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\DOI{10.5802/crchim.306}
\datereceived{2023-09-30}
\daterevised{2024-02-07}
\dateaccepted{2024-03-14}
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\dateposted{2024-04-25}
\begin{document}

\begin{noXML}

%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{GDR Prom\'eth\'ee}
%\def\cdr@specialtitle@french{GDR Prom\'eth\'ee}
%\makeatother

\CDRsetmeta{articletype}{research-article}

\title{A sustainable solvothermal process extracting critical elements
from Li-ion batteries}

\alttitle{Un proc\'{e}d\'{e} solvothermal durable pour extraire les
\'{e}l\'{e}ments critiques des batteries Li-ion}

\author{\firstname{Virginie} \lastname{Nachbaur}\CDRorcid{0000-0002-8971-0089}\IsCorresp}
\address{GPM, Normandie Univ, UNIROUEN, INSA Rouen, CNRS, 76000 Rouen, France}
\email[V. Nachbaur]{virginie.nachbaur@univ-rouen.fr}

\author{\firstname{Firas} \lastname{Ayadi}}
\addressSameAs{1}{GPM, Normandie Univ, UNIROUEN, INSA Rouen, CNRS,
76000 Rouen, France}
%\email{ayadifiras11@yahoo.fr}

\author{\firstname{B\'eatrice} \lastname{Foulon}}
\addressSameAs{1}{GPM, Normandie Univ, UNIROUEN, INSA Rouen, CNRS,
76000 Rouen, France}
%\email{beatrice.foulon@univ-rouen.fr}

\author{\firstname{Malick} \lastname{Jean}}
\addressSameAs{1}{GPM, Normandie Univ, UNIROUEN, INSA Rouen, CNRS,
76000 Rouen, France}
%\email{malick.jean@univ-rouen.fr}

\author{\firstname{Val\'erie} \lastname{Pralong}\CDRorcid{0000-0003-4644-8006}}
\address{CRISMAT, Univ. Caen, ENSICAEN, CNRS, 14050 Caen, France}
%\email{valerie.pralong@ensicaen.fr}

\author{\firstname{Samuel} \lastname{Jouen}\CDRorcid{0009-0003-9199-6252}\IsCorresp}
\addressSameAs{1}{GPM, Normandie Univ, UNIROUEN, INSA Rouen, CNRS,
76000 Rouen, France}
\email[S. Jouen]{samuel.jouen@univ-rouen.fr}

\shortrunauthors

\keywords{\kwd{Li-ion
batteries}\kwd{Solvothermal}\kwd{Polyol}\kwd{Critical
elements}\kwd{Recycling}}

\altkeywords{\kwd{Batteries
Li-ion}\kwd{Solvothermal}\kwd{Polyol}\kwd{\'{E}l\'{e}ments
critiques}\kwd{Recyclage}}

\thanks{ESP (Energy and Propulsion Systems) Carnot Institute, European
Union and the Normandy region (FEDER/FSE 2014-2020) and CNRS (French
National Center for Scientific Research)}

\begin{abstract}
A solvothermal process was developed for extracting cobalt, nickel,
lithium and manganese from LiCoO\textsubscript{2} (LCO) or
Ni\textsubscript{(1--\textit{x}--\textit{y})} \text{Mn}\textsubscript{\textit{x}}\text{Co}\textsubscript{\textit{y}}\text{O}\textsubscript{2} (NMC) cathode active materials,
without using any acid or base, neither high temperature. The solvent
used, ethylene glycol, is a mild reducing agent and does not pose any
unacceptable environmental risk. Structural characterization of the
recovered products showed that cobalt and nickel are together in a
metallic state (Co or Ni--Co alloy), lithium and manganese are in the
form of carbonates easily separable due to their different solubilities
in water. Magnetic separation was used to isolate Ni--Co alloy from
MnCO\textsubscript{3}. The purities of the recovered products depend on the
quantities of LCO powder processed and fall within the range of 
90.5--97\% for lithium carbonate and 96.7--99.9\% for cobalt. In the
case of NMC, the purities of Li\textsubscript{2}CO\textsubscript{3}, MnCO\textsubscript{3} and
Ni\textsubscript{\textit{x}}Co\textsubscript{(1--\textit{x})} are respectively 94.4\%, 95.2\% and 91\%. The
process demonstrated its effectiveness when treating NMC-type black
mass extracted from a mobile phone battery.
\end{abstract}

