Nail Penetration Test Research Articles

Regulating chiral nematic liquid crystal of hydroxypropyl methylcellulose coating on separator for High-Safety Lithium-Ion batteries

The conventional commercial polypropylene separator (PP) struggles to inhibit the thermal runaway triggered by dendrite short circuits, and its safety cannot meet the needs of the development of high-energy–density batteries. Herein, an easily commercializable design strategy was employed to induce the aqueous solution of natural polymer hydroxypropylmethylcellulose (HPMC) to self-assemble on the surface of the PP separator by Na2SO4, MnSO4, and Al2(SO4)3 to form the chiral nematic liquid crystal (CLC) with excellent performance, and finally the thermally stable separators (H-Na@PP, H-Mn@PP, and H-Al@PP) were obtained. Theoretical calculations and experiments demonstrate that the CLC induced by Na+, Mn2+, and Al3+ can interact with the electrolyte solvent to form a desolvation structure of Li+, which reduces the migration barrier of Li+ through the separator and accelerate the Li+ transport. Furthermore, the ordered CLC structure can ensure uniform electric field and Li+ flux. Hence, Li//LFP, Li (50 μm)//LFP, and Li//NCM811cells are assembled using these modified separators, featuring remarkable cycling stability and high Coulombic efficiency. As the result, H-Na@PP, H-Mn@PP, and H-Al@PP separators in Li//LFP cell display a high initial capacity of 141.2 mAh/g, 146.7 mAh/g and 130.7 mAh/g at 1C, respectively and stable cycling performance over 1000 cycles. Notably, the capacity retention rate remains high at 87 %, 69 %, and 59 % even after 700 cycles, respectively, which are higher than the 21 % capacity retention rate of PP separator. Meanwhile, the pouch cells equipped with these separators deliver exceptional electrochemical performance and show a lower temperature distribution without thermal runaway behavior under the Φ3 mm nail penetration test, indicating its feasibility for high-safety energy storage systems.

Study on the Commercial Metalized Plastic Current Collector PET-Cu and PP-Cu Toward High-Energy Lithium-Ion Battery.

Commercial metalized plastic current collector (MPCC) is receiving widespread attention from the business and academic communities, due to its properties of excellent electrical conductivity and low mass density. However, the application of MPCC on the side of copper is rarely studied. Herein, sandwich-like polyethylene terephthalate-based (PET) and polypropylene-based (PP) copper (Cu) current collectors via magnetron sputtering and electroplating are fabricated. Most importantly, the electrical performance, mechanical safety quality, and revealed the corresponding failure mechanism for the MPCC cells are first systematically evaluated. First, during the 45°C electrical cycling tests, PET-Cu CC (82.67%) and PP-Cu CC (82.32%) cells both have comparable capacity retention with the traditional Cu CC (Tra-Cu CC) cell (84.55%) after 500 cycles. The slight reduction in the cycling performance is induced by the crack of the Cu layer around the embedded SiO2 particle for PET-Cu CC cell and the detachment of Cu layer for PP-Cu CC cell. Second, during the nail-penetration test, MPCC cells maintain no fire and explosion for more than 5min, since the heat-shrinkable function of polymeric film can interrupt the continuous Joule heat released by internal short-circuit. This work provides important guidance for the large-scale application of MPCC in the field of lithium-ion batteries.

A lightweight carbon-incorporated polymer current collector for improving the performance and safety of lithium-ion batteries

The current collector plays a crucial role in transporting electrons and mechanical support of electrodes for lithium-ion batteries (LIBs). Commercial Cu and Al foil with high density deliver no capacity, so decreasing the weight of the metallic current collector is an effective method for enhancing the energy density of batteries. In this study, we prepared a carbon-incorporated polyimide (CIPI) current collector with a lightweight areal density of 1.41 mg cm−2 and voltage window of 0–5 V. CIPI can suppress the current collector corrosion and improve the long-term cycling performance of high-voltage cathode. Besides, CIPI decreases polarization resistance and relieves the volume expansion of SiOx particles, demonstrating a high capacity retention after 200 cycles. In nail penetration tests, the maximum temperature of the pouch battery with CIPI is only 46.73 % of the battery using Cu and Al foil. In addition, the mechanical flexibility makes CIPI suitable for flexible batteries, delivering ultra-stable voltage even when repeatedly folded in half. The CIPI opens a new pathway to develop high energy density and high safety LIBs and promotes the development of flexible and wearable storage devices.

