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MnO2 primary batteries
Journal Pre-proof Thermodynamics, kinetics and crystal structure of γ/β-MnO2 in Li/MnO2 primary batteries
Yali Chang, Hao Zhang, Weijun Xiang, Shengping Wang, Xiaoyan Zhu, Jingxian Yu PII:
S0013-4686(20)30310-8
DOI:
https://doi.org/10.1016/j.electacta.2020.135918
Reference:
EA 135918
To appear in:
Electrochimica Acta
Received Date:
09 December 2019
Accepted Date:
16 February 2020
Please cite this article as: Yali Chang, Hao Zhang, Weijun Xiang, Shengping Wang, Xiaoyan Zhu, Jingxian Yu, Thermodynamics, kinetics and crystal structure of γ/β-MnO2 in Li/MnO2 primary batteries, Electrochimica Acta (2020), https://doi.org/10.1016/j.electacta.2020.135918
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Journal Pre-proof
Thermodynamics, kinetics and crystal structure of γ/β-MnO2 in Li/MnO2 primary batteries
Yali Chang1, Hao Zhang1, Weijun Xiang1, Shengping Wang1, *, Xiaoyan Zhu1, *, Jingxian Yu2, *
1
Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074,
China 2 ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP), School of Chemistry and Physics,
The University of Adelaide, Adelaide, SA 5005, Australia *
Corresponding Author. [email protected] (S. Wang), [email protected] (X. Zhu), [email protected] (J. Yu)
Abstract：γ/β-MnO2 as a cathode material is a key factor for the performance of Li/MnO2 primary batteries. To clarify the thermodynamic and kinetic characteristics of γ/β-MnO2, its approximate entropy (ΔS) at various depths of discharge (DOD) in a quasi-equilibrium state is studied. Based on ΔS, the reduction process of γ/β-MnO2 with Li+ intercalation is divided into three stages (singlephase, two-phase and single-phase reaction), but these three stages are hard to distinguish by X-ray diffraction (XRD). The above result is different from an earlier conclusion of only a single-phase reaction. The two-phase reaction of γ/β-MnO2 has a low charge transfer impedance (Rct) and solid phase diffusion impedance (Zw). This is advantageous for providing a high electrochemical reduction reaction rate of MnO2 and a high power output of the battery. The functional relationship between x in LixMnO2 and its corresponding open circuit voltage (OCV, the stable potential of MnO2 vs Li/Li+) is a mathematical formula similar to the Nernst equation. According to the formula parameters (m and n), the state of charge (SOC) of γ/β-MnO2 can be determined. From the thermodynamics of γ/β-MnO2 during discharge, the rate controlling steps (RCSs) are ion solid diffusion and electron transport, which is different from previous reports stating solid phase diffusion as a single RCS. Keywords: thermodynamics; entropy; rate controlling step; lithium manganese dioxide primary battery
Journal Pre-proof 1. Introduction Lithium secondary batteries, such as lithium air batteries [1] and lithium sulfur batteries [2], have attracted the attention of many scholars due to their good cycling performances
[3-5].
However,
because of their high energy density, low self-discharge rate and long shelf life, lithium primary batteries are still irreplaceable. The Li/MnO2 primary battery is one of the most produced versions of lithium metal batteries [6, 7]. It is well known that γ/β-MnO2 as an active cathode material is the most important factor for the capacity density, power density, and storage performance of Li/MnO2 primary batteries
[8-12].
To improve the electrochemical performance of batteries, considerable
research on MnO2 has been conducted [13-18]. Studies on the kinetics of MnO2 as a cathode in Li/MnO2 primary batteries have been reported frequently, but its thermodynamic characteristics have been less researched [19-21], which is due to the thermodynamic studies that need to be carried out in equilibrium or quasi-equilibrium states; additionally, the test conditions are harsh. The MnO2 cathode material has good stability and can meet the thermodynamic test conditions [19, 21, 22]. Thermodynamic research has mostly been used to study the safety of batteries [23, 24]. In fact, the thermodynamic characteristics of MnO2 are related to electrochemical performance, such as shelf life, self-discharge rate, and over discharge performance [25-27].
Similar to the kinetics behavior, the entropy (ΔS) of γ/β-MnO2 as the cathode in Li/MnO2
primary batteries is related to its electrochemical activity, for instance, the number of activation sites and the free energy of lithium intercalation
[28-32].
The relationships between the
thermodynamic characteristics and the state of charge (SOC) of MnO2 are studied [33]. The SOC of the MnO2 and Li/MnO2 batteries can be calculated based on the ΔS and the enthalpy (ΔH), but they are not easy to obtain. In addition, the functional relationships of the open circuit voltage (OCV) at various depths of discharge (DOD) are similar to the Nernst equation. However, the values and physical significance of each parameter has not been determined. Thus, the association between the thermodynamics and kinetics of MnO2 has not been established. In this paper, the thermodynamic parameters and kinetic behavior of γ/β-MnO2 in Li/MnO2 primary batteries are explored. The aims are to clarify the lithium intercalation mechanism of γ/βMnO2 and seek alternative solutions to improve its electrochemical performance. The processes of lithium dissolution or deposition will not change the composition of the lithium electrode, and the
Journal Pre-proof anode has excellent electrochemical activity. Therefore, the thermodynamic characteristics of the Li/MnO2 primary batteries are mainly attributed to the MnO2 electrode
[28].
Therefore, the
relationships between the diffusion coefficient of lithium ions (DLi+), the charge transfer impedance (Rct) and the ΔS of γ/β-MnO2 at various DODs are established by using Li/MnO2 primary batteries, and the discharge mechanism of γ/β-MnO2 and the evaluation basis for the SOC of batteries are explored.
2. Experimental 2.1. Preparation of the cathodes and the batteries Heat-treated electrolytic manganese dioxide (HEMD, γ/β-MnO2) was prepared by thermally treating electrolytic manganese dioxide (EMD, γ-MnO2, Xiangtan China, purity 91 wt%) at 375 °C for 15 h in air. HEMD (80 g) was mixed thoroughly with 5 g Super P, 5 g colloidal graphite, 10 g polytetrafluoroethylene and 35 ml absolute ethyl alcohol to form a slurry. The slurry was pressed into a film by rolling. The film was subsequently punched into a disc, and the disc was pressed on a current collector and dried at 65 °C in air. The current collector of the MnO2 electrode was 304 stainless steel woven mesh with a thickness of 0.1 mm and 40 mesh. The γ/β-MnO2 electrode had a thickness of 0.73 mm, a diameter of 15.5 mm, and a mass of 0.32 g (without a current collector). The MnO2 electrode was dried under vacuum at 210 °C for 8 h prior to assembling the test battery. The CR2016 test batteries were assembled in an argon-filled glove box. The electrolyte was 1 M LiClO4 dissolved in propyl carbonate (PC), 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DIOX) at a 5:3:2 volume ratio. Lithium foil (a thickness of 0.5 mm and a diameter of 16 mm) served as the anode, and a Celgard 2400 film was employed as a separator.
