Exploring the electrochemical activity of bismuth

Exploring the electrochemical activity of bismuth oxychloridefor rechargeable aqueous aluminium–metal battery and a methodfor enhanced performance

SUNNY NANDI and SHYAMAL KUMAR DAS*

Department of Physics, Tezpur University, Assam 784028, India

*Author for correspondence ([email protected])

MS received 5 April 2021; accepted 3 July 2021

Abstract. This report illustrates an investigation on the electrochemical stability of bismuth oxychloride (BiOCl) for

rechargeable aluminium–metal battery for the first time. It is shown that Al3? ion could be stored in bismuth oxychloride

in aqueous electrolyte. A stable discharge capacity of 145 mAh g-1 was achieved over 200 cycles at a current rate of

2.5 A g-1. It is found that capacity decline was a major concern. However, a simple and innovative strategy was adopted

to dramatically improve the electrochemical stability by synthesizing a binder-free BiOCl electrode on a thermally

exfoliated graphite current collector. The use of such current collector is also shown for the first time. The facile method

of processing the exfoliated graphite is also discussed.

Keywords. Aluminium–metal battery; bismuth oxyhalides; bismuth oxychloride; bismuth oxyiodide; exfoliated

graphite.

1. Introduction

The possibility of direct use of aluminium (Al) metal as

anode has uniquely placed rechargeable aluminium (Al)-

metal batteries in the league of emerging energy storage

devices [1–9]. There are manifold advantages of utilizing Al

resources in Al-ion or metal batteries. The exceptionally

low cost of Al metal and Al salts, and the ease in their

handling and processing are a few prime reasons for the

exploration of Al-metal batteries. Another important reason

for Al being a favorite for electrochemists is due to an

electrochemical redox reaction, which can release three

electrons per Al3? cation. These three electrons transfer

could essentially boost the power density of an Al-metal

battery. The research works on Al-metal batteries with

various types of graphene cathodes are a testimony of it

[3–8]. For example, Chen et al [3] reported an Al-graphene

cell, which shows a power density of 175 kW kg-1 and an

energy density of 66 Wh kg-1. There are also enough

examples of other non-graphitic cathodes for Al-metal

battery [10–16]. These works mostly took the advantage of

Al stripping and plating phenomenon in chloroaluminate-

based ionic liquid electrolytes in order to achieve a

rechargeable system [3–16]. However, reaping the benefits

of Al metal would be a daunting task considering the

incompatibility of chloroaluminate electrolytes in terms of

cost and need for inert and corrosion-free environment. As a

substitute, Al3? ion conducting aqueous electrolytes may

help in mitigating these challenges. The rechargeability of

Al-metal cells in aqueous electrolytes has recently been

demonstrated with MnO2, AlxMnO2�nH2O and graphite

[17–20].

As a semiconducting layered material, bismuth oxy-

chloride (BiOCl) has been investigated for varieties of

photocatalytic and energy storage applications in recent

times [21]. BiOCl could be visualized as a sandwich

structure where interlacing [Bi2O2] layers are placed

between two chloride layers [21]. There exists weak van der

Waals interaction between the consecutive interlayers,

which can facilitate easy transport of charged species in two

dimensions. Ye et al [22] investigated the suitability of

BiOCl nanosheets as anode in non-aqueous lithium-ion

battery and it was found that it can show discharge capacity

of 254 mAh g-1 over 15 cycles at a current rate of

500 mA g-1. Similarly, Zhang et al [23] demonstrated the

Na? ion storage capability of ultra-thin BiOCl nanoplates in

non-aqueous electrolyte with an initial discharge capacity of

1050 mAh g-1 at a current rate of 10 mA g-1. Li et al [24]

also reported the K? ion storage by BiOCl, where it was

shown that discharge capacity of *200 mAh g-1 could be

achieved over 50 cycles at current density of 50 mA g-1 in

non-aqueous electrolyte. However, the poor cycling

Supplementary Information: The online version contains supplementary material at https://doi.org/10.1007/s12034-021-02530-2.

