Anisotropic ion transport in nanostructured solid polymer electrolytes

<p indent="0mm">Since the commercialization of lithium-ion batteries in the 1990s, lithium-ion batteries have been successfully applied in portable electronics, electric vehicles, and grid energy storage. Although current organic liquid electrolytes have high ionic conductivities, they are inherently flammable, volatile, and prone to leakage. Moreover, severe side reactions and dendrite growth on the surface of the lithium anode during the charge-discharge process can cause safety hazards, which greatly impede their applications in lithium metal batteries. Solid electrolytes, including inorganic solid electrolytes and polymer electrolytes, are regarded as effective alternatives to organic liquid electrolytes for the construction of lithium metal batteries with high energy density and safety. Among them, solid polymer electrolytes offer excellent flexibility, processability, and interfacial compatibility over inorganic solid electrolytes, and they are extraordinarily promising for lithium metal batteries with high energy density and safety. Ideal solid polymer electrolytes should have following features: (1) High ionic conductivity (&gt; <sc>10 ^–4 S cm ^–1) </sc> at room temperature; (2) high lithium ion transference number (~1) to reduce the concentration polarization and improve the rate performance of batteries; (3) intimate contact at the electrode/electrolyte interfaces; (4) wide electrochemical window <sc>(&gt;4.5 V</sc> vs. Li/Li ⁺) to match high-voltage cathodes and improve the energy density of batteries; (5) good mechanical stability to resist processing, buffer electrode volume change and inhibit dendrite growth; (6) good thermal stability to withstand environmental changes. Generally, the ionic conductivity of pure solid polymer electrolytes at room temperature is low <sc>(~10 ^–6 S cm ^–1). </sc> Researchers have tried to improve the ionic conductivities by adjusting the lithium salt concentration, such as developing “polymer-in-salt” solid electrolytes. However, increasing the concentration of lithium salt leads to the deterioration of the mechanical strength. Strategies such as developing novel lithium salts, modifying polymer matrix, and incorporating inorganic fillers into solid polymer electrolytes are proposed to promote ionic conductivities of solid polymer electrolytes. In particular, composite polymer electrolytes, fabricated by dispersing a certain amount of inorganic fillers into solid polymer electrolytes, have improved ionic conductivities without sacrificing their mechanical performances. Poor interfacial property between electrodes and electrolytes is also a critical issue for solid polymer electrolytes. On one hand, poor and uneven solid/solid contacts at the electrode/electrolyte interfaces lead to high resistance and sluggish ionic transport kinetics. Furthermore, the volume change of the positive and negative electrodes in the charge/discharge process deteriorates the interfacial contacts, blocks the ion and electron transport through the interfaces, and greatly reduces the electrochemical reaction kinetics. On the other hand, the electrochemical windows of solid polymer electrolytes are usually narrow <sc>(＜4.5 V).</sc> During cycling, redox reactions are prone to occur at the electrode/electrolyte interfaces, causing battery failure. Solid polymer electrolytes have also poor thermal and mechanical stabilities. Therefore, design and synthesis of polymer-based solid electrolytes with excellent comprehensive performances and construction of fast and stable ion transport channels at the electrolyte/electrode interfaces are of great significance for the successful development of solid-state lithium metal batteries. This paper presents a brief review of the research progress in solid polymer electrolytes from two aspects: Improving the ionic conductivities of solid polymer electrolytes and enhancing the interfacial performance at electrolyte/electrode interfaces. First, targeted optimization strategies on ionic conductivities of solid polymer electrolytes, including constructing continuously aligned ionic transport paths and shortening the ionic transport distance, are summarized. Second, interface optimization strategies, including constructing wetting interfaces and synthesizing asymmetric electrolytes, are presented to reduce the interface resistance and improve the interfacial contact. Finally, perspectives on the development of solid polymer electrolytes and high-performance solid-state lithium metal batteries are discussed, and key research directions and advanced test methods are proposed. This review may provide a comprehensive understanding and further guidance for not only the material design of solid polymer electrolytes, but also the structural design of lithium metal batteries with favorable electrochemical and interfacial performances.

