Research Article | Open Access
Hongcai Gao, Nicholas S. Grundish, Yongjie Zhao, Aijun Zhou, John B. Goodenough, "Formation of Stable Interphase of Polymer-in-Salt Electrolyte in All-Solid-State Lithium Batteries", Energy Material Advances, vol. 2021, Article ID 1932952, 10 pages, 2021. https://doi.org/10.34133/2021/1932952
Formation of Stable Interphase of Polymer-in-Salt Electrolyte in All-Solid-State Lithium Batteries
The integration of solid-polymer electrolytes into all-solid-state lithium batteries is highly desirable to overcome the limitations of current battery configurations that have a low energy density and severe safety concerns. Polyacrylonitrile is an appealing matrix for solid-polymer electrolytes; however, the practical utilization of such polymer electrolytes in all-solid-state cells is impeded by inferior ionic conductivity and instability against a lithium-metal anode. In this work, we show that a polymer-in-salt electrolyte based on polyacrylonitrile with a lithium salt as the major component exhibits a wide electrochemically stable window, a high ionic conductivity, and an increased lithium-ion transference number. The growth of dendrites from the lithium-metal anode was suppressed effectively by the polymer-in-salt electrolyte to increase the safety features of the batteries. In addition, we found that a stable interphase was formed between the lithium-metal anode and the polymer-in-salt electrolyte to restrain the uncontrolled parasitic reactions, and we demonstrated an all-solid-state battery configuration with a LiFePO4 cathode and the polymer-in-salt electrolyte, which exhibited a superior cycling stability and rate capability.
The ever-increasing demand for long-lasting, portable electronic devices and long-range electric vehicles has stimulated extensive efforts to increase the safety and energy density of rechargeable batteries [1–3]. Methods to enable a safe lithium-metal anode are being explored actively to replace the graphite anode in conventional lithium-ion batteries [4–9]. However, lithium-metal anodes with organic-liquid electrolytes are impeded by the formation of an unstable solid-electrolyte interphase (SEI), the extensive growth of lithium dendrites, and a low plating/stripping efficiency during charge/discharge cycles [10–13]. The integration of solid-polymer electrolytes into all-solid-state cell configurations represents an intriguing strategy to suppress safety hazards associated with highly flammable organic electrolytes; the affordable cost and mechanical flexibility also allow solid-polymer electrolytes to be processed into desirable sizes and shapes with a broad range of fabrication techniques [14–17].
Departing from the numerous investigations on solid-polymer electrolytes with poly(ethylene oxide) and its derivatives, recent attention has shifted focus to polymers with nitrile groups, such as polyacrylonitrile (PAN) . The appealing properties of PAN that make it a promising matrix for solid-polymer electrolytes include a high dielectric constant, high oxidation potential, and strong coordination ability with lithium salts [19–21]. However, the ionic conductivity of PAN solid-polymer electrolytes is usually too low to meet practical requirements even at elevated temperatures. For conventional salt-in-polymer electrolytes, the cation-coordinating macromolecule is the primary component. In these electrolytes, migration of mobile ions occurs in amorphous regions and is assisted by segmental motions of the macromolecule host [22–24]. High ionic conductivities of such electrolytes can be obtained only at temperatures above their glass-transition temperature [25–27].
The related system of polymer-in-salt electrolyte with lithium salt as the major component exhibits much higher ionic conductivities than salt-in-polymer scheme by creating a rubbery version of a glass-like polymer electrolyte [28, 29]. The polymer-in-salt electrolytes, therefore, provide an ideal platform to combine the desirable mechanical properties of rubbery polymers with the fast-ionic conduction of glassy electrolytes. The extremely high salt content in the polymer-in-salt electrolytes promotes the formation of an ionic conducting network consisting of aggregated cation/anion clusters that are interconnected to provide percolation pathways for fast ion migration [30–32]. As a promising strategy to increase the ionic conductivity with respect to the salt-in-polymer scheme, the design of polymer-in-salt electrolytes has been actively explored recently, but very limited attention has been focused on the PAN system [33–35].
