Self-assembly of core–shell structures driven by low doping limit of Ti in LiCoO₂: first-principles thermodynamic and experimental investigation

Abstract

Introducing additives is a general method of performance improvement in materials engineering, but details regarding whether the additive is doped in the host crystal or present as a secondary phase are usually examined from experimental experience, with a systematic theoretical prediction lacking, which sometimes causes controversy on the role of additives. In this study, the dopability of Ti in crystalline LiCoO (LCO) is investigated by a first-principles simulation method, and the doping limit is quantitatively calculated. The probability of Ti substitution for Co is examined and related to point-defect formation in LCO as a function of the general experimental variables of temperature and gas-phase partial pressures, enabling practical use of the theoretical model for real experiments. It was found that Ti substitution for Co, accompanied by the formation of a Li vacancy, is the most probable Ti doping form in LCO, but the doping limit is very low and most Ti would segregate into secondary phases. The theoretical prediction showed good agreement with the experimental results. Based on theoretical predictions, particles having LCO cores and Ti-rich shells are obtained from a simple sol-gel route followed by one-step firing without additional surface treatment. The high-voltage cyclability of LCO is greatly improved. The method demonstrated in this study may be a useful tool for screening suitable coating or doping elements for various material systems and provide a guide for designing simple spontaneous coating processes, as in this study.

An Overview on the Advances of LiCoO2 Cathodes for Lithium‐Ion Batteries

“…[ 69a ] Recent results indicate that only small amount of Ti (<0.15%) can be doped into LiCoO 2 lattice and most of them enrich on the surface/interface of LiCoO 2 . [ 26,77 ] Al is one of the most attractive dopants due to strong AlO bond, similar valence and ionic radius with Co in LiCoO 2 . Ceder et al first predicted theoretically and demonstrated experimentally that the Al doping is able to increase the open‐circuit voltage and working voltage of LiCoO 2 .…”

Surface enrichment and diffusion enabling gradient-doping and coating of Ni-rich cathode toward Li-ion batteries

“…Because structure disintegration usually commences around the crystal surface 17 , 18 , and considering the specific capacity, an effective surface-enrichment gradient doping is required for this cathode. Theoretical calculations revealed that the doping efficiency is generally low because of easily formed dopant-containing electrochemical inert compounds on the particle surface 19 , 20 . Because of the sensitivity of the Ni-rich oxide precursor to moisture and air, and abundant Li source (i.e., Li 2 CO 3 , LiOH) 21 , 22 , the traditional multistep coating and doping approach is undesirable for industrial production.…”

Thermochemical investigation of Zr doping in LiNi8/12Co2/12Mn2/12O2 based on phase equilibria simulation

“…In our previous reports, an entire crystal was considered as the combination of unit supercells containing each type of defect. The probability of each defect type, or the proportion of each supercell unit in the entire crystal, was calculated by considering the mixing entropy of the units, which demonstrated that the proportion of each unit was determined by the Boltzmann factor. Therefore, in this study, the macroscale LNCM crystal is considered as an ensemble system comprising a combination of supercell units that may be Zr‐doped or undoped, and the ratio between the doped and undoped units is determined by the Boltzmann factors.…”