Effect of Solid Electrolyte Interphase of Amorphous Si Flake Anode on Cycle Performance in Solvate Ionic Liquid Electrolytes

Abstract

Introduction Si is expected to apply for lithium ion batteries as an anode material with a high capacity. However, Si anodes show poor cycleability caused by a pulverization of the Si particles due to large volume changes during charge and discharge cycles. Hence, we designed a nano-flake Si called Si LeafPowder® (Si-LP), which can relieve the physical stress caused by expansion and shrinkage of Si during alloying/dealloying reactions with lithium. A good cycleability of Si-LP with a thickness of 100 nm has been reported.1) On the other hand, S is a promising cathode material with a high capacity. Therefore, Si-S batteries are expected as a next generation electrochemical energy storage device. However, when S cathode is charged and discharged in a conventional ethylene carbonate (EC) based electrolyte, polysulfide anions dissolve into the electrolyte resulting in a poor cycleability. Recently, the dissolution of lithium polysulfide anions was successfully suppressed by using a solvate ionic liquid electrolyte, tri- or tetra-glyme and lithium salt (1:1) complex electrolyte2). We recently reported that cycleability of the Si-LP electrodes in the glyme-based electrolyte was improved by the addition of fluoroethylene carbonate (FEC), which has been recognized as an effective additive that forms a stable solid electrolyte interphase (SEI) on Si electrodes. 3). However, the capacity retention was inferior to the case of using EC based electrolyte. Therefore, we tried to form a pre-film on the Si-LP electrode with the FEC electrolyte prior to cycling to improve the cycleability in the glyme based electrolyte. Experimental The Si-LP composite electrodes consisted of 83.3 wt.% Si-LP, 5.6 wt.% Ketjen black, and 11.1 wt.% carboxymethyl cellulose sodium salt. The loading of the Si-LP composite on a Cu foil with the thickness of 20 mm was approximately 0.3 mg/cm2. The electrolyte was prepared by mixing a purified tetraglyme (G4) and lithium bis(trifluoromethansulfonyl)amide (Li[TFSA]) at the equimolar ratio in an Ar-filled glovebox (referred to as [Li(G4)][TFSA]). [Li(G4)][TFSA] was mixed with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE) in a molar ratio of 1:4 to reduce the viscosity of the electrolyte (referred to as [Li(G4)][TFSA]/HFE). 10 wt.% FEC-added [Li(G4)][TFSA]/HFE was also prepared. The pre-film on the Si-LP electrode was formed by a charge and discharge pre-cycle in the 1 M LiPF6/FEC electrolyte with a coin-type two electrodes half-cell at C/6 rate (1C = 4200 mAh g-1), 30°C. The working and counter electrodes were the Si-LP electrode and a Li metal, respectively. Charge and discharge properties of the Si-LP electrodes with the pre-film were investigated in the additive-free [Li(G4)][TFSA]/HFE electrolytes. For comparison, the pristine Si-LP electrodes were cycled in the [Li(G4)][TFSA]/HFE with and without FEC at C/2 rate. The surface of the Si-LP electrodes after cycling was evaluated by SEM/EDX. Results and discussion Figure 1(a) and 1(b) show charge and discharge curves of the Si-LP electrodes in [Li(G4)][TFSA]/HFE without and with FEC, respectively. By the addition of FEC, the capacity retention at the 100th cycle was improved and polarization was reduced. However, the discharge capacity of the Si-LP electrode with FEC-addition decreased quickly after 100th cycle and the capacity retention at the 200th cycle was 38.4%. On the other hand, the Si-LP electrodes with the pre-film achieved a superior cycleability in [Li(G4)][TFSA]/HFE as shown in Figure 1(c); the capacity retention at the 200th cycle was 72.8%. This result implies that a stable SEI was formed by the pre-cycle in 1 M LiPF6/FEC electrolyte. From SEM/EDX analysis, the reductive decomposition of the electrolyte was suppressed by the presence of the pre-film, whereas on the Si-LP electrodes without the pre-film, SEI film with a large amount of F content was confirmed, indicating the decomposition of HFE. Formation of stable SEI is necessary to achieve a long cycle-life of Si anodes in the glyme-based electrolytes. References 1) M.Saito et al., Solid State Ionics, 225, 506 (2012). 2) K. Dokko et al., J. Electrochem. Soc., 160,A1304 (2013). 3) M. Haruta et al., Electrochemistry, 83,837 (2015). Acknowledgment This study was supported by JST-ALCA SPRING and JSPS-KAKENHI Grant Number 16H04649. Figure 1