\begin{altabstract}
Un proc\'{e}d\'{e} solvothermal a \'{e}t\'{e} mis au point pour
extraire le cobalt, le nickel, le lithium et le mangan\`{e}se des
mat\'{e}riaux actifs des cathodes LiCoO\textsubscript{2} (LCO) ou
Ni\textsubscript{(1--\textit{x}--\textit{y})} \text{Mn}\textsubscript{\textit{x}}\text{Co}\textsubscript{\textit{y}}\text{O}\textsubscript{2} (NMC), sans utiliser d'acide ou de
base, ni de haute temp\'{e}rature. Le solvant utilis\'{e},
l'\'{e}thyl\`{e}ne glycol, est un agent r\'{e}ducteur qui ne
pr\'{e}sente aucun risque av\'{e}r\'{e} pour l'environnement. La
caract\'{e}risation structurale des produits r\'{e}cup\'{e}r\'{e}s
\`{a} l'issue du traitement a montr\'{e} que le cobalt et le nickel
sont tous deux \`{a} l'\'{e}tat m\'{e}tallique (Co ou alliage Ni--Co),
et que le lithium et le mangan\`{e}se sont sous la forme de carbonates,
facilement s\'{e}parables en raison de leurs diff\'{e}rences de
solubilit\'{e}s dans l'eau. Une s\'{e}paration magn\'{e}tique permet
d'isoler l'alliage Ni--Co du MnCO\textsubscript{3}. Pour LCO, les puret\'{e}s des
produits extraits d\'{e}pendent des quantit\'{e}s de poudre de LCO
trait\'{e}es et sont comprises entre 90,5 et 97 \% pour le carbonate de
lithium et entre 96,7 et 99,9~\% pour le cobalt. Dans le cas des NMC,
les puret\'{e}s de Li\textsubscript{2}CO\textsubscript{3}, MnCO\textsubscript{3} et Ni\textsubscript{\textit{x}}Co\textsubscript{(1--\textit{x})} sont
respectivement de 94,4~\%, 95,2~\% et 91~\%. Le proc\'{e}d\'{e} a
\'{e}galement d\'{e}montr\'{e} son efficacit\'{e} lors du traitement
d'une black mass extraite d'une batterie de t\'{e}l\'{e}phone portable
dont la cathode est de type NMC.
\end{altabstract}

\maketitle
{\vspace*{.55pc}}

\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1}

Mineral resources play an essential role in the technologies associated
with energy transition. These resources contain elements designated as
critical by many countries. In particular, the growing deployment of
electric vehicles requires considering the use of certain elements as a
geopolitical issue. This is particularly the case for elements present
in lithium-ion batteries (LiB) such as cobalt, lithium, nickel,
manganese, or graphite, whose increasing demand has revealed
interdependencies and vulnerabilities~\cite{1}. In response to the
urgency of the situation, the European Union is seeking to reduce its
dependence while initiating a more environmentally friendly cycle by
proposing a revision of the old Battery Directive~\cite{2}. The newly
proposed directive~\cite{3} sets very ambitious recycling rate targets
which must increase, between 2027 and 2031, from 90 to 95\% for Co and
Ni, and from 50 to 80\% for Li, but also establishes, for the first
time, requirements for recycled content in new batteries. These
recycled contents must reach, by 2036, 26\% for Co, 15\% for Ni and
12\% for Li. Recycling is, therefore, a strategic aspect since, by
2027, it~is estimated that around 50,000 tons of used batteries will
need to be processed in Europe, allowing for the recovery of 25,000 to
30,000 tons of metals of interest. Currently, recycling rates struggle
to increase (less than 1\% of Li is actually recycled globally), as
recycling faces many barriers that hinder its development. Among these,
the consumption of energy and chemicals is a significant factor in the
cost of operations. This is particularly the case for pyrometallurgical
and hydrometallurgical methods which are used at the industrial level
to recycle elements such as cobalt, lithium, nickel, copper or
manganese~\cite{4,5,6}. Pyrometallurgy, although it remains a
frequently used process for the extraction of high-value metals such as
cobalt and nickel, suffers from several drawbacks such as high
operating temperature, production of toxic gases, high energy costs and
the limited number of materials recovered (Li and Al are lost in the
slag). Although hydrometallurgy (dissolution in acidic media in the
presence of a reducing agent) is the preferred process for LiB
recycling~\cite{7,8} due to high purity and recovery rate of metals,
this process suffers from high consumption of quite aggressive media,
unavoidably leading to the production of acidic effluents, and from the
use of expensive extractants~\cite{9}. In order not to add acid,
interesting studies have been carried out using subcritical water and
an addition of chlorinated polyvinyl chloride (CPVC) or polyvinylidene
fluoride (PVDF)~\cite{10,11}. The acid formed during the degradation of
CPVC at 290~{\textdegree}C or PVDF at 350~{\textdegree}C (HCl or HF)
induces the leaching of the valuable metals. Furthermore, several
researches have been conducted using organic acids, ionic liquids (ILs)
and deep eutectic-solvents (DESs), with the promise of a green
approach~\cite{11,12,13,14}. However, despite a great deal of research
in these areas, it has proved impossible to transpose the studies
carried out on a laboratory scale to an industrial scale. Among the
probable causes of this failure, the excessive viscosity of these
solvents, their difficulty in being recycled, their limited chemical
stability and their prohibitive prices are frequently
mentioned~\cite{15}.\looseness=-1