2.2. Electrochemical test Galvanostatic discharge measurements of the test batteries were performed at 0.5 mA cm-2 with a cut-off voltage of 1.5 V using a LAND CT2001A instrument. Electrochemical impedance spectroscopy (EIS) of γ/β-MnO2 at various DODs was performed on a VMP3 electrochemical workstation with an amplitude of 5 mV from 105 Hz to 10-2 Hz. The solid phase diffusion coefficient of the lithium ion was calculated from Eqs. 1-2.
Journal Pre-proof ZRe = Re + Rct + σww -0.5 𝐷 = 0.5(
R𝑇
(1)
2
) 𝐴F2𝜎w𝐶
(2)
2.3. Measurement and calculation of thermodynamic parameters The test batteries were discharged to the desired SOC with a current density of 0.5 mA cm-2 at 25 °C, and the interval discharge capacity of MnO2 was 15.4 mAh g-1 (5% of the theoretical capacity density of MnO2). The OCVs of the stabilized test batteries at various temperatures were measured. The test batteries were allowed to rest for 72 h at each setting temperature to reach a quasi-equilibrium state, and the setting temperatures were 25, 20, 15, 10 and 5 °C. The test battery was considered to be in a quasiequilibrium state when the change value of its OCV was less than 40 µV min-1 [26]. The Li+ intercalation amount x (x in LixMnO2) was calculated by Q/Qth. Q and Qth are the actual capacity density and theoretical capacity density (308 mAh g-1) of MnO2, respectively. The ΔS, ΔH and Gibbs free energy (ΔG) of MnO2 of the Li/MnO2 primary batteries at various DODs were calculated according to Eqs. 3-5: dOCV (𝑥) ∆𝑆 (𝑥) = F ) d𝑇 ∆𝐻 (𝑥) = ―F(OCV ― 𝑇
dOCV (𝑥) ) d𝑇
∆𝐺 (𝑥) = ∆𝐻 (𝑥) ― 𝑇∆𝑆 (𝑥)
(3)
(4) (5)
2.4. Materials characterization The test batteries were disassembled in an argon-filled glove box. The MnO2 electrodes at various DODs were removed and washed three times with dimethyl carbonate (DMC) and then dried under vacuum at 65 °C for 2 h. The X-ray diffraction (XRD) patterns of MnO2 at various DODs were obtained using a Bruker D8 Advance at 40 kV with Cu Kα radiation (λ=0.1542 nm). The electron conductivity of the MnO2 electrode was measured by an Agilent 4294A precision impedance analyzer with a four-probe
Journal Pre-proof method. The surface morphology of the MnO2 material was measured by SU8010 high resolution field emission scanning electron microscopy (FE-SEM), and the average grain size was evaluated from SEM by the linear intercept mathod, using Nano Measurer System 1.2.
3. Results and discussion 3.1. Structural evolution of MnO2 during discharge 3.1.1. Electron conductivity of the MnO2 electrode The discharge curve of MnO2 with a stable discharge plateau is shown in Fig. 1a. The discharge capacity of MnO2 was 293 mAh g-1, and its maximum lithium intercalation amount was 0.95 (Li0.95MnO2). The electrochemical performances of MnO2 were limited by electron transport in the cathode, ion transport and diffusion in the electrolyte, ion solid-state diffusion in MnO2 and charge transfer at the electrode/electrolyte interface [34, 35]. It is generally believed that the solid diffusion of Li+ in MnO2 is the rate controlling step (RCS) [34, 36]. However, there have been some reports that electron transport is also an important factor limiting the electrochemical performance of MnO2 [34]. The electron conductivities of the MnO2 electrode at various DODs are shown in Fig. 1a. The change in electron conductivity could be divided into three stages. At the initial discharge (x in LixMnO2 was 0-0.2), the electron conductivity decreased sharply. When x was 0.2-0.5, the electron conductivity decreased slowly. The electron conductivity tended to be stable near the end of discharge (x was 0.5-0.95). The electron conductivity of MnO2 was larger than that of LixMnO2, and the electron conductivity of the MnO2 electrode decreased with DOD. The molar ratio of Mn3+/Mn4+ in MnO2 increased as the amount of intercalated Li+ increased. The transfer path for electrons decreased; thus, a decrease in electron conductivity was alleviated. At a later stage of discharge, the electron conductivity of the MnO2 electrode was no longer affected by 𝐶Li + , 𝐶Mn3 + and that tended to be stable.
3.1.2. Crystal structure of MnO2 The SEM image and particle size distribution pattern of the MnO2 samples for testing are shown in Figs. 1b, c. The average particle size with Gaussian distribution was ~120 nm. The XRD spectra of MnO2 at various DODs are shown in Fig. 1d. As x in LixMnO2 increased, the peak at ~28.8°
Journal Pre-proof gradually disappeared, the peak strength at ~56.6° decreased, the peaks at ~37°, ~42.8° and ~56.6° shifted to small angles, new peaks at ~22° and ~54° appeared, and the intensity of the last two peaks increased gradually. The HEMD was randomly stacked with [1×1] pyrolusite MnO2 and [1×2] ramsdellite MnO2 [21]. The peak at ~28.8° was the characteristic peak of pyrolusite MnO2, which almost disappeared when x was 0.60. At this SOC, pyrolusite MnO2 was almost completely filled with lithium. Some new peaks of a spinel-like phase, i.e., (a) and (e), subsequently appeared. Li+ preferentially intercalated in ramsdellite MnO2, then in pyrolusite, and finally in the spinel-like phase
[21].
However, in the XRD spectrum, only two lithium intercalation stages were observed.
When x in LixMnO2 was 0-0.60, the MnO2 cell expanded gradually with increasing DOD, which is when the new phase appeared. The Li+ intercalation process in ramsdellite and pyrolusite MnO2 could not be distinguished by XRD. This was because the structures of ramsdellite and pyrolusite MnO2 were not obviously changed by lithium intercalation.
