Li-O2

www.afm-journal.de www.MaterialsViews.com FEA 1. Intro The elec renewab strategie security, this tran (EES) sy able batt their hig art batte and grid in electr storage cles ( < 10 density of Li-ion batteries increased ≈ 15% per year and may reach the theoretical limits soon. [ 4–8 ] In this context, in addition to the Making Li-Air Batteries Rechargeable: Material Challenges Yuyan Shao , * Fei Ding , Jie Xiao , Jian Zhang , Wu Xu Yong Wang , * and Jun Liu * Li-air batteries, the d positive electrode is shown in term “desired electrochemica Li-O 2 reactions at oxygen elec lead to various discharge pro only the one in Equation (4) w could enable truly rechargea words, the successful forma the prerequisite for a practica will be discussed in details in Anode: Li ↔ Li+ + e− Cathode: Alkaline O2 + 2H2O + 4e − ↔ 0 A Li-air battery could potentially provide three to fi ve times higher ener sity/specifi c energy than conventional batteries and, thus, enable the d range of an electric vehicle to be comparable to gasoline vehicles. How making Li-air batteries rechargeable presents signifi cant challenges, m related to the materials. Here, the key factors that infl uence the recharg ability of Li-air batteries are discussed with a focus on nonaqueous sys The status and materials challenges for nonaqueous rechargeable Li-ai batteries are reviewed. These include electrolytes, cathode (electrocata lithium metal anodes, and oxygen-selective membranes (oxygen suppl air). A p DOI: 1 Dr. Y. Y. Shao , Dr. F. Ding , Dr. J. Xiao , Dr. J. Zhang , Dr. W. Xu , Dr. S. Park , Dr. J.-G. Zhang , Dr. Y. Wang , Dr. J. Liu Pacifi c Northwest National Laboratory Richland WA 99352, USA E-mail: yuyan.shao@pnnl.gov; jiguang.zhang@pnnl.gov; yong.wang@pnnl.gov; jun.liu@pnnl.gov Dr. F. Ding National Key Laboratory of Power Sources Tianjin Institute of Power Sources Tianjin 300381, P. R. China Prof. Y. Wang The Ge and Bio Washin Pullma Adv. Fun 987wileyonlinelibrary.com© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Acid O2 + 4e − + 4H+ ↔ 2H2O (E0= 4.26 V vs Li /Li+ (3) Nonaqueous 2Li + + 2e− + O2 ↔ Li2O2 (E0= 2.96 V vs Li/ Li+ (4) 0.1002/adfm.201200688 ne and Linda Voiland School of Chemical Engineering engineering gton State University n, WA 99164, USA ct. Mater. 2013, 23, 987–1004 duction trifi cation of transportation [ 1 ] and large-scale deployment of le energy (e.g., solar, wind) [ 1 ] have been the indispensable s to address the issues with global climate change, energy and sustainability. One of the most diffi cult challenges for sformation is the unsatisfactory electrical energy storage stems. [ 1–3 ] Among various EES technologies, recharge- eries, especially lithium-ion batteries are attractive due to h energy density and effi ciency. However, the state-of-the- ries fall far behind the requirements for electric vehicles energy storage. For example, current traction batteries ic vehicles (e.g., Li-ion batteries) are still short of energy capacity, which severely limits the range of electric vehi- 0 miles). After several decades of development, the energy in recent The s aqueous tively. [ 24 ] cally rech negative The elec the envir potential the electr Equation tively. Th (zinc-air aqueous erspective for the future of rechargeable Li-air batteries is provid esired electrochemical reaction on Equation (4) . [ 32–40 ] Here we use the l reaction” because there are other trode in nonaqueous systems that ducts of Li-air battery, [ 37 , 40–42 ] but ith Li 2 O 2 as the discharge product ble Li-air batteries; [ 40 , 42 ] in other tion/decomposition of Li 2 O 2 [ 38 ] is l rechargeable Li-air batteries. This the Section 3.1. (1) 4OH−(E = 3.43 V vs Li/ Li+ ) (2) TU R E A R TIC LE , Sehkyu Park , Ji-Guang Zhang , * continuous investigations on advanced Li-ion batteries, [ 9–13 ] there have been great efforts in new transformational “beyond Li-ion” battery technologies [ 14–19 ] that can provide suffi cient energy storage capacity for practical electric vehicles (without “range anxiety”). [ 20 ] Lithium-air batteries seem to be one of the most promising battery tech- nologies that could provide signifi cantly enhanced energy storage capability that would be suffi cient to drive electric vehi- cles of more than 300 miles (per charge), which is comparable to gasoline vehicles. [ 21 ] Therefore, there has been strong interest in Li-air battery technology around the world years. [ 20 , 21 ] chematics of nonaqueous Li-air batteries [ 22 ] and Li-air batteries [ 23 ] are shown in Figure 1 a,b, respec- Both of these systems have been reported to be electri- argeable. The fundamental electrochemical reaction at electrode is the same for both systems ( Equation (1) ). trochemical reactions at positive