Building Safe Lithium-Ion Batteries for Electric Vehicles: A Review

4.1

Cathode Materials

Presently, as shown in Table 3 [33], three types of cathode materials are widely used, which are LFP, LMO and layered oxide of LiMO2 (M = Ni, Co, Mn and Al). Among them, LFP is commonly used for its low cost, the highest thermal stability and long cyclability. It could keep stable at a temperature of 350 °C [70], which is attributed to its strong P=O covalent bond in PO43− octahedral structure [71]. The equation shows the decomposition for the delithiated Li0FePO4 [72].

$$2{\text{Li}}_{0} {\text{FePO}}_{4} \to {\text{Fe}}_{2} {\text{P}}_{2} {\text{O}}_{7} + 0.5{\text{O}}_{2}$$

(1)

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However, it shows a low theoretical specific capacity of 170 mAh g−1 and low volume energy density due to its low tap density of about 2.2 g cm−3.

In contrast to LFP, LMO possesses better manufacturing performance. However, it suffers from low energy density and poor cycling stability due to Mn dissolution during the charge/discharge process, especially at evaluated temperature. Compared with other cathode materials, Ni-based layered oxides have been widely investigated because of their higher specific energy. Among them, pure LiNiO2 suffers from poor electrochemical properties and thermal stability [73,74,75]. By replacing Ni with Mn, Co or Al into lattice structure, typical LiNixCoyMnzO2 (0 < x < 1, 0 < y < 1, 0 < z < 1, x + y + z = 1) including NCM333 (x:y:z = 3:3:3), NCM433 (4:3:3), NCM532 (5:3:2), NMC622 (6:2:2) and NCM811 (8:1:1), has been developed to enhance electrochemical properties [76, 77]. Among these materials, higher utilization of lithium at the same voltage range would be obtained with the increasing Ni content. Thus, the cathode materials with a higher Ni content such as LiNi0.8Co0.15Al0.05O2 (NCA) and NCM811 have been regarded as promising candidates to meet requirements of energy, power and cost for application of EVs [78, 79]. However, it is reported that these materials with larger specific energy would present a lower onset temperature of phase transition and are easy to release a large amount of oxygen during phase transition. Figure 2 shows the guidelines for the temperature ranges of phase transitions for the four charged NMC433, NMC532, NMC622 and NMC811 [76]. It could be seen that the onset temperature of NMC111 is about 260 °C, but the first phase transition of NMC 811 starts at about 135 °C [76]. It was reported that the onset temperature of NCA is around 170 °C [80].

$${\text{Li}}_{0.36} {\text{Ni}}_{0.8} {\text{Co}}_{0.15} {\text{Al}}_{0.05} {\text{O}}_{2} \to 0.18{\text{Li}}_{2} {\text{O}} + 0.8{\text{NiO}} + 0.05{\text{Co}}_{3} {\text{O}}_{4} + 0.025{\text{Al}}_{2} {\text{O}} + 0.372{\text{O}}_{2}$$

(2)

Reproduced with permission from Ref. [76]. Copyright 2014, American Chemical Society

a Onset temperature and b the corresponding temperature region of the phase transition for NMC.

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To effectively enhance the safety and electrochemical performances of LIBs, the mechanism of capacity degradation and poor thermal stability for these cathode materials should be deeply understood. With the aids of various advanced characterization tools, it is convenient for us to explore and optimize cathode materials of safe LIBs for EVs.

4.1.1

Failure Mechanism of Cathode

In general, poor thermal stability and cycle performance of nickel-rich layered oxide mainly stem from chemical reactions between delithiated cathodes and nonaqueous electrolytes at elevated temperature, which cause decomposition of cathode material and oxidation of electrolytes. The overdelithiated cathode would release oxygen from its lattice due to its high oxidability. Further, the electrolyte could react with oxygen to generate heat. If the heat generation and accumulation are more than its dissipation, a catastrophic failure of the cell will happen [81]. For the sake of designing safe cathode materials with high capacity, mechanisms of structure evolution, thermal stability and oxygen release in cathode materials during cycling should be thoroughly demonstrated. For example, some researchers investigated the structure evolution mechanism of NCM111 during cycling by in situ high-resolution synchrotron radiation diffraction and neutron powder diffraction. As shown in Fig. 3a and b, when the charge voltage is below 4.2 V, NCM111 maintains layered hexagonal phase H1 structure with slightly increased c and decreased a lattice parameters. When the voltage region is from 4.2 to 4.4 V, a new hexagonal phase H2 is detected and gradually intensified. When it is charged to a high voltage above 4.6 V, irreversible structure change appears from the original layered structure phase to a layered hexagonal phase H3 and a cubic spinel phase. After full lithiation, lattice parameters do not go back to its original ones, indicative of an irreversible structure evolution after charging to high voltage [82].

Reproduced with permission from Ref. [85]. Copyright 2017, American Chemical Society

In situ/operando techniques for the in-depth understanding of LIBs. a In situ synchrotron radiation powder diffraction (SRD) of the NCM111 electrode: the contour plot of reflection evolution of 003, 101, 104, 018 and 110 during the first delithiation–lithiation. b Lattice parameter changes of NCM111 during the first charge–discharge process. Reproduced with permission from Ref. [82]. Copyright 2019, The Electrochemical Society. In situ HEXRD patterns of delithiated Li1−x(Ni1/3Mn1/3Co1/3)0.9O2 during thermal abuse for c the dry delithiated cathode and d the delithiated cathode with the presence of EC/EMC (3:7, in weight ratios). e The delithiated cathode with the presence of 1.2 M LiPF6 in EC/EMC (3:7, in weight ratios). Reproduced with permission from Ref. [83]. Copyright 2013, Wiley–VCH. In situ SEM image for showing the progression of particle fracture and fragmentation during cycles of f the as-prepared sample and g the sample after three complete charge/discharge cycles. Reproduced with permission from Ref. [84]. Copyright 2013, Wiley-VCH. h Changes in secondary particle volume from in situ light microscopy for (left) NCM111 and (right) NCM811 cathodes cycled against metallic lithium anodes. Only the second cycle is shown for clarity. Shaded areas represent the error margin. The corresponding voltage profiles and the evolution of the lithium content [x(Li)] with cycling time are shown as well.

