**4. Results and discussions**

## **4.1. Surface morphology**

#### *4.1.1. Transmission electron microscopy (TEM)*

**Figure 1a–d** shows the TEM micrographs of the crystalline LiMn<sup>2</sup> O4 spinel nanowires in the range of 5–20 nm without agglomeration. The wires appear to be highly crystalline and moderately dispersed, which causes the material to possess a larger surface area. These nanowires have diameters of tens of nanometers and lengths up to several micrometers and basically retains the morphology of the precursor MnO<sup>2</sup> nanowires. At higher magnification, **Figure 1c**, nanowires adhering together are observed, which leads to the notion that these nanowires have a fiber-brush aspect. **Figure 1b** is a typical high-resolution electron microscopy (HREM) image clearly displaying the lattice fringes of the material. The SAED patterns along [100] direction of the single crystals are showed in the inset. Bright diffraction spots including (022), (222) and (004) in the [100] zone are generated from the spinel structure with *Fd3m* space group. The images also show single LiMn<sup>2</sup> O4 nanowires with distinctive lattice fringes. The discriminable lattice fringes illustrate that the prepared nanowires are single crystals in the area shown. **Figure 2** shows a typical TEM image at a magnification of 2 and 20 nm of LiMn<sup>2</sup> O4 . The primary particle size of the as-synthesized powders is around 10 nm with visible agglomerations. The as-synthesized and calcined powders typically have surface areas of 18.0 m<sup>2</sup> g−1 measured by BET method [11].

LiMn<sup>2</sup> O4

nano-LiMn<sup>2</sup>

O4

**Figure 1.** High resolution TEM image of crystalline LiMn<sup>2</sup>

nanopowders in Ar atmosphere. The images reveal apparent changes in roughness

nanowires.

Analysis of Electrochemical and Structurally Enhanced LiMn2O4 Nanowire Cathode System

http://dx.doi.org/10.5772/0

43

particles (a–b). Evidently, a rough structure with closely distributed micro-

O4

between the films. It is known that the distribution of particles influences the cyclability and discharge capacity [12]. Tapping mode AFM images displayed the surface morphology of

O4

pores of less than 5 nm in diameter was observed. From the two-dimensional image, it was evident that this surface yields a large degree of surface roughness (a). A more detailed analysis of the particle size is shown in the histogram of (a, b-i). Particle sizing and metrics play a critical role in determining battery capacity and performance. Therefore, the typical size of materials used for battery construction is >1 μm [13]. Here, the size distribution moved toward an average diameter of 60 nm aiding the high-rate capabilities of the cathode. For LiMn<sup>2</sup>

several "cauliflower-like" areas are observed with a rms (root mean square) roughness of

#### *4.1.2. Atomic force microscopy (AFM)*

The surfaces of the cathode were observed by high-resolution atomic force microscopy in dried state. **Figure 3** shows 2 × 2 μm<sup>2</sup> dimensional AFM images of LiMn<sup>2</sup> O4 nanowires and the Analysis of Electrochemical and Structurally Enhanced LiMn2O4 Nanowire Cathode System http://dx.doi.org/10.5772/0 43

**Figure 1.** High resolution TEM image of crystalline LiMn<sup>2</sup> O4 nanowires.

external reference for chemical. XRD Measurements were carried out with a D8 ADVANCE diffractometer from BRUKER axs using an X-ray tube with copper K-alpha radiation operated at 40 kV and 40 mA and a position sensitive detector, Vantec\_1, which enables fast data acquisition. Measurement range: [12–90° in 2 theta], Step size: 0.027° in 2 theta, Measurement

mounted on coin-cells operating at 30°C. Electrode mixtures were prepared by mixing the oxide powder (70 wt%) with acetylene black (current collector) (20 wt%) and polyvinylidene fluoride (PVdF) binder (10 wt%) in N-methylpyrrolidone (NMP) solvent to form a mixed slurry. The slurry was coated on an aluminum foil, followed by drying in a vacuum oven at

