多功能添加剂PFPN对可充锂硫电池的影响

沈 旺，雷智鸿，谢李生，杨 军，努丽燕娜，王久林



多功能添加剂PFPN对可充锂硫电池的影响

沈 旺，雷智鸿，谢李生，杨 军，努丽燕娜，王久林

（上海电化学能源器件工程技术研究中心，上海交通大学化工系，上海 200240）

由于具有极高的比容量和丰富的硫资源储量，锂硫电池已成为下一代可充电池研究的热点之一。但锂硫电池中存在较大的安全性隐患，这将阻碍其实际应用。一种高效的阻燃添加剂，乙氧基五氟环磷腈（PFPN）被首次用于锂硫电池。添加5%质量分数的PFPN使得高度易燃的碳酸酯电解液完全不燃，同时减小极化电压，并显著提高硫基复合材料的倍率性能。上述结果表明，PFPN是一种适用于可充锂硫电池的多功能添加剂。

阻燃添加剂；PFPN；功能性电解液；锂硫电池；复合正极

Currently, rechargeable lithium ion batteries (Lion) are being widely adopted as power sources for portable devices, and for electric vehicles (or hybrid) at a relatively small scale; however, its limited energy density and highly flammable organic liquid electrolytes make Lion possibly unable to meet the demand of next generation of safe and long-driving range electric vehicles [1]. Li/S battery has become a hot research topic due to its high theoretical energy density up to 2600 W·h·kg–1, 3—5 folds higher than those of state-of-the-art Lion batteries [2].Compared to Lion, Li/S battery might possess more serious hazard [3], because ① the lithium dendrites; ② the high volatility and flammability of the electrolytes; ③ the cathode generally composed of elemental sulfur and conductive carbon, which are combustible and explosive.

In traditional Lion, lots of flame inhibitors have been reported to improve the thermal stabilities of conventional organic electrolytes, which can be divided into four main types according to their molecular structures: Alkyl phosphates [4-5],fluorinated alkyl phosphates [6], ionic liquids [7], and phosphazenes [8]. Although most of these inhibitors can effectively improve the safety properties of Lion at a certain extent, the amount of additive required to achieve nonflammability obviously destroys the electrochemical performances of Lion, for example, deteriorating cycling stability or weakening power rate capability [9-10]. Ethoxypentafluorocyclotriphosphazene (N3P3F5OCH2CH3, PFPN), has moderate viscosity ( 1.2 mPa·s ) and high boiling point (125 ℃), which was reported as a novel flame inhibitor for Lion [11].Moreover, the flame inhibiting efficiency of PFPN is the highest among flame-retarding additives ever synthesized and reported in the literatures, because of its high phosphorus (P) content up to 33.8% (weight ratio).

Considering the extensive usage of Li/S battery as large size, it is important to figure out the safety issues of Li/S battery before its practical applications. In our previous work, safe sulfur cathodes and stable cycle performance have been achieved by creatively combining elemental sulfur with non-flammable pyrolyzed polyacrylonitrile, abbreviated as S@pPAN [12-15]. Therefore, we proposed a concept in this work to build up a safe Li/S battery with both a nonflammable electrolyte with PFPN as flame-retardant and a nonflammable cathode with S@pPAN composite materials.

1 Experimental

1.1 Sulfur-based composite cathode materials preparation

Elemental sulfur and polyacrylonitrile (PAN) were simply mixed with ethanol served as dispersant, then ball milling for 12 h at 120 r·min–1. After drying and heating at 300 ℃ under N2atmosphere, the target material S@pPAN composite was obtained. The sulfur content in S@pPAN was ca. 46%, which was measuredVario EL III elemental analyzer.

1.2 Coin-cell assembling and performance characterizations

The cathode was prepared by mixing S@pPAN, super P and carbonyl-β-cyclodextrin as a binder at the ratio of 8∶1∶1 [15]. Distilled water was adopted as dispersant to make a slurry, which was stirred, and then pasted onto carbon coated Al foil. After drying at 80 ℃ under vacuum, and the cathode was cut into small pieces, with diameter of 12 mm and cathode loading ca. 2.0 mg·cm–2. Li foil with a thickness of 0.2 mm was used as anode. The blank electrolyte was composed of 1 mol·L–1LiPF6(EC∶DMC=1∶1, volume ratio). PFPN was directly added into the blank electrolyte at the ratios of 0, 1%, 3%, 5% (weight ratio). PE film was adopted as separator, and 2016-type coin cells were prepared in a glove box (MBraun, Germany). The charge/discharge cycles of coin cells were measured with the voltage range of 1.0—3.0 VLAND cycler (Wuhan, China) at room temperature.

