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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 NiC2O4·2H2O 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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