Galvanostatic discharge method was used to reveal the power performance of SLHC and SC. The second discharge profiles of the SLHC and SC at a current density of 0.1 mA∙cm⁻² (Figure 1) are illustrated since LiM_xO_y need “activation” during the first charging process ( LiM x O y − zLi + − ze − → Li 1 − z M x O y ). The second discharge process of the SLHC consists of two kinds cathodic reactions. The first one is conventional Li⁺-insertion reaction

( L i 1 − z M x O y + z L i + + z e − → L i M x O y ) at different voltage (~3.8 V for Li_1-zMn₂O₄, ~3.6 V for Li_1-zNi_1/3Co_1/3Mn_1/3O₂ and ~3.4V for Li_1-zFePO₄) and the second one is ORR ( 2 L i + + O 2 + 2 e − → L i 2 O 2 ) at about 2.5 - 2.6 V. Obviously, under 0.1 mA∙cm⁻², Li⁺-insertion reaction dominates the initial parts (ORR dominates the rest) of discharge processes of SLHC, which are different from those of SC. Since

most parts of discharge processes of SLHC are ORR, the specific capacities of SLHC are a little larger than those of SC. In addition, under 0.1 mA∙cm⁻², ORR voltage plateaus are about 2.7 V vs. Li/Li⁺ (2.96 V in theory), which reveals the a dynamic characteristic of ORR.

As for the power performances (P = V*I) of SLHC and SC, it is vital important to keep high voltages output at large discharge currents. Figure 2 compares the 600 s polarization curves for both the SLHC and SC at three current densities. During the short test process, Li⁺-insertion reaction are dominative for SLHC and while, ORR still dominate most parts of discharge processes of SC. Furthermore, due to the collaborative advantage of specific capacities and Li⁺-insertion potentials for LiMn₂O₄, LiFePO₄ and LiNi_1/3Co_1/3Mn_1/3O₂, the polarization of the SLHC is much smaller than that of SC at all current densities. It is worth noting that, under high current density, the average voltage difference of SLHC and SC is larger in shorter test process. For example, when the current density rises from 0.1 to 0.3 mA∙cm⁻², the average voltage (E_50s) differences of SLHC and SC increase from about 0.7 to 1.4 V vs. Li/Li⁺ This excellent depolarization effect of SLHC should provide a substantially enhanced power output. As is shown in Figure 3, at low current density (0.1 mA∙cm⁻²), the weight specific power (P_w, Figure 3(a)) and volume specific power (Pv, Figure 3(b)) of SLHC in 50 s is 13.2 W∙kg⁻¹_cathode and 2685 W∙m⁻³_cathode, respectively, which is a little higher than that of SC (11.5 W∙kg⁻¹_cathode and 2129 W∙m⁻³_cathode). However, with the increase of current density from 0.1 mA∙cm⁻² to 0.3 mA∙cm⁻², obviously, the P_w and P_v differences in 50 s between SLHC and SC become larger. At 0.3 mA∙cm⁻², P_w and P_v of SLHC are 36.9 W∙kg⁻¹_cathode and 7507 W∙m⁻³_cathode, respectively, which is much higher than that of SC (25.4 W∙kg⁻¹_cathode and 4712 W∙m⁻³_cathode). In addition, similar results can be observed in 600 s as shown in Figure 3. More

importantly, it can be speculated that at much higher current density, these advantages in power performances of SLHC should be more remarkable.

