**4. Material effects**

## **4.1. Introduction**

In the preceding discussion we found the incremental iso-conversional approach for analysing thermogravimetric data to be the favourable method to investigate the kinetics of water loss during the heat treatment of a γ-MnO2 sample. Using the kinetic parameters from this analysis, particular heating regimes at a selection of temperatures were devised to prepare materials with a theoretical complete water loss (*α*=1), thereby avoiding the use of excessive temperatures and/or times. We now consider the effects of these optimised thermal treatment regimes on the heat treated material structure, composition and morphology. The electrochemical characteristics of these heat treated EMD samples are examined and the observed changes in the material properties used to relate the electrochemical performance to the material properties.

## **4.2. Heat treated material properties**

The XRD patterns for the heat treated MnO2 samples are shown in Figure 1. The most obvious changes in these XRD patterns as a result of heat treatment are (i) the merging of the (110) (~22° 2θ) and the (130) (~36° 2θ) lines in the starting γ-MnO2 to form a single peak at ~29° 2θ in the sample heat treated at 400°C; (ii) clearer separation of the (221) and (240) peaks at ~56° 2θ by 400°C; and (iii) disappearance of the peak at ~68° 2θ and the emergence of two peaks at ~66° 2θ and ~73° 2θ. The extent of conversion from the γ-MnO2 phase, which predominantly displays a ramsdellite composition, to the more thermodynamically stable pyrolusite structure can be determined quantitatively by calculation of the fraction of pyrolusite (Pr) in the samples using the method outlined by Chabre and Pannetier [9]. These results are listed in Table 1. Interestingly, these changes are consistent with literature investigating γ-MnO2 heat treated at various temperatures for 24 hours [18,19], indicating a progressive structural conversion with increasing heat treatment temperature. This is despite the largely different experimental heating times used. The similarities indicate that this conversion is mostly influenced by the thermodynamics of the heating process rather than the kinetics. For instance, samples heated at lower temperatures (i.e., 200°C and 250°C) have begun the conversion from γ→β-MnO2, although this process is clearly retarded by insufficient thermal energy to drive this process to completion. This is further supported by comparing the data recorded at 400°C using this analysis method (16 mins heating time) with a sample heated at the same temperature for 7 days. In both cases, a Pr value of 0.84 was calculated from the XRD data suggesting few or no kinetic limitations for this process at the elevated temperature.

The structural changes can be further elucidated by consideration of the unit cell parameters (determined from the XRD patterns of these materials assuming an orthorhombic unit cell) as a function of heat treatment temperature. The unit cell parameters for the starting EMD were a0 = 4.468 Å, b0 = 9.554 Å and c0 = 2.833 Å, which is an expansion in the *a*-*b* plane, but a slight contraction in the *c* direction, compared with ramsdellite [20]. The unit cell parameters for the heat treated materials are shown in Table 1. The decrease shown for both the a0 and b0 parameters represents a structural contraction in these directions, while the steady increase in the c0 parameter indicates lattice expansion in this direction. By considering the differences in the crystal structures of γ-MnO2 and pyrolusite, it is clear that the main differences are found in the *a*-*b* plane. Since the unit cell is found to contract along both these directions, this suggests that ion (Mn(IV)) movement predominates in these directions during heat treatment. The excess of edge sharing octahedra in a uniform array in the *c* direction, without any vacancies present to compensate or provide a buffer for the close proximity of the Mn(IV) ions, results in expansion in this direction [18].

286 Heat Treatment – Conventional and Novel Applications

electrochemical performance to the material properties.

**4.2. Heat treated material properties** 

the elevated temperature.

approach is superior.

**4. Material effects** 

**4.1. Introduction** 

circumstances, while the data we have collected shows a nice asymptotic change with heating rate (Figure 9), the assumptions made in the numerical analysis do not lead to an accurate estimate of the activation energy, again implying that the incremental integral

In the preceding discussion we found the incremental iso-conversional approach for analysing thermogravimetric data to be the favourable method to investigate the kinetics of water loss during the heat treatment of a γ-MnO2 sample. Using the kinetic parameters from this analysis, particular heating regimes at a selection of temperatures were devised to prepare materials with a theoretical complete water loss (*α*=1), thereby avoiding the use of excessive temperatures and/or times. We now consider the effects of these optimised thermal treatment regimes on the heat treated material structure, composition and morphology. The electrochemical characteristics of these heat treated EMD samples are examined and the observed changes in the material properties used to relate the

