Electrochemical performances over a wide temperature range
Undoubtedly, LIBs operating over a wide temperature range presents great importance in practical applications such as those in electric vehicles, space and military missions. For wide temperature LIBs, the most challenging work is to get a compromise between subzero temperature performance and high temperature performance. The most direct and efficient strategy is formulating wide temperature electrolyte by dissolving thermally stable lithium salts in low melting point and high boiling point solvents with low viscosity.7, 37–43 Herein, a dual-salt electrolyte of 0.6 M LiTFSI + 0.4 M LiDFOB EC/PC/EMC (1:1:3, by volume) is developed for 5 Ah
NCM523/G pouch cell. Wherein, carbonate solvents possessing low melting point (PC, Tm = -48.8℃; EMC, Tm = -53℃) and high boiling point (EC, Tb =243℃; PC, Tb = 242℃) are selected for formulating the wide temperature range electrolyte. In the 1st formation cycle at room temperature (RT), 5 Ah NCM523/G pouch cell with the dual-salt electrolyte demonstrated high initial Coulombic efficiency of 85.8% (5.14/5.99 Ah) than that (47.6%, 2.18/4.58 Ah) of the pouch cell with 1 M LiPF6 EC/PC/EMC (1:1:3, by volume) (Fig. 1a). Moreover, severe gas swelling of pouch cell with 1 M LiPF6 EC/PC/EMC is observed (Left inset in Fig. 1a), due to the PC-induced formation of unstable solid-electrolyte-interphase (SEI) layer on graphite anode.44–45 As a sharp contrast, pouch cell with the dual-salt electrolyte shows no obvious swelling (Right inset in Fig. 1a), benefiting from the formation of stable SEI layer by LiDFOB salt.46–47 Therefore, 5 Ah NCM523/G pouch cell with the dual-salt electrolyte is employed for the following electrochemical and thermal safety study.
After the vacuum degassing procedure of formation process, the determined gravimetric energy density of 5 Ah NCM523/G pouch cell with dual-salt electrolyte is highly up to 208.8 Wh kg− 1. At RT at 0.5 C rate for 400 cycles, the pouch cell presents a high average Coulombic efficiency of 99.88% and delivers a discharge capacity retention of 92.72%, 84.10%, 74.90% and 68.77%, at the 100th, 200th, 300th, and 400th cycle, respectively (Fig. 1b-c). When the temperature is increased up to 60 ℃, the discharge capacity retention of the pouch cell at 0.2 C rate is 82.23%, 79.02%, 76.56% and 74.48% at the 100th, 200th, 300th and 400th cycle, respectively, with an average Coulombic efficiency reaching to 99.92% (Fig. 1d-e). In subsequent, subzero temperature performance is revealed by discharging the room-temperature fully charged 5 Ah NCM523/G pouch cell, at varied subzero temperatures of -10 °C, -20 °C, -30 °C, and − 40 °C, respectively (Fig. 1f). The corresponding discharge capacity retention is 85.79%, 75.76%, 64.46%, and 40.81%. In general, the high-energy 5 Ah NCM523/G pouch cell using the formulated dual-salt electrolyte is very competent for wide temperature range applications. Moreover, it is noted that, to the best of our knowledge,29 this is the first case to evaluate the application of dual-salt electrolytes in LIBs over a wide temperature range, which is significant for the commercialization process of dual-salt electrolytes.
Heat generation during charge-discharge operation
Thermal anomalies inside the cell or pack in the absence of proper thermal management can instigate accelerated cell capacity degradation and even hazardous thermal runaway. Therefore, the heat generation accompanied with battery charge-discharge operation is receiving more and more attention. Generally, heat generated from battery cycling can be divided into reversible heat and irreversible heat. Irreversible heat refers to the ohmic heat from the polarization or overpotential of the cell, while reversible heat is determined by the measurement of cell entropic coefficient depending on the intrinsic nature of the electrode materials (relating with the atom arrangement in the crystal lattice).48 Deciphering the heat generation law of a single cell is essential to design and optimize the battery thermal management system, which ensures batteries in a pack or module running in an ideal temperature range. Here, two boundary scenarios of adiabatic and isothermal environment, representing the worst and best case for the heat management, respectively, are considered for heat determination of the pouch cell with dual-salt electrolyte.