Implementation of solid polymer electrolytes (SPEs) is believed to be one pathway forward in the development of lithium-metal batteries (LMBs) that are both safe and possesses high capacities. This combination is possible since the free-standing SPE maintains its mechanical integrity during electrochemical and thermal exposure while utilizing lithium metal as the negative electrode, thus accessing the extremely high theoretical capacity of lithium metal. The main drawback for SPEs are however the poor ionic conductivity compared to the liquid electrolytes used in lithium-ion batteries today.[1] The low ionic conductivity for SPEs stems from the conduction mechanism of the ions, which is a slower process in SPEs compared to in liquid electrolytes. In general, the conductivity in SPEs are confined to the amorphous regions, where segmental motion of the polymer chains facilitates the ion transport; while the crystalline regions restrict the movements of the polymer chains and impairs the ion transport.[2] The highly crystalline polyketone poly(1-oxoheptamethylene) (POHM) is a material that has shown interesting characteristics for implementation as a SPE in batteries. In earlier studies, the polyketone revealed an extensive improvement of the ionic conductivity upon increasing the salt concentration, although the crystallinity remained with a high melting point.[3] Herein this study, the effect of the crystallinity on the characteristics and the performance was investigated further. To understand its effect, a second less hydrogenated version of POHM was synthesized, POHM-75, which is fully amorphous at high salt concentrations. The characteristics of the polyketones were in addition also compared with the more well-studied SPEs, that is polyethylene oxide (PEO) and poly(trimethylene carbonate) (PTMC). Focus of the presentation is on the interplay between the ionic conductivity and mechanical stability for the polyketones, for which something different to the normally observed tradeoff between the two properties is observed. The electrochemical performance of the polyketones are furthermore evaluated when implemented as SPEs in LMBs at extreme temperature environments, a possible application area for mechanically stable SPEs.[1] D. Zhou, D. Shanmukaraj, A. Tkacheva, M. Armand, G. Wang, Chem 2019, 5, 2326-2352.[2] M. A. Ratner, P. Johansson, D. F. Shriver, MRS Bulletin 2011, 25, 31-37.[3] T. Eriksson, H. Gudla, Y. Manabe, T. Yoneda, D. Friesen, C. Zhang, Y. Inokuma, D. Brandell, J. Mindemark, Macromolecules 2022, 55, 10940-10949. Figure 1

Cubic garnet solid polymer electrolyte for room temperature operable all-solid-state-battery

Lithium dendrite formation and low ionic conductivity of solid polymer electrolytes (SPEs) are the main challenges that plague the long cycle life and capacity retention of lithium‐metal batteries. To address these challenges, poly(crown ether piperidinium) (PCEP)/poly(ethylene oxide) (PEO) based SPEs with synergistic regulation of cation−anion functions are designed and prepared. The crown ether moieties within PCEP enable coordination with Li + , reducing the Li + coordination number and enhancing the ionic conductivity of SPEs to 4.2 × 10 −4 S cm −1 at 60°C. Concurrently, piperidinium units effectively immobilize TFSI − ions through electrostatic adsorption, resulting in an elevated Li + transference number (0.7) and a concomitant reduction in lithium dendrite growth. Consequently, a coin cell using a PCEP‐based SPE with a lithium iron phosphate cathode displayed a high cycle life of 2000 cycles at 1.0C with a commendable final discharge capacity of 117.9 mAh g −1 , corresponding to 79.4% capacity retention. Furthermore, lithium−sulfur coin cells utilizing PCEP‐based SPEs exhibit a specific discharge capacity of 671.8 mAh g −1 after 80 cycles at 0.2C, corresponding to 80.7% of the initial capacity, outperforming SPEs without PCEP. This research work introduces a synergistic cation−anion regulation strategy for the advancement of solid‐state energy conversion/storage systems.

