Electrode Materials of Sodium-Ion Batteries toward Practical

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Electrode Materials of Sodium-Ion Batteries toward Practical Application Yangyang Huang,†,§ Yuheng Zheng,† Xiang Li,† Felix Adams,‡ Wei Luo,*,† Yunhui Huang,*,† and Liangbing Hu*,‡ †

Institute of New Energy for Vehicles, School of Materials Science and Engineering, Tongji University, Shanghai 201804, China Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States § Dongguan McNair New Power Co., Ltd, Dongguan, Guangdong 523800, China

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‡

ABSTRACT: Advances in developing affordable batteries are vital for integrating renewable and environmentally friendly energy sources into the power grid. Benefiting from the abundance of sodium resources, sodium-ion batteries (SIBs) have attracted great attention as one of the most promising energy storage and conversion devices for grid-scale energy storage systems. From this perspective, we present a succinct and critical survey of the emerging electrode materials, such as layered transition-metal oxides, polyanionic compounds, Prussian blue analogue cathode materials, and hard carbon anode materials, that have potential value for large-scale applications.

A

the cost of the former is about $0.11/W h, while for the latter the cost is $0.14/W h.5,6 Although the latter is higher at this stage, the cost of SIB is potentially lower upon the further development of SIBs. In the past decade, significant progress has been made on electrode material, electrolyte, current collector, and cell configuration for SIBs. The cost of the electrode material accounts for a large part of the SIB cost. As a result, electrode material is still a key topic, especially in regard to large-scale applications. In this Perspective, we focus on cathode and anode materials with these characteristics: high performance, low cost, abundance, low environmental impact, long lasting, and safe. On the cathode side, we introduce layered metal oxides, polyanionic compounds, and prussian blue analogues (PBAs). For anodes, we focus on hard carbon materials. Because there are many comprehensive review papers on some other electrode materials,4,5 such as alloy compounds, TiO2, and quaternary oxides, details on them will not be repeated because of space limitations. Layered Metal Oxides. Layered metal oxides with the general formula NaxMO2 (x ≤ 1; M represents one or more metal ions, Ni, Co, Fe, Mn, Cu, etc.) are the most studied cathodes because of their high capacity and working voltage. Typical layered metal oxides can be classified into two main groups, P2 and O3 types, depending on the exact position of the alkali metal and the number of alkali metal layers in the structure perpendicular

s the global demand for energy increases rapidly and it becomes clear that the use of fossil fuels has substantial consequences for our climate, developing green, renewable, and carbon-neutral energy sources is critical. However, renewable energy sources, such as solar and wind, are typically intermittent. Therefore, developing sustainable electrical energy storage (EES) technologies for stationary applications is crucial and has attracted tremendous research attention. Secondary batteries among various EES technologies present the most suitable pattern in terms of energy density and conversion efficiency.1,2 Lithium-ion batteries (LIBs) with high energy density and long cycle life have been successfully developed as power sources for portable electronic devices. The application of LIBs in electric vehicles (EVs) led to an increase in cost due to the limited and uneven distribution of lithium reserves.3 Low-cost EES that uses naturally abundant raw materials is urgently required, especially for large-scale stationary applications. Similar to LIBs, SIBs are a class of rechargeable batteries in which sodium ions are used as charge carriers. Considering the physical and chemical properties that are similar to those of LIBs, along with the huge abundance and low-cost of Na, SIBs have recently been considered as an ideal stationary energy storage technology.4 SIBs can be manufactured in the same way as LIBs, which makes it possible for the LIB manufacturers to produce SIBs using the existing production lines. Furthermore, the overall production costs and risk factors can be reduced using low-cost aluminum as current collectors for both cathodes and anodes. Recent cost analysis on LiNi0.8Co0.1Mn0.1O2/graphite cells and NaNi0.6Co0.05Mn0.35O2/hard carbon cells shows that © 2018 American Chemical Society

