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      "text": "Abstract The ever-growing market of electrochemical energy storage impels the advances on cost-effective and environmentally friendly battery chemistries. Lithium- ion batteries (LIBs) are currently the most critical energy storage devices for a variety of applications, while sodium-ion batteries (SIBs) are expected to complement LIBs in large-scale applications. In respect to their con- stituent components, the cathode part is the most signif- icant sector regarding weight fraction and cost. Therefore, the development of cathode materials based on Earth’s abundant elements (Fe and Mn) largely determines the prospects of the batteries. Herein, we offer a comprehen- sive review of the up-to-date advances on Fe- and Mn- based cathode materials for LIBs and SIBs, highlighting some promising candidates, such as Li- and Mn-rich layered oxides, LiNi0.5Mn1.5O4, LiFe1-xMnxPO4,",
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      "text": "diffusion (Kim et al. 2016a). Herein, this review summarizes the research ad- vances on Fe- and Mn-based cathode materials for LIBs and SIBs, respectively, which are categorized into ox- ides, polyanion compounds, and hexacyanometalates (for SIBs). With a grasp of the inherent properties and up-to-date achievements of these candidates, especially high-energy LIB cathode candidates and highly stable SIB cathodes, we further discuss the impendent chal- lenges and prospects in this field and put forward some insights into the opportunity of Fe- and Mn-based cath- ode materials. We hope that this review can inform readers of the rationality and priority of Fe- and Mn- based cathode materials as candidates for future LIBs and SIBs, and call for further efforts to fulfill this goal.",
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      "text": "The introduction of LCO cathode has empowered the commercialization of the first LIB. LCO also guides the investigation of a family of α-NaFeO2-type layered oxides LiMO2, where M can be a TM or a mixture of several (Chen et al. 2016). As shown in Fig. 2a, they have a rhombohedral structure with a space group of R-3m, in which the M-O octahedral slabs and Li-O octahedral layers follow O3 stacking along the c axis. Though LCO has a high theoretical capacity of 282 mAh g−1, the structure will undergo an irreversible phase transition when more than half of the Li ions are removed (Xia et al. 2007). Moreover, LCO also suffers from safety issues due to its thermal instability at the highly charged states (MacNeil et al. 2002). Therefore, motivation exists for Co to be replaced by abundant, inexpensive, and non-toxic transition metals.",
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      "text": "the use of an ion-exchange process to prepare high- quality LiNi0.5Mn0.5O2 with very low little intralayer disordering, which could maintain a capacity of ∼ 180 mAh g−1 even at a high rate of 6 °C (Kang et al. 2006). However, the extra cost from the method pre- vents it from practical application. Meanwhile, researchers investigated the co- substitution of Co and Mn for better structural stability. In 2001, Ohzuku et al. reported the isometric LiNi1/3Mn1/3Co1/3O2 (NMC111) cathode, which demon- strated a rechargeable capacity of 150 mAh g−1 in 3.5– 4.2 V or 200 mAh g−1 in 3.5–5.0 V (Tsutomu and Yoshinari 2001). The appreciable capacity and cycling stability of NMC soon attracted considerable interest from the science community as well as industry. It is widely recognized that high Ni content in the layered NMC as the active redox species contributes to a higher capacity but at the expense of safety and difficult preparation. High Mn content existing as inactive Mn4+ enhances the structural stability, and high Co content improves the rate perfor- mance and processing ability. As the NMC materials still consume Ni and Co largely, the next-generation cathodes should be Ni and Co poor or free materials.",
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      "text": "In principle, the structure of LiMO2 is dependent on the size of the M cation (Shirane et al. 1995). When Co is substituted by similarly smaller M cations, such as V3+, Cr3+, Ni3+, the layered rock-salt structure can be easily preserved, whereas the same structure can hardly be inherited when accommodating Fe3+ with much larger size. In fact, the structural complexity of LiFeO2 is much higher than other LiMO2. Around ten phases of LiFeO2 have been reported under different preparation",
      "category": "scientific_body",
