Acta Physico-Chimica Sinica  2017, Vol. 33 Issue (3): 486-499   (6235 KB)    
Recent Advances in Li Anode for Aprotic Li-O2 Batteries
ZHANG Yan-Tao1,2, LIU Zhen-Jie1,2, WANG Jia-Wei1, WANG Liang1,2, PENG Zhang-Quan1,*   
1 State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China;
2 University of Chinese Academy of Sciences, Beijing 100049, P. R. China
Abstract: The aprotic Li-O2 battery has attracted considerable interest in recent years because of its high theoretical specific energy that is far greater than that achievable with state-of-the-art Li-ion technologies. To date, most Li-O2 studies, based on a cell configuration with a Li metal anode, aprotic Li+ electrolyte and porous O2 cathode, have focused on O2 reactions at the cathode. However, these reactions might be complicated by the use of Li metal anode. This is because both the electrolyte and O2 (from cathode) can react with the Li metal and some parasitic products could cross over to the cathode and interfere with the O2 reactions occurring therein. In addition, the possibility of dendrite formation on the Li anode, during its multiple plating/stripping cycles, raises serious safety concerns that impede the realization of practical Li-O2 cells. Therefore, solutions to these issues are urgently needed to achieve a reversible and safety Li anode. This review summarizes recent advances in this field and strategies for achieving high performance Li anode for use in aprotic Li-O2 batteries. Topics include alternative counter/reference electrodes, electrolytes and additives, composite protection layers and separators, and advanced experimental techniques for studying the Li anode|electrolyte interface. Future developments in relation to Li anode for aprotic Li-O2 batteries are also discussed.
Key words: Lithium-oxygen battery     Lithium metal anode     Li anode stability     Solid electrolyte interphase layer    

1 Introduction

Confronted with the rapidly growing demands for high density energy storage applications, the aprotic Li-O2 batteries, which are based on the Li-O2 reaction of $2\text{Li+}{{\text{O}}_{2}}\rightleftharpoons \text{L}{{\text{i}}_{\text{2}}}{{\text{O}}_{2}}$ (EӨ=2.96 V (vs Li/Li+)) with an ultra-high theoretical specific energy of 3500 Wh∙ kg-1, have been proposed to be a holy grail for electric energystorage applications1-8. As early as 1996, the aprotic Li-O2 battery was devised by Abraham and Jiang9. Subsequently, incessant efforts have been made to contribute its further research and development. Despite considerate progresses, a bunch of scientific and technological issues still impede the realization of practical Li-O2 cells, which include degraded capacity, limited cycle life, low energy efficiency, and instability of cell components.

So far, the majority of Li-O2 studies, using a standard configuration of Li|aprotic Li+ electrolyte|porous cathode (O2), focus on the O2 reactions in the cathode. To alleviate the above mentioned problems, novel catalysts10-16, electrolytes17-22 and binders23, 24 have been used. In addition, fundamental studies of understanding of the reaction mechanisms under various operating conditions25-28 have been reported. However, the stability of the Li anode is still another crucial issue and should not be ignored. Especially, the reactions arising from the metallic Li become much more complicated when oxygen and water are involved in the electrolyte. Therefore, O2 shuttle is one of the major culprits that can cause passivation/parasitic reaction on Li metal anode. Moreover, some of the side reaction products could cross over to the cathode then interfere with O2 electrochemistry, and degrade the overall performance of Li-O2 cells29-31. In this light, stabilizing the Li metal anode interface could be as challenging as addressing the reversibility in air-cathode. Besides, Li metal electrode has been plagued for decades with the problem of the dendrite formation during repeated stripping/plating, which lead to serious safety concerns and poor Coulombic efficiency32, 33.

