Introduction
A solid-state lithium-ion battery uses a solid-state material as the electrolyte portion of the battery replacing the traditional liquid electrolyte. All solid-state lithium-ion batteries are defined by a solid-state electrolyte and its anode material, which is commonly a solid lithium metal. The working principle of solid-state lithium-ion batteries is the same as that of liquid batteries.
The technical development of solid-state batteries adopts a gradual conversion strategy according to the contained electrolyte mass percentage, the liquid electrolyte content gradually decreasing to the final product all-solid-state battery. Solid-state batteries include semi-solid batteries, quasi-solid batteries and all-solid batteries.
Semi-solid batteries are the stage of transition from liquid lithium batteries to all-solid-state batteries. The mass production of semi-solid batteries can provide a buffer stage and lay the foundation for the early layout of the all-solid-state battery industry chain. Specifically, lithium-ion batteries can be divided into the following five categories:
- Liquid lithium battery: During the manufacturing process, the batteries only contain liquid electrolytes and do not contain solid electrolytes, including liquid lithium-ion batteries and liquid metal lithium batteries.
- Gel electrolyte lithium battery: Belongs to the category of liquid lithium-ion batteries. The liquid electrolyte in the battery exists in the form of gel electrolyte and does not contain solid electrolyte.
- Semi-solid battery: Both solid electrolyte and liquid electrolyte exist in the cell, the mass or volume ratio of the liquid electrolyte is relatively large, and the electrolyte content accounts for 5%-10%.
- Quasi-solid-state battery: The quantity of the solid electrolyte in the battery is higher or the volume is larger in the battery, and the electrolyte content accounts for 0%-5%.
- Hybrid solid-liquid lithium battery: There are both liquid and solid electrolytes in the cell. The aforementioned semi-solid and quasi-solid batteries are all types of hybrid solid-liquid lithium batteries.
- All solid-state battery: The battery core is composed of solid electrodes and solid electrolyte materials. Within the working temperature range, the battery cell does not contain any mass and volume fraction of liquid electrolyte.
Solid-State-Battery Component Materials
The main components of the solid-state battery cell are the anode and cathode active materials and the solid electrolytes:
- Anode: Active materials are lithium metal and silicon. Lithium metal enables the highest possible energy density, but the technology for processing them is not yet well-established for largescale manufacturing. The energy densities of silicon materials being achieved are lower than that for lithium metal anode.
- Cathode: Active materials are transition metal-based oxides (NMC, NCA) and lithium iron phosphate (LFP). These materials are used in liquid lithium-ion batteries and supply chains, and processing routes are already established. Currently, the highest energy densities can be achieved by using NMC/NCA layered oxides.
- Solid electrolytes: The three groups of solid electrolyte materials are oxides, sulfides and polymers. The oxide solid electrolyte group consists of materials all containing lithium and oxygen as the main components as well as various other elements. They generally exhibit high mechanical and chemical stability, but are brittle and have a relatively poor ionic conductivity. The sulfide solid electrolyte group consists of materials all containing lithium and sulfur as the main components as well as other elements. They are mechanically softer and more malleable than oxide materials, but the chemical compatibility to lithium metal and high-potential cathode active material is limited. The polymer solid electrolyte group is dominated by polyethylene oxide, which is in commercial use. However, they have limited ionic conductivities at room temperature, poor chemical compatibility with high-potential cathode active material and a low limiting current density due to the ionic conduction mechanism.
Product Performance Comparison
Compared with traditional liquid batteries, semi-solid batteries have higher energy density. The current material system of liquid lithium batteries is gradually reaching the upper limit. The solid-state electrolytes can match high-energy cathode (high-nickel cathode material, nickel-manganese spinel cathode material, etc.) and metal lithium metal anode, greatly increasing the theoretical energy density up to 500Wh/kg from 350Wh/kg for liquid electrolyte lithium-ion batteries. The electrochemical window of all-solid-state batteries can reach more than 5V, which is higher than that of liquid lithium batteries (4.2V).
Commercialisation
It will take time for all-solid-state batteries to be put into use, and semi-solid-state batteries are an intermediate solution for the transition from liquid-state batteries to all-solid-state batteries. All-solid-state batteries have the advantages of high energy density and good safety performance, but now the implementation of all-solid-state batteries will encounter challenges, including the following three aspects:
- Solid-solid interface contact leads to large internal resistance of the battery;
- The ionic conductivity is not high, and the conductivity of the existing solid-state electrolyte is 1-2 orders of magnitude lower than that of the liquid electrolyte; and
- Due to the lack of commercialisation (inorganic solid-state electrolytes and raw materials have not yet been mass-produced to form a supply chain), the cost of all-solid-state electrolytes is higher than that of traditional liquid lithium-ion batteries, and the product yield is low.
Semi-solid batteries are an intermediate solution for the transition to all-solid batteries.
- Semi-solid batteries retain a certain amount of electrolyte, and their cycle performance and rate performance are better than those of all-solid batteries;
- Semi-solid battery electrode materials are soaked in electrolyte, which can improve the problem of low conductivity of solid-state batteries; and
- The current cost of semi-solid batteries is slightly higher than that of traditional lithium batteries, but is at a lower position than all-solid-state batteries.
Although Japan, the United States, and South Korea were relatively early in the research and development and industrial layout of all-solid-state batteries, China took the lead in realising the mass production of solid-state batteries because it chose the route of solid-liquid hybrid batteries.
The solid-state battery production lines of enterprises such as Ganfeng Lithium Co., Ltd, Qingtao Energy Development Co., Ltd and Weilan New Energy have been completed and put into production. From 2022 to 2025, a number of leading semi-solid battery companies in China will gradually release car-grade batteries for auto companies. For example, Dongfeng will release E70 model, Lantu will release Chasing Wind and other models, and NIO will release ET7 model equipped with semi-solid batteries. During the iterative process of solid-state batteries, the liquid electrolyte content will drop from 20wt% to 0wt%, and the lithium content of the anode material of the battery will gradually increase until it is replaced by metal lithium sheets. The battery density is expected to gradually increase to 500Wh/kg.
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