range of 5–20 nm without agglomeration. The wires appear to be highly crystalline and moderately dispersed, which causes the material to possess a larger surface area. These nanowires have diameters of tens of nanometers and lengths up to several micrometers and basically

nanowires adhering together are observed, which leads to the notion that these nanowires have a fiber-brush aspect. **Figure 1b** is a typical high-resolution electron microscopy (HREM) image clearly displaying the lattice fringes of the material. The SAED patterns along [100] direction of the single crystals are showed in the inset. Bright diffraction spots including (022), (222) and (004) in the [100] zone are generated from the spinel structure with *Fd3m*

O4

. The primary particle size of the as-synthesized powders is around 10 nm with vis-

dimensional AFM images of LiMn<sup>2</sup>

The discriminable lattice fringes illustrate that the prepared nanowires are single crystals in the area shown. **Figure 2** shows a typical TEM image at a magnification of 2 and 20 nm of

ible agglomerations. The as-synthesized and calcined powders typically have surface areas of

The surfaces of the cathode were observed by high-resolution atomic force microscopy in

nanowire cathode, were carried out with electrodes

O4

nanowires. At higher magnification, **Figure 1c**,

nanowires with distinctive lattice fringes.

O4

nanowires and the

spinel nanowires in the

O4

time: 1 s/step.

**3.1. Electrochemical tests**

The Electrochemistry of the LiMn<sup>2</sup>

42 Nanowires - Synthesis, Properties and Applications

**4. Results and discussions**

*4.1.1. Transmission electron microscopy (TEM)*

retains the morphology of the precursor MnO<sup>2</sup>

space group. The images also show single LiMn<sup>2</sup>

18.0 m<sup>2</sup> g−1 measured by BET method [11].

*4.1.2. Atomic force microscopy (AFM)*

dried state. **Figure 3** shows 2 × 2 μm<sup>2</sup>

**Figure 1a–d** shows the TEM micrographs of the crystalline LiMn<sup>2</sup>

**4.1. Surface morphology**

LiMn<sup>2</sup> O4

120°C for 48 h and a cathode electrode was formed.

LiMn<sup>2</sup> O4 nanopowders in Ar atmosphere. The images reveal apparent changes in roughness between the films. It is known that the distribution of particles influences the cyclability and discharge capacity [12]. Tapping mode AFM images displayed the surface morphology of nano-LiMn<sup>2</sup> O4 particles (a–b). Evidently, a rough structure with closely distributed micropores of less than 5 nm in diameter was observed. From the two-dimensional image, it was evident that this surface yields a large degree of surface roughness (a). A more detailed analysis of the particle size is shown in the histogram of (a, b-i). Particle sizing and metrics play a critical role in determining battery capacity and performance. Therefore, the typical size of materials used for battery construction is >1 μm [13]. Here, the size distribution moved toward an average diameter of 60 nm aiding the high-rate capabilities of the cathode. For LiMn<sup>2</sup> O4 several "cauliflower-like" areas are observed with a rms (root mean square) roughness of

*4.1.3. Vibrational structure analysis (Raman/SS-NMR)*

**Figure 4** shows the Raman spectra (RS) of LiMn<sup>2</sup>

active tetragonal hausmannite (Mn<sup>3</sup>

containing lithium is characteristic of the <sup>7</sup>

**Figure 4.** Vibrational spectra of LiMn<sup>2</sup>

O4

nanowires and LiMn<sup>2</sup>

O4

precursor (inset).

