Surface engineering of nanomaterials for improved energy storage – A review

Chemical Engineering Science 154 (2016) 3–19

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Surface engineering of nanomaterials for improved energy storage – A review Keith Share a,b, Andrew Westover a,b, Mengya Li a, Cary L. Pint a,b,n a b

Department of Mechanical Engineering, Vanderbilt University, Nashville, TN 37235, USA Interdisciplinary Materials Science Program, Vanderbilt University, Nashville, TN 37235, USA

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T


Surface engineering decouples bulk and surface properties relevant to energy storage.
Surface engineering is critical for nanostructured materials in energy storage.
Recent advances in energy storage build from four mechanistic roles of surface engineering.
Future innovation in energy storage centers on advances in surface engineering methods.

art ic l e i nf o

a b s t r a c t

Article history: Received 10 March 2016 Received in revised form 19 May 2016 Accepted 23 May 2016 Available online 24 May 2016

Nanomaterials bring extreme promise for a future wave of energy storage materials with high storage capacity, fast recharging capability, and better durability than bulk material counterparts. However, this promise is often overshadowed by greater surface area and higher reactivity of nanostructured active materials - obstacles that must be overcome to be practical. Specifically for energy storage systems, many materials that exhibit promise in bulk form for high capacity or energy density exhibit surfaces that are unstable or reactive in electrochemical environments when downsized to nanometer length scales. As a result, surface engineering can be a powerful tool to decouple bulk material properties from surface characteristics that often bottleneck energy storage applications of nanomaterials. This review discusses advances made toward the surface engineering of nanostructures in the context of four mechanistic roles that surface engineering can play. This includes (i) chemical activation, where the surface layer plays the active role in facilitating a Faradaic chemical process, (ii) solid electrolyte interphase (SEI) control, where a surface layer can lead to a stable artificial interface for Faradaic processes to occur, (iii) chemical passivation, where near atomically thin surface protective layers can protect from corrosion or unwanted electrochemical reactions at interfaces, and (iv) mechanical stability, where a thin layer can provide mechanical support to inhibit fracturing or mechanical failure. This review elucidates surface engineering as a multi-faceted tool for engineering materials for energy storage that intersects the quest for new materials and the rediscovery of old materials to break new ground in energy storage applications. The discussion concludes by highlighting key current challenges in surface engineering for pure metal anodes in metal-ion batteries and polysulfide immobilization in lithium-sulfur batteries. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Surface engineering Batteries Supercapacitors Pseudocapacitors Nanomaterials Energy storage

n Corresponding author at: Department of Mechanical Engineering, Vanderbilt University, Nashville, TN 37235, USA. E-mail address: [email protected] (C.L. Pint).

http://dx.doi.org/10.1016/j.ces.2016.05.034 0009-2509/& 2016 Elsevier Ltd. All rights reserved.

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1. Introduction In 1959, renowned physicist Richard Feynman gave a historical talk entitled “There's Plenty of Room at the Bottom,” where he proceeded to predict how systems engineered at the nanoscale can radically change the landscape of scientific research efforts (Feynman, 1960). Just short of seven decades later, the average researcher whose efforts overlap with nanoscience has the capability at their fingertips to engineer materials and their surfaces with atomic scale precision. Whether it is recognized as such or not, the abundance of research tools that can engineer surfaces of materials and still maintain the integrity of the bulk interior material has unlocked a whole new frontier of research in the area of surface engineering. From this concept emerges an engineering approach for heterogeneous material systems where the surface properties can be engineered separately from the bulk material properties to harness the true application potential of material systems. This approach can be synergistically applied alongside the search and discovery of new nanoscale materials or can be a technique to rediscover materials with extraordinary bulk properties that were overlooked many decades ago due to the inability to overcome limitations of reactive or poorly suited surface/interface characteristics. For the specific case of energy storage applications with nanomaterials, surface engineering becomes a critical component of functional electrode design. Despite years of research on nanoscale materials for energy storage, commercial batteries still make use of micro-scale materials for electrodes. This is due to a combination of both (1) manufacturing challenges for nanoscale materials, but also (2) the reactive nature of nanoscale materials that leads to high irreversible capacities associated with solid electrolyte

interphase (SEI) formation (Armand and Tarascon, 2008). New nanoscale materials exhibiting improved performance compared to bulk materials often lead to lower performance at the cell-level due to the increased electrolyte consumed while forming a stable electrode-electrolyte interface. This is one of many challenges that can be addressed and overcome through surface engineering. However, beyond the limits of this one challenge, surface engineering is a tool highly applicable to a broad scope of energy storage materials due to the native function of energy storage devices that require an electrode-electrolyte interface that is stable. In the case of electrochemical supercapacitors, this interface must be stable to prevent Faradaic charge transfer reactions and sustain an electric double layer. In the case of batteries and pseudocapacitors, this interface must facilitate Faradaic charge transfer reactions that store and release energy without degradation. The premise of engineering electrode materials for these applications without tools to perform surface engineering experiments requires a researcher to isolate materials among a limited set where the bulk properties are favorable for high performance (storage capacity, internal resistance, etc.), but in addition to this, the electrochemical interface between these materials and the electrolyte is also favorable. Surface engineering therefore dramatically decreases the complexity of the engineering challenge by allowing a researcher to identify a material with ideal bulk properties, and then independently engineer the surface properties to be stable. This opens a trajectory for faster, more efficient, and more intelligently designed progress toward improved energy storage systems that can overcome many of the key bottlenecks limiting their advancement today. This review will first lead into advancements in surface engineering by providing a brief introduction into the most common

Fig. 1. Schematic representing the different properties of nanostructures that can be controlled through surface engineering.