The other critical challenge towards the application of PAN solid-polymer electrolytes is the uncontrolled passivation reactions against lithium-metal anodes at the interface that lead to the formation of poor quality SEI layers, and consequently, inferior electrochemical performance of the all-solid-state batteries, including unsatisfactory coulombic efficiency and rapid capacity fading [36–39]. Nevertheless, the interfacial chemistry of the PAN polymer-in-salt electrolyte against a lithium-metal anode remains an important, but unexplored topic. We herein report a PAN polymer-in-salt electrolyte that facilitates the generation of a stable interphase layer against a lithium-metal anode owing to the high concentration of lithium salt. This interphase layer prevents successive parasitic reactions after formation, enabling long-term, dendrite-free plating/stripping of a lithium-metal anode. By integrating the polymer-in-salt electrolyte with a lithium-metal anode and a LiFePO4 cathode, an all-solid-state lithium battery possessing stable cycling performance and high rate capability is demonstrated.
2. Results and Discussion
The polymer-in-salt electrolyte membrane was prepared with a solution-casting method. A mixture of PAN, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and SiO2 nanoparticles in dimethylformamide was cast on a porous cellulose membrane; the molar ratio of the acrylonitrile monomer in PAN and the lithium salt of LiTFSI was kept at 1 : 1, which stabilized the prepared polymer electrolyte in the polymer-in-salt state [40, 41]. The SiO2 nanoparticles accounted for 5% of the total weight of the solid-polymer electrolyte. The thickness of the resulting polymer-in-salt electrolyte membranes was 100-120 μm, having a relatively smooth surface when analyzed with scanning electron microscopy (SEM), and X-ray diffraction verified that the polymer-in-salt electrolyte was amorphous (Figure S1). With the resulting polymer-in-salt electrolyte membrane, we fabricated all-solid-state lithium cells with LiFePO4 as the cathodes and lithium metal as the anodes (Figure 1(a)), and we found that a stable interphase can be formed between the electrolyte and the lithium-metal anode during repeated charge/discharge cycles to enable dendrite-free plating/striping of lithium across the polymer-in-salt electrolyte (Figure 1(c)).
Symmetric cells with stainless-steel blocking electrodes were used for electrochemical impedance spectroscopy (EIS) measurements to obtain the ionic conductivities of the polymer electrolytes. The influence of lithium salt concentration on ionic conductivity of the polymer electrolytes was first investigated. The ionic conductivity of the PAN polymer electrolytes steadily increased at 60°C with increasing the lithium salt concentration (Figure S2). A polymer-in-salt electrolyte with 84% wt.% LiTFSI and 16 wt.% PAN exhibited an ionic conductivity that was about 5 orders of magnitude higher than that of a salt-in-polymer electrolyte with 21% wt.% LiTFSI and 79 wt.% PAN. Compared to the conventional polymer electrolyte with a low ionic conductivity arising from the segmental motion of polymer chains, the increased ionic conductivity in the polymer-in-salt electrolyte can be attributed to the motion of Li+ ions through the ion-conducting network formed by aggregated ionic clusters arising from a high lithium salt concentration (Figure 1(b)) [42, 43]. As an ionic conductivity in the range of 10-4 S cm-1 is needed for an electrolyte to power a battery , the salt-in-polymer electrolyte has an ionic conductivity too low to enable the operation of an all-solid-state cell. The polymer-in-salt electrolyte with 84% wt.% LiTFSI and 16 wt.% PAN was, therefore, used in the following experiments. The temperature dependence of ionic conductivity for the polymer-in-salt electrolyte was further investigated, which follows an Arrhenius behavior , exhibiting an ionic conductivity of S cm-1 at 30°C and progressively increasing to S cm-1 at 60°C (Figure 2(a)). According to the Arrhenius equation, the activation energy was calculated to be 23.1 kJ mol-1, which is lower than that of solid-polymer electrolytes with poly(ethylene oxide) (~38 kJ mol-1) . This lower activation energy indicates the formation of conduction network consisting of aggregated ionic clusters which can promote the transport of Li+-ions in the polymer-in-salt electrolyte, whereas Li+-ion transport in conventional polymer electrolytes is assisted by the segmental motion of polymer chains in the amorphous regions [47, 48].
