Constructing LiCl‐Rich Solid Electrolyte Interphase by High Amine‐Containing 1,2,4,5‐Benzenetetramine Tetrahydrochloride Additive

Strategies that aim to achieve highly stable lithium metal batteries (LMBs) are extensively explored. To date, the controlled formation of high‐quality inorganic SEI is still quite challenging, which requires a deep understanding and hence the fine‐tuning of solvation chemistry by using functional additives in the electrolyte. In this work, a high amine‐containing 1,2,4,5‐benzenetetramine tetrahydrochloride (BHCL) is developed as a dual‐function electrolyte additive for LMBs. The amine group with a high donor number increases the lithium affinity, while the phenyl group with a strong inductive effect prevents the decomposition of solvents, and the free chloride ions replace anions mediating the formation of the rigid inorganic LiCl‐rich SEI layer. The experimental results corroborate the theoretical findings. The modified Li||Li symmetric battery is stably cycled for over 2500 h at 1 mA cm−2 current density with an overpotential of ≈45 mV. The performances of the Li||Cu and Li||LFP cells are also significantly enhanced. Therefore, this work provides a promising design principle of multifunctional electrolyte additive.


Introduction
Lithium-ion batteries are widely used in various portable devices and being promoted in the next generation vehicles. [1]However, even after decades of development, their energy density (≈250 Wh kg −1 ) is still relatively low for many important applications.Lithium metal has a low redox potential (−3.04 V vs DOI: 10.1002/aelm.202300772standard hydrogen electrode) and a high theoretical specific capacity (3860 mAh g −1 ), making it an ideal anode material in next-generation batteries. [2]Despite the promises, unfortunately, it falls short in practical applications due to the low Coulombic efficiency, the severe lithium dendrite growth, and hence the limited lifespan. [3]he main mechanism behind the instability of lithium metal anodes is well, if not completely, understood.The highly reactive lithium metal can react with the non-aqueous liquid electrolyte, and a solid-electrolyte interface (SEI) is formed instantaneously on the metal surface. [4]Volume expansion of lithium during cycling causes the nonuniform SEI to crack.The exposed fresh lithium undergoes side reactions, together with the formation of new SEI.Eventually the electrolyte is depleted, and the electrodes are severely corroded, causing degradation and instability (such as thermal runaway) over time. [5]n recent year, it is further understood that the interaction between lithium ions, solvent molecules and anions will form a solvent sheath that might be a key to stabilizing lithium metal anodes. [6]The solvent sheath is reduced on diffusion to the lithium metal surface, leading to the formation of SEI dominated by organic lithium salts.The heterogeneous composition, poor lithium conductivity, and high-brittleness of the newly formed layer result in uneven lithium deposition and lithium dendrite growth. [7]trategies that aim to regulate the composition of solvent sheath have been extensively explored.One successful example is to introduce functional additives in the electrolyte, with a primary goal to form inorganic SEI layers that are generally more stable and effective at preventing dendrite growth and improving battery safety compared to organic SEI layers.For example, the addition of small amounts of LiNO 3 with high Li + affinity will lead to more NO 3− anions in the solvation sheath and hence a more favorable formation of inorganic SEI with abundant LiN x O y species. [8]1,3,5-triformylphloroglucinol can effectively adjust the structure of the solvent sheath, due to the hydrogen bonding between TFP and the anion of the lithium salt and the solvent. [9]With high donor number, N,N-diethyl-2,3,3,3tetrafluoropropionamide can introduce fluorinated chains into the solvation sheath and may decompose to form fluorides SEI. [10]hese successful examples demonstrate the feasibility of tailoring the solvent shell to achieve inorganic SEI.However, most inorganic SEI components are still dominated by LiF and Li 2 CO 3 . [11]In comparison, LiCl has a lower lithium ion diffusion barrier. [12]It has been reported that Cl atoms tend to float on the surface of lithium crystals rather than participate in the formation of dendrites. [13]By adding chloride to lithium metal batteries, dendrite growth can be effectively hindered and a stable SEI layer can be prepared to achieve long-cycle performance. [14]ere we report the first use a high-amine content 1,2,4,5benzenetetramine tetrahydrochloride (BHCL) that can efficiently regulate the solvent sheath and construct LiCL-rich inorganic SEI.With the four high donor number amine group, BHCL has a strong coordination ability with Li + and is involved in the electrical double layer near Li metal. [15]The chloride ion attached to the amine group then enters the solvent sheath and decomposes to form an inorganic LiCl-rich SEI layer.In addition, the-oretical calculations prove that the lowest unoccupied molecular orbital (LUMO) is reduced due to the strong inductive electronwithdrawing effect of the phenyl group. [16]herefore, a flat and dense inorganic LiCl-rich Li deposition layer can be achieved (see Figures 1 and 4).The improved Li||Li symmetric cell has a remarkable cycling stability for over 2500 h at a current density of 1 mA cm −2 with an overpotential of ≈45 mV.At a current density of 0.5 mA cm −2 , the Li||Cu battery still achieves a coulombic efficiency (CE) of 98.1% after 100 cycles, extending the cycle life of a lithium metal anode.The designed additive also shows great promise in Li||LFP full cells, where both the cycling stability and the cell polarization are significantly improved.