The XRD patterns for the heat treated MnO2 samples are shown in Figure 1. The most obvious changes in these XRD patterns as a result of heat treatment are (i) the merging of the (110) (~22° 2θ) and the (130) (~36° 2θ) lines in the starting γ-MnO2 to form a single peak at ~29° 2θ in the sample heat treated at 400°C; (ii) clearer separation of the (221) and (240) peaks at ~56° 2θ by 400°C; and (iii) disappearance of the peak at ~68° 2θ and the emergence of two peaks at ~66° 2θ and ~73° 2θ. The extent of conversion from the γ-MnO2 phase, which predominantly displays a ramsdellite composition, to the more thermodynamically stable pyrolusite structure can be determined quantitatively by calculation of the fraction of pyrolusite (Pr) in the samples using the method outlined by Chabre and Pannetier [9]. These results are listed in Table 1. Interestingly, these changes are consistent with literature investigating γ-MnO2 heat treated at various temperatures for 24 hours [18,19], indicating a progressive structural conversion with increasing heat treatment temperature. This is despite the largely different experimental heating times used. The similarities indicate that this conversion is mostly influenced by the thermodynamics of the heating process rather than the kinetics. For instance, samples heated at lower temperatures (i.e., 200°C and 250°C) have begun the conversion from γ→β-MnO2, although this process is clearly retarded by insufficient thermal energy to drive this process to completion. This is further supported by comparing the data recorded at 400°C using this analysis method (16 mins heating time) with a sample heated at the same temperature for 7 days. In both cases, a Pr value of 0.84 was calculated from the XRD data suggesting few or no kinetic limitations for this process at The changes in BET surface area for the HEMD samples are shown in Table 1. The relatively high surface area for these materials indicates that they are quite porous. Most evident from this data is the general decrease in surface area as temperature increases, barring a slight increase between the 350°C and 400°C samples. Given the structural changes occurring during heat treatment, the decreasing surface area suggests that the pores in EMD are removed as Mn(IV) ions diffuse through the structure, creating a more defect free and crystalline material [18]. The slight increase in surface area for the 400°C material is likely to be caused by slow kinetics for this process (relating to the much shorter heating period applied to this material), thus not allowing for the completion of pore collapse. Noticeably, at the lower temperatures (e.g., 200°C), the surface area has not decreased significantly from the original value. This is likely to be connected to insufficient activation energy at the relatively low temperature to drive Mn(IV) diffusion, a factor responsible for pore closure. Table 1 also lists the changes in micro- (<2 nm) and meso-pore (2-50 nm) structures as a result of heat treatment. Clearly, heat treatment causes the collapse of micro-pores, while an increase in the meso-pore volume was observed. The increase in the micro-pore volume for the 400°C material with respect to the 350°C sample indicates that kinetic limitations in the collapse of these pores is responsible for the increase in BET surface area for this material.

A comparison of how the structural changes relate to morphological changes clearly portrays the key differences in the HEMD materials prepared. Figure 10 compares the changes in the orthorhombic unit cell volume with BET surface area. From this data, it is evident that between the temperatures tested, small changes in BET surface area can relate to large structural changes (e.g., between the standard EMD and 200°C material), or vice versa. Generally, however, the interplay of the thermodynamics and kinetics influencing the variation in these parameters leads to an approximately exponential decrease in unit cell volume with respect to BET surface area. Decreases in the unit cell volume can be attributed to manganese ions within the structure having sufficient thermal energy to move to positions consistent with pyrolusite, thus causing intra-crystallite rearrangement within the material. Conversely, changes in BET surface area relate to either the sintering of crystallites (both intra- and inter-) by joining together across the pores thus resulting in pore closure, or the opening of existing pores.

Monitoring the Effects of Thermal

Treatment on Properties and Performance During Battery Material Synthesis 289

increase due to the increase in the total manganese content. Additionally, although limited, some thermal reduction of the material is possible at the higher temperatures considered, thereby causing slight increases to Mn(III) at the expense of Mn(IV). These processes superimpose to give the relatively steady Mn(IV) and slightly increasing Mn(III) content

The proportions of water removed above and below 110°C '(%H2O(>110°C)) and (%H2O(<110°C)) respectively, further support these findings. The %H2O(<110°C), which is water adsorbed on the surface of the material, generally decreases as heat treatment temperature increases, barring slightly higher values for the 300°C and 400°C materials relative to those around them. Clearly, the amount of water adsorbed to the surface of the material is largely determined by the available surface area and therefore it is no surprise that this result reflects the relative BET surface area for these samples (Table 1). The steady decrease in %H2O(>110°C) (i.e., structural water associated with Mn(III) and vacancy defects) with heat treatment temperature provides a second, independent measure of the extent of structural water removal from the HEMD samples. By 350°C it appears that all structural defects that will be removed in the heat treatment have been, evidenced by the

approximately constant %H2O(>110°C) value for the 350°C and 400°C materials.

shown these relationships to hold for a broad range of EMD materials [21].