Bull. Mater. Sci. (2021) 44:234 � Indian Academy of Scienceshttps://doi.org/10.1007/s12034-021-02530-2Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

stability of BiOCl, as was observed in all these studies, due

to pulverization is a matter of concern. These studies cer-

tainly put forward a question on the viability of BiOCl as an

electrode material for rechargeable Al-batteries. With this

motivation, we embarked on to investigate the Al3? ion

electrochemistry of BiOCl in aqueous electrolyte. Herein,

we show that BiOCl can work as a cathode in rechargeable

aqueous Al-metal battery. The cell shows a stable discharge

capacity of 145 mAh g-1 over 200 cycles at a current rate of

2.5 A g-1 with a clear flat discharge potential plateau at 0.6

V and polarization as low as 0.17 V. The issue of severe

capacity decline of BiOCl is also tackled with an innovative

and simple strategy of preparing a binder-free BiOCl elec-

trode in thermally exfoliated graphite current collector. We

would like to mention here that our previous work exclu-

sively reported the electrochemistry of graphite as a cathode

rather than a current collector in Al-metal battery [20]. It

was found that Al3? ion could be reversibly intercalated in

graphite in aqueous electrolyte. The difference here is that

graphite is being used as a current collector. It is to be noted

that the voltage range used in the present case is 0–1.2 V

(vs. Al), whereas it was 0–2 V (vs. Al) in the previous case.

It was noticed that graphite shows considerable Al3? ion

storage only above 1.5 V (vs. Al) [20]. Therefore, it is

assumed to be safe to use graphite as current collector while

evaluating the electrochemical activity of BiOCl. To the

best of our knowledge, this study is the first to offer evi-

dence of stable electrochemical behavior of BiOCl in

rechargeable Al-metal battery configuration.

2. Experimental

One of the objectives of this study is to improve the elec-

trochemical stability of BiOCl. For fulfilling this purpose,

optimization of the graphite current collector has been

performed. Three different types of graphite films were used

as current collector in this study. These are labelled as

pristine graphite film, exfoliated graphite film and thermally

exfoliated graphite film. Pristine graphite film of thickness

0.5 mm was used to obtain the exfoliated and thermally

exfoliated graphite films. The exfoliation process of the

pristine graphite film was adopted from our previous work

[25]. Briefly, to mention, a dc potential of 3 V was applied

for 5 min between two pieces of pristine graphite film in

1 M AlCl3 aqueous electrolyte, which resulted in the

exfoliation of the pristine graphite film. This exfoliated

graphite film was further heat treated at 800�C for less than

a 1 min in air in order to obtain the thermally exfoliated

graphite film.

BiOCl was synthesized by a procedure as reported in

reference [26]. In a typical synthesis, 0.97 g of Bi(NO3)3�5H2O and 0.268 g of NH4Cl was dissolved in 50 ml of

ethylene glycol. The mixture was poured in a Teflon

autoclave and heated at 180�C for 20 h. The white product

was recovered by centrifugation after washing with

water/ethanol and dried at 100�C for 12 h. In order to obtain

a binder-free BiOCl electrode, few pieces of thermally

exfoliated graphite films were immersed in the above

mixture and performed the heat treatment as described. The

binder-free electrodes were collected after the heat treat-

ment and flushed with water/ethanol and dried at 100�C for

12 h.

The crystallographic phase identification was performed

by using powder X-ray diffraction (XRD, Bruker Axs D8

Focus; Cu-Ka radiation, k = 1.5406 A). The morphology

was observed by scanning electron microscopy (SEM,

JEOL JSM 6390LV and Zeiss, Sigma). X-ray photoelectron

spectroscopy (XPS) was recorded by Physical Electronics

(PHI 5000 Versa Probe III). Electrode slurry was made

from BiOCl, PVDF and N-methyl-2-pyrrolidone. The

weight ratio of BiOCl and polyvinylidene fluoride is 80:20.

