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NMF has been synthesized with several different techniques including solid-state synthesis (Uzun et al., 2013), sol-gel (Karthikeyan et al., 2013, 2012; Kiziltas-Yavuz et al., 2016; Li et al., 2014; Lin et al., 2017; Zhao et al., 2022), molten salt (Zhao et al., 2018), mixed hydroxide (Cheng et al., 2019; Karthikeyan et al., 2013), co-precipitation (Karthikeyan et al., 2013; Liu et al., 2013; Tabuchi et al., 2011; Xi et al., 2021) and reverse micelle synthesis (Penki et al., 2016). Co-precipitation is considered to be the state-of-the-art and thus it is the one, which will be briefly discussed. The method is a two-step process, where firstly the precursor metal hydroxide is formed through a precipitation technique. The precursor is synthesized by dissolving transition metal salts, typically sulfates, and precipitating them with a suitable base such as LiOH or NaOH. The final material is formed by lithiating the sample between 700 °C and 900 °C with LiOH. The method yields particles with a favorable spherical morphology, which improves the electrochemical performance of the material. Additionally, the particles exhibit a lower surface area compared to nanomaterials, which reduced unwanted surface reactions. The main drawback of the synthesis is its considerable complexity. For repeatable results, variables such as temperature, rotation speed, pH, solution concentrations, addition rates, additives and reactions times need to be carefully controlled (Liu et al., 2013).

 Structure

In NMF Ni, Mn and Fe exist in oxidation states of 2, 4 and 3 respectively (Liu et al., 2013). This suggests that the Ni²⁺/Ni⁴⁺  redox pair is the most electrochemically active, being responsible for most of the electrochemical storage. Mn remains mostly unchanged during the charging, which indicates to its role of stabilizing the structure. Similarly, the Fe³⁺/Fe⁴⁺ redox couple is considered mostly electrochemically inactive contributing to the stability of the material during repeated cycles (Karthikeyan et al., 2013). It should be noted that the roles of Mn and Fe are still under research with varying views. The roles of each cation may also be dependent on the stoichiometric ratio of each element, which hinders the formation of a general view on their roles.   

The structure of NMF consists out of repeating transition metal oxide (TMO) layers with lithium intercalated in between them with a stacking sequence of AB CA BC. The material exhibits the structure type of α-NaFeO₂ with a space group of R-3m (166) (Lin et al., 2017; Liu et al., 2013; Xi et al., 2021). The oxygen atoms are packed in a face centered cubic structure and transition metal cations are in the octahedral sites. This structure is often denoted as O3, where “O” refers to octahedral coordination and “3” to the number of lithium layers in a unit cell.

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Figure 2: A typical XRD pattern observed between 2-theta values of 15 and 70 for NMF 811 with the relevant reflections marked. Reflections related to another phase (Li₅FeO₄) can be ignored between 2-theta values of 19 and 25. (Figure by Akseli Rautakorpi)

 Electrochemistry

The performance of batteries is most commonly studied with two methods: cycling and rate capability. Cycling studies the degradation of the batteries performance over repeated cycles. Rate capability studies how much capacity can be delivered under specific discharge rates and its effects on the performance of the battery. Additionally, cycling data can be further studied with differential capacity analysis (dQ/dV), which enables the study of capacity change as a function of voltage. NMF has practically an equal theoretical capacity to NMC. Importantly, several papers have reported high reversible specific capacities during the initial cycles reaching values above 200 mAh/g (Cheng et al., 2019) (Uzun et al., 2013) (Zhao et al., 2018) (Tabuchi, 2011). Thus, NMF can be seen as a promising material for the transition to Co-free battery chemistries.

In NMF Ni, Mn and Fe exist in oxidation states of 2, 4 and 3 respectively (Liu et al., 2013). This suggests that the Ni²⁺/Ni⁴⁺  redox pair is the most electrochemically active, being responsible for most of the electrochemical storage. Mn remains mostly unchanged during the charging, which indicates to its role of stabilizing the structure. Similarly, the Fe³⁺/Fe⁴⁺ redox couple is considered mostly electrochemically inactive contributing to the stability of the material during repeated cycles (Karthikeyan et al., 2013). It should be noted that the roles of Mn and Fe are still under research with varying views. The roles of each cation may also be dependent on the stoichiometric ratio of each element, which hinders the formation of a general view on their roles. 

Interestingly, Xi et al. (Xi et al., 2021) studied the capacity of NMF 811 as a function of cell voltage and found that the material stores more energy at higher voltages (> 4.1 V), while storing less energy at lower potentials (< 3.9 V) when compared to NMC. This would suggest that the material reaches its full potential only at higher voltages. Thus, increasing the cut-off potential from the common 4.6 V to 4.8 V or higher could yield in a notable capacity increase. Additionally, as higher potentials are used more power can be extracted from the battery. 

Over 1 stoichiometric ratio of Li

stability, optimization the most important things to improve. coatings, doping, etc.

references

Reference style will be changed/updated. One reference included just as an example and to see the reference list.

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