Unveiling the Role and Mechanism of Mechanochemical Activation on

respectively; ka is the slope of the fitted straight line or the apparent rate constant of the leaching reaction (min–1); R2 is the coefficient ...
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Environmental Processes

Unveiling the Role and Mechanism of Mechanochemical Activation on Lithium Cobalt Oxide Powders from Spent Lithium-ion Batteries Mengmeng Wang, Quanyin Tan, and Jinhui Li Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03469 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018

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Unveiling the Role and Mechanism of

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Mechanochemical Activation on Lithium Cobalt Oxide

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Powders from Spent Lithium-ion Batteries

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Mengmeng Wang†, Quanyin Tan†, Jinhui Li†,*

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†State Key Joint Laboratory of Environment Simulation and Pollution Control,

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School of Environment, Tsinghua University, Beijing 100084, China

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* Corresponding Author

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Mailing address: Room 804, Sino-Italian Environmental and Energy-efficient

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Building, School of Environment, Tsinghua University, Haidian District, Beijing

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100084, China

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E-mail address: [email protected]

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Tel.: +86-10-62794143

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Fax: +86-10-62772048

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TABLE OF CONTENTS

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ABSTRACT

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This research presented the impacts of mechanochemical activation (MCA) on

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the physiochemical properties of lithium cobalt oxide (LiCoO2) powders of cathode

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materials from spent lithium-ion batteries, and analyzed the relevant effects of these

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changes on the leaching efficiency of lithium (Li) and cobalt (Co) and the leaching

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kinetics of LiCoO2 powders. The results revealed the superiority of MCA in the

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following levels of changes in the LiCoO2 powders: first, the physical properties

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included a decrease in the average particle size, an increase in the specific surface

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area, and the appearance of a mesoporous structure change; second, changes in

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crystal-phase structures were reflected in the grain refinement of LiCoO2 powders,

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lattice distortions, lattice dislocations, storage and increment of internal energy; third,

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the surface characteristics included a chemical shift of lithium element electrons, a

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reduction in Co3+ concentration, and an increment in the surface hydroxyl oxygen

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concentration. These changes in physiochemical properties and structures enhanced

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the hydrophilicity and interface reactivity of the activated LiCoO2 powders, and

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significantly improved the leaching efficiencies of Li and Co in organic acid solutions.

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The rate-limiting step of metal leaching was also altered, from a surface chemical

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reaction-controlled one before MCA to an ash layer diffusion-controlled one after

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MCA.

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INTRODUCTION

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With the increased production and use of portable electronic devices and

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electric-vehicle power batteries, the amount of spent lithium-ion batteries (LIBs) has

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also been rapidly increasing, and both academic researchers and industrial analysts

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have focused more attention on recycling LIB metals.1, 2 The valuable LIB-cathode

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metals lithium (Li) and cobalt (Co), in the form of lithium cobalt oxide (LiCoO2),

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provide a major economic incentive for the recovery of spent LIBs.3-10 Although

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several recovery methods have been used, hydrometallurgy has always been the major

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one, because of its high metal recovery rate, mild reaction conditions, controllability,

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and low environmental load.11-14 As for environmentally friendly reaction reagents,

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organic acids are generally the first choice for Li and Co leaching. A series of organic

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acids, including L-tartaric,15 aspartic,16 malic,17 oxalic,18 formic,19 ascorbic,20

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succinic,21 and citric22 acids have been employed to leach Li and Co from LiCoO2

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powders. Yet despite excellent technical prospects and environmental benefits, the

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industrial application of organic acid leaching has still been limited by technical

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bottlenecks, such as long reaction time and high reagent cost. Thus, significantly

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shortening the metal leaching time and further reducing the costs of reagents are the

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current challenges for the hydrometallurgic recovery of metals from spent LIBs.15, 23,

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24

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In recent years, mechanochemical activation (MCA) technology has been widely

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applied for enhancing the recovery efficiency of valuable metals from solid wastes.25,

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During MCA, the solid materials, under the actions of friction, collision, impact, 4

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cutting, and other mechanical forces, would obviously experience changes in

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physiochemical properties and material structures, even as those forces efficiently

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convert a portion of mechanical energy into internal energy, thereby improving the

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chemical reaction activity of solid materials.27, 28 MCA, therefore—because of its

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unique reaction mechanism, kinetics, thermodynamics, low operating costs, and ease

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of use—is a good candidate for metal recovery from e-waste.29, 30

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Yuan et al.31 used MCA to activate waste cathode ray tube glass, and discovered

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that mechanical forces could significantly enhance the lead leaching efficiency in

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nitric acid solution by destroying the Si-O bonds of network structures in the glass.

