Full picture discovery for mixed-fluorine anion effects on high-voltage spinel lithium nickel manganese oxide cathodes

Abstract

Small amounts of fluorine substituting for oxygen deficiencies could reduce Mn dissolution, enhancing the cyclability in spinel-type lithium nickel manganese oxides (LiNi0.5Mn1.5O4). Fluorine anion incorporation simultaneously enhances the C-rate capability and specific capacity fading. We used experimental and theoretical approaches to obtain a full picture of the mixed-anion effects for LiNi0.5Mn1.5O4−xFx cathode materials. The fluorine anion reduced the activation barrier for lithium-ion hopping along the most energetically preferable 8a-16c-8a route, enhancing the C-rate capability. Simultaneously, the coordination bond of the linear F−–Mn3+–F− (Mn@2F diagonal) arrangement increased the oxidation potential to 5.1 V (vs Li+/Li). This hampered full extraction of Li+ from the spinel lattice, which was triggered by the oxidation of Mn3+ below the cutoff voltage (3.5–4.8 V (vs Li+/Li)), leading to a capacity loss. Fluorine improves battery performance by lowering the activation barrier for lithium ion hopping, scientists in Japan have shown. To make batteries both smaller and able to store more energy requires better materials, especially those used for the electrodes. One promising option is lithium nickel manganese oxide to which fluorine has been added. The addition of fluorine had been shown to improve the discharge rate and cyclability of batteries, but the reason for this improvement was unclear. Now, in an experimental and theoretical study, Nobuyuki Zettsu and Katsuya Teshima from Shinshu University in Japan and co-workers have demonstrated that fluorine anions reduce the activation barrier for lithium ion hopping along the most energetically preferable crystallographic direction, thus improving electrode performance. This opens up a route to develop high-capacity, high-voltage lithium-ion batteries. We demonstrated experimental and theoretical approaches to obtain a full picture of the mixed-anion effects for LiNi0.5Mn1.5O4−xFx cathode materials. The fluorine anion reduced the activation barrier for lithium (Li)-ion hopping along the most energetically preferable 8a-16c-8a route, enhancing the C-rate capability. Simultaneously, the coordination bond of the linear F−–Mn3+–F− (Mn@2F diagonal) arrangement increased the oxidation potential to 5.1 V (vs Li+/Li). This hampered full extraction of Li+ from the spinel lattice, which was triggered by the oxidation of Mn3+ below the cutoff voltage (3.5–4.8 V (vs Li+/Li)), leading to a capacity loss.