Single step synthesis of W-modified LiNiO₂ using an ammonium tungstate flux

Abstract

Modification of LiNiO2 with small amounts of W in a simple one-step synthesis process leads to changes in the crystal structure and electrochemical behavior, but it is also consequential for physical features such as the materials' morphology and thermal stability.

Preventing Interdiffusion during Synthesis of Ni-Rich Core–Shell Cathode Materials

“…Coatings of high-valence metal compounds have been explored in Ni-rich materials, including those with W, Ti, Ta, Sb, Nb, etc. − Tungsten addition has been reported previously to improve the performance and recently published work shows that W does not substitute into the TM layer but rather stays in the grain boundaries in the form of Li x W y O z secondary phases. − This behavior is not unique to W. Mo, Nb, and Sb are also observed to be enriched at grain boundaries. , The formation of secondary phases in the grain boundaries can potentially act as a barrier to prevent interdiffusion of transition metals during synthesis, therefore, making it possible to synthesize core–shell and other microstructures even with elements like Mg and Al which ordinarily diffuse rapidly. Stable thin shells may also be possible to produce without compromising the optimum synthesis temperature.…”

Oxygen-redox activity in non-Li-excess W-doped LiNiO2 cathode

“…1b) of pristine WLNO can be fit with a rhombohedral R3 ̅ m unit cell with the refined lattice parameters comparable to those in literature (Table S1). [47,49,50] No traces of unreacted precursors are seen. However, there is compelling evidence that true bulk W doping (i.e., W occupying Li and/or Ni sites) does not occur in WLNO, and that amorphous LixWyOz phases are formed along grain boundaries and particle surfaces.…”

Oxygen-redox activity in non-Li-excess W-doped LiNiO2 cathode

“…1b) of pristine WLNO can be fit with a rhombohedral R3 ̅ m unit cell with the refined lattice parameters comparable to those in literature (Table S1). [33,35,36] No traces of unreacted precursor are seen. However, there is compelling evidence that true bulk W doping (i.e., W occupying Li/Ni sites) does not occur in WLNO, and that amorphous LixWyOz (x/y > 1) phases are formed along grain boundaries and particle surfaces.…”

“…These values are lower than those previously reported for comparable W-doped LiNiO2 samples (~5%). [33,35,36] This is most likely a consequence of the solid-state doping approach, which can impart a higher degree of heterogeneity in the W doping due to its inherent sluggish kinetics. [43] Taken together, the refinements provide strong evidence that the WLNO exhibits some offstoichiometry with an appreciable amount of Ni present in the Li layer.…”

“…To probe the nature of oxidized O 2− species in non-Li-excess cathodes, we performed an O K-edge hr-RIXS study using 2% tungsten doped LiNiO2 (LiNi0.98W0.02O2) as a model system. In addition to its superior electrochemical performance compared to undoped LiNiO2 [33][34][35][36], it is a Mn-free system, which removes the possibility of the theoretically predicted [37] but experimentally unverified [38,39] Mn 4+ to Mn 7+ oxidation and the stabilization of orphaned O 2p orbitals by Mn. [40][41][42] The crystallographic and electronic structure changes in the LiNi0.98W0.02O2 cathode were investigated as a function of delithiation using X-ray diffraction and spectroscopy methods.…”