Novel thermoresponsive ether-based electrolyte for wide-temperature operating lithium metal batteries

doi:10.21203/rs.3.rs-5101221/v1

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Novel thermoresponsive ether-based electrolyte for wide-temperature operating lithium metal batteries

https://doi.org/10.21203/rs.3.rs-5101221/v1

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Developing wide-temperature range and safety electrolytes for lithium metal batteries (LMBs) is expected to possess high redox interfacial stability, rapid kinetics and intrinsic safety. However, traditional electrolytes are rarely able to satisfy all of these characteristics simultaneously, often exhibiting preference for one over the other. Herein, we present a novel ether-based thermoresponsive electrolyte, that are designed by temperature-dependent Li⁺ solvation structure and forming polycrystalline electrode/electrolyte interface, can achieve the above characteristics at conventional salt concentration. The solvation sheath in the novel electrolyte is reconstructed by 1,3,5-trioxane (TO), accelerating the dissociation and charge transfer kinetics of anions. TO also induces cationic-ring-opening polymerization of tetrahydrofuran solvent molecules at 60 ^oC to produce oxidation-resistant ether-based polymers, which enhances the high-temperature performance and safety of LMBs. Consequently, the Li||LiNi_0.8Co_0.1Mn_0.1O₂(NCM811) cells using this thermoresponsive electrolyte operate well over a wide temperature range (from −60 to 60 ^oC). Besides, the Li||NCM811 pouch cell (1.5 Ah) achieve a high capacity-retention of 74.7% after 60 cycles at −40 °C, accompanied by an impressive energy density of 317.1 Wh kg⁻¹.

Physical sciences/Chemistry/Energy

Physical sciences/Energy science and technology/Energy storage/Batteries

Wide temperature

Thermoresponsive electrolyte

Interfacial chemistry

Li-metal batteries

With the global community pursuit of carbon neutrality, there is an immediate requirement for high-specific-energy batteries to promote energy storage and conversion efficiency^1–3. The energy density of advanced Lithium metal batteries (LMBs) has exceeded 400 Wh kg^− 1, becoming a strong contender for next-generation secondary batteries^4,5. However, the practical application of LMBs is limited by persistent parasitic reactions, disordered dendrite growth and low ionic conductivity, further resulting in rapid capacity decay and battery failure^6–9. In theory, the homogeneous and stable solid electrolyte interface (SEI) and cathode electrolyte interface (CEI) enhance the electrochemical performance of LMBs^10,11. For commercial electrolyte, the unstable organic components, including lithium dicarbonate ((ROCO₂Li)₂), semicarbonate (ROCO₂Li) and alcohol salts (ROLi), leading to slow transfer rate of Li⁺ and harmful side reactions^3,12. Notably, the viscosity of electrolyte will form local anion agggregation at ultra-low temperature (below − 40 ^oC)^13,14. Therefore, developing robust interfaces and adaptable solvation structure through electrolyte engineering is benefical for the rapid desolvation process and stable operation of LMBs across a wide temperature range¹⁵.

Recently, researchers have proposed a series of electrolyte strategies to improve the cycling stability, temperature adaptability, and safety of LMBs. The novel liquefied gas electrolytes^16,17, (locally) highly concentrated electrolytes^18,19, mixed salt systems^20,21 have achieved significant results at ultra-low temperatures, but due to the thermodynamically unstable SEI and the presence of low flash point free-solvent molecules, the battery still experiences thermal runaway. The cyclophosphazene-based flame-retardant electrolyte system²² is capable of ensuring the safe and stable operation of batteries at high temperatures. However, the increased viscosity of the electrolyte at low temperatures poses a challenge to Li⁺ desolvation, leading to battery failure. Moreover, realizing stable ultra-low-temperature ( ≤ − 40°C) performance with lean electrolyte (≤ 2.5 g Ah^− 1) and conventional salt concentration still faces challenges^6,23. Compared with other solvents (Supplementary Table 1), tetrahydrofuran (THF) is an optimal solvent for low-temperature LMBs due to its exceptionally low freezing point (− 108.4 ^oC), excellent Li⁺ transport kinetics, and extremely low viscosity^24,25.

In this work, we developed a novel thermoresponsive electrolyte comprising THF, 1,3,5-trioxane (TO), and lithium bis(fluorosulfonyl)imide (LiFSI) to ensure the safe and stable operation of LMBs over wide temperature (from − 60 to 60 ^oC). The addition of TO can not only weaken the binding energy of Li⁺-THF and improve the charge transport kinetics at low temperatures, but also form Li-polyoxymethylene (LiPOM) in the CEI and SEI, promoting uniform Li⁺ flux and impeding the electrochemical corrosion by the electrolyte, and realize the suitability for high-voltage cathode and conventional salt concentration. More importantly, TO can also trigger cationic-ring-opening polymerization of THF molecules at high temperatures, resulting in the formation of ion-aggregates-dominated solvated structure and polyether, which increases the high-temperature performance and safety of LMBs. As a result, the 1.5 Ah pouch cell demonstrates an impressive energy density of 386.8 Wh kg^− 1 at 25 ^oC. Even at − 40 ^oC, the energy density of LMBs reaches 317.1 Wh kg^− 1, accompanied by a high capacity-retention of 74.7% after 60 cycles. This work presents a novel approach to design thermoresponsive electrolyte for up-and-coming LMBs over a wide temperature range.