Results and discussion
We begin by discussing the design concept through theoretical calculations.It is common to describe the thermodynamic ability to accept a new electron by using LUMO energy.A low LUMO energy means that the additive molecule is more likely to accept electrons from the anode and helps to form a stable SEI layer.Figure 2a shows the LUMO energy of DOL (1,3-dioxolane), DME (1,2-dimethoxyethane), BHCL, and benzene in the electrolyte.The LUMO energy of benzene is the lowest, followed by that of BHCL, both of which are far lower than that of the typical solvent molecules DME and DOL.This proves that the introduction of a phenyl group can effectively reduce the LUMO energy.
Another important parameter is the donor number (DN).Solvents with a high DN are typically more polar and have a higher ability to donate electrons, and hence a higher solubility of the lithium salt in the electrolyte.Figure 2b exhibits the DN of the four materials.In our design, BHCL has four amine groups and hence has a much higher DN than that of the other three substances.The calculation results of the binding energy of Li + and four materials further confirmed the appeal argument (Figure 2c; Figure S1, Supporting Information).The binding energy of Li + with DME, DOL, and benzene ranged from −1.85 to −1.7 ev.After introducing several amine groups, Li + binding energy with BHCL decreased to −2.398 ev, indicating that amine group improve the coordination ability with Li + .Due to higher binding energy with Li + , BHCL can replace part of the solvent molecules in the original Li + solvent sheath and reduce the solvent molecules decomposition.
Figure 2e and Figure S2 (Supporting Information) is the electrostatic potential energy diagram of the four materials.Compared with the conventional solvents DME and DOL, the lowest electrostatic potential energy value of benzene is significantly higher, indicating that benzene belongs to an anti-solvent structure, [17] while BHCL has the lowest electrostatic potential energy after the introduction of four amine groups.The value is significantly reduced, and the amine groups on both sides have binding sites for Li + , indicating that the amine group is a key part of the introduction of BHCL into the Li + solvent sheath.
This result is further confirmed by the Li + radial distribution function (g(r), RDF) and the coordination numbers in the molecular dynamics simulations.We simulated the solvation environment with 1 m LiTSFI in DME/DOL (1:1 by volume) as the blank electrolyte (Figure 2g), and then added 0.5 wt% benzene and 0.5 wt% BHCL as the control group, respectively (Figure 2h,i; Figure S3, Supporting Information).In the blank electrolyte, the clear sharp peak corresponding to the DME-O-Li + pair appears at 2 Å.This first coordination sheath is also related to the formation of the SEI layer on the lithium metal surface.The peaks corresponding to the DOL-O-Li + and the TFSI − -O-Li + pairs are also shown but barely visible.The coordination numbers are also shown for reference.The RDF peaks show almost no change after the addition of the benzene.After the addition of the BHCL, two new RDF peaks (BHCL-N-Li pair, BHCL-C-Li pair) appear at 2.1 and 2.4 Å, respectively, and the coordination number of DME decreased from 5.8 to 5.5.
The snapshot of first sheath of Li + in different electrolyte is shown in Figure 2e and Figure S4 (Supporting Information), one BHCL can bind two Li + to form a stable solvation structure.In addition, we calculated the binding energies of Li + and anions under the three solvation structures (Figure 2d;Figure S5, Supporting Information), it can be seen that adding benzene has basically no effect on the solvation structure, and the binding energy of Li-TFSI is also maintained at −4.961 ev.However, after the addition of BHCL, the binding energy of Li + and TFSI − in the new solvent sheath structure is increased to −3.840 ev, and the binding energy of lithium ions and chloride ions under the structure is as high as −6.863 ev, indicating that the introduction of BHCL causes some chloride ions to replace TFSI − and participate in the SEI formation process (Figure S6, Supporting Information).Therefore, we can expect the formation of a rigid inorganic LiCl-rich SEI layer.
From the theoretical investigations, we can see that the LUMO energy, donor number, binding energy and hence the Li + solvation sheath in the electrolyte can be synergistically optimized by using the amine and phenyl groups in the BHCL additive.This design will therefore help to promote the formation of a stable and uniform LiCl-rich SEI layer, and to improve the solubility and ionic conductivity of the electrolyte.The experimental results in the following sections will strongly support these hypotheses.
Lithium symmetric cells were fabricated and tested to determine the optimal amount of BHCL as an electrolyte additive under various current capacity conditions (Figure S7, Supporting Information).As little as 0.5 wt% of BHCL in the electrolyte can result in significant increases in the cycling stability and stable hysteresis voltage.The overpotential began to increase when the BHCL additive content was increased to 2 wt%, and the electrolyte viscosity also increased significantly (Figure S8, Supporting Information).Therefore, the optimum content of BHCL additive in this work was set at 0.5 wt%.
To obtain the first impression of the overall effect of BHCL, Li||Li symmetric batteries were tested at different current densities.At a current density of 1 mA cm −2 and a deposition capacity of 1 mAh cm −2 , the Li||Li symmetric battery with conventional electrolyte exhibited significant fluctuations in its overpotential voltage.The voltage dropped abruptly after ≈1100 h of cycling, mainly due to the rapid formation of lithium dendrites and the internal short circuit in the battery.In comparison, the Li||Li symmetric cell with BHCL additive in the electrolyte demonstrates a stable voltage profile and an overpotential of 45 mV after 2500 h of cycling (Figure 3a).After cycling for 1000 h of cycling at a current density of 2 mA cm −2 , the cell still showed stable voltage curves with an overpotential of 76 mV (Figure 3b).On the contrary, the conventional electrolyte-based cell started to degrade after only 350 cycles.For the BHCL-based cell, it is even possible to cycle for 1000 h with an overpotential of ≈200 mV at a high current density of 5 mA cm −2 and a high deposition rate of 5 mAh cm −2 (Figure 3c).This is in our knowledge much better than the symmetrical cells based on any conventional electrolyte.In Table S1 (Supporting Information) the performance of BHCL was compared with that of other reported additives.It is clear to see that BHCL improves the overpotential and cycle stability significantly.
