Ex-situ XRD and corresponding refinement are also carried out for structure evolution analysis of LCO after 500 cycles in full cells and the results are shown in
Figs. 3a and
b. Although LCO-2 outperforms LCO-1 in discharge capacity, cycling stability and rate performance, their XRD patterns after 500 cycles are still both well assigned to the standard PDF card without any extra peaks, indicating that the LCO phase is maintained. Compared with the pristine state before cycling, the peak intensity after cycled is significantly weakened especially for (003) peak, indicating the reduced degree of crystallization. The refinement results (Table S6 in Supporting information) show that the cell lattice parameter
a increases while
c decreases after cycling for both LCO-1 and LCO-2. As a result, the
c/
a ratio decreases compared to that before the long cycle. All these refinement parameters exhibit a consistent trend of change, but the
c/
a ratio for LCO-1 decreases more than that of LCO-2, meaning that the deterioration of layered structure is more serious within LCO-1 than LCO-2, which reconfirms the structural stability effect of homogeneous Al doping [
34]. EIS tests of half cells before and after 100 cycles at both 25 ℃ and 45 ℃ were conducted to provide detailed electrode/electrolyte interface kinetic information [
36].
Figs. 3c and
d show the Nyquist plots of LCO-1 and LCO-2 tested under 4.65 V at 25 ℃ and 4.6 V at 45 ℃, respectively. It can be seen that all Nyquist plots consist of one or two semicircles in the high frequency region and one diagonal line in the low frequency region, corresponding to the electrode/electrolyte interface resistance (
Rsei), the charge transfer resistance (
Rct) and the Warburg resistance related to the lithium ions diffusion ability in the electrode bulk, respectively [
37]. The values of each impedance are summarized in Table S7 (Supporting information). The
Rsei values of LCO-1 after 100 cycles are 85.7 Ω tested at 25 ℃ and 398.8 Ω tested at 45 ℃, whereas the values of LCO-2 are 55.5 Ω and 248.3 Ω, respectively. The lower
Rsei values indicate that LCO-2 has a more favorable electrode/electrolyte surface for lithium ion transport [
38]. In addition, for the group tested at 25 ℃, the
Rct value of LCO-1 increases 84.8 Ω to 179.5 Ω after 100 cycles with a growth rate of 112%. In contrast, this resistance of LCO-2 increases from 107.5 Ω to 142.5 Ω with a smaller growth of 33%. As for the samples tested at 45 ℃, the
Rct value of LCO-1 increases 154.4 Ω to 457.7 Ω after 100 cycles, while
Rct value of LCO-2 increases from 199.5 Ω to 404.2 Ω, of which the growth rate is consistent with the trend of the results tested at 25 ℃. Above all, both the
Rsei values and
Rct increases after 100 cycles of LCO-2 are smaller than that of LCO-1 both at 25 ℃ and 45 ℃. The comparison strongly demonstrates that LCO-2 has more favorable electrode/electrolyte interface stability and smaller charge transfer impedance than LCO-1, indicating that the homogeneous Al doping is conductive to facilitating the interface reaction kinetics and stabilizing the electrode/electrolyte interface impedance. To further verify the structure stabilizing effect of homogeneous Al doping, the LCO electrodes before and after 500 cycles test in full cells under 4.5 V at 45 ℃ were characterized by the cross-sectional SEM. High temperature and long cycling are set to intensify the failure behavior. As shown in
Figs. 3e and
f, the fresh particles in both sampels are tightly packed, and the sections of the particles are intact and clean with almost no cracks. However, after 500 cycles under high cut-off voltage and high temperature, the structural collapse and interior cracks are observed within LCO-1 (
Fig. 3g). For the crack formation mechanism, it is believed closely related to the irreversible phase transition of O
3 to H
1–3 occurring over 4.5 V, which will bring about huge and heterogeneous microscopic stress distribution [
39,
40]. On the contrary, the cross section of LCO-2 after 500 cycles remains intact with only a few visible cracks (
Fig. 3h), proving the mechanical stability of LCO-2 is obviously enhanced. Such results are consistent with the front
in-situ XRD analysis and reconfirms that the homogeneous Al doping can effectively stabilize the structure and suppress the detrimental phase transition, thus boosting the electrochemical performance.