Received: April 15, 2018 Accepted: June 6, 2018 Published: June 6, 2018 1604

DOI: 10.1021/acsenergylett.8b00609 ACS Energy Lett. 2018, 3, 1604−1612

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voltage of 2.75 V based on the Fe2+/Fe3+ and Mn3+/Mn4+ redox couples, of which the energy density is estimated to be 520 Wh/kg versus Na metal (Figure 1a).12 Unfortunately, the cycling stability of P2-Na2/3[Fe1/2Mn1/2]O2 is relatively poor because of the irreversible phase transitions during electrochemical Na insertion− extraction cycles. In general, the P2 phase could transform to the OP4 phase upon charging above 3.8 V. Compared with the P2−O2 phase transition, the P2−OP4 phase transformation is more reversible.13,14 Nazar et al. found that the formation of OP4 phase and a new poor crystalline “Z” phase (NaxFe0.5Mn0.5O2 x ≤ 0.25) resulted from migration of Fe3+ into tetrahedral sites,15 and this migration is highly reversible. In addition, cation doping is an effective way to improve the structural stability of P2-type cathodes by suppressing the phase transition and Jahn−Teller structural distortion. For example, P2-Na2/3Mn0.8Fe0.1Ti0.1O2 shows improved cycling life and a fast Na+ mobility.16 Another shortcoming of P2 phase materials is the small proportion of Na ions, resulting in a low desodiation capacity in the initial cycle. Armand et al. reported that the Na deficiency issue can be effectively relieved by the introduction of Na3N (Figure 1c).17 However, the formation of N2 gas due to the decomposition of Na3N would result in potential safety issues.

The key issue for SIBs toward largescale storage applications is the search for suitable electrode materials with characteristics of high performance, low cost, abundance, low environmental impact, long-term cyclability, and high safety. to the layering.7 “P” and “O” represent prismatic and octahedral sites occupied by Na ions, respectively. In a lattice made up of hexagonal sheets, there are three possible positions for the oxygen atoms, conventionally named A, B, and C. For example, when the layers are packed in the order ABCABC, an O3 structure is obtained. Among all layered metal oxides, the Co- and Ni-containing materials exhibit large reversible capacities and high voltages.8−11 However, they cannot satisfy the cost requirements for large-scale batteries. Fe- and Mn-based layered oxides are appealing owing to their natural abundance and potential low cost. P2-Na2/3[Fe1/2Mn1/2]O2 delivers a large specific capacity of 190 mAh/g with an average

Figure 1. Charge−discharge curves for (a) Na2/3Mn1/2Fe1/2O2 and (b) NaMn1/2Fe1/2O2. Panels a and b are reprinted with permission from ref 12. Copyright 2012 Nature Publishing Group. (c) Cycle life evolution of Na0.67[Fe0.5Mn0.5]O2 with various Na3N contents. Reprinted with permission from ref 17. Copyright 2017 Elsevier. (d) XRD patterns and (e) cycling performance of the O3-Na0.9Cu0.22Fe0.3Mn0.48O2 stored in air for one month. (f) The schematic and charge−discharge curves of pouch cells using O3-Na0.9Cu0.22Fe0.3Mn0.48O2 cathode and hard carbon anode. (g) Cycling performance of the pouch cells in panel f. Panels d−g are reprinted with permission from ref 19. Copyright 2015 Wiley. (h) Galvanostatic charge−discharge profiles of 1.0 Ah pouch cells using O3-NaNi1/3Mn1/3Fe1/3O2 cathode and hard carbon anode. Reprinted with permission from ref 20. Copyright 2016 the Electrochemical Society. (i) Cycling performance of Na[Li0.05Ni0.3Mn0.5Cu0.1Mg0.05]O2 at various current densities. Reprinted with permission from ref 21. Copyright 2018 Wiley. 1605

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Figure 2. Polyanionic compounds as cathode for SIBs. (a) The structure of the Na3V2(PO4)2F1+2xO2−2x (0 ≤ x ≤ 1). (b) Potentials of different redox couples in different polyanionic frameworks. Reprinted with permission from ref 24. Copyright 2018 Wiley. (c, d) The voltage profiles of Na3V2(PO4)2F1+2xO2−2x (0 ≤ x ≤ 1). Reprinted with permission from ref 25. Copyright 2014 Wiley. (e) The structure and (f) voltage profiles of Na2Fe2(SO4)3. Reprinted with permission from ref 28. Copyright 2014 Nature Publishing Group.

