of the GD-OES method by well-established analytical techniques, aged anodes from commercial state-of-the-art Li-ion cells are analyzed. The results show through-plane and in-plane inhomogeneity in surface film growth: local island-like Li deposition is revealed for 16Ah pouch cells cycled at 45 °C and high charging current density while a more homogeneous Li plating gradient is found for cycling 26650-type cells at −20 °C.

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