Elsevier

Energy Storage Materials

Volume 15, November 2018, Pages 407-414
Energy Storage Materials

Ternary lithium-salt organic ionic plastic crystal polymer composite electrolytes for high voltage, all-solid-state batteries

https://doi.org/10.1016/j.ensm.2018.07.017Get rights and content

Abstract

Organic ionic plastic crystals (OIPCs) are a promising class of solid electrolyte material and recent reports of high concentration Li-containing OIPCs are drawing attention due to their enhanced performance. In this report, nano-sized poly(vinylidene difluoride) (PVdF) particles were incorporated with 1:1 (mole ratio) LiFSI : [C2mpyr][FSI], (C2mpyr = N-ethyl-N-methylpyrrolidinium, FSI = bis(fluorosulfonyl)imide) in a ternary composite, and developed as free-standing solid electrolytes using a facile powder pressing strategy. The electrolytes exhibited relatively high ionic conductivity, up to 10-4 S cm-1 at 30 °C, resulting from an interphase consisting of the OIPC-LiFSI coating the individual, closely packed PVdF particles. The Li+ transference number was 0.44±0.02 at 50 °C, the highest value reported so far for a plastic crystal based electrolyte. Li | LiNi1/3Mn1/3Co1/3O2 cells charged to 4.6 V sustained up to 1300 charge-discharge cycles at 50 °C, highlighting the exceptional stability of these electrolytes. The cells exhibited better capacity retention than a comparative liquid LP30 electrolyte (1 M LiPF6 in ethylene carbonate : dimethyl carbonate (vol. 1:1)) when cycled at a rate of 1 C at 50 °C.

Introduction

Addressing safety concerns for consumer electronics requires the implementation of safer energy storage devices, particularly to allow practical increases in energy density and higher power, which pose increased safety risks. The current commercial liquid electrolytes used in Li-ion technology are composed of flammable organic solvents such as ethylene carbonate (EC), or dimethyl carbonate (DMC) and pose ignition, explosion, and leakage threats [1]. Beyond Li-ion technologies have driven extensive research into new electrode and electrolyte materials and great efforts have been expended to develop batteries with intrinsically safe solid electrolytes [2], [3], [4], [5], [6], [7], [8], [9], [10] or safer battery designs [11], [12].

As one promising class of solid electrolyte material, organic ionic plastic crystals (OIPCs) are solid-state analogues of ionic liquids (ILs) that display significant structural disorder. They are plastic, non-flammable, non-volatile and enable fast transport of target ions, such as Li+, rendering them appealing for use as safe electrolyte materials for batteries [13]. In common with ILs, there are huge varieties of combinations of cations and anions to design specific OIPCs [13]. One drawback of neat OIPCs is the relatively low ionic conductivity at ambient temperature, at around 10-7 S cm-1, [14], [15] compared to 10-3 S cm-1 for ionic liquids, [16], [17] and 10-2 to 10-3 S cm-1 for organic solvent electrolytes [18].

To increase the ionic conductivity and improve the mechanical properties of OIPCs, ceramic nano-particles have been incorporated with different OIPCs and found to exhibit a non-predictable relationship of ionic conductivity enhancement with different particle-OIPC combinations [19]. Li salt doping also enhances OIPC conductivity and has allowed good electrochemical performance in Li cells using a variety of anion-cation and composite combinations. [3], [20], [21], [22] Traditionally, only a low concentration of Li salt doping (for example,<10 mol%) was considered due to the formation of secondary liquid or solid phases beyond that concentration threshold, which negated the solid-state advantages of OIPCs [20] or led to reduced conductivity [23]. However, recently, the unique physico-chemical and electrochemical properties of high salt-containing electrolytes have attracted attention as a possible pathway to higher energy density alkali metal electrode batteries. [3], [23], [24], [25], [26], [27], [28], [29], [30] This approach has been investigated for molecular solvents (e.g., glymes [24], ethers [25], carbonates [26], water [27]) and for ionic liquids (pyrrolidinium [20], [28], phosphonium [29], or imidazolium [30] cations with the [FSI]- [20] (bis(trifluoromethanesulfonyl)imide) or [FTFSI]- [31] (fluorosulfonyl(trifluoromethanesulfonyl)imide) anions). In general, these high lithium salt concentration electrolytes have demonstrated inhibition of corrosion of the Al current collector at the cathode [26], [32], higher Li+ transference number and wider electrochemical window [29] and better cycling performance than traditional organic liquid electrolytes at high rates for Li-metal full cells [20], [28], [33], for Na-metal full cells [34], [35], and for Li – oxygen cells [36].

