Electrochemical impurity removal from spent ternary lithium battery graphite anodes and their performance study

Abstract

With the rapid development of new energy vehicles, power lithium-ion batteries are increasingly widely used, and a large number of lithium batteries are also ushering in a peak period of retirement. The comprehensive recycling and utilization of spent lithium batteries has attracted high attention from various countries. The layered structure of the graphite anode in spent lithium batteries is basically unchanged, so high-temperature graphitization is not required during recycling, and only the removal of internal impurities needs to be considered. In this paper, the spent graphite anode was treated by heat treatment, ultrasonic separation, and acid leaching, and then an innovative electrochemical treatment was adopted to deeply remove internal metal impurities. Comparing graphite at different recycling stages, it was found that the presence of organic impurities in graphite seriously affects various electrochemical properties, while trace amounts of inorganic impurities such as Cu and Fe have little effect on the initial discharge specific capacity but reduce the cycling stability of graphite. The final recovered graphite had internal major metal impurity content below 20 mg/kg, with a discharge specific capacity of 358.7 mAh/g at 0.1 C rate, and a capacity retention rate of 95.85% after 150 cycles. Compared with reported spent graphite recycling methods, this method can deeply remove impurities inside the graphite anode, solving the problems of large acid-base consumption, incomplete impurity removal, and high energy consumption. The recovered regenerated graphite anode has good electrochemical performance, providing a new recycling and regeneration path for spent lithium battery graphite anodes.

1. Introduction

With the rapid development of technology and industry, the demand for power lithium-ion batteries for new energy vehicles is increasing. Lithium-ion batteries have advantages such as easy availability of raw materials, rechargeability, long battery life, high capacity, and good electrochemical performance, and are widely used in national defense, aerospace, marine, and daily life fields [1]. In recent years, multiple national ministries have issued policies to strongly support the new energy industry, leading to a yearly increase in the use of lithium-ion batteries [2]. Typically, the lifespan of power lithium-ion batteries for new energy vehicles is about 5 years. After prolonged use, battery performance degrades, and when capacity decays to below 80%, the battery faces retirement [3–8]. It is predicted that by 2030, the retirement volume of spent lithium batteries in China will reach 3.9×10^9 kg, and globally it will exceed 1.0×10^10 kg [9]. Therefore, the recycling of spent lithium-ion batteries is urgent.

In recent years, spent lithium battery recycling has attracted widespread attention. Since cathodes contain valuable metal elements such as Li, Co, Ni, and Mn, previous research has focused on recovering high-value metals from this 'urban mine', while studies on recycling the lower-value anode graphite are relatively scarce. Since battery failure is generally due to structural changes in cathode materials [10] and continuous growth of the SEI film on the graphite anode surface [11], the graphite structure itself is not destroyed. Therefore, recycling anode graphite only requires removal of internal impurities, omitting the high-temperature graphitization step. Compared to discarding or incinerating graphite, recycling spent graphite after impurity removal not only reduces environmental pollution but also has good economic benefits.

There are many methods to recover graphite from spent lithium batteries, which can be classified into three categories: mechanical separation, pyrometallurgy, and hydrometallurgy. Mechanical separation involves no heating or acid-base treatment, only mechanical crushing, sieving, and other methods to complete separation [12]. In the laboratory, mechanical separation typically involves manually breaking the discharged battery shell, separating components, and scraping off the graphite from the anode sheet; in factories, it generally involves large crushing devices to break the discharged battery, followed by air separation and gravity separation to separate cathode and anode powders and metal powders. Pyrometallurgy involves heat treatment of graphite at high temperatures in air or inert gas environments to remove impurities and improve graphite performance [13–14]. Hydrometallurgy removes impurities from graphite using acid or alkali solutions [15–16]. Each method has its advantages and disadvantages, and appropriate selection is needed to achieve the goal of graphite recovery. Yang et al. [17] heat-treated spent battery anode sheets under different atmospheres, then acid-leached in 1 mol/L hydrochloric acid and 4% H2O2, obtaining recovered graphite that exhibited good electrochemical performance when assembled into coin cells. Zhang et al. [18] used a H2SO4+H2O2 system for acid leaching to remove impurities, followed by heat treatment to remove organic impurities, and finally coated with phenolic resin to improve graphite performance, achieving first-cycle Coulombic efficiency and discharge specific capacity meeting mid-end graphite requirements. The above studies successfully recovered anode graphite with good electrochemical performance, but high concentrations of acid-base solutions and oxidants were used in the hydrometallurgical processes. Additionally, lithium batteries have high requirements for metal impurity content in anode graphite; small amounts of metals can cause side reactions in the battery, reducing capacity, and even piercing the separator leading to thermal runaway. Therefore, further research is needed on impurity removal and recycling of spent graphite.