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Chalcogenide materials are emerging as promising anode materials for sodium-ion batteries because of their reasonable capacity, which stems from two consecutive conversion and de/alloy reactions. Herein, we introduce the layer-structured Bi2S3 that shows long-term cyclability for sodium storage. Since deep sodiation induces the de/alloy reaction accompanied by a large volume change, we intentionally limit the reaction to the conversion process, resulting in long-term stable cyclability followed by a two-step conversion reaction: Bi2S3 + 3Na+ + 3e− → NaBiS2 + Bi0 + Na2S followed by NaBiS2 + 3Na+ + 3e− → Bi0 + 2Na2S, which was confirmed by operando X-ray diffraction, X-ray photoelectron spectroscopy and time-of-flight secondary-ion mass spectroscopy. During the electrochemical reaction, the presence of reduced graphene oxide (rGO) anchored with Bi2S3 assists to improve the charge transfer and buffers the electrode integrity, resulting in enhanced electrode performance of the Bi2S3/rGO composite compared with that of bare Bi2S3. The feasibility of using the Bi2S3/rGO composite is further confirmed in a full cell by pairing it with a Na0.67[Ni0.1Fe0.1Mn0.8]O2 cathode, resulting in reasonable capacity retention of ~74 % of the initial capacity for 300 cycles.

Conversion anode materials have been applied in lithium and sodium secondary batteries owing to their high capacities; however, there are limited reports on their use in potassium-ion batteries. He...

We report the nano-sized CuSO4–carbon nanotube composite (nano-CuSO4/C) as a novel conversion-based cathode material for Na-ion batteries (NIBs). The nano-CuSO4/C undergoes a conversion reaction during the charge/discharge process with a high redox potential of ~2.7 V (vs. Na+/Na) and the highest reported energy density for NIB cathode materials. Nano-CuSO4/C exhibits excellent electrochemical performance, with a specific capacity of ~335 mAh g−1 at a rate of C/30 (1C = 335 mA g−1), and even at 5C, its capacity is maintained up to ~204 mAh g−1, corresponding to ~61% of the theoretical capacity. Furthermore, nano-CuSO4/C delivers outstanding capacity retention of ~72% over 300 cycles at 2C with high coulombic efficiency of more than 99%. We confirm the reversible sodium storage mechanism on nano-CuSO4/C under Na-ion battery system using various analyses, such as operando/ex situ X-ray diffraction, X-ray absorption near edge structure spectroscopy, extended X-ray absorption fine structure spectroscopy, transmission electron microscopy, and time-of-flight secondary-ion mass spectroscopy. CuSO4 is transformed into Cu0 and Na2SO4 during the discharge (reduction) process, and the original CuSO4 is recovered during the charge (oxidation) process. This fundamental understanding of CuSO4 provides insight for the use of high-capacity conversion-based cathode materials in NIBs.

Recycling of waste electronics to recover raw materials is beneficial for the environment, particularly for electronic devices containing expensive metals such as lithium-ion batteries, which use lithium and cobalt as the electro-active materials. Recycling of these resources is a critical issue for both environmental reasons and cost reduction of lithium-ion batteries. In this study, we introduce a combination of efficient direct physical and intermediate recycling processes to minimize the environmental burden of lithium-ion batteries. High-purity materials are required in the fabrication of lithium-ion batteries to avoid unwanted side reactions stemming from impurities that may lead to safety issues. To achieve this purification, we employ a method that uses the difference in the solubility of materials in several solvents at different temperatures. Using this method, we successfully refine lithium and cobalt compounds with high purity from waste electrodes. The obtained high-purity materials are compared with commercial materials to ensure that their physical and chemical properties are comparable. Finally, it is observed that the electrochemical performance of the electrode material prepared from the purified material is similar to that of the commercially available electrode materials.

