Potassium Fluoride and Carbonate Lead to Cell Failure in Potassium-Ion Batteries

Recent advances in rational design for high-performance potassium-ion batteries

The growing global energy demand necessitates the development of renewable energy solutions to mitigate greenhouse gas emissions and air pollution. To efficiently utilize renewable yet intermittent energy sources such as solar and wind power, there is a critical need for large-scale energy storage systems (EES) with high electrochemical performance. While lithium-ion batteries (LIBs) have been successfully used for EES, the surging demand and price, coupled with limited supply of crucial metals like lithium and cobalt, raised concerns about future sustainability. In this context, potassium-ion batteries (PIBs) have emerged as promising alternatives to commercial LIBs. Leveraging the low cost of potassium resources, abundant natural reserves, and the similar chemical properties of lithium and potassium, PIBs exhibit excellent potassium ion transport kinetics in electrolytes. This review starts from the fundamental principles and structural regulation of PIBs, offering a comprehensive overview of their current research status. It covers cathode materials, anode materials, electrolytes, binders, and separators, combining insights from full battery performance, degradation mechanisms, in situ/ex situ characterization, and theoretical calculations. We anticipate that this review will inspire greater interest in the development of high-efficiency PIBs and pave the way for their future commercial applications.

In Situ Ions Induced Formation of KxF-Rich SEI Layers toward Ultrastable Life of Potassium-Ion Batteries

Engineering F-rich solid electrolyte interphase (SEI) layers is regarded as an effective strategy to enable the long-term cycling stability of potassium-ion batteries (KIBs). However, in the conventional KPF6 carbonate electrolytes, it is challenging to form F-containing SEI layers due to the inability of KPF6 to decompose into KxF. Herein, AlCl3 is employed as a novel additive to change the chemical environment of the KPF6 carbonate electrolyte. First, due to the large charge-to-radius ratio of Al3+, the Al-containing groups in the electrolyte can easily capture F from PF6 - and accelerate the formation of KxF in SEI layer. In addition, AlCl3 also reacts with trace H2O or solvents in the electrolytes to form Al2O3, which can further act as a HF scavenger. Upon incorporating AlCl3 into conventional KPF6 carbonate electrolyte, the hard carbon (HC) anode exhibits an ultra-long lifespan of 10000 cycles with a high coulombic efficiency of ≈100%. When coupled with perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), the full cell exhibits a high capacity retention of 81% after 360 cycles-significantly outperforming cells using conventional electrolytes. This research paves new avenues for advancing electrolyte engineering towards developing durable batteries tailored for large-scale energy storage applications.

Electrolyte Design Strategies for Non-Aqueous High-Voltage Potassium-Based Batteries

High-voltage potassium-based batteries are promising alternatives for lithium-ion batteries as next-generation energy storage devices. The stability and reversibility of such systems depend largely on the properties of the corresponding electrolytes. This review first presents major challenges for high-voltage electrolytes, such as electrolyte decomposition, parasitic side reactions, and current collector corrosion. Then, the state-of-the-art modification strategies for traditional ester and ether-based organic electrolytes are scrutinized and discussed, including high concentration, localized high concentration/weakly solvating strategy, multi-ion strategy, and addition of high-voltage additives. Besides, research advances of other promising electrolyte systems, such as potassium-based ionic liquids and solid-state-electrolytes are also summarized. Finally, prospective future research directions are proposed to further enhance the oxidative stability and non-corrosiveness of electrolytes for high-voltage potassium batteries.

Less is More: Trace Amount of a Cyclic Sulfate Electrolyte Additive Enable Ultra-Stable Graphite Anode for High-Performance Potassium-Ion Batteries

Graphite can be successfully used as an anode for potassium-ion batteries (PIBs), while its conversion to KC₈ leads to huge volume expansion, destruction of solid electrolyte interphase (SEI), and thus poor cycling stability. Incorporating additives into electrolytes is an economical and effective way to construct robust SEI for high-performance PIBs. Herein, we developed a series of sulfur-containing additives for PIB graphite anodes, and the impacts of their molecular structure and contents on the SEI are also systematically investigated. Compared with butylene sulfites and 1,3-propane sultone, the 1,3,2-dioxathiolane 2,2-dioxide (DTD) additive endows the graphite electrode (GE) with a higher reversible capacity, and better cycling stability in both the dilute potassium bis(fluorosulfonyl)imide (KFSI)- and potassium hexafluorophosphate (KPF₆)-based carbonate electrolyte, as a result of a thinner and sulfate-enriched SEI. Moreover, the addition of a trace amount (0.2 wt %) DTD to the electrolyte can effectively protect the GE running over 800 cycles at 1 C. Excessive additives in the electrolyte will induce continuous SEI growth and render a rapid capacity fading of the GE. This strategy using the electrolyte additive paves the way for the design of novel PIB electrolytes and thus provides a great opportunity for commercial PIBs.

The Quest for Stable Potassium-Ion Battery Chemistry

Potassium-ion batteries (KIBs) have attracted wide interest for energy storage because of the abundance of the electrode materials involved; however, their electrochemical performances are far behind what can be achieved from lithium-ion batteries (LIBs) or sodium-ion batteries (SIBs). Herein, key promising electrode and electrolyte materials for potassium-ion batteries are identified, the coupled electrochemical reactions in the cell are investigated, and the compatibility between different materials is demonstrated to play the most important role. K₂ Mn[Fe(CN)₆ ] cathode can deliver a high capacity of ≈125 mAh g^-1 and exceptional cycling stability over 61 000 cycles (≈9 months) if the side reactions from the anode can be prevented. Graphite is a good anode material but is subjected to degradation in traditional carbonate electrolytes. New concentrated electrolytes are developed and evaluated. A stable KIB system is demonstrated by coupling a stable K₂ Mn[Fe(CN)₆ ] cathode, a prepotassiated graphite anode with a concentrated electrolyte to achieve a high energy density of ≈260 Wh kg^-1 (based on the active mass of cathode and anode) and good cycling of over 1000 cycles.