Conversely, HEMD is limited by the intrinsic material properties.

this being especially noticeable at the higher discharge rates.

The trends in material properties discussed above are specific to a single EMD heat treated under the temperature/time determined from kinetic analysis. However, our investigation of the influence of the starting EMD properties on the resultant HEMD properties has

Due to the large number of cells tested (i.e., five HEMD materials each discharged at four currents), only the discharge characteristics of each HEMD at 2 mA/g are shown in Figure 11. The typically flat discharge curve of Li/MnO2 at 2.8-3.0 V is evident from this plot. The primary discharge capacity (in mAh/g of MnO2) for the cells tested was calculated by using a 2.0 V cut-off point. This result is shown in Table 4. As expected, the capacity of a given material decreases with higher discharge currents. On average the capacity for unheated EMD was 104 mAh/g (or 34% utilisation), which was less than half that of HEMD, with an average of 238 mAh/g (77% utilisation) at the 2 mA/g discharge rate. This clearly demonstrates the importance of the heat treatment process, as has been previously reported in the literature [22-24]. This vast difference in performance also suggests that EMD based cells are failing in a different way to HEMD cells. The high water content of the EMD likely leads to destructive side reactions with the electrolyte and anode causing cell failure.

Considering the HEMD materials in greater detail, we note that manganese dioxide heat treated at 250°C and above maintained relatively good capacity at the higher discharge rates. Also of note are the excellent performance characteristics of the 300°C and 350°C materials particularly at the lower discharge rates. It was also observed that of the various HEMD samples, the material treated at 200°C exhibited relatively poor discharge capacities,

with respect to heat treatment temperature.

**4.3. Electrochemical performance** 

The underlying assumption in the preparation of these samples is that structural water, which is associated with defects (i.e., Mn(III) and cation vacancies), has been completely removed from the material. There is an expectation then that each of the HEMD samples will have no Mn(III), cation vacancies or water removed above 110°C (%H2O(>110°C)). However, the compositional data for the HEMD samples (Table 1) shows that this is not the case. In light of this, the trends evident in these parameters provide important insights into the kinetic and thermodynamic limitations of the heat treatment under the set conditions and at these temperatures. The data for these samples show that the total manganese content (%Mn(T)) increases with heating temperature to an optimum at 300°C. This increase relates to the rearrangement of Mn(IV) ions to form a more defect free structure as cation vacancies are annealed via loss of structural protons. As a consequence, a steady decrease in the cation vacancy fraction is observed with heating temperature. The gradual decline in this parameter suggests that as the thermal energy required for this process is met by the higher temperatures, vacancy removal is able to proceed to a greater degree. After heat treatment at 350°C and 400°C, vacancy defects and associated water have been completely removed from the structure, indicating that these heating regimes have provided sufficient thermal energy to complete this process, without any kinetic limitations.

**Figure 10.** Relationship between BET surface area and unit cell volume for heat treated EMD samples.

The trends in Mn(III) and Mn(IV) percentages are confused to some degree due to a number of processes taking place during the heating experiment. First, the oxidation of Mn(III) to Mn(IV) takes place as a consequence of the oxidising air atmosphere present, thus lowering Mn(III) and increasing Mn(IV). Further, both Mn(III) and Mn(IV) experience a proportionate increase due to the increase in the total manganese content. Additionally, although limited, some thermal reduction of the material is possible at the higher temperatures considered, thereby causing slight increases to Mn(III) at the expense of Mn(IV). These processes superimpose to give the relatively steady Mn(IV) and slightly increasing Mn(III) content with respect to heat treatment temperature.

The proportions of water removed above and below 110°C '(%H2O(>110°C)) and (%H2O(<110°C)) respectively, further support these findings. The %H2O(<110°C), which is water adsorbed on the surface of the material, generally decreases as heat treatment temperature increases, barring slightly higher values for the 300°C and 400°C materials relative to those around them. Clearly, the amount of water adsorbed to the surface of the material is largely determined by the available surface area and therefore it is no surprise that this result reflects the relative BET surface area for these samples (Table 1). The steady decrease in %H2O(>110°C) (i.e., structural water associated with Mn(III) and vacancy defects) with heat treatment temperature provides a second, independent measure of the extent of structural water removal from the HEMD samples. By 350°C it appears that all structural defects that will be removed in the heat treatment have been, evidenced by the approximately constant %H2O(>110°C) value for the 350°C and 400°C materials.

The trends in material properties discussed above are specific to a single EMD heat treated under the temperature/time determined from kinetic analysis. However, our investigation of the influence of the starting EMD properties on the resultant HEMD properties has shown these relationships to hold for a broad range of EMD materials [21].