This slurry was cast on pristine graphite, exfoliated graphite

and thermally exfoliated graphite foils, and all of them were

dried at 110�C for 12 h. Hence, three different types of

electrodes were obtained. These were BiOCl coated on

pristine graphite, exfoliated graphite and thermally exfoli-

ated graphite current collectors, which are respectively

called as BiOCl/G, BiOCl/EG and BiOCl/TEG hereafter.

The binder-free BiOCl electrode is abbreviated as BiOCl/

TEG (binder free).

Cyclic voltammetry (CV) and galvanostatic discharge/

charge experiments were performed in a conventional three-

electrode electrochemical glass cell. Pt electrode and

aqueous Ag/AgCl electrode were used as the counter and

reference electrodes, respectively, while BiOCl/G, BiOCl/

EG and BiOCl/TEG act as the working electrodes. The

discharge/charge and CV experiments were performed in

the voltage range of –0.8 to 0.3 V and -1 to 0.3 V,

respectively. Electrochemical impedance spectra were

obtained by scanning in the frequency range of 1 MHz–200

kHz at signal amplitude of 10 mV.

An aqueous Al-metal full cell was tested with Al metal

as anode and BiOCl as cathode. Typically used Al foil

(Hindalco Freshwrap) for household purposes was utilized

for this purpose. The utilized electrolyte was 1 M AlCl3�6H2O aqueous solution. The galvanostatic discharge/

charge and CV experiments were performed in the voltage

range of 0–1.2 V, considering BiOCl/G, BiOCl/EG and

BiOCl/TEG as the working electrodes. Al was used as

both the reference and counter electrodes. For ex-situXRD, SEM and XPS experiments, the electrodes were

harvested after required number of discharge/charge cycles

and dried at 110�C for 24 h. All the electrochemical

experiments were conducted at room temperature (25�C)

and ambient atmosphere.

Bismuth oxyiodide (BiOI) was prepared by direct thermal

treatment of bismuth (III) iodide (BiI3) as reported by Chen

et al [27]. Briefly, 1 g of BiI3 was put in a crucible container

and heated at 300�C for 8 h. The red product was collected

by washing with ethanol and distilled water for a few times,

and dried at 60�C for 12 h.

234 Page 2 of 9 Bull. Mater. Sci. (2021) 44:234

3. Results and discussion

Figure 1a–c shows the SEM images of pristine graphite,

exfoliated graphite and thermally exfoliated graphite films.

It is evident that the surface of the electrochemically

exfoliated graphite and thermally exfoliated graphite are

covered with nanoflakes (figure 1b and c). The digital

photographs of these graphite films are shown in figure 1d–

f. It is observed that the thicknesses of the exfoliated gra-

phite and the thermally exfoliated graphite are quite larger

than the pristine graphite film (0.5 mm). The respective

thickness values are 0.78 and 2.02 mm. It signifies that

thermally exfoliated graphite is more porous in nature,

which is quite beneficial to enhance the electrode and

electrolyte contact area. The XRD patterns (supplementary

figure S1) indicate the graphitic structure of all these gra-

phite films.

The XRD pattern (figure 2a) shows that the synthesized

material is crystallized in tetragonal crystal phase of BiOCl

(JCPDS 06-0249). The binder-free BiOCl electrode also

possesses the similar crystal structure. FESEM investigation

(figure 2b and c) shows spherical clusters of BiOCl, which

is composed of nanoflakes.