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Besides structure degradation of cathode materials, the parasitic reaction arising from interactions between electrolytes and highly reactive delithiated cathodes is also one main reason of failure for Ni-rich cathodes during cycling above 4.2 V. Chen et al. [83] developed an in situ high-energy X-ray diffraction (HEXRD) technique as an alternative to differential scanning calorimetry (DSC) and used it to investigate the chemical reactions between electrode materials and electrolytes during thermal ramping. Figure 3c–e shows the in situ XRD patterns of pure delithiated Li1.1(Ni1/3Mn1/3Co1/3)0.9O2 (NMC), the delithiated cathode in the presence of EC/EMC (3:7, by weight) and mixed delithiated NMC with an equivalent amount of 1.2 M LiPF6 in EC/EMC (3:7, in weight ratios) during thermal ramping. Compared to pure NMC, the mixed one in the presence of electrolytes decomposed much earlier with the onset temperature reducing from 278 to 197 °C. This result indicates that the thermal decomposition pathway of delithiated cathode materials, Li1−x(Ni1/3Mn1/3Co1/3)0.9O2, strongly depends on the exposed chemical environment, revealing the necessity of precise surface design.

In the microlevel, Ni-rich cathode materials consist of some secondary micrometer particles, which is aggregated by primary micrometer particles. The microstructural evolution of the single Li(Ni0.8Co0.15Al0.05)O2 particle during electrochemical cycling was further demonstrated by using in situ electron microscopy (Fig. 3f, g). Compared with the as-prepared particles, the particles after three cycles showed obvious cracks, which allow penetration of electrolytes between primary particles. The above result suggests that loss of grain-to-grain connectivity between particles would result in capacity fading and performance degradation [84]. Furthermore, more exposed cathode surface during cycling will intensify reactions between cathodes and electrolytes, which would aggravate capacity fade.

The generation of cracks between micrometer-sized secondary particles is due to anisotropic lattice change of primary nanoparticles during repeated Li+ insertion/extraction [85]. Kondrakov et al. [85] compared volumetric changes between NCM111 and NCM811 during cycling from 3.0 to 4.3 V versus Li/Li+ via in situ XRD, which were 1.2% and 5.1%, respectively (Fig. 3h). The Ni-rich cathode with a higher Ni content will utilize more lithium than a lower one, which means a deeper delithiated state. Volume change of NCM811 secondary particles (7.8% ± 1.5%) is even larger than that of NCM111 (3.3% ± 2.4%), which could be attributed to obvious decrease in the interlayer lattice parameter c from 14.467 to 14.030 Å.

Above all, with the aid of advanced characterization techniques, the reasons, for structure evolution as well as side reaction between the electrolytes and the primary particles of cathode materials, have been deeply understood in the microlevel, which would guide us to design safe cathodes.

4.1.2

Cathode Material Design for Safe LIBs

As mentioned above, overlithiated cathodes will lead to irreversible structure change to nonelectrochemically active structure, large volumetric expansion and oxygen release during cycling. To solve these problems, many efforts have been made by controlling cation mixing [86], doping [87], surface coating [88], constructing core–shell structure and designing concentration-gradient composition with a decreasing Ni content and an increasing Mn content from the inner part to the outer part of cathode particles [89].

Doping Introducing extrinsic cations or anions into layered structure has proved to be an effective way to address structural instability of Ni-rich cathode materials. In general, doping in cathode materials can suppress the order–disorder transition by preventing Ni2+ ions migration to the undesired Li slab during cycling, stabilize structure and avoid oxygen release by increasing the bonding strength between anions and transition-metal ions. The effective dopant ions include Al, Mg, Zr, Ti, Ga and F [87]. Kam et al. [90] used Ti4+ to partially substitute Co3+ at transition-metal sites. The valence difference between Ti4+ and Co3+ is compensated by the reduction of Mn from + 4 to + 3 to stabilize the layered structure. Ti4+ substitution can prevent phase change during cycling by restricting Ni migration to the Li slab, which is helpful to improve structural stability even with cycling to high voltage. With increasing the amount of cation dopants in transition layered structure, the specific capacity of the cathode material will generally decrease due to inert properties of the dopants. Thus, an ideal dopant would largely improve the structure stability but at the same time not lower specific capacity with a small amount of doping. The Al-based dopant is a good example. The cathode material with less than 5% Al doping almost remains unchanged specific capacity as compared with the original one [91].

For anion doping, F-doping has been considered as a promising approach for stabilizing the layered structure and optimizing cycle stability, since F is the most electronegative element and can form strong bonding with transition metals in the layered cathodes [92]. With the partial replacement of oxygen-metal bonds by the fluorine-metal bond, the initial Columbic efficiency, cycle performance and voltage degradation have been effectively improved, for example, in Li(Li0.133Mn0.467Ni0.2Co0.2)O1.95F0.05 (LMR). Additionally, the structure stability of LMR is improved upon heating, which is confirmed by in situ X-ray diffraction [93].

Surface/structure modification During charge/discharge, side reactions between cathodes and electrolytes would be accelerated by reactive Ni4+ and Co4+ ions, which will deteriorate the cathode surface and make the cathode electrolyte interface (CEI) become thicker. Surface/structure modification is effective to solve the above problem of cathodes. To date, three typical methods have been widely used to achieve high-capacity Ni-rich cathodes, which are summarized in Table 4, including surface coating, core–shell and concentration-gradient cathode fabrications [89]. In the following section, recent advances are discussed in detail by describing the representative examples.

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(1) Surface coating Surface coating is an effective way to suppress the side reactions between cathode materials and electrolytes without sacrificing capacity. The surface coating layer could act as a physical protection barrier to reduce side reactions between electrolytes and active cathode materials, especially at highly lithiated states. Additionally, the coating layer could serve as the scavenger of hydrogen fluoride (HF). Without the proper coating, HF will attack the cathode surface, making the transition-metal ions dissolved into electrolytes. Taking the TiO2 coating layer as an example, as metal fluorides present much lower Gibbs formation energy than the corresponding metal oxides, acidic HF will react with TiO2 as TiO2 + 4HF → TiF4 + 2H2O. The formed TiF4 layer will further prevent the surface from being attacked by the acidity of electrolyte. Furthermore, the coating layer could help to suppress the phase transition of cathodes from a layer structure to a rock-salt phase during lithiation/delithiation.

Recently, many efforts have been focused on constructing a coating layer. The layers of TiO2 [94, 95], Al2O3 [96], MgO [97] and ZrO2 [98] are generally much stable but exhibit low Li-ion conductivity. The thickness and homogeneity of the coating layer is vital to balance stability and rate capability. One of the promising techniques for achieving uniform thin coating layers on cathode materials is atomic layer deposition (ALD), which features low growth temperature, atomic-scale/stoichiometric deposition and excellent uniformity [99]. Xie et al. took advantage of ALD to construct a uniform, thin, stable and Li-ion conductive LiAlF4 interfacial coating layer on an NCM811 electrode and obtained improved stability and comparable rate performance. Zhang et al. employed ALD to coat the layered oxide cathode with Al2O3 and significantly improved the cycle ability of cathodes. Systematic STEM atomic-level imaging and nanoscale chemical composition analysis reveals that uniform Al2O3 coating acts as a physical barrier between cathodes and electrolytes, prevents surface structure transformation and suppresses Mn dissolution in the electrolyte [100]. However, the ALD method is not efficient enough for large-scale production.