Dynamic thermal runaway evolution of Li-ion battery during nail penetration

The nail penetration test is widely adopted to evaluate the safety of lithium-ion batteries by triggering the internal short circuit. This work compares the phenomena, temperature rise rate, surface temperature, and mass loss under the penetration test for different nail materials, nail diameters, and cell SOCs. The thermal runaway behaviour and characteristics of these test cells are studied in depth. The results show that the maximum temperature and temperature rise rate of the cell increase with the increase of SOC. The maximum temperature and the temperature rise rate of the cell decrease with the increase of the nail diameter. Under high SOC conditions (greater than 75 % SOC), the more conductive nail increases the release of Joule heat generation inside the battery, which leads to a more intense thermal runaway behaviour. At low SOC (less than 50 % SOC), the more conductive nail promote cooling, which reduces the intensity of thermal runaway. However, non-conductive nails show the exact opposite effect. This dual effect of different nail materials was elucidated by a simplified heat transfer analysis. By calculating the heat production and heat dissipation during the thermal runaway process, the effects of the nail material and the cell SOCs on the thermal runaway behaviour of the battery can be summarized. This work deepens the understanding of the safety of batteries under external shocks and provides new scientific insights into battery design and its safety testing.

Self-conductive organic quaternary ammonium lithium salt-based ultra-concentrated electrolyte for safe lithium metal batteries

Electrolytes used in lithium batteries (LBs) have been the subject of considerable research interest, given their pivotal role in determining the energy density, cycle stability, charge/discharge rate, and safety of LBs. However, the safety concerns associated with leakage-prone and unstable electrolytes and lithium dendrites have been a significant focus of attention. Herein, an ultra-concentrated electrolyte named UC-QLi is presented, consisting of 80 wt% of a new self-conductive organic lithium bicarbonate dimethyl ammonium chloride (LiBDAC) and 20 wt% of fluoroethylene carbonate. The prepared quasi-solid electrolyte shows a high ionic conductivity (0.51 × 10−3 S cm−1) and an outstanding electrochemical stability window (above 6 V). Molecular dynamic, electron probe microanalysis, and X-ray photoelectron spectroscopy techniques have been used to investigate the potential chemical structure and formation mechanism of the solid-electrolyte-interface layer. The Li|UC-QLi|Li symmetric cell can be cycled stably at various current densities (0.5–2 mA cm−2) for 1000 h without producing lithium dendrites. The discharge capacity of the Li|UC-QLi|NCM coin cell at 0.1C reaches 164.1 mAh/g, with a Coulombic efficiency of 99 % throughout the entire cycles. The Li|UC-QLi|NCM cell can continuously and repeatedly light a commercial LED for at least 1 h without performance degradation. Most importantly, UC-QLi-based pouch cells can deliver capacity and are extremely safe. In the nail penetration test, no leakage, thermal runaway, combustion, or explosion phenomena were detected, which fully demonstrates the high safety of the real pouch cell. The herein prepared quasi-solid organic electrolyte provides a new strategy for the development of next-generation high safety LBs.

Design of double layer cathode electrode for improving the safety and stability of lithium-ion batteries

LiNi0.8Co0.1Mn0.1O2 (NCM) is a widely used cathode material for lithium-ion batteries (LIBs). However, the poor cycle performance and safety issue remains a huge challenge for its practical applications. Here we show a simple double layer strategy to improve the electrochemical characteristics and safety performance. NCM was coated by LiFePO4 (LFP), a safe cathode material with long cycle stability, exhibited more excellent cycling stability and similar rate properties at room temperature (called NCM/LFP). Under 45 ℃, the NCM/LFP (6:1)||graphite pouch cell shows excellent 83% capacity retention after 600 cycles at 1C, which compared to the sharp attenuation of pristine NCM. Furthermore, NCM/LFP (6:1)||graphite cell could pass the nail penetration tests and the thermal runaway temperature (TTR) delayed by 193℃, which proved the double layer design has better safety performance. The current work might pave an avenue for cathode materials, as a sustainable solution to high energy density LIBs for applications.

