Tungsten Infused Grain Boundaries Enabling Universal Performance Enhancement of Co-Free Ni-Rich Cathode Materials

Abstract

Ni-rich cathode materials suffer from poor capacity retention due to micro-cracking and interfacial reactivity with electrolyte. Addition of tungsten (W) to some Ni-rich materials can improve capacity retention. Here, a WO3 surface coating is applied on Ni-rich hydroxide precursors before heating with lithium hydroxide. After heating in oxygen, Ni-rich materials with any of the commonly used dopants (magnesium, aluminum, manganese, etc.) show a “universal” improvement in capacity retention. Experimental characterization and theoretical modelling showed W was concentrated in the grain boundaries between the primary grains of secondary particles of the layered oxides, and W is incorporated in amorphous LixWyOz phases rather than as a substituent in the LiNiO2 lattice. This self-infusion of W in the grain boundaries during synthesis also significantly restricts primary crystallite grain growth. Along with smaller primary grain size, the LixWyOz phases in the grain boundaries lead to improved resistance to microcracking and reduced surface or interfacial reactivity. Improving the intrinsic properties of primary grains through doping of Mg, Al, or Mn and reinforcing the secondary particle structure mechanically and chemically using W or a similar element, M, that forms LixMOy phases and does not substitute into LiNiO2 is a universal strategy to improve polycrystalline Ni-rich materials.

Electrochemical Characterization and Microstructure Evolution of Ni-Rich Layered Cathode Materials by Niobium Coating/Substitution

“…The slightly expanded lattice parameters can be explained by two possible reasons: (1) the larger radius of Nb 5+ than Ni 3+ , Co 3+ , and Mn 4+ , indicating that Nb has been successfully participated into NMC 9055 hexagonal structure; (2) temperature effect, which was supported by W modification did not significantly affect the lattice parameters and the lattice parameters (W-NiMnAl heated at 750 °C) were slightly increased in Dahn's study. 38 Figure 1d shows the relative intensity between (003)/(104) peaks, which is often used as an indicator of Li + and Ni 2+ cation mixing. With increasing Nb concentration, I(003)/ I(104) of the samples slightly changed from 1.59 (NMC 9055) to 1.68 (0.7% Nb-coated/substituted NMC 9055), 1.53 (1.4% Nb-coated/substituted NMC 9055), and 1.49 (2.1% Nbcoated/substituted NMC 9055).…”

High‐Energy Ni‐Rich Cathode Materials for Long‐Range and Long‐Life Electric Vehicles

“…Microstructural manipulation can be achieved by compositional gradient design of transition metal ions [ 19,24,25,31 ] or by doping with various atoms (e.g., B, Ta, Mo, W, and Sb) during lithiation. [ 18,26–30,32,33 ] Furthermore, a reduction in volume deformation and internal stress inside the particle via Al doping can enhance the structural integrity and robustness of cathode. [ 30,34,35 ]…”

“…Microstructural manipulation can be achieved by compositional gradient design of transition metal ions [19,24,25,31] or by doping with various atoms (e.g., B, Ta, Mo, W, and Sb) during lithiation. [18,[26][27][28][29][30]32,33] Furthermore, a reduction in volume deformation and internal stress inside the particle via Al doping can enhance the structural integrity and robustness of cathode. [30,34,35] In addition to microcracking, the time during which highly reactive Ni 4+ ions are exposed to the electrolyte when the cathode is in a highly charged state also affects the deterioration of the cathode.…”

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

“…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. [34,35] This makes the bulk Li-deficient, leading to the formation of off-stoichiometric phases. Evidence for amorphous LixWyOz phases can be seen in the XRD data (Fig.…”

“…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.…”