electrode depend on onment (electrolyte) around it and so do the electrode s. [ 21 , 25 ] In the case of aqueous Li-air batteries, [ 21 , 26–31 ] ochemical reactions at positive electrodes are shown in (2, 3) for alkaline (neutral) and acidic systems, respec- ey are similar to cathode reactions of alkaline fuel cells batteries) and acidic fuel cells, respectively. In non- gy den- riving ever, ostly e- tems. r lysts), y from ed. 988 www.afm-journal.de www.MaterialsViews.com wileyonli FE A TU R E A R TI C LE . KGaA, Weinheim The extremely high specifi c energy/energy density of Li-air batteries mainly comes from two factors. First, in contrast to most o inside active Instead reduce tical Li also ca to prov and ho rial (lit capacit (–3.04 when Northw nonaqu weight of 20% optimi curren energie oxygen also b from t could p which teries. [ 4 Li-air b efforts ability batterie avoid c air” an are usu Mak cant ch will dis nents s able L (both s anodes from a other, w compo electro with th In t batterie tives. [ 21 that m teries t challen this re ability electro 4) lithi We un and co Dr. Jun Liu is a Laboratory Fellow at the Pacifi c Northwest National Laboratory and leader for the Transformational Materials Science Initiative. Dr. Liu’s main research interest is the synthesis of functional nanomaterials for energy storage, catalysis, environ- mental separation, and health care. Dr. Ji-Guang (Jason) Zhang is a Laboratory Fellow at the Energy ces. stage, they are not the core research activities in the herefore, these are not included here. The last section review gives a perspecitive on the future challenges and nites for rechargeable Li-air batteries. llenges in Aqueous Li-air Batteries ous Li-air batteries, water and/or H + /OH − also partici- the reactions as shown in Equation (2, 3) . Considering reactants shown in the left side of the Equation (2– 4) , Yong Wang graduated with PhD in Chemical Engineering at Washington State University (WSU) in 1993, and joined PNNL in 1994. He recently assumed a joint position at WSU and PNNL, continuing to be a Laboratory Fellow at PNNL and being the Voiland Distinguished Professor in Chemical Engineering at WSU. Dr. Wang is best known for his ership in the development of novel catalytic materials reaction engineering for the conversion of fossil and ass feedstock. Adv. Funct. Mater. 2013, 23, 987–1004 nelibrary.com © 2013 WILEY-VCH Verlag GmbH & Co ther batteries that must carry both the anode and cathode a storage system, Li-air batteries are unique in that the cathode material (oxygen) is not stored in the battery. , oxygen can be absorbed from the environment and d by catalytic surfaces inside the air electrode. In prac- -air batteries, carbon-based air electrodes are sometimes lled the cathode for convenience. Its main function is ide a triple phase region to facilitate the Li-O 2 reaction st the reaction products. [ 43–45 ] Second, the anode mate- hium metal) of Li-air batteries has the highest specifi c y (3862 mAh/g) and the lowest electrochemcial potential V vs. SHE [ 46 ] ), which corresponds to the highest voltage reacting with oxygen. Recently, a team at the Pacifi c est National Laboratory (PNNL) demonstrated a primary eous Li-air battery of 362 Wh/kg based on the whole cell and working for 33 days in ambient air with a humidity RH (there is still much room for improvement by the zation of the cell structure and materials), [ 47 ] while the t well-developed Li-ion batteries have cell-based specifi c s of only ≈ 200 Wh/kg. A hierarchically porous graphene electrode with a high capacity of 15 000 mAh/g has een reported for primary nonaqueous Li-air battery his team. [ 48 ] A well-developed nonaqueous Li-air battery otentially provide a specifi c energy of 500–1000 Wh/kg, is at least 2–5 times greater than the present Li-ion bat- 9 , 50 ] However, in order to compete with Li-ion batteries, atteries need to be rechargeable. Therefore, intensive have been made worldwide to explore the recharge- of Li-air batteries. We want to point out that most Li-air s in research labs are tested in pure oxygen in order to ontaminants from air (especially water). Therefore, “Li- d “Li-oxygen”, and, “air cathode” and “oxygen cathode” ally used without differentiation unless specifi ed. ing a Li-air battery truely rechargeable presents signifi - allenges, especially for cell component materials. As we cuss in the following sections, most of the key compo- till cannot meet the requirements for practical recharge- i-air batteries. These components include electrolytes olvents and lithium salts), cathodes/catalysts/binders, and oxygen-selective membranes for oxygen supply ir. Furthermore, these components interplay with each hich further complicates the