Nanostructured Electrode Materials for High-Energy Rechargeable Li

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Nanostructured Electrode Materials for HighEnergy Rechargeable Li, Na and Zn Batteries YAMIN ZHANG, and Nian Liu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b03839 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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Chemistry of Materials

Nanostructured Electrode Materials for HighHigh-Energy Rechargeable Li, Na and Zn Batteries Yamin Zhang1 and Nian Liu1,* ABSTRACT: Fossil fuel is the main energy resource currently. The continuous consumption of this non-renewable resource has caused very serious environment problem, which has motivated tremendous research efforts in this century. Energy storage is critical to alleviate the current energy and environmental problems. Comparing to mechanical energy storage, rechargeable batteries allow energy storage in smaller footprint. The intersection of rechargeable battery with nanomaterials have been a booming research topic recently and yielded new applications of nanomaterials as well as new solutions to many long-lived problems in battery science and technology. In this perspective, we highlight the most recent (2015~2017) examples across lithium, sodium and zinc battery chemistries, where nanoscale materials tailoring and design addresses the intrinsic problems and limitations at both the materials level and device level. And a few principles are generalized at the end.

TOC

KEYWORDS: Nanomaterials; Lithium battery; Zinc battery; Sodium battery; Electrocatalyst

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Nanostructured Electrode Materials for HighHigh-Energy Rechargeable Li, Na and Zn Batteries Yamin Zhang1 and Nian Liu1,* 1School

of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA KEYWORDS: Nanomaterials; Lithium battery; Zinc battery; Sodium battery; Electrocatalyst

ABSTRACT: Fossil fuel is the main energy resource currently. The continuous consumption of this non-renewable resource has caused very serious environment problem, which has motivated tremendous research efforts in this century. Energy storage is critical to alleviate the current energy and environmental problems. Comparing to mechanical energy storage, rechargeable batteries allow energy storage in smaller footprint. The intersection of rechargeable battery with nanomaterials have been a booming research topic recently and yielded new applications of nanomaterials as well as new solutions to many long-lived problems in battery science and technology. In this perspective, we highlight the most recent (2015~2017) examples across lithium, sodium and zinc battery chemistries, where nanoscale materials tailoring and design addresses the intrinsic problems and limitations at both the materials level and device level. And a few principles are generalized at the end.

Introduction Fossil fuel is the main energy resource currently, but it is non-renewable. The consumption of this resource has caused some serious problems, such as climate change, air and water pollution.1 In the past few decades, researchers have achieved big progress on the sustainable energy development, including photothermal receivers, wind turbines and photovoltaic cells. However, compared with energy generators, energy storage technology is also critical to alleviate the energy and environmental problems. Finding low-cost, high efficiency and environmentally friendly energy storage devices is important for the development of economy and society, which can be used in small electronic devices (smart phones, laptops, etc.), electric vehicles and even grid energy storage.2 Among energy storage technologies, rechargeable batteries have advantages of small footprint, fast response, and location-independence, which give them great potential. The recent success of entirely battery-powered electric vehicles is an important intermediate step towards even larger scale energy storage for the grid.3 Energy density, power density, safety and cost are the most important factors for batteries, including rechargeable lithium batteries4,5, sodium batteries6 and zinc batteries7. Lithium batteries are widely used currently. Lithium has the highest energy density and it is the lightest metal on periodic table. However, organic electrolyte is used for

lithium batteries, which is not safe. Besides, lithium electrodes are less stable with limited capacity, which limits its development for various different applications. Sodium is more abundant compared with lithium and it can be a cheaper option for large-scale energy devices. Sodium possesses high electrochemical capacity, which is good to be a candidate for rechargeable batteries. But the ionic radius of sodium is bigger than lithium’s ionic radius, which makes it harder to find the reversible intercalation electrodes. Also, compared with lithium battery, sodium battery has smaller obtainable voltage window due to the higher standard electrode potential of sodium. Zinc has various advantages such as abundance, low cost, lower equilibrium electrochemical potential, a flat discharge profile, a long storage life and environmental benignity. Zinc is the most active metal that is stable with water. By using aqueous electrolyte, Zinc battery is safe and can be manufactured in ambient condition. Also, Zn has high volumetric capacity with two valence electrons and high density. However, ZnO passivation and zincate ion dissolution limit the reversibility of zinc battery. Besides, hydrogen evolution makes it difficult to realize sealed cell geometry. Nanomaterials have at least one dimension in the nanoscale, which include nanoparticles, nanowires, nanorods, and so on. Nanostructured materials offer unusual chemical, mechanical, and electrical properties that are

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absent in the bulk, and have found applications in various different energy storage devices.8 For example, cycling life of lithium batteries can be improved by using nanoelectrodes thanks to the better strain accommodation during lithium insertion/de-insertion steps. Also, higher contact area between electrode and electrolyte and shorter path lengths of nanoelectrodes can lead to higher charge/discharge rates.9 Most importantly, nanomaterials are highly tunable to be fabricated and designed for different purposes, which is beneficial for solving complex problems in batteries. Many technical approaches have been explored to fabricate nanomaterials, including vapor phase growth, liquid phase growth, solid phase formation and hybrid growth. Besides, most commonly used characterization methods have matured, which include structural characterization methods (scanning electron microscope (SEM), transmis-

sion electron microscope (TEM), X-ray diffraction (XRD), and various scanning probe microscopy) and chemical characterization methods (electron spectroscopy, ion spectroscopy, and optical spectroscopy). These make it possible for nanomaterials to be used and evaluated in batteries. In this perspective, we will focus on nanostructured electrodes for lithium, sodium and zinc batteries. Due to limited space, we will use a few examples in each battery chemistry to illustrate the design principles. In lithium battery, we will focus on silicon anode, lithium metal anode, sulfur cathode, and oxygen cathode. In sodium battery, we will focus on anode materials, sulfur and metal sulfide cathodes. In zinc battery, we will focus on zinc anode, MnO2, NiOOH, and oxygen cathodes. Figure 1 shows calculated energy densities of Li, Na, and Zn secondary battery systems.10

