Lithium oxalate as capacity and cycle-life enhancer in LNMO/Graphite and LNMO/SiG full cells

In the present study, we explore the use of lithium oxalate as a “sacrificial salt” in combination with lithium nickel manganese spinel (LNMO) cathodes. By online electrochemical mass spectrometry (OEMS), we demonstrate that the oxidation of lithium oxalate to CO₂ (corresponding to 525 mAh/g) occurs quantitatively around 4.7 V vs. Li/Li⁺. LNMO/graphite cells containing 2.5 or 5 wt% lithium oxalate show an up to ∼11% higher initial discharge capacity and less capacity fade over 300 cycles (12% and 8% vs. 19%) compared to cells without lithium oxalate. In LNMO/SiG full-cells with an FEC-containing electrolyte solution, lithium oxalate leads to a better capacity retention (45% vs 20% after 250 cycles) and a higher coulombic efficiency throughout cycling (∼1%) compared to cells without lithium oxalate. When CO₂ from lithium oxalate oxidation is removed after formation, a similar capacity fading as in LNMO/SiG cells without lithium oxalate is observed. Hence, we attribute the improved cycling performance to the presence of CO₂ in the cells. Further analysis (e.g., FEC consumption by ¹⁹F-NMR) indicate that CO₂ is an effective SEI-forming additive for SiG anodes, and that a combination of FEC and CO₂ has a synergistic effect on the lifetime of full-cells with SiG anodes.