## 1 Introduction  
In the early industrial development, lithium (Li) was used in many industries. For example, lithium acted as a fluxing agent in the ceramic industry and as a deoxidizer and dechlorination agent in the metallurgical industry. In addition, lithium has attracted more and more attention, worldwide, in the recent years, because of its application in the battery industry [1; 2]. Lithium is mainly supplied to electronic industrial products, especially the power battery market. Lithium-ion batteries (LIBs) are widely used in electric vehicles and hybrid vehicles, due to their high energy density and high tolerance of a wide range of temperatures [3; 4].  
According to the report of the US Geological Survey [5; 6; 7; 8; 9], the market for lithium is mainly 39% for the battery industry, 30% for the ceramics and glass industry, and 8% for the lubrication industry. Lithium is usually extracted from brine [10; 11; 12] and mineral [13] and the world's annual crude lithium production is about 35,000 tons. Among all countries, Australia and Chile are the largest and second largest crude lithium exporters, respectively. In order to meet the needs of the battery industry, lithium production increased by, approximately, 12% in 2016. It is expected that the lithium-ion battery industry will flourish in the future and the demand for lithium metal will become more and more urgent. The refore, it is important to recycle lithium metal or its compounds from LIBs.  
It not only achieves the goal of waste reduction but also improves the secondary resource utilization of waste LIBs.  
In the LIBs, lithium, cobalt (Co), nickel (Ni), and manganese (Mn) are the main materials that are needed to recycle. Among them, Lithium is usually obtained in the form of lithium carbonate. Lithium carbonate has become a very important material in recent years. The global demand for lithium carbonate has gradually increased, due to its versatility, and the price will rise significantly in the future [14]. According to the report of Sociedad Quimica y Minera de Chile, the demand for lithium carbonate will increase six hundred thousand tons to eight hundred thousand tones. For industrial activities, lithium carbonate is not only used as cathode materials in the LIBs but is also used to create other compounds, such as lithium chloride (LiCl), lithium bromide (LiBr) and lithium oxide (Li\({}_{2}\)O). LiCl, LiBr, and Li\({}_{2}\)O all can be raw materials for other industries. For example, LiBr can be used as an absorbent and a refrigerant. On the other hand, in the medical industry, lithium carbonate can also be used as a treatment for bipolar disorder [15; 16]. Due to the unlimited development of lithium carbonate, a lot of countries try various methods to purify lithium carbonate to apply in different industries.  
In the purification process, the methods used mainly are the causticization method, electrolysis, recrystallization, hydrogenation-decomposition, and the hydrogenation-precipitation method [17]. As the hydrogenation-decomposition method generates little waste of materials and solutions and can achieve a higher purification of lithium carbonate, it was chosen for this experiment. During the decomposed process, calcium and lithium, both, separate out in the form of carbonates, due to their common properties of precipitation with an increase in temperature. In addition, sodium is also a critical problem in the purification process. To deal with these problems, the ion-exchange resin was chosen to remove the impurities. Comparing the literature, it could be observed that as a resin, IRC-748 has a great adsorption efficiency on Ca\({}^{2+}\), but insufficient efficiency on Na\({}^{+}\); Dowex G26 has the better effect on both Ca\({}^{2+}\) and Na\({}^{+}\). In order to reduce the impurities, efficiently, Dowex G26 was used in this experiment. To sum up, the hydrogenation-decomposition method and the Dowex G26 resin were used to get a high purity of the lithium carbonate.