Systematic “Apple‐to‐Apple” Comparison of Single‐Crystal and Polycrystalline Ni‐Rich Cathode Active Materials: From Comparable Synthesis to Comparable Electrochemical Conditions

Abstract

State‐of‐the‐art ternary layered oxide cathode active materials in Li‐ion batteries (LIBs) consist of polycrystalline (PC), i.e., micron‐sized secondary particles, which in turn consist of numerous nanosized primary particles. Recent approaches to develop single crystals (SCs), i.e., single and separated micron‐sized primary particles, appear promising in terms of cycle life given their mechanical stability. However, a direct and systematic (“fair”) comparison of SC with PC in LIB cell application remains a challenge due to both differences on material level and state‐of‐charge (SoC), as SCs typically have slightly lower delithiation capacities/Li+ extraction ratios. In this work, PC and SC Li[Ni0.8Mn0.1Co0.1]O2 (NMC811) are synthesized with comparable bulk and surface characteristics from identical self‐synthesized precursors. Indeed, the cycle life of SCs is not only superior, when conventionally charged to equal upper cutoff voltage (UCV), as shown in NMC||Li and NMC||graphite cells, but also after adjusting UCVs to similar SoCs, where bigger SCs counterintuitively have even a better rate performance and cycle life.