## **4.3. Electrochemical performance**

288 Heat Treatment – Conventional and Novel Applications

the opening of existing pores.

(both intra- and inter-) by joining together across the pores thus resulting in pore closure, or

The underlying assumption in the preparation of these samples is that structural water, which is associated with defects (i.e., Mn(III) and cation vacancies), has been completely removed from the material. There is an expectation then that each of the HEMD samples will have no Mn(III), cation vacancies or water removed above 110°C (%H2O(>110°C)). However, the compositional data for the HEMD samples (Table 1) shows that this is not the case. In light of this, the trends evident in these parameters provide important insights into the kinetic and thermodynamic limitations of the heat treatment under the set conditions and at these temperatures. The data for these samples show that the total manganese content (%Mn(T)) increases with heating temperature to an optimum at 300°C. This increase relates to the rearrangement of Mn(IV) ions to form a more defect free structure as cation vacancies are annealed via loss of structural protons. As a consequence, a steady decrease in the cation vacancy fraction is observed with heating temperature. The gradual decline in this parameter suggests that as the thermal energy required for this process is met by the higher temperatures, vacancy removal is able to proceed to a greater degree. After heat treatment at 350°C and 400°C, vacancy defects and associated water have been completely removed from the structure, indicating that these heating regimes have provided sufficient

thermal energy to complete this process, without any kinetic limitations.

**Figure 10.** Relationship between BET surface area and unit cell volume for heat treated EMD samples.

20 25 30 35 40

**BET SA (m2/g)**

350oC

115

116

117

118

119

**Unit Cell Volume (Å3)**

120

121

122

400oC

Starting EMD

200oC

250oC

300oC

The trends in Mn(III) and Mn(IV) percentages are confused to some degree due to a number of processes taking place during the heating experiment. First, the oxidation of Mn(III) to Mn(IV) takes place as a consequence of the oxidising air atmosphere present, thus lowering Mn(III) and increasing Mn(IV). Further, both Mn(III) and Mn(IV) experience a proportionate Due to the large number of cells tested (i.e., five HEMD materials each discharged at four currents), only the discharge characteristics of each HEMD at 2 mA/g are shown in Figure 11. The typically flat discharge curve of Li/MnO2 at 2.8-3.0 V is evident from this plot. The primary discharge capacity (in mAh/g of MnO2) for the cells tested was calculated by using a 2.0 V cut-off point. This result is shown in Table 4. As expected, the capacity of a given material decreases with higher discharge currents. On average the capacity for unheated EMD was 104 mAh/g (or 34% utilisation), which was less than half that of HEMD, with an average of 238 mAh/g (77% utilisation) at the 2 mA/g discharge rate. This clearly demonstrates the importance of the heat treatment process, as has been previously reported in the literature [22-24]. This vast difference in performance also suggests that EMD based cells are failing in a different way to HEMD cells. The high water content of the EMD likely leads to destructive side reactions with the electrolyte and anode causing cell failure. Conversely, HEMD is limited by the intrinsic material properties.

Considering the HEMD materials in greater detail, we note that manganese dioxide heat treated at 250°C and above maintained relatively good capacity at the higher discharge rates. Also of note are the excellent performance characteristics of the 300°C and 350°C materials particularly at the lower discharge rates. It was also observed that of the various HEMD samples, the material treated at 200°C exhibited relatively poor discharge capacities, this being especially noticeable at the higher discharge rates.

Monitoring the Effects of Thermal

Treatment on Properties and Performance During Battery Material Synthesis 291

**2 mA/g 5 mA/g 10 mA/g 20 mA/g** 

25C 200C 250C 300C 350C 400C

rates, with the 300°C and 350°C materials exhibiting superior performance. Further, at high rates heat treatment at 250°C and above has resulted in materials exhibiting virtually equivalent performance. Comparison with literature Ragone diagrams [4] reveals that the performance of our materials is superior in terms of specific energy, and comparable in

> 25 0.39 0.23 0.22 - 200 0.77 0.69 0.63 0.39 250 0.87 0.75 0.77 0.66 300 0.94 0.85 0.73 0.66 350 0.94 0.83 0.82 0.64 400 0.89 0.86 0.85 0.71

**Table 5.** Utilization of active material (Mn(IV) percentage) for heat treated EMD samples at various

**Figure 12.** Ragone diagram showing points corresponding to energy delivered by the cell by the cut-off voltage and the corresponding power value for the heat treated EMD samples with respect to discharge