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tools and techniques used by researchers. This includes gas phase, wet chemical, and electrochemical approaches. Following this brief introduction, recent literature advances in surface engineering techniques will be parsed and discussed based on the four primary mechanistic roles of surface engineering approaches in energy storage, as illustrated in Fig. 1. This includes surface engineering for (1) chemical activation of surfaces, (2) solid electrolyte interphase (SEI) control, (3) chemical passivation of surfaces, and (4) controlled mechanical properties. A general visual overview into the role of surface passivation in the context of each of these mechanisms is illustrated in Fig. 1. Finally, this review closes by highlighting some of the most dynamic current application areas for surface engineering in energy storage materials ranging from stabilizing soluble polysulfides in lithium-sulfur batteries, to preventing dendrite formation in pure metal anodes for highly optimal lithium-ion battery configurations. This review consistently emphasizes the principle that many of the most rapid advancements in nanostructured energy storage materials are directly tied to the ability to engineer atomic-scale control on functional surfaces.

2. Surface engineering approaches 2.1. Chemical vapor deposition and atomic layer deposition Gas phase processing of materials is by far the most appealing technique to perform surface engineering efforts. These approaches generally involve a mixture of precursor gases that flow through a high temperature chamber to react with the surface of the substrate. In the case of chemical vapor deposition (CVD) the reactions between the gas phase precursor and the substrate are controlled by the precursor partial pressure, the temperature, and the reactivity of the electrode. For atomic layer deposition (ALD), the precursors react in a self-limiting process enabling atomiclevel control. As a result, ALD involves the alternate pulsing of two or more precursors. CVD is a promising method to engineer the physical, chemical, and mechanical properties of surfaces, although precise thickness control can be difficult to achieve. Carbon nanomaterials deposited by CVD on various substrates can improve the conductivity (Su et al., 2011), mechanical properties (Taylor, 1991), hydrophobicity (Liu et al., 2004), and serve as a corrosion-protective coating layer (Oakes et al., 2013). Another feature of CVD is the easy removal of gaseous impurities which could enable high-purity film deposition (Lock et al., 2006). However, CVD uses high temperatures which

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can damage or decompose some materials and limits the choice of stable precursors. Conventional CVD processes suffer from low deposition rates, high reaction temperatures, and low-pressure operation. To break the processing limits of conventional CVD, several assistive tools have been developed for enhanced CVD processes such as plasma (Chhowalla et al., 2001), laser (Aoyagi et al., 1985), and hot-filaments (Dupuie and Gulari, 1991). Some derivations of CVD such as metal-organic chemical vapor deposition (MOCVD) have also been developed to deposit new materials (Berry et al., 1988; Zanella et al., 1991). On the other hand, ALD can provide near room temperature processing for some depositions, providing favorable conditions for thermally fragile substrates (Groner et al., 2004; Kukli et al., 2015). ALD involves a two-step process with an organometallic precursor and an oxidizing or reducing agent. Inert gas, such as Ar or N2, carries away the unreacted residual precursor molecules to avoid a CVD reaction between precursors. A schematic of how ALD works is presented in Fig. 2a (Leskelä and Ritala, 2003; Tiznado et al., 2008). Fig. 2b shows an ALD alumina coating on a Si wafer with a high aspect ratio trench structure, emphasizing the precise conformity of the coating (Ritala et al., 1999). The high vapor pressure and diffusion coefficients of some precursors enable conformal and pinhole-free coatings on substrates with ultra-high aspect ratios or complex 3D architectures (Elam et al., 2003; Groner et al., 2006). Compared to CVD, ALD provides more precise thickness control and can be operated at much lower temperatures. However, only limited materials can be deposited by ALD process mainly because of the high requirement of the precursor, which should have a self-limiting reaction and be volatile. 2.2. Chemical surface modification via wet chemistry Wet chemistry surface engineering uses reagents or solvents to chemically modify or functionalize the surface of materials. Wet chemical routes provide an additional level of control on the engineered interface in that the coating can be produced based on either non-covalent interactions (Georgakilas et al., 2012) or covalent functionalization (Englert et al., 2011). One example of a non-covalent surface engineered system is a carbon nanotube coated with an ionic surfactant, such as sodium dodecylbenzene sulfonate (SDBS) (Moore et al., 2003) that can maintain the integrity of the carbon nanotube but enable dispersion in liquid media and prohibit bundling (O'connell et al., 2002). Covalent interactions are achieved by chemically grafting reactive functional groups to the surface. Strong oxidizers such as nitric acid or

Fig. 2. (a) Schematic of a typical ALD process. (b) Cross-sectional SEM image of an Al2O3 ALD film with a thickness of 300 nm on a Si wafer with a trench structure. Reprinted with permission from Wiley.

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hydrogen peroxide (Moreno-Castilla et al., 1995), have been reported as chemical reagents for grafting hydroxyl and carboxyl groups on the surface, increasing the wettability (Rupp et al., 2004) and creating highly-reactive defect sites (Li et al., 2014). As a straightforward method of engineering the surface properties of nanomaterials, wet chemistry methods are easy to achieve once the process has been developed. However, the most difficult part is chemical selection for the targeted reaction. Wet chemistry-based chemical modification can result in functionalized surfaces that partially corrode or dissolve in the solution (Mougenot et al., 1996). Covalent wet chemistry processes use strong oxidizers that can be dangerous, especially when the reaction is performed under elevated temperature. After functionalization, several time-consuming rinsing steps are necessary to get rid of harmful chemical residue. Although reactive ion etching (RIE) is a dry chemical process, the control of the surface properties are similar to wet chemical modification. In addition to surface control, RIE can be used to produce finely controlled nanoscale features and arrays that are difficult to achieve with wet chemical processing (Lammertink et al., 2000; Marty et al., 2005). Surface properties such as the hydrophobicity (He et al., 2011b; Kim et al., 2013) and reflectivity (Park et al., 2013) can be controlled.

difficult to achieve because of side reactions such as water electrolysis and the balance of diffusion into the pores and reactivity with the pore edge (Nielsch et al., 2000). Similar to ED is electrophoretic deposition (EPD), a colloidal process for producing films or coatings from a stable suspension. Unlike ED, there are no chemical reactions occurring in the EPD process. When a DC electric field is applied to a colloidal suspension, the charged particles migrate toward and deposit on the oppositely charged electrode. A scheme showing the basic setup for an EPD system is shown in Fig. 3. Surfactants are usually added to stabilize the dispersed particles in the solvent and provide charged sites to facilitate migration, however recent studies have demonstrated surfactant-free EPD of nanostructures using polar solvents (Carter et al., 2014; Oakes et al., 2015). A broad range of materials can be deposited by EPD, including metals (Teranishi et al., 1999), oxides (Ducheyne et al., 1990), and nitrides (Ohgi et al., 2008). Uses of EPD include the engineering of 3-D foam materials with mechanical integrity (Carter et al., 2014). The scalable and inexpensive EPD process enables densely packed coatings (Hayward et al., 2000), homogeneous microstructure (Kaya, 2008), and deposition into porous materials (Haber and Gal-Or, 1992).

