Electrochemical Lithium Recycling System Using High Dense Li-Ion Conducting Solid Electrolyte Toward Renewable and Sustainable Energy Technology
Authors: Bae Hyuntae , Kim Youngsik 
Published: December 2020

As the lithium-ion battery (LIB) has been widely commercialized for energy storage systems in portable and stationary devices, it’s production and disposal have dramatically increased over the past few decades. In addition, increasing the number of electric vehicles (EVs) causes a more dramatic increase in waste LIBs. As lithium is the key material for LIB, this increasing market trend of LIB leads to lithium source depletion which is an urgent problem for the future LIB market. In addition, waste LIBs can cause serious environmental problems as it contains flammable and explosive materials.
To circumvent these issues, various LIB recycling technologies have been researched over the world. The most used hydrometallurgy method leaches waste battery electrode materials into acidic or alkaline conditions to form ionized lithium solution. After leaching, precipitation and extraction methods can be used to obtain lithium chemicals. However, these methods have a problem of using a large amount of toxic acid or alkali chemicals to leach the waste battery materials. This leads to complicate procedure high chemical cost and requires environmental cost for recycling waste LIBs.
A new type of lithium recycling system using a solid electrolyte has been proposed to overcome these issues. This electrochemical system is named waste-to-lithium (WTL) recycling system and can simply extract lithium from the waste lithium materials. This system consists of three compartments: the waste cathode compartment, the harvesting anode compartment, and the recycling cathode compartment. Each compartment is physically separated with Li-ion conducting ceramic membrane (LATP), and this membrane can selectively penetrate lithium ions. Waste cathode compartment stores waste LIB materials in the water. By electrochemically charging the WTL system, lithium ions from the waste LIB materials are extracted and transfer to the harvesting anode compartment where the lithium ions form lithium metal by reacting with the electrons. After the charging process, lithium metal can be transformed into lithium precursor chemicals such as LiOH and Li2CO3 by electrochemically discharging it into a recycling cathode compartment that is filled with water.
The main key component of the WTL system is Li-ion conducting ceramic membrane. WTL system applied Li1.4Al0.4Ti1.6(PO4)3 (LATP) solid electrolyte as Li-ion conducting ceramic membrane due to the characteristic of high ionic conductivity and high chemical stability in water. However, on account of the poor sinterability of LATP, it is difficult to sinter dense LATP pellet that is enough to block the water ions. Japan company OHARA corporation has sold dense glass Li-ion conducting glass-ceramic (LICGC) pellets for Li-air batteries. However, OHARA employs a melting quenching method to synthesize LICGC glass-ceramic, which melts all the precursor materials at extremely high temperature. In addition, this glass-ceramic includes high-cost germanium materials for high dense glass sintering. These two critical factors lead to the extremely high cost of the OHARA LICGC glass-ceramic pellet. For higher lithium recycling efficiency of the WTL system, water-stable, low-cost solid electrolyte must be applied to the WTL system.
For low-cost solid electrolytes, the water-base synthesis method was used to form the LATP pellet, which only uses water during the synthesis process that can lower the price of the procedure. However, this synthesized LATP pellet had low density with a porosity of 6.6%, which permits the water ions through the pellet. To circumvent this porosity issue, pores of the LATP pellet was filled with epoxy-resin polymer. Epoxy-resin has strong adhesion with substrates and has excellent mechanical properties and strong chemical stability. This polymer fills the pores of the LATP and blocks the water penetration through the pellet. The Epoxy-LATP solid electrolyte showed high ionic conductivity over 2×10-4 S/cm, the high bending strength of 47 MPa and stable cycle performance in aqueous solution for 300 hours.
By applying the low cost, high dense LATP pellet, the concept of the WTL system is proved using well-known cathode materials from Li-ion batteries such as LiFePO4, LiMnO2, LiNi0.3Mn0.3Co0.3O2 and a commercial Li-ion battery pack. The harvested Li metal shows a purity near 99%, and the produced material after discharging showed a pure Li2CO3 phase. This WTL system will be a promising system for the low-cost and environmentally friendly lithium recycling system for the increasing future lithium demand.