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\DOI{10.5802/crchim.316}
\datereceived{2023-09-29}
\daterevised{2024-01-23}
\datererevised{2024-04-23}
\dateaccepted{2024-05-15}
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\dateposted{2025-01-13}
\begin{document}

\begin{noXML}

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%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{GDR Prom\'eth\'ee - French Research Network on \textit{Hydrometallurgical Processes for Primary and Secondary Resources}}
%\def\cdr@specialtitle@french{GDR Prom\'eth\'ee - Groupement de Recherche sur les Proc\'ed\'es Hydrom\'etallurgiques pour la Gestion Int\'egr\'ee des Ressources Primaires ...}
%\makeatother

\title{Lithium recovery from secondary sources: A review in battery
recycling with emphasis on chemical precipitation}

\alttitle{R\'{e}cup\'{e}ration du lithium \`{a} partir de sources
secondaires : une revue compl\`{e}te du recyclage des batteries avec un
accent particulier sur la pr\'{e}cipitation chimique}

\author{\firstname{Lorena E.} \lastname{Ramirez Velazquez}\IsCorresp}
\address{Universit\'{e} de Lorraine, Laboratoire R\'{e}actions et
G\'{e}nie des Proc\'{e}d\'{e}s (LRGP) UMR 7274 CNRS, 1 rue Grandville
BP20451, 54001 Nancy, France}
\email[L. E. Ramirez Velazquez]{lorena.ramirez@univ-lorraine.fr}

\author{\firstname{Herv\'{e}} \lastname{Muhr}\IsCorresp}
\addressSameAs{1}{Universit\'{e} de Lorraine, Laboratoire R\'{e}actions et
G\'{e}nie des Proc\'{e}d\'{e}s (LRGP) UMR 7274 CNRS, 1 rue Grandville
BP20451, 54001 Nancy, France}
\email[H. Muhr]{herve.muhr@univ-lorraine.fr}

\keywords{\kwd{Precipitation}
\kwd{Lithium carbonate}
\kwd{Hydrometallurgy}
\kwd{Recycling}
\kwd{Lithium-ion batteries}}

\altkeywords{\kwd{Pr\'{e}cipitation}
\kwd{Carbonate de lithium}
\kwd{Hydrom\'{e}tallurgie}
\kwd{Recyclage}
\kwd{Batteries lithium-ion}}

\begin{abstract}
The growing demand and rising prices of lithium have been driven by the
expanding market of electrical vehicles and electronic devices.
Therefore, the recovery of lithium from Li-ion batteries has gained
enormous relevance. This document provides an overview of lithium's
role in the battery industry in the ongoing energy transition. It also
explores the various processes used over the years to extract valuable
metals from spent lithium-ion Batteries and provides an up-to-date
review of the current recycling methodologies applied to batteries.
This work particularly emphasizes the hydrometallurgical process
involving the chemical precipitation of lithium carbonate. Indeed it is
the most sought-after form in the lithium value chain. This route had
demonstrated good recovery yields as well as high purity levels. This
method stands out from other recycling methods since it is
environmentally friendly, consumes little energy, and does not require
a large number of chemical reagents. This work encourages further
exploration and refinement of hydrometallurgical practices to make
recycled lithium a viable source for the battery supply chain.  
\end{abstract}

\begin{altabstract}
La demande croissante et l'augmentation des prix du lithium sont dues
\`{a} l'expansion du march\'{e} des v\'{e}hicules \'{e}lectriques et
des appareils \'{e}lectroniques. Par cons\'{e}quent, la
r\'{e}cup\'{e}ration du lithium des batteries Li-ion a pris une
importance consid\'{e}rable. Ce document pr\'{e}sente une vue
d'ensemble du r\^{o}le du lithium dans l'industrie des batteries dans
le cadre de la transition \'{e}nerg\'{e}tique en cours. Il explore
\'{e}galement les diff\'{e}rents processus utilis\'{e}s au fil des ans
pour extraire les m\'{e}taux critiques des batteries lithium-ion
usag\'{e}es et fournit une analyse actualis\'{e}e des m\'{e}thodologies
de recyclage actuellement appliqu\'{e}es aux batteries. Ce travail met
particuli\`{e}rement l'accent sur le processus hydrom\'{e}tallurgique
impliquant la pr\'{e}cipitation chimique du carbonate de lithium. Il
s'agit en effet de la forme la plus recherch\'{e}e dans la cha\^{i}ne
de valeur du lithium. En outre, cette voie a d\'{e}montr\'{e} des
rendements de r\'{e}cup\'{e}ration notables ainsi que des niveaux de
puret\'{e} \'{e}lev\'{e}s. Parmi les autres avantages, citons le
respect de l'environnement, la faible consommation d'\'{e}nergie et la
r\'{e}duction de la consommation de r\'{e}actifs chimiques. Ces travaux
encouragent \`{a} poursuivre l'exploration et le perfectionnement des
pratiques hydrom\'{e}tallurgiques afin de faire du lithium recycl\'{e}
une source viable pour la cha\^{i}ne d'approvisionnement des batteries.
\end{altabstract}

