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 the electrode 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 electrode with a-LLTO through magnetron sputtering. The sputter-deposited a-LLTO would form a conformal, dense, and very thin overburden on the surface of LCO electrode. The following merits can be reasonably expected. First, the potential damages to the modification layer during the preparing process of electrodes are avoided. Second, the mass fraction of a-LLTO in the modified electrode is very small. Third, the undesirable interactions between LCO and electrolyte can be suppressed effectively. Last and most importantly, the deposited a-LLTO will not undermine the transport pathways for Li+ and e- in the electrode because it mainly exists near the top surface of the electrode.
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 a-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 a-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 a-LLTO thin-film solid electrolytes [32-33]. Additionally, the previous literatures have demonstrated a-LLTO thin-film solid electrolytes are with excellent chemical and electrochemical stabilities [31]. Therefore, it is potential to construct a highly stable and conductive LCO-electrolyte interface via deposition of a-LLTO on the surface of LCO electrode.
A conformal coating of a-LLTO on LCO electrode can be realized by sputtering deposition. As seen in Figure 1e, pristine LCO electrode is with a porous microstructure. The surface modification by depositing a-LLTO for 10 min doesn’t change the surface morphology of electrode significantly (Figure 1f). When the deposition time is extended to 100 min, an overburden can be observed because of the accumulation and agglomeration of a-LLTO particles (Figure 1g). However, the distribution of La and Ti observed in EDS mapping images (Figure 1h) suggests that LCO electrode has been evenly and conformally covered by a-LLTO after 10 min deposition. Co and C are also detected by the surface composition analysis, which indicates the thickness of deposited a-LLTO is much thinner than the probing depth of EDS (~ 1 um). The XRD patterns of LCO electrodes with and without a-LLTO modification are similar and in good agreement with the standard pattern of LCO (PDF#44-0145), even though the deposition time of LLTO is extended to 100 min (green, yellow, pink, red, and black lines in Figure 1b). This is consistent with the fact that the as-deposited LLTO is in an amorphous form.
It is difficult to directly determine the thickness of LLTO layer on the porous LCO electrode because the surface of porous LiCoO2 electrode is rough. However, the thickness of LLTO thin films deposited on a flat substrate with the same processing parameters should be a useful reference. Thus, we deposited LLTO thin films on silicon wafers, and determined the thickness using a profilometer. The thicknesses of LLTO thin films deposited by 10 min, 30 min, 60 min, and 100 min sputtering, are 11 nm, 24 nm, 52 nm, and 80 nm, respectively (Figure S1).
The cycling stability and rate capacity of LCO can be effectively improved via depositing a-LLTO on the made-up electrodes (Figure 2). The LCO electrodes with and without a-LLTO coating were cycled in the cut-off voltage ranging from 2.75 to 4.5 V vs. Li+/Li at various cycling rates. In the initial charge/discharge cycles at high voltages, the capacity decay is dominated by irreversible phase transitions and the destruction of crystal structure of LCO [34-38]. Meanwhile, the negative effects of undesired side reactions on the cycling performance of LiCoO2 become more and more pronounced in the later charge/discharge cycles, because of the sluggish kinetics of the interfacial side reactions. The LLTO surface modification mainly address the undesired side reactions at the LiCoO2-electrolyte interface. Thus, all the samples show similar initial discharge capacities (~ 195 mAh/g) at 0.2 C. And their capacity retention in the first 20 cycles is closed, as shown in Figure 2a. In the sequential cycles, the positive effect of the deposited a-LLTO is gradually emerged. After 100 cycles, the discharge capacity of pristine LCO drops to 108 mAh/g. The capacity retention is only 55.4%, comparing with its initial discharge capacity. Meanwhile, LCO-LLTO-10 presents a stable discharge capacity as high as 150 mAh/g after 100 cycles, corresponding to a capacity retention rate of 76.9%. However, a thicker overburden on the surface of porous electrodes will increase the interfacial impedance (Figure S2). The discharge capacities of LCO-LLTO-30, LCO-LLTO-60, and LCO-LLTO-100 after 100 cycles fall in between the pristine LCO and LCO-LLTO-10, which are 122, 124, and 123 mAh/g, respectively. In the later cycles, LCO-LLTO-10 performs a superior capacity retention, since and it may achieve an optimized balance between the cycling stability and charge carrier transport at cathode-electrolyte interface in this study.