3.2. Thermodynamic characteristics 3.2.1. ΔS The curve of ΔS vs. x in LixMnO2 can show information for the component and phase change of MnO2, which are difficult to detect by other physical characterization methods. Based on ΔS, the phase transition process of MnO2 could be identified. The curve of ΔS vs. x had an inflection point, which indicated that a phase transition occurred [32]. Based on the slope of the curve (Fig. 2c), the process of lithium intercalation into MnO2 could be divided into three stages. These three stages were a single-phase reaction (x in LixMnO2 was 0.10-0.45), a two-phase reaction (x was 0.45-0.60), and a single-phase reaction (x was 0.60-0.90) [21, 22]. XRD could only observe two stages. Similar to ΔS, Rct and DLi+ were also closely related to x in LixMnO2 (Fig. 2c). In stage I (x in LixMnO2 was 0.10-0.45), Rct decreased and DLi+ increased with increasing DOD. In this region, ΔS became negative, and γ/β-MnO2 transitioned toward an orderly state. The degree of irregularity was a key influence on the electrochemical performance of MnO2. The ordered structure of γ/β-MnO2 was beneficial to the electrochemical performance of batteries. The length and width of the Li+ intercalation tunnel of ramsdellite MnO2 were larger than those of pyrolusite MnO2, and thus, the insertion and diffusion impedances of Li+ were smaller. Therefore, Li+ was first
Journal Pre-proof intercalated into ramsdellite MnO2 to occupy the active site, resulting in a single-phase reaction. In this stage, the lattice of MnO2 expanded due to Li+ intercalation. No phase disappeared, but a new phase formation was observed (Fig. 1b). In stage II (x in LixMnO2 was 0.45-0.60), the reduction of MnO2 was a two-phase reaction. Li+ intercalated in MnO2 and LixMnO2 at the same time. In this region, Rct still decreased and DLi+ increased until the end of the two-phase reaction. The two-phase reaction facilitated the insertion and diffusion of Li+. The electrochemical activity of MnO2 was closely related to its ΔS. When some active sites of γ/β-MnO2 were about to be filled, the crystal structure of γ/β-MnO2 tended to be ordered. However, as other new active sites began to be filled, the degree of irregularity for the crystal structure gradually increased [23]. Some equivalent active sites of MnO2 were available to be filled with lithium, and the other active sites began to be used for intercalation; ΔS changed from a negative value to a positive value. Therefore, Li+ began to intercalate in pyrolusite MnO2 at x in LixMnO2 was 0.54. In the two-phase reaction, the slope of the curve of ΔS vs. x increased, and the irregularity of the MnO2 crystal structure increased rapidly. There were two main reasons for the increase in irregularity. First, the pyrolusite MnO2 structure collapsed. This was confirmed by XRD spectra, i.e., the peak at ~28.8° gradually disappeared in this region. Second, due to the two-phase reaction, there was internal stress generated at the interface of the two phases, which led to the disorder of the crystal structure. In summary, both the phase transition and irregularity had effects on the electrochemical properties of γ/β-MnO2, but the phase transition influence was greater. In stage III (x in LixMnO2 was 0.60-0.90), the two-phase reaction of MnO2 mainly occurred, and it was a solid solution reaction. The (a) and (e) peaks appeared in the XRD spectrum when x was 0.60, which was attributed to the formation of a spinel-like phase. In this region, ΔS gradually decreased and eventually became negative, which meant that the crystal structure of MnO2 tended to be ordered. However, Rct increased gradually, DLi+ decreased and reached its lowest value near the end of the discharge. This indicated again that the influence of the phase change on the electrochemical performance of γ/β-MnO2 was greater than that of irregularity. ΔS was positive when x was 0.60-0.70, which meant that the degree of irregularity was still increasing, perhaps because a small amount of pyrolusite-MnO2 was still intercalated. At the later stage (x was 0.700.90), Li+ only intercalated in the spinel-like phase. The system was gradually ordered when the
Journal Pre-proof active sites were about to be fully filled, and then the ΔS became negative.
3.2.2. OCV (the stable potential of γ/β-MnO2 vs Li/Li+) According to the curve of ΔS vs. x in LixMnO2, the whole discharge process of γ/β-MnO2 was divided into three stages. The x values in LixMnO2 for the three stages were 0.10-0.45, 0.45-0.60 and 0.60-0.90. The curves of OCV vs. x in LixMnO2 were fitted mathematically (Fig. 3). The fitting formula is shown as Eq. 6, An the values of the parameters obtained by fitting are listed in Table 1. 1―𝑥 OCV = m + nln 𝑥
(6)
The function of OCV vs. x in MnO2 fit well with Nernst-like (Eq. 7): 𝐸 = 𝐸θ +
R𝑇 𝐶Mn4 + ln 𝑒F 𝐶Mn3 +
(7)
The m in Eq. 6 could be understood as a standard potential, and n was RT/eF. The theoretical value of RT/eF was 0.0257 at 25 °C when the number of electrons in the reaction was 1. The calculated n was slightly different from the theoretical value because the test batteries were in a quasi-equilibrium state and not an equilibrium state. However, the formula could still be used for a thermodynamic analysis. The value of m was related to the ratio of
𝐶Mn4 +
𝐶Mn3 + .
With increasing x, Mn4+ was reduced to Mn3+,
so the value of m decreased gradually. The transfer path of electrons in γ/β-MnO2 could be shortened by appropriately increasing the 𝐶Mn3 + . This would increase the electrical conductivity of the MnO2 electrode. Therefore, the Rct decreased gradually at the initial stage of discharge. Near the end of discharge (x in LixMnO2 was 0.5-0.95), the electrical conductivity of the MnO2 electrode tended to be stable due to the poor electrical conductivity of LixMnO2. Although the electron conductivity of the MnO2 electrode continued to decrease throughout the discharge process, the decline rates were variable at various discharge stages. The electron conductivity decreased sharply during the initial stage of discharge (x was 0-0.20) and then decreased slowly (x was 0.20-0.50). The increase in 𝐶Mn3 + could improve the electron conductivity of the MnO2 electrode and alleviated the decreasing
Journal Pre-proof electron conductivity to a certain extent. At a constant temperature, the value of n was related to the number of electrons participating in the electrochemical reaction. During the whole discharge process, n was greater than 0.0257, indicating that the charge transfer number of the reaction was less than 1. In stage I, the MnO2 electrode had high electron conductivity and DLi+; however, the Rct was the smallest. It could be considered that the electrochemical performance of γ/β-MnO2 was limited by ion diffusion in electrolyte-filled pores. During the initial discharge, Li+ preferentially intercalated in the [1×2] tunnel, which was easy for Li+ intercalation and diffusion, but the Rct was the largest during the entire discharge process. This indicated that the ion diffusion in the electrolyte during the initial discharge was the RCS. When x increased, the value of n increased, and the charge transfer number decreased. The values of n for stage II and stage III were similar. Interestingly, the electron conductivity of the MnO2 electrode remained unchanged during the later discharge, which confirmed that n was related to the electrochemical charge transfer behavior. However, the electrochemical parameters still changed; in particular, the DLi+ decreased to its lowest value. The solid-state diffusion of Li+ during the late discharge was the RCS. The slow solid-state diffusion of Li+ was mainly due to the availability of less active sites in MnO2 and the increase in the degree of disorder caused by the collapse of the MnO2 crystal structure. In conclusion, the electron transfer was the RCS during the initial discharge. With the increase in DOD, the solid phase diffusion of Li+ gradually became the RCS. The Rct at initial discharge was the largest, which was not only due to the high irregularity of γ/β-MnO2 but also the low electron transfer. Both of them affected the electrochemical performance of γ/β-MnO2.