2.3. Electrochemical or electrophoretic surface modification Electrodeposition (ED), also called electrochemical deposition, is a chemical precipitation process triggered by electrochemistry. Applied electrical current attracts ionic species in solution to the electrode where they are reduced/oxidized and deposited. ED can be used to deposit a variety of materials including metals (Endres, 2002), ceramic particles (Simunkova et al., 2009), and polymers (Beck, 1988). The surface morphology, crystallinity, hardness, and adhesion properties can all be controlled during ED (Green et al., 2008; Morales et al., 2005). ED offers several advantages such as low cost, fast deposition rates (Könenkamp et al., 2000), large area deposition (Yin et al., 2001), and no post-deposition treatment (Pauporté and Lincot, 2000). However, there are a limited number of materials that can be electrodeposited from aqueous solutions (Yin et al., 2001). Moreover, ED with uniform penetration into porous structures are

3. Surface engineering mechanisms in energy storage materials 3.1. Chemical activation Chemical activation in the context of this review article refers to the ability for a surface engineering process to transform a material into a functional template for energy storage. This approach takes a number of embodiments – in many cases it involves the coating of active material onto a template that is highly optimized for surface area, electrical conductivity, and/or ionic transport so that the physical properties and electrochemical properties can be engineered independently. This review will distinguish recent advances in chemical activation in the context of two separate device platforms – pseudocapacitive electrodes and battery electrodes.

Fig. 3. Schematic demonstrating the experimental setup and mechanistic process during electrophoretic deposition. Right side panel shows charged nanoparticles migrating toward an electrode with an opposite charge.

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3.1.1. Hybrid supercapacitors and pseudocapacitors Pseudocapacitors and hybrid supercapacitors are based on a form of Faradaic storage that involves faster charging and discharging rates than typical batteries and energy densities that are increased relative to non-Faradaic supercapacitors. For the purpose of this review, a hybrid supercapacitor is coined as a Faradaic storage material with an obvious redox pair where the active layer is thin enough that the total capacity is rapidly accessible, making the Faradaic storage mechanism difficult to distinguish from a battery. On the other hand, a pseudocapacitor is a material where the Faradaic storage mechanism is distinguished from a battery, leading to charge storage over a range of voltages instead of at a fixed redox couple. This distinction is based on recent discussion of such terminology (Brousse et al., 2015). Common hybrid supercapacitor electrodes include Ni(OH)2 (Hu et al., 2015), cobalt oxides (Dong et al., 2012), mixed metal oxides (Li et al., 2015a), or organic materials (Bachman et al., 2015), while common pseudocapacitor electrodes are MnO2 (Gao et al., 2012; He et al., 2014; Lu et al., 2011; Toupin et al., 2002) and RuO2 (Hu and Huang, 1999; McKeown et al., 1999). Due to the surface and near surface storage mechanism of these devices, only thin layers of the active material are needed to store significant energy (Park et al., 2004). Boukhalfa et al. (2012) used ALD to deposit vanadium oxide on CNTs and showed an optimum thickness after only 100 cycles. Additional cycles lowered the performance because the increased thickness impeded the transport of the charge carriers to the surface and ion diffusion through the vanadium oxide. The limitations in pseudocapacitors and hybrid supercapacitors can be overcome through surface engineering by separately optimizing the structure and surface to avoid diffusion limitations while providing high surface area. Metal oxides commonly used in these devices have intrinsically high electrical resistances and have shown tremendous benefits

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from smartly engineered scaffolds (Brezesinski et al., 2009; Li et al., 2015b). The scaffolds must provide conductive pathways with short diffusion lengths for efficient charge transfer, high surface area, and no irreversible reactivity. Some shortcomings of the substrate architecture can be overcome during deposition of the active material with additional nanostructuring to increase the surface area and complete coverage of the substrate to prevent undesirable side reactions (Simon and Gogotsi, 2008). Common ways to achieve these goals include nanowire core/shell arrays or nanostructured conductive carbon templates. The scaffold and active material are commonly made in 2 step processes allowing for separate optimization of the structure (surface area, short diffusion lengths) and chemical reactivity. One example of this is work by Lu et al. (2013) which takes advantage of 2 step processing to improve the carrier density of the TiO2 NWs by 3 orders of magnitude by hydrogenating the surface before depositing MnO2 (Fig. 4). The increased conductivity of the substrate significantly improved the capacitance, retention, and decreased the IR drop in galvanostatic testing. Other substrate architectures are useful as well (Chen et al., 2012) displayed by Cao et al. who deposited Co(OH)2 onto a high surface area ultra-stable Y zeolite (Cao et al., 2004). The co-precipitation mechanism provided control over the mass loading of Co(OH)2 with an optimal value around 48 wt% leading to a capacitance of 1492 F/g based on total mass at a discharge current of 4 mA/cm2. Common processing techniques for depositing the faradaic material have advantages and disadvantages but conformal and complete coatings of the substrate by the active material can mitigate capacity fade during cycling. Ziliong et al. (2014) electrodeposited a conformal coating of MnO2 onto ZnO NWs that was less than 5 nm thick and were able to achieve over 5000 cycles with only 1.5% loss in capacitance. Dubal et al. (2015) deposited

Fig. 4. a) Schematic diagram illustrating the growth process for H-TiO2@MnO2 and H-TiO2@C core–shell NWs on a carbon cloth substrate. b) CV curves collected for H-TiO2, MnO2, TiO2@MnO2, and H-TiO2@MnO2 electrodes at the scan rate of 100 mV s  1. c) Specific capacitance of these electrodes calculated from CV curves as a function of scan rate. Reproduced with permission from Wiley.