Toward a better understanding of the Li-O2 reactions and realization of practical aprotic Li-O2 batteries, it is critically necessary to develop novel strategies to improve the stability of lithium metal electrode especially in the presence of O2. In this paper, the recent advances in fundamental science of improving the metallic lithium stability have been reviewed, including (ⅰ) alternative counter/reference electrodes that react with neither electrolyte nor O2; (ⅱ) solid electrolyte interphase (SEI) modification, including in situ SEI formation and various artificial SEI; (ⅲ) experimental techniques. Finally, we propose a perspective on challenges and opportunities for Li metal electrode used in aprotic Li-O2 batteries.

2 Alternative counter/reference electrodes

Li metal is recognized as the most promising anode material due to its high theoretical specific capacity (3860 mAh∙g-1) as well as the lowest electrochemical potential (-3.04 V, vs SHE (the standard hydrogen electrode)). The use of the Li metal electrode accompanied by safety concerns associated to its well-known reactivity, possible dendrite formation leading to short circuit34-36, remains great challenge on commercialization of Li metal batteries (LMBs), Li-sulfur (Li-S) batteries and Li-oxygen (Li-O2) batteries2, 4, 37. Moreover, for Li-O2 batteries operated under ambient atmosphere, the parasitic reactions of Li metal and the permeated O2 in the electrolyte can not be eliminated completely. The lithium metal can, in fact, react with the O2 solubilized in the electrolyte to form lithium oxide or hydroxide at the metal surface, resulting in the increase of the cell resistance and polarization38, 39. The replacement of the lithium metal by alternative lithium ion/alloy material represents one of the possible solutions to overcome the aforementioned issues.

To identify the alternative counter/reference electrode materials, several works have been reported. In 2012, Bruce et al.40 initially introduced lithium iron phosphate (LiFePO4) as counter electrode to replace Li metal used in Li-O2 batteries due to instability of metallic lithium toward dimethylformamide (DMF) electrolyte. For the similarly reason, McCloskey et al.41 also adopted LiFePO4 as the anode to conduct the quantitative analysis of O2 using differential electrochemical mass spectrometry (DEMS) in acetonitrile (CH3CN) electrolyte. As shown in Fig. 1a, the partially charged Li1-xFePO4 acting as not only counter electrode (Li+ rich) but also reference electrode (stabilized at~3.45 V, vs Li/Li +), involving in a two-phase reaction of Li1-xFePO4 and FePO4, can significantly reduce the contaminations from the electrolyte decomposition on Li anode42. Thus, the obtained clean electrochemical system of Li-O2 batteries can ensure a better understanding of the reaction mechanisms and quantitative measurement of the O2 reversibility43, 44. From this view, the Li-ion intercalation materials, which react with neither electrolyte nor O2, display significant promise as alternative anode materials used in Li-O2 batteries. In the study reported by Lee et al.45, graphite and Li4Ti5O12 showing high Coulombic efficiencies in Li-ion batteries, were successfully demonstrated their cyclability and reversibility as anode in Li-O2 batteries (Fig. 1b and 1c). But such kinds of Liion intercalation materials are not suitable to replace metallic Li for rechargeable Li-O2 cells, because the high energy density of Li-O2 battery was seriously discounted.

Fig. 1 Galvanostatic discharge-charge curves of Li ion-O2 batteries fabricated with (a) LiFePO4 42, (b) graphite and (c) Li4Ti5O12 45, (d) LixSi-C46, (e) LixSn-C38 and (f) LixAl-C47 as counter/reference electrode instead of Li metal

Moreover, Hassoun et al.46, 38 have reported lithium-alloying composites, LixSi-C and LixSn-C electrodes, to replace the unsafe lithium metal anode. As shown in Fig. 1d and 1e, the LixSi-O2 and LixSn-O2 batteries discharged around 2.7 and 2.4 V respectively, attributing to the lower anode potential of LixSi (~0.3 V (vs Li/Li+)) and LixSn (~0.5 V (vs Li/Li+)), unlike the LiFePO4 (~3.45 V (vs Li/ Li +)). Although the alloy materials show up big promise to the practical lithium ion-O2 batteries, slight voltage decay is observed only after several cycles in Fig. 1d and 1e. A functional uniform SEI film that can alleviate the attack of O2, was formed on lithiated Al-carbon (LixAl-C) anode47. As shown in Fig. 1f, the invariable discharge and charge profiles also verified the interfacial stability between LixAl-C and electrolyte. We speculate that the interface stability toward the permeated O2 in the electrolyte and the volume expansion during repeated stripping/plating still need to be further improved.

