Structural evolution of plasma sprayed amorphous Li4Ti5O12 electrode and ceramic/polymer composite electrolyte during electrochemical cycle of quasi-solid-state lithium battery

A quasi-solid-state lithium battery is assembled by plasma sprayed amorphous Li4Ti5O12 (LTO) electrode and ceramic/polymer composite electrolyte with a little liquid electrolyte (10 µL/cm2) to provide the outstanding electrochemical stability and better normal interface contact. Scanning Electron Microscope (SEM), Scanning Transmission Electron Microscopy (STEM), Transmission Electron Microscopy (TEM), and Energy Dispersive Spectrometer (EDS) were used to analyze the structural evolution and performance of plasma sprayed amorphous LTO electrode and ceramic/polymer composite electrolyte before and after electrochemical experiments. By comparing the electrochemical performance of the amorphous LTO electrode and the traditional LTO electrode, the electrochemical behavior of different electrodes is studied. The results show that plasma spraying can prepare an amorphous LTO electrode coating of about 8 µm. After 200 electrochemical cycles, the structure of the electrode evolved, and the inside of the electrode fractured and cracks expanded, because of recrystallization at the interface between the rich fluorine compounds and the amorphous LTO electrode. Similarly, the ceramic/polymer composite electrolyte has undergone structural evolution after 200 test cycles. The electrochemical cycle results show that the cycle stability, capacity retention rate, coulomb efficiency, and internal impedance of amorphous LTO electrode are better than traditional LTO electrode. This innovative and facile quasi-solid-state strategy is aimed to promote the intrinsic safety and stability of working lithium battery, shedding light on the development of next-generation high-performance solid-state lithium batteries.


Compared with traditional rechargeable lithium batteries
Frequency Magnetron Sputtering Deposition (RFMSD) [2], Pulsed Laser Deposition (PLD) [3], Electron Beam Evaporation (EBE) [4], Chemical Vapor Deposition (CVD) [5], Molecular Beam Epitaxy (MBE) [6], and other methods [7]. However, these methods are not conducive to the high-efficiency preparation of macroscopic large-capacity lithium solid-state batteries used in electric vehicles. The plasma spraying process can realize the structural design and control of complex functional coatings, prepare functional coatings with good stress tolerance and high deposition efficiency, and then can prepare macroscopic large-capacity single battery. Li 4 Ti 5 O 12 (LTO) electrode for lithium-ion batteries is usually prepared by mechanically mixing highly viscous materials, coating, and drying [8][9][10]. This traditional method has some inherent shortcomings: (1) There are many process steps, a long process, low production efficiency, and high cost. (2) It is inevitable to use volatile and toxic organic solvents, such as N-methylpyrrolidone (NMP). (3) The size of the prepared pole piece is limited by the coating machine, and electrodes of any size cannot be customized. But the plasma spraying process can overcome the above shortcomings.
In recent years, the preparation of amorphous modified electrodes or electrolytes through processes can improve their electrochemical performance [1115]. Amorphous Li 2 WO 4 protective layer helps to inhibit the degradation of LiCoO 2 [16]. It can effectively improve the rate performance of the LiCoO 2 material. Amorphous modified silyl-terminated 3D polymer electrolyte enhances thermal stability and cycle performance for lithium metal battery [17]. The remarkable improvements of the amorphous Li 3 PO 4 -coated Li electrode are mainly attributed to high chemical stability and inhibiting the growth of lithium dendrite [18].
In solid-state lithium batteries, ceramic electrolytes have received widespread attention, such as garnet-type Li 7 La 3 Zr 2 O 12 (LLZO) [19] and NASICON-type Li 1+x Al x Ti 2-x (PO 4 ) 3 (LATP) [20] or Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 (LAGP) [21]. Due to its excellent ion conductivity, chemical stability, and wide electrochemical window relative to lithium metal, some of them have become a new class of solid-state electrolyte system (SSES) [22]. However, when the ceramic solid electrolyte is in contact with the metal lithium negative electrode, electrochemical reactions are prone to occur, which reduce the battery performance. A feasible solution strategy is to use ceramic/polymer based composite electrolyte. The ceramic electrolyte channels provide continuous pathways, which help in maintaining a high ionic conductivity between the electrodes, while the polymer channels permit improvement of the mechanical properties compared to those of the ceramic alone, in particular, mitigation of the brittleness of ceramics.
In this contribution, the plasma spraying and fast cooling methods were used to prepare the amorphous thin LTO electrode on the perforated copper foil. Our previous works showed that LTO electrode with excellent electrochemical performance can be prepared by suspension plasma spraying [23]. The flexible electrolyte film with ceramic/polymer based composite was prepared by mechanical mixing and mold casting. And then, we attempted to construct a quasi-solid-state lithium ion battery by adopting flexible film as the solid electrolyte, LTO electrode as cathode, and Li metal as anode. Structural evolution of plasma sprayed amorphous LTO electrode after 200 cycles and cyclic characteristics were investigated for quasi-solid-state lithium ion battery. In order to compare the electrochemical behavior of the two electrodes, traditional LTO electrode was prepared by method of coating slurry on current collector and drying. The plasma sprayed method of preparing amorphous electrodes and applying them to quasi-solid batteries was adopted in this work to improve electrochemical stability. This innovative and quasi-solid-state lithium battery strategy aims to provide a new way to enhance electrochemical cycle performance of quasi-solid-state lithium battery.