Mn-O vibration of MnO6

**Figure 5** shows the <sup>7</sup>

LiMn<sup>2</sup> O4 O4

of photons and a noticeable decrease in the peak width is observed with modification. The bands at ~560 and ~660 cm−1 are attributed to the O-Mn-O bending and stretching modes, respectively. Some vibrational or rotational transitions, which exhibit low polarizability, becomes Raman inactive therefore sharper peaks signifies better crystallinity and less cation mixing [14]. The observation of narrower bands and less modes may be the result of no translation invariance and lattice distortion around the Mn3+ and Mn4+ cations. The catalytically

633 cm−1, conforming to the Mn-O breathing vibration of Mn2+ ions in tetrahedral coordination

and 492 cm−1 respectively. As discussed by Julien et al. [16], the A1g mode correlated with

Nuclear magnetic resonance (NMR) spectroscopy has been employed as a significant tool to probe the local structure and dynamics of these nano materials. Here we show how this technique help understand the origins of the performance of the given nanomaterial [17].

scale is fast between these ions in comparison to the NMR time scale (ca. 10−5 s) and therefore the lithium spins detect a manganese oxidation state corresponding to 3.5 (i.e., "Mn3.5+" ions), pertaining to only one magnetically in equivalent lithium site (the 8a site) [12]. This permits the NMR spectroscopy to be used as a means to follow the partial charge-ordering process. The hyperfine shift of >500 ppm from the chemical shift range of diamagnetic compounds

is a hopping semiconductor containing both Mn3+ and Mn4+ ions. The hopping time

O4

(A1g mode) [15]. The T2 g (1), Eg, and T2 g (2) modes of Mn<sup>3</sup>

increases. The Raman data are in agreement with diffraction analysis.

Li-NMR spectrums of both spinel LiMn<sup>2</sup>

and LiMn<sup>2</sup>

Analysis of Electrochemical and Structurally Enhanced LiMn2O4 Nanowire Cathode System

O4

) spinel is acknowledged by the solid peak erected at

and LiMn<sup>2</sup>

O4

O4

groups will shift to lower energies as the average Mn oxidation state

Li MAS NMR spectrum of LiMn<sup>2</sup>

nanowires.

is shown by peaks 300, 347,

nanowires.

O4

O4 . A Blue shift

45

http://dx.doi.org/10.5772/0

**Figure 2.** TEM images of LiMn<sup>2</sup> O4 nanoparticles.

**Figure 3.** AFM images of LiMn<sup>2</sup> O4 shown in tapping mode in three-dimensional views at 600 nm and particle size distribution histogram (i) of LiMnO<sup>4</sup> (a) and LiMn<sup>2</sup> O4 nanowires (b), respectively.

73.5 Å, whilst LiMn<sup>2</sup> O4 nanowires topography appears smoother and homogeneous with rms roughness of 26.3 Å which can be calculated using Eq. (1). These features constitute the stability of the spinel structure, which enhances the electrochemical properties.

$$R\_{\mathbf{q}} = \sqrt{\frac{1}{n} \sum\_{i=1}^{n} y\_i^2} \tag{1}$$

#### *4.1.3. Vibrational structure analysis (Raman/SS-NMR)*

**Figure 4** shows the Raman spectra (RS) of LiMn<sup>2</sup> O4 and LiMn<sup>2</sup> O4 nanowires. A Blue shift of photons and a noticeable decrease in the peak width is observed with modification. The bands at ~560 and ~660 cm−1 are attributed to the O-Mn-O bending and stretching modes, respectively. Some vibrational or rotational transitions, which exhibit low polarizability, becomes Raman inactive therefore sharper peaks signifies better crystallinity and less cation mixing [14]. The observation of narrower bands and less modes may be the result of no translation invariance and lattice distortion around the Mn3+ and Mn4+ cations. The catalytically active tetragonal hausmannite (Mn<sup>3</sup> O4 ) spinel is acknowledged by the solid peak erected at 633 cm−1, conforming to the Mn-O breathing vibration of Mn2+ ions in tetrahedral coordination (A1g mode) [15]. The T2 g (1), Eg, and T2 g (2) modes of Mn<sup>3</sup> O4 is shown by peaks 300, 347, and 492 cm−1 respectively. As discussed by Julien et al. [16], the A1g mode correlated with Mn-O vibration of MnO6 groups will shift to lower energies as the average Mn oxidation state increases. The Raman data are in agreement with diffraction analysis.

