A lithium ion manganese oxide battery (LMO) is a lithium-ion cell that uses manganese dioxide ( MnO
2), as the cathode material. They function through the same intercalation/de-intercalation mechanism as other commercialized secondary battery technologies, such as lithium cobalt oxide ( LiCoO
2). Cathodes based on manganese-oxide components are earth-abundant, inexpensive, non-toxic, and provide better thermal stability. [1]
One of the more studied manganese oxide-based cathodes is LiMn
2O
4, a cation ordered member of the spinel structural family (space group Fd3m). In addition to containing inexpensive materials, the three-dimensional structure of LiMn
2O
4 lends itself to high rate capability by providing a well connected framework for the insertion and de-insertion of Li+
ions during discharge and charge of the battery. In particular, the Li+
ions occupy the tetrahedral sites within the Mn
2O
4 polyhedral frameworks adjacent to empty octahedral sites. [2] [3] As a consequence of this structural arrangement, batteries based on LiMn
2O
4 cathodes have demonstrated a higher rate-capability compared to materials with two-dimensional frameworks for Li+
diffusion. [4]
LiMn2O4→Li1−xMn2O4+xLi++xe−
| Theoretical capacity (mAh/g) | 148 |
| Practical Capacity (mAh/g) | 110 |
| Operating Voltage (V) | 4 |
| Galvanometric energy density (Wh kg−1) | 410 |
| Volumetric energy density (Wh/L) | 1300 |
| Cost | Low |
| Advantage | Cheap and good performance |
| Disadvantage | Poor cycling stability and performance at high temperature |
The voltage, capacity, and current density that are practically reached in real batteries are significantly impacted by the contact potential and kinetic effects. Kinetic variables, which frequently arise at the contact and continuously change over time as a result of degradation events, thus govern the electrochemical performance. Critical issues are presented by these events, especially with regard to high voltage stability.
Applying a high voltage to a spinel-structured cathode may induce partial spinel-to-layered transformation on the surface region. The distortion of the surface structure can be extended to the bulk level. [6] [7]
Since layered structures and spinel share an oxygen sub-lattice of the cubic close-packed structure, the evolution of layered structure from spinel only necessitates the migration of Mn from 16d octahedral interstices to another, passing through the tetrahedral interim site without the need for oxygen reordering.
Face-sharing cations have a strong repulsion and a little separation, making this an adverse energy process that is unlikely to occur unless another event facilitates it. Mn can only migrate across these tetrahedral interstices if a tiny quantity of oxygen is removed. [8]
Mn4+ ions are reduced to Mn3+ ions or even lower as a result of oxygen non-stoichiometry because of the required charge adjustment. Mn3+ and Mn4+ ions are dispersed throughout half of the octahedral site in LiMn2O4. In the spinel structure, the Mn3+ ions have demonstrated a predilection for tetrahedral 8a sites with high mobility. [9]
Because of their strong nucleophilic and Lewis's base characteristics, cathode materials can interact with oxygen atoms in the electrolyte to cause redox reactions. The electrolyte will be reduced on the anode surface and oxidized on the cathode surface once the electrode potential exceeds the typical electrolyte stability window. This will result in the formation of solid electrolyte interphase (SEI) and cathode-electrolyte interface (CEI) layers, respectively. By passivating the electrode and blocking electron transport, the CEI layer would stabilize the electrode and electrolyte. [5]
It is commonly known that oxygen is necessary for the formation of the CEI layer and electrolyte breakdown. At high voltage, the electrolytes containing LiPF6 salts become unstable. They can create a highly concentrated acidic species (HF) when they combine with a small amount of water, which acts as an oxygen source. [10] By causing a disproportionate reaction, the HF damages the electrode materials' surface, which is the main reason why Mn dissolves. The dissolved Mn2+ may either settle on the anode surface and spontaneously reduce to metallic Mn, or it could mix with F− to create MnF2. [11]
Stress is produced in the LiB system by the anisotropic lattice expansion and contraction along with the repetitive Li-insertion and extraction. Stress can cause cracking, which can start at the grain boundaries and spread until the particles are completely ground up.
