Sulfate-Containing Composite Based on Ni-Rich Layered Oxide LiNi_0.8Mn_0.1Co_0.1O₂ as High-Performance Cathode Material for Li-ion Batteries

Composite positive electrode materials (1-x) LiNi_0.8Mn_0.1Co_0.1O₂∙xLi₂SO₄ (x = 0.002-0.005) for Li-ion batteries have been synthesized via conventional hydroxide or carbonate coprecipitation routes with subsequent high-temperature lithiation in either air or oxygen atmosphere. A comparative study of the materials prepared from transition metal sulfates (i.e., containing sulfur) and acetates (i.e., sulfur-free) with powder X-ray diffraction, electron diffraction, high angle annular dark field transmission electron microscopy, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy revealed that the sulfur-containing species occur as amorphous Li₂SO₄ at the grain boundaries and intergranular contacts of the primary NMC811 crystallites. This results in a noticeable enhancement of rate capability and capacity retention over prolonged charge/discharge cycling compared to their sulfur-free analogs. The improvement is attributed to suppressing the high voltage phase transition and the associated accumulation of anti-site disorder upon cycling and improving the secondary agglomerates' mechanical integrity by increasing interfacial fracture toughness through linking primary NMC811 particles with soft Li₂SO₄ binder, as demonstrated with nanoindentation experiments. As the synthesis of the (1-x) LiNi_0.8Mn_0.1Co_0.1O₂∙xLi₂SO₄ composites do not require additional operational steps to introduce sulfur, these electrode materials might demonstrate high potential for commercialization.