**Figure 5** shows the <sup>7</sup> Li-NMR spectrums of both spinel LiMn<sup>2</sup> O4 and LiMn<sup>2</sup> O4 nanowires.

Nuclear magnetic resonance (NMR) spectroscopy has been employed as a significant tool to probe the local structure and dynamics of these nano materials. Here we show how this technique help understand the origins of the performance of the given nanomaterial [17]. LiMn<sup>2</sup> O4 is a hopping semiconductor containing both Mn3+ and Mn4+ ions. The hopping time scale is fast between these ions in comparison to the NMR time scale (ca. 10−5 s) and therefore the lithium spins detect a manganese oxidation state corresponding to 3.5 (i.e., "Mn3.5+" ions), pertaining to only one magnetically in equivalent lithium site (the 8a site) [12]. This permits the NMR spectroscopy to be used as a means to follow the partial charge-ordering process. The hyperfine shift of >500 ppm from the chemical shift range of diamagnetic compounds containing lithium is characteristic of the <sup>7</sup> Li MAS NMR spectrum of LiMn<sup>2</sup> O4 .

**Figure 4.** Vibrational spectra of LiMn<sup>2</sup> O4 nanowires and LiMn<sup>2</sup> O4 precursor (inset).

73.5 Å, whilst LiMn<sup>2</sup>

**Figure 3.** AFM images of LiMn<sup>2</sup>

distribution histogram (i) of LiMnO<sup>4</sup>

**Figure 2.** TEM images of LiMn<sup>2</sup>

44 Nanowires - Synthesis, Properties and Applications

O4

nanoparticles.

O4

O4

nanowires topography appears smoother and homogeneous with rms

nanowires (b), respectively.

shown in tapping mode in three-dimensional views at 600 nm and particle size

(1)

roughness of 26.3 Å which can be calculated using Eq. (1). These features constitute the stabil-

ity of the spinel structure, which enhances the electrochemical properties.

O4

(a) and LiMn<sup>2</sup>

**Figure 5.** LiMn<sup>2</sup> O4 nanowire NMR spectrum at 13 kHz (a) and 16 kHz (b) spinning speed, magnetic field strength of 11.7 T and resonance frequency of 194.29 MHz. LiMn<sup>2</sup> O4 nanopowder (inset).

unchallenging Li<sup>+</sup>

**Figure 6.** XRD spectra of LiMn<sup>2</sup>

**4.2. Electrochemical analysis**

*4.2.1. Redox reaction analysis*

O4

O4

curve of LiMn<sup>2</sup>

[31]. The LiMn<sup>2</sup>

) depicts high crystallinity [24].

O4

oxide particles and reduce the polarization of the LiMn<sup>2</sup>

ciencies after cycling for the nanowires and LiMn<sup>2</sup>

shows an increased discharge current densities due to improvement of Li<sup>+</sup>

LiMn<sup>2</sup> O4 pass through the coating layer more during charging and discharging pro-

Analysis of Electrochemical and Structurally Enhanced LiMn2O4 Nanowire Cathode System

(b).

O4

electrode. The nanowire cathode

O4

were 146 mAh g−1 / 99% and 122 mAh g−1/

diffusion pathway

http://dx.doi.org/10.5772/0

47

spinel, due to the lower

cess as the diffusion path is unhindered [22, 23]. The strong peak at 2θ = 18. 76°, corresponds to a (111) peak with an interplanar distance of *d* = 0.476 nm. The full width at half-maximum (FWHM) becoming narrower is also due to higher synthesis temperatures which help to enhance the mobility of atoms. Sharp and relatively high intensity peaks (as compared to

nanowires (a) and alfa-MnO<sup>2</sup>

To clarify the kinetic behavior of lithium-ion transfer, the discharge tests were carried out at. **Figure 7** shows the effect of discharge current densities on the capacities of the two types of electrodes. It is well known that the nanowire electrodes have good electronic conductivity; therefore, they can greatly increase the electrical conductivity among the transition metal

