# Electrochemical Ion Pumping Device for Blue Energy Recovery: Mixing Entropy Battery
## 5 Intercalation Materials as Cathode Electrode in MEB
### Na\({}_{4}Mn_{9}O_{18}\)  
The sodium manganese oxide has been considered as a promising cathode material in the Na\({}^{+}\) ion battery. The orthorhombic structure of sodium manganese oxides is dependent on the sodium content, and it is made of \(MnO_{5}\) square pyramids and \(MnO_{6}\) octahedral as shown in Figure 7. The manganese ions are located in two different environments, leading to three available sites in which two \(Na^{+}\) ions occupy the large s-shaped tunnels while another \(Na^{+}\) ion occupies a smaller tunnel [51], according to the follow equation [22]:  
\[18 Na_{2}Mn_{5}O_{10}+4 Na^{+}+4e^{-}\leftrightarrow 10 Na_{4}Mn_{9}O_{18} \tag{48}\]  
Table 3 shows the most typical battery materials and the principal parameters used in the MEB for operation and design of the electrochemical device. These parameters deliver the constructive form of the MEB. The volume of the test battery indicates that the technology is of laboratory scale and has not yet been tested on a pilot scale. All MEB use parallel plate electrodes in their design, and the electrode distance is also important because the internal resistance of electrolytes is reduced by decreasing the distance between electrodes [22].  
Table 4 shows information about the suspension preparation procedure for the active material in the cathode electrodes. The active material is mixed with a polyvinylidene fluoride (PVDF) as a binder, carbon black or graphite and N-Methyl-2-pyrrolidone (NMP), 1-Methyl-2-pyrrolidinone or dimethyl sulfoxide as solvent.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & \multirow{2}{*}{**BM**} & **Electrode** & **Volume** & **Range** & **Electrode** & **Range Density** \\  & & **Area [cm\({}^{2}\)]** & **MEB** & **Potential** & **Distance** & **Current [A/cm\({}^{2}\)]** & **N\({}^{\circ}\) Cycle** \\  & & & **[cm\({}^{3}\)]** & **[V]** & **[mm]** & & \\ \hline
[5] & LiFePO4 & 2 & 0.35 & 0–0.5 & 10 & \(\pm\)0.5 & 100 \\
[5] & Na\({}_{2}\)Mn\({}_{5}\)O\({}_{10}\) & 2 & 0.35 & - & 10 & \(\pm\)0.25 & 100 \\
[1] & Na\({}_{2}\)Mn\({}_{5}\)O\({}_{10}\) & 1 & - & - & 0.6 & - & - \\
[22] & Na\({}_{4}\)Mn\({}_{9}\)O\({}_{18}\) & 9 & 1.5 & - & 1.7 & \(\pm\)0.03 & 12 \\
[3] & Na\({}_{4}\)Mn\({}_{9}\)O\({}_{18}\) & 3.01 & 0.06 & 0–0.6 & 0.2 & \(\pm\)0.5 & 3 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Battery materials used as cathodes in MEBs.  
Figure 7: Schematic illustration of the crystal structure of \(Na_{4}Mn_{9}O_{18}\). The \(Mn\) atoms form alternated octahedron \(MnO_{6}\) and tetrahedron \(MnO_{4}\) to form a several channels where diffusion of \(Li^{+}\) ions occurs (MP-1094147).  
Table 5 shows information regarding the blue energy recovery obtained by various researchers using different active materials for the MEB.  
Many groups are focusing on a synthesis of new material electrodes with high specific capacity and cycling steadiness. Also, efforts have been directed to simplify electrode manufacturing processes. The most commonly used electrode materials have been modified by elemental doping, coating and composting with other materials like LiNiMnCoO\({}_{2}\)[52]. These imply high ion diffusivity, enhancement in conductivity, ionic mobility and high retention capacity during the charging/discharging process. According to the reaction mechanisms of electrode materials, the materials can be divided into two groups, namely insertion and conversion type materials [53]. The literature shows that insertion materials are more promising materials because a robust crystalline structure, enlarged diffusion channel and easier pathways for ion insertion and removal are achieved. These provide a long-term cycling stability and high rate capability in comparison with conversion electrodes like _Ag or Bi_. These characteristics imply excellent stability and good safety performance for the intercalation material, and therefore are a preferred choice in commercial batteries [54].