PNNL

Abstract

Rechargeable Li-ion batteries (LIBs) have attracted considerable attention and already been widely used in portable electronic devices and electric vehicles due to their unparalleled advantages (high energy density, high voltage, etc.). As one of the most promising cathode materials, the Ni-rich LiNixMnyCo1-x-yO2 (x=0.6) cathodes (Ni-rich NMC) have been extensively investigated because of their high specific capacity (more than 200 mAh/g). While high Ni content in Ni-rich NMC cathodes leads to their high specific capacity, their structural instability results in their capacity loss and poor cyclability, thereby hindered their large-scale commercial applications. So far, there have been several mechanisms to dissect the performance degradation caused by the unstable structure in Ni-rich NMC.(Figure 1) (1) The closed ion size between Ni2+ (0.069 nm) and Li+ (0.076 nm) brings irreversible phase transformation in Ni-rich NMC from layered structure to spinel structure or rock-salt structure owing to the cation migration during deintercalation. The lack of electrochemical activity from the phase transformation leads to capacity fading. It is noted that the irreversible phase transformation is usually initiated from the particle surface and propagated into the bulk. (2) Side reaction between Ni-rich NMC and electrolyte is another pragmatic mechanism. Inevitably, it forms undesirable surface film on the surface of cathode materials causing irreversible capacity loss and high impedance. (3) Dissolution and corrosion of transition metal components at the particle surface, which is originated from the interaction between cathode and electrolyte, bring non-trivial effect on structure stability. Moreover, oxygen evolution reaction also happens at high voltage during charging. (4) Intergranular and intragranular crack triggered by volume change during (de)intercalation breaks the formed cathode electrolyte interphase (CEI) and exposes fresh surface again which consumes active lithium by forming new CEI. Based on the failure mechanism discussed above, it is clear that the majority of structural instability and capacity loss originates from the cathode surface or the newly generated surface. Hence, surface engineering is highly desired to suppress the surface reaction to realize high electrochemical performance.