3.2.3. ΔH and ΔG In Fig. 4, the curve of ΔH vs. x in LixMnO2 was almost monotonic compared with that of ΔS. ΔH was too large to obtain slight change information. ΔS could be used to infer the reaction mechanism of the electrochemical system. ΔG showed that the reaction of lithium intercalation into MnO2 was spontaneous, and the reverse reaction was nonspontaneous. Therefore, the battery system (Li|PC+DME+LiClO4|γ/βMnO2) was a good primary battery; however, it was not suitable for secondary batteries.
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4. Conclusion Based on the ΔS of γ/β-MnO2 in a Li/MnO2 primary battery, its electrochemical reduction reaction could be divided into single-phase, two-phase and single-phase reactions. However, only two stages could be observed in the XRD results. According to the kinetic parameters of γ/β-MnO2, the two-phase reaction was beneficial to the electrochemical performance. It could reduce the intercalation impedance of Li+ and facilitate the solid phase diffusion of Li+. The irregularity of γ/βMnO2 also affected the electrochemical performance, and an ordered state was beneficial for lithium intercalation. However, the influence of irregularity was not as great as that of phase transition. The m and n in the fitting formulas from the curves of OCV vs. x in LixMnO2 could determine the influence of ion diffusion and electron transport on the electrochemical performance of γ/βMnO2. The results showed that the electrochemical reaction of γ/β-MnO2 during stage I was controlled by electron transport, and these reactions during stages II and III were controlled by the solid-state diffusion of Li+. Exploring the RCS of the reaction from a thermodynamic perspective could be applied to other electrochemical systems. The main contradictions affecting electrochemical performance could be found, and the electrochemical performance could then be specifically improved. Reports on the thermodynamics of γ/β-MnO2 electrodes in Li/MnO2 primary batteries are rare. In this paper, the thermodynamic characteristics for the intercalation and solid diffusion of lithium ions in MnO2 are revealed. The methods and preliminary conclusions can be used as a reference for other batteries. Many characteristics of the electrochemical reactions of electrode materials can be revealed by analyzing the thermodynamics; interestingly, some of these cannot be observed by physical characterization or electrochemical measurements. By combining these three methods, a deeper understanding of the structural changes in the process of lithium insertion can be obtained. The electron number in the electrochemical reaction of the electrode materials also affects the thermodynamic parameters (m, n), so a multi-electron reaction is also worth studying. Furthermore, the scientific law of correlation between the kinetic performance and the thermodynamic parameters of electrode materials can be used to accurately evaluate the SOCs of batteries, which are very difficult to solve in practical work.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (21173198).
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4915-4920. M. Minakshi, K. Nallathamby, D.R.G. Mitchell, Electrochemical characterization of an aqueous lithium rechargeable battery: the effect of CeO2 additions to the MnO2 cathode, J. Alloy. Compd. 479 (2009) 87-90. W.M. Dose, S.W. Donne, Optimizing Li/MnO2 batteries: relating manganese dioxide properties and electrochemical performance, J. Power Sources 221 (2013) 261-265. J.-G. Wang, Y. Yang, Z.-H. Huang, F. Kang, Interfacial synthesis of mesoporous MnO2/polyaniline hollow spheres and their application in electrochemical capacitors, J. Power Sources 204 (2012) 236-243. X.Y. Zhan, J.J. Tang, Z.H. Li, D.S. Gao, P. Chen, Q. Wu, Nanometer Cr2O3-doped MnO2 spheres for rechargeable lithium batteries, J. Solid State Electrochem. 14 (2010) 1007-1011. M. Zhi, A. Manivannan, F. Meng, N. Wu, Highly conductive electrospun carbon nanofiber/MnO2 coaxial nano-cables for high energy and power density supercapacitors, J. Power Sources 208 (2012) 345-353. T. Ohzuku, M. Kitagawa, T. Hirai, Electrochemistry of manganese dioxide in lithium nonaqueous cell II. X-ray diffractional and electrochemical characterization on deep discharge products of electrolytic manganese dioxide, J. Electrochem. Soc. 137 (1990) 40-46. Y. Shao-Horn, S.A. Hackney, B.C. Cornilsen, Structural characterization of heat-treated electrolytic manganese dioxide and topotactic transformation of discharge products in the LiMnO2 cells, J. Electrochem. Soc. 144 (1997) 3147-3153. W. Bowden, C.P. Grey, S. Hackney, F. Wang, Y. Paik, N. Iltchev, R. Sirotina, Lithiation of ramsdellite-pyrolusite MnO2; NMR, XRD, TEM and electrochemical investigation of the discharge mechanism, J. Power Sources 153 (2006) 265-273. T. Ohzuku, M. Kitagawa, T. Hirai, Electrochemistry of manganese dioxide in lithium nonaqueous cell I. X-ray diffractional study on the reduction of electrolytic manganese dioxide, J. Electrochem. Soc. 136 (1989) 3169-3174. K.E. Thomas, J. Newman, Heats of mixing and of entropy in porous insertion electrodes, J. Power Sources 119-121 (2003) 844-849. K.E. Thomas, C. Bogatu, J. Newman, Measurement of the entropy of reaction as a function of state of charge in doped and undoped lithium manganese oxide, J. Electrochem. Soc. 148 (2001) A570-A575. K. Maher, R. Yazami, Effect of overcharge on entropy and enthalpy of lithium-ion batteries, Electrochim. Acta 101 (2013) 71-78. K. Maher, R. Yazami, A thermodynamic and crystal structure study of thermally aged lithium ion cells, J. Power Sources 261 (2014) 389-400. K. Maher, R. Yazami, A study of lithium ion batteries cycle aging by thermodynamics techniques, J. Power Sources 247 (2014) 527-533. K. Jalkanen, T. Aho, K. Vuorilehto, Entropy change effects on the thermal behavior of a LiFePO4/graphite lithium-ion cell at different states of charge, J. Power Sources 243 (2013) 354-360. Y. Reynier, R. Yazami, B. Fultz, The entropy and enthalpy of lithium intercalation into graphite, J. Power Sources 119-121 (2003) 850-855. Y. Reynier, R. Yazami, B. Fultz, I. Barsukov, Evolution of lithiation thermodynamics with the graphitization of carbons, J. Power Sources 165 (2007) 552-558.
