First Exploration on Electrochemical Activation of Low‐Cost Albite

First Exploration on Electrochemical Activation of Low-cost Albite Mineral for Boosting Lithium Storage Capability

Jun Mei, Tiantian Wang, Hong Peng, Godwin A. Ayoko, Jianjun Liu, Ting Liao*, Ziqi Sun*

Dr. J. Mei, Prof. G. A. Ayoko, Prof. Z. Sun

School of Chemistry and Physics, Queensland University of Technology, 2 George Street, Brisbane, QLD 4000, Australia E-mail: [email protected]

T. Wang, Prof. J. Liu

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

T. Wang, Prof. T. Liao

School of Mechanical, Medical and Process Engineering, Queensland University of Technology, 2 George Street, Brisbane, QLD 4000, Australia

E-mail: [email protected]

Dr. H. Peng

School of Chemical Engineering, The University of Queensland, Brisbane, Australia

T. Wang

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

This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/adsu.202000057.

Keywords: albite, mineral, capacitive, batteries, carbon coating

Abstract: The direct use of natural minerals for low-cost energy storage is a promising solution towards large-scale and affordable sustainable energy supply, but it is usually impeded by their inert electrochemical activity. In this work, electrochemically inert albite mineral layered particles were first activated as a promising low-cost anode material for electrochemical Li+ ions storage devices through a

facile thermal reduction treatment technology. Via this strategy, partial SiO2 reduction within the albite mineral particles and conductive carbon layer on the surface were simultaneously realized, which effectively address the issues of inactive lithium storage and poor electric conductivity of albite minerals. Via theoretical density functional theory (DFT) calculations and molecular dynamics (MD) simulations, the activation mechanism of lithium storage achieved by silica reduction was understood. This innovatively activated albite mineral delivered a maximum specific capacity of >250 mAh g-1 based on a dominant surface-driven capacitive storage mechanism, and performed excellent capacity and cycling stability at high charging/discharging rates. This design opens a new pathway to address the current cost issue in energy devices, provides insights into producing cost-effective anodes by using silica-rich minerals, and gives an alternative solution for improving ions chemical reactivity of inorganic natural minerals towards low-cost and large-scale energy storages.

1. Introduction

Even though rechargeable batteries have been widely used in smart electricity grids and electric vehicles, the high cost of active electrode materials is still a great challenge for a more widespread application of this class of sustainable power source. The direct use of low-cost natural minerals recently has been found to be a promising alternative to develop cheaper but high-performance batteries.[1–5] Albite mineral, as one of typical tectosilicate mineral member in plagioclase feldspar family, is a common constituent in siliceous aggregates, such as granite and rhyolite. Chemical component analysis confirms that the primary constitutes of albite mineral include silicon dioxide

[6] (SiO2) and aluminium oxide (Al2O3). As a type of silicon-rich minerals, albite mineral has the

potential to be utilized as anode material candidates for lithium ion batteries (LIBs). Unfortunately, it exhibits nearly inert activity in the electrochemical reactions towards Li+ ions and poor electrical conductivity, which impede efficient ions transport within in the crystals.[7] From this pointview, for energy storage application of albite mineral material, functional activation through electronic structure alteration and/or chemical modification are significantly essential, for example, for the possible use as an anode material candidate for Li-based secondary batteries.

As a silicon-rich mineral, albite is actually very promising to be used as an anode for LIBs, based on some existing knowledge on silicon-containing electrode materials. First, silicon is one promising candidate to replace the predominately graphite due to its strikingly high theoretical capacity

-1 [8–10] (~4,200 mAh g ) for lithium storage. Second, it has demonstrated that SiO2 powders treated by mechanical milling possess a significantly enhanced activity towards Li+ ions storage.[7,11,12] Of course, silicon-based electrodes suffer from obvious volume changes (>400%) upon lithiation/delithiation, which often leads to an unsatisfying cycling stability.[10,13,14] This disadvantage, however, can be solved by amphorization and/or nanosizing of silicon/silica. For example, amorphous SiOx can deliver buffered volume expansion coefficiencies (< 200%), and thus lead to an enhanced cycling stability for