In order to identify the Al3? ion electrochemical activity

in BiOCl, CV and galvanostatic charge–discharge experi-

ments were initially performed on BiOCl/G and binder-free

BiOCl (i.e., BiOCl/TEG (binder free)) electrodes in 1 M

AlCl3 aqueous electrolyte in a conventional three-electrode

electrochemical cell. Figure 3a shows the CV profile of

BiOCl/G at a scan rate of 2.5 mV s-1. It shows two

prominent cathodic redox peaks, designated as A and B in

the figure, at -0.35 and -0.72 V, respectively, and one

anodic peak at 0.05 V (peak C) in the first cycle. The cur-

rent response for peak B severely decreases with increasing

cycle number. A decrease in current response for peaks A

and C is also noticed. Similarly, as shown in figure 3b,

redox peaks A and C could also be profoundly noticed in

case of BiOCl/TEG (binder free). However, peak B could

be marginally observed and the current response is not

severely decreasing unlike BiOCl/G electrode. Galvanos-

tatic charge–discharge experiments also corroborate the CV

results. Figure 3c and d shows the discharge/charge profiles

of BiOCl/G and BiOCl/TEG (binder free), respectively, in 1

M AlCl3 aqueous electrolyte in the voltage range of –0.8 to

0.3 V (vs. Ag/AgCl) at a current rate of 2.5 A g-1. In both

cases, flat charge–discharge profiles could be observed.

Figure 1. SEM and digital images of (a, d) pristine graphite, (b, e) exfoliated graphite and (c, f) thermally exfoliated graphite films.

Bull. Mater. Sci. (2021) 44:234 Page 3 of 9 234

The discharge and charge plateaus are located at -0.22 and

-0.11 V, respectively. However, noticeable difference in

the storage capacities could be observed. As seen from the

variation of capacity with cycle number (supplementary

figure S2), the binder-free electrode outperforms the BiOCl/

G electrode. The first cycle discharge and charge capacities

are 805 and 490 mAh g-1, respectively, for the BiOCl/TEG

(binder free) electrode, whereas these values are only 250

and 195 mAh g-1 for BiOCl/G. The 80th cycle discharge

capacities for BiOCl/TEG (binder free) and BiOCl/G are

316 and 3.5 mAh g-1, respectively. Another important

feature is that almost 77% of discharge capacity is seen in

the plateau region in the first discharge cycle for BiOCl/

TEG (binder free), whereas only 57% could be seen for

BiOCl/G. It is to be noted that a flat discharge plateau is

very much important for maintaining a stable energy den-

sity for batteries. A comparison of the CV profiles (sup-

plementary figure S3) indicates that neither the pristine

graphite film nor the thermally exfoliated graphite film

exhibits any electrochemical activity in the measured

potential range. CV experiments were also performed with

0.5 M Al2(SO4)3 and 1 M Al(NO3)3 aqueous electrolytes

with BiOCl/TEG (binder free) electrode. The CV profiles

(supplementary figure S4) are almost identical for AlCl3 and

Al2(SO4)3 aqueous electrolytes. However, the CV profile

(supplementary figure S4b) obtained in Al(NO3)3 aqueous

electrolyte shows sharp decrease in current in the negative

potential with difficulty to observe the cathodic peak.

Similar trend was also noticed for previously reported other

electrode materials [28–31]. Again, galvanostatic charge–

discharge experiment (supplementary figure S5) performed

with BiOCl/TEG (binder free) in Al2(SO4)3 aqueous elec-

trolyte at a current rate of 2.5 A g-1 shows very low specific

capacities in comparison to AlCl3 aqueous electrolyte.

To verify the effect of other cations, CV experiments

were performed with 1 M LiCl, 1 M NaCl and 1 M MgCl2aqueous electrolytes at scan rate of 2.5 mV s-1. The CV

profiles (supplementary figure S6) obtained in LiCl and

NaCl aqueous electrolytes are almost similar. One pair of

prominent cathodic and anodic redox peaks could be

observed at -0.83 and 0.06 V, respectively. On the other

hand, two major anodic peaks at -0.29 and -0.1 V, and

Figure 2. (a) XRD patterns of BiOCl and BiOCl/TEG (binder free) electrodes. SEM images of (b) BiOCl/G and

(c) BiOCl/TEG (binder free) electrodes.


one cathodic peak at -0.9 V could be seen in case of MgCl2aqueous electrolyte. However, a comparison of the CV

profiles indicates that the potential positions of the cathodic

peaks observed with LiCl, NaCl and MgCl2 electrolytes do

not match with the potential observed for AlCl3 electrolyte

(supplementary figure S7). It thus suggests the electroac-

tivity of Al3? ion in BiOCl.