The so-called polyol process has been used for the synthesis of finely
divided metal particles from their oxides, hydroxides, or salts in
polyalcohols~\cite{16,17,18}. The polyol (e.g., ethylene glycol,
diethylene glycol) acts as both solvent and reducing agent. This method
can also be performed in a solvothermal way, which is a treatment
performed in a closed reactor, above the boiling temperature of the
solvent. In this case, a reduction of metal oxides is also observed,
leading to the obtaining of several morphologies of the nanoparticles
related to the concentration of the precursor~\cite{19}. Although
ethylene glycol is often used as a component in mixtures (associated to
a hydrogen bond acceptor) in DESs~\cite{20} or to a mild organic acid
in an eco-friendly leaching system~\cite{21,22}, it has never been used
alone to extract metals for recycling purposes.\looseness=-1

Ethylene glycol (EG) is a major industrial organic compound used in the
production of polyester fibers and films, antifreeze, coolant, and
numerous other applications. This solvent presents favorable features
like low toxicity and noncorrosiveness, and it is environmentally
benign and readily biodegradable~\cite{23}.

In this study, a new original approach with a low environmental
footprint was developed to recycle lithium-ion batteries
cathodes~\cite{24}. This process is based on a solvothermal treatment
of the cathode material in the presence of EG and carbonate ions and
without any acid. Performed at low temperature (225~{\textdegree}C),
the process converts the phase present in the cathode compound into
several products easily separable. Ni and Co ions are reduced to the
zero valent state, together in an alloy. Manganese cations and lithium
cations combine with carbonate ions to form manganese carbonate and
lithium carbonate. The resulting powder (which is a mixture of Ni--Co
alloy, lithium carbonate and manganese carbonate) may be separated from
the solvent by centrifugation, and the several solid compounds
isolated, thanks to their solubilities, by a succession of washings and
magnetic separation. In this work, the characterization of the
recovered compounds was performed, their purities were determined, and
the recovery rates by elements were calculated. As some of the Li$^+$
cations remain in EG after treatment, our research aims at showing that
this solvent can be reused several times with the same efficiency, and
that this reuse allows to recover all Li$^+$ cations coming from the
previous steps. The combination of solvent and source of carbonate ions
was first ascertained in the case of LCO in terms of recovery rate and
purity of the recovered powders. The possibility of reusing the solvent
in order to reduce the environmental impact of the process was then
assessed. Subsequently, the process was investigated both on NMC 111
cathode powder and on a black mass (BM) containing NMC cathode active
material.

\section{Materials and methods} \label{sec2}
\subsection{Materials} \label{sec2.1}

The cathode powders (LiCoO$_2$: LCO or
LiCo$_{0.33}$Ni$_{0.33}$Mn$_{0.33}$O$_2$: NMC 111) were either obtained
from MSE Supplies or synthesized through the ceramic route. X-ray
diffraction (XRD) characterizations and chemical analyses of these
powders are presented in Supporting Information Figures~S1 and~S2. The
black mass (BM) was obtained from mobile phone batteries (Xiaomi model
BN44), by discharging, opening in a glovebox, shredding in a cutting
mill (Retsch SM 300), and sieving in a vibratory sieve shaker (Fritsch
Analysette~3 Spartan) using a 100~$\upmu$m grid size. XRD
characterization of the sieved BM and inductively coupled plasma (ICP)
analysis are shown in Supporting Information Figure~S3. As graphite
peaks are observed (ICDD file No.~65-6212), one can conclude that
sieving was not efficient to remove graphite present in the BM. In
fact, other separation techniques exist or are currently being
developed to extract graphite from black mass before any processing,
such as froth flotation~\cite{25,26}. Furthermore, ICP and
Energy-dispersive X-ray spectroscopy (EDX) analysis (Supporting
Information Figure~S4) indicate that the cathode active material is a
NMC-type.