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      "text": "(Hirayama et al. 2011). As shown in Fig. 3, large voltage hysteresis is identified in the first cycle of LiFeO2, and the following profiles are similar with initially reduced LiFeO2-x. X-ray diffraction (XRD) patterns also clearly revealed the degradation of the structure. To address the intrinsic redox issue, the possible route is to incorporate other materials to form solid solutions or nanocomposites. For example, LiFeO2-Li2MnO3 solid solution (Li1+",
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      "text": "With the low-cost constitution and very high deliverable capacities exceeding 250 mAh g−1, Li2MnO3-based lay- ered oxides are considered as one of the most promising cathode candidates for next-generation LIBs (Thackeray et al. 2007; Yabuuchi et al. 2011). Bare Li2MnO3 possesses a layered structure that can be represented in conventional- layered LiMO2 notation as Li(Li1/3Mn2/3)O2, in which Li+",
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      "text": "concentration of Li ions in the spinel body (Li/Mn = 1:2) results in only half of the theoretical specific capacity (146 mAh g−1) of layered LiMO2, attributing to the low- cost, stable, and kinetic Mn-based 3D spinel framework, LiMn2O4 has progressively substituted LCO in some large-scale and high-power applications. Another pro- nounced feature from the spinel configuration is the flat working voltage plateau at 4.0 V. In contrast, layered structured cathodes display sloping potential profiles and significantly lower potentials based on the same Mn3+/Mn4+ redox couple. As an explanation, the 3D spinel framework experiences negligible structural dis- tortion during the insertion/deinsertion of Li ions, there- by displaying an almost constant electrochemical poten- tial due to the consistent site energy. However, the layered structures are greatly distorted, resulting in the gradual change in site energy and hence sloping poten- tial profiles for the insertion of Li-ions. The Li ions in the spinel structure are located in the tetrahedrons, experiencing less repulsive forces from the local envi- ronment compared with the Li ions in the octahedrons of layered oxides, and the reduced site energy contributes to a higher potential (Liu et al. 2016a). Apart from the relatively low energy density, the other most claimed issue is the limited cycle life of LiMn2O4 due to Mn ions’ dissolution, especially under elevated temperatures (Jang et al. 1996; Xia et al. 1997). Previously, the cause of the degradation was ascribed to the dissolution of Mn2+ ions, originating from the dis- proportionation of Mn3+ ions LiMn2O4 (2Mn3+ ⇆Mn2+",
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      "text": "is the dominant dissolved Mn cation in LiPF6-based organic electrolytes, and the Mn3+ can stably exist in the electrolyte instead of suffering disproportionation (Banerjee et al. 2017). Even though the metal dissolu- tion is not negligible and haunts all the metal oxides, the practical cycling performance depends on its micro- structure and physico-chemical properties. In general, hierarchical microstructures are considered as optimal choices, which can shorten the ion diffusion pathway compared with bulk materials. Meanwhile, in contrast to nanosized particles, the primary micron particles can be densely packed for high volumetric energy density and show better structural integrity over repeated charge/ discharge cycles (Zhou et al. 2017). For example, as shown in Fig. 5b–e, Lee et al. recently reported densely",
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      "text": "Spinel LiMn2O4 is another classic cathode material for LIBs, of which Thackeray et al. have studied the lithium insertion/deinsertion properties since the 1980s (Thackeray et al. 1983, 1984). Different from the lay- ered structure, Li ions in the spinel structure occupy 1/8 tetrahedral sites, and the MnO6 octahedrons form a three-dimensional (3D) framework, leaving vacant tet- rahedral and octahedral interstitial sites for the highly efficient diffusion of Li+ (Fig. 5a). Even though the low",
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      "text": "(Ein-Eli et al. 1998), etc.) was initially designed to enhance the cycling stability by elevating the valance of Mn ions. Among the doped spinels, LiNi0.5Mn1.5O4 (LNMO) surprisingly displays one dominant plateau at around 4.7 V along with good electrochemical activity, thus presenting it as the most attractive spinel cathode for high-energy and high-power densities (Ohzuku et al. 1999). The massive substitution of Mn by 1/4 Ni adds complexity to the spinel structure, which is divided into two different symmetries: Fd-3m and P4332. The former one is isostructural to LiMn2O4 with Ni and Mn ions randomly distributed in the 16d octahedral sites, named the disordered phase. The later one has distinct 4a and",
