A High Power Carbon / LiMxOy Hybrid Cathode for Non-Aqueous LiO 2 Battery

Among all the issues that restrict the application of Li-air battery, poor power performances of O2 cathode comes first. In this paper, we establish carbon (Super P)/LiMxOy (LiMn2O4/LiFePO4/LiNi1/3Co1/3Mn1/3O2) hybrid cathode to promote the power output of conventional carbon cathode through continuous Li+-insertion reaction of LiMxOy and Li+ transportation in bulk LiMxOy during the discharging process. Weight and volume specific power performances of the hybrid cathode are much higher than those of traditional Super P carbon cathode. The mechanism of improving power performance of O2 cathode has also been discussed through electrochemical impedance spectroscopy and cyclic voltammetry method in this paper.


Introduction
The non-aqueousLi-air (Li-O 2 ) batteries have attracted great attention owing to the highest theoretical specific energy (3505 Wh•kg −1 ) among various energy storage systems [1] [2] [3].However, the development of Li-O 2 batteries is largely lagged by low round-trip efficiency [4] [5] [6] [7] (caused by decomposition of non-aqueous electrolyte and carbon based oxygen electrode), short cycle life [8] [9] [10] (caused by non-recovery of reaction surface/interface), and poor power capability [11] [12] [13] (caused by low kinetics of electron, Li + and O 2 transport) during oxygen reduction reaction (ORR) and oxygen evolution reaction (OER).The low round-trip efficiency and short cycle life issues mentioned above have attracted great attentions in recent years [14] [15] [16].More importantly, the power ability of the non-aqueous Li-O 2 battery (no advantage com-pare to Li-ion battery) [17], should be improved as well since high power output are required if it is expected to be developed for portable device and electric transportation.
For non-aqueous Li-O 2 battery, sluggish ORR process of the O 2 cathode, during which O 2 is, principally, reduced to produce Li 2 O 2 on cathode surface, gives rise to the poor power output.Insolubility and low ionic/electronic conductivity of Li 2 O 2 [18] [19] [20] mean it is difficult to transport electron and Li + to the reaction interface through bulk Li 2 O 2 .Low O 2 solubility and transportation in electrolyte mean it is hard to supply sufficient O 2 , especially under high current density [11] [21] [22] [23].
To improve the poor power ability of O 2 cathode mentioned above, researchers have focused on how to promote the ORR catalysis, oxygen and ionic/electronic transport.Although the catalysis mechanism is still in dispute, ORR catalysts have been found to play a key role in improving the power ability of the O 2 cathode [24] [25] [26].In addition, continuous passage construction for gaseous O 2 through cathode design could provide a specific power more than 1600 W•kg −1 [21]
The Li-O 2 battery configuration used in this paper has been described elsewhere [29], including a lithium foil (1.6 cm in diameter), one pieces of Whatman glass microfibre filters separator (1.9 cm in diameter), and a SC or SLHC cathode (1.4 cm in diameter).The cell was assembled in a glove box with water contents of <0.1 ppm, and about 300 μl prepared electrolyte was added in each cell.After standing for at least 24 h at room temperature, cells were discharged and charged under O 2 with a 1.1 atm pressure using a New are Battery Testing System (CT-3008, Shenzhen Newear Co., Ltd.).
Electrochemical impedance spectroscopy (EIS) was measured by a CHI660E (CH Instruments, Inc.) and the spectra were obtained in the frequency range from 1 MHz to 100 mHz with an AC amplitude of 5 mV at 0% deep of discharge (DOD).
Cyclic voltammetry (CV) was also measured by a CHI660E with the scan speed of 10 mV•s −1 from the open circuit potential (OCP) to 2 V (vs.Li + /Li) and then, 4.5 V (vs.Li + /Li).

Power Performances of SLHC and SC
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 x O y need "activation" during the first charging process ( 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 LiMn 2 O 4 , LiFePO 4 and LiNi 1/3 Co 1/3 Mn 1/3 O 2 , 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 36.9W•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.

High Power Output Mechanism of SLHC
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 x O y ( 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-z Mn 2 O 4 , ~3.6 V for Li 1-z Ni 1/3 Co 1/3 Mn 1/3 O 2 and ~3.4 V for Li 1-z FePO 4 ).Furthermore, addition of LiM x O 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, Li 2 O 2 (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 x O y in SLHC plays two vital roles.First, Li + -insertion reaction of LiM x O y during the discharging process could benefit the power output of SLHC (Figure 3).Second, Li + could diffuse through bulk LiM x O 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-O 2 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 x O y compared with C. At very low frequencies, there is a re-gion 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).

Figure 1 .
Figure 1.Discharge profiles of the SLHC and SC at 0.1 mA•cm −2 .

Figure 2 .
Figure 2. Galvanostatic polarization curves of the SLHC and SC.
. A gentle anodic peak (P a ) around 3.5 V vs. Li/Li + is observed in Figure4reflects the OER process ( no sharp P a

Figure 4 .
Figure 4. Cyclic voltammograms for SLHC and SC in the voltage range between 2 and 4.5 V.The potential was swept at a scan rate of 10 mV/s.

Figure 5 .
Figure 5. Schematic representation of the discharging process of SLHC and SC.

Figure 7 .
Figure 7.The plot of the imaginary resistance as a function of the inverse square root of angular speed for SLHC and SC.Data obtained from impedance spectroscopy.
[27]faster Li + transport in Li 2 O 2 could also promote the power ability of O 2 cathode[27].However, awkward problems (e.g., catalysts are easily deactivated and O 2 /electrolyte/Li 2 O 2 reaction interfaces are limited when Li 2 O 2 deposit on the cathode surface) still exist and need to be tackled.