A cost-effective alternative to accelerating rate calorimetry: Analyzing thermal runaways of lithium-ion batteries through thermocouples

In order to facilitate safety of heavy-duty batteries, approaches for studying thermal runaway (TR) need to be developed. So far, these have relied on accelerating rate calorimetry as a standard technique. This method, however, is costly, generally has size limitations, and is therefore of limited use for large format batteries. In this study, we examined the TR behavior of battery cells through a thermal propagation test at module level employing 157 Ah battery cells, using simple thermocouples. This constitutes one of the largest prismatic cell format analyzed to date, while the utilization of thermocouples enables a cost-effective method to study its TR. Parameters such as TR onset temperature, maximum temperature, heat release, and trigger time of the cells were comprehensively evaluated and compared, using this method. An onset temperature for TR at around 144 °C and a maximum temperature from 757 °C to 863 °C were observed. Heat release was estimated as 1.59 MJ per battery cell, deviating within ∼1 % compared to nail penetration tests. Moreover, six distinct stages during TR could be observed, in accordance with literature. This shows that the thermal propagation test using thermocouples is able to align well with other methods such as accelerating rate calorimetry, but is considerably easier to employ.

Glycerol Triacetate-Based Flame Retardant High-Temperature Electrolyte for the Lithium-Ion Battery.

Rechargeable batteries that can operate at elevated temperatures (>70 °C) with high energy density are long-awaited for industrial applications including mining, grid stabilization, naval, aerospace, and medical devices. However, the safety, cycle life, energy density, and cost of the available high-temperature battery technologies remain an obstacle primarily owing to the limited electrolyte options available. We introduce a flame-retardant electrolyte that can enable stable battery cycling at 100 °C by incorporating triacetin into the electrolyte system. Triacetin has excellent chemical stability with lithium metal, and conventional cathode materials can effectively reduce parasitic reactions and promises a good battery performance at elevated temperatures. Our findings reveal that Li-metal half-cells can be made that have high energy density, high Coulombic efficiency, and good cycle life with triacetin-based electrolytes and three different cathode chemistries. Moreover, the nail penetration test in a commercial-scale pouch battery using this new electrolyte demonstrated suppressed heat generation when the cell was damaged and excellent safety when using the triacetin-based electrolyte.

Polymer based multi-layer Al composite current collector improves battery safety

The safety problem obstructs the large-scale application of high energy batteries. Instantaneous short circuit at millisecond triggers uncontrollable thermal runaway in high energy batteries. Herein, to shut down the short circuit at early stage, we accomplish the safety design using composite current collector that spontaneously breaks due to its heterogeneous ductility. The composite current collector has a core with high ductility (the polyethylene terephthalate base), and a skin with low ductility (multi-layer metal). Controlled oxygen is introduced when evaporating aluminum atoms onto the polyethylene terephthalate base, therefore the skin layer will be composed of aluminum and aluminum-oxide layers in staggered rows. The multi-layer skin is more susceptible to crack under the mesoscopic deformation than the plastic core, thereby preventing short circuit by cutting off the pathway of electrons. The cell assembled by the multi-layer composite current collector can pass the nail penetration test without spark, fire and explosion, while the cell with pure current collector and composite current collector with single layer skin cannot. The assembled cell has comparable electrochemical performance (88% capacity retention after 450 cycles) with the reference cell. This work affords novel ideas for the high safety structural design of high energy batteries.