Furthermore, the charge-discharge curves of the BHCL-based symmetric cell exhibited a constant and symmetrical behavior (Figure 3d,e), a clear indication that the BHCL additive is beneficial for the Li + ionic transport and the suppression of lithium dendrites.Figure 3f compares the rate performance of the Li||Li symmetric cells.The stable overpotential at various current densities further illustrates the superiority of the BHCL additive.
The electrochemical impedance measurement of the symmetrical battery under different cycles is shown in Figure S9 (Supporting Information).The BHCL-based Li||Li symmetric cells demonstrated the lower interfacial resistance (Rs) and charge transfer resistance (Rct) than the conventional electrolytes-based cells.This means that the BHCL-induced SEI film enhances the interfacial stability and Li-ion transport during the repeated Li metal plating/stripping process.
The top-view and cross-sectional morphologies of the lithium metal anodes (from the Li||Li symmetric cells with different electrolytes) were obtained after 50 cycles and after 200 cycles (Figure 4;Figure S10, Supporting Information).The BHCL additive induces a very smooth surface of the lithium metal anode.Figure 4b, on the other hand, shows a rough surface in the case without the BHCL additive.According to the cross-sectional view (Figure 4c), there are no cracks in the lithium metal after the addition of BHCL additives.This is a clear indication that the BHCL-induced SEI layer experiences much less side reactions and its rigid structure effectively prevents excessive consumption of electrolytes as well as lithium metal.It is further proved by the shiny surface after cycling (see the inset photo in Figure 4b) in the case with BHCL-based electrolyte.Figure 4d,f shows the morphology of lithium metal anodes with conventional electrolyte.A large number of pore structures can be observed on the surface, which are associated with the formation of lithium dendrites.From the cross-sectional view, the lithium metal was severely corroded by the electrolyte and eventually collapsed (Figure 4f).To supplement the information, Figure 4g demonstrates the uniform distribution of Cl, N, and F elements on the lithium metal surface.These results unambitiously corroborate the theoretical findings that BHCL promotes the formation of SEI layers that are structurally stable and effective at preventing dendrite growth.
Our theoretical study of the solvent sheath suggests the formation of SEI layer with an inorganic-dominant character.To prove this, the lithium metal anodes of Li||Li symmetric cells (after 10 cycles) were analyzed by using XPS.The C 1s spectra (Figure 5a) showed the peaks at 284.8, 287, 289.7, and 290 e V, which belong to C─C, COR, COOR, and CO 3 2− , respectively.They are related to the derivatives of organic electrolytes.It is clear that the amount of organic components in the SEI layer is significantly reduced by the BHCL additive.The high-resolution XPS spectra of Li 1s (Figure 5b) and Cl 2p (Figure 5d) showed distinct peaks at 56.3 and 198.3 eV, respectively, illustrating the successful formation of the LiCl-rich SEI layer.As shown in the F 1s spectrum (Figure 5c), the SEI formed in the presence of the BHCL additive exhibited a strong Li─F peak (684.8 eV) but a negligible S─F peak (684.8 eV).In contrast, a prominent S─F peak was observed in the presence of BHCL additive, indicating less TFSI − involvement in SEI construction. [18]The TFSI − anions that originally participated in the SEI forming reaction will be replaced by Cl − ions.Therefore, the incorporation of BHCL indeed results in a LiCl-rich inorganic SEI layer, which, again, corroborates the theoretical findings.This is expected to suppress the Li dendrite growth and extend the lifetime of Li metal anodes.
Coulombic efficiency (CE) is defined as the ratio of lithium stripped from copper foil to lithium plated onto copper foil in the same cycle.It is a key indicator for measuring the reversibility and utilization of lithium metal during long-term cycling.Figure 6a shows the CE of Li||Cu cells with different electrolytes at a current density of 0.5 mA cm −2 and a deposition capacity of 1 mAh cm −2 .A remarkable difference here could be seen.The Li||Cu cell with conventional electrolyte showed a low and rapid decay CE (85.4% after 100 cycles).With the addition of BHCL, the CE was improved significantly to 98.1% and remained constant over 100 cycles.
Figure 6b shows the Li plating/stripping experiments for 40 h, using Aurbach's method [19] at a current density of 0.5 mA cm −2 and a deposition capacity of 2.5 mAh cm −2 .The average CE of the battery with BHCL additive (98.4%) was higher than that without additive (92.4%).The inset of Figure 6c depicts the initial Li plating/stripping curves of Li||Cu cells with different electrolytes.The lower lithium stripping voltage was observed for the Li||Cu batteries with BHCL additive, demonstrating that BHCL can effectively enhance the uniform deposition of lithium.Fur-thermore, during the Li plating/stripping process, the hysteresis voltage of Li||Cu cells with conventional electrolytes increased sharply to ≈590 mV after 100 cycles, while the overpotentials of Li||Cu cells with BHCL additive were at remained stable at ≈48 mV for 100 cycles (Figure 6c).This much reduced hysteresis voltage is attributed to the BHCL additive, which induces the formation of a stable and ionically conductive SEI on the Li surface, thus promoting the homogeneous deposition of Li metal.
To investigate the stability of BHCL additive at high voltages, the measured anodic polarization curves are shown in Figure 6d.In the case with the BHCL additive, the onset potential of electrochemical oxidation of aluminum foil was ≈5 V (vs to Li/Li + ) -significantly higher than that of the battery without additive.This suggests that BHCL increases the voltage window and prevents the corrosion of aluminum current collectors.The full-cell Li||NMC data further confirms this conclusion.(Figure S11, Supporting Information) When the voltage exceeds 4.3 V, the charging curve of the battery without BHCL becomes unstable, the capacity decays rapidly and the Coulombic efficiency fluctuates greatly.
Finally, to demonstrate the viability of BHCL additive in practical applications, Li||LiFePO 4 (LFP) full cells were assembled and tested at a current rate of 1 C (1 C = 170 mA g −1 ).As illustrated in Figures 6e,f, the performances of Li||LFP full cells were also substantially improved by the BHCL additive in the electrolyte.In the case with conventional electrolyte, the discharge capacity of the Li||LFP full cell was 128.8 mAh g −1 in the first cycle and showed a continuous decay over 200 cycles.With the BHCL additive, the discharge capacity of the first cycle was 135.1 mAh g −1 , and the capacity retention rate reached 90.2% after 200 cycles.