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Yang et al.32 used MCA to activate spent lithium iron phosphate battery cathode

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materials, and found that this activation could significantly enhance the leaching of

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lithium and iron. While MCA has proved to be an effective approach for accelerating

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the recycling performance of valuable metals from e-waste, the role and mechanism

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of MCA has seldom been fully investigated. Moreover, there is still a lack of

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systematic research concerning the effects of MCA on the physiochemical properties

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(physical properties, crystal-phase structures, surface characteristics, and solid-liquid

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interfacial behaviors) and structures of solid waste materials. Yet changes in these

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properties and structures could profoundly affect the efficiency and behaviors of metal

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leaching, making MCA an important area for further research.

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This study therefore focused on the role of MCA and its effects on the physical

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properties and material structures of LiCoO2 powders in spent LIBs. Using acetic acid

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leaching as a probe reaction, the effects of these physiochemical properties and 5

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structural changes on the metal leaching rate, leaching efficiency, and metal leaching

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kinetics were fully investigated. This study aimed to (1) discover the effects of MCA

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on the physical properties, crystal-phase structures, surface characteristics, and

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solid-liquid interfacial behaviors of LiCoO2 powders; (2) clarify the strengthening

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effect and mechanism of MCA on metal leaching; (3) investigate the effects of MCA

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on metal leaching kinetics; and (4) evaluate the economic and environmental benefits

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of MCA combined with acetic acid leaching for metal recovery from spent LIBs.

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MATERIALS AND METHODS

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Materials and reagents Waste LiCoO2 powders were provided by Huaxin

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Environmental Co. Ltd, Beijing, China. The Li and Co contents in the LiCoO2

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powders were 3.4 wt%, and 29.1 wt%, respectively. The acetic acid and hydrogen

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peroxide used were all of analytical grade and were purchased from the Chemical

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Reagent Company of Beijing, China. Deionized water was used for the preparation

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and dilution of chemical solutions.

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Experimental procedure The schematic diagram for this study is shown in Figure

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1. In the MCA stage, 0.5 g of LiCoO2 powders were placed in a zirconia pot with an

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inner volume of 45 mL and ground with 14 zirconia balls of 10 mm diameter. The

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LiCoO2 powders were then activated using a planetary ball-mill apparatus

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(Pulverisette-7, Fritsch, Idar-Oberstein, Germany) at several different rotary speeds (0,

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100, 200, 300, 400, and 500 rpm, where 0 rpm represents non-activated LiCoO2

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powders) for 30 min. The LiCoO2 powders were leached with acetic acid in a closed

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100-mL three-necked flask placed in a water bath with a magnetic stirrer. Various 6

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acetic acid concentrations (1, 5, 10, and 20 vol.%), H2O2 concentrations (0, 1, 3, 5,

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and 7 vol.%), and leaching temperatures (25, 35, 45, 55, and 65 oC) were analyzed for

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their effects on metal leaching from the LiCoO2 powders. Lithium cobalt oxide powders (C/LiCoO2) Mechanochemical activation Material characterization

Physical properties

Surface characteristics

Phase structure

Interfacial behavior

Organic acid leaching (Acetic acid) Leaching rate of metals

Parameter optimization

Leaching kinetics of metals

Economic assessment

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Figure 1. Schematic diagram of this study.

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Analytical methods The metal contents of the LiCoO2 powders were determined

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using an inductively coupled plasma-optical emission spectrometer (ICP-OES,

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OPTIMA 2000, PerkinElmer, USA) after digestion with an HNO3-HCl mixture. The

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contents of Li and Co in the acetic acid leaching solution were measured with

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ICP-OES and the leaching percentages expressed by the following equation:

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WLi/Co (wt%) =

C ×100% C0

Eq. (1)

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where C is the final mass of metal ions in the filtrate after leaching and C0 is the

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original mass of metal ions in the LiCoO2 powders before leaching.

