Li y (Ni 0.425 Mn 0.425 Co 0.15 ) 0.88 O 2 materials were synthesized by a slow rate electrochemical deintercalation from Li 1.12 (Ni 0.425 Mn 0.425 Co 0.15 ) 0.88 O 2 during the first charge and the first discharge in order to study the structural modifications occurring during the first cycle and especially during the irreversible "plateau" observed in charge at 4.5 V vs Li + /Li. Chemical Li titrations showed that the lithium ions are actually deintercalated from the material during the entire first charge process, excluding the possibility that electrolyte decomposition causes the "plateau". Redox titrations revealed that the average transition metal oxidation state is almost constant during the "plateau", despite further lithium ion deintercalation. 1 H MAS NMR data showed that no Li + /H + exchange was associated to the "plateau" itself. Rietveld refinement of the XRD pattern for a material reintercalated after being deintercalated at the end of the "plateau", as well as redox titrations, revealed an M/O ratio larger than that of the pristine material, which is consistent with the oxygen loss proposed by Dahn and coauthors for the LiNi x Li (1/3-2x/3) Mn (2/3-x/3) O 2 materials to explain the irreversible overcapacity phenomenon observed upon overcharge. X-ray and electron diffraction showed that the transition metal ordering initially present within the slabs is lost during the "plateau" due to a cation redistribution. To explain this behavior a cation migration to the vacancies formed by the lithium deintercalation from the transition metal sites (3a) is assumed, leading to a material densification.

Layered Li 1+x (Ni 0.425 Mn 0.425 Co 0.15 ) 1-x O 2 materials (0 x 0.12) were prepared at 1000°C for 12 h in air by a coprecipitation method. As x increased in Li 1+x (Ni 0.425 Mn 0.425 Co 0.15 ) 1-x O 2 , the substitution of x Li + ions for x transition metal ions induced for charge compensation an increase in the average transition metal oxidation state. X-ray photoelectron spectroscopy analyses showed that cobalt and manganese were present in these materials in the trivalent and tetravalent states, respectively, and that increasing overlithiation led to the oxidation of Ni 2+ ions into Ni 3+ ions. The refinement of the crystal structure of these materials in the R m space group and magnetic measurements showed a decrease in the Ni occupancy in the Li layers with increasing overlithiation. From an electrochemical point of view, the reversible capacity in the 2-4.3 V range decreased with overlithiation Keywords :Although LiCoO 2 is suitable for the lithium-ion battery application, its high cost and toxicity prevent its use in low-price or large devices. Positive electrodes with LiNiO 2 revealed an attractive reversible capacity 1 but suffered from a quite poor capacity retention 2 and also from a low thermal stability of their deintercalated phases. 3,4,5,6 Partial substitution for nickel allowed an optimization of these properties for compositions such as LiNi (1-x-y) Co x Al y O 2 . 7,8,9 Nevertheless, there is still a need for cheaper and safer positive electrode materials with higher electrochemical performances. Recently lithium-rich manganese-based materials such as Li[Ni x Li (1/3-2x/3) Mn (2/3-x/3) ]O 2 and Li[Co x Li (1/3x/3) Mn (2/3-2x/3) ]O 2 were extensively studied by various research groups. 10,11,12,13 Interesting results were obtained, for instance for the Li[Ni 1/3 Li 1/9 Mn 5/9 ]O 2 phase with a capacity of 230 mAh/g between 2.0 and 4.6 V at 55°C. 14 In all these materials, the manganese ions are in the tetravalent state in the pristine material 15 so that they are electrochemically inactive. Because there are no Mn 3+ ions, no structural evolution to the spinel structure is expected to occur upon cycling, on the contrary to what was observed for the layered LiMnO 2 . 16,17,18,19 Furthermore, the presence of a large amount of manganese ions at the stable tetravalent oxidation state is thought to be responsible for a higher thermal stability. Differential scanning calorimetry experiments (DSC) on charged electrodes of Li[Ni x Li (1/3-2x/3) Mn (2/3-x/3) ]O 2 for x=5/12 indicate that this material should be thermally safer than LiCoO 2 . 10 The DSC profiles of the fully oxidized Li x [Li 0.12 Ni z Mg 0.32-z Mn 0.56 ]O 2 (z=0.3) material also demonstrate much higher thermal stability than Li x CoO 2 . 20 Note also that most of these overlithiated materials exhibit an irreversible plateau at around 4.5 V/Li during the first charge. The origin of this plateau was attributed by Lu and Dahn to be due to an irreversible oxygen loss. 14