Lithium Magnesium Tungstate Solid as an Additive into Li(Ni1/3Mn1/3Co1/3)O2 Electrodes for Li-Ion Batteries

Abstract

Addition of 1 wt% Li4MgWO6 (LMW) solid in a positive electrode is effective to improve battery performance of Li(Ni1/3Mn1/3Co1/3)O2 (NMC)//graphite lithium-ion cell. The batteries with LMW additive show better capacity retention and lower resistance than those of the additive-free batteries. To elucidate the improvement mechanism, the NMC and graphite electrodes after long-term cycle are examined by means of electrochemical tests in Li half-cells, electron microscopy, synchrotron X-ray diffraction, and photoelectron spectroscopy. The results reveal that the addition of LMW suppresses imbalanced state of charge (SOC) between the positive and negative electrodes during cycles whereas evident imbalanced SOC causes the capacity degradation during long cycling under the LMW-free condition. The imbalanced SOC results in overcharge of NMC which is proved by formation of severe micro-cracks in NMC particles. Crystallinity of LMW added in the positive composite electrode is lost after 2000 cycles and tungsten is detected on the graphite electrode, which suggests that the additive is gradually dissolved in electrolyte and tungsten is deposited on the graphite. Because of the formation of surface layer containing tungsten ingredient on the graphite, irreversible side-reactions on the graphite electrode should be suppressed, which contributes to successfully suppress the SOC-unbalancing as well as capacity fading.