(Figure 1i).21 Faradion Co. Ltd. fabricated NaaNi1−x−y−zMnxMgyTizO2 via a solid-state method, which is already available in the market.22,23 Although this material exhibited excellent electrochemical performance, the usage of Ni leads to a high cost. Polyanionic Compounds. Several research groups highlighted polyanionic compounds as high-voltage cathode materials. The formula of these polyanionic compounds could be expressed as NaxMM′(XO4)3 (M = V, Ti, Fe, Tr, Al, or Nb etc.; X = P or S, x = 0−4). These polyanionic compounds are embedded with Mn+/(n+1)+ redox in a framework of strongly covalent bonded polyanion units (XO4n−) and are thus highly stable in terms of structure rearrangement during Na+ insertion−extraction and thermal abuse tolerance. Figure 2a demonstrates a series of Na3V2(PO4)2F1+2xO2−2x (0 ≤ x ≤ 1). The redox in these compounds usually provides higher voltages and could be tuned by replacing the XO4 with F− or O− depending on their difference in electronegativity, as shown in Figure 2b.24 Among various polyanionic compounds, Na3V2(PO4)3 provides stable cycling and high Na+ diffusion rate within its three-dimensional ion transportation channels. It exhibits a NASICON (Na super ionic conductor, originated from Na1+xZr2P3−xSixO12, which has superior Na-ion conductivity) type structure with relatively high voltage and long-term cycling capability. To improve the energy density, PO4 polyanion could be replaced by other groups, such as PO4F, P2O7, and F. These novel compounds usually provide V3+/V4+ redox with higher voltage, indicating higher energy densities. The Na3V2(PO4)2F1+2xO2−2x (0 ≤ x ≤ 1) family is another promising one among these voltage-tunable NASICON type polyanionic compounds.25 As shown in Figure 2a, the O and F substitute each other with a different stoichiometric ratio and would provide V3+/V4+ redox with higher potential along with F element (more electronegativity) increasing and O decreasing consequently (Figure 2c,d). This is a fantastic trait because it

O3-phase Na[Mn1/2Fe1/2]O2 can be synthesized by controlling the ratio of Na/[Mn and Fe]. As shown in Figure 1b, O3-phase Na[Mn1/2Fe1/2]O2 exhibits a larger polarization than that of P2-phase because the O3-phase suffers from more complex irreversible phase transitions.12 Moreover, Nax[Mn1/2Fe1/2]O2 is sensitive to air. In particular, the uptake of CO2 and oxidation by water lead to the formation of inactive Mn4+ and NaOH, respectively.18 To improve the air stability and its cycling performance, Hu et al. designed a new air-stable O3-Na0.9Cu0.22Fe0.3Mn0.48O2, showing unexpectedly superior stability against moisture.19 Even after the sample is soaked in water and stored in air for one month, there is no change in the structure and cycling performance (Figure 1d,e). Moreover, a prototype SIB has been fabricated with this cathode and hard carbon anode, which demonstrated a high energy density of 210 Wh/kg (Figure 1f) and excellent cycling stability (Figure 1g). Typically, three synthetic methods including sol−gel method, solid-state method, and coprecipitation method are used to prepare layered metal oxides. Sol−gel methods require an additional separate gel-forming step at elevated temperature, which increases the difficulty of industrialization. At this stage, solid-state and coprecipitation methods are the preferred ways to produce layered metal oxides for commercial applications. Ma et al. synthesized O3-NaNi1/3Fe1/3Mn1/3O2 in large scale by using the modified hydroxide coprecipitation technique combined with a solid-state method, showing good electrochemical performance under various temperature conditions.20 The 1.0 Ah pouch cells using the prepared cathode and hard carbon anode were manufactured, which exhibited excellent cycling performance with capacity retention over 73% after 500 cycles at 1.0 C rate (Figure 1h). Dou et al. used a coprecipitation method to prepare a novel airstable O3-type Na[Li0.05Mn0.5Ni0.3Cu0.1Mg0.05]O2, which exhibited remarkable capacity retention of 81.6% after 400 cycles at 1.0 C 1606

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Figure 3. (a) Galvanostatic charge−discharge curves of Na1.92FeFe(CN)6. Reprinted from ref 29. Copyright 2015 American Chemical Society. (b) Galvanostatic charge−discharge curves of the removal of H2O in Na2MnFe(CN)6 frameworks. Reprinted from ref 31. Copyright 2015 American Chemical Society. (c) Scale-up of PBA synthesis from 10 to 100 kg per batch. (d) Cycling performance of PBA cathode. (e−h) Pouch cells and corresponding performance at different conditions with PBA cathode and hard carbon anode. Panels c−h are reprinted with permission from ref 32. Copyright 2018 Wiley. (i) Symmetric open-framework cell schematic. Reprinted with permission from ref 33. Copyright 2014 Nature Publishing Group.