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Fig. 5. SEM images of a) A-3/CFP, b) A-4/CFP, c) A-6/CFP and the insets show the high magnified SEM images of Ni(OH)2. d) Schematic diagram showing the Ni(OH)2 nanowalls growth process. e) CV curves ranging from  0.2 to 0.6 V at a scan rate of 5 mV s  1. f) capacity at different current densities. Reproduced with permission from Nature Publishing Group.

MnO2 on Si NWs using chemical bath deposition and fully coated the NW array, preventing the Si NWs from reacting with the electrolyte. This gave them the opportunity to optimize the ionic liquid electrolyte achieving a 2.2 V range. Control of the crystal structure of the active material during deposition can also significantly enhance the ionic conductivity and reactivity (Devaraj and Munichandraiah, 2008; Ghodbane et al., 2009). Separate processing of the substrate and surface allows for optimization of the active material during its deposition. Ke et al. (2015) used self-supported TiO2 NW arrays but with a TiO2 /Ni(OH)2 core–shell (Fig. 5). The Ni(OH)2 morphology, crystallinity, and surface area were optimized during the chemical bath deposition process using different amounts of aqueous ammonia. Less ammonia produced thinner nanowalls with higher surface area and lead to higher capacities and a lower charge transfer resistance. The TiO2 scaffold was able to provide a 3D

nanostructure with efficient electron transport while the Ni(OH)2 acted as the optimized active material. Graphene (Sun et al., 2012), CNTs (Arabale et al., 2003; Kim et al., 2005) and other conductive carbon substrates provide a high surface area conductive substrate for faradaic metal oxides (Lai and Lo, 2015; Wu et al., 2010; Yu et al., 2011b). The conductivity and morphology of the substrate can still be tuned separately from the active material but with less precise control than NW arrays. The carbon materials act as supercapacitor materials themselves but a synergistic effect between the metal oxides and carbon improves the capacitance beyond the sum of the individual materials (Chen et al., 2010a; Wu et al., 2012d). A thin layer of the carbon can also be deposited on top of the pseudocapacitve material to provide a conductive wrapping that can increase the capacitance by  20% (Yu et al., 2011a).

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3.1.2. Batteries Surface engineering for batteries enables separate optimization steps for substrate morphology and active material deposition in order to improve battery performance. Many structural requirements for batteries are similar to those for pseudocapacitors, except batteries are generally optimized for total storage capacity instead of power capability. Controlled porous features in the active layer help produce high capacities and fast rate capabilities by providing short ion and electron diffusion lengths. However, unlike pseudocapacitors, redox reactions in batteries access deep bulk material and not just the surface, potentially causing large volume expansion of the electrode. Solid electrolyte interphase (SEI) formation also occurs upon charging and discharging a battery. High surface area is not necessarily a requirement for batteries because more SEI will form, irreversibly consuming electrolyte, electrons, and ions. Surface engineering to control SEI formation will be discussed on its own. Traditional ALD of metal oxides and sulfides like TiO2, V2O5 (Badot et al., 2000), and Cu2S (Meng et al., 2015) can be used as active layers but recent work has focused on lithiated electrode materials such as LiCoO2 (Donders et al., 2013), LixMn2O4 (Miikkulainen et al., 2013), and LiFePO4 (Nilsen et al., 2014). Miikulainen et al. (2013) used 2 different ALD techniques to produce lithiated Li xMn2O4 films, although it is not a traditional ALD process because it is not a self-limited reaction. The first method used a typical ALD process recipe for ternary materials. They also demonstrated that films of MnO2 or V2O5 could be deposited and then lithiated post deposition. These processing techniques open a lot of experimental parameters for optimizing lithium content and crystal structure. Liu et al. (2014a) developed an ALD process for the quaternary compound LiFePO4 on CNTs. The conductive CNT network allowed for charging rates up to 60 C with decent capacities around 150 mAh/g at 0.1 C. Beyond traditional LIBs, newer technologies such as Li–S also greatly benefit from the process control afforded by ALD. Meng et al. (2014) deposited Li2S onto high aspect ratio silicon trenches and mesocarbon microbeads, demonstrating the advantages of ALD onto multiple 3D substrates. ALD process development of battery specific materials is in its infancy but combining the high potential capacities of these materials with the self-limiting ALD reaction and optimized substrate architectures has a huge potential to produce high capacity and high rate batteries with long lifetimes. Systematic studies of the relationship between electrode architecture and electrode thickness shows that either factor can

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limit the performance, emphasizing the importance of separate optimization (Ye et al., 2014). Tang et al. (2014) made TiO2 nanotubes with varying aspect ratios but similar surface areas allowing a fair comparison between samples (Fig. 6). They showed that higher aspect ratios have higher capacities, especially at particularly fast rates (30 C) due to better electron/ion transport. These studies point to the importance of controlling the substrate morphology (Groult et al., 2007). Sacrificial templates require extra processing to remove but can allow for unique and controllable architectures (Hassoun et al., 2007). Wang et al. (2012b) used anodic alumina templates to compare a 3D NW template with a clustered NW template on a Ni current collector with ALD deposited TiO2 (Fig. 7). Due to the unique template that forms multiple interconnects in the 3D NW network, agglomeration of the NWs was prevented resulting in much higher areal capacities and rate capabilities. The TiO2 thickness was varied during ALD deposition where an 8 nm film had a higher capacity and better rate capability than a 16 nm film that was limited by the ion diffusion rate through the TiO2. Multi-valent ion batteries such as Al3 þ and Mg2 þ bring potential for high capacity batteries because each intercalated ion can react with multiple electrons. The diffusion of these ions through electrodes is particularly limiting due to the multi-valent state. Designing electrodes with short diffusion lengths is important to achieve good rate capability, meaning that chemical activation processes are increasingly important (Lin et al., 2015). NW arrays of V2O5 and TiO2 have shown promise as Al ion electrodes (Jayaprakash et al., 2011; Liu et al., 2012b). These newer battery technologies still suffer from irreversible reactions between the electrolyte and electrode/current collector but complete coverage of engineered high aspect ratio current collectors using techniques discussed earlier could mitigate these issues. 3.2. Surface engineering for SEI control The term solid electrolyte interphase (SEI) (Peled, 1979) refers to an irreversible layer that forms along the interface between the electrode and the electrolyte in a battery. The SEI primarily forms from reduction of the electrolyte on the electrode surface (Delpuech et al., 2013; Lee et al., 2007). The composition and function of the SEI varies depending on the type and morphology of the electrode (Ein-Eli, 1999; He et al., 2013b) and testing parameters (He et al., 2011a, 2008). The SEI layer can provide a stable interface but it can also impede ion diffusion. Volumetric expansion of the