\maketitle

\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1}

\begin{figure*}
{\vspace*{1pt}}
\includegraphics{fig01}
{\vspace*{2pt}}
\caption{\label{fig1}Global lithium product by source in million metric
tons lithium carbonate equivalent. Data extracted from~\cite{1}.}
{\vspace*{2pt}}
\end{figure*}

Lithium is a non-substitutable element of Li-ion batteries (LIBs) whose
technology is indispensable in electromobility and energy transition.
The worldwide usage for LIBs nowadays is about 700 GWh and is expected
to increase in the next decade to around 4.7 TWh. The lithium demand
for LIBs represents 60--66\% of the market and is anticipated~to rise
to 95\% by 2030. To satisfy this demand, around 120 to 150 new battery
factories should be built around the globe~\cite{1}. Currently, the
main supply of lithium are primary sources from Australia and~Latin
America. Primary sources include ores (pegmantite and granite) and
salted waters such as continental and geothermal brines, volcano
sediments, and sea water. Lithium content tends to be higher in mineral
ore deposits compared to salted waters. However, access to these
deposits is expensive since they must be mined from hard rock
formations. Lithium extraction from continental brines is extremely
energy-intensive, as the evaporation \mbox{process} in ponds can take up to 24
months. Additionally, the water consumption stands at 800 m$^{3}$ per
ton of lithium carbonate. Geothermal brines represent just 3\% of the
global lithium reservoirs with no significant commercial plans to
exploit these reserves. Today, lithium extraction from seawater remains
in the research phase mainly because researchers are exploring various
methods to elevate lithium concentration in seawater, which typically
ranges from\break 0.1 to 0.2 ppm~\cite{2}.

Nowadays, world powers like eastern and western Europe, Russia, and
some~independent state members of the Commonwealth are developing
projects to recover lithium by their own means and sources in order to
gain lithium supply independence. Figure~\ref{fig1} illustrates the
forecast of lithium production by country in comparison with lithium
recovery from secondary sources.  It shows that recycling represents a
small fraction when compared with the larger volume of lithium obtained
from primary sources. At present, only 5\% of discarded batteries are
\unskip\break recycled~\cite{3}.

Nevertheless, while there is a slight improvement in the projected
growth of lithium recycling from 2020 to 2030, it is still far from
being able to compete with primary sources. Indeed the recycling
industry~faces numerous challenges, primarily related to waste
management, engineering and \mbox{commercialization}. One of the technical
difficulties lies in the design of LIBs. Batteries come in different
sizes, forms, and chemistries, making the \mbox{battery} \mbox{disassembly} and
materials separation extremely challenging. Therefore, scaling up pilot
tests presents an arduous task, mainly because of the lack of
regulation and standardization for the recovered materials~\cite{4}.
Given the huge potential of battery recycling, ongoing advancements in
recycling technologies are essential. This article aims to provide an
overview of existing methods in metal recycling, including direct
physical methods, pyrometallurgy, bio- and hydrometallurgy, with a
specific focus on chemical precipitation.



%tab1
\begin{table*}
\caption{\label{tab1}Features of commercial Li-ion batteries~\cite{6,7}}
\begin{tabular}{lcccccc}
\thead
{Composition}
 &amp; {LiCoO$_{2}$}  &amp; {LiFePO$_{4}$}  &amp; {LiMn$_{2}$O$_{4}$}
 &amp; {LiAl$_{x}$Co$_{y}$Ni$_{xy}$O$_{2}$}  &amp; 
 {LiCo$_{x}$Mn$_{y}$Ni$_{xy}$O$_{2}$}\\
\endthead
Material price/\dollar(kWh$^{-1}$)
 &amp; 88 &amp; 32 &amp; 26 &amp; 39 &amp; 40--50 \\
Battery price/\dollar(kWh$^{-1}$)
 &amp; 357 &amp; 222 &amp; 251 &amp; 199 &amp; 145--230 \\
{\parbox[t]{4.8cm}{\raggedright\hangindent.8pc Annual global demand for cathode materials in 2022 (kt)}}
 &amp; 23.1 &amp; 58.5 &amp; 9.0 &amp; 20.1 &amp; 259.9\vspace*{2pt}
\botline
\end{tabular}
{\vspace*{2pt}}
\end{table*}