Li+ ions storage.[13,15–20] Inspired by these mentioned above, to modify the physical and chemical properties of the pristine albite mineral for effective lithium storage, reduction of Si-O bonds within albite structures to create partly SiOx species and spontaneous coating of a conductive carbon layer on albite particles to address the issues of electrically non-conductive and electrochemically inactive albite minerals are proposed in this work. It is expected to effectively enhance its chemical reactivity towards Li+ ions and to realize the direct utilization of low-cost albite as an anode material for rechargeable batteries.

Herein, a facile thermal reduction treatment has been performed to modify the activity and conductivity of albite mineral particles. During the synthetic process, sugar, which acts as the carbon source, was initially pyrolyzed into carbon at an elevated temperature, and at the same time, inactive SiO2 species in albite were partly reduced into SiOx compounds with the presence of carbon in a non-oxidizing environment. Using this strategy, the deposition of an uniform conductive carbon layer onto albite particles and the formation of active partly reduced SiOx compounds are simultaneously realized, which are highly desired for ions storage behaviours. As expected, electrochemical characterization suggests that the obtained electrochemically activated albite (EAA) presented great potential as anode materials for high-rate LIBs. Further in-depth evaluation on ionic storage mechanism of the EAA anode reveals that the origin of fast Li+ ions kinetics was primarily contributed by partially reduced SiO2 together with surface-driven capacitive behaviours, which involves surface redox pseudocapacitance (faradaic) and double-layer capacitance (non-faradaic), particularly at high scan rates (~86% at 1.0 mV s-1). It is anticipated that our elaborate design on the

EAA anode can offer some useful clues for improving the ion reactivity of low-cost inorganic natural minerals by boosting surface capacitive contribution and provide an effective solution for the direct use of abundant natural minerals for low-cost and large-scale energy storages.

Scheme 1. Schematic illustration of synthesis of electrochemically activated albite (EAA).

2. Results and Discussion

Electrochemically activated albite (EAA) innovated in this wok was fabricated through a facile thermal reduction technique. Specifically, as illustrated in Scheme 1, white albite mineral powder was first dispersed into sugar water under continuously rigorous stirring. After dried in an oven in air, the resulting sugar-coated albite mineral powder was transferred into a tube furnace using a customized quartz boat, and then heated into 700 oC and maintained for 2 h under a continuous flowing of Ar and H2 gas mixture to produce final black EAA product. This synthetic approach is quite simple, and it is easy for massive production in industrial scale. During the synthetic process, it should be noteworthy that sugar plays crucial roles as both the carbon source for coating and the reducing agent for activating SiO2 species in an oxygen-free environment. It is clearly observed that the as-synthesized EAA exhibited a porous foam-like appearance with various pore sizes (Figure 1a and b), which were formed by the pyrolysis process of sugar on the albite particle surfaces at an

elevated temperature. High-magnification scanning electron microscope (SEM) image revealed that these albite mineral particles showed a sheet-like morphology with a planer size of several micrometre (Figure 1c). Bright-field Transmission electron microscope (BF-TEM) image (Figure 1d) and the corresponding element mapping patterns (Figure 1e-j) of EAA revealed that albite mineral particles, which were mainly composed of Si (36.6 %), Al (12.4 %) and Na (7.7 %), K (0.6 %) and Ca

(0.4 %) (Figure S1 and S2), were fully covered by an outer carbon box (Figure 1d-j). Furthermore, the element distribution scan along the marked line in Figure 1d verified the uniform distribution of carbon element on albite mineral particle surfaces and the thickness of the carbon layer was around

25 nm (Figure 1k). In the meanwhile, a visual comparison on high-magnification TEM images of albite mineral particles (Figure 1l) and EAA (Figure 1m) also clearly evidenced that a thin carbon layer with a thickness of ~25 nm was uniformly coated onto albite particles in EAA. These results clearly verify that carbon was uniformly coated onto albite mineral particles and this is expected to considerably improve the overall conductivity of albite mineral, which is highly desired for electrochemical ions storage. Recently, it has revealed that the layered structure is very critical to improve the ionic storage of the materials by providing significant interfacial or capacitive storages particularly at high rates.[21,22] It is also expected that the layered structure of albite mineral will also contribute to the surface/interface related capacitive lithium storages.

