Junhyeok Kim1 Jaephil Cho1

1, Ulsan National Institute of Science and Technology, Seoul, , Korea (the Republic of)

The layered nickel-rich materials have attracted extensive attention as a promising cathode candidate for high-energy density lithium-ion batteries (LIBs). However, they have been suffering from inherent structural and electrochemical degradation including severe capacity loss at high electrode loading density (>3.0 g cm3) and high temperature cycling (>60°C). The structural issues originate from the phase transition (from layered to rock-salt like structure) of nickel-rich cathode upon electrochemical cycling. The ionic radius between lithium ion (0.76 Å) and divalent nickel ion (0.69 Å) is substantially similar, thus divalent nickel ion can easily migrate into lithium slabs.<span style="font-size:10.8333px"> </span>This cation mixing layer consisted of inactive NiO-like phase deteriorates the lithium ion transport and causes poor thermal stability. In terms of chemical stability, the residual lithium compounds such as LiOH and Li2CO3 in the nickel-rich cathode are strongly related to safety issues. The unstable trivalent nickel ions in host structure can increase these unwanted residual lithium compounds during synthesis process or storage period in air, resulting in electrochemical decomposition upon cycling and preventing lithium ion transport with the increase in charge transfer resistance.[13-17] In addition, the electrocatalytic side reaction between cathode and electrolyte severely increases the non-conducting and unstable solid-electrolyte interphase (SEI) layer on the cathode surface for long-term cycles, thus resulting in poor cycle performance. In this regard, it is highly desired to construct the uniform SEI layer having high ionic conductivity and electrochemical/thermal stability on the cathode at early stage.
Herein, we report a high performance nickel-rich LiNi0.84Co 0.14Al0.02O2 cathode with an artificial SEI layer. We discovered that initially formed SEI compounds effectively enhance the electrode wettability. Notably, these compounds were electrochemically rearranged by interacting with hydrolysis by-products, constructing homogeneous, artificial SEI layer along the grain boundaries of primary particle during the formation cycle. The prepared cathode with homogeneous LixPOy layer and TM concentration gradient demonstrated extremely high thermal stability at 60°C with high structural and morphological integrity. When used at high electrode density of ~3.3 g cm3, this cathode also outperforms in any nickel-rich cathodes (See Table S1). This work suggests that our strategy can simultaneously address two issues related to chemical and structural stability by the newly developed surface engineering method.