\section{Types of Li-ion batteries}\label{sec2}

LIBs are built with an anode, a cathode, an organic electrolyte, and a
separator. The electrochemical properties of these batteries make them
unique in the mobility market, characterized by their high specific and
high-density energy. In addition, they can operate between 500 and 1000
cycles of charge-discharge, reaching at least 80\% of their initial
charge. On the other hand, LIBs require high-safety protocols during
their handling to prevent overcharge and instability. At high
temperatures and voltage, degradation may occur, and the potential fire
hazard always exists~\cite{5}. Various cathode chemistries include
commercial lithium cobalt oxide (LCO), lithium iron phosphate (LFO),
lithium manganese oxide (LMO), lithium nickel cobalt aluminum oxide
(NCA), and nickel manganese cobalt (NCM). Depending on their
composition, different recycling methods are preferred. For instance,
LCO, NCA, and NCM can be treated with hydrometallurgical,
pyrometallurgical, and direct pathways, while LFP and LMO are usually
directly recycled~\cite{6}. Table~\ref{tab1} provides an outlook for
the battery market, showing the interaction between global demand,
materials and battery prices. Notably, lithium and cobalt, recognized
as critical raw materials, influence battery manufacturing\break costs.

\section{Pre-treatment methods for spent Li-ion batteries} \label{sec3}

Typically, LIBs retain some residual energy inside and for that reason
a 10~wt\% NaCl solution is used to \mbox{discharge} them. For this, the
battery is submerged entirely into the electrolytic solution, inducing
a controlled short-circuit. Thus, the risk of explosion and the
emission of toxic gases are prevented. Other discharge approaches
include electronic techniques and the use of conductive liquids.
Nevertheless, the NaCl solution has proved the best compromise between
cost and efficiency~\cite{8,9}. Once the battery is discharged, it
undergoes separation and dismantling methods with the aim of separating
the different materials according to their composition~\cite{9}. The
choice of pre-treatment methods, whether mechanical, solvent-based, or
involving calcination, depends on factors such as size, composition,
and metal extraction processes. Pre-treatment methods can also be
combined as needed. 

Mechanical pre-treatment methods include crushing, sieving, particle
size separation, flotation, gravity, and magnetic separation. Particle
size separation aims to separate the minerals according to their
particle size distribution after crushing and sieving. The studies with
a commercial granulator showed that electrode materials have a cutoff
\mbox{diameter} of less than 0.5~mm, copper pieces around 1--6~mm, and plastics
greater than 6~mm~\cite{10}. The flotation method uses a chemical agent
which follows the principles of hydrophilicity and hydrophobicity. This
agent is able to trap hydrophobic metals inside bubbles of a foam and
these will float to the surface while the hydrophilic ones will remain
in the mixture~\cite{10}. During magnetic separation, steel pieces are
recovered with almost 100\% efficiency. Finally, gravity separation
aims to separate different materials according to their density, for
example, plastic, composites, and aluminum foils~\cite{10}.

The solvent pre-treatment makes use of solutions and solvents to
isolate active materials from the foils usually composed of copper or
aluminum. This process essentially functions by dissolving the binder
\mbox{allowing} to pull out the active materials~\cite{9}.

The calcination pre-treatment aims to separate carbon and other organic
material from the battery. The temperature range is 150 to
500~\textdegree C~\cite{9}.

\section{Methods of lithium extraction from spent Li-ion batteries}
\label{sec4}

There are three main methods of recovering metals from LIBs: the bio-
and hydrometallurgical, pyrometallurgical and direct recycling
(Figure~\ref{fig2}). These methods can also be~combined or integrated for
a more efficient metal recovery.

\begin{figure*}
{\vspace*{1pt}}
\includegraphics{fig02}
{\vspace*{1pt}}
\caption{\label{fig2}Methods of lithium extraction from spent
lithium-ion batteries. Data extracted from~\cite{11}.}
{\vspace*{1pt}}
\end{figure*}

Hydrometallurgy relies in extraction, purification, and recovery.
Selective separation technologies are chosen according to the
properties of products and impurities, requiring a leaching
step~\cite{12}. While this method has low energy requirements and
minimal gas emissions, wastewater treatment is essential~\cite{11}. In
the bio-hydrometallurgy, bioleaching involves an interaction between
microorganisms and metals for dissolution and then
extraction. \mbox{Bio-hydrometallurgy} offers innovative contributions for a
greener process~\cite{13}. 

