Unveiling the structure, chemistry, and formation mechanism of an in-situ phosphazene flame retardant-derived interphase layer in LiFePO₄ cathode

Highlights

• Phosphazene-based fire retardant gel electrolyte is prepared.

• The mechanism of fire retardant additive on CEI layer formation on LFP electrode is clarified.

• TOF-SIMS and XPS are employed to clarify the fire-retardant CEI layer chemistry.

• HR-TEM is employed to revealed the structure of the CEI layer.

Abstract

The safety risks posed by the liquid electrolytes of lithium-ion batteries have drawn large-scale concern recently due to the increased reports of fires in electric vehicles and portable devices fuelled by these electrolytes. Efforts to address the fire safety of electrolytes have predominantly focused on the incorporation of flame retardant additives which generally lead to poor electrochemical properties when used in large quantities. Conversely, low amounts (∼5 vol%) of fluorinated phosphazene-based flame retardants have been reported to yield non-flammable electrolytes while improving the electrochemical properties. Herein, an advanced fire testing method, cone calorimetry, is employed to analyze a model electrolyte (ethoxy (pentafluoro) cyclotriphosphazene (EPCP) based electrolyte), which reveals that the flourinated phosphazene-based flame retardant electrolyte with up to 6 vol% only exhibits ignition delay rather than the widely acknowledged non-flammable behavior. Besides, for the first time, by employing a time-of-flight secondary-ion mass spectrometry (TOF-SIMS) and transmission electron microscopy, we clarify the chemistry and structure of a flame retardant-derived cathod electrolyte interphase (CEI) layer formed on the LiFePO₄ cathode surface. The CEI layer is characterized by a phosphorus and nitrogen (PN) rich layer, which inhibits the formation of a thick parasitic LiF layer, which improves the electrochemical integrity of cells.

Introduction

The development of fire-safe, reliable, and low-cost rechargeable Li-ion batteries to cater for the ever-growing demands of numerous emerging fields, such as electric vehicles, grid storage, and advanced robotics, has recently received considerable attention from researchers in academia and the industry [1], [2], [3], [4], [5], [6]. This has motivated extensive investigation of flame retardant electrolytes, which impart fire safety to lithium-ion batteries compared to the standard electrolytes, which readily burn on exposure to heat and a sufficient amount oxygen [7], [8], [9]. The main strategies employed to achieve this objective above include the use of intrinsic flame retardant electrolytes [10], [11], [12], ionic liquids [13], [14], and the introduction of flame retardant additives into the electrolytes [15], [16], [17], [18], [19]. Among the aforementioned strategies, the introduction of flame retardant additives into the electrolytes holds great prospect from an industrial perspective due to the maintenance of current production lines [20]. Unfortunately, when used in large quantities, flame retardant additives often lead to poor electrochemical performance due to the negative impact on transport properties of the electrolytes and parasitic electrochemical reactions that occur at the electrode–electrolyte interface [16], [21], [22]. Hence, the development of new electrolytes that can simultaneously improve both fire resistance and electrochemical integrity of the batteries is crucial and urgent. Phosphorus and fluorine-rich compounds, can impart fire resistance and have attracted increased interest over the years [17], [3], [23], [24], [25], [26], [27], [28]. Nevertheless, various drawbacks, such as rapid capacity fade, poor cycling performance, low rate capability, and low coulombic efficiency (CE), remain obstacles towards the practical application of the majority phosphorus-containing compounds [29], [30], [31]. The electrochemical unstable nature of the additives, negative effect on ion transport, destructive co-intercalation, depletion of electrolytes, and parasitic reactions with the electrolyte during cycling are widely considered as the primary sources of these drawbacks [32], [33], [34], [35].