best choice because of the nontoxic and low-cost priorities. Generally, the electrochemical performance of PBAs is influenced by its phase purity, crystallinity, defects, and water content. The presence of vacancies and coordinating water in the structural framework can cause a distorted lattice, resulting in lower efficiency and poor cycling life. Thus, it is important to fabricate a PBA framework with low defects and water content. Recently, Wang et al. reported a high Na concentration Na1.92FeFe(CN)6 with a rhombohedral structure, which delivered a high reversible capacity of 160 mAh/g and a high energy density of 490 Wh/kg,29 as shown in Figure 3a. The high Na concentration can overcome the Na-deficiency problem and reduce the water insertion. The inset of Figure 3a shows much flatter two-phase voltage profiles and sharper peaks, which indicates the first-order transition between these structures during the charge−discharge. Goodenough’s group reported Na1.72MnFe(CN)6 with a reversible capacity of 134 mAh/g and high rate stability.30 The same group continued to remove interstitial H2O in the Na2MnFe(CN)6 framework either thermally or electrochemically.31 As shown in Figure 3b, the dehydrated Na2MnFe(CN)6 exhibited a reversible capacity of 150 mAh/g with flat plateaus at ∼3.5 V, which delivered excellent rate capability and superior cycling performance with 75% capacity retention over 500 cycles.

creates the potential for tuning the positive electrode materials, which could fit the specific highest occupied molecular orbital− lowest unoccupied molecular orbital (HOMO−LUMO) gap of the electrolyte. All of these compositions exhibit exceptional volume changes upon Na+ uptake and removal (∼2%) with energy density of about 500 Wh/kg. Most recently, it was found that Na3V2(PO4)2FO2 could uptake additional Na atoms, to achieve Na4V2(PO4)2FO2 with a plateau potential at about 1.5 V,26 which further extends the energy density to about 600 Wh/kg. Although the above-mentioned V-based polyanionic compounds provide impressive energy density, they exhibit low electron conductivity and elemental toxicity. There are several 3.0 V Fe-based phosphate PO4 insertion compounds that have been reported.27 When the phosphate (PO4) is replaced with SO4, the potential of Fe2+/Fe3+ redox couple in Na2Fe2(SO4)3 reaches about 3.8 V against Na/Na+.28 Figure 2e presents the structure of this earthabundant composition, which exhibits a high energy density (540 Wh/kg), high rate capability (Figure 2f), and excellent capacity retention at 20 C. Prussian Blue Analogues. PBAs with the general chemical formula NaxM1M2(CN6) (M1 and M2 = Fe, Mn, Ni, Co) have been investigated as promising cathodes in both aqueous and nonaqueous batteries. Among all PBAs, NaxMFe(CN)6 is the 1607

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Figure 4. (a) Voltage profiles of glucose pyrolyzed hard carbon anodes (1000 and 1150 °C). Reprinted with permission from ref 34. Copyright 2000 the Electrochemical Society. (b) Discharge−charge curves of hard carbon anodes obtained with carbonization temperature ranging from 1200 to 2500 °C. Reprinted with permission from ref 35. Copyright 2015 Wiley. (c−e) The schematic and performance of pouch cells. Panels c−e are reprinted with permission from ref 36. Copyright 2016 Elsevier. (f) Initial discharge−charge profiles of commercially available hard carbon anode from Kuraray Co. Ltd. Na ion storage mechanism of hard carbon anode: (g) “intercalation−adsorption” mechanism. (h) “Adsorption−intercalation” mechanism. Reprinted with permission from ref 40. Copyright 2015 American Chemical Society. (i) “Absorption-filling” mechanism of hard carbon anode for Na ion storage. Reprinted with permission from ref 43. Copyright 2016 Wiley.

