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\begin{document}

\title{Dual additives for stabilizing Li deposition and SEI formation in
anode-free Li metal batteries}



\author[1]{Baolin Wu}%
\author[2]{Chunguang Chen}%
\author[3]{Dmitri Danilov}%
\author[3]{Zhiqiang Chen}%
\author[4]{Ming Jiang}%
\author[1]{Rüdiger-A. Eichel}%
\author[3]{Peter Notten}%
\affil[1]{Forschungszentrum Jülich GmbH}%
\affil[2]{Chinese Academy of Sciences Institute of Mechanics}%
\affil[3]{Eindhoven University of Technology}%
\affil[4]{Hangzhou Dianzi University College of Electronics and Information}%


\vspace{-1em}



  
  \date{May 27, 2024}


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\selectlanguage{english}
\begin{abstract}
Anode-free Li-metal batteries (AFLBs) are of significant interest to
energy storage industries due to their intrinsically high energy.
However, the accumulative Li dendrites and dead Li continuously consume
active Li during cycling. That results in a short lifetime and low
Coulombic efficiency (CE) of AFLBs. Introducing effective electrolyte
additives can improve the Li deposition homogeneity and solid-state
interphase (SEI) stability for AFLBs. Herein, we reveal that introducing
dual additives, composed of LiAsF6 and FEC, into a commercial carbonate
electrolyte will boost the cycle life and average CE of
NCM\textbar{}\textbar{}Cu AFLBs. The NCM\textbar{}\textbar{}Cu AFLBs
with the dual additives exhibit a capacity retention of about 75\% after
50 cycles, much higher than those with bare electrolytes (35\%). The
average CE of the NCM\textbar{}\textbar{}Cu AFLBs with additives can
maintain 98.3\% over 100 cycles. In contrast, the average CE without
additives rapidly decline to 97\% after only 50 cycles. In situ Raman
measurements reveal that the prepared dual additives facilitate denser
and smoother Li morphology during Li deposition. The dual additives
significantly suppress the Li dendrite growth, enabling stable SEI
formation on anode and cathode surfaces. Our results provide a broad
view of developing low-cost and high-effective functional electrolytes
for high-energy and long-life AFLBs.%
\end{abstract}%



\sloppy


Article category: Full Paper

Subcategory: Lithium Ion Batteries

Dual additives for stabilizing Li deposition and SEI formation in
anode-free Li metal batteries

Baolin Wu, Chunguang Chen,\textsuperscript{*} Dmitri L.
Danilov,\textsuperscript{*} Zhiqiang Chen, Ming Jiang, R\selectlanguage{ngerman}üdiger-A. Eichel
and Peter H.L. Notten\textsuperscript{*}

Baolin Wu, Dr. Chunguang Chen, Dr. Dmitri L. Danilov, Prof. Rüdiger-A.
Eichel, Prof. Peter H.L. Notten

Forschungszentrum Jülich (IEK-9), D-52425, Jülich, Germany;

Baolin Wu, Prof. Rüdiger-A. Eichel

RWTH Aachen University, D-52074 Aachen, Germany.

Dr. Chunguang Chen

LNM, Institute of Mechanics, Chinese Academy of Sciences, Beijing
100190, China;

School of Engineering Sciences, University of Chinese Academy of
Sciences, Beijing 100049, China.

Dr. Dmitri L. Danilov, Dr. Zhiqiang Chen, Prof. Peter H.L. Notten

Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The
Netherlands.

Dr. Ming Jiang

Insitute of Crabon Neutrality and New Energy, School of Electronics and
Information, Hangzhou Dianzi University, Hangzhou 310018, China.

Prof. Peter H.L. Notten

University of Technology Sydney, Broadway, Sydney, NSW 2007, Australia

E-mail: p.h.l.notten@tue.nl ; chenchunguang@imech.ac.cn;
D.Danilov@tue.nl

\textbf{Keywords} : Dual-additives, Li growth, SEI formation, anode-free
lithium metal battery, in situ Raman

\textbf{Abstract.} Anode-free Li-metal batteries (AFLBs) are of
significant interest to energy storage industries due to their
intrinsically high energy. However, the accumulative Li dendrites and
dead Li continuously consume active Li during cycling. That results in a
short lifetime and low Coulombic efficiency (CE) of AFLBs. Introducing
effective electrolyte additives can improve the Li deposition
homogeneity and solid-state interphase (SEI) stability for AFLBs.
Herein, we reveal that introducing dual additives, composed of
LiAsF\textsubscript{6} and FEC, into a commercial carbonate electrolyte
will boost the cycle life and average CE of NCM\textbar{}\textbar{}Cu
AFLBs. The NCM\textbar{}\textbar{}Cu AFLBs with the dual additives
exhibit a capacity retention of about 75\% after 50 cycles, much higher
than those with bare electrolytes (35\%). The average CE of the
NCM\textbar{}\textbar{}Cu AFLBs with additives can maintain 98.3\% over
100 cycles. In contrast, the average CE without additives rapidly
decline to 97\% after only 50 cycles. \emph{In situ} Raman measurements
reveal that the prepared dual additives facilitate denser and smoother
Li morphology during Li deposition. The dual additives significantly
suppress the Li dendrite growth, enabling stable SEI formation on anode
and cathode surfaces. Our results provide a broad view of developing
low-cost and high-effective functional electrolytes for high-energy and
long-life AFLBs.

