2019 年 3 月 31 日
報告書
実施期間 2018 年 12 月 4 日~2019 年 3 月 1 日
容量低下バッテリーの再生技術に関する
共同研究
“FM Lab” Co Ltd Dr. Daniil Itkis
概要
名称
容量低下バッテリーの再生技術に関する共同研究
実施期間
2018 年 12 月 4 日~2019 年 3 月 1 日
開発/調査 代表者
“FM Lab” Co Ltd Dr. Daniil Itkis
実施者
“FM Lab” Co Ltd Dr. Daniil Itkis “SC-Tek” LLC Dr. Mikhail Kondratenko
目的
使用済み自動車より回収された容量低下したリチウムイオンバッテリーを再
生し、定置用電源または自動車用電源としてリサイクルをする為に必要な高度
リサイクル技術について研究を行う。
実施内容
リチウムイオン電池の主要な劣化要因として、電極内での固体-電解質界面物
質(SEI)の生成が想定されている。超臨界流体は表面張力がほぼゼロであり、溶
剤を取り込むことができることから、多孔質の電極内に溶剤を浸透させ、セルを
非破壊で SEI を除去できる可能性がある。SEI の除去後は、新しい電解液を再注
入することで、セルを破壊することなく、容量低下した電池の性能を回復させる
ことを目的としている。本件研究では、超臨界流体による SEI の除去について
検証を行い、低下容量回復への有効性を見極めた。
成果
グラファイト電極に発生した SEIの除去を効率的に確認するために、対極 Li金
属のハーフセルを用いて実験を行った。また、超臨界液体による洗浄とバッテリ
ーからの抽出物の分析が可能な新システムを作製した。室温で 100 サイクルの
1C 充放電試験後によりバッテリーを劣化させ、その後、バッテリーを解体して
超臨界液体による洗浄を実施した。洗浄後電極の容量-電圧測定を実施したとこ
ろ、大きな不可逆容量が観測された。これは新たに SEI を形成するためにリチ
ウムが消費されたことを示し、SEI が効果的に除去されたことを示唆する結果を
得た。
(詳細は以下の技術報告書参照)
01 March 2019
Final Report
Research period: 4th December 2018 to 1st March 2019
Title:
“Searching the Approaches for Lithium-Ion Battery (LIB)
Second Life Performance Improvement”
“FM Lab” Co Ltd
Daniil Itkis
Summary
Title
“Searching the Approaches for Lithium-ion Battery (LIB) Second Life
Performance Improvement”
Research period
4th December 2018 to 1st of March 2019
Delegate of the research
Company: “FM Lab”Co Ltd Name of delegate: Dr. Daniil Itkis
Prosecutor
Company: Nissan Motor Co., Ltd. Name of prosecutor: Mr. Masanori Nakamura
Subcontractor/delegate
Company: “SC-Tek” LLC Name of delegate: Dr. Mikhail Kondratenko
Purpose
The main aim of the project is to monitor and to evaluate promising approaches,
which can enable enhancement of the LIB performance during its second life (i.e.
approaches enabling recovery of battery capacity and/or power after it was used
for some time and has been degraded).
Project Tasks
Task 1. Literature survey on possible types of non-destructive approaches for cell
properties recovery
1.1 Survey on the reasons for battery capacity/power loss during operation
1.2 Survey on possible types of non-destructive (i.e. without cell disassembly)
approaches for cell recovery, including evaluation of their effectiveness
Task 2. Experimental studies of the possibility of SEI removal by supercritical
fluids
2.1 Preparation of individual components of SEI for solubility studies
2.2 Selection of supercritical fluids
2.3 Tests of individual SEI components solubility
2.4 Preparation and cycling of graphite electrodes
2.5 Testing of SEI removal by selected supercritical fluids
2.6 Testing of battery components stability in supercritical fluids (separator,
cathode material, current collectors, binders, carbon additives)
2.7 Testing of SEI removal with a model pouch cell
Task 3. Studies of the electrolytes for lithium inventory refilling
3.1 Selection of lithium salts, with easily oxidizable anions
3.2 Electrochemical studies the selected salts stability in alkylcarbonate solvents
3.3 Test lithiation of graphite negative electrode
Task 4. Literature survey on polymer binder for enhancing LIB second life
performance
4.1 Survey on conventional polymer binders and their degradation in LIBs
4.2 Search for ideas on new polymer binder chemistries for second LIB life
performance improvement
Task 4. Report preparation
Results summary
A two-stage approach is suggested for improvement of the performance of spent
lithium-ion batteries. The approach requires development of SEI removal and
lithium inventory refilling technique. The possibility of SEI removal by supercritical
fluids was demonstrated. The candidate electrolytes for lithium inventory refilling
were selected. Further challenges are identified.
