Extending the low temperature operational limit of Li-ion battery to −80 °C
Introduction
Li-ion batteries (LIBs) are extensively used in portable electronics and electric vehicles because of their high energy density, long cycle life, low self-discharge and long shelf life [[1], [2], [3]]. Their performance is little affected when the temperature increases from room temperature to 60 °C; however, when the temperature falls below 0 °C, LIBs suffer from both severe energy and power losses [[4], [5], [6]]. For example, at −40 °C, commercial LIBs can only deliver up to 10% of its room-temperature capacity [[7], [8], [9], [10]]. This temperature dependent performance limits their application in extremely cold environment, such as in high altitude and aerospace applications [[10], [11], [12]]. Recent literature shows there is a significant effort towards improvement of LIB design to extend their use in low temperature. One method is to add ancillary devices for heating or insulation [[13], [14], [15], [16]]. This approach can improve the local temperature but the use of additional components affects portability and undesirable for use in small devices. Another method is the tailoring of electrolyte composition to prevent freezing and allow Li diffusion [[10], [11], [12],[17], [18], [19], [20]].
Previous works ascribed the poor low-temperature performance of LIB to the low ionic conductivity of electrolyte, which led to the study of different solvent mixtures, admixtures and novel electrolytes [7,13,17]. Although the conductivities of these electrolytes at low temperature are improved, both the rate performance and capacity of LIBs drop significantly when the temperature decreases, especially during charging [10,21,22]. Recent studies show that the low-temperature performance of LIBs can be improved greatly by using liquefied gas [23] or ethyl acetate [24] as electrolyte solvent. These electrolytes do not have high ionic conductivity, suggesting that, besides ionic conductivity, some other key factors contribute to the performance improvement for the LIB at low temperature. The vastly different performance of LIBs in the electrolytes with similar ionic conductivity further validates this point [25,26].
Here, we used a holistic approach in studying the effect of low temperature on Li+ migration. We found that the poor performance of LIB at low temperature was mainly caused by the low migration of Li+ at liquid-solid interface, which is related to the sluggish desolvation of solvated Li+ during Li+ intercalation. Though some previous works have referred that desolvation process would limit the migration of Li+ at liquid-solid interface, the influence degree of this process on the low-temperature performance of LIB is not well investigated [24,27,28]. Thus, it would be hard to provide a good guidance for the selection of low temperature electrolyte. In this work, we found that the difficulty of the desolvation increases exponentially with the decrease of temperature. A small decrease of desolvation between ion and solvent would greatly improve the desolvation rate at liquid-solid interface. Besides the above factor, we also found that it is very important to increase lithium migration rate in the active material. The understanding of these enabled us to demonstrate a LIB that can deliver ∼60% of its room-temperature capacity (0.1 °C rate) even at −80 °C, comparable with the performance that measured at −60 °C in liquefied gas-based electrolyte [23]. These findings pave an effective way for the design of lithium-ion batteries for ultralow temperature application.
Section snippets
Results and discussions
During Li+ intercalation, the following steps are believed to occur (Scheme 1): (I) migration of solvated Li+ to the surface of the active material from electrolyte (liquid phase diffusion), (II) removal of Li+ solvation shell and passing through the liquid-solid interface [[27], [28], [29], [30], [31]] (desolvation), (III) migration of desolvated Li+ through the solid electrolyte interphase (SEI, including or excluding) layer and into the active material (solid-state diffusion). Temperature
Conclusions
In this study, we found that, the poor performance of LIBs at low temperature is mainly caused by the sluggish desolvation at the liquid-solid interface and the reduced lithium migration rate in the active material. Based on the temperature dependence of the Li+ desolvation, the DIOX-based electrolyte was selected because of its low binding energy between Li+ and DIOX solvent, thus improving ion migration at the liquid-solid interface at low temperature. Further, nanoscale lithium titanate was
Preparation of materials
Lithium titanate (LTO), LiCoO2 (LCO) and LiNi1/3Co1/3Mn1/3O2 (NCM111) were purchased from Hefei KEJING Materials Technology Co. Ltd., P.R. China and used as received. Nano-LTO was synthesized using a one-pot hydrothermal procedure following previous reports [40,41]. Briefly, 3.4 ml (10 mM) of tetrabutyl titanate and 0.378 g (9 mM) of LiOH·H2O were thoroughly mixed in 45 ml ethanol using a magnetic stirrer in a closed container for 12 h at room temperature. 50 ml of deionized water was added to
Acknowledgement
This work was financially supported by the National Key Research and Development Program of China (2017YFB0307001), the National Natural Science Foundation of China (91648109, 51675236).
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