Fig. 6. a) Correlation between the aspect ratio and the capacity of different NTs at various discharging rates (1 C ¼ 168 mA g  1). b) Scheme of the electron and Li þ transport pathways. Reproduced with permission from Wiley.

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Fig. 7. a) Schematic fabrication process flow for straight Ni/TiO2 nanowire arrays (top) and 3-D Ni/TiO2 nanowire network (bottom), which uses Al foils with different impurity. The side holes in the 3-D PAA template result in interconnections in 3-D nanowire network. b) Rate capabilities of 32 mm long 3-D Ni/TiO2 nanowire network structure with different TiO2 coating thicknesses. c) SEM images of Straight Ni/TiO2 nanowire arrays and d) as-prepared 3-D Ni/TiO2 nanowire network. Reproduced with permission from American Chemical Society.

electrode can expose additional material causing more SEI to form, irreversibly consuming ions and electrolyte while possibly producing gas (Nadimpalli et al., 2012; Tokranov et al., 2014). Additives such as vinylene carbonate (Aurbach et al., 2002; El Ouatani et al., 2009) and CO2 can facilitate formation of SEI with lower interfacial resistance than additive free electrolytes (Aurbach et al., 1999). The end goal for SEI engineering is to form a stable and ionically conductive surface that enables high rate capability and little degradation over thousands of cycles. Controlling SEI is particularly important for nanomaterials due to the high surface area and electrolyte exposure in a battery cell. 3.2.1. Manipulating the SEI in batteries ALD again immerges as an important surface deposition technique when manipulating the SEI (Chen et al., 2010b; Liu et al., 2014b). ALD coatings are most effectively deposited onto the assembled electrode as opposed to the powder itself in order to maintain rapid electron transfer (Jung et al., 2010; Jung et al., 2011). Typical ALD materials, such as Al2O3 and ZnO, have lower ionic conductivity but can protect the electrode and control SEI formation. Ion conducing surfaces and multifunctional surfaces can provide even greater enhancements (Cheng et al., 2013; Li et al., 2013). Park et al. (2014) ALD deposited LiAlO2 which has a

much higher conductivity than Li2O–Al2O3. Application of this surface coating to only the anode in a full cell achieved 90– 100 mAh/g and drastically improved the cyclability. It also protected the anode from chemical attack by dissolved transition metals from the cathode. Memarzadeh et al. used surface coatings of sputtered Al on Si NWs for multiple purposes (Fig. 8) (Memarzadeh et al., 2012). The Al compressed the Si preventing mechanical breakdown, acted as a Li intercalation material, and changed the SEI composition. The best surface coating of 3 wt% Al showed the highest coulombic efficiency over 100 cycles. Even simpler process steps like annealing can drastically change the surface of the electrodes leading to better SEI formation (Abel et al., 2012; Wu et al., 2012a). Han et al. (2015) annealed TiO2 hollow spheres to remove residual chemisorbed water and hydroxyl groups from the surface resulting in a less resistive SEI and over 2  improvement in the capacity (Fig. 9). Additional passivating materials with higher ionic conductivity still need to be explored. 3.2.2. Characterization of the SEI Characterizing the composition of the SEI can elucidate the reactions that occur and provide guidance toward the ideal SEI, which in turn directs surface engineering efforts. Typical

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Fig. 8. SEM images of the a) SiNWs/TiN, b) 1 Al/SiNWs, c) 3 Al/SiNWs, d) 8 Al/SiNWs, and e) 13 Al/SiNWs. f) cycle life, g) discharge capacity retention, and h) coulombic efficiency of the uncoated and aluminum-coated SiNWs/TiN at 0.1 C rate. Reproduced with permission from Royal Society of Chemistry.

Fig. 9. a) Rate capabilities and b) cycling performances at 1 C of annealed TiO2 solid nanoparticles, TiO2 hollow spheres, and annealed hollow spheres. Note that C and D represent the specific charge capacity and discharge capacity, respectively. Charge/discharge curves at various rates of c) annealed TiO2 solid nanoparticles, d) TiO2 hollow spheres, and e) annealed TiO2 hollow spheres. f) EIS measurements of annealed TiO2 solid nanoparticles, TiO2 hollow spheres, and annealed TiO2 hollow spheres (experiments are in symbols and fittings are in solid curves). Reproduced with permission from Royal Society of Chemistry.

