Christian Jordy

Layered Li1 + x ( Ni0.425Mn0.425Co0.15 ) 1 − x O2 Positive Electrode Materials for Lithium-Ion Batteries

Layered Li 1 + x (Ni 0 . 4 2 5 Mn 0 . 4 2 5 Co 0 . 1 5 ) 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 . 4 2 5 Mn 0 . 4 2 5 Co 0 . 1 5 ) 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 R3m 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.

Synthesis of Li1.1(Ni0.425Mn0.425Co0,15)0.9O1.8F0.2 Materials by Different Routes : Is There Fluorine Substitution for Oxygen?

A one-step synthesis method was used, with LiF or NiF 2 as fluorine precursor, to prepare Li 1.1 (Ni 0.425 Mn 0.425 Co 0.15 ) 0.9 O 1.8 F 0.2 materials. 7 Li and 19 F magic angle spinning NMR analyses revealed the presence of fluorine as LiF at the surface of the Li(Ni 0.425 Mn 0.425 Co 0.15 )O 2 particles, rejecting the formation of fluorine-substituted Li 1.1 (Ni 0.425 Mn 0.425 Co 0.15 ) 0.9 O 8 F 0.2 materials. These results highlighted that change in cell parameters with increasing fluorine content is not by itself proof for effective fluorine substitution for oxygen in layered oxides and that heterogeneity in the transition metal and fluoride-ion distribution at the crystallite scale can be at the origin of these modifications. LiF was shown to be present as small particles in some grain boundaries but not as a continuous layer covering the particles surface. Improved cycling stability was observed for these LiF-coated materials, showing that effective fluorine substitution for oxygen is not required for improvement of the cyclability of these layered oxides; a surface modification can be sufficient and can also have a huge impact.

A combined Mössbauer spectroscopy and x-ray diffraction operando study of Sn-based composite anode materials for Li-ion accumulators

The reaction mechanisms of Li with Sn/BPO4 composites to be used as negative electrode materials for Li-ion batteries were studied during electrochemical cycling by operando Mössbauer spectroscopy and X-ray diffraction using a specifically conceived in situ electrochemical cell. The starting composites consist of three main components: β-Sn particles as the electrochemically active species, an inactive matrix of BPO4 and an amorphous SnII-borophosphate interfacial phase linking the two former components and improving the cohesion of the composite. During the first discharge, the latter Sn(II) species are first reduced to zerovalent tin forming Li-poor Li–Sn alloys. After its complete reduction, the reaction of Li continues with β-Sn leading to Li–Sn alloys increasingly rich in Li, with a final composition between those of Li7Sn2 and Li13Sn5. X-ray diffraction shows a progressive loss of long range order of the composites with the suppression of the diffraction peaks of the initial β-Sn and the formation of an ill-defined mixture of Li–Sn alloys. The evolution of this mechanism is investigated on going from a reference Sn/BPO4 composite prepared by conventional ceramic methods with common micrometric BPO4 to a new improved material prepared by carbothermal synthesis starting from nanometric BPO4. With the new composite prepared by carbothermal synthesis, a significant improvement of the reversible capacity at the first cycle is obtained together with a slight improvement of the cycling behaviour. An additional improvement can be obtained by increasing the rate of the first discharge, and thus hampering the formation of the thermodynamically stable LiSn intermetallic.

Structure and Electrochemical Performances of the Li 1+x (Ni 0.425 Mn 0.425 Co 0.15 ) 1−x O 2 Materials

Layered Li1 +x (Ni 0.425 Mn 0.425 Co 0.15 ) 1−x O 2 materials (x = 0 and 0.12) have been prepared by a coprecipitation method. The substitution of × Li + ions for × metal ions in the transition metal layers leads to an increase in the average transition metal oxidation state for charge compensation, which was found to be due to the oxidation of Ni 2+ ions into Ni 3+ ions. The refinement in the R-3m space group of the X-ray diffraction data has shown an increasing lamellar character for the structure of these materials, with increasing overlithiation. Electron diffraction has revealed the presence of a √3.a hex . × √3.a hex . superstructure due to a cationic ordering in the transition metal layers, those being packed with faults along the c hex. -axis. From an electrochemical point of view, the reversible capacity in the 2–4.3V decreased with overlithiation, in agreement with a decreasing amount of exchangeable electrons.