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      "text": "and Mn3+ oxalates and carbonates, accompanying the oxidation of electrolyte (Jarry et al. 2015). The prob- lematic Ni2+/Ni4+ redox is also reflected in the electro- chemical process. Song et al. investigated a series of spinel cathodes with compositions of LiNi0.5-xMn1.5+",
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      "text": "Represented by LiFePO4, polyanion cathode materials are typically 3D structured compounds constructed by corner- and/or edge-shared M-O and X-O (X = P, S, As, Mo, or W) polyhedrons. These frameworks afford much better structural stability toward lithium (de)insertion compared with layered oxides, and the covalently bond- ed oxygen atoms prevent the oxygen loss that haunts lithium metal oxides. Also, the strong X-O bond can pull some charge density out of M–O bonds, and as a result, the M-O bond exhibits increased ionicity, leading to higher redox potential as a result of the inductive effect (Gong and Yang 2011; Masquelier and Croguennec 2013).",
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      "text": "xO4 (x = 0, 0.05, and 0.08) and found the removal of the Ni4+/Ni2+ redox reactions from the surface stabilizes the electrochemical performance at 55 °C. Doping with alien ions and surface coating are widely used strategies to improve the cyclability of cathode materials, which have also been extensively studied on LNMO. In gen- eral, elemental doping aims at improving the electronic conductivity of LNMO by altering the conduction band of the whole bulk, whereas the goal of the surface",
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      "text": "structural stability. The key drawbacks of LiFePO4 are inferior electric conductivity (ca. 10−9 S cm−1), the selective one-dimensional (1D) lithium pathway (Fig. 7a), and low gravimetric and volumetric ener- gy densities. Intensive studies in the last 20 years have effectively addressed the kinetic issues by size tailoring, crystal facet controlling, bulk doping, and surface conducting modification. In light of the 1D lithium ion pathway along [010], constructing nano- structures, especially 2D nanosheets with exposed (010) facets and shortened diffusion length, is an effective solution (Zhao et al. 2014). Surface modi- fication with conductive layers, mainly carbona- ceous materials, is widely used to improve the con- ductivity. However, common procedures easily re- sult in uneven and/or incompact coating layers due to the lack of affinity between the two substances. Constructing high-quality coating layers requires",
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      "text": "220 mAh g−1 (Legagneur et al. 2001). However, the early practice only realized the reversible capacity of ~ 8 mAh g−1 at C/250, suggesting deficient electrochem- ical activity. Until 2010, Yamada et al. obtained an attractive capacity of 200 mAh g−1 by introducing Ketjen black and vapor grown carbon fibers to increase its electrical conductivity (Yamada et al. 2010). They also pointed out the moisture sensitivity of this material, of which the surface degradation happens after exposure to air. Considering the appreciable electrical conductiv- ity of LiFeBO3 (reported to be 3.9 × 10−7 S cm−1) to- gether with the negligible volume change of ~ 2%, the inferior performance may be due to the constricted Li+",
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      "text": "Sodium metal oxides have been intensively studied as SIB cathodes, which mimic the lithium metal oxides, but the considerably different physico-chemical proper- ties (e.g., ionic size, electron configuration) renders some disparities on the crystal chemistry. First, the vast Na+ can hardly be hosted in tetrahedral sites. Therefore, there are no isostructural spinel cathodes for SIBs. Sec- ond, layered sodium metal oxides (NaxMO2) have a richer polymorphism due to the varied Na-O coordina- tion and oxygen stacking rules. Figure 9a shows the crystal structures of the most common polymorphs, O3 and P2, which are present in layered NaxMO2 (Yabuuchi et al. 2012a). The notions were introduced by Delmas et al. (Delmas et al. 1980), where the O or P stands for the octahedral (O) or a prismatic (P) sites accommodat- ing Na+ ions. The number n (n = 1, 2, 3, etc.) represents",