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characterization techniques include FTIR (Talyosef et al., 2007), EIS (He et al., 2012), XPS (Philippe et al., 2012), NMR (Trill et al., 2011), and AFM (Lacey et al., 2015; Tian et al., 2009). Spectroscopic studies on the SEI have shown the presence of ionic insulators such as Li2CO3 and LiF (Arreaga-Salas et al., 2012; Cheng et al., 2012). Xiao et al. (2011) compared the SEI composition of ALD deposited Al2O3/Si with bare Si. SIMS and XPS revealed a much thinner SEI on the Al2O3 coated electrode and the presence of LiAlO2 which has been shown to possess excellent Li þ conductivity. Lipson et al. (2014) used a unique striped Al2O3 pattern on MnO and investigated the different SEI formation using SICM topography. They showed that 3 Å of Al2O3 can partially prevent thick SEI formation while only 9 Å can completely prevent thick SEI formation (Fig. 10). SEI formed on Si NWs with hydride and methyl terminated surfaces were investigated with FTIR and XPS (Xu et al., 2011). The methyl termination on the Si surface showed fewer new peaks, suggesting the surface is relatively unreactive. The mechanical properties of the SEI are also important to investigate (Xu et al., 2011; Zhang et al., 2012). 3.3. Surface engineering for chemical passivation Due to their reduced dimensions, nanomaterials possess high surface free energy, and this drives their reactivity in many standard environments (Zhou et al., 2011). Whereas the surface reactivity can be controlled by engineering the SEI layer in battery electrodes, non-Faradaic electrochemical supercapacitors are an example of an energy storage device that requires the electrodes to be chemically inert. Due to this, most supercapacitor electrodes are produced from nanoscale forms of carbon. Supercapacitors (or electric double layer capacitors) are symmetrical devices composed of 2 electrodes with an electrolyte between. Unlike other energy storage devices discussed so far that store charge through Faradaic electron transfer reactions, supercapacitors store energy due to electrostatic attraction between opposite charges. Ions in the electrolyte are attracted to the electrode of opposite charge and form a double layer along the surface (Wang et al., 2012a). The electrostatic nature of these devices permits very fast charge and discharge. Chemical passivation of nanomaterials can broaden supercapacitors to include a whole range of new materials with tailored

chemical compatibility with electrolytes that leads to high voltage windows and durability that is often quoted to be over 1 million cycles (Lu et al., 2014, 2012). It can also be used to engineer the sub-structure of highly porous nanoscale electrodes to increase capacitance since pore size and capacitance are correlated (Galhena et al., 2015; Huang et al., 2008; Largeot et al., 2008). Surface engineering with atomic-scale precision can potentially lead to pore sizes that lead to partial desolvation of ions in solvents (Chmiola et al., 2008, 2006b; Vix-Guterl et al., 2005) and hence increased capacitance (Feng and Cummings, 2011). One platform where such control has been demonstrated has been carbide derived carbons (Chmiola et al., 2006a). Outside of traditional carbon electrodes, other materials such as porous silicon can provide the high surface area required for a supercapacitor electrode (Desplobain et al., 2007; Rowlands et al., 1999; Westover et al., 2014b). Specifically in porous silicon, pore size and porosity can be controlled using electrochemical etching to optimize electrodes for surface area, electrolyte penetration, and conductivity (Sailor, 2012). Oakes et al. (2013) demonstrated that porous silicon is a poor stand-alone electrode for supercapacitors because it reacts with ionic liquid electrolytes near 1.3 V (Fig. 11). To overcome this, a few-layer coating of graphene material on the surface of the porous silicon through CVD enabled the voltage window to be extended to  2.7 V, with discharge times increased by over 20  . The carbon coating also stabilizes nanostructured silicon in aqueous and organic media (Carter et al., 2015; Chatterjee et al., 2014). Alternately, the porous silicon can be passivated by TiN using ALD (Grigoras et al., 2014). Since porous silicon is made from a silicon wafer, energy storage devices can be integrated into silicon electronics such as solar cells (Cohn et al., 2015; Westover et al., 2014a). Mesoporous silicon can also serve as a template for porous carbon with sub nanometer pores (Jurewicz et al., 2004; Vix-Guterl et al., 2004). Researchers have also shown that various coatings on Si NWs can prevent undesirable reactions and increase the voltage window (Alper et al., 2012; Berton et al., 2014), but CVD of an ultra-thin carbon coating seems to be the most promising (Alper et al., 2014). These efforts, combining CVD and/or ALD processes to surface engineer materials such as silicon to exhibit stability for energy storage in electrolytes corrosive to the native silicon material represents the power of surface

Fig. 10. a) SICM topography images of samples that have undergone one electrochemical cycle with lithographically defined Al2O3 stripes on the surface of MnO. a,b) Two different regions of a sample with stripes of 3 Å thick ALD Al2O3 on the surface of MnO. c) 9 Å thick and d) 90 Å thick stripes of ALD Al2O3. Reproduced with permission from American Chemical Society.

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Fig. 11. a) Scheme of the effect of coating P-Si on the capacitive charge storage properties. b) Nyquist plot for graphene-coated and pristine P-Si based on EIS sample characterization, with knee frequencies labeled in the plot. c) Cyclic voltammetry measurements for graphene-coated and pristine P-Si, with approximate electrochemical windows in EMIBF4 electrolyte environment labeled. Reproduced with permission from Nature Publishing Group.

engineering approaches, and can lead to enabling new heterogeneous surfaces for non-Faradaic energy storage systems. 3.4. Mechanical properties Surface engineering of the mechanical properties of electrodes has had a large impact enabling new higher energy density battery technologies. Mechanical properties become an important part of an energy storage system that exhibits high storage capacity, since volumetric changes lead to cracking or pulverization of materials without robust mechanical features. These mechanical stresses can lead to the loss of active material due to cracking and mechanical failure (McDowell et al., 2013; Zhang, 2011). Surface engineering is a tool that can, in addition to chemical modification, provide a mechanical support structure to mitigate the loss of active material. Since the early stages of Li ion battery research it has been well known that Li alloying materials such as Si, Sn, Ge (Bourderau et al., 1999; Larcher et al., 2007; Li et al., 1999) as well as metal oxide materials (Reddy et al., 2013; Wu et al., 2012c) and elemental S (Qiu et al., 2014; Yang et al., 2011) have extremely high Li ion storage capacities, especially in the case of Si. Such high Li ion storage capacity inevitably necessitates high volume expansions, upwards of 300–400% in the case of Si. In traditional bulk materials these volume expansions cause mechanical fractures which lead to degradation of the active electrode materials and battery

