The Effect of Lithium Content and Core to Shell Ratio on Structure and Electrochemical Performance of Core-Shell Li_(1+x)[Ni_0.6Mn_0.4]_(1−x)O₂Li_(1+y)[Ni_0.2Mn_0.8]_(1−y)O₂Positive Electrode Materials

Abstract

Core-shell positive electrode materials with a core:shell mass ratio of 2:1 and 4:1 were synthesized in a two-step reaction. Powder X-ray diffraction, SEM and spatial EDS measurements were used to characterize the core and shell phases in the precursors and lithiated products. It was determined using EDS that the precursor and lithiated products are both core-shell and the two phases can be easily resolved with laboratory grade XRD equipment. Two phase Rietveld refinement was completed on the core-shell lithiated products. The results of these refinements in conjunction with contour plots of the lattice parameters within the Li-Ni-Mn oxide layered single phase region were used to position the core and shell of each sample on the Li-Ni-Mn-O phase diagram as a function of the amount of Li 2 CO 3 used in synthesis. The shell phase retained an approximately fixed amount of Li while the Li content of the core phase increased as the overall Li content of the core-shell sample increased. Both the core and shell were electrochemically active. A specific capacity of 220 mAh/g was achieved in a core shell material between 2.5-4.6 V vs. Li/Li + . © The Author ( There has been an extraordinary effort to synthesize and characterize different materials for the positive electrode of lithium-ion batteries.1,2 A primary motivation of this research is to improve the performance of lithium-ion cells to meet requirements for electric vehicle (EVs) and grid storage applications. As the choices of positive electrode materials move away from current commercial staples such as LiFePO 4 5 the cell must be charged to higher potentials, sometimes as high as 4.8 V vs. Li/Li + , to access the additional reversible capacity. At higher potentials, degradation of the electrolyte through oxidation on the positive electrode may vastly reduce the lifetime of the cell. For the high energy density of new materials to be fully utilized, oxidation of the electrolyte must be minimized. Otherwise, these materials are of little commercial interest for EVs and grid storage where lifetime requirements of decades of useful service would be difficult to achieve.The rate of parasitic reactions that degrade the electrolyte at the positive electrode's surface are affected by the composition of the electrolyte, the potential of the positive electrode, the composition of the positive electrode and many other parameters in the construction and operation of the cell. Commonly, researchers are addressing this by improving electrolyte formulations via additives and solvent blends.6,7 A complementary approach is examining the positive electrode surface composition and its effect on the degradation of the electrolyte. [8][9][10][11][12] Rowe et al. 13 showed via high precision charger measurements that the columbic efficiency and charge end point capacity slippage were dependent on the composition of the positive electrode of various LiNi-Mn layered oxides. Samples in that study were cycled up to 4.6 V vs. Li/Li + with the same additive-free electrolyte formulation. ...

Study of the Failure Mechanisms of LiNi0.8Mn0.1Co0.1O2Cathode Material for Lithium Ion Batteries

“…The calendar and cycle lifetimes of cells are affected by the structural stability of the active electrode materials as well as the parasitic reactions that occur in lithium ion batteries. The degree of lithium utilization of LiCoO 2 is limited to ∼70% in order avoid the O3 -H1-3 -O1 phase transformation when charged above 4.45 V.1 Parasitic reactions such as electrolyte oxidation at the cathode-electrolyte interface can ultimately cause cell failure.2-5 The rate of the parasitic reactions is related to both the catalytic role of the cathode surface which depends on its composition and surface area, 3,6 as well as on the stability of the electrolyte.2-5 Methods such as the use of electrolyte additives 7-11 and core-shell positive electrode materials [12][13][14] have been developed and studied to reduce the rate and extent of parasitic reactions, and hence increase capacity retention and lifetime of high-voltage Li-ion cells. The layered lithium Ni-Mn-Co oxides Li 1+x (Ni y Mn z Co (1-y-z) ) 1-x O 2 (NMC) are considered to be promising positive electrode materials.…”

“…2-5 Methods such as the use of electrolyte additives 7-11 and core-shell positive electrode materials [12][13][14] have been developed and studied to reduce the rate and extent of parasitic reactions, and hence increase capacity retention and lifetime of high-voltage Li-ion cells. The layered lithium Ni-Mn-Co oxides Li 1+x (Ni y Mn z Co (1-y-z) ) 1-x O 2 (NMC) are considered to be promising positive electrode materials.…”

“…[2][3][4][5] The rate of the parasitic reactions is related to both the catalytic role of the cathode surface which depends on its composition and surface area, 3,6 as well as on the stability of the electrolyte. [2][3][4][5] Methods such as the use of electrolyte additives [7][8][9][10][11] and core-shell positive electrode materials [12][13][14] have been developed and studied to reduce the rate and extent of parasitic reactions, and hence increase capacity retention and lifetime of high-voltage Li-ion cells. The layered lithium Ni-Mn-Co oxides Li 1+x (Ni y Mn z Co (1-y-z) ) 1-x O 2 (NMC) are considered to be promising positive electrode materials.…”

In Situ Powder Diffraction Studies of Electrode Materials in Rechargeable Batteries

“…Typical electrodes for lithium-and sodium-ion batteries contain am ixture of the active material, ab inder to hold the particlest ogether and onto the current collector,a nd ac arbon-based conductive matrix to ensure conductivity between active materialp articles and the current collector. [9] Althoughr esearchers are workingo nc omplex core-shell morphologies of the active materials [10] or composite electrodes [11] and binder-free approaches, [12] constructing the electrode at the macroscopic level plays as ignificant role in battery performance. Thus, incorporating macroscopic and atomic-level detail from crystallographic studies in ah olistica pproach will lead to better batteries.…”

The Effect of Interdiffusion on the Properties of Lithium-Rich Core-Shell Cathodes

“…[18][19][20] These have a high Ni content in the core and increasing Mn content from the core to the surface, with a maximum Mn content on the surface of ∼50%. In our previous report, 17,21 Li-rich and Mn-rich materials 22,23 were used as the protecting shell for voltages above 4.5 V. It was shown that the Li-rich and Mn-rich shell protected the Ni-rich core from reactions with the electrolyte while the Ni-rich core rendered a high and stable average voltage. 21 Diffusion of the cations between the core and shell phases occurs during sintering to prepare the NMC core-shell oxide.…”