Materials in nature have evolved to the most efficient forms and have adapted to various environmental conditions over tens of thousands of years. Because of their versatile functionalities and environmental friendliness, numerous attempts have been made to use bio-inspired materials for industrial applications, establishing the importance of biomimetics. Biomimetics have become pivotal to the search for technological breakthroughs in the area of rechargeable secondary batteries. Here, the characteristics of bio-inspired materials that are useful for secondary batteries as well as their benefits for application as the main components of batteries (e.g., electrodes, separators, and binders) are discussed. The use of bio-inspired materials for the synthesis of nanomaterials with complex structures, low-cost electrode materials prepared from biomass, and biomolecular organic electrodes for lithium-ion batteries are also introduced. In addition, nature-derived separators and binders are discussed, including their effects on enhancing battery performance and safety. Recent developments toward next-generation secondary batteries including sodium-ion batteries, zinc-ion batteries, and flexible batteries are also mentioned to understand the feasibility of using bio-inspired materials in these new battery systems. Finally, current research trends are covered and future directions are proposed to provide important insights into scientific and practical issues in the development of biomimetics technologies for secondary batteries.

Since Ni-rich cathode material is very sensitive to moisture and easily forms residual lithium compounds that degrade cell performance, it is very important to pay attention to the selection of the surface modifying media. Accordingly, hydroxyapatite (Ca5 (PO4 )3 (OH)), a tooth-derived material showing excellent mechanical and thermodynamic stabilities, is selected. To verify the availability of hydroxyapatite as a surface protection material, lithium-doped hydroxyapatite, Ca4.67 Li0.33 (PO4 )3 (OH), is formed with ≈10-nm layer after reacting with residual lithium compounds on Li[Ni0.8 Co0.15 Al0.05 ]O2 , which spontaneously results in dramatic reduction of surface lithium residues to 2879 ppm from 22364 ppm. The Ca4.67 Li0.33 (PO4 )3 (OH)-modified Li[Ni0.8 Co0.15 Al0.05 ]O2 electrode provides ultra-long term cycling stability, enabling 1000 cycles retaining 66.3% of its initial capacity. Also, morphological degradations such as micro-cracking or amorphization of surface are significantly suppressed by the presence of Ca4.67 Li0.33 (PO4 )3 (OH) layer on the Li[Ni0.8 Co0.15 Al0.05 ]O2 , of which the Ca4.67 Li0.33 (PO4 )3 (OH) is transformed to CaF2 via Ca4.67 Li0.33 (PO4 )3 F during the long term cycles reacting with HF in electrolyte. In addition, the authors' density function theory (DFT) results explain the reason of instability of NCA and why CaF2 layers can delay the micro-cracking during electrochemical reaction. Therefore, the stable Ca4.67 Li0.33 (PO4 )3 F and CaF2 layers play a pivotal role to protect the Li[Ni0.8 Co0.15 Al0.05 ]O2 with ultra-long cycling stability.

The effect of Nb5+ doping on the electrochemical and structural characteristics of NaTi2(PO4)3 was investigated. The Nb5+ substitution lowered the band gap energy from 2.8 to 1.4 eV according to density functional theory calculations. Subsequent carbonization of pitch carbon on the surface of NaNbxTi2−x(PO4)3 (x = 0.05) significantly increased the electrical conductivity, thereby improving the capacity and high-rate capability. The excellent cyclability and superior electrode performance result from facile Na+ insertion and extraction that occur via a biphasic redox mechanism, namely, the Ti4+/3+ redox couple for the cathode and a sequence of two biphasic redox reactions associated with the Ti3+/2+ redox couple for the anode, as revealed by operando X-ray diffraction and ex situ X-ray absorption spectroscopic studies. Moreover, the possibility of application of Na2.9Nb0.05Ti1.95(PO4)3 as both a cathode and anode material was demonstrated in a symmetric cell, which delivered a capacity of 105 mA h g−1 after 100 cycles at 0.2C with a capacity retention of 83%.

We propose the coating of tunnel-type Na0.44MnO2 cathode materials with multi-functional Na2MoO4 nanolayers for use in rechargeable sodium batteries. Electro-conducting Na2MoO4 nanolayers (electrical conductivity of ∼103 S cm−1) are autogenously formed on the surface of Na0.44MnO2 particles through the reaction of (NH4)2MoO4 with surface sodium residues via melt impregnation at 350 °C. The Na2MoO4-modified Na0.44MnO2 electrode delivers discharge capacities of ∼120.4 mA h (g-oxide)−1 at 0.1C (12 mA g−1) and 79.7 mA h g−1 at 50C (6 A g−1). Moreover, with continuous cycling at a rate of 60C (7.2 A g−1), the Na2MoO4-coated Na0.44MnO2 electrode is able to retain a capacity of approximately 56 mA h g−1 without notable capacity fading for 1000 cycles. This achievement is attributed to the presence of Na2MoO4 on the active materials, which facilitates electron transfer during electrochemical reaction in Na cells. More interestingly, Na2MnO4 undergoes two-step HF scavenging to finally form MoO3−xF2x layers via an intermediate of H2MoO4 (MoO3·H2O) layers. The surface layers protect the active materials from HF attack in the electrolyte. These multi-functional effects of the Na2MoO4 and MoO3−xF2x surface layers are responsible for the long-term cycle stability of the cathode material for ultra-high-rate sodium storage applications.