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The average sizes of the LiCoO2 particles before and after MCA were measured 7

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with a Microtrac particle size analyzer (MT3300, Tokyo, Japan). The specific surface

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areas (SSAs) of the LiCoO2 powders were measured with a nitrogen gas adsorption

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instrument

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Brunauer-Emmett-Teller (BET) method. The crystal structures of the LiCoO2 powders

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were characterized by X-ray diffraction (XRD; Philips PW 1700, USA) using Cu Kα

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radiation (γ=1.5418Å) with 30 kV voltage and 30 mA current. Analysis of the X-ray

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diffraction data was carried out using MDI Jade 6.5 software. The morphologies and

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structures of the LiCoO2 powders were characterized with field emission scanning

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electron microscopy (FESEM; Carl Zeiss MERLIN Compact, Germany) and a

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transmission electron microscope (TEM; FEI, Tecnai G2 Spirit). X-ray photoelectron

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spectroscopy analysis was conducted using a PHI Quantera SXM (XPS;

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PHI-5300/ESCA, ULVAC-PHI, Inc, Japan). Thermogravimetric analysis combined

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with the differential scanning calorimetry (TGA-DSC) was conducted using a thermo

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gravimetric analyzer (TGA/DSC 1, Mettler Toledo, Switzerland) under argon

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atmosphere. The contact angle of the LiCoO2 powders was measured with the sessile

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drop method using a contact angle meter (OCA15Pro, Dataphysics Instrument Co.,

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Ltd., Germany). The zeta potential was measured via dynamic light scattering using

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particle size analyzer (SZ-100Z, Horiba Ltd., Japan).

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RESULTS AND DISCUSSION

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Physical properties of the LiCoO2 powders The properties and structures of

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LiCoO2 powders will affect the powders’ chemical reaction activity. Figure 2 shows

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the physical property changes of the activated LiCoO2 powders at different rotary

(ASAP2010,

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speeds. Figure 2a shows that the average particle sizes of LiCoO2 powders decreased

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as the rotary speed increased. At the rotary speed of 0 rpm, the average particle size

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was about 13 µm, but when the rotary speed exceeded 300 rpm, the average particle

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size was 0.206 µm. Yet as the rotary speed continued to increase to 500 rpm, the

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downward trend in average particle size was barely perceptible, possibly because at

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low rotary speeds, MCA had a significant effect on the crushing and grinding of

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LiCoO2 powders, whereas at high rotary speeds, agglomeration occurred among the

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LiCoO2 powder particles, and when the particle crushing speed and aggregation were

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balanced, the average particle sizes no longer decreased significantly. In contrast to

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the average particle size results, the specific surface areas of the LiCoO2 powders

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were gradually enlarged with an increase in the rotary speed. At 0 rpm, the LiCoO2

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powders had a specific surface area of 0.61 m2/g, but at 500 rpm, the specific surface

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area was enlarged by 40 times, to 24.56 m2/g. The combined results indicate that with

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an increase in rotary speed, MCA led to a decline in the particle sizes of LiCoO2

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powders, and gradually exposed numerous fresh surfaces and enlarged the specific

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surface areas. Moreover, the pore volumes of LiCoO2 powders were also enlarged

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with increases in the rotary speed (Figure 2b).

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Figure 2c indicates that at 0 and 100 rpm, the N2 adsorption/desorption curves of

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LiCoO2 powders at low-pressure clung to the X-axis, with little adsorption and weak

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interaction between nitrogen molecules and LiCoO2 powders, and no evident

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hysteretic loop was observed. These results indicate that the LiCoO2 powders did not

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develop pore structures after activation at low rotary speeds. With the rise of rotary 9

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speed to 300, 400, and 500 rpm, the N2 adsorption/desorption curves of the activated

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LiCoO2 powders all belonged to IV-type curves, indicating that the LiCoO2 powders

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began to develop pore structures after activation at high rotary speeds. Moreover, a

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clearly evident H3 hysteretic loop appeared at high relative pressures. Thus, it could

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be concluded that the LiCoO2 powders activated at high rotary speeds developed

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seam-shaped pore structures accumulated from sheet-like particles. The pore size

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distributions in Figure 2d also show that the LiCoO2 powders at 0 and 100 rpm had no

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pore structure, while with the rise of rotary speed, mesoporous structures sized at

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2.5-5 nm began to appear in the LiCoO2 powders.

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Figure 2. Physical properties of the LiCoO2 powders activated at different rotary

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speeds: (a) average size and specific surface area, (b) pore volume and pore size, (c)

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N2 absorption-desorption curves, and (d) pore diameter distribution.

