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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