Contents
Abbreviations ...................................................................................................................... 5
Introduction ......................................................................................................................... 6
Literature survey ................................................................................................................ 8
LIB performance degradation ......................................................................................... 8
The ideas of LIB performance recovery ........................................................................ 10
Using of supercritical CO2 for electrolyte and graphite recovery from LIBs .............. 11
The methodology of experimental studies ........................................................................ 13
Cloud point measurements ........................................................................................... 13
Mass spectroscopy.......................................................................................................... 14
Preparation of the electrodes and cell assembly .......................................................... 14
Electrochemical measurements .................................................................................... 15
Results ............................................................................................................................... 15
Solubility of electrolyte in supercritical fluids ............................................................. 15
Extraction of electrolyte from high-pressure cell ......................................................... 17
Graphite anode cycling and washing ............................................................................ 20
Selecting the electrolytes for refilling lithium inventory ................................................ 23
Challenges for the future .................................................................................................. 24
Survey on polymer binders ............................................................................................... 25
References .......................................................................................................................... 27
Abbreviations
ACN acetonitrile
DMC dimethyl carbonate
EC ethylene carbonate
LIB lithium-ion battery
NMP N-methyl-2-pyrrolidone
PC propylene carbonate
PVDF polyvinylidene difluoride
scCO2 supercritical carbon dioxide
SCF supercritical fluid
SEI solid-electrolyte interphase
Introduction
Today lithium-ion batteries (LIB) are used in numerous applications. Such power sources
have lots of advantages, such as high specific energy and energy density, high specific
power, long shelf life, etc. Today the market of electric vehicles actively develops
increasing the demand for LIBs. According to various estimates [1,2], the annual amount
of LIB waste is ca. 200-500 million tons, of which 5-15% belong to cobalt - an expensive
and toxic element. Due to expected even more rapid growth of LIB production the battery
recycling [1 – 4] and “second life” became quite hot topics driven by both economic and
environmental factors. Although battery recycling is rather cost- and labour-consuming,
increasing amount of spent batteries pushes active investments into this field. Reusing
spent batteries in the applications not requiring high performance – battery “second life”
– is on the contrary much less expensive. However, there are serious limitations on
energy and power of the spent batteries so the number of applications is quite limited.
In this project we aimed at improving the performance of the spent lithium-ion batteries,
to make it possible to use recovered ones during its “second life” in demanding
applications such as electric vehicles. Although recovery of initial properties of the new
battery seems to be impossible, we believe that the battery capacity and/or peak power
can be partially restored. The main aim of the project is to monitor and to evaluate
promising approaches, which can enable enhancement of the LIB performance during
its second life.
It is known, that there are many reasons for the loss of battery capacity and power during
its discharge/recharge cycling [5]. One of the most serious and universal (can be found
in majority of LIBs, which have graphite negative electrode) reasons for degradation is
damaging of initial SEI (solid-electrolyte interphase) layer and its re-formation leading
to active lithium inventory loss. SEI films form on the surface of the negative electrode
from the products of electrolyte reduction and decomposition. During the cycling of
battery the film of SEI can be damaged, and a new protective layer is formed. Thus, the
number of active charge carriers in the cell decreases.
We proposed an idea for restoring the spent battery capacity, based primarily on
removing the SEI layer from the surface of the negative electrode and further “refilling”
of lithium inventory in the cell. During this project stage we investigated removal of
electrolyte from the cell by complete dissolution with supercritical fluids (sc-CO2) and co-
solvents (ACN (acetonitrile) and mixture of ACN:PC (acetonitrile : propylene carbonate),
EC:DMC (ethylene carbonate : dimethyl carbonate)). The best results were obtained by
using a mixture of ACN and sc-CO2. We also addressed the removal of SEI from the
negative electrode by treatment of the cycled graphite electrode with CO2 and ACN in
the high-pressure reactor. We showed that the graphite electrodes are still
electrochemically active after the treatment with supercritical fluids, and irreversible
capacity appears again after “washing” of the cycled electrode. Although current results
indicate the possibility to use the suggested approach, lots of further studies are required
for proof-of-concept.
