Simple way of making free-standing cathode electrodes for flexible lithium-ion batteries

The flexible electrodes used in the lithium-ion battery (LIB) offer an excellent opportunity to be bent and folded without deforming their electrochemical characteristics. However, a flexible electrode does not include metal foil as a current collector, limiting the LIB's flexibility and weakening the mechanical strength. This study fabricates flexible LiFePO4 (LFP) free-standing electrodes by a scalable and straightforward solution-based etching process. The obtained free-standing electrodes show capacities and bending performances that are similar to the conventional electrodes with aluminum current collectors. This study opens a new avenue for developing a free-standing electrode for low-cost and flexible lithium-ion batteries.


Introduction
In recent years within a short period, the demand for exible lithium-ion batteries (LIBs) with high-energy capacity vastly increased due to the rapid development of foldable and wearable devices such as roll-up displays, active radio-frequency identify cation tags, and integrated circuit smart cards. 1-3 LIBs can be exible or even foldable via the conventional engineering approach by making thinner ones, but LIBs with lighter congurations will signicantly reduce the overall capacity. Therefore, different methods need to develop to manufacture exible and high-capacity LIBs. 4,5 Conventional LIB metal current collectors such as copper (Cu) and aluminum (Al) are typically used as anode and cathode, respectively. Moreover, these rigid and non-exible metal current collectors have been considered dead weight in LIBs. Hence they signicantly reduce the overall energy density of LIBs and limit their exibility. [6][7][8] In a new approach to fabricate lighter, more secure, and more efficient LIBs, a variety of carbon-based current collectors, including carbon nanotubes, [9][10][11] carbon paper, 12-14 graphene paper, [15][16][17] and carbon ber 18,19 have been developed to replace the traditional metal foils. For instance, Chen et al. developed a highly conductive ($3000 S cm À1 ) reduced graphene oxide lm by the current-induced annealing method and demonstrated its applicability as a lightweight current collector. 20 Liu et al. reported TiO 2 and activated carbon-ber composite as light and ultra-long life (2000 cycles at 20C) of LIB performance where the intrinsic open channel structure and large surface area of the ultra-thin TiO 2 nano-sheets possessed the fast charge and discharge. 21 However, the higher resistance of the electrodes and lower loading level of the active material of the previous approaches have created a signicant obstacle for making exible LIBs. Moreover, the previously reported methods require a modied fabrication process and an extensive investigation of the reliability before the exible applications. Practically, suppose we could re-engineer one of the LIBs components (metal foil current collectors) with the freestanding exible electrodes. In that case, it immediately overcame all the fabrication issues of efficient LIBs. Therefore, we believe further investigation and the feasibility study of utilizing free-standing exible electrodes for LIBs which are necessary for the conguration of exible LIBs.
Herein, in this study, we prepare a free-standing and exible electrode (cathode) by a conventional fabricating process with LiFePO 4 (LFP) for LIBs to maximize the reliability for exible device application. Additionally, we introduce two different etchants for the as-prepared exible electrode compared to the physical and electrochemical properties of the cathode material. This novel lightweight free-standing cathode electrode possesses an extremely high energy density than the conventional LFP/Al electrode for exible LIB application. pyrrolidone (NMP) solvents were purchased from Alfa Aesar. Al foil was purchased from Ubiq Tech Co., Ltd. Two etchants, H 3 PO 4 -based etchant (Al Etch-M1) and TMAH-based etchant (Al Etch-M2), were obtained from Hai-Bo Advanced Chem-Materials Co., Ltd.

Preparation of free-standing electrode
The slurry was composed of LFP (80%) with PVDF (10%) and Super P (10%) in NMP, then coated on carbon-coated aluminum foil and dried in a vacuum at 120 C for 12 h. The thickness and loading levels were 36 mm and 1.65 mg cm À2 , respectively. To prepare the exible LFP free-standing lm, the prepared LFP/Al lm electrode was directly placed in two different undiluted etchants: Al Etch-M1 etchant and Al Etch-M2 etchant to etch the Al foil within 1 minute at 25 C, respectively. All the chemicals and solvents are used at their original concentrations without further dilution. Finally, the free-standing LFP lm was collected from the respective etchants, washed with DI water, and dried at 60 C for 30 min under vacuum. The complete schematic preparation route for LFP free-standing electrode (LFP-Al) is illustrated in Fig. 1. For comparison study, a conventional cathode with carbon-coated Al-foil current collector was also prepared by using a similar slurry solution composed of LFP (80%) with PVDF (10%) and Super P (10%) in NMP.