0 100 200 300 400 500 600

**Energy (Wh/kg)**

2 mA/g

20 mA/g

5 mA/g

0

5

10

15

20

25

**Power (W/kg)**

30

35

40

45

10 mA/g

As the electrochemical behaviour of manganese dioxide is largely dependent on the structure, composition and morphology of the material, the changes in these parameters with heat treatment temperature are invoked in explaining their relative electrochemical performance. Since the performance is not directly proportional to the heating temperature, it is immediately evident that there are a number of factors influencing material performance. From the current understanding of the Li/MnO2 system, these factors are

**Temperature (oC) Utilization of Active Material**

terms of specific power.

discharge currents (2.0 V cut-off).

rates.

outlined as follows:

**x in LixMnO2**

**Figure 11.** Discharge curves for heat treated EMD samples discharged at 2 mA/g.


**Table 4.** Discharge capacity for heat treated EMD samples at various discharge currents (2.0 V cut-off).

Normalising the capacity of the cell in terms of the proportion of electrochemically active material (i.e., Mn(IV) percentage) reveals the extent to which the available material is being used. This will have a significant effect on the relative capacity of these HEMD materials since the proportion of Mn(IV) to Mn(III) has been seen to vary depending on the heat treatment conditions. The normalised capacities are listed in Table 5, and demonstrate that heat treatment at 300°C and 350°C has resulted in the highest proportion of electrochemically active Mn(IV) being utilised at the at 2 mA/g rate (94%).

Another useful way in which to present the data from these tests is through the use of a Ragone diagram, comparing the relative specific energy and power output of the materials. Due to the number of cells tested, only the points corresponding to the energy delivered by the cell by the cut-off voltage, and the corresponding power value, are shown in Figure 12. This diagram clearly shows a greater differentiation in energy delivered at the low discharge rates, with the 300°C and 350°C materials exhibiting superior performance. Further, at high rates heat treatment at 250°C and above has resulted in materials exhibiting virtually equivalent performance. Comparison with literature Ragone diagrams [4] reveals that the performance of our materials is superior in terms of specific energy, and comparable in terms of specific power.

290 Heat Treatment – Conventional and Novel Applications

**Temperature (oC)** 

**Figure 11.** Discharge curves for heat treated EMD samples discharged at 2 mA/g.

Increasing heat treatment temperature

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0

**Potential (V)**

Standard EMD 200°C

electrochemically active Mn(IV) being utilised at the at 2 mA/g rate (94%).

**Capacity (mAh/g) 2 mA/g 5 mA/g 10 mA/g 20 mA/g** 

250°C 300°C 350°C 400°C

25 104.1 62.9 59.8 - 200 207.5 185.0 169.4 103.8 250 242.9 208.4 215.1 183.8 300 252.9 229.7 195.4 178.4 350 252.0 221.5 218.9 170.5 400 233.4 226.0 223.7 185.4 **Table 4.** Discharge capacity for heat treated EMD samples at various discharge currents (2.0 V cut-off).

0 0.2 0.4 0.6 0.8 1

**x in LixMnO2**

Normalising the capacity of the cell in terms of the proportion of electrochemically active material (i.e., Mn(IV) percentage) reveals the extent to which the available material is being used. This will have a significant effect on the relative capacity of these HEMD materials since the proportion of Mn(IV) to Mn(III) has been seen to vary depending on the heat treatment conditions. The normalised capacities are listed in Table 5, and demonstrate that heat treatment at 300°C and 350°C has resulted in the highest proportion of

Another useful way in which to present the data from these tests is through the use of a Ragone diagram, comparing the relative specific energy and power output of the materials. Due to the number of cells tested, only the points corresponding to the energy delivered by the cell by the cut-off voltage, and the corresponding power value, are shown in Figure 12. This diagram clearly shows a greater differentiation in energy delivered at the low discharge


**Table 5.** Utilization of active material (Mn(IV) percentage) for heat treated EMD samples at various discharge currents (2.0 V cut-off).

**Figure 12.** Ragone diagram showing points corresponding to energy delivered by the cell by the cut-off voltage and the corresponding power value for the heat treated EMD samples with respect to discharge rates.

As the electrochemical behaviour of manganese dioxide is largely dependent on the structure, composition and morphology of the material, the changes in these parameters with heat treatment temperature are invoked in explaining their relative electrochemical performance. Since the performance is not directly proportional to the heating temperature, it is immediately evident that there are a number of factors influencing material performance. From the current understanding of the Li/MnO2 system, these factors are outlined as follows:

#### 292 Heat Treatment – Conventional and Novel Applications

i. It is well known that the removal of both surface and structural water from the manganese dioxide structure is paramount, and that higher heating temperatures perform this function most completely and effectively [22].