Recently, the introduction of fluorinated phosphazene-based flame retardant additives into flammable carbonate electrolytes has been reported in LIBs to simultaneously improve fire safety, cycling performance, rate capability, and CE [18], [36], [37], [38], [39], [40], [41], [42], [43]. A considerable number of works reported that a low amount (∼5%, by weight/volume) of this additive can render the standard carbonate electrolytes non-flammable due to the ability of the flame retardant additive to act in the gas phase by releasing fluorine and phosphorus radical which bind to O• and OH• radicals that are released during combustion reactions [38], [40], [37]. This is not surprising since the commonly used fire testing methods, such as the direct burning test of the electrolyte using lighters, cannot provide sufficient heat flux to the electrolyte under study and hence may lead to unreliable conclusions. Besides, the battery cells fabricated with fluorinated phosphazene-containing electrolytes have also been shown to exhibit lower interfacial resistance compared to the cells based on standard carbonate electrolytes, which is beneficial to the electrochemical performance [37], [44].

Despite some modest success, the studies of electrolyte fire behavior, flame retardant mechanism, and the influence of flame retardant additives on the electrochemical properties remain challenging in the battery community. Although the high fire resistance of fluorinated phosphazene-containing electrolytes has been widely proven by researchers by employing rather simplistic approaches such as direct flame tests and self-extinguishing tests using common lighters [40], [42]. No measurements have been reported using a reliable fire testing method to directly verify the improved safety of electrolytes of the fluorinated phosphazene-containing electrolytes. Thus, a measurement of the fire safety of the electrolytes using a more reliable method is necessary to clarify the real fire behavior of the fluorinated phosphazene-containing electrolytes. In addition, the investigation of the flame retardant mechanism will deepen our understanding of their mode of action to suppress fire in lithium-ion batteries.

Furthermore, there has been a limited understanding of the interphases generated on the surface of cathode materials in the reported cell compositions consisting of flame retardant electrolytes in general, owing partly to the complex influence of the flame retardant additives, which have not been recognized until recently [15], [44], [45]. Besides, the commonly employed surface-sensitive diagnostic equipment, such as X-ray and infrared spectroscopies to study the surface of electrodes, cannot spatially separate different components in electrodes. Undoubtedly, studies on the fire behavior of flame retarded electrolytes and flame retardant mechanism using advanced fire testing methods and evolved gas analysis, respectively, along with the studies on the influence of flame retardant additive on the composition of the CEI layer, are critical to the design and optimization of a new generation of batteries with simultaneously enhanced fire-safety and electrochemical integrity.

Herein, we employ cone calorimetry test (CCT) and thermogravimetry coupled to Fourier Transform Infrared (TG-FTIR) spectroscopy to clarify the real fire behavior of the fluorinated phosphazene-based electrolytes and fire suppression mechanism that takes place in the modified electrolyte respectively. Moreover, we employ time-of-flight secondary-ion mass spectrometry (TOF-SIMS) for the first time to investigate the electrode–electrolyte interphases and the impact of the CEI layer on the overall cell performance in cells based on lithium iron phosphate LFP cathode, with a particular focus on the role of the fluorinated phosphazene based flame retardant additive. We clearly elucidate that the fluorinated phosphazene-based electrolyte with up to 6 % of ethoxy (pentafluoro) cyclotriphosphazene (EPCP) exhibits a delayed ignition rather than a non-flammable behavior as widely acknowledged and propagated. On the other hand, we use the TG-FTIR technique to further investigate the fire suppression mechanism of the fluorinated phosphazene based electrolyte. By employing techniques, such as TOF-SIMS, X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM), we also observed a non-uniform and fragile CEI with rich carbonate solvent-derived species and thick LiF deposits formed on the surface of the electrode cycled in the standard carbonate electrolyte, which aids the surface deterioration of the cathode. In contrast, a flame retardant-derived CEI composed of rich phosphorus (P) and nitrogen (N) species forms on the electrode surface cycled in the fluorinated phosphazene based electrolyte, whose uniform and robust structure contribute to the prevention of corrosion on the surface of LFP, hence leading to the enhanced cycling performance, rate capability and CE. This work not only provides a better insight into the fire behavior of fluorinated phosphazene based electrolytes and their fire suppression mechanism but also sheds light on the role of fluorinated phosphazene additives in the generation of a uniform and robust CEI, which can inhibit the parasitic electrochemical reactions triggered by the HF attack.