Literature survey
LIB performance degradation Cyclic charging and discharging of LIBs leads to deterioration of the electrochemical
properties of the cell, e.g. capacity. There are numerous reasons for the battery
degradation which are depicted in Fig. 1 [5].
The main mechanisms of LIB properties degradation can be divided into three main
groups: corrosion of current collectors, destruction of electrode materials, cracking and
the formation of new SEI layers. These groups are described in more detail below.
Fig. 1 Degradation mechanisms in lithium-ion cells [5]
The most common materials that are used in LIB as current collectors for the positive
and negative electrodes are aluminum and copper, respectively. Both materials undergo
degradation. The main problems associated with the degradation of the current
collectors are poor adhesion, the formation of a passivating film and various types of
corrosion. Taken together, these factors lead to an increase in the internal resistance of
the cell due to the formation of an insulating film of degradation products on the current
collector surface during the charge-discharge cycles [6].
Aluminum is damaged more by pitting corrosion under the action of some electrolyte
degradation products, such as HF [7]. Deep discharge (overdischarge) can lead to a
possible dissolution of copper Cu ⇌ Cu+ + e- [6]. Cu+ ions formed during this process can
later be reduced on the surface of the negative electrode with the formation of copper
dendrites.
The cathode material also can limit the battery cycle life. Many different degradation
processes affect the positive electrode, such as the destruction of an active material,
degradation of the inactive components of the electrode, the interaction of aging products
with negative electrode, oxidation of electrolyte and the formation of a surface film.
Together these mechanisms, which are summarized in Fig. 2, can significantly reduce
the LIB cycle life [5, 8].
Fig. 2 The mechanisms of positive electrode degradation [8]
Indeed, the mechanisms of degradation of the positive electrode strongly depend on the
active materials used. Below few examples of the mechanisms of transition metal oxide-
based electrodes are mentioned. E.g. Ni-rich layered oxide electrodes tend to lose oxygen
at high potentials: LiNiO2 ⇌ 0.5x Li2O + 0.25x O2 + {Li1-xNi(II)x/2}Ni(III)1-x}O2-x [8, 9].
LCO materials does not behave in this way. However, at potentials greater than 4.2 V
(vs. Li+/Li) an insignificant dissolution of Co, which can later be detected at negative
electrode surface, is observed [8]. Mixed layer cobalt/nickel oxides also lose its capacity
due to the formation of a film on the surface, which is formed due to oxidation of the
electrolyte solvent at high potentials, as well as partial decomposition of the lithium salt.
The instability of the SEI at the negative electrode is probably one of the most important
factors affecting the lifespan of the battery. Fig. 3 presents the main mechanisms for
changing the passivating film composition and structure during LIB operation [8].
Fig. 3 SEI evolution modes during battery cycling [8]
As known, the formation of SEI during the first charge / discharge cycles (formation cycle
and ageing) leads to the loss of some lithium inventory due to immobilization of Li+ in
SEI. The composition and stability of the resulting film depends on many factors,
including the electrolyte composition and additives, temperature, working potentials and
currents. During LIB operation, which can sometimes involve high operating potentials,
excessively low or high temperatures, mechanical deformation of the cell, SEI layer can
be damaged, which leads to loss of charge carriers due to the formation of a new portions
of SEI [5, 10].
The ideas of LIB performance recovery There are not so many experimental reports on the recovery of LIB electrochemical
performance. Table 1 summarizes the search terms that were used for seeking some
ideas on the improvement of the electrochemical performance of the spent lithium-ion
batteries. As can be seen, “battery second life” is now understood mainly as the activity
focused on using the spent LIBs in the application, not demanding the ultimate
performance. The word “recovery” is typically used to denote recovery (extraction) of
some components from spent battery for refabricating LIBs or for reusing it in new
batteries (e.g. nickel and cobalt recovery). Some studies involving supercritical fluids,
which were suggested by our team as a media to remove SEI from the anodes, were found.
Mainly such works are aimed at extraction of the electrolyte from used cells, however,
the improvement of the graphite anode performance is mentioned in one of the works
[11].