Among the coating materials, inorganic species have different kinds of crystal structure as compared with transition layered oxides, which makes them difficult to realize uniform coating. Further, the adhesion between coating materials and cathodes is relatively weak. During repeat cycling, the coating layer might detach from cathode particles. At this point, organic conductive polymers are attractive since they possess better elasticity, which can buffer the volume change of cathodes during cycling. However, polymers show poor thermal stability and good porosity, which will decrease the thermal stability of cathodes. A combination of organic conductive polymers and inorganic materials might be a good choice.

(2) Core–shell cathode Compared with doping and coating with inert materials, coated materials with electrochemically active and fast Li+ insertion/delithiation properties are used, which would not sacrifice the high capacity of Ni-rich cathode. As shown in Fig. 4a, b, transition layered oxides with a higher Ni content show worse thermal stability [101]. In order to effectively increase thermal stability and maintain high capacity at the same time, constructing “core–shell” structured materials with a higher Mn content in the shell and a higher Ni content in the core, is proven effective (Fig. 4c) [102]. For example, a material with a Li(Ni0.8Co0.2)O2 Ni-rich core encapsulated by a 1-μm-thick Mn-rich Li(Ni0.5Mn0.5)O2 shell was prepared (Fig. 4d) [103]. The Mn-rich shell shows good thermal and structural stability, while the Ni-rich core contributes to high specific capacity. The primary exothermic temperature of Li1−δ(Ni0.5Mn0.5)O2 began at approximately 300 °C [104], which is about 100 °C higher than that of Li1−δ(Ni0.8Co0.2)O2. The core material delivers a capacity of 220 mAh g−1, much higher than that of the shell material (140 mAh g−1). Compared with pure Li(Ni0.8Co0.2)O2 (Fig. 4e), the core–shell Li[(Ni0.8Co0.2)0.8(Ni0.5Mn0.5)0.2]O2 material exhibits improved thermal stability. The pouch cell with core–shell Li[(Ni0.8Co0.2)0.8(Ni0.5Mn0.5)0.2]O2 as the cathode shows excellent nail penetration performance [103]. Sun et al. further explored the effect of shell thickness on the thermal stability based on core–shell Li[(Ni0.8Co0.1Mn0.1)1−x(Ni0.5Mn0.5)x]O2 particles. With the increasing thickness of the shell layer, the thermal stability of the core–shell material is enhanced (Fig. 4f) [105]. However, due to the mismatch of the crystal structure of shell and core materials, volume change during repeated charge/discharge is different, which will create voids between cores and shells, even leading to primary partial cracks. The voids between the core and the shell may hinder the Li-ion migration, and the cracks will make the fresh surface being exposed to the electrolyte, causing severe side reaction. And the low-capacity shell materials will decrease the whole capacity.

Reproduced with permission from Ref. [106]. Copyright 2012, Nature Publishing Group

a A map of the relationship between discharge capacity, thermal stability and capacity retention. b The DSC data of Ni-rich materials charged to 4.3 V. Reproduced with permission from Ref. [101]. Copyright 2015, Wiley-VCH. c Schematic drawings of Ni-rich core and Mn-rich shell structure. Reproduced with permission from Ref. [102]. Copyright 2016, Wiley-VCH. d SEM images of core–shell Li[(Ni0.8Co0.2)0.8(Ni0.5Mn0.5)0.2]O2 powders after 150 cycles. e Voltage versus cell surface temperature plots in the Li-ion cells of core–shell C/Li[(Ni0.8Co0.2)0.8(Ni0.5Mn0.5)0.2]O2 cells during nail penetration tests at 4.3 V. Insets show the Li-ion cell images of spherical C/core–shell Li[(Ni0.8Co0.2)0.8(Ni0.5Mn0.5)0.2]O2 cells after nail penetration at 4.3 V. Reproduced with permission from Ref. [103]. Copyright 2006, American Chemical Society. f DSC profiles of Li[(Ni0.8Co0.1Mn0.1)1–x(Ni0.5Mn0.5)x]O2 (x = 0–1) electrodes charged to 4.5 V versus Li. Reproduced with permission from Ref. [105]. Copyright 2006, American Chemical Society. g Schematic diagram of the FCG lithium transition-metal-oxide particle with the Ni concentration decreasing from the center toward the outer layer and the concentration of Mn increasing accordingly. h EPMA line scan of the integrated atomic ratio of transition metals as a function of the distance from the particle center to the surface for the lithiated FCG material. i DSC profiles of the delithiated FCG material, the delithiated Li1–xNi0.86Co0.10Mn0.04O2 and the delithiated Li1−xNi0.7Co0.1Mn0.2O2 with a scanning rate of 1 °C min−1.

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(3) Concentration-gradient cathode In order to eliminate the large difference between cores and shells, concentration-gradient structure is introduced to further enhance the capacity of cathode materials. With chemical composition continuously varying from the core/shell interface to the outermost surface of the shell, the structural inconsistency of the interface is eliminated. A full concentration-gradient (FCG) cathode, in which the concentration of the main species is fully gradient changeable, is developed for ideal performance. Unlike the core–shell materials with concentration gradients, the FCG cathode shows more flexibility in adjusting gradients (Fig. 4g). A pioneering work was reported by Amine et al. [106] on a Ni-rich layered oxide with a nominal composition of LiNi0.75Co0.10Mn0.15O2. As shown in Fig. 4h, the concentrations of Ni and Mn vary continuously from central to surface areas within a primary particle of ~ 5 μm, with LiNi0.9Co0.1O2 in the core and LiNi0.7Co0.1Mn0.2O2 on the surface. Due to a large distance for gradient change, the structural inconsistency is minimized. This advanced FCG material delivers a specific capacity up to 215 mAh g−1 with outstanding cycling stability in a full-cell configuration and maintains 90% capacity even after 1000 cycles. These results reveal that the chemical compositions of outer shells play a vital role in both electrochemical performance and thermal stability of Ni-rich materials (Fig. 4i). Sun et al. recently designed a Ni-rich concentration-gradient Li(Ni0.865Co0.12Al0.015)O2 (NCA) cathode, with a Ni-rich core and a Co-rich particle surface. With a much more stable surface shell, this gradient NCA cathode showed improved high-temperature cycle performance and maintained 90% of its initial capacity after 100 cycles at 60 °C and excellent thermal stability. The Co-rich surface provides a physical barrier between Ni-rich core materials and electrolytes. With the concentration-gradient shell, the inherent internal strain is largely reduced, ensuring a stable particle during lifetime [107]. The concentration-gradient-type materials are promising for practical application, but they still face many challenges such as controlling coverage and the consistent gradient of batches when extending to a 1000-fold and even larger scale.