Armed with this knowledge of Al3? ion electrochemistry

of BiOCl, further electrochemical investigation of BiOCl

was made in a full-cell configuration by pairing Al metal as

anode in 1 M AlCl3 aqueous electrolyte. Our previous

investigation indicates that aqueous electrolyte of AlCl3 is

beneficial for better functioning of Al-metal batteries [20].

It is because AlCl3 aqueous electrolyte can effectively

remove the inherently present passivating oxide layer from

the Al metal surface, which is otherwise difficult in case of

Al2(SO4)3 and 1 M Al(NO3)3 aqueous electrolytes. Four

full-cells were assembled with BiOCl/G, BiOCl/EG, BiOCl/

TEG and BiOCl/TEG (binder free) electrodes. Figure 4a–d

shows the respective galvanostatic charge/discharge profiles

at a current rate of 1 A g-1. It could be observed that all the

cells exhibit good reversibility with varying degree of

specific capacities. The highest specific capacities are

shown by BiOCl/TEG (binder free) with first cycle dis-

charge and charge capacity values of 687 and

514 mAh g-1, respectively (figure 4d). On the other hand,

these values are only 119 and 90 mAh g-1 for BiOCl/G

(figure 4a), 229 and 200 mAh g-1 for BiOCl/EG (figure 4b),

235 and 172 mAh g-1 for BiOCl/TEG (figure 4c). The

electrochemical cycling stability was investigated at a high

current rate of 2.5 A g-1. As shown in figure 4e, Al-BiOCl/

TEG (binder free) cell showed the best performance.

Although there is an initial decline of specific capacities,

these values got stabilized after 50th cycle with discharge

capacity value of 145 mAh g-1 at the 200th cycle. Con-

trarily, rest of the Al cells could not sustain the current rate

of 2.5 A g-1. Al-BiOCl/TEG cell could show specific

capacities but these values are inferior in comparison to Al-

BiOCl/TEG (binder free). The discharge capacity values are

only 36 and 12 mAh g-1 at the 50th and 100th cycle,

respectively. The high degree of reversibility is also

reflected from the CV profiles (figure 4f). A pair of highly

reversibly redox peaks could be seen at 0.4 and 1 V. It is

noted here that the contribution of the graphite/exfoliated

graphite/thermally exfoliated graphite current collector in

estimating the specific capacities of BiOCl is very

Figure 3. CV profiles of (a) BiOCl/G and (b) BiOCl/TEG (binder free) electrodes at a scan rate of 2.5 mV s-1.

Galvanostatic discharge/charge profiles of (c) BiOCl/G and (d) BiOCl/TEG (binder free) electrodes at a specific

current rate of 2.5 A g-1 in 1 M AlCl3 aqueous electrolyte.


negligible. It should not be confused with our previous work

since the voltage range and current rate are completely

different in the present work [20]. The inactivity of these

current collectors is well evidenced from the charge–dis-

charge experiments performed with the pure current col-

lectors (i.e., without any BiOCl) at the same current rate

(supplementary figure S8). The inactivity was also evi-

denced from the CV profiles as discussed earlier (supple-

mentary figure S3). Another interesting feature of the

charge–discharge profiles of the Al-BiOCl cells is the

existence of flat discharge and charge plateaus in all cells.

