Probing the Degradation Mechanism of Li₂MnO₃ Cathode for Li-Ion Batteries

Abstract

Capacity and voltage fading of Li2MnO3 is a major challenge for the application of this category of material, which is believed to be associated with the structural and chemical evolution of the materials. This paper reports the detailed structural and chemical evolutions of Li2MnO3 cathode captured by using aberration corrected scanning/transmission electron microscopy (S/TEM) after certain numbers of charge–discharge cycling of the batteries. It is found that structural degradation occurs from the very first cycle and is spatially initiated from the surface of the particle and propagates toward the inner bulk as the cyclic number increases, featuring the formation of the surface phase transformation layer and gradual thickening of this layer. The structure degradation is found to follow a sequential phase transformation: monoclinic C2/m → tetragonal I41 → cubic spinel, which is consistently supported by the decreasing lattice formation energy based on DFT calculations. For the first time, high spatial resolution quantitative chemical analysis reveals that 20% oxygen in the surface phase transformation layer is removed and such a newly developed surface layer is a Li-depleted layer with reduced Mn cations. This work demonstrates a direct correlation between structural degradation and the cell’s electrochemical degradation, which enhances our understanding of Li–Mn-rich (LMR) cathode materials.

On the Localized Nature of the Structural Transformations of Li2MnO3 Following Electrochemical Cycling

“…Lattice parameters of the pristine and cycled samples. [010]-irradiated sample demonstrates a region which is close to a rock-salt crystal structure in the middle of the expected monoclinic structure and its FFT also reveals the ±(002) forbidden refl ection, seen following cycling (Figure 6 c) Contrary to previous reports suggesting that structural transformations are confi ned to, or occur predominately at, the near-surface regions, [ 13,23,24,33 ] our STEM analysis reveals signifi cant areas with local spinel ordering far away from the surface of the electrochemically cycled particles. The STEM image contrast that would result from a Mn 3 O 4 -type spinel is essentially indistinguishable from that which would arise from a LiMn 2 O 4 spinel, differing only in the intensity of the tetrahedral Li site columns that results from partial occupancy by Mn.…”

“…That the transitional regions are not necessarily confi ned to the edges is somewhat contrary to previously published papers on similar oxide materials. [ 13,[21][22][23][24] We note that, away from the particle surface, increased sample thickness and potential overlap of several structural domains result in reduced image contrast (compare, for example, Figure 5 a with the top-left corner of Figure 5 c). In these conditions, FFTs are useful to identify the emerging phases and symmetries in the HR-STEM images.…”

Review—Recent Advances and Remaining Challenges for Lithium Ion Battery Cathodes

“…[58][59][60][61] Additionally a slow, irreversible transition metal migration to the Li + slab occurs at these oxygen deficient sites, since the coordination of transition metals by only 5 O 2− ions is instable, this transition metal migration ultimately resulting in a defect spinel phase formation at the surface of the material. 62,63 These surface reactions are irreversible, causing the large irreversible capacity loss during this cycle, but additional reversible capacity is made possible through charge acceptance by the oxide itself in the material, described above in greater detail.…”

Ni and Co Segregations on Selective Surface Facets and Rational Design of Layered Lithium Transition‐Metal Oxide Cathodes

“…On the other hand, when cathode materials are subjected to battery cycling, material degradation often initiates from the surface layer, as evidenced by recent characterization studies. [17][18][19][20][21][22] For example, a surface reconstruction layer (SRL) was frequently observed on cathode particle surface after battery cycling, which was believed to act as a barrier for Li-ion transport and thus contribute to battery' high polarization and poor rate capability. [17][18][19][20][21][23][24][25] Moreover, such SRL keeps growing from particle surface into inner bulk as battery cycling continues, which is believed to contribute to battery's capacity and voltage decay.…”