Lithium Ion Transference Number Research Articles

A polydopamine-modified garnet–based polymer-in-ceramic hybrid solid electrolyte membrane for high-safety lithium metal batteries

Composite solid electrolyte (CSE) membranes combining the attractive properties of ceramic and polymer-based electrolytes have emerged as preferred electrolytes for all-solid-state lithium metal batteries (ASSLMBs). In this study, we used solution-casting to prepare a CSE membrane from a suspension of polydopamine (PDA)-modified Li6.28La3Zr2Al0.24O12 (LLZAO) filler (PDA@LLZAO), poly(vinylidene fluoride–co–hexafluoropropylene) (PVDF-HFP), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and succinonitrile (SN) as the middle layer and a matrix of PVDF-HFP, LiTFSI, and SN as both the top and bottom layers. The presence of PDA on the surface of LLZAO enabled the filler to percolate well within the polymer matrix. Consequently, a membrane based on PDA@LLZAO (CSE1) exhibited high ionic conductivity (4.01 × 10–4 S cm−1), a high lithium transference number (ca. 0.76), high tensile strength (29.09 MPa), and a stable electrochemical window (ca. 5.01 V vs Li/Li+) relative to those properties of a counterpart membrane (CSE0) having the same constituents as CSE1, but an unmodified LLZAO filler. The interfacial stability developed by the synergetic effect of the CSE1 membrane and Li metal anode enhanced the corresponding Li plating/stripping performance (2000 h) and critical current density (2.0 mA cm−2). Benefiting from this stable interfacial contact, an Al2O3@NCM811/CSE1/Li coin-type cell provided a discharge specific capacity of 136.46 mAh/g at a rate of 0.5C after 300 cycles, with a capacity retention of 86.22 % and a average coulombic efficiency of 99.16 % at 25 °C. Hence, our proposed strategy for preparing CSEs appears to be promising for use in ASSLMB applications.

Constructing artificial solid electrolyte interphase by facile chemical reaction for stable lithium metal anodes

Lithium metal is considered one of the most promising anode materials because of its high theoretical capacity and the lowest electrode potential, but practical application is hampered by high reactivity and volume variation in circulation. As a key component, the artificial solid electrolyte interphase (SEI) is designed to effectively stabilize the lithium anode. Here, the chemical reaction between dimethyl sulfoxide (DMSO) and lithium is accelerated in the presence of potassium hydroxide (KOH). The sulfur-containing organic compounds, lithium sulfate (Li2SO4), lithium hydroxide (LiOH) and other favorable components are introduced into SEI, which enable the modified lithium metal anode to have lower interfacial impedance, higher lithium ion transference number, and more uniform Li-ion deposition. The assembled Li-Li symmetric battery realizes a stable cycle of more than 800 h at a current density of 0.5 mA cm−1 and an areal capacity density of 1 mAh cm−1. The full cell coupled with LiNi0.6Co0.2Mn0.2O2 (NCM622) still maintains 113 mAh g−1 discharge capacity with 63% capacity retention rate after 400 cycles at a current density of 0.5 C. The simple construction of artificial SEI strategy of this study provides an effective strategy for LMBs with superior electrochemical performance.

Cellulose Acetate-Based High-Electrolyte-Uptake Gel Polymer Electrolyte for Semi-Solid-State Lithium-Oxygen Batteries with Long-Cycling Stability.

Lithium-oxygen batteries have received great research interest owing to their ultrahigh theoretical energy density and are considered as one of the promising secondary batteries. However, there are still some challenges in their practical application, like liquid organic electrolyte evaporation in the semi-open system and instability in the high-voltage oxidizing environment. In this work, a cellulose acetate-based gel polymer electrolyte (CA@GPE) is proposed, whose cross-linked microporous structure ensures the ultrahigh liquid electrolyte uptake of 2391%. The prepared CA@GPE exhibits a high lithium-ion transference number of 0.595, a satisfying ionic conductivity of 0.47 mS cm-1 and a wide electrochemical stability window up to 5.0 V. The Li//Li symmetric cell employing CA@GPE could cycle stably over 1200 h. The lithium-oxygen battery with CA@GPE presents a superb cycling lifetime of 370 cycles at 0.1 mA cm-2 under 0.25 mAh cm-2 . This work offers a possible strategy to realize long-cycling stability lithium-oxygen batteries.

