Elemental Sulfur as a Cathode Additive for Enhanced Rate Capability of Layered Lithium Transition Metal Oxides

Layered lithium transition metal oxides face two intrinsic problems of (1) oxygen evolution at high potentials or high temperatures and (2) the presence of alkaline Li residual compounds, to which nearly all known concerns with this class of cathode materials can be attributed. By selecting LiNi 0.80 Co 0.10 Mn 0.10 O 2 (NCM811) as an example, here we dissolve small amount of elemental sulfur into the cathode slurry, finding that sulfur provides two beneficial functions in improving the electrochemical performance of NCM811 cathode. In the slurry-making process, sulfur reduces Ni 3+ ions to produce a protective NiO layer and Li 2 S 2 O 3 . In the initial charging process, the resultant Li 2 S 2 O 3 is electrochemically oxidized to Li 2 SO 4 , and meanwhile the Li residual compounds on the particle surface are removed. It is found that adding 0.25∼0.50 wt% (vs. NCM811) sulfur into the cathode slurry significantly enhances rate capability of Li/NCM811 cells although improvement on capacity retention is not visible. Impedance analysis explains that the enhanced rate capability is attributed to considerable reduction in both of the surface layer resistance and charge-transfer resistance. The results of this work indicate that sulfur additive provides a simple and practically feasible strategy for improving performances of the layered cathode materials.