Correlations among the structure, thermal properties, and ionic conductivity of solid polymer electrolytes (SPEs) were studied using a ureasil–polyethylene oxide (U-PEO) organic–inorganic hybrid prepared according to a simple sol–gel route, employing a low molecular weight PEO macromer (Mw = 1900 g mol−1). The behavior of an amorphous sample loaded with lithium triflate (LiTFSI) at an optimum ratio between ether oxygen and lithium (EO/Li+ = 15) was compared with that of a semicrystalline sample prepared without salt loading. The temperature range investigated by differential scanning calorimetry (DSC), Raman spectroscopy, small angle X-ray scattering (SAXS), and complex impedance spectroscopy covered both the glass transition and the melting temperature of the U-PEO. The gauche to trans conformational transformation of the (O–C–C–O)Li+ sequence showed similarity between the temperature evolution of the semi-crystalline U-PEO and amorphous U-PEO:Li+ samples, providing an indication of the local structural memory of crystalline state in the amorphous SPE. The linear thermal expansion of the average correlation distance between the siloxane crosslink nodes and the long-distance period of the lamellar semi-crystalline edifice were determined by SAXS. Comparison of the expansion curves suggested that although the siloxane nodes were excluded from the PEO crystalline edifice, the sharp expansion of the amorphous region between the lamellae during melting permitted modulation of the free volume of the hybrid network. In addition, the temperature-induced Li+-EO decomplexation observed by Raman spectroscopy explained the change of the average activation energy of the conduction process revealed by the different Arrhenius regimes. These results evidence the key role of the ionic conductivity decoupling from the segmental motion of chain pair channels on the improvement of ion mobility through the free volume between chains. This concept may inspire materials chemistry researchers to design optimized structures of polymer electrolytes with minimized structural memory of crystaline building blocks and improved ionic conductivity.

Control of ionic conductivity in solid polymer electrolyte by photoirradiation

A simple low-cost method to prepare gel electrolytes incorporating graphene oxide with increased ionic conductivity and electrochemical stability

The transparent and flexible solid polymer electrolytes (SPEs) were fabricated from polyacrylonitrile‐polyethylene oxide (PAN‐PEO) copolymer which was synthesized by methacrylate‐headed PEO macromonomer and acrylonitrile. The formation of copolymer is confirmed by Fourier‐transform infrared spectroscopy (FTIR) measurements. The ionic conductivity was measured by alternating current (AC) impedance spectroscopy. Ionic conductivity of PAN‐PEO‐LiClO4 complexes was investigated with various salt concentration, temperatures and molecular weight of PEO (Mn). And the maximum ionic conductivity at room temperature was measured to be 3.54 × 10−4 S/cm with an [Li+]/[EO] mole ratio of about 0.1. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 461–464, 2006

The development of high-performance solid polymer electrolytes is crucial for producing all-solid-state lithium metal batteries with high safety and high energy density. However, the low ionic conductivity of solid polymer electrolytes and their unstable electrolyte/electrode interfaces have hindered their widespread utilization. To address these critical challenges, a strong Lewis acid (aluminum fluoride (AlF3)) with dual functionality is introduced into polyethylene oxide) (PEO)-based polymer electrolyte. The AlF3 facilitates the dissociation of lithium salt, increasing the iontransfer efficiency due to the Lewis acid-base interaction; further the in-situ formation of lithium fluoride-rich interfacial layer is promoted, which suppresses the uneven lithium deposition and continuous undesired reactions between the Li metal and PEO matrix. Benefiting from our rational design, the symmetric Li/Li battery with the modified electrolyte exhibits much longer cycling stability (over 3600 h) than that of the pure PEO/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte (550 h). Furthermore, the all-solid-state LiFePO4 full cell with the composite electrolyte displays a much higher Coulombic efficiency (98.4% after 150 cycles) than that of the electrolyte without the AlF3 additive (63.3% after 150 cycles) at a large voltage window of 2.4–4.2 V, demonstrating the improved interface and cycling stability of solid polymer lithium metal batteries.

