2We report the electrochemical study of cubic and monoclinic NiP 2 polymorphs towards Li, as candidate for anodic applications for Li-ion batteries. We found that the monoclinic form is the most attractive one performance-wise. Monoclinic NiP 2 can reversibly uptake 5 lithium per formula unit leading to reversible capacities of 1000 mAh/g at an average potential of 0.9 V vs Li + /Li°. From complementary XRD and HREM measurements it was shown that during the first discharge the cubic phase undergoes a pure conversion process (NiP 2 + 6 Li + + 6e - Ni° + 2Li 3 P) as opposed to a sequential insertionconversion process for monoclinic NiP 2 . Such a different behaviour rooted in subtle structural changes was explained through electronic structure calculations. Once the first discharged is achieved, both phases were shown to react with Li through a classical conversion process. More importantly, we report a novel way to design NiP 2 electrodes with enhanced capacity retention and rate capabilities. It consists in growing the monoclinic NiP 2 phase, through a vapour phase transport process, on a commercial Nifoam commonly used in Ni-based alkaline batteries. These new self-supported electrodes, based on chemically made interfaces, offer new opportunities to fully exploit the capacity gains provided by conversion reactions.

International audienceThe use of the high energy Li-ion battery technology for emerging markets like electromobility requires precise appraisal of their safety levels in abuse conditions. Combustion tests were performed on commercial pouch cells by means of the Fire Propagation Apparatus also called Tewarson calorimeter in the EU, so far used to study flammability parameters of polymers and chemicals. Well-controlled conditions for cell combustion are created in such an apparatus with the opportunity to analyse standard decomposition/combustion gases and therefore to quantify thermal and toxic threat parameters governing the fire risk namely the rate of heat release and the effective heat of combustion as well as the toxic product releases. Using the method of O2 consumption, total combustion heats and its kinetic of production were determined as a function of the cell state of charge unveiling an explosion risk in the case of a charged cell. The resulting combustion heat is revealed to be consistent with cumulated contribution values pertaining to each organic part of the cell (polymers and electrolytes) as calculated from thermodynamic data. The first order evaluation of the dangerousness of toxic gases resulting from fire induced combustion such as HF, CO, NO, SO2 and HCl was undertaken and stressed the fact that HF is the most critical gas originating from F-containing cell components in our test conditions

FeP y (y = 1, 2, 4) anodes all react with lithium through a conversion reaction FeP y + 3yLi → yLi3P + Fe0 in their first discharge, leading to nanocomposite discharged electrodes described by nanosized Fe0 particles embedded in yLi3P matrixes. From electrochemical and complementary in situ X-ray diffraction and high-resolution transmission electron microscopy studies, we deduce that the conversion reaction occurring during the first discharge is followed by two successive insertion and conversion processes in further cycles for the FeP electrode. The insertion process is highly reversible, leading to a capacity retention of 300 mA h g-1 and 1900 mA h cm-3 after 100 cycles, and corresponds to the formation of an intermediate tetragonal LiFeP phase as deduced from first-principles T = 0 K phase diagram calculations and preliminary Mössbauer analyses. We expect the kinetics of this reaction to be strongly limited by the increase in y, thus leading to an increasing capacity fading when increasing the y P/Fe ratio.