It should be note that a considerable decay of capacity can be observed in Figure 2a, which should ascribe to the irreversible phase transition and the crystal structure destruction of LCO, although the presented study demonstrated the positive effects of electrode-level LLTO coating on the cycling stability of LCO at high voltages. On the other hand, the efforts of improving the structure stability of LCO cycled at high voltages through foreign elements doping have made significant progress recently [39-41]. It is promising to develop the strategies for promoting the performance of LiCoO2 at high voltages based on the synergetic effect of coating and doping.
Table 1. The average discharge capacities (mAh/g) of pristine LCO and LCO-LLTO-10 at different cycling rates
Cycling Rate
|
0.1 C
|
0.2 C
|
0.5 C
|
1 C
|
2 C
|
5 C
|
Pristine LCO
|
200
|
195
|
184
|
163
|
107
|
39
|
LCO-LLTO-10
|
200
|
194
|
187
|
175
|
142
|
72
|
As cycling rate increases, the positive effect of the a-LLTO modification becomes more and more notable (Figure 2b). 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. For example, the discharge capacity of LCO-LLTO-10 reaches 72 mAh/g at 5 C, which is 84.6% higher than that of pristine LCO (39 mAh/g). In addition, the discharge voltage platform of pristine LCO has almost disappeared at 2 C (Figure 2c), which indicates the aggravated interfacial polarization due to the limited charge transfer kinetics. Meanwhile, LCO-LLTO-10 performs an observable discharge voltage platform (~3.76 V) at 2 C (Figure 2d). The superior rate capacity of LCO-LLTO-10 indicates that the surface modification with a proper a-LLTO deposition time, would not only retain the conductive network for Li+ and e- in the electrodes, but also somehow enhance its charge transport and/or transfer.
It is worthy to note that the cells used for testing rate performance didn’t exhibited the fast capacity decay in the initial 20 cycles (Figure 2b), which is markedly different from the trend shown in Figure 2a. It should ascribe to the two activation cycles at 0.1 C before the rate performance testing, because these two cycles may help to form a uniform and dense cathode electrolyte interphase (CEI) layer.
Table 2 The polarization voltage (DV) of pristine LCO and LCO-LLTO-10 in 5 cycles
DV
|
DV1st (V)
|
DV2nd (V)
|
DV3rd (V)
|
DV4th (V)
|
DV5th (V)
|
Pristine LCO
|
0.210
|
0.206
|
0.207
|
0.209
|
0.210
|
LCO-LLTO-10
|
0.230
|
0.232
|
0.232
|
0.232
|
0.232
|
To investigate the effects of the deposited a-LLTO on the conduction in LCO electrodes, EIS measurements of pristine LCO and LCO-LLTO-10 are conducted at room temperature with a cathode-liquid electrolyte-lithium metal configuration (Figure 3a). The different intercepts on Zre axis of their EIS curves indicate that tested cell with LCO-LLTO-10 possesses a much smaller total resistance than the counterpart with pristine LCO. The inclined line at low frequency is derived from Warburg impedance, which is related to Li+ diffusion within LCO electrodes [42]. The Li+ diffusion coefficient can be calculated by Equation 1 [43]: (see Equation 1 in the Supplementary Files)
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, F is the Faraday constant (C/mol)), C is the Li+ concentration in the unit cell volume (mol/cm3), and d is Warburg coefficient (Ω*(rad/s)0.5). The Warburg coefficient d can be determined by Equation 2 [43]:(see Equation 2 in the Supplementary Files)
where Zre is the Warburg impedance, Rtotal is the start resistance of the oblique line, and w is the angular frequency corresponding to the impedance sweep frequency f (w=2pf). According to the Zre vs w-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 electrodes is enhanced by the deposited a-LLTO.
The influence of the deposited a-LLTO on the Li+ transport across LCO-electrolyte interface should be more pronounced. The measured impedance spectra are further fitted with the equivalent circuit model shown in the inset of Figure 3a. Here, Rs, Rf, and Rct represent the bulk resistance of electrolyte, the resistance of the solid electrolyte interphase on the surface of cathode, and the charge-transfer resistance, respectively. Consequently, LCO-LLTO-10 exhibits a much lower Rct (40.9 W) than LCO (101.8 W), indicating the deposited a-LLTO drastically enhanced Li+ transport across LCO-electrolyte interface.
It is generally believed that the added interfaces will enhance the difficulties in charge transport and transfer. However, the above EIS analyses suggest the Li+ transport in bulk and charge transfer across interface are both improved via the deposition of a-LLTO on the surface of LCO electrode. This well agrees with the observed excellent rate capacities of LCO-LLTO-10 (Figure 2c and 2d), and can be attributed to the following facts. First, the ionic conductivity of deposited a-LLTO is much higher than that of LCO (~10-8 S/cm) [44]. Second, the sputtering process, which is a physical vapor deposition, enables the deposited a-LLTO to form well-contacted interfaces with LCO particles [19,20]. Third, the deposited a-LLTO may provide additional Li+ transport pathways in the electrodes [19,43].