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      "text": "redox couple has been realized in the electrochemical cycling of α-NaFeO2. Later studies revealed the crit- ical role of the cut-off voltage in the electrode per- formance of α-NaFeO2 in the sodium cells. When a cut-off voltage was raised over 3.5 V, the cathode experienced irreversible structural transition coupled with significant capacity decay (Yabuuchi et al. 2012b). The fade of the electrode after heavy removal of Na+ should also be related to increasing amount of highly active Fe4+. A recent study disclosed the chemical instability of Fe4+ in batteries which spon- taneously oxidized the electrolyte to reduce back to Fe3+ at charged state (Lee et al. 2015). α-NaFeO2",
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      "text": "was also found to show an additional advantage of good thermal stability. As reported by Zhao et al. (Zhao et al. 2013), the highly charged Na0.58FeO2 powder decomposed thermally at a temperature higher than 300 °C, and NaFeO2 showed better ther- mal stability in the electrolyte than LCO in LIBs. The development of NaFeO2 is restricted by the low re- versible capacity of 80–100 mAh g−1 and the inferior cycling performance (Fig. 10).",
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      "text": "NaxMnO2 has a variety of polymorphs, which can be divided into two large groups: 2D layered structures consisting of slabs of edge-sharing MO6 octahedra at high x (e.g., Na2/3MnO2, NaMnO2) and 3D tunnel-like structures at low x (e.g., Na0.2MnO2, Na0.4MnO2, Na0.44MnO2) (Clément et al. 2015; Parant et al. 1971). For NaMnO2, monoclinic α-NaMnO2 (O′3 structure) and orthorhombic β-NaMnO2 (consisting of zig-zag layers) show high specific capacities. However, similar to the case of its lithium counterpart, NaxMnO2 is sub- jected to severe capacity decay when cycled in SIBs. As",
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      "text": "(de)insertion. Such merit is more demanded in SIB chemistry, as the movement of the larger Na ions puts forward higher requirement for structural stability. Giv- en the poor cycling stability haunting most of the lay- ered oxides, much interest has shifted to develop polyanion compounds as long-life cathodes for SIBs.",
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      "text": "Olivine LFP is the most successful polyanion cath- ode for LIBs. Unfortunately, its sodium counterpart NaFePO4 (NFP) simply cannot duplicate its success, as the thermodynamically stable phase of NFP is a maricite structure (Bridson et al. 1998), in which Na+ and Fe2+ ions occupy the opposite sites of those in olivine LiFePO4. Due to the absence of Na+",
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      "text": "The stronger inductive effect of the SO4 2- group can elevate the redox potential when compared with common PO4 3. In 2014, Barpanda et al. reported the use of a new Na2Fe2(SO4)3 with the alluaudite-type as a cathode mate- rial for SIB. The sulfate showed the highest-ever Fe3+/Fe2+",
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      "text": "The combination of different anion groups (F−, PO4 3−, P2O7 4−, CO3 2−, etc.) offers new opportunities toward desirable electrode materials. The induction of highly electronegative F−ions enables higher redox potentials due to the stronger inductive effect. Na2FePO4F is a representative Fe-based fluorophosphate, which was first introduced by Nazar’s group in 2007 (Ellis et al. 2007). As shown in Fig. 18a, the structure is assigned to the orthorhombic Pbcn space group, in which the bioctahedral Fe2O7F2 chains are connected by PO4 tet- rahedra to constitute 2D [FePO4F] layers. In this way,",
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      "text": "1) The extensively investigated strategies of constructing conductive composites and tailoring particle size eas- ily cause insufficient gravimetric and volumetric en- ergy densities as well as additional materials and processing costs. 2) Due to the large size of the Na ion, conventional host structures do not favor efficient and repeatable Na ion (de)intercalation. A critical step to realizing large-scale SIB applications is to search stable and kinetic SIB cathode materials. 3) Instead of emphasizing the electrode performance of a cathode material in a half cell, the practical performance should be demonstrated in full battery level, which requires innovation in the design and optimization of both the cathode and the anode materials.",