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The above results indicate that the overall structure of LiCoO2 powders was 10

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destroyed after MCA, as the average particle sizes decreased, further facilitating the

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dispersion and mass transfer of LiCoO2 powders in the liquid phase. A large specific

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surface area will provide more active sites for chemical reactions, and well-developed

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pore structures will facilitate the solvent and solute molecules to infiltrate and diffuse

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into the inner part of the activated LiCoO2 powders. Changes in these physical

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properties favor a liquid-phase reaction of LiCoO2 powders.33

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Phase structure of the LiCoO2 powders XRD patterns in Figure 3a show that

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the LiCoO2 powders at 0 rpm exhibited major diffraction peaks at the (003), (101),

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(104), (015), (107), (018), (110), and (113) planes of the LiCoO2 crystals,

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accompanied by numerous impurity peaks—the amorphous peaks of carbon. With an

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increase in the rotary speed, the diffraction peak intensities of major crystal planes

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(003), (101), and (104) were weakened (Figure 3b), while the full widths at half

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maximum (FWHM) of the diffraction peaks were enlarged (Figure 3c). These results

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indicate that the mechanical impact, friction, and shear force during MCA led to grain

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refinement and lattice distortion of the LiCoO2 crystals.32

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SEM results (Figure S1) showed that the LiCoO2 powders at 0 rpm were

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irregular complete particles, and increase in the rotary speed induced the increasing

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destruction of LiCoO2 crystal particles. After activation at 500 rpm, no obvious

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interfaces were observed between LiCoO2 powder particles, and the surfaces of the

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activated LiCoO2 powders became loose and cloud-like. TEM results showed that the

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LiCoO2 powders at 0 rpm were large irregularly shaped spherical particles (Figure 3d).

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The LiCoO2 powders activated at 500 rpm were loose, layer-like, stacked structures 11

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with significantly reduced particle sizes (Figure 3e). HRTEM results showed that the

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lattices of the LiCoO2 crystals changed from an orderly state to a disorderly state after

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MCA, further confirming that the mechanical forces destroyed the LiCoO2 crystal

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lattices, thereby inducing lattice dislocation and distortion. Selected area electron

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diffraction (SAED) showed that the LiCoO2 crystals changed from layer-like

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hexahedron single crystals to polycrystals after activation.

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TGA-DSC results confirmed that the LiCoO2 powders activated at 500 rpm were

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more prone to pyrolysis than those at 0 rpm (Figure S2). From the perspective of

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energy, the MCA subjected LiCoO2 crystals to grain refinement and lattice distortion,

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and a portion of the mechanical energy was stored in the LiCoO2 powders in the form

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of internal energy. The LiCoO2 powders were therefore more susceptible to pyrolysis,

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since they were in a metastable state after MCA.28, 34

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The above results indicate that during MCA, mechanical impact, shear force, and

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friction led to the lattice deformation and defects of LiCoO2 crystalline internal

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structures. After MCA, the LiCoO2 powders’ chemical energy was obtained from the

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external environment, and their crystalline interior underwent lattice distortion and

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defects, increasing the internal energy. As a result, the LiCoO2 powder surfaces were

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manifested as a layer of high-activity and high-energy floccules. The existence of

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floccules was the major cause of the intensified leaching reaction of solid materials

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after MCA.

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Figure 3. (a) XRD patterns, (b) XRD peak intensity, and (c) FWHM of the LiCoO2

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powders activated at different rotary speeds; TEM, HRTEM, and SAED of the

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LiCoO2 powders activated at (d) 0 rpm and (e) 500 rpm.

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Surface characteristics of the LiCoO2 powders XPS survey scan spectra of the

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LiCoO2 powders at different rotary speeds (Figure S3) show that the surface

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characteristic peaks of LiCoO2 powders were mainly Co2p, O1s, and C1s, while the

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characteristic peaks of Li1s were not obvious, due to the low Li concentration. The

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high-resolution XPS spectra of Li1s (Figure 4a) show that the binding energy of Li1s

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peak shifted about ∆E=0.61 eV to the low-energy field with an increase in the rotary

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speed, indicating that the Li element experienced a chemical shift during MCA, and

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the bonding interaction was destroyed. When the rotary speed exceeded 300 rpm, a 13