Design better particles The layered oxide materials are commonly prepared by the co-precipitation method with round-shape primary particles randomly aggregating into large secondary particles. Anisotropic lattice volume expansion or contraction between the primary particles during cycling will result in microcracks, and electrolytes will penetrate into microcracks, causing severe side reaction.

Another way to eliminate cracks in the secondary particles is directly using single-crystal cathodes. Dahn et al. compared single-crystal and polycrystalline NCM523-positive electrode materials for high-voltage LIBs. The results show that single-crystal materials yield to longer lifetime for LIBs at both 40 and 55 °C when tested at an upper cutoff potential of 4.4 V. The reasons for superior performance of the single-crystal-based cells were explored by using thermogravimetric analysis/mass spectrometry experiments on the charged electrode materials, showing that single-crystal materials are extremely resistant to oxygen loss below 100 °C compared with the polycrystalline materials [108].

Kim et al. [109] reported a new concept to overcome the crack problem in the secondary particles of Ni-rich cathode materials via the nanoscaled surface treatment of the primary particles. As presented in the schematic diagram (Fig. 5a), NCM622 cathode materials with the surface treatment of its primary particles were prepared with the solution of lithium acetate and cobalt acetate (final products marked as ST-NCM). The surface of ST-NCM primary particles consists of the cation-mixing layer as a pillar layer, which effectively improves the structural stability and electrochemical performance of the cathode by suppressing the microcracks in the particles during cycling. As shown in Fig. 5b, c, compared with bare materials, the ST-NCM second particles remain tightly bound after 150 cycles. On the contrary, the untreated cathode materials appear obvious microcracks. Moreover, due to the cation-mixing layer on the surface with strong Mn–O bonding, oxygen release would be prevented at elevated temperatures in ST-NCM. Such a creative treating method of primary particles provides us a way to improve both the electrochemical properties and thermal stability of cathodes.

Reproduced with permission from Ref. [111]. Copyright 2013, American Chemical Society

a Schematic diagram of a coating method for alleviating the surface degradation of LiNi0.6Co0.2Mn0.2O2 cathode materials and the comparison of particle cross sections. b Before and c after 150 cycles before and after the modification. Reproduced with permission from Ref. [109]. Copyright 2015, American Chemical Society. d Schematic illustration of the structure of radially aligned primary particles. SEM images of radially aligned primary particles e before and f after cycle. Reproduced with permission from Ref. [110]. Copyright 2019, Wiley-VCH. g Integrated atomic ratio of transition metals as a function of the distance from the center of the particle for the lithiated FCG–Mn–F material. h TEM image, along with EDS data along the single elongated FCG–Mn–F primary particle. i Cycling performance of the laminated-type Al-pouch cell (35 mAh) by using mesocarbon microbead (MCMB) graphite as the anode and FCG–Mn–F as the cathode at a rate of 1 C, corresponding to 200 mA g−1 (an upper cutoff voltage of 4.4 V).

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Sun et al. [110] synthesized micrometer-sized Ni-rich NCM811 secondary cathode materials consisting of radially aligned single-crystal primary particles to address the issue of particle cracks during cycles (Fig. 5d). The aligned single-crystal secondary particles packed tightly (Fig. 5e), achieving coordinated charge–discharge volume change, which could alleviate the volume change-induced intergrain stress. This material with unique structure exhibits excellent cycling performance without any degradation after 300 cycles. The primary particles maintain origin shapes without obvious cracks (Fig. 5f). Sun et al. [111] applied this method into concentration-gradient-type cathodes with Ni concentration decreasing linearly and Co concentration increasing from the center to the outer surfaces (Fig. 5g). The primary particles are composed of the rod-shaped secondary particles of ~ 2.5 μm grown in the radial direction (Fig. 5h). This cathode is paired with mesocarbon microbead graphite to assemble into an aluminum pouch-type full cell (~ 35 mAh). The full cell shows excellent cycle performance and maintains 70.3% capacity after 1000 cycles even at 55 °C (Fig. 5i). Such good cycle performance is mainly attributed to the stable surface and well-combined second particles, which can reduce the internal stress.

4.2

Anode Materials

Based on the lithiation/delithiation mechanism, anode materials can be classified into three types. (1) Intercalation anodes include carbon-based materials and lithium titanium oxide (LTO), in which lithium ions are inserted into crystals with low volume expansion [112, 113]. Graphite has been commercially used as LIB anode for about 20 years due to its low costs, high electrical conductivity and low-potential plateau [112]. LTO has also been used in LIB markets for its high rate charge/discharge and long cycle life [114]. However, the shortcomings of LTO are its high lithiation/delithiation plateau (≈ 1.55 V vs. Li/Li+) and low specific capacity (175 mAh g−1) [113]. (2) Conversion reaction anodes consist of MX, where M would be transition metals such as Fe, Co and Cu, and X may be O, F or even H. Hence, MX could be compounds like Fe2O3, Co2O3 and CuO. During charge/discharge, these materials show higher specific capacity than intercalation anodes. However, their poor electronic conductivity may result in large irreversible capacity and voltage hysteresis and low rate performance [115]. (3) Alloying-type anode materials mainly refer to the IV and V groups, including Si, Ge, Sn, Sb metal and metal oxides, sulfides or phosphides. The alloying-type anodes could theoretically alloy with x Li+ to be LixM and offer discharge capacity almost four times higher than that of graphite. However, the huge volume change (> 300%) during alloying and dealloying would result in serious pulverization and poor cycle life, which severely restricts their practical application [116].

Although alternative anode materials such as the conversion type and the alloying type have been explored for high energy density, many researchers focus on how to improve reversible capacity, effectively alleviate large volume expansion and control capacity fading [117]. Hence, the safety issues of these materials have not received much concern [118]. Therefore, we only concentrate on the safety problems of the commercial graphite anode in this review.

4.2.1

Failure Mechanism of Anodes

As shown in Fig. 6 [119], during the initial charging process of a fresh LIB in the carbonate-based nonaqueous electrolyte, the SEI is firstly formed in a potential window range of 0.6–1.3 V versus Li/Li+ in the anode side [120]. It is well known that the conventional SEI is mainly composed of stable inorganic components (such as LiF, Li2O and Li2CO3) and some metastable organic components [such as polymers, ROCO2Li, (CH2OCO2Li)2 and ROLi] [121]. Li ions intercalate into graphite in the range of 0.065–0.2 V over charging. As the potential is below 0 V, Li ions would deposit on graphite and grow to Li dendrites on the anode surface, which may impale separators and hence lead to short circuit and heat generation [122].

Reproduced with permission from Ref. [119]. Copyright 2013, Elsevier

Illustration of aging effects on graphite anodes.