In-situ construction of high-temperature-resistant 3D composite polymer electrolyte membranes towards high-performance all-solid-state lithium metal batteries

Enabling solid polymer electrolytes (SPEs) with enhanced electrochemical properties and high-temperature-resistance is highly desirable for developing high-performance lithium metal batteries (LMBs) with high-safety. In this contribution, single-ion conducting polymers and SiO2 particles, as advanced fillers, are in-situ cross-linked in poly(ethylene oxide) (PEO) based SPEs to form three-dimensional (3D) cross-linking inorganic/organic composite SPEs. Benefiting from the hydrogen bond interaction between the additives and PEO chains, the crystallinity of PEO polymers in the resultant SPEs is greatly decreased, giving rise to high conductivity (3.3 × 10−4 S cm−1) and lithium ion transference number (0.60) at 60 °C. The Li/Li symmetrical cells with the composite SPEs can be run stably more than 400 h at 60 °C and 550 h at 120 °C under 0.1 mA cm−2, respectively. Remarkably, the favorable thermal stability of composite SPEs leads to safer solid-state LMBs operating at the high temperature up to 120 °C, where it reaches an outstanding rate performance up to 4C and displays impressive cycling performances with 89.9% retention over 110 cycles at 1C. This work offers tremendous potential in the practical application of dendrite-free and high-performance LMBs.

In situ mineralized Ca3(PO4)2 inorganic coating modified polyethylene separator for high-performance lithium-ion batteries

• Flower-like calcium phosphate coating is successfully formed on the surface of modified PE separator. • The obtained composite separator exhibits stronger affinity to electrolyte and absorbs more liquid electrolyte. • CaP@PE separator possess an improved ionic conductivity (0.52 mS cm −1 ) and lithium-ion transference number (0.36). • Cell assembled with CaP@PE separator exhibits excellent cycle performance and rate performance. The organic–inorganic composite separator possesses great thermal stability and electrolyte wettability, which is normally prepared via the slurry containing binder. However, this preparation is involved with large amounts of organic solvent that is harmful to health. In this paper, we design a novel method that combines the crosslinking technology and biomimetic mineralization process to prepare a Ca 3 (PO 4 ) 2 inorganic coating modified polyethylene separator (CaP@PE). The obtained composite separator exhibits stronger affinity to electrolyte, and its porous coating structure can store more liquid electrolyte, thus the ionic conductivity is promoted from 0.27 mS cm −1 to 0.52 mS cm −1 and the lithium-ion transference number is increased from 0.26 to 0.36. Compared with PE separator, CaP@PE separator shows better thermal stability at high temperature. Due to the improved ionic transport performance and reduced charge transfer impedance, LiCoO 2 /Li half-cell employing CaP@PE separator displays superior cycle stability and capacity retention ability after 150 cycles at a current density of 1C. Even at a high rate of 5C (7.5 mA cm −2 ), the cell with CaP@PE separator still exhibits a discharge capacity of 0.80 mAh. This work provides a promising separator to optimize the electrochemical performance and safety performance in lithium-ion battery.