3 SEI modification

As early as in 1979, Peled49 firstly identified the concept of SEI, which constitutes of various organic and inorganic components and displays the properties of electrically insulating and ionically conducting. The SEI layer with a thickness of~20 nm is formed from the spontaneous reactions between alkaline metal and solvent/ slat anions in liquid electrolyte48. Although the consumed Li+ used for the formation of SEI leads to a low efficiency, the SEI can effectively prevent the further physical contact and chemical reaction between the Li and electrolyte. In addition, Li ions can be evenly distributed via functionalized SEI during repeated stripping/ plating and the dendrite-free morphology can be obtained. In order to protect the Li anode for Li-O2 battery better, it is significantly important to modify SEI. The following summarized the advanced strategies to modify SEI layer including adopting new electrolyte systems, additives and some artificial composite protect layers (CPLs).

3.1 Electrolyte and additives

The conventional electrolyte systems in Li-O2 battery mostly contain solvents and Li salts3-5. The widely used organic alkyl carbonate-based and esters-based electrolytes were demonstrated susceptible to the attack of the superoxide radical anion and severe decomposed during the charge and discharge processes. So it is critically necessary to adopt more chemically/electrochemically stable ether and glymes-based, e.g. dimethoxyethane (DME), tetraglyme (TEGDME); nitrile-based, e.g. acetonitrile (CH3CN), trimethylacetonitrile (TMA); sulfone-based, e.g. dimethyl sulfoxide (DMSO), amide-based, e.g. N, N-dimethylacetamide (DMA), dimethylformamide (DMF); ionic liquid-based (IL) and gel/solid state-based electrolytes5. Although the above-mentioned solvents showed relative stability towards oxygen reduction species, the unstable interphase, such as the SEI formed on the surface of the Li metal anode, will result in the deterioration of battery performance.

In order to enhance the interfacial stability of the straight-chain alkyl amides with the Li anode, lithium nitrate (LiNO3), which previously was shown to improve battery performance by formation of a stable SEI in Li-S battery50-52, has been applied to enhance the interphase compatibility between Li metal and DMA electrolyte in Li-O2 systems. Walker et al.53 showed that some electroactive species generated from the reaction of Li and DMA, while the inclusion of LiNO3 can substantially inhibit this reaction. The compatibility of this system with Li metal is attributed to the inertness of the amide core combined with the nitrate anion contributing to the formation of a protective SEI. This assembled Li-O2 battery shows comparatively high efficiency and stability over 80 cycles (> 2000 h) with minimal changes in the voltage in Fig. 2a. Subsequently, Giordani et al.29 demonstrated the synergistic effect of the O2 and LiNO3 on the formation of a stable SEI. The low overpotential and durable cyclic performance in symmetric Li-Li cells under O2 is better than Ar as depicted in Fig. 2b. Combined Fig. 2b with 2c, we can conclude that neither O2 nor LiNO3 alone is able to stabilize the Li metal interface in DMA. The further studies about the effect of O2 combining with additives LiNO3 and vinylene carbonate (VC) have been investigated on the typical 1 mol∙L-1 LiClO4/DMSO electrolyte by Roberts et al.30. As displayed in Fig. 2d, the improved cycle numbers of 25, 27 and 33 were achieved respectively with the addition of O2, LiNO3 and VC. It is obvious that the use of oxygen as an additive is sufficient to stabilize the surface, but this result is a little different from that of Giordani et al.29, in DMA electrolyte. The positive effective about the O2 stabilizing effect is consistent with Aurbach′s report54.

