Towards a calcium-based rechargeable battery
1
A. Ponrouch,
1
C. Frontera,
1
F. Bardé,
2
M.R. Palacín
1
*
2
1
Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) Campus UAB, E-08193
3
Bellaterra, Catalonia (Spain)
4
2
Toyota Motor Europe, Research & Development 3, Advanced Technology 1,Technical Centre,
5
Hoge Wei 33 B, B-1930 Zaventem, (Belgium).
6
Corresponding author: M. R. Palacín (e-mail: rosa.palacin@icmab.es)
7
8
The development of a rechargeable battery technology using light electropositive
9
metal anodes would bring in a breakthrough in energy density.
1
For multivalent charge
10
carriers (M
n+
), the number of ions that must react to achieve a certain electrochemical
11
capacity is diminished by two (n=2) or three (n=3) when compared to Li
+
.
2
While proof-of-
12
concept has been achieved for magnesium,
3
,
4
,
5
the electrodeposition of calcium was thought
13
to be impossible to date
6
and research restricted to non rechargeable systems.
7
,
8
,
9
,
10
Here we
14
demonstrate the feasibility of calcium plating at moderate temperatures using conventional
15
organic electrolytes, such as those used for the Li-ion technology. The reversibility of the
16
process upon cycling has been ascertained and thus the results presented here constitute
17
the first step towards the development of a new rechargeable battery technology using
18
calcium anodes.
19
20
2
Amongst multivalent electropositive metals, an aluminium based cell
11
has been recently
21
reported which, in spite of limited potential (2 V) and capacity (70 mAh/g) values, does exhibit
22
fast rate capability. Calcium is an especially attractive alternative as it is the fifth most abundant
23
element on earth crust and its standard reduction potential is only 170 mV above that of lithium,
24
enabling significantly larger cell potential than that achievable with magnesium or aluminium
25
(Table S1). Moreover, Ca
2+
would hold promise for faster reaction kinetics than Mg
2+
(and thus
26
better power performance) due to its lower polarizing character. Pioneering research work by
27
Aurbach et al.
6
allowed to conclude that the electrochemical behavior of calcium electrodes in
28
conventional organic electrolytes is surface-film controlled, as is the case for lithium
12
but that
29
calcium deposition was virtually impossible, which was attributed to the lack of calcium ion
30
transport through the surface passivation layer formed.
31
In order to develop viable calcium metal anodes, the electrolyte must contain Ca
2+
ions and allow
32
reversible calcium metal plating/stripping (upon reduction/oxidation). Considering an electrode
33
covered with a surface passivation layer, the electrodeposition of a metal M is only enabled (see
34
Figure S1), if all the following requisites are fulfilled: 1) solvated M
x+
ions can diffuse/migrate
35
within the electrolyte, 2) the desolvation energy barrier at the electrolyte/passivation layer
36
interface is low, 3) the desolvated M
x+
ions can migrate through the passivation layer and 4) the
37
energy barrier for nucleation and growth of M at the electrode substrate interface is low. A
38
number of factors can influence the feasibility of one, or most commonly more, of the above
39
processes including the the composition of the electrolyte solvent and salt, its concentration, and
40
temperature and the nature of the substrate. These will determine the tendency of ion pairing (in
41
turn influencing diffusion of M
x+
ions within the electrolyte), the desolvation energy, the
42
3
composition of the passivation layer (and its ionic conductivity) and the nucleation energy
43
barrier.
12,
13
,
14
44
In view of the chemical similarity between calcium and magnesium, a first approach to develop
45
calcium based batteries might have based on a concept analogous to that developed for
46
magnesium, using electrolytes in which no surface layer is developed.
15
Nonetheless, the limited
47
redox stability and intrinsic complexity of the electrolyte formulations used in magnesium
48
batteries prompted us to follow a radically different approach. Considering the ideal properties
49
of any electrolyte in terms of stability, viscosity and ability to dissociate salts, we decided to
50
reinvestigate conventional polar aprotic solvents, such as alkyl carbonates, as potential
51
electrolytes to enable the development of calcium based batteries. These solvents can exhibit
52
high dielectric constants (ɛ) to dissolve salts to sufficient concentration and low viscosity to
53
enhance ionic conductivity and display good thermal (liquidus range) and electrochemical
54
stabilities. Such factors are at the origin of their generalized use in Li-ion cells
16
and their
55
consideration for the emerging Na-ion technology.
17
In both cases, degradation reactions
56
involving electrolyte solvents and salts take place at the interface with the highly reducing
57
negative electrode, which result in insoluble products adhering to its surface and forming a
58
protective solid passivation layer which enables Li
+
and Na
+
migration and is thus usually termed
59
Solid Electrolyte Interphase (SEI).