The process is performed in a Teflon-lined bomb provided by Parr
Instruments (4744 model, 45~mL) or by Top Industrie (2148 5000, 500~mL
or 2148 4000, 1~L), depending on the amount of solvent used. The
solvent used is ethylene glycol (Merck~99\%) in the presence of
carbonate ions (e.g. potassium carbonate Alfa Aesar 99.0\% min), with
an amount corresponding to 1.2$\times$ the stoichiometric amount of
carbonate ions necessary to form Li$_2$CO$_3$ from Li$^+$ cations
contained in the cathode powder. A pressure calculation based on the
saturation vapor pressure of ethylene glycol and the air pressure at
225~{\textdegree}C, indicates that in the 45~mL reactor using 30~mL of
ethylene glycol, the pressure is expected to be 3.9~bars (i.e. an
overpressure of 2.9~bars). Table~\ref{tab1} summarises the operating
conditions for the treatments performed in this study.

\begin{table}%tab1
\caption{\label{tab1}Conditions applied for the treatments of LCO and
NMC cathode powders and for BM}
\begin{tabular}{ccc}
\thead
\parbox[t]{1.5cm}{\centering Sample name\raisebox{-.4pc}{}} &
\parbox[t]{2.3cm}{\centering Mass of powder treated (g)} &
\parbox[t]{1.7cm}{\centering EG volume (mL)}\\
\endthead
LCO1 & 1.5 & v-EG 30\\
LCO2 & 15 & v-EG 300\\
LCO3 & 30 & v-EG 600\\
LCO4 & 15 & r-EG 300\\
NMC & 1.5 & v-EG 30\\
BM Xiaomi & 2 & v-EG 30
\botline
\end{tabular}
\tabnote{v-EG: virgin ethylene glycol; r-EG: reused ethylene glycol.}
\end{table}

\subsection{Experimental procedure} \label{sec2.2}

After introducing the cathode powder or the black mass, the reactor was
filled with ethylene glycol in which potassium carbonate was previously
dissolved. The reactor was then closed and heated to 225~{\textdegree}C
for 10~h in an oven. This choice of experimental conditions comes from
the conditions usually applied during polyol reduction treatments of
metal oxides under solvothermal conditions (temperature higher than the
boiling point of the solvent). After cooling, the reaction mixture
comprises of both powder and liquid, and several steps of
centrifugation, rinsing and magnetic separation (if needed) were
carried out. These steps are illustrated in Figures~\ref{fig1}a
and~b for the LCO and NMC cathode powder treatments,
respectively. For LCO (Figure~\ref{fig1}a), the solvent was first
separated from the powder by centrifugation and stored for possible
reuse. Then, the extracted powder was rinsed with distilled cold water
while undergoing ultrasound dispersion, and separated again from water
by a second step of centrifugation (P2). The rinse water was evaporated
in an oven to recover a white powder (P1).

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}Summary diagram of the different stages of the
solvothermal process for LCO cathode powder~(a) and for NMC cathode
powder and Black Mass~(b).}
\end{figure*}

For NMC (Figure~\ref{fig1}b), a subsequent step was added, in which the
rinsed black powder was magnetically separated to isolate magnetic and
non-magnetic fractions, each of which was dried. Magnetic separation
was performed by mixing, under ultrasonic dispersion, the powder to be
separated with water or ethanol in a beaker, and by approaching a
Nd$_2$Fe$_{14}$B magnet close to the side of the beaker, which attracts
magnetic part of the powder (this operation was repeated twice). The
fraction that was not attracted to the magnet was a beige powder
(P2$'$), while the fraction that was attracted to the magnet was a
black powder (P3$'$).

\subsection{Characterization methods} \label{sec2.3}

The crystallinity and purity of the obtained materials were
characterized using an Empyrean diffractometer (PAnalytical) or a D8
diffractometer (Bruker AXS), using Co K${\upalpha}$ radiations. The
intensity was measured with a 2${\uptheta}$ step of 0.01{\textdegree}.
Phase identification was done using DIFFRAC.EVA (Bruker AXS). Powders
were observed by Scanning Electron Microscopy (SEM) equipped with an
Energy-dispersive X-ray spectroscopy (EDX) detector (Zeiss 1530).
Infrared spectroscopy analyses were performed with a Shimadzu IR
affinity-1 spectrometer. ATR spectra of liquid samples were obtained
using a GladiATR instrument equipped with a monolithic diamond by
accumulating 32 scans in the spectral range from [4000--400] cm$^{-1}$
with a resolution of 4~cm$^{-1}$. \mbox{Elemental} quantification was
performed by Crealins (Lyon, France) using inductively coupled
plasma-atomic emission spectrometry (ICP-AES) iCAP 6500 Duo (Thermo
Scientific) after acidic dissolution of the samples in the presence of
sulfuric acid/nitric acid mixtures in an open environment at
220--240~{\textdegree}C. The relative error on the mass \% of the
elements is~5\%.

