DOI: 10.31038/NAMS.2019244

Introduction

The non-food valorization of biomass represents a major axis of research currently animating a large number of scientists. Whether for energy purposes, replacing fossil fuels such as oil, or as innovative strategies for access to new bio-sourced products, modern valorization approaches are above all respectful of the principles of green chemistry, and generally refer to processes or to the use of eco-compatible products. Renewable polymers have emerged as an attractive alternative to conventional metallic and organic materials for a variety of different applications, due to their biocompatibility, biodegradability and low cost of production [1].Renewable biomass can provide many industrial solutions as it is composed primarily of cellulose (30–35%), lignin (15–30%) and hemicellulose (20–35%).Among them, lignin is known as one of the main bio-resource raw material that can be used for the synthesis of environmentally friendly polymers, thus a good candidate for replacing regular industrial aromatic polymers and fine chemicals. Due to its chemical and structural diversity, lignin valorization remains a major challenge for achieving a viable biomass-based economy [2]. Lignins are composed of polymerized monolignols and their derivatives. Particularly grass lignins contain important amounts of alkenes yielding up to 10–15% by weight aldehydes [3].

Lignin extraction from lignocellulosic biomass

The lignocellulosic biomass has to be treated before any widespread utilization of its components. As a decrease in the lignin content in plants results in an increase in biodegradability, lignin removal from this biomass is a crucial pretreatment step [4,5]. A large number of chemophysical pretreatment approaches has been investigated on a wide variety of feedstock [6]. These methods require the use of hazardous materials such as acids, alkalis and/or organic solvents. They are currently four industrial processes to extract pure lignin: sulfite, kraft, organosolv and soda processes [7, 8]. Kraft pulping accounts for approximately 85% of the produced lignin. The delignification process is performed at high temperatures (170°C) and high pH 13 or 14, during which the lignin is dissolved in sodium hydroxide and sodium sulfide (white liquor) [9].

The sulfite process involves the reaction between lignin and a metal sulfite and sulfur dioxide, with calcium, magnesium or sodium acting as counter ions; the pH can vary between 2 and 12, and the temperature between 120 and 180 °C, with a digestion time of 1–5 h [10]. The soda process is typically used for the treatment of grass, straw and sugarcane bagasse, which accounts for 5% of the total pulp production [11]. The biomass is digested at temperatures that vary between 140 and 170 °C in the presence of 13–16% by weight of aqueous solution of sodium hydroxide. The soda lignin contains no sulfur which is not the case of the kraft and sulfite processes.

The organosolv process is based on the treatment of the biomass using organic solvents including ethanol, methanol, acetic and formic acid that are usually mixed with water at temperatures that range from 170 to 190°C [12]. The recovery and separation of the dissolved lignin and hemicelluloses can be done by precipitation of lignin or evaporation of the organic solvent, after adjusting the temperature, pH and concentration of the organic solvent. Organosolv pulping is one of the most efficient options for the further valorization of lignin and it also preserves the native structure of the lignin [13]. The obtained lignin is sulfur-free, with lower ash-content, and it has higher purity.

The removal of lignin by different technologies originates different product streams. Beyond the production costs and the environmental impact the lignin extraction method has to be selected depending on the use that will be given to the extracted lignin. In general, the kraft and sulfite processes allow extracting lignin at reasonable costs, while the organosolv method continues to be an expensive technology but still with high quality extracted lignin. However, the soda extraction process generates lower production costs and low environmental impact.

Ozonation of lignocellulosic biomass

Ozonation can circumvent the different issues of the above extraction processes that require the use of hazardous materials (acids, alkalis and/or organic solvents), as it is considered as a green process. Ozone (O₃) is a powerful oxidizing agent (E° = 2.07 V). It is one of the most promising lignocellulosic biomass oxidative pretreatment for selective lignin degradation with minimal effects on the hemicellulose and cellulose contents [14]. It provides low production of inhibitory compounds such as furfural and HMF (Hydroxymethylfurfural), and more importantly it requires no chemical additives during all the pretreatment process. However, ozonation demands high energy generation costs but this aspect can be avoided by optimizing the ozonation process. Ozone has a high affinity for phenol and polyphenols such as lignin and tannic acid. During ozonation lignin is converted to soluble products which to a great extent are biodegradable and thus yield a useful byproduct [15].

It has been reported the ozonolysis of grass lignin to selectively cleave aromatic aldehydes by limiting the reaction residence time to a few minutes for preventing over oxidation of targeted products [16], the ozonation was done in acidic media to avoid the production of secondary ozonides usually done at neutral pH media [17]. Ozone has been used to remove lignin from different biomass such as wheat and rye straw [18], cotton stalk [19], magazine pulps [20], among others.