Figure 1. Calculated energy densities of Li, Na and Zn secondary battery systems

Li battery Silicon anodes Lithium ion batteries (LIBs) is widely used in small electronic devices (smart phones, laptops, etc.) and electric vehicles.11–14 However the increasing energy storage demands continues to request more operating cycles, better stability, higher power and energy densities. The theoretical specific capacity of commercialized graphite anode is 372 mAh g−1, which is far from the energy storage requirement .15 There are many alloy16,17 and conversionreaction anode materials including Si18, Sn19, Al20, metal oxides21, etc. Silicon is a very attractive material to be the anode in lithium batteries. It has a theoretical specific capacity of 4200 mAh g−1, which is more than 10 times that of graphite.22,23 As anode, silicon could be used in traditional LIBs, recent Li–S and Li–O2 batteries. Silicon anode is a good replacement for the lithium metal, which have potential to form dendrite. However, during the cycling, silicon

anode has a big volume change (∼300%), which causes the structural degradation of silicon, instability of solidelectrolyte interphase (SEI) and further limits the development of silicon anodes. Nanostructured electrode design has been proven an effective approach to solve these challenges. For nanoparticle design, Liu et al.24 developed a hierarchical structured silicon anode (Figure 2A, 2B) which successfully tackled problems including structural degradation, unstable SEI and side reactions. In addition, the problem that nanoelectrode usually has low volumetric capacity was also solved thanks to their design. Their inspiration came from the structure of a fruit - pomegranate. In their design, every single silicon nanoparticle was encapsulated by a carbon shell (Figure 2E), which provided enough free space for silicon anode expansion and contraction resulting from lithiation and delithiation. Then an ensemble of these hybrid nanoparticles was further encapsulated by a

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thicker conductive carbon layer and formed a micrometersized pomegranate. This thick carbon layer served as an electrolyte barrier. This nanostructured silicon anode has superior cycle stability with a capacity retention of 97% after 1,000 cycles (Figure 2F). It also shows 1,270 mAh cm-3 volumetric capacity and high Coulombic efficiency of almost 100%. The areal capacity of this anode can even reach 3.7 mAh cm-2, which is the areal capacity of commercial LIBs, and remained stable over 100 cycles. This was attributed to the stable and spatially confined solidelectrolyte interphase and the lower electrode–electrolyte contact area. Besides, Xu et al. 25 synthesized the watermelon-inspired Si/C microspheres to get better electrochemical performance of densely compacted Si/C nanostructured anodes. This densely compacted Si/C anodes exhibited superior cycle stability, as well as excellent rate performance and Coulombic efficiency. In contrast to impregnation coating, Lu et al.26 designed a different structured nanoelectrode, nonfilling carbon-coated porous silicon microparticles (nC-pSiMP), and solved the problems including structural degradation and the low volumetric capacity. In their design, many interconnected silicon nanoparticles firstly formed a porous silicon microparticle, then the outer surface of this microparticle was coated with the conductive carbon. This composite structure demonstrated high volumetric capacity (∼1,000 mAh cm3), excellent cycling stability and reversibility (∼1,500 mAh g-1 capacity retention for 1,000 cycles). For nanomaterials, low tap density is another major issue needed to be addressed. Lin et al.27 demonstrated a mechanical approach to assemble nanostructured Si secondary clusters (nano-Si SC) and produced 420 g (>95% yield) of nano-Si SC in lab. They achieved much denser packing of nanostructures (1.38 g cm-3) and 0.91 g cm-3 high tap density. As a result, nano-Si SC showed excellent cycle stability with more than 95% capacity retention after 1,400 cycles. As a substitute for carbon, graphene can also be used as coating to encapsulate Si particles. Li et al.28 introduced conformal graphene cages (Figure 2C, 2D), which acted as a mechanically strong and flexible buffer, on micrometer-sized silicon particles as lithium battery anodes (SiMP@Gr). Such cage (Figure 2G) allows the expansion and contraction of microparticles inside the cage during the cycling. The electrical connectivity was retained as well. It showed stable cycling and high Coulombic efficiency due to a stable SEI and the minimization of irreversible consumption of lithium ions. Besides, silicon-nanolayer-embedded graphite also exhibited a high Coulombic efficiency.29 In another case, Ryu et al. 30 successfully prepared Si nanosheets from inexpensive natural clays. This scalable carbon-coated Si nanosheet anodes showed a higher reversible capacity of 865 mAh g−1 at 1.0 A g−1 rate with 92.3% capacity retention after 500 cycles. After 200 cycles, it only showed 42% nominal volume expansion. It also delivered higher rate capability with 60% capacity at rate of 20 A g−1. Although coating the nanostructured Si with conductive carbon layer or some other functional conductive materials can significantly improve the performance of silicon anode, the mechanism and relative performance of

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different type of coatings are incompetently understood by researchers. Luo et al.31 tried to use thin alucone and Al2O3 as coatings to understand the mechanism of coatings on the Si nanowires (SiNWs). They focused on the lithiation step of SiNWs and summarized the effect of these two coatings. They found two types of lithiation fronts of the SiNWs through a nanoscale half-cell battery by using in situ TEM and chemo-mechanical simulation, including “V-shaped” and “H-shaped” fronts. “V-shaped” front was produced by alucone coating, while “H-shaped” front was leaded byAl2O3 coating. Their results indicate that morphological evolution during lithiation step is determined by the lithium diffusivity on the surface and bulk lithiation reaction rate of different coating layers on the SiNWs. Besides, the rate performance of the battery can be determined by the coating layer as a result that the lithiation rate in the coatings could be limiting step for lithiation reaction. Lithium metal anodes Although nanostructured Si materials are promising candidates as anodes in LIBs, lithium metal anode is considered as other promising candidate anodes for rechargeable LIBs thanks to the highest theoretical capacity (3,860 mAh g–1)32 and lowest electric potential (–3.04 V versus SHE) of lithium metal. However, during charge/discharge cycles, uneven lithium dendrite deposition, unstable SEI and relatively big dimension change problems, leading to low Coulombic efficiency and serious safety concerns, which limits its application. As a result, many efforts have been put on advanced characterization in order to elucidate lithium growth process, and on structure design of lithium metal anode to improve cycling performance. Coating is one of the earliest explored strategies. Nanomaterials coating can achieve the goal with minimum increase of the electrode weight. Applying thin chemical protection layers directly on Li metal is a promising solution to protect Li metal, such as protective polymer layer, atomic layer deposition (ALD) of 14 nm thick Al2O3 layer33 and the artificial LiF-containing protection film34. Besides, ultrathin two-dimensional (2D) atomic layers, which consists of hexagonal boron nitride (h-BN) and graphene were grown directly on Cu metal current collectors.35 This 2D atomic crystal structured electrode showed a superior protection for lithium metal and smooth lithium deposition was realized. Zheng et al.36 used amorphous hollow conductive carbon monolayers to coat the Li metal and this anode achieved a significantly higher Coulombic efficiency (more than 99% for more than 150 cycles) than the bare one. They attribute this good performance to the carbon nanospheres, which could help to isolate the lithium deposition and facilitate stable SEI formation. Minimizing volume change by using stable hosts is also a good way to improve lithium metal anode cycling performance.37 Lin et al. 38 developed LixSi–Li2O as a stable host for lithium ion insertion/de-insertion. As a conductive framework, it blocks embedded lithium from direct contact with electrolyte and keeps constant volume in electrode-level. It exhibited high-power output and showed a stable cycle performance. Besides, Lin et al 39 reported layered reduced gra-