Sodium-ion batteries (SIBs) are emerging power sources for the replacement of lithium-ion batteries. Recent studies have focused on the development of electrodes and electrolytes, with thick glass fiber separators (~380 μm) generally adopted. In this work, we introduce a new thin (~50 μm) cellulose-polyacrylonitrile-alumina composite as a separator for SIBs. The separator exhibits excellent thermal stability with no shrinkage up to 300°C and electrolyte uptake with a contact angle of 0°. The sodium ion transference number, t Na + , of the separator is measured to be 0.78, which is higher than that of bare cellulose ( t Na + : 0.31). These outstanding physical properties of the separator enable the long-term operation of NaCrO2 cathode/hard carbon anode full cells in a conventional carbonate electrolyte, with capacity retention of 82% for 500 cycles. Time-of-flight secondary-ion mass spectroscopy analysis reveals the additional role of the Al2O3 coating, which is transformed into AlF3 upon long-term cycling owing to HF scavenging. Our findings will open the door to the use of cellulose-based functional separators for high-performance SIBs.

In this study, we report the synthesis and electrochemical reactions of nanosized SnO2/reduced graphene oxide (rGO), as well as its cell performance that implement full cells coupled with carbon-coated NaCrO2 cathodes. We synthesize nanosized SnO2/rGO composites to mitigate main drawback that conversion and alloy reaction materials suffer from self-pulverization on discharge (reduction). Hydrothermally produced SnO2 nanoparticles are simultaneously attached onto rGO sheets via a self-assembly process, in which rGO sheets provide sufficient electron conduction paths (∼10−3 S cm−1) during electrochemical reactions. As anticipated, this technique results in satisfactory cell performance with help from the effect mentioned above. For the first time, we apply the SnO2/rGO composite materials to a full cell, adopting a carbon-coated NaCrO2 (110 mAh (g-NaCrO2)−1 cathode. The full cell demonstrates an excellent capacity retention, approximately 84% of the initial capacity (88 mAh (g-NaCrO2)−1) for 300 cycles, and is active even at a rate of 10C (1.05 A g−1), delivering 87 mAh (g-NaCrO2)−1. This result demonstrates the feasibility of using carbon-coated NaCrO2//SnO2/rGO sodium-ion cells for energy storage.

Recent lithium-ion battery (LIB) technologies power electric vehicles (EVs) to run approximately 220 miles in a single charge, and further effort to increase the energy density of LIBs is being made to run LIB-mounted EVs up to 300 miles in the next few years. Among several important components of LIBs, cathode materials play a significant role in contributing to cost, safety issues, and more importantly energy density. For this concern, Ni-rich cathode materials are indispensable because of their high capacity, reaching over 200 mAh g−1. To commercialize Ni-rich cathode material, tremendous work has been carried out to stabilize the crystal structure and minimize the side reaction with electrolytes, namely, doping, surface modification from nano- to microscale, densification of secondary particles, morphological alternation of primary particles in a secondary particle, and so on. The approaches that have pursued will be discussed in this chapter followed by a perspective.

The re-heating effect for Ni-rich Li[Ni 0.7 Mn 0.3 ]O 2 is investigated because the process is required in surface modification and removal of adhered water molecules. A representative binary Ni-rich Li[Ni 0.7 Mn 0.3 ]O 2 (in which cationic distribution in Li layers is not affected by heteroelements) is selected and synthesized via co-precipitation. The as-synthesized Ni-rich Li[Ni 0.7 Mn 0.3 ]O 2 is re-heated at 200 °C, 400 °C, and 600 °C, so that the resulting structural and electrochemical properties are compared by means of X-ray diffraction, transmission electron microscopy, time of flight-secondary ion spectroscopy, thermogravimetric analysis, high temperature X-ray diffraction, and electrochemical tests. Raising the re-heating temperature increases the occupancy of Ni 2+ in Li layers and accelerates the aggregation of lithium-related compounds such as Li 2 CO 3 and LiOH towards the particle surface. Among the several conditions tested, re-heating at 200 °C results in a negligible change in the crystal structure; specifically, Ni 2+ occupation in Li layers, higher capacity with good reversibility upon cycling tests, better rate capability, and thermal properties. Therefore, re-heating of cathode active materials, in particular Ni-rich compositions, should be considered to stabilize both electrode performances and thermal properties.