Lignin presents a very complex assembly which limits enormously their interaction with host polymer matrices for industrial applications. It has been reported that only 2% of the annually extracted lignin from paper and pulp industry is used for applications such as fillers, adhesives and dispersants; the remaining lignin is burned as industrial waste for energy generation [21].

Synthesis of lignin nanoparticles

One way to overcome the limitations of lignin pointed out in the above section is to reduce the size of lignin particles until the nanometric size (less than 100 nm). At this scale, new functionalities and properties of materials are observed and used for a wide range of novel applications. As the size of the particles is reduced to the nanoscale range, the surface to volume ratio of the particles gradually increases which in turn increases the reactivity of the particles and changes their mechanical, electrical and optical properties [22] (Adusei-Gyamfi and Acha, 2016).These lignin nanoparticles hold huge potential for downstream valorization due to their unique morphology and abundant multifunctional groups. Thus the idea is to prepare lignin nanoparticles, which will greatly improve their reactivity and solubility with host matrices, and will provide a morphological and structural control of these structures for different high-value applications [21].

Several different methods have been published to synthesize nanolignin. Frangville et al. reported nanolignin obtained by precipitation in HCl. The resulting nanoparticles were crosslinked with glutaraldehyde, getting good stability over a wide range of pH [23]. Gilca et al. prepared nanolignin by sonication, they identified two main reaction patterns resulting in chain cleavage (depolymerization) and oxidative coupling (polymerization), both probably promoted by the hydroxyl and superoxy radicals generated by ultrasound [24]. Hydroxypropyl lignin nanoparticles were reported to be prepared by reacting an alkaline lignin solution with propylene oxide, then acidifying and centrifuging the mixture to precipitate the nanoparticles [25]. It has been reported also the use of a solution precipitation from alkaline lignin with either ethylene glycol or alkaline solution, which resulted in smaller nanoparticles [26]. Of all the methods mentioned above, the precipitation method in HCl seems to be the easiest for the synthesis of lignin nanoparticles.

Applications of lignin nanoparticles

As for the applications for lignin nonmaterial’s, lignin shows unique properties such as antioxidant and antibacterial properties, ultraviolet absorption, and high toughness [27]. To produce novel materials with improved properties, nanolignin particles can be incorporated in polymers for use in food packaging with the properties mentioned above, such as polyvinyl alcohol/Chitosan (providing UV absorbance, antioxidant and antibacterial properties), glycidyl methacrylate grafted polylactic acid (providing UV-absorbance and antibacterial properties), polylactic acid(providing UV-absorbance and antibacterial properties) [28], among others.

Lignins are known by their free radical scavenging activity due to their complex phenolic structure, which make them recognized as efficient natural antioxidants [29]. The incorporation of natural antioxidants to food packaging materials has been widely studied in order to improve protection of light and/or oxygen sensitive products [30]. Lignins have been proposed as antioxidants for polylactic acid films [31]. Lignin nanoparticles have been reported to exhibit higher antioxidant activity than neat lignin [26, 32, 33].

Nanolignin particles have also been involved in production of antimicrobial materials. Richter et al. [34] produced nanolignin loaded with silver ions and coated with a cationic polyelectrolyte layer capable of adhering to bacterial cell membranes. The nanoparticles killed both Gram-negative and Gram-positive bacteria while using at least 10 times less silver than conventional silver nanoparticles, thus reducing the environmental impact produced by silver nanoparticles. Films based on polylactic acid [35], chitosan, and/or polyvinyl alcohol [36] added with nanoparticles of lignin presented activity against Gram-negative bacteria, indicating that the films containing nanolignin particles could be used as antibacterial food packaging.

UV radiation accelerates oxidation rates in food [37] and also photodegradation of organic polymers [38]. The UV-absorbing capacity of lignin was tested by Yearla and Padmasree [32] by monitoring survival rates of UV-irradiated E. coli. In the absence of lignin compounds, the mortality of E. Coli was 100% after 5 minutes of UV exposure, while their survival was improved in the presence of lignin in proportion to their concentration. This study was done based on the fact that Escherichia coli suffer intracellular oxidative damage and death caused by UV radiation. In addition, because lignin nanoparticles have superior UV protection over ordinary lignin, the survival rate results were much better when nanolignin was used. Bionanocomposite films made of gluten-lignin nanoparticles have been reported to absorb UV radiation [99], thus these materials could be applied in food packaging with UV protective features, such as nuts and other food products that are prone to lipid oxidation.