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phene oxide (rGO) as a stable host for lithium (Li–rGO, Figure 2H-2J) because rGO has nanoscale interlayer gaps. Such designed electrode limited electrode thickness

change to only ∼20% during cycling, avoided Li dendrite deposition and unstable SEI problems of lithium foil.

Figure 2. Structure and performance of silicon pomegranate, SiMP@Gr and Li–rGO as Li-ion battery anodes. (A) Three-dimensional view of structure change of a pomegranate-like microparticle before and after battery cycling. (B) Simplified 2D cross-section view of structure change of a pomegranate-like microparticle before and after battery cycling. Reproduced with permission.24 Copyright 2014, Nat. Nanotechnol. (C) Simplified 2D cross-section view of SiMP before and after battery cycling. (D) Simplified 2D crosssection view of SiMP@Gr before and after battery cycling. Reproduced with permission.28 Copyright 2016, Nat. Energy. (E) SEM image of a pomegranate-like microparticle with a 40 nm void space. (F) Reversible delithiation capacity of the silicon pomegranate anode for the first 1,000 cycles. (G) TEM image of an individual SiMP@Gr particle. (H) SEM image of pristine GO film. (I) SEM image of sparked rGO films. (J) SEM image of layered Li–rGO films. Reproduced with permission.39 Copyright 2016, Nat. Nanotechnol.

On the other hand, the heterogeneous nucleation of Li metal has been utilized in nanoscale design. Yan et al.40 explored the nucleation of lithium on various metal substrates and found that nucleation barrier exists on metals showing negligible lithium solubility, whereas no nucleation barrier is present for metals exhibiting a definite solubility. Furthermore, they designed a nanocapsule structure with Au nanoparticle seeds inside and hollow carbon

spheres outside for lithium metal anode. As a result, the lithium metal predominantly grew inside the hollow carbon spheres during deposition. Introducing new electrolyte system and additives is also an efficient method to modify solid electrolyte interphase layer41 and minimize the lithium dendrite growth. Qian et al.42 reported a highly concentrated electrolyte, which minimized the lithium dendrite deposition and showed a high-

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rate cycling performance with more than 99% Coulombic efficiency. This electrolyte was consisted of lithium bis(fluorosulfonyl)imide salt and ether solvents. This concentrated electrolyte can increase both the solvent coordination and Li ion availability in the electrolyte. Sulfur cathodes Among the various electrochemical energy storage systems, rechargeable lithium-sulfur (Li-S) battery has drawn considerable attention because of their higher specific energy (~2,500 W h kg-1) compared to lithium-ion batteries.43–46 However, Li-S system has fundamental challenges to be solved, for example, the low electrical conductivity of S and sulfides, and solubility of intermediate lithiated sulfur compounds. 47 An efficient route to enhance cycling performance of Li– S battery is confining soluble sulfur species by coating, including carbon, polymer, metal oxide and metal sulfide. Wu et al. 48 reported a stable double shelled sulfur cathode, carbon black decorated polydopamine-coated hollow nanosulfur spheres (CB-HNSs-PD) (Figure 3A-3D), with stable cycling over 2500 cycles. They used polydopamine as “nano-binder” to glue carbon black and sulfur at the nanoscale, which was different from those only used additional binder to fabricate cathode in the coating process. This approach with void space inside successfully prevented carbon black’s detachment from cathode surface during cycling, provided conductive agents for their reutilization, trapped the polysulfides and reutilized the migrating polysulfides. Li et al. 49 published a pomegranate-like carbon cluster–encapsulated sulfur as the cathode. Their inspiration came from the design of silicon–carbon pomegranatelike cluster structure.24 Besides, graphene-based carbon50, which can expectedly yield a strong interaction between S and functional group in graphene is also a functional coating for S cathode.51 For example, Li et al.52 achieved ordered sulfur-graphene (S-G) nanowalls by using a simply electrochemical assembly strategy. The S-G nanowalls were aligned onto electrically conductive substrates. Such cathode had excellent cyclability and high-rate performance. Specifically, graphene arrays arrange perpendicularly to the substrates and sulfur nanoparticles are homogeneously anchored between graphene layers. This structure improves the accommodation of sulfur volume change and diffusivity of lithium and electron. Interconnected graphene/carbon nanotube (CNT)/sulfur (denoted GCS) hybrid53 can also improve the accommodation of sulfur volume variation and the transfer of Li and electron during cycling, which can be attributed to the conductive network formed by graphene and CNT. In this structure, graphene/carbon nanotube has ~70 wt% uniform loading of sulfur. Such GCS hybrid exhibited capacity of 657 mAh g−1 for more than 450 cycles with 0.04% capacity decay per cycle. In addition, carbon nanofibers, carbon nanotubes, carbon nanocages and C-wood matrix54 are also promising sulfur hosts for Li-S batteries. Li et al. 55 filled MnO2 nanosheets into hollow carbon nanofibers (MnO2@HCF,