failure (Zhang, 2011). Primarily there have been two strategies developed to overcome these difficulties, (1) developing nanoscale materials that can accommodate the large volume expansions (Chan et al., 2008; Lee and Cho, 2011) and (2) the use of thin surface coatings to act as an elastic glue which holds the material together as it stores Li. The most effective materials combine both of these strategies to overcome these mechanical challenges. There have been several different methods in which researchers have surface engineered electrodes which help to limit the amount of mechanical expansion and degradation. The first of these consists of a bulk coating on the outside of the entire electrode (Fu et al., 2013; Yesibolati et al., 2014). One of the best examples of this was presented by Lu et al. (2015) who showed that by coating a Si NP electrode with a conductive carbon, 1000 cycles of stable capacity could be reached at 1500 mAh/g. They noted that when individual nanoparticles or microparticles were coated the expansion of the active materials upon cycling would still cause fracturing of the coating material leading to quicker degradation. The second strategy consists of using a core–shell principle where each individual active material particle or wire has a protective coating such as that shown by Nguyen et al. (2012) They first grew NiSix NW via a silane CVD technique, followed by another silane CVD to grow an amorphous Si layer. Finally, an alumina ALD coating was deposited. They show impressive cyclability with 150 cycles at a full discharge capacity of 3000 mA h/g, and up

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These yolk/shell structures are typically developed by applying an external coating on a multilayer nanoparticle and then removing the outer layer of the nanoparticle. The most successful implementation of this strategy has been a study by Liu et al. (2014b) which builds on one of their previous studies (Liu et al., 2012a). They developed a pomegranate like design (Fig. 12) that showed impressive performance with a total capacity of about 3000 mA h/ g and minimal degradation over 1000 cycles at a rate of C/20. Similar yolk shell strategies have enabled impressive performance in S/Carbon yolk-shell (Zhou et al., 2015), Sn/C yolk/shells (Zhang et al., 2008) and S/TiO2(Seh et al., 2013) yolk shells. In addition to the coating methods and materials presented here there have been numerous different material coatings ranging from Au (Thakur et al., 2012), Ag (Yu et al., 2010) Ni (Zhang et al., 2007), exotic oxides (Yesibolati et al., 2014) and various forms of carbon (Wu et al., 2012b), with all giving essentially the same results. This emphasizes that rather than the specific methods and chemical compositions of the coatings, what is of primary importance is the coating architecture utilized. Although the studies mentioned above are all targeted for Li ion batteries, the same principles apply for use in sodium and other metal ion systems. For example there have been several studies in the last few years showing the effectiveness of yolk-shell and core–shell architectures on improving the cycling performance of Na ion batteries (Choi and Kang, 2015; Liu et al., 2015; Wang et al., 2015).

4. Emerging topics in surface engineering 4.1. Surface engineering sulfur cathodes to prevent polysulfide dissolution

Fig. 12. Three-dimensional view a) and simplified two-dimensional cross-section view b) of one pomegranate microparticle before and after electrochemical cycling (in the lithiated state). The nanoscale size of the active-material primary particles prevents fracture on (de)lithiation, whereas the micrometer size of the secondary particles increases the tap density and decreases the surface area in contact with the electrolyte. The self-supporting conductive carbon framework blocks the electrolyte and prevents SEI formation inside the secondary particle, while facilitating lithium transport throughout the whole particle. The well-defined void space around each primary particle allows it to expand without deforming the overall morphology, so the SEI outside the secondary particle is not ruptured during cycling and remains thin. c) Calculated surface area in contact with electrolyte (specific SEI area) and the number of primary nanoparticles in one pomegranate particle versus its diameter. Reproduced with permission from Nature Publishing Group.

to 700 cycles at a partial discharge of 1200 mA h/g, which was notably greater than an uncoated sample which showed up to 500 cycles of partial discharge up to 1200 mA h/g. They note however that even with this impressive cycling, the alumina coating still fractured upon repeated charge discharge as was mentioned by Lu et al. (2015). Similar trends were shown for core shell nanoparticles of Si with in-situ polymerized C coatings (Wu et al., 2013) and Fe3O4 with carbon coatings (He et al., 2013a). It was noted in the majority of core/shell structures that although the coatings did provide improved performance they still fractured during full lithiation/delithiation cycles. This fracturing motivated a now widely developed technique, the use of a yolk/ shell structure. This method is similar to the core/shell method, but instead of the interior core completely filling the shell, there is additional void space. This void space accommodates significant volume expansion which is then supported by the shell structure.

Lithium–sulfur batteries are a grand challenge to the battery community offering many times the capacity of conventional Liion batteries, but a series of key research limitations before commercial systems can be realized. An ideal lithium-sulfur battery utilizes a lithium metal anode and a sulfur cathode that usually involves carbon networks to immobilize sulfur and provide electrical connectivity across the electrode. Although sulfur cathodes also experience similarly large volume expansions as other high capacity electrodes, there are several unique challenges that arise in these electrodes. As a sulfur cathode reacts with Li, there are three things that happen, a) the active material has large volume expansions, b) there are intermediate phases in the Li/S reaction chain that are soluble in most electrolytes leading to the dissolution of polysulfides causing loss of the active material, and c) large volume expansions cause agglomeration of the active S resulting in significant losses in conductivity (Ji and Nazar, 2010). Although the latter two challenges are unique to sulfur cathodes, the strategies employed to address these challenges with regards to surface engineering are more or less the same, utilizing full electrode coatings (Yang et al., 2011), core–shell (Liang et al., 2014) and yolkshell (Zhou et al., 2013) morphologies with the most effective strategy also being the use of yolk-shell structures. One excellent example of this is the development of S/TiO2 yolk/shell structures for Li/S batteries. Seh et al. (2013) developed S/TiO2 yolk/shell electrodes using wet chemical processing. These resulting yolk/ shell particles showed excellent cycling performance of about 800 mA h/g S for 1000 cycles. This could potentially be paired with pre-lithiated Si/C yolk-shell particles to form a super high energy density Li ion battery. Additionally, as one of the key challenges in practical lithiumsulfur batteries is overcoming polysulfide shuttling – a mechanism that reduces the total active sulfur in the cathode through dissolution of polysulfides into the electrolyte, surface engineering has been shown to be a critical tool (Peng and Zhang, 2015).