Design of the thermoresponsive electrolyte and temperature-dependent solvated structures

The primary requirements for stable operation of LMBs at ultra-low temperatures are high ionic conductivity, low viscosity, and low freezing point²⁶. In contrast, THF has excellent physical properties in all aspects and is the optimal solvent for ultra-low temperature LMBs (Supplementary Table 1). The results of quantum chemical calculations indicate that THF exhibits the highest lowest unoccupied molecular orbital (LUMO) energy level with a high propensity to supply power, enabling excellent reduction stability to the anode. In comparison, LiFSI exhibits the lowest LUMO and the highest highest Occupied Molecular Orbital (HOMO), which suggests that it will be preferentially reduced and oxidized to form an inorganic-rich inner SEI and CEI. The additive TO exhibits a lower LUMO and higher HOMO than THF, and is capable of preferential redox to form LiPOM, which serves to protect against the decomposition of the solvent THF (Fig. 1a and Supplementary Fig. 1a). The reduction potentials of the three compounds were quantified, leading to the same conclusion. The reduction potential of TO (0.57 V) was higher than that of THF (0.38 V), while both were significantly lower than that of FSI- (1.64 V) (Fig. 1d) ^3,27. Furthermore, electrostatic potential (ESP) simulation revealed that the ESP of the entire solvated structure was transferred from FSI⁻ to the solvent upon TO addition. The ESP of the solvent was markedly reduced, indicating that TO can balance the energy in the solvated sheath and inhibit the continuous decomposition of the lithium salt. This is anticipated to result in excellent low-temperature performance at conventional Li-salt concentration (Supplementary Fig. 2). Based on the above design ideas, we selected 1M LiFSI THF as the reference electrolyte (denoted as THF-based electrolyte). The additive TO with 20% molar ratios was introduced into the THF-based electrolyte denoted as TO-based electrolyte.

Molecular dynamics (MD) simulations and the corresponding radial distribution functions (RDFs) were employed to evaluate the coordination structures of solvents, cations, and anions in THF-based and TO-based electrolytes^8,28. The calculations indicate that the average coordination in THF- and TO-based electrolytes is Li⁺(THF-O)_2.8(FSI-O)_0.9 and Li⁺(THF-O)_1.9(TO-O)_0.7(FSI-O)_1.3, respectively. These findings show that TO can engage in the solvation of the Li⁺ solvation sheath within the electrolyte (Li-OTO peaks at ~ 2.1 Å) and diminish the coordination of Li⁺-THF, while enhancing the coordination of Li⁺- FSI⁻ (Fig. 1b, c). The blue shift of the S-N-S peaks observed in the Raman spectra, along with the shift of the ⁷Li NMR spectra to the lower field, provide further support for the aforementioned conclusion (Fig. 1e-g). The fitted curves of the 700–780 cm^− 1 Raman spectral bands indicate that the TO-based electrolyte forms a solvated structure dominated by contact ion pairs (CIPs) at − 40 ^oC. This enhanced anionic interfacial chemistry ensures high charge-transfer kinetics at low temperatures and enhances the electrochemical performance (Fig. 1f).

Variable-temperature Raman and NMR spectroscopy tests can directly reveal changes in the solvated structure as a function of temperature. In the TO-based electrolyte, the contents of AGG I (one FSI⁻ coordinating to two Li⁺) and AGG II (one FSI⁻ coordinating to more than two Li⁺)²⁹ increased with temperature, indicating a significant reduction in free solvent molecules and more anions involved in the composition of the solvated sheath at high temperature (Fig. 1f). Furthermore, the shift of variable-temperature ⁷Li NMR to higher field provides additional evidence for the enhancement of Li⁺ coordination with anions (Fig. 1g). Additionally, the MD snapshots illustrate the strong aggregation of anions and the formation of ionic clusters at 60 ^oC, and the corresponding RDFs demonstrate that the coordination of Li-O_THF becomes weaker and the coordination of Li-O_FSI⁻ stronger with increasing temperature (Supplementary Fig. 3). The above results indicate that the thermal motion of the polar THF solvent molecules increases during the temperature increase, thereby modifying the original ion-dipole mode of action and prompting the formation of temperature-dependent solvated structures.

In general, the optimal electrolyte for high-temperature LMBs operation should possess a stable solvated structure, a robust redox-stable interface, and a high safety solvent³⁰. By means of MD simulations conducted at 60 ^oC, three typical solvated structures (with a ratio higher than 75%) were extracted from each electrolyte, and their binding energies were calculated by density functional theory (DFT). The results show that the solvated structures of the TO-based electrolyte have higher binding energies than those of the THF-based electrolyte, which indicates that it has high thermal and oxidative stability (Supplementary Fig. 4). As shown in Fig. 1h and Supplementary Fig. 5, the characteristic peaks of THF in the TO-based electrolyte were markedly diminished, and novel NMR peaks of hydrogen and carbon were discerned, culminating in the formation of a polyether (PTHF) with enhanced thermal stability. According to gel permeation chromatography, PTHF has high number average molecular weight (M_n) and high weight average molecular weight (M_w) of 1.82×10⁵ and 3.47×10⁵ g mol^− 1, respectively. The polymer dispersity index is approximately 2, which corresponds to a homogeneous molecular weight distribution, and effectively enhances the high temperature performance and safety of the batteries (Fig. 1i).

The thermal polymerization mechanism and the evolution of solvated structure of TO-based electrolyte are described in detail in Fig. 1j, Supplementary Figs. 6–9 and Supplementary Note 1. The thermal motion of the molecules is insignificant at − 40 ^oC, and THF, TO maintains relatively high ionic dipole interactions with Li⁺ to form a CIP-dominated solvated structure, which accelerates the Li⁺ desolvation process and achieves fast charge transfer kinetics at low temperatures. The thermal motion and kinetic energy of THF molecules increase at high temperature and undergo cationic ring-opening polymerization reaction in the presence of primary TO-oxonium-ions to form PTHF (Supplementary Fig. 9). This will drastically reduce the free solvent molecules in the electrolyte and decouple the solvents in the solvated structure, resulting in the formation of a more thermally stable anion-rich solvated sheath (i.e., the solvated structure is transformed from CIP to AGG I and AGG II). Notably, in contrast to the polymerization mechanism of THF at high temperatures, TO is in contact with Li-metal and undergoes a ring-opening polymerization reaction initiated by a minute quantity of H⁺(FSIOH)⁻ to form a robust and compact LiPOM layer³¹, which effectively enhances the redox stability of SEI and CEI (Supplementary Fig. 7, 8). Consequently, the TO-based electrolyte demonstrates excellent safety and wide-temperature performance, overcoming the limitations of conventional electrolytes to achieve the compatibility of thermodynamically stable interface, high safety and fast charge transfer kinetics at low temperatures.