Experimental study on behaviors of lithium-ion cells experiencing internal short circuit and thermal runaway under nail penetration abuse condition

This work compared the macro-phenomenon, voltage response, surface temperature, mass loss and material characteristics of LiCoO2 (LCO), LiMn2O4 (LMO), Li(NixCoyMnz)O2 (NCM), LiFePO4 (LFP) cells with the same capacity under penetration tests. The tests provided an in-depth study on behaviors and properties of internal short circuit (ISC) and thermal runaway (TR). From the results, a feature was discovered that the parameters of different types of cells showed different tendencies when penetrated with a series of diameter nails, determined by cathode materials. The LFP cell experienced a slight ISC. Temperature rise rate and maximum temperature increased with larger diameter of nails. The NCM, LCO and LMO cells experienced a severe ISC. The maximum temperature of cells increased, however, the temperature rise rate reduced and macro-phenomena delayed and weakened with larger diameter of nails. For different type of cells, the order of ISC sensitivity was as follows: LCO > LMO > NCM > LFP. As state of charge (SOC) increased, the cells had a higher risk of TR, evidenced by higher surge current and temperature rise rate. The results of nail penetration tests at 100 % SOC indicated that the hazard of TR was arranged in the order of LCO > NCM > LMO > LFP. Due to the fragmentation of secondary particles during the extrusion of nail, the cathode composed of secondary particles suffered more damage than composed of primary particles. The research can provide support for the safety design of cells.

Ultrafast Carbothermal Shock Synthesis of Wadsley–Roth Phase Niobium‐Based Oxides for Fast‐Charging Lithium‐Ion Batteries

AbstractWadsley–Roth phase niobium‐based oxides show potential as anode candidates for fast‐charging lithium‐ion batteries. Traditional synthesis methods, however, usually involve a time‐consuming calcination process, resulting in poor production efficiency. Herein, a novel carbothermal shock (CTS) method that enables the ultra‐fast synthesis of various Wadsley–Roth phase Nb‐based oxides within seconds is introduced. The extremely rapid heating rates enabled by CTS alter the reaction mechanism from a sluggish solid‐state process to a swift liquid‐phase assisted one and drive the chemical reactions away from equilibrium, thereby generating abundant oxygen vacancies and dislocations. Theoretical calculations reveal that oxygen vacancies significantly lower the energy barrier for Li+ diffusion and enhance the intrinsic electronic conductivity. Moreover, dislocations help convert the surface tensile stress arising from Li+ intercalation into compressive stress, effectively improving the structural integrity during cycling. Notably, this approach can also be applied to synthesize LiFePO4 cathode materials under ambient conditions, eliminating the requirement for inert atmospheres. Consequently, the CTS‐synthesized Nb14W3O44||LiFePO4 battery demonstrates reversible structural evolution validated by in situ XRD and exceptional cycling ability (e.g., 0.0065% capacity decay per cycle at 4 A g−1 over 3000 cycles). Importantly, the Nb14W3O44||LiFePO4 configuration also shows enhanced thermal stability in the Ah‐level pouch cell nail penetration test, confirming its feasibility.

Experimental and numerical modeling of the heat generation characteristics of lithium iron phosphate battery under nail penetration

This study conducted nail penetration tests on 20 Ah prismatic LiFePO4 batteries and simulated the slow release of Joule heat and side reaction heat by combining a new thermal model with a parameter optimization method. The results indicate that the 50% and 80% SOC LiFePO4 batteries only release Joule heat under penetration, while the side reaction heat is acquired under 100% SOC besides Joule heat. Moreover, approximately 56.4% of the stored electrical energy is converted into Joule heat, which accounts for the majority of the total heat production of 100% SOC LiFePO4 battery under penetration, while side reaction heat accounts for only 6.4%. Furthermore, the exothermic side reactions of 100% SOC LiFePO4 battery under penetration can be effectively suppressed when the electrical energy release ratio is less than 0.52, or the convection coefficient between the battery and its surroundings exceeds 12 W/m2K. This numerical study expands the analysis of the heat generation characteristics of LiFePO4 batteries during penetration and provides practical guidance for system safety design.