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Generally, the SEI film is relatively stable, which can permeate Li+ to react with graphite. However, during operation, under some abuse conditions such as excessive currents, overcharging, internal/external short circuiting or high temperature, LIBs would be ignited to generate heat. Some researches show that SEIs could decompose at a relatively low temperature of 69 °C [123], and the metastable organic components could exothermically decompose at 90–120 °C to some gases like carbon dioxide and ethylene [124]:

$$\left( {{\text{CH}}_{2} {\text{OCO}}_{2} {\text{Li}}} \right)_{2} \to {\text{Li}}_{2} {\text{CO}}_{3} + {\text{C}}_{2} {\text{H}}_{4} + {\text{CO}}_{2} + 0.5{\text{O}}_{2}$$

(3a)

$$2{\text{Li}} + \left( {{\text{CH}}_{2} {\text{OCO}}_{2} {\text{Li}}} \right)_{2} \to 2{\text{Li}}_{2} {\text{CO}}_{3} + {\text{C}}_{2} {\text{H}}_{4}$$

(3b)

As shown in Fig. 6, if the SEI layer is broken down, lithiated graphite anodes would exothermally react with electrolytes, which would further increase system temperature. If the intercalated lithium is exposed to electrolytes, it could react with carbonate-based organic solvents at about 100 °C [64, 125]:

$$2{\text{Li}} + {\text{C}}_{3} {\text{H}}_{4} {\text{O}}_{3} \left( {\text{EC}} \right) \to {\text{Li}}_{2} {\text{CO}}_{3} + {\text{C}}_{2} {\text{H}}_{4}$$

(4a)

$$2{\text{Li}} + {\text{C}}_{4} {\text{H}}_{6} {\text{O}}_{3} \left( {\text{PC}} \right) \to {\text{Li}}_{2} {\text{CO}}_{3} + {\text{C}}_{3} {\text{H}}_{6}$$

(4b)

$$2{\text{Li}} + {\text{C}}_{3} {\text{H}}_{6} {\text{O}}_{3} \left( {\text{DMC}} \right) \to {\text{Li}}_{2} {\text{CO}}_{3} + {\text{C}}_{2} {\text{H}}_{4}$$

(4c)

The releasing flammable hydrocarbon gases, such as ethane and methane, would yield a pressure inside the cell. What’s more, they will burn if the cathode releases oxygen and temperature provides suitable conditions, which is a huge risk for LIBs. Above all, side reactions at the graphite/electrolyte interface as well as the poor thermal stability of lithiated graphite and SEIs have been regarded as the main sources for safety issues of anodes.

4.2.2

Anode Modification for Safe Batteries

According to the failure mechanism of the anode, the thermal stability of SEI plays an important role in battery safety and how to improve it is a critical way to enhance LIBs safety. The alternative method is mainly focused on forming a stable artificial SEI layer with high thermal stability to protect lithiated graphite from being directly exposed to the electrolyte. As shown in Fig. 7a, compared with the rapid capacity decay of natural graphite, the graphite anode coated with Al2O3, prepared by Jung et al. [126] via ALD, showed excellent cycling stability with 98% capacity retention after 200 cycles, indicating that the artificial SEI of Al2O3 can effectively protect lithiated graphite from undesirable reactions. Additionally, as shown in Fig. 7b, DSC results indicate that the SEI film formed on NG anodes coated with Al2O3 is stabler than that without Al2O3. Cui et al. [127] reported that a thermoresponsive polymer switching (TRPS), consisting of graphene-coated spiky nickel nanoparticles mixed in a polymer composite, exhibited a high thermal expansion coefficient (σ). It could be used as a current collector coating layer in the anode (Cu). In Fig. 7c, d, the safe batteries with such TRPS materials show excellent battery performance at normal temperature and they can shut down rapidly under abnormal conditions like overheating and shorting due to the excellent mechanical flexibility, high room-temperature σ and large operating voltage windows. Another approach to build thermally stable artificial SEIs is to add functional electrolyte additives, which can modify the surface and formation process of SEIs. The SEI supporting additives will be introduced in the part of safer electrolyte design. For the reduction additives [128], their reductive potentials are higher than electrolyte solvents, so the additives would be reduced to form an insoluble solid layer before the reduction of solvents. Therefore, they could reduce gases generation and improve the thermal stability of SEIs. However, the artificial SEI film would sacrifice the theoretical specific capacity of anode materials. Thus, it is necessary to develop some active artificial SEI layers in the future.

Reproduced with permission from Ref. [127]. Copyright 2016, Nature Publishing Group

a Cycling properties at 50 °C of NG electrodes with and without ALD coated. b DSC curves of fully lithiated graphite electrodes with (the solid line) and without ALD coated (the dotted line). Reproduced with permission from Ref. [126]. Copyright 2010, Wiley-VCH. c COMSOL simulation of two-dimensional temperature profiles on the cross section of normal and safe batteries after shorting for 30 s. d COMSOL simulation of battery temperature dependence on shorting time.

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4.3

Separator Materials

In LIBs, the separator is a critical component with the function of preventing electronic contact and ensuring Li-ion transport between two electrodes during battery running. Except the demand for wettability and porous properties, the desired membrane should have high chemical/electrochemical stability, high thermal stability and excellent mechanical properties to ensure building a safe and reliable LIB.

Generally, the common separators for LIBs are polyolefin membranes such as polyethylene (PE) and polypropylene (PP), but they would be melted down at the temperatures of 130 and 170 °C, respectively [129]. During the melting process, the micropores of separators will be destroyed, which will isolate the pathway of Li ions between the electrodes and drastically increase the internal resistance of LIBs [130]. If the separator is melted, the internal short circuit will occur, which may lead to a large amount of heat generation and trigger the thermal runaways [129, 131].

4.3.1

Modification of Traditional Polyolefin Membranes

By taking advantages of different melting points of PE and PP, a modified polyolefin membrane with self-shut capability was developed [92]. As shown in Fig. 8a, a tri-layer structured PP/PE/PP separator is constructed, in which PP layers with higher melting points serve as the frame to prevent separator collapse, while the middle PE layer plays the role of shutting down the agent. However, this kind of separators may still fail if the temperature continues to increase above the melting point of PP. Further improving the thermal stability of polyolefin membranes is desirable. One effective way is to coat a layer of other polymers with high temperature and voltage tolerance. Roh et al. [132] improved the thermal stability of PE separators by coating a layer of macroporous polyarylate (PAR) via the physical method. The melting temperature of the modified separator is enhanced up to 194 °C [132]. However, the physical bond is so weak that the coated layer may peel off during cycling [133]. To overcome the above issue, dopamine was used to modify polyolefin membranes due to its strong self-polymerization and high hydrophilicity [133,134,135]. For example, Hu et al. [133] used dopamine as a mediator to achieve tightly coating aramid nanofiber (ANF) on PP separators and obtained stable cycle performance and improved thermal stability. Besides that, other polymerizable monomers were grafted to polyolefin separator with chemical bonds by UV, electron beams and γ-ray irradiation [136,137,138]. Lee et al. [139] prepared PE membranes grafted with siloxane by electron beam irradiation technique. Besides excellent cycle performance and improved thermal stability, the grafted PE separator shows enhanced electrochemical stability. What’s more, it could remain stable up to 5.2 V, which is helpful to design high-voltage LIBs and to improve overcharge endurance [139].