A trace amount of Fe (0.25%) is incorporated in Ni-rich Li[Ni1-x-yCoxMny]O2 cathode active material at an impurity level, Li[Ni0.78Co0.11Mn0.11]Fe0.0023O2 (NCMF), to investigate the effects of Fe on structural and electrochemical properties. The modified structure of NCMF shows a lithium-deficient composition with a reduced cation mixing ratio compared to the bare sample, Li[Ni0.78Co0.11Mn0.11]O2 (NCM), notwithstanding the tendency of Fe3+ ions that easily migrate into Li layers. NCMF electrochemically outperforms NCM including the increase in the initial charge/discharge capacity with higher Coulombic efficiency, cycleability at high charging cut-off voltage, and rate performance with relatively lowered overpotentials. These enhanced electrochemical properties of NCMF are analyzed by in-situ X-ray diffraction and post-mortem high angle annular dark field scanning transmission electron microscopy. Mitigated structural changes in lattice parameters and lattice volume during the second charging are observed in NCMF, and the cation-disordered region of NCMF becomes narrower than that of NCM after cycling. Incidentally, NCMF exhibits a smaller amount of residual lithium compounds than NCM, of which rate performance is inferior to NCMF. Although the improved cycleability of NCMF is consistent with the result of our previous report [Park et al., Electrochim. Acta, 296 (2019) 814–822] adopting NCM111, a different mechanism in Ni-rich NCM is herein proposed.

The thermal stability of electrochemically delithiated bare, silica-coated and silicon phosphate-coated Li0.3[Ni0.7Mn0.3]O2 is studied with in-situ high temperature X-ray diffraction (HT-XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and time of flight-secondary ion spectroscopy (ToF-SIMS). For the three delithiated materials, gradual phase transformation appears in the crystal structure in the temperature range of 25–600 °C: rhombohedral to salt structure via cubic spinel phase, which results from oxygen evolution from the active materials as noticed in TGA. Coating evidently retards the above phase transition toward a high temperature approximately over 40 °C owing to less amount of oxygen release from the crystal structure. This effect appears more prominent in the presence of the silicon phosphate coating layer relative to the silica, presumably due to presence of the Si–P–O covalent character. Also, the surface layer remains up to 600 °C, showing original smooth edges. Therefore, thermal degradation of the active materials is delayed when their surfaces are modified by nanoscale coating layers.

The Ni-rich Li[Ni0.6Co0.2Mn0.2]O2 surface has been modified with H3PO4. After coating at 80 °C, the products were heated further at a moderate temperature of 500 °C in air, when the added H3PO4 transformed to Li3PO4 after reacting with residual LiOH and Li2CO3 on the surface. A thin and uniform smooth nanolayer (< 10 nm) was observed on the surface of Li[Ni0.6Co0.2Mn0.2]O2 as confirmed by transmission electron microscopy (TEM). Time-of-flight secondary ion mass spectroscopic (ToF-SIMS) data exhibit the presence of LiP+, LiPO+, and Li2PO 2 + fragments, indicating the formation of the Li3PO4 coating layer on the surface of the Li[Ni0.6Co0.2Mn0.2]O2. As a result, the amounts of residual lithium compounds, such as LiOH and Li2CO3, are significantly reduced. As a consequence, the Li3PO4-coated Li[Ni0.6Co0.2Mn0.2]O2 exhibits noticeable improvement in capacity retention and rate capability due to the reduction of residual LiOH and Li2CO3. Further investigation of the extensively cycled electrodes by X-ray diffraction (XRD), TEM, and ToF-SIMS demonstrated that the Li3PO4 coating layers have multi-functions: Absorption of water in the electrolyte that lowers the HF level, HF scavenging, and protection of the active materials from deleterious side reactions with the electrolyte during extensive cycling, enabling high capacity retention over 1,000 cycles.

