Lithium-ion battery performance enhanced by the combination of Si thin flake anodes and binary ionic liquid systems

An appropriate combination of Si anodes and binary bis(fluorosulfonyl)amide-based ionic liquid electrolytes significantly improves Li-ion battery performances.


Introduction
A secondary battery with high capacity and favorable rate capability that exceeds the current Li-ion battery (LIB), which consists of a graphite anode and a lithium-transition metal oxide cathode, is expected to be the future energy storage device supporting smart grid communities, electric vehicles, and Internet of Things (IoT) technology. 1,2 One such battery is the Si anode type LIB as Si has a very high theoretical capacity (3579 mA h g À1 ), which is comparable to that of the Li metal anode (3860 mA h g À1 ). However, during the charge-discharge processes, the Si anode suffers from a dramatic volume change that often causes Si pulverization accompanied by the conductive path loss, resulting in the rapid fading of battery capacity. Several approaches for overcoming this issue have already been proposed. Nanoscale morphology designs of Si materials, such as nanowires, 3,4 nanotubes, 5 thin films, 6 and thin flakes, 7,8 are highly effective. Recently, it was revealed by in situ and operando scanning electron microscopy (SEM) 9 that the addition of flexibility to the Si material by nanoscale material tailoring is an important factor for suppressing the degradation. Another approach for improving capacity fading is to use organic additives, such as 4-fluoroethylene carbonate (FEC) and vinylene carbonate (VC), as a component of the battery electrolyte. For example, the capacity retention rate was improved from 17% without an additive to 60% with FEC and 74% with VC due to the variation in chemical species and their compositions in the solid electrolyte interface (SEI), which form a physicochemically stable and good Li-ion conducting SEI layer on the Si anode. 10 Ionic liquids (ILs), which are liquid salts at room temperature, have great potential as advanced electrolytes for LIBs because of their various unique properties including wide electrochemical window, negligible vapor pressures, nonflammability, and good thermal stability. 11,12 In particular, bis((fluorosulfonyl)amide) anion ([FSA] À )-based IL electrolytes have been widely used for Si anode LIBs, as [FSA] À contributes to the formation of a favorable SEI layer onto the Si anode. [13][14][15][16] However, there is insufficient information on the effect of the Li(I) and [FSA] À concentration in the IL electrolyte on the battery performance although it is known that the aqueous and organic battery electrolytes become more favorable at higher Li salt concentrations (in many cases, up to 50 mol%). [17][18][19][20][21] In the present study, in order to improve the cycle characteristics and high-rate charge-discharge performance of the Si anode LIBs, the effects of the Li 9,22 and ex situ X-ray photoelectron spectroscopy (XPS). The combination of the Si anode and IL electrolyte is proposed for designing a high-performance Si anode LIB that has the potential to be the future of energy storage devices.

Preparation and evaluation of Si composite anode LIBs
A Si thin flake (Si-LeafPowder s , OIKE & Co., Ltd (Japan)), with vertical and horizontal sizes of 3-5 mm (Fig. 1a) and a thickness of 100 nm (Fig. 1b), was used as an active anode material. A composite electrode containing an 83.3 wt% Si thin flake, a 5.6 wt% conductive additive (Ketjen Black (KB), EC600JD, Lion Corp. (Japan)), and a 11.1 wt% water soluble binder (carboxymethyl cellulose sodium salt (NaCMC), Nacalai Tesque (Japan)) was prepared by the procedure used in a previous study. 7 The mass loading of the Si-LPs on the current collector was 0.4-0.5 mg cm À2 . CR2032type coin full cells were constructed by stacking the Si composite anode, a polyolefin separator (Nippon Sheet Glass Co., Ltd (Japan)), and a LiCoO 2 composite cathode with an Al foil current collector (3.0 mA h cm À2 , Hohsen (Japan)) in an Ar-filled glove box (Omni-Lab, Vacuum Atmospheres Co. (USA), O 2 and H 2 O o 1 ppm). The quantitative ratio between the cathode and the anode materials was set so that the actual capacity of the LiCoO 2 electrode was more than twice the theoretical capacity of the Si composite electrode. The electrolytes used were binary IL systems: 83.3-16. ) in an Ar-filled glove box. The charge-discharge tests were conducted at 298 K in a constant current/constant voltage (CC/CV) mode between À3.88 and À2.40 V (vs. LiCoO 2 ) using a battery test system (HJ1001SD8, Hokuto Denko (Japan)). The cut-off current for the CV mode was 1/10, the same as that for the CC mode. Unless otherwise noted, the discharge capacity per Si weight is represented as mA h g À1 in this paper.