The performance of an electrolyte for Li metal batteries depends on a combination of its ionic conductivity, target ion transference number, shear modulus to resist dendrite growth, and electrochemical stability window [1]. OIPCs are usually powdery or waxy materials and as such are difficult to apply as solid electrolytes in their neat form. Previous work has developed OIPC composites as solid, free-standing electrolytes and promoted their use towards all-solid-state battery applications. Composites of LiBF4 doped [C2mpyr][BF4], (N-ethyl-N-methylpyrrolidinium tetrafluoroborate), together with electrospun PVdF nanofibers, not only showed enhanced ion conduction compared to the neat plastic crystal, but also supported stable Li | LiFePO4 cell cycling for the first time [37], although at relatively high temperatures: 50 °C and 80 °C. In another study, a branch-structured polymer, hyperbranched bis-MPA polymer, generation 4 (dendrimer) was introduced to modify the microstructure and phases of Li-doped [C2mpyr][BF4] and showed enhanced mechanical and ion transport properties [38]. A comprehensive study of the effects of electrospun PVdF nanofibers on Li-doped [C2mpyr][FSI] was reported in terms of the phase behavior, ion conduction, crystal structure, and ion dynamics [3]. This composite solid electrolyte supported Li | Li symmetric cell cycling at 0.13 mA cm-2 for 500 cycles at room temperature. Most recently, high concentration, 50 mol%, Li salt-containing plastic crystal electrolytes were investigated for the first time, using polymer nanofibers as the matrix to produce self-standing composites that sustained stable Li | LiNi1/3Mn1/3Co1/3O2 cell cycling at room temperature [20]. This is an important step towards the ambient temperature use of OIPCs in energy storage devices and in particular highlights the electrochemical and interfacial stability of a variety of OIPC composites with Li metal anodes and intercalation cathode materials.

However, in the composites prepared with the electrospun PVdF nanofibers, a rather high loading, up to 90 wt%, of the relatively expensive plastic crystal was required to fill the porous matrix. Additionally, the random distribution of the fibers make it harder to control the interfacial area interacting with the plastic crystal, which was determined to be important in governing ion transport in the composites. [39] The fiber composite membranes also exhibited a soft nature and were prone to allow short circuits due to Li dendrite growth. [3] Furthermore, ionic conductivities of the [C2mpyr][FSI]-PVdF fiber composites at 20 °C were still rather low, at 10-6 S cm-1. Considering the Li+ transference number of 0.1 [3] and 0.36 [20] measured for the fiber composites with different contents of Li salt (10 mol% and 50 mol% respectively), the actual Li+ ion conductivity was even lower.

In addition, the interactions between the –CF2 dipoles on the PVdF polymer chain and [C2mpyr]+ cations in the plastic crystal, [C2mpyr][BF4], were proposed to result in the partial amorphization and enhanced ion dynamics of the plastic crystal. [40] It was realized that these ion-dipole interactions could be better understood by using PVdF polymer particles with spherical shape to achieve better control of the surface area and easier synthesis procedure. A highly conductive interfacial area was discerned in the composite comprised of PVdF nano-particles and [C2mpyr][FSI] [39]. The ionic conductivity of the optimized composite, 60 wt% PVdF and 40 wt% (10 mol% Li-doped [C2mpyr][FSI]), even surpassed that of the 10 mol% Li-doped [C2mpyr][FSI] bulk material. The enhancement was proposed to be due to the formation of a percolated and highly conductive Li ion pathway at the polymer|OIPC interface. Long-term cycling (1000 cycles) of this electrolyte was demonstrated with LiFePO4 cathode, which was charged to 4.2 V.