Table 1 Summary of the literature search results, obtained using Google Scholar and Scopus databases.
The table provides only general ide bout the keywords – a lot of combinations were tested (e.g. “li-ion”
instead of battery etc.)
Keywords Results
“battery” AND “second life”;
“battery” AND “second life” AND improvement”
Only papers & patents about using
spent batteries in less demanding
applications
“battery” AND “recovery”
“battery” AND “regeneration”
Completely non-relevant results
“supercritical” AND “battery” Only 9 papers/1 book chapter by 3
scientific groups are found (years
2013-2018). Works are mainly
devoted to electrolyte extraction
from the lithium-ion cells
Using of supercritical CO2 for electrolyte and graphite recovery from LIBs Supercritical fluids are substances at a temperature and pressure above their critical
point, where liquid and gas phase cannot be distinguished [12]. On the phase diagrams,
the region of existence of the supercritical state typically looks as shown in Fig. 4 [13].
Fig. 4 An example of a phase diagram of pure substance [13].
Typical advantages of using SCFs over liquid solvents are high mobility of SCFs, the
absence of surface tension and corresponding capillary effects in SCFs and, therefore,
their outstanding ability to easily penetrate even the smallest pores of the materials.
This makes SCFs especially beneficial when treating porous materials with high specific
surface area. Moreover, the solubility of substances in supercritical fluids strongly
depends on SCF density and can be easily tuned by variation of the fluid pressure.
Carbon dioxide (CO2) is one of the most widely used SCFs. . Its critical point is at 304.1
K and 73.8 atm [12, 13]. Safety, the absence of toxic properties, low cost, chemical
inertness, relatively low values of the critical temperature and pressure – all these
properties make scCO2 a very attractive solvent for a wide range of applications:
synthesis, impregnation, extraction. Among the most well-known examples of scCO2
applications there are the decaffeination of coffee and supercritical dyeing of polyester
fabrics with disperse dyes.
It was already proposed to use scCO2 for extraction of LIB electrolytes based on organic
carbonates with conducting LiPF6 salt [14]. After conducting a series of experimental
studies, the authors concluded that using scCO2 enables extraction of various organic
solvents used in battery electrolytes (DMC, EC, and EMC). However, it was shown that
LiPF6 has a low solubility in scCO2, so scientists were able to extract only trace amounts
of the salt. Also, in addition to organic carbonates, the authors were able to extract some
decomposition products of the solvent, the concentration of which depended on the cell
prehistory.
To increase the efficiency of LiPF6 extraction using of scCO2 with various co-solvents
(propylene carbonate, acetonitrile, diethyl carbonate) was suggested [15]. It was
demonstrated that both liquid and supercritical CO2 can be effectively used for extraction
of EC and LiPF6 if ACN/PC mixture is used as a co-solvent.
There were also some efforts taken to recover graphite from spent LIBs [11]. The authors
tested three different methods of graphite recovery: a heat treatment process, electrolyte
extraction using liquid CO2 with the addition of acetonitrile, and electrolyte extraction
with supercritical CO2. It was demonstrated the graphite recovered from the used
battery with a help of liquid CO2 and acetonitrile mixture has the best electrochemical
performance. At the same time the authors found that treatment of anodes with
supercritical carbon dioxide leads to a deterioration of electrochemical performance of
the material.
The methodology of experimental studies
Cloud point measurements The high-pressure regions of phase diagrams were investigated to evaluate the
electrolyte solubility in pure supercritical CO2 or in scCO2 with addition of cosolvents.
Acetonitrile, propylene carbonate, dimethyl carbonate and ethylene carbonate were used
as-received from Sigma-Aldrich (all solvents were anhydrous). A high-pressure setup
was assembled (see Fig. 5) for cloud-point observations and determination of the
homogeneity regions. 33 l of electrolyte was poured into a thoroughly cleaned and dried
3.3 ml high-pressure optical cell (Sitec) equipped with two sapphire observation windows
(8). The cell filling was performed inside an argon-filled glovebox with oxygen and
humidity concentrations below 5 and 0.1 ppm, respectively. A photodiode was attached
to the bottom window providing a backlight. The cell (8) and a high-pressure vessel (3)
with a freely-moving piston (4) were then filled with CO2 (99.995% purity) to 600 bar at
22ºC (room temperature) using a high-pressure generator (model #81-5.75-10, High
Pressure Equipment Company) (2). After that the valve (9) was opened. The system was
then heated up to the desired temperatures in 35 – 75ºC range using external heating
system (13, 14) controlled with two thermocouples and left for 1 hour to reach
thermodynamical equilibrium. The pressure in the system was then decreased
quasistatically and recorded using the back-pressure regulator (ABPR-20/200, Waters)
(7) connected to the PC (12) and additionally monitored by a pressure gauge (5). The
cloud point was determined by the naked eye observation of the media inside the cell
interior. Then the entire reactor volume began to cloud (at pressure P1) and after that
the entire reactor volume was completely cloudy (at pressure P2) (we cannot see
photodiode light).