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portion of the lithium oxide in the LiCoO2 powders reacted with carbon black to form

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lithium carbonate (Table S1).35 The high-resolution XPS spectra of C1s also

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confirmed that a portion of the C element was converted to carbonate after activation

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at 300, 400, and 500 rpm (Figure 4b and Table S2). The FT-IR results in Figure S4a

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further confirmed the appearance of carbonate characteristic peaks (864.7, 1435.3,

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and 1487.5 cm-1) in the MCA products. The possible formation mechanism of Li2CO3

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is as follows (Figure S4b):

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3LiCoO2 + C → 3Li + Co3O4 + CO2 ↑

(R1)

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12LiCoO2 + 6CO2 → 6Li2CO3 + 4Co3O4 + O2 ↑

(R2)

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4Li + 2CO2 + O2 → 2Li2CO3

(R3)

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Figure 4c shows that each high-resolution XPS spectrum of Co2p was

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composed of a Co2p3/2 main peak, a satellite peak (S1), a Co2p1/2 main peak, and

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another satellite peak (S2). The energy difference between Co2p3/2 and Co2p1/2

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splitting is approximately 15 eV, which indicates the presence of both Co2+/Co3+

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species in all LiCoO2 samples.36 The peak fitting results indicated that the main peak

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of Co2p3/2 could be split into Peak 1 and Peak 2, while the Co2p1/2 main peak could

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be split into Peak 3 and Peak 4. Since the FWHM of the Co2+ peak was broader than

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that of the Co3+ peak (Table S3), and the Co2+ had a larger spin-splitting distance ∆E

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(∆E2>∆E1) than the Co3+ one (Table S4), it can be deduced that the fitted Peaks 1 and

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3 correspond to Co3+, while Peaks 2 and 4 correspond to Co2+.

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rotary speed, the I(Co3+/Co2+) values decreased to varying degrees, indicating that the

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Co3+ proportion declined during MCA, while the Co2+ proportion increased, due to 14

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With the rise in

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the reduction effect of carbon black on Co3+ under the action of mechanical forces

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(Table S4). Compared with Co3+, Co2+ reacted faster during leaching in organic acid,

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and the usage of the reductant could be therefore reduced.39

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The high-resolution XPS spectra of O1s (Figure 4d) show that the oxygen

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species on the surface of LiCoO2 crystals were mainly lattice oxygen (Olatt) and

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surface hydroxyl oxygen (Osur); the Olatt originated from the internal structure of

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LiCoO2 crystals, and the Osur resulted from the hydrothermal synthesis of LiCoO2

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crystals.37 With an increase in the rotary speed, the I(Olatt/Osur) values continually

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decreased, implying that the proportion of Olatt on the LiCoO2 crystal surface declined,

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while the proportion of Osur increased (Table S5). The decrease in the Olatt proportion

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implied an increase in oxygen vacancy concentration, which improved the surface

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hydroxylation of the activated LiCoO2 powders. Moreover, the inner electron binding

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energy of Li1s (from 0 to 100 rpm), Co2p, and O1s all shifted to the high-energy

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fields with an increase in the rotary speed, a result that could be attributed to the

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differences in the effects of extra-atomic relaxation.40

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Figure 4. High-resolution XPS spectra of (a) Li1s, (b) C1s, (c) Co2p, and (d) O1s of

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the LiCoO2 powders activated at different rotary speeds.

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Interface behavior of the LiCoO2 powders The contact angles of water and

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ethanol on the surface of LiCoO2 powders activated at different rotary speeds were

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measured using the contour image method (Figure S5). The values of the contact

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angles were calculated according to the Young-Laplace equation (γSV = γSL + γLV ×

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cosθe). Table 1 shows that the contact angles of water on the surface of the LiCoO2

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powders decreased with an increase in the rotary speed, indicating that the surfaces of

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LiCoO2 powders became more hydrophilic after MCA. Combined with the O1s XPS

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results, it can be concluded that the enhancement of the surface hydrophilicity can be

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attributed to the increasing proportion of Osur on the surfaces of the LiCoO2 powders.