1. Introduction

Lithium-ion batteries (LIBs) have become the critical component and
technology limiting the driving range of electric vehicles
(EVs).\textsuperscript{{[}1{]}} Boosting the energy density of LIBs will
certainly lengthen EVs' range. Li metal is regarded as the most
attractive negative electrode for high-energy batteries due to its high
theoretical specific capacity (3860 mAh·g\textsuperscript{-1}) and low
redox potential (-3.04 V vs. standard hydrogen
electrode).\textsuperscript{{[}2, 3{]}} However, Li metal anode commonly
suffers from intensive side reactions, low Coulombic efficiency (CE),
and rapid capacity fading. These problematic issues originate from Li
dendrite growth during Li plating/stripping and the continuous formation
of solid electrolyte interphase (SEI), which consumes cyclable
lithium.\textsuperscript{{[}4-6{]}} Additionally, Li metal batteries
often use thick Li metal foils at the anode side, which reduces
batteries' energy density and causes safety
concerns.\textsuperscript{{[}7{]}}

Anode-free Li metal batteries (AFLBs) with current-collector
(Cu)\textbar{}\textbar{}electrolyte\textbar{}\textbar{}cathode\textbar{}\textbar{}current-collector
cell configuration can be used to overcome the drawbacks of conventional
Li metal battery systems.\textsuperscript{{[}8, 9{]}} In AFLBs, the
cathode offers active Li source for being plated on the Cu or other type
of current-collector substrate. In this sense, no excess of Li metal is
present in pristine AFLBs.\textsuperscript{{[}10, 11{]}} The absence of
Li metal anode leads to less weight, less volume, lower manufacturing
costs, and therefore higher energy density than conventional Li metal
batteries.\textsuperscript{{[}12, 13{]}} The anode-free design was
previously considered impractical due to the short cycle life and low
CE, caused by unstable SEI formation, Li dendrites, and dead Li
metal.\textsuperscript{{[}9{]}} The Li source in AFLBs is only provided
by the cathode material. Therefore the formation of SEI and dead Li
metal rapidly consume cyclable Li-ions, drastically decreasing battery
capacity.\textsuperscript{{[}8{]}} The CE of Li deposition must be
considered while developing practical AFLBs to achieve high energy
density and recyclability.

Electrolyte engineering effectively improves the CE by manipulating the
Li deposition growth and the SEI
composition.\textsuperscript{{[}14{]}}In fact, engineering electrolyte
is an elaborate combination of several methods, including altering
organic solvents and salt chemistry, adding additives, and manipulating
concentrations of all components. Introducing a functional additive is
the most promising among these electrolyte engineering methods. This
method can effectively control lithium metal's plating/exfoliation
behavior and modify the SEI layer's chemistry. Additives are applied in
trace doses (usually less than 5\%), making balancing the performance
and cost easier.\textsuperscript{{[}15, 16{]}} Electrolyte additives can
be categorized into two groups: inorganic additives such as
LiNO\textsubscript{3}\textsuperscript{{[}17{]}} and
AlCl\textsubscript{3}\textsuperscript{{[}18{]}}, and organic additives
like vinylene carbonate (VC)\textsuperscript{{[}19{]}} and
fluoroethylene carbonate (FEC).\textsuperscript{{[}20{]}} The
fluorine-containing electrolyte additives have superior kinetic
reactivity, which facilitates the formation of more LiF components in
the SEI layers to improve stability.\textsuperscript{{[}21{]}} FEC is
one of Li metal batteries' most celebrated electrolyte additives. It
forms a stable yet elastic LiF-rich SEI on Li
metal.\textsuperscript{{[}22{]}} Recently, many fluorine-containing
salts have also been explored as additives, such as
LiBF\textsubscript{4} and
LiAsF\textsubscript{6}.\textsuperscript{{[}23{]}} However, in contrast
with Li metal batteries, a single additive is insufficient for effective
application in AFLBs due to the complexity of electrochemical processes
involved. Therefore, exploring suitable additives for anode-free systems
is essential for developing high-performance batteries.

