Stabilizing LiNi_0.8 Co_0.15 Mn_0.05 O₂ Cathode by Doping Sulfate for Lithium-Ion Batteries

Residual sulfate (SO₄ ^2- ) in precursor Ni_0.8 Co_0.15 Mn_0.05 (OH)₂ (pre-NCM) is commonly regarded as being harmful to Li[Ni_0.8 Co_0.15 Mn_0.05 ]O₂ (NCM) performance, leading to significant performance losses and also hampering the electrode fabrication. Therefore, manufacturers try their best to lower sulfate contents in pre-NCM. However, how the sulfate affects the cathode materials is not systematically studied. To address these issues, NCM was synthesized with different amounts of intentionally added sulfate (NH₄ )₂ SO₄ in pre-NCM. It was demonstrated that anionic SO₄ ^2- doped in NCM could influence the grain size in sintering process and stabilize the layer structure during the charge-discharge process at a certain doping amount. The first-principles calculations suggested that the SO₄ ^2- doped in the transition metal layer could effectively facilitate Li⁺ diffusion in the lattice. SO₄ ^2- doping could reduce the energy barrier for Li⁺ migration and then suppress drastic contraction of the c axis during cycling. The phase transition of H2 to H3 caused by c axis contraction was suppressed and the cycling performance was enhanced. The capacity retention could reach 80.9 (0.2 C) and 80.4 % (1 C) after 380 and 240 cycles in coin cells, respectively. These findings illustrate that a certain amount of sulfate could be beneficial to NCM cathodes.