Observation of anomalous capacity hysteresis in commercial sodium ion batteries at low temperatures

Abstract

We report on a surprising low-temperature phenomenon that is unique to Na-ion batteries (NIBs), where the capacity hysteresis ratio, defined as charge capacity divided by discharge capacity, exceeds unity. Specifically, we compare degradation behaviors between LIBs (Molicel) and NIBs (Hakadi, NaxNiMnFeO2|hard carbon|~57% carbonate electrolyte abbreviated as NMF|HC|SC57) batteries at –20°C, 0°C, and 20°C with similar charge/discharge rates at each temperature. At -20°C, the measured capacity hysteresis ratio is ~207% higher for NIBs; at 0°C, it is ~60% higher than LIBs. Differential capacity analysis reveals temperature-dependent voltage shifts, with minimal polarization during charging for NIBs, suggesting unusual electrochemical kinetics. This behavior exists even under slow cycling, indicating it is not rate limited. To better understand the origins of this behavior, we characterized the cathode, anode, and electrolyte components which revealed sodium metal deposition and stable SEI formation on the anode along with distinctive electrolyte features. These insights along with the anomalous hysteresis suggests that new physics is needed to fully explain the low-temperature behavior of NIBs.