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As a hydrophilic functional group, the Osur can easily bind with water molecules to 16

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form hydrogen bonding, thus enhancing the hydrophilicity of the powders.35 Using the contact angles of water and ethanol, the surface free energy of LiCoO2

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powders

activated

at

different

rotary

speeds

were

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Owens-Wendt-Rabel-Kaelble method. Table 1 shows that the surface free energy of

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the LiCoO2 powders increased gradually with the increase in rotary speed. During

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MCA, the LiCoO2 powders were crushed into smaller sizes under the action of

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mechanical forces, and developed numerous fresh surfaces, thus enlarging the specific

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surface area and increasing the surface free energy and interface activity of the

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LiCoO2 powders. Zeta potential results showed that MCA could significantly enhance

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the surface negative charge of the LiCoO2 powders. During liquid-phase leaching, H+

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could be better diffused via electrostatic attraction and combined with the surface

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hydroxyl of the LiCoO2 powders. These changes were all favorable for the

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liquid-phase interfacial infiltration and dispersion of the activated LiCoO2 powders,

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and accelerated the liquid-phase reactions of the activated LiCoO2 powders.

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Table 1. Contact angle, surface free energy, and zeta potential of the LiCoO2 powders

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activated at different rotary speeds. Rotary speed

Contact angle (°)

Surface free energy

Zeta potential

(rpm)

Water

Ethanol

(mN/m2)

(mV)

0

37.0

22.0

97.7

-0.6

100

30.0

21.8

110.7

-3.9

200

22.5

20.7

122.2

-43.6

300

19.8

21.0

126.0

-45.4

400

19.1

26.5

130.4

-47.0

500

17.6

25.5

131.6

-48.7

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Analysis of metal leaching and kinetics Figures 5a and 5b show that the

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leaching percentages of Li and Co varied with leaching time from the LiCoO2

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powders activated at different rotary speeds. The following results can be observed: (1)

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under identical conditions, the Li leaching percentage was always higher than that of

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Co, indicating that Li was preferentially leached out due to its higher activity; (2) the

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leaching percentages of both Li and Co were enhanced with increasing activation

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rotary speed. These results indicate that the LiCoO2 powders were more activated at

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higher rotary speeds. Combined with the characterization results, it was obvious that

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the average particle sizes of the LiCoO2 powders decreased after MCA, thus

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improving both the specific surface areas and surface activity. More importantly, the

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LiCoO2 crystals were lattice-deformed and experienced numerous structural defects,

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increasing the internal energy and enhancing the chemical reaction activity of the

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solid materials. This is the core influencing factor for the rapid leaching of metals in

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LiCoO2 powders after MCA. During the leaching reactions, the highly activated 18

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surface layer of the LiCoO2 powders was rapidly dissolved in acetic acid solution, and

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the metal leaching rate and percentage were therefore improved within a very short

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time. The effects of volume concentrations of acetic acid and hydrogen peroxide

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(H2O2) on the metal leaching percentages of Li and Co are shown in Figure S6.

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The effects of MCA on the metal leaching kinetics were investigated by

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recording Li and Co leaching percentages varied within 5 min, at five different

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temperatures, from LiCoO2 powders activated at 0 rpm (Figures 5c and 5d) and at 500

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rpm (Figures 5e and 5f), respectively. The contrasting experiments showed that the

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increase in leaching temperature significantly accelerated the metal leaching under

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identical conditions, possibly because the temperature increase accelerated the mass

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transfer behaviors. With an increase in the leaching temperature, more energy was

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accumulated in the LiCoO2 powders, enhancing their ability to weaken or destroy the

320

chemical bonds in the LiCoO2 particles and increasing the number of molecules

321

containing kinetic energy equal to or greater than the leaching apparent activation

322

energy. As a result, the metal leaching rate was enhanced and the metal leaching

323

percentage per unit time was significantly improved.

19

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Figure 5. Effect of rotary speed on the leaching percentages of (a) Li and (b) Co

326

(conditions: 25 oC, 20 vol.% acetic acid, and 5 vol.% H2O2); effect of temperature on

327

the leaching percentages of (c) Li and (d) Co (conditions: 0 rpm, 20 vol.% acetic acid,

328

and 5 vol.% H2O2); effect of temperature on the leaching percentages of (e) Li and (f)

329

Co (conditions: 500 rpm, 20 vol.% acetic acid, and 5 vol.% H2O2). 20

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Since the LiCoO2 powder particles before the leaching reaction were dense and

331

poreless, the reaction interface gradually contracted toward the cores after the acetic

332

acid molecules reacted with the LiCoO2 powder particles. Thus, metal leaching from