Monitoring the Effects of Thermal

Treatment on Properties and Performance During Battery Material Synthesis 293

comparing the structural and morphological properties. Here, the sample heated at 400°C has maintained a larger unit cell and higher surface area compared to the 350°C material, most likely as a consequence of the shorter heating time applied to this material. This difference is likely responsible for its slightly lower performance at 2 mA/g (233 mA/g compared to 252-3 mA/g for the 300°C and 350°C materials). It is also possible that the conversion of γ-MnO2 to a more pyrolusite-like structure as a consequence of the high heating temperature has been limiting on the performance. In conjunction with the low performance for the 200°C material, this may suggest that the optimum γ/β-MnO2 structure for lithium insertion is not highly ramsdellitic or pyrolusitic structures, but rather some intermediate structure. From this, we propose an optimum value of Pr for HEMD at low discharge rates, lying somewhere between 0.5 and 0.84, which leads to superior performance. At high rate discharge however, the 400°C material capacity is similar to the other materials, suggesting the differences in material properties have become more or less

The capacity of the material heat treated at 250°C (243 mAh/g) is higher the 400°C material (233 mAh/g) and slightly below the 300°C and 350°C materials (252-3 mAh/g) at the 2 mA/g discharge rate, but is essentially equal to them at high rates (~180 mAh/g at 20 mA/g). This is despite the fact that this material still contains reasonable amounts of structural water associated with both cation vacancies and Mn(III). The decreased capacity at the low rate can likely be attributed to the moderate retention of the γ-MnO2 structure (Pr=0.52) which may limit the diffusion of lithium through the structure, although evidently to a lesser degree than in the 200°C material or the highly pyrolusitic 400°C structure. Additional differences between the 250°C material and those around it can be clearly seen in Figure 10. Noticeably, although the unit cell volume has not varied significantly between the 200°C and 250°C materials, the surface area has decreased by around 10%. This is largely due to a lower micropore volume in the 250°C material (Table 1). This feature would likely aid the discharge of this material since the small micro-pores can only accommodate a limited number of lithium ions. Hence, when the cell is discharged, the few ions in these pores are soon inserted into the structure. The subsequent deficiency of lithium ions in this locality

limits the discharge capabilities of the cell, especially at high discharge rates.

performance at high rates.

The materials heat treated at 300°C and 350°C demonstrated the highest capacity over the range of discharge rates tested. Relating this back to the properties, the 300°C material had a moderate amount of defects remaining in the structure, with a pyrolusite to ramsdellite ratio of 0.73. The particular structural arrangement is likely of particular significance in light of the poorer performance of materials with low or high Pr values (e.g., 200°C and 400°C materials, respectively). Also of interest are the relationships between the structural and morphological features of the 300°C sample (Figure 10). This data suggests that this material has undergone significant amounts of structural rearrangement (shown by a smaller unit cell), but maintained a relatively high surface area (31 m2/g). The combination of the structural arrangement, composition and surface area has resulted in this material delivering one of the highest capacities at low discharge rates, with comparable

immaterial in determining the electrochemical performance.


In explaining the relative performance of the HEMD tested, each material will be considered separately and the above factors used to explain the resulting electrochemical performance. Some comparison will also be made highlighting differences between materials heat treated at consecutive temperatures and the effect these differences have had on the electrochemical performance. In the proceeding discussion, those materials exhibiting poorest performance will be discussed first, followed by the HEMD samples with better electrochemical characteristics.

From the physical characterisation of the 200°C material, it is clear that it has retained much of its original BET surface area, experienced some removal of cation vacancies, lost only a small amount of structural water, and maintained to a large extent the γ-MnO2 parent structure. Of these features, the presence of structural water associated with cation vacancies and Mn(III) will be a limiting factor on the electrochemical performance. The water present evidently makes for poor performance at all discharge rates (on average 15% lower capacity than the 250°C material), highlighting the importance of its removal prior to use in Li/MnO2 system. Perhaps an additional feature that limits the performance of this material is the relative ease of lithium ion movement in γ/β-MnO2. While fundamental work has been performed analysing the mechanism and activation energy for proton diffusion in MnO2 structures [25], these relationships are currently unknown for lithium diffusion in EMD/HEMD. The data presented here may indicate slower lithium ion diffusion through less pyrolusitic structures.

