The development of wireless devices, electric vehicles, 5G technology and other directions has put forward higher requirements for battery performance. At present, with the increase in energy density, commercial liquid lithium-ion batteries have more serious safety hazards. Therefore, the development of traditional liquid lithium-ion batteries has encountered bottlenecks and it is difficult to meet the safety and energy density requirements of new battery application scenarios. All-solid-state batteries are expected to achieve high safety under high energy density battery systems, and therefore have received widespread attention.
The technical route of all-solid-state batteries is mainly divided into oxide all-solid-state batteries, sulfide all-solid-state batteries and polymer all-solid-state batteries according to different electrolyte materials. Among them, sulfide solid electrolytes have the highest lithium ion conductivity, excellent mechanical properties and a wide operating temperature range, and are currently very practical all-solid-state battery systems.
However, the electrochemical stability window of sulfide solid electrolytes is relatively narrow; when sulfide solid electrolytes are matched with lithium metal, interfacial side reactions are serious and there is the problem of lithium dendrite growth. The performance of the assembled batteries is difficult to have competitive advantages over current commercial lithium-ion batteries in terms of positive electrode active material loading, battery cycle life, and charge and discharge rate. Therefore, sulfide all-solid-state battery systems need to find alternative negative electrodes to metallic lithium.
At present, there are few effective modification methods for silicon-based negative electrode materials in all-solid-state batteries. The key factors affecting their electrochemical performance mainly include external pressure, binders and conductive agents, silicon particle size, structural design and surface modification.
External pressure
The external pressure refers to the pressure applied to the battery during the operation of the all-solid-state battery, which can effectively ensure the solid-solid contact between the electrode and the electrolyte, and ensure the effective electron and ion conduction inside the silicon negative electrode during the cycle.
Binder
In liquid batteries, binders suitable for silicon negative electrodes have been widely studied. They have special properties such as self-healing, high mechanical strength, high elasticity and good electronic or ion conduction ability, which can effectively improve the cycle stability and rate performance of silicon negative electrodes.
Conductive agent
The conductive agent is also an important component in the silicon electrode, which can promote the electron conduction inside the silicon electrode. However, for sulfide all-solid-state batteries, the conductive agent carbon will promote the decomposition of sulfide solid electrolytes.
Silicon particle size
Nano silicon has better stress release than micro silicon during lithiation, and it has a shorter lithium ion diffusion path, thus having better cycle and rate performance. Therefore, nanostructured silicon has been widely used in liquid lithium-ion batteries, such as nanoporous particles, nanowires and nanofibers. However, researchers have found that when Si is used in sulfide all-solid-state systems, the active material and the electrolyte are in solid-solid contact, and nm-Si is difficult to disperse evenly in the solid electrode, thereby affecting the utilization rate of the active material of the composite negative electrode. The performance of μm-Si in solid-state batteries may be due to the more uniform electrode morphology, and the electrochemical performance may be better than that of nm-Si, while also effectively reducing costs.
Silicon anode structure design
Reasonable structural design can provide buffer space for the volume expansion of silicon anode, prevent the reaction between silicon anode and electrolyte, and thus improve its cycle stability. However, the compaction density of the material should also be considered when designing the structure. The pores of porous materials will reduce the volume energy density of the battery. It is necessary to comprehensively evaluate the two and select the most appropriate range.
Surface modification
Surface modification is one of the common methods for modifying silicon anode materials in liquid batteries. It can maintain the structural stability of silicon, improve the electronic or ionic conduction of the silicon anode surface, and reduce the formation of SEI. However, there are relatively few reports on the surface modification of silicon anode materials in sulfide all-solid-state batteries.