LiMn2O4 cathode materials suffer ca. 8% lattice volume expansion and contraction for each charge and discharge cycle between LiMn2O4 and λ-MnO2. [12] [13]
The orientation of Mn–O bonding sublattices regulates the production of LiMn2O4 cracks, according to the first principles-based study. [14] Because of Mn oxidation and the lower radius of Mn4+ ions compared to Mn3+ ions, the delithiation state of LiMn2O4 encourages strain-driven Mn–O bond compression. These surface bonds are unable to completely shorten to the equilibrium bond lengths due to the fixed lattice parameters of the LiMn2O4 host structure. Tensile surface tension is thus the result of these conflicting pressures acting on the near-surface Mn–O bond. The production of surface cracks that spread into bulk LiMn2O4 may be initiated by these high tensile surface stresses. [15]
A workable method to prevent Jahn-Teller distortion, stabilize the crystal lattice, and improve LiMn2O4's electrochemical performance is to dope or partially substitute pure LiMn2O4 with other elements. A number of doping elements, including Al, Ni, Fe, Mg, Si, and B, have been shown to be effective.
For instance, the Al3+ occupation in sites 8a and 16d prevented Mn2+ ions from moving to dissolve in the electrolyte and reduced the likelihood of Mn disproportion. Modification of Al3+ ions also decreased the electrolyte breakdown products that were deposited on LiMn2O4's surface. The surface may be significantly shielded from HF assaults by the Al3+ ions. [16]
It is a controllable and effective way of preserving the CEI layer and reducing the side reactions. The presence of the metal oxide coatings can minimize the direct contact area of the LiMn2O4/electrolyte interface and suppress the dissolution of Mn3+ ions. [17]
The coating materials comprise principally various metal fluoride, metal oxide, carbon, lithium-ion conductor, and polymer. Among them, TiO2 was claimed to have channels that could be utilized to store small Li+. Spinel LiMn2O4 surface modification may further limit Mn dissolution over cycles and postpone electrode-electrolyte side reactions.
The incoherent of lattice contraction and expansion between coating layer and core structure would lead to the formation of voids and cracks during repeated cycle at high cut-off voltage. Therefore, designing concentration gradient structure without any apparent gap between the shell and core structure has been proposed to enhance the stability. [5]
Li
2MnO
3 is a lithium rich layered rocksalt structure that is made of alternating layers of lithium ions and lithium and manganese ions in a 1:2 ratio, similar to the layered structure of LiCoO
2. In the nomenclature of layered compounds it can be written Li(Li0.33Mn0.67)O2. [18] Although Li
2MnO
3 is electrochemically inactive, it can be charged to a high potential (4.5 V v.s Li0) in order to undergo lithiation/de-lithiation or delithiated using an acid leaching process followed by mild heat treatment. [19] [20] However, extracting lithium from Li
2MnO
3 at such a high potential can also be charge compensated by loss of oxygen from the electrode surface which leads to poor cycling stability. [21] New allotropes of Li
2MnO
3 have been discovered which have better structural stability against oxygen release (longer cycle-life). [22]
The layered manganese oxide LiMnO
2 is constructed from corrugated layers of manganese/oxide octahedra and is electrochemically unstable. The distortions and deviation from truly planar metal oxide layers are a manifestation of the electronic configuration of the Mn(III) Jahn-Teller ion. [23] A layered variant, isostructural with LiCoO2, was prepared in 1996 by ion exchange from the layered compound NaMnO2, [24] however long term cycling and the defect nature of the charged compound led to structural degradation and cation equilibration to other phases.
The layered manganese oxide Li
2MnO
2 is structurally related to Li
2MnO
3 and LiCoO2 with similar transition metal oxide layers separated by a layer containing two lithium cations occupying the available two tetrahedral sites in the lattice rather the one octahedral site. The material is typically made by low voltage lithiation of the parent compound, direct lithiation using liquid ammonia, or via use of an organic lithiating reagent. [25] Stability on cycling has been demonstrated in symmetric cells although due to Mn(II) formation and dissolution cycling degradation is expected. Stabilization of the structure using dopants and substitutions to decrease the amount of reduced manganese cations has been a successful route to extending the cycle life of these lithium rich reduced phases. These layered manganese oxide layers are so rich in lithium.
One of the main research efforts in the field of lithium-manganese oxide electrodes for lithium-ion batteries involves developing composite electrodes using structurally integrated layered Li
2MnO
3, layered LiMnO2, and spinel LiMn
2O
4, with a chemical formula of xLi
2MnO
3 • yLi
1+aMn
2-aO
4 • z LiMnO2, where x+y+z=1. The combination of these structures provides increased structural stability during electrochemical cycling while achieving higher capacity and rate-capability. A rechargeable capacity in excess of 250 mAh/g was reported in 2005 using this material, which has nearly twice the capacity of current commercialized rechargeable batteries of the same dimensions. [26] [27]