Journal Pre-proof [31] R. Yazami, Y. Reynier, Thermodynamics and crystal structure anomalies in lithiumintercalated graphite, J. Power Sources 153 (2006) 312-318. [32] Y.F. Reynier, R. Yazami, B. Fultz, Thermodynamics of lithium intercalation into graphites and disordered carbons, J. Electrochem. Soc. 151 (2004) A422-A426. [33] Y. Manane, R. Yazami, Accurate state of charge assessment of lithium-manganese dioxide primary batteries, J. Power Sources 359 (2017) 422-426. [34] R. Tian, S.-H. Park, P.J. King, G. Cunningham, J. Coelho, V. Nicolosi, J.N. Coleman, Quantifying the factors limiting rate performance in battery electrodes, Nat. Commun. 10 (2019) 1933. [35] F. Jiang, P. Peng, Elucidating the performance limitations of lithium-ion batteries due to species and charge transport through five characteristic parameters, Sci. Rep. 6 (2016) 32639. [36] J. Wang, Y. Xia, Y. Liu, W. Li, D. Zhao, Mass production of large-pore phosphorus-doped mesoporous carbon for fast-rechargeable lithium-ion batteries, Energy Storage Mater. 22 (2019) 147-153.
Journal Pre-proof Figure captions
Fig. 1 Discharge curve with a current density of 0.5 mA cm-2 at 25 °C and the electron conductivities (a), SEM image (b), particle size distribution pattern (c), and XRD spectra (d) of the MnO2 samples. In Fig. 1d, the (a)-(f) XRD patterns indicate the peaks of MnO2, the (a) and (e) peaks indicate the spinel-like phase, and the (b), (c), (d), and (f) peaks are indexed to the (110), (101), (111), and (211) planes of β-MnO2 (JCPDS 24-0735), respectively. The (g) peak indicates graphite.
Fig. 2 EIS (a); the relationship curves between ZRe and ω-0.5 in the low-frequency range and equivalent circuit diagram (b); and ΔS, Rct, DLi+ (c) of γ/β-MnO2 at various DODs. R1 represents the ohmic resistance, R2 represents the interfacial impedance, R3 is the charge transfer resistance, and Zw is the Warburg impedance. Q1 and Q2 represent the electric double-layer capacitance of the interface. The points are test data, and the lines are fitting data.
Fig. 3 The curves of OCV vs. x in LixMnO2. The points are experimental data, and the lines represent fitting data.
Fig. 4 The curves of ΔH and ΔG vs. x in LixMnO2.
Table captions
Table 1 Fitting parameter values from Eq. 6 of γ/β-MnO2 at various DODs.
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Table 1 Fitting parameter values from Eq. 6 of γ/β-MnO2 at various DODs. x in LixMnO2
m
n
0.15-0.45
3.04
0.0396
0.45-0.60
3.02
0.0996
0.60-0.90
2.99
0.0850
Yali Chang, Hao Zhang, Weijun Xiang, Shengping Wang, Xiaoyan Zhu, Jingxian Yu PII:
S0013-4686(20)30310-8
DOI:
https://doi.org/10.1016/j.electacta.2020.135918
Reference:
EA 135918
To appear in:
Electrochimica Acta
Received Date:
09 December 2019
Accepted Date:
16 February 2020
Please cite this article as: Yali Chang, Hao Zhang, Weijun Xiang, Shengping Wang, Xiaoyan Zhu, Jingxian Yu, Thermodynamics, kinetics and crystal structure of γ/β-MnO2 in Li/MnO2 primary batteries, Electrochimica Acta (2020), https://doi.org/10.1016/j.electacta.2020.135918
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Journal Pre-proof
Thermodynamics, kinetics and crystal structure of γ/β-MnO2 in Li/MnO2 primary batteries
Yali Chang1, Hao Zhang1, Weijun Xiang1, Shengping Wang1, *, Xiaoyan Zhu1, *, Jingxian Yu2, *
1
Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074,
China 2 ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP), School of Chemistry and Physics,
The University of Adelaide, Adelaide, SA 5005, Australia *
Corresponding Author. [email protected] (S. Wang), [email protected] (X. Zhu), [email protected] (J. Yu)
Abstract：γ/β-MnO2 as a cathode material is a key factor for the performance of Li/MnO2 primary batteries. To clarify the thermodynamic and kinetic characteristics of γ/β-MnO2, its approximate entropy (ΔS) at various depths of discharge (DOD) in a quasi-equilibrium state is studied. Based on ΔS, the reduction process of γ/β-MnO2 with Li+ intercalation is divided into three stages (singlephase, two-phase and single-phase reaction), but these three stages are hard to distinguish by X-ray diffraction (XRD). The above result is different from an earlier conclusion of only a single-phase reaction. The two-phase reaction of γ/β-MnO2 has a low charge transfer impedance (Rct) and solid phase diffusion impedance (Zw). This is advantageous for providing a high electrochemical reduction reaction rate of MnO2 and a high power output of the battery. The functional relationship between x in LixMnO2 and its corresponding open circuit voltage (OCV, the stable potential of MnO2 vs Li/Li+) is a mathematical formula similar to the Nernst equation. According to the formula parameters (m and n), the state of charge (SOC) of γ/β-MnO2 can be determined. From the thermodynamics of γ/β-MnO2 during discharge, the rate controlling steps (RCSs) are ion solid diffusion and electron transport, which is different from previous reports stating solid phase diffusion as a single RCS. Keywords: thermodynamics; entropy; rate controlling step; lithium manganese dioxide primary battery
Journal Pre-proof 1. Introduction Lithium secondary batteries, such as lithium air batteries [1] and lithium sulfur batteries [2], have attracted the attention of many scholars due to their good cycling performances
[3-5].
However,
because of their high energy density, low self-discharge rate and long shelf life, lithium primary batteries are still irreplaceable. The Li/MnO2 primary battery is one of the most produced versions of lithium metal batteries [6, 7]. It is well known that γ/β-MnO2 as an active cathode material is the most important factor for the capacity density, power density, and storage performance of Li/MnO2 primary batteries
[8-12].
To improve the electrochemical performance of batteries, considerable
research on MnO2 has been conducted [13-18]. Studies on the kinetics of MnO2 as a cathode in Li/MnO2 primary batteries have been reported frequently, but its thermodynamic characteristics have been less researched [19-21], which is due to the thermodynamic studies that need to be carried out in equilibrium or quasi-equilibrium states; additionally, the test conditions are harsh. The MnO2 cathode material has good stability and can meet the thermodynamic test conditions [19, 21, 22]. Thermodynamic research has mostly been used to study the safety of batteries [23, 24]. In fact, the thermodynamic characteristics of MnO2 are related to electrochemical performance, such as shelf life, self-discharge rate, and over discharge performance [25-27].
Similar to the kinetics behavior, the entropy (ΔS) of γ/β-MnO2 as the cathode in Li/MnO2
primary batteries is related to its electrochemical activity, for instance, the number of activation sites and the free energy of lithium intercalation
[28-32].
