The Contradiction Between the Half-Cell and Full-Battery Evaluations on the Tungsten-Coating LiNi_0.5Co_0.2Mn_0.3O₂ Cathode

A one-step synthesis method is developed to prepare the Li[Ni_0.5Co_0.2Mn_0.3]O₂ (NCM 523) cathode with uniform tungsten-coating layer for lithium-ion battery. Such modified material shows similar properties with the bare NCM 523 in terms of the primary particles, secondary structure, and specific capacity. With W coating layer, the NCM 523 gives remarkable improvement in the long-term capacity retention in the half-cell testing, which is sharply inconsistent with the result from the full-battery tests, indicating a contradiction between the half-cell and full-battery tests in some case. After further investigations, such conflict between the half-cell and full-cell tests in evaluating the W-coating strategy is attributed to the dissolution of Li₂WO₄ layer in the electrolyte, which probably destroys the solid electrolyte interface (SEI) on the graphite anode and irreversibly consumes the active lithium ions for renovating SEI in full-cell testing. These results will benefit researchers in the area of lithium-ion batteries to fully understand the differences between the half-cell and full-cell testing, and develop the effective strategies for cathode modifications.