Reproduced with permission from Ref. [149]. Copyright 2018, Elsevier

a Closing pore processes and shutdown behaviors of PP/PE/PP separators. Reproduced with permission from Ref. [148]. Copyright 2015, Elsevier. b The grafted mechanism of TiO2-modified PE separators with strong chemical bonds. Reproduced with permission from Ref. [140]. Copyright 2016, Elsevier. c The thermal behavior of PI/PE separators. Reproduced with permission from Ref. [148]. Copyright 2015, Elsevier. d Photograph, voltage and temperature profiles of cells assembled with PIC separators after a penetration test. e Photograph, voltage and temperature profiles of cells assembled with PE separators after a penetration test.

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Besides polymers, ceramic materials with high thermal stability and excellent mechanical properties, such as aluminum oxide (Al2O3), titanium dioxide (TiO2) and silicon dioxide (SiO2), have also been applied to coat polyolefin membranes [138, 140]. In order to achieve better coating effect, polymer binders are usually used as mediators to coat ceramic materials onto the membranes [141], which have a crucial effect on the final performance of the modified membrane. Lee et al. [142] found that Al2O3/poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) composites would reduce the thermal stability of PP separators though the coating agent Al2O3 is thermally stable. When the temperature rises to about 140 °C, the melting of the PVDF-HFP binder makes the Al2O3 particles agglomeration, which produces a strong driving force for the shrinkage of PP membranes. On the contrary, polyimide (PI) P84 [142] and phenolphthalein polyetherketone (PEK) [143] with higher thermal stability are proven as better choices. However, the traditional coating method is easy to obtain thick membranes due to excessive polymer binders, which would block the pores of separators and hence increase the cell internal resistance [140]. To decrease the interfacial resistance of separators and enhance cycle stability, binder-free coating methods have been proposed, such as grafting [140, 144, 145], ALD and magnetron sputtering deposition (MSD) [146, 147]. Zhu et al. [140, 144] developed novel SiO2-grafted PE separators and TiO2-grafted PE separators by electron beams. Figure 8b shows the mechanisms of preparing TiO2-grafted PE separators; electron beam irradiation is used to create defects, which accelerate uniform coating of TiO2 [140]. The chemical bonds between coating materials and separators are the critical factor for improving stability of TiO2-grafted PE separators [140]. Chen et al. [146] and Peng et al. [147] deposited ultrathin TiO2 films on PP membranes by the ALD or MSD method. The modified PP separator could effectively avoid thermal shrinkage and poor wettability. However, the demand for special equipment and techniques makes chemical grafting difficult for large-scale application.

4.3.2

Novel Separators with High Thermal Stability

To overcome the congenital shortage of polyolefin membranes with poor thermal stability, new kinds of separators have been proposed.

The nonwoven separators based on high-heat-resistant polymer materials have been paid considerable attentions. For instance, poly(m-phenylene isophthalamide) (PMIA), polyimide (PI), poly(ethylene terephthalate) (PET) and poly(phthalazinone ether sulfone ketone) (PPESK) have extremely high thermal stability up to 250–500 °C [150], showing an obvious advantage over traditional polyolefin membranes. However, the intrinsic disadvantages of the large pore size and nonuniform pore distribution of nonwoven separators could result in serious self-discharge and lithium dendrites, which will hinder their extensive application in LIBs [151, 152]. Modifying nonwoven separators has been put forward, such as PVDF/PMIA/PVDF, PE/PI and PPESK/PVDF membranes [148, 153]. The excellent performance of PE/PI membrane is shown in Fig. 8c, as compared with PP/PE/PP separators [148]. Some inorganic particles, like Al2O3, SiO2 and TiO2, are also used to coat the nonwoven membrane for improving its electrochemical stability [152, 154, 155].

Cellulose paper-based separators are also promising because of their low production costs, good mechanical properties and high electrochemical stability [156, 157]. However, macro-/microfiber cellulose papers meet the intrinsic disadvantages of improper porous structure and poor mechanical/physicochemical properties [158]. Cladding is proposed as an effective way to overcome these disadvantages. Wang et al. [149] prepared paper-supported inorganic composite (PIC) membranes by spraying Al2O3 powders onto the surface of commercial paper substrates, which is helpful to improve the penetration test. As shown in Fig. 8d, during the test, the temperature of graphite/LiCoO2 full cells with PIC separators slowly increases to 180 °C and the voltage remains 3.78 V [149]. Additionally, the cell did not get fire after the nail test. By contrast, graphite/LiCoO2 full cells with PE separators reacted violently, and the temperature sharply increased to 329 °C, while the voltage dropped quickly down to 0 V (Fig. 8e) [149]. Furthermore, the PIC membrane exhibited high electrochemical stability with an oxidation decomposition voltage of 4.5 V (vs. Li+/Li), higher than 4.2 V of the PE [149].

Employing inorganic separator materials with “absolutely” thermal stability and strong electrolyte wettability is another good way. Pure inorganic separators composed of Al2O3 and SiO2 show extremely high thermal stability up to 2054 and 1650 °C, respectively [159, 160]. Such separators also exhibit excellent electrochemical performance and strong compressive strength. But the poor flexibility and tensile strength may restrict their large-scale application [159]. To improve the tensile strength of inorganic separators, polymer binders are introduced although they will sacrifice some thermal performance. Raja et al. [161] prepared MgAl2O4-based ceramic membranes (PCMs) with PVDF-HFP as the binder. Although the thermal stability has been reduced by polymers, the PCM membrane with a low binder ratio could resist the temperature up to 350 °C [161]. Similarly, Zhang et al. [162] prepared Al2O3-based ceramic membranes with styrene–butadiene rubber (SBR) as the binder. Then, He et al. [163] developed a pure ceramic membrane composed of Al2O3 nanowires with impressive flexibility by a filtration procedure, which provides a new mentality on overcoming the fragility of pure inorganic membranes.