The electrochemical window of the polymer-in-salt electrolyte was evaluated with cyclic voltammetry and linear sweep voltammetry measurements of cells consisting of a stainless-steel working electrode and a lithium-metal reference electrode. The plating/stripping behavior of lithium metal can be seen in the cyclic voltammogram (Figure 2(b)). The strong reduction peak at -0.47 V (vs. Li+/Li) corresponds to lithium plating on the stainless-steel working electrode and the oxidation peak centered at 0.32 V (vs. Li+/Li) corresponds to lithium stripping from the working electrode. The comparable peak intensities of the reduction and oxidation currents suggests that lithium can be plated/stripped reversibly across the polymer-in-salt electrolyte onto/from the working electrode . As indicated in the linear sweep voltammetry profile, the polymer-in-salt electrolyte possesses an electrochemical window stable up to 4.8 V (vs. Li+/Li), only showing an oxidation current upon reaching that voltage. Thus, the electrochemical stability window of the polymer-in-salt electrolyte is wide enough to meet the requirements of most prime cathode candidates for lithium-based batteries in conjunction with a lithium-metal anode [50, 51]. This oxidation stability can be attributed to the high oxidation potential of PAN and its strong complexation interactions with the lithium salts within the polymer-in-salt electrolyte.
Most solid-polymer electrolytes used in lithium batteries exhibit a low Li+-ion transference number (typically smaller than 0.3) . When polarizing a cell with a binary electrolyte, the Sand’s time, which indicates the ionic concentration of the electrolyte dropping to zero on the electrode surface to induce parasitic reactions of the electrolyte and nucleation of lithium dendrites by the formation of a local space charge, is inversely proportional to the anion transference number squared [53, 54]. The depletion of Li+ ions on the electrode surface because of a low Li+-ion transference number will induce decomposition reactions of the electrolyte rather than the plating/stripping reactions of the lithium-metal anode. Therefore, it is a promising strategy to reduce the decomposition of electrolyte and suppress the nucleation of lithium dendrites by using an electrolyte with a high Li+-ion transference number [55, 56]. The potentiostatic polarization method was used to obtain the Li+-ion transference number of the polymer-in-salt electrolyte , with EIS measurements performed immediately before and after a small polarization of 10 mV was applied to a lithium-lithium symmetric cell. The results of this experiment are presented in Figure 2(c), and the Li+-ion transference number of the polymer-in-salt electrolyte was calculated to be 0.47, which is higher than that of the typical solid-polymer electrolytes . This high Li+-ion transference number may be attributed to the formation of aggregated ionic clusters within the polymer-in-salt electrolyte, significantly restricting the movement of the large [TFSI]- anions and promoting only Li+-ion transfer owing to its smaller size [59, 60]. With this high Li+-ion transference number, the polymer-in-salt electrolyte is expected to reduce the polarization and increase the rate performance in all-solid-state lithium batteries .
Interfacial stability of the polymer-in-salt electrolyte against the lithium-metal electrode was investigated with EIS of a lithium-lithium symmetric cell at different time intervals. The intercepts at the real-axis in the EIS spectra at high frequencies represent the bulk resistances of the polymer-in-salt electrolyte, which remained constant throughout 30 days of storage (Figure S3). The semicircles at low frequencies represent the interfacial resistances between the polymer-in-salt electrolyte and the lithium-metal electrodes . Initially, the initial interfacial resistance was 257 Ω cm2 for a fresh lithium-lithium symmetric cell. After 10 days, it increased to 293 Ω cm2 before asymptotically stabilizing in the following days (Figure 2(d)). The stabilization of interfacial resistance after 10 days of storage suggests the formation of a stable interphase layer on lithium-metal surface in contact with the polymer-in-salt electrolyte.