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Figure 3E, 3G) as sulfur host. Both the lithium and electron transfer can be facilitated by using this MnO2@HCF hybrid host. Besides, such host can prevent polysulfide dissolution (Figure 3F) by physically entrapping polysulfides into carbon shell and chemically binding them to the MnO2. The sulfur loading was as high as of 71 wt% in electrode (Figure 3H). The theMnO2@HCF/S MnO2@HCF/S electrode showed a good performance with specific capacity of 1,161 mAh g−1 at 0.05 C rate and stable cycling for more than 300 cycles at 0.5 C rate. Zhu et al.56 rationally integrated high conductive super-long carbon nanotubes (CNTs) and nano-sized hollow graphene spheres (GSs) and successfully constructed a free-standing paper electrode. The hollow GSs not only accommodate sulfur species and sustain the volume fluctuation during cycling, but also retard the dissolution of polysulfides and parasitic shuttle. The graphene walls of GSs and super-long CNTs synergistically constructed hierarchical short-/longrange electron/ion pathways. As a result, this cathode exhibited a high-rate capacity retention of 40% at 16.7 A g−1 and a superior capacity retention of 89.0% over 500 cycles with a high sulfur utilization of 81%. Lyu et al.57 reported a new kind of carbon–sulfur composites with a high sulfur loading of 79.8 wt% by infusing sulfur into the novel hierarchical carbon nanocages (hCNC) with high pore volume, network geometry and good conductivity. The designed S@hCNC composite presented a large capacity, high current density capability and long cycling life, which could shorten the charging time for mobile devices from hours to minutes. The good performance of this cathode is also due to the alleviation of polysulfide dissolution, the enhancement of electron and Li-ion diffusion. The cathode architecture with natural, three-dimensionally (3D) aligned microchannels filled with rGO also achieved good cycling performance. 54 Metal oxide and sulfide, such as TiO2, MoS2 and LiS258, can also effectively solve the insulating problem and enhance the capacity of sulfur cathode. Yan et al.59 prepared mesoporous titanium dioxide sub-microspheres (MS TiO2) by triggering self-assembly of primary TiO2 nanoparticles during hydrothermal synthesis. This unique 3D structure can realize effective contact between host material and lithium polysulfide intermediates, physically confine the intermediates to the electrode, and accommodate the volume change of sulfur upon cycling. As a result, this cathode exhibited superior rate capacity and cycling stability than that of commercial TiO2 as well as sulfur composite cathode. Zhang et al.60 synthesized hierarchical MoS2/SnO2 nanocomposites by uniformly loading SnO2 onto the surface of MoS2 nanosheets. This nanocomposite exhibited high capacity (up to 900 mAh g−1 for more than 80 cycles) and cycling capability because of the inhibition of lithium polysulfide diffusion by the loading of SnO2 nanoparticles and the increasing conductance of the composites due to the presence of metallic Mo particles. Incorporating fewlayer nanosheets of phosphorene, which served as immobilizer and catalyst in Li-S batteries, into a porous carbon nanofiber network (cathode matrix) could also obviously improve the cycling performance of Li-S batteries.61

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Figure 3. Various nanostructured sulfur cathodes. (A) 3D view of CB-HNSs-PD double-shelled cathode before and after being discharged. (B) Implified 2D cross-section view of CB-HNSs-PD double-shelled cathode before and after being discharged. (C) SEM image of CB-HNSs-PD cathode. (D) TEM image of CB-HNSs-PD showing CB stick on the HNSs-PD. Reproduced with permission.48 Copyright 2017, Adv. Energy Mater. (E) Synthesis method of the MnO2@HCF/S composites. (F) Mechanism of advantages of MnO2@HCF/S composites over HCF/S. (G) SEM and TEM images of MnO2 @HCF. (H) SEM and TEM images of MnO2@HCF/S composite. Reproduced with permission.55 Copyright 2015, Angew. Chemie - Int. Ed.

Oxygen cathodes Rechargeable lithium oxygen (Li–O2) battery has drawed increasing attention recently. The energy density of Li–O2 battery is 5,217 Wh kg−1, which is the highest among all

type of rechargeable batteries.62–64 However, limitations of Li–O2 battery, such as corrosion of cell components by reactive oxygen species, large voltage hysteresis, low current density, and unstable cycling performance, hinder the pos-

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sible applications of Li-O2 battery.65 As published, the discharge process of Li-O2 batteries is oxygen reduction reaction (ORR, O2 (g) + 2Li++ 2e− → Li2O2(s)), which forms solid Li2O2 on the surface of cathode. The decomposition of the solid Li2O2 is oxygen evolution reaction (OER, Li2O2(s)→2Li++O2(g) + 2e−), which requires higher potentials during the recharge process.66 Such required high recharge potential causes big energy loss of Li–O2 batteries. Using catalyst in the air cathode is an efficient way to suppress the energy loss. The designed heterogeneous electrocatalysts should be able to decrease overpotentials of both O2 reduction reaction and O2 evolution reaction because Li2O2 is an electronic insulator and has low solubility in organic electrolytes.61,67 Nanostructured electrocatalysts have received much interest because of their high surface area and excellent catalyst performance. Here we will discuss nanostructured metal oxide, carbon and carbon based electrocatalysts. Metal oxide catalysts in the form of powder can be used in cathode by mixing with carbon black and binder. For example, manganese oxides (MnOx) have attracted intensive attention due to their abundance in the earth's crust, low-price, nontoxicity and high catalytic activity toward oxygen reduction reaction. Jang et al. 68 synthesized sea urchin shaped α-MnO2/RuO2 mixed oxides as the air cathode. MnO2 and RuO2 nanoparticles were homogeneously distributed on several straight and radially grown nanorods. This nanostructure showed excellent stable cycling performance by decreasing the overpotential as air cathode catalyst. Huang et al.69 prepared Ag nanoparticles decorated β-MnO2 nanorods (Ag/β-MnO2) as cathode catalyst for rechargeable Li-O2 batteries, which showed superior rate capability and cycling stability. Furthermore, the results showed that the electrochemical performance of βMnO2 was greatly enhanced by the presence of Ag nanoparticles. Co3O4 also has relatively better catalytic activity towards both ORR and OER than other transition metal oxides. Liu et al. 70 synthesized hierarchical Co3O4 porous nanowires (NWs, Figure 4A) as cathode catalyst, which showed improved efficiency and stable cycling performance in organic Li-O2 batteries. All the nanostructured catalysts showed above exhibited excellent bifunctional electrocatalytic activity for ORR and OER, which accounted for the superior performance when they served as the catalysts in the cathode. As effective electrocatalysts for the ORR, carbon and carbon based nanostructured materials have also attracted much attention in Li–O2 batteries.71 Sun et al. 72 reported

