Synthesis of Tantalum Doped NMC811 and Its Impact on Crystal Structure and Electrochemical Performance at Higher Upper Cut-off Voltage

Abstract

Ni-rich, cathode active materials such as Li[Ni0.8Mn0.1Co0.1]O2 (NMC811) suffer from capacity fade, especially at higher upper cut-off voltages. There is an increasing interest on dopants such as Mo or Ta as a promising dopants for high-Ni materials. In this study four cathode active materials with Li[(Ni0.78Mn0.11Co0.11)1−xMx]O2 (NMC811, x = 0, 0.005, 0.007, 0.013; M = Ta) were synthesized via a batch co-precipitation synthesis followed by calcination. To increase the probability of homogeneous distribution of the dopant, the highly porous hydroxide precursor was impregnated with Ta-ethoxide and afterwards calcined. Focus of this study is the effect of Ta on the crystal structure and its influence on stability at high voltage operation. Using Rietveld method, trends for several unit cell parameters, such as c-parameter, crystallite size, microstrain, transition metal-oxygen bond lengths) were found. Doping NMC811 with 0.5 mol% Ta leads to <3% lower initial capacity (205 mAh g−1 at C/10), >5% higher capacity retention (>85%), and >7% higher accumulated energy output over 100 cycles at 1 C (66.3 kWh kg−1 for 3.0–4.5 V vs Li+/Li). This performance enhancement is attributed to the high transition metal—oxygen bond dissociation energy, which is thought to significantly suppress surface phase transformations, and hence, reduce the vulnerability towards material degradation.