\section{Results and discussion} \label{sec3}
\subsection{Application of the process to LCO material} \label{sec3.1}
\subsubsection{Use of virgin ethylene glycol} \label{sec3.1.1}

For this first experiment (LCO1), XRD analysis and Scanning Electron
Microscopy results of P1 and P2 are presented in Figure~\ref{fig2}. P1
powder (Figure~\ref{fig2}a) consists of grains of few tens of
micrometers and contains only one phase of lithium carbonate
(monoclinic, ICDD file No.~22-1141). P2 powder (Figure~\ref{fig2}b)
contains particles of a few hundreds of nanometers which are formed
through the aggregation of smaller particles. Phase analysis shows that
two phases are present: a face-centered cubic (fcc) phase and a
hexagonal close-packed (hcp) corresponding to metallic cobalt (ICDD
files No.~15-0806 and~\mbox{89-4308} respectively). This morphology and
the coexistence of both phases is classically observed in the case of
synthesis of cobalt nanoparticles by polyol way from cobalt
salts~\cite{17,27}.

\begin{figure*}
\includegraphics{fig02}
{\vspace*{-.4pc}}
\caption{\label{fig2}XRD patterns and SEM micrographs of P1~(a) and
P2~(b) obtained after treating LCO compound.}
{\vspace*{-.6pc}}
\end{figure*}

Chemical analysis of the residual EG, after separation from the solid
by centrifugation, shows that it contains dissolved lithium, with an
amount corresponding to approximately 30~wt\% of the initial lithium
present in the LCO compound.

At this stage, the mechanism involved in the process can be explained
as follows: LiCoO$_2$ phase is destroyed by the joint action of EG
which reduces Co$^{2+}$ cations to Co$^0$, and carbonate ions which
extract lithium cations to form lithium carbonate, according
to~(\ref{eq1})

{\begin{eqnarray}\label{eq1}
&& \text{LiCoO}_{2} + \text{CH}_{2} \text{OHCH}_{2} \text{OH} + 
0.5 \text{CO}_{3}^{2-}\nonumber\\
&&\quad \rightarrow 0.5\text{LiCO}_{3} + \text{Co} +
[\text{EG}]_{\mathrm{ox}{\cdot}\mathrm{product}} 
\end{eqnarray}}\unskip
where [EG]$_{\mathrm{ox}{\cdot}\mathrm{product}}$ represents the
possible oxidation products of EG as it will be discussed in
Section~\ref{sec3.1.2}.

The reduction is performed without any dissolution of cobalt cations,
as cobalt was not detected in the residual EG. Lithium carbonate is
partially soluble in EG, its solubility has been evaluated, under our
solvothermal experimental conditions, to be approximately 3~g/L. After
separation of the residual EG by centrifugation, the solid contains
Co$_{(s)}$ and Li$_2$CO$_{3(s)}$. Li$_2$CO$_3$ is soluble in water,
especially cold water (13.1~g/L at 20~{\textdegree}C and 15.2~g/L at
0~{\textdegree}C~\cite{24}), so it dissolves in water during the
rinsing, while Co remains as a powder (P2). Lithium carbonate can be
recovered by evaporating water, this is P1.

Recovery rates for lithium and cobalt are calculated according to
Equations~(\ref{eq2}) and~(\ref{eq3}) where m$_{\mathrm{LCO~treated}}$
is the mass of LCO treated (1.5, 15 or 30~g), m$_{\mathrm{P1}}$ and
m$_{\mathrm{P2}}$ are the recovered mass of P1 and P2 powders,
\%mass(Li) is the Li content in P1 and \%mass(Co) is the Co content in
P2, these two values being determined by ICP analysis. The values 7.4\%
and 62.1\% are the content of respectively Li and Co in the initial
cathode powder obtained by ICP analysis (Supporting Information
Figure~S1).
{\begin{eqnarray}
\label{eq2} \text{Li recovery rate} &=& \frac{\text{\%mass (Li)} \times
\mathrm{m}_{\mathrm{P1}}}{7.4\% \times \mathrm{m}_{\mathrm{LCO~treated}}}
\times 100\quad\Seqnsplit
\label{eq3} \text{Co recovery rate} &=& \frac{\text{\%mass (Co)} \times
\mathrm{m}_{\mathrm{P2}}}{62.1\% \times \mathrm{m}_{\mathrm{LCO~treated}}}
\times 100\quad
\end{eqnarray}}
\noindent Recovery rates of Co and Li (respectively 84.1\% and 40\%), and
purities of Co and Li$_2$CO$_3$ (respectively 96.7\% and 90.5\%) are
indicated in Table~\ref{tab2} for this first experiment. Concerning the
recovery rate of lithium in the form of carbonate powder, it should be
remembered that part of this carbonate is dissolved in EG (around
30\%). This part will be recovered during subsequent solvent reuse (see
Section~\ref{sec3.1.2}). These \mbox{values}, especially for the
recovery rate of lithium, are quite low, if one examines the new
directive concerning battery recycling~\cite{3}. This value of around
70\% is below the threshold to be reached for lithium (80\%).