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the mesoporous carbon nanocubes (MCCs, Figure 4B) as the catalyst for Li–O2 batteries, which delivered discharge capacities of 26,100 mAh g−1 at rate of 200 mA g−1. Compared with commercial carbon black catalysts, this value is promising. MCCs have a lot of hierarchical mesopores and macropores, which could increase O2 diffusion rate and give space for accommodation of insoluble Li2O2. As a result, MCCs could be used as a conductive host for other highly efficient catalysts. For example, Ru functionalized MCCs showed excellent OER catalytic activity. Guo et al. 73 synthesized uniform ordered mesoporous carbon nanofiber arrays coated with RuO2, which also displayed good cycling performance, such as high capacity, high rate capability and long-term cycling. The explanation for the high performance of this material is similar to that of above Ru functionalized mesoporous carbon nanocubes. Mesopores facilitated both the electron and ion transfer, while the macropores provided void space to accommodate O2/Li2O2 and improve O2 diffusion. Moreover, the RuO2 coating alleviated the decomposition of electrolyte and enhanced the electronic conductivity of surface. As reviewed above, many catalysts can be used in the cathode to improve battery performance. However, there are drawbacks because catalysts were usually loaded on carbon substrates. The discharge product Li2O2 is a strong oxidizer. The carbon cathode can be attacked by Li2O2, which is thermodynamically favored.

→ ∆ = −. + → + ∆ = −. + +

For this reason, the cathodes with carbon have shorttime cycling performance, which will cause ambiguity if investigators want to study the stability of electrolyte when Li2O2 is present. As a result, it is very important to investigate carbon-free and binder-free electrodes. Kim et al.74 coated Ni nanowire substrate with 80% capacity retention for more than 100 cycles (Figure 5B). The large capacity of their anode could be attributed to the structure of the hybrid material. On the one side, Na can be intercalated along the x axis of the P layers to form Na3P. On the other side, graphene layers serve as electrical highway and provided buffer space to accommodate expansion on the y/z axis.

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Figure 5. Structure and electrochemical performance of phosphorus-graphene anode for sodium battery. (A) Structural evolution of the sandwiched phosphorene–graphene structure during sodiation. (B) Reversible desodiation capacity and Coulombic efficiency for the first 100 galvanostatic cycles of the phosphorene/graphene (48.3 wt% P) anode tested under different current densities. Reproduced with permission.77 Copyright 2015, Nat. Nanotechnol.

Transition metal oxides, including V2O578, MnO279, TiO280 have also been used as anode materials in SIBs. For example, TiO2 is chemically stable, abundant and nontoxic, which makes it a possible candidate to be an anode for SIBs. However, TiO2 is less conductive, which limits its rate capability and therefore its electrochemical performance. Yeo et al.81 proposed a functional TiO2 composite with good performance. In their design, TiO2 nanofibers was wrapped by rGO (rGO@TiO2 NFs). This composite exhibited a significantly high initial capacity. Besides, it showed more than 80% capacity retention over 200 cycles and >90% Coulombic efficiency. Comparing with pristine TiO2 NFs, this excellent performance could be attributed to the highly conductive properties of the rGO anchored to the TiO2 NFs. Highly ordered 3D Ni-TiO2 core−shell nanoarrays fabricated by Xu et al.82 also delivered a high reversible capacity, fast ion and electron transfer, and good electrode integrity as an anode material. Materials with layered structure have been successful as host for Li-ion batteries, because of facile ion intercalation and the interlayer spacing accommodates volume expansion. They have been explored for SIB electrodes as well. Tin disulfide (SnS2) has high theoretical capacity, big interlayer distance and is not expensive. Conventionally, SnS2 nanomaterials were usually hydrothermally synthesized and took a lot of time, which is hard to utilize in a big scale. Wang et al.83 developed a straightforward solid-state reaction method to synthesize carbon-coated SnS2 (SnS2/C) nanosphere as anode, which showed only 0.14% decay per cycle and exhibited excellent cycling performance with a high reversible capacity. MoS2 has large interlayer distance along c-axis, which is good for accommodation of sodium

ions. Su et al. 84 successfully synthesized few-layer ultrathin MoS2 nanosheets (~10 nm), which exhibited a superior cycling performance. This is because that MoS2 is ultra-thin and has large interlayer space, which provide a short ion diffusion path. Xiong et al.85 fabricated a flexible membrane by encapsulating MoS2 with interconnected carbon nanofibers as a binder-free electrode. Such membrane demonstrated superior performances, such as high reversible capacity as well as superior rate capability, showing promise as long-life SIB anode. Wang et al. 86 developed a new anode electrode with pseudocapacitive charge storage to enhance the energy and power of SIBs. The nanosheet compound MXene Ti2C showed pseudocapacitance, which accounted for the higher specific capacity relative to double-layer capacitor electrode and higher rate capability relative to ion intercalation electrode. Also, it can be operated even at a relatively high voltage of 2.4V, which was not possible for conventional energy storage systems. Sulfur and metal sulfide cathodes Sodium−sulfur (Na-S) batteries are triggering tremendous research interest as candidates to power our future society as a result of the high theoretical capacity (1672 mAh g−1, based on the formation of Na2S) of a Na-S battery. Na-S battery at room temperature is safe, simple, low-cost and potentially offer high energy densities.87,88 However, operating Na–S battery at room temperature is more challenging than Li–S batteries. Na-S batteries is limited by the low conductivity of sulfur, large volumetric variation (≈170%), slow reaction kinetics between S and Na, and high reactivity or solubility of polysulfides in most com-