Herein, a facile and effective dual additive of
LiAsF\textsubscript{6}and FEC in a commercial carbonate electrolyte (1M
LiPF\textsubscript{6}-EC/DEC/DMC (1:1:1 vol\%)) is reported. The
proposed dual additive enhances the electrochemical performance of the
AFLBs using NCM532 as the cathode and bare Cu foil as the anode current
collector. The NCM\textbar{}\textbar{}Cu cell possesses an excellent
electrochemical performance with high-capacity retention of
\textasciitilde{}75\% after 50 cycles and an average CE of
\textasciitilde{}98.3\% in the presence of dual additives. \emph{In
situ} Raman measurements were performed to investigate the effect of the
dual additives of LiAsF\textsubscript{6} and FEC on the morphology of
deposited Li metal and formed SEI layers. The synergistic effect of
LiAsF\textsubscript{6} and FEC contributes to smoother lithium plating.
It suppresses the formation of Li dendrites and dead Li metal.
Furthermore, F-rich SEI and CEI layers were observed to form on the Li
anode and NCM-cathode surfaces, facilitating the stability of anode-free
cells.

\textbf{2. Results and discussion}

The Cu\textbar{}\textbar{}Li half-cell test was electrochemically cycled
to understand polarization, capacity retention, and CE. The cyclic CE
indicates the rate of Li consumption during the Li plating/stripping
process and the degree of electrolyte decomposition. The electrochemical
performance of Cu\textbar{}\textbar{}Li cell cycled at 0.5
mA·cm\textsuperscript{-2} with the plating stripping capacity of 2
mAh·cm\textsuperscript{-2} is shown in Figure 1a. Figure 1b and c
illustrates the polarization voltages of the cells with E1 and E2
electrolytes, respectively. The cells with E2 electrolyte charged and
discharged at 0.5 mA·cm\textsuperscript{-2} show a 22 mV and 29 mV
polarization voltage for the 2\textsuperscript{nd} and
10\textsuperscript{th} cycles, respectively.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1.} Electrochemical performance of
Cu\textbar{}\textbar{}Li cells at 0.5 mA\selectlanguage{ngerman}·cm\textsuperscript{-2} current
density with E1 and E2. (a) Voltage profiles with E1 and E2;
Polarization profile of plating/stripping process using E1 (b) and E2
(c); (d) Coulombic efficiency curves.

In contrast, under the same current density, the polarization voltage
was 37 mV and 45 mV at the 2\textsuperscript{nd} and
10\textsuperscript{th} cycles for the cells with E1 electrolyte. The
cells with E1 exhibit more significant polarization than E2 due to the
faster growth of Li dendrites.\textsuperscript{{[}24, 25{]}} The Li
dendrites growth increases resistance due to an additional amount of SEI
and dead Li metal formation. That leads to a decline in CE and capacity.
Because the Li metal anode side has a surplus of Li-ions in the half
Cu\textbar{}\textbar{}Li cell, the CE indicates the Li loss on the Cu
current collector due to the decomposition of electrolytes. The CE
performance of Cu\textbar{}\textbar{}Li cells at 0.5
mA·cm\textsuperscript{-2} for both E1 and E2 electrolytes is shown in
Figure 1d. The initial CE of the cells with E2 electrolytes was 90\% at
0.5 mA·cm\textsuperscript{-2}, which is higher than the initial CE of
the cells with E1 electrolytes (80\%). Furthermore, after a few cycles,
the CE of the cells with E1 electrolytes declined, and fluctuations
started. In comparison, the CE is more stable and higher in cells with
E2 electrolytes. Additionally, the capacity of Cu\textbar{}\textbar{}Li
cells with E2 electrolytes is more stable than that of cells with E1
electrolytes.

The NCM\textbar{}\textbar{}Cu full AFLBs were also assembled with bare
Cu as the anode current collector and
LiNi\textsubscript{0.5}Co\textsubscript{0.3}Mn\textsubscript{0.2}O\textsubscript{2}(NCM532)
as the cathode. E1 and E2 electrolytes were used to evaluate the
full-cell performance. The tests show that the capacity loss of AFLBs is
only around \selectlanguage{english}~14\% in the first cycle, regardless of the electrolytes E1
and E2 (Figure 2a). This performance is much higher than the results
with carbonate solvent (EC/EMC (3:7 wt \%)), giving 77\% capacity loss
after the first charge,\textsuperscript{{[}26{]}}. The initial discharge
capacity of NCM\textbar{}\textbar{}Cu cells with E2 is 3.98
mAh*cm\textsuperscript{-2}. It is higher than the cells with E1
electrolytes (3.85 mAh*cm\textsuperscript{-2}). With cycling, the
capacity of NCM\textbar{}\textbar{}Cu cells with E1 drops quickly. After
100 cycles, the discharge capacity decreased to 0.5
mAh*cm\textsuperscript{-2}, much lower than that of cells with E2
(\textasciitilde{}2 mAh*cm\textsuperscript{-2}). The capacity retention
(Figure 2b) in E1 is below 15\% after 100 cycles, comparable with
earlier reported results.\textsuperscript{{[}25{]}} The low CE of the
cells with E1 electrolyte can be explained by the depletion of active Li
metal due to the presence of free solvents. These solvents continuously
react with active Li-ions during the continuous plating/stripping
processes. Consumption of electrolytes could be another reason because
the SEI layer formed in traditional carbonate electrolytes originates
from the decomposition of electrolyte
solvents.\textsuperscript{{[}27{]}} On the contrary, the
NCM\textbar{}\textbar{}Cu cells with optimized electrolyte (E2) retained
75\% after 50 cycles (Figure 2b) and even higher than 50\% after 100
cycles. The E2-based cells exhibit a higher average CE of 98.3\% after
100 cycles. From the first CV curves (Figure 2c), it was found that the
current in the CV curve of E2 is higher than for E1 for the same voltage
range (3 to 4 V). That confirms the presence of electrochemical
reactions of the dual additives in this voltage range. The decomposition
of the dual additives facilitates the formation of a more stable SEI
layer and, therefore, a significant improvement in the stability of the
anode-free cell. Figure 2d shows the influence of the current (0.1, 0.2,
0.5, 0.75, and 1 C-rate) on the discharge capacity of the anode-free
cells with two different electrolytes. The initial discharge capacity of
4 mAh*cm\textsuperscript{-2} for the NCM electrode at 0.1 C-rate is
reduced to 2 mAh*cm\textsuperscript{-2} at 1 C-rate. However, the
capacity can almost be fully recovered to 3.5
mAh*cm\textsuperscript{-2}at 0.1 C-rate after 15 cycles, indicating a
better rate capability than the cells using E1 electrolytes. It is worth
noting that the rate capability of the sample with E2 electrolytes is
systematically better than for E1 electrolytes.