Quasi-solid polymer electrolytes with fast interfacial transport for lithium metal batteries

The quasi-solid electrolyte based on Poly(vinylidene fluoride-co-hexafluoropropylene) has drawn great interest owing to good wettability and compatibility with lithium metal anode. However, the low ionic conductivity and lithium-ion transfer number limit the practical application of Poly(vinylidene fluoride-co-hexafluoropropylene). Herein, quasi-solid electrolyte is prepared by adding NASICON-type lithium-ion conductor Li1.3Al0.3Ti1.7(PO4)3 to Poly(vinylidene fluoride-co-hexafluoropropylene). The Li1.3Al0.3Ti1.7(PO4)3 not only reduces the crystallinity of the polymer, but also participates in the conduction process of Li+. Through the synergistic effect of Li1.3Al0.3Ti1.7(PO4)3 and Poly(vinylidene fluoride-co-hexafluoropropylene), the quasi-solid polymer electrolyte has a high ionic conductivity (3.64 × 10−3 S cm−1) at room temperature and large lithium-ion transfer number (0.64). The symmetric cell exhibits a small overpotential (18 mV) as well as a stable cycle (3000 h) at 0.2 mA cm−2 with areal capacity of 0.2 mAh cm−2. The full cell with LiFePO4 cathode keeps a discharge capacity of 143.4 mAh g−1 after 600 cycles with capacity retention of 94.7%, and also performs good electrochemical performance with high voltage cathode (LiNi0.85Mn0.1Co0.05O2). Furthermore, the pouch cell assembled by quasi-solid polymer electrolyte have good flexibility, which can be used normally after folding and shearing. Our work supplies a feasible solution for the design of quasi-solid lithium metal batteries.

Polyacrylonitrile Porous Membrane-Based Gel Polymer Electrolyte by In Situ Free-Radical Polymerization for Stable Li Metal Batteries.

Because of their high ionic conductivity, utilizing gel polymer electrolytes (GPEs) is thought to be an effective way to accomplish high-energy-density batteries. Nevertheless, most GPEs have poor adaptability to Ni-rich cathodes to alleviate the problem of inevitable rapid capacity decay during cycling. Therefore, to match LiNi0.8Co0.1Mn0.1O2 (NCM811), we applied pentaerythritol tetraacrylate (PETEA) monomers to polymerize in situ in a polyacrylonitrile (PAN) membrane to obtain GPEs (PETEA-TCGG-PAN). The impedance variations and key groups during the in situ polymerization of PETEA-TCGG-PAN are investigated in detail. PETEA-TCGG-PAN with a high lithium-ion transference number (0.77) exhibits an electrochemical decomposition voltage of 5.15 V. Noticeably, the NCM811|PETEA-TCGG-PAN|Li battery can cycle at 2C for 120 cycles with a capacity retention rate of 89%. Even at 6C, the discharge specific capacity is able to reach 101.47 mAh g-1. The combination of LiF and Li2CO3 at the CEI interface is the reason for the improved rate performance. Moreover, when commercialized LFP is used as the cathode, the battery can also cycle stably for 150 cycles at 0.5C. PETEA and PAN can together foster the transportation of Li+ with the construction of a fast ion transport channel, making a contribution to stable charge-discharge of the above batteries. This study provides an innovative design philosophy for designing in situ GPEs in high-energy-density lithium metal batteries.

Sustainable Lithium‐Ion Battery Separator Membranes Based on Carrageenan Biopolymer

Aiming to improve the sustainability of materials for lithium‐ion batteries (LIBs), this work reports on the development of novel membranes based on iota‐carrageenan biopolymer and their suitability as separator in LIBs applications. The membranes are prepared by freeze‐drying and its morphology, thermal, mechanical, and electrochemical properties are evaluated as a function of polymer concentration in the solution. Porous membranes with a degree of porosity above 90% and different interconnected pore sizes between 50 and 70 µm for both polymer concentrations are obtained. The electrochemical parameters of the membranes are influenced by the carrageenan polymer concentration, being 1.34 mS cm‐1, 3, 7, and 0.48 for ionic conductivity, tortuosity, MacMullin number, and lithium transference number, respectively, for the membrane prepared from the 4 wt% carrageenan content solution. The half‐cells cathodic prepared with the developed membranes show good cyclability and rate capability. The discharge capacity values obtained with the membrane prepared from 4 wt% carrageenan content are 145 and 25 mAh g‐1 at C/10‐ and 1C‐rates, respectively, demonstrating excellent battery cycling performance and long‐term stability. Thus, this work demonstrates that iota‐carrageenan biopolymer membranes developed by freeze‐drying can be used as separators for a next generation of more sustainable batteries.