1 Preparation of amorphous and traditional LTO electrodes
Our previous work prepared spherical LTO powder material by spray granulation method [23]. The spherical LTO powder was shotted at high speed onto the perforated copper foil by plasma spraying. The perforated copper foil with a thickness of 45 μm and an area of 20 cm 2 was fixed on a copper plate (1 m 2 ) with good heat dissipation. The main gas in the plasma was argon, and hydrogen was used as auxiliary gas. The effective spraying distance was 110 mm, and the plasma current varied between 400 and 500 A. The weight of active material load on the copper foil with holes was 1.1 mg. The schematic diagram of the LTO electrode preparation process is shown in Fig. 1. The traditional LTO electrode was prepared by the method of Ref. [24]. www.springer.com/journal/40145

2 Preparation of ceramic/polymer based composite electrolyte
Ceramic/polymer electrolyte film (CPCEF) was prepared by solution cast method. Before the experiment, the PVDF, LATP, and LiClO 4 powders were vacuum-dried for 24 h at 60 ℃. The corresponding PVDF, LATP, LiClO 4 , and DMF at a mass ratio of 10:1:0.124:80 were weighed. Firstly, 0.5 g PVDF was dissolved in 40 mL DMF with stirring at 65 ℃ for 0.5 h. After forming a transparent viscous solution, 0.5 g LATP and 0.062 g LiClO 4 were added to continue heating and stirring for 0.5 h to make the mixture even. Finally, the evenly stirred solution was casted into a polytetrafluoroethylene mold and placed under vacuum drying at 60 ℃ for 24-72 h. Flexible electrolyte film with ceramic/ polymer composite could be obtained after the solvent was evaporated. The specific illustration is shown in Fig. 2.

3 LTO|ceramic/polymer composite electrolyte|Li quasi-solid battery assembly
CR2025 button battery in a glove box filled with argon was assembled. The plasma sprayed LTO electrode was to cut out the pole piece as the cathode, the Li metal as the anode, and the CPCEF as the solid electrolyte. Then, a given volume (10 μL/cm 2 ) of liquid electrolyte with 1 M LiPF 6 EC/EMC/DEC was injected into flexible electrolyte film to get the quasi-solid-state composite electrolyte. 1-2 spacers were added and assembled with all materials to form a quasi-solid battery (Fig. 3).

4 Characterization and electrochemical measurements
The scanning electron microscope (SEM; GeminiSEM 500, ZEISS) was applied to observe the micromorphology of the plasma sprayed LTO electrode before and after the charge-discharge cycle test. Auxiliary focused ion beam (FIB; 450S, FEI) was used to etch the sample surface for characterizing the microstructure of the sample by transmission electron microscope (TEM; Titan Themis 200, FEI). The prepared quasi-solid battery samples were subjected to charge and discharge test by NEWARE battery test system (CT-4008). At room temperature, the samples were tested by electrochemical impedance spectroscopy (EIS) in the frequency range of 10 5 -10 -3 Hz with electrochemical workstation (CHI660E).

