In situ initiator-free gelation of highly concentrated lithium bis(fluorosulfonyl)imide-1,3-dioxolane solid polymer electrolyte for high performance lithium-metal batteries

Highlights

• The initiator-free in situ 3.5 M LiFSI-DOL SPE integrates a high ionic conductivity (7.9 mS cm⁻¹ at room temperature) and high Li⁺ transference number (0.82).

• The Li|Li symmetric battery with the in situ 3.5 M LiFSI-DOL can be stably stripped/plated for over 1000 cycles with a low overpotential of 45 mV at 5 mA cm⁻² & 5 mAh cm⁻².

• The Li-S battery with the in situ 3.5 M LiFSI-DOL SPE shows good compatibility and stability.

Abstract

The safety concern on the uncontrollable growth of lithium dendrites in liquid electrolytes is the main challenge for high performance lithium-metal batteries. The traditional ex situ solid electrolytes can solve these issues to a great extent but are still plagued by low ionic conductivity, low Li⁺ transference number, and high interfacial impedance. Herein, a solid polymer electrolyte (SPE) with highly concentrated (3.5 M) lithium bis(fluorosulfonyl)imide (LiFSI) in 1,3-dioxolane (DOL) is developed via a simple in situ initiator-free gelation route at room temperature. This simple and ingeniously designed in situ SPE integrates a high ionic conductivity (7.9 mS cm⁻¹ at room temperature), high Li⁺ transference number (0.82), and low interface impedance. As a result, the in situ 3.5 M LiFSI-DOL SPE demonstrates a superior lithium dendrite-free behavior, which enables the Li|Li symmetric battery to be stably stripped/plated for more than 1000 cycles with a low overpotential of 45 mV at 5 mA cm⁻² and 5 mAh cm⁻². Moreover, the Li|Cu battery with the in situ 3.5 M LiFSI-DOL SPE also exhibits a high coulombic efficiency (CE) up to 98.0% at 5 mA cm⁻². In addition, the in situ 3.5 M LiFSI-DOL SPE used in the Li|LiFePO₄ battery leads to superior cycle life (500 cycles), CE (98.8%), rate performance, and good mechanical flexibility. Remarkably, the Li-S batteries with the in situ 3.5 M LiFSI-DOL SPE also show good compatibility and stability. This work shows tremendous prospect toward high performance solid-state lithium-metal batteries by combining the in situ polymerization and high concentration electrolyte.

Introduction

The state-of-the-art lithium-ion batteries (LIBs) are facing a critical challenge to follow and meet the rapid development and high demand of high energy storage systems owing to the limitation of their low theoretical specific capacity [[1], [2], [3], [4]]. Attention has been drawn to lithium-metal battery (LMB) because of its much higher theoretical specific capacity (3860 mAh g⁻¹) and the reported lowest oxidation-reduction potential (−3.04 V vs. SHE) [[5], [6], [7], [8]]. Unfortunately, the practical application of LMB with liquid electrolytes (LEs) is still hindered by the safety hazards due to the uncontrollable growth of lithium dendrites [[9], [10], [11]], as illustrated in Fig. 1a. Solid polymer electrolytes (SPEs) have attracted wide attention because of their superior stability toward lithium [[12], [13], [14]]. The traditional ex situ SPEs have made great progress in inhibiting the growth of lithium dendrites but still suffer from low ionic conductivity, low Li⁺ transference number, and high interfacial impedance (Fig. 1b) [[15], [16], [17], [18]].

Researchers have proposed in situ SPEs to mitigate these problems because the in situ SPEs can greatly reduce the interface impedance between the electrodes (Fig. 1c) [14,19,20]. Nevertheless, most previous in situ strategies require undesirable external driving conditions such as non-electrolytic monomers, initiators, high temperature, and long gelation time [[21], [22], [23], [24], [25]]. From this perspective, to develop a facile preparation method of in situ SPEs showing high stability of lithium, high ionic conductivity, and high transference number of Li⁺ is still critical and indispensable. Notably, most of the reported batteries along this line are concomitant with the relatively low current density (e.g. <2 mA cm⁻²) and low stripping/plating capacity (e.g. <2 mAh cm⁻²), which are failed to meet the practical requirements (e.g. higher than 3 mA cm⁻² and 4 mAh cm⁻²) [[26], [27], [28]].

In 2018, we studied the electrochemical behaviors of lithium bis(fluorosulfonyl)imide (LiFSI)-1,3-dioxolane (DOL) electrolytes with LiFSI concentrations less than 3 M. In these cases, the electrolytes are mainly liquid or viscous due to the slow polymerization process [29]. Herein, a novel SPE with highly concentrated (3.5 M) LiFSI in DOL is developed via a simple in situ initiator-free gelation routine at room temperature. The highly concentrated LiFSI can effectively induce DOL ring-opening polymerization. In contrast with the conventional in situ SPEs, our in situ 3.5 M LiFSI-DOL SPEs are prepared free from any external driving conditions. We find that our in situ 3.5 M LiFSI-DOL SPEs showing high ionic conductivity (7.9 mS cm⁻¹ at room temperature) and high Li⁺ transference number (0.82) can effectively inhibit the growth of lithium dendrites. The in situ 3.5 M LiFSI-DOL SPE demonstrates a resultant superior lithium dendrite-free behavior, and it enables our Li|Li symmetric battery to be stably stripped/plated over 1000 cycles showing a low polarization voltage of 45 mV at a high current density of 5 mA cm⁻² and a high platting/striping capacity of 5 mAh cm⁻². Moreover, our Li|Cu battery with the in situ 3.5 M LiFSI-DOL SPE also exhibits a high coulombic efficiency (CE) up to 98.0% at 5 mA cm⁻². Impressively, the Li|LiFePO₄ and the Li|S batteries coupled with the in situ 3.5 M LiFSI-DOL SPE demonstrate enhanced electrochemical behaviors in capacity property, cycle stability, and CE, compared with the LiFSI-DOL LEs with low concentration (1 M).