Structure and ionic conductivity of the solid electrolyte interphase layer on tin anodes in Na-ion batteries.

Structure, stability, and ionic conductivity of the SEI layer on Sn anodes in Na-ion batteries (NIBs) are studied using experimental and theoretical methods. Raman spectra show that the SEI layer consists of Na 2 O and Na 2 CO 3 , the latter becoming more dominant close to the discharged state (at 0.3 V). According to our theoretical phase diagrams, Na 2 O can be stable in the whole voltage range of charge/discharge (from 0.0 V to 1.90 V), but Na 2 CO 3 can decompose under carbon and/or oxygen poor conditions, leading to the formation of Na 2 O. These findings are in agreement with our experimental cyclic voltammetry and Raman spectra as function of voltage. Both compounds of the SEI layer have very low ionic conductivity close to the discharge state (0.2–0.3 V), but the ionic conductivity of Na 2 O is much larger than that of Na 2 CO 3 for a wide range of voltages from 0.4 V to the charge state (∼1.5 V). This work suggests that engineered artificial SEI with Na 2 O or naturally formed SEI in a carbon and/or oxygen poor environment can improve the conductivity of the SEI layer in NIBs. [ABSTRACT FROM AUTHOR]