Electrolyte properties and interfacial dynamics

Although the increase in TO content enhances the degree of electrolyte polymerization at high temperatures, it also forms an excessively thick LiPOM layer at the interface, which impedes the interfacial charge transfer kinetics. To obtain the LiPOM layer with an optimal thickness, we configured TO-based electrolytes with varying molar ratios. Figure 2a shows the variation of ionic conductivity with temperature for the five electrolytes. The TO-base (20%) electrolyte exhibits the highest ionic conductivity of 3.928 mS cm^− 1 at − 40°C. It is noteworthy that the ionic conductivity of the TO-based electrolytes exhibited a decreasing trend as the temperature was increased to 60°C. This is primarily due to the ring-opening polymerization of electrolyte solvent molecules and the formation of strong interactions between cations and anions in the solvated structure. The Arrhenius equation was employed to calculate the activation energy (Ea) of the electrolyte at low temperatures (Fig. 2b)³², resulting in a lower Ea value for the TO-based electrolyte in comparison to the THF-based electrolyte. Furthermore, the TO-based (20%) electrolyte exhibited an Ea of only 12.1 kJ mol^− 1. The lower Ea indicates accelerated ion diffusion kinetics, resulting in the highest Li⁺ transference number of 0.629 (Supplementary Fig. 10). This enhances the prolonged sand time and is advantageous for inhibiting dendrite growth³³. The results of linear scanning voltammetry (LSV) also indicated a positive shift in the oxidation onset potential to 4.73 V with increasing TO content (Fig. 2c). Although it has the potential to considerably enhance the oxidation window of the electrolyte, it also causes a deterioration in the wettability of the electrolyte with the separator (Supplementary Fig. 11). Overall, the TO-base (20%) electrolyte was observed to effectively enhance the ionic conductivity, ion transference number, and oxidative decomposition potential, while also exhibiting moderate viscosity at low temperatures.

An in situ IR cell device was constructed for the purpose of monitoring the dynamic alterations in the interfacial electrolyte conformation of copper electrodes throughout the processes of plating/stripping (Supplementary Fig. 12 and Supplementary Note 2) ^34,35. This device was designed to facilitate insight into the modifications in solvation structure that occur in THF-based and TO-based electrolytes during the processes of charging/discharging, and to elucidate the impact of these modifications on the SEI derivatization. Figures 2d-f show the raw IR transmission spectra of in situ FTIR using TO-based electrolytes. The data demonstrate a decline in the peak of ROLi (835 ~ 845 cm^− 1) and an increase in the peak of C = O (1725 ~ 1825 cm^− 1)³⁶, indicating that the organic SEI formed is a robust and dense LiPOM layer derived from TO, as opposed to an unstable ROLi layer derived from THF. For comparison, the THF-based electrolyte exhibited strong ROLi and C-S (~ 800 cm^− 1) signals³⁷ (lithium salt decomposition), and no C = O signal was observed (Supplementary Fig. 13). These findings indicate that the TO-derived LiPOM layer can act as a barrier to impede the inward diffusion of LiFSI to the Li anode, thereby slowing the successive decomposition of LiFSI and reducing the occurrence of side reactions.

The impact of TO on the electrode interface kinetics was further elucidated through the examination of the distribution of relaxation times (DRT) (Fig. 2g, h and Supplementary Fig. 14)^38,39. This analysis classifies distinct electrochemical processes based on local maxima in a continuous distribution function⁴⁰. Li⁺ diffusion and interfacial charge transfer demonstrate a pronounced temperature dependence, with these processes dominating at low temperatures. It is noteworthy that the resistance values of these two processes for the TO-based electrolyte are only one-tenth of those for the THF-based electrolyte. This highlights the crucial role of TO in enhancing Li⁺ desolvation and electrode interface charge transfer at ultra-low temperatures.

Effect of the designed electrolyte on Li plating/stripping at ultra-low temperatures

To evaluate the impact of the designed LiPOM layer on Li plating/stripping, the Coulombic efficiency (CE) of THF-based and TO-based electrolytes in Li||Cu cells was initially assessed using the Aurbach method⁴¹. As shown in Fig. 3a, the TO-based electrolyte exhibits an exceptional CE of 99.06% at − 40°C and 0.5 mA cm^− 2, which is considerably higher than THF-based electrolyte (76.03%), and its initial lithium nucleation overpotential is also only 278 mV. This indicates that the addition of TO effectively improves the Li plating/stripping efficiency and kinetics. Furthermore, long cycling tests were conducted at a current density of 0.5 mA cm^− 2 and a capacity of 1 mAh cm^− 2. The 250-cycle average CE of the Li||Cu cell utilizing TO-based electrolytes reached 98.56% (Fig. 3b), exhibiting a comparatively lower and more stable polarization voltage in its corresponding voltage profile (Fig. 3c). Moreover, the Li||Li symmetric cell demonstrated stable operation for 450 h under these conditions (Fig. 3d). In contrast, the CE of the THF-based electrolyte exhibited pronounced fluctuations during the initial stage, and a notable short-circuit phenomenon was observed after 20 cycles. This proves that the TO-derived LiPOM layer can effectively safeguard the Li anode from solvent molecule erosion, impede the growth of lithium dendrites and the formation of dead lithium, and demonstrate excellent Li plating/stripping reversibility and long-term stability at ultra-low temperature. As shown in Fig. 3e, the Li||Cu cell assembled by TO-based electrolyte exhibits remarkable rate performance at − 40°C, with a CE reaching 95.97% at a high current density of 1 mA cm^− 2. The cell is able to maintain stability when the current density is returned to 0.1 mA cm^− 2, and the corresponding voltage profile is consistent with the initial state (Fig. 3f). In contrast, the THF-based electrolyte demonstrated a notable decline in CE at 0.5 mA cm^− 2, resulting in direct short-circuiting and failure at 1 mA cm^− 2. This fully demonstrates that the addition of TO improves the interfacial charge transfer kinetics of the cell at ultra-low temperature.