[25–27]. This coincides with prior research that has recognized the ability of nanowire architecture to enhance the stability or cyclability of electrode materials [28]. The initial discharge

ratio of Mn3+/Mn4+ ions [29]. The initial discharge capacities and corresponding coulombic effi-

70%, respectively. The decrease in capacity over subsequent cycles is explained by the change in surface area [30]. Three-dimensional porous nanostructures with large surface area could exhibit higher durability in the lithium insertion/ extraction process at a high current density, owing to the short lithium ion diffusion lengths in the 3-D channels of the electrode

The large capacities are due to the nanowire morphology, stability and the high quality of the single crystal, which can shorten the diffusion lengths of both the lithium and electrons [32].

O4

nanowire electrode exhibited excellent rate capability as shown in **Figure 7**.

nanowire electrode was similar to that of LiMn<sup>2</sup>

The spinel LiMn<sup>2</sup> O4 nanowires have a distinctly different spectral line shape as compared to their powder form [18]. The isotropic resonance at 511 ppm for LiMnO<sup>4</sup> is assigned to lithium ions in the tetrahedral 8a site, whilst the isotropic resonances at ~680 and 835 ppm for LiMn<sup>2</sup> O4 nanowires is assigned to lithium present in the proximity of higher oxidation state manganese ions (Mn4+). The latter is generally originated from vacancies on both the lithium and manganese sites (i.e., Mn 16*d* sites and the interstitial 16*c* sites or Li-for-Mn substitutions) [19]. The resonance at 445 ppm is ascribed to the presence of Mn3+ ions in the Li local coordination sphere. Mn3+ is a *Jahn–Teller (distortion)* active ion; therefore there will be a distortion of the octahedron in this case [20]. The enhancement of spinning sideband manifold for LiMn<sup>2</sup> O4 nanowires is caused by the increased portion of paramagnetic manganese around lithium [21]. Hence, it can be suggested that the lithium atoms are interacted with the manganese and cause for better electrochemical performance.

#### *4.1.4. X-ray diffraction microscopic analysis (XRD)*

The lattice constant was calculated from the corresponding diffraction pattern using XRD spectra, in relation to the crystal structure and is reported in **Figure 6**. All diffraction peaks can be assigned to the diffraction indices of LiMn<sup>2</sup> O4 spinel (JCPDS file no. 35-782), indicating that the structure of the spinel was maintained. The majority diffraction peaks of LiMnO<sup>2</sup> are observed, and closely correspond to layered LiMnO<sup>2</sup> (011), (202) and (111) planes. The intensive diffraction peaks appeared at 21.92, 36.92, 42.42 and 55.96°, respectively, should be assigned to the characteristic peaks for γ-MnO<sup>2</sup> , and the peaks occurred at 17.94, 28.78, 66.0°, respectively, should be ascribed to the characteristic peaks for α-MnO<sup>2</sup> . Hence, the sample appears to be composed of both γ and α-MnO<sup>2</sup> . From the broadening of XRD peaks, it can be said that nanowires are being formed. This suggest that the nanowires would render Analysis of Electrochemical and Structurally Enhanced LiMn2O4 Nanowire Cathode System http://dx.doi.org/10.5772/0 47

**Figure 6.** XRD spectra of LiMn<sup>2</sup> O4 nanowires (a) and alfa-MnO<sup>2</sup> (b).

unchallenging Li<sup>+</sup> pass through the coating layer more during charging and discharging process as the diffusion path is unhindered [22, 23]. The strong peak at 2θ = 18. 76°, corresponds to a (111) peak with an interplanar distance of *d* = 0.476 nm. The full width at half-maximum (FWHM) becoming narrower is also due to higher synthesis temperatures which help to enhance the mobility of atoms. Sharp and relatively high intensity peaks (as compared to LiMn<sup>2</sup> O4 ) depicts high crystallinity [24].