The pyrometallurgical method involves the incineration and~smelting of
LIBs in a plasma arc furnace at temperatures above 700~\textdegree C.
Metals such as copper, cobalt, iron, and nickel are recovered in the
form~of alloys, while plastics and all organic compounds are
incinerated. Aluminum, manganese, silicon, calcium, lithium, and traces
of iron are found in the slag, requiring a hydrometallurgical process
for metal recovery~\cite{11,14,15} (Figure~\ref{fig2}). Although a simple
operation, it requires costly equipment, excessive energy and is
polluting~\cite{12}.

Lately there has been a growing exploration of the direct physical
recycling process. This method begins with a mechanical pretreatment
method leading to individual cells. Then, these cells undergo treatment
using supercritical CO$_{2}$, enabling the extraction and treatment of
electrolytes. After reducing temperature and pressure, CO$_{2}$ can be
separated from the \mbox{electrolytes.} Finally, the cathode material can be
reused (Figure~\ref{fig2}). This method has demonstrated several
advantages, including low energy consumption, a high recovery rate and
it is environmentally friendly. Nevertheless, it has high operational
and equipment requirements. Additionally, not all materials can be
successfully recovered through this process~\cite{11}, similar to the
pyrometallurgical method which requires a hydrometallurgical method in
order to recover metals effectively. 

\section{Hydrometallurgy} \label{sec5}

This section will provide a deep description of the hydrometallurgical
process which is commonly \mbox{combined} with the pyrometallurgical process.
This technique primarily involves two steps: leaching and purification.
This method has a great worldwide industrial potential and is the
predominant approach to extract lithium. Hydrometallurgy is more
suitable in recycling lithium when LIBs have a single chemical
composition~\cite{16}.

\begin{figure*}
\includegraphics{fig03}
{\vspace*{2pt}}
\caption{\label{fig3}Mechanism of the leaching process of cathode
materials from Li-ion batteries. Extracted from~\cite{17}.}
{\vspace*{2pt}}
\end{figure*}

\subsection{Leaching process}

The objective of leaching is to obtain metal ions in an aqueous
solution from the cathodes of the spent LIBs. The mechanism involves
the following steps: initially, the reagent diffuses and adheres to the
\mbox{material's} surface~\cite{17}. Then, an \mbox{oxidoreduction} reaction on the
surface, results in the formation of highly soluble species. Reaction
products detach from the surface and disperse (Figure~\ref{fig3}).
Choosing the right leaching agent is crucial to achieve high \mbox{efficiency}
in the material dissolution. High selectivity towards the target metal
is desired as well as the regeneration ability of the leaching agent.
Leaching agents can be acids (inorganic or organic) or alkaline
solutions. Acid leaching is the most common method since it can
dissolve the cathode materials effectively. The most commonly used
acids for leaching are \mbox{hydrochloric} acid, nitric acid, and sulfuric
acid~\cite{8}. Sulfuric acid shows a non-oxidizing nature in contrast
to HCl and HNO$_{3}$. When HCl or HNO$_{3}$ reacts with cathode
materials, production of NO$_{x}$ or Cl$_{2}$ gases
is observed. Cl$_{2}$ is hazardous for the environment as well as
SO$_{x}$ and NO$_{x}$. As a
result, HCl is avoided because of its high cost and corrosive
properties. Inorganic acids have
demonstrated high efficiency for lithium and cobalt with rates of 98
and 95\%, respectively~\cite{16}.

Table~\ref{tab2} shows various leaching agents reported in the
literature, along with their corresponding leaching rates. The data
indicate a consistent approximate yield of 99\% for lithium, while the
yield can vary from 70 to 90\% for cobalt.

