Multi-Dopant Influence on the Volume Changes of Layered-Oxide Cathodes for Lithium-Ion Batteries

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The growing global market of electric vehicles demands performance improvement of lithium-ion batteries (LIBs). Layered transition-metal oxide (LiMO2) is widely used as cathode materials for LIBs owing to its large energy densities. However, LiMO2 exhibits a large volume change upon (de)lithiation, which induces a mechanical stress inside a particle.1 Consequently, interparticle cracking occurs with repeating charge/discharge cycles, resulting in severe performance degradation. Introducing redox-inactive cations within and between transition-metal layers has been widely employed to suppress such volume changes. For example, Zhang et al. proposed that doping Ti, Mg, Mo, and Nb into LiMO2 could improve the cycle performance by reducing the volume change upon charge/discharge.2,3 In this study, the roles of multiple dopants in suppressing volume changes of LiNi0.5Mn0.5O2 are investigated using operando X-ray diffraction measurements.LiNi0.5Mn0.5O2 and four multi-doped samples (LiNi0.5Mn0.43Ti x Mg y Mo z Nb0.07–x–y–z O2) were synthesized by the co-precipitation methods. Working electrodes were prepared by coating an Al current collector with a slurry of active material, acetylene black, and polyvinylidene fluoride at a weight ratio of 80:10:10. 2032-type coin cells were fabricated using the working electrode, Li metal as a counter electrode, and 1 mol L–1 LiPF6 ethylene carbonate/diethyl carbonate (1:1 v/v%) + 2 wt% vinylene carbonate as an electrolyte. Galvanostatic charge/discharge measurements were performed at a C-rate of 0.1C (28 mA g–1). Laminated cells with the LiMO2 electrodes and 1 mol L–1 LiPF6 ethylene carbonate/diethyl carbonate (1:1 v/v%) electrolyte were fabricated for operando X-ray diffraction measurements with a transmission mode.Powder X-ray diffraction measurements confirm the successful synthesis of all LiMO2. Galvanostatic charge/discharge measurements show that LiNi0.5Mn0.5O2 (non doped), LiNi0.5Mn0.43Ti0.03Mg0.02Mo0.02Nb0.01O2 (4-element doped), LiNi0.5Mn0.43Ti0.03Mg0.02Mo0.02O2 (3-element doped (w/o Nb)) deliver larger reversible capacities (~190 mAh g–1) than LiNi0.5Mn0.43Mg0.02Mo0.02Nb0.01O2 (3-element doped (w/o Ti)) and LiNi0.5Mn0.43Ti0.01Mg0.02Nb0.04O2 (3-element doped (w/o Mo)). After the 70th cycle, non-doped LiNi0.5Mn0.5O2 exhibits greater capacity retention (90%) than four-element doped and three-element doped (w/o Nb) LiNi0.5Mn0.5O2 (86% and 85%, respectively). Operando X-ray diffraction measurements show that the volume change (ΔV/V) at 80% depth of charge was −4.2%, −1.8%, and –3.3% for no-doped, four-element doped, and three-element doped (w/o Nb) LiMO2. These results indicate that a Nb dopant plays an important role in the suppression of a volume change during the first charge. Yan, P., et al., Nat Commun. 8, 14101 (2017).Zhang, R., et al., Nature 610, 67–73 (2022).Zhang, R. et al., Energy. 8, 695–702 (2023). Figure 1