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mon liquid electrolytes which leads to poor cycle stability. So in this section we focus mostly on the nanostructured sulfur cathode of Na–S battery. Sodium polysulfides have high reactivity and solubility in common liquid electrolytes such as carbonates or glycols, respectively, which leads to short cycle life. Qiang et al. 89 addressed this problem by demonstrating a highly doped sulfur/carbon nanoporous cathode to inhibit reactivity of the sulfides with carbonate electrolytes and also inhibit the diffusion of polysulfides dissolved in tetraethylene glycol dimethyl ether (TEGDME). This electrode showed ultrahigh stability at room temperature with < 3% degradation of discharge capacity for more than 8,000 cycles. This combination of low cost and excellent cycle stability is promising for stationary, grid-level energy storage. Wang et al.88 also reported an interconnected mesoporous hollow carbon nanospheres as a sulfur host to realize superior electrochemical cycling performance of Na-S battery. In this structure, the C backbone was continuous and interlaced, which ensured high tap density, high structural intimacy, and the integrity of the nanocomposite after S loading. Besides, the inner hollow nanospaces encapsulated a high sulfur ratio and could tolerate the volume changes of the interior sulfur. The outer carbon nanoshells could confine the polysulfides, thereby inhibiting the shuttle effect. Therefore, the mesoporous carbon shells not only served as a highly conductive network for S and electron transport, but also supplied open active diffusion channels. Furthermore, they proposed the mechanism of their Na/S battery, with reversible conversion between S8 and Na2S4. Zheng et al. 90 successfully loaded 10% of Cu nanoparticles into high-surface-area mesoporous carbon (HSMC) cathode, which can effectively stabilize the sulfur (50%) loaded in HSMC-Cu-S . The special structured HSMC-Cu-S compo-

site showed a Coulombic efficiency of 100%, which was attributed to the synergistical effects of Cu nano-inclusions and HSMC host. Nanoscale metal sulfides have reversible conversion reactions with lithium or sodium. FeS2 is promising because of its earth abundance, low toxicity, and low raw material cost, but has not been widely studied for secondary SIBs. 91 Douglas et al.92 demonstrated that FeS2 nanoparticles possessed ultrafine sizes (Figure 6A), which were favorable to maintain reversible reactions compared to bulk-like electrode materials in sodium ion batteries. This is a result that nanoparticle size was similar to or smaller than the diffusion length of Fe. The ultrafine FeS2 nanoparticles showed improved cycling, rate capability and reversible capacity over 500 mAh g−1 for sodium storage (Figure 6B). FeS also has high theoretical capacity and is low cost, environmental friendly and abundant. However, low electrical conductivity, inactive kinetics and severe volume change during sodiation/desodiation hinder its application. To solve these problems, Wang et al.93 designed yolk-shell FeS@carbon nanospheres (FeS@C) as the cathode (Figure 6C), which achieved high capacity of ~545 mAh g-1 over 100 cycles at 0.2C (100mA g-1), and delivered high energy density of ~438Wh kg-1. Such nanostructural design, including nanosized FeS yolks (~170nm) with porous conductive carbon shells (~30nm) and buffer space (~20nm), has been widely used to get satisfactory cycle stability. Na2S could be directly used as a cathode material in the discharged state, yet it has a low conductivity and sodium ion diffusivity. To overcome these problems, Yu et al. 94 wrapped Na2S particles with multi-walled carbon nanotube (MWCNT) and then spread them onto MWCNT fabrics. This unique cathode provided a high capacity of 500 mAh g-1 over 50 cycles.

Figure 6. Schematic and electrochemical performance of nanostructured metal sulfide for sodium battery cathodes. (A) FeS2 nanoparticle and its conversion upon sodiation. (B) Cycling performance and charge efficiency of bulk and ultrafine FeS2 nanoparticle cathodes, upon galvanostatic cycling at 0.1 A g−1. Reproduced with permission.92 Copyright 2015, ACS Nano. (C) Schematic of the synthesis of yolk-shell FeS@C as SIB cathode. Reproduced with permission.93 Copyright 2015, Nat. Commun.

Rechargeable Zn battery Electric vehicles (EVs) are expected to replace internal combustion engine vehicles in the coming years. However,

Most EVs today use lithium-ion batteries, which have high cost and concerns regarding both their safety and the supply of lithium and cobalt. These factors have led to intense

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research on alternative rechargeable battery technologies. Compared with lithium, zinc is more stable, inexpensive, and compatible with aqueous electrolytes.95 There are various energy storage systems including Zn–MnO2 batteries, Zn–Ni batteries96 and Zn–air batteries, among which Zn-air has comparable volumetric energy density as Li-S batteries. However, the irreversibility of Zn anode limit all of their cycle life. Also, the power performance of Zn-air battery has historically been a drawback due to sluggish oxygen reduction and evolution at the air electrode. Recently progress has been made on using nanostructured electrode materials to improve performance of Zn-based batteries. Zn metal and zinc oxide anodes Although Zinc has relatively high specific energy and can be charged more efficiently in aqueous electrolytes, the performance of a zinc electrode is limited by dendrite (defined as sharp, needle-like metallic protrusions) growth, shape change, passivation and internal resistance, and hydrogen evolution. A multitude of strategies have been investigated to increase the performance of zinc electrodes,

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such as high surface area/3D electrode structure design and electrode additives. Nanomaterials have the potential to minimize the anode passivation and dendrite growth problems. Besides, shape change and hydrogen evolution problems can also be solved with the properly nanostructured design. The electrode design can be started with Zn or fully discharged state ZnO. Zn foil has widely been used as the anode. However, the low utilization of Zn foil and poor rechargeability due to dendrite formation have prevented its wider practical applications. Very recently, Parker et al.97 designed a monolithic, porous, aperiodic Zn anode structure. Even at a deep discharge, the inner core of Zn is still remained. This 3D microstructure design minimizes the dendrite formation problem due to the large surface area and porous property. They demonstrated that the microstructure 3D zinc (Figure 7) elevated the performance of Ni-Zn battery in following fields: (i) more than 90% theoretical depth of discharge (DODZn) in primary cells, (ii) more than 100 highrate cycles at 40% DODZn, and (iii) more than ten thousand duty cycles required for start-stop microhybrid vehicles.

Figure 7. Schematic of the effect of 3D Zn sponge compared with conventional powder zinc anodes. Reproduced with permission.97 Copyright 2017, Science.

Nanostructured ZnO also has attracted much attention. Liu et al.98 developed ZnO nanoparticles deposited on hierarchical carbon cloth-carbon nanofiber (CC-CF) (Figure 8A) as the anode. The conformally assembled ZnO nanoparticles (replacing traditional Zn plate) are essential to buffer the shape change and suppress the Zn dendrite growth. This anode enables fast electron transport and rapid ion diffusion due to the 3D highly conductive and flexible current collector and the intimate deposition of nanosized ZnO. The established full-cell device delivered high capacity (Figure 10B), high energy and power densities, and superior cycling stability with absence of Zn dendrite when integrated with polymer electrolyte. Their work shed light on the development of next-generation

flexible solid-state electrochemical devices. Unlike 3D structure design, Liu et al.99 successfully increased the performance by adding discharge-trapping electrode additives. As shown in Figure 8B, they synthesized Zn/Al layered double oxides (LDOs) from Zn/Al layered double hydroxides (LDHs). The LDHs were decomposed to ZnO and aluminum species were dispersed in the lattice. Such anode showed a good electrochemical cycling performance with good reversible capability and low polarization (Figure 8C). Adding heavy metal additives is also an effective way to improve the performance of anode. Yuan et al.100 achieved Bi based compound film-coated ZnO with improving electrochemical activity of ZnO. Bi based compound film was composed of Bi6(NO3)4(OH)2O6, BiO and

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Bi2O3 nanoparticles, which contained many micropores. The improvement of cycling performance was attributed to indirect contact between ZnO/Zn with the electrolyte, sup-

pression of ZnO/Zn dissolution, and adequate diffusion of H2O, OH− and zincates ions.