Needle Penetration Studies on the Dynamics during Internal Short Circuits in Automotive Lithium-Ion Cells: Measurement of Internal Temperature and Short Circuit Current

Lithium-ion batteries (LIBs) have been widely applied in consumer electronic devices and electric vehicles for energy storage. Especially for the battery electric vehicles (BEVs), it is desirable to increase the volumetric energy density of LIB cells due to the limitation of a further volumetric expansion of the battery pack, which amounts to currently 220-400 L depending on the size of the BEV [1]. As an example, the capacity of the Li-ion battery pack in BMW i3s has been improved from 60 Ah over 94 Ah to 120 Ah over the course of a few years. Along with this trend of increasing energy density, however, the reactivity of the cells is increasing. Therefore, the potential safety issues are becoming more important. Between 2015 and 2018, a total of 15 fire incidents of BEVs were reported in Europe. In 2019, 113 fire incidents took place only within China [2]. Considering the decision by the European Union to ban the sale of new petrol and diesel vehicles from 2035, this concern with regards to the safety of LIBs would be growing progressively with the accelerated switch to BEVs.One of the main causes for a thermal runaway is an internal short circuit (ISC) [1,3]. Under many different abuse scenarios, ISC appears to trigger the thermal runaway (TR); (i) mechanical abuse (deformation of separator leading to ISC),(ii) electrical abuse (separator pierced by dendrite resulting to ISC) or iii) thermal abuse (shrinkage or melting of separator causing ISC) [1]. Nail penetration tests have been widely used to reproduce ISCs. Only the temperature on the cell surface and the cell voltage are measured in a conventional nail penetration test. However, there is also a challenge to understand the internal dynamics in a Li-ion cell in which an ISC takes place [4]. This study introduces an advanced needle penetration test that enables the measurement of both the internal temperature and the ISC current.As seen in Figure 1 (a), the amount of ISC current can be measured indirectly by replenishing the lost cell capacity from ISC with a power supply that is set to constant voltage mode (in this case, 4.2 V) and connected to the cell via a shunt. Additionally, the internal temperature can be measured using a thermocouple which is embedded in a specially crafted probe. The needle penetrates Li-ion cells with a slow penetration speed of 0.01 mm/s to understand the effect of piercing individual layers on the ISC with regard to electrical and thermal behaviour.Figure 1 (b) presents the ISC current and the internal temperature measured using this method. In the unstable ISC current signal, sharp periodical peaks are detectable. It is assumed that the ISC current is constantly increasing by several layer-to-layer shorts adding up and that its periodically decreasing profile originates from the increasing contact resistance by cell layers melting or being ruptured. Since thermal energy emerges from the ISC current, the internal temperature fluctuates dynamically corresponding to the ISC current profile, while surface temperature signals do not show any changes. This advanced needle penetration test offers the great research possibility to understand the internal dynamics of ISC that has been barely studied and to determine the internal onset cell temperature for TR.[1] X. Feng et al Energy Storage Mater., 10, 246 (2018).[2] Y.S. Duh et al J. Energy Storage, 41, 102888 (2021).[3] D.P. Finegan et al J. Electrochem. Soc., 164 , A3285 (2017).[4] S. Huang et al J. Electrochem. Soc., 167, 90526 (2020). Figure 1

Using Artificial Solid-Electrolyte Interphase Coatings for Enhancing Safety of High-Energy Li-Ion Batteries from Material Level

The development of high-energy-density Li-ion batteries (LIBs) to meet the demand for electric vehicles and sustainable energy storage applications simultaneously brings about more safety issues. On the anode side, graphite has been a major anode material for high-energy LIBs and is anticipated to continuously play an important role even for the high-capacity Si-graphite (Si-Gr) composite anodes in the near future. The low lithiation electrochemical potential of graphite enables a lower anode potential allowing higher energy for a full cell but is vulnerable to the plating of metallic Li dendrite, which could easily arise from either over-lithiation, due to heterogeneity in the negative electrode, or fast charging. Li dendritic deposits could penetrate through the separator to trigger cell short-circuit and eventually thermal runaway. On the cathode side, the high surface reactivity and low oxygen lattice stability of the Ni-rich layered oxide cathodes lower the thermal stability. This study focuses on enhancing the safety of LIB full-cells by modifying the properties and stability of the solid-electrolyte interphases (SEIs) respectively on the anode and cathode electrodes. A polymeric artificial SEI for the anode is developed not only to improve the electrode performance but also effectively suppress SEI and Li dendrite formation. Meanwhile, a composite coating for the cathode is derived to simultaneously enhance the cycle stability and thermal stability, due to the altered thermal decomposition pathway, of an NCM811 cathode. The overall enhanced safety of the full cells is illustrated with the nail penetration test, and the safety-enhancing mechanisms are revealed by conducting post-mortem and synchrotron-based operando studies.