Fig. 5 The schematic of the high-pressure setup used for experiments on solubility and electrolyte
extraction. (1) CO2 cylinder, (2) high-pressure generator, (3) high-pressure reactor with a piston, (4)
piston, (5) pressure gauge, (6) outlet high-pressure valve, (7) back-pressure regulator, (8) high-pressure
optical reactor, (9) connecting high-pressure valve, (10) light-emitting diode, (11) photodiode (optional;
observation by eye also possible), (12) data collection system, (13) and (14) external heating system),
controlled by thermocouples, (15) vacuum leak valve, (16) high vacuum chamber, (17) molecular turbo-
pump, (18) quadrupole mass-spectrometer.
Mass spectroscopy Mass spectroscopy experiments were performed using Residual Gas Analyzer XT100
(Extorr). The gas analyzer was connected to a vacuum chamber pumped by HiCube 80
Eco (Pfeiffer Vacuum). The base pressure inside vacuum chamber was kept at 10-8 mbar.
The exhaust from a back-pressure regulator (7 in Fig. 5) was introduced into UHV
chamber through a leak valve (CVT) (15). Mass spectra were collected while the pressure
inside the chamber was maintained at 10-5 mbar level. Each spectrum was obtained at a
speed of 3 scans/min.
Preparation of the electrodes and cell assembly The electrode slurry consisted of 95 mass. % of natural graphite (Gelon) and 5 mass. %
of PVDF binder (Solvay Solef 5130). For slurry preparation 0.9 g of PVDF was mixed
with 12ml NMP (BASF, battery grade) and stirred for 3 hours. After that, 17.1 g of
graphite was added to the polymer solution, the mixture was sonicated to disaggregate
large graphite agglomerates and then it was strirred by high-shear mixer with a
dissolver stirrer for 15 hours. The density of the obtained slurry was 1.5 g/ml. The slurry
was casted onto the copper foil (20 m, Gelon) using automatic film applicator coater
(Zehntner ZAA 2300) operating at a coating speed of 20 mm/s. Wet coating thickness was
set to be 100, 200, 300 or 400 m. The electrodes were dried in air at 80ºC shrinking by
about two times. Prepared electrodes were calandered using hot rolling press (MTI corp.)
at 80ºC. The initial dry thickness of the coating (excluding the foil thickness) was
compressed by 5, 10 or 15%. Further, the electrodes were cut into 15 mm diameter discs,
dried at 105ºC in vacuum for 12 hours and transferred to glove box without exposition to
air.
Coin cells battery were assembled inside an argon-filled glove box with oxygen and water
content below 5 and 0.1 ppm, respectively. The graphite electrodes played a role of
working electrode, metallic lithium served as a counter electrode (Li discs, 110 m
thickness, China Energy Lithium). The electrodes were separated by a single-layer
polypropylene separator (Celgard 2500). 1 M LiPF6 in EC:DMC 1:1 (vol.) (Sigma-Aldrich)
served as an electrolyte.
Electrochemical measurements The electrodes were analysed in galvanostatic charge-discharge experiments. Lower
voltage cut-off was set to 5 mV, the higher one – to 1.5 V for all experiments. All
measurements were carried out using Biologic SAS MPG-2 multichannel potentiostat.
Results
Solubility of electrolyte in supercritical fluids We focused at analysis of electrolyte solubility in scCO2 as the residual electrolyte as well
as SEI should be removed from the cell (and from the electrodes, respectively). To study
the solubility of the electrolyte in scCO2 we performed a series of model experiments, in
which we evaluated the homogeneity of the media inside a high-pressure vessel equipped
with an optical window at a set of temperatures in the pressure range up to 600 atm.