%tab2
\begin{table*}
\caption{\label{tab2}Research results for the highest leaching rates of
spent Li-ion batteries in inorganic acids}
\begin{tabular}{ccccccc}
\thead
\parbox[t]{2cm}{\centering Cathode materials}  &amp; Leaching agent  &amp; 
$T$ (\textdegree C)  &amp; 
\parbox[t]{1.5cm}{\centering S/L ratio (g/L)}  &amp; 
\parbox[t]{1.3cm}{\centering Time (min)} &amp; 
\parbox[t]{2cm}{\centering Leaching rate (\%)}\vspace*{2pt} &amp; {Ref.}  \\
\endthead
{LCO}  &amp; 2 M H$_{2}$SO$_{4}$ + 10 vol\% H$_{2}$O$_{2}$ &amp; 70 &amp; \030 &amp; 60 &amp; 
\parbox[t]{1.3cm}{\centering Co 98.5\Lbreak  Li 99.8}\vspace*{3pt} &amp; \cite{18} \\
{LCO}  &amp; 2 M H$_{2}$SO$_{4}$ + 5 vol\% H$_{2}$O$_{2}$ &amp; 80 &amp; \050 &amp; 60 &amp; 
\parbox[t]{1.3cm}{\centering Co $\sim$ 99\Lbreak  Li $\sim$ 99}\vspace*{3pt} &amp; \cite{19} \\
{LCO}  &amp; 1 M HNO$_{3}$ + 1.7 vol\% H$_{2}$O$_{2}$ &amp; 75 &amp; \010 &amp; 30 &amp;
\parbox[t]{1.3cm}{\centering Co $\sim$ 99\Lbreak  
{Li $\sim$ 99}}\vspace*{2pt} &amp; \cite{20} \\
{LCO}  &amp; 0.7 M H$_{3}$PO$_{4}$ + 4 vol\% H$_{2}$O$_{2}$ &amp;  &amp;  &amp;  &amp; 
\parbox[t]{1.4cm}{\centering Co $\sim$ 99.7\Lbreak  Li $\sim$ 99.9}\vspace*{3pt} &amp; \cite{21} \\
{LCO}  &amp; 2 M H$_{2}$SO$_{4}$ + 5 vol\% H$_{2}$O$_{2}$ &amp; 75 &amp; 100 &amp; 60 &amp; 
\parbox[t]{1.4cm}{\centering Co $\sim$ 70\Lbreak  Li $\sim$ 99.1} &amp; \cite{22}\vspace*{3pt}
\botline
\end{tabular}
\end{table*}

The use of organic acids has been also explored, such as oxalic acid
and citric acid among others. Organic acids exhibit notable
characteristics such as their propensity for easy degradation,
recyclability, and eco-friendliness. On the other hand, they are more
expensive than the inorganic acids and their leaching rates are slower.
As a result, the industry has not yet employed organic acids for the
recovery of metals in spent LIBs~\cite{8}.

Alkaline leaching is a well-known method in metal extraction, involving
the interaction between hydroxide ions and metals. Sodium hydroxide and
{ammonium} species are the most common leaching agents. Sodium hydroxide
can dissolve the aluminum efficiently and it is slightly corrosive.
\mbox{Ammonia} is used mainly in spent LIBs containing copper, cobalt, nickel,
and zinc. Normally, for cathode leaching, a solution containing
ammonium sulfite, ammonia, ammonium carbonate and deionized water is
prepared. During the leaching process, ammonium carbonate acts as a
buffer and ammonium sulfite serves as a reductant. Afterwards, ammonium
sulfite converts nickel and cobalt from their insoluble high oxidation
states of to their soluble lower oxidation states~\cite{8}. 

\subsection{Purification process}

The purification process aims to separate valuable metals from the
spent LIB solution after the \mbox{leaching} process. This leaching solution
includes elements such as iron, aluminum, copper, manganese, nickel,
cobalt, and lithium. This process could \mbox{include} solvent extraction,
electrochemical extraction, and chemical precipitation. To enhance
metal recovery, these techniques usually are used in
combination~\cite{8}. For example, the use of solvent extraction
combined with chemical precipitation allows the selective separation of
copper, nickel, and lithium through chemical precipitation, while
cobalt and manganese are efficiently separated using solvent
extraction.

\subsubsection{Solvent extraction}

Solvent extraction uses a two-phase system to separate lithium from the
leached cathode material, resulting in an efficient method since it
demonstrates to be highly selective for metal ions. This technique
relies on relative solubility to separate ions from polar and non-polar
liquid. Nonpolar extractants are mainly used to separate valuable
metals such as cobalt, nickel, magnesium, and lithium. Common
extractants include di-(2-ethylhexyl) \unskip\break{phosphoric} acid (D2EHPA),
diethylhexyl phosphoric acid (DEHPA), bis-(2,4,4-tri-methyl-pentyl)
phosphinic acid (Cyanex 272), trioctylamine (TOA), and 2-ethylhexyl
phosphonic acid mono-2-ethylhexyl ester (PC-88A). In some cases, a
mixture of two or more extractants is employed to enhance
selectivity~\cite{8,9}. Table~\ref{tab3} displays a compilation of
solvent extraction processes conducted over recent years, employing
various spent materials. The extraction efficiency and purity of
Li$_{2}${CO}$_{3}$ vary depending on the spent material and the
extractant employed.