Fig. 2 Voltage profiles of (a) Li-O2 cells53 in 1 mol∙L-1 LiNO3/DMA; (b) symmetric Li/Li cell under O2 and Ar (inset) in 1 mol∙L-1 LiNO3/DMA, (c) pressure and voltage profiles of symmetric Li/Li cell cycled under O2 in 1 mol∙L-1 LiTFSI/DMA29; (d) Li-Cu cells in 1 mol∙L-1 LiClO4/DMSO electrolyte with and without additives in the presence and absence of oxygen30

However, contradictory results were proposed by Assary39 and Younesi55 et al., Assary et al.39 observed LiOH as well as carbonates rooting in the O2 crossover from the cathode to anode in tetraglyme-based electrolyte. Younesi et al.55 discussed the SEI components on the Li anode with 1 mol∙L-1 LiPF6/PC electrolyte used in Li-O2 cell for the first time. The X-ray photoelectron spectra (XPS) analysis concludes that the SEI layer on the Li anode constantly changes during the charge and discharge processes in the present O2, namely the only existed O2 is unable to stabilize the SEI. These worse impacts are based on the unstable ether or carbonate-based electrolytes. The mechanism and reliability of oxygen stabilizing the SEI on the Li metal is complicated and is not fully understand yet. We speculate that the partial pressure of oxygen, the identity of solvents, salts and additives act essential role on fabricating a stable SEI.

Except those additives mentioned above, Bryantsev et al.56 has demonstrated 2% N, N-dimethyltrifluoroacetamide (DMTFA) as additives which can effectively stabilize the Li electrode in 0.5 mol∙L-1 LiTFSI and 98% DMA. Five different types of fluorinated amides solvents were investigated to stabilize the lithium/ electrolyte interface as shown in Fig. 3a and 3b. The LiTFSI/ DMTFA electrolyte shows the lowest interfacial resistance and exhibits the best cyclic performance. Combined the calculated energy barriers with XPS analysis, LiF was recognized to be formed on the Li metal surface with little or no activation energy. Although DMTFA shows reduced ability to form an effective SEI upon extended cycling due to the instability to the O2 reduction reactions, the presence of DMTFA significantly improves the stability of the unprotected Li anode in rechargeable Li-O2 battery. Zhang et al.57 also discovered LiF in an artificial SEI layer synthesized by pre-charging a symmetric Li| 1 mol∙L-1 LiF3SO3-TEGDME-fluoroethylene carbonate|Li cell within a voltage window of 0-0.7 V. The cyclic performance of Li-O2 batteries assembled with thus obtained Li metal significantly improved. Both of the results reveal that LiF is essential composition for stabilizing SEI.

Fig. 3 Time evolution of the interfacial resistance (a) and cyclic performance (b) of symmetric Li/electrolyte/Li cells with 0.5 mol∙L-1 LiTFSI in five different solvents at 30 ℃ under Ar56

Zhang and co-workers58 discovered a self-healing electrostatic shield mechanism by employing the Cs+ additives in 1 mol∙L-1 LiPF6/PC electrolyte. These additive cations form a positively charged electrostatic shield around the initial growth tip of the protuberances to alter dendrite formation used in LMBs. On the basic of the above ingenious ideas, Lee and Park59 successfully introduced CsI as a multifunctional redox medium, which coupled with the synergistic function of I- and Cs+, to reduce overpotential and improve the cyclic performance of Li-O2 cells. Besides, the addition of the LiNO3 salt in the electrolyte activates these multieffects and strengths the protection of metallic Li. The obviously smooth surface without cracks and holes even Li dendrites can be observed in Fig. 4a-4d after disassembled the cell cycling with CsI. This work proposed a strategy by integrating the redox medium, addictives or salt to facilitate the decomposition of the discharge product (e.g. Li2O2) and improve the stability of the metallic Li. Ishikawa et al.60 provided a novel approach that the additive aluminum iodide (AlI3, small Al3+ concentration of 600 × 10-6 (w, mass fraction)) was utilized as a pre-modification reagent for Li metal anode in binary electrolytes of propylene carbonate (PC) and dimethyl carbonate (DMC). Such type additive contributes to fabricating a durable Li-alloy layer on Li metal interface. But this formula still need to be proved whether it is feasible in Li-O2 battery.