The relationships between the
thermodynamic characteristics and the state of charge (SOC) of MnO2 are studied [33]. The SOC of the MnO2 and Li/MnO2 batteries can be calculated based on the ΔS and the enthalpy (ΔH), but they are not easy to obtain. In addition, the functional relationships of the open circuit voltage (OCV) at various depths of discharge (DOD) are similar to the Nernst equation. However, the values and physical significance of each parameter has not been determined. Thus, the association between the thermodynamics and kinetics of MnO2 has not been established. In this paper, the thermodynamic parameters and kinetic behavior of γ/β-MnO2 in Li/MnO2 primary batteries are explored. The aims are to clarify the lithium intercalation mechanism of γ/βMnO2 and seek alternative solutions to improve its electrochemical performance. The processes of lithium dissolution or deposition will not change the composition of the lithium electrode, and the
Journal Pre-proof anode has excellent electrochemical activity. Therefore, the thermodynamic characteristics of the Li/MnO2 primary batteries are mainly attributed to the MnO2 electrode
[28].
Therefore, the
relationships between the diffusion coefficient of lithium ions (DLi+), the charge transfer impedance (Rct) and the ΔS of γ/β-MnO2 at various DODs are established by using Li/MnO2 primary batteries, and the discharge mechanism of γ/β-MnO2 and the evaluation basis for the SOC of batteries are explored.
2. Experimental 2.1. Preparation of the cathodes and the batteries Heat-treated electrolytic manganese dioxide (HEMD, γ/β-MnO2) was prepared by thermally treating electrolytic manganese dioxide (EMD, γ-MnO2, Xiangtan China, purity 91 wt%) at 375 °C for 15 h in air. HEMD (80 g) was mixed thoroughly with 5 g Super P, 5 g colloidal graphite, 10 g polytetrafluoroethylene and 35 ml absolute ethyl alcohol to form a slurry. The slurry was pressed into a film by rolling. The film was subsequently punched into a disc, and the disc was pressed on a current collector and dried at 65 °C in air. The current collector of the MnO2 electrode was 304 stainless steel woven mesh with a thickness of 0.1 mm and 40 mesh. The γ/β-MnO2 electrode had a thickness of 0.73 mm, a diameter of 15.5 mm, and a mass of 0.32 g (without a current collector). The MnO2 electrode was dried under vacuum at 210 °C for 8 h prior to assembling the test battery. The CR2016 test batteries were assembled in an argon-filled glove box. The electrolyte was 1 M LiClO4 dissolved in propyl carbonate (PC), 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DIOX) at a 5:3:2 volume ratio. Lithium foil (a thickness of 0.5 mm and a diameter of 16 mm) served as the anode, and a Celgard 2400 film was employed as a separator.
2.2. Electrochemical test Galvanostatic discharge measurements of the test batteries were performed at 0.5 mA cm-2 with a cut-off voltage of 1.5 V using a LAND CT2001A instrument. Electrochemical impedance spectroscopy (EIS) of γ/β-MnO2 at various DODs was performed on a VMP3 electrochemical workstation with an amplitude of 5 mV from 105 Hz to 10-2 Hz. The solid phase diffusion coefficient of the lithium ion was calculated from Eqs. 1-2.
Journal Pre-proof ZRe = Re + Rct + σww -0.5 𝐷 = 0.5(
R𝑇
(1)
2
) 𝐴F2𝜎w𝐶
(2)
2.3. Measurement and calculation of thermodynamic parameters The test batteries were discharged to the desired SOC with a current density of 0.5 mA cm-2 at 25 °C, and the interval discharge capacity of MnO2 was 15.4 mAh g-1 (5% of the theoretical capacity density of MnO2). The OCVs of the stabilized test batteries at various temperatures were measured. The test batteries were allowed to rest for 72 h at each setting temperature to reach a quasi-equilibrium state, and the setting temperatures were 25, 20, 15, 10 and 5 °C. The test battery was considered to be in a quasiequilibrium state when the change value of its OCV was less than 40 µV min-1 [26]. The Li+ intercalation amount x (x in LixMnO2) was calculated by Q/Qth. Q and Qth are the actual capacity density and theoretical capacity density (308 mAh g-1) of MnO2, respectively. The ΔS, ΔH and Gibbs free energy (ΔG) of MnO2 of the Li/MnO2 primary batteries at various DODs were calculated according to Eqs. 3-5: dOCV (𝑥) ∆𝑆 (𝑥) = F ) d𝑇 ∆𝐻 (𝑥) = ―F(OCV ― 𝑇
dOCV (𝑥) ) d𝑇
∆𝐺 (𝑥) = ∆𝐻 (𝑥) ― 𝑇∆𝑆 (𝑥)
(3)
(4) (5)
2.4. Materials characterization The test batteries were disassembled in an argon-filled glove box. The MnO2 electrodes at various DODs were removed and washed three times with dimethyl carbonate (DMC) and then dried under vacuum at 65 °C for 2 h. The X-ray diffraction (XRD) patterns of MnO2 at various DODs were obtained using a Bruker D8 Advance at 40 kV with Cu Kα radiation (λ=0.1542 nm). The electron conductivity of the MnO2 electrode was measured by an Agilent 4294A precision impedance analyzer with a four-probe
Journal Pre-proof method. The surface morphology of the MnO2 material was measured by SU8010 high resolution field emission scanning electron microscopy (FE-SEM), and the average grain size was evaluated from SEM by the linear intercept mathod, using Nano Measurer System 1.2.
3. Results and discussion 3.1. Structural evolution of MnO2 during discharge 3.1.1. Electron conductivity of the MnO2 electrode The discharge curve of MnO2 with a stable discharge plateau is shown in Fig. 1a. The discharge capacity of MnO2 was 293 mAh g-1, and its maximum lithium intercalation amount was 0.95 (Li0.95MnO2). The electrochemical performances of MnO2 were limited by electron transport in the cathode, ion transport and diffusion in the electrolyte, ion solid-state diffusion in MnO2 and charge transfer at the electrode/electrolyte interface [34, 35]. It is generally believed that the solid diffusion of Li+ in MnO2 is the rate controlling step (RCS) [34, 36]. However, there have been some reports that electron transport is also an important factor limiting the electrochemical performance of MnO2 [34]. The electron conductivities of the MnO2 electrode at various DODs are shown in Fig. 1a. The change in electron conductivity could be divided into three stages. At the initial discharge (x in LixMnO2 was 0-0.2), the electron conductivity decreased sharply. When x was 0.2-0.5, the electron conductivity decreased slowly. The electron conductivity tended to be stable near the end of discharge (x was 0.5-0.95). The electron conductivity of MnO2 was larger than that of LixMnO2, and the electron conductivity of the MnO2 electrode decreased with DOD. The molar ratio of Mn3+/Mn4+ in MnO2 increased as the amount of intercalated Li+ increased. The transfer path for electrons decreased; thus, a decrease in electron conductivity was alleviated. At a later stage of discharge, the electron conductivity of the MnO2 electrode was no longer affected by 𝐶Li + , 𝐶Mn3 + and that tended to be stable.