Conclusion
Designing multifunctional additives is a viable strategy to improve the structural integrity of SEI layers in lithium metal batteries.An ideal choice of additive should simultaneously improve the electrolyte stability, the lithium ionic conductivity, and promote the formation of a stable and uniform inorganic SEI layer on the lithium metal anode surface.However, this task is complex and challenging, hence requiring a deep understanding and fine-tuning of the solvation chemistry.
In this work, we report the first use of a combination of amine and phenyl functional groups that can efficiently regulate the solvent sheath and form inorganic LiCl-rich SEI.Theoretical studies unveil the different functionalities of amine and phenyl groups in the solvent sheath.The amine group utilizes its high donor number to tightly bind the Li + , while the phenyl group helps to lower the LUMO and induce the formation of chloride components.Their synergistic effect can promote the formation of an inorganic LiCl-rich SEI layer, reduce the number of free solvent molecules, and improve electrolyte stability.
The experimental results corroborate the theoretical findings.As expected, an inorganic-rich LiCl SEI was formed on Li metal due to the BHCL additive, which greatly enhanced the electrochemical performance of the Li metal anode.At 1 mA cm −2 current density, Li||Li symmetric cells can be stably cycled for more than 2500 h with a low overpotential of ≈45 mV, and 100 cycles with a coulombic efficiency of 95% at 0.5 mA cm −2 .
This work demonstrates the encouraging potential of multifunctional additives in tuning the solvation chemistry and hence, in stabilizing the lithium metal anodes.Among the vast library of functional groups, the selected combination of amine and phenyl groups is successful but not necessarily an optimum choice.This points to the importance of pre-screening and a priori prediction, that are assisted by theoretical investigations, [20] in the design of multifunctional electrolyte additive for highly stable lithium metal batteries.

