Abstract
Nickel-rich layered materials are emerging as cathodes of choice for next-generation high energy density lithium ion batteries intended for electric vehicles. This is because of their higher practical capacities compared to compositions with lower Ni content, as well as the potential for lower raw materials cost. The higher practical capacity of these materials comes at the expense of shorter cycle life, however, due to undesirable structure and chemical transformations, especially at particle surfaces. To understand these changes more fully, the charge compensation mechanism and bulk and surface structural changes of LiNi0.6Mn0.2Co0.2O2 were probed using synchrotron techniques and electron energy loss spectroscopy in this study. In the bulk, both the crystal and electronic structure changes are reversible upon cycling to high voltages, whereas particle surfaces undergo significant reduction and structural reconstruction. While Ni is the major contributor to charge compensation, Co and O (through transition metal-oxygen hybridization) are also redox active. An important finding from depth-dependent transition metal L-edge and O K-edge X-ray spectroscopy is that oxygen redox activity exhibits depth-dependent characteristics. This likely drives the structural and chemical transformations observed at particle surfaces in Ni-rich materials.
Highlights
The need for lithium-ion batteries with higher energy density and lower cost than currently available, for transport applications, has led to intensified interest in Ni-rich NMC (LiNixMnyCozO2; x+y+z≈1, where x>y) cathode materials.[1,2,3,4,5] These materials deliver higher practical capacities in a typically used voltage range than NMCs with lower Ni content (e.g., LiNi1/3Mn1/3Co1/3O2 or NMC-333), and most formulations contain less of the expensive Co component, reducing raw material costs
We employed a combination of synchrotron in-operando X-ray diffraction (XRD), soft X-ray absorption spectroscopy with different detection modes to probe different depths, annular dark-field scanning transmission electron microscopy (ADF-STEM) and electron energy loss spectroscopy (EELS)
STEM-EELS analysis was performed for the particle shown in Figure 1d and show that the oxidation states of three transition metals (i.e., Mn, Co and Ni) and oxygen remain almost unchanged from the surface to the bulk (Figure 1e)
Summary
The need for lithium-ion batteries with higher energy density and lower cost than currently available, for transport applications, has led to intensified interest in Ni-rich NMC (LiNixMnyCozO2; x+y+z≈1, where x>y) cathode materials.[1,2,3,4,5] These materials deliver higher practical capacities in a typically used voltage range than NMCs with lower Ni content (e.g., LiNi1/3Mn1/3Co1/3O2 or NMC-333), and most formulations contain less of the expensive Co component, reducing raw material costs. Changes in bulk and surface electronic structures during the initial cycle.—NMC electrodes are commonly cycled to 4.3 V in lithium half-cells, with practical capacities rising as the Ni content is increased, the theoretical limit of ∼280 mAh/g is not attained.[2,6,32] Raising the charge voltage limit to 4.7 V vs Li+/Li results in increased utilization,[9] but lower capacity retention upon cycling due to rising cell impedance.[17,18] Figure S2 shows capacity retention data for half cells containing NMC-622 cycled to various voltage limits.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