333

the LiCoO2 powders could be described by the shrinking unreacted-core model

334

(Figure S7). The metal leaching percentage versus leaching time at different leaching

335

temperatures was fitted with the use of LiCoO2 powders activated at 0 and 500 rpm,

336

respectively; ka is the slope of the fitted straight line, or the apparent rate constant of

337

the leaching reaction (min-1); R2 is the coefficient of determination, and a larger R2

338

means the linear relationship of the regression equation is better and the fitting result

339

is more reliable (Tables S6 and S7).41 At 0 rpm, both Li and Co, 1-(1-x)1/3 were well

340

linearly related with the leaching time t (R2 > 0.98), indicating that the Li/Co leaching

341

from the LiCoO2 powders followed the shrinking unreacted-core model with surface

342

chemical-reaction-control as the rate-limiting step (Table S6). At 500 rpm, for both Li

343

and Co, 1-3(1-x)2/3+2(1-x) is well linearly related with the leaching time t (R2 > 0.98),

344

indicating that the Li/Co leaching from the LiCoO2 powders followed the shrinking

345

unreacted-core model with ash layer diffusion control as the rate-limiting step (Table

346

S7). The contrast analysis showed that MCA profoundly affects the Li/Co leaching

347

kinetics from LiCoO2 powders. The rate-limiting step of metal leaching changed from

348

a surface chemical reaction-controlled one before MCA to an ash layer

349

diffusion-controlled one after MCA.

350

Apparent activation energy of the metal leaching reaction On the basis of

351

the ka from Tables S6 and S7, the Arrhenius equation (Eq. (2)) was employed to 21

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determine the apparent activation energy:

k a = Ae

353



Ea RT

Eq. (2)

354

where A is the pre-exponential factor (min-1); Ea is the apparent activation energy of

355

reaction (kJ·mol-1); R is the molar gas constant (R=8.3145, J·mol-1·K-1); and T is the

356

reaction absolute temperature (K). Logarithm of both sides of Eq. (2) yields:

ln ka = ln A −

357

Ea RT

Eq. (3)

358

Obviously, lnka and 1/T are linear relations, and the slope of the straight line is -Ea/R.

359

Thus, according to the rate constant at different reaction temperatures, the Ea can be

360

calculated from Eq. (3).

361

The leaching rate constant ka at different reaction temperatures in Tables S6 and

362

S7 was substituted into Eq. (3). Arrhenius plots were plotted with the intercept lnka as

363

the Y-axis and 1/T as the X-axis, and the slope of the straight line was exactly -Ea/R.

364

The leaching apparent activation energies of Li and Co before and after the MCA

365

were then obtained (Figure S8). The apparent activation energy of Li decreased from

366

43.09 to 11.68 kJ·mol-1 after MCA (Figure S8a), while that of Co declined from 47.74

367

to 22.92 kJ·mol-1 (Figure S8b). These results indicated that MCA enhanced the

368

chemical reaction activity of the LiCoO2 powders, reduced the Ea of the metal

369

leaching reaction, and lowered the temperature and time dependence of the leaching

370

reactions.

371

Environmental implications A comparison of the methods revealed that MCA

372

combined with acetic acid leaching could achieve the goal of metal leaching at room

373

temperature in a shorter time (Table S8). Further economic analysis showed that 22

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acetic acid was more effective as a leaching reagent for Li and Co leaching than other

375

traditional inorganic or organic acids (Table S9). And in terms of environmental

376

benefits, acetic acid is much safer, in both transportation and storage, than high-risk

377

HCl and H2SO4. In addition, the acetic acid reagent can be regenerated into sodium

378

acetate, as depicted in a designed flow chart (Figure S9), to further reduce the reagent

379

cost and complete the separation of Li and Co.30 Economic assessment (Figure S10

380

and Table S10) for the recovery of 1 kg spent LIBs at the laboratory scale, based on

381

MCA combined with an acetic acid leaching process, demonstrated economic

382

feasibility. Thus, acetic acid is a better reagent for metal recovery from spent LIBs, in

383

terms of leaching percentage, security, and economy, than other chemical reagents.