%tab3
\begin{table*}
\caption{\label{tab3}Research of solvent extraction process of spent
Li-ion batteries (LIBs) in the literature~\cite{10}{\vspace*{-2pt}}}
\begin{tabular}{cccccc}
\thead
{Spent materials}  &amp; {Residue}  &amp; {Efficiency (\%)}  &amp; {Purity (\%)}  &amp; {Extractants}  &amp; {REF} \\
\endthead
{LIB scraps}  &amp; Li$_{2}$CO$_{3}$ &amp; 80 &amp; ${&gt;}$98 &amp; DEHPA Cyanex 272 &amp; \cite{23} \\
{LiCoO$_{2}$} &amp; Li$_{2}$SO$_{4}$ solution &amp; Leaching 100 &amp; ${&gt;}$99.5 &amp; PC-88A &amp; \cite{24} \\
{LiNi$_{0.5}$Co$_{0.2}$Mn$_{0.3}$O$_{2}$}
 &amp; Li$_{2}$CO$_{3}$ &amp; Leaching 94 &amp; - &amp; Cyanex 272 &amp; \cite{25} \\
{LIB scraps}
 &amp; Li$_{2}$CO$_{3}$ &amp; 72 &amp; 99.7 &amp; DEHPA Cyanex 272 &amp; \cite{26} \\
{LiCoO$_{2}$}
 &amp; Li$_{2}$CO$_{3}$ &amp; 86 &amp; 74.2 &amp; Cyphos II-101 &amp; \cite{27}\vspace*{2pt}
\botline
\end{tabular}
\end{table*}

\subsubsection{Electrochemical extraction} 

Electrochemical deposition had demonstrated to be an effective method
since it can separate the metals from a solution based on the potential
difference between the electrodes. For example, Chu et~al.~\cite{28}
built a system using aluminum foil as cathode, the active material from
the spent LIB as anode, and sulfuric acid as the electrolyte. Through
electrolysis, a successful separation between the aluminum foil and the
active material is achieved. The process can be explained as follows:
copper enters the electrolyte as impurity and is electrodeposited on
the cathode, while nickel, cobalt, and manganese remain undissolved.
The aluminum foil remains insoluble within the solution~\cite{16}. This
method proved to have an extraction efficiency for lithium ranging
between 75--95\% and the capability to extract other metals beyond
Li$_{2}$CO$_{3}$. The recycled lithium exhibited a high purity level of
99\%.

Other electrochemical methods extract lithium using a lithium-selective
transmissive membrane where spent LIBs are placed in a system using a
Li-ion--conductive ceramic solid electrolyte. When active material is
placed in water, lithium in the charged cathode does not dissolve while
the lithium in the anode dissolves in the water forming lithium
hydroxide~\cite{9}.

\subsubsection{Chemical precipitation}

Chemical precipitation is the most commonly used method for separating
lithium from solutions after leaching the spent LIB cathodes. This
method uses the pH-dependent difference of solubility in the active
material to extract valuable metals. 
Chemical precipitation is used to
remove ionic constituents from water by the addition of counterions to
reduce solubility. Most metal are precipitated as hydroxides, sulfides,
and carbonates~\cite{28}. Chemical precipitation generally involves
simultaneous and rapid occurrence of nucleation, crystal growth, and
other mechanisms, such as Ostwald ripening and agglomeration~\cite{29}.
The main reactants include NaOH, Na$_{3}$PO$_{4}$ and Na$_{2}$CO$_{3}$ 
among others. They are used to precipitate valuable metals ions
followed by lithium recovery, mainly in the form of Li$_{2}$CO$_{3}$ or
Li$_{3}$PO$_{4}$ (Table~\ref{tab4}). Li$_{2}$CO$_{3}$ is the most
sought-after form of lithium as it consistently yields the highest
purity \mbox{levels.}