#### **4.2. Electrochemical analysis**

#### *4.2.1. Redox reaction analysis*

The spinel LiMn<sup>2</sup>

O4

**Figure 5.** LiMn<sup>2</sup>

LiMn<sup>2</sup> O4 O4

46 Nanowires - Synthesis, Properties and Applications

11.7 T and resonance frequency of 194.29 MHz. LiMn<sup>2</sup>

cause for better electrochemical performance.

*4.1.4. X-ray diffraction microscopic analysis (XRD)*

can be assigned to the diffraction indices of LiMn<sup>2</sup>

be assigned to the characteristic peaks for γ-MnO<sup>2</sup>

sample appears to be composed of both γ and α-MnO<sup>2</sup>

are observed, and closely correspond to layered LiMnO<sup>2</sup>

nanowires have a distinctly different spectral line shape as compared

nanowire NMR spectrum at 13 kHz (a) and 16 kHz (b) spinning speed, magnetic field strength of

nanopowder (inset).

is assigned to

O4

to their powder form [18]. The isotropic resonance at 511 ppm for LiMnO<sup>4</sup>

O4

lithium ions in the tetrahedral 8a site, whilst the isotropic resonances at ~680 and 835 ppm for

manganese ions (Mn4+). The latter is generally originated from vacancies on both the lithium and manganese sites (i.e., Mn 16*d* sites and the interstitial 16*c* sites or Li-for-Mn substitutions) [19]. The resonance at 445 ppm is ascribed to the presence of Mn3+ ions in the Li local coordination sphere. Mn3+ is a *Jahn–Teller (distortion)* active ion; therefore there will be a distortion of the octahedron in this case [20]. The enhancement of spinning sideband manifold for LiMn<sup>2</sup>

nanowires is caused by the increased portion of paramagnetic manganese around lithium [21]. Hence, it can be suggested that the lithium atoms are interacted with the manganese and

The lattice constant was calculated from the corresponding diffraction pattern using XRD spectra, in relation to the crystal structure and is reported in **Figure 6**. All diffraction peaks

that the structure of the spinel was maintained. The majority diffraction peaks of LiMnO<sup>2</sup>

intensive diffraction peaks appeared at 21.92, 36.92, 42.42 and 55.96°, respectively, should

it can be said that nanowires are being formed. This suggest that the nanowires would render

66.0°, respectively, should be ascribed to the characteristic peaks for α-MnO<sup>2</sup>

O4

spinel (JCPDS file no. 35-782), indicating

, and the peaks occurred at 17.94, 28.78,

. From the broadening of XRD peaks,

(011), (202) and (111) planes. The

. Hence, the

nanowires is assigned to lithium present in the proximity of higher oxidation state

To clarify the kinetic behavior of lithium-ion transfer, the discharge tests were carried out at. **Figure 7** shows the effect of discharge current densities on the capacities of the two types of electrodes. It is well known that the nanowire electrodes have good electronic conductivity; therefore, they can greatly increase the electrical conductivity among the transition metal oxide particles and reduce the polarization of the LiMn<sup>2</sup> O4 electrode. The nanowire cathode shows an increased discharge current densities due to improvement of Li<sup>+</sup> diffusion pathway [25–27]. This coincides with prior research that has recognized the ability of nanowire architecture to enhance the stability or cyclability of electrode materials [28]. The initial discharge curve of LiMn<sup>2</sup> O4 nanowire electrode was similar to that of LiMn<sup>2</sup> O4 spinel, due to the lower ratio of Mn3+/Mn4+ ions [29]. The initial discharge capacities and corresponding coulombic efficiencies after cycling for the nanowires and LiMn<sup>2</sup> O4 were 146 mAh g−1 / 99% and 122 mAh g−1/ 70%, respectively. The decrease in capacity over subsequent cycles is explained by the change in surface area [30]. Three-dimensional porous nanostructures with large surface area could exhibit higher durability in the lithium insertion/ extraction process at a high current density, owing to the short lithium ion diffusion lengths in the 3-D channels of the electrode [31]. The LiMn<sup>2</sup> O4 nanowire electrode exhibited excellent rate capability as shown in **Figure 7**. The large capacities are due to the nanowire morphology, stability and the high quality of the single crystal, which can shorten the diffusion lengths of both the lithium and electrons [32].