The discharge and charge potential plateaus are,

Figure 4. Galvanostatic discharge/charge profiles of (a) BiOCl/G, (b) BiOCl/EG, (c) BiOCl/TEG, (d) BiOCl/TEG

(binder free) electrodes at a specific current rate of 1 A g-1. (e) Variation of discharge capacities with cycle number at

a current rate of 2.5 A g-1. (f) CV profile of Al-BiOCl/TEG (binder free) electrode at a scan rate of 2.5 mV s-1 in 1 M

AlCl3 aqueous electrolyte. The potential is measured with respect to Al electrode.


respectively, observed at 0.6 and 0.8 V (vs. Al) for Al-

BiOCl/TEG (binder free) cell. The polarization (estimated

as the difference between the discharge and charge poten-

tials at half of the capacity values) is found to be lowest

(= 0.17 V) for Al-BiOCl/TEG (binder free) cell. These

values are in the range of 0.3–0.4 V for rest of the Al-BiOCl

cells. An estimation of charge transfer resistance by elec-

trochemical impedance spectroscopy (EIS) shows the low-

est resistance for Al-BiOCl/TEG (binder free) cell. The EIS

spectra are shown in figure 5a. The decreasing order of the

charge transfer resistance is as follows: BiOCl/G[BiOCl/

EG[BiOCl/TEG[BiOCl/TEG (binder free). This is very

much consistent with the overall electrochemical stability

data. It is expected that the direct growth of BiOCl on the

thermally exfoliated graphite could provide better electrical

contact and easy pathways for Al3? ion transport.

Structural evolution of the BiOCl electrodes before and

after discharge was investigated by ex-situ XRD analysis.

Figure 5b (and supplementary figure S9) shows the ex-situXRD patterns of the electrodes. Titanium (Ti) was used as

the current collector for performing this measurement so

that the intense (002) peak from graphite current collector

could be avoided. It is noted here that the BiOCl peaks are

not so clearly visible due to high intensity (002) diffraction

peak from the graphite substrate (figure 2a). The analysis of

the patterns was difficult due to the superimposition of large

number of diffraction peaks of BiOCl, Bi2O3, Bi, Al2O3 and

Ti. However, few of the peaks 2h = 13.74�, 16.45�, 18.15�,41.67� and 57.27o seen after discharge could be identified

from the Al2O3 phase (JCPDS nos. 16-0394 and 23-1009).

The Bi peaks (2h = 37.98� and 39.68�) are superimposed

with Ti phase. The absence of major Bi2O3 peaks suggest it

could be in amorphous or nanocrystalline phase. Ex-situSEM images of the discharge state electrode show the

morphological change of BiOCl after discharge (figure 5c).

As shown in figure 5c, tiny flakes could be seen in com-

parison to spherical clusters as was observed for pristine

BiOCl. During the charged state, the intensity of the

prominent peaks from Al2O3 phase diminishes. Ex-situ XPS

investigation was carried out to further understand the

electrochemical process. As show in figure 5d, the Bi 4f

peaks could be located at 160.15 and 165.43 eV after dis-

charge. Whereas these peaks are located at 158.02 and

163.27 eV in case of pristine BiOCl. This shift may be

attributed to the interaction of Al3? ions with BiOCl during

discharge similar to Li?, Na? and K? ions [22–24]. During

charge, the Bi 4f peaks again shift to lower binding ener-

gies, although these could not be seen overlapping with the

Figure 5. (a) Electrochemical impedance spectroscopy for BiOCl/G, BiOCl/EG, BiOCl/TEG and BiOCl/TEG (binder free)

electrodes. (b) Ex-situ XRD patterns before discharge, after 5th discharge and charge. (c) SEM image of BiOCl/TEG (binder free)

electrode after discharge, and ex-situ XPS spectra for (d) Bi 4f, (e) O 1s, (f) Al 2p of BiOCl/TEG (binder free) electrode before

discharge, after 5th discharge and charge, respectively.


pristine BiOCl spectrum. The XPS survey spectrum of

BiOCl/TEG (binder free) is also shown in supplementary

figure S10. There is also a change in the O 1s spectrum

(figure 5e). The peaks shift to higher binding energies

during discharge. The new peaks are at 530.8 and 533.3 eV.