Figure 1. Morphologies characterizations of albite mineral particles and EAA particles. (a) Photograph of EAA product and (b) optical picture of EAA surface showing the porous structure. (c) SEM image of EAA particles. (d) BF-TEM image of EAA particles and the corresponding element mapping patterns of (e) C, (f) Si, (g) Al, (h) Na, (i) K and (j) O. Inset in d shows the schematic illustration of EAA particles. (k) Element distribution patterns along the marked line in d. (l, m) TEM images of (l) pristine albite mineral particles and (m) EAA particles. Inset in m shows the thickness of carbon layer coated on albite particles.

Figure 2a presents thermal gravimetric analysis (TGA) of the as-synthesized EAA conducted under air flow, where dramatic weight loss occurred at the temperature higher than 700 oC and a carbon percentage of ~17.6 % in EAA was calculated based on the weight loss. Structural changes of albite mineral crystals over reactions were characterized by X-ray diffractometer (XRD), Fourier transform infra-red spectroscopy (FT-IR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) techniques. As known, standard albite mineral crystals exhibit a triclinic crystal system (Inset in

Figure 2a) with the lattice constants of a = 8.16 Å, b = 12.82 Å, c = 7.13 Å (α = 93.96°, β = 116.47°, γ =

88.63°). For EAA, however, the intensities of all peaks attributed to albite were obviously decreased

(Figure 2b), which is primarily due to the presence of coated carbon layer on the surfaces of albite mineral particles. Besides, compared to pristine albite mineral particles, it is clearly detected that slight changes of some main peaks occur in EAA (Figure 2c), which is possibly caused by the partly breakdown of numerous surface Si-O bonds and the further migration of Si atoms with the presence of carbon at an elevated temperature. In the FT-IR pattern of pristine albite mineral (Figure 2d), these main peaks located on 464.8, 534.7, 581.3, 643.6, 761.4, and 990.1 cm-1 are ascribed to Si-O-Si rocking, O-Si-O bending/Na-O stretching, Al-O bending, tetrahedral ring vibration, Si-O-Si bending, and bridging-stretching of [SiO4], respectively. After thermal reduction of EAA, red-shift phenomena were detected, suggesting possible structural variations, including the reduction of SiO2 into SiOx in

EAA. This can be further revealed by Raman spectra in Figure 2e. By comparison, most of these peaks belonging to albite mineral became widening with greatly reduced intensities. The peaks in low- and middle wavenumber regions can be attributed to Al/Si-O-Al/Si or Na/K-O lattice modes and

O-Al/Si-O deformation or Si/Al-O-Si/Al lattice modes, respectively. As the wavenumber increased, two obvious peaks were found, and they should be attributed to Al-Si-O stretching and O-Si/Al deformation in [TO4] lattice. This comparison between albite mineral and EAA verifies the main framework of albite crystal structure remained unchanged after surface carbon coating and thermal reduction, and partly structural distortions may only appear on the surface of EAA during high- temperature thermal treatment process. To further confirm the possible surface structural changes related to silicon over thermal treatment, high-resolution XPS patterns of Si 2p in albite mineral and

EAA was provided in Figure 2f and g, respectively. In pristine albite mineral particles, the existence

form of silicon is similar to silicon dioxide (Si4+), which is evidenced by a solely obvious peak centred at ~102 eV. For EAA, however, the case becomes complex with multiply peaks determined, which can be attributed to Si0 (100.5 eV), Si1+ (101.5 eV), Si2+ (103.2 eV), Si3+ (104.2 eV), and Si4+ (105.3 eV).