All the as-prepared powders were identified as a single phase of cubic spinel structure indicative of an unobstructed Li ion diffusion pathway. The results have shown that thin-nanowire precursor morphology is preserved after the solid-state reaction. Such morphology improves the kinetic properties at very high current rate and was capable of the facile structural transformation of the cubic and tetragonal phase in the large compositional range. The LiMn<sup>2</sup>

Analysis of Electrochemical and Structurally Enhanced LiMn2O4 Nanowire Cathode System

nanowires showed a decrease in potential difference, indicating an improved charge transportation process. The nanostructures aid structural stability, reduction of side reactions and Mn dissolution between the interface of the cathode and electrolyte, which contributes to the recovering performance. Moreover, the nanowire cathode system has great potential for improving the electrode-filled ratio and safety in lithium ion battery–operating electronic

Natasha Ross is grateful to the National Research Foundation (NRF) for the award of the Department of Science and Technology's Innovation Postgraduate Scholarship for the

[1] Marom R, Amalraj SF, Leifer N, Jacob D, Aurbach D. A review of advanced and practical

[2] Kim YS, Kanoh H, Horotsu T, Ooi K. Chemical bonding of ion-exchange type sites in

[3] Thackeray MM, David WIF, Goodenough JB. Lithium insertion into manganese spinels.

. Materials Research Bulletin. 2002;**37**:391-396

lithium battery materials. Journal of Materials Chemistry. 2011;**21**:9938

devices, in transportation applications, and in applications on the electric grid.

**Acknowledgements**

**Conflict of interest**

**Author details**

**References**

The author declares that there is no conflict of interest.

Natasha Ross\*, Shane Willenberg and Emmanuel Iwuoha

University of the Western Cape, Cape Town, South Africa

spinel-type manganese oxides Li1.33Mn1.67O4

Materials Research Bulletin. 1983;**18**:641-647

\*Address all correspondence to: nross@uwc.ac.za

research grant.

O4

49

http://dx.doi.org/10.5772/0

**Figure 7.** Discharge current densities at 0.1 mV s−1 for LiMn<sup>2</sup> O4 nanowires (a) and LiMn<sup>2</sup> O4 (b) in EC: DMC, 1 M LiPF6 for the (1st (a, b) and 50th (a-i, b-i) cycle).

**Figure 8.** Nyquist plot of LiMn<sup>2</sup> O4 (a) and LiMn<sup>2</sup> O4 nanowires (b).

This was further corroborated by electrochemical impedance spectroscopy results shown in **Figure 8**. The nanowires show a significant decrease in impedance due to their enhanced electrochemical diffusion processes.

#### **5. Conclusion**

In summary, the LiMn<sup>2</sup> O4 nanowires have proven excellent thermal stability for a hightemperature sintering process as well as a charge-discharge reversible stability and improved conductivity attained by their architecture, excellent crystallinity and decreased impedance. All the as-prepared powders were identified as a single phase of cubic spinel structure indicative of an unobstructed Li ion diffusion pathway. The results have shown that thin-nanowire precursor morphology is preserved after the solid-state reaction. Such morphology improves the kinetic properties at very high current rate and was capable of the facile structural transformation of the cubic and tetragonal phase in the large compositional range. The LiMn<sup>2</sup> O4 nanowires showed a decrease in potential difference, indicating an improved charge transportation process. The nanostructures aid structural stability, reduction of side reactions and Mn dissolution between the interface of the cathode and electrolyte, which contributes to the recovering performance. Moreover, the nanowire cathode system has great potential for improving the electrode-filled ratio and safety in lithium ion battery–operating electronic devices, in transportation applications, and in applications on the electric grid.