The properties of the 400°C material are very similar to the sample heat treated at 350°C; i.e., both samples have had basically all cation vacancies removed, with minor, but similar, amounts of structural water remaining, and both have experienced a high structural conversion to pyrolusite. However, an interesting difference is apparent in Figure 10, when comparing the structural and morphological properties. Here, the sample heated at 400°C has maintained a larger unit cell and higher surface area compared to the 350°C material, most likely as a consequence of the shorter heating time applied to this material. This difference is likely responsible for its slightly lower performance at 2 mA/g (233 mA/g compared to 252-3 mA/g for the 300°C and 350°C materials). It is also possible that the conversion of γ-MnO2 to a more pyrolusite-like structure as a consequence of the high heating temperature has been limiting on the performance. In conjunction with the low performance for the 200°C material, this may suggest that the optimum γ/β-MnO2 structure for lithium insertion is not highly ramsdellitic or pyrolusitic structures, but rather some intermediate structure. From this, we propose an optimum value of Pr for HEMD at low discharge rates, lying somewhere between 0.5 and 0.84, which leads to superior performance. At high rate discharge however, the 400°C material capacity is similar to the other materials, suggesting the differences in material properties have become more or less immaterial in determining the electrochemical performance.

292 Heat Treatment – Conventional and Novel Applications

i. It is well known that the removal of both surface and structural water from the manganese dioxide structure is paramount, and that higher heating temperatures

ii. A greater amount of the more electrochemically active Mn(IV) species (compared to Mn(III)) is advantageous, and therefore materials in which oxidation of the Mn(III) to

iii. Samples retaining the γ-MnO2 structure, with larger [1×2] tunnels in ramsdellite regions of the structure, are thought to provide both a greater degree of mobility and a larger number of insertion sites for ions introduced into the structure during discharge [25]. These relationships certainly hold for the aqueous system, although as yet there is currently no clear indication in the literature relating these structural features to the ease of lithium diffusion through materials used in the non-aqueous Li/MnO2 system. iv. The effect of sample porosity on the discharge characteristics of Li/MnO2 cells remains relatively unknown. However, it has been previously determined that during heat treatment micro-pores within the EMD are sintered shut as a result of manganese ion movement, while the structural contraction connected with the phase transition from γ→β-MnO2 and the increase in material density, induces cracking [18]. This results in an increase in the number of larger pores. These factors will be especially influential on

In explaining the relative performance of the HEMD tested, each material will be considered separately and the above factors used to explain the resulting electrochemical performance. Some comparison will also be made highlighting differences between materials heat treated at consecutive temperatures and the effect these differences have had on the electrochemical performance. In the proceeding discussion, those materials exhibiting poorest performance will be discussed first, followed by the HEMD samples with better electrochemical

From the physical characterisation of the 200°C material, it is clear that it has retained much of its original BET surface area, experienced some removal of cation vacancies, lost only a small amount of structural water, and maintained to a large extent the γ-MnO2 parent structure. Of these features, the presence of structural water associated with cation vacancies and Mn(III) will be a limiting factor on the electrochemical performance. The water present evidently makes for poor performance at all discharge rates (on average 15% lower capacity than the 250°C material), highlighting the importance of its removal prior to use in Li/MnO2 system. Perhaps an additional feature that limits the performance of this material is the relative ease of lithium ion movement in γ/β-MnO2. While fundamental work has been performed analysing the mechanism and activation energy for proton diffusion in MnO2 structures [25], these relationships are currently unknown for lithium diffusion in EMD/HEMD. The data presented

here may indicate slower lithium ion diffusion through less pyrolusitic structures.

The properties of the 400°C material are very similar to the sample heat treated at 350°C; i.e., both samples have had basically all cation vacancies removed, with minor, but similar, amounts of structural water remaining, and both have experienced a high structural conversion to pyrolusite. However, an interesting difference is apparent in Figure 10, when

perform this function most completely and effectively [22].

Mn(IV) has occurred will exhibit better performance.

the rate capabilities and total capacity of the cell [26].

characteristics.

The capacity of the material heat treated at 250°C (243 mAh/g) is higher the 400°C material (233 mAh/g) and slightly below the 300°C and 350°C materials (252-3 mAh/g) at the 2 mA/g discharge rate, but is essentially equal to them at high rates (~180 mAh/g at 20 mA/g). This is despite the fact that this material still contains reasonable amounts of structural water associated with both cation vacancies and Mn(III). The decreased capacity at the low rate can likely be attributed to the moderate retention of the γ-MnO2 structure (Pr=0.52) which may limit the diffusion of lithium through the structure, although evidently to a lesser degree than in the 200°C material or the highly pyrolusitic 400°C structure. Additional differences between the 250°C material and those around it can be clearly seen in Figure 10. Noticeably, although the unit cell volume has not varied significantly between the 200°C and 250°C materials, the surface area has decreased by around 10%. This is largely due to a lower micropore volume in the 250°C material (Table 1). This feature would likely aid the discharge of this material since the small micro-pores can only accommodate a limited number of lithium ions. Hence, when the cell is discharged, the few ions in these pores are soon inserted into the structure. The subsequent deficiency of lithium ions in this locality limits the discharge capabilities of the cell, especially at high discharge rates.

