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Phosphorous-Based Materials for K-Ion Batteries

Maryam Meshksar, Fatemeh Afshariani, and Mohammad Reza Rahimpour*

School of Chemical and Petroleum Engineering, Shiraz University, Mollasadra Ave., Shiraz, Iran

Abstract

Due to the fact that potassium-ion batteries (KIBs) have the advantageous properties of lithium-ion batteries (LIBs) and sodium-ion batteries (NIBs) without having their individual drawbacks, they meet fast growing interest recently. One of the major challenges in KIBs, is using a suitable materials as anodes and cathodes in which large size of potassium-ions can accommodate with reasonable voltage, capacity, life cycle, kinetics, cost, etc. Phosphorous as an anode material which operates via alloying/dealloying reaction mechanism is well known for LIBs and NIBs. Phosphorous is also attractive for KIBs because of its high energy density compared with C-based anodes, based on K3P formation with 2596 mA h/g theoretical capacity. However, low electrical conductivity, large volume expansion and extreme electrolyte decomposition as a result of high reactivity of phosphide surface, causes poor performance of KIBs. For overcoming this issue, P alloy with other elements like Sn4P3 or incorporated P elements in carbon matrix should be applied as electrode material.

Keywords: Potassium-ion batteries, cathode, phosphorous materials, battery, anode

1.1 Introduction

As the consumption of fossil fuels causes serious concerns due to their environmental impacts on human, environment and animal life and also their rinsing costs, the utilization of renewable and clean energy sources like water, wind, and solar energies has become a growing interest. Therefore, there is a strong demand for sustainable and environmentally friendly energy storage devices for storing and utilizing these intermittent and renewable energy sources efficiently. The use of rechargeable batteries as a portable energy devices is became an interesting method due to their adjustable shapes and sizes, high energy efficiencies and densities, pollution-free operations and long-cycle life [1, 2].

As lithium has the lowest standard hydrogen potential compared with other alkaline elements (-3.04 V vs. Eº) which lead to its highest energy density, It was attracted more attention to be used in alkaline-ion batteries [3]. Lithium-ion batteries (LIBs) are first became commercialized by Sony in 1991 for applying in portable electronic devices including mobile phones, laptops, and cameras [4]. Since the demand of LIBs is increasing in the last decades and the Li+ reserves are limited in the earth (only 20 ppm lithium is present within the crust of earth), lithium costs is becoming increased which is a main concern of large-scale companies [5]. It was reported that about 20% of all produced lithium is applied in LIB industries and it is predicted that by 2025 lithium reserves are became depleted. So, it is important to use alternatives to LIBs in less than six years [6, 7].

Sodium-ion batteries (NIBs) and potassium-ion batteries (KIBs) are started to attract much attention because of their advantages including low costs and abundant reserves. The physical properties as well as economic characteristics of lithium, sodium and potassium are presented in Table 1.1 [2]. Both Na+ and K+ ions are placed after lithium in the alkali series which have same chemical properties to Li+. NIBs are started to be explored earlier than KIBs. It is initially illustrated that NIBs could operate at room temperatures by applying similar principles to LIBs. Higher temperature sodium-based batteries have been commercialized, specially molten salt electrolytes like ZEBRA batteries [8].

Besides abundant potassium reserves and low costs, see Table 1.1, potassium ions have extremely high hydration and oxidation rates, have lower melting points than sodium (Na: 97.72°C and K: 63.38°C) and also KIBs have more negative standard electrode potential than NIBs which is more closed to LIBs (Na: -2.71 V, K: -2.93 V vs. E0 and Li: -3.04 V vs. Eº). This fact causes preferable rate performance and high energy densities of KIBs [9-11]. As K+ ions are larger and heavier than Na+, much smaller solvated ions based on Stock's radius will form by K+-ions in the propylene carbonate (PC) as solvent (Na+: 4.6 Ao, Li+: 4.8 Ao and K+: 3.6 Ao) as a result of their weaker Lewis acidity which causes fast K+ transportation in the electrolyte and higher ionic conductivity [12]. Similar to Na, K does not form alloys electrochemically with Al. So, low-cost aluminum foil could be used as collectors for both cathodes and anodes in NIBs and KIBs instead of more expensive copper foil which is normally applied in anode of LIBs [13]. Therefore, KIBs attract more attention to be used in large scale as low cost energy storage devices. In order to design high performance KIB, better understanding of its challenges is undeniable. These challenges which are actually based on large atomic size of potassium and its relatively active surface, are summarized below [3]:

Table 1.1 Physical and economic properties of lithium, sodium, and potassium.

1. Large Volume Changes During Cycling

   Regarding to the large size of K+-ion in comparison with Li+ and Na+, the volume change of the electrode as a result of structural damage is conceivable during the cycling process. For instance, in the case of LIBs, the volume of graphite anode is expanded near 10% after discharge is taking place completely. However, with the same anode material (graphite), the volume expansion is around 60% for the case of KIBs when the discharge process is finished [14].

   In addition, alloy-based materials are regarded as favorable anode materials exhibiting high energy density and theoretical capacity, but similar to graphite anodes, they also suffer from considerable volume expansion. To illustrate, the potassiation process can lead to 680% volume expansion of Sn4P3 alloy as a conventional anode material. The volume expansion can result in a series of problems as a consequence of releasing the induced stress [15].

2. Kinetics of Sluggish Reaction

   Despite the small Stoke's radius of K+-ions relative to Li+ and Na+ which presumably contributes to its faster transportation within the electrolyte, some serious problems associated with the chemical diffusivity into the solid as well as slow diffusion coefficient can occur along the surface electrolyte interphase (SEI) layer. In fact, both ionic diffusion capacity in the electrolyte and chemical diffusion into the solid simultaneously affect the rate capacity. It is generally known that the rate capacity is controlled by this comparatively sluggish step [16].

3. Extreme Electrolyte Reduction and Metal Dendrites

   One the most important issues which generally limits the life cycle is the decomposition of electrolytes. Indeed, during the electrochemical charge/discharge processes, decomposition of the electrolyte mainly arises from reduction of electrolyte along with its consumption for the formation of SEI layer. Typically, the reduction of the electrolyte which is controlled by both thermodynamic and kinetic processes is depends on the reduction potential of used solvents. Besides, in order to form the SEI layer on both cathode and anode sides, the electrolyte consumption is vitally required. During the electrochemical cycling the SEI layer is formed sequentially in each cycle. Actually, in addition to formation of SEI layer in the initial cycle, it is also formed in the next cycles as a result of newly produced interface due to the electrode pulverization as well as electrolyte decomposition by-products. Potassium exhibits higher reactivity in relation with lithium and therefore growth of K dendrite must be considered as a result of half-cells and also extended systems of K-O2 and K-S [17, 18].

Such mentioned problems should all be taken into consideration to aim for enhancing the performance of KIBs. The huge volume expansion which occurs during the cycling process can probably contribute to electrode pulverization, leading to...