1.3 Flame inhibiting and conductivity measurements

Self-extinguishing time (SET) was adopted to evaluate the combustion properties of the electrolytes. A cotton wick was immersed to absorb 0.2—0.3 g electrolyte liquid, and was subsequently ignited. The time for the flame to self-extinguish was recorded. The SET tests were repeated for six times.

FE30 conductivity meter and Inlab 710 conductivity measurement cell (Mettler Toledo, Switzerland) was applied to measure the ionic conductivities of the electrolytes with different PFPN contents by inserting the InLab 710 conductivity electrode into the electrolyte solution at room temperature.

1.4 Electrochemical Impedance

Electrochemical Impedance Spectra (EIS) were measured using a Solartron FRA 1250 frequency response analyzer in combination with a Solartron SI 1287 electrochemical interface. The frequency ranged from 100 kHz to 0.01 Hz, and the amplitude was set at 5 mV.

2 Results and Discussion

2.1 Self-extinguishing time and the ionic conductivities of the electrolytes

Safety is a critical property for a battery, which cannot be over emphasized. Because of the high volatile organic solvents, the electrolytes composed of carbonates such as EC (ethylene carbonate) and DMC (dimethyl carbonate) are highly flammable. Here we adopted the method called self-extinguishing time (SET) to characterize the flammability of the liquid electrolytes. Fig.1(a) shows the effect of PFPN addition on both SET and ionic conductivity of the electrolytes. The SET curve of PFPN-based electrolyte shows a sharp decrease with the content of PFPN which demonstrates an excellent flame-retarding efficiency. At 1% PFPN addition, the SET value of the electrolyte evidently decreased from ca. 100 s·g–1to 52.6 s·g–1, then sharply decreased to 7.5 s·g–1at 3% PFPN addition. When the addition of PFPN increased to 5% or more, the SET remarkably decreased close to zero, indicating the electrolyte can not be ignited. Fig.1(a) demonstrates that just a small addition of flame-retarding additive PFPN can effectively make the carbonate-based electrolytes totally non- flammable. Some studies suggested that when phosphorus and nitrogen both existed in the molecular of flame-retardant, the flame-retardant efficiency can be greatly improved [16]. When fluorinate compounds being used as flame-retarding additives, it decomposes and generates free radicals (F·), which capture other free radicals (usually H·and OH·) derived from the burning electrolyte and stop the combustion [17]. Thus, the flame retardancy can be further increased by combining the halogen with the phosphazene backbones. Fig.1(a) also shows the influence of PFPN addition on the ionic conductivity of the as-prepared electrolytes. It is clear that the addition of PFPN hardly affect the conductivity of the electrolytes, just a slight decrease along with the PFPN addition increase, partially because of the relatively high viscosity of PFPN (1.2 mPa·s at 25 ℃). It is interesting to find that elemental sulfur reacts with polyacrylonitrile (PAN) to form a nonflammable composite S@pPAN [12]. Combined with aforementioned nonflammable electrolyte, it is feasible to construct safe Li/S batteries.

2.2 Electrochemical performances of the batteries with different PFPN contents

Besides the function to prevent the combustion of the electrolyte, its influence on the electrochemical performances of the batteries is a critical factor to evaluate the comprehensive functions of the flame inhibitors. The following investigations mainly lie in the impact of PFPN addition on the electrochemical performances of S@pPAN composite cathode. Charge/discharge behaviors of coin cells with various additions of PFPN were shown in Fig.1(b). Two types of specific capacities calculated based on the S@pPAN composite and sulfur mass, respectively, were presented in this work. It is evident that PFPN additive hardly impact the charge/discharge profiles of the S@pPAN composite material. The initial discharge specific capacity of S@pPAN based on sulfur mass was about 1844 mA·h·g–1(sulfur), which exceeded the theoretical capacity of sulfur 1672 mA·h·g–1. The possible explications lie in two aspects, one is that the conjugated backbone of pPAN reacts with Li ion and delivers irreversible capacity; another is interfacial reactions also providing partial capacity [12]. In the following second cycle, the reversible capacity was about 1512 mA·h·g–1(sulfur), which means a sulfur utilization ca. 90.4%.