ARC (an adiabatic calorimeter) is a pivotal integrated technology to study the thermal safety of LIBs at multilevel, and research progress on ARC is reviewed by us very recently.22 In general, ARC will simulate an accurate adiabatic condition by keeping the cavity temperature consistent with the sample temperature, preventing the self-generated heat loss of sample. Therefore, ARC is the worse-case scenario. Here, the 5 Ah NCM523/G pouch cell with the dual-salt electrolyte is placed in the adiabatic cavity of ARC (BTC500, HEL, Figure S1) and connected with a charge/discharge apparatus. Obviously, the surface temperature of pouch cell increases during both charge and discharge process (Fig. 2a and Figure S2a-c). For example, at 0.5 C rate, the overall heat generation (19.5℃, 1.9 kJ) during the charge process is much higher than that (6.3℃, 0.6 kJ) of the discharge process. At varied C-rates, the self-heating rate curves during charge and discharge process are symmetrical (Figure S2d-f), evidencing that the heat generation mainly consisted of the irreversible joule heat and the reversible electrochemical reaction heat.49–50 Obviously, the reversible electrochemical reaction heat dominates the total heat at low rates and irreversible joule heat dominates the total heat at high rates. These results clearly tell us that, at the worst case of adiabatic condition, the 5 Ah NCM523/G pouch cell at higher rates will easily get thermal runaway in few cycles, revealing the importance of battery thermal management. As for the battery thermal management, it is necessary to determine the heat generation of a single cell at a constant temperature. Here, as a best-case scenario, IMC (isoBTC, HEL) is used to test the heat generation of the 5 Ah NCM523/G pouch cell at the isothermal condition (Figure S3). At 0.5 C rate at 30℃, the overall heat generation (1.2 kJ) during the charge process is much higher than that (0.4 kJ) of the discharge process (Fig. 2b). Clearly, the released energy during 1 charge-discharge cycle at isothermal condition (1.6 kJ) is lower than that at the adiabatic condition (2.5 kJ). Interestingly, at 0.5 C rate, more heat is generated at both 10℃ and 50℃ when compared to 30℃, suggesting the importance of selecting work temperature for battery (Figure S4). The reversible and irreversible heat are determined based on the ohmic resistance and total heat generation.51 The internal resistance of the pouch cell is calculated by the hybrid pulse power characterization method (Figure S5). On the whole, the reversible heat dominates the total heat generation at 0.5 C rate, and the endothermic and exothermic reactions are distinguishable and transformable during the charge or discharge process (Fig. 2c). Compared to fresh pouch cell, long-term (400 cycles) cycled pouch cell demonstrates much higher heat release power at higher rates, especially at 1 C rate (Fig. 2d). In summary, the design of an efficient and smart battery thermal management system must comprehensively consider the effects of work temperature, state of charge (SOC), charge-discharge current rate, and charge-discharge protocol on heat generation, and IMC testing will give the answers.
Thermal runaway feature and mechanism
As a high-energy storage reservoir, LIBs easily get thermal runaway when operated under mechanical, electrical, and thermal abuse conditions. To develop an efficient battery safety risk controlling strategy, it is necessary to obtain some critical parameters, such as onset temperature for self-heating (Tonset), self-heating rate (SHR), thermal runaway temperature (Ttr), and maximum temperature (Tmax), etc. Here, 5 Ah NCM523/G pouch cell using dual-salt electrolyte is placed in the cavity of ARC (BTC500, HEL, Fig. 3a and Figure S6), and the typical heat-wait-search (HWS) mode is used to study thermal runaway features of the pouch cell. Under the HWS mode of ARC, the built-in camera in cavity captures that smoke and flame is rapidly spurted out of the pouch cell (100% SOC, after the formation process) (Fig. 3b). Obviously, the Tonset, Ttr (SHR over 1℃ min− 1 as the criteria) and Tmax is determined to be 91℃, 171℃, and 516℃, respectively (Fig. 3c). After the thermal runaway process, the aluminum plastic film is severely damaged (inset in Fig. 3c). It is noted here, that most previous battery thermal runway investigations focus on the fully charged cell (100% SOC) because of its violent thermal runway hazardous, while the fact that fully discharged cell (0% SOC) also undergoes thermal runway is neglected. Here, to explore the rooted mechanism for the triggering of thermal runway behavior, 5 Ah NCM523/G pouch cell without dual-salt electrolyte and 5 Ah NCM523/G pouch cell with dual-salt electrolyte but without formation process are fabricated and tested by ARC under same conditions, and interestingly, both pouch cells do not present severe exothermic reactions related to the thermal runway of pouch cell below 250℃ (Fig. 3d). However, when the 5 Ah NCM523/G pouch cell with dual-salt electrolyte is cycled for one formation cycle (0% SOC), the thermal runaway occurs under the same testing condition in ARC (Tonset = 141℃, Ttr = 199℃, Tmax = 280℃). These results clearly tell us that the formed interfacial layer components between the electrolyte and electrodes play a crucial role in triggering the thermal runaway process of pouch cell. Therefore, we confirm that pouch cell after formation process will go thermal runaway anyway regardless of the SOC, and pouch cell with higher SOC demonstrates faster and more severe thermal runaway process (Figure S7).