These binding energies correspond to Al2O3, which again

signifies the possibility of formation of Al-O bond [32,33].

Similarly, there is a pronounced shift in the Cl 2p peaks

(supplementary figure S11). Al 2p peak at 75.6 eV could

also be observed for the discharged state BiOCl, which also

confirms the presence of Al [32]. Therefore, based on these

observations, it could be commented that there is a con-

version type of electrochemical reaction that undergoes

while Al3? ion electrochemically react with BiOCl. Similar

type of conversion reaction was also observed for Li?, Na?

and K? ions [22–24]. The following two types of reversible

electrochemical reactions are possible during the discharge

process at the BiOCl electrode:

(i) 3BiOCl ? Al3? ? 3e- ? Bi2O3 ? Bi ? AlCl3(ii) Bi2O3 ? 2Al3? ? 6e- ? 2Bi ? Al2O3.

It is noted here that the potential plateaus of BiOCl is

almost identical to Bi2O3, which signifies similar kind of

storage mechanism [25]. Complete reversible conversion of

Al2O3 may not be possible as indicated by the existence of

Al 2p during charging, which was observed at slightly lower

binding energy (74.6 eV). As evidenced in our previous

work, it is found that there is no occurrence of plating and

stripping of Al in AlCl3 aqueous electrolyte unlike

chloroaluminate electrolytes [20]. However, there is con-

tinuous corrosion of Al electrode during repeated cycling.

Based on it we proposed the following irreversible elec-

trochemical reaction at the Al anode: Al ? Al3? ? 3e-

[20]. Therefore, we comment that Al-BiOCl cell is a

rechargeable system but the longevity of the cell depends on

the availability of Al anode in the system. However, as

proposed in earlier work, this system could also be

mechanically recharged by replacing the corroded Al

electrode with a fresh piece of Al. CV experiments were

also performed at different scan rates (supplementary

figure S12) to determine any relationship between the peak

current responses (I) for both the cathodic and anodic redox

peaks and the scan rates (c), according to the equation

I = kc0.5 (k is a constant). However, it does not follow any

linear relationship ruling out the possibility of diffusion-

controlled Al3? ion insertion/extraction process [34,35]. In

addition to BiOCl, other bismuth oxyhalides may also act as

an electrode material for aqueous Al-metal battery. We also

attempted to check the Al3? ion electroactivity of BiOI. The

XRD pattern (supplementary figure S13a) of BiOI matches

well with the tetragonal phase of BiOI (JCPDS no.

10-0445). As shown in supplementary figure S13b, the CV

profiles clearly exhibit redox peaks for BiOI. The charge–

discharge profiles (supplementary figure S13c) are similar

to BiOCl. However, the specific capacity values are lower

in comparison to BiOCl.

4. Conclusions

In summary, the Al3? ion storage behaviour of BiOCl in

aqueous electrolyte is demonstrated. When coupled with an

Al anode, BiOCl delivers a stable specific capacity of 145

mAh g-1 over several cycles. It was also shown that an

optimization of the graphite current collector is necessary

for an improved electrochemical performance of BiOCl. A

binder-free BiOCl electrode on thermally exfoliated gra-

phite current collector could significantly stabilize the

cycling performance of the investigated Al-metal cells. We

also investigated the electrochemical performance of BiOCl

using various aqueous electrolytes and found that AlCl3-

based aqueous electrolyte is more suitable for enhanced

capacity and long-term electrochemical stability.

Acknowledgements

SKD acknowledges the financial support received from the

Science and Engineering Research Board, Department of

Science and Technology, Government of India (Grant No.:

CRG/2018/000263). We acknowledge the facilities avail-

able at SAIC, Tezpur University. FESEM and XPS facilities

at IIT Guwahati and IIT Roorkee are also acknowledged.

We also thank Tezpur University for Innovation and

Research Grant 2020-2021.

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