%tab4
\begin{table*}
\caption{\label{tab4}Research of precipitation processes of spent
Li-ion batteries (LIBs) in the literature{\vspace*{-2pt}}}
\begin{tabular}{lcccc}
\thead
{Spent materials} &amp; {Residue} &amp; {Efficiency (\%)} &amp; {Purity (wt\%)} &amp; {REF} \\
\endthead
{LiCoO$_{2}$}  &amp; Li$_{2}$SO$_{4}$ &amp; 90 &amp; - &amp; \cite{30} \\
{LIB scraps}    &amp; LiF &amp; 50 &amp; ${&gt;}$99 &amp; \cite{31} \\
{Cathode materials}  &amp; Li$_{2}$CO$_{3}$ &amp; 80  &amp; 96.97 &amp; \cite{32} \\
{Cathode materials}  &amp; Li$_{2}$CO$_{3}$ &amp; 71 &amp; - &amp; \cite{33} \\
{Mixed cathode materials}  &amp; Li$_{2}$CO$_{3}$ &amp; 80 &amp; - &amp;~\cite{34} \\
{LiNi$_{0.3}$Co$_{0.3}$Mn$_{0.3}$O$_{2}$} &amp; Li$_{2}$CO$_{3}$ &amp; Leaching 99.7  &amp; - &amp; \cite{35} \\
{Cathode materials}  &amp; Li$_{3}$PO$_{4}$ &amp; 89 &amp; - &amp; \cite{36} \\
{LiNi$_{0.3}$Co$_{0.3}$Mn$_{0.3}$O$_{2}$} &amp; Li$_{2}$CO$_{3}$ &amp;  98 &amp; 99.9 &amp; \cite{37} \\
{LiFePO$_{4}$} &amp; Li$_{3}$PO$_{4}$ &amp; 82.6 &amp; 96.5 &amp; \cite{38} \\
{Cathode material and graphite} &amp; Li$_{2}$CO$_{3}$ &amp; Leaching 99.9 &amp; 99.4 &amp; \cite{39} \\
{LiFePO$_{4}$}  &amp; Li$_{2}$CO$_{3}$ &amp; 95.1 &amp; 99.95 &amp; \cite{40} \\
{Cathode materials}  &amp; Li$_{2}$CO$_{3}$ &amp; ${&gt;}$90 &amp; 99.93 &amp; \cite{41} \\
{Cathode materials}  &amp; Li$_{3}$PO$_{4}$ &amp; 85.6 &amp; - &amp; \cite{42} \\
{Anode}  &amp; Li$_{2}$CO$_{3}$ &amp; Leaching 100  &amp; ${&gt;}$99 &amp; \cite{43} \\
{LiNi$_{0.3}$Co$_{0.3}$Mn$_{0.3}$O$_{2}$}  &amp; Li$_{2}$CO$_{3}$ &amp; Leaching 99 &amp; - &amp; \cite{44} \\
{LiNi$_{0.5}$Co$_{0.2}$Mn$_{0.3}$O$_{2}$} &amp; Li$_{2}$CO$_{3}$ &amp; 76 &amp; ${&gt;}$99.5 &amp; \cite{45} \\
{LiFePO$_{4}$}  &amp; Li$_{2}$CO$_{3}$ &amp; 99.4 &amp; - &amp; \cite{46} \\
{Cathode} &amp; Li$_{2}$CO$_{3}$ &amp; 38 &amp; ${&gt;}$98.48 &amp; \cite{47} \\
{LiFePO$_{4}$}  &amp; Li$_{2}$CO$_{3}$ &amp; 80 &amp; - &amp; \cite{48} \\
{LiNi$_{0.5}$Co$_{0.2}$Mn$_{0.3}$O$_{2}$}  &amp; Li$_{2}$CO$_{3}$ &amp; 91.2  &amp; 99 &amp; \cite{49} \\
{LiNi$_{x}$Co$_{1-x}$\ $_{-y}$Mn$_y$O$_{2}$}  &amp; Li$_{3}$PO$_{4}$ &amp; Leaching 99.1 &amp; - &amp; \cite{50}\vspace*{2pt}
\botline
\end{tabular}
{\vspace*{-2pt}}
\end{table*}

\subsection{Lithium precipitation in the form of $\mathrm{Li}_2\mathrm{CO}_3$}


Carbonate precipitation is normally used in water treatment to reduce
calcium hardness but nowadays it is a great option for lithium recovery
from spent LIBs. It has been demonstrated that carbonate is able to
decrease the solubility of some metals, while \mbox{increasing} that of
others. The effect depends on the target metal and the pH at which the
treatment is carried out~\cite{18,51}. Lithium carbonate exhibits
inverse solubility, thus the higher the temperature, the lower the
residual content of dissolved lithium and \mbox{therefore} the higher the
quantity of precipitated lithium carbonate~\cite{28,52}
(Figure~\ref{fig4}). 

\begin{figure}
{\vspace*{2pt}}
\includegraphics{fig04}
\caption{\label{fig4}Evolution of the lithium recovery rate as a
function of temperature. Data extracted from~\cite{34}.}
{\vspace*{2pt}}
\end{figure}

In the recycling industry, Li$_{2}$CO$_{3}$ is precipitated after a
saturated solution of sodium carbonate is added into the mixture
resulting from the leaching process. The precipitation reaction can be
described as follows:
{\begin{equation}\label{eq1}
2\,{\mathrm{Na}}_{(\mathrm{aq})}^{+}+
{\mathrm{CO}}_{3(\mathrm{aq}) }^{2-}+2\,{\mathrm{Li}}_{(\mathrm{aq})}^{+}
\rightarrow {\mathrm{Li}_{2}}
{\mathrm{CO}_{3}}_{(\mathrm{s})}+2\,{\mathrm{Na}}_{(\mathrm{aq})}^{+}
\end{equation}}\unskip
In this reaction, lithium ions in the solution react with sodium
carbonate, resulting in the formation of lithium carbonate along with
sodium ions. 