To elucidate the substantial discrepancies in CE and cycling stability among different electrolytes, scanning electron microscopy (SEM) and Kelvin probe force microscopy (KPFM) were employed to examine the morphology and surface potential of Li deposits, thereby facilitating an initial assessment of the underlying causes of these discrepancies from a morphological perspective. As shown in Fig. 3g, the quantity of Li plating on the Cu collector was markedly diminished in the THF-based electrolyte, and a considerable amount of whisker-like Li generation was also observed under SEM, which resulted in a notable increase in Li plating porosity, surface roughness, and surface potential. This is attributed to the slow Li⁺ desolventization and interfacial charge transfer process of the THF-based electrolyte at ultra-low temperatures. This ultimately results in tip-driven Li-deposition, which leads to severe short-circuiting and failure of the cell⁸. In contrast, following the introduction of TO, the Li deposits exhibited a dense and uniform bulk deposition morphology, with a surface roughness and surface potential of only 823.1 nm and 115 mV (Fig. 3h), respectively. This contributes to the achievement of higher CE and longer cycle life, indicating a significant enhancement of the solvation process and SEI kinetics in the TO-based electrolyte.

Notably, the Li||Cu and Li||Li cells assembled by TO-based electrolyte also exhibit outstanding CE and long-cycle stability at room temperature. Specifically, the Li||Cu cell cycled 400 turns with an average CE of 99.38%, the Li||Li cell cycled stably for 1400 h at a current density of 1 mA cm^− 2 and a capacity of 2 mAh cm^− 2. Furthermore, the Li||Cu cell still has a CE of 96.36% at a high current density of 5 mA cm^− 2 (Supplementary Fig. 15). The Li-deposits formed in the TO-based electrolyte were also more homogeneous and flat, forming larger bulk deposits with a surface roughness and surface potential of only 504.7 nm and 11.9 mV, respectively. This was observed from SEM and KPFM (Supplementary Fig. 16). This was corroborated by in-situ observations made with polarized light microscopy (Fig. 3i). The TO-based electrolyte, formed a denser and more homogeneous Li layer, which enhanced the efficiency of Li deposition and effectively suppressed the growth of lithium dendrites. In contrast, the Li deposition of the THF-based electrolyte was sparse, exhibiting the formation of lithium dendrites and dead lithium.

Interfacial Chemistry Study

To obtain the nanostructures of SEI formed at ultra-low temperatures, we first observe the Li deposits on Cu grid via high-resolution transmission electron microscopy (HRTEM). As shown in Supplementary Fig. 17, the thickness of the TO-based electrolyte-derived SEI is mere 9.74 nm. It comprises an inner inorganic phase comprising LiF, Li₂O, and Li₂CO₃, and an outer organic layer of a "mosaic" type amorphous organic SEI. This structure of SEI exhibits high strength and elasticity, which effectively inhibit the electron tunneling effect and homogenize the Li⁺ flux⁴². Consequently, the electrolyte is capable of achieving high CE and dense and uniform lithium deposition at ultra-low temperatures. In contrast, the SEI in the THF-based electrolyte is 22.3 nm thick, which impedes Li⁺ diffusion. Furthermore, the outer organic layer of SEI is distributed intermittently, indicating severe erosion by the electrolyte and a lack of stability and robustness.

To gain further insight into the composition and relative content of the structure of dual-layered SEI, depth profiling XPS tests were conducted (Fig. 4a-d and Supplementary Fig. 18). According to the fine fitting results of XPS C 1s spectra, the presence of additional O-C-O and C = O signals was observed in TO-based electrolyte, which provided insight into the pathways and products of TO decomposition³. The O-C-O signal was attributed to the TO-derived LiPOM layer, while the C-O signal was attributed to the THF derivative, which formed a short-chain organic SEI. As Ar⁺ sputtering deepens, the relative content of organic species declines precipitously, while the relative content of inorganic components, including LiF, Li₂O, and Li₃N, rises markedly. This increase in inorganic components will facilitate the enhancement of ionic conductivity in the SEI and reduce kinetic barriers for Li⁺ passage through the SEI. Figure 4d depicts the percent composition of each species in the SEI derived from both electrolytes at ultra-low temperatures. The SEI derived from the THF-based electrolyte exhibit elevated C-O (27%) in the surface layer and augmented Li₂CO₃ (23%) in the interior. This suggests that the formed SEI are of low strength and poorly stabilized, with the solvent molecules persistently eroding and undergoing side reactions with the Li metal. In contrast, the TO-based electrolyte derived SEI exhibits higher O-C-O (19%) in surface layer and a greater abundance of LiF (64%) and LiO₂ (8%) in internal layer. This suggests that the dense LiPOM layer effectively inhibits the erosion of Li metal by the solvent.

The time-of-flight secondary ion mass spectrometry (TOF-SIMS) also provided dependable supporting information for the SEI derived from TO-based electrolytes⁴³. The initial detection of ion fragments pertaining to SEI components is presented in Supplementary Fig. 19. The LiF₂⁻ and C₂HO⁻ ionic fragments are characteristic of LiF and organic components, respectively. The presence of abundant organic components in SEI indicates that TO can decompose to form LiPOM and participate in the formation of SEI. The three-dimensional structure of SEI and the spatial distribution of each component were further resolved by depth profile analysis of TOF-SIMS (Fig. 4e-g). The organic component of SEI is dense in the surface layer, which is capable of forming a solid layer. With the increase of the sputtering time, the content of the organic component starts to decrease, and the SEI gradually transitions to a LiF-dominated structure. By combining evidence from TOF-SIMS, HR-TEM, and XPS, we conclude that TO in the electrolyte is able to form a dense LiPOM outer layer on conventional anion-derived SEI, which protects the Li anode and enables the cell to have high CE and long-cycle stability.