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Recent efforts have indicated that thin coatings of metal oxides, such as indium-tin oxide (ITO), vanadium oxide, and even alumina can lead to strong surface binding of soluble polysulfides, thus mitigating effects of polysulfide shuttling (Yao et al., 2014; Zhang et al., 2015). This recently emerging research area has shown great promise in the goal of achieving cathodes where 100 cycles can be reached with over 90% of the initial sulfur capacity – a key benchmark for lithium-sulfur battery research efforts. 4.2. Surface engineering to overcome dendrite growth for metal anodes The first reports on rechargeable Li ion batteries were enabled by using Li metal as the anode. At the beginning there was much hope in being able to use Li metal as the anode material in commercial devices but it quickly became apparent that quick mechanical degradation due to dendrite growth made this unfeasible. Initially there was much research done on coating methods for preventing dendrite growth and enabling the use of Li metal as an anode (Catanzarite, 1979; De Jonghe et al., 1990), but they failed to show significant enough cycle stability to motivate their use in commercial devices. Although there have been consistent studies on dendrite formation and efforts to overcome them (Bhattacharyya et al., 2010; Scrosati and Garche, 2010; Xu et al., 2014), until recently, research on the high capacity low voltage anode materials such as Si has taken center stage. In the last couple of years however the prospect of Li–S batteries with potential energy densities 3–4 times greater than that of current Li ion battery systems has provided increased motivation for the use of Li metal as an anode material. The reason for this is in a Li–S battery configuration, the initial Li is supplied by the anode material instead of by the cathode material as in traditional Li ion battery systems.

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The last few years have thus seen a rapid jump in the number of publications attempting to utilize Li metal as an anode (Khurana et al., 2014; Mukherjee et al., 2014; Tu et al., 2015; Yan et al., 2014). There are two methods in particular that are very promising. The first by Zheng et al. (2014) produced a conductive carbon yolk-shell design that allowed them to develop highly improved cycling for Li metal anode systems. They initially drop cast deposited polystyrene NPs on a Cu foil current collector and then applied a conductive carbon coating by flash evaporation of carbon fibers. The polystyrene NPs were then burned away at 400 °C leaving a conductive carbon film. This film was peeled back slightly to allow for Li electrodeposition. This composite electrode material showed excellent cyclability with 99.9% columbic efficiency up to 150 cycles. The second strategy for developing stable Li metal anodes was recently presented in a pair of studies that involves the use of ALD to coat thin layers of alumina on Li metal foil (Kazyak et al., 2015; Kozen et al., 2015). In both cases this approach allowed significantly improved Li metal anode performance. In particular the work of Kazyak et al. (2015) showed that with a 2–3 nm (20 cycles) coating of alumina the Li metal anode was stable for an average of over 700 cycles with the best device stable for over 1200 cycles (Fig. 13). Together these two approaches strongly suggest that further research into thin coatings that can mechanically inhibit dendrite growth shows exceptional promise for finally realizing a viable highly stable Li metal anode. Although yet to be shown, it is feasible that a similar method of coating other metals for use in their respective metal-ion systems could show similarly impressive results. In particular Na ion systems which suffer from the same problems as metallic Li (Slater et al., 2013) would be worth exploring. It could potentially also benefit the use of metallic Mg (Shterenberg et al., 2014) and Ca

Fig. 13. a) Constant current charge/discharge voltage profiles for Li symmetric cells showing the effects of ALD coatings on overpotential losses at 1 mA/cm2. Each half cycle represents 0.25 mA h/cm2. In this plot, the dotted lines represent the point of failure for each cell. For the control sample, this was 711 cycles for the 20  this point was at 1259 cycles. All cells were tested to failure. EIS measurements taken after 0, 300, and 700 cycles are shown in the insets. b) Zoom in on stable voltage overpotential regime. c) Zoom in after failure of control and 30x samples, showing erratic voltage behavior while the 20  sample remains stable. Reproduced with permission from American Chemical Society.

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(Ponrouch et al., 2016) for exploration of their respective battery systems. 4.3. Surface engineering to prevent micro-Cracks in high voltage cathode materials The vast majority of cathode materials are made of ceramics with Li intercalation potentials near 4 V with respect to Li/Li þ . Many of these are highly stable and are commonly used in commercial batteries, however these LiMOx materials are brittle ceramics so repeated lattice distortions due to Li intercalation often cause microcracks which results in the degradation of the electrodes ability to intercalate lithium. There is an active area of research that effectively uses surface coatings to prevent these microcrack formations and improve the stability of the cathodes (Chen et al., 2010b, 2014).

5. Conclusion Nanomaterials have unveiled a trajectory for energy storage research that gives promise toward faster charging, electrode materials with improved durability, and materials with greater energy density. In this review, we have discussed advances in the area of surface engineering, which is an area of research that we argue provides a foundation for many of the recently discussed innovations and breakthroughs in energy storage devices and materials. By separately optimizing the characteristics of the surface coating and the underlying material, many of the challenges associated with the incorporation of nanostructures into high performance energy storage systems can be overcome. Here we isolate recent advances in the context of four leading mechanistic roles of surface engineering: (1) Activation of surfaces for energy storage capability by coating active material onto predefined substrates, (2) Passivation of surfaces from unwanted chemical reactions, (3) Control over the solid electrolyte interphase to sustain stable Faradaic storage, and (4) Control over the mechanical properties of nanostructures to mitigate degradation and active material loss. We further demonstrate surface engineering to be a relevant theme underlying three of the most dynamic topics in current battery research, such as cathode design for lithium-sulfur batteries, dendrite mitigation in metal anodes, and micro-crack prevention in high voltage cathodes. Moving forward continued advancement of surface engineering techniques such as atomic layer deposition enables a near limitless area of innovation for next-generation energy storage technology. Decoupling the engineering problem for energy storage materials by modifying surfaces independently from bulk materials adds a dimension of control to unlock the promise of future advanced energy storage systems.

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