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. Unexpectedly, LCO-LLTO-10 exhibits larger ΔV than pristine LCO for the 5 sweeping cycles. The contradiction between the smaller Rct and larger ΔV for LCO-LLTO-10 can be explained as follows. The experimentally determined Rct for LCO-LLTO-10 may be derived from the LCO-LLTO interface, while the Rct for pristine LCO is derived from the interface between LCO and liquid electrolyte. As above mentioned, the physical vapor deposition leads to a well-contacted LCO-LLTO interface, in turn bring about the decreased Rct for LCO-LLTO-10. Meanwhile, ΔV is derived from the total impedance from liquid electrolyte to LCO. The introduction of a-LLTO may add two interfaces in the LCO-LLTO-electrolyte configuration, which are LCO-LLTO and LLTO-electrolyte interfaces. In addition, the ionic conductivity of a-LLTO is much lower than that of liquid electrolyte, even though it is one of the most conductive solid electrolytes. Thus, larger ΔV are observed in the sample with LCO-LLTO-10. Since the charge transfer between cathode and electrolyte generally is the rate-limiting step, and the Li+ transport between a-LLTO solid electrolyte and LiPF6-based liquid electrolyte should be relative fast [43], the better rate capacity and larger ΔV observed for LCO-LLTO-10 is reasonable. More importantly, LCO-LLTO-10 maintains a constant ΔV in the 5 sweeping cycles, while the ΔV for pristine LCO varies and shows an increasing trend (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.
As mentioned above, a coating layer of non-conductive materials on individual particles may impede the interfacial charge transfer. Meanwhile, the analysis on Rct and ΔV indicates that the charge transfer between LCO electrode and liquid electrolyte is promoted by the deposited very thin layer of a-LLTO. Thus, it is reasonable to expect that the cycling stability and rate performance of LiCoO2 should be further improved if the individual particles are well-coated by an ultrathin layer of conductive a-LLTO.
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 [45], oxygen loss of LCO [46,47], Co dissolution [48], and HF corrosion at the cathode-electrolyte interface [49]. To reveal how the deposited a-LLTO helps to stabilize the interface, the surface chemistry of the electrodes based on pristine LCO and LCO-LLTO-10 are analyzed by XPS after 100 charge-discharge cycles. The signals of La, Ti, and O are much strong in the spectrum derived from the cycled LCO-LLTO-10 (Figure 4a), indicating that La and Ti are not dissolved in the electrolyte during cycling. In addition, the peak at 530 eV, which ascribes to the titanate oxygen in LLTO, is observed in the O 1s spectrum derived from cycled LCO-LLTO-10 (Figure S3). Moreover, the F 1s spectrum only show two peaks derived from PVDF and absorbed liquid electrolyte, and no any peaks related to other fluorides (LaF3, TiF3, or TiF4) are observed (Figure 4b). These observations suggested that the deposited a-LLTO on the LCO electrode surface is stable during long-term charge-discharge cycling.
For LCO-LLTO-10, LCO particles would be protected from HF corrosion by the deposited a-LLTO, if HF forms due to undesirable side reactions. The stronger O signal derived from LCO-LLTO-10 (Figure 4a) implies that the oxygen loss of LCO are potentially prevented by the deposited a-LLTO. Additionally, LCO-LLTO-10 presents a much stronger peak ascribed to the absorbed liquid electrolyte solute, comparing with pristine LCO (Figure 4b). This indicates that the deposited a-LLTO leads to a better wettability of liquid electrolyte on the surface of electrodes, or restrains the decomposition of the liquid electrolyte. The peaks corresponding to Co 2p are almost unobserved for LCO-LLTO-10, while that for pristine LCO is obvious (Figure 4c). The deposited a-LLTO may prevent the diffusion of Co3+/4+ to the surface of the electrode and its dissolution into liquid electrolyte. The peaks around 283 eV, 285 eV, 286 eV, and 289 eV observed in C1s spectra (Figure 4d), are associated with carbon black, PVDF, polyether carbon (O-C-O), and carbonyl group (C=O), respectively [34,50]. Generally, the polyether carbon and carbonyl group are considered to result from electrolyte decomposition [34,50]. The absence of C=O peak in the curve derived from LCO-LLTO-10 demonstrates that at least part of the cacoethic side reactions are blocked by the deposited a-LLTO. Based on the above analysis, most of the stability issues of LCO cathode at 4.5 V should be addressed by the deposited a-LLTO.