      "category": "scientific_body",
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      "text": "Considering the technology advances achieved on ener- gy conversion and storage and the pressing concerns of carbon emission and environmental pollution, the tran- sitions to clean, green, and sustainable development are practically logical and urgent. LIBs, as the state-of-the- art energy storage system, are starting to support green vehicles and residential energy storage. To meet the enlarging market, batteries themselves should adopt green and low-cost chemistries. As the costliest sector of a LIB, cathode materials involve removable Li ions as charge carriers, and transition metal ions serving as redox centers, which account for most of the cost. Given their elemental abundance, Mn- and Fe-based cathode materials are therefore preferable choices, and their so- dium analogs are also attracting much attention, as they might enable the future of Li-free SIBs, which would be ideal choices for large-scale applications. This review involves the discussion of most of the Fe- and Mn-based cathode materials for LIBs and SIBs, in- cluding oxides, polyanion compounds, and hexacyanometalates (for SIBs). We especially emphasize their merits, drawbacks, and up-to-date advances. To gain a direct view of the recently achieved electrochemical properties of those materials, we list their average operat- ing potentials, discharge capacities (at low and high rates), and cycling performances in Table 1 (LIBs) and Table 2 (SIBs). The energy densities of those Fe- and Mn-based cathode materials are plotted in Fig. 20. From the",
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      "text": "However, to compete with the benchmark LCO and NMC, efforts should be made to improving the gravimetric and volumetric energy densities at reduced cost. In comparison to the Bexternal^ modifications, governing the crystal and electronic structures of the host materials can alter the fundamental ionic and electronic conductivities, which can be considered as a research focus toward more practi- cal cathode materials. Concerning SIB cathodes, which are experiencing even quicker advances with a range of can- didatures being proposed, instead of simply mimicking the host structures of Li+ during synthesis, some new complex structures have been introduced to cope with the chal- lenges due to the distinct ionic size and electron configu- ration of Na+. Although the kinetics are reduced when compared with their LIB counterparts, there are some exciting results identified during the investigation of SIB cathodes. For example, the reversible high-energy Fe3+/ Fe4+ redox couple is observed in Fe-contained layered oxides for SIBs, which is absent for LIBs. The Jahn–Teller effect of Mn3+, which is always blamed for issues in LIB cathodes, is found to be Bcooperative^ in some SIB cath- odes. These phenomena are worthwhile to be further stud- ied, and the outputs may, in turn, stimulate the advances of LIBs. In addition, the development of a low-cost and high- performance battery requires the advance of the anode part. Current graphite anode suffers from poor ionic conductiv- ity and poses safety concerns due to its low lithiation potential. Zero-strain hosts and dendrite-free anodes are promising candidates. We believe better understanding and development of Fe- and Mn-based cathode materials will help to make rechargeable Li-ion and Na-ion batteries cheaper, greener, and better.",
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      "text": "ion batteries. Adv Energy Mater 8(6):1701415. https://doi. org/10.1002/aenm.201701415 Zhu Y, Xu Y, Liu Y, Luo C, Wang C (2013) Comparison of electrochemical performances of olivine NaFePO4 in sodium-ion batteries and olivine LiFePO4 in lithium-ion batteries. Nanoscale 5:780–787. https://doi.org/10.1039/C2 NR32758A Zhu X, Li X, Zhu Y, Jin S, Wang Y, Qian Y (2014a) LiNi0.5Mn1.5O4 nanostructures with two-phase intergrowth",
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      "text": "ion batteries. Adv Energy Mater 8(6):1701415. https://doi. org/10.1002/aenm.201701415 Zhu Y, Xu Y, Liu Y, Luo C, Wang C (2013) Comparison of electrochemical performances of olivine NaFePO4 in sodium-ion batteries and olivine LiFePO4 in lithium-ion batteries. Nanoscale 5:780–787. https://doi.org/10.1039/C2 NR32758A Zhu X, Li X, Zhu Y, Jin S, Wang Y, Qian Y (2014a) LiNi0.5Mn1.5O4 nanostructures with two-phase intergrowth",
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