To decipher the rooted mechanism for triggering the thermal runaway of NCM523/G pouch cell, ARC (BTC130, HEL) equipped with a small bomb chamber is used to study the thermal compatibility of battery components (Fig. 4a and Figure S8). In the Ar-filled glove box, the anode and cathode are carefully and separately dissembled from fully charged (100% SOC) and fully discharged (0% SOC) 5 Ah NCM523/G pouch cell. The as-formulated fresh dual-salt electrolyte presents a high Tonset of 218℃, suggesting its high thermal stability (Fig. 4b). The fully delithiated cathode/electrolyte (100% SOC) and lithiated cathode/electrolyte (0% SOC) demonstrate Tonset of 134℃ (Fig. 4c) and 171℃ (Fig. 4d), respectively. And the fully lithiated anode/electrolyte (100% SOC) and delithiated anode/electrolyte (0% SOC) exhibit the Tonset of 95℃ (Fig. 4e) and 128℃ (Fig. 4f), respectively. These results reveal that, at different SOCs, cathode/electrolyte and anode/electrolyte will inevitably go thermal runaway and anode/electrolyte is easier to get thermal runaway than cathode/electrolyte. In addition, higher thermal reactivity is presented at higher SOCs. Therefore, it is concluded here that high SOC is just the accelerating factor of battery thermal runaway, but not the root cause for triggering it. After thermal runaway of the dual-salt electrolyte, anode/electrolyte, and cathode/electrolyte, the remained incondensable gas species in the sealed bomb chamber is collected and analyzed by the mass spectrometer (MS), respectively (Insets in Fig. 4b-e). Wherein, the remained gas species for thermal runaway of sole as-formulated dual-salt electrolyte is mainly consisted of CO2 (51.8%) and CH4 (24.7%) (Insets in Fig. 4b). For fully delithiated NCM523 cathode/electrolyte (100% SOC), O2 (87.9%) dominates the collected gas species after thermal runaway (Insets in Fig. 4c). Revealed by temperature-resolved XRD equipped with an in-situ heating module, when the temperature exceeds 200℃, the fully delithiated NCM523 cathode will release O2 by phase transformation of layered structure ((003)R) to spinel structure ((111)S) and disordered spinel structure ((108)R and (110)R) to the NiO-like rock-salt structure ((440)S) (Fig. 4g). As an interest comparison, lithiated NCM523 cathode (0% SOC) reacting with electrolyte produces main gas species of CO2 (48.7%), CH4 (26.8%) and O2 (13.4%). The dramatic decrease of O2 percentage is possibly attributed to the well-preserved crystal structure of lithiated NCM523 cathode (0% SOC) (Fig. 4h). To further verify this, XRD
patterns of the powders collected after thermal runaway of 0 and 100% SOC 5 Ah NCM523/G pouch cell are tested. It is presented that the fully delithiated NCM523 cathode (100% SOC) undergoes phase transformation and crystal structure collapse while the main layered structure of lithiated NCM523 cathode (0% SOC) is preserved (Figure S9). These results indicate that O2 releasing accompanying with phase transformation of delithiated NCM523 cathode at high temperature around 200℃ may aggravate the burning or explosion, but is not the rooted mechanism for triggering thermal runaway. Obviously, the rooted factor for triggering the thermal runaway still depend on anode side. After thermal runaway of fully lithiated anode/electrolyte (100% SOC), the dominated gas species is H2 (68.2%), while CO2 (39%), CH4 (26.5%) and H2 (18.8%) are the main gases determined for thermal runaway of unlithiated anode/electrolyte (0% SOC). Except the evolution of CO2 and CH4 from thermal decomposition of electrolyte, H2 gas constitutes the most dominating gas species in the thermal runaway of anode/electrolyte at 0% and 100% SOC. This implies that, clarifying the origin of H2 evolution is of great significance to understand the rooted triggering factor for thermal runaway. Temperature-resolved XRD equipped with an in-situ heating module is also utilized to analyze the bulk phase change of graphite anode at different SOC. For lithiated graphite anode (100% SOC), the exothermic phase transformation of LiC6 (001) to LiC12 (002), and LiC12 (002) to C (002) occurs at ca. 100℃ and ca. 200℃, respectively (Fig. 4i). But, even if there is no such exothermic phase transformations for delithiated