These results suggest that silicon species on the surface of albite mineral particles are reduced to more active SiOx with the presence of carbon, and this change are expected to be beneficial to accelerating ion reaction kinetics when the material is used in energy storage devices.

Except for the structural changes, physical properties are obviously altered for facilitating ions anchoring and transport after carbon coating. As illustrated in Figure 2h and i, pristine albite mineral exhibits a quite low surface area of only 1.79 m2 g-1 and nearly no pores can be detected. After coating carbon at a reducing environment, the surface area of EAA can be greatly increased into

64.41 m2 g-1, and the mesoporous structure can be also clearly identified in a pore size range of 2-16 nm. The variations of the physical and chemical properties mentioned above are fully desired for electrochemical ions storage mechanism in terms of increasing effective active sites, improving conductivity, and buffering structural changes upon charging/discharging, which endow EAA promising as a good anode material candidate for rechargeable batteries.

Figure 2. Structural characterizations of albite mineral particles and EAA particles. (a) TGA curve of EAA showing 17.6 % weight loss under air flow. Inset shows the crystal structure of pristine albite mineral. (b) XRD and (c) enlarged XRD patterns, (d) FT-IR and (e) Raman spectra of albite mineral and EAA. (f, g) High-resolution XPS pattern of Si 2p in (f) albite mineral and (g) EAA. (h) Nitrogen adsorption-desorption plots and (i) the corresponding pore size distribution plots of albite mineral and EAA.

To evaluate the possibility of albite as anode material for electrochemical Li+ ions storage devices, cyclic voltammetric (CV) technique was first applied to reveal the possible reaction pathways over charging/discharging. As for the pristine albite electrode, a broad cathodic peak appeared at ~0.8 V in the first four cycles (Figure 3a) indicate the formation of relatively stable solid electrolyte interphase (SEI) layer on the surface of albite mineral particles [12,23,24]. As the potential decreases

over discharging, the lithiation reactions occur, which is possibly accompanied by the formation of

Li-Si system alloy. Upon charging in the initial cycle, it is clearly detected that there is a prominent peak located at ~0.2 V and a broad peak at ~0.9 V, which should be attributed to the reversible alloying/dealloying reaction with Li+ ions [25]. These anodic peaks remain nearly unchanged in the following three oxidation cycles, indicating good electrochemical reaction stability. Compared to pristine albite anode, EAA electrode manifests quite similar CV curves (Figure 3b). However, the anodic peaks become broader. This is related to the presence of coated carbon layers onto albite mineral particles, which will first react with active Li+ ions and contributes to increasing lithium storage capacity. As shown in Figure 3c, the initial discharge capacities of albite mineral and EAA anodes were 263.1 and 86.6 mAh g-1, respectively. The comparison of cycling stabilities in Figure 3d suggest that the capacity remain over 140 mAh g-1 for EAA anode, which is still superior to the pristine albite electrode. This excellent cycling stability is highly associated to the unique merits from the EAA structure. For example, (i) the existing carbon layer can effectively prevent the albite mineral particles from serious aggregation, improve the electric conductivity, and buffer the structural fluctuation upon cycling; (ii) the produced porous structure can efficiently promote the insertion/extraction of Li+ ions; (iii) the partly reduction of Si-O bonds can increase the reactivity of silica with Li+ ions and accelerate the reaction kinetics.

As a further important indicator for high-performance LIBs, rate capability of EAA anode was measured at various current densities ranging from 5 to 5,000 mA g-1. As presented in Figure 3e and f, the average discharge capacities are 175, 160, 145, 132, 93, and 73 mAh g-1 at the current densities of 5, 10, 20, 50, 500, and 1,000 mA g-1. Even at a rate as high as 2,000 and 5,000 mA g-1, the capacity of EAA anode still maintains at around 56 and 32 mAh g-1, respectively. Attractively, the coulombic

efficiencies of EAA anode keep close to 100% after the first cycles regardless of rate, suggesting a high degree of reversibility upon lithiation and delithiation processes contributed by the synergic effects raised from the outer carbon layer and the inner partly reduced albite mineral particles.