Cyclic voltammetry is carried out to study the electrochemical process of SLHC since it is a useful technique for discerning kinetics and mechanisms of electrochemical reactions. The potential for full-range cyclic voltammograms (CVs) is first swept from OCP of around 2.9 V to 2.0 V vs. Li/Li⁺, and then it is reversed to anodic direction (Figure 4). During the first cathodic scan process, no obvious cathodic peaks (P_c) are observed for both SLHC and SC, which indicates a poor kinetic characteristic of ORR. When the scan reverses to anodic direction, the current of SLHC responds more strongly than that of SC, which may attribute to the “activation” of LiM_xO_y ( L i M x O y − z L i + − z e − → L i 1 − z M x O y ) mentioned above. A gentle anodic peak (P_a) around 3.5 V vs. Li/Li⁺ is observed in Figure 4 reflects the OER process ( L i 2 O 2 → 2 L i + + 2 e − + O 2 ) and no sharp P_a exists because of the solution-like delithiation and two-phase oxidation processes [30] . It is worth to note that, during the second cathodic scan, the cathodic current below 4.0 V vs. Li/Li⁺ may come from the Li⁺-insertion reaction at different voltage (~3.8 V for Li_1-zMn₂O₄, ~3.6 V for Li_1-zNi_1/3Co_1/3Mn_1/3O₂ and ~3.4 V for Li_1-zFePO₄). Furthermore, addition of LiM_xO_y in SLHC also benefit ORR process since an obvious P_c around 2.5 V is observed for SLHC, which is different from that of SC (Figure 4).

The discharging processes of SLHC and SC are schematically displayed in Figure 5. For SC, Li₂O₂ (insolubility and low ionic/electronic conductivity) covers on the outer space of C/electrolyte interface during the discharging process, which gives rise to the poor kinetic characteristic of ORR. However, the addition of LiM_xO_y in SLHC plays two vital roles. First, Li⁺-insertion reaction of LiM_xO_y during the discharging process could benefit the power output of SLHC (Figure 3). Second, Li⁺ could diffuse through bulk LiM_xO_y to the inner reaction interface,

which benefit the ORR process (Figure 4) and electrochemical performance (Figure 1) of SLHC.

EIS is further introduced to the study of the kinetic properties of SLHC and SCat 0% DOD (Figure 6). Laoire et al. [31] interpreted the impedance spectra of Li-O₂ batteries and proposed the equivalent circuit (Rs (C (RpW)), where C is the capacitive contributions of the two electrodes, R_s is the electronic resistance of the electrodes and their contacts to the current collectors, and electrolyte resistance, R_p is the charge transfer resistance at the two electrodes, W is the linear Warburg element that may be attributed to the diffusion of the electroactive species to the electrode. As shown in Figure 6, the charge-transfer resistance R_p of SLHC is a little larger than that of SC, probably because of the lower electronic conductivity of LiM_xO_y compared with C. At very low frequencies, there is a region in which a typical Warburg behaviour, related to the diffusion of lithium ions in the cathode active material, is seen. By using the model proposed by Ho et al. [32] , the diffusion coefficient for SLHC and SC are calculated by using Equation (1).

D L i = 1 / 2 [ ( V M / S F A ) ( δ E / δ x ) ] 2 (1)

where V_M is the molar volume (SLHC-10.23 cm³∙mol⁻¹, SC-5.28 cm³∙mol⁻¹), S is the contact area between electrolyte and sample (1.54 cm²), F is the Faraday constant (96,486 C∙mol⁻¹), A is the plot slope of imaginary resistance (Zim.) vs. inverse square root of angular frequency ( 1 / 2 π f ) , which can be obtained from

the Warburg impedance, and δE/δx is the slope of galvanostatic charge-discharge curves (SLHC, −1.284, SC, −0.424).

Figure 7 shows the plot of the imaginary resistance determined by impedance spectroscopy as a function of the inverse square root of the angular frequency for SLHC and SC. Linear behaviors of SLHC and SC are observed for frequency values ranging from 100 m∙Hz to 178 m∙Hz with a slope of 236.8 Ω∙s⁻¹ and 338.2 Ω∙s⁻¹, respectively. The diffusion coefficients of lithium for SLHC (5.43 × 10⁻¹⁵ cm²∙s⁻¹) and SC (2.33 × 10⁻¹⁵ cm²∙s⁻¹) obtained by substitution of the curve slopes in Equation (1). This result of D_Li is fairly in agreement with that of CV (Figure 4) and galvanostatic charging (Figure 1).