A molecular thermodynamic model based on the theory of melting point depression, the Adam–Gibbs configurational entropy model, and the modified lattice‐fluid model was developed to interpret ionic conductivities of solid polymer electrolytes with various pressures. A new configurational entropy, S was introduced in this model. Quantitative descriptions according to the proposed model agreed with the experimental data for the given systems. Our results showed that a eutectic point moves toward higher melting temperatures, whereas a lower weight fraction of salt and the ionic conductivity significantly decrease with pressure. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 706–713, 2002

In this work, we have investigated the effects of 2-amino-4,6-dimethoxypyrimidine on polyvinylidene fluoride/potassium iodide/iodine (PVDF/KI/I2) in dye-sensitized solar cells (DSSC). Different weight ratios (0%, 10%, 20%, 30%, 40% and 50%) of 2-amino-4,6-dimethoxypyrimidine-doped PVDF/KI/I2-based solid polymer electrolytes (SPEs) were prepared by solution-casting method. The prepared 2-amino-4,6-dimethoxypyrimidine-doped PVDF/KI/I2-based SPEs were characterized by powder x-ray diffraction (PXRD) analysis, AC impedance analysis and scanning electron microscopy (SEM) analysis. The crystallinity of 2-amino-4,6-dimethoxypyrimidine-doped PVDF/KI/I2-based SPEs was confirmed by PXRD measurement. The ionic conductivity of 2-amino-4,6-dimethoxypyrimidine-doped PVDF/KI/I2-based SPEs was calculated using AC impedance analysis. The ionic conductivity values of different weight ratios (0%, 10%, 20%, 30%, 40% and 50%) of 2-amino-4,6-dimethoxypyrimidine-doped PVDF/KI/I2-based SPEs are 5.50 × 10−6 S cm−1, 1.74 × 10−5 S cm−1, 4.91 × 10−5 S cm−1, 2.04 × 10−5 S cm−1, 1.58 × 10−5 S cm−1 and 1.04 × 10−5 S cm−1, respectively. Ionic conductivity studies revealed that the 20% 2-amino-4,6-dimethoxypyrimidine-doped PVDF/KI/I2-based SPE showed the highest ionic conductivity value. The SEM images show the surface morphology of 2-amino-4,6-dimethoxypyrimidine-doped PVDF/KI/I2-based SPEs. The power conversion efficiency (PCE) of DSSCs utilizing different weight ratios (0%, 10%, 20%, 30%, 40% and 50%) of 2-amino-4,6-dimethoxypyrimidine-doped PVDF/KI/I2-based SPEs are 1.4%, 2.0%, 2.5%, 2.3%, 1.9% and 1.6%, respectively. These results revealed that the DSSC using 20% 2-amino-4,6-dimethoxypyrimidine-doped PVDF/KI/I2-based SPE exhibited the highest PCE.

Solid polymer electrolytes (SPEs)─polymer melts with added salts─exhibit ion conduction and high mechanical properties, and are thus promising materials for future energy storage devices. The ion conductivity in an SPE is intricately connected to the salt ion distribution in the polymer matrix. The relationship between ion diffusion and ion distribution in SPEs remains unresolved. Here, we conduct coarse-grained molecular dynamics simulations and establish correlations between ion distribution and transport for a phenomenological SPE model. We propose phase diagrams of SPEs as a function of ion pair size, ion concentration, and the Bjerrum length of the material. A crossover from a discrete ion aggregate to a percolated ion aggregate is demonstrated as a function of ion pair size for low ion concentration in the SPE. The ion diffusion shows a strong correlation with its size, as has been found experimentally. The work provides important design strategies for controlling the ion distribution and enhancing ion conductivity in a polymer matrix.