In our work we considered one of the conventional electrolytes used in LIBs – 1 M
solution of LiPF6 in EC:DMC mixture (1:1 vol.). First, we analyzed the system
EC:DMC:scCO2 (without lithium salt). The obtained phase diagram is shown in Fig. 6.
Fig. 6 Phase diagram of EC:DMC:scCO2 system obtained using cloud point measurements. Black line
shows the pressure, at which phase separation becomes visible, red line shows the cloud point.
As seen from the Fig. 6 EC:DMC solvent mixture can form homogeneous phase with
scCO2 at reasonable pressures at 40 – 50ºC. Adding the salt, however, complicates the
system. Using even 10-fold excess of EC:DMC (adding 300 L of EC:DMC solvent
mixture to 33 L of 1M LiPF6 solution poured into high-pressure reactor) resulted in
easily visible precipitate on the optical window of the high-pressure vessel, which is
shown in the photographs in Fig. 7. Although it was possible to determine the cloud
points in these systems, precipitate was detected at all the temperature and pressure
range. Presumably, LiPF6 was not dissolved in supercritical fluid in this case.
Fig. 7 (left) Optical photographs showing LiPF6 crystallization on the optical windows of the high-
pressure reactor vessel. (right) Phase diagram of LiPF6:EC:DMC:scCO2 system obtained using cloud
point measurements. Black line shows the pressure, at which phase separation becomes visible, red
line shows the cloud point.
1 2
3 4
600 atm 240 atm
150 atm 110 atm
45 °C
45 °C
45 °C
45 °C
To overcome such solid phase precipitation the co-solvent was added. According to Ref.
[15], adding of ACN:PC mixture to scCO2 is an efficient way to wash the electrolyte out
of the electrochemical cells. Fig. 8 shows the phase digraph for the system “electrolyte
solution – ACN:PC – scCO2”. Using the co-solvents allowed to obtain a homogeneous
phase at pressures below 200 atm in 50 – 65ºC temperature range. Such result is
surprising as dielectric permittivity for ACN:PC (3:1 vol.) mixture is expected to be close
to that of EC:DMC (1:1 vol.), while better solubility of ionic salts can be expected for
higher polar media.
Fig. 8 Phase diagram of LiPF6:EC:DMC:ACN:PC:scCO2 system obtained using cloud point
measurements. ACN:PC (3:1 vol.) mixture was used as a co-solvent for scCO2. Black line shows the
pressure, at which phase separation becomes visible, red line shows the cloud point.
Extraction of electrolyte from high-pressure cell Further research was aimed at understanding whether the electrolyte residuals can be
extracted from the high-pressure reactor. Mass spectral analysis was employed for that
purpose. While such a tool is not suitable for analysis of ionic compounds (namely, LiPF6)
as the latter have too low vapor pressure at room temperature, it can effectively monitor
organic solvents in the exhaust from the rector. It was shown that both electrolyte
solvents can be detected in the exhaust vapors of the high-pressure system (see Fig. 9).
Fig. 9 Mass spectrum of the exhaust from high-pressure system filled with 10 vol. % 1 M LiPF6 solution
in EC:DMC (1:1 vol.) and 90 vol. % scCO2.
However, we found that PC added as a co-solvent along with ACN for better dissolution
of LiPF6, is not effectively extracted from the reactor up to 60ºC. Fig. 10 shows the mass
spectra of the exhausts from the reactor filled with electrolyte solution, scCO2 and
ACN:PC mixture (3:1 vol.) as a co-solvent. While EC and DMC are easily detected in the
mass spectrum, no traces of PC were detected meaning that PC resides inside the high-
pressure system.
Fig. 10 Mass spectra of the exhaust from high-pressure system filled with 1 vol. % 1 M LiPF6 solution
in EC:DMC (1:1 vol.), 9 vol. % of ACN:PC mixture (3:1 vol.) and 90 vol. % scCO2. Spectra were obtained
while the reactor was held at 70ºC.
To check the possibility of PC extraction we further recorded the mass spectra of the
exhausts from the reactor filled with ACN:PC mixture (3:1 vol.) and scCO2 (no electrolyte
solution was added). The spectra are shown in Fig. 11 for temperatures from 40 to 70ºC.