Figure 3. Electrochemical properties of albite mineral and EAA anodes for LIBs. (a, b) CV curves of (a) albite and (b) EAA anodes in the first four cycles. (c) Initial discharge-charge profiles of albite and EAA anodes. (d) Cycling performance for 100 cycles at a current density of 50 mA g-1. Inset shows the

supressed volume changes over cycling. (e) Rate capability of EAA anode between 5 and 5,000 mA g- 1. (f) Discharge-charge profiles of EAA anode at different rates from 5 to 5,000 mA g-1.

To further understand the attractive rate capability behaviour of EAA anode in LIBs, electrochemical kinetics was then studied using CV technique in the potential window of 0-3.0 V. As shown in Figure 4a, a series of similar CV curves at various scan rates between 0.1 and 1.0 mV s-1 are clearly identified, which indicates a small polarization phenomenon of the as-obtained EAA anode for LIBs. Figure 4b illustrates the power-law relationship between peak current (i, mA) and scan rate

(v, mV s-1) according to the Equation (1) and (2):

(1)

(2) where b value is the calculated slope of the fitted linear curve, which is an effective indicator for the charge storage mechanisms. It should be noteworthy that the b-value of 0.5 indicates a completely diffusion-controlled behaviour while that of 1.0 corresponds to a surface/interface-controlled capacitive behaviour.[26,27] For the EAA anode, the b value is calculated to be around 0.97, indicating a dominant surface/interface-controlled capacitive Li+ ions storage behaviour during the electrochemical redox process. Furthermore, the capacitive current can be separated from the diffusion-controlled current according to Equation (3) and (4):

(3)

⁄ ………………………………………………………..……………(4)

where i and v refer to the current at a specific potential and scan rate, respectively; and can be obtained from the slope and intercept, respectively, by plotting ⁄ versus under various

[28,29] potentials. Notably, represents the surface and/or interface-controlled capacitive

contribution, and stands for the diffusion-controlled process. As presented in Figure 4c, over

65% of the total capacity is related to a capacitive behaviour at a low scan rate 0.1 mV s-1 for EAA anode. As the scan rate increases, the capacitive contribution ratios increase, achieving 85.8% at a high rate of 1.0 mV s-1 (Figure 4d). As a result, the desired fast Li+ ions transport kinetics can be easily achieved based on a high ratio of capacitive-controlled process, particularly at high rates, significantly improving the rate capability of EAA anode for LIBs.

Figure 4. Kinetics analysis of Li+ ions storage behaviour in EAA anode. (a) CV curves of EAA anode at a scan rate range of 0.1-1.0 mV s-1. (b) Relation between Log(peak current, mA) and Log(scan rate. mV s-1) curve for the determination of b value based on the slop of the fitted linear curve. (c) Normalized contribution ratios of diffusion and capacitive storage mechanisms in EAA anode at various scan rates from 0.1 to 1.0 mV s-1. d) CV curve at a scan rate of 1.0 mV s-1 showing the ratio between surface-driven capacitive contribution and diffusion-controlled contribution.

Besides the significantly enhanced surface capacitive storage, the partly reduced SiO2 inside the crystals must also play key roles in the realized Li ion storage. The presence of reduced SiO2 in the

