Phytic Acid Customized Hydrogel Polymer Electrolyte and Prussian Blue Analogue Cathode Material for Rechargeable Zinc Metal Hydrogel Batteries

Zinc anode deterioration in aqueous electrolytes, and Zn dendrite growth is a major concern in the operation of aqueous rechargeable Zn metal batteries (AZMBs). To tackle this, the replacement of aqueous electrolytes with a zinc hydrogel polymer electrolyte (ZHPE) is presented in this study. This method involves structural modifications of the ZHPE by phytic acid through an ultraviolet (UV) light-induced photopolymerization process. The high membrane flexibility, high ionic conductivity (0.085 S cm-1), improved zinc corrosion overpotential, and enhanced electrochemical stability value of ≈2.3 V versus Zn|Zn2+ show the great potential of ZHPE as an ideal gel electrolyte for rechargeable zinc metal hydrogel batteries (ZMHBs). This is the first time that the dominating effect of chelation of phytic acid with M2+ center over H-bonding with water is described to tune the gel electrolyte properties for battery applications. The ZHPE shows ultra-high stability over 360 h with a capacity of 0.50 mAh cm-2 with dendrite-free plating/stripping in Zn||Zn symmetric cell. The fabrication of the ZMHB with a high-voltage zinc hexacyanoferrate (ZHF) cathode shows a high-average voltage of ≈1.6 V and a comparable capacity output of 63 mAh g-1 at 0.10 A g-1 of the current rate validating the potential application of ZHPE.

Asymmetric Lithium Extraction and Insertion in High Voltage Spinel at Fast Rate

Spinel-structured ordered-LiNi0.5Mn1.5O4 (o-LNMO) has experienced a resurgence of interest in the context of reducing scarce elements such as cobalt from the lithium-ion batteries. O-LNMO undergoes two two-phase reactions at slow rates. However, it is not known if such phenomenon also applies at fast rates. Herein, we investigate the rate-dependent phase transition behavior of o-LNMO through in operando time-resolved X-ray diffraction. The results indicate that a narrow region of the solid solution reaction exists for charge and discharge at both slow and fast rates. The overall phase transition is highly asymmetric at fast rates. During fast charge, it is a particle-by-particle mechanism resulting from an asynchronized reaction among the particles. During fast discharge, it is likely a core-shell mechanism involving transition from Li0+xNi0.5Mn1.5O4 to Li1+xNi0.5Mn1.5O4 in the outer layer of particles. The Li0.5Ni0.5Mn1.5O4 phase is suppressed during fast discharge and appears only through Li redistribution upon relaxation.

Elevating Energy Density for Sodium-Ion Batteries through Multielectron Reactions

It remains a great challenge to explore desirable cathodes for sodium-ion batteries to satisfy the ever-increasing demand for large-scale energy storage systems. In this Letter, we report a NASICON-structured Na₄MnCr(PO₄)₃ cathode with high specific capacity and operation potential. The reversible access of the Mn²⁺/Mn³⁺ (3.75/3.4 V), Mn³⁺/Mn⁴⁺ (4.25/4.1 V), and Cr³⁺/Cr⁴⁺ (4.4/4.3 V vs Na/Na⁺) redox couples in a Na₄MnCr(PO₄)₃ cathode endows a distinct three-electron redox reaction during the insertion/extraction process. The highly stable NASICON structure with a small volume variation upon cycling ensures long-time cycling stability (73.3% capacity retention after 500 cycles within the potential region of 2.5-4.6 V). The impedance analysis and interface characterization indicate that the evolution of a cathode electrolyte interphase at high potential is correlated with the capacity fading, while the robustness of the NASICON framework is redemonstrated.

Co-Substitution Enhances the Rate Capability and Stabilizes the Cyclic Performance of O3-Type Cathode NaNi0.45- xMn0.25Ti0.3Co xO2 for Sodium-Ion Storage at High Voltage

O3-type NaNiO₂-based cathode materials suffer irreversible phase transition when they are charged to above 4.0 V in sodium-ion batteries. To solve this problem, we partially substitute Ni²⁺ in O3-type NaNi_0.45Mn_0.25Ti_0.3O₂ by Co³⁺. NaNi_0.45Mn_0.25Ti_0.3O₂ with co-substitution possesses an expanded interlayer and exhibits higher rate capability, as well as cyclic stability, compared with the pristine cathode in 2.0-4.4 V. The optimal NaNi_0.4Mn_0.25Ti_0.3Co_0.05O₂ delivers discharge capacities of 180 and 80 mA  h  g^-1 at 10 and 1000 mA g^-1. At 100 mA g^-1, NaNi_0.4Mn_0.25Ti_0.3Co_0.05O₂ exhibits 152 mA h g^-1 in the initial cycle and maintains 91.4 mA h g^-1 after 180 cycles. Through ex situ X-ray diffraction, co-substitution is demonstrated to be effective in enhancing the reversibility of P3-P3″ phase transition from 4.0 to 4.4 V. Electrochemical impedance spectroscopy indicates that higher electronic conductivity is achieved by co-substitution. Moreover, cyclic voltammetry and the galvanostatic intermittent titration technique demonstrate faster kinetics for Na⁺ diffusion due to the co-substitution. This study provides a reference for further improvement of electrochemical performance of cathode materials for high-voltage sodium-ion batteries.