Characterization of the SEI layer formed on Si anodes
A SEI layer formed on Si anodes was characterized by an XPS system (AXIS Ultra DLD, Kratos (UK)). Ex situ XPS analyses were conducted using the Si thin flake composite anodes after five charge-discharge processes. The process was performed in CC mode of 1/2C in the coin-type full cell described above. The resultant Si anodes were rinsed with a battery-grade diethyl carbonate (DEC, Wako (Japan)) in an Ar-filled glove box. XPS measurements were carried out using an airtight transfer vessel to prevent contamination from moisture and oxygen in air.

Preparation of Si anodes for operando SEM observations
A binder-free Si thin flake anode with a conductive additive (acetylene black (AB), Strem Chemicals (USA)) was prepared using an electrophoretic deposition (EPD) method, as depicted in Fig. 1c. 9,23 The solution for the EPD process was a dry acetone solution containing a 1.00 g L À1 Si thin flake, 0.40 g L À1 AB, and 1.00 g L À1 citric acid monohydrate (Wako (Japan)) which was sonicated for 20 min prior to use. A Cu mesh (20 mesh, Nilaco Co. (Japan))and a Pt plate were used as the anode and cathode, respectively. A voltage of 100 V was applied between the electrodes

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This journal is © The Royal Society of Chemistry 2020 Mater. Adv., 2020, 1, 625--631 | 627 that were set 2 cm apart, for 60 s to carry out EPD. Si thin flakes and AB co-deposited Cu mesh electrodes were dried at 353 K under a vacuum over 6 hours to strengthen the adhesion of the Si materials.

Operando SEM experiments
An operando SEM observation of the binder-free Si anodes was performed using a two-electrode type full cell with a mesh-type anode, a LiCoO 2 composite cathode with an Al foil current collector (3.0 mA h cm À2 , Hohsen (Japan)), and glass microfiber filter separators (GF/A, Whatman (UK)) with IL electrolytes (Fig. 1d). The three types of binary ILs described above were used as the electrolyte. A commonly used SEM observation system (S-3400N, Hitachi (Japan)) was converted into an operando system by equipping a feed-through terminal, which enables electrochemical measurements in the SEM chamber, with an original SEM system. All the operando experiments were conducted in the vacuum chamber of the SEM system. 24 The variation in morphology in the Si anode during the charge-discharge processes in CC/CV mode was observed by a secondary electron image mode. The charge-discharge rate was 1/2C. The cut-off voltages were À3.88 and À2.40 V (vs. LiCoO 2 ), and at the cut-off voltages, the cut-off current was maintained until the 1/20C rate was achieved. The electrochemical experimental conditions were controlled with a potentiostat/galvanostat (VersaSTAT 4, Princeton Applied Research (USA)).