EAA material is crucial for the increased chemical reactivity of silica with Li+ ions and the enhanced ions diffusion and transport behaviours for batteries. To reveal more details on the effects of the

presence of the reduced Si-O bonds, theoretical density functional theory (DFT) calculations and molecular dynamics (MD) simulations were conducted to explore the possible reduction sites, understand the electronic changes after reduction, and identify the lithium ions diffusion trajectories. As shown in Figure 2a, in an ideal albite crystal structure, both Al and Si (Na and Ca) atoms partially occupy the same lattice sites (Table S1). To build a practical albite crystal structure model for DFT calculation, Monte Carlo sampling electrostatic potential calculations were first utilized to identify the top 12 low-energy atom distribution sites via the Supercell Program, which were further relaxed by DFT calculations to obtain the albite structure with the lowest bonding energy (Figure S3). Similarly, the reduced albite crystal structure with the lowest energy was confirmed from these top 20 low-energy structures (Figure S4), as illustrated in Figure 5a. By systematically analysing the bond lengths between the oxygen atoms and their nearest surrounding cations (Na+, Al3+, Si4+, and Ca2+) in all 20 low-energy sites (Table S2), it can be concluded that the O-

13 site (Figure S5) with a lowest vacancy formation energy tends to be preferentially lost, and this is accompanied by the reduction of Si-O bonds. Figure 5b and c give a direct comparison on the density of states (DOS) and the projected DOS (PDOS) in the eigenstate and reduced state of albite cells, in which an obvious electrons localization below Fermi level in the reduced albite structure was observed. Further integrated analysis with the Bader Charges (Figure S6) of each surrounding cation, it can be inferred that this distinct localized electron phenomenon is mainly caused by the reduction of the Si-18 site (Figure S7). Meanwhile, the specific comparison on the PDOS of other local Si, Al, Ca,

Na can O atoms involved in the reductive reaction of Si-O bonds (Figure S8-12) verifies that the Si absorbs almost the total charges that are transferred from the anions, which is also distinguished

from the visualized charge density distributions in the eigenstate and reduced state of the albite cells (Figure S13).

Figure 5. (a) DFT relaxation energies of the top 20 low-energy states in the reduced albite structure. Inset shows the lowest bonding energy state among these low-energy structures. (b) The DOS and PDOS comparison in eigenstate and reduced state of albite cells. (c) The PDOS comparison of Si in eigenstate and reduced state of albite cells. (d) Average and single MSD curves of the interstitial lithium ions in eigenstate and reduced state of albite structure. (e-h) Crystal structures with the interstitial lithium sites labelled in eigenstate and reduced state of albite structure, and their corresponding lithium ions diffusion trajectories in both states.

To investigate the transport of lithium within the reduced albite, MD simulations were applied to track the changes of the lithium ions diffusion trajectories after the reduction of Si-O bonds. As displayed in the mean-square displacements (MSD) curves of the average and the interstitial lithium ions in Figure 5d, the slope of the average MSD plot for the reduced albite structure is much larger as compared to the original eigenstate, which fully demonstrates a higher migration rate for these interstitial lithium ions within the reduced albite cells. It should be further noteworthy that the Li-3 and Li-8 sites (Figure S14) manifest obvious migration behaviours compared to other interstitial lithium sites. Moreover, the corresponding slops of the average MSD plots for other atoms (Al, Si,

Ca, Na, and O) were close to zero (Figure S15 and S16), suggesting slight migration distance and good structural integration for albite cells in both the eigenstate and the reduced state. Via more detailed investigations on each interstitial lithium diffusion trajectories before and after reduction, it is clearly detected that more ions pathways are created with the presence of reduced Si-O bonds.

Compared to the ion diffusion trajectories in the eigenstate (Figure 5e and g), the interstitial lithium ions, especially the Li-3 and Li-8 sites (labelled in Figure 5e and f), are further activated by the loss of oxygen atoms in the reduced state (Figure 5f and h), and these lithium ions will undergo additional diffusion pathways (Figure S17). Therefore, with the presence of the reduced Si-O bonds in the EAA material, the electrochemical lithium storage can be significantly enhanced. These results are in well consistence with the experimental results.

Figure 6. (a) Nyquist plots of albite and EAA-based cells after cycles. (b) XRD analysis of EAA anodes at different discharging or charging states. (c-f) SEM analysis of EAA anodes at different discharging or charging states. (g) Schematic illustration of the stress release states with a carbon coating layer onto albite particles. (h) TEM images of EAA particles after cycles.