In-situ X-ray diffraction (XRD) was employed to investigate the impact of two electrolyte-derived CEIs on the structural transformation of the NCM811 cathode during the charging/discharging processes (Fig. 5a, b). In 3.0 ~ 4.5 V, the NCM811 cathode underwent a series of phase transitions: H1→M→H2→H3a, which corresponded to the expansion and contraction of the lattice volume (Supplementary Fig. 20a, b)^44,45. Notably, the intensity and Brag angle of the (003) and (101) peaks of the NCM811 cathode were significantly changed throughout the phase transition using the THF-based electrolyte (the H1→M phase transition was particularly pronounced). In comparison, the volume expansion of the NCM811 cathode and the trailing of the peaks were significantly suppressed in the TO-based electrolyte, indicating a reversible phase transition and excellent structural stability. The fully charged Li||NCM811 cell was subjected to accelerated degradation tests at 4.5 V (Supplementary Fig. 20c). The results demonstrated that the leakage current density of the cell with TO-based electrolyte was only 5.44 µA cm^− 2, indicating that the TO-derived CEI effectively reduced the interfacial side reaction rate and passivated the cathode. Furthermore, to examine the safeguarding of the CEI formed by the two electrolytes against the structural transformation of the cathode at − 40 ^oC, the NCM811 cathode after 100 cycles was dismantled for XRD pattern analysis (Supplementary Fig. 21a-f). The separation of the (006)/(102) peaks was observed to be more pronounced in the TO-based electrolyte, which suggests that the structural transition from the layered to the rock salt phase was effectively suppressed⁴⁶. Furthermore, the SEM of the cathode, which was circulated in the TO-based electrolyte, did not develop intergranular cracks and exhibited a stable structure (Supplementary Fig. 22). Collectively, these findings illustrate that the TO-based electrolyte is effective in forming a dense and robust CEI, which serves to safeguard the NCM811 electrode from structural deterioration during the phase transition.

To gain further insight into the chemical state and compositional information of the CEI, the NCM811 cathode was subjected to a depth profiling XPS test following 100 cycles. From the C 1s spectrum, it can be observed that the THF-based electrolyte-derived CEI has a strong C-O signal, indicating that its surface consists of short-chained and unstable organics, which is the main reason why it exhibits poor cathodic reversibility and structural disruption (Fig. 5c). As a comparison, the TO-based electrolyte-derived CEI surface displays a greater abundance of O-C-O and C = O signals, indicative of the successful formation of the surface LiPOM layer (Fig. 5d)³. As the Ar⁺ sputtering progresses, there is a notable decline in the relative content of organic species, accompanied by a pronounced increase in the relative content of LiF. This shift is expected to significantly enhance the kinetics of Li⁺ transport in CEI. The organic-inorganic bilayer CEI was also demonstrated with high reliability using TOF-SIMS. The initial detection of LiF₂⁻, CO₃⁻ and C₂HO⁻ indicated the presence of characteristic ion fragments of LiF, Li₂CO₃ and organic components, respectively (Supplementary Fig. 23a-c). The 3D structure of CEI and the spatial distribution of each component were further resolved by TOF-SIMS depth profiling (Supplementary Fig. 23d-f and 24). CEI consists of a dense organic layer on the surface and a LiF-dominated inorganic layer in the interior, and the content of Li₂CO₃ in the surface layer is extremely low, which suggests that CEI effectively inhibits the occurrence of the side reactions¹⁰. In light of the evidence derived from in situ XRD, XPS and TOF-SIMS, it can be proved that the CEI formed by TO-based electrolyte is constituted by a dense and homogeneous LiPOM outer layer and a LiF-rich inner layer. This structure inhibits interfacial side reactions and the dissolution of TMs, preserves the initial structure of NCM811, and enhances interfacial stability.

From the experimental and theoretical results, the function of TO is shown in Fig. 5e. TO is capable of participating in the Li⁺ solvation structure, weakening the binding energy of Li⁺-THF, promoting Li⁺ desolvation, and enhancing the charge transfer kinetics at the interface. More importantly, TO is able to prioritize THF reduction and oxidation to build a uniform, thin and robust LiPOM layer on the surface of Li-Metal and NCM electrodes. The double-layer SEI inhibits interfacial side reactions and lithium dendrite growth, while the double-layer CEI inhibits the dissolution of transition metal (TM) ions. This electrolyte design effectively enhances the cycling performance of the battery under high voltage and wide-temperature. In contrast, in the THF-based electrolyte, the inability to form a dense polymer layer to block the solvent attack on the both electrodes resulted in the growth of lithium dendrites, the dissolution of TMs, and the rupture of NCM particles (Fig. 5f).