To further understand the synergetic effect of the dual additives of
LiAsF\textsubscript{6} and FEC on stabilizing the electrochemical
performance of AFLBs, the Li nucleation overpotentials were measured.
The Li nucleation overpotential probes the Li metal nucleation and
growth behavior on Cu current collectors. The three-electrode system was
used for measuring Li nucleation overpotentials with Cu as the negative
electrode, NCM532 as the positive electrode, and Li foil as the
reference electrode. The voltage profiles of Li metal deposited onto
bare Cu foil during the first charge process in two different
electrolytes are shown in Figure 2e-f. There is a significant voltage
dip at the beginning of Li metal deposition (blue dashed circle marked),
which is the Li nucleation overpotential. A flat voltage plateau follows
this dip. The Li nucleation overpotential is defined as the difference
between the bottom of the voltage dip and the flat part of the voltage
plateau.\textsuperscript{{[}28{]}} The voltage difference represents the
energy necessary to overcome the heterogeneous nucleation barrier due to
the large thermodynamic mismatch between Li metal and Cu. For E1, the
overpotential is around 38 mV. In contrast, the overpotential for E2 is
only 30 mV, indicating a lower nucleation barrier for Li metal with the
dual additives of LiAsF\textsubscript{6} and
FEC.\textsuperscript{{[}29{]}} The dual additives facilitate the
deposition of Li metal on the Cu surface. That is also beneficial to
form more compact Li dendrites.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure 2.} Electrochemical performance of
NCM\textbar{}\textbar{}Cu cells at 0.5 mA\selectlanguage{ngerman}·cm\textsuperscript{-2} current
density with E1 and E2. (a) Voltage profile using E1 and E2; (b)
Capacity retention and CE curves; (c) CV curves; (d) Rate capability at
various indicated C-rates; Voltage profiles of galvanostatic Li
deposition on a copper substrate at mA·cm\textsuperscript{-2} with E1(e)
and E2 (f). \(E_{\text{we}}\) refers to the potential of the Cu current
collector. A clear overpotential was observed, as circled by the dashed
line. The inset shows the overpotentials of Li nucleation.

The top and cross-sectional morphological analyses of the deposited Li
metal were performed. Figure 3a shows the morphology of the deposited Li
metal on Cu with E1 electrolyte. The top and cross-section show the
presence of tiny block crystal structures. After discharging, the
crystal structures disappeared, and the residual Li metal became porous
and non-compact (Figure 3b). After 100 cycles, many cracks are observed
on the surface of the residual Li metal (Figure 3c). Such cracks suggest
that the instability of the SEI is related to electrolyte composition.
The thickness of the Li residues is increasing, which is attributed to
the formation of dead Li metal and SEI layers, resulting in rapid
capacity loss.

In contrast, introducing dual additives makes the deposited Li
morphology after the initial charge more uniform (Figure 3d). It can be
related to the lower overpotential of Li nucleation, which is favorable
for the larger size of Li nuclei. The formed Li-metal structures in
E2-based cells are more compact than those with E1 (Figure 3e). More
importantly, the residual Li-metal structures in E2 cells exhibited
fewer cracks on the surface and were more compact after 100 cycles. In
addition, the thickness of the residue for E2 is smaller than for E1
(Figure 3f), suggesting that introducing dual additives improves the
stability of the formed SEI layers and suppresses the formation of dead
Li metal.