There are two synthesis methods for PBAs: coprecipitation method and hydrothermal method. Recently, Novasis Energy Inc. successfully synthesized Mn-based PBAs with a quantity of 100 kg per batch (Figure 3c).32 They demonstrated that the synthesis of PBAs was carried out by coprecipitation method based on a low-temperature aqueous chemistry. A high-temperature calcination step is not needed, which can decrease the manufacturing cost and carbon emissions. The as-synthesized PBAs exhibited excellent cycling stability with capacity retention ∼90% after 700 cycles at 1.0 C (Figure 3d). Pouch cells with PBA cathode and hard carbon anode were also fabricated (Figure 3e). As shown in Figure 3f−h, the cell delivers both high Coulombic efficiency and capacity retention of 98.6% over 500 cycles at 1.0 C. Even at temperatures as low as −20 °C, 83% capacity can be maintained, which is higher than that of LiFePO4 under the same conditions in LIBs. Furthermore, the cells can deliver a good cycling life at an elevated testing temperature (Figure 3h). On the other hand, Cui et al.33 developed a symmetric Na-based open framework cell (Figure 3i), which was composed of a copper hexacyanoferrate (CuHCF) cathode and a manganese hexacyanoferrate (KMnHCMn) anode. This symmetric cell provided a specific energy of 27 Wh/kg at 1.0 C based on the mass of the active materials. It showed nearly no capacity loss after 1000 cycles,

which suggested its potential for practical applications. In summary, PBAs are promising cathode candidates for commercial SIBs. However, the waste solutions resulting from the synthesis processes should also be taken into consideration because of the existence of toxic cyanide CN−. Carbon Anodes. Recent research on anode materials is focusing on carbonaceous materials, alloy compounds, oxides, and organic compounds. Among them, carbon-based anodes are considered as the most promising one in the short-term. In 2000, Dahn et al. first reported the electrochemical Na storage behavior of hard carbon prepared by the pyrolysis of sucrose.34 As shown in Figure 4a, a high reversible capacity of 300 mAh/g can be obtained with two distinct regions: a sloping region and a lowvoltage plateau region. To date, efforts have been made to further enhance the reversible capacity of hard carbon through structural investigations. Hasegawa et al. reported that hard carbon structure depended on the synthesis conditions.35 They explored the relationship between physical properties and electrochemical performance via controlling the carbonization temperature from 800 to 3000 °C. As shown in Figure 4b, hard carbon carbonized at 1600−2500 °C delivered the highest capacity with an initial Coulombic efficiency over 90%. The choice of carbon precursor is another key issue. As shown in Table 1, hard 1608

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environment-friendly, and high performance for SIBs. Layered metal oxides, polyanionic compounds, and PBAs possess the most promising application prospects because of their acceptable cycling performance and energy density. The Co- and Ni-free layered metal oxides are suitable cathode candidates for low-cost SIBs, but the structural and moisture stability still need to be improved for practical applications. The introduction of extra cations, such as Al3+, Cu2+, Mg2+, Mn4+, Ti4+, etc., into the transition-metal layers is an effective strategy to improve the cycling performance by suppressing the irreversible phase transitions and enhancing the air stability. Polyanionic compounds can provide higher voltage, but optimization on reducing the particle size and carbon coating is usually needed to improve its relatively poor electron conductivity. Another type of cathode, PBAs, also shows good electrochemical performance due to abundant redoxactive sites and strong structural stability. Among all PBA materials, Mn-based and Fe-based PBA frameworks exhibit the advantages for practically viable nonaqueous SIBs due to its low-cost, low environmental impact, and high electrochemical performance. However, suitable synthesis methods are critical. Compared with their organic electrolyte counterparts, aqueous SIBs (ASIBs) with high rate capability, strict safety, long cyclability, and low cost can fulfill the requirements of large-scale EES applications. A comparison of ASIBs and lead−acid battery for energy storage is shown in Table 2. Here, ASIBs using NaxCuFe(CN)6 as cathodes and NaTi2(PO4)3/C as anodes would be a choice for large-scale stationary storage to substitute lead−acid batteries. Unfortunately, the current development of aqueous NaxCuFe(CN)6 cathodes is still in its early stages. Therefore, it is crucial to improve the performance of NaxCuFe(CN)6 by tailoring the synthesis conditions.

Table 1. Electrochemical Performance Comparison of Hard Carbon Anodes Derived from Various Precursors precursor

temperature (°C)

surface area (m2/g)

Commercial Hard Carbons 240 −

Carbotron P(J) (Kureha)39 Type-2 (Kurary)37

−

glucose34 sucrose40 wood41

1100 1400 1100

pitch/lignin42 coal36

capacity (mAh/g)

