Electrochemical characterization of polypyrrole–LiNi1/3Mn1/3Co1/3O2 composite cathode material for aqueous rechargeable lithium batteries

Interfacial Chemistry in Aqueous Lithium-Ion Batteries: A Case Study of V2O5 in Dilute Aqueous Electrolytes

Aqueous lithium-ion batteries (ALIBs) are promising for large-scale energy storage systems because of the cost-effective, intrinsically safe, and environmentally friendly properties of aqueous electrolytes. Practical application is however impeded by interfacial side-reactions and the narrow electrochemical stability window (ESW) of aqueous electrolytes. Even though higher electrolyte salt concentrations (e.g., water-in-salt electrolyte) enhance performance by widening the ESW, the nature and extent of side-reaction processes are debated and more fundamental understanding thereof is needed. Herein, the interfacial chemistry of one of the most popular electrode materials, V2O5, for aqueous batteries is systematically explored by a unique set of operando analytical techniques. By monitoring electrode/electrolyte interphase deposition, electrolyte pH, and gas evolution, the highly dynamic formation/dissolution of V2O5/V2O4, Li2CO3 and LiF during dis-/charge is demonstrated and shown to be coupled with electrolyte decomposition and conductive carbon oxidation, regardless of electrolyte salt concentration. The study provides deeper understanding of interfacial chemistry of active materials under variable proton activity in aqueous electrolytes, hence guiding the design of more effective electrode/electrolyte interfaces for ALIBs and beyond.

Layered transition metal oxides (LTMO) for oxygen evolution reactions and aqueous Li-ion batteries

This perspective paper comprehensively explores recent electrochemical studies on layered transition metal oxides (LTMO) in aqueous media and specifically encompasses two topics: catalysis of the oxygen evolution reaction (OER) and cathodes of aqueous lithium-ion batteries (LiBs). They involve conflicting requirements; OER catalysts aim to facilitate water dissociation, while for cathodes in aqueous LiBs it is essential to suppress water dissociation. The interfacial reactions taking place at the LTMO in these two distinct systems are of particular significance. We show various strategies for designing LTMO materials for each desired aim based on an in-depth understanding of electrochemical interfacial reactions. This paper sheds light on how regulating the LTMO interface can contribute to efficient water splitting and economical energy storage, all with a single material.

Voltage issue of aqueous rechargeable metal-ion batteries

Over the past two decades, a series of aqueous rechargeable metal-ion batteries (ARMBs) have been developed, aiming at improving safety, environmental friendliness and cost-efficiency in fields of consumer electronics, electric vehicles and grid-scale energy storage. However, the notable gap between ARMBs and their organic counterparts in energy density directly hinders their practical applications, making it difficult to replace current widely-used organic lithium-ion batteries. Basically, this huge gap in energy density originates from cell voltage, as the narrow electrochemical stability window of aqueous electrolytes substantially confines the choice of electrode materials. This review highlights various ARMBs with focuses on their voltage characteristics and strategies that can effectively raise battery voltage. It begins with the discussion on the fundamental factor that limits the voltage of ARMBs, i.e., electrochemical stability window of aqueous electrolytes, which decides the maximum-allowed potential difference between cathode and anode. The following section introduces various ARMB systems and compares their voltage characteristics in midpoint voltage and plateau voltage, in relation to respective electrode materials. Subsequently, various strategies paving the way to high-voltage ARMBs are summarized, with corresponding advancements highlighted. The final section presents potential directions for further improvements and future perspectives of this thriving field.

Conducting Polymer Coating on a High-Voltage Cathode Based on Soft Chemistry Approach toward Improving Battery Performance

The surface of a 5 V class LiNi_0.5Mn_1.5O₄ particle is modified with poly(3,4-ethylenedioxythiophene) (PEDOT) conducting polymer by utilizing the hydrophobic characteristics of the 3,4-ethylenedioxythiophene (EDOT) monomer and the tail group of cetyl trimethyl ammonium bromide (CTAB) surfactants, in addition to the electrostatic attraction between cationic CTAB surfactant and cathode materials with a negative ζ potential in aqueous solution. With this novel concept, we design and prepare a uniform EDOT monomer layer on the cathode materials, and chemical polymerization of the EDOT coating layer is then carried out to achieve PEDOT-coated cathode materials via a simple one-pot preparation process. This uniform conducting polymer layer provides notable improvement in the power characteristics of electrodes, and stable electrochemical performance can be obtained especially at severe operating conditions such as the fully charged state and elevated temperatures owing to the successful suppression of the side reaction between the oxide particle and the electrolyte as well as the suppression of Mn dissolution from the oxide material.

Ni-Rich LiNi0.8Co0.1Mn0.1O2 Oxide Coated by Dual-Conductive Layers as High Performance Cathode Material for Lithium-Ion Batteries

Ni-rich materials are appealing to replace LiCoO₂ as cathodes in Li-ion batteries due to their low cost and high capacity. However, there are also some disadvantages for Ni-rich cathode materials such as poor cycling and rate performance, especially under high voltage. Here, we demonstrate the effect of dual-conductive layers composed of Li₃PO₄ and PPy for layered Ni-rich cathode material. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy show that the coating layers are composed of Li₃PO₄ and PPy. (NH₄)₂HPO₄ transformed to Li₃PO₄ after reacting with surface lithium residuals and formed an inhomogeneous coating layer which would remarkably improve the ionic conductivity of the cathode materials and reduce the generation of HF. The PPy layer could form a uniform film which can make up for the Li₃PO₄ coating defects and enhance the electronic conductivity. The stretchy PPy capsule shell can reduce the generation of internal cracks by resisting the internal pressure as well. Thus, ionic and electronic conductivity, as well as surface structure stability have been enhanced after the modification. The electrochemistry tests show that the modified cathodes exhibited much improved cycling stability and rate capability. The capacity retention of the modified cathode material is 95.1% at 0.1 C after 50 cycles, whereas the bare sample is only 86%, and performs 159.7 mAh/g at 10 C compared with 125.7 mAh/g for the bare. This effective design strategy can be utilized to enhance the cycle stability and rate performance of other layered cathode materials.