EDX measurements on the residual Li metal after 100 cycles were
performed to investigate further the effect of the dual additives on SEI
formation. As shown in Figure S1, C, O, F, and P elements were found in
the residuals (Figure S1a). These are attributed to the components of
formed SEI, such as Li\textsubscript{2}O,
Li\textsubscript{2}CO\textsubscript{3}, LiF,
LiPO\textsubscript{2}F\textsubscript{2}, and carbonate-based Li
salts.\textsuperscript{{[}30, 31{]}} However, in comparison with the E1
electrolytes, the residues with dual additives contain arsenic, and the
content of F is significantly higher (Figure S1b). This observation
indicates that the additives also participate in forming SEI layers
during cycling. The dual additives promote the formation of
Li\textsubscript{x}As alloy phase and LiF on the formed Li-metal
surface,\textsuperscript{{[}32{]}} enhancing the mechanical properties
of the formed SEI layers, improving the stability.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure 3.} Top (left panel) and cross-sectional (right panel)
SEM images of electrodeposited Li films on Cu substrates in different
states with two electrolytes: pristine electrolyte (E1) after initial
charged (a), first discharged (b) and 100 cycles (c); optimized
electrolyte (E2) after initial charged (d), first discharged (e) and 100
cycles (f).

EIS test was performed to understand the electrolyte/electrode
interfacial behavior. The EIS spectra of the cells with E1 and E2
electrolytes were measured before cycling, after first charging, and
after the 1\textsuperscript{st} and 10\textsuperscript{th} cycles. The
Nyquist plots of the cells with E1 and E2 electrolytes are shown in
Figure 4. They are with one semicircle at the high frequency, attributed
to the Li\textsuperscript{+} diffusion through the SEI on the surface of
the electrode.\textsuperscript{{[}33{]}} Figure 4a-d shows the impedance
of the cells with E1 and E2 electrolytes increase continuously with
cycling. The impedance increase is due to the continuous generation of
SEI as well as the formation of dead Li metal. However, the cell with
the E2 electrolyte has a lower impedance than that with the E1
electrolyte (Figure 4c-d). The lower impedance of E2-based cells
suggests that the dual additives can effectively reduce the cell's
impedance and thus improve cell stability. Dual additives reduce the
barrier of Li nucleation and guide the Li metal to grow in structures
with a smoother and denser morphology. Moreover, introducing F atoms
makes the SEI layer contain more LiF inorganic components. That makes
the formed SEI layer more stable and contributes to reducing the cell's
impedance, thus effectively improving the stability of the anode-free
cell.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 4.} EIS curves of anode-free NCM\textbar{}\textbar{}Cu
full cells that contains E1 and E2 before cycling (a), first charged
(b), after first(c) and 100\textsuperscript{th} (d) cycles.

To further demonstrate the effect of dual additives on the Li
deposition/dissolution and SEI formation, \emph{in situ} Raman was
performed on the NCM\textbar{}\textbar{}Cu cells with two different
electrolytes (E1 and E2). The geometry of the \emph{in situ} Raman cell
is illustrated in Figure S2. NCM532, with a 10 mm diameter, was used as
a cathode. A glass fiber separator soaked with 50 \selectlanguage{greek}μ\selectlanguage{english}L E1 electrolytes was
placed on top of the cathode. 16 mm Cu mesh was placed on top of the
separator. Then the glass window was pressed on the top. The cell was
cycled at C/10 between 4.3 V and 3.0 V. Figure 5a shows the morphology
of the formed Li metal structures during the first cycle. Initially, the
Cu surface is shiny and smooth. After the first 30 min of plating, it
was found that the deposited Li nuclei on the upper Cu surface were
inhomogeneous (30-min image in Figure 5a). With Li plating proceeding,
the plated Li metal show a porous structure (2h and 10 h image in Figure
5a). During the Li deposition process, the growth of inhomogeneous Li
nuclei results in different Li growth rates at different locations. Then
the Li metal grows into the free space with time. The deposited Li metal
exhibit a highly porous structure due to the uniform Li growth process
when the formed Li-metal structures fill the free space. As the Li
stripping process begins, the dissolution rate of Li metal is also
inconsistent at various points. That facilitates the formation of Li
dendrites and dead lithium. It can be observed that when discharged to
3V, there is still a lot of Li metal on the copper surface as well as
dendritic Li metal. That causes a significant initial capacity loss.

