Contents lists available at ScienceDirect Journal of Energy Storage journal homepage: www.elsevier.com/locate/est Battery heating for lithium-ion batteries based on multi-stage alternative currents Lei Zhang a , Wentao Fan a , Zhenpo Wang a, ⁎ , Weihan Li b , Dirk Uwe Sauer b a Collaborative Innovation Center for Electric Vehicles in Beijing & National Engineering Laboratory for Electric Vehicles, Beijing Institute of Technology, Beijing, 100081, China b Electrochemical Energy Conversion and Storage Systems, Institute for Power Electronics and Electrical Drives (ISEA), RWTH Aachen University, Jaegerstrasse 17/19, 52066 Aachen, Germany ARTICLEINFO Keywords: Lithium-ion batteries Internal heating Alternative current EIS tests ABSTRACT This paper presents a multi-stage alternative current (AC) strategy for internally hearting lithium-ion batteries. To this end, the influence of the amplitude and frequency of ACs is first examined. Electrochemical Impedance Spectroscopy (EIS) tests under different temperatures and frequencies are also conducted to obtain battery impedance spectra that are used to derive the maximum permissible current amplitude under different tem- peratures. An equivalent circuit model is then proposed and parameterized to predict battery heat generation based on the EIS test datasets. Finally, a multi-stage alternative current strategy is proposed for battery heating, in which the magnitude of the imposed AC is maintained unchanged for a constant time. The effects of different time durations are also examined. The results show that the proposed battery heating strategy can heat the tested battery from -20 °C to above 0 °C in less than 5 minutes without incurring negative impact on battery health and a small current duration is beneficial to reducing the heating time. 1. Introduction Lithium-ion batteries are being extensively used as energy sources that enable widespread applications of consumer electronics and bur- geoning penetration of electrified vehicles [1]. They are featured with high energy and power density, long cycle life and no memory effect relative to other battery chemistries [2]. Nevertheless, lithium-ion batteries suffer from significant loss of energy and power capability at subzero temperatures, limiting their applications in cold environments [3,4]. The underlying factors can be ascribed to sluggish charge-transfer kinetics, slow solid-state diffusion of lithium ions in electrode materials, reduced electrolyte diffusivity and conductivity, and high solid-elec- trolyte interface (SEI) resistance [5]. In particular, lithium ions may fail to intercalate into the anode during high-current charging at subzero temperatures due to poor diffusivity of lithium ions and charge transfer properties of electrodes [6], leading to Li-plating at the anode surface. The deposited metallic lithium is irreversibly consumed in side reac- tions with the electrolyte to form the undesired dendrites, which results in capacity fade of lithium-ion batteries [7]. Besides, the uncontrolled dendrite development would possibly pierce the separator and cause the internal short-circuit of two electrodes, which may evolve to severe safety accidents such as thermal runaway. In order to improve the battery performance at subzero tempera- tures, substantial efforts have been made through materials innovation and design optimization at the cell level and thermal management at the system level. At the cell level, electrolyte additives are developed to improve the electrolyte performance at low temperatures [8]; but this inevitably compromises battery cell performance at normal tempera- tures. Alternatively, it is meaningful to preheat batteries to a certain temperature so that charging protocol can be implemented without sacrificing battery health as well as to shorten charging time [9]. Ex- isting preheating approaches can be grouped into two categories, i.e., external and internal heating, according to their heat generation pat- terns. Generally, external heating approaches employ a separate heating element for preheating batteries, where the generated heat is funneled to the exteriors of battery cells via conduction and/or con- vection. In this regard, Rao et al. [10] systematically analyzed and compared the performance of disparate battery thermal management techniques including air, liquid, phase change and heat pipe, etc. Nevertheless, there is significant heat loss along heat transfer routes in addition to complex system design as well as added weight, cost and space requirements. In contrast, internal heating has the advantages of high efficiency, short time, structural simplicity and uniform tempera- ture distribution. For example, Wang et al. [11] proposed a novel cell https://doi.org/10.1016/j.est.2020.101885 Received 22 February 2020; Received in revised form 9 September 2020; Accepted 11 September 2020 ⁎ Corresponding author: National Engineering Laboratory for Electric Vehicles, 5 Zhongguancun South St. Haidian District, Beijing 100081, China. E-mail address: wangzhenpo@bit.edu.cn (Z. Wang). Journal of Energy Storage 32 (2020) 101885 2352-152X/ © 2020 Published by Elsevier Ltd. T