3.1.2. Crystal structure of MnO2 The SEM image and particle size distribution pattern of the MnO2 samples for testing are shown in Figs. 1b, c. The average particle size with Gaussian distribution was ~120 nm. The XRD spectra of MnO2 at various DODs are shown in Fig. 1d. As x in LixMnO2 increased, the peak at ~28.8°
Journal Pre-proof gradually disappeared, the peak strength at ~56.6° decreased, the peaks at ~37°, ~42.8° and ~56.6° shifted to small angles, new peaks at ~22° and ~54° appeared, and the intensity of the last two peaks increased gradually. The HEMD was randomly stacked with [1×1] pyrolusite MnO2 and [1×2] ramsdellite MnO2 [21]. The peak at ~28.8° was the characteristic peak of pyrolusite MnO2, which almost disappeared when x was 0.60. At this SOC, pyrolusite MnO2 was almost completely filled with lithium. Some new peaks of a spinel-like phase, i.e., (a) and (e), subsequently appeared. Li+ preferentially intercalated in ramsdellite MnO2, then in pyrolusite, and finally in the spinel-like phase
[21].
However, in the XRD spectrum, only two lithium intercalation stages were observed.
When x in LixMnO2 was 0-0.60, the MnO2 cell expanded gradually with increasing DOD, which is when the new phase appeared. The Li+ intercalation process in ramsdellite and pyrolusite MnO2 could not be distinguished by XRD. This was because the structures of ramsdellite and pyrolusite MnO2 were not obviously changed by lithium intercalation.
3.2. Thermodynamic characteristics 3.2.1. ΔS The curve of ΔS vs. x in LixMnO2 can show information for the component and phase change of MnO2, which are difficult to detect by other physical characterization methods. Based on ΔS, the phase transition process of MnO2 could be identified. The curve of ΔS vs. x had an inflection point, which indicated that a phase transition occurred [32]. Based on the slope of the curve (Fig. 2c), the process of lithium intercalation into MnO2 could be divided into three stages. These three stages were a single-phase reaction (x in LixMnO2 was 0.10-0.45), a two-phase reaction (x was 0.45-0.60), and a single-phase reaction (x was 0.60-0.90) [21, 22]. XRD could only observe two stages. Similar to ΔS, Rct and DLi+ were also closely related to x in LixMnO2 (Fig. 2c). In stage I (x in LixMnO2 was 0.10-0.45), Rct decreased and DLi+ increased with increasing DOD. In this region, ΔS became negative, and γ/β-MnO2 transitioned toward an orderly state. The degree of irregularity was a key influence on the electrochemical performance of MnO2. The ordered structure of γ/β-MnO2 was beneficial to the electrochemical performance of batteries. The length and width of the Li+ intercalation tunnel of ramsdellite MnO2 were larger than those of pyrolusite MnO2, and thus, the insertion and diffusion impedances of Li+ were smaller. Therefore, Li+ was first
Journal Pre-proof intercalated into ramsdellite MnO2 to occupy the active site, resulting in a single-phase reaction. In this stage, the lattice of MnO2 expanded due to Li+ intercalation. No phase disappeared, but a new phase formation was observed (Fig. 1b). In stage II (x in LixMnO2 was 0.45-0.60), the reduction of MnO2 was a two-phase reaction. Li+ intercalated in MnO2 and LixMnO2 at the same time. In this region, Rct still decreased and DLi+ increased until the end of the two-phase reaction. The two-phase reaction facilitated the insertion and diffusion of Li+. The electrochemical activity of MnO2 was closely related to its ΔS. When some active sites of γ/β-MnO2 were about to be filled, the crystal structure of γ/β-MnO2 tended to be ordered. However, as other new active sites began to be filled, the degree of irregularity for the crystal structure gradually increased [23]. Some equivalent active sites of MnO2 were available to be filled with lithium, and the other active sites began to be used for intercalation; ΔS changed from a negative value to a positive value. Therefore, Li+ began to intercalate in pyrolusite MnO2 at x in LixMnO2 was 0.54. In the two-phase reaction, the slope of the curve of ΔS vs. x increased, and the irregularity of the MnO2 crystal structure increased rapidly. There were two main reasons for the increase in irregularity. First, the pyrolusite MnO2 structure collapsed. This was confirmed by XRD spectra, i.e., the peak at ~28.8° gradually disappeared in this region. Second, due to the two-phase reaction, there was internal stress generated at the interface of the two phases, which led to the disorder of the crystal structure. In summary, both the phase transition and irregularity had effects on the electrochemical properties of γ/β-MnO2, but the phase transition influence was greater. In stage III (x in LixMnO2 was 0.60-0.90), the two-phase reaction of MnO2 mainly occurred, and it was a solid solution reaction. The (a) and (e) peaks appeared in the XRD spectrum when x was 0.60, which was attributed to the formation of a spinel-like phase. In this region, ΔS gradually decreased and eventually became negative, which meant that the crystal structure of MnO2 tended to be ordered. However, Rct increased gradually, DLi+ decreased and reached its lowest value near the end of the discharge. This indicated again that the influence of the phase change on the electrochemical performance of γ/β-MnO2 was greater than that of irregularity. ΔS was positive when x was 0.60-0.70, which meant that the degree of irregularity was still increasing, perhaps because a small amount of pyrolusite-MnO2 was still intercalated. At the later stage (x was 0.700.90), Li+ only intercalated in the spinel-like phase. The system was gradually ordered when the
Journal Pre-proof active sites were about to be fully filled, and then the ΔS became negative.
3.2.2. OCV (the stable potential of γ/β-MnO2 vs Li/Li+) According to the curve of ΔS vs. x in LixMnO2, the whole discharge process of γ/β-MnO2 was divided into three stages. The x values in LixMnO2 for the three stages were 0.10-0.45, 0.45-0.60 and 0.60-0.90. The curves of OCV vs. x in LixMnO2 were fitted mathematically (Fig. 3). The fitting formula is shown as Eq. 6, An the values of the parameters obtained by fitting are listed in Table 1. 1―𝑥 OCV = m + nln 𝑥
(6)
The function of OCV vs. x in MnO2 fit well with Nernst-like (Eq. 7): 𝐸 = 𝐸θ +
R𝑇 𝐶Mn4 + ln 𝑒F 𝐶Mn3 +
(7)
The m in Eq. 6 could be understood as a standard potential, and n was RT/eF. The theoretical value of RT/eF was 0.0257 at 25 °C when the number of electrons in the reaction was 1. The calculated n was slightly different from the theoretical value because the test batteries were in a quasi-equilibrium state and not an equilibrium state. However, the formula could still be used for a thermodynamic analysis. The value of m was related to the ratio of
𝐶Mn4 +
𝐶Mn3 + .