Improving the Ionic Conductivity of PEGDMA-Based Polymer Electrolytes by Reducing the Interfacial Resistance for LIBs.

Polymer electrolytes (PEs) based on poly(ethylene oxide) (PEO) have gained increasing interest in lithium-ion batteries (LIBs) and are expected to solve the safety issue of commercial liquid electrolytes due to their excellent thermal and mechanical stability, suppression of lithium dendrites and shortened battery assembly process. However, challenges, such as high interfacial resistance between electrolyte and electrodes and poor ionic conductivity (σ) at room temperature (RT), still limit the use of PEO-based PEs. In this work, an in situ PEO-based polymer electrolyte consisting of polyethylene glycol dimethacrylate (PEGDMA) 1000, lithium bis(fluorosulfonyl)imide (LiFSI) and DMF is cured on a LiFePO4 (LFP) cathode to address the above-mentioned issues. As a result, optimized PE shows a promising σ and lithium-ion transference number (tLi+) of 6.13 × 10−4 S cm−1 and 0.63 at RT and excellent thermal stability up to 136 °C. Moreover, the LiFePO4//Li cell assembled by in situ PE exhibits superior discharge capacity (141 mAh g−1) at 0.1 C, favorable Coulombic efficiency (97.6%) after 100 cycles and promising rate performance. This work contributes to modifying PEO-based PE to force the interfacial contact between the electrolyte and the electrode and to improve LIBs’ performance.

Solvate ionic liquid-based ionogels for lithium metal batteries over a wide temperature range

Ionogels for lithium metal batteries are usually obtained by immobilization of lithium salt-containing ionic liquids using linear polymers as gelator, which are thermally unstable and lack of adhesion. Meanwhile, the two-cation (ionic liquid cation and lithium ion) competition results in low lithium-ion transference number. In this work, we designed novel ionogels by in situ UV radiation-induced chemically crosslinked polymerization of butyl acrylate in a solvate ionic liquid ([Li(G4)][TFSI]) without additional lithium salt. The obtained ionogels (IG-m/n, m/n is the weight ratio between butyl acrylate and solvate ionic liquid) exhibit nonflammability, favorable thermal and mechanical features, high ionic conductivity, wide electrochemical window, and interestingly adhesive ability that provides an intimate contact with the Li anode. Consequently, one of the ionogels, IG-1/2, owing to its optimized adhesion, ionic conductivity, and mechanical property, could endow the as-fabricated Li|IG-1/2|LiFePO4 cells with high cyclability and Coulombic efficiency at room temperature, 90 °C, and 150 °C.

In situ formed self-healable quasi-solid hybrid electrolyte network coupled with eutectic mixture towards ultra-long cycle life lithium metal batteries

The exploration of high-performance polymer-based electrolyte have great promise for the development of next-generation lithium metal batteries. In this study, a self-healable quasi-solid hybrid electrolyte network coupled with deep eutectic solvent (X-PPS-D4) was prepared by polyaddition reaction, photo-initiated free-radical crosslinking, and physical blending. X-PPS-D4 not only possessed high ionic conductivity (σ) of 2.03✗10−4 S cm−1 at 30 ℃ but also wide electrochemical stability window (0∼5.0 V vs. Li+|Li). The enhanced lithium ion transference number (tLi+= 0.44) for X-PPS-D4 resulted from reduction of anion mobility that was proved by spectral analysis together with nuclear magnetic resonance study and theoretical calculation. Under testing temperature of 30 ℃, X-PPS-D4 could support stable long-term (>1300 h) plating/stripping tests of lithium symmetrical cell at current density of 0.1 mA cm−2. More importantly, the in situ formed X-PPS-D4 on LiFePO4 cathode within the coin cell facilitated excellent cell performance with specific capacity over 100 mA h g−1 at 5 C and ultra-long cycle life (>1000 cycles) with high delivered specific capacity at 1 C (>116.1 mA h g−1 after 1000 cycle). X-PPS-D4 also supported coin cells with high voltage cathode to perform decently. This research provides new insights into structure-property relationship of quasi-solid hybrid electrolyte with eutectic mixture and offered an effective way to develop high-performance polymer-based electrolytes for LMBs.

