344
P.-E. Lippens
of a solid solution (R1) followed by a two-phase reaction (R2). This mechanism is
reversible for desodiation. The voltage curve differs from that of NaTi 2 (PO 4 ) 3 formed
by a plateau at 2.2 V, which was assigned to a two-phase reaction corresponding to
the redox couple Ti
4+ /Ti
3+ [73]. The operando XRD patterns in region R1 are almost
identical to the XRD pattern of the pristine material with R ¯
3c structure while the
sodiation in R2 shows two phases with R ¯
3c and P ¯
1c structures. These results are
consistent with the mechanism suggested by the voltage profile but do not reveal the
redox mechanism.
Operando
57 Fe Mössbauer measurements were carried out with the Cell 2
described in Sect. 7.3.7 and two different protocols. In the first protocol, the spectra
were recorded continuously during the first cycle in galvanostatic regime with a
current of 0.04 Na per fu and per hour and an acquisition time of 2 h per spectrum. In the second protocol, a periodic current was imposed. Each 16-h period
consisted of a constant current of 0.1 Na per fu and per hour during 4 h and a zero
current (open circuit condition) during 12 h for recording the spectrum in order to
improve the signal-to-noise ratio. The spectra show that in R1 the Fe
3+ doublet of
Na 1.5 Fe 0.5 Ti 1.5 (PO 4 ) 3 (δ = 0.41 mm s
−1 , = 0.34 mm s
−1 ) is transformed into a
Fe
2+ doublet (δ ≈ 1.2 mm s
−1 , ≈ 2.0 mm s
−1 ) (Fig. 7.9).
The total area of the spectra does not vary noticeably during the sodiationdesodiation process, indicating that the recoil-free fractions of Fe
2+ and Fe
3+ have
close values. Thus, the relative contributions of the Fe
2+ and Fe
3+ subspectra to the
Mössbauer spectra can be regarded as the relative amounts of Fe
2+ and Fe
3+ ions in
the electrode material. The amount of Fe
3+ linearly decreases in R1 while the amount
of Fe
2+ increases (Fig. 7.10). In R2, the variations are not significant and the amounts
of Fe
3+ and Fe
2+ are almost constant, showing that sodiation does not change the
oxidation state of iron anymore. The variations are reversible for desodiation with
strong changes at the end of the process. It should be noted that the values of the relative contributions of Fe
2+ and Fe
3+ are more dispersed for the galvanostatic protocol,
which confirms the better quality of the data obtained with the other protocol. These
results show that the mechanism of sodiation consists in the formation of a solid
solution coming with the reduction of Fe
3+ into Fe
2+ , followed by a two-phase reaction leading to the reduction of Ti
4+ into Ti
3+ . The mechanism is reversible for
desodiation. The cyclability and rate capability are better for Na 1.5 Fe 0.5 Ti 1.5 (PO 4 ) 3
compared to NaTi 2 (PO 4 ) 3 , which can be attributed to enhanced Na diffusion due to
the single-phase reaction in R1.
The operando Mössbauer spectroscopy provides a quantitative characterization
of the redox reactions but cannot distinguish between solid-solution and two-phase
reactions. This is obtained by operando XRD. Thus, the combined use of these two
techniques successfully explains the two-step mechanism by the reactions
N a 1.5 Fe
3+
0.5 T i
4+
1.5 (P O 4 ) 3
R3c
+ 0.5N a = N a 2 Fe
2+
0.5 T i
4+
1.5 (P O 4 ) 3
R3c
(7.19)
N a 2 Fe
2+
0.5 T i
4+
1.5 (P O 4 ) 3
R3c
+ 1.5N a = N a 3.5 Fe
2+
0.5 T i
3+
1.5 (P O 4 ) 3
P1c
(7.20)
P.-E. Lippens
of a solid solution (R1) followed by a two-phase reaction (R2). This mechanism is
reversible for desodiation. The voltage curve differs from that of NaTi 2 (PO 4 ) 3 formed
by a plateau at 2.2 V, which was assigned to a two-phase reaction corresponding to
the redox couple Ti
4+ /Ti
3+ [73]. The operando XRD patterns in region R1 are almost
identical to the XRD pattern of the pristine material with R ¯
3c structure while the
sodiation in R2 shows two phases with R ¯
3c and P ¯
1c structures. These results are
consistent with the mechanism suggested by the voltage profile but do not reveal the
redox mechanism.
Operando
57 Fe Mössbauer measurements were carried out with the Cell 2
described in Sect. 7.3.7 and two different protocols. In the first protocol, the spectra
were recorded continuously during the first cycle in galvanostatic regime with a
current of 0.04 Na per fu and per hour and an acquisition time of 2 h per spectrum. In the second protocol, a periodic current was imposed. Each 16-h period
consisted of a constant current of 0.1 Na per fu and per hour during 4 h and a zero
current (open circuit condition) during 12 h for recording the spectrum in order to
improve the signal-to-noise ratio. The spectra show that in R1 the Fe
3+ doublet of
Na 1.5 Fe 0.5 Ti 1.5 (PO 4 ) 3 (δ = 0.41 mm s
−1 , = 0.34 mm s
−1 ) is transformed into a
Fe
2+ doublet (δ ≈ 1.2 mm s
−1 , ≈ 2.0 mm s
−1 ) (Fig. 7.9).
The total area of the spectra does not vary noticeably during the sodiationdesodiation process, indicating that the recoil-free fractions of Fe
2+ and Fe
3+ have
close values. Thus, the relative contributions of the Fe
2+ and Fe
3+ subspectra to the
Mössbauer spectra can be regarded as the relative amounts of Fe
2+ and Fe
3+ ions in
the electrode material. The amount of Fe
3+ linearly decreases in R1 while the amount
of Fe
2+ increases (Fig. 7.10). In R2, the variations are not significant and the amounts
of Fe
3+ and Fe
2+ are almost constant, showing that sodiation does not change the
oxidation state of iron anymore. The variations are reversible for desodiation with
strong changes at the end of the process. It should be noted that the values of the relative contributions of Fe
2+ and Fe
3+ are more dispersed for the galvanostatic protocol,
which confirms the better quality of the data obtained with the other protocol. These
results show that the mechanism of sodiation consists in the formation of a solid
solution coming with the reduction of Fe
3+ into Fe
2+ , followed by a two-phase reaction leading to the reduction of Ti
4+ into Ti
3+ . The mechanism is reversible for
desodiation. The cyclability and rate capability are better for Na 1.5 Fe 0.5 Ti 1.5 (PO 4 ) 3
compared to NaTi 2 (PO 4 ) 3 , which can be attributed to enhanced Na diffusion due to
the single-phase reaction in R1.
The operando Mössbauer spectroscopy provides a quantitative characterization
of the redox reactions but cannot distinguish between solid-solution and two-phase
reactions. This is obtained by operando XRD. Thus, the combined use of these two
techniques successfully explains the two-step mechanism by the reactions
N a 1.5 Fe
3+
0.5 T i
4+
1.5 (P O 4 ) 3
R3c
+ 0.5N a = N a 2 Fe
2+
0.5 T i
4+
1.5 (P O 4 ) 3
R3c
(7.19)
N a 2 Fe
2+
0.5 T i
4+
1.5 (P O 4 ) 3
R3c
+ 1.5N a = N a 3.5 Fe
2+
0.5 T i
3+
1.5 (P O 4 ) 3
P1c
(7.20)