Galvanostatic cycling of a lithium-lithium symmetric cell at varying current densities was performed to further probe the interfacial stability of the polymer-in-salt electrolyte against lithium-metal electrodes upon repeated plating/stripping. A considerable amount of lithium was transported in the symmetric cell during each cycle to increase the possibility of an internal short-circuit caused by the lithium dendrite proliferation. At a current density of 0.1 mA cm-2, the symmetric cell with the polymer-in-salt electrolyte exhibits stable lithium plating/stripping behavior for 200 h without a short circuit (Figures 3(a) and 3(b)). The slight decrease in polarization voltage in the initial cycles indicates the in situ formation of a Li+-conducting electrode/electrolyte interphase that allows a stable cycling voltage for the duration of the experiments [63–65]. When the current density increased to 0.2 mA cm-2, the symmetric cell retains stable lithium plating/stripping behavior without a short circuit for 200 h (Figures 3(c) and 3(d)). These results demonstrate the effectiveness of the polymer-in-salt electrolyte in preventing lithium dendrite growth, thus providing a safe pathway for implementation of lithium-metal anodes in all-solid-state cell configurations.
To evaluate full-cell performance of the polymer-in-salt electrolyte, all-solid-state cell configurations with lithium-metal as the anodes and LiFePO4 as the cathodes were fabricated. Electrochemical performance of these cells was evaluated at 60°C with galvanostatic charge/discharge cycling. The charge/discharge profiles of the resulting cell exhibit the typical voltage profile of the LiFePO4 cathode with a plateau centered at 3.4 V (vs. Li+/Li) . The high reversibility of the all-solid-state cell is indicated by the equivalence of the charge and discharge capacities at each current rate (Figure 4(a)). The specific discharge capacity of the cell with respect to the cathode was 142 mAh g-1 at 0.05 C with a coulombic efficiency of 98.6%. A coulombic efficiency below 100% indicates the formation of a passivation layer on the electrode surface in the initial cycles. This interfacial layer could prevent further parasitic reactions during the prolonged cycling of the cell but also adds an additional element to the cell impedance, causing a relatively lower coulombic efficiency upon extended cycling than desired [67, 68]. After increasing the rate to 1 C, the all-solid-state cell possessed a specific discharge capacity of 83 mAh g-1, keeping 58.5% of the capacity from 0.05 C (Figure 4(b)). The specific discharge capacity of the all-solid-state cell was 56 mAh g-1 after increasing the rate to 2 C, demonstrating a good rate capability compared to all-solid-state lithium cells with other solid-polymer electrolytes [69, 70]. Moreover, the all-solid-state battery showed stable long-term cycling at 0.2 C rate with 95.1% first-cycle capacity retention after 100 charge/discharge cycles (Figure 4(c)).
The integrity of the electrode/electrolyte interface upon repeated cycling was investigated with EIS before and after undergoing 100 charge/discharge cycles (Figure 4(d)). An equivalent circuit was used to simulate the EIS spectra (Figure S4) . Only a slight increase of interfacial resistance was observed after 100 charge/discharge cycles (Figure S5). This result corroborates the formation of a mechanically robust and electrochemically stable interphase between the polymer-in-salt electrolyte and the electrodes that does not deteriorate upon repeated cycling. The surface morphology of the lithium-metal anode after 100 charge/discharge cycles in the all-solid-state cell was investigated with SEM (Figure S6), showing a relatively smooth surface without the formation of dendrites. The formation of a stable interphase between the polymer-in-salt electrolyte and the lithium-metal electrode is proven to reduce the resistance of Li+-ion transfer across the interface and enable a uniform distribution of Li+-ion flux to inhibit the generation of a localized electric field and suppress the nucleation of lithium dendrites [72, 73].