The materials heat treated at 300°C and 350°C demonstrated the highest capacity over the range of discharge rates tested. Relating this back to the properties, the 300°C material had a moderate amount of defects remaining in the structure, with a pyrolusite to ramsdellite ratio of 0.73. The particular structural arrangement is likely of particular significance in light of the poorer performance of materials with low or high Pr values (e.g., 200°C and 400°C materials, respectively). Also of interest are the relationships between the structural and morphological features of the 300°C sample (Figure 10). This data suggests that this material has undergone significant amounts of structural rearrangement (shown by a smaller unit cell), but maintained a relatively high surface area (31 m2/g). The combination of the structural arrangement, composition and surface area has resulted in this material delivering one of the highest capacities at low discharge rates, with comparable performance at high rates.

Finally, as we have already noted the material properties of the 350°C material are similar to that of the 400°C sample in terms of composition, although key differences are noted in the structure and morphology, as highlighted in Figure 10. These differences are significant in light of the superior electrochemical characteristics of the 350°C material. Figure 10 shows that heat treatment at 350°C has resulted in the greatest contraction of the unit cell and also a lowering of the BET surface area relative to the other HEMD materials. The smaller unit cell is the main structural difference between the 350°C and 400°C materials, which otherwise have very similar Pr values (0.83 and 0.84, respectively). This suggests that the proportion of each phase is not the only structural feature influencing the movement of lithium ions through the material, and that the unit cell volume is also significant. The high performance of the 350°C material is an indication that lower unit cell volumes are favoured for lithium diffusion in HEMD. The morphology differences between these materials are also likely to play a significant role in determining their relative electrochemical performance. As previously noted, the lower BET surface area for the 250°C sample due to a lower volume of micropores, was found to enhance its capacity over the 200°C material. Similarly, the change in morphology brought about by the removal of these micropores in the 350°C material has clearly improved the material capacity across discharge rates.

Monitoring the Effects of Thermal

Treatment on Properties and Performance During Battery Material Synthesis 295

conversion from γ-MnO2 towards β-MnO2. The material surface area generally decreases with heating temperature, although noticeably, the pore collapse process is thermally limited at lower temperatures, while at the higher temperatures the reaction kinetics dictate the extent to which this process proceeds. Despite the theoretical expectation of complete water removal from the EMD, experimental results demonstrate that thermal and kinetic limitations present under these heating conditions prevent the completion of the water loss process. In general, however, as heating temperature increases there is a decrease in cation vacancy fraction and amount of structural water, while Mn(III) and Mn(IV) increase and

Electrochemical performance of the HEMD materials generated in this work revealed they are superior in terms of specific energy, and comparable in terms of specific power, to recent reports in the literature for the primary Li/MnO2 system. Greatest differentiation of materials was noted at the low discharge rates, where materials prepared at 300°C and

Finally, it is important to note that the method of kinetic analysis outlined here is not limited to only mass loss processes. It can in fact be employed to monitor the progress of any thermally based reaction which undergoes changes measurable by TG, DTA, DSC, etc. Therefore, the method of kinetic analysis outlined here could be applied in optimising the preparation and electrochemical performance of numerous other positive electroactive materials with similar

The authors would like to acknowledge the University of Newcastle EM-X-ray Unit and Ms. Jenny Zobec for assistance in obtaining the XRD data. WMD acknowledges the University of

[1] Lewis N.S (2005) Scientific Challenges in Sustainable Energy Technology, Keynote

[4] Simon P, Gogotsi Y (2008) Materials for Electrochemical Capacitors. Nat. mater. 7: 845-854. [5] Linden D, Reddy T.B, editors (2001) Handbook of Batteries. 3rd ed. USA: McGraw-Hill. [6] Whittingham M.S (2004) Lithium Batteries and Cathode Materials. Chem. rev. 104:

[2] Ristinen R.A, Kraushaar J.P (2005) Energy and the Environment. 2nd Ed. USA: Wiley. [3] Ragone D (1968) Review of Battery Systems for Electrically Powered Vehicles. SAE

remain relatively steady, respectively.

350°C demonstrated best performance.

improvements to performance anticipated.

*Discipline of Chemistry, University of Newcastle, Callaghan, Australia* 

address at 208th meeting elecrochem. soc., Los Angeles.

Newcastle for the provision of an APA PhD scholarship.

Technical Paper 680453. doi:10.4271/680453.

Wesley M. Dose and Scott W. Donne

**Author details** 

**Acknowledgement** 

**6. References** 

4271-4301.
