Influences of direction and magnitude of Mg2+ doping concentration gradient on the performance of full concentration gradient cathode material

Achieving a highly stable structure is the key to successfully enhancing the cycling stability and safety performance of LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode material for Li-ion batteries at a high cutoff voltage (4.5 V). In this work, the co-precipitation method is applied to prepare the precursor of the full concentration gradient (FCG) NCM811, and Mg2+ is doped into bulk phase to prepare LiNi0.8-xCo0.1Mn0.1MgxO2 (x = 0.01, 0.02, 0.03, and 0.04) for modification. Influences of magnesium doping direction and magnitude on electrochemical performance are investigated. It is found that Li(Ni0.8Co0.1Mn0.1)1-xMgxO2 shows low Li+/Ni2+ cation mixing in the voltage of 2.7–4.5 V with high structural stability and improves cycling stability because of the pillaring effect of inactive Mg in the crystal structure. In terms of the doping direction, compared with the uniform magnesium doping material, the specific discharge capacity with a forward concentration gradient doping is increased by 5.96%, and the cycle stability can be promoted as high as 6.33%. For the magnitude of the doping concentration gradient, the specific discharge capacity compared to pristine (Ni0.8Co0.1Mn0.1O2) material can be increased by 10.1%, and correspondingly the cycle stability can be promoted as high as 13.7%. After doping 3% Mg2+ with the optimal forward concentration gradient direction and cycling for 200 cycles at 2.7 ~ 4.5 V and 1 C, the specific discharge capacity is up to 163.5 mAh/g, and the capacity retention rate can keep 82.99%. Additionally, LiNi0.8Co0.1Mn0.1O2 doped with 3% Mg2+ along the forward concentration gradient direction has a positive effect on reducing the material impedance.