(Invited) Thermal Stability and Thermal Energy Measurements for a Lithium-Ion Pouch Cell with Different State of Health

Lithium-ion batteries have developed into one of the most popular secondary batteries on the market today due to high voltage, long lifetime and high energy density. However, lithium-ion cells have issues related to safety. Since the development of an internal short in a lithium-ion battery at present cannot be detected [1, 2], the physical reactions of a lithium-ion battery must be managed by a robust battery design. Lithium-ion batteries used in the Norwegian maritime sector need to be approved by thermal propagation tests as verification of design [3]. During a propagation test, one or several lithium-ion cells are forced into thermal runaway either by heating, overcharge or nail penetration. The acceptance criterion is either; no spread of thermal runaway between cells in a module, or between modules. Despite these rules, several fires have been reported. Of the more well-known are the fire/gas explosion onboard the electric car ferry ‘MF Ytterøyningen’ [4] and the fire on board ‘MS Brim’ [5].A commercial 64 Ah lithium-ion pouch cell was studied by forcing thermal runaway with nail penetration, heating and overcharge to evaluate the energy release based on the abuse method. Two cells aged to 80% state of health (SoH) were also studied with accelerating rate calorimetry and nail penetration to see the effect of cell ageing on thermal stability, critical temperature, energy release and jet flame severity. These methods were combined with diagnostics and cell disassembly to relate the differences found in degradation and safety to the diagnostic data. The safety results will be highlighted in this presentation.The uncycled cells safety properties were rated by fire and mass losses. All cells caught fire, and the mass loss ranged from 38.8% for nail penetration, 55.4% for heat ramp and 72.3% for overcharge. Based on mass loss only, overcharge is the most severe abuse method. Contrary to this, energy released to the solid surroundings during thermal runaway (internal thermal energy) decreased with increasing mass loss. Mass loss could be a key feature in explaining energy release and abuse method severity, see Figure 1. As a result, an uncycled cell in a battery module may withstand the heat from a neighboring thermal runaway during overcharge but may be driven into thermal runaway by a nail penetration test.Cyclic ageing at 25 °C to 80% SoH had a minor impact on abuse severity and mass loss. However, cycled cells are found to lose more electrical energy than internal thermalenergy. Cell electrical energy loss is not directly related to a change in the cell's internal thermal energy. Thermal runaway temperature was found to decrease for a cycled cell, from 204°C to 182°C.Dependent on abuse method, a false sense of security may result from the propagation test. Possibly, a module could show no cell-to-cell propagation with the overcharge method but be driven into thermal runaway propagation by a nail penetration. A decrease in thermal stability combined with a change in internal thermal energy could be hazardous and might be an end-of-life criteria. It is of vital importance to develop diagnostic tools capable of detecting a potentially hazardous drop in thermal stability for aged lithium-ion cells.Acknowledgements: This work was part of the BattMarine project (281005), funded by the Research Council of Norway and Norwegian industry.

Firefighting Gel Polymer Electrolyte for Non-Flammable Li-Ion Batteries Based on Extremely Low Amount of a Cross-Linkable Polymer