The potential curve of the first cycle and the corresponding \emph{in
situ} Raman spectra are shown in Figure 5b. With cycling, a peak at 1846
cm\textsuperscript{-1} starts to be observed, which is formed due to the
symmetric stretching vibration of the triple bond of the acetylide anion
in Li\textsubscript{2}C\textsubscript{2}.\textsuperscript{{[}34,
35{]}}In addition, the signal at 1070 cm\textsuperscript{-1} is
attributed to the symmetric stretching vibration of the carbonate anion
from
Li\textsubscript{2}CO\textsubscript{3}.\textsuperscript{{[}36{]}}Li\textsubscript{2}CO\textsubscript{3}
is an SEI product originating from the decomposition of the organic
carbonates in the electrolyte.\textsuperscript{{[}37{]}} During the
charging process, the intensity of SEI-induced spectra (1070 and 1846
cm\textsuperscript{-1}) continuously grows. The intensity increase is
due to the continuously formed SEI caused by the reaction between the
electrolyte and newly-deposited Li metal. More importantly, during the
discharge process, the SEI does not disappear, even when discharged to
3.0 V, which is another significant cause of the large decay of initial
capacity and CE. The broad signals at 470 and 910
cm\textsuperscript{-1}might be attributed to various origins. These
signals could be from the electrolyte, \emph{e.g.} the ring breathing
modes of EC-lithium complexes R-O and C-C
stretching.\textsuperscript{{[}35{]}} Another possibility could be the
decomposition of the polymer-based SEI, known as organic
SEI.\textsuperscript{{[}37{]}}

Figure 5c shows the optical microscopy images of Li metal
plating/stripping behavior with E2 electrolyte. In the first 30 min of
Li plating (Figure 5c), the Li nuclei were preferably deposited on the
bottom of the copper surface. However, when charging to 4.3V, only a
small amount of Li metal was found on the upper blank parallelogram
area. That indicates more compact Li-metal structures formed on the
bottom of the copper surface. In the stripping process, less rod-shaped
Li metal was left. In addition, when discharged to 3V, the
earlier-formed Li-metal structures on the upper Cu surface disappeared.
This observation confirms that the dual additives improve Li plating and
stripping reversibility. Dual additives facilitate the Li metal to form
a more compact and smooth Li morphology.

Figure 5d shows \emph{in situ} Raman spectra recorded during the first
cycle. Similarly, SEI peaks at 1070 and 1846
cm\textsuperscript{-1}appeared during the charging process. The SEI
peaks can still be observed after discharge to 3.0 V, indicating that
the formed SEI layers are stable. That could be well-protected plated Li
metal. Compared to the E1 electrolyte, E2 can increase LiF content in
the SEI components, thus improving stability. The theoretical Raman
signal of LiF is at 228 cm\textsuperscript{-1}. However, the signal is
very weak and can hardly be traced in the Raman spectra because the
formed LiF layer is very thin.\textsuperscript{{[}38{]}} Nevertheless,
\emph{in situ} Raman measurements could effectively resolve the origins
of the high-capacity loss and low CE of NCM\textbar{}\textbar{}Cu cells
by observing the formation of dead Li metal and SEI layers. The dual
additives effectively guide the Li deposition in planar surfaces to form
a more compact and smoother Li metal anode, which improves the stability
of AFLBs.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Figure 5.} \emph{In situ} Raman measurements of
NCM\textbar{}\textbar{}Cu cells with E1 (a-b) and E2 (c-d) electrolytes.
Microscopic images on the surface of copper current collector at
different cycling states with E1 (a) and E2 (c) electrolytes; \emph{In
situ} Raman spectra for the copper current collector with E1 (b) and E2
(d) electrolytes during the first cycle. The potential curves (left) and
pseudocolor plots (right) are shown together.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 6.} Top (left panel) and cross-sectional (right panel)
SEM images of NCM with two electrolytes: fresh NCM cathode (a-b), after
100 cycles with pristine electrolyte(c-d) and optimized electrolyte
(e-f).

In addition to the positive effects of the dual additives on the
deposited Li metal, the effect on the NCM cathodes was also explored.
Top and cross-sectional morphological analysis of the NCM cathodes after
100 cycles with two different electrolytes (Figure 6). It was shown that
NCM developed many cracks after 100 cycles with E1 electrolytes (Figure
6c). In contrast, the morphology of NCM is better preserved after 100
cycles with E2 electrolytes (Figure 6e). According to the
cross-sectional SEM images (Figure 6b, d, f), the NCM cycled with E2
electrolytes is more compact. In addition, the existence of the fluorine
on the surface of the NCM cycled with E2 electrolytes was confirmed by
EDX measurements (Figure S3). This observation indicates that
introducing dual additives of LiAsF\textsubscript{6} and FEC facilitates
the formation of fluorine-containing composites on the surface of the
NCM cathode. These composites protect the NCM and improve its cycling
stability. Raman tests were performed on the NCM cathodes at different
cycling states, as shown in Figure 7. Bands between 300 and 750
cm\textsuperscript{-1} correspond to metal-oxygen vibrations. Bands at
1350 and 1600 cm\textsuperscript{-1} refer to the Raman shifts of the
carbon black conductive additive.\textsuperscript{{[}39, 40{]}} For the
NCM cathode cycled with E1 electrolyte, the peak of at 450 and 580
cm\textsuperscript{-1} decreased after charge to 4.3V. It is likely that
a kind of structural (dis-)ordering transition (hexagonal phases H1,
H1+H2) happens during the charging process when the electrode is
partially delithiated.\textsuperscript{{[}41{]}} Subsequently, the peaks
increased after full discharge. However, after 100 cycles, these peaks
become weak, indicating that NCM cathode materials' structure
drastically degraded. In contrast, with the E1 electrolyte, the Raman
peaks of the NCM cathode are higher at 450 and 580
cm\textsuperscript{-1} (Figure 7b), indicating that more NCM materials
are active with E2 after 100 cycles. It is confirmed that the dual
additives help form more stable CEI layers on the NCM surface, improving
stability. Therefore, the dual additives synergistically improve the
stability of both positive and negative electrodes.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{Figure 7.} Raman spectra measured fresh NCM cathodes and cycled
at different cycling states with E1 (a) and E2 (b).