Conclusion

Lignin as a renewable polymer is an attractive alternative to conventional metallic and organic materials due to its biocompatibility and biodegradability. Ozone has proved its efficiency as pretreatment method of lignocellulosic biomass for removing lignin. Ozonation is a green method requiring no hazardous compounds such as acids or alkalis, and needs moderate reaction conditions such as room temperature and atmospheric pressure. Lignin is a very complex polymer limiting its applications. One way to solve this problem is to reduce the size of ordinary lignin particles at the nanoscale, where new properties are being harnessed for novel applications. Lignin nanoparticles are good candidates for the next generation functional nanocomposites as they present very interesting properties such as UV light blockers, radical scavengers, and antioxidants very promising for potential applications in the food sector such as packaging.

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DOI: 10.31038/NAMS.2019243

It is well known to synthetic organic chemists that ball milling can be used for the synthesis of new compounds [1]. On the other hand, it is well known that ball milling of complex natural materials such as lignin and pollen grains can lead to alteration of their original structure and may produce new compounds that originally do not exist [2]. For example, Milled wood Lignin (MWL) contains more hydroxyl groups than the native one due to the extensive depolymerization during the ball milling process [2]. The presence of more hydroxyl groups indicated the homolytic bond cleavage between lignin monomers, which in turn produce reactive radicals that can create new compounds [2].

It is well known that sporopollenin is composed of carbon, hydrogen, and oxygen in the form of a cross-linked polymer that is amazingly stable [3]. It makes up the outer wall of pollen grains, and when extracted it is in the form of an empty exine or microcapsule [3]. During the late past century sporopollenin exine resistance against chemical treatments has been well documented [3]. It was found that the rigidity of sporopollenin restricted its practical analytical analysis to a limited number of techniques, such as Fourier transform infrared (FTIR) spectroscopy and analytical pyrolysis combined with gas chromatography–electron ionization-mass spectrometry (GC–EI-MS) [4,5]. Based on those outdated and old studies, there is still the belief that sporopollenin contains an aromatic identified as p-coumaric acid and, to a lesser extent, ferulic acid [4,5]. Some recent studies, which still champion this outdated theory was published in nature plants implying a resemblance between sporopollenin and lignin. In this study, Li et al.[6] reported the structure elucidation of sporopollenin extracted from ball-milled pollen grains. It should be noted that in 1966, Gordon Shaw, one of the earliest pioneers in sporopollenin, withdraw his proposal that sporopollenin contains lignin because it does not give any positive test for lignin’s [7].

Needless to say that the pollen grain contains proteins and genetic material that is enclosed by the intine composed of carbohydrates followed by the exine composed of sporopollenin [8]. Logically, high energy ball milling of the pollen grains could produce new artefactual compounds through the reaction between all these pollen grains components together, which in turn could alter the structure of the studied target material sporopollenin.

The effect of ball milling on the structure of these complex natural polymeric biomaterials is perhaps comparable to using specifically pyrolysis GC-MS for their structural analysis [9,10]. Pyrolysis GC-Ms can lead to the identification of compounds that initially does not exist in your sample [9,10]. As an example, pyrolysis GC-MS of unsaturated fatty acids leads to the production of aromatic compounds and, even linear saturated polymers such as polyethylene can produce aromatics during pyrolysis GC-MS analysis [9,10]. Overall,  as a mass spectrometrist, firstly, it is recommended to avoid any procedures that could alter the structure of these complex natural materials before using any mass spectrometric techniques for structure elucidation and/or sequencing purposes. Secondly, using soft ionization methods such as electrospray ionization (ESI-MS) and matrix-assisted laser desorption/ionization (MALDI-MS) is more advantageous than pyrolysis GC-MS. The use of these soft ionization methods allows the analysis of the native form of these complex materials without any alteration in their structure that could lead to misleading results [4,5,9,10]. We are in the process of reporting new finding on the structure of the hollow empty clean sporopollenin exine using state of the art analytical experiments, such as high-resolution X-ray-photoelectron spectroscopy, TOF-SIMS, TOF-SIMS, MALDI-TOF-MS, OF-SIMS (FIB) MALDI-Tandem Mass spectrometry analysis and Solid-State ¹H and ¹³C-NMR (1D and 2D experiments) [11]. We can state that sporopollenin exine does not contain any aromatics and bear no resemblance to lignin [11].

References

1. Stolle A, Szuppa T, Leonhardt SE, Ondruschka B (2011) Ball milling in organic synthesis: solutions and challenges. Chemical Society Reviews 40: 2317–2329.
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DOI: 10.31038/NAMS.2019242

Abstract

The Lithium-ion battery (LIB) has been utilized in many applications for thirty years, from personal electronics to electric vehicles. Many developments during the past decades resulted in high energy density and capacity in LIBs. However, battery safety still remains an issue alongside the continuous growth in the LIB market. Recent high-profile hazardous incidents gained considerable attention from the public as well as among researchers. This short review summarizes the recent efforts in improving LIB safety on three fronts: (1) materials advancement, (2) early monitoring and detection of thermal runaway events, and (3) fault diagnosis and fault-tolerance controls.