Performance of practical Li-metal cells at wide temperature

In order to demonstrate the effectiveness of the designed electrolyte operating over a wide temperature range, Li||NMC811 full cells were assembled to fully evaluate the TO-based electrolyte system. As shown in Fig. 6a, b, the TO-based electrolyte full cell exhibits excellent long-cycle performance, with a capacity retention of 87.3% after 500 cycles at 25°C and 1C. Furthermore, the dQ/dV curves exhibit a high degree of overlap, with only a slight voltage change of 16 mV, indicating phase change reversibility and structural stability of the cathode (Fig. 6c). More importantly, the Li||NMC811 full cell employing a TO-based electrolyte still exhibits a capacity residual of 176.8 mAh g^− 1 after 200 cycles, with a capacity retention of 90% and an average CE of 99.4% at an ultra-low temperature of − 40°C (0.2C charge/discharge) (Fig. 6d, e). It is worthy of note that the polarization of the cell increases significantly at ultra-low temperatures. Thanks to the improved solvation structure of TO and the formation of dual-layered CEI and SEI, which effectively enhances the Li⁺ diffusion and interfacial charge transfer kinetics at low temperatures. Thus, a reversible phase transition of the anode was achieved (especially for the H1→M phase change process, where the potential increased by only 48 mV after 200 cycles), reducing the polarization and the loss of cell capacity (Fig. 6f). In contrast, the initial capacity of the cell with THF-based electrolyte was only 120 mAh g^− 1, which decayed to 41% of the initial capacity after 200 cycles. Furthermore, the TO-based electrolyte is capable of achieving 400 cycles at 60 ^oC with a capacity retention rate of up to 88.9% (Fig. 6g, h), due to the solvation structure transformation and solvent molecule reconfiguration at elevated temperatures. As shown in Fig. 6i, the "six-star diagram" visualization demonstrates that the TO-based electrolyte system exhibits significant advantages in terms of Li anode stability, wide temperature discharge, multiplicity performance, thermal stability, desolvation, and cycling stability.

Excellent wide-temperature performance and long cycle stability make advanced TO-based electrolytes promising for use in practical LMBs. Li||NCM811 pouch cells (1.5 Ah) with lean electrolytes (2 g Ah^− 1) and a N/P ratio of 2.2 were assembled, allowing an initial specific energy density of 386.8 Wh kg^− 1(Supplementary Fig. 25). The energy density of a cell is calculated by taking the masses of all components in pouch cells into consideration (Supplementary Table 2). Surprisingly, the LMB pouch cell was able to achieve an unprecedented initial energy density of 317.1 Wh kg^− 1 at − 40 ^oC, and exhibited remarkable cycling stability, retaining 74.7% of its capacity after 60 cycles (Fig. 6j). In contrast, the discharge capacity of THF-based electrolytes decays to 0 after 10 cycles. The disassembly of the cycled Li||NCM811 pouch cells was employed to investigate the underlying cause of the notable discrepancy in cycling performance at low temperatures between two electrolytes. The Li anodes that were cycled in the THF-based electrolyte exhibited a notable depletion and displayed considerable quantities of dead lithium and whisker-like formations (Supplementary Fig. 26a, b). Conversely, in the TO-based electrolyte, the lithium remained in the form of lumpy deposits, although the distribution of lithium on the Cu collector exhibited slight inhomogeneity after repeated cycling (Supplementary Fig. 26c, d). Further, the temperature adaptability of Li||NCM811 pouch cells was evaluated. The cells were observed to operate stably at − 60 ~ 60 ^oC, exhibiting excellent room temperature capacity retention of 81.5% and 61.7% at − 60 and − 40 ^oC, respectively (Fig. 6k). Furthermore, our designed TO-based electrolyte exhibits the most optimal overall performance in terms of wide-temperature performance, capacity retention, and energy density when compared with advanced electrolytes designed in the literature (Fig. 6i and Supplementary Table 3).

To further demonstrate the practical application potential of the TO-based electrolyte, it was assembled into a pouch cell and assembled on a robot, which was able to drive the robot to work stably under the harsh temperature conditions of − 40 ^oC (Supplementary Fig. 27 and Supplementary Movie 1). The safety of LMB is typically determined by the combination of the intrinsic safety of the electrolyte and the thermal runaway temperature of the cathode material in a fully charged state^47,48. As shown in Supplementary Fig. 28a, b and Supplementary Movie 2, the polymerization products of the TO-based electrolyte did not exhibit combustion in ignition test, and the thermal decomposition temperature of the electrolyte reached 211°C with minimal heat release. In-situ variothermal XRD demonstrated that the formation of dual-layered CEI effectively suppressed the release of oxygen from the cathode during charging and enhanced the thermodynamic stability (Supplementary Fig. 28c, d). Consequently, the thermal safety of LMBs utilising a TO-based electrolyte was markedly enhanced.

In summary, we present a novel thermoresponsive electrolyte design strategy that enables the safe and stable operation of LMBs at − 60 ~ 60°C. The theoretical and experimental evidence demonstrates that the additive TO is able to participate in the Li⁺ solvation structure, thereby promoting Li⁺ transport and charge transfer kinetics. Combined with the evidence from HRTEM, XPS, TOF-SIMS and in-situ XRD, the SEI, CEI derived from TO-based electrolyte is composed of a uniform, thin and robust outer layer of LiPOM and an inner layer enriched with LiF, Li₂O, which is able to effectively prevent the degradation of both electrodes. More importantly, when the temperature rises, the solventized structure of the electrolyte undergoes a transformation, with solvent molecules reorganizing to form a polyether with enhanced oxidation resistance. This significantly enhances the safety of the battery. Consequently, the practical 1.5 Ah Li||NCM811 pouch cell with optimized electrolyte is able to operate at − 60 ~ 60°C. Specifically, the pouch cell is capable of achieving 100 cycles at 25°C with an initial energy density of 386.8 Wh kg^− 1. Surprisingly, it still has a room temperature discharge capacity of 61.7% at − 60°C, and achieves an unprecedented initial energy density of 317.1 Wh kg^− 1 at − 40°C with a capacity retention of 74.7% for 60 cycles.

Materials

Active materials (LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) powders), polyvinylidene difluoride (PVDF), Super P, carbon-coated aluminum foil (Al, 15 µm-thick), separator (Celgard 2325, thickness: 25 µm, PP/PE/PP three-layer membrane), Coin cell components (CR2032, spacer: 15.8 × 1 mm, spring: 15.4 × 1.1 mm) were purchased from Nanjing Mojiesi Energy Technology. Lithium Bis(fluorosulfonyl)imide (LiFSI, > 98%), tetrahydrofuran (THF, ≥ 99.9%), N-methyl-2-pyrrolidone (NMP, ≥ 99.5%) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. 1,3,5trioxane (TO, > 99.0% (GC)) were purchased from Tokyo Chemical Industry Co., Ltd. The 450 µm Li-metal sheets were procured from Beijing InnoChem Science & Technology Co., Ltd. The NCM811 cathodes (22 mg cm^− 2) were procured from Guangdong Canrd New Energy Technology Co., Ltd.