Only at 70ºC small intensity peaks at M/Z 57 and 58 a.m.u. were detected in mass-
spectra evidencing that PC can be removed from the system by supercritical fluid,
however, the rate is probably too low. Fig. 12 reveals a close-up of the mass-spectrum
registered when high-pressure reactor was maintained at 70ºC. Such a temperature,
however, requires higher pressure to form a homogenous phase with electrolyte (see Fig.
8).
Fig. 11 Mass spectra of the exhaust from high-pressure system filled with 1 vol. % 1 M LiPF6 solution
in EC:DMC (1:1 vol.), 9 vol. % of ACN:PC mixture (3:1 vol.) and 90 vol. % scCO2. Spectra were obtained
while the reactor was held at 70ºC.
Fig. 12 Region of the mass spectrum of the exhaust from high-pressure system filled with 1 vol. % 1 M
LiPF6 solution in EC:DMC (1:1 vol.), 9 vol. % of ACN:PC mixture (3:1 vol.) and 90 vol. % scCO2 at 70ºC
showing the trace amount of PC can be detected.
Graphite anode cycling and washing Also, our efforts were directed on analysis of the electrochemical behavior of the graphite
electrode after washing with scCO2 and ACN as a co-solvent. For this purpose, graphite
electrodes were prepared and cycled in two-electrode half-cells (with Li counter
electrode). After the formation cycle, the cells were cycled for 100 cycles at higher current,
then 1 cycle was performed at low current density. The graphite electrodes were then
treated in the high-pressure reactor and used for assembly of the new cells, which were
again discharged/recharged. This experimental sequence is depicted in Fig. 13.
Fig. 13 Schematic of the cell testing algorithm.
Fig. 14 Discharge/recharge galvanostatic voltage profiles for graphite electrodes. As-deposited electrode
thickness was 105 m. The graphite coating thickness was reduced by 5 (left) and 15% (right) by
calandering. The current density was 25 mA/g.
The voltage profiles for the formation cycle are shown in Fig. 14. The irreversible
capacity did not exceed 10%. We observed strong capacity fade for both electrodes
compressed by 5 and 15% at higher current (ca. 1C-rate, see Fig. 15), however, after 100
cycles low current Li+ insertion/extraction demonstrated good capacity retention (red
curves in Fig. 14). So, it can be concluded that while the capacity retention is reasonable
for both electrodes, rate capability is poor. As the optimization of negative electrode
performance is outside the scope of work, we used such graphite electrodes for testing
the possibility of its treatment by supercritical fluids.
Fig. 15 The capacity fade of the electrodes during cycling at 185 mA/g. Voltage cutoffs were set to 5 mV
and 1.5 V. The cells were cycled at room temperature with no additional efforts taken for temperature
stabilization.
Fig. 16 Typical mass spectrum of the exhaust from high-pressure (500 atm) system during washing of
the graphite electrodes (discs 15 mm in diameter, the spectrum is shown for the electrode compressed
by 15%) by scCO2 (91 vol. %) and ACN (9 vol. %) as a co-solvent. Spectra were obtained while the reactor
was held at 40ºC.
Washing of the electrodes after its removal from the disassembled cells were done at 500
atm and 40ºC. Fig. 16 shows the mass spectra of the high-pressure system exhausts
during washing of the electrode by scCO2 and ACN. Both electrolyte solvents (EC and
DMC) and ACN characteristic peaks are visible in the spectrum. After keeping the
electrode in the reactor at 500 atm and 40ºC for 1 h the reactor was flushed with
supercritical fluid for 1 h at 1 ml/min. The electrode was then used for the assembly of a
new electrochemical cell. Fig. 17 compares lithium insertion/extraction into/from the
electrode after cycling and after washing. It is seen that irreversible capacity appeared
after washing the electrode, which can evidence that SEI was removed from the surface.
Fig. 17 Discharge/recharge voltage profiles for the electrode compressed by 15% during calandering
before (left) and after (right) washing in the high-pressure reactor. The current was 25 mA/g, voltage
range 5 mV – 1.5 V.
However, the appeared irreversible capacity is much higher than that of a pristine
electrode after calandering. It can occur due to an increase in surface area of the
electrode. In addition, total capacity became twice lower giving a hint that delamination
or loss of the active material (graphite) occurred due to supercritical treatment of some
other manipulations during cell disassembly and reassembly.