Safety issues of lithium-ion batteries(LIBs) have caught a priority of concerns as the electric vehicle(EV) market expands dramatically and requires higher energy densities. To be free from a variety of unpredictable situations and environments triggering such a safety issue, materials that provide a mechanism to control the factors causing fire and explosion are required to be involved in LIB cells.In this work, we present a gel polymer electrolyte(GPE) characterized by nonflammability and fire-extinguishing capability by radical scavenging(Rs). This GPE was realized within battery cells by in situ crosslinking a small amount of a cross-linkable fluoro-functionalized cyanoethyl polyvinyl alcohol(F7) in conventional carbonate electrolytes having LiPF6. The cyanoethyl functional groups(-CN) is responsible for gelation by cross-linking while the fluorine-containing moiety terminated radical chain reactions triggering thermal runaway by Rs. The strong interaction between the polymer and solvent molecules restricted solvent volatility. The tiny solid content at 2 wt. % in GPE guaranteed a liquid-level ionic conductivity at 9 mS/cm, disallowing battery performances to be sacrificed. Formation of the LiF-dominant solid-electrolyte-interphase(SEI) layer on anodes was encouraged by the fluoro-moiety of F7.In previous research of our group, PVA-CN was used as cross-linkable polymer for enhancement of mechanical property with additional metal-chelating functional group. Hydroxy group of PVA-CN is useful to modify the functional group by DCC coupling mechanism. Herein, as same as previous modification, the Heptafluorobutyric acid(HFB) was used for non-flammability and battery performance. F7 was expected to not only increase thermal stability but also scavenge the radicals. In addition, LiF-rich SEI layers would be made by fluorine groups.First, GPE increase its thermal stability by strong interaction between cross-linked polymer matrix and organic solvents. The binding energy of ethylene carbonate (EC) and hydrogen of cyanoethyl side chain was 0.4 eV which is stronger compared with general hydrogen bond(0.2 eV). Therefore, the flammable gas generation was suppressed by strong interaction between solvents and polymer matrix.In addition, the flash point demonstrated the strong interaction in GPE. The flash point of GPE which contained different polymer contents was decreased as the ratio of polymer content was increased. The composition of the atmosphere in the devices changed into more flammable because of the higher ratio of EMC gas in the atmosphere within enough time to equilibrate. In other words, EC forms a greater number of strong interactions with the polymer matrix while the polymer contents increased. The SET test of 2 wt.% PVA-CN-based GPE(G2)(3.36 s/g) and 5 wt.% PVA-CN-based GPE(G5)(0 s/g) proved that higher contents of PVA-CN in GPE ensure the non-flammability.Second, to overcome the poor electrochemical ability of G5, Rs functionalized F7 was used instead of PVA-CN. Unlike G2, only with 2 wt. % of F7 guaranteed the 0 s/g of SET. Non-flammability of 2 wt.% F7-based GPE(FG2) is due to the fluorine moiety scavenge the O• or H•. In the nail penetration test, only the FG2 was not exploded and maintained charged voltage while the nail penetrates the whole pouch cell vertically because of its elasticity.The Rs capability was demonstrated by cyclic voltammetry(CV) with 2,2’-azobisisobutyronitrile(AIBN) and alpha-tocopherol(α-TOH). After α-TOH react with AIBN radicals at the 70 ℃, the oxidation current decreased because of consumption of α-TOH. The Rs capability was calculated by ratio of current retention of α-TOH oxidation current with a same amount of among scavenger additives. The Rs capability of F7 was 214%.Lastly, our findings suggest sufficient non-flammability without battery performances degradation. The capacity retention of FG2 was higher than LE at the 300th Cycles. This is because the fluorine moiety contributed to make LiF-rich SEI layers. The rigid SEI layers and GPE suppressed volume expansion of graphite. Also, the capacity fade was not observed in FG2 with higher current density. The reason for great rate capability is higher lithium-ion conductivity which is a multiplication of ionic conductivity and lithium-ion transference number(tLi+). FG2has 4.82 of lithium-ion conductivity, while 3.63 in LE. Therefore, the mass transfer resistance was not large on the electrode surface due to the higher lithium-ion mobility in FG2.In this work, the non-flammability and battery performance were successfully demonstrated by thermal stability and Rs capability without any battery performances degradation. The thermal stability was enhanced by strong intermolecular forces between polymer chain and polar organic solvents. The radical chain reaction was terminated by fluorine moiety of GPEs matrix. The capacity retention of FG2 was outstanding compared with LE. Therefore, due to the FG2, the battery will be able to ensure non-flammability without degradation of performance. Figure 1

A Thermal-Ball-Valve Structure Separator for Highly Safe Lithium-Ion Batteries.