Keywords

lithium-ion battery; battery safety; battery management system; fault diagnosis; fault-tolerance control

Introduction

Lithium-Ion Battery (LIB) technology and the industry experienced rapid development in the past three decades, due to the advantages of LIBs over other energy storage systems, including: high energy density, strong stability, low maintenance, and low self-discharge. The LIB is the predominant power source in both consumer electronics and Electric Vehicles (EVs). The global market of LIBs (Figure 1) increased from $18.8 billion in 2014 to $28.5 billion in 2018. It is expected to increase by 11.1% (compound annual growth rate, CAGR) to $53.3 billion by 2024 [1]. The continuous growth in the global EV market from 2014 to 2024, at a rate of 21.1% (CAGR), is anticipated to contribute to the growth in the LIB market [2]. Although the LIB offers many advantages, safety still remains as an issue. According to a report issued by the Federal Aviation Administration (FAA), the number of air/airline incidents involving the battery – lithium-battery-induced smoke, fire, extreme heat, or explosion – increased from 9 to 50 incidents annually from 2014 to 2018 [3]. There are many reports about EV fire accidents caused by LIBs. For example, an all-electric compact car BYD e6 (BYD Auto company, China) caught fire after being hit by a Nissan GTR, an accident that led to the deaths of three passengers in Shenzhen, China, in May 2012 [4]. The battery pack of a Tesla Model S immediately caught fire after the driver hit debris on a highway in Washington State in 2013 [5]. Improving LIB safety is an important issue that requires research into new materials for the device, as well as structure optimization, and system design.

A typical EV battery is a three-level assembly: battery cell, battery module, and battery pack (Figure 2). The battery cell is a basic unit, consisting of an anode, a cathode, a separator, and liquid electrolyte. Multiple battery cells are connected and placed into a frame, called a battery module. Finally, the several battery modules are assembled into a battery pack, along with a control system, and system protection. The battery pack is a complete system that can be installed in an EV. Fires in a battery cell occur because of physical and electrical faults [6]. A chain reaction fire in the battery cells results in an explosion of the battery pack, called a cascading thermal runaway event. As mentioned in a review by Mauger et al., a gasoline fire can only be ignited when the gas tank’s air level is between 1.4 and 7.6%, and temperature is above 200^oC [7]. The gas tank design of the vehicle ensures that gasoline cannot self-ignite in a tank. Unlike gasoline, the cascading thermal runaway event can happen in the LIB. This review summarizes the recent research efforts to improve LIB safety on three fronts: (1) materials advancement, (2) early monitoring and detection of thermal runaway events, and (3) fault diagnosis and fault-tolerance controls.

Current efforts in battery safety improvements

1 . Materials advancements

Cathode (positive electrode) material can be categorized into three groups, based on crystal geometry:

• Lamellar compounds (lithium cobalt oxide – LCO; lithium nickel oxide – LNO; lithium nickel cobalt aluminum oxide – NCA; and lithium nickel manganese cobalt oxide – NMC)
• Spinel lithium manganese oxide – LMO
• Olivine lithium iron phosphate – LFP

Figure 1. Global history and forecast data for lithium battery, including lithium battery global market size; electric vehicle sales globally; and numbers of problem incidents related to lithium battery reported by FAA [1–3].

Figure 2. A typical battery pack assembly for EV.

LCO is the conventional cathode material used in the invention of LIBs by Sony. Because of materials shortages, cost, and the toxicity of cobalt (Co), transition metals (Mn and Ni) are used to partially substitute for Co. However, Huggins’ study showed that the partial pressure of oxygen at equilibrium varies exponentially, with a redox potential of the transition metal oxide vs lithium [8]. At 25^oC, the equilibrium oxygen pressure for cathode materials is 1 atm, at a potential of about 3V. This oxygen pressure increases to greater than 50 atm at higher potentials. The lamellar compounds tend to lose oxygen (migrating to the counter-electrode, graphite) and produce carbon dioxide (exothermic reaction), which results in a thermal runaway and release of the emission gases.

LFP exhibits remarkable thermal stability because the oxygen is covalently bonded with the phosphorous atoms. Despite the fact that the operating voltage of the LFP with a graphite anode (3.2V) is lower than that of LCO (3.7–3.9V) and LMO (4.0V), the LFP cell passes the mechanical stress tests and short circuit tests without thermal runaway [7]. Hence, the LFP is commonly used for safety purposes in applications such as EVs and in industrial applications. Although the demand for the LFP cathode reached ~100 Gg (100,000 tons) in 2017, the forecast for LFP’s market share (along with LCO, NMC, NCA, and LMO) will decrease from 38% in 2017 to 15% in 2025 because of its low energy density [9]. Further developments of cathode materials with moderate to high energy density and safety are still needed.