18
,
19
,
20
The intrinsic properties of solvents and the cumulated
60
know how in the field of Li-ion and Na-ion batteries led us to the selection of a mixture of
61
ethylene carbonate (EC, ɛ=89.78 and known to build very stable passivation layers but melting at
62
36.4 ºC) and propylene carbonate (PC, ɛ=64.92, melting at -48.8ºC) for the electrolyte
63
formulation. This mixture has been shown to exhibit wide liquidus range (ca. -90ºC to 240ºC)
64
and electrochemical operation window, and to dissolve sodium salts yielding high ionic
65
4
conductivities.
21
The feasibility of reversible calcium electrodeposition in such electrolytes
66
containing salts with known stable anions has been assessed through cyclic voltammetry. No
67
redox processes in the potential window investigated (from -1.5 to 2 V vs. Ca
2+
/Ca
passivated
) could
68
be detected at room temperature. Nonetheless, at higher temperatures (50-100ºC) and for
69
electrolytes containing Ca(ClO
4
)
2
and Ca(BF
4
)
2
a redox process is observed, with intensity
70
dependent on salt concentration and increasing with temperature (see Figure 1). No similar
71
process was observed in any of the experiments carried out with Ca(TFSI)
2
.
72
73
74
75
76
77
78
79
80
81
82
Figure 1. Cyclic voltammograms of EC:PC
based electrolytes (0.5 mV/s scan rate) with (a) 0.3 M
83
concentration of different salts at 100ºC, (b) with 0.65 M Ca(BF
4
)
2
at 75ºC or 100ºC and (c) with diverse Ca(BF
4
)
2
84
concentrations from 0.3M to 0.8M at 100ºC. Insets depict an expanded scale for (a) and onset potentials for the
85
redox process observed in (b) and (c).
86
-0.04
-0.02
0
0.02
0.04
-1 0 1 2
Intensity (mA)
Potential (V vs Ca
2+
/Ca
passivated
)
Ca(ClO
4
)
2
Ca(BF
4
)
2
Ca(TFSI)
2
a
-0.005
0
0.005
-1 0 1 2
-0.015
-0.01
-0.005
0
0.005
0.01
-1 0 1
Intensity (mA)
Potential (V vs Ca
2+
/Ca
passivated
)
b
75˚C
100˚C
-0.04
-0.02
0
0.02
0.04
-1 0 1 2
Intensity (mA)
Potential (V vs Ca
2+
/Ca
passivated
)
0.3 M
0.45 M
0.65 M
0.8 M
c
Plating
Stripping
75˚C
-0.85
-0.46
100˚C
-0.65
-0.51
Plating
Stripping
0.3 M
-0.65
-0.52
0.45 M
-0.52
-0.42
0.65 M
-0.65
-0.51
0.8 M
-0.88
-0.48
5
While the reversibility of this redox process is poor for the electrolytes containing Ca(ClO
4
)
2
,
87
(inset in Figure 1a) voltammograms typical of reversible metal plating/stripping are observed in
88
the case of Ca(BF
4
)
2
. The onset potential depends on both the temperature and salt concentration,
89
with the smaller differences between oxidation and reduction (0.10 V) being found for 0.45 M
90
Ca(BF
4
)
2
at 100ºC (Figures 1b and 1c).
91
In order to ascertain whether the reversible redox process observed in electrolytes
92
containing Ca(ClO
4
)
2
and Ca(BF
4
)
2
was due to calcium metal plating/stripping, copper substrate
93
disks were polarized at low potential (between -1 and -1.5V vs. Ca
2+
/Ca
passivated
for 200h) and
94
75ºC and further characterized. After disassembling the electrochemical cells, grey deposits are
95
visible that are thicker for Ca(BF
4
)
2
containing electrolytes. This is in agreement with the much
96
larger current being observed in cyclic voltammograms and also consistent with scanning
97
electron microscopy images (see Figures 2 and S2). The deposit grown using Ca(BF
4
)
2
was
98
dense and thick enough to be scratched from the substrate and sealed inside a borosilicate
99
capillary to perform synchrotron radiation diffraction. The corresponding pattern does exhibit
100
reflexions corresponding to Ca metal and CaF
2
as major phases. Rietveld refinements allow
101
determining that these are present in equimolar ratios with crystallite sizes close to 15nm in both
102
cases (Figure 2c). These results confirm that the deposit contains calcium metal and that the
103
redox process observed does indeed correspond to reversible calcium plating/stripping, which is
104
possible in conventional alkyl carbonate electrolytes under our operation conditions. CaF
2
is
105
certainly derived from electrolyte decomposition and most likely part of the surface passivation
106
layer, in agreement with similar studies conducted on Li-ion batteries reporting presence of LiF
107
in passivation layers formed in LiBF
4
containing electrolytes.
22
As scratching of the deposits did
108
not enable its full quantitative transfer to the capillary (residues always remain which lead to an
109
