Recycling acidic iron wastewater for the production of an iron oxalate anode material with superior long-cycling lithium storage ability

Abstract

With its considerable high capacity and prolonged cycle life, iron oxalate is regarded as a noteworthy anode material for lithium-ion batteries. The solvothermal technique is frequently employed for the synthesis of iron oxalate; however, it results in the generation of acidic iron wastewater, thereby complicating subsequent treatment processes and elevating production costs. In this work, the impact of a wastewater recycling procedure on the morphological characteristics and crystalline phases of iron oxalate was investigated. The alterations of c(Fe²⁺) and c(H⁺) impede the hydrolysis process of oxalic acid and inhibit the complexation of iron oxalate complexes ([Fe(C₂O₄)₂]²⁻), thus inhibiting precipitation nucleation/crystal growth. By controlling wastewater recycling, the water usage per unit mass of iron oxalate dropped by 31%, from 2.97 to 2.03 mL g⁻¹. Furthermore, the crystal structure of iron oxalate produced by the wastewater recycling process demonstrates a preference for growth on the (002) crystal plane and exhibits stability following loss of crystal water. The morphologies of iron oxalates undergo a gradual transition from irregular rod-like particles to a more regular polyhedral structure. Based on the intact particle structure facilitating stable diffusion channels for Li⁺, the ferrous oxalate dihydrate, synthesized via the first acidic water recycling process and subsequently subjected to thermal treatment at 290 °C, exhibits superior rate capability, with a discharge specific capacity of 840 mA h g⁻¹ at 5 A g⁻¹ after 200 cycles. These findings offer a theoretical framework for investigating the growth process of iron oxalate during the treatment of regenerated iron oxalate wastewater and its effects on electrochemical properties.