Introducing low-tortuosity channels in thick electrode for high-areal-capacity solid polymer battery

All-solid-state lithium battery (ASSLB) is the most promising candidate of next generation lithium ion batteries. However, the construction of high loading composite electrode is a bottleneck of ASSLB. Herein, low-tortuosity LiFePO4 (LFP-LT) composite electrodes with high mass loadings are prepared by a phase inversion and in-situ polymerization method. The in-situ formed polymer electrolyte possesses a wide electrochemical window (0–4.9 V vs Li+/Li), high lithium-ion transference number (0.73) and a high Li+ conductivity (0.23 mS cm−1) at room temperature. In addition, the low-tortuosity channels can serve as fast electron/ion pathways. Consequently, the LFP-LT composite electrode with a loading of 15 mg cm−2 shows an excellent cycling stability with a capacity retention of 89.0 % after 250 cycles at 1C. Notably, an ultra-high loading LFP-LT composite electrode (35 mg cm−2) displays a high areal capacity of 5 mAh cm−2 and an enhanced cycling performance with a capacity retention of 94.0 % after 150 cycles at 0.5C. This work presents an efficient strategy to construct low-tortuosity and high loading solid polymer batteries.

Increasing the ionic conductivity and lithium-ion transport of photo-cross-linked polymer with hexagonal arranged porous film hybrids

SummaryHigh ionic conductivity, suitable mechanical strength, and electrochemical stability are the main requirements for high-performance poly(ethylene oxide)-based electrolytes. However, the low ionic conductivity owing to the crystallinity of the ethylene oxide chain that limits the discharge rate and low-temperature performance has restricted the development and commercialization of these electrolytes. Lithium electrolytes that combine high ionic conductivity with a high lithium transference number are rare and are essential for high-power batteries. Here, we report hexagonal arranged porous scaffolds for holding prototype polyethylene glycol-based composite electrolytes containing solvate ionic liquid. The appealing electrochemical and thermal properties indicate their potential as electrolytes for safer rechargeable lithium-ion batteries. The porous scaffolds in the composite electrolytes ensure better electrochemical performance towing to their shortened pores (sizes of 3-14 μm), interconnected pathways, and improved lithium mobility. We demonstrate that both molecular design and porous microstructures are essential for improving performance in polymer electrolytes.

All-cellulose gel electrolyte with black phosphorus based lithium ion conductors toward advanced lithium-sulfurized polyacrylonitrile batteries

Sulfurized polyacrylonitrile (SPAN) have been regarded as a promising cathode in high-energy-density lithium-sulfur batteries. However, severe safety issues derived from the electrolyte leakage and the uncontrollable lithium dendrite growth have seriously hindered the practical usage of Li-SPAN batteries. To address these issues, an eco-friendly and porous cellulose gel electrolyte (GE) was designed and prepared by UV photopolymerization and phase inversion methods. Also, covalent functional black phosphorous nanosheets (denoted as BP-Li) were fabricated and chosen as superior Li+ conductors in cellulose GE, ensuring the rapid ion transmission and suppressing the growth of lithium dendrites. As expected, the cellulose/BP-Li GE exhibited satisfactory ionic conductivity up to 5.21 × 10−3 S cm−1 with the high lithium ion transference number of 0.72. The Li-SPAN battery assembled with cellulose/BP-Li GE delivered high capacity of 938.8 mAh g−1 after 500 cycles with the capacity retention of 72.8 %. Hence, the cellulose/BP-Li GE displayed its promising usage in Li-SPAN batteries.