−

280

8.7

Biomass/Biomass Waste 307 27 311 8 295 − Pitch Mixture 1400 254 1.3 1200 222 −

initial coulombic efficiency (%) 78 86.4

88 82 80 82 81

carbon showed reversible capacities mainly from 280 to 330 mAh/g and exhibited high initial Coulombic efficiency when the surface area is low. Among various carbon precursors, pitch mixture and biomass as well as biomass waste are attractive for large-scale applications because of their low cost and relatively high carbon yield. Hu et al. reported a coal-derived hard carbon.36 With such hard carbon anodes, a prototype Na pouch cell was fabricated (Figure 4c), which exhibited a practical energy density of 100 Wh/kg (Figure 4d,e). Kuraray Co. Ltd. has also produced hard carbon from biomass using various temperatures.37 As shown in Figure 4f, the carbon material delivered a reversible capacity of about 280 mAh/g and a high initial Coulombic efficiency of ∼90%. There has been speculation that hard carbon anodes in SIBs exhibit poor rate capability. Recent work showed that this is caused by the poorly designed half-cell test protocol.38 The rigid 0 V cutoff voltage together with the sensitivity of the hard carbon capacity against polarization causes premature truncation of the sodiation process of the hard carbon anode, which would be negligible in full cells. In the work of Zheng et al.,38 the full cell cycled 1300 times (2020 h) at 1.0 C before the capacity retention hit 70%, while the half cell with the same anode survived no more than 100 h with low capacity retention. This indicates that hard carbon itself could serve as a high-rate anode material. Rational designing of a hard carbon anode for Na storage is the pursuit of many groups. Unfortunately, the Na storage mechanism is still not clear, and some controversies still exist. Dahn and Stevens reported that the sloping and plateau regions correspond to the insertion of Na+ into the graphene layers and the adsorption of Na+ in the nanopores of hard carbon, respectively (Figure 4g).34 Ji et al. proposed that the sloping part of the potentiogram curve can be better explained through storage at defect sites and that the low-voltage plateau region resulted from the intercalation between graphene sheets, as well as minor phenomenon of Na adsorption on pore surfaces (Figure 4h).40 Recently, Hu’s group found that the sloping region corresponds to adsorption of Na in defected sites, edges, and the surface of nanographitic domains while the plateau region is contributed by the nanovoids’ filling (Figure 4i).43 Xiao et al.44 reported that the in-plane defects on the nanosections of graphene are tunable by means of the temperature increasing rate during the pyrolysis process. Low temperature-increasing rate would induce less defect concentration in hard carbon, thus resulting in higher reversible specific capacity. Summary and Future Outlook. In this Perspective, we focus on the development of electrode materials with low-cost,

Cathode materials such as Mn- and Febased layered metal oxide, Fe-based polyanionic compounds, and Fe−Mnbased PBAs possess promising application prospects, while hard carbon derived from biomass or pitch mixtures as the anode is the best choice for SIBs in the current stage. Regarding the anode side, hard carbon is the most promising choice in the near term. Exploring methods for producing hard carbon using low-cost precursors with high yield is important for manufacturing. Understanding of the solid electrolyte interphase (SEI) between anodes and electrolytes needs to be improved to obtain higher initial Columbic efficiency and decrease the selfdischarge of hard carbon for practical application. On the electrolyte side, electrolyte design and functional development is very critical. In general, NaClO4 and NaPF6-containing carbonated based electrolytes are widely used. The lower-purity NaPF6 salt may lead to poor electrochemical performance, while NaClO4based electrolytes relate to its safety concerns. Thus, the development of high-purity NaPF6 is required. To date, some academic and industrial organizations have provided a series of commercialized prototypes of SIBs. Recently, over 5.0 kWh of prototype Na-ion pouch cells were fabricated with layered metal oxide cathode and hard carbon anode by Faradion Co. Ltd.45 In 2015, CNRS and RS2E46 launched their first commercial cylindrical 18650 SIBs, which offered a life 1609

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Table 2. Comparison of ASIBs and Lead−Acid Batteries for Energy Storage cell components

safety

cathode anode separator electrolyte voltage energy density toxicity

characteristics

caustic spill prevention redundancy

cell properties

temperature deep discharge cycle life

ASIBs

lead−acid batteries

layered metal oxides or PBAs C@NaTi2(PO4)3 synthetic cotton separator alkali-ion salt−water electrolyte ∼2 V 25−35 Wh/kg ASIBs are made from nontoxic materials. They are safe to install in homes or businesses and in close proximity to people. ASIBs use a saltwater-based, neutral-PH electrolyte. ASIBs are completely sealed. ASIBs can be built at 48 V nominal. To improve energy of a system, more batteries are put in parallel. If a single battery fails, the system still functions. ASIBs can operate from −5 to 40 °C without negatively impacting the life of the batteries. Exceed 2000 and 3500 cycles discharge at 100% and 70% DOD, respectively.