The separator located between the positive and negative electrodes not only provides a lithium-ion transmission channel but also prevents short circuits for direct contact of electrodes. The inferior dimension thermostability of commercial polyolefin separators intensifies the thermal runaway of batteries under abuse such as short circuits, overcharge, and so on. a polyvinylidene fluoride/polyether imide (PVDF/PEI) separator with high thermal stability in which the high thermostable PEI microspheres are evenly dispersed in the PVDF film matrix and also located in the micro holes of the PVDF film is developed. They not only function as strong skeleton that enables the rare shrink of the separator at 200°C avoiding short circuit but also act as ball valve that blocks the lithium ion transmission channel at 150°C interrupting the further heat aggregation. Thus, the LiNi0.6Co0.2Mn0.2O2/Li batteries exhibit high cycle stability of 96.5% capacity retention after 100 cycles at 0.2C and 80°C. Further, the LiNi0.6Co0.2Mn0.2O2/graphite pouch cells are constructed and deliver good safety performance without smoke release and catching fire after the nail penetration test.

Constructing electronic and ionic conductive double coating to achieve superior structure stability and electrochemical properties of LiNi0.90Co0.04Mn0.03Al0.03O2

In the work, an improvement strategy for surface covering the quaternary layered oxide LiNi0.90Co0.04Mn0.03Al0.03O2 with the ionic conductor (Li3BO3) and electronic conductor (polypyrrole) is put forward to relieve the interface troubles and ameliorate the electrochemical properties of Ni-rich cathode. Benefiting from the multiple advantages of the double conductors, surface residual alkali and cation mixing of LiNi0.90Co0.04Mn0.03Al0.03O2 are remarkably decreased, as well as the surface side reactions, which are helpful to enhance the interfacial and crystal structure stability during long-term working. As a result, the PPy-LBO coated cathode delivers the large discharge capacity of 149.5 mA h g−1 with 10.0C high rate, outstanding capacity of 167.5 mA h g−1 with −30 °C discharging, and prominent cyclic stability with 90.5 % capacity conservation rate after 200 cycles at 1.0C with 45 °C. Meanwhile, the full batteries prepared with PPy-LBO coated sample also demonstrate the brilliant performance including the safety capability (Nail penetration, Overcharging and Impact test) and long-term cycling when compared to the pristine cathode.

Analysis of the thermal runaway phenomenon of Nickel-Manganese-Cobalt cells induced by nail penetration through high-speed infrared imaging

Gradual replacement of fossil fuels by clean and renewable energies is crucial to minimize the greenhouse effect from the transport sector. The electric vehicle is intended to be the leading technology for this purpose, for which clean and renewable energy must be stored as chemical energy in the battery. From the safety point of view, the main issue of this technology is related to the risk of thermal runaway. Among other factors, a collision can be one of the causes of a battery electric vehicle explosion, as damage to the surface of the battery can generate a short circuit, resulting in a battery thermal runaway. To replicate this situation at a system level, the nail penetration test is reproduced in laboratories. In this work, the effect of the nail penetration position, battery state of charge (SOC) and initial ambient temperature on the thermal runaway behavior of a Nickel-Manganese-Cobalt 811 cell is investigated. Infrared images were used to evaluate the temperature distribution in the battery surface, revealing the root causes for the scattering observed in the surface temperature when measured through thermocouples. Furthermore, a methodology to correct the thermographic images is described, abording the main uncertainties related to the characteristics of the optical access. The results show that the battery state of charge has a strong effect on the heat release than nail position and initial ambient temperature, increasing in 20 % from SOC 50 % to SOC 100 %. Regarding the position, nail penetration tests at the top of the battery lead to lower temperatures when compared to the other positions, representing 10 % less heat release than middle and bottom nail position. Furthermore, the thermographic images showed a variability in the spatial distribution of temperature between the test cells of up to 40 °C, with case rupture being mainly responsible for the higher values.

Mechanistic understanding of reproducibility in nail penetration tests

Mechanistic understanding of reproducibility in nail penetration tests