The preparation of electrolyte

The THF-based electrolyte was prepared by dissolving 1M LiFSI (0.187 g) into the 1 mL THF. Next, 0.018g TO was dissolved in THF-based electrolyte to prepare the TO-based electrolyte (LiFSI:TO molar ratios of 1:0.2). All the preparation steps mentioned above were conducted at room temperature (~ 25°C) in an Ar-filled glove box (H₂O < 0.1 ppm and O₂ < 0.1 ppm).

Coin cells and pouch cells assembling

All coin cells were assembled in the Argon-filled glove box with H₂O < 0.1 ppm and O₂ < 0.1 ppm. Li||Li CR-2032 type coin cells were assembled in the following order: cathode case, Li-metal sheet, Celgard 2325 separator (18 mm), Li-metal sheet, spacer, spring, and anode case. Li||Cu CR-2032 type coin cells were assembled in the following order: cathode case, Cu foil (14mm), Celgard 2325 separator, Li sheet, spacer, spring, and anode case. Li||NCM811 full cell were assembled in the following order: cathode case, aluminum disk (19 mm), NCM811 cathode disk, Celgard 2325 separator, Li anode sheet, spacer, spring, and anode shell. All coin cells were added with 75 µL electrolyte. Additionally, to prevent corrosion of the stainless-steel case under high voltages, an additional piece of Al foil must be placed between the cathode case and NCM cathode disk.

For Li||NCM811 pouch cell (10.0 × 7.0 cm²), Li anode (composed of Cu current collector and double-layer Li foil) and cathode of double-layer active material were stacked one by one and separated by Celgard 2325, the electrolyte volume is 3.0 mL. The thickness and mass loading of anode, cathode and other components are provided in Supplementary Table 3. The Li||NCM811 pouch cell was assembled in a dry room whose dew point is − 50°C.

Electrochemical Testing

All electrochemical data presented in this work were generated from CR-2032 coin cells. Low temperature galvanostatic tests were performed on a Neware BTS 4000 system, while room temperature galvanostatic tests were performed on a LANHE-CT 2001A system. The Low temperature tests were measured in a JHY-H-50L temperature chamber to maintain the cell at a set temperature. All potentiostatic tests were performed on a CHI-760E. The electrochemical impedance spectroscopy (EIS), from 1 MHz to 100 mHz with an amplitude of 5 mV was tested using a Chenhua electrochemical workstation. In the LSV tests, a carbon-coated aluminium foil (c-Al foil) was used as a working electrode with a sweep rate of 1 mV s^− 1. Round cycles from 3 to 6 V (vs. Li/Li⁺) were performed, and then the stable recorded cathodic scan for each Li||c-Al foil cell with different electrolytes was recorded as the LSV curve. The electrochemical floating test was performed in Li||NCM811 cells with different electrolyte. The cells were first charged to 4.5 V at 0.1C and then maintained for 12 h with the current monitored by LANHE-CT 2001A system.

The ionic conductivity of the electrolyte was measured by a customized two-electrode coin cell. The two stainless steel electrodes were placed symmetrically between a polytetrafluoroethylene disc with a thickness of 0.027 inches. A glass-fiber separator soaked with electrolyte was placed inside the washer, which constrained its surface area to a known value. The electrolyte conductivity values were then obtained with electrochemical impedance spectroscopy (EIS) using the following equation:

𝜎 =$\:\frac{\varvec{L}}{\varvec{A}\ast\:\varvec{R}}$

where R is the measured ionic resistance and A and L are the area of the electrodes and the space between the electrodes, respectively. The data points from 40°C to − 60°C were measured in a JHY-H-50L temperature chamber to maintain the cell at a set temperature for 2h intervals before each measurement.

The transfer numbers of the electrolytes were determined by a commonly used potentiostatic polarisation technique on a CHI-760E, and the transfer coefficient was then calculated using the following equation:

$$\:{\varvec{t}}_{+}=\frac{{\varvec{I}}_{\varvec{S}\varvec{S}}(\varDelta\:\varvec{V}-{\varvec{I}}_{0}{\varvec{R}}_{0})}{{\varvec{I}}_{0}(\varDelta\:\varvec{V}-{\varvec{I}}_{\varvec{S}\varvec{S}}{\varvec{R}}_{\varvec{S}\varvec{S}})}$$

where 𝛥𝑉 is the applied bias, R₀ is the initial cell impedance and R_SS is the steady-state cell impedance.

To evaluate the CE of Li||Cu cells, Aurbach’s method was applied. The overall CE was calculated by the following Eq. 3⁸

$$\:{\varvec{C}\varvec{E}}_{\varvec{a}\varvec{v}\varvec{g}-\varvec{n}}=\frac{\varvec{n}{\varvec{Q}}_{\varvec{c}}+{\varvec{Q}}_{\varvec{s}}}{\varvec{n}{\varvec{Q}}_{\varvec{c}}+{\varvec{Q}}_{\varvec{t}}}$$

Q_c represents the deposition or dissolution capacity in n cycles. Its fixed value is 1 mAh cm^− 2. Q_t is the initial Li reservoir capacity deposited on the Cu foil and Q_s indicates the capacity finally stripped from the Cu foil.

Pouch cells were cycled in the voltage range of 3.0–4.3V at 0.1C at 25°C. Pouch cells were cycled in the voltage range of 2.8–4.3 V at 0.05C at − 40°C. The cycling performance of the pouch cells was tested under a fixing device to provide 1.0 MPa external pressure.