Cycling performances of the batteries composed of S@pPAN composite cathode and as-prepared electrolytes with various amounts of PFPN are shown in Fig.1(c). Fig.1(c) demonstrates that PFPN additive effectively improved the cycling stability of the S@pPAN cathode. With the blank electrolyte, the specific capacity of the S@pPAN cathode gradually decreased, only 1292 mA·h·g–1(sulfur)left after 50 cycles. On the contrary, the specific capacity of the S@pPAN cathode remained at 1351 mA·h·g–1(sulfur)after 50 cycles in the electrolyte with 3% PFPN addition.

The rate performances of cells were also investigated to further evaluate PFPN additive. The batteries with various PFPN additions were cycled at various rates, namely, 0.2C, 0.5C, 1C, 2C, 3C, 5C, 7C, 0.2C. It is clear that the rate capability of the composite cathode also benefits from the addition of PFPN. In the blank electrolyte, the performances of the battery were very sensitive to rate variations. As demonstrated in Fig.1(d), at low rates (<1 C), the effect of PFPN additions were negligible. Remarkable difference emerged at high rates. In the blank electrolyte, the specific capacity of S@pPAN cathode material decreased to 660 mA·h·g–1(sulfur)at 7 C, while S@pPAN cathode material in the electrolyte with 3% PFPN addition delivered a capacity of 1100 mA·h·g–1(sulfur)at same rate. The battery with 3% PFPN addition in the electrolyte illustrates the best rate performance, coinciding with the recharge- discharge patterns where the battery with 3% PFPN in the electrolyte reduces the voltage polarization. However, S@pPAN cathode material in the electrolyte with 5% PFPN addition also exhibited a rapid capacity loss at high rates. It is well known that the rate capability of the batteries is mainly depending on the ion transportation within the electrode and electrolyte, and on the interface. The electrolyte containing 5% PFPN has a relatively high viscosity, which prevents lithium ion transportation in the electrolyte, leading to relatively poor rate performance.

Cycled at low rate of 0.2 C again, the specific capacity of S@pPAN cathode with PFPN addition recovered well, meaning that the reversibility of the electrode is well enhanced by PFPN. The charge/discharge profiles were compared between blank electrolyte and the electrolyte with 3% PFPN addition, as shown in Fig.1(e) and (f), respectively. At the same cycling rate, the S@pPAN composite cathode material exhibited a higher discharge voltage and a larger specific capacity after the addition of PFPN into the electrolyte. In the blank electrolyte, the initial discharge voltage of the battery sharply decreased from 2.24 V (0.5C) to 1.74 V (7C), whereas with 3% PFPN additive the discharge voltage the batteries changed from 2.33 V (0.5C) to 1.95 V (7C), as shown in Fig.1 (e, f).

In order to explain the influence of PFPN on the electrochemical performances of the S@pPAN composite materials, the electrochemical impedance spectra (EIS) of the cathode were investigated at various discharge voltages, that is, 2.3, 2.0, 1.7 and 1.0 V, respectively. At open circuit voltage of 2.3 V, the cathode was in a fully charged state. Only one semicircle and a sloping line were observed in the high/medium-frequency regions and low-frequency region, respectively. There are two semi-circles followed by a sloping line for other impedance spectra measured at lower battery voltages (Fig.2). The EIS were fitted and the results are given in Fig.2 and Table 1. The presence of PFPN obviously reduced both the Li ion transfer resistance (f) in the interface and the electrochemical reaction resistance (ct).fdecreased

Table 1 Electrochemical parameters at various discharge voltages

to about one fifth of the blank electrolyte andctwas reduced even 10 times, after the addition of PFPN. The explanation might be the participation of PFPN in the interfacial reaction on the cathode, resulting in a novel interface with low Li ion transfer resistance and high electrochemical reaction rate.

