Evgeny V. Antipov

The effectiveness of introducing succinic anhydride as an electrolyte additive for high-energy lithium-ion battery cells based on LiNi_0.6Mn_0.2Co_0.2O₂/graphite (NMC622/graphite), which provide an optimal balance between specific capacity and calendar life, is demonstrated. The use of vinylene carbonate and fluoroethylene carbonate as functional additives for the formation of protective layers in lithium-ion batteries based on NMC622/graphite is particularly important for operational stability and improvement of calendar life. A comparative evaluation of the effectiveness of succinic anhydride as a replacement for vinylene carbonate and fluoroethylene carbonate is carried out for the first time. A recrystallization procedure for technical-grade succinic anhydride was developed to remove critical impurities of water and succinic acid. The decomposition of electrolyte additives is found to occur in the range of 2.4–2.8 V prior to the decomposition of carbonate solvents, ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate, resulting in the formation of protective interphase layers on the electrodes. A key result is the demonstration that an electrolyte containing 1 vol % succinic anhydride after recrystallization increases capacity retention to 92% after 200 cycles in lithium-ion batteries based on NMC622/graphite compared with an additive-free electrolyte. The effectiveness of succinic anhydride as an alternative to widely used vinylene carbonate and fluoroethylene carbonate is also demonstrated. The improvement in the calendar life of lithium-ion batteries based on NMC622/graphite is associated with the formation of protective interphase layers on the electrodes.

Through operando / ex situ methods and theoretical calculations, this work reinterprets the data, proves intercalation along the whole voltage profile, and identifies curvature as a very important descriptor for sodium storage in hard carbon.

The low-temperature properties of sodium-ion electrolytes based on 1 M NaPF ₆ solutions in EC-DEC-DMC (2:1:1 and 1:1:1 by volume) were studied. The data on phase transitions obtained from visual experiments and DSC studies were compared with the results of electrochemical experiments using steel (St), metallic sodium (Na), and NASICON-type active materials (NVP, NTP) electrodes. The specific conductivity of the “2:1:1” electrolyte, although initially slightly higher than that of the “1:1:1,” decreases sharply upon cooling and at −40 °C is ≈0.5 mS cm ⁻¹ vs ≈1.5 mS cm ⁻¹ for the “1:1:1.” However, the capacities of NVP-Na half-cells in both electrolytes are almost identical down to −40 °C. Studies of symmetrical cells showed that the main contributor to polarization loss is the sodium-electrolyte interface. Both electrolytes behave similarly with respect to metallic sodium. However, in the case of NVP-NVP cells, the charge transfer resistance in case of “1:1:1” electrolyte was lower on cooling than for the “2:1:1.” NVP-NTP full cells additionally demonstrated an advantage of the “1:1:1” system compared to “2:1:1” at low temperatures. At −40 °С, the cell with “1:1:1” electrolyte demonstrated retention of 75% of the room-temperature capacity, while in the case of “2:1:1” this value was slightly more than 50%.

Hydrothermal synthesis of LiFePO ₄ without using the traditional three-fold lithium excess revealed unexpected alluaudite-triphylite phase transformation.