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[Ni_0.8Mn_0.1Co_0.1]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.