384

In summary, MCA can obviously change the physical properties, crystal-phase

385

structure, surface characteristics, and solid-liquid interfacial behaviors of LiCoO2

386

powders and significantly improve the leaching efficiencies of Li and Co in acetic

387

acid solution. The rate-limiting step of metal leaching from LiCoO2 powders was

388

altered from a surface chemical reaction-controlled one before MCA to an ash layer

389

diffusion-controlled one after MCA, and the apparent activation energies of the

390

leaching reactions were significantly lowered. Rapid leaching of Li (99.8 wt%) and

391

Co (99.7 wt%) was achieved within a short time of 15 min at room temperature. MCA

392

combined with acetic acid leaching presents outstanding practical application

393

prospects and may be an environmentally friendly technology worth popularizing for

394

the recovery of valuable metals from spent LIBs.

395

SUPPORTING INFORMATION 23

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SEM images of the LiCoO2 powders activated at different rotary speeds (Figure S1);

397

TGA-DSC curves of the LiCoO2 powders activated at 0 and 500 rpm (Heating rate of

398

15 °C/min in argon atmosphere) (Figure S2); XPS survey scan spectra of the LiCoO2

399

powders activated at different rotary speeds (Figure S3); (a) FT-IR spectra of the

400

LiCoO2 powders activated at different rotary speeds, and (b) the possible formation

401

mechanism of Li2CO3 (Figure S4); Contact angle images of (a) water and (b) ethanol

402

on the surface of the LiCoO2 powders activated at different rotary speeds, and (c)

403

schematic of a liquid drop showing the quantities for the Young–Laplace equation

404

(Figure S5); Effects of different acetic acid concentrations on the leaching

405

percentages of (a) Li and (b) Co (conditions: rotary speed of 500 rpm, acetic acid

406

concentration of 20 vol.%, and H2O2 concentration of 5 vol.%); and effects of

407

different H2O2 concentrations on the leaching percentages of (c) Li and (d) Co

408

(conditions: rotary speed of 500 rpm, acetic acid concentration of 20 vol.%, and H2O2

409

concentration of 5 vol.%) (Figure S6); Schematic diagram of shrinking

410

unreacted-core model (Figure S7); Arrhenius plots for leaching of (a) Li and (b) Co

411

from the LiCoO2 powders activated at 0 and 500 rpm (Figure S8); Designed flow

412

chart for Li and Co recovery from spent LIBs (Figure S9); Materials balance and cost

413

assessment for spent LIBs recovery based on MCA combined with acetic acid

414

leaching (Figure S10); Fitting peaks of the Li1s XPS spectra of the LiCoO2 powders

415

activated at different rotary speeds (Table S1); Fitting peaks of the C1s XPS spectra

416

of the LiCoO2 powders activated at different rotary speeds (Table S2); FWHM of the

417

Co2p fitting peaks of the LiCoO2 powders activated at different rotary speeds (Table 24

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S3); Fitting peaks of the Co2p XPS spectra of the LiCoO2 powders activated at

419

different rotary speeds (Table S4); Fitting peaks of the O1s XPS spectra of the

420

LiCoO2 powders activated at different rotary speeds (Table S5); The rate constant (ka)

421

and the coefficient of determination (R2) for Li and Co leaching from the LiCoO2

422

powders activated at 0 rpm for different temperatures (Table S6); The rate constant

423

(ka) and the coefficient of determination (R2) for Li and Co leaching from the LiCoO2

424

powders activated at 500 rpm for different temperatures (Table S7); Key operational

425

parameters of different methods used for Li and Co recovery from the LiCoO2

426

powders (Table S8); Economic comparison of the Li and Co leaching from the

427

LiCoO2 powders by various inorganic acids and organic acids (Table S9); Economic

428

benefit assessment of the proposed MCA combined with acetic acid leaching process

429

(Table S10). Supporting Information is available free of charge via Internet at

430

http://pubs.acs.org.

431

AUTHOR INFORMATION

432

Corresponding author

433

Jinhui Li*,

434

Tel.: +86-10-62794143.

435

Fax: +86-10-62772048.

436

E-mail address: [email protected]

437

Notes

438

The authors declare no competing financial interest.

439

ACKNOWLEDGEMENTS 25

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440

This study was financially supported by major project of “The National Social

441

Science Fund of China” (16ZDA071) and the China Postdoctoral Science Foundation

442

(2016M601051). The authors thank Prof. Joseph F. Chiang from State University of

443

New York College at Oneonta for his valuable advice. The authors also thank Dr.

444

Kang Liu from Huazhong University of Science and Technology for his valuable

445

advice.

446

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