Characterizations
A eld-emission scanning electron microscope (FE-SEM, JSM-6500) analyzed all samples' surface morphology and crosssection image. All samples' short-range order crystal structure from Raman spectra were obtained using MRS5000 Micro Raman Spectrometer 532 nm with 125 mW DPSS laser (Rayleytek Co., Ltd). Different etchant solutions were determined iron (Fe) using inductively coupled plasma optic emission spectrometry (ICP-OES, Spectro Arcos, SPECTRO model Analytical Devices, PerkinElmer OPTIMA-7300DV). First, the etchant solution was dried on the dish, and then add the 6% HNO 3 to cover the samples. Finally, the solution was injected into the vial and lid on the ICP-OES for detection. Typically, the samples prepared for the iron dissolution experiments were injected into an air/acetylene ame. Aer 10 s of stabilization, the measurement was recorded. In ICP-OES, three measures (5 s each) were recorded for iron absorbance at the wavelength of 372 nm. The iron contents of the samples were obtained from the calibration curve of various Fe concentrations prepared by using an iron standard solution (Sigma-Aldrich [Fe] ¼ 1000 ppm).

LIB fabrication
The LFP/Al electrode and LFP free-standing electrode were introduced in the typical CR2032 coin-type cells. In the CR2032 coin-type cells fabrication, lithium metal was used as a counter electrode. As an electrolyte, 1 M LiPF 6 in a mixture of 1 : 1 (v/v) ethylene carbonate (EC) and diethyl carbonate (DEC) was used as a counter electrode and an electrolyte, respectively. A Celgard 2325 triple layer of PP/PE/PP membrane was used as a separator was soaked in electrolyte for 24 h before cell assembly. The cells were assembled inside an Ar-lled glove box where the H 2 O and O 2 were lower than 1 ppm. The charge-discharge cycling properties of the as-prepared CR2032 coin-type cells were measured in the voltage region of 2.5-4.2 mV (vs. Li/Li + ) under a constant current using an Ubiq BAT-750B battery test system. Aer fabricating the LFP/Al and LFP free-standing lm battery, all battery coin-cells were stored for 8 h before the chargingdischarging process for stabilization. The cyclic voltammogram (CV) was performed at a scan rate of 0.1 mV s À1 over a range of 2.5 to 4.2 mV (vs. Li/Li + ) on the Autolab electrochemical station. The electrochemical impedance spectroscopy measurements were also carried out from 0.1 Hz to 100 kHz using a 10 mV AC signal.

Characterization
Field-emission scanning electron microscopy (FE-SEM) was used to clarify the microstructure differences on the surface and cross-section of all prepared LFP/Al electrodes before and aer etching processes, as shown in  well dispersed and bounded with PVDF and SP. The surface morphology of LFP/Al-etched M1 electrode clearly shows the interwoven network and smooth structure. In contrast, LFP/Al electrode and LFP/Al-etched M2 electrode surfaces only have small inter-particle connected void space. The observed pores and voids between LFP particles can provide channels for effective electrolyte diffusion and absorption, which possesses an essential impact on the electrochemical performance for improving the rate performance of the battery. In Fig. 2d Fig. 3a and c. [22][23][24] Additionally, the other observed bands approximately in the region from 300-1100 cm À1 correspond to the intra-molecular stretching modes of the specic -Fe-O (300-600 cm À1 ) and -PO 4 (900-1100 cm À1 ) of LFP. The observed broad peak of PO 4 stretching bands is well matched with the other reports where three different regions appear at 950 cm À1 , 986 cm À1 , and 1058 cm À1 , respectively. 25