The precipitation of Li$_{2}${CO}$_{3}$ using carbon dioxide
represents a promising and sustainable approach for lithium recovery.
The carbonation process begins with the dissolution of CO$_{2}$ {in
water:}
{\begin{equation}\label{eq2}
{\mathrm{H}_{2}}\mathrm{O}_{(\mathrm{aq})}+
{\mathrm{CO}_{2}}_{(\mathrm{g})}\rightarrow {\mathrm{H}_{2}}
{\mathrm{CO}_{3}}_{(\mathrm{aq})}
\end{equation}}\unskip

For precipitation to occur, the pH of the system should be alkaline, at
least 8 and optimally higher than 10. This is mainly because the
dominant species in the system shifts to $\mathrm{CO}_{3}^{2-}$ at pH above
10, as indicated in the following reaction:
{\begin{equation}\label{eq3}
{\mathrm{H}_{2}\mathrm{CO}_{3}}_{(\mathrm{aq})}
\leftrightarrow {\mathrm{H}^{+}}_{(\mathrm{aq})}+
{\mathrm{CO}_{3}^{2-}}_{(\mathrm{aq})}
\end{equation}}\unskip
Lithium and bicarbonate ions coexist in the system. By maintaining an
alkaline pH, lithium precipitation occurs, as shown in the next
equation:
{\begin{equation}\label{eq4}
{\text{LiHCO}_{3}}_{(\mathrm{aq})}+{\mathrm{OH}^{-}}_{(\mathrm{aq})}
+{\mathrm{Li}^{+}}_{(\mathrm{aq})}
\rightarrow {\mathrm{Li}_{2}\mathrm{CO}_{3}}_{(\mathrm{s})}+
{\mathrm{H}_{2}\mathrm{O}}_{(\mathrm{aq})}
\end{equation}}\unskip
Different studies had explored this pathway (Table~\ref{tab4}). For
example, Han et~al.~\cite{53} obtained a Li$_{2}${CO}$_{3}$ precipitate
after a leaching process of Li$_{2}${SO}$_{4}$ using CO$_{2}$ gas in
an alkaline solution. After analyzing different samples performed at
various operational parameters, it was found that temperature and pH
play a key role in the process. A substantial concentration of
$\mathrm{CO}_{3}^{2-}$ ions within the system is critical to achieve a state
of supersaturation, which enables the processes of nucleation and
crystal growth. This study reveals a lithium recovery rate between 41.6
and 45.5\% at an operational temperature of 50~\textdegree C. Matsumoto
et~al.~\cite{54} managed to precipitate lithium in the form of
Li$_{2}$CO$_{3}$ by injecting CO$_{2}$ microbubbles where it was
demonstrated that bubble size influences the particle size of the
precipitate. The effect of supersaturation and temperature in the
precipitation process were noted as well. In the study of Ramirez
et~al.~\cite{52}, a Li$_{2}$CO$_{3}$ precipitation was carried out
using a NaOH solution and injecting a gas flow of CO$_{2}$ followed by
a CO$_{2}$ purification to increase lithium purity, resulting in a
lithium recovery yield of 48.3\% and purity of 99.9\%. Carbonate
precipitation was demonstrated to be the safest, most economical, and
most efficient method compared to the other methods discussed
previously. However, the primary drawback lies in the low lithium
concentration remaining in the solution after leaching~\cite{8,9}.
Hence, the implementation of additional techniques, such as membrane
technologies, should be considered. Nevertheless, it is important to
acknowledge that this would increase operational complexity and costs.

\section{Conclusion}\label{sec6}

Nowadays, lithium recycling is becoming an economic, environmental, and
governmental issue due to the growing demand of e-mobility and
electronic devices. For that reason, finding novel routes in the
development of greener and low-cost processes is indispensable in the
recycling of Li-ion batteries (LIBs). The recycling industry has
encountered numerous drawbacks and challenges in the extraction of
valuable metals from LIBs. Because of this, secondary sources of
lithium are currently considered as unreliable for its integration into
the lithium supply chain. However, promising progress has been made in
the past years with regards to recycling LIBs. Different treatments and
processes are being explored and, among the methods studied,
hydrometallurgy involving carbonate precipitation has proved to be the
most affordable and eco-friendly approach for lithium extraction.
Additionally, using CO$_{2}$ not only produces a precipitate with
superior purity but also plays a significant role in \mbox{enhancing} the
\mbox{sustainability} of the process. Although this method offers several
advantages, it is crucial to combine chemical precipitation with other
techniques to enhance metal recovery yields. Further research should
focus on improving and integrating hydrometallurgy with other
technologies to advance the production of the next-generation batteries
from recycled lithium.\looseness=-1


\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

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