A flame retardant sandwiched separator coated with ammonium polyphosphate wrapped by SiO₂ on commercial polyolefin for high performance safety lithium metal batteries

Highlights

• A facile but effective approach of coating is introduced to prepare advanced sandwiched separator.

• The sandwiched separator with absorbing electrolyte shows excellent flame retardant property.

• Thermal stability, ionic conductivity and electrochemical performances have been improved.

• The sandwiched separator has excellent capability to inhibit lithium dendrites.

Abstract

Lithium metal batteries are regarded as the “Holy Grail” among varieties of energy storage systems. Nevertheless, the development and practical application of lithium metal batteries have been seriously hindered by the safety issues, which are generated from threatening lithium dendrites, flammable separator and electrolytes. In this work, a flame retardant sandwiched separator has been prepared by simply coating ammonium polyphosphate wrapped by SiO₂ (APP@SiO₂) on the surface of commercial polyolefin, thus significantly improving the safety of lithium metal batteries. The APP@SiO₂ separator impregnated with sufficient electrolyte exhibits excellent flame retardant performance, which manifests self-extinguishing performance. In addition, thermal stability, ionic conductivity, and electrochemical performances have been also improved obviously. More importantly, the APP@SiO₂ separator possesses a satisfactory ability to suppress lithium dendrites, owing to the fact that the high ionic conductivity of modified separator can promote uniform deposition of Li⁺ and the dangerous lithium dendrites contacted with separator are consumed by the reaction with SiO₂. Therefore, a facile but effective approach can endow the multifunctional sandwiched separator with a favorable potential application for high performance and safety lithium metal batteries.

Introduction

Rechargeable lithium-based batteries have achieved revolutionary development and extensively applied in energy storage devices and electrochemical research areas in recent years [1], [2], [3], [4], [5]. Due to the pressure of increasingly depleted fossil fuels and ever-growing energy demands for application, the development and utilization of higher energy density batteries become particularly urgent and the traditional lithium-ion batteries no longer meet the demands. Lithium mental is considered as the most potential anode material as the terminal goal of high energy density storages, owing to the fact that it possesses the highest specific capacity (3860 mAh g^–1), the lowest redox potential (−3.04 V vs standard hydrogen electrode) and the lowest density in metal materials (0.53 g cm^–3) [6], [7], [8]. Unfortunately, the safety conundrum induced by sharp lithium dendrites generated from the charge process is still an enormous challenge to hinder the practical application of lithium metal batteries. The formation and growth of dendrites will pierce separator and then directly contact with cathode materials resulting in short circuit, and then initiating thermal runaway or even fire safety accidents [9], [10], [11].

Up to know, extensive efforts have been contributed to improve the safety issues and electrochemical performances, which mainly focus on improving the electrode materials, separators and electrolytes. Snehashis Choudhury designed crosslinked-nanoparticle-polymer-composites separator to internalize in the pores for regulating ion and mass transport to inhibit the growth of lithium dendrites [12]. Wang utilized Lorentz Force established a magnetic field in the vertical aspect of the battery pole piece to eliminate tip dendrite growth [13]. Wu employed the sensing terminal to acquire short-circuit warning by adding a conductive copper mesh in the middle of the separator when the lithium dendrites spread to the copper mesh layer of the intelligent separator [14]. However, those complex methods with high cost are not conducive to large-scale applications. To our surprise, Cui prepared a sandwich-like separator with a simple approach by sandwiching SiO₂ nanoparticle between two layers of commercial separator to suppress the growth of lithium dendrites by reacting with SiO₂ [15]. Hence, it provides a novel and feasible idea to suppress lithium dendrites by using designed separator.

In addition, poor thermal stability of separator and flammable organic esters-based or ethers-based electrolytes also are significant factors influencing the safety of lithium metal batteries. When the internal temperature of the battery rises due to unreasonable utilization, the separator will shrink and then result in short circuit, which will cause thermal runaway, and then the flammable electrolyte will exacerbate the fire accident [16], [17], [18], [19]. Some works have improved battery safety by replacing inflammable carbonate electrolytes with non-combustible ionic electrolytes, but the high price makes it even more confusing [20], [21], [22]. Adding flame retardant additive to the commercial electrolyte is another commonly method to enhance the safety of lithium batteries, but high viscosity of flame retardant additive could deteriorate electrochemical performance [23], [24], [25]. Nevertheless, it is worth mentioning that high efficiency flame retardant is mixed into the separator to prepare a flame retardant separator, which can improve electrochemical and flame retardant performance simultaneously [26], [27], [28].

Based on the above two points, ammonium polyphosphate with high flame retardant efficiency, environmentally friendly and low cost is adopted to endow the lithium metal battery with flame retardant. However, the fatal flaw of ammonium polyphosphate is easily hydrolyzed when exposed in atmosphere containing moisture, which is inevitable during the preparation, transportation and storage of the separator [29, 30]. Hence, it is extremely necessary to modify the ammonium polyphosphate into hydrolysis resistance. Wrapped by hydrolysis resistance materials is one of the most common and effective method. Therefore, in this work, the SiO₂ nanoparticles, which possess the properties of hydrolysis resistance, favourable electrolyte affinity and reacting with lithium dendrites, are employed to wrap ammonium polyphosphate forming SiO₂ nanoparticles/ammonium polyphosphate composite structure, and then coat on commercial Celgard separator surface both sides to form sandwich-like separator. The advanced separator with flame retardant, excellent electrochemical performance and suppression of lithium dendrites characteristic was acquired successfully. Herein, it should be noted that there are four properties which make the sandwich-like separator novel and efficient: (1) The sandwiched separator possesses outstanding flame retardant, which manifest the separator with filling liquid electrolyte can achieve self-extinguishing property. Compared with traditional flammable separator and electrolyte, the safety performance of lithium battery is obviously improved by utilizing flame retardant separator. (2) The existence of APP and SiO₂ with well electrolyte affinity can facilitate the transport of Li⁺ and regulate uniformly distribution of Li⁺to alleviate the growth rate of lithium dendrites. What's more, the SiO₂ nanoparticles on the surface of APP play a role in preventing APP hydrolysis, and more importantly, it also can react with dangerous lithium dendrites to guide lithium dendrites growing along the cross-sectional direction of separator, thereby avoiding the lithium dendrite piercing the separator and causing the short circuit. (3) In corporation with APP@SiO₂ layers, the thermal stability of separator is also improved. Moreover, the electrochemical performances are elevated after assembled to coin cells, owing to the remarkable electrolyte affinity of SiO₂ nanoparticles. (4) Besides, this advanced separator also has many advantages such as easy preparation, low cost, unharmful to matrix performance, and ease of commercialization.