\begin{table*}%tab2
\tabcolsep=9.2pt
\caption{\label{tab2}Recovery rates of Li, Co, Ni and Mn and purity of
P1, P2, P2$'$ and P3$'$ powders, calculated from ICP-AES analyses of
the different powders extracted{\vspace*{-.3pc}}}
\begin{tabular}{ccccccccc}
\tbody
\xmorerows{1}{Sample} & \multicolumn{8}{c}{{LCO cathode}}\\\cline{2-9} 
& \multicolumn{4}{c}{Recovery rate (\%)} && \multicolumn{3}{c}{Purity (\%)}\\
\cline{2-5}\cline{7-9} 
&\multicolumn{2}{c}{{Li}$^{\mathrm{a}}$} &
\multicolumn{2}{c}{{Co}} &&
{Li}$_{{2}}${CO}$_{{3}}$ & {Co metal}&\\
LCO1 & \multicolumn{2}{c}{40.0} & \multicolumn{2}{c}{84.1} && 90.5 & 96.7&\\
LCO2 & \multicolumn{2}{c}{69.7} & \multicolumn{2}{c}{99.1} && 95.2 & 99.9&\\
LCO3 & \multicolumn{2}{c}{55.9} & \multicolumn{2}{c}{98.1} && 97.1 & 99.9&\\
LCO4 & \multicolumn{2}{c}{101.3$^{\mathrm{b}}$} &
\multicolumn{2}{c}{99.3} && 95.4 & 99.9&\\\xxbotline
\xmorerows{1}{Sample}
& \multicolumn{8}{c}{{NMC 111 cathode}}\raisebox{1.2pc}{}\\
\cline{2-9} 
& \multicolumn{4}{c}{Recovery rate (\%)} &&
\multicolumn{3}{c}{Purity (\%)}\\\cline{2-5}\cline{7-9} 
& {Li}$^{\mathrm{a}}$ & {Co} & {Ni} & {Mn} && {Li}$_{{2}}${CO}$_{{3}}$ & 
{Ni}$_{{x}}${Co}$_{{1}-{x}}$ {alloy} & {MnCO}$_{{3}}$\\
NMC & 24 & 79 & 80.5 & 72 && 94.4 & 91 & 95.2
\botline
\end{tabular}
\tabnote{$^{\mathrm{a}}$Considering only P1 powder, without lithium
dissolved in EG, $^{\mathrm{b}}$up to 100\% due to the presence of Li
ions in EG coming from the previous treatment.{\vspace*{-.5pc}}}
\end{table*}

As this first experiment was performed on a small amount of powder
(1.5~g), unavoidable materials losses during the several steps
(centrifugation, drying) have a strong influence on the recovery rates.
The impact of these losses on the recovery rate is minimized by
increasing the quantity of powder treated. The recycling process was
performed on higher amounts of powder (15~g and 30~g, samples LCO2 and
LCO3), with the same steps as described previously. As for the first
experiment, a powder of Co was isolated, and a powder of Li$_2$CO$_3$
was recovered after evaporating the rinsing water. As it can be seen in
Table~\ref{tab2}, values of recovery rates and purity are greatly
improved. The recovery rate of lithium in Li$_2$CO$_3$ form is between
55.9 and 69.7\% with a purity between 95.2 and 97.1\%. The recovery
rate of cobalt is between 98.1 and 99.1\%, i.e. identical for LCO2 and
LCO3 taking into account measurement uncertainties, and with a purity
of 99.9\%. 

\subsubsection{Ethylene glycol reuse} \label{sec3.1.2}

In order to save EG and to recover more lithium, the solvent was reused
(up to 4 times, but the result of only 1 time is shown here, sample
LCO4). The phases obtained are the same that in Section~\ref{sec3.1.1},
and the values of recovery rates and purities are indicated in
Table~\ref{tab2}. In this case, the recovery rate of lithium
(calculated by comparing with the lithium amount present in 15~g of LCO
treated) is higher than 100\% because Li$^+$ cations present in the
solvent (and coming from a previous treatment of LCO) have
\mbox{precipitated} as carbonate during the second treatment as
saturation was reached more easily for this salt. This experiment also
shows that EG is not degraded during the process, and that it remains
efficient as a reducer for possible reuse. 

