Recent advances in producing hollow carbon spheres for use in sodium−sulfur and potassium−sulfur batteries

Abstract

Sodium-sulfur (Na-S) and potassium-sulfur (K-S) batteries for use at room temperature have received widespread attention because of the abundance and low cost of their raw materials and their high energy density. However, their development is restricted by the shuttling of polysulfides, large volume expansion and poor conductivity. To overcome these obstacles, an effective approach is to use carbon-based materials with abundant space for the sulfur that has sulfiphilic sites to immobilize it, and a high electrical conductivity. Hollow carbon spheres (HCSs) with a controllable structure and composition are promising for this purpose. We consider recent progress in optimizing the electrochemical performance of Na-/K-S batteries by using these materials. First, the advantages of HCSs, their synthesis methods, and strategies for preparing HCSs/sulfur composite materials are reviewed. Second, the use of HCSs in Na-/K-S batteries, along with mechanisms underlying the resulting performance improvement, are discussed. Finally, prospects for the further development of HCSs for metal−S batteries are presented.

1. Introduction

The lithium-sulfur (Li-S) battery, with its exceptional energy density of 2,600 Wh kg−1 and remarkable theoretical specific capacity of 1,675 mAh g−1, represents an attractive option for next-generation energy storage. Nevertheless, substantial technological hurdles remain to be overcome for its widespread commercialization. The slow redox kinetics caused by the unsatisfactory conductivity of sulfides and lithium sulfides (Li2S/Li2S2) has led to challenges such as lower actual capacity than theoretical capacity and poor stability[1,2]. Furthermore, as the sulfur cathode expands during cycling, it may cause the active materials to detach from the current collector. This detachment leads to the loss of close electrical contact between the current collector and sulfur cathode, resulting in deactivation and rapid capacity decay[3]. More importantly, the dissolution of soluble lithium polysulfides (LiPSs) into the electrolyte results in a detrimental “shuttle effect” during cycling, compromising the electrochemical cycling performance[4–6].

Although the theoretical gravimetric and volumetric energy density of Li-S batteries is approximately twice that of advanced sodium-sulfur (Na-S) batteries, their sustainable development is hindered by the low abundance of Li on Earth (0.0017%) and the consequent hike in lithium source prices[7]. Additionally, the uneven distribution and mining limit the wide application of lithium metal resources. For instance, 80% of China’s lithium consumption depends on imports because there is only around 13.6% of the whole resources of Li in China. Therefore, it is critical to develop the performance of advanced batteries that are rich in raw materials, cost-effective and environmentally friendly to ensure the sustainable development of key industries such as grid energy storage and electric vehicles. A good alternative is sodium (Na). Na resources (Na2CO3) is abundant (2.3%), widely distributed, cost-effective, and their physicochemical properties are similar to Li[8]. These advantages predict that sodium-based batteries have a long-term development prospect. Among them, room-temperature (RT) sodium-sulfur (Na-S) batteries as potential candidates have garnered increasing attention for sizeable energy storage devices, owing to superior energy density (1,274 Wh kg−1) and specific capacity (1,675 mAh g−1). Similarly, potassium-sulfur (K-S) batteries also show potential to replace Li-S cells. K, compared to Li and Na, not only boasts abundant availability, widespread distribution, and cost-effectiveness, but also exhibits incomparable benefits[9]. The redox voltage (−0.09 V) of potassium is naturally lower than that of Li (0 V) and Na (0.23 V) in organic electrolyte, which facilitate higher voltage in full battery applications. Despite its larger atomic radius (Na: 1.90 Å; K: 2.35 Å; Li: 1.55 Å), the Stokes’ radius of K in carbonate-based solvents is smaller than that of Na (4.6 Å) and Li (4.8 Å), because of weaker Lewis acidity[10]. When carbonate solvents are used as electrolytes, K ions show higher ion mobility, ionic conductivity, and higher ion transport numbers. Therefore, the K-S batteries are poised to exhibit superior power density. With both cost-effectiveness and high energy density, K-S batteries hold the potential to become a promising alternative.