Thus, inspired by the study of Wang et al. [39], here a novel solid composite electrolyte, prepared using a high concentration 50 mol% Li-containing [C2mpyr][FSI], together with nano-sized PVdF powder, is reported. [C2mpyr][FSI] is a recently discovered OIPC with a wide plastic phase range from -22 °C to 203 °C [15]. The phase behavior of binary [C2mpyr][FSI] / LiFSI systems were detailed by Zhou et al. [20] The new 50 mol% composite electrolyte supports extraordinary Li metal battery performance using a high voltage NMC based cathode. The novelty of the work reported here is the first use of a high concentration Li salt-doped OIPC as a PVdF powder nano-composite, to produce a highly conductive ternary solid state electrolyte with advanced Li metal cell performance comparable or superior to the best reported thus far for any solid state electrolyte. [41], [42] Previous reported OIPC composites, based on a fiber matrix with 50 mol% Li salt [20] or a powder composites prepared with 10 mol% Li salt, [39] were derived from similar compositions but did not demonstrate the high current density, high areal capacity nor high voltage stability that is reported here. Uniquely, the composite combines high ionic conductivity, of up to 10-4 S cm-1 at 30 °C, and high Li+ transference number, close to 0.5. The robustness of the composite electrolytes is demonstrated in high voltage (4.6 V), all-solid-state Li | LiNi1/3Mn1/3Co1/3O2 cells, with better capacity retention than liquid LP30 electrolyte, and over 1300 cycles at a rate of 1 C at 50 °C. Superior rate performance and cycling stability at room temperature is also demonstrated for the first time using these composite electrolytes, bringing these systems closer to practical application.

Section snippets

Electrolyte preparation

The composite electrolyte was prepared following a facile powder pressing method [39], as shown in the schematic in Fig. 1(a). 50 mol% LiFSI (CoorsTek, Inc.,>99.5%) was mixed with [C2mpyr][FSI], to prepare a clear, viscous liquid. [C2mpyr][FSI] was synthesized by the metathesis reaction between [C2mpyr]Br and KFSI [3], [15]. This liquid was mixed with PVdF nano-particles (Sigma-Aldrich) and made up to 60 wt% of the total weight of the composite [39]. The mixture was then dispersed into dry

Sample preparation and morphology

The PVdF nano-particles, with average diameters of 200 nm (Fig. S1) or 362 nm, were used to compare the influence of particle size on the thermal and transport properties of the composites. The Fourier transform infrared spectroscopy (FTIR) results in Fig. S2 showed the two different PVdF particles had similar α and β phase distribution [45], [46], allowing direct comparison of the effects of PVdF particle size.

Although mixing the plastic crystal with 50 mol% LiFSI produced a viscous clear

Conclusions

A straightforward powder pressing method is used for the preparation of high performance, high lithium-salt content [C2mpyr][FSI] – PVdF powder composite electrolytes. The advantages of this novel solid electrolyte are demonstrated by excellent capacity retention in all-solid-state high voltage Li metal batteries. Compared to liquid LP30 electrolyte, which decline to 80% of initial capacity after 21 cycles, over 1300 cycles at a rate of 1 C are shown. In Li metal symmetrical cells studies, up

Acknowledgements

Y.D.Z., X.E.W., J.M.P., G.W.G. and P.C.H. thank the Australian Research Council for support from the Discovery Projects program (DP140101535). M.F. acknowledges ARC for Australian Laureate Fellowship (FL110100013). The NMR experiments were supported via the ARC LIEF grant LE11010014. Y.D.Z. thank Australian Synchrotron Powder Diffraction beamtime and Dr. Helen Brand for technical help. Y.D.Z. thanks Dr. John Ward for the help with SEM. Y.D.Z. thank Prof. Masahiro Yoshizawa-Fujita and Yukari

Data availability

The raw data required to reproduce these findings are available to download from https://data.mendeley.com/datasets/msz9y9d72m/1. The processed data required to reproduce these findings are available to download from https://data.mendeley.com/datasets/msz9y9d72m/1.

Declarations of interest

None.

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