The practical application of lithium (Li) metal battery is impeded by the Li dendrite growth and unstable solid electrolyte interphase (SEI) layer. Herein, an ultra‐stretchable and ionic conducting chemically crosslinked pressure‐sensitive adhesive (cPSA) synthesized via the copolymerization of 2‐ethylhexyl acrylate and acrylic acid with poly(ethyleneglycol)dimethacrylate as crosslinker (short for 70cPSA), is developed as both artificial SEI layer and solid polymer electrolyte (SPE) for stable Li‐metal electrode, enabling all‐solid‐state Li metal batteries with excellent cycling performance. As an artificial SEI layer, the 70cPSA‐modified electrodes exhibit excellent electrochemical performance in Li|70cPSA@Cu half cells and 70cPSA@Li|70cPSA@Li symmetric cells. In full cells with LiFePO4 (LFP) as cathode, the 70cPSA@Li|LFP cell exhibits stable cycling performance over 250 cycles. Utilized as SPE, the all‐solid‐state Li|SPE|LFP cell delivers excellent cycling stability with a capacity retention of 86% over 500 cycles. With high‐voltage LiNi0.8Mn0.1Co0.1O2 (NMC811) as cathode, the Li|SPE|NMC811 cell exhibits a discharge capacity of 124.3 mAh g−1 with a capacity retention of 71% after 200 cycles. The rational design of PSAs and investigation of their dual role for stable and safe Li‐metal batteries may shed a light on adhesive polymers for battery applications.

Jeffamine® based polymers as highly conductive polymer electrolytes and cathode binder materials for battery application

Rechargeable lithium-ion batteries take advantage of solid-state ionic conductors, such as solid polymer electrolytes (SPEs) [1, 2]. Despite the large technological potential of solvent-free SPEs, they suffer from a serious problem: a relatively low ionic conductivity. The addition of ionic liquids (ILs) to SPEs appears as a possible solution due to the enhancement of electrochemical stability (~4-6 V) and ionic conductivity (~10-3 – 10-2 S.cm-1) [3].ILs has two important features for their specific use in battery applications: high concentration of ions and high ion mobility at room temperature. The use of PVDF polymer and its copolymers together with ILs as SPEs still has to be thoroughly explored.This work shows the thermal and electrochemical properties of SPEs based on poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE), and specific amounts (4wt% to 32wt%) of the ionic liquid N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, [N1 1 1 2(OH)][NTf2]. The SPE was prepared by thermaly induced phase separation (TIPS).The addition of ILs in P(VDF-TrFE) affects the microstructure, thermal stability and ionic conductivity of the polymer membrane.The ionic conductivity increases as the ionic liquid (IL) content increases with a maximum value at room temperature of 1.7x10-5 S.cm-1for an IL composition of 32wt%. The temperature behavior of the ionic conductivity follows the Arrhenius equation and the electrical response of the membrane is well represented by a Randles modified equivalent circuit. The results prove that the developed SPEs are adequate for energy storage devices. Acknowledgments This work was supported by FEDER through the COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Project PEST-C/FIS/UI607/2011, PEST-C/QUI/UI0686/2011, PEST-OE/EQB/LA0004/2011, projects PTDC/CTM-NAN/121274/2010 and PTDC/QUI-QUI/117340/2010, grants SFRH/BD/68499/2010 (C.M.C.) and SRFH/BD/90366/2012 (R.L.) and FCT Investigator grant (J.M.S.S.E.). The authors also thank funding from Matepro –Optimizing Materials and Processes”, ref. NORTE-07-0124-FEDER-000037”, co-funded by the “Programa Operacional Regional do Norte” (ON.2 – O Novo Norte), under the “Quadro de Referência Estratégico Nacional” (QREN), through the “Fundo Europeu de Desenvolvimento Regional” (FEDER). The authors thank Solvay for kindly supplying the high quality materials.