Figure 1 .
Figure 1.Molecular structure of BHCL and its designing principle.

Figure 2 .
Figure 2. a) LUMO energy levels of DME, DOL, benzene, and BHCL molecules.b) Donor number of DME, DOL, benzene, and BHCL molecules.c) Binding energy of Li + -solvent molecules.d) Binding energy of Li + -anions molecules in different solvent structure.e) Electrostatic potential maps of BHCL molecule in vacuum conditions.f) Snapshot of the first sheath of Li + in BHCL electrolyte.The radical distribution function (continuous line) and the coordination number (dash line) in d) blank electrolyte, e) benzene electrolyte and f) BHCL electrolyte.

Figure 3 .
Figure 3. Cycling stability of Li||Li symmetric cells without and with BHCL as the electrolyte additive at current density of a) 1 mA cm −2 ; b) 2 mA cm −2 with deposition capacity of 1 mAh cm −2 and c) 5 mA cm −2 with deposition capacity of 5 mAh cm −2 .Voltage profiles of the Li||Li cells with d) BHCL and e) without any additive at certain cycles.f) The rate capacity of Li||Li cell without and with BHCL as the electrolyte additive with deposition capacity of 1 mAh cm −2 .

Figure 4 .
Figure 4. a-c) Top and cross-sectional SEM image of the Li metal anode with and d-f) without BHCL as the electrolyte additive after 50 cycles.g) the corresponding EDS mapping of the Li metal anode with BHCL as the electrolyte additive.

Figure 6 .
Figure 6.a) Coulombic efficiency of Li||Cu cells with or without BHCL electrolyte additive at current density of 0.5 mA cm −2 and deposition capacity of 1 mAh cm −2 .b) The Li plating/stripping voltage during long term cycling at 0.5 mA cm −2 and 2.5 mAh cm −2 .c) Comparison of the initial voltage profiles (inset figure) and voltage hysteresis profiles of the Li plating/stripping for the Li||Cu cells with different electrolytes at a current density of 0.5 mA cm −2 .d) Oxidation stability in Li||Al half cells detected by LSV.e-f) Performances of Li ||LFP full cells.e) Initial charging/discharging profiles; f) Cycling performance at current density of 1C.