investigation on individual nents. For example, in a nonaqueous Li-air battery, some lytes are stable with the anode, but they are not stable e cathode, especially in an oxygen-rich environment. he last a few years, several review articles on Li-air s have been published from different perspec- , 28 , 44 , 45 , 49–52 ] Here, we will focus on the critical issues ust be understood and addressed to make Li-air bat- ruly rechargeable. Except for a briefl y summary on the ges in aqueous Li-air batteries (Section 2), most part of view will discuss various issues related to the recharge- of nonaqueous Li-air bateries. These issues include: 1) lytes, 2) air electrode material and strucure, 3) catalysts, um metal anodes, and 5) oxygen selective membranes. derstand that other components such as seperators [ 53–55 ] rrent collectors [ 56 ] are also important, but, at least at the batte devi present fi eld. T of this opportu 2. Cha In aque pate in all the lead and biom and Environment Directorate of the Pacifi c Northwest National Laboratory (PNNL). Currently, he is the group leader for PNNL’s efforts in the area of Energy Storage for Mobil Applications. His research inter- ests are in the development of energy storage and energy effi cient devices, including lithium-air batteries, lithium-ion ries, thin-fi lm solid-state batteries, and electrochromic www.afm-journal.de www.MaterialsViews.com FEA T Based on the PLE concept, several groups [ 24 , 31 , 60 ] used an organic liquid or pol- lytes (LiOH electrolyte) have been exten- nona dens effor short batte teries Fo have leagu lently (Li 1 + x (Japa are im when this p Lipon form Figur stabi layer. Figure an aq thin fi layer. Adv. Fu © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim sively inv will be c electrolyt the neut 10). The alkaline the LATP suggeste electrolyt glass. W pH diagr an aqueo that a sim cannot a Li-ion co environm Since the naked proton transfer in LATP does not i cern that a proton may tran be slow) and be reduced b which is detrimental for lo issues in Li-air batteries. that the H + ion cannot go ther confi rmed the stability electrolytes. [ 66 ] On the cathode side, Zh alysts and cabon materials nitride catalyst, [ 68 ] and grap as a catalyst in air electr mechanism. They also re using a graphene-based a performances. Meanwhile, a titanium electrode for th reduces the charge overp fundamental reaction at a to that in a Zn-air battery queous Li-air batteries exhibit higher theoretical energy ity than aqueous systems [ 29 , 57 ] and have attracted the most t worldwide to date. [ 21 , 29 , 57 ] Therefore, in addition to the discussion on the status and challenges of aqueous Li-air ries in this section, we will focus on nonaqueous Li-air bat- in the following sections. r aqueous Li-air batteries, great effort and progress been made since the pioneering work by Visco and col- es. [ 58 ] It is well known that lithium metal will react vio- with water. Solid electrolytes, such as NASICON glass + y Al x Ti 2 − x Si y P 3 − y O 12 or LATP) made by Ohara Inc. n) have good lithium-ion conductivity ( ≈ 10 − 4 S cm − 1 ) and permeable to water. However, this material is not stable in contact with lithium metal. Visco et al. [ 58 ] fi rst solved roblem by depositing a solid-state interfacial layer (Cu 3 N, , etc.) between lithium metal and NASICON glass, thus ing a protected lithium electrode (PLE) as shown in e 1 c. Imanishi’s group also did extensive studies on the lity of such a lithium anode using LiPON as the interfacial [ 59 ] 1 . a) Structure scheme of a nonaqueous Li-air battery system. b) Structure scheme of ueous Li-air battery. c) Original Polyplus protected lithium electrode structure with solid lm as interlayer. d) Protected lithium electrode with organic electrolytes as the interfacial Reproduced with permission. [ 24 ] Copyright 2010, Elsevier. nct. Mater. 2013, 23, 987–1004 989wileyonlinelibrary.com estigated. It is found that LATP glass orroded rapidly in acidic or alkaline es, while this glass is more stable in ral electrolytes (pH value from 4 to buffer acid electrolyte and saturated electrolyte are both useful to protect glass. [ 62 , 64 ] Zhou and co-workers [ 60 ] d a fuel cell design to control the e pH value and to protect the LATP olfenstine [ 65 ] tried to use potential- am to predict LATP glass stability in us electrolyte and it was discovered ple potential–pH diagram for water ccurately predict the stability of solid nducting membranes in aqueous ents. (H + ) is smaller than Li + ion (the ion nvolve solvation shell), there is a con- sfer through LATP (although it may y lithium metal to form hydrogen, ng term stability and presents safety Our recent inveistigation indicated through the LATP