A flexible Li2SnO3-coupled PEO-based single-ion conducting composite solid-state electrolyte for highly-stable Li metal batteries

Development of all-solid-state lithium-ion batteries (ASSLBs) with potentially high energy density and safety is critical for next-generation energy storage devices. In this work, a low-cost flexible Li2SnO3(LSO)-coupled PEO-based composite single-ion conducting polymer solid-state electrolyte (CSIPE) was prepared by a simple solvent-free solid-phase method. The obtained LSO-coupled CSIPE exhibited a high ionic conductivity of 5.59 × 10−4 S cm−1 at 60 °C with a high lithium ion transference number of 0.54 and an electrochemical stability window as high as 5.28 V. In addition, the results based on Vogel-Tammann-Fulcher (VTF) model and differential scanning calorimetry (DSC) measurements indicated that the presence of LSO reduced the activation energy for lithium ion migration and the crystallinity of the polymer matrix to provide channels and flexible polymer chains for the transportation of lithium ions. Moreover, the fabricated lithium metal anode based full cell worked stably for more than 100 cycles maintaining a high specific capacity of 127.1 mAh g−1 and a specific capacity retention ratio of 96.5% at a high cycling rate of 0.5 C. While, the CSIPE also demonstrated an excellent rate performance and outstanding adaptation to cathodes. Furthermore, the obtained LSO-coupled CSIPE remained stable even at a temperature as high as 322 °C without thermal runaway, and worked under different current densities for 450 h without short-circuiting, indicating good interfacial stability to ensure the safety of the batteries. It is expected that this LSO-coupled CSIPE is a promising substitute for liquid electrolytes with broad commercial prospects.

High-Rate Solid Polymer Electrolyte Based Flexible All-Solid-State Lithium Metal Batteries.

A flexible poly(vinylidene fluoride)-polyetherimide@poly(ethylene glycol) (PVDF-PEI@PEG) solid composite polymer electrolyte is prepared by an in situ thermal curing approach. The homogeneous PVDF-PEI composite porous membrane with an optimized PVDF and PEI weight ratio increases the amorphous phase, while the fast lithium ion transport channels are formed through the filled PEG electrolytes. The optimized polymer electrolyte exhibits high ionic conductivity of 2.36 × 10-4 S cm-1 at 60 °C and lithium ion transference number of 0.578 as well as excellent electrochemical stability window of 5.5 V. Moreover, the superior stability toward lithium metal anode enables over 3600 h cycling of the Li//Li symmetric cell at 0.1 mA cm-2. In particular, the LiFePO4//Li battery delivers high specific capacities of 132.4 and 111.5 mAh g-1 with a retention of 86.6% and 85.9% after 200 cycles at 2 C and 100 cycles at 3 C rate under 60 °C, respectively, demonstrating the feasibility as an energy storage device with high rate capability.

Scalable fabrication of Solvent‐Free composite solid electrolyte by a continuous Thermal-Extrusion process

Composite solid-state electrolytes (CSEs) are regarded as a promising alternative for the next‐generation lithium-ion batteries because they integrate the advantages of inorganic electrolytes and organic electrolytes. However, there are two issues faced by current CSEs: 1) a green and feasible approach to prepare CSEs in large scales is desired; and 2) the trace solvents, remaining from the preparation processes, lead to some serious concerns, such as safety hazard issues, electrolyte–electrode interfacial issues, and reduced durability of batteries. Here, a continuous thermal-extrusion process is presented to realize the large-scale fabrication of solvent‐free CSE. A 38.7-meter CSE membrane was prepared as a demonstration in this study. Thanks to the elimination of residual solvents, the electrolyte membrane exhibited a high tensile strength of 3.85 MPa, satisfactory lithium transference number (0.495), and excellent electrochemical stability (5.15 V). Excellent long-term stability was demonstrated by operating the symmetric lithium cell at a stable current density of 0.1 mA cm−2 for over 3700 h. Solvent-free CSE lithium metal batteries showed a discharge capacity of 155.7 – 25.17 mAh g−1 at 0.1 – 2.0C, and the discharge capacity remained 78.1% after testing for 380cycles.