Fig. 4 SEM and optical images of the metallic Li anode before and after treated with different electrolytes (a) pristine, after cycling in (b) 1 mol∙L-1 LiTFSI, (c) mixture of LiNO3 and LiTFSI (50 : 50), (d) 0.05 mol∙L-1 CsI with 0.5 mol∙L-1 LiNO3 and 0.5 mol∙L-1 LiTFSI in TEGDME59; (e) from left to right shows the lithium foil, PEO-based membrane and super P carbon grid after CV test in Li-O2 cell67; Li sheet (f) with a water drop, (g) exposed in air with (left) and without (right) gel electrolyte75

Currently, the solid-state electrolytes (SSEs), such as ceramic solid electrolyte (CSE) and solid polymer electrolyte (SPE), are considered as competitive alternatives to the flammable and volatile liquid electrolytes in terms of safety issues. In contrast, unwanted chemical cross-over actions, severe anion and cation concentration gradient and dendrite formation can be effectively overcome by making use of the SSE. In addition, the robust mechanical strength, elevated thermal stability, uneasy-leaked and lower cost properties are pushing its widely application5. Now, more and more works have been done to integrate the SSE into LiO2 batteries.

The lithium-ion conducting glass ceramic of Li-Al-Ge-PO4 (LAGP)61, 62 and Li-Al-Ti-PO4 (LATP)63, 64 systems with high ionic conductivity (in the range of 10-4-10-3 S∙cm-1) have been employed in Li-O2 battery. Most lithium solid electrolytes are thermodynamically unstable against Li metal. Various kinds of Liion conductors, such as lithium-nitride (Li3N), lithium phosphorous oxynitride (LiPON) and poly (ethylene) oxide (PEO), are applied as the protective interlayer to separate the CSE and Li metal electrode. Kumar et al.61 investigated one kind of solid state electrolyte combined LAGP with polymer-ceramic (PC) membranes comprised of PC (BN), PC (AIN), PC (Si3N4) and PC (Li2O) in Li-O2 batteries. Using PC material can reduce the cell impedance and enhance the electrochemical compatibility with lithium. Another similar optimized component and configuration of SSE fabricated from glass-ceramic (GC) LAGP and polymer-ceramic (PC) containing Li2O and BN was assembled in Li-O2 cells as shown in Fig. 5a65. This sandwiched SSE exhibited excellent thermal stability and cyclic performance used in Li-O2 cells operated in the temperature range of 30-105 ℃ and physically isolated lithium from air and moisture. However, the limited cycle life (less than 40 cycles) needs to improve further. A long-term cyclic performance (150 cycles) super-hydrophobic quasi-solid electrolyte Li-O2 cell coupled with RuO2/MnO2/SP cathode was proposed as depicted in Fig. 5b66. This special super-hydrophobic SiO2-based composite solid-state electrolyte can resist humid atmosphere (RH of 45%), which also displayed higher thermal tolerance, wider operation electrochemical window (> 5.5 V) and better mechanical flexibility in contrast with the traditional liquid electrolyte.

