Since their market introduction in 1991, lithium (Li)-ion battery is the main solution to power electronic portable devices, and after almost three more decades of development this technology is embedded in modern electric and hybrid cars. However, its maturity leaves little gap to increase its energy density in the order of 250 Wh.kg-1. In order to increase their energy density, a promising solution is to use positive materials with high potential (up to 5V vs Li + / Li) [1] . The spinel LiNi_1/2Mn_3/2O₄ which owns a capacity of 147 mAh/g and a potential of 4.8 V vs Li + / Li, enables to reach a high energy density (at least 20% higher than the state of the art Li-ion technology), However, this operating potential is largely above the electrochemical stability range of conventional electrolytes based on carbonate solvents [2] . Cycling tests reveals a low coulombic efficiency induced by electrolyte oxidation and capacity loss induced by material degradation [3] . A simple model based on coulombic
efficiency has been developed to quantify the solvent decomposition current. By comparing values obtained on thin film and composite electrode, we proved that the electrolyte oxidation mainly happen on the surface of the LNMO, with a main reaction that can simply be written as: Li + + EC + NMO → EC + + LiNMO where EC stand for a solvent electrolyte molecule and EC+ the product of the solvent oxidation.

The rate of solvent decomposition is kinetically controlled by the charge-transfer, therefore a simple anodic Tafel expression has been used to model the current density due to this mechanism and has been compared to our model based on coulombic efficiency. Tests at different C rates indicate that the longer the electrode stays at high potential, the higher is the capacity fading, showing that the product of electrolyte oxidation seems to be involved into material degradation. In order to get a more deep insight into these parasitic reactions (oxidation of the electrolyte, active material degradation), cells were tested at different temperatures (10, 25 and 50°C). Especially, we have obtained that coulombic efficiency is strongly improved when the operation temperature is decreased, in full agreement with thermal activation of the electrolyte decomposition Inter estingly, we obtained a lower capacity fading which demonstrates the strong link between faradic efficiency (electrolyte oxidation) and material degradation.

[1] Tarascon et al. Nature Mater. 2011
[2] Yi et al. J. Power Sources, 2016
[3] Pieczonka al. J. of Phys. Chemistry, 2013