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 commercial-off-the-shelf (COTS) Na-ion batteries (NIBs) tested in this work, where the capacity hysteresis ratio, defined as charge capacity divided by discharge capacity, exceeds unity. Specifically, we compare degradation behaviors between LIBs (Molicel) and COTS NIBs (Hakadi, Na_xNiMnFeO₂|hard carbon|∼57% carbonate electrolyte abbreviated as NMF|HC|SC57) 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 no voltage shifts as a function of temperature in these NIBs unlike LIBs, suggesting unusual 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 positive electrode, negative electrode, and electrolyte components which revealed sodium metal deposition and stable SEI formation on the negative electrode along with excess electrolyte within the battery can. These insights along with the anomalous hysteresis suggests that new physics is needed to fully explain the low-temperature behavior of commercial NIBs.