Anode (negative electrode): spinel lithium titanate -LTO (theoretical capacity ~175 mAh/g) has been studied as an alternative to graphite (theoretical capacity ~372 mAh/g) [10–11]. Belharouak’s and Chen’s studies of lithiated LTO and lithiated graphite by differential scanning calorimetry showed that LTO exhibits higher exothermic onset temperature than that of graphite (130^oC vs 100^oC, respectively) and generates much less heat (383 J/g vs 2,750 J/g, respectively) [12–13]. In nail penetration tests, the temperature of the cells after penetration increased by only 5^oC in a LIB with the LTO anode but rapidly increased by 130^oC with the graphite anode [13]. Although the LTO has much lower theoretical capacity, it shows high thermal stability and capability to delay a thermal runaway event and may serve as a potentially safer alternative to graphite.

Separator is a porous polymeric membrane that prevents internal short-circuit events between the anode and cathode. Typical commercial separators are polypropylene PP (T_m ~165^oC) and polyethylene PE (T_m ~130^oC), configured as a single layer, or bi- and tri-layers [14]. Uniform and appropriate porosity (40–60%) is necessary to have uniform current density, retain a sufficient amount of liquid electrolyte, and maintain mechanical strength [15]. High porosity separators tend to shrink, which results in pore distortion and leads to an internal short circuit as the temperature increases. Zhang’s study showed that the mechanical properties of the PP and PE separators exhibited low punch strength and anisotropy in uniaxial tensile tests [16]. The separator failed easily under tension and punch. Recent separator improvements include surface coating of polydopamine (PDA), to inhibit Li-dendrite growth, and gamma irradiation, to enhance cross-linking of the PE polymer chains and enhance thermal stability of the PE separator, in order to improve safety [15].

Table 1. Typical materials in lithium-ion battery technology [7].

Liquid electrolyte includes ethylene carbonate (EC), diethyl carbonate (DEC), ethyl-methyl carbonate (EMC), and dimethyl carbonate (DMC). Different additives are added to improve safety, reduce gas generation, provide overcharge protection, and serve as fire-retardant. One drawback of liquid electrolyte is its flammable nature, due to the low flash point of each component. To overcome this hazard, ionic liquid (IL) has been added to reduce the flammability. However, the use of IL is limited in the commercial electrolyte because of its high cost. Current liquid electrolyte developments aim for low flammability, fire suppression, electrochemical stability, and performance. For examples, Zeng et al. reported a stable and electrode-compatible, non-flammable phosphate electrolyte by adjusting the Li salts-to-solvent molar ratio [17]. Wang et al. demonstrated that the concentrated electrolyte contains lithium salt and a flame-retardant solvent to suppress fires and permit stable charge-discharge cycling [18].

Solid-state electrolyte (SSE) is considered as a safe alternative to the organic liquid electrolyte and separator. SSE must demonstrate high ionic conductivity (greater than 10^–4 S/cm at room temperature RT), negligible electron conductivity, and a broad electrochemical stability window [19]. There are three types of inorganic oxide-based SSE: a sodium super ion conductor (NASICON), with Li⁺ replacement; a garnet type; and a perovskite type. They have comparable bulk ionic conductivity (10^–5-10^–3 S/cm at RT) with the organic liquid electrolyte and show good chemical stability. However, there are several problems with the SSE: poor electrolyte/electrode interface in the NASICON-type; Li dendrite problems in the garnet type; and high interfacial resistance in the perovskite type. Sulfide-based SSE is another inorganic SSE with an ionic conductivity (10^–2 S/cm) higher than that of the oxide-based SSE. However, it suffers from the generation of flammable hydrogen sulfide (H₂S) upon exposure to the ambient atmosphere. The solid-state, hybrid electrolyte consists of a soft and flexible polymer electrolyte and a rigid inorganic SSE. It demonstrates good compatibility with the anode because of the intimate contacts. It is safer than the rigid inorganic SSE, because the uniform interfacial Li⁺ distribution also inhibits lithium dendrite formation [20]. Although the SSE has attractive properties, especially superior thermal stability and low flammability, problems such as lower bulk ionic conductivity at RT and interfacial mismatch between the solid-state electrolyte and the electrode materials still prevent widespread commercial application.

Beyond LIB: other battery chemistries – several non-lithium chemistries (Na, K, Mg, Ca, and Al) have been studied as potential alternative batteries to the LIB [21]. The Na⁺ and K⁺ in propylene carbonate (PC) exhibit higher mobility and ionic conductivity than that of Li⁺ because the Stoke’s radii in PC are in the order sequence of K⁺ < Na⁺ < Li⁺. Na-ion and K-ion batteries are expected to be a low-cost alternative to the LIB due to the abundance of Na and K resources, in addition to a similar energy density, similar battery design, and the same production process as the LIB [22]. Currently, studies of Na- and K-ion batteries and their safety are still in the development stage at research institutes.