Introduction Sodium-deficient P2 or P′2 type layered materials are known to deliver high capacity with acceptable capacity retention. However, the initial charge capacity is substantially lower than the discharge capacity because of the insufficient amount of sodium in their crystal structure, hindering practical application of these materials as cathodes in sodium-ion batteries (SIBs). This limitation can be overcome by introducing a sacrificial salt additive, which participates in the electrochemical oxidation reaction by releasing enough sodium ions to compensate for the insufficient sodium content in the cathode material. Herein, the sacrificial salt NaNO2 was blended with a high-capacity orthorhombic P′2 type Na2/3[Co0.05Mn0.95]O2 cathode material, increasing the initial charge capacity from 154 to 210 mA h g-1. During electrochemical oxidation, the NaNO2 was oxidatively decomposed by the following reaction: NaNO2 → NO2 + Na+ + e-, where NO2 is an oxidizer that enables full desodiation to Na0[Co0.05Mn0.95]O2. The first coulombic efficiency of Na2/3[Co0.05Mn0.95]O2 was improved from 1.38 to 0.98 by virtue of the sacrificing and oxidizing roles of NaNO2. Experimental P’2-type layered Na2/3[Co0.05Mn0.95]O2 was synthesized via spray pyrolysis. Stoichiometric amounts of manganese nitrate tetra-hydrate, cobalt nitrate hexahydrate, and sodium nitrate were dissolved in distilled water at 25 °C. Citric acid as a chelating agent and sucrose as a particle agglomeration inhibitor were added to the prepared aqueous solution at a molar ratio of the starting material : citric acid : sucrose of 1 : 0.2 : 0.05. The concentration of the starting solution was 0.08 M. The resulting aerosol stream was introduced into a vertical quartz reactor heated to 400 °C. The as-received precursor powders were calcined at 1200 °C for 10 h in a furnace in a dry air atmosphere with a flow rate of 300 mL min-1 and subsequently cooled to 25 °C. NaNO2 was intimately blended with the conducting agents by ball-milling at a weight ratio of 1 : 1, yielding a black-colored powder. The as-synthesized Na2/3[Co0.05Mn0.95]O2 and the mixture of NaNO2 and conducting agents were homogeneously blended in an agate mortar. Results and Discussion The introduction of a sacrificing agent, NaNO2, into sodium-deficient P2- and P’2-type layered cathode materials helped to overcome their intrinsic drawback of low charge capacities. The additive caused the release of additional sodium ions during electrochemical oxidation, which, in turn, provided additional capacity upon charging, bringing the first charge capacity closer to the discharge capacity. As a result, a high energy density of approximately 300 Wh kg-1 (calculated based on the cathode material) was achieved for the NaNO2/Na2/3[Co0.05Mn0.95]O2//hard carbon full cell. Supplementary experimental studies revealed that additional sodium ions were intercalated into the hard carbon. Hence, the high-capacity P’2 layered cathode can be used as a cathode material for high-energy-density SIBs. Details will be mentioned at the conference site. Figure 1

In order to confirm reasons that deteriorate cathode performances, Ni-rich Li[Ni0.7Mn0.3]O2 is modified by lithium isopropoxide to artificially provide lithium excess environment by forming Li2O on the surface of active materials. X-ray diffraction patterns indicate that the lithium oxide coating does not affect structural change comparing to the bare material. Scanning electron microscopy and transmission electron microscopy data show the presence of coating layers on the surface of Li[Ni0.7Mn0.3]O2. Electrochemical tests demonstrate that the Li2O-coated Li[Ni0.7Mn0.3]O2 exhibits a greater irreversible capacity with a small capacity because of the presence of insulating layers composed of lithium compounds on the active materials since these layers delay facile Li+ diffusion. Also, the Li2O layer forms byproducts such as Li2CO3, LiOH, and LiF, as are proved by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. The presence of residual lithium tends to bond with hydrocarbons induced from decomposition of electrolytic salt during electrochemical reactions. And the reaction, accelerated by the decomposition of electrolytic salt that produces the byproducts, causes the formation of passive layers on the surface of active material. As a result, the new layers consequently impede diffusion of lithium ions that deteriorate electrochemical properties.