The effect of various cycling conditions was further explored. It has
long been established that using lower Li plating current density help
improve AFLBs' lifetime by forming a compact Li metal
anode.\textsuperscript{{[}42{]}} For this, an asymmetric cycling
protocol was designed with 0.1 C-rate for charge and 0.25 C-rate for
discharge. This cycling protocol help to minimize lithium inventory loss
during Li plating and stripping.\textsuperscript{{[}43{]}} As shown in
Figure 8a, the discharge capacity decreased. However, higher capacity
retention of 60\% after 100 cycles was demonstrated, with a similar
average CE of 98.2\% (Figure 8b). Therefore, using the asymmetric
cycling protocol helps to improve the stability of the battery capacity.
Then, we examine the influence of the depth of discharge on the lifetime
of anode-free cells. Limited depth of discharge has previously been
shown to be beneficial to the lifetime of Li metal
cells.\textsuperscript{{[}44{]}} Therefore, a limited cutoff voltage
(3.6-4.3 V) was applied together with an asymmetric cycling protocol,
which may improve the NCM cathodes' stability. The specific capacity
further decreased when a limited cutoff voltage was used (Figure 8a).
However, the capacity retention and CE are similar to the cutoff voltage
of 3.0-4.0V (Figure 8b).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

\textbf{Figure 8.} Electrochemical performance of
NCM\textbar{}\textbar{}Cu cells at 0.1C-rate charge/0.25C-rate discharge
with E2 from 3-4.3V and 3.6-4.3V. (a) Voltage profile; (b) Capacity
retention and Coulombic efficiency.

\textbf{3. Conclusion}

The dual-additives of LiAsF\textsubscript{6} and FEC in commercial
carbonate electrolytes (1 M LiF\textsubscript{6} in EC/DMC/DEC) for
anode-free lithium-ion batteries (AFLBs) are introduced to enhance the
CE and capacity stability. Benefiting from the synergistic effect of the
dual additives, the assembled AFLBs (NCM\textbar{}\textbar{}Cu geometry)
could deliver a high average CE of up to 98.3\% when cycled at 0.5
mA\selectlanguage{ngerman}·cm\textsuperscript{-2}. It shows a reasonably high discharge capacity
retention of 75\% after 50 cycles. In contrast, the pristine commercial
electrolyte exhibited a lower CE of 97\% and low-capacity retention of
35\% after 50 cycles. \emph{In situ} optical observation indicates that
the introduction of dual additives induces the plating of Li metal into
a more compact and conformal morphology, resulting in a reduced battery
residence. Furthermore, \emph{in situ} Raman measurements revealed the
synergistic effect of LiAsF\textsubscript{6} and FEC additives in
controlling the Li nucleation and growth to form Li columns with
self-alignment in the plane. That results in more compact, uniform, and
less-dendritic Li metal anodes. It also was observed that
LiAsF\textsubscript{6} and FEC could be reduced to form the
Li\textsubscript{x}As alloy phase and LiF. That facilitates forming of
solid and rigid LiF-rich interphase layers on negative and positive
electrode surfaces. A deep understanding of the positive impact of
additives on controlling Li growth and stable SEI layers opens up the
potential for developing low-cost and efficient functional electrolytes
for long-life AFLBs.

\textbf{4. Experimental Section}

\textbf{4.1 Electrolyte preparation}

Commercially available electrolyte containing 1M LiPF\textsubscript{6}in
a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and
dimethyl carbonate (DMC) (1:1:1, by volume ratio) was used as the
standard electrolytes. 2 wt \% LiAsF\textsubscript{6} (99.9\%) and 2 wt
\% fluoroethylene carbonate (FEC) (99\%) were added and gently stirred
for 24 h to prepare the optimized electrolytes. All the materials are
purchased from Sigma-Aldrich.

