KTi2 (PO4 )3 Electrode with a Long Cycling Stability for Potassium-Ion Batteries.

In this work, rhombohedral KTi2 (PO4 )3 is introduced to investigate the related theoretical, structural, and electrochemical properties in K cells. The suggested KTi2 (PO4 )3 modified by electro-conducting carbon brings about a flat voltage profile at ≈1.6 V, providing a large capacity of 126 mAh (g-phosphate)-1 , corresponding to 98.5% of the theoretical capacity, with 89% capacity retention for 500 cycles. Structural analyses using electrochemical performance measurements, first-principles calculations, ex situ X-ray absorption spectroscopy, and operando X-ray diffraction provide new insights into the reaction mechanism controlling the (de)intercalation of potassium ions into the host KTi2 (PO4 )3 structure. It is observed that a biphasic redox process by Ti4+/3+ occurs upon discharge, whereas a single-phase reaction followed by a biphasic process occurs upon charge. Along with the structural refinement of the electrochemically reduced K3 Ti2 (PO4 )3 phase, these new findings provide insight into the reaction mechanism in Na superionic conductor (NASICON)-type KTi2 (PO4 )3 . The present approach can also be extended to the investigation of other NASICON-type materials for potassium-ion batteries.

Nature-Derived Cellulose-Based Composite Separator for Sodium-Ion Batteries.

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.

Layered K0.28MnO2·0.15H2O as a Cathode Material for Potassium-Ion Intercalation.

Here, we present K0.28MnO2·0.15H2O, which has a two-dimensional open framework, as an intercalation host for potassium ions. K0.28MnO2·0.15H2O has a layered structure consisting of edge-sharing MnO6 octahedra with a large basal spacing of ∼7.3 Å, which facilitates K+-ion mobility. Water molecules in the interlayers between the MnO2 layers play an important role as a pillar to support the structure during repetitive de/potassiation cycles, as confirmed by an operando X-ray diffraction study. As a result, the large K+ ions readily migrate into the crystal structure, resulting in satisfactory electrochemical performance in K-cells. With the aid of the structural pillar, the K0.28MnO2·0.15H2O cathode delivers a high reversible capacity of 150 mA h g-1 over 100 cycles at a rate of 0.1 C (15 mA g-1), with acceptable power capability up to 5 C-rates.

The Conversion Chemistry for High-Energy Cathodes of Rechargeable Sodium Batteries.

Herein, the Cu2P2O7/carbon-nanotube nanocomposite is reported as a cathode material based on a conversion reaction for rechargeable sodium batteries (RSBs). The nanocomposite electrode exhibits the large capacity of 355 mAh g-1, which is consistent with the 4 mol Na+ storage per formula unit determined by first-principles calculation. Its average operation voltage is approximately 2.4 V (vs Na+/Na). Even at 1800 mA g-1, a capacity of 223 mAh g-1 is maintained. Moreover, the composite electrode exhibits acceptable capacity retention of over 75% of the initial capacity for 300 cycles at 360 mA g-1. The overall conversion reaction mechanism on the Cu2P2O7/carbon-nanotube nanocomposite is determined to be Cu2P2O7 + 4Na+ + 4e- → 2Cu + Na4P2O7 based on operando/ex situ structural and physicochemical analyses. The high energy density of the Cu2P2O7/carbon-nanotube nanocomposite (720 Wh kg-1) supported by this conversion chemistry indicates a high possibility of application of this material as a promising cathode candidate for RSBs.

Cycling Stability of Layered Potassium Manganese Oxide in Nonaqueous Potassium Cells.

Potassium-ion batteries have emerged as an alternative to lithium-ion batteries as energy storage systems. In particular, KxMnO2 has attracted considerable attention as a cathode material because of its high theoretical capacity and low cost. In this study, partial substitution of Mn in P3-type K0.5MnO2 with divalent Ni is performed, resulting in a first discharge capacity of approximately 121 mAh (g-oxide)-1 with 82% retention for 100 cycles. Operando synchrotron X-ray diffraction analysis reveals the occurrence of phase transition from P3 to O3 on charge and O3-P3-P'3 transition on discharge at the first cycle, where P'3 is a new distorted form of the P3 phase, accompanied by reversible Mn4+/3+ and Ni3+/2+ redox pairs, as evidenced by X-ray absorption spectroscopy. The reduced variation in the lattice parameters during de/potassiation for P3-K0.5[Ni0.1Mn0.9]O2 relative to P3-K0.5MnO2 is suggested as a possible reason for the enhanced electrochemical performance of K0.5[Ni0.1Mn0.9]O2. These results open the possibility of using inexpensive and high-capacity Mn-based cathode active materials for potassium-ion batteries.