LIB performance
To understand the various factors contributing to electrode performances, Fig. 4 shows the LIB performances of the cell using LFP/Al electrode, LFP/Al-etched M1 electrode, and LFP/Aletched M2 electrode. The initial three cycles were carried out at 0.1C and the following 50 cycles at 0.5C. The batteries using LFP/Al electrode and LFP/Al-etched M2 electrode show stable cycling up to 50 cycles, as shown in Fig. 4a and c, respectively. They exhibit excellent coulombic efficiencies over 99% aer the second cycle. However, the LIB using LFP/Al-etched M1 electrode demonstrates almost zero capacity. The observed battery cycle performance reveals that the deterioration in the LFP crystal structure during the Al Etch-M1 etching affects the performance of free-standing FEA/Al-etched M1 electrodes.
The free-standing cathode's LFP/Al-etched M2 capacity is between 135 and 137 mA h g À1 . In comparison, the supported electrode capacity drops from 137 mA h g À1 of C-LFP to only 56 mA h g À1 of the electrode (empty symbols in Fig. S4 †). The collector-free demonstrated here leads to an apparent increase of the gravimetric specic capacity by reducing the amount of inert material within the entire electrode. Additionally, to   evaluate the electrochemical performance, the cyclic voltammetry (CV) between 2.0-4.5 V (versus Li + /Li) at a scan rate of 0.1 mV s À1 was conducted using 1 M LiPF 6 in 1 : 1 (v/v) EC/DEC as an electrolyte, as shown in Fig. 5a. The CV curves for LFP/Aletched M2 electrode (black line) and LFP/Al electrode (red line) indicate that the Al Etch-M2 etchant does not adversely affect the electrochemical properties of the LFP electrode because the polarization (DE) of the two electrodes are similar. Furthermore, the observed potential gap changed from 0.34 V to 0.31 V for the LFP/Al-etched M2 electrode, smaller than 0.03 V than the LFP/Al electrode, indicating that LFP/Al-etched M2 electrode has a better kinetics process and stability than LFP/Al electrode. The electrochemical impedance study was performed in Fig. 5b. The Nyquist plot shows an intercept on the real axis at high frequencies attributed to the electrolyte resistance. A semicircle was observed in the high-middle frequency region for two electrodes. The diameter of this semicircle on the Z re axis is approximately equal to the charge-transfer resistance through the electrode/electrolyte interface. The observed straight line at the lower frequency region reveals the diffusion of lithium ions into the bulk of the electrode. The calculated tting values of the charge-transfer resistance R ct of LFP/Al electrode is 59 U, which is higher than that of LFP/Al-etched M2 electrode (R ct ¼ 73 U). The lower R ct value is attributed to the better charge transfer characteristics for LFP/Al-etched M2 electrode than LFP/Al electrode. Notably, the LFP/Al-etched M2 electrode is more exible and lighter than the LFP/Al electrode. Aer carefully evaluating all the results, LFP/Al-etched M2 electrode fabricated by a novel method could be an excellent candidate for highly exible LIBs.

Bending stress test
To investigate the cycling stability of the cell under bending stress for LFP/Al-etched M2 electrode, the galvanostatic charging/discharging curves of the LIB are assembled aer the electrode bending condition (50 times at bending radius 3 mm), as shown in Fig. 6. The initial discharge capacity of the battery is around 154 mA h g À1 with coulombic efficiency of 98% at 0.2C. The detail of capacity changes at different cycles is shown in Table 2. The observed galvanostatic charging/discharging curves under bending stress reveal the superior exibility and electrochemical performance of LFP/Al-etched M2 electrode.

Mechanism insight of different etchant
The wet chemical aluminum etching process using the Al Etch-M1 etchant a well-established for many electronics industries. 28 The functions of each component in this specic etchant are different, like the nitric acid used for alumina oxidation reaction; phosphoric acid removes the reaction products away from the surface of the substrate, and acetic acid is used as a diluent to maintain a low etching concentration. The primary mechanism of wet chemical aluminum etching reaction steps are as follows: 29 Anode: Al / Al 3+ + 3e À Cathode: Precipitation reaction of metal hydroxides: Alumina or aluminum hydroxide dissociates as trivalent aluminum ions in acidic solutions with a pH less than 4. The reactions steps are as follows Al(OH) 3    perform the charging and discharging process during galvanostatic charge-discharge measurement. The Al Etch-M1 etchant contains a certain percentage of HNO 3 ; therefore, the highly oxidative HNO 3 effectively attacks the Fe in the LFP electrode material under an acidic medium followed by oxidation, hydrolysis, and crystallization reaction iron convert into hematite. 29 The complete reaction steps are as follows. 30 Oxidation: On the other hand, the Al Etch-M2 etchant shows high selectivity without affecting the iron content of the LFP cathode material. The etching mechanism of the Al Etch-M2 etchant is as follows. 30 Anode: Al / Al 3+ + 3e À

Conclusion
This study demonstrates the successful fabrication of a highenergy-density battery based on a free-standing and exible electrode by using a simple etching process to remove aluminum foil. This free-standing LFP/Al-etched M2 electrode has good exibility, high capacity, and stable cyclic performance even under the bending stress tests. Moreover, this simple etching technique is versatile for fabricating a broad class of exible free-standing anode and cathode materials for LIBs, which is applied in portable or wearable electronic devices.

Conflicts of interest
There are no conicts to declare.