2.3 Mechanism research through XPS

X-ray photoelectron spectroscopy (XPS) spectra were measured to investigate the effect of PFPN on the interfacial components, as shown in Fig.3. The cathode cycled in electrolyte with 5% PFPN demonstrated a new peak at 135.3 eV representing phosphazene of PFPN, which confirmed the aforementioned speculation that PFPN participated in the interfacial reactions. The residual PFPN in the interface with electron-rich phosphorus structure could accelerate the formation of solid electrolyte interface (SEI) [18-19]. Although the detailed mechanism related to the formation of SEI is yet not completely clear as it remarkably differs with various systems [20], it is generally accepted that interfacial reactions are related to a reductive decomposition of the electrolyte components, including solvents, salts and additives [21-22]. PFPN consists of three phosphorus and five fluorine atoms, which probably facilitates the formation of a novel SEI possessing low interfacial resistance, leading to good cycling stability and high power rate capability for S@pPAN composite cathode material.

3 Conclusions

（1）Summarily, a safe Li/S battery was reported in this work, which demonstrated good cycling stability and outstanding power rate capability. The safety, particularly flammability issue of the Li/S battery was effectively eliminated via combining flame-inhibiting S@pPAN composite cathodes and nonflammable carbonate electrolytes derived from the presence of PFPN.

（2）PFPN not only works as a flame inhibitor, but also modifys the interface on the S@pPAN composite cathode and accelerates the electrochemical reactions.

（3）After optimized the addition of PFPN (3 %) in the carbonate electrolyte, the S@pPAN composite cathode materials demonstrated stable specific capacity up to 1500 mA·h·g–1(sulfur)and remarkable high power rates with a specific capacity of 1100 mA·h·g–1(sulfur)at 7 C.

（4）During cycling, the lithium dendrites and the holes will lead the pulverization of the lithium metal anode, and the risk is increased at this time. PFPN might not work to improve the safety in this case. Therefore, more efforts are required to address the dendrite issue of lithium metal anode.

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社区自治的路径强调社区治理的本位是社会自治，主张社区治理体制改革，推动“政社分开”，例如实行社区的“议行分设”体制、“一会两站”模式等。社区自治应当基于社区居民的共同利益和诉求，尤其是业主对于自身物权维护的需要。鉴于现实中居民自治的衰落和业主自治的兴起，近年来社区自治研究开始偏重业主自治，并有逐步取代居民自治研究的趋势。但是，研究者也指出了当前社区自治的局限及其原因，即受制于国家结构性约束的偏态自治和居民主观态度的无序自治与低度自治。⑨

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Multi-functional additive PFPN for rechargeable lithium sulfur battery with composite cathode materials

,,,,,

(Shanghai Electrochemical Energy Devices Research Center, Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China)

Lithium sulfur (Li/S) is a promising candidate of next generation of rechargeable batteries because of its extremely high specific capacity and abundant natural resources. However, the safety issue might be one of the most important factors which hinder their practical applications. Here a highly effective flame-retarding additive, ethoxypentafluorocyclotriphosphazene (PFPN), is firstly applied in Li/S batteries to improve their thermal safety properties. The addition of about 5% (weight ratio) PFPN makes the highly flammable liquid electrolyte totally nonflammable. Furthermore, the addition of PFPN simultaneously reduces the polarization voltage and obviously enhances the rate capability of sulfur-based composite materials. All these results indicate that PFPN is a favorable flame inhibiting additive in conventional liquid electrolytes for rechargeable lithium sulfur batteries.

flame retardant; PFPN; functional electrolyte; lithium sulfur batteries; composite cathode

10.12028/j.issn.2095-4239.2016.04.001

TM 911

A

2095-4239（2016）04-397-07

2016-03-15；修改稿日期：2016-05-06。

国家自然科学基金（51272156，21333007），上海市科委（14JC1491800）及教育部新世纪优秀人才（NCET-13-0371）项目。

沈旺（1990—），男，硕士研究生，E-mail：sjtusw@sjtu.edu.cn；通讯联系人：王久林，博士，研究员，主要研究方向为锂硫电池及其关键材料，E-mail：wangjiulin@sjtu.edu.cn。