4.3.3

Functionalized Separators

Although the separator with high thermal stability and strong mechanical properties could greatly inhibit the occurrence of an internal short circuit, some special functions have been introduced into the separator for enhancing the safety of LIBs. Adding fire retardants into the electrolytes has proven as an effective way to suppress the combustion of LIBs [164]. But in most cases, the addictive fire retardant will result in the poor electrochemical performance of LIBs [165]. Incorporating fire-extinguishing agents into separators as an automatic fire-extinguishing device has been proposed. Yim et al. [165] put forward a novel separator modified by integrating temperature-responsive microcapsules with fire-extinguishing agents, which could realize the self-extinguishing capability of LIBs. They chose (1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-(trifluoromethyl)-pentane) (DMTP) as the extinguishing agent and PMMA as the temperature-responsive polymeric layer [165]. Figure 9a illustrates the extinguishing mechanism of this microcapsule modified separator. The subsequent standard nail penetration test shows that the temperature-responsive microcapsules could suppress the temperature change [165]. Liu et al. [166] developed novel separator integrating temperature-responsive microfibers with core–shell structure (Fig. 9b). They employed triphenyl phosphate (TPP) as the extinguishing agent (core) and PVDF-HFP as the temperature-responsive shell. If the temperature increases to the melting point of the temperature-responsive shell, the encapsulated extinguishing agent is released into the electrolyte, effectively suppressing temperature rise [165, 166].

Reproduced with permission from Ref. [166]. Copyright 2017, Wiley-VCH

a Schematic for the coating of microcapsules on a PE separator by the PVDF-HFP binder. Once the temperature surpasses the melting point of the PMMA shell, the internal extinguishing agent DMTP will release into the electrolyte, suppressing the burning of the electrolytes. Reproduced with permission from Ref. [165]. Copyright 2015, American Chemical Society. b Structure of core–shell structure microfibers. Once thermal triggering, the TPP agent will release into the electrolyte, suppressing the ignition of electrolytes.

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Some electroactive redox polymers, such as polypyrrole, polythiophene and polytriphenylamine, can be p-doped into an electronically conductive phase during oxidation and reversibly n-doped into the neutral isolating state over reduction [167, 168]. This characteristic provides a new strategy to prevent LIBs from overcharging by developing potential-sensitive separators, which can produce an internal conducting bypass for shunting the charge current at the overcharged state [168]. The common way to prepare potential-sensitive separators is to incorporate the electroactive redox polymers into the pores of separator membranes [168, 169]. Zhang et al. introduced poly(4-methoxytriphenylamine) (PMOTPA) to modify the commercial PE separator. The subsequent overcharge test shows that the voltage of the LFP/LTO full cell with this potential-sensitive separator could keep safe at about 2.6 V [168]. Li et al. [169] prepared a poly(3-decyl thiophene)-modified separator, which could maintain the voltage of LiFePO4/C cells at a safe value below 4 V.

The reliability and safety problems caused by dissolved Mn cation in the electrolyte solution have received great attention for ternary oxide cathodes. Some experts proposed that modifying separators with specific chemical functionality is an effective way to shut down the migration path of Mn cations [170]. Ziv et al. proposed that polymeric materials functionalized with macrocycles or the alkali metal salts of organic acids could effectively trap the Mn cation [170,171,172,173]. The separators modified by poly(vinylbenzo-18-crown-6) (PVB-18C6) [170] and poly(divinylbenzene-(vinylbenzyl-aza-15-crown-5)-vinylbenzylchloride)) (PVB-A15C5) [172] ethers, as well as the dilithium salt of poly(ethylene-alternate-maleic acid) (PE-alt-MaLi2) [171], show an brilliant ability to chelate Mn species. Compared with the formers, PE-alt-MaLi2 exhibits better functionality [171]. It is reported that the amount of Mn on the graphite anode could be reduced by over 80% in the cells with the functional separator [171].

4.4

Electrolyte Materials

The electrolytes serve as a medium for ion charge transfer. Although nonaqueous organic electrolytes including carbonate solvents and lithium hexafluorophosphate (LiPF6) have been used for decades since the invention of LIBs [174], they are essentially not safe. The instability of electrolytes origins from both solvents and lithium salt.

Presently, as shown in Table 5 [175], cyclic carbonates (e.g., EC, PC) are commonly used as electrolyte solvents, which possess high dielectric constants to ensure high-concentration lithium salts dissociation capability. However, their high melting points and high viscosity will hinder ion transportation during the storage process. In order to satisfy practical application, linear carbonates (e.g., DMC, DEC and EMC) with low viscosity are mixed with cyclic carbonates to become composite electrolytes with proper chemical/electrochemical stability and good interfacial properties [176]. To be mentioned, cyclic ethylene carbonate (EC) could electrochemically decompose at about 0.8 V versus Li/Li+ to form protective SEI films at the surface of low-potential anodes [177]. Although the carbonates have been considered as the solvents with almost the best comprehensive properties, their low flash points make them easy to decompose to gases, which are inflammable and explosive [178]. Thus, constructing safer carbonate-based composite solvents and exploring innovative solvents would be important to design safe LIBs.

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Another important component of electrolytes is the lithium salt that provides Li ions. At present, as shown in Table 6, the common lithium salts used in LIBs contain LiPF6, lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6) and lithium perchlorate (LiClO4). Among these salts, LiPF6 is commercially used due to its good ionic conductivity and ability to passivate aluminum current collectors, but it is sensitive to a trace amount of water and would easily decompose to toxic and corrosive gases at high temperature [124]. LiAsF6 seems to be an ideal candidate due to good electrochemical properties and thermal stability, but its toxicity limits its practical application [179]. The fluoride-free alternative salt, LiClO4, exhibits comparable properties to LiPF6, and it is not sensitive to moisture and has no toxic decomposition products; however, there is a severe risk of explosion due to the high oxidizability of ClO4− and interaction with organic compounds. Thus, researchers pay more attention to developing novel lithium salts such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) [180], lithium bis(fluorosulfonyl) imide (LiFSI) [181, 182], lithium bis(oxalate)borate (LiBOB) [183], lithium difluoro(oxalate) borate (LiODFB) [184] and lithium bis(fluorosulfonyl) amide [185].

