Theoretical-molar Fe³⁺ recovering lithium from spent LiFePO₄ batteries: an acid-free, efficient, and selective process

Highlights

• Two methods for recovering spent LFP batteries using Fe3+ as leaching agent are proposed;

• The amount of Fe2(SO4)3 is theoretical-molar dosage(method-1 Fe2(SO4)3:LFP  = 1:2, method-2 Fe2(SO4)3:LFP = 1:6);

• The Li leaching efficiency is remarkable in method-1 and 2, at 500 and 400 g/L solid-liquid ratio, respectively;

• No acid is introduced and thus avoiding the possible secondary pollution;

• CuSO4, NiSO4, and MgSO4 demonstrate the leaching-reaction of LFP needing a driving force and suitable pH.

Abstract

In spent lithium iron phosphate batteries, lithium has a considerable recovery value but its content is quite low, thus a low-cost and efficient recycling process has become a challenging research topic. In this paper, two methods about using the non-oxidizing inorganic iron salt - Fe₂(SO₄)₃ to recover lithium from LiFePO₄ are proposed. The method-1 is theoretical-molar Fe₂(SO₄)₃ (Fe₂(SO₄)₃ : LiFePO₄ =1:2) dosage is added and more than 97% of lithium can be leached in just 30 min even under a pretty high solid-liquid ratio of 500 g/L. Spectrophotometry provides the evidence of Fe²⁺/Fe³⁺ substitution in the leaching process. In the method-2, the generated Fe²⁺ originating from LiFePO₄ is fully utilized with the addition of H₂O₂, and the dosage of Fe₂(SO₄)₃ is decreased by two thirds (Fe₂(SO₄)₃ : LiFePO₄ =1:6). Several sulphates (CuSO₄, NiSO₄, MgSO₄) are employed to explore the leaching mechanism. All the results reveal that the reaction of Fe³⁺ substituting Fe²⁺ has a powerful driving force. In addition, these two leaching processes simultaneously present superior selectivity for the impurities. The Fe₂(SO₄)₃ in two methods does not cause pollution and is easily regenerated by adding H₂SO₄. The proposed rapid, efficient and selective leaching thought would be a competitive candidate for recycling spent LiFePO₄ batteries.

Introduction

In recent years, lithium-ion batteries (LIBs), as a high-performance secondary rechargeable green power, have been widely used in various portable electronic devices, electric vehicles and renewable energy storages.(Harper et al., 2019; Li et al., 2018a; Wang et al., 2018a; Yu et al., 2018) Common LIBs include nickel-cobalt-manganese ternary lithium batteries, and lithium iron phosphate batteries (LiFePO₄, LFP).(Manthiram et al., 2017; Zheng et al., 2018a; Zeng et al., 2014) With the increase of market demand for LIBs, the consumption of lithium will quickly grow with each passing day.(Shi et al., 2018) However, the limited natural lithium resources such as salt lakes and lithium minerals, are low lithium content and insufficient for the gradually market demand.(Chen et al., 2016) But at the same time, a large number of spent LIBs with abundant valuable elements have been produced in the past few years.(Sonoc et al., 2015) It is common knowledge that the heavy metals and hazardous organic chemicals in these end-of-life LIBs will pose serious threats to the ecological system and human health.(Harper et al., 2019; Al-Thyabat et al., 2013; Georgi-Maschler et al., 2012; Meng et al., 2018; Li et al., 2018b) Hence, recycling spent LIBs has become an important research topic for solving the shortage of lithium resources and the pollution of used batteries. (Wang et al., 2009; Zou et al., 2013; Sencanski et al., 2017; Hu et al., 2017; Heydarian et al., 2018; Sun et al., 2015)

Various kinds of spent LIBs, such as nickel-cobalt-manganese ternary lithium batteries and lithium cobaltate batteries which contain many valuable metals, have been extensively studied.(Gao et al., 2018; Zeng et al., 2015; Zhang et al., 2018; Wang et al., 2018b) However, LiFePO₄ batteries do not attract enough attention due to their low valuable metals, even though they are extensively used in electric buses, communication base stations, etc. owing to their superior reversibility, excellent thermal safety, low toxicity, and low-cost.(Li et al., 2018c; Li et al., 2017a; Talebi-Esfandarani et al., 2016) Therefore, how to efficiently and low-cost treat the increasing spent LiFePO₄ batteries has become an extremely important study theme.

Generally, there are two ways about recovering spent LFP batteries: direct regeneration process and leaching-resynthesizing process. (Zhang et al., 2019) The principle of direct regeneration process is to require the lithium to be replenished to compensate for the loss during use.(Harper et al., 2019; Song et al., 2017) The advantage of this process is direct and simple, while much obtained regeneration LFP batteries often have poor electrochemical performances because of no rigorously impurity control.(Wang et al., 2018c) Song et al. Song et al. (2019) reported a hydrothermal reaction process to get regenerated LFP with the help of graphene oxide, but yet it was high-cost and difficult for industrialization limited by the complicated synthesis process of graphene oxide. For the second way, various acids are used to leach spent LFP cathode materials, including sulfuric acid (Li et al., 2017b), phosphoric acid(Bian et al., 2015; Yang et al., 2017a), oxalic acid(Li et al., 2018c; Fan et al., 2018), acetic acid(Yang et al., 2018), citric acid(Li et al., 2019), and so on. Li et al.(Li et al., 2017b) revealed a leaching method with stoichiometric H₂SO₄ and H₂O₂. However, the inorganic acids frequently do not have sufficient selectivity for impurities such as Al, so it is necessary to separate Al foil before leaching. In general, organic acids could selectively leach lithium, but they are difficult to be applied in practice due to their high-price and difficulty in recycling. Besides, an oxidation (Na₂S₂O₈) leaching (Zhang et al., 2019) was proposed to recovery spent LFP batteries, in which lithium can be efficiently leached out at ambient temperature. Usage-cost owing to the high-price, high-insecurity, and non-recycling of Na₂S₂O₈ impedes its application in the recycling process of spent LFP. Not long ago, Liu et al.(Liu et al. (2019)) proposed a mechanochemically induced substitution by co-grinding LiFePO₄ and NaCl, which is simple but must use much excessive materials and needs a long time to activate. Thusly, it is imperative to find a more efficient, low-cost and environmentally friendly way to dispose of spent LFP batteries.

In this paper, two leaching methods using Fe₂(SO₄)₃ to selectively leach Li from the spent LFPs based on a spontaneous substitution reaction were reported, in which Fe³⁺ was employed to exchange the Li⁺ and Fe²⁺ in LFP. It was inspired by the similar olivine-type structure between LiFePO₄ and FePO₄. When only Fe₂(SO₄)₃ was used, a high Li leaching efficiency was acquired even under a high solid − liquid ratio and a short leaching time. Furthermore, H₂O₂ was introduced to oxide the generated Fe²⁺ and thus the new generated Fe³⁺ could circularly substitute the Fe²⁺ in LFP, which also obtained a good leaching efficiency of Li. The main advantages for the two routings could be summarized as follow: (1) Li can be efficiently recovered in a short time; (2) Fe₂(SO₄)₃ is theoretical-molar dosage, recyclable, selective and low-cost; (3) no acid is introduced and thus avoid the possible secondary pollution. More importantly, it will provide a new thought relating to recovery of valuable elements in the spent battery materials.