2. Early monitoring and detection of thermal runaway events

Four different methods to monitor and detect thermal runaway events include (1) monitoring terminal voltage and surface temperatures, (2) an embedded optical fiber sensor, (3) electrochemical impedance analysis (EIS), and (4) a gas sensor monitor [23].

Terminal voltage and surface temperature monitoring method – This method uses the voltage and temperature sensors in real-time measurements of state of health (SOH), state of charge (SOC), and location of the faulty battery. Disadvantages of this method include high cost, low accuracy in thermal runaway prediction, and complexity of voltage sensor setup [23].

Embedded optical fiber sensor method – In this method, several types of fiber are used to set up optical sensors. For example, fiber Bragg grating arrays are attached on the surface of the cathode to record temperature and strain [24–26]. A nickel-coated fluorescent fiber is used for fluorescence lifetime measurements [23]. This technique can predict a thermal runaway event with high accuracy and directly monitor the internal temperature of the battery. However, the cost to set up the optical fibers and modify battery packaging is significant.

EIS method – The EIS technique uses an electrochemical impedance meter and a frequency response analyzer to determine the relationships between internal temperature and impedance phase shift or ohmic resistance [23, 27]. This method is able to predict the state of battery and thermal runaway temperatures with high accuracy. A drawback is the complex calibration process due to the fact that different battery systems have variant impedance parameters.

Gas sensing method – This simple and inexpensive method offers high accuracy and rapid detection, and functions by detecting vented gas concentration, because air flows faster than the speed of heat propagation in solid materials [23, 28]. Recently, Cai et al. validated the gas-sensing-based method by simulations [29]. In the simulation, the detection of the thermal runaway can be made at 85 seconds by the gas-sensing method, while the surface temperature measurement detected the thermal runaway propagation to neighboring cells at 710 seconds. Although the gas sensing-based method has many advantages, the sensor faults such as gas-sensor poisoning and gas cross-interference still persist.

3. Fault diagnosis and fault-tolerance control

A battery management system (BMS) consists of sensors, controllers, and computational algorithms. The BMS is designed to function in several ways: detect malfunctions and ensure battery safety, maintain accuracy and reliability, and predict and maximize battery life [30]. Battery faults are typically detected by data-driven approaches. Sensor faults are commonly diagnosed by model-based approaches, i.e. comparing the actual outputs to the estimated or nominal outputs (residual generation). A statistical cumulative-sum test is applied, rather than the selection of a fixed threshold, for high accuracy. The detected faults are then distinguished and monitored by fault-tolerant control (FTC) [31]. Most current and voltage sensors use Hall effect sensors and are usually subjected to bias and gain faults [32]. Many FTC methods for both the battery and sensors have been proposed and validated, including: re-arranging voltage measurement topology to distinguish between sensor and cell faults, without false detection and additional sensor [33–34]; nonlinear observability analysis for the sensor-biased fault-tolerance [35]; and active FTC to maintain battery temperature and deenergize the cell under faulty conditions [36].

Conclusion

Safety of LIBs has attracted considerable attention of researchers worldwide, as the incidence of LIB fires and explosions increases. SSE is a safe alternative but exhibits low ionic conductivity. Other battery chemistries have been studied but still remain in the development stage. BMS, the sensor system, and FTC are the most suitable measures to monitor and detect malfunctions of LIBs. Further developments of diagnostic schemes for fault detection and FTC for battery and sensors, together with novel materials developments, are needed to improve the safety of lithium-ion batteries.

References

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DOI: 10.31038/NAMS.2019241

We are currently living in the “Plastic age” as rightfully predicted in the classic movie “The graduate” in the 60’s where main character young Benjamin, just graduated and asking himself what to do next gets the famous advice: “I just say one word – Plastics. There is a great future in Plastics!” This was certainly correct; our daily life relies on synthetic polymers for all aspects of modern life. Going back in history, one of the first synthetic resins, a thermoset produced by a catalyzed reaction of phenol and formaldehyde, was introduced already 1909 under the name Bakelite. The creation of a synthetic plastic was revolutionary for its electrical non conductivity and chemical, solvent and heat-resistant properties in electrical insulators, radio and telephone casings and such diverse products as kitchenware, jewelry, pipe stems, children’s toys, and firearms. Bakelite was particularly suitable as a molding compound, an adhesive or binding agent, a varnish, and a protective coating and as such extremely useful for the emerging electrical and automobile industries. In the following years a number of new polymeric materials, thermoplastics and thermosets were developed, mainly formed by linking small molecules – monomers – together in a repetitive formation. Plastics are today available with extremely versatile properties ranging from, resistance to corrosion, low density, high strength, transparency, low toxicity, durability and a remarkable affordability and therefore used by almost every industry in the world, from food packaging to space exploration. Plastic is the ultimate commodity of convenience.