P2-Cathode materials for sodium battery are usually active in the range of 2–4.3 V, but the decomposition of the electrolytic salt above 4 V versus Na+/Na is common. Arguably, the one of the concerns is the formation of HF after the reaction of the salts with water molecules, which are present as an impurity in the electrolyte. This HF ceaselessly attacks the active materials and gradually causes the failure of the electrode via electric isolation of the active materials. In this study, we report a bio-inspired b-NaCaPO4 nanolayer on a P2-type layered Na2/3[Ni1/3Mn2/3]O2 cathode material. The coating layers successfully scavenge HF and H2O, and excellent capacity retention was achieved with the b-NaCaPO4-coated Na2/3[Ni1/3 Mn2/3]O2 electrode. This retention was possible because a less acidic environment was produced in the Na cells during prolonged cycling. The intrinsic stability of the coating layer also assisted in delaying the exothermic decomposition reaction of the de-sodiated electrodes. We suggest formation and reaction mechanisms for the coating layers responsible for the excellent electrode performance. The suggested technology is promising for use with cathode materials in rechargeable sodium batteries to mitigate the effects of acidic conditions in Na cells. P2-type layered Na2/3[Ni1/3Mn2/3]O2 powders were synthesized by a conventional solid-state method. Na2CO3, Mn2O3, and NiO were thoroughly mixed. And then it was calcined at 1,000 °C for 12 h in air and then quenched, after which the pellet was immediately transferred into an Ar-filled glove box to minimize the adsorption of water and exposure to the CO2 in the air. Calcium nitrate and phosphoric acid were first dissolved in anhydrous ethanol at room temperature, and the as-synthesized active materials were added slowly to the solution. Then, the solution containing the active material was stirred continuously at 80 °C in a dry room, accompanied by the slow evaporation of the solvent. The as-prepared and bare Na2/3[Ni1/3Mn2/3]O2 powders were heated at 700 °C for 2 h in air. Also, the coated powders were characterized by XRD, SEM, and HR-TEM. Electrochemical properties of coated powders were examined by galvanostatic cycle test and electrochemical impedance spectroscopy. b-NaCaPO4 coatings shows uniform coating layers, as observed by TEM. Thickness of coating layer is of about 10nm. Electrochemical test with half cells in voltage range of 2.5 - 4.3 V at 25 oC indicates that coated materials have better capacity retention and coulombic efficiency, rate capability, and low resistance. Thin coating layers seem to effective to improve the electrochemical properties. Also, b-NaCaPO4 coatings give decrease of residual sodium and byproduct on the surface of particle, as confirmed by ToF-SIMS. Details will be mentioned at the conference site. Figure 1

In the search for high-capacity anode materials, a facile hydrothermal route has been developed to synthesize phase-pure NiC2O4 center dot 2H(2)O nanorods, which were crystallized into the orthorhombic structure without using templates. To ensure the electrical conductivity of the nanorods, the produced NiC2O4 center dot 2H(2)O nanorods were attached to reduced graphene oxide (rGO) sheets via self-assembly layer-by-layer processes that utilize the electrostatic adsorption that occurs in a poly(diallyldimethylammonium chloride) solution. The high electrical conductivity aided by the presence of rGO significantly improved the electrochemical properties: 933 mAh g(-1) for the charge capacity (oxidation), which showed 87.5% efficiency at the first cycle with a retention of approximately 85% for 100 cycles, and 586 mAh g(-1) at 10 C-rates (10 A g(-1)) for the NiC2O4 center dot 2H(2)O/rGO electrode. The lithium storage processes were involved in the conversion reaction, which were fairly reversible via a transformation to Ni metal accompanied by the formation of a lithium oxalate compound upon discharge (reduction) and restoration to the original NiC2O4 center dot 2H(2)O upon charging (oxidation); this was confirmed via X-ray diffraction, transmission electron microscopy, X-ray photoelectron microscopy and time-of-flight secondary ion mass spectroscopy. We believe that the high rate capacity and rechargeability upon cycling are the result of the unique features of the highly crystalline NiC2O4 center dot 2H(2)O nanorods assisted by conducting rGOs.

