We present a novel concept to achieve high performance and high safety simultaneously by passivating a Li-ion cell and then self-heating before use. By adding a small amount of triallyl phosphate in conventional electrolytes, we show that resistances of the passivated cells can increase by ~5×, thereby ensuring high safety and thermal stability. High power before battery operation is delivered by self-heating to an elevated temperature such as 60°C within tens of seconds. The present approach of building a resistive cell with highly stable materials and then delivering high power on demand through rapid thermal stimulation leads to a revolutionary route to high safety when batteries are not in use and high battery performance upon operation.

To break away from the trilemma among safety, energy density, and lifetime, we present a new perspective on battery thermal management and safety for electric vehicles. We give a quantitative analysis of the fundamental principles governing each and identify high-temperature battery operation and heat-resistant materials as important directions for future battery research and development to improve safety, reduce degradation, and simplify thermal management systems. We find that heat-resistant batteries are indispensable toward resistance to thermal runaway and therefore ultimately battery safety. Concurrently, heat-resistant batteries give rise to long calendar life when idling at ambient temperatures and greatly simplify thermal management while working, owing to much enlarged temperature difference driving cooling. The fundamentals illustrated here reveal an unconventional approach to the development of current and future battery technologies as society moves toward ubiquitous electrified transportation.

Li metal batteries (LMBs) employing high voltage cathodes are necessary to attain high energy density. Although highly concentrated ether-based electrolytes (e.g. 4 M LiFSI/DME) can yield stable cycling of Li metal anodes, their high voltage instability fosters incompatibility with high voltage cathodes. In this work, the temperature dependence of fresh cell performance, Li Coulombic efficiency (CE), and cycling stability of LMBs in highly concentrated LiFSI/DME electrolytes was explored. Elevated temperature operation was deemed essential for highly concentrated electrolytes to achieve practical rate capability. Moreover, at 60 °C, the cycling stability of Li metal anodes with a Li CE as high as 99.2% was demonstrated in a highly concentrated LiFSI-1.2DME electrolyte (LiFSI: DME = 1: 1.2 mol.). At room temperature, the LiFSI-1.2DME electrolyte enabled stable LMBs with NMC622 cathodes. However, due to the high temperature and high voltage instability of the LiFSI-1.2DME electrolyte in contact with NCM622, a small amount of TAP (∼1 wt.%) was added, significantly enhancing the cycling stability at 60 °C. This newly developed LiFSI-1.2DME electrolyte with 1 wt.% TAP ultimately enabled LMBs with NMC622 cathodes and minimal excess Li metal anode to be cycled stably for 200-300 cycles at 60 °C.

Li metal batteries (LMBs) employing high voltage cathodes show promise as next generation batteries for electric vehicles due to their high energy density. However, the cycling stability of such energy-dense LMBs at high charge rates has seldom been explored. In this work, Li Coulombic efficiency (CE) as a function of current density (i.e., C-rate) and temperature was investigated in ether-based highly concentrated electrolytes (HCEs). We found that Li metal anodes can be stably cycled in the HCEs at elevated temperatures with a high Li CE of >99.4% for a plating capacity of 3 mAh cm−2 at a high C-rate greater than 1 C. The cycling stability of LMB full cells with Li(Ni0.8Co0.1Mn0.1)O2 (NMC811) cathodes was also investigated at different charge rates (1 C to 3 C) at 40 and 60 °C. The LMBs with high areal capacity (∼3.2 mAh cm−2) NMC811 cathodes and minimal Li excess anodes (negative/positive capacity ratio (N/P ratio) <3.1) can achieve ∼189 stable cycles with an average efficiency of ∼99.86% at 1 C charge and C/3 discharge at 60 °C.

No abstract