Especially hydrocarbon polymers like polyethylene and polypropylene, which account for more than half of the world plastics production and more than 90% of packaging materials are ubiquitous in single-use and short-term applications because their starting materials are abundant and inexpensive. In addition, we have learned to vary the chemical structure of the polymer chains (such as branching, molecular weight, and dispersity) through catalysis, and in such to alter their physical properties. The single-use nature of plastics is essential in sterile packaging for foods, strong-but in expensive materials for transportation and storage, and safe and disposable components in medical devices, leading to their manufacture in tremendous quantities. Three hundred and eighty million tons (380 Mt) of plastics are created worldwide each year, which corresponds to roughly 7% of crude oil and natural gas produced. Moreover, the plastic market is currently increasing, and some analysts predict quadrupled production by 2050 (~1100 to 1500 Mt per year).

However, the good properties of plastics, namely the mechanical performance connected with low density, are intrinsically connected with the existence of large molecules in the material. This long chain molecular nature turned out to be a major draw-back in the circularity and recycling. Mixtures of polymers are thermodynamically not stable; in fact only a few polymers are miscible at all and thus small impurities (not speaking of non-polymeric ones) deteriorate the functional properties significantly! Processing of plastic waste is limited by technical challenges, which include contamination from mixtures of polymers and additives as well as oxidative degradation during melt re-processing. Current recycling processes rely mainly on primary recycling (termed closed-loop recycling, reprocessing an uncontaminated, single plastic to give a product used for the same purpose as the original plastic) and secondary (mechanical) recycling –down cycling- results in lower-in-value materials with different uses compared to the original material. Both primary and secondary recycling involve sorting, grounding, washing, and extruding, which cause varying degrees of polymer degradation, resulting in a limited number of reprocessing cycles of polymers. This accounts even more for cross-linked polymers, often referred to as thermosets and another class of plastics comprising ca. 15−20% of polymers produced. The outstanding performance of conventional thermosets like the first material Bakelite originates from their covalently cross-linked networks but results directly in a limited recyclability. The available recycling techniques include mechanical, thermal, and chemical processing. These methods typically require a high energy input and do not take the recycling of the thermoset matrix itself into account but focus on retrieving the more valuable fibers, fillers, or substrates. Thermoset materials are in particular amongst the most difficult materials to recycle, and in most cases even considered impossible to recycle. Most attention is given to the recycling of fiber-reinforced composites, since the fibers are generally more valuable than the matrix material, especially when carbon fibers are used. The most favorable method of recycling, the direct re-use of components in similar or lower performance applications without any form of reprocessing is virtually impossible for the existing thermoset materials.

The downside of the plastic age is the massive quantity of waste, pollution and lost value associated with single-use plastics. Over 75% of materials produced each year, 300 Mt, are discarded after a single use. Currently, most of this waste is either lost to landfills and the environment, or inefficiently incinerated in power plants to produce electricity, generating greenhouse gases (e.g., CO₂) and toxic by-products in the process. Inefficient recycling and extremely slow environmental degradation of plastics are causing increasing concern about their widespread use. After a single use, 90 % of these materials are currently treated as waste creating a global environmental crisis, despite of their inherent chemical and energy value. Plastics have got themselves a bad name, mainly for two reasons: most are made from petroleum and they persist in the environment for decades or centuries beyond their functional lifetimes and end up as litter in the environment. Durability, one of plastic’s greatest assets is now its curse. Its robustness means that plastics stay in our environment for hundreds of years turning them into a persistent part of the landscape, and more importantly of the seascape. Once discarded, bulk plastics are polluting the oceans. Converging sea currents are accumulating plastic waste in a floating island known as the Great Pacific Garbage Patch, which now covers an area larger than Greenland. The bigger bits of plastic are life-threatening to marine life and sea birds, they can strangle marine animals and birds or build up in their stomachs. More recently, the awareness of the presence and danger of micro plastics has raised concern about their presence in the food chain. If nothing changes, by 2050 there will be as much plastic in the sea as there is fish. Who wants to eat plastic then?