Figure 8. Structure, schematic and electrochemical performance of nanostructured ZnO for zinc battery anodes. (A) SEM image of ALD ZnO coated CFs. Reproduced with permission.98 Copyright 2016, Adv. Mater. (B) Schematic of the transformation process of ZnAl-LDHs precursor to LDOs material. (C) Cycle performance of calcined LDH materials, precursor LDHs and commercial ZnO. Reproduced with permission.99 Copyright 2015, Electrochim. Acta.

MnO2 cathodes Zinc/manganese oxide batteries have low cost and high safety, and they have been commercial as primary batteries. Making them rechargeable is challenging yet important for wider applications. The overall reaction of Zn–MnO2 battery is: + + → + . The MnO2 cathode suffers significant capacity fading during the initial 20 cycles, and their utilization remains low.101,102 Recently, Pan et al.103 demonstrated a highly reversible zinc/manganese oxide system with a capacity of 285 mAh

g−1MnO2, and capacity retention of 92% for more than 5,000 cycles (Figure 9B). α-MnO2 nanowires (Figure 9A) were used as the cathode and ZnSO4-based aqueous solution was used as the electrolyte. This system showed that a conversion reaction between α-MnO2 and H+ was mainly responsible for the good performance of the system. Tang et al.104 successfully synthesized single-crystalline α-MnO2 nanorods with surface area ~95.2 m2 g-1 and diameters ranging from 10 to 20 nm, which showed improving discharge capacity compared with commercial MnO2.

Figure 9. Structure and electrochemical performance of α-MnO2 cathode for Zn-MnO2 battery. (A) TEM image of α-MnO2 nanowires. (B) Electrochemical performance of Zn-MnO2 batteries in ZnSO4 aqueous electrolyte with MnSO4 additive. Reproduced with permission.103 Copyright 2016, Nat. Energy.

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Flexible/bendable electronics are favorable currently. Flexible energy storage devices are critical to develop that type of electronics. The relatively inexpensive MWCNTs is promising to be used in flexible MnO2 composite cathode compared with graphite. Wang et al.105 successfully demonstrated that the implementation of MWCNTs and conductive polymer poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS) can help to develop the highly conductive MnO2 composite electrode for flexible Zn/MnO2 batteries. It’s worth noting that more dispersible, carboxylated MWCNTs would decrease the cycling performance of Zn/MnO2 batteries by increasing the resistance of the electrode. Nickel cathodes Rechargeable Zn-Ni batteries is promising to become a substitute for LIBs due to their high energy density and stable cycling performance in aqueous electrolyte. 96 However, Ni oxides and Ni hydroxides have some drawbacks, including low conductivity, high rate unaffordability and bad cycling performance. In recent decades, many investigators have demonstrated a lot of solutions to improve the performance of Zn-Ni batteries. The reversible reactions of two electrodes are showed below. Cathode reaction:

!""# + $ + ⇔ & + Anode reaction: ' + ( ⇔ ( + $ In the cathode reaction, there are two main redox pairs, including α-Ni(OH)2/γ-NiOOH and β-Ni(OH)2/β-NiOOH,

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respectively. α-Ni(OH)2/γ-NiOOH couple has high theoretical capacity than the other one because the oxidation state of nickel in γ-NiOOH can be larger than 3 with the existence of Ni4+ defect. However, α-Ni(OH)2 has the potential to be transformed to β-Ni(OH)2 and it is not stable in aqueous electrolyte. Ion and electron transfer is another problem needed to be solved. There are two approaches can be used to overcome these problems: (1) nanostructured design to stabilize α-Ni(OH)2; and (2) combine Ni oxides or Ni hydroxides with other conductive materials. Kiani et al.106 compared the performance of nanostructured β-Ni(OH)2 and microstructured β-Ni(OH)2. Cyclic voltammetric (CV) results showed that nanostructured β-Ni(OH)2 has larger proton diffusivity and higher OER potential compared with microstructured β-Ni(OH)2. Besides, nanostructured βNi(OH)2 exhibited an excellent cycling performance with high reversibility and capacity. Liu et al.98 combined two approaches and deposited porous ultrathin NiO nanoflakes conformally on hierarchical CC-CF (Figure 10A) as the cathode, which enabled fast electron transport and rapid ion diffusion and delivered high capacity, high energy and power densities as well as superior cycling stability (Figure 10B) in full cells. Besides, the device can also work well even with severe bending, which makes it possible for flexible batteries. Apart from Ni(OH)2 and NiOOH, Ni3S2 can also be the cathode material to pair with Zn anode. Hu et al.107 synthesized self-supported Ni3S2 ultrathin nanosheets grown on the surface of nickel foam. This Ni3S2/Ni composite showed a high capacity and slight capacity decay for more than 100 cycles as cathode material for Zn-Ni battery, and showed extraordinary rate capability and high operating voltage.

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Figure 10. Nanostructured Ni cathode for Zn-Ni battery. (A) Schematic illustration of the flexible Ni–Zn battery using 3D hierarchical metal oxide (NiO and ZnO) coated N–doped CFs on CC as the electrodes. (B) Cycling test of flexible quasi–solid–state Ni–Zn battery. Reproduced with permission.98 Copyright 2016, Adv. Mater. (C) SEM image of AgNW–GA. Reproduced with permission.108 Copyright 2017, Advanced Functional Materials.

