Introduction

Lithium-ion manganese oxide (LMO) battery belongs to the most popular rechargeable lithium electrode technology initiated in the early 1980s and employed commercially after 15 years^[1]. LMO battery adapts LiMn₂O₄ as cathode material. Due to the specific three-dimensional Mn₂O₄ spinel framework, this variety of lithium batteries enables rapid Li-ion diffusion and thus decreases the resistance of ion flow between electrodes. Low inner resistance enables fast charging and discharging of a battery and advances the ability to transfer a higher density of power in a short time. Therefore, the LMO battery provides a nominal voltage of 4.0 V. The constant development of the LMO (LiMn₂O₄) electrode structure has provided a long life span, high-density power, thermal stability, and safety^[2]. Moreover, LiMn₂O₄ has been proven non-toxic and environmentally friendly in comparison to LiCoO₂ for instance. Considering these properties, LMO batteries have been adapted to a wide range of applications, including electric vehicles, power tools, medical devices and consumer electronics^[2]. 

Battery structure and operation

The architecture of a typical lithium-ion battery includes two current collectors and two electrodes; a positive cathode and a negative anode that are separated by a microporous separator. ^[3][4]. The construction is filled up with an acidic electrolyte solvent^[4]. The anode and cathode are responsible for storing lithium, while the electrolyte facilitates the movement of positively charged lithium ions from the anode to the cathode and back again in reverse through the separator. The separator serves to prevent the flow of electrons inside the battery. The constant movement of lithium ions generates free electrons in the anode, which in turn creates a positive charge at the positive current collector. The electrical current flows from the current collector through the device being powered.^[1][5].

The charging-discharging cycle is caused by the added potential difference between electrodes leading to the movement of lithium ions and hence electrochemical reaction in electrodes^[1]. While discharging, Li-ions transfer from the anode to the cathode and during charging, lithium ions move opposite; from the cathode to the anode. This movement results in an oxidation-reduction reaction in electrodes.^[5]

During discharge oxidation reaction of lithium in the lithium-graphite anode can be described as follows^[6]:

C₆Li → 6C (graphite) + Li⁺ + e^-

And respectively reduction reaction of manganese in LMO cathode:

Li_x-1Mn₂O₄ (s) + xLi+ + xe- → LiMn₂O₄ (s),

where manganese undergoes reduction from Mn⁴⁺-ion to Mn³⁺.

During charging, the electrodes undergo the same reactions but in reverse.

LMO cathode materials

LiMn₂O₄ spinel

LiMn₂O₄ spinel structure belongs to the cubic Fd-3m space group, where diffused lithium-ions are surrounded by the Mn₂O₄ spinel framework with an equal amount of Mn³⁺ and Mn⁴⁺ -ions. The LMO unit cell structure is presented in Figure 2 and the octahedral coordination of Mn-ions shown in Figure 3. Li ions are tetrahedrally coordinated.^[7]

Figure 2. The crystal structure of LiMn₂O₄ illustrated along different directions a) [100], b) [111] and c) [110]. The lithium atoms are presented as green, oxygen as red and manganese as purple. (Figure: Ariana Voitsekhovski)^[8]

Figure 3. Octahedral coordination of Mn ions created. (Figure: Ariana Voitsekhovski) ^[8]

The addition of Li-ions to the manganese oxide framework determines Mn³⁺ and Mn⁴⁺ concentrations in the crystal structure as Li⁺ ions reduce Mn⁴⁺ ions to Mn³⁺ science the overall charge stays neutral during charging. The radius of the Mn³⁺-ion is larger than the radius of Mn⁴⁺, thus leading to a larger cell volume. If Mn3+ concentration exceeds 50%, the LiMn₂O₄ spinel shows Jahn–Teller distortion, where octahedral Mn-O structure stretches among c-axis^[7]. During oxidation-reduction reactions of Mn^4+/3+ -cations take place, the spinel structure can transform between cubic and octahedral LiMn₂O4 leading to the degradation process.^[6]