graphite anode (0% SOC) (Fig. 4i), the delithiated graphite anode/electrolyte (0% SOC) still exhibits thermal runaway and the self-heating happens earlier than lithiated NCM523 cathode/electrolyte (0% SOC). Hence, the LiC6-LiC12-graphite phase transformation is also excluded to be the rooted factor for triggering thermal runaway. In subsequent, it is not difficult to infer that the thermal runaway of pouch cell is induced by the exothermic reactions related to the broken of the formed SEI layer on graphite anode. This is also evidenced by the fact that, cycled graphite anode (delithiated state, 0% SOC) reacting with electrolyte shows exothermic peak at ca. 80 ~ 170℃, which is not appeared for the uncycled pristine graphite anode and electrolyte, during the DSC test (Figure S10). Conventionally, SEI layer on graphite anode are typically determined to be consisted of inorganic species (such as LiF, Li2CO3, Li2O, Li2C2, Li2C2O4, LiOH, etc.) and organic species (such as ROCO2Li, (CH2OCO2Li)2, ROLi, CH3Li, etc.).52–57 Although the broken of SEI layer is always identified as the initial stage during the thermal runaway process, but the exothermic reactions related to the thermal decomposition of listed inorganic and organic species are not presented in detail to date. Moreover, the underlying cause for large amounts of H2 evolution at graphite anode side is still remained unclear. Therefore, some unknown species must be still unidentified in the SEI layer of graphite anode. In some reports, it is suggested that the graphite anode surface is enriched of hydrogen,26, 58–59 but no further detailed evidence is provided. Herein, by employing a delicately-designed deuterium-oxide (D2O) titration device connecting with an on-line gas analysis mass spectrometry (MS) system (Fig. 5a), we unprecedentedly identify the existence of H− containing species on the surface of the graphite anode. Fully-lithiated graphite (100% SOC) and fully-delithiated graphite (0% SOC) is titrated by deuterium-oxide (D2O) with the following guideline reactions: LixC6 + xD2O → xLiOD + C6 + x/2 D2↑, (x ≤ 1); H− + D2O →OD− + HD↑. Surprisingly, HD (m/z = 3) signal is observed for both fully-lithiated graphite (100% SOC) and fully-delithiated graphite (0% SOC). In addition, the mole of HD and D2 (m/z = 4) at fully-lithiated graphite (100% SOC) is 0.14 µmol mg− 1 and 4.26 µmol mg− 1, respectively (Fig. 5b), which decrease to 0.014 µmol mg− 1 and 0.029 µmol mg− 1, respectively, when the graphite anode is fully-delithiated (0% SOC) (Fig. 5c). It is, the first time, discovered that H− containing species, most probably in the form of LiH, do exist in the SEI layer of graphite anode, and the H− containing species exhibit highly electrochemical reversibility at graphite anode surface during cycling. In another delicately-designed experiment, fully-lithiated graphite (100% SOC) is heated at 90℃, in the titration vessel of abovementioned on-line gas analysis mass spectrometry (MS) system. H2 signal (m/z = 2) is detected after
heating to 90℃, confirming that the broken of SEI layer is accompanied by evolution of H2 (Figure S11). Obviously, the released amount of H2 (Fig. 4e-f) is highly correlated with the determined amount of H− containing species. Highly lithiated graphite (high SOC) contains more H− containing species and releases more H2 when heating. Moreover, it is previously proposed that the recombination of the active hydrogen to form H2 will lead to huge generation of heat, which is even larger than that of H2 burning in oxygen,26 implying that the H−-induced H2 evolution accompanies with large amounts of heat releasing. Furthermore, the presence of H− is also confirmed in graphite anode disassembled from NCM523/G and NCM811/G pouch cells (using conventional LiPF6 based carbonate electrolyte), regardless of SOC, indicating this phenomenon is universal applicable for varied NCM/G cells (Figure S12). These amazing results indicate that the H− induced H2 releasing at the graphite anode side may be the rooted thermal runaway trigger of NCM523/G pouch cell, while the phase transformation of lithiated graphite anode and the O2-releasing by delithiated NCM cathode are just accelerating factors for thermal runaway.