Effect of Alkyl Side Chain Length on the Lithium-Ion Conductivity for Polyether Electrolytes.

The design guidelines of polymer structure to effectively promote lithium-ion conduction within the polymer electrolytes (PEs) are crucial for its practical use. In this study, the electrolyte properties of a simple polyether having alkyl side chains with varied lengths (−(CH2)m−H, m = 1, 2, 4, 6, 8, and 12) were compared and established a valid design strategy based on the properties of the alkyl side chain. Various spectro-electrochemical measurements successfully connected the electrolyte properties and the alkyl side chain length. Steric hindrance of the alkyl side chain effectively suppressed the interaction between ether oxygen and lithium-ion (m ≥ 2), decreasing the glass transition temperature and the activation energy of lithium-ion transfer at the electrode-electrolyte interface. The strong hydrophobic interactions aligned and/or aggregated the extended alkyl group (m ≥ 8), creating a rapid lithium-ion transport pathway and enhancing lithium-ion conductivity. A clear trend was observed for the following three crucial factors determining bulk lithium-ion transport properties along with the extension of the alkyl side chain: 1) salt dissociability decreased due to the non-polarity of the alkyl side chain, 2) segmental mobility of polymer chains increased due to the internal plasticizing effect, and 3) lithium-ion transference number increased due to the inhibition of the bulky anion transport by its steric hindrance. The highest lithium-ion conductivity was confirmed for the PEs with an alkyl side chain of moderate length (m = 4) at 70°C, indicating the optimized balance between salt dissociability, polymer segmental mobility, and selective lithium-ion transfer. The length of an alkyl side chain can thus be a critical factor in improving the performance of PEs, including thermal stability and lithium-ion conductivity. Precise tuning of the alkyl side chain-related parameters such as steric hindrance, polarity, internal plasticizing effect, and self-alignment optimizes the polymer segmental mobility and salt dissociability, which is crucial for realizing high lithium-ion conductivity for PEs.

Passerini polymerization as a novel route for high ionic conductivity solid polymer electrolyte

Polyethylene glycol (PEG) used as solid polymer electrolyte has shown disadvantages of low ionic conductivity and lithium ion transference number due to its high crystallinity. In this study, we propose Passerini three-component polymerization (P-3CP) to modify PEGs for solid polymer electrolytes (SPEs). Thus, a series of PEG-based polyesters (PGPEs) with molar masses between 14,400 and 30,200 gmol−1 are synthesized by P-3CP. The crystallinity of obtained PGPEs is successfully suppressed by the introduction of side groups via P-3CP. SPEs consisting of PGPEs as matrix and bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) with different concentrations were investigated in detail. The SPE composed of PGPE-4000 with 30 wt% LiTFSI presented the best ionic conductivities of 5.85 × 10-5 Scm−1 and 1.94 × 10-3 Scm−1 at room temperature and 80 °C, respectively, which are significantly higher than the commonly used PEG-based SPE. The LiFePO4/Li solid-state lithium-ion battery assembled with PGPE-based SPE showed excellent cycle stability. After 100 cycles at 0.5C at 60 °C, the discharge specific capacity remains 133 mAhg−1. Furthermore, due to the addition of amide side groups, the flame retardant property of PGPE-based SPEs is improved. Compared to PEG, the limiting oxygen index (LOI) values increase from 21.0 to 25.5 after modification by P-3CP. Therefore, the P-3CP is an attractive method to obtain polyesters from dicarboxylic acid or diol monomers for the application in the lithium battery field.