In addition, charge-transfer resistance (Rct) and structural stability of EAA was evaluated by using electrochemical impedance spectroscopy (EIS) analysis and examining morphology changes after lithiation/delithiation cycling.[30] Figure 6a describes the Nyquist plots of the EAA-based cells, where

Rct is directly related to the semicircle at the high-frequency regions. It is clearly observed that the

diameter of the semicircle for the EAA-based cell is much smaller than that for the pristine albite- based cell, indicating a lower Rct and a faster reaction efficiency of the reduced and coated albite electrode. Based on the equivalent circuit analysis (inset of Figure 6a), the charge transfer resistance is 199.6 Ω and 63.8 Ω, respectively, for albite and EAA-based cells. This lower resistance of EAA- based cell is primarily due to the presence of conductive carbon coating on the surfaces of albite particles. Moreover, the major crystal structures and surface morphologies of the EAA anode possesses no obvious changes via ex situ XRD (Figure 6b) and SEM (Figure 6c-f) analysis at different states (e.g. 1.5 V, 0.6 V, 0.0 V, and 3.0 V) in a discharging or charging cycle, indicating that the structures of EAA particles with carbon coating layers are stable and the Li+ ions storage mechanisms mainly depend on the surface capacitance. Besides, it is expected that this core-shell structure can well relieve the stress from the volume variations of albite particles upon discharging and charging, as illustrated in Figure 6g, which was evidenced by TEM analysis after cycles (Figure 6h), which showed limited volume changes of the outmost coated carbon layers on the albite particles. The superior Li+ ions storage behaviour of the EAA can be contributed by the key factors as follows: (i) the reduced resistance and the promoted Li+ ions and/or electron transfer after reduction and carbon coating, (ii) the restriction of undesired volume expansion or shrinkage of albite particles by the outer carbon coatings, (iii) the increased active sites for anchoring more ions contributed by silica reduction, and (iv) the satisfying electrode stability upon cycling.

3. Conclusion

In summary, electrochemically inert natural silicon-rich albite mineral was first explored as a potential anode material for electrochemical Li+ ions storages through a facile thermal reduction

activation process. This work offers a new pathway to utilize the low-cost natural minerals for addressing the current cost issues in rechargeable batteries. We revealed that the partial silica reduction and surface carbon coating of the albite anode can achieve fast Li+ ions kinetics and confined volume variations during charging/discharging. Moreover, the layered structure of albite particles can further provide surface-driven capacitive lithium storage (~86% at 1.0 mV s-1), which offers the materials superior storage capacity and cycling stability particularly at high rates. It is anticipated that the electrochemical activation strategy achieved on low-cost and inert albite mineral in this work give us insights into improving ions reactivity of inorganic natural minerals and reaching low-cost high-performance rechargeable batteries by directly utilizing abundant natural minerals.

Experimental Section

Fabrication of Electrochemically Activated Albite (EAA): Pristine albite mineral powder was purchased from National Institute of Standards and Technology (NIST, USA). White sugar (cane sugar, 100%) was purchased from a local supermarket in Australia. All other chemicals were purchased from Sigma-Aldrich and used without any further purification. In a typical synthesis, albite mineral was dispersion into sugar water with a weight ratio of 1:1 between albite and sugar. After completely dried at 90 oC under air, the mixture was transferred into a tube furnace, heated into 700 o o -1 C with a heating rate of 5 C min and maintained for 2 h under continuous Ar/H2 (95:5) flow to produce final EAA product.