glass, and fur- of the PLE structure in the neutral ou’s group studied several novel cat- , including Cu catalyst, [ 67 ] titanium hene. [ 26 ] Cu metal was fi rst suggested ode based on the copper-corrosion ported that aqueous Li-O 2 batteries ir electrode exhibit good discharge Zhou and co-workers also suggested e cathode charging process, which otential of the air cathode. [ 27 ] The n aqueous Li-air cathode is similar (or a fuel cell). Therefore the widely U R E A R TIC LE ymer electrolyte as the interfacial layer between lithium metal and LATP glass, forming a triple electrolyte structure (organic electrolyte/LATP glass/aqueous electrolyte) or hybrid electrolyte (Figure 1 d). Both organic liquid electrolytes and polymer electrolytes have demonstrated similar performances to those with LiPON or Cu 3 N fi lms. Since the structure in Figure 1 d is easier to prepare in research laboratories, this PLE architecture has been used in most aqueous Li-air battery studies. For long-term cycling of a rechargeable Li-air battery, LATP glass (and other water- impermeable solid state electrolyte) needs to be highly stable in aqueous electrolytes with a large range of pH values. Therefore, the chemical stability of LATP glass in neu- tral (LiCl or LiNO 3 electrolyte), [ 61 ] acidic (HCl, lithium acetate electrolyte and phosphate buffer electrolyte), [ 30 , 61–63 ] and alkaline electro- [61,64] 990 www.afm-journal.de www.MaterialsViews.com wileyon FE A TU R E A R TI C LE characteristics: [ 21 , 49 ] 1) high stability in oxygen-rich electro- chemical conditions, [ 35 , 37 , 41 , 76–78 ] 2) high boiling point/low vapor [79] developed oxygen electrocatalysts in the later systems can be used in aqueous Li-air batteries [ 26 , 60 , 69–71 ] and there are already severa Alt much for th term Sever value LATP can s these LATP glass. LATP > 1 ye Th tery r nonaq LATP few m impro comp to op atures high trolyt Th tation will b catho of cat trode area a Oth includ and 3 oxyge issues 3. Ch 3.1. E Altho inves most oxyge eral n nonaq chem in an have other trolyt electr electr linelibrary.com © 2013 WILEY-VCH Verlag GmbH & Co pressur ally, the ucts (su Sinc opmen the req electrol thesis t oxygen does n tates e is iden study fo out; oth electrol co-work based) ( α -MnO section rich el electrol side rea solvent of this electrol 3.1.1. In Carbon bonate because solvent ture ra assume publica that the trolytes Li 2 O 2 . [ 4 and rep used in electroc the pos has bee later ex Kuboki dischar Before γ -butyro reducti Aurbac The e.g., O 2 many d base, a easily d dischar carbona l excellent reviews on this topic. [ 72–74 ] hough aqueous Li-air batteries show good promise and progress has been made, signifi cant challenges still exist eir practical applications. The fi rst challenge is the long- stability of protected lithium anode in aqueous electrolytes. al methods have been proposed to keep the electrolyte pH stable (close to neutral) to minimize the corrosion of the glass. [ 60 , 62 , 64 ] Since cation/anion exchange membranes uppress the anion/cation going through the membrane, membranes can be used to control the pH value on the glass surface if such a fi lm can be attached to the LATP However, even in a neutral electrolyte, slight corrosion of glass has already been observed. [ 61 ] The long-term (e.g., ar) stability of LATP glass has not been demonstrated. e second challenge in aqueous Li-air batteries is the bat- ate performance (this is also true or even worse for ueous Li-air batteries). The limited conductivity of the glass restricts the current densities of anodes to only a A per cm 2 at room temperatures. In addition to further ve the conductivity of the LATP glass by optimizing their osition and synthesis conditions, Yamamoto [ 31 ] proposed erate Li-air batteries with LATP glass at elevated temper- to increase the conductivity of the glass. However, the temperature system has additional problems, such as elec- e evaperation and quicker corrosion of LATP glass. e third challenge in aqueous Li-air batteries is the precipi- of LiOH in cathode. During the discharge process, LiOH ecome saturated in electrolyte and precipitate on the de. It will block the air channel and reduces the active area hodes. Novel cathode design that could minimize the elec- blocking is needed. More air channels, higher effective nd proper pore size of air cathodes are all very important. er challenges for rechargeable aqueous Li-air batteries e 1) water balance, 2) low discharge/charge effi ciency, ) CO 2 contamination from air. As discussed in Section 3.5, n-selective membranes might be applicable to address the with water evaporation and CO 2 contamination. allenges in Nonaqueous Li-Air Batteries lectrolytes ugh various nonaque