With increasing x, Mn4+ was reduced to Mn3+,
so the value of m decreased gradually. The transfer path of electrons in γ/β-MnO2 could be shortened by appropriately increasing the 𝐶Mn3 + . This would increase the electrical conductivity of the MnO2 electrode. Therefore, the Rct decreased gradually at the initial stage of discharge. Near the end of discharge (x in LixMnO2 was 0.5-0.95), the electrical conductivity of the MnO2 electrode tended to be stable due to the poor electrical conductivity of LixMnO2. Although the electron conductivity of the MnO2 electrode continued to decrease throughout the discharge process, the decline rates were variable at various discharge stages. The electron conductivity decreased sharply during the initial stage of discharge (x was 0-0.20) and then decreased slowly (x was 0.20-0.50). The increase in 𝐶Mn3 + could improve the electron conductivity of the MnO2 electrode and alleviated the decreasing
Journal Pre-proof electron conductivity to a certain extent. At a constant temperature, the value of n was related to the number of electrons participating in the electrochemical reaction. During the whole discharge process, n was greater than 0.0257, indicating that the charge transfer number of the reaction was less than 1. In stage I, the MnO2 electrode had high electron conductivity and DLi+; however, the Rct was the smallest. It could be considered that the electrochemical performance of γ/β-MnO2 was limited by ion diffusion in electrolyte-filled pores. During the initial discharge, Li+ preferentially intercalated in the [1×2] tunnel, which was easy for Li+ intercalation and diffusion, but the Rct was the largest during the entire discharge process. This indicated that the ion diffusion in the electrolyte during the initial discharge was the RCS. When x increased, the value of n increased, and the charge transfer number decreased. The values of n for stage II and stage III were similar. Interestingly, the electron conductivity of the MnO2 electrode remained unchanged during the later discharge, which confirmed that n was related to the electrochemical charge transfer behavior. However, the electrochemical parameters still changed; in particular, the DLi+ decreased to its lowest value. The solid-state diffusion of Li+ during the late discharge was the RCS. The slow solid-state diffusion of Li+ was mainly due to the availability of less active sites in MnO2 and the increase in the degree of disorder caused by the collapse of the MnO2 crystal structure. In conclusion, the electron transfer was the RCS during the initial discharge. With the increase in DOD, the solid phase diffusion of Li+ gradually became the RCS. The Rct at initial discharge was the largest, which was not only due to the high irregularity of γ/β-MnO2 but also the low electron transfer. Both of them affected the electrochemical performance of γ/β-MnO2.
3.2.3. ΔH and ΔG In Fig. 4, the curve of ΔH vs. x in LixMnO2 was almost monotonic compared with that of ΔS. ΔH was too large to obtain slight change information. ΔS could be used to infer the reaction mechanism of the electrochemical system. ΔG showed that the reaction of lithium intercalation into MnO2 was spontaneous, and the reverse reaction was nonspontaneous. Therefore, the battery system (Li|PC+DME+LiClO4|γ/βMnO2) was a good primary battery; however, it was not suitable for secondary batteries.
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4. Conclusion Based on the ΔS of γ/β-MnO2 in a Li/MnO2 primary battery, its electrochemical reduction reaction could be divided into single-phase, two-phase and single-phase reactions. However, only two stages could be observed in the XRD results. According to the kinetic parameters of γ/β-MnO2, the two-phase reaction was beneficial to the electrochemical performance. It could reduce the intercalation impedance of Li+ and facilitate the solid phase diffusion of Li+. The irregularity of γ/βMnO2 also affected the electrochemical performance, and an ordered state was beneficial for lithium intercalation. However, the influence of irregularity was not as great as that of phase transition. The m and n in the fitting formulas from the curves of OCV vs. x in LixMnO2 could determine the influence of ion diffusion and electron transport on the electrochemical performance of γ/βMnO2. The results showed that the electrochemical reaction of γ/β-MnO2 during stage I was controlled by electron transport, and these reactions during stages II and III were controlled by the solid-state diffusion of Li+. Exploring the RCS of the reaction from a thermodynamic perspective could be applied to other electrochemical systems. The main contradictions affecting electrochemical performance could be found, and the electrochemical performance could then be specifically improved. Reports on the thermodynamics of γ/β-MnO2 electrodes in Li/MnO2 primary batteries are rare. In this paper, the thermodynamic characteristics for the intercalation and solid diffusion of lithium ions in MnO2 are revealed. The methods and preliminary conclusions can be used as a reference for other batteries. Many characteristics of the electrochemical reactions of electrode materials can be revealed by analyzing the thermodynamics; interestingly, some of these cannot be observed by physical characterization or electrochemical measurements. By combining these three methods, a deeper understanding of the structural changes in the process of lithium insertion can be obtained. The electron number in the electrochemical reaction of the electrode materials also affects the thermodynamic parameters (m, n), so a multi-electron reaction is also worth studying. Furthermore, the scientific law of correlation between the kinetic performance and the thermodynamic parameters of electrode materials can be used to accurately evaluate the SOCs of batteries, which are very difficult to solve in practical work.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (21173198).
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Journal Pre-proof Figure captions
Fig. 1 Discharge curve with a current density of 0.5 mA cm-2 at 25 °C and the electron conductivities (a), SEM image (b), particle size distribution pattern (c), and XRD spectra (d) of the MnO2 samples. In Fig. 1d, the (a)-(f) XRD patterns indicate the peaks of MnO2, the (a) and (e) peaks indicate the spinel-like phase, and the (b), (c), (d), and (f) peaks are indexed to the (110), (101), (111), and (211) planes of β-MnO2 (JCPDS 24-0735), respectively. The (g) peak indicates graphite.
Fig. 2 EIS (a); the relationship curves between ZRe and ω-0.5 in the low-frequency range and equivalent circuit diagram (b); and ΔS, Rct, DLi+ (c) of γ/β-MnO2 at various DODs. R1 represents the ohmic resistance, R2 represents the interfacial impedance, R3 is the charge transfer resistance, and Zw is the Warburg impedance. Q1 and Q2 represent the electric double-layer capacitance of the interface. The points are test data, and the lines are fitting data.
Fig. 3 The curves of OCV vs. x in LixMnO2. The points are experimental data, and the lines represent fitting data.
Fig. 4 The curves of ΔH and ΔG vs. x in LixMnO2.
Table captions
Table 1 Fitting parameter values from Eq. 6 of γ/β-MnO2 at various DODs.
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Table 1 Fitting parameter values from Eq. 6 of γ/β-MnO2 at various DODs. x in LixMnO2
m
n
0.15-0.45
3.04
0.0396
0.45-0.60
3.02
0.0996
0.60-0.90
2.99
0.0850