The chemical composition of lithium-metal anode surface in contact with the polymer-in-salt electrolyte during cycling was investigated after 100 charge/discharge cycles with X-ray photoelectron spectroscopy (XPS). The all-solid-state cell was disassembled in an argon-filled glove box, and the lithium-metal electrode was transferred to the XPS analysis chamber through a sample-transfer device without exposing the sample to air . The C 1s spectrum (Figure 5(a)) of the cycled lithium-metal electrode suggests the presence of C-F (288.9 eV), C-O (286.3 eV), and C-C (285.1 eV) species, which are generally derived from the degradation of PAN and the LiTFSI salt . The Li 1s spectrum (Figure 5(b)) indicates the presence of LiF (55.4 eV) and Li2O (54.1 eV) species . In accordance with the Li 1s and C 1s spectra, the F 1s spectrum (Figure 5(c)) confirms the presence of LiF (684.9 eV) and C-F (688.2 eV) species . The N 1s spectrum (Figure 5(d)) suggests the formation of Li-N (398.7 eV) and N-S (396.8 eV) bonds, which are related to the decomposition products of LiTFSI salt in the forms of Li3N and Li2NSO2CF3 [78, 79]. The formation of LiF and Li3N in the interphase may play an important role in modulating the plating/stripping behavior of lithium, taking the advantages of the high surface energy and/or low activation barrier for Li+ ions to migrate across the interphase, enabling dendrite-free operation of the electrode [80–83]. In addition to preventing lithium dendrites, this interphase layer also inhibits successive deterioration of the polymer-in-salt electrolyte by the lithium-metal anode once it has formed. As a result, the all-solid-state battery exhibits stable cycling even after an extended number of charge/discharge cycles.
In order to overcome the challenges of conventional salt-in-polymer electrolytes based on PAN that have low ionic conductivity and experience severe parasitic reactions with lithium-metal electrode at the interface, we have investigated polymer-in-salt electrolytes consisting of PAN for all-solid-state battery configurations. The polymer-in-salt electrolyte with the lithium salt of LiTFSI as the major component exhibits a high ionic conductivity and an increased lithium-ion transference number compared to traditional solid-polymer electrolytes. Additionally, the polymer-in-salt electrolyte suppresses effectively nucleation and proliferation of lithium dendrites through the formation of a stable interphase layer. By integrating the polymer-in-salt electrolyte with a lithium-metal anode and LiFePO4 cathode, the resulting all-solid-state cell operating at 60°C exhibited good rate capability up to 2 C rate and excellent capacity retention after an extended number of galvanostatic charge/discharge cycles at 0.2 C rate. These performance metrics make this PAN-based polymer-in-salt electrolyte a promising strategy to enable the all-solid-state configuration of lithium-metal batteries of increased safety and energy density.
4. Materials and Methods
Experimental details including the preparation and characterization of the polymer electrolytes and all-solid-state lithium batteries can be found in Supplementary Materials.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this article.
H.G. and J.B.G. conceived this project. H.G. designed experimental procedures. H.G., N.S.G., Y.Z., and A.Z. contributed to the materials preparation, experimental measurements, and data analysis. H.G., N.S.G., and J.B.G. cowrote the manuscript. J.B.G. supervised the project.
The preparation and characterization of the polymer electrolytes and the all-solid-state lithium batteries was supported by the United States Department of Energy, Office of Basic Energy Sciences (DE-SC0005397). J.B.G. also acknowledges support from the Robert A. Welch Foundation (F-1066).
Materials and methods. Figure S1: characterization of the polymer-in-salt electrolyte membrane. Figure S2: ionic conductivities of the PAN-based polymer electrolytes as a function of the concentration of lithium salt LiTFSI. Figure S3: electrochemical impedance spectra of the lithium-lithium symmetric cell with the polymer-in-salt electrolyte at different time of storage. Figure S4: equivalent electric circuit used to fit the electrochemical impedance spectra of the all-solid-state battery. Figure S5: the interface resistance and charge-transfer resistance of the all-solid-state battery before and after the charge/discharge cycling test. Figure S6: SEM image of the surface morphology of lithium-metal anode from the all-solid-state cell after the charge/discharge cycling test. (Supplementary Materials)
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