Novel Acrylonitrile-Based Polymers for Solid–State Polymer Electrolyte and Solid-State Lithium Ion Battery

Rechargeable lithium-ion batteries (LIBs) involving lithium metal oxides, liquid electrolyte and graphite have been widely used in portable electronic devices due to their relatively high energy density and long cycle life. These desirable features make LIBs very attractive as the power source for electronic devices, hybrid electric vehicles (HEVs) and electric vehicles (EVs) applications [1, 2]. For future EV applications, higher energy density of LIBs up to 360 Wh kg-1 is required. Currently, the energy density of the state-of-the-art LIBs using conventional graphite anode, LiFePO4 (denoted as LFP) or LiNi0.5Co0.2Mn0.3O2 (NCM523) cathodes and 1-1.2 M LiPF6 in organic carbonate electrolytes provide practically achievable energy densities of up to around 200-260 Wh kg−1 [3]. When commercial graphite anodes are used, LiNi0.8Co0.15Al0.05O2 (NCA), LiNi0.8Co0.1Mn0.1O2 (NCM811), LiNi0.5Mn1.5O4 (LNMO) and LiNiPO4 (LNP) cathode based batteries with high-voltage provide energy densities of 354, 338, 351 and 414 Wh kg-1, respectively. However, LIBs using these high-voltage cathode materials and the organic carbonate electrolytes exhibit quite low thermal stability and tend to catch fire or even explode when abnormal charge/discharge cycling or accidental penetration of cells occurs, which greatly limits the automotive applications. When replacing graphite with a Li metal anode, the energy densities of all battery systems can be enhanced significantly due to the highest theoretical specific energy density (3860 mAh g-1) among all anode materials for rechargeable LIBs. Nevertheless, commercial LIBs are prone to cause safety problems due to the safety concern arising from Li dendrite growth in liquid organic electrolytes [4-6].The promising solid-state LIBs offer high thermal stability (i.e., low risk in catching fire), high energy density, wide electrochemical stability window and less environmental impact. A competent electrolyte is the key component of solid-state LIBs. The solid-state electrolyte materials are mainly classified as solid polymer electrolytes (SPEs), inorganic solid electrolytes (ISEs), and organic/inorganic composite electrolytes. ISEs include oxide-based and sulfide-based materials [7, 8], which show very high ionic conductivity (10-2 – 10-3 S cm-1). Furthermore, the lithium ion transference number is close to 1. However, the major limitation factors of practical solid-state LIB applications are the large interfacial impedance between electrode and ISE and the difficulty of processing [9]. Considering processability, mechanical flexibility, interfacial compatibility and electrochemical stability, one prefers SPEs to the inorganic ceramic electrolytes. Nevertheless, SPEs have low ion conductivities (10−7 − 10−5 S cm−1 near room temperature) and most of the Li+ transference numbers are lower than 0.5 [10, 11]. The major requirements for SPEs include high ionic conductivity and transference number at room temperature, wide electrochemical potential window, high mechanical strength and excellent thermal stability. However, the ion conductivity is the most important (> 10-4 S cm-1 at room temperature desired) and should be considered first. The coordinating groups of a good polymeric host are expected to interact with Li+ and facilitate dissociation.In this study, we prepared various novel acrylonitrile-based polymers (e.g., acrylonitrile/acrylate copolymer and polymer with two pendant groups b-cyano ethyl ether (-O-CH2CH2-CN) sulfonate alkyl ether (-O-(CH2)3SO3Li). The corresponding SPEs comprising acrylonitrile-based polymer and ca. 50 wt.% lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) with high ionic conductivity (up to 10-3 S cm-1) at room temperature, high ion transfer number (up to 0.45) and large electrochemical potential window (oxidation stability > 5 V vs. Li+/Li) achieved. The selected SPEs were used as the separator in solid-state batteries with LiFePO4 as the cathode and Li foil as the anode; and long-term cycle stability of solid-state LIB was achieved. The polymers and corresponding SPEs were characterized by using DSC, SEM, XRD and FTIR measurements. Ionic conductivities of SPEs were determined from electrochemical impedance spectroscopy results. The linear sweep voltammetry technique was adopted to measure the oxidation stability window of SPE, and the Evans-Vincent-Bruce method was used to determined ion transfer number.