1 Electrochemical performance of amorphous and traditional LTO electrodes
As can be seen from Fig. 4 This shows that the amorphous structure improves the cycle stability of the battery, which is consistent with the conclusion of Ref. [25]. Figure 4(b) shows the cycle performance of amorphous and traditional LTO electrodes at a charge-discharge rate from 0.1 to 0.5 C.
The capacity retention rate is the discharge capacity of each cycle relative to the first cycle. Under 0.1 C charge and discharge cycles, the capacity retention rate of amorphous electrode decreased slowly, but that of traditional electrode decreased rapidly. Clear differences are observed between these two electrodes. The capacity retention rate of the traditional LTO electrode dropped to 9% at a rate of 0.2 C, dropped to 0% when the rate exceeded 0.3 C, and finally returned to 13% at a rate of 0.1 C. In contrast, the amorphous LTO electrode still maintained a high capacity retention rate of 41% at 0.5 C. After 30 cycles, the capacity retention rate of the amorphous LTO electrode recovered to 78% at 0.1 C. The results indicated that the amorphous LTO electrode shows excellent rate performance in terms of high-power durability. Figures 4(c) and 4(d) are the EIS between amorphous and traditional LTO electrodes, and the illustration is its equivalent circuit. R Ω is the internal resistance of the solid electrolyte and electrodes in the circuit, R ct is the charge transfer resistance in the solid electrolyte, and the total resistance R is the sum of R Ω and R ct . Figure shows that the interface impedance of the amorphous LTO electrode|ceramic/polymer composite electrolyte (525 Ω) is smaller than that of the traditional LTO electrode|ceramic/polymer composite electrolyte (3600 Ω), indicating that the amorphous structure interface has excellent electrical conductivity and facilitates the migration of lithium ions to obtain better electrochemical performance, thereby improving the electrochemical performance of the amorphous LTO electrode. This is consistent with the conclusion of Ref. [26].

2 Structural evolution of amorphous LTO electrode
Structural evolution of plasma sprayed amorphous LTO electrode before and after the cycle test is analysed by SEM, STEM, TEM, and EDS to explore the root cause of the difference in electrochemical performance. Amorphous LTO electrode surface micromorphology was investigated using SEM, as shown in Figs. 5(a)-5(d). Spherical LTO powder was sprayed on the perforated copper foil to form a full coverage layer (Fig. 5(a)). After partial magnification, the typical stack forming method of thermal spray coating was found [27] (Fig.  5(b)), and the unmelted spherical LTO particles could also be observed in the figure. When further zoomed into the nanoscale (Fig. 5(c)), local nano-structured tiny particle protrusions were discovered. These protrusions combined with the flexible CPCEF could increase the interface contact between the electrode and the electrolyte film, thereby promoting the electrochemical reaction at the interface. Figure 5(d) is the surface of the amorphous LTO pole piece after the cycle test. It could be seen that the surface protrusions were reduced, and cracks were found in the coating. The appearance of these cracks caused the electrochemical reaction to penetrate deep into the coating. Figure 6 shows the XRD of the LTO coating. The LTO powder has a very strong diffraction characteristic peak, while the coating cannot detect the sharp characteristic peak, indicating that the coating has been transformed into amorphous state. Studies in Refs. [13,16,17] show that the sharp characteristic peaks of amorphous materials cannot be measured accurately.  In order to obtain the structural evolution information inside the amorphous LTO electrode after the electrochemical cycle test, the FIB was used to cut the samples before and after the cycle test, as shown in Fig.  7. It can be known from the figure that the thickness of the thin coating after spraying is about 5-8 μm.
From Figs. 8(a)-8(c), it could be seen that the main elements of the original LTO electrode are Ti and O. Although Li cannot be distinguished in EDS, after the quantitative EDS test, the composition of sample are Ti (53.8 wt%) and O (46.2 wt%) roughly, which can confirm that the coating is LTO. The magnified TEM images of Fig. 8(a) are Figs. 8(d) and 8(e); only the amorphous diffraction ring was observed, indicating that the coating had an amorphous structure. The electrode coating with amorphous structure had good electrochemical stability, high coulomb efficiency, and good rate performance [16,28], which explained the good cycle stability and low interface impedance of the above sprayed LTO electrode.  After 200 cycles of test, microcracks ( Fig. 9(a)) appeared in the coating. Since the quasi-solid electrolyte film contains about 10 mL/cm 2 of the liquid electrolyte whose main inorganic salt is LiPF 6 , these electrolytes penetrate into the amorphous LTO electrode through surface cracks (Figs. 5(c) and 5(d)) and react with the internal electrode (Figs. 9(b)-9(d)). In order to explore the reaction inside the coating, the magnified partially coating electrode (Fig. 10) showed that the crystalline phase of LTO was formed at the interface between rich fluorine compounds and amorphous LTO electrode, which indicated that LTO recrystallization occurred under the action of electrochemical reaction. The crystallization behavior caused the fracture and crack propagation inside the amorphous LTO electrode coating, increased the active sites of the electrochemical reaction, and improved the overall cycle performance.

