Commercial lithium-ion batteries have become the most important part of the secondary battery system due to their own advantages such as good safety, high working voltage, no memory effect, and long cycle life. However, with the increasingly functionalization of portable electronic products, the vigorous development and utilization of clean energy, and the vigorous development of new energy vehicles, people have put forward increasingly stringent requirements for the energy density of secondary batteries. Lithium-ion batteries face many difficulties in achieving breakthroughs in energy density, which has led to the arrival of the “post-lithium-ion battery” era. Based on the principle of thermodynamics, the energy density (ED, the unit is W·h/kg or W·h/L) of the battery is affected by the total molecular weight of the active material (∑Mw), the working voltage (E) and the number of electrons transferred in the electrochemical process (n), as shown in formula (1-1). Therefore, exploring a battery system with both light weight and multiple electron transfer reactions has become a solution to improve battery energy density.
In the formula, F is Faraday’s constant.
Lithium-sulfur battery is a battery system with elemental sulfur as the positive electrode and metallic lithium as the negative electrode. Among them, the two active materials used as positive and negative electrodes have the characteristics of light weight; at the same time, sulfur and lithium can theoretically undergo an electrochemical reaction that transfers two electrons. Therefore, the energy density of lithium-sulfur batteries can reach up to 2600W·h/kg, which is nearly 10 times the energy density of current commercial lithium-ion batteries, as shown in Figure 1. In addition, the elemental sulfur used as the cathode has a wide range of sources and is environmentally friendly, which can alleviate the cost and pollution problems of traditional transition metal cathode materials. As a result, lithium-sulfur batteries have gradually become a typical representative of the next generation of high-energy-density battery systems.
The research on lithium-sulfur batteries began in the 1960s, mainly based on the in-depth research and great success of metal lithium primary batteries. The prototype of the battery structure consists of a positive electrode containing elemental sulfur, a conductive agent and a binder, an organic electrolyte and a metal lithium negative electrode. The poor electrochemical performance of sulfur electrodes, poor electrolyte compatibility, and the destruction of the dendrites and structure of lithium metal have caused the research on lithium-sulfur batteries to fall into a “bottleneck period”. However, research on lithium-sulfur batteries has not stalled, but has made progress in exploring electrolyte systems, electrochemical reaction mechanisms, and high-performance electrode materials. Especially in the 21st century, researchers have gradually deepened the research on lithium-sulfur batteries, and achieved breakthroughs in the construction of electrode materials, the rational optimization of electrolyte and battery structure, and the mechanism analysis of the electrochemical reaction process. From 2007 to 2011, the Chemical Defense Research Institute has developed a lithium-sulfur soft-package battery (3A·h) with an energy density of 320W·h/kg; in 2015, the Dalian Institute of Chemical Physics, Chinese Academy of Sciences developed a single lithium-sulfur battery (30A·h) with an energy density of up to 520W·h/kg and a 330W·h/kg lithium-sulfur battery pack (1kW·h); in 2016, the Dalian Institute of Chemical Physics, Chinese Academy of Sciences increased the energy density of single lithium-sulfur batteries to 566W·h/kg (35A·h, measured at 25°C) and 616W·h/kg (39A·h, measured at 50°C) , The specific energy of the lithium-sulfur battery pack is increased to 332W·h/kg (1kW·h), the lithium-sulfur battery developed has passed the power battery standard for electric vehicles, and has been successfully applied to a 12kW·h lithium-sulfur battery-photovoltaic battery combined system. The British company Oxis has developed a lithium-sulfur battery (10~35A·h) with an energy density of more than 400W·h/kg, and is expected to increase the energy density of lithium-sulfur batteries to 500W·h/kg in 2019; US Sion Power has developed a lithium-sulfur battery (20A·h) with an energy density of 400W·h/kg using its own patented technology, and has successfully applied it to unmanned aircraft; the energy density of the lithium-sulfur secondary battery developed by Polyplus also exceeds 400W·h/kg, and can achieve long cycle and low cost; Moltech and Samsung are also actively developing lithium-sulfur battery product research and development. At the same time, many countries have also formulated a blueprint for the development of lithium-sulfur batteries. Among them, Japan’s goal is to make the energy density of lithium-sulfur batteries reach 500W·h/kg by 2020; while the United States expects to increase the energy density of lithium-sulfur batteries to 600W·h/kg and achieve 1,000 cycles. Improving the utilization rate of sulfur and improving the stability of the battery in complex or even extreme environments have become the core work of lithium-sulfur battery research. With the rational design of material structure, the advancement of battery manufacturing technology, the flexible application of analysis and detection technology and simulation calculation, the research work of lithium-sulfur batteries has made a series of substantial progress, laying a solid foundation for the practical use of lithium-sulfur batteries.