The switch from a linear economy with its throwaway culture to a circular economy with efficient reuse of waste plastics is therefore mandatory. An increased focus on bio-derived and degradable composites as well as recycling could lower the degree of pollution. Reduce, reuse and recycle have been embraced as the common approach to tackle the escalating plastic waste problem. The goal is to create a circular plastic economy where products are 100%recyclable, used for as long as possible, and their waste is minimized. Compared to plastics, massive recycling of much older man-made materials like iron and steel (> 70 %!), copper, glass, aluminum and paper is the current technology and already since a long time developed and optimized. For a circular economy, the vast majority of plastic materials should be recyclable and the materials entering the chain should be bio-based. Although biodegradable polymers and in particular PLA have been the focus of much research over the last decades, only ~1% of plastics are currently produced from renewable resources. Besides, polymers derived from bio-renewable resources, commonly referred to as renewable polymers, bio-based polymers, or even sustainable polymers in the literature, are not necessarily sustainable or degradable, where as degradable polymers are not necessarily recyclable. For example, 100% bio-based polythene (bio-PE) and bio-based polyethylene terephthalate (bio-PET) are not biodegradable. Dreams and reality of green polymer chemistry have no match, conflicts and competition with food production and unrealistic high CO₂ production are consequences. Biopolymers are also not a realistic alternative to synthetic polymers (properties and processing). Recycling is costly, reliant on changes in human behavior and produces partially lower quality materials, in terms of both thermal and mechanical properties. Additionally, recycling does not change our plastic addiction; if we want to maintain our current lifestyles, modification to plastic manufacture needs to go hand in hand with effective recycling. So a change in mind-setting is mandatory!

A already developed way to reduce the demand for finite raw materials and to minimize the negative impact on the environment involves chemically recyclable polymers which are capable of being returned to the corresponding monomers in a depolymerization process ready for repolymerization to virgin-quality polymers. This seemingly ideal strategy has motivated the research on the exploration of chemically recyclable polymers and also the mild processes for the catalytic conversion of the recyclable polymers to monomers or new polymers, namely chemolysis (by depolymerizing or decomposing the polymer in the presence of a chemical catalyst, typically needing relatively low temperature or even ambient temperature). Consequently, to advance plastic recycling practices, improving chemical recycling selectivity and efficiency through monomer and polymer design and catalyst development is mandatory, minimizing the need for sorting and expanding recycling beyond polyesters, polyamide and polyurethanes [1]. Recent advances report on the catalytic selective hydrogenolysis of PE at moderate conditions (300 °C, 9 bar H₂) on nanoparticles in a solvent free process offering an option to obtain high value re-usable materials out of waste-PE[2] showing as such an opportunity to deal with the already existing large quantities of plastic waste in an economic and ecological way. Key feature of circular economy is preventing waste by making products and materials more efficient and reusing them. The challenge in a circular economy is the development of environmentally friendly polymers with better properties and at the same time facilitating reuse of plastics. This can be e.g. realized by polymers custom designed for recycling such as e.g. all-polymer (one component) composites, self-reinforcing polymers (molecular composites) [3] or the use of reversible chemistry [4]. Starting with the already about 15 year development of instruction of self-repair functionalities [5] in plastics and as such enabling these materials to react early on a developing damage or defect and reducing over-design of systems and realizing weight and cost reduction in a new way the future for plastics and other materials is not only a design or use (self-repair) but a design for recycling. This starts with a design of products to encourage a re-use as well as in a material choice for less impacting and recyclable materials, for reduction of quantities and variations in one product and an optimalisation of process technology. In the design phase, material sorting processes, separation and dismantling considerations are as important as the introduction of extension of lifetime and preservation of functional property technologies like self-repair. Consequently, there is a need for a new generation of materials that can be reprocessed like thermoplastics that still retain the beneficial properties of high performance thermoplastic or thermoset materials⁴. Such a material can be realized by the incorporation of dynamic interactions and dynamic covalent bonds into linear polymers and networks. In such plastics, thermal depolymerization and solvent assisted depolymerization and especially dissociative depolymerization and adaptable cross-links can transfer the non-recyclable plastics to small thermoplastic piece and thus enabling complete recycling and re-use. Alternatively (and parallel)it is the intrinsic production of cost-, resource-, eco- and energy efficient high performance polyolefin’s using modern multisite polymerization catalysts as reported recently by Mühlhaupt et al. resulting in all polyolefin injection moldable composites with a unique combination of high toughness, stiffness and strength which show virtually no change in properties during 7 cycles of remolding one of the most promising trends³. While following these ideas, the future of Plastics and ourselves will be still great. The need is to shift now from the Design-for-Use –self repair – strategies to the Design for recycling–multiple use- strategies to guarantee a bright Future for Plastics.

References

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2. GokhanCelik, Robert M Kennedy, Ryan A Hackler, MagaliFerrandon, AkalankaTennakoon,  et al. (2019) Upscaling Single-Use Polyethylene into High Quality Liquid Products. ACS Central Science DOI: 10.1021/asccentsci.9b00722.
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