The aforementioned released gases and temperature profiles during thermal runaway of anode/electrolyte and cathode/electrolyte are separately determined and analyzed, not considering of anode/cathode crosstalk. However, in practical cases, despite anode and cathode is physically separated by the separator, the porosity of separator always allow the crosstalk of byproducts.10, 60 Then the question comes: during the triggering process of thermal runway, is there any crosstalk effect when the released gas species migrate through porous separators? To answer this question, a self-made two bomb chamber testing system is delicately fabricated, where the anode and cathode is placed separately, but the released gas species can flow freely by the connected pipeline, and two bomb chambers are heated by one same heating wire in the quasi-adiabatic cavity of ARC (Figure S13, and insets in Fig. 6a-b). Connected with the bomb with fully delithiated NCM523 cathode/electrolyte, the fully-lithiated graphite/electrolyte reaction delivers a same Tonset of 95℃ and a lower Ttr (from 306℃ to 247℃) (Fig. 6a), compared to the one bomb test of the fully-lithiated graphite/electrolyte mentioned above (Fig. 4e). This clearly tell us that the triggering of anode/electrolyte thermal runaway at low temperatures is not affected by gas species generated by cathode/electrolyte reactions. But at high temperatures, the released gas species (especially O2) will accelerate the thermal runaway of anode/electrolyte. Another two-bomb testing reveals that the gas species (especially H2) produced by anode/electrolyte decrease the thermal stability of cathode/electrolyte (Fig. 6b). In specific, Tonset drops from 136℃ to 116℃, and a sharp temperature rise from 159℃ to 285℃ is appeared in the HWS curve. Furthermore, gas species collected from the two-bomb system (100% SOC electrodes) are mainly consisted of CO2 (28.5%), CH4 (32.7%), H2 (15.2%), and O2 (12%), suggesting that H2 from anode and O2 from cathode side is consumed, and subsequently, evidencing the occurrence of the crosstalk of the released gas species during thermal runaway (Figure S14). Moreover, H2 is preliminarily calculated to have higher affinity to NCM (Figure S15), suggesting that the released H2 has high tendency to react with the NCM in elevated temperatures. In summary, it is concluded here that the H−-induced H2 releasing at anode side and H2 migration to cathode side is the rooted thermal runaway trigger of NCM523/G pouch cell, while the phase transformation of lithiated graphite anode and the O2-releasing by delithiated NCM523 cathode are just accelerating factors for thermal runaway. Based on all the analysis and experiments, a modified and upgraded thermal runaway route map for 100% SOC NCM523/graphite pouch cell is depicted here (Fig. 6c): 1) under abuse (mechanical, electrical, or thermal) conditions, when the battery temperature increases to ca. 100℃, mild exothermic reactions related to phase transformation of LiC6 to LiC12 and SEI layer destruction happens; The SEI layer broken is accompanied by H−-induced H2 releasing and corresponding large amounts of heat releasing; 2) Parts of released H2 will diffuse to cathode side, possibly reacting with the fully delithiated NCM to release heat; The reactions in step 1 and step 2 will raise the temperature to ca. 200℃, at which the polyolefin separators have been melted and partial short circuit between the cathode and anode poles will continue to push up the temperature; 3) when the temperature is pushed up to the range of 200–250℃, three severe exothermic reactions happens (electrolyte decomposition; phase transformation of LiC12 to graphite; and O2-releasing by delithiated NCM523 cathode) and the released large amounts of gases (O2, H2, CH4, CO, C2H4, etc.) lead to the final severe thermal runaway (smoke, fire, and even explosion).