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4.4.1

Failure Mechanism of Electrolytes

Nonaqueous electrolytes account for a high percentage of the whole battery weight in a LIB. Almost all of the chemical reactions in the battery are related to the electrolyte. The carbonate solvents are usually flammable, and lithium salts are unstable. When LIBs are under abuse conditions, they could be easily ignited to generate a lot of heat and cause thermal runaways [120]. For the traditional graphite/LiCoO2 battery, if the temperature reaches 69 °C, the SEI film on the anode surface would be destroyed and produce initial reaction heat, and then the exposed lithiated graphite will react with the organic solvent (EC) [186]:

$$2{\text{Li}} + {\text{C}}_{3} {\text{H}}_{4} {\text{O}}_{3} \to {\text{Li}}_{2} {\text{Co}}_{3} + {\text{C}}_{2} {\text{H}}_{4}$$

(5)

For Li0.5CoO2 cathode, Li0.5CoO2 will react with EC at 70 °C to generate water [187]:

$${\text{Li}}_{0.5} {\text{CoO}}_{2} + 0.1{\text{C}}_{3} {\text{H}}_{4} {\text{O}}_{3} \to 0.5{\text{LiCoO}}_{2} + 0.5{\text{CoO}} + 0.3{\text{CO}}_{2} + 0.2{\text{H}}_{2} {\text{O}}$$

(6)

Kerr et al. [188] reported that the initial decomposition temperature of the electrolyte (LiPF6/EC:DMC) is about 80 °C. Once decomposition happens, complicated reactions in electrolyte would be triggered as follows [114, 189, 190]:

The decomposition of LiPF6:

$${\text{LiPF}}_{6} \leftrightarrow {\text{LiF}} + {\text{PF}}_{5}$$

(7a)

$${\text{PF}}_{5} + {\text{H}}_{2} {\text{O}} \to {\text{PF}}_{3} {\text{O}} + 2{\text{HF}}$$

(7b)

The series of reactions between strong Lewis acid PF5 and the organic solvent:

$${\text{C}}_{2} {\text{H}}_{5} {\text{OCOOC}}_{2} {\text{H}}_{5} + {\text{PF}}_{5} \to {\text{C}}_{2} {\text{H}}_{5} {\text{OCOOPF}}_{4} + {\text{HF}} + {\text{CH}}_{2} = {\text{CH}}_{2}$$

(8a)

$${\text{C}}_{2} {\text{H}}_{5} {\text{OCOOC}}_{2} {\text{H}}_{5} + {\text{PF}}_{5} \to {\text{C}}_{2} {\text{H}}_{5} {\text{OCOOPF}}_{4} + {\text{C}}_{2} {\text{H}}_{5} {\text{F}}$$

(8b)

$${\text{C}}_{2} {\text{H}}_{5} {\text{OCOOPF}}_{4} \to {\text{PF}}_{3} {\text{O}} + {\text{CO}}_{2} + {\text{C}}_{2} {\text{H}}_{5} {\text{F}}$$

(8c)

$${\text{C}}_{2} {\text{H}}_{5} {\text{OCOOPF}}_{4} \to {\text{PF}}_{3} {\text{O}} + {\text{CO}}_{2} + {\text{C}}_{2} {\text{H}}_{4} + {\text{HF}}$$

(8d)

$${\text{C}}_{2} {\text{H}}_{5} {\text{OCOOPF}}_{4} + {\text{HF}} \to {\text{PF}}_{4} {\text{OH}} + {\text{CO}}_{2} + {\text{C}}_{2} {\text{H}}_{5} {\text{F}}$$

(8e)

$${\text{C}}_{2} {\text{H}}_{5} {\text{OH}} + {\text{CH}}_{2} = {\text{CH}}_{2} \to {\text{C}}_{2} {\text{H}}_{5} {\text{OC}}_{2} {\text{H}}_{5}$$

(8f)

The oxidation reactions of various organic solvents:

$$2.5\,{\text{O}}_{2} + {\text{C}}_{3} {\text{H}}_{4} {\text{O}}_{3} \left( {\text{EC}} \right) \to 3{\text{CO}}_{2} + 2{\text{H}}_{2} {\text{O}}$$

(9a)

$$6\,{\text{O}}_{2} + {\text{C}}_{5} {\text{H}}_{10} {\text{O}}_{3} \left( {\text{DEC}} \right) \to 5{\text{CO}}_{2} + 5{\text{H}}_{2} {\text{O}}$$

(9b)

$$3\,{\text{O}}_{2} + {\text{C}}_{3} {\text{H}}_{6} {\text{O}}_{3} \left( {\text{DMC}} \right) \to 3{\text{CO}}_{2} + 3{\text{H}}_{2} {\text{O}}$$

(9c)

$$4{\text{O}}_{2} + {\text{C}}_{4} {\text{H}}_{6} {\text{O}}_{3} \left( {\text{PC}} \right) \to 4{\text{CO}}_{2} + 3{\text{H}}_{2} {\text{O}}$$

(9d)

$${\text{O}}_{2} + {\text{C}}_{3} {\text{H}}_{4} {\text{O}}_{3} \left( {\text{EC}} \right) \to 3{\text{CO}} + 2{\text{H}}_{2} {\text{O}}$$

(9e)

$$3.5\,{\text{O}}_{2} + {\text{C}}_{5} {\text{H}}_{10} {\text{O}}_{3} \left( {\text{DEC}} \right) \to 5{\text{CO}} + 5{\text{H}}_{2} {\text{O}}$$

(9f)

$$1.5\,{\text{O}}_{2} + {\text{C}}_{5} {\text{H}}_{10} {\text{O}}_{3} \left( {\text{DMC}} \right) \to 3{\text{CO}} + 3{\text{H}}_{2} {\text{O}}$$

(9g)

$$2\,{\text{O}}_{2} + {\text{C}}_{4} {\text{H}}_{6} {\text{O}}_{3} \left( {\text{PC}} \right) \to 4{\text{CO}} + 3{\text{H}}_{2} {\text{O}}$$

(9h)

Equation (7) presents the decomposition of LiPF6 in LiPF6-based electrolytes; Eqs. (8) and (9) show further reactions between strong Lewis acid phosphorus pentafluoride (PF5) and organic solvents or water; PF5 may attack the electron lone pair of oxygen in the solvent molecule and then make it decompose. In the meantime, as shown in Eq. (9), the oxygen generated by cathodes would react with carbonate solvents to release CO2 or CO. Kawamura et al. [189] found that the heat of 1 M LiPF6 was 375 J g−1 in 1:1 PC: DMC (or EC: DMC) and 515 J g−1 in 1:1 PC: DEC (or EC: DEC), and the exothermic peaks were located at 230 and 280 °C, respectively. The above reactions could produce flammable gases and generate large amounts of heat, which would cause the flatulence and leakage of LIBs. Once the electrolytes leak out and contact with air, the batteries would be fired and probably exploded.

With increasing requirements on safety for LIBs used in EVs, developing extraordinary electrolytes with good thermal stability and excellent electrochemical properties is desirable and urgent. On the basis of the failure mechanism of traditional electrolytes and advanced in situ characterization techniques, many researchers are focusing on designing novel electrolytes.

4.4.2

Novel Electrolytes for Safe LIBs

The improvement of electrolyte thermal stability is significantly important to the safety of LIBs. The above failure mechanisms reveal that the thermal stability of electrolytes is closely related to the components including organic carbonate solvents and lithium salts. Presently, many efforts have been devoted to designing stabler lithium salts, adding various functional additives for nonflammable electrolyte solvents and exploring nonflammable solvents, as shown in Fig. 10. In the following contents, detailed introduction and discussion will be presented.

Approaches to design safer lithium-ion battery electrolytes

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