3 Structural evolution of ceramic/polymer composite electrolyte
The initial state of the CPCEF had a smooth surface with irregular holes (Fig. 11(a)). These holes provided channels for the liquid electrolyte to penetrate, and its surface state was consistent with that reported [24,29]. After 200 cycles, as shown in Fig. 11(b), the number of holes on the surface of the CPCEF increased showing a worm-like connection, which indicated that the structure of the film had changed after the electrochemical reaction. The solid electrolyte composed of polymer and ceramic can give full play to the good mechanical properties of polymer and high ionic conductivity of ceramic, and overcome their respective shortcomings [30,31]. Figure 12 shows the XRD of the CPCEF, which implies the characteristic diffraction peaks of ceramic LATP and PVDF are both obvious before the cycle test, indicating that CPCEF is a mechanical mixture. The CPCEF material retains the respective characteristics of polymer and ceramic, but the crystallinity of the polymer matrix and the conduction of lithium ions will be reduced through the active ceramic filler [32]. The diffraction peak of the ceramic material (LATP) was reduced to a very low level, and the polymer matrix (PVDF) is further amorphized after 200 cycles. The results show that the viscoelastic film of CPCEF has changed after 200 cycles, and the www.springer.com/journal/40145  physical and chemical properties have transformed as well. It indicates that the transformation of physical and chemical properties of the composite electrolyte is one of the reasons for the capacity degradation of the quasi-solid-state battery, but the exact mechanism needs rigorous improvements further.

Conclusions
In this paper, the plasma spraying method was used to prepare amorphous LTO electrode for the study of structural evolution during electrochemical cycle of quasi-solid-state lithium battery. The flexible electrolyte film with ceramic/polymer based composite was prepared by mechanical mixing and mold casting. The electrochemical behavior of amorphous and traditional LTO electrodes has been studied, and the conclusions are as follows: (1) The plasma spraying process conditions can prepare the coating of amorphous LTO electrode of about 8 μm, with internal dense structure. Mechanical mixing and mold casting method can prepare ceramic/ polymer composite electrolyte with good flexibility.
(2) Compared with the traditional LTO electrode, the capacity attenuation rate of the amorphous LTO electrode battery at 0.1 C after 200 cycles is about 68%, and coulomb efficiency is about 99.8%, better than traditional LTO electrode. ( 3) The rate test shows that the amorphous LTO electrode battery can withstand a rate of 0.5 C chargedischarge, while the traditional LTO electrode can only withstand at 0.2 C. The capacity retention and rate performance of the amorphous LTO electrode is better than that of the traditional LTO electrode.
(4) The structure of the amorphous LTO electrode and ceramic/polymer composite electrolyte has evolved after 200 cycles, fracture and cracks have occurred inside the electrodes, and the electrolyte film has changed from an irregular porous state to an ordered hole. The transformation of physical and chemical properties of the composite electrolyte is one of the reasons for the capacity degradation of the quasi-solid-state lithium battery.
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