Jatinkumar Rana

On the structural integrity and electrochemical activity of a 0.5Li2MnO3·0.5LiCoO2 cathode material for lithium-ion batteries

Structural changes in a 0.5Li2MnO3·0.5LiCoO2 cathode material were investigated by X-ray absorption spectroscopy. It is observed that both Li2MnO3 and LiCoO2 components of the material exist as separate domains, however, with some exchange of transition metal (TM) ions in their slab layers. A large irreversible capacity observed during activation of the material in the 1st cycle can be attributed to an irreversible oxygen release from Li2MnO3 domains during lithium extraction. The average valence state of manganese ions remains unchanged at 4+ during charge and discharge. In the absence of conventional redox processes, lithium extraction/reinsertion from/into Li2MnO3 domains occurs with the participation of oxygen anions in redox reactions and most likely involves the ion-exchange process. In contrast, lithium deintercalation/intercalation from/into LiCoO2 domains occurs topotactically, involving a conventional Co3+/Co4+ redox reaction. The presence of Li2MnO3 domains and their unusual participation in electrochemical processes enable LiCoO2 domains of the material to sustain a higher cut-off voltage without undergoing irreversible structural changes.

An investigation of the electrochemical delithiation process of carbon coated α-Fe2O3 nanoparticles

The electrochemical lithiation–delithiation of iron oxide is a rather complex process, which is still not fully understood. In this study we investigated the electrochemical lithiation–delithiation mechanism of hematite by means of X-ray diffraction (XRD), 57Fe Mossbauer spectroscopy, high-resolution transmission electron microscopy (HRTEM) and X-ray absorption spectroscopy (XAS). Since the delithiation process has been so far less investigated, particular attention was dedicated to the characterization of the chemical species that are formed during this process. The results of this investigation indicated that at the end of the delithiation process lithium iron oxide α-LiFeO2 is formed. The formation of this compound may be the explanation for the irreversible capacity loss in the first cycle, which is usually assigned to the formation of an organic gel-like layer. Based on these results a new charge–discharge mechanism of hematite in lithium-ion batteries (LIBs) is proposed and discussed.

Unravelling the mechanism of lithium insertion into and extraction from trirutile-type LiNiFeF6 cathode material for Li-ion batteries

LiNiFeF6 was used as cathode material in lithium-ion cells and studied by in situ X-ray diffraction (XRD), in operando X-ray absorption spectroscopy (XAS) and 7Li MAS NMR spectroscopy. An optimised electrochemical in situ cell was employed for the structural and electrochemical characterisation of LiNiFeF6 upon galvanostatic cycling. The results for the first time reveal the lithium insertion process into a quaternary lithium transition metal fluoride with a trirutil-type host structure (space group P42/mnm). The in situ diffraction experiments indicate a preservation of the structure type after repeated lithium insertion and extraction. The lithium insertion reaction can be attributed to a phase separation mechanism between Li-poor Li1+x1NiFeF6 and Li-rich Li1+x2NiFeF6 (x1 ≲ 0.16 ≲ x2), where not only the weight fractions, but also the lattice parameters of the reacting phases change. The insertion of Li ions into [001]-channels of the trirutile structure causes an anisotropic lattice expansion along the tetragonal a-axes. An overall increase in the unit cell volume of ~6% and a reduction in the c/a ratio of ~4% are detected during discharge. Changes of atomic coordinates and distances suggest the accommodation of intercalated lithium in the empty six-fold coordinated 4c site. This is confirmed by 7Li MAS NMR spectroscopy showing two Li environments with similar intensities after discharging to 2.0 V. Furthermore, in operando XAS investigations revealed that only Fe3+ cations participate in the electrochemical process via an Fe3+/Fe2+ redox reaction, while Ni2+ cations remain electrochemically inactive.