\textbf{4.2 Electrochemical Measurements}

Commercial 18650-type cylindrical batteries manufactured by Tianjin
LiShen Battery Co., Ltd were dismantled in an argon-filled glove box,
and pieces of the
LiNi\textsubscript{0.5}Co\textsubscript{0.3}Mn\textsubscript{0.2}O\textsubscript{2}(NCM532)
cathodes were taken out. A bare Cu foil was cut into a 12 mm diameter
disc used as the anode current collector. A 2400-type Celgard separator
(25 \selectlanguage{greek}μ\selectlanguage{english}m thick) and a glass fiber separator were used as the separator.
Cu\textbar{}\textbar{}Li half-cells and full AFLBs with NCM532 cells
were investigated in Swagelok cells with 75 \selectlanguage{greek}μ\selectlanguage{english}L of electrolytes. The pure
commercial electrolyte of LiPF\textsubscript{6}in EC/DEC/DMC 1:1:1 v/v/v
ratio is denoted as E1 electrolyte. The optimized electrolyte by adding
2 wt \% LiAsF\textsubscript{6} and FEC into the pristine E1 electrolyte
(LiPF\textsubscript{6} in EC/DEC/DMC 1:1:1 v/v/v ratio) was named E2
electrolyte. All measurements were performed in an argon-filled glove
box (\textless{}0.1 ppm oxygen and \textless{}0.1 ppm
H\textsubscript{2}O). All electrochemical tests were carried out in an
MKF120 climate chamber (Binder, Germany) using a VMP3 potentiostat
(Bio-Logic, France). The amount of Li metal to be plated onto the copper
surface was controlled by restricting the time. The stripping was a
voltage-controlled (0.1 V) process with a constant current density of
0.5 mA\selectlanguage{ngerman}·cm\textsuperscript{-2}. Galvanostatic (dis-)charge cycling of
NCM532\textbar{}\textbar{}Cu cells was performed in the voltage range of
3.0--4.3 V at charge-discharge rates of 0.1C. Cyclic voltammogram (CV)
tests were operated in a voltage window of 3.0--4.3 V vs.
Li\textsuperscript{+}/Li at a scan rate of 0.1 mV
s\textsuperscript{--1}. Electrochemical impedance spectroscopy (EIS)
tests were conducted with an amplitude of 10 mV in the frequency range
of 10--100 kHz.

\textbf{4.3 Material Characterization}

Scanning electron microscopy (SEM) and energy-dispersive X-ray
spectroscopy (EDX) experiments were carried out using a Quanta FEG 650
(FEI) environmental scanning electron microscope operated at a voltage
of 20 kV.

All Raman measurements were carried out using a Bruker Senterra Raman
microscope (Bruker, Germany). Experiments were performed using a 532 nm
green laser at a spectral resolution of 9 to 18
cm\textsuperscript{--1}and a confocal setup. The laser power was set to
10 mW to prevent sample damage. For the microscope, a 10×objective was
used. Five integrations were carried out with an integration time of 2 s
to collect the spectra. All material surfaces were scanned by recording
multiple Raman spectra pointwise on a grid with an isotropic spatial
resolution of 2 \selectlanguage{greek}μ\selectlanguage{english}m using a 50\selectlanguage{ngerman}×magnification.

For \emph{in situ} measurements, a Cu/NCM full cell was set up inside an
ECC-Opto-Std (EL-CELL, Germany) cell.\textsuperscript{{[}41,
45{]}}NCM532 electrode with a diameter of 10 mm was used as a cathode. A
1 mm thick glass-fiber separator (EL-CELL, Germany) soaked with ca. 75
\selectlanguage{greek}μ\selectlanguage{english}L of E1 and E2 electrolytes was placed on top of the cathode. 16 mm Cu
mesh was distributed on top of the separator and pressed onto the glass
window of the cell. The cell was cycled at a constant rate of 300 \selectlanguage{greek}μ\selectlanguage{english}A
(C/10) between 4.3 V and 3.0 V using a BioLogic SP-200 potentiostat
(BioLogic Science Instruments, France). During cycling, Raman spectra
were recorded with a time step of 2h at the Cu mesh surface with an
isotropic spatial resolution of 2 \selectlanguage{greek}μ\selectlanguage{english}m under 50 times magnification.

\textbf{Supporting Information}

Supplementary material (further details of EDX measurement results
and\emph{in situ} Raman cell) is available in the online version of this
article.

Acknowledgements

Baolin Wu gratefully acknowledges fellowship support from the China
Scholarship Council.

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Supporting Information

Dual additives for stabilizing Li deposition and SEI formation in
anode-free Li metal batteries

Baolin Wu, Chunguang Chen,\textsuperscript{*} Dmitri L.
Danilov,\textsuperscript{*} Zhiqiang Chen, Ming Jiang, Rüdiger-A. Eichel
and Peter H.L. Notten\textsuperscript{*}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

\textbf{Figure S1.} EDX spectra of electrodeposited Li metal on Cu
substrates after 100 cycles with two electrolytes: pristine electrolyte
(a) and optimized electrolyte (b).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}

\textbf{Figure S2.} Schematic image of the In-situ Raman cell
configuration.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}

\textbf{Figure S3.} EDX spectra of NCM cathodes. (a) Fresh sample. After
100 cycles with E1 (b) and E2 (c) electrolytes.

\selectlanguage{english}
\FloatBarrier
\end{document}