Fig. 5 Schematics of (a) the sandwiched PC/GC/PC SSE model65 and (b) the proposed super-hydrophobic quasi-solid electrolyte66 used in Li-O2 cells

Solvent-free polymer electrolyte with a high molecular weight PEO-LiCF3SO3 was reported by Scrosati et al.67. The dissembled cell (Fig. 4e) shows that the electrodes especially the lithium is still in excellent condition, which confirms that the PEO matrix can effectively protect lithium metal. Electrolytes with polymer additives are reported as promising electrolyte systems that possess many advantages over non-aqueous liquid or ionic liquid-based electrolytes in terms of electrochemical stability. Recently Tokur et al.68 reported that the selective amount of poly (vinylidene fluoride) (PVDF) and PEO additives can significantly increase the cyclability of Li-O2 batteries. Another feature is visible from Fig. 6a and 6b, in which PEO-based electrolyte exhibits more stable cyclic performance with lower overpotential than that of PVDF. It was suggested that polymer additives not only provided high conductivity but also protected the Li metal against the corrosion. In order to improve the cyclic life, they69 developed another highly reversible Li-O2 battery with addition of polymer (PEO) and ceramic (Al2O3) fillers in 1 mol∙L-1 LiPF6/NMP-based composite polymer electrolyte. The voltage of the discharge scarcely changes particularly after 35 cycles with a deep cycling capacity of 2.54 mAh in Fig. 6c and 6d. The excellent electrochemical properties indicate the stabile SEI formed on the Li anode. This result further demonstrates that the polymer electrolyte is promising to overcome the problem of anode corrosion and dendrite formation70. Such ceramic fillers include Al2O3 71, TiO2 72, SiO2 73 and ZrO2 74 etc. Peng et al.75 proposed a coaxialflexible-fiber architecture of all-solid-state Li-O2 batteries coupled with gel electrolyte which mixed with lithium triflate (LiTF), tetraethylene glycol dimethyl ether (TEGDME), poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), N-methyl-2-pyrrolidinone (NMP), 2-hydroxy-2-methyl-1-phenyl-1-propanone (HMPP), and tri-methylolpropane ethoxylate triacrylate (TMPET). Fig. 4f and 4g present that this coating layer with gel electrolyte can effectively prevent the lithium anode from corrosion by water, oxygen, etc.

Fig. 6 Discharge and charge profiles of Li-O2 cells in (a) 1 mol∙L-1 EMITFSI-LiTFSI-0.1% PVDF, (b) 1 mol∙L-1 EMITFSI-LiTFSI-0.1% PEO68, (c) 1 mol∙L-1 LiPF6 (PEO 0.5% and Al2O3 1%, mass fraction), (d) 1 mol∙L-1 LiPF6 without any additives in NMP69

The exploration of stable and compatible interphase established between SSE/CSE and electrodes becomes significant critical. Zhou et al.76 reported a composite polymer electrolyte synthesized as polypropylene (PP)-supported poly (methyl methacrylate) (PMMA)-blend-poly (styrene) (PSt) with doping nanofumed SiO2. This novel gel polymer electrolyte displayed smaller interfacial resistance with Li metal than that of the liquid electrolyte. The better compatibility with Li metal anode contributes to durable cycling life compared with the traditional liquid one in Fig. 7a and 7b. Besides, this polymer system can alleviate the corrosion of Li metal caused by air and H2O. Subsequently, in order to improve the mechanical strength and ionic conductivity of SSE, Yi and Zhou77 proposed a plausible solution by designing a hybrid quasisolid-state electrolyte combing a polymer electrolyte of poly (methylmethacrylate-styrene) with a ceramic amorphous LiNbO3 electrolyte. This hybrid electrolyte offers a stable interface toward Li anode and cathode electrode. The enhanced cycling stability shown in Fig. 7c and 7d can be attributed to the stable interfacial resistance between hybrid quasi-solid-state electrolyte (HQSSE) and electrodes.