PbO2 Pb grass fiber 4−7 M H2SO4 1.5−2 V 20−40 Wh/kg Lead metal, which composes over 70% of the weight of a lead−acid battery, is a known carcinogen.

span of over 2000 cycles with an energy density of 90 Wh/kg. CNRS further fabricated a new start-up named “Tiamat” with polyanionic cathode and hard carbon anode.47 Recently, new SIB prototypes using Prussian white as cathode and hard carbon as anode have also been developed by Altris.48 For ASIBs, Aquion Energy Co. Ltd.49 produced its first aqueous hybrid ion battery using a manganese oxide cathode, a C@ NaTi2(PO4)3 anode, and a saltwater and synthetic cotton separator. In 2015, the first production line of ASIBs was built by Enpower Energy Corp.50 in China. We believe that low-cost and high-performance SIBs will play an important role in future grid energy storage systems.

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Sulfuric acid electrolyte is caustic and harmful to skin. Flooded lead acid batteries have the risk of jar leakage. Lead−acid batteries are installed in either 2 or 12 V increments, which must be connected with each other to reach the target voltage. A single failure will destroy the whole system. The corrosion rate of positive plates doubles for every 10 °C rise in temperature. 150−350 cycles charged at 100% DOD;250−300 cycles charged at 70% DOD

Felix Adams is an undergraduate Materials Science and Engineering student at the University of Maryland College Park. He is interested in learning more about new forms of sustainable energy storage. Wei Luo is a Professor in the School of Materials Science and Engineering at Tongji University. He received his Ph.D. from Huazhong University of Science and Technology in 2012. Prior to his current position, Luo worked as a postdoctoral researcher at Oregon State University (2012−2014) and University of Maryland (UMD, 2014−2016) before being promoted to Assistant Research Professor at UMD (2016−2017). His research interests include energy storage and conversion devices, biomass materials, and low-dimensional nanomaterials. Yunhui Huang received his Ph.D. from Peking University in 2000. From 2004 to 2007, he worked with Prof. John Goodenough in the University of Texas at Austin. In 2008, he became a Chair Professor of materials science in Huazhong University of Science and Technology. He is now the Director of the Institute of New Energy for Vehicles in Tongji University. His research group works on rechargeable batteries for energy storage and their electrode materials. http://nev.tongji.edu.cn/

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiang Li: 0000-0002-1302-7136 Wei Luo: 0000-0002-4019-4634 Liangbing Hu: 0000-0002-9456-9315

Liangbing Hu is an associate professor at University of Maryland College Park. His research interests include nanomaterials and nanostructures, roll-to-roll nanomanufacturing, energy storage focusing on solid-state batteries and Na ion batteries, and printed electronics. He is the (founding) director of the Center for Advanced Renewable Biomaterials (CARB) at the University of Maryland College Park (www.carb.umd.edu). He is also the cofounder of Inventwood, Inc. with efforts to further commercialize nanocellulose-based nanotechnologies (https://www.bingnano.com/).

Notes

The authors declare no competing financial interest. Biographies Yangyang Huang is now a Ph.D. candidate in the School of Materials Science and Engineering at Tongji University. His research mainly focuses on materials for sodium-ion batteries and solid-state lithium batteries.

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ACKNOWLEDGMENTS We are grateful for the financial support by National Key R&D Program of China (No. 2018YFB0905400), the National Natural Science Foundation of China (No. 51632001), and the project of Innovative Group of Guangdong Province (No. 2014ZT05N013).

Yuheng Zheng is a postdoctoral researcher in the School of Materials Science and Engineering at Tongji University. He received his Ph.D. in Frontier Institute of Science and Technology, Xi’an Jiaotong University. He focused on low cost and long-life electrode materials for sodium-ion batteries, lithium metal anode, and other techniques for electrochemical energy storage systems.

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Xiang Li is a postdoctoral researcher in the School of Materials Science and Engineering at Tongji University. He obtained his Ph.D. in 2018 from Huazhong University of Science and Technology. His research focuses on metal oxides and phosphates for Li-ion and Na-ion batteries.

REFERENCES

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DOI: 10.1021/acsenergylett.8b00609 ACS Energy Lett. 2018, 3, 1604−1612