The J-T effect can add structural instability by affecting the octahedral Mn-O structure and thus providing degradation of the spinel cathode. However, the J-T distortion depends on  Mn³⁺-ions and therefore can be minimized by reducing Mn³⁺ -cations' high-spin state. To proceed with the avoidance of high-spin Mn³⁺-cations and cathode degradation, the spinel structure is usually modified with the addition of nickel cations Ni²⁺.^[6]

Layered LMO 

A layered LMO structure has been employed as one of the solutions for more stable structures and improved performance. Mainly layered lithium-ion manganese oxide structures differ from LiMn₂O₄ spinel by obtaining different and layered crystal structures. Layered LMOs include, for instance, LiMnO₂, Li₂MnO₂ and Li₂MnO₃ structure forms which can be adapted separately or combined. The combination of different lithium-ion manganese oxide structures has shown improvement in performance.^[9][10]Li₂MnO₃ crystallizes in the monoclinic crystal system (C2/m) whereas LiMnO₂ is orthorhombic (Pmnm). Li₂MnO₂ crystallizes in a trigonal crystal system (P-3m1). Of these named LMO structures, Li₂MnO₃ provides poor electrical cycle stability due to the oxygen gas release at higher voltages. Additionally, the manganese ion in Li₂MnO₃ has an oxidation state of +IV and thus Li₂MnO₃ is usually adapted as a precursor instead of a cathode.^[10]

As Li₂MnO₄, LiMnO₂ (Figure 4) experiences J-T distortion due to the structure destabilization caused by Mn³⁺-cations and performs electrochemically unstable. Here, the J-T effect can be decreased with a combination of layered and spinel LMO which increase structural stability^[11]

Figure 4. The layer-type structure of LiMnO₂.^[12](Figure: Ariana Voitsekhovski)

LMO Anode materials

The anode of a battery should advance inert nature and stability regarding surface as it contacts with electrolyte and lithium cations but at the same time provide sufficient lithium ion intercalation capacity. Today, researchers have explored several anode materials, such as graphitic carbon, amorphous carbon, nitrides, tin oxides, and tin-based alloys. Nevertheless, graphitic carbon has remained the primary commercial anode material for lithium batteries as the nature of graphitic carbon enables a light passivation of inserted Li-cations providing the safety of a battery by decreasing the flammability. However, this aspect decreases the energy density as well and hence, carbon-based anodes develop weaker lithium ion intercalation capacity in comparison to lithium ion alloys^[13].  Fortunately, recent technological advancements have brought new perspectives on the modification and preparation of carbon anode materials. ^[14]

Graphite-based anodes include sheets of hexagonal graphite or rhombohedral graphite which reorganize on top of each other during charging^[13]. The surface properties of graphite still requires more examination but studies have shown that rhombohedral graphite increases endurance of the graphitic anode^[15]. 

As an alternative for graphitic carbon, lithium tin oxide has contributed excellent qualities regarding stability and lack of expansion during the charging-discharging cycle. Although, the use of this type of anode is compatible only with applications that manage with lower energy density.^[13]

Electrolytes

The electrolyte allows lithium ions to flow between the cathode and the anode, enabling the battery to generate an electrical current through reversible Li⁺-ion movement. Due to this connection between electrodes and the cathode-electrode interface, the design of the electrolyte significantly affects the performance of a high-voltage battery. The applied high voltage at the cathode results in various degradation reactions that develop at the cathode-electrolyte interface. The interface should perform electronically insulating but enable conductive Li⁺-ion transportation at the same time. Hence, usually electrolyte for lithium battery consists of lithium salt dissolved into an organic solvent with the possible addition of organic additives. The common lithium salt-based electrolyte includes lithium hexafluorophosphate (LiPF₆) dissolved into a solvent mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and tetrafluoroethyl trifluoroethyl ether (TFE).^[16]

Due to the contact of the electrolyte with the cathode material, the cathode surface undergoes electrolyte decomposition. To prolong the cycle life of a battery and prevent the cathode from corrosion and destructive reactions, such as polymerization, acid-base reactions, and nucleophilic reactions, for instance, the selection of electrolyte for lithium cathode should be based on the redox potential of both electrodes and electrolyte.^[16]

References