Fig. 7 Voltage profiles of Li-O2 cells at different cycles using (a) liquid electrolyte, (b) as-synthesized polymer electrolyte76; (c) the time-dependent EIS of the Li/HQSSE/Li cell at open circuit potential, (d) the discharge-charge profiles of Li-O2 cell assembled with HQSSE at different cycles77

Although the liquid or IL electrolyte own high ionic conductivity, many safety issues still limited their application. Herein, the CSE, SPE or their mixture systems display satisfied advantages. Nevertheless, problems arising from all of electrolyte systems associated with the interphase stability will be exposed when permeated into O2, H2O, CO2 used in lithium-air batteries. So much efforts should be paid to explore and develop more compatible and durable electrolyte systems.

3.2 Composite protect layer and separator

Except for the in situ SEI, some artificial SEI layers, such as some kinds of polymer membranes or separators which can store electrolyte and provide pathway for ion transportation, have been synthesized via skillful mixture of ceramic fillers and polymer additives. For example, a polymer membrane consisting of PVDF, LiTFSI and ZrO2 exhibits obvious advantages to prevent the direct reaction between the LixSn-C anode and the liquid electrolyte (Fig. 8a and 8b)78. Fig. 8b also verifies the instability of the Li-SnC anode with the permeated O2 in the electrolyte, which we have discussed above.

Fig. 8 Comparison of the third cycle discharge-charge behavior (a) and time-dependent voltage evolution curves (b) of LixSn-C/O2 cells with and without the gel polymer membrane78; discharge-charge profiles of Li-O2 cells fabricated using Li electrode (c) without CPL, (d) with CPL, and SEM and optical images of Li electrode (e) without CPL, (f) with CPL after 80 cycles79

Lee et al.79 reported a composite protective layer comprising Al2O3 and PVDF-HFP coated on the Li metal. The CPL closely contacts with Li metal and lowers the resistance of the cell, which makes a great contribution to the cyclic performance (Fig. 8c and 8d). The shiny silver and smooth surface of the disassembled Li foil in Fig. 8f demonstrates that the CPL can effectively suppress the dendrite formation and separate the liquid electrolyte corrosion. Besides, they also demonstrated a high-performance nonaqueous Li-O2 battery through synergistic combination of 2, 2, 6, 6-tetramethylpiperidinyl-1-oxyl (TEMPO) as redox mediator (RM) with the abovementioned CPL80. As reported81, the RM reacts with the Li metal in the electrolyte, which is described as self-discharge process. Their experimental results explain the stability of Li interface with CPL when it is in contact with the TEMPO-containing electrolyte with increasing storage time as depicted in Fig. 9a-9d.

Fig. 9 Charge/discharge profiles of Li-O2 cells (a) without CPL and (b) with CPL at 250 mA∙g-1 (based on the mass of carbon), Nyquist plots of the Li/Li symmetric cell (c) without CPL and (d) with CPL in 1 mol∙L-1 LiClO4 /TEGDME containing 0.05 mol∙L-1 TEMPO80

To our knowledge, the widely-used separators are usually porous such as polyethylene (PE). Kim et al.82 synthesized the poreless polyurethane (PU) separator resulting from the high chain packing density. The poreless PU can prevent water and oxygen from penetrating to Li metal while Li ion can diffuse freely. They also use redox mediator LiI to test the protective ability in comparison with commercial porous polyethylene (PE) for Li-O2 cell (Fig. 10a and 10b). The absence of LiOH and uncorroded Li metal indicate that the PU separator can block the diffusion of the LiI and alleviate the side reaction. Besides, the smooth surface of Li metal was obtained by using a two-dimensionally ordered nanoporous aluminium oxide separator after 15 cycles in Li-O2 cell (Fig. 10c and 10d)83. The flat and smooth surface can be attributed to the uniform distribution Li+ through porous separator during the repeated charge and discharge processes.