Selecting the electrolytes for refilling lithium inventory
As lithium is trapped in SEI, removal of old SEI leads to active lithium ions inventory
loss. To “refill” lithium inventory we need to add some Li+ to the system and then
intercalate it into one of the electrodes. To intercalate Li+ into one of the electrodes (e.g.
negative electrode) we need another reaction to proceed on the other electrode (e.g.
positive electrode). At the same time, it should not be transition metal oxidation in the
active material at positive electrode (we should not affect the electrode balancing. We
suggest adding a lithium salt (LiX) with an anion (X-), which can be easily decomposed.
this idea is illustrated in Fig. 18.
Fig. 18 The idea of refilling of lithium inventory after SEI removal
During this stage of the project we have selected and ordered a number of salts, which
can potentially be used for lithium inventory refilling. Such salt should have easily
oxidizable (or easily reduceable, but this option is not preferable as reduction of anions
is more problematic). We performed a survey on possible mechanisms of anion reduction
in aprotic media. The results are summarized in Table 2.
Table 2 Summary of that lithium salts, which can be suggested as an electrolyte for refilling lithium
inventory in the cell after SEI removal.
Salt Oxidation mechanism Reduction
mechanism
Reference
Li2C2O4
Li+ + e-→ Lio
[16]
C2O42- -e- → C2O4·-
C2O4·-→ CO2 + CO2·-
C2O4·--e-→ C2O4 → 2CO2
CO2·--e-→ CO2
Li+ + e-→ Lio [17]
CH3COOLi 2CH3COO-→ 2CO2 + C2H6 +
2e-
Li+ + e-→ Lio [18]
LiNO3
NO3- -e-→ NO3·
2NO3· → N2O6
N2O6 → N2O5 + O2
Li+ + e-→ Lio [19]
NO3- ↔ NO2 + O2 + e-
or
2NO3- ↔ N2O5 + O2 + 2e-
Li+ + e-→ Lio [20]
Challenges for the future
Although a promising result was obtained showing that graphite electrode can be active
after washing by supercritical fluid, there is a still long way to demonstrate the proof-of-
concept for the suggested idea of spent LIBs performance recovery. Main challenges for
further work are summarized below.
The capacity of graphite electrode was much lower after washing compared to
that for pristine electrode. The irreversible capacity vice versa significantly
increased. In-depth analysis (including SEM, AFM, probably FTIR and XPS) is
required to understand the reasons for that. Further tuning the experimental
conditions of washing is needed to enable the “refreshment” of the negative
electrodes without sacrificing its performance.
Washing by supercritical fluids should be tested on the electrodes inside the cell
(first – coin cells, then – pouch cells). Full lithium-ion cells are to be used for this
purpose. The stability of positive electrodes and separators in supercritical CO2
should be proven by additional experiments.
The ability to refill lithium inventory should be experimentally checked using
pristine and washed graphite electrodes. The voltammetry in 3-electrode cells is
required to understand the potentials, at which the anions of the selected
electrolytes (salts) can be decomposed. The electrolytes with enough high
solubility and oxidation potential within the solvent stability window should be
found.
Survey on polymer binders
No papers disclosing any ideas on special polymer binders for improving LIB second life
performance were found. Even more surprising result is that actually there was no
comprehensive review on binders published before 2014 [21]. Binders make a small
contribution to the electrode composition, but they play an important role in affecting
the cycling stability and rate capability for Li-ion batteries. However, all the reports on
this topic are devoted to enhancing the battery cycle life, but not to the attempts to affect
the properties of a spent battery. Binders, which are typically used are summarized in
Table 3 [21].
Table 3 Typical binders, their molecular structures and the corresponding electrode materials and
references. The table is adopted from [21]
We considered the idea that some “smart” binder can help in enhancing the battery life.
Such a binder should have an ability to restore the electric contacts between active
material particles and conducting agent inside the electrodes after electrode degradation
as schematically shown in Fig. 19. Although we found no reports with similar approach
in literature, new generation of binders with advanced properties are being developed
nowadays. The closest example is reported in [22], where polymer chains are cross-linked
into gel state to keep active material particles together during their expansion and
contraction upon cycling.
Fig. 19 Illustration of the idea to use “smart” polymer binder for the LIB electrodes.
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