Material characterizations

The Nuclear Magnetic Resonance (NMR) spectrometer was used to characterize the organic solvents or electrolytes in this work, and each test dissolved a 20 µL sample in deuterated chloroform. The ¹H and ⁷Li Nuclear Magnetic Resonance (NMR) spectra were acquired using the Bruker 400 MHz and Bruker AVANCE NEO 600 MHz, respectively. The gel permeation chromatography measurements (1260 Infinity II, Agilent Technologies) were performed by dissolving the polymerization product in THF. The Contact Angle (CA) measuring instrument (JC2000DS2) was used to characterise the wettability of the four electrolytes studied. The amount of electrolyte is controlled at 10 uL at a time. The Raman spectroscopy data of the electrolytes were obtained using a LabRAM HR Evolution Raman micro-spectrometer equipped with 633 nm laser. The morphologies of Li deposition in different electrolytes were collected by FESEM (JSM-7800F, JEOL), HR-TEM (JEM-2100F, JEOL) was used to observe the thickness and morphology of CEI and SEI films. The roughness and potential of the lithium-embedded negative electrode surface were obtained by KPFM (SPM-9700HT). XPS data was collected using Thermo Scientific K-Alpha⁺. The analysis chamber has a vacuum degree of approximately 2×10^− 7 mbar, X-ray source: monochromatic Al Kα source, energy: 1486.6 eV, voltage: 12 kV, beam current: 6 mA, analyzer scanning mode: CAE, work function: 4.2 eV. The depth-profiling XPS was carried out at an etching rate of 0.2 nm s^− 1. ToF-SIMS (PHI nano ToF II, ULVAC-PHI) also investigated SEI and CEI components. The area of analysis is 100 µm × 100 µm, while the sputtering area is 400 µm × 400 µm.

Data for X-ray diffraction was acquired using a Bruker D8 Advance X-ray diffractometer outfitted with a LynxEye 1-dimensional detector with Cu-Ka radiation at 40 kV and 40 mA (λ = 1.5418 Å) with a step increment of 0.02 and a duration per step of 0.1 s. The operando XRD test during charging and discharging rate of 0.5 mA cm^− 2 were performed at 25°C and diffraction patterns were collected every 8 min. The LIB-MS-R is provided by the Beijing Scistar Technology Co. Ltd. The device simulates a lithium-ion coin cell to observe the growth of lithium dendrite and its corresponding changes.

Density functional theory calculations

The DFT calculations were performed using Gauss 16 quantum chemistry software. All the molecules were pre-optimised at the B3LYP/6-31G* level. Next, the Molclus program was used to search for configurations of the complex. The optimized geometry was obtained using the DFT-D3 van der Waals (vdW) correction proposed by Grimme. During the correction process, all atoms were allowed to relax until the atomic force on each atom was below 0.005 eV Å⁻¹ and the energy was less than 1.0 × 10^− 6 eV. A total of 200 initial configurations were generated and each configuration was optimised at the B3LYP/3-21G* level. The configuration with the lowest energy was then further optimised to the B3LYP/6-311G(d) level. The intermolecular interactions were described using the Grimme d3bj dispersion. The binding energy was calculated according to the following equation.

Binding energy = Ecomplex–(Efragment₁ + Efragment₂)⁴⁹

Molecular dynamics simulations

All molecular dynamics (MD) simulations were performed by the GROMACS 2023 simulation package^50,51. The systems were described by the OPLS-AA force field⁵². The parameters of THF and TO molecules were generated by the LigParGen web server and that of Li⁺ and FSI⁻ were obtained from Jensen et al⁵³and Lopes et al⁵⁴, respectively. The Lorentz-Berthelot mixing rules were chosen to calculate the LJ parameters of the cross interactions. The molar ratios of the electrolytes were obtained from the experimental part of this work. The periodic boundary condition was applied to all three dimensions. The Particle Mesh Ewald (PME) method was used to calculate the long-range electrostatic interaction with a cut-off for a real space of 1.2 nm. The short-range van der Waals cut-off was set to be 1.2 nm.

The initial simulation boxes were constructed by Packmol⁵⁵. All the cases, after energy minimization, were first equilibrated in 20 ns NPT ensemble and 35 ns NVT ensemble, respectively, and every production run was performed for 5 ns in the NVT ensemble with 2 fs time step and saved every 0.2 ps. The temperature of the system was controlled by the Nosé-Hoover thermostat, and the simulated temperature was maintained at 233.15 K. The Berendsen pressure coupling regulated the system at 1 bar⁵⁶. The visualization was generated via the VMD package. The binding energy of Li⁺ containing internal term (ΔE_int), van der Waals (ΔE_vdW) and electrostatic (ΔE_ele) energies was calculated with Generalized Born model by gmx_MMPBSA⁵⁷.

Acknowledgements

This work was supported by National Natural Science Foundation of China (22479094, 22075174), the Science and Technology Commission of Shanghai Municipality (20520740900 and 19DZ2271100), and International Joint Laboratory on Resource Chemistry.

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Novel thermoresponsive ether-based electrolyte for wide-temperature operating lithium metal batteries

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Abstract

Figures

Introduction

Results

Design of the thermoresponsive electrolyte and temperature-dependent solvated structures

Electrolyte properties and interfacial dynamics

Effect of the designed electrolyte on Li plating/stripping at ultra-low temperatures

Interfacial Chemistry Study

Performance of practical Li-metal cells at wide temperature

Discussion

Methods

Materials

The preparation of electrolyte

Coin cells and pouch cells assembling

Electrochemical Testing

𝜎 =\(\:\frac{\varvec{L}}{\varvec{A}\ast\:\varvec{R}}\)

Material characterizations

Density functional theory calculations

Molecular dynamics simulations

Declarations

Acknowledgements

References

Additional Declarations

Supplementary Files

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