Material Characterizations: Field-emission scanning electron microscope (FE-SEM, Zeiss, Germany) equipped with an energy dispersive spectroscopic detector (EDS, Oxford XMax, UK) was used to examine surface morphology and element mapping patterns of albite and EAA. X-ray diffractometer (XRD) patterns was collected on a Panalytical X'Pert MPD, diffractometer (Netherlands). Ex situ XRD

tests were carried out on a Rigaku SmartLab machine with copper radiation (λ=1.54 Å). Transmission electron microscope (TEM, JEOL 2100F, Japan) was applied to present more details on structure and composition of albite and EAA. Thermal gravimetric analysis (TGA) was conducted on a Netzsch thermal analyser (STA 449 F3, Germany) to analyse the specific carbon percentage in EAA. Chemical compositions of albite and EAA and valence states of Si, Al and Na elements were confirmed by X-ray photoelectron spectroscopy (XPS, Kratos, UK). Raman spectra of albite and EAA were recorded on an inViaTM Raman microscope (Renishaw, UK) using a laser source of 532 nm and a laser power of 0.5 %. Optical photograph of EAA was captured on a Leica DM6000 microscope (Germany). Fourier transform infra-red spectra (FT-IR) spectra were obtained on a Bruker spectrometer (ALPHA, Germany). Surface area and pore size distribution of albite and EAA were examined by a TriStar II 3020 surface area analyser (Micromeritics, USA) at 77 K. The samples were degassed at 150 oC under vacuum for 24 h before tests, and the specific surface area and pore size distribution were calculated based on Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) models, respectively.

Electrochemical measurements: CR 2032-coin cells were assembled in Ar-filled glove box. Lithium was utilized as the counter and reference electrode and 1 M LiPF6 solution (EC/DEC, 1:1, v/v) was chosen as the electrolyte. CV curves were collected on a CHI 760 electrochemical workstation (CH Instruments, USA) in the potential range of 0-3.0 V (versus Li/Li+). Galvanostatic charge-discharge profiles were obtained on the high-performance Neware battery tester systems. Electrochemical impedance spectra (EIS) were obtained on a Biologic workstation with a frequency range between 100 kHz and 5 mHz.

Theoretical calculation methods: The ab initio theoretical computations were applied within the formalism of spin-polarization density functional theory (DFT) and the generalized gradient approximation (GGA) of the exchange-correlation function. The projector augmented wave (PAW) potential was used to treat the valence electron-ion interaction in the Vienna Ab initio Simulation Package (VASP). The wave functions were expanded based on plane-wave and the energy cutoff was set up to 500 eV. Brillouin-zone integrations were applied through using the K-point sampling of the Monkhorst-Pack scheme with a 3×2×2 grid. The energy convergence and structural relaxation were kept less than 1.0×10-6 eV and 0.01 eV/Å, respectively. The partial occupation of Na(Ca) and Al(Si) in the albite lattice was solved by using the Supercell Program through Monte Carlo sampling

electrostatic potential calculations. The trajectories of lithium ions diffusion pathways are obtained by the Molecular Dynamics (MD) method. For ab initio MD procession, considering both the calculation time and calculation precision, the kinetic energy cutoff was set as 350 eV. The K-point sampling of the Monkhorst-Pack scheme was set as 1×1×1 grid. 25ps 0.3 fs/step NVT. The temperature was set as 500 K.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

J. Mei and T. Wang contributed equally to this work. This work was supported by Australian Research Council (ARC) Future Fellowship projects (FT180100387 and FT160100281) and an ARC Discovery Projects (DP160102627 and DP200103568). The authors acknowledge the support from the Central Analytical Research Facility (CARF) of QUT, which is hosted by generous funding from the Science and Engineering Faculty (QUT). The authors also acknowledge the generous grants of CPU time from the Australian National Computational Infrastructure Facility and the high-performance computing centre at the Queensland University of Technology.

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Electrochemically inert natural silicon-rich albite mineral was first explored as a potential anode material for electrochemical lithium ions storages through a facile thermal reduction activation process. This nature-derived low-cost material delivered superior storage capacity and cycling stability particularly at high rates when utilized for rechargeable batteries.

Keyword: Albite mineral

Jun Mei, Tiantian Wang, Hong Peng, Godwin A. Ayoko, Jianjun Liu, Ting Liao*, Ziqi Sun*

Title: First Exploration on Electrochemical Activation of Low-cost Albite Mineral for Boosting Lithium Storage Capability