Results and discussion
Performance of Si thin flake composite anode LIBs with different binary IL electrolytes The first three cycles of the charge-discharge profiles recorded at the Si thin flake composite anodes in the coin-type cells with (a) 83.  ILs at a 3C rate. It should be noted that the first five cycles were conducted at 1/6C to form a better SEI layer on the Si anode. Although their discharge capacities differ by ca. 500 mA h g À1 , the discharge capacity profile itself is very similar. A higher capacity was attained in the cell with the 50.0-50.0 mol% IL and a favorable discharge capacity of over 1100 mA h g À1 was maintained even after 500 cycles. Except the first five cycles conducted at 1/6C, the average values of the coulomb efficiency were 99.4% and 99.7% for the 83.3-16.7 mol% IL and 50.0-50.0 mol% IL, respectively. These outstanding performances seem to meet the requirements of the LIB industry.  In order to clarify the reason for this, further information on the SEI layer formed on the Si thin flake anode was collected via ex situ XPS for finding out information on the SEI layer formed on a Si anode. [26][27][28] In the IL electrolytes used in the present study, the SEI layer is believed to be generated by the decomposition of FSA and TFSA anions. 14,16,29-33 Fig. 5 1 eV), which is known as one of the SEI components having high Li-ion conductivity. 30,31 The fact that the abundance    Fig. 6 and Table S2, ESI †), indicates that such SEI composition is important to get a higher performance out of the full cell with this IL electrolyte, as will be discussed later. Besides, -SO 2 R (168.9 eV and 170.1 eV), 33 Li 2 SO 4 (167.2 and 168.1 eV), 16,33 and Li 2 S based components (162.1-165.3 eV), 16 that have no clear role on battery performance improvement, were also confirmed (Fig. S1, ESI †). Fig. 6 shows the relationship between the chemical species and their composition ratios contained in the SEI layers generated in each IL electrolyte. The detailed data are given in Table S2 (ESI †) and their physical properties are summarized in Table S3 (ESI †) Li[FSA] concentration in the IL system, the abundance ratio of Li 3 N, which has both a superior Li ion conductivity and mechanical strength, slightly increased, while that of LiF decreased. Interestingly, the Li ion conductivity for Li 3 N is more than 150 times higher than that for LiF.
Operando SEM observation of Si thin flake anodes IL, however it was difficult to obtain clear SEM images because the SEM image quality of the IL-covered specimen strongly depends on the density of the ILs, i.e., at higher density, the image quality degrades. 34 As indicated in Table S1 (ESI †), the 50.0-50.0 mol% electrolyte has a higher density than the other two electrolytes. Thus, only two results obtained in the 83.3-16.7 mol% IL electrolytes including the movies by operando SEM observation during the charge process (Movies S1 and S2, ESI †) are further discussed in this paper. As shown in a previous paper, there are two types of changes in morphology, ribbon type lithiation and flat-plate type lithiation, during the charge process of the Si thin flakes. 9 In this study, operando SEM observations reveal that the lithiation behavior depends on the IL electrolyte species. In the 83.3-16.7 mol% [C 2 mim][FSA]-Li[TFSA] electrolyte, many wrinkles appeared after the lithiation (ribbon type lithiation, Fig. 7a). On the other hand, when 83.3-16.7 mol% [C 2 mim][FSA]-Li [FSA] was used as the electrolyte, and even though the Si flakes were temporarily bent during the lithiation process, they eventually expanded in the two-dimensional direction with almost no wrinkles after the process (flat-plate type lithiation, Fig. 7b). Considering these new findings obtained from the ex situ XPS and the operando SEM observation, when the major components in the SEI layer are good Li ion conductors with a sufficient shear modulus (e.g., LiF and Li 3 N), that is, [C 2 mim][FSA]-Li[FSA] IL is used as an electrolyte, the change in morphology tends to be controlled by the flat-plate type lithiation. This means that the lithiation process uniformly proceeds during the charge process due to the SEI layer containing more LiF and Li 3    like structure (Fig. 8a). Meanwhile, in the [C 2 mim][FSA]-Li[FSA] electrolyte, since the major components are LiF and Li 3 N, Li(I) can uniformly diffuse via those components to the Si flakes and the lithiation reaction proceeds homogeneously. Consequently, the Si thin flakes isotropically expand during the charge process (Fig. 8b). It should be remembered that these components also have a desirable shear which can contribute to the improvement of the SEI layer strength. showing outstanding battery performances, such as good cyclability and high rate capability. The differences of morphological variations in Si thin flakes during the charge process are attributable to the chemical species contained in the SEI layer and their abundance ratios. The approach reported in this paper can effectively enhance the battery performance through the combination of active materials and IL electrolytes and is a novel LIB design methodology.

Conflicts of interest
There are no conflicts to declare.