Fig. 10 (a) Voltage profiles in the first cycles for PE, PU, PE + LiI and PU + LiI cells in 1 mol∙L-1 LiClO4/TEGDME, (b) XRD patterns and digital photograph images of the cyclic Li metal82; SEM images of the Li metal after 15 cycles (c) without AAO and (d) with AAO of Li-O2 cell in 1 mol∙L-1 LiNO3/DMAc electrolyte cycled at 0.5 mA for 1000 s charge or discharge83

The abovementioned artificial SEI (CPL and separators), synthesized via the integrating the liquid electrolyte with ceramic solid or solid polymer fillers, can be widely broadened. The properties e.g. size of the pore, elastic or mechanical modulus, ion conductivity, can be tuned or tailored via optimizing the composition and content. Although, the CPL or separators can suppress the volume change of the Li metal and weaken the diffusion of O2, water and some other parasitic substances, their durability especially the electrochemical stability against the Li metal and oxygen reduction species (e.g. O2-) is not clear and need to be improved because of the lack of long-term cyclic performance in the reported studies of Li-O2 batteries.

4 Experimental techniques

In order to fully understand and solve the problems arising from Li anode, the investigations on the nature of Li anode interphase should rely on the rapid development of modern advanced characterization techniques. At present, the spectroscopic investigations of SEI on negative and positive electrodes for lithium-ion batteries have widely developed with Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Xray diffraction (XRD), Raman and surface enhanced Raman scattering (SERS), scanning electron microscopy (SEM) and highresolution transmission electron microscopy (HRTEM) etc.84, 85 These techniques can be used to study the structure, composition and evolution of the bulk, surface and interphase of SEI. However, ex situ analysis of the SEI (or Li metal) requires to be separated and isolated, this disassemble process may change the surface morphology, chemical content and even the structure of SEI. Herein, in situ space-resolution, time-resolution and energy-resolution technologies are favored to provide more reliable and accurate information.

Liu and his collaborators86 reported an operando, spatially resolved phase and structural investigation on the lithium anode in an operating Li-O2 cell using a micro-focused synchrotron X-ray diffraction (μ-XRD) technique. The experimental set-up and cell design are shown in Fig. 11a and 11b. The special polyimide tube of the design cell can pass through the high-energy X rays. The results of this time and space-resolution μ-XRD reveal the conversion of metallic lithium into LiOH in 1 mol∙L-1 LiCF3SO3/ TEGDME electrolyte. The thickness of the LiOH layer increased as the cycle progressed, possibly resulting from the reaction of Li with H2O formed through the TEGDME decomposition. It preliminary discloses the reason for the failure of Li-O2 cell. The in situ XRD characterization can directly observe phase transition during the electrochemical reaction.

Fig. 11 Schematic of operando micro-focused synchrotron X-ray diffraction (μ-XRD) technique (a) and the mode design of Li-O2 cell (b)86

Scanning probe microscopy (SPM) in different variations, for example, the atomic force microscopy (AFM) is well known for its high-resolution topographic imaging capabilities of characterizing thin film and quantifying 3D roughness and texture, has been used to study evolution of SEI in Li-ion batteries87-90. Above all, the characterization of SEI on Li metal is a challenge because of the variety of chemically similar components, enclosed electrolyte species and the sensitive interphase. Herein, the advanced in situ/operando combination techniques, e.g. in situ SERS-DEMS etc. are powerful tools to understand the nature and quantitatively assess the relationship between Li metal and O2 upon forming SEI. Further works are urgent to be conducted.

5 Conclusions and outlook

In this review, we summarize the current development on protecting the Li metal anode in aprotic Li-O2 battery. Although parasitic reaction and dendrite can be avoided as long as the Li metal is replaced by the alternative counter electrodes, this sacrifices the high energy of Li anode. The integration of organic liquid, polymer, ceramic and gel electrolytes with robust additives shows promising to constructive electrochemical and mechanical stable SEI on the Li metal. While the interface stability between the Li metal and the electrolytes, O2 etc. still lack definite explanation. Besides, the thermodynamics and kinetics properties, such as thickness, ion conductivity, mechanical modulus of durable SEI have not be concluded. Fundamental understand the mechanisms of SEI formation are critical to develop safe, longterm rechargeable Li-O2 batteries.

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