Corrosion/Fragmentation of Layered Composite Cathode and Related Capacity/Voltage Fading during Cycling Process

Abstract

The Li-rich, Mn-rich (LMR) layered structure materials exhibit very high discharge capacities exceeding 250 mAh g(-1) and are very promising cathodes to be used in lithium ion batteries. However, significant barriers, such as voltage fade and low rate capability, still need to be overcome before the practical applications of these materials. A detailed study of the voltage/capacity fading mechanism will be beneficial for further tailoring the electrode structure and thus improving the electrochemical performances of these layered cathodes. Here, we report detailed studies of structural changes of LMR layered cathode Li[Li0.2Ni0.2Mn0.6]O2 after long-term cycling by aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The fundamental findings provide new insights into capacity/voltage fading mechanism of Li[Li0.2Ni0.2Mn0.6]O2. Sponge-like structure and fragmented pieces were found on the surface of cathode after extended cycling. Formation of Mn(2+) species and reduced Li content in the fragments leads to the significant capacity loss during cycling. These results also imply the functional mechanism of surface coatings, for example, AlF3, which can protect the electrode from etching by acidic species in the electrolyte, suppress cathode corrosion/fragmentation, and thus improve long-term cycling stability.

Surface reconstruction and chemical evolution of stoichiometric layered cathode materials for lithium-ion batteries

“…Previous studies have attributed the surface reduced layer mainly to electrochemical processes with little attention dedicated to the effects of electrode-electrolyte reactivity 4,5,17 . Here we found that a surface reduced layer similar to the ones observed during high-voltage cycling is also created after an NMC electrode was immersed in the electrolyte (Fig.…”

“…Structural and/or chemical rearrangements at surfaces determine the way a material interacts with its surrounding environment, thus controlling the functionalities of the material [6][7][8][9][10] . Specifically, the surfaces of lithium-ion battery electrodes evolve simultaneously with charge-discharge cycling (that is, in situ surface reconstruction and formation of a surface reaction layer (SRL)) that can lead to deterioration of performance 4,5,11 . An improved understanding of in situ surface reconstruction phenomena imparts knowledge not only for understanding degradation mechanisms for battery electrodes but also to provide insights into the surface functionalization for enhanced cyclability 12,13 .…”

“…C hemical evolution and structural transformations at the surface of a material directly influence characteristics relevant to a wide range of prominent applications including heterogeneous catalysis [1][2][3] and energy storage 4,5 . Structural and/or chemical rearrangements at surfaces determine the way a material interacts with its surrounding environment, thus controlling the functionalities of the material [6][7][8][9][10] .…”

Study of Cathode Materials for Lithium-Ion Batteries: Recent Progress and New Challenges

“…Three main reduction processes (which accompany Li-intercalation) can be identified: Re1 at about 4.5 V is related to Li occupation in the tetrahedral sites; Re2 at 3.7 V can be ascribed to Li occupation in the octahedral sites corresponding to Ni 4+ /Ni 2+ and Co 4+ /Co 3+ ; and Re3 at <3.5 V can be explained by Li occupation in the octahedral sites associated with Mn 4+ /Mn 3+ redox [83,84]. Further delithiation beyond 4.5 V during the first cycle leads to oxygen vacancies and the subsequent migration of transition metal ions into lithium sites and lithium ions into tetrahedral sites, resulting in a distorted oxygen lattice [85][86][87][88]. Figure 11B,D shows that all the reduction peaks shift with cycling, but the shifting is more serious for 0.5Li 2 migration of transition metal ions into lithium sites and lithium ions into tetrahedral sites, resulting in a distorted oxygen lattice [85][86][87][88].…”

High‐Performance Heterostructured Cathodes for Lithium‐Ion Batteries with a Ni‐Rich Layered Oxide Core and a Li‐Rich Layered Oxide Shell

“…To overcome this problem, many studies have focused on stabilizing the structure with various efforts, such as surface modifications and transition‐metal‐ion substitutions,2, 5, 6, 7, 8 but none of the efforts could completely eliminate the phase transition. In this regard, new approaches are needed to develop high‐capacity cathodes.…”