In the search for high capacity anode materials, a facile hydrothermal route has been developed to synthesize phase-pure NiC2O4·2H2O nanorods, which were crystallized to orthorhombic structure without using templates. To ensure the electric conductivity of the nanorods, the produced NiC2O4·2H2O nanorods were attached to reduced graphene oxide sheets via self-assembly layer-by-layer processes utilizing the electrostatic adsorption that occurs in a poly(diallyldimethylammonium chloride) solution. High electric conductivity aided by the presence of reduced graphene oxide (rGO) significantly improved the electrochemical properties: 933 mAh g-1 for the charge capacity (oxidation), which showed 87.5% efficiency at the first cycle with its retention approximately 85% for 100 cycles, and 586 mAh g-1 at 10 C-rates (10 A g-1) for the NiC2O4·2H2O/rGO electrode were measured. We determined the details of the lithium storage processes involved with the conversion reaction, which were fairly reversible via a transformation to Ni metal accompanied by the formation of a lithium oxalate compound on discharge (reduction) and restoration to the original NiC2O4·2H2O on charge (oxidation); this was confirmed via X-ray diffraction, transmission electron microscopy, X-ray photoelectron microscopy, and time-of-flight secondary ion mass spectroscopy. We believe that the high rate capacity and rechargeability upon cycling are a result of the unique features of the highly crystalline NiC2O4·2H2O nanorods assisted by conducting rGOs.

Lithium secondary batteries have played important roles as energy storage and conversion devices EV(electric vehicle) and ESS(energy storage system). In this respect, improvement of electrodes is being progressed towards ensure long cycling performances, stability, safety, and so on. For anode materials, in particular, conversion reaction materials need to be further developed because alloy and conversion reaction enabled high capacity delivery though the cyclability should be improved. However, severe volume expansion impedes the utilization of those electrode materials in commercial batteries. Tin oxide is composited with rGO to ensure structural and cycle stability in both lithium and sodium systems. In this study, we synthesized SnO2 nanoparticle was anchored on rGO sheet by layer by layer self-assembly method. For synthesize SnO2 nanoparticle, we used hydrothermal method. To confirm the physical and electrochemical properties, XRD, SEM, TEM, RAMAN, galvanostatic test. The synthesized SnO2 and rGO/SnO2 had same crystal structure and particle size was about 10-20nm. Both materials exhibited high charge capacity of about 1000mAh/g and much improved rate capability was achieved by rGO sheets in Li cell. Similarly, much improved electrochemical properties were found for SnO2/rGO composite in Na cells as well. It is thought that rGO sheet assisted electro chemical stability by their 2D network during repetitive volume expansion.

Lithium secondary battery is promising as a power backup for energy storage system. In this respect, studies towards improvement in performances of electrode are being progressed to assure long cycling performances, thermal stability, safety, and so on. In particular, positive electrode materials are need to be further investigated to accomplish the above- mentioned properties. Several kinds of 4 V class positive electrode materials are commercially available. Among them, LiCoO2 is one of the most common electrode materials in rechargeable lithium batteries. Although the LiCoO2 has many advantages, this material has poor capacity retention because of dissolution of Co and structural instability at deeply charged state. To improve the nature of active material, surface modification is effective to overcome those limited properties of active materials. Actually, Metal oxides as coating materials have been extensively studied. In this study, Metal phosphate coatings are applied on the surface of Li[Ni0.7Co0.2Mn0.1]O2. We, here, report the resulting structural, electrochemical and thermal properties of the surface-modified Li[Ni0.7Co0.2Mn0.1]O2. H3PO4 and metal salts were selected as starting materials for surface modification of Li[Ni0.7Co0.2 Mn0.1]O2. A solution anhydrous ethanol containing metal phosphates was stirred at 30 oC for 5 hours. Then, active materials were added into the solution. And the solution was evaporized at 80 oC in air and the resulting precipitates were then heated at 500 oC in air. Also, the coated powders were characterized by XRD, SEM, and HR-TEM. Electrochemical properties of coated powders were examined by galvanostatic cycle test and electrochemical impedance spectroscopy. Metal phosphates coatings shows uniform coating layers, as observed by TEM. TG and DSC analyses were conducted for the materials. Electrochemical test with half cells in voltage range of 3 - 4.3 V at 25 oC indicates that coated materials have better capacity retention and coulombic efficiency, rate capability and thermal properties. Thin coating layers were effective in improvement of the electrochemical, structural and thermal properties. Also, metal phosphates coatings give decrease in the residual lithium content and byproduct on the surfaces of particles, as confirmed by ToF-SIMS. And the materials were analyzed into XPS and ToF-SIMS to confirm surface byproducts of the materials itself at high temperature. Details will be mentioned at the conference site.

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