Oxygen cathodes Zinc–air batteries have the highest specific energy among all Zn-based batteries.109–112 Comparing to Li-air batteries, Zn−air is tolerant to moisture in air and is potentially a simpler system. Moreover, the solid discharge product of Zn-air, ZnO or Zn(OH)2, forms on the anode side, and will not clog the air cathode. Below are the reactions of Zn-air batteries. The air electrode reaction:

" + + ($ ⇔ ( ) = *. (*+ ,- .) The zinc electrode reaction: ' + ⇔ + + $ ) = −. + ,- .) The overall reaction: ' + ⇔ ) = . + ,- .) Among many challenges for the development of Zn–air batteries, it is crucial to develop highly efficient and low cost catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in the air electrode because both the ORR and the OER show considerably high overpotentials.113 Up to now, Pt and Pt-based alloys are known as the most efficient catalysts. However, the high cost and poor durability of Pt-based catalysts hinder their broad applications.114 Dhavale and Kurungot111 prepared well organized Cu−Pt nanocage intermetallic structures, which has high density of active site to facilitate dissociative oxygen adsorption, and improve ORR activity compared to the commercially available 20 wt% Pt/C. Nanostructured catalysts are favorable due to their unique properties, however, most of them show poor conductivity and suffer from agglomeration when used in batteries.115 Composition and structure control would help to increase the stability and activity of catalysts. One main approach is to grow electrocatalysts on graphene nanosheet. For example, metal nanoparticles, metal oxide and magnetic nanoparticles. Zhou et al.116 synthesized an efficient and economical hybrid electrocatalyst: cobalt nanoparticles encapsulated in N-doped graphene nanoshells. It presented equivalent performance to the commercial Pt/C electrocatalyst in alkaline system but it was synthesized with cheaper and more abundant raw materials. Also, it was robust to allow the replacement of Zn anode and electrolyte periodically, which could be an ideal cathode electrocatalyst for EVs. Xie et al.117 reported a catalyst consisting of Co3O4 nanoparticles encapsulated in a graphene supported carbon matrix (Co3O4@C/graphene) with higher electrocatalytic activity and better stability compared with commercial 20 wt % Pt/C. They proved the synergistic coupling between Co3O4 and nitrogen-doped graphene. The carbon encapsulation structure would be more important on the performance of catalyst if the Co3O4 nanoparticle sizes become smaller. Also, Prabu118 synthesized CoMn2O4 nanoparticles supported on N-doped re-

duced graphene oxide (CoMn2O4/N-rGO), which showed improved and excellent zinc–air battery cycle performance. They attributed the improved battery performance to the coupling effect between CoMn2O4 and graphene hybrid materials as well. Most recently, a 3D graphene aerogel catalyst engineered with a silver nanowire network (AgNW–GA, Figure 10C) was presented by Hu et al.108, which showed high ORR performance and superior stability. Due to its 3D sponge structure, the electrocatalyst loading was increased and the mass transport was effectively enhanced. Anchoring catalysts on the other hosts has also reported, such as nickel–cobalt nanoparticles on porous fibrous carbon aerogels119 and cobalt–manganese oxide on N–doped carbon nanotubes120. Apart from anchoring electrocatalyst on graphene to improve the kinetics, another approach is decorating the electrocatalyst with other catalysts. Goh et al. 121 presented an inexpensive yet efficient bifunctional catalyst consisting of MnO2 nanorods decorated with Ag nanocrystals (∼11 nm) (Ag–MnO2) as air cathode. In aqueous alkaline electrolyte, it was successfully cycled for 270 cycles and showed improved ORR and OER catalytic activity. The ORR mechanism of Ag–MnO2 is direct 4e− reaction, which was proved by RDE results that 3.7 electrons were transferred. This was an evidence for the high ORR kinetics and catalytic performance of Ag–MnO2. Core shell nanostructure alloys have received considerable attention recently because of high electron conductivity.122 Zhang et al.123 produced core–shell AgCu bimetallic nanoparticles. This catalyst showed enhanced ORR activity, highest working potential and lowest overpotential.

Conclusion and Outlook In this perspective, we summarized the recent progress of nanostructured electrode materials for reversible Li, Na, Zn batteries. Nanostructured materials and their composite possess unique properties, including high surface area and shorter pathways for ions and electrons, and could provide additional knobs to break the intrinsic limitation of bulk materials, such as poor electrical conductivity, dissolution and volume expansion of electrode. However, there are still some unsolved problems with nanostructured electrodes. First, high surface area of nanostructured electrode can enhance electrochemical performance, but also lead to severe side reactions, which is unfavorable for electrode stability and reversibility. Second, the morphology and structure of nanostructured electrode are sometimes hard to maintain over cycling, due to weak mechanical and chemical characteristics. Third, advanced nanoscale characterization tools are to be developed to understand electrochemical phenomena at the nanoscale, guide and verify the design of nanostructured electrode materials. High fabrication cost, low volumetric energy density also need to be solved.

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Looking forward, difficult challenges in the battery field should continue to be attempted with nanostructure design of electrode materials. Meanwhile, new nanomaterials should be tested in battery applications. Finally, new tools should be developed to allow mechanistic studies of the electrochemical reactions and phenomena at the nanoscale. Collectively these could further the application of nanostructured electrode materials in batteries.

AUTHOR INFORMATION Corresponding Author * Correspondence: [email protected]

Author Contributions Yamin Zhang and Nian Liu co-wrote the manuscript.

ACKNOWLEDGMENT N. L. acknowledges support from faculty startup funds from the Georgia Institute of Technology.

Biographies Yamin Zhang is pursuing her Ph.D. in chemical engineering under the supervision of Prof. Nian Liu at Georgia Institute of Technology, School of Chemical and Biomolecular Engineering (since Aug. 2016). Her current research interests include the development of advanced nanostructured electrode materials and membrane separators for rechargeable zinc batteries. She received her B.S. from Tianjin University (China), where she specialized in nanostructured catalysts. Nian Liu is an Assistant Professor at Georgia Institute of Technology, School of Chemical and Biomolecular Engineering (since Jan. 2017). His research group focuses on materials engineering for reversible high-energy chemical reactions, and understanding their fundamental science using advanced light microscopy. Liu received his B.S. from Fudan University (China), and Ph.D. from Stanford University, where he worked with Prof. Yi Cui on the structure design for Si anodes for highenergy Li-ion batteries. He then worked with Prof. Steven Chu at Stanford University as a postdoc, where he developed in situ optical microscopy to probe beam-sensitive battery reactions.

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Nanostructured Electrode Materials for High-Energy Rechargeable Li

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