New Insight into Ethylenediaminetetraacetic Acid Tetrasodium Salt as a Sacrificing Sodium Ion Source for Sodium-Deficient Cathode Materials for Full Cells.

Sacrificing sodium supply sources is needed for sodium-deficient cathode materials to achieve commercialization of sodium-ion full cells using sodium-ion intercalation anode materials. Herein, the potential of ethylenediaminetetraacetic acid tetrasodium salt (EDTA-4Na) as a sacrificing sodium supply source was investigated by intimately blending it with sodium-deficient P2-type Na0.67[Al0.05Mn0.95]O2. The EDTA-4Na/Na0.67[Al0.05Mn0.95]O2 composite electrode unexpectedly exhibited an improved charge capacity of 177 mA h (g-oxide)-1 compared with the low charge capacity of 83 mA h (g-oxide)-1 for bare Na0.67[Al0.05Mn0.95]O2. The reversible capacity of an EDTA-4Na/Na0.67[Al0.05Mn0.95]O2//hard carbon full-cell system increased to 152 mA h (g-oxide)-1 at the first discharge with a Coulombic efficiency of 89%, whereas the Na0.67[Al0.05Mn0.95]O2 without EDTA-4Na delivered a discharge capacity 51 mA h g-1 because of the small charge capacity. The EDTA-4Na sacrificed itself to generate Na+ ions via oxidative decomposition by releasing four sodium ions and producing C3N as a decomposition resultant on charge. It is thought that the slight increase in discharge capacity is associated with the electroconducting nature of the C3N deposits formed on the surface of the Na0.67[Al0.05Mn0.95]O2 electrode. We elucidated the reaction mechanism and sacrificial activity of EDTA-4Na, and our findings suggest that the addition of EDTA-4Na is beneficial as an additional source of Na+ ions that contribute to the charge capacity.

Unraveling the Role of Earth-Abundant Fe in the Suppression of Jahn-Teller Distortion of P'2-Type Na2/3MnO2: Experimental and Theoretical Studies.

Layered Na2/3MnO2 suffers from capacity loss due to Jahn-Teller (J-T) distortion by Mn3+ ions. Herein, density functional theory calculations suggest Na2/3[Fe xMn1- x]O2 suppresses the J-T effect. The Fe substitution results in a decreased oxygen-metal-oxygen length, leading to decreases in the b and c lattice parameters but an increase in the a lattice constant. As a result, the capacity retention and rate capability are enhanced with an additional redox pair associated with Fe4+/3+. Finally, the thermal properties are improved, with the Fe substitution delaying the exothermic reaction and reducing exothermic heat.

Synthesis and Electrochemical Reaction of Tin Oxalate-Reduced Graphene Oxide Composite Anode for Rechargeable Lithium Batteries.

Unlike for SnO2, few studies have reported on the use of SnC2O4 as an anode material for rechargeable lithium batteries. Here, we first introduce a SnC2O4-reduced graphene oxide composite produced via hydrothermal reactions followed by a layer-by-layer self-assembly process. The addition of rGO increased the electric conductivity up to ∼10-3 S cm-1. As a result, the SnC2O4-reduced graphene oxide electrode exhibited a high charge (oxidation) capacity of ∼1166 mAh g-1 at a current of 100 mA g-1 (0.1 C-rate) with a good retention delivering approximately 620 mAh g-1 at the 200th cycle. Even at a rate of 10 C (10 A g-1), the composite electrode was able to obtain a charge capacity of 467 mAh g-1. In contrast, the bare SnC2O4 had inferior electrochemical properties relative to those of the SnC2O4-reduced graphene oxide composite: ∼643 mAh g-1 at the first charge, retaining 192 mAh g-1 at the 200th cycle and 289 mAh g-1 at 10 C. This improvement in electrochemical properties is most likely due to the improvement in electric conductivity, which enables facile electron transfer via simultaneous conversion above 0.75 V and de/alloy reactions below 0.75 V: SnC2O4 + 2Li+ + 2e- → Sn + Li2C2O4 + xLi+ + xe- → LixSn on discharge (reduction) and vice versa on charge. This was confirmed by systematic studies of ex situ X-ray diffraction, transmission electron microscopy, and time-of-flight secondary-ion mass spectroscopy.