In order to confirm the integrity of the ethylene glycol, infrared
spectrometry was carried out on the solvent after extraction of the
solid phase by centrifugation. Figure~\ref{fig3} shows the infrared
spectra of virgin ethylene glycol, of the solvent after extraction of
samples LCO1 and LCO4, and of a solvent after three successive reuses.
The spectrum of the ethylene glycol presents absorption bands which
correspond perfectly to what is expected~\cite{28}. After treatment,
the solvent spectra still show all the absorption bands characteristic
of ethylene glycol. Additional absorption bands appear at 1595, 1355
and 1125~cm$^{-1}$, whose intensities increase with the number of times
the solvent is reused. The two bands located at 1595, 1355~cm$^{-1}$
could be attributed to asymmetric and symmetric elongation vibrations
of the COO$^-$ group in a glycolate (CH$_2$OHCOO$^-$), or glyoxylate
(CHOCOO$^-$), both of which are oxidation products of ethylene
glycol~\cite{29,30,31}. Doubt remains as to the attribution of the weak
absorption band at 1125~cm$^{-1}$, whose wavenumber is slightly higher
than already observed for these two compounds~\cite{29}. These results
confirm that the reduction of Co(III) to cobalt metal during treatment
does indeed involve a redox mechanism with ethylene glycol. The exact
nature of the ethylene glycol oxidation product(s) remains to be
confirmed. In addition, the solvent being in very large excess compared
with the cathode compound (EG/LCO molar ratio $=$ 36/1 in the case of
LCO1, LCO2 and LCO3), it can be reused several times before being
significantly oxidized.

\begin{figure}
{\vspace*{.4pc}}
\includegraphics{fig03}
\caption{\label{fig3}Infrared spectra of virgin ethylene glycol, EG
after treatment and extraction of samples LCO1 and LCO4, and of EG
after three successive reuses in the process.}
{\vspace*{-.3pc}}
\end{figure}

\subsection{Application of the process to NMC material} \label{sec3.2}

\looseness=-1
The process applied to NMC 111 powder, and following the scheme of
Figure~\ref{fig1}b, leads to the obtaining of 3 fractions of powder
named P1, P2$'$ and P3$'$. XRD analysis and Scanning Electron
Microscopy results of these 3 powders are presented in
\mbox{Figure}~\ref{fig4}.\ As in the treatment of LCO, P1
(Figure~\ref{fig4}a) only contains Li$_2$CO$_3$, with the same
morphology that was previously observed (grains of few tens of
micrometers). P2$'$ powder (Figure~\ref{fig4}c) consists in
octahedral-shaped grains of around 10 micrometers and contains MnCO$_3$
as a single phase (rhombohedral, ICDD file No.~44-1472).\ P3$'$ powder
(Figure~\ref{fig4}b) mainly contains spherical particles with
heterogeneous sizes between few hundreds of nanometers and several
micrometers, which are formed through the aggregation of smaller
grains.\ Some octahedral grains are also observed.\ Concerning P3$'$
powder, the XRD structural characterization shows the presence of
diffraction peaks whose positions are between those of cobalt (ICDD
file No.~15-0806) and nickel (ICDD files No.~87-0712), indicating the
presence of a single-phase Ni--Co alloy. No hcp structure was
identified, as usually observed while forming Ni--Co alloys by polyol
way~\cite{17}. Peaks of MnCO$_3$ with a low intensity were also
observed in P3$'$ due to an incomplete magnetic separation, which
explains the observation of octahedral-shaped grains in the SEM picture
(Figure~\ref{fig4}b).

\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}XRD patterns and SEM micrographs of P1~(a),
P3$'$~(b) and P2$'$~(c) after treating NMC 111 compound. A small amount
of MnCO$_3$ is present in P3$'$ due to an incomplete magnetic
separation.}
\end{figure*}

The mechanism explained in Section~\ref{sec3.1.1} can be completed as
follows for the treatment of NMC. After extraction of Li$_2$CO$_3$ by
its dissolution in water, the remaining powder contains MnCO$_3$ (which
is insoluble in water~\cite{32}) and the Ni--Co alloy. As the alloy is
ferromagnetic, it is attracted by permanent magnet, whereas MnCO$_3$ is
not.

Recovery rates of Co, Ni, Mn and Li are reported in Table~\ref{tab2}
(respectively 79\%, 80.5\%, 72\% and 24\%). The value for cobalt is of
a similar magnitude as when treating an equivalent quantity of LCO
(84.1\%). The purity of lithium carbonate (94.4\%) is also comparable
to what was obtained for LCO (90.5\%). The low recovery rate for
lithium means that Li$^+$ cations remain in EG, due to the
precipitation of MnCO$_3$. These results show that, in EG, manganese
carbonate precipitates preferentially and that it is therefore
necessary to increase the amount of carbonate ions in order to improve
the lithium recovery rate. The purity in the Ni--Co alloy (91\%) is
lower than for metallic cobalt in LCO, which can be explained by the
presence of MnCO$_3$ in the alloy. The magnetic separation, which was
performed by a very simple method, is the reason of this low value. The
purity could be considerably increased by using more suitable
techniques adapted for magnetically separating powders, such as
high-gradient magnetic separator (HGMS) which creates strong magnetic
forces developed in association with the intense fields delivered by a
superconducting magnet~\cite{33,34}.\looseness=-1

\subsection{Application of the process to a black mass} \label{sec3.3}

Despite the presence of graphite in addition to the active cathode
material, the recycling treatment was applied to this BM to ensure that
the graphite did not affect the process. It should be noted that in
this case, magnetic separation was not carried out, which leads to the
isolation of only two fractions instead of three. The first fraction is
obtained after centrifugation and rinsing with water, and should
contain the Ni--Co alloy and manganese carbonate (mixture of P2$'$ and
P3$'$) if we refer to the Figure~\ref{fig1}b. The second fraction is
recovered by evaporating the rinsing water (P1) and should contain
lithium carbonate. XRD characterization of these two fractions is shown
in Figure~\ref{fig5}.

\begin{figure*}
{\vspace*{.8pc}}
\includegraphics{fig05}
\caption{\label{fig5}XRD patterns of the mixture $\text{P2}' +
\text{P3}'$~(a), and P1~(b) after treating the BM extracted from a
Xiaomi battery.}
\end{figure*}

As expected, the powder fraction ($\text{P2}' + \text{P3}'$) contains
the alloy and MnCO$_3$, but it also contains graphite, which obviously
was not affected by the process (Figure~\ref{fig5}a).\ Concerning the
powder P1, it contains Li$_2$CO$_3$, but this phase is not the main one
(Figure~\ref{fig5}b).\ Another phase, identified as Li$_3$PO$_4$ (ICDD
file No.~25-1030) is present with a higher content by regarding its
diffraction peak intensities. But the main phase (by intensity) is a
third compound, identified as LiF (ICDD file No.~45-1460). The
formation of these two phases could be due to the presence of PVDF
and/or the electrolyte in the BM before the treatment, although we did
not identify these two compounds by the techniques we used. Concerning
PVDF, its degradation seems unlikely at the temperature applied in the
present study. Indeed, PVDF undergoes thermal decomposition only in the
range 400--500~{\textdegree}C under atmospheric pressure~\cite{35}, and
under supercritical conditions in water it decomposes only above
290~{\textdegree}C~\cite{11}. Furthermore, it was shown that the PVDF
present on cathodes was not degraded when treated in ethylene glycol at
a temperature close to its boiling point
(197~{\textdegree}C)~\cite{36}. A~recent study shows that the
conversion of LiPF$_6$ into LiF and Li$_3$PO$_4$ is possible by water
extraction using a hydrothermal way~\cite{37}.\ In this study,
the~authors show that the interest of treating LiPF$_6$ lies in the
fact that this compound is an environmental pollutant, and that
producing LiF and Li$_3$PO$_4$ is economically profitable. The
possibility that the process developed in the present study could be
another way of treating LiPF$_6$ needs to be supported by additional
characterisation, particularly spectroscopic, in order to clearly
highlight the presence of the electrolyte before
treatment.\looseness=-1

\section{Conclusions} \label{sec4}

A new method for recycling lithium-ion battery cathodes has been
developed. This method enables the extraction and the separation of
valuable elements (cobalt, nickel, lithium, manganese) without the need
for high temperatures or the consumption of acids or bases. The solvent
used, with low environmental impact, is reusable. Its continuous reuse
allows for the accumulation of lithium as lithium carbonate, which
partially dissolves during treatment, until EG becomes saturated,
thereby increasing the lithium recovery rate.

\looseness=-1
Furthermore, it has been shown that recovery rates, especially for
cobalt, are significantly improved when processing large quantities of
active cathode materials. A magnetic separation step is necessary when
the processed material contains cobalt, nickel, and manganese
simultaneously. This step strongly influences the purities of the
Ni--Co alloy and manganese carbonate collected after treatment. The use
of more efficient magnetic separation devices than the one used in this
study could greatly enhance these purities.

The application of this treatment to NMC-type black mass from a battery
has demonstrated that the presence of graphite does not affect this
process. Additionally, it has been shown that the recovered powder also
contains LiF. Further investigations need to be conducted to determine
if this compound originates from the decomposition of LiPF$_6$ present
in the electrolyte.

\section*{Declaration of interests}

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

\section*{Funding}

The authors would like to thank the ESP (Energy and Propulsion Systems)
Carnot Institute for their financial support in the context of the
NeoBATT project. This work was additionally supported by the European
Union and the Normandy region as part of the operational program
FEDER/FSE 2014-2020 through the RECYLION project, and by the CNRS
(French National Center for Scientific Research) in the frame of the
prematuration CARY-ION project.

\CDRGrant[EU]{FEDER/FSE 2014-2020}

\back{}

\section*{Supplementary data}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crchim-306-suppl.pdf}}

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