Improved cycling stability of LiCoO 2 at 4.5 V via surface modification of cathode plates with conductive LLTO

The stability issue of LiCoO 2 cycled at high voltages is one of the burning questions for the development of lithium ion batteries with high energy density and long cycling life. Although it is effective to improve the cycling performance of LiCoO 2 via coating individual LiCoO 2 particles with another metal oxides or fluorides, the rate capacity is generally compromised because the typical coating materials are poor conductors. Herein, amorphous Li 0.33 La 0.56 TiO 3 , one of the most successful solid electrolytes, was directly deposited on the surface of made-up LiCoO 2 cathode plates through magnetron sputtering. Not only the inherent conductive network in the made-up LiCoO 2 cathode plates was retained, but also the Li + transport in bulk and across the cathode-electrolyte interface was enhanced. In addition, the surface chemical analysis of the cycled LiCoO 2 cathode plates suggests that most of the stability issues can be addressed via the deposition of amorphous Li 0.33 La 0.56 TiO 3 . With an optimized deposition time, the LiCoO 2 cathode plates modified by Li 0.33 La 0.56 TiO 3 performed a steady reversible capacity of 150 mAh/g at 0.2 C with the cut-off voltage from 2.75 to 4.5 V vs. Li + /Li, and an 84.6% capacity gain at 5 C comparing with the pristine one.


Introduction
Lithium ion batteries (LIBs) have been urged for high energy density, high rate capability, and long cycling life, with increasing energy storage demands in portable electronics, electrical vehicles, and stationary power sources [1][2][3]. The most direct way to increase LIBs' energy density is to apply cathode materials with higher capacities and/or higher working voltages [4][5][6][7][8]. LIBs with LiCoO2 (LCO) cathode has gained great commercial success in the past 3 decades, especially as the power source for portable electronics, benefiting from its high specific capacity, high redox potential, and long cycling life [9][10][11][12]. However, the generally utilized specific capacity of LCO can only reach 140 mAh/g, roughly half of its theoretical capacity of 272 mAh/g, with the upper cut-off voltage of 4.2 V vs. Li+/Li [11][12][13]. Theoretically, the utilized specific capacity can be improved by increasing the cut-off voltage. However, the cycling stability of LCO is poor when the cut-off voltage exceeds 4.2 V vs. Li+/Li [1]. In addition, it is demonstrated that the capacity decay of LCO below 4.5 V vs. Li+/Li is mainly due to the cacoethic side reactions, Co dissolution, and HF corrosion at the liquid-solid interface between LiPF6-based organic electrolyte and LCO cathode [14,15]. Therefore, great efforts have been made to realize a stable cathode-electrolyte interface at 4.5 V vs. Li+/Li via surface modifications of LCO [16][17][18].
In terms of structural feature, the surface modifications can be divided into two types. In one type, the modification layer is coated on individual LCO particles before casting the cathode plates [16][17][18]. In the other type, the modification layer is deposited on the surface of made-up cathode plates [19,20]. Although the surface modification on individual LCO particles is effective to improve its cycling stability [16][17][18], and can be easily realized via low-cost wet chemical routes [21][22][23][24], there are some disadvantages limit its application. For example, the modification layer on particles may break due to the severe mechanical impacts during slurry mixing and electrode calendering [13]. In addition, the modification layer on individual particles may tip the balance of ionic conductivity and electronic conductivity in the bulk of cathode plates [1]. Alternatively, the surface modification of made-up cathode plates, which is carried out after LCO granulating, slurry mixing, and electrode calendaring, and only introduces a thin layer of modification materials on the surface of cathode plates, is potential to addressing the above issues [13,19,20,25].
Herein, amorphous Li0.35La0.56TiO3 (-LLTO), which is one of the most successful solid electrolytes [29][30][31], was directly deposited on the surface of made-up LCO cathode plates through magnetron sputtering (Figure 1a). The sputter-deposited -LLTO doesn't require high temperature heat treatment, and performs a high ionic conductivity (1.54 × 10-5 S/cm at room temperature). It is inspiring that the electrode level surface modification by -LLTO not only doesn't impair the bulk conduction in LCO cathode, but also enhances the charge transfer kinetics at LCO-electrolyte interface, which is favorable for rate capacity. In addition, the deposited -LLTO effectively prevents Co dissolution, HF corrosion, and other side reactions at LCOelectrolyte interface. The LCO-LLTO-electrolyte configuration leads to a relatively stable interfacial polarization. As a result, the presented surface modification of cathode plates with -LLTO, enables LCO steady operates for more than 100 cycles with an upper cut-off voltage of 4.5 V vs. Li+/Li, and a reversible capacity of 150 mAh/g at 0.2 C.

The preparation of LCO cathode and surface modification by -LLTO
LCO cathode plates were prepared by spreading well-mixed commercial LCO powders (Aladdin, 80 wt%), acetylene black (MTI KJ Group, 10 wt%) and PVDF (Arkema, 10 wt%) on the surface of Al foil. N-methyl-pyrrolidone was used as solvent to form the slurry. The as-casted LCO cathode plates were dried in dynamic vacuum overnight at 110 C to remove the solvent and trace water after calendering. The statistic thickness of the casted LCO cathode is ~ 40 m, which is determined by a screw micrometer. -LLTO was deposited on the surface of Si substrates or LCO cathode plates by magnetron sputtering. The cavity was evacuated to 5  10-4 Pa or less.
The LCO cathode plates were pre-heated at 120 C for 30 min in vacuum to remove the trapped moisture and air. The Li0.33La0.56TiO3 target was pre-sputtered for 5 min to remove dust and foreign particles on the surface. The distance between target and substrate was 15 cm. The sputtering power was 120 W. The working pressure was 1 Pa.
The argon and oxygen ratio was 70 : 30 (sccm). The substrate temperature was kept at 120 C. To obtain the modification layers with different thickness, the sputtering time

Materials characterizations
The thickness of LLTO films on Si substrate were determined using cross-section scanning electron microscopy (SEM). The phase analysis was performed by X-ray diffraction (XRD) using CuKα radiation. The surface morphology of LCO electrodes were observed by SEM. The elemental distributions of Co, C, La and Ti were analyzed by Energy Dispersive Spectrometer (EDS). X-ray photoelectron spectroscopy (XPS, Thermo Fisher Escalab Xi+) was used to analyze the surface chemical compositions of the electrodes.

Results and discussion
Energy density and rate capability are the two core requirements for cathode technologies. It raises the challenge for the surface modification of LCO cathode. The conductive network in cathode plate for Li+ and e-should be maintained, while the LCO-electrolyte interface is stabilized via introducing inactive materials as little as possible. As shown in Figure 1a, we propose coating the made-up LCO cathode plate with -LLTO through magnetron sputtering. The sputter-deposited -LLTO would form a conformal, dense, and very thin overburden on the surface of LCO cathode plate.
The following merits can be reasonably expected. First, the potential damages to the modification layer during the preparing process of cathode plates are avoided. Second, the mass fraction of -LLTO in the modified cathode plate is very small. Third, the undesirable interactions between LCO and electrolyte can be suppressed effectively.
Last and most importantly, the deposited -LLTO will not undermine the transport pathways for Li+ and e-in the cathode plate because it mainly exists near the top surface of the cathode plate. The X-ray diffraction peaks derived from the Li0.35La0.56TiO3 target used here are well identical with crystalline LLTO (PDF # 46-0465) (blue line in Figure 1b).
However, no diffraction peaks belonging to crystalline LLTO can be observed in the XRD pattern of the LLTO thin-film on Si substrate (purple line in Figure 1b). The diffraction peak at 28.48 should ascribe to the Si substrate. It is reasonable to conclude the as-deposited LLTO thin-film is amorphous. As shown in Figure 1c, the as-deposited LLTO thin-film is homogeneous, dense, and without any crystalline grains, which further confirms it is amorphous. The ionic conductivity of -LLTO thin-film is calculated based on its bulk resistance determined by the intercept on Zre axis of EIS curve ( Figure 1d) and its thickness determined in the side-view SEM image (insert in Figure 1c). The as-deposited -LLTO thin-film performs an ionic conductivity of 1.54 × 10-5 S/cm at room temperature, which is comparable to the reported values for -LLTO thin-film solid electrolytes [32][33]. Additionally, the previous literatures have demonstrated -LLTO thin-film solid electrolytes are with excellent chemical and electrochemical stabilities [31]. Therefore, it is potential to construct a highly stable and  Figure 1b). This is consistent with the fact that the as-deposited LLTO is in an amorphous form. Fortunately, LCO-LLTO-10 may achieve an optimized balance between the cycling stability and charge carrier transport at cathode-electrolyte interface in this study. As cycling rate increases, the positive effect of the -LLTO modification becomes more and more notable (Figure 2b-2d). The average discharge capacities of pristine LCO and LCO-LLTO-10 at the different cycling rates are listed in Table 1. After two activating cycles at 0.1 C, the specific capacities of pristine LCO are slightly lower but very closed to that of LCO-LLTO-10 at 0.2 C and 0.5 C. However, LCO-LLTO-10 exhibits remarkably higher capacities than pristine LCO when the cycling rate surpassed 1 C. Especially, the discharge capacity of LCO-LLTO-10 reaches 72 mAh/g at 5 C, which is 84.6% higher than that of LCO (39 mAh/g). The superior rate capacity of LCO-LLTO-10 indicates that the surface modification with a proper -LLTO deposition time, would not only retain the conductive network for Li+ and e-in the cathode plates, but also somehow enhance its charge transport and/or transfer.  Table 2 The polarization voltage (V) of pristine LCO and LCO-LLTO-10 in 5 cycles impedance, which is related to Li+ diffusion within LCO cathode plates [34]. The Li+ diffusion coefficient + can be calculated by Equation 1 [35]: where R is the ideal gas constant (J/(mol*K)), T is the Kelvin temperature (K), A is the effective electrochemical interfacial area (cm2), n is the charge number of carrier ions, where Zre is the Warburg impedance, Rtotal is the start resistance of the oblique line, and  is the angular frequency corresponding to the impedance sweep frequency f (=2f).
According to the Zre vs -1/2 plots shown in Figure 3b, the + of LCO-LLTO-10 is determined to be 7.52  10-12 cm2/s, which is much higher than that of pristine LCO (2.32  10-12 cm2/s). This suggests the bulk ionic conduction in the LCO cathode plates is enhanced by the deposited -LLTO, which can be attributed to the following facts.
First, the ionic conductivity of deposited -LLTO is much higher than that of LCO (~10-8 S/cm) [36]. Second, the sputtering process, which is a physical vapor deposition, enables the deposited -LLTO to form well-contacted interfaces with LCO particles [19,20]. Third, the deposited -LLTO may provide additional Li+ transport pathways in the cathode plates [19,35]. This well agrees with the observed excellent rate capacities of LCO-LLTO-10.
It is generally believed that a decreased Rct would lead to a smaller polarization voltage (ΔV), which is the difference between redox peaks in CV profile. Figure 3c and   3d show the CV profiles of pristine LCO and LCO-LLTO-10 for 5 sweeping cycles.
The values of ΔV are summarized in Table 2 (Table 2). This implies that the interface between pristine LCO and liquid electrolyte is continuously degrading, while the LCO-LLTO-electrolyte configuration leads to an excellent interfacial stability. The potential mechanisms causing the degradation of LCO cycled at high cut-off voltages include, but are not limited to, electrolyte oxidation by delithiated LCO [37], oxygen loss of LCO [38,39], Co dissolution [40], and HF corrosion at the cathodeelectrolyte interface [41]. To reveal how the deposited -LLTO helps to stabilize the  [42,43].
Generally, the polyether carbon and carbonyl group are considered to result from electrolyte decomposition [42,43].

Conclusions
In summary, the cycling stability and rate capacity of LCO at high cut-off voltage

Availability of Data and Materials
The data supporting the conclusions of this article are included within the article and its additional files.

Competing Interests
The authors declare no competing financial interest.

Authors' Contributions
Shipai Song and Xiaokun Zhang conceived the experiment and carried out data analysis.
Shipai Song carried out the samples fabrication with assistance from Kai Huang, Hao Zhang, and Fang Wu. Shipai Song and Xiang Peng performed